Intramolecular NâB Coordination as a Stabilizing Scaffold for Ï

Article pubs.acs.org/Organometallics

Intramolecular N→B Coordination as a Stabilizing Scaffold for π‑Conjugated Radical Anions with Tunable Redox Potentials Markus Grandl,† Benjamin Rudolf,† Yu Sun,‡ Dominique F. Bechtel,‡ Antonio J. Pierik,‡ and Frank Pammer*,† †

Institute of Organic Chemistry II and Advanced Materials, Ulm University, Albert-Einstein-Allee 11, 89081 Ulm, Germany Fachbereich Chemie, Technische Universität Kaiserslautern, Erwin-Schrödinger-Strasse 54, 67663 Kaiserslautern, Germany

‡

S Supporting Information *

ABSTRACT: In this work we report on a preparative strategy that allows the facile tuning of the electron affinities of π-conjugated organic n-type materials via intramolecular N→B coordination: 2fold hydroboration of a pyrazine-derived substrate with 9Hborabicyclo[3.3.1]nonane (9H-BBN) and Piers’ borane ((C6F5)2BH) furnishes the intramolecularly N→B-coordinated compounds PzBBN and PzBPF in high yield. For each borane racemic mixtures of the respective chiral derivatives and achiral meso compounds have been isolated, and their electrochemical, optical, and structural properties have been investigated and complemented by DFT calculations. The boranes exhibit high electron affinities with electrochemical LUMO energy levels of −3.69 and −4.30 eV, respectively. The corresponding radical anions PzBBN•− and PzBPF•− can be reversibly generated and are stable in solution at ambient temperature. Analysis of the radical species by EPR spectroscopy and single-crystal analysis of [PzBPF]•−[CoCp2]+ allowed further corroboration of the π-delocalized nature of these radicals.

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INTRODUCTION Conjugated organic materials that can serve as electrontransporting/accepting (n-type) materials are of great interest in emerging applications ranging from organic photovoltaics to n-channel field effect transistors, electroluminescent devices, and energy storage.1 Research in the field of organic n-type materials is dominated by fullerene-2 and perylene-derived3 lead structures, as these systems provide electron affinities in the range of −3.5 to −4.0 eV that is of interest e.g. for electron acceptor materials in organic photovoltaics4 and allow stable ntype device operation under ambient conditions.5 Comparable electron affinities can alternatively be reached with preparative ease in ionic compounds such a N-alkylated viologens,6 while synthetic strategies to increase the electron affinity of uncharged n-type materials involves perhalogenation of acenes7 or incorporation of heteroatomic functional groups featuring such as phosphino oxides.8 Likewise, n-type materials can be prepared by decoration of conjugated π systems and tricoordinate boron centers9,10 or by substitution of carbon by electron negative imine nitrogen centers in N-heteroacenes.11 Radical anions of triarylboranes have been studied intensely12 and could in several instances be structurally characterized.13 Perhalogenated triarylboranes exhibit electrochemical LUMO levels of −3.1 to −3.6 eV14 (Chart 1), while annulated tetraazapentacenes reach substantially higher electron affinities.15 What these systems have in common, however, is that the redox potential of a given lead © XXXX American Chemical Society

Chart 1. Examples of High-Electron-Affinity Scaffolds

structure cannot be easily varied without substantial preparative effort. This may be alleviated by a combination of these two Special Issue: Tailoring the Optoelectronic Properties of Organometallic Compounds with Main Group Elements Received: December 19, 2016

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DOI: 10.1021/acs.organomet.6b00916 Organometallics XXXX, XXX, XXX−XXX

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Organometallics strategies: i.e. intermolecular16 and intramolecular17−19 coordination of N-heteroarenes to tricoordinate boron centers. Intramolecular N→B ladder formation has been used to increase the performance of organic photovoltaic cells20 and to prepare n-type acceptor polymers.21 In our group we have a standing interest in tailoring the electronic properties of conjugated π systems via intramolecular N→B coordination22−24 and have explored the potentialities of efficiently generating such N→B ladder structures by hydroboration.23−25 Herein, we report the hydroboration of a pyrazine-based substrate which yields N→B ladder boranes with high electron affinities, and highly variable reduction potential. The π-conjugated scaffold thus formed can reversibly host a negative charge in solution at ambient temperature.

preparative HPLC to quantitatively remove residual amounts of the respective meso derivatives. With authentic samples of both pairs of diastereomers in hand, the diastereoselectivity could also be estimated via NMR experiments at higher dilution that prevented precipitation of the products. According to these experiments, 9H-BBN effects a selectivity of 57:43 for the meso over the rac form, while with Piers’ borane the rac form is preferentially formed in a ratio of 61:39 (see Figures S1 and S2 in the Supporting Information). PzBBN and PzBPF could be unambiguously identified and characterized by 1H, 13C, and 11B NMR and elemental analysis.23 Furthermore, single crystals suitable for analysis by X-ray diffraction could be obtained for meso-PzBBN (Figure 1a) and for the isomers rac-PzBPF (Figure 1b) and meso-

