J. Phys. Chem. 1082, 86,149-152
149
Examinatlon of the Conditions for ENDOR-in-Solution Experiments on Transition Metal Complexes W. Mohl,t C. J. Wlnscom,t M. Plato,t K. Moblus," and W. Lubitzz Freie UniversPt Berlin, West Berlin, West Qermny (Received: July 1. 1981)
First proton ENDOR experiments have been performed on a transition metal complex in solution (chromyl ethylene glycolate anion in ethanol). The microwave, rf, and temperature dependences of the ENDOR signal amplitude and line width have been measured. The results are in good agreement with the predictions of a general theory. From these findings, limiting values of the various magnetic interactions could be derived for successful ENDOR-in-solution experiments on transition metal complexes.
Introduction Transition metal complexes (TMC's) with organic ligands play an important role in many fundamental reactions in chemistry and biology. For a better understanding of the function of TMC's, it is clearly of interest to first understand their electronic structure. Many TMC's exist in stable paramagnetic ground states; others exist as relatively long-lived paramagnetic intermediates during a reaction sequence. An example of the latter is the reduction of Cr"' to Cr"' via CrV,whose electronic state is of doublet character. Such CrVintermediates play a crucial role in all known oxidations by Crw. ESR spectroscopy has been widely used to learn about the electronic structure from studies of the hyperfine (hf) structure of the metal ion and ligand nuc1ei.l In complexes of low symmetry, however, where hyperfine interactions with many nonequivalent ligand nuclei occur, ESR suffers from problems of resolution, and ENDOR would be the appropriate spectroscopic method. So far, ENDOR on TMC's has been restricted to solid-state experiments,"'O and no ENDORin-solution experiments have been yet reported. Liquidsolution experiments would be very valuable, however, since in solid solutions small ligand hyperfine couplings are often masked by anisotropic broadening. In liquid solutions, the situation is very different for organic radicals where there exists already a wealth of experience with ENDOR in solution on a variety of different nuclei in different molecular environments." The obvious question, therefore, arose whether this experience could be applied to study the hyperfine structure of ligand nuclei of a TMC in solution. As a test, the complex that we have chosen is the chromyl ethylene glycolate anion, whose structure is depicted in Figure l. The following reasoning guided us in selecting this particular TMC: (i) This complex is known to be particularly suited as a polarized proton target in high-energy experiment~,'~J~ and a better understanding of the electronic structure would certainly help to rationalize why for this TMC dynamic nuclear polarization is so efficient. (ii) Earlier ESR experiments and sophisticated MO calculations on this complex by Winscom14gave already a rather deep insight into the electronic structure which could serve as a solid basis for the ENDOR experiments. (iii) From inspection of Winscom's ESR results,'* the feasibility of ENDOR in solution of this TMC looked very promising. Firstly, the ESR could be saturated. This Institut ftir Molekiilphysik.
* Institut ftir Organische Chemie.
*Address correspondence to this author at the following address: Institut ftir Molekiilphysik, Freie Universitat Berlin, West Berlin, West Germany. 0022-3654/82/20%6-0149$01.25/0
means that the electron spin relaxation rate must be small relative to other TMC's. This is a result of the g factor having a value quite close to the free-electron value (acceptable spin-rotation relaxation rates) and showing very little anisotropy (small Zeeman relaxation rates). Secondly, only the less dominant (10%) of the two isotopes, 52Cr and 53Cr, has a nonvanishing nuclear spin of I = 3/2. As a consequence, ENDOR spectroscopy on molecules containing 52Crwill not suffer from any relaxation contributions caused by a large metal hf anisotropy. Experimental Section The ESR part of our broad-band ENDOR setup consists basically of