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University of the Pacific Theses and Dissertations Graduate School

2018 Bimetallic complexes: The fundamental aspects of metal-metal interactions, ligand sterics and application Michael Bernard Pastor University of the Pacific, [email protected]

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BIMETALLIC COMPLEXES: THE FUNDAMENTAL ASPECTS OF METAL- METAL INTERACTIONS, LIGAND STERICS AND APPLICATION

by

Michael B. Pastor

A Dissertation Submitted to the

In Partial Fulfillment of the

Requirements for the Degree of

DOCTOR OF PHILOSOPHY

Department of Chemistry Pharmaceutical and Chemical Sciences

University of the Pacific Stockton, CA

2018

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BIMETALLIC COMPLEXES: THE FUNDAMENTAL ASPECTS OF METAL- METAL INTERACTIONS, LIGAND STERICS AND APPLICATION

by

Michael B. Pastor

APPROVED BY:

Dissertation Advisor: Qinliang Zhao, Ph.D

Committee Member: Anthony Dutoi, Ph.D.

Committee Member: Xin Guo, Ph.D.

Committee Member: Jianhua Ren, Ph.D.

Committee Member: Balint Sztáray, Ph.D.

Department Chairs: Jianhua Ren, Ph.D.; Jerry Tsai, PhD.

Dean of Graduate Studies: Thomas Naehr, Ph.D.

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BIMETALLIC COMPLEXES: THE FUNDAMENTAL ASPECTS OF METAL- METAL INTERACTIONS, LIGAND STERICS AND APPLICATION

Copyright 2018

by

Michael B. Pastor

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ACKNOWLEDGMENTS

My sincere gratitude goes out to Qinliang Zhao who welcomed me into her laboratory and allowed me freedom in the direction of the research. She is a strong woman, mother, professor, and scientist. Thank you for believing in me, pushing me to strive for more, and supporting me during my graduate studies.

Thank you to my committee members for their helpful suggestions regarding my dissertation but also for aiding me in my coursework and research along the way.

I would like to thank Professor O. David Sparkman for teaching such a rigorous course and providing advice that I will never forget. Thank you to the Pacific Mass

Spectrometry Facility, Patrick Batoon, Sven Hackbusch, and Krege Christison for helping me to make the transition into mass spectrometry and for your guidance and assistance throughout the years.

I would also like to thank my manager Andre Randall of Agilent Technologies and all of the wonderful Agilent employees for allowing me to work part time in Santa Clara as I completed my dissertation.

Lastly, I would like to thank my loving girlfriend Melissa, parents, family, and bandmates for their undying support. The support system I have outside of the lab has proven to be invaluable and key to my success.

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BIMETALLIC COMPLEXES: THE FUNDAMENTAL ASPECTS OF METAL- METAL INTERACTIONS, LIGAND STERICS AND APPLICATION

Abstract

by Michael B. Pastor

University of the Pacific 2018

Metal containing complexes have been used to catalyze various organic transformations for the past few decades. The success of several mononuclear catalysts led to transition metal catalysts used in pharmaceuticals, environmental, and industrial processes. While mononuclear complexes have been used extensively, bimetallic systems have received far less attention. Bimetallic or polynuclear sites are commonly found in metalloenzymes that perform elegant transformation in biological systems, underlying their significance. Inorganic chemists take inspiration from nature and design model bimetallic complexes to further study this cooperativity effect. A bimetallic platform offers many structural and functional differences such as the identity of the metal atoms and the bonding interactions between metals, which have been reflected in their unique catalytic ability and reactivity.

This dissertation encompasses work related to the computational study of metal- metal interactions of bimetallic systems, the 1H NMR study of stereochemical and conformational changes in solution of N,Nʹ-diarylformamidines, the synthesis of dizinc 6 formamidinate complexes, and the synthesis and catalytic ability of dicopper formamidinate complexes.

In the first part, DFT calculations are used to study factors that influence metal- metal bond lengths in various complexes. Several experimentally obtained X-ray crystal structures were used as the basis for the study. Differences in metal-metal separations were investigated through various functionals, indicating the importance of charge, orbital interactions, and formal bond order. BH&HLYP SDD/aug-CC-PVDZ geometry

n- 2- optimizations of octahalodimetalate anions Tc2X8 (X = Cl, Br; n=2, 3), Re2X8 (X = Cl,

4- Br), and Mo2Cl8 reproduced M-M bond distance trends observed experimentally. The study demonstrated that the increase in σ and π bond strength resulted in the shortening in

2- 3- Tc-Tc bond distance from Tc2X8 to Tc2X8 , which was further supported by the short

4- Mo-Mo bond in the Mo2Cl8 ion. This study was expanded further through the inclusion

n+ n- of [M2Cl4(PMe3)4] (M = Tc, Re, n = 0-2) and [Mo2E4] (E = HPO4 or SO4, n = 2-4), allowing a systematic study on the role of charge on the metal atoms. PBEO SDD/aug-

CC-PVDZ calculations revealed that both formal bond order and formal charge on the metal atoms dictate the trends in M-M bond strength.

The second half of this dissertation focuses on the synthesis and characterization of bimetallic Zn- and Cu-formamidinate complexes. The stereochemical exchange of substituted N,Nʹ-diarylformamidines were studied through 1H NMR in various solvents.

Alkyl substituents placed on the ortho positions were found to shift the isomeric equilibrium in solution through destabilization of the hydrogen-bond dimer evident in X- ray crystal structures. The Z-isomer of substituted N,Nʹ-diarylformamidines is observed in CDCl3, C6D6, and DMSO-d6 when the ligands feature significant steric hinderance.

Similar ortho substituted N,Nʹ-diarylformamidines were also used to enforce steric 7 interactions to limit the nuclearity of Zn-formamidinate complexes. Various dizinc formamidinate complexes were synthesized through direct and transmetalation routes.

NMR and mass spectrometry were used alongside X-ray crystal structures to fully characterize the dizinc complexes. Dicopper formamidinates formed through a transmetallation route were synthesized and feature distinct short Cu…Cu separations thought to be brought about by metalophillic interactions. Preliminary results suggest catalytic ability of dicopper formamidinates in cyclopropanation and aziridination of styrene with various diazo compounds. The catalytic activity suggests the formation of dicopper carbene and nitrene intermediates, of which only few published experimentally observed examples exist in the literature.

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TABLE OF CONTENTS

LIST OF TABLES ...... 10

LIST OF FIGURES ...... 12

LIST OF ABBREVIATIONS ...... 17

The Utility of Bimetallic Complexes in Nature, Metal-Metal Bonded Structures, and Catalysis ...... 19

Metal Cooperativity in Nature ...... 24

Metal-Metal Bonding and Interactions ...... 27

Bimetallic Catalysis ...... 30

DFT Investigation of Multiply Bonded Bimetallic Systems ...... 35

Introduction ...... 35

Experimental ...... 40

Results and Discussion ...... 41

Conclusions ...... 54

Probing Factors Influencing the Metal-Metal Bond Strength in Bimetallic

Systems ...... 55

Introduction ...... 55

Experimental ...... 58

Results and Discussion ...... 59

Conclusions ...... 67 9

Steric-Induced Stereochemical Exchange of N,Nʹ-Diarylformamidines in

Solution ...... 69

Introduction ...... 69

Experimental ...... 71

Results and Discussion ...... 75

Conclusions ...... 86

Fine-tuning the Geometry of Dizinc Complexes through Ligand Sterics ...... 87

Introduction ...... 87

Experimental ...... 89

Results and Discussion ...... 96

Conclusions ...... 103

Weakly Interacting Dicopper Formamidinate Complexes ...... 104

Introduction ...... 104

Experimental ...... 109

Results and Discussion ...... 114

Conclusions ...... 123

REFERENCES ...... 125

APPENDIX A. COMPUTATIONAL DATA ...... 162

APPENDIX B. NMR DATA ...... 184

APPENDIX C. MASS SPECTRAL DATA ...... 217

APPENDIX D. X-RAY DATA ...... 239

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LIST OF TABLES

Table Page

TABLE 1.1 COMPOUND NUMBER, NAME, AND CHAPTER IN WHICH IT IS DISCUSSED IN THIS

DISSERTATION...... 34

TABLE 2.1 ELECTRONIC CONFIGURATION OF DIMETAL UNITS (EC), FORMAL BOND ORDER

(FBO), FORMAL CHARGE OF EACH METAL ATOM (FC), SELECTED BOND DISTANCES (Å)

AND ANGLES (º) OF MULTIPLY BONDED BIMETALLIC COMPOUNDS...... 40

TABLE 2.2 EXPERIMENTAL (EXP) AND CALCULATED TC-TC BOND DISTANCES (Å) FOR THE

N- TC2X8 ANIONS (X = CL, BR; N=2, 3), DERIVED FROM CALCULATIONS USING DIFFERENT

FUNCTIONALS.A ...... 46

TABLE 2.3 EXPERIMENTAL (EXP) AND CALCULATED (CAL) TC-TC AND TC-X BOND DISTANCES,

FORMAL CHARGE ON EACH METAL ATOM (FC), FORMAL BOND ORDER (FBO), NATURAL

N- BOND ORDER (NBO), AND EFFECTIVE BOND ORDER (EBO) FOR THE M2X8 ANIONS (M =

TC, RE, MO; X = CL, BR; N=2-4), DERIVED FROM CALCULATIONS USING BH&HLYP

FUNCTIONAL AND BASIS SETS SDD/AUG-CC-PVDZ...... 49

TABLE 3.1 ELECTRONIC CONFIGURATION OF DIMETALLIC UNITS (EC), FORMAL BOND ORDER

(FBO), FORMAL CHARGE OF EACH METAL ATOM (FC), SELECTED BOND LENGTHS (Å) AND

ANGLES (º) OF MULTIPLY BONDED BIMETALLIC COMPOUNDS...... 57

TABLE 3.2 EXPERIMENTAL (EXP) AND CALCULATED (CAL) M-M AND M-X BOND LENGTHS, AND

FORMAL CHARGE ON EACH METAL ATOM (FC). IN THE DIMETALLIC UNITS: ELECTRONIC

CONFIGURATION (EC), FORMAL BOND ORDER (FBO), CALCULATED NATURAL BOND ORDER 11

(NBO), AND CALCULATED EFFECTIVE BOND ORDER (EBO) FOR MODEL COMPOUNDS 1-8.

DERIVED FROM CALCULATIONS USING PBE0 FUNCTIONAL...... 67

TABLE 4.1 CRYSTALLOGRAPHIC DATA FOR COMPOUNDS 9-12...... 85

TABLE 4.2 SELECTED BOND LENGTHS (Å), BOND ANGLES (°) AND TORSION ANGLES (°)...... 86

TABLE 5.1 SELECTED BOND LENGTHS (Å) AND ANGLES (º) IN COMPLEXES 13, 14, AND 15 ...... 95

TABLE 5.2 SELECTED BOND LENGTHS (Å) AND ANGLES (º) IN COMPLEXES 16 AND 17 ...... 96

12

LIST OF FIGURES

Figure Page

FIGURE 1.1 GENERAL CATALYTIC CYCLE FOR THE CONVERSION OF A AND B INTO PRODUCT C.

BOUND SUBSTRATE SPECIFIED IN PARENTHESES...... 20

FIGURE 1.2 SCHEMATIC DIAGRAM FOR THE HABER-BOSCH PROCESS...... 21

FIGURE 1.3 CATALYTIC CYCLE FOR CROSS METATHESIS (LEFT) AND RELEVANT MOLYBDENUM

AND RUTHENIUM CARBENE CATALYSTS (RIGHT)...... 22

FIGURE 1.4 GENERAL MECHANISM OF THE NEGISHI AND SUZUKI PALLADIUM-CATALYZED

CROSS-COUPLING REACTIONS...... 23

FIGURE 1.5. BIMETALLIC ENZYMES IN BIOLOGICAL SYSTEMS: (A) NI/FE AND MO/CU SITES OF

CARBON MONOXIDE DEHYDROGENASE ENZYMES (B) ACTIVE SITE OF CCO FROM THE BOVINE

HEART...... 25

2- FIGURE 1.6 FAMOUS METAL-METAL BONDED COMPLEXES. FROM LEFT TO RIGHT: RE2CL8 ,

RH2(OAC)4, AND MO2(OAC)4...... 28

FIGURE 1.7. MIXED DONOR LIGANDS FOR SUPPORTING METAL-METAL BONDS DEVELOPED BY

BERRY, THOMAS, AND LU...... 29

FIGURE 1.8 LEFT: DIRHODIUM TETRACETATE DIMER WITH AXIAL COORDINATION SITE SHOWN.

RIGHT: PROPOSED REACTION INTERMEDIATES IN CARBENE AND NITRENE TRANSFER

REACTIONS FEATURING AXIAL COORDINATION TO A SINGLE RH CENTER...... 31

FIGURE 1.9 STRUCTURAL AND FUNCTIONAL DIFFERENCES IN THE DESIGN OF MONONUCLEAR

AND BIMETALLIC CATALYSTS...... 32 13

2– FIGURE 2.1 RE2CL8 AND ORBITAL INTERACTIONS THAT MAKE UP THE QUADRUPLE BOND...... 35

FIGURE 2.2 SYNTHESIZED M2X8 ANIONS WITH ECLIPSED AND STAGGERED GEOMETRIES...... 37

FIGURE 2.3 A REPRESENTATION OF METAL D-BASED ORBITALS IN THE MOLECULAR ORBITAL

DIAGRAM FOR OCTAHALODIMETALATE SPECIES AT AN ECLIPSED GEOMETRY (TOP) AND AT A

STAGGERED GEOMETRY (BOTTOM)...... 38

FIGURE 2.4 A SIMPLE ILLUSTRATION OF CHARGE EFFECT ON ORBITAL OVERLAP. BONDING

ATOMS WITH HIGHER CHARGE ON THE LEFT (III) AND LOWER CHARGE ON THE RIGHT (IV). 44

2- FIGURE 2.5 SELECTED MO BONDING ORBITALS WITH 0.05 CONTOUR SURFACE FOR THE TC2CL8

ANION, DERIVED FROM CALCULATIONS USING THE FUNCTIONAL BH&HLYP. POSITIVE AND

NEGATIVE VALUES ARE REPRESENTED AS YELLOW AND RED SURFACES, RESPECTIVELY.

VALUES UNDERNEATH EACH MO FIGURE IN THE LAST ROW ARE GIVEN AS THE PERCENT OF

ELECTRON DENSITY...... 47

FIGURE 2.6 GRAPHICAL REPRESENTATION OF SPIN DENSITY WITH 0.006 CONTOUR SURFACE FOR

3- 3- THE TC2CL8 (LEFT) AND TC2BR8 (RIGHT) ANIONS...... 48

FIGURE 2.7 THE ORBITALS BETWEEN THE TC IONS WITH 0.05 CONTOUR SURFACE FOR THE

2- TC2BR8 ANION, DERIVED FROM NBO ANALYSES USING THE FUNCTIONAL BH&HLYP.

POSITIVE AND NEGATIVE VALUES ARE REPRESENTED AS BLUE AND GREEN SURFACES,

RESPECTIVELY. THE ORBITAL TYPE BETWEEN THE TWO TC ATOMS AND THEIR ELECTRON

OCCUPATION NUMBERS IN PARENTHESES ARE PROVIDED BELOW EACH ORBITAL...... 50

FIGURE 2.8 THE ORBITALS BETWEEN THE TC IONS WITH 0.05 CONTOUR SURFACE FOR THE

3- TC2BR8 ANION, DERIVED FROM NBO ANALYSES USING THE FUNCTIONAL BH&HLYP.

POSITIVE AND NEGATIVE VALUES ARE REPRESENTED AS BLUE AND GREEN SURFACES,

RESPECTIVELY. THE ORBITAL TYPE BETWEEN THE TWO TC ATOMS AND THEIR ELECTRON

OCCUPATION NUMBERS IN PARENTHESES ARE PROVIDED BELOW EACH ORBITAL...... 52

2- FIGURE 3.1 ILLUSTRATIONS OF THE RE2CL8 ANION WITH AN ECLIPSED GEOMETRY (I) AND D-D

ORBITAL OVERLAP (II), AND A REPRESENTATION OF ITS METAL D-BASED ORBITALS IN THE 14

MOLECULAR ORBITAL DIAGRAM (III). ORBITALS ORDER IN ENERGY OFTEN CHANGES FROM

THOSE SHOWN IN THE DIAGRAM BECAUSE OF THEIR SIMILARITY IN ENERGY...... 56

FIGURE 3.2 MODEL COMPOUNDS SYMBOLIZED AS 1-8 USED IN DFT CALCULATIONS...... 62

FIGURE 3.3 SELECTED BONDING MOS WITH 0.05 CONTOUR SURFACE FOR TC2CL4(PME3)4 (1)

DERIVED FROM CALCULATIONS USING THE FUNCTIONAL PBE0. POSITIVE AND NEGATIVE

VALUES ARE REPRESENTED AS RED AND YELLOW SURFACES, RESPECTIVELY. VALUES

UNDERNEATH EACH MO FIGURE IN THE LAST ROW ARE GIVEN AS A PERCENTAGE OF THE

ELECTRON DENSITY DIVIDED AMONG THE TC ATOMS, PME3 GROUPS AND FOUR CL ATOMS,

RESPECTIVELY...... 62

FIGURE 3.4 GRAPHICAL REPRESENTATION OF SPIN DENSITY WITH 0.006 CONTOUR SURFACE FOR

+ + THE PARAMAGNETIC MODEL COMPOUNDS [TC2CL4(PME3)4] (2), [RE2CL4(PME3)4] (4) AND

3- [MO2 (SO4)4] (7) USING BH&HLYP FUNCTIONAL...... 64

FIGURE 3.5 THE ORBITALS BETWEEN THE TC CENTERS WITH 0.05 CONTOUR SURFACE FOR THE

TC2CL4(PME3)4 (1) DERIVED FROM NBO ANALYSES USING THE FUNCTIONAL PBE0.

POSITIVE AND NEGATIVE VALUES ARE REPRESENTED AS BLUE AND GREEN SURFACES,

RESPECTIVELY. THE ORBITAL TYPE BETWEEN THE TWO TC ATOMS AND THEIR ELECTRON

OCCUPATION NUMBERS IN PARENTHESES ARE PROVIDED BELOW EACH ORBITAL...... 66

FIGURE 4.1 N,Nʹ-DIARYLFORMAMIDINE LIGANDS IN THIS STUDY. NAME AND COMPOUND

NUMBERING ARE SPECIFIED BELOW EACH STRUCTURE...... 70

FIGURE 4.2 SYNTHESIS OF VARIOUS N,Nʹ-DIARYLFORMAMIDINES FROM TRIETHYL

ORTHOFORMATE AND SUBSTITUTED ANILINES...... 72

FIGURE 4.3 DIARYLFORMAMDINE ISOMERS IN THIS STUDY. CONFORMERS ARE DENOTED BY THE

CURVED ARROWS...... 76

1 FIGURE 4.4 LEFT: STACKED H NMR SPECTRA (AROMATIC REGION) OF 11 IN DMSO-D6, C6D6,

CDCL3, AND CDCL3 + 1.0 EQ. ACOH (BOTTOM TO TOP). Z ISOMER SIGNALS ARE DENOTED

WITH RED BARS. RIGHT: SPECIES PRESENT IN SOLUTION...... 77 15

1 FIGURE 4.5 LEFT: STACKED H NMR SPECTRA (AROMATIC REGION) OF 12 IN DMSO-D6, C6D6,

CDCL3, AND CDCL3 + 1.0 EQ. ACOH (BOTTOM TO TOP). Z ISOMER SIGNALS ARE DENOTED

WITH RED BARS. RIGHT: SPECIES PRESENT IN SOLUTION...... 79

1 FIGURE 4.6 LEFT: STACKED H NMR SPECTRA (AROMATIC REGION) OF 9 IN DMSO-D6, C6D6,

AND CDCL3, (BOTTOM TO TOP). Z ISOMER SIGNALS ARE DENOTED WITH RED BARS. RIGHT:

SPECIES PRESENT IN SOLUTION...... 81

FIGURE 4.7 HYDROGEN BOND DIMER FORMATION OF COMPOUND 9. ELLIPSOIDS ARE SHOWN AT

THE 50% PROBABILITY LEVEL...... 82

FIGURE 4.8 HYDROGEN BOND DIMER FORMATION OF ACIDIFIED COMPOUNDS 9H (LEFT) AND 11H

(RIGHT). ELLIPSOIDS ARE SHOWN AT THE 50% PROBABILITY LEVEL...... 83

FIGURE 4.9 HYDROGEN BOND DIMER FORMATION OF COMPOUNDS 10 (LEFT), 11 (MIDDLE), AND

12 (RIGHT). ELLIPSOIDS ARE SHOWN AT THE 50% PROBABILITY LEVEL...... 84

FIGURE 5.1 DIRECT METALATION OF N,N'-DIARYLFORMAMIDINES WITH DIETHYL ZINC TO YIELD

DIZINC-FORMAMIDINATES (13-15)...... 90

FIGURE 5.2 TRANSMETALLATION OF N,N'-DIARYLFORMAMIDINES WITH LIN(TMS)2 AND ZNCL2

TO YIELD DIZINC-FORMAMIDINATES (16-17)...... 90

FIGURE 5.3 X-RAY CRYSTAL STRUCTURES OF 13 (LEFT), 14 (MIDDLE), AND 15 (RIGHT).

