J. Phys. Chem. 1984, 88, 4351-4354
4351
Electron Spin Resonance Spectra of Dloxygen Complexes of Group 1B Metal Atomsii2 J. A. Howard,* R. S~tcliffe,~ National Research Council of Canada, Ottawa, Ontario, Canada K I A OR9
and B. Mile Department of Chemistry and Biochemistry, Liverpool Polytechnic, Liverpool, England L3 3AF (Received: September 26, 1983)
Adducts of group 1B metal atoms to molecular oxygen (CuO,, Ago2, and AuO,) have been prepared in a rotating cryostat at 77 K and isolated in adamantane. Electron spin resonance spectra at 10 and 77 K indicate that most of the unpaired spin population in these complexes resides in a a* orbital on the two oxygen nuclei with low but measurable unpaired s spin population on the metal atom. ESR spectra of mono(dioxygen) complexes enriched in ”0 suggest that CuOzhas end-on bonding with magneticallyand spacially nonequivalent oxygen atoms and Ago2and Au02 have side-on bonding with equivalent oxygen atoms.
Introduction Transition-metal atoms (M) in their ground electronic state and zerovalent transition-metal complexes (ML,, where n is the number of ligands L) react with molecular oxygen to give mono(dioxygen)-transition-metal complexes which have either side-on 1 or end-on bonding 2 between the oxygen and metal at~rn.~-’~ 0-0 \ I
M
1
0-0
/
M
2
Naked metal atom-dioxygen complexes are only stable at low temperature and have to be prepared by cryogenic techniques and isolated in an inert matrix.18 The interaction of oxygen with group 1B metal atoms is, however, of interest for the light it might throw on the catalysis of the epoxidation of ethylene by molecular oxygen which occurs exclusively over metallic silver. It has been concluded from infrared spectroscopic studies of group 1B metal atom-dioxygen complexes that Cu0217 and are tight ion pairs, Le., (Cu)+(02)-and (Ag)’(O,)- with side-on bonding and nonequivalent oxygen atoms. The nature of the bonding and the structure of these species is, however, still in doubt. Thus it has been suggested that A g o 2 has equivalentg ( 1 ) Issued as NRCC No. 23540. (2) Cryochemical Studies Part 13. For part 12 see Howard, J. A,; Sutcliffe, R.; Mile, B. J. Phys. Chem. 1984, 88, 2183-5. (3) NRCC Research Associate. (4) Huber, H.; Klotzbiicher, W.; Ozin, G. A , ; Vander Voet, A. Can. J. Chem. 1973, 51, 2722-36. (5) Darling, J. H.; Garton-Sprenger, M. B.; Ogden, J. S. Faraday Symp. 1973, 8, 75-82. (6) Fieldhouse, S.A,; Fullam, B. W.; Neilson, G. W.; Symons, M. C. R. J. Chem. SOC.,Dalton Trans. 1974, 567-9. (7) Klotzbucher, W. E.; Ozin, G. A. J. Am. Chem. SOC.1973,95, 3790-2, 1975, 97, 3965-74. (8) McIntosh, D.;Ozin, G. A. Inorg. Chem. 1976, 15, 2869-71. (9) McIntosh, D.; Ozin, G. A. Inorg. Chem. 1977, 16, 59-63. (10) Huber, H.; Ozin, G. A. Inorg. Chem. 1977, 16, 64-9. (1 1) Huber,H.; McIntosh, D.; Ozin, G. A. Inorg. Chem. 1977, 16,975-9. (12) Lindsell, W. E.; Preston, P. N. J. Chem. SOC.,Dnlton Trans. 1979, 1105-8. (13) Lever, A. B. P.; Ozin, G. A,; Gray, H. B. Inorg. Chem. 1980, 19, 1823-4. (14) Carlton, L.; Lindsell, W. E.; Preston, P. N. J. Chem. SOC.,Chem. Commun. 1981, 531-2. (15) Chang, S.; Blyholder, G.; Fernandez, J. Inorg. Chem. 1981, 20,
--_- . . 78 1 1-7
(16) Tevault, D. E.; Smardzewski, R. R.; Urban, M. W.; Nakamato, K. J . Chem. Phys. 1982, 77, 577-8. (17) Tevault, D. E. J. Chem. Phys. 1982, 76, 2859-63. (1 8) Klabunde, K. J. “Chemistry of Free Atoms and Particles”;Academic Press: New York, 1980. Blackborow, J. R.; Young, D.“Metal Vapor Synthesis in Organometallic Chemistry”; Springer-Verlag: New York, 1979. Moskovits, M.; Ozin, G. A. “Cryochemistry”;Wiley-Interscience: New York, 1976.
