J. Phys. Chem. 1996, 100, 13569-13574
13569
Photochemistry of Nitrosyl Metal Complexes. Laser Photolysis Studies of Nitrosylcobalt Schiff Bases in Ethanol and Toluene in the Temperature Range 160-300 K Mikio Hoshino,* Reiko Konishi, Norichika Tezuka, Ikuko Ueno, and Hiroshi Seki The Institute of Physical and Chemical Research, Wako, Saitama 351-01, Japan ReceiVed: March 6, 1996; In Final Form: May 21, 1996X
(N,N′-Disalicylideneethylenediaminato)cobalt(II), CoIIsalen, and (N,N′-bis(1-methyl-3-oxobutylidene)ethylenediaminato)cobalt(II), CoIIacacen, in both ethanol and toluene react with nitric oxide to give the nitric oxide adducts, (NO)CoIIsalen and (NO)CoIIacacen. The laser photolysis studies revealed that (NO)CoIIsalen photodissociates NO to yield CoIIsalen with the quantum yields 1.0 ( 0.1 in ethanol and 0.6 ( 0.1 in toluene at 300 K. The transient product, CoIIsalen, undergoes the recombination reaction with NO to regenerate (NO)CoIIsalen with the rate constant 1.1 × 109 M-1 s-1. The quantum yields, φ, for photodissociation of NO from (NO)CoIIacacen are obtained as φ ) 1.0 ( 0.1 and φ < 0.01 in toluene and ethanol, respectively. The quantum yields φ for both (NO)CoIIsalen and (NO)CoIIacacen are found to be markedly dependent on temperature: the yield decreases with a decrease in temperature. The photodissociation mechanism of NO from (NO)CoIIsalen and (NO)CoIIacacen is discussed on the basis of the laser photolysis studies carried out in the temperature range 160-300 K. Bunsen absorption coefficients of NO in ethanol and toluene at 300 K were determined from kinetic analysis of the recombination reaction between NO and cobalt(II) Schiff bases.
Introduction Metal complexes interact with simple diatomic molecules, O2, CO, and NO, resulting in the formation of their adducts.1-10 In particular, the interaction between O2 and iron(II) metalloporphyrin has been the subject of extensive studies for elucidation of the biological functions of hemoglobin and myoglobin in vivo.11-14 In the latter half of the 1980’s, NO was found to be produced in vivo by the enzymatic oxidation of arguinine.15,16 It has been revealed that NO plays important physiological roles as a vascular regulator,17,18 in neuronal communication,19,20 and in nonspecific defense against bacterial infection.21,22 NO molecules produced in vivo are expected to react with hemoproteins and metalloenzymes. In fact, the enzymatic reaction of catalase23 has been found to be inhibited by NO. Furthermore, the formation of the nitrosylhemoproteins would relate to transportation of NO in vivo. With this background, we have studied the photochemistry of nitrosylhemoproteins and NO adducts of synthetic metalloporphyrins.24,25 Recently, a light-sensitive nitrile hydratase from Rhodococcus sp. N-771 was found to have an NO molecule bound to the nonheme iron center.26 This enzyme is activated upon irradiation followed by release of NO from the iron center. These findings suggest that the interaction between nonheme metal complexes and NO would become an important subject of further studies for the understanding of the biological roles of NO in life. In the present study, we have carried out laser photolysis studies of nitric oxide adducts of nonheme metal complexes, cobalt(II) Schiff bases, in the temperature range 160-300 K. The results are compared with those of nitrosylmetalloporphyrins. Experimental Section Reagent grade salicylic acid, acetyl acetone, and ethylenediamine were supplied from Wako Pure Chem. Ind. (N,NX
Abstract published in AdVance ACS Abstracts, July 1, 1996.
