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UNSATURATED Pt PINCER COMPLEXES and ATTEMPTS TOWARD THE UNSATURATED Pt PINCER COMPLEXES AND ATTEMPTS TOWARD THE ISOLATION OF A METHANE -COMPLEX A Dissertation by JESSICA CARMAN DEMOTT Submitted to the Office of Graduate and Professional Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Chair of Committee, Oleg V. Ozerov Committee Members, Kevin Burgess Mark T. Holtzapple Matthew Sheldon Head of Department, François Gabbaï December 2015 Major Subject: Chemistry Copyright 2015 Jessica Carman DeMott ABSTRACT Alkane -complexes have been identified as key intermediates in C-H bond activation and subsequent functionalization of alkanes. Methane and larger saturated alkanes, however, are sterically and electronically disinclined to bind to transition metals. In particular, a thermodynamically stable methane σ-complex intermediate has not been attained or structurally characterized. This research aimed to make progress toward the development and isolation of stable organometallic methane σ-complexes. 16-electron d8 Pt(II) complexes are typically stable square planar, 4-coordinate species that require ligand dissociation in order to provide an open coordination site for methane. Accessing stable three coordinate Pt(II) species, however, is a challenge. Novel Pt silylene pincer ligand scaffolds, however, were found to undergo a net 1,2-migration of R from Pt to Si, yielding highly unsaturated Pt cations with weakly coordinating carborane anions. Apparent protodesilylation of mesitylene resulted in two Pt cationic units linked together by a siloxane bridge where unsaturation at the Pt centers was retained. Attempts to isolate unsaturated three-coordinate Pt species with bis(phosphine)diarylamido-based (PNP) pincer ligands were frustrated by rapid C-H activation of various arene reaction solvents. When dichloromethane was used as a solvent, formation of the paramagnetic species [(PNP)PtCl]+ was observed. This is consistent with the radical abstraction of a chlorine atom and oxidation of the ligand, demonstrating the system’s potential for metalloradical behavior. Overall, the suspected ii (PNP)Pt+ intermediate is a highly reactive species that exhibits both strongly electrophilic and metalloradical behavior. Exploring alternative pincer ligand scaffolds for generating stable methane - complexes lead us to the use of carbazole pincer ligands that have lower oxidation potentials, a less basic central nitrogen, and a new steric profile. In contrast to previously synthesized Pt triflate complexes, facile displacement of triflate by water was observed for a Pt complex of a bis(imino)carbazolyl (NNN) pincer ligand containing mesityl sidearms. The steric bulk projected by the mesityl groups of the NNN ligand may favor the coordination of smaller ligands such as water. Unexpectedly facile cyclometallation of the mesityl sidearm was also observed. iii DEDICATION To Mom and Dad iv ACKNOWLEDGEMENTS First and foremost I would like to thank my advisor, Dr. Oleg V. Ozerov. It has been a long path and certainly an impossible one without you. Thank you for your contagious curiosity, your inspiration, and your challenges. I’ve deeply appreciated the opportunities you’ve built for me over the last several years. Thank you to my committee members: Dr. Kevin Burgess, Dr. Mark Holtzapple, Dr. Matthew Sheldon, and Dr. Hong-Cai Zhou for their support and guidance. I would also like to acknowledge the various funding sources that supported this work: US National Science Foundation, the Welch Foundation, IPMI, and BASF. I would like to thank all the members of the Ozerov group (past, present, and honorary) that have made this journey all the more interesting, fun, and endurable: Sam Timpa, Jillian Davidson, Chun-I Lee, Loren Press, Billy McCulloch, Chandra Palit, Chris Pell, Wei-Chun Shih, John Dekarske, Linda Redd, Rita Silbernagel, Alex Kosanovich, Patrick Hubbard, Qingheng Lai, Soomin Park, Rafael Huacuja, Rodrigo Ramirez, Jose Veleta, Chelsea Mandell, Yanjun Zhu, Weixing Gu, Justin Walensky, Dan Smith, Dave Herbert, Jia Zhou, Samantha Yreugas, Claudia Fafard, Sophia Koziatek, Sylvain Gatard, Wei Weng, and Lei Fan. I would like to thank Dr. Joanna Goodey-Pellois for all the mentorship over the last five years. Thank you to the Chemistry Department, PLU, WISE, and GSAC for support, inspiration, and patience. I deeply appreciated the help from Philip Wymola and Melvin Williams for supplies for the troops; Greg Wylie and Steve Silber for NMR v support; Dr. Nattamai Bhuvanesh for X-ray solutions; Prof. Dmitry G. Gusev and Panida Surawatanawong for computational contributions; Yohannes Rezenom and Vanessa Santiago for Mass Spec; Ron Carter and Curtis Lee for lab repairs; and of course the Graduate Advising and Business offices: Sandy Manning, Valerie McLaughlin, Crystal King, Simon North, and François