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. 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-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 F² data by full-matrix least-squares techniques using SHELXL-2016/6 [20] 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 (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).

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.

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.

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 [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.

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.

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 ΔH_f = −1.55 KJ.mol⁻¹, is an exothermic process. Beyond 220˚C appear some perturbations on the heat change behavior of the complex salt.

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 [25] . The pair of 4,5-dihydro-1H-benzo[g]indazole

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˚ [26] . 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.

The DFT calculations were performed at the B3LYP level in the gas phase using

Symmetry transformations used to generate equivalent atoms: #1 ?x + 1, y-1/2, −z + 1/2.

the LanL2DZ basis set, in order to understand the structural preferences of the atoms in the [Ag(N₂C₁₁H₁₀)₂]NO₃ complex salt. The resulting optimized structure is shown in Figure 6.

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

[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 115^th MO (Figure 7(a)) and has an energy of −129.85 Kcal/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 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.