C 2005) Journal of Chemical Crystallography, Vol. 35, No. 12, December 2005 ( DOI: 10.1007/s10870-005-5176-y
Crystal and molecular structure of [5,10,15,20tetrakisphenyl-2-ethoxy-3-oxa-chlorinato]Ag(II) Matthias Zeller,(1)∗ Allen D. Hunter,(1) Jason R. McCarthy,(2) (2) ¨ Steven H. Capetta,(2) and Christian Bruckner Received December 1, 2004; accepted April 12, 2005
The silver(II) complex of the chlorin-like derivative [5,10,15,20-tetrakisphenyl-2-ethoxy3-oxa-chlorinato]Ag(II) (1Ag), formally formed by substitution of one pyrrolic subunit in silver meso-tetraphenylporphyrin by one ethoxy-substituted hydrooxazole moiety, crystal˚ c = 9.7361(13) A, ˚ and lizes in the tetragonal space group I 4/m with a = 13.4811(10) A, Z = 2. The molecule, which is asymmetric, accommodates the high-symmetry space group by rotational disorder with the silver atom located on the four-fold axis. KEY WORDS: Porphyrin-analogs; chlorins; silver(II) complexes; disorder.
of the resulting diol chlorin, and subsequent ringclosure reactions.5–10 Several of these pyrrolemodified porphyrins showed dramatically altered photophysical and coordination properties as compared to ‘regular’ porphyrins and chlorins. Key to the understanding of the spectroscopic properties of these pyrrole-modified porphyrins is the study of their conformation and degree of flexibility,6 as any distortion of a porphyrinic chromophore from planarity profoundly influences its optical spectra.11 We found the silver(II) complexes of diol chlorins to be unique starting materials for the synthesis of pyrrole-modified porphyrins.8 One of these pyrrole-modified porphyrins was [5,10,15,20-tetrakisphenyl-2-ethoxy-3-oxachlorinato]Ag(II) (1Ag). The identification of 1Ag could convincingly be deduced on the basis of its spectroscopic and analytical properties. This report will provide the ultimate proof of its unusual structure. Moreover, several solidstate structures were reported for silver(II) complexes of porphyrins,12,13 but silver(II) chlorins
Introduction One of the main driving forces behind current synthetic porphyrin chemistry is the synthesis of chromophores with designed optical properties. Chromophores which absorb or fluoresce red and near-infrared wavelengths are, for instance, of potential use in photomedicine and synthetic light harvesting devices.1,2 Much of this work has focused on the synthesis of expanded and isomeric porphyrins,3 heteroporphyrins and porphyrinanalogs containing non-pyrrolic heterocycles.4 We recently reported the formal replacement of a pyrrolic subunit in meso-tetraarylporphyrins by a non-pyrrolic subunit using a step-wise approach: Dihydroxylation of the porphyrin β,β positions, followed by an oxidative ring opening (1)
Department of Chemistry, Youngstown State University, One University Plaza, Youngstown, Ohio 44555-3663. (2) Department of Chemistry, University of Connecticut, Unit 3060, Storrs, Connecticut 06269-3060. ∗ To whom correspondence should be addressed; e-mail: mzeller@ cc.ysu.edu.
935 C 2005 Springer Science+Business Media, Inc. 1074-1542/05/1200-0935/0
936 or chlorin-like compounds are little studied, and no solid-state structure of a silver complex of a chlorin was hitherto reported. Experimental Synthesis of the title compound [5,10,15,20-Tetrakisphenyl-2-ethoxy-3-oxachlorinato]Ag(II) (1Ag) was synthesized by periodate-mediated diol cleavage of [5,10,15,20tetrakisphenyl-2,3-cis-dihydroxy-chlorinato] Ag (II) (2Ag) in the presence of EtOH, as previously described.8 Single crystals of 1Ag were grown by slow evaporation of a CHCl3 /EtOH solution.
¨ Zeller, Hunter, McCarthy, Capetta, and Bruckner C3B, and C3A have been restrained to be isotropic within a standard deviation of 0.01, and each C2 and O2 and C3A and C3B have been restrained to have identical anisotropic displacement parameters. Both the bond distance as well as the thermal ellipsoid restraints did not result in a significant increase of the R-values or the Goof. Crystal data and experimental details are listed in Table 1.
