New synthesis, reinvestigation of the solid-state structure, and NMR analysis of the unsaturated anion tetrakis(μ-hydrido)decacarbonyl-triangulo-trirhenate(1-)

The unsaturated anion [Re3(μ-H)4(CO)10]- has been synthesized by reaction of [Re3(μ-H)2(CO)12]- under high pressure of H2; the reaction is reversed under CO pressure. The solid-state structure of the anion has been reinvestigated on its bis(triphenylphosphine)nitrogen(1+), PPN+, salt. This salt crystallizes in the monoclinic space group P21/c, with cell constants a = 11.327 (3) Å, b = 12.672 (2) Å, c = 32.183 (6) Å, β = 95.08 (2)°, and Z = 4. The structure, solved by Patterson and Fourier methods, has been refined by least squares, on the basis of 4622 significant counter data, to a final R value of 0.042. The anion, of idealized C2v, symmetry, contains an isosceles triangle of rhenium atoms, with two longer hydrogen-bridged edges of 3.189 (1) and 3.190 (1) Å and one shorter edge (formally a Re-Re double bond), bridged by two hydrides, of 2.789 (1) Å. Each rhenium atom exhibits an almost octahedral coordination geometry, one with four carbonyls and two hydrides and the other two with three CO groups and three hydrides. NMR analysis at variable temperatures (-90 to +50°C) has shown that the solid-state structure is retained in solution and the four carbonyl resonances (intensity ratio 2:1:1:1) have been unambiguously assigned. Selective population transfer experiments have pointed out the absence of any scrambling of the ligands. The two carbonyls mutually trans in the Re(CO)4 moiety are selectively enriched under 13CO exchange.