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Supramolecular solid-state patterns generated by hydrogen bonding and π–π stacking interactions in the mononuclear Cr(III) complexes
Polyhedron 102 (2015) 410–416
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Supramolecular solid-state patterns generated by hydrogen bonding and p–p stacking interactions in the mononuclear Cr(III) complexes Elena Melnic a, Ecaterina Tocana b, Anatolii V. Siminel a, Lilia Croitor a,⇑ a b
Institute of Applied Physics, Academy of Sciences of R. Moldova, Academy Str., 5, MD2028 Chisinau, Republic of Moldova Tiraspol State University, Iablocikin Str., 5, MD2069 Chisinau, Republic of Moldova
a r t i c l e
i n f o
Article history: Received 29 June 2015 Accepted 28 September 2015 Available online 13 October 2015 Keywords: X-ray Cr3+ Aromatic ligand Supramolecular pattern Luminescence
a b s t r a c t Three new mononuclear Cr(III) complexes of formulas [Cr(acac)2(bipy)]NO32H2O (1), [Cr(acac)2(phen)] NO32H2O (2) and [Cr(acac)2(phen)]ClO40.25H2O (3), (Hacac = acetylacetonate, bipy = 2,20 -bipyridine and phen = 1,10-phenanthroline) were synthesized to illustrate the role of the non-covalent interactions in sustaining supramolecular solid-state architectures. In all structures the Cr3+ cation is coordinated by one bidentate N-donor ligand and two acetylacetonate anions within a slightly distorted N2O4 octahedron. These ionic complexes are arranged into 1D supramolecular structures by combination of p–p aromatic face-to-face and offset stacking interactions, as well as O–H O hydrogen bond interactions. The luminescence studies show that compounds 1–3 display blue luminescence upon excitation with ultraviolet light. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays a pronounced interest has been focused on the crystal engineering of supramolecular ensembles established by combination of coordinative and weaker non-covalent interactions (e.g. hydrogen bonds, p–p stacking), which play an essential role in the fields from molecular biology to crystal engineering [1–6]. The design and construction of innovative metallo-supramolecular assemblies based on the non-covalent interactions such as p–p stacking and hydrogen bonding may adjust the dimensionality and lead to new topologies with desired functions [3,4,7,8]. It is known that the introduction of polycyclic aromatic chelating ligands containing N-donors, like 2,20 -bipyridine (bipy) and 1,10-phenanthroline (phen) into the metal-acetylacetonato systems prevent the formation of higher dimensional coordination networks and may provide supramolecular recognition sites for p–p, C–H–p stacking and C–H O hydrogen bonding to form supramolecular structures [8–12]. The phen molecule coordinating to a metal ion forms a large planar system of four fused rings: two pyridine fragments, one C6-ring and one chelate ring [13], the smaller bipy ligand, which comprises three rings with metal ion (two pyridine and one chelate rings) have a tendency to form stacking interactions with the p-systems of various aromatic groups. The tendency for stacking interactions to occur is an ⇑ Corresponding author. Tel.: +373 22 738154; fax: +373 22 725887. E-mail address: [email protected] (L. Croitor). http://dx.doi.org/10.1016/j.poly.2015.09.069 0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.
important issue when using phenanthroline complexes in various fields. For example, it is known that these complexes interact with DNA by intercalating between base pairs of DNA [14]. Another crucial factor for introducing in synthesis of these chelating ligands is defined by the conjugated p-systems containing aromatic rings that are currently of interests in the development of fluorescent materials [15–18]. The solvent molecules and the counterions influence also the final supramolecular architecture. The noncoordinating anions play not only a charge-compensating role, but they can dramatically influence the overall solid-state architecture through their templating function [5,8]. Chromium is an important heavy metal that is widely used in industrial processes. It is also an essential micronutrient for proper glucose metabolism that stimulates the enzyme system and stabilizes nucleic acids [19]. It has been suggested that some coordinative complexes are able to migrate from the cytoplasm to the nucleus and attack the DNA structure [20]. For example, Cr in the most stable oxidation state Cr(III), is considered to interact with the DNA molecule [21–23]. Also, chromium(III) polypyridines are excellent materials for luminescence probes [24]. Here we report the supramolecular systems constructed by combining two or three organizing forces: metal-coordination, hydrogen bonds and p–p stacking interactions. Three Cr(III) mononuclear complexes with the compositions [Cr(acac)2(bipy)] NO32H2O (1), [Cr(acac)2(phen)]NO32H2O (2) and [Cr(acac)2(phen)] ClO40.25H2O (3), Hacac = acetylacetone, bipy = 2,20 -bipyridine and phen = 1,10-phenanthroline (Scheme 1), were synthesized and
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for C22H26N3O9Cr: C, 49.99; H, 4.96; N, 7.95. Found: C, 49.92; H, 4.88; N, 7.87%.
Scheme 1. Schematic illustration of the ligands with acronyms used in this study.
studied by X-ray method. The luminescence properties of compounds 1–3 were investigated.
2.2.3. Synthesis of [Cr(acac)2(phen)]ClO40.25H2O (3) Cr(ClO4)36H2O (35.2 mg, 0.1 mmol) and 1,10-phenanthroline (18.02 mg, 0.1 mmol) were dissolved in 6 mL of methanol. Then 8 drops of acetylacetone were added. The reaction mixture was stirred for 5 min, filtered off and then slowly cooled to room temperature giving brown crystals. Yield: 27%. Anal. Calc. for Table 2 Selected bond lengths (Å) and angles (°) in coordination metal environment in 1–3.
2. Experimental 2.1. Materials and methods All reagents and solvents were obtained from commercial sources and were used without further purification. Caution! Perchlorate salts are dangerous and should be handled with care and in only small quantities. Elemental analysis was performed on an Elementar Analysensysteme GmbH Vario El III elemental analyzer. Emission spectra were measured for monocrystals at room temperature on an Excitation YAG:Nd3+ laser, third harmonic generation, k = 355 nm, duration = 10 ns, time repetition 10 Hz. 2.2. Synthesis 2.2.1. Synthesis of [Cr(acac)2(bipy)]NO32H2O (1) Cr(NO3)39H2O (40.01 mg, 0.1 mmol) and 2,20 -bipyridine (15.61 mg, 0.1 mmol) were dissolved in 6 mL of methanol. Then 4 drops of acetylacetone were added. The reaction mixture was stirred for 5 min, filtered off and then slowly cooled to room temperature giving violet crystals. Yield: 80%. Anal. Calc. for C20H26N3O9Cr: C, 47.62; H, 5.19; N, 8.33. Found: C, 47.56; H, 5.11; N, 8.27%. 2.2.2. Synthesis of [Cr(acac)2(phen)]NO32H2O (2) Cr(NO3)39H2O (40.01 mg, 0.1 mmol) and 1,10-phenanthroline (18.02 mg, 0.1 mmol) were dissolved in 6 mL of methanol. Then 8 drops of acetylacetone were added. The reaction mixture was stirred for 5 min, filtered off and then slowly cooled to room temperature giving cherry-colored crystals. Yield: 75%. Anal. Calc.
