Aqueous Transformation of a Metal Diformate to a Metal Dihydride Carbonyl Complex Accompanied by H₂ Evolution from the Formato Ligands

The rhodium diformate complex [RhIII(tacn)(HCOO)2](OTf) (2, tacn = 1,4,7-triazacyclononane, OTf- = CF3SO3-) reacts with HCOONa at pH 6−7 at 80 °C in water to form a rhodium dihydride CO complex, [RhIII(tacn)(H)2(CO)](OTf) (5), with H2 evolution from the formato ligands of 2. This is the first example of H2 evolution from hydrogen atoms of formato ligands of a metal diformate complex. Complex 2 was synthesized from the reaction of a rhodium aqua complex, [RhIII(tacn)(H2O)3](OTf)3 (1), with HCOONa in water. Recrystallization of 2 with HCOONa in water gave crystals of a rhodium triformate complex, [RhIII(tacn)(HCOO)3] (3), whose structure was unequivocally determined by X-ray analysis. The key intermediate in the transformation from 2 to 5 is a rhodium hydride formate complex, [RhIII(tacn)H(HCOO)](OTf) (4), which was obtained by heating of an aqueous solution of the rhodium diformate complex 2 with 10 equiv of dimethyl sulfoxide (DMSO) at pH 6−7 in water. Complex 4 was well characterized by isotopic labeling measurements of 1H NMR, electrospray ionization mass spectrometry (ESI-MS), and GC.