HYDROGEN ATOMS ARE OMITTED FOR CLARITY. ELLIPSOIDS ARE SHOWN AT THE 50%

PROBABILITY LEVEL...... 97

FIGURE 5.4 X-RAY CRYSTAL STRUCTURES OF 16 (LEFT) AND 17 (RIGHT). HYDROGEN ATOMS ARE

OMITTED FOR CLARITY. ELLIPSOIDS ARE SHOWN AT THE 50% PROBABILITY LEVEL...... 100

FIGURE 5.5 CID STUDY OF COMPOUND 13. PRECURSOR/PRODUCT IONS (BOXED IN BLACK) AND

NEUTRAL LOSSES (RED) ARE SPECIFIED...... 102

FIGURE 6.1.1 DICOPPER FORMAMIDINATE COMPLEX (MIDDLE), ONE AND TWO ELECTRON

OXIDIZED COMPLEXES (LEFT), AND TARGETED REACTIVE INTERMEDIATES (RIGHT)...... 106

FIGURE 6.1.2 GENERAL MECHANISM FOR COPPER CATALYZED CYCLOPROPANATION ...... 107 16

FIGURE 6.1.3 STRUCTURALLY CHARACTERIZED DICOPPER CARBENES ...... 108

FIGURE 6.1.4 TRANSMETALLATION OF N,N'-DIARYLFORMAMIDINES WITH LIN(TMS)2 AND CUI

TO YIELD DICOPPER-FORMAMIDINATES (18-24)...... 110

FIGURE 6.1.5 X-RAY CRYSTAL STRUCTURES OF COMPOUNDS 18-23. CU-CU SEPARATIONS ARE

SPECIFIED BELOW...... 115

FIGURE 6.1.6 CYCLIC VOLTAMMETRY OF 24 IN ACETONITRILE...... 116

FIGURE 6.1.7 X-RAY CRYSTAL STRUCTURE OF PH2C=N-N=CPH2 AZINE BYPRODUCT ...... 118

FIGURE 6.1.8 INFRARED SPECTROSCOPY SPECTRUM OF 24 WITH 10 EQ. OF PH2CN2 ...... 119

FIGURE 6.1.9 CYCLOPROPANATION OF STYRENE WITH EDA AND 10 MOL-% 24...... 120

FIGURE 6.1.10 AZIRIDINATION OF STYRENE WITH PHINTS AND 5 MOL-% 24...... 121

FIGURE 6.1.11 B3LYP/6-31G*, SDD GEOMETRY OPTIMIZATIONS OF A SIMPLIFIED DICOPPER

FORMAMIDINATE (LEFT), CARBENE (2ND FROM THE LEFT), NITRENE (2ND FROM THE RIGHT),

AND OXO (RIGHT) INTERMEDIATES. A LOSS IS PLANARITY SEEN IN ALL INTERMEDIATES IS

ILLUSTRATED IN THE DICOPPER CARBENE (BOTTOM)...... 122

FIGURE 6.1.12 POSSIBLE STRUCTURES FOR THE METAL CARBENE AND NITRENE INTERMEDIATES

INVOLVED IN CATALYTIC CYCLOPROPANATION AND AZIRIDINATION REACTIONS...... 123

17

LIST OF ABBREVIATIONS

 AcOH acetic acid

 BO bond order

 CcO Cytochrome c Oxidase

 CID Collision-induced dissociation

 CHCl3 chloroform

 COSY Homonuclear Correlation Spectroscopy

 DART Direct Analysis in Real Time

 DCM dichloromethane

 DFT density functional theory

 EC electronic configuration

 EBO effective bond order

 ESI electrospray ionization

 EtOH ethanol

 FBO formal bond order

 FC formal charge

 HMBC Heteronuclear Multiple-Bond Correlation spectroscopy

 MS mass spectrometry

 NBO natural bond orbital 18

 NHC N-Heterocyclic Carbene

 NMR Nuclear Magnetic Resonance

 NOE nuclear overhauser effect

 QM quantum mechanics

 rt room temperature, 25 °C (also RT)

 THF tetrahydrofuran

 TMS trimethylsilyl 19

The Utility of Bimetallic Complexes in Nature, Metal-Metal Bonded

Structures, and Catalysis

Metal-containing complexes have been used as catalysts in various organic transformations for the past decade. Catalysts are used to reduce the activation energy for a given process, which can lead to quicker turnover, less chemical waste, as well as more environmentally friendly synthetic routes.1 Catalysts are classified as either homogeneous or heterogeneous catalysts. Homogeneous catalysts are present in the same phase as the reactants while heterogeneous catalysts are present in a different phase or have reactants adsorbed to its surface. By definition, the catalyst is not consumed in the reaction of interest although it may be inhibited, deactivated, or destroyed through unwanted side reactions. Transition metals are capable of catalyzing various reactions due to the presence of d orbitals. The available electrons held in unfilled d orbitals allow for reduction/oxidation of the metal center, which is a process that is vital for most chemical transformations. The metal center is also capable of binding multiple substrates through its d-electrons leading to the formation of intermediate metal complexes. These intermediates can release the combined substrates to reveal a new product while regenerating its original form in the process (Figure 1.1). This general mechanism is what allows chemists to utilize metal-based catalysts to perform numerous organic transformations in the laboratory. Recent advancements in the field of transition metal catalysis have enabled applications in pharmaceuticals and other specialty chemicals.1

Understanding the role of metals in catalytic cycles including the mechanistic details, 20 allows chemists to design new catalysts to improve catalyst efficiency, selectivity, and minimize unwanted byproducts.

Figure 1.1 General catalytic cycle for the conversion of A and B into product C. Bound substrate specified in parentheses.

Between 1901 and 2012, the Nobel Foundation has recognized achievements related to chemical and enzymatic catalysis at least 15 times.2 This accounts for about

14% of the chemistry prizes and 19% of all Nobel Prize winners. Some of the most notable winners include Fritz Haber (1918, Haber-Bosch Process), Robert. H. Grubbs and

Richard Schrock (2005, Olefin Metathesis), as well as Richard F. Heck, Ei-ichi Negishi, and Akira Suzuki (2010, Pd-Catalyzed Cross Coupling). The Haber-Bosch process involves the reaction of nitrogen from air with hydrogen gas to form ammonia. The reaction proceeds at high temperature and pressure and is aided by an iron catalyst. The high temperature assists the metal catalyst while high gas pressures ensure recycling of the reactants (Figure 1.2). The iron-based catalyst binds the substrates allowing for a significant drop in the activation energy for this process. The gases are cryogenically cooled, but only ammonia will liquefy at these temperatures, allowing recycling of 21 unreacted hydrogen and nitrogen. This process remains one of the most economical routes toward nitrogen fixation and continues to be used in chemical industry across the world.3

Figure 1.2 Schematic diagram for the Haber-Bosch process.

Schrock and Grubbs were recognized for their work on various olefin metathesis reactions promoted by mononuclear molybdenum and ruthenium carbene complexes, respectively. In the presence of these carbene complexes, olefins exchange their substituents about the double bonds resulting in the straight swap of groups (cross metathesis, Figure 1.3), closure of rings (ring-closing metathesis), as well as various types of metathesis polymerization.4 Cross metathesis has been shown to occur via the

[2+2] cycloaddition of an alkene to a metal carbene, forming a metallacyclobutane intermediate. The metallacyclobutane can then coeliminate its ligands to form the original 22 or new carbene. The introduction of a new alkene to the generated metal carbene will follow a similar cycloaddition to produce the new olefin. The work of Grubbs and

Schrock has allowed for new synthetic routes toward olefins for chemical synthesis as well as pharmaceutical drugs.5

Figure 1.3 Catalytic cycle for cross metathesis (left) and relevant molybdenum and ruthenium carbene catalysts (right).

Most recently Heck, Negishi, and Suzuki developed a family of cross-coupling reactions in which two hydrocarbon fragments are coupled with the aid of a transition metal catalyst.6 The general mechanism involves the oxidative addition of an organic halide to the catalyst, followed by a transmetallation step introducing a new coupling partner, followed by reductive elimination to regenerate the catalyst and yield the organic product (Figure 1.4). Another metal alkyl reagent is typically used for the transmetallation step, which generates a metal halide byproduct. The introduction of a 23 new metal allows for more possibilities and new mechanistic insight. The ease and utility of coupling reactions have found these reactions being extensively used in the pharmaceutical industry as well as in various C-C bond forming reactions.7,8 Although the catalytically relevant species in these processes are thought of to be mononuclear by nature, the most recent work highlights how multiple metals can function concurrently to unveil previously undiscovered synthetic avenues.

Figure 1.4 General mechanism of the Negishi and Suzuki palladium-catalyzed cross- coupling reactions.

While mononuclear complexes (complexes containing a single metal atom) have been studied extensively and applied in various fields and disciplines, bimetallic or multimetallic systems have received far less attention. The inclusion of another metal atom and its ability to influence reactivity and catalysis has intrigued chemists for quite some time. Synthetic inorganic chemists are interested in bimetallic complexes to help 24 answer fundamental questions: How do the metals interact on an electronic level if placed in close proximity? Can they better stabilize bridging or terminal atoms or groups such as oxygen, nitrenes, carbenes, or even gases like hydrogen? Do distinct bimetallic pathways exist? These are just a few curiosities that have motivated inorganic chemists to design and synthesize new bimetallic complexes and catalysts for the past half a century.

Metal Cooperativity in Nature

Biological catalysts present in living systems are capable of promoting various elegant transformations that allow organisms to function on a daily basis. Several metalloenzyme catalysts incorporate multiple metals, many of which are thought to act through bimetallic or polynuclear pathways.9 For example, several metalloenzymes are involved in the redox chemistry of carbon dioxide. Multielectron reduction reactions of

CO2 can lead to the formation of a diverse group of products. Although quite challenging, selectively forming one reduction product can be vital in specific situations.

Metalloenzyme catalysts can dictate the product distribution through the effect of catalyst nuclearity. Formate dehydrogenase enzymes feature mononuclear Mo and W active sites

- that interconvert HCO2 and CO2. Alternatively, carbon monoxide dehydrogenase enzymes contain polynuclear active sites, which interconvert CO and CO2 through distinct binuclear Cu/Mo10 and Ni/Fe11 catalytic sites. The Cu/Mo pair operates in the oxidative direction while the Ni/Fe core is capable of both CO oxidation and CO2 reduction. Figure 1.5 a illustrates the proposed binding modes of CO2 in bimetallic binding sites of biologically relevant enzymes. This example highlights how catalyst 25 nuclearity can greatly influence product selectivity even in simple two-electron reduction reactions.

Cyctochrome c oxidase (CcO) is also thought to reduce O2 through a bimetallic

Cu/Fe pathway in which oxygen only binds to the fully reduced CuI/FeII state (Figure 1.5 b).12,13 It is assumed that the copper site and modified tyrosine each provide an electron to the bound oxygen in addition to two electrons from the iron. This four-electron reduction must occur without forming toxic partially reduced oxygen species. The specificity of the bimetallic active site is thought to prevent the formation of toxic species and the active species is regenerated by ferrous cytochrome c. The natural occurrence of several multimetallic bioenzymes in which metal atoms are placed in close proximity implies an advantage when using multiple metals cooperating simultaneously for multielectron redox processes.

Figure 1.5. Bimetallic enzymes in biological systems: (a) Ni/Fe and Mo/Cu sites of carbon monoxide dehydrogenase enzymes (b) Active site of CcO from the bovine heart.

26

Biological systems often serve as inspiration to synthetic inorganic chemists although the synthetic analogues may never reach the level of selectivity and efficiency achieved in nature. Neal Mankad’s group designed their heterobimetallic and monometallic N-heterocyclic carbenes (NHCs) to test if bimetallic systems can give catalytic control like that seen in nature. Through variation of the pairing metal with Cu in (NHC)Cu-[M] ([M] = metal carbonyl anion) and comparison to the monometallic analogue, they exhibited drastic differences in the product distribution.14 The monometallic Cu system catalyzed the hydroboration of CO2 with pinacolborane to yield formate exclusively. The introduction of a bimetallic system with Cu-Fe, Cu-W, or Cu-

Mo moieties resulted in formation of formate and CO. These results demonstrate that the production of CO required both metal sites and confirms a cooperative effect of the two metal units.

Synthetic Fe/Cu analogues of CcOs have received significant attention as the four-electron reduction of O2 is of great biological and technological importance.

Although the mechanism in which the enzyme reduces O2 to H2O open for debate, it is assumed that this four-electron reduction is enabled through one electron each from Cu and the modified tyrosine residue as well as two electrons from the Fe in the heme. Due to the complexity of testing this in the native enzyme, chemists have designed their own catalysts which mimic this metal cooperativity effect. A similar four-electron reduction of

II oxygen was achieved through a mononuclear copper complex [(tmpa)Cu ](ClO4)2 with one electron reductants in the presence of HClO4, forming a bimetallic

II II 2+ 15 [tmpaCu (O2)Cu (tmpa)] peroxo species. Additional studies include synthetic CcO models that are attached to self-assembled monolayer films on gold electrodes to control the electron delivery to copper and phenol of the modified tyrosine residue.16 This 27 synthetic model mimics one of the key features of CcO while achieving similar selectivity. The study of synthetic models allows us to probe the synergistic effect of multiple metals in biological processes, many of which place the metal atoms in close proximity.

Metal-Metal Bonding and Interactions

2- The structural elucidation of Re2Cl8 by F. Albert Cotton in 1964 was a significant discovery not only in the field of inorganic chemistry but changed the way all chemists think of bonding.17 This unique compound (Figure 1.6 A) features an eclipsed geometry and an exceedingly short Re-Re bond of 2.24 Å, which served as the first example of a metal-metal quadruple bond. Prior to 1964, the concept of a quadruple bond was entirely unknown and there were several skeptical organic chemists who found it difficult to grasp that d-orbitals could partake in bonding interactions that s- and p- orbitals could not. Cotton developed a bonding model that introduced a new type of bonding interaction made possible through d-orbitals: the δ bond. Cotton proposed that the δ-interaction is unique to transition metals with no corresponding analogue in the main group elements.18 The pioneering work by Cotton and his group laid foundation for the synthesis and characterization of metal-metal bonded complexes. Although the concept of quadruple bonds is not considered groundbreaking in chemistry today, there 28 has been a resurgence in the field of metal-metal bonded complexes primarily among younger inorganic chemists.

2- Figure 1.6 Famous metal-metal bonded complexes. From left to right: Re2Cl8 , Rh2(OAc)4, and Mo2(OAc)4.

Synthetically, there are several ways to arrive at a metal-metal bonded complex.

Early work by Cotton typically involved harsh reaction conditions, use of strong acids, and elevated temperatures to arrive at thermodynamically stable products such as the dirhenium and dimolybdenum species (Figure 1.6 B and C).19 Ligand exchange reactions can then be used to further modify the complex while keeping the metal-metal bond intact. These ligand exchanges allow for systematic studies of ligand effects on metal- metal bond lengths.

Modern metal-metal bonded complexes utilize carefully designed ligands that encourage metal-metal interactions. Berry utilizes similar mixed donor ligands to form homobimetallic ruthenium complexes (Figure 1.7).20 The mixed donor ligands make the dinuclear core more susceptible to axial ligand binding, evident through chloro and nitride complexes. Thomas and Lu utilize ligands designed to yield heterobimetallic 29 complexes with 3-fold symmetry, followed by Lu (Figure 1.7). The Thomas group takes advantage of the mixed-donor-ligand strategy to form Ti-M and V-M heterobimetallic complexes (M = Fe, Co, Ni, and Cu)21 as well as perform catalytic hydrosilation of ketones via a Co/Zr complex.22 On account of the difficulty in assigning distinct oxidation states of both metals, Thomas invokes the use of [MM']n notation in which n is the number of d electrons available from both metals. Their group has synthesized a variety of [MM'] complexes with n ranging from 8 to 12 and covering various bond orders. The chelating set of N- and P- atoms allows for the installation of a first anchor metal (M) followed by the introduction of new metal (M'). Notable bimetallic examples include the first trigonally symmetric heterobimetallic complex FeCrL (L = [N(o-C6H4-

i 23 24 NCH2P Pr2)3]), weakly interacting heteronuclear LFeCoCl, and NiGaL capable of

25 hydrogenating CO2 to formate at ambient temperature.

Figure 1.7. Mixed donor ligands for supporting metal-metal bonds developed by Berry, Thomas, and Lu.

Apart from formally bonded metal-metal complexes, certain complexes exhibit nonbonding interactions between metals. The phenomenon known as metalophillicity describes the interaction between closed shell metal centers such as Au, Ag, and Cu. 30

These interactions are so common that they are often referred to as aurophilicity, argentophilicity, and cuprophilicity respectively. These d10-d10 interactions are similar to the strength of a hydrogen bond reaching as high as 11 kcal/mol.26 The weakly interacting coinage metals allow for the facile synthesis of bimetallic complexes. Homobimetallic copper(I) and silver (I) N,N'-Di-p-tolylformamidinato complexes were originally structurally characterized by Cotton.27 They feature particularly short metal-metal separations despite the lack of any formal bond. Fackler also characterized similar gold(I) complexes with analogous metallophilic interactions.28 This phenomenon, specifically cuprophillicity, is discussed in closer detail in Chapter 6.

Another synthetic route toward bimetallic complexes involves tuning the ligand sterics to manipulate the nuclearity of the resulting complex. A recent publication from our group,29 offers a direct metalation route with diethyl zinc to form bimetallic complexes preferentially. The substituents on the ortho positions of N,N'- diarylformamidine ligands greatly influence the nuclearity of the zinc complexes.

Chapter 5 discusses this publication and recent efforts related to manipulating complex nuclearities through ligand sterics.

Bimetallic Catalysis

Catalysis brought about by a bimetallic complex is an area that has grown tremendously over the past fifty years. Dirhodium paddlewheel complexes are the most recognizable bimetallic catalysts utilized in carbene and nitrene transfer reactions (Figure

1.8 left). Teyssie first reported the catalytic decomposition of ethyl diazoacetate using

30,31 Rh2(OAc)4 in 1973. Since this discovery, Rh2 systems have received widespread attention in organic synthesis and are found to outperform mononuclear catalysts in terms 31 of efficiency.32,33 Dirhodium catalyzed carbene transfer reactions are thought to proceed via axial carbene intermediates in which the carbene is connected to a single Rh center with the other metal atom providing electronic support.34 The work of Doyle,34 Davies,35 and Du Bois36 rely on the dirhodium paddlewheel complexes but have utilized carefully designed ligands to achieve enantioselectivity and new catalytic avenues. The presence of two rhodium centers help facilitate the stabilization of reactive intermediates (Figure 1.8 right) that are transferred to organic substrates.

Figure 1.8 Left: Dirhodium tetracetate dimer with axial coordination site shown. Right: Proposed reaction intermediates in carbene and nitrene transfer reactions featuring axial coordination to a single Rh center. 32

Figure 1.9 Structural and functional differences in the design of mononuclear and bimetallic catalysts.

This emerging area of small molecule catalysis focuses on the use of catalysts featuring two bonded or closely situated metal atoms. This unique approach expands upon the work done in single-site catalysis but also introduces new parameters which could give rise to new catalytic activity and product distribution (Figure 1.9). 33

Mononuclear catalysts are typically optimized through varying the identity of the metal and sterics and electronic properties of the ligands. In bimetallic catalysis, other factors including catalyst nuclearity and metal pairing must also be considered. Bimetallic complexes also typically offer more available d electrons which allow for twice the number of redox events compared to mononuclear catalysts. Several chemists are currently exploring this concept of bimetallic cooperativity in catalysis. Often times, bimetallic catalysts outperform mononuclear analogues 37,38 suggesting competing catalytic cycles, the need for two metal centers, or lower energy barriers due to low oxidation states of metals achieved through cooperative redox reactions. Bimetallic asymmetric catalysis in which chiral ligands are utilized result in the favored formation of one stereoisomer.39,40 This is very important in the pharmaceutical industry in which only a specific isomer is desired. In addition to homobimetallic catalysis, many groups focus exclusively on heterobimetallic catalysis.41,42

Bimetallic complexes, reactivity, and catalysis is an area that is not yet well understood and has room for growth. These bimetallic moieties are often found in biological processes in metalloenzymes as well as synthetic analogues that have been used in chemical industry. The success of these complexes likely lies in the metal cooperativity which can better stabilize reactive intermediates and lower the overall activation energy for a given process. The scope of this dissertation will cover fundamentals of metal-metal bonding in homobimetallic complexes, the synthetic design and characterization of dizinc complexes, and preliminary catalytic activity of a dicopper system. It is important to note that no direct metal-metal bonded complexes will be discussed in chapters 5 and 6, although some metal-metal interactions are believed to 34 occur in the dicopper formamidinates presented in chapter 6. The analyzed compounds to be discussed are specified in Table 1.1.

Table 1.1 Compound number, name, and chapter in which it is discussed in this dissertation.

Compound # Name Chapter

1 [Tc2Cl4(PMe3)4] 3 1+ 2 [Tc2Cl4(PMe3)4] 3

3 [Re2Cl4(PMe3)4] 3 1+ 4 [Re2Cl4(PMe3)4] 3 2+ 5 [Re2Cl4(PMe3)4] 3 2- b 6 [Mo2(E)4] 3 3- b 7 [Mo2(E)4] 3 4- b 8 [Mo2(E)4] 3 9 D(3,5-Xyl)FH 4

9H [D(3,5-Xyl)FH2]BF4 4 10 D(o-Tolyl)FH 4 11 D(2-iPrPh)FH 4 i 11H [D(2- PrPh)FH2]ClO4 4 12 D(2-tBuPh)FH 4

13 [D(o-Tolyl)F]3Zn2Et(THF) 5

14 [D(2-FPh)F]3Zn2Et(THF) 5

15 [B(2,6-FPh)F]2Zn2Et2(THF) 5

16 [D(2,6-Xyl)F]2Zn2Cl2(THF) 5 i 17 [Li(THF)4]{[B(2,6- Pr2Ph)F]2Zn2(µ-Cl)Cl2} 5

18 [B(2,6-F2Ph)F]2Cu2 6

19 [D(2,6-Xyl)F]2Cu2 6 i 20 [D(2- PrPh)F]2Cu2 6

21 [D(2-FPh)F]2Cu2 6

22 [D(2-MePh)F]2Cu2 6 t 23 [D(2- BuPh)F]2Cu2 6 i 24 [B(2,6- PrPh)F]2Cu2 6

35

DFT Investigation of Multiply Bonded Bimetallic Systems

Introduction

Complexes containing metal-metal multiple bonds have been of great interest to

2– chemists since the discovery of Re2Cl8 , an anion containing the first quadruple bond

(Figure 2.1) in 1964.17,18,43,44 When two transition metal atoms approach one another, d orbitals from each metal center are able to generate σ, π, and δ interactions. When the two metal atoms are bound with a fixed metal-metal distance, the overlap between the d orbitals diminishes from σ, to π, and then to δ bonds as shown in Figure 2.1, therefore, bond strength generally decreases from σ, to π, and then to δ.45

2– Figure 2.1 Re2Cl8 and orbital interactions that make up the quadruple bond.

n- In octahalodimetalate anions M2X8 , a fundamental paradigm in understanding metal- metal multiple bonds, no single specific orientation around the M-M axis needs to be 36 imposed to form σ or π bonds, however, δ bonds involving the face-to-face overlap of

푑푥2−푦2 orbitals restrict rotation about the M-M axis thus enforces an eclipsed rotomeric

18 relationship between the two square MX4 halves (I in Figure 2.2). Octahalodimetalate anions with no δ bond character, where both δ and δ* orbitals are completely empty or fully filled, can exist in either confirmation (I or II in Figure 2.2).46–53 Molecular orbital (MO) interaction diagrams of the metal d-based orbitals for octahalodimetalate anions in both

2– conformations are shown in Figure 2.3. For the archetypal Re2Cl8 anion, each Re at 3+ oxidation state, has a d4 configuration, thus the eight d electrons will fill in one σ, two π, and one δ bonding orbitals leading to a quadruple Re-Re bond and an electronic configuration of σ2π4δ2. The existence of a δ bond accounts for its eclipsed geometry.

2- However, in the cases of Os2X8 (X = Cl, Br), where both for δ and δ* bonds are filled with electrons, each individual OsX4 unit can freely rotate about the Os-Os axis, resulting in both staggered and eclipsed rotomeric forms (Figure 2.2 and Table 2.1).48–53 37

Figure 2.2 Synthesized M2X8 anions with eclipsed and staggered geometries.

Metal-metal bond distances in dimetallic units, observed by X-ray crystallography with high accuracy, have been used as an essential parameter to determine bond order and electronic configuration accordingly.45 Generally, the metal-metal distance increases as the

n- bond order decreases (Table 2.1). Octahaloditechnetate anions Tc2X8 (X = Cl, Br; n = 2,

3) serve as exceptions to this general trend. The Tc-Tc bond distance of 2.147(4) Å in

2- 2 4 2 54 Tc2Cl8 , which is in an eclipsed geometry and bear a formal quadruple bond (σ π δ ), is

3- considerably longer than the observed Tc-Tc distance of 2.117(2) Å in Tc2Cl8 with a formal bond order of 3.5 (σ2π4δ2δ*1).55,56 Similar unexpected shortening in Tc-Tc distance

2- 57 was observed in the bromine analogues (2.1625(9) Å in Tc2Br8 and 2.1265(9) Å in

3- 58 2- 3- [Tc2Br8] ). The charge of each Tc atom (3+ for Tc in Tc2X8 , 2.5+ for Tc in Tc2X8 ), 38 the only difference between the two octahaloditechnetate anions, has been proposed as the intrinsic cause for this “anomaly”, however it has not been thoroughly investigated through advanced computational theory.59,60

* d (b2u) 2 2 dx -y dx2-y2 n- d(b1g) X X * s (a2u) X X M M * p (eg) X X dz2 dz2 X X * d (b1u)

D4h dxy, dxz, dyz dxy, dxz, dyz

d(b2g)

p(eu) s(a1g)

dx2-y2 dxy n- nb(e2) * X X s (b2) X X M M * p (e3) X X dz2 dz2 x X X y D4d dxy, dxz, dyz dx2-y2, dxz, dyz z nb(e2)

p(e1) s(a1)

Figure 2.3 A representation of metal d-based orbitals in the molecular orbital diagram for octahalodimetalate species at an eclipsed geometry (top) and at a staggered geometry (bottom).

