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Journal of the Chilean Chemical Society

versão On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.52 n.3 Concepción set. 2007

http://dx.doi.org/10.4067/S0717-97072007000300017 

 

J. Chil. Chem. Soc, 52, N° 3 (2007) págs.: 1266-1270

 

ORGANOMETALLIC 1,5-BENZODIAZEPINE AND 1,5-BENZODIAZE PINIUM COMPOUNDS: SYNTHESIS, CHARACTERIZATION, X-RAY DIFFRACTION STRUCTURES AND THEORETICAL INVESTIGATION

 

CRISTINA GALLARDOa, ALEXANDER TRUJILLOa, MAURICIO FUENTEALBAa, ANDRES VEGAb, DAVID CARRILLOa, CAROLINA MANZURa*

aLaboratorio de Química Inorgánica, Instituto de Química, Pontificia Universidad Católica de Valparaíso, Avenida Brasil 2950, Valparaíso, Chile;
b Facultad de Ecología y Recursos Naturales, Universidad Nacional Andrés Bello, Avenida República 275, Santiago, Chile.


ABSTRACT

The organometallic tridentate ketoamine or enaminone compound, (η5-Cp)Fe(η5-CJH4)-C(=0)-CH=C(Me)-NH-C6H4-o-NH2, undergoes an intramolecular cyclocondensation promoted by Cu(C104)2-6H20 (2:1 molar ratio) affording the neutral 2-ferrocenyl-4-methyl-1,5-benzodiazepine, 1. However, when the molar ratio used is 1:1, the ketoamine or enaminone compound transforms into the 2-ferrocenyl-4-methyl-1,5-benzodiazepinium cation, [2]+. The X-ray molecular structure of 1 exhibits a seven-membered ring with a boat conformation, and two folding dihedral angles along the N(l)-N(2) and C(11)-C(13) axes. In the case of [2]+, the structure shows only one folding dihedral angle along the N(l)-N(2) axis. A rationalization of the properties of 1 and [2]+ is provided through DFT calculations.

Keywords: Organometallic diazepine, organometallic diazepinium, DFT calculations, X-ray structures, ferrocenyl ketoamine, ferrocenyl enaminone.


INTRODUCTION                                                   

It is well known in the literature that reactions of ß-diketones with 1,2-diamines generally yield symmetrical 2:1 tetradentate Schiff bases whatever their proportion.1 However, relatively few studies on tridentate 1:1 condensation products have been reported.2 These compounds, synthesized by a single condensation reaction of a carbonyl function of a ß-diketones with only one amine group of an aliphatic or aromatic 1,2-diamine, are referred to as "half units" or "hemi-Schiff bases" and in solution they can exist as a tautomeric mixture of the keto-amine (enaminone) and keto-imine (iminone) forms (Scheme 1, a and b, respectively).


Nowadays, the interest on the development of these "half units", containing a free NH2 group at one end of the molecule (Scheme 1), stems from their synthetic potential as precursors for the preparation of unsymmetrical tetradentate Schiff base ligands and their corresponding metal complexes.3 On the other hand, considering the importance of the ferrocenyl fragment as a strong electron-releasing group" and the interesting electronic features exhibited by tridentate ferrocene-based keto-amines and their metal complexes,5 we have initiated a systematic study on the reactivity of ONN tridentate "half units" synthesized by reaction of ferrocenoylacetone, Fc-C(=0)-CH2-C(=0)-Me, Fc= η5-C5H5Fe( η5-C5H4)-, with aromatic 1,2 diamines. As part of this work we report herein (i) the cyclocondensation of Fc-C(=0)-CH=C(Me)-NH-C6H4-o-NH2 into 2-ferrocenyl-4-methyl-1,5-benzodiazepine, 1, and 2-ferrocenyl-4-methyl-1,5-benzodiazepinium cation, [2]+, promoted by copper(II), (ii) the molecular structures of complexes 1 and [2 ]+ solved by single crystal X-ray diffraction analysis and, (iii) a DFT calculation which provides a rationalization of the electrochemical and spectroscopic properties of complexes 1 and [2 ]+. The synthesis of these compounds represents an important entry toward the preparation of organometallic 1,5-benzodiazepine and 1,5-benzodiazepinium compounds with potential pharmacological properties.6 In fact, many classical members of organic 1,5-benzodiazepines are widely used as antianxiety, analgestic, sedative, antidepresive and hypnotic agents.7

