Syntheses and coordination isomerism of heteroleptic divalent-metal (M = Co, Zn) carbazate complexes

Three heteroleptic divalent-metal alkyl-carbazate-thiocyanate complexes, [M(NCS)2(NH2NHCOOR)2] [M = Co and R = CH2CH3 (1); M = Co and R = CH3 (2); M = Zn and R = CH3 (3)], have been prepared and characterized, and their crystal structures determined. In 1, Co(II) adopts a fairly regular centrosymmetric trans-CoO2N4 octahedral geometry arising from its coordination by two N,O-bidentate ethylcarbazate ligands and two trans N-bonded thiocyanates. In isostructural 2 and 3, the metals adopt distorted cis-MO2N4 octahedral geometries arising from two N,O-bidentate methylcarbazate ligands with cis N-bonded thiocyanates. The crystal structures feature N–H⋯O and N–H⋯S interactions. Thermal analysis data show that these compounds begin to decompose at temperatures between 130 and 160 °C. Crystal data: 1, CoC8H16N6O4S2, Mr = 383.32, P21/n (No. 14), a = 5.2599(3) Å, b = 7.4209(4) Å, c = 20.1948(12) Å, β = 94.070(1)°, V = 786.28(8) Å3, Z = 2, R(F) = 0.028, wR(F2) = 0.073; 2, CoC6H12N6O4S2, Mr = 355.27, P21/n (No. 14), a = 7.8663(3) Å, b = 10.5804(3) Å, c = 17.6313(5) Å, β = 102.019(10)°, V = 1435.26(8) Å3, Z = 4, R(F) = 0.036, wR(F2) = 0.097; 3, ZnC6H12N6O4S2, Mr = 361.71, P21/n (No. 14), a = 7.8883(2) Å, b = 10.5756(3) Å, c = 17.5827(5) Å, β = 101.676(1)°, V = 1436.46(7) Å3, Z = 4, R(F) = 0.031, wR(F2) = 0.084. Graphical Abstract


Introduction
A variety of functional materials have been synthesized containing different types of molecular and supramolecular architectures incorporating strong metal-ligand covalent bonds and weak non-covalent interactions [1,2]. Extensive investigations have been made in the roles of different types of organic and inorganic ligands in establishing these networks [3,4]. An important strategy for the generation of new supramolecular architectures involves the use of neutral ligands having both hydrogen bond donor and acceptor capabilities [5,6].
In this context, alkyl carbazates, the alkyl esters of hydrazine carboxylic acid (NH 2 NHCOOH), are of interest. These molecules contain NH 2 and NH donor groups, potential N-and O-hydrogen-bond acceptors, and at the same time, they can act as N,Ochelating ligands. The coordination chemistry of alkyl carbazates has not been much explored, but recently, we have reported various complexes and coordination polymers using ethyl carbazate (NH 2 NHCOOCH 2 CH 3 , Ec), as a co-ligand [7]. The SCN − thiocyanate anion, in combination with different chelating and bridging ligands, has been used to construct numerous molecular and extended crystalline structures [8]. Complexes of the type [M(NCS) 2 (L) 2 ] (L = unsymmetrical bidentate, chelating ligand) can, in principle, give rise to 15 distinct structures when considering geometrical and linkage isomerism [9].
In this manuscript, we describe the syntheses, characterization, and crystal structures of

Materials and methods
All chemicals were of analytical grade and used as received. The hydrazine and thiocyanate contents were determined titrimetrically using standard KIO 3, and metal contents were determined using EDTA solutions [10,11]. Elemental analyses were carried out using a Perkin-Elmer-240B CHN analyzer. IR spectra were recorded as KBr pellets with a Perkin-Elmer Pyris Diamond spectrophotometer from 4000 to 400 cm −1 . UV/visible spectra were recorded in methanol solution on a Shimadzu UV-168 spectrophotometer. 1 H and 13 C{ 1 H} NMR spectra were obtained on a Bruker 400 MHz instrument. Magnetic susceptibility measurements for 1 and 2 were made using a Lakeshore VSM 7410 vibrating sample magnetometer. Thermogravimetric analyses were carried out under oxygen on a Pyris Diamond thermal analyzer with a heating rate of 20°C min -1 and~5 mg of sample used for each experiment.

Syntheses
A 10 mL aqueous solution of the appropriate metal nitrate (0.001 M) was added to 10 mL of an aqueous solution containing 0.002 M of ammonium thiocyanate and 0.002 M of Mc or Ec. The total volume was increased to 60 mL by adding distilled water. The resulting solutions (pale pink for 1 and 2; colorless for 3) of pH~6 were concentrated on a water bath to about one-third of their initial volumes and left for crystallization at room temperature. After a few days, the solid products formed were filtered off, washed with ethanol, and dried in air. All three syntheses yielded single crystals suitable for diffraction studies. The same products arise if the corresponding metal chlorides are used as starting materials.

X-ray crystallography studies
Single-crystal X-ray diffraction data for 1, 2, and 3 were collected on a Bruker Smart Apex CCD diffractometer equipped with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). Selected crystal, data collection, and refinement parameters are listed in table 1. Multi-scan absorption corrections were performed using SADABS [12] during data reduction. The structures were solved by direct methods with SHELXS-97 and refined by full-matrix least squares against F 2 using SHELXL-97 [13]. Hydrogens were located in different maps, relocated to idealized geometries, and refined as riding (N-H = 0.86-0.90 Å; C-H = 0.97-0.98 Å) with U iso (H) = 1.2U eq (N atom) or 1.5U eq (C atom). The methyl groups were allowed to rotate, but not to tip, to best fit the electron density.

