Pendant Bases as Proton Relays in Iron Hydride and Dihydrogen Complexes

Abstract

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.

Pendant bases as proton transfer relays in diiron dithiolate complexes inspired by [Fe–Fe] hydrogenase active site

“…An understanding of proton transfer precess will be necessary for developing efficient hydrogen production/hydrogen oxidation catalysts. Recently, DuBois and co-workers reported that trans Fe(II) complexes containing the diphosphine ligand with a pendant nitrogen base as the potential model of the iron-iron hydrogenase enzymes can be successively protonated in two steps using increasingly strong acids and a nitrogen atom in a six-membered chelate ring can promote very rapid intra-and intermolecular proton/hydride exchange in octahedral Fe(II) PNP complexes and function as a proton relay for oxidized Fe(III) hydrides as well [24]. Incorporating good donor ligands (cyanide, tertiary phosphines and isonitrile) to the propanedithiolato-bridged dinuclear complex [(l-pdt)Fe 2 (CO) 6 ] (1) that bears remarkable structural similarities with the active site of Fe-Fe hydrogenase renders the iron atoms more electron-rich and more protophilic [17,[25][26][27][28].…”

Coordination Studies of a New Nonsymmetric Ditertiary Phosphane Bearing a Single Phosphaadamantane Cage

“…We, and others, have found that ditertiary phosphanes such as those of general formula (R 2 PCH 2 ) 2 N(R) have interesting coordination properties [7,8] including their use in selfassembly reactions through covalent bonds or H-bonding contacts. [2][3][4][5]9] The tetraphenyl-substituted phosphane (Ph 2 PCH 2 ) 2 N(R), closely related to the three-carbon spacer (Ph 2 PCH 2 ) 2 CH 2 [1,3-bis(diphenylphosphanyl)propane, dppp], has been widely utilised in several metal-catalysed reactions.…”

Mononuclear and Heterodinuclear Metal Complexes of Nonsymmetric Ditertiary Phosphanes Derived from R2PCH2OH

“…In contrast, bridge cleavage of the dimers {RuCl 2 (η 6 -p-cym)} 2 or {IrCl 2 (η 5 -Cp*)} 2 with 3a gave the κ 1 -P-monodentate complexes RuCl 2 (η 6 -p-cym)(3a) (6) and IrCl 2 (η 5 -Cp*)(3a) (7), respectively, in which the -PAd group is noncoordinating. Reaction of 6 or 7 with AuCl(tht) (tht = tetrahydrothiophene) gave the mixed-metal complexes κ 2 -P,PЈ-µ-RuCl 2 (η 6 -pcym){Ph 2 PCH 2 N(Ph)CH 2 PAd(AuCl)} (8) and κ 2 -P,PЈ-µ-IrCl 2 (η 5 -Cp*){Ph 2 PCH 2 N(Ph)CH 2 PAd(AuCl)} (9). All new compounds have been fully characterised by spectroscopic and analytical methods.…”