New Triazaborine Chromophores: Their Synthesis via Oxazaborines

Article pubs.acs.org/Organometallics

New Triazaborine Chromophores: Their Synthesis via Oxazaborines and Electrochemical and DFT Study of Their Fundamental Properties František Josefík,† Tomás ̌ Mikysek,*,‡ Markéta Svobodová,† Petr Šimůnek,† Hana Kvapilová,§ and Jiří Ludvík§ †

Institute of Organic Chemistry and Technology, Faculty of Chemical Technology and ‡Department of Analytical Chemistry, Faculty of Chemical Technology University of Pardubice, Studentská 573, CZ-53210 Pardubice, Czech Republic § J. Heyrovský Institute of Physical Chemistry ASCR, v.v.i., Dolejškova 3, CZ-182 23 Prague 8, Czech Republic S Supporting Information *

ABSTRACT: Eight new and stable triazaborine chromophores featuring various substituents as donor and acceptor moieties were prepared and investigated. An interpretation of the measured electrochemical data (CV, RDV, and dc polarography) and UV−vis spectra and quantum chemical calculations are presented. In the homologous series the first reduction proceeds as a one-electron reversible process localized at the −NC−CN− part of the central heterocycle being in conjugation with the attached carbonyl. The first oxidation of triazaborines proceeds as a two-electron irreversible process, most probably of the ECE type, localized at the negatively charged boron atom and surrounding unsaturated structures, including the substituted phenyl ring. For a better understanding of the relationship between the structure and redox properties, the LFER approach was applied for the first oxidation as well as reduction potential using the Hammett σ (para) substituent constants. The energies of the longestwavelength absorption bands taken from UV−vis spectra were compared with the experimentally found differences Eox − Ered and with calculated HOMO−LUMO gaps, and no systematic influence of substitution was found. The calculated optimized structures and displacement of the frontier orbitals confirmed the interpretations presented above.

■

INTRODUCTION In recent years, part of materials chemistry has been devoted to the development of new organic compounds for modern technologies.1 In general, charge-transfer chromophores that exhibit solid-state fluorescence, good optical transparency, solubility, thermal stability, or biological activity are currently in demand.2,3 The history of boron-containing heterocyclic compounds started in 1958, when Dewar reported on 9-aza-10boraphenanthrene and related derivatives.4,5 Such compounds exhibited high chemical stability in warm acidic and alkaline media. Later on, boron derivatives of purine and quinazoline were reported,6 initially targeted to their use in boron-neutron capture therapy (BNCT).7 At the beginning of new millennium various types of boron heterocycles with common features started to appear; boron was coordinated between two heteroatoms, mainly nitrogen and oxygen, and moreover substituted by two phenyl groups or two fluorine atoms. Such an arrangement is often called “super-acceptor” thanks to the deficit of electrons at the nitrogen atom due to boron coordination and the presence of electron-acceptor groups at the boron atom. An increase of publications was also registered after the year 2000, when modern technologies dealing with boron-based materials started to play a significant role. Papers related to boron heterocyclic compounds were more focused on descriptions of their physicochemical and chemical properties, and they were tested for photoluminescence, electroluminescence, and nonlinear optical properties.8−11 © XXXX American Chemical Society

Some of them are applicable to the construction of organic light-emitting devices (OLED) and organic thin-film transistors (OTFT).12−14 The research on dipyrromethane boron derivatives (BODIPY) which exhibit excellent thermal, chemical, and photochemical stability and high quantum yields of fluorescecence9,15−17 also played an important role. Another wide group of boron heterocycle structures consists of the scorpionates, which showed suitable properties for OLED.18 In addition to optoelectronics some of the boron heterocycles were also tested for their biological activity. Notably, the derivatives of diazaborine exhibited antibacterial and anticancer activity.3,7,19,20 This paper continues previously published work21−24 which relates to triazaborine and oxazaborine compounds. The present contribution deals with synthesis, electrochemical characterization of redox properties, and HOMO−LUMO characterization of a new series of triazaborine chromophores recently synthesized via oxazaborines (Scheme 1). The experimental data and their interpretation can be further employed in the design and development of new structures of organic boron compounds and in “tuning” of their properties. Special Issue: Organometallic Electrochemistry Received: March 3, 2014

A

dx.doi.org/10.1021/om500219g | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

Scheme 1. Triazaborines Prepared and Studied in This Work

Scheme 3. Synthesis of Oxazaborines 3

■

RESULTS AND DISCUSSION Synthesis. The starting β-enaminone 1 (1-azepan-2ylideneacetone) was prepared by a two-step synthesis, starting from acetylacetone and 7-methoxy-3,4,5,6-tetrahydro-2H-azepine. Acetylacetone reacts with the 7-methoxy-3,4,5,6-tetrahydro-2H-azepine in the presence of a catalytic amount of Ni(acac)2 at 100 °C to form 3-azepan-2-ylidenepentane-2,4dione,25 which undergoes deacetylation by sodium methanolate in methanol26 to form 1-azepan-2-ylideneacetone (1) (Scheme 2). Two strategies (A and B) were used for the synthesis of oxazaborines 3 (Scheme 3). Procedure A is suitable for the oxazaborines having X as electron donating or slightly electron withdrawing (halogen) groups.24 The method, however, cannot be used for the synthesis of oxazaborines bearing electronwithdrawing groups due to the instability of the corresponding diazonium tetraphenylborates. In this case, the two-step reaction (B) has to be applied (Scheme 3). The approach consists of the preparation of the intermediary azo compound 2, which can be transformed to oxazaborine 3 via its reaction with triphenylborane.21 This trick allowed us to prepare oxazaborines having X = COOEt, CN, NO2 in acceptable yields of 50−68%. The mechanism of the formation of oxazaborines is suggested in ref 21. Oxazaborines 3 were then subjected to thermal rearrangement to the corresponding triazaborines 4 (Scheme 4). The rearrangement is discussed in more detail in ref 21. Electrochemistry. The fundamental electrochemical characterization of oxidation and reduction abilities of the newly synthesized triazaborine molecules 4 were performed in dried dimethylformamide (DMF). 27 To discern the possible influence of electrode material on the redox processes, platinum and mercury electrodes were used and the data were compared. To identify reversible and irreversible processes, “steady-state” techniques such as rotating disk voltammetry (RDV) and dc polarography (dc-P) as well as a “transient” method (cyclic voltammetry, CV) were applied. Four types of electrodes were used: Pt stationary electrode and hanging mercury drop electrode (HMDE) for CV, Pt rotating disk electrode (PtRDE) for RDV, and dropping mercury electrode (DME) for dc-P. To distinguish diffusion-controlled processes from adsorption phenomena, dependences of limiting or peak currents on concentration, scan rate (in CV), and rotation speed (in RDV) were determined and evaluated.