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RESULTS AND DISCUSSION Synthesis and Structure. The substrate 2,5-bis(o-styryl)pyrazinylene (Pz) has been prepared by Negishi coupling of 2,5-dibromopyrazine with 2 equiv of 2-(β,β-dimethylstyryl) zinc(II) chloride.24 Pz was then hydroborated 2-fold with a slight excess of 9H-BBN and Piers’ borane (HB(C6F5)2)26 to give the corresponding ladder boranes PzBBN and PzBPF, in 89% and 81% yields, respectively (Scheme 1). Hydroboration Figure 1. Crystal structures of (a) meso-PzBBN and (b) rac-PzBPF (R,R enantiomer shown). Ellipsoids are drawn at the 65% probability level. Hydrogen atoms and 1 equiv of n-hexane in the structure of racPzBPF have been omitted for clarity.

Scheme 1. Synthesis of Boranes PzBBN and PzBPF

PzBPF (not shown). Analysis of the boranes by 11B NMR in THF solution showed sharp resonances at 0.2 and 0.3 ppm for rac- and meso-BPF, respectively, and at −1.2 ppm for racPzBBN. 23 These values are in the typical range for tetracoordinate N,C-chelated boranes9,18−24 and indicate N→ B coordination to be predominant in solution. The crystal structures showed meso-PzBBN, meso-PzBPF, and rac-PzBPF to all adopt N→B-coordinated structures in the solid state with the newly formed isopropyl groups occupying axial28 positions. The centrosymmetrical meso-PzBBN exhibits a short N→B distance of 1.639(2) Å, in comparison to that for meso-PzBPF of 1.652(1) Å, with corresponding biaryl torsion angles of ±24.8(2) and ±26.5(1)°, respectively. rac-PzBPF is less symmetrical, with torsion angles of 24.2(3) and 17.9(3)° and corresponding N→B bond lengths of 1.630(3) and 1.656(3) Å. The asymmetry of the racaxax configuration of PzBPF and the preferred axial orientation of the iPr groups appear to be intrinsic, rather than being packing effects. This can be concluded from the DFT study, which yielded geometries in good agreement with the crystal data for the meso diastereomers, correctly reproduced the unequal N→ B coordination in racaxax-PzBPF, and showed the axial conformations to be energetically favorable due to reduced steric strain (see Section 1.3 and Tables S1 and S2 in the Supporting Information). Electronic Properties. Borylation strongly affects the electrochemical properties, as determined by cyclic voltammetry (CV, Figure 2a) and square-wave voltammetry (SVW, Figure S3 in the Supporting Information): Pz undergoes reversible one-electron reduction with a peak potential at −2.52 eV (SWV) relative to the ferrocene/ferrocenium redox couple (FcH/FcH+). The reduction potentials of rac-PzBBN and racPzBPF appear anodically shifted and show reversible oneelectron reductions to the radical anions with peak potentials at −1.41 and −0.80 V, along with additional, irreversible

with 9H-BBN is sluggish and requires prolonged heating in tetrahydrofuran to achieve full conversion. In contrast, the reaction with Piers’ borane proceeds much more quickly and reaches completion overnight at ambient temperature. Two stereogenic centers are generated in the course of the reactions, and consequently mixtures of rac and meso stereoisomers are formed. The less soluble meso stereoisomers27 already precipitate from the reaction mixtures and can be isolated in high purity by filtration and washing. The more readily soluble rac isomers could also be isolated but required purification by B


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Organometallics

in solution at 540 and 520 nm, respectively. This observation indicates a predominantly N→B coordinated structure in solution, since noncoordinated “open” conformers are expected to exhibit much larger band gaps, similar to those of the substrate (see Table S2 and Figure S8 in the Supporting Information). Slight differences in the optical absorption are observable between the individual diastereomers but appear rather insignificant in comparison to the differences originating from changing the substituents on boron. A comparison of the Kohn−Sham frontier orbitals shows that the chemical nature of the boryl-groups directly affects the electronic situation of the boranes (Figure 3). The HOMO-1, HOMO, and LUMO of PzBPF as well as the LUMO of PzBBN represent π- and π*-type orbitals, respectively, that are delocalized mainly throughout the central diphenylpyrazinylene system, while the HOMO and HOMO-1 of PzBBN are dominated by contributions from σ-type orbitals localized in the 9BBN frames. The spectra of all four boranes appear to be composed of three main absorption bands (λ1−3, Table 1) of comparable energy, which have been individually discerned by deconvolution of the experimental spectra (see Figures S4 and S5 in the Supporting Information) and could also be reproduced by time-dependent DFT (TD-DFT) calculations in reasonable agreement with the experimental data (see Table S2 and Figure S8 in the Supporting Information). The TDDFT calculations consistently indicated that the lowest energy transitions in both types of compounds are mainly HOMO− LUMO in character and are of comparable energy (PzBPF (calcd), 96% HOMO−LUMO, 3.11 eV/399 nm, f = 0.188; PzBBN (calcd), 93% HOMO−LUMO, 4% HOMO-2− LUMO, 3.06 eV/405 nm, f = 0.086). However, the σ character of the HOMO in PzBBN renders this transition partially forbidden and hence weakened in comparison to PzBPF. Instead, the absorption of PzBBN in the visible range is dominated by a more intense transition (91% HOMO-2− LUMO, 4% HOMO−LUMO, 3.39 eV/366 nm, f = 0.151) that primarily involves HOMO-2, which represents a “purer” π orbital and hence enables more effective electronic coupling to the LUMO. Properties of Radical Anion Species. The large separation of the first reversible one-electron reductions of PzBBN and PzBPF from the second, irreversible reductions indicated that the corresponding radical anions should be stable toward decomposition and disproportionation. This was confirmed by reduction of rac-PzBBN and meso-PzBPF to rac-PzBBN•− and meso-PzBPF•− with decamethylcobaltocene (CoCp*2) and cobaltocene (CoCp2), respectively.30 Under exclusion of air and moisture the radical salts proved to be stable over days in THF solution at ambient temperature

