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Photoaquation and aquation of trans-dichlorobis (bipyridine)ruthenium(III)
163
1. Photochem. Photobiol. A: Chem., 79 (1994) 163-166
Photoaquation and aquation of transdichlorobis(bipyridine)ruthenium(III) Liangshiu
Lee’
and
Che-Hung
Wang
Department of Chem&y, National Sun Yat-Sen Universi@,Kaohsing (Taiwan) (Received November 2, 1993; accepted November 11, 1993)
Abstract The quantum yield for photoaquatkn of mms-[Ru(bipy),Cl;I+, where bipy = 2,2’-bipyridine, is found to be 1.5 X lO_’ mol Einstein-‘, when irradiated at its ligand field band. The estimated activation enthalpy for the thermoaquation of this complex is 20 kcal mol- I. In comparison with tmnr-[Ru(en)&lJ +, where en 5 ethylenediamine, the bipy complex has a higher quantum efficiency than the en complex. The difference between the reactivities in the ground state and excited state can be understood within the framework of the ligand field model. The electronic interaction between Ru(II1) and ligands that stabilizes the ground state also stabilizes the intermediate in the photoaquation reaction. Hence the bipy complex is found to be less reactive toward aquation in the ground state than the en complex is but to be more reactive in the excited state.
1. Introduction The synthesis and characterization of photochemical properties of transition metal complexes allow a detailed quantitative description of the excited-state manifolds and their dynamics [l]. An interesting feature of photochemistry is the altered substitution behavior of metal complexes between their ground and excited states. The ligand field model for the photosubstitution of Ru(II1) (d5) complexes has been established by Vanquickenborne and Ceulemans (VC) and modified by others [3]. Because of the results of the stereochemistry of photoaquation of Ru(en),Cl*+ (en = ethylenediamine) [4], the validity of the VC model is controversial [5-71. As the determination of the geometries of the photoproducts in these reactions is generally difficult and the uncertainty associated with their ratio is normally great, we sought an alternative viewpoint for these reactions. In the course of our investigation, we found that the substitutional behavior of Ru(II1) complexes can be understood within the framework of the VC model. Such an example is reported herein. The thermal aquation and photoaquation reactions of Ru(en),Cl,+ (en = ethylenediamine) and Ru(bipy),Cl,+ (bipy = 2,2’-bipyridine) are contrasting. The former complex is found to be more ‘Author
to whom
correspondence should be addressed.
lOlO-6030/94/$07.00 8 1994 Elsevier Sequoia. All rights reserved SSDI 1010-6030(93)03759-A
labile; the rate constant for aquation near 313 K is 2.4 x 10m5 ma1 s-’ [8]. The bipy complex undergoes aquation only above 320 K. The quantum yields for photoaquation of Ru(en),Cl,+ and are 5.0 x lop3 mol Einstein- ’ [4] Ru(bipy),Cl,+ and 1.5 x lo-* mol Einstein-l respectively. There is substantial “activation” of the bipy complex, whereas the excitation of the en complex increases the rate of aquation only slightly. We believe that it is the electronic effect which stabilizes the bipy complex in the ground state and the same ainteraction that reduces the energy barrier for aquation in the excited state. The preliminary study on [Ru(tn)Cl,] +, where tn = 1,3propanediamine, shows that the tn complex is more labile than the en counterpart in the thermal reaction owing to its steric mobility.
