6-Amino-3-(prop-2-en-1-yl)-9 H -purin-3-ium Tetracopper(I) Hexabromide: Synthesis and X-ray Structure Determination

: 6-Amino-3-(prop-2-en-1-yl)-9 H -purin-3-ium tetracopper(I) hexabromide [(C 5 H 5 N 5 (C 3 H 5 )) 2 Cu 4 Br 6 ] ( 1 ) is synthesized in this paper. The title compound is characterized by Raman spectroscopy and single crystal X-ray diffraction: monoclinic, Sp.gr. P 2 1 /c, Z = 2, unit cell dimensions: a = 8.6870(3), b = 9.7447(2), c = 16.4418(4) Å, β = 100.681(3) ◦ , V = 1367.72(7) Å 3 at 150 K. The peculiarities of the metal coordination and distribution of weak interactions are analyzed. The signiﬁcant role of hydrogen bonds in the construction of 1 crystal structure is shown.


Introduction
Both metallic copper and copper compounds are widely used in many branches of modern science and technology [1]. In turn, copper-containing complexes with unsaturated olefin ligands are attractive as catalysts in fine organic synthesis [2,3] in terms of their unique biochemical role [4] and interesting physical properties [5], as well as productive tools in the crystal engineering of coordination compounds [6,7], etc. In recent years, close attention has been paid to the crystal structure and properties of Cu(I) π-complexes with allyl-substituted heterocyclic ligands due to the possibility of the synchronous coordination of one metal atom with several organic linkers and the subsequent emergence of unique topological units [8][9][10].
However, adenine as a purine base is directly involved in the construction of nucleotides of nucleic acids (DNA and RNA). The mentioned heterocyclic nucleus incorporated in various derivatives, primarily attracts the attention of researchers due to its indirect participation in the transmission of hereditary information [11], its possible control of microorganisms, and the therapeutic effect of these microorganisms in medical practice [12,13]. Moreover, due to the presence of several coordination centers, the adenine nucleus is very interesting in the context of implementing diverse possibilities for coordination with metal ions. It is possible for all donor N atoms of N-substituted adenine to act as Lewis bases and to form bonds with metal ions. However, in the coordination compounds of copper(II), cobalt(III) and other d-metal ions with N3-substituted adenine ligands, the coordination of the metal atom is involved only in the N7 atom of the heterocyclic nucleus [14,15]. A modification of the adenine ring by the allyl group would also significantly extend its coordination behavior regarding Cu(I), forming a strongly directed Cu(I)-(C=C) interaction.
The linkage role of the two halogen atoms Br(3) causes the formation of dimers {Cu2Br6}. This is quite common among inorganic fragments in copper halide coordination compounds. These units occurs as isolated anions of the same composition [17] or as part of polyanions (for example, linear {CuBr2}n n− [18], or more complex 2D, or even 3D skeletons [19]). The arrangement of metal coordination tetrahedra in these kind of building blocks, in our opinion, depends mainly on steric hindrances. In the case of isolated anions [Cu2Br6] 2− , the copper polyhedra are as far apart as possible and the torsion angle Br-Cu-Br-Cu is 0.0° (the same value is observed in the structure of the discussed compound). Instead, the inclusion of {Cu2Br6} as the topological block in the three-dimensional framework slightly shifts the tetrahedra relative to each other (the value of the torsion angle increases, for example, to about 15 ° in the complicated fragment {Cu9Cl16} [20]).
Another halogen atom-Br(1)-acts as a bridge between the polyhedra of the crystallographically independent Cu(1) and Cu(2) atoms. As a result, the topological unit of the composition {[C5H5N5(C3H5)Cu2Br3]2} ( Figure 1) plays a key role in constructing the crystal structure of the discussed compound.

