Unsymmetric Dinuclear Rh<sup>I</sup><sub>2</sub> and Rh<sup>I</sup>Rh<sup>III</sup> Complexes Supported by Tetraphosphine Ligands and Their Reactivity of Oxidative Protonation and Reductive Dechlorination
Takayuki Nakajima, Mami Maeda, Aya Matsui, Mariko Nishigaki, Momoko Kotani, Tomoaki Tanase
Abstract
Two linear tetradentate phosphine ligands, meso-Ph2PCH2P(Ph)CH2XCH2P(Ph)CH2PPh2 (X = CH2 (meso-dpmppp), NBn (meso-dpmppmNBn; Bn = benzyl)) were utilized to synthesize unsymmetrical dinuclear RhI complexes, [Rh2Cl2(meso-dpmppp)(L)] (L = XylNC (1a), CO (1b)) and [Rh2Cl2(meso-dpmppmNBn)(L)] (L = XylNC (1c), CO (1d)), where electron-deficient RhI → RhI centers with 30 valence electrons are supported by a tetraphosphine in an unusual cis-/trans-P,P coordination mode. The RhI dimers of 1a–d were treated with HCl under air to afford the RhI → RhIII dimers with 32 e–, [Rh2Cl4(meso-dpmppp)(L)] (L = XylNC (4a), CO (4b)) and [Rh2Cl4(meso-dpmppmNBn)(L)] (L = XylNC (4c), CO (4d)), via intermediate hydride complexes, [{RhCl2(μ-H)RhCl(L)}(meso-dpmppp)] (L = XylNC (2a), CO (2b)) and [{RhCl2(μ-H)RhCl(L)}(meso-dpmppmNBn)] (L = XylNC (2c), CO (2d)), and [{Rh(H)Cl2(μ-Cl)Rh(L)}(meso-dpmppp)] (L = XylNC (3a), CO (3b)) and [{Rh(H)Cl2(μ-Cl)Rh(L)}(meso-dpmppmNBn)] (L = XylNC (3c), CO (3d)). The hydride intermediates 2 and 3 were monitored under nitrogen by 1H{31P} and 31P{1H} NMR techniques to reveal two reaction pathways depending on the terminal auxiliary ligand L. Further, the reductive dechlorination converting RhIRhIII (4b,d) to RhI2 (1b,d) was accomplished with a CO terminal ligand by reacting with various amines that acted as one-electron reducing agents through an inner-sphere electron transfer mechanism. DFT calculations were performed to elucidate the electronic structures of 1a–d and 4a–d and to estimate the structures of the hydride intermediate complexes 2 and 3.