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Original article
12 (
5
); 729-738
doi:
10.1016/j.arabjc.2016.04.006

Luminescent activity of metallosupramolecular Cd(II) complexes containing dimethylterpyridine ligand

, , , , , , ,
a Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61-614 Poznań, Poland
b Institute of Technology and Life Science, Biskupińska 67, 60-463 Poznań, Poland
c Institut de Science et d’Ingénierie Supramoléculaires (ISIS), Université de Strasbourg, 8 allée Gaspard Monge, 67083 Strasbourg, France
d Wielkopolska Center for Advanced Technologies, Umultowska 89c, 61-614 Poznań, Poland

⁎Corresponding authors. zbychuh@amu.edu.pl (Zbigniew Hnatejko), violapat@amu.edu.pl (Violetta Patroniak)

Received: , Accepted: ,
© The Author(s) 2016
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
1 Adam Gorczyński and Damian Marcinkowski contributed equally to the experimental results given in this work.

Abstract

Abstract

Four complexes of different Cd(II) salts with 6,6″-dimethyl-2,2′:6′,2″-terpyridine L have been synthesized of the following structural formulae: [CdL(CH3OH)(ClO4)2] (1) [CdL(Cl/Br)2] (2) [CdLCl2] (2a) [CdL(CH3CO2)2] (3). Their properties have been established through analytical and spectroscopic (ESI-MS, IR, 1H NMR and UV–Visible absorption and emission) methods as well as by X-ray structure determinations. Quite high quantum yield values were obtained for the solution luminescence, despite the fact that presented compounds are ‘open species’ i.e. are susceptible to the effect of external environment. Titration experiments proved speciation i.e. formation of both 2:1 and 1:1 (L:Cd2+) species in MeCN, yet only the latter ones can be isolated in their crystalline form. In the solid state, there appears to be a correlation between the emission intensity and the stacking arrays in the lattice. There is no evidence that the methyl substituents exert a major influence upon the properties of the complexes, and it is implied however that they might also be responsible for preferential formation of 1:1 complexes in the solid state due to observed intermolecular packing lattices.

Keywords

Terpyridine
Cadmium complexes
Luminescent materials
Hirshfeld surfaces
Self-assembly
1

1 Introduction

Interest in the self-assembly of well-defined architectures from organic ligands and specific metal ions for the development of new materials with unique properties is long-established (Lunn et al., 2015; Ulijn, 2015; Z. Zhang et al., 2015; Silva et al., 2015; He et al., 2015; Faiz et al., 2009; Champin et al., 2007; Nitschke, 2007; Gianneschi et al., 2005; Fujita et al., 2005; Steel, 2005; Albrecht, 2004; Seidal and Stang, 2002; Swiegers and Malefetse, 2000; Leininger et al., 2000; Saalfrank et al., 2008; Rosi et al., 2003; Whitesides and Grzybowski, 2002; Ward, 2002; Caulder and Raymond, 1999; Piguet et al., 1997; Baxter, 1996; Lehn, 1995). Efficient manipulation of desired structure may involve exploitation of metal–ligand interactions, H-bonding and/or host–guest interactions, with an outcome centred upon tuning of their behaviour, particularly focusing on emissive properties, as a function of solvent, anions, pH, guests and many others (Yan et al., 2015; Y. Zhang et al., 2015; Zhou et al., 2016; Yan et al.; Yamashina et al., 2015). Of the many ligands which have been exploited in such research, 2,2′:6′,2″-terpyridine (tpy) and its derivatives (Schubert et al., 2006) are particularly notable for their use as building blocks in supramolecular chemistry due to their special spectroscopic properties, relatively strong non-covalent interactions and coordination capacity as strong, tridentate N-donors (Wild et al., 2011; Alexeev et al., 2010; McMurtrie and Dance, 2005; Scudder et al., 1999). In particular, transition metal ion complexes of terpyridines have been widely studied and various applications have been investigated (Lunn et al., 2015; Ulijn, 2015; Z. Zhang et al., 2015; Silva et al., 2015; He et al., 2015; Faiz et al., 2009; Champin et al., 2007; Nitschke, 2007; Gianneschi et al., 2005; Fujita et al., 2005; Steel, 2005; Albrecht, 2004; Seidal and Stang, 2002; Swiegers and Malefetse, 2000; Leininger et al., 2000; Saalfrank et al., 2008; Rosi et al., 2003; Whitesides and Grzybowski, 2002; Ward, 2002; Caulder and Raymond, 1999; Piguet et al., 1997; Baxter, 1996; Lehn, 1995; Yan et al., 2015; Y. Zhang et al., 2015; Zhou et al., 2016; Yan et al.; Yamashina et al., 2015; Schubert et al., 2006; McMurtrie and Dance, 2005; Scudder et al., 1999; Sakamoto et al., 2015; Fihey et al., 2015; Gao et al., 2014; Rajwar et al., 2014; Numata et al., 2013; Fillaud et al., 2013; Kashif et al., 2013; Schott et al., 2013; Hayami et al., 2011; Winter et al., 2011; Eyrazici et al., 2008). Many of these complexes, as well as closed-shell metal ion analogues, are highly luminescent (Yan et al., 2015; Y. Zhang et al., 2015; Zhou et al., 2016; Yan et al.; Yamashina et al., 2015; Schubert et al., 2006; Wild et al., 2011; Fihey et al., 2015; Eyrazici et al., 2008; Puntoriero et al., 2008; Wong and Yam, 2007; Medlycott and Hanan, 2005; Thangavelu et al., 2013). In general, the properties of terpyridine complexes can be usefully modified by functionalization of the parent ligand, one important example of this being the incorporation of an M(tpy)2 unit into “expanded ligand” species open to involvement in, for example, metallopolymer formation (Byrne et al., 2014; Constable, 2008).

