Synthesis, Structures and Chemical Reactivity of Dithiolato-Bridged Ni-Fe Complexes as Biomimetics for the Active Site of [NiFe]-Hydrogenases

Abstract

To develop the structural and functional modeling chemistry of [NiFe]-H2ases, we have carried out a study regarding the synthesis, structural characterization and reactivity of a new series of [NiFe]-H2ase model complexes. Thus, treatment of diphosphine dppb-chelated Ni complex (dppb)NiCl2 (dppb = 1,2-(Ph2P)2C6H4) with (dppv)Fe(CO)2(pdt) (dppv = 1,2-(Ph2P)2C2H2, pdt = 1,3-propanedithiolate) and NaBF4 gave dicarbonyl complex [(dppb)Ni(pdt)Fe(CO)2(dppv)](BF4)2 ([A](BF4)2). Further treatment of [A](BF4)2 with Me3NO and Bu4NCN or KSCN afforded t-cyanido and t-isothiocyanato complexes [(dppb)Ni(pdt)Fe(CO)(t-R)(dppv)]BF4 ([1]BF4, R = CN; [2]BF4, R = NCS), respectively. While azadiphosphine MeN(CH2PPh2)2-chelated t-hydride complex [MeN(CH2PPh2)2Ni(pdt)Fe(CO)(t-H)(dppv)]BF4 ([3]BF4) was prepared by treatment of dicarbonyl complex [MeN(CH2PPh2)2Ni(pdt)Fe(CO)2(dppv)](BF4)2 ([B](BF4)2) with Me3NO and 1.5 MPa of H2, treatment of dicarbonyl complex [B](BF4)2 with Me3NO (without H2) in pyridine resulted in formation of a novel monocarbonyl complex [MeN(CH2PPh2)2Ni(SCHCH2CH2S)Fe(CO)(dppv)]BF4 ([4]BF4) via the unexpected sp3 C-H bond activation reaction. Furthermore, azadiphosphine PhN(CH2PPh2)2-chelated µ-mercapto complex [PhN(CH2PPh2)2Ni(pdt)Fe(CO)(µ-SH)(dppv)]BF4 ([5]BF4) was prepared by treatment of dicarbonyl complex [PhN(CH2PPh2)2Ni(pdt)Fe(CO)2(dppv)](BF4)2 ([C](BF4)2) with Me3NO and H2S gas, whereas treatment of azadiphosphine Ph2CHN(CH2PPh2)2-chelated dicarbonyl complex [Ph2CHN(CH2PPh2)2Ni(pdt)Fe(CO)2(dppe)](BF4)2 ([D](BF4)2, dppe = 1,2-(Ph2P)2C2H4) with Me3NO⋅2H2O gave rise to µ-hydroxo complex [Ph2CHN(CH2PPh2)2Ni(pdt)Fe(CO)(µ-OH)(dppe)]BF4 ([6]BF4). All the possible pathways for formation of the new model complexes are briefly discussed, and their structures were fully characterized by various spectroscopic techniques and for six of them by X-ray crystallography.

Functionalized nickel(ii)–iron(ii) dithiolates as biomimetic models of [NiFe]-H2ases

“…Indeed, this molecule contains one sp 3 C–Fe bond, namely the C29–Fe1 bond formed by the interaction of one sp 3 C–H bond connected directly to the S atom in the pdt ligand with the pyridine N atom and the positively-charged Fe atom. Just like the previously crystallographically characterized analogue [ A ]BF 4 , 34 the Ni center of [ 3 ]BF 4 adopts a distorted square-planar geometry, whereas the Fe center adopts a pseudo-octahedral geometry. The sp 3 C29–Fe1 bond length (2.047 Å) and Ni1⋯Fe1 distance (2.8019 Å) in [ 3 ]BF 4 are very close to the corresponding sp 3 C–Fe bond length (2.067 Å) and Ni⋯Fe distance (2.9751 Å) in its MeN(CH 2 PPh 2 )-chelated [ A ]BF 4 analogue, 34 respectively.…”

“…To show the generality of the synthetic method for the preparation of the previously reported monocarbonyl/sp 3 C–Fe bond-containing complex [MeN(CH 2 PPh 2 ) 2 Ni(SCH 2 CH 2 CHS)Fe(CO)(dppv)]BF 4 ([ A ]BF 4 ) 34 from the dicarbonyl complex [MeN(CH 2 PPh 2 ) 2 Ni(pdt)Fe(CO) 2 (dppv)](BF 4 ) 2 ([ B ](BF 4 ) 2 ), 32 we carried out the reaction of the dicarbonyl complex [ 1 ](PF 6 ) 2 or [(dppe)Ni(pdt)Fe(CO) 2 (dppv)](BF 4 ) 2 ([ C ](BF 4 ) 2 ) 32 with an equimolar decarbonylation agent Me 3 NO in pyridine at room temperature; as a result, the expected [ 2 ]PF 6 and [ 3 ]BF 4 were produced in 88% and 78% yields, respectively (Scheme 3).…”

“…35 The third step results in the formation of the sp 3 C–Fe bond-containing complexes [ 2 ]PF 6 and [ 3 ]BF 4 by the loss of one molecule of the pyridine salt [C 5 H 5 NH]X from the transition state. 34…”

“…35 The third step results in the formation of the sp 3 C-Fe bond-containing complexes [2]PF 6 and [3]BF 4 by the loss of one molecule of the pyridine salt [C 5 H 5 NH]X from the transition state. 34 Interestingly, the suggested pathway shown in Scheme 3 could be supported by observing the changes of the terminal CO absorption bands of the starting material [1](PF 6 ) 2 or [C](BF 4 ) 2 and product [2]PF 6 or [3]BF 4 determined in situ during the reaction of [1](PF 6 ) 2 or [C](BF 4 ) 2 with Me 3 NO in pyridine (see Fig. 2 and Fig.…”

“…As shown in Fig. 3, complex [3] 34 respectively. Electrochemical properties and electrocatalytic H 2 -producing ability of [2]PF 6 .…”