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Platinum and Palladium Isocyanides as Platforms on which to Study Metal/Lewis Acid Cooperativity and Ligand-based Redox-Noninnocence

  • Author(s): Barnett, Brandon Richard
  • Advisor(s): Figueroa, Joshua S
  • et al.
Abstract

This dissertation describes various investigations into the reactivity and electronic structure of platinum and palladium complexes supported by m-terphenyl isocyanides. Use of these encumbering ligands facilitated the formation of two-coordinate Pt(CNArDipp2)2, which serves an isolable mimic of the unstable carbonyl Pt(CO)2. Importantly, Pt(CNArDipp2)2, along with its palladium congener Pd(CNArDipp2)2, represent the only monomeric and zero-valent binary isocyanide complexes of these metals. These complexes can act as the Lewis basic component of Metal-only Lewis Pairs (MOLPs), binding thallium(I) and silver(I) via retrodative σ-bonds. Reactivity studies of Pt(CNArDipp2)2 culminated in the discovery of a very rare singly-buttressed metal-borane adduct Pt(κ2-N,B-Cy2BIM)(CNArDipp2), which is synthesized via hydroboration of a coordinated isocyanide ligand to form an ambiphilic (boryl)iminomethane (BIM) ligand. This complex exhibits rich reactivity with small molecules via metal/borane cooperation. It is shown to effect various E-H and E-X bond activations, as well as oxidative insertions of organoazides, organocarbonyls and organonitriles, most of which represent unprecedented reactivity modes for metal-borane adducts. Unligated Cy2BIM can also be synthesized upon hydroboration of CNArDipp2 with dicyclohexylborane. It is shown to be monomeric in solution, allowing it to act as a highly competent Frustrated Lewis pair despite bearing a Lewis acid of only moderate acidity.

Two vignettes of ligand-based redox-noninnocence can be found in Chapters 5 and 6. The trinuclear dianion K2[Pt3(µ-CO)3(CNArDipp2)3] and radical anion K(THF)4[Pt3(µ-CO)3(CNArDipp2)3] were synthesized, notable as the all-carbonyl variants [Pt3(CO)6]2–/1– are unstable and have never been crystallographically characterized. Most importantly, it is shown that the highest occupied molecular orbital in these complexes is primarily CO/CNR π*, producing the first example of an ensemble of CO and isocyanide ligands exhibiting redox-noninnocence. Finally, solution dynamics of the palladium bis-nitroxide diradicals trans-Pd(κ1-N-ArNO)2(CNArDipp2)2 were examined to gain insights into their stability and mode of decomposition. Judicious electronic modulation of the redox-active nitrosoarene ligands revealed that installation of para-formyl or para-cyano substituents greatly increased the kinetic stability of the corresponding diradicals, signaling a potentially general strategy for the stabilization of inherently short-lived classical nitroxide spin adducts.

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