Mono- vs Bis(carbene) Complexes: A Detailed Study on Platinum(II)−Benzimidazolin-2-ylidenes

The reaction of PtBr2 with NaOAc and 1,3-diisopropylbenzimidazolium bromide (A) in DMSO afforded
the mixed monocarbene−DMSO complex cis-[PtBr2(DMSO)(iPr2-bimy)] (cis-1) and the bis(carbene)
complex trans-[PtBr2(iPr2-bimy)2] (trans-2). The DMSO ligand in cis-1 can be easily replaced by stronger
donors such as triphenylphosphine and pyridine to give novel benzannulated monocarbene complexes
trans-[PtBr2(iPr2-bimy)(PPh3)] (trans-3), cis-[PtBr2(iPr2-bimy)(PPh3)] (cis-3), and trans-[PtBr2(iPr2-bimy)(Pyridine)] (trans-4), respectively. All compounds have been fully characterized by multinuclei NMR
spectroscopies and mass spectrometry (ESI, FAB). X-ray diffraction studies on single crystals of cis-1,
trans-2, cis-3, and trans-4 revealed a square-planar geometry and a fixed orientation of the N-isopropyl
substituents with the C−H protons pointing to the metal center to maximize interesting and rare C−H···Pt preagostic interactions. These interactions are also retained in solution, as indicated by the large
downfield shift of the isopropyl C−H protons in the 1H NMR spectrum compared to that in the precursor
salt A.

将溴化铂(II)与乙酸钠、1,3-二异丙基苯并咪唑鎓溴化物（化合物A）在二甲基亚砜（DMSO）中反应，得到混合单卡宾-二甲基亚砜配合物顺式-[PtBr₂(DMSO)(iPr₂-bimy)]（顺式-1，其中iPr₂-bimy为二异丙基苯并咪唑卡宾配体）以及双卡宾配合物反式-[PtBr₂(iPr₂-bimy)₂]（反式-2）。顺式-1中的二甲基亚砜配体可被三苯基膦（PPh₃）、吡啶等更强的给体配体取代，分别得到新型苯并稠合单卡宾配合物反式-[PtBr₂(iPr₂-bimy)(PPh₃)]（反式-3）、顺式-[PtBr₂(iPr₂-bimy)(PPh₃)]（顺式-3）以及反式-[PtBr₂(iPr₂-bimy)(吡啶)]（反式-4）。所有化合物均通过多核核磁共振波谱法以及电喷雾电离（ESI）、快原子轰击（FAB）质谱完成了完整表征。对顺式-1、反式-2、顺式-3以及反式-4的单晶进行X射线衍射分析，结果显示其均具有平面正方形几何构型，且N-异丙基取代基呈现固定取向，其C-H质子指向金属中心，以最大化这种罕见且有趣的C-H···Pt预agostic相互作用。与前体盐A相比，异丙基C-H质子在1H核磁共振谱中出现显著的低场位移，这表明此类相互作用在溶液中同样存在。