Cationic Re(III) complexes [ReX2(PMe3)2(biimH2)]X (where biimH2
= 2,2â²-biimidazole; X = Cl, Br, I) are prepared by phosphine displacement in the corresponding PPh3 complexes in the presence of excess PMe3. The NâH protons of co-ordinated biimidazole are moderately acidic and the monodeprotonated form [ReX2(PMe3)2(biimH)] is isolated after addition of one equivalent of NaOCH3. Crystal structures determined for two compounds of each series reveal that strong hydrogen bonding takes place between the NâH groups and the halide counter-ion in the cationic complexes, whereas the neutral molecules crystallise as dimeric units tightly associated via two complementary NâHâ¯N interactions. The acidities of the two NâH groups of [ReCl2(PMe3)2(biimH2)]+
in CH2Cl2 are estimated from UV-visible spectra and found to correspond approximately to those of formic acid and 3-chlorophenol, respectively. Electrochemical data on the series of PPh3 and PMe3 complexes with the three halides indicate that the redox potentials of both the metal and the counter-ion are affected by ion pairing in solution.
阳离子 Re(III) 复合物 [ReX2(PMe3)2(biimH2)]X(其中 biimH2 = 2,2â²-
双咪唑;X = Cl、Br、I)是在过量 PMe3 的存在下,通过膦置换相应的 PPh3 复合物制备的。配位
双咪唑的 NâH 质子呈中度酸性,加入一个当量的 NaOCH3 后,分离出单去质子化形式 [ReX2(PMe3)2(biimH)]。为每个系列中的两种化合物测定的晶体结构显示,阳离子配合物中的 NâH 基团和卤化物反离子之间有很强的氢键作用,而中性分子则结晶为二聚体单元,通过两个互补的 NâHâ¯N 相互作用紧密结合在一起。根据紫外-可见光谱估算出了
CH2Cl2 中 [ReCl2(PMe3)2(biimH2)]+ 的两个 NâH 基团的酸度,发现它们分别与
甲酸和
3-氯苯酚的酸度近似。PPh3 和 PMe3 与三种卤化物的系列配合物的电
化学数据表明,
金属和反离子的氧化还原电位都受到溶液中离子配对的影响。