Highly Reactive Cyclic(alkyl)(amino) Carbene- and N-Heterocyclic Carbene-Bismuth(III) Complexes: Synthesis, Structure, and Computations.

Abstract

Cyclic(alkyl)(amino) carbene (CAAC)-stabilized complexes of phosphorus, one of the lightest group 15 elements, are well-established and can often be obtained in high yields. In contrast, analogous CAAC compounds of bismuth, the heaviest nonradioactive member of group 15, are unknown. Indeed, reactivity increases as you descend the group, and as a result there are only a few examples of N-heterocyclic carbene (NHC)-bismuth complexes. Moreover, activated bismuth compounds often readily extrude bismuth metal, making isolation of stable complexes highly challenging. We report that CAACs react with phenylbismuth dichloride (PhBiCl2) to afford Et2CAAC-Bi(Ph)Cl2 and CyCAAC-Bi(Ph)Cl2. Significantly, these complexes represent the first structurally characterized examples of CAAC-coordination to bismuth. The CAAC-stabilized bismuth compounds can also be obtained from air-stable salts, [Et2CAAC-H]22+ [Cl2(Ph)Bi(μ-Cl2)Bi(Ph)Cl2]2- and [CyCAAC-H]22+ [Cl2(Ph)Bi(μ-Cl2)Bi(Ph)Cl2]2-, by deprotonation with potassium bis(trimethylsilyl)amide, K[N(SiMe3)2]. The electronic effects of the ligand on the bismuth center were investigated by comparing the CAAC-Bi(Ph)Cl2 complexes to the NHC analogues, SIPr-Bi(Ph)Cl2(THF) and IPr-Bi(Ph)Cl2 (SIPr = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazole-2-ylidene; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene). Interestingly, the "normal" IPr-Bi(Ph)Cl2 slowly isomerizes to the "abnormal" carbene complex, Cl2(Ph)Bi-IPr-H, at -37 °C. In the solid-state, the CAAC-, NHC-, and abnormal NHC-bismuth compounds exhibit Bi atomic centers in unique coordination environments. The complexes were fully characterized by NMR, elemental analysis, and single crystal X-ray diffraction studies. In addition, the bonding was probed by natural bond orbital (NBO) calculations.