an AEG 20 XT magnet and power supply, a Varian E 102-03 microwave bridge with maximum power output of ca. 400 mW (corresponding to a microwave HI field strength of 140 mG in the rotating frame at the sample position), and conventional components for the phase-sensitive detection channel (Ithaco Dynatrac 391 A). The design of the rf channel and of the cavity with the rf solenoid is similar to that already described.15 The rf source comprised an hp 8660B signal generator followed by an IF1 404A distributed power amplifier with a maximum output of 500 W (ca. 19 G in the rotating frame at the sample location). ENDOR signals were singly coded by frequency modulating (rate, 10 kHz) the rf field. The temperature was controlled by an AEG instrument to within f 2 K and calibrated by means of a thermocouple. Spin concentrations of samples were determined by on-line, computerassisted comparison of doubly integrated ESR signal intensities with a ruby standard. This standard was cali(1)B. A. McGarvey, Transition Met. Chem., 3, 89 (1966). (2)H.G. Rist and J. S. Hyde, J. Chem. Phys., 52, 4633 (1970). (3)H. van Williien, C. F. Mulks, A. Bonhaouss, M. Ferhat, and A. H. Roufwse, J. Am. Chem. SOC., 102, 4846 (1980). (4)H.van Willigen, Chem. Phys. Lett., 65, 490 (1979). (5) M. Rudin, E. JBrin, A. Schweiger, and Hs. H. Giinthardt, Chem. Phya. Lett., 67, 374 (1979). (6)E. JBrin, M. Rudin, A. Schweiger, and Ha. H. Giinthardt, Chem. Phys. Lett., 69, 1 (1980). (7)N. M. Atherton and A. J. Horsewill. Mol. Phvs.. 37. 1349 (1979). (8)G . Feher, R. A. Isaacaon, C. P. Scholes, and R.Nagel, Ann: N.Y. Acad. Sci., 222,86 (1973). (9)C.A. Hutchison, Jr., and D. B. McKay, J. Chem. Phya., 66, 3311 (1977). (10)R. de Beer, W. de Beer, C. A. van't Hoff, and D. Ormondt, Acta Crystallogr., Sect. B,29, 7 (1973). (11)M. Plato, W. Lubitz, and K. MBbius, J.Phvs. Chem., 85. 1202 (1981). (12)H. Glattli, M. Odehnal, J. Ezratty, A. Malinovski, and A. Abragam, Phys. Lett. A, 29, 250 (1969). (13)A. Masaike, H.Glattli,J. Ezratty, and A. Malinovski, Phys. Lett. A, 30, 63 (1963!. (14)C. J. Wmacom, Mol. Phys., 28, 1579 (1974). (15)K. MBbius and R. Biehl in 'Multiple Electron Resonance Spectroscopy", M. M. Dorio and J. H. Freed, Eds., Plenum Press, New York, 1979,p 475 ff.
0 1982 American Chemical Society
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The Journal of Physical Chemistty, Vol. 86, No. 1, 1982
1 H
H
Mohl et al.
I
Figure 1. Proposed structure of the chromyl ethylene glycolate anion. "I4
I T = 190K
'L L
11
13
15
17 M H z
Flgure 2. Typical proton ENDOR spectrum at optimum conditions.
Flgure 3. Plots of the experimental ENDOR signal amplitude (plot 1) and line wklth M o t 2) as functions of temaerature. (Note that the freezing point of the solvent, ethanol, is ca. 158 K.)
brated against an aqueous MnS04 solution.16 The estimated overall accuracy of measuring the spin concentration of the dilute chromyl species is *30%. This rather large error is due to the poor signal-to-noise ratio obtained for these low concentrations (10-4-10-5 M). Samples of the chromyl ethylene glycolate complex17i18 were prepared by dissolving potassium dichromate in ethylene glycol (1 g/10 mL) followed by dilution with ethanol. Finely ground potassium dichromate (Merck) was dried for 2 h at 110 "C under reduced pressure (-1 torr). Ethylene glycol (p.a. Merck) was used without further purification. Ethanol (Uvasol, Merck) was distilled and stored over molecular sieves (3 A) on the vacuum line. The ethylene glycol solution of the dichromate was carefully degassed under high-vacuum conditions (ca. torr); a side arm of the sample tube containing ethanol was previously filled. Thereby, a considerable amount of the water formed during the dichromate reduction by ethylene glycol was removed; the presence of a water byproduct would otherwise decrease the lifetime of the intermediate CrV complex.ls In samples prepared in this way, the Cr" species persisted for almost 1year (dark storage at 255 K) without significant loss of ESR signal intensity. The concentration of the complex could be varied by ethanol distillation within the sealed tube.