1 Cr(1)AO(2) Cr(1)AO(3) Cr(1)AO(1) O(2)ACr(1)AO(3) O(2)ACr(1)AO(1) O(3)ACr(1)AO(1) O(2)ACr(1)AO(4) O(3)ACr(1)AO(4) O(1)ACr(1)AO(4) O(2)ACr(1)AN(1) O(3)ACr(1)AN(1)
1.926(3) 1.931(3) 1.954(3) 92.29(12) 91.72(13) 89.29(13) 89.38(13) 91.90(13) 178.36(13) 172.54(14) 94.80(14)
Cr(1)AO(4) Cr(1)AN(1) Cr(1)AN(2) O(1)ACr(1)AN(1) O(4)ACr(1)AN(1) O(2)ACr(1)AN(2) O(3)ACr(1)AN(2) O(1)ACr(1)AN(2) O(4)ACr(1)AN(2) N(1)ACr(1)AN(2)
1.959(3) 2.058(4) 2.073(3) 90.69(14) 88.07(14) 94.42(14) 173.18(14) 89.30(13) 89.39(13) 78.55(15)
2 Cr(1)AO(3) Cr(1)AO(2) Cr(1)AO(4) O(3)ACr(1)AO(2) O(3)ACr(1)AO(4) O(2)ACr(1)AO(4) O(3)ACr(1)AO(1) O(2)ACr(1)AO(1) O(4)ACr(1)AO(1) O(3)ACr(1)AN(2) O(2)ACr(1)AN(2)
1.931(2) 1.939(2) 1.949(2) 92.88(10) 91.84(10) 88.99(9) 89.32(10) 91.60(9) 178.66(9) 172.56(10) 94.21(10)
Cr(1)AO(1) Cr(1)AN(2) Cr(1)AN(1) O(4)ACr(1)AN(2) O(1)ACr(1)AN(2) O(3)ACr(1)AN(1) O(2)ACr(1)AN(1) O(4)ACr(1)AN(1) O(1)ACr(1)AN(1) N(2)ACr(1)AN(1)
1.955(2) 2.073(3) 2.079(3) 90.55(10) 88.21(10) 93.74(10) 173.27(10) 89.57(10) 89.71(10) 79.23(10)
3 Cr(1)AO(1) Cr(1)AO(2) Cr(1)AO(3) O(2)ACr(1)AO(3) O(2)ACr(1)AO(4) O(3)ACr(1)AO(4) O(2)ACr(1)AO(1) O(3)ACr(1)AO(1) O(4)ACr(1)AO(1) O(2)ACr(1)AN(2) O(3)ACr(1)AN(2)
1.955(4) 1.928(4) 1.931(4) 92.84(18) 88.70(16) 91.83(16) 91.93(16) 89.95(16) 178.08(16) 94.14(19) 172.81(18)
Cr(1)AO(4) Cr(1)AN(2) Cr(1)AN(1) O(4)ACr(1)AN(2) O(1)ACr(1)AN(2) O(2)ACr(1)AN(1) O(3)ACr(1)AN(1) O(4)ACr(1)AN(1) O(1)ACr(1)AN(1) N(2)ACr(1)AN(1)
1.947(4) 2.079(5) 2.084(4) 90.09(17) 88.05(17) 173.20(19) 93.77(19) 89.47(16) 89.70(16) 79.3(2)
Table 1 Crystal and structure refinement data for compounds 1–3.
Empirical formula Formula weight Crystal system Space group Z a (Å) b (Å) c (Å) b (°) V (Å3) Dcalc (g/cm3) l (mm1) F(0 0 0) Crystal size (mm) Reflections collected/unique (Rint) Reflections with [I > 2r(I)] Data/restraints/parameters GOF on F2 R1, wR2 [I > 2r(I)] R1, wR2 (all data)
1
2
3
C20H26N3O9Cr 504.44 monoclinic P2/c 4 14.0923(8) 7.6578(5) 23.717(2) 116.248(6) 2295.5(3) 1.460 0.554 1052 0.30 0.12 0.02 7433/4022 (0.0616) 2030 4022/13/302 0.967 0.0728, 0.0714 0.1562, 0.0878
C22H26N3O9Cr 528.46 monoclinic P2/c 4 15.0152(5) 7.6084(3) 23.3883(9) 116.651(3) 2388.04(15) 1.470 0.536 1100 0.50 0.20 0.08 7756/4169 (0.0269) 3317 4169/16/313 0.999 0.0483, 0.1278 0.0639, 0.1394
C22H22N2O8.50ClCr 537.87 monoclinic P21/c 4 7.5199(6) 21.456(2) 15.3344(8) 93.467(5) 2469.6(3) 1.447 0.622 1108 0.45 0.06 0.04 8474/4306 (0.0380) 2879 4306/82/336 0.999 0.0802, 0.2306 0.1211, 0.2607
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C22H22N2O8.50ClCr: C, 59.72; H, 5.01; N, 6.33. Found: C, 59.67; H, 4.97; N, 6.29%.
3. Results and discussion 3.1. Structural description
2.3. X-ray crystallography Diffraction measurements for 1–3 were carried out at room temperature on a Xcalibur ‘‘Oxford Diffraction” diffractometer equipped with CCD area detector and a graphite monochromator utilizing Mo Ka radiation. Final unit cell dimensions were obtained and refined on an entire data set. All calculations to solve the structures and to refine the proposed models were carried out with the programs SHELXS97 and SHELXL97 [25]. The structures were solved by direct methods and refined by full-matrix least-squares methods on F2 by using the SHELXL97 program package. All non-hydrogen atoms were refined anisotropically. For 3 the perchlorate anion is disordered over two positions (75/25% occupation). For the disordered solvent water molecule, hydrogen atoms were not localized. Hydrogen atoms attached to carbon atoms were placed in geometrically idealized positions and refined by using a riding model. In compounds 1 and 2 hydrogen atoms of solvated water molecules are disordered over two positions with the equal occupancies and the O–H and H H distances were restricted by the DFIX command. The disordered fragments were refined in anisotropic approximation using equal anisotropic displacement parameters for the similar atoms. The X-ray data and the details of the refinement for 1–3 are summarized in Table 1. Selected geometric parameters for 1–3 are given in Tables 2 and 3. The figures were produced using the MERCURY program [26].
