High‐Yield Synthesis of a Hybrid 2,3,4,5‐Tetracarba‐1,6‐nido‐hexaborane(6) Cluster with an exo‐Polyhedral Boracycle

Abstract

Dedicated to Prof. Dr. Hansgeorg Schnçckel on the occasion of his 70th birthdaySince the pioneering work of Stock et al., [1] the chemistry of boron hydrides has evolved into a fascinating area comprising different classes of compounds such as organoboranes, polyhedral boranes, and carborane clusters, to name only a few. [2][3][4][5] In polyhedral carborane chemistry, several approaches towards the construction of clusters have been developed. [6, 7] For example, i) cage or polyhedron growth by carbon or boron insertion, [8] ii) intercluster fusion, with two or more atoms connecting the individual subclusters, [9] and iii) selective cage degradation and cage coupling.[10] Even though several reports on the synthesis of carboranes and their derivatives have appeared in the literature, the number of carborane clusters prepared by organoboranes still remains rather limited. With the appropriate choice of the substituents at boron, which allows a selective fine-tuning of the electronic and/or steric properties, small organoboranes can serve as useful and versatile starting materials for the generation of carborane clusters.[11] However, these protocols tend to provide the targeted species only in moderate yields and with limited selectivity, experimental drawbacks that have been observed so far when novel cluster types are to be generated.As part of our ongoing studies on boroles and their derivatives, [12][13][14] we recently described the unusual dimerization/ rearrangement of 1-chloro-2,3,4,5-tetraphenylborole at slightly elevated temperatures (40 8C) to afford the [4.5]decatriene derivative 1.[15] Since boroles still represent a rather uncommon class of main group element species with only few structurally characterized examples, we became interested in the reactivity of 1 towards different reducing agents, which might be useful in the generation of new structural motifs.[16] Herein, we report the synthesis and structural characterization of a novel 2,3,4,5-tetracarba-1,6-nido-hexaborane(6) derivative, 2, which was obtained from the reaction of 1 and an equimolar amount of dilithio-1,2,3,4-tetraphenylbuta-1,3-diene.[17] The reaction proceeded with high selectivity, and the exocyclic nido-carborane derivative 2 was isolated in excellent yield (Scheme 1). The nature of the carbaborane body in 2 was deduced from 11 B NMR spectroscopy in solution, which revealed two distinct resonances at d = À40.3 ppm and d = 13.4 ppm in the typical range for 2,3,4,5-tetracarba-1,6-nido-hexaboranes(6) (d = À51 to À37 ppm and d = 9 to 28 ppm) for apical and basal boron atoms, respectively. [18, 19] The 11 B NMR chemical shifts of 2 proved temperature independent in the inspected range between 75 and À80 8C. The number of signals determined by 13 C NMR spectroscopy indicates rotational hindrance of several phenyl groups around the cluster framework, which results in chemical inequivalence of the respective phenyl carbon atoms and concomitant line broadening of some resonances.Single crystals of 2 suitable for X-ray diffraction anal...

A theoretical study on the stability difference of the borane BnHn2− and carborane C2Bn−2Hn(5 ≤ n ≤ 7) clusters

“…By the way, it should be mentioned that the relative stability of various isomers of carborane C 2 B n −2 H n (5 ≤ n ≤ 7) had ever been studied by Williams, Jemmis, Gimarc, and McKee . The geometries of (5 ≤ n ≤ 7), 1,5‐C 2 B 3 H 5 , 1,6‐C 2 B 4 H 6 , and 1,7‐C 2 B 5 H 7 displayed in Figure are optimized and confirmed by vibration frequency analysis at B3LYP/def2‐TZVPP level with Gaussian 09 package, where the B3LYP/def2‐TZVPP method is always used as structure optimization method for boranes and borane analogs . Based on the geometry optimization results, the precise frontier orbital energy levels E SCF (∞) are extrapolated according to Eq.…”

Oligo(borolyl)benzenes—Synthesis and Properties

“…Oxidative addition reactions of haloboroles to transition‐metal complexes, as well as the reduction of a carbene‐stabilized haloborole leading to a nucleophilic π‐boryl anion, have received considerable attention 8g. Other new reactivity patterns, such as the spontaneous conversion of haloboroles to spiro compounds and the additional reduction‐induced rearrangement to nido‐carborane clusters, have been reported recently 8h,k. The substituents on the butadiene backbone have been limited to phenyl, pentafluorophenyl, tolyl, and, as recently reported, thienyl groups 6.…”

Recent developments in the chemistry of antiaromatic boroles

“…46 Reductive treatment of 42 with either 1,4-dilithio-1,2,3,4tetraphenylbutadiene or magnesium anthracene enabled the selective generation of the unusual hybrid 2,3,4,5-tetracarba-1,6-nido-hexaborane(6) cluster 44 featuring an exo-polyhedral boracycle in excellent yield. 48 Even though nido-C 4 B 2 clusters have been known for more than 40 years, this experimental approach and the uncommon structural motif of a bridging 1,3-butadiene fragment between B apical and C basal of the perphenylated cluster framework are noteworthy and have been without precedence in the carborane chemistry.…”