H/D exchange via reversible pyridine ortho-metallation, and competition between CH oxidative addition and CO coordination in hydrido-carboxyl triangular rhenium clusters: a 1H-NMR investigation. X-ray crystal structure of the anion [Re 3( μ-H) 2(CO) 11(Py)] −

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The triangular cluster anion [Re 3(μ-H) 3(μ-NC 5H 4)(CO) 10] − ( 2), containing an ortho-metallated pyridine bridging a cluster edge, undergoes in acetone solution a selective H/D exchange between the basal hydridic site and the α-position of the ortho-metallated pyridine, with k = 1.9(1) × 10 −6 s −1, at room temperature. The process was studied by NMR on the partially deuterated derivative [Re 3(μ-H) 2(μ-D)(μ-NC 5D 4)(CO) 10] − and an equilibrium isotope effect was observed, the equilibrium concentration of the isotopomer containing C-bound deuterium being about double that of the isotopomer containing Re-bound deuterium. In pyridine- d 5 solutions, in contrast, the preferential deuteration of the hydrides bridging the lateral edges of [Re 3(μ-H) 3(μ-NC 5H 4)(CO) 10] − was observed, with a rate about half that of the deuteration of the ortho-metallated pyridine ( k = 2.9(1) × 10 −5 s −1 at 302 K for the latter process). The H/D exchange processes in 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(μ-H) 2(CO) 12] − ( 4, t 1 2 ≈ 48 h ) in acetone solution, while in pyridine it gave the novel anion [Re 3(μ-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 the absence of CO. The competition ratio r between CO coordination and pyridine ortho-metallation on the intermediate generated by CH reductive elimination has been estimated as 0.45(15). The reaction of [Re 3(μ-H) 4(CO) 10] − ( 1) with CO in pyridine follows two parallel paths, affording both compounds 3 and 2, the latter, in turn, converting into 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) × 10 −4 s −1 at 302 K) and the value of the competition ratio r between the two pathways leading to 3 and 2, respectively, was found to be 0.5(1). Under high H 2 pressure, 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(μ-H) 2(CO) 11(Py)] − ( 3), as [NEt 4] + salt, has been investigated by X-ray analysis. The crystals are monoclinic, space group P2 1/c, with a = 14.327(3), b = 12.310(2), c = 17.437(3) A ̊ , β = 91.67(2)° and Z = 4 . The refinements, performed by full-matrix least-squares on the basis of 2064 significant [I > 2σ(I)] reflections, gave final agreement indices R and w R 2 of 0.0397 and 0.0835, respectively. The anion contains an isosceles triangle of rhenium atoms, with one shorter edge [3.025(1) Å], and two longer hydrogen-bridged edges [3.202(1) and 3.209(1) Å]; it bears eleven terminal CO groups and an N-bonded pyridine molecule [ReN 2.21(2) Å].