0-0 Bond Formation and the Transition Metal Chemistry of p-Diketiminate, Siloxide, and Triamide Ligands by MASSACHUSETTS INSTITUTE Michael Pesek Marshak OF TECHNOLOG Y B. A. Cornell University | JUN 1 5 2112 Ithaca, NY 2006 LIBRARIES SUBMITTED TO THE DEPARTMENT OF CHEMISTRY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTORATE OF PHILOSOPHY AT THE ARCHIVES MASSACHUSETTS INSTITUTE OF TECHNOLOGY JUNE 2012 @ 2012 Massachusetts Institute of Technology All rights reserved Signature of Author................................ ... .... .. .. .. - . ... Michael P. Marshak Department of Chemistry Ar 120, 2012 C ertified by........................................... Ihaniel G. Nocera The Henry Dreyfus Professor of Energy and Professor of Chemistry 7) Thesis Supervisor A ccepted by ............................................................. V.............. ......................... Robert W. Field Chairman, Departmental Committee on Graduate Students This doctoral thesis has been examined by a Committee of the Department of Chemistry as follows: Richard R. Schrock Frederick G. Keyes Professor of Chemistry Committee Chairman ~A/ 7 Daniel G. Nocer The Henry Dreyfus Professor of Energy andProfessor of Chemistry Thesis Supervisor Christopher C. Cummins Professor of Chemistry 2 0-0 Bond Formation and the Transition Metal Chemistry of p-Diketiminate, Siloxide, and Triamide Ligands by Michael Pesek Marshak Submitted to the Department of Chemistry on May 11, 2012 in Partial Fulfillment of the Requirements for the Degree of Doctorate of Philosophy in Chemistry Abstract The electrochemical splitting of water into hydrogen and oxygen has been proposed as an alternative means to store electrical energy. The limiting aspect of this reaction is the oxygen forming reaction, which can be catalyzed by transition metal species with varying degrees of efficiency. This thesis examines the characteristics of oxygen bonding that complicate the 0-0 bond formation reaction, and examines ligand platforms that can stabilize high valent metal oxo intermediates. Siloxide ligands were used to generate a series of 4-coordinate Cr, Mn, Fe, and Co complexes, but these could not support the corresponding high valent oxo species. Instead, a remarkably stable 4-coordinate Crv tetrasiloxide complex was isolated. Chelating triamide ligands were explored, which might generate pseudotetrahedral metal oxo species, but complexes of various metal ions could not be reliably isolated. Planar bis-p- diketiminate (NacNac) complexes were synthesized by reaction of acetonitrile with Fe and Co mesityl species. The Co complex undergoes a ligand centered oxidation event to yield the first structurally characterized NacNac radical cation. In contrast to known redox noninnocent ligand platforms, no significant changes in C-C or C-N bond lengths are observed by X-ray crystallography. DFT calculations and the electronic and structural characterization of the oxidized and reduced Co complex confirm these conclusions. Thesis Supervisor: Daniel G. Nocera Title: The Henry Dreyfus Professor of Energy and Professor of Chemistry 3 This thesis is dedicated to my parents Daniel and Joan, and to my brother Andrew, for the endless love and support they have given me throughout my lfe. 4 Table of Contents A bstract..........................................................................................................................................3 Table of C ontents...........................................................................................................................5 List of Figures..............................................................................................................................10 List of Tables................................................................................................................................13 List of Schem es.............................................................................................................................15 Chapter 1: The Complexity of 0-0 Bond Formation ........................................................ 16 1.1 Oxygen ...................................................................................................................... 17 1.2 Biological and Synthetic W ater Oxidation .......................................................... 19 1.2.1 Photosystem II.......................................................................................... 19 1.2.2 Synthetic W ater Oxidation Catalysts......................................................... 19 1.3 The M echanism of 0-0 Bond Form ation ................................................................ 21 1.3.1 a V ersus 7 Bonding in 02......................................................................... 21 1.3.2 "Sphixis".................................................................................................. 21 1.3.3 Electrochemical Aspects and Electron Transfer....................................... 25 1.3.4 Therm odyam ics ........................................................................................ 26 1.3.5 Role of the Catalyst ................................................................................. 27 1.4 Ligand Design and Scaffolds for Oxidizing Species ............................................. 32 1.5 References ................................................................................................................. 34 Chapter 2: Synthesis and Redox Activity of bis-p-Diketiminate Cobalt............................38 5 2.1 Introduction...............................................................................................................39 2.2 Results ....................................................................................................................... 41 2.2.1 Reaction with Acetonitrile........................................................................ 41 2.2.2 Characterization of 1 ................................................................................. 42 2.2.3 Electrochem istry and Synthesis of 2 ........................................................ 50 2.2.4 Synthesis of Fe(1-mesitylbutane-1,3-diimine) 2 (3) ................................. 55 2.3 Discussion ................................................................................................................. 57 2.4 Conclusions ............................................................................................................... 61 2.5 Experim ental Section ............................................................................................. 64 2.5.1 General Considerations............................................................................. 64 2.5.2 Physical M ethods...................................................................................... 64 2.5.3 X-ray Crystallographic Details................................................................. 65 2.5.4 Computational M ethods .......................................................................... 65 2.5.5 Preparation of Co(1-mesitylbutane-1,3-diimine) 2 (1). ............................. 66 2.5.6 Preparation of [Co(1 -mesitylbutane- 1,3-diimine) 2(THF)2] [PF 6] (2)........67 2.5.7 Preparation of Fe(1 -mesitylbutane- 1,3-diim ine) 2 (3)............................... 67 2.5.8 X-ray Crystallographic Tables.................................................................. 68 2.5.9 Cartesian Coordinates for Optim ized Structures ...................................... 70 2.6 References ................................................................................................................. 74 Chapter 3: First Row Transition Metal Siloxide Chemistry of the "DTBMS" Ligand........79 3.1 Introduction...............................................................................................................80 3.1.1 Electronic Considerations........................................................................ 80 6 3.1.2 G eom etric Considerations ........................................................................ 82 3.1.3 Synthetic Considerations.......................................................................... 84 3.2 Titanium /V anadium Siloxides .................................................................................. 86 3.2.1 Attem pted Syntheses ............................................................................... 86 3.3 Chrom ium Siloxides..................................................................................................87 3.3.1 Synthesis of Chrom ium Siloxide Species.....................................................87 3.3.2 Redox Chem istry ...................................................................................... 92 3.3.3 Spectroscopy............................................................................................. 92 3.3.4 D FT Calculations of 3 ............................................................................... 95 3.3.5 D FT calculations of hypothetical (M e3 SiO)3 CrO ......................................... 96 3.3.6 Comparison to Solid State Cr4. ................................. ........................ .... 97 3.4 M anganese Siloxides............................................................................................. 99 3.4.1 Synthesis.................................................................................................... 99 3.5 Iron Siloxides..........................................................................................................101