Amino acid complexes of metal carbonyls: Mechanistic aspects of the CO-labilizing ability of glycinate ligands in zero-valent chromium and tungsten 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)(O2CCH2PPh2)] (5), and [Et4N][Cr(CO)(4)(O2CCH2-PPh2)] have been synthesized from the reaction of the M(CO)(5)THF adduct with the tetraethylammonium salt 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 geometry of the metal anion is, in each case, that of a distorted octahedron consisting of four carbonyl ligands and a puckered five-membered glycinate chelate ring, bound through the nitrogen atom and one of its oxygen atoms. Notable about complex 1 is that the crystal obtained exhibited 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 in 1 relative to the corresponding orientation in the W(CO)(4)(O2CCH2NH2)(-) 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-assisted removal of a proton from the amine ligand leading to a substitutionally labile amide transient species, The tungsten analogue of complex 1 was used to obtain evidence in support of this mechanism. The isotope 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 the intermolecular exchange of CO in the [Et4N][W(CO)(4)(O2CCH2NH2)] salt 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 consistent with the proposed mechanism. In addition, the effect of substitution of electron-donating (C(CH3)(3)) and electron-withdrawing (C6H5) substituents on the methylene carbon was evaluated. There was little change in the rate of CO exchange observed for W(CO)(4)(O2CCH(C(CH3)(3)NH2)(-) (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 responsible for the observed CO liability. As anticipated on the basis of the proposed substitutional pathway, the phosphino acid metal carbonyl derivatives did not exhibit facile intermolecular CO exchange.