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Chemistry of Mangana- and Rhenatricarbadecaboranyl Tricarbonyl Complexes:  Evidence for an Associative Mechanism of Ligand Substitution Involving an η6−η4 Cage-Slippage Process Analagous to η5−η3-Cyclopentadienyl Ring-Slippage

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posted on 05.07.2006, 00:00 by Robert Butterick, Bhaskar M. Ramachandran, Patrick J. Carroll, Larry G. Sneddon
The reaction of the tricarbadecaboranyl anion, 6-Ph-nido-5,6,9-C3B7H9-, with M(CO)5Br [M = Mn, Re] or [(η6-C10H8)Mn(CO)3+]BF4- yielded the half-sandwich metallatricarbadecaboranyl analogues of (η5-C5H5)M(CO)3 [M = Mn, Re]. For both 1,1,1-(CO)3-2-Ph-closo-1,2,3,4-MC3B7H9 [M = Mn (2) and Re (3)], the metal is η6-coordinated to the puckered six-membered open face of the tricarbadecaboranyl cage. Reactions of 2 and 3 with isocyanide at room temperature produced complexes 8-(CNBut)-8,8,8-(CO)3-9-Ph-nido-8,7,9,10-MC3B7H9 [M = Mn (4), Re (5)], having the cage η4-coordinated to the metal. Photolysis of 4 and 5 then resulted in the loss of CO and the formation of 1-(CNBut)-1,1-(CO)2-2-Ph-closo-1,2,3,4-MC3B7H9 [M = Mn, Re (6)], where the cage is again η6-coordinated to the metal. Reaction of 2 and 3 with 1 equiv of phosphine at room temperature produced the η6-coordinated monosubstituted complexes 1,1-(CO)2-1-P(CH3)3-2-Ph-closo-1,2,3,4-MC3B7H9 [M = Mn (7), Re (9)] and 1,1-(CO)2-1-P(C6H5)3-2-Ph-closo-1,2,3,4-MC3B7H9 [M = Mn (8), Re (10)]. NMR studies of these reactions at −40 °C showed that substitution occurs by an associative mechanism involving the initial formation of intermediates having structures similar to those of the η4-complexes 4 and 5. The observed η6−η4 cage-slippage is analogous to the η5−η3 ring-slippage that has been proposed to take place in related substitution reactions of cyclopentadienyl−metal complexes. Reaction of 9 with an additional equivalent of P(CH3)3 gave 8,8-(CO)2-8,8-(P(CH3)3)2-9-Ph-nido-8,7,9,10-ReC3B7H9 (11), where the cage is η4-coordinated to the metal. Photolysis of 11 resulted in the loss of CO and the formation of the disubstituted η6-complex 1-CO-1,1-(P(CH3)3)2-2-Ph-closo-1,2,3,4-ReC3B7H9 (12).