Synthesis of the First Heteroaryl-Substituted Boryl Complexes: Strong

Article pubs.acs.org/IC

Synthesis of the First Heteroaryl-Substituted Boryl Complexes: Strong Stabilizing Effects of Boron−Aryl π‑Conjugation Holger Braunschweig,* Rian D. Dewhurst, and Thomas Kramer Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany S Supporting Information *

ABSTRACT: The first examples of heteroarylboryl complexes were prepared and have been found to be unreactive toward a range of strong reductants, strong Lewis bases, and halide-abstraction reagents. The inertness of the complexes is attributed to strong π-conjugation between the π-basic heteroaryl groups and the π-acidic boron atom. This hypothesis is supported by comparison of the structural and spectroscopic properties of the heteroarylboryl complexes with analogous previously reported homoarylboryl complexes, with the former showing greater coplanarity of the aryl ring with the boron atom and much less facile reactivity.

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INTRODUCTION The ability of so-called π-excessive heterocyclic groups, particularly thiophenes, to adopt coplanar geometries with bound tricoordinate boron groups began to be recognized around a decade ago and is now quite well-established.1 This coplanarity facilitates π-electronic communication between boron and other groups, making heteroaryl groups indispensable as linkers in push−pull and other types of π-conjugated systems. A fascinating nuance of this phenomenon that is only now emerging is that such heteroaryl groups and linkers appear to adopt coplanar arrangements almost regardless of the electronic properties of the trigonal planar boron group attached. Heterocycle−boron coplanarity appears to be maintained when the boron group is strongly π-acidic (e.g., diarylboryls, -BAr2;1a−e,g−i,m,n,t boroles, -BC4R41o,p) and surprisingly also when it is weakly π-acidic (e.g., boronate esters -B(OR)21k,l,r). More surprising still is that these groups also stay coplanar when the boron group is weakly (e.g., 1,3-diaza-2boroles, -BN2C2R41f,j,m,n,q) or even strongly π-basic (e.g., borolyl anions, -[BC4R4]−;1p diborenes, -B(L)BAr(L),1s L = neutral Lewis donor). Heteroaryl substitution is thus emerging as a truly universal strategy for π-conjugation with tricoordinate boron groups. One option for the modulation of the electronics of such a tricoordinate boron group is the ligation of the boron atom to a transition metal as a boryl (-BR2) ligand. Our growing interest in studying the electronics of π-conjugated systems containing unusual boron-containing groups thereby led us to consider boryl complexes of the form [LnM-BR(Het)] (Het = 5membered heteroaryl) as interesting synthetic targets. We were also surprised to discover that while cationic base-stabilized © XXXX American Chemical Society

borylene complexes containing aromatic nitrogen bases of the form [LnM{BR(py)}]+ (py = pyridine derivative)2 are known in the literature, the Cambridge Crystallographic Database contains no structurally authenticated examples of heteroarylsubstituted boryl complexes. Given the rarity of nucleophilic boryl species, the ironcentered nucleophilicity of (and the relatively convenient access to) well-defined salts of the organometallic anions [(η5C5R5)Fe(CO)2]− has meant that they are now some of the most reliable and frequently used reagents for the synthesis of transition metal boryl complexes.3,4 The electron-rich and sterically shielding nature of the [(η5-C5R5)Fe(CO)2]− fragment additionally leads to relatively stable corresponding boryl complexes. By combining this anion with dihalo(heteroaryl)boranes, we were able to prepare the first examples of heteroarylboryl complexes, which show distinct boron−heterocycle coplanarity and surprising inertness under a range of conditions. Our results are presented herein.

