Gone with the Wind—Adducts of Volatile Pyridine Derivatives and Copper(II) Acetylacetonate

Abstract

The investigation of adducts of weakly coordinating pyridine ligands with copper acetylacetonate is more arduous in the presence of volatile pyridine derivatives. The present study reports the synthesis of new adducts, including those with volatile ligands. Furthermore, the formation of one-dimensional coordination polymers is observed when bidentate ligands are used. The synthesis and characterization of the adduct formed by pyridine is particularly noteworthy, which despite its simplicity has not yet been structurally elucidated. A total of four pentacoordinate complexes, one oligomer and two coordination polymers are synthesized and discussed in this study. The obtained structures of the complexes complement the spectrum of known adducts due to the substituents on the pyridines, and allow conclusions to be drawn about the cause of the different structures based on the electronic properties of the substituents. Furthermore, intermolecular interactions are discussed using Hirshfeld surface analysis and attributed to the pyridine derivatives present.

1. Introduction

In its early stages, coordination chemistry underwent significant conceptual evolution, necessitating the reassessment and reinterpretation of several fundamental principles of chemistry. For example, the investigation of complex geometries became a central focus, as established coordination numbers were found to exhibit atypical geometrical configurations: Whether and to what extent tetracoordinate metal ions could adopt tetrahedral or planar geometries and if these coordination numbers can be increased. Copper(II) complexes with four ligating atoms were intensively examined, leading to the synthesis of numerous novel species that substantially contributed to the expanding body of knowledge in the field.

The structural elucidation of the coordination geometry of copper(II) acetylacetonate ([Cu(acac)₂]) underlined initial considerations of the formation of a square-planar arrangement of the ligands. After initial theoretical crystallographic analyses [1], the first experimentally determined structures were published by different research groups [2,3], which confirmed its square-planar structure. In addition to structural aspects, [Cu(acac)₂] has also gained considerable attention in spectroscopic studies, particularly due to its paramagnetic properties, which have furthered the understanding of these complexes [4]. Attempts to extend the initial square-planar coordination geometry were made, and pentacoordinate complexes have been generated by additionally coordinating N-heterocyclic bases to the square-planar complexes [5]. In the absence of structural evidence, tentative suggestions were made on the basis of spectroscopy, and pyridine and its derivatives were frequently used to generate the pentacoordinate geometry [6,7,8,9]. Due to the lack of crystallographic evidence, both penta- and tetracoordinate geometries have been discussed for these adducts, with several spectroscopic experiments suggesting a pentacoordinate structure [5,6,10]. The high volatility and weak coordination of the pyridine derivatives prevented the isolation of stable adducts, so quinoline was used as an alternative and the first example of a pentacoordinate [Cu(acac)₂] adduct with an additional heterocycle was isolated [11]. The structure of this adduct was later elucidated and shown to feature a square-pyramidal coordination geometry [12]. Subsequently, other pentacoordinate adducts with pyridine derivatives were reported [13,14,15,16,17,18,19,20] as well as hexacoordinate adducts [21,22,23,24], some of which were found to crystallize as one-dimensional [25,26,27,28] or two-dimensional [29] coordination polymers. It is noteworthy that these structurally elucidated examples are based almost exclusively on substituted pyridine derivatives with comparatively low volatility. Adducts of 4-methylpyridine or pyridine were found to slowly decompose with ready reformation of the initial [Cu(acac)₂] complex when, for example, the isolated adduct is washed with cold benzene or simply stored in air [6,30]. This is supported by studies on the equilibrium of adduct formation and the stability of these adducts in solution, which show that the bond formed by the pyridine derivative and the copper(II) ion is rather weak [31,32]. In general, studies on the stability of these adducts carried out by different groups show that the stability of the adducts correlates with the inherent basicity of the investigated nitrogen bases [6,7,8,9,31].

The use of coordinatively unsaturated complexes in the construction of larger supramolecular structures through coordinative saturation using suitable ligands has since become firmly established. This method is not only used in crystal engineering, but also, for example, in the field of metal–organic frameworks or coordination polymers [33,34,35]. Using volatile, coordinating solvent molecules (e.g., water or methanol), coordinatively unsaturated metal complexes can be generated by thermal activation whose free coordination sites are saturated by bridging ligands, thus building the corresponding framework [36,37]. The high flexibility in adopting different coordination numbers and geometries of copper(II) ions, as well as the diversity of pyridine-based ligands, allows for the generation of a wide variety of responsive MOFs [38,39,40,41,42,43].

In this paper we report the synthesis, spectroscopic properties and structural characterization of adducts of copper(II) acetylacetonate with pyridine, 4-methylpyridine, 4-bromopyridine, 4-pyrrolidinopyridine, 4,4’-bipyridine and 1,2-di-(4-pyridyl)ethylene.

2. Materials and Methods

2.1. General Aspects

All chemicals used in this study were of p.a. grade and were obtained from ABCR (Karlsruhe, Germany), Acros Organics (Geel, Belgium), Alfa Aesar (Haverhill, MA, USA), AppliChem (Darmstadt, Germany), BLDpharm (Shanghai, China), Carl Roth (Karlsruhe, Germany), Fisher Scientific (Hampton, NY, USA), Fluorochem (Hadfield, UK), Merck (Darmstadt, Germany), Sigma-Aldrich (St. Louis, MI, USA), TCI (Tokyo, Japan) or VWR (Radnor, PE, USA) and used, if not stated otherwise, without further purification.

