Rhenium-Silver Bicyclic “Spiro” Hydrido-Carbonyl Clusters : NMR Investigation of Their Formation and Reversible Fragmentation

The reactions of the hydrido complexes [Re2H2(μ-H) (CO)8]- (1) and [Re4H2(μ-H) 3(CO)16]- (2) with 0.5 equivalents of AgOTf (OTf-=CF3SO3-) afforded rapidly and selectively the novel cluster anions [Ag(μ- H)4{Re 2(μ-H)(CO)8}2]- (3) and [Ag(μ-H)4{Re4(μ-H)3(CO) 16}2]- (4), respectively, formed by donor-ec-ceptor interactions between the rhenium hydrides and the Ag- cation. 1H and 109Ag NMR spectroscopies showed that each Ag atom is surrounded by four hydrido ligands, bridging Re-Ag interactions. The Re2AgRe2 cluster 3 therefore adopts the well known "bow-tie" geometry, constituted by two triangular clusters sharing the Ag vertex (spiro bis-cyclopropane). The Re4AgRe4 anion 4 has instead an unprecedented spiro bis-cyclopentane-like structure, which is stable in solution only at temperatures lower than 220 K. The NMR investigation of the behavior in solution of these complexes revealed different intra- and inter-molecular dynamic processes, involving reversible fragmentation of the cyclic structures, with elimination of neutral or anionic hydrido-carbonyl fragments and interconversion among the different spiro derivatives. In particular, at temperature higher than 220 K the onset of a dissociative equilibrium was observed, involving the egress of a Re2(μ-H) 2(CO)8 fragment from 4, with formation of the [Ag(μ- H)4{Re2(μ-H)(CO)8}{Re4(μ-H) 3(CO)16}]- anion 5, with a mixed spiro cyclopropaneecyclopentane-like structure, never previously observed. The fragmentation reaction was completely reversible on lowering the temperature. The mixed spiro derivative 5 was obtained (in mixture with the two symmetrical spiro derivatives 3 and 4) by either adding 1 equivalent of [Re 2(μ-H)2(CO)8] to 3, or by reacting 1 equivalent of Ag+ with a 1:1 mixture of the free hydrido complexes 1 and 2. Intermolecular exchanges between the Re4H5 fragments of 4 and 5, and between the Re2H3 fragments of 3 and 5 were revealed by 2D 1H EXSY NMR experiments, which also showed exchange between the spiro bis-cyclopropane 3 and the free hydrido complex 1. Furthermore, a higher binding ability towards Ag+ of 1 with respect to 2 was proved, since 1 was found to be able to displace 2 from the spiro bis-cyclopentane adduct 4 and from the mixed spiro cyclopropaneecyclopentane 5, leading to free hydrido complex 2 and the spiro bis-cyclopropane 3.