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RESULTS AND DISCUSSION Toluene suspensions of anionic complex Na[(η5-C5Me5)Fe(CO)2] were separately treated with 5-dichloroboryl-2methylfuran (1a), 5-dichloroboryl-2-trimethylsilylthiophene (1b), and 3-dichloroboryl-1-methylpyrrole (1c). After removal of solvent and extraction with hexane, concentrating the supernantant and cooling led to precipitation of red or orange solids 2a−c (Figure 1) in moderate-to-good yields. The compounds were found to have characteristic 11B NMR signals Received: January 27, 2015

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DOI: 10.1021/acs.inorgchem.5b00192 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Heterocyclic boryl complexes prepared in this study.

for transition metal boryl complexes (2a: 96.5; 2b: 100.9; 2c: 99.3 ppm); however, these signals are significantly shifted to low frequency from those of comparable aryl(chloro)boryl complexes [(η5-C5Me5)(OC)2Fe(BClPh)] (A: 111 ppm) and [(η5-C5Me5)(OC)2Fe(BClMes)] (B: 112 ppm) prepared by Aldridge and co-workers.5,6 This difference is an indicator of higher electron density at the boron atom in the heteroaryl derivatives, which likely stems from π donation from the electron-rich heteroaryl groups. The two CO vibration bands in each of the infrared spectra of the prepared compounds (2a: 1997, 1913; 2b: 1998, 1914; 2c: 1992, 1922 cm−1) were found at lower frequency than those of A (1995, 1929 cm−1) and B (1996, 1936 cm−1), indicating weaker C−O stretches and thus greater electron density at the Fe center of the heteroarylboryl complexes. Solid-state structures of 2a and 2c were determined by single-crystal X-ray crystallography (Figure 2). Unfortunately, despite much effort, we were unable to grow single crystals of 2b of suitable quality for structural analysis. Complexes 2a and 2c show near-coplanar arrangements of the FeBCl and aryl planes (angle between planes for 2a: 17.79°; for 2c: 8.93°), in contrast to the more tilted A5 (28.27, 32.52°) and the effectively perpendicular arrangement in chloro(mesityl)boryl complex B6 (88.36°). The B−C distances of the heteroaryl complexes (2a (furyl): 1.523(3); 2c (pyrrolyl): 1.529(3) Å) are significantly shorter than those of the homoaryl derivatives of Aldridge (A: 1.56(2), 1.59(2); B: 1.569(2) Å). The most salient comparison is between complexes 2a/c and the more accurately determined mesityl structure B, in which the B−C bond is ca. 3% longer than those of the heteroaryl complexes, a persuasive indicator of increased aryl−boron conjugation in the latter. Figure 2 shows significant (up to 2.7%) disparities between corresponding single and double bonds in the heteroaryl groups of 2a/c. These differences, taken together with the

Figure 2. Crystallographically determined structures of 2a (top) and 2c (bottom), with analysis of lengths of comparable bonds shown below each structure. Thermal ellipsoids represent 50% probability. Hydrogens and thermal ellipsoids of the cyclopentadienyl ligands have been omitted for clarity. Selected bond lengths (Å) and angles (deg) for 2a: Fe−B 1.971(3), B−Cl 1.809(3), B−C5 1.523(3), O1−C2 1.354(3), C2−C3 1.345(3), C3−C4 1.412(3), C4−C5 1.362(3), C5− O1 1.389(3); angle between FeBCl and furyl planes: 17.79°. For 2c: Fe−B 2.001(2), B−Cl 1.834(2), B−C3 1.529(3), N1−C2 1.351(2), C2−C3 1.395(3), C3−C4 1.436(3), C4−C5 1.358(3), C5−N1 1.376(2); angle between FeBCl and pyrrolyl planes: 8.93°.

relatively short B−C bonds in 2a/c, suggest significant contributions from alternative ylidic canonical forms of the “BHeteroaryl” fragments consisting of borataalkene and either oxocarbenium (2a) or iminium (2c) groups, with attendant BC double bonds. The phenyl moiety of the analogous chlorophenylboryl complex A5 shows a similar pattern of relatively long C−C bonds near the boron atom and shorter C−C bonds on the opposite side of the ring (d(C1−C2,6)avg is 2.8% longer than d(C4−C3,5)avg). This suggests that although the Ph and FeBCl planes are considerably less coplanar than the corresponding planes of 2a and 2c, some conjugation may also be present in A (although it should be noted that greater experimental uncertainty exists in this structure). In contrast, this effect is much less significant in the chloromesitylboryl derivative B (d(C1−C2,6)avg is 1.5% longer than d(C4−C3,5)avg), B

