ADDITION-REACTIONS OF THE UNSATURATED CLUSTER ANION [RE4(MU-H)3(MU-3-H)2(CO)12]- WITH CO, PPH3, AND MECN AND CHARACTERIZATION OF AN UNSTABLE ADDUCT WITH WATER - X-RAY CRYSTAL-STRUCTURE OF THE TETRAETHYLAMMONIUM SALT OF THE DERIVATIVE [RE4(MU-H)4(MU-3-H)(CO)12(PPH)3]-

The tetrahedral cluster anion [Re4(mu-H)3(mu3-H)2(CO)12]- (1) easily adds donor molecules L, due to its unsaturation (58 cluster valence electrons). The reaction process changes significantly on varying the nature of L. In the presence of CO, two carbonyls are added, leading to the previously known butterfly derivative [Re4(mu-H)5(CO)14]- (6). Analogously, in the presence of MeCN, at 193 K, the novel [Re4(mu-H)5(CO)12(NCMe)2]- (4) is formed, which, on the basis of the NMR data, is supposed to contain a butterfly cluster, with trans-diaxial nitriles coordinated on the wingtips of the butterfly. The reaction with PPh3, at 193 K, leads on the contrary to a monoaddition derivative [Re4(mu-H)4(mu3-H)(CO) 12(PPh3)]- (3), containing a tetrahedral cluster, as shown by the X-ray analysis of its NEt4+ salt. Crystal data: triclinic, space group P1BAR (No. 2), a = 12.330 (2) angstrom, b = 13.202 (2) angstrom, c = 15.503 (3) angstrom, a = 92.73 (1)-degrees, beta = 98.02 (1)-degrees, gamma = 96.92 (1)-degrees, Z = 2. The refinements, based on 3711 significant reflections, gave final R and R, indices of 0.033 and 0.033. The same compound has also been obtained upon treating [Re4(mu-H)s(CO)12(NCMe)2]- (4) with 1 equiv of PPh3, at 243 K. Both compounds 3 and 4 decompose at room temperature, giving the previously known unsaturated triangular cluster anions [Re3(mu-H)4(CO)9(PPh3)]- (9) and [Re3(mu-H)4(CO)9(NCMe)]- (10), respectively. H-1 NMR evidence has been obtained which shows that at low temperature compound 1 is able to reversibly coordinate one H2O molecule, giving the adduct [Re4(mu-H)5(CO)12(H2O)]- (5), in which water is supposed to act as a bridging ligand. On increasing the temperature, the equilibrium between compound 1 and the adduct with water progressively favors compound 1. Approximate values of the equilibrium constants have been estimated from the NMR data at temperatures higher than 240 K (where the equilibrium is fast on the NMR time scale), allowing an evaluation of the thermodynamic parameters (DELTAH-degrees = -59 +/- 2 kJ mol-1, DELTAS-degrees = -215 +/- 8 J K-1 mol-1). The activation parameters for the dissociation of the aquo ligand (DELTAH(double dagger) = 71 +/- 1 kJ mol-1, DELTAS(double dagger) = 111 +/- 3 J K-1 mol-1) have been estimated from the rate constants obtained through a band-shape analysis in the range 202-234 K. The same computer simulations gave also the activation energy of the mutual exchange of the hydrides in the adduct with H2O (E(a) = 52 +/- 1 kJ mol-1). Compound 1, at T > 270 K, is unstable in solution in the presence of water and transforms slowly into the triangular cluster anion [Re3(mu-H)3(mu3-OH)(CO)9]- (11), likely via the unsaturated anion [Re3(mu-H)4(CO)9(H2O)- (12).