Compound [Cr(acac)2(bipy)]NO32H2O (1) crystallizes in the monoclinic centrosymmetric P2/c (No. 13) space group. The crystal structure consists of the mononuclear [Cr(acac)2(bipy)]+ cation (Fig. 1a), NO 3 anion and two crystallization water molecules. The chromium(III) ion displays a distorted octahedral coordination involving two nitrogen atoms of the chelating bipy group and four oxygen atoms from two bidentate chelating acac ligands. The N–Cr–N bite angle in the chelating bipyridine moiety is 78.57 (15)°, and the average O–Cr–O bite angle in the acac chelate rings are equal to 91.71(13)° and 91.90(13)°. The four C@Oacac bond distances are experimentally equivalent, having values in range 1.275(5)–1.286(5) Å, and are consistent with significant electron delocalization over these bonds. The Cr(1)AN(1) bond distance (2.058(4) Å) is shorter than the Cr(1)AN(2) bond (2.073(3) Å) in the same ligand, suggesting that there is some twist in the bipy ring, the dihedral angle between the two pyridine rings equals to 3.05°. This ligand forms dihedral angles of 81.98° and 87.23° with the acac planes. In the crystal structure of 1 the chelate bipy ligands are stacked by the p–p interactions along the crystallographic b axis, giving supramolecular polymer (Fig. 1b). The centroid centroid separation between bipy rings (3.917 Å) and the interplanar separations indicate the offset stacking (3.468 and 3.469 Å). The Cr Cr separation through the stacked ligands is 7.854 Å. Crystal packing reveals
Table 3 Hydrogen bond distances (Å) and angles (°) for 1 and 2. DAH A
d(DAH)
d(H A)
d(D A)
<(DHA)
Symmetry transformation for acceptor
1 O(1w)AH(3w1) O(2w) O(1w)AH(2w1) O(1w) O(1w)AH(1w1) O(6) O(2w)AH(1w2) O(6) O(2w)AH(1w2) O(5) O(2w)AH(2w2) O(1w) O(2w)AH(3w2) O(2w) C(4)AH(4) O(1) C(14)AH(14C) O(2w) C(15)AH(15A) O(6) C(19)AH(19B) O(5)
0.85 0.88 0.88 0.87 0.87 0.84 0.86 0.93 0.96 0.96 0.96
1.95 2.06 2.07 2.20 2.45 2.11 1.82 2.62 2.65 2.61 2.47
2.748(6) 2.762(7) 2.828(7) 2.878(7) 3.309(7) 2.748(6) 2.670(7) 3.393(6) 3.416(5) 3.441(7) 3.358(6)
157.0 136.0 142.8 134.8 170.2 132.8 170.2 140.6 137.3 144.5 152.9
x, y + 1, z x, y, z + 1/2 x, y, z x, y, z x, y, z x, y 1, z x, y, z + 1/2 x, y, z x, y, z 1/2 x, y + 1, z 1/2 x + 1, y, z + 1/2
2 O(1w)AH(1w1) O(5) O(1w)AH(2w1) O(1w) O(1w)AH(3w1) O(2w) O(2w)AH(1w2) O(5) O(2w)AH(2w2) O(1w) O(2w)AH(3w2) O(2w) C(1)AH(1) O(5) C(9)AH(9) O(6) C(16)AH(16A) O(6)
0.86 0.96 0.89 0.81 1.00 1.13 0.93 0.93 0.96
2.22 1.85 1.84 2.25 1.90 1.97 2.61 2.60 2.57
2.915(6) 2.755(5) 2.700 2.901(6) 2.700 2.683(5) 3.367(6) 3.336(7) 3.491(6)
137.3 154.9 163.1 137.5 134.9 117.2 139.5 136.3 159.8
x, y, z x + 1, y, z + 1/2 x, y 1, z x, y, z x, y + 1, z x + 1, y, z + 1/2 x, y + 1, z + 1/2 x, y, z x, y 1, z
3 C(2)AH(2) O(7A) C(9)AH(9) O(6A) C(10)AH(10) O(5A) C(10)AH(10) O(6A) C(16)AH(16B) O(8) C(17)AH(17C) O(5A) C(19)AH(19) O(8A) C(21)AH(21A) O(6) C(21)AH(21A) O(7A) C(21)AH(21C) O(1) C(22)AH(22B) O(8A)
0.93 0.93 0.93 0.93 0.96 0.96 0.93 0.96 0.96 0.96 0.96
2.48 2.69 2.69 2.89 2.56 2.03 2.65 2.56 2.35 2.64 2.45
3.09(3) 3.36(3) 3.56(5) 3.47(3) 3.310(15) 2.83(3) 3.45(4) 3.260(10) 3.29(5) 3.593(8) 3.36(3)
123.6 130.1 155.5 122.0 135.0 138.4 144.6 130.0 166.4 176.0 158.0
x 1, y, z x + 1, y 1/2, z + 1/2 x + 1, y 1/2, z + 1/2 x + 1, y 1/2, z + 1/2 x, y 1/2, z + 1/2 x + 1, y 1/2, z + 1/2 x, y + 3/2, z 1/2 x, y, z x, y, z x + 1, y, z x, y + 3/2, z 1/2
E. Melnic et al. / Polyhedron 102 (2015) 410–416
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Fig. 1. View of the molecular structure of mononuclear [Cr(acac)2(bipy)]+ cation with partial atomic numbering scheme (a); the stacking of the mononuclear cations linked by p–p interactions between the aromatic systems (b); crystal packing in 1 (c) and the infinite one-dimensional hydrogen-bonding tape combining the tetrameric water clusters and NO 3 anions along b axis. Only one position is shown for the disordered H-atoms in the water molecules (d). C-bound H atoms are omitted for clarity.
Fig. 2. The molecular structure of mononuclear [Cr(acac)2(phen)]+ cation in 2 and 3 with atomic numbering scheme.
that the hydrophilic outer sphere groups (nitrate anion and water molecules) are interconnected through the hydrogen bonds (Fig. 1c, Table 3). The hydrogen bonds between the two water molecules generate a tetrameric water cluster, where each water
Fig. 3. View of the stacking of the mononuclear [Cr(acac)2(phen)]+ cations linked by p–p interactions of the aromatic systems in 2 (a) and 3 (b). C-bound H atoms are omitted for clarity.
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Table 4 Selected geometric parameters for stacking modes of 1,10-phenanthroline rings including centroid-to-centroid (C-to-C), plane-to-plane (P-to-P) and centroid-to-plane (C-to-P) distances (Å) in reported compounds. Compound
p–p stacking interaction
C-to-C distance
P-to-P distance
C-to-P distance (av)
Reference
Present work Present work
2
Ph–Ph
3.738
3.458
3.465
3
Ph–Ph
3.835, 3.713 3.813, 3.846 3.772, 3.851 3.764 3.776, 3.810
3.465, 3.446
3.455
3.117, 3.311 3.440 3.111, 3.268
3.063
[21]
3.614 3.055
[22] [23]
Py–Ph a
HPENCR
Py–Py
SACSULa XPHOCRa
Py–Ph Py–Py
a HPENCR = bis(l2-hydroxo)-tetrakis(1,10-phenanthroline-N,N0 )-di-chromium(III) tetrachloride hexahydrate; SACSUL = (l2-oxalato-O,O0 ,O00 )-azido-(oxalato-O,O0 )-tris (1,10-phenanthroilne)-chromium(III)-manganese(II) monohydrate; XPHOCR = bis (l2-hydroxo)-bis(bis(1,10-phenanthroline)-chromium(III)) tetraiodide tetrahydrate.
