Inorg. Chem. 2008, 47, 4520-4530
Iron Tetraanthracenotetraazaporphyrins: Synthesis, Structural Characterization, Ligand Binding Properties, and Unexpected Selectivity of a Bis-“Bowl” Tetraazaporphyrin Jeffrey P. Fitzgerald,* Joshua R. Lebenson, Guangbin Wang, Gordon T. Yee, Bruce C. Noll, and Roger D. Sommer Departments of Chemistry, United States NaVal Academy, Annapolis, Maryland 21402, Virginia Polytechnic Institute and State UniVersity, Blacksburg, Virginia 24061, DePaul UniVersity, Chicago, Illinois 60614, and Department of Chemistry and Biochemistry, UniVersity of Notre Dame, Notre Dame, Indiana 46556 Received November 1, 2007
The synthesis and characterization, by optical spectroscopy, mass spectrometry, superconducting quantum interference device (SQUID) magnetometry, and single-crystal X-ray diffraction, of six iron complexes of tetraanthracenotetraazaporphyrin (TATAP) are reported. Eight benzo groups, flanking the macrocycle periphery, form a nonpolar “bowl” on each face of the porphyrazine and prevent µ-oxo dimer formation. Fe(TATAP) readily binds THF, a variety of neutral nitrogenous axial ligands, and carbon monoxide. The equilibrium binding constants for the first two are higher than those of analogous porphyrins while those of the latter are smaller. We attribute these differences to the higher π acidity of the porphyrazine ligand. Fe(TATAP) also shows different relative magnitudes of the successive equilibrium binding constants, K1 and K2, for hindered nitrogenous ligands when compared to those of porphyrin analogues. Surprisingly, Fe(TATAP), in toluene solution, shows no affinity for O2 when exposed to 1 atm partial pressure of O2 at 25 °C. These results are explained in terms of an unusually positive iron(III/II) redox potential when coordinated by the TATAP ligand.
Introduction The reversible binding of oxygen and carbon monoxide by ferrous porphyrins and the factors affecting the kinetics and thermodynamics of these processes have been extensively studied since Perutz’s1 and Kendrew’s2 Nobel prizewinning structural characterization of hemoglobin. Over the intervening decades, successive generations of porphyrinbased biomimetic models have explored the effects of solvent polarity, axial ligand basicity, axial ligand strain, distal-side * To whom correspondence should be addressed. E-mail: fitzgera@ usna.edu. Fax: 410-293-2218. (1) (a) Perutz, M. Nature 1970, 228, 726–734. (b) Perutz, M. F. Br. Med. Bull. 1976, 32, 193–208. (c) Perutz, M. F. Sci. Am. 1978, 239, 92– 125. (d) Perutz, M. F. Annu. ReV. Biochem. 1979, 48, 327–386. (2) Nobbs, C. L.; Watson, H. C.; Kendrew, J. C. Nature 1966, 209, 339– 341. (3) (a) Momenteau, M.; Reed, C. A. Chem. ReV. 1994, 94, 659–698. (b) Collman, J. P.; Fu, L. Acc. Chem. Res. 1999, 32, 455–463. (c) Collman, J. P.; Boulatov, R.; Sunderland, C. J.; Fu, L. Chem. ReV 2004, 104, 561–588, and references therein. (4) This ligand is also known as dibenzobarrelenoporphyrazine.