DOI: 10.1021/acs.inorgchem.5b00192 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry again suggesting less conjugation with the boron atom in this compound. Our initial aim in this work was to explore the reactivity of heteroaryl complexes toward reduction, halide abstraction, and Lewis-base addition, similar to a recent study of Lewis base adducts of a dihaloboryl complex of FeII, which uncovered an interesting C−C radical homocoupling reaction at the C4 atoms of boron-bound 3,5-lutidine bases.7 To this end, 2a was treated in a variety of solvents with the strong reductants lithium powder, potassium graphite, sodium naphthalenide, the TiIII reagent [Ti(NtBuAr)3] (Ar = 3,5-C6H3Me2) of Cummins et al.,8 and the MgIMgI reagent [Mg(MesNacnac)]2 (MesNacnac = (MesNCMe)2CH) of Jones and Stasch et al.9 To our surprise, in the less-polar solvents benzene, fluorobenzene, toluene, diethyl ether, and hexane, no reaction was observed with any of the reductants. In the polar solvents THF and 1,2-dimethoxyethane, complete consumption of 2a and unselective reactivity was observed, and we were unable to isolate or identify any of the products. Complexes 2a−c were additionally treated with halide-abstraction reagents aluminum trichloride, Na[BArCl4] (ArCl = 3,5-C6H3Cl2), and Na[BArF4] (ArF = 3,5-C6H3(CF3)2) and the strong Lewis bases 3,5-lutidine, trimethylphosphine, and 1,3-dimethylimidazol-2-ylidene (an N-heterocyclic carbene). Surprisingly, no reaction was observed in any of these 18 reactions. This is in marked contrast to the analogous mesityl-substituted complex B, which underwent clean halide abstraction with Na[BArF4] to form the cationic borylene complex [(η5-C5Me5)(OC)2Fe(BMes)][BArF4].5,6 The unwillingness of the heteroarylboryl complexes to undergo halide abstraction is surprising given that the π-donating heteroaryl groups would presumably serve to stabilize any resulting electron-poor cationic species.

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CONCLUSIONS As is clear from Figure 3, there are distinct spectroscopic and structural differences between the heteroaryl boryl complexes presented in this study (2a−c) and the previously reported analogous homoaryl derivatives (A, B).5,6 The heteroaryl complexes show (a) lower-frequency 11B NMR signals, (b) lower-frequency CO infrared bands, (c) more coplanarity between the aryl ring and the FeBCl planes, and (d) considerably shorter B−C bonds. These indicators are in accordance in suggesting that the heteroaryl substituents engage in significantly more π conjugation with the attached boron atom than do homoaryl substituents. This is particularly dramatic in the case of the mesityl-substituted complex, which due to steric reasons, is unable to adopt a coplanar arrangement. The conjugation observed in complexes 2a−c may also contribute to their inertness when treated with strong reductants, halide abstraction reagents, and strong Lewis acids and bases. This pattern of coplanarity, conjugation, and communication echoes that observed in organic π-conjugated compounds featuring five-membered heterocycles as linkers between boron atoms and other groups and may be useful in the search for boron-containing compounds with interesting electronic properties.1

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Figure 3. Comparison of spectroscopic and structural metrics of chloroheteroaryl (2a−c) and chlorohomoaryl (A, B) boryl complexes. thaw cycles and stored over molecular sieves. NMR spectra were acquired on a Bruker Avance 500 (1H, 500.1 MHz; 11B, 160.5 MHz; 13 C, 125.8 MHz) FT-NMR spectrometer. 1H NMR and 13C{1H} NMR spectra were referenced to external TMS via residual protons of the solvent (1H) or the solvent itself (13C). 11B{1H} NMR spectra were referenced to external BF3·Et2O. IR spectra were acquired on a JASCO FT/IR-6200 type A spectrometer. Elemental analysis was performed with a Leco CHNS-932 elemental analyzer. Reagents Na[(η5-C5Me5)Fe(CO)2],10 dichlorofur-2-ylborane,11 dichloro-5-trimethylsilylthien-2-ylborane,1p dichloropyrrol-3-ylborane,11 potassium graphite,12 sodium naphthalenide,13 Cummins’ TiIII reagent,8 [Mg(MesNacnac)]2,9 Na[BArCl4],14 Na[BArF4],15 PMe3,16 and IMe17 were prepared according to published procedures. 3,5-Lutidine was used after storing over molecular sieves, and AlCl3 was used after sublimation. The crystal data of 2a and 2c were collected on a Bruker X8-APEX 2 (APEX2 CCD-detector, Nonius FR-591 rotating anode generator) diffractometer with multilayer mirror monochromated Mo Kα radiation. The structure was solved using intrinsic phasing methods (ShelXT), refined with the ShelXL software package (see CIF files in the Supporting Information for details on software version) and expanded using Fourier techniques.18 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included in the structure