0022-3654/84/2088-4351$01.50/0
and nonequivalentI6 oxygen atoms although both groups agree that it is a tight ion pair. Electron spin resonance spectroscopic studies of these complexes should provide a more definitive picture of their structure because metal atom and oxygen- 17 hyperfine interactions should enable the number of metal atoms to be established and the number and equivalency of the oxygen atoms to be determined. The metal carbonyls (CO)5Mn12and (CO),CO,~for example, have been shown to react with oxygen to give the peroxo complexes (CO),MnO, and (C0)4C002with magnetically nonequivalent oxygen atoms and a SOMO composed mainly of dioxygen a* orbitals with little contribution from metal d orbitals. There have been no detailed ESR studies of dioxygen-group 1B metal atom complexes which would help to clarify the structure and bonding in these complexes although Mattar, McIntosh, and Ozin19 have reported that ESR studies of CuO, isolated in rare gas matrices indicate that it has a nonlinear end-on bonded structure. We, therefore, report here the results of such a study on complexes prepared in a rotating cryostat at 77 K. Experimental Section The rotating cryostat, furnace used to vaporize Cu, Ag, and Au, and equipment used to record and calibrate ESR spectra have been described in previous publications from these laboratories.20 Oxygen enriched to 70% in I7O was purchased from Merck Sharpe and Dohme Canada Ltd. lo7Agand 63Cu0were obtained from Oak Ridge National Laboratory, TN. 63Cu0was reduced to 63Cuwith H, at 500 OC. Au was kindly provided by Dr. C. M. Hurd (NRC Ottawa). Adamantane (Aldrich) was sublimed before use. In a typical experiment 5-10 mg of metal and 1 g of matrix were deposited in 0.5 h and oxygen was bled into the cryostat at 0.1 torr so that the pressure in the cryostat was -2 x torr. Results CuO,. Reaction of 63Cu atoms with molecular oxygen in adamantane at 77 K gave the anisotropic ESR spectrum shown in Figure la. This spectrum can be analyzed in terms of one quartet of absorption features centered at 31436 and two quartets of almost first-derivative features centered at 3259 and 3270 G. This pattern of lines is indicative of a species with an orthorhombic g tensor. Analysis of the spectrum gave g , = 2.000, gyy= 2.007, g,, = 2.081, and g,, = 2.029. The quartet spacings are similar, although not identical, indicating an almost isotropic hyperfine interaction with one copper nucleus ( I = 3/2). The distances between the MI = h3/,transitions, divided by 3, gave A,,(Cu) (19) Mattar, S.;McIntosh, D.F.; Ozin, G. A,, personal communication. (20) Buck, A,; Mile, B.; Howard, I. A. J. Am. Chem. SOC.1983, 105, 3381-7.
Published 1984 American Chemical Society
4352 The Journal of Physical Chemistry, Vol. 88, No. 19, 1984 I
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I
Howard et al.
m I
V = 9153.0 MHz II
1
V -9153.8MHZ
w 3226G
U
Figure 1. (a)
ESR spectrum of 63Cu02 in adamantane at 77 K. (b) First-order computer simulation of the spectrum of one unpaired electron and one 63Cunucleus for parameters given in the text.