 EXPERIMENTAL SECTION

General data

Solvents were dried and distilled under dinitrogen by standard methods prior to use. Reagents were purchased from commercial suppliers and used without further purification. Ferrocenoylacetone, Fc-C(=0)-CH2-C(=0)-Me, was synthesized according to published procedures with some slight modifications.8 Ferrocenyl-ketoamine ("half unit"), Fc-C(=0)-CH=C(Me)-NH-C6H4-o-NH2, was synthesized according to a procedure described in our Laboratory.9 Microanalytical data were obtained on a Perkin Elmer model 2400 elemental analyzer. IR spectra were obtained as KBr disks on a Perkin Elmer model 1600 FT-IR spectrophotometer, in the range of 4000-450 cm-1. Electronic spectra were recorded in CH2C12 and MeOH solutions with a Spectronic, Genesys 2, spectrophotometer. 1H-NMR spectra were acquired at 297 K on a multinuclear Bruker AC 400 spectrometer in (CD3)2CO; chemical shifts were referenced using the chemical shifts of residual solvent resonances. Variable-temperature 1H-NMR spectra were recorded in the 223-323 K range. Electrochemical measurements were performed using a Radiometer Analytical model PGZ 100 all-in-one potentiostat, using a standard three-electrode setup with a vitreous carbon working electrode, platinum wire auxiliary electrode and Ag/Ag+ (0.1 M AgN03 in MeCN) as the reference electrode. MeCN solutions were 1.0 mM in the compound under study and 0.1 M in the supporting electrolyte n-Bu4N+PF6- with the voltage scan rate = 100 mVs-1. Under these experimental conditions the Cp2Fe0/+ couple was located at 0.089 V (Δ E = 70 mV). E1/2 is defined as equal to (Epa+ Epc)/2, were Epa and Epc are the anodic and cathodic peak potentials, respectively. Melting points were determined in evacuated capillaries and were not corrected.

Caution: complexe 1 and [2]+ were synthesized using Cu(ClO4)2 -6H2O as reagent; complex [2]+ was isolated as perchlorate salt. Complex [2]+ClO2- •CH2 Cl2 was dissolved in different organic solvents (UV-Visible spectroscopy, 1H NMR spectrommetry, Cyclic Voltammetry) and also handled in solid state (IR) without any incident. In spite of this the impredictable behavior of perchlorate salts necessitates extreme care in their handling.

2-ferrocenyl-4-methyl-1,5-benzodiazepine, 1: A Schlenk tube was charged with a magnetic stirring bar and a solution of 100 mg (0.28 mmol) of the ferrocenyl keto-amine in 20 mL of MeOH, then a solution of 51.5 mg(0.14 mmol) of Cu(C104)2-6H20 dissolved in 10 mL of MeOH, was added drop wise developing immediately a dark red color. The reaction mixture was stirred for 1 h. The resulting solution was concentrated under vacuum until a solid was formed. The brown reddish crude solid was dissolved in 10 mL of CH2C12, filtrated and evaporated to dryness. Suitable single crystals for X-ray diffraction studies were obtained from MeCN. Yield: 58.1 mg (67%). Mp: 157°C. Anal. Caled for C20H18FeN2 (342.26 gmol-1): C, 70.19; H, 5.30; N, 8.19. Found: C, 69.96; H, 5.28; N, 7.98. UV-vis [(λmax, nm (logε)]; CH2C12: 231 (4.03); 284 (3.06); 320 (3.61); 377 (2.81); 462 (2°74). MeOH: 239 (4.00); 280 (2.91); 310 (3.71); 370 (3.00); 453 (2.88). IR (cm-1, KBr): 3102 (vw), 3078 (w), 3052 (w), v(C-H) arom; 2963 (w), 2922 (w), 2853 (vw), v(C-H) aliph; 1634 (m), 1602 (m), 1575 (m), v(C=N) and v(C=C). Other very strong bands not attributed are also observed at 1264, 1106, 818, 798 and 758 cm1. 1H-NMR [(CD3)2CO, ε ppm]: 2.37 (s, 3H, CH3); 3.15 (bs, 2H, CH2); 4.20 (s, 5H, C5H3); 4.53 (s, 2H, C5H4); 5.02 (s, 2H, C5H6); 7.20 (m, 2H, o-C6H4); 7.33 (m, 2H, C6H4).