Multi-configurational CASSCF/CASPT2 calculations have shown that the stronger

3- 2- effective π bond character in Tc2X8 , relative to Tc2X8 , may result in the shorter Tc-Tc distance.58 However, if the increase in π bond strength is due to the change in charge, an 39 increase in the σ character should be observed as well. No detailed explanation regarding the increase in π bond strength was provided in the multi-configurational calculations.

Here, we carried out thorough DFT calculations using different functionals and basis sets and natural bond orbital (NBO) analyses to comprehend how the change in formal charge on the metal atoms in octahalodimetalate anions influences bond strength and bond distance in gas phase.

40

Table 2.1 Electronic Configuration of Dimetal Units (EC), Formal Bond Order (FBO), Formal Charge of Each Metal Atom (FC), Selected Bond Distances (Å) and Angles (º) of Multiply Bonded Bimetallic Compounds.

Compounds EC FBO FC M-M M-X X-M- Ref (av.) M-X (av.) n 2 4 2 [ Bu4N]2[Tc2Cl8] σ π δ 4 +3 2.147(4) 2.320(4) 59 2 4 2 1 [PyH]3[Tc2Cl8] σ π δ δ* 3.5 +2.5 2.1185(5) 2.364(2) 55 2 4 2 1 [NH4]3[Tc2Cl8] σ π δ δ* 3.5 +2.5 2.13(1) 2.36(2) 79-80 2 4 2 1 K3[Tc2Cl8] σ π δ δ* 3.5 +2.5 2.117(2) 2.36(2) 81-82 2 4 2 1 Y[Tc2Cl8] σ π δ δ* 3.5 +2.5 2.105(2) 2.364(2) 15.75 83 a 2 4 2 2 K2n[Tc2Cl6]n σ π δ δ* 3 +2 2.044(1) 2.40(2) 45 46-47 n 2 4 2 [ Bu4N]2[Tc2Br8] σ π δ 4 +3 2.1625(9) 2.473(2) 0.55 57 2 4 2 1 Cs2.221(H3O)0.779[Tc2Br8] σ π δ δ* 3.5 +2.5 2.1265(9) 2.515(2) 4.86 58 b 2 4 2 (pipzH2)2[Mo2Cl8] σ π δ 4 +2 2.129(3) 2.455(4) 84-86 2 4 2 K4[Mo2(SO4)4] σ π δ 4 +2 2.111(1) 2.139(5) 89-90 2 4 1 K3[Mo2(SO4)4] σ π δ 3.5 +2.5 2.164(3) 2.064(5) 91 2 4 Cs2[Mo2(HPO4)4] σ π 3 +3 2.223(2) 2.01(1) 92 c 2 4 2 1 [Tc2Cl4(PR3)4][PF6] σ π δ δ* 3.5 +2.5 2.107(1) 2.332(3) 87 c 2 4 2 2 Tc2Cl4(PR3)4 σ π δ δ* 3 +2 2.127(1) 2.394(2) 88 c 2 4 2 [Re2Cl4(PR3)4][PF6]2 σ π δ 4 +3 2.208(1) 2.290(8) 59 c 2 4 2 1 [Re2Cl4(PR3)4][PF6] σ π δ δ* 3.5 +2.5 2.218(1) 2.330(5) 59 c 2 4 2 2 Re2Cl4(PR3)4 σ π δ δ* 3 +2 2.241(1) 2.38(1) 59 d 2 4 2 [dpae]2[Re2Cl8] σ π δ 4 +3 2.212 2.319 96 n 2 4 2 [ Bu4N]2[Re2Br8] σ π δ 4 +3 2.226(4) 2.473(5) 97 n 2 4 2 [ Bu4N]2[Re2I8] σ π δ 4 +3 2.245(3) 2.709(3) 50 n 2 4 2 2 [ Bu4N]2[Os2Cl8] σ π δ δ* 3 +3 2.182(1) 2.322(6) 49.0 53 2 4 2 2 [PMePh3]2[Os2Cl8] σ π δ δ* 3 +3 2.211(2) 2.326(4) 51 n 2 4 2 2 [ Bu4N]2[Os2Br8] σ π δ δ* 3 +3 2.196(1) 2.444(4) 46.7 53 2 4 2 2 [C5Me4H)2OsBr]2[Os2Br8] σ π δ δ* 3 +3 2.224(5) 2.465(4) 52 2 4 2 2 [C5Me5)2OsH]2[Os2Br8] σ π δ δ* 3 +3 2.220(3) 2.458(2) 49 n 2 4 2 2 [ Bu4N]2[Os2I8] σ π δ δ* 3 +3 2.217(1) 2.640(2) 46.7 51 a A chloride-bridged polymetallic chain b pipz = piperazine c PR3 = PMe2Ph d dpae = chloro-(dinitrogen-N)-bis(1,2-bis(diphenylarsino)ethane-As,As')-rhenium

Experimental

DFT gas phase calculations61,62 were performed in the Gaussian 03 (Revision C.02) program suite,63 using different functionals BH&HLYP64–68 and B3LYP68,69 with the same basis sets SDD/aug-CC-PVDZ: SDD70 for the metal elements Tc, Re, and Mo with an effective core potential, and aug-CC-PVDZ for Cl and Br.71 Geometry optimizations of the 41

n- M2X8 anions were carried out using the coordinates from the crystallographic data in the literature as the starting points. To ensure better accuracy in the calculations, no symmetry constraints were applied in all calculations. All results were verified by frequency calculations to ensure that geometry with at least a local energy minimum was achieved.

2- Selected molecular orbitals for Tc2Cl8 are depicted in Figure 2.5 as well as those for

2- Tc2Br8 in Appendix A. NBO analyses were performed by using the method built into the

Gaussian 03 program.64 Molecular orbitals and spin density plots were generated from

Gaussian cube files using Visual Molecular Dynamics (VMD 1.8.6)76 and rendered using

POV-Ray v3.677 for Windows. NBO orbitals were generated from Gaussian fchk files using Multiwfn v3.2.78 Selected bonding and anti-bonding orbitals between Tc atoms in

n- Tc2X8 from NBO analyses are in Figure 2.7, Figure 2.8, and Appendix A, together with bond-order analysis in Table 2.3. All computations were carried out on a SGI Altix 350

32-processor computer located at the University of the Pacific.

Results and Discussion

Comparison of bond order, formal charge and bond distances. The principal goal of this project is to locate the cause of the irregular decrease in Tc-Tc distance from

2- 3- Tc2X8 to Tc2X8 , even when the formal bond order decreases from 4 to 3.5. Electronic properties and selected bond distances for octahaloditechnetate anions, their analogues

3- and derivatives were summarized in Table 2.1. Detailed comparisons of the four Tc2Cl8 anions with different counter cations79–83 confirmed that there is no direct correlation between the size of the cations and the Tc-Tc distances. All four Tc-Tc distances, with

3- small variations, reinforced the fact that the Tc-Tc distance in the Tc2X8 ion with a 42

2- formal bond order of 3.5 is indeed shorter than those in the Tc2X8 ion with a formal quadruple bond. Thus, the unexpected shortening in Tc-Tc distance could not be a result of external electrostatic forces from the packing of anions and cations. Irregular decrease in the Tc-Tc distance was also observed in the polymeric chain K2n[Tc2Cl6]n in which the

4- Tc-Tc bond distance in each Tc2Cl8 unit, bearing a formal triple bond, shrank further down to 2.044(1) Å.46,47 Another example from the list in Table 2.1 worth comparing is

4- 84–86 the Mo2Cl8 ion which also contains a formal quadruple bond. Mo and Tc are side by side on the periodic table and feature similar electronic configurations. If only the size of

4- 2- metal atoms is considered, a longer M-M distance in Mo2Cl8 than that in Tc2X8 should be predicted based on the larger atomic radius of Mo. However, a shorter Mo-Mo distance of 2.129(3) Å was observed again. This example suggests that considering solely the formal bond order is not sufficient to explain the irregularities in metal-metal separations; perhaps other factors are to also be considered. Because the selection of cation does not influence the shortening and there are no axial ligands present in any crystal structure, the only easily conceptualized difference that could be potentially used to explain the phenomenon is the change in formal charge on the bonding metal atoms,

3+ 2- 2.5+ 3- 2+ 4- 2+ namely from Tc in Tc2X8 to Tc in Tc2X8 , and then to Tc in Tc2Cl8 and Mo in

4- Mo2X8 . The change in effective charge on the metal atoms is also reflected in M-X distances, in which a continuous increase in averaged M-X distances was detected when

2- the charge on M atoms decreased, for example 2.320(4) Å in Tc2Cl8 , 2.364(2) Å in

3- 4- Tc2Cl8 and 2.40(2) Å in the unit Tc2Cl8 of K2n[Tc2Cl6]n.

A simple illustration of how the change in charge of the two atoms bound together affect bond strength is shown in Figure 2.4. As the effective charge decreases, the orbitals 43 expand from III to IV, leading to better orbital overlap in IV. Furthermore, the electrostatic repulsion between the two atoms decreases as well. Hence stronger bonding and shorter bond distances could be expected from the decrease in formal charge of the two atoms bound together. The effect of charge change on metal-metal bond distance is normally not observed experimentally because in most cases it is being dominated by the effect from the change in formal bond order. A change in formal bond order has been used as the main variable in analyzing variations in metal-metal bond distances such as those from

87 88 n+ 59 [Tc2Cl4(PMe2Ph)4][PF6] to Tc2Cl4(PMe2Ph)4 and in [Re2Cl4(PMe2Ph)4] (n = 2, 1, 0) where an increase in the M-M bond distance (0.020 Å for Tc, 0.023 and 0.010 Å for Re in

4- 89,90 Table 1) was observed. In the case of the molybdenum analogues from [Mo2(SO4)4] ,

3- 91 2- 92 to [Mo2(SO4)4] , and then to [Mo2(HPO4)4] , electrons were removed from the bonding

δ orbital concurrently with the formal charge on each Mo atom increasing from 2+, to 2.5+ and then to 3+, considerably large increases in the Mo-Mo bond distance (0.053 and 0.059

Å) were observed. Even though the M-M distance difference in the two examples listed above is mainly controlled by the change in formal bond order, it may be worthwhile to point out that that a small change in M-M distance in the Tc and Re systems was observed when the two players, namely formal bond order and charge, have opposite effects on the

M-M distance, whereas a much bigger change in the Mo system was observed when both have increasing effect on the M-M distance. It appears that when both formal charge and bond order work cooperatively to strengthen bonds, the net change is significant, and the net change is minor when the two factors work against each other. 44

d

p

s

III IV

Figure 2.4 A simple illustration of charge effect on orbital overlap. Bonding atoms with higher charge on the left (III) and lower charge on the right (IV).

To summarize this section, both changes in formal bond order and formal charge of the atoms will influence the total bond strength and thus bond distance. When both influence bond strength in the same direction, an expected increase or decrease should be obtained. When both influence bond strength in opposite directions, the actual direction for the change in M-M bond distance will be determined by the dominant factor, which is

n- probably the charge change in the abnormal Tc2X8 system. Both formal bond order and effective charge on the metal atoms seem to contribute to the observed change in bond distance due to coulombic repulsion and bonding interactions. Here, a thorough DFT study has been conducted to verify this hypothesis. 45

Electronic structures. In order to find a functional which can provide optimized geometries consistent with the experimental data, geometry optimizations on the anions

n- Tc2X8 (X = Cl, Br, n = 2, 3) were carried out using BH&HLYP and B3LYP (Table 2.2) and with the same basis sets SDD/aug-CC-PVDZ: SDD for Tc, Re and Mo with an effective core potential, and aug-CC-PVDZ for Cl and Br. Detailed comparisons between the calculated and the experimental geometric data (Table 2.2 and Appendix A) suggest that calculations with the functional BH&HLYP and the basis sets SDD/aug-CC-PVDZ provide reasonable and appropriate geometries. Moreover, the calculations reproduced

2- the experimentally observed shortening of the Tc-Tc bond distance from Tc2X8 to

3- Tc2X8 anions. The c.a. 0.04 Å decrease in Tc-Tc distance is in excellent agreement with those measured experimentally, especially in the case of the bromine derivatives (0.029 Å

n- for Tc2Cl8n-, 0.036 Å for Tc2Br8 ). Similar Tc-X bond distances and the same trend in change upon reduction of the anion were also obtained from the calculations (Appendix

A). Electronic structures from the selected calculations using the functional BH&HLYP and basis sets SDD/aug-CC-PVDZ were used for all further analyses.

46

n- Table 2.2 Experimental (exp) and Calculated Tc-Tc Bond Distances (Å) for the Tc2X8 Anions (X = Cl, Br; n=2, 3), Derived from calculations using different functionals.a

2- 3- 2- 3- Tc2Cl8 Tc2Cl8 Tc2Br8 Tc2Br8

exp 2.147(4) 2.1185(5) 2.1625(9) 2.1265(9)

BH&HLYP 2.166 2.12 2.163 2.126 (C1)

BH&HLYP 2.166 2.12 2.163 2.127 (D4h)

B3LYP 2.159 2.168 2.17 2.174 a All calculations were computed with the same basis sets SDD for Tc and aug-CC- PVDZ for Cl and Br. The empty boxes indicated those calculations were not completed.

2- A detailed evaluation on the frontier MOs of Tc2Cl8 (Figure 2.5 and Appendix

2- A) and Tc2Br8 (Appendix A) indicated that there were one σ, two π, and one δ bonding interaction resulting from the d orbitals on the Tc atoms, which contribute to the strong Tc-

2- Tc bonds. The order of the orbitals in energy for Tc2Cl8 corresponds to the eclipsed

2- geometry in Figure 2.1, whereas in Tc2Br8 , the order between one of the virtual σ* and the virtual δ orbital was switched. A large percentage of ligand character was observed in the π components and the top δ and δ* pair (Figure 2.5, Appendix A). A breakdown of

2- 2- the Tc contribution in the frontier MOs for Tc2Cl8 and Tc2Br8 , provided in Appendix

A, indicates that metal d orbitals are the main components in the metal-metal bond formation, but s and p orbitals also contribute to the σ and π interactions. A comparison of the chlorine and bromine analogues suggests that the electronic structure is not sensitive to the nature of the coordinating ligands. The assignment of an electronic configuration of

2 4 2 2- σ π δ for Tc2X8 is consistent with the description carried out by Self-Consistent Field

Theory (SCF-X-SW) calculations93 and also comparable to the ones determined with the 47

Amsterdam Density Functional (ADF 2004) package.75 Similar molecular orbitals were

3- observed in the trinegative anions Tc2X8 , to which the electronic configuration of

σ2π4δ2δ*1 was assigned.

2- Figure 2.5 Selected MO bonding orbitals with 0.05 contour surface for the Tc2Cl8 anion, derived from calculations using the functional BH&HLYP. Positive and negative values are represented as yellow and red surfaces, respectively. Values underneath each MO figure in the last row are given as the percent of electron density.

Spin density on the trinegative anions. Special attention should be given to the nature of the orbital occupied by the odd or extra electron located in a singly occupied 48 molecular orbital (SOMO). Molecular orbital analysis from the DFT calculations indicate

3- that the odd electron in Tc2X8 is located in a δ* orbital. This can also be seen from the spin density diagram in which the primary spin density is on two Tc d-shaped orbitals for

3- 3- 2 4 2 1 both Tc2Cl8 and Tc2Br8 (Figure 2.6). The electronic configuration of σ π δ δ* is consistent with the experimental data obtained through EPR in which g of 1.912, g of

2.096 and a hyperfine structure due to two equivalent 99Tc nuclei (I = 9/2) were observed.94 This has also been supported by self-Consistent Field Theory (SCF-X-SW) calculations93 and a brief report of a molecular orbital calculation in the CNDO approximation that provided the same orbital assignment for the odd electron.95 Because of the odd electron occupies the anti-bonding δ* orbital, the overall δ orbital strength in

3- 2- Tc2X8 is weaker than that in Tc2X8 (Table 2.3), consistent with the data obtained from

NBO analyses below.

Figure 2.6 Graphical representation of spin density with 0.006 contour surface for the 3- 3- Tc2Cl8 (left) and Tc2Br8 (right) anions.

49

Table 2.3 Experimental (exp) and Calculated (cal) Tc-Tc and Tc-X Bond Distances, Formal Charge on Each Metal Atom (FC), Formal Bond Order (FBO), Natural Bond n- Order (NBO), and Effective Bond Order (EBO) for the M2X8 Anions (M = Tc, Re, Mo; X = Cl, Br; n=2-4), derived from calculations using BH&HLYP functional and basis Sets SDD/aug-CC-PVDZ.

2- 3- 2- 3- 2- 2- 4- Orbital Tc2Cl8 Tc2Cl8 Tc2Br8 Tc2Br8 Re2Cl8 Re2Br8 Mo2Cl8

exp 2.147(4) 2.1185(5) 2.1625(9) 2.1265(9) 2.212 2.226(4) 2.129(3) Tc-Tc cal 2.166 2.120 2.163 2.126 2.198 2.208 2.121

exp 2.320(4) 2.364(2) 2.473(2) 2.515(2) 2.319 2.473(5) 2.455(4) Tc-X cal 2.353 2.421 2.509 2.573 2.369 2.522 2.566

FC 3+ 2.5+ 3+ 2.5+ 3+ 3+ 2+

FBO 4 3.5 4 3.5 4 4 4

NBO 2.243 2.551 2.272 2.489 2.846 2.775 3.517

EBO σ 0.71 0.75 0.67 0.72 0.76 0.72 0.82

π 1.43 1.57 1.40 1.53 1.51 1.48 1.75

δ 0.44 0.26 0.56 0.26 0.60 0.58 0.82

NBO analyses. From the qualitative MO diagram in Figure 2.3 and the MO

2- analysis from DFT calculations, one would expect the Tc2X8 ions to have four electron pairs available for metal-metal bonding and hence the anions are stabilized in an eclipsed conformation with a σ2π4δ2 configuration. To further probe the bonding characters between the two Tc atoms in octahaloditechnetate anions, NBO analyses were carried out

2- for the anions. The results from these computations, depicted for Tc2Br8 in Figure 2.7

2- 2 and Tc2Cl8 in Appendix A, show a σ bond formation from two dz orbitals from Tc atoms, two π interactions from the dxz and dyz orbitals, and a δ bond between two dxy orbitals. It is consistent with the postulated formal quadruple bond formulation. A bond- order analysis using Wiberg bond indices from the NBO analyses (Table 2.3) produced a 50

2- 2- natural Tc−Tc bond order of 2.243 for Tc2Cl8 and 2.272 for Tc2Br8 . The trend in

n- Wiberg bond indices for Tc2X8 correlates well with the experimental bond distances, where a larger value, indicating a stronger bonding, was observed in the trinegative

3- 3- anions (2.551 for Tc2Cl8 and 2.489 for Tc2Br8 ). The values are much smaller than the formal bond order of 4 or 3.5, which is most likely due to the large percentage of ligand character in the MOs obtained from DFT calculations (Figure 2.5). In comparison to the multi-configurational calculations, more electron population was observed in the anti- bonding orbitals obtained from the NBO analysis.58

2- Figure 2.7 The orbitals between the Tc ions with 0.05 contour surface for the Tc2Br8 anion, derived from NBO analyses using the functional BH&HLYP. Positive and negative values are represented as blue and green surfaces, respectively. The orbital type between the two Tc atoms and their electron occupation numbers in parentheses are provided below each orbital.

51

When compared to the corresponding dianions, the same types of NBO bonding

3- 3- and anti-bonding orbitals were obtained for Tc2Br8 (Figure 2.8) and Tc2Cl8 (Appendix

n- A), Detailed analyses of the electron occupation numbers between the four Tc2X8 anions showed that the electrons filling in the anti-bonding σ* and π* orbitals decreased

2- 3- considerably from Tc2X8 to Tc2X8 , which resulted in the increase in natural bond order.

The change in the individual type of bond strength can be determined through effective bond order (EBO). EBO is defined as (ηb-ηa)/(ηb+ηa) in which ηb is the occupation number for the bonding natural orbital and ηa is the occupation number for the corresponding anti-

58 2- bonding natural orbital. In comparison to the EBO values for Tc2X8 (0.71 and 1.43 for

2- 2- Tc2Cl8 , 0.67 and 1.40 Tc2Br8 in Table 2.3), an increase in σ and π components (0.75 and

3- 3- 1.57 for Tc2Cl8 , 0.72 and 1.53 Tc2Br8 ) for the trinegative anions were observed. This correlates well with our earlier hypothesis that the decrease in the charge on the Tc atoms

2- 3- from 3+ in Tc2X8 to 2.5+ in Tc2X8 most likely causes an expansion of the valence orbitals of the metal atoms, which consequently leads to the observed shortening in the Tc-

Tc bond because it strengthens the σ and π bonds.59,60 52

3- Figure 2.8 The orbitals between the Tc ions with 0.05 contour surface for the Tc2Br8 anion, derived from NBO analyses using the functional BH&HLYP. Positive and negative values are represented as blue and green surfaces, respectively. The orbital type between the two Tc atoms and their electron occupation numbers in parentheses are provided below each orbital.

A more detailed rationale is as the charge on each Tc atom decreases from 3+ to

2.5+, the resulting expansion in d orbital size will provide better orbital overlap. Stronger orbital overlap will lead to an increase in the energy gap between the bonding and anti- bonding orbitals, thus less electron populations will be in the anti-bonding orbitals, and larger EBO values will be obtained. This rationale correlates well with the computational results from NBO analysis shown in Figure 2.7 and Figure 2.8. The decrease in the positive charge of each Tc atom could also reduce the electrostatic repulsion between the two Tc atoms, providing extra incentive for the Tc-Tc shortening. Moreover, the decrease in the charge could reduce the interactions between Tc and the halogen ligands, thus more 53 electrons could be potentially filled in the metal-metal bonding orbitals, leading to strong

Tc-Tc bond.

2- 3- The charge decrease on the Tc atoms from Tc2X8 to Tc2X8 should also strengthen

2- the δ bond, however, an extra electron from the reduction of the Tc2X8 anion is placed in the δ* orbital, causing a larger decreasing effect on the δ bond strength. Because the occupation numbers in the δ* NBO orbitals nearly doubled (Figure 2.8 and Appendix A),

2- 2- the δ bond strength significantly decreased from 0.44 for Tc2Cl8 and 0.56 Tc2Br8 to 0.26

3- 3- in both Tc2Br8 and Tc2Br8 . Even though there is a decrease in δ orbital character, the major components controlling the Tc-Tc bond strength are the σ and π bonds of much higher strength.

Similar calculations were also carried out for the formally quadruply-bonded

4- 84–86 2- 96 2- 97 analogues Mo2Cl8 , Re2Cl8 , and Re2Br8 . When comparing each EBO component

4- in Table 2.3, much stronger σ, π, and δ bonding formed in Mo2Cl8 , which could be rationalized by the same explanation that the decrease in charge of the metal core, +2 for

4- Mo in Mo2Cl8 , resulted in better orbital overlap thus an increase in natural bond order and accordingly a decrease in Mo-Mo bond distance. Much stronger σ, π, and δ bonding in the

Re analogues were also observed. Since each Re atom is at the same oxidation state as the

2- Tc atoms in Tc2X8 , the stronger bonding could be explained by the fact that 5d orbitals were involved in the metal-metal bonding in the Re systems instead of the 4d orbitals in

Tc and Mo. Overall, simply considering bond order when analyzing bond lengths is not sufficient. We have shown that the formal charge on the metal atoms should also be considered when studying bond lengths and bond strengths.

