The Redox Chemistry of [Co6C(CO)15]2–: A Synthetic Route to New Co-Carbide Carbonyl Clusters

Pre-Equilibrium Reaction Mechanism as a Strategy to Enhance Rate and Lower Overpotential in Electrocatalysis

Pre-equilibrium reaction kinetics enable the overall rate of a catalytic reaction to be orders of magnitude faster than the rate-determining step. Herein, we demonstrate how pre-equilibrium kinetics can be applied to breaking the linear free-energy relationship (LFER) for electrocatalysis, leading to rate enhancement 5 orders of magnitude and lowering of overpotential to approximately thermoneutral. This approach is applied to pre-equilibrium formation of a metal-hydride intermediate to achieve fast formate formation rates from CO₂ reduction without loss of selectivity (i.e., H₂ evolution). Fast pre-equilibrium metal-hydride formation, at 10⁸ M^-1 s^-1, boosts the CO₂ electroreduction to formate rate up to 296 s^-1. Compared with molecular catalysts that have similar overpotential, this rate is enhanced by 5 orders of magnitude. As an alternative comparison, overpotential is lowered by ∼50 mV compared to catalysts with a similar rate. The principles elucidated here to obtain pre-equilibrium reaction kinetics via catalyst design are general. Design and development that builds on these principles should be possible in both molecular homogeneous and heterogeneous electrocatalysis.

Transition Metal Carbide Complexes

Carbide complexes remain a rare class of molecules. Their paucity does not reflect exceptional instability but is rather due to the generally narrow scope of synthetic procedures for constructing carbide complexes. The preparation of carbide complexes typically revolves around generating L_nM-CE_x fragments, followed by cleavage of the C-E bonds of the coordinated carbon-based ligands (the alternative being direct C atom transfer). Prime examples involve deoxygenation of carbonyl ligands and deprotonation of methyl ligands, but several other p-block fragments can be cleaved off to afford carbide ligands. This Review outlines synthetic strategies toward terminal carbide complexes, bridging carbide complexes, as well as carbide-carbonyl cluster complexes. It then surveys the reactivity of carbide complexes, covering stoichiometric reactions where the carbide ligands act as C₁ reagents, engage in cross-coupling reactions, and enact Fischer-Tropsch-like chemistry; in addition, we discuss carbide complexes in the context of catalysis. Finally, we examine spectroscopic features of carbide complexes, which helps to establish the presence of the carbide functionality and address its electronic structure.

Metal carbonyl clusters of groups 8-10: synthesis and catalysis

In this review article, we discuss advances in the chemistry of metal carbonyl clusters (MCCs) spanning the last three decades, with an emphasis on the more recent reports and those involving groups 8-10 elements. Synthetic methods have advanced and been refined, leading to higher-nuclearity clusters and a wider array of structures and nuclearities. Our understanding of the electronic structure in MCCs has advanced to a point where molecular chemistry tools and other advanced tools can probe their properties at a level of detail that surpasses that possible with other nanomaterials and solid-state materials. MCCs therefore advance our understanding of structure-property-reactivity correlations in other higher-nuclearity materials. With respect to catalysis, this article focuses only on homogeneous applications, but it includes both thermally and electrochemically driven catalysis. Applications in thermally driven catalysis have found success where the reaction conditions stabilise the compounds toward loss of CO. In more recent years, MCCs, which exhibit delocalised bonding and possess many electron-withdrawing CO ligands, have emerged as very stable and effective for reductive electrocatalysis reactions since reduction often strengthens M-C(O) bonds and since room-temperature reaction conditions are sufficient for driving the electrocatalysis.

