Pyrazole (Hpz) or 1,2-diazacyclopenta-2,4-diene is a heterocyclic five-membered ring compound containing three carbon atoms with two nitrogen atoms in adjacent positions. Over the past two decades, pyrazole-containing compounds have received considerable attention owing to the fact that pyrazoles exhibit antimicrobial   , anticancer  , antibacterial   , antipyretic  and analgesic  activities. Moreover, they are suitable agents for investigating the active sites of biomolecules and for modeling the biosystems of oxygen transfer  . The pyrazole moiety shows a broad game of chemical reactivity due to the presence of both the pyridine- and the pyrole-type nitrogen atoms, enabling it to act both as a Lewis acid and as a Lewis base. Electronic and steric effects can therefore be fine-tuned nearly at will by introducing various substituents on different carbon atoms on the ring or by substituting hydrogen atoms    to generate new pyrazole derivatives. Some of these pyrazoles are very promising for the synthesis of inorganic materials with particular properties such as luminescence and collective magnetic phenomena  -  . In particular, Trofimenko et al.  synthesized and characterized the pyrazole, 4,5-dihydro-1H-benzo[g]indazole, whose coordination chemistry is still less developed.
In the present work, the complex salt bis(4,5-dihydro-1H-benzo[g]indazole)silver(I) nitrate, [Ag(N2C11H10)2]NO3, was synthesized and characterized. The optimized structure, frontier molecular orbitals (HOMO and LUMO) and global reactivity descriptors were investigated by performing DFT calculations.
2. Experimental Section
2.1. Materials and Experimental Procedures
All chemicals were purchased from Aldrich and used as received. The ligand, 4,5-dihydro-1H-benzo[g]indazole was prepared following Trofimenko reported procedure  . The synthesis of the complex was carried out in air. Melting point was uncorrected and measured using an SMP3 Stuart Scientific instrument operating at a ramp rate of 1.5˚C /min. Elemental analysis (C, H, N) was performed with a Fisson Instrument 1108 CHNS-O elemental analyzer, while the thermogravimetric analysis was obtained using a Perkin-Elmer STA 6000 thermo-balance. The IR spectrum was recorded from 4000 - 650 cm−1 with a Perkin-Elmer System 100 FT-IR spectrophometer. 1HNMR spectrum was recorded on a Mercury Plus Variant 400 spectrophotometer operating at room temperature. Proton chemical shift (δ) values are reported in parts per million (ppm) from SiMe4 (calibrating by internal deuterium solvent lock). Peak multiplicities are abbreviated as: singlet, s; doublet, d; triplet, t; quartet, q and multiplet, m. Crystal of the new compound coated with dry perfluoropolyether were mounted on a glass fiber and fixed under a cold nitrogen stream. The intensity data were collected on a Bruker-Nonius X8ApexII CCD area detector diffractometer using Mo-Kα-radiation source (λ = 0.71073 Å) fitted with a graphite monochromator. The data collection strategy used was ω and φ rotations with narrow frames (width of 0.50 degree). Instrument and crystal stability were evaluated from the measurement of equivalent reflections at different measuring times and no decay was observed. The data were reduced using SAINT  and corrected for Lorentz and polarization effects, and a semiempirical absorption correction was applied (SADABS)  . The structure was solved by direct methods using SIR-2002  and refined against all F2 data by full-matrix least-squares techniques using SHELXL-2016/6  minimizing w[Fo2-Fc2]2. All the non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms of the compound were included from calculated positions and allowed to ride on the attached atoms with isotropic temperature factors (Uiso values) fixed at 1.2 times those Ueq values of the corresponding attached atoms. The DFT calculations were performed using the Gaussian 09 Revision ? A.02-SMP program  . The vibrational frequencies, electronic structure and geometries of the isolated compound were computed within the density functional theory (DFT) at the B3LYP level, using the LanL2DZ basis set for all the atoms. Molecular orbitals (MO) were visualized using the GaussView 5.0.8 program. Global reactivity descriptors―the chemical potential (μ), chemical hardness (η), molecular electrophilicity (ω), and chemical softness which indicate the overall stability and reactivity of the system  were computed directly from the energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).
