Synthesis of rhenium–platinum complexes bearing carbaboraneligands and their unusual dynamic behavior

Abstract

Treatment of Cs[3,3,3-(CO)₃-closo-3,1,2-ReC₂B₉H₁₁] 1 with [Pt(NCMe)₂{Ph₂P(CH₂)₂PPh₂}][BF₄]₂ yielded [3,3,3-(CO)₃-3,8-(Pt{Ph₂P(CH₂)₂PPh₂})-8-(μ-H)-closo-3,1,2-ReC₂B₉H₁₀][BF₄] 2b as the sole product. The cation of salt 2b is a bimetallic species with a Re–Pt bond bridged by a nido-7,8-C₂B₉H₁₁ ligand pentahapto coordinated to the rhenium and with the β-B–H bond in the ligating CCBBB ring of the carbaborane cage forming a 3-centre 2-electron B–H⇀Pt bond. Deprotonation of 2b with strong bases gives an isomeric mixture of the complexes [3,3,3-(CO)₃-3,n-(Pt{Ph₂P(CH₂)₂PPh₂})-closo-3,1,2-ReC₂B₉H₁₀] 3a (n = 4), 3b (n = 8). The isomers are readily separated and the proportion of 3a to 3b formed is solvent- as well as base-dependent. In THF solutions of 2b afford a 1∶3∶1 equilibrium mixture of 2b, 3a, and 3b. Refluxing THF solutions of 1 with [Pt(H)(THF)(PEt₃)₂][BF₄] for several days yields a separable mixture of the isomers [3,3,3-(CO)₃-3,n-{Pt(PEt₃)₂}-closo-3,1,2-ReC₂B₉H₁₀] 4a (n = 4), 4b (n = 8). The same reactants, when stirred together in CH₂Cl₂ at ambient temperatures, give 4a and [3,3,3-(CO)₃-3,8-{Pt(H)(PEt₃)}-8-(μ-H)-closo-3,1,2-ReC₂B₉H₁₀] 5. Complex 4a, not 5, is the precursor to 4b, the interconversion occurring at elevated temperatures in solution by an intramolecular exchange of B–Pt σ bonds between α-B and β-B vertices in the coordinating CCBBB face of the cage. Treatment of 1 with [Pt(Me)(THF)(PMe₂Ph)₂][BF₄] at room temperature initially gives [3,3,3-(CO)₃-3,4-{Pt(PMe₂Ph)₂}-closo-3,1,2-ReC₂B₉H₁₀] 6a. However, 6a isomerizes in CH₂Cl₂ solutions at ambient temperatures to give a 1∶1 mixture of 6a and [3,3,3-(CO)₃-3,8-{Pt(PMe₂Ph)₂}-closo-3,1,2-ReC₂B₉H₁₀] 6b. The NMR data (¹H, ¹³C, ¹¹B and ³¹P) for the new complexes are discussed.