Rhodium(I) and Iridium(I) Complexes of the Conformationally Rigid IBioxMe₄ Ligand: Computational and Experimental Studies of Unusually Tilted NHC Coordination Geometries

Computational methods have been used to analyze distorted coordination geometries in a coherent range of known and new rhodium­(I) and iridium­(I) complexes containing bioxazoline-based NHC ligands (IBiox). Such distortions are readily placed in context of the literature through measurement of the Cnt­(NHC)–CNCN–M angle (ΘNHC; Cnt = ring centroid). On the basis of restricted potential energy calculations using cis-[M­(IBioxMe4)­(CO)2Cl] (M1; M = Rh, Ir), in-plane (yawing) tilting of the NHC was found to incur significantly steeper energetic penalties than orthogonal out-of-plane (pitching) movement, which is characterized by noticeably flat potential energy surfaces. Energy decomposition analysis (EDA) of the ground-state and pitched structures of M1 indicated only minor differences in bonding characteristics. In contrast, yawing of the NHC ligand is associated with a significant increase in Pauli repulsion (i.e., sterics) and reduction in M→NHC π back donation, but is counteracted by supplemental stabilizing bonding interactions only possible due to the closer proximity of the methyl substituents with the metal and ancillary ligands. Aided by this analysis, comparison with a range of carefully selected model systems and EDA, distorted coordination modes in trans-[M­(IBioxMe4)2­(COE)Cl] (M2; M = Rh, Ir) and [M­(IBioxMe4)3]+ (M3; M = Rh, Ir) have been rationalized. Steric interactions were identified as the major contributing factor and are associated with a high degree of NHC pitching. In the case of Rh3, weak agostic interactions also contribute to the distortions, particularly with respect to NHC yawing, and are notable for increasing the bond dissociation energy of the distorted ligands. Supplementing the computational analysis, an analogue of the formally 14 VE Rh­(I) species Rh3 bearing the cyclohexyl-functionalized IBiox6 ligand ([Rh­(IBiox6)3]+, Rh3-Cy) was prepared and found to exhibit an exceptionally distorted NHC ligand (ΘNHC = 155.7(2)°) in the solid state.本研究采用计算方法，对一系列包含基于双噁唑啉的氮杂环卡宾（N-Heterocyclic Carbene, NHC）配体（IBiox）的已知及新型铑(I)和铱(I)配合物中扭曲的配位几何构型开展分析。通过测量NHC的环质心（ring centroid, Cnt）-CNCN-金属角（记为ΘNHC），可将此类扭曲结构与已有文献研究置于统一的对比框架中。以顺式-[M(IBioxMe4)(CO)₂Cl]（M1，M为铑、铱）为对象开展限制性势能计算，结果发现，NHC的平面内（偏航）倾斜所带来的能量惩罚显著高于正交平面外（俯仰）运动；后者的势能面呈现明显平坦的特征。对M1的基态及俯仰构型开展能量分解分析（Energy Decomposition Analysis, EDA），结果显示二者的成键特征仅存在细微差异。与之形成对比的是，NHC配体的偏航倾斜会导致泡利排斥（即空间位阻）显著升高，同时金属到NHC的π反馈配位作用减弱；但由于甲基取代基与金属及辅助配体的距离更近，可形成额外的稳定化成键相互作用，从而抵消了上述不利影响。借助上述分析，并结合一系列精心筛选的模型体系与能量分解分析结果，反式-[M(IBioxMe4)₂(COE，环辛烯，cyclooctene)Cl]（M2，M为铑、铱）及[M(IBioxMe4)₃]⁺（M3，M为铑、铱）中的扭曲配位模式得到了合理解释。研究发现空间位阻是主要贡献因素，且与NHC的高程度俯仰倾斜相关。对于Rh3而言，弱agostic相互作用（agostic interaction）也对配位扭曲有所贡献，尤其在NHC偏航倾斜过程中，此类相互作用可显著提升扭曲配体的键解离能，这一点值得关注。作为计算分析的补充验证，本研究合成了形式价电子数为14（valence electrons, VE）的Rh(I)物种Rh3的环己基官能化IBiox6配体衍生物[Rh(IBiox6)₃]⁺（记为Rh3-Cy），并通过实验发现其固态结构中存在极度扭曲的NHC配体（ΘNHC = 155.7(2)°）。