J . Phys. Chem. 1990, 94, 3502-3508
3502
acetone. The maximum value observed in the present experiments is approximately 2 a t the lowest temperatures. We ascribe the remaining difference to the influence of the spin-lattice relaxation (TlpH)which has been neglected in eq 21.
4
5
7
6
io3 K -T
Figure 4. Ratio of the maximum measured I3C CP NMR intensity of the methyl line LpmX and of the intensity Ispof a single-pulse experiment, both corrected by using Curie’s law, as a function of the temperature.
theory. In order to compare the efficiency of the cross-polarization method with respect to single-pulse experiments as a function of the correlation time of the molecules under study, we have plotted in Figure 4 for the methyl carbon of the adsorbed acetone the ratio and of the corresponding quantity Ispfor a single-pulse of ICpmaX experiment (also corrected by using Curie’s law) as a function of temperature. One can see a dramatic decrease of the efficiency of the cross-polarization method even to values less than 1 which means that the single-pulse signal intensity is larger than that of a cross-polarization experiment. The theoretical enhancement of the I3C N M R signal intensity due to cross polarization is given byI3
which is ca. 3 for the carbon of the methyl group of I3C-enriched
Conclusions Both from a theoretical treatment and experiments performed on a system consisting of molecules in an adjustable state intermediate between a solid and a liquid it follows that the efficiency of cross-polarization transfer depends strongly on the mobility of the molecules under study. Therefore, N M R techniques based on cross polarization are only applicable to systems where the correlation time is larger than a certain critical value. Below that value which is of the order of some 10 ~s for typical organic compounds under the experimental conditions given above, the N M R signal intensity is strongly reduced. Therefore, the results of CP experiments performed on samples containing components of different mobility ( e g , coal and other heterogeneous systems) must be taken with care. If there are molecular species with high thermal mobility it is in principle not possible to determine quantitatively their concentration from N M R signal intensities under the conditions of cross polarization. The P M S method proposed by Tekely et al. leads to an underestimation or even a neglect of an unknown amount of mobile species. In our opinion this can account for some of the unexpected and suprising results of PMS studies, e.g., that most of the protons in the detected “mobile component” should be bonded to aromatic structures whereas it is well-known that the extracts from coals of various rank contain predominantly aliphatic hydrogen.l5.l6 (15) Grint, A.; Mehani, S.; Trewhalla, M.; Crook, M. J. Fuel 1985, 64, 1355. (16) Moinelo, S. R.; Garcia, A,; Bermejo, J.; Menendez, R. J . Mol. Struct. 1986, 143, 545.
Observation of Discrete Trimethylsllylnitrene by Matrix- Isolation Spectroscopy Robert F. Ferrante Department of Chemistry, US.Naval Academy, Annapolis, Maryland 21402 (Received: August 4 , 1989; In Final Form: November 15, 1989)
The trimethylsilylnitrene radical, (CH3)&N, has been produced by the interaction of (CH,),SiN, or (CH3)3SiNC0with metastable molecular nitrogen or argon and has been trapped and observed directly in nitrogen and argon matrices. Electron spin resonance (ESR) data indicate that trimethylsilylnitrene has a triplet ground state, with D = 1.57 cm-l and AN = 12.0 G. The zero-field splitting and I4N hyperfine coupling constants are consistent with a high concentration of spin density on the nitrogen atom, as in other nonaromatic nitrenes. A number of infrared (IR) absorptions have been observed and preliminary assignments made, including the Si-N stretch at 871.4 cm-I, CH3 vibrations at 1248.0, 850, and 746.6 cm-I, and the antisymmetric Si-C, stretching mode at 736.1 cm-I. There is no strong evidence in either the ESR or IR data for pr-dr bonding between the nitrogen and silicon atoms. The metastable energy transfer fragmentation method gives a product distribution different than direct photolysis or pyrolysis of the same precursors, favoring production of triplet molecules. However, singlet nitrenes are probably produced as well. The latter molecules may be involved in reactions that produce a silylated imine, as seen in direct photolysis of the azide, or in reactions with CO to yield the trimethylsilyl isocyanate observed in other experiments.
Introduction In a search for molecules containing double bonds to silicon, a number of reports have focused on the decomposition of silyl azides, R3SiN3,in the gas phase and solution, by photolytic and pyrolytic means.!+ Much of this work was directed toward the ~~~
~~
~
generation of silaimines, of the form R,Si=NR. While the silaimines were not observed directly, their existence was implied by product analysis in many experiments utilizing chemical trapping However, there is ample precedent for double-bonded silicon intermediates in the form of the silaethenes, R2Si=CR2,’-14 and silanones, R2Si=0.I5-I9 In addition, the
~~
( I ) Reichle, W. T. Inorg. Chem. 1964, 3, 402. (2) Klein, D. W.; Connolly, J. W. J . Orgunomet. Chem. 1971, 33, 311. (3) Golino. C. M.; Bush, R. D.; Sommer, L. H. J . Am. Chem. Sor. 1974, 96,614. (4) Parker. D. R.; Sommer, L. H. J . Am. Chem. SOC.1976, 98, 618.
(5) Parker, D. R.; Sommer, L. H. J . Orgunomet. Chem. 1976, 110, CI. (6) Guimon, C.; Pfister-Guillouzo, G. Orgunomem(1ics 1987, 6, 1387. (7) Barton, T. J.; McIntosh, C. L. J . Chem. Sor., Chem. Commun. 1972, 861.
