ORTHO-METALATED PYRIDINE-DERIVATIVES OF THE UNSATURATED ANION [RE3(MU-H)4(CO)10]- - SYNTHESES AND X-RAY CRYSTAL-STRUCTURES OF THE TETRAETHYLAMMONIUM SALTS OF THE ANIONS [RE3(MU-H)3(MU-ETA(2)-NC5H4)(CO)10]- AND[RE3(MU-H)3(MU-ETA(2)-NC5H4)(CO)9(NC5H5)]-

The unsaturated cluster anion [Re3 (mu-H)4(CO)10]- (1) reacts overnigh t in pyridine solution, at room temperature, giving the A-pyridyl deri vative [Re3(mu-H)3(mu-eta2-NC5H4)(CO)10]- (2). The ortho-metalation pr oceeds with H-2 evolution, which derives from the elimination of two h ydrides of the parent cluster, as shown on performing the reaction wit h NC5D5. When the above reaction, in pyridine solution, is accomplishe d at 85-degrees-C, within 2 h another product is obtained, namely [Re3 (mu-H)3(mu-eta2-NC5H4)(CO)9(NC5H5)]- (3), containing the mu-pyridyl li gand and a pyridine molecule replacing a carbonyl ligand. On the contr ary, in tetrahydrofuran solution, in the presence of only a moderate e xcess of pyridine (approximately 5 equiv), the reaction at 85-degrees- C gives the unsaturated anion [Re3(mu-H)4(CO)9(NC5H5)]- (4), previousl y known. This species has been proven to be unable to further react wi th pyridine, and therefore the formation of 3 from compound 1 occurs v ia intermediate 2 and not 4. Both complexes 2 and 3, as their NEt4+ sa lts, have been investigated by X-ray analysis. Complex 2 gives orthorh ombic crystals, space group Pbca (No. 61), with a = 13.297 (4) angstro m, b = 18.343(7) angstrom, c = 24.432(12) angstrom, and Z = 8; the str ucture was refined to R = 0.028 for 1829 significant observed reflecti ons. Complex 3 crystallizes in the monoclinic space group P2(1)/c (No. 14), with a = 13.363(1) angstrom, b = 13.143(1) angstrom, c = 18.955( 3) angstrom, beta = 100-09(1)-degrees, and Z = 4; the refinements, bas ed on 3924 significant observed data, gave a final R value of 0.027. B oth 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 mu-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. H-1 NMR analysis of 2 in solutio n at different temperatures reveals a fluxional behavior, leading to t he exchange of the two hydrides bridging the longer triangular edges ( E(a) = 72 +/- 1 kJ/mol).