Results and discussion Synthesis of [5,10,15,20-Tetrakisphenyl2-ethoxy-3-oxa-chlorinato]Ag(II) (1Ag) [5,10,15,20-Tetrakisphenyl-2-ethoxy-3-oxachlorinato]Ag(II) (1Ag) was prepared by
Single crystal X-ray analysis Diffraction data of 2 were collected on a Bruker AXS SMART APEX CCD diffractometer at 100(2) K using monochromatic Mo Kα radiation with omega scan technique. The unit cell was determined using SAINT+.14 The structure was solved by direct methods and refined by full matrix least squares against F2 with all reflections using SHELXTL.14 Refinement of an extinction coefficient was found to be insignificant. All nonhydrogen atoms were refined anisotropically. Hydrogen atoms were placed in calculated positions and were refined with an isotropic displacement parameter 1.2 (C --- H and CH2 ) or 1.5 (CH3 ) times that of the adjacent carbon atom. The hydrooxazole and pyrrole rings are disordered to simulate tetragonal symmetry for the whole molecule. The occupancy ratio, taking special positions into account, has been fixed to reflect the 3:1 ratio for hydrooxazole-to-pyrrole rings, respectively, that is found in an individual molecule. Due to the severe overlap of most atoms in the disordered parts of the molecule, their atomic positions are not very accurate and the bond distances within the ethoxy-hydrooxazole moiety have been restrained to reasonable values.15 The thermal displacement parameters of C11, C2, O2,
Table 1. Crystal Data and Structure Refinement of 1Ag Empirical formula Formula weight Solvent Crystal habit, color Temperature Crystal system Space group ˚ Unit cell dimensions (A) a c ˚ 3) Volume (A Z Density (calculated) Absorption coefficient (mm−1 ) F(000) Crystal size θ range for data collection Index ranges Reflections collected Independent reflections Reflections with I > 2 σ (I) Absorption correction Maximum and minimum transmission Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2σ (I)] R indices (all data) Largest difference peak and hole
C45 H32 N4 O2 Ag1 768.62 CHCl3 /EtOH Block, purple 100(2) K Tetragonal I 4/m 13.4811(10) 9.7361(13) 1769.4(3) 2 1.441 0.615 784 0.425 mm × 0.415 mm × 0.38 mm 2.14◦ –28.28◦ −17 ≤ h ≤ 17, −17 ≤ k ≤ 17, −12 ≤ l ≤ 12 14207 1167 1160 Multi-scan 0.790 and 0.722 1167/50/109 1.211 R1 = 0.0346, wR2 = 0.0916 R1 = 0.0349, wR2 = 0.0922 1.631 and −0.332 e A˚ −3
[2-ethoxy-3-oxa-chlorinato]Ag(II)
937
Scheme 1. Routes of formation of 1Ag.
periodate-mediated diol cleavage of diol chlorin 2Ag in the presence of EtOH (Scheme 1). Thus, one pyrrolic unit of the ultimate starting material meso-tetraphenylporphyrin was formally replaced by an ethoxy-substituted hydrooxazole unit. The initial isolable periodate oxidation product on oxidation of 2Ag in the absence of EtOH is the hydroxy-substituted porpholactol 3Ag.8 This hemiacetal can, in an acid-catalyzed nucleophilic substitution reaction with EtOH, be converted to acetal 1Ag.8 The overall yield for the preparation of 1Ag from 2Ag is 60%. One alternative, but lower yielding, synthesis of 1Ag is by reduction of the corresponding porpholactone 4H2 (overall yield from the ultimate starting material 1H2 is ∼40%).9,10 Since the DIBAL-H reduction of free base 4H2 proceeds only smoothly as its zinc(II) chelate, 4H2 needs to be first converted to its zinc(II) complex, followed by reduction, acetalization under acidic conditions (this step also removes the central metal zinc).10 Silver insertion is accomplished using a well known disproportionation reaction.8
A number of syntheses were reported for porpholactones of type 4H2 .9,16 In the context of the preparation of 1Ag by oxidative cleavage of diol chlorin 2Ag, the permanganate-mediated oxidation of 2H2 is the most relevant.10 Though no mechanisms were proven for the formation of either 1 or 4 by oxidation of 2, the outcome of the reaction and the general reactivity of the oxidants used, allow the conclusion that the intermediates are the corresponding bisaldehyde and biscarboxylate secochlorins, respectively.5 These intermediates presumably leave then the framework carbon as CO or CO2 , respectively, to form the oxazole moiety. Considering the metric parameters of the silver complex 1Ag, it is closely related to mesotetraphenylporphyrin. The bulk of the molecule is determined by the four meso-phenyl groups attached to the planar chromophore (see Fig. 2). From a crystallographic point of view, the main difference between both compounds lies in the loss of symmetry on replacement of one pyrrole by a hydrooxazole unit. A metalloporphyrin, such as
¨ Zeller, Hunter, McCarthy, Capetta, and Bruckner
938
Fig. 1. ORTEP representation of the asymmetric unit of 1Ag (ellipsoids at 30% probability).
