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J. Phys. Chem. B 2001, 105, 7554-7563
A Continuous Wave and Pulse EPR and ENDOR Investigation of Oxygenated Co(II) Corrin Complexes Sabine Van Doorslaer,*,† Arthur Schweiger,† and Bernhard Kra1 utler‡ Physical Chemistry Laboratory, Swiss Federal Institute of Technology, 8092 Zurich, Switzerland, and Institute for Organic Chemistry, UniVersity of Innsbruck, A-6020 Innsbruck, Austria ReceiVed: NoVember 22, 2000; In Final Form: May 1, 2001
Heptamethyl cobyrinate perchlorate, [Cob(II)ester]ClO4, has the relevant structural features of base-off B12r, the reduced Co(II) form of vitamin B12. The reversible oxygenation behavior of this complex in different solvents is investigated using continuous wave (cw) EPR at X-band and compared with that of Co(II) porphyrin complexes. Furthermore, the influence of the addition of a nitrogen base (pyridine or 1-methylimidazole) to the solutions is investigated. To determine the electronic structure of the oxygenated complexes, different pulse electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) techniques are applied. The g and cobalt hyperfine matrix and their principal axes are determined using a combination of cw-EPR at X- and Q-band, ESE (electron spin-echo)-detected EPR at W-band and Davies-ENDOR at Q-band. The experimental g and cobalt hyperfine values are found to be sensitive to the change of solvent, addition of a nitrogen base, and change in the ring structure. From the HYSCORE (hyperfine sublevel correlation) spectra measured at X-band, the interactions with the corrin nitrogen nuclei and the nitrogens of the axial base are deduced. Comparison of these data with those of oxygenated base-on Co(II) porphyrin complexes revealed shorter axial cobalt-nitrogen bonds in the Co(II) corrin than in the porphyrin case. On the other hand, the nitrogen atoms of the corrin and of the porphyrin ligands show similar, small interactions, which is due to the fact that the unpaired electron resides mainly on the O2 fragment.
1. Introduction The ability of Co(II) complexes to reversibly bind dioxygen has been studied extensively during the past three decades.1-3 A large variety of Co(II) compounds can reversibly bind dioxygen, including Schiff base complexes,4,5 cobaloximes(II),6 Co(II) porphyrins7 and Co(II) corrins.8,9 The structures of the different dioxygen adducts and the dynamics of binding of the O2 moiety have mainly been investigated by using electronic paramagnetic resonance (EPR) spectroscopy.1,2,10 In several cases, X-ray analyses have provided accurate three-dimensional structures in single crystals of Co(II)-dioxygen adducts.9,11 The kinetics of dioxygen binding and dissociation and the thermodynamics of the redox process have been studied in great detail.12,13 Different models have been proposed to describe the electronic structure of the oxygen carriers and the nature of the bound O2.14,15 Cobalt oxygen carriers have also been studied extensively for their use in industrial applications, in particular in gas separation processes, because of the highly selective transport of oxygen through polymer membranes16 and their ability to act as electrocatalysts for the reduction of O2 at graphite electrodes17 and as electrochemical dioxygen generators.18 The unique oxygen binding properties of Co(II) porphyrins have obtained considerable attention in biochemical studies, mainly because these complexes can be used as model systems for the iron-containing heme proteins hemoglobin and myoglobin.2,19 The analysis of the electronic and geometric structure of the oxygenated native heme proteins is found to be very * Corresponding author. † Swiss Federal Institute of Technology. ‡ University of Innsbruck.
difficult because of their fast autoxidation rates and the fact that they are diamagnetic and therefore EPR silent. Through the chemical substitution of ferrous protoporphyrin IX with cobaltous porphyrin in hemoglobin and myoglobin,19 both the oxy and deoxy species of the proteins could be studied with EPR and electron nuclear double resonance (ENDOR) spectroscopy.20-24 The naturally occurring cobalt(III) corrins, which are related to vitamin B12, are indispensable cofactors in a variety of biological processes, including some in human metabolism.25,26 As a rule, the B12 derivatives are involved in unique organometallic biological reactions. Frequently, cobalt(II) corrins, such as cob(II)alamin (or B12r), are relevant intermediates in such enzymatic processes but are formed transiently and are difficult to observe. EPR and, more recently, ENDOR spectroscopy have provided a unique opportunity to study these paramagnetic Co(II) species.2,27 Using specific experimental precautions, the Co(II) corrin cob(II)alamin (B12r) was found to bind dioxygen reversibly.8,9 In solution, vitamin B12r is immediately oxidized to B12a when brought in contact with oxygen at room temperature, but at lower temperatures all of the autoxidation steps, except the first, proceed slowly.28 Heptamethyl cobyrinate perchlorate, [Cob(II)ester]ClO4 (1),29,30 has the relevant structural features of base-off B12r, the reduced Co(II) form of vitamin B12 in which the intramolecular dimethylbenzimidazole is not coordinated to the cobalt (hence base-off). The corrin ligand of (1) is derived directly from B12 and differs from that of base-off B12r in the periphery only, where the six carboxamide substituents and the characteristic nucleotide function of base-off B12r are replaced by methyl ester groups. However, the essential features of the geometry of the corrin ligand are retained. In solution, the Co(II)corrinate (1) is
10.1021/jp004270f CCC: $20.00 © 2001 American Chemical Society Published on Web 07/12/2001
EPR and ENDOR Study of Co(II) Corrin Complexes SCHEME 1
J. Phys. Chem. B, Vol. 105, No. 31, 2001 7555 TABLE 1: Overview of the Different Complexes and Their Numeration Mentioned in This Papera number
name of complex
(1) (1′) (2) (3) (4) (5) (6)
[Cob(II)ester]ClO4 [Cob(II)ester]+ [Cob(II)ester‚O2]+ [Cob(II)ester‚py‚O2]+ [Cob(II)ester‚1MeIm‚O2]+ [Co(II)TPP‚py‚O2] [Co(II)TPP‚1MeIm‚O2]
a TPP ) tetraphenylporphyrin, Cob(II)ester ) heptamethyl cobyrinate, 1MeIm ) 1-methylimidazole, py ) pyridine.