UV/Vis spectra were recorded on a V-750 spectrophotometer (Jasco, Tokyo, Japan) within a range of 250–800 nm. All measurements were recorded in a quartz cuvette (path length 1 cm) with dichloromethane as a solvent. FT-IR spectra were recorded on an ATR-Alpha II IR-spectrometer (Bruker Optic GmbH, Bremen, Germany). NMR spectra were recorded on an Avance Neo 400 NMR spectrometer (Bruker BioSpin, Ettlingen, Germany). Chemical shifts are reported as δ, parts per million (ppm), and referenced to internal solvent residues. Coupling constants J are stated in Hertz (Hz). The following abbreviations were used for spin multiplicities: d (doublet). High-resolution ESI-MS were recorded on an Impact II VIP QTOF ESI-MS (Bruker Daltonics, Bremen, Germany).

2.2. Synthesis

• General procedure A for the preparation of Cu(II) adducts using solid heterocycles

A suspension of [Cu(acac)₂] (100 mg, 0.382 mmol, 1.00 equiv.) in benzene (15 mL) was heated at 75 °C. The solid heterocycle (0.764 mmol, 2.00 equiv.) was then added to the clear solution and the mixture stirred for a further 30 min at 75 °C. The adducts were obtained by cooling the solution to 8 °C, washing the crystals with pentane and drying in vacuo.

• General procedure B for the preparation of Cu(II) adducts using liquid heterocycles

[Cu(acac)₂] (100 mg, 0.382 mmol, 1.00 equiv.) was dissolved in a minimal amount of the liquid heterocycle at 75 °C. The adducts were obtained by cooling the solution to −30 °C.

2.2.1. Synthesis of [Cu(acac)₂]

[Cu(acac)₂] was synthesized according to a literature procedure [44]. Acetylacetone (2.45 g, 2.5 mL, 24.5 mmol, 2.05 equiv.) was dissolved in a minimal amount of an aqueous ammonia solution (2.5%). The resulting solution was then added slowly to an ice-cooled solution of CuSO₄ (3.00 g, 12.0 mmol, 1.00 equiv.) in water (50 mL) yielding a blue precipitate, which was subsequently filtered and washed with water until the filtrate was colorless. The blue precipitate was further washed with acetone (100 mL) and Et₂O (10 mL). After removing the volatiles in vacuo, the product was obtained as a blue-violet solid (1.71 g, 6.50 mmol, 54%). IR (ATR): ṽ/cm⁻¹ = 429 (w), 453 (vs), 611 (m), 652 (w), 684 (w), 781 (vs), 936 (m), 1017 (m), 1188 (w), 1274 (m), 1350 (s), 1406 (s), 1520 (vs), 1550 (s), 1574 (vs). ε₅₅₄/(L·mol⁻¹·cm⁻¹) = 28.23 (DCM), ε₆₅₈/(L·mol⁻¹·cm⁻¹) = 32.49 (DCM).

2.2.2. Synthesis of 4-Bromopyridine

4-Bromopyridine was synthesized according to a literature procedure [45]. An aqueous NaOH solution (5M, 4 mL) was added slowly to a solution of 4-bromopyridine hydrochloride (4.00 g, 20.6 mmol, 1.00 equiv.) in water (20 mL). The resulting yellowish solution was stirred for 10 min. Subsequently, the aqueous layer was extracted with Et₂O (3 × 30 mL). The combined organic layers were dried over MgSO₄ and the solvent was removed in vacuo to give 4-bromopyridine as a colorless oil (3.08 g, 19.5 mmol, 95%). ¹H NMR (400 MHz, DMSO-d₆): δ (ppm) = 8.47 (d, ³J_HH = 4.4 Hz, 1H, H2), 7.68 (d, ³J_HH = 4.4 Hz, 1H, H3). ESI-MS (m/z) [C₅H₄NBr+H]⁺: calcd.: 157.9600, found: 157.9599. The analytical data are in agreement with the literature [46].

2.2.3. Synthesis of [Cu(acac)₂(py)] (1)

General procedure B: [Cu(acac)₂] and pyridine (py) were converted to adduct 1 and isolated as blue crystals. IR (ATR): ṽ/cm⁻¹ = 431 (s), 451 (s), 558 (w), 597 (m), 612 (s), 653 (m), 683 (m), 702 (vs), 750 (m), 779 (vs), 934 (s), 1016 (s), 1067 (w), 1146 (w), 1188 (m), 1213 (w), 1273 (s), 1350 (vs), 1403 (vs),1434 (s), 1516 (vs), 1549 (s), 1574 (vs), 2922 (vw), 2999 (vw), 3022 (vw), 304 (vw). ε₆₅₈/(L·mol⁻¹·cm⁻¹) = 74.37 (DCM).

2.2.4. Synthesis of [Cu(acac)₂(4-MePy)] (2)

General procedure B: [Cu(acac)₂] and 4-methylpyridine (4-MePy) were converted to adduct 2 and isolated as blue crystals. IR (ATR): ṽ/cm⁻¹ = 428 (vs), 486 (s), 529 (vw), 593 (m), 654 (w), 676 (w), 711 (vw), 772 (m), 798 (m), 813 (w), 930 (m), 1011 (s), 1193 (w), 1222 (w), 1266 (m), 1332 (m), 1350 (s), 1402 (vs), 1518 (vs), 1578 (vs), 2921 (vw), 2997 (vw). ε₆₆₄/(L·mol⁻¹·cm⁻¹) = 58.82 (DCM). Elemental analysis: C₁₆H₂₁NO₄Cu: calcd. C 54.2, H 6.0, N 4.0; found C 54.1, H 6.0, N 3.9.