silver(II) meso-tetraphenylporphyrin, will exhibit both a four-fold axis as well as a perpendicular mirror plane. On the other hand, the pyrrole-
modified complex 1Ag will be void of any elements of symmetry. Thus, 1Ag might be expected to crystallize in a low-symmetry space group. In contrast to expectations, however, silver complex 1Ag crystallizes in the tetragonal space group I 4/m, with the central silver atom located on the four-fold axis. To accommodate for the overall tetragonal symmetry, each one quarter of the asymmetric molecule has to be rotated by 90◦ , 180◦ or 270◦ around the central silver atom. This disorder is also found when solving the structure of 1Ag in lower symmetry space groups. Based on this observation, it has to be assumed that the degree of rotation of individual molecules is not systematically dependent on each other, but that it is random at least with respect to the whole crystal. This is not totally surprising. A survey of more than 200 structures of meso-tetraphenylporphyrins andmetalloporphyrins demonstrates that the crystal packing of these derivatives is governed by interactions of the periphery of the large and rigid
Fig. 2. ORTEP representation of 1Ag, expanded to show one not disordered molecule (ellipsoids at 30% probability).
[2-ethoxy-3-oxa-chlorinato]Ag(II)
939
meso-phenylporphyrin structure that is nearly invariant with the type of coordinated metals, metal coordination numbers, or clathrated solvent molecules.17 The porphyrins generally pack in body-centered tetragonal or triclinic arrangements. Hence, the modification in 1Ag only affects the filling of the extended channel structure pre-determined by tetragonal packing of the large meso-tetraphenylporphyrinoid structure. The meso-phenyl group flanking the oxazole ring shields this moiety effectively from undergoing inter-molecular interactions and, consequently, the relative orientation of the 1Ag is (nearly) random and the packing unaffected by the flexible ethoxy group filling the otherwise often solventfilled extended channels. The resulting asymmetric unit defining one quarter of an individual molecule is shown in
Fig. 1, and an expanded view of a full molecule is shown in Fig. 2. The asymmetric unit is best described as a superposition of the hydrooxazole with the pyrrole units in a ratio of 1:3. The overall spatial requirements of the two heterocyclic moieties are similar, thus the overlap of the ring atoms is nearly perfect. Only slightly larger than usual thermal ellipsoids are an indication of the disorder present. For this reason, the atoms C1, C4, C5, and N1 as well as the phenyl rings have not been separated into individual contributions for the disordered parts of the molecule. Severe overlap is also observed for the other atoms in the heterocyclic rings and the atom positions for the minor dihydrooxazole ring are not very reliable. Thus, the bond lengths within this ring have been restrained to reasonable values (Table 2).15
˚ and Angles (◦ ) for 1Ag Table 2. Selected Bond Lengths (A) Bond Lengths Ag(1) ---- N(1) N(1) ---- C(4) N(1) ---- C(1) C(1) ---- O(2) C(1) ---- C(5)#1 C(1) ---- C(2) C(2) ---- C(3A) C(3A) ---- C(4) C(4) ---- C(5) C(4) ---- C(3B) O(2) ---- C(3B) C(3B) ---- O(1) O(1) ---- C(10) C(10) ---- C(11) C(5) ---- C(6)
2.079(2) 1.367(3) 1.373(3) 1.36(4) 1.392(4) 1.458(16) 1.407(17) 1.455(7) 1.405(4) 1.41(3) 1.34(4) 1.343(19) 1.440(16) 1.51(2) 1.505(3)
Bond angle N(1)#1 ---- Ag(1) ---- N(1) N(1) ---- Ag(1) ---- N(1)#3 C(4) ---- N(1) ---- C(1) C(4) ---- N(1) ---- Ag(1) C(1) ---- N(1) ---- Ag(1) O(2)#2 ---- C(1) ---- N(1) O(2) ---- C(1) ---- N(1) O(2) ---- C(1) ---- C(5)#1 N(1) ---- C(1) ---- C(5)#1 N(1) ---- C(1) ---- C(2) C(5)#1 ---- C(1) ---- C(2) C(3A) ---- C(2) ---- C(1) C(2) ---- C(3A) ---- C(4) N(1) ---- C(4) ---- C(5) N(1) ---- C(4) ---- C(3B)#2 N(1) ---- C(4) ---- C(3B) C(5) ---- C(4) ---- C(3B) N(1) ---- C(4) ---- C(3A) C(5) ---- C(4) ---- C(3A) C(3B) ---- O(2) ---- C(1) O(2) ---- C(3B) ---- O(1) O(2) ---- C(3B) ---- C(4) O(1) ---- C(3B) ---- C(4) C(3B) ---- O(1) ---- C(10) O(1) ---- C(10) ---- C(11) C(1)#4 ---- C(5) ---- C(4)