This article not subject to U S .Copyright. Published 1990 by the American Chemical Society
Observation of Discrete Trimethylsilylnitrene carbon analogues of silaimines are well-known; (CHJ2C=NCH, is the sole product in the photolysis of tert-butyl azide, (CH,),CN,, in solution.20 Matrix-isolation techniques were applied in the photolysis of trimethylsilyl azide, (CH,),SiN,, in hopes of directly observing the supposed silaimine intermediate. Instead, only a C- or Nor (Csilylated carbon imine, either (CH3)2HSi-CH=NH H3),HSi-N=CH2, was observed.21 Aside from the loss of N2, formation of this isomer requires migration and rearrangement of a methyl group, while the solution photolysis of the carbon azide analogue causes methyl migration alone. A subsequent study of the matrix photolysis of tert-butyl azideZ2yielded the same product as observed in solution, demonstrating that the difference between the photolytic behavior of (CH,),SiN, and (CH3),CN, was not merely a function of the temperature or medium. Nitrene intermediates have often been implicated in the decomposition of organic azide^.^^-^^ However, the involvement of silylnitrenes, R,SiN, in many of the experiments described above was considered of secondary importance, even in cases where the silaimine and silylnitrene could conceivably lead to the same p r o d ~ c t . ~Recent ,~ work, though, both experimental and theoretical, has renewed interest in silylnitrenes. Matrix photolysis of trimethylsilyl azide in the presence of carbon monoxide was reported to produce trimethylsilyl isocyanate, (CH,),SiNCO, presumably by reaction of trimethylsilylnitrene, (CH,),SiN, with C0.26 Ab initio calculations6 on silyl azide model systems also considered the involvement of a silylnitrene intermediate; these calculations reproduced some features of related experiments.6q21 Although trimethylsilylnitrene is expected to have a ground triplet the latter results are concerned with singlets, in observation of spin conservation rules. Extensive calculations on triplet (CH,),SiN have not been performed, and the molecule was not previously observed. This is in contrast to the isoelectronic trimethylsilylcarbene, (CH,),SiCH, which has been studied directly by e ~ p e r i m e n tand ~ ~ ,modeled ~~ the~retically.~~
(8) Sommer, L. H.; McLick, J. J . Organomet. Chem. 1975, 101, 171. (9) Chapman, 0.L.; Chang, C.-C.; Kolc, J.; Jung, M. E.; Lowe, J . A,; Barton, T. J.; Tumey, M. L. J . Am. Chem. Soc. 1976, 98, 7844. (IO) Nefedov, 0. M.; Maltsev, A. K.; Khabashesku, V. N.; Korolev, V. A. J . Organomet. Chem. 1980, 201, 123. Gusel’nikov, L. E.; Volkova, V. V.; Avakvan. V. G.: Nametkin. N . S. J . Oreanomet. Chem. 1980. 201. 137. (Ii) Mahaffy, P. G.; Gutowsky, R.; Montgomery, L. K. J . Am. Chem. Soc. 1980, 102, 2854. (12) Maier, G.; Mihm, G.; Reisenauer, H. P. AnEew. Chem., Int. Ed. Engl. 1981, 20, 597. (13) Maltsev, A. K.; Khabashesku, V. N.; Nefedov, 0. M. J . Organomet. Chem. 1982, 226, 11. (14) Arrington, C . A.; Klingensmith, K. A,; West, R.; Michl, J . J . Am. Chem. Soc. 1984, 106, 525. ( I 5 ) Golino, C. M.; Bush, R. D.; Sommer, L. H. J . Am. Chem. Soc. 1975, 97, 7371. (16) Schnockel, H. Z . Anorg. Allg. Chem. 1980, 460, 37. (17) Arrington, C. A.; West, R.; Michl, J . J . Am. Chem. Soc. 1983, 105, 6176. (18) Kudo, T.; Nagase, S. J . Phys. Chem. 1984.88, 2833. (19) Withnall, R.; Andrews, L. J . Am. Chem. SOC.1985, 107, 2567. Withnall, R.; Andrews, L. J . Phys. Chem. 1985, 89, 3261. (20) Solar, S.; Koch, E.; Leitich, J.; Margaretha, P.; Polansky, 0. E. Uonatsch. Chem. 1973, 104, 220. (21) Perutz, R. N . J . Chem. Soc., Chem. Commun. 1978, 762. (22) Dunkin, I. R.; Thomson, P. C. P. Tetrahedron Lett. 1980,21, 3813. (23) Gilchrist, T. L.; Rees, C. W. Carbenes, Nitrenes, and Arynes; Appleton-Century-Crofts: New York, 1969. (24) Isaacs, N. S.Reactive Intermediates in Organic Chemistry; Wiley: London, 1974. (25) Azides and Nitrenes; Scriven, E. F. V., Ed.; Academic: Orlando, 1984. (26) Posey, J. M.; Arrington, C. A. Abstracts ofPapers, 38th Southeast Regional Meeting of the American Chemical Society, Louisville, KY; American Chemical Society: Washington, DC, 1986; Abstract 318. (27) Wasserman, E. In Progress in Physical Organic Chemistry; Streitwieser, A., Taft, R. W., Eds.; Wiley-lnterscience: New York, 1971; Vol. 8, p 319. (28) Wentrup, C. Reactiue Molecules: The Neutral Reactive Intermediates in Organic Chemistry; Wiley: New York, 1984; Chapter 4. (29) Kreeger, R. L.; Shecther, H. Tetrahedron Lett. 1975, 25, 2061. (30) Chedekel, M. R.; Skoglund, M.; Kreeger, R. L.; Shechter, H. J . Am. Chem. SOC.1976, 98, 7846.
The Journal of Physical Chemistry, Vol. 94, No. 9, I990 3503
I
I
I
8000
i
8100
MAGNETIC FIELD [GI
Figure 1. ESR spectrum of (CH3),SiN in solid nitrogen at 12 K (u = 9.100 GHz).
Some triplet alkylnitrenes have been readily produced when the parent alkyl azide is allowed to interact with metastable particles generated in a microwave-powered discharge through nitr~gen.,~-,~ This method, in conjunction with matrix-isolation spectroscopy, has allowed direct characterization of such radicals even when they could not be produced by p h o t o l y ~ i s .This ~~~~~ work represents a similar study, aimed at isolation and direct characterization of the trimethylsilylnitrene radical. In addition to confirming the existence and examining the structure of this molecule, it was hoped that some implications for the mechanisms described above could be explored.
Experimental Section Samples of trimethylsilyl azide, (CH,),SiN,, in nitrogen (Matheson) or argon (Matheson) were prepared from the commercially available liquid (Aldrich) with no additional purification. To minimize inclusion of water and subsequent hydrolysis, liquid samples were transferred under nitrogen and degassed by several freeze (77 K)-pump-thaw cycles. The vapor over the degassed liquid was then expanded into a sample bulb, and the matrix gas was added to produce mixtures approximately 0.9% in the azide. To some samples, an equal amount or a 10-fold excess of carbon monoxide (Linde) was added to the azide vapor before filling with the matrix gas. Trimethylsilyl isocyanate, (CH,),SiNCO, was prepared by the procedure of Neville and M C G ~ through ~ , ~ the reaction of trimethylsilyl chloride, (CH3),SiC1 (Aldrich), with a suspension of lead cyanate in mixed xylenes (Aldrich) dried over molecular sieves. The fresh Pb(OCN), was precipitated from aqueous solutions of Pb(N03)2 (Aldrich) and KOCN (Fisher) and vacuum-dried before the final reaction. The liquid (CH,),SiNCO was used after a single distillation from the solvent and handled as described for the azide samples above. The trimethylsilylnitrene radical was produced by the reaction of the parent azide or isocyanate with metastable argon or molecular nitrogen generated in a microwave discharge. The matrix-isolation system and metastable energy transfer (MET) discharge source utilized in this study have been described prev i o ~ s l y .A~ ~pure nitrogen discharge was employed to clean the tube prior to sample preparation; this gas was pumped off continuously before the substrate achieved deposition temperatures. In a typical run, the 0.9% mixture of parent compound in nitrogen or argon was mixed with an equal or greater quantity of matrix gas in which a microwave-powered discharge had been established. To minimize sample disruption or photolysis from the discharge plasma, the glowing region of the discharge itself was limited to the volume of the discharge tube away from the reagent gas inlet and, in most cases, out of direct line of sight of the interaction region or deposition target. Downstream from the interaction region, the mixed gas streams were condensed onto the appropriate (31) Goddard, J . D.; Yoshioka, Y.; Schaefer, H. F. J . Am. Chem. Soc. 1980, 102, 7644.
