K. Besancon et al., SOLUTION EQUILIBRIA IN TRIALKYL-PHOSPHITE DERIVATIVES OF [IR4(CO)12] - CRYSTAL-STRUCTURE OF [IR4(CO)11(P(OCH2)3CET)], Helvetica Chimica Acta, 76(8), 1993, pp. 2926-2935
The monosubstituted [Ir4(CO)11L] clusters (L = P(OPh)3, 1; L = (P(OMe)
3, 2; L = P(OCH2)3CEt, 3) were obtained in good yields by the reaction
of [Ir4(CO)11I]-with the corresponding phosphite. In the solid state,
cluster 3 has a C(s) geometry with all terminal ligands as shown by a
n X-ray analysis. Three isomers are present in solution: one with term
inal ligands (A) and two with three edge-bridging CO's and with L in a
xial (B) or radial (C) position (see Scheme). The thermodynamic and ki
netic parameters of isomerisations B reversible A and A reversible C w
ere determined by simulation of the variable-temperature P-31-NMR spec
tra. The three isomers correspond to three minima on the kinetic pathw
ay of CO scrambling, whose relative energies vary independently within
a small range (1-9 kJ mol-1 at 298 K). At low temperature, isomer C i
s always the least stable and is not observed for 1 which bears most b
ulky phosphite ligand. The isomerisations are due to two intramolecula
r merry-go-rounds of CO groups about two unequivalent faces of the unb
ridged species A.