Scheme 4. Thermal Rearrangement of Oxazaborines 3 to Triazaborines 4

As a result, electrochemical data from dc-P, RDV, and CV on Pt as well as on HMDE for each respective compound are consistent: that is, the material of the electrode plays no significant role and the first reduction and oxidation processes are transport-controlled. The complete electrochemical data are summarized in Table 1. The main attention was given to the first oxidation and reduction potentials and their reversibility, since their values can be related to HOMO and LUMO energies; their difference reflects the HOMO−LUMO gap and influences the low-energy band of the UV−vis spectra. For an understanding of the structure−redox properties relationship and for confirmation/exclusion of the analogous redox mechanisms, the electronic influence of individual substituents on the reduction and oxidation potential was followed. For this reason the LFER (linear free energy relationship) approach of the Hammett type was applied28 for the first oxidation and reduction potentials with utilization of the σ (para) substituent constants (Table 1 and Figure 1). Reduction. The first reduction of all compounds 4a−h proceeds at potentials of −1.0 to −1.55 V (vs SCE) as a oneelectron reversible process. The cathodic/anodic peak separation 60−75 mV, the cathodic/anodic peak current ratio close to unity, and the independence of the peak potential on scan rate (in CV) as well as the half-wave potentials on rotation speed (in RDV) confirm this interpretation.

Scheme 2. Synthesis of the Starting β-Enaminone 1

B


Organometallics

Article

Table 1. Electrochemical Data of Studied Compounds oxidationg compd 4a 4b 4c 4d 4e 4f 4g 4h

a

Epa(CV), V 0.64 1.25 1.42 1.51 1.54 f f f

b

reductiong

E1/2(RDV), V

E°(CV), V

0.65 1.20 1.33 1.41 1.445 f 1.62 1.65

−1.53 −1.47 −1.435 −1.39 −1.325 −1.23 −1.165 −1.00

c

d

ΔEp, mV

E°(HMDE), V

75 65 75 65 70 90 65 60

−1.52 −1.47 −1.44 −1.39 −1.33 −1.23 −1.16 −0.99

c

ΔEp, mV

E1/2(RDV), V

E1/2(dc-P), V

σpe

65 60 70 75 80 90 75 60

−1.52 −1.47 −1.44 −1.39 −1.32 −1.022 −1.17 −1.00

−1.53 −1.47 −1.43 −1.39 −1.32 −1.20 −1.17 −1.00

−0.63 −0.28 −0.13 0 0.23 0.43 0.71 0.81

Anodic peak. bE° of the reversible couple ((Epa + Epc)/2). c(Epa + Epc)/2. dEpa − Epc eHammett σ (para) constant. fNonevaluable shoulder. gVs SCE.

a

Figure 1. Dependence of the first reduction (a) and oxidation potentials (b) of compounds 4a−h on the respective Hammett σ (para) substituent constants.

From the Ered − σp plot (Figure 1a) it is evident that the reduction potentials of 4a−g follow well a straight line with the slope 0.29 V/σp unit; this means that all of these compounds are reduced according to an analogous mechanism, the reduction centers in the molecules are the same, and the localizations of the LUMO should be similar. However, the only exception is compound 4h, which is reduced at a much less negative potential; thus, the data for 4h do not fit this graph. It is evident that the nitro group is reduced more easily than the rest of the molecule; the nitro group itself represents the reduction center, and the reduction mechanism is therefore completely different. A typical electrochemical behavior, analogous for compounds 4b−g, is presented in Figure 2, and the CV and RDV curves of 4a are shown in Figure 3. Oxidation. From a comparison of the limiting oxidation current with the one-electron-reduction current of the same compound recorded at the Pt-RDE (Figure 2b), it follows that the first oxidation of triazaborines 4b−h proceeds as a twoelectron irreversible process, most probably of the ECE type. The LFER treatment revealed that the first oxidation potentials follow a linear dependence on σp (Figure 1b), pointing to the fact that the oxidation of 4b−e,g,h proceeds analogously at the same oxidation center and therefore a similar displacement of HOMO can be expected. (In the case of 4f, only a nonevaluable anodic shoulder in CV and RDV was obtained in the same potential region.) The anomalous case not fitting the Hammett plot is the diethylamino derivative 4a, where a reversible one-electron oxidation proceeds at much lower potentials (Figure 3). This oxidation was attributed to the one-electron reversible

oxidation of the diethylaniline moiety preceding the oxidation of the core. This type of reversible oxidation was observed for example in the case of bis(dimethylanilino)imidazoles.29 In all cases, the oxidation behaviors (on the cyclic voltammogram) are identical when the potential is first scanned to positive potentials and when the reduction proceeds first. The same situation occurs with reductions. This means that the oxidation and reduction occur independently at different parts of the triazaborine molecules. In addition to this, a higher slope (reaction constant, Hammett ρ value) is observed for oxidation than for reduction. The substitution thus influences the oxidation center more strongly: in other words, the oxidation center should be closer to the substituent than the reduction center and/or some kind of delocalized system must interconnect the oxidation center and the substituent. Quantum Chemical Calculations. In order to correlate the experimental electrochemical data with the theory, quantum chemical calculations of HOMO and LUMO energies were performed. The results are collected in Table 2. Since the experimental values of the first reduction potential should correlate with the LUMO energy and the first oxidation potential should correlate with the HOMO energy, the calculated values were plotted versus the experimental data (Figure 4). The good correlation confirms the credibility and reliability of the calculations. (This is especially important in the anomalous cases of compounds 4a,h.) UV−Vis Spectrometry. Optical properties of the target compounds were investigated by the absorption electron spectra measured in acetonitrile in the UV−vis region (Figure 5). The wavelengths of absorption maxima and their energies in C


Organometallics

Article

Figure 2. Electrochemical behavior of 4b: (a) cyclic voltammetry, scan rate v = 100 mV/s; (b) Pt-RDV, rotation frequencies f = 500 and 1500 min−1; (c) dc polarography, concentrations 0.5 and 1 mM.