EXPERIMENTAL SECTION

General Information. All manipulations were performed either under an atmosphere of dry argon or in vacuo using standard Schlenk line or glovebox techniques. All solvents were purified by distillation from appropriate drying agents and stored under argon over molecular sieves. Deuterated solvents were degassed by several freeze−pump− C

DOI: 10.1021/acs.inorgchem.5b00192 Inorg. Chem. XXXX, XXX, XXX−XXX

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factor calculations. All hydrogen atoms were assigned to idealized geometric positions. ShelXL was interfaced with ShelXLE GUI for most of the refinement steps.19 Pictures of molecules were prepared using POV-RAY 3.6.2.20 Crystallographic data can be obtained from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac. uk/data_request/cif. Synthesis of [(η5-C5Me5)(OC)2Fe{5-BCl(OC4H2-2-Me)}] (2a). A suspension of Na[(η5-C5Me5)Fe(CO)2] (130 mg, 0.48 mmol) in toluene was treated dropwise with 5-dichloroboryl-2-methylfuran (1a, 78 mg, 0.48 mmol) in toluene (2 mL) at −70 °C. The mixture was allowed to warm to room temperature over 1 h, and the solvent was removed under vacuum. After extraction of the remaining solid with hexane, the solution was concentrated and stored at −35 °C. After 12 h, a red solid was isolated (130 mg, 0.35 mmol, 72%). Crystals suitable for X-ray diffraction were obtained from a saturated solution of 2a in toluene at −35 °C. 1H NMR (500.13 MHz, C6D6): δ 1.56 (s, 15H, C5Me5), 2.06 (s, 3H, OC4H2-5-Me), 5.87 (d, 3JHH = 3.29 Hz, 1H, OC4H2-5-Me), 7.35 (d, 3JHH = 3.29 Hz, 1H, OC4H2-5-Me) ppm. 11 1 B{ H} NMR (160.47 MHz, C6D6): δ 96.5 ppm (br, width at halfmaximum ca. 414 Hz). 13C{1H} NMR (125.77 MHz, C6D6): δ 9.60 (C5Me5), 13.92 (OC4H2-5-Me), 95.51 (C5Me5), 108.55 (OC3H-3CH), 123.05 (OC3H-4-CH), 160.20 (OC3H-5-CMe), 216.41 (CO) ppm. IR (hexane): 2024, 1997, 1913 cm−1. Anal. Calcd: C, 54.53; H, 5.38. Found: C, 54.25; H, 5.42. Crystal data for 2a: C17H20BClFeO3, Mr = 374.44, yellow block, 0.50 × 0.39 × 0.11 mm3, monoclinic space group P21/n, a = 7.578(8) Å, b = 14.719(15) Å, c = 15.415(15) Å, β = 97.71(2)°, V = 1704(3) Å3, Z = 4, ρcalcd = 1.460 g·cm−3, μ = 1.052 mm−1, F(000) = 776, T = 100(2) K, R1 = 0.0441, wR2 = 0.0763, 3663 independent reflections [2θ ≤ 53.694°], and 214 parameters. CCDC 1037424. Synthesis of [(η5-C5Me5)(OC)2Fe{5-BCl-2-SiMe3-(SC4H2)}] (2b). To a suspension of Na[(η5-C5Me5)Fe(CO)2] (120 mg, 0.44 mmol) in toluene was added 5-dichloroboryl-2-trimethylsilylthiophene (1b, 71 mg, 0.44 mmol) in toluene (2 mL) dropwise at −70 °C. The mixture was allowed to warm to room temperature over 1 h, and the solvent was removed under vacuum. After extraction of the remaining solid with hexane, the solution was concentrated and stored at −35 °C. After 48 h, an orange solid was isolated (140 mg, 0.39 mmol, 88%). 