2.2. Synthesis of [Ag(N2C11H10)2]NO3
The compound Bis(4,5-dihydro-1H-benzo[g]indazole)silver(I) Nitrate, [Ag(N2C11H10)2]NO3, was synthesized by the reaction of 4,5-dihydro-1H-benzo[g]indazole with silver(I) nitrate in methanol, at ambient temperature according to Equation (1).
In a 50 mL round bottom flask containing 25 mL of methanol was introduced 0.05 g (0.29 mmol) of silver nitrate (AgNO3) which dissolved upon magnetic agitation at ambient temperature (AT) giving a colorless solution. To this solution was added 0.10 g (0.58 mmol) of 4,5-dihydro-1H-benzo[g]indazole (C11H10N2) in the 1:2 ratio, which also dissolved after few minutes of agitation giving a yellow limpid solution. The resulting solution was stirred overnight and then filtered. The complete evaporation of the solvent from the mother liquor at ambient temperature (AT) gave the non-hygroscopic and air-stable yellowish crystals of [Ag(N2C11H10)2]NO3 in an 80% yield.
3. Results and Discussion
3.1. Physical Properties and Elemental Analysis
The synthesized complex salt, [Ag(N2C11H10)2]NO3 is yellowish in color and melts between 210˚C - 212˚C. The experimental values from the analysis of elements present are in conformity with the theoretical values as summarized on Table 1.
3.2. IR Spectrum of [Ag(N2C11H10)2]NO3
The FT-IR spectrum of [Ag(N2C11H10)2]NO3 displays a characteristic broad (br) absorption band at 3214 cm−1 attributed to the stretching vibration of N?H of the pyrazole unit  . The shift to higher frequencies with respect to the spec-
Table 1. Percentage of analyzed elements (C, H, N) in [Ag(N2C11H10)2]NO3.
trum of the free 4,5-dihydro-1H-benzo[g]indazole ligand (3159 - 3062 cm−1) is due to the interaction between the silver metal and the ligand molecule. The variable weak (v, w) band between 2947 cm−1 and 2897 cm−1 can be assigned to the stretching vibrations of C?H of both saturated and unsaturated carbon atoms of the ligand. The variable weak (v, w) bands between 1585 cm−1 and 1540 cm−1 can be assigned to vibrations of the C=N of the pyrazole unit and the C=C stretching of the aromatic ring  . The broad (br) intense absorption band at 1372 - 1283 cm−1 is attributable to the stretching of the non coordinated nitrate ion.
3.3. 1H Nuclear Magnetic Resonance Spectrum (1HNMR)
The 1HNMR spectrum shows five families of protons appearing from the weak field towards the strong field as follows: a singlet at δ = 7.9 ppm (2H, s) is attributable to the N?H group of the pyrazole ring, a singlet at δ = 7.5 ppm (2H, s) is attributed to the N=CH- of the pyrazole ring, a multiplet at 7.2 ppm (8H, m) is characteristic of aromatic protons and two triplets at δ = 2.9 ppm (4H, t) and δ = 2.7 ppm (4H, t) attributable to two methylene (CH2-CH2) groups of the cyclohexane ring.
3.4. Thermogravimetric Analysis
Thermal stability of the complex salt [Ag(N2C11H10)2]NO3 was measured from room temperature to 250˚C (Figure 1) under dinitrogen atmosphere. The TG analysis (curve 1) shows that [Ag(N2C11H10)2]NO3 is thermally stable right up to 210˚C and progressively loses weight till 250˚C. This weight loss of 27.6% cannot be attributed to a particular fragment of this complex salt molecule. In fact, at this temperature range (210˚C - 250˚C), the complex melts and decomposes. The heat change (curve 2) confirms that the melting, occurring at 210˚C with formation enthalpy of ΔHf = −1.55 KJ.mol−1, is an exothermic process. Beyond 220˚C appear some perturbations on the heat change behavior of the complex salt.
