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J.W. Dethlefsen et al. / Inorganica Chimica Acta 362 (2009) 1585–1590
During the photolysis of
[Cr2+] = [NO] and this gives
a
[Cr(NO)]2+ complex we have
d½NOꢄ
¼ 0:05 Mꢀ1 ½NOꢄ
ð5Þ
dt
d½Cr2þꢄ
dt
By assuming that the values of the rate constants in dmso
solution do not differ dramatically from those in aqueous
solution and since [NO] ꢅ 1 M, we can conclude that the dom-
inating trapping mechanism is the reaction between dioxygen
and the [Cr(dmso)6]2+ complex (Eq. (3)) and not the reaction
between dioxygen and NO (Eq. (1)). In Table 4 the observed
quantum yields,
U, defined as the amount of decomposed 1 rel-
ative to the amount of absorbed photons, are listed. Based on
the orbital splitting diagram in Fig. 3 it would be expected that
~
irradiation in the m2 band would lead to a weakening of the
Cr–N(O) bond with a possible release of NO as a result. It is
therefore surprising that the photorelease of NO occurs to the
same extent at wavelengths as high as 580 nm. Furthermore,
Fig. 6. Experimental (—) and simulated (----) EPR spectra of [Cr(dmso)5(NO)]2+
The spectrum at 298 K was taken of solution of [Cr(NCCH3)5(NO)](PF6)2
equilibrated in neat dmso for 3.5 h. For the low-temperature spectrum the same
volume of butane-2,3-diole was added to the equilibrated solution and the mixture
was cooled to 66 K.
.
a
U
at 404 nm in the analogous complex of [Cr(NCCH3)5(NO)]2+
in MeCN was found to be as high as 0.55 mol Einsteinꢀ1, one
order of magnitude higher that in 1. We have no explanation
for this behaviour.
2.5. EPR spectra
positions of the ligands in the spectrochemical series are
similar. As mentioned earlier the cation in 1 has been prepared
by McCain in solution with EPR parameters close to the values
in Table 5.
EPR spectra of 1 were recorded in dmso solution at room
temperature and in a frozen glass at 66 K as shown in Fig. 6.
At room temperature the spectrum is typical for an S = ½ system
with superhyperfine coupling to 14N (I = 1). This gives the in-
tense three line splitting and additional less intense lines coming
from hyperfine coupling to 53Cr (9.5% natural abundance, I = 3/2).
A simulation of the spectrum gave the values for giso and Aiso
shown in Table 5 [25]. From the frozen glass spectrum the
In recent years the bis(dithiocarbamato)iron(II) complexes
[Fe(S2CNR2)2] have been used as trapping agents for in vivo
detection of nitric oxide [27]. With hydrophilic R-groups, the
complexes are water soluble, and very low concentrations of
NO have been detected in vivo through the formation of the
complexes [Fe(S2CNR2)2(NO)] which have a characteristic three-
line EPR spectrum. The advantage of using [Fe(S2CNR2)2] com-
plexes as trapping agents is that they are X-band EPR silent. In
an attempt to confirm that one of the products in the photode-
composition of complexes containing a Cr(NO)2+ core is NO, we
have on this background photolysed a deoxygenated MeCN solu-
tion of [Cr(NCCH3)5(NO)](PF6)2 in the presence of excess of
[Fe(S2CNEt2)2]. We chose MeCN as a solvent since the other pho-
tolysis product [Cr(NCCH3)6]2+ is X-band EPR silent (S = 2) and
since this product does not react further, in contrast to
[Cr(dmso)6]2+ as discussed above. In Fig. 7 EPR spectra taken
during a photolysis are shown. The EPR spectrum recorded be-
fore the photolysis is the spectrum of [Cr(NCCH3)5(NO)]2+ which
consist of a broad band located at g = 1.98 without structure be-
cause of superhyperfine coupling to 14N in the five MeCN li-
gands. During the photolysis new bands indeed grow up as the
result of the formation of [Fe(S2CNEt2)2(NO)] as shown in
Fig. 7, whereas the broad band from the [Cr(NCCH3)5(NO)]2+
complex eventually disappears. From the final spectrum the
anisotropic parameters g , g\ and A\(14N) were calculated by a
k
least-squares fit [25]. The parameters A\(53Cr) and A (53Cr) were
k
calculated from the positions of the low intensity lines around
3420 and 3650 G, and A (14N) was calculated from A (14N) =
k
k
3Aiso
(
14N) ꢀ 2A\(14N). For comparison the EPR parameters for
other [CrL5(NO)] type complexes with L being an oxygen donor
ligand are listed in Table 5. It is noted that the values of the
hyperfine and superhyperfine constants are quite similar. As
seen in Fig. 3 the unpaired electron resides in the dxy orbital,
which is not involved in bonding to the NO ligand. Superhyper-
fine interaction to 14N occurs through a spin orbit coupling
matrix element between the dxy and the {dyz,dzx} set of orbitals
[26]. The fact that the hyperfine constants Aiso(
53Cr), A (53Cr), and
k
A\(53Cr) are very similar in the four complexes indicates that the
degree of the covalency of the metal–ligand bonds is similar. The
values of the g parameters, or rather the shift (
electron value:
excited states [26]. Again we see that the g values are very
similar in the four complexes, which is not surprising since the
Dg) from the free
D
g = 2.002319 ꢀ g, reflect the energies of the
superhyperfine constant Aiso(
14N) in [Fe(S2CNEt2)2(NO)] can be
calculated to Aiso
(
14N) = 12.2 ꢁ 10ꢀ4 cmꢀ1 with giso = 2.0400, very
close to the published values being 12.2 ꢁ 10ꢀ4 cmꢀ1 and 2.0388
[28].
Table 4
Observed quantum yields,
U, for the photodecomposition of 1 in dmso solution at
298 K defined as the amount of decomposed 1 relative to the amount of absorbed
photons
3. Experimental
k (nm)
U )
(mol Einsteinꢀ1
3.1. Reagents and materials
365
405
436
546
580
0.108(6)
0.070(1)
0.046(2)
0.034(5)
0.049(1)
The complexes [Cr(NCCH3)5(NO)](PF6)2, [Fe(S2CNEt2)2], and
[Cr(NCCH3)4(BF4)2] were prepared according to the literature
methods [9,29,30]. Solvents were dried with molecular sieves
(4 Å) prior to use.