Synthesis and characterization of mixed-valent complexes containing the Rh25+ core. X-ray crystal structure of [Rh2(form)4(H2O)]O2CCF3 (form = N,N′-di-p-tolylformamidinate anion)

The chemical oxidation of the Rh2(4+) formamidinate derivative Rh2(forM)4 (form = NN'-di-p-tolylformamidinate) with the appropriate silver salts leads to the formation of the paramagnetic complexes [Rh2(form)4(H2O)]X (X = CF3COO (1), NO3 (2)). The complexes were characterized by EPR spectroscopy and X-ray analysis. The EPR spectra in frozen solution consist of two or three absorption lines. The low-field one (or two) is unresolved while the high-field one is split into a triplet. The pattern is interpreted in terms of hyperfine magnetic interaction between the unpaired electron and two equivalent Rh-103 nuclei (I = 1/2). Complex 1 crystallizes in the tetragonal P4/ncc space group, with a = 14.101(2), C = 29.235(3) angstrom, V = 5813.0(4) angstrom3, Z = 4 and D(calc) = 1.24 g cm-3. The mono-electron oxidation does not cause appreciable variation of the structural parameters of the radical species with respect to those of the parent neutral complexes. The only detectable effect is a more favourable orientation of the tolyl group attached to N(2) which allows the axial coordination of a water molecule. The oxidation results also in a slight lengthening of the Rh-Rh bond distance (2.452(2) angstrom) rather than shortening as expected on simple MO grounds.