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Can. J. Chem. Vol. 81, 2003
Table 1. Electrospray mass spectral data for [(me4-salen)CrV(O)]+.
Scheme 1.
Relative abundance (%)
Mass
Theoretical
Experimental
5.18
1.21
100.00
34.73
8.69
5.41
2.13
100.00
35.03
9.02
388.11
389.11
390.10
391.11
392.11
Scheme 2.
salen)CrIII(H2O)2]CF3SO3 dissolved became greenish-black,
indicative of [(me4-salen)CrV(O)]+. Prior to rate measure-
ments, any unreacted iodosobenzene was removed by filtra-
tion, and pyridine N-oxide in acetonitrile was added to
produce [(me4-salen)CrV(O)(pyO)]+. UV–vis spectroscopic
analysis indicated that this was the same complex that had
been previously characterized (6).
In addition to UV–vis spectroscopy, the [(me4-
salen)CrV(O)]+ solution was analyzed using electrospray
mass spectrometry (ESI-MS) (10). When the expanded ex-
perimental spectrum of the M+ region (m/z 390) was com-
pared with the theoretical spectrum for C20H22N2O3Cr+, the
isotopic distribution and relative intensities in Table 1 were
obtained.
The only mass with a relative abundance that is not within
5% of the predicted value is that of m/z 389.11. That the re-
sulting abundance, 2.13%, is much greater than the theoreti-
cal value of 1.21% is not entirely surprising, since the area
of a small peak on the shoulder of a very large peak can be
substantially altered by isobaric interferences (11). Over-
lapping peaks with the same nominal mass can result in a
large deviation from the expected value for a small peak.
The other isotopic distribution peaks, all within 5% of the
expected values, along with a good comparison between the
UV–vis spectrum of this complex with that reported for the
complex previously characterized (6) provide convincing
evidence for its structure.
well with that previously reported for chromium(II) com-
pounds (5). There was no maximum at 580 nm correspond-
ing to chromium(III). Since this compound is extremely air-
sensitive, it was stored and transferred under argon.
Chromium(II) hexahydrate was ligated with dihydro
8,8,8′,8′-tetramethylsalen (6, 7) and oxidized to the corre-
sponding chromium(III) complex as follows: (me4-salen)H2
(3.570 g, 11.00 mmol) was placed in a 100 mL round-
bottom flask fitted with a stopcock. The flask was evacuated
and back-filled with argon. Methanol (45 mL), freshly dis-
tilled under an argon atmosphere, was added to the flask,
and [CrII(H2O)6](CF3SO3)2 (3.23 g, 7.05 mmol) in an argon-
filled Schlenk tube was dissolved in methanol (5 mL) and
added dropwise to the round-bottom flask through a cannula.
The mixture, initially blue, turned brown. Stirring was con-
tinued for 5 h at room temperature. The flask was opened to
the atmosphere and heated to reflux for 1 h. The mixture
was cooled and the solvent removed on a rotoevaporator.
Distilled water (80 mL) was added and the mixture stirred
for 2 h. The rusty-orange solid that formed was collected in
a Buchner funnel and placed in a 70°C oven to dry over-
night. This solid was recrystallized from water giving 2.59 g
(4.63 mmol, 65.6%) purified product. The UV and IR spec-
tra compared satisfactorily with literature spectra for [(me4-
salen)CrIII(H2O)2]CF3SO3 (6, 8). UV–vis (CH3CN) (nm) (ε
(L mol–1 cm–1)): 418 (3049), 320 (8376), 286 (15 000), 230
(36 000), 202 (25 000). IR (Nujol) (cm–1): 3389–3553 (b),
2954 (s), 2923 (s), 2854 (s), 1618 (s), 1602 (m), 1551 (m),
1465 (s), 1443 (s), 1401 (w), 1379 (m), 1338 (w), 1284 (m),
1238 (s), 1224 (m), 1173 (m), 1146 (m), 1128 (m), 1027
(m), 958 (w), 907 (w), 880 (w), 843 (w), 797 (w), 752 (m),
639 (m).
Kinetic methods
The visible spectrum of
a solution of [(me4-sa-
len)CrV(O)]CF3SO3 in acetonitrile decreases in intensity uni-
formly from 350 to 800 nm. However, when pyridine N-ox-
ide (pyO) is added to the solution, a broad band centered at
628 nm appears (in accordance with work previously
described in the literature (6)). Data summarized in Fig. 1
indicate that the intensity of this band increases as the con-
centration of pyO is increased. The most reasonable expla-
nation for this observation is reversible ligation with
formation of a new complex, [(me4-salen)CrV(O)(pyO)]+,
when pyridine N-oxide is present. Since the absorbance pla-
teaus when the concentration of pyO is 0.1 M or greater, it
can be assumed that the equilibrium expressed in eq. [1] lies
substantially to the right under such conditions.
This chromium(III) complex was oxidized to the corre-
sponding chromium(V) complex by iodosobenzene as fol-
lows: [(me4-salen)CrIII(H2O)2]CF3SO3 (1.21 g, 0.37 mmol)
was placed in a 100 mL round-bottom flask, and 80 mL of
HPLC-grade acetonitrile (CH3CN), obtained from Fisher
Scientific, was added along with iodosobenzene (0.60 g,
2.7 mmol), prepared as described in the literature (9). The
flask was sealed and the solution stirred. The orange
solution that formed initially when the [(me4-
[1]
[(me4-salen)CrV(O)(CH3CN)]+ + pyO
ꢀ [(me4-salen)CrV(O)(pyO)]+ + CH3CN
When a reductant is added to these solutions, the band at
628 nm diminishes with time, thus providing a convenient
physical change that can be used to monitor reaction rates.
© 2003 NRC Canada