Investigation of structure-directing interactions within copper(I) thiocyanate complexes through X-ray analyses and non-covalent interaction (NCI) theoretical approach

Abstract

Herein, we reported the synthesis of copper(I) thiocyanate complexes with ortho-pyridinyl carbohydrazones containing a thiophene (L₁) or a furyl ring (L₂) as a mixture of two different crystals for each compound, linkage isomers of C_1N, [Cu(NCS)(L₁)PPh₃] and C_1S, [Cu(SCN)(L₁)PPh₃], for L₁, whereas monomeric and polymeric structures C_2N, [Cu(NCS)(L₂)PPh₃], and C_2P, [–(NCS)Cu(L₂)–]_n, for L₂. Crystallographic information and theoretical calculations, mainly noncovalent interaction reduced density gradient (NCI-RDG) analyses, were pursued to generate a profound understanding of the structure-directing interactions in these complexes. The supramolecular assemblies are first driven by cooperative π⋯π interactions and hydrogen bonds followed by CH⋯π, S⋯S and S⋯π linkages. In the case of the linkage isomers, intermolecular interactions may have a significant role in the formation of the less stable S-bound isomer C_1S.

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Introduction

Copper(I) compounds have attracted a growing interest because of their high structural diversity,^1–4 catalytic activity^5–7 and photophysical properties.^3,8–10 They have applications in different areas such as organic light-emitting diodes (OLEDs),^11–22 supramolecular assemblies, oxygen sensors and biological probes. The rich structural features and utilitarian considerations have motivated researchers to focus on the synthesis and characterization of Cu(I) complexes with various donor ligands. The formation of structural variations is greatly influenced by several parameters such as the synthetic conditions or steric/electronic effects exerted by the ligand.

The triatomic pseudohalide, thiocyanate anion (SCN⁻), is an excellent versatile ambidentate ligand with two donor atoms, S or N.²³ It can coordinate to metal ions both in terminal and bridging coordination modes and potentially provide fascinating examples of linkage isomerism.^24–26 When SCN⁻ acts as a terminal ligand, it affords potential interaction sites to generate non-covalent intermolecular interactions and, accordingly, can direct the crystal packing. Controlling the self-assemblies in the solid state on the basis of molecular structures and through the use of weak interactions is a long-standing goal of supramolecular chemistry.²⁷

Very recently in our previous work, cuprous halide complexes of ortho-, meta- and para-pyridinyl carbohydrazones were introduced.²⁸ The influence of ligand structure and halide variations on the molecular structures and supramolecular arrays of the complexes were studied both experimentally and theoretically. In the following, we employed cuprous pseudohalide, CuSCN, for the synthesis of complexes with two ortho-pyridinyl carbohydrazones. Copper(I) thiocyanate compounds are very interesting in solar cell applications as a p-type semiconductor.^29–31

In this contribution, we report the structural characteristics of complexes from the reaction of CuSCN with PPh₃ and ortho-pyridinyl carbohydrazones containing a thiophene (L₁) or a furyl ring (L₂); see Scheme 1. The former resulted in two linkage isomers: C_1N [Cu(NCS)(L₁)PPh₃] and C_1S [Cu(SCN)(L₁)PPh₃], while the later afforded monomeric and polymeric complexes of C_2N [Cu(NCS)(L₂)PPh₃] and C_2P [–(NCS)Cu(L₂)–]_n. We have also used a recently introduced alternative interpretive technique, the non-covalent interaction (NCI) approach, to manifest the diverse NCIs at the crystal packing structures. This method is based on the analysis of the electron density and enables us to identify and visualize the interactions.³² Various non-covalent interactions, including hydrogen bonding,³³ S⋯S, S⋯π,³⁴ π⋯π³⁵ and CH⋯π³⁶ interactions, have been investigated in this work.

Scheme 1

Results and discussion

Synthesis

Ligands L₁ and L₂ were prepared by mixing equivalent amounts of 2-thiophenecarboxylic acid hydrazide (1) or furoic hydrazide (2) and 2-pyridinecarboxaldehyde in methanol solution.

A solution of the ligand in CHCl₃ was added dropwise to a mixture of copper(I) thiocyanate and PPh₃ while stirring in CH₃CN and then the mixture was filtered off. After slow diffusion of diethyl ether in the filtered solutions, two different crystals were obtained for each compound including the light orange needle crystals (C_1N) and clear light red irregular crystals (C_1S) for L₁ and orange needle crystals (C_2N) and dark orange hexagonal crystals (C_2P) for L₂. A mixture of the isomeric crystals C_1N and C_1S is shown in Fig. 1.

