Organometallics 1995,14, 5377-5381
5377
Multiple Bonds between Main Group Elements and Transition Metals. 144.' Photoreactivity of Organorhenium(VI1) Oxides Wolfgang A. Herrmann,*lt Fritz E. Kuhn,t>tDirk A. Fiedler,? Mike R. Mattner,tlg Martin R. Geisberger,+Horst Kunkely,l' Arnd Vogler,*pIl and Steen Steenkenl Anorganisch-chemisches Institut der Technischen Universitat Munchen, Lichtenbergstrasse 4, 0-85747 Garching, Germany, Znstitut f i r Anorganische Chemie der Universitat Regensburg, Universitatsstrasse 31, 0-93040 Regensburg, Germany, and Max-Planck-Znstitut f i r Strahlenchemie, Stiftsstrasse 34-36, 0-45470 Miilheim I Ruhr, Germany Received November 21, 1994@ Ligand-to-metal charge transfer excitation of organorhenium(VI1) oxides R-Re03 [R = CH3, C2H5, ql-mesityl, CsH5, v5-C5H5, v5-C5H4(CH3), and v5-C5(CH3)5] in acetonitrile or dichloromethane solution leads to photoredox decomposition. It is shown by EPR spectroscopy that radical pairs RPRe03 are generated in the primary step. The overall efficiency is determined by the reactivity of the radicals R , which decreases in the order CH3 > C2H5 > C5H4(CH3) > C5(CH3)5. The regeneration of R-Re03 is favored with mesityl > C5H5 increasing stability of R. Accordingly the quantum yield varies with R.
Introduction Organometallic oxides containing transition metals in high oxidation states have been attracting much attention.2 However, very little is known about their photoreactivity . Recently, we reported some qualitative observations regarding the light sensitivity of CH3Re03 (la).3The present work is an extension of the previous
2a,b, the ligands are coordinated t o the metal via R-C a-bonds, while complexes 3a-c adopt "half-sandwich" structures. It is known that organorhenium(VI1)oxides decompose more quickly in light than in the dark.4-8 Since the formula R-ReO3 implies heptavalent rhenium with a do electron configuration, all low-energy electronic transitions must be of the ligand-to-metal charge transfer type (LMCT). The radical pair RVRe03 (ReV1) is expected to be generated in the primary photochemical step.
Results and Discussion
la:R=H lb:R=C+b
za
3a:R=R'=H 3b:R = H , R ' = M 3C:R = R'=CH,
study, and it includes a variety of other complexes of the type R-Re03,4 with R = C2H5 (lb), mesityl (2a), C6H5 (2b), Cp (v5-C5H5)(3a),Cp' [v5-C5H4(CH3)1(3b), and Cp* [v5-C5(CH3)5I( 3 ~ ) . In ~ the - ~ case of la,b and
* To whom correspondence should be addressed.
' Technische Universitiit Miinchen.
* Fellow of the Hermann Schlosser-Foundation, 1992- 1994.
