Organometallics 1995, 14, 780-788
780
Photochemistry of [Ru(I)(iPr)(CO)2(iPr=DAB)] (iPr-DAB = NJV-Diisopropyl-1,4=diaza1,3=butadiene):Homolysis of the Metal-Alkyl Bond from the q,(Ru-iPr)+ State. Crystal Structure of the Photoproduct [Ru(I)~(CO)~(~P~-DAB)I Heleen A. Nieuwenhuis,? Maartje C. E. van de Ven,? Derk J. Stufkens,*i? Ad Oskam,? and Kees Goubitz$ Anorganisch Chemisch Laboratorium, Universiteit van Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands, and Laboratorium voor Kristallografie, Universiteit van Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands Received July 11, 1994@ The complex [Ru(I)(iPr)(CO)2(iPr-DAB)I undergoes a very efficient photodecomposition (@ = 1.2 at rt), whereas the corresponding complexes [Ru(I)(R)(CO)z(iPr-DAB)I(R = Me, Et)
are photostable. ESR, using nitrosodurene and tBuNO as radical scavengers and timeresolved absorption spectroscopy showed that the primary photoprocess is a homolytic splitting of the ruthenium-isopropyl bond. This reaction most probably proceeds from the 3ot,.7r* state of the complex, in which ob represents the (Ru-iPr) bonding orbital and n* the lowest unoccupied orbital of the iPr-DAB ligand. By using IR, W-vis and lH-NMR was identified as the final product. spectroscopy the bis-iodide complex [Ru(I)z(CO)~(~F'~-DAB)] ~ I ~=R551.1) U, was The single crystal X-ray structure of this photoproduct ( C ~ O H I , ~ N ~ O M, determined. The crystal is tetragonal, space group 141/a,with unit cell dimensions a = 11.552 (l),c = 24.489 (3)A, 2 = 8. The structure refinement converged to R = 0.044 for 978 observed reflections. The very high quantum yields indicate that this photoproduct is formed by an electron transfer chain reaction of the radical [Ru(I)(CO)diPr-DAB)Ywith the parent complex. The temDerature has a large influence on this reaction since its quantum yield decreases by a factor '3 when the temperature is lowered from 293 to 273 K.Introduction Low-valent transition metal a-diimine complexes such as [M(CO)r(a-diimine)l(M = Cr, Mo, W),1-3 [Re(L)(co)3or (a-diimine)]" ( n = 0, l+; L = halide, N-d~nor),~-ll [LnM-M(C0)3(a-diimine)l (L,M = (C0)5Mn, (C0)5Re, (CO)&o, Ph3Sn etc.; M = Mn, Re),lJ2-l7are characterized by strongly allowed metal to a-diimine charge ~
* To whom correspondence should be addressed.
t Anorganisch Chemisch Laboratorium.
* Laboratorium voor Kristallografie. Abstract published in Advance ACS Abstracts, December 15,1994. @
(1) Stufkens, D. J. Coord. Chem. Rev. lgW),104,39. (2)Balk, R.W.; Stufkens, D. J.;Oskam, A. h o g . Chim. Acta 1978, 28, 133. (3)Balk, R. W.; Snoeck, T.; Stufkens, D. J.;Oskam, A. Imrg. Chem. 1980,19,3015. (4)Stufkens, D. J. Commun. Imrg. Chem. 1992,13,359. (5)Wrighton, M.; Morse, D. L. J . Am. Chem. SOC.1974,96,998. (6)Geofioy, G.L.; Wrighton, M. In Organometallic Photochemistry; Academic Press: New York, 1979. (7)Juris, A.; Campagna, S.; Bidd, I.; Lehn, J-M.; Ziessel, R. Znorg. Chem. 1988,27,4007. (8)Kaim, W.; Kramer, H. E. A.; Vogler, C.; Rieker, J. J. Orgammet. Chem. 1989,367,107. (9)Kalyanasundaram, K. J. Chem. SOC.Faraday Trans. 2, 1986, 82,2401. (10)Sullivan, P.J . Phys. Chem. 1989,93,24. (11)Worl, L. A.; Duesing, R.; Chen, P.; Della Ciana, L.; Meyer, T. J. J . Chem. SOC.Dalton Trans. 1991,849. (12)Stufkens, D. J. Steric and Electronic effeds on the Photochemical Reactions of Metal-Metal Bonded Carbonyls. In Stereochemistry of Organometallic and Inorganic Compounds Vol 3; Bernal, I., Ed.; Elsevier: Amsterdam, 1989;p 226. (13)Morse, D. L.;Wrighton, M. S. J . Am. Chem. Soc. 1976,98,3931. (14)Kokkes, M. W.; Stufkens, D. J. Oskam, A. Inorg. Chem. 1986, 24,2934. (15)Kokkes, M. W.; de Lange, W. G. L.; Stufkens, D. J.; Oskam, A. J . Organomet. Chem. 1986,294,59.
transfer (MLCT) transitions in the visible region. In particular the Re-complexes have been studied in detail, since the properties of their MLCT states appeared to be strongly affected by variation of L. In order to achieve an even greater diversity with respect to excited state properties, we have started an investigation of a related series of complexes [Ru(X)(R)(CO)n(a-diimine)I(X = halide, R = alkyl). A detailed study of the absorption and emission spectra of these complexes has already shown that X and R strongly influence the characters of the charge transfer transitionsls and the emission properties of the lowest-excited states.lg Thus, the complexes show at least two absorption bands in the visible region, their relative intensities depending on X and R. These bands have been assigned to charge transfer transitions to the a-diimine ligand from two sets of orbitals, which are metal-d,-halide-p, bonding and antibonding, respectively. Upon variation of X from C1 to I, the X-p, orbitals increase in energy, resulting in a change of character of the lowest-energy transitions from MLCT into XLCT. This effect was evident from the resonance Raman spectra and was also reflected in drastic changes in emission lifetimes and (16)Kokkes, M. W.; Stufkens, D. J.; Oskam, A. Znorg. Chem. 1985, 24.4411. (17)van der Graaf, T.; Stufkens, D. J.; Oskam, A,; Goubitz, K. Inorg. Chem. 1991,30,299. (18)Nieuwenhuis, H.A.; Stufkens, D. J.; Oskam, A. Inorg. Chem. 1994,33,3212. (19)Nieuwenhuis, H.A.;Stufkens, D. J.; Week, A., Jr. Imrg. Chem., submitted for publication. ~~
?
