Laser flash photolysis study of Jacobsen catalyst and related manganese(III) salen complexes. Relevance to catalysis

Article abstract of DOI:10.1021/ja003371o

The laser flash photolysis and emission properties of a set of five-coordinate manganese(III) Schiff-base complexes have been examined. In contrast to the intramolecular electron transfer between Mn³⁺ and the equatorial salen ligand reported to occur in the absence of axial coordination, our laser flash photolysis study has shown that the reactivity of the respective excited states is appreciably influenced by the electron donor strength of the apical ligand at the metal center. In fact, homolytic and heterolytic photocleavage of the metal-ligand apical bond can be the most important processes upon laser excitation, their relative contribution being influenced by medium effects and the δ-charge donation of the axial ligand. On the other hand, the detection of reactive intermediates such as the oxomanganese(V) salen complex (λ_max 530 nm) by laser flash photolysis opens the way to apply this fast detection technique to the study of reaction mechanisms in catalysis by metallic complexes. As a matter of fact, quenching of oxomanganese(V) salen by simple alkenes has been observed by laser flash.

Full text of DOI:10.1021/ja003371o

7074
J. Am. Chem. Soc. 2001, 123, 7074-7080
Laser Flash Photolysis Study of Jacobsen Catalyst and Related
Manganese(III) Salen Complexes. Relevance to Catalysis
Mar´ıa J. Sabater,† Mercedes AÄ lvaro,† Hermenegildo Garc´ıa,*,† Emilio Palomares,† and
J. C. Scaiano*,‡
Contribution from the Departamento de Qu´ımica, Instituto de Tecnolog´ıa Qu´ımica UPV-CSIC,
UniVersidad Polite´cnica de Valencia, Apartado 22012, 46071 Valencia, Spain, and Department of
Chemistry, Centre for Catalysis Research and InnoVation, UniVersity of Ottawa,
Ottawa, Ontario, K1N 6N5, Canada
ReceiVed September 13, 2000. ReVised Manuscript ReceiVed April 23, 2001
Abstract: The laser flash photolysis and emission properties of a set of five-coordinate manganese(III) Schiff-
base complexes have been examined. In contrast to the intramolecular electron transfer between Mn3+ and the
equatorial salen ligand reported to occur in the absence of axial coordination, our laser flash photolysis study
has shown that the reactivity of the respective excited states is appreciably influenced by the electron donor
strength of the apical ligand at the metal center. In fact, homolytic and heterolytic photocleavage of the metal-
ligand apical bond can be the most important processes upon laser excitation, their relative contribution being
influenced by medium effects and the σ-charge donation of the axial ligand. On the other hand, the detection
of reactive intermediates such as the oxomanganese(V) salen complex (λmax 530 nm) by laser flash photolysis
opens the way to apply this fast detection technique to the study of reaction mechanisms in catalysis by metallic
complexes. As a matter of fact, quenching of oxomanganese(V) salen by simple alkenes has been observed by
laser flash.
Introduction
salen complexes. Thus, the photoproducts would be formed
through an initial intramolecular metal-ligand electron transfer
that can lead either to oxidative cyclization or to the dimerization
of the salen ligand.5,6
Manganese(III) salen complexes have been the subject of
intensive investigations over the past years, reflecting that they
are excellent catalysts for the chiral and achiral epoxidation of
alkenes.1,2 In sharp contrast, there is a paucity of reports dealing
with the photochemistry of manganese complexes in general,
and specifically the photochemical and photophysical properties
of the pentacoordinate manganese salen commonly used in
catalysis remain unexplored. From the limited information
available on related complexes, it has been rationalized that the
emission mechanism of Mn(II)-bipyridine complexes immobi-
lized in different zeolites involves a charge-transfer [Mn(III)-
bipyridine•-] excited state from which luminescence is ob-
served.3 Excited states of similar nature, generated upon
irradiation of chiral Mn(II) salen complexes within the cavities
of zeolite Y, are reported to be enantioselectively quenched by
chiral 2-butanols.4 In both cases, light is able to instantaneously
generate a reactive metallo fragment within the polar environ-
ment of the zeolite.
Interestingly, deactivation of electronic excited states of azido
pentacoordinated Mn(III) salen complexes occurs through a
redox alternate pathway in which only the metal and the apical
azido ligand are mechanistically implicated, while the robust
equatorial salen ligand remains unaltered.7 In fact, photolysis
of azidomanganese complexes is a general reaction route to
obtain salen-derived nitridomanganese(V) complexes by azide
photodenitrogenation and simultaneous oxidation of the metal.7
Chiral nitridomanganese(V) salen are also relevant complexes
in asymmetric catalysis because they can effect the enantiose-
lective aziridation of alkenes.