S0022-3654(96)00687-9 CCC: $12.00
disalicylideneethylenediaminato)cobalt(II), CoIIsalen, was synthesized from salicylic acid, ethylenediamine, and cobalt(II) acetate according to the literature.27 The crude CoIIsalen obtained was purified three times by recrystallization from benzene solutions. (N,N′-bis(1-methyl-3-oxobutylideneethylenediaminato)cobalt(II), CoIIacacen, was synthesized from bis(acetylacetoneethylenediamine) and cobalt(II) acetate.28,29 Purification of CoIIacacen was made by recrystallization from benzene solutions. Reagent grade toluene and ethanol from Wako Pure Chem. Ind. were used as solvents for laser photolysis without further purification. NO gas (99.9%) was purchased from Takachiho Chem. Co. Ltd. Absorption spectra were recorded on a Hitachi 330 spectrophotometer. The laser photolysis studies were carried out with the use of a Nd:YAG laser (HY 500 from JK Lasers Ltd.) equipped with second (532 nm), third (355 nm), and fourth (266 nm) harmonic generators. Duration of a laser pulse is ca. 20 ns. The detection system of the transient absorption spectra was described in an earlier paper.30 The temperature of the sample solutions for laser photolysis was controlled with a cryostat (Model DN1007 from Oxford Instruments). Nitric oxide adducts31 were prepared by introduction of NO gas into the solutions of CoIIsalen and CoIIacacen on a vacuum line. NO pressures were measured by a mercury manometer. Results Absorption Spectra. Figure 1 shows the absorption spectra of CoIIsalen and the nitric oxide adduct of CoIIsalen, (NO)CoIIsalen, in ethanol. The spectrum of CoIIsalen exhibits absorption peaks at 343 and 403 nm with an absorption shoulder around 480 nm. The molar absorption coefficient, , was obtained as (9.15 ( 0.05) × 103 M-1 cm-1 at 403 nm. When the ethanol solution of CoIIsalen is exposed to NO gas at 200 Torr, a new absorption spectrum with a peak at 390 nm was obtained. Since no recovery of CoIIsalen is observed after removal of excess NO, (NO)CoIIsalen is concluded to be irreversibly produced.31 The molar absorption coefficient of (NO)CoIIsalen in ethanol was determined as (6.12 ( 0.05) × 103 M-1 cm-1 at 390 nm. © 1996 American Chemical Society
13570 J. Phys. Chem., Vol. 100, No. 32, 1996
Hoshino et al.
Figure 1. Absorption spectra of (A) CoIIsalen and (B) (NO)CoIIsalen at PNO ) 25 Torr in ethanol at 300 K.
Figure 3. Plot of pseudo-first-order rate constant, k, for the decay of the transient Co(II)salen, represented as a function of NO pressures, PNO.
after 355 nm laser pulsing was identical with that measured before pulsing. Thus, the decay of the transient is explained by regeneration of (NO)CoIIsalen according to eq 2. kNO(1)
CoIIsalen + NO 98 (NO)CoIIsalen
Figure 2. Transient absorption spectrum observed for an ethanol solution of 7.0 × 10-5 M (NO)Co(II)salen at 20 ns after 355 nm laser pulsing.
The absorption spectra of both CoIIsalen and (NO)CoIIsalen in toluene are slightly red-shifted compared with those in ethanol owing to the effects of solvent: the absorption peaks of CoIIsalen are located at 353 and 415 nm ( ) (1.07 ( 0.05) × 104 M-1 cm-1 ) and those of (NO)CoIIsalen at 310 and 405 nm ( ) (4.94 ( 0.05) × 103 M-1 cm-1). The spectrum of CoIIacacen in ethanol has absorption peaks at 293 and 333 nm with a long absorption tail toward 550 nm. When the solution was exposed to NO, new absorption peaks appear at 247 nm owing to the formation of (NO)CoIIacacen. In toluene solution, CoIIacacen has absorption peaks at 381 ( ) (4.18 ( 0.05) × 103 M-1 cm-1) and 337 nm. The spectrum of (NO)CoIIacacen shows an absorption shoulder around 340 nm with a broad absorption band centered at 480 nm. The molar absorption coefficient of (NO)CoIIacacen was obtained as (1.42 ( 0.05) × 103 M-1 cm-1 at 390 nm. Laser Photolysis. Figure 2 shows the transient absorption spectrum observed with 355 nm laser photolysis for (NO)CoIIsalen in ethanol at an NO pressure of 25 Torr. Since the spectrum is in good accord with the difference spectrum obtained by subtracting the spectrum of (NO)CoIIsalen from that of CoIIsalen in ethanol, the photochemical reaction of (NO)CoIIsalen is expressed as
