Comparison of [Ni(P^Ph₂N^Ph₂)₂(CH₃CN)]²⁺ and [Pd(P^Ph₂N^Ph₂)₂]²⁺ as electrocatalysts for H₂ production

The complexes [Ni(P^Ph₂N^Ph₂)₂(CH₃CN)]²⁺ and [Pd(P^Ph₂N^Ph₂)₂]²⁺, where P^Ph₂N^Ph₂ is 1,5-diphenyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane, are compared as electro-catalysts for H₂ production under identical experimental conditions. With [(DMF)H]⁺ as the acid in acetonitrile solution, [Pd(P^Ph₂N^Ph₂)₂]²⁺ afforded a turnover frequency (TOF) of 230 s^-1 for formation of H₂ under dry conditions and a TOF of 640 s^-1 when H₂O was added. These rates are similar to the TOFs of 590 s^-1 (dry) and 720 s^-1 (wet) that were previously measured for [Ni(P^Ph₂N^Ph₂)₂(CH₃CN)]²⁺ using [(DMF)H]⁺. The [Ni-(P^Ph₂N^Ph₂)₂(CH₃CN)]²⁺ and [Pd(P^Ph₂N^Ph₂)₂]²⁺ complexes both exhibited large current enhancements when treated with trifluoroacetic acid (TFA). At a TFA concentration of 1.8 M, TOF values of 5670 and 2060 s^-1 were measured for [Ni(P^P ₂N^P ₂)2(CH₃CN)]²⁺ and [Pd(P^Ph ₂N^P 2)₂]²⁺, respectively. The fast rates observed using TFA are, in part, attributed to homoconjugation of TFA in acetonitrile solutions, which decreases the effective pK_a^e of the acid. In support of this hypothesis, dramatically lower rates of H₂ production were observed using p-anisidinium, which has a pK_a^Me value comparable to that of TFA but does not homoconjugate significantly in acetonitrile solutions. (Figure Presented)