Figure 3. Electrochemical behavior of 4a: (a) cyclic voltammetry, scan rate v = 100 mV/s; (b) Pt-RDV, rotation frequencies f = 500 and 1500 min−1.

Table 2. Quantum Chemical Calculations and Optical and Electrochemical Data of Compounds 4a−h calcd values

a

exptl data

compd

HOMO, eV

LUMO, eV

HOMO−LUMO gap, eV

Eox − Ered, V

4a 4b 4c 4d 4e 4f 4g 4h

−5.21 −5.80 −5.96 −6.05 −6.08 −6.16 −6.22 −6.29

−2.26 −2.36 −2.41 −2.44 −2.52 −2.63 −2.71 −3.02

2.95 3.44 3.55 3.61 3.56 3.53 3.51 3.27

2.17 2.67 2.77 2.80 2.77 2.79 2.65

a

λmax, nm (eV) 470 425 420 417 420 422 427 436

(2.64) (2.92) (2.95) (2.97) (2.95) (2.94) (2.90) (2.84)

E1/2(ox1) − E1/2(red1) based on the data from Table 1. D


Organometallics

Article

Figure 4. Correlations between (a) calculated HOMO energy and first oxidation potential and (b) calculated LUMO energy and first reduction potential.

Figure 6. Influence of the substitution of compounds 4b−g on the calculated HOMO−LUMO gap (▲), energy of the longest-wavelength band in the UV−vis spectrum (■), and experimental difference Eox − Ered (●). The respective data for compounds 4a,h are represented by the open symbols ○, △, and □.

Figure 5. UV−vis absorption spectra of compounds 4a (1), 4d (2), and 4h (3) measured in acetonitrile. The spectra of 4b,c,e−g are very similar to that of 4d.

has been studied previously.24 No fluorescence in the solution state at room temperature has been observed. The calculated optimized structures and displacement of the frontier orbitals shown in Figure 7 confirmed the interpretations presented above. (In addition to the anomalous compounds 4a,h, the localization of the HOMO and LUMO of 4d is presented for comparison as a typical case.) It follows from the HOMO and LUMO “maps” that the oxidation center in the 4a is localized prevalently at the diethylaniline moiety (Figure 7a), whereas the oxidation center in all other derivatives involves the negatively charged boron atom and surrounding unsaturated structures including the substituted phenyl ring (Figure 7c,e). The reduction center in 4h is localized at the nitro group and the π-electron system is extended to the phenyl ring under formation of a quinone-like system (Figure 7f). The other derivatives, however, have their reduction center localized at the −NC−CN− part of the central heterocycle being in conjugation with the attached carbonyl (acetyl) (Figure 7b,d). The reduction center is distant and separate from the substituents X; therefore, their influence on the reduction potential is low. On the other hand, the oxidation center is closer to the substitution and is interconnected through the phenyl ring. This explains the higher Hammett ρ value for reductions. It is noteworthy that the two phenyl rings attached at the boron atom are not involved in any π system due to their nearly perpendicular position toward the rest of the molecule.

eV are summarized in Table 2. The intense longest-wavelength absorption bands with λmax between 417 and 470 nm dominate in the spectra. The most bathochromically shifted band was obtained for compound 4a (λmax 470 nm). It reflects the presence of the strong N,N-diethylamino donor which is engaged more efficiently in D−A (donor−acceptor) interactions than the other substituents. The energies of the longestwavelength absorption bands were compared with the calculated HOMO−LUMO gaps and with the experimentally acquired difference between the first oxidation and reduction potentials, respectively (Table 2). The plot of the three aforementioned energies versus the Hammett σ constants (Figure 6) shows two results. (a) In the series 4b−g, the substitution has no systematic influence on the λmax, HOMO− LUMO gap and Eox − Ered values (solid symbols in Figure 6). On the other hand, compounds 4a,h having a different oxidation (diethylaniline) or reduction center (nitro group), exhibit different energies and spectra (open symbols in Figure 6). (b) The energies of the λmax, HOMO−LUMO gap, and Eox − Ered values correlate mutually very well, differing only in additive constants. Structural Aspects. X-ray diffraction data of some relevant oxazaborines and triazaborines have been reported previously.24 In addition to this, all of the prepared triazaborines fluoresce in the solid state upon illumination with a UV lamp (254/366 nm). The fluorescence behavior of some compounds (4b,c,e) in both the solid state and frozen 2-Me THF solution (at 77 K) E


Organometallics

Article

Figure 7. Calculated distribution of the HOMO and LUMO in molecules 4a,d,h.