Fellow of the Fonds der Chemischen Industrie, 1994-1996. Universitat Regensburg. MPI Mu1heim.Abstract published in Advance ACS Abstracts, September 15,1995. (1)Preceding paper of this series: Herrmann, W: A,; Kuhn, F. E.; Romlo, C. C. J. Organomet. Chem., in press. (2)Reviews on organometallic oxides in general: (a) Nugent, W. A.; Mayer, J. M. Metal to Metal Multiple Bonds; Wiley-Interscience: New York, 1988. (b) Bottomley, F.; Sutin, L. Adu. Orgunomet. Chem. 1988, 28,339. (3)Kunkely, H.; Turk, T.; Teixeira, C.; de MBric de Beilefon, C.; Herrmann, W. A.; Vogler, A. Organometallics 1991,10, 2090. (4)Comprehensive review articles: (a) Herrmann, W. A. J. Organomet. Chem. 1986,300, 111. (b) Herrmann, W. A.; Herdtweck, E.; Floel, M.; Kulpe, J.; Kiisthardt, U.; Okuda, J. Polyhedron 1987,6,1165. (c) Herrmann, W.A. Angew. Chem., Int. Ed. Engl. 1988,27,1297. (d) Herrmann, W. A. J . Organomet. Chem. 1990,382,1. (5)(a) Herrmann, W. A.; Kuhn, F. E.; RomHo, C. C.; Tran Huy, H.; Wang, M.; Fischer, R. W.; Kiprof, P.; Scherer, W. Chem. Ber. 1993, 126,45. (b) Herrmann, W. A.; RomHo, C. C.; Fischer, R. W.; Kiprof, P.; de Meric de Bellefon, C. Angew. Chem., Int. Ed. Engl. 1991,30, 185. 8
I'
@
0276-7333/95/2314-5377$09.QQIQ
(1)Electronic Spectra. The absorption spectra of all hitherto examined complexes of formula R-Re03 (atype ligands R) exhibit characteristic intense bands. They differ in wavelength and intensity with the organic ligands R (Table 1). However, the absorption spectra of the (naryl)rhenium(VII)oxides 3a-c in CH3CN and CH2C12 are insensitive with respect to R. The W l v i s spectrum of 3c is shown in Figure 1. In contrast t o the parent compound la3 other complexes R-Re03 do not show an emission in solution or in the solid state, neither at room temperature nor a t 77 K. In analogy t o la, the longest-wavelength absorption of 2a at I,,, = 334 nm is assigned to the LMCT transition which involves promotion of an electron from the Re-C a-bond to empty d orbitals of Re (a1 e* in
-
(6)(a)Kiihn, F. E.;Herrmann, W. A.; Hahn, R.; Elison, M.; Bliimel, J.; Herdtweck, E. Organometallics 1994,13,1601. (b) Herrmann, W. A,; Taillefer, M.; de MBric de Bellefon, C.; Behm, J. Inorg. Chem. 1991, 30, 3247. (c) Thiel, W. R.; Fischer, R. W.; Herrmann, W. A. J. Organomet. Chem. 1993,459,C9. (d) Herrmann, W. A.; Serrano, R.; Bock, H. Angew. Chem., Int. Ed. Engl. 1984,23,383.(e) Klahn-Oliva, A. H.; Sutton, D. Organometallics 1984,3,1313. (7)(a) de Meric de Bellefon, C.; Herrmann, W. A.; Kiprof, P.; Whitaker, C. Organometallics 1992,11, 1072. (b) Herrmann, W. A.; Ladwig, M.; Kiprof, P.; Riede, J. J.Orgunomet. Chem. 1989,371,C13. (8)(a) Herrmann, W. A.; Kuhn, F. E.; Fischer, R. W.; Thiel, W. R.; RomHo, C. C. Inorg. Chem. 1992,31, 4431. (b) Herrmann, W. A.; Kuchler, J. G.; Felixberger, J. K.; Herdtweck, E.; Wagner, W. Angew. Chem., Int. Ed. Engl. 1988,27, 394. (c) Beattie, I. R.; Jones, P. J. Inorg. Chem. 1979,18,2318.
0 1995 American Chemical Society
Herrmann et al.
5378 Organometallics, Vol. 14, No. 11, 1995
Table 1. Characteristic Maxima in the Absorption Suectra of Selected Organorheniu") Oxides no. A,,
compd CHsReOs . .
la
CzHsRe03
lb
[r]'-CsHz(CH3)31Re03 2a ( ~ ~ ~ - C ~ I - I ~ ) R ~3a O~
(nm)
E
205 231 260 210 240 262 334 212 297 344" 377"
346"
380" 357" 402" a
(M-l cm-')
solvent
1600 1500 1020 2300 1250 1040
n-hexaneb
8550 6700 3700 1200 800 7600 4100 1040 670 6500 3900 1050 750
CH&N CHjCN
CH3CN
CH3CN
CH3CN
Shoulder. Taken from ref 3.
