Hydrido-Carbonyl Rhenium Clusters with a Square Geometry of the Metal Core. Synthesis and X-Ray Characterization of the Novel [Re4(μ-H)3(CO)16]- Anion

Three and tetranuclear ring clusters have been obtained by treatment of [Re2(CO)8(THF)2] with carbonyl-rhenates containing two terminal hydrides. The reaction with [ReH2(CO)4]- provided a selective route to the previously known [Re3(μ-H)2(CO)12]- triangular cluster anion 1. The reaction with [Re2H2(μ-H)(CO)8]- gave the novel [Re4(μ-H)3(CO)16]- anion 2, containing a rare example of a puckered-square metal cluster. Protonation of 1 is known to afford the neutral [Re3(μ-H)3(CO)12] species 3. Analogously the reaction of 2 with a strong acid afforded the previously known square metal clusters [Re4(μ-H)4(CO)16] 4. The reaction could not be reversed by treatment with bases. Photolysis of 4 gave the unsaturated complex [Re2(μ-H)2(CO)8] 5: this is the reverse of the dimerization reaction, that in THF at room temperature produces 4 from 5. Thermal treatment (reflux in cyclohexane for 24 h) left 4 almost unchanged. A single crystal X-ray analysis of [NEt4]2 showed a s/e/s/s (e = eclipsed, s = staggered) conformation of the Re(CO)4 units, leading to a puckered geometry of the ring, at variance with the square-planar geometry of 4 (all eclipsed). Two of the three hydrides of 2 have been located as bridging the Re-Re edges from inside the metal ring, as previously observed in 4. Density functional computations indicated a puckered conformation as the most stable for both 2 and 4, with very low activation energies for ring inversion (6.6 and 2.2 kcal·mol-1, respectively), but ruled out solid state fluxionality for 4, whose observed planar geometry must be attributed to packing stabilization.