Figure 2. (a) Cyclic voltammograms of Pz, rac-PzBBN, and racPzBPF, recorded in THF solution with 0.1 M nBu4NPF6 as supporting electrolyte at a scan rate of 100 mV s−1. (b) UV−vis absorption spectra of Pz and individual diastereomers of PzBBN and PzBPF, recorded in ca. 10−4 M THF solution.

reductions to the dianions at −2.93 and −2.21 V, respectively. On the basis of the first reduction, these potentials correspond to electrochemical LUMO levels of −3.69 and −4.30 eV, relative to the HOMO of ferrocene (−5.1 eV29). Notably, these results demonstrate that the electron affinity can be readily varied over a range of ca. 0.6 eV, in a single high-yield reaction, without altering the underlying π-conjugated framework. The optical absorption spectra also immediately reflect the effect of borylation: with respect to the substrate (Pz, λmax 310 nm, λonset 364 nm; Table 1) both BBN- and BPF-functionalized systems exhibit drastic bathochromic shifts in their absorption spectra, with the longest wavelength absorption maxima at ∼400 nm (BBN) and ∼465 nm (BPF) and absorption onsets

Table 1. Electronic and Optical Properties of Pz and Ladder Boranes Epred1a (V)

Epred2a (V)

−2.52

Pz BBN

LUMOb (eV)

Egoptc (nm (eV))

λ1d (nm)

−2.58

364 (3.41)

307

λ2d (nm)

λ3d (nm)

rac meso

−1.41

−2.93

−3.69

543 (2.28) 539 (2.30)

333 331

399 388

452 448

rac meso

−0.80

−2.21

−4.30

521 (2.38) 516 (2.40)

320 318

388 390

460 455

BPF

a

Peak potential determined by SWV in THF with 0.1 M nBu4NPF6 relative to FcH/FcH+. bRelative to the HOMO of ferrocene (−5.1 eV29). cBased on the absorption onset in THF solution. dDerived by deconvolution; see the Supporting Information for details. C


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Organometallics

limited resolution to the presence of different conformers in solution combined with multiple hyperfine interactions of the delocalized charge.31 However, the structure of meso-PzBPF•− could be further elucidated by single-crystal X-ray crystallography and comparison of this structure with crystal data for neutral meso-PzBPF and structures simulated by DFT calculations of both the radical and neutral compounds. Single crystals of the cobaltocenium salt [meso-PzBPF]•−[Cp2Co]+ were obtained in the form of green plates by diffusion of nhexane into a solution of the radical in THF (Figure 5a,b).

Figure 5. (a, b) Crystal structure and packing of [mesoPzBPF]•−[Cp2Co]+. Ellipsoids are drawn at the 55% probability level. Hydrogen atoms and the counterion in (a) have been omitted for clarity. (c) SOMO plot of meso-PzBPF•−, isovalue 0.03. (d, e) Mulliken spin densities of meso-PzBPF•− and rac-PzBBN•−. ax/eq refers to the ipso carbon atoms next to boron.

Figure 3. Frontier orbital plots for (a) mesoaxax-PzBBN and (b) mesoaxax-PzBPF. Orbital plots were generated using GaussView 5.0, with isovalue 0.035.

