EPR characterization of the products formed after photolysis of [Co(III)(salen)(CH3)(H2O)] and [Co(III)(SaltMe)(CH3)(H2O)] in the presence of N- and P-donor bases

Article abstract of DOI:10.1021/ic990919n

Anaerobic photolysis of [Co(III)(salen)(CH₃)(H₂O)] (1) and [Co(III)(saltMe)(CH₃)(H₂O)] (2) induces homolytic cleavage of the cobalt-carbon bond and formation of cobalt(II) five-coordinate species and organic rad

Full text of DOI:10.1021/ic990919n

1994
Inorg. Chem. 2000, 39, 1994-1997
Experimental Section
EPR Characterization of the Products Formed
after Photolysis of [CoIII(Salen)(CH3)(H2O)] and
[CoIII(SaltMe)(CH3)(H2O)] in the Presence of N-
and P-donor Bases
Solvents were purchased from Merck, and all other chemicals from
Aldrich. All solvents used in EPR experiments were purified by standard
methods8 and stored under nitrogen. The cobalt(II) complexes were
prepared from CoCl2 and the corresponding Schiff base ligand using
published procedures.9 All manipulations were carried out under
nitrogen; the solvents were deoxygenated with nitrogen, and all solids
were degassed under vacuum. The samples to be irradiated were
prepared by adding the N- or P-donor base to solutions of the aqua-
(methyl)cobalt(III) complexes in toluene/dichloromethane (2:1). Pho-
tolysis was performed with a Hg lamp (250 W Philips HP/T; λ ) 560-
580 nm), and X-band EPR spectral measurements were made on a
Bruker ESP E-300 instrument. EPR spectral simulations were performed
using a program based on Pilbrow’s formalism.10 The best fit was
observed for collinear g and A(Co) tensors, neglecting any quadrupolar
interactions and assuming g and A(P) to have the same principal axes.
The spin Hamiltonian parameters were calculated assuming an 2A1
ground state.11
Joa˜o Gomes and Baltazar de Castro*
CEQUP/Departamento de Qu´ımica, Faculdade de Cieˆncias,
Universidade do Porto, 4169-007 Porto, Portugal
ReceiVed August 2, 1999
Introduction
Organocobalt(III) complexes with Schiff base equatorial units
have been extensively used as models of organocobalamins,1,2
which are important cofactors in many enzymatic processes.
Structural, kinetic, and spectroscopic studies have been per-
formed in organocobalamins and in several model compounds
to assess the steric and electronic contributions that labilize the
cobalt-carbon bond.1-5 Organocobalt(III) complexes with 3-
and 4-substituted pyridines have been extensively studied to
probe electronic effects, and those with phosphines to probe
steric contributions.1a,3
Previous studies concerning EPR characterization of pho-
tolysis products of cobalamins and model compounds have
shown that the reactivity of the cobalt(II) fragments depends
largely on the equatorial moiety, and an understanding of the
determining factors in the formation of 1:1 or 1:2 adducts has
been the objective of intense research.4-7 The purpose of this
work is to use EPR spectroscopy to understand the photolysis
mechanism of anaerobic solutions of [CoIII(Schiff base)(CH3)-
(H2O)] in aprotic solvents and in the presence of several N-
and P-donor bases and to characterize the behavior of the cobalt-
(II) fragments formed upon photolysis. Two Schiff base ligands
were used, salen ) N,N′-ethylenebis(salicylideneiminato) and
saltMe ) N,N′-(1,2-dimethyl)butanebis(salicylideneiminato), in
an attempt to correlate the reactivity of the cobalt(II) complexes
formed upon irradiation with the electronic and steric effects
of the axial ligands.
