Rhodium(III) and Iridium(III) Complexes of a Neopentyl-Substituted PNP Pincer Ligand that Feature Agostic Interactions

The synthesis and characterization of five-coordinate rhodium­(III) and iridium­(III) complexes of the form [M­(PNP-Np)­(biph)]­[BArF4] are described, where PNP-Np is the neopentyl-substituted pincer ligand 2,6-(Np2PCH2)2C5H3N (Np = CH2tBu), biph = 2,2′-biphenyl, and ArF = 3,5-(CF3)2C6H3. These complexes are notable for the adoption of δ-agostic interactions in the solid state, as evidenced by X-ray crystallography (50–150 K) and ATR-IR spectroscopy, but are structurally dynamic in solution, exhibiting pseudorotation of the biph ligand on the 1H NMR time scale (185–308 K). The strength of the agostic interactions is discussed with reference to the known tert-butyl-substituted analogues [M­(PNP-tBu)­(biph)]­[BArF4], probed by reaction with carbon monoxide, and quantified computationally through NBO analysis, from which the conclusion is that 3-center–2-electron bonding increases in the order M = Ir > Rh (cf. 1.5× greater perturbation energy) and pincer ligand = PNP-Np > PNP-tBu (cf. 3.3× greater perturbation energy).

本文报道了通式为[M(PNP-Np)(biph)][BArF₄]的五配位铑(III)与铱(III)配合物的合成与表征。其中，PNP-Np为新戊基取代的钳形配体2,6-(Np₂PCH₂)₂C₅H₃N（Np = CH₂tBu），biph为2,2′-联苯(2,2′-biphenyl)，ArF为3,5-双(三氟甲基)苯基(3,5-(CF₃)₂C₆H₃)。
该类配合物的突出特点是在固态下可形成δ-agostic相互作用，该结论经X射线晶体学(X-ray crystallography，测试温度范围50~150 K)与衰减全反射红外光谱(ATR-IR)证实；但在溶液中其结构具有动态性，在氢核磁共振(¹H NMR)时间尺度(185~308 K)下可观测到联苯配体的假旋转现象。
本文结合已报道的叔丁基取代类似物[M(PNP-tBu)(biph)][BArF₄]，通过与一氧化碳的反应探究其agostic相互作用的强度，并通过自然键轨道(NBO)分析开展量化计算，最终得出结论：三中心两电子键的强度遵循M=Ir > Rh的顺序（微扰能量高出约1.5倍），且钳形配体的强度顺序为PNP-Np > PNP-tBu（微扰能量高出约3.3倍）。