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J. Am. Chem. Soc. 1999, 121, 1870-1878
A Study of Solid [{Cu(MePY2)}2O2]2+ Using Resonance Raman and X-ray Absorption Spectroscopies: An Intermediate Cu2O2 Core Structure or a Solid Solution? Elna Pidcock,† Serena DeBeer,† Honorio V. Obias,‡ Britt Hedman,*,†,§ Keith O. Hodgson,*,†,§ Kenneth D. Karlin,*,‡ and Edward I. Solomon*,† Contribution from the Department of Chemistry, Stanford UniVersity and Stanford Synchrotron Radiation Laboratory, SLAC, Stanford, California 94305, and Department of Chemistry, Johns Hopkins UniVersity, Baltimore, Maryland 21218 ReceiVed September 28, 1998 Abstract: Solid [{Cu(MePY2)}2O2]2+ is spectroscopically characterized using resonance Raman and X-ray absorption spectroscopy, for which the former technique probes the nature of the O-O bond and the latter defines the Cu-Cu interaction. In contrast to the crystal structure obtained for [{Cu(MePY2)}2O2]2+, which shows an “intermediate” Cu2O2 core (Cu-Cu ) 3.4 Å and O-O ) 1.6 Å), resonance Raman peaks characteristic of both a side-on peroxide-bridged dicopper(II) core and bis-µ-oxo dicopper(III) core are observed. The bisµ-oxo isomer is estimated to be present at approximately 5-20%. A good fit is obtained for EXAFS data for solid [{Cu(MePY2)}2O2]2+ using an 80:20 ratio of Cu-Cu separations of 3.6 Å (characteristic of a side-on peroxide-bridged copper core) and 2.8 Å (associated with a bis-µ-oxo dicopper core). Analysis of the edge region places an upper limit on the amount of bis-µ-oxo isomer present in the solid at 40%. The factors governing the presence of bis-µ-oxo and/or side-on peroxide cores in solution for differing ligand systems are considered, and the contribution of the bite angle of the equatorial nitrogen atom donors is explored. The reactivity of [{Cu(MePY2)}2O2]2+ in solution is correlated with the presence of the bis-µ-oxo core, using frontier molecular orbital theory.
Introduction The oxygen transport protein hemocyanin and the monooxygenase enzyme tyrosinase have been studied extensively.1,2 A crystal structure obtained of the oxy form of hemocyanin (oxyHc) showed oxygen bound as peroxide in a side-on, µ-η2:η2 fashion between the two copper(II) centers.3 The electronic absorption spectrum of oxyHc has bands at 345 nm ( ) 20 000 M-1 cm-1) and a weak band at 570 nm ( ) 1000 M-1 cm-1), and the peroxide O-O stretch is observed at 740 cm-1.4 The absorption and resonance Raman spectra collected for oxy-tyrosinase are very similar to those obtained for oxyhemocyanin, and it is generally accepted that tyrosinase also binds peroxide in a [Cu2(µ-η2:η2)(O2)]2+ core. Tyrosinase activates O2 for reaction with phenols, oxygenating them to o-quinones.5 It has been shown that the monophenol substrate * To whom correspondence should be addressed at Stanford University. Prof. E. I. Solomon: FAX: (650) 725-0259. E-mail: solomon@ chem.stanford.edu. †Department of Chemistry, Stanford University. ‡Department of Chemistry, Johns Hopkins University. § Stanford Synchrotron Radiation Laboratory, SLAC, Stanford University. (1) (a) Eickman, N. C.; Himmelwright, R. S.; Solomon, E. I. Proc. Natl. Acad. Sci. U.S.A. 1979, 76, 2094-2098. (b) Lowery, M. D.; Solomon, E. I. Science 1993, 259, 1575-1581. (c) Solomon, E. I.; Sundaram, U. M.; Machonkin, T. E. Chem. ReV. 1996, 96, 2563-2605. (d) Magnus, K. A.; Ton-That, H.; Carpenter, J. E. Chem. ReV. 1994, 94, 727-735. (2) (a) Lerch, K. Enzymatic Browning and its PreVention; American Chemical Society: Washington, DC, 1995. (b) Sa´nchez-Ferrer, A.; Rodrı´guez-Lo´pez, J. N.; Garcı´a-Ca´novas, F.; Garcı´a-Carmona, F. Biochim. Biophys. Acta 1995, 1247, 1-11. (3) Magnus, K. A.; Hazes, B.; Ton-That, H.; Bonaventura, C.; Bonaventura, J.; Hol, W. J. G. Proteins 1994, 19, 302-309. (4) Solomon, E. I.; Baldwin, M. J.; Lowery, M. D. Chem. ReV. 1992, 92, 521-542.
