Synthesis and crystal structure of poly[(2,6-di­methyl­py­ra­zine-κN4)(μ3-thiocyanato-κ3N:S:S)copper(I)]

Näther, C.

doi:10.1107/S2056989026001866

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ISSN: 2056-9890

Volume 82| Part 3| March 2026| Pages 305-308

https://doi.org/10.1107/S2056989026001866

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Synthesis and crystal structure of poly[(2,6-dimethylpyrazine-κN⁴)(μ₃-thiocyanato-κ³N:S:S)copper(I)]

Christian Näther ^a ^*

^aInstitut für Anorganische Chemie, Universität Kiel, Max-Eyth.-Str. 2, 24118 Kiel, Germany
^*Correspondence e-mail: [email protected]

Edited by W. T. A. Harrison, University of Aberdeen, United Kingdom (Received 17 February 2026; accepted 19 February 2026; online 24 February 2026)

Crystals of the title compound, [Cu(NCS)(C₆H₈N₂)]_n (C₆H₈N₂ = 2,6-dimethylpyrazine), were prepared by the reaction of CuNCS and 2,6-dimethylpyrazine in acetonitrile. The asymmetric unit consists of one Cu^I cation, one thiocyanate anion and one 2,6-dimethylpyrazine ligand with all atoms lying on general positions. The copper cations are tetrahedrally coordinated by two S- and one N-bonded thiocyanate anions and one 2,6-dimethylpyrazine ligand, which s coordinated to the metal center with the N atom that is not adjacent to the methyl groups. The copper cations are linked by the μ-1,3,3 (N,S,S) bridging thiocyanate anions into layers that lie parallel to the ac plane. The layers are stacked perpendicular to the b-axis direction and are separated by the 2,6-dimethylpyrazine ligands. The title crystal structure is compared with those of related CuNCS compounds with isomeric dimethylpyrazine ligands.

Keywords: crystal structure; coordination polymer; copper(I) thiocyanate; 2,6-dimethylpyrazine.

CCDC reference: 2532015

1. Chemical context

Coordination compounds based on copper(I) halides with chloride, bromide and iodide anions and N-donor coligands show an extremely large structural variability (e.g., Kromp & Sheldrick, 1999 ; Näther et al., 2001 , 2002 ; Li et al., 2005 ; Peng et al., 2010 ). They usually consist of a variety of CuX substructures such as monomeric and dimeric units, chains or layers that can be further linked if bridging ligands are used. This might also be one reason why several polymorphs or isomers are reported (Näther & Jess, 2003 ; Park et al., 2012 ; Peng et al., 2010; Näther et al., 2003 ). For many of these compounds, a different ratio between the copper(I) halide and the N-donor coligands is observed and thermal treatment of the coligand-rich compounds usually leads to the transformation into coligand-deficient compounds that show more condensed CuX substructures (Näther & Jess, 2001 , 2002 ).

Such coordination compounds can also be prepared with copper(I) pseudohalides such as cyanide, azide or thiocyanate anions and many of them are reported in the literature because of their luminescence properties (Chesnut et al., 1999 ; Lemos et al., 2001 ; Starosta et al., 2012 ; Nitsch et al., 2015 ). As is the case for the copper(I) halide coordination compounds, they also show typical CuX substructures (X = pseudohalide), which, especially for cyanides, are very often more complicated than those in copper(I) halides. The different CuX substructures can further be connected into more condensed networks if bridging coligands such as, for example, pyrazine derivatives are used. If a database search is limited to the isomeric dimethylpyrazine ligands and copper(I), a number of compounds with cyanide, azide and thiocyanate anions are reported in the CSD (Version 5.43, 2025; Groom et al., 2016 ) using CONQUEST (Bruno et al., 2002 ).

With azide anions, no copper(I) compounds with 2,3-dimethylpyrazine are reported but one three-dimensional compound with the composition Cu₂(N₃)₂(2,5-dimethylpyrazine) is known that shows a complicated Cu–azide substructure, in which the azide anions act as μ-1,1,3 bridging ligands (Guang et al., 2012 ). Furthermore, Cu₂(N₃)₂(2,6-dimethylpyrazine) is also found (Fan et al., 2015a ,b ).

