Chalcogen Impact on Covalency within Molecular [Cu3(μ3-E)]3+ Clusters (E = O, S, Se): A Synthetic, Spectroscopic, and Computational Study

Metal-Tuned Ligand Reactivity Enables CX2 (X = O, S) Homocoupling with Spectator Cu Centers

Ligand non-innocence is ubiquitous in catalysis with ligands in synthetic complexes contributing as electron reservoirs or co-sites for substrate activation. The latter chemical non-innocence is manifested in H+ storage or relay at sites beyond the metal primary coordination sphere. Reaction of a competent CO2-to-oxalate reduction catalyst, namely, [K(THF)3](Cu3SL), where L3- is a tris(β-diketiminate) cyclophane, with CS2 affords tetrathiooxalate at long reaction times or at high CS2 concentrations, where otherwise an equilibrium is established between the starting species and a complex-CS2 adduct in which the CS2 is bound to the C atom on the ligand backbone. X-ray diffraction analysis of this adduct reveals no apparent metal participation, suggesting an entirely ligand-based reaction controlled by the charge state of the cluster. Thermodynamic parameters for the formation of the aforementioned Cligand-CS2 bond were experimentally determined, and trends with cation Lewis acidity were studied, where more acidic cations shift the equilibrium toward the adduct. Relevance of such an adduct in the reduction of CO2 to oxalate by this complex is supported by DFT studies, similar effects of countercation Lewis acidity on product formation, and the homocoupled heterocumulene product speciation as determined by isotopic labeling studies. Taken together, this system extends chemical non-innocence beyond H+ to effect catalytic transformations involving C-C bond formation and represents the rarest example of metal-ligand cooperativity, that is, spectator metal ion(s) and the ligand as the reaction center.

Metal Site-Specific Electrostatic Field Effects on a Tricopper(I) Cluster Probed by Resonant Diffraction Anomalous Fine Structure (DAFS)

Studies of multinuclear metal complexes are greatly enhanced by resonant diffraction measurements, which probe X-ray absorption profiles of crystallographically independent metal sites within a cluster. In particular, X-ray diffraction anomalous fine structure (DAFS) analysis provides data that can be interpreted akin to site-specific XANES, allowing for differences in metal K-edge resonances to be deconvoluted even for different metal sites within a homometallic system. Despite the prevalence of Cu-containing clusters in biology and energy science, DAFS has yet to be used to analyze multicopper complexes of any type until now. Here, we report an evaluation of trends using a series of strategically chosen Cu(I) and Cu(II) complexes to determine how energy dependencies of anomalous scattering factors are impacted by coordination geometry, ligand shell, cluster nuclearity, and oxidation state. This calibration data is used to analyze a formally tricopper(I) complex that was found by DAFS to be site-differentiated due to the unsymmetrical influence on different Cu sites of the electrostatic field from a proximal K+ cation.

Tuning metal-metal interactions for cooperative small molecule activation

Cluster complexes have attracted interest for decades due to their promise of drawing analogies to metallic surfaces and metalloenzyme active sites, but only recently have chemists started to develop ligand scaffolds that are specifically designed to support multinuclear transition metal cores. Such ligands not only hold multiple metal centers in close proximity but also allow for fine-tuning of their electronic structures and surrounding steric environments. This Feature Article highlights ligand designs that allow for cooperative small molecule activation at cluster complexes, with a particular focus on complexes that contain metal-metal bonds. Two useful ligand-design elements have emerged from this work: a degree of geometric flexibility, which allows for novel small molecule activation modes, and the use of redox-active ligands to provide electronic flexibility to the cluster core. The authors have incorporated these factors into a unique class of dinucleating macrocycles (nPDI2). Redox-active fragments in nPDI2 mimic the weak-overlap covalent bonding that is characteristic of M-M interactions, and aliphatic linkers in the ligand backbone provide geometric flexibility, allowing for interconversion between a range of geometries as the dinuclear core responds to the requirements of various small molecule substrates. The union of these design elements appears to be a powerful combination for analogizing critical aspects of heterogeneous and metalloenzyme catalysts.

Encapsulation of tricopper cluster in a synthetic cryptand enables facile redox processes from CuICuICuI to CuIICuIICuII states