2. Experimental
section
2.1. Materials Unless otherwise stated, all chemicals and solvents used in this work were of analytical reagent grade and used without further treatment or purification. 2.2. Synthesis 2.2.1. K,[RuCI, -H,O] Because pentachlororuthenium(II1) is much more reactive than ruthenium trichloride 191, we chose it as the starting material. The synthesis of
164
12. Lee, C.-H. Wang / Photoaquation and aquation of dich~~b~(b~~d~~)~~nium(lll)
this compound was first reported by Mercer and Buckley [lo]. We report here a modified method. RuCI,.xH,O (0.26 g; Janssen) was dissolved in ethanol (30 ml; 95%). The residue was titered off. Ethanol was then removed by evaporation and 25 ml of concentrated HCl was added to the dried solid. After reflux for 5 h during which period the solution turned red, 0.1 g of KC1 and a trace amount of NaBI& were added. When the effervescence ceased, the mixture was further refluxed for 8 h. The volume was then reduced to 3 ml and cooled. The product was collected by vacuum filtration and washed several times with ethanol and then ether (yield, 86%). UV-visible A,, = 327 nm, l=2200 cm-’ M-’ (in 0.1 M HCl). Anal. Found: Cl, 47.34; Ru, 28.0 [ 11J. K,RuCl,H,O talc.: Cl, 47.33; Ru, 26.97%. 2.2.2. [RuCl,(bipy)),]C101~3H20 The preparation of the bipy complex was modeled after the procedure of Poon and Che [12]. K,RuCl,H,O (0.3 g) was placed in a flask in an ice bath and a 50 ml acetone solution of 0.25 g of 2,2’-bipyridine was added to the flask dropwise. The solution was kept at 0 “C and stirred overnight. The volume was then reduced to 10 ml without heating, and 5 ml of 1 M HClO, was added. The resulting precipitate was collected and washed with acetone and ethanol. The product was then dissolved in 2 ml of 1 M HC104 and passed through a 10 cm column of Dowex-50 H-form cation exchange resin. The first portion collected gave an orange product found to be an Ru(Il) compound; the second portion after work-up gave a red product of [Ru(bipy)zClz]C10,*3H,0. Anal. Found: N, 9.09; C, 37.33; H, 3.47. RuCl,O,C,&H,, talc.: N, 8.78; C, 37.66; H, 3.47%. 2.2.3. Ru(bipy)2Cl(H20)2+ Equimolar amounts of dichlororuthenium(lI1) compound and AgClO, were mixed in water and stirred without heating for several hours until no further spectral change was observed. The precipitate was collected and the weight showed an equimolar amount of AgCl was formed. The resulting solution was then taken as the solution of aquachlororuthenium(II1) species without further purification, and the spectra were taken as such. The optical density was calculated on the basis of the quantity of starting material. 2.3. Instrumentation All absorption spectra and absorbance measurements were recorded on a Hitachi U-3200 spectrophotometer. The elemental analysis was
done with a Heraeus CHN-Q rapid analyzer at the Southern Instrumental Center, National Chen Kung University, Tainan, Taiwan, with the exception of the Ru analysis. ‘H and 13C nuclear magnetic resonance (NMR) spectra were recorded on a Varian VXR300 spectrometer. Quantum yields for irradiation at the wavelengths of the mercury lines were determined using a continuous-beam photolysis apparatus consisting of an Oriel universal arc source lamp equipped with a 500 W high pressure Hg lamp. The light beam passed through a 10 cm water cell to remove IR components, a fused silica collimating lens (Orielfl2.6; focal length, 5.1 cm), an Orielmercuryline interference filter of 1 in diameter and a hollow brass thermostated cell compartment controlled by a Neslab RTB-9DD constant-temperature circulating bath. These components were all mounted on an Oriel 1 m optical bench. The incident intensity of the irradiation beam was determined by ferrioxalate [13] actinometty. 2.4. Photolysis procedure The photolysis of the Ru(III) complexes was carried out at 25 “C in a 2 cm cylindrical quartz cell. The samples were irradiated Eor time intervals ranging from 90 to 300 min. The period between spectral monitoring during photolysis was 3 min in the first 25% of the reaction and 20 min toward the end of photolysis. The wavelength of the incident light was chosen to be 313 nm for both Ru(III) complexes. Water used as solvent for photolysis was obtained with a Fison Fi-Stream Distillatory and treated with a Millipore Mill&Q system to remove all the trace impurities. All spectroscopic measurements were corrected for thermal reaction. Quantum yields were determined by plotting quantum yields [14] at each monitoring instant vs. time and extrapolating to time zero. 2.5. Kinetic experiments The thermal aquation reaction was carried out in dark at temperatures ranging from 25 to 80 “C and monitored with a spectrophotometer. The results were treated as first-order kinetics and plots of ln(A, -A,) vs. time were used for calculation of the rate constants [15]. The activation enthalpies were estimated by In k and l/T plots. 3. Results and discussion Electronic absorption spectra for the respective dichloro and aquachloro complexes are presented in Fig. 1 and Table 1 with Ru(en),Cl,?+ as reference. The bipy complex shows a stronger absorption in
L. Lee, C.-H. Wang / Photoaquation and equation of dichiombir(bipyridine)pyridine)nrrhenium(lll)
the spectral region between 300 and 400 nm, which is mainly Cl to Ru(lI1) charge transfer band [16] and the ligand field band is obscured. The spectral changes upon irradiation in this spectral region in the photolysis of Ru(bipy)&l,+ are shown in Fig. 2. The distinct isosbestic points indicate that no secondary reactions have taken place. The resulting spectra are compared with spectra of aquachloro species. Proton NMR measurements were carried out on the resulting solution after exhaustive photolysis. The results indicate that the trans-aquachloro compound was the sole product
0.6
165
.