1
I
,
\
,
CsUsymmetry). The assignments of the absorptions of lb correspond to that of laq3 Band assignments are based on recent MO calculationsga in the case of the half-sandwich complexes 3ac. The HOMO is bonding and derived from the doubly degenerate el" n-orbitals of the aromatic ligands. The precise nature of the LUMO is not clear, because two types of antibonding d-based MOs are close in energy.g One of them results from the ninteraction of the e* orbitals (&OS) with the aromatic ligand, while the other one is generated by the a-type overlap of the aromatic ligand with the al* orbital of the Reo3 moiety. The similarity of the absorption spectra of all three complexes suggests that the sequence of the e* and al* orbitals is invariant with R. According t o the calculations, the two longest-wavelength absorptions of 3a-c near 350 and 390 nm are assigned to the el" e* and a1* LMCT transitions from the n-orbitals of the el aromatic ligands t o the Reo3 group. The more intense band at shorter wavelength (ca. 300 nm) very likely arises from a LMCT process, too. (2) Photolysis Studies. Solutions of la398and lb are sensitive to short-wavelength light. On photolysis of l a with 254 nm light in aqueous solution (eq l),
-
-
(9) (a) Szyperski, T.;Schwerdtfeger, P. Angew. Chem.,Int. Ed. Engl. 1989,28,1128.(b) Herrmann, W. A.; Riisch, N.; Bock, H.; Kostlmeier, S.; Kiihn, F. E.; Solouki, B. Unpublished results, 199311994,
la
photochemical products observed are CH4 (by GC and NMR) and C2H6 (by GC); see Figure 2. In the presence of small concentrations of 0 2 , the formation of C2H6 is inhibited, while that of CH4 is only reduced (see Figure 3). From this it is concluded that CH4 is formed in a cage reaction, while C2H6 is produced by dimerization of freely diffusing methyl radicals. Experiments with solutions of l a (ca. M) in D20 (purity 99.3%)show the formation of CH3D and CH4 (detected by lH-NMR). A mechanism suggested for the formation of CH3D is D' abstraction from the solvent. Photolysis generates a radical pair inside a solvent cage. Coordination of D2O to an electron deficient metal center may lower the bonding energy in the D20 molecule and facilitate deuterium abstraction by methyl radicals, an otherwise thermodynamically not expected reaction. Concerning C&, it is probably the product of H abstraction by CH3' from l a or photolysis products. The kinetics of the photolysis were studied by GC analysis. The rates are influenced by the nature of the atmosphere in which the experiments are conducted. If aqueous solutions of l a are saturated with oxygen-a typical radical scavengerthe formation of C2H6 exhibits an induction period of 3-4 min (Figure 3). No inhibition effect is observed in argon atmosphere (Figure 2). Additionally in comparison to argon atmosphere the absolute concentrations of C& and C2H6 are lower in the presence of oxygen. These results establish further evidence for the suggested radical mechanism. The photochemical decomposition of lb is similar. Ethane and perrhenate are formed in the presence of traces of water (Figure 4). In concentrated, absolutely dry solutions in CH3CN formation of ethane and low amounts of ethylene and n-butane is observed (GC) upon photolysis; a dark residue consisting mainly of Reo3 (IR, elemental analysis) is concomitantly formed. The quantum yield (4) for the disappearance of lb was determined by measuring the change of extinction a t 254 nm: 4 = 0.53 & 0.03 at Aim = 254 nm; see Table 2. Photolysis of the a-mesityl complex 2a in CH3CN was accompanied by spectral changes (Figure 5) that are characterized by the occurrence of two new absorption bands attributed to mesitylene, A, = 265 and 272 nm. The mesitylene was also detected by GCMS. The increase of the optical density was caused by light scattering of colloidal ReO3, which is formed in analogy to the photolysis of lb (vide supra). The quantum yield for the disappearance of 2a was determined by measuring the change of extinction at 334 nm: = 0.18 f 0.01 at Aim = 333 nm. When the photolysis was carried out in CH2C12, the spectral changes were similar to those observed in CH3CN (Figure 5). Solutions of the n-complexes 3a,b in CH3CN are also light-sensitive. The spectral changes occurring upon photolysis are similar in both cases (Figure 6). The same type of photoreaction should thus be involved. EPR studies were performed to obtain evidence of free radicals R' occurring during photolysis. Solutions of la,b, 2b, and 3a and nitrosodurene (ND) or 5,5-
Organorhenium(VII) Oxides
Organometallics,Vol. 14,No.11, 1995 5379
Table 2. Quantum Yield $I of Disappearance of OrganorheniumWI) Oxides during Irradiation compd no. L,,(nm) @(%I solvent
.-
f
0.0003
I
U
0.0002
la lb 2a 3a 3b 3~
CH3b03 CzHsRe03 [v1-CsHz(CH3)31Re03 (v5-C5HdRe03 Cv5-CsH4(CH3)1Re03 [v5-Cs(CH3)51b03
1
0.0001 71
/
f
.o..
0 ......0......@ o.""' 0..._..
/&"'
.....' 0
254 254 333 313 313 313
0.58 0.53 0.18 0.028 0.013 0.0001
H2O" CH3CNb CH3CNb CH3CNb CH3CNb CH&Nb
a Taken from ref 3. Photolysis in CH2Cl2 led to the same results.
C,H,
0
0
I
0
I
a
4
16
12
20
time (min.)
Figure 2. Photolytic formation of CH4 and CzHs in aqueous solution of la (c = 1.64 x M, argon atmosphere).
I
250
I
,
,
I
1
I
I
t
,
1
1
350
300
,
Fnm a LOO
Figure 5. Spectral changes during photolysis of (VImesity1)ReOs(2a)in acetonitrile at (a) 0 min and (e) 5 min irradiation time with Aim = 333 nm (1cm-cell, c = 1.76 x 10-4 MI.
I
2
0
8
6
4
10
12
time (min.)
Figure 3. Photolytic formation of CH4 and CzH6 in aqueous solution of la (c = 1.64 x M, oxygen atmosphere). 200
300
400
nm
Figure 6. Spectral changes during the photolysis of (q5C5H,&H3)Re03in acetonitrile at (a) 0 min and (e) 8 min irradiation time with Ai, = 313 nm (1cm cell, c = 3.44 x 10-4 MI.
.u,0 . 7 U
C0.6 f0.5 i0.4 P0.3 0.2
0.1 200
220
240
260
280
300
Wavslength(nm1
320
340
360
Figure 4. Spectral changes during photolysis of CzHSRe03 (lb)in acetonitrile at (0) 0 min and (5) 5 min irradiation time with Aim = 254 nm (1cm cell, c = 1.97 x M). dimethyl-1-pyrroline 1-oxide (DMPO) as radical scavengers were irradiated in the cavity of an EPR spectrometer. The hyperfine splitting constants and the g-values of simultaneously recorded EPR spectra are in excellent agreement with reported data (cf Figure 7 and Table 31, especially if one considers a possible solvent effect on the hyperfine splitting constants.1° A control (10)(a) Janzen, E. G.; Coulter, G. A,; Oeler, U. M.; Bergsma, J. P. Can. J . Chem. 1982,60,2725.Rehorek, D.Chem. SOC.Rev. 1991,2, 341. (b) Terabe, S.;Kuruma, K.; Konaka, R. J. Chem. SOC.,Perkin Trans. 2 1973,1252. ( c ) Barker, P.J.; Stobart, S. R.; West, P. R. J. Chem. Soc., Perkin Trans. 2 1986,127.
