Abstract/Summary

We report the synthesis and molecular structures of [FeFe]-ase biomimics [Fe 2 (CO) 4 {μ-S(CH 2 ) n S}{ κ2 - (Ph 2 PCH 2 ) 2 NR}] ( 1–4 ) ( n = 2, 3; R = Me, Bn) and a comparative study of their protonation and re- dox chemistry, with the aim of assessing their activity as catalysts for H 2 oxidation. They are prepared in good yields upon heating the hexacarbonyls and PCNCP ligands in toluene, a minor product of one reaction ( n = 3, R = Bn) being pentacarbonyl [Fe 2 (CO) 5 (μ-pdt){Ph 2 PCH 2 N(H)Bn}] ( 5 ). Crystal structures show short Fe-Fe bonds (ca. 2.54 ˚A) with the diphosphine occupying basal-apical sites. Each undergoes a quasi-reversible one-electron oxidation and IR-SEC shows that this results in formation of a semi-bridging carbonyl. As has previously been observed, protonation products are solvent dependent, nitrogen being the favoured site of protonation site upon addition of one equivalent of HBF 4 .Et 2 O in d 6 -acetone, while hydride formation is favoured in CD 2 Cl 2 . However, the rate of N to Fe 2 proton-transfer varies greatly with the nature of both the dithiolate-bridge and amine-substituent. Thus with NMe complexes ( 1–2 ) N- protonation is favoured in acetone affording a mixture of endo and exo isomers, while for NBn complexes ( 3–4 ) proton-transfer to afford the corresponding μ-hydride occurs in part (for 3 edt) or exclusively (for 4 pdt). In acetone, addition of a further equivalent of HBF 4 .Et 2 O generally does not lead to hydride for- mation, but in CD 2 Cl 2 dications [Fe 2 (CO) 4 {μ-S(CH 2 ) n S}( μ-H){ κ2 -(Ph 2 PCH 2 ) 2 NHR}] 2 + result, in which the diphosphine can adopt either dibasal or basal-apical positions. Proton-transfer from Fe 2 to N has been previously identified as a required transformation for H 2 oxidation, as has the accessibility of the all- terminal carbonyl isomer of cations [Fe 2 (CO) 4 {μ-S(CH 2 ) n S}{ κ2 -(Ph 2 PCH 2 ) 2 NR}] + . We have carried out a preliminary H 2 oxidation study of 3, oxidation by Fc[BF 4 ] in the presence of excess P(o-tolyl) 3 affording [HP(o-tol) 3 ][BF 4 ], with a turnover of ca. 2.3 ±0.1 mol of H 2 consumed per mole of 3.