Fig. 1 50× magnification photo of a mixture of orange rectangular cube C_1N and red cube C_1S.

Various ratios of acetonitrile and chloroform solvents were assessed in the crystallization of C₁. Upon using more chloroform, the percentage of C_1N was dominant, whereas a higher amount of acetonitrile in the reaction pot increased the percentage of C_1S. For C₂, the formation of crystals depended on the concentration of the reaction mixture. At a high concentration, the polymeric compound precipitated fast and we only obtained crystals of C_2N, but slower diffusion of diethyl ether in the more dilute solution afforded crystals of both C_2P (as the dominant product) and C_2N suitable for X-ray diffraction.

ORTEP diagrams of the molecular structures are shown in Fig. 2. The crystallographic data of the complexes are listed in Table 1. Selected bond distances and angles are summarized in Table 2.

Fig. 2 ORTEP diagrams of C_1N, C_1S, C_2N and C_2P with displacement ellipsoid at the 50% level. Hydrogen atoms were omitted for clarity. Symmetry codes: (i) x, 1/2 − y, 1/2 + z; (ii) x, 1/2 − y, −1/2 + z.

Table 1 Crystal data and structural refinement parameters for compounds C_1N, C_1S, C_2N and C_2P

Compound	C1N	C1S	C2N	C2P
Formula	C30H24CuN4OPS2	C30H24CuN4OPS2	C30H24CuN4O2PS	C12H9CuN4O2S
Fw	615.16	615.16	599.10	336.83
λ/Å	1.54184	0.71073	1.54184	1.54184
T/K	173	173	173	173
Crystal system	Triclinic	Triclinic	Triclinic	Monoclinic
Space group	P1̄	P1̄	P1̄	P21/c
a/Å	9.8334(5)	10.7768(9)	9.8386(7)	15.7005(7)
b/Å	10.2673(6)	11.0362(8)	10.1356(6)	8.0099(4)
c/Å	15.0986(9)	12.2584(5)	14.9963(10)	10.3633(5)
α/°	87.742(5)	85.195(5)	90.004(5)	90
β/°	89.510(4)	78.024(5)	90.051(6)	94.374(4)
γ/°	66.545(5)	80.116(6)	113.105(7)	90
V/Å^3	1397.31(15)	1403.30(17)	1375.48(17)	1299.49(11)
D calc/Mg m^−3	1.462	1.456	1.447	1.722
Z	2	2	2	4
μ/mm^−1	3.292	1.015	2.666	3.948
F(000)	632	632	616	680
2θ/°	142.128	59	143	142
R(int)	0.025	0.030	0.030	0.041
GOOF	1.022	1.11	1.04	1.03
R 1 (I > 2σ(I))	0.0295	0.0444	0.0373	0.0356
wR2 (I > 2σ(I))	0.0778	0.1072	0.1078	0.0955

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Table 2 Selected bond lengths (Å) and angles (°) around copper(I) for complexes C_1N, C_1S, C_2N and C_2P

C1N
Cu1–P1	2.1880(6) Å	Cu1–N2	2.1367(16) Å
Cu1–N1	1.979(18) Å	Cu1–N3	2.1501(16) Å
P1–Cu1–N1	119.83(5)°	N1–Cu1–N2	99.81(7)
P1–Cu1–N2	121.68(5)°	N1–Cu1–N3	101.96(7)
P1–Cu1–N3	126.46(4)°	N2–Cu1–N3	77.56(6)
C1S
Cu1–S1	2.2936(9) Å	Cu1–N2	2.050(2) Å
Cu1–P1	2.1990(7) Å	Cu1–N3	2.181(2) Å
S1–Cu1–P1	117.60(3)°	P1–Cu1–N2	117.49(6)°
S1–Cu1–N2	107.38(6)°	P1–Cu1–N2	103.74(6)°
S1–Cu1–N3	127.28(6)°	N2–Cu1–N3	77.65(8)°
C2N
Cu1–P1	2.1893(6) Å	Cu1–N2	2.1547(19) Å
Cu1–N1	2.1411(19) Å	Cu1–N4	1.983(2) Å
P1–Cu1–N1	121.44(6)°	N1–Cu1–N2	77.23(7)°
P1–Cu1–N2	126.25(5)°	N1–Cu1–N4	98.52(8)°
P1–Cu1–N4	121.53(7)°	N2–Cu1–N4	101.32(8)°
C2P
Cu1–N1	2.105(2) Å	Cu1–N4	1.906(2) Å
Cu1–N2	2.131(2) Å	Cu1–S1	2.2971(8) Å
N1–Cu1–N2	77.74(9)°	N2–Cu1–N4	130.03(9)°
N1–Cu1–N4	109.43(9)°	N2–Cu1–S1	103.43(7)°
N1–Cu1–S1	111.59(7)°	N4–Cu1–S1	117.54(7)°