■

CONCLUSIONS A new series of substituted triazaborine chromophores was synthesized via oxazaborines and their thermal rearrangement. Their oxidation and reduction abilities were investigated electrochemically using rotating-disk voltammetry, cyclic voltammetry, and dc polarography. The electrode material plays no role, and all processes are transport-controlled. The main attention was given to the reversibility of the process and to the first oxidation and reduction potentials, since their values can be related to HOMO and LUMO energies. For an understanding of the relationship between the structure and redox properties, the electronic influence of individual substituents on the reduction and oxidation potentials was followed. For this reason the LFER approach was applied to the first oxidation as well as the reduction potential using the Hammett σ (para) substituent constants. The first reduction of all compounds 4a−h proceeds at potentials of −1.0 to −1.55 V (vs SCE) as a one-electron reversible process. It was found from the Hammett plot that the reduction mechanism is analogous for all studied compounds, except for the nitro derivative, where the nitro group itself is reduced at less negative potentials. The first oxidation of triazaborines 4b−h proceeds as a twoelectron irreversible process, most probably of the ECE type. The only anomalous case is for the diethylamine derivative, where the one-electron reversible oxidation of the diethylaniline moiety precedes the oxidation of the core. Using the DFT method, the energies of the HOMO and LUMO were calculated and compared with experimental values of oxidation and reduction potentials, respectively. The good correlation confirms the credibility and reliability of the calculations. The energies of the longest-wavelength absorption bands taken from UV−vis spectra were compared with the experimentally found differences Eox − Ered and with calculated HOMO−LUMO gaps. The plot of the three aforementioned energies versus the Hammett σ constants shows that in the

series 4b−g no systematic influence of the substitution exists and the energies of the λmax, HOMO−LUMO gap, and Eox − Ered values correlate mutually very well. On the other hand, the compounds 4a,h, having a different oxidation (diethylaniline) or reduction center (nitro group), exhibit bathochromically shifted spectra and lower HOMO−LUMO gaps. The calculated optimized structures and displacement of the frontier orbitals confirmed the interpretations presented above that the oxidation center of 4a is localized prevalently at the diethylaniline moiety, whereas the reduction center of 4h is localized at the nitro group. The oxidation center in all other derivatives involves the negatively charged boron atom and surrounding unsaturated structures, including the substituted phenyl ring; the reduction center is localized at the −NC− CN− part of the central heterocycle being in conjugation with the attached carbonyl. All aforementioned derivatives of triazaborine exhibit solid-state fluorescence, which points to their potential use in optoelectronics.

■

EXPERIMENTAL SECTION

NMR spectra were measured in CDCl3 using a Bruker AVANCE III spectrometer operating at 400.13 (1H) and 100.62 MHz (13C). All of the pulse sequences were taken from the Bruker software library. Proton spectra measured were calibrated on the basis of internal TMS (δ 0.00). Carbon NMR spectra were calibrated on the middle signal of the solvent multiplet (δ 77.16). Elemental analyses were performed on a Flash 2000 CHNS Elemental Analyzer. Melting points were measured on a Kofler Boetius PHMK 80/2644 hot-stage microscope and were not corrected. MALDI HRMS were measured using a MALDI LTQ Orbitrap XL instrument (Thermo Fisher Scientific) with DCTB (trans-2-[3-(4-tert-butylphenyl)-2-methylprop-2-en-1-ylidene]malononitrile) as the matrix dissolved in acetonitrile. UV−vis spectra were recorded on a Shimadzu UV-2450 spectrophotometer in acetonitrile. All solvents and reagents were purchased from commercial suppliers and were used without further purification. Diazonium tetrafluoroborates were prepared according to an ordinary procedure (diazotization using cold NaNO2/HCl with subsequent precipitation, from the filtered solution of the diazonium chloride, by aqueous sodium tetrafluoroborate). Precipitated diazoF