3.5. Crystal Structure of [Ag(N2C11H10)2]NO3
Single-crystal X-ray structural analysis reveals that the title compound is a complex salt of formula, [Ag(N2C11H10)2]NO3, having [Ag(N2C11H10)2]+ as complex cation and as counter anion. Figure 2 shows the MERCURY and the ORTEP views of the crystal structure of the title compound. Crystal data and structure refinement details are summarized in Table 2 and selected bond lengths and angles are in Table 3.
Each cationic entity, [Ag(N2C11H10)2]+, consists of a central silver ion coordinated to two nitrogen atoms (N2, N4) from the pyrazole unit of two ligand molecules. As a result, the silver ion has a pseudo-linear AgN2 coordination mode, with an N2?Ag1?N1 angle of 154.2˚ and a bond distance of N2?Ag1 = 2.194 Å, Ag1?N4 = 2.127 Å. This Ag-N bond length is similar to those found in other silver-pyrazole type complexes  . The pair of 4,5-dihydro-1H-benzo[g]indazole
Figure 1. Thermogravimetric curve of [Ag(N2C11H10)2]NO3 under N2 atmosphere.
Figure 2. (a) Mercury view (ball and sticks) of the molecular structure of [Ag(N2C11H10)2]NO3 Ag (pink), N (blue), O (red); (b) ORTEP view of the molecular structure of [Ag(N2C11H10)2]NO3 showing the numbering and labeling of the different atoms (hydrogen atoms have been omitted for the sake of clarity).
Table 2. Crystallographic data and structure refinement details of [Ag(N2C11H10)2]NO3.
Table 3. Selected bond lengths and angles in the title compound.
ligands bonded to the Ag center is arranged such that the nitrogens bearing H atoms in each ligand lie on the same side of the pseudo-linear Ag?N bonds. This geometry adopted by silver is similar to that observed by Crawford and his coworkers in the complex salt, bis(3,5-dimethyl-1H-pyrazole-κN2)silver(I) hexafluoridoantimonate ([Ag(N2C5H8)2]SbF6) in which N2?Ag1?N2 angle is rather 176.54˚  . Detailed analysis of the crystal packing of the salt [Ag(N2C11H10)2]NO3 reveals that the NO3− anion does not act as coordinated ligand, but rather is involved in hydrogen bonds N?H・・・O: 2.07Å and weak interactions: Ag・・・O: 3.0 Å (Figure 3(a) and Figure 3(b) and Table 4. In addition to these interactions, there are other non-covalent intermolecular interactions such as Ag・・・π: 3.4 Å and C?H・・・π: 2.8 Å as depicted in Figure 4(a) and Figure 4(b) involving the π?electron of cyclohexene ring and either silver metal or the hydrogen atom belonging to the neighboring pyrazole.
The anion is linked to the cation complex through electrostatic interactions, intermolecular N?H・・・O and Ag・・・O interactions. The bulk structure is consolidated by N?H・・・O, C?H・・・π Ag・・・π and Ag・・・O intermolecular interactions, thus generating a helical crystalline network, as shown in Figure 5.
3.6. DFT Studies
The DFT calculations were performed at the B3LYP level in the gas phase using
Table 4. Hydrogen bond lengths (Ǻ) and angles (˚) for the title compound.
Symmetry transformations used to generate equivalent atoms: #1 ?x + 1, y-1/2, −z + 1/2.
Figure 3. (a) N?H・・・O (2.07Å) intermolecular hydrogen bonds; (b) weak Ag・・・O (3.0 Å) interactions.
Figure 4. (a) Ag・・・π (3.4 Å) and (b) C?H・・・π (2.8 Å) intermolecular interactions.
Figure 5. Pseudo-helical crystalline network in [Ag(N2C11H10)2]NO3.
Figure 6. Optimized structure of [Ag(N2C11H10)2]NO3.