molecule acts as a single double hydrogen bond donor and single acceptor. The nitrate anions interconnect further water clusters in one-dimensional chain forming through the alternating R44(8) and R46(12) H-bonded patterns [27] (Table 3, Fig. 1c and d). Weak C–H O intermolecular hydrogen bonds make additional impact in stabilization of crystal structure. The principal of them are presented in Table 3. Notably, the following criteria were used for search in the Cambridge Structural Database (ConQuest Version 1.17): (a) centroid–centroid distance between two pyridine fragments was constrained to limits 3.3–3.9 Å; (b) bond types – aromatic; (c) H atoms were not specified; (d) no polymer structures. Only in three examples from 140 discrete compounds built up from Cr(III) atom
and 2,20 -bipyridine have been found p–p interactions between delocalized p systems, namely (l3-oxo)-bis(l2-benzoato-O,O0 )-bis (l2-ethanolato)-tris(isothiocyanato)-bis(2,20 -bipyridine)-tri-chromium(III) [28], (l3-chromato)-hexakis(2,20 -bipyridyl)-tri-copper (II) tetraperchlorate monohydrate [29] and (l2-oxido)-tetrakis (2,20 -bipyridine)-bis(isothiocyanate)-di-chromium diperchlorate hydrate [30]. In all these compounds the interplanar separations between bipy rings indicate the offset stackings. In the first two examples, the stacking is observed among one pyridine ring, while in the last example and our compound 1 p–p interactions have been found between both delocalized p systems. Compounds [Cr(acac)2(phen)]NO32H2O (2) and [Cr(acac)2 (phen)]ClO40.25H2O (3) were obtained similarly and crystallize in the monoclinic centrosymmetric P2/c (No. 13) (2) and P21/c (No. 14) (3) space groups. As it is evidenced from comparison of unit cell dimensions and crystal systems compounds 1 and 2 are isomorphous with consequential increase of unit cell volume in 2 against 1 (Table 1). The crystal structures of 2 and 3 contain the similar mononuclear [Cr(acac)2(phen)]+ cation (Fig. 2) and nitrate anion and two solvated water molecules in 2 and perchlorate ion and solvated water molecule in 3 refined with 0.25 occupancy. Similar to 1, the chromium atoms coordinate with two nitrogen atoms of a chelating phen group and four oxygen atoms from two bidentate chelating acac ligands to form a distorted N2O4 octahedral geometry. The N–Cr–N bite angle in the phen moiety is 79.23(10)° in 2 and 79.3(2)° in 3, and the interval of O–Cr–O bite angle in the acca chelate rings is in the range 91.60(9)–91.93 (16)°. The four C@Oacac bond distances are experimentally equivalent, having values in range 1.267(7)–1.285(7) Å, and are consistent with significant electron delocalization over these bonds. The phen ligand is nearly planar, the maximum atomic deviation being 0.017 Å for C2 in 2 and 0.0292 Å for C9 in 3. This ligand forms dihedral angles of 83.98° and 87.49° in 2 and 83.30° and 88.28° in 3 with the chelate acac planes.
Fig. 4. Crystal packing in compounds 2 (a) and 3 (b). C-bound H atoms are omitted for clarity.
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Ph-Ph stacking
Ph-Ph and Py-Ph stackings
Py-Py stacking
(a)
(b)
(c)
Fig. 5. Schematic illustration of p–p stacking interactions of the aromatic systems in the reported 2 (a) and 3 (b), and previously reported and deposited at the Cambridge Structural Database, HPENCR and XPHOCR (c) Cr(III) 1,10-phenantroline complexes; view perpendicular to the plane of the overlapping phen ligands. Mode Ph–Ph has been found for the first time, to our knowledge, in the structures 2 and 3.
Fig. 6. Solid-state luminescence emissions recorded at room temperature for compounds 1 (blue) (a), 2 (red), 3 (green) and free phen ligand (magenta) (b). (Color online.)
In the crystal structure of compounds 2 and 3 p–p stacking interactions are noted between neighboring hydrophobic phen molecules of mononuclear cations, yielding one-dimensional supramolecular zipper-like chains along the b direction in 2 similar to 1 and a direction in 3 (Fig. 3). The centroid centroid separation between phen rings and the interplanar separations indicate the face-to-face and offset stackings (Table 4). The metal metal separations through the stacked ligands between neighboring complexes are 7.608 (2) and 7.520 Å (3), across diagonal are 8.884 and 9.223 Å (2) and 9.125 and 9.180 Å (3). The supramolecular motifs in 2 reproduce those in 1, since the crystal packing in 2 reveals that the hydrophilic outer sphere groups (nitrate anion and water molecules) are interconnected through the hydrogen bonds generated a tetrameric water cluster, that are further organized in a 1D chain with nitrate anions via hydrogen bond interactions (Fig. 4a, Table 3). The disordered ClO4 anions and water molecules in crystal packing of 3 are accumulated in the interchain space, held there by the electrostatic, van der Waals forces and weak C–H O intermolecular hydrogen bonds (Fig. 4b, Table 3). Our survey of the Cambridge Structural Database (ConQuest Version 1.17) reveals 94 discrete Cr(III) 1,10-phenanthroline compounds. By using the above-mentioned criteria, only in three of them, bis(l2-hydroxo)-tetrakis(1,10-phenanthroline-N, N0 )-di-chromium(III) tetrachloride hexahydrate, (l2-oxalato-O,O0 , O00 )-azido-(oxalato-O,O0 )-tris(1,10-phenanthroline)-chromium(III)manganese(II) monohydrate and bis(l2-hydroxo)-bis(bis(1, 10-phenanthroline)-chromium(III))tetraiodide tetrahydrate, the aromatic groups of phen molecules are stacked through the p–p
interactions [31–33] (Table 4). In these compounds there are two modes of stacking, between pyridine–pyridine (Py–Py) and pyridine–phenyl (Py–Ph) rings. The reported structures 2 and 3 show exclusive examples of p–p interactions between phenyl–phenyl (Ph–Ph) rings in Cr(III)–phen compounds (Fig. 5). 3.2. Luminescence The luminescence properties of complexes 1–3, and pure phen ligand were studied in the solid state at room temperature (Fig. 6). Irradiation of crystalline samples 1–3 with ultraviolet light (kex = 337.1 nm) resulted in a broad emission with the maximum at 465 nm for compound 1 and 440 nm for compounds 2 and 3, as well as weak emission at 650 and 675 nm for compounds 2 and 3, respectively. The results indicate that the monomeric complexes have blue light emissions in the solid state and are predominantly originate from metal-to-ligand charge transfer (MLCT) transition. The bipy and phen ligands are coordinated with Cr(III) ions to form additional five-member rings, which increase the p–p⁄ conjugation length and the conformational coplanarity, accordingly reduces the energy gap between the p and p⁄ molecular orbitals of the ligand [34–37]. 4. Conclusion Three new Cr(III) mononuclear complexes constructed from acetylacetonate and bipyridyl-like ligands (bipy and phen), namely [Cr(acac)2(bipy)]NO32H2O (1), [Cr(acac)2(phen)]NO32H2O (2) and
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[Cr(acac)2(phen)]ClO40.25H2O (3), (Hacac = acetylacetone, bipy = 2,20 -bipyridine and phen = 1,10-phenanthroline) were prepared and their structures were determined by single-crystal X-ray diffraction. The p–p stacking interactions between neighboring bipy and phen molecules of mononuclear cations of compounds 1–3, yield one-dimensional supramolecular zipper-like chains. Crystal packing of 1 and 2 reveals that the hydrophilic outer sphere species (nitrate anion and water molecules) are interconnected through the hydrogen bonds into one-dimensional ladder-like chain. Compounds 1–3 reveal ligand-based luminescent properties. Acknowledgment
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
The authors acknowledge the financial support from the Grant for Young Scientists (Project 15.819.02.03F). Appendix A. Supplementary data
[19] [20] [21] [22] [23]
CCDC 1407066–1407068 contains the supplementary crystallographic data for 1–3. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected].