known to be less readily oxidized in air than is B12r. Its crystal structure showed a pentacoordinate Co(II) center to which the perchlorate counterion is axially coordinated.30 In an earlier exploratory investigation, a cold solution of heptamethyl cob(II)yrinate iodide in toluene31 was left in contact with air and the EPR spectrum of the oxygenated Co(II) complex could be observed.32 The present work is part of a larger study of Co(II) porphyrins, Co(II) corrins, and their dioxygen complexes with modern pulse EPR and ENDOR spectroscopy.33,34 It reports on the oxygenation behavior of solutions of (1) in methanol and in toluene as investigated using continuous-wave (cw) EPR at X-band frequencies (≈9 GHz). Furthermore, the effect of the addition of the nitrogen bases pyridine and 1-methylimidazole is studied. For all of these complexes, the g and cobalt hyperfine matrices are determined at low temperature by means of cwEPR at X- and Q-band (35 GHz), of echo-detected EPR at W-band (95 GHz) and of Davies-ENDOR at Q-band. The hyperfine and nuclear quadrupole interactions of the nitrogens of the corrin ring and of the axial bases are measured at X-band using two-dimensional hypefine sublevel correlation (HYSCORE) spectroscopy. The influence of the solvent and the axial ligand on these parameters is discussed. Comparisons are made with our previous study on (oxyCo)TPP(L) systems (TPP ) tetraphenylporphyrin, L ) pyridine, 1-methylimidazole).33 The studies of dioxygen complexes of the Co(II) corrin (1) by continuous wave and pulse EPR and ENDOR spectroscopy provide the first detailed insights into the redistribution of the electron spin into the corrin ring system and in the axial ligand, upon binding of dioxygen. Such studies on Co(II) corrins by modern EPR spectroscopic means are important to evaluate the specific bonding characteristics of the unique corrin ligand Vis a Vis the more symmetric (and more “common”) porphyrin ligand in their respective Co(II) complexes. The extension of such investigations to the cases of protein-bound Co(II) corrins will help to solve questions concerning the structure and dynamics of the radical(oid) intermediates in enzymatic reactions catalyzed by coenzyme B12 and related organometallic B12 derivatives (for review, see refs 25 and 26). 2. Materials and Methods 2.1. Sample Preparation. Heptamethyl cobyrinate perchlorate,[Cob(II)ester]ClO4 (1), was prepared as described in ref 30. As solvents, purified absolute toluene and methanol (Fluka, puriss., absolute) were used. Pyridine (py) and 1-methylimidazole (1MeIm) were purchased from Fluka (pro analysis). [Cob(II)ester]ClO4 was dissolved in the two solvents to a final
concentration of 5 × 10-4 M. To obtain the base-on complexes (axial ligation of a nitrogen base to the cobalt), pyridine or 1MeIm was added to the solutions (5 × 10-3 M of the nitrogen base). After mixing the components, the solutions were transferred to EPR tubes and exposed to air for 10 min to obtain the oxygenated complexes. The samples were then frozen for storage. Table 1 gives an overview of all the complexes and their corresponding numeration that will be mentioned in this manuscript. 2.2. Spectroscopy. The X-band cw-EPR spectra were recorded on a Bruker ESP300 spectrometer (microwave (mw) frequency 9.427 GHz) equipped with a liquid nitrogen cryostat. All the spectra were measured with a mw power of 20 mW, a modulation frequency of 100 kHz, and a modulation amplitude of 0.5 mT. The X-band pulse EPR and ENDOR spectra (15 K throughout) were recorded on a Bruker ELEXSYS spectrometer (mw frequency 9.72 GHz) equipped with an Oxford liquid helium cryostat. The magnetic field was measured with a Bruker ER 035M NMR gaussmeter. All pulse EPR measurements were conducted at a repetition rate of 1 kHz. The observer positions were carefully chosen in order to scan through all the molecular orientations that contribute to the cw-EPR spectrum (orientation selectivity35). The HYSCORE (hyperfine sublevel correlation)36 experiments were carried out with the pulse sequence, π/2-τ-π/2-t1-π-t2-π/2-τ-echo, with pulse length tπ/2 ) 24 ns and tπ ) 16 ns. The time intervals t1 and t2 were varied from 96 to 8272 ns in steps of 16 ns. Three τ values (96, 176, and 344 ns) were used to reduce the blind spots. An eight-step phase cycle was employed to eliminate unwanted echo contributions.37 The Q-band cw-EPR and pulse ENDOR spectra were recorded on a home-built spectrometer (mw frequency 35.39 GHz) equipped with a Bruker ENDOR ER5106 QTE probe head and an Oxford liquid helium cryostat. The EPR spectra were measured with a modulation amplitude of 0.2 mT and an mw power of 10 mW. All pulse ENDOR measurements were conducted at a repetition rate of 0.3 kHz and a temperature of 15 K. The Davies-ENDOR38 spectra were measured using the pulse sequence, π-T-π/2-τ-π-τ-echo, with mw pulse lengths tπ/2 ) 100 ns and tπ ) 200 ns, and a time interval τ ) 400 ns. A radio frequency (rf) π pulse of variable frequency νrf and length 15 µs was applied during the time interval T of 17 µs. The W-band echo-detected EPR spectra were measured at 15 K on a Bruker ELEXSYS680 W-band spectrometer (mw frequency 94.1 GHz) with a critically coupled Bruker TetraFlex probe head and cooling equipment from Oxford and Cryogenics. A π/2-τ-π-τ-echo sequence was used, with pulse lengths tπ/2 ) 200 ns and tπ ) 400 ns, and a τ value of 600 ns. A supercon sweep over 250 mT was performed. The data were processed with the program MATLAB 5.1. (The MathWorks, Inc., Natick, MA). The time traces of the HYSCORE spectra were baseline corrected with a third-order polynomial, apodized with a Hamming window, and zero filled.
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Figure 2. ln(K′) as a function of 1/T for the oxygenation reactions of [Cob(II)ester]+ in toluene and methanol.
Figure 1. Temperature of the X-band cw-EPR spectra of a 5 × 10-4 M toluene solution of Cob(II)ester open to air.
