School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia.
Dalton Trans. 2009 Sep 21(35):7029-38. doi: 10.1039/b906724h. Epub 2009 Jul 21.
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.
一系列双齿 1-(1-吡唑基甲基)-取代的 NHC 配体（13a-c、14a-c 和 15a-c）被合成，其中取代基具有不同的空间位阻，位于供电子原子的相邻位置。这些配体与铑（I）配位，得到一系列通用式为[Rh(L)(COD)]BPh4（其中 L 为混合供电子吡唑基-NHC 配体，COD=1,5-环辛二烯）的配合物。[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）的固体结构通过单晶 X 射线衍射确定。配合物[Rh(15a)2(COD)]BPh4（18a）很不寻常，因为两个吡唑基-NHC 配体（15a）通过 NHC 供电子体而不是一个配体与金属配位，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的有效催化剂。催化活性与配合物结构相关，发现与金属结合的配体的空间位阻越大，加氢反应的速率越慢。