Abs.
16.0
00 ~1103
3iD
400
360 shm
Fig. 2. Spectra 0.1 M HClO,.
6.0
during
photolysis
of Ru(bipy)2C12+
at 2.5 “C in
TABLE 2. Quantum yields for photolysis and rate constants the aquation of dichloro-Ru(II1) complexes Compound
mznr-Ru(tn)zClt+ franr-Ru(bipy),Cl,f trans-Ru(en),Clz+ 0.0 200
300
thin
500
4ao
Fig. 1. The electronic absorption spectra of bymr-[Ru@ipy)zCIJ+ (-) and harr.v[Ru(bipy)@I,O)a]2+ (- - -) complexes.
TABLE LBand maxima in the reaction monitoring range (300-400 nm) of dicbloro- and aquachloro-Ru(III) complexes where the optical densities were determined in 0.1 M HQ Compound ;XIOr tranr-Ru(en)&l,+ [8] nzma-Ru@ipy),Cla+ tMns-Ru@ipy)2Cl(H,0)Z’
354 372 370
1.7 5.2 3.7
cm-’
M-l)
[9] [4]
9 (X10-’ mol Einstein-‘)
kb (X10--’
1.5 O..SoC
8,8 0.0075 0.75 [S]
Srradiated at 313 nm in 0.1 M HClO, bin 0.1 M HCIOI at 50 “C. irradiated at 345 nm.
for
s-1)
at 25 “C.
of photolysis. The quantum yields are listed in Table 2. The thermal aquation of the complex in acidic solution also produces the aquachloro species and the rate constants for aquation at 50 “C are listed in Table 2 with data for the en complex for comparison. A kinetic plot of the aquation reaction appears in Fig. 3. The estimated activation enthalpy for the aquation of Ru(bipy)$&+ is 20 kcal mol - ’ . It is believed that both thermal aquation and photoaquation of the Ru(II1) compIexes go through
166
L. Lee, C.-H. Wang I Photoaquation and aquation of dich~~~(~d~e)nuheniwn(lll)
Fig. 3. Kinetic plot for the thermal aquation of Ru@ipy)#&* in 0.1 M HCtO,.