2-ferrocenyl-4-inethyl-1,5-benzodiazepinlum perchlorate methylene chloride, [2]+2 C104 CH2C12: To a Schlenk tube charged with a solution of 58.9 mg (0.16 mmol) of the ferrocenyl keto-amine in 10 mL of MeOH, was added drop wise a solution of 59.3 mg (0.16 mmol) of Cu(C104)2-6H20 dissolved in 10 mL of MeOH, developing, immediately, a deep violet color. The reaction mixture was stirred for 1 h. The resulting solution was concentrated under vacuum until a solid was formed. The crude solid was dissolved in 10 mL of CH2C12, filtrated and allowed to stand at room temperature giving, by slow evaporation of the solvent, suitable single crystals for X-ray diffraction studies. Yield: 49.2 mg (57%). Mp: 145°C dec. Anal. Caled for C21H21Cl3FeN20„ (527.61 gmol1): C, 47.81; H, 4.01; N, 5.31. Found: C, 47.65; H, 3.95; N, 5.25. UV-vis [(λmax, nm (logε)]; CH2C12: 263 (4.22); 314 (3.86); 338 sh (3.93); 393 (3.64); 570m(3.57). MeOH: 265 (4.22); 308 (4.05); 337 sh (4.02); 403 (3.37); 562 (3.70). IR (cm-1, KBr): 3311 (m), 3241 (w), v(N-H); 3164 (w), 3000 (w), v(C-H) arom; 2962 (w), 2925 (w), v(C-H) aliph; 1622 (s), 1579 (vs), 1518 (s), v(C-N) or v(O-C); 1108(vs), V3(T2) and 758(m), V4(T2) of CÍO;. 1H-NMR [(CD3)2CO, ε ppm: 2.10 (s, 3H, CH3); 4.51 (s, 5H, Cp); 4.94 (br s, 2H, C5H4); 5.18 (br s, 2H, C5H4); 5.22 (s, IH, CH); 5.63 (s, 2H, CH2CL2)); 6.78 (m, IH, C6H4)); 6.95 (m, IH, C6H4)); 7.10 (m, 2H, C6H4); 8.68 (br s, IH, NH); 9.01 (br s, IH, NH).

X-ray crystal structure determination of 1 and [2]+Cl04-•CH2Cl2

A summary of the crystallographic data is given in Table 1. Complete details of the crystal, X-ray data collection, and structure solution are provided as Supporting Information. Semi-empirical corrections, via ψ-scans, were applied for absorption. Intensity data were collected on a Bruker Smart Apex diffractometer equipped with a bidimensional CCD detector using graphite monochromated Mo-Ka radiation (λ=0.71073 A). The diffraction frames were integrated using the SAINT package,10 and corrected for absorption with SADABS.11 The structure was solved using XS in SHELXTL-PC12 by direct methods and completed (non-H atoms) by difference Fourier techniques. Refinement was performed by the full-matrix least-squares method based on F2. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in their calculated positions, assigned fixed isotropic thermal parameters and allowed to ride on their respective parent atoms. The perchlorate anion in [2]+C104- CH2C12 was disordered over two positions, with occupation multiplicity µ = 0.53/0.47.


