Golden Opportunity: A Clickable Azide-Functionalized [Au25(SR)18]-Nanocluster Platform for Interfacial Surface Modifications

Ultrasmall atomically precise monolayer-protected gold thiolate nanoclusters are an intensely researched nanomaterial framework, but there is a lack of a system that can be directly synthesized and undergo interfacial surface chemistry. We report an [Au₂₅(SCH₂CH₂-<i>p</i>-C₆H₄-N₃)₁₈]^- nanocluster platform with azide moieties appended onto <i>each </i>surface ligand. The structure of this surface reactive cluster has been confirmed by single-crystal X-ray crystallography, mass spectrometry and UV-Vis, IR and NMR spectroscopies. We show that <i>all</i> surface azide moieties are amenable to cluster-surface strain-promoted alkyne-azide cycloaddition (CS-SPAAC) chemistry with a strained cyclooctyne, opening this as a new <i>platform </i>to allow functional, post-assembly surface modifications to this very prominent nanocluster.

Golden Opportunity: A Clickable Azide-Functionalized [Au25(SR)18]-Nanocluster Platform for Interfacial Surface Modifications

Ultrasmall atomically precise monolayer-protected gold thiolate nanoclusters are an intensely researched nanomaterial framework, but there is a lack of a system that can be directly synthesized and undergo interfacial surface chemistry. We report an [Au₂₅(SCH₂CH₂-<i>p</i>-C₆H₄-N₃)₁₈]^- nanocluster platform with azide moieties appended onto <i>each </i>surface ligand. The structure of this surface reactive cluster has been confirmed by single-crystal X-ray crystallography, mass spectrometry and UV-Vis, IR and NMR spectroscopies. We show that <i>all</i> surface azide moieties are amenable to cluster-surface strain-promoted alkyne-azide cycloaddition (CS-SPAAC) chemistry with a strained cyclooctyne, opening this as a new <i>platform </i>to allow functional, post-assembly surface modifications to this very prominent nanocluster.

Multistep Electrochemical Reduction Path of Clusters [Os3(CO)10(α-diimine)]: Comparison of Electrochemical and Photochemical Os-Os(α-diimine) Bond Cleavage

Electrochemical reduction of the triangular clusters [Os3(CO)10(α-diimine)] (α-diimine = 2,2'-bipyridine (bpy), 2,2'-bipyrimidine (bpym)) and [Os3(CO)10(μ-bpym)ReBr(CO)3] produces primarily the corresponding radical anions. Their stability is strongly determined by the π-acceptor ability of the reducible α-diimine ligand, which decreases in the order μ-bpym > bpym >> bpy. Along this series, increasing delocalisation of the odd electron density in the radical anion over the Os(α-diimine) chelate ring causes weakening of the axial (CO)4Os-Os(CO)2(α-diimine) bond and its facile cleavage for α-diimine = bpy. In contrast, the cluster radical anion is inherently stable for the bridging bpym ligand, the strongest π-acceptor in the studied series. In the absence of the partial delocalisation of the unpaired electron over the Re(bpym) chelate bond, the Os3-core of the radical anion remains intact only at low temperatures. Subsequent one-electron reduction of [Os3(CO)10(bpym)]•- at T = 223 K gives the open-triosmium core (= Os3*) dianion, [Os3*(CO)10(bpym)]2-. Its oxidation leads to the recovery of parent [Os3(CO)10(bpym)]. At room temperature, [Os3*(CO)10(bpym)]2- is formed along a two-electron (ECE) reduction path. The chemical step (C) results in the formation of an open-core radical anion that is directly reducible at the cathodic potential of the parent cluster in the second electrochemical (E) step. In weakly coordinating tetrahydrofuran, [Os3*(CO)10(bpym)]2- rapidly attacks yet non-reduced parent cluster molecules, producing the relatively stable open-core dimer [Os3*(CO)10(bpym)]22- featuring two open-triangle cluster moieties connected with an (bpym)Os-Os(bpym) bond. In butyronitrile, [Os3*(CO)10(bpym)]2- is stabilised by the solvent and the dimer [Os3*(CO)10(bpym)]22- is then mainly formed by reoxidation of the dianion on reverse potential scan. The more reactive cluster [Os3(CO)10(bpy)] follows the same reduction path, as supported by spectroelectrochemical results and additional valuable evidence obtained from cyclic voltammetric scans. The ultimate process in the reduction mechanism is fragmentation of the cluster core triggered by the reduction of the dimer [Os3*(CO)10(α-diimine)]22-. The products formed are [Os2(CO)8]2- and {Os(CO)2(α-diimine)}2. The latter dinuclear fragments constitute a linear polymeric chain [Os(CO)2(α-diimine)]n that is further reducible at the α-diimine ligands. For α-diimine = bpy, the charged polymer is capable of reducing carbon dioxide. The electrochemical opening of the triosmium core in the [Os3(CO)10(α-diimine)] clusters exhibits several common features with their photochemistry. The same Os-α-diimine bond dissociates in both cases but the intimate mechanisms are different.