One-pot reaction of tris(2-aminoethyl)amine (TREN), [Cu^I(MeCN)₄]PF₆, and paraformaldehyde affords a mixed-valent [TREN4Cu^IICu^ICu^I(μ₃-OH)](PF₆)₃ complex. The macrocyclic azacryptand TREN4 contains four TREN motifs, three of which provide a bowl-shape binding pocket for the [Cu₃(μ₃-OH)]³⁺ core. The fourth TREN caps on top of the tricopper cluster to form a cryptand, imposing conformational constraints and preventing solvent interaction. Contrasting the limited redox capability of synthetic tricopper complexes reported so far, [TREN4Cu^IICu^ICu^I(μ₃-OH)](PF₆)₃ exhibits several reversible single-electron redox events. The distinct electrochemical behaviors of [TREN4Cu^IICu^ICu^I(μ₃-OH)](PF₆)₃ and its solvent-exposed analog [TREN3Cu^IICu^IICu^II(μ₃-O)](PF₆)₄ suggest that isolation of tricopper core in a cryptand enables facile electron transfer, allowing potential application of synthetic tricopper complexes as redox catalysts. Indeed, the fully reduced [TREN4Cu^ICu^ICu^I(μ₃-OH)](PF₆)₂ can reduce O₂ under acidic conditions. The geometric constraints provided by the cryptand are reminiscent of Nature's multicopper oxidases (MCOs). For the first time, a synthetic tricopper cluster was isolated and fully characterized at Cu^ICu^ICu^I (4a), Cu^IICu^ICu^I (4b), and Cu^IICu^IICu^I (4c) states, providing structural and spectroscopic models for many intermediates in MCOs. Fast electron transfer rates (10⁵ to 10⁶ M^-1 s^-1) were observed for both Cu^ICu^ICu^I/Cu^IICu^ICu^I and Cu^IICu^ICu^I/Cu^IICu^IICu^I redox couples, approaching the rapid electron transfer rates of copper sites in MCO.

σ-Noninnocence: Masked Phenyl-Cation Transfer at Formal NiIV

Reductive elimination is an elementary organometallic reaction step involving a formal oxidation state change of -2 at a transition-metal center. For a series of formal high-valent NiIV complexes, aryl-CF3 bond-forming reductive elimination was reported to occur readily (Bour et al. J. Am. Chem. Soc. 2015, 137, 8034-8037). We report a computational analysis of this reaction and find that, unexpectedly, the formal NiIV centers are better described as approaching a +II oxidation state, originating from highly covalent metal-ligand bonds, a phenomenon attributable to σ-noninnocence. A direct consequence is that the elimination of aryl-CF3 products occurs in an essentially redox-neutral fashion, as opposed to a reductive elimination. This is supported by an electron flow analysis which shows that an anionic CF3 group is transferred to an electrophilic aryl group. The uncovered role of σ-noninnocence in metal-ligand bonding, and of an essentially redox-neutral elimination as an elementary organometallic reaction step, may constitute concepts of broad relevance to organometallic chemistry.

Cyclophanes as Platforms for Reactive Multimetallic Complexes

Multimetallic cofactors supported by weak-field donors frequently function as reaction centers in metalloproteins, and many of these cofactors catalyze small molecule activation (e.g., N₂, O₂, CO₂) with prominent roles in geochemical element cycles or detoxification. Notable examples include the iron-molybdenum cofactor of the molybdenum-dependent nitrogenases, which catalyze N₂ fixation, and the NiFe₄S₄ cluster and the Mo(O)SCu site in various carbon monoxide dehydrogenases. The prevailing proposed reaction mechanisms for these multimetallic cofactors relies on a cooperative pathway, in which the oxidation state changes are distributed over the aggregate coupled with orbital overlap between the substrate and more than one metal ion within the cluster. Such cooperativity has also been proposed for chemical transformations at the surfaces of heterogeneous catalysts. However, the design details that afford cooperative effects and allow such reactivity to be harnessed effectively in homogeneous synthetic systems remain unclear. Relatedly, hydride donors ligated to these metal cluster cofactors are suggested as precursors to the state that reacts with substrates; here too, however, the reactivity of hydride-decorated clusters supported by weak-field ligands is underexplored. Inspired by the reactivity potential of multimetallic assemblies evidenced in biological systems, approaches to design, synthesize, and evaluate reactivity of polynuclear metal compounds have been actively explored. In a similar vein to the templating function afforded by enzyme active sites, a carefully engineered organic ligand can be employed to control metal nuclearity of the complex and the local coordination environment of each metal center. This Account presents our efforts within this field, beginning with ligand design considerations followed by a survey of observed small molecule activation by trimetallic cyclophanates. We highlight the distinct reactivity outcomes accessed by multimetallic compounds as compared to aggregates that assemble in reaction mixtures from monometallic precursors. Contributing to the opportunity for programmed cooperativity in these designed multimetallic compounds, the cyclophane also dictates the orientation of substrate binding and metal-substrate interactions, which has a prominent influence on reactivity. For example, the dinitrogen-tricopper(I) cyclophanate reacts with dioxygen with markedly different results as compared to monocopper compounds. As an unexpected outcome, one series of tricopper compounds were discovered to be competent catalysts for carbon dioxide reduction to oxalate-a formally one-electron process-hinting at an inherently broader reaction scope for weak-field clusters at lowering the barrier for one-electron pathways as well as multielectron redox transformations. Further reflecting the role of the ligand in tuning reactivity, the trimetallic trihydride cluster compounds, [M₃(μ-H)₃]³⁺ (M = Fe^II, Co^II, Zn^II), demonstrate substrate specificity for CO₂ over various other unsaturated molecules and surprising stability toward water. This series reflects the role of the local environment of a shallow ligand pocket to control substrate access. Summed together, the systems described here evidence the anticipated cooperative reactivity accessed in designed multimetallic species vs self-assembled monometallic systems (e.g., O₂ activation and O atom transfer) as well as control of substrate access by seemingly subtle structural effects. Indeed, future efforts aim to interrogate the limits of cooperativity in these systems as well as the role of ligand dynamics and sterics on reactivity.