STEREOCHEMICAL CONTROL OF REACTIVITY IN CO(III) ALKYL COMPLEXES OF THE TROPOCORONAND LIGAND SYSTEM

Reduction of the macrocyclic CoIIN4 Complexes [Co(TC-3,3)] and [Co(TC- 3,4)] with 40% Na/Hg followed by addition of alkyl halide gave the fiv e-coordinate diamagnetic Co(III) alkyl compounds, [CoR(TC-3,3)] (R = M e, Et, n-Pr) and [Co(n-Pr)(TC-3,4)]. Addition of MeLi to a cold THF so lution of [CoCl(TC-4,4)] yielded the five-coordinate complex [CoMe(TC- 4,4)). X-ray structural analyses of [CoR(TC-3,3)] (R = Et, n-Pr) and [ CoMe(TC-4,4)] revealed that both complexes have square-pyramidal coord ination geometries. This result may be contrasted with the finding tha t the structures of the corresponding [CoCl(TC-n,n)] complexes change from square pyramidal to trigonal bipyramidal as n changes from 3 to 4 . Square-pyramidal stereochemistry for [CoMe(TC-4,4)] is attributed to the stronger sigma-donating character of the methyl group compared to chloride ion. All three Co(III) alkyl complexes homolyze in solution in room light, or at elevated temperatures in the dark, to afford the corresponding Co(II) tropocoronand complex and alkyl radical. The Co-C bond in [CoMe(TC-4,4)] homolyzes more than 13 times faster than that in [CoMe(TC-3,3)], reflecting the torsional strain within the methylen e linker chains of the ligand. Significantly, [CoMe(TC-4,4)] reacts ra pidly with 1-2 atm of CO at 40-degrees-C to yield the CO insertion pro duct [Co(COMe)(TC-4,4)], whereas [CoEt(TC-3,3)] does not react with CO under identical conditions. An X-ray structural study revealed that t he acyl complex is five coordinate, with a stereochemistry that is bet ween square pyramidal and trigonal bipyramidal. This structure further displays the dual control of metal geometry by the steric constraints of the tropocoronand and the donor property of the fifth ligand. CO i nsertion probably proceeds for [CoMe(TC-4,4)] because the more flexibl e [TC-4,4]2- ligand affords a coordination site cis to the methyl grou p to which the CO can transiently bind.