After 2D Fourier transformation, the absolute-value spectra were calculated. To get rid of τ-dependent blind spots HYSCORE spectra recorded with different τ values were added. The echodetected EPR spectra were first smoothed and then the first derivative was computed. The cw-EPR and echo-detected EPR spectra were simulated using MAGRES.39 Simulations of the HYSCORE spectra were done using TRYSCORE.40 3. Results 3.1. Temperature-Dependent Reversible Oxygenation. When methanol or toluene solutions of (1) are evacuated by the usual freeze-pump-thaw method, an X-band EPR spectrum is observed, which can be ascribed to the [Cob(II)ester]+ (1′) complex. Exposure of the sample tubes to air at room temperature for 10 min and then slowly lowering the temperature results in an additional EPR spectrum (Figure 1). The intensity of this spectrum increases when the temperature is reduced, while the intensity of the spectrum of (1′) decreases. The new spectrum has the typical features of an oxygenated Co(II) complex1 and is ascribed to [Cob(II)ester‚O2]+ (2). Upon degassing the solution, the EPR spectrum of (1′) is again obtained and the spectrum of (2) disappears. After four months of storage of both the methanol and toluene solution in contact with O2 at room temperature, no significant autoxidation took place. Since the cw-EPR spectra of (1′) and (2) are observed at the same time (see Figure 1), the equilibrium constants K(T) ) [(2)]/[(1′)][O2] of the oxygenation reaction
[Cob(II)ester]+ (1′) + O2 / [Cob(II)ester‚O2]+ (2) can be estimated. It is difficult to determine the concentration of O2. Consequently, the measurements were done under a surplus of O2, so that [O2] is constant (5 × 10-4 M of (1)). The
new equilibrium constant K′(T) ) [(2)]/[(1′)] can then be determined from the ratio of the double integrated EPR spectra of (1′) and (2). As can be recognized from Figure 1, the EPR spectra of (1′) and (2) overlap. To determine the relative intensities of the two spectra, first the intensity of spectrum (2) was determined by integrating the whole EPR spectrum, considering only the magnetic field region where the spectrum (2) appears, baseline correcting for the underlying spectrum of (1′), and integrating again the remaining spectrum. To obtain the intensity of spectrum (1′), the corrected doubly integrated spectrum of (2) was subtracted from the double integral of the whole spectrum of (1′+2). Both a linear baseline correction and a baseline obtained through spectral simulation were used, but the resulting values of K′(T) did not differ significantly. From the values of ln K′(T) as a function of 1/T, we got an estimate for ∆H (Figure 2). For the toluene solution of (1), the cw-EPR spectra of (1′) and (2) could simultaneously be observed in the temperature range 140-230 K (Figure 1). Below 140 K, only species (2) is present. To exclude the effect of the phase transition of the solvent, the data below 178 K (melting point of toluene) were not taken into account in the calculation of ∆H in order. Freezing of the solvent can hamper the O2 transport, which probably explains the deviation of the linear relation between ln K′ and 1/T below the melting point (Figure 2). In the temperature range 185-230 K, ∆H was found to be -34.4 ( 2.5 kJ mol-1. For the methanol solution of (1), the cw-EPR spectra of (1′) and (2) could only be observed simultaneously in the temperature range 150-185 K. Above 185 K, the spectrometer could no longer be tuned due to the high polarity of the solvent (melting point of methanol, 175.5 K), and below 150 K, only the EPR spectrum of (2) could be observed. Figure 2 shows that the values of ln K′(T) agree well with the values found for the toluene solution in this temperature range. An appearent solvent effect could not be noticed. For the formation enthalpy of Cob(II)ester‚I‚O2 in toluene, von Zelewsky32 found a value of -30.96 ( 6.28 kJ mol-1, which is comparable to the value we obtained for the formation enthalpy of (2). The same procedure was applied to toluene and methanol solutions of (1) (5 × 10-4 M) containing 5 × 10-3 M pyridine or 1-methylimidazole, respectively. For these low concentra-
EPR and ENDOR Study of Co(II) Corrin Complexes
Figure 3. (a) First derivative of the echo-detected W-band EPR spectrum of [Cob(II)ester‚O2]+ in methanol taken at 15 K. The peaks indicated with arrows arise from Mn(II) in the cavity. (b) Simulation of (a). (c) First derivative of the echo-detected W-band EPR spectrum of [Cob(II)ester‚py‚O2]+ in toluene taken at 15 K. (d) Simulation of (c).
tions, only the cw-EPR spectra of [Cob(II)ester‚py‚O2]+ (3) and [Cob(II)ester‚1MeIm‚O2]+ (4) were observed, even at the highest temperatures. To obtain the cw-EPR spectra of the corresponding oxygen-free complexes, the concentration of (1) and of the nitrogen bases had to be increased to values at which the O2 concentration could no longer be considered to be constant. This demonstrates that the coordination of the nitrogen of the bases has a strong influence on the oxygenation equilibria and stabilizes the oxygenated form of the cobester complexes. The oxygenation behavior of the base-on cobester complexes is also different from that of [Co(p-MeO)TPP‚py] and [Co(pMeO)TPP‚1MeIm], where the cw-EPR spectra of both the oxygenated and oxygen-free complexes could already be observed in toluene solutions 8 × 10-4 M in Co(p-MeO)TPP, with corresponding enthalpies ∆H ) -38.2 ( 4.6 kJ mol-1 and ∆H ) -37.2 ( 2.1 kJ mol-1.41 3.2. Determination of the g Matrix and Cobalt Hyperfine Interactions. Due to strong overlap of the different spectral features, it is difficult to determine the g and cobalt hyperfine (ACo) principal values and the relative orientation of their principal axes from the X-band powder cw-EPR spectrum (for a typical spectrum see Figure 1, 140 K, (2)). To facilitate the analysis, echo-detected EPR experiments at W-band (95 GHz) were performed at 15 K. Figure 3 shows the first derivation of the smoothed echo-detected EPR spectra of [Cob(II)ester‚O2]+ (2) in methanol and [Cob(II)ester‚py‚O2]+ (3) in toluene, together with the simulated spectra. It clearly illustrates the advantage of high-frequency EPR. The spectral features representing the three principal g values are nicely separated in the W-band EPR spectra, whereas they overlap in the X-band spectra. Because of large g strain effects, the cobalt hyperfine splittings are no longer resolved in the W-band spectra. To determine the principal values of the ACo matrix and the relative orientation of the principal axes with respect to the g principal axes, Q-band cobalt Davies-ENDOR spectra were measured at different observer positions. In the X-band DaviesENDOR spectra, the cobalt and proton ENDOR signals strongly overlap. At Q-band, the proton signals are shifted to higher frequency (the proton Zeeman frequency, νH, is 14.9 MHz for
J. Phys. Chem. B, Vol. 105, No. 31, 2001 7557
Figure 4. Q-band cobalt ENDOR spectra of [Cob(II)ester‚O2]+ in toluene at different observer positions. (a) Experimental spectra. (b) Simulations. The inset shows the echo-detected Q-band EPR spectrum.
Figure 5. Q-band cobalt ENDOR spectra of [Cob(II)ester‚1MeIm‚ O2]+ in methanol at different observer positions. (a) Experimental spectra. (b) Simulations. The inset shows the echo-detected Q-band EPR spectrum.
B0 ) 350 mT (typical field for X-band) and 53.2 MHz for B0 ) 1250 mT (typical field for Q-band)). Figures 4a and 5a show Davies-ENDOR spectra for [Cob(II)ester‚O2]+ in toluene and [Cob(II)ester‚1MeIm‚O2]+ in methanol recorded at different field positions. The ENDOR spectra of [Cob(II)ester‚py‚O2]+ (not shown) are very similar to the ones of [Cob(II)ester‚1MeIm‚ O2]+. The cobalt ENDOR signals are weak, and only those belonging to one mS manifold are visible. The signals are observed from 30 to 50 MHz. Since the cobalt Zeeman frequency, νCo, is 12.57 MHz at B0 ) 1250 mT, we expect the magnitude of the cobalt hyperfine interaction to vary from 35 to 75 MHz or from 85 to 125 MHz, depending on whether we are observing the upper frequency or the lower frequency set of ENDOR signals, respectively. From the partially resolved cobalt hyperfine splittings in the CW-EPR spectra at X-band (see Figure 1, 140 K, (2)), upper limits for the magnitude of the cobalt hyperfine couplings can be estimated, which confirm the first assumption (observation of upper-frequency set).