Computational Details

DFT13 calculations were carried out using the Amsterdam Density Functional (ADF) program.14 The Vosko-Wilk-Nusair parametrization15 was used to treat electron correlation within the local density approximation, with gradient corrections added for exchange16 and correlation,17 respectively. The numerical integration procedure applied for the calculation was developed by te Velde.18

The standard ADF TZP basis set was used for all the atoms. The frozen core approximation was used to treat core electrons at the following level: Fe, 3p; C, Is; N, Is and O, ls.1M Full geometry optimizations were carried out on each complex using the analytical gradient method implemented by Verluis and Ziegler.19 The geometry for all the model compounds discussed in the text was fully optimized, with a good agreement between the computed geometric parameters and the available structural data. Unrestricted formalism was used for the odd electron number compounds.

RESULTS AND DISCUSSION

Complexes 1 and [2]+C104-CH2Cl2 were synthesized in MeOH, at room temperature, by a cyclocondensation type reaction of the ferrocenylketoamine derivative ("half unit"), Fc-C(=0)-CH=C(Me)-NH-C6H4-o-NH2, promoted by Cu(C104)2-6H20 in a 2:1 and 1:1 molar ratio, respectively (Scheme 2). Probably, factors such as (i) the flexible sp3 character adopted by the nitrogen atom in the -NH- group of the ketoamine tautomeric form9 and (ii) the activation of the carbonyl carbon center provoked by the direct coordination of the electrophilic copper(II) cation at the carbonyl oxygen atom, -C=0...Cu2+, could explain the easy nucleophilic attack of the terminal amine group to form an intermediate containing the >C(OH)-NH- moiety, in a first step, which would lead to the formation of the >C=N- (in 1) or >C=NH- (in [2]+) bonds, in a second step. An alternative procedure to synthesize 1,5-benzodiazepines derivatives has also been reported in the literature by condensation of 1,2-diaminobenzene with carbonyl compounds in the presence of transition metal cations,20 although the mechanism remains unknown. In the present case complexes 1 and [2]+C104-•CH2C12 were isolated as orange reddish and deep purple crystalline solids in 67 and 57% yields based on the ferrocenylketoamine, respectively. The complexes exhibit a good solubility in polar solvents such as MeOH, CH2C12 and Me2CO but they are slightly soluble in non-polar solvents such as toluene and petroleum ether, and were fully characterized by ■H-NMR, IR and UV-Vis spectroscopies. Additionally, the molecular structures of complexes 1 and [2]+Cl4-•CH2C12 were solved by single crystal X-ray diffraction analysis (vide infra).


The most peculiar feature observed in the IR spectrum of [2]+Cl4--CH2Cl2 is the presence of two sharp bands of medium and weak intensities at 3309 and 3243 cm-, respectively, corresponding to the stretching modes of the NH groups of the 1,5-benzodiazepinium cation. These bands are, obviously, absent in complex 1. Likewise, a very large and strong band at 1108 cm- along with the medium intensity band at 758 cm-, attributed to the V3(T2) and V4(T2) modes of the Cl4- counteranion21 have also been observed. On the other hand, complex 1 exhibits three characteristic bands of medium intensity at (i) 1634 and 1602 and (ii) 1575 cm-, assigned to the v(C=N) and v(C=C) stretching modes, respectively. The C=N bonds, according to the X-ray structure, are localized in the seven-membered ring (vide infra).

The 1H-NMR spectra of complexes 1 and [2]+ClO4-•CH2Cl2, recorded in (CD3)2CO, at room temperature, are also consistent with their molecular structures. One of the more remarkable features of complex 1 is the presence of a broad singlet at 3.15 ppm, attributed to the methylene proton resonances of the 1,5-diazepine ring. In order to get a deeper insight into this peculiarity, the variable-temperature 1H-NMR spectroscopic study was carried out in the temperature range 223-323 K. In the low temperature (223 K) the two doublets at 8 2.13 and 4.08 (JH-H= 10.86 Hz) are attributable to the geminal protons of the CH2 group. When the temperature is raised the two doublets undergo coalescence at 268 K and then, at 323 K, transform into a sharp singlet at 8 3.11 (Figure 1). The free energy of activation associated to the fluxional process involving the methylene group of the seven-membered ring was calculated at the coalescence temperature22 and found to be 48.7 kJmol-1.


On the other hand, the 1H-NMR spectrum of complex [2]+ClO4-•CH2Cl2 is characterized by the presence of two broad down-field singlets corresponding to the NH proton resonances at 8 9.01 and 8.68 showing the different electronic effects of the methyl and ferrocenyl groups, respectively.