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Structural analysis

Cu(NCS)(L₁)PPh₃] (C_1N). The title compound crystallizes in the triclinic space group P1̄. Ligand L₁ binds to the copper atom in a bidentate chelating manner via N2 (pyridine) and N3 (imine). An isothiocyanate anion (N-donor) and one PPh₃ occupy the other coordination sites (Fig. 1). Houser and co-workers suggested an angular index (τ₄) which determines the geometry of the four-coordinate metal centres as follows: τ₄ = [360 − (α + β)]/141] (α and β are the two largest angles around a four-coordinate metal centre). The values of τ₄ will range from 1.00 for a perfect tetrahedral geometry to zero for a perfect square planar environment. Intermediate structures including trigonal pyramid and seesaws fall within the range of 0 to 1.00.³⁷ According to the τ₄ value for C_1N (0.79), the coordination polyhedra of the copper centre can be described as trigonal pyramid.

In the structure of C_1N, each molecular unit of the complex is joined to the neighbouring unit by means of three 2-fold interactions including classical and non-classical hydrogen bonds N4–H4⋯S1 and C7–H7⋯S1, respectively (Table 3), and π_py–π_thiophene interactions (Table 4).

Table 3 Hydrogen bond geometries for compounds C_1N, C_1S, C_2N and C_2P

Structure	D–H⋯A	d D–H	d H⋯A	d D⋯A	∠ D–H⋯A	Symm. codes
C1N	N4–H4⋯S1	0.880	2.6800	3.5042(18)	157.00	1 − x, 1 − y, 1 − z
	C7–H7⋯S1	0.950	2.8700	3.6680(2)	143.00	1 − x, 1 − y, 1 − z
	C3–H3⋯S1	0.930	2.8754	3.7910(2)	162.00	−x, 2 − y, 1 − z
	C21–H21⋯S2	0.930	3.0230	3.5480(2)	116.00	1 − x, 1 − y, 2 − z
C1S	C10–H10⋯N1	0.950	2.5580	3.4540(4)	157.30	1 − x, 1 − y, 1 − z
	C7–H7⋯N1	0.950	2.7990	3.5690(5)	138.80	1 − x, 1 − y, 1 − z
	N4–H4⋯N1	0.880	2.2330	3.0760(3)	163.16	1 − x, 1 − y, 1 − z
C2N	N3–H3⋯S1	0.879	2.6810	3.4930(2)	153.90	1 − x, 1 − y, 1 − z
	C6–H6⋯S1	0.949	2.8469	3.6410(3)	141.80	1 − x, 1 − y, 1 − z
	C10–H10⋯N4	0.950	2.7230	3.5320(3)	143.60	1 + x, 1 + y, z
	C2–H2⋯S1	0.950	2.8812	3.8020(2)	163.60	2 − x, 2 − y, 1 − z
	C15–H15⋯S1	0.950	2.9839	3.8270(3)	148.60	x, −1 + y, z
C2P	C9–H9⋯O2	0.950	2.4970	3.3940(4)	157.40	x, 1/2 − y, −1/2 + z
	N3–H3⋯O1	0.880	2.3140	3.1000(3)	148.70	x, 1/2 − y, −1/2 + z
	C2–H2⋯N4	0.950	2.6850	3.5830(4)	157.90	2 − x, 1/2 + y, 1/2 − z
	C11–H11⋯N4	0.950	2.5310	3.3480(4)	144.10	1 − x, −1/2 + y, 1/2 − z
	C11–H11⋯O1	0.950	2.6870	3.1830(4)	113.10	1 − x, −1/2 + y,1/2 − z

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Table 4 π. interaction geometries for compounds C_1N, C_1S, C_2N and C_2P