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TABLE 2: Principal Values of the g and ACo Matrices for [Cob(II)ester·O2]+ (2), [Cob(II)ester·py·O2]+ (3), and [Cob(II)ester·1MeIm·O2]+ (4)a
2 (a) 2 (b) 3 (a) 3 (b) 4 (a) 4 (b) 5 (b) 6 (b)
gx′ (( 0.0005)
gy′ (( 0.0005)
gz′ (( 0.0010)
AxCo [MHz] (( 1.5)
AyCo [MHz] (( 1.5)
AzCo [MHz] (( 1.5)
β1 [o] (( 5)
2.0073 2.0065 2.0064 2.0067 2.0062 2.0060 2.0020 2.0029
1.9840 1.9835 1.9890 1.9890 1.9898 1.9898 1.9827 1.9836
2.0740 2.0740 2.0728 2.0723 2.0740 2.0740 2.0705 2.0729
-67.0 -72.0 -53.0 -54.0 -51.0 -51.0 -53.0 -52.5
-39.0 -43.0 -29.0 -28.0 -29.0 -27.0 -21.4 -21.5
-35.0 -35.0 -28.0 -27.0 -27.0 -26.0 -22.7 -22.5
75 75 65 65 65 65 64 65
a (a) and (b) denote the used solvents methanol and toluene, respectively. The data are derived from the cw-EPR spectra recorded at X-, Q-, and W-band and the Q-band Davies-ENDOR spectra. The Euler angles for the g principal axes are R1 ) 0°((5°) and γ1 ) 0°((5°), β1 is given in the table. The Euler angles for the ACo principal axes are R2 ) β2 ) γ2 ) 0°((5°). The molecular axes indicated in Figure 6 are taken as the reference frame. The values are compared with the ones found for [Co(II)TPP·py·O2] (5) and [Co(II)TPP·1MeIm·O2] (6).33
Inspection of Figures 4a and 5a also shows that the cobalt nuclear quadrupole interaction is not resolved (59Co, I ) 7/2). From the ENDOR line width, a maximum nuclear quadrupole coupling (e2qQ/2I(2I - 1)h) of 700 kHz for the base-off complexes and of 400 kHz for the base-on complexes is estimated. In the simulations of the spectra shown in Figure 4b and 5b, a large line width (2 MHz) was used to account for this line broadening. The value of the line width was taken so as to give acceptable agreement with the experiment, although it should be clearly noted that an isotropic line width can never completely simulate the effect of the nuclear quadrupole interaction. The spin Hamiltonian then used to model the system involved only the electron Zeeman interaction and the hyperfine interaction between the unpaired electron and the nuclear spin of cobalt. The “noise” in the simulated spectra results from the relative low number of steps taken for the integration. An increase of the number of integration points only led to smoother spectra, but did not change the overall conclusion and significantly increased the computation time. The used simulation program MAGRES39 is developed so as to determine automatically the magnetic field orientations contributing to the ENDOR spectrum at a given observer position and for a given spin system. The increase in signal intensity close to 50 MHz in Figure 4a is due to the proton interactions. The g and ACo principal values and the Euler angles with respect to the molecular frame, obtained from these simulations, are collected in Table 2. Figure 6 shows schematically the directions of the g and ACo principal axes. The reference frame was chosen so that x and y are in the plane of the macrocycle and bisect the N-Co-N angle. EPR measurements of vitamin B12r in a B12b single crystal show that the largest (in absolute value) cobalt hyperfine interaction is directed approximately along the C2V axis of the corrin ring.8 Since this information is lost in frozen solutions, we use the same assignment. The assignment of the principal g axes was done in the same way as in our earlier study on oxygenated Co(II) porphyrin complexes.33 It is obvious from Figures 4 and 5 that the ENDOR simulations are less satisfying for the base-off than for the baseon complexes. We ascribe this to the effect of the nuclear quadrupole interaction, which is larger in the former case. 3.3. Interactions with the Surrounding Nitrogens. HYSCORE,36 a two-dimensional experiment in which a mixing π pulse creates correlations between the nuclear coherences in two different electron spin (mS) manifolds, is found to be the most appropriate method to study the magnetic parameters of the ligand nitrogens. The spin Hamiltonian of an S ) 1/2, I ) 1 system (e.g., 14N) can be described in terms of the g matrix, the hyperfine matrix A, and the nuclear quadrupole tensor Q. The principal values Qx, Qy, and Qz of the traceless Q tensor
Figure 6. Schematic representation of the principal axes of the observed g, A, and Q tensors.
are usually expressed by the quadrupole coupling constant K ) e2qQ4h and the asymmetry parameter η, with Qx ) - K(1 - η), Qy ) - K(1 + η), and Qz ) 2K. HYSCORE spectra of disordered S ) 1/2, I ) 1 systems are dominated by the crosspeaks between the double-quantum (DQ) frequencies42,43
νDQR,β ) 2
x(2a ( ν ) + K (3 + η ) 2
I
2
2
(1)
where a is the hyperfine coupling at a particular observer position. Since the solvent was found to have no influence on the HYSCORE spectra of the different Co(II)complexes under study, we focus on the methanol solutions only. Figure 7a-c shows the HYSCORE spectra of the base-off complex [Cob(II)ester‚O2]+ (2) and the base-on complexes [Cob(II)ester‚ 1MeIm‚O2]+ (4) and [Cob(II)ester‚py‚O2]+ (3) at the same observer positions. For (2), the DQ cross-peaks of the corrin nitrogens occur in the (+,+) quadrant (Figure 7a, arrows). This implies that for these hyperfine interactions |a| < 2νN, where νN is the nuclear Zeeman frequency of 14N. Comparison of Figure 7a and 7b simplifies the assignment of signals to the
EPR and ENDOR Study of Co(II) Corrin Complexes
Figure 7. X-band nitrogen HYSCORE spectra at B ) 344.3 mT. (a) [Cob(II)ester‚O2]+. The arrows indicate the DQ cross-peaks of the interaction with the corrin nitrogens. (b) [Cob(II)ester‚1MeIm‚O2]+. The arrows indicate the DQ cross-peaks assigned to the binding nitrogen of 1MeIm, the stars indicate the DQ cross-peaks of the interaction with the corrin nitrogens and the diamonds denote the peaks due to the interaction with the remote nitrogen of 1MeIm. (c) [Cob(II)ester‚ py‚O2]+. The arrows indicate the DQ cross-peaks assigned to the nitrogen of pyridine, the stars indicate the DQ cross-peaks of the interaction with the corrin nitrogens.
nitrogens of the imidazole base. New DQ cross-peaks (Figure 7b, arrows) arise in the (-,+) quadrant indicating a hyperfine interaction |a| > 2νN. Since similar peak positions were found for complex (3) (Figure 7c, arrows), these signals are ascribed to the binding nitrogen of the base. The positions of the DQ cross-peaks are found to vary only slightly upon change of the observer position, as in the case for the oxygenated base-on Co(II)TPP complexes.33 From the positions of the DQ crosspeaks and using eq 1, a first idea about the magnitude of the hyperfine interaction and of K2(3 + η2) can be obtained, which can then be optimized by simulating the spectra. The nuclearquadrupole-dependent contribution is larger for (3) than for (4), hence the DQ cross-peaks appear at higher frequencies in the HYSCORE spectrum 7c than in 7b. In the (-,+) quadrant of the HYSCORE spectrum in Figure 7b, a cross-peak appears at (-5.3,1.8 MHz) that is not visible in the spectrum of Figure 7c. This peak might be part of the DQ ridge, which would indicate a considerably larger anisotropy of the hyperfine interaction with the nitrogen of 1MeIm than with the one of pyridine. However, since six nitrogen nuclei are contributing to the spectrum in 7b, it cannot be excluded that the (-5.3,1.8
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Figure 8. Simulated nitrogen HYSCORE spectra at B ) 344.3 mT. (a) [Cob(II)ester‚O2]+. 14N nuclei of the corrin ring. (b) [Cob(II)ester‚ 1MeIm‚O2]+. Directly binding 14N nucleus of 1MeIm. (c) [Cob(II)ester‚1MeIm‚O2]+. 14N nuclei of the corrin ring.