(32) Carrick, P. G.; Engelking, P. C. J . Chem. Phys. 1984, 81, 1661. (33) Ferrante, R. F. J . Chem. Phys. 1987, 86, 25. (34) Ferrante, R. F.; Erickson, S. L.; Peek, B. M. J . Chem. Phys. 1987, 87, 2421. (35) Neville, R. G.; McGee, J . L. In Inorganic Syntheses; Holtzlaw, H. F., Ed.; McCraw-Hill: New York, 1966; Vol. 8, p 23.
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The Journal of Physical Chemistry. Vol. 94, No. 9, I990
substrate, yielding a final matrix/parent mole ratio >200. During deposition, the sample window was maintained near 20 K; the high temperature improved the optical quality of the matrices. All samples were cooled to I O K before spectra were recorded. Most samples were deposited over a period of 30 min. at a total gas deposition rate of approximately 0.1 mmol/min or less. I n some experiments, samples were photolyzed in situ with a 100-W medium-pressure mercury arc lamp (Ealing) for periods ranging from 15 min to 5 h. I n most of these cases, the unfocused lamp radiation was passed through a water filter and directed onto the sample through quartz or CaF, windows. The electron spin resonance spectrometer has been described in previous work.,, Infrared spectra were recorded at a nominal resolution of 0.5 cm-' with a Digilab FTS-65 Fourier transform infrared spectrometer, employing a mercury cadmium telluride (MCT) detector. Based on the absorption of matrix-isolated CO, frequency accuracy is estimated to be 0.2 cm-' or better; reported frequencies were reproducible to within 0.4 cm-'. Electronic absorption spectra were obtained with a Gilford Response 11 UV/vis spectrometer with a band-pass of 0.2 nm.
Results The X-band (9.100 G H z ) ESR spectrum of trimethylsilylnitrene in solid nitrogen appears in Figure 1. It consists of a single strong asymmetric line at 8036 G (measured by the criteria of Wasserman et al.36),showing poorly resolved triplet structure with a spacing of 12.0 f 0.4 G . Annealing to 35 K had no effect on the structure of the line. This signal could also be produced with comparable intensity in mixed-matrix experiments, where Ar was used in preparation of the reagent gas sample, which was then allowed to interact with metastables produced in a discharge through NZ. Samples produced in that fashion were approximately equimolar in argon and nitrogen. The high field ESR signal could be produced only weakly when argon was used as both the reagent diluent and the metastable particle source. No structure was discernible in experiments involving pure Ar or mixed-matrix samples. Hg arc lamp photolysis, either unfiltered or employing a UG-5 ultraviolet-transmitting filter, strongly bleached the signal after only 15 min. Annealing of samples to temperatures greater than 32 K also led to a very substantial reduction in signal intensity. Figure 2 shows segments of the infrared spectra of nitrogen matrices containing the parent trimethylsilyl azide and its photolysis and MET fragmentation products. If a sample of the parent azide (Figure 2a) is exposed to Hg arc irradiation through a water filter for 5 h, the new lines of a single photolysis product marked P in Figure 2b appear. These features have been identified by Perutz2' as a C- or N-silylated imine, either (CHJ2HSi-CH= calculations6 suggest that the latter N H or (CH,),HSi-N=CH,; is the more probable isomer. Brief ( < I h) unfiltered photolysis completely destroys the parent and the imine photoproduct. If the discharge source is employed, rather than photolysis, the spectrum appearing in Figure 2c is generated. New features at 1248.0, 871.4, 746.6, and 736.1 cm-' generated only with the discharge are marked by D in that figure; these lines are attributed to trimethylsilylnitrene, (CH3),SiN. As with the ESR signal, these features are destroyed if the sample is briefly exposed to the unfiltered radiation of the Hg arc lamp and also disappear on annealing the sample to temperatures near 35 K. Also concordant with the ESR result is the observation of these lines, with much lower intensities, in samples prepared from (CH,),SiN, in Ar by using an argon discharge. A comparison of traces b and c of Figure 2 shows that the strongest features of the photolysis product at 1667.4, 1257.3, 893.4, and 779.9 cm-' are also produced in the discharge source. although only weakly. The doublet at 2286.7 cm-', marked I in Figure 2c, is the strongest band of the spectrum of trimethylsilyl isocyanate, (CH,),SiNCO, and is probably produced by reaction26of the nitrene with C O impurity generated in the discharge source. The C O line itself is concealed by the (36) Wasserman. E.; Snyder. L C.; Yager, W . A . J . Chem. Phys. 1964. 41, 1763
Ferrante
'C -
t
c
k)
I
i
I
t
Figure 2. IR spectra of (CH,),SiN, samples in solid nitrogen at IO K:
(a) deposit without discharge or photolysis; (b) after 5-h Hg arc lamp photolysis in situ; (c) sample deposited with discharge source but without photolysis. Symbols: D, discharge product, (CHJ3SiN; I, isocyanate product, (CH,),SiNCO; P, photolysis product, (CHJ2HSi-N=CH2; S, siloxane impurity, ((CH,),Si),O; W, water. strong azide stretch near 21 50 cm-]. If the samples are purposely doped with CO, the isocyanate feature becomes extremely strong, and the nitrene lines are not observed. The strong lines of the parent molecule are broadened in these cases as well. The lines assigned to trimethylsilylnitrene can also be produced when trimethylsilyl isocyanate is used as the precursor and exposed to interaction with metastables generated in a nitrogen discharge. In that case, the nitrene lines are produced with an intensity only one-third to one-half that observed with the azide parent. The characteristic line of the C O product resulting from fragmentation of the isocyanate is observed as well. While that feature is relatively strong, its intensity suggests that well below 1% of the parent molecule is decomposed in the MET process. The silylated imine that is generated in photolysis and (weakly) in discharged trimethylsilyl azide also appears to be produced in these experiments with MET fragmentation of the isocyanate, but the lines are quite weak. As with the nitrene, their intensities are only one-third to one-half that observed for the discharged azide and very much weaker than those generated by simple photolysis of the azide. The silylated imine is not produced, however, when (CH,),SiNCO is photolyzed under the same conditions (irradiation with an unfocused Hg arc lamp for 5 h through a water filter) that generated strong (CHJ2HSi-N=CH2 (or (CH,),HSi-CH=NH) signals with the azide parent. Despite precautions to exclude water while handling the precursors, many samples were contaminated to various degrees by hexamethyldisiloxane, (CH3),SiOSi(CH3),, a product resulting from hydrolysis of the parent compounds. In Figure 2, the three sharp lines near 1060 cm-' (labeled S), characteristic of the Si-0-Si unit, are associated with this molecule. None of the features assigned to the silylated imine or the nitrene could be correlated with the intensity of this contaminant band. A number of other lines were observed in the infrared spectra of samples prepared with the discharge source, but these were eliminated from consideration by comparison with spectra generated from discharged nitrogen, hydrogen azide, or trimethylsilyl chloride. None
Observation of Discrete Trimethylsilylnitrene of the lines discussed above appeared in these control experiments. N o new signals in the visible or ultraviolet regions could be clearly assigned either to the silylated imine photolysis product or to the trimethylsilylnitrene radical. In addition to the broad bands of the parent,37 a single weak, sharp line at 380 nm did appear in the MET fragmentation of (CH3)3SiN3and (CH3)3SiNCO but was also produced with discharged (CH3),SiCI. It may be due to the trimethylsilyl radical, (CH3)3Siror a rearrangement product thereof.
