Two CuII complexes of 3,4,5-trimethyl-1H-pyrazole

The crystal structures of two CuII complexes of 1-H-3,4,5-trimethylpyrazole, cis-[{CuCl[3,4,5-(CH3)3(C3H2N2)]2}2(μ-Cl)2] and [Cu{3,4,5-(CH3)3(C3HN2)}4(H2O)](NO3)2, are presented and compared to the 3-methyl-1H-pyrazole and 3,5-dimethyl-1H-pyrozole analogues.


Chemical context
Pyrazoles are a useful class of molecules because they coordinate with metal ions, form conjugated -systems, and can be tuned electronically and sterically through a number of possible substituent groups. It is therefore important to gain a better understanding of how changes in reaction conditions, including solvent, substituents, and counter-ions, affect the structures of compounds incorporating pyrazole and its derivatives. Previous work using mono-and dimethyl pyrazole ligands demonstrated the effect of the counter-ion on the final structure and electronic properties of their respective Cu II complexes from water (Giles et al., 2015). Absent in this analysis were complexes incorporating 1-H-3,4,5-trimethylpyrazole. Work presented herein adds structural determinations of complexes of 1-H-3,4,5-trimethylpyrazole under the same reaction conditions to complete the series. Complexes incorporating this final ligand are important to obtain a complete understanding of how different pyrazole substituents and their locations affect the coordination environment about the central Cu II atom. CuCl 2 and Cu(NO 3 ) 2 were used to assess counter-ion effects on the crystal structure in a manner consistent with the previous work. ISSN 2056-9890

Structural commentary
In the CuCl 2 complex with 1-H-3,4,5-trimethylpyrazole (1, Fig. 1), there are two trimethylpyrazole ligands and three chloride ions bound to each Cu II center. Two of the chloride ions bridge asymmetrically to a second copper(II), which is related to the first Cu II by an inversion center. The overall geometry around each Cu II center is square pyramidal, with the axial position occupied by the elongated bridging chloride contact, and the equatorial positions occupied by the two trimethylpyrazole ligands in a cis configuration, one terminal chloride ion, and the shorter bridging chloride contact. In 1, the trimethylpyrazole ligands are tilted off-perpendicular from the basal plane of the square-pyramidal Cu II coordination environment. The dihedral angles of the pyrazole ligands to the basal plane and to each other are as follows: between the mean N9/N10/C11-C13 plane and the mean Cl2/Cl1/N2/N10 plane, 53.9 (2) ; between the mean N2/N1/C3-C5 plane and the mean Cl2/Cl1/N2/N10 plane, 47.1 (2) ; between the mean N9/N10/C11-C13 and the mean N2/N1/C3-C5 plane, 51.5 (2) . The trimethylpyrazole ligand is not deprotonated in the complex as there are two chloride ions per Cu II ion. Additionally, the bond distances within the trimethylpyrazole ring are more characteristic of a non-aromatic, conjugated ring [C3-C4, C11-C12, 1.403 (6) and 1.410 (6) Å ; C4-C5, C12-C13, 1.383 (6)-1.388 (6) Å ; C13-N9, C11-N10, 1.341 (5) Å , C3-N2, C5-N1, 1.335 (5) and 1.339 (5) Å ; N9-N10, N1-N2, 1.353 (5) and 1.356 (5) Å ], rather than the heteroaromatic species obtained upon deprotonation (Allen et al., 1987). The structure produced is similar to the previously reported 1-H-3,5-dimethylpyrazole Cu II complex. These structures differ from the 1-H-3-methylpyrazole-Cu II complex primarily through the positioning of the pyrazole ligands, which are Molecular structure of 2, showing the atom-labeling scheme. Displacement ellipsoids are drawn at the 50% probability level. Only one set of disordered methyl protons are shown. The Cu-OH 2 bond resides on a twofold rotation axis, with half of the molecule generated by the symmetry operator 1 2 À x, y, 1 À z. Hydrogen-atom labels are omitted for clarity.

Figure 1
Molecular structure of 1, showing the atom-labeling scheme. Displacement ellipsoids are drawn at the 50% probability level. The complex resides on an inversion center, with half of the molecule generated by the symmetry operator Àx, 1 À y, 1 À z. Only one set of disordered methyl protons are shown. Hydrogen-atom labels are omitted for clarity.
oriented trans to one another in the monomethylpyrazole complex rather than cis as in the di-and trimethylpyrazole complexes (Giles et al., 2015).

