acetylthioester formation ͉ dinuclear nickel-site model ͉ N2S2 ligand ͉ nickel acyl complex A cetyl-CoA synthase/CO dehydrogenase (ACS/CODH), a bifunctional metalloenzyme found in acetogenic, methanogenic, and sulfate-reducing bacteria, has the important function of fixing CO and CO 2 in the global carbon cycle (1-3). Whereas CODH catalyzes the reversible conversion of CO 2 to CO, ACS assembles acetyl-CoA from CO, CoA, and a methyl moiety that is derived from the methylcobalamin of the corrinoid iron-sulfur protein (CFeSP). Recently, the crystal structures of the ACS/ CODH system from the bacteria Moorella thermoacetica and Carboxydothermus hydrogenoformans have been elucidated (4-6); the ACS active site denoted as the A-cluster contains a [Fe 4 S 4 ] cubane cluster and a Ni d -Ni p dinuclear site as shown in Fig. 1, where the 2 nickels designated as Ni d and Ni p occupy distal and proximal positions, respectively, to the [Fe 4 S 4 ] cluster. The geometry around Ni d is square planar, composed of 2 cysteine sulfurs and 2 carboxyamide nitrogens of the tripeptide Cys-GlyCys from the protein backbone. Ni p carries an unidentified ligand X and 3 bridging cysteine sulfurs, 2 from the aforementioned tripeptide, and 1 from the [Fe 4 S 4 ] cluster.Biological investigations suggest that the oxidized state of the cluster A ox should be formulated as {Ni d 2ϩ -Ni p 2ϩ -[Fe 4 S 4 ] 2ϩ } (7-9). However, the electronic configuration of the active reduced state, probably reduced by 2 electrons from A ox , which is proposed to be {Ni d 2ϩ -Ni p 1ϩ -[Fe 4 S 4 ] 1ϩ }, has not yet been firmly established (10-11). The order of the addition of the substrates, Me, CO, and CoA, to the active site is also controversial, and several mechanisms have been proposed on the basis of biological and computational studies (12)(13)(14). On the other hand, synthetic studies that probe the reaction mechanisms are limited in number, although various thiolato-bridged dinuclear nickel complexes have been reported as A-cluster models (15-23).Proposed ACS mechanisms commonly involve the following steps: (i) methyl transfer from methylcobalamin to the reduced Ni p (0) [or Ni p (I)] site, (ii) CO insertion into the Ni p -Me bond, and (iii) formation of acetyl-CoA via the reductive elimination at the Ni p site (1-3, 7-14). These reactions had been studied by using mononuclear nickel complexes. CO insertion into the Ni-Me bonds of Ni(NS 3 R )Me [NS 3 R is N(CH 2 CH 2 SR) 3 ; R is i-Pr, t-Bu], investigated by Stavropoulos et al. (24) , resulted in the formation of acetylthioesters CH 3 C(O)SR and Ni(0) species in high yield upon treatment with CO. Likewise, the reaction of [Ni(bpy)(SR)(Me)] with Ͼ3 equiv of CO was found to yield [Ni(bpy)(CO) 2 ] and acetylthioesters (25). Before these reports, Kim et al. (26) had shown that the reaction of Ni(dppe-)(SAr)(Me) with CO generated CH 3 C(O)SAr. Recently, Rampersad et al. (27) demonstrated the acetylthioester formation in the reaction of Ni-Pd dinuclear complex, Ni(bme-daco)Pd(CO-)(COMe), with NaSMe. Eckert ...

The dinuclear Ni(II)-Ni(I) complex Ni(II)(dadt(Et))Ni(I)(SDmp)(PPh(3)) was synthesized as a Ni(II)(d)-Ni(I)(p) model of the A-cluster in acetyl CoA synthase. This complex was reacted with Co(dmgBF(2))(2)(Me)(Py) and KSDmp successively to afford Ni(dadt(Et))Ni(Me)(SDmp), which further reacts with CO to afford the acetylthioester CH(3)C(O)SDmp via reductive elimination.

