Alkyne coupling at a rhenium–monocarborane substrate: synthesis of Re,B-η2:σ-butadienyl complexes

Abstract

Treatment of 7-NH₂Bu^t-nido-7-CB₁₀H₁₂ in tetrahydrofuran (THF) with LiBuⁿ (3 equiv) and then [ReBr(CO)₃(THF)₂] gives the rhenacarborane dianion [1-NHBu^t-2,2,2-(CO)₃-closo-2,1-ReCB₁₀H₁₀]²⁻, isolated as the bis-[N(PPh₃)₂]⁺ salt (4). Iodine oxidation of this Re^I intermediate gives the Re^III complex [1,2-µ-NHBu^t-2,2,2-(CO)₃-closo-2,1-ReCB₁₀H₁₀] 6 in which the carborane functions formally as an 8-electron (6π + 2σ) donor. Reaction of 6 with ligands L in the presence of Me₃NO gives substituted products [1,2-µ-NHBu^t-2,2-(CO)₂-2-L-closo-2,1-ReCB₁₀H₁₀] [L = PPh₃ (7a), CNXyl (7b; Xyl = C₆H₃Me₂-2,6), or Bu^tCCH (7c)]. Formation of complex 7c is unexpectedly accompanied by [1,2-µ-NHBu^t-2,2-(CO)₂-3,2-σ:η²-{C(CHBu^t)–CHCHBu^t}-closo-2,1-ReCB₁₀H₉] 8a, in which an alkyne-derived dienyl group is bound to both the rhenium centre and to an adjacent boron vertex. Complex 8a is also obtained from 7c with Bu^tCCH and Me₃NO. The same reaction of 7c, using PhCCH or CNXyl instead of Bu^tCCH, gives, respectively, [1,2-µ-NHBu^t-2,2-(CO)₂-3,2-σ:η²-{C(CHBu^t)–CHCHPh}-closo-2,1-ReCB₁₀H₉] 8b and [1,2-µ-NHBu^t-2-Bu^tCCH-2-CO-2-CNXyl-closo-2,1-ReCB₁₀H₁₀] 9. Addition of donors L to 8a results in displacement from rhenium of the pendant dienyl moiety, yielding [1,2-µ-NHBu^t-2,2-(CO)₂-2-L-3-{C(CHBu^t)–CHCHBu^t}-closo-2,1-ReCB₁₀H₉] [L = PMe₃ (10a), CNBu^t (10b)]. Single-crystal X-ray diffraction analyses have confirmed the novel structural features of compounds 6, 7c, 8b and 9.