(NO)CoIIsalen + hν f CoIIsalen + NO
(1)
The transient spectrum decays over the whole wavelength region studied with the pseudo-first-order rate constant of 9 × 105 s-1. The absorption spectrum of (NO)CoIIsalen in ethanol observed
(2)
Figure 3 shows the pseudo-first-order rate constant, k, for regeneration of (NO)CoIIsalen, represented as a function of NO pressure, PNO, at 300 K. The plot of k vs PNO gives a straight line. From the slope of the line, the bimolecular rate constant, kNO(1) in eq 1 is determined as kNO(1) ) (3.5 ( 0.1) × 104 Torr-1 s-1. Laser photolysis of (NO)CoIIsalen in ethanol was carried out in the absence of excess NO. The transient, CoIIsalen, decays according to second-order kinetics. The bimolecular rate constant, kbi(1), is determined as (1.51 ( 0.05) × 109 M-1 s-1. On the assumption that Henry’s law holds in the NO pressure range studied, the relationship between kNO(1) and kbi(1) is expressed by
kNO(1)PNO ) kbi(1)[NO]
(3)
where PNO is in units of Torr. The Bunsen absorption coefficient, β, of NO in ethanol is given by
β ) 760 × 22.4 × 103× [kNO(1)/kbi(1)] × 10-3
(4)
With the use of eq 4 and the values for kNO(1) and kbi(1) obtained above, the value of β in ethanol is calculated as 0.39 ( 0.05. Figure 4 shows the transient absorption spectra observed for (NO)CoIIsalen in ethanol without excess NO at 280 K after 355 nm laser pulsing. The spectrum observed at 20 ns after pulsing agrees well with the difference spectrum (CoIIsalen - (NO)CoIIsalen). At 25 µs after a pulse, CoIIsalen completely disappears, leaving a long-lived species having a small negative absorption around 395 nm. The species decays according to first-order kinetics with a rate constant 8.3 × 103 s-1, returning to (NO)CoIIsalen. The same species is found to be produced by photolysis of (NO)CoIIsalen in ethanol below 280 K. However, no detection of such species was made by the laser photolysis of (NO)CoIIsalen at 300 K probably because of its short lifetime at elevated temperatures. From analysis of the transient spectrum, the new transient species is considered to have an absorption spectrum similar to that of (NO)CoIIsalen. It is therefore suggested that the new species is a nitric oxide
Nitrosyl Metal Complexes
J. Phys. Chem., Vol. 100, No. 32, 1996 13571 the absorption spectrum of (NO)CoIIacacen in toluene was observed before and after photolysis. Thus, the decay of the transient is explained by regeneration of (NO)CoIIacacen. kNO(2)
CoIIacacen + NO 98 (NO)CoIIacacen
Figure 4. Transient absorption spectra observed for an ethanol solution of 7.0 × 10-5 M (NO)Co(II)salen at (A) 20 ns and (B) 25 µs after 355 nm laser pulsing at 280 K.
Figure 5. Transient absorption spectrum observed for a toluene solution of 8.0 × 10-4 M (NO)Co(II)acacen at PNO ) 20 Torr at 300 K.
adduct of CoIIsalen with a structure slightly different from that of (NO)CoIIsalen. A toluene solution of (NO)CoIIsalen also affords CoIIsalen as the transient by 355 nm laser photolysis. The bimolecular rate constant, kbi(1), for the regeneration of (NO)CoIIsalen was determined as (1.1 ( 0.1) × 109 M-1 s-1 from the secondorder decay of CoIIsalen detected in the absence of excess NO. The plot of the pseudo-first-order decay rate constant, k, vs PNO gives a straight line with an intercept at the origin. The slope gives a bimolecular rate constant kNO(1) between NO and CoIIsalen as (1.54 ( 0.05) × 104 Torr-1 s-1. From this value and kbi(1), the Bunsen absorption coefficient of NO in toluene is determined as 0.24 ( 0.02. The transient spectrum detected in the absence of excess NO uniformly decays in the whole wavelength region studied in the temperature range 300-240 K. In contrast to the ethanol solution, the long-lived transient could not be observed after the decay of CoIIsalen even below 280 K. Figure 5 shows the transient absorption spectrum observed for (NO)CoIIacacen in toluene at PNO ) 20 Torr at 20 ns after 355 nm laser pulsing . The spectrum is almost identical with the difference spectrum (CoIIacacen - (NO)CoIIacacen), indicating that the photodissociation of NO occurs from (NO)CoIIacacen in toluene.