Taking into consideration the importance in catalysis of
pentacoordinate Mn(III) salen complexes, the present work
reports the first laser flash photolysis study of the transient UV-
vis absorption spectra of a series of salen-derived Mn(III)
complexes including the so-called Jacobsen catalyst. Assignment
of the photogenerated intermediates is mainly based on the
influence of the axial coordinating ligand around the Mn(III)
salen moiety on the transient absorption spectra.
Likewise, a product study on the photochemical decomposi-
tion of four-coordinate cationic Mn(III) Schiff base complexes
has been rationalized assuming a photochemical pathway closely
related to those mentioned above for the tetradentate Mn(II)
Results and Discussion
* To whom correspondence should be addresed.
† Universidad Polite´cnica de Valencia.
This study deals with the photochemistry of a set of five-
coordinate Mn(III) Schiff base complexes, namely Mn(III)-
‡ University of Ottawa.
(1) (a) Jacobsen, E. N.; Zhang, W.; Muci, A. R.; Ecker, J. R.; Deng, L.
J. Am. Chem. Soc. 1991, 113, 7063-7064. (b) Jacobsen, E. N.; Zhang, W.
J. Org. Chem. 1991, 56, 2296-2298.
(5) Fukuda, T.; Sakamoto, F.; Sato, M.; Nakano, Y.; Tan, X. S.; Fujii,
Y. Chem. Commun. 1998, 1391-1392.
(6) Garc´ıa-Deibe, A.; Sousa, A.; Bermejo, M. R.; Mac Rory, P. P.;
McAuliffe, C. A.; Pritchard, R. G.; Helliwell, M. J. Chem. Soc., Chem.
Commun. 1991, 728-729.
(7) (a) Du Bois, J.; Tomooka, C. S.; Hong, J.; Carreira, E. M. Acc. Chem.
Res. 1997, 30, 364-372. (b) Sriram, R.; Endicott, J. Inorg. Chem. 1977,
16, 2766-2772. (c) Forment´ın, P.; Folgado, J. V.; Forne´s, V.; Garc´ıa, H.;
Ma´rquez, F.; Sabater, M. J. J. Phys. Chem. B. 2000, 104, 8361-8365.
(2) Zhang, W.; Loebach, J. L.; Wilson, S. R.; Jacobsen, E. N. J. Am.
Chem. Soc. 1990, 112, 2801-2803.
(3) Knops-Gerrits, P. P. H. J. M.; De Schryver, F. C.; Van der Auweraer,
M.; Mingroot, H. V.; Li, X. Y.; Jacobs, P. A. Chem. Eur. J. 1996, 5, 592-
597.
(4) Sabater, M. J.; Garc´ıa, S.; AÄ lvaro, M.; Garc´ıa, H.; Scaiano, J. C. J.
Am. Chem. Soc. 1998, 120, 8521-8522.
10.1021/ja003371o CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/27/2001

Photolysis of Jacobsen Catalyst and Mn Complex
J. Am. Chem. Soc., Vol. 123, No. 29, 2001 7075
Scheme 1. Preparation of Five-Coordinate Salen-Derived Mn(III) Complexes with Different Axial Ligands
(salchd)Cl 1 [salchd ) 1,2-bis(salicylidenimino)cyclohexane],
Mn(III)(salchd)(4-phenylpyridine-N-oxide)ClO4 2, Mn(III)-
(salchd)CH3CO2 3, and Mn(III)(salchd)PhCH2CO2 4 (Scheme
1). Complex 1 was synthesized by adding Mn(CH3CO2)2 to
previously obtained salchd ligand followed by air oxidation in
the presence of chloride ions according to the general procedure
reported by Jacobsen for the tert-butyl analogues (Scheme 1).8
The new complexes 2-4 were prepared starting from complex
1 by precipitating the apical chloride ligand as AgCl and
replacing it by good coordinating anions such as 4-phenylpy-
ridine-N-oxide (4PPNO), acetate, and phenylacetate.9 It is worth
remarking that other alternate routes for the synthesis of
complexes 2-4 proved to be much less efficient. For example,
direct reaction of Mn(II) salen complex with 4PPNO, or with
the sodium salts of acetate and phenylacetate, led predominantly
to decomposition of the complexes into brown insoluble solids.
