Article pubs.acs.org/Organometallics
Synthesis and Characterization of 14-Vertex Carboranes Jian Zhang, Fangrui Zheng, and Zuowei Xie* Department of Chemistry and State Key Laboratory of Synthetic Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, People’s Republic of China S Supporting Information *
ABSTRACT: Twelve new 14-vertex closo-carboranes, including CAd (carbon atoms adjacent), CAp (carbon atoms apart), and cage B-substituted species, were synthesized and structurally characterized via either a [13 + 1] or a [12 + 2] polyhedral expansion protocol. Treatment of various 13-vertex nido-carborane dianionic salts with HBBr2 gave the corresponding 14-vertex carboranes. Mono-B-substituted 14-vertex carboranes 1-R-2,3-(CH2)3-2,3-C2B12H11 (R = Ph, Cl, Br, I) were prepared from the reaction of nido-[8,9-(CH2)3-2,3-C2B11H11]2− with the dihaloborane reagents RBX2. CAd 14-vertex closocarboranes were also synthesized by treatment of 12-vertex arachno-carborane tetraanionic salts with HBBr2 reagent. These new 14-vertex closo-carboranes were fully characterized by 1H, 13C, and 11B NMR spectroscopy as well as high-resolution mass spectrometry. Their structures were further confirmed by single-crystal X-ray analyses.
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INTRODUCTION The chemistry of 12-vertex o-carborane has been extensively studied owing to its commercial availability1 and applications in medicine as boron neutron capture therapy agents,2 in supramolecular design as building blocks,3 and in coordination/ organometallic chemistry as ligands.1,4 In general, o-carboranes can be prepared through the reaction of alkynes with B10H12·L2 (L = Lewis base).5 The icosahedral framework can also be constructed via a polyhedral expansion methodology,6 from the reaction of 11-vertex nido-carborane dianions with dihaloborane reagents.7 Such a methodology has been successfully extended to the synthesis of supercarboranes.8 Accordingly, 12 13-vertex closocarboranes9−15 and 2 CAd (carbon atoms adjacent) 14-vertex closo-carborane isomers10,12 have been prepared and structurally characterized using CAd nido-carborane anions as starting materials.16 These supercarboranes show interesting properties. They share some chemical properties with those of icosahedral species; on the other hand, they have their own unique characteristics. For example, the 13-vertex carborane 1,2-(CH2)3-1,2-C2B11H11 (1) can accept either two electrons from group 1 metals to form a nido-carborane dianion10,11 or a single electron to give a stable carborane radical anion with [2n + 3] framework electrons,17 whereas 12-vertex carboranes can undergo 2e or 4e reduction to yield nido-16,18 or arachnocarborane tetraanions.16a,c,d,19 Compound 1 can undergo αdeprotonation of the methylene chain, giving the monoanion [1,2-CH(CH2)2-1,2-C2B11H11]− with exo CC π bonding,15 while this is not feasible for the corresponding 12-vertex species.20 In addition, unlike 12-vertex carboranes, 1 reacts with various nucleophiles to give the cage boron and/or carbon extrusion products closo-CB11−, nido-CB10−, closo-CB10−, and closo-C2B10, depending on the nature of the nucleophiles.21−23 © 2013 American Chemical Society
In contrast, the knowledge about 14-vertex carboranes is very limited, due partially to the existence of only two known examples of 14-vertex carborane isomers, 2,3-(CH2)3-2,3C2B12H12 and 2,8-(CH2)3-2,8-C2B12H12. In this connection, we expanded the synthetic scope to include carbon atoms apart (CAp) and cage-boron-substituted 14-vertex carboranes. These results are reported in this article.
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RESULTS AND DISCUSSION Synthesis of 13-Vertex nido-Carborane Dianions. 13-Vertex nido-carborane dianions serve as crucial synthons for the preparation of 14-vertex closo-carboranes.10 Following the synthetic motif of [1,2-(CH2)3-1,2-C2B11H11][Na2(THF)4] (6) and [1,2-C6H4(CH2)2-1,2-C2B11H11][Na2(THF)4] (9), prepared from the corresponding closo species 1,2-(CH2)31,2-C2B11H11 (1) and 1,2-C6H4(CH2)2-1,2-C2B11H11 (4),11 treatment of the CAd 13-vertex carboranes 1,2-Me2Si(CH2)21,2-C2B11H11 (2), 1,2-(CH2)4-1,2-C2B11H11 (3), and 1,2-Me21,2-C2B11H11 (5a) and CAp 13-vertex carborane 1,6-Me2-1,6C2B11H11 (5b) with excess Na metal in THF gave, after recrystallization, the corresponding 13-vertex nido-carborane salts [1,2-Me2Si(CH2)2-1,2-C2B11H11][Na2(THF)4] (7), [1,2(CH 2 ) 4 -1,2-C 2 B 11 H 11 ][Na 2 (THF) 4 ] (8), [1,2-Me 2 -1,2C2B11H11][Na2(THF)4] (10a), and [1,3-Me2-1,3-C2B11H11][Na2(THF)4] (10b) as colorless crystals in 80−90% isolated yields (Scheme 1). The CAd 13-vertex nido-carborane salts 7, 8, and 10a exhibited very similar 11B NMR spectra with peaks at about −10, −15, and −26 ppm in an intensity ratio of 1:5:5, regardless of the substituents on the cage carbons, which was similar to the peaks of 6 and 9 reported in the literature.11 Received: September 16, 2013 Published: November 6, 2013 7399
dx.doi.org/10.1021/om4009235 | Organometallics 2013, 32, 7399−7406
Organometallics
Article
Scheme 1. Synthesis of 13-Vertex nido-Carborane Dianionic Salts
Figure 1. Molecular structure of the anion in [1,2-Me2Si(CH2)2-1,2C2B11H11][Na2(THF)4] (7). Selected bond distances (Å) and angles (deg): C1−C1A = 1.568(6), C1−B2 = 1.562(4), B2−B5 = 1.863(5); C1A−C1−B2 = 109.8(2), C1−B2−B5 = 109.6(2), B2−B5−B2A = 89.7(3).
The high-field boron signals indicated the high charge density of nido-carborane dianions after taking up 2e. A similar phenomenon was also observed in the 13C NMR spectra, in which the cage carbons were shifted significantly upfield (