This is in accordance with previous ESR studies,14in which all eight protons were found to be equivalent within the ESR line width. The ENDOR signal amplitude and line width as functions of temperature are shown in Figure 3. The maximum ENDOR signal was observed at the optimum temperature !PPt N 193 K. We believe that this value is almost entirely determined by relaxation from rotational diffusion and not by exchange effects. Heisenberg spin exchange can most certainly be ruled out at M. The the measured spin concentration of 1 X corresponding Heisenberg exchange rate, om, is estimated to be ca. 3 kI-Iz,ll which is negligibly small compared with the observed relaxation rates (see below). At higher spin concentrations, where Heisenberg exchange can still be neglected (om N 20 kHz at M), a strong decrease of the ENDOR signal was observed, which can only be attributed to the increasing electron dipole-other dipole interaction between the observed CrVcomplex and other paramagnetic products of the reaction sequence, e.g., Cr"' species.18 It should be mentioned that, as a result of the anticipated quartet ground state of the final Crm product, the ENDOR-diminishing effect is observed at lower concentrations than for organic doublet radicals. From the Einstein-Debye (ED) relation"
Discussion of Experimental Results Proton ENDOR spectra of the chromyl complex were observed at different temperatures and at different microwave and rf power settings. A typical ENDOR spectrum is shown in Figure 2. In all cases only a single proton hyperfine coupling of 1.74 f 0.02 MHz could be resolved.
in which q is the viscosity of the solvent and Veffthe effective molecular volume, the optimum rotational correlation time, 7R0Pt(ED),can be estimated. Assuming the complex to be of ellipsoidal shape with semiaxes a = 4, b = 3, and c = 2 A, estimated from standard bond lengths and van der Waals radii, we obtain Veff 31 100 A3. This yields fRoPt(ED) = 0.7 ns with an error of ca. *30%. Theoretically, for a single proton, fRoPt is related to the magnetic properties of the complex by'l
(16)F.Schneider and M. Plato, 'Elektronenspinresonanz", Thiemig Verlag, Mtjnchen, 1971,pp 161-5. (17)N. S. Garifyanow, B. M. Kozyrev, and V. N. Fedotov, Sou. Phys.-Dokl. (Engl. Transl.),13, 107 (1968). (18)P. R. Bontchev, A. Malinowski, M. Mitewa, and K. Kabassonov, Inorg. Chim. Acta, 6,499 (1972).
rR
rRoPt(theor)
= Vef#/kT
200(B/Tr AH^)^'^^ ns
(1)
(2)
where AH is the dipolar (traceless) part of the proton hfs
The Journal of Physical Chemistry, Vol. 86, No. 1, 1982
ENDOR In Solution on Transition Metal Complexes
tensor in MHz and B is a molecular constant originating from the coupling of the electron spin with the rotational motion of the tumbling molecule. In the case of several equivalent p r ~ t o mTr , ~AH2 ~ in eq 2 has to be multiplied by an average squared transition matrix element (IIJ correctly weighted over all spin subgroups. It follows that (11*1),,2 = n where n is the number of equivalent protons. In our case this introduces an additional factor of 8-'12 on the right-hand side of eq 2. The value of Tr AH2 is obtained by using the experimental evidence14that ca. 90% of the unpaired electron spin is localized at the chromium atom. It therefore appears justified to use a point-dipole approximation to calculate AH. Using the geometrical configuration and standard bond lengths, we obtain for each proton T r AH2 3: 21 MHz2. The value of B can be estimated to within f50% from the g tensor of the complex" A
N
f/z Tr (g -
(3)
From the experimental data in ref 14 we obtain B = 8 X lo4. Inserting these values of Tr AH2 and B into eq 2, and including the factor 8-1/2, yields TRoPt(theor)N 0.9 ns which-within the error of f25%-is in good agreement with the Einstein-Debye estimate. Measurements of the ENDOR line width as a function of the rf field yielded after extrapolation to zero microwave field the following relaxation times at P P t (assuming homogeneous broadening and taking (lI+l),,2 = 8 for the evaluation of T1ef?:19
Tlneff= (1.0 & 0.3) X
lo4
s
T2, = (7.0 f 0.7) X lO-'s The error in TI,& is mainly determined by the inaccuracy of the rf magnetic field measurement at the sample location in the NMR coil. The value of Tzncorresponds to an unsaturated line width of 260 kHz. The measured T2, is practically equal to the electron spin-lattice relaxation time, which is predominantly determined by spin-rotational coupling and given by T,, = (2B/7R)-'.l1 Inserting the values for B = 8 X lo4 and rR = 0.7 ns given above yields T1, = 5 X lo-' s. Since TznN- TI,, it may be concluded that the ENDOR lines are essentially lifetime broadened by electron spin-lattice relaxation. According to ref 11, the optimum microwave and rf radiation field strengths, H,"Pt and HnoPt, can be estimated from
Heopt3: 0.3(B T r AH2(lIkl)2)1/2 G
HnoPtN 100(B Tr AH2)1/2G
(44 (4b)
where Tr AH2 is given in MHz2. I t has to be pointed out that the average transition moment 0: (11k1)2 does not appear in relation 4b to first order; HnOpt is therefore independent of the number of equivalent nuclei involved in the NMR transitions. Inserting values of B and Tr AH2 into eq 4, we obtain H,"Pt = 110 mG, H,"Pt = 13 G. These values are still within the power limitation of our experimental setup. Using the requirements of the chromyl complex test case as a "yardstick", we are now able to define the practical limits for the magnitude of the various magnetic interactions within which solution ENDOR of ligand nuclei would still appear possible. Using the full rf power capability of 500 W of our setup, corresponding to ca. 20 G in the ro(19)D.S.Leniart, H. D. Connor, and J. H. Freed, J. Chem. Phys., 63, 166 (1975).