According to the computational study, the highest singly occupied molecular orbital (SOMO) of meso-PzBPF•− strongly resembles the LUMO of the neutral species (Figure 5c) and should therefore exhibit a quinoidal electron distribution in the central pyrazinylene ring with additional, smaller contributions from the terminal phenylene rings. This is consistently reflected by both the simulated structure and the crystal structure of meso-PzBPF•−, which show the two types of aromatic C−N bonds to elongate upon reduction, while the C−C bonds appear shortened relative to the neutral species (Table 2). The DFT study also predicted a strengthening of the ladder structure in the reduced state, which manifests itself in a strongly increased stabilization of closed over open conformers of radical anions (mesoaxax-PzBPF•−, ΔGo/c = −317 kJ/mol; meso-PzBPF, ΔGo/c = −189 kJ/mol; racaxax-PzBBN•−, ΔGo/c = −330 kJ/mol; racaxax-PzBBN, ΔGo/c = −111 kJ/mol) and in a concurrent contraction of the coordinative N→B bonds by 0.04−0.06 Å. These results have been almost quantitatively confirmed by the crystal structures of meso-PzBPF and [mesoPzBPF]•−[Cp2Co]+ (Table 2). Furthermore, calculated Mulliken spin densities of meso-PzBPF•− and rac-PzBBN•− (Figure 5d,e), as well as Mulliken and natural bond orbital

and were studied by EPR spectroscopy (Figure 4) and UV−vis spectroscopy (Figures S6 and S7 in the Supporting Information). The EPR spectrum of [meso-PzBPF]•[Cp2Co] shows a broadened resonance with a g factor of giso = 2.0029. Weak shoulder bands can be discerned that may be attributable to hyperfine coupling, but these did not allow us to deduce further structural information. We tentatively attribute the

Figure 4. Continuous-wave X-band EPR spectra of (a) meso-PzBPF•− (∼5 × 10−3 M in THF at room temperature) and b) rac-PzBBN•− (∼3 × 10−4 M in THF at room temperature). D


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Organometallics Table 2. Structural Data of meso-PzBPF, meso-PzBPF•−, and Simulated Structuresa compd

SG

N→B (Å)

N−C (Å)

N−CH (Å)

C−CH (Å)

meso-PzBPF meso-PzBPF•− meso-PzBPF meso-PzBPF•− rac-PzBBN rac-PzBBN•−

P1̅ P1̅ calcd calcd calcd calcd

1.652(1) 1.575(2) 1.647 1.585 1.644 1.600

1.364(1) 1.401(2) 1.353 1.390 1.353 1.389

1.333(1) 1.361(2) 1.330 1.360 1.332 1.361

1.392(1) 1.350(2) 1.393 1.365 1.395 1.370

absorption band. Broad near-IR absorption bands typically associated with radical species were not observed within the investigated spectral window, up to 1100 nm.

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CONCLUSIONS We have demonstrated that well-defined ladder boranes capable of intramolecular N→B coordination can be generated by 2fold hydroboration of a pyrazine-derived substrate. This transformation allows a substantial increase in the electron affinity of the conjugated system and a variance in this affinity over a range of 0.6 eV from −3.69 to −4.30 eV, through the choice of the borane employed. Furthermore, the ladder structure renders the corresponding radical anions stable at ambient temperature in solution. Analyses of the radical species by EPR spectroscopy, structural analysis of single crystals of [meso-PzBPF]•−[Cp2Co]+, and computational studies indicate that, while the excess charge is to a large part delocalized within the central pyrazinylene ring, the peripheral phenylene ings also contribute to the stabilization. The boranes reported herein exhibit electron affinities suitable for use in n-channel field effect transistors or as acceptor component in organic photovoltaics. Notably, the synthetic strategy outlined here allows modification of not only the electron affinity but also other properties important for device optimization, such as solubility and bulk packing, in a single high-yield preparative step. Furthermore, the clean conversion of the bifunctional Pz substrate indicates that this approach should also be suitable for the postfunctionalization of polymeric substrates. Further explorations of the preparative scope of this method and of possible device applications are currently in progress in our group.

Calculated data are given for “axax” conformers only. SG: space group.