2.2.5. [Cu(acac)₂(4-PyrPy)] (3)

General procedure A: [Cu(acac)₂] and 4-pyrrolidinopyridine (4-PyrPy) (113 mg, 0.764 mmol) were converted to adduct 3 and isolated as green crystals (80.0 mg, 0.195 mmol, 51%). IR (ATR): ṽ/cm⁻¹ = 427 (s), 472 (vw), 531 (m), 589 (m), 653 (w), 660 (w), 676 (w), 765 (s), 799 (vw), 828 (s), 925 (m), 1001 (vs), 1018 (w), 1159 (w), 1186 (w), 1230 (m), 1267 (m), 1340 (m), 1398 (vs), 1450 (m), 1487 (m), 1512 (vs), 1577 (s), 1596 (m), 2865 (vw), 2965 (vw). ε₆₉₉/(L·mol⁻¹·cm⁻¹) = 73.81. Elemental analysis: C₁₉H₂₆N₂O₄Cu: calcd. C 55.7, H 6.4, N 6.8; found C 55.8, H 6.4, N 6.8.

2.2.6. Synthesis of [Cu(acac)₂(4-BrPy)] (4)

General procedure B: [Cu(acac)₂] and freshly synthesized 4-bromopyridine (4-BrPy) were converted to adduct 4 and isolated as blue crystals. IR (ATR): ṽ/cm⁻¹ = 426 (s), 437 (s), 482 (s), 598 (m), 676 (s), 721 (w), 771 (m), 803 (m), 820 (m), 932 (m), 1017 (m), 1060 (m), 1088 (w), 1190 (w), 1214 (w), 1217 (m), 1314 (w), 1399 (s), 1477 (m), 1516 (m), 1558 (s), 3049 (vw). ε₆₄₆/(L·mol⁻¹·cm⁻¹) = 85.20 (DCM).

2.2.7. Synthesis of ${}_{\mathrm{\infty }}{}^1\left[\text{Cu}\right(\text{acac}{)}_2\left(\text{bpy}\right)]$ (5)

General procedure A: [Cu(acac)₂] and 4,4′-bipyridine (bpy) (119 mg, 0.764 mmol) were converted to the coordination polymer 5 and isolated as green-blue crystals (29.1 mg, 69.6 mmol, 18%). IR (ATR): ṽ/cm⁻¹ = 426 (vs), 498 (m), 573 (w), 592 (m), 613 (s), 659 (w), 671 (w), 737 (vw), 764 (s), 821 (s), 861 (w), 926 (m), 995 (w), 1015 (m), 1039 (vw), 1078 (w), 1163 (vw), 1189 (w), 1217 (w), 1267 (m), 1399 (vs), 1421 (m), 1455 (m), 1514 (vs), 1580 (vs), 2165 (vw), 2907 (vw), 3048 (vw). ε₆₄₈/(L·mol⁻¹·cm⁻¹) = 104.59 (DCM). Elemental analysis: C₂₀H₂₂N₂O₄Cu: calcd. C 57.5, H: 5.3, N: 6.7; found 57.2, H: 5.4, N: 6.4.

2.2.8. Synthesis of ${}_{\mathrm{\infty }}{}^1\left[\text{Cu}\right(\text{acac}{)}_2\left(\text{TDPE}\right)]$ (6)

A suspension of [Cu(acac)₂] (50 mg, 0.191 mmol, 1.00 equiv.) in benzene (15 mL) was heated at 75 °C. trans-1-(2-Pyridyl)-2-(4-pyridyl)ethylene (TDPE) (174 mg, 0.955 mmol, 5.00 equiv.) was added to the clear solution, which was stirred for further 30 min at 75 °C. Compound 6 was obtained by cooling the solution to 8 °C, washing the crystals with pentane, and drying in vacuo (28.8 mg, 64.8 µmol, 34%). IR (ATR): ṽ/cm⁻¹ = 409 (m), 429 (s), 467 (m), 487 (m), 501 (m), 514 (m), 539 (s), 552 (vs), 593 (s), 655 (m), 669 (m), 764 (s), 832 (vs), 927 (m), 994 (vs), 1016 (m), 1073 (w), 1192 (m), 1265 (s), 1347 (s), 1403 (vs), 1516 (vs), 1578 (vs), 2023 (w) 3043 (vw). ε₆₅₇/(L·mol⁻¹·cm⁻¹) = 64.76 (DCM). Elemental analysis: C₂₂H₂₄N₂O₄Cu: calcd. C 59.5, H 5.5, N 6.3; found C 59.5, H 5.5, N 6.0.

2.2.9. Synthesis of [(Cu(acac)₂)₃(TDPE)₂] (7)

General procedure A: [Cu(acac)₂] and trans-1-(2-pyridyl)-2-(4-pyridyl)ethylene (TDPE) (139 mg, 0.764 mmol) were converted to adduct 7 and isolated as pale blue crystals (26.7 mg, 23.2 µmol, 18%). IR (ATR): ṽ/cm⁻¹ = 426 (s), 441 (vs), 489 (w), 552 (s), 600 (m), 659 (vw), 676 (vw), 704 (vw), 767 (m), 836 (m), 930 (w), 1002 (w), 1018 (m), 1190 (w), 1272 (m), 1348 (s), 1403 (vs), 1516 (vs), 1545 (w), 1580 (s), 1593 (s), 1976 (vw), 2166 (vw). Elemental analysis: C₅₄H₆₂N₄O₁₂Cu₃: calcd. C 56.4, H 5.5, N 4.9; found C 56.7, H 5.5, N 4.7.