151
tating frame as an upper limit for H,,we obtain Tr (g
=3
X
Tr Amax2= 40 MHz2
(54 (5b)
if either Tr A2 (case 5a) or B (case 5b) is kept constant and equal to the values for the chromyl complex. In the case of an isotropic g tensor and axially symmetrical A tensor, these values correspond to a maximum g shift k - gelmax of 0.03 or a maximum anisotropy of lA,,- A I1- of 8 MHz. The condition expressed by eq 5b is rather restrictive and shows that ENDOR in solution on ligand nitrogen nuclei, which often have Tr A2 2 100 MHz2, will be difficult to perform. This limitation is even more serious for cases in which the transition metal has I # 0 unless the spin density in the metal atom is very small or unless the g shift is considerably smaller than in the chromyl complex. For example, in order to allow for Tr A2 = 4000 MHz2, the value of - gel, should not exceed 0.003 ( B = 2 X 10") which is a very strong requirement for TMC's. Condition 5b is even more restrictive in cases where there is a significant ligand or metal quadrupole coupling, since this interaction produces additional nuclear spin-lattice relaxation. So far, we have not considered any anisotropy of the g tensor which produces electron spin relaxation via modulation of the electron Zeeman interaction. It can, however, be shown that the associated relaxation rate W,C is at most on the order of 5% of the spin-rotational relaxation rate W,SR and can therefore be neglected in this context. Conclusion and Discussion of F u t u r e TMC Candidates for ENDOR in Solution In this work a transition metal complex, whose static spin Hamiltanian parameters suggested the possibility of detecting ENDOR in solution, was chosen as a test case. Indeed, as it turns out, the chromyl ethylene glycolate anion is the only TMC on which successful solution ENDOR experiments have been so far performed. By restricting sample concentrations to less than lo4 M, Heisenberg exchange and dipole-other dipole interactions were effectively eliminated. The remaining relaxation processes caused by Brownian rotational diffusion generally comprise contributions from the spin-rotational coupling and from the Zeeman, hyperfine, and quadrupole terms of the spin Hamiltonian. In the present case of the chromyl complex, only contributions from the spin-rotational coupling and from the ligand dipolar hyperfine term are important, since the g tensor is almost isotropic and, for 52Cr,I = 0. On this basis the optimum rotational correlation time , from the for a maximum ENDOR signal, T ~ O P ~derived static spin Hamiltonian parameters was found to be in excellent agreement with that estimated from the Einstein-Debye relationship. The optimum values of the microwave and rf H1 field strengths derived from the magnetic interaction parameters were confirmed experimentally. They are close to the power limits of our experimental setup. On the basis of our test case, practical limits regarding the magnitudes of the various spin Hamiltonian parameters could be established for ENDOR-in-solution experiments on TMC's. Provided that the required value of TR"@ can be estimated by appropriate choice of solvent and temperature for a given effective TMC volume, the classes of TMC's having doublet ground states, for which solution ENDOR might be possible, may be proposed. Complexes formed from first-row transition metal ions may be divided into two groups: those formed from
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The Journal of Physical Chemistry, Vol. 86, No. 1, 1982