Copper(I) compounds with cyanide anions are reported with all three isomers of dimethylpyrazine. These include Cu₃(CN)₃(2,3-dimethylpyrazine) (Greve & Näther, 2004 ), Cu₆(CN)₆(2,3-dimethylpyrazine) (Chesnut et al., 2001 ) and Cu₂(CN)₂(2,5-dimethylpyrazine) (Chesnut et al., 2001). Finally, two isomers of Cu₂(CN)₂(2,6-dimethylpyrazine) are reported. In one of them, the copper cations are linked by bridging cyanide anions into Cu₄(CN)₄ units that condense into layers by way of Cu₂(CN)₂ four-membered rings (Näther, 2025 ), whereas in the second modification a one-dimensional copper(I) cyanide network is found that consists of alternating twelve- and four-membered rings (Chesnut et al., 2001).

With thiocyanate anions and 2,5-dimethylpyrazine, a compound with the composition Cu₂(NCS)₂(2,5-dimethylpyrazine) is found, in which CuNCS layers are observed, that are linked by the 2,5-dimethylpyrazine ligands into a three-dimensional network (Näther et al., 2003; Otieno et al., 2003 ). A three-dimensional structure is also found for Cu₂(NCS)₂(2,3-dimethylpyrazine), even if the layer topology is different from that of the 2,5-dimethylpyrazine compound (Näther et al., 2003). Compounds with a 1:1 ratio of copper(I) thiocyanate and 2,6-dimethylpyrazine are unknown and we therefore tried to prepare such compounds by the reaction of CuNCS and 2,6-dimethylpyrazine. In the course of these investigations we obtained crystals of the title compound, (I), that were characterized by single-crystal X-ray diffraction.

2. Structural commentary

The asymmetric unit of (I), Cu(NCS)(C₆H₈N₂) (C₆H₈N₂ = 2,6-dimethylpyrazine), consists of one copper(I) cation, one thiocyanate anion and one 2,6-dimethylpyrazine ligand, with all of the atoms located in general positions (Fig. 1) in space group P2₁/c. The metal cations are fourfold coordinated by one N- and two S-bonded thiocyanate anions and one 2,6-dimethylpyrazine ligand. Because of steric repulsion between the metal cation and the methyl groups of the 2,6-dimethylpyrazine ligand, this ligand is only coordinated with the N atom that does not lie between the two methyl groups (Fig. 1). The two Cu—S bond lengths are only slightly different and the bond angles deviate from the ideal values, which shows that the tetrahedra are slightly distorted (Table 1). As expected, the C—N—Cu angle is close to linearity, whereas the C—S—Cu angles roughly correspond to a tetrahedral angle (Table 1).

Table 1
Selected geometric parameters (Å, °)

Cu1—N11ⁱ	1.963 (2)	Cu1—S1	2.3238 (8)
Cu1—N2	2.094 (2)	Cu1—S1ⁱⁱ	2.3809 (8)

N11ⁱ—Cu1—N2	105.53 (10)	S1—Cu1—S1ⁱⁱ	114.74 (3)
N11ⁱ—Cu1—S1	118.22 (7)	C11—S1—Cu1	107.96 (9)
N2—Cu1—S1	110.30 (7)	C11—S1—Cu1ⁱⁱⁱ	96.46 (10)
N11ⁱ—Cu1—S1ⁱⁱ	105.40 (8)	Cu1—S1—Cu1ⁱⁱⁱ	105.06 (3)
N2—Cu1—S1ⁱⁱ	100.90 (7)	C11—N11—Cu1^iv	168.0 (3)
Symmetry codes: (i) $[x-1, -y+{\script{1\over 2}}, z-{\script{1\over 2}}]$ ; (ii) $[x, -y+{\script{1\over 2}}, z-{\script{1\over 2}}]$ ; (iii) $[x, -y+{\script{1\over 2}}, z+{\script{1\over 2}}]$ ; (iv) $[x+1, -y+{\script{1\over 2}}, z+{\script{1\over 2}}]$ .