Discussion That the ESR and infrared spectra discussed above are associated with the same chemical species is apparent by the correspondence of the conditions for their generation and by their identical photolysis and annealing behavior. The assignment of these lines to trimethylsilylnitrene is based on the characteristics of the ESR signal, the positions of the infrared lines, and the fact that these signals can be produced by the MET fragmentation of two different precursors, trimethylsilyl azide and trimethylsilyl isocyanate. Interpretations based on the ESR and the IR data, and some mechanistic implications, will be considered in more detail below. ESR Evidence. The characteristic line shape and high resonant field of the ESR signal are indicative of randomly oriented, axially symmetric triplet-state molecules with a high zero-field splitting ( D ) value.38 It is assigned to the xy2 transition of the ground 3Al state of (CH,),SiN. As is commonly the case in such systems, the weak z lines were not observed, Thus, the computed zero-field splitting value, D = 1.57 f 0.01 cm-I in a nitrogen matrix, is calculated assuming g,, = g, = g, and D > 0. The assignment of the feature to trimethylsilylnitrene is strengthened by consideration of the structure of the signal; the three lines, split by 12.0 G, are due to the hyperfine interaction of the unpaired electrons with the single I4N (I = 1) nucleus. In this simple chemical system, only a nitrene is consistent with the symmetry, spin multiplicity, and zero-field and hyperfine splitting constants indicated by this signal. A comparison of the parameters observed for trimethylsilylnitrene with those reported for related species further supports the assignment. The D value for (CH,),SiN is close to that observed for its carbon analogue tert-butylnitrene, (CH3)3CN,27 where D = 1.63 cm-I, and for other simple alkylnitrenes such as methylnitrene, CH,N,39 D = I .72 cm-', and ethylnitrene, CH3CH2N,34D = 1.67 cm-'. It is substantially larger than the zero-field splitting observed for arylnitrenes, such as phenylnitrene, C6H5N,27where D = 0.9978 cm-l, but the extensive delocalization that reduces the interaction of the unpaired spins in the aromatics cannot occur for the other molecules. Even limited delocalization might suggest that the zero-field splitting in the silicon-containing molecule should be much larger than that in the carbon alkylnitrenes, because the spin-orbit coupling constant of Si is greater than that For C. For example, the zero-field splitting for CNN is 1.15 while D for SiNN is 2.21 ~ m - ' . ~ Of ' course, some caution must be exercised in the rationalization of zero-field splittings on atomic substitution; the series of triplet molecules C 2 0 ( D = 0.74 cm-I), Sic0 (2.28 cm-I), and S i 2 0 (1.91 cm-I, all in Ne) provide a prime example.42 However, most of the unpaired spin density in trimethylsilylnitrene, as in other nitrenes,24*27134 is on the N atom, so the effect of the silicon atom on the spin-orbit contribution to D is minimal here. Apparently, the excited states are not sufficiently low-lying to make the significant contributions reported in a few organic n i t r e n e ~either. ~ ~ Ab initio (37) Thayer. J . S.; West, R. Inorg. Chem. 1964, 3, 889. (38) Weltner, W. Magnetic Atoms and Molecules; Van Nostrand Reinhold: New York, 1983; pp 156-218, 341-348. (39) Carrick, P. G.; Brazier, C. R.; Bernath. P. F.; Engelking, P. C. J . Am. Chem. SOC.1987, 109. 5100. (40) Smith, G. R.; Weltner, W. J . Chem. Phys. 1975, 62, 4592. (41) Lembke. R. R.; Ferrante, R. F.; Weltner, W. J . Am. Chem. SOC. 1977, 99, 416. (42) Van Zee. R. J.; Ferrante, R. F.; Weltner, W. Chem. Phys. Lett. 1987, 139, 426. (43) Alvarado, R.; Grivet, J.-Ph.; Mijoule, C. Chem. Phys. Leu. 1975.35, 403.
The Journal of Physical Chemistry, Vol. 94, No. 9, 1990 3505
calculation^^^ with the STO-3G basis set (energy-minimized for the Si-N bond length, but assuming C,, symmetry and standard bond lengths and angles for the rest of the molecule) confirm that virtually all of the unpaired spin density in (CH3),SiN is on the nitrogen p orbitals. Observed hyperfine coupling constants are also consistent with this interpretation; the N hyperfine splitting for (CH,),SiN is 12.0 G, which is close to the value for ethylnitrer~e,,~ 16.0 G. A crude calculation utilizing the experimental splitting and atomic data for p orbitals on nitrogen38indicates an unpaired electron spin density on the nitrene N greater than 60% in trimethylsilylnitrene. Thus, this quantity is comparable to that of an alkylnitrene and is significantly above the value calculated for a wide range of aromatic n i t r e n e ~in, ~support ~ of the view of limited delocalization. It is also interesting to compare the zero-field splitting observed for (CH,),SiN, 1.57 cm-', with that reported for the corresponding carbene,,O (CHJ3SiCH, where D = 0.613 an-'. This large disparity between nitrenes and their related carbenes is observed in all cases where comparable data exist. Examination of some tabulated values2* shows that, for aromatic radicals, the ratio Dnitrene/Dcarkne = 2.0. The data on alkyl radicals are quite limited, but DNH/DCH2 = 2.7, and for the trimethylsilylnitrene-trimethylsilylcarbene pair, Dnitrene/pcarbene = 2.6. The much larger zero-field splitting value for the nitrenes is attributed to the higher electronegativity of the N atom compared to C, resulting in greater interaction between the unpaired spins because of their smaller average separation and thus an increased value of D.28 The observed value for trimethylsilylnitrene, compared to that of trimethylsilylcarbene, is in agreement with this trend and suggests that the trimethylsilyl radicals are much more like typical alkyl species, with minimal spin delocalization, than like the aromatic systems. As noted above, calculations on (CH,),SiN indicate that most of the spin density is located in nitrogen p orbitals. There is further agreement in the report that atomic d orbital populations in H3SiCH, a suitable model for trimethylsilylcarbene, are small.31 Thus, theory and experiment form a consistent, but possibly surprising, portrait of d-orbital involvement in these radicals. p,-d, bonding has long been considered an important feature in the structure of stable silicon-containing molecules, including substituted silane^^^-^' and trimethylsilyl halogen^^^,^^ and pseudo(N3, NCO, NCS). Chedekel et aL30 used similar arguments to rationalize the near-linear structure of trimethylsilylcarbene. The evidence presented above, however, suggests little d-orbital participation. In contrast to the stable compounds, it seems reasonable that the electron-deficient centers in these radicals would have a markedly reduced propensity for electron donation to the neighboring silicon, thus limiting the extent of the pr-dT bonding interaction. A parallel is found in the decreased back-bonding for more electronegative halogens in related species." IR Euidence. The new set of bands in the infrared spectrum formed by the MET fragmentation of (CH3),SiN3 mirrors the behavior of the ESR signal in terms of conditions of generation and photolysis and annealing behavior. As with the ESR signal, (44) Frisch, M . J.; Binkley, J . S.; Schlegel, H . B.; Raghavachari, K.; Melius, C. F.; Martin, R. L.; Stewart, J. J. P.; Bobrowicz, F. W.; Rohlfing, C . M.; Kahn, L. R.; Defrees, D. J.; Seeger, R.; Whiteside, R. A,; Fox, D.J.; Fleuder, E. M.; Pople, J . A. GAUSSIAN86; Carnegie-Mellon Quantum Chemistry Publishing Unit: Pittsburgh, 1984. (45) Frost, D. C.; Herring, F. G.; Katrib, A,; McLean, R. A. N.; Drake, J. E.; Westwood, N. P. C. Can. J . Chem. 1971, 49, 4033. (46) Frost, D. C.; Herring, F. G.; Katrib, A,; McLean, R. A. N.; Drake, J. E.; Westwood, N. P. C. Chem. Phys. Lett. 1971, I O , 347. (47) Egorochkin, A. N.; Vyazankin, N . S.; Ostasheva, N. S.; Kuz'min, 0. V.: Nametkin, N . S.; Kovalev, I. F.; Voronkov, M. G. J . Organomet. Chem. 1973, 59, 1 1 7 (48) Koehler. P.; Licht. K.; Kriegsmann, H. Z . AnorR. - All& - Chem. 1977, 433, 47. (49) Paliani, G.; Sorriso, S.; Cataliotti, R.; Danieli, R. 2. Phys. Chem. ( Wiesbaden) 1977, 107, 18 1, (50) Goubeau, J.; Heubach, E.; Paulin, D.;Widmaier, I . 