54

Conclusions

n- Electronic properties of the M2X8 (M = Tc, Re, Mo; X = Cl, Br; n=2-4) anions have been evaluated by DFT calculations using a series of functionals and basis sets. The study indicated that DFT calculations using the functional BH&HLYP and basis sets

SDD/aug-CC-PVDZ provided optimized geometries consistent with the experimental data.

Molecular orbital and NBO analyses provided electronic configurations of σ2π4δ2 for the

2- 2 4 2 1 3- Tc2X8 anions and σ π δ δ* for Tc2X8 . The study demonstrated that the increase in σ and

2- 3- π bond strength resulted in the shortening in Tc-Tc bond distance from Tc2X8 to Tc2X8 ,

4- which was further supported by the short Mo-Mo bond in the Mo2Cl8 ion. Here the effect of the charge played a dominant role in controlling the Tc-Tc distance, instead of the formal bond order in other normal cases. The study provides strong evidence for the argument that changes in charge on the bonding atoms could have a pronounced effect on bond strength, natural bond order, and thus the bond distance. All variables, such as formal bond order, atom size, ligand environment, packing conditions and formal charge, should be used together to analyze variations in metal-metal bond distances. The study also indicated that metal-metal bond distances are sensitive to the choice of functionals and basis sets in DFT calculations. 55

Probing Factors Influencing the Metal-Metal Bond Strength in

Bimetallic Systems

Introduction

Metal-metal multiple bonds, observed in nearly all metal atoms, have attracted great interest from inorganic chemists45 since the discovery of the first quadruple bond in the

2– 17 Re2Cl8 anion (I in Figure 3.1). In addition to σ and π interactions between d orbitals, unique δ interactions are possible when two d orbitals are in a face-to-face orientation. In

n- the archetypal octahalodimetalate anions (M2X8 ), no specific orientation about the M-M axis is needed to form σ or π bonds, however, an eclipsed rotomeric relationship between

43,44,98 the two square MX4 halves is essential in the presence of δ components (II). The

2– occurrence of the quadruple bond in the Re2Cl8 anion can be clearly rationalized by a simplified molecular orbital (MO) interaction diagram in which the four bonding orbitals are completely filled up with eight d electrons (III).

56

2- Figure 3.1 Illustrations of the Re2Cl8 anion with an eclipsed geometry (I) and d-d orbital overlap (II), and a representation of its metal d-based orbitals in the molecular orbital diagram (III). Orbitals order in energy often changes from those shown in the diagram because of their similarity in energy.

Due to the great success of X-ray crystallography, metal-metal bond lengths in bimetallic units have been measured to the nearest thousandth of an Å. The precise bond lengths have been widely used as a crucial piece of experimental data in analyzing formal bond orders and electronic configurations.99 Metal-metal bond length largely depends on the size and oxidation state of the metals, as well as the ligand environment of the metal atoms involved in bonding. For complexes of similar geometry within the same ligand environment, the metal-metal bond length generally increases as the formal bond order

n- 54–58,79–83 84–86 decreases. However, the octahalodimetalate anions M2X8 (M = Tc, Mo; X =

Cl, Br; n = 2, 3, 4 as shown in Table 3.1) serve as exceptions to this general trend. Our recent theoretical study on the system100 supported the hypothesis in the literature60 that the charge change of the bonding metal centers serves as the dominant effect on the change 57 in bond strength. The decrease in the charge of the bonding atoms led to an increase in the strength of both σ and π bonds, resulting in the unexpected decrease in bond length.

Table 3.1 Electronic Configuration of Dimetallic Units (EC), Formal Bond Order (FBO), Formal Charge of Each Metal Atom (FC), Selected Bond Lengths (Å) and Angles (º) of Multiply Bonded Bimetallic Compounds.

Compounds FC EC FBO M-M M-X M-P Ref (av.) (av.) n 2 4 2 [ Bu4N]2[Tc2Cl8] +3 σ π δ 4 2.147(4) 2.320(4) 54 2 4 2 1 [PyH]3[Tc2Cl8] +2.5 σ π δ δ* 3.5 2.1185(5) 2.364(2) 55, 56 n 2 4 2 [ Bu4N]2[Tc2Br8] +3 σ π δ 4 2.1625(9) 2.473(2) 57 2 4 2 1 Cs2.221(H3O)0.779[Tc2Br8] +2.5 σ π δ δ* 3.5 2.1265(9) 2.515(2) 58 b 2 4 2 (pipzH2)2[Mo2Cl8] +2 σ π δ 4 2.129(3) 2.455(4) 92 a 2 4 2 2 Tc2Cl4(PR3)4 +2 σ π δ δ* 3 2.127(1) 2.394[2] 2.446[2] a 2 4 2 1 [Tc2Cl4(PR3)4][PF6] +2.5 σ π δ δ* 3.5 2.1073(8) 2.332[2] 2.483[2] a 2 4 2 2 Re2Cl4(PR3)4 +2 σ π δ δ* 3 2.241(1) 2.387[3] 2.418[15] 59 a 2 4 2 1 [Re2Cl4(PR3)4][PF6] +2.5 σ π δ δ* 3.5 2.218(1) 2.330[3] 2.460[6] 59 a 2 4 2 [Re2Cl4(PR3)4][PF6]2 +3 σ π δ 4 2.208(2) 2.291[5] 2.508[5] 59

2 4 2 2 Re2Cl4(PEt3)4 +2 σ π δ δ* 3 2.232(6) 2 4 2 Re2Cl6(PEt3)2 +3 σ π δ 4 2.222(3) 2.315[8] 2 4 2 K4[Mo2(SO4)4] +2 σ π δ 4 2.111(1) 2.139[5] 2 4 1 K3[Mo2(SO4)4] +2.5 σ π δ 3.5 2.164(3) 2.064(5) 2 4 Cs2[Mo2(HPO4)4] +3 σ π 3 2.223(2) 2.01(1) a PR3 = PMe2Ph b pipz = piperazine

In order to understand whether charge plays a similar role in controlling bond length in other systems, it is necessary to continue our study in strategically selected bimetallic

n+ 87,101 59 complexes with minimal structural differences. [M2Cl4(PMe3)4] (M = Tc, Re, n =

n- 92 89,91,94 0-2) and [Mo2E4] (E = HPO4 or SO4, n = 2-4) were chosen to be analyzed theoretically as the formal charges of the metal centers vary upon redox reactions while maintaining similar ligand sets. DFT and Multi-configurational CASSCF/CASPT2

88 calculations have been performed for the neutral precursors Tc2X4(PMe3)4 (X = Cl, Br), 58 but not the complete series or the Re analogues. Here, density functional theory (DFT) calculations using functionals PBE0, BH&HLYP, and B3LYP with basis sets SDD/aug-

CC-PVDZ/D95 were conducted on the three bimetallic systems. Natural bond orbital

(NBO) analyses were also applied in effort to comprehend how individual bond strength varied in gas phase. An additional goal of the study was to verify whether or not the functional BH&HLYP with 50% of HF term68,100 is indeed a better choice over the common functional B3LYP for complexes containing metal-metal multiple bonds.

Experimental

DFT calculations in the gas phase61,62 were performed using the Gaussian 03

(Revision E.01) program suite,63 with different functionals BH&HLYP, B3LYP,64–67

PBE0102 and the same basis sets SDD/aug-CC-PVDZ/D95: SDD70 for the metal elements

Re, Tc and Mo with an effective core potential, aug-CC-PVDZ for Cl, P and S,71 and

Double-ζ quality basis sets (D95) for C and H.71 Geometry optimizations of model

101 + 101 n+ compounds Tc2Cl4(PMe3)4 (1), [Tc2Cl4(PMe3)4] (2), and [Re2Cl4(PMe3)4] (3, n =0;

4, n = 1; 5, n = 2)59 were carried out using the coordinates from the crystallographic data of the PMe2Ph analogues with the PMe2Ph groups being simplified to PMe3 and counter

2- 92 anions were removed. Starting point of model compounds [Mo2(HPO4)4] (6),

3- 89 4- 91 [Mo2(SO4)4] (7), and [Mo2(SO4)4] (8) were derived from crystallographic data with the water molecules or sulfate anions in the axial positions and counter cations were removed. To ensure better accuracy in the calculations, no symmetry constraints were applied in all calculations. All results were verified by frequency calculations to ensure that geometry with at least a local energy minimum was achieved. Selected molecular orbitals 59 for 1 are depicted in Figure 3.3 as well as those for 3 (Appendix A). NBO analyses were performed by using the method built in Gaussian 03 program.63 Molecular orbitals were generated from Gaussian cube files using Visual Molecular Dynamics (VMD 1.8.6)76 and rendered using POV-Ray v3.677 for Windows. Spin density plots and NBO orbitals were generated from Gaussian fchk files using Multiwfn v3.2.78 Selected bonding and anti- bonding orbitals between M atoms in compounds 1-8 from NBO analyses are in Figure

3.5, and Appendix A together with bond-order analysis in Table 3.2. All computations were carried out on a SGI Altix 350 32-processor computer located at the University of the

Pacific.

Results and Discussion

Metal-Metal and Metal-Ligand Bond Lengths. In general, bond strength of metal- metal multiple bonds is directly related to the bond length obtained experimentally. In order to study how factors, including formal bond order and charge of the bonding atoms, influence the bond strength, it is important to study systems that possess identical ligand sets but differ only in the oxidation states of the bimetallic units or population of the

n+ molecular orbitals. The series of [M2Cl4(PMe3)4] (M = Tc, Re; n = 0, 1, 2) complexes serves as an ideal choice for this detailed examination. From the neutral dimer

101 87 Tc2Cl4(PMe2Ph)4 , to the singly oxidized [Tc2Cl4(PMe2Ph)4][PF6] , as well as those in

n+ 59 the [Re2Cl4(PMe2Ph)4] (n = 0, 1, 2) system, a drop in the M-M bond length (-0.020 Å for Tc, -0.023 and -0.01059 Å for Re in Table 3.1) was observed. The molybdenum

2- 92 3- 89,90 4- 91 analogues from [Mo2(HPO4)4] , [Mo2(SO4)4] , to [Mo2(SO4)4] , exhibit considerably large shortenings in the Mo-Mo bond length (-0.059 and -0.053 Å). In all 60 three systems, formal bond order of each bimetallic unit, derived from the formal charge and electronic configuration, increase from 3, to 3.5, then to 4 upon redox reaction. The change in formal bond order alone could not rationalize the significant contrast among the changes in the M-M bond lengths between the Tc2 or Re2 system and the Mo2 system. Since the compounds in each system possess nearly the same ligand set, the difference in formal charge of the metal atom may be one of the only factors governing the variations in M-M bond length.

Our study on the octahalodimetalate anions100 has indicated that a decrease in formal charge of the bonding atom resulted in an increase in individual σ and π bond strength, and vice versa. Upon oxidation of Tc2Cl4(PMe2Ph)4, the formal charge of each Tc changed from 2+ to 2.5+, thus a decrease in σ and π strength could be expected. While the increase in formal bond order tends to shorten the Tc-Tc bond length, the decrease in σ and π strength should counteract this effect resulting in a smaller net change in Tc-Tc bond length. A similar explanation could be given for the Re system. In the Mo analogues from

2- 92 3- 89,90 4- 91 [Mo2(HPO4)4] , to [Mo2(SO4)4] , and then to [Mo2(SO4)4] , the formal charge of each Mo center decreased from 3+, to 2.5+ then to 2+, suggesting an increase in σ and π strength to be expected. In this case an increase in formal bond order and bond strength, displayed significant loss in the Mo-Mo bond strength.

The trends in metal-ligand distances are also worth analyzing. As we go through the series from the neutral Re(I)-Re(I) compound to the Re(II)-Re(II) ion, the Re-C1 distances decrease first by 0.057 (4) Å and then 0.039 (6) Å. Each of these changes is statistically significant and the average value of the two is 0.048 Å. This trend presumably reflects the decreasing bond radius of the metal atoms as their formal charge changes from +2.0 to +2.5 61 to +3.0. The decreases in bond radius could be attributed to contraction of the metal valence orbitals, the increased electrostatic attraction as the metal charge increases, or both. For the

Re-P bonds the opposite trend is seen. The mean bond lengths increase, by 0.042 (16) Å from 1 to 2 and by 0.048 (8) Å from 2 to 3. Presumably this trend reflects the fact that Re

P K bonding is important for 1, is less so but still exists for 2, and is still less so for 3.

Electronic Structures and M-M Bond Strength. Thorough theoretical analysis of all model compounds 1-8 depicted in were carried out in an attempt to assign the contributors involved in the observed variations in M-M bond lengths. Selected molecular orbitals for the model compounds Tc2Cl4(PMe3)4 (1) and Re2Cl4(PMe3)4 (3) derived from the d-d interactions are shown in Figure 3.3 and Appendix A. In both cases, the HOMO is the metal based δ* orbital. A σ, two π and one δ orbital were observed below the HOMO, indicating the electronic configuration of σ2π4δ2δ*2 and a formal bond order of 3. There are ten orbitals overall, generated from the interactions between metal d orbitals, which are consistent with the MO diagram derived in Figure 3.1. As the neutral compounds (1 and

3) are oxidized (2, 4, and 5) electrons from the HOMO anti-bonding δ* orbital are removed, resulting in a formal bond order increase of 3, to 3.5 (compounds 2 and 4), to 4 (compound

5). A shortening in the M-M bond is to be expected and is observed in the experimental

2- data. MO analysis of the model compound [Mo2(SO4)4] (6, Appendix A) suggests an electronic configuration of σ2π4, in which the HOMO and HOMO-1 is a set of two degenerate π orbitals and a Mo-Mo bond order of three. The LUMO is the bonding δ orbital. As the compound 6 is reduced to 7, and consequently to 8 (Appendix A), electrons fill the bonding δ orbital, increasing the bond order from 3, to 3.5, to 4. A shortening in 62

Mo-Mo bond length should also be expected, which is consistent with experimental findings.

Figure 3.2 Model compounds symbolized as 1-8 used in DFT calculations.

Figure 3.3 Selected bonding MOs with 0.05 contour surface for Tc2Cl4(PMe3)4 (1) derived from calculations using the functional PBE0. Positive and negative values are represented as red and yellow surfaces, respectively. Values underneath each MO figure in the last row are given as a percentage of the electron density divided among the Tc atoms, PMe3 groups and four Cl atoms, respectively.

63

Spin Density on the Singly Oxidized Species. In all three cases, (1-2, 3-5, and 6-

8) the formal bond orders increase as a shortening in M-M bond length is observed.

However, the net changes vary greatly (-0.020, -0.023, and -0.010 Å, -0.059 and -0.053 Å in the Tc2, Re2, and Mo2 systems respectively). The only other major change in the systems is the changes in oxidation states. It is necessary to confirm that the change in oxidation states was only localized on the M-M bonds. Special attention should be given to the singly

+ 3- oxidized species [M2Cl4(PMe3)4] (M = Tc, Re) and [Mo2(SO4)4] , because the location of the charge could be located experimentally through EPR studies as well as theoretically

+ through spin density analysis. EPR studies of the [Re2Cl4(PMe3)4] analogue indicated that the odd electron was located on the δ* orbital. This is consistent with the observation from the aforementioned DFT study in which the SOMO was assigned as a symmetric metal-

* 101 3- based δ orbital. EPR studies of [Mo2(SO4)4] , also showed well-defined hyperfine structure due to the fact that the odd electron is located on the molybdenum unit. Upon oxidation, the increase in charge was localized between the two Re centers whereas the supporting ligands did not share much load of the charge increase. Further evidence came from the spin density diagram in Figure 3.4, in which most of the spin density resided symmetrically on the two Re centers.

64

Figure 3.4 Graphical representation of spin density with 0.006 contour surface for the + + paramagnetic model compounds [Tc2Cl4(PMe3)4] (2), [Re2Cl4(PMe3)4] (4) and [Mo2 3- (SO4)4] (7) using BH&HLYP functional.

The Re analogue gave similar experimental results suggesting the increase in charge was also localized on the Re atoms.59 Further oxidation of the Re bimetallic complex to 2+ oxidation state completely removed both electrons on the metal-based δ* orbital, resulting in an increase in charge primarily on the metal centers. These examples indicate the increase in charge is mainly localized on the metal centers, allowing the study of the influence of charge change on the bond strength.

NBO Analyses. After determining the change in formal charge is localized on the metal atoms, how can the change in formal charge influence the individual bond strength?

NBO analysis is a tool that has demonstrated that the changes in individual bond strength, due to changes in formal charge, have been the main contributor in the exceptional trend of the M-M bond lengths for the octahalodimetalate anions.100 Similar NBO analyses have been conducted for all model compounds with their corresponding NBO diagrams shown in Figure 3.5 and Appendix A. Detailed analyses of the electron occupation numbers in all of the metal based orbitals demonstrate that upon oxidation of the Tc2 and Re2 systems

(1 to 2, 3 to 4, and 5 respectively), the electron occupation numbers in the σ and π bonding 65 orbitals decrease while those in the σ* and π* orbitals increase. Alternatively, the electron occupation numbers in the σ and π bonding orbitals upon oxidation in the Mo2 system (6 to 7, and 8) increase while those in the σ* and π* orbitals decrease. Small changes but in a different direction in individual σ and π bond strength could be expected between the Tc2 or Re2 systems and Mo2 system. Individual bond strength can be quantified using effective bond order (EBO) defined as (ηb-ηa)/(ηb+ηa) where ηb is the occupation number for the bonding natural orbital and ηa is the occupation number for the corresponding anti-bonding

58 natural orbital. The EBO values of the σ and π bond strength for the Tc2 and Re2 systems

(Table 3.2) decrease as the neutral compound 1 or 3 is oxidized, which supports the fact that the shortening in metal-metal bond length is greatly diminished (only -0.020 Å in the

Tc2 system, -0.023 and -0.010 Å in the Re2 system). The EBO values of the Mo2 systems show an increase in σ and π bond strength increases as the compound 6 is reduced, resulting in a larger net difference in the Mo-Mo distance (-0.059 and -0.053 Å).

66

Figure 3.5 The orbitals between the Tc centers with 0.05 contour surface for the Tc2Cl4(PMe3)4 (1) derived from NBO analyses using the functional PBE0. Positive and negative values are represented as blue and green surfaces, respectively. The orbital type between the two Tc atoms and their electron occupation numbers in parentheses are provided below each orbital.

67

Table 3.2 Experimental (exp) and calculated (cal) M-M and M-X bond lengths, and formal charge on each metal atom (FC). In the dimetallic units: electronic configuration (EC), formal bond order (FBO), calculated natural bond order (NBO), and calculated effective bond order (EBO) for model compounds 1-8. Derived from calculations using PBE0 Functional.

n+ n+ n- [Tc2Cl4(PR3)4] [Re2Cl4(PR3)4] [Mo2(E)4] 1 2 3 4 5 6 7 8 M–M (Å) 2.127(1) 2.1073(8) 2.241(1) 2.218(1) 2.208(2) 2.223(2) 2.164(3) 2.111(1) Δ in M–M –0.020 –0.023 –0.010 –0.059 –0.053 (in Å ) FC on M +2 +2.5 +2 +2.5 +3 +3 +2.5 +2 σ2π4δ2δ σ2π4δ2δ EC σ2π4δ2δ*2 σ2π4δ2δ*1 σ2π4δ2 σ2π4 σ2π4δ1 σ2π4δ2 *2 *1 FBO 3 3.5 3 3.5 4 3 3.5 4 NBO 2.231 2.222 2.254 2.356 2.657 2.597 2.745 3.527 EBO σ 0.741 0.711 0.778 0.754 0.735 0.815 0.849 0.843 π 1.519 1.430 1.571 1.504 1.432 1.688 1.808 1.809 δ 0.000 0.144 0.000 0.252 0.586 0.001 0.434 0.799

The changes in electron populations in δ and δ* orbitals are mainly due to the redox process, which can be explained separately from that of the σ and π orbitals. As compound

1 is oxidized to 2, electron population in HOMO δ* orbital decreases from 1.96 to 1.14.

Similar results are observed in the Re2 system. On the other hand, when compound 6 is reduced to 7, and then to 8, the electron population in the LUMO δ orbital increases from

0.49, to 1.15 and then to 1.98. Both analyses suggest that as the compounds undergo redox reactions, the location of the charge change remains on the metal-based δ or δ* orbital, consistent with the finding from spin density analysis. The change in electron population in δ or δ* orbital was also reflected in the EBO values.

Conclusions

Theoretical studies of the Tc, Re and Mo systems with minimal changes in the ligand environments indicated that the formal charge of the bonding metal centers exhibits a direct influence on the individual σ and π bond strength and ultimately the experimental 68 bond length. The two factors, namely changes in bond strength and formal bond order, play significant roles in governing the experimental bond length. In the Tc2 and Re2 systems, the two factors have opposite effect on the bond length resulting in smaller differences in the M-M bond lengths. In contrast, the significant contraction of the Mo-Mo bond in the

Mo2 system is since both players have the same shortening effect on the Mo-Mo bond.

NBO analysis is valuable in evaluating the change in individual σ, π and δ bond strengths through EBO values clearly rationalizing this phenomenon. Bothe the formal charge on the metal atoms involved as well as the electronic configuration and bond order should be considered when analyzing trands in metal-metal bond distances in bimetallic systems. 69

Steric-Induced Stereochemical Exchange of N,Nʹ-Diarylformamidines in

Solution

Introduction

The formamidine structural moiety, N=C(H)-NH, has been studied extensively due to its resemblance to bioactive molecules,103,104 use as synthetic intermediates,105–108 pesticides,109–111 and rate promoters in oxidative polyphenylene ether formation.112

Formamidines and formamidinates are also common ligands used in coordination chemistry in which they bind to various transition metals through multiple modes.113–117

N,Nʹ-Diarylformamidines in particular have been used in the preparation of dinuclear paddle-wheel and lantern type complexes, permitting analysis of the metal-metal interactions.118–122 These ligands are typically prepared through the reaction of various anilines with triethyl orthoformate under refluxing conditions.123 Since the time of the first documented synthesis, there have been various modifications involving milder reaction conditions and facile synthetic routes.124–127

Diarylformamidines gained interest in the late 80s as the first crystal structures were published for N,Nʹ-diarylformamidines (Figure 4.1) and related structures.128–130 The molecular structures revealed two formamidine ligands bound together through intermolecular hydrogen bonds about the formamidine backbone with the aryl rings twisted out of the formamidine planes. This dimer formation was rationalized through a 70 dynamic proton exchange process in which two tautomers coexist. The amine proton is rapidly exchanged with the imine nitrogen, which is facilitated through the dimer formation. This fast double proton transfer has been studied through many 1H and 15N

NMR studies to elucidate the species involved.131–133 Evidence from these and previous studies indicate only the s-trans (E) species are capable of forming cyclic hydrogen- bonded dimers. Formamidine hydrogen bonding interactions, proton exchange, and dimer formation have also been studied computationally.134,135 It has been shown that formamidines are also capable of forming cyclic dimers with water, formic acid, and carboxylic acids in a similar fashion.136,137

Figure 4.1 N,Nʹ-diarylformamidine ligands in this study. Name and compound numbering are specified below each structure.

Recent 1H NMR and crystallographic studies on similar ligands investigated the formation of the rare s-cis (Z) isomer.138,139 The inclusion of pyridine rather than phenyl substituents allows for new hydrogen bonding potential. Although the pyridine nitrogens 71 stabilize the dimer formation by increasing hydrogen-bond contacts, it also introduces the potential for new intramolecular hydrogen bonds resulting in a stabilization of the s-cis

(Z) isomer. The 1H spectra showed two species in solution which could be manipulated by varying the deuterated solvent or through the addition of a hydrogen bond acceptor.