Results and Discussion
The Cu1 atom in the crystal structure of [(C 5 H 5 N 5 (C 3 H 5 )) 2 Cu 4 Br 6 ] (1) is located in a planar-trigonal coordination environment built from a nitrogen atom of a five-membered heterocyclic core of adenine ligands (distance Cu-N 1.978(4) Å), C=C bond of the allyl group of the same cation (Cu-C distances are 2.039(4)-2.059(5) Å, Cu-m-1.932(1) Å (m-midpoint of C=C bond of allyl group), Table 1), and bromine atoms (Cu-Br-2.321(1) Å). Another crystallographically independent Cu2 metal atom forms a slightly deformed tetrahedral environment (τ 4 = 0.93, [16])) from the bromine atoms (Cu-Br-2.462(1)-2.499(1) Å).  (1) Br (2) The linkage role of the two halogen atoms Br(3) causes the formation of dimers {Cu 2 Br 6 }. This is quite common among inorganic fragments in copper halide coordination compounds. These units occurs as isolated anions of the same composition [17] or as part of polyanions (for example, linear {CuBr 2 } n n− [18], or more complex 2D, or even 3D skeletons [19]). The arrangement of metal coordination tetrahedra in these kind of building blocks, in our opinion, depends mainly on steric hindrances. In the case of isolated anions [Cu 2 Br 6 ] 2− , the copper polyhedra are as far apart as possible and the torsion angle Br-Cu-Br-Cu is 0.0 • (the same value is observed in the structure of the discussed compound). Instead, the inclusion of {Cu 2 Br 6 } as the topological block in the three-dimensional framework slightly shifts the tetrahedra relative to each other (the value of the torsion angle increases, for example, to about 15 • in the complicated fragment {Cu 9 Cl 16 } [20]).
Another halogen atom-Br(1)-acts as a bridge between the polyhedra of the crystallographically independent Cu(1) and Cu(2) atoms. As a result, the topological unit of the composition {[C 5 H 5 N 5 (C 3 H 5 )Cu 2 Br 3 ] 2 } (Figure 1) plays a key role in constructing the crystal structure of the discussed compound. work slightly shifts the tetrahedra relative to each other (the value of the torsion angle increases, for example, to about 15 ° in the complicated fragment {Cu9Cl16} [20]).
Another halogen atom-Br(1)-acts as a bridge between the polyhedra of the crystallographically independent Cu(1) and Cu(2) atoms. As a result, the topological unit of the composition {[C5H5N5(C3H5)Cu2Br3]2} ( Figure 1) plays a key role in constructing the crystal structure of the discussed compound.  Isolated topological units with formal stoichiometry L 2 Cu 4 Hal 6 are also common among coordination compounds of copper and, for example, were observed in the structure of the [(3,2-pypzpym) 2 (Cu 4 I 6 )] n compound [21] based on 1-methyl-3-(1-(pyridin-2-yl)-1Hpyrazol-3-yl)pyridin-1-ium. However, according to the Cambridge Structural Database [22], compound 1 is the first known example of a structure with the {Cu 4 Hal 6 } motif, which is π-attached (through the Cu I -(C=C) interaction) to the olefin group of the π,σ-ligand. A somewhat similar copper(I) halide fragment has previously been observed only in π-ligands, such as N-allylpiperazinium [23] and N-propargylamonium [24].
Based on the presence in the crystal structure of the compound [AllAd 2 Cu 4 Br 6 ] isolated topological units with {[C 5 H 5 N 5 (C 3 H 5 )Cu 2 Br 3 ] 2 } composition are important to explore the supramolecular interactions that bind these blocks in a three-dimensional framework. The analysis of the geometry of hydrogen contacts (Table 2) displays the presence of quite noticeable interactions where the acceptors are nitrogen (of the adenine nucleus of the ligand) and carbon atoms (from the allyl group of the ligand), and donor-bromine anions. The corresponding bonds are formed both within the {[C 5 H 5 N 5 (C 3 H 5 )Cu 2 Br 3 ] 2 } dimerand between individual topological blocks by connecting them into a three-dimensional grid. The Hirschfield surface analysis confirms the conclusion that they play an important role in the construction of the structure, since the total contribution to the surface area for X-H···Br contacts is 39.2% (Figure 2).
Crystals of the title compound (1) were obtained under the alternating-current electrochemical technique [26] starting from an acetone solution of copper(II) bromide and 3-(2-propen-1-yl)-3H-purin-6-amine, which was acidified by two drops of 48% hydrobromic acid. The mixture was carefully stirred and then was placed into a small test tube with subsequent alternating-current reduction (frequency 50 Hz, voltage 1.0 V) and after 10 days only few yellowish crystals of the title compound appeared on the copper wire electrodes (Scheme 1).
Diffraction data for the 1 single crystal were collected on an Agilent Gemini A fourcircle diffractometer with an Atlas CCD detector. The collected data were processed with the CrysAlis PRO program (version 1.171.40.57a) [27]. The structure was solved by ShelXT and refined by the least squares method on F 2 by ShelXL software with the following graphical user interface of OLEX 2 [28][29][30]. Atomic displacements for non-hydrogen atoms were refined using an anisotropic model. N-bound hydrogen atoms were derived from difference Fourier synthesis and refined isotropically. Other hydrogen atoms were placed on geometrically calculated positions and refined as riding atoms with relative isotropic displacement parameters. The figures were prepared using DIAMOND 3.1 software (Bonn, Germany).
The Platon program was used to analyze the geometric features of the distribution of weak interactions [31]. The Hirschfield surface for the structure [C5H5N5(C3H5)Cu2Br3] was constructed using the Crystal Explorer software package [32].
Crystals of the title compound (1) were obtained under the alternating-current electrochemical technique [26] starting from an acetone solution of copper(II) bromide and 3-(2-propen-1-yl)-3H-purin-6-amine, which was acidified by two drops of 48% hydrobromic acid. The mixture was carefully stirred and then was placed into a small test tube with subsequent alternating-current reduction (frequency 50 Hz, voltage 1.0 V) and after 10 days only few yellowish crystals of the title compound appeared on the copper wire electrodes (Scheme 1).
Diffraction data for the 1 single crystal were collected on an Agilent Gemini A fourcircle diffractometer with an Atlas CCD detector. The collected data were processed with the CrysAlis PRO program (version 1.171.40.57a) [27]. The structure was solved by ShelXT and refined by the least squares method on F 2 by ShelXL software with the following graphical user interface of OLEX 2 [28][29][30]. Atomic displacements for non-hydrogen atoms were refined using an anisotropic model. N-bound hydrogen atoms were derived from difference Fourier synthesis and refined isotropically. Other hydrogen atoms were placed on geometrically calculated positions and refined as riding atoms with relative isotropic displacement parameters. The figures were prepared using DIAMOND 3.1 software (Bonn, Germany).
Raman spectrum from 1 (see supplementary materials) was recorded with a Horiba Jobin-Yvon LabRAM HR spectrometer with the use of the 632.81 nm excitation line of a He-Ne laser (17 mW). Before data recording, the instrument scale was calibrated using a silicon polycrystalline plate (a characteristic band at 521 cm −1 ).