Ortho-metalated pyridine derivatives of the unsaturated anion [Re3(μ-H)4(CO)10]-. Syntheses and X-ray crystal structures of the tetraethylammonium salts of the anions [Re3(μ-H)3(μ-η2-NC5H 4)(CO)10]- and [Re3(μ-H)3

Full title: Ortho-metalated pyridine derivatives of the unsaturated anion [Re3(μ-H)4(CO)10]-. Syntheses and X-ray crystal structures of the tetraethylammonium salts of the anions [Re3(μ-H)3(μ-η2-NC5H 4)(CO)10]- and [Re3(μ-H)3(μ-η2-NC5H 4)(CO)9(NC5H5)]-. The unsaturated cluster anion [Re3(μ-H)4(CO)10]- (1) reacts overnight in pyridine solution, at room temperature, giving the μ-pyridyl derivative [Re3(μ-H)3(μ-η2-NC5H 4)(CO)10]- (2). The ortho-metalation proceeds with H2 evolution, which derives from the elimination of two hydrides of the parent cluster, as shown on performing the reaction with NC5D5. When the above reaction, in pyridine solution, is accomplished at 85°C, within 2 h another product is obtained, namely [Re3(μ-H)3(μ-η2-NC5H 4)(CO)9(NC5H5)]- (3), containing the μ-pyridyl ligand and a pyridine molecule replacing a carbonyl ligand. On the contrary, in tetrahydrofuran solution, in the presence of only a moderate excess of pyridine (∼5 equiv), the reaction at 85°C gives the unsaturated anion [Re3(μ-H)4(CO)9(NC5H 5)]- (4), previously known. This species has been proven to be unable to further react with pyridine, and therefore the formation of 3 from compound 1 occurs via intermediate 2 and not 4. Both complexes 2 and 3, as their NEt4+ salts, have been investigated by X-ray analysis. Complex 2 gives orthorhombic crystals, space group Pbca (No. 61), with a = 13.297 (4) Å, b = 18.343(7) Å, c = 24.432(12) Å, and Z = 8; the structure was refined to R = 0.028 for 1829 significant observed reflections. Complex 3 crystallizes in the monoclinic space group P21/c (No. 14), with a = 13.363(1) Å, b = 13.143(1) Å, c = 18.955(3) Å, β = 100.09(1)°, and Z = 4; the refinements, based on 3924 significant observed data, gave a final R value of 0.027. Both the anions contain an isosceles triangle of metal atoms, with the two longer edges bridged by a hydrido ligand and the shorter one by a hydride and a μ-pyridyl ligand. In 3 an axial carbonyl of the Re(CO)4 unit of 2 has been substituted by a pyridine molecule whose aromatic ring faces that of the pyridyl group. 1H NMR analysis of 2 in solution at different temperatures reveals a fluxional behavior, leading to the exchange of the two hydrides bridging the longer triangular edges (Ea, = 72 ± 1 kJ/mol). © 1993 American Chemical Society.