MHz) cross-peak arises from combinations between different nuclear frequencies. The fact that the general appearance of the DQ-peaks in the HYSCORE spectra of (4) does not change with changing magnetic field setting (not even at the so-called “single-crystal-like” positions) seems to be in favor of the latter explanation. Figure 8b shows a simulation of the HYSCORE spectrum for the directly binding 14N nucleus of 1MeIm. It has to be compared with Figure 7b, considering that in the simulation the corrin nitrogens (labeled by * in Figure 7b) are not included. The simulation was done assuming ideal pulses, which explains the differences in the relative peak intensities between experiment and simulation. The parameters were optimized so as to reproduce the main features of the HYSCORE spectra taken at different observer positions. Special attention was also payed to the reproduction of the cross-peaks between the basic frequencies. The parameter set used for this simulation is given in Table 3. For the simulation of the couplings of the pyridine nitrogen in [Cob(II)ester‚py‚O2]+ (not shown), the same procedure was applied. The simulations confirmed our preliminary finding that the nuclear quadrupole interaction is larger for the pyridine nitrogen than the 1MeIm nitrogen. Figure 6 shows the orientation of the principal axes of AN and QN for the binding nitrogen of the base.
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TABLE 3: Principal Values of the 14N Hyperfine Matrix and the Nuclear Quadrupole Interaction of the Binding Nitrogen of the Base of [Cob(II)ester·py·O2]+ (3) and [Cob(II)ester·1MeIm·O2]+ (4) Derived from the X-band HYSCORE Spectraa ANx [MHz] ANy [MHz] ANz [MHz] |e2qQ/h| [MHz] η (( 0.1) (( 0.1) (( 0.1) (( 0.05) (( 0.02) 3 (a), (b) 4 (a), (b) 5 (b) 6 (b)
3.5 3.3 3.4 3.2
3.5 3.3 3.4 3.2
3.8 3.9 3.7 3.8
2.65 2.05 2.95 2.25
0.25 0.08 0.23 0.10
a The Euler angles for both AN and QN are R3 ) β3 ) γ3 ) 0° ((5°). The data are compared with those found for the porphyrin complexes [Co(II)TPP·py·O2] (5) and [CoTPP·1MeImy·O2] (6);33 (a) in methanol, (b) in toluene.
TABLE 4: Principal Values of the 14N Hyperfine Matrix and the Nuclear Quadrupole Tensor of the Corrin Nitrogens of [Cob(II)ester·O2]+ (2), [Cob(II)ester·py·O2]+ (3), and [Cob(II)ester·1MeIm·O2]+ (4) Derived from the X-band HYSCORE Spectra and the Corresponding Euler Anglesa N Ax′′ [MHz] (( 0.2)
N Ay′′ [MHz] (( 0.2)
N Az′′ [MHz] (( 0.2)
R4 [°] (( 20)
β4 [°] (( 20)
γ4 [°] (( 20)
2 (a), (b)
-1.3
-1.3
-0.1
3 (a), (b) 4 (a), (b) 5 (b) 6 (b)
-1.2 -1.2 -1.3 -1.2
-1.2 -1.2 -1.3 -1.2
-0.3 -0.3 -0.4 -0.3
45 135 225 315 id. id. id. id.
45 45 45 45 id. id. id. id.
0 0 0 0 id. id. id. id.
|e2qQ/h| [MHz] (( 0.1)
η (( 0.2)
R5 [°] (( 20)
β5 [°] (( 20)
γ5 [°] (( 20)
45 135 225 315 id. id. id. id.
90 90 90 90 id. id. id. id.
0 0 0 0 id. id. id. id.
2 (a), (b)
1.4
0.3
3 (a), (b) 4 (a), (b) 5 (b) 6 (b)
1.6 1.6 1.8 1.7
0.3 0.3 0.3 0.5
a The same Euler angles are used in all simulations, so they are mentioned only once and are further indicated by id. The data are compared with those obtained for the interactions of the nitrogens of the porphyrin ring for the complexes [Co(II)TPP·py·O2] (5) and [Co(II)TPP·1MeIm·O2] (6);33 (a) in methanol, (b) in toluene.
It is difficult to determine exactly the whole set of spin Hamiltonian parameters for the corrin nitrogens. The interactions are weak, the four corrin nitrogens are not geometrically equivalent, and, for each nucleus, the A and Q principal axes will be noncoinciding. The ridge form of the DQ cross-peaks is found to depend more strongly upon the observer position than was the case for the DQ cross-peaks arising from the interaction with the nitrogen of the axial base. This suggests already a larger hyperfine anisotropy in the former case. Table 4 shows for the three complexes under study the values of the hyperfine and nuclear quadrupole interactions obtained from the simulations of the HYSCORE spectra at different observer positions. It is assumed that the four nitrogen ligands have the same set of principal values and that for each nucleus the Az axis is pointing approximately along the direction of the dioxygen fragment and the axis of the largest Q value is pointing along the N-Co axis (Figure 6). The errors on the derived angles are, however, rather large, and an inequivalence of the four-parameter sets is expected. Simulations assuming coaxial A and Q tensors failed to reproduce the experimental features,
as we already observed earlier in our study of oxygenated Co(II) porphyrins.33 Figure 8a and 8c show the simulations of the interaction with the 14N corrin nuclei for [Cob(II)ester‚O2]+ and [Cob(II)ester‚1MeIm‚O2]+. The figures should be compared with Figure 7a and 7b (transitions labeled by *). 4, Discussion The temperature-dependent cw EPR measurements indicate that the addition of a nitrogen base to a methanol or toluene solution of (1) has a considerable influence on the oxygenation equilibrium. Table 2 shows that the difference between the baseoff and base-on complexes expresses itself also in the gy′ and ACo values of the oxygenated complexes (2), (3), and (4). Although no solvent effect was apparent in the temperaturedependent EPR study (Figure 2), the change of the solvent has a significant influence on the cobalt hyperfine values of the baseoff complex (2). This complex still has a free sixth coordination site, so that the ligation capacity of the solvent plays an important role. In the toluene solution, [Cob(II)ester‚O2]+ is expected to form a contact ion pair with the counterion ClO4-, whereas in methanol, a solvent molecule will act as the sixth ligand. This is confirmed by the observation that the experimental cobalt hyperfine values become smaller (in absolute values) when toluene instead of methanol is used. This observation agrees with the change of the ACo values upon addition of a nitrogen base. Finally, a comparison between the oxygenated base-on cobester and CoTPP complexes (Table 2) shows that the g and cobalt hyperfine values also sense the change in ring structure. The cobalt hyperfine interactions found for the oxygenated Co(II) complexes (2), (3), and (4) (Table 2) are comparable in magnitude to those reported for other oxygenated Co(II) complexes.1,2,7,8,10,20-24, 33 The cobalt hyperfine interactions of oxygenated Co(II) complexes are considerably less anisotropic than those of oxygen-free Co(II) complexes (as an example: Co for CoTPP(py) in toluene ACo x,y e 40 MHz and Az ) 236 34 MHz ). This change is related to the electron-withdrawing effect of the O2 fragment. The electronic structure of oxygenated Co(II) complexes is usually discussed in terms of the superoxide formulation (Co(III)O2-)44 or by the spin-pairing model.2,45 In the spinpairing model, it is assumed that the two molecular orbitals (MOs) of the Co-O2 part