Finally, the UV-vis spectroscopic studies of complexes 1 and [2]+Cl4- •CH2C12 are consistent with the behavior of most ferrocenyl monosubstituted compounds reported in the literature.23 The spectra of these complexes, performed in solvents of different polarity, such as CH2C12 (ε=8.90) and MeOH (ε=32.63), exhibit five absorption bands. In CH2C12, the complexes exhibit a prominent absorption band in the range 310-340 nm, assigned to π-π* transitions (ILCT). In the case of complex 1 this band is blue-shifted in MeOH by 10 nm, while in complex [2]+ this band is not affected. On the other hand, the broad lower energy band, at 462 and 570 nm, respectively, is attributed to a metal-to-ligand charge-transfer (MLCT) transition and it is strongly influenced by the nature of the acceptor. This assignment is in accordance with the latest theoretical treatment reported by Barlow et al.24 and with the previously cited experimental work, although the assignment may be controversial.25 In both complexes this band is blue-shifted by 8-9 nm in MeOH.

Electrochemical studies

In order to determine the electronic effects of the organic moieties on the electrochemical features of the ferrocenyl group in complexes 1 and [2]+, Cyclic Voltammetry experiments were performed in MeCN with 0.1 Mn-Bu4N+PF6- at room temperature. In both cases an anodic reversible wave, unequivocally attributed to the ferrocenyl group, was observed (Figure 2). While in the neutral 1,5-benzodiazepine 1 the E½= 0.256 V (ΔE = 65 mV), in the case of complex [2]+, the redox potential of the ferrocenyl group is strongly shifted to the anodic region. The high potential, E½ = 0.450 V (ΔE = 70 mV), reflects the strong electron-accepting capability of the positive delocalized seven-membered ring. In addition, an irreversible one-electron reduction wave was also observed in complex [2]+ at Epc = -1.168 V which correspond to the NH proton reduction and the simultaneous generation of the neutral 1,5-benzodiazepine. The absence of this cathodic wave when NaOMe is added to the cell solution, is an empirical demonstration of this attribution.


X-ray crystallographic studies

The crystal and molecular structures of complexes 1 and [2]+Cl4-•CH2Cl2, were carried out by single crystal X-ray diffraction analysis, as outlined in the Experimental Section. The molecular plots of complexes 1 and [2]+, along with the atom labeling schemes, are presented in Figures 3 and 4, and selected bond lengths and angles are listed in Table 2. Compound 1 crystallizes in the orthorombic space group Pbca with eigth molecules in the unit cell, while [2]+Cl4-•CH2Cl2 crystallizes in the triclinic space group P-l with two molecules in the unit cell. In the sandwich fragment the metrical parameters are typical of T|5-Fe-r|5 coordination with the ring centroid-iron-ring centroid vectors of 1.642 and 1.643 A for 1 and 1.660 and 1.638 A for [2 ]+. These vectors are almost collinear being the carbocyclic rings coordinated to the iron center essentially parallel with one another.




The X-ray molecular structure of 1 exhibits a seven-membered ring with a boat conformation, and two folding angles along the N(1)-N(2) and C(ll)-C(13) axes of 145.2 and 125.7°, respectively. Two localized double bonds for C(11)-N(1) = 1.281(4) and C(13)-N(2) = 1.277(4) Á, are also observed. On the other hand, the more remarkable structural feature of cation [2]+ is the non-planar seven-membered ring with a folding dihedral angle of 14.7° along the N(l)-N(2) axis, a probable consequence of the contribution of the following resonance structures: -NH+=C(Me)-CH=C(Fc)-NH- and -NH-C(Me)=CH-C(Fc)=NH+-. To the best of our knowledge, the X-ray molecular structures of complexes 1 and [2 ]+ represent the first examples reported in the literature for organometallic 1,5-benzodiazepine and 1,5-benzodiazepinium species.

