Communication pubs.acs.org/crystal
Role of Fluorine Interactions in the Solid State Structure and Photophysical Properties of 3,5-Disubstituted-2-(2′-pyridyl)pyrrole Pd(II) Complexes Teresa F. Mastropietro,†,‡ Alessia Aprea,†,‡ Massimo La Deda,‡,§ Iolinda Aiello,†,‡ Mauro Ghedini,†,‡ and Alessandra Crispini*,‡,§ †
Centro di Eccellenza CEMIF.CAL, LASCAMMCNR INSTM, Unità INSTM della Calabria and LiCryl, CNR-INFMUniversità della Calabria, I-87036 Arcavacata di Rende (CS), Italy ‡ Dipartimento di Chimica, Università della Calabria, I-87036 Arcavacata di Rende (CS), Italy § Dipartimento di Scienze Farmaceutiche, Università della Calabria, I-87036 Arcavacata di Rende (CS), Italy S Supporting Information *
ABSTRACT: The analysis of the crystal structures of two new mononuclear Pd(II) compounds, containing 3,5-disubstituted-2-(2′pyridyl)pyrrole ligands, reveals that the use of fluoroaliphatic (1) versus aliphatic (2) substituting groups on the HLn ligand induces substantial changes in their supramolecular architectures. Despite the common molecular geometry suitable for the formation of mixed “aryl embraces”, “tetris-like” columnar heterochiral (1) and helicoidal chiral (2) organizations are formed. The resulting supramolecular packing affects their solid state photophysical properties.
as emission can be quenched or enhanced by aggregation effects.12 In this work, we report on two new mononuclear Pd(II) compounds [(HL1)(L1)PdCl] (1) and [(HL2)(L2)PdCl] (2), containing fluoroalipathic (HL1) or aliphatic (HL2) 3,5disubstituted-2-(2′-pyridyl)pyrrole ligands (Chart 1), coordinated in both chelate and monodentate modes. Five-membered metallacycles are typical of many stable photoactive molecular materials, where the different ligand structures or the nature of the ancillary ligands often affects their chemical−physical properties.13 Moreover, the demonstrated versatile coordination ability of these ligands14 yields the possibility to obtain complexes such as 1 and 2, with molecular geometries suitable for the formation of mixed “aryl pyridyl” embraces in their supramolecular architectures. The effect produced in these systems by the introduction of perfluorinated groups is investigated in this light, proving the ability of fluorine segregation effects to direct the self-assembly of the derived metal complexes and the formation/lack of new mixed aryl embraces. In addition, the impact of the different supramolecular architectures on the solid state photophysical properties of the new complexes is explored, enlightening un
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ntermolecular interactions are of particular significance in many areas of modern chemistry, especially in the field of supramolecular chemistry1 and material design.2 Hydrogen bonds,3 π···π stacking,4 and C−H···π,5 interactions occupy prominent positions among noncovalent interactions, and they have been studied over many years. More recently, this family has been extended to other weak interactions, such as halogen bonding,6 including, among others, C−H···X−C and X···X interactions (X = halogen). With respect to the other halogens, fluorine has a unique behavior. Along with C−F···H−C, C− F···π, and C−F···F−C interactions,7 organic fluorine can offer additional segregation effects8,9 which can be used to modulate the supramolecular structure or generate smart properties in the solid state. Moreover, when a concerted set of π···π stacking and C−H···π interactions occurs between phenyl or, more generally, aryl groups, in systems with aromatic moieties connected by flexible spacers, another supramolecular motif called an “aryl embrace” has been introduced.10 Recent studies have demonstrated that these aromatic interactions can be stabilized by fluorine substitution and used for generating molecular assemblies of growing hierarchical complexity.11 In the field of molecular based materials, all these types of noncovalent interactions can be used to control both the resulting molecular packing and solid state properties. In particular, it has been demonstrated that supramolecular organizations can have a significant impact on the solid state photophysical properties of luminescent inorganic complexes, © 2012 American Chemical Society
Received: March 20, 2012 Revised: April 13, 2012 Published: April 16, 2012 2173
