KINETICS AND MECHANISM OF A CO(III) COMPLEX CATALYZED HYDRATION OF NITRILES

[Co(cyclen)(OH2)2]3+ (1) efficiently catalyzes hydration of acetonitri le to acetamide in a three-step cycle at pH 7, 40-degrees-C (cyclen = 1,4,7,11-tetraazacyclododecane). The mechanism involves (a) equilibriu m coordination of acetonitrile to 1 (K1 = 2.5 M-1) followed by (b) int ramolecular metal hydroxide attack on the coordinated nitrile resultin g in formation of the chelated acetamide (k2 = 4.7 x 10(-3) s-1) and ( c) dissociation of the chelated amide (k3 = 3.3 x 10(-4) s-1). Both in termediates in the catalytic cycle (coordinated acetonitrile and chela ted acetamide) have been detected by visible absorbance spectroscopy a nd C-13 NMR. The crystal structure of the chelated benzamide has been determined. Joint Lewis acid activation and metal hydroxide activation provides over a 10(10)-fold rate acceleration for the hydration react ion.