binds directly to copper at the active site,6 but it has not been established whether O-O bond scission occurs prior to, or concomitant with, substrate activation. This question has become increasingly important with the isolation of [Cu(III)2(µ-O)2]2+ cores, (characterized by intense absorption bands at 300-350 nm and 400-450 nm and an intense resonance Raman peak assigned as a Cu-O stretch at ∼600 cm-1),7,8 and the demonstration that interconversion of the [Cu2(µ-η2:η2)(O2)]2+ and [Cu(III)2(µ-O)2]2+ cores can be effected by changing the solvent.9 Studies performed on a series of complexes, [Cu2(NnPY2)(O2)]2+ (where two bis(2-pyridylethyl) amine units (PY2) are linked via the amino nitrogen by a -(CH2)n- chain (where n ) 3,4,5)) showed that dioxygen is bound in a side-on, µ-η2:η2 fashion.10 The absorption spectra of the peroxide-bridged binuclear copper cores have bands characteristic of a planar sideon peroxide core at 360 nm and 520-600 nm (depending on the length of the alkyl chain) as well as an additional band in the region 420-490 nm.11,12 It was established that the “extra” (5) Jolley, R. L.; Evans, L. H.; Makino, N.; Mason, H. S. J. Biol. Chem. 1974, 249, 335-345. (6) Wilcox, D. E.; Porras, A. G.; Hwang, Y. T.; Lerch, K.; Winkler, M. E.; Solomon, E. I. J. Am. Chem. Soc. 1985, 107, 4015-4027. (7) Mahapatra, S.; Halfen, J. A.; Wilkinson, E. C.; Pan, G.; Wang, X.; Young, V. G.; Cramer, C. J.; Que, L., Jr.; Tolman, W. B. J. Am. Chem. Soc. 1996, 118, 11555-11574. (8) Mahadevan, V.; Hou, Z. G.; Cole, A. P.; Root, D. E.; Lal, T. K.; Solomon, E. I.; Stack, T. D. P. J. Am. Chem. Soc. 1997, 119, 1199611997. (9) Halfen, J. A.; Mahapatra, S.; Wilkinson, E. C.; Kaderli, S.; Young, V. G.; Que, L., Jr.; Zuberbu¨hler, A. D.; Tolman, W. B. Science 1996, 271, 1397-1400. (10) Pidcock, E.; Obias, H. V.; Abe, M.; Liang, H.-C.; Karlin, K. D.; Solomon, E. I. J. Am. Chem. Soc. 1999, 121, 1299-1308.