Results and Discussion
Reaction of [CoIII(Schiff Base)(CH3)(H2O)] with N- and
P-Donor Bases. The addition of N- and P-donor bases to
solutions of [CoIII(Schiff base)(R)(H2O)] results in the rapid
replacement of water molecules,1,12 and with excess base,
typically only base-bound alkylcobalt(III) species are present
in solution.13 This methodology provides a means for obtaining
base-bound alkylcobalt(III) complexes that avoids the need to
isolate these compounds.1-5 In the present study, the extent of
water replacement was found to depend on the type and amount
of base and on the equatorial ligand. For solutions with a less
than 10-fold molar excess of base, the optical electronic spectra
are consistent with partial or no water replacement, and the
solutions contain, most often, a mixture of [CoIII(Schiff base)-
(CH3)(H2O)], [CoIII(Schiff base)(CH3)], and [CoIII(Schiff base)-
(CH3)(L)]. In the presence of base:metal complex ratios greater
than 10:1, water replacement is complete for pyridines; for
phosphines, even with a base:metal ratio of 20:1, water
replacement is not complete.
Photolysis in Frozen Matrix (77 K). Solutions of [CoIII-
(Schiff base)(CH3)(H2O)] irradiated and observed at 77 K are
EPR silent, even after relaxation to fluid solutions and re-cooling
to 77 K. These results suggest that, under these conditions,
homolysis of the cobalt-carbon bond is not accomplished by
photolysis with visible light.
EPR spectra, recorded at 77 K, of irradiated frozen solutions
in toluene/dichloromethane of [CoIII(Schiff base)(CH3)(H2O)]
in the presence of a 10-fold excess of base show one signal
that is ascribed to a five-coordinate cobalt(II) complex, [CoII-
(Schiff base)(L)], and a strong and much narrower signal
centered at g ≈ 2.00 that is attributed to an alkyl radical or to
* To whom correspondence should be addressed: Baltazar de Castro,
Departamento de Qu´ımica, Faculdade de Cieˆncias da Universidade do Porto,
R. Campo Alegre, 687, 4169-007 Porto, Portugal. E-mail: bcastro@fc.up.pt.
Fax: 011-351-22 608 2959. Phone: 011-351-22 608 2892.
(1) (a) Randaccio, L.; Bresciani-Pahor, N.; Zangrando, E.; Marzilli, L.
G. Chem. Soc. ReV. 1989, 18, 225. (b) Summers, M. F.; Marzilli, L.
G.; Bresciani-Pahor, N.; Randaccio, L. J. Am. Chem. Soc. 1984, 106,
4478. (c) Toscano, P. J.; Marzilli, L. G. Prog. Inorg. Chem. 1984, 31,
105.
(2) (a) Vitamin B12 and B12-proteins; Kra¨utler, B., Golding, B. T., Arigoni,
D., Eds.; Wiley-VCH: Weinheim, Germany, 1998. (b) B12; Dolphin,
D., Ed.; Wiley: New York, 1982; Vol. 1, p 2. (c) Finke, R. G.;
Schiraldi, D. A.; Mayer, B. J. Coord. Chem. ReV. 1984, 54, 1.
(3) Pahor, N. B.; Forcolin, M.; Marzilli, L. G.; Randaccio, L.; Summers,
M. F.; Toscano, P. J. Coord. Chem. ReV. 1985, 63, 1.
(4) (a) de Castro, B.; Pereira, J.; Rangel, M. Organometallics 1991, 10,
3848. (b) de Castro, B.; Rangel, M.; Raynor, J. B. J. Chem. Soc.,
Dalton Trans. 1990, 3311.
(8) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon Press: Oxford, 1988.
(9) Schrauzer, G. N.; Silbert, J. W.; Windgassen, R. J. J. Am. Chem. Soc.
1968, 90, 6681.
(5) (a) Arcos, T.; de Castro, B.; Ferreira, M. J.; Rangel, M.; Raynor, J.
B. J. Chem. Soc., Dalton Trans. 1994, 369. (b) Rangel, M.; Arcos,
T.; de Castro, B. Organomettalics 1999, 18, 3451.
(6) (a) Labauze, G.; Raynor, J. B. J. Chem. Soc., Dalton Trans. 1980,
2388. (b) Labauze, G.; Raynor, J. B. J. Chem. Soc., Dalton Trans.
1981, 590. (c) Raynor, J. B. Inorg. Chim. Acta 1977, 22, L28.