a five-coordinated trigonal bipyramidal (CQ transition state [7]. The angular overlap model stabilization energies (AOMSEs) [17] are calculated for both ground- and excited-state species using the angular overlap model parameters e, and err, for various ligands [18]. The trans complex in this work has D,, symmetry. For. the electronic ground state, the AOMSEs are AOMSE (D,+,,)= &,
+ 4eWc,+ ~zY,~,- 14e,
(1)
AOMSE(&)=~e,N+~~+e.l,cl-~e,
(2) The difference between the AOMSEs in eqns. (1) and (2) is taken to be the activation barrier [19] for the aquation reactions, denoted AAOMSE. For complexes which have no r-bonding capability, e& = 0 and AAOMSE = ge, + 2e, + e,,. For the bipy complex, the ligand field band is buried in the Cl-Ru(II1) charge transfer band, but a charge transfer excited state is generally substitutionally inert; we assume that the reactive state which leads to photoaquation is the ligand field state and that the ligand field model is applicable to the bipy complex. The dominant factor in AAOMSE is p eWN_The difference between the activation enthalpies of the en and the bipy complexes indicates that bipy is a better u donor toward Ru(II1) than is en and that eobiWis greater than e_,. As for the excited-state behavior, the AOMSEs are AOMSE*(D,,) = 7e, + 2e,,cl + ercl - 12e, AOMSE*(&)
=$$eUN + eocl + 2erc, - Fe,
(3) (4)
The difference between eqns. (3) and (4) is regarded as the barrier for photoaquation reactions,
denoted AAOMSE*. For complexes in which e rrN=O, AAOMSE* = -g eUN+e&f%&,. It is obvious that ligands with a greater eoN value have a smaller barrier in excited-state reactions. These simplified VC calculations reveal that a good v-donating ligand stabilizes the Ru(II1) complex in its ground state to a greater extent than a poorer u-donating ligand does. However the very same electronic effect also contributes to the stability of the five-coordinated intermediate in the excited-state reaction and reduces the barrier for photoaquation of Ru(II1) complexes. A preliminary study on tn complexes shows that it is more susceptible to the substitution reaction than is the en complex in both excited and ground states. The reason, we believe, is not electronic but the difference between the steric mobilities of the two complexes. Proceeding from Da,, symmetry to trigonal bipyramidal (TBP) C, symmetry requires a bidentate ligand to open to accommodate the expansion of the angle in the equatorial plane. For a tn ligand with its longer carbon chain, this opening is more easily accomplished than for en ligands. References 1 A. Juris, V. Batzani, P. Belser and A. von Zelewsky, Coord. Chem. Rev., 84 (1988) 85. 2 L.G. Vanquickenborne and A. Ceulemans, 1 Am. Chem. See., 100 (1978) 475; Znorg. Chem., 17 (1978) 2730. 3 K.F. Purcell, S.F. Clark and J.D. Petersen, Ino%. Chem, 19 (1980) 2183. 4 M.E. Rerek and P.S. Sheridan, Inorg. Chem., 19 (1980) 2646. 5 J.D. Petersen, Inotg. Chem., 20 (1981) 3123. 6 M.E. Rerek and P.S. Sheridan, Inorg. C/rem., 23 (1984) 2198. 7 A. Cetdemans, D. Beyens and L.G. Vanquickenbome, Itto~K. Chem., 22 (1983) 1113. 8 LA. Broomhead, L. Kane-Maguire and D. Wilson, Inorg ckem., 14 (1975) 2575. 9 L. Lee, unpublished results, 1990. 10 BE. Mercer and R.R. Buckley, Inorg. Chem., 4 (196.5) 1692. 11 K.J. Moore, Ph.D. Diwertation, Clemson University, Clemsen, SC, 1983. 12 C.K. Poon and CM. Che, J. Chem. Sot., Dalton Truer., (1980) 756. 13 C.G. Hatchard and C.A. Parker, Proe. R Sot. London, Ser. A, 235 (1956) 518. 14 A.W. Adamson and P.D. Fleischauer, Conceptr of Inorganic Photochem&y, Wiley, New York, 1975, Chapter 2. 15 D. Katakis and G. Gordon, Mechanirmr of InorganicReacrions, Wiley, New York, 1987, Chapter 4. 16 H. Nagao, H. Nishimura, H. Funato, Y. Icbikawa, F.S. Howell, M. Mukaida and H. Kakihana, Inorg. C&m, 28 (1989) 3955. 17 KF. Purcell and J.C. Katz, Inorganic C/rem&y, Saunders, Philadelphia, PA, 1977, Chapter 9. 18 E. Larsen and G.N. LaMar, 1. Chcm. Educ., 51 (1974) 633. 19 L. Lee, SF. Clark and J.D. Petersen, fnorg Chem., 24 (1985) 3558.