~~~~
0276-7333/95/2314-0780$09.Q0f0 0 1995 American Chemical Society
Photochemistry of [Ru(I)(iPr)(CO)z(iPr-DAB)]
ocI Nn N=
ipT-N-‘N-iPr iPr-DAB
Figure 1. Structure of [Ru(I)(iPr)(CO)2(iPr-DAB)](l) and of t h e iPr-DAB ligand. quantum yields. Variation of R from Me to Et and iPr increased the metal character of the highest-filled orbitals and therefore also of the lowest-energy charge transfer transitions. Variation of R did not only affect the character of the electronic transitions, it also influenced the photostability of the complexes. Contrary to the methyl- and ethyl-complexes,the isopropyl-complex [Ru(I)(iPr)(CO)z(iPr-DAB)](iPr-DAB = N,”-diisopropyl-1,4-diaza-1,3butadiene) appeared to be photolabile. In this respect, it resembles the complexes [(CO)5Mn-Ru(Me)(CO)z(adiimine)],which undergo homolytic splitting of the MnRu bond upon irradiation into their MLCT absorption band.20 Similar light-induced homolysis reactions have been observed for series of other metal-metal bonded complexes [4M-M(C0)3(a-de)1,l2-l8for the metalalkyl complexes [Zn(R)2(R-DAB)Iz1and [Re(R)(CO)s(adiimine)1,22,23 and recently also for the metal-halide complexes mer-[Mn(X)(C0)3(a-diimine)1.24 In order to clarify the photolability of [Ru(I)(iPr)(CO)z(iPr-DAB)] (1) and the relationship with the abovementioned complexes, we have studied in more detail the photochemical properties of this complex. The structure of 1 is shown in Figure 1.
Experimental Section Materials. Solvents for synthetic purposes were of reagent grade and dried on sodium wire (THF, n-hexane). For spectroscopic measurements solvents of analytical grade (THF, MeCN, 2-MeTHF, MeOH, CHzC12) or UVASOL quality (toluene) were used, dried on sodium wire, except for CH2C12, MeOH and MeCN, which were dried using CaClZ, MgSOa, and PzO5, respectively. All solvents were freshly distilled under Nz atmosphere prior to use. 2,3,5,6-Tetramethylnitrosobenzene (nitrosodurene) and tBuNO were commercially obtained and used as received. All preparations were performed under an atmosphere of purified nitrogen, using Schlenk techniques. was The photosensitive complex [Ru(I)(iPr)(CO)z(iPr-DAB)] carefully handled under exclusion of light. Apparatus and Photochemistry. Infrared spectra were recorded on a BioRad FTS-7 FTIR spectrometer. Electronic absorption spectra were recorded on a Perkin-Elmer Lambda 5 UV-vis spectrophotometer, equipped with a 3600 data station or a Varian Cary 4E spectrophotometer. Low-temperature UV-vis and IR measurements were carried out using an Oxford (20)Nieuwenhuis, H.A,; van Loon, A.; Moraal, M. A.; Stufkens, D. J; Oskam, A.; Goubitz, K. J . Orgunomet. Chem., in press. (21) Kaupp, R; Stoll, H.; Preuss, H.; Kaim, W.; Stahl,T.;van Koten, G.; Wissing, E.; Smeets, W. J.; Spek, A. L. J. Am. Chem. SOC.1991, 113, 5606.
(22)Rossenaar, B. D.; Kleverlaan, C. J.;Stufkens, D. J.; Oskam, A. J . Chem. SOC.Chem. Commun. 1994,63. (23)Lucia, L. A,; Burton, R. D.; Schanze, K. S. Inorg. Chim. Acta 1993,208,103. (24)Stor, G. J.;Morrison, S. L.; Stufkens, D. J. Oskam, A. Orgunometullics 1994,13, 2641.
Organometallics, Vol. 14, No. 2, 1995 781 Instruments DN 1704/54 liquid-nitrogen cryostat. ESR measurements were performed on a Varian E6 ESR spectrometer equipped with a temperature-control accessory. Coupling constants were obtained by computer simulation. Resonance Raman measurements were performed on a Dilor XY spectrometer, using a SP 2016 argon ion laser as excitation source. Because of the photolability of the complex, the sample solution (concentration of ca. 0.01 M complex in CHZC12) was pumped through a home built, air tight flow-cell,in which the sample was kept under nitrogen. To study the photochemical reactions of the complex, sample solutions were irradiated by one of the lines of a SP 2025 argon ion laser or a Philips HPK 125 W high pressure mercury lamp provided with the appropriate interference filter. Quantum yields of the disappearance of the parent complex were determined by measuring the decay of its visible absorption band on a Varian Cary 4E spectrophotometer following automatized procedures. The formula used for the calculation of the quantum yields included a correction for the increasing absorption of the p h o t o p r o d u ~ t .During ~~ the measurements the sample solutions were kept in thermostated cuvettes within the UV-vis apparatus. The sample was irradiated while stirred by one of the laser lines of a SP 2025 argon ion laser, via an optical fibre and a computer controlled mechanical shutter. Light intensities were measured with a power-meter, which was calibrated with an Aberchrome 540 solution according t o literature methods.26 In situ ‘H-NMR spectra of photolyzed solutions were recorded on a Bruker AMX 300 spectrometer, using a special Bruker CIDNP 300 MHz lH Probe, equipped with a glass fiber (4 = 8 mm). Via this fiber the solutions were irradiated with an Oriel AG 150 W high pressure Xe lamp provided with a water cooling and a 530 nm cutoff filter. For these experiments typical sample concentrations of W3-10-* M were used in deuterated solvents. For the nanosecond flash photolysis studies the sample was excited by 10 ns pulses of the 532 nm line of a Nd:YAG-laser (Spectra Physics GCR-3). A 450 W high pressure Xe lamp pulsed with a Muller Elektronik MSPO5 pulser, was used as probe light. After passing the sample the probe light was collected into a fiber and transferred t o a spectrograph (EG&G Model 1234) equipped with a 150 g/mm grating and a 250 pm slit resulting in a resolution of 6 nm. This spectrograph was coupled to a gated, intensified diode array detector (EG&G Model 1421) which was part of an EG&G OMA I11 handling system and a 1304 gate pulse amplifier with variable time windows of 5 ns. The programming of the OMA afforded a time-resolved way of measuring. During the photolysis experiment the sample flowed through a home-built cell, specially constructed for the study of short-lived intermediates under inert gas atmosphere. Afterwards the spectra were corrected for the bleaching of the parent, by importing these spectra together with the ground state spectra in the computer program Grams. The subtraction factor was then varied until no bleaching was observed. Preparation of the Complexes. The complex [Ru(I)(iPr)(CO)z(iPr-DAB)I(1)and the ligand NJV’-diisopropyl-l,4-diaza1,3-butadiene (iPr-DAB) were synthesized according to literature method^.^^^^' The photoproduct [RU(I)Z(CO)Z(~P~-DAB)] was prepared by irradiation of a 1 mmol solution of 1 in 100 mL THF or CHzClz under nitrogen atmosphere with a high pressure mercury lamp provided with a 550 nm interference filter. The reaction was followed with IR spectroscopy and the irradiation was stopped before follow-up reactions started. The solution was evaporated until dryness. The product was purified by washing with hexane. Recrystallization took place (25)Vichovl, J.; Hartl,F.; VlEek, A,, Jr.; J . Am. Chem. SOC.1992, ” -774. - -, i- n- -c- -. (26)Aberchromics LTD,School of chemistry and applied chemistry, College of Cardiff, University of Wales. (27)Bock, H.; tom Dieck, H. Chem. Ber. 1967,100, 228.