experiment with benzoyl peroxide as a source of phenyl radicals confirmed the generation of CsHg upon irradiation of 2b in solution. Photolysis of 3a,b yields dark-colored precipitates that mostly consist of Re03 (IR, elemental analyses). Black precipitates of Re& (elemental analyses) are obtained upon dissolution in concentrated HC1 and addition of H2S. The n-cyclopentadienyl complexes 3a,b decomposed with quantum yields of 4 = 0.028 f 0.001 and 0.013 f 0.001%, respectively, at Ai, = 313 nm. When the irradiation was carried out in CHzC12, the spectral changes were quite similar to those in CH3CN. The fully methylated derivative 3c was almost insensitive to light. Only slow photodecomposition took place with 4= This very low quantum yield explains why radicals were not detectable by means of EPR spectroscopy. Irradiation causes (radical)reactions between ND and the solvent before a sufficient Cp* radical concentration is established.
Herrmann et al.
5380 Organometallics, Vol. 14,No. 11, 1995
L--=z 313 80
-
~
-_
-
_I-
3 2 1 30
F
.
~
Field [mT]
_--.328 80
Figure 7. EPR spectrum of the CH3-ND radical at 20 "C generated during irradiation at A,m = 254 nm (1mM CH3Reo3 and 10 mM ND in CH2C12; ND = nitrosodurene; asterisks indicate solvent background signals). According t o the LMCT character of the low-energy excited states and as shown by EPR experiments, organorhenium(VI1) oxides undergo a photoredox decomposition uia generation of R' and 'Reo3 radicals. The photolytic efficiency depends on the competition of several processes. The Reo3 radicals aggregate mainly t o bulk, insoluble ReO3, which is a well-known compound.ll The organic radicals R' can recombine with the (monomeric)Re03 fragment, if not yet aggregated to regenerate R-Re03. They can also abstract (mostly) hydrogen from solvent molecules coordinated t o the metal center or can dimerize to R-R.12 The occurrence of the latter reactions has been shown by GC, GC/MS, and NMR techniques (see above). It is known that such processes also take place in the thermal decomposition of organorhenium(VI1)o ~ i d e s . ~ - ~ J ~ The majority of carbon-based radicals are very reactive, readily abstracting hydrogen from various solvents including CH3CN and CH2C12. Less reactive radicals facilitate recombination and dimerization. Although the latter process is generally rapid,12 it is hampered by the low steady-state concentration of R' in a conventional stationary photolysis. In analogy to phenyl radicals,12 the mesityl radical undergoes facile hydrogen abstraction from the solvent (GC/MS and W/vis detection of mesitylene) while the recombination with Re03 and the dimerization are expected t o be slow due to the steric situation of the mesityl radical. This is in accordance with the observation that the photolysis of 2a yielding mesitylene is rather efficient (4 = 0.18). The radicals Cp and Cp' generated in the photolysis of 3a,b are so stable14-16 that they dimerize in the absence of suitable scavengers (no H' abstraction from ( l l ) ( a )Kiprof, P.; Herrmann, W. A.; Kiihn, F. E.; Scherer, W.; Kleine, M.; Elison, M.; Rypdal, K.; Volden, H. V.; Gundersen, S.; Haaland, A. Bull. SOC.Chim. Fr. 1992,129,655. (b) Krebs, B.; Miiller, A. 2.Naturforsch. 1968,23b, 415. (c) Nechamkin, H.; Hiskey, C. F. Inorg. Synth. 1950, 3, 168. (12) Monograph on organic radicals: Kochi, J. K., Ed. Free Radicals; CRC Press: Boca Raton, FL, 1989. (13) Herrmann, W. A.; Kiihn, F. E.; RomSio, C. C.; Tran Huy, H. J . Organomet. Chem. 1994,481,227. (14)(a) Barker, P. J.; Davies, A. G.; Fisher, J. D. J. Chem. SOC., Chem. Commun. 1979, 587. (b) Barker, P. J.; Davies, A. G.; Tse, M. W. J . Chem. SOC.,Perkin Trans. 2 1980, 941. (15) Davies, A. G.; Giles, J. R.; Lusztyk, J. J . Chem. SOC., Perkin Trans. 2 1981, 747. (16)Davies, A. G.; Lusztyk, J. J . Chem. SOC.,Chem. Commun. 1980, 554.