[24] [25] [26]
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Polyhedron journal homepage: www.elsevier.com/locate/poly
Supramolecular solid-state patterns generated by hydrogen bonding and p–p stacking interactions in the mononuclear Cr(III) complexes Elena Melnic a, Ecaterina Tocana b, Anatolii V. Siminel a, Lilia Croitor a,⇑ a b
Institute of Applied Physics, Academy of Sciences of R. Moldova, Academy Str., 5, MD2028 Chisinau, Republic of Moldova Tiraspol State University, Iablocikin Str., 5, MD2069 Chisinau, Republic of Moldova
a r t i c l e
i n f o
Article history: Received 29 June 2015 Accepted 28 September 2015 Available online 13 October 2015 Keywords: X-ray Cr3+ Aromatic ligand Supramolecular pattern Luminescence
a b s t r a c t Three new mononuclear Cr(III) complexes of formulas [Cr(acac)2(bipy)]NO32H2O (1), [Cr(acac)2(phen)] NO32H2O (2) and [Cr(acac)2(phen)]ClO40.25H2O (3), (Hacac = acetylacetonate, bipy = 2,20 -bipyridine and phen = 1,10-phenanthroline) were synthesized to illustrate the role of the non-covalent interactions in sustaining supramolecular solid-state architectures. In all structures the Cr3+ cation is coordinated by one bidentate N-donor ligand and two acetylacetonate anions within a slightly distorted N2O4 octahedron. These ionic complexes are arranged into 1D supramolecular structures by combination of p–p aromatic face-to-face and offset stacking interactions, as well as O–H O hydrogen bond interactions. The luminescence studies show that compounds 1–3 display blue luminescence upon excitation with ultraviolet light. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays a pronounced interest has been focused on the crystal engineering of supramolecular ensembles established by combination of coordinative and weaker non-covalent interactions (e.g. hydrogen bonds, p–p stacking), which play an essential role in the fields from molecular biology to crystal engineering [1–6]. The design and construction of innovative metallo-supramolecular assemblies based on the non-covalent interactions such as p–p stacking and hydrogen bonding may adjust the dimensionality and lead to new topologies with desired functions [3,4,7,8]. It is known that the introduction of polycyclic aromatic chelating ligands containing N-donors, like 2,20 -bipyridine (bipy) and 1,10-phenanthroline (phen) into the metal-acetylacetonato systems prevent the formation of higher dimensional coordination networks and may provide supramolecular recognition sites for p–p, C–H–p stacking and C–H O hydrogen bonding to form supramolecular structures [8–12]. The phen molecule coordinating to a metal ion forms a large planar system of four fused rings: two pyridine fragments, one C6-ring and one chelate ring [13], the smaller bipy ligand, which comprises three rings with metal ion (two pyridine and one chelate rings) have a tendency to form stacking interactions with the p-systems of various aromatic groups. The tendency for stacking interactions to occur is an ⇑ Corresponding author. Tel.: +373 22 738154; fax: +373 22 725887. E-mail address: [email protected] (L. Croitor). http://dx.doi.org/10.1016/j.poly.2015.09.069 0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.
important issue when using phenanthroline complexes in various fields. For example, it is known that these complexes interact with DNA by intercalating between base pairs of DNA [14]. Another crucial factor for introducing in synthesis of these chelating ligands is defined by the conjugated p-systems containing aromatic rings that are currently of interests in the development of fluorescent materials [15–18]. The solvent molecules and the counterions influence also the final supramolecular architecture. The noncoordinating anions play not only a charge-compensating role, but they can dramatically influence the overall solid-state architecture through their templating function [5,8]. Chromium is an important heavy metal that is widely used in industrial processes. It is also an essential micronutrient for proper glucose metabolism that stimulates the enzyme system and stabilizes nucleic acids [19]. It has been suggested that some coordinative complexes are able to migrate from the cytoplasm to the nucleus and attack the DNA structure [20]. For example, Cr in the most stable oxidation state Cr(III), is considered to interact with the DNA molecule [21–23]. Also, chromium(III) polypyridines are excellent materials for luminescence probes [24]. Here we report the supramolecular systems constructed by combining two or three organizing forces: metal-coordination, hydrogen bonds and p–p stacking interactions. Three Cr(III) mononuclear complexes with the compositions [Cr(acac)2(bipy)] NO32H2O (1), [Cr(acac)2(phen)]NO32H2O (2) and [Cr(acac)2(phen)] ClO40.25H2O (3), Hacac = acetylacetone, bipy = 2,20 -bipyridine and phen = 1,10-phenanthroline (Scheme 1), were synthesized and
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for C22H26N3O9Cr: C, 49.99; H, 4.96; N, 7.95. Found: C, 49.92; H, 4.88; N, 7.87%.
Scheme 1. Schematic illustration of the ligands with acronyms used in this study.
studied by X-ray method. The luminescence properties of compounds 1–3 were investigated.
2.2.3. Synthesis of [Cr(acac)2(phen)]ClO40.25H2O (3) Cr(ClO4)36H2O (35.2 mg, 0.1 mmol) and 1,10-phenanthroline (18.02 mg, 0.1 mmol) were dissolved in 6 mL of methanol. Then 8 drops of acetylacetone were added. The reaction mixture was stirred for 5 min, filtered off and then slowly cooled to room temperature giving brown crystals. Yield: 27%. Anal. Calc. for Table 2 Selected bond lengths (Å) and angles (°) in coordination metal environment in 1–3.
2. Experimental 2.1. Materials and methods All reagents and solvents were obtained from commercial sources and were used without further purification. Caution! Perchlorate salts are dangerous and should be handled with care and in only small quantities. Elemental analysis was performed on an Elementar Analysensysteme GmbH Vario El III elemental analyzer. Emission spectra were measured for monocrystals at room temperature on an Excitation YAG:Nd3+ laser, third harmonic generation, k = 355 nm, duration = 10 ns, time repetition 10 Hz. 2.2. Synthesis 2.2.1. Synthesis of [Cr(acac)2(bipy)]NO32H2O (1) Cr(NO3)39H2O (40.01 mg, 0.1 mmol) and 2,20 -bipyridine (15.61 mg, 0.1 mmol) were dissolved in 6 mL of methanol. Then 4 drops of acetylacetone were added. The reaction mixture was stirred for 5 min, filtered off and then slowly cooled to room temperature giving violet crystals. Yield: 80%. Anal. Calc. for C20H26N3O9Cr: C, 47.62; H, 5.19; N, 8.33. Found: C, 47.56; H, 5.11; N, 8.27%. 2.2.2. Synthesis of [Cr(acac)2(phen)]NO32H2O (2) Cr(NO3)39H2O (40.01 mg, 0.1 mmol) and 1,10-phenanthroline (18.02 mg, 0.1 mmol) were dissolved in 6 mL of methanol. Then 8 drops of acetylacetone were added. The reaction mixture was stirred for 5 min, filtered off and then slowly cooled to room temperature giving cherry-colored crystals. Yield: 75%. Anal. Calc.
