Transition-Metal Carbonyl Complexes and Electron-Donating Properties of N-Heterocyclic-Carbene-Phosphinidene Adducts

Rhodium(I), tungsten(0), and molybdenum(0) carbonyl complexes of the N-heterocyclic-carbene-phosphinidene adducts IPr center dot PR [1a, R = H; 1b, R = Ph; 1c, R = Mes; IPr = 1,3-bis( 2,6-diisopropylphenyl) imidazolin-2-ylidene; Mes = 2,4,6-tri-methylphenyl] were prepared. The reaction of 1b with [Rh( mu-Cl)(CO)(2)](2) afforded the dicarbonyl rhodium(I) complex cis[(IPr center dot PPh) RhCl(CO)(2)] (2) as the major product together with the tetranuclear complex [{mu-(IPr center dot PPh)}(2)Rh-4(mu-Cl)(4)(CO)(4)] (3). The latter was characterized by X-ray diffraction analysis. The tungsten pentacarbonyl complexes [(IPr center dot PR)W(CO)(5)] (4a, R = H; 4b, R = Ph; 4c, R = Mes) were isolated from the reactions of 1a-1c with [(Me3N)W(CO)(5)], whereas the reactions of 1a and 1b with [(thf)Mo(CO)(5)] (thf = tetrahydrofuran) gave the corresponding molybdenum complexes [(IPr center dot PR)Mo(CO)(5)] (5a, R = H; 5b, R = Ph). The molecular structures of the five carbonyl complexes were established by X-ray diffraction analyses. The IR spectroscopic analysis of the CO stretching frequencies of the carbonyl complexes 2, 4, and 5 revealed the strong electron-donating abilities of the phosphorus(I) ligands.