The complexes were isolated in high yields, recrystallized,
and characterized by analytical and spectroscopic techniques
(see Experimental Section). Thus, the infrared spectra of
complexes 1-4 showed prominent absorptions in the region
around 2900 and 2800 cm-1. This is due to ν(C-H) vibrations
of the alkyl groups and two characteristic bands around 1620
and 1540 cm-1 that were assigned to CdN and CsO stretching
vibrations, respectively.10 The sharp and intense absorption at
1094 cm-1 in the case of complex 2 is within the wavenumber
range expected for a ClO4- anion not coordinated to manganese
(reported strong Cl-O stretching absorption at 1085 cm-1).11
Thus, the most likely structure of complex 2 can be described
as a cationic five-coordinate complex, presumably square
pyramidal, where the N-oxide is axially coordinated to the metal,
and the ClO4- is acting as a noncoordinating counteranion. This
structure is analogous to those obtained by X-ray diffraction
for other related (salen)Mn(III) perchlorate complexes.12
The electronic spectra of the complexes were recorded in
dichloromethane solutions. The UV-vis spectra of complexes
1, 3, and 4 exhibited a broad and featureless absorption that
resembled that of the Jacobsen catalyst, with shoulders at 228,
255, and 318 nm (Figure 1). In striking contrast, complex 2
showed three well-defined bands with absorption maxima at
235, 305, and 420 nm, reflecting the stronger contribution of
the ligand-centered absorption bands to the global UV-vis
absorption spectrum of the complex (Figure 1, spectrum e).
Figure 1. UV-vis absorption spectra of 10-3 M dichloromethane
solutions of the Jacobsen catalyst (a) and complexes 1 (b), 3 (c), 4 (d),
and 2 (e).
The complexes were photolyzed in solution (∼10-4 M) using
the 266 nm output of a pulsed Nd:YAG laser (10 ns, energy/
pulse ∼14 mJ). Hence, laser flash excitation of a nitrogen-purged
dichloromethane solution of 1 or tert-butylated Jacobsen catalyst
did not lead to the observation of any transient. In contrast,
upon 266 nm excitation, the time-resolved changes in the UV-
vis spectrum of a deaerated dichloromethane solution of 2
(∼10-4 M) resulted in the prompt appearance of a transient
absorption band with maximum at 410 nm, which decayed
following a first-order kinetics (Figure 2). In agreement with
literature data for aromatic N-oxides, direct laser flash photolysis
of 4PPNO did not lead to the observation of any transient.13
This rules out the possibility that the observed transient spectrum
would correspond to a 4PPNO localized state. However, given
that intramolecular electron transfer between the metal center
and the ligand is a common process in the known photochem-
istry of metallic complexes, we explored the possibility that the
transient observed in the photolysis of complex 2 would
correspond to an N-oxyl radical generated through electron
transfer from the N-oxide to Mn(III).14
(8) Larrow, J. F.; Jacobsen, E. N. J. Org. Chem. 1994, 59, 1939-1942.
(9) Leighton, J. L.; Jacobsen, E. N. J. Org. Chem. 1996, 61, 389-390.
(10) De Vos, D. E.; Jacobs, P. A. In Proceedings of the 9th International
Zeolite Conference, Montreal; von Ballmoos, R., et al., Eds.; 1992; pp 615-
622.
(11) Hathaway, B. J.; Underhill, A. E. J. Chem. Soc. 1961, 3091-3094.
(12) Pospisil, P. J.; Carsten, D. H.; Jacobsen, E. N. Chem. Eur. J. 1996,
2 (8), 974-980.
To provide support to the assignment of Figure 2 as a 4PPNO•
radical, a photoinduced electron transfer from 4PPNO to the
(13) (a) Bucher, G.; Scaiano, J. C. J. Phys. Chem. 1994, 98, 12471-
12473. (b) Ogawa, Y.; Iwasaki, S.; Okuda, S. Tetrahedron Lett. 1981, 22,
2277-2280.
(14) Suslick, K. S.; Watson, R. A. New J. Chem. 1992, 16, 633-642.

7076 J. Am. Chem. Soc., Vol. 123, No. 29, 2001
Sabater et al.
Figure 2. Decay traces of (b) complex 2 in dichloromethane solution
under nitrogen monitored at 410 nm after 266 nm excitation and (0)
of a dichloromethane solution under nitrogen containing triphenylpy-
rilium tetrafluoroborate and 4-phenylpyridine-N-oxide monitored at the
same wavelength after 355 nm excitation. The inset shows the transient
spectrum recorded 2 µs after 266 nm laser-pulse excitation of 2 in
dichloromethane under nitrogen.
2,4,6-triphenylpyrylium (TP+) excited state was carried
out (eq 1).
Figure 3. Transient spectra recorded under nitrogen 0.3 µs after 266
nm laser-pulse excitation of complex 3 in dichloromethane (full dots)
and acetonitrile (open circles). The spectrum in dichloromethane has
been vertically shifted.
not obtained. In this regard, it should be noted that the typical
UV-vis absorption spectra of salen manganese complexes are
characterized by very broad bands with low extinction coef-
ficients (see for instance Figure 1, spectra a-d, and the
explanatory comments above). This type of spectra may be
difficult to detect by time-resolved techniques in the presence
of other transients having much more intense absorption bands.