Figure 1
The asymmetric unit of (I) expanded to show the full metal coordination sphere with labeling of selected atoms and displacement ellipsoids drawn at the 50% probability level. Symmetry codes: (i) x − 1, −y + $[{1\over 2}]$ , z - 1/2; (ii) x, −y + $[{1\over 2}]$ , z − $[{1\over 2}]$ .

In the extended structure, the copper(I) cations are connected by μ-1,1,3(S,S,N)-bridging thiocyanate anions into ten-membered rings built up of three cations and three thiocyanate anions, condensing into corrugated layers that lie parallel to the ac plane (Fig. 2). It is noted that this layer topology is completely different from that in Cu₂(NCS)₂(2,3-dimethylpyrazine) (Näther et al., 2003), where tetranuclear units built up of four copper(I) cations and four thiocyanate anions are observed, which condense into layers by way of Cu₂S₂ rings (Fig. 3: top). In contrast, in Cu₂(NCS)₂(2,5-dimethylpyrazine) (Näther et al., 2003; Otieno et al., 2003), ten-membered rings are also found but the orientation of the two thiocyanate anions within these rings is reversed and the rings are therefore more distorted (Fig. 3: bottom).

Figure 2
Crystal structure of (I) with a view onto the CuNCS layers along the crystallographic b-axis direction. The 2,6-dimethylpyrazine ligands are omitted for clarity.

Figure 3
View of the CuNCS networks in Cu₂(NCS)₂(2,3-dimethylpyrazine) (top) and Cu₂(NCS)₂(2,5-dimethylpyrazine) (bottom) reported in the literature (Näther et al., 2003

; Otieno et al., 2003

Concerning the overall structural discussion, it must be kept in mind that in the 2,3- and 2,5-dimethylpyrazine compounds, the ratio between the CuNCS component and the dimethylpyrazine derivative is different, but this difference only originates from the fact that the coligand is only terminally coordinating in (I), whereas it act as a bridging ligand in the compounds with the two other isomers. In this context, it is noted that no CuNCS compounds with 2,3- or 2,5-dimethylpyrazine are reported, in which the ratio between CuNCS and coligand is identical to that in (I), which indicates that for these ligands a terminal coordination is not favored. In contrast, in (I), one of the coordinating N atoms of the 2,6-dimethylpyrazine ligand is shielded by the two neighbouring methyl groups, which means that a compound with a bridging coordination of the neutral coligand, similar to that in the thiocyanate compounds with 2,3- and 2,5-dimethylpyrazine, might not exist.

3. Supramolecular features

The layers in (I) are stacked perpendicular to the b-axis direction and are separated by the 2,6-dimethylpyrazine ligands (Fig. 4). The coligands of neighboring layers point towards each other, which means that the layers are only linked by van der Waals interactions. There are no directional intermolecular interactions. This is completely different to the 2,3- and 2,5-dimethylpyrazine compounds Cu₂(NCS)₂(2,3-dimethylpyrazine) and Cu₂(NCS)₂(2,5-dimethylpyrazine) in which the layers are connected by bridging 2,3- and 2,5-dimethylpyrazine ligands into a three-dimensional network (Näther et al., 2003; Otieno et al., 2003). As mentioned above, this might be traced back to the fact that in 2,3- and 2,5-dimethylpyrazine, only one methyl group is adjacent to the coordinating N atom, whereas in 2,6-dimethylpyrazine the two methyl groups effectively shield one of the N atoms, which makes metal coordination much more difficult.

Figure 4
Crystal structure of (I) with a view along the crystallographic a-axis direction.

4. Database survey

As mentioned above, with 2,6-dimethylpyrazine and copper(I) thiocyanate no compounds are reported but there is one mixed copper(I/II) pseudohalide compound with the composition [Cu₈^ICu₂^II(CN)₄(NCS)₈(2,6-dimethylpyrazine)₇] that shows a three-dimensional coordination network (Jess & Näther, 2006 ).

