Photoreactivity of the ruthenium nitrosyl complex, Ru(salen)(Cl)(NO). Solvent effects on the back reaction of NO with the Lewis acid Ru(III)(salen)(Cl) [1]

Full text of DOI:10.1021/ja000137p

7592
J. Am. Chem. Soc. 2000, 122, 7592-7593
Communications to the Editor
Photoreactivity of the Ruthenium Nitrosyl Complex,
Ru(salen)(Cl)(NO). Solvent Effects on the Back
Reaction of NO with the Lewis Acid RuIII(salen)(Cl)1
Carmen F. Works and Peter C. Ford*
Department of Chemistry and Biochemistry
UniVersity of California, Santa Barbara
Santa Barbara, California 93106
ReceiVed January 11, 2000
ReVised Manuscript ReceiVed June 19, 2000
Nitric oxide has been broadly established as having important
roles in mammalian biology as a bioregulatory molecule and as
a toxic agent produced in immune response to pathogen invasion.2
Furthermore, numerous disease states have been coupled to the
over- or under-production of NO.3 In this context, ongoing studies
here have been concerned with the preparation and mechanistic
evaluation of compounds potentially usable for photochemically
activated delivery of NO to specific physiological targets.4 Among
such compounds have been various metal nitrosyl complexes,
including those of ruthenium porphyrins5 and ammines,6 which
are thermally stable yet are photochemically active toward NO
release.7 In this context, we have begun to explore a different
synthetic platform for ruthenium nitrosyls, namely, the salen-type
complexes Ru(R-salen)(X)(NO) where R-salen is a derivative of
the N,N′-bis(salicylidene)ethylenediamine dianion. Additional
interest in the nitrosyl ruthenium salen complexes derives from
recent demonstrations that these are precursors of oxene and
carbene transfer catalysts for the asymmetric epoxidations and
cyclopropanations of alkenes as well as of Lewis acid catalysts
for asymmetric hetero-Diels-Alder reactions.8,9 These catalysts
are reportedly activated by light,8 so quantitative evaluation of
their photochemical properties has multidimensional interest.
Described here is the photoreactivity of a representative member
of this family, Ru(salen)(Cl)(NO) (1, chloro(nitrosyl)(N,N′-bis-
(salicylidene)ethylenediaminato)ruthenium(II)) in various media.
These results demonstrate the NO is photolabilized (reversibly)
to give solvento ruthenium(III) analogues and provide a logical
model to explain the photochemical activation and solvent-
dependent reactivities of the catalysts noted above.
Figure 1. Temporal spectral changes following the reaction of photo-
chemically generated Ru(salen)(Cl)(Sol) (2) with NO (9.2 mM) to
regenerate Ru(salen)(Cl)(NO) (1) in THF. The spectra were recorded at
intervals of 0, 150, 300, 450, 750, 1050, and 4380 s. The first of these (0
s), that with the highest absorbances at the λmax indicated by the arrows,
was recorded immediately after terminating the photolysis. Inset: Plot
of kobs vs [NO] for the reaction of 2 with NO in THF solution at 23 °C.
The slope (kNO) ) 0.22 M-1 s-1 under these conditions.
with 365 nm light, a new species was formed with UV-vis bands
at 354 (ꢀ ) 1.1 × 104 M-1 cm-1), 376 (1.2 × 104), 404 (1.4 ×
104), and 516 nm (1.8 × 103) (isosbestic point at 325 nm). In
addition, a broad band centered at 646 nm (3.3 × 103), assigned
as a ligand (phenoxo) to metal (RuIII) charge transfer (LMCT)
band characteristic of Ru(III) salen complexes,11 was also seen.
The spectrum of the photoproduct and its decay back to that of
1 are shown in Figure 1. Similar spectral changes were seen with
deaerated solutions of 1 (5 × 10-5 M) in acetonitrile and CH2Cl2
as well as in aerated toluene (the air was present to trap the NO
generated). The IR spectra of product solutions showed disap-
pearance of the νNO band and shifts to lower frequencies of the
salen bands consistent with NO photodissociation. Neither
chloride nor chlorine atom photodissociation would explain the
(4) (a) Ford, P. C.; Bourassa, J.; Miranda, K.; Lee, B.; Lorkovic, I.; Boggs,
S.; Kudo, S.; Laverman, L. Coord. Chem. ReV. 1998, 171, 185-202. (b)
Bourassa, J.; DeGraff, W.; Kudo, S.; Wink, D. A.; Mitchell, J. B.; Ford, P. C.
