A singular feature of the catalytic C-cluster of carbon monoxide dehydrogenase is a sulfide-bridged Ni⋯Fe locus where substrate is bound and transformed in the reversible reaction CO + H2O ⇌ CO2 + 2H+ + 2e−. A similar structure has been sought in this work. Mononuclear planar NiII complexes [Ni(pyN2Me2)L]1− (pyN2Me2 = bis(2,6-dimethylphenyl)-2,6-pyridinedicarboxamidate(2-)) derived from a NNN pincer ligand have been prepared including L = OH− (1) and CN− (7). Complex 1 reacts with ethyl formate and CO2 to form unidentate L = HCO2− (5) and HCO3− (6) products. A binucleating macrocycle was prepared which specifically binds NiII at a NNN pincer site and five-coordinate FeII at a triamine site. The NiII macrocyle forms hydroxo (14) and cyanide complexes (15) analogous to 1 and 7. Reaction of 14 with FeCl2 alone and with ethyl formate and 15 with FeCl2 affords molecules with the NiII-L-FeII bridge unit in which L = µ2:η1-OH− (17) and µ2:η2-HCO2− (18) and -CN− (19). All bridges are non-linear (17, 140.0°; 18, M-O-C 135.9° (Ni), 120.2° (Fe); 19, Ni-C-N 170.3°, Fe-N-C 141.8°) with Ni⋯Fe separations of 3.7–4.8 Å. The NiIIFeII complexes, lacking appropriate Ni-Fe-S cluster structures, are not site analogues but their synthesis and reactivity provide the first demonstration that molecular NiII…FeII sites and bridges can be attained, a necessity in the biomimetic chemistry of C-clusters.

Carbon dioxide may react with free or metal-bound hydroxide to afford products containing bicarbonate or carbonate, often captured as ligands bridging two or three metal sites. We report the kinetics and probable mechanism of an extremely rapid fixation reaction mediated by a planar nickel complex ½Ni II ðNNNÞðOHÞ 1− containing a tridentate 2,6-pyridinedicarboxamidate pincer ligand and a terminal hydroxide ligand. The minimal generalized reaction is M-OH þ CO 2 → M-OCO 2 H; with variant M, previous rate constants are ≲10 3 M −1 s −1 in aqueous solution. For the present bimolecular reaction, the (extrapolated) rate constant is 9.5 × 10 5 M −1 s −1 in N, N′-dimethylformamide at 298 K, a value within the range of k cat ∕K M ≈10 5 -10 8 M −1 s −1 for carbonic anhydrase, the most efficient catalyst of CO 2 fixation reactions. The enthalpy profile of the fixation reaction was calculated by density functional theory. The initial event is the formation of a weak precursor complex between the Ni-OH group and CO 2 , followed by insertion of a CO 2 oxygen atom into the Ni-OH bond to generate a four center Niðη 2 -OCO 2 HÞ transition state similar to that at the zinc site in carbonic anhydrase. Thereafter, the Ni-OH bond detaches to afford the Niðη 1 -OCO 2 HÞ fragment, after which the molecule passes through a second, lower energy transition state as the bicarbonate ligand rearranges to a conformation very similar to that in the crystalline product. Theoretical values of metric parameters and activation enthalpy are in good agreement with experimental values [ΔH ‡ ¼ 3.2ð5Þ kcal∕mol].nickel hydroxide | carbon dioxide-bicarbonate conversion | reaction mechanism R ecent research in this laboratory has been directed toward the attainment of synthetic analogues of the NiFe 3 S 4 active site of the enzyme carbon monoxide dehydrogenase (1, 2), which catalyzes the interconversion reactionIn the course of this work, we have prepared binuclear Ni II ∕Fe II bridged species with the intention of simulating the Ni…Fe component of the enzyme site that is the locus of substrate binding, activation, and product release (3). The nickel site in the binuclear species has been prepared separately in the form of the planar hydroxide complex ½NiðpyN 2 Me2 ÞðOHÞ 1− containing a N,N′-2,6-dimethylphenyl-2,6-pyridinedicarboxamidate dianion. Whereas bridging hydroxide ligation is common, terminal binding is not and in general is stabilized in divalent metal complexes by hydrogen bonding or by steric shielding, as is the case here. As reported recently (3), exposure of a solution of ½NiðpyN 2 Me2 ÞðOHÞ 1− to the atmosphere results in an instantaneous color change from red to red-orange. This results in the fixation of CO 2 as the bicarbonate complex ½NiðpyN 2 Me2 ÞðHCO 3 Þ 1− accompanied by the spectral changes in Fig. 1. Several features of the CO 2 fixation reaction 1 are noteworthy. The reactant is a terminal hydroxide species affording a unidentate bicarbonate (η 1 -OCO 2 H) product. Far more common is the reaction of bridged M II 2 ðμ-OHÞ 1;2 pr...

The planar complexes [Ni(II)(pyN(2)(R2))(OH)](-), containing a terminal hydroxo group, are readily prepared from N,N'-(2,6-C(6)H(3)R(2))-2,6-pyridinedicarboxamidate(2-) tridentate pincer ligands (R(4)N)(OH), and Ni(OTf)(2). These complexes react cleanly and completely with carbon dioxide in DMF solution in a process of CO(2) fixation with formation of the bicarbonate product complexes [Ni(II)(pyN(2)(R2))(HCO(3))](-) having η(1)-OCO(2)H ligation. Fixation reactions follow second-order kinetics (rate = k(2)'[Ni(II)-OH][CO(2)]) with negative activation entropies (-17 to -28 eu). Reactions were monitored by growth and decay of metal-to-ligand charge-transfer (MLCT) bands at 350-450 nm. The rate order R = Me > macro > Et > Pr(i) > Bu(i) > Ph at 298 K (macro = macrocylic pincer ligand) reflects increasing steric hindrance at the reactive site. The inherent highly reactive nature of these complexes follows from k(2)' ≈ 10(6) M(-1) s(-1) for the R = Me system that is attenuated by only 100-fold in the R = Ph complex. A reaction mechanism is proposed based on computation of the enthalpic reaction profile for the R = Pr(i) system by DFT methods. The R = Et, Pr(i), and Bu(i) systems display biphasic kinetics in which the initial fast process is followed by a slower first order process currently of uncertain origin.

The mononuclear Pd(ii) complex cis-[PdCl(2)([9]aneS(3))]([9]aneS(3)= 1,4,7-trithiacyclononane) converts at 44 kbar pressure into an intensely coloured chain polymer, which exhibits distorted octahedral co-ordination at the metal centre and an unprecedented [1233] conformation for the thioether crown.

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