The reaction of NiBr2(EtOH)4 with a 1:2-3 mixture of FeBr2(CO)4 and Na(SPh) generated a linear trinuclear Fe-Ni-Fe cluster (CO) 3Fe(-SPh) 3Ni(-SPh)3Fe(CO)3, 1, whereas the analogous reaction system FeBr 2(CO)4/Na(S t Bu)/NiBr2(EtOH)4 (1:2-3:1) gave rise to a linear tetranuclear Fe-Ni-Ni-Fe cluster [(CO) 3Fe(-S t Bu)3Ni(-Br)]2, 2. By using this tetranuclear cluster 2 as the precursor, we have developed a new synthetic route to a series of thiolate-bridged dinuclear Fe(CO) 3-Ni complexes, the structures of which mimic [NiFe] hydrogenase active sites. The reactions of 2 with SC(NMe 2)2 (tmtu), Na{S(CH2)2SMe} and ortho-NaS(C 6H4)SR (R ‫؍‬ Me, t Bu) led to isolation of (CO)3Fe(-S t Bu)3NiBr(tmtu), 3, (CO)3Fe(S t Bu)(-S t Bu)2Ni{S(CH2)2SMe}, 4, and (CO) 3Fe(S t Bu)(-S t Bu)2Ni{S(C6H4)SR}, 5a (R ‫؍‬ Me) and 5b (R ‫؍‬ t Bu), respectively. On the other hand, treatment of 2 with 2-methylthiophenolate (ortho-O(C 6H4)SMe) in methanol resulted in (CO)3Fe(-S t Bu)3Ni(MeOH){O(C6H4)SMe}, 6a. The methanol molecule bound to Ni is labile and is readily released under reduced pressure to afford (CO) 3Fe(S t Bu)(-S t Bu)2Ni{O(C6H4)SMe}, 6b, and the coordination geometry of nickel changes from octahedral to square planar. Likewise, the reaction of 2 with NaOAc in methanol followed by crystallization from THF gave (CO) 3Fe(-S t Bu)3Ni(THF)(OAc), 7. The dinuclear complexes, 3-7, are thermally unstable, and a key to their successful isolation is to carry out the reactions and manipulations at ؊40°C.

A [NiFe] hydrogenase model compound having a distorted trigonal-pyramidal nickel center, ðCOÞ 3 Feðμ-S t BuÞ 3 NiðSDmpÞ, 1 (Dmp¼ C 6 H 3 -2;6-ðmesitylÞ 2 ), was synthesized from the reaction of the tetranuclear Fe-Ni-Ni-Fe complex ½ðCOÞ 3 Feðμ-S t BuÞ 3 Ni 2 ðμ-BrÞ 2 , 2 with NaSDmp at −40°C. The nickel site of complex 1 was found to add CO or CN t Bu at −40°C to give ðCOÞ 3 FeðS t BuÞðμ-S t BuÞ 2 NiðCOÞ ðSDmpÞ, 3, or ðCOÞ 3 FeðS t BuÞðμ-S t BuÞ 2 NiðCN t BuÞðSDmpÞ, 4, respectively. One of the CO bands of 3, appearing at 2055cm −1 in the infrared spectrum, was assigned as the Ni-CO band, and this frequency is comparable to those observed for the CO-inhibited forms of [NiFe] hydrogenase. Like the CO-inhibited forms of [NiFe] hydrogenase, the coordination of CO at the nickel site of 1 is reversible, while the CN t Bu adduct 4 is more robust.iron | nickel | thiolates B iological hydrogen evolution and uptake are mediated by hydrogenase enzymes (1-7). The most prevalent family are the [NiFe] hydrogenases, and various forms have been identified (8)(9)(10)(11)(12)(13)(14)(15). A unique feature of the [NiFe] hydrogenase is the common organometallic dinuclear Ni-Fe frame at the active site ( Fig. 1), which has attracted inorganic and organometallic chemists attempting to model both the structure and physicochemical properties. Thiolate-bridged Ni-Fe complexes have been synthesized as structural models of the active site (16-26), and sulfurligated mono-and dinuclear transition metal complexes have been reported to promote H 2 activation mimicking the function of [NiFe] hydrogenase (27-41). However, synthesis of better structural/functional models remains challenging.One interesting aspect of the [NiFe] hydrogenase is the reversible inhibition of the active site by CO. According to a crystallographic analysis of the CO-inhibited form of the [NiFe] hydrogenase obtained from Desulfovibrio vulgaris Miyazaki F (D. v. Miyazaki F), a CO molecule is coordinated at the nickel center (42). It has been suggested that this nickel-bound CO can be liberated and the catalytic activity recovered, upon flushing with a stream of N 2 (43), or by white-light irradiation at 20 K (44). Herein we report the synthesis of a thiolate-bridged dinuclear Ni-Fe complex ðCOÞ 3 Feðμ-S t BuÞ 3 NiðSDmpÞ, 1, carrying a bulky thiolate SDmp (Dmp¼C 6 H 3 -2;6-ðmesitylÞ 2 ) at Ni, and its CO and CN t Bu adducts, ðCOÞ 3 FeðS t BuÞðμ-S t BuÞ 2 NiðCOÞðSDmpÞ, 3, and ðCOÞ 3 FeðS t BuÞðμ-S t BuÞ 2 NiðCN t BuÞðSDmpÞ, 4. The reaction of 1 with CO occurs reversibly to give 3, while the analogous CN t Bu adduct 4 is robust. We also report the x-ray crystal structures of 1, 3, and 4. Results and DiscussionSynthesis and Structure of ðCOÞ 3 Feðμ-S t BuÞ 3 NiðSDmpÞ, 1. We have reported that a linear tetranuclear Fe-Ni-Ni-Fe complex ½ðCOÞ 3 Feðμ-S t BuÞ 3 Ni 2 ðμ-BrÞ 2 , 2, which was synthesized from FeBr 2 ðCOÞ 4 þNaðS t BuÞþNiBr 2 ðEtOHÞ 4 (1∶2∼3∶1), serves as a convenient precursor for the synthesis of thiolate-bridged dinuclear Ni-Fe complexes (17). The penta-coo...