H D EXCHANGE VIA REVERSIBLE PYRIDINE ORTHO-METALATION, AND COMPETITION BETWEEN C-H OXIDATIVE ADDITION AND CO COORDINATION IN HYDRIDO-CARBONYL TRIANGULAR RHENIUM CLUSTERS - A H-1-NMR INVESTIGATION - X-RAY CRYSTAL-STRUCTURE OF THE ANION [RE-3(MU-H)(2)(CO)(11)(PY)](-)/

The triangular cluster anion [Re-3(mu-H)(3)(mu-NC5H4)(CO)(10)](-) (2), containing an ortho-metallated pyridine bridging a cluster edge, unde rgoes in acetone solution a selective H/D exchange between the basal h ydridic site and the alpha-position of the ortho-metallated pyridine, with k = 1.9(1) X 10(-6) s(-1), at room temperature. The process was s tudied by NMR on the partially deuterated derivative [Re-3(mu-H)(2)(mu -D)(mu-NC5D4)(CO)(10)](-) and an equilibrium isotope effect was observ ed, the equilibrium concentration of the isotopomer containing C-bound deuterium being about double that of the isotopomer containing Re-bou nd deuterium. In pyridine-d(5) solutions, in contrast, the preferentia l deuteration of the hydrides bridging the lateral edges of [Re-3(mu-H )(3)(mu-NC5H4)(CO)(10)](-) was observed, with a rate about half that o f the deuteration of the ortho-metallated pyridine (k = 2.9(1) X 10(-5 ) s(-1) at 302 K for the latter process). The H/D exchange processes i n both solvents have been rationalized as arising from the reversible reductive elimination of the ortho-metallated pyridine. The different behavior in the two solvents is discussed. In the presence of CO, the anion 2 gave [Re-3(mu-H)(2)(CO)(12)](-) (4, t(1/2) approximate to 48 h ) in acetone solution, while in pyridine it gave the novel anion [Re-3 (mu-H)(2)(CO)(11)(Py)](-) (3). In py-d(5), under CO, the resonances of the ortho-metallated pyridine of 2 decreased at the same rate as in t he absence of CO. The competition ratio r between CO coordination and pyridine ortho-metallation on the intermediate generated by C-H reduct ive elimination has been estimated as 0.45(15). The reaction of [Re-3( mu-H)(4)(CO)(10)](-) (1) with CO in pyridine follows two parallel path s, affording both compounds 3 and 2, the latter, in turn, converting i nto 3 at a slower rate. The rate of disappearance of 1 was the same as that measured in the absence of CO (k = 2.1(2) X 10(-4) s(-1) 302 K) and the value of the competition ratio r between the two pathways lead ing to 3 and 2, respectively, was found to be 0.5(1). Under high H-2 p ressure, 2 slowly transforms back into the unsaturated parent anion 1, showing the complete reversibility of the ortho-metallation reaction. The structure of the anion [Re-3(mu-H)(2)(CO)(11)(Py)](-) (3), as [NE t(4)](+) salt, has been investigated by X-ray analysis, The crystals a re monoclinic, space group P2(1)/c, with a = 14.327(3), b = 12,310(2), c = 17.437(3) Angstrom, beta = 91.67(2)degrees and Z = 4. The refinem ents, performed by full-matrix least-squares on the basis of 2064 sign ificant [I > 2 sigma(I)] reflections, gave final agreement indices R a nd wR(2) Of 0.0397 and 0.0835, respectively. The anion contains an iso sceles triangle of rhenium atoms, with one shorter edge [3.025(1) Angs trom], and two longer hydrogen-bridged edges [3.202(1) and 3.209(1) An gstrom]; it bears eleven terminal CO groups and an N-bonded pyridine m olecule [Re-N 2.21(2) Angstrom].