782 Organometallics, Vol. 14,No. 2, 1995
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Table 1. Crystallographic Data of [Ru(I)z(CO)z(iPr-DAB)] formula molecular weight space group
CioHi6NzOzIzRU 551.1 141/a 11.552(1), 24.489(3) 3268.0(6) 8 2.24 0.71069 46.8 2048 293 0.044,0.078 978
a,c (A)
v ('43)
Z
Dx(g cm-9 A(Mo Ka) (A) p(Mo Ka) (cm-') F(000) temp (K) final R, Rw observed reflections
from CHzClz at -50 "C. IR v(C0) (CHZClZ): 2058,2004 cm-'; UV-vis (CHzClz): 375, 398, 460, 500(sh) nm. 'H-NMR (CDC13): 8.16 (s) (2 imine-H), 4.63 (m) (2 iPr-CH), 1.67 (d) (12 iPr-CH3). Crystal Structure Determination of [Ru(I)&O)a(iPrDAB)]. Crystals were grown from a saturated CHzClz solution at 243 K. Crystal data and numerical details of the structure determination are listed in Table 1. A crystal with dimensions 0.25 x 0.40 x 0.50 mm approximately was used for data collection on an Enraf-Nonius CAD-4 diffractometer with graphite-monochromated Cu Ka radiation and w-28 scan. A total of 1363 unique reflections was measured within the range 0 Ih 5 16, 0 c k I13, 0 I 1 c 34. Of these, 978 were above the significance level of 2.5 dn. The maximum value of (sin @)/,I was 0.70 A-l. Two reflections (024, 031) were measured hourly and showed no decrease during the 16 h collecting time. Unit-cell parameters were refined by a least-squares fitting procedure using 23 reflections with 40 < 2 0 < 42". Corrections for Lorentz and polarization effects were applied. The asymmetric unit contains half a molecule with the Ru at a special position. The structure was solved by direct methods. The hydrogen atoms were calculated. Full-matrix least squares refinement on F, anisotropic for the non-hydrogen atoms and isotropic for the hydrogen atoms, restraining the latter in such a way that the distance of their carrier remained constant a t approximately 1.09 A, converged t o R = 0.044, R , = 0.078, NU),,,^ = 0.31. A weighting scheme w = (6.0 + Fobs 0.0093F0b,2)-1was used. An empirical absorption correction (DIFABS)28was applied with coefficients in the range of 0.71-1.42. A final difference Fourier ma revealed a residual electron density between -0.8 and 0.7 e ifw3in the vicinity of the heavy atoms. Scattering factors were taken from Cromer and MannZ9and from the International Tables for X-ray Cry~tallography.~~ The anomalous scattering of Ru and I was taken into account. All calculations were performed with XTAL,31 unless stated otherwise.
+
Results The complex [Ru(I)(iPr)(C0)2(iPr-DAB)I (1) exhibits at least two composite absorption bands i n t h e visible region (Table 2, Figure 2). Both bands are solvatochromic, which indicates that they belong to charge transfer transitions. In a previous article we have explained t h e occurrence of these bands for t h e [Ru(X)(R)(CO)Z(adiimine)] (X = halide; R = alkyl) complexes by a mixing of t h e metal-d, a n d halide-p, orbitals, leading t o t h e formation of two sets of two mixed metal-halide orbitals (bonding a n d antibonding) from which these CT transitions originate.l8 (28) Walker, N.;Stuart, D. Acta Crystallogr. 1983,A39, 158. (29) Cromer, D.T.;Mann, J. B. Acta Crystallogr. 1968,A24, 321. (30) International Tables for X-ray Crystallography Vol. N; Kynoch Press: Birmingham, 1974;p 55. (31)Hall, S. R.; Flack, H. D.; Stewart,J. M.; Eds.XTAL3.2Reference Manual; Universities of Western Australia, Geneva and Maryland, 1992.
400
300
500 wavelength [nm]
600
700
Figure 2. The UV-vis absorption spectra of 1 in MeCN, THF, and toluene at room temperature.
abs
A
___
V&O
+
V& N
+
- - -- - - - _
I
300
400
500 600 wavelength in [nm]
700
Figure 3. Resonance Raman excitation profiles of v,(CN) and vs(CO) of 1 in CHZC12 a t room temperature; the intensities are relative to the 708 cm-l Raman band of CH2Clz.
