Pendant bases as proton relays in iron hydride and dihydrogen complexes.
The complex trans-[HFe(PNP)(dmpm)(CH(3)CN)]BPh(4), 3, (where PNP is Et(2)PCH(2)N(CH(3))CH(2)PEt(2) and dmpm is Me(2)PCH(2)PMe(2)) can be successively protonated in two steps using increasingly strong acids. Protonation with 1 equiv of p-cyanoanilinium tetrafluoroborate in acetone-d(6) at -80 degrees C results in ligand protonation and the formation of endo (4a) and exo (4b) isomers of trans-[HFe(PNHP)(dmpm)(CH(3)CN)](BPh(4))(2). The endo isomer undergoes rapid intramolecular proton/hydride exchange with an activation barrier of 12 kcal/mol. The exo isomer does not exchange. Studies of the reaction of 3 with a weaker acid (anisidinium tetrafluoroborate) in acetonitrile indicate that a rapid intermolecular proton exchange interconverts isomers 4a and 4b, and a pK(a) value of 12 was determined for these two isomers. Protonation of 3 with 2 equiv of triflic acid results in the protonation of both the PNP ligand and the metal hydride to form the dihydrogen complex [(H(2))Fe(PNHP)(dmpm)(CH(3)CN)](3+), 11. Studies of related complexes [HFe(PNP)(dmpm)(CO)](+) (12) and [HFe(depp)(dmpm)(CH(3)CN)](+) (10) (where depp is bis(diethylphosphino)propane) confirm the important roles of the pendant base and the ligand trans to the hydride ligand in the rapid intra- and intermolecular hydride/proton exchange reactions observed for 4. Features required for an effective proton relay and their potential relevance to the iron-only hydrogenase enzymes are discussed.
Henry
Renee M
RM
Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA.
Shoemaker
Richard K
RK
DuBois
Daniel L
DL
DuBois
M Rakowski
MR
D000143:Acids / Q000737:chemistry; D004563:Electrochemistry; D005290:Ferric Compounds / Q000737:chemistry*; D005296:Ferrous Compounds / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry*; D006864:Hydrogenase / Q000737:chemistry* / Q000378:metabolism; D009682:Magnetic Resonance Spectroscopy; D011522:Protons; D013816:Thermodynamics
D000143:Acids; D005290:Ferric Compounds; D005296:Ferrous Compounds; D011522:Protons; D006859:Hydrogen; D006864:Hydrogenase