Perhaps the simplest form of functionalization of a ligand such as 2,2′:6,2″-terpyridine is the introduction of a methyl group. It is not necessarily expected that this should be innocuous, given the very well-known example of the dramatic effects of 2,9-dimethylation of 1,10-phenanthroline on the coordination properties of this bipyridine derivative e.g. (O’Reilly and Plowman, 1960; Fannizzi et al., 2004, 1991; ten Brink et al., 2003). We have, however, shown (Gorczyński et al., 2014; Wałęsa-Chorab et al., 2011a,b,c; Fik et al., 2014, 2015) that the analogous terpyridine derivative, 6,6″-dimethyl-2,2′:6,2″-terpyridine (L, Fig. 1), also with methyl groups adjacent to two N-donors, readily forms 1:1 complexes with a variety of transition metal ions and forms a 2:1 complex with Cu(II) at least.

Schematic representation of ligand L used in present work.
Figure 1
Schematic representation of ligand L used in present work.

Apart from well-known structural motifs, interesting properties of those complexes have been shown, Cu(II) congener with H-bond mediated spin canting or helical Ag(I) species of in vitro antiproliferative activity in front. In all complexes with divalent metal ions, the bond between the metal ion and the central pyridine-N is, if anything, slightly shorter than that in the analogous or closely similar complexes of 2,2′:6,2″-terpyridine but the difference in bond lengths to the outer and central donors is consistently slightly greater (supplementary material; Table S1) for the 6,6″-dimethyl-2,2′:6,2″-terpyridine complexes, indicating that the methyl groups do have a significant effect on the coordination geometry. To explore the possibility that they might also have a protective function with respect to the metal ion/ligand chromophore and thus influence their luminescent properties – quantum yields in particular –, we have extended our studies of complexation of 6,6″-dimethyl-2,2′:6,2″-terpyridine to Cd(II), a metal ion known to give luminescent complexes with the parent 2,2′:6,2″-terpyridine ligand and some derivatives (Lee et al., 2013a,b, 2012; Kubota et al., 2013; Ghosh et al., 2015; Yan et al., 2011; Nawrot et al., 2015; Machura et al., 2011).

2

2 Experimental section

2.1

2.1 Materials and physical measurements

6,6″-Dimethyl-2,2′:6,2″-terpyridine, L, was prepared in our laboratory as reported previously (Fik et al., 2014). The metal salts were used as supplied from Aldrich. ESI mass spectra for acetonitrile solutions ∼10−4 M were measured using a Waters Micromass ZQ spectrometer. Microanalyses were performed using a Perkin–Elmer 2400 CHN microanalyser. All NMR Spectra were recorded on Bruker Fourier 300 MHz spectrometer. NMR solvents were purchased from Euriso-Top or Deutero GmbH, and used as received. Spectra were referenced at solvent residual peaks (1H: MeCN d-3: 1.94 ppm). IR spectra were obtained with a Perkin–Elmer 580 spectrophotometer and peak positions are reported in cm−1. All electronic absorption spectra were recorded with a Shimadzu UV-2401 PC spectrophotometer, between 220 and 800 nm, in 10 × 10 mm quartz cells using 1 × 10−5 M solutions with respect to the metal ions. Excitation and emission spectra were measured at room temperature on a Hitachi F-7000 spectrofluorimeter with excitation and emission slits of 2.5 nm. For determination of luminescence quantum yields, tryptophan in water (luminescence quantum yield Φ = 0.14) was used as a reference standard (Eaton, 1988).

