STRUCTURAL AND REACTIVITY STUDIES OF A CYANOACETIC ACID-DERIVATIVE OF TUNGSTEN PENTACARBONYL - X-RAY STRUCTURE OF W(CO)5NCCH2COOH

The new cyanoacetic acid complex W(CO)5NCCH2COOH (1) has been synthesized from the reaction of W(CO)5THF and NCCH2COOH. The molecular structure of 1 has been determined by X-ray diffraction methods. The compound crystallizes in the triclinic space group P1BAR with two molecules in a cell of dimensions a = 6.7390 (10) angstrom, b = 9.745 (2) angstrom, c = 10.241 (2) angstrom, alpha = 63.57 (2)-degrees, beta = 81.71 (2)-degrees, gamma = 76.22 (2)-degrees. Full-matrix least-squares refinement gives final R and R. on F of 0.026 and 0.037 for 2030 observed [F > 4sigma(F)] data. The structure confirms the presence of the nitrile ligand located 2.17 8 (7) A from the tungsten center, with the carboxyl groups of two molecules coupled by strong intermolecular hydrogen-bonding (O-H...O = 2.627 (3) angstrom). This hydrogen-bonding structure was observed as well in solution as indicated by infrared spectroscopy. The displacement of the cyanoacetic acid ligand in 1 by carbon monoxide was shown to proceed via a solvent-assisted I(d) process in tetrahydrofuran and by a D process in the less-interacting solvent methylene chloride. The activation parameters for these processes are consistent with the proposed mechanism, with DELTAH(double dagger) and DELTAS(double dagger) for the displacement of the NCCH2COOH ligand determined to be 19.4 kcal/mol and -14.7 eu in THF and 29.2 kcal/mol and 11.8 eu in CH2Cl2, respectively. Comparable kinetic data were observed for the tungsten complexes containing the electronically similar ligands NC(CH2)10COOH and CH3CN. The relevance of this study to the catalytic decarboxylation reaction of cyanoacetic acid in the presence of W(CO)5O2CCH2CN- is discussed.