2.2.10. Synthesis of [(Cu(acac)₂)₂(TDPE)] · 3 CHCl₃ (8)

A suspension of [Cu(acac)₂] (200 mg, 0.764 mmol, 1.00 equiv.) in CHCl₃ (3.5 mL) was heated to 55 °C. trans-1-(2-Pyridyl)-2-(4-pyridyl)ethylene (TDPE) (279 mg, 1.53 mmol, 2.00 equiv.) was added to the clear solution which was stirred for 30 min at 55 °C. Compound 8 was obtained by cooling the solution to −30 °C, washing the crystals with pentane and drying in vacuo (171 mg, 0.161 mmol, 42%). IR (ATR): ṽ/cm⁻¹ = 433 (vs), 489 (w), 551 (vs), 597 (m), 659 (m), 739 (s), 834 (m), 931 (m), 970 (w), 1003 (m), 1018 (m), 1193 (w), 1271 (m), 1350 (s), 1393 (vs), 1516 (vs), 1574 (vs), 1597 (vs), 2994 (vw). Elemental analysis: C₃₂H₃₈N₂O₈Cu: calcd. C 54.5, H 5.4, N 4.0; found C 54.6, H 5.4, N 3.9.

2.3. X-Ray Crystallography

Crystals suitable for X-ray crystallography were obtained from benzene (8 °C, 2, 3, 5, 6, 7), CHCl₃ (−30 °C, 8) or from the respective liquid donor ligand (−30 °C, 1, 4). Adducts of liquid donor ligands were found to easily dissociate into the free ligand and solid [Cu(acac)₂]. Crystals of complex 1 so rapidly lost pyridine when put into oil that they had to be picked from a solution of pyridine. Intensity data was collected with monochromated MoKα radiation on a Bruker D8-Venture diffractometer (Bruker AXS, Karlsruhe, Germany) with Photon III detector using the APEX5 version 2023.9.2 software suite [47,48,49]. The collection method involved ω scans. Data reduction was carried out using the program SAINT [48]. The crystal structures were solved by Intrinsic Phasing using SHELXT [50]. Non-hydrogen atoms were first refined isotropically followed by anisotropic refinement by full matrix least-squares calculation based on F² using SHELXL [51]. H atoms were positioned geometrically and allowed to ride on their respective parent atoms. Compound 2 crystallized with two half molecules of the adduct in the asymmetric unit, the other halves are related by a mirror plane. The H-atoms on the methyl group of the 4-picolin substituent are disordered across the mirror plane. Compound 8 crystallized with half a molecule of [Cu(acac)₂] and 2.5 molecules of CHCl₃ in the asymmetric unit. Two CHCl₃ molecules are rotationally disordered (0.49:0.51; 0.55:0.45); half a molecule is disordered across the inversion center. The TDPE ligand is rotationally disordered (0.78:0.22). Further details of the crystal structure determinations are available from

Table 1. Crystallographic data of compounds 1–8.

	Compound 1	Compound 2	Compound 3	Compound 4	Compound 5	Compound 6	Compound 7	Compound 8
Empirical formula	C15H19CuNO4	C16H24CuNO4	Cu19H26CuN2O4	C15H18BrCuNO4	C20H22CuN2O4	C22H24CuN2O4	C54H62Cu3N4O12	C37H43Cl15Cu2N2O8
Formula weight	340.85	354.88	409.96	419.75	417.93	443.97	1149.69	1302.56
Crystal system	Triclinic	Monoclinic	Monoclinic	Triclinic	Monoclinic	Triclinic	Triclinic	Triclinic
Space group	P-1	C2/m	C2/m	P-1	P21	P-1	P-1	P-1
a, Å	7.7444(3)	21.7043(4)	10.9402(3)	8.6807(3)	11.1929(4)	8.2115(3)	12.4530(10)	9.1804(8)
b, Å	8.1821(3)	9.5368(2)	12.2115(3)	10.2923(5)	14.2606(6)	8.6888(3)	14.0998(11)	11.2336(10)
c, Å	13.7360(5)	17.9698(4)	15.0631(4)	11.4344(5)	11.9295(5)	8.7538(3)	15.3886(12)	13.2041(11)
α, deg.	93.9659(10)	90	90	112.0530(10)	90	69.7339(10)	88.207(3)	78.857(3)
β, deg.	98.1740(10)	116.6720(10)	107.4790(10)	110.5830(10)	92.6190(10)	62.2510(10)	74.608(3)	86.404(2)
γ	117.6440(10)	90	90	96.890(2)	90	76.4940(10)	89.364(3)	86.933(2)
V, Å3	754.10(5)	3325.07(12)	1919.46(9)	848.21(6)	1902.06(13)	516.55(3)	2603.8(4)	1332.2(2)
Z	2	8	4	2	4	1	2	1
ρcalc, g cm−3	1.501	1.418	1.419	1.643	1.459	1.427	1.466	1.624
μ (MoKα), mm−1	1.462	1.330	1.164	3.658	1.176	1.087	1.280	1.597
Crystal size, mm3	0.44 × 0.17 × 0.12	0.18 × 0.17 × 0.12	0.09 × 0.08 × 0.04	0.27 × 0.20 × 0.18	0.09 × 0.05 × 0.02	0.18 × 0.12 × 0.07	0.13 × 0.12 × 0.11	0.45 × 0.11 × 0.09
Temperature (K)	100(2)	100(2)	100(2)	100(2)	100(2)	100(2)	100(2)	100(2)
θ range, deg	2.844–30.082	1.905–30.057	2.567–28.707	2.130–30.060	2.227–30.054	2.508–30.017	2.225–30.055	2.646–28.719
hkl range, deg	−10:10, −11:11, −19:19	−30:30, −13:13, −25:25	−14:14, −16:16, −20:20	−12:12, −14:14, −16:16	−15:15, −20:20, −16:16	−11:11, −12:12, −12:12	−17:17, −19:19, −21:21	−12:12, −15:15, −17:17
Total/unique data/Rint	11181/4425/0.0162	29205/5151/0.0190	14383/2482/0.0305	14932/4948/0.0174	32060/11140/0.0363	8874/3011/0.0369	44994/15126/0.0213	14488/6850/0.0258
Observed data [I > 2σ(I)]	4291	4783	2284	4668	9753	2790	12894	6140
Nref/Npar	4425/194	5151/229	2482/122	4948/203	11,140/496	8874/135	3011/673	6850/440
R1/wR2 [I > 2σ(I)]	0.0233/0.0651	0.0241/0.0731	0.0334/0.0808	0.0235/0.0607	0.0389/0.0931	0.0389/0.0887	0.0379/0.1102	0.0527/0.1529
R1/wR2 [all data]	0.0241/0.0656	0.0258/0.0743	0.0369/0.0826	0.0252/0.0615	0.0461/0.0968	0.0418/0.0912	0.0444/0.1143	0.0586/0.1600
Flack parameter	n/a	n/a	n/a	n/a	0.203(17)	n/a	n/a	n/a
S	1.073	1.035	1.088	1.044	1.033	1.057	1.046	1.083
Min./max. res. dens., eÅ−3	0.698/−0.577	0.440/−0.317	2.116/−0.733	1.342/−1.005	0.685/−0.693	0.584/−0.451	1.547/−0.634	1.848/−1.648

