Introduction

Numerous compounds containing the 1,2,4–triazine moieties are well known in natural materials and show interesting biological, pharmacological and medicinal properties. 4-Amino-5-oxo-3-phenylamino-1,2,4-triazine has a powerful inhibiting effect on the cell wall lignification catalysed by peroxidases, and alters the integrity of chloroplast. Some of the 3,5,6-trisubstituted-1,2,4-triazines can be active as blood platelet aggregation inhibitors and others exhibit antiviral inhibitory activity (against influenza viruses for example), significant activity towards leukaemia and ovarian cancer, and anti-HIV activity [1]. More recently, it has been revealed that a zinc–dppt complex (dppt = 5,6-diphenyl-3-(2-pyridyl)-1,2,4-triazine) associated to a biocide could have significant biocidal effects on living cells including those of microorganisms (bacteria and fungi), cell culture system, plants and animals [2]. Another interesting property of the derivatives of 1,2,4-triazine compounds is formation of coloured complexes when they are coordinated to a metal ion. In particularly, the ligand 5,6-diphenyl-3-(2-pyridyl)-1,2,4-triazine has been widely used as an sensitive reagent for the determination of Fe(II) by spectrophotometric methods, in natural and waste waters [3, 4].

On the other hand, investigations on the syntheses, crystal, molecular and electronic structures of metal complexes containing ambidentate ligands are of great interest in connection with the accumulation of metal complexes having ambidentate ligands and with regulation the reactivities of active sites on metal complexes. Many transition metal complexes containing ambidentate ligands have been synthesized, and their structures, physical properties and linkage isomerization reactions of ambidentate units have been investigated. Among the ambidendate ligands the pseudohalide ions (N3 , NCS, NCO, N(NC) 2 ) are versatile ligands that can bind transition metal ions in a variety of ways. These anions can act as monodentate ligands and as bridging ligands leading to the formation of mononuclear and polynuclear species with different dimensionalities and nuclearities. Among these, the azide- and thiocyanate-containing metal complexes are considered to be the most investigated systems because of their diverse structures and applications in magnetic materials. Having two different donor atoms SCN can coordinate to metals through either the nitrogen or the sulphur atom, or both. In agreement with the hard soft acid base (HSAB) SCN ion coordinate to hard acids (Mn2+, Co2+ and Ni2+) through nitrogen atom, and the uncoordinated sulphur atom is involved in hydrogen bonds and sometimes involved in S···S interactions. If the transition metal centre is soft acid (Cd2+, Cu+ and Hg2+) SCN ligand binds to central ion through sulphur atom. Different bridging modes of the thiocyanato ligand can generate various types of supramolecular structures with particular properties. Thiocyanato bridges play an important role in the magnetic exchange pathways between paramagnetic centres. SCN ion possesses polarizable π system, the electron is dislocated and it may act as a mediator for the magnetic interaction between the paramagnetic transition metal centres. The geometry and coordination mode of NCS in 3d metal complexes is strongly influenced by the electronic and steric effects around central ion [511]. Thiocyanate complexes with the formula of [M(NCS)2L4], where L is a N-heteroaromatic ligand such as pyridine, are called Werner-type complexes and are well known. The complexes are interesting because of their architectures organised by non-covalent contacts as hydrogen bonds, π–π stacking interaction and simplicity of preparation and their catalytic efficiency [12, 13]. Their interesting structural properties and potential application are attractive in studies of magnetism or magnetic exchange ions.

In this article, we present the synthesis, crystal, molecular, electronic structures and the spectroscopy characterisation of the manganese(II), nickel(II) and copper(II) complexes with 5,6-diphenyl-3-(2-pyridyl)-1,2,4-triazine ligands. The electronic structure of the studied complexes has been determined with the density functional theory (DFT) method, and employed for discussion of the bonding properties. Currently, density functional theory (DFT) is commonly used to examine the electronic structure of transition metal complexes. It meets with the requirements of being accurate, easy to use and fast enough to allow the study of relatively large molecules of transition metal complexes.

Experimental

All reagents used for the synthesis of the complex are commercially available and were used without further purification.