10.1021/ja983444s CCC: $18.00 © 1999 American Chemical Society Published on Web 02/20/1999
Study of Solid [{Cu(MePY2)}2O2]2+
J. Am. Chem. Soc., Vol. 121, No. 9, 1999 1871
Scheme 1
band is an O22- f Cu(II) CT band, and its energy and intensity are correlated with the degree of “butterflying” of the Cu2O2 plane due to the short methylene linker.10 The more planar the core, the higher the energy and the lower the intensity of the O22- f Cu(II) CT band at 420-490 nm. In the planar limit, e.g., [Cu(HB(3,5-i-Pr2pz)3)]2(O2),13 this band has no intensity. A related series of complexes, [Cu2(XYL)]2+, where the Cu(PY2) centers are linked, not by an alkyl chain but by a xylyl unit, react with dioxygen and hydroxylate the arene ring of the ligand.14,15 The absorption spectrum of the O2 bound intermediate in [Cu2(NO2-XYL)(O2)]2+ is very similar to those obtained for the [Cu2(NnPY2)(O2)]2+ series, with bands at 358 nm ( ) 20 000 M-1 cm-1), 435 nm ( ) 5000 M-1 cm-1), and 530 nm ( ) 1200 M-1 cm-1).14 It was determined, using resonance Raman spectroscopy, that dioxygen binds as peroxide in a sideon geometry and that it is this species and not an unobservable amount of bis-µ-oxo isomer which is likely to be the reactive intermediate in the electrophilic attack of the aromatic ring in the hydroxylation reaction.16 A mononuclear complex [Cu(I)(MePY2)]+, (Scheme 1) where the amine nitrogen of the PY2 unit is substituted with a methyl group (cf. the Nn series where the PY2 units are linked by an alkyl chain) reacts with dioxygen to form a binuclear complex [{Cu(MePY2)}2O2]2+ with an absorption spectrum similar to those observed for both [Cu2(NnPY2)(O2)]2+ and [Cu2(NO2XYL)(O2)]2+, with bands at 355 nm ( (M-1 cm-1) 14 700), 410 nm (2500) and 530 nm (400).17 The geometry at the two copper centers of the binuclear [{Cu(MePY2)}2O2]2+ complex is unconstrained by the presence of a linker, and hence the Cu2O2 unit is expected to be close to planar; the additional band observed in the absorption spectra obtained for the linked Cu(PY2)-containing complexes should have no intensity in the planar Cu2O2 core (vide supra). Resonance Raman spectroscopic studies for solutions of [{Cu(MePY2)}2O2]2+ have shown that the extra band at 410 nm is not due to a butterfly Cu2O2 core, but rather to the presence of a small amount (1-10%) of the bis-µ-oxo isomer, in contrast to the results obtained for [Cu2(NnPY2)(O2)]2+ and [Cu2(NO2XYL)(O2)]2+.18 It has been observed that [{Cu(MePY2)}2O2]2+ in solution can effect clean H-atom abstraction reactions with exogenously added hydrocarbon substrates, reactivity which is (11) Karlin, K. D.; Tyekla´r, Z.; Farooq, A.; Haka, M. S.; Ghosh, P.; Cruse, R. W.; Gultneh, Y.; Hayes, J. C.; Zubieta, J. Inorg. Chem. 1992, 31, 1436-1451. (12) Karlin, K. D.; Haka, M. S.; Cruse, R. W.; Meyer, G. J.; Farooq, A.; Gultneh, Y.; Hayes, J. C.; Zubieta, J. J. Am. Chem. Soc. 1988, 110, 1196-1207. (13) Kitajima, N.; Fujisawa, K.; Fujimoto, C.; Moro-oka, Y.; Hashimoto, S.; Kitagawa, T.; Toriumi, K.; Tatsumi, K.; Nakamura, A. J. Am. Chem. Soc. 1992, 114, 1277-1291. (14) Karlin, K. D.; Nasir, M. S.; Cohen, B. I.; Cruse, R. W.; Kaderli, S.; Zuberbu¨hler, A. D. J. Am. Chem. Soc. 1994, 116, 1324-1336. (15) Karlin, K. D.; Hayes, J. C.; Gultneh, Y.; Cruse, R. W.; McKown, J. W.; Hutchinson, J. P.; Zubieta, J. J. Am. Chem. Soc. 1984, 106, 21212128. (16) Pidcock, E.; Obias, H. V.; Xin Zhang, C.; Karlin, K. D.; Solomon, E. I. J. Am. Chem. Soc. 1998, 120, 7841-7847. (17) Sanyal, I.; Mahroof-Tahir, M.; Nasir, M. S.; Ghosh, P.; Cohen, B. I.; Gultneh, Y.; Cruse, R. W.; Farooq, A.; Karlin, K. D.; Lui, S.; Zubieta, J. Inorg. Chem. 1992, 31, 4322-4332. (18) Obias, H. V.; Lin, Y.; Murthy, N. N.; Pidcock, E.; Solomon, E. I.; Ralle, M.; Blackburn, N. J.; Neuhold, Y.-M.; Zuberbu¨hler, A. D.; Karlin, K. D. J. Am. Chem. Soc. 1998, 120, 12960-12961.