Structure	Interaction	C–C (Å)	P–P°	P–CC°	CH⋯CgI	C⋯Cg (Å)	C–H–Cg (°)	Symm. code
Cg stands for the centre of gravity of the mentioned ring: for C1N: Cg2: S2, C9–C12; Cg3: N2, C2–C6; Cg4: C13–C18; Cg6: C25–C30; for C1S: Cg7: C25–C30; for C2N: Cg2: O2, C8–C11; Cg4: C13–C18; Cg6: C25–C30; for C2P: Cg2: O2, C8–C11; Cg3: N1, C1–C5.
C1N	πpy–πthiophene	3.749	7.52	19.63	—	—	—	1 − x, 1 − y, 1 − z
				27.14
	πpy–πpy	3.486	0.0	7.88	—	—	—	−x, 1 − y, 1 − z
	CH⋯πPPh3	—	—	—	C4–H4A⋯Cg6	3.725	165.63	−x, 1 − y, 1 − z
	CH⋯πPPh3	—	—	—	C5–H5⋯Cg4	2.689	145.87	−x, 1 − y, 1 − z
	CH⋯πpy	—	—	—	C11–H11⋯Cg3	3.737	134.12	1 + x, −1 + y, z
	CH⋯πthiophene	—	—	—	C14–H14⋯Cg2	3.134	145.84	−1 + x, 1 + y, z
	CH⋯πthiophene	—	—	—	C2–H2⋯Cg2	3.592	134.97	−1 + x, 1 + y, z
C1S	Amide⋯πpy	3.403	—	—	—	—	—	1 − x, 1 − y, 1 − z
	CH⋯πPPh3	—	—	—	C11–H11⋯Cg7	2.749	141.19	1 + x, y, z
	CH⋯πPPh3	—	—	—	C17–H17⋯Cg7	3.275	132.77	1 − x, −y, 2 − z
C2N	πpy–πfuryl	3.719	11.74	17.28	—	—	—	1 − x, 1 − y, 1 − z
				28.73
	πpy–πpy	3.461	0.0	5.15	—	—	—	2 − x, 1 − y, 1 − z
	CH⋯πfuryl	—	—	—	C1–H1⋯Cg2	3.602	135.83	1 + x, 1 + y, z
	CH⋯πPPh3	—	—	—	C4–H4⋯Cg6	2.721	145.84	2 − x, 1 − y, 1 − z
	CH⋯πPPh3	—	—	—	C3–H3A⋯Cg4	3.761	166.03	2 − x, 1 − y, z
C2P	S⋯πpy	3.882	—	19.60	—	—	—	x, y, 1 + z
	πpy–πpy	3.801	0.0	28.19	—	—	—	2 − x, 1 − y, −z
	πfuryl–πfuryl	3.583	0.0	17.13	—	—	—	1 − x, −y, −z

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The dimers are further connected to each other through π_py–π_py and C5–H5⋯π_PPh3 interactions along the a-direction (Fig. 3a) to afford chains which are laterally linked together via various intermolecular interactions to generate a 3D network. The interactions which link the chains along the b-axis include (i) S1⋯S1, (ii) C3H3⋯S1, (iii) C11–H11⋯π_py, (iv) C2–H2⋯π_thiophene and (v) C14–H14⋯π_thiophene linkages (Fig. 3b). In addition, C21H21⋯S2 H-bonds plus weak C27–H27⋯π_PPh3 interactions (C⋯Cg: 4.090 Å) connect them along the c-direction (Fig. 3c). The distance of the S⋯S interaction was found to be about 3.456 Å which is 4% shorter than the sum of the van der Waals radii of two sulfur atoms. A summary of the parameters for the other interactions mentioned above are presented in Tables 3 and 4.

Fig. 3 (a) Representation of dimeric units in C_1N and their association through π–π and C–H⋯π interactions, creating the [100] chain; (b and c) side views of the crystal packing in the ac and ab planes, respectively, which show how the chains are connected in the 3D network.

[Cu(SCN)(L₁)PPh₃] (C_1S). The red irregular crystals of C_1S are the second form resulting from the reaction of a 1 : 1 molar ratio of L₁ and the mixture of cuprous thiocyanate and PPh₃. X-ray diffraction analysis reveals that it crystallizes in the triclinic space group P1̄. The central copper atoms are again in trigonal pyramidal environments (τ₄ = 0.82, exactly equal to that for C_1N) formed by N_py and N_im (from the chelating ligand), PPh₃ moiety and thiocyanate anion (this time as an S-donor) (Fig. 1). Changing the coordination site of the ambidentate ligand, NCS⁻, alters the supramolecular architecture of C_1S compared to that of the N-bound isomer C_1N.

The crystal structure of C_1S contains hydrogen bonded dimers generated by three pairwise interactions, N4–H4⋯N1, C7–H7⋯N1 and C10–H10⋯N1 (Table 3 and Fig. 4a). It is worth noting that the coordination of sulfur to the Cu(I) atom and consequently the orientation of the N1 atom direct the formation of hydrogen bonds and lead to supernumerary slippage of molecules on each other. The offset of pyridine and thiophene rings prevents the formation of a π_py–π_thiophene interaction, unlike in the structure C_1N. Thus, instead of a π_py–π_thiophene interaction, a C=O⋯π interaction is established between the discrete molecules in the dimers. In addition, S⋯S interactions connect the dimers to form [001] chains. The distance between two sulfur atoms is equal to 3.594 Å. These chains are held together by C11–C11⋯π_PPh3 and C17–H17⋯π_thiophene linkages (Table 4) along the a- and b-directions, respectively, which complete a 3D network (Fig. 4b–d).