Table 2. IR and UV-vis Data of Complex 1 and Its Photoproducts in Various Solvents (v(C0) in [cm-'1, L9 ab$ in m) complex
solvent
[Ru(I)(~R)(CO)Z(~P~-DAB)I (1) MeCN THF CHzClz toluene [Ru(I)z(CO)Z(~P~-DAB)] MeCN THF CHzClz toluene [Ru(iPr)(MeCN)(CO)z(iPr-DAB)]I MeCN
v(C0)
am=.abs
2024, 1962 2021, 1958 2027, 1963 2022, 1956 2054, 1999 2050, 1996 2058,2004 2054,2000 2038, 1969
412,452 429,480 425,479 433,496 380, 460 (br) 400,480 (br) 400,460 (br) 381,460 (br)
The resonance R a m a n (rR) spectra of 1,obtained by excitation into t h e lowest energy band, showed t h e strongest resonance enhancement for vs(CN) of iPr-DAB at 1537 cm-'. This result is in accordance with transfer of negative charge t o the lowest n* orbital of t h e iPrDAB ligand which causes a weakening of t h e CN bonds. The intensity of the v,(CN) R a m a n band normalized to t h e intensity of a solvent band is depicted in Figure 3 as a function of t h e wavelength of excitation. This excitation profile shows a maximum within t h e first absorption band at about 500 nm. The corresponding excitation profile of v,(CO), also presented in this figure, shows that this vibration is only weakly coupled t o t h e lowest-energy electronic transitions. The CO bonds a r e therefore only weakly influenced by the lowest energy transitions, which means that these transitions have more XLCT (I iPr-DAB) than MLCT character.
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Photochemistry of [Ru(I)(iPr)(C0)2(iPr-D~)]
Organometallics, Vol. 14, No. 2, 1995 783
t
2150
2100
2050
2000
1950
1900
1850
wavenumbers [cm-l
A
300
400
500 600 wavelength [nm]
700
800
Figure 4. IR 4CO) (CHZCld (a) and UV-vis (THF) (b) spectral changes during the photolysis of 1 at room temperature. Photochemistry. Contrary to the corresponding complexes [Ru(I)(R)(C0)2(iPr-DAl3)1 (R = Me, Et), complex 1 is very photolabile, even upon irradiation into the low-energy CT bands. The photodecomposition has been followed by IR, W-vis, and 1H-NMR spectroscopy. The IR and W-vis data of the parent complex and its photoproducts are collected in Table 2. To check its thermal stability, a THF solution of 1 was refluxed for several hours in the dark. Several samples were taken and analyzed by IR spectroscopy, but the complex appeared to be thermally completely stable under these conditions. The complex 1 has two strong IR bands in the v(C0) region at 2021 and 1958 cm-l (THF),respectively. Upon irradiation with ;1 > 500 nm these bands disappeared and two strong bands showed up at 2050 and 1996 cm-l, respectively (Figure 4a). Prolonged irradiation caused the disappearance of the two strong bands while at the same time new weak bands appeared at 1946,1930, and 1779 cm-l. This means that the initially formed photoproduct is not photostable. The appearance of the weaker bands was not the result of a secondary thermal reaction since no IR changes were observed when the solution was kept in the dark after a short time of irradiation. Although the two v(C0) bands of the first photoproduct were strong, they never reached the intensity of the parent bands before the followup reaction started. Upon irradiation of a solution of 1 in THF, the Wvis spectrum showed a regular decrease of the 480 nm band. A very broad composite band remained with an apparent maximum at 400 nm (Figure 4b). The iso-
sbestic points, observed initially, disappeared upon prolonged irradiation, due to a secondary photochemical reaction. The solvent did not influence the photoreaction since the same photoproduct with IR bands at ca. 2050 and 1996 cm-l was formed in CH2C12, MeCN, THF, MeOH, and toluene. This result was confirmed by the W-vis spectral changes which were very similar in these solvents both with respect to band shapes and maxima. Only in case of MeCN two biscarbonyl products were formed. One had its IR frequencies at 2055 and 1999 cm-l, similar to those observed in the other solvents. The IR frequencies of the second product at 2038 and 1969 cm-l closely resemble those of the products [Ru(R)(MeCN)(CO)z(iPr-DAB)]I, obtained by reduction of the complex [Ru(I)(R)(CO)2(iPr-DAB)1in MeCN.32 Increasing the concentration of the starting complex did also not influence the nature of the photoproduct. Even exposure of the solid complex t o sunlight afforded similar photoproducts. In order t o study the effect of the viscosity of the solvent, comparative experiments performed in MeOH ( q = 0.6 cp at rt) and in a 5% solution of MeOH in ethylene glycol ( q = 19.9 cp at rt) showed that also the viscosity of the solvent did not influence the kind of product nor the rate of its formation. Addition of the radical scavenger CC14 to a THF solution of 1 initially gave rise to a reaction with less side products. Only the two characteristic IR bands of the first product were then observed. It cannot, however, be excluded that in this case [Ru(I)(Cl)(CO)z(iPrwere formed, since DAB)] or [Ru(C1)2(C0)2(iPr-DAl3)1 these complexes are expected to have comparable IRfrequencies as the analogous diiodide complex. Also in this case, however, prolonged irradiation led t o decomposition of the product. 'H-NMR. In order to identify the first photoproduct, the photolysis of 1 was followed in situ with lH-NMR, using a special CIDNP probe in which the sample was irradiated within the NMR machine. Immediately after irradiation a spectrum which did not show any linebroadening was obtained which means that radicals, if formed at all, were only short-lived. Figure 5 shows the lH-NMR spectra obtained after different intervals of irradiation in a THF-ds solution. The most striking feature is the disappearance of a doublet at 1.51 ppm belonging to the CH3-protons of the isopropyl ligand. The positions of the other peaks hardly changed, although their intensities decreased to half the original values. The imine protons of the iPrDAB ligand shifted from 8.38 to 8.36 ppm and the septet at 4.54 ppm, belonging to the CH of the iPr-group of iPr-DAB, remained a t the same position. The doublets of the CH3-protons of this ligand shifted from 1.64 and 1.31 ppm to 1.60 ppm. As in the lH-NMR spectra of a solution of the pure, crystalline [RU(I)~(CO)~(~P~-DAB)], only one doublet was observed at 1.60 ppm for all CH3protons of the iPr-group with a corresponding integral. At 1.28 ppm a broad singlet was found which can be assigned to hexane molecules and isomers, formed by dimerization of the iPr-radicals. The reaction in CDC13 led to similar results, although more side-reactions took place. (32)Nieuwenhuis, H. A.; Hartl, F.; Stufiens, D. J. To be submitted for publication.
