A model for the CO-inhibited form of [NiFe] hydrogenase: synthesis of CO3Fe(micro-StBu)3Ni{SC6H3-2,6-(mesityl)2} and reversible CO addition at the Ni site.

A [NiFe] hydrogenase model compound having a distorted trigonal-pyramidal nickel center, (CO)(3)Fe(micro-S(t)Bu)(3)Ni(SDmp), 1 (Dmp = C(6)H(3)-2,6-(mesityl)(2)), was synthesized from the reaction of the tetranuclear Fe-Ni-Ni-Fe complex [(CO)(3)Fe(micro-S(t)Bu)(3)Ni](2)(micro-Br)(2), 2 with NaSDmp at -40 degrees C. The nickel site of complex 1 was found to add CO or CN(t)Bu at -40 degrees C to give (CO)(3)Fe(S(t)Bu)(micro-S(t)Bu)(2)Ni(CO)(SDmp), 3, or (CO)(3)Fe(S(t)Bu)(micro-S(t)Bu)(2)Ni(CN(t)Bu)(SDmp), 4, respectively. One of the CO bands of 3, appearing at 2055 cm(-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.

Thiolate-bridged dinuclear iron(tris-carbonyl)-nickel complexes relevant to the active site of [NiFe] hydrogenase.

The reaction of NiBr(2)(EtOH)(4) with a 1:2-3 mixture of FeBr(2)(CO)(4) and Na(SPh) generated a linear trinuclear Fe-Ni-Fe cluster (CO)(3)Fe(mu-SPh)(3)Ni(mu-SPh)(3)Fe(CO)(3), 1, whereas the analogous reaction system FeBr(2)(CO)(4)/Na(S(t)Bu)/NiBr(2)(EtOH)(4) (1:2-3:1) gave rise to a linear tetranuclear Fe-Ni-Ni-Fe cluster [(CO)(3)Fe(mu-S(t)Bu)(3)Ni(mu-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(CH(2))(2)SMe} and ortho-NaS(C(6)H(4))SR (R = Me, (t)Bu) led to isolation of (CO)(3)Fe(mu-S(t)Bu)(3)NiBr(tmtu), 3, (CO)(3)Fe(S(t)Bu)(mu-S(t)Bu)(2)Ni{S(CH(2))(2)SMe}, 4, and (CO)(3)Fe(S(t)Bu)(mu-S(t)Bu)(2)Ni{S(C(6)H(4))SR}, 5a (R = Me) and 5b (R = (t)Bu), respectively. On the other hand, treatment of 2 with 2-methylthio-phenolate (ortho-O(C(6)H(4))SMe) in methanol resulted in (CO)(3)Fe(mu-S(t)Bu)(3)Ni(MeOH){O(C(6)H(4))SMe}, 6a. The methanol molecule bound to Ni is labile and is readily released under reduced pressure to afford (CO)(3)Fe(S(t)Bu)(mu-S(t)Bu)(2)Ni{O(C(6)H(4))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)(3)Fe(mu-S(t)Bu)(3)Ni(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 degrees C.