1. Photochem. Photobiol. A: Chem., 79 (1994) 163-166
Photoaquation and aquation of transdichlorobis(bipyridine)ruthenium(III) Liangshiu
Lee’
and
Che-Hung
Wang
Department of Chem&y, National Sun Yat-Sen Universi@,Kaohsing (Taiwan) (Received November 2, 1993; accepted November 11, 1993)
Abstract The quantum yield for photoaquatkn of mms-[Ru(bipy),Cl;I+, where bipy = 2,2’-bipyridine, is found to be 1.5 X lO_’ mol Einstein-‘, when irradiated at its ligand field band. The estimated activation enthalpy for the thermoaquation of this complex is 20 kcal mol- I. In comparison with tmnr-[Ru(en)&lJ +, where en 5 ethylenediamine, the bipy complex has a higher quantum efficiency than the en complex. The difference between the reactivities in the ground state and excited state can be understood within the framework of the ligand field model. The electronic interaction between Ru(II1) and ligands that stabilizes the ground state also stabilizes the intermediate in the photoaquation reaction. Hence the bipy complex is found to be less reactive toward aquation in the ground state than the en complex is but to be more reactive in the excited state.
1. Introduction The synthesis and characterization of photochemical properties of transition metal complexes allow a detailed quantitative description of the excited-state manifolds and their dynamics [l]. An interesting feature of photochemistry is the altered substitution behavior of metal complexes between their ground and excited states. The ligand field model for the photosubstitution of Ru(II1) (d5) complexes has been established by Vanquickenborne and Ceulemans (VC) and modified by others [3]. Because of the results of the stereochemistry of photoaquation of Ru(en),Cl*+ (en = ethylenediamine) [4], the validity of the VC model is controversial [5-71. As the determination of the geometries of the photoproducts in these reactions is generally difficult and the uncertainty associated with their ratio is normally great, we sought an alternative viewpoint for these reactions. In the course of our investigation, we found that the substitutional behavior of Ru(II1) complexes can be understood within the framework of the VC model. Such an example is reported herein. The thermal aquation and photoaquation reactions of Ru(en),Cl,+ (en = ethylenediamine) and Ru(bipy),Cl,+ (bipy = 2,2’-bipyridine) are contrasting. The former complex is found to be more ‘Author
to whom
correspondence should be addressed.
lOlO-6030/94/$07.00 8 1994 Elsevier Sequoia. All rights reserved SSDI 1010-6030(93)03759-A
labile; the rate constant for aquation near 313 K is 2.4 x 10m5 ma1 s-’ [8]. The bipy complex undergoes aquation only above 320 K. The quantum yields for photoaquation of Ru(en),Cl,+ and are 5.0 x lop3 mol Einstein- ’ [4] Ru(bipy),Cl,+ and 1.5 x lo-* mol Einstein-l respectively. There is substantial “activation” of the bipy complex, whereas the excitation of the en complex increases the rate of aquation only slightly. We believe that it is the electronic effect which stabilizes the bipy complex in the ground state and the same ainteraction that reduces the energy barrier for aquation in the excited state. The preliminary study on [Ru(tn)Cl,] +, where tn = 1,3propanediamine, shows that the tn complex is more labile than the en counterpart in the thermal reaction owing to its steric mobility.