Synthesis of [M(SCN)2(pytz)2], where M = Mn(II) (1), Cu(II) (3) and [NiCl2(pytz)2]·2CH3OH (2) complexes

These complexes were synthesized in the reaction between MnCl2·4H2O, NiCl2·6H2O, CuCl2·2H2O, NH4SCN (except the nickel(II) complex) and stoichiometric volumes of 5,6-diphenyl-3-(2-pyridyl)-1,2,4-triazine (pytz) in methanolic solution (50 cm−3). The mixtures of the compounds were refluxed for 0.5 h. After this time, the volume of the solvent was reduced to about 20 cm3; the solution was cooled and left out to slow evaporation.

[Mn(SCN)2(pytz)2] (1)

Yield 84%. IR (KBr): 3064 νArH; 2068 ν(CN from SCN); 1599 νCN, 1572 νC=C; 1481, 1374 δ(C–CH in the plane); 1446 νArH; 1121 δ(C–CH in the plane); 1009 δ(C–H out of the plane); 798 δ(C–C out of the plane); 770 ν(SC from SCN), 698 δ(C–C in the plane); 538 δ(NCS), 414 ν(Mn–Npy). UV–vis (methanol; logε): 498.4 (0.98), 439.0 (1.01), 360.5(1.24), 283.0 (3.76), 234.0 (2.11), 206.5 (4.27).

[NiCl2(pytz)2]·2CH3OH (2)

Yield 91%. IR (KBr): 3359 νOH; 3060 νArH; 1601 νCN, 1580 νC=C; 1517, 1255 δ(C–CH in the plane); 1446 νPh; 1013 δ(C–CH in the plane); 924 δ(C–H out of the plane); 770 δ(C–C in the plane). UV–vis (methanol; logε): 887.0 (0.78), 790.2 (0.81), 494.0 (1.32), 331.3 (2.79), 300.0 (3.89), 235.0 (2.13), 205.4 (4.28).

[Cu(SCN)2(pytz)2] (3)

Yield 62%. IR (KBr): 3097 νArH; 2080 ν(CN from SCN); 1612 νCN, 1591 νC=C; 1483, 1368 δ(C–CH in the plane); 1445 νPh; 1039 δ(C–CH in the plane); 1007 δ(C–H out of the plane); 811 δ(C–C out of the plane), 760 ν(SC from SCN); 693 δ(C–C in the plane); 536 δ(NCS). UV–vis (methanol; logε): 752.5 (1.03), 433.5 (1.16), 360.5 (1.21), 300.0 (1.71), 232.5 (2.17), 207.0 (4.32).

Physical measurements

Infrared spectra were recorded on a Nicolet Magna 560 spectrophotometer in the spectral range of 4000 ÷ 400 cm−1 with the sample in the form of KBr pellet. Electronic spectra were measured on a Lab Alliance UV–VIS 8500 spectrophotometer in the range of 1100 ÷ 180 nm in methanol solution. EPR spectra were recorded in powder sample at 295 K on a Bruker EMX-10 spectrometer using 100 kHz field modulation.

DFT calculations

The calculations were carried out using Gaussian09 [14] program. The DFT/B3LYP [15, 16] method was used for the geometry optimisation and electronic structure determination. The calculations were performed using the polarisation functions for all atoms: 6–311 g**—manganese, nickel, copper, 6–31 g**—carbon, nitrogen, sulphur and 6–31 g—hydrogen. Natural bond orbital (NBO) calculations were performed with the NBO code [17] included in Gaussian09. The contribution of a group to a molecular orbital was calculated using Mulliken population analysis. GaussSum 2.2 [18] was used to calculate group contributions to the molecular orbitals and to prepare the density of states (DOS) and overlap population density of states (OPDOS) spectra. The OPDOS spectra were created by convoluting the molecular orbital information with Gaussian curves of unit height and full width at half maximum (FWHM) of 0.3 eV. Mayer bond orders were calculated with use of QMForge program [19].