Examples of photocleavages where only a single fragment is
spectroscopically detected are not uncommon in laser flash
photolysis.16
In view of the photochemical behavior of complex 2 that
undergoes homolytic photocleavage of the apical 4PPNO ligand,
it is of interest to determine whether this process may be a
general pattern for related (salen)Mn(III) complexes coordinated
to ligands with a negative oxygen terminus, and if this
photoprocess can lead to the generation of proposed intermedi-
ates in Mn-salen-catalyzed epoxidations.
The transient spectra of complex 3 in dichloromethane,
recorded at two different times after 266 nm laser flash
excitation, are shown in Figure 3. At shorter time scales, this
spectrum exhibits two absorption bands at 430 and 630 nm,
and a shoulder at 360 nm. All these bands decay in the
microsecond time scale with different kinetics, indicating that
the spectrum corresponds to more than one species. The
disappearance kinetics of the 430 nm band can be fitted to a
first-order decay with a lifetime of 4.9 µs. In oxygen-saturated
dichloromethane, the decay kinetics of the 430 nm band also
followed first-order decay kinetics with a much shorter lifetime
(0.24 µs), giving an approximate oxygen quenching rate constant
of 3.9 × 109 M-1 s-1. Interestingly, a different transient
spectrum with absorption maxima centered at 360 and 430 nm
was recorded in acetonitrile (Figure 3). The presence of these
two bands indicates that the same two transients observed in
dichloromethane are also generated in acetonitrile, but in
different proportions. This must reflect the difference in polarity
between CH2Cl2 and CH3CN, the more polar intermediate being
presumably that characterized by the 360 nm band.
TP+ tetrafluoroborate is a good electron-acceptor photosen-
sitizer upon selective 355 nm laser excitation (a control
experiment established that the direct 355 nm photolysis of
4PPNO does not give any transient).15 Importantly, this TP+-
photosensitized photolysis of 4PPNO leads to the same transient
spectrum observed for complex 2. Furthermore, the normalized
decays of the transient obtained in dichloromethane through both
alternate procedures superimpose exactly. According to eq 1,
these results strongly support that the 4PPNO• radical is the
most reasonable assignment for the species observed in the direct
photolysis of complex 2 (Figure 2).
Thus, these experimental observations firmly indicate that the
prevalent pathway after light absorption is an unprecedented
homolytic photocleavage of the exchangeable 4PPNO axial
ligand, yielding the corresponding oxygen-centered radical, plus
the concurrent (salen)Mn(II) (eq 2).
We note that the behavior of 4PPNO in complex 2 contrasts
sharply with the tendency of aromatic N-oxides to lose atomic
oxygen upon direct excitation with no detectable formation of
any transient.13 We note that even though a (salen)Mn(II)
complex must be formed concomitantly with the pyridinyloxyl
radical, the transient UV-vis spectrum is dominated by the
latter, and spectroscopic evidence for the Mn(II) moiety was
(15) (a) Garc´ıa, H.; Miranda, M. A. Chem. ReV. 1994, 94, 1063-1089.
(b) Mattay, J.; Vodenhof, M.; Denig, R. Chem. Ber. 1989, 122, 951-958.
(16) Cozens, F. L.; Ortiz, W.; Schepp, N. P. J. Am. Chem. Soc. 1998,
120, 13543-13544.

Photolysis of Jacobsen Catalyst and Mn Complex
J. Am. Chem. Soc., Vol. 123, No. 29, 2001 7077
that laser flash photolysis of 4 at 266 nm could lead to the
spectroscopic observation of the benzyl anion and/or the benzyl
radical by heterolytic or homolytic photodetachment of the
apical ligand and CO2 loss as depicted in eqs 4 and 5.
Figure 4. Transient spectrum of Jacobsen catalyst in hexane under
nitrogen recorded 1.5 µs after 266 nm laser excitation. The inset shows
an initial growth followed by a decay of the ∆O.D. monitored at 450
nm.
It is known that tetracoordinated cationic Mn(III) complexes
having the apical position free are characterized by an absorption
band at 360 nm tailing beyond 400 nm.17 Hence, a reasonable
assignment for the transient species with absorption maximum
at 360 nm observed predominantly in acetonitrile is the cationic
Mn(III) complex formed by heterolytic photodetachment of the
acetoxy ligand present in complex 3 (eq 3).
Both benzylic intermediates have characteristic absorption
bands at 350 and 310 nm for the anion and the radical,
respectively.16 In fact, after laser excitation of complex 4 in
dichloromethane or acetonitrile, an absorption band at 350 nm,
attributable to the benzyl anion, was clearly detected, decaying
with first-order kinetics and a lifetime of 0.15 µs. This benzyl
anion is quenched by ethanol and other protic solvents due to
protonation of the carbanionic center.