2. Anorg. Allg. Chem. 1959. 300, 194. ( S I ) Kimura, K.; Katada, K.; Bauer, S. H . J . Am. Chem. SOC.1966.88, 416. (52) Dakkouri, M.; Oberhammer, H. 2. Naturforsch., A 1972, 27, 1331.
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they are by themselves consistent with the assignment to the trimethylsilylnitrene radical, (CH3)3SiN. The fact that they are also produced in the fragmentation of a different precursor, (CH,),SiNCO, along with the expected CO byproduct, is a very strong corroboration of the a s ~ i g n m e n t . ~Furthermore, ~ the generation of intense isocyanate bands, and the concomitant absence of nitrene bands, in discharged (CH3),SiN3/CO/N2 mixtures provides additional support as well. Because of the limited number of bands observed, interpretation of the spectrum in terms of specific vibrations is more difficult. Still, a preliminary assignment can be made by comparison with related molecules. At the low concentrations produced in these experiments, only the most intense absorptions would be observed; as seen in Figure 2c, most of the new bands are closely associated with strong absorptions due to skeletal motions of the parent molecule. The line at 1248.0 cm-' is the easiest to assign and probably corresponds to the symmetric deformation of the CH, group. This vibration appears at 1262 cm-I in the gas phase for the trimethylsilyl pseudohalogens (CH3),SiN3, (CH,),SiNCO, and (CH,),SiNCS; its intensity is very strong in all three molecules.s4 In the nitrogen matrix, the azide and isocyanate exhibit a strong, broad absorbance centered at about 1260 cm-I. As expected for a methyl vibration, the frequency of this line is only slightly dependent on the group attached to the silicon. For trimethylsilyl chloride,50 this vibration is assigned to a line at 1256 cm-l, and it drops in frequency to 1247 cm-' if the CI is replaced with H in trimethyl~ilane.~~ In terms of mass (but not bonding), the closest model to (CH,),SiN may be (CH,),SiOH. The reported value of the CH3 symmetric deformation for that molecules6 is 1445 cm-l. However, if the assignments of the symmetric and antisymmetric methyl deformations in trimethylsilanolS6are reversed, the new assignment of 1257 cm-' is much more in keeping with the whole family of compounds. It is most likely that the line reported by Perutz2' for the silylated imine photoproduct, and observed i n these experiments as well at 1257.3 cm-I, is also the C H 3 symmetric deformation of that species, analogous to the 1248-cm-' line of trimethylsilylnitrene. The trimethylsilyl pseudohalogens also exhibit strong absorptions associated with methyl rocking modes. The strongest of these appear at 854 cm-l for the azide,S4853 cm-' for the i s o c ~ a n a t e , ~ ~ and 849 cm-' for the isothiocyanateS4in the gas phase. In nitrogen matrices, (CH3&3iN, and (CH,),SiNCO both show very intense, broad (about 2 4 - ~ m -full ~ width at half-maximum) bands centered at 850 cm-I. While the nearest line observed for (CH,),SiN is at 871.4 cm-I, the best assignment of this band is probably not to the CH, rocking vibration but to the Si-N stretch, as discussed below. As with the methyl deformation treated earlier, the rocking motion is largely insensitive to the group attached to the silicon. For (CH3)3SiCI,S0this line appears at 846 an-'; for (CH3)3SiOH,56 the absorption is at 850 cm-I. Even for the silane analogue (CH,)3SiH,5Sit only moves to 831 cm-I. In all likelihood, the strongest of the methyl rocking modes for trimethylsilylnitrene also appears near 850 cm-' and is obscured by the very broad and intense parent absorptions. The assignment of the 87 1.4-cm-' vibration in trimethylsilylnitrene to the Si-N stretching mode is also supported by consideration of analogous compounds. The closest models for (CH,),SiN in the literature are trimethylsilylamine, (CH3),SiNH2, and triethyisilylamine, (CH3CH2),SiNH2, where the pertinent vibrations appear at 882 and 828 cm-l, respe~tively.~'Absorption intensities associated with the Si-N stretching mode in these compounds are strong, as are the intensities of the Si-X stretching
Ferrante vibrations for the trimethylsilyl halides and trimethylsilanol used as models here. Assignment of the 871-cm-' line of (CH,),SiN to the Si-N stretch implies that such is also the case for that molecule, since it is likely that only the strongest absorptions of trimethylsilylnitrene are observed here, given the low concentrations. The assignment is thus consistent with the models but in contrast to the Si-N absorptions reported for the parent compounds, which are only weak to moderate in intensity. As expected, these occur at much lower frequencies, specifically 538 and 537 cm-' for trimethylsilyl azide and isocyanate, respecti~ely.~~ A simple diatomic harmonic oscillator treatment of this vibration in the parent compounds can be used to remove the mass effect of the relatively rigid pseudohalide (-N=X=Y) unit. Thus, substituting N for N, or NCO, a frequency of 810 cm-' is predicted for (CH,)3SiN. The approximations inherent in that calculation are exacerbated because of differences in bonding between an electron-deficient radical and these stable compounds; the accuracy of such a transference of force constants may be suspect. For example, bonds adjacent to a radical center have been s h o ~ n ~to* be * ~shorter ~ than normal bonds in a closed shell system (although some caution must be exercised in that assessment@). This might tend to increase the expected frequency. More importantly, the M-X stretch has been shown to be quite sensitive to electronic effects in a family of stable compounds of the type (CH3)3MX,while other vibrations were not so affected.49 However, this sensitivity can provide some interesting clues to the nature of the bonding in the nitrene. In consideration of the Si-N stretching vibration, the trialkylsilylamines mentioned above, (CH3)3SiNH2and (CH3CH2)3SiNH2,can be included in a group with other ammonia derivatives where only one hydrogen has been replaced with a trialkylsilyl group, R3Si. Other members of this class, and the Si-N stretching frequencies associated with those molecules, are 851 