Crystal structures determination and refinement

The crystals of [Mn(SCN)2(pytriaz)2] (1), [NiCl2(pytriaz)2]·2CH3OH (2) and [Cu(SCN)2(pytriaz)2] (3) were mounted in turn on a Xcalibur, Atlas, Gemini ultra Oxford Diffraction automatic diffractometer equipped with a CCD detector, and used for data collection. X-ray intensity data were collected with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at temperature of 298.0(2) K (1, 2) or 150.0(2) K (3), with ω scan mode. Ewald sphere reflections were collected up to 2θ = 50.10. The unit cell parameters were determined from least-squares refinement of the setting angles of 8420, 9214 and 5146 strongest reflections for complexes (1), (2) and (3), respectively. Details concerning crystal data and refinement are shown in Table 1. During the data reduction, the decay correction coefficient was taken into account. Lorentz, polarisation and numerical absorption corrections were applied. The structures were solved by direct method. All the non-hydrogen atoms were refined anisotropically using full-matrix, least-squares technique on F 2. All the hydrogen atoms were found from difference Fourier synthesis after four cycles of anisotropic refinement, and refined as “riding” on the adjacent atom with individual isotropic temperature factor equal to 1.2 times the value of equivalent temperature factor of the parent atom, with geometry idealisation after each cycle. Olex2 [20] program was used for all the calculations. Atomic scattering factors were those incorporated in the computer programs.

Table 1 Crystal data and structure refinement details of [Mn(SCN)2(pytriaz)2] (1), [NiCl2(pytriaz)2].2CH3OH (2) and [Cu(SCN)2(pytriaz)2] (3) complexes

Results and discussion

The reactions of the chloride salts of manganese(II), nickel(II) and copper(II) with 5,6-diphenyl-3-(2-pyridyl)-1,2,4-triazine (pytz) and ammonium thiocyanate in the case of complexes 1 and 3 have been carried out. Refluxing the starting metal(II) salt with the ligands in methanol leads to hexacoordinate complexes with good yields. Infrared spectra of the complexes present characteristic bands due to ligands vibrations.

The stretching modes of the aryl C–H bond display maxima at 3064, 3060 and 3097 cm−1 for complexes (1), (2), and (3) respectively. In the spectrum of complex (2) band with maximum at 3359 cm−1 is assigned to hydroxyl group of the methanol molecules. The νC=N band of pyridine ring appears at about 1600 cm−1. The νCN, νCS and δNCS frequencies of isothiocyanato ligands present maxima at 2068 cm−1 (1), 2080 cm−1 (3), 770, 760 cm−1 and 538, 536 cm−1, respectively, and it is in good agreement with the end-on NCS coordination. The coordination modes of thiocyanate ligands in the studied complexes are indeterminable from the IR spectral data of these compounds. For N-bonded complexes, generally the C–N stretching band is in a lower region around 2050 cm−1 than that of 2100 cm−1 for S-bonded complexes. However, the frequencies of the bands are sensitive to other factors like coexisting ligands and the structure of the compounds were determined using X-ray analysis. While the M–S–C angles of S-bonded thiocyanato ligand in complexes are bent around 110°, the M–N–C angles of N-bonded isothiocyanato ligands are close to linear.

The complexes (1) and (3) crystallize in the monoclinic space group C2/c. The complex (2) crystallizes in triclinic P-1 space group. Details concerning crystal data and refinement are shown in Table 1. The molecular structures of the studied compounds are shown in Fig. 1. The selected bond lengths and angles are listed in Table 2.

Fig. 1
figure 1

ORTEP drawing of [Mn(SCN)2(pytriaz)2] (1), [NiCl2(pytriaz)2]·CH3OH (2), [Cu(SCN)2(pytriaz)2] (3) with 50% probability displacement ellipsoids. Hydrogen atoms are omitted for clarity

Table 2 Selected bond lengths (Å) and angles (°) for [Mn(SCN)2(pytriaz)2] (1), [NiCl2(pytriaz)2] (2) and [Cu(SCN)2(pytriaz)2] (3) complexes