2.2

2.2 Preparation of the complexes: general procedures

All complexes were prepared under similar conditions. A mixture of the appropriate metal salts (78 μmol) and the ligand L (20.4 mg, 78.2 μmol) in MeOH:CH2Cl2 1:1 (10 mL) was stirred at room temperature for 48 h under the normal atmosphere to give a colourless solution. The complexes were isolated by evaporation of the solvent and recrystallization of the residue from a minimum volume of MeOH by the gradual addition of ether to provide all as white solids. Although complex 2 was expected to be the simple bromo species, the sample of CdBr2 used in the synthesis was apparently impure and both the elemental analysis and structural data were consistent with the product containing both chloride and bromide.

[CdL(CH3OH)(ClO4)2] (1) Yield 67% (17.5 mg); 1H NMR (300 MHz, Acetonitrile-d3) δ(ppm) = 8.78–8.66 (m, 2H), 8.63–8.52 (m, 1H), 8.49–8.40 (m, 2H), 8.08 (t, J = 7.9 Hz, 2H), 7.38 (ddd, J = 7.8, 1.1, 0.6 Hz, 2H), 1.78 (s, 6H). ESI-MS: m/z (%) = 473 (100) [CdL(ClO4)]+, 574 (90) [HCdL(ClO4)2]+, 262 [HL]+ IR (KBr, cm−1): ν(C—H)ar = 3069, 3035, νas(CH3) = 2922, νs(CH3) = 2856, γ(C—H)py overtones = 2019–1655, ν(C⚌C)py = 1593, 1569, 1472, 1457, σ(CH3) = 1395, ν(C⚌N)py = 1375, 1258, ρ(C—H)py = 1197, 1182, 1142, γ(C—H)py = 1041, 1016, 790, 650. Anal. Calc. for [Cd(C17H15N3)(ClO4)2] (572.63): C, 35.66; H, 2.64; N, 7.34; Found: C, 35.65; H, 2.57; N, 7.38%.

[CdL(Cl/Br)2] (2) Yield 72% (17.3 mg); 1H NMR (300 MHz, Acetonitrile-d3) δ(ppm) = 8.53–8.45 (m, 2H), 8.39–8.32 (m, 1H), 8.31–8.23 (m, 2H), 8.07 (t, J = 8.0 Hz, 2H), 7.58 (d, J = 7.8 Hz, 2H), 3.04 (s, 6H). ESI-MS: m/z (%) = 409 (100) [CdL(Cl)]+, 446 (90) [HCdL(Cl)2]+, 262 [HL]+ IR (KBr, cm−1): ν(C—H)ar = 3063, 3039, νas(CH3) = 2923, νs(CH3) = 2859, γ(C—H)py overtones = 2023–1657, ν(C⚌C)py = 1589, 1572, 1475, 1453, σ(CH3) = 1391, ν(C⚌N)py = 1372, 1259, ρ(C—H)py = 1194, 1185, 1144, γ(C—H)py = 1041, 1018, 791, 652. Anal. Calc. for [Cd(C17H15N3)ClBr] (489.09): C, 41.75; H, 3.09; N, 8.59. Found: C, 41.77; H, 3.07; N, 8.59%.

[CdLCl2] (2a) Yield 69% (13.8 mg), 1H NMR (300 MHz, Acetonitrile-d3) δ(ppm) = 8.56–8.46 (m, 2H), 8.41–8.24 (m, 3H), 8.18–8.03 (m, 2H), 7.68–7.53 (m, 2H), 1.72 (s, 6H). ESI-MS: m/z (%) = 187 (30) [CdL]2+; 262 (50) [LH]+; 472 (45) [CdL(ClO4)]+ IR (KBr, cm−1): ν(C—H)ar = 3066, 3034, νas(CH3) = 2925, νs(CH3) = 2857, γ(C—H)py overtones = 2023–1650, ν(C⚌C)py = 1588, 1572, 1472, 1451, σ(CH3) = 1394, ν(C⚌N)py = 1374, 1256, ρ(C—H)py = 1191, 1182, 1142, γ(C—H)py = 1041, 1019, 792, 654. Anal. Calc. for [Cd(C17H15N3)Cl2] (444.64): C, 45.92; H, 3.40; N, 9.45. Found: C, 45.92; H, 3.41; N, 9.47%.