(7) (a) Pezeshk, A.; Greenway, F. T.; Vincow, G. Inorg. Chem. 1978,
17, 3421. (b) Pezeshk, A.; Greenway, F. T.; Dabrowiak, J. C.; Vincow,
G. Inorg. Chem. 1978, 17, 1717. (c) Pezeshk, A. Inorg. Chem. 1992,
31, 2282.
(10) Pilbrow, J. R.; Winfield, M. E. Mol. Phys. 1973, 25, 1073.
(11) In all cases, the symmetry point group was taken as C2V; for compounds
with zero or two bound pyridine molecules it is C2V(x), but for the
five-coordinate adducts it is Cs. This difference is reflected solely in
the symmetry labels of two excited states that are not discussed in
this work. Furthermore, as no splitting from the equatorial nitrogen
atoms could be observed, the contribution of the ligand term to the
spin Hamiltonian includes only the axial ligand component.
(12) Kennedy, B. J.; Murray, K. S. Inorg. Chim. Acta 1987, 134, 249.
(13) Tsou, T.; Loots, M.; Halpern, J. J. Am. Chem. Soc. 1982, 104, 4, 623.
10.1021/ic990919n CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/05/2000

Notes
Inorganic Chemistry, Vol. 39, No. 9, 2000 1995
Figure 1. Experimental and simulated EPR spectra in frozen solution
(77 K) observed after room-temperature photolysis of a toluene/
dichloromethane solution of [CoIII(salen)(CH3)(H2O)] to which were
added in a 20:1 molar ratio (a) phenyldimethylphosphine and (b)
triphenylphosphine.
a product of the reaction of the alkyl radicals with solvent
molecules.14 This latter signal is lost just above the softening
point of the glass, and the spectra of re-cooled frozen solutions
exhibit only the signal of the five-coordinate cobalt(II) complex.
In the high-field region, spectra with phosphines exhibit
hyperfine coupling to cobalt (59Co, I ) 7/2) and to one
phosphorus atom (31P, I ) 1/2) (Figure 1), implying coordination
of one molecule of the P-donor base (type B) and formation of
five-coordinate complexes. In spectra with pyridines, each cobalt
hyperfine line is split into three lines with relative intensity 1:1:1
(type C), implying formation of a five-coordinate compound
as each cobalt atom interacts with one molecule of nitrogen
base (14N, I ) 1) (Figure 2a).
Figure 2. Experimental and simulated EPR spectra in frozen solution
(77 K) observed after room-temperature photolysis of a toluene/
dichloromethane solution of [CoIII(salen)(CH3)(H2O)] to which pyridine
was added in (a) a 1:1 molar ratio and (b) a 10:1 molar ratio.
identical to spectra attributed to the species [CoII(Schiff base)],
probably weakly solvated.15
Pyridine Adducts. For photolyzed solutions of [CoIII(saltMe)-
(CH3)(H2O)] with pyridines, the type of complex formed is
independent of the amount of N-donor base added (Table 1).
EPR spectra are identical to those observed after photolysis at
77 K (type C), implying the formation of a five-coordinate
compound.
Photolysis in Fluid Solutions. Room-temperature irradiation
of [CoIII(Schiff base)(CH3)(H2O)] solutions results in frozen EPR
spectra with large g-tensor anisotropy (type A), which are
(14) (a) Rao, D. N. R.; Symons, M. C. R. J. Chem. Soc., Faraday Trans.
1 1983, 79, 269. (b) Rao, D. N. R.; Symons, M. C. R. J. Chem. Soc.,
Faraday Trans. 1 1984, 80, 423.
