Nature of hydrogen interactions with Ni(II) complexes containing cyclic phosphine ligands with pendant nitrogen bases

Abstract

Studies of the role of proton relays in molecular catalysts for the electrocatalytic production and oxidation of H(2) have been carried out. The electrochemical production of hydrogen from protonated DMF solutions catalyzed by [Ni(P(2)(Ph)N(2)(Ph))(2)(CH(3)CN)](BF(4))(2), 3a (where P(2)(Ph)N(2)(Ph) is 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane), permits a limiting value of the H(2) production rate to be determined. The turnover frequency of 350 s(-1) establishes that the rate of H(2) production for the mononuclear nickel catalyst 3a is comparable to those observed for Ni-Fe hydrogenase enzymes. In the electrochemical oxidation of hydrogen catalyzed by [Ni(P(2)(Cy)N(2)(Bz))(2)](BF(4))(2), 3b (where Cy is cyclohexyl and Bz is benzyl), the initial step is the reversible addition of hydrogen to 3b (K(eq) = 190 atm(-1) at 25 degrees C). The hydrogen addition product exists as three nearly isoenergetic isomers 4A-4C, which have been identified by a combination of one- and two-dimensional (1)H, (31)P, and (15)N NMR spectroscopies as Ni(0) complexes with a protonated amine in each cyclic ligand. The nature of the isomers, together with calculations, suggests a mode of hydrogen activation that involves a symmetrical interaction of a nickel dihydrogen ligand with two amine bases in the diphosphine ligands. Single deprotonation of 4 by an external base results in a rearrangement to [HNi(P(2)(Cy)N(2)(Bz))(2)](BF(4)), 5, and this reaction is reversed by the addition of a proton to the nickel hydride complex. The small energy differences associated with significantly different distributions in electron density and protons within these molecules may contribute to their high catalytic activity.

Fig. 1.

Structural features of the active site of Fe-only hydrogenases and the proposed activation of hydrogen.

Fig. 2.

Structure 3.

Fig. 3.

Cyclic voltammograms of a 3.2 × 10⁻⁴ M solution of [Ni(P^Ph₂N^Ph₂)₂(CH₃CN)](BF₄)₂, 3a, with increasing concentrations of H⁺-DMF(OTF)/DMF (1:1) in acetonitrile. Conditions were as follows: scan rate of 50 mV/s, acetonitrile solvent, 0.3 M NEt₄BF₄ as supporting electrolyte, glassy carbon working electrode. Inset shows values of i_c/i_p vs. the concentration of the buffer H⁺-DMF(OTF)/DMF in acetonitrile.

Scheme 1

Fig. 4.

400-MHz ³¹P NMR spectra of 4 formed at room temperature in acetone-d₆ (a) and the same sample solution cooled to −80°C (b).

Fig. 5.

³¹P NMR spectra of 4A(H₂) (a), 4A(D₂) (b), and 4A(HD) (c) recorded at 400 MHz in acetone-d₆ at −70°C.

Fig. 6.

Proposed structures for isomers 4B and 4C.

Fig. 7.

Structure 8.