REACTIONS OF A SULFIDO-BRIDGED DINUCLEAR MOLYBDENUM COMPLEX WITH NITRILES AND ISONITRILES UNDER HYDROGEN - FACILE C=N BOND-CLEAVAGE

The dinuclear complex [(CPMO)2(S2CH2)(µ-S)(µ-SH)]+X- (X = SO3CF3, BF4; Cp = C5H5; 1(X)) reacted with nitriles RCN under 1‒2 atm of H2 at 25‒50 °C to form the new cationic complex [inline formula omitted] and ammonia. The reaction has been characterized for HCN as well as for a series of alkyl- and aryl-substituted nitriles. Complex 1 reacted with isonitriles (RNC) and hydrogen under similar conditions to form the cationic complex [inline formula omitted] and the primary amine RNH2. The reactions were inhibited by excess acid. In order to probe the mechanisms of these reactions, the interactions of nitriles and isonitriles with 1 in the absence and presence of a protic acid and in the absence and presence of hydrogen have been studied. In the absence of hydrogen, 1 reacted with isonitriles to form [(CpMo)2(S2CH2)(S2CNH(R))]+ (2), but no reaction of 1 with nitriles was detected spectroscopically. Under similar conditions in the presence of excess protic acid, 1 reacted with n-butyl isocyanide to form [(CPMo)2(S2CH2)(µ-S)(µ-SCH═NHBu)]2+ (4) and with acetonitrile to form [(CPMo)2(S2CH2)(µ-S)(µ-SC(CH3)═NH2)]2+ (5). These S‒H insertion products were identified by spectroscopic methods. The reaction of 4 and 5 with hydrogen appeared to involve carbon‒sulfur bond hydrogenolysis; e.g., in the reaction of 5 with H2, ethylammonium ion was identified as a product. The C‒N bond cleavage reactions are proposed to involve the deprotonated forms of the insertion products 4 and 5, [(CpMo)2(S2CH2)(µ-S)(µ-SCH═NBu)]+ and [(CPMo)2(S2CH2)(µ-S)(µ-SC(CH3)═NH)]+. The former complex was tentatively identified by NMR spectroscopy. Reactions of these derivatives with hydrogen are proposed to lead to intramolecular S‒H insertion products [(CpMo)2(S2CH2)(S2CH(NH2Bu))]+ (6) and [(CpMo)2(S2CH2)(S2C(CH3)NH3)]+ (7). A precedent for this type of reaction has been characterized in a related molybdenum system. Complexes 6 and 7 are expected to eliminate BuNH2 and NH3, respectively, to form the final molybdenum products. The possible relevance of these reactions to the reductions of nitriles and isonitriles effected by the nitrogenase enzymes is discussed. © 1990, American Chemical Society. All rights reserved.