Chemistry of C-trimethylsilyl-substituted heterocarboranes. 26. Further investigation of oxidative cage closure, cage fusion, and cage isomerizations: Synthetic, structural, and bonding studies on "carbons adjacent" and "carbons apart" tetracarba-nido-dodecaborane(12) derivatives

A number of tetracarbon carboranes of the general form (SiMe3)(2)(R)(2)C4B8 H8 (R = SiMe3 (III and IV) Me (XIII and XIV), n-Bu (VII, XI, and XII), t-Bu (VIII)) were synthesized by the reactions of the corresponding mono- or di anions of nido-2-(SiMe3)-3-(R)-2,3-C2B4H6 with NiCl2 in n-hexane. The nido- carboranes where R = n-Bu (I), t-Bu (II) are newly reported and were obtain ed from the reaction of Me3SiC=CR with B5H9. The tetracarbon compounds were minor products of the reactions, with yields ranging from 13% for VIII to 37% for XI; the major products were the corresponding closo-1-(SiMe3)-2-(R) -1,2-C2B4H4 compounds; which were obtained in yields from 69% (R = t-Bu (VI )) to 32% (R =SiMe3). The closo-carboranes where R = n-Bu(V), t-Bu (VI) cou ld be converted to the more stable closo-1-(SiMe3)-6-(R)-1,6-C2B4H4 (R = n- Bu (IX), t-Bu(X)) by heating to 250-255 degrees C for 4-5 h; this agrees wi th previous studies on the compounds where R = SiMes(3), Me. To aid in cage structure determination a monobrominated, tetracarbon carborane, XV, was p repared from the reaction of VII with Br-2 in n-hexane, in 46% yield. All c ompounds were characterized by their H-1, B-11, and C-13 NMR spectra and el emental analyses and by the following: I-XIV by their IR spectra; III, IV, VII-X, XIII, and XIV by mass spectrometry; and III, IV, XI, XII, and XV by single-crystal X-ray crystallography. Depending on the structures of the C4 B8 cages, the compounds could be classified as having "carbons apart" struc tures, in which all cage carbons were separated by at least one boron atom (III, IV,VII, VIII, XIV, and XV), or "carbons adjacent" structures, in whic h the carbons were localized on the same side of the clusters (XI-XIII). Co mpound III is unique in that its structure is that of a distorted cuboctahe dron, while IV and XV are nido-carboranes having a C3B3 face that surmounts a Bg ring and an apical cage carbon atom; none of the rings are planar, an d the apical carbon is unequally bonded to the B-5 ring atoms. Similarities in their NMR spectra indicate that VII and VIII have structures similar to IV and XV. Two types of "carbons adjacent" cages were identified: one typi fied by XI, in which the four cage carbons are bonded contiguously, and a m ore open cage, in which the bond between the middle two carbons is broken ( MI). All of the C4B8 compounds were found to have nonrigid stereochemistrie s; both the 11B and 13C NMR spectra of the "carbons apart'' compounds consi sted of single resonance peaks, indicating rapid atom equilibration on the NMR time scale. The "carbons adjacent" compounds showed B-11 NMR spectra of six resonance peaks; each had a different chemical shift and peak-area rat io pattern. The 11B NMR spectra of the "carbons apart" isomers could be suc cessfully accounted for by using GIAO ab initio molecular orbital chemical shift calculations at the B3LYP/6-311G** level of theory on the B3LYP/6-31G * optimized geometries of a set of model compounds, III', IV', and XV', whi ch had H's as substituents on the cage carbons. However, neither similar ca lculations on the "carbons adjacent" carboranes nor comparisons with the sp ectra of carboranes having similar structures could successfully account fo r the B-11 NMR spectra of XI-XIII. It was speculated that the presence of two different substituents on the ca ge carbons of these compounds gives rise in solution to multiple isomers th at would greatly complicate their NMR spectra.