and the Cambridge Crystallographic Data Center on quoting the depository number CCDC 2471909–2471916. Molecular graphics were created using the program ORTEP-3 for Windows [52,53].

3. Results

3.1. Synthesis

The synthesis of copper(II) acetylacetonate has been extensively documented in the scientific literature [6,44,54,55,56,57]; it was conveniently prepared from acetylacetone and copper(II) sulfate in aqueous ammonia [44]. The adducts were then obtained by dissolving [Cu(acac)₂] in a minimal quantity of the liquid heterocycle and stirring the reaction mixture at an elevated temperature to induce adduct formation. In case of solid pyridine derivatives, a suspension of the two starting materials was heated in benzene instead (Scheme 1). The progress of the reaction was evident from a color change in the reaction solutions from blue to green indicative of the formation of the adduct. The crystalline products were obtained by storing the reaction mixtures at −30 °C (neat ligands) or 8 °C (benzene solutions), respectively, in low to moderate yields of 18–42%. In the case of trans-vinylene dipyridine, the stoichiometry of the starting materials and the choice of solvent were found to significantly influence product formation. The expected 1:1 adduct 6 was only isolated when [Cu(acac)₂] was heated in benzene with an excess of ligand (5 eq.) (Figure 1). Using a stoichiometry of 1:2 in the reaction under similar conditions instead compound 7, a 3:2 adduct of [Cu(acac)₂]:ligand (Figure 1) was isolated in low yield. This reaction was reproducible and confirmed by microanalysis from an experiment where 7 was frozen out of the reaction mixture by storing the mixture at 8 °C, and then warming it to room temperature. Changing the solvent to CHCl₃ resulted in the formation of crystallographically characterized compound 8 (see Section 2.2.10) despite a stoichiometric excess of heterocycle to [Cu(acac)₂] of 2:1. The presence of acidic hydrogen atoms in the solvent that are able to engage in hydrogen bonding interaction with the oxygen atoms of the acetylacetonate ligands (vide infra) may hereby influence the structural properties of the isolated complex and the stoichiometry of the crystallized product.

The instability of adducts with simple pyridine ligands has already been discussed. Consequently complexes 1–4 were found to be very labile and could only be obtained at low temperatures in the presence of excess ligand. Attempts to isolate bulk amounts of the complexes resulted in rapid decomposition due to the evaporation of the ligand and formation of uncoordinated [Cu(acac)₂] (see SI videos).

3.2. Structures of the Complexes

Pyridine, 4-methylpyridine, 4-pyrrolidinepyridine and 4-bromopyridine form with copper(II) acetylacetonate the monomeric, pentacoordinate adducts 1–4 that were all characterized by X-ray crystallography. 1,2-Di-(4-pyridyl)-ethylene forms a 3:2 adduct 7 with copper(II) acetylacetonate in which a central copper(II) acetylacetonate unit is coordinated to two molecules of 1,2-di-(4-pyridyl)-ethylene and another unit of copper(II) acetylacetonate is attached to the free coordination site of each ligand. Furthermore, both 1,2-di-(4-pyridyl)-ethylenes and 4,4′-bipyridines have been shown to form the one-dimensional coordination polymers 5 and 6, in which the central copper ion is hexacoordinate.

Compounds 1–4 show structural and spectroscopic features analogous to those of previously reported pentacoordinate adducts (Table S1). The coordination of the acetylacetonate ligands is preserved in all cases. The distance of the N atom of the pyridine derivatives to the central copper ion ranges from 2.206(2) Å to 2.323(1) Å for the pentacoordinate adducts and from 2.409(3) Å to 2.477(1) Å for the hexacoordinate species (

Table 2. Overview of pentacoordinate adducts of copper(II) acetylacetonate with investigated pyridine-based ligands. Cu-N distances and geometry index τ₅ [58] are included (τ₅ = 0: square-pyramidal, τ₅ = 1: trigonal-bipyramidal).