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Urganometallics, Vol. 14, N O . 2, 1YYa
x
II
5
4
I
I
2
0.5
chemical shift [ppm] Figure 5. lH-NMR spectral changes during the in situ photolysis of 1in THF-ds solution at room temperature after 0,3, 8, and 15 min (a-d) of irradiation with 530 nm light. Table 3. Selected Bond Distances (A) in [Ru(I),(CO)z(iPr-DAB)] with ESD’s in Parentheses bond
distance
bond
distance
Ru-I Ru-I* Ru-C(5) Ru-C(5)* Ru-N Ru-N* C( 1)-C( 1)*
2.71 12(8) 2.71 12(8) 1.90(1) 1.90(1) 2.14(1) 2.14( 1) 1.42(2)
C(l)-N C( I)*-N* c(2)-c(3) c(2)-c(4) C(2)-N W)-W)
1.28(1) 1.28(1) 1.46(3) 1.53(3) 1.49(2) 1.10(2)
Table 4. Selected Bond Angles (deg) in [Ru(I)~(CO)~(~~-DAB)] with ESD’s in Parentheses atoms
Figure 6. ORTEP drawing of the photoproduct [Ru(I)z(CO)z(iPr-DAB)I.
The observed intensity decrease of the NMR signals cannot be due to the formation of a solid product since no precipitate was observed in the NMR tube. In fact, careful examination of the spectra revealed a broad, structureless band of overlapping signals at about 1.31 ppm. In addition, some weak multiplets could be distinguished at ca. 4.3 ppm. Their integral, although hard to be determined quantitatively because of overlap between signals of the photoproduct and the decomposition products, might account for the missing amount of material. Crystal Structure of [Ru(I)z(C0)2(iPr-DAB)l. In order to establish the structure of the first photoproduct, the reaction was performed on preparative scale and the photoproduct was isolated. The X-ray crystal structure analysis of this product confirmed that the first photoproduct was in fact [Ru(I)z(CO)z(iPr-DAB)](Figure 6), its bond lengths and angles (Tables 3 and 4) closely resembling those of the analogous complex [Ru(I)2(CO)z(pTol-DAB)](pTol = p-toluidine), investigated by tom (33) tom Dieck, H; Kollvitz, W.; Kleinwachter, I; Rohde, W.; Stamp, L. Trans. Met. Chem. 1986, 11, 361.
I-Ru-I* I-Ru-C(5) I -Ru -C(5)* I-Ru-N I-Ru-N* I*-Ru-C(S) I*-Ru-C(S)* I*-Ru-N I*-Ru-N* C(5)-Ru-C(5)* C(S)-Ru-N C(5)-Ru-N*
angle 179.63(5) 90.2(4) 89.5(4) 89.3(3) 91.0(3) 89.5(4) 90.2(4) 91.0(3) 89.3(3) 90.2(4) 173.9(5) 97.5(5)
atoms C(S)*-Ru-N C(S)*-Ru-N* N-Ru-N* C(l)*-C(1)-N C(1)-C(l)*-N* C(3)-C(2)-(C4) C(3)-C(2)-N C(4)-C(2)-N Ru-N-C(l) Ru-N-C(2) Ru-N*-C(l)* C(1)-N-C(1)*
angle 97.5 (5 173.9(5) 76.4(4) 119(4) 119(1) 117(2) 112(1) 109(2) 113.1(8) 132.0(9) 113.1(8) 115(1)
Dieck and c o - ~ o r k e r s .The ~ ~ complex has a slightly distorted octahedral geometry in which the iodide ions occupy an axial position. The Ru-I bond lengths are shorter (2.711 A) than in case of [Ru(I)(Me)(CO)z(iPrDAB)] (2.81 A).34 Although the Ru-CO bond lengths of [Ru(I)2(CO)z(iPr-DAB)Iand [Ru(I)(Me)(CO)z(iPrDAB)] are comparable, the C-0 bond lengths in the diiodide complex (1.10 A) are sli htly shorter than in [Ru(I)(Me)(C0)2(iPr-DAB)I (1.15 ), indicating a somewhat decreased n-backbonding. ESR Spectroscopy. Although radicals did not disturb the ‘H-NMR spectra, their photochemical forma-
1
(34) Kraakman, M. J. A.; de Klerk-Engels, B.; de Lange, P. P.M.; Vrieze, K.; Smeets, W. J. J.; Spek, A. L. Organometallics 1992, 11, 3774.
Photochemistry of [Ru(I)(iPr)(CO)$iPr-DAB)]
Organometallics, Vol. 14, No. 2, 1995 785 , 2
I
abs
___
t
\
A
\ 1
iI
ffI t ,\ '
300
-'a\,, 293 K
,1
I
400
-
4
!
- 1.5 -
,
273K
+-
\
\
,,
500 wavelength [nm]
253K
600
a
-
700-
Figure 8. Wavelength dependence of the disappearance quantum yield upon irradiation of 1 in THF at 293, 273, 253, and 233 K. Table 5. Quantum Yields for the Photoreaction of 1 in THF at Different Temperatures (Aexc in nm,Temperature in K)
I I
10 G
I
Figure 7. ESR spectra obtained by irradiation of 1 in THF in the presence of (a) nitrosodurene and (b) tBuNO. tion could be established with ESR spectroscopy when a spin trap'was added to the solution. Irradiation of a THF solution of 1 a t room temperature with a 10-fold excess of nitrosodurene afforded the ESR spectrum of the radical adduct [(CH3)4(C6H)N(o.)CH(CH3)21.The spectrum (Figure 7a) showed six lines due to the hyperfine coupling of one nitrogen nucleus (IN = 1)and one hydrogen nucleus (IH = l/2) of the iPr-group. The coupling constants, a~ (NO) = 14.10 G and UH ( i b ) = 7.11 G, determined by computer simulation, correspond to the literature data.35 These ESR spectra did not show any signal due to the metal fragment, and tBuNO was therefore used instead of nitrosodurene to trap the ruthenium radicals. The ESR spectrum of the irradiated THF solution of 1 and tBuNO in a 1:1.8 concentration ratio (Figure 7b) showed a more complicated pattern than the spectrum of Figure 7a. However, this is also due to the trapped iPr-radical. The ESR spectrum could be simulated for [(tBu)N(O')CH(CHg)21,with a coupling Of aN (NO) = 15.6 G, UH (CH(iPr)) = 4.0 G and UH (CHs(tBu, iPr)j = 0.6 G. No evidence was obtained for the two possible Ru-radicals [Ru(I)(CO)2(iPr-DAB)(tBu)N(0')1 and [Ru(iPr)(CO)p(iPrDAB)(tBu)N(O')I. This is noteworthy since the radical scavenger (tBuNO) was completely consumed during the ESR-measurement, and no characteristic signals were observed for (tBu)aNO',which is normally the case when tBuNO is used in excess.36 Quantum Yields. The efficiency of the photolysis in THF was established by determining the quantum yields (a)for the disappearance of 1 a t various temperatures and wavelengths of irradiation. The results, (35) Terabe, S.;Kuruma, K.; Konaka, R. J.Chem. SOC.Perkin Trans. 2 1973, 1252.