Figure 3. (a) ESR spectrum of 170-enrichedlo7Ag02in adamantane at 77 K. (b) The wings of the spectrum at higher gain (c) a stick diagram for Ag’70170with equivalent oxygen nuclei and (d) a stick diagram for singly labeled Ago,.
oxygen nuclei is obtained from the M,(Cu) = 3/2, MXO’) = -5/2, MXO”) = -5/1 and MXCu) = 3/2, MXO’) = -3/2, M,(O’/) = -5/2 lines. This splitting must be associated with the axis parallel to the p, orbital of the unpaired electron, i.e., the x axis and A,(O’) = -61 G. The width of the spectrum is 5A(O’) + 5A(O”) + 3A(Cu) and solving this equation, using A ( 0 ’ ) = -61 G and A(&) = 38 G, gives A,,(O”) = -85 G. The values of A, and A,, cannot be obtained from the spectrum because of overlapping lines in the center of the spectrum. These parameters are, however, probably less than 10 G. The isotropic and anisotropic components of the 170hyperfine interactions must, therefore, be of comparable magnitude, i.e., &,(O’) = -20.3 G (-57.6 MHz), Aiso(0”)= -28 G (-79.5 MHz), Aanlw(O’)= -20 G (-56.8 MHz), A,,,,(O”) -28 G (-79.5 MHz). Ago,. Io7Agatoms and molecular oxygen in adamantane at 77 K gave a characteristic peroxyl powder spectrum with g,, = 2.003, gyy= 2.010, g,, = 2.056, and g,, = 2.023. There was a resolved 10-G doublet associated with the z axis and the hint of 2-G doublets associated with the x and y axes. This spectrum is assigned to Ago2 on the basis of g,, and the resolved hyperfine interaction from a nucleus with I = Peroxyls derived from the matrix can be eliminated because they would have a much smaller g factor (-2.015). Furthermore in the absence of 0, an Figure 2. (a) ESR spectrum of a mixture of s3Cu’70170, 63Cu1700, intense doublet from Io7Agatoms was observedZowhich was absent 63Cu0170, and CuOO in adamantaneat 77 K. The stick diagrams show the high- and low-field transitions for C U ’ ~ O ’ with ~ O two inequivalent in the presence of oxygen. oxygen atoms. The lines labeled * and + are from CuO170and C U ~ ~ O O , The ESR spectrum given by Io7Agatoms and 1702 at 77 K is respectively. (b) The wings of the spectrum at higher gain. shown in Figure 3 and is a composite spectrum containing contributions from Agl7Ol7O (49%), Ag170160 (21%), Ag160170 = 38 G, A,(Cu) = 43.6 G, and A,,(Cu) = 45.7 G. A computer (21%), and Ag‘60160(9%). It has a strong and poorly resolved simulation using a first-order treatrnent2l for these parameters central feature plus a set of 11 equally spaced lines centered about is shown in Figure 1b. g,, which emerge at higher gain. These lines can be assigned to Reaction of 63Cuwith O2 enriched to 70 at.% in I7O ( I = 5 / 2 ) Ag170170with two magnetically equivalent oxygen nuclei and gives 9% unlabeled, 42% singly labeled, and 49% doubly labeled a hyperfine interaction of -75 G. In addition to the undecet, lines radicals. The anisotropic ESR spectrum of this mixture was appear in the spectrum closer to the center which can be assigned anticipated to be complex and this was confirmed experimentally to the singly labeled species. This hyperfine interaction must be (Figure 2). The extremes of this spectrum, especially the low-field associated with axes parallel to the p, orbital containing the lines are, however, well resolved and give the exact pattern exunpaired electron, Le., A,,(O’) = A,,(O”) = -75 G. Oxygen pected for 63Cu170170 with two magnetically inequiualent oxygen hyperfine interactions perpendicular to this direction could not nuclei. Such a pattern is readily distinguished from a species with be resolved but must be small (
