Mixed Ruthenium - Rhodium Carbonyl Cluster Complexes. Synthesis of the Anions [Ru3Rh(CO)13]- and [RuRh3(CO)12]- and Crystal Structures of Their [N(PPh3)2]+ Salts

The new anion [Ru3Rh(CO)13]- (1) has been obtained by reaction of [Rh(CO)4]- ([N(PPh3)2]- or [PPh4]+ salt) with Ru3(CO)12; [RuRh3(CO)12]- (2) has been derived by oxidative degradation of [RuRh4(CO)12]2-. Their salts, [N(PPh3)2][Ru3Rh(CO)13] (1) and [N(PPh3)2][RuRh3(CO)12] (II), were characterized by single-crystal X-ray diffraction. Data for I: space group P1̄, a = 9.827(2) Å, b =14.911(2) Å, c = 18.735(3) Å, α = 110.53(1)°, β = 99.79(2)°, γ = 91.85(2)°, R1 = 0.0342, wR2 = 0.0926 for 7257 independent reflections with I > 3σ(I). Data for II: space group P21/c. a = 14.746(3) Å, b = 21.395(4) Å, c = 16.140(4) Å, β= 102.95(2)°, R1 = 0.0582, wR2 = 0.1383 for 4381 independent reflections with I > 3σ(I). Both anions have a tetrahedral metal frame; I has an idealized C3 symmetry, with the Rh atom and its unique terminal carbonyl on the 3-fold axis; nine more carbonyls are terminally bonded to the three Ru atoms, while three bridge the Ru-Rh edges. In 2, which has an idealized Cs symmetry, three carbonyls are terminally bound on the Ru atom, and one, one, and two CO, respectively, on Rh1, Rh2, and Rh3; five more CO bridge all edges but the Ru-Rh3 edge. It is worthy of note that subtle details of the CO ligands stereochemistry allowed the correct labeling of metal centers, otherwise indistinguishable on the basis of good quality X-ray diffraction data only. Compound 1 reacts with PPh3, yielding the rhodium-substituted [Ru3Rh(CO)12(PPh3)]- (1a) (31P NMR; 56.9 ppm (d), JP-Rh = 188 Hz). 13C NMR spectrum of 1 is a doublet (207 ppm, JC-Rh = 17.5 Hz) consistently with a fluxional behavior with complete CO scrambling, from 295 down to 170 K.