(36) AndrBa, R. R.; de Lange, W. G. L.; van der Graaf, T.; Rijkhoff, M.; Stufiens, D. J.; Oskam, A. Organometallics 1988, 7 , 1100.
T
514.5
501.7
488.0
476.5
457.9
293 283 273 263 253 243 233 223
1.35 0.82 0.41 0.27 0.21 0.14 0.07 0.03
1.27
1.16 0.48 0.40 0.29 0.18 0.12 0.07 0.03
1.19
1.15
0.38
0.48
0.16
0.20
0.08
0.08
0.45 0.20 0.08
presented in Table 5, show that at room temperature the quantum yields are larger than unity at all wavelengths of irradiation. This means that the reaction is photocatalytic since more than one molecule of the starting complex is consumed per one photon absorbed. Figure 8 presents the quantum yields at different temperatures and wavelengths of irradiation. At all temperatures used no regular wavelength dependence of was observed. The irregular behavior at 293 K is most likely due to the sensitivity of the radical chain reaction, taking place at this temperature, to small amounts of impurities. The quantum yield of the photoreaction was strongly temperature dependent and appeared to decrease by a factor 3 going from 293 t o 273 K. Transient Absorption Spectra. Nanosecond flash photolysis experiments were carried out on solutions of complex 1 in MeCN, THF, and toluene. Figure 9 shows the transient spectra obtained for the complex in toluene within the laser flash and at several delay times. The spectra recorded in THF within the laser pulse showed depletion of the ground state absorption and formation of a transient with broad absorptions above 500 and near 430 nm. Immediately after the laser pulse, these features disappeared and the strong band at 340 nm remained essentially unchanged during the whole time-window of the apparatus (0.1 ms). In toluene and MeCN the same observations were made. No solvent dependence was observed. The transient absorption band at 430 nm is assigned to the excited state of the complex, because of the large similarities of these features with those of the timeresolved absorption spectra of the photostable complexes
786 Organometallics, Vol. 14, No. 2, 1995
*
Nieuwenhuis et al.
propyl and [Ru(I)(C0)2(iPr-DAB)I'radicals. The iPrradicals could be observed with ESR by using a radical I trap. In the transient absorption spectra the [Ru(I)(C0)2(iPr-DAB)I0radicals were observed as intermediA ates. The question remains how these radicals react after homolytic cleavage of the ruthenium-alkyl bond. Obviously, the spectral data do not provide any indication for an internal alkyl transfer to the iPr-DAB ligand. Such a reaction was found by Kaupp et a1.21 for the complexes [Zn(R)2(R-DAB)Iwhich undergo both thermally and photochemically transfer of R to the R-DAB ligand, leading to C and N alkylation. When the R 300 400 500 600 700 radical escaped from the solvent cage, a C-C coupled wavenumbers [cm-l] dimer of the [Zn(R)(R-DAB)]'radicals was obtained. In of complex 1,the 'H-NMR spectra did not provide case * laser pulse at 532 nm any evidence for the formation of a N- or C-alkylated Figure 9. Transient absorption spectra of 1 measured in product or a C-C-coupled dimer. toluene at 0, 10, and 20 ns after laser excitation with 532 Thus, the [Ru(I)(CO)2(iPr-DAB)l'radicals, observed nm (corrected for bleaching of the parent). in the transient absorption spectra, are not converted into a C-C coupled dimer or alkylated products, but [Ru(I)(R)(CO)2(iPr-DAB)l (R = Me, Et).37 From this afford the complex [Ru(I)2(CO)z(iPr-DAB)Ias the only excited state there is no recovery of the ground state carbonyl-containing photoproduct. Only when the reacwithin the time-domain of the experiment. Instead, a tion was performed in a coordinating solvent (S) such complete conversion occurs into the photoproduct charas acetonitrile, a second stable photoproduct [Ru(iPr)acterized by an absorption a t 360 nm. This latter band (S)(C0)2(iPr-DAB)lIwas obtained. Based on this prodis assigned to the radical [Ru(I)(CO)2(iPr-DAB)I',beuct formation, the nature of the primary photoprocess cause of its close resemblance with the spectrum of the and the high quantum yields, the photoreaction is electrochemically generated radical [Ru(Me)(PPh)(C0)2proposed to proceed according to the mechanism pre(iPr-DAB)I'(Amm = 357 nm).32The transient absorption sented in Scheme 1. spectrum of the radical [Re(CO)a(bpy)l'also showed a After homolysis of the Ru-iPr bond, the coordinastrong band near 360 nm, which lasted for at least 20 tively unsaturated radical [Ru(I)(CO)z(iPr-DAB)?formed ps.23 will take up a solvent molecule (S) t o give [Ru(I)(S)(C0)2(iPr-DAB)I'. This radical will then reduce the Discussion parent complex with formation of [Ru(I)(iPr)(C0)2(iPrThe high photoreactivity of complex 1 is noteworthy This reaction DAB)]- and [Ru(I)(S)(CO)2(iPr-DAB)l+. since the corresponding ethyl- and methyl-complexesare is in line with the observation of similar photodisprophotostable. According t o the spectral data, the same portionation reactions of metal-metal bonded comThe first photoproduct is formed in different solvents and plexes such as [(CO)sMn-Mn(C0)3(~-diimine)l.