The addition of a hydrogen bond acceptor results in the elimination of the Z isomer contribution as the ligand is locked into the E form through the hydrogen bond adduct.

This study aims to investigate the formation of the rare s-cis (Z) isomer in ortho substituted N,Nʹ-diarylformamidines by hindering the formation of hydrogen-bond dimers through steric repulsion. N,Nʹ-Diarylformamidines with alkyl substituents at the ortho positions were studied through 1H NMR in various deuterated solvents along with subsequent AcOH addition. The introduction of sterically demanding substituents at the ortho position seems to destabilize the formamidine dimer formation resulting in the appearance of proton signals indicative of the rare Z isomer.

Experimental

General procedures. Syntheses were conducted in a regular wet-lab setting, using chemicals and solvents directly from commercial vendors and glassware dried at 100 oC overnight. The substituted anilines and triethyl orthoformate were purchased from Sigma

Aldrich. Chloroform-d1, C6D6, and DMSO-d6 were purchased from Cambridge Isotope

Labs and were treated with 4Å molecular sieves to remove trace water content. All other reagents were purchased from commercial vendors and used without further purification unless explicitly stated. 72

Synthesis of diarylformamidines. Compounds 9-12 were synthesized according to a published procedure (Figure 4.2).124 The aniline and triethyl orthoformate (2:1) were heated at 120 ºC overnight as ethanol distilled as a byproduct. The crude solid was washed with hexanes (3 x 10 mL) to remove residual starting materials. The crude products were further purified through recrystallization in THF and hexanes.

Figure 4.2 Synthesis of various N,Nʹ-diarylformamidines from triethyl orthoformate and substituted anilines.

Acidified molecules (9H and 11H) were prepared through treating the appropriate diaryl formamidine with 1 eq. of perchloric acid. The formamidine was dissolved in

CH3CN while an acid solution was prepared in water. The two solutions were combined and stirred at room temperature for 24 h. The solvent was removed through vacuum to give a crude white solid product. Vapor-diffusion crystallization with acetonitrile and diethyl ether diffusing in yielded colorless crystals.

Ligand Characterization:

1 D(3,5-Xyl)FH (9). Yield = 83%. H NMR (CDCl3, 600 MHz, δ, ppm): 8.26 (s, 1H, HN-

CH=N), 6.75 (s, 2H, aromatic C-H), 6.70 (s, 4H, aromatic C-H), 2.30 (s, 12H, C-CH3).

DART-MS: [M+H]+, 253.1639 (Calcd 253.1705), [2M+H]+, 505.3268 (Calcd 505.3331). 73

1 [D(3,5-Xyl)FH2]BF4 (9H). Yield = 56%. H NMR (CDCl3 + 1.0 eq AcOH, 600 MHz, δ, ppm): 13.17 (br, 2H, CH-NH-C), 8.28 (s, 1H, HN-CH-NH), 6.84 (s, 6H, aromatic C-H),

2.31(s, 12H, C-CH3).

1 D(o-Tolyl)FH (10). Yield = 83%. H NMR (CDCl3, 600 MHz, δ, ppm): 8.07 (s 1H, HN-

+ CH=N), 7.21-7.01 (m, 8H, aromatic C-H), 2.33 (s, C-CH3). DART-MS: [M+H] ,

225.1332 (Calcd 225.1392), [2M+H]+, 449.2678 (Calcd 449.2705).

i 1 D(2- PrPh)FH (11). Yield = 74%. H NMR (CDCl3, 600 MHz, δ, ppm): 8.01 (s, 1H,

HN-CH=N), 7.30-7.29 (d, 4H, aromatic C-H, J = 7.56 Hz), 7.17 (t, 2H, aromatic C-H, J =

7.56 Hz), 7.11 (t, 2H, aromatic C-H, J = 7.55 Hz), 7.03 (s, CH-NH-C) 3.30 (sep, 2H, C-

+ CH-(CH3)2, J = 6.87 Hz), 1.26 (d, 12H, CH-(CH3)2, J = 6.87 Hz). DART-MS: [M+H] ,

281.2019 (Calcd 281.2018), [2M+H]+, 561.3904 (Calcd 561.3957).

i 1 [D(2- PrPh)FH2]ClO4 (11H). Yield = 72%. H NMR (CDCl3 + 1.0 eq AcOH, 600 MHz,

δ, ppm): 12.40 (br, 2H, CH-NH-C), 7.82 (s, 1H, HN-CH-NH), 7.34 (d, 2H, aromatic C-

H), 7.24 (t, 2H, aromatic C-H), 7.19 (t, 2H, aromatic C-H), 7.01 (d, 2H, aromatic C-H),

3.34 (sep, 2H, C-CH-(CH3)2), 1.26 (d, 12H, CH-(CH3)2).

t 1 D(2- BuPh)FH (12). Yield = 38%. H NMR (CDCl3, 600 MHz, δ, ppm): 7.88 (s, 1H,

NH-CH=N), 7.42 (d, 2H, aromatic C–H, J = 7.90 Hz), 7.22 (t, 2H, aromatic C–H, J =

7.55 Hz), 7.10 (t, 2H, aromatic C–H, J = 7.73 Hz) 1.53 (s, 18H, –C–CH3). DART-MS:

[M+H]+, 309.2301 (Calcd 309.2331), [2M+H]+, 617.4558 (Calcd 617.4583).

Determination of X-ray structures. Single crystals for all samples were coated with deoxygenated Paratone-N oil and mounted on Kaptan loops. Crystallographic data for compound 9 and 10 were collected on Beamline 11.3.1 at the Advanced Light Source, 74

Lawrence Berkeley National Lab using monochromatic radiation (λ = 0.7749 Å).

Crystallographic data for compound 9H, 11, 11H and 12 was collected at 100(2) K on a

Siemens (Bruker) SMART CCD area detector instrument with Mo Kα radiation. Raw data was integrated and corrected for Lorentz and polarization effects using Bruker APEX2 v.

2009.1.140 Absorption corrections were applied using SADABS.141 Space group assignments were determined by examination of systematic absences, E-statistics, and successive refinement of the structures. The program PLATON142 was employed to confirm the absence of higher symmetry for any of the crystals. The program

CELL_NOW143 was employed to check and solve twining problems of the crystals. The position of the heavy atoms was determined using direct methods in the program

SHELXTL.144 Subsequent cycles of least-squares refinement followed by difference

Fourier syntheses revealed the positions of the remaining non-hydrogen atoms. Non- hydrogen atoms were refined with anisotropic displacement parameters, and hydrogen atoms were added in idealized positions. Selected bond lengths and parameters of hydrogen bonding interactions are shown in Table 4.1 and Table 4.2. X–ray crystal structures of 9H,

11H, and compounds 9-12 (Figure 4.7, Figure 4.8, and Figure 4.9) were generated from

Ortep-3 for Windows145 and rendered using POV-Ray v3.6 for Windows.77 X-ray structures showing hydrogen bonds (Figure 4.7, Figure 4.8, and Figure 4.9) were generated from Mercury 3.3.146

Other physical measurements. 1H NMR and 13C NMR spectra were acquired on a JEOL ECA-600 NMR-spectrometer (600 MHz for 1H and 150 MHz for 13C) with spinning at r.t. HRMS spectra were obtained from a JEOL DART AccuTOF mass spectrometer. 75

Results and Discussion

NMR study. The conformation and stereochemistry of N,Nʹ-diarylformamidines

9-12 have been poorly characterized in solution. Published examples only include characterization for the E-isomer in CDCl3 and C6D6 despite the presence of additional

13,103,124,125,147 signals or are solely reported in DMSO-d6 to simplify the spectra. While these are commonly utilized as ligands in many metal complexes, the complexity of the

1H NMR spectra of these neutral molecules seem to have been overlooked.

N,Nʹ-Diarylformamidines 9-12 (Figure 4.3) were studied through NMR spectroscopy to elucidate the isomerization process. Molecules 10-12 include alkyl substituents on the ortho positions while 9 includes methyl substituents on the meta positions. These molecules were selected to investigate the effect of large substituents on the formamidine dimer formation in solution and its role in E:Z equilibrium in N,Nʹ- diarylformamidines. 76

Figure 4.3 Diarylformamdine isomers in this study. Conformers are denoted by the curved arrows.

Ortho substituted diarylformamidines. 11 exhibited good solubility in various solvents and provided multiple 1H signals through the iPr group for analysis. 10 yielded similar results (Appendix B) to 11 but provides less information with a methyl substituent, therefore the 1H NMR analysis of 11 will be discussed and compared to 9 and

12. 77

1 Figure 4.4 Left: stacked H NMR spectra (aromatic region) of 11 in DMSO-d6, C6D6, CDCl3, and CDCl3 + 1.0 eq. AcOH (bottom to top). Z isomer signals are denoted with red bars. Right: Species present in solution.

Figure 4.4 illustrates the behavior of 11 in solution. In DMSO-d6, the spectrum suggests a symmetric species consistent with the literature148. The high chemical shift of the formamidine proton (H5) and multiplicity suggests an exchange process in the amine proton as seen in the broad amine signal (not shown). This proton exchange is found to be facilitated by the molecule’s ability to form hydrogen bond dimers with itself or other proton donors/acceptors. Interestingly, the chemical shift value of H5 decreases as the electron donating ability of the ortho substituents increase (10 to 12) consistent with previous findings.149 The aromatic signals indicate an exchange between rotamers about the N-CPh single bond. This is reflected in the broad aromatic H4 signal as its local environment changes from an electron rich (amine/imine) to a less electron rich

(formamidine) environment. This broadening of ortho H4 was also observed by Xing et 78

138 al. in their N,Nʹ-di(2-pyridyl)formamidines due to rotation about the N-Cpy bond. The aromatic signals were assigned through HMBC experiments and coupling constant analysis (Appendix B).

Interestingly, the same ligands produced significantly different spectra in CDCl3 and C6D6. The spectra included signals representing a symmetric species (similar to the

DMSO-d6 spectra) but now also included minor contributions from a dissymmetrical species (denoted by red bars). Inspection of the formamidine proton H5 shows a distinct upfield doublet which is indicative of the uncommon Z formamidine isomer.138 The formamidine doublet suggests a much more localized amine proton due to steric inhibition of the hydrogen-bond dimer formation. The Z-isomer shows unique proton signals for the aromatics and is also made clear through the signals of the alkyl substituents (Appendix B). In CDCl3, 11 shows two unique sets of methyl doublets as well as methine septets alongside the symmetric species, suggesting chemical inequivalence between the two phenyl rings and the alkyl substituents. Non hydrogen bonding solvents such as CDCl3 and C6D6 allow for the exchange of geometrical isomers

(E and Z) in N,Nʹ-Diarylformamidines as evidenced through their 1H spectra.

As shown by Xing et al., The equilibrium of this isomerization process can be shifted through the addition of hydrogen-bonding solvent (e.g., AcOH) to the CDCl3 and

C6D6 solutions. Addition of acetic acid to 11 in CDCl3 results in the disappearance of the

Z-isomer contribution (Figure 4.4 top) as a hydrogen-bond adduct is formed between the formamidine and acetic acid effectively locking it into the E form. Competitive hydrogen bonding to the amine proton is a potential explanation for the absence of the Z-isomer in 79

DMSO-d6 in 10 and 11; however, 12 displays the Z-isomer even when analyzed in

DMSO-d6.

1 Figure 4.5 Left: stacked H NMR spectra (aromatic region) of 12 in DMSO-d6, C6D6, CDCl3, and CDCl3 + 1.0 eq. AcOH (bottom to top). Z isomer signals are denoted with red bars. Right: Species present in solution.

12 in DMSO-d6 (Figure 4.5), shows a similar spectrum to that in CDCl3 or C6D6.

The formamidine doublet is present in addition to two sets of inequivalent methyl singlets. In addition, the aromatic signals show unique broadening patterns suggesting hindered N-Cph single bond rotation, resulting in a non-symmetric species. This phenomenon is also observed in the 2 nonequivalent methyl signals (Appendix B). 1.0 eq. of acetic acid was added to the sample to shift the equilibrium toward the E-isomer, however the Z-isomer methyl peaks persist (Appendix B). It seems that addition of a hydrogen bond solvent does shift the equilibrium but the adduct formation is made more difficult due to the steric demands of the tBu group. When acetic acid is added to the 80

CDCl3 sample, complete removal of the Z isomer is achieved (Top spectrum in Figure

4.5). It seems that this equilibrium is somewhat solvent dependent however, it is interesting to see the Z-isomer present in DMSO-d6. To our knowledge, this is the first example of the presence of a Z formamidine isomer in DMSO-d6.

E:Z isomeric ratios. E:Z Isomeric ratios were calculated through comparison of integration values for 1H NMR signals for each isomer. The isomeric ratios for 10-12 were determined by the inegration of the formamidine proton in CDCl3. The calculated ratios were 10:1, 8.3:1, and 5:1 respectively. A general trend in which the Z contribution increases as the size of the alkyl susbstituents increases from 10 to 12 is observed.

Molecules 10-12 were studied in various deuterated solvents to analyze their isomeric exchange in solution while 9 did not appear to exhibit this isomerization. Diaryl formamidines 10 and 11 proved to behave similar to previously published N,Nʹ-di(2- pyridyl)formamidines in the sense that E and Z isomers were observed in non hydrogen bonding solvents while solely the E isomer was observed in DMSO-d6. This phenomenon was attributed to the molecule’s ability to hydrogen bond to itelf or solvent (DMSO and

AcOH). In the absence of a hydrogen bonding solvent, the alkyl substituents at the ortho positions seem to disrupt the dimer formation and lead to an isomerization process to form the rare Z-isomer. Xing et. al. demonstrated that the Z-isomer could be stabilized through the introduction of pyridine nitrogens which partake in intermolecular hydrogen bonding to the formamidine proton. Despite the lack of an extra intermolecular hydrogen bond contact to stabilize the Z form, The Z-isomer can be observed also through destabilization of the formamidine dimer (E form) through simple steric interactions. 81

1 Figure 4.6 Left: stacked H NMR spectra (aromatic region) of 9 in DMSO-d6, C6D6, and CDCl3, (bottom to top). Z isomer signals are denoted with red bars. Right: Species present in solution.

The need for alkyl substituents at the ortho positions to form the Z-isomer is obvious when considering the completeley unsubstituted diarylformamidine and the 3,5 dimethyl diaryl formamidine (9). Figure 4.6 shows the simple NMR spectra of 9 indicating a single symmetric species in CDCl3. The unsubstituted formamidine allows for unhindered rotation about the N-CPh bond and forms a hydrogen bond dimer in solution resulting in a clean and symmetric NMR spectrum in CDCl3. In 9, methyl groups are installed at the meta positions but do not seem to disrupt the intramolecular hydrogen bonding involved in the dimer formation, thus only the E form is observed in CDCl3. It is not until a methyl group is placed at the ortho position, hindering the dimer formation through steric interactions, that the Z-isomer is visible by 1H NMR.

Solid state structural analysis. X-ray crystal structures of compounds 9-12 exhibit distinct hydrogen bond dimers through intramolecular hydrogen exchange about 82 the formamidine backbone. All crystal structures feature dimers in the E-isomeric form.

The Z-isomer would not allow for this dimer formation likely due to steric constraints.

Figure 4.7 illustrates the hydrogen bond dimer of compound 9. The crystal structure features a nearly planar dimer with the phenyl rings rotating out with an averaged rotation angle of 32.2º (Table 4.2). The methyl groups on the meta positions of the phenyl ring to not provide much steric hinderance as they point away from the formamidine core. The ortho aromatic C-H still introduces some steric hinderance, forcing the dimer to break planarity. In N,Nʹ-di(2-pyridyl)formamidines, steric interactions are minimized through the inclusion of a pyridine nitrogen resulting in rotation angles of 19.7º.138 The acidified

9H features the E-isomeric form with a more planar structure due to essentially no steric interactions with the ortho aromatic hydrogens (Figure 4.8 left).

Figure 4.7 Hydrogen bond dimer formation of compound 9. Ellipsoids are shown at the 50% probability level. 83

Figure 4.8 Hydrogen bond dimer formation of acidified compounds 9H (left) and 11H (right). Ellipsoids are shown at the 50% probability level.

Compounds 10-12 incorporate alkyl substituents on the ortho position of the phenyl rings causing greater steric hinderance in the hydrogen bond dimers. In an effort to minimize steric interactions, the methyl groups are placed away from the formamidine proton and are forced toward the center of the dimer. This seems to cause a destabilization of the dimer as the molecules reorient themselves causing a break from

i planarity. Through the increase in the size of the alkyl substituent (i.e. –CH3, - Pr, to – tBu), more steric interactions are introduced resulting in a larger rotation angle (43.30º,

48.32º, to 51.82º, Table 4.2). Figure 4.9 illustrates the hydrogen bond dimers of compounds 10-12. It is evident that the formamidine core is forced to rotate to maintain hydrogen bond contacts in midst of the steric interactions. Again, the acidified 11H features a smaller rotation angle than its neutral counterpart but is still sizeable due to the relatively large isopropyl substituents (Figure 4.8 right). 84

Figure 4.9 Hydrogen bond dimer formation of compounds 10 (left), 11 (middle), and 12 (right). Ellipsoids are shown at the 50% probability level.

85

Table 4.1 Crystallographic data for compounds 9-12.

9•0.5C6H b a b b b a 9H 10 11 11H 12 14 D(2- B(3,5- B(3,5- D(o- i i t Name D(2- PrPh)FH PrPh)FH2(ClO D(2- BuPh)FH Xyl)FH Xyl)FH2 BF4 Tolyl)FH 4) chemical C H N C H N BF C H N C H N C H N Cl O C H N formula 20 27 2 17 21 2 4 15 16 2 19 24 2 19 25 2 4 21 28 2 formula 295.43 340.17 224.30 280.40 380.86 308.45 weight T, K 150(2) 100(2) 100(2) 100(2) 100(2) 100(2) space group P42/nbc Pī Pī Pī P21/n C2/c

Z 16 2 2 2 4 8 a, Å 21.1615(8) 7.2602(7) 7.335(10) 8.1970(5) 13.6336(12) 23.2594(13) b, Å 21.1615(8) 8.3157(9) 8.371(10) 8.7421(5) 8.3723(7) 8.4609(5) c, Å 15.3065(7) 14.8098(15) 10.433(13) 12.0737(7) 17.1837(16) 18.9018(11)

α, deg 90 74.338(6) 74.786(12) 95.095(3) 90 90

β, deg 90 77.827(6) 83.376(16) 102.729(3) 104.689(5) 99.302(3)

γ, deg 90 79.196(6) 80.425(17) 100.872(3) 90 90

V, Å3 6854.4(6) 833.44(15) 607.9(14) 821.07(8) 1897.3(3) 3670.9(4)

-3 Dc, g cm 1.145 1.355 1.225 1.134 1.333 1.116 R c/wR d (all 0.0702/0.1 0.0586/0.13 1 2 0.689/0.1361 0.0557/0.1234 0.0828/0.1215 0.0930/0.1313 data) 697 97 c d R1 /wR2 0.0581/0.1 0.0466/0.13 0.495/0.1246 0.0432/0.1139 0.0464/0.1068 0.0522/0.1141 [I>2(I)] 605 05 Goodness of 1.085 1.099 1.043 1.032 1.030 1.013 fit CCDC deposition 1857008 1857009 1857010 1857011 1857012 1857013 No. a monochromatic radiation (λ = 0.7749 Å). b Mo Kα radiation.

c d 2 2 2 2 2 1/2 R1 = Σ||Fo|  |Fc||/ Σ|Fo|. wR2 = { Σ[w(Fo  Fc ) ]/ Σ[w(Fo ) ]} .

86

Table 4.2 Selected bond lengths (Å), bond angles (°) and torsion angles (°).

Compound 9 9H 10 11 11H 12 C1–N1 1.3119(16) 1.318(3) 1.2936(16) 1.2878(10) 1.3152(14) 1.2788(15) C1–N2 1.304(3) 1.3411(15) 1.3490(10) 1.3168(15) 1.3537(15) C2–N2 1.3128(16) N1–C11 1.406(2) 1.423(3) 1.4154(16) 1.4168(10) 1.4272(15) 1.4227(14) N2–C21 1.4098(19) 1.432(3) 1.4102(17) 1.4160(10) 1.4283(14) 1.4267(15) N1–C1–N2 122.35(19) 123.63(9) 121.44(7) 122.62(11) 122.02(11) N1–C1–N1A 123.0(2) N2–C2–N2A 123.1(2) C1–N1–C11–C12 -150.15(14) -18.4(3) 135.35(10) 127.40(8) -143.43(11) 124.57(12) C1–N2–C21–C22 9.8(3) -138.05(8) -135.96(8) 137.94(12) -131.79(12) C2–N2–C21–C22 -34.6(2) Ave Rotation Angle 32.2 14.1 43.30 48.32 39.32 51.82 (α)

Conclusions

Several N,Nʹ-Diarylformamidines were synthesized and characterized through

NMR, mass spectrometry, and X-ray crystallography. 1H NMR studies revealed the

occurrence of the rare z-isomeric form of the formamidines. While the X-ray crystal

structures revealed only the E-isomeric form stabilized in hydrogen bond dimers,

increasing sterics on the ortho position causes slight distortions to the dimers. The

increase in sterics causes an increase in the Z-isomeric contribution reflected in the

increased rotation angles and 1H NMR spectra. The isomeric distribution can be

manipulated through the use of different deuterated solvents and acid addition. DMSO-d6

was shown previously to eliminate the Z-isomer entirely, but with large steric demands in

12 it is still present. Acetic acid acts as a hydrogen bond acceptor and further stabilized

the E-isomer, completely removing all Z-contribution. The incorporation of alkyl

substituents on the ortho positions of N,Nʹ-diarylformamidines allows for a shift in the

isomerization equilibrium induced by steric destabilization. 87

Fine-tuning the Geometry of Dizinc Complexes through Ligand Sterics

Introduction

Amidines (R1NC(R2)NHR3), a class of N,Nʹ-chelating ligands, have been used extensively in the field of coordination chemistry. They have been used to study s, p, d,

150–158 2 and f block complexes. This class of bidentate ligands includes amidines (R = CR2),

2 2 guanidines (R = NR2) and formamidines (R = H). Phenyl rings have been implemented as R1 and R3 in formamidine ligands to introduce steric interactions and limit the number of metal atoms for complexing. These ligands known as N,Nʹ-diarylformamidines

(DArFHs), have gained popularity due to their facile synthesis and great tunability through substituents on the aryl ring. These ligands are typically synthesized through a 2:1 reaction of substituted anilines and triethyl orthoformate, although slight modifications have been made since one of the earliest reported synthesis in 1963.124,126,159–161

DArFHs have been used to form stable main group complexes primarily used in salt elimination reactions.113,119,162–169 These useful precursors are commonly used to form transition metal and f block formamidinate complexes.170,171 The steric demands of

DArFHs make them well suited for isolating low nuclearity complexes as well as introducing metal-metal interactions in the classic ‘Paddlewheel’ or ‘Lantern type’ complexes.172–176 Studies have shown that formamidinates can bind metals in various binding modes which can be influenced through the metal choice as well as sterics introduced through the ligand; These changes due to steric interactions have been shown 88

to alter the reactivity and catalytic ability of complexes.177–181 Despite the large number of published metal-amidinate complexes, there are few structurally characterized examples of zinc-formamidinates.

Zinc-amidinates are attractive candidates for catalysts due to their low toxicity, abundance, and low cost. Zinc-amidinate complexes have been reported for their catalytic activity in a wide range of chemical transformations including ring opening polymerization of cyclic esters and ε-caprolactone, hydrosilylation reactions, and the Tishchenko reaction.182–185 These complexes are typically synthesized through a salt elimination or hydrocarbon elimination route, although both methods can yield similar products.