[CdL(CH3CO2)2] (3) Yield 67% (17.5 mg), 1H NMR (300 MHz, Acetonitrile-d3) δ(ppm) = 8.40 (d, J = 7.9 Hz, 2H), 8.27 (d, J = 7.8 Hz, 1H), 8.20 (d, J = 8.0 Hz, 2H), 7.99 (t, J = 7.8 Hz, 2H), 7.50 (d, J = 7.7 Hz, 2H), 2.80 (s, 6H), 1.72 (s, 6H). ESI-MS: m/z (%) = 473 (100) [CdL(ClO4)]+, 574 (90) [HCdL(ClO4)2]+, 262 [HL]+ IR (KBr, cm−1): ν(C—H)ar = 3069, 3035, νas(CH3) = 2922, νs(CH3) = 2856, γ(C—H)py overtones = 2019–1655, ν(C⚌C)py = 1593, 1569, 1472, 1457, σ(CH3) = 1395, ν(C⚌N)py = 1375, 1258, ρ(C—H)py = 1197, 1182, 1142, γ(C—H)py = 1041, 1016, 790, 650. Anal. Calc. for [Cd(C17H15N3)(CH3CO2)2] (491.82): C, 51.28; H, 4.30; N, 8.54; Found: C, 51.28; H, 4.31; N, 8.55%.

2.3

2.3 X-ray crystallography

Diffraction data were collected by the ω-scan technique at 100(1) K on an Agilent Technologies four-circle Xcalibur diffractometer, with Eos CCD detector and graphite-monochromated Mo Kα radiation (λ = 0.71069 Å). The data were corrected for Lorentz-polarization as well as for absorption effects (Agilent Technologies, 2011). Precise unit-cell parameters were determined by a least-squares fit of 3436 (1), 2992 (2), 3082 (2a), and 3336 (3) reflections of the highest intensity, chosen from the whole experiment. The structures were solved with SIR92 (Agilent Technologies, 2011) and refined with the full-matrix least-square procedure on F2 by SHELXL-2013 (Altomare et al., 1993). All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were placed in idealized positions and refined as ‘riding model’ with isotropic displacement parameters set at 1.2 (1.5 for methyl groups) times Ueq of their carrier atoms. In complex 2, problems with refinement led to the model with Cl/Br disorder, that is with chloride contamination of the supposed bromide. For the crystal selected for the X-ray structure determination, the optimum solution was one for a mixture of di-Cl, di-Br and Cl-Br complexes in an approximately 4:1:1 ratio, i.e. Cl:Br 3:1, although elemental analyses (see above) on the bulk were consistent with Cl:Br 1:1. The displacement parameters of disordered fragments as well as the Cd–Cl and Cd–Br distances in 2 were subjected to appropriate restraints (EADP, DFIX Altomare et al., 1993). The crystals of 2 and 2a (Cl/Br and Cl) are isostructural, which suggests that in fact this system forms a solid solution.

3

3 Results and discussion

The synthesis of the complexes followed well-established procedures for a labile metal ion such as Cd(II). Crystallization was in all cases readily achieved by gradual addition of diethylether to solutions in methanol and gave colourless materials characterized as [CdL(CH3OH)(OClO3)2], 1, [CdL(Cl/Br)2], 2, [CdLCl2], 2a, and [CdL(κ2-O2CCH3)2], 3. Elemental analyses and basic spectroscopic measurements (Supporting Material) were fully consistent with these compositions and X-ray structure determinations (Table 1) were used to fully define the coordination modes in the solids.