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unexpected aggregation-induced emission generated with fluorination. Although complexes 1 and 2 are not intrinsically chiral, the rigidity of the molecular conformation provided by the crystal lattice generates a sort of conformational enantiomorphism. The chelate and monodentate ligands “wrap” around the metal center, conceiving, in principle, two possible enantiomers, namely Λ and Δ forms (Figure 1a). The enantiomeric pair crystallizes as a racemic mixture in the case of complex 1, in the monoclinic P21/c space group, with two crystallographically independent molecules in the asymmetric unit. On the contrary, 2 crystallizes in the chiral orthorhombic P2(1)2(1)2(1) space group, made up of a single crystallographically independent complex of Λ configuration (Figure 1b).15 In both compounds, two HLn ligands and a peripheral chloride coordinate to the Pd(II) metal center, in a slightly distorted square planar geometry. The neutral monodentate ligand coordinates by means of the N-pyridyl donor atom. The electronic effects exerted by the CF3 substituents in 1 reduce the N(2) electron availability for coordination. A consequent slight elongation of the Pd−N(2) bond distance is observed in 1 (2.025(4) and 2.030(4) Å), if compared with 2 (2.008(2) Å) (Supporting Information Table S2). The metal−chloride bond compensates the reduction in the donation from N(2) to the metal center (2.312(2) and 2.318(2) Å in 1; 2.342(1) Å in 2). In both cases, the pendant pyridylpyrrole bound trans to the Npyridyl atom of the chelated pyridylpyrrolate, with the pyridyl ring almost perpendicular to the Pd(II) coordination plane [88.8(1)° at Pd(1) for 1, 88.4(1)° Pd(2) for 2], as reported in Figure 2 for complex 2. The Cl−Pd−N(3) bond angle in 1 (mean 85.8(1)°) is significantly smaller than that observed in 2 (87.6(1)°), causing a different dihedral angle between the two planes associated to both heterocycle rings of the monodentate ligand (py and pyr: 49.7(2)° in 1 and 35.1(1)° in 2, respectively). In both complexes, the coordinated chloride is involved in an intramolecular interaction with the N−H pyrrolic group, contributing to stabilize the molecular conformation (Figure 1b, Supporting Information Table S3). Despite their similar molecular backbone, the supramolecular organizations of 1 and 2 are very different. As a result of fluorination effects, complex 1 tends to form inversion pairs (of
Chart 1
Figure 1. Possible Λ and Δ enantiomers in complexes 1 and 2 (a) and view of the asymmetric unit in 2 (Λ form) with the atomic numbering scheme (b).
Figure 2. View of the orientation of the pyridine ring of the monodentate ligand with respect to the Pd(II) coordination plane in 2.
Figure 3. Views of the aryl-embrace motif (a) and the columnar organization (b) in 1. 2174
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Figure 4. View of the relative orientation of two consecutive molecular complexes in 1 (a) and 2 (b). The existence of π···π stacking interactions evidenced for 1.
Figure 5. Side (a) and prospective (b) views of the helicoidal motif along the a crystallographic direction in complex 2.
Λ and Δ form) in the solid state (Figure 3a). Indeed, each molecule in 1 assembles into a repetitive dimer motif which, for the mutual disposition of the aryl components, resembles the parallel 4-fold phenyl embrace (P4AE) secondary motif wellknown in metal coordination complexes.10 In this case, two pendant pyridyl rings face each other in an offset face-to-face interaction, fairly displaced by the presence of the CF3 groups [interplanar distance of 3.88 Å and offset of 2.7 Å]. On the other hand, these groups collaborate into the formation of the embrace through F···F interactions [shortest F···F distance of 2.907(6) Å] along the same direction. Orthogonally, the presence of edge-to-face C−H···π interactions between the hydrogen atom of the pendant ligand and the π-system of the coordinated pyridyl rings lying above and below is characterized by very short H---phenyl plane distances (2.82 Å). The
so formed dimer piles up by means of π···π interactions [interplanar distance of 3.41 Å between mean planes N(1)/ C(5) and [N(5)/C(27)]i, i = 1 − x, 1 − y, 1 − z], assembling a columnar motif through a sort of “tetris-like” aryl-embrace organization (Figure 3b). Indeed, the L-shaped molecular complexes work as pieces of a tile-matching puzzle game. On the contrary, π···π interactions are totally absent in 2, as shown in Figure 4. The supramolecular assembly of the complexes originates a helicoidal chiral motif along the a crystallographic direction (Figure 5a and 5b), with the CH3 groups pointing in opposite directions at the outer surface of the helix. Weak C− H···Cl intermolecular interactions [Pd−Cl(1)···H(12a)i 2.856 Å, Cl(1) ···H(12a)−C(12) 138.0°, i = 0.5 + x, 0.5 − y, −z] involving the pendant pyridyl rings can be recognized. 2175
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Figure 6. Absorption spectra in CH2Cl2 of 1 (a) and 2 (b).