Examples of zinc-amidinate complexes include a homoleptic mono zinc complex

{HC(NDipp)2}2Zn (Dipp = 2,6-diisopropylphenyl), a bisformamidinate tetrazinc oxide

i [{HC(NDipp)2}2Zn2Et2]2O, an amide-bridged dizinc complex [(IZn)2(l-N Pr2) {µ-N,N'-

(NDipp)2CH}], and two oxygenated tetranuclear zinc clusters [HC(NR)2]6Zn4O (R = Ph, p-Tol).186–188

In a previous study on Zn-formamidinates,189 our group showed that the nuclearity of zinc-formamidinate complexes can be directly influenced through the introduction of bulky groups on the ortho positions of the aryl ring. Bimetallic systems can be synthesized preferentially depending on the steric demands of the ligand. This work expands on the previous findings regarding the correlation between DArFH sterics and the inclusion of metal atoms but also provides an alternative synthetic route toward dinuclear zinc formamidinate complexes. 89

Experimental

General procedures. All manipulations were carried out using glove-box techniques under nitrogen atmosphere. Glassware was oven-dried at 150 ºC for a minimum of 10 h and cooled in an evacuated antechamber prior to use in the glove box. Hexanes,

THF, and acetonitrile were dried and deoxygenated on a Glass Contour System (SG Water

USA, Nashua, NH) and stored over 4 Å molecular sieves (Strem) prior to use. Chloroform- d and benzene-d6 were purchased from Cambridge Isotope Labs, degassed and stored over

4 Å molecular sieves in the glove box prior to use. Non-halogenated solvents were tested with a standard purple solution of sodium benzophenone ketyl in THF in order to confirm effective oxygen and moisture removal. All N,N'-diarylformamidine ligands were prepared, following a procedure in the literature.124 All other reagents were purchased from commercial vendors and used without further purification unless explicitly stated.

Synthesis. All DArFH ligands were synthesized through the combination of corresponding anilines and triethyl orthoformate (2:1 ratio) via a condensation reaction.124 Complexes 13-15 were formed after reacting corresponding DArFH ligands with commercially available diethyl zinc solution (1.0 M diethylzinc in hexanes, Sigma

Aldrich) at -30 ºC (Figure 5.1). Direct metalation with diethyl zinc offers a simple synthesis route as a single reagent deprotonates the DArFH ligand to generate the corresponding diarylformamidinate (DArF-) in situ as well as provides metal atoms for complexation. The resulting zinc formamidinate complexes are isolated and crystallized for further analysis. 90

Figure 5.1 Direct metalation of N,N'-diarylformamidines with diethyl zinc to yield dizinc-formamidinates (13-15).

Complexes 16 and 17 utilize a slightly different synthetic approach (Figure 5.2).

The appropriate DArFH is first treated with lithium bis(trimethylsilyl)amide to form a lithium formamidinate complex. Once the lithium complex is formed, often evident in the dramatic yellow color, transmetallation is achieved through the addition of anhydrous zinc (II) chloride. The resulting chloride-bridged zinc formamidinates offer a different route toward dizinc complexes.

Figure 5.2 Transmetallation of N,N'-diarylformamidines with LiN(TMS)2 and ZnCl2 to yield dizinc-formamidinates (16-17).

Complex Characterization

[D(o-Tolyl)F]3Zn2Et(THF) (13). 0.89 mL of 1.0 M diethylzinc in hexanes (0.89 mmol) was added to a solution of D(o-Tolyl)FH (0.300 g, 1.34 mmol) in THF (10 mL) at -35 ºC, 91

producing a pale yellow solution upon mixing. Evolution of gas was also observed upon addition of diethylzinc. The solution was allowed to stir overnight at room temperature.

The solvent was removed under vacuum to give a pale yellow solid. This solid was washed with hexanes (3 x 5.0 mL) and dried under vacuum to yield 13 as a white solid. Crystal yield: 0.172 g, 43%. X-ray quality crystals were grown through diffusion-evaporation crystallization with a saturated THF solution of 13 with hexanes diffusing in. Globular

1 crystals were produced in a few days. H NMR (CDCl3, 600 MHz, δ, ppm): 8.03(s, 3H,

N=CH–NH), 7.10 (m, 12H, aromatic C-H), 7.01 (m, 12H, aromatic C-H), 3.47 (br, 4H, O-

CH2-CH2 in THF), 2.30 (s, 18H, Ar-CH3), 1.65 (br, 4H, CH2-CH2-CH2-O in THF), -0.37

13 (t, 3H, Zn–CH2–CH3), -1.16 (q, 2H, Zn–CH2–CH3). C NMR (CDCl3, 150 MHz, δ, ppm):

166.07, 148.87, 132.22, 130.76, 126.79, 124.89, 124.30, 68.47, 25.48, 18.85, 10.97, 0.25.

+ + ESI-MS, m/z: 797.2 ([M-(THF)-Et] , [C45H45N6Zn2] ), theoretical 797.2289.

[D(2-FPh)F]3Zn2Et(THF) (14). 0.89 mL of 1.0 M Diethylzinc in hexanes (0.86 mmol) was added to a solution of D(2-FPh)FH (0.300 g, 1.29 mmol) in THF (10 mL) at -35 ºC, producing a pale yellow solution upon mixing. Evolution of gas was also observed upon addition of diethylzinc. The solution was allowed to stir overnight at room temperature.

The solvent was removed under vacuum to give a pale yellow solid. This solid was washed with hexanes (3 x 5.0 mL) and dried under vacuum to yield 14 as a white solid. Crystal yield: 0.259 g, 65%. X-ray quality crystals were grown through diffusion-evaporation crystallization with a saturated THF solution of 2 with hexanes diffusing in. Colorless

1 blocky crystals were produced in a few days. H NMR (CDCl3, 600 MHz, δ, ppm): 8.48

(s, 3H, N=CH–NH), 7.21 (t, 6H, aromatic C-H), 7.07 (m, 12H, aromatic C-H), 6.98 (m,

6H, aromatic C-H), 3.74 (br, 4H, O-CH2-CH2 in THF), 1.84 (br, 4H, CH2-CH2-CH2-O in 92

13 THF), -0.34 (t, 3H, Zn–CH2–CH3), -1.18 (q, 2H, Zn–CH2–CH3). C NMR (CDCl3, 150

MHz, δ, ppm): 163.97, 156.67, 155.05, 137.25, 124.77, 123.95, 115.90, 68.16, 25.72,

+ + 10.92, 1.49. ESI-MS, m/z: 821.1 ([M-(THF)-Et] , [C39H27N6F6Zn2] ), theoretical

821.0784.

[B(2,6-FPh)F]2Zn2Et2(THF) (15). 1.12 mL of 1.0 M in hexanes (1.12 mmol) was added dropwise to a solution of B(2,6-FPh)FH (0.300 g, 1.12 mmol) in THF (10 mL) at -35 ºC.

White solids formed upon addition of diethyl zinc but were dissolved with stirring. The solution was allowed to stir overnight at room temperature. The solvent was removed under vacuum to give a white solid. The solid was washed with hexanes (3 x 5.0 mL) and dried under vacuum to yield 15 as a white solid. Yield: 0.241 g, 54%. X-ray quality crystals were grown through diffusion-evaporation crystallization with a saturated THF solution of 15 with hexanes diffusing in. Colorless block crystals were produced in a few

1 days. H NMR (C6D6, 600 MHz, δ, ppm): 7.65 (s, 2H, N=CH–NH), 6.57 (m, 8H, aromatic C-H), 6.43 (m,4H, aromatic C-H), 4.01 (br, 4H, O-CH2-CH2 in THF), 1.37 (br,

4H, CH2-CH2-CH2-O in THF), 1.19 (t, 6H, Zn–CH2–CH3), 0.39 (q, 4H, Zn–CH2–CH3).

13 C NMR (C6D6, 150 MHz, δ, ppm): 167.95, 159.24, 157.62, 128.35, 124.36, 111.78,

111.62, 70.47, 31.97, 25.58, 23.06, 14.37, 12.75. ESI-MS, m/z: 929.0 ([M + L]+),

+ [C39H21N6F12Zn2] , theoretical 929.0219.

[D(2,6-Xyl)F]2Zn2Cl2(THF) (16). 0.219 g of lithium bis(trimethylsilyl)amide (1.308 mmol) was added to a solution of B(2,6-Me2Ph)FH (1.189 mmol) in THF (10 mL) at 25

ºC, producing a pale yellow solution upon mixing. The colored solution was allowed to stir for 1 hour at room temperature. 0.243 g of anhydrous ZnCl2 (1.783 mmol) was added and was allowed to stir overnight. The resulting pale yellow solution was dried under vacuum 93

to give a sticky pale yellow precipitate. The crude solids were washed with hexanes (5 x

5.0 mL) until a white powder was obtained. The dried powder was dissolved in 10 mL of toluene and filtered through Celite to remove any undissolved solids. Crystal yield: 0.232 g, 52%. The clear filtrate was used to produce X-ray quality crystals through diffusion- evaporation crystallization with hexanes diffusing in. Blocky colorless crystals of 16 were

1 grown in week. H NMR (C6D6, 600 MHz, δ, ppm): 6.97 (d, 8H, aromatic C-H), 6.93(t,

4H, aromatic C-H), 6.84(s, 2H, N=CH–NH), 3.44 (br, 4H, O-CH2-CH2 in THF), 2.48(s,

13 24H, Ar-CH3), 1.26 (br, 4H, CH2-CH2-CH2-O in THF). C NMR (C6D6, 150 MHz, δ, ppm): 166.98, 146.99, 135.08, 128.55, 128.35, 127.98, 125.44, 19.68. ESI-MS, m/z:

+ + 779.0831 ([M + Cl2 + H2 + Li] , [C34H40N4Zn2Cl4Li] ), theoretical 779.0750.

i [Li(THF)4]{[B(2,6- Pr2Ph)F]2Zn2(µ-Cl)Cl2} (17). 0.112 g of lithium

i bis(trimethylsilyl)amide (0.905 mmol) was added to a solution of D(2,6- Pr2Ph)FH (0.823 mmol) in THF (10 mL) at 25 ºC, producing a bright yellow solution upon mixing. The colored solution was allowed to stir for 1 hour at room temperature. 0.168 g of anhydrous

ZnCl2 (1.23 mmol) was added and was allowed to stir overnight. The resulting pale yellow solution was dried under vacuum to give a pale yellow precipitate. The solids were washed with hexanes (3 x 5.0 mL) to remove the color. The washed solids were then dissolved in 15 mL of benzene and filtered through Celite to remove any undissolved solids. The filtrate was lyophilized to yield 17 as a fluffy white solid.

Isolated yield: 0.346 g, 67%. X-ray quality crystals were grown through diffusion- evaporation crystallization with a saturated THF solution of 17 with hexanes diffusing in.

1 Cylindrical crystals were produced in a week. H NMR (C6D6, 600 MHz, δ, ppm): 7.28(s,

3H, N=CH–NH), 7.13-7.15 (m, 18H, aromatic C-H), 3.86 (sep, 12H, C-CH-(CH3)2), 1.54 94

13 (d, 36H, -CH-(CH3)2), 1.28(d, 36H, -CH-(CH3)2). C NMR (C6D6, 150 MHz, δ, ppm):

165.44, 145.20, 143.62, 128.59, 128.35, 126.53, 123.70, 68.53, 28.37, 25.47, 24.81,

+ + 24.49. ESI-MS, m/z: 1003.3418 ([M + Cl + H2 + Li] , [C50H72N4Zn2Cl4Li] ), theoretical

1003.4296.

X-ray structure determinations. Crystals were carefully sealed with Teflon and

Para-film in a capped vial and stored in a N2 filled zip-lock bag before transportation.

Single crystals suitable for X-ray analysis were coated with deoxygenated Paratone-N oil and mounted on Kaptan loops. Crystallographic data were collected on Beamline 11.3.1 at the Advanced Light Source, Lawrence Berkeley National Lab using monochromatic radiation (λ = 0.7749 Å) at 150(2) K. Raw data was integrated and corrected for Lorentz and polarization effects using Bruker APEX2 v. 2009.1.140 Absorption corrections were applied using SADABS.141 Space group assignments were determined by examination of systematic absences, E-statistics, and successive refinement of the structures. The program

PLATON142 was employed to confirm the absence of higher symmetry for any of the crystals. The position of the heavy atoms was determined using direct methods in the program SHELXTL.144 Subsequent cycles of least-squares refinement followed by difference Fourier syntheses revealed the positions of the remaining non-hydrogen atoms.

Non-hydrogen atoms, excluding the THF solvent molecules in compound 17 were refined with anisotropic displacement parameters, and hydrogen atoms were added in idealized positions. Selected distances, bond angles and torsion angles are shown in Table 5.1 and

Table 5.2. X–ray crystal structures of complexes 13, 14, 15, 16, and 17 (Figure 5.3 and

Figure 5.4) were generated from Ortep-3 for Windows and rendered using POV-Ray v3.6 for Windows.145 95

Table 5.1 Selected bond lengths (Å) and angles (º) in complexes 13, 14, and 15

13 14 15 Zn1Zn2 3.230(4) 3.23457(11) Zn1Zn2 3.455(3) Zn1-O1S 2.105(7) 2.12750(7) Zn1-O1S 2.211(2) Zn2-C61 2.032(11) 2.00586(7) Zn1-C41 2.003(3) Zn1-N1 2.028(7) 1.95581(7) Zn2-O1S 2.233(2) Zn1-N3 1.973(7) 1.98530(6) Zn2-C43 1.996(3) Zn1-N5 1.971(7) 1.98867(5) Zn1-N1 2.032(3) Zn2-N2 2.111(7) 2.08126(6) Zn1-N3 2.044(2) Zn2-N4 2.156(7) 2.10417(7) Zn2-N2 2.019(3) Zn2-N6 2.121(7) 2.09202(8) Zn2-N4 2.046(2) C1-N1 1.342(10) 1.32200(3) C1-N1 1.319(3) C1-N2 1.302(12) 1.32181(4) C1-N2 1.318(3) C21-N3 1.346(11) 1.32752(5) C21-N3 1.322(3) C21-N4 1.292(9) 1.31228(4) C21-N4 1.323(3) C41-N5 1.352(10) 1.31640(4) C41-N6 1.312(10) 1.32130(4) N1-C1-N2 123.1(8) 122.834(2) N1-C1-N2 125.3(2) N3-C21-N4 123.0(8) 122.587(2) N3-C21-N4 124.3(2) N5-C41-N6 123.3(7) 122.837(2) N1-Zn1-N3 113.8(3) 113.8925(12) N1-Zn1-N3 125.3(2) N1-Zn1-N5 114.9(3) 117.9564(13) N2-Zn2-N4 103.1(3) 105.796(2) N2-Zn2-N4 124.3(2) N2-Zn2-N6 109.5(3) 107.7630(13) N1-Zn1Zn2- N1-Zn1Zn2- 34.8(3) -33.6011(14) 0.69(9) N2 N2 N3-Zn1Zn2- N3-Zn1Zn2- 33.4(3) -33.7264(4) -0.39(8) N4 N4 N5-Zn1Zn2- 28.6(3) -32.6517(13) N6

96

Table 5.2 Selected bond lengths (Å) and angles (º) in complexes 16 and 17

16 17 Zn1Zn2 2.911 Zn1Zn2 3.092 Zn1-Cl1 2.312 Zn1-Cl1 2.219 Zn1-O1S 2.036 Zn2-Cl2 2.361 Zn2-Cl1 2.428 Zn2-Cl3 2.218 Zn2-Cl2 2.199 Zn1-N1 1.961 Zn1-N1 1.998 Zn1-N3 1.962 Zn1-N3 2.014 Zn2-N2 1.997 Zn2-N2 2.007 Zn2-N4 1.985 Zn2-N4 2.012 C1-N1 1.324 C1-N1 1.319 C1-N2 1.318 C1-N2 1.332 C21-N3 1.315 C31-N3 1.319 C21-N4 1.310 C31-N4 1.32 N1-C1-N2 113.885 N1-C1-N2 126.257 N3-C21-N4 114.554 N3-C31-N4 127.621 N1-Zn1-N3 127.984 N1-Zn1-N3 123.390 N2-Zn2-N4 122.577 N2-Zn2-N4 122.532 N1-Zn1Zn2- N1-Zn1Zn2- -127.478 -13.471 N2 N2 N3-Zn1Zn2- N3-Zn1Zn2- 129.681 -12.091 N4 N4

Results and Discussion

Structural analysis. Molecular structures of dizinc complexes 13-17 were determined through single crystal X-ray diffraction. The following structural comparison highlights the similarities of complexes 13-15 and compares them to complexes 16 and

17. Complexes 13-15 were synthesized via a direct metallation route with diethyl zinc while complexes 16 and 17 were synthesized through an alternative transmetallation route. Dizinc complexes were formed exclusively through tailoring the amount of sterics on the aryl rings. A detailed structural analysis is provided to identify factors influencing the geometries of resulting complexes. 97

Dinuclear diethyl zinc complexes. Figure 5.3 illustrates the similarity of [D(o-

Tolyl)F]3Zn2Et(THF) (13) and [D(2-FPh)F]3Zn2Et(THF) (14) respectively. Compound 13 crystalized in a P21/c space group while 14 in a P21/n space group. A detailed comparison of the structures and bond matrices of 13 and 14 (Figure 5.3 and Table 2.1) indicate that the two complexes are nearly identical. All zinc centers in 13 and 14 reside in a pseudo- tetrahedral environment with connections to each formamidine N as well as an ethyl or

THF ligand. The complexes feature a pseudo-lantern-type geometry with each formamidine ligand bridging the two zinc centers.

Figure 5.3 X-ray crystal structures of 13 (left), 14 (middle), and 15 (right). Hydrogen atoms are omitted for clarity. Ellipsoids are shown at the 50% probability level.

NMR analysis (Appendix B) of 13 and 14 suggest symmetric formamidine environments as evidenced by the single formamidine N=CH–NH (complexes 13 and 14) and methyl signals (complex 13). This indicates labile THF ligands in solution. N-C bond lengths in the N-C-N formamidine backbone range from 1.30 to 1.34 Å suggesting bond delocalization across the formamidinate binding site. The torsion angles (Table 5.1) in 13 98

and 14 are fairly similar which is likely due to their similar geometries influenced by the similar size of the methyl and fluoro substituents.

Figure 5.3 shows the crystal strucure of 15. This compound crystallized in a P21/c space group with a single molecule in the asymetric unit. X-ray diffraction reveals N-C-N bond lengths all ~1.32 Å suggesting bond delocalization. Each Zn center exhibits a pseudo-tetrahedral environment and is connected to each of the two formamidine ligands.

The torsion angles in 15 are very close to 1º due to only the presence of two ligands because of increased steric demands with diortho fluorine substituents. The complex features a nearly planar structure with a bridging THF molecule. Proton NMR studies suggests a symmetric formamidine environment as evidenced by a single formamidine proton signal. This is in agreement with the crystal structure of 15.

Complexes 13, 14, and 15 feature Zn…Zn separations of 3.230(4), 3.235(11), and

3.455(3) Å respectively which are well beyond the Zn-Zn single bond realm (~2.3 Å)190 suggesting no net bonding between the zinc atoms. This is further confirmed by the d10 electronic configuration of the Zn2+ atoms. Despite not featuring any metal-metal bonds, the dizinc platform allows for reactivity studies. Unfortunately, very limited reaction products were obtained. One of the successful reactions included the formation of chloride bridged species, which promped us to synthesize zinc chloride complexes via a transmetallation route.

13 and 14 utilize similar ligand sterics which ultimately resulted in similar complex geometries. Ortho substituents -CH3 and -F have comparable atomic radii thus resulting in analogous complexes. Comparing these complexes to previously published 99

i 29 examples such as [D(2- PrPh)F]2Zn2Et2(THF) highlights the significant influence of ligand sterics on complex geometry. A change from an ortho iPr to less sterically demanding substituents results in the incorporation of a third formamidine ligand while maintaining two zinc centers.

Complex 15 utilizes two ortho -F substituents which introduces more sterics to the zinc formamidinate. The result is a complex with the inclusion of only two formamidinate ligands. Analogous complexes [D(2,6- Xyl)F]2Zn2Et2(THF) and [D(2- i PrPh)F]2Zn2Et2(THF) feature a similar geometry. This suggests that similar steric demands can be achieved through the use of diortho -F or -CH3 substituents as when a single -iPr substituent is used. There appears to be a direct correlation between the amount of sterics implemented at the ortho positions of the aryl ring and the geometry of resulting zinc-formamidinates.

Chloride-bridged complexes. Figure 5.4 shows the X-ray crystal structures of chloride-bridged complexes 16 and 17. Both structures feature bridging chloride atoms across the two metal centers. Compound 16 utilizes a fairly bulky D(2,6- Xyl)FH ligand to form a bimetallic system. There is one bridging chloride, a terminal chloride, and a terminal labile THF molecule. Both Zn centers remain in the 2+ oxidation state with the inclusion of chloride ligands from the original ZnCl2 reagent. 16 crystallizes in a P21/n space group.

Proton NMR experiments reveal a symmetric formamidine environment due to the presence of a single formamidine and methyl signal. Each Zn center exhibits a pseudo- tetrahedral environment and is connected to each of the two formamidine ligands. Torsion angles for 16 are 129.681º and -127.478º. This large torsion angle is likely due to the presence of a bridging and terminal chloride ligand. 100

Figure 5.4 X-ray crystal structures of 16 (left) and 17 (right). Hydrogen atoms are omitted for clarity. Ellipsoids are shown at the 50% probability level.

Bond metrics reveal a delocalized system with bond equalization about the N-C-N back bone (~1.32 Å, Table 5.2). Once again the Zn…Zn separation is outside of the single bond region with a separation of 2.911 Å. It appears that the bridging chloride causes a smaller metal-metal separation although it is ultimately impossible due to electronic configurations in the Zn2+ atoms.

i Compound 17 utilizes the most sterically demanding ligand in the D(2,6- Pr2Ph)FH ligand. The result is a chloride bridged complex with three chloride ligands. This crystal structure is unique in the sense that a counter ion is required for stabilization. The unit cell features a lithium cation with four THF molecules bound. In this structure, no THF molecule is bound to the Zn atoms likely due to increased sterics from the ligand preventing access to the Zn atoms. 17 crystallizes in a P1 space group. Each unit cell contains 2 molecules of 17 with two lithium counterions. Proton NMR experiments reveal an asymmetric formamidine environment observed in the formamidine and methine signals. 101

In this case, there are two distinct methyl signals caused by the presence of the chloride atoms which create two methyl chemical environments.

Bond metrics also show a delocalized system with similar C-N bond lengths of ~

1.32 Å (Table 5.2). The Zn…Zn is 3.092 suggesting no metal-metal bond. The larger separation can be rationalized through increased ligand-ligand repulsion caused by the large isopropyl groups.

Mass spectrometry studies. Complexes 13-17 were characterized through ESI mass spectrometry despite their sensitivity to air and moisture. Dilute acetonitrile solutions of the dizinc complexes were prepared in a glovebox and placed in a sealed septum-capped GC vial to limit exposure to air. The solution was taken up via syringe and introduced to the mass spectrometer through an ESI source. Ions formed through the loss of an anionic ligand such as an ethyl group forming a positively charged ion from the formal 2+ charge on each Zn atom. Lithium adducts are also observed in the chloride bridged species 16 and 17 due to the small amount of Li from the lithium chloride reagent. Ions containing zinc are easily identifiable due to the five isotopes of zinc. All spectra feature good agreement with the theoretical isotopic distributions191 and exact masses.