Table 1 Crystal data and refinement details.
Compound 1 2 2a 3
Formula C18H19CdCl2N3O9 C17H15Br0.5Cl1.5CdN3 C17H15Cl2CdN3 C21H21CdN3O4·CH3OH
Formula weight 604.66 466.85 444.62 523.85
Crystal system Monoclinic Monoclinic Monoclinic Triclinic
Space group P21/c P21/c P21/c P-1
a (Å) 9.1548(9) 7.3577(2) 7.3693(3) 8.6144(5)
b (Å) 29.0364(16) 17.9905(4) 18.0205(7) 11.1565(6)
c (Å) 8.8010(6) 12.7106(3) 12.6629(4) 11.6459(6)
α (°) 90 90 90 85.743(4)
β (°) 115.005(9) 100.361(2) 100.504(4) 81.641(4)
γ (°) 90 90 90 76.735(5)
V (Å3) 2120.2(3) 1655.05(7) 1653.43(11) 1076.84(10)
Z 4 4 4 2
dx (g cm−3) 1.89 1.87 1.79 1.62
F(0 0 0) 1208 916 880 532
μ (mm−1) 1.34 2.77 1.65 1.05
Θmax (°) 28.00 28.18 26.60 26.65
Reflections
Collected 8802 6651 6090 5992
Unique (Rint) 4347 (0.021) 3413 (0.022) 3151 (0.019) 3887 (0.012)
With I > 2σ(I) 3888 3082 2808 3718
R(F) [I > 2σ(I)] 0.0323 0.0201 0.0331 0.0189
wR(F2) [I > 2σ(I)] 0.0689 0.0485 0.0863 0.0476
R(F) [all data] 0.0387 0.0238 0.0385 0.0202
wR(F2) [all data] 0.0710 0.0497 0.0901 0.0482
Goodness of fit 1.11 1.05 1.05 1.03
Max/min Δρ (e Å−3) 0.78/−1.02 0.35/−0.42 0.92/−0.58 0.56/−0.28

3.1

3.1 Description of the structures

Figs. 2–5 show perspective views of the complexes 1–3 as found in the crystal lattices.

Complex 1: ellipsoids are drawn at the 50% probability level, and hydrogen atoms are depicted as spheres of an arbitrary radius of 0.1 Å.
Figure 2
Complex 1: ellipsoids are drawn at the 50% probability level, and hydrogen atoms are depicted as spheres of an arbitrary radius of 0.1 Å.
Complex 2a: ellipsoids are drawn at the 50% probability level, and hydrogen atoms are depicted as spheres of an arbitrary radius of 0.1 Å. The structure of the mixed Cl/Br complex 2 is essentially identical.
Figure 3
Complex 2a: ellipsoids are drawn at the 50% probability level, and hydrogen atoms are depicted as spheres of an arbitrary radius of 0.1 Å. The structure of the mixed Cl/Br complex 2 is essentially identical.
Complex 3: ellipsoids are drawn at the 50% probability level, and hydrogen atoms are depicted as spheres of an arbitrary radius of 0.1 Å.
Figure 4
Complex 3: ellipsoids are drawn at the 50% probability level, and hydrogen atoms are depicted as spheres of an arbitrary radius of 0.1 Å.
CH⋯O interactions (dashed lines) of a complex unit in the lattice of 1 (all units being equivalent). For clarity, only the perchlorate ligands of complex units which would otherwise project along the line of view of the central unit are shown.
Figure 5
CH⋯O interactions (dashed lines) of a complex unit in the lattice of 1 (all units being equivalent). For clarity, only the perchlorate ligands of complex units which would otherwise project along the line of view of the central unit are shown.

The coordination modes observed in these complexes fit well within the context of those found for Cd(II) complexes of terpyridines in general in the Cambridge Structural Database (CSD), where coordination numbers between 5 and 7 are observed. (For a close comparison with 2a, for example, see Pickardt et al., (1999).) Thus, there is no evidence that the methyl substituents induce significant changes in the ability of the Cd(II) centre to bind ligands in addition to the 6,6″-dimethyl-2,2′:6,2″-terpyridine. Hirshfeld surfaces for the complex molecules in each of the crystal lattices were calculated using CrystalExplorer (Wolff et al., 2012) (excluding 2 because of uncertainty as to how to deal with the mixed halide sites). The form of these surfaces can be used to assess the presence of weak interactions beyond those of dispersion forces (Wolff et al., 2012; Spackman and Jayatilaka, 2009) and quite significant differences between the three complexes are apparent (see Fig. S1 in the ESI). Thus, while so-called π-stacking (Waters, 2013; Ehrlich et al., 2013) is a common feature of lattices of complexes of terpyridine and its derivatives (McMurtrie and Dance, 2005; Scudder et al., 1999), it is not apparent for complex 1, where the labile interactions appear to be largely of the aromatic-CH⋯O-perchlorate type (Fig. 5). In contrast, there do appear to be intermolecular interactions between 6,6″-dimethyl-2,2′:6,2″-terpyridine ligands lying in parallel planes (“stacked”) in complexes 2a and 3. Although in the lattices of all four complexes it is possible to find terpyridine ligand units lying in parallel planes only ∼3.1 Å apart, the overlap in projection of a pair of adjacent units in 1 is very minor, whereas in 2, 2a and 3 it is substantial, with stacked arrays evident down a in 2 and 2a and down c in 3, and, as indicated by the Hirshfeld surfaces for 2a and 3, it involves specific interactions beyond those of dispersion. The major overlap projections for 1, 2a and 3 are shown in Fig. 6, with the specific interactions between terpyridine units in 2a and 3 shown as dashed lines between the atoms involved.