Complex	Coordinating Pyridine Ligand	Cu-N [Å] [a]	Cu-O [Å] [a]	τ5 [a]
1		2.216(1)	1.948(4)	0.04
2		2.27(2)	1.943(5)	0
3		2.206(2)	1.95(2)	0.41
4		2.292(1)	1.940(4)	0.03
7 (CN=5)		2.323(1)	1.938(5)	0.02
5		2.42(1)	1.959(8)	/
6		2.477(1)	1.955(4)	/
7 (CN=6)		2.467(5)	1.95(2)	/

^[a] mean value.

). All values are in the range of related pyridine adducts of copper(II) acetylacetonate (Tables S1 and S2). The distances of the oxygen atoms of the acetylacetonate ligand to the copper(II) ion are in all cases very similar ranging from 1.926(1) Å to 1.974(1) Å and close to the value of [Cu(acac)₂] (1.92 Å) consistent with an only weakly coordinated pyridine ligand. Larger deviations are also caused by the transition from square-pyramidal to trigonal-bipyramidal coordination geometry, whereby an oxygen atom from an acac ligand is positioned opposite to a pyridine derivative. Depending on the ligand, different coordination polyhedra are observed for the pentacoordinate species, which may be characterized by the geometry index τ₅ [58], which is a measure for the distortion of the coordination polyhedron, and will be discussed later. The complexes are all blue-green to green with absorption maxima in solution between 646 nm and 699 nm (Figures S1–S7).

One frequently discussed aspect of pentacoordinate copper(II) acetylacetonate adducts is the transition between a square-pyramidal and a trigonal-bipyramidal coordination geometry. As noted by other authors, predicting this structural preference is highly dependent on multiple factors, making a reliable assessment challenging without computational methods [20].

In general, while the trigonal-bipyramidal arrangement is often considered the energetically most favorable conformation [59], the energy difference between this geometry and the square-pyramidal configuration is typically small. As a result, a rapid interconversion between the two geometries is possible [60,61].

An examination of the previously reported pentacoordinate complexes and the newly presented adducts reveals that, in most cases, a square-pyramidal coordination geometry is observed (Table S1). However, as the Cu–N bond length decreases, the coordination environment gradually distorts from a square-pyramidal to a trigonal-bipyramidal arrangement. The most pronounced distortion is observed in adducts formed by 4-dimethylaminopyridine, 4-pyrrolidinylpyridine and 4-aminopyridine. The increased electron density and basicity of these derivatives (

Table 3. Summary of selected pyridine ligands with their structural and chemical properties: Cu–N distance within [Cu(acac)₂] adduct and temperature of experiment, geometry parameter τ₅ and donor strength of nitrogen ligand N.

	Cu-N [Å]	T [K]	τ5 [a]	N [b]	Reference (CCDC Code)
	2.12	150	0.86	15.8	[16] (608513)
	2.01	150	1.12	15.2	[14] (769010)
	2.21	100	0.41	15.9	this work
	2.22	100	0.04	12.9	this work
	2.27	100	0	13.7	this work

^[a] [58] ^[b] [62].

), attributed to their electron-donating substituents on the aromatic ring, enhance their nucleophilicity compared to pyridine or 4-picoline, thereby influencing the coordination geometry.

When relating this phenomenon to the original geometry of the [Cu(acac)₂] complex, the effect can be understood as follows: At large Cu–N distances, the additional lone pair of the donor ligand has little influence on the geometry of the central copper ion, resulting in a minimal distortion of the inherently preferred square-planar coordination geometry of [Cu(acac)₂].

As the Cu–N distance decreases (around 2.20 Å), the overlap with the d-orbitals of the metal ion increases progressively. This transition occurs via the square-pyramidal geometry, which is structurally closest to the fundamental square-planar coordination environment of [Cu(acac)₂]. Ligands bearing electron-donating substituents enhance electron density at the metal center, thereby facilitating greater electron donation. Consequently, these ligands promote a further shift in the coordination geometry toward the optimal arrangement for pentacoordinate metal ions (Figure 2).

In the following, representative structures will be discussed in more detail. The structurally analogous pentacoordinate adducts are exemplified by compound 1. Additionally, the structures of the 3:2 adduct 7 and the polymers 5 and 6 are discussed. Furthermore, the intermolecular interactions in the solid state are identified and compared.

3.3. Structure of Complex 1 to 4

The pyridine complex 1 crystallizes in the triclinic space group P-1 with one molecule in the asymmetric unit (Figure 3). The complex is composed of one pyridine molecule and one [Cu(acac)₂] molecule, with the coordination of pyridine to the copper ion resulting in a pentacoordinate geometry. The geometry index τ₅ = 0.038 confirms the presence of a slightly distorted square pyramid. The copper atom is located 0.23 Å above the basal mean plane formed by the O atoms of the acac ligands. In addition to the previously discussed distinctions between trigonal-bipyramidal and square-planar geometries of the respective adducts, the structure of complex 1 presented here exemplifies the structural characteristics of pentacoordinate complexes 2–4. Notably, all complexes exhibit distinct intermolecular interaction patterns, which are analyzed in the following section.