Synthesis
The reaction of divalent Co and Zn salts with ammonium thiocyanate and Mc or Ec in water led to compounds of the general formula [M(NCS) 2 (L) 2 ]. All the compounds are highly soluble in water. The following general scheme represents the synthesis of 1, 2, and 3:

Spectroscopic studies
IR spectra of 1, 2, and 3 (see Supplementary material) each show a strong band at 2100 cm −1 , which can be assigned to ν asym (CN) stretch of the thiocyanate. The peak position strongly suggests N-coordination of this anion in these complexes [14]. The spectra also exhibit a band at 1670-1690 cm −1 , which is characteristic of coordinated C=O stretching frequency of carbazate ligands, compared to 1735 and 1724 cm −1 in the spectra of Ec and Mc, respectively [7]. Absorptions around 3180-3250 cm −1 and 1120 cm −1 for all three complexes are in accord with N-H and N-N stretching vibrations, respectively, of carbazate ligands, signifying coordination of nitrogen to the metal.
The UV/visible spectra of 1 and 2 show broad absorptions with λ max = 510 nm (19,600 cm −1 ) and 515 nm (19,400 cm −1 ), respectively. These can both be assigned to the 4 T 1g (F) → 4 T 1g (P) d-d electronic transition of an octahedral Co 2+ d 7 system [15], although the colors perceived by the eye are distinctly different for the two compounds, which can be ascribed to the different λ max values arising from the different dispositions of the ligands in 1 and 2.
In the 1 H NMR spectrum of the Mc ligand, the methoxy protons are a singlet at δ = 3.72 ppm, whereas the 13 C{ 1 H} NMR spectrum shows two peaks at δ = 52.52 and 159.45 ppm due to the methoxy and carbonyl carbon, respectively. In both the 1 H and 13 C{ 1 H} NMR spectra of 3, the downfield shift of all these peaks indicates coordination of the carbonyl O of Mc to Zn. In addition, the 13 C{ 1 H} spectrum shows a peak at 133.23 ppm, which can be assigned to the carbon of coordinated thiocyanate.
The room temperature magnetic susceptibility measurements showed that the effective magnetic moments for 1 and 2 are 4.9 and 4.1 μ B , respectively, which are higher than the spin-only value of 3.88 μ B . This may be ascribed to the orbital angular momentum contribution in d 7 systems and is typical of distorted octahedral cobalt(II) complexes.
3.4. X-ray crystal structures of 1-3 The molecular structures of 1, 2, and 3 are depicted in figures 1, 2, and 3, respectively; selected geometrical data are given in tables 2, 3, and 4, respectively. All these complexes consist of neutral mononuclear units An overall comparison of the MN 4 O 2 polyhedra in 1, 2, and 3 reveals that the octahedron in 1 has a volume of 12.39 Å 3 , a quadratic elongation of 1.016, and an angular variance, ζ [18] of 53.7°2. The corresponding data for 2 are 12.50 Å 3 , 1.018, and 57.2°2, respectively, and for 3, 12.68 Å 3 , 1.023, and 70.5°2, respectively. Apart from the slightly larger value of ζ for 3, these values are similar, indicating that the degrees of distortion of the octahedra are more or less equal.

Crystal packing
The crystal packing in 1, 2, and 3 is largely achieved through N-H⋯O and N-H⋯S hydrogen bonds (table 5). In 1, the NH 2 group of each symmetry-equivalent Ec ligand forms one     coordinated Mc ligands of the adjacent molecule. This results in [0 1 0] chains generated by a crystallographic 2 1 axis incorporating unusual R 2 2 (7) loops ( figure 6). Compounds 2 and 3 also feature N-H⋯S hydrogen bonds, which lead to a 3-D network in the crystal. [Co(NCS) 2 (C 7 H 9 NO 2 ) 2 ] [C 7 H 9 NO 2 = 2,6-bis(hydroxymethyl)pyridine] [21] [27], the three chelate rings are close to planar, and the ligand bite angles are very similar (77.28°, 78.34°, and 78.36°; mean = 78.00°). For the two crystallographically distinct [Zn(Ec) 3 ] 2+ cations described in reference [28], the ligand bite angles are clustered in the narrow range 77.05°-78.17°(mean = 77.69°), but the chelate rings adopt a variety of conformations including near planar, an envelope with the NH 2 as the flap, and twisted about the Zn-N and C-O bonds. All these examples indicate that the carbazate chelate ring is relatively flexible and can adopt different conformations to presumably optimize crystal packing effects.

Conclusion
By varying the pendant alkyl group of the carbazate ligand (L) from methyl to ethyl, we were able to isolate cis and trans pseudo-isomers with equivalent M(NCS) 2 L 2 formulas. These lead to completely different packing motifs in the crystals, although the same types of hydrogen bonds connect the molecules. Their physical properties are consistent with their crystal structures and corresponding data for related compounds reported previously.

Supplemental data
Supplemental data for this article can be accessed here.