Article pubs.acs.org/accounts
Synthesis, Structure, and Reactivity of 13- and 14-Vertex Carboranes Jian Zhang 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 CONSPECTUS: Carboranes are a class of polyhedral boron hydride clusters in which one or more of the BH vertices are replaced by CH units. Their chemistry has been dominated by 12-vertex carboranes for over half a century. In contrast, knowledge regarding supercarboranes (carboranes with more than 12 vertices) had been limited merely to possible cage geometries predicted by theoretical work before 2003. Only in recent years has significant progress been made in synthesizing supercarboranes. Such a breakthrough relied on the use of Carbon-Atoms-Adjacent (CAd) nido-carborane dianions or arachno-carborane tetraanions as starting materials. In this Account, we describe our work on constructing and elucidating the chemistry of supercarboranes. Earlier attempted insertions of the formal [BR]2+ unit into Carbon-Atoms-Apart (CAp) 12-vertex nido-[7,9-C2B10H12]2− did not produce the desired 13-vertex carboranes. Such failure is often attributed to the extraordinary stability of the B12 icosahedron (the “icosahedral barrier”). However, the difference in reducing power between CAp and CAd 12-vertex nido-carborane dianions had been overlooked. Our results have shown that CAd nido-carborane dianions are weaker reducing agents than the CAp isomers, allowing a capitation to prevail over a redox reactivity. This finding provides an entry point to the synthesis of supercarboranes and a series of 13- and 14-vertex closo-carboranes have been prepared and structurally characterized. They share some chemical properties with those of 12-vertex carboranes; on the other hand, they have their own unique characteristics. For example, a 13vertex closo-carborane can undergo single electron reduction to give a stable carborane radical anion with [2n + 3] framework electrons, which can accept one additional electron to form a 13-vertex CAd nido-carborane dianion. 13-Vertex closo-carborane can also react with various nucleophiles to afford the cage carbon and/or boron extrusion products closo-CB11−, nido-CB10−, closoCB10−, and closo-C2B10, depending on the nature of the nucleophiles. Studies of supercarboranes remain a relative young research area, particularly in comparison to the rich literature of icosahedral carboranes with 12-vertices. Other supercarboranes are expected to be prepared and structurally characterized as the field progresses, and the results detailed here will further these efforts.
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insert boron is often attributed to “icosahedral barrier” (the extraordinary stability of the B12 icosahedron).2 We revisited the aforementioned reactions and found that 12-vertex CAp nido-[7,9-C2B10H12]2− is a very strong reducing agent, whereas 11-vertex CAd nido-[7,8-C2B9H11]2− is almost redox inactive, which is supported by literature work.4 Such a dramatic difference in reducing power between isomers of nidocarborane dianions had been largely overlooked. With this in mind, we speculated that there are two competitive reactions between CAp nido-[7,9-C2B10H12]2− and RBX2: capitation versus redox reaction. The former leads to the formation of a 13-vertex carborane C2B11H12(R), while the latter gives oC 2 B 10 H 12 , the starting material. In other words, the experimentally observed o-C2B10H12 in the above reactions appears to be the result of a redox reaction, rather than the degradation of the expected 13-vertex carborane by losing a BH vertex.2 If this is true, lowering the reducing power of nido[R2C2B10H10]2− dianions should promote the capitation, leading to the formation of the desired supercarboranes.
INTRODUCTION
Carboranes are a class of polyhedral boron hydride clusters in which one or more of the BH vertices are replaced by CH units. They constitute a class of structurally unique molecules with exceptionally thermal and chemical stabilities and the ability to hold various substituents. These properties have made them useful basic units in supramolecular design, medicine, catalysts and materials.1 Their chemistry has been dominated by icosahedral carboranes with 12 vertices for over half a century.1 On the other hand, carboranes with more than 12 vertices (supercarboranes), although studied theoretically, were experimentally unknown until 2003.2 Many attempts to prepare 13-vertex carboranes using “polyhedral expansion” methodology3 by insertion of [BR]2+ (R = alkyl, H, halide) into a 12-vertex Carbon-Atoms-Apart (CAp) nido-[7,9-C2B10H12]2− failed,2 although this method works well in the reconstruction of o-carboranes via reaction of 11-vertex Carbon-Atoms-Adjacent (CAd) nido-[7,8-C2B9H11]2− with RBX2.1a It also works well in the synthesis of a series of 13vertex metallacarboranes via the insertion of a metal fragment into a 12-vertex CAp nido-[7,9-C2B10H12]2−.1a The failure to © 2014 American Chemical Society
Received: March 1, 2014 Published: April 29, 2014 1623
dx.doi.org/10.1021/ar500091h | Acc. Chem. Res. 2014, 47, 1623−1633
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Article
Electrochemical studies show that the redox potentials of carboranes depend upon the relative positions of the two cage carbons atoms.5 The easiest way to control the relative positions of such cage carbons during the cage-opening process is to introduce an appropriate linker between two cage carbon atoms. To this end, we developed a method to prepare three regioisomers of nido-[R2C2B10H10]2− in a controlled manner, in which the two cage carbons are in 7,8-, 7,9-, and 7,10-positions, respectively.6 Reactivity studies on these isomers show that CAd nido-[7,8-R2C2B10H10]2− has the weakest reducing power among them. This finding offered a critical entry point into the synthesis of supercarboranes.7 Since then, significant progress has been made in this research area.7,8 This Account summarizes these recent developments in the synthesis, structure, and reactivity of 13- and 14-vertex carboranes. Their numbering systems and color codes are illustrated in Chart 1.