ψ1 ) R′dz2 + γ 4s + β π*(x′) + R′′′dxz ψ2 ) R′′dyz + π*(y′)
(2)
affect the cobalt hyperfine interaction. The doubly occupied MO, ψ1, describes the overlap of a π*(x′) orbital of O2 with the cobalt 3dz2, 3dxz, and 4s orbital. The second MO, ψ2, contains the unpaired electron. Since the two MOs are close in energy,45 ψ2 can polarize ψ1 resulting in a negative spin density in the 3dz2, 3dxz, and 4s orbitals. The principal values of the cobalt hyperfine matrix are then found to be
1 2 Ax ) - T|| + aCo iso + (f - 2l - h) 2 7 1 2 Ay ) - T|| + aCo iso + (f + l + 2h) 2 7 2 Az ) + T|| + aCo iso + (- 2f + l - h) 7
(3)
EPR and ENDOR Study of Co(II) Corrin Complexes TABLE 5: Values of aiso, f + h and l + h Derived from the Experimental Cobalt Hyperfine Couplings Given Table 2 Using the Spin-Pairing Modela 2 (a) 2 (b) 3 (a) 3 (b) 4 (a) 4 (b)
aiso [MHz]
f + h [MHz]
l + h [MHz]
upper limit of R′′2
-47.0 -50.0 -36.7 -36.3 -35.7 -34.7
0.6 -4.1 4.1 4.1 2.9 4.1
32.7 33.8 28.0 30.3 25.7 28.0
0.054 0.056 0.047 0.051 0.043 0.047
a An upper limit for R′′2 is calculated assuming h ) 0; (a) in methanol, (b) in toluene.
where T|| ) 3 MHz is the direct point dipole-dipole interaction between the unpaired electron in the π*(y′) orbital and the cobalt nuclear spin,45 and f ) PFOUCo-OR′2, h ) PFOUCo-OR′′′2, and l ) PR′′2 describe the indirect spin polarization of dz2 and dxz and the direct hyperfine contribution from dyz, respectively. UCo-O is the spin-polarization constant, FO is the spin density on the adjacent oxygen, and P ) geβegnβn < r-3 > 3d ) 600 MHz.45 Hoffman et al.,44 on the other hand, proposed a mechanism for the description of the anisotropic cobalt hyperfine interaction in a MO (ψ1) involving the antibonding π*(y′) oxygen orbital and the cobalt 3dyz orbital (π back-bonding). In eq 3, this implies that f ) h ) 0. This assumption leads to R′′2 values of 0.054 to 0.059 for the base-off complex (2), and of 0.040 to 0.043 for the base-on complexes (3) and (4). This essentially means that almost complete electron transfer from cobalt to oxygen has taken place (superoxo formulation). However, the model does not fully describe the anisotropy of the cobalt hyperfine matrix. The spin-pairing model does not allow for a large transfer of the unpaired electron to the oxygen. Equation 3 leads to the values of f + h and l + h listed in Table 5. In addition, upper limits for R′′2 are listed for which h is taken positive (f and h are expected to be positive in a spin-pairing model). Since the individual values of f, h, and l may not be derived from eq 3, f or h can actually be negative implying direct interaction rather than spin polarization. In fact, for complex (2) in toluene, a slightly negative value of f + h is found. Such negative values have also been reported for other oxygenated Co(II) complexes.2,8 Thus, the spin-pairing model, as described by Trovog et al.45 appears to be incomplete for the description of the cobalt hyperfine structure. For the oxygen adducts of bis(dimethylglyoximato)cobalt(II) complexes in an X-zeolite matrix, Lubitz et al.46 gave a more rigorous analysis of the spin-pairing model and showed that, within the confines of the 3-orbital restriction, ca. 0.9 electrons are transferred to the O2 fragment, consistent with the formal Co(III)O2- description of Hoffman et al.44 Furthermore, an X-ray study showed that B12rO2 has the structural features of a Co(III) corrin, i.e., that the structure of B12rO2 can be best described as a superoxo-cob(III)alamin.9 The finding that the cobalt hyperfine interaction is virtually independent of the axial base is another indication against the spin-pairing model of Trovog et al.45 Only for the base-off complex (2), significantly larger (absolute) values are found for this interaction. This can be explained as follows. Upon addition of a nitrogen base to [Cob(II)ester]+, the unpaired electron resides in the 3dz2(Co) + pσ(N) orbital. The 3dyz (3dxz) orbital combines with the pπ(N) orbital, but the energy shift is smaller than the one for the 3dz2 orbital. If spin polarization of the 3dz2 orbital plays an important role for the oxygenated complexes, a change of the base strength would have a significant influence on the cobalt hyperfine interaction, which is not observed. The difference in the cobalt hyperfine interaction for [Cob(II)ester‚
