Synthesis and Characterization of Catecholato Copper(II) Complexes with Sterically Hindered Neutral and Anionic N3 Type Ligands: Tris(3,5-diisopropyl-1-pyrazolyl)methane and

: Three catecholato copper(II) complexes, [Cu(catCl 4 )(L1 (cid:48) )] , [Cu(catBr 4 )(L1 (cid:48) )] , and [Cu(catCl 4 )(L1H)] , supported by sterically hindered neutral and anionic N3 type ligands: tris(3,5-diisopropyl-1-pyrazolyl)methane (referred to as L1 (cid:48) ) and hydrotris(3,5-diisopropyl-1-pyrazolyl)borate (referred to as L1 − ), are synthesized and characterized in detail. Their X-ray structures reveal that both [Cu(catCl 4 )(L1 (cid:48) )] and [Cu(catBr 4 )(L1 (cid:48) )] complexes have a ﬁve-coordinate square-pyramidal geometry and [Cu(catCl 4 )(L1H)] complex has a four-coordinate square-planar geometry. The L1H is unusual protonated ligand that controls its overall charge. For the three catecholato copper(II) complexes, the oxidation state of copper is divalent, and catechol exists in catecholate as two minus anion. This di ﬀ erence in coordination geometry a ﬀ ects their d-d and CT transitions energy and ESR parameters. = 1,4,7-tribenzyl-1,4,7-triazacyclononane, DPyA pyridin-2-amine, , N N’ , N’ Tp hydrotris(3-cumenyl -5-methylpyrazol-1-yl)borate, NH(Py) 2 = di-2-pyridylamine, b two crystallographically independent molecules.


Introduction
Transition metal complexes ligated by the hydrotris(pyrazolyl)borate as an anionic nitrogen-containing tripod ligand, as first prepared by Professor S. Trofimenko in 1966, are widely studied compounds [1]. An important advance in this chemistry is the introduction of alkyl substitutions of the pyrazolyl rings at the 3 (and 5) position(s) to prevent the formation of an inert hexa-coordinate compound [2]. The coordination behavior of transition metal complexes can easily be changed by introducing substituents with different electronic and steric properties on the pyrazolyl rings. Therefore, transition metal complexes based on these ligands have attracted a great deal of interest, and are still undergoing many investigations [3,4].
On the other hand, isoelectronic tris(pyrazolyl)methane ligands have received less attention. This ligand is formally derived from hydrotris(pyrazolyl)borate ligand, in which the central boron atom is replaced by a carbon atom. This tris(pyrazolyl)methane was also prepared by Professor S. Trofimenko in 1970 as a neutral nitrogen-containing tripod ligand [5]. Some researchers have improved the synthetic method of this ligand, including Elguero and co-workers [6], Reger and co-workers [7], and us [8,9]. We found that tris(pyrazolyl)methane could be synthesized in high yields using an autoclave [8,9].

Synthesis
All of the four complexes prepared as shown in Scheme 4 gave a satisfactory elemental analysis. The reaction of mononuclear chlorido copper(II) [CuCl 2 (L1 )] with suitable catechols, catH 2 X 4 (X = Cl and Br) and NEt 3 at −50 • C yielded green colored mononuclear catecholato complexes,

Structure
Successful single-crystal X-ray structural analyses were performed on compounds
In contrast to the L1′ complexes, the L1 − complex is something different. Its structure revealed that hydrotris(pyrazolyl)borate adopts an unusual bidentate mode of coordination mode with one dangling pyrazole ring. The apical nitrogen (N21) is located away from the copper(II) center; the distance between Cu1 and N21 is 2.991(6) Å. The apical deviation of the copper(II) ion from the corresponding least-squares N2O2 basal plane is 0.11 Å, indicating that the coordination geometry around the copper(II) ion in [Cu(catCl4)(L1H)] is essentially a four-coordinate square-planar geometry whose basal plane consists of two nitrogen atoms (N11 and N31) from hydrotris(pyrazolyl)borate and two oxygen atoms (O40 and O45) from catechol. The dihedral angle between apical pyrazole and catechol ring is 71.2°. The distance of N21···O40 is 4.112(7) Å and that of