2. Experimental
section
2.1. Materials Unless otherwise stated, all chemicals and solvents used in this work were of analytical reagent grade and used without further treatment or purification. 2.2. Synthesis 2.2.1. K,[RuCI, -H,O] Because pentachlororuthenium(II1) is much more reactive than ruthenium trichloride 191, we chose it as the starting material. The synthesis of
164
12. Lee, C.-H. Wang / Photoaquation and aquation of dich~~b~(b~~d~~)~~nium(lll)
this compound was first reported by Mercer and Buckley [lo]. We report here a modified method. RuCI,.xH,O (0.26 g; Janssen) was dissolved in ethanol (30 ml; 95%). The residue was titered off. Ethanol was then removed by evaporation and 25 ml of concentrated HCl was added to the dried solid. After reflux for 5 h during which period the solution turned red, 0.1 g of KC1 and a trace amount of NaBI& were added. When the effervescence ceased, the mixture was further refluxed for 8 h. The volume was then reduced to 3 ml and cooled. The product was collected by vacuum filtration and washed several times with ethanol and then ether (yield, 86%). UV-visible A,, = 327 nm, l=2200 cm-’ M-’ (in 0.1 M HCl). Anal. Found: Cl, 47.34; Ru, 28.0 [ 11J. K,RuCl,H,O talc.: Cl, 47.33; Ru, 26.97%. 2.2.2. [RuCl,(bipy)),]C101~3H20 The preparation of the bipy complex was modeled after the procedure of Poon and Che [12]. K,RuCl,H,O (0.3 g) was placed in a flask in an ice bath and a 50 ml acetone solution of 0.25 g of 2,2’-bipyridine was added to the flask dropwise. The solution was kept at 0 “C and stirred overnight. The volume was then reduced to 10 ml without heating, and 5 ml of 1 M HClO, was added. The resulting precipitate was collected and washed with acetone and ethanol. The product was then dissolved in 2 ml of 1 M HC104 and passed through a 10 cm column of Dowex-50 H-form cation exchange resin. The first portion collected gave an orange product found to be an Ru(Il) compound; the second portion after work-up gave a red product of [Ru(bipy)zClz]C10,*3H,0. Anal. Found: N, 9.09; C, 37.33; H, 3.47. RuCl,O,C,&H,, talc.: N, 8.78; C, 37.66; H, 3.47%. 2.2.3. Ru(bipy)2Cl(H20)2+ Equimolar amounts of dichlororuthenium(lI1) compound and AgClO, were mixed in water and stirred without heating for several hours until no further spectral change was observed. The precipitate was collected and the weight showed an equimolar amount of AgCl was formed. The resulting solution was then taken as the solution of aquachlororuthenium(II1) species without further purification, and the spectra were taken as such. The optical density was calculated on the basis of the quantity of starting material. 2.3. Instrumentation All absorption spectra and absorbance measurements were recorded on a Hitachi U-3200 spectrophotometer. The elemental analysis was
done with a Heraeus CHN-Q rapid analyzer at the Southern Instrumental Center, National Chen Kung University, Tainan, Taiwan, with the exception of the Ru analysis. ‘H and 13C nuclear magnetic resonance (NMR) spectra were recorded on a Varian VXR300 spectrometer. Quantum yields for irradiation at the wavelengths of the mercury lines were determined using a continuous-beam photolysis apparatus consisting of an Oriel universal arc source lamp equipped with a 500 W high pressure Hg lamp. The light beam passed through a 10 cm water cell to remove IR components, a fused silica collimating lens (Orielfl2.6; focal length, 5.1 cm), an Orielmercuryline interference filter of 1 in diameter and a hollow brass thermostated cell compartment controlled by a Neslab RTB-9DD constant-temperature circulating bath. These components were all mounted on an Oriel 1 m optical bench. The incident intensity of the irradiation beam was determined by ferrioxalate [13] actinometty. 2.4. Photolysis procedure The photolysis of the Ru(III) complexes was carried out at 25 “C in a 2 cm cylindrical quartz cell. The samples were irradiated Eor time intervals ranging from 90 to 300 min. The period between spectral monitoring during photolysis was 3 min in the first 25% of the reaction and 20 min toward the end of photolysis. The wavelength of the incident light was chosen to be 313 nm for both Ru(III) complexes. Water used as solvent for photolysis was obtained with a Fison Fi-Stream Distillatory and treated with a Millipore Mill&Q system to remove all the trace impurities. All spectroscopic measurements were corrected for thermal reaction. Quantum yields were determined by plotting quantum yields [14] at each monitoring instant vs. time and extrapolating to time zero. 2.5. Kinetic experiments The thermal aquation reaction was carried out in dark at temperatures ranging from 25 to 80 “C and monitored with a spectrophotometer. The results were treated as first-order kinetics and plots of ln(A, -A,) vs. time were used for calculation of the rate constants [15]. The activation enthalpies were estimated by In k and l/T plots. 3. Results and discussion Electronic absorption spectra for the respective dichloro and aquachloro complexes are presented in Fig. 1 and Table 1 with Ru(en),Cl,?+ as reference. The bipy complex shows a stronger absorption in
L. Lee, C.-H. Wang / Photoaquation and equation of dichiombir(bipyridine)pyridine)nrrhenium(lll)
the spectral region between 300 and 400 nm, which is mainly Cl to Ru(lI1) charge transfer band [16] and the ligand field band is obscured. The spectral changes upon irradiation in this spectral region in the photolysis of Ru(bipy)&l,+ are shown in Fig. 2. The distinct isosbestic points indicate that no secondary reactions have taken place. The resulting spectra are compared with spectra of aquachloro species. Proton NMR measurements were carried out on the resulting solution after exhaustive photolysis. The results indicate that the trans-aquachloro compound was the sole product
0.6
165
.