90.0 180.0 108.9(2) 125.72(17) 125.40(17) 109.8(16) 109.8(16) 122.5(16) 126.7(2) 108.1(7) 125.2(7) 107.8(11) 105.0(7) 126.3(2) 100.1(12) 100.1(12) 133.1(12) 110.2(3) 123.4(3) 100(3) 122(3) 113(2) 113(2) 117.4(18) 106(2) 125.8(2)
Note. Symmetry transformations used to generate equivalent atoms: #1: −y + 1, x, z; #2: −x + 1, −y + 1, −z + 1; #3: y, −x + 1, −z + 1; #4: x, y, −z + 1.
¨ Zeller, Hunter, McCarthy, Capetta, and Bruckner
940 The central macrocycle of the molecule is planar, with the silver atom 1Ag and the atoms N1, C1, C4, C5, and C6 located on the crystallographic mirror plane perpendicular to the four-fold axis. This finding is the experimental proof for the UV-vis spectroscopically derived conclusion of planarity.8 The atoms C2 and C3a of the pyrrole ring are slightly tilted out of this plane by 0.11(3) ˚ respectively, an observation made and 0.17(2) A, as well for silver meso-tetraphenylporphyrin.13 The largest deviation from planarity in 2 is found for the oxygen atom O2 of the hydrooxazole moi˚ above the mirety, which is located 0.23(6) A ror plane. The hydrooxazole carbon atom C3b
lies basically within the crystallographic mirror ˚ below). The overall rms of plane (0.01(5) A ˚ which the C19 N4 OAg macrocycle is 0.0551 A, ˚ for the is comparable to the value of 0.0681 A C20 N4 Ag moiety in silver tetraphenylporphyrin.13 The ethoxy group, which as a result of the disorder is only one-eighth occupied and therefore not very well defined, shows no unexpected properties. The methyl carbon atom is located very closely to the second four-fold axis and is superimposed with its symmetry-related counterparts. An overall packing diagram showing both disorder and overlap for several molecules is shown in Fig. 3. The phenyl substituents are exhibiting the
Fig. 3. Packing diagram of 1Ag, view along the c-axis. Hydrogen atoms are omitted for clarity. All orientations for the disordered pyrrole and hydrooxazole are shown. Each pyrrole unit above and below the 001 mirror plane is three-eighth occupied, each hydrooxazole above and below the 001 mirror plane is one-eighth occupied.
[2-ethoxy-3-oxa-chlorinato]Ag(II)
941 Supplementary material CCDC-256361 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data request/cif, by e-mailing data
[email protected] or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44(0)1223-336033.
Acknowledgments Scheme 2. Structure of compound 5Ag.
expected geometry and are with a dihedral angle of 89.80(15)◦ nearly perpendicular to the plane of the chlorin molecule. The silver–nitrogen distance observed in ˚ This is slightly shorter than 1Ag is 2.078(2) A. those observed for silver(II) porphyrin complexes, which have Ag–N distances between 2.082(3) and ˚ 12,13 Carbaporphyrinato silver com2.098(5) A. plexes in which the silver atom is N3 C coordinated, on the other hand, exhibit consistently slightly shorter Ag–N bond distances (2.031(5) to ˚ 18,19 as does the single known Ag(II) 2.076(11) A), ˚ 20 corrolato complex (1.944(2) to 1.966(2) A). The structure resembling most closely 1Ag is that of the pyrrole-modified carbaporphyrin 5Ag (Scheme 2).19 Here, the Ag–N bond distances of the N3C coordination sphere are between 2.03(2) ˚ Notably, this structure crystallizes and 2.08(2) A. in the space group P -1 and the AgN3 C macrocycle is not disordered with respect to the position of the pyrrole and dehydrofurane moiety. Evidently, the 2-pyrrolyl group attached to the dehydrofuran ring in combination with having two phenyl and two tolyl substituents imposes enough steric strain to orient the molecule in a unique position in the crystal. A disorder of the pyrrole and the ethoxy groups was reported for 5Ag, but the overall hightemperature factors did not allow it to be properly resolved. In conclusion, we have presented the structural proof for the silver(II) complex of hydrooxazole-modified porphyrin 1Ag, a member of a rare but growing class of pyrrolemodified porphyrins which are distinguished by their chlorin-like spectroscopic properties.