J. Phys. Chem. B, Vol. 105, No. 31, 2001 7561 O2]+ and [Cob(II)ester‚B‚O2]+ (B ) nitrogen base) seems mostly to be due to the change in the MO with most 3dyz character; spin-polarization of the 3dz2 orbital plays only a minor role for this interaction. The same was observed for oxygenated Co(II)TPP complexes.33 From the ENDOR line width, a maximum cobalt nuclear quadrupole coupling of 700 kHz for the base-off complexes and of 400 kHz for the base-on complexes was estimated. There exists relatively few cobalt nuclear quadrupole data for Co(II) complexes. In their single-crystal ENDOR study of N,N′ethylenebis(acetylacetonatiminato)Co(II), Rudin et al. determined the cobalt nuclear quadrupole principal values to be Qx ) -2.76 MHz, Qx ) 2.61 MHz, and Qz ) 0.15 MHz.47 N,N′ethylenebis(acetylacetonatiminato)Co(II) has no axial ligands, and the unpaired electron resides mainly in the cobalt dyz orbital. The different electronic structure is probably responsible for the difference in magnitude between the cobalt nuclear quadrupole couplings reported for N,N′-ethylenebis(acetylacetonatiminato)Co(II) and the couplings estimated for the complexes under study. For both complexes (3) and (4), the hyperfine interaction of the binding base nitrogen is found to be dominated by the isotropic part (aiso ) 3.6 MHz (3), 3.5 MHz (4)). Only a small dipolar part is observed, which agrees with the withdrawal of the unpaired electron by the O2 fragment. The spin-pairing model would lead to a larger dipolar contribution. The fact that the hyperfine interactions of the directly binding nitrogen of the base do not differ significantly for (3) and (4) is in agreement with the similarity between the cobalt hyperfine interactions of these complexes. The differences in the nuclear quadrupole parameters of these nitrogens are governed by the differences in the structure of the base. For the pyridine molecule, Hsieh et al.48 found from nuclear quadrupole resonance (NQR) experiments the values |e2qQ/h| ) 4.584 MHz and η ) 0.396. No NQR data are available for 1MeIm, but for imidazole the parameters |e2qQ/h| ) 3.220 MHz and η ) 0.119 are reported.49 DFT calculations using a continuous model50 gave values of |e2qQ/h| ) 3.607 MHz and η ) 0.047 for the bicoordinated nitrogen of 1MeIm. The corresponding calculated values for this nitrogen in imidazole were |e2qQ/h| ) 3.591 MHz and η ) 0.039. Hsieh et al.48 observed that upon coordination of pyridine with a Lewis acid, the electric field gradient at the nitrogen nucleus decreases. This is illustrated here, where [Cob(II)ester‚O2]+ acts as the Lewis acid. Comparison with our previous data for the base-on Co(II) porphyrin complexes (Table 3, ref 33) shows that the values of |e2qQ/h| are significantly smaller for the corrin complexes. A combined three-pulse ESEEM and EXAFS study of (oxyCo)[(o-R)TPP](1MeIm) (whereR)-H,-NHCOC(CH3)3,-NHCOCH3,-NHCONHC6H5, an ortho substitutent on one of the four meso phenyls of TPP) in different solvents51 showed a relation between |e2qQ/h| and the Co-N distance. For the same nitrogen base, a lower value of |e2qQ/h| of the directly binding nitrogen corresponds to a shorter Co-N distance. This suggests that, for the corrin complexes under study, the nitrogen base is closer to cobalt than in the porphyrin case. This shortening of the Co-N distance may be due to the overall positive charge of the Co(II) corrin complexes or to the different ring structure. The deduced short axial Co-N bond in (2) and (3) would also be consistent with the X-ray analytical result9 where the axial Co-N distance was found to be 0.206 nm. The hyperfine interactions with the corrin nitrogens of (2), (3), and (4) can be split into an isotropic part with aiso ) -0.9 MHz and a dipolar part of (-0.4, -0.4, 0.8) MHz for (2) and
7562 J. Phys. Chem. B, Vol. 105, No. 31, 2001 (-0.3, -0.3, 0.6) MHz for (3) and (4). The dipolar part corresponds to a value of r ) 0.24 (( 0.04) nm and r ) 0.27 (( 0.04) nm, where r is the distance between the nitrogen and the unpaired electron. The X-ray analysis of B12rO2 reports a distance of approximately 0.27 nm between the corrole nitrogens and the oxygen atom coordinated to the cobalt. The second oxygen was found at a distance of 0.29 nm from the nearest ring nitrogens and 0.38 nm from the other two nitrogen nuclei.9 The orientation of the hyperfine principal axes and the values of r support again the assumption that an almost complete transfer of the unpaired electron from cobalt to the dioxygen has taken place, as suggested by Hoffman.44 The negative aiso value indicates that the s-spin density is mainly due to polarization effects.33 The hyperfine and nuclear quadrupole parameters of the corrin nitrogens are similar to those observed for the porphyrin nitrogens in (5) and (6) (Table 4, ref 33). This similarity is due to the fact that the unpaired electron resides mainly on the O2 fragment. Finally, we mention the interaction with the remote nitrogens of 1MeIm. Based on the data we found for the directly binding nitrogen of 1MeIm, it is expected that the hyperfine interaction with the remote nitrogen is very small. For the amino nitrogen of imidazole, the experimental NQR data |e2qQ/h| ) 1.391 MHz and η ) 0.930 are reported.49 DFT calculations predict for the nuclear quadrupole parameters of the remote nitrogen of 1MeIm |e2qQ/h| ) 2.420 MHz and η ) 0.211 and for the remote nitrogen of imidazole |e2qQ/h| ) 2.097 MHz and η ) 0.229.50 Simulations of HYSCORE spectra using an isotropic hyperfine interaction of 100 kHz and the experimental nuclear quadrupole data from the free imidazole predict two broad diagonal peaks at ≈1.2 MHz and ≈2.3 MHz in the (+,+) quadrant. Indications of these peaks are observed in the experimental spectra of [Cob(II)ester‚1MeIm‚O2]+ (Figure 7b, ]). These peaks are missing in the corresponding HYSCORE spectrum of [Cob(II)ester‚py‚O2]+ (Figure 7c). Conclusion Toluene and methanol solutions of [Cob(II)ester·O2]+ (2), [Cob(II)ester·py·O2]+ (3), and [Cob(II)ester·1MeIm·O2]+ (4) are studied by means of different one- and two-dimensional EPR and ENDOR methods at microwave frequencies between 9.7 and 95 GHz, which allow a detailed analysis of these compounds. The temperature dependence of the oxygenation reaction of the base-off complex [Cob(II)ester]+ was studied with cw EPR. An enthalpy ∆H ) -34.4 ( 2.5 kJ mol-1 was obtained, and it was shown that the addition of a nitrogen base changes the oxygenation equilibrium considerably and stabilizes the oxygenated form of the cobester complexes. The g and ACo matrices and the direction of their principal axes are determined by EPR at X-, Q- and W-band and DaviesENDOR at Q-band. It is shown that the g and cobalt hyperfine matrices of the Co(II) corrin complexes are sensitive to the solvent, to the addition of a nitrogen base, and to the ring structure. The hyperfine and nuclear quadrupole interactions of the nitrogen(s) of the axial ligand and the nitrogens in the corrin ring are investigated with HYSCORE at X-band. The hyperfine interaction of the binding nitrogen of the axial ligand is found to be predominately isotropic. The nuclear quadrupole tensor points along the Co-N bond direction. The values for |e2qQ/h| are found to be smaller for (3) and (4) than for the corresponding oxygenated base-on Co(II)TPP complexes. This difference can be explained by a shorter Co-N distance for the former