a

charges (see Table S3 in the Supporting Information), also consistently indicate that the excess charge is to a large part localized on the nitrogen centers and the carbon atoms of the pyrazinylene ring. Still, appreciable delocalization into the ortho and para positions of the phenylene rings is also present. These results could be further corroborated with the EPR data of rac-PzBBN•−. This radical also showed a broadened resonance with a g factor of giso = 2.0031 and also allowed us to resolve a hyperfine structure that corresponds to coupling of the unpaired electron to two nitrogen centers (a(14N) = 7.17 G) and the two pyrazinyl hydrogen atoms (a(1H) = 1.10 G), as well as to 11B and 10B centers (a(11B) = 2.28 G, a(10B) = 0.77 G).31 The coupling constants are somewhat lower than those reported for neat pyrazinyl radicals32 ([Pz]•−K+,32a a(14N) = 7.22 G, a(1H) = 2.66 G), and for radical anions of intermolecular borane−pyrazine adducts (e.g., [C5H4N2· (BEt3)2]•−K+,33 a(14N) = 8.02 G, a(11B) = 2.59 G, a(1H) = 2.61 G; [C5H4N2·(BPh3)2]•−[nBu4N]+,34 a(14N) = 7.99 G, a(11B) = 2.51 G, a(1H) = 2.65 G). The 14N coupling constants in radical anions of borane adducts are generally larger than those in the neat N-heteroarenes, presumably because the electron-withdrawing effect of the N→B coordination stabilizes increased charge localization at the nitrogen atoms. The lower coupling constants derived for [rac-PzBBN]•− therefore indicate delocalization of the excess charge into the periphery of the molecule. Notably, coupling to nitrogen is stronger than that in radical anions that feature N,N2-chelated tetracoordinate boron, either in negatively charged (formazanate)boron complexes (a(14N) = 4.48−5.17 G, a(11B) = 0.75−0.98 G)35 or in neutral radicals derived from 2,2′-bipyridyl-containing boronium salts36 (e.g., [(bipy)-9,10-dihydro-9-boraanthryl]•,36b a(14N) = 3.58−4.11 G, a(11B) = 3.89−4.36 G), while coupling to 11B varies. Hyperfine coupling to boron is generally stronger in radicals containing tricoordinate boron (typically 5−10 G,12,13 up to 46.3 G for [(MeO)3B·B(OMe)3]•− 37). Weaker coupling has been reported, however, for borole-derived (a(11B) = 2.94,12d 3.73 G13f) and individual borafluorenederived radicals (a(11B) = 3.1−4.5 G12j), while extended delocalization can prevent its observation altogether.13a,c The optical properties of rac-PzBBN•− and meso-PzBPF•− were studied by monitoring the chemical reduction of the neutral boranes with sodium metal (Figures S6 and S7 in the Supporting Information). Gradual reduction of rac-PzBBN led to marginal weakening of the main absorption band at ca. 400 nm and emergence of a new band centered at 545 nm without substantial changes to the optical absorption onset. Generation of meso-PzBPF•−, likewise, resulted in weakening of the band at 380 nm and increased absorption of the longest wavelength

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EXPERIMENTAL SECTION

Materials and Instrumentation. All reactions and manipulations of sensitive compounds were carried out under an atmosphere of prepurified argon using either Schlenk techniques or an inertatmosphere glovebox (MBraun Labmaster). Toluene, Et2O, THF, DMF, and dichloromethane were purified using a solvent purification system (MBraun; alumina/copper columns for hydrocarbon solvents). Hexane and benzene were dried by distillation from CaH2 under an argon atmosphere prior to use. 1-Bromo-2-(2-methylprop-1-en-1yl)benzene38 and bis(pentafluorophenyl)borane26a were prepared according to literature procedures. Other reagents were commercially available (Aldrich, Acros, Alfa Aesar) and were either used as obtained or purified by standard procedures.39 Preparative HPLC was performed on a Shimadzu CBM-20A instrument equipped with an SPD-20A UV−vis detector and an LC-8A solvent delivery system using LiChrospher columns (Nucleosil 100-5 NO2). 1H, 13C, and 11B NMR spectra were recorded at 293 K on a Bruker Avance DRX 400 (400 MHz) spectrometer or a Bruker Avance 500 AMX (500 MHz). Solution 1H and 13C NMR spectra were referenced internally to the solvent residual signals.40 Solution 11B NMR spectra were referenced externally to BF3·Et2O (10% in CHCl3). Individual signals are referred to as a singlet (s), doublet (d), pseudodoublet (psd), triplet (t), multiplet (m), centrosymmetric multiplet (mc), and broadened (br). The following abbreviation are used for signal assignment: Ph (phenyl), Pz (pyrazinylene), BBN (9-borabicyclo[3.3.1]non-9-yl), PF (2,3,4,5,6-pentafluorophenyl). High-resolution mass spectrometry measurements were performed on a Bruker SolariX FTMS using MALDI (matrix assisted laser desorption ionization). trans-2-[3-(4tert-Butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) was used as the matrix in MALDI measurements. UV−visible absorption spectra and photoluminescence spectra were acquired on a PerkinElmer Lambda 19 UV/vis/NIR spectrometer and a PerkinElmer LS 55 fluorescence spectrometer, respectively. Spectroscopic data of chemically reduced species were recorded with a E


