Transition Metal Complexes of PNP and PNN Ligands: Synthesis and Catalytic Applications in Hydrophosphination of Terminal Alkynes

In this study, we report the synthesis and structural characterization of a series of transition metal complexes derived from two pincer-type ligands: {(o-PPh₂)C₆H₄C(O)NH(C₉H₆N)} (1) and {(o-Ph₂PC₆H₄C(O)N(H)C₆H₄PPh₂-o)} (2). Treatment of Ligand 1 with [Rh (acac)(CO)₂] afforded rhodium pincer complex [Rh (CO){(o-PPh₂)C₆H₄C(O)N(C₉H₆N)}κ³-P,N,N] (3). Ligand 1, upon reaction with [Ru(H)(CO)(Cl)(PPh₃)₃] in THF, yielded the ruthenium complex [Ru (PPh₃)(CO)(Cl){(o-PPh₂)C₆H₄C(O)N(C₉H₆N)}κ³-P,N,N] (4), while its reaction with [Ru (Cl)₂(PPh₃)₃] in methanol afforded a Ru^III complex, [Ru (CH₃OH)(Cl)₂{(o-PPh₂)C₆H₄C(O) N(C₉H₆N)}κ³-P,N,N] (5). Ligand 2, when reacted with CuI in a 1:1 M ratio in a CH₂Cl₂/CH₃CN (1:1 v/v) mixture, yielded the dimeric complex [CuI(o-PPh₂)C₆H₄C(O) N(H)CH₂C₆H₄(o-PPh₂)]₂ (6). Increasing the CuI to ligand ratio to 2:1 yielded the tetrameric complex [Cu₄(μ₂-I)₂(μ₃-I)₂{(o-PPh₂)C₆H₄C(O)NCH₂C₆H₄ (o-PPh₂)}₂-P,P] (7). The reaction of Ligand 2 with two equivalents of [AuCl (SMe₂)] produced a binuclear gold complex, [AuCl{(o-PPh₂)C₆H₄C(O)N(H)CH₂C₆H₄{(o-PPh₂)}AuCl] (8). Furthermore, Ligand 2 underwent complexation with [Rh (acac)(CO)₂] and [Mn₂(CO)₁₀] to afford PNP-type pincer complexes [RhCO{(o-PPh₂)C₆H₄C(O)NCH₂C₆H₄(o-PPh₂)}κ³-P,N,P] (9) and [Mn (CO)₃{(o-PPh₂)C₆H₄ C(O)NCH₂C₆H₄(o-PPh₂)}κ³-P,N,P] (10) respectively. Notably, the rhodium PNN pincer Complex 3 was successfully employed as a catalyst in the additive-free hydrophosphination of phenylacetylenes, providing a more straightforward and operationally simple methodology for this transformation.