transition metals whose naturally abundant nuclei have # 0. In the first group (Ti, Cr, Fe, Ni), the typically large metal hf anisotropy is absent, allowing for a less stringent requirement on the deviation of g from g,*l. (In the second group metal hf interactions will generally be present, necessitating a harder requirement for e.) For both groups, deviations from g,.l will be more marked for the heavier members of each series (the spin-orbit coupling constant for Cu2+is approximately 6 times larger than that for Ti3+). On this basis, complexes formed from Ti and Cr will be the most amenable to study by ENDOR in solution, whereas complexes of Co ( W o , I = 7/2) and Cu (s3W~, I = 3/2) will be the least likely candidates. As an example from the first group, the complex TimHz(cp),-MgC1(cp = cyclopentadienyl) suggests itself, where the published datam indicate that B (eq 3) is ca. 2 X Analogues of this complex involving different halides and with Mg replaced by A1 are known. Other aryktitanium(II1) complexes, e.g., Tin1(C8H8).cp2'(B = 2.2 X might still be possible. Similarly aryl-chromium(1) complexes, such as Cr(c&&'+(ref 22 and 23), B N 1.6 X and its methyl-substituted analogues,23suggest themselves for ENDOR-in-solutionstudies. A different type of chromium(1) complex, [Cr(CN),NO]* (B 9 X 10-9 (ref 24), provided the ground state is a doublet, would also seem to be a possibility. Analogues of the chromyl bis(ethy1ene glycolate) anion, having a Crv oxidation state, have been well documented by Garifyanov and co-workers." It is worth
I = 0, and those with I
(20)J. G. Kenworthy, J. Myatt, and M. C. R. Symons, J. Chem. SOC. A , 3428 (1971). (21)J. L. Thomaa and R. G. Hayes, Znorg. Chem., 11, 348 (1972). (22)W. Kuthe and W . Kleinwichter, Ann. Phys., 21, 137 (1968). (23)W.Karthe and W. Klehwichbr, Z. Phys. Chem., 247,241(1971). (24)B. A. Goodman, J. B. Raynor, and M. C.R. Symons, J. Chem. SOC. A, 994 (1966).
Additions and Corrections
mentioning that (CrOC14)l+(ref 25) falls into an acceptable Another interesting Cr" exrange for B (B = 5 X ample might be Cr(S2C2(CF3)2)31(B = 1.5 X 10-4).26 From the heavier members of the first group, suitable examples are difficult to find, although undoubtedly exceptions exist. ' h o examples might be [Fe(CN)SNO]3-(ref and Ni(pc)'- (pc = phthalocya27) where B N 1.7 X mine)28where the isotropic g value lies so close to g, (suggesting that the unpaired electron occupies an orbital of mainly ligand character) that the value of B is anticipated to be acceptable. Very few suitable examples of complexes formed from the second group of TMC's (V, Mn, Co, Cu) exist. Such possibilities as there are will generally come from species where the unpaired electron is mainly located on the organic ligand. Analogues to the Ni(pc)'- complex in the foregoing paragraph, the complexes Co(p~)~-, Co(p~)~-, and ~~ themselves. CU(PC)~from ref 28 and C U ( P C )suggest Using similar reasoning, certain isolated examples of O2 adducts of cobalt-Schiff base complexes, where it is well-known that the Co hf interaction is both small in magnitude and almost i s o t r o p i ~ may , ~ ~ be suitable candidates. Acknowledgment. We are grateful to Professor H. van Willigen (University of Massachusetts, Boston) and Dr. E. Boroske (Free University Berlin) who have participated in an earlier stage of these studies. This work was supported by the Deutsche Forschungsgemeinschaft(Sfb161). (25)H. Kon. J. Inora. Nucl. Chem.. 25. 933 (1963). (26)A. Davison, N. Edelstein, R. H', Holm, and A: H. Maki, J. Am. Chem. Soc., 86,2799 (1964). (27)D.A. C. McNeil, J. B. Raynor, and M. C. R. Symons, J. Chem. Soc., 410 (1965). (28)D.W.Clack, N. S. Hush, and J. R. Yandle, Chem. Phys. Lett., 1. 157 (1967). ' (29)'C.M.Guzy, J. B. Raynor, L. P. Stodulski, and M. C. R. Symons, J. Chem. SOC.A, 997 (1969). (30)F.Basolo, Chem. Reo., 79, 175 (1979).
ADDITIONS AND CORRECTIONS 1981, Volume 85
Hiroshi Yamataka* and Takashi Ando*: Model Calculations of Kinetic Isotope Effects in the SN2 Reaction of Benzyl Arenesulfonates with NJV-Dimethyl-p-toluidine. Page 2281. In eq 1,45 "C should be changed to 35 "C. Page 2284. In the footnote of Table II,45 "C should be changed to 35 "C.