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Organometallics

Hz, 6H; CH3), 1.69 (d, 4J(H,H) = 1.5 Hz, 6H; CH3) ppm. 13C NMR (101 MHz, CDCl3): δ 151.9 (s; Ph-2/5), 145.1 (s; Pz-3/6), 137.3 (s; Ph-1), 136.8 (s; CHC), 136.3 (s; Ph-2), 130.7 (s; Ph-3), 130.0 (s; Ph-6), 128.9 (s; Ph-5), 127.1 (s; Ph-4), 124.2 (s; CHC), 26.3 (s; CH3), 19.5 (s; CH3) ppm. HR-FTMS (ESI, positive mode): m/z calcd for C24H24N2 [M + H]+, 341.20123 Da; found, 341.20148 Da. Anal. Calcd for C24H24N2: C, 84.67; H, 7.11; N, 8.23. Found: C, 85.06; H, 6.83; N, 8.10. Preparation of rac-/meso-PzBBN. A thick-walled pressure tube was charged with Pz (250 mg, 735 μmol), 9H-BBN dimer (359 mg, 1.47 mmol), and THF (8 mL) inside the glovebox. The solution was stirred for 7 days at 75 °C. The resulting red precipitate was separated by centrifugation of the reaction mixture and washed with diethyl ether to provide the compound meso-PzBBN (210 mg, 359 μmol, 49%) as a red crystalline solid. The supernatant of centrifugation was concentrated under reduced pressure and purified by silica gel column chromatography using petroleum ether/ethyl acetate (20/1) as eluent. Further purification by preparative HPLC (n-hexane/methylene chloride 8/2) furnished the compound rac-PzBBN (174 mg, 298 μmol, 40%) as a red solid. Data for meso-PzBBN are as follows. Mp: 292.4−294.2 °C. 1H NMR (400 MHz, THF-d8): δ 9.42 (s, 2H; Pz-3/ 6), 7.89 (mc, 2H; Ph-6), 7.47 (mc, 2H; Ph-4), 7.35 (mc, 2H; Ph-5), 7.31 (mc, 2H; Ph-3), 2.31−2.20 (m, 7H; Ph-CH-BBN, BBN-H), 1.98−1.79 [m, 13H; BBN-H, CH(CH3)2], 1.64−1.59 (mc, 2H; BBNH), 1.49−1.33 (m, 6H; BBN-H), 0.99 (d, 3J(H,H) = 6.8 Hz, 6H; CH3), 0.95−0.91 (m, 2H; BBN-H), 0.37 (br s, 2H; BBN-bridgehead), 0.30 (d, 3J(H,H) = 6.9 Hz, 6H; CH3) ppm. 13C NMR (101 MHz, THF-d8): δ 148.1, 146.4, 139.7, 133.3, 132.3, 129.0, 126.1, 125.1, 35.3, 31.4, 30.4, 29.5, 28.9, 24.8, 23.7, 23.6, 19.3 ppm. Note: due to low solubility and signal broadening through 13C−11B coupling, no signals of nuclei bound to boron were observed. Anal. Calcd for C40H54B2N2: C, 82.20; H, 9.31; N, 4.79. Found: C, 82.15; H, 9.27; N, 4.73. Data for rac-PzBBN are as follows. Mp: 249.7−255.2 °C. 1H NMR (400 MHz, THF-d8): δ 9.40 (s, 2H; Pz-3/6), 7.81 (mc, 2H; Ph-6), 7.48 (mc, 2H; Ph-4), 7.39 (mc, 2H; Ph-5), 7.31 (mc, 2H; Ph-3), 2.48 (m, 2H; BBNH), 2.26 (mc, 2H; Ph-CH-BBN), 2.23−2.18 (m, 2H; BBN-H), 2.04− 1.76 [mc, 12H; BBN-H, CH(CH3)2], 1.69−1.60 (m, 4H; BBN-H); 1.40 (mc, 6H; BBN-H), 1.00 (mc, 2H; BBN-H), 0.94 (d, 3J = 6.9 Hz, 6H; CH3), 0.39 (mc, 2H; BBN-H), 0.27 (d, 3J = 6.8 Hz, 6H; CH3) ppm. 13C NMR (101 MHz, THF-d8): δ 149.4 (s, Pz-1), 147.6 (s, Ph2), 141.5 (s, Pz-C2), 134.3 (s, Ph-3), 133.5 (Ph-4), 130.5 (s, Ph-1), 127.2 (s, Ph-6), 127.1 (s, Ph-5), 40.0 (s, CHB), 36.2 (s, BBN-CH), 32.8 (s, BBN-CH), 31.7 (s, BBN−CH), 31.5 (s, BBN−CH), 30.4 (s, CHMe2), 26.0 (br s, BBN-CB), 25.3 (s, CH3), 25.1 (s, BBN-CH), 25.0 (s, BBN-CH), 23.1 (s, BBN-CB), 20.7 (s, CH3) ppm. 11B NMR (128 MHz, C6D6, 41 °C): δ −1.2 (br s) ppm. Anal. Calcd for C40H54B2N2: C, 82.20; H, 9.31; N, 4.79. Found: C, 82.19; H, 9.18; N, 4.92. Preparation of rac- and meso-PzBPF. Compound Pz (100 mg, 294 μmol), and HB(C6F5)2 (264 mg, 764 μmol) were dissolved in toluene (5 mL). The solution was stirred for 18 h at room temperature. The resulting orange precipitate was separated by centrifugation. Recrystallization from n-hexane/benzene furnished meso-PzBPF (115 mg, 111 μmol) as an orange crystalline solid in 38% yield. The supernatant was concentrated under reduced pressure and purified by silica gel column chromatography using petroleum ether/ethyl acetate (12/1) as eluent. Further purification by preparative HPLC (n-hexane/methylene chloride 8/2) furnished 131 mg (127 μmol) of the compound rac-PzBPF in 43% yield. Data for rac-PzBPF are as follows. Mp: 288.0−297.0 °C. 1H NMR (400 MHz, THF-d8): δ 9.43 (s, 2H; Pz-3/6), 7.71 (mc, 2H; Ph-6), 7.53 (mc, 2H; Ph-4), 7.45 (mc, 2H; Ph-3), 7.37 (mc, 2H; Ph-5), 2.98 (mc, 2H; PhCHB), 1.87 [mc, 2H; CH(CH3)2], 0.99 (d, 3J(H,H) = 6.6 Hz, 6H, CH3), 0.40 (d, 3J(H,H) = 6.9 Hz, 6H, CH3) ppm. 13C NMR (101 MHz, THF-d8): δ = 149.8, 149.5 (br), 148.0 (br), 147.5 (br), 146.6, 143.0 (s; Pz-3), 142.3 (br), 141.1 (br), 140.3 (br), 139.9 (br), 139.2 (br), 138.6 (br), 137.8 (br), 136.7 (br), 134.5 (s; Ph-4), 133.0 (s; Ph3), 128.4, 128.0 (s; Ph-5/6), 120.0, 116.7, 39.0 (br s; Ph-CHB), 31.8 [s; CH(CH3)2], 25.0 (s; CH3), 20.4 (s; CH3) ppm. Note: due to signal broadening and 13C−19F coupling not all carbons of the pentafluorophenyl moiety could be detected. 11B NMR (128 MHz, THF-