In all the complexes, central ions have octahedral environment and lie on the inversion centre in the structures of the complexes. The coordination octahedra have angular deviations (maximum 12.5°) connected with N–M–N angles. The C–N and C–S bond length values fall in the 1.153(2)–1.158(2) Å and 1.620(2)–1.632(19) Å ranges for the complexes (1) and (3), similar to those observed for isothiocyanate complexes. The M–N(pytz) and Ni–Cl distances are normal and comparable with distances in other complexes containing the N–heterocyclic ligands. The Mn–N–C angles about 162° are in a good agreement with those found for M(3d)2+ complexes having bent terminally bonded NCS ligand (141°–174°). The isothiocyanate ligands are almost linear (178°). The conformations of molecules (2) and (3) are stabilized by intra- and intermolecular hydrogen bond, and the hydrogen bonds are shown in Table 3. Additionally, the π-stacking interactions in the crystal structures of the complexes are observed. In Fig. 2, the networks formed by the hydrogen bonds and π-stacking interactions in the complexes are shown.

Table 3 Hydrogen bonds for [NiCl2(pytz)2]·2CH3OH (2) and [Cu(SCN)2(pytriaz)2] (3) complexes (Å and °)
Fig. 2
figure 2

The crystal packing of complexes 13 viewing down the a axis with hydrogen bonds indicated by dotted lines

To form an insight in the electronic structures and bonding properties of the complexes, the DFT calculations were carried out. Before the calculations of electronic structures of the complexes, their geometries were optimised in triplet states using the DFT method with the B3LYP functional. From the data shown in Table 2, one may see that the elongation of bonds in gas phase optimised geometry do not outrun 0.1 Ǻ and the largest differences in angles were found for Mn(1)–C(1)–N(1) 3.12°. Based on the optimised geometry of complex (3) the IR spectrum was calculated and compared with the experimental one. As one can see from Fig. 3, the experimental and calculated IR spectra are close each other besides the hydrogen bonds range (~3500 cm−1) which are poorly interpreted by the single point calculations in Gaussian.

Fig. 3
figure 3

Experimental IR spectrum and calculated IR transitions of [Cu(SCN)2(pytriaz)2] (3) complex

The formal charges of central ions are +2 in the complexes and the calculated charges on the central atoms, obtained from natural population analysis, are approximately 1.61 on Mn2+, 1.40 on Ni2+ and 1.45 on Cu2+. The charges on the N-donor atoms from isothiocyanate ligands are −0.8 in complexes (1) and (3), chloride ions in complex (2) have charges close to −0.8 and those on the pytz are −0.5 and −0.3 on pyridine and triazole nitrogen donor atoms, respectively. These are the results of charges donation from the ligands to the metal centre. The conclusion confirms the second-order perturbation analysis from NBO. The stabilization energy calculated in this analysis shows that the lone pairs localised on the N-donor atoms of thiocyanate ligands donate the charge to the Mn2+ and Cu2+ ions d orbitals, and the stabilization energy (ΔE ij ) is 69.56 and 92.14 kcal/mol, respectively. The back donation charges form metal to NCS ligands stabilise the molecules (1) and (3) by 37.92 and 46.65 kcal/mol. The energies in the complex (2) connected with charge flow between chloride ions and central nickel(II) atom averaging to 12.04 (Ni → Cl) and 99.78 (Cl → Ni). The stabilisation energies associated with the charge donation from central ions to the N-donor ligands (pytz) are equal to 35.92 kcal/mol for (1), 29.82 for (2) and 66.76 kcal/mol for complex (3). The stabilization energy of the charge donation from the pytz to the antibonding d orbitals is 49.61 kcal/mol for complex (1), 91.10 for (2) and 77.35 kcal/mol for (3). The NBO analyses suggest the ionic character of bonds between central ions and ligands in the complexes. The calculated Mayer bond orders for M–NCS/Cl are equal to 0.30 and the values for bonds between pytz ligands and manganese(II) are 0.14, 0.23 for Ni–N(pytz) and in complex (3) the order of bond between pyridine nitrogen donor and copper(II) is more covalent (0.28) that that between triazole moiety and Cu(II) (0.10). The bond orders pointed out the ionic character bonds in the complexes.

