Transition metal (Rh and Fe) complexes and main-group (Se and B) adducts with N,N′-diphosphanyl NHC ligands: A study of stereoelectronic properties

Abstract

Attempts to evaluate experimentally the donor characteristics of the N,N′-bis(di-tert-butylphosphanyl)-imidazole-2-ylidene (PCNHCP) hybrid ligand are described. Thus, reactions of PCNHCP with [Rh(μ-Cl)(COD)]2 and [Rh(μ-Cl)(CO)2]2 led to the formation of the mononuclear and dinuclear complexes, [Rh(PCNHCP,κP,κCNHC)2]Cl (PCNHCP-RhCl) and [Rh(CO)(PCNHC,κP,κCNHC,κN)]2 (PCNHC-RhCO), respectively, the latter resulting after in situ cleavage of one (t-Bu)2P-Nimid bond of PCNHCP. With ligands acting as a P,C-chelate, a straightforward evaluation of the Tolman electronic parameter (TEP) of the CNHC donor is problematical; the viability of dangling P- and bound CNHC-donors (i.e. κCNHC) has been observed in the trinuclear Fe(ii) chain complex [Fe3Cl2(μ-Cl)4(THF)2(PCNHCP,κCNHC)2] (PCNHCP-Fe), obtained by the reaction of PCNHCP with [Fe4Cl8(THF)6] and, recently, established on CrII, CoII and AuI centres. Evaluation of the π-accepting properties of the PCNHCP (and the related Dipp-PCNHC) was based on the 77Se NMR chemical shifts of the corresponding NHC-Se adducts, PCNHCP-Se (and Dipp-PCNHC-Se), which were prepared from the free PCNHCP (and Dipp-PCNHC) and Se. The π-acidity of PCNHCP is found to be higher than that of Dipp-PCNHC but lower than that of SIPr. The donor ability of the CNHC in PCNHCP was explored by its reaction with the Lewis acids tris(pentafluorophenyl)borane (B(C6F5)3) and tris(pentafluorophenyl)boroxine ([(C6F5)BO]3), which resulted in stable donor-acceptor adducts with no FLP reactivity. The steric properties of PCNHCP and Dipp-PCNHC are conformation dependent, with the percent buried volume (%Vbur) of PCNHCP in the structurally characterised conformer calculated at 67.6, the largest value currently reported for NHC ligands.