withdrawing action raises the energy of the orbital localized on the pyrrole ring, and the resulting blue-shift accounts for the LUMO localization on this fragment. Complexes 1 and 2 are not luminescent in solution, although the respective ligands are found to be emissive (see Supporting Information Figures S3 and S4). However, while complex 1 shows a good solid state luminescence, with a maximum centered at 520 nm and an emission quantum yield of 2.4%, 2 is not emissive in the solid state (Figure 7). Although in the solid state the molecular rotational modes are strongly attenuated with respect to solution, allowing radiative pathways, the lack of emission in the case of 2 indicates that the mechanisms present in solution are still active. Most likely, the distortion of the excited state geometry induced by the rotation of the monodentate ligand favors nonradiative paths in the deactivation processes. Nevertheless, and differently from the case of 2, the radiative deactivation is allowed in 1, probably due to the presence of intermolecular π···π stacking interactions involving the N∧N ligands and exerting a stabilization effect of the electronic state localized onto this fragment.16 In conclusion, the two Pd(II) complexes of general formula [(Ln)(HLn)PdCl] (1 and 2) described here proved to have an overall geometry suitable for the generation of the two Λ and Δ enantiomeric forms, according to the orientation of the monodentate HLn ligand. The existence of secondary packing motifs in 1, linking pairs of molecules in centrosymmetric and, therefore, heterochiral mixed “aryl embraces”, is mainly driven by fluorine interaction effects. In the absence of these types of segregation effects, complex 2 gives rise to chiral crystals, being isolated and analyzed in its Λ form. Consequently, helicoidal chiral motifs are formed, connected by weak C−H---Cl intermolecular interactions involving the pendant pyridyl rings. Moreover, an interesting solid state luminescence is observed in 1. This phenomenon can be attributed to the intermolecular packing of 1, causing a switch from the nonemissive in solution excited state to a solid state emissive transition. These results show that fluoroaliphatic, rather than aliphatic, groups can be fundamental for the induction of otherwise nonfavorable interactions and for the generation of reliable supramolecular synthons and smart solid state properties.
Figure 7. Emission (on the right) and excitation (on the left) spectra of complex 1 recorded from a solid sample.
The photophysical properties of 1 and 2 were explored, at room temperature, in CH2Cl2 solution and in solid state. While the differences in the photophysical properties of 1 and 2 in solution can be attributed to the electronic effects of the CF3 substituents on the single molecule, the spectroscopic features recorded from the solid samples show superimposed packing effects. The absorption spectrum of 2 in CH2Cl2 shows two partially fused bands at 330 and 375 nm, with some shoulders, and a very weak band at 500 nm. When the CF3 groups are introduced, the absorption spectrum suffers a hypsochromic shift: the electronic bands of 1 appear distinct, positioned at 290 and at 345 nm, each having a shoulder, while a lowintensity band shows a maximum at 405 nm (Figure 6). The attribution of these transitions takes advantage of the absorption spectra of the free ligands (see Supporting Information Figures S1 and S2). On this basis, the intense bands of 1 and 2 originate from π−π transitions on the aromatic rings of the N̂ N coordinated ligand, while the shoulders matching every band are due to the same transitions localized on the N-coordinate ligand; the low-intensity bands at 405 and at 500 nm, in 1 and 2, respectively, are due to a π−π excitation on the cyclopalladated ring. The blue-shift of the absorption spectrum of 1 with respect to 2 results from the electronic effect exerted by the fluorine on the pyrrole ring, as is clearly visible in the free ligands spectra: the electron 2176