cm-I. (CH3),SiNHCH3,61866 cm-I, and (CH3)3SiN(CH3)2,61 For the mono(trimethylsilyl)amines,then, the Si-N stretch ranges from about 850 to 880 cm-I. Higher substitution, however, leads to higher frequencies: in the bis(trimethylsily1)amines these lines appear at 924 cm-' for ((CH3)3Si)2NH61and 906 cm-' for ((CH3)3Si)2NCH3.6'The Si-N stretch in tris(trimethylsilyl)amine, ((CH3),Si)3N, is in the same range, at 916 cm-1.61 In the latter compound, the high stretching frequency, and other properties, are ascribed to partial multiple-bond character61 stemming from p,-d, bonding, in analogy to the well-known trisilylamine molecule (H3Si)3N.62 Similar parallels can be made between the bis(trimethylsilyl)amines and (H3Si)2NH.62 While the complete planarity of the bis- and tris(trimethylsily1)amines could not be proven spectroscopically,61 there was strong evidence for this increased bond strength. The mono(trimethylsily1)amines have significantly lower Si-N stretching frequencies, indicating reduced bond strength due to decreased multiple-bond character. The 87 I-cm-' line of trimethylsilylnitrene clearly falls in the range for these model compounds and well below that resulting from multiple-bonding interactions. No bands in the 880-1000-cm-' range could consistently be assigned to (CH,),SiN. The correspondence of the observed vibration with expectations based on these model compounds further supports the contention from ESR data that (CH,),SiN contains a normal Si-N single bond; there is little p,-d, bonding in this molecule. The remaining vibrations observed for (CH3),SiN, at 746.6 and 736.1 cm-', might be assigned, by proximity to parent vibrations, either as additional C H 3 rocking motions or as Si-C3 stretching motions, or to both. As discussed above in regard to the 87 1-cm-' line, the methyl rocking vibrations are remarkably consistent for the trimethylsilyl halogens and pseudohalogens. All of these
(53) Dunkin, I . R.; Thomson, P. C. P. J . Chem. Soc., Chem. Commun. 1982, 1192.
(54) Durig, J. R.; Sullivan, J . F.; Cox, A . W.; Streusand, B. J. Spectrochim.Acta, Part A 1978, 34, 719. ! 5 5 ) Marchand, A.; Valade, J.; Forel, M.-T.; Josien, M.-L.; Calas, R. J . C h m Phys. 1962, 59, 1142. (56) Rouviere, J.; Tabacik, V.; Fleury, G.Spectrochim. Acta, Part A 1973, 29, 229. (57) Wiberg. N.: Uhlenbrock, W. Chem. Ber. 1971, 104. 2643.
( 5 8 ) Pacansky, J.; Chang, J . S. J . Chem. Phys. 1981, 74, 5539. ( 5 9 ) Pacansky, J.; Dupuis, M. J . Am. Chem. SOC.1982, 104, 415.
(60) Xie, Y.: Scuseria, G.E.; Yates. B. F.; Yamaguchi, Y.; Schaefer, H. F . J . Am. Chem.Soc. 1989, 111. 5181. (61) Goubeau, J.; Jimenez-Barbera, J. Z . Anorg. Allg. Chem. 1960, 303. 217. (62) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry; Wiley: New York, 1972; Chapter 1 1 , and references therein.
Observation of Discrete Trimethylsilylnitrene species exhibit an additional absorption of moderate to strong intensity near 760 cm-’ assigned to this vibration. It again varies from this value by less than IO cm-I for the molecules (CH3)3SiN3,S4(CH3)3SiNC0,s4(CH3)3SiNCS,S4(CH3)3SiC1,50and (CH3)3SiH.55 For the group of trimethylsilyl-substituted amines discussed above, this vibration appears on the low side of this value, in the range of 746-758 cm-’.61 Such constancy among all of these compounds suggests again that this vibration for (CH&SiN will be in the same region, although possibly obscured by bands of the precursor. In the set of lines between 740 and 765 cm-’ observed for the parent molecule, all of the features are quite sharp and well resolved. The discharged samples appear identical in this region, except for the addition of a new line at 746.6 cm-I. This line is therefore assigned as a methyl rocking mode of the nitrene. The moderate intensity is also consistent with this assignment. The assignment of the strong 736.1-cm-’ line in the trimethylsilylnitrene spectrum is more difficult. Vibrations that might be expected in this region include additional methyl rocking modes or the antisymmetric Si-C3 stretch. The line is not an artifact of the matrix, since it was unchanged by mild annealing in nitrogen matrices and appeared in mixed nitrogen/argon samples as well. Since the frequency seems too low for a rocking mode, by comparison with the models discussed earlier, the 736-cm-I line is best assigned as the antisymmetric Si+ stretch. (It is well above values observed for the symmetric Si-C stretch in the models.) Bands attributed to this mode cover a large frequency range and exhibit highly variable intensities. Examples include the trimethylsilyl pseudohalogenss4 (7 10-690 cm-l, strong-very weak), trimethylsilyl and substituted trimethylsilyl halogen^^^^^^ (750-650 cm-l, mostly weak), trimethylorganosilanes,55 (CH3),SiR (near 725 cm-I, strong-weak), trimethylalkoxysilane~,~~ (CH3)3SiOR (725-707 an-’, mostly weak), and trialkylsil~xanes,~~ R3SiOSiR3 (727-7 12 cm-l, strong-weak). While toward the high end of the range, the assignment of the 736. I-cm-’ line of (CH3),SiN to the Si-C3 antisymmetric stretching vibration best accommodates the alternatives among a wide variety of chemical models. Reactions. Having established the existence of the trimethylsilylnitrene radical, and some features of its structure, it is of interest to consider some mechanistic implications. First, it is obvious that the metastable energy transfer fragmentation employed here leads to a different product mix, in either composition dr quantity, or both, compared to photolysis or pyrolysis. This was also the case in the formation of methyl- or ethylnitrene from their respective azide^.^^,^^ In these systems, theory and experiment indicate that the imine isomer is the more stable final product (other than N,) in the degradation of the azide. Production of the imines could involve a singlet nitrene intermedia t e ? ~however, ~~ in the silicon system, that may not be the most energetically favorable route.6 While the triplet nitrene is clearly the ground state, its participation in photolytic or pyrolytic conversions is limited because of spin conservation consideration^.^,^^ The MET fragmentation apparently involves no such constraint. While the discharge is a very complicated system, generating atoms, molecules, and ions of varied concentrations and lifetimes, there is much to suggest that the A3Z,+ state of N2 plays an important role in the reactivity of the system.@ Collisional energy transfer from this excited molecule to a singlet precursor could then lead to a triplet product while remaining in the same spin manifold of the collision system throughout the encounter. The net result is that the MET process favors formation of the triplet nitrene, either in its ground state or in a state that rapidly decays to the ground state. This scheme is currently being employed to produce other nitrenes of theoretical and experimental interest. The redistribution of energy deposited into the parent azide (or isocyanate) is also a matter of interest. Certainly, the efficacy (63) Demuynck, J.; Fox, D. J.; Yamaguchi, Y.; Schaefer, H . F. J . Am. Chem. SOC.1980, 102, 6204. (64) Wright, A. N.; Winkler, C. A. Actiue Nitrogen; Academic: New York, 1969; Chapter 3.
