This article describes synthesis and transition metal complexes of ethylenediamine-derived potential tetradentate PNNP type bisphosphine ligand.

The synthesis of the ethylenediamine-derived bisphosphine ligand, (o-PPh₂ (C₆H₄)C(O)NH(CH₂))₂ (1) (hereafter referred to as PN^HN^HP), is described via two methods distinct from previously reported procedures. Treatment of ligand 1 with Pd(COD)Cl₂ afforded the trans-dichloropalladium complex [PdCl₂(PN^HN^HP)-κ²-P,P] (2). In contrast, the reaction of 1 with Pd(COD)Cl₂ in the presence of a base led to the formation of the tetradentate cis-complex [Pd(PNNP)-κ⁴-P,N,N,P] (3). Similarly, ligand 1 reacted with Pt(COD)Cl₂ to yield the corresponding cis-complex [Pt(PNNP)-κ⁴-P,N,N,P] (4). Reaction of 1 with NiCl₂(DME) in the presence of a base resulted in the formation of [Ni(PNNP)-κ⁴-P,N,N,P] (5). Notably, exposure of the cis-palladium complex 3 to HCl gas led to its conversion to the trans-complex 2, while the cis-platinum complex 4 remained unaffected. Density functional theory (DFT) calculations confirm that the conversion of 3 to 2 is thermodynamically favorable, with a Gibbs free energy change (ΔG) of −29.7 kcal mol. Furthermore, treatment of ligand 1 with cuprous halides yield [CuX(PN^HN^HP)-κ²-P,P] (6: X = Cl; 7: X = Br; 8: X = I). Reaction with Cu(NCCH₃)₄BF₄ produced the dinuclear complex [Cu₂(PN^HN^HP)₂-κ⁴-P,O,O,P]BF₄ (9).