(15) (a) Daul, C.; Schla¨pfer, C. W.; von Zelewsky, A. Struct. Bonding
(Berlin) 1979, 36, 129. (b) Engelhardt, L. M.; Duncan, J. D.; Green,
M. Inorg. Nucl. Chem. Lett. 1972, 8, 725.

1996 Inorganic Chemistry, Vol. 39, No. 9, 2000
Notes
Table 1. Stoichiometry of the Cobalt(II) Complexes Formed after Room-Temperature Photolysis of [CoIII(Schiff base)(CH3)(H2O)] in Toluene/
Dichloromethane Solutions to Which a Lewis Base Was Added in Different Molar Ratios
number of bases coordinated axially to Co(II)
1:1
1:2
1:10
Co:axial ligand
1:30
Co:axial ligand
complex
axial ligand
PAa
TCAb
Co:axial ligand
Co:axial ligand
[CH3Co(salen)H2O]
py
924
882
940
942
970
882
899
900
937
938
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
4-CNpy
4-Mepy
4-Etpy
4-N(Me)2py
3-CNpy
3-Clpy
3-Brpy
3-Mepy
3-Etpy
PMe2Ph
PEt3
1 + 2
1
1
1
1 + 2
2
2
2
1 + 2
1 + 2
1
1
1
1
1
1
1 + 2
2
2
2
1 + 2
1 + 2
1
1
1
1 + 2
1 + 2
1 + 2
1
1
1
1
1
1
1
1
125
130
132
137
145
152
PBu3
PMePh2
PBuPh2
PPh3
1
1
1
[CH3Co(saltme)H2O]
py
924
882
940
942
970
882
899
900
937
938
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
4-CNpy
4-Mepy
4-Etpy
4-N(Me)2py
3-CNpy
3-Clpy
3-Brpy
3-Mepy
3-Etpy
PMe2Ph
PEt3
125
130
132
137
145
152
PBu3
PMePh2
PBuPh2
PPh3
0 + 1
0 + 1
0 + 1
0 + 1
0 + 1
a Proton affinity (PA), values in kJ mol-1, from ref 21. b Tolman cone angle (TCA), from ref 26.
In contrast, solutions of [CoIII(salen)(CH3)(H2O)] with
pyridines yield three types of EPR spectra, depending on the
amount of added base (Table 1). With equimolar ratios of [CoIII-
(salen)(CH3)(H2O)]:base, spectra of type C were always ob-
served (Figure 2a), whereas for higher amounts of base, different
results were obtained. For a 10-fold excess of base [except for
3-Etpy, 3-Mepy, and 4-(NMe2)py], each cobalt hyperfine line
in the EPR spectra is split into five lines with relative intensity
1:3:5:3:1 (type D), a pattern indicative of an interaction of the
cobalt atom with two pyridine molecules (Figure 2b), thus
supporting formation of [CoII(salen)(L2)]. For intermediate molar
ratios, the observed spectra exhibit a superposition of two
signals, one of type C and another of type D, implying the
simultaneous existence of complexes with one and two pyridines
bound to the metal. However, for 3-Mepy, 3-Etpy, and
4-(NMe2)py, even in the presence of a 20-fold excess of base,
the EPR spectra always show a mixture of signals arising from
[CoII(salen)(L)] and [CoII(salen)(L2)].
the presence of a 20-fold excess of base, only type B spectra
are observed. EPR data provide a clear indication that cobalt-
(II) complexes with the Schiff bases studied do not form species
with two axially bound phosphines.
Photololysis Mechanism. Photolysis of frozen solutions of
alkylcobalamins9,14,16 and model compounds4,5,14-19 with bound
pyridine or phosphine molecules has been extensively studied.
The resulting products have been found to originate from
homolytic fission of the Co-C bond after light absorption that
moves an electron from the equatorial ligand into the antibond-
ing Co(dz ) σ* orbital.14 Photolyzed solutions of the alkyl-Schiff
2
base-cobalt(III) system studied in this work exhibit EPR spectra
similar to those reported, thus implying that irradiation with
visible light induces homolytic cleavage of the Co-C bond.
