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

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 g_eff = 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 A_iso (−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.

Keywords:

• H₂
• Fe²⁺
• Fe¹⁺
• formic acid
• dual mode
• 4-pulse
• HYSCORE

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.