Synthesis and Structural Characterization of Carbene-Stabilized

Aug 12, 2016 - The carbene-stabilized electron-rich iminocarboranyl boron(I) compound [(Dipp)NC(But)C2B10H10]B(NHC) (1) (Dipp = 2,6-diisopropylphenyl,...
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Synthesis and Structural Characterization of Carbene-Stabilized Carborane-Fused Azaborolyl Radical Cation and Dicarbollyl-Fused Azaborole Hao Wang,† Jiji Zhang,† Zhenyang Lin,*,‡ and Zuowei Xie*,†,§ †

Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, People’s Republic of China Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China § State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: The carbene-stabilized electron-rich iminocarboranyl boron(I) compound [(Dipp)NC(But)C2B10H10]B(NHC) (1) (Dipp = 2,6diisopropylphenyl, NHC = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) is an excellent precursor for the preparation of the unprecedented carborane-fused azaborolyl radical cation [{(Dipp)NC(But)C2B10H10}B(NHC)]•+ ([2]•+) and dicarbollyl-fused azaborole 7,8-nido-C2B9H9-7,8C(But)N(Dipp)B(NHC) (3) via controlled single-electron oxidation and two-electron oxidative deboration, respectively. Singlecrystal X-ray analyses and DFT calculations indicate that the unpaired electron in [2]•+ is delocalized over the BNC unit, and the two fused five-membered rings in 3 form a sort of π-conjugated system.

B

Chart 2. Representative Examples of Lewis Base Stabilized Cationic Boron Radicals

oron-centered radicals have recently received much attention, which has not only promoted a broader understanding of boron chemistry but also led to wide applications of these radicals in synthetic chemistry.1 Among these, anionic boron radicals have been the most studied (Chart 1), which are generally obtained from the chemical reduction of Chart 1. Boron-Centered Radicals

containing radicals,31 the corresponding boron-containing heterocyclic radical cations remain elusive thus far. Very recently, we reported the preparation of the carbenestabilized iminocarboranyl boron(I) compound [(Dipp)NC(But)C2B10H10]B(NHC) (1; Dipp = 2,6-diisopropylphenyl, NHC = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene), which underwent 2e− oxidation with AgOAc to give the corresponding trivalent boron compound [(Dipp)NC(But )C 2B10 H10 ]B(OAc)2.32 We wondered whether this reaction could proceed via stepwise two-electron oxidation, involving an azaborolyl radical cation intermediate. With this in mind, we carried out a controlled single-electron oxidation of 1 and two-electron oxidative deboration of 1, leading to the preparation and structural characterization of unprecedented examples of a carborane-fused azaborolyl radical cation and a dicarbollyl-fused azaborole. These results are reported in this communication. The oxidation of 1 was first assessed electrochemically. Cyclic voltammetry of a THF solution of 1 (scan rate 50 mV/s, supporting electrolyte 0.1 M Bu4NPF6) showed stepwise twoelectron oxidation, with the first wave being reversible at E1/2 =

boranes or diboranes.2−17 Subsequently, several examples of Lewis base stabilized neutral boron radicals have been prepared and structurally characterized via the reduction of haloboranes or frustrated Lewis pair hydrogenation.18−27 On the other hand, cationic boron radicals have been much less studied due to the electron-deficient nature of boron. More recent reports have shown that Lewis base stabilized borylenes and diborenes are proper precursors for the synthesis of these species.15,28−30 Representative examples are compiled in Chart 2. All isolated cationic boron radicals are carbene- or phosphinestabilized species that are prepared from either one-electron oxidation of the corresponding Lewis base stabilized electronrich borylenes and diborenes15,28,30 or single-electron reduction of the cationic borane (type II in Chart 2).29 Though considerable progress has been recently made in the preparation and structural characterization of boron radicals and boroncontaining heterocyclic neutral radicals, as well as carborane© XXXX American Chemical Society

Received: July 6, 2016

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DOI: 10.1021/acs.organomet.6b00545 Organometallics XXXX, XXX, XXX−XXX

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Organometallics −1.09 V (versus Fc/Fc+) and the second irreversible at a potential of 0.46 V. This measured value of the first oxidation potential is comparable to that observed in (CAAC)2BH (E1/2 = −0.94 V); CAAC = (alkyl)(amino)carbene)28 and bisphosphinestabilized diborene (E1/2 = −1.05 V)30 but is more positive than that found in biscarbene-stabilized diborenes (E1/2 = −1.55 to −1.95 V).15,30 These data indicate that 1 may be a good neutral reductant and potential precursor for the generation of a boroncontaining (azaborolyl) radical cation, which was subsequently confirmed by the reaction of 1 with CuI or CuCl. Treatment of 1 with an excess amount of CuI in THF at room temperature gave the NMR-silent compound [2]2[Cu2I4] as dark red crystals in 39% isolated yield. Similarly, another NMR-silent compound [2][CuCl2] was isolated as dark red crystals in 44% isolated yield via the reaction of 1 with an excess amount of CuCl in THF (Scheme 1). It was noteworthy that both [2]2[Cu2I4] and [2][CuCl2] were fully converted back to 1 by the reaction with K[CpFe(CO)2] in THF, as confirmed by 11B NMR spectra.