The same mechanism must be operative for photolysis in a
fluid matrix, as solutions irradiated at 77 K, warmed to room
temperature, and re-cooled exhibit only EPR signals due to
cobalt(II) species, which are identical to those obtained when
the photolysis is performed in a fluid matrix. For fluid solutions
of salen complexes with excess N-donor base, the five-
coordinate cobalt(II) binds a second base molecule, whereas
for all other cases, five-coordinate cobalt(II) species do not react
Phosphine Adducts. For [CoIII(salen)(CH3)(H2O)], the spec-
tra are independent of the amount of base added (Table 1) and
are identical to those observed for photolyzed solutions at 77
K (type B). Solutions of [CoIII(saltMe)(CH3)(H2O)] irradiated
in the presence of any amount of PMe2Ph, PEt3, PBu3, or
PMePh2 exhibit EPR spectra of type B, thus indicating formation
of [CoII(saltMe)(L)]. However, EPR spectra of photolyzed
solutions of [CoIII(saltMe)(CH3)(H2O)] with equimolar ratios
of the bulkier phosphines, PBuPh2 and PPh3, exhibit signals
assigned to [CoII(saltMe)(L)] (type B) and to a complex with
no axially bound phosphine (type A). Increasing the amount of
phosphine decreases the intensity of the type A signal, and in
(16) Hay, B. P.; Finke, R. G. J. Am. Chem. Soc. 1986, 108, 482.
(17) Maillard, Ph.; Giannotti, C. Can. J. Chem. 1982, 60, 1402.
(18) Tada, M.; Nakamura, T.; Matsumoto, M. J. Am. Chem. Soc. 1988,
110, 4647.
(19) (a) Magnuson, V. E.; Weber, J. H. J. Organomet. Chem. 1975, 92,
233. (b) Witman, M. W.; Weber, J. H. Inorg. Nucl. Chem. Lett. 1975,
11, 591. (c) Finke, R. G.; Smith, B. L.; McKenna, W. A.; Christian,
P. A. Inorg. Chem. 1981, 20, 687. (d) Tamblyn, W. H.; Kochi, J. K.
J. Inorg. Nucl. Chem. 1981, 43, 1385.

Notes
Inorganic Chemistry, Vol. 39, No. 9, 2000 1997
with excess base in solution. This latter behavior is similar to
that observed for the final photolysis products of alkyl-
cobaloximes in the presence of phosphines, for which stable
five-coordinate cobalt(II) complexes were observed, even in the
presence of large excesses of base.4,5 In contrast, solutions of
alkyl(pyridine)cobaloximes exhibit EPR bands due to a Co(II)
complex with two bound pyridine molecules.4,5 The different
behavior of cobalt(II) phosphine and pyridine complexes was
attributed to strong steric interactions between the phosphine
and the equatorial system, which induce structural modifications
that hinder binding of a second molecule of base.
Structural Information from EPR Data. The X-ray struc-
tures of [CoII(salen)(py)],20a [CoII(salen)(NO)],20b [CoII(salen)-
(py)],20c and [CoII(3-MeOsalen)(H2O)]20d have been reported.
In all of these structures, the cobalt atom lies 0.20-0.43 Å above
the equatorial plane, and the two salicylaldehyde fragments are
bent away from the axial ligand. On the basis of structural data,
it can be suggested that stabilization of five-coordinate species
occurs by moving the metal atom away from the equatorial plane
and by bending the equatorial ligand away from the axial ligand.
obtained for 4-substituted pyridines, and the ratios FR/FI were
close to 1. This ratio for proton affinity values is 1.16, which
indicates that inductive and resonance effects are of similar
magnitude in proton affinities and in unpaired spin densities.
Application of the above equation to 3-substituted pyridines
failed for salen complexes and was only marginally acceptable
for Aav(Co) of saltMe complexes. Introduction of the steric
parameter proposed by Brown25 provides the more complex
equation ∆R ) FIσI + FRσR+ + FSσS, which does fit our data.
The results show that steric effects are important and that they
are markedly different for saltMe and salen complexes, with
the paradoxical observation that the latter are more sensitive to
steric effects. Possibly, saltMe complexes with one bound
pyridine are already extensively distorted, and as such, the
possibility of being good indicators of subtle steric effects is
lost.