Figure 1. X-ray crystal structure of [{Cu(MePY2)}2O2](BArF)2. The bond distances and angles for the Cu2O2 core are as follows: Cu1-Cu2 ) 3.445(2) Å, Cu1-O1 ) 1.905(5) Å, O1-Cu2 ) 1.922(6) Å, O1-O2 ) 1.666(12) Å, O1-Cu1-O2 ) 51.6(3)°, Cu1-O1-Cu2 ) 128.4(3)°.
not observed for binuclear copper complexes with linked PY2 units, [Cu2(NnPY2)(O2)]2+ and [Cu2(NO2-XYL)(O2)]2+. An X-ray crystal structure has been obtained for [{Cu(MePY2)}2(O2)]2+.19,20 Surprisingly, the copper-copper separation was found to be 3.45 Å, and the O-O distance was 1.67 Å (Figure 1); a structure which is intermediate between that of a side-on peroxide core (Cu-Cu ) 3.6 and O-O ) 1.41 Å) and a bis-µ-oxo core (Cu‚‚‚Cu ) 2.9 and O-O ) 2.3 Å). We have studied the solid form of [{Cu(MePY2)}2O2]2+ using resonance Raman spectroscopy to probe the nature of the bound dioxygen and X-ray absorption spectroscopy (XAS) to examine the copper-copper interaction to establish whether solid [{Cu(MePY2)}2O2]2+ comprises a mixture of the side-on peroxide and bis-µ-oxo isomers, analogous to the solution, or if the “intermediate” core dimensions given by the crystal structure represent the true structure. A discussion of the reactivity observed for [{Cu(MePY2)}2O2]2+ in solution and the factors which govern the formation of side-on peroxide and bis-µ-oxo isomers is presented. Experimental Section Preparation of [{Cu(MePY2)}2(O2)]2+. Under a blanket of argon, 1 mmol of [(MePY2)Cu(I)(CH3CN)][BArF]18 (where BArF is B[3,5-(CF3)2C6H3]4-) and a spin bar were transferred to a 100-mL flame-dried Schlenk flask. Then, 40 mL of freshly distilled, degassed dichloromethane was added. The yellow solution was stirred for 15 min and filtered through a mediumsized frit into another flame-dried 100-mL Schlenk flask. This solution was cooled to -95 °C (methanol/liquid nitrogen bath) and stirred at this temperature for 15 min. The cold, yellow solution was bubbled through with ultrahigh purity dioxygen (99.999%), passed through two cold traps maintained at -78 and -10 °C for about 1-5 min. Formation of a dark colored solution was observed. The solution was allowed to stand at this temperature for 30 min, whereupon 80 mL of precooled (-10 °C) freshly distilled degassed heptane was added dropwise. The resulting mixture was kept at this temperature for a few hours until the supernatant was almost clear. The brown precipitate was filtered through a precooled coarse frit maintained at -100 °C. Precooled heptane was used to wash the precipitate, followed by precooled pentane. Resonance Raman Spectroscopy. Resonance Raman data were obtained using a Princeton Instruments ST-135 back(19) H. V. Obias, 1998, Ph.D. Dissertation, Johns Hopkins University (20) Obias, H. V.; Lin, Y.; Murthy, N. N.; Neuhold, Y.-M.; Zuberbu¨hler, A. D.; Karlin, K. D., manuscript in preparation.
1872 J. Am. Chem. Soc., Vol. 121, No. 9, 1999 illuminated CCD detector on a Spex 1877 CP triple monochromator with 1200, 1800, and 2400 grooves/mm holographic spectrograph gratings. The excitation was provided by Coherent I90C-K Kr+ and Innova Sabre 25/7 Ar+ CW lasers. A polarization scrambler was used between the sample and the spectrometer. Spectral resolution was