Figure 1. Molecular structure of the radical cation [2]•+ in [2][CuCl2]. Selected bond lengths (Å): C(1)−C(2) 1.638(4), C(1)−C(11) 1.502(4), N(1)−C(11) 1.426(4), N(1)−B(13) 1.426(4), C(2)− B(13) 1.578(5), C(28)−B(13) 1.585(5).

reasonable that the bond distances in cationic species are slightly longer than those in their neutral counterpart. The B(13)−N(1) distances of 1.419(14) Å in [2]2[Cu2I4] and 1.426(4) Å in [2][CuCl2] are also comparable to those (1.40−1.48 Å) observed in boron heterocyclic compounds.21,33 To understand the electronic structure of the radical cation [2]•+, density functional theory (DFT) calculations at the B3LYP-D3/6-31G(d,p) level of theory were performed. The total spin density distribution and the corresponding singly occupied molecular orbital (SOMO) together with the LUMO (the lowest unoccupied molecular orbital) are shown in Figures 2

Scheme 1. Synthesis of Azaborolyl Radical Cation and Its Resonance Forms

Figure 2. Spin density plot of the radical cation [2]•+ calculated at the B3LYP-D3/6-31G(d,p) level of theory.

and 3, respectively. The spin population is mainly localized at the C(11) atom (∼73% spin density) and the B(13) atom (∼25% spin density). The SOMO and LUMO orbitals display features observed in those of 1 and the allyl unit, suggesting that one

The UV−vis spectra of [2]2[Cu2I4] and [2][CuCl2] displayed an intense absorption band centered at λmax 517 nm for [2]2[Cu2I4] and 514 nm for [2][CuCl2], which were shifted hypsochromically in comparison to their neutral parent compound 1 (λmax 577 nm). Their THF solutions exhibited EPR signals with g = 1.99 (line width 17 G) for [2]2[Cu2I4] and g = 1.99 (line width 16 G) for [2][CuCl2], respectively, at room temperature. The molecular structures of both [2]2[Cu2I4] and [2][CuCl2] have been confirmed by single-crystal X-ray analyses. They consist of well-separated, alternating layers of the discrete radical cations [2]•+ and the complex anion [Cu2I4]2− or [CuCl2]−. In the radical cation [2]•+, the exo boron atom is bound to one cage carbon, one nitrogen atom, and one carbene in a trigonal-planar geometry (Figure 1), and the resulting C(1)C(2)B(13)N(1)C(11) five-membered ring is coplanar, similar to that of azaborole. The dihedral angles of this plane and NHC plane are 67.4° in [2]2[Cu2I4] and 67.9° in [2][CuCl2], which are very close to that of 65.3° observed in 1.32 These data strongly suggest that NHC serves solely as a σ donor, not a π acceptor. The bond distances in the five-membered plane C(1)C(2)B(13)N(1)C(11) in [2]2[Cu2I4] and [2][CuCl2] are very close to each other (see the Supporting Information), which are barely longer than the corresponding values observed in 1. It is

Figure 3. Plot of frontier molecular orbitals for the radical cation [2]•+ calculated at the B3LYP-D3/6-31G(d,p) level of theory. B

DOI: 10.1021/acs.organomet.6b00545 Organometallics XXXX, XXX, XXX−XXX

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Organometallics electron is removed from the 4π BNC-allyl unit in 1 to form the corresponding radical cation, and such a radical cation is stabilized via delocalization over the BNC moiety.31a This argument is also supported by the changes in the atomic charges calculated for the C(11), N(1), and B(13) ring atoms from the NBO (natural bond orbital) analyses (see the Supporting Information for details). Compounds [2]2[Cu2I4] and [2][CuCl2] represent the first examples of azaborolyl radical cationic salts. On the other hand, we wondered if 1 could be oxidized by elemental sulfur via two-electron oxidation to give “BS”containing compounds.34,35 To this end, treatment of 1 with 0.25 equiv of elemental sulfur in THF at room temperature afforded the unprecedented zwitterionic salt 3 as orange-red crystals in 31% yield (Scheme 2). The 11B NMR spectrum of the cage

(dihedral angle 64.4°), again indicating that NHC serves solely as a σ donor. The above structural features are confirmed by DFT calculations at the B3LYP-D3/6-31G(d,p) level of theory. The HOMO (highest occupied molecular orbital) clearly shows the π conjugation between two fused five-membered rings (Figure 5).

