Uriah Kilgore - [Ni(PPh2NC6H4X2)2]2+ Complexes as Electrocatalysts for H2 Production: Effect of Substituents, Acids, and Water on Catalytic Rates

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      [Ni(PPh2NC6H4X2)2]2+  Complexes as Electrocatalysts for H2 Production:   Effect of Substituents, Acids, and Water on Catalytic   Rates


      J. Am. Chem. Soc.,  2011, 133 (15), pp 5861–5872


      A series of mononuclear nickel(II) bis(diphosphine) complexes   [Ni(PPh2NC6H4X2)2](BF4)2  (PPh2NC6H4X2 =   1,5-di(para-X-phenyl)-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane;   X = OMe, Me, CH2P(O)(OEt)2, Br, and   CF3) have been synthesized and characterized. X-ray   diffraction studies reveal that   [Ni(PPh2NC6H4Me2)2](BF4)2  and   [Ni(PPh2NC6H4OMe2)2](BF4)2  are tetracoordinate with distorted square planar geometries. The   Ni(II/I) and Ni(I/0) redox couples of each complex are   electrochemically reversible in acetonitrile with potentials that   are increasingly cathodic as the electron-donating character of X   is increased. Each of these complexes is an efficient   electrocatalyst for hydrogen production at the potential of the   Ni(II/I) couple. The catalytic rates generally increase as the   electron-donating character of X is decreased, and this   electronic effect results in the favorable but unusual situation   of obtaining higher catalytic rates as overpotentials are   decreased. Catalytic studies using acids with a range of   pKa values reveal that turnover frequencies   do not correlate with substrate acid pKa  values but are highly dependent on the acid structure, with this   effect being related to substrate size. Addition of water is   shown to dramatically increase catalytic rates for all catalysts.   With   [Ni(PPh2NC6H4CH2P(O)(OEt)22)2](BF4)2  using [(DMF)H]+OTf as the acid and with   added water, a turnover frequency of 1850 s−1 was   obtained.

      Address (URL): http://http://pubs.acs.org/doi/abs/10.1021/ja109755f