Carborane Substituents Promote Direct Electrophilic Insertion over Reduction–Metalation Reactions

Abstract

Tw o-electron reduction of 1,1'-bis(o-carborane) followed by reaction with [Ru(h-mes)Cl 2 ] 2 affords [8-(1'-1',2'-closo-C 2 B 10 H 11 )-4-(h-mes)-4,1,8-closo-RuC 2 B 10 H 11 ]. Subsequent two-electron reduction of this species and treatment with [Ru(h-arene)Cl 2 ] 2 results in the 14-vertex/12-vertex species [1-(h-mes)-9-(1'-1',2'-closo-C 2 B 10 H 11 )-13-(h-arene)-1,13,2,9-closo-Ru 2 C 2 B 10 H 11 ]b yd irect electrophilic insertion, promoted by the carborane substituent in the 13-vertex/12-vertex precursor.W hen arene = mesitylene (mes), the diruthenium species is fluxional in solution at room temperature in aprocess that makes the metal-ligand fragments equivalent. A unique mechanism for this fluxionality is proposed and is shown to be fully consistent with the observed fluxionality or nonfluxionality of as eries of previously reported 14-vertex dicobaltacarboranes. Thechemistryofbis(carboranes),twocarboraneunitsjoinedtogether by ac onventional two-center two-electron bond, is highly topical. In principle,bis(carboranes) have the capacity to undergo similar reactions to single-cage carboranes but with two important differences,s pecifically 1) reactivity at as ingle cage is likely to be significantly influenced by the presence of al arge,e lectron-withdrawing substituent and 2) the reaction could occur at both cages and such reactivity could be either isolated or cooperative.A ni nteresting example of this second point is provided by the following: four-electron reduction of 1,1'-bis(o-carborane), the trivial name for [1-(1'-1',2'-closo-C 2 B 10 H 11 )-1,2-closo-C 2 B 10 H 11 ], followed by metalation with {CoCp + }f ragments (Cp = cyclopenadienyl), and finally Co II !Co III oxidation, leads to the anticipated racemic and meso diastereoisomers of the 13-vertex cobaltacarborane/13-vertex cobaltacarborane species [1-(1'-4'-Cp-4',1',6'-closo-CoC 2 B 10 H 11 )-4-Cp-4,1,6-closoCoC 2 B 10 H 11 ].[1] In marked contrast, four-electron reduction of 1,1'-bis(o-carborane) followed by metalation with {Ru(pcymene) 2+ }f ragments (p-cymene = 1-i Pr-4-Me-C 6 H 4 )a ffords au nique 13-vertex ruthenacarborane/12-vertex carborane species.Inthis molecule,the Ru atom in the expanded cage is bonded to an exopolyhedral {Ru(p-cymene)} unit by aRu-Ru bond bridged by a[ m-s,h 3 :h 3 ,s-C 6 ] 2À "flyover" ligand formed by the room-temperature reductive cleavage of an arene,the origin of this reduction being the pendant carborane dianion.[2]References [1]a nd [2] represent the only reports to date of the reduction and subsequent metalation of bis(carboranes) and, although they describe very different outcomes,in both cases both carborane units were fully reduced (by the addition of two electrons) before metalation. Herein we describe the results of the reduction of only as ingle cage of 1,1'-bis(o-carborane) followed by metalation, and the consequences of repeating this process.W ef ound that unique linked heteroborane species are produced, one subset of which displays unexpected fluxionality that is traced to the formation ...