Acceptor PCP Pincer Iridium(I) Chemistry: Stabilization of Nonmeridional PCP Coordination Geometries

Abstract

The preparation of a series of four-coordinate complexes (CF3PCP)Ir(L) (L = CO, DBU, nbe, coe, MeP(C2F5)2 (dfmp)) and five-coordinate complexes (CF3PCP)Ir(L)(L′) (L = L′ = CO, dfmp, nbd, cod, (C2F5)2PCH2CH2P(C2F5)2 (dfepe); L = PhCN, L′ = C2H4) from dehydrohalogenation of (CF3PCP)Ir(C2H4)(H)Cl with Et3N in the presence of trapping ligands is reported. (CF3PCP)Ir(L) and (CF3PCP)Ir(L)2 for L = CO, dfmp have been structurally characterized and establish a distorted-trigonal -bipyramidal coordination geometry for (CF3PCP)Ir(L)2 with a bent PCP unit and inequivalent axial and equatorial L coordination sites. (CF3PCP)Ir(L)(L′) systems (L = L′ = CO, C2H4; L = PhCN, L′ = C2H4) are highly fluxional, with ligand site interconversion free energy barriers determined by VT NMR of 9.7 kcal mol−1 (L = L′ = CO), 12.2 kcal mol−1 (L = L′ = C2H4), and 16.1 kcal mol−1 (L = C2H4, L′ = PhCN). A dissociative site exchange mechanism is proposed. (CF3PCP)Ir(L) complexes readily undergo oxidative addition reactions. Addition of H2 to (CF3PCP)Ir(CO) reversibly forms trans-(CF3PCP)Ir(CO)(H)2 at ambient temperatures. In contrast, addition of H2 to (CF3PCP)Ir(dfmp) affords fac,cis-(CF3PCP)Ir(dfmp)(H)2 as the major product, with an unusual facially coordinated pincer group. VT NMR monitoring of the reaction of (CF3PCP)Ir(CO) with H2 established the initial formation of fac,cis-(CF3PCP)Ir(CO)(H)2 followed by conversion to mer,cis-(CF3PCP)Ir(CO)(H)2 prior to isomerization to mer,trans-(CF3PCP)Ir(CO)(H)2. The unusual stability of (CF3PCP)Ir(L)2 and fac,cis-(CF3PCP)Ir(L)(H)2 complexes is attributable to the increased stability of nonplanar (PCP)M moieties possessing strongly π-accepting phosphorus groups.