1 Cr(1)AO(2) Cr(1)AO(3) Cr(1)AO(1) O(2)ACr(1)AO(3) O(2)ACr(1)AO(1) O(3)ACr(1)AO(1) O(2)ACr(1)AO(4) O(3)ACr(1)AO(4) O(1)ACr(1)AO(4) O(2)ACr(1)AN(1) O(3)ACr(1)AN(1)
1.926(3) 1.931(3) 1.954(3) 92.29(12) 91.72(13) 89.29(13) 89.38(13) 91.90(13) 178.36(13) 172.54(14) 94.80(14)
Cr(1)AO(4) Cr(1)AN(1) Cr(1)AN(2) O(1)ACr(1)AN(1) O(4)ACr(1)AN(1) O(2)ACr(1)AN(2) O(3)ACr(1)AN(2) O(1)ACr(1)AN(2) O(4)ACr(1)AN(2) N(1)ACr(1)AN(2)
1.959(3) 2.058(4) 2.073(3) 90.69(14) 88.07(14) 94.42(14) 173.18(14) 89.30(13) 89.39(13) 78.55(15)
2 Cr(1)AO(3) Cr(1)AO(2) Cr(1)AO(4) O(3)ACr(1)AO(2) O(3)ACr(1)AO(4) O(2)ACr(1)AO(4) O(3)ACr(1)AO(1) O(2)ACr(1)AO(1) O(4)ACr(1)AO(1) O(3)ACr(1)AN(2) O(2)ACr(1)AN(2)
1.931(2) 1.939(2) 1.949(2) 92.88(10) 91.84(10) 88.99(9) 89.32(10) 91.60(9) 178.66(9) 172.56(10) 94.21(10)
Cr(1)AO(1) Cr(1)AN(2) Cr(1)AN(1) O(4)ACr(1)AN(2) O(1)ACr(1)AN(2) O(3)ACr(1)AN(1) O(2)ACr(1)AN(1) O(4)ACr(1)AN(1) O(1)ACr(1)AN(1) N(2)ACr(1)AN(1)
1.955(2) 2.073(3) 2.079(3) 90.55(10) 88.21(10) 93.74(10) 173.27(10) 89.57(10) 89.71(10) 79.23(10)
3 Cr(1)AO(1) Cr(1)AO(2) Cr(1)AO(3) O(2)ACr(1)AO(3) O(2)ACr(1)AO(4) O(3)ACr(1)AO(4) O(2)ACr(1)AO(1) O(3)ACr(1)AO(1) O(4)ACr(1)AO(1) O(2)ACr(1)AN(2) O(3)ACr(1)AN(2)
1.955(4) 1.928(4) 1.931(4) 92.84(18) 88.70(16) 91.83(16) 91.93(16) 89.95(16) 178.08(16) 94.14(19) 172.81(18)
Cr(1)AO(4) Cr(1)AN(2) Cr(1)AN(1) O(4)ACr(1)AN(2) O(1)ACr(1)AN(2) O(2)ACr(1)AN(1) O(3)ACr(1)AN(1) O(4)ACr(1)AN(1) O(1)ACr(1)AN(1) N(2)ACr(1)AN(1)
1.947(4) 2.079(5) 2.084(4) 90.09(17) 88.05(17) 173.20(19) 93.77(19) 89.47(16) 89.70(16) 79.3(2)
Table 1 Crystal and structure refinement data for compounds 1–3.
Empirical formula Formula weight Crystal system Space group Z a (Å) b (Å) c (Å) b (°) V (Å3) Dcalc (g/cm3) l (mm1) F(0 0 0) Crystal size (mm) Reflections collected/unique (Rint) Reflections with [I > 2r(I)] Data/restraints/parameters GOF on F2 R1, wR2 [I > 2r(I)] R1, wR2 (all data)
1
2
3
C20H26N3O9Cr 504.44 monoclinic P2/c 4 14.0923(8) 7.6578(5) 23.717(2) 116.248(6) 2295.5(3) 1.460 0.554 1052 0.30 0.12 0.02 7433/4022 (0.0616) 2030 4022/13/302 0.967 0.0728, 0.0714 0.1562, 0.0878
C22H26N3O9Cr 528.46 monoclinic P2/c 4 15.0152(5) 7.6084(3) 23.3883(9) 116.651(3) 2388.04(15) 1.470 0.536 1100 0.50 0.20 0.08 7756/4169 (0.0269) 3317 4169/16/313 0.999 0.0483, 0.1278 0.0639, 0.1394
C22H22N2O8.50ClCr 537.87 monoclinic P21/c 4 7.5199(6) 21.456(2) 15.3344(8) 93.467(5) 2469.6(3) 1.447 0.622 1108 0.45 0.06 0.04 8474/4306 (0.0380) 2879 4306/82/336 0.999 0.0802, 0.2306 0.1211, 0.2607
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C22H22N2O8.50ClCr: C, 59.72; H, 5.01; N, 6.33. Found: C, 59.67; H, 4.97; N, 6.29%.
3. Results and discussion 3.1. Structural description
2.3. X-ray crystallography Diffraction measurements for 1–3 were carried out at room temperature on a Xcalibur ‘‘Oxford Diffraction” diffractometer equipped with CCD area detector and a graphite monochromator utilizing Mo Ka radiation. Final unit cell dimensions were obtained and refined on an entire data set. All calculations to solve the structures and to refine the proposed models were carried out with the programs SHELXS97 and SHELXL97 [25]. The structures were solved by direct methods and refined by full-matrix least-squares methods on F2 by using the SHELXL97 program package. All non-hydrogen atoms were refined anisotropically. For 3 the perchlorate anion is disordered over two positions (75/25% occupation). For the disordered solvent water molecule, hydrogen atoms were not localized. Hydrogen atoms attached to carbon atoms were placed in geometrically idealized positions and refined by using a riding model. In compounds 1 and 2 hydrogen atoms of solvated water molecules are disordered over two positions with the equal occupancies and the O–H and H H distances were restricted by the DFIX command. The disordered fragments were refined in anisotropic approximation using equal anisotropic displacement parameters for the similar atoms. The X-ray data and the details of the refinement for 1–3 are summarized in Table 1. Selected geometric parameters for 1–3 are given in Tables 2 and 3. The figures were produced using the MERCURY program [26].
