Thermal and photochemical reactions of bis(diamine)