solvent). The recombination of R' and *Re03 should thus determine the photoreactivity. As a matter of fact, the photochemical quantum yields are low. GC/MS analyses have shown that various isomers of dihydrofulvalene are formed during photolysis. They can participate in subsequent reactions including p ~ l y m e r i z a t i o n . l ~It- ~is~ interesting to note that the formation of Cp radicals was also confirmed by EPR spectroscopy for the photolysis of (Cp),S&-, and HgCp2.14 The S n ( W and Hg(I1) complexes, like the Re(VII) species 3a, possess only LMCT states at low energies due to their so configuration.l* No radicals were detected during the (inefficient) photochemical decomposition of 3c. By way of contrast, exposition of 3a,b in any organic solvent to daylight effects darkening of the yellow colored solutions within ca. 30 min. 3c does not show a significant color change within 1day. It remains unclear whether this different behavior is due to an efficient radical recombination process or to the high photostability.
Conclusion The present study has revealed that organorhenium(VII) oxides of formula R-Re03 undergo photolytic homolysis of the carbon-rhenium single bonds, thus resembling the thermal decomposition pathway^.^-^ Subsequent reactions of the radicals are listed in Scheme 1. Light in the range of 1= 200-400 nm is most efficient for homolysis. Most of these compounds thus decompose slowly in daylight. Temperature sensitive organorhenium(VI1)oxides show a t room temperature both light- and temperature-induced decomposition. Decomposition by light is in most cases quicker than thermal d e g r a d a t i ~ n . ~While - ~ thermal degradation takes weeks, complete photolysis can be achieved within minutes. Interestingly the thermally most stable compound CH3Re03 (dec > 250 "C) shows the highest quantum yield during irradiation and thus is the most light-sensitive of the examined compounds. The most light-stable derivatives of this class are the naromatic complexes of the cyclopentadienyl type.
Experimental Section (1) Materials. The organorhenium(VI1) complexes 1-3 were prepared according t o published Acetonitrile, CH2C12, and DzO were of at least spectrograde quality. The isotopic purity of D2O was 99.3%. Water was triply distilled. (2) Photolytic Investigations. Photolyses were carried out at room temperature in 1 cm spectrophotometer cells. The light source was a Hanovia Xe/Hg 977 B-1 (1000 W) lamp. Monochromatic light (1, = 254, 313, 333, and 436 nm) was obtained by means of a Schoeffel GM 250-1 high-intensity monochromator. For quantum yield determinations, the concentrations were such that complete light absorption (A 2 2) was ensured. Absorbed light intensities were determined by a Polytec pyroelectric radiometer which was calibrated and equipped with an Rkp-345 detector. The progress of photolysis was monitored by UV/vis spectral measurements with an Uvikon 860 double-beam spectrophotometer or a Hewlett Packard 8452A diode array spectrometer. All photolytic (17) Moulton, R. D.; Farid, R.; Bard, A. J. Electroanal. Chem. 1988, 256, 309. (18)(a) Vogler, A,; Paukner, A.; Kunkely, H. Coord. Chem. Rev. 1990, 97, 285. (b) Vogler, A.; Nikol, H. Pure Appl. Chem. 1992, 64, 1311.
Organometallics, Vol. 14, No. 11, 1995 5381
OrganorheniumWII) Oxides
Table 3. Conditions, EPR Parameters, and Comparison with Known Data (in Parentheses) CH3" C2H5" C5H5" C6H5' a
254 254 300-340 300-340
1.37 (1.37) 1.37 (1.37) 1.33 (1.32-1.38) 1.42
1.24 (1.22) 1.13 (1.10) 0.50 (0.53-0.73) 2.05
2.0057 (2.0060) 2.0067 (2.0061) 2.0057 (2.0060-2.0072) 2.0055
1oc 1oc
10d
Spin trap: ND. Solvent: CH2C12. Spin trap: DMPO. Solvent: CH3CN.