The coordination geometry around the copper(II) ion in [Cu(catCl 4 )(L1 )] and [Cu(catBr 4 )(L1 )]
is essentially a five-coordinate square-pyramidal geometry with the basal plane comprising two nitrogen atoms (N11 and N31) from tris(pyrazolyl)methane and two oxygen atoms (O40 and O45) from catechol and whose axial site is occupied by the remaining nitrogen atom (N21) (Figure 1). This coordination geometry is supported by the structural parameter In contrast to the L1 complexes, the L1 − complex is something different. Its structure revealed that hydrotris(pyrazolyl)borate adopts an unusual bidentate mode of coordination mode with one dangling pyrazole ring. The apical nitrogen (N21) is located away from the copper(II) center; the distance between Cu1 and N21 is 2.991(6) Å. The apical deviation of the copper(II) ion from the corresponding least-squares N 2 O 2 basal plane is 0.11 Å, indicating that the coordination geometry around the copper(II) ion in [Cu(catCl 4 )(L1H)] is essentially a four-coordinate square-planar geometry whose basal plane consists of two nitrogen atoms (N11 and N31) from hydrotris(pyrazolyl)borate and two oxygen atoms (O40 and O45) from catechol. The dihedral angle between apical pyrazole and catechol ring is 71.2 • . The distance of N21···O40 is 4.112(7) Å and that of N21···O45 is 2.903(7) Å. Therefore, the dangling pyrazole ring is tilted toward N31 pyrazole ring. From the averaged Cu-O distances of 1.921(4) Å and the averaged C-O distances of 1.335(8) Å, the coordinated catechol group is best described as catCl 4 2− rather than either sqCl 4 •− or qCl 4 (Scheme 1 and Table 1) as well as the L1 complexes and this copper is also divalent. This oxidation assignment is also consistent with the UV-Vis and ESR data (vide infra). The charge consideration of the catechol and the copper oxidation state indicates the anionic hydrotris(pyrazolyl)borate should be neutral. This unusual behavior suggests the dangling pyrazole must be protonated at the apical nitrogen (N21). This behavior is supported by ν(N-H) in its IR spectrum (vide infra) and intramolecular hydrogen bond between N21 and O45 (2.903(7) Å). This protonated hydrotris(pyrazolyl)borate is very rare. The first example of copper(II) complex was reported by Professor S. Trofimenko in 1994 [39].

IR Spectroscopy
IR spectra of the three catecholato complexes were measured using KBr pellets as shown in Figure 3.

In the L1 complexes [Cu(catCl 4 )(L1 )] and [Cu(catBr 4 )(L1 )]
, two broad characteristic bands were observed at 1466 and 1400 cm −1 , corresponding to the ring stretching (ν(C-C)) and the CO group stretching (ν(C-O)), respectively. In the L1 − complex [Cu(catCl 4 )(L1H)], the ν(C=N) band split to 1569 and 1537 cm −1 due to different pyrazole ring environment. Intensity and stretching energy indicate that former is derived from the protonated pyrazole ring. Catechol ring stretching was also observed at 1453 and 1377 cm −1 . Moreover, the ν(N-H) band was observed at 3142 cm −1 . This is the first observation of the protonated pyrazole by IR spectroscopy. N21···O45 is 2.903(7) Å. Therefore, the dangling pyrazole ring is tilted toward N31 pyrazole ring. From the averaged Cu-O distances of 1.921(4) Å and the averaged C-O distances of 1.335(8) Å, the coordinated catechol group is best described as catCl4 2− rather than either sqCl4 •− or qCl4 (Scheme 1 and Table 1) as well as the L1′ complexes and this copper is also divalent. This oxidation assignment is also consistent with the UV-Vis and ESR data (vide infra). The charge consideration of the catechol and the copper oxidation state indicates the anionic hydrotris(pyrazolyl)borate should be neutral. This unusual behavior suggests the dangling pyrazole must be protonated at the apical nitrogen (N21). This behavior is supported by ν(N-H) in its IR spectrum (vide infra) and intramolecular hydrogen bond between N21 and O45 (2.903(7) Å). This protonated hydrotris(pyrazolyl)borate is very rare. The first example of copper(II) complex was reported by Professor S. Trofimenko in 1994 [39]. Other reported examples include V(IV) complex [40], Mn(II) complex [41], Pt(II) complexes [42][43][44], and Pt(IV) complex [45].

IR Spectroscopy
IR spectra of the three catecholato complexes were measured using KBr pellets as shown in Figure 3.

