Base-mediated transformation of the agostic (π-allyl)-closo-rhodacarboranes into complexes with an open (1-5-η5)-pentadienyl ligand.

The reactions of agostic (CH3···Rh) complexes [3-{(1-3-η(3))-1,1,2-trimethylallyl}-1-R-2-R'-closo-3,1,2-RhC2B9H9] [1: R = R' = Me; 2: R,R' = μ-(ortho-xylylene); 3: R = Ph, R' = Me] in a cooled (+5 °C) toluene solution with the strong non-nucleophilic base N,N,N',N'-tetramethylnaphthalene-1,8-diamine (tmnd) formally involve a linear coupling of the π-allyl ligand of one molecule of the complex to the π-allyl ligand of the second molecule of the complex, ultimately forming the structurally novel mononuclear species [3-{(1-5-η(5))-2,3-dimethyl-5-(3-methylbuten-2-yl)pentadienyl}-1-R-2-R'-closo-3,1,2-RhC2B9H9] (4, 5) and (6a,b, a mixture of diastereomers). Complexes 5 and 6a,b were also prepared using weaker bases such as PPh3 or even EtOH instead of tmnd. All new complexes 4-6 were characterized by a combination of analytical and multinuclear NMR data, as well as by single-crystal X-ray diffraction studies of two selected species 5 and the major diastereomer of 6, which revealed an open η(5)-pentadienyl coordination mode of the hydrocarbon ligands in these complexes.