Synthesis and Characterization of Diiron Diselenolato Complexes Including Iron Hydrogenase Models

Abstract

Diiron diselenolato complexes have been prepared as models of the active site of [FeFe]-hydrogenases. Treatment of Fe 3 (CO) 12 with 1 equiv of 1,3-diselenocyanatopropane (1) in THF at reflux afforded the model compound Fe 2 (µ-Se 2 C 3 H 6 )(CO) 6 (2) in 68% yield. The analogous methyl-substituted complex, Fe 2 (µ-Se 2 C 3 H 5 CH 3 )(CO) 6 (3), was obtained from the reaction of Fe 3 (CO) 12 with the in situ generated compound 3-methyl-1,2-diselenolane (4). In contrast, the reaction of Fe 3 (CO) 12 with 1,3,5-triselenacyclohexane (5) produced a mixture of Fe 2 (µ 2 ,κ-Se,C-SeCH 2 SeCH 2 )(CO) 6 (6), Fe 2 [(µ-SeCH 2 ) 2 Se](CO) 6 (7), and Fe 2 (µ-Se 2 CH 2 )(CO) 6 (8). Compounds 2, 3, 6, and 7 were characterized by IR, 1 H, 13 C, and 77 Se NMR spectroscopy, mass spectrometry, elemental analysis, and X-ray single-crystal structure analysis. The He I and He II photoelectron spectra for 3 are reported, and the electronic structure is further analyzed with the aid of DFT computations. The calculated reorganization energy of the cation of 3 to the "rotated" structure, which has a semibridging carbonyl ligand, is less than that of the analogous complexes with sulfur instead of selenium. Complexes 2 and 3 have been proved to be catalysts for electrochemical reduction of protons from the weak acids pivalic and acetic acid, respectively, to give hydrogen.

Diiron Dichalcogenolato (Se and Te) Complexes: Models for the Active Site of [FeFe] Hydrogenase

“…The computational methodology utilized by Weigand and co-workers to investigate these catalysts (12, 13, 23-27) has previously been shown to give good quantitative agreement with the structures, CO stretching frequencies (νCO), oxidation and reduction potentials, bond energies, and pK a values of a variety of catalysts inspired by [FeFe] hydrogenase active sites. [56,59,60,[101][102][103] The optimized geometries and computed νCO values agree very well with the experimental values, suggesting that the computations are able to yield sufficiently reasonable electron distributions and energies for catalysts 12, 14, and 23-27.…”

“…A lengthening of the Fe-Fe bond is notable for all these complexes. For example, whereas the Fe-Fe bond length is 2.5610(8) Å in 13, [56] compound 12 displays a distance of 2.5103(11) Å. [97] As already described for complexes 6-8 and 10, no distinct deviation of the peak potentials can be observed for 13 and 14, relative to those of their sulfur-containing analogues.…”

“…Following these initial experiments, more selenium-containing complexes based on the exhaustively investigated sulfur complex, [Fe 2 (μ-SC 3 H 6 S)(CO) 6 ] (PDT) (12), were synthesized to obtain more detailed information on [2Fe2Se] cluster compounds. [Fe 2 (μ-SeC 3 H 6 Se)(CO) 6 ] (PDSe) (13), [56,57] [Fe 2 (μ-Se 2 C 3 H 5 CH 3 )(CO) 6 ] (14), and [Fe 2 {(μ-SeCH 2 ) 2 Se}(CO) 6 ] (15) (Scheme 6) were prepared subsequently. A lengthening of the Fe-Fe bond is notable for all these complexes.…”

“…[58] A significant shift of both the oxidation [+0. 35 V (18) [58] Ͻ +0.76 V (14) [56] ] and reduction potential [-2.00 V (18) [58] Ͻ -1.83 V (14) [56] ] was observed. This trend is not unexpected, and similar shifts were already observed for sulfur-containing compounds.…”

“…[56,[58][59][60] These active site catalysts typically exhibit extensive overlap of six Fe 3d orbitals and range up to about 9 eV with a weak shoulder on the low ionization energy side corresponding to ionization from the HOMO. The HOMO is calculated to be predominantly the Fe-Fe σ bond (vide infra), with an adiabatic ionization (IE A ) energy around 7.5 eV.…”