Theoretical studies

The study of complex [2 ]+ indicates that the delocalized form of 2-ferrocenyl-4-methyl-1,5-benzodiazepinium cation, [2]+, Scheme 3a, is 0.83 eV (19 kcal mol-1) and 0.67 eV (16 kcal mol-1) more stable than the hypothetical localized iminium forms of the cations showed in Schemes 3b and 3c. Although the slight difference in terms of energy between the three forms of the molecule the most stable delocalized cationic form is observed both in solid state and solution.


In order to get a deeper insight into the electrochemical behavior of the neutral 1,5-benzodiazepine, 1, and the related cationic 1,5-benzodiazepinium, [2]+, we have optimized their structures and the corresponding to their one-electron oxidized ([1]+, [2]2+) and reduced species ([1]-, 2). We have also computed their electronic structures. Figure 5 shows an energy levels diagram for 1 and [2 ]+. It is clear that the HOMO has, in both cases, a highly ferrocenyl character, which agrees with the experimental observation of an oxidation wave mainly related to this molecular fragment. On the other hand, LUMO has, in both cases, a main benzodiazepine ring character, relating the reduction wave to this molecular fragment. The computed HOMO-LUMO gap is almost the half on going from the neutral benzodiazepine, 1, to the protonated benzodiazepinium cation, [2]+, (2.09 vs 1.10 eV). Just considering in a first approach the gap as an indicative of the ease or difficulty to reduce a molecule, these results suggest the reduction of 1 is more expensive in energy compared to [2 ]+. Accordingly, it is observed that the reduction of [2]+ occurs under the experimental conditions utilized while the reduction corresponding for 1 is not observed into the solvent window limit. The disappearance of the reduction wave of [2 ]+ after the addition of a strong base (removal of H+ from [2 ]+ gives 1) also agrees with these results. Computed adiabatic oxidation/reduction potentials for 1 and [2]+ are 6.54/0.89 and 9.68/4.95 eV, respectively, which confirms the gap previously suggested.


Finally, it must be commented that the composition of HOMO and LUMO supports the assignment of a MLCT for both compounds, as discussed previously. The computed HOMO-LUMO gap is also consistent with a lower energy value for this transition in [2]+ when compared to 1.

Concluding remarks

To summarize, as a result of our studies, we have found that the organometalhc tridentate ketoamine or enaminone compound, Fc-C(=0)-CH=C(Me)-NH-C6H4-o-NH2, undergoes, under mild conditions, an intramolecular cyclocondensation promoted by copper(II) perchlorate hexahydrate into the neutral 2-ferrocenyl-4-methyl-1,5-benzodiazepine complex, 1, and into the cationic 2-ferrocenyl-4-methyl-1,5-benzodiazepinium species, [2]+, provided that the molar ratio is 2:1 or 1:1, respectively. To the best of our knowledge, complexes 1 and its protonated species [2]+ are the first organometalhc 1,5-benzodiazepines structurally characterized by X-ray diffraction analysis. Likewise, complex 1 and [2 ]+ represent the first members of a new family of 2-ferrocenyl,4-methyl- 1,5-benzodiazepines.

Supplementary material

Crystallographic data (excluding structure factors) for the structural analyses of complexes 1 and [2]+Cl4-•CH2Cl2 have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication CCDC-619984 and 601778, respectively. Copies of the data may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44-1223-336-033; e-mail: deposit@ccdc.cam.ac.uk or ). Cartesian coordinates fot the optimized molecular geometries are available as XYZ formatted file.

ACKNOWLEDGEMENTS

C. M. and D. C. gratefully acknowledge Fondo Nacional de Desarrollo Científico y Tecnológico, FONDECYT (Chile), Grant no. 1040851 and the Vicerrectoría de Investigación y Estudios Avanzados, Pontificia Universidad Católica de Valparaíso, Chile, for financial support of this work. The authors also acknowledge Dr. M. T. Garland, Laboratorio de Cristalografía, Departamento de Física, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile. M. F. and A. T. thank the CONICYT, Chile, for support of a graduate fellowship.

 

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(Received 5th May 2007 - Accepted 25th June 2007)

Corresponding author: e-mail: cmanzur@ucv.cl.

 

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