Pyrazolyl-N-heterocyclic carbene complexes of rhodium as hydrogenation catalysts: The influence of ligand steric bulk on catalyst activity
作者:Michael J. Page、Jörg Wagler、Barbara A. Messerle
DOI:10.1039/b906724h
日期:——
A series of bidentate 1-(1-pyrazolylmethyl)-substituted NHC ligands (13a–c, 14a–c and 15a–c) were synthesised with substituents of varying steric bulk incorporated adjacent to the donor atoms. These ligands were coordinated to rhodium(I) to give a series of complexes of the general formula [Rh(L)(COD)]BPh4 (where L = a mixed-donor pyrazolyl-NHC ligand and COD = 1,5-cyclooctadiene). The solid state structures of [Rh(13b)(COD)]BPh4 (16b), [Rh(13c)(COD)]BPh4 (16c), [Rh(14a)(COD)]BPh4 (17a), [Rh(14b)(COD)]BPh4 (17b), [Rh(15a)2(COD)]BPh4 (18a), and [Rh(15b)(COD)]BPh4 (18b) were determined by single crystal X-ray diffraction. The complex [Rh(15a)2(COD)]BPh4 (18a) is unusual in that two of the pyrazolyl-NHC ligands (15a) are coordinated to the metal through the NHC donor instead of one ligand forming the expected chelate. These complexes (with the exception of 18a) were found to be effective catalysts for the hydrogenation of styrene. The catalytic activity was correlated with complex structure, and it was found that the greater the steric bulk of the metal bound ligand, the slower the rate of the hydrogenation.
合成了一系列双齿的1-(1-吡唑基甲基)-取代的NHC配体(13a–c、14a–c和15a–c),这些配体的取代基在供体原子旁边具有不同的空间体积。这些配体与铑(I)配位,形成了一系列具有一般分子式[Rh(L)(COD)]BPh4的配合物(其中L为混合供体吡唑基-NHC配体,COD为1,5-环辛二烯)。通过单晶X射线衍射确定了[Rh(13b)(COD)]BPh4(16b)、[Rh(13c)(COD)]BPh4(16c)、[Rh(14a)(COD)]BPh4(17a)、[Rh(14b)(COD)]BPh4(17b)、[Rh(15a)2(COD)]BPh4(18a)和[Rh(15b)(COD)]BPh4(18b)的固体结构。配合物[Rh(15a)2(COD)]BPh4(18a)不同寻常之处在于,两个吡唑基-NHC配体(15a)是通过NHC供体与金属配位,而不是一个配体形成预期的螯合物。这些配合物(除18a外)被发现是苯乙烯加氢反应的有效催化剂。催化活性与配合物结构相关,发现金属配合的配体的空间体积越大,加氢反应的速率越慢。