Collison-induced dissociation (CID) studies on a linear ion trap were conducted to explore the fragmentation pathways of the zinc containing ions. Complex 13 exhibited an ion formed through the loss of a labile THF molecule and an ethyl ligand. This ion,

+ denoted as [L3Zn2] , was selected and fragmented at 20 a. u. to form a new positively

2- + charged ion [Zn2LL ] (Figure 5.5). This new ion is formed through the loss of a neutral ligand. The nuetral ligand (LH) is formed through the loss of an anionic ligand (L) which 102

then deprotonates a bound anionic ligand, generating a dianionic ligand (L2-). Selecting

2- + the [Zn2LL ] ion as the new precursor ion, it is fragmented at 30 a. u. promoting the loss of a neutral Zn atom to form a new ion [ZnLCDI]+. This loss was confirmed by expanding the isolation width of the precursor and the product ion and revealed a new isotopic distribution with only one zinc atom present. This can also be rationalized through the dianionic ligand (L2-) reducing the Zn2+ to Zn0 as it donates two electrons and forms a stable carbodiimide adduct (CDI). The new precursor ion [ZnLCDI]+ is fragmented at 20 a. u. resulting in the loss of the neutral carbodiimide generating the mononuclear [ZnL]+ ion.

Figure 5.5 CID study of compound 13. Precursor/product ions (boxed in black) and neutral losses (red) are specified.

103

Complexes 14 and 15 follow similar fragmentation mechanisms when analyzed

+ through CID (Appendix C). Interestingly, Complex 15 forms similar [L3Zn2] despite having only 2 bound formamidine ligands in the crystal structure and NMR. Perhaps ion molecule reactions occur between the anion ethyl ligands and neutral formamidine ligands which lead to this ion formation in the gas phase. Complexes 16 and 17 were not able to be studied on the Thermo LTQ so no CID studies were conducted.

Conclusions

A series of stable bimetallic Zn complexes (13-17) supported by DArF ligands were synthesized and characterized. Structural analysis indicates the sterics of ortho substituents greatly influence the resulting nuclearity and geometry of the resulting Zn complexes.

Studies show that DArFs with no or less bulky substituents tend to stabilize higher nuclearity (>2) complexes, while very bulky substituents can form mononuclear species.

Through fine tuning of ligand sterics, bimetallic complexes can be synthesized preferentially. CID studies of diethyl zinc formamidine complexes suggest gas phase reactions through anionic ligands and the zinc centers to yield various neutral losses, which is not commonly explored. This work indicates that sterics dictate the number of metal atoms, number of DArF ligands, and bond metrics of the resulting Zn formamidinate complexes. Both direct metalation and transmetallation syntheses offer facile routes toward dizinc complexes. 104

Weakly Interacting Dicopper Formamidinate Complexes

Introduction

Dicopper formamidinate complexes are an attractive platform to study due to their relatively low cost, potential to form reactive intermediates, and catalytic ability. Group

11 metals are unique in the sense that they form metal-metal interactions classified as metalophilic interactions despite formal bond orders of zero.192,193 These d orbital interactions result in metal-metal separations that are shorter than the sum of their van der

Waals radii, which can help stabilize metal complexes. Aurophilic interactions (Au/Au) have been estimated to have the strength of a typical hydrogen bond.194 Several copper complexes replicate this phenomenon, known as cuprophilicity, seeming to promote the formation of dinuclear and polyatomic copper clusters despite its true origin remaining unknown.195–197

Copper is widely used as a metal cofactor to promote specific functions in biology. Dicopper moieties are relevant in both cyctochrome c oxidase and nitrous oxide reductase (N2OR), which mediate electron transfer in biological systems. Dinuclear mixed-valence Cu2(I,II) in the CuA site of both CcO and N2OR have been identified through X-ray crystallography with short Cu-Cu distances of ~2.5 Å and feature completely delocalized class III systems by electron paramagnetic resonance (EPR) spectroscopy.198–200 Several synthetic analogues have been made featuring similar nitrogen-based ligands and Cu-S bonds to investigate the role of metal-metal interactions 105 in these biological systems.201–205 The proximity of the copper atoms is a feature that many chemists hope to replicate in synthetic analogues and probe through reactivity studies.

Dicopper formamidinate complexes (Figure 6.6.1 middle) were originally synthesized by Cotton’s group in 1988.27 The electronic configuration of Cu1+ prohibited the formation of direct metal-metal bonding. Although the complexes feature short Cu-

Cu separations, Cotton concluded that the close proximity was due to the constraint of the ligand rather than metalophilic interactions. Being primarily interested in complexes featuring metal-metal bonds, the reactivity of dinuclear silver and copper complexes were not investigated further until several decades later. Recent studies on dicopper formamidinates include reactions with CS2 and formations of mixed valence species through the reduction of iodine.206,207 With the limited reactivity studies on this system, our group sought out to stabilize reactive intermediates such as oxo-, nitrene-, and carbene intermediates. Despite not being able to isolate these reactive intermediates, preliminary results in catalytic cyclopropanation and aziridination reactions indicate the possible formation of dicopper carbenes and nitrenes respectively. 106

Figure 6.6.1 Dicopper formamidinate complex (middle), one and two electron oxidized complexes (left), and targeted reactive intermediates (right).

Copper-catalyzed cyclopropanation reactions of olefins using diazoalkanes has been known for more than 45 years. Various copper containing species including copper bronze, copper(II) sulfate, and copper(II) oxide are known for catalytically decomposing diazo compounds primarily forming alkenes derived from alkane coupling of the diazo reagent following dinitrogen extrusion.208 When this decomposition is carried out in the presence of olefins, cyclopropanes are one of the dominant organic products. It is widely agreed that the species responsible for this catalytic transformations are reactive carbene intermediates generated in the diazo decomposition (Figure 6.1.2).209 Catalytic cyclopropanation reactions require the use of a transition metal complex to promote the removal of N2 from the diazo reagent while simultaneously stabilizing transient carbene species and preventing the formation of unwanted carbene dimerization products.210–213

Although various transition metals are capable of performing catalytic cyclopropanation and forming isolatable metal carbenes,214–219 copper is attractive due to its relatively low cost compared to other active transition metals including rhodium and ruthenium. Copper 107 catalysts are also capable of enantioselective C-H insertion,220 enantioselective cycopropanation,221 as well as site selective C-H insertion.222

Figure 6.1.2 General Mechanism for copper catalyzed cyclopropanation

Extensive effort has been dedicated to structurally characterize the proposed copper carbenes involved in cyclopropanation reactions. Despite the long history of diazo decomposition mediated by simple Cu reagents, the copper carbenes went undetected until the early 2000s. Straub and Hofmann utilized an extremely basic and sterically demanding iminophosphanamide ligand to stabilize unique copper coordination modes through strong metal-to-ligand back donation as well as shielding to the copper provided by the bulky ligand.223 They later took advantage of this ligand platform and provided the first experimental evidence of a terminal copper carbene through the use of NMR spectroscopy.224 This work sparked the interest of many inorganic chemists including Dai and Warren. They were able to structurally characterize both monocopper and dicopper carbenes (Figure 6.1.3 left) through the use of a similar copper(I)ethylene complex.225

Hofmann was later able to isolate monocopper and dicopper carbenes (Figure 6.1.3 right) with their iminophosphanamide ligands.226 The X-ray crystal structures revealed structural similarities in the metal “carbenoid” and bridging dicopper carbene intermediates. Although both groups reported catalytic cyclopropanation of olefins 108 through the use of various diazo reagents, studies suggested that only the monocopper carbenes are relevant for cyclopropanation reactions.

Figure 6.1.3 Structurally characterized dicopper carbenes

Analogous to cyclopropanation reactions, copper nitrenes can be transferred to alkenes to form aziridine products. Aziridines are key intermediates in the synthesis of pharmaceuticals and agrochemicals.227 Kwart and Kahn first discovered that copper promoted the decomposition of benzenesulfonyl azide through the extrusion of dinitrogen to form aziridines in the presence of alkenes in 1967.228,229 While azides can react with copper complexes to form copper nitrenes, [N-(p-toluenesulfonyl)imino]phenyliodinane, or PhI=NTS, is one of the most commonly used nitrene transfer reagents. The hypervalent iodine compound readily transfers the nitrene unit while forming stable phenyliodine as a byproduct. While most studies are performed using dirhodium(II) tetracarboxylates and other similar systems,230 copper complexes are an attractive alternative for use in natural product synthesis, medicinal chemistry and materials science. 109

Using a dicopper formamidinate platform, we have shown that these complexes are capable of cyclopropanating styrene via ethyldiazoacetate as well as the aziridination of styrene via PhI=NTS. The formamidinate supporting ligands are much less likely to dissociate into monomeric complexes due to Cu-Cu interactions existing in the starting complex. Unlike the copper complexes utilized by Warren and Hoffmann, we utilize a dicopper species rather than a monocopper ethylene complex which may illustrate that dicopper systems can be relevant in cyclopropanation and aziridination reactions. Similar dicopper systems have been used in cyclopropanation and aziridination of olefins with various reagents, but lack any evidence for a catalytically relevant dicopper system.221,231–

233 More work needs to be devoted to elucidate this interesting phenomenon.

Experimental

Synthesis of dicopper formamidinate complexes. Dicopper formamidinate complexes 18-24 were synthesized through a transmetallation route (Figure 6.1.4) with

LiN(TMS)2 and CuI. 100 mg of various formamidine ligands were dissolved in 10 mL of

THF. 1.2 eq of LiN(TMS)2 were added to the THF solution producing a yellow color.

The yellow solutions were stirred for 1 hour at room temperature. 1.5 eq of CuI were added to the mixture and allowed to stir overnight. The pale yellow solutions were filtered through Celite and the filtrate was pumped down to remove the THF. The obtained pale yellow solids were washed with hexane (3 x 5 mL) to remove byproducts and to remove the yellow color. The crude product was then dried under vacuum before being dissolved in 10 mL of benzene. The benzene solution was filtered through Celite to remove the LiI byproduct. The benzene filtrate was frozen at -30 ºC and lyophilized overnight. Fluffy white solids were obtained in varying yields. Crystals of 18-23 were 110 obtained by dissolving either the crude or purified products in a small amount of THF and placed in a vapor diffusion crystallization set up with hexanes diffusing in. colorless crystals can be obtained in as little as a few days at room temperature or a few weeks at

-30 ºC.

Figure 6.1.4 Transmetallation of N,N'-diarylformamidines with LiN(TMS)2 and CuI to yield dicopper-formamidinates (18-24).

An alternative route to dicopper formamidinates was explored utilizing a direct metalation route with mesitylcopper(I). mesitylcopper was synthesized via a modified published method.234 1.09 g of CuCl was dissolved in 10 mL THF and allowed to cool at

-30 ºC. 10 mL of a 1.0 M solution of 2-mesitylmagnesium bromide in THF was added to the reaction mixture also after cooling at -30 ºC. The solution of 2-mesitylmagnesium bromide was added to the CuI solution and allowed to stir at room temperature for 12 hours. The solution turned yellow and a white precipitate formed. 10 mL of dioxane was then added to the reaction mixture and stirred for an additional 2 hours. The solution was filtered to remove solids and pumped down to remove the solvent. The crude solids were 111 taken up in 10 mL of benzene and filtered again. The filtrate was frozen and pumped down to give a pale yellow product (1.3 g, 65% yield).

Direct metalation with mesitylcopper can be performed through the reaction of

1.0 eq. of formamidine ligand with 1.1 eq. mesitylcopper in THF. The reaction is allowed to take place overnight and is filtered and pumped down. The yellow crude solid can be washed with cold hexane to help remove the yellow color. Crystals can be obtained in

THF/hexane at -30 ºC. The direct metalation route generates mesitylene as a byproduct rather than LiI, which is very difficult to be removes completely. If the cyclopropanation and aziridination catalysis is to be pursued further, the direct metalation route should be used to eliminate the possibility of Li catalyzed reactions.

Preparation of 1H NMR samples. 20-30 mg of the purified complexes were dissolved in deuterated solvent in a glovebox. The solution was placed in a J. Young tube with a Teflon cap before being taken out of the glovebox. The samples were analyzed immediately to prevent decomposition.

Preparation of MS samples. A few milligrams of the purified complexes were placed in plastic capped crystallization vials before being removed from a nitrogen atmosphere. The caps were removed and quickly applied to a glass capillary tube for

DART-MS analysis. Dilute acetonitrile solutions were prepared for ESI as discussed in

Chapter 5.

112

Complex Characterization

[B(2,6-F2Ph)F]2Cu2 (18)

+ + Yield = 16%. DART-MS, m/z: 660.9732 [M+H] , [C26H14N4Cu2F8 + H] , theoretical

660.9761.

[B(2,6-Xyl)F]2Cu2 (19)

1 Yield = 76%. H NMR (CDCl3, 600 MHz, δ, ppm): 7.02 (s, 2H, N=CH–NH), 6.99 (d,

8H, aromatic C-H), 6.89 (t, 4H, aromatic C-H), 3.35 (s, 24H, Ar-CH3). DART-MS, m/z:

+ + 629.1768 [M+H] , [C34H38N4Cu2 + H] , theoretical 629.1767.

i [D(2- PrPh)F]2Cu2 (20)

1 Yield = 53%. H NMR (CDCl3, 600 MHz, δ, ppm): 7.60 (s, 2H, N=CH–NH), 7.21 (dd,

4H, aromatic C-H), 7.09 (dt, 8H, aromatic C-H), 6.97 (dd, 4H, aromatic C-H), 3.75 (sep,

+ 2H, -CH-(CH3)2), 1.17 (s, 24H, Ar-CH3). DART-MS, m/z: 685.2385 [M+H] ,

+ [C38H54N4Cu2 + H] , theoretical 685.2393.

[D(2-FPh)F]2Cu2 (21)

+ + Yield = 15%. DART-MS, m/z: 589.0132 [M+H] , [C26H18N4Cu2F4 + H] , theoretical

589.0138.

[D(2-MePh)F]2Cu2 (22)

1 Yield = 66%. H NMR (CDCl3, 600 MHz, δ, ppm): 7.66 (s, 2H, N=CH–NH), 7.13 (m,

8H, aromatic C-H), 6.97 (m, 8H, aromatic C-H), 2.43 (s, 12H, Ar-CH3). DART-MS, m/z:

+ + 573.1137 [M+H] , [C30H30N4Cu2 + H] , theoretical 573.1141. 113

t [D(2- BuPh)F]2Cu2 (23)

1 Yield = 41%. H NMR (CDCl3, 600 MHz, δ, ppm): 7.63 (s, 2H, N=CH–NH), 7.33 (dd,

4H, aromatic C-H), 7.05 (dt, 4H, aromatic C-H), 7.01 (dt, 4H, aromatic C-H), 6.80 (dd,

+ 4H, aromatic C-H), (s, 36H, Ar-CH3). DART-MS, m/z: 741.3022 [M+H] , [C42H54N4Cu2

+ H]+, theoretical 741.3019.

Synthesis of diphenyldiazomethane. Diphenyldiazomethane was synthesized via a published method.235 A mixture of 13 g of benzophenone hydrazine, 15 g of anhydrous sodium sulfate, 200 mL of ether, 5mL of ethanol saturated with potassium hydroxide, and

35 g of mercuric oxide was placed in a bomb flask and shaken for 75 min while wrapped in a wet towel. The reaction mixture was filtered and the filtrate was concentrated on a rotary evaporator. The resulting oil was frozen on dry ice to produce pink-purple crystals.

Synthesis of PhINTS. PhINTs was synthesized via a published method.221 The nitrene donor was synthesized by adding 3.2 g of iodobenzenediacetate to a methanolic solution of p-toluenesulfonamide (1.73 g) and potassium hydroxide (1.53 g) in an ice bath. Upon addition, the solution turned yellow. The yellow solution was allowed to stir at room temperature for 3 hours and then it was poured into 250 mL of distilled water. A yellow precipitate was formed and the solution was left in the refrigerator overnight. The yellow solid was filtered and washed with distilled water before being dried under vacuum.

General procedure for the cyclopropanation of styrene. 20 mg of [B(2,6- i PrPh)F]2Cu2 (24) (10 mol-%) was dissolved in 5 mL of toluene. 5.0 eq. of styrene was 114 added to the reaction mixture. 200 µL of a 15% solution of EDA in toluene (1.0 eq.) was added to the reaction mixture over a 20 hour period via a syringe pump. The reaction was allowed to stir for 24 hours ensuring complete addition of the diazo compound. Aliquots of the reaction mixture were filtered through alumina to remove the catalyst and any insoluble material prior to gas chromatography analysis. The aliquots were further diluted with methanol before analysis. Identification of products was performed via DART-MS and GC-MS while the quantitation was performed using GC-FID with naphthalene as an internal standard.

General procedure for the aziridination of styrene. 5 mg of compound 24 (5 mol-%) was dissolved in 5 mL of toluene. 10.0 eq. of styrene was added to the reaction mixture. 20 mg of PhINTS was added directly to the reaction mixture due to its low solubility. Small aliquots were analyzed via DART-MS and revealed the presence of the cyclopropanation product. GC-MS and GC-FID studies were not performed on this reaction mixture.

Results and Discussion

Dicopper formamidinates 18-24 (Figure 6.1.5) were synthesized via a transmetallation route by treating the neutral formamidine ligands with lithium bis(trimethylsilyl)amide or LiN(TMS)2 followed by transmetallation with Cu(I) iodide.

The crude product is washed with hexane to remove the bis(trimethylsilyl) amine byproduct. The crude product was then taken up in benzene and filtered to remove the

LiCl byproduct. Crystals were grown in THF/hexanes and produce colorless crystals. 115

The X-ray crystal structures indicate a planar dicopper core with short Cu-Cu separations of ~2.5 Å. The formamidinate complexes feature two Cu1+ centers with electron configurations of [Ar] 3d10. With no d electrons available for metal-metal bonding, the net bond order is zero. With Cu-Cu separations well below the sum of copper’s van der Waals radii (2.8 Å), there appears to be some cuprophilic interactions caused by the full d orbitals.

Figure 6.1.5 X-ray crystal structures of compounds 18-23. Cu-Cu separations are specified below.

Due to the lack of reactivity studies, our group sought out to explore the realm of reactive substrates that may bind or react with this planar dicopper platform. As an initial screening, cyclic voltammetry was used to determine the redox activity of this system.

Cyclic voltammetry was performed on 24 in acetonitrile and suggests one reversible and 116 one quasi-reversible redox event (Figure 6.1.6). A mixed valence and a dicupric species are formed following one and two electron oxidations respectively. Moving to negative potential, the mixed valence species is reformed followed by a stretched reduction to reform the initial Cu2(I,I) species. The stretched redox event is characteristic of a minor structural reorganization. In a similar dicopper system, the binding of coordinating solvent was observed when the Cu2(I,I) system is oxidized to the mixed valence

197 species. It seems that the bound acetonitrile upon oxidation of the Cu2(I,I) core, causes a stretch in the CV trace but is a quasi-reversible process.

Figure 6.1.6 Cyclic Voltammetry of 24 in acetonitrile.

In addition to the X-ray crystal structures, dicopper formamidinates were also studied through NMR and mass spectrometry. Diamagnetic complexes 18-23 are easily 117 characterized through 1H resembling the neutral formamidine ligands in most cases

(Appendix B).

Surprisingly, these complexes ionize well through DART (Appendix C) and ESI.

DART ionization produces both protonated molecules ([M + H]+) and molecular ions

([M+●]) likely through oxidation of one of the Cu atoms. DART ionization typically forms protonated molecules through a proton transfer from protonated water clusters formed through the penning ionization of atmospheric water by metastable helium atoms

(ionization energy 19.8 eV) .236 An alternative mechanism involves direct penning ionization of an analyte whose ionization energy is less than 19.8 eV leading to the formation of molecular ions.237 Another possibility includes trace impurities such as NO+

+● or O2 formed from atmospheric gases which may result in ion-molecule reactions including charge transfer, addition, and oxidation.238 In an attempt to form solely molecular ions, dopant-assisted DART with argon gas (ionization energy 11.55 eV) was performed with dichlorobenzene as a dopant but resulted in very low ion counts despite removing the contribution of protonated molecules. Perhaps this could be revisited using a proper dopant assisted DART setup and different dopant.

ESI was also used to produce solely molecular ions of the dicopper formamidinates. Unlike DART, ESI requires a dilute solution of the copper complexes in a suitable solvent. Dilute solutions were prepared in 2 mL septum capped GC vials under an inert atmosphere. The solutions were then introduced to the mass spectrometer through direct infusion via a syringe pump. Molecular ions are detected along with various adducts. The intensities of the molecular ions are quite low likely due to the low efficiency of oxidation of one of the Cu centers within the ESI source. 118

With two electrons available, several substrates were screened in attempt to form reactive dicopper intermediates. One and two electron oxidations were explored with

FcPF6, FcBPh4, and NOSbF6 resulting in drastic color changes however, no crystals were obtained. Reactions with reagents such as trityl chloride and iodosobenzene displayed promising color changes as well, but also failed to produce crystals. The most promising reagent turned out to be diphenyldiazomethane (Ph2CN2). When reacted with the bright pink Ph2CN2, gas formation was observed along with a dramatic color change to yellow/green. This result was very encouraging and suggested the release of nitrogen gas induced through the coordination to the dicopper platform resulting in some sort of metal carbene. In an attempt to isolate a dicopper carbene, crystals were grown in toluene/hexane. The crystal structure revealed the Ph2C=N-N=CPh2 azine byproduct

(Figure 6.1.7) which is formed through a metal carbene and a diazo reagent. While this may not have been the desired result, it suggests that a copper carbene is formed but is reactive even toward excess diazo compounds in solution.

Figure 6.1.7 X-ray crystal structure of Ph2C=N-N=CPh2 azine byproduct 119

Metal carbenes are extremely reactive species and are typically isolated or detected spectroscopically at low temperatures. Despite efforts to cool reaction mixtures while also ensuring no excess Ph2CN2, the elusive dicopper carbene was not detected by

13C NMR or isolated as a crystalline product. Interestingly, the dicopper formamidinates were able to fully react with 10 eq. of Ph2CN2 at RT in benzene confirmed by infrared spectroscopy (Figure 6.1.8). The N=N diazo stretch is very prominent at 2032 cm-1 for the Ph2CN2 starting material, but is completely removed in the product spectra, suggesting catalytic decomposition of the diazo starting material.

Rather than isolating the copper carbene, catalytic transfer of the carbene unit to alkenes forming cyclopropanes was explored. Preliminary reactions were carried out with

Ph2CN2 and styrene as the carbene acceptor. Reactions were carried out in non- coordinating solvents with slow addition of the diazo compound to minimize the formation of azine and carbene dimer (Ph2C=CPh2) byproducts. Cyclopropanation products were detected through DART-MS alongside the azine and carbene dimer.

Figure 6.1.8 Infrared spectroscopy spectrum of 24 with 10 eq. of Ph2CN2 120

Ethyldiazoacetate (EDA) is a commonly used diazo reagent due to the incorporation of an α-carbonyl which are less prone to catalytic dimerization. Slow addition of EDA to toluene solution of 10 mol % 24 with 5.0 eq. of styrene, produced cis and trans cyclopropanation products at a combined yield of 63% with 8% combined yield of carbene dimer (E and Z) byproducts (Figure 6.1.9). The trans cyclopropane dominates over the cis cyclopropane likely due to steric effects. Enantiomeric excesses within each cyclopropane were not explored and are not expected to be significant due to lack of chirality in the dicopper complex. Reaction conditions including temperature, solvent, catalyst loading, diazo reagent, varying alkene, have not yet been explored but preliminary catalytic cyclopropanation reactions are promising.