Compound [Cr(acac)2(bipy)]NO32H2O (1) crystallizes in the monoclinic centrosymmetric P2/c (No. 13) space group. The crystal structure consists of the mononuclear [Cr(acac)2(bipy)]+ cation (Fig. 1a), NO 3 anion and two crystallization water molecules. The chromium(III) ion displays a distorted octahedral coordination involving two nitrogen atoms of the chelating bipy group and four oxygen atoms from two bidentate chelating acac ligands. The N–Cr–N bite angle in the chelating bipyridine moiety is 78.57 (15)°, and the average O–Cr–O bite angle in the acac chelate rings are equal to 91.71(13)° and 91.90(13)°. The four C@Oacac bond distances are experimentally equivalent, having values in range 1.275(5)–1.286(5) Å, and are consistent with significant electron delocalization over these bonds. The Cr(1)AN(1) bond distance (2.058(4) Å) is shorter than the Cr(1)AN(2) bond (2.073(3) Å) in the same ligand, suggesting that there is some twist in the bipy ring, the dihedral angle between the two pyridine rings equals to 3.05°. This ligand forms dihedral angles of 81.98° and 87.23° with the acac planes. In the crystal structure of 1 the chelate bipy ligands are stacked by the p–p interactions along the crystallographic b axis, giving supramolecular polymer (Fig. 1b). The centroid centroid separation between bipy rings (3.917 Å) and the interplanar separations indicate the offset stacking (3.468 and 3.469 Å). The Cr Cr separation through the stacked ligands is 7.854 Å. Crystal packing reveals
Table 3 Hydrogen bond distances (Å) and angles (°) for 1 and 2. DAH A
d(DAH)
d(H A)
d(D A)
<(DHA)
Symmetry transformation for acceptor
1 O(1w)AH(3w1) O(2w) O(1w)AH(2w1) O(1w) O(1w)AH(1w1) O(6) O(2w)AH(1w2) O(6) O(2w)AH(1w2) O(5) O(2w)AH(2w2) O(1w) O(2w)AH(3w2) O(2w) C(4)AH(4) O(1) C(14)AH(14C) O(2w) C(15)AH(15A) O(6) C(19)AH(19B) O(5)
0.85 0.88 0.88 0.87 0.87 0.84 0.86 0.93 0.96 0.96 0.96
1.95 2.06 2.07 2.20 2.45 2.11 1.82 2.62 2.65 2.61 2.47
2.748(6) 2.762(7) 2.828(7) 2.878(7) 3.309(7) 2.748(6) 2.670(7) 3.393(6) 3.416(5) 3.441(7) 3.358(6)
157.0 136.0 142.8 134.8 170.2 132.8 170.2 140.6 137.3 144.5 152.9
x, y + 1, z x, y, z + 1/2 x, y, z x, y, z x, y, z x, y 1, z x, y, z + 1/2 x, y, z x, y, z 1/2 x, y + 1, z 1/2 x + 1, y, z + 1/2
2 O(1w)AH(1w1) O(5) O(1w)AH(2w1) O(1w) O(1w)AH(3w1) O(2w) O(2w)AH(1w2) O(5) O(2w)AH(2w2) O(1w) O(2w)AH(3w2) O(2w) C(1)AH(1) O(5) C(9)AH(9) O(6) C(16)AH(16A) O(6)
0.86 0.96 0.89 0.81 1.00 1.13 0.93 0.93 0.96
2.22 1.85 1.84 2.25 1.90 1.97 2.61 2.60 2.57
2.915(6) 2.755(5) 2.700 2.901(6) 2.700 2.683(5) 3.367(6) 3.336(7) 3.491(6)
137.3 154.9 163.1 137.5 134.9 117.2 139.5 136.3 159.8
x, y, z x + 1, y, z + 1/2 x, y 1, z x, y, z x, y + 1, z x + 1, y, z + 1/2 x, y + 1, z + 1/2 x, y, z x, y 1, z
3 C(2)AH(2) O(7A) C(9)AH(9) O(6A) C(10)AH(10) O(5A) C(10)AH(10) O(6A) C(16)AH(16B) O(8) C(17)AH(17C) O(5A) C(19)AH(19) O(8A) C(21)AH(21A) O(6) C(21)AH(21A) O(7A) C(21)AH(21C) O(1) C(22)AH(22B) O(8A)
0.93 0.93 0.93 0.93 0.96 0.96 0.93 0.96 0.96 0.96 0.96
2.48 2.69 2.69 2.89 2.56 2.03 2.65 2.56 2.35 2.64 2.45
3.09(3) 3.36(3) 3.56(5) 3.47(3) 3.310(15) 2.83(3) 3.45(4) 3.260(10) 3.29(5) 3.593(8) 3.36(3)
123.6 130.1 155.5 122.0 135.0 138.4 144.6 130.0 166.4 176.0 158.0
x 1, y, z x + 1, y 1/2, z + 1/2 x + 1, y 1/2, z + 1/2 x + 1, y 1/2, z + 1/2 x, y 1/2, z + 1/2 x + 1, y 1/2, z + 1/2 x, y + 3/2, z 1/2 x, y, z x, y, z x + 1, y, z x, y + 3/2, z 1/2
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Fig. 1. View of the molecular structure of mononuclear [Cr(acac)2(bipy)]+ cation with partial atomic numbering scheme (a); the stacking of the mononuclear cations linked by p–p interactions between the aromatic systems (b); crystal packing in 1 (c) and the infinite one-dimensional hydrogen-bonding tape combining the tetrameric water clusters and NO 3 anions along b axis. Only one position is shown for the disordered H-atoms in the water molecules (d). C-bound H atoms are omitted for clarity.
Fig. 2. The molecular structure of mononuclear [Cr(acac)2(phen)]+ cation in 2 and 3 with atomic numbering scheme.
that the hydrophilic outer sphere groups (nitrate anion and water molecules) are interconnected through the hydrogen bonds (Fig. 1c, Table 3). The hydrogen bonds between the two water molecules generate a tetrameric water cluster, where each water
Fig. 3. View of the stacking of the mononuclear [Cr(acac)2(phen)]+ cations linked by p–p interactions of the aromatic systems in 2 (a) and 3 (b). C-bound H atoms are omitted for clarity.
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Table 4 Selected geometric parameters for stacking modes of 1,10-phenanthroline rings including centroid-to-centroid (C-to-C), plane-to-plane (P-to-P) and centroid-to-plane (C-to-P) distances (Å) in reported compounds. Compound
p–p stacking interaction
C-to-C distance
P-to-P distance
C-to-P distance (av)
Reference
Present work Present work
2
Ph–Ph
3.738
3.458
3.465
3
Ph–Ph
3.835, 3.713 3.813, 3.846 3.772, 3.851 3.764 3.776, 3.810
3.465, 3.446
3.455
3.117, 3.311 3.440 3.111, 3.268
3.063
[21]
3.614 3.055
[22] [23]
Py–Ph a
HPENCR
Py–Py
SACSULa XPHOCRa
Py–Ph Py–Py
a HPENCR = bis(l2-hydroxo)-tetrakis(1,10-phenanthroline-N,N0 )-di-chromium(III) tetrachloride hexahydrate; SACSUL = (l2-oxalato-O,O0 ,O00 )-azido-(oxalato-O,O0 )-tris (1,10-phenanthroilne)-chromium(III)-manganese(II) monohydrate; XPHOCR = bis (l2-hydroxo)-bis(bis(1,10-phenanthroline)-chromium(III)) tetraiodide tetrahydrate.