One typical C=N stretching vibration in both [Cu(catCl4)(L1′)] and [Cu(catBr4)(L1′)]
complexes was shifted to 1557 and 1556 cm −1 from the corresponding L1′ ligand at 1553 cm −1 , respectively [9]. Coordinated catechols typically show two strong stretching bands attributed to the ring stretching (ν(C-C)) and the CO group stretching (ν(C-O)) around ~1500 cm −1 and ~1350 cm −1 , respectively [46][47][48][49][50]. In the L1′ complexes [Cu(catCl4)(L1′)] and [Cu(catBr4)(L1′)], two broad characteristic bands were observed at 1466 and 1400 cm −1 , corresponding to the ring stretching (ν(C-C)) and the CO group stretching (ν(C-O)), respectively. In the L1 − complex [Cu(catCl4)(L1H)], the ν(C=N) band split to 1569 and 1537 cm −1 due to different pyrazole ring environment. Intensity and stretching energy indicate that former is derived from the protonated pyrazole ring. Catechol ring stretching was also observed at 1453 and 1377 cm −1 . Moreover, the ν(N-H) band was observed at 3142 cm −1 . This is the first observation of the protonated pyrazole by IR spectroscopy. Far-IR spectra of the three catecholato complexes were measured using CsI pellets as shown in Figure 4. From the literature, the M-O stretching bands of the coordinated catechol with transition metals were observed between 500 and 600 cm −1 [48][49][50]. However, the M-O stretching band values for the three catecholato complexes shown in Figure 4 are so broad and complicated that these assignments are very difficult at this stage and require more experiments and DFT calculations to make reliable assignments. Far-IR spectra of the three catecholato complexes were measured using CsI pellets as shown in Figure 4. From the literature, the M-O stretching bands of the coordinated catechol with transition metals were observed between 500 and 600 cm −1 [48][49][50]. However, the M-O stretching band values for the three catecholato complexes shown in Figure 4 are so broad and complicated that these assignments are very difficult at this stage and require more experiments and DFT calculations to make reliable assignments.

ESR Spectroscopy
The frozen glass ESR spectra of the complexes at 137 K are presented in Figure 6. The ESR spectra show that these complexes have an S = 1/2 ground state. The order of g|| > g⊥ > 2.0023 is satisfied in all the complexes, confirming the presence of unpaired electron of the copper(II) ion in dx 2 −y 2 orbital.

ESR Spectroscopy
The frozen glass ESR spectra of the complexes at 137 K are presented in Figure 6. The ESR spectra show that these complexes have an S = 1/2 ground state. The order of g|| > g⊥ > 2.0023 is satisfied in all the complexes, confirming the presence of unpaired electron of the copper(II) ion in dx 2 −y 2 orbital.

ESR Spectroscopy
The frozen glass ESR spectra of the complexes at 137 K are presented in Figure 6. The ESR spectra show that these complexes have an S = 1/2 ground state. The order of g|| > g⊥ > 2.0023 is satisfied in all the complexes, confirming the presence of unpaired electron of the copper(II) ion in d x Inorganics 2020, 8, 37 9 of 15 as catecholate two minus anion. Some differences in the ESR parameters in all the complexes are caused by different coordination geometries: five-coordinate square-pyramidal and four-coordinate square-planar. The ESR parameters of the reported Cu(II)-catBu 2 or Cu(II)-catCl 4 complexes are also consistent with an S = 1/2 ground state [27][28][29][30]33]. On the other hand, the reported magnetism of Cu(II)-sqBu 2 or sqCl 4 complexes are diamagnetism (ESR silent) [27,33,34,36,37] or ferromagnetism [35]. These observations are consistent with the above consideration that catechol is coordinated as catecholate two minus anion. Some differences in the ESR parameters in all the complexes are caused by different coordination geometries: five-coordinate square-pyramidal and four-coordinate squareplanar. The ESR parameters of the reported Cu(II)-catBu2 or Cu(II)-catCl4 complexes are also consistent with an S = 1/2 ground state [27][28][29][30]33]. On the other hand, the reported magnetism of Cu(II)-sqBu2 or sqCl4 complexes are diamagnetism (ESR silent) [27,33,34,36,37] or ferromagnetism [35].

Material and General Techniques
Preparation and handling of all the complexes was performed under an argon atmosphere using standard Schlenk tube techniques or in a VAC inert atmosphere glovebox containing argon gas. Dichloromethane and acetonitrile were carefully purified by refluxing and distilling under an argon atmosphere over phosphorous pentoxide and calcium hydride prior to use, respectively [54]. Other reagents are commercially available and were used without further purification. [CuCl2(L1′)] [8,11] and [{Cu(L1)}(μ-OH)]2 [55] were prepared using the published methods.

