The Mechanism of Reduction and Metalation of para Carboranes: The Missing 13‐Vertex MC₂B₁₀ Isomer

Abstract

Unanticipated complexity: The first example of a 4,1,11‐MC2B10 metallacarborane (see picture; Ru magenta, B pink, C gray, H white) is reported. Reduction and metalation of 1,12‐Ph2‐1,12‐closo‐C2B10H10 at both room temperature and low temperature yields no fewer than five isomeric supraicosahedral metallacarborane products. Calculations identify two initial nido‐[C2B10H12]2− species, each with one C atom that is not in the open face.

Isomerisation of nido-[C2B10H12]2− dianions: unprecedented rearrangements and new structural motifs in carborane cluster chemistry

“…3). This features a p-antibonding interaction along the C 1 -B 6 connection and a further antibonding interaction between B 3 and B 4 . Therefore the 2e occupation of this orbital is consistent with the breaking of these interactions to give a 4-membered C 1 -B 3 -B 8 -B 4 and a 5-membered C 1 -B 3 -B 2 -B 6 -B 5 face in Int(A).…”

“…1 ). 4 This implies the presence of five isomeric nido fragments following reduction, i.e. 1,7 -, 3,7 -, 4,7 -, 7,9 - and [ 7,10 -Ph 2 - 7,10 - nido -C 2 B 10 H 10 ] 2– .…”

“…1,7-, 3,7-, 4,7-, 7,9-and [7,10-Ph 2 -7,10-nido-C 2 B 10 H 10 ] 2À . A concurrent computational study on [1,12-closo-C 2 B 10 H 12 ] (para-carborane) suggested the rst nido-fragments formed upon reduction were [1,7- (4,7). At the time, the further isomerisation of these species to the remaining nido species was not considered, however computing these directly provided relative energies, H (enthalpies, 0 K) of +29.5, +24.0, +22.0 and +1.4 kcal mol À1 for 1,7, 3,7, 4,7 and 7,10 respectively, all relative to 7,9 at 0.0 kcal mol À1 .…”

Boron

“…55 The related 4,1,11-MC 2 B 10 (M = {(arene)Ru}) isomer has also been reported and the reductive pathway from 1,12-Ph 2 C 2 B 10 H 10 explored computationally. 56 The 13-vertex bimetallic metallocarborane species, [4,5,2,3-M 2 C 2 B 9 H 9 ] (M = {(p-cymene)Ru}, or {CpCo}) have also been described and their thermal isomerisation described and studied computationally. 54 In independent studies Xie and co-workers have published the synthesis and structural studies on a series of 13-vertex cobaltacarboranes formed by the reduction of the octamethylated carborane ortho-C 2 B 10 Me 8 H 4 and subsequent reaction with CoCl 2 /CpNa mixtures (Cp = C 5 H 5 , MeC 5 H 4 , BuC 5 H 4 ).…”

Untethered 4,1,2-MC2B10 supraicosahedral metallacarboranes, their C,C′-dimethyl 4,1,6-, 4,1,8- and 4,1,12-MC2B10 analogues, and DFT study of the (4,)1,2- to (4,)1,6-isomerisations of C2B11 carboranes and MC2B10 metallacarboranes

“…E-mail: d.ellis@hw.ac.uk, d.mckay@hw.ac.uk, a.j.welch@hw.ac.uk; Fax: +44 (0)131 451 3180; Tel: +44 (0)131 451 8038+44 (0)131 451 3259+44 (0)131 451 3217 with {Ru( p-cymene) 2+ } (p-cymene = η-C 6 H 4 Me i Pr-1,4) was later confirmed as a rather complex process involving the isolation of 4,1,10-, 4,1,6-, 4,1,8-, 4,1,12-and, uniquely, a 4,1,11-RuC 2 B 10 metallacarborane. 6 If the cage C atoms of 1,2-closo-C 2 B 10 carborane are tethered, reduction and metallation affords (tethered) 4,1,2-MC 2 B 10 species, which can either have docosahedral or henicosahedral cage geometries (Fig. 1), 7 the latter being the shape of tethered 13-vertex carboranes, 8 but, with only two exceptions (cobaltacarboranes afforded either by direct insertion of a highly nucleophilic Co 0 fragment into 1-Me-1,2-closo-C 2 B 10 H 11 9 or by direct reaction between [8-CN-7,8-arachno-C 2 B 10 H 14 ] − , Na[Cp] and CoCl 2 followed by aerial oxidation 10 ) untethered 4,1,2-MC 2 B 10 species remain unknown.…”