Electronic Properties of N-Heterocyclic Carbene (NHC) Ligands: Synthetic, Structural, and Spectroscopic Studies of (NHC)Platinum(II) Complexes

N-Heterocyclic carbene complexes of platinum(II) have been synthesized, notably monocarbene
complexes cis-[(IPr)Pt(dmso)(Cl)2], 6, cis-[(IMes)Pt(dmso)(Cl)2], 7, cis-[(SIPr)Pt(dmso)(Cl)2], 8, cis-[(SIMes)Pt(dmso)(Cl)2], 9, and cis-[(TTP)Pt(dmso)(Cl)2], 10. All complexes have been fully characterized
by multinuclear NMR spectroscopy. Complex 7, 9, and 10 have been characterized by X-ray
crystallography. The data obtained have allowed for the differentiation between electronic contributions
(σ and π) present in the Pt−NHC bond. Supported by computational analyses, the percentage of π back-donation from the metal to the NHC is found to be on the order of 10%. More interestingly, we find that
saturated NHC (SIPr and SIMes) are more efficient π back-acceptors than their unsaturated NHC congeners
(IPr and IMes). The synergistic effect between π back-donation and σ donation present in the saturated
NHC systems results in increased electron density at the platinum center compared to the bonding situation
in the unsaturated NHC examples.

本研究合成了一系列铂(II)的氮杂环卡宾（N-Heterocyclic carbene, NHC）配合物，其中代表性单卡宾配合物包括cis-[(IPr)Pt(dmso)(Cl)₂]（化合物6）、cis-[(IMes)Pt(dmso)(Cl)₂]（化合物7）、cis-[(SIPr)Pt(dmso)(Cl)₂]（化合物8）、cis-[(SIMes)Pt(dmso)(Cl)₂]（化合物9）以及cis-[(TTP)Pt(dmso)(Cl)₂]（化合物10）。所有配合物均通过多核核磁共振波谱法（multinuclear NMR spectroscopy）完成全面表征；其中化合物7、9与10进一步通过X射线晶体衍射法（X-ray crystallography）完成表征。所获得的实验数据可用于区分Pt-NHC键中存在的σ与π电子相互作用组分。结合计算分析结果，研究发现金属中心向NHC配体的π反馈电子占比约为10%。更值得关注的是，饱和型NHC（SIPr与SIMes）相较于其不饱和同系物（IPr与IMes），是更高效的π反馈电子受体。饱和NHC体系中存在的π反馈与σ给电子协同效应，使得铂中心的电子密度相较于不饱和NHC配合物体系有所提升。