Instrumentation
IR spectra (4000-400 cm −1 ) and far-IR (650-150 cm −1 ) spectra were recorded on KBr pellets and on CsI pellets, respectively, using a JASCO FT/IR-550 spectrophotometer (JASCO, Tokyo, Japan). Abbreviations used in the description of the vibration data are as follows: vs, very strong; s, strong; m, medium; w, weak. UV-Vis spectra at low temperature were measured on an Otsuka Electronics MCPD-2000 system (Otsuka Electronics, Tokyo, Japan) with an optical fiber attachment (300-1100 nm). ESR spectra as frozen solutions (dichloromethane/1,2-dichloroethane) were recorded on a Bruker EMX-T ESR spectrometer (Bruker Japan, Yokohama, Japan) at 137 K in quartz tubes (diameter 5 mm) with a liquid nitrogen temperature controller BVT 3000. The elemental analyses (C, H, N) were performed by the Chemical Analysis Center at the University of Tsukuba.

Material and General Techniques
Preparation and handling of all the complexes was performed under an argon atmosphere using standard Schlenk tube techniques or in a VAC inert atmosphere glovebox containing argon gas. Dichloromethane and acetonitrile were carefully purified by refluxing and distilling under an argon atmosphere over phosphorous pentoxide and calcium hydride prior to use, respectively [54]. Other reagents are commercially available and were used without further purification.

Instrumentation
IR spectra (4000-400 cm −1 ) and far-IR (650-150 cm −1 ) spectra were recorded on KBr pellets and on CsI pellets, respectively, using a JASCO FT/IR-550 spectrophotometer (JASCO, Tokyo, Japan). Abbreviations used in the description of the vibration data are as follows: vs, very strong; s, strong; m, medium; w, weak. UV-Vis spectra at low temperature were measured on an Otsuka Electronics MCPD-2000 system (Otsuka Electronics, Tokyo, Japan) with an optical fiber attachment (300-1100 nm). ESR spectra as frozen solutions (dichloromethane/1,2-dichloroethane) were recorded on a Bruker EMX-T ESR spectrometer (Bruker Japan, Yokohama, Japan) at 137 K in quartz tubes (diameter 5 mm) with a liquid nitrogen temperature controller BVT 3000. The elemental analyses (C, H, N) were performed by the Chemical Analysis Center at the University of Tsukuba.

[Cu(catCl 4 )(L1 )]
To a solution of [CuCl 2 (L1 )] (160 mg, 0.266 mmol) in dichloromethane (40 cm 3 ) was added tetrachlorocatechol (81.5 mg, 0.329 mmol) and triethylamine (72.6 mg, 0.717 mmol) dissolved in dichloromethane (10 cm 3 ) at −50 • C and the solution was stirred at −50 • C for 30 min. During the reaction, the color of the solution gradually turned from yellow-green to green. After it was stirred at 0 • C for 30 min, the solvent was evaporated under vacuum. The resulting solid was extracted with acetonitrile (40 mL  The diffraction data were measured on a Rigaku/MSC Mercury CCD system (Rigaku, Tokyo, Japan) with graphite monochromated Mo Kα (λ = 0.71070 Å) radiation at low temperature. The unit cell parameters of each crystal were determined using CrystalClear [56] from 6 images. The crystal to detector distance was ca. 45 mm. Data were collected using 0.5 • intervals in ϕ and ω to a maximum 2θ value of 55.0 • . A total of 744 oscillation images were collected. The highly redundant data sets were reduced using CrystalClear and corrected for Lorentz and polarization effects [56]. An empirical absorption correction was applied for each complex. Structures were solved by direct methods (SIR92 and SIR97) [57,58] and heavy-atom Patterson methods [59]. The position of the copper ions and their first coordination sphere were located from a direct method E-map; other non-hydrogen atoms were found in alternating difference Fourier syntheses, and least squares refinement cycles. During the final refinement cycles the temperature factors were refined anisotropically. Refinement was carried out by a full matrix least-squares method on F 2 . All calculations were performed with the CrystalStructure [60] crystallographic software package except for refinement, which was performed using SHELXL 2013 [61]. Hydrogen atoms were placed in calculated positions. Sheldrick weighting scheme was used. Crystallographic data and structure refinement parameters including the final discrepancies (R and Rw) are listed in Table 2. The crystals of [Cu(catCl 4 )(L1H)] show a slightly lower quality of diffraction and some carbon atoms were disordered. Moreover, the solvent molecules in this crystal were highly disordered. Therefore, PLATON SQUEEZE was used to account for severely disordered solvent molecules [62].