The π-electron system in the pyrazole unit and the benzene fraction of the ligands observed in the experimental structure are well localized in the optimized structure. The reproducibility of the experimental geometry is quite satisfactory. A comparison of the experimental and theoretical geometric parameters of the compound is illustrated on Table 5.
It is observed that some slight changes occurred in the geometry of [Ag(N2C11H10)2]NO3 after optimization. The Ag1?N2 and Ag1?N4 bond lengths increase from 2.127 Å to 2.211 Å and from 2.194 Å to 2.211 Å respectively. The computed C11?N1, C3?C4, C10?C11, N5?O2 and N5?O3 bonds appear overestimated with a discrepancy ranging from 0.102 to 0.149 Å while the remaining bond lengths agree within 0.073 Å. Selected dihedral angles between various atomic planes are presented in Table 6 and FT-IR vibrational frequencies of
Table 5. Comparison between experimental and theoretical geometric parameters of [Ag(N2C11H10)2]NO3.
Table 6. Dihedral angles between some atomic planes in [Ag(N2C11H10)2]NO3.
[Ag(N2C11H10)2]NO3 in Table 7.
The DFT studies show that Ag(N2C11H10)2]NO3 has 332 molecular orbitals (MOs), with 115 occupied MOs and 217 unoccupied MOs. The highest occupied molecular orbital, HOMO is the 115th MO (Figure 7(a)) and has an energy of −129.85 Kcal/mol while the lowest unoccupied molecular orbital, LUMO which is the 116th MO (Figure 7(b)), has an energy of -37.71 Kcal/mol. The red regions represent the positive phases of the molecular orbitals while the green ones represent the negative phases. Significant contributions to the highest occupied molecular orbitals (HOMOs) come from the orbitals of the metal and the nitrate ion, with some small contributions coming from the nitrogen atoms of the ligands which are bonded to the metal center. Also, the orbitals of the ligand molecules make the main contributions to the lowest unoccupied molecular orbitals (LUMOs) with small contributions from the orbitals of the central metal.
Table 7. Calculated IR vibrational frequencies (cm−1) of [Ag(N2C11H10)2]NO3 complex salt and their assignment.
Figure 7. Pictural view of (a) highest occupied molecular orbitals and (b) lowest unoccupied molecular orbitals of [Ag(N2C11H10)2]NO3.
Table 8. Global reactivity descriptors for the title complex.
Some global reactivity descriptors of the complex salt obtained from theoretical calculations are summarized on Table 8.
The new complex salt, bis(4,5-dihydro-1H-benzo[g]indazole)silver(I) nitrate, [Ag(N2C11H10)2]NO3, has been synthesized and characterized. This compound presents an interesting two-dimensional pseudo-helical network based on the self-assembly of ionic units through coulombic and N?H・・・O, C?H・・・π, Ag・・・π and Ag・・・O intermolecular interactions. Thermal analysis reveals that the compound is stable up to ca. 210˚C. DFT results show some discrepancies between the X-ray and the optimized structures in terms of bond lengths and angles. The Ag+ cation and the ion make the major contributions to HOMO, while the C11H10N2 ligands make significant contributions to LUMO. However, the reactivity of this complex salt cannot be directly induced, but can be compared with the reactivity of other related complexes.
Preliminary observations from our laboratory promisingly suggest that a well-conceived and systematically conducted preparative procedure may be applied generally to fabricate a whole range of homologous materials.
The authors are grateful to Prof E. Alvarez of Instituto de Investigaciones Quimicas (IIQ)-Universidad de Sevilla (Spain) for X-ray facilities and to Prof C. Pettinari of the University of Camerino (Italy) for spectroscopic and Thermogravimetric analyses facilities.
Detailed crystallographic data in CIF format for this paper were deposited with the Cambridge Crystallographic Data Centre (CCDC-1547647). The data can be obtained free of charge at http://www.ccdc.cam.ac.uk/conts/retrieving.html [or from Cambridge Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge CB2 IEZ, UK; fax: +44 (0) 1223-336033; e-mail: email@example.com].
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