Figure 1. Molecular structure of 1,2-(CH2)3-1,2-C2B11H11.
Chart 1. Numbering System and Color Code in 13- and 14Vertex closo-Clusters
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observed in other carboranes (Table 1), and diagnostically, the C NMR signals of cage carbons are significantly downfield shifted to about 140 ppm (Table 2). Most of CAd 13-vertex closo-carboranes without B-substituents exhibit similar 11B NMR spectra spanning a narrow range 5 to −2 ppm in a 1:5:5 pattern (Figure 2). 13-Vertex closo-carboranes without a C,C′-linkage were also prepared. 1,2-Me2Si(CH2)2-1,2-C2B11H11 undergoes facile desilylation on silica gel to afford 1,2-Me2-1,2-C2B11H11.10 It does not degrade to a 12-vertex carborane in solution but isomerizes upon heating to its CAp isomer, 1,6-Me2-1,6C2B11H11 (Scheme 2). This result suggests that the C,C′linkage does not have any obvious effects on the stability of 13vertex closo-carboranes and that the CAp isomer is thermodynamically more stable than its CAd one. These results support our earlier assumption that the reducing power of nido-carborane dianions plays a critical role in the synthesis of supercarboranes. The role of C,C′-linkage is to lock the two cage carbon atoms in ortho-positions during the reduction, leading to the formation of CAd nido-carborane dianions with lower reducing power, which facilitates the capitation reaction to yield 13-vertex closo-carboranes. 13
13-VERTEX CARBORANES
Synthesis and Characterization
13-Vertex closo-carboranes can be prepared by reaction of CAd 12-vertex nido-carborane dianions with dihaloboranes via a [12 + 1] protocol. Treatment of [7,8-(CH2)3-7,8-C2B10H10][Na2(THF)x], [7,8-Me2Si(CH2)2-7,8-C2B10H10][Na2(THF)x], or [7,8-(CH2)4-7,8-C2B10H10][Na2(THF)x] with HBBr2·SMe2 in toluene afforded CAd 13-vertex closo-carboranes 1,2-(CH2)31,2-C2B11H11,9 1,2-Me2Si(CH2)2−1,2-C2B11H11,10 or 1,2(CH2)4-1,2-C2B11H1111 in about 40% isolated yields (Scheme 1).
Electrophilic Substitution
Treatment of 1,2-(CH2)3-1,2-C2B11H11 with excess MeI, Br2, or I2 in the presence of AlCl3 generated hexa-substituted 13-vertex closo-carboranes 8,9,10,11,12,13-X6-1,2-(CH2)3-1,2-C2B11H5 (X = Me, Br, I; Scheme 3).9 Similar to the 12-vertex carboranes, the substituted B(H) vertices are those farthest from the cage carbons, that is, the most electron-rich. These B-substituted species have similar structural features to the parent 13-vertex closo-carborane, but the substituents significantly influence the cage boron chemical shifts due to electronic effects (Figure 3).
