Synthesis and crystal structure of copper ( II ) complexes with pyridoxal S-methylisothiosemicarbazone bearing a new coordination mode

The reaction of copper(II) nitrate and pyridoxal S-methylisothiosemicarbazone (PLITSC) in the presence of ammonium thiocyanate resulted in the formation of two types of complexes. A dimeric complex of the formula [{Cu(μ-PLITSC)NCS}2](NCS)2·2MeOH was obtained in the presence of excess of ammonium thiocyanate, while in the presence of its nearly equimolar amount, crystals of a polymeric complex {[Cu(μ-PLITSC)NCS]NO3∙MeOH}n were formed. The complexes were characterized by elemental analysis, conductometry, IR spectroscopy, and their structures are unequivocally determined by single crystal X-ray crystallography. In both complexes, the copper atom is five-coordinated, situated in a deformed square-pyramidal environment. The basal plane of the square pyramid is defined by the oxygen atom of the deprotonated phenol group and terminal nitrogen atoms of the isothiosemicarbazide moiety of the Schiff base, as well as the nitrogen atom of the coordinated thiocyanate ion, while the apical site is occupied by the oxygen atom of the hydroxymethyl group of the neighboring asymmetric unit. This coordination mode of the PLITSC was hitherto unknown. A survey of the Cambridge Structural Database showed that the coordination of the hydroxymethyl group is rare within pyridoxal-based Schiff base metal complexes.

(Received 14 January, accepted 28 January 2019) Abstract: The reaction of copper(II) nitrate and pyridoxal S-methylisothiosemicarbazone (PLITSC) in the presence of ammonium thiocyanate resulted in the formation of two types of complexes.A dimeric complex of the formula [{Cu(μ-PLITSC)NCS} 2 ](NCS) 2 •2MeOH was obtained in the presence of excess of ammonium thiocyanate, while in the presence of its nearly equimolar amount, crystals of a polymeric complex {[Cu(μ-PLITSC)NCS]NO 3 •MeOH} n were formed.The complexes were characterized by elemental analysis, conductometry, IR spectroscopy, and their structures are unequivocally determined by single crystal X-ray crystallography.In both complexes, the copper atom is five-coordinated, situated in a deformed square-pyramidal environment.The basal plane of the square pyramid is defined by the oxygen atom of the deprotonated phenol group and terminal nitrogen atoms of the isothiosemicarbazide moiety of the Schiff base, as well as the nitrogen atom of the coordinated thiocyanate ion, while the apical site is occupied by the oxygen atom of the hydroxymethyl group of the neighboring asymmetric unit.This coordination mode of the PLITSC was hitherto unknown.A survey of the Cambridge Structural Database showed that the coordination of the hydroxymethyl group is rare within pyridoxal-based Schiff base metal complexes.Keywords: dimeric and polymeric Cu(II) complexes; Schiff bases; tetradentate coordination mode; X-ray crystallography.

INTRODUCTION
Thiosemicarbazones and their S-alkyl derivatives (isothiosemicarbazones) represent a very important group of organic compounds.2][3] In addition, these compounds, especially thiosemicarbazones and their metal complexes, have shown a great variety of biological activities, such as antiviral, antibacterial, antimalarial, antitumor, etc. [4][5][6][7][8] The mutual property of thiosemicarbazones is that the sulfur atom of the thioureido fragment, -NH-C(NH 2 )=S, is used for coordination, differing from isothiosemicarbazones, where coordination through the nitrogen atom of the NH 2 -group of the -N=C(-SR)-NH 2 fragment occurs, rather than through the sulfur atom of S-alkyl moiety.[3]9 However, some very rare coordination modes of isothiosemicarbazones are also known, e.g., coordination through the sulfur atom of the S-alkyl moiety, 10 or lack of coordination of the isothioureido fragment. 11Coordination of the mentioned NH 2 -group in tridentate isothiosemicarbazones results in its frequent condensation with, for example, salicyl-and pyridine-2-aldehyde 12 or 2-(diphenylphosphino)benzaldehyde, 13 thus forming complexes with tetradentate N 2 O 2 , N 3 O and N 2 OP donors, respectively.It was also found that bis(isothiosemicarbazones) of pentan-2,4-dione are able to stabilize some less usual oxidation states: Fe(IV), 14,15 Ni(III) 16 and Cu(III). 17he focus of this research was pyridoxal S-methylisothiosemicarbazone (PLITSC), one of the isothiosemicarbazone ligands.Complexes of some 3d metals with this ligand have been synthesized and structurally characterized, 9,18,19 and it was found that, without exception, this ligand acts as a tridentate N 2 O donor, which is coordinated to the metal ion through the azomethine and isothioamide nitrogen atoms and the phenolic oxygen atom.It should be mentioned that this ligand can be coordinated in three different forms: neutral (zwitterionic), monoanionic (in which deprotonation of isothioureido fragment occurs) and dianionic (with the additional deprotonation of the pyridine nitrogen atom). 9erein, the molecular and crystal structures of dimeric and polymeric complexes of Cu(II) with PLITSC, of the formulas [{Cu(μ-PLITSC)NCS} 2 ](NCS) 2 • •2MeOH (1) and {[Cu(μ-PLITSC)NCS]NO 3 •MeOH} n (2) are presented.In the complexes, the tetradentate N 2 O 2 manner of coordination of the PLITSC was found, with additional oxygen atom of the hydroxymethyl group as a fourth donor and bridging ligator.