1H NMR (500.13 MHz, C6D6): δ 0.26 (s, 9H, SiMe3), 1.51 (s, 15H, C5Me5), 7.34 (t, 3JHH = 3.31 Hz, 1H, SC4H-3-H), 8.13 (d, 3JHH = 3.31 Hz, 1H, SC4H-4-H) ppm. 11B{1H} NMR (160.47 MHz, C6D6): δ 100.9 ppm (br, width at half-maximum ca. 610 Hz). 13C{1H} NMR (125.77 MHz, C6D6): δ −0.15 (SiMe3), 9.63 (C5Me5), 95.83 (C5Me5), 135.90 (SC3H-3-CH), 140.71 (SC3H-4-CH), 151.31 (SC3H-2-CSi), 216.62 (CO) ppm. 29Si{1H} NMR (99.36 MHz, C6D6): δ = −6.35 ppm. IR (hexane): 2025, 1998, 1914 cm−1. Anal. Calcd: C, 50.86; H, 5.84; S, 7.15. Found: C, 51.09; H, 6.09; S, 7.17. Synthesis of [(η5-C5Me5)(OC)2Fe{3-BCl(NC4H3-1-Me)}] (2c). A suspension of Na[(η5-C5Me5)Fe(CO)2] (130 mg, 0.48 mmol) in toluene was treated dropwise with 3-dichloroboryl-1-methyl-pyrrole (1c, 77 mg, 0.48 mmol) in toluene (2 mL) at −70 °C. The mixture was warmed to room temperature, and the solvent was removed in vacuum. Extraction with hexane and storage of the concentrated solution at −35 °C yielded a red solid (100 mg, 0.27 mmol, 56%). Crystals suitable for X-ray diffraction were obtained from a saturated solution of 2c in toluene at −35 °C. 1H NMR (500.13 MHz, C6D6): δ 1.63 (s, 15H, C5Me5), 2.74 (s, 1H, NC4H3-1-Me), 6.31 (m, 1H, NC4H3-1-Me), 7.07 (m, 1H, NC4H3-1-Me), 7.23 (m, 1H, NC4H3-1Me). 11B{1H} NMR (160.47 MHz, C6D6): δ 99.3 ppm (br, width at half-maximum ca. 495 Hz). 13C{1H} NMR (125.77 MHz, C6D6): δ 9.81 (C5Me5), 35.21 (N-Me), 95.45 (C5Me5), 117.33 (NC3H-4-CH), 123.10 (NC3H-5-CH), 130.84 (NC3H-2-CH), 217.44 (CO) ppm. IR (hexane): 2025, 1992, 1922 cm−1. Anal. Calcd: C, 54.67; H, 5.67; N, 3.75. Found: C, 53.88; H, 5.70; N, 3.30. Crystal data for 2c: C17H21NBClFeO2, Mr = 373.46, yellow block, 0.32 × 0.18 × 0.10 mm3, triclinic space group P1̅, a = 7.5571(4) Å, b = 8.5345(5) Å, c = 15.0614(9) Å, α = 93.376(3)°, β = 98.979(2)°, γ = 114.711(2)°, V = 863.25(9) Å3, Z = 2, ρcalcd = 1.437 g·cm−3, μ = 1.035 mm−1, F(000) = 388, T = 100(2) K, R1 = 0.0369, wR2 = 0.0708, 3627 independent reflections [2θ ≤ 53.54°], and 214 parameters. CCDC 1037425.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental and crystallographic details. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the European Research Council. REFERENCES

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DOI: 10.1021/acs.inorgchem.5b00192 Inorg. Chem. XXXX, XXX, XXX−XXX