Synthesis and x-ray analysis of the dirhodium(II,II) carboxylate complex Rh2(O2CCH3)4(formH)2 (form H=N,N′-di-p-tolylformamidine)

The tetrakisacetate dirhodium(II,II) complex reacts with N,N′-di-p-tolylformamidine (formH) giving the bisadduct Rh2(O2CCH3)4(formH)2. The complex crystallizes in the monoclinic space group P21/c, with a=11.638(2), b=12.464(3), c=17.303(4) Å, β=98.09(2)°9(9), V=2484.Å3, Z=2 and Dcalc=1.506 g cm-3. The structure reveals that bond lengths and angles within the Rh2(O2CCH3)4 fragment are in the range quoted for similar complexes. It also reveals that the two formamidines are axially bonded to the dirhodium core through the imino nitrogen. The NCN formamidine planes are rotated by 100.00(4)° with respect to the equatorial planes of the two rhodium atoms, leading to several hydrogen-bonding interactions. The 1H NMR spectrum exhibits features that do not correspond to the solid-state geometry. It shows in fact only one signal for the methyl groups of the formamidine, suggesting that the title complex is subject to a fluxional process that involves fast exchange of the coordination sites, amino and imino, of formamidine.