Synthesis, structure, and nonrigidity of Os4(CO)15, Os4(CO)13(PMe3)[P(OMe)3], and Os4(CO)14(CNBut)

Abstract

The binary carbonyl Os4(CO)15, 1, has been prepared by the addition of Os(CO)5 to Os3(CO)10(cyclooctene)2 at −15 °C. The related clusters Os4(CO)13(PMe3)[P(OMe)3], 2, and Os4(CO)14(CNBu′), 3, have been synthesized from Os4(CO)13(PMe3) and Os4(CO)15(CNBu′), respectively. The crystal structures of 1, 2, and 3 reveal similar planar metal skeletons with short (2.774 (1) − 2.793 (2) Å) and long (2.978 (2) − 3.019 (2) Å) peripheral Os—Os bonds; the hinge Os—Os bond in 1–3 ranges in length from 2.936 (2) to 2.948 (1) Å. The variable temperature 13C nuclear magnetic resonance spectra of 1 and 3 show that both are highly nonrigid in solution even at −120 °C. The mechanism of nonrigidity is believed to be an all-equatorial, merry-go-round carbonyl exchange. The variable temperature 13C nmr spectra of 2 indicate it is rigid on the nmr time scale in solution at −45 °C. Carbonyl exchange is, however, observed in the spectrum at −6 °C. From the mode of collapse of the signals it is believed that the lowest energy exchange processes in 3 involve axial-equatorial, merry-go-round CO exchanges in the two planes that each contain a short Os—Os bond. Crystallographic data for compound 1: space group C2/c; a = 12.802 (3) Å, b = 10.217 (3) Å, c = 16.380 (5) Å, β = 91.39 (2)°; R = 0.044, 1204 observed reflections. For compound 2: space group P21/c; a = 11.106 (7) Å, b = 16.931 (5) Å, c = 16.481 (5) Å, β = 97.71 (5)°; R = 0.051, 2117 observed reflections. For compound 3: space group P21/n; a = 11.747 (3) Å, b = 18.009 (5) Å, c = 12.448 (2) Å, β = 92.65 (2)°; R = 0.054, 2131 observed reflections. Keywords: osmium, carbonyl, cluster, nonrigidity.