The Journal of Physical Chemistry, Vol. 94, NO. 9, 1990 3507
of the method lies in the fact that sufficient energy migrates to the N-N (or N-C) bond to cause scission, with formation and loss of molecular nitrogen (or CO) a significant driving force. While spectroscopically useful concentrations of nitrenes were produced in this work, and related MET fragmentation^,^^,^^ the overall product yield is quite low. Compared to photolysis, where near-quantitative c o n ~ e r s i o n ~ofl , the ~ ~ azide to imines may be observed in matrices, the MET process is not very efficient. This probably results from the limited number of energy-transfer collisions that can occur between the interaction region and the deposition target. Other products, besides the nitrene, are produced as well. As mentioned above, IR and UV bands of another transient species were observed in these experiments. Production of these same lines with three different (CHJ3SiX parent molecules (X = N,, NCO, CI) suggests that fragmentation to the trimethylsilyl radical, (CH3),Si, may occur. A multiplet pattern reminiscent of the solution ESR spectrum of this radicalM was observed near g,, but positive identification was complicated by poor resolution and the added features of the powder pattern. As in the cases of methyland e t h ~ l n i t r e n e , ~there ~ ” ~ was no direct ESR evidence for the isolation of N atoms in these samples. These experiments also suggest that the singlet nitrene is produced and undergoes additional reactions. The absence of nitrene bands in samples doped with CO, and the production of the isocyanate by reaction with C O contaminant, indicate that the nitrene is scavenged by carbon monoxide. This lends credence to the mechanism proposed for the generation of trimethylsilyl isocyanate in photolyzed (CH3)3SiN3/CO/Armixtures.26 That photochemistry may depend on singlet trimethylsilylnitrene, for spin conservation reasons; the behavior in this system appears similar in some respects to matrix reactions involving pentafluorophenylnitrene, C6F5N,S3and CO. However, radiation was excluded for most depositions in this work, so there must be some thermal reaction as well. This implies that (CH3)$iN is more reactive than C6F5N,a common observation in the comparison of alkyl- and arylnitrenes. Since the thermal reaction between the nitrene and CO to produce the isocyanate is also spin-forbidden for the ground triplet state, that process, too, is presumed to occur through an excited singlet state of the nitrene. The corresponding reaction of trimethylsilylcarbene with CO has not been reported, but high-temperature reactions observed for that molecule were presumed to occur via the singlet state as well.3o It is possible that such reactions can proceed through triplet states of the product, with subsequent conversion to the singlet ground state. Indeed, an analogous reaction between the triplet carbene cyclopentadienylidene, C5H4,and C O proceeds readily toward the ketene product even at 25 K.67 However, the annealing behavior of IR bands in the trimethylsilylnitrene system examined here showed no strong evidence for low-temperature thermal conversion of the triplet nitrene into the isocyanate. In strongly CO-doped samples, the nitrene was absent, probably being consumed in the gas phase before deposition; in matrices contaminated with a trace of CO, such conversions were not detected. This situation also parallels that observed for C6FSN,53except that the less reactive arylnitrene was isolated in the presence of high C O concentrations. The singlet nitrene may also be involved as an intermediate in the generation of the silylated imine photolysis product even in ((CH3)2HSi-CH=NH or (CH3),HSi-N=CH2), discharge runs where radiation from the plasma was excluded. Since conversions between the nitrene singlet and triplet states are probably not facile, and triplet rearrangements are uncommon, it is likely that the singlet (CH,),SiN is produced in the discharge directly, probably by fragmentation from an excited singlet state of the azide.68 This could occur in the manner suggested above, but involving a different metastable particle in the energy transfer. (65) Stolkin, I.; Ha, T.-K.; Gunthard, H. H. Chem. Phys. 1977, 21,327. (66) Bennett, S. W.; Eaborn, C.; Hudson, A.; Hussain, H. A.; Jackson, R. A. J . Organomet. Chem. 1969, 16, P36. (67) Baird, M. S.;Dunkin, I. R.; Hacker, N.; Poliakoff, M.; Turner, J. J . J . Am. Chem. SOC.1981, 103, 5190. (68) Lewis, F. D.;Saunders, W. H. J . Am. Chem. SOC.1968, 90, 7031.
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Some singlet metastables do exist in these discharge^.^^ Without the ability to experimentally "filter" discharge metastables, full elucidation of the mechanisms of production and conversion of singlet or triplet trimethylsilylnitrene in these experiments will probably await a thorough theoretical treatment.