J. Am. Chem. Soc. 1997, 119, 2853-2860.
(5) Miranda, K. M.; Bu, X.; Lorkovic, I.; Ford, P. C. Inorg. Chem. 1997,
36, 4838-4848.
(6) Boggs, S., Ph.D. Dissertation, University of California, Santa Barbara,
1996.
(7) Lorkovic, I. M.; Miranda, K. M.; Lee, B.; Bernhard, S.; Schoonover,
J. R.; Ford, P. C. J. Am. Chem. Soc. 1998, 120, 116474-11683.
(8) (a) Takede, T.; Irie, R.; Shinoda, Y.; Katsuki, T. Synlett 1999, 7, 1157-
1159. (b) Mihara, J.; Hamada, T.; Takede, T.; Irie, R.; Katsuki, T.; Synlett
1999, 7, 1160-1162. (c) Uchida, T.; Irie, R.; Katsuki, T. Synlett 1999, 7,
1163-1165.
The optical spectrum of Ru(salen)(Cl)(NO)10 displays a band
at λmax 384 nm (ꢀ ) 6.3 × 103 M-1 cm-1) in tetrahydrofuran,
and the FTIR spectrum shows a NO stretch at νNO ) 1844 cm-1
(dichloromethane). When a solution of 1 in THF was irradiated
(9) Odenkirk, W.; Rheingold, A. L.; Bosnich, B. J. Am. Chem. Soc. 1992,
114, 6392-6398.
(1) Reported in part at the 217th National Meeting of the American
Chemical Society, Anaheim, CA, March 1999, INORG 164.
(10) 1 was prepared in accordance with a previously published procedure.9
1
(2) (a) Moncada, S.; Palmer, R. M. J.; Higgs, E. A. Pharmacol. ReV. 1991,
43, 109-142. (b) Feldman, P. L.; Griffith, O. W.; Stuehr, D. J. Chem. Eng.
News 1993, 71, 10, 26-38. (c) Wink, D. A.; Hanbauer, I.; Grisham, M. B.;
Laval, F.; Nims, R. W.; Laval, J.; Cook, J.; Pacelli, R.; Liebmann, J.; Krishna,
M.; Ford, P. C.; Mitchell, J. B. Curr. Top. Cell. Regul. 1996, 34, 159-187.
(d) Feelish, M.; Stamler, J. S., Eds. Methods in Nitric Oxide Research: John
Wiley and Sons: Chichester, England, 1996, and references therein.
(3) E.g.: (a) Wiseman, H.; Halliwell, B. Biochem. J. 1996, 313, 17-29.
(b) Wink, D. A.; Vodovotz, Y.; Laval, J.; Laval, F.; Dewhirst, M. W.; Mitchell,
J. B. Carcinogenesis 1998, 19, 711-721
The compound was characterized by H NMR, UV-vis, IR, and FAB mass
spectrometry.
(11) Leung, W. H.; Che, C. M. Inorg. Chem. 1989, 28, 4619-4622.
(12) Photodissociation of Cl- would give [Ru(salen)(sol)(NO)]+ with a νNO
band and electronic spectrum analogous to those of 1. Photodissociation of
Cl. would be accompanied by metal center reduction; in contrast the observed
LMCT band indicates oxidation to Ru(III). Preliminary studies show that
photolysis of the cationic complex Ru(salen)(H2O)(NO)+ in aqueous solution
gives spectral changes analogous to those reported here, again consistent with
+
NO labilization to give Ru(salen)(H2O)2
.
10.1021/ja000137p CCC: $19.00 © 2000 American Chemical Society
Published on Web 07/21/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 31, 2000 7593
spectral changes12,13 and both processes are inconsistent with the
NO-dependent kinetics for regeneration of 1 described below.
Thus, we conclude that the photoreaction is represented by eq 1.
The product spectra, especially the LMCT bands, are solvent
dependent consistent with the formulation of this species as
Ru(salen)(Cl)(Sol) (2, Sol ) solvent).
