The Synthesis And Reaction Of 17-electron Cobalt Complexes Containing B(3/6)-norbornyl Substituted Carborane

Dicarba-closo-dodecarboranes have been used for decades as versatile building blocks for construction of functional materials and pharmaceuticals owing to their useful properties such as thermal stability,unique structures and electronic effects,enriched boron content and low bio-toxicity.To enable those diverse applications,structural modifications of carboranes have been the first concern.Substitution at carbon was readily achieved via deprotonation of C-H as early as the discovery of carborane.Substitution at boron remains much less explored owing to the less acidic B-H bonds.In particular,selective boron-functionalization at carborane cluster is still a synthetic challenge because of the chemically close B-H bonds and limited boron functional groups.This situation seriously restricts further exploration of advanced applications of carborane derivatives.Untill now boron-substitution of o-carborane was mainly accomplished by electrophilic substitution to give,for example,B-alkylation,B-halogen and B-chalcogen,but the reactive sites of these examples are mostly localized on B(9/12)which is attributed to the inherent charge distribution of o-carborane cage.The similar substitution in the alternative B(3/6)positions suffered from a major restriction as the key starting material 3-I-o-carborane was typically synthesized by deboronation to form nido-carborane,followed by cage reconstruction using BI3,thus limiting options for further substitution in these two positions.Moreover,metal-promoted selective boron substitution has proven difficult although significant advances have been accomplished,among which boron functionalization by organic group was rare.On the other hand,through intramolecular metal-induced B-H activation at B(3/6)sites of carborane and further functionalization with organic substrates,selective B-M and B-C(B-alkyl and B-vinyl)formation has been achieved.Recent advance on cobalt-mediated both B-H and C-H activation and B-C formation has brought renewed interest.Within those diverse examples,an unprecedented boron-norbornyl group at carborane was observed and preliminarily docu-mented,but the possible formation appears far from a conventional route.These have inspired us to make further exploration.In this study we have extended the scope of reactions,investigated the complicated mechanism by both experiments and theoretical calculations,as well as exam-ined the reactivity of unsaturated intermediates that has led to finding an effective pathway to numerous novel boron-functionalized carborane derivatives.The in-depth study demonstrates the feasibility of cobalt-induced selective B-H functionalization that can provide variety of bo-ron-functionalized carborane derivatives bearing unique in situ generated norbornyl or norborna-dienyl functional group.In this paper,the 16-electron complex CPCoS2C2B10H10(1)is found to react with the alkynes HC=CC(O)R[R= methyl(Me),phenyl(Ph),styryl(St),ferrocenyl(Fc)]at ambient temperature to give two types of 17-electron cobalt complexes 2a-d and 3a-d,both containing unique B(3/6)-norbornyl carborane moieties,as confirmed by the X-ray crystallography.The possible reactive sites of complex 1 and the alkyne,including Co-S bond,Cp ligand,carboranyl group,C?C bond,are unprecedentedly involved in the formation of boron-norbornyl moiety at the carborane.A plausible reaction mechanism via a tandem sequence of alkyne insertion,metal-induced B-H activation,B-Cp formation,Cp delivery,intra-molecular electron transfer and Diels-Alder addition is proposed on the basis of DFT calculations.The reactivity of these paramagnetic 17-electron complexes has been studied:Exposed to a combination of air,moisture and silica,complexes 2a-d undergo redox reactions to give the 16-electron complexes 4a-c containing an unusual boron-norbomadienyl moiety as well as the cobalt-free carborane derivatives 5a-d containing a boron-norbornyl unit.In this reaction,a single 17-electron precursor undergoes two types of C-S(both alkyl C-S and carboranyl C-S)cleavage reactions,thus opening a way to novel B-H functionalized carborane derivatives.Furthermore,both 2a-d and 3a-d allow further alkyne insertion into the Co-S bond to generate the cobalt-free B-H functionalized carborane derivatives(Z/E)-7a-d and(Z/E)-8a-d,both containing a vinyl sulfido group.Addition of AICl3 not only promotes the conversion of 2a-d,but also leads predominantly to(E)-9a-d as retro-Diels-Alder products.Especially,one-pot reactions of complex 1 and selected alkynes were also examined to lead in good yields to those products.Throughout this study,the 17-electron complexes 2a-d and 3a-d serve as intermediates for synthesis of variety of boron-functionalized carborane derivatives.Upon heating,isomerization of(Z/E)-7a-d of the vinyl group(from E to Z)and reorganization of the norbornyl moiety occur lead to(Z)-9a-d as well as to the unexpected[1,2]-H shifted products(Z)-10b,c.In this study,efficient routes have been developed through cobalt-mediated B-H activation to prepare boron-functionalized carborane derivatives that are unavailable by conventional routes.