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steric hindrance, distal-side polarity, and distal-side hydrogen bonding on the differential binding of dioxygen and carbon monoxide.3 Discrimination between the gaseous ligands dioxygen and carbon monoxide in these complexes is given by M, defined as the ratio P1/2(O2)/P1/2(CO), where P1/2(O2) and P1/2(CO) are the half-saturation pressures for oxygen and carbon monoxide, respectively. Large values of M indicate a preference for CO binding over O2. M values for biological heme-based oxygen carriers range from 20 (myoglobin) to 150 (T-state hemoglobin) while M values for synthetic analogues range from K2, and no evidence is seen for the formation of the six-coordinate adduct, 6. As expected, a larger K1 value is observed for the (25) (a) Lever, A. J. Chem. Soc. 1965, 1821–1829 and references therein. (b) Dale, B.; Williams, J.; Johnson, C.; Thorp, T. J. Chem. Phys. 1968, 49, 3441–3444. (26) Collman, J.; Hoard, J.; Kim, N.; Lang, G.; Reed, C. J. Am. Chem. Soc. 1975, 97, 2676–2681. (27) Collman, J. P.; Reed, C. A. J. Am. Chem. Soc. 1973, 95, 2048–2049. (28) Collman, J. P.; Gagne, R. R.; Reed, C. A.; Halbert, T. R.; Lang, G.; Robinson, W. T. J. Am. Chem. Soc. 1975, 97, 1427–1439. (29) Jaffe, H.; Orchin, M. Theory and Applications of UltraViolet Spectroscopy; Wiley: New York, 1962; pp 578–583.
Figure 5. Spectroscopic titration of 4, Fe(TATAP), with pyridine in toluene at 25 °C. [Fe(TATAP)]tot ) 1.1 × 10-5 M. [pyr] ) 0–1.0 × 10-3 M. Table 3. Axial Ligation and Equilibrium Binding Constants to 4 at 25 °C
more basic quinuclidine ligand. Brault and Rougee have examined THF binding to ferrous porphyrins and observed only the formation of the five-coordinate adduct with K values ∼ 5.30 Thus, THF binds to Fe(TATAP) several orders of magnitude more tightly than it does to iron(II) porphyrins. Larger binding constants to ferrous porphyrazines have been observed with nitrogenous ligands and are attributed to the higher effective charge on iron due to stronger iron-tomacrocycle backbonding in the porphyrazine complexes.8 The formation of the five-coordinate monoquinuclidine adduct of Fe(TATAP) is unexpected, since, as discussed below, treatment of ferrous porphyrins with strong field ligands such as amines usually yields six-coordinate lowspin complexes, such as Fe(TPP)(piperidine)2, which is characterized by X-ray diffraction.31 These results, however, do not necessarily indicate a fundamental difference in the ligand binding by Fe(TATAP) and Fe(TPP). Quinuclidine, a tertiary amine, may have a larger steric requirement than (30) Brault, D.; Rougee, M. Biochemistry 1974, 13, 4591–4597. (31) Radonovich, L. J.; Bloom, A.; Hoard, J. L. J. Am. Chem. Soc. 1972, 94, 2066–2078.
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Fitzgerald et al. the secondary amine piperidine. Indeed, the crystal structure referenced above shows unexpectedly long Fe-Npip bonds, which are attributed to close contacts of piperidine hydrogen atoms with the porphyrin core. In addition, as mentioned above for the bis-THF complex, formation of the sixcoordinate bis-piperidine complex may be driven by solubility and/or crystal packing forces. We are aware of no solution-phase binding studies of iron(II) porphyrins with quinuclidine. Spectrophotometric titrations of 4 with unhindered aromatic amines show very different behavior. As shown in Figure 5, new bands appear near 355 and 595 nm, and multiple isosbestic points are maintained throughout the titration. Analysis of the absorbance changes as a function of ligand concentration indicates coordination of two axial bases and, therefore, proves the equilibrium is between the four-coordinate and the six-coordinate complexes (i.e., K2 . K1). The five-coordinate complex, Fe(TATAP)L, is not observed, and only the product of the successive equilibrium binding constants, K1K2, can be determined. Similar behavior is observed for ferrous porphyrins32 and tetraazaporphyrins8 and has been explained by a decrease