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Molecular Physics
An International Journal at the Interface Between Chemistry and Physics
Volume 111, 2013 - Issue 18-19: Modern EPR Spectroscopy
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Invited Articles

Back-clocking of Fe2+/Fe1+ spin states in a H2-producing catalyst by advanced EPR

, , , &
Pages 2942-2949 | Received 10 Mar 2013, Accepted 15 Apr 2013, Published online: 22 May 2013
 

Abstract

A mononuclear Fe-(P(PPh2)3) ((P(PPh2)3) = tris[2-diphenylphospino)ethyl]phosphine) catalyst was studied in situ under catalytic conditions using advanced electron paramagnetic resonance (EPR) techniques. Fe-(P(PPh2)3) efficiently catalyses H2 production using HCOOH as substrate. Dual-mode continuous-wave (CW) EPR, used to study the initial Fe2+(S = 2) state, shows that the complex is characterised by a – rather small – zero field splitting parameter Δ = 0.45 cm−1 and geff = 8.0. In the presence of HCOOH substrate the complex evolves and a unique Fe1+(S = 1/2) state is trapped. The Fe1+ atom is coordinated by four 31P nuclei in a pseudo-C3 symmetry. Only a small fraction of the Fe1+ spin density is delocalised onto the 31P atoms. Four-pulse electron spin echo envelope modulation (ESEEM) and two-dimensional hyperfine sublevel correlation spectroscopy (2D-HYSCORE) data reveal the existence of two types of 1H couplings. One corresponds to weak, purely dipolar coupling, tentatively assigned to phenyl protons. The most important is a – rather unusual – 1H coupling with negative Aiso (−2.75 MHz) and strong dipolar part (T = 5.5 MHz). This 1H is located on the pseudo-C3 symmetry axis of the Fe1+-(P(PPh2)3-HCOO complex where one substrate molecule, formate anion, is coordinated on the Fe1+ atom.

Acknowledgements

This research has been co-financed by the European Union (European Social Fund – ESF) and Greek National Funds through the operational programme ‘Education and Lifelong Learning’ of the National Strategic Reference Framework (NSRF) – Research Funding Program: THALIS. Investing in knowledge society through the European Social Fund.

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