Compound 1 forms a supramolecular network in the solid state, that is stabilized not only by van der Waals interactions but also by intermolecular C–H···O contacts (Figure 4A). Two strands of the adducts align in an antiparallel, head-to-tail fashion. Within each strand, the adducts are held together by interactions between the para-C–H bond of a pyridine ligand and two oxygen atoms of an adjacent acac ligand (2.69 Å and 2.63 Å; dashed green lines). The interaction between the two strands is mediated by short contacts between the meta-C–H bond of a pyridine ligand and an oxygen atom of a neighboring acac ligand (2.52 Å; dashed orange line). Even though the planes of the pyridine ligands are parallel to each other, they are too far apart for π–π interactions (centroid···centroid: 6.13 Å, shift distance: 6.03 Å).

In adduct 2, substitution of the H atom of the para-C–H bond of pyridine with a methyl group eliminates the bridging interactions with adjacent acac ligands observed in the previous structure. Instead, the molecules are arranged in layers (Figure 4B), in which intermolecular interactions within each layer are mediated by C–H···O (2.71 Å; orange) and C–H···N (2.78 Å; green) contacts between the meta-C–H bond of the picoline ligand and neighboring oxygen atoms of the acac ligand. Notably, the aromatic ligands within a single layer are oriented orthogonally to one another and each adduct is oriented head-to-tail (centroid···centroid: 4.82 Å, shortest C–H···plane: 2.74 Å). The layers themselves are predominantly connected via van der Waals interactions of the methyl groups.

In the solid state, the molecules of compound 3 are arranged in layers, which are stabilized by van der Waals interactions between the methyl groups of the acac ligands. The molecules within a single layer are aligned head-to-tail, and the substituents of the pyridine ligands are arranged parallel to each other. Within a single layer (Figure 5A), C–H···O interactions are observed between the meta-C–H protons of the pyridine ligand and the oxygen atoms of the acac ligands (2.59 Å; orange). Additional C–H···O interactions are formed between the methylene groups of the pyrrolidine moiety and the oxygen atoms of the acac ligands (2.40 Å; green). Despite the nearly parallel alignment of the heteroarenes, the distances between them are too large for π–π interactions (centroid···centroid: 8.20 Å, shift distance: 8.03 Å).

The intermolecular interactions in the solid-state structure of compound 4 differ markedly from those in complex 1, primarily due to the absence of the para-C–H bond in the pyridine ligand. Instead, C–H···O interactions (2.73–3.06 Å) involving the meta-C–H bonds of the pyridine ligand and oxygen atoms of the acac ligands generate a three-dimensional network linking the individual adducts (Figure 5B). Furthermore, the slightly offset arrangement of the adducts allows the methyl groups of the acac ligands to orient around the bromo substituent, giving rise to additional stabilizing C–H···Br interactions. Although the planes of the aromatic compounds are parallel to each other, they are too far apart to allow interaction (centroid···centroid: 5.96 Å, shift distance: 5.72 Å).

3.4. Structure of Coordination Polymer 5

Although one-dimensional coordination polymers with bipyridine ligands have been reported previously [25,28,63], the structure was re-determined at 100 K to allow for a better comparison of experimental data. Polymer 5 crystallizes in the monoclinic space group P2₁ with two independent molecules in the asymmetric unit (Figure 6). Each nitrogen atom of the bipyridine ligands coordinates to a copper(II) ion, forming one-dimensional polymer chains. The average Cu–N bond length is 2.42(1) Å, similar to that observed for other hexacoordinate complexes (see SI).

In the solid-state structure of polymer 5, the polymer chains are aligned parallel to each other forming an approximate plane perpendicular to the [Cu(acac)₂]-fragment. The chains are arranged in layers, which are within the plane stabilized by van der Waals interactions involving the methyl groups of the acac ligands. The two shortest interchain Cu···Cu’ distances are 7.9717(5) Å and 8.0002(5) Å. Interchain connectivity (Figure 7) is further supported by short contacts between the meta-C–H bonds of the bipyridine ligands and the oxygen atoms of adjacent acac ligands (2.46–2.77 Å, orange). Although the two pyridine rings within each bipyridine ligand adopt a twisted conformation, neighboring pyridine rings from adjacent chains are nearly parallel, exhibiting a modest dihedral angle of approximately 11°, which may facilitate weak π–π interactions (centroid···centroid: 5.67 Å, shift distance: 5.53 Å). Adjacent planes are connected via interactions between the methyl groups of the acac ligand and the pyridyl groups.

3.5. Structure of Coordination Polymer 6

Coordination polymer 6 crystallizes in the triclinic space group P-1 with half a molecule in the asymmetric unit (Figure 8). The central copper(II) ion features an octahedral coordination environment. The Cu–N bond length is 2.477(1) Å, which is comparable to the Cu–N distances observed in coordination polymer 5 and other related hexacoordinate [Cu(acac)₂] complexes.

In the solid-state structure, the polymer chains are arranged parallel to each other (Figure 9). The shortest Cu···Cu’ distance is equivalent to the cell parameter a. Interchain interactions are mediated by van der Waals forces and also involve contacts between the C–H bonds of the ethylene linkers and the oxygen atoms of the acac ligands (2.53 Å and 2.80 Å, green). These interactions effectively link the polymer chains. Interestingly, aside from van der Waals interactions, no significant short contacts are observed between the C–H bonds of the pyridine moieties and the heteroatoms of the polymer, which is a distinct contrast to the other presented compounds, especially complex 5. It appears that the interaction of the C-H bonds of the ethylene unit with adjacent oxygen atoms is favored in this instance. Moreover, the pyridine rings of neighboring polymer chains are parallel, with a short interplanar distance of 2.34 Å (centroid···centroid: 5.44 Å, shift distance: 4.91 Å) and a larger interplanar distance of 2.98 Å (centroid···centroid: 4.66 Å, shift distance: 3.58 Å), indicating the potential for π–π stacking interactions between the aromatic systems.