On the other hand, although the laser flash photolysis of
complex 1 in dichloromethane or acetonitrile did not lead to
the observation of any transient, we undertook the same study
using the Jacobsen catalyst (the tetra-tert-butyl derivative of
complex 1) but also failed to detect any transient in either
dichloromethane or acetonitrile. However, since the tert-butyl
groups of the Jacobsen catalyst confer it some solubility in
apolar solvents, the laser flash photolysis of Jacobsen catalyst
was also pursued in hexane. Unexpectedly, 266 or 355 nm laser
excitation in hexane led to observation of two bands at 360
and 420 nm (see Figure 4) which are compatible with the
formation of the (salchd)Mn(II) complex.
In light of these results, we propose that excited states of
pentacoordinate Mn(III) Schiff base complexes can undergo
heterolytic and homolytic axial ligand photodissociation as a
general process. The relative efficiency of these two photodis-
sociations depends not only on the medium polarity but also
on the σ electron-donor character of the coordinating axial base
and the stability of the resulting radical or ionic species.
Effectively, the prevalence of the homolytic pathway with the
N-oxide derivative as axial ligand can be related to the lower
oxidation potential of this compound relative to poorer donors
such as acetate and phenylacetate.19
It is worth noting the different photochemical reactivities of
the five-coordinate complexes studied here compared to those
reported for four-coordinate Mn(salen), either Mn(II) or Mn(III).
In both cases four-coordinated Mn(II) or Mn(III) complexes
undergo intramolecular metal-to-ligand electron transfer to
generate a transient species having a ligand with radical anion
character. For tetracoordinated Mn complexes this transient
species can be considered as an excited state, since light
emission repopulates the ground state of manganese salen
complexes. According to this, observation of photoluminescence
will provide sound evidence that the same process occurs for
Contrary to what is observed for complex 2, the fact that the
acetoxy ligand or any of the species derived from it does not
absorb in the monitored region of the UV-vis spectrum permits
recording the absorption of the (salen)Mn fragments without
interference. In this regard, it is worth noting that either the
+II or +III oxidation state of manganese-in-salen complexes
can be easily distinguished by its electronic spectra, which are
very different depending on the metal oxidation state. Hence,
the transient with an absorption band at 430 nm would be the
Mn(II) salen complex arising from a competitive homolytic
photodissociation of the metal-acetoxy bond. In support of this
assignment is the decrease in the quantum yield observed for
the generation of the 430 nm transient when increasing the
solvent polarity. On the other hand, the fact that the transient
characterized by the 430 nm band is quenched by oxygen
indicates that this species reacts to give oxygenated products, a
chemical behavior that is compatible with salen Mn(II) com-
plexes, but not with cationic Mn(III)salen+ that is air stable.
This scheme parallels the work of Steenken, McClelland, and
co-workers on the heterolytic versus homolytic cleavage of
benzylic derivatives in various solvents.18
To seek support for the above mechanistic rationalization of
the photochemistry of salen manganese complexes 2 and 3,
complex 4 having the UV-detectable chromophore phenylacetate
as apical ligand was prepared and photolyzed under the same
conditions. According to the previous discussion we anticipated
(17) Srinivasan, K.; Michaud, P.; Kochi, J. K. J. Am. Chem. Soc. 1986,
108, 2309-2320.
(18) (a) McClelland, R. A.; Kanagasabapathy, V. M.; Banait, N. S.;
Stenken, S. J. Am. Chem. Soc. 1992, 114, 1816. (b) McClelland, R. A.;
Kanagasabapathy, V. M.; Banait, N. S. J. Am. Chem. Soc. 1991, 113, 1009.
Bartl, J.; Steenken, S.; Mayr, H.; McClelland, R. A. J. Am. Chem. Soc.
1990, 112, 6918.
(19) The oxidation potential of 4PPNO in acetonitrile using 0.1 M
tetrabutylammonium perchlorate as electrolyte was 1.45 V vs saturated
calomel electrode (SCE). In contrast, acetate and phenylacetate do not
undergo anodic oxidation under these conditions in the range of potentials
available in acetonitrile.

7078 J. Am. Chem. Soc., Vol. 123, No. 29, 2001
Sabater et al.
Figure 6. Transient spectrum recorded 0.2 µs after 266 nm laser-
pulse excitation of binuclear complex 5 in dichloromethane. The arrow
shows the band that has been previously assigned to (salchd)Mn(V)d
O. The inset shows the decays monitored at 530 (b) and 710 (O) nm.
Figure 5. Emission spectra in dichloromethane solution under nitrogen
of (a) Jacobsen catalyst and complexes (b) 2 and (c) 4.
above the equatorial salen plane and the resulting geometry is
roughly square pyramidal, in which the manganese atoms are
located above the main salen plane.12 This geometry disfavors
the metal-salen orbital interaction, making less favorable the
manganese salen single-electron transfer, and shortening the
lifetime of the intramolecular charge-separated species. From
our laser flash photolysis study, it appears that the essential role
of the labile axial ligand in the photochemistry of pentacoor-
dinate Mn(III) salen complexes is to provide an efficient
pathway for competitive excited-state deactivation.