spectrometer setup by Mountain Photonic GmbH consisting of an AVA Avaspec-ULS2048XL-USB2-RS and an AVA AvaLight-DH-S (Deuterium-Halogen) light source, linked by AVA FC-UVIR600 glass fibers. Samples were prepared inside an inert gas glovebox and measured in quartz cuvettes with a 1 mm path length. Elemental analyses were performed on an Elemental Vario EL analyzer. Melting points were measured on a Büchi M-565 melting point apparatus with a heating rate of 2 K/min. Electrochemical analyses were performed both in cyclic voltammetry and square-wave voltammetry mode with an Autolab potentiostat−galvanostat with a three-electrode system, consisting of a Pt working electrode (0.785 mm2), a Pt counter electrode, and an Ag/AgCl reference electrode. The measurements were carried out in THF or CH2Cl2 with [N(n-Bu)4][PF6] (0.1 M) as supporting electrolyte and were internally referenced against the ferrocene/ferrocenium redox couple. EPR spectra were recorded on a Bruker ELEXSYS E580 EPR X-band spectrometer equipped with an ER4102ST cavity at ambient temperature. Samples were measured as ca. 0.005 M (meso-PzBPF•−) and 2.5 × 10−4 M (rac-PzBBN•−) solutions in dry THF. The spectra were corrected relative to a graphene reference sample. EPR conditions for meso-PzBPF•−: microwave frequency, 9.774 GHz; microwave power, 0.40 mW; modulation frequency, 100 kHz; modulation amplitude, 0.1 mT. EPR conditions for rac-PzBBN•−: microwave frequency, 9.766 GHz; microwave power, 0.40 mW; modulation frequency, 100 kHz; modulation amplitude, 0.1 mT. The EPR spectrum of rac-PzBBN•− was simulated with EasySpin.41 X-ray diffraction intensities were collected on an Agilent Technologies SuperNova single-crystal X-ray diffractometer at 150 K with Mo Kα radiation. The structures were solved using direct methods (SIR9242 or Shelxs-201443), completed by subsequent difference Fourier syntheses, and refined by full-matrix least-squares procedures. Semiempirical absorption corrections from equivalents (Multiscan) were carried out. CCDC 1513370−1513373 contain supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif and are also included in the Supporting Information. Computational Details. Quantum chemical calculations were performed on the Campus Ulm Shared Science Cluster (CUSS) computer server and on the bwForCluster JUSTUS at the University of Ulm, using release D.01 of the Gaussian09 program package. Sets of axax, axeq, and eqeq conformers of the rac and meso diastereomers and one noncoordinated “open” conformer of each of the synthesized boranes, as well structures of racaxax-PzBPF•−, open-PzBPF•−, mesoaxax-PzBBN•−, and open-PzBBN•− have been optimized at the M06-2X/6-31G(d,p) level of theory, in the gas phase. Frequency calculations confirmed the optimized structures to be local minimum structures (no imaginary frequencies). Frontier orbital levels were derived from single-point calculations on the optimized structures at the M06-2X/TZVP level. The first 15 excited states were simulated by time-dependent DFT (TD-DFT) at the same level. The M06-2X44 functional was chosen, as it accounts well for dispersion interactions and has been shown to accurately represent coordination complexes of organoboranes.45 Preparation of 2,5-Bis(2-(2-methylprop-1-en-1-yl)phenyl)pyrazine (Pz). A solution of 1-bromo-2-(2-methylprop-1-en-1yl)benzene (5.30 g, 25.1 mmol) in THF (60 mL) was cooled to −78 °C, and a solution of n-BuLi (1.6 M in n-hexane, 15.7 mL, 25.1 mmol) was added within 60 min. The pale yellow solution was stirred for 3 h at −78 °C before a solution of zinc chloride (3.42 g, 25.1 mmol) in THF (30 mL) was added slowly over 30 min. The colorless solution was stirred for 45 min at −78 °C before it was warmed to room temperature. Addition of PdCl2(PPh3)2 (734 mg, 1.05 mmol), and 2,5-dibromopyrazine (2.49 g, 10.5 mmol) was followed by stirring of the solution at room temperature for 18 h. The solvent was removed, and the crude product was purified by column chromatography over silica gel using petroleum ether/ethyl acetate (15/1) as eluent to yield 3.26 g (9.57 mmol, 92%) of compound Pz as colorless needles. Mp: 126.2−127.3 °C. 1H NMR (400 MHz, CDCl3): δ 8.83 (s, 2H; Pz-3/6), 7.73 (mc, 2H; Ph-6), 7.41 (mc, 4H; Ph-4/5), 7.33 (mc, 2H; Ph-3), 6.29 (br s, 2H; CCH), 1.84 (d, 4J(H,H) = 1.7 F