(NO)CoIIacacen + hν f CoIIacacen + NO
(5)
The transient decays with a pseudo-first-order rate constant of 4.0 × 106 s-1 according to first-order kinetics. No change in
(6)
Laser photolysis studies of (NO)CoIIacacen in toluene without excess NO confirmed that CoIIacacen and NO undergo the recombination reaction with the second-order rate constant kbi(2) ) (1.7 ( 0.1) × 109 M-1 s-1. The pseudo-first-order rate constant, k, for nitrosylation of CoIIacacen was measured as a function of NO pressure, PNO. The plot of k vs PNO gave a straight line with an intercept at the origin. From the slope of the line, the bimolecular rate constant kNO(2) was obtained as (2.4 ( 0.1) × 104 Torr-1 s-1. With the use of kbi(2) and kNO(2) mentioned above, the Bunsen absorption coefficient of NO solubility in toluene is calculated as 0.24 ( 0.02, in good agreement with that measured by the laser photolysis of (NO)CoIIsalen in toluene. The transient spectra for the toluene solution of (NO)CoIIacacen were measured by 355 nm laser photolysis in the temperature range 300-200 K. The species detected was solely CoIIacacen. Laser photolysis of (NO)CoIIacacen in ethanol gave results markedly different from those in toluene. The transient observed after laser pulsing has a very small absorption and completely decays within 2 µs. Since the decay behavior is independent of NO pressure, the transient is not ascribed to CoIIacacen. Furthermore, the transient spectrum is different from the difference spectrum (CoIIacacen - (NO)CoIIacacen). It is therefore concluded that photodissociation of NO from (NO)CoIIacacen hardly occurs in ethanol solutions. Quantum Yield Measurements. The quantum yield for the photodissociation of NO from nitrosyl cobalt(II) Schiff bases was obtained with the use of the laser photolysis method.24,25,32 When a molecule A undergoes photoreaction to yield a molecule B, the quantum yield φ for the formation of B is expressed as
φ ) ∆D(λ)/(∆ABIabs/NA)
(7)
where ∆D(λ) is the absorbance change at a wavelength λ observed after laser pulsing, ∆AB is the difference in the molar absorption coefficient between molecules A and B at λ, Iabs is the number of photons absorbed by molecule A, and NA is the Avogadro number. Iabs can be determined with the use of a benzene solution of benzophenone. When the benzene solution has the same absorbance as that of the solution of molecule A at the wavelength of laser light, the yield φT of the triplet benzophenone is given by
φT ) DT/(TIabs/NA)
(8)
Here, DT and T are, respectively, the absorbance of the triplet benzophenone measured after laser pulsing and the molar absorption coefficient of the triplet benzophenone at 530 nm. From eqs 7 and 8, eq 9 is derived.
φ ) ∆D(λ)TφT/DT∆AB
(9)
For benzophenone in benzene, φT and T have already been determined as 1.0 and 7.6 × 103 M-1 cm-1 at 530 nm, respectively.33,34 The quantum yields, φNO, for photodissociation of NO from (NO)CoIIsalen in ethanol and toluene were determined by the
13572 J. Phys. Chem., Vol. 100, No. 32, 1996
Hoshino et al. plot of ln Γ vs 1/T gave knr/kr0 ) 3.1 × 10-5 and ∆Er ) 3.9 kcal mol-1 for the toluene solution. In the case of (NO)CoIIacacen in toluene, we obtained φi ) 1.0, knr/kr0 ) 8.43 × 10-4 and ∆Er ) 2.1 kcal mol-1. The bimolecular rate constants, kbi, for the recombination reaction between NO and the cobalt(II) Schiff bases were measured in the temperature range 300-200 K. The rate constants decrease with the decrease in temperature, suggesting that kbi follows the Arrhenius expression.
kbi ) kbi0 exp(-∆Ebi/RT)
Figure 6. Quantum yields, φ, for photodissociation of NO from (NO)Co(II)salen in ethanol, represented as a function of temperature. The solid line is the calculated values of φ with the use of eq 13, φi ) 1.0, knr/kr0 ) 4.0 × 10-5, and ∆Er ) 4.0 kcal mol-1 (see text).