Orthogonal views of the most strongly overlapped pairs of complexes lying in parallel planes in (a) [CdL(CH3OH)(ClO4)2], 1, (b) [CdLCl2], 2a, and (c) [CdL(CH3CO2)2], 3, with interactions beyond dispersion forces, as identified using CrystalExplorer, are shown as dashed lines.
Figure 6
Orthogonal views of the most strongly overlapped pairs of complexes lying in parallel planes in (a) [CdL(CH3OH)(ClO4)2], 1, (b) [CdLCl2], 2a, and (c) [CdL(CH3CO2)2], 3, with interactions beyond dispersion forces, as identified using CrystalExplorer, are shown as dashed lines.

The conformations of the terpyridine ligand, described by the dihedral angles between the aromatic ring planes, are similar in all complexes and the twists between the terminal rings are small: 2.9(2)° in 1, 6.61(10)° in 2, 6.77(17)° in 2a, and 9.86(10)° in 3. As expected for their coordination in a tridentate species, the outer N-atoms of the ligand are in cisoid arrangements relative to the central N-donor. Geometrical parameters defining the primary coordination sphere of each complex are given in Table 2. None can be regarded as exceptional within the general context of Cd(II)/terpyridine coordination. It must be noted however, that M—N bond distances in 1–3 were found to be one of the longest in the series of transition metal complexes of ligand L and its non-methylated derivative terpy (Table S1). This finds reflection in solution behaviour of cadmium species, where partial solvolysis occurs. One should also highlight that 2 and 2a are isostructural, which suggests that in fact this system forms a solid solution (cf. Section 2.3), albeit their luminescence quantum yields were profoundly different.

Table 2 Selected geometrical parameters (Å, °) with s.u.’s in parentheses.
1
Cd1—N1 2.352(3) Cd1—O1B 2.428(2)
Cd1—N8 2.248(3) Cd1—O1C 2.314(2)
Cd1—N14 2.334(3) Cd1—O1D 2.244(2)
O1D—Cd1—N8 161.00(9) O1B—Cd1—O1C 154.80(8)
N1—Cd1—N14 143.65(9)
2
Cd1—N1 2.3872(18) Cd1—Cl1 2.462(2)
Cd1—N8 2.3049(17) Cd1—Cl2 2.453(2)
Cd1—N14 2.3872(18) Cd1—Br1 2.562(2)
Cd1—Br2 2.557(2)
N1—Cd1—N14 140.55(6)
2a
Cd1—N1 2.389(6) Cd1—Cl1 2.4597(10)
Cd1—N8 2.309(3) Cd1—Cl2 2.4816(38)
Cd1—N14 2.379(5)
N1—Cd1—N14 140.36(6)
3
Cd1—N1 2.3848(16) Cd1—O1B 2.2458(14)
Cd1—N8 2.2971(15) Cd1—O2B 2.6740(15)
Cd1—N14 2.3932(15) Cd1—O1C 2.2953(14)
Cd1—O2C 2.5319(15)
N1—Cd1—N14 140.46(5)

3.2

3.2 Absorption and emission spectroscopy

The electronic absorption spectra of the complexes were recorded for 10−5 M solutions in acetonitrile. At such a concentration in a moderately good coordinating solvent such as CH3CN, it could be expected that at least partial solvolysis of the simple ligands would occur. Were the solvent to have been water, where a stability constant value ∼105 M−1 would be anticipated (Hamilton et al., 2011), and at least partial loss of the terpyridine would be expected and to a lesser degree this could occur in CH3CN, so that the solution electronic spectra must be those of various mixed species. The apparent absorption maxima and molar absorptivities for these solution spectra are given in Table 3.

