Relationships between structure and ligand dynamics: II. Alkyne and carbonyl dynamics in Os 3(CO) 9(alkyne)(L) (L = CO, PR 3)

Abstract

The synthesis of the phosphine substituted complexes Os 3(CO) 9(μ 3-η 2-CH 3CH 2-CCCH 2CH 3)L (L = P(C 6H 5) 3 (III), P(CH 3) 3 (IV)) and Os 3(CO) 9(μ 3-η 2-CH 3CCCH 3)L (L = P(OCH 3) 3 (V)) are reported. A detailed analysis of the VT- 1H and VT- 13C NMR of these complexes is presented and compared with the same studies on the parent complexes Os 3(CO) 9(μ 3-η 2-RCCR)(μ-CO) (R = CH 2CH 3 (I), CH 3 (II)). In the parent complexes I and II a two stage exchange process is observed: (1) a low energy process involving axial radial exchange at the carbonyl bridged osmium atoms, (2) a higher energy exchange process in which alkyne motion over the face of the metal triangle is coupled with bridge-terminal exchange of the carbonyls, and with axialradial exchange at the unique osmium atom. The phosphine derivatives III–V, all show a three stage exchange process: (1) a localized axial-radial exchange at the unsubstituted osmium atoms; (2) a semibridging terminal carbonyl exchange at the phosphine substituted osmium coupled with a restricted oscillation of the alkyne, pivoted on this osmium atom; (3) free motion of the alkyne and averaging of all the carbonyl groups. The relationship between the differences in the observed dynamic processes are understood by a comparison of the solid state structures of I and III which are also reported. Compound I crystallizes in the triclinic space group P 1 , with unit cell parameters a 9.292(2), b 15.340(3), c 8.391(2) Å, α 91.27(2), β 116.70 (1), γ 105.24(2)°, V 1017.3(4) Å 3, and Z = 2. Compound III belongs to the monoclinic space group P2 1/c, with a 14.271(4), b 11.370(2), c 21.192(5) Å, β 104.43(2) 3, V 3330(1) Å 3, and Z = 4. The structures were refined by full matrix least squares to R F = 0.044, R w F = 0.058 for I, and R F = 0.033, R w F = 0.041 for III, respectively.