(1)
To confirm photodissociation of NO as shown in eq 1, the
photolysis of 1 in acetonitrile was carried out in the presence of
added Co(TPP) (TPP ) tetraphenylporphyrin). Co(TPP) has a
Soret band at λmax 412 nm and reacts rapidly with NO to give
Co(TPP)(NO) with a λmax at 432 nm. Accordingly, when this
solution was irradiated at 365 nm, the Ru(III) photoproduct bands
at ∼700 nm grew as did the 432 nm absorbance of Co(TPP)-
(NO).14
Quantum yields for eq 1 were determined from spectral changes
in aerated acetonitrile to be 0.13 ( 0.01 and 0.09 ( 0.01 for the
irradiation wavelengths λirr ) 365 and 436 nm, respectively.13
The photoreactivity of 1 in other solvents was comparable, but
quantitative measurements in some cases were complicated by
fast back reactions.
The kinetics for the back reaction (eq 2) were studied by
photochemical generation of 2 under excess NO and following
the reformation of 1 according to changes in the optical spectra.
In donor solvents, the reaction was sufficiently slow to follow
using conventional spectrometers; however, in toluene it was
necessary to use laser flash photolysis techniques (Figure 2).13
The transient spectrum recorded 300 ns after 355 nm flash
photolysis in that solvent clearly shows formation of 2. In each
solvent, the back reaction kinetics are first order in 2, and kobs is
linearly dependent on [NO] (eq 3).
Figure 2. NO concentration dependence of the kobs values for the reaction
of 2 with NO in toluene solution at 25 °C after 355 nm flash photolysis
of 1 (kNO ) 3.7 × 107 M-1 s-1). Inset: Transient spectrum of 2 recorded
300 ns after the flash using a CCD camera (500 ns gate width) under
649 Torr of NO.
The second-order rate constants obtained from these experi-
ments are as follows: (3.7 ( 0.4) × 107 M-1 s-1 in toluene, 6.4
( 0.6 M-1 s-1 in CH2Cl2, 0.25 ( 0.05 M-1 s-1 in THF,15 and
∼5 × 10-4 M-1 s-1 in acetonitrile (25 °C). The NO dependence
of the back reaction, the lack of a NO band in the IR, and the
formation of a Ru(III) species are consistent with the photoreaction
depicted in eq 1. The dramatic variation of rate constants measured
in different solvents strongly supports the proposed formation of
a solvento species after the loss of NO. These observations also
suggest a dissociative or dissociative interchange mechanism for
eq 2.
In summary, the ruthenium nitrosyl complex Ru(salen)(Cl)-
(NO) 1 undergoes NO labilization upon near-UV irradiation to
give the solvento species Ru(salen)(Cl)(Sol). Thus, this system
has the potential to serve as a precursor for photochemical NO
delivery to various targets, although (ongoing) structural modi-
fications to tune spectroscopic and solubility properties are in
order. Furthermore, these studies indicate that the Lewis acid,
oxene transfer, etc. catalysts formed upon photoactivation of
Ru(R-salen)(X)(NO) precursors are the solvento species analogous
to 2. The back reaction to reform 1 from 2 and excess NO varies
by 11 orders of magnitude for the solvents probed, and this
observation suggests that ligand substitution occurs by a dis-
sociative or dissociative interchange mechanism.
kNO
Ru(salen)(Cl)(Sol) + NO 8 Ru(salen)(Cl)(NO) + Sol
(2)
d[1]/dt ) -d[2]/dt ) kNO[NO][2]
(3)
(13) (a) Flash and continuous photolysis experiments were carried out in
solutions deaerated by 3 freeze-pump-thaw cycles except for continuous
photolysis experiments in cyclohexane where the oxidation of NO by O2 from
air was used to make that reaction irreversible for spectral analysis. (b) Light
intensities for continuous photolysis experiments were determined using
ferrioxalate actinometry according to: Calvert, J. G.; Pitts, J. N. Photochem-
istry; J. Wiley and Sons: New York, 1966. (c) Flash experiments used time-
resolved optical detection techniques described in ref 7.
Acknowledgment. This work was supported by the National Science
Foundation (CHE 9726889). We thank Leroy Laverman and Steve
Massick for help with flash photolysis experiments and Ivan Lorkovic′
for valuable discussions.
JA000137P
(14) Since both Co(TPP) and Co(TPP)NO absorb strongly in the photolysis
wavelength regions, this experiment confirms NO formation but only
qualitatively. We are currently working on reliable protocols for quantitative
determination of very low NO concentrations in nonaqueous media.
(15) (a) The kNO in THF experiment was estimated from the assumption
that the solubility of NO in 25 °C THF is approximately that of CO (0.011
mol L-1 atm-1).14b (b) Payne, M. W.; Leussing, L. L.; Shore, S. G. J. Am.
Chem. Soc. 1987, 109, 617-618.