A phosphomide based PNP ligand, 2,6-{Ph2PC(O)}2(C5H3N), showing PP, PNP and PNO coordination modes

Abstract

A new class of PNP pincer ligands, pyridine-2,6-diylbis(diphenylphosphino)methanone, 2,6-{Ph₂PC(O)}₂(C₅H₃N) (1) (hereafter referred to as “bis(phosphomide)”), was prepared by the reaction of picolinoyldichloride with diphenylphosphine in the presence of triethylamine. The bis(phosphomide) 1 shows symmetrical PNP, unsymmetrical PNO and simple bidentate PP coordination modes when treated with various transition metal precursors. The reaction between 1 and [Ru(p-cymene)Cl₂]₂ in a 1 : 1 molar ratio yielded a binuclear complex [Ru₂Cl₄(NCCH₃)(p-cymene){2,6-{Ph₂PC(O)}₂(C₅H₃N)}] (2) containing an unsymmetrical PNO pincer cage around one of the ruthenium centers, whereas the second ruthenium is bonded to the other phosphorus atom along with cymene and two chloride atoms. Symmetrical pincer complexes [RuCl(NCCH₃)₂{2,6-{Ph₂PC(O)}₂(C₅H₃N)}](ClO₄) (3), [Ru(η⁵-C₅H₅){2,6-{Ph₂PC(O)}₂(C₅H₃N)}](OTf) (4) and [RhCl{2,6-{Ph₂PC(O)}₂(C₅H₃N)}] (5) were obtained in the respective reactions of 1 with [RuCl(NCCH₃)₂(p-cymene)](ClO₄), [Ru(η⁵-C₅H₅)Cl(PPh₃)₂] and [Rh(COD)Cl]₂. Group 10 metal complexes [NiCl{2,6-{Ph₂PC(O)}₂(C₅H₃N)}](BF₄) (6), [PdCl{2,6-{Ph₂PC(O)}₂(C₅H₃N)}]ClO₄ (7) and [PtCl{2,6-{Ph₂PC(O)}₂(C₅H₃N)}]ClO₄ (8) were obtained by transmetallation reactions of in situ generated Ag^I salts of 1 with Ni(DME)Cl₂ or M(COD)Cl₂ (M = Ni, Pd and Pt). The reactions between 1 and CuX or [Cu(NCCH₃)₄](BF₄) produced mononuclear complexes of the type [CuX{2,6-{Ph₂PC(O)}₂(C₅H₃N)}] (9, X = Cl; 10, X = Br; 11, X = I), [Cu(NCCH₃){Ph₂C(O)}₂(C₅H₃N)}](BF₄) (12) and [Cu{Ph₂C(O)}₂(C₅H₃N)}₂](BF₄) (13). Similarly, the silver complexes [AgX{2,6-{Ph₂PC(O)}₂(C₅H₃N)}] (14, X = ClO₄; 15, X = Br) were obtained by the treatment of 1 with AgClO₄ or AgBr in 1 : 1 molar ratios. Treatment of 1 with AuCl(SMe₂) in 1 : 1 and 1 : 2 molar ratios produced mono- and binuclear complexes, [AuCl{2,6-{Ph₂PC(O)}₂(C₅H₃N)}] (16) and [Au₂Cl₂{2,6-{Ph₂PC(O)}₂(C₅H₃N)}] (17), in good yield. The structures of ligand 1 and complexes 2, 5 and 17 were confirmed using single-crystal X-ray diffraction studies. DFT calculations were carried out to gain more insights into the structure and bonding features as well as feasibility of some key chemical transformations.