The atomic charge calculations may describe the relocation of electron density of the compounds. The electron distribution is not apparent from the partial atomic charges, so the plot of the electrostatic potential for the complex 3 is shown in Fig. 4. The isoelectronic contours are plotted at 0.05 a.u. (31 kcal/mol). The colour code of these maps is in the range of 0.05 a.u. (deepest red) to −0.005 a.u. (deepest blue), where blue indicates the strongest attraction and red indicates the strongest repulsion. Regions of negative V(r) are usually associated with the lone pair of electronegative atoms. The negative potential in the compounds wrap thiocyanate ligands and nitrogen atoms in the 5,6-diphenyl-3-(2-pyridyl)-1,2,4-triazine ligands. As one can see from Fig. 4, negative potentials on sulphur atoms in the complex are smaller than the ones on nitrogen atoms. The natural charges obtained from NBO analysis are N(NCS) −0.7, S(NCS) −0.3 and N(py) −0.5, N(tz) −0.3. That is why, and additionally because of steric hindrance exerted by phenyl rings in the ligands, the studied complexes do not form polymeric systems with NCS connector.

Fig. 4
figure 4

Electrostatic potential (ESP) surface of complex 3. ESP surface is shown both in space (with positive and negative regions shown in red and blue, respectively) and mapped on electron densities (isovalue = 0.05) of the molecule (ESP colour scale is such that δ+ → δ in the direction red → blue) (Color figure online)

In all the complexes, the HOMO orbitals are localised on thiocyanate or chloride ligands with contribution of d metal orbitals (13%). The d orbitals of central ions play a significant role (32–55%) in the lower HOMO–4/5 MOs in the complexes. The LUMO orbitals are localised on the pytz ligands in all complexes. In virtual molecular orbitals the d cental ions orbitals are visible in the (β spin) Lumo + 14/15 (28–90%) in complex (1), LUMO + 10/12 (36%, 77%) in (2) and Lumo + 4 (70%) in (3). Because in the frontier region, neighbouring orbitals may be a quasi-degeneracy of the energetic levels, consideration of only the HOMO and LUMO may not yield a realistic description of the frontier orbitals. For this reason, the density of states (DOS) in terms of Mulliken population analysis was calculated using the GaussSum program. The results provide a pictorial representation of MOs compositions. The DOS diagrams for complexes (2) and (3) are shown in Fig. 5, and they may enable us to ascertain the orbital composition characteristics with respect to the particular fragments. As one may see from the DOS diagrams, pytz ligands play significant role in the virtual frontier and in the lower occupied molecular orbitals. The chloride contributes in wider range HOMO orbitals than pseudohalide NCS ligands. Additionally, the complexes with isothiocyanate ligands have smaller HOMO–LUMO gap when compared with chloride ligands (1: 2.08 eV; 2: 2.40 eV; 3: 2.22 eV). The HOMO term is shifted to higher energy by pseudohalide ligands which have an impact on the contribution of d orbitals of central ions in the HOMO orbitals. The values are small for SNC complexes indicating that the M–NCS bonds are more covalent than M–Cl bond. This is because of the difference in bonding of Cl and NCS. On the M–NSC complexes, the d 2 z orbital (assuming z-axis coincides with M–Cl or M–NCS are filled) can donate electron density to the metal whereas the back donation to chloride ligands is difficult.

Fig. 5
figure 5

The density of states (DOS) diagrams for [NiCl2(pytriaz)2] (2) and [Cu(SCN)2(pytriaz)2] (3) complexes

Electronic spectrum of the Mn(II) complex (1) exhibits weak intensity absorption bands with maxima at ν1: 20064, ν2: 22779, ν3: 27739, ν4: 35335 cm−1. These bands may be assigned to transitions: ν1: 6A1g  → 4T1g (4G), ν2: 6A1g  → 4E g /4A1g (4G)(10B + 5C), ν3: 6A1g  → 4 E g (4D)(17B + 5C) and ν4: 6A1g  → 4T1g (4P)(7B + 7C). The parameters B and C were calculated from the second and third transitions because these transitions are free from the crystal field splitting and depend only on B and C parameters. The values of Racah parameters are equal to B = 709 cm−1 and C = 3138 cm−1. Based on the maximum of first transition the crystal field splitting parameters 10 Dq has been calculated and amount to 10558 cm−1.

