Coordination Studies of a New Unsymmetrical κ⁴-PNN‘N‘ ‘-Tetradentate Ligand: Stepwise Formation and Structural Characterization

The two-step synthesis of a new unsymmetrical ligand 2-{Ph2PC6H4C(H)N}C6H4{N(H)COCH2N(H)CO2Bz}, 2·HH, via acid-catalyzed Schiff base condensation of 2-(H2N)C6H4{N(H)COCH2N(H)CO2Bz}, 1, with 2-Ph2PC6H4(CHO) in refluxing EtOH is reported. The multidentate ligand 2·HH, isolated in ca. 60% yield, exhibits an array of ligation modes, as exemplified by coordination studies with NiII, PdII, PtII, and AuI mononuclear metal precursors. Hence, reaction of 2·HH with AuCl(tht) (1:1 molar ratio, tht = tetrahydrothiophene) affords AuCl(2·HH), 3, in which the ligand behaves as a classic, neutral two-electron phosphorus donor. In contrast, reaction with MCl2(cod) (M = Pt, Pd; cod = cycloocta-1,5-diene) affords the corresponding dichloro complexes MCl2(2·HH) (4a M = Pt; 4b M = Pd) in which κ2-P/N-chelation through both P and imino N-donor atoms is observed. Likewise, treatment of Pd(CH3)Cl(cod) with 2·HH gave Pd(CH3)Cl(2·HH), 4c, in which the imino nitrogen is trans to the methyl ligand. Cycloocta-1,5-diene elimination from, and single methyl protonation of, Pt(CH3)2(cod) with 1 equiv of 2·HH in toluene at ambient temperature affords the neutral complex Pt(CH3)(2·H-), 5a, in which 2·H- functions effectively in a κ3-PNN‘ coordination mode. The dichloro compounds 4a or 4b undergo smooth N(H) deprotonation with tBuOK to give 6a\6a‘ and 6b\6b‘ in which 22- acts as a dianionic κ4-PNN‘N‘ ‘ ligand. The corresponding square-planar, diamagnetic, nickel(II) complex 6c\6c‘ was prepared in excellent yield from NiCl2·6H2O, 2·HH, and tBuOK. Variable-temperature NMR experiments confirm 6a\6a‘ and 6b\6b‘ exist, in solution, as a pair of conformational (anti and syn) isomers due to restricted rotation about the N−CO2Bz group. This feature is also borne out by single-crystal X-ray studies of anti-6a·CHCl3, syn-6a‘·H2O, anti-6b·CHCl3, and anti-6c·CH2Cl2. To the best of our knowledge, we believe these constitute the first examples of crystallographically characterized conformers of a tetradentate ligand incorporating a P-donor center. All new compounds reported have been fully characterized by a combination of spectroscopic (NMR, FT-IR, ES-MS) and analytical methods. Furthermore, single-crystal X-ray studies have also been undertaken on compounds 2·HH, 3, 4a, and 5a·Et2O.