Abs.
16.0
00 ~1103
3iD
400
360 shm
Fig. 2. Spectra 0.1 M HClO,.
6.0
during
photolysis
of Ru(bipy)2C12+
at 2.5 “C in
TABLE 2. Quantum yields for photolysis and rate constants the aquation of dichloro-Ru(II1) complexes Compound
mznr-Ru(tn)zClt+ franr-Ru(bipy),Cl,f trans-Ru(en),Clz+ 0.0 200
300
thin
500
4ao
Fig. 1. The electronic absorption spectra of bymr-[Ru@ipy)zCIJ+ (-) and harr.v[Ru(bipy)@I,O)a]2+ (- - -) complexes.
TABLE LBand maxima in the reaction monitoring range (300-400 nm) of dicbloro- and aquachloro-Ru(III) complexes where the optical densities were determined in 0.1 M HQ Compound ;XIOr tranr-Ru(en)&l,+ [8] nzma-Ru@ipy),Cla+ tMns-Ru@ipy)2Cl(H,0)Z’
354 372 370
1.7 5.2 3.7
cm-’
M-l)
[9] [4]
9 (X10-’ mol Einstein-‘)
kb (X10--’
1.5 O..SoC
8,8 0.0075 0.75 [S]
Srradiated at 313 nm in 0.1 M HClO, bin 0.1 M HCIOI at 50 “C. irradiated at 345 nm.
for
s-1)
at 25 “C.
of photolysis. The quantum yields are listed in Table 2. The thermal aquation of the complex in acidic solution also produces the aquachloro species and the rate constants for aquation at 50 “C are listed in Table 2 with data for the en complex for comparison. A kinetic plot of the aquation reaction appears in Fig. 3. The estimated activation enthalpy for the aquation of Ru(bipy)$&+ is 20 kcal mol - ’ . It is believed that both thermal aquation and photoaquation of the Ru(II1) compIexes go through
166
L. Lee, C.-H. Wang I Photoaquation and aquation of dich~~~(~d~e)nuheniwn(lll)
Fig. 3. Kinetic plot for the thermal aquation of Ru@ipy)#&* in 0.1 M HCtO,.
