Active-site models for the nickel-iron hydrogenases: Effects of ligands on reactivity and catalytic properties

Maria E. Carroll, Bryan E. Barton, Danielle L. Gray, Amanda E. MacK, Thomas B. Rauchfuss

Research output: Contribution to journalArticlepeer-review

Abstract

Described are new derivatives of the type [HNiFe(SR) 2(diphosphine)(CO) 3] +, which feature a Ni(diphosphine) group linked to a Fe(CO) 3 group by two bridging thiolate ligands. Previous work had described [HNiFe(pdt)(dppe)(CO) 3] + ([1H] +) and its activity as a catalyst for the reduction of protons (J. Am. Chem. Soc.2010, 132, 14877). Work described in this paper focuses on the effects on properties of NiFe model complexes of the diphosphine attached to nickel as well as the dithiolate bridge, 1,3-propanedithiolate (pdt) vs 1,2-ethanedithiolate (edt). A new synthetic route to these Ni-Fe dithiolates is described, involving reaction of Ni(SR) 2(diphosphine) with FeI 2(CO) 4 followed by in situ reduction with cobaltocene. Evidence is presented that this route proceeds via a metastable μ-iodo derivative. Attempted isolation of such species led to the crystallization of NiFe(Me 2pdt)(dppe)I 2, which features tetrahedral Fe(II) and square planar Ni(II) centers (H 2Me 2pdt = 2,2-dimethylpropanedithiol). The new tricarbonyls prepared in this work are NiFe(pdt)(dcpe)(CO) 3 (2, dcpe = 1,2-bis(dicyclohexylphosphino)ethane), NiFe(edt)(dppe)(CO) 3 (3), and NiFe(edt)(dcpe)(CO) 3 (4). Attempted preparation of a phenylthiolate-bridged complex via the FeI 2(CO) 4 + Ni(SPh) 2(dppe) route gave the tetrametallic species [(CO) 2Fe(SPh) 2Ni(CO)] 2(μ-dppe) 2. Crystallographic analysis of the edt-dcpe compund [2H]BF 4 and the edt-dppe compound [3H]BF 4 verified their close resemblance. Each features pseudo-octahedral Fe and square pyramidal Ni centers. Starting from [3H]BF 4 we prepared the PPh 3 derivative [HNiFe(edt)(dppe)(PPh 3)(CO) 2]BF 4 ([5H]BF 4), which was obtained as a ∼2:1 mixture of unsymmetrical and symmetrical isomers. Acid-base measurements indicate that changing from Ni(dppe) (dppe = Ph 2PCH 2CH 2PPh 2) to Ni(dcpe) decreases the acidity of the cationic hydride complexes by 2.5 pK a PhCN units, from ∼11 to ∼13.5 (previous work showed that substitution at Fe leads to more dramatic effects). The redox potentials are more strongly affected by the change from dppe to dcpe, for example the [2] 0/+ couple occurs at E 1/2 = -820 for [2] 0/+ vs -574 mV (vs Fc +/0) for [1] 0/+. Changes in the dithiolate do not affect the acidity or the reduction potentials of the hydrides. The acid-independent rate of reduction of CH 2ClCO 2H by [2H] + is about 50 s -1 (25 °C), twice that of [1H] +. The edt-dppe complex [2H] + proved to be the most active catalyst, with an acid-independent rate of 300 s -1.

Original languageEnglish (US)
Pages (from-to)9554-9563
Number of pages10
JournalInorganic Chemistry
Volume50
Issue number19
DOIs
StatePublished - Oct 3 2011

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

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