PNN′ & P2NN′ ligands via reductive amination with phosphine aldehydes: synthesis and base-metal coordination chemistry

Abstract

Novel PNN′ & P₂NN′ ligands based on 2-aminopyridine (APyPNN-R) R = Ph (1a), Cy (1b), ⁱBu (1c), 8-aminoquinoline (AQPNN-R) R = Ph (2a), Cy (2b), ⁱBu (2c), ⁱPr (2d), and 2-picolylamine (P₂NN-R) R = Ph (3a), Cy (3b), ⁱBu (3c), have been synthesized via a versatile, one-pot, single-step, reductive amination of tertiary phosphine acetaldehydes with the amine by reaction with STAB (where STAB is sodium(triacetoxy)borohydride). Ligands 1b and 1c bridge between paramagnetic Co(II) and form dimeric complexes Co₂Cl₄(APyPNN-R)₂ (4 and 5) when reacted with cobalt dichloride. Ligands 2a–c coordinate in a tridentate fashion forming chelate complexes MCl₂(AQPNN-R) M = Co(II) (6–8), and, for 2d, the Fe(II) complex FeCl₂(AQPNN-ⁱPr) (9). A solution magnetic susceptibility value for 9 of 3.9μ_B is consistent with a monomer–dimer equilibrium. The synthesis of the dimeric complex [FeCl₂(AQPNN-Ph)]₂ (10) using 2a as well as solid state magnetic susceptibility measurements on 9 and 10 confirm this phenomenon. Ligand 3a coordinates to Fe(II) in an interesting tetradentate fashion despite bearing a tertiary amine moiety giving octahedral FeCl₂(P₂NN′) (11). All of the metal complexes have been characterized by elemental analysis, paramagnetic ¹H NMR spectroscopy, solution magnetic susceptibility, and single crystal X-ray diffraction (XRD).