MZ was supported by NSF grant 0111511, and the diffractometer was funded by NSF grant 0087210, by Ohio Board of Regents grant CAP491, and by YSU. CB acknowledges support from the Petroleum Research Fund, administered by the ACS. References 1. Sternberg, E.D.; Dolphin, D.; Br¨uckner, C. Tetrahedron 1998, 54, 4151. 2. Jun-Hong, C.; Nalwa, H.S.; Kosal, M.E.; Rakow, N.A.; Suslick, K.S. In The Porphyrin Handbook, Vol. 6; Kadish, K.M.; Smith, K.M.; Guilard, R., Eds.; Academic: San Diego, 2000, pp. 43– 132. 3. (a) Sessler, J.L.; Gebauer, A.; Vogel, E. In The Porphyrin Handbook, Vol. 2; Kadish, K.M.; Smith, K.M.; Guilard, R., Eds.; Academic: San Diego, 2000, pp. 1–54. (b) Sessler, J.L.; Gebauer, A.; Weghorn, S.J. In The Porphyrin Handbook, Vol. 2; Kadish, K.M.; Smith, K.M.; Guilard, R., Eds.; Academic: San Diego, 2000, pp. 55–124. 4. (a) Lash, T.D. In The Porphyrin Handbook, Vol. 2; Kadish, K.M.; Smith, K.M.; Guilard, R., Eds.; Academic: San Diego, 2000, pp. 125–200. (b) Latos-Grazynski, L. In The Porphyrin Handbook, Vol. 2; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, 2000, pp. 361–416. 5. (a) Br¨uckner, C.; Rettig, S.J.; Dolphin, D. J. Org. Chem. 1998, 63, 2094. (b) Br¨uckner, C.; Sternberg, E.D.; MacAlpine, J.K.; Rettig, S.J.; Dolphin, D. J. Am. Chem. Soc. 1999, 121, 2609. 6. Campbell, C.J.; Rusling, J.F.; Br¨uckner, C. J. Am. Chem. Soc. 2000, 122, 6679. 7. (a) Daniell, H.W.; Br¨uckner, C. Angew. Chem., Int. Ed. 2004, 43, 1688. (b) McCarthy, J.R.; Hyland, M.A.; Br¨uckner, C. Chem. Commun. 2003, 1738. (c) McCarthy, J.R.; Hyland, M.A.; Br¨uckner, C. Org. Biomol. Chem. 2004, 2, 1484. 8. McCarthy, J.R.; Melfi, P.J.; Capetta, S.H.; Br¨uckner, C. Tetrahedron 2003, 59, 9137. 9. McCarthy, J.R.; Jenkins, H.A.; Br¨uckner, C. Org. Lett. 2003, 5, 19. 10. Br¨uckner, C.; McCarthy, J.R.; Daniell, H.W.; Pendon, Z.D.; Ilagan, R.P.; Francis, T.M.; Ren, L.; Birge, R.R.; Frank, H.A. Chem. Phys. 2003, 294, 285. 11. (a) Shelnutt, J.A.; Song, X.-Z.; Ma, J.-G.; Jentzen, W.; Medforth, C.J. Chem. Soc. Rev. 1998, 27, 31. (b) Haddad, R.E.; Gazeau, S.; P´ecaut, J.; Marchon. J.-C.; Medforth, C.J.; Shelnutt, J.A. J. Am. Chem. Soc. 2003, 125, 1253. 12. (a) Wong, W.-K.; Zhang, L.; Wong, W.-T.; Xue, F.; Mak, T.C.W. J. Chem. Soc., Dalton Trans. 1999, 615. (b) Senge, M.O.; Runge, S.; Speck, M.; Ruhlandt-Senge, K. Tetrahedron 2000, 56, 8927.
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