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Organometallics d8): δ = 0.2 (br s) ppm. 19F NMR (376 MHz, THF-d8): δ −126.1 (br s), −129.8 (br s), −132.5 (br s), −155.9 (mc), −157.6 (mc), −162.0 (mc), −163.5 (br s) ppm. Anal. Calcd for C48H26B2F20N2: C, 55.85; H, 2.54; N, 2.71. Found: C, 55.95; H, 2.73; N, 2.75. Data for meso-PzBPF are as follows. Mp: 288.8−297.3 °C. 1H NMR (400 MHz, THF-d8): δ 9.37 (s, 2H; Pz-3/6), 7.75 (d, 3J(H,H) = 7.9 Hz, 2H; Ph-6), 7.54 (mc, 2H; Ph-4), 7.43 (mc, 2H; Ph-3), 7.35 (mc, 2H; Ph-5), 3.00 (mc, 2H; Ph−CHB), 2.06 [mc, 2H; CH(CH3)2], 1.04 (d, 3J(H,H) = 6.6 Hz, 6H, CH3), 0.32 (d, 3J(H,H) = 6.9 Hz, 6H, CH3) ppm. 13C NMR (101 MHz, THF-d8): δ 150.0 (br s; C-F), 149.6 (br s; C-F), 149.2 (s; Ph-1), 148.1 (br s; C-F), 147.6 (br s; C-F), 146.2, 142.8, 142.5 (br s; C-F), 141.1 (br s; C-F), 140.4 (br s; C-F), 139.8 (br s; C-F), 138.7 (br s; CF), 137.9 (br s; C-F), 136.7 (br s; C-F), 134.7 (s; Ph-4), 133.1 (s; Ph3), 128.3 (s; Pz-3), 128.1 (s; Ph-5), 127.2 (s; Ph-6), 39.1 (br s, PhCHB), 31.1 [s; CH(CH3)2], 25.6 (s; CH3), 19.8 (s; CH3) ppm. Note: due to signal broadening and 13C−19F coupling not all carbons of the pentafluorophenyl moieties could be detected. 11B NMR (128 MHz, THF-d8): δ 0.3 (br s) ppm. 19F NMR (376 MHz, THF-d8): δ = −126.3 (br s), −129.9 (br s), −133.7 (br s), −155.5 (mc), −157.6 (m c ), −162.0 (m c ), −164.0 (br s) ppm. Anal. Calcd for C48H26B2F20N2: C, 55.85; H, 2.54; N, 2.71. Found: C, 56.04; H, 2.92; N, 2.57. Preparation of Radical Species. To solutions of the respective borane in THF was added 0.95 equiv of either Cp2Co or Cp*2Co from stock solutions in THF. For EPR measurements the solutions were then used as obtained. A crystal structure of [mesoPzBPF]•−[Cp2Co]+ was obtained by precipitating the radical salt from the THF solution into hexanes, followed by washing with hexanes. The precipitate was filtered off, dried, redissolved in THF, and crystallized by diffusion of hexanes.

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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.6b00916. Details on instrumentation, preparative and computational procedures, analytical data, and crystallographic information (PDF) Crystallographic data (CIF) Calculated structures (MOL)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for F.P.: [email protected]. ORCID

Frank Pammer: 0000-0002-3869-0196 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the German Chemical Industry Fund (FCI) (doctoral scholarship for M.G. and Liebig scholarship for F.P.) and the German Science Foundation (DFG) for financial support and Bernhard Müller for support with X-ray data collection.

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REFERENCES

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