Pyrazole is a five-membered heterocycle, made up of three carbon atoms and two nitrogen atoms in adjacent positions. Numerous compounds containing this organic molecule are known to exhibit anti-hyperglycemic, analgesic, anti-inflammatory, antipyretic, antibacterial, antimicrobial, antihypertensive and antidepressant activities [1] [2] [3] . The coordination chemistry of pyrazole derivatives continues receiving considerable attention, due to the ability of these molecules to act as remarkably flexible ligand systems in complexation with a wide range of metal ions [4] [5] [6] . They easily coordinate metal centers through their N2 atoms and also form interesting hydrogen interactions. Metal complexes of pyrazole derivatives often yield unusual electronic and steric properties that can be fine-tuned nearly at will [7] [8] . They find applications in antipyretics and antirheumatics, in herbicides, in fungicides, and also as metal ion extractants [9] [10] . Furthermore, pyrazole complexes exhibit luminescence, anticancer and anti-HIV activities [11] [12] [13] . In living organisms, metal ions are usually bonded to the imidazole unit of hystidine, which is a part of proteins. In view of the similarity of pyrazole and imidazole, pyrazole metal complexes are suitable agents to mimic enzymatic reactions [14] .

The pyrazole derivative, 4,5-dihydro-1H-benzo[g]indazole, is a bulky ligand synthesized and characterized by Trofimenko and co-workers [15] . Very recently, investigation of the coordination reactivity of this versatile pyrazole ligand towards silver(I) nitrate in methanol led to formation of a pseudo linear bis(4,5-dihydro-1H-benzo[g]indazole)silver(I) nitrate ([Ag(N₂H₁₀C₁₁)₂]NO₃) adduct [16] . In continuation of this research program which aims to deepen the coordination ability of pyrazole derivatives, we report herein the synthesis, characterization and DFT studies of bis(4,5-dihydro-1H-benzo[g]indazole)silver(I) hexafluorophosphate complex salt, [Ag(N₂H₁₀C₁₁)₂]PF₆, the crystallographic structure of which was assessed through Rietveld refinement from powder diffraction data.

All chemicals were purchased from Aldrich and used as received. The ligand, 4,5-dihydro-1H-benzo[g]indazole was prepared following Trofimenko’s reported procedure [15] . The synthesis of the complex was carried out in air. Melting point was measured and uncorrected 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. The X-ray powder diffraction (XRPD) data were measured at room temperature, in air. Fortunately, the compound gave traces of decent quality, allowing a fruitful structural determination. Some amount of the sample was gently ground in an agate mortar, and then deposited in the hollow of a 0.2 mm deep aluminum sample holder, equipped with a quartz monocrystal zero background plate (supplied by the Gem Dugout, State College, PA). Diffraction data were collected in the 5 - 105˚ 2θ range, sampling at 0.02˚, on a θ:θ vertical scan Bruker AXS D8 Advance diffractometer, equipped with a linear Lynxeye position sensitive detector, set at 300 mm from the sample. Ni-filtered Cu Kα_1,2 radiation, λ = 1.5418 Å. Standard peak search methods, followed by indexing by TOPAS-R [17] , allowed the determination of approximate cell parameters; systematic absences allowed the detection of the probable space groups, later confirmed by successful structure solutions and refinements. Structure solution was initiated by employing a semi-rigid molecular fragment for 4,5-dihydro-1H-benzo[g]indazole, flexible about two torsion angles, and free silver, phosphorus and fluoride atoms. Hydrogen atoms have been positioned in idealized locations as they have been included in the definition of the rigid bodies. Simulated annealing (occasionally helped by soft restraints) allowed the location and orientation of the used fragments, later refined by the Rietveld method. The fundamental parameter approach in describing the peak shapes was employed, the background contribution was modeled by a polynomial fit, and preferred orientation effects (010 pole) were described by the March-Dollase formulation [18] [19] . A single isotropic thermal parameter was adopted for the Ag atom, while lighter atoms were assigned B = B_Ag + 2.0 Ų. Structure solution and refinements were performed by TOPAS-R.

The optimization of the experimental structure at the B3LYP/LanL2DZ level was performed using the Gaussian 09 program [20] and was followed by the vibrational frequency and the HOMO-LUMO gap calculations. In the whole, HOMO-LUMO gap is used to measure the stability indicators. The electron affinity (A) and ionization potential (I) respectively defined by Equation (1)-(2) are used as global reactivity descriptors of the synthesized complexes.

I = − E HOMO (1)

A = − E L U M O (2)

where, E_HOMO and E_LUMO are respectively energy values of HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital). Additional global quantities can be built from these previous parameters such as chemical hardness ( η ) [21] , chemically soft (S), chemical potential (μ) and global electrophilicity index (ω) respectively defined as followed.

η = I − A 2 (3)

S = 1 2 η (4)

μ = − ( I + A ) 2 (5)

ω = μ 2 2 η (6)

It is important to mention that chemical hardness ( η ) is associated with the stability of the compound because it measures the ability of the compound to resist changes in its electron distribution or charge transfer. High values of chemical hardness indicate more stability and low reactivity of the compound. On the other hand, molecules with small values of η are said to be chemically soft and are highly polarizable and more chemically reactive. Chemical potential (μ) is a quantity that measures the escaping tendency of electrons from this compound in its ground state. High values of chemical potential signify that the molecule is less stable and more reactive [22] . The global electrophilicity index (ω) measures the stabilization in energy of a system, when it acquires an additional electronic charge from the environment [23] .

The compound [Ag(N₂H₁₀C₁₁)₂]PF₆ was synthesized through the reaction of 4,5-dihydro-1H-benzo[g]indazole with silver(I) hexafluorophosphate in methanol, at ambient temperature according to Equation (7).

AgPF 6 + 2C 11 H 1 0 N 2 → AT MeOH [ Ag ( N 2 H 1 0 C 11 ) 2 ] PF 6 (7)

In a 50 mL round bottom flask containing 25 mL of methanol was introduced 0.05 g (0.29 mmol) of silver hexafluorophosphate (AgPF₆) which was dissolved upon magnetic agitation at ambient temperature. To this stirred solution, 0.10 g (0.58 mmol) of 4,5-dihydro-1H-benzo[g]indazole (C₁₁H₁₀N₂) was added in successive small portions. The mixture was stirred overnight and then filtered. The resulting colorless solution was allowed to evaporate in a hood at room temperature. A colorless microcrystalline material unsuitable for single crystal X-ray diffraction was harvested within few days in 82.5% yield.

The synthesized complex salt is insoluble in H₂O and slightly soluble in CH₂Cl₂ or CHCl₃. This colorless solid melts at 247 - 249˚C. The experimental values obtained from elemental analysis are in agreement with theoretical values as illustrated on Table 1 and confirms that the synthesized compound is actually bis(4,5-dihydro-1H-benzo[g]indazole)silver(I) hexafluorophosphate, [Ag(N₂H₁₀C₁₁)₂]PF₆.