The lowest state of the nickel(II) ion is 3F term, which splits into 3A2g , 3T2g and 3T1g terms in the field with O h symmetry. The studied complexes have D4h point group symmetry, and in this symmetry 3T2g and 3T1g terms split into singlet 3A2g and 3B2g and doublet 3E g levels each. As a consequence, the first and second spin allowed transition bands observed on the UV–vis spectra of the complexes are assigned to 3T2g (3F) → 3A2g (3F) and 3T1g (3F) → 3A2g (3F), respectively. The third absorption band results from a spin allowed transition 3T1g (3P) → 3A2g (3F). The value of the ligand field parameter 10 Dq, calculated on the basis of electronic bands positions for the complex (given in “Experimental”), is equal to 10Dq = 11234 cm−1. Racah parameters B and C are equal to 674 and 3174 cm−1, respectively. The Racah parameter B for a metal ion varies as a function of the ligand bound to the ion. The value of this parameter will be always lower for the complexed ion than that for the free ion. The reduction of the value of B is related to the extent of metal–ligand bond covalency. The metal–ligand bond becomes partially covalent when the d-orbitals overlap with the ligand orbitals. As a result, the interelectronic repulsion within the d-orbitals decreases and B value is lowered. The ratio B/B 0 = β gives a measure of covalency in the metal–ligand bond. Taking into account that the B 0 for Mn(II) free ion is 786 cm−1 and for Ni(II) is 1041 cm−1 the nepheloauxetic parameters calculated for the complexes are β = 0.90 and 0.65 for complexes (1) and (2), respectively. The values confirm ionic character of manganese/nickel–ligands σ bonds in the complexes and are in accordance with the calculated Mayer bond orders.

On the UV spectrum of complex (2) was recorded the band with maximum at 790.2 nm. It is ascribed to 1E g (1D) term in O h symmetry, which separates in the D4h symmetry into 1A1g and 1B1g levels. This indicates that the band correspond to spin forbidden 1A1g /1B1g (1D) → 3A2g (3F) transitions. For the calculated Dq/B ratio (1.67) in the Tanabe–Sugano diagram for d 8 ions in octahedral coordination one can see, in Fig. 6, that the 1E g (1D) singlet state is lower than the triplet 3T1g (3F) term which confirms the band position in the spectra. Additionally close to the Dq/B ratio of the complex the singlet and triplet terms intersect which explain the higher intensity of the spin forbidden band than that 3T2g (3F) → 3A2g (3F) transition.

Fig. 6
figure 6

The part of UV–vis spectrum of [NiCl2(pytriaz)2] (2) complex with Tanabe–Sugano diagram for d 8 ions in octahedral coordination

The electronic spectrum of copper(II) complex (3) shows three bands with maxima at 13289, 23068 and 27738 cm−1 which may be considered to the following spin allowed transitions: 2B1g  → 2A1g (d x 2 y 2 → d z 2), 2B1g  → 2B2g (d x 2 y 2 → d zy ) and 2B → 2E g (d x 2 y 2 → d zy , d yz ) and suggest D4h symmetry. For this complex the nature of the transitions observed in the UV–Vis spectrum has been studied by the time-dependent density functional (TDDFT) method based on the optimised geometry, in gas phase without any symmetry restrictions, in the doublet state. The experimental spectrum with the calculated transitions is depicted in Fig. 6. The TDDFT/PCM calculations show that the two longest wavelength experimental bands of (3) at 752.5 (13 289 cm−1) and 433.5 nm (23 068 cm−1) originate from the transitions between β-spin HOMO, HOMO-1, HOMO-23 orbitals and β-spin LUMO orbital. As can be seen from Fig. 5, in the LUMO orbital with β spin the Cu d x 2 y 2 orbital participates in character and the β-spin HOMO and HOMO-1 orbitals are composed of the π-bonding orbitals of isothiocyanate with some contributions of 5,6-diphenyl-3-(2-pyridyl)-1,2,4-triazine orbitals. The HOMO-23 is ligand (SCN and pytz) π-bonding orbitals with some contribution of d Cu orbitals. Accordingly, the transitions assigned to the longest wavelength experimental band can be seen as mixed SCN/pytz → Cu (LMCT) and d → d (LF). Thus, no “clear” dd transition bands occur in the low-lying energy region for the complex. The reason for this is probably because of the pytz ligand provides high-lying occupied π-bonding orbitals, and as a result the high-lying occupied orbitals are composed of the π-bonding orbitals of 5,6-diphenyl-3-(2-pyridyl)-1,2,4-triazine. The experimental bands with maxima at 232.5 and 207.0 nm are attributed to the intraligand (IL) (π(pytz) → π *(pytz) ) and ligandligand charge transfer(SCN) → π *(pytz) ) transitions (Fig. 7).

