Protophilicity, electrochemical property, and desulfurization of diiron dithiolate complexes containing a functionalized C 2 bridge with two vicinal basic sites

Abstract

Two diiron dithiolate complexes [{μ-SC(NBn)CH(NHBn)S-μ}Fe 2(CO) 5L] (L = PPh 3, 2; P(Pyr) 3, 3) containing a functionalized C 2 bridge with two vicinal basic sites were prepared and characterized as models of the FeFe-hydrogenase active site. The molecular structures of 2 and its N-protonated form [( 2H N)(OTf)] were determined by X-ray analyses of single crystals. In the solid state of [( 2H N)(OTf)]. Each asymmetric unit contains a molecule of [( 2H N)(OTf)] and a molecule of water. The molecule of water is close to the iron atom of the [Fe(CO) 3] unit (Fe···O(H 2O), 4.199 Å). The complexes 2 and 3 are relatively protophilic. 31P NMR spectra and cyclic voltammograms show that they can be protonated by the mild acids CCl 3COOH and CF 3COOH. Electrochemical studies show that the first reduction peak of 3 at −1.51 V versus Fc +/Fc is 110 mV more positive than that (−1.62 V) found for the analogous diiron azadithiolate complex [{(μ-SCH 2) 2N(CH 2C 6H 5)}Fe 2(CO) 5P(Pyr) 3] ( 7). Protonation of 2 and 3 leads to the anodic shifts of 610–650 mV for the Fe IFe I/Fe IFe 0 reduction potentials. The shifts are apparently larger than that (450 mV) for protonation of 7. The reaction of the all-carbonyl complex [{μ-SC(NBn)CH(NHBn)S-μ}Fe 2(CO) 5L] with two equivalents of bis(diphenylphosphino)methane (dppm) in refluxing toluene affords a desulfurized complex [(μ-S)(μ-dppm) 2Fe 2(CO) 4] ( 6). The reaction process was studied. A dppm mono-dentate intermediate [{μ-SC(NBn)CH(NHBn)S-μ}Fe 2(CO) 5(κ 1-dppm)] ( 4) and a dppm μ-bridging species [{μ-SC(NBn)CH(NHBn)S-μ}Fe 2(CO) 4(μ-dppm)] ( 5) have been isolated and spectroscopically characterized.