Table 3 Characteristics of the CH3CN solution spectra of complexes 1, 2, 2a and 3.
Compound λmax/nm (ε/104 M−1 cm−1)
1 241 (10.95), 274 (5.97), 292 (7.44), 325 (11.30), 337 (10.98)
2 241 (8.98), 273 (4.97), 283 (6.44), 291,5 (7.78), 323 (10.37), 335 (10.22)
2a 241 (2.45), 273 (1.22), 291 (1.57), 321 (2.35), 332 (2.34)
3 241 (2.66), 284 (1.90), 293 (2.11), 331 (3.18)

The bands are assigned to π–π∗ transitions within the terpyridine ligand as modified by the Cd(II) binding in the different species present. Titration studies performed for solution of L (c = 1 × 10−5 M, MeCN) with cadmium(II) perchlorate were carried out similar to the ones previously done for manganese(II) ions (Wałęsa-Chorab et al., 2011a). This system has been chosen for the sake of comparison with the results of emission studies – solubility issues precluded titration within the given concentration range for cadmium(II) halides and cadmium(II) acetate (supporting information – Fig. S12 and Fig. S13). Speciation of cadmium(II) complexes in coordinating solvent is in line with results from other groups and indeed such was observed herein, i.e. a mixture of 1:2 and 1:1 species is observed in the solution (Lee et al., 2013b). It must be underlined though that attempts to obtain 1:2 species from such titrations or separate synthetic protocols always resulted in formation of monometallic 1:1 complexes as revealed by single crystal X-ray analyses. This could imply that methyl substituents in ligand L might be responsible for favourization of such species in the solid state due to observed intermolecular packing lattices (compare with Figs. 5 and 6) Emission spectra were recorded for both the solid complexes (Fig. 7) and for their solutions in acetonitrile (Fig. 8).

Solid state emission spectra for the free ligand, L, and its complexes 1, 2, 2a and 3, recorded for an excitation wavelength of 292 nm. Inset presents normalized spectra in the 330–450 nm region.
Figure 7
Solid state emission spectra for the free ligand, L, and its complexes 1, 2, 2a and 3, recorded for an excitation wavelength of 292 nm. Inset presents normalized spectra in the 330–450 nm region.
Emission spectra of the complexes 1, 2, 2a and 3 as 10−5 M solutions in acetonitrile (λex 292 nm).
Figure 8
Emission spectra of the complexes 1, 2, 2a and 3 as 10−5 M solutions in acetonitrile (λex 292 nm).

Filled d10 shell of studied cadmium(II) complexes precludes any redox activity or electron transfer processes; therefore, observed fluorescence is a result of ILCT transitions from excited states to the ground state (Hayami et al., 2011). The more complicated forms of the solution spectra are consistent with the presence of multiple species in solution, so that it is difficult to interpret the impressively high values (similar to those of their Zn(II) analogues Wałęsa-Chorab et al., 2011a) of the quantum yields of 0.30, 0.37 and 0.37, respectively, obtained for complexes 1, 2a and 3. All the bands appear to be blue-shifted with respect to the free ligand emission (at 384 nm for excitation at 324 nm (Wałęsa-Chorab et al., 2011a), which may in fact be detectable as a shoulder in the spectra of the complexes, but again no simple comparison is possible, since the free ligand would be expected to adopt a cisoid and not a transoid conformation. Interestingly, the solid state emissions of the complexes are red-shifted compared to the free ligand. Although a precise quantitative comparison has not been made, it appears that the emission intensity is significantly greater in the complexes where significant interactions occur within stacked arrays of the complexes in their lattices. A similar situation has been observed in the complexes M2[UO2(dipic)2] (M = alkali metal, dipic = pyridine-2,6-dicarboxylate), where the unusually strong green emission is associated with extended stacking of the dipicolinate units (Harrowfield et al., 2006). Comparison with related cadmium (II) 2,2′:6′,2″-terpyridine (terpy) compounds seems difficult, due to different measurement conditions as well as noted speciation in solution. It should be however noted that mono-, di- and polymeric cadmium architectures of 1:1 Cd:terpy synthesized by Nawrot et al. showed quantum yields of solid state emission in the range from 1% to 3%, with 2.87% value for monometallic Cd(terpy)(NO3)(N3)(H2O) complex (λem = 589 nm) (Nawrot et al., 2015). Related monometallic [Cd(terpy)(NO3)2(H2O)] species were also found to exhibit emissive properties (Yan et al., 2011). Despite the fact that their quantum yields were not measured, emission from acetonitrile solutions was centred around 350 nm and was solvent dependent, whereas solid state emission at room temperature resulted in green-yellow response at 569 nm.

