A combined experimental and theoretical study of bimetallic bis- and tris-homocubane analogues

Abstract

Synthesis of various metal-incorporated bis- and tris-homocubane analogues has been reported. Room temperature reactions of [Cp*MCl₂]₂ (Cp* = η⁵-C₅Me₅, M = Ir or Rh) with chalcogenated borohydride reagents, Li[BH₂E₃] (E = S or Se), yielded a series of bimetallic bis- and tris-homocubane derivatives (1–7). The bishomocubane analogues belong to the 1,3-bishomocubane family with the general formula [(Cp*M)₂(μ-E)₂(μ₃-E)₄(μ₃-BH)₂] (1: M = Ir, E = S; 2: M = Ir, E = Se; 5: M = Rh, E = S and 6: M = Rh, E = Se), and [(Cp*Ir)₂(μ-S)₂(μ₃-S)₄(μ-BH₂)₂], 3, can be described as an unusual bishomocubane having two (μ-BH₂) units with a missing-bond. In addition to these bishomocubanes, two trishomocubane derivatives [(Cp*M)₂(μ-E)₃(μ₃-E)₄(μ₃-BH)₂] (4: M = Ir, E = S and 7: M = Rh, E = Se) were isolated from the above reactions. Trishomocubane 4 adopts a 1,2,4-trishomocubyl structure, whereas 7 is a D₃-trishomocubyl analogue. In a similar fashion, thermolysis of [Cp*CoCl]₂ with Li[BH₂E₃] (E = S or Se) led to the formation of Co-1,3-bishomocubane analogues, [(Cp*Co)₂(μ-E)₂(μ₃-E)₄(μ₃-BH)₂] (8: E = S and 9: E = Se). All the compounds were characterized by multinuclear NMR and IR spectroscopies and mass spectrometric analysis. The core geometries of 1–4 and 8 were unequivocally established by single-crystal X-ray diffraction studies. Density functional theory (DFT) computations further demonstrated that metals and chalcogen atoms play an important role in determining the thermodynamic stability of the bis- and tris-homocubane species.