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

Treatment of 7-NH2But-nido-7-CB10H12 in tetrahydrofuran (THF) with LiBun (3 equiv) and then [ReBr(CO)3(THF)2] gives the rhenacarborane dianion [1-NHBut-2,2,2-(CO)3-closo-2,1-ReCB10H10]2−, isolated as the bis-[N(PPh3)2]+ salt (4). Iodine oxidation of this ReI intermediate gives the ReIII complex [1,2-µ-NHBut-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-NHBut-2,2-(CO)2-2-L-closo-2,1-ReCB10H10] [L = PPh3 (7a), CNXyl (7b; Xyl = C6H3Me2-2,6), or ButCCH (7c)]. Formation of complex 7c is unexpectedly accompanied by [1,2-µ-NHBut-2,2-(CO)2-3,2-σ:η2-{C(CHBut)–CHCHBut}-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 ButCCH and Me3NO. The same reaction of 7c, using PhCCH or CNXyl instead of ButCCH, gives, respectively, [1,2-µ-NHBut-2,2-(CO)2-3,2-σ:η2-{C(CHBut)–CHCHPh}-closo-2,1-ReCB10H9] 8b and [1,2-µ-NHBut-2-ButCCH-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-NHBut-2,2-(CO)2-2-L-3-{C(CHBut)–CHCHBut}-closo-2,1-ReCB10H9] [L = PMe3 (10a), CNBut (10b)]. Single-crystal X-ray diffraction analyses have confirmed the novel structural features of compounds 6, 7c, 8b and 9.