Fig. 4 Self-assembly of C_1S: (a) dimeric aggregates formed by pairwise H-bonds and C=O⋯π linkages; (b) S⋯S interactions generating [001] chains; (c and d) C–H⋯π interactions which link the chains in the ac and ab planes, respectively.

[Cu(NCS)(L₂)PPh₃] (C_2N). This compound crystallizes in the triclinic space group P1̄. A trigonal pyramidal configuration of the Cu(I) centres (τ₄ = 0.80) has been formed by the bidentate chelating ligand, PPh₃ and isothiocyanate (N-donor). C_2N has a similar coordination environment to that of C_1N leading to its being isostructural with this complex. The supramolecular organization of C_2N is also disciplined by the formation of dimers through the same kind of interactions in C_1N: N3–H3⋯S1, C6–H6⋯S1 and π_py–π_furyl stacking interactions. Pyridine rings participate in the other π–π interactions (π_py–π_py) which build up a chain of the dimers directed along the c-axis. The chains are further reinforced by C3–H3A⋯π_PPh3 and C4–H4⋯π_PPh3 contacts (see Table 4). On the other hand, S⋯S synthons accompanied by C2–H2⋯S1, C15–H15⋯S1 and C10–H10⋯N4 hydrogen bonds as well as C1–H1⋯π_furyl interactions link the chains to create (110) sheets. The third dimension of the supramolecular assembly results from the connection of the layers via C21–H21⋯π_furyl linkages. Crystal packing diagrams of C_2N are presented in Fig. 5.

Fig. 5 (a) Representation of dimeric units in C_2N; (b) association of these units through π–π and C–H⋯π interactions, creating the [100] chain; (c and d) side views of the crystal packing in the ab and ac planes, respectively; which show how the chains are connected in the 3D construction (PPh₃ moieties in (c) were omitted for more clarity).

[–(NCS)Cu(L₂)–]_n (C_2P). The reaction of a 1 : 1 molar ratio of L₂ with the mixture of CuSCN and PPh₃ results in two kinds of crystals: orange needle crystals, C_2N, and dark orange hexagonal ones, C_2P. X-ray diffraction analysis confirms that C_2P crystallizes in the monoclinic space group P21/c. Unlike the former complexes, PPh₃ does not coordinate to copper(I). In return, the trigonal pyramid geometry of the Cu(I) centre (τ₄ = 0.80) consists of the chelating ligand (L₂), thiocyanate (−SCN) and isothiocyanate (−NCS). Indeed, each thiocyanate anion acts as a bridge between the Cu(I) ions through simultaneous binding from S and N atoms. This coordination pattern leads to the formation of infinite polymeric chains extended along the c-axis (Fig. 6a). The Cu⋯Cu distance within the metal chain is 5.323 Å. The chains are further stabilized via intrachain hydrogen bonds (N3–H3⋯O1, C9–H9⋯O2 and C4–H4⋯S1).

Fig. 6 (a) Intrachain interactions within the coordination polymeric structure of C_2P and the side view of the [001] chains to show (b) an overall supramolecular array containing two-fold layers connected to each other; (c) π–π stacking and H-bonding interactions linking a chain to four others and (d) S⋯π_py interactions which connect the chains laterally.

Although the coordination structure of C_2P is different from the others, π–π interactions still have an important contribution in the crystal packing. Herein, π_furyl–π_furyl interactions establish two-fold sheets of the neighbouring chains which are also fortified by bifurcated hydrogen bonds. The other side of the chains in the sheets is involved in π_py–π_py stacking and H-bonding interactions, leading to the connection of the (011) layers along the a-direction to complete the overall supramolecular association (Fig. 6b). In other words, each coordination chain is associated with four other chains in a 3D arrangement, from one side through π_furyl–π_furyl, C11–H11⋯N4 and C11–H11⋯O1 linkages and from the other side by π_py–π_py stacking and C2–H2⋯N4 interactions (Fig. 6c). In addition, these chains are laterally linked through interesting S⋯π_py interactions in the b-direction (Fig. 6d).