Molecular Nickel Phosphide Carbonyl Nanoclusters: Synthesis, Structure, and Electrochemistry of [Ni11P(CO)18]3- and [H6-nNi31P4(CO)39]n- (n = 4 and 5)

The reaction of [NEt₄]₂[Ni₆(CO)₁₂] in thf with 0.5 equiv of PCl₃ affords the monophosphide [Ni₁₁P(CO)₁₈]^3- that in turn further reacts with PCl₃ resulting in the tetra-phosphide carbonyl cluster [HNi₃₁P₄(CO)₃₉]^5-. Alternatively, the latter can be obtained from the reaction of [NEt₄]₂[Ni₆(CO)₁₂] in thf with 0.8-0.9 equiv of PCl₃. The [HNi₃₁P₄(CO)₃₉]^5- penta-anion is reversibly protonated by strong acids leading to the [H₂Ni₃₁P₄(CO)₃₉]^4- tetra-anion, whereas deprotonation affords the [Ni₃₁P₄(CO)₃₉]^6- hexa-anion. The latter is reduced with Na/naphthalene yielding the [Ni₃₁P₄(CO)₃₉]^7- hepta-anion. In order to shed light on the polyhydride nature and redox behavior of these clusters, electrochemical and spectroelectrochemical studies were carried out on [Ni₁₁P(CO)₁₈]^3-, [HNi₃₁P₄(CO)₃₉]^5-, and [H₂Ni₃₁P₄(CO)₃₉]^4-. The reversible formation of the stable [Ni₁₁P(CO)₁₈]^4- tetra-anion is demonstrated through the spectroelectrochemical investigation of [Ni₁₁P(CO)₁₈]^3-. The redox changes of [HNi₃₁P₄(CO)₃₉]^5- show features of chemical reversibility and the vibrational spectra in the ν_CO region of the nine redox states of the cluster [HNi₃₁P₄(CO)₃₉]^n- (n = 3-11) are reported. The spectroelectrochemical investigation of [H₂Ni₃₁P₄(CO)₃₉]^4- revealed the presence of three chemically reversible reduction processes, and the IR spectra of [H₂Ni₃₁P₄(CO)₃₉]^n- (n = 4-7) have been recorded. The different spectroelectrochemical behavior of [HNi₃₁P₄(CO)₃₉]^5- and [H₂Ni₃₁P₄(CO)₃₉]^4- support their formulations as polyhydrides. Unfortunately, all the attempts to directly confirm their poly hydrido nature by ¹H NMR spectroscopy failed, as previously found for related large metal carbonyl clusters. Thus, the presence and number of hydride ligands have been based on the observed protonation/deprotonation reactions and the spectroelectrochemical experiments. The molecular structures of the new clusters have been determined by single-crystal X-ray analysis. These represent the first examples of structurally characterized molecular nickel carbonyl nanoclusters containing interstitial phosphide atoms.

Octahedral co-carbide carbonyl clusters decorated by [AuPPh₃]⁺ fragments: synthesis, structural isomerism, and aurophilic interactions of Co₆C(CO)₁₂(AuPPh₃)₄

The Co6C(CO)12(AuPPh3)4 carbide carbonyl cluster was obtained from the reaction of [Co6C(CO)15](2-) with Au(PPh3)Cl. This new species was investigated by variable-temperature (31)P NMR spectroscopy, X-ray crystallography, and density functional theory methods. Three different solvates were characterized in the solid state, namely, Co6C(CO)12(AuPPh3)4 (I), Co6C(CO)12(AuPPh3)4·THF (II), and Co6C(CO)12(AuPPh3)4·4THF (III), where THF = tetrahydrofuran. These are not merely different solvates of the same neutral cluster, but they contain three different isomers of Co6C(CO)12(AuPPh3)4. The three isomers I-III possess the same octahedral [Co6C(CO)12](4-) carbido-carbonyl core differently decorated by four [AuPPh3](+) fragments and showing a different Au(I)···Au(I) connectivity. Theoretical investigations suggest that the formation in the solid state of the three isomers during crystallization is governed by packing and van der Waals forces, as well as aurophilic and weak π-π and π-H interactions. In addition, the closely related cluster Co6C(CO)12(PPh3)(AuPPh3)2 was obtained from the reaction of [Co8C(CO)18](2-) with Au(PPh3)Cl, and two of its solvates were crystallographically characterized, namely, Co6C(CO)12(PPh3)(AuPPh3)2·toluene (IV) and Co6C(CO)12(PPh3)(AuPPh3)2·0.5toluene (V). A significant, even if minor, effect of the cocrystallized solvent molecules on the structure of the cluster was observed also in this case.