Scheme 1. Synthesis of CAd 13-Vertex closo-Carboranes
One-Electron Reduction
Carboranes with odd skeletal electron counts are very rare. The only characterized example is neutral radical CB11Me12· with [2n + 1] framework electrons.13 We found that 13-vertex carborane radical anion [{1,2-(CH2)3-1,2-C2B11H11}·][Na(18crown-6)(THF)2] could be isolated in 80% yield as brown crystals from the reaction of 1,2-(CH2)3-1,2-C2B11H11 with 1 equiv of Na metal in THF, followed by recrystallization in the presence of 18-crown-6 (Scheme 4).14 It has a very similar cage structure to that of its parent species but with elongated bond lengths. This radical species exhibits an EPR signal with g = 1.994 at room temperature, which is similar to that observed in
Mono-B-substituted 13-vertex closo-carboranes 3-R-1,2(CH 2 ) 3 -1,2-C 2 B 1 1 H 1 0 [R = Ph, (Z)-EtCHC(Et), (E)-tBuCHCH],9 3-Ph-1,2-(CH2)4-1,2-C2B11H10,11 and 3Ph-1,2-o-C6H4(CH2)2-1,2-C2B11H1012 were synthesized in the same manner using RBX2 (X = Cl, Br) as capping agents. CAd 13-vertex closo-carboranes have a henicosahedral cage geometry with one trapezoidal C2B2 open face, and thus, the cage C atoms are less connected. A typical structure is shown in Figure 1. The Ccage−Ccage distances are much shorter than those 1624
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Table 1. Cage C−C Bond Lengths (Å) in CAd Carboranes R2C2BnHn (n = 11, 12) 14-vertex closoR2
13-vertex closo-
13-vertex nido-
2,3-isomer
2,8-isomer
(CH2)3 Me2Si(CH2)2 (CH2)4 o-C6H4(CH2)2 Me2
1.421(3) 1.439(3) 1.425(4) 1.427(2) 1.421(5)
1.529(8) 1.568(6)
1.608(4) 1.648(3)
1.599(3) 1.663(5) 1.622(3)
1.553(5) 1.556(11)
1.594(4)
Table 2. 13C Chemical Shifts of Cage Carbons in Carboranes R2C2BnHn (n = 10−12)a
a
R2
12-vertex closo-
13-vertex closo-
13-vertex nidob
(CH2)3 Me2Si(CH2)2 (CH2)4 o-C6H4(CH2)2 Me2
84.0 87.5 73.2 71.2 73.5
136.4 144.5 142.5 138.9 140.7 (1,2-) 120.6, 83.2 (1,6-)
16.0 11.2 10.3 9.2 7.3 (1,2-) 44.4, 35.7 (1,6-)
14-vertex closo72.3 76.2 66.0 65.0
(2,3-) (2,3-) (2,3-) (2,3-)
63.7 (2,4-)
90.0 (2,8-) 90.9 (2,8-) 78.4 (2,8-) 77.7 (2,8-)c 78.6 (2,9-)
In CDCl3, in units of ppm. Numbers in parentheses refer to the cage carbon positions of the compounds. bIn d5-pyridine. cIn CD2Cl2.
Scheme 4. One-Electron Reduction of 1,2-(CH2)3-1,2C2B11H11
CB11Me12·.13 It can be viewed as [2n + 3] system that is an intermediate between the two well-established and abundant [2n + 2] and [2n + 4] systems. Cyclic voltammetry of 1,2(CH2)3-1,2-C2B11H11 shows stepwise two-electron reduction, with the first wave reversible and the second quasi-reversible (Figure 4). It is noteworthy that the corresponding 12-vertex
Figure 2. Stick presentation of chemical shifts and relative intensities of 11B{1H} spectra of (CH2)3C2BnHn (n = 10−12).
Scheme 2. Synthesis of CAd and CAp 13-Vertex closoCarboranes without C,C′-Linkage
Scheme 3. Electrophilic Substitution Reaction of 1,2-(CH2)31,2-C2B11H11
Figure 4. Cyclic voltammetry of 1,2-(CH2)3-1,2-C2B11H11. Reproduced with permission from ref 14. Copyright 2007 American Chemical Society
carborane 1,2-(CH2)3-1,2-C2B10H10 only undergoes a simultaneous two-electron reduction to give nido[(CH2)3C2B10H10]2−.15 These results suggest that larger cages may enhance the stability of clusters with [2n+3] framework electrons. Two-Electron Reduction
Figure 3. Stick presentation of chemical shifts and relative intensities of 11B{1H} spectra of 8,9,10,11,12,13-X6-1,2-(CH2)3-1,2-C2B11H5. The solid and hollow lines represent the BH and BX vertices, respectively.
13-Vertex closo-carboranes are readily reduced to the corresponding nido-carborane dianions by excess group 1 1625
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metals. Treatment of 1,2-(CH2)3-1,2-C2B11H11, 3-Ph-1,2(CH2)3-1,2-C2B11H10, 1,2-Me2Si(CH2)2-1,2-C2B11H11, 1,2(CH2)4-1,2-C2B11H11, 1,2-o-C6H4(CH2)2-1,2-C2B11H11, or 1,2Me2-1,2-C2B11H11 with excess Na metal in THF afforded 13vertex [1,2-(CH2)3-1,2-C2B11H11][Na2(THF)4],9 [3-Ph-1,2(CH2)3-1,2-C2B11H10][Na2(THF)4],9 [1,2-Me2Si(CH2)2-1,2C 2 B 11 H 11 ][Na 2 (THF) 4 ], 16 [1,2-(CH 2 ) 4 -1,2-C 2 B 11 H 11 ][Na 2 (THF) 4 ], 1 6 [1,2-o-C 6 H 4 (CH 2 ) 2 −1,2-C 2 B 1 1 H 1 1 ][Na2(THF)4],9 or [1,2-Me2-1,2-C2B11H11][Na2(THF)4],16 respectively, in high yields (Scheme 5).
Scheme 7. Reduction of 1,2-(CH2)3-1,2-C2B11H11 by Li, Mg and Ca Metal
Scheme 5. Reduction of CAd 13-Vertex Carboranes by Na Metal −10, −15, and −26 ppm in an intensity ratio of 1:5:5. These high-field shifted signals indicate the increased charge density of the cage after taking up two electrons. This phenomenon is also reflected in the upfield-shifted cage-carbon signals (