in iron spin state from S ) 1 or 2 to S ) 0, which causes stronger binding of the second axial ligand compared to that of the first. The K1K2 values reported in Table 3 are similar to those reported in the literature for iron(II) octaethyltetraazaporphyrin but are larger than those reported for iron(II) porphyrins. For example, Brault and Rougee have reported K1K2 values of 4 × 107 and 1.3 × 108 for imidazole and pyridine, respectively, binding to deuteroheme in benzene.30,33 The different behavior of Fe(TATAP) with aromatic versus nonaromatic nitrogenous bases is surprising; quinuclidine is several orders of magnitude more basic than both 1-methylimidazole and pyridine34 yet binds only once. This may be a steric effect similar to that observed with 2-substituted imidazoles and ferrous porphyrins.27 The existence of vacant π* orbitals on the aromatic amines suggests that iron-toaxial ligand backbonding may also play a role. Electron donation from the iron dxz and dyz orbitals is further consistent with the change in iron spin state on ligand coordination, as discussed above. Titrations of toluene solutions of 4 with hindered nitrogenous axial ligands (those bearing methyl groups in the 2 position) reveal nonisosbestic changes in the visible spectrum.35 Nevertheless, at higher concentrations, limiting spectra consistent with the six-coordinate bis-ligated complex, Fe(TATAP)L2, 6, are observed. These observations are interpreted as successive binding of two axial ligands where K2 is only slightly greater than K1. Steric interactions between the flanking methyl group and the porphyrazine plane result (32) (a) Collman, J. Acc. Chem. Res. 1977, 10, 265–272. (b) Ellis, P.; Linard, J.; Szymanski, T.; Jones, R.; Dudge, J.; Basolo, F. J. Am. Chem. Soc. 1980, 102, 1889–1896. (33) (a) Brault, D.; Rougee, M. Biochem. Biophys. Res. Commun. 1974, 57, 654–659. (34) Handbook of Chemistry and Physics, 80th ed.; David, R. L., Ed.; CRC Press: New York, 1999. (35) See Figure 8 in the Supporting Information for changes in the visible spectrum of 4 on titration with 1,2-dimethylimidaole.
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in smaller values of both K1 and K2 but a greater reduction in the latter.26 In the five-coordinate complex, the iron atom can move out of the macrocycle plane toward the axial ligand, thereby reducing repulsions between the methyl group and the macrocycle. However, such relief is not possible in the symmetrical six-coordinate complex. Without the visible spectrum of the pure five-coordinate complex, we were unable to determine individual values of K1 and K2, although an estimate was made of their product, K1K2 (see Table 3). Unlike Fe(TATAP), where K2 > K1, ferrous porphyrins bind two equivalents of hindered nitrogenous ligand with successively smaller binding constants. For example, Basolo reports K1 and K2 for the binding of 1,2-dimethylimidazole to iron(II) tetraarylporphyrins as 2.5 × 104 and 2.0, respectively.36 Because K2 1.3 × 10-3 0.0089 g 0.067 c PocPiv 1,2-Me2Im 0.3 C2-Cap 1,2-Me2Im 1 0.20 h a Rougee, M.; Brault, D. Biochemistry, 1975, 14, 4100–4106. b Strauss, S. H.; Holm, R. H. Inorg. Chem. 1982, 21, 863–868, at 20 °C. c Collman, J. P.; Brauman, J. I.; Iverson, B. L.; Sessler, J. L.; Morris, R. M.; Gibson, Q. H. J. Am. Chem. Soc. 1983, 105, 3052–3064. d Wayland, B. B.; Mehne, L. F.; Swartz, J. J. Am. Chem. Soc. 1978, 100, 2379–2383. Value of K, which was converted to P1/2 using the referenced value of Henry’s constant for CO in toluene. e Hashimoto, T.; Dyer, R. L.; Crossley, M. J.; Baldwin, J. E.; Basolo, F. J. Am. Chem. Soc. 1982, 104, 2101–2109, at 23 °C. f Suslick, K. S.; Fox, M. M.; Reinert, T. J. J. Am. Chem. Soc. 1984, 106, 4522–4525. [1,2-Me2Im] calculated based on log K1 ) 4.70 and 99% fivecoordinate Fe(TTPPP). g Collman, J. P.; Brauman, J. I.; Doxsee, K. M. Proc. Natl. Acad. Sci. U.S.A. 1979, 76, 6035–6039. h Linard, J. E.; Ellsi, P. E., Jr.; Budge, J. R.; Jones, R. D.; Basolo, F. J. Am. Chem. Soc. 1980, 102, 1896–1904.
