Diversity of Metal−Ligand Interactions in Halide (X = I, Br, Cl, F) and Halide-Free Ambiphilic Ligand Rhodium Complexes

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• 作者：Bradley E. Cowie ; David J. H. Emslie ; Hilary A. Jenkins ; James F. Britten
• 刊名：Inorganic Chemistry
• 出版年：2010
• 出版时间：May 3, 2010
• 年：2010
• 卷：49
• 期：9
• 页码：4060-4072
• 全文大小：1131K
• 年卷期：v.49,no.9(May 3, 2010)
• ISSN：1520-510X

文摘

Reaction of the neutral ambiphilic ligand 2,7-di-tert-butyl-5-diphenylboryl-4-diphenylphosphino-9,9-dimethylthioxanthene (TXPB) with [{Rh(μ-Cl)(CO)₂}₂] yields [RhCl(CO)(TXPB)] (1) (Emslie et al. Organometallics 2006, 25, 5835). Complex 1 is square planar with the TXPB ligand bound to rhodium via the phosphine and thioether donors (these are features common to complexes 2−5, vide infra). Treatment of 1 with Me₃SiBr and Me₃SiI allowed for halide substitution to afford [RhBr(CO)(TXPB)] (2) and [RhI(CO)(TXPB)] (3), respectively. The halide co-ligands in complexes 1 and 2 form a strong bridging interaction between rhodium and the borane group in TXPB. The presence of stronger borane−halide coordination in 1 is clearly illustrated by an ¹¹B NMR chemical shift of 12 ppm versus 27 ppm in 2. In contrast, the iodide ligand in 3 forms only a weak bridging interaction to boron, leading to a B···I distance of 3.125(7) Å, and an ¹¹B NMR chemical shift of 56 ppm (versus 69 ppm for free TXPB). A lower carbonyl stretching frequency in 3 (2002 cm⁻¹) versus 1 or 2 (2008 and 2013 cm⁻¹, respectively) could be attributed to weakening of the Rh−X bond in 1 and 2 as a consequence of halide−borane coordination and/or a shorter Rh−S bond in complex 3. [Rh(CO)(TXPB−F)] (4) and the halide-free cation [Rh(CO)(TXPB)][PF₆] (5) were accessed by reaction of 1 with [NMe₄]F and Tl[PF₆], respectively. Complex 4 is zwitterionic with fluoride bound to boron [¹¹B NMR δ 4 ppm; B−F = 1.445(6) Å; Rh···F = 3.261(3) Å] and an η²-interaction between the cationic rhodium center and the ipso- and ortho-carbon atoms of a B-phenyl ring in TXPB−F. By contrast, rhodium in 5 engages in an η²-interaction with boron and the ipso-carbon of one B-phenyl ring; Rh−B and Rh−C_ipso bond lengths in 5 are 2.557(3) and 2.362(2) Å, respectively. The long Rh−B distance and an ¹¹B NMR chemical shift of 57 ppm are consistent with only a weak Rh−B interaction in 5, and a CO stretching frequency of 2028 cm⁻¹ (Nujol), versus 2004−2013 cm⁻¹ for complexes 1−4, is indicative of greatly reduced electron density in 5, relative to 1−4.