Dynamic disorder in crystalline [Fe2Os(CO)(12)] and direct evidence for rotation of the Fe2Os triangle in the solid state from variable temperature X-ray diffraction and C-13 MAS NMR studies

Single-crystal X-ray diffraction data have been collected for [Fs(2)Os(CO)(12)] at 120, 223, 288, 292 and 323 K. The two studies at ambient temperature (288 and 292 K) reveal a approximate to 12:1 disorder of the metal triangle as previously reported. At the two lowest temperatures there is no evidence of disorder, while at 323 K the ratio of the major:minor component of disorder decreases significantly to approximate to 1.4.:1. Data collected on the same crystal specimens indicate unequivocally that this disorder is dynamic in nature. Two-dimensional exchange and one-dimensional variable-temperature C-13 magic angle spinning (MAS) NMR spectroscopy showed that carbonyl exchange is rapid above 306 K in the crystalline solid. Two independent exchange processes of similar energy are observed. The first is consistent with the crystallographic evidence, and involves an 'in-plane' rotation of the Fe2Os triangle in steps of 60 degrees within a relatively rigid icosahedral carbonyl manifold. The second involves localised axial-equatorial exchange in the Os(CO)(4) group. The Os L(III) and Fe K edge X-ray absorption fine structure spectra are consistent with identical structures being present in tetrahydrofuran solution and in the solid phase.