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8, 265. (c) Chopra, D.; Cameron, T. S.; Ferrara, J. D.; Guru Row, T. N. J. Phys. Chem. A 2006, 110, 10465. (d) Schwarzer, A.; Seichter, W.; Weber, E.; Evans, H. S.; Losada, M.; Hulliger, J. CrystEngComm 2004, 6, 567. (e) Choudhury, A. R.; Guru Row, T. N. Cryst. Growth Des. 2004, 4, 47. (f) Halper, S. R.; Cohen, S. M. Inorg. Chem. 2005, 44, 4139. (10) (a) Dance, I.; Scudder, M. CrystEngComm 2009, 11, 2233. (b) Reger, D. L.; Semeniuc, R. F.; Smith, M. D. Cryst. Growth Des. 2005, 5, 1181. (c) Russel, V.; Scudder, M.; Dance, I. J. Chem. Soc., Dalton Trans. 2001, 789. (d) Dance, I.; Scudder, M. J. Chem Soc., Daton Trans. 1998, 1341. (11) (a) Lorenzo, S.; Lewis, G. R.; Dance, I. New J. Chem. 2000, 24, 295. (b) Rasmusson, T.; Martyn, L. J. P.; Chen, G.; Lough, A.; Oh, M.; Yudin, A. K. Angew. Chem., Int. Ed. 2008, 47, 7009. (12) (a) Yam, V. W.-W.; Chan, K. H.-Y.; Wong, K. M.-C.; Zhu, N. Chem.Eur. J. 2005, 11, 4535. (b) Wadas, T. J.; Wang, Q.-M.; Kim, Y.-j.; Flaschenreim, C.; Blanton, T. N.; Eisenberg, R. J. Am. Chem. Soc. 2004, 126, 16841. (c) Yam, V. W.-W.; Wong, K. M.-C.; Zhu, N. J. Am. Chem. Soc. 2002, 124, 6506. (13) (a) Palladacycles: Synthesis, Characterization and Applications; Dupont, J.; Pfeffer, M., Eds.; Wiley-VCH: Weinheim, Germany, 2008. (b) Ghedini, M.; Aiello, I.; Crispini, A.; Golemme, A.; La Deda, M.; Pucci, D. Coord. Chem. Rev. 2006, 250, 1373. (c) Donnio, B.; Guillon, D.; Deschenaux, R.; Bruce, D. W. In Crabtree, R. H., Mingos, D. M. P., Eds.; Comprehensive Organometallic Chemistry III; Elsevier: Oxford, 2006; Vol. 12. (14) (a) Flores, J. A.; Andino, J. G.; Tsvetkov, N. P.; Pink, M.; Wolfe, R. J.; Head, A. R.; Lichtenberger, D. L.; Massa, J.; Caulton, K. G. Inorg. Chem. 2011, 50, 8121. (b) Pucci, D.; Aiello, I.; Aprea, A.; Bellusci, A.; Crispini, A.; Ghedini, M. Chem. Commun. 2009, 1550. (15) The X-ray data were collected on a Bruker-Nonius X8APEXII CCD area detector diffractometer, using Mo Kα radiation (λ = 0.71073 Å). Data were processed through the SAINT17 reduction and SADABS18 absorption software. The structures were solved with the SHELXTL software package19 by standard direct methods and subsequently completed by Fourier recycling. The final geometrical calculations and the graphical manipulations were performed using the XP utility of the SHELXTL system and the DIAMOND program.20 Crystal data for 1: C44H22Cl2F24N8Pd2, Mr = 1402.40, monoclinic, space group P21/c, a = 20.244(12), b = 18.179(9), c = 13.766(7) Å, β = 99.48(2)°, V = 4997(5) Å3, Z = 4, μ = 0.960 cm−1, θmax = 23.68°, 719 parameters, 0 restraints, 7544 independent reflections, 5715 with I > 2, σ(I), R = 0.0365, wR = 0.0870 (R = 0.0583, wR = 0.1005 for all data), GOF = 0.982. Crystal data for 2: C22H23ClN4Pd, Mr = 485.29, orthorhombic, space group P212121, a = 8.8491(5), b = 9.8091(5), c = 23.6058(13) Å, V = 2049.0(2) Å3, Z = 4, μ = 1.051 cm−1, θmax = 28.28°, 253 parameters, 0 restraints, 5086 independent reflections, 4669 with I > 2, σ(I), R = 0.0238, wR = 0.0578 (R = 0.0293, wR = 0.0607 for all data), GOF = 1.025. (16) Riesgo, E. C.; Hu, Y.-Z.; Bouvier, V.; Thummel, R. P.; Scaltrito, D. V.; Meyer, G. J. Inorg. Chem. 2001, 40, 3413. (17) SAINT, Ver. 6.45; Bruker Analytical X-ray Systems, Inc.: Madison, WI, USA, 2003. (18) Sheldrick, G. M. SADABS Program for Absorption Correction, Version 2.10; Analytical X-ray Systems: Madison, WI, USA, 2003. (19) SHELXTL; Bruker Analytical X-ray Instruments: Madison, WI, USA, 1998. (20) DIAMOND, 3.1b; Crystal Impact GbR, CRYSTAL IMPACT K; Brandeburg & H. Putz GBR: Bonn, Germany, 2006.