Table 1. Wavelength for the Maximum Intensity of the Excitation
and Emission Spectra of Some of the Salen Complexes (2 × 10-7
M)
compound
λ
ex (nm)
λem (nm)
Jacobsen catalysta
340
375
365
420 and 530
468
440
2b
3b
a Hexane. b Dichloromethane.
Given the relevance of these types of complexes as oxidation
catalysts, we were interested in applying the laser flash
photolysis technique to the generation of transients proposed
as intermediates in epoxidation reactions. Aimed at this purpose,
and having determined the photoreactivity of pentacoordinate
Mn salen complexes, we prepared a binuclear oxobridged salen
complex [Mn(III)(salchd)-O-Mn(IV)(salchd)]+ (5) and submit-
ted it to photolysis in dichloromethane solution. The mixed III/
IV valence was deduced from the magnetic susceptibility
measurements (see the Experimental Section). Laser flash
photolysis led to the formation of a transient absorption band
with maximum at 530 nm which decayed with a first-order
kinetics with a lifetime of 1.56 µs (Figure 6). On the basis of
the reported UV spectrum of Mn(III)salen in highly concentrated
oxidizing media, this transient was assigned to the reactive
oxomanganese(V) complex that has been postulated as the
oxygen-transferring species in the epoxidation reaction with
manganese salen systems.17,20,21 This intermediate may have
been formed through homolytic photocleavage of the oxygen-
metal bond as depicted in eq 6. There is also a different transient
(λmax 710 nm) that accompanies the oxomanganese but decays
differently (see Figure 6). This second transient could be formed
through a photocleavage (either homo- or heterolytic) different
than that indicated in eq 6.
pentacoordinate Mn(III) salen complexes. In fact, the Jacobsen
catalyst and complexes 2 and 4 exhibit a broad featureless
photoluminescence band characteristic of the metal-to-ligand
transition in the 500-600 nm region. Although these emissions
could be easily recorded with good signal-to-noise ratio (see
Figure 5), the corresponding photoluminescence quantum yields
were low and could not be measured (<0.01). In some cases, a
ligand-centered emission at much shorter wavelength showing
vibrational fine structure was also observed upon excitation at
the ligand-localized absorption bands (see spectrum a in Figure
5). Table 1 summarizes the wavelength maxima for excitation
and emission for the above-mentioned complexes. Concerning
the laser flash photolysis study discussed above, what is relevant
from the luminescence spectra is that the emission-time profile
is too rapid to be monitored within the time resolution of our
system. This implies that the emitting excited-state lifetime is
too short (<1-2 ns), and cannot be responsible for any of the
transients detected in the laser flash photolysis experiments that
typically expand in the microsecond time scale. Thus, the
emitting excited state that probably corresponds to the intramo-
lecular metal-ligand electron transfer (the only photochemical
process yet reported for other manganese complexes) is also
generated in pentacoordinate Mn(III) salen complexes, but it
does not correspond to any of the transient species observed
by laser flash photolysis that are much longer lived.
We suggest that the different photochemical behavior of
pentacoordinate Mn(III) salen complexes can be related to the
relative energy of the new orbital system having a labile apical
ligand that is reflected in the coordination geometry. Effectively,
in four-coordinate complexes the manganese is centered in the
basal salen plane. In this geometry there is a stronger metal-
salen orbital overlap favoring a more efficient intramolecular
metal-ligand electron transfer than when the manganese ion
moves away from the main salen plane. However, axial
complexation induces the displacement of the manganese ion
Related binuclear oxobridged salen complexes have already
been proposed in equilibrium with the active oxomanganese-
(V) during the thermal alkene epoxidation catalyzed by Mn(III)
(20) Chellamine, A.; Kulanthaipandi, P.; Rajagopal, S. J. Org. Chem.
1996, 64, 2237-2239.
(21) Sevvel, R.; Rajagopal, S.; Srinivasan, C.; Alhaji, N. I.; Chellamani,
A. J. Org. Chem. 2000, 65, 3334-3340.

Photolysis of Jacobsen Catalyst and Mn Complex
J. Am. Chem. Soc., Vol. 123, No. 29, 2001 7079
ment of the easily exchangeable axial ligand in pentacoordinate
Mn(III) salen complexes is the predominant photoprocess arising
from the initial charge-separated excited state.