Several new mono- and dinuclear eta (5)-pentamethylcyclopentadienyl (Cp*) iridium(III) complexes bearing 5-methyltetrazolate (MeCN 4 (-)) have been synthesized and their molecular and crystal structures have been determined. For complexes incorporating 2,2'-bipyridine (bpy) or 1,10-phenanthroline (phen), both mononuclear kappa N (2)-coordinated and dinuclear mu-kappa N (1):kappa N (3)-bridging MeCN 4 complexes were obtained: [Cp*Ir(bpy or phen)(MeCN 4-kappa N (2))]PF 6 ( 1 or 3) and [{Cp*Ir(bpy or phen)} 2(mu-MeCN 4-kappa N (1):kappa N (3))](PF 6) 3 ( 2 or 4), respectively. It was confirmed by X-ray analysis that the dinuclear complex in 2 has a characteristic structure with a pyramidal pocket constructed from a mu-kappa N (1):kappa N (3)-bridging MeCN 4 (-) and two bpy ligands. In the case of analogous complexes with N, N-dimethyldithiocarbamate (Me 2dtc (-)), yellow platelet crystals of mononuclear kappa N (1)-coordinated complex, [Cp*Ir(Me 2dtc)(MeCN 4-kappa N (1))].HN 4CMe ( 5.HN 4CMe), and yellow prismatic crystals of dinuclear mu-kappa N (1):kappa N (4)-bridging one, [{Cp*Ir(Me 2dtc)} 2(mu-MeCN 4-kappa N (1):kappa N (4))]PF 6 ( 6), were deposited. The kappa N (1)- and kappa N (1):kappa N (4)-bonding modes of MeCN 4 (-) in these complexes presumably arise from the compactness of the Me 2dtc (-) coligand. 6 is the first example in which tetrazolates act as a mu-kappa N (1):kappa N (4)-bridging ligand. Furthermore, the molecular and crystal structures of dinuclear complexes having mu-kappa (2) S, N:kappa S-bridging 2-pyridinethiolate (2-Spy (-)) or 8-quinolinethiolate (8-Sqn (-)) ligands have been determined: [(Cp*Ir) 2(mu-2-Spy or 8-Sqn-kappa (2) S, N:kappa S) 2] ( 7 or 8). These thiolato-bridging complexes were stable toward the addition of 5-methyltetrazole (HN 4CMe), owing to the characteristic intramolecular stacking interaction between the pyridine or the quinoline rings. The 2-Spy complex of 7, however, reacted with an excess amount of Na(N 4CMe), resulting in cleavage of the IrN(py) bond and coordination of MeCN 4 (-) in the mu-kappa N (2):kappa N (3)-bridging mode: [(Cp*Ir) 2(mu-2-Spy-kappa S:kappa S) 2(mu-MeCN 4-kappa N (2):kappa N (3))]PF 6 ( 9). This bridging mode of MeCN 4 (-) was also observed in the triply bridging MeCN 4 complex: [(Cp*Ir) 2(mu-MeCN 4-kappa N (2):kappa N (3)) 3]PF 6 ( 10). In these various MeCN 4 complexes, the structural parameters of the MeCN 4 moiety were not perturbed by the difference in the bonding modes.

The trinuclear nickel complex [{Ni(dadt(Et))}(2)Ni](NiBr(4)) (dadt(Et) = N,N'-diethyl-3,7-diazanonane-1,9-dithiolate) (1a), prepared by the reaction of Ni(dadt(Et)) and Ni(EtOH)(4)Br(2), was found to serve as a useful synthetic precursor of various dinuclear nickel complexes modeling the active site of acetyl-CoA synthase (ACS). The reactions of 1a with 4 equiv of the potassium salts of arenethiolates in ethanol produced a series of dinuclear nickel thiolate complexes Ni(dadt(Et))Ni(SAr)(2) (Ar = Ph (2a), p-Tol (2b), 2,4,6-triisopropylphenyl (Tip) (2c)) in good yields. The analogous reactions of 1a with Ag(OTf) in the presence of (t)BuNC and (NMe(2))(2)CS (tmtu) generated the dicationic dinuclear nickel complexes [Ni(dadt(Et))Ni((t)BuNC)(2)](OTf)(2) (3) and [Ni(dadt(Et))Ni(tmtu)(2)](OTf)(2) (4), respectively. The molecular structures of 1a, 2a-c, 3, and 4 determined by X-ray analysis compare well with that of A-cluster in ACS.

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