ASSOCIATED CONTENT
S Supporting Information *
Synthetic details; crystallographic data and tables; absorption spectra of 1 and 2; absorption and emission of HL1 and HL2; and CCDC reference numbers 826042−826043. For ESI and crystallographic data in CIF other electronic format see DOI: 10.1039/b000000x/. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the MiUR for the financial support through the PRIN 2007 (2007WJMF2W) project. REFERENCES
(1) (a) Lehn, J. M. Supramolecular chemistry: concepts and perspectives; VCH: Weinheim, 1995. (b) Steed, J. W.; Atwood, J. L. Supramolecular chemistry; John Wiley and Sons: Chichester, England, 2000. (c) Comprehensive Supramolecular Chemistry; Atwood, J. L., Davies, J. E. D., Mac Nicol, D. D., Vogtle, F., Lehn, J. M., Eds.; Pergamon: Oxford, U.K., 1996; Vol. 6.G. (d) Desiraju, R. Nature 2001, 412, 397. (2) (a) Crystal Engineering: From Molecules and Crystals of Materials; Braga, D., Grepioni, F., Orpen, A. G., Eds.; NATO ASI Series; Kluwer Academic: Dordrecht, The Netherlands, 1999. (b) Moulton, B.; Zaworotko, M. J. Curr. Opin. Solid State Mater. Sci. 2002, 6, 117. (c) Crystal Engineering: The Design and Applications of Functional Solids; Seddon, K. R., Zaworotko, M. J., Eds.; NATO ASI Series; Kluwer Academic: Dordrecht, The Netherlands, 1998. (3) (a) Aakeröy, C. B.; Seddon, K. R. Chem. Soc. Rev. 1993, 22, 397. (b) Desiraju, G. R. Acc. Chem. Res. 2002, 35, 565. (c) Braga, D.; Maini, L.; Polito, M.; Grepioni, F. Struct. Bonding (Berlin) 2004, 111, 1. (4) (a) Janiak, C. J. Chem. Soc., Dalton Trans. 2000, 3885. (b) Roesky, H. W.; Andruh, M. Coord. Chem. Rev. 2003, 236, 91. (5) The Importance of π-Interactions in Crystal Engineering: Frontiers in Crystal Engineering, 2nd ed.; Edward, R. T., Tiekink, J., ZukermanSchpector, Eds.; John Wiley and Sons: 2012. (6) (a) Santis, A.; Forni, A.; Liantonio, R.; Metrangolo, P.; Pilati, T.; Resnati, G. Chem.Eur. J. 2003, 9, 3974. (b) Metrangolo, P.; Neukirch, H.; Pilati, T.; Resnati, G. Acc. Chem. Res. 2005, 38, 386. (c) Reddy, C. M.; Kirchner, M. T.; Gundakaram, R. V; Padmanabhan, K. A.; Desiraju, G. R. Chem.Eur. J. 2006, 12, 2222. (d) Awwadi, F. F.; Willett, R. D.; Peterson, K. A.; Twamley, B. Chem.Eur. J. 2006, 12, 8952. (7) (a) Thalladi, V. R.; Weise, H.-C.; Boese, R.; Nangia, A.; Desiraju, G. R. Acta Crystallogr., Sect. B 1999, 55, 1005. (b) Prasanna, M. D.; Guru Row, T. N. CrystEngComm 2000, 25. Prasanna, M. D.; Guru Row, T. N. CrystEngComm 2000, 3, 135. (c) Choudhury, A. R.; Urs, U. K.; Smith, P. S.; Goddard, R.; Howard, J. A. K.; Guru Row, T. N. J. Mol. Struct. 2002, 641, 225. (d) Choudhury, A. R.; Guru Row, T. N. CrystEngComm 2006, 8, 265. (8) (a) Chopra, D.; Guru Row, T. N. CrystEngComm 2011, 13, 2175. (b) Berger, R.; Resnati, G.; Metrangolo, P.; Weber, E.; Hulliger, J. Chem. Soc. Rev. 2011, 40, 3496. (c) Reichenbächer, K.; Süss, H. I.; Hulliger, J. Chem. Soc. Rev. 2005, 34, 22. (d) Awwadi, F. F.; Willett, R. D.; Peterson, K. A.; Twamley, B. Chem.Eur. J. 2006, 12, 8952. (e) Thakur, T. S.; Kirchner, M. T.; Bläser, D.; Boese, R.; Desiraju, G. R. CrystEngComm 2010, 12, 2079. (f) Jeannin, O.; Fourmigùe,́ M. Chem.Eur. J. 2006, 12, 2994. (g) Halper, S. R.; Cohen, S. M. Inorg. Chem. 2005, 12, 4139. (9) (a) Rybalova, T. V.; Bagryanskaya, I.; Yu., J. Struct. Chem. 2009, 50, 741. (b) Choudhury, A. R.; Guru Row, T. N. CrystEngComm 2006, 2177
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