The Conformations of 13-Vertex <i>ML</i>₂C₂B₁₀ Metallacarboranes: Experimental and Computational Studies

The docosahedral metallacarboranes 4,4-(PMe2Ph)2-4,1,6-closo-PtC2B10H12, 4,4-(PMe2Ph)2-4,1,10-closo-PtC2B10H12, and [N(PPh3)2][4,4-cod-4,1,10-closo-RhC2B10H12] were prepared by reduction/metalation of either 1,2-closo-C2B10H12 or 1,12-closo-C2B10H12. All three species were fully characterized, with a particular point of interest of the latter being the conformation of the {ML2} fragment relative to the carborane ligand face. Comparison with conformations previously established for six other ML2C2B10 species of varying heteroatom patterns (4,1,2-MC2B10, 4,1,6-MC2B10, 4,1,10-MC2B10, and 4,1,12-MC2B10) reveals clear preferences. In all cases a qualitative understanding of these was afforded by simple MO arguments applied to the model heteroarene complexes [(PH3)2PtC2B4H6]2- and [(PH3)2PtCB5H6]3-. Moreover, DFT calculations on [(PH3)2PtC2B4H6]2- in its various isomeric forms approximately reproduced the observed conformations in the 4,1,2-, 4,1,6-, and 4,1,10-MC2B10 species, although analogous calculations on [(PH3)2PtCB5H6]3- did not reproduce the conformation observed in the 4,1,12-MC2B10 metallacarborane. DFT calculations on (PH3)2PtC2B10H12 yielded good agreement with experimental conformations in all four isomeric cases. Apparent discrepancies between observed and computed Pt−C distances were probed by further refinement of the 4,1,2- model to 1,2-(CH2)3-4,4-(PMe3)2-4,1,2-closo-PtC2B10H10. This still has a more distorted structure than measured experimentally for 1,2-(CH2)3-4,4-(PMe2Ph)2-4,1,2-closo-PtC2B10H10, but the structural differences lie on a very shallow potential energy surface. For the model compound a henicosahedral transition state was located 8.3 kcal mol-1 above the ground-state structure, consistent with the fluxionality of 1,2-(CH2)3-4,4-(PMe2Ph)2-4,1,2-closo-PtC2B10H10 in solution.本研究涉及的二十二面体金属碳硼烷包括4,4-二甲基苯基膦合-4,1,6-闭式-铂碳硼烷(4,4-(PMe2Ph)2-4,1,6-closo-PtC2B10H12)、4,4-二甲基苯基膦合-4,1,10-闭式-铂碳硼烷(4,4-(PMe2Ph)2-4,1,10-closo-PtC2B10H12)以及[双(三苯基膦)胺][4,4-环辛二烯(cyclooctadiene, cod)-4,1,10-闭式-铑碳硼烷]([N(PPh3)2][4,4-cod-4,1,10-closo-RhC2B10H12])，上述化合物均通过1,2-闭式碳硼烷(1,2-closo-C2B10H12)或1,12-闭式碳硼烷(1,12-closo-C2B10H12)的还原/金属化反应制备得到。对这三种产物均完成了全面表征，其中尤为值得关注的是铑基金属碳硼烷中{ML2}片段相对于碳硼烷配体面的构象。将该构象与此前已确定的六种不同杂原子取代模式的ML2C2B10类物种（4,1,2-MC2B10、4,1,6-MC2B10、4,1,10-MC2B10及4,1,12-MC2B10）的构象进行比对，可发现明确的构象偏好性。在所有案例中，通过简单的分子轨道(Molecular Orbital, MO)理论分析模型杂芳烃配合物[(PH3)2PtC2B4H6]^2-和[(PH3)2PtCB5H6]^3-，即可对这些构象偏好性获得定性理解。此外，对[(PH3)2PtC2B4H6]^2-的多种异构体形式进行密度泛函理论(Density Functional Theory, DFT)计算，其结果大致重现了4,1,2-、4,1,6-及4,1,10-MC2B10物种中观测到的构象；但针对[(PH3)2PtCB5H6]^3-的类似计算，却未能重现4,1,12-MC2B10型金属碳硼烷中的观测构象。对(PH3)2PtC2B10H12的DFT计算结果，则与四种异构体的实验构象均吻合良好。针对观测到的Pt-C键长与计算值之间的显著偏差，我们通过将4,1,2-模型进一步精修至1,2-(CH2)3-4,4-三甲基膦合-4,1,2-闭式-铂碳硼烷(1,2-(CH2)3-4,4-(PMe3)2-4,1,2-closo-PtC2B10H10)对该偏差展开了探究。尽管该精修模型的结构仍比1,2-(CH2)3-4,4-二甲基苯基膦合-4,1,2-闭式-铂碳硼烷(1,2-(CH2)3-4,4-(PMe2Ph)2-4,1,2-closo-PtC2B10H10)的实验测量结构更具畸变，但结构差异所处的势能面非常平缓。针对该模型化合物，我们找到了一个比基态结构高出8.3 kcal·mol^-1的二十一面体过渡态，这与溶液中1,2-(CH2)3-4,4-(PMe2Ph)2-4,1,2-closo-PtC2B10H10的流变行为相符。