Reagents
All chemicals used were commercially available products of analytical reagent grade, except for the ligand PLITSC, the preparation of which was described earlier. 20YRIDOXAL S-METHYLISOTHIOSEMICARBAZONE COPPER(II) COMPLEXES 469 Synthesis of the complexes [{Cu(μ-PLITSC)NCS} 2 ](NCS) 2 ⋅2MeOH (1).A mixture of PLITSC (0.060 g, 0.22 mmol) and Cu(NO 3 ) 2 ⋅3H 2 O (0.045 g, 0.19 mmol) was poured over by 10 cm 3 MeOH and heated.After 15 min, a solution of NH 4 NCS (0.030 g, 0.4 mmol) in 2 cm 3 MeOH was added to the cooled solution, the color of which became intense green.The resulting solution was left at room temperature to evaporate.The obtained dark-green single crystals were filtered and washed with MeOH.Yield: 0.05 g (45 %).

Analytical methods
Elemental analyses (C, H, N and S) of air-dried compounds were realized by standard micro-methods.Molar conductivity measurements of freshly prepared MeOH solutions (c = = 1.0 mmol dm -3 ) were performed on a Jenway 4010 conductivity meter.IR spectra were recorded on a Nicolet Nexus 670 (Thermo Scientific) FTIR spectrophotometer, in the range of 400-4000 cm -1 using the KBr pellet technique.

Crystal structure determination
Diffraction experiments were conducted on a Gemini S (Oxford Diffraction) four-circle diffractometer equipped with a Mo-anode sealed tube (Mo K α X-radiation, λ = 0.71073 Å) and a Sapphire3 CCD detector.Diffraction data were processed with the CrysAlis Pro . 21Analytical absorption corrections were performed based on face indexed crystal models.Structure solution and space group determination were performed by SHELXT-2018. 22Full-matrix least--squares refinements were performed by SHELXL-2018, 23 , using ShelXle 24 as a graphical interface.All non-hydrogen atoms were refined with anisotropic displacement parameters.Hydrogen atoms bonded to carbon atoms were introduced in the idealized positions and refined by a riding model.Their displacement parameters were approximated as U iso = kU eq of the parent atoms (k = 1.2 and 1.5 for sp 2 and sp 3 hybridized carbon atoms, respectively).Disorder of methyl group of the pyridoxal moiety was treated by the AFIX 123 command in SHELXL-2018.Hydrogen atoms bonded to heteroatoms were discovered in the residual density map and were refined by application of distance restraints (N-H = 0.86 Å; O-H = = 0.82 Å), with their U iso = 1.2U eq of the parent atoms.In 2, this treatment was used for hydrogen atoms bonded to N1 and O2, while for the other heteroatom-bonded hydrogen atoms, a riding model was used.Structure validation and additional geometrical calculations were performed by PLATON. 25The graphics were plotted with ORTEP3 for Windows. 26rystallographic and refinement details are listed in Table S-I of the Supplementary material to this paper and are deposited in The Cambridge Crystallographic Data Centre under CCDC 1890436-1890437.

