Synthesis and characterization of Co and Ni complexes stabilized by keto- and acetamide-derived P,O-type phosphine ligands

Abstract

The coordination properties of the β-keto phosphine ligands R₂PCH₂C(O)Ph (HL¹1, R = i-Pr; HL²2, R = Ph), of the new acetamide-derived phosphine ligand (i-Pr)₂PNHC(O)Me (HL³3) and of Ph₂PNHC(O)Me (HL⁴4) have been examined towards Ni(II) complexes. Comparisons are made between systems in which the PCH₂ function of the ketophosphine has been replaced with an isoelectronic PNH group in amide-derived ligands, or the PCH functionality of phosphinoenolates with a PN group in phosphinoiminolate complexes. Furthermore, ligands HL²2 and HL⁴4 reacted with [(η⁵-C₅H₅)CoI₂(CO)] to afford the phosphine mono-adducts [(η⁵-C₅H₅)CoI₂{Ph₂PCH₂C(O)Ph}] (1) and [(η⁵-C₅H₅)CoI₂{Ph₂PNHC(O)Me}] (3), respectively, which upon reaction with excess NEt₃ yielded the phosphinoenolate complex [(η⁵-C₅H₅)CoI{Ph₂PCHC(O)Ph}] (2) and the phosphinoiminolate complex [(η⁵-C₅H₅)CoI{Ph₂PNC(O)Me}] (4), respectively. The complexes cis-[Ni{(i-Pr)₂PNC(O)Me}₂] (6) and cis-[Ni{Ph₂PNC(O)Me}₂] (7) were obtained similarly from NiCl₂ and HL³ and HL⁴, respectively, in the presence of a base. The phosphinoenolate complex [Ni{(i-Pr)₂PCHC(O)Ph}₂] (5) exists in ethanol as a mixture of the cis and trans isomers, in contrast to cis-[Ni{(Ph₂PCHC(O)Ph}₂], and the solid-state structure of the trans isomer of 5 was established by X-ray diffraction. The structures of the ligand HL³ and of the complexes 1, 3 in 3·3/2CH₂Cl₂, 4, 6 and 7 have also been determined by X-ray diffraction and are compared with those of related complexes. Complexes 4, 6 and 7 contain a five-membered heteroatomic metallocyclic moiety, which is constituted by five different chemical elements. The structural consequences of the steric bulk of the P substituents and of the electronic characteristics of the P,O chelates are discussed.