3.6. Structure of Oligomer 7

Oligomer 7 crystallizes in the triclinic space group P-1 and consists of two half independent molecules, each composed of three molecules [Cu(acac)₂] and two molecules 4,4′-vinylenedipyridine (Figure 10). 4,4′-Vinylenedipyridine acts as a bridging ligand, with the central [Cu(acac)₂] molecule linked to each of the terminal [Cu(acac)₂] molecules by a single bridging ligand.

The geometry index τ₅ for the pentacoordinate copper centers are similar for both independent molecules (τ₅ = 0.00 and 0.05) and indicates always a distorted square-pyramidal geometry. The average Cu-N distance involving the pyridine nitrogen atoms is 2.323(1) Å, which is longer than that observed for the unsubstituted pyridine adduct. The Cu–N bond lengths in the hexacoordinate central unit are significantly longer (2.462(1) Å and 2.472(1) Å) than those in the pentacoordinate sites, consistent with the larger coordination number of the central copper(II) ion.

In the solid-state structure of complex 7, the molecules are aligned in parallel strands that are stabilized primarily by van der Waals interactions involving the methyl groups of the terminal [Cu(acac)₂] units (Figure 11). The van der Waals forces are not confined to interactions within the strands but are also part of the interstrand forces. Furthermore, significant interstrand interactions are mediated by the oxygen atoms of the acac ligands. There are short contacts between the ortho- and meta-C–H bonds of the pyridine rings (2.78–3.06 Å, orange), as well as the C–H bonds of the ethylene linkers (2.62 Å and 2.63 Å, green) and nearby oxygen atoms. For π–π interactions to form, the nearest aromatic groups are too far apart, and the planes are too close together (centroid···centroid: 6.84 Å, shift distance: 6.84 Å).

3.7. Hirshfeld Surface Analysis

To further characterize the intermolecular interactions, both Hirshfeld surface analysis and the corresponding fingerprint plots were employed. Representative structures with a single molecule in the asymmetric unit, namely complexes 1, 3 and 4, were selected for a detailed discussion. The surfaces and plots were generated using the CrystalExplorer 21 software [64].

The d_norm surface (Figure 12A) of compound 1 reveals the presence of C–H···O contacts between the meta-positioned C–H groups and the oxygen atoms of the acac ligand (Figure 12A, i and ii). These interactions, as previously illustrated in Figure 4A, govern the molecular packing in the solid state. The fingerprint plots indicate that H···H interactions are predominant (Figure 12D), primarily arising from contacts between methyl groups. In addition to the characteristic alkane pattern (d_e = d_i ≈ 1.2 Å) the plot also displays shorter, less intense, contacts around 1.1 Å, a feature commonly associated with polycyclic aromatic systems. The absence of significant π–π interactions is evident in the C···C plot (Figure 12C), where interactions near d_e = d_i ≈ 1.8 Å are negligible. The C···H plot (Figure 12E), while exhibiting a typical wing-like pattern suggestive of C–H···π interactions, lacks the prominence usually observed in aromatic systems or ethenes. The plot corresponding to H···O contacts (Figure 12F) reflects the previously discussed interactions involving the meta- and para-C–H groups of the pyridine ring and the oxygen atoms of the acac ligand.

In complex 3, the d_norm surface (Figure S18A) highlights not only the C–H···O interactions already identified in compound 1 but also shows the involvement of the methylene group of the pyrrolidine substituent. As was the case for compound 1, the C···C fingerprint plot (Figure S18C) reveals, in agreement with earlier observations, no significant evidence for π–π stacking interactions. The intermolecular interactions in complex 3 are likewise dominated by H···H contacts (Figure S18D), that may be attributed to van der Waals forces from either the methyl groups of the acac ligands or the methylene groups of the pyrrolidine moiety. The features typically associated with C–H···π interactions are essentially absent from the C···H plot (Figure S18E). The H···O plot (Figure S18F) corroborates the contacts already inferred from the d_norm surface, particularly those involving the meta-C–H bonds of the pyridine ring and the methylene groups of the pyrrolidine substituents.

The fingerprint plots and the d_norm surface of complex 4 closely resemble those of the two previously discussed structures (Figure S19A). A strong dominance of H···H contacts (Figure S19D) is again evident, underscoring their primary role in governing the intermolecular interactions. C–H···O interactions, mediated by the oxygen atoms of the acac ligand and the C–H groups of the pyridine ring, are also present. Notably, a more pronounced contribution (Figure S19C) from C···C contacts (d_e = d_i ≈ 1.8 Å) may indicate the presence of π–π interactions, consistent with the layered architecture of the compound. However, even this contribution remains comparatively minor when considered in the broader context of all interactions.

A comprehensive analysis (Figure 13) of the relative contributions of each contact type to the overall intermolecular interaction profile reveals that H···H contacts are by far the most dominant. This is unsurprising given the abundance of potential van der Waals interactions. Although C···H and O···H contacts also make significant contributions, their combined presence still falls short of the proportion attributed to H···H contacts. In the structure of adduct 4 are additional Br···H interactions from the spatial arrangement of methyl groups around the bromine substituent. All other interaction types contribute for less than 10% of the total and were therefore not further subdivided. The share of C···C contacts, potentially indicative of π–π stacking, accounts for less than 1%, and may therefore be ignored in the context of intermolecular binding.