tetraazaporphyrin, the carbon monoxide ligand is known to be a strong σ donor and π acceptor. Thus, the lower CO affinity for iron porphyrazines may be due to the lower electron density on the metal available for backbonding to the carbonyl group. Unfortunately, we have not been able to measure the IR stretching frequencies of the iron tetraazaporphyrin carbonyl complexes to further examine this proposal. However, the IR stretching frequencies of the less labile ruthenium(II) carbonyl complexes are known, and these correlate well with the higher π acidity of the porphyrazine ligand compared to that of the porphyrin ligand.38 Alternatively, the difference in CO affinities could be due to the higher oxidation potential of Fe coordinated by a porphyrazine compared to porphyrin. CO is a strong π (38) ν(CO)’s for Ru(OEP)CO, Ru(OETAP)CO, and Ru(TATAP)CO have been measured at 1976, 2003, and 2027 cm-1, respectively. Similar values have been reported previously for the first two complexes (Whitten et al. J. Am. Chem. Soc. 1975, 97, 277-281 and Collman et al. J. Am. Chem. Soc. 1993, 115, 9309-9310).
acceptor ligand, and any metal that holds onto its electrons more tightly will bind CO less readily. However, the similar CO affinities of Fe(OETAP) and Fe(TATAP) do not correlate with the metal oxidation potential (vide infra). As can be seen in Table 4, the presence of small amounts of aromatic amines raises the CO affinities of Fe(TATAP), as indicated by slight decreases in the P1/2(CO) values. The nitrogenous ligand concentrations were selected based on data in Table 3 such that 90% or more of the Fe(TATAP) remained four-coordinate in the absence of CO. At high nitrogenous ligand concentrations, the CO affinity became negligible due to the lack of an open coordination site. These data are consistent with our earlier observation that Fe(TATAP) binds a second axial ligand more tightly than the first, due to a change in iron spin state. In effect, the nitrogen ligand “primes” the Fe(TATAP) to bind carbon monoxide. Similar results are seen in the literature where comparable CO binding studies have been done (OEP and TPP), although it should be noted that these are slightly different experiments. In the case of the porphyrins, the concentration of 1,2-Me2Im is adjusted such that CO is binding to a fivecoordinate species. In the case of Fe(TATAP), CO is binding to a four-coordinate iron in the presence of a sixth ligand. Oxygen Reactivity. Exposure of a toluene solution of 4 to air or pure oxygen (1 atm at room temperature) results in no reaction on the basis of the lack of changes in the visible spectrum. In an effort to enhance its O2 affinity, Fe(TATAP) was exposed to oxygen in the following: (1) toluene containing 2.0 × 10-5 M 1,2-Me2Im and (2)o-dichlorobenzene, a noncoordinating polar solvent (vide infra). In the former case, no changes were observed in the visible spectrum, and in the latter, Fe(TATAP) extracted Cl from the solvent and formed Fe(TATAP)Cl. The oxygen reactivity of Fe(TATAP), 4, is in sharp contrast to the rapid ( 10σ(I), harvested from the entire data collection. All data were corrected for Lorentz and polarization effects, as well as for absorption. Structure solution by direct methods in space group P1j revealed the non-hydrogen atoms of the Fe(TATAP) moiety. Subsequent difference maps following cycles of least-squares refinement indicated unresolved residual density that could not be modeled into sensible solvent molecules. Using the SQUEEZE routine of PLATON, void space and residual density were found that were consistent with two molecules of benzene and six of chloroform per unit cell. Details of the structure determination are given in the cif file, available as Supporting Information. Supporting Information Available: Visible spectrum changes (nonisosbestic) for 4 when titrated with a hindered nitrogenous ligand, 1,2-dimethylimidazole. X-ray crystallographic results for 8 in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org. IC702149Z