Organometallics

Article

nium tetrafluoroborates were dried in vacuo in a desiccator and stored in a refrigerator. Diazonium tetraphenylborates were prepared immediately before use according to the literature procedure.21 Caution! Diazonium tetraphenylborates in the crystalline state can undergo explosive decomposition even with very slight mechanical stimulation! Enaminone 1 was prepared according to the procedures reported in ref 24. General Procedures for the Preparation of Oxazaborines 3. Method A. This methodology was adopted from ref 24. Method B. To a cold (5 °C) mixture of β-enaminone 1 (5 mmol) and sodium acetate (5 mmol) in dry dichloromethane (15 mL) was added benzenediazonium tetrafluoroborate (5 mmol). The reaction mixture was stirred at room temperature for 24 h. Sodium acetate was removed by suction, and to the crude reaction mixture was added BPh3 (10 mmol). The solution was stirred for 24 h at room temperature. The solvent was evaporated in vacuo. The crude residue was chromatographed on silica gel with dichloromethane as the mobile phase to give the corresponding oxazaborines 3. Compound 3h was too unstable to be characterized properly. It decomposed upon an attempt to purify it; hence, the crude reaction mixture was subjected to thermal rearrangement (vide infra) and the corresponding triazaborine 4h was then purified and identified. The following compounds were prepared according to the aforementioned procedure. 4-(4-Diethylaminophenyldiazenyl)-3-methyl-1,1-diphenyl6,7,8,9-tetrahydro-5H-[1,3,2 λ4]oxazaborino[3,4-a]azepine (3a). Yield: 26%. Mp: 193−195 °C. 1H NMR (400 MHz, CDCl3): δ 7.61−7.52 (m, 2H), 7.40−7.34 (m, 4H), 7.29−7.17 (m, 6H), 6.70− 6.60 (m, 2H), 3.63−3.53 (m, 2H), 3.45−3.31 (m, 4H), 3.22−3.16 (m, 2H), 2.48 (s, 3H), 1.80−1.65 (m, 4H), 1.40−1.30 (m, 2H), 1.23−1.12 (m, 6H). 13C NMR (101 MHz, CDCl3): δ 175.8, 173.3, 148.8, 143.4, 134.8, 133.3, 131.8, 127.2, 126.4, 123.6, 111.2, 51.1, 44.7, 29.8, 29.8, 26.3, 23.7, 22.3, 12.8. Anal. Calcd for C31H37BN4O (492.46): C, 75.61; H, 7.57; N, 11.38. Found: C, 75.42; H, 7.51; N 11.20. 4-(4-Methoxyphenyldiazenyl)-3-methyl-1,1-diphenyl-6,7,8,9-tetrahydro-5H-[1,3,2 λ4]oxazaborino[3,4-a]azepine (3b). Yield: 42%. Mp: 160−162 °C. 1H NMR (400 MHz, CDCl3): δ 7.65−7.57 (m, 2H), 7.40−7.35 (m, 4H), 7.30−7.20 (m, 6H), 6.96−6.89 (m, 2H), 3.83 (s, 3H), 3.64−3.57 (m, 2H), 3.28−3.18 (m, 2H), 2.53 (s, 3H), 1.80−1.70 (m, 4H), 1.41−1.32 (m, 2H). 13C NMR (101 MHz, CDCl3) δ 177.8, 173.4, 160.6, 147.5, 147.3, 133.3, 131.6, 127.2, 126.6, 123.2, 114.2, 55.6, 51.2, 29.8, 29.8, 26.2, 23.6, 22.7. Anal. Calcd for C28H30BN3O2 (451.37): C, 74.51; H, 6.70; N, 9.31. Found: C, 74.79; H, 6.76; N, 9.02. 3-Methyl-4-(4-methylphenyldiazenyl)-1,1-diphenyl-6,7,8,9-tetrahydro-5H-[1,3,2 λ4]oxazaborino[3,4-a]azepine (3c). Yield: 29%. Mp: 135−137 °C. 1H NMR (400 MHz, CDCl3): δ 7.56−7.51 (m, 2H), 7.39−7.33 (m, 4H), 7.30−7.17 (m, 8H), 3.64−3.54 (m, 2H), 3.26− 3.20 (m, 2H), 2.55 (s, 3H), 2.37 (s, 3H), 1.78−1.68 (m, 4H), 1.40− 1.30 (m, 2H). 13C NMR (101 MHz, CDCl3) δ 178.4, 173.5, 151.1, 139.4, 134.8, 133.3, 131.6, 129.7, 127.3, 126.6, 121.7, 51.2, 29.9, 29.8, 26.2, 23.5, 22.8, 21.5. Anal. Calcd for C28H30BN3O (435.37): C, 77.24; H, 6.95; N, 9.65. Found: C, 77.49; H, 7.03; N, 9.51. 3-Methyl-4-(phenyldiazenyl)-1,1-diphenyl-6,7,8,9-tetrahydro-5H[1,3,2 λ4]oxazaborino[3,4-a]azepine (3d). Yield: 57%. Mp: 137−139 °C. 1H NMR (400 MHz, CDCl3): δ 7.66−7.61 (m, 2H), 7.42−7.35 (m, 7H), 7.30−7.21 (m, 6H), 3.70−3.54 (m, 2H), 3.32−3.23 (m, 2H), 2.57 (s, 3H), 1.78−1.70 (m, 4H), 1.42−1.32 (m, 2H). 13C NMR (101 MHz, CDCl3) δ 179.0, 173.5, 153.1, 134.9, 133.3, 131.7, 129.0, 127.3, 127.1, 126.7, 121.8, 51.3, 29.9, 29.8, 26.1, 23.5, 22.9. HRMS (MALDI): calcd for C27H28BN3O [M + H]+ 422.23982 Da, [M + Na]+ 444.22177 Da; found [M + H]+ 422.24027 Da, [M + Na]+ 444.22214 Da. 4-(4-Bromophenyldiazenyl)-3-methyl-1,1-diphenyl-6,7,8,9-tetrahydro-5H-[1,3,2 λ4]oxazaborino[3,4-a]azepine (3e). Yield: 48%. Mp: 163−165 °C. 1H NMR (400 MHz, CDCl3): δ 7.55−7.47 (m, 4H), 7.38−7.34 (m, 4H), 7.30−7.20 (m, 6H), 3.68−3.52 (m, 2H), 3.30− 3.19 (m, 2H), 2.56 (s, 3H), 1.80−1.65 (m, 4H), 1.40−1.33 (m, 2H). 13 C NMR (101 MHz, CDCl3): δ 179.6, 173.5, 151.9, 133.2, 132.2, 131.6, 128.1, 127.3, 126.7, 123.2, 122.9, 51.3, 29.9, 29.7, 26.1, 23.5,