NCI approach

Principle. The non-covalent interaction (NCI) reduced density gradient (RDG) method has been recently developed as a theoretical strategy to visualize weak interactions. Investigation of the interactions using NCI-RDG analysis is quite concordant with the traditional method that recognizes them according to distances and angles. However, the NCI-RDG technique has more accuracy and precision, as it is based on fundamental computation. It provides a rich illustration of strong attractive, van der Waals interactions and also steric repulsions. The theory rests on the analysis and the graphical interpretation of two scalar properties, charge density ρ and its derivatives, namely the λ eigenvalue of its Hessian and its reduced gradient s(ρ),²² defined as:
where ∇ρ is the gradient of ρ. The non-covalent interactions are located in the regions with low RDG and density. Analysis of sign(λ₂) of the electron density Hessian can be used to discern different types of interactions. For the strong ones such as H-bonds, sign(λ₂)ρ < 0; for the weak van der Waals types, sign(λ₂)ρ ≈ 0; and for the non-bonded interactions like steric repulsion, sign(λ₂)ρ > 0.

As the sign of λ₂ describes the essence of the interaction, 2D plots comprising sign(λ₂) × ρ versus RDG s would indicate a non-covalent interaction near-zero area in the horizontal axis.^38–44 Close contacts between atoms change the behaviour of the reduced gradient signal more compared to the contacts among the atoms present in the tails, leading to troughs in the 2D NCI plots. These troughs, specially the ρ value at the troughs, are the basis of the NCI approach. The 2D NCI plots are then applied as inputs to construct 3D NCI plots, including isosurfaces of the reduced gradient of the density enabling the spatial visualization of the close contacts.

We applied this method to unravel the nature of supramolecular interactions in the title complexes. NCI analysis has been performed on the structure of complexes including the diverse noncovalent interactions. The considered structures were cut out directly from the CIF data. Since dimerization is the prominent feature of the crystal packing in the monomeric complexes (C_1N, C_1S and C_2N), the main NCIs are related to the interactions involved in the formation of dimers. The 2D and 3D NCI plots of dimers are shown in Fig. 7. Accordingly, we have done calculation once on the dimers including only the carbohydrazone ligands (Cu⁺ and SCN⁻ ions and PPh₃ moieties have been eliminated) and again for the whole dimeric units of complexes. Some of the other interesting intermolecular interactions in the crystal structures have also been investigated by the NCI method. The presence of noncovalent interactions is characterized by spikes at negative to near-zero sign of λ₂, whereas the peaks at positive sign indicate the repulsive steric contacts due to the ring formation.⁴⁵ The spikes at the zero area (sign(λ₂)ρ between ±0.015 a.u.) show vdW interactions. Notable points of the NCI calculations have been illustrated in the following:

Fig. 7 Left: The NCI RDG s vs. sign(λ₂)ρ plots for dimers of ligands and complexes. Right: Coloured RDG-based NCI isosurfaces for the dimers of complexes.

(i) As shown in Fig. 7, for the ligand dimers, the spikes that appeared at 0.024 a.u. belong to the pyridine ring closure. These spikes shift to lower values (less repulsion) in the whole dimeric units of complexes. It can be explained by the effect of metal ion in the charge redistribution as well as the electrostatic interaction between atoms within the rings.

(ii) In the case of C_1N, the thiophene ring closure spike (sign(λ₂)ρ) is located between 0.042 and 0.044 a.u. while in the 2D plot and the 3D isosurface of C_2N, the furyl ring has a much lesser repulsion of ring closure than the thiophene alternative. It may be caused by the greater charge perturbation due to the presence of an oxygen atom which leads to more electrostatic interactions. Consistent with this, natural bond orbital (NBO) analysis also reveals the stabilizing energy of 54 kcal mol⁻¹ for the electronic delocalization “lone pair (O) → π* (C–C) orbital” which is more than that for the corresponding charge transfer energy in the thiophene ring (LP(S) → π*(C–C): 48 kcal mol⁻¹).

(iii) C_1N and C_1S compounds are linkage isomers, in a way that SCN⁻ is coordinated from N or S atoms, respectively. Optimization of the isomers, in the gas phase and also acetonitrile and chloroform solutions, indicates that the stability of C_1S is approximately 3–4 kcal mol⁻¹ less than that of C_1S; however, it has been also formed in the solid state. The formation of C_1S can be attributed to stronger intermolecular attractions particularly those involved in dimerization which compensate for the lesser stability of the discrete units of C_1S. RDG isosurfaces show stronger interactions in the C_1S dimer rather than in C_1N. Counterpoise calculations at the M06-2X/6-311G* level indicate that the binding energy of two complexes in a dimer, ΔE_dimer, for C_1S is 2.8 kcal mol⁻¹ more than for C_1N as well.