Van Doorslaer et al. complexes. For the corrin nitrogens, AN and QN are not coaxial. The hyperfine interactions have a considerable anisotropic contribution which arises from the dipolar interaction with the unpaired electron on the dioxygen fragment, and the isotropic hyperfine coupling is found to be negative. This work demonstrates the potential of cw and pulse EPR and ENDOR techniques at different microwave frequencies to study in great detail the electronic and geometric structure of biologically relevant paramagnetic metal complexes in frozen solutions, such as Co(II) forms of vitamin B12 derivatives. Acknowledgment. This research has been supported by the Swiss National Science Foundation. Dr. Gunnar Jeschke and Prof. Dr. W. Spiess of the Max-Planck Institut fu¨r Polymerforschung in Mainz are gratefully thanked for the W-band measurements. Dr. Igor Gromov is thanked for his helpful assistance with the Q-band spectrometer. Dr. R. Bachmann and S. Bonjour are acknowledged for their helpful discussions. References and Notes (1) (a) Jones, R. D.; Summerville, D. A.; Basolo, F. Chem. ReV. 1979, 79, 139. (b) Dago, R. S.; Corden, B. B. Acc. Chem. Res. 1980, 13, 343. (2) Smith, T. D.; Pilbrow, J. R. Coord. Chem. ReV. 1981, 39, 295. (3) Busch, D. H.; Alcock, N. W. Chem. ReV. 1994, 90, 8234. (4) Niswander, R. H.; Taylor, L. T. J. Magn. Reson. 1977, 26, 491. (5) De Vos, D. E.; Feijen, E. J. P.; Schoonheydt, R. A.; Jacobs, P. A. J. Am. Chem. Soc. 1994, 116, 4746. (6) Schrauzer, G. N.; Lee, L. P. J. Am. Chem. Soc. 1970, 92, 1551. (7) Walker, F. A. J. Am. Chem. Soc. 1970, 92, 4235. (8) Jo¨rin, E.; Schweiger, A.; Gu¨nthard Hs. H. J. Am. Chem. Soc. 1983, 105, 4277. (9) Hohenester, E.; Kratky, C.; Kra¨utler, B. J. Am. Chem. Soc. 1991, 113, 4523. (10) Bowen, J. H.; Shokhirev, N. V.; Raitsimring, A. M.; Buttlaire, D. H.; Walker, F. A. J. Phys. Chem. B 1997, 101, 8683. (11) Brucker, E. A.; Olson, J. S.; Phillips, G. N.; Dou, Y.; Ikeda-Saito, M. J. Biol. Chem. 1996, 271, 25419. (12) Rybak-Akimova, E. V.; Marek, K.; Masarwa, M.; Busch, D. H. Inorg. Chim. Acta 1998, 270, 151. (13) Rybak-Akimova, E. V.; Otto, W.; Deardorf, P.; Roesner, R.; Busch, D. H. Inorg. Chem. 1997, 36, 2746. (14) Dedieu, A.; Rohmer, M.-M.; Veillard, A. J. Am. Chem. Soc. 1976, 98, 5789. (15) Bytheway, I.; Hall, M. B. Chem. ReV. 1994, 94, 639. (16) Nishide, H.; Kawakami, H.; Suzuki, T.; Azechi, Y.; Soejima, Y.; Tsuchida, E. Macromolecules 1994, 24, 6306. (17) Song, E.; Shi, C.; Anson, F. C. Langmuir 1998, 14, 4315. (18) Granier, C.; Guilard, R. Microchem. J. 1996, 53, 109. (19) Hoffman, B. M.; Petering, D. H. Proc. Natl. Acad. Sci. U.S.A. 1970, 67, 637. (20) Ikeda-Saito, M.; Brunori, M.; Yonetani, T. Biochim. Biophys. Acta 1978, 533, 173-180; and further references herein. (21) Dickinson, L. C.; Chien, J. C. W. Proc. Natl. Acad. Sci. U.S.A. 1980, 77, 1235. (22) Chien, J. C. W.; Dickinson, L. C. Proc. Natl. Acad. Sci. U.S.A. 1972, 69, 2783. (23) Hori, H.; Ikeda-Saito, M.; Yonetani, T. Nature 1980, 288, 501. (24) Hori, H.; Ikeda-Saito, M.; Yonetani, T. J. Biol. Chem. 1982, 257, 3636. (25) Vitamin B12 and B12-Proteins; Kra¨utler, B.; Arigoni, D.; Golding, B. T., Eds.; Wiley-VCH: Weinheim, 1998. (26) Chemistry and biochemistry of B12; Banerjee, R. Ed., J. Wiley & Sons: New York, 1999. (27) Gerfen, G. J. In Chemistry and biochemistry of B12; Banerjee, R. Ed., J. Wiley & Sons: New York, 1999; p 165. (28) Bayston, J. H.; Kelso King, N.; Looney, F. D.; Winfield, M. E. J. Am. Chem. Soc. 1969, 91, 2776. (29) Murakmai, Y.; Hisaeda, Y.; Kajihara, A. Bull. Chem. Soc. Jpn. 1983, 56, 3642. (30) Kra¨utler, B.; Keller, W.; Hughes, M.; Caderas, C.; Kratky, C. J. Chem. Soc., Chem. Commun. 1987, 1678. (31) Werthemann, L.; Keese, R.; Eschenmoser, A. In Wertheman L., doctoral thesis, ETH no. 4097, 1968. (32) von Zelewsky, A. HelV. Chim. Acta 1972, 55, 2941. (33) Van Doorslaer, S.; Schweiger, A. J. Phys. Chem. B 2000, 104, 2919. (34) Van Doorslaer, S.; Bachmann, R.; Schweiger, A., J. Phys. Chem. A 1999, 103, 5446. (35) Rist, G.; Hyde, J. J. Chem. Phys. 1970, 52, 4633.
EPR and ENDOR Study of Co(II) Corrin Complexes (36) Ho¨fer, P.; Grupp, A.; Nebenfu¨hr, H.; Mehring, M. Chem. Phys. Lett. 1986, 132, 279. (37) Gemperle, C.; Aebli, G.; Schweiger, A.; Ernst, R. R. J. Magn. Reson. 1990, 88, 241. (38) Davies, E. R. Phys. Lett. 1974, A 47, 1. (39) Keijzers, C. P.; Reijerse, E. J.; Stam, P.; Dumont M. F.; Gribnau, M. C. M J. Chem. Soc., Faraday Trans. 1 1987, 83, 3493. (40) Szosenfogel, R.; Goldfarb, D.; Mol. Phys. 1998, 95, 1295. (41) Walker, F. A. J. Am. Chem. Soc. 1973, 95, 1154-1159. (42) Dikanov, S. A.; Xun, L.; Karpiel, A. B.; Tyryshkin, A. M.; Bowman, M. K. J. Am. Chem. Soc. 1996, 118, 8408. (43) Dikanov, S. A.; Tsvetkov, Yu. D.; Bowman, M. K.; Astashkin, A. V. Chem. Phys. Lett. 1982, 90 149. (44) Hoffman, B. M.; Diemente, D. L.; Basolo, F., J. Am. Chem. Soc. 1970, 92, 61.
J. Phys. Chem. B, Vol. 105, No. 31, 2001 7563 (45) Trovog, B. S.; Kitko, D. J.; Drago, R. S. J. Am. Chem. Soc. 1976, 98, 5144. (46) Lubitz, W.; Winscom, C. J.; Diegruber, H.; Mo¨seler, R. Z. Naturforsch. 1987, 42a, 970. (47) Rudin, M.; Schweiger, A.; Berchten, N.; Gu¨nthard, Hs. Mol. Phys. 1980, 41, 1317. (48) Hsieh, Y.-N.; Rubenacker, G. V.; Cheng, C. P.; Brown, T. L. J. Am. Chem. Soc. 1977, 99, 1384. (49) Garcia, M. L. S.; Smith, J. A. S.; Bavin, P. M. G., Ganellin, C. R. J. Chem. Soc., Perkin Trans. 2, 1983, 1391. (50) Torrent, M.; Musaev, D. G.; Morokuma, K.; Ke S-C.; Warncke, K. J. Phys. Chem. B 1999, 103, 8618. (51) Lee, H. C.; Scheuring, E.; Peisach, J.; Chance, M. R.; J. Am. Chem. Soc. 1997, 119, 12201.