Syntheses and characterization
Both complexes were obtained in the reaction of MeOH solutions of Cu(NO 3 ) 2 ⋅3H 2 O, PLITSC and NH 4 NCS.In the presence of excess NH 4 NCS, the R O D I Ć et al.
dimeric complex 1 was formed, otherwise, the obtained complex was polymeric complex 2.
The complexes are crystal substances, soluble in MeOH and DMF, while poorly soluble in H 2 O and Me 2 CO.Molar conductivities of MeOH solutions of complex 1 and 2 are in concordance with the values characteristic for 1:1 type of electrolytes. 27n the IR spectra of both complexes, bands in the region 2780-2850 cm -1 could be ascribed to ν(NH + ) vibrations of the protonated pyridine nitrogen atom of the zwitterionic ligand form, which is formed by migration of the hydrogen atom located on a phenolic oxygen atom to a pyridinic nitrogen atom. 9,19The azomethine band is located at ca. 1560 cm -1 in the spectra of both complexes.In comparison to the same band in the spectrum of the free ligand (1655 cm -1 ), it is shifted to a lower energy, due to coordination of the azomethine nitrogen atom.Coordination of oxygen atom of the deprotonated phenolic OH-group leads to a positive shift of the ν(C-O) band.Namely, this band is located at 1255 cm -1 in the spectra of the free ligand, while its positions in the spectra of complexes 1 and 2 are at 1305 and 1329 cm -1 , respectively. 19inally, in the spectrum of complex 1, the doublet band at 2115 and 2075 cm -1 , corresponds to ν(C-N) vibrations of coordinated and non-coordinated NCS-groups, respectively.Unlike this, in the spectrum of complex 2, only one band is found in the same region (2058 cm -1 ). 28In addition, in the spectrum of the latter complex, very strong band at 1385 cm -1 undoubtedly originates from ν(NO) vibrations of the NO 3 -group. 28nalytical and IR spectral data for the complexes are given in the Supplementary material to this paper.