Gabbaï. I would like to thank all of my family for selflessly supporting this journey, often from behind the scenes and without acknowledgement. I’d like to thank all the women in the family who stepped in as mom. Thank you for the love, the insanity, the memories, and the wisdom: Melissa, Vanessa, Aunt Betty, Grandma Carmen, Aunt Doreen, Aunt Edie, Holly, Heather, Aunt Linda, Jennifer, Shannon, Elizabeth, Amanda, Janet, Jill and Christina Brammell. Thank you to Morgan MacInnis. Special thanks to my mom for her creativity, artistic nature, and bravery and special thanks to my dad for introducing me to Einstein. vi TABLE OF CONTENTS Page ABSTRACT .......................................................................................................................ii DEDICATION .................................................................................................................. iv ACKNOWLEDGEMENTS ............................................................................................... v TABLE OF CONTENTS .................................................................................................vii LIST OF FIGURES ............................................................................................................ x LIST OF TABLES .........................................................................................................xxii LIST OF CHARTS ........................................................................................................ xxiv I. INTRODUCTION .......................................................................................................... 1 1.1 Overview .................................................................................................................. 1 1.1.1 Industrial Uses of Methane ................................................................................ 1 1.1.2 Platinum-Catalyzed Oxidation of Methane: The Shilov System ...................... 4 1.1.3 Improved Catalysts for Platinum-Catalyzed Methane Conversion ................... 8 1.2 Coordination of C-H σ-Bonds to Transition Metals .............................................. 10 1.2.1 Impetus for Improving the Understanding of Alkane -Complexes............... 10 1.2.2 E-H Bonds as Ligands for Transition Metals .................................................. 11 1.3 Transition Metal σ-Alkane Complexes in the Literature ....................................... 25 1.3.1 Observation of Alkane σ-Complexes in Matrices ........................................... 25 1.3.2 Transition Metal σ-Alkane Complexes Characterized in Solution ................. 26 1.3.3 Transition Metal σ-Alkane Complexes Characterized in the Solid-State ....... 31 1.3.4 Direct Observation of a Rhodium -Methane and -Ethane Complexes ....... 40 1.4 Isolation of Unsaturated Pt Complexes and the Role of Trans-Influence.............. 42 II. SILYL-SILYLENE INTERPLAY IN CATIONIC PSiP PINCER COMPLEXES OF PLATINUM ..................................................................................................................... 46 2.1 Introduction ............................................................................................................ 46 2.2 Results and Discussion ........................................................................................... 47 2.2.1 Synthesis of Neutral Pt Complexes ................................................................. 47 2.2.2 Synthesis of Cationic Pt Complexes ............................................................... 48 2.2.3 Isomerization of Silylene Complex ................................................................. 49 vii 2.2.4 DFT Calculations ............................................................................................ 51 2.2.5 Hydrolysis of Silylene ..................................................................................... 55 2.3 Conclusions ............................................................................................................ 57 2.4 Experimental .......................................................................................................... 58 2.4.1 General Considerations ................................................................................... 58 2.4.2 Synthesis and Characterization of Ligand and Pt Complexes ......................... 59 2.4.3 X-Ray Structural Determination Details ......................................................... 85 2.4.4 Computational Details ..................................................................................... 91 III. FRUSTRATED LEWIS PAIR-LIKE SPLITTING OF AROMATIC C–H BONDS AND ABSTRACTION OF HALOGEN ATOMS BY A CATIONIC [(FPNP)Pt]+ SPECIES .........................................................................................................................
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