(iv) It was thought to be of interest to further investigate the sulfur interactions to figure out their nature in the solid state structures. The sulfur atom, due to its large van der Waals radius and high polarizability, is able to establish several interactions with its local environment.⁴⁶ Morgan and co-workers first proposed the hypothesis that a strong interaction exists between aromatic rings and divalent sulfur atoms.⁴⁷ The importance of the S⋯π aromatic interaction is revealed in the high degree of its conservation across members in protein folding and stabilization.^34,48

Fig. 8 represents the 3D plots of S⋯S and S⋯π interactions in the solid state structures. The compact and small, flat, pill-shaped isosurfaces, concentrated on the NCI critical points indicate that these interactions are significantly attractive and contribute to the crystal packing stability.⁴⁵

Fig. 8 RDG-based NCI surfaces for S⋯S and S⋯π interactions.

Conclusions

Diverse coordination structures from the reaction of CuSCN with PPh₃ and ortho-pyridinyl carbohydrazone containing a thiophene (L₁) or furyl ring (L₂) were presented. A mixture of thiocyanate linkage isomers, C_1N and C_1S, was obtained for L₁, while the reaction with L₂ rendered two monomeric and polymeric compounds, C_2N and C_2P, respectively. The molecular and supramolecular structures of these systems were elucidated using X-ray diffraction. The structure-directing interactions were also investigated by NCI-RDG calculations.

Pyridine, thiophene and furyl rings, the polarized aromatic systems, have an important role in governing the supramolecular assembly of the complexes by establishing π–π interactions. However, the coordination of sulfur to the Cu(I) atom in C_1S leads to supernumerary slippage of the neighbouring molecules which prevents the formation of π_py–π_thiophene connections. CH–π interactions between PPh₃ moieties contribute to further stabilization of the self-association in the monomeric complexes (C_1N, C_1S and C_2N).

A prominent feature of the crystal packing in the monomeric complexes is the formation of the dimeric motifs via hydrogen bonding and π–π stacking interactions. Formation of the less stable S-bound isomer C_1S can be attributed to stronger intermolecular attractions particularly those involved in dimerization which compensate for the lesser stability of the discrete units of C_1S compared to C_1N.

Another interesting feature of the solid state structures is the presence of the lesser known S⋯S and S⋯π interactions. NCI-RDG analysis clearly indicates the significant contribution of these interactions in maintaining favourable packing interactions in the complex.

Experimental

Materials and methods

All chemicals and solvents used in the syntheses were of reagent grade and were used without further purification. ¹H and ¹³C NMR spectra were recorded on a Bruker (Avance DRS) 250 MHz spectrometer. IR spectra were recorded on a Nicolet 510P spectrophotometer using KBr disks. Elemental analysis was performed using a Heraeus CHN-O-RAPID apparatus.

Synthesis procedures

The ligands were prepared from the reaction of two equivalent amounts of 2-thiophenecarboxylic acid hydrazide or 2-furoic hydrazide and 2-pyridinecarboxaldehyde in methanol solution.

A solution of ligand (0.20 mmol) in CHCl₃ (4 mL) was added dropwise to a stirred solution of a mixture of copper(I) thiocyanate (0.20 mmol) and triphenylphosphine (0.20 mmol) in CH₃CN (2 ml). The colour of the reaction mixture turned from orange to red. The reaction mixture was filtered; slow diffusion of diethyl ether in the filtered solution afforded suitable single crystals (total yields for the mixture of C_1N and C_1S: 81%, and for that of C_2N and C_2P: 76%). The complexes were obtained in good yields. Physical and spectroscopic data of the compounds are presented below:

(ortho-Thiophene)C(O)NHNCH(ortho-pyridine) (L₁). Mp: 177–179 °C. Selected IR peaks (cm⁻¹): 3433 w, 3065 w, 1641 s, 1584 m, 1374 s, 1157 m, 733 m. ¹H NMR (CDCl₃, δ ppm): 7.20–7.35 (m, 2H), 7.73–7.81 (m, 2H), 8.18–8.26 (m, 3H), 8.66 (s, 1H), 10.52 (s, 1H).

(ortho-Furyl)C(O)NHNCH(ortho-pyridine) (L₂). Mp: 157–160 °C. Selected IR peaks (cm⁻¹): 3422 w, 3005 w, 1662 s, 1579 s, 1468 m, 1308 m, 1187 m, 756 m. ¹H NMR (CDCl₃, δ ppm): 6.49 (s, 1H), 7.25–7.53 (m, 3H), 7.69 (s, 1H), 8.10 (d, 1H), 8.41–8.55 (d, 2H), 10.64 (s, 1H). ¹³C NMR (CDCl₃, δ ppm): 112.23, 116.32, 121.16, 124.26, 125.69, 136.48, 137.68, 144.82, 148.07, 148.27, 152.81.