Crystal structure description
Complex 1 consists of dinuclear centrosymmetric complex ion [{Cu(μ-PLITSC)NCS} 2 ] 2+ and NCS -, while complex 2 is comprised of polymeric complex ions {[Cu(μ-PLITSC)(NCS)] + } n and NO 3 -.Both structures contain methanol molecules as a solvent of crystallization.Molecular structures of the dinuclear centrosymmetric complex cation of 1 and polymeric complex cation of 2 are given in Fig. 1, and selected structural parameters are noted in Table S-II of the Supplementary material.
In both complexes, the ligand is coordinated as a N 2 O 2 tetradentate donor, in a way that the isothioamide and azomethine nitrogen atoms and phenoxide oxygen atom make bonds to one copper atom, while the oxygen atom of the hydroxymethyl group binds a second copper atom.In 1, this results in a dimeric complex, while it results in polymeric catenation in 2.
A survey of the Cambridge Structural Database (CSD) 29 version 5.40 (Nov, 2018) revealed over 200 structures with a pyridoxal-based Schiff base metal complex.It should not escape notice that in a significant number of these (80), the ligands are pyridoxal semi-, thiosemi-or isothiosemicarbazones (and their derivatives), as well as pyridoxal-aminoguanidine.Furthermore, only 16 of structurally characterized pyridoxal-based Schiff base metal complexes contain a coordinated hydroxymethyl function, illustrating how this coordination mode is rare.In a sub-set of these containing copper(II) as a central atom, the hydroxymethyl group plays the role in dimerization of Cu(II) atoms in six complexes (CSD refcodes: ARALAG, 30 IVEPAC, 31 KAHGIM, 32 KAHGOS, 32 PEBWEA 33 and QEKRII 34 ), while in three complexes, the hydroxymethyl group plays the role in polymerization of Cu(II) atoms (CSD refcodes: QEKREE, 34 EBAQOQ 35 and FIVKAW 36 ).The intra-ligand bond lengths have typical values in accordance with those found in structurally related complexes. 9The bond lengths of the C1-N1 and C2-N3 bonds are typical for localized double bonds, and the C4-O1 bond length clearly indicates electron delocalization of the phenoxide function.Similarly, the valence angles of the isothiosemicarbazide moieties of the two complexes match within 1-2°.Finally, it is worth noting that the C5-N4-C7 angles have values greater than 120° (ca.124°) which is representative for a protonated pyridine moiety. 9,37n 1 and 2, the copper atom is five-coordinated, situated in a deformed square-pyramidal environment.The basal plane of the square pyramid is defined by Schiff base donor atoms and the nitrogen atom of the coordinated thiocyanate ion, while the apical site is occupied by the oxygen atom of the hydroxymethyl group of the neighboring asymmetric unit.Both coordination polyhedrons deviate from the ideal square-pyramid towards trigonal-bipyramid.The application of the Holmes method 38  and 84 % for 2, meaning that the coordination polyhedron of 2 is slightly more deformed.
It has been shown 39 that in certain classes of copper(II) complexes, formally five-coordinate (square-pyramidal) copper complexes could alternatively be described as six-coordinate (octahedral) if the sum of corresponding covalent radii is increased by ca.0.1 Å.As the copper atom of the dimeric complex 1 has an additional short contact (2.9879( 16) Å) with the phenoxide oxygen atom O1 ii (symmetry code (ii) -x, -y, -z+1) of the neighboring asymmetric unit, the coordination polyhedron of copper atom in 1 could be described as quasi-octahedral (5+1 coordination) in lieu of square-pyramidal.That would also render the complex cation of 1 1-D polymeric, with the repeating unit being a "dimer" and with the crystallographic axis a as the propagation direction.It is of interest to note that the bridging role of phenoxide oxygen in pyridoxal Schiff base complexes is remarkably rare, as it is found only in one complex (CSD refcode ELUVIR 40 ).
The metal-ligand bond lengths are in accordance with those observed in similar Cu(II) complexes with pyridoxal-based thiosemi-, isothiosemi-, and semicarbazones. 9As expected, the phenoxide oxygen O1 participates in the shortest metal-ligand bond, which is in accordance with the formal negative charge located on this atom.In general, the bond lengths within the coordination polyhedron are comparable in 1 and 2, with the exception of the apical Cu1-O2 bonds.Namely, this bond is by 0.1 Å longer in complex 1.
Cis-valence angles within the basal plane of the coordination polyhedron are very similar (within a 2° tolerance), while the cis-valence angles involving the apical O2 are significantly different for the two complexes.Among these, O2-Cu1-N3 and O2-Cu-O1 are the smallest in 1, while the O2-Cu1-N5 and O2-Cu1-N1 are smallest in 2, which reveal that Cu1-O2 apical bond is inclined in opposite directions with respect to the basal plane in these complexes.The magnitude of inclination is rather similar, as the acute angles between the Cu1-O2 apical bond vector and the least squares plane through the basal atoms (O1, N1, N3, N5) are 83.05(6)° for 1, and 82.06(7)° for 2.
The rationale for the difference in coordination polyhedra geometry could be expressed as follows.As the Cu1-O2 bond is longer in 1, and inclined away from the coordinated thiocyanate ion, the steric repulsion between these two groups is lower in 1.This allows for a wider trans basal angle N3-Cu1-N5 in 1 (178.64(7)°),whereas in 2, this angle is narrower (169.32(8)°)due to the repulsion of the coordinated water molecule and the thiocyanate ion.Due to the larger N3-Cu1-N5 angle in 1, the Cu(II) ion is less displaced from the mean basal plane in 1 (0.0602(3) Å) than in 2 (0.1254(3) Å).An additional reason for the wider trans basal angle N3-Cu1-N5 in 1 may be found in the existence of the O1 ii atom occupying the sixth quasi-octahedral site.
Both structures have developed complex hydrogen bonding networks that connect structural units in a 3-D network (1) or double layers parallel to the bc plane (2).Common to both hydrogen-bonding patterns is that all hydrogen donors are involved in hydrogen-bonding interactions (with the exception of the N4H4 group in 2) and that N2H2 groups and MeOH molecules donate hydrogen bonds to the uncoordinated anion.The hydrogen bonding geometry is presented in Table I.

CONCLUSIONS
The obtained results show that the ligand, pyridoxal S-methylisothiosemicarbazone, besides the usual tridentate N 2 O mode of coordination (terminal nitrogen atoms of the isothiosemicarbazide moiety, and the oxygen atom of deprotonated phenolic OH-group), can also be coordinated as a tetradentate ligand.In this R O D I Ć et al.
case, the oxygen atom of the hydroxymethyl group acts as an additional donor atom of the chelate ligand.In 1, this resulted in a dimeric complex, while in 2, it resulted in polymeric catenation.A perusal of the Cambridge Structural Database showed that coordination of the hydroxymethyl group is rare within pyridoxalbased Schiff base metal complexes.
(using 174° as the value of the idealized trans-basal angles) indicates that transition along the Berry pseudorotation coordinate (D 3h → C 2v → C 4v ) for the coordination polyhedrons of the complexes is 89 % for 1, R O D I Ć et al.