With copper(I) halides, two compounds with 2,6-dimethylpyrazine are known. This includes Cu₂Cl₂(2,6-dimethylpyrazine), in which the copper cations are tetrahedrally coordinated by three chloride anions and one 2,6-dimethylpyrazine ligand and are linked by μ-1,1 bridging chloride anions into double chains that are further connected into layers by bridging 2,6-dimethylpyrazine ligands (Fan et al., 2015 ). CuI(2,6-dimethylpyrazine) shows a structure similar to that of Cu₂Cl₂(2,6-dimethylpyrazine) mentioned above, but in this compound, the 2,6-dimethylpyrazine ligand is only terminally coordinated (Kitada & Ishida, 2014 ; Zhang et al., 2014 ).

Finally, it is noted that with divalent copper(II) cations, two different polymorphs with the composition CuBr₂(2,6-dimethylpyrazine) are reported, in which the copper cations are linked into chains by bridging 2,6-dimethylpyrazine ligands (Ding et al., 2021 ).

5. Synthesis and crystallization

Copper(I) thiocyanate and 2,6-dimethylpyrazine were purchased from Sigma-Aldrich: 1.000 mmol (121.6 mg) of copper(I) thiocyanate and 1.000 mmol (108.1 mg) of 2,6-dimethylpyrazine were reacted in 3 ml of acetonitrile. Within 3 d, colourless blocks of (I) suitable for single crystal X-ray diffraction were obtained.

6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. The C—H hydrogen atoms were positioned with idealized geometry (methyl H atoms allowed to rotate but not to tip) and were refined isotropically with U_iso(H) = 1.2U_eq(C) (1.5 for methyl H atoms).

Table 2
Experimental details

Crystal data
Chemical formula	[Cu(NCS)(C₆H₈N₂)]
M_r	229.76
Crystal system, space group	Monoclinic, P2₁/c
Temperature (K)	200
a, b, c (Å)	5.6765 (4), 23.4382 (14), 6.9655 (6)
β (°)	104.620 (9)
V (Å³)	896.73 (12)
Z	4
Radiation type	Mo Kα
μ (mm⁻¹)	2.61
Crystal size (mm)	0.20 × 0.19 × 0.15

Data collection
Diffractometer	Stoe IPDS-II
Absorption correction	Numerical (X-RED and X-SHAPE; Stoe, 2008 )
T_min, T_max	0.470, 0.620
No. of measured, independent and observed [I > 2σ(I)] reflections	6594, 2163, 1704
R_int	0.047
(sin θ/λ)_max (Å⁻¹)	0.661

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.036, 0.096, 1.03
No. of reflections	2163
No. of parameters	112
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.49, −0.62
Computer programs: X-AREA (Stoe, 2008), SHELXT (Sheldrick, 2015a ), SHELXL (Sheldrick, 2015b ), DIAMOND (Brandenburg, 1999 ) and XP in SHELXTL-PC (Sheldrick, 2008 ) and publCIF (Westrip, 2010 ).

Supporting information

CCDC reference: 2532015

Crystal structure: contains datablock I. DOI: https://doi.org/10.1107/S2056989026001866/hb8197sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989026001866/hb8197Isup2.hkl

checkCIF report

Computing details top

Poly[(2,6-dimethylpyrazine-κN⁴)(µ₃-thiocyanato-κ³N:S:S)copper(I)] top

Crystal data top

[Cu(NCS)(C₆H₈N₂)]	F(000) = 464
M_r = 229.76	D_x = 1.702 Mg m⁻³
Monoclinic, P2₁/c	Mo Kα radiation, λ = 0.71073 Å
a = 5.6765 (4) Å	Cell parameters from 5598 reflections
b = 23.4382 (14) Å	θ = 6.3–56.0°
c = 6.9655 (6) Å	µ = 2.61 mm⁻¹
β = 104.620 (9)°	T = 200 K
V = 896.73 (12) Å³	Block, colorless
Z = 4	0.20 × 0.19 × 0.15 mm

