TY - JOUR
T1 - [Ni(PPh 2NC6H4X 2) 2]2+ Complexes as electrocatalysts for H2 production
T2 - Effect of substituents, acids, and water on catalytic rates
AU - Kilgore, Uriah J.
AU - Roberts, John A.S.
AU - Pool, Douglas H.
AU - Appel, Aaron M.
AU - Stewart, Michael P.
AU - Dubois, M. Rakowski
AU - Dougherty, William G.
AU - Kassel, W. Scott
AU - Bullock, R. Morris
AU - Dubois, Daniel L.
PY - 2011/4/20
Y1 - 2011/4/20
N2 - A series of mononuclear nickel(II) bis(diphosphine) complexes [Ni(P Ph2NC6H4X2)2](BF 4)2 (PPh2NC6H4X 2 = 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(PPh2NC6H4Me 2)2](BF4)2 and [Ni(P Ph2NC6H4OMe2)2](BF 4)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.
AB - A series of mononuclear nickel(II) bis(diphosphine) complexes [Ni(P Ph2NC6H4X2)2](BF 4)2 (PPh2NC6H4X 2 = 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(PPh2NC6H4Me 2)2](BF4)2 and [Ni(P Ph2NC6H4OMe2)2](BF 4)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.
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U2 - 10.1021/ja109755f
DO - 10.1021/ja109755f
M3 - Article
C2 - 21438562
AN - SCOPUS:79954566267
VL - 133
SP - 5861
EP - 5872
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
SN - 0002-7863
IS - 15
ER -