23.1. Anal. Calcd for C27H27BBrN3O (500.24): C, 64.83; H, 5.44; N, 8.40. Found: C, 64.93; H, 5.47; N, 8.19. 4-(4-Ethoxycarbonylphenyldiazenyl)-3-methyl-1,1-diphenyl6,7,8,9-tetrahydro-5H-[1,3,2 λ4]oxazaborino[3,4-a]azepine (3f). Yield: 50%. Mp: 158−160 °C. 1H NMR (400 MHz, CDCl3): δ 8.10−8.06 (m, 2H), 7.66−7.62 (m, 2H), 7.40−7.20 (m, 10H), 4.42− 4.34 (m, 2H), 3.69−3.57 (m, 2H), 3.30−3.26 (m, 2H), 2.60 (s, 2H), 1.80−1.70 (m, 4H), 1.43−1.30 (m, 5H). 13C NMR (101 MHz, CDCl3): δ 180.5, 173.6, 166.4, 155.8, 133.2, 131.9, 130.9 130.6, 130.3, 127.4, 126.8, 121.5, 61.2, 51.4, 29.9, 29.7, 26.0, 23.4, 23.3, 14.5. Anal. Calcd for C30H32BN3O3 (493.40): C, 73.03; H, 6.54; N, 8.52. Found: C, 72.81; H, 6.59; N, 8.29. 4-(4-Cyanophenyldiazenyl)-3-methyl-1,1-diphenyl-6,7,8,9-tetrahydro-5H-[1,3,2 λ4]oxazaborino[3,4-a]azepine (3g). Yield: 69%. Mp: 213−215 °C. 1H NMR (400 MHz, CDCl3): δ 7.72−7.61 (m, 4H), 7.38−7.33 (m, 4H), 7.33−7.20 (m, 6H), 3.71−3.58 (m, 2H), 3.34− 3.22 (m, 2H), 2.61 (s, 3H), 1.80−1.70 (m, 4H), 1.42−1.32 (m, 2H). 13 C NMR (101 MHz, CDCl3): δ 181.5, 173.6, 155.3, 134.9, 133.2, 131.9, 128.1, 127.4, 126.9, 122.2, 119.0, 111.6, 51.5, 30.0, 29.6, 26.0, 23.5, 23.3. Anal. Calcd for C28H27BN4O (446.35): C, 75.34; H, 6.10; N, 12.55. Found: C, 75.27; H, 6.18; N, 12.29. 3-Methyl-4-(4-nitrophenyldiazenyl)-1,1-diphenyl-6,7,8,9-tetrahydro-5H-[1,3,2 λ4]oxazaborino[3,4-a]azepine (3h). Yield: 61%. 1H NMR (400 MHz, CDCl3): δ 8.29−8.27 (m, 2H), 7.71−7.69 (m, 2H), 7.39−7.33 (m, 4H), 7.33−7.22 (m, 6H), 3.70−3.61 (m, 2H), 3.36− 3.26 (m, 2H), 2.63 (s, 3H), 1.80−1.74 (m, 4H), 1.40−1.35 (m, 2H). HRMS (MALDI): calcd for C27H27BN4O3 [M + H]+ 467.22490 Da; found [M + H]+ 467.22547 Da. General Procedure for the Preparation of Triazaborines 4.24 Oxazaborines 3 (2 mmol) were heated at reflux in DMF (10 mL) for the appropriate time (shown for the particular compound). The reaction was monitored by TLC. The solvent was evaporated in vacuo, and the crude reaction mixture was chromatographed on silica gel with dichloromethane as the mobile phase to give the corresponding triazaborines. The following compounds were prepared using the aforementioned procedure. 4-Acetyl-2-(4-diethylaminophenyl)-1,1-diphenyl-2,5,6,7,8,9-hexahydro-1H-[1,2,4,3 λ4]-triazaborino[4,5-d]azepine (4a). Yield: 1% (10 h). Mp: 194−196 °C. 1H NMR (400 MHz, CDCl3): δ 7.43−7.38 (m, 4H), 7.25−7.17 (m, 6H), 7.14−7.09 (m, 2H), 6.38−6.28 (m, 2H), 3.53−3.46 (m, 2H), 3.28−3.20 (m, 6H), 2.52 (s, 3H), 1.65−1.55 (m, 4H), 1.28−1.23 (m, 2H), 1.13−1.02 (m, 6H). 13C NMR (101 MHz, CDCl3): δ 196.4, 164.3, 146.3, 137.6, 134.1, 134.1, 133.1, 127.5, 126.5, 124.5, 110.3, 51.5, 44.4, 29.5, 29.5, 27.5, 27.0, 23.1, 12.7. HRMS (MALDI): calcd for C31H37BN4O [M]+ 492.30549 Da, [M + Na]+ 515.29526 Da; found [M]+ 492.30507 Da, [M + Na]+ 515.29486 Da. 4-Acetyl-2-(4-methoxyphenyl)-1,1-diphenyl-2,5,6,7,8,9-hexahydro-1H-[1,2,4,3 λ4]-triazaborino[4,5-d]azepine (4b). Yield: 6% (20 h). Mp: 143−145 °C. 1H NMR (400 MHz, CDCl3): δ 7.42−7.38 (m, 4H), 7.26−7.16 (m, 8H), 6.59−6.52 (m, 2H), 3.66 (s, 3H), 3.56−3.50 (m, 2H), 3.25−3.19 (m, 2H), 2.52 (s, 3H), 1.66−1.59 (m, 4H), 1.31− 1.22 (m, 2H). 13C NMR (101 MHz, CDCl3): δ 196.3, 164.8, 157.9, 145.5, 141.8, 134.0, 133.8, 127.6, 126.7, 124.4, 113.0, 55.4, 51.8, 29.8, 29.4, 27.4, 26.9, 22.8. Anal. Calcd for C28H30BN3O2 (451.37): C, 74.51; H, 6.70; N, 9.31. Found: C, 74.30; H, 6.69; N, 9.10. 4-Acetyl-2-(4-methylphenyl)-1,1-diphenyl-2,5,6,7,8,9-hexahydro1H-[1,2,4,3 λ4]-triazaborino[4,5-d]azepine (4c). Yield: 18% (9 h). Mp: 141−143 °C. 1H NMR (400 MHz, CDCl3): δ 7.43−7.38 (m, 4H), 7.25−7.17 (m, 6H), 7.14−7.10 (m, 2H), 6.87−6.82 (m, 2H), 3.58−3.50 (m, 2H), 3.24−3.19 (m, 2H), 2.52 (s, 3H), 2.19 (s, 3H), 1.65−1.58 (m, 4H), 1.31−1.24 (m, 2H). 13C NMR (101 MHz, CDCl3): δ 196.4, 165.0, 145.8, 145.5, 136.0, 134.1, 128.4, 127.6, 126.7, 123.0, 51.9, 29.8, 29.5, 27.4, 26.9, 22.7, 21.0. Anal. Calcd for C28H30BN3O (435.37): C, 77.24; H, 6.95; N, 9.65. Found: C, 77.41; H, 7.02; N, 9.39. 4-Acetyl-2-phenyl-1,1-diphenyl-2,5,6,7,8,9-hexahydro-1H-[1,2,4,3 λ4]-triazaborino[4,5-d]azepine (4d). Yield: 7% (6 h). Mp: 159−161 °C. 1H NMR (400 MHz, CDCl3): δ 7.44−7.39 (m, 4H), 7.25−7.21 (m, 6H), 7.20−7.17 (m, 2H), 7.08−6.99 (m, 3H), 3.60−3.49 (m, 2H), 3.24−3.19 (m, 2H), 2.54 (s, 3H), 1.65−1.59 (m, 4H), 1.32−1.23 (m, G


