Mono- and Hetero-Dinuclear Complexes of Janus-Head NHC Ligands Possessing Backbone Phosphinoyl Groups: the Case of Soft and Hard Metal Centers

The synthesis and structures of RhI and IrI complexes bearing 4‐phosphinoyl‐ and 4,5‐bis(phosphinoyl)imidazol‐2‐ylidene ligands (NHCP) is reported. Deprotonation of the corresponding imidazolium hydrogen sulfate salts by KOtBu in the presence of [M(cod)Cl]2 afforded complexes of the formula [M(cod)Cl(NHCP)] [5a/5b: NHCP = 4‐Ph2P(O)‐IiPrMe, 5a: M = Rh, 5b: M = Ir and 7a/7b: NHCP = 4,5‐{Ph2P(O)}2‐InBuMe]. The RhI complexes 5a and 7a were then converted into [Rh(CO)2Cl(NHCP)] 8a and 9a via reaction with carbon monoxide; the average CO stretching frequencies [8a: $\tilde {\nu}$(CO)av = 2036 cm–1 and 9a: $\tilde {\nu}$(CO)av = 2038.4 cm–1] led to the conclusion that mono‐ and bis‐functionalized NHCPs act as relatively weaker donors than backbone unsubstituted NHC. A first examination of Janus‐head ligand properties of complex 7a revealed that reaction with TiCl4 afforded hetero‐dinuclear 10a,a′ as a mixture of isomeric monodentate phosphinoyl titanium(IV) adducts. All products were characterized by elemental analyses, spectroscopic and spectrometric methods and, in addition, complexes 7a and 7b by single‐crystal X‐ray structure analysis. A family of hybrid ligands composed of hard phosphine oxides and soft N‐heterocyclic carbene donor sites have been employed to synthesize [M(cod)Cl(NHCP)] complexes I, which were further converted into the corresponding dicarbonyl complexes in order to study their donor properties. The advantage of backbone phosphinoyl groups was exploited to generate hetero‐dinuclear TiIV, RhI complexes 10a,a′.