AMINO-ACID COMPLEXES OF METAL-CARBONYLS - MECHANISTIC ASPECTS OF THE CO-LABILIZING ABILITY OF GLYCINATE LIGANDS IN ZERO-VALENT CHROMIUM ANDTUNGSTEN DERIVATIVES

The amino and phosphino acid derivatives of chromium(0) and tungsten(0 ), [Et4N][Cr(CO)(4)(O2CCH2NH2)] (1), [Et4N] [Cr(CO)(4)(O2CCH2NHMe)] (2 ), [Et4N][Cr(CO)(4)(O2CCH2NMe2)] (3), [Et4N][W(CO)(4)(O2CCH(C(CH3)(3)) -NH2)] (4), [Et4N][W(CO)(4)(O2CCH(C6H5)NH2)], [Et4N][W(CO)(4)(O2CCH2PP h2)] (5), and [Et4N][Cr(CO)(4)(O2CCH2-PPh2)] have been synthesized fro m the reaction of the M(CO)(5)THF adduct with the tetraethylammonium s alt of the corresponding amino or phosphino acid in THF solution. The complexes have been characterized in solution by C-13 NMR and infrared spectroscopies and in the solid state by X-ray crystallography. The g eometry of the metal anion is, in each case, that of a distorted octah edron consisting of four carbonyl ligands and a puckered five-membered glycinate chelate ring, bound through the nitrogen atom and one of it s oxygen atoms. Notable about complex 1 is that the crystal obtained e xhibited both a different morphology and a different space group than its tungsten analogue. Examination of the packing diagram reveals that this change is due to the different orientation of the chelate ring i n 1 relative to the corresponding orientation in the W(CO)(4)(O2CCH2NH 2)(-) anion. Complexes 1 and 2 exhibit intermolecular hydrogen-bonding interactions between the amine N-H group and the distal oxygen on an adjacent molecule, with N ... O distances of 2.828 and 2.821 Angstrom, respectively. Investigations of the lability of the carbonyl ligands have been carried out, The lability is proposed to be due to base-assi sted removal of a proton from the amine ligand leading to a substituti onally labile amide transient species, The tungsten analogue of comple x 1 was used to obtain evidence in support of this mechanism. The isot ope effect (k(H)/k(D)) was measured for W(CO)(4)(O2CCH2NH2)(-) using d (5)-glycine and was found to be 2.34. The activation parameters for th e intermolecular exchange of CO in the [Et4N][W(CO)(4)(O2CCH2NH2)] sal t were determined and found to be Delta H-double dagger = 15.4 +/- 1.0 kcal/mol and Delta S-double dagger = -23.2 +/- 3.2 eu, values consist ent with the proposed mechanism. In addition, the effect of substituti on of electron-donating (C(CH3)(3)) and electron-withdrawing (C6H5) su bstituents on the methylene carbon was evaluated. There was little cha nge in the rate of CO exchange observed for W(CO)(4)(O2CCH(C(CH3)(3)NH 2)(-) (5) and W(CO)(4)(O2CCH(C6H5)(NH2)(-) vs W(CO)(4)(O2CCH2NH2)(-), showing that steric or electronic away from the N center are not respo nsible for the observed CO liability. As anticipated on the basis of t he proposed substitutional pathway, the phosphino acid metal carbonyl derivatives did not exhibit facile intermolecular CO exchange.