Organometallics

Article

2H). 13C NMR (101 MHz, CDCl3): δ 196.4, 165.1, 148.0, 134.3, 134.1, 127.8, 127.6, 126.7, 126.1, 123.2, 52.0, 29.9, 29.4, 27.3, 26.8, 22.7. Anal. Calcd for C27H28BN3O (421.34): C, 76.97; H, 6.70; N, 9.97. Found: C, 77.10; H, 6.68; N, 9.79. 4-Acetyl-2-(4-bromophenyl)-1,1-diphenyl-2,5,6,7,8,9-hexahydro1H-[1,2,4,3 λ4]-triazaborino[4,5-d]azepine (4e). Yield: 20% (14 h). Mp: 136−137 °C. 1H NMR (400 MHz, CDCl3): δ 7.43−7.38 (m, 4H), 7.27−7.18 (m, 6H), 7.17−7.09 (m, 4H), 3.58−3.51 (m, 2H), 3.22−3.16 (m, 2H), 2.53 (s, 3H), 1.65−1.59 (m, 4H), 1.33−1.22 (m, 2H). 13C NMR (101 MHz, CDCl3): δ 196.3, 165.4, 147.0, 144.9, 134.4, 134.0, 130.9, 127.8, 126.9, 124.5, 119.6, 52.1, 30.0, 29.4, 27.3, 26.7, 22.5. Anal. Calcd for C27H27BBrN3O (500.24): C, 64.83; H, 5.44; N, 8.40. Found: C, 65.03; H, 5.44; N, 8.12. 4-Acetyl-2-(4-ethoxycarbonylphenyl)-1,1-diphenyl-2,5,6,7,8,9hexahydro-1H-[1,2,4,3 λ4]-triazaborino[4,5-d]azepine (4f). Yield: 30% (4 h). Mp: 131−133 °C. 1H NMR (400 MHz, CDCl3): δ 7.77−7.70 (m, 2H), 7.45−7.40 (m, 4H), 7.33−7.28 (m, 2H), 7.27− 7.17 (m, 6H), 4.35−4.23 (m, 2H), 3.67−3.49 (m, 2H), 3.23−3.15 (m, 2H), 2.56 (s, 3H), 1.68−1.60 (m, 4H), 1.39−1.21 (m, 5H). 13C NMR (101 MHz, CDCl3): δ 196.5, 166.3, 165.6, 151.3, 144.8, 134.8, 134.0, 129.4, 127.8, 127.3, 127.0, 122.5, 60.9, 52.3, 30.1, 29.4, 27.3, 26.7, 22.5, 14.4. Anal. Calcd for C30H32BN3O3 (493.40): C, 73.03; H, 6.54; N, 8.52. Found: C, 72.73; H, 6.45; N, 8.22. 4-Acetyl-2-(4-cyanophenyl)-1,1-diphenyl-2,5,6,7,8,9-hexahydro1H-[1,2,4,3 λ4]-triazaborino[4,5-d]azepine (4g). Yield: 50% (4 h). Mp: 136−138 °C. 1H NMR (400 MHz, CDCl3): δ 7.45−7.40 (m, 4H), 7.34−7.29 (m, 4H), 7.27−7.20 (m, 6H), 3.62−3.51 (m, 2H), 3.20−3.15 (m, 2H), 2.56 (s, 3H), 1.66−1.61 (m, 4H), 1.30−1.22 (m, 2H). 13C NMR (101 MHz, CDCl3): δ 196.3, 165.9, 151.0, 144.5, 135.1, 133.9, 131.9, 128.0, 127.2, 123.0, 119.0, 108.4, 52.5, 30.3, 29.4, 27.3, 26.6, 22.3. Anal. Calcd for C28H27BN4O: C, 75.34; H, 6.10; N, 12.55. Found: C, 75.60; H, 6.17; N, 12.30. 4-Acetyl-2-(4-nitrophenyl)-1,1-diphenyl-2,5,6,7,8,9-hexahydro1H-[1,2,4,3 λ4]-triazaborino[4,5-d]azepine (4h). Yield: 50% (4 h). Mp: 189−191 °C. 1H NMR (400 MHz, CDCl3): δ 7.93−7.88 (m, 2H), 7.47−7.43 (m, 4H), 7.41−7.36 (m, 2H), 7.30−7.20 (m, 6H), 3.64−3.52 (m, 2H), 3.20−3.15 (m, 2H), 2.58 (s, 3H), 1.68−1.60 (m, 4H), 1.30−1.24 (m, 2H). 13C NMR (101 MHz, CDCl3): δ 196.3, 166.0, 152.6, 144.5, 135.3, 133.9, 128.0, 127.3, 123.6, 122.6, 52.6, 30.4, 29.4, 27.3, 26.6, 22.2. HRMS (MALDI): calcd for C27H27BN4O3 [M + H]+ 467.22490 Da, [M + Na]+ 489.20684 Da; found [M + H]+ 467.22551 Da, [M + Na]+ 489.20731 Da. Electrochemistry. Electrochemical measurements were carried out in N,N-dimethylformamide dried by azeotropic distillation (in benzene) followed by fractionation at reduced pressure27 containing 0.1 M Bu4NPF6. dc polarography, cyclic voltammetry (CV), and rotating disk voltammetry (RDV) were used in a three-electrode arrangement. The working electrode was a dropping mercury electrode (DME) for polarographic measurements (drop time τD = 1 s, scan rate v = 5 mV s−1), a hanging mercury drop electrode (HMDE) for CV, and a platinum disk (2 mm in diameter) for oxidation and RDV experiments. As the reference and auxiliary electrodes, a saturated calomel electrode (SCE) separated by a bridge filled with supporting electrolyte and a Pt wire were used, respectively. All potentials are given vs SCE. Voltammetric measurements were performed using a PGSTAT 128 potentiostat (AUTOLAB, Metrohm Autolab BV, Utrecht, The Netherlands) operated via NOVA 1.9 software. Quantum Chemical Calculations. Geometric and electronic structures of the studied compounds were computed using the Gaussian 0930 (G09) software package. Density functional theory (DFT) was employed with Becke’s three-parameter hybrid functional B3LYP31,32 and triple-ζ polarized 6-311G(d)33 basis sets for all atoms. To account for the effect of solvation, the polarizable continuum model34 (PCM) with parameters of the solvent used in experiments was included in all the performed calculations. Geometries of all the molecules were fully optimized with no symmetry constraints; vibrational analyses were carried out on the optimized structures to ensure proper character of the stationary

states. Maps of the frontier molecular orbitals were constructed using GaussView software.