Figure 6.1.9 Cyclopropanation of Styrene with EDA and 10 mol-% 24.

121

In addition to carbene transfer reactions, nitrene transfer reactions were also explored. Dicopper formamidinate 24 (5 mol %) was reacted with PhINTs in the presence of excess styrene (10 eq.) in toluene at RT (Figure 6.1.10). The aziridination product was detected through DART-MS. The yields for this reaction have not been determined due to instability of the PhINTS reagent. These preliminary results suggest the formation of a dicopper nitrene in situ, which is then transferred to an alkene.

Figure 6.1.10 Aziridination of Styrene with PhINTS and 5 mol-% 24.

Computational modeling of intermediates. Computational modeling was utilized to investigate the possible structures of the reactive carbene and nitrene intermediates. First, a simplified model of the dicopper formamidinate (Figure 6.1.11 left) was build replacing the phenyl rings with methyl groups. A B3LYP/6-31G*, SDD optimization was performed giving a stationary point with a Cu-Cu separation of 2.577

Å, in good agreement with the obtained x-ray crystal structures. Optimizations with 122 bridging carbene, nitrene, and oxo groups were built from the optimized planar dicopper complex. Similar B3LYP/6-31G*, SDD optimizations were conducted on the intermediates. All calculations gave stationary points, which feature a loss in planarity of the dicopper formamidinate (Figure 6.1.11 bottom). The Cu-Cu separations vary slightly with the different bridging groups. Optimizations on terminally bound carbene, nitrene, and oxo species were also performed but all faced convergence issues or optimized to the bridging species. These preliminary calculations suggest that the carbene, nitrene, and oxo groups could be stabilized by the dicopper platform through a bridging binding mode although more evidence is needed.

Figure 6.1.11 B3LYP/6-31G*, SDD geometry optimizations of a simplified dicopper formamidinate (left), carbene (2nd from the left), nitrene (2nd from the right), and oxo (right) intermediates. A loss is planarity seen in all intermediates is illustrated in the dicopper carbene (bottom).

123

These calculations do not exclude the possibility for a terminal carbene or even a terminal biscarbene. The proposed intermediates are depicted as bridging carbenes based on reviewing the literature and structures of similar systems although there are several possibilities. Figure 6.1.12 depicts several conceivable structures for the key intermediates involved in cyclopropanation and aziridination reactions. Additional work is necessary to determine which structure or structures are involved in the catalysis.

Figure 6.1.12 Possible structures for the metal carbene and nitrene intermediates involved in catalytic cyclopropanation and aziridination reactions.

Conclusions

Several dicopper formamidinate complexes were synthesized and characterized.

The dicopper system is capable of two electron oxidations with evidence suggesting the incorporation of coordinating solvent. The dicopper complexes are able to catalytically decompose diazo reagents including EDA and Ph2CN2. Analysis by mass spectrometry revealed carbene dimerized products as well as cyclopropanation products. Preliminary results suggest that the dicopper formamidinates may be used to catalytically 124 cyclopropanate styrene with EDA. Similarly, evidence indicates their ability to perform aziridination of styrene when treated with a nitrene donor (PhINTS). Although there is no direct evidence for the formation of a dicopper carbene or nitrene, the chemical transformations suggest the need for the reactive intermediates. Mass spectrometry analysis offers a unique avenue to study the reactivity in the gas phase. Future work may include ion molecule reactions in an ion trap mass spectrometer. 125

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APPENDIX A. COMPUTATIONAL DATA

163

Chapter 2: DFT investigation of multiply bonded bimetallic systems

2- Selected MO diagrams with 0.05 contour surface for the Tc2Cl8 anion, derived from calculations using the functional BH&HLYP. Positive and negative values are represented as red and yellow surfaces, respectively. Values underneath each MO figure in the last row are given as the percent of electron density divided among the Tc atom on the left, the Tc atom on the right and eight Cl atoms, respectively. 164

2- Selected MO diagrams with 0.05 contour surface for the Tc2Br8 anion, derived from calculations using the functional BH&HLYP. Positive and negative values are represented as red and yellow surfaces, respectively. Values underneath each MO figure in the last row are given as the percent of electron density divided among the Tc atom on the left, the Tc atom on the right and eight Br atoms, respectively.

165

2- The orbitals between the Tc centers with 0.05 contour surface for the Tc2Cl8 anion, derived from NBO analyses using the functional BH&HLYP. Positive and negative values are represented as blue and green surfaces, respectively. The orbital type between the two

Tc atoms and their electron occupation numbers in parentheses are provided below each orbital. The LP and LP* NBO orbitals were tentatively assigned as the δ and δ*orbitals.

166

3- The orbitals between the Tc centers with 0.05 contour surface for the Tc2Cl8 anion, derived from NBO analyses using the functional BH&HLYP. Positive and negative values are represented as blue and green surfaces, respectively. The orbital type between the two

Tc atoms and their electron occupation numbers in parentheses are provided below each orbital. 167

n- Calculated average Tc-X bond lengths (Å) for the Tc2X8 anions (X = Cl, Br; n=2, 3), derived from calculations using different functionals.a

2- 3- 2- 3- Tc2Cl8 Tc2Cl8 Tc2Br8 Tc2Br8

exp 2.320(4) 2.364(2) 2.473(2) 2.515(2) BH&HLYP 2.353 2.421 2.509 2.573 B3LYP 2.375 2.438 2.531 2.592 OPBE 2.338 2.484 2.538 SVWN 2.316 2.361 2.459 2.500 BP86 2.369 2.426 2.522 2.575 BLYP 2.400 2.462 2.560 2.621 HCTH 2.372 2.530 2.597 O3LYP 2.363 2.516 2.578 X3LYP 2.372 2.435 2.528 2.588 TPSS 2.364 2.421 2.513 2.567 OLYP 2.373 2.438 2.527 2.591 PBE 2.365 2.421 2.517 2.569

TPSSh 2.355 2.505 2.559

a All calculations were carried out with the same basis set SDD for Tc and aug-CC-

PVDZ for Cl and Br. The empty boxes indicated those calculations were not completed. 168

2- a Selected frontier orbitals in the Tc2Cl8 ion.

bonding breakdown of Tc energy Level type in %2Tc %8Cl contribution, % (eV) Tc–Tc s p d

LUMO+9 δ* 5.96 21.85 78.15 0.00 0.00 100 LUMO+5 δ 4.83 22.3 77.70 0.09 0.01 99.90 LUMO+4 σ* 4.66 97.41 2.59 88.04 11.16 0.80 LUMO+3 σ* 4.50 97.98 2.02 88.02 11.55 0.43 LUMO+2 π* 4.40 69.20 30.80 0.11 0.30 99.59 LUMO+1 π* 4.40 69.22 30.78 0.00 0.28 99.72 LUMO δ* 2.46 77.04 22.96 0.00 0.00 100 HOMO δ -1.19 64.43 35.57 0.00 0.00 100 HOMO-2 π -2.13 33.16 66.84 0.00 17.22 82.78 HOMO-3 π -2.13 33.17 66.83 0.00 17.29 82.71 HOMO-14 σ -4.09 49.02 50.98 32.10 12.90 55.00 HOMO-21 σ -5.00 60.51 36.49 75.81 1.18 23.01

a Values for the charge distribution are given as the percent of charge in each level divided among the two Tc atoms and eight Cl atoms. 169

2- a Selected frontier orbitals in the Tc2Br8 ion.

bonding breakdown of Tc energy Level type in %2Tc %8Br contribution, % (eV) Tc–Tc s p d

LUMO+6 δ* 5.15 21.27 78.73 0.00 0.00 100 LUMO+5 σ* 4.49 98.40 1.60 87.86 11.74 0.40 LUMO+4 σ* 4.14 98.73 1.27 86.67 12.36 0.97 LUMO+3 δ 4.07 27.21 72.79 0.00 0.00 100 LUMO+2 π* 4.04 67.76 32.24 0.00 0.45 99.54 LUMO+1 π* 4.04 67.76 32.24 0.00 0.46 99.54 LUMO δ* 2.00 79.05 20.95 0.00 0.00 100 HOMO-1 δ -1.44 64.23 35.77 0.00 0.00 100 HOMO-8 π -2.33 21.24 78.76 0.00 41.11 58.89 HOMO-9 π -2.33 21.23 78.77 0.00 41.12 58.88 HOMO-14 σ -3.86 36.34 63.66 19.31 21.15 59.54 HOMO-22 σ -5.23 88.66 11.34 80.44 5.88 13.68

a Values for the charge distribution are given as the percent of charge in each level divided among the two Tc atoms and eight Br atoms.

170

Chapter 3: Probing Factors Influencing the Metal-Metal Bond Strength in

Bimetallic Systems

Selected bonding MOs with 0.05 contour surface for Re2Cl4(PMe3)4 (3) derived from calculations using the functional BH&HLYP. Positive and negative values are represented as red and yellow surfaces, respectively. Values underneath each MO figure in the last row are given as the percent of electron density divided among the Re atoms,

PMe3 groups and four Cl atoms, respectively.

171

2- Selected bonding MOs with 0.05 contour surface for [Mo2(HPO4)4] (6) derived from calculations using the functional BH&HLYP. Positive and negative values are represented as red and yellow surfaces, respectively. Values underneath each MO figure in the last row are given as the percent of electron density divided among the Mo atoms,

PMe3 groups and four Cl atoms, respectively.

172

4- Selected bonding MOs with 0.05 contour surface for [Mo2(SO4)4] (8) derived from calculations using the functional BH&HLYP. Positive and negative values are represented as red and yellow surfaces, respectively. Values underneath each MO figure in the last row are given as the percent of electron density divided among the Mo atoms,

PMe3 groups and four Cl atoms, respectively.

173

+ The orbitals between the Tc centers with 0.05 contour surface for the [Tc2Cl4(PMe3)4]

(2) derived from NBO analyses using the functional BH&HLYP. Positive and negative values are represented as blue and green surfaces, respectively. The orbital type between the two Tc atoms and their electron occupation numbers in parentheses are provided below each orbital.

174

The orbitals between the Re centers with 0.05 contour surface for the Re2Cl4(PMe3)4 (3) derived from NBO analyses using the functional BH&HLYP. Positive and negative values are represented as blue and green surfaces, respectively. The orbital type between the two Re atoms and their electron occupation numbers in parentheses are provided below each orbital.

175

+ The orbitals between the Re centers with 0.05 contour surface for the [Re2Cl4(PMe3)4]

(4) derived from NBO analyses using the functional BH&HLYP. Positive and negative values are represented as blue and green surfaces, respectively. The orbital type between the two Re atoms and their electron occupation numbers in parentheses are provided below each orbital.

176

2+ The orbitals between the Re centers with 0.05 contour surface for the [Re2Cl4(PMe3)4]

(5) derived from NBO analyses using the functional BH&HLYP. Positive and negative values are represented as blue and green surfaces, respectively. The orbital type between the two Re atoms and their electron occupation numbers in parentheses are provided below each orbital.

177

2- The orbitals between the Mo centers with 0.05 contour surface for the [Mo2(HPO4)4] (6) derived from NBO analyses using the functional BH&HLYP. Positive and negative values are represented as blue and green surfaces, respectively. The orbital type between the two Mo atoms and their electron occupation numbers in parentheses are provided below each orbital.

178

3- The orbitals between the Mo centers with 0.05 contour surface for the [Mo2(SO4)4] (7) derived from NBO analyses using the functional BH&HLYP. Positive and negative values are represented as blue and green surfaces, respectively. The orbital type between the two

Mo atoms and their electron occupation numbers in parentheses are provided below each orbital.

179

3- The orbitals between the Mo centers with 0.05 contour surface for the [Mo2(SO4)4] (8) derived from NBO analyses using the functional BH&HLYP. Positive and negative values are represented as blue and green surfaces, respectively. The orbital type between the two

Mo atoms and their electron occupation numbers in parentheses are provided below each orbital.

180

Experimental (exp) and Calculated M-M Bond Lengths (cal in Å) for Model Compounds

1-8 (M = Tc, Re; n = 0-2, E=HPO4 or SO4). Derived from calculations using functionals

BH&HLYP and B3LYP.a

Bond n+ n+ n- b lengt [Tc2Cl4(PMe3)4] [Re2Cl4(PMe3)4] [Mo2(E)4] h Comp 1 2 3 4 5 6 7 8 ound n = 0 1 0 1 2 2 3 4 FC of 2+ 2.5+ 2+ 2.5+ 3+ 3+ 2.5+ 2+ M 2.127 2.1073( 2.241 2.218 2.208 2.223 2.164 2.111 exp (1) 8) (1) (1) (2) (2) (3) (1) 2.165 2.143 2.271 2.233 2.216 2.209 2.161 2.142 BPE0 M-M BH&H 2.154 2.117 2.268 2.228 2.203 2.197 2.126 2.063 LYP 2.180 B3LYP 2.184 2.156 2.292 2.258 2.239 2.232 2.159 Mo- 2.064 2.394 2.387 2.330 2.291 O 2.01 2.139 exp 2.332[2] [2] [3] [3] [5] (av.) (1) (5) [5]

M-Cl 2.399 2.344 2.408 2.355 2.307 2.004 2.068 2.063 BPE0 (av.) BH&H 2.41 2.35 2.42 2.36 2.32 2.01 2.07 2.16 LYP 2.11 B3LYP 2.43 2.37 2.44 2.38 2.34 2.02 2.09 1.521 2.446 2.418 2.460 2.508 1.55 1.510 exp 2.483[2] [2] [15] [6] [5] (1) (1) [5] O-P M-P or O- 2.434 2.478 2.431 2.480 2.522 1.610 1.596 1.596 (av.) BPE0 S (av.) BH&H 2.47 2.50 2.46 2.50 2.54 1.60 1.58 1.56 LYP B3LYP 2.47 2.52 2.46 2.52 2.57 1.62 1.60 1.61 a All calculations were computed with the same basis sets SDD for metal elements, aug- CC-PVDZ for Cl, P and S, and D95 for C and H. b Water molecules or sulfate anions in the axial positions of the dimolybdenum units were omitted to simplify the calculation.

181

Averaged Mulliken Atomic Charge of Each Element in the Model Compounds 1-8

Obtained from Theoretical Calculations Using Functionals BH&HLYP.

n+ n+ n- [Tc2Cl4(PR3)4] [Re2Cl4(PR3)4] [Mo2(E)4] 1 2 3 4 5 6 7 8 n = 0 1 0 1 2 2 3 4 FC on +2 +2.5 +2 +2.5 +3 +3 +2.5 +2 each M M 1.49 1.67 1.30 1.52 1.68 1.12 1.14 0.94 Cl -0.75 -0.673 -0.761 -0.716 -0.64 P 0.227 0.155 0.315 0.242 0.19 0.755 C -0.850 -0.829 -0.841 -0.821 0.80 -0.539 O -0.573 (O1- -0.536 O3) -0.413 -0.418 -0.479 -0.265 S 0.597 0.561

Averaged Mulliken Atomic Charge of Each Element in the Model Compounds 1-8

Obtained from Theoretical Calculations Using Functionals PBE0.

n+ n+ n- [Tc2Cl4(PR3)4] [Re2Cl4(PR3)4] [Mo2(E)4] 1 2 3 4 5 6 7 8 n = 0 1 0 1 2 2 3 4 FC on +2 +2.5 +2 +2.5 +3 +3 +2.5 +2 each M M 1.47 1.66 1.27 1.48 1.65 0.958 0.971 0.402 - Cl -0.704 -0.632 -0711 -0.657 0.594 P 0.158 0.087 0.244 0.181 0.124 0.574 C -0.062 -0.011 -0.056 -0.001 0.048 O -0.389 -0.413 -0.418 S 0.416 0.438

182

Experimental (exp) and Calculated (cal) M-M and M-X Bond Lengths, and Formal Charge on Each Metal Atom (FC). In the dimetallic units: Electronic Configuration (EC), Formal

Bond Order (FBO), calculated Natural Bond Order (NBO), and calculated Effective Bond

Order (EBO) for model compounds 1-8. Derived from Calculations Using BH&HLYP

Functional and Basis Sets SDD/aug-CC-PVDZ/D95.

n+ n+ n- [Tc2Cl4(PR3)4] [Re2Cl4(PR3)4] [Mo2(E)4] 1 2 3 4 5 6 7 8 2.127(1 2.1073(8 2.241( 2.218( 2.208( 2.223( M–M (Å) 2.164(3) 2.111(1) ) ) 1) 1) 2) 2) Δ in M–M –0.020 –0.023 –0.010 –0.059 –0.053 (in Å ) FC on M +2 +2.5 +2 +2.5 +3 +3 +2.5 +2 σ2π4δ2δ σ2π4δ2δ* σ2π4δ2 σ2π4δ2 EC σ2π4δ2 σ2π4 σ2π4δ1 σ2π4δ2 *2 1 δ*2 δ*1 FBO 3 3.5 3 3.5 4 3 3.5 4 NBO 2.278 2.340 2.293 2.404 2.744 2.636 2.962 3.680 EBO σ 0.782 0.723 0.796 0.769 0.743 0.816 0.848 0.859 π 1.564 1.432 1.576 1.503 1.451 1.684 1.833 1.891 δ 0.00 0.267 0.00 0.266 0.637 0.00 0.528 0.795

183

Calculated Natural Bond Order (NBO in average), and calculated Effective Bond Order

(EBO) for model compounds 1-5. Derived from Calculations Using PBE0 Functional.

n+ n+ [Tc2Cl4(PR3)4] [Re2Cl4(PR3)4] 1 2 3 4 5 M-Cl (Å) 2.399 2.344 2.408 2.355 2.307 NBO (M- 0.465 0.509 0.483 0.526 0.616 Cl) EBO (σ) 0.864 0.842 0.890 0.795 0.856 M-P (Å) 2.434 2.478 2.431 2.480 2.522 NBO (M- 0.390 0.388 0.406 0.398 0.398 P) EBO σ 0.780 0.765 0.805 0.870 0.780

184

APPENDIX B. NMR DATA 185

Chapter 4: N, N′-diarylformamidine ligands

Stacked 1H NMR spectra (methyl region) of compound 9 in various deuterated solvents

186

Stacked 1H NMR spectra (aromatic region) of compound 10 in various deuterated solvents

187

Stacked 1H NMR spectra (methyl region) of compound 10 in various deuterated solvents

188

Zoom in of HMBC spectrum of compound 10 in CDCl3 + 1.0 eq AcOH

189

Full HMBC spectrum of compound 10 in CDCl3 + 1.0 eq AcOH

190

Stacked 1H NMR spectra (methyl region) of compound 11 in various deuterated solvents

191

Zoom in of HMBC spectrum of compound 11 in CDCl3 + 1.0 eq AcOH

192

Full HMBC spectrum of compound 11 in CDCl3 + 1.0 eq AcOH

193

Stacked 1H NMR spectra (methyl region) of compound 12 in various deuterated solvents

194

1 Full H NMR spectrum of DPhFH in CDCl3

195

1 Full H NMR spectrum of 9 in CDCl3

196

Full

1 H NMR spectrum of 10 in CDCl3

197

1 Full H NMR spectrum of 11 in CDCl3

198

1 Full H NMR spectrum of 12 in CDCl3

199

1 Full H NMR spectrum of B(2,6-Xyl)FH + acetic acid in CDCl3

200

1 i Full H NMR spectrum of B(2,6- PrPh)FH + acetic acid in CDCl3

201

1 Full H NMR spectrum of D(2-FPh)FH in CDCl3

202

1 Full H NMR spectrum of B(2-FPh)FH in CDCl3

203

Chapter 5: Dizinc complexes supported by formamidinates

1 H NMR spectrum of compound 13 in CDCl3

204

13 C NMR spectrum of compound 13 in CDCl3

205

1 H NMR spectrum of compound 14 in CDCl3

206

13 C NMR spectrum of compound 14 in CDCl3

207

1 H NMR spectrum of compound 15 in C6D6

208

13 C NMR spectrum of compound 15 in C6D6

209

1 H NMR spectrum of compound 16 in C6D6

210

13 C NMR spectrum of compound 16 in C6D6

211

1 H NMR spectrum of compound 17 in C6D6

212

13 C NMR spectrum of compound 17 in C6D6

213

Chapter 6: Weakly interacting dicopper formamidinate complexes

1 H NMR spectrum of compound 19 in CDCl3

214

1 H NMR spectrum of compound 20 in CDCl3

215

1 H NMR spectrum of compound 22 in CDCl3

216

1 H NMR spectrum of compound 23 in CDCl3

217

APPENDIX C. MASS SPECTRAL DATA

218

Chapter 4: N, N′-diarylformamidine ligand

Positive DART-MS spectrum of compound 9. [M + H]+ and [2M + H]+ are shown.

219

Positive DART-MS spectrum of compound 10. [M + H]+ and [2M + H]+ are shown.

220

Positive DART-MS spectrum of compound 11. [M + H]+ and [2M + H]+ are shown.

221

Positive DART-MS spectrum of compound 12. [M + H]+ and [2M + H]+ are shown.

222

Positive DART-MS spectrum of DPhFH. [M + H]+ and [2M + H]+ are shown.

223

Positive DART-MS spectrum of B(3,5-Xyl)FH. [M + H]+ and [2M + H]+ are shown.

224

Positive DART-MS spectrum of B(2,6-iPrPh)FH. [M + H]+ and [2M + H]+ are shown.

225

Positive DART-MS spectrum of D(2-FPh)FH. [M + H]+ and [2M + H]+ are shown

226

+ + Positive DART-MS spectrum of B(2,6-F2Ph)FH. [M + H] and [2M + H] are shown.

227

Chapter 5: Dizinc complexes supported by formamidinates

Experimental (black) and theoretical (red) mass spectra of compound 13. Observed ion, m/z (black), and exact mass (red) are specified.

228

Experimental (black) and theoretical (red) mass spectra of compound 14. Observed ion, m/z (black), and exact mass (red) are specified.

229

Experimental (black) and theoretical (red) mass spectra of compound 15. Observed ion, m/z (black), and exact mass (red) are specified.

230

Experimental (black) and theoretical (red) mass spectra of compound 16. Observed ion, m/z (black), and exact mass (red) are specified.

231

Experimental (black) and theoretical (red) mass spectra of compound 17. Observed ion, m/z (black), and exact mass (red) are specified.

232

CID study of compound 21. Precursor/product ions (boxed in black) and neutral losses (red) are specified.

233

Chapter 6: Weakly interacting dicopper formamidinate complexes

Positive DART-MS spectrum of 18. [M + H]+ and [M+•] are shown.

234

Positive DART-MS spectrum of 19. [M + H]+ and [M+•] are shown.

235

Positive DART-MS spectrum of 20. [M + H]+ and [M+•] are shown.

236

Positive DART-MS spectrum of 21. [M + H]+ and [M+•] are shown.

237

Positive DART-MS spectrum of 22. [M + H]+ and [M+•] are shown.

238

Positive DART-MS spectrum of 23. [M + H]+ and [M+•] are shown.

239

APPENDIX D. X-RAY DATA

240

Chapter 5: Dizinc complexes supported by formamidinates

X-ray crystal structure of 13 with atom labels. Hydrogen atoms are omitted for clarity. Ellipsoids are shown at the 50% probability level.

241

X-ray crystal structure of 14 with atom labels. Hydrogen atoms are omitted for clarity. Ellipsoids are shown at the 50% probability level.

242

X-ray crystal structure of 15 with atom labels. Hydrogen atoms are omitted for clarity. Ellipsoids are shown at the 50% probability level.

243

X-ray crystal structure of 16 with atom labels. Hydrogen atoms are omitted for clarity. Ellipsoids are shown at the 50% probability level.

244

X-ray crystal structure of 17 with atom labels. Hydrogen atoms are omitted for clarity. Ellipsoids are shown at the 50% probability level.