measurements of the absorbance changes at 410 nm after 355 nm laser pulsing. The yields are 1.0 ( 0.1 in ethanol and 0.6 ( 0.1 in toluene. A marked solvent effect is found for the quantum yields of NO photodissociation from (NO)CoIIacacen. The yields were obtained as φNO < 0.01 in ethanol and φNO ) 1.0 ( 0.1 in toluene. Photodissociation of NO from nitrosyl complexes is suppressed at low temperatures. Figure 6 shows the quantum yields for photodissociation of NO from (NO)CoIIsalen in ethanol, represented as a function of temperature. The reaction scheme for photodissociation is shown by
(NO)CoIIsalen + hν f **(NO)CoIIsalen kr
**(NO)CoIIsalen 98 CoIIsalen + NO knr
**(NO)CoIIsalen 98 (NO)CoIIsalen
(10) (11) (12)
Here, **(NO)CoIIsalen is an intermediate from which NO dissociation occurs. The quantum yield, φNO, for photodissociation of NO is expressed as
φNO)φikr(kr+ knr)-1
(13)
Here, φi is the quantum yield for the formation of **(NO)CoIIsalen and kr and knr are, respectively, the rate constants for NO dissociation and nonradiative transition at **(NO)CoIIsalen. The nature of **(NO)CoIIsalen will be discussed later. On the assumption that eq 11 is an activation process, kr is formulated as
kr ) kr0 exp(-∆Er/RT)
(14)
Equations 13 and 14 provide
Γ ) φi/φNO - 1 ) (knr/kr0) exp(∆Er/RT)
(15)
As mentioned above, φNO ) 1.0 at 300 K. Thus, φi ) 1.0 is safely assumed for (NO)CoIIsalen. The plot of ln Γ vs 1/T gives a straight line. The slope and the intercept of the line give knr/ kr0 ) 4.0 × 10-5 and ∆Er ) 4.0 kcal mol-1. For (NO)CoIIsalen in toluene, φNO increases with the increase in temperature and levels off around 260 K; φNO ) 0.6 in the region 260-300 K. From this result, φi ) 0.6 is assumed. The
(16)
The plots of ln(kbi(1)) and ln(kbi(2)) vs 1/T gave good straight lines in the temperature range studied. From the slopes and the intercepts of the lines, the activation energies and the preexponential factors are obtained. The Arrhenius expressions of kbi(1) for (NO)CoIIsalen is given by
kbi(1) ) 1.16 × 1012 exp(-2.8 × 103/T) M-1s-1
(17)
in ethanol and
kbi(1) ) 2.11 × 1011 exp(-2.4 × 103/T) M-1s-1
(18)
in toluene. The bimolecular rate constant, kbi(2), for (NO)CoIIacacen in toluene is expressed as
kbi(2) ) 1.10 × 1010 exp(-1.02 × 103/T) M-1s-1
(19)
In Table 1 are summarized φNO and kbi at 300 K, knr/kr0, ∆Er, kbi and ∆Ebi obtained for (NO)CoIIsalen and (NO)CoIIacacen in both ethanol and toluene. 0,
Discussion Cobalt(II) Schiff bases have been well recognized to form the dioxygen complexes. Because of the importance of these complexes as a model of mono-oxygenases and dioxygenases, the oxidation of organic molecules with the use of the dioxygen complexes has been extensively investigated.35-37 In relation to the chemical reactivity, the electronic structure of the dioxygen complexes has been studied by ESR spectroscopy; the unpaired electron is principally located at the dioxygen group of the dioxygen complexes.38-40 In contrast to the dioxygen complexes, the NO complexes of cobalt(II) Schiff bases have received less attention. However, as mentioned above, the studies of the interaction between NO and nonheme metal complexes would become a significantly important subject for further studies of NO chemistry and biology. CoIIsalen and CoIIacacen are found to form their nitrosyl complexes. The chemical bond NO-Co is made of the dz2 orbital of the cobalt atom and the π* orbital of the NO molecule. The ESR measurements confirmed that (NO)CoIIsalen and (NO)CoIIacacen are diamagnetic. The NO-Co bonds of both (NO)CoIIsalen and (NO)CoIIacacen are stable in a degassed solution. No thermal dissociation of the bonds has been observed. The laser photolysis studies revealed that both (NO)CoIIsalen and (NO)CoIIacacen photodissociate NO to yield CoIIsalen and CoIIacacen, respectively. The transients, CoIIsalen and CoIIacacen, observed in the absence of excess NO decay according to second-order kinetics. In the presence of excess NO, the decay of the transient follows first-order kinetics. The pseudofirst-order rate constant was found to increase linearly with an increase in NO pressure in the range 10-180 Torr. These
Nitrosyl Metal Complexes
J. Phys. Chem., Vol. 100, No. 32, 1996 13573
TABLE 1: Kinetic Parameter for Photodissociation of NO IIsalen
(ethanol) Co CoIIsalen (toluene) CoIIacencen (ethanol) CoIIacencen (toluene)
∆Er (kcal/mol)
kbi° (M-1 s-1)
∆Ebi (kcal/mol)
4.0 × 3.1 × 10-5
4.0 3.9
1012
1.16 × 2.11 × 1011
2.8 2.4
8.4 × 10-4
2.1
1.10 × 1010
1.0
ΦNO
kNO at 300K (M-1 s-1)
knr/kr0
1.1 ( 0.1 0.6 ( 0.1