For five-coordinate adducts with phosphines, no correlation
between spin Hamiltonian parameters and Tolman’s electronic
parameter, ø, could be found. However, these parameters
correlate with Tolman’s cone angle (TCA),26 an indication that
steric effects are dominant. For complexes with salen, the values
of g increase with phosphine bulkiness, thus implying longer
Co-P bonds for the bulkier axial ligands. This interpretation
is supported by the observation that bulkier phosphines exhibit
smaller phosphorus hyperfine coupling constants: as A(P)
decreases, so does the unpaired spin density on the phosphorus
s orbital. A similar behavior was observed for complexes with
saltMe, but only for phosphines with TCA smaller than ∼140°;
above this value, an increase in TCA does not change g or A,
an apparent indication that the Co-P bond length does not
increase above a certain limit. A rationale for the observed
behavior can be obtained by proposing that the deformation
imposed on the equatorial ligand has reached a limit and that
longer bonds would preclude bond formation. Bulky phosphines
induce more extensive steric repulsions with the equatorial
moiety, resulting in greater displacements of the cobalt atom
toward the axial base and/or higher tilt of the equatorial moiety
and, consequently, less reactive five-coordinate adducts.
Our results on photolysis of [CoIII(Schiff base)(CH3)(H2O)]
in the presence of N-donor and P-donor bases indicate that the
reactivity and stability of the homolysis fragments is largely
dependent on electronic and steric effects for N-donor bases
and on steric effects for P-donor bases. The stronger 4-substi-
tuted pyridines, pyridines with alkyl groups in position 3, and
bulky phosphines induce distortions in the equatorial moiety
that promote stabilization of the five-coordinate species formed
upon homolysis.
For five-coordinate complexes with pyridines, a plot of g
or gz against base strength, as quantified by the base proton
affinities (PA),21 shows different relations for 3- and 4-substi-
tuted pyridines. For 4-substituted pyridines, we observed a linear
decrease of g and a linear increase of gz with increasing PA.
This result implies shorter Co-N bond lengths for the stronger
bases, as a decrease in the axial bond length increases the
antibonding dz orbital energy.6,22 In contrast, for 3-substituted
2
pyridines, a linear dependence of both g and gz with PA is
observed only for the weaker bases; 3-methyl- and 3-ethyl-
pyridines do not fall in line with the other pyridines. This
behavior can be ascribed to steric repulsions of the alkyl groups
with the equatorial moiety, which do not allow the expected
short Co-N distances as observed for sterically nondemanding
4-substituted pyridines.
EPR data are thus compatible with a decrease in the Co-
Naxial bond length for stronger bases, which must impose
structural modifications in the complex. The proximity of the
two atoms must promote a longer displacement of the cobalt
atom from the equatorial plane, thus rendering the sixth position
less accessible for binding a second molecule. Stronger steric
repulsions are expected for compounds with the bulkier equato-
rial ligand, [CoII(saltMe)L], arising from interactions between
the axial base and the methyl groups on the equatorial ligand
ethylenic bridge, thus implying weaker axial base interactions
and more extensive structural distortions.
We stress that there is a relationship between the total spin
density on cobalt (or on the axial nitrogen) and the axial base
strength for pyridines, and it is of interest to assess the
applicability of the LFER models developed in organic chem-
istry. We used the dual parameter approach (DSP) and tried to
fit the values of Aav(Co) and Az(N) to an equation of the form
∆R ) FIσI + FRσR+, where ∆R is the difference between
Aav(Co), or Az(N), in pyridine and substituted pyridines, and σI
and σR+ have their usual meaning.23,24 Straight lines were
Supporting Information Available: Details of the DSP analysis
of spin density data; EPR parameters for the photolysis products in
the presence of N-donor (Table S1) and P-donor (Table S2) bases; EPR
spectra, 77 K, of [CoIII(salen)(CH3)(H2O)] after room-temperature
photolysis (Figure S1); plots of g and gz vs proton affinity of pyridines
(Figure S2); and plots of g and Aav(P) vs proton Tolman’s cone angle
(Figure S3). This material is available free of charge via the Internet at
http://pubs.acs.org.
IC990919N
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Randaccio, L. J. Chem. Soc., Dalton Trans. 1978, 498. (d) Calligaris,
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1984, 13, 1630.
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(27) The quantity f represents the “goodness of fit” of the data to eq 1 or
2; f test values of less than 0.1 indicate an excellent fit and those
between 0.1 and 0.2, an acceptable fit. See: Arnold, D. P.; Bennett,
M. A. Inorg. Chem. 1984, 23, 2117.