[Cu(NCS)(L₁)PPh₃] (C_1N). Orange crystals. Mp: 211–214 °C. Anal. calcd for C₃₀H₂₄CuN₄OPS₂: C, 58.57; H, 3.93; N, 9.11. Found: 58.49; H, 3.96; N, 9.17. Selected IR peaks (cm⁻¹): 3437 s, 2925 s, 2003 s, 1670 m, 1540 m, 1431 w, 1264 s, 1142 w, 696 m, 517 w.

[Cu(SCN)(L₁)PPh₃] (C_1S). Red crystals. Mp: 211–213 °C. Anal. calcd for C₃₀H₂₄CuN₄OPS₂: C, 58.57; H, 3.93; N, 9.11. Found: 58.41; H, 3.90; N, 9.15. Selected IR peaks (cm⁻¹): 3437 s, 2927 w, 2074 m, 1665 m, 1545 m, 1426 w, 1226 m, 1126 w, 703 m, 512 m.

[Cu(NCS)(L₂)PPh₃] (C_2N). Orange crystals. Mp: 204–206 °C. Anal. calcd for C₃₀H₂₄CuN₄O₂PS: C, 60.14; H, 4.04; N, 9.35. Found: 60.21; H, 4.05; N, 9.41. Selected IR peaks (cm⁻¹): 3435 w, 2083 s, 1686 s, 1533 m, 1469 s, 1284 s, 1179 s, 752 m, 696 m, 516 m.

[–(NCS)Cu(L₂)–]_n (C_2P). Dark orange crystals. Mp: 238–240 °C (decomp.). Anal. calcd for C₁₂H₉CuN₄O₂S: C, 42.79; H, 2.69; N, 16.63. Found: 42.68; H, 2.69; N, 16.60. Selected IR peaks (cm⁻¹): 3277 w, 2112 s, 1672 s, 1543 m, 1464 m, 1295 m, 1186 m, 771 w.

Crystal structure determination

Single crystal X-ray diffraction data were collected for all compounds on an Agilent Gemini Ultra diffractometer equipped with an Eos CCD area detector and using either Mo-Kα radiation (λ = 0.71073 Å) or Cu-Kα radiation (λ = 1.5418 Å). The data were collected at 173 K using an Oxford Cryosystems Cryostream 600. The data were processed with CrysAlisPro.⁴⁹ Semi-empirical absorption corrections were carried out using the Multi-Scan⁵⁰ program. The structures were solved by direct methods using SHELXT⁵¹ and refined with full matrix least squares refinement using SHELXL-2013 (ref. 52) within Olex2.⁵³ All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were added at calculated positions and refined using a riding model based on the parent atom. The CIF files have been deposited with the CCDC and have been given the deposition numbers 1401344, 1401345, 1469718 and 1469717 for C_1N, C_1S, C_2N and C_2p, respectively.

Computational details

The NCI technique was carried out through the analysis of the reduced density gradient (RDG) with low densities³² at the ωB97XD⁵⁴/6-311+G** level using the Gaussian 09 package⁵⁵ and Multiwfn program.⁵⁶ The calculated grid points are plotted for a defined real space function, sign(λ₂(r))ρ(r) and reduced density gradient (RDG) and a visualization of the gradient isosurface was depicted using the VMD 1.9.2 software.⁵⁷ The colour of the isosurfaces was decided using the value of sign(λ₂)ρ. Blue, green and red colour codes are commonly used to describe stabilizing H-bonding, van der Waals and steric interaction, respectively. Pictures are provided for an isosurface value of s = 0.5.

Natural bond orbital (NBO) analysis⁵⁸ was performed on the crystal structure of the complexes using the NBO 3.1 module in Gaussian 09 at the B3LYP/6-311+G** level of theory. The binding energy of two complexes in a dimer, ΔE_dimer, for C_1N, C_1S and C_2N were calculated at the M062X/6-311G* level based on the energy difference between the dimer and its units. The interaction energies have been corrected for the basis set superposition error (BSSE) using the counterpoise (CP) procedure.⁵⁹

Acknowledgements

Financial support of this work by Tarbiat Modares University is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: IR and NMR spectra of all compounds and crystallographic files in CIF format for structural determination of complexes. CCDC 1401344, 1401345, 1469718 and 1469717. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ce01339b

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