molecule acts as a single double hydrogen bond donor and single acceptor. The nitrate anions interconnect further water clusters in one-dimensional chain forming through the alternating R44(8) and R46(12) H-bonded patterns [27] (Table 3, Fig. 1c and d). Weak C–H O intermolecular hydrogen bonds make additional impact in stabilization of crystal structure. The principal of them are presented in Table 3. Notably, the following criteria were used for search in the Cambridge Structural Database (ConQuest Version 1.17): (a) centroid–centroid distance between two pyridine fragments was constrained to limits 3.3–3.9 Å; (b) bond types – aromatic; (c) H atoms were not specified; (d) no polymer structures. Only in three examples from 140 discrete compounds built up from Cr(III) atom
and 2,20 -bipyridine have been found p–p interactions between delocalized p systems, namely (l3-oxo)-bis(l2-benzoato-O,O0 )-bis (l2-ethanolato)-tris(isothiocyanato)-bis(2,20 -bipyridine)-tri-chromium(III) [28], (l3-chromato)-hexakis(2,20 -bipyridyl)-tri-copper (II) tetraperchlorate monohydrate [29] and (l2-oxido)-tetrakis (2,20 -bipyridine)-bis(isothiocyanate)-di-chromium diperchlorate hydrate [30]. In all these compounds the interplanar separations between bipy rings indicate the offset stackings. In the first two examples, the stacking is observed among one pyridine ring, while in the last example and our compound 1 p–p interactions have been found between both delocalized p systems. Compounds [Cr(acac)2(phen)]NO32H2O (2) and [Cr(acac)2 (phen)]ClO40.25H2O (3) were obtained similarly and crystallize in the monoclinic centrosymmetric P2/c (No. 13) (2) and P21/c (No. 14) (3) space groups. As it is evidenced from comparison of unit cell dimensions and crystal systems compounds 1 and 2 are isomorphous with consequential increase of unit cell volume in 2 against 1 (Table 1). The crystal structures of 2 and 3 contain the similar mononuclear [Cr(acac)2(phen)]+ cation (Fig. 2) and nitrate anion and two solvated water molecules in 2 and perchlorate ion and solvated water molecule in 3 refined with 0.25 occupancy. Similar to 1, the chromium atoms coordinate with two nitrogen atoms of a chelating phen group and four oxygen atoms from two bidentate chelating acac ligands to form a distorted N2O4 octahedral geometry. The N–Cr–N bite angle in the phen moiety is 79.23(10)° in 2 and 79.3(2)° in 3, and the interval of O–Cr–O bite angle in the acca chelate rings is in the range 91.60(9)–91.93 (16)°. The four C@Oacac bond distances are experimentally equivalent, having values in range 1.267(7)–1.285(7) Å, and are consistent with significant electron delocalization over these bonds. The phen ligand is nearly planar, the maximum atomic deviation being 0.017 Å for C2 in 2 and 0.0292 Å for C9 in 3. This ligand forms dihedral angles of 83.98° and 87.49° in 2 and 83.30° and 88.28° in 3 with the chelate acac planes.
Fig. 4. Crystal packing in compounds 2 (a) and 3 (b). C-bound H atoms are omitted for clarity.
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Ph-Ph stacking
Ph-Ph and Py-Ph stackings
Py-Py stacking
(a)
(b)
(c)
Fig. 5. Schematic illustration of p–p stacking interactions of the aromatic systems in the reported 2 (a) and 3 (b), and previously reported and deposited at the Cambridge Structural Database, HPENCR and XPHOCR (c) Cr(III) 1,10-phenantroline complexes; view perpendicular to the plane of the overlapping phen ligands. Mode Ph–Ph has been found for the first time, to our knowledge, in the structures 2 and 3.
Fig. 6. Solid-state luminescence emissions recorded at room temperature for compounds 1 (blue) (a), 2 (red), 3 (green) and free phen ligand (magenta) (b). (Color online.)
In the crystal structure of compounds 2 and 3 p–p stacking interactions are noted between neighboring hydrophobic phen molecules of mononuclear cations, yielding one-dimensional supramolecular zipper-like chains along the b direction in 2 similar to 1 and a direction in 3 (Fig. 3). The centroid centroid separation between phen rings and the interplanar separations indicate the face-to-face and offset stackings (Table 4). The metal metal separations through the stacked ligands between neighboring complexes are 7.608 (2) and 7.520 Å (3), across diagonal are 8.884 and 9.223 Å (2) and 9.125 and 9.180 Å (3). The supramolecular motifs in 2 reproduce those in 1, since the crystal packing in 2 reveals that the hydrophilic outer sphere groups (nitrate anion and water molecules) are interconnected through the hydrogen bonds generated a tetrameric water cluster, that are further organized in a 1D chain with nitrate anions via hydrogen bond interactions (Fig. 4a, Table 3). The disordered ClO4 anions and water molecules in crystal packing of 3 are accumulated in the interchain space, held there by the electrostatic, van der Waals forces and weak C–H O intermolecular hydrogen bonds (Fig. 4b, Table 3). Our survey of the Cambridge Structural Database (ConQuest Version 1.17) reveals 94 discrete Cr(III) 1,10-phenanthroline compounds. By using the above-mentioned criteria, only in three of them, bis(l2-hydroxo)-tetrakis(1,10-phenanthroline-N, N0 )-di-chromium(III) tetrachloride hexahydrate, (l2-oxalato-O,O0 , O00 )-azido-(oxalato-O,O0 )-tris(1,10-phenanthroline)-chromium(III)manganese(II) monohydrate and bis(l2-hydroxo)-bis(bis(1, 10-phenanthroline)-chromium(III))tetraiodide tetrahydrate, the aromatic groups of phen molecules are stacked through the p–p
interactions [31–33] (Table 4). In these compounds there are two modes of stacking, between pyridine–pyridine (Py–Py) and pyridine–phenyl (Py–Ph) rings. The reported structures 2 and 3 show exclusive examples of p–p interactions between phenyl–phenyl (Ph–Ph) rings in Cr(III)–phen compounds (Fig. 5). 3.2. Luminescence The luminescence properties of complexes 1–3, and pure phen ligand were studied in the solid state at room temperature (Fig. 6). Irradiation of crystalline samples 1–3 with ultraviolet light (kex = 337.1 nm) resulted in a broad emission with the maximum at 465 nm for compound 1 and 440 nm for compounds 2 and 3, as well as weak emission at 650 and 675 nm for compounds 2 and 3, respectively. The results indicate that the monomeric complexes have blue light emissions in the solid state and are predominantly originate from metal-to-ligand charge transfer (MLCT) transition. The bipy and phen ligands are coordinated with Cr(III) ions to form additional five-member rings, which increase the p–p⁄ conjugation length and the conformational coplanarity, accordingly reduces the energy gap between the p and p⁄ molecular orbitals of the ligand [34–37]. 4. Conclusion Three new Cr(III) mononuclear complexes constructed from acetylacetonate and bipyridyl-like ligands (bipy and phen), namely [Cr(acac)2(bipy)]NO32H2O (1), [Cr(acac)2(phen)]NO32H2O (2) and
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[Cr(acac)2(phen)]ClO40.25H2O (3), (Hacac = acetylacetone, bipy = 2,20 -bipyridine and phen = 1,10-phenanthroline) were prepared and their structures were determined by single-crystal X-ray diffraction. The p–p stacking interactions between neighboring bipy and phen molecules of mononuclear cations of compounds 1–3, yield one-dimensional supramolecular zipper-like chains. Crystal packing of 1 and 2 reveals that the hydrophilic outer sphere species (nitrate anion and water molecules) are interconnected through the hydrogen bonds into one-dimensional ladder-like chain. Compounds 1–3 reveal ligand-based luminescent properties. Acknowledgment
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
The authors acknowledge the financial support from the Grant for Young Scientists (Project 15.819.02.03F). Appendix A. Supplementary data
[19] [20] [21] [22] [23]
CCDC 1407066–1407068 contains the supplementary crystallographic data for 1–3. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected].
[24] [25] [26]
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