Synthesis , Structural Characterization and DFT Studies of Silver ( I ) Complex Salt of Bis ( 4 , 5-dihydro-1 H-benzo [ g ] indazole )

A new silver complex salt [Ag(N2C11H10)2]NO3 (where N2C11H10 = 4,5-dihydro-1H-benzo[g]indazole), has been synthesized and characterized by elemental and thermal analyses, IR and HNMR spectroscopies, single crystal X-ray structure determination and DFT studies. Its molecular structure comprises of a silver center coordinated to two nitrogen atoms from two 4,5-dihydro-1H-benzo[g]indazole molecule giving rise to a cationic complex entity, [Ag(N2C11H10)2] with 3 NO − as counter ion. The bulk structure is consolidated by N–H···O, C–H···π, Ag···π and Ag···O intermolecular interactions, thus generating a pseudo-helical network. The optimized structure, frontier molecular orbitals (HOMO and LUMO) and global reactivity descriptors were investigated by performing DFT calculations.

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 [9] [10] [11] 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 [12]- [15].In particular, Trofimenko et al. [16] synthesized and characterized the pyrazole, 4,5-dihydro-1H-benzo[g]indazole, whose coordination chemistry is still less developed.

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 [16].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. 1 HNMR 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 SiMe 4 (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-K α -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 [17] and corrected for Lorentz and polarization effects, and a semiempirical absorption correction was applied (SADABS) [18].The structure was solved by direct methods using SIR-2002 [19] and refined against all F 2 data by full-matrix least-squares tech-niques using SHELXL-2016/6 [20] minimizing w[Fo 2 -Fc 2 ] 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 (U iso values) fixed at 1.2 times those U eq values of the corresponding attached atoms.The DFT calculations were performed using the Gaussian 09 Revision -A.02-SMP program [21].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 [22] were computed directly from the energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).( )

Synthesis of [Ag(N2C11H10)2]NO3
In a 50 mL round bottom flask containing 25 mL of methanol was introduced 0.05 g (0.29 mmol) of silver nitrate (AgNO 3 ) 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 (C 11 H 10 N 2 ) 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(N 2 C 11 H 10 ) 2 ]NO 3 in an 80% yield.

Physical Properties and Elemental Analysis
The synthesized complex salt, [Ag(N 2 C 11 H 10 ) 2 ]NO 3 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.

IR Spectrum of [Ag(N2C11H10)2]NO3
The FT-IR spectrum of [Ag(N 2 C 11 H 10 ) 2 ]NO 3 displays a characteristic broad (br) absorption band at 3214 cm −1 attributed to the stretching vibration of N-H of the pyrazole unit [23].The shift to higher frequencies with respect to the spec- 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 [24].The broad (br) intense absorption band at 1372 -1283 cm −1 is attributable to the stretching of the non coordinated nitrate ion.

1 H Nuclear Magnetic Resonance Spectrum ( 1 HNMR)
The 1 HNMR 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 (CH 2 -CH 2 ) groups of the cyclohexane ring.

Thermogravimetric Analysis
Thermal stability of the complex salt [Ag(N 2 C 11 H 10 ) 2 ]NO 3 was measured from room temperature to 250˚C (Figure 1) under dinitrogen atmosphere.The TG analysis (curve 1) shows that [Ag(N 2 C 11 H 10 ) 2 ]NO 3 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 ΔH f = −1.55KJ.mol −1 , is an exothermic process.Beyond 220˚C appear some perturbations on the heat change behavior of the complex salt.

Crystal Structure of [Ag(N2C11H10)2]NO3
Single-crystal X-ray structural analysis reveals that the title compound is a com-   Table 3. Selected bond lengths and angles in the title compound.
thus generating a helical crystalline network, as shown in Figure 5.

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.
It is observed that some slight changes occurred in the geometry of   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.Some global reactivity descriptors of the complex salt obtained from theoretical calculations are summarized on Table 8.

Conclusions
The new complex salt, bis( 4 NO − ion make the major contributions to HOMO, while the C 11 H 10 N 2 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.

3 NO
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-κN 2 )silver(I) hexaf-luoridoantimonate ([Ag(N 2 C 5 H 8 ) 2 ]SbF 6 ) in which N2-Ag1-N2 angle is rather 176.54˚ [26].Detailed analysis of the crystal packing of the salt [Ag(N 2 C 11 H 10 ) 2 ]NO 3 reveals that the NO 3 − 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

) 2 ]
NO 3 has 332 molecular orbitals (MOs), with 115 occupied MOs and 217 unoccupied MOs.The highest occupied molecular orbital, HOMO is the 115 th MO (Figure 7(a)) and has an energy of −129.85Kcal/mol while the lowest unoccupied molecular orbital, LUMO which is the 116 th 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
,5-dihydro-1H-benzo[g]indazole)silver(I) nitrate, [Ag(N 2 C 11 H 10 ) 2 ]NO 3 , 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 the3

Table 6
and FT-IR vibrational frequencies of

Table 8 .
Global reactivity descriptors for the title complex.