Fig. 7
figure 7

The UV–vis spectrum of [Cu(SCN)2(pytriaz)2] (3) complexes with calculated transitions

An emission property of the complex (2) has been examined in the methanol solution (with concentration of 5 × 10−4 mol/dm3) at room temperature. The excitation was executed at 494 nm, i.e., for the maximum of absorption band which may be assigned to the combination of d → d, MLCT (dπ(Ni) → π *(pytz) ) and 1ClLCT (n(Cl) → π*) transitions. The low intense emission with maximum at 581 nm was observed (Fig. 8).

Fig. 8
figure 8

The emissions spectrum of the methanolic solution (c = 5 × 10−4 mol/dm3) of complex 2 with excitation at 494 nm

The X-band EPR spectra of the complexes were recorded at room temperature at frequency 9.8 GHz, in polycrystalline form (Fig. 9). On the spectrum of manganese(II) complex (1) is visible two broad signals with g values of 3.40 and 1.96. The EPR spectrum of polycrystalline sample of complex 3 exhibit signals with g || = 2.20 and g  = 2.08. The ordering of g values g || > g  > 2.0023 observed for the copper(II) complex indicates that the unpaired electron most likely resides in their d x 2 y 2 orbital which is consistent with 2B1g ground state. The fact that the g || is little smaller than 2.3 is an indication of some ionic character of bonding in the complex as in the case of complexes (1) and (2) [2123]. According to Hathaway and Billing [24], if the geometric parameter G, calculated by the expression G = (g || − 2.0023)/(g − 2.0023), is greater than 4 the exchange interaction is negligible, but G < 4 indicates considerable exchange interaction in the solid complexes. In the complex (3), the G value is about 2.5 indicating exchange interactions in solid complex.

Fig. 9
figure 9

The EPR spectra of solid state [Mn(SCN)2(pytriaz)2] (1) and [Cu(SCN)2(pytriaz)2] (3) complexes

Summarising, in the simple one-pot syntheses isothiocyanate or chloride complexes of manganese(II), nickel(II) and copper(II) with 5,6-diphenyl-3-(2-pyridyl)-1,2,4-triazine (pytz) and SCN/Cl ligands were obtained. The complexes were characterised by IR, UV–vis, EPR spectroscopy, and their crystal structures were determined by X-ray diffraction. The experimental studies on the complexes incorporating 5,6-diphenyl-3-(2-pyridyl)-1,2,4-triazine have been accompanied computationally by the density functional theory (DFT) and time-dependent DFT calculations. X-ray studies revealed six-coordinate geometry about the metal centre with two bounded pytz molecules in trans bis-chelate configuration in the equatorial plane, and two thiocyanate ions in axial positions. The nickel(II) complex shows a weak luminescent properties. In solid state of copper(II) complex the exchange interactions are expected and the differences between chloride and NCS ligands are demonstrated.

Supplementary data

CCDC 785978, CCDC 773120 and CCDC 791533 contain the supplementary crystallographic data for complexes [Mn(SCN)2(pytz)2], [NiCl2(pytz)2].2CH3OH and [Cu(SCN)2(pytz)2] respectively. These data can be obtained free of charge from http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk Calculations have been carried out in Wroclaw Centre for Networking and Supercomputing (http://www.wcss.wroc.pl).