Synthesis, Structure, and Spectroscopic Characterization of [H8-nRh22(CO)35]n- (n=4, 5) and [H2Rh13(CO)24{Cu(MeCN)}2]- Clusters: Assessment of CV and DPV As Techniques to Circumstantiate the Presence of Elusive Hydride Atoms

The previously ill-characterized [HxRh22(CO)(35)](4-/5-) carbonyl cluster has been obtained as a byproduct of the synthesis of [H3Rh13(CO)(24)](2-) and effectively separated by metathesis of their sodium salts with [NEt4]Cl. Although the yields are modest and never exceed 10-15% (based on Rh), this procedure affords spectroscopically pure [H3Rh22(CO)(35)](5-) anion. Formation of the latter in mixture with other Rh clusters was also observed by electrospray ionization-mass spectrometry (EST-MS) in the oxidation of [H2Rh13-(CO)(24)](3-) with Cu2+ salts. The recovery of further amounts of [H3Rh22(CO)(35)](5-) was hampered by too similar solubility of the salts composing the mixture. Conversely, the reaction in CH3CN of [H2Rh13(CO)(24)](3-) with [Cu(MeCN)(4)](+)[BF4](-) leads to the [H2Rh13(CO)(24){Cu(MeCN)}(2)](-) bimetallic cluster. The X-ray crystal structures of [H4Rh22(CO)(35)](4-), [H3Rh22(CO)(35)](5-), and [H2Rh13(CO)(24){Cu(MeCN)}(2)](-) are reported. From a formal point of view, the metal frame of the former two species can be derived by interpenetration along two orthogonal axes of two moieties displaying the structure of the latter. The availability of [H8-nRh22(CO)(35)](n-) salts prompted their detailed chemical, spectroscopic, and electrochemical characterization. The presence of hydride atoms has been directly proved both by ESI-MS and H-1 NMR Moreover, both [H4Rh22(CO)(35)](4-) and [H3Rh22(CO)(35)](5-) undergo distinctive electrochemically reversible redox changes. This allows to assess electrochemical studies as indisputable though circumstantial evidence of the presence of H-1 NMR-silent hydride atoms in isostructural anions of different charge.