Data collection top

Stoe IPDS-II diffractometer	1704 reflections with I > 2σ(I)
ω scans	R_int = 0.047
Absorption correction: numerical (X-Red and X-Shape; Stoe, 2008)	θ_max = 28.0°, θ_min = 3.2°
T_min = 0.470, T_max = 0.620	h = −7→7
6594 measured reflections	k = −29→30
2163 independent reflections	l = −9→9

Refinement top

Refinement on F²	Hydrogen site location: inferred from neighbouring sites
Least-squares matrix: full	H-atom parameters constrained
R[F² > 2σ(F²)] = 0.036	w = 1/[σ²(F_o²) + (0.0586P)²] where P = (F_o² + 2F_c²)/3
wR(F²) = 0.096	(Δ/σ)_max = 0.001
S = 1.03	Δρ_max = 0.49 e Å⁻³
2163 reflections	Δρ_min = −0.62 e Å⁻³
112 parameters	Extinction correction: SHELXL-2016/6 (Sheldrick 2015b), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.010 (3)

Special details top

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Cu1	0.69476 (6)	0.27873 (2)	0.52022 (5)	0.02275 (15)
N1	0.7931 (5)	0.43852 (12)	0.1025 (4)	0.0296 (6)
C1	0.5987 (6)	0.40441 (13)	0.0613 (4)	0.0281 (6)
C2	0.5679 (6)	0.36229 (13)	0.1940 (4)	0.0252 (6)
H2	0.423880	0.339862	0.163200	0.030*
N2	0.7354 (4)	0.35266 (11)	0.3629 (3)	0.0226 (5)
C3	0.9327 (5)	0.38613 (14)	0.4014 (4)	0.0259 (6)
H3	1.057102	0.379901	0.519348	0.031*
C4	0.9611 (6)	0.42982 (13)	0.2736 (4)	0.0253 (6)
C5	0.4140 (8)	0.41223 (19)	−0.1321 (6)	0.0507 (11)
H5A	0.396766	0.452951	−0.164523	0.076*
H5B	0.256982	0.396923	−0.121909	0.076*
H5C	0.467254	0.391925	−0.236799	0.076*
C6	1.1765 (7)	0.46912 (17)	0.3216 (5)	0.0373 (8)
H6A	1.226703	0.477786	0.200078	0.056*
H6B	1.311239	0.450682	0.417527	0.056*
H6C	1.132326	0.504568	0.378469	0.056*
S1	0.88125 (12)	0.29030 (3)	0.85550 (10)	0.02064 (18)
C11	1.1536 (5)	0.25913 (13)	0.9018 (4)	0.0187 (5)
N11	1.3454 (4)	0.23893 (12)	0.9428 (3)	0.0243 (5)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Cu1	0.01201 (19)	0.0303 (2)	0.0246 (2)	−0.00085 (13)	0.00210 (13)	0.00651 (13)
N1	0.0388 (15)	0.0256 (14)	0.0223 (12)	−0.0007 (11)	0.0039 (11)	0.0035 (9)
C1	0.0351 (16)	0.0231 (16)	0.0208 (13)	0.0000 (13)	−0.0028 (12)	0.0016 (11)
C2	0.0276 (14)	0.0204 (15)	0.0236 (13)	0.0012 (11)	−0.0012 (11)	0.0008 (11)
N2	0.0247 (12)	0.0197 (12)	0.0200 (11)	−0.0019 (9)	−0.0005 (9)	0.0043 (9)
C3	0.0243 (14)	0.0271 (16)	0.0218 (13)	−0.0043 (11)	−0.0023 (11)	0.0030 (10)
C4	0.0263 (15)	0.0246 (15)	0.0246 (14)	−0.0034 (12)	0.0053 (11)	−0.0017 (11)
C5	0.057 (2)	0.049 (2)	0.0310 (18)	−0.008 (2)	−0.0170 (17)	0.0144 (16)
C6	0.0353 (18)	0.038 (2)	0.0378 (18)	−0.0144 (15)	0.0071 (14)	0.0014 (13)
S1	0.0139 (3)	0.0280 (4)	0.0194 (3)	0.0041 (2)	0.0030 (2)	0.0007 (2)
C11	0.0148 (12)	0.0262 (14)	0.0146 (11)	−0.0048 (10)	0.0031 (9)	−0.0029 (9)
N11	0.0114 (10)	0.0392 (15)	0.0212 (11)	0.0020 (10)	0.0021 (8)	−0.0027 (10)

