MULTISTEP ELECTROCHEMICAL REDUCTION PATH OF CLUSTERS [Os_3(CO)_(10)(alpha-DIIMINE)]:COMPARISON OF ELECTROCHEMICAL AND PHOTOCHEMICAL Os-Os(alpha-DIIMINE)BOND CLEAVAGE

摘要

Electrochemical reduction of the triangular clusters [Os_3(CO)_(10)(alpha-diimine)](alpha-diimine = 2,2'-bipyridine(bpy),2,2'-bipyrimidine(bpym))and [Os_3(CO)_(10)(mu-bpym)ReBr(CO)_3] produces primarily the corresponding radical anions.Their stability is strongly determined by the pi-acceptor ability of the reducible alpha-diimine ligand,which decreases in the order mu-bpym > bpym bpy.Along this series,increasing delocalisation of the odd electron density in the radical anion over the Os(alpha-diimine)chelate ring causes weakening of the axial(CO)_4Os-Os(CO)_2(alpha-diimine)bond and its facile cleavage for alpha-diimine = bpy.In contrast,the cluster radical anion is inherently stable for the bridging bpym ligand,the strongest pi-acceptor in the studied series.In the absence of the partial delocalisation of the unpaired electron over the Re(bpym)chelate bond,the Os_3-core of the radical anion remains intact only at low temperatures.Subsequent one-electron reduction of [Os_3(CO)_(10)(bpym)]~(centre dot-)at T = 223 K gives the open-triosmium core(= Os_3*)dianion,[Os_3*(CO)_(10)(bpym)]~(2-).Its oxidation leads to the recovery of parent [Os_3(CO)_(10)(bpym)].At room temperature,[Os_3*(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,[Os_3*(CO)_(10)(bpym)]~(2-)rapidly attacks yet non-reduced parent cluster molecules,producing the relatively stable open-core dimer [Os_3*(CO)_(10)(bpym)]_2~(2-)featuring two open-triangle cluster moieties connected with an(bpym)Os-Os(bpym)bond.In butyro-nitrile,[Os_3*(CO)_(10)(bpym)]~(2-)is stabilised by the solvent and the dimer [Os_3*(CO)_(10)(bpym)]_2~(2-)is then mainly formed by reoxidation of the dianion on reverse potential scan.The more reactive cluster [Os_3(CO)_(10)(bpy)] follows the same reduction path,as supported by spectro-electrochemical 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 [Os_3*(CO)_(10)(alpha-diimine)]_2~(2-).The products formed are [Os_2(CO)_8]~(2-)and{Os(CO)_2(alpha-diimine)}_2.The latter dinuclear fragments constitute a linear polymeric chain [Os(CO)_2(alpha-diimine)]_n that is further reducible at the alpha-diimine ligands.For alpha-diimine = bpy,the charged polymer is capable of reducing carbon dioxide.The electrochemical opening of the triosmium core in the [Os_3(CO)_(10)(alpha-diimine)] clusters exhibits several common features with their photochemistry.The same Os-alpha-diimine bond dissociates in both cases but the intimate mechanisms are different.