Normalized emission spectrum (inset of Figs. 7 and S14 supporting information) clarifies that shift towards lower energy is dependent on counterion in the following order: Br−/Cl− (2) > Cl− (2a) > AcO− (3) > ClO4− (1), with ca. 14 nm, 12 nm, 6 nm, and 3 nm respectively with regard to the emission of free ligand. Note that in the solid state, the complexes 2 and 2a show similar behaviour, whereas the quantum yield in solution for 2 was only 0.04, possibly reflecting a quenching effect of free bromide present due to solvolysis. Those have been well established to display efficient intersystem crossing from S1 to the triplet state of aromatic molecules, plausibly attributed to heavy-atom effect (Najbar and Mac, 1991).

In fact, such mixed halide complexes have rarely been investigated – as a consequence of serendipity rather than planning – alas dealing with just syntheses and crystallographic characterization (Chattopadhyay et al., 1992; Park et al., 2001; Hu et al., 2005; Hoogervorst et al., 2004; Momeni et al., 2009). Nag and co-workers though reported detailed studies that concern complexation of 6,6′-bis(bromo/chloromethyl)-2,2′-bipyridines with cobalt(II), nickel(II) and copper(II) halides to produce [M(bpy-Br2−xClx)Cl2−yBry] complexes (Ghosh et al., 2008). Copper(II) analogues were subjected to cyclic voltammetry measurements which revealed solvent as well as structural dependence on E1/2 values of Cu(II)/Cu(I) redox couples. Up to our knowledge, such luminescent effect observed herein within 2/2a systems has not been observed before for tpy related species; therefore, further spectroscopic studies are anticipated with regard to complexes of variously substituted derivatives of ligand L as a function of different counterions.

4

4 Conclusions

Considering the present results within the general context of terpyridine coordination chemistry, it does appear that the introduction of methyl substituents in the 6,6″ positions of 2,2′:6′,2″-terpyridine has certain influence upon the coordination behaviour of the ligand. Although spectroscopic analyses involve existence of both 2:1 and 1:1 (L:Cd(II)) species in solution, only the latter metalloarchitectures form in the solid state as revealed by single crystal X-ray analyses. A rare example of mixed halide species [CdL(Cl/Br)2] (2) should also be emphasized, which exhibits ca. a magnitude lower luminescence quantum yield than its isostructural analogue [CdLCl2] (2a). In fact, crystallization of the complex from 2:1 (L:M) mixture also leads to the formation of monometallic cadmium(II) congeners, what would imply that the role of the methyl groups might be not only responsible for solid-state emission, but also facilitates the formation of 1:1 coordination compounds. The Cd(II) complexes in particular show very similar properties to Cd(II) complexes of both terpyridine itself and some of its derivatives. There is nonetheless the prospect that by influencing the disposition of co-ligands, the methyl groups could indirectly influence the suitability of the complexes for stacking in their crystalline lattices and thus, as it seems from the present results, control the efficiency of emission from the excited solids.

Acknowledgements

This research was carried out as a part of the National Science Centre – Poland project (Grant No. 2011/03/B/ST5/01036) and the National Center for Research and Development – Poland (LIDER/024/391/L-5/13/NCBR/2014).

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Appendix A

Supplementary material

Crystallographic data (excluding structure factors) for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, Nos. 1045909 (1), CCDC-1005802 (2), CCDC-1400326 (2a) and CCDC-1045908 (3). Copies of this information may be obtained free of charge from: The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK. Fax: +44 (1223)336 033, e-mail:deposit@ccdc.cam.ac.uk, or www: www.ccdc.cam.ac.uk. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.arabjc.2016.04.006.

Appendix A

Supplementary material

Supplementary data 1

Supplementary data 1

Supplementary data 2

Supplementary data 2

Supplementary data 3

Supplementary data 3

Supplementary data 4

Supplementary data 4

Supplementary data 5

Supplementary data 5


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