Unprecedented steric deformation of ortho-carboranew Brian W. Hutton, Fraser MacIntosh, David Ellis, Fabien Herisse, Stuart A. Macgregor,* David McKay, Victoria Petrie-Armstrong, Georgina M. Rosair, Dmitry S. Perekalin, Hugo Tricas and Alan J. Welch* Received (in Cambridge, UK) 25th June 2008, Accepted 15th August 2008 First published as an Advance Article on the web 19th September 2008 DOI: 10.1039/b810702e The reduction and subsequent oxidation of meta-carboranes containing bulky groups attached to the cage C atoms affords sterically-crowded ortho-carboranes with unprecedentedly long C–C connectivities. Although it has been appreciated for more than 45 years that ortho-carborane, 1,2-closo-C 2 B 10 H 12 , has a distorted icosahedral geometry, 1 its accurate structure has been determined only relatively recently. In 1996 Davidson, Wade et al. 2 used the acidic nature of the carborane CH units to form a supra- molecular dimer with hexamethylphosphoramide, locking the CH units in C–HO hydrogen bonds and so overcoming the disorder that had frustrated previous attempts to obtain an accurate structure crystallographically; the two indepen- dent C–C distances measured at 150 K were 1.629(6) and 1.630(6) A ˚ . Later, Rankin and co-workers 3 redetermined the gas phase structure of ortho-carborane by electron diffraction, improving on the accuracy of previous determinations 4 and affording C1–C2 1.624(8) A ˚ , in excellent agreement with the results of a parallel computational study, 1.621 A ˚ . The C–C distance in ortho-carborane can be lengthened to ca. 1.8 A ˚ by attaching bulky substituents to the cage C atoms, 5 or to nearly 1.9 A ˚ by incorporating the cage C atoms in suitable exopolyhedral cycles. 6 Alternatively, the C–C distance can be extended electronically by p-donor substituents at one cage C atom to distances 42A ˚ , 7 but such species can be argued to have formal skeletal electron counts in excess of the (2n + 2) (n = no. of vertices) normally associated with closo species 8 and so bear closer comparison with (2n + 3) carborane radical anions 9 or (2n + 4) nido dianions. 8 The electronic causes for long C–C distances in neutral and anionic ortho-carboranes as a function of various substituents have been explored computationally. 10 Herein we report unprece- dented lengthening of the C1–C2 connectivity of ortho- carborane, to 2.15 A ˚ , and related deformation of the carbo- rane icosahedron by intramolecular steric crowding alone. When ortho-carborane is doubly deprotonated and allowed to react with two equivalents of the substituted fulvene 1 (a, R = Me) the known, 11 singly-substituted, product 1-CMe 2 (C 5 H 5 )-1,2-closo-C 2 B 10 H 11 is afforded on aqueous work-up, implying that the CMe 2 (C 5 H 5 ) substituent is already too bulky to 1,2-disubstitute by this approach. However, double deprotonation of meta-carborane, 1,7-closo-C 2 B 10 H 12 (I), followed by treatment with two equivalents of 1 readily yields the disubstituted species 1,7-{CR 2 (C 5 H 5 )} 2 -1,7-closo- C 2 B 10 H 10 (2a, R = Me; 2b,R 2 = Pm) (Pm = pentamethy- lene) on work-up (Scheme 1).z In 2 there are two isomeric forms of the cyclopentadienyl ring, with the CH 2 function a or b; by 1 H NMR spectroscopy the ratio a : b is 1 : 2 for 2a and 2 : 9 for 2b. Compounds 2 are converted to the corresponding bis-ferrocenyl species 1,7-(CR 2 Fc) 2 -1,7-closo-C 2 B 10 H 10 3a and 3b {Fc = (Z-C 5 H 4 )Fe(Z-C 5 H 5 )} by standard means. It is well established that 2e reduction of meta-carborane followed by reoxidation affords ortho-carborane. 12 Similarly, reduction (Na, THF) then reoxidation of compounds 3 produces the crowded substituted ortho-carboranes 4a and 4b. By this (indirect) method may be prepared ortho-carborane derivatives with substantially sterically demanding groups attached to the two cage carbon atoms that are not possible starting from ortho-carborane directly. Crystallographic studyy of 4a and 4b reveals the extent of the steric crowding within. Molecules of 4a have effective C 2 symmetry about an axis from the mid-point of C1–C2 to the mid-point of B9–B12, with the Me groups of the CMe 2 Fc substituents interdigitated about this axis and the Fc units oriented away from both the C1/C2 region and the BH cage vertices. The C1–C2 distance is 1.9378(16) A ˚ . In 4b (Fig. 1) the CPmFc substituent on C2 has an orientation that corresponds to the CMe 2 Fc substituent in 4a, but the other CPmFc substituent (on C1) has rotated so that its Pm group is side-on to the Pm group on C21 not face-on (which presumably would be unten- able). Nevertheless, the C1–C2 distance in 4b is 2.156(4) A ˚ , easily the longest such distance experimentally recorded for a true (2n + 2) ortho-carborane derivative. Further evidence for the severe intramolecular crowding in 4b is the presence of two HH distances of 2 A ˚ (H12BH26B 1.995 A ˚ , H12BH22A 1.997 A ˚ ) and distortion of the Fc group on C211 as a result of the forced orientation of the adjacent Pm group; the C211–C215 ring is tilted at C211 (the C21–C211 bond meets the plane at 11.381) and the two Cp rings on Fe2 subtend a dihedral angle of 11.651. As the cage carbon atoms C1 and C2 are progressively forced apart by steric crowding, the boron atoms to which they are both connected, B3 and B6, are pulled together, the B3B6 separation reducing from 2.886 A ˚ (average) in ortho-carborane, 2 to 2.756 A ˚ in 4a to 2.663 A ˚ in 4b. Concomitantly, a smaller, Department of Chemistry, School of Engineering & Physical Sciences, Heriot–Watt University, Edinburgh, UK EH14 4AS. E-mail: a.j.welch@hw.ac.uk, s.a.macgregor@hw.ac.uk; Fax: +44 131 451 3180/+ 44 131 451 3180; Tel: +44 131 451 3217/+44 131 451 8031 w Electronic supplementary information (ESI) available: Experimental procedures; views of all structures; computational details. CCDC 688279–688284. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b810702e This journal is  c The Royal Society of Chemistry 2008 Chem. Commun., 2008, 5345–5347 | 5345 COMMUNICATION www.rsc.org/chemcomm | ChemComm Published on 19 September 2008. 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