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 (PC_NHCP) hybrid ligand are described. Thus, reactions of PC_NHCP with [Rh(μ-Cl)(COD)]₂ and [Rh(μ-Cl)(CO)₂]₂ led to the formation of the mononuclear and dinuclear complexes, [Rh(PC_NHCP,κP,κC_NHC)₂]Cl (PC_NHCP-RhCl) and [Rh(CO)(PC_NHC,κP,κC_NHC,κN)]₂ (PC_NHC-RhCO), respectively, the latter resulting after in situ cleavage of one (t-Bu)₂P–N_imid bond of PC_NHCP. With ligands acting as a P,C-chelate, a straightforward evaluation of the Tolman electronic parameter (TEP) of the C_NHC donor is problematical; the viability of dangling P- and bound C_NHC-donors (i.e. κC_NHC) has been observed in the trinuclear Fe(II) chain complex [Fe₃Cl₂(μ-Cl)₄(THF)₂(PC_NHCP,κC_NHC)₂] (PC_NHCP-Fe), obtained by the reaction of PC_NHCP with [Fe₄Cl₈(THF)₆] and, recently, established on Cr^II, Co^II and Au^I centres. Evaluation of the π-accepting properties of the PC_NHCP (and the related Dipp-PC_NHC) was based on the ⁷⁷Se NMR chemical shifts of the corresponding NHC–Se adducts, PC_NHCP-Se (and Dipp-PC_NHC-Se), which were prepared from the free PC_NHCP (and Dipp-PC_NHC) and Se. The π-acidity of PC_NHCP is found to be higher than that of Dipp-PC_NHC but lower than that of SIPr. The donor ability of the C_NHC in PC_NHCP 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 PC_NHCP and Dipp-PC_NHC are conformation dependent, with the percent buried volume (%V_bur) of PC_NHCP in the structurally characterised conformer calculated at 67.6, the largest value currently reported for NHC ligands.