Aqueous transformation of a metal diformate to a metal dihydride carbonyl complex accompanied by H₂ evolution from the formato ligands

The rhodium diformate complex [Rh^III(tacn)(HCOO) ₂](OTf) (2, tacn = 1,4,7-triazacy-clononane, OTf^- = CF₃SO₃^-) reacts with HCOONa at pH 6-7 at 80°C in water to form a rhodium dihydride CO complex, [Rh ^III(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, [Rh ^III(tacn)(H₂O)₃](OTf)₃ (1), with HCOONa in water. Recrystallization of 2 with HCOONa in water gave crystals of a rhodium triformate complex, [Rh^III-(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, [Rh^III(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.