INTRAMOLECULAR C-H BOND ACTIVATION - PREPARATION, STRUCTURE, AND PROPERTIES OF A UNIQUE COBALT(III) COMPLEX, K[COIII(DACODA)(SO3)].5H2O, CONTAINING A WEAK AGOSTIC INTERACTION IN AQUEOUS-SOLUTION

Oxidation of [Co(II)(dacoda)(H2O)] (dacoda = 1,5-diazacyclooctane-N,N'-diacetate) with PbO2 in the presence of 1 equiv of SO3(2-) affords the classically five-coordinate Co(III) complex K[Co(III)(dacoda)(SO3)].5H2O (3), which contains a three-center, two-electron Co..HC (agostic) interaction in the sixth coordination position. 3 crystallizes in the monoclinic space group P2(1)/c with a = 7.500 (2) angstrom, b = 16.5469 (15) angstrom, c = 30.978 (4) angstrom, beta = 89.91 (2)-degrees, V = 3844 (1) angstrom3, with Z = 8, and a final unweighted R value of 0.051. The structural and spectroscopic results indicate that a weak agostic interaction exists between the Co center and the C(2)-H(2a) bond. The agostic Co(1)-H(2a) and Co(1)-C(2) distances were found to be 2.31 (5) and 2.531 (4) angstrom, respectively. C-13 NMR studies reveal that the agostic C(2)-H(2a) group shows only a slightly reduced coupling constant, J(C-13-H-1) = 122 Hz, versus the value of 129 Hz observed for similar methylene C-H groups within the metal complex. The ligand field parameter, Dq, of the "agostic ligand" is calculated to be only approximately 600 cm-1, reflecting the weakness of the agostic interaction. The weakness of the Co..HC interaction and the reason for the surprising stability of 3 in aqueous solution are attributed to the strong trans influence of the sulfite ligand. However, this complex can be deprotonated in methanol to form the Co-alkyl complex [Co(III)(dacoda-C(2))(H2O)], where dacoda-C(2) refers to dacoda coordinated in a quinquedentate fashion with the C(2) carbon of the daco portion of the ligand directly bonded to the cobalt center. These results demonstrate the activation of a C-H bond via electrophilic attack of Co(III) in a relative simple but sterically constrained molecule, which may be inherent to C-H activation in enzymes.