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 [Re-2(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 [Re(mu -H)(2)(CO)(12)](-) triangular cluster anion 1. The reaction with [Re2H2(mu -H)(CO)(8)](-) gave the novel [Re-4(mu -H)(3)(CO)(16)](-) anion 2. containing a rare example of a puckcred-square metal cluster. Protonation of 1 is known to afford the neutral [Re-3(mu -H)(3)(CO)(12)] species 3. Analogously the reaction of 2 with a strong acid afforded the previously known square metal clusters [Re-4(mu -H)(4)(CO)(16)] 4. The reaction could not be reversed by treatment with bases. Photolysis of 4 gave the unsaturated complex [Re-2(mu -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 hi 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 tall 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 1. 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.