a five-coordinated trigonal bipyramidal (CQ transition state [7]. The angular overlap model stabilization energies (AOMSEs) [17] are calculated for both ground- and excited-state species using the angular overlap model parameters e, and err, for various ligands [18]. The trans complex in this work has D,, symmetry. For. the electronic ground state, the AOMSEs are AOMSE (D,+,,)= &,
+ 4eWc,+ ~zY,~,- 14e,
(1)
AOMSE(&)=~e,N+~~+e.l,cl-~e,
(2) The difference between the AOMSEs in eqns. (1) and (2) is taken to be the activation barrier [19] for the aquation reactions, denoted AAOMSE. For complexes which have no r-bonding capability, e& = 0 and AAOMSE = ge, + 2e, + e,,. For the bipy complex, the ligand field band is buried in the Cl-Ru(II1) charge transfer band, but a charge transfer excited state is generally substitutionally inert; we assume that the reactive state which leads to photoaquation is the ligand field state and that the ligand field model is applicable to the bipy complex. The dominant factor in AAOMSE is p eWN_The difference between the activation enthalpies of the en and the bipy complexes indicates that bipy is a better u donor toward Ru(II1) than is en and that eobiWis greater than e_,. As for the excited-state behavior, the AOMSEs are AOMSE*(D,,) = 7e, + 2e,,cl + ercl - 12e, AOMSE*(&)
=$$eUN + eocl + 2erc, - Fe,
(3) (4)
The difference between eqns. (3) and (4) is regarded as the barrier for photoaquation reactions,
denoted AAOMSE*. For complexes in which e rrN=O, AAOMSE* = -g eUN+e&f%&,. It is obvious that ligands with a greater eoN value have a smaller barrier in excited-state reactions. These simplified VC calculations reveal that a good v-donating ligand stabilizes the Ru(II1) complex in its ground state to a greater extent than a poorer u-donating ligand does. However the very same electronic effect also contributes to the stability of the five-coordinated intermediate in the excited-state reaction and reduces the barrier for photoaquation of Ru(II1) complexes. A preliminary study on tn complexes shows that it is more susceptible to the substitution reaction than is the en complex in both excited and ground states. The reason, we believe, is not electronic but the difference between the steric mobilities of the two complexes. Proceeding from Da,, symmetry to trigonal bipyramidal (TBP) C, symmetry requires a bidentate ligand to open to accommodate the expansion of the angle in the equatorial plane. For a tn ligand with its longer carbon chain, this opening is more easily accomplished than for en ligands. References 1 A. Juris, V. Batzani, P. Belser and A. von Zelewsky, Coord. Chem. Rev., 84 (1988) 85. 2 L.G. Vanquickenborne and A. Ceulemans, 1 Am. Chem. See., 100 (1978) 475; Znorg. Chem., 17 (1978) 2730. 3 K.F. Purcell, S.F. Clark and J.D. Petersen, Ino%. Chem, 19 (1980) 2183. 4 M.E. Rerek and P.S. Sheridan, Inorg. Chem., 19 (1980) 2646. 5 J.D. Petersen, Inotg. Chem., 20 (1981) 3123. 6 M.E. Rerek and P.S. Sheridan, Inorg. C/rem., 23 (1984) 2198. 7 A. Cetdemans, D. Beyens and L.G. Vanquickenbome, Itto~K. Chem., 22 (1983) 1113. 8 LA. Broomhead, L. Kane-Maguire and D. Wilson, Inorg ckem., 14 (1975) 2575. 9 L. Lee, unpublished results, 1990. 10 BE. Mercer and R.R. Buckley, Inorg. Chem., 4 (196.5) 1692. 11 K.J. Moore, Ph.D. Diwertation, Clemson University, Clemsen, SC, 1983. 12 C.K. Poon and CM. Che, J. Chem. Sot., Dalton Truer., (1980) 756. 13 C.G. Hatchard and C.A. Parker, Proe. R Sot. London, Ser. A, 235 (1956) 518. 14 A.W. Adamson and P.D. Fleischauer, Conceptr of Inorganic Photochem&y, Wiley, New York, 1975, Chapter 2. 15 D. Katakis and G. Gordon, Mechanirmr of InorganicReacrions, Wiley, New York, 1987, Chapter 4. 16 H. Nagao, H. Nishimura, H. Funato, Y. Icbikawa, F.S. Howell, M. Mukaida and H. Kakihana, Inorg. C&m, 28 (1989) 3955. 17 KF. Purcell and J.C. Katz, Inorganic C/rem&y, Saunders, Philadelphia, PA, 1977, Chapter 9. 18 E. Larsen and G.N. LaMar, 1. Chcm. Educ., 51 (1974) 633. 19 L. Lee, SF. Clark and J.D. Petersen, fnorg Chem., 24 (1985) 3558.