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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...

A variety of transition-metal cluster structures are found within the extended arrays of solidstate materials. Although molecular analogues of some of these clusters have been synthesized-ither by the self-assembly of smaller components in solution or by the excision of intact cluster units directly from the solid-state phase-molecular chemists are often unaware of the rich structural variety expressed in solid-state clusters. This article presents these diverse structural types as potential targets for synthetic molecular chemistry by describing fundamental solid-state cluster topologies in transition-metal chalcogenide/halide phases and detailing cases where molecular counterparts have been prepared. Particular emphasis is placed on cluster excision as a method of molecular cluster synthesis.

Tetrahedral FeCl[N(SiMe(3))(2)](2)(THF) (2), prepared from FeCl(3) and 2 equiv of Na[N(SiMe(3))(2)] in THF, is a useful ferric starting material for the synthesis of weak-field iron-imide (Fe-NR) clusters. Protonolysis of 2 with aniline yields azobenzene and [Fe(2)(mu-Cl)(3)(THF)(6)](2)[Fe(3)(mu-NPh)(4)Cl(4)] (3), a salt composed of two diferrous monocations and a trinuclear dianion with a formal 2 Fe(III)/1 Fe(IV) oxidation state. Treatment of 2 with LiCl, which gives the adduct [FeCl(2)(N(SiMe(3))(2))(2)](-) (isolated as the [Li(TMEDA)(2)](+) salt), suppresses arylamine oxidation/iron reduction chemistry during protonolysis. Thus, under appropriate conditions, the reaction of 1:1 2/LiCl with arylamine provides a practical route to the following Fe-NR clusters: [Li(2)(THF)(7)][Fe(3)(mu-NPh)(4)Cl(4)] (5a), which contains the same Fe-NR cluster found in 3; [Li(THF)(4)](2)[Fe(3)(mu-N-p-Tol)(4)Cl(4)] (5b); [Li(DME)(3)](2)[Fe(2)(mu-NPh)(2)Cl(4)] (6a); [Li(2)(THF)(7)][Fe(2)(mu-NMes)(2)Cl(4)] (6c). [Li(DME)(3)](2)[Fe(4)(mu(3)-NPh)(4)Cl(4)] (7), a trace product in the synthesis of 5a and 6a, forms readily as the sole Fe-NR complex upon reduction of these lower nuclearity clusters. Products were characterized by X-ray crystallographic analysis, by electronic absorption, (1)H NMR, and Mössbauer spectroscopies, and by cyclic voltammetry. The structures of the Fe-NR complexes derive from tetrahedral iron centers, edge-fused by imide bridges into linear arrays (5a,b; 6a,c) or the condensed heterocubane geometry (7), and are homologous to fundamental iron-sulfur (Fe-S) cluster motifs. The analogy to Fe-S chemistry also encompasses parallels between Fe-mediated redox transformations of nitrogen and sulfur ligands and reductive core conversions of linear dinuclear and trinuclear clusters to heterocubane species and is reinforced by other recent examples of iron- and cobalt-imide cluster chemistry. The correspondence of nitrogen and sulfur chemistry at iron is intriguing in the context of speculative Fe-mediated mechanisms for biological nitrogen fixation.

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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.