The role of binuclear iridium complexes in the activation of hydrogen and carbon-dioxide and the formation of carbon-carbon bonds

Jianxin Wu, Purdue University

Abstract

The binuclear iridium complex (Ir$\sb2(\mu$-CNR)(CNR)$\sb4$(dmpm)$\sb2$) (PF$\sb6$) $\sb2$ (1, R = 2,6-Me$\sb2$C$\sb6$H$\sb3$) has been synthesized and structurally characterized. Complex 1 is fluxional in solution. Variable low-temperature $\sp1$H NMR studies of 1 reveal a dynamic process involving the exchange of isocyanide ligands between bridging and terminal positions. Hydrogenation of 1 leads to displacement of one isocyanide ligand and formation of the dihydride (Ir$\sb2(\mu$-H)$\sb2$(CNR)$\sb4$(dmpm)$\sb2$) (PF$\sb6$) $\sb2$ (2). The kinetics of the hydrogenation of 1 to 2 has been studied over the temperature range 0 $\leq$ T $\leq$ 36$\sp\circ$C. The hydrogenation of 1 is first order in (1) and zero order in (H$\sb2$). Reduction of 1 with sodium amalgam leads to the formation of Ir$\sb2(\mu$-CNR)$\sb2$(CNR)$\sb2$(dmpm)$\sb2$ (3). The X-ray structure of 3 reveals a cis, cis-M$\sb2$(dmpm)$\sb2$ and a very short non-bonded C$\cdots$C contact of 2.37(2) A for the $\mu$-isocyanide ligands. Addition of BH$\sb3\sp.$THF to 3 results in a bis-$\mu$-aminocarbyne complex Ir$\sb2\{\mu$-CN(BH$\sb3$)R$\}\sb2$(CNR)$\sb2$(dmpm)$\sb2$ (4). This reaction does not induce carbon-carbon coupling. Reaction of 3 with Al$\sb2$Et$\sb6$ forms Ir$\sb2\{\eta\sp2$-(CNR)$\sb2$AlEt$\sb2\}$(CNR)$\sb2$(dmpm)$\sb2$ (5). The single crystal structure of 5 reveals the coupling of the two bridging isocyanide ligands (d$\sb{\rm C-C}$ = 1.48(1) A) and their condensation with one AlEt$\sb2$ fragment to form a planar C$\sb2$N$\sb2$Al ring. The newly formed carbon-carbon bond in 5 is parallel to the Ir-Ir bond, and can be described as "1,2-dimetallated" olefin. Complex 5 has an unpaired electron and exhibits an isotropic EPR spectrum at $-150\sp\circ$C with g = 2.005. Complex 5 exhibits a reversible one-electron oxidation at $-0.22$ V vs. SCE. This oxidation is accompanied by an isomerization from "cradle" (5) to "A-frame" (5$\sp+$) structure. Complex 3 reacts with carbon dioxide under mild conditions, forming Ir$\sb2\{$CN(CO$\sb2$)R$\}\sb2$(CNR)$\sb2$(dmpm)$\sb2$ (6). Complex 6 is labile and releases two equivalents of carbon dioxide under heating. Complex 6 reacts with methylene chloride to form a carbamoyl complex (Ir$\sb2(\mu$-H)($\mu$-CO)$\{$C(O)NHR$\}\sb2$(CNR)$\sb2$-(dmpm)$\sb2$) Cl (7). Hydride and carbamoyl ligands are confirmed unambiguously by NMR and FTIR spectroscopies. The $\sp{13}$CO$\sb2$ labelling studies indicate an unprecedented oxygen atom transfer route. Complex 7 ultimately results from an oxygen atom transfer from carbon dioxide to a coordinated isocyanide ligand. (Abstract shortened with permission of author.)

Degree

Ph.D.

Advisors

Kubiak, Purdue University.

Subject Area

Chemistry

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