Figure 3
Packing of 1 viewed down the crystallographic b-axis direction, highlighting the alignment of the copper complexes in the same orientation throughout the crystal. Hydrogen atoms are omitted for clarity.  (3) 174 (3) Symmetry codes: (i) Àx þ 1 for the comparable CuÁ Á ÁCu distance in the dimethylpyrazole complex, suggesting additional steric crowding due to the third methyl group. The packing ( Fig. 3) is also different in that the trimethylpyrazole complexes are oriented in the same direction within the crystal, whereas the dimethylpyrazole complexes alternate their orientation. Both the di-and trimethylpyrazole complexes pack in space group P1.
In 2, there is hydrogen bonding present between the oxygen atoms of the nitrate ions and both the pyrazole N-H and coordinated water O-H atoms on the complex, limiting the positional disorder of the nitrate ions (Table 2). Surprisingly, 2 packs more closely together ( Fig. 4) than its congener incorporating 1-H-3,5-dimethylpyrazole (Giles et al., 2015). The positioning of the pyrazole ligands in the trimethylpyrazole complex is such that the pyrazole-pyrazole overlap occurs between two different portions of the pyrazole ring, allowing for a closer contact [centroid-centroid distance between N9/ N10/C11-C13 rings = 4.49 (2) Å , distance between ring planes = 3.35 (2) Å ], likely the result of pyrazole ring polarization that leaves one region electron-withdrawn while the other is more electron-rich. In the dimethylpyrazole complex, the pyrazole ligands overlap with the same region of the ring, which have similar electronic properties and therefore are more repulsive, increasing the ring-ring overlap distance as measured between the comparable ring centroids [4.98 (2) Å ] and the interplane distance [3.97 (2) Å ]. The result is closer packing for the trimethylpyrazole complex [10.06 (2) Å between Cu II centers of complexes in adjacent columns, 7.89 (2) Å between Cu II centers of complexes within stacked columns] when compared to the dimethylpyrazole [10.15 (2) Å between Cu II centers in adjacent complexes, 8.23 (2) Å between Cu II centers in stacked complexes]. Both structures crystallize in space group I2/a (reported as C2/c for the dimethylpyrazole complex).

Database survey
A search of the CSD (Groom et al., 2016;Version 5.38, May 2017 update) for structures containing 1-H-3,4,5-trimethylpyrazole yields only 17 entries. The structure with CSD refcode FITQEE is of the neutral ligand only (Infantes et al., 1999). In this structure, one trimethylpyrazole molecule resides on a twofold rotation axis, with positional disorder of the pyrazole N-H proton, and a C-C bond length of 1.389 Å , a C-N bond length of 1.341 Å , and a N-N bond length of 1.346 Å . The C-C and C-N bond lengths are equivalent because of the twofold rotation. The other trimethylpyrazole molecule does not reside on a symmetry element, but still contains C-C and C-N bonds that are close in distance (C-C range from 1.385 to 1.388 Å and C-N is 1.336 Å ), with an N-N distance of 1.357 Å . These distances are comparable to those seen in 1 and 2, although both 1 and 2 show wider ranges of bond lengths than the free ligand.
The closest match to 1, CSD refcode CENJIO, is a fluoridebridged Cu II complex containing three 1-H-3,4,5,-trimethylpyrazole ligands per copper(II) center (Rietmeijer et al., 1984) and tetrafluoroborate as counter-ion. In this complex, the Packing of 2 viewed down the crystallographic c-axis direction, highlighting the alternating columns of stacked copper complexes. Hydrogen atoms are omitted for clarity. bond lengths are within expected lengths for a neutral pyrazole group and are comparable to, but cover a wider range of distances than, those in 1 and 2.

Synthesis and crystallization
All manipulations were carried out in air at room temperature with reagents as obtained from the manufacturer, unless otherwise stated. 3-Methyl-2,4-pentanedione was purchased from Alfa-Aesar, while the hydrazine monohydrate was purchased from Sigma-Aldrich. CuCl 2 Á2H 2 O was purchased from Aldrich, and Cu(NO 3 ) 2 Á2.5H 2 O was purchased from Fisher. Deionized water was used in all reactions.
further addition of the Cu II solution, reaching dark green. Upon complete addition of Cu II , the solution became teal green with a small amount of precipitate. The reaction was stirred overnight, filtered, and the solvent slowly evaporated to yield dark-green crystals.
[Cu{3,4,5-(CH 3 ) 3 (C 3 HN 2 )} 4 (H 2 O)](NO 3 ) 2 (2) 1-H-3,4,5-trimethylpyrazole (0.16601 g, 1.5070 mmol) was dissolved in 5 mL of H 2 O, with 1 mL of acetone added to aid dissolution. A light-blue solution of 0.18296 g (0.78655 mmol) Cu(NO 3 ) 2 Á2.5 H 2 O in 5 mL H 2 O was added via pipette to this solution while stirring. A 1 mL rinse of the Cu II vessel with H 2 O was added to the reaction. There was an immediate change of color to light green as the Cu II solution was added, which darkened upon further addition of the Cu II solution, reaching dark green. Upon complete addition of Cu II , the solution became teal green with a small amount of precipitate. The reaction was stirred overnight, filtered, and the solvent slowly evaporated to yield dark-blue crystals.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. For 1, five reflections (110)

Di-µ-chlorido-bis[chloridobis(3,4,5-trimethyl-1H-pyrazole-\ κN 2 )copper(II)] (1)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Reflections omitted from final refinement on account of beamstop truncation: (h k l) 1 1 0; 1 0 0; 0 1 0; 0 0 1; 1 1 1