MU-HYDRIDE GEOMETRY AND DYNAMICS IN THE PROTIC ACID ADDUCTS OF TRIOSMIUM IMIDOYL CLUSTERS

The mu(3)-imidoyl cluster, (mu-H)Os-3(CO)(9)(mu(3)-eta(2)-C=NCH2CH2CH2) (1) forms the neutral acid adducts (mu-H)(2)Os-3(CO)(9)(mu-eta(2)-C=NCH2CH2CH2)X (X = Cl(2), Br (3), CF3CO2 (4), CF3SO3 (6)), upon addition of the corresponding acids HX to chloroform solutions of the mu(3)-imidoyl cluster at room temperature. Solid state structures of 3 and 4 reveal that X is in an axial position on the third Os atom and on the same face of the cluster as the mu-imidoyl ligand which bridges two Os atoms. Thermolysis, at 40-80 degrees C under CO, of 3 isomerizes it to 8 where the bromide has migrated 180 degrees to occupy an axial position in the opposite face of the cluster. In addition, loss of HBr to form 1 and competitive formation of (mu-H)(mu-Br)Os-3(CO)(10) (7) occurs. For X = CF3CO2 and CF3SO3 partial dissociation of the neutral adduct into (mu-H)(2)Os-3(CO)(9)(mu(3)-imidoyl)(+) (5) and X(-) is observed in solution, while for X = Cl and Br this cation is only observed when the acid adduct is treated with AgSbF6. The same cation can be generated by treatment of 1 with the noncoordinating acid HBF4. The location of the metal-bound hydrogens in the solid state structures of 3 and 8 reveals that the halogen atom is loosely associated with one of the metal-bound hydrogens while this is not the case for the trifluoroacetate derivative 4. An investigation of the variable-temperature H-1- and C-13-NMR using one and two-dimensional methods reveals the presence of three isomers in solution for 2, 3, and 8 but only two for 4 and 6. For 2 and 3, exchange between two of the three isomers precedes direct exchange of the two hydride ligands, while for 8 direct exchange of the inequivalent hydrides is the lowest energy process. Mechanisms for these diverse exchange processes are presented in the light of their solid state structures and the two-dimensional NMR results. Compound 3 crystallizes in the orthorhombic space group Pna2(1) with unit cell parameters a = 29.608(6) Angstrom, b = 7.687(2) Angstrom, c = 17.121(6) Angstrom, V = 3.897(3) Angstrom(3), and Z = 4. Least squares refinement of 4051 observed reflections gave a final agreement factor of R = 0.044 (R(W) = 0.045). Compound 4 crystallizes in the trigonal space group P3(1) with unit cell parameters a = 8.976(3) Angstrom, c = 23.27(1) Angstrom, V = 1624(2) Angstrom(3), and Z = 3. Least squares refinement of 2973 observed reflections gave a final agreement factor of R = 0.030 (R(W) = 0.036). Compound 8 crystallizes in the monoclinic space group P2(1)/c with a 18.253(8) Angstrom, b = 14.129(5) Angstrom, c = 17.728(6) Angstrom, beta = 116.32(3)degrees, V = 4098(6) Angstrom(3), and Z = 8. Least squares refinement of 2948 observed reflections gave a final agreement factor of R = 0.056 (R(W) = 0.050).