Autoionization of homogeneous nickel(II) diphosphane hydrogenation catalysts. An NMR study and crystal structures of [Ni(o-MeO-dppe)I-2] and [Ni(o-MeO-dppe)(2)](PF6)(2)

The synthesis of a number of nickel(II) complexes containing the didentate phosphane ligand 1,2-bis(di(o-methoxyphenyl)phosphino)ethane (o-MeO-dppe) i s reported. Two types of complexes have been synthesized, i.e., the mono(ch elate! complex (I) of the general formula [Ni(o-MeO-dppe)X-2] (where X = Cl , Br or I) and the bis(chelate) complex (2) of the general formula [[Ni(o-M eO-dppe)(2)]Yr (where Y = PF6 or trifluoroacetate (TFA)). These complexes h ave been characterized using electronic absorption and NMR spectroscopy. Th e structures of the mono(chelate) complex [Ni(o-MeO-dppe)I-2] (1c) and of t he bis(chelate) complex [Ni(o-MeO-dppe)(2)](PF6)(2) (2e) have been determin ed by X-ray crystallography. [Ni(o-MeO-dppe)I-2] crystallizes in the monocl inic space group P2(1)/c with Z = 4, a = 12.1309(1) Angstrom, b = 16.5759(3 ) Angstrom, c = 17.6374(2) Angstrom, beta = -119.3250(10)degrees. [Ni(oMeO- dppe)(2)](PF6): crystallizes in the monoclinic space group C2/c with Z = 4, a = 22.5326(3) Angstrom, b = 13.6794(2) A, c = 21.7134(3) Angstrom, beta = 107.1745(7)degrees; In both structures the nickel ion is in a square-plana r geometry with a NiP2I2 and NiP4 chromophore, respectively. Using H-1 and P-31{1H} NMR spectroscopy the behavior-of the complexes in various solvents has been studied. It appears that in solution these nickel complexes are i nvolved in an autoionization equilibrium: 2[Ni(o-MeO-dppe)X-2] reversible a rrow [Ni(o-MeO-dppe)(2)](2+) + ["NiX4"](2-). The ionized complex (3) consis ts of a cationic unit in which a nickel atom is surrounded by two didentate phosphane ligands, and an anionic unit that stoichiometrically consists of a nickel atom and four anions. The position of the autoionization equilibr ium is highly dependent on the anion and the solvent used. In a polar solve nt in combination with weakly coordinating anions only the ionized complex is observed, whereas in;an apolar solvent in combination with coordinating anions only the mono(chelate) complex occurs. A comparison of the behavior of o-MeO-dppe with its unsubstituted analogue dppe in combination with nick el(II) acetate using P-31{H-1} NMR spectroscopy shows that the latter is mo re readily oxidized.