■

ASSOCIATED CONTENT

S Supporting Information *

Figures giving 1H and 13C NMR spectra for 3a−h and 4a−h and an xyz file giving molecular coordinates for the calculated structures. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail for T.M.: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank Prof. Vladimı ́r Machácě k for consultations during synthesis. J.L. is grateful for institutional support from RVO 61388955. This research was supported by the project CZ.1.07/2.3.00/30.0021 (Ministry of Education, Youth and Sports of the Czech Republic).

■

REFERENCES

(1) He, G. S.; Tan, L. S.; Zheng, Q.; Prasad, P. N. Chem. Rev. 2008, 108 (4), 1245. (2) Li, D.; Zhang, H. Y.; Wang, Y. Chem. Soc. Rev. 2013, 42 (21), 8416. (3) McClary, C. A.; Taylor, M. S. Carbohydr. Res. 2013, 381, 112. (4) Dewar, M. J. S.; Kubba, V. P.; Pettit, R. J. Chem. Soc. 1958, 3073. (5) Dewar, M. J. S.; Kubba, V. P.; Pettit, R. J. Chem. Soc. 1958, 3076. (6) Chissick, S. S.; Dewar, M. J. S.; Maitlis, P. M. J. Am. Chem. Soc. 1961, 83 (12), 2708. (7) Pozzi, E. C. C.; Trivillin, V. A.; Colombo, L. L.; Hughes, A. M.; Thorp, S. I.; Cardoso, J. E.; Garabalino, M. A.; Molinari, A. J.; Heber, E. M.; Curotto, P.; Miller, M.; Itoiz, M. E.; Aromando, R. F.; Nigg, D. W.; Schwint, A. E. Radiat. Environ. Biophys. 2013, 52 (4), 481. (8) Chen, H. Y.; Chi, Y.; Liu, C. S.; Yu, J. K.; Cheng, Y. M.; Chen, K. S.; C hou, P. T.; Peng, S. M.; Lee, G. H.; Carty, A. J.; Yeh, S. J.; Chen, C. T. Adv. Funct. Mater. 2005, 15 (4), 567. (9) Khan, T. K.; Rao, M. R.; Ravikanth, M. Eur. J. Org. Chem. 2010, 2010 (12), 2314. (10) Yakubovskyi, V. P.; Shandura, M. P.; Kovtun, Y. P. Eur. J. Org. Chem. 2009, 2009 (19), 3237. (11) del Rey, B.; Keller, U.; Torres, T.; Rojo, G.; Agulló-López, F.; Nonell, S.; Martí, C.; Brasselet, S.; Ledoux, I.; Zyss, J. J. Am. Chem. Soc. 1998, 120 (49), 12808. (12) Wakamiya, A.; Taniguchi, T.; Yamaguchi, S. Angew. Chem., Int. Ed. 2006, 45 (19), 3170. (13) Zhang, H.; Huo, C.; Ye, K.; Zhang, P.; Tian, W.; Wang, Y. Inorg. Chem. 2006, 45 (7), 2788. (14) Massue, J.; Frath, D.; Ulrich, G.; Retailleau, P.; Ziessel, R. Org. Lett. 2011, 14 (1), 230. (15) Kollmannsberger, M.; Rurack, K.; Resch-Genger, U.; Daub, J. J. Phys. Chem. A 1998, 102 (50), 10211. (16) Rurack, K.; Kollmannsberger, M.; Daub, J. Angew. Chem., Int. Ed. 2001, 40 (2), 385. (17) Khan, T. K.; Pissurlenkar, R. R. S.; Shaikh, M. S.; Ravikanth, M. J. Organomet. Chem. 2012, 697 (1), 65. (18) Yang, G.; Su, T.; Shi, S.; Su, Z.; Zhang, H.; Wang, Y. J. Phys. Chem. A 2007, 111 (14), 2739. (19) Davis, M. C.; Franzblau, S. G.; Martin, A. R. Bioorg. Med. Chem. Lett. 1998, 8 (7), 843. (20) Levy, C. W.; Baldock, C.; Wallace, A. J.; Sedelnikova, S.; Viner, R. C.; Clough, J. M.; Stuitje, A. R.; Slabas, A. R.; Rice, D. W.; Rafferty, J. B. J. Mol. Biol. 2001, 309 (1), 171. H


Organometallics

Article

(21) Pešková, M.; Šimůnek, P.; Bertolasi, V.; Machácě k, V.; Lyčka, A. Organometallics 2006, 25 (8), 2025. (22) Svobodová, M.; Bárta, J.; Šimůnek, P.; Bertolasi, V.; Machácě k, V. J. Organomet. Chem. 2009, 694 (1), 63. (23) Svobodová, M.; Šimůnek, P.; Machácě k, V.; Štruncová, L.; Růzǐ čka, A. Tetrahedron 2012, 68, 2052. (24) Josefík, F.; Svobodová, M.; Bertolasi, V.; Šimůnek, P.; Machácě k, V.; Almonasy, N.; Č ernošková, E. J. Organomet. Chem. 2012, 699, 75. (25) Bruneire, P.; Célérier, J.-P.; Petit, H.; Lhommet, G. J. Heterocycl. Chem. 1986, 23 (4), 1183. (26) Ilvespäa,̈ A.; Fuhrer, W. (Ciba-Geigy AG) DE 2707658, 1975; Chem. Abstr. 1978, 88, 22609. (27) Liška, A.; Vojtíšek, P.; Fry, A. J.; Ludvík, J. J. Org. Chem. 2013, 78, 10621. (28) Zuman, P. Substituent Effects in Organic Polarography; Plenum Press: New York, 1967. (29) Kulhánek, J.; Bureš, F.; Pytela, O.; Mikysek, T.; Ludvík, J. Chem. −Asian J. 2011, 5, 1604. (30) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.;Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J., Gaussian 09, Revision C.01, Gaussian, Inc., Wallingford, CT, 2009. (31) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (32) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (33) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650. (34) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem. 2003, 24, 669.

■

NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on July 9, 2014, with an error in the third paragraph from the end. The corrected version was reposted on July 16, 2014.

I


New Triazaborine Chromophores: Their Synthesis via Oxazaborines

Recommend Documents