Geometric parameters (Å, º) top

Cu1—N11ⁱ	1.963 (2)	C3—C4	1.393 (4)
Cu1—N2	2.094 (2)	C3—H3	0.9500
Cu1—S1	2.3238 (8)	C4—C6	1.500 (4)
Cu1—S1ⁱⁱ	2.3809 (8)	C5—H5A	0.9800
N1—C1	1.334 (4)	C5—H5B	0.9800
N1—C4	1.341 (4)	C5—H5C	0.9800
C1—C2	1.393 (4)	C6—H6A	0.9800
C1—C5	1.495 (5)	C6—H6B	0.9800
C2—N2	1.332 (4)	C6—H6C	0.9800
C2—H2	0.9500	S1—C11	1.666 (3)
N2—C3	1.338 (4)	C11—N11	1.155 (4)

N11ⁱ—Cu1—N2	105.53 (10)	N1—C4—C6	117.5 (3)
N11ⁱ—Cu1—S1	118.22 (7)	C3—C4—C6	121.9 (3)
N2—Cu1—S1	110.30 (7)	C1—C5—H5A	109.5
N11ⁱ—Cu1—S1ⁱⁱ	105.40 (8)	C1—C5—H5B	109.5
N2—Cu1—S1ⁱⁱ	100.90 (7)	H5A—C5—H5B	109.5
S1—Cu1—S1ⁱⁱ	114.74 (3)	C1—C5—H5C	109.5
C1—N1—C4	117.7 (3)	H5A—C5—H5C	109.5
N1—C1—C2	121.2 (3)	H5B—C5—H5C	109.5
N1—C1—C5	118.3 (3)	C4—C6—H6A	109.5
C2—C1—C5	120.5 (3)	C4—C6—H6B	109.5
N2—C2—C1	121.7 (3)	H6A—C6—H6B	109.5
N2—C2—H2	119.2	C4—C6—H6C	109.5
C1—C2—H2	119.2	H6A—C6—H6C	109.5
C2—N2—C3	116.9 (3)	H6B—C6—H6C	109.5
C2—N2—Cu1	117.0 (2)	C11—S1—Cu1	107.96 (9)
C3—N2—Cu1	125.3 (2)	C11—S1—Cu1ⁱⁱⁱ	96.46 (10)
N2—C3—C4	122.0 (3)	Cu1—S1—Cu1ⁱⁱⁱ	105.06 (3)
N2—C3—H3	119.0	N11—C11—S1	176.6 (3)
C4—C3—H3	119.0	C11—N11—Cu1^iv	168.0 (3)
N1—C4—C3	120.6 (3)

C4—N1—C1—C2	1.8 (5)	C2—N2—C3—C4	1.2 (4)
C4—N1—C1—C5	−177.9 (3)	Cu1—N2—C3—C4	170.5 (2)
N1—C1—C2—N2	−2.9 (5)	C1—N1—C4—C3	0.7 (5)
C5—C1—C2—N2	176.7 (3)	C1—N1—C4—C6	−178.6 (3)
C1—C2—N2—C3	1.3 (4)	N2—C3—C4—N1	−2.3 (5)
C1—C2—N2—Cu1	−168.9 (2)	N2—C3—C4—C6	177.0 (3)

Symmetry codes: (i) x−1, −y+1/2, z−1/2; (ii) x, −y+1/2, z−1/2; (iii) x, −y+1/2, z+1/2; (iv) x+1, −y+1/2, z+1/2.

Acknowledgements

Financial support by the State of Schleswig-Holstein is gratefully acknowledged.

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