Scheme 2. Reaction of 1 with Elemental Sulfur

Figure 5. Plot of the HOMO (−4.64 eV) of 3 calculated at the B3LYPD3/6-31G(d,p) level of theory.

NBO analyses indicate a significant enhancement of B(12)− C(2)/C(1)−C(11)/C(1)−C(2) bond strength in comparison with those observed in [2]•+. Compound 3 can be viewed as a zwitterionic salt in which the C3BN ring formally bears a charge of +2 and the C2B3 face is formally dinegative. Such a dicarbollylfused azaborole has never been reported before. Compound 3 may be formed via 2e− oxidation of 1 by S8, with the formation of S2−, followed by nucleophilic attack of S2− on the most electrophilic cage B(3) (deboration reaction). The nucleophile-initiated deboration process is commonly observed in o-carboranes, though S2− promoted selective deboration has not been reported in the literature.37,38 In summary, a carborane-fused azaborolyl radical cation has been prepared and structurally characterized by single-electron oxidation of the carbene-stabilized iminocarboranyl boron(I) compound 1 with CuCl or CuI. DFT calculations indicate that the radical is delocalized over the BNC unit, suggesting that the imine moiety is an excellent π acceptor in the stabilization of highly reactive boron radicals. On the other hand, the reaction of 1 with elemental sulfur led to the isolation of an unprecedented carbene-stabilized dicarbollyl-fused azaborole via a two-electron oxidative deboration reaction. Both X-ray data and DFT calculations show that the two fused five-membered rings form a sort of π-conjugated system, in which the dicarbollyl unit is formally dinegative and the azaborolyl fragment bears formally a charge of +2. These results suggest that electron-rich boron(I) compounds can serve as good starting materials for the preparation of azaborolyl radical cations.

exhibited a 1:1:1:1:1:1:1:1:1 pattern, ranging from 5.3 to −25.1 ppm, indicating the formation of a nido-C2B9 species. The chemical shift of the exo boron atom in 3, as confirmed by the 1 H-coupled 11B NMR spectrum, was observed at 40.5 ppm as a broad singlet, which was significantly shifted downfield in comparison to that of 17.7 ppm in 1.32 Such data indicate the enhancement of Lewis acidity of the exo boron atom. The molecular structure of 3 has been confirmed by singlecrystal X-ray analyses (Figure 4), showing the formation of the

Figure 4. Molecular structure of 3. Selected bond lengths (Å): C(1)− C(2) 1.515(5), C(1)−C(11) 1.414(6), N(1)−C(11) 1.398(5), N(1)− B(12) 1.462(6), B(12)−C(2) 1.495(6), B(12)−C(28) 1.564(6), B(5)−C(1) 1.758(6), B(4)−B(5) 1.711(8), B(3)−B(4) 1.719(8), B(3)−C(2) 1.719(6).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00545. Experimental details and complete characterization data (PDF) Cartesian coordinates for the calculated structures (XYZ) Crystallographic data for [2]2[Cu2I4], [2][CuCl2], and 3 (CIF)

nido structure. The open C2B3 face is fused with another C3BN five-membered ring with a dihedral angle of 122.2°. The B(12)− C(28) bond length of 1.564(6) Å in 3 is slightly shorter than that of 1.585(4) Å in 1. Except for the lengthening of the N(1)− B(12) bond distance in 3, all four other bonds in the C3BN plane are shorter than those observed in 1. The C(1)−C(2) distance of 1.515(5) Å is also much shorter than that of 1.635(6) Å found in closo-[3-Co(η5-NC4H4)-1,2-(μ-CH2)3-1,2-C2B9H9].36 These data suggest the overlapping of 2p orbitals among the two fused five-membered rings. In addition, similar to that observed in 1, the NHC ligand is not coplanar with the C3BN plane

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for Z.L.: [email protected]. C

DOI: 10.1021/acs.organomet.6b00545 Organometallics XXXX, XXX, XXX−XXX

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Organometallics *E-mail for Z.X.: [email protected].

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from the RGC of The Hong Kong SAR (Project No. 14306114) and State Key Laboratory of Elemento-Organic Chemistry, Nankai University (Project No. 201321). We are grateful to Prof. Liang Deng (SIOC) for the CV measurements and Prof. Hung Kay Lee (CUHK) for EPR experiments.

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DOI: 10.1021/acs.organomet.6b00545 Organometallics XXXX, XXX, XXX−XXX