On the other hand, laser flash photolysis of a binuclear mixed-
valence complex [Mn(III)(salchd)-O-Mn(IV)(salchd)]+ leads
to the formation of the reactive oxo(salchd)manganese(V)
complex (λmax 530 nm), a species that has been proposed as
key intermediate in the epoxidation reaction with manganese
salen systems,17,23-25 and we have observed that this species
indeed reacts with aliphatic mono- and disubstituted alkenes.
The fact that the quenching constant of 1-methylcyclohexene
is much lower than those of cyclohexene and 1-alkenes agrees
with the reported data that chiral induction of trisubstituted
alkenes is much lower due to the higher steric encumbrance of
these alkenes toward the side-on approach to the active MndO
bond.26,27 The possibility of generating intermediates proposed
in the Mn(III)-salen-catalyzed epoxidation using the laser flash
technique would enable the study of their reactivity and could
open the way to the elucidation of reaction mechanisms in
transition metal catalysis.
Figure 7. Plot of 1/τ versus the concentration of cyclohexene for the
decay of the oxomanganese(V)salen transient monitored at 530 nm.
The inset shows three representative decay without and after addition
of 1.5 × 10-6 and 2 × 10-5 M cyclohexene.
Experimental Section
salen complexes.17,22 However, as far as we know, the photo-
chemical generation of a representative oxomanganese(V) salen
complex is unprecedented and opens the way to study the
chemical reactivity of these short-lived intermediates under
reactive conditions through fast detection techniques. We note
that the presence of a second transient does not interfere with
the kinetic study of the oxomanganese salen complex.
In fact, the decay kinetics of the 530 nm band was
significantly faster in the presence of alkenes. Thus, under the
conditions of the laser flash photolysis experiments, the pho-
togenerated oxomanganese(V) transient reacts with 1-hexene
(kq ) 5.3 × 105 M-1 s-1), 1-octene (kq ) 6.4 × 105 M-1 s-1),
and cyclohexene (kq ) 2.0 × 106 M-1 s-1), but not with
1-methylcyclohexene (kq < 105 M-1 s-1). As an example, Figure
7 presents the quenching for cyclohexene.
To confirm that the observed alkene quenching of the
photogenerated oxomanganese(V) transient leads to the forma-
tion of epoxide, a preparative photolysis was carried out.
Equivalent amounts of oxo dimer 5 and cyclohexene in
dichloromethane were photolyzed under nitrogen using the 308
nm output of an excimer laser. After percolation of the solution
through basic alumina, gas chromatography (GC) analysis
reveals the presence of 1,2-cyclohexanediol (conversion 16%,
selectivity > 95%). This diol is the known ring-opening product
of the expected epoxide. This shows that laser flash photolysis
can be a valuable tool in the investigation of reaction mecha-
nisms in the field of metal complex catalysis.
Fourier transformed infrared (FT-IR) spectra were recorded at room
temperature in a KBr disk using a spectrophotometer (Nicolet 710 FT).
Room-temperature transmission UV-vis spectra of transparent dichlo-
romethane solutions were recorded in a UV-vis scanning spectropho-
tometer (Shimadzu). The emission spectra of pure complexes were
recorded with a spectrofluorimeter (FS900 Edinburgh) with a mono-
chromator (Czerny-Turner) in the 200-800 nm range. Excitation
wavelengths were those corresponding to the charge transfer (CT)
maxima. Samples dissolved in dichloromethane (10-2 M) contained in
7 × 7 quartz cuvettes were sealed with septum caps and purged with
nitrogen at least 15 min before recording the luminescence spectra.
Laser flash experiments were performed at concentrations in the 10-4
M range with a pulsed Nd:YAG laser (pulse width, 10 ns; energy pulse,
∼14 mJ).
Preparation of Manganese Salen Complexes 1-5. (1) Preparation
of 1. Complex 1 was prepared according to a general procedure reported
by Jacobsen.7 Basically, this procedure consists of heating at reflux
‚
temperature a suspension of 6.72 g (0.027 mol) of Mn(OAc)2 4H2O in
60 mL of ethanol, and a solution of 8.69 g of salchd ligand (0.027
mol) in 20 mL of toluene, for 2 h. A gas dispersion tube was placed,
and air was bubbled through the reaction mixture for 1 h. After this,
heating and air bubbling were stopped, and a saturated NaCl solution
(10 mL) was added. The mixture was cooled to room temperature, and
1 precipitated as a brown solid.
Characterization data for complex 1. IR(KBr): 1620, 1603, 1541,
1447, 1305, 740 cm-1. UV-vis (CH2Cl2): 246, 288, 325 426, 500
nm. MS(FAB): m/z 375 (M - Cl-)+. Anal. Calcd for C20H20N2O2-
ClMn (%): C, 55.92; H, 4.96; N, 6.31; Measured: C, 55.91; H, 4.76;
N, 6.73.
