Tuning of PCP Pincer Ligand Electronic and Steric Properties

Abstract

A survey of the electronic and steric characteristics of anionic (^RPCP, ^RPOCOP, ^RPNCNP) and neutral (^RPNP, ^RPONOP, ^RPNNNP) phosphine terdentate ligands is presented. A review of the scope of syntheses, particularly in regard to the variation of phosphine PR₂ substituents, is followed by a general discussion of pincer steric variation, terdentate ligand conformations, and coordination geometries. Buried volume parameters, %V_bur, provide a measure of overall steric influence, while the concepts of cis and trans ligand void space and PR₂ steric influence are used to more completely define asymmetric steric influence. Pincer arm conformations are categorized in terms of C₂ twist, C_s “gull wing,” and asymmetric bending, which accommodate a range of non-meridional P–M–P bending geometries. For 5-coordinate metal complexes, geometries can vary between two square pyramidal and two trigonal bipyramidal geometrical extremes and are distinguished using subtending angle parameters α, β, and γ. A key feature to note is that pincer ligands often assume non-meridional geometries with P–M–P angles as small as 107°. DFT studies on P–Ir–P bending energetics for (^RPCP)Ir(L) (L = CO, NH₃) with a wide range of R substituents show that destabilization from bending is primarily steric in origin and is particularly significant (+12 to 30 kcal mol⁻¹) for ^tBuPCP systems. Pincer electronic effects are surveyed using comparative IR, electrochemical, and binding affinity data. While R electronic effects are significant, particularly for our R = CF₃ systems, DFT calculations of ΔG _diss(Ir–CO) for (^RPCP)Ir(CO)₂ systems show that reduced Ir–CO binding correlates mostly with steric destabilization in non-meridional tBu-substituted pincer systems.