Conclusions The trimethylsilylnitrene radical, (CHJ3SiN, has been produced by the interaction of (CH3)$iN3 or (CH,),SiNCO with metastable molecular nitrogen or argon and has been trapped and observed directly in nitrogen and argon matrices. Electron spin resonance data indicate that trimethylsilylnitrene has a triplet ground state, and the zero-field splitting and I4N hyperfine are consistent with a high concentration of spin density on the nitrogen
atom, as in other nonaromatic nitrenes. A number of infrared absorptions have been observed and preliminary assignments made. There is no strong evidence in either the ESR or IR data for pT-dT bonding between the nitrogen and silicon atoms. The metastable energy transfer fragmentation method gives a different product distribution than direct photolysis or pyrolysis of the same precursors, favoring production of triplet molecules. However, singlet nitrenes are probably produced as well. The latter molecules may be involved in reactions that produce a silylated imine, as seen in direct photolysis of the azide, or in reactions with CO to yield the trimethylsilyl isocyanate observed in other experiments. Acknowledgment. The support of the Naval Academy Research Council is gratefully acknowledged.
Picosecond Relaxation of Strongly Coupled Porphyrin Dimers Oman Bilsel, Juan Rodriguez, and Dewey Holten* Department of Chemistry, Washington University. S t . Louis, Missouri 63130 (Received: August 21, 1989; In Final Form: Noaember 21, 1989)
Subpicosecond time-resolved absorption studies have been carried out on several lanthanide porphyrin complexes consisting of a Ce( IV) ion sandwiched between two strongly coupled cofacial porphyrin macrocycles. Following excitation, two relaxation processes having time constants of 1.5 and 10 ps are observed. The 1.5-ps process is attributed to excited-state deactivation to the electronic ground state via a low-lying ring-to-metal charge-transfer state. Relaxation of the resultant vibrationally excited ground electronic state can account for the complex spectral dynamics observed at longer times.
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Introduction Understanding the photophysical behavior of porphyrins and other chromophores within van der Waals distance is not only of much general interest but also may provide insights into how the electronic properties of the dimeric primary electron donor in bacterial photosynthetic reaction centers influence the initial stages of the charge separation The lanthanide porphyrin dimers provide an ideal series of molecules for studying the electronic structure and dynamics of strongly interacting porphyrins. These complexes, consisting of a lanthanide ion sandwiched between two porphyrin macrocycles, are distinguished from most previously studied dimers by their rigid structure, -3-A inter-ring separation, unusual electronic absorption spectrum, and, for the oxidized species, a prominent near-infrared absorption Recently, time-resolved absorption experiments on the sandwich dimers CelV(OEP), and CerV(TTP),using 30-ps flashes showed that the photoexcited complexes largely decay in 110 PS.~.' It was proposed that this extremely fast nonradiative decay involved low-energy exciton and/or charge-transfer states, although the ( I ) Kirmaier, C.; Holten, D. Photosynth. Res. 1987, 103, 225-260. (2) Boxer, S.G.Biochim. Biophys. Acta 1983, 726, 265-292. (3)(a) Buchler, J. W.: Kapellman, H.-G.; Knoff, M.; Lay, K.-L.; Pfeifer, S . Z . Naturforsch. B: Anorg. Chem., Org. Chem. 1983,388, 1339-1345. (b) Buchler. J. W.; Decian, A.; Fischer, M.; Kihn-Botulinski, M.; Paulus, H.; Weiss, R. J . Am. Chem. SOC.1986, 108, 3652-3659. (c) Buchler, J. W.; EIJsser, K.: Kihn-Botulinski, M.; Scharbert, B. Angew. Chem., Int. Ed. Engl. 1986, 25, 286-287. (d) Buchler, J. W.; Elsasser, K.; Kihn-Botulinski, M.; Scharbert, B.; Transil, S. In Porphyrins: Excited States and Dynamics; Gouterman, M.. Rentzepis. P. M., Straub, K. D., Eds.; ACS Symposium Series 321;American Chemical Society: Washington, DC, 1986;pp 94-104. ( e ) Buchler, J. W.; Scharbert, B. J . Am. Chem. SOC.1988, 110, 4272-4276. (4)(a) Buchler, J. W.; DeCian, A.; Fischer, J.; Loffler, J.; Weiss, R. Chem. Ber. 1989, 122, 2219-2228. (b) Loffler, J. Doctoral Thesis, Technische Hochschule Darmstadt, 1989. (5) (a) Donohoe, R. J.; Duchowski. J. K.; Bocian, D.F. J . Am. Chem. Soc. 1988, 110, 6119-6124. (b) Duchowski, J. K.; Bocian, D. F. J . Am. Chem. Soc., submitted for puulication. ( 6 ) Yan, X.: Holten, D. J . Phys. Chem. 1988, 92,409-414. (7)Ce(OEP), is cerium(1V) bis( 1,2,4,5,7,8,IO,l I-octaethylporphyrin), Ce(TTP), is cerium(1V) bis(5,10.15,20-tetratolylporphyrin),and TPP is 5. IO, 15,20-tetraphenylporphyrin.
0022-3654/90/2094-3508$02.50/0
limited time resolution of the experiments did not permit identification of the key transient states or their kinetics. In this report, we present the results of steady-state and subpicosecond timeresolved absorption experiments on CelV(OEP),, Ce'V(TTP)2,and a newly synthesized asymmetric dimer, CeIV(OEP)(TTP). These studies provide fundamental new information on the electronic properties and photophysical behavior of strongly coupled porphyrin dimers.
Experimental Section Ce"(OEP), and CeIV(TTP), were prepared according to the procedure of Buchler and c o - ~ o r k e r s . A ~ ~modification was developed to yield the mixed dimer, CetV(OEP)(TTP),as follows. A 186.4-mg sample of H2TTP (Porphyrin Products), 148.6 mg of H,OEP (Aldrich), and 1 g of Ce111(acac),.3H20 (Strem) were refluxed in 50 mL of trichlorobenzene for 20 h. The trichlorobenzene was removed by rotary evaporation, and the crude product mixture was chromatographed on predried (160 "C overnight) activity 1 basic alumina (2.5 cm X 25 cm column) with CH2CI2 as the eluent. A small yellow/brown band of Ce(TTP), was closely followed by a larger dark red band of Ce(OEP)(TTP). The third and largest of the dimer bands was the brown/red Ce(OEP),. The Ce(OEP)(TTP) fraction was rechromatographed in the same fashion. Smaller amounts of highly pure material also were obtained from the crude reaction product mixture by chromatography on predried (1 60 OC for 30 min) alumina G TLC plates (20 X 20 cm2: Analtech, spotted in CHzC12 and eluted with toluene). The room-temperature ' H N M R data for Ce(OEP),, Ce(TTP)*, and Ce(OEP)(TTP), recorded on a Varian XL-300 or VXR-500 spectrometer, are summarized in Table I. The NMR spectra of the two symmetric dimers agree with the literature data for these c o m p ~ u n d s . ~The ~ - ~peak assignments for the new compound, Ce(OEP)(TTP), are based on integrated intensity ratios, proton decoupling experiments, and chemical shift comparisons with the symmetric complexes. During the course of our studies, we learned that Buchler et al. have synthesized the related molecule Ce(OEP)(TPP).4 Since our N M R data on Ce(OEP)(TTP) are in accord with the N M R results of Buchler et
0 1990 American Chemical Society