(2) Preparation of 2-4. Complexes 2-4 were prepared according
to a protocol reported for related complexes.8 Thus, a 50-mL round-
bottom flask fitted with a dropping funnel was charged with 0.269 g
(1.3 mmol) of AgClO4 and 5 mL of dry CH3CN. The dropping funnel
was charged with a solution of 0.5 g (1.219 mmol) of complex Mn-
(III)(salchd)Cl in 5 mL of dry CH3CN, and this solution was added
dropwise. The mixture was stirred for 24 h at room temperature, filtered
Conclusions
Laser flash photolysis of a set of five-coordinate manganese-
(III) salen complexes leads as general photoprocesses to the
homolytic and heterolytic photodetachment of the axially ligated
σ-electron donor from the salen manganese core. The relative
efficiency of these processes can be related to the polarity of
the solvent and to the oxidation potential of the apical
substituent.
(23) Dickson, F. E.; Gowling, E. W.; Bentley, F. F. Inorg. Chem. 1967,
6, 1099-1101.
(24) Adam, W.; Mock-Knoblanch, C.; Saha-Mo¨ler, Ch. R.; Herderich,
M. J. Am. Chem. Soc. 2000, 122, 9685-9691.
Comparison of our spectroscopic results with previous product
studies reported in the literature for related four-coordinate
cationic salen Mn(III) enables us to conclude that photodetach-
(25) Cavallo, L.; Jacobsen, H. Angew. Chem., Int. Ed. 2000, 39, 589-
592.
(26) Groves, J. T.; Myers, R. S. J. Am. Chem. Soc. 1983, 105, 5791-
5796.
(22) Jacobsen, E. N.; Deng, L.; Furukawa, Y.; Martinez, L. E. Tetra-
hedron 1994, 50, 4323-4334.
(27) Brandes, B. D.; Jacobsen, E. N. J. Org. Chem. 1994, 59, 4378-
4380.

7080 J. Am. Chem. Soc., Vol. 123, No. 29, 2001
Sabater et al.
through Celite, and washed with 10 mL of acetonitrile. The filtrate
was concentrated to 15 mL, and 1.219 mmol of each specific ligand
(4-phenylpyridine-N-oxide, sodium acetate, and sodium phenylacetate)
was added. The solutions were stirred at room temperature for 24 h to
give the complexes 2-4 as brown solids.
Characterization data for complex 2. IR(KBr): 1623, 1600, 1544,
1472, 1447, 1395, 1311, 1223, 1094, 908 cm-1. UV-vis (CH2Cl2):
242, 311, 423 nm. MS(FAB): m/z 546 (M - ClO4-)+. Anal. Calcd
for C31H31N3O7ClMn·1H2O: C, 55.92; H, 4.96; N, 6.31. Found: C,
55.91; H, 4.76; N, 6.73.
(2) Preparation of 5. Complex 5 was prepared by adding 2 mL of
NaOCl (4% active chlorine) to 5 mL of a dichloromethane solution
containing 0.5 g (1.22 mmol) of complex 1. The resulting biphasic
system was stirred at room temperature for 24 h. A brown solid
precipitated. The solid was filtered off, washed with dichloromethane
and n-pentane and dried.
Characterization data for complex 5. IR(KBr): 1616, 1605, 1542,
1447, 1305, 1142, 910, 747 cm-1. UV-vis (CH2Cl2): 246, 288, 325,
426, 500 nm. MS(FAB): m/z 375, 391 (M - Mn(salchd)Cl - ClO4-)+.
Magnetic susceptibility: X ) 5.1 emu‚K/mol.
Characterization data for complex 3. IR(KBr): 1625, 1610, 1565,
1537, 1447, 1305, 1142, 910 cm-1. UV-vis (CH2Cl2): 246, 290, 325,
426 nm. MS(FAB): m/z 375 (M - CH3CO2-)+. Anal. Calcd for
C22H23N2O4Mn: C, 60.83; H, 5.38; N, 6.45. Found: C, 60.75; H, 5.41;
N, 6.43.
Characterization data for complex 4. IR(KBr): 1614, 1593, 1568,
1531, 1444, 1301, 1117, 906 cm-1. UV-vis (CH2Cl2): 246, 290, 325,
426 nm. MS(FAB): m/z 375 (M - PhCH2CO2-)+.
Acknowledgment. Financial support by the Spanish DGI-
CYT (H.G., MAT97-1016-CO2) and Generalitat Valenciana
(M.J.S, GVDOC99-0203) is gratefully acknowledged. J.C.S.
acknowledges support from the Natural Sciences and Engineer-
ing Research Council of Canada.
JA003371O