12‐Vertex Zwitterionic Bis‐phosphonium‐nido‐carborates through Ring‐Opening Reactions of 1,2‐Diphosphetanes

Abstract

Carborane-substituted 1,2-diphosphetanes (Ia,b) react with elemental lithium in THF with cleavage of the P-P bond to give a deep red solution from which, in the case of Ia, red crystals of a lithiated intermediate, [{1-Li(THF)PtBu-6-PtBu-4,1,6-closo-Li(THF)C B H }{Li(THF) }] ⋅2 THF (2 a), are obtained. The compound is dimeric, C -symmetric and contains six lithium and four phosphorus atoms. Two lithium atoms cap the six-membered C B faces, resulting in two 13-vertex closo-clusters (according to Wade's rules) with docosahedral geometry. The addition of methyl iodide resulted in the formation of zwitterionic bis-phosphonium-nido-carborates 7,10-bis(tert-butyldimethylphosphonium)dodecahydro-7,10-dicarba-nido-dodecaborate(2-) (1 a) and 7,10-bis(N,N-diisopropylaminodimethylphosphonium)dodecahydro-7,10-dicarba-nido-dodecaborate(2-) (1 b) in moderate to good yields. Compounds 1 a and 1 b exhibit short C -P bonds and large C ⋅⋅⋅C distances in the solid state. Further insight into the ring opening and reduction potential of the alkyl halide was obtained from methylation reactions of different 1,2-bis-phosphinocarboranes. The reaction of rac-/meso-1,2-bis(tert-butylmethylphosphino)-1,2-dicarba-closo-dodecaborane(12) (3 a) with two equivalents of methyl iodide also resulted in the formation of 1 a (as shown by NMR spectroscopy), whereas the reaction of 1,2-bis(diphenylphosphino)-1,2-dicarba-closo-dodecaborane(12) with methyl triflate afforded the phosphonium salt 1-methyl-diphenylphosphonium-2-diphenylphosphino-1,2-dicarba-closo-dodecaborane(12) triflate (4) without reduction of the cluster.

Redox-active carborane clusters in bond activation chemistry and ligand design

“…The Hey-Hawkins group reported a related carboranyl diphosphetane that displays similar oxidative addition-type reactivity in the presence of MeI. 69 The synthesized carboranyl diphosphetane undergoes P–P bond activation and cluster opening upon reduction with lithium metal. The reaction of the resulting Li intermediate with excess MeI induces the methylation of both phosphorus centers, generating a zwitterionic bis-phosphonium- nido -carborane (Fig.…”

Metal-Free Bond Activation by Carboranyl Diphosphines

“…The carbon atoms of ortho -C 2 B 10 H 12 form single bonds to exohedral substituents, and π-back-donation from those groups to the cluster can increase the exohedral bond order and weaken the intracluster C–C bond. , Recently, there was a report of opening of the ortho -{C 2 B 10 } cage upon the reaction of carboranyl diphosphines with HCl, leading to the formation of a zwitterionic nido -cluster that contains two phosphonium centers (Scheme I) . Albeit complicated by side reactions and only partial conversion, similar reactivity has been observed between a carboranyl diphosphine and methyl iodide: the phosphorus centers become methylated and the carborane cage opens . Concomitant formation of I 2 has been proposed as a source of electrons for the cluster reduction (Scheme J).…”

“…28 and only partial conversion, similar reactivity has been observed between a carboranyl diphosphine and methyl iodide: the phosphorus centers become methylated and the carborane cage opens. 29 Concomitant formation of I 2 has been proposed as a source of electrons for the cluster reduction (Scheme 1J). Conceptually, carboranyl diphosphines represent a metalfree cluster-containing molecular framework that combines two important features normally reserved for transition metals: the simultaneous ability to accept electrons to cycle through two-electron redox states and the ability to donate electrons through Lewis basic binding sites (Scheme 1K).…”

Activation of Alkynes by a Redox-Active Carboranyl Diphosphine and Formation of Boron-Containing Phosphacycles

“…A single crystal X-ray diffraction study revealed the structure of P­(H)­( i Pr 2 )­P­(CHCPh)­( t Bu 2 )­(C 2 B 10 H 9 ) ( 2 ) (Figure ) and confirmed the carborane cage is an open 12-vertex nido -cluster, which is reduced by two electrons. The cluster is C 2 -symmetric, − giving rise to two enantiomers of 2 that co-crystallized as a racemate. The P­( i Pr 2 ) group is protonated, and the P­( t Bu 2 ) group is converted to an alkenylphosphonium center through the attachment of phenylacetylene.…”