~~ reaction will, however, proceed further since a recent at different temperatures. In all cases two strong v(C0) spectroelectrochemical study has shown that the rebands were observed at about 2050 and 1990 cm-l, duced complexes [Ru(X)(R)(CO)z(iPr-DAB)]- (X = harespectively. Solvent coordination can therefore be lide; R = alkyl) immediately lose X- and transform into excluded. Complete conversion of complex 1 was not the radicals [Ru(R)(C0)2(iPr-DAB)I',which dimerize to possible due to the photolability of the first photoproduct. This secondary photoreaction started when about give the metal-metal bonded dimer [Ru(R)(CO)a(iPr50% of the starting complex had been converted. DAB)12.32 Similarly, the reduced 1 will decompose into [Ru(iPr)(S)(CO)2(iPr-DAB)l* and I-. The iodide will In order to obtain the spectral data of this first to give the react with the cation [Ru(I)(CO)z(iPr-DAB)]+ photoproduct as a pure compound, complex 1 was photoproduct [Ru(I)~(CO)~(~P~-DAB)I. irradiated on a preparative scale and the photoproduct was isolated and purified. Its spectral data and phoContrary to the spectroelectrochemical experiments, tolability closely resemble those of [Ru(I)z(CO)z(iPrdid not, howthe radicals [Ru(iPr)(S)(CO)2(iPr-DAB)l* DAB)] (see Table 2). Besides, the crystallographic data ever, produce the dimer [Ru(iPr)(CO)2(iPr-DAB)l2. This of the complex confirmed the assignment of this phois most likely due to the fact that, in the absence of toproduct as [Ru(I)~(CO)~(~P~-DAB)I. From this crystal excess electrons, these radicals act instead as reducing structure, it was clear that the photoproduct was neither agents with respect to the parent complex 1 and [Ru(C1)2(C0)2(iPr-DA)lnor [Ru(I)(Cl)(CO)2(iPr-DAB)], transform into the cation [Ru(iPr)(S)(CO)2(iPr-DAB)l+. although it had been formed in CHzClz solution. This In noncoordinating solvents, this cation will react back is noteworthy since other studies have shown that CH2with I- to give the parent complex. In a coordinating Cl2 can be used as a radical scavenger.20 solvent, the I- will not replace the solvent molecule, but The ESR and transient absorption spectra showed act as a counter ion. This explains the formation of [Ruthat the primary photoprocess is homolysis of the (iPr)(MeCN)(C0)2(iPr-DAB)II upon irradiation in MeCN. metal-alkyl bond, giving rise t o the formation of isoThis electron transfer reaction between the radicals [Ru(iPr)(S)(C0)2(iPr-DAB)I' and 1 starts an electron (37) Nieuwenhuis, H. A.; Stuflcens, D. J.; McNicholl, R.-A.; Altransfer chain (ETC) reaction, explaining the high Obaidi, A. H. R.; Coates, C. G.;McGarvey, J. J.; Westwell, J.; George, M. W.; Turner, J. J. To be submitted for publication. quantum yields (CD > 1)observed at room temperature. n
Photochemistry of [Ru(I)(iPr)(CO)2(iPr-DAB)/
Organometallics, Vol. 14,No. 2, 1995 787
Scheme 1. Proposed Reaction Routes for the Photoreaction of 1 [Ru(l)(iPr)(CO)diPr-DAB)) (1)
P
[Ru(I)(S)(CO)~(~P~-DAB))’ + [iPrr
+I. (I)
c--
[Ru(iPr)(S)(C0)2(iPr-DM)I*
[Ru(iPr)( S)(CO)*(iPr-OAB)]I (S = MeCN)
(1)
u t
[Ru(iPr)(CO)l(iPr-DAB)]z
A similar ETC mechanism has been proposed for the photoinduced disproportionation of [(C0)5Mn-Mn(C0)3(a-diimine)l.17,38 According to this mechanism, the iPr-radicals are not involved in the reaction sequence and scavenging these radicals is therefore not expected to influence the product formation. Just as Lucia et al.,23 we used MeOH as a trap for these alkyl radicals and this indeed did not influence the product formation. In addition, the ESR experiments have shown that both nitrosodurene and tBuNO only trap the iPr-radicals and not the [Ru(I)(CO)2(iPr-DAB)l’ radicals. Probably the [Ru(I)(CO)Z(iPr-DAB)yradicals are too reactive at room temperature to be detected. Irradiation of a solution of 1 in THF both in the absence and presence of a 5- or 10-fold excess of nitrosodurene gave rise to the same IR spectral changes. This means that the iPr-radicals are indeed not involved in the product formation. In situ lH-NMR experiments showed that these radicals dimerize. The formation of radicals by irradiation of 1 closely resembles the homolysis reactions observed for the metal-metal bonded complexes [L,M-M(CO)s(a-diimine)] (M = Mn, Re; L,M’ = (C0)5Mn, (Co)&O, Cp(C0)2Fe, Ph3Sn)12-18 and t(C0)5Mn-Ru(Me)(CO)z(adiimine)],Z0the metal-alkyl complexes [Re(R)(CO)s(adiimine)]22,23and [Zn(R)2(R’-DAB)1,21 and the metalhalide complexes mer-[Mn(X)(CO)3(bpy)](X = halide)24 and several N,Si-chelated complexes of 1r.39-42In case of [Zn(R)2(R’-DAB)],the metal orbitals are too low in energy to be involved in the electronic transitions and the reaction proceeds from the 3up*state by irradiation into the spin-allowed ub(Zn-R) n*(R’-DAB)transition. For the other complexes, the situation is more comn* transition is either not plicated since the Ob allowed or at least coincides with the much stronger d, n* (MLCT)transitions in the visible region. For these complexes, the homolysis reaction from the 30b7c* state
-
-
-
(38)van der Graaf, T.; Hofstra, R.; Schilder, P.G. M.; Rijkhoff, M.; Stufltens, D. J.; van der Linden, J. G . M. Organometallics 1991,10, 3668. (39)Carlson, G. A.;Djurovich, P. I.; Watts, R. J. Inorg. Chem. 1993, 32,4483. (40)Djurorvich, P.I.; Watts, R. J. Inorg. Chem. 1998,32,4681. (41)Djurorvich, P.I.; Cook, W.; Josh, R.; Watts, R. J. J . Phys. Chem. 1994,98,398. (42) Djurorvich, P. I.; Watts, R. J. J . Phys. Chem. 1994,98,396.
(Ru(I)z( C0)2(iPr-DAB)]
Scheme 2. Schematic Energy Level Diagram of la XLCT / MLCT
hv
I
The CT state has mixed MLCTEUCT character (see text). has therefore been proposed t o occur via MLCT excitation followed by surface crossing to the reactive 3UbZ* state (Scheme 2). Evidence for this mechanism has been provided by the complexes [Re(R)(CO)3(a-diimine)l, which undergo homolysis of the Re-R bond with varying quantum yield depending on R.22 For R = Me, Q, is only ca. and this result agrees with the rather low energy of the Ub(Re-Me) orbital with respect to the metald, orbitals as derived from the UV-photoelectron spectra. For R = Et or benzyl, or if R represents a metal fragment such as Mn(C0)5, the quantum yields are close t o unity since the Ob orbital is then the HOMO. As a * is lower in energy than the MLCT result, the 3 u ~state states. Apparently, the [Ru(X)(R)(CO)z(a-diimine)lcomplexes behave similarly, since they are photostable for R = Me, Et, but photodecompose for R = iPr. Occupation of a 30b37* state, after excitation into a charge transfer state, is closely related to the behavior of chromophore-quencher (C-Q) complexes, such as [Re(D)(CO)3(a-diimine)l+,in which D represents an of these C-Q organic donor m o l e ~ u l e . ~Irradiation ~-~~ complexes into a MLCT band is followed by electron transfer from D to the metal by which the complex arrives in a LL’CT state. From this state the complex returns to the ground state within 10-100 ns depending on D. In the metal-metal and metal-alkyl bonded (43)Perkins, T.A.;Humer, W.; Netzel, T. L.; Schanze, K. S. J . Phys Chem. 1990,2229. (44)Schanze, K.S.;MacQueen, D. B.; Perkins, T. A,; Cabana, L. A. Coord. Chem. Rev. 1993,122,63. (45) Chen, P.; Duesing, R.; Graff, D. K.; Meyer, T. J. J.Phys. Chem. 1991,95,5850.
Nieuwenhuis et al.
788 Organometallics, Vol. 14, No. 2, 1995
complexes MLCT excitation is followed by electron transfer from a bonding orbital and the LL'CT state is now a reactive 3q,n* state from which radicals are formed. As shown in Table 5 and Figure 8, the quantum yield of the homolysis reaction strongly depends on the temperature. In order to find out if this influence of the temperature could be due to a solvent cage effect, the photoreaction has been studied in solvents of different viscosity. For this purpose, the reaction was followed for 1 dissolved in MeOH (q = 0.6 cp at rt) and in ethylene glycol (q = 19.9 cp a t rt) containing 5% MeOH. No difference in reaction rate was then observed, which means that the observed influence of the temperature on this rate is not due t o a cage effect. the quanUnlike the complex [Re(Me)(CO)3(iPr-DAB>1,22 tum yields in the present study do not show a significant wavelength dependence. This means that the temperature dependence is most likely due t o an energy barrier between the MLCTKLCT state and the reactive 3Ubn* state. Two effects may be responsible for the observed drastic influence of the temperature on the quantum yield. First of all, a lowering of temperature will cause a decrease of reaction rate of the catalytic cycle since the thermal reaction between the primary radical product and the parent compound will be slowed down. At a certain low temperature the catalytic cycle will be stopped and the quantum yield is then determined by the primary photoprocess itself. This primary photoprocess may also be temperature dependent due to the presence of a barrier for the homolysis reaction from the 3UbZ* state. This is not unlikely since in this state the complex still contains one electron in its ub(Ru-iPr) orbital. Cleavage of the metal-alkyl bond from this state may therefore also be accompanied by an activation energy. An attempt to confirm the presence of this latter effect by measuring the occurrence of the weak emission from the 3uan*state at lower temperatures failed because of interference from the much stronger emission of the photoproduct [RU(I)~(CO)~(~P~-DAB)]. However, a recent study has shown that the related complexes [(CO)sMn-Ru(Me)(CO)z(a-diimine)l,which also decompose into radicals
a t room temperature from a 3q9c* state, emit from this state in a 2-MeTHF glass at 77 K.19 Similarly, the presence of such a barrier for decomposition from the ot,.z* state can also not be excluded for complex 1. It is therefore tentatively concluded that the influence of the temperature on the quantum yield is primarily caused by a slowing down of the catalytic cycle, but at lower temperatures possibly also by a small barrier for the homolysis reaction.
Conclusions Photodecompositionof [Ru(I)(iPrXCO)2(iPr-DAEVl leads t o the formation of [RU(I)~(CO)~(~P~-DAB)~. If the reaction is performed in a coordinating solvent (S) such as acetonitrile, [Ru(iPr)(S)(CO)2(iPr-DAB)IIis also produced. ESR and transient absorption spectra have shown that the primary photoprocess is homolysis of the radimetal-alkyl bond. The [Ru(I)(S)(CO)~(~P~-DAB)~' cals formed reduce the parent complex, which then releases I-. The high quantum yields observed for the photodecompositionof the parent complex are explained in terms of an electron transfer chain reaction. The temperature dependence of the quantum yields is tentatively attributed to a decrease in reaction rate of the catalytic reaction of the primary photoproduct and to a barrier for the reaction from the 3abn*state.
Acknowledgment. J. M. Ernsting is thanked for technical assistance with the NMR experiments and J. Fraanje for the measurement of the X-ray structure. The Netherlands Foundation for Chemical Research (SON) and the Netherlands Organisation for Pure Research (NWO) are thanked for financial support. A. VlEek Jr. is thanked for critically reading the manuscript. Supplementary Material Available: Listings of the atomic coordinates of non-hydrogen (Table S1)and hydrogen (Table S2) atoms, the anisotropic thermal parameters of the non-hydrogen atoms (Table S3),the bond lengths of the nonhydrogen atoms (Table S4) and hydrogen atoms (Table S5), and the bond angles of the non-hydrogen atoms (Table S6)and hydrogen bond angles (Table S7) (7 pages). Ordering information is given on any current masthead page. OM9405403