Scheme 1 R
I
o+o
R-R
+
0
I 2/n
R
(Reo3),
H-R
+ '/n(R003), + *RWI
I
o+o 0
studies, except for la, were carried out in air-saturated solutions since deaeration of solvents gave identical results. (3) GC and G C M S Analyses. The composition of the gas phase was analyzed with a Hewlett Packard capillary gaschromatograph (HP 5890) with a flame ionization detector and a HP 3394-A integrator. The liquid phase and the labeling experiments were analyzed with a HP 5890-A gas chromatograph coupled with a HP 5970 mass detector, using a HP, BP5, Scientific Glass Engineering GmbH column (95% methylpolysiloxane/5% phenylpolysiloxane; 1 = 25 m, = 0.22 mm, thickness 25 mm). (4) EPR Studies. EPR spectra were recorded on a Jeol RE2X spectrometer equipped with a T E ~ H cylindrical cavity resonator and an Advantec frequency counter. The temperature was kept at 20 "C in all experiments. Test solutions containing 1-2.0 mM la,b, 2b, and 3a,b and 5-20 mM 2,3,5,64etramethylnitrosobenzene(nitrosodurene, ND) or 10 mM 5,5-dimethyl-l-pyrroline1-oxide (DMPO) were prepared using dichloromethane and acetonitrile, respectively. CHzClz (Merck, pa; saturated with argon) was purified by passing it through a column filled with basic activated alumina (Aldrich; activity grade I). CH3CN (Sigma-Aldrich, HPLC grade, satu-
+
rated with argon) was similarly purified using neutral alumina of equal activity. ND was prepared according to the method of Smith and Tay10r.l~A 0.18 mL volume of solution were transferred under argon into a cylindrical EPR tube (Jeol no. ES-LC+2) or a flat cell (Jeol no. ES-LCM). Spectra were then accumulated by means of the ESPRIT data acquisition and manipulation package (version 3.4) connected to the spectrometer in the fast-sweep mode. For simultaneous irradiation of the sample tube, either a Katadyn mercury low-pressure lamp (TiO2-doped quartz cylinder, 8 W nominal power) or a HBO 200 W/4 Xe lamp was used. The former only emits the 254 nm Hg line, whereas the latter required the use of an appropriate filter t o prevent side reactions. In this work, a solution of 1.75 M NiS04-6HzO 0.50 M CoS04.7H20 in water was positioned between the lamp and the sample tube as close to the cavity as possible in either a 1 cm wide quartz cuvette or a 2 cm wide Schlenk tube with flattened walls. Thus, irradiation could be carried out at 331 nm or 300-340 nm, respectively. g-values were calculated relative to the two central lines of a Mn2+-doped sample of MgO located in the same cavity during subsequent irradiation experiments. (5) NMR Studies. The NMR spectra were obtained on a Bruker DPX 400 NMR spectrometer, using a 5-mm quartz tube.
+
Acknowledgment. Support of this research by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie (graduate fellowship for M.R.M.) is gratefully acknowledged. We thank the Hermann Schlosser-Foundation (graduate fellowship for F.E.K.), the DEGUSSA AG (Prof. Offermanns and Dr. Gerhartz) for a generous gift of Re2O7, and Prof. J. F. Endicott for valuable comments. Dr. I. Wurdack is thanked for providing DMPO. OM940889Q (19) (a)Smith, L. I.; Taylor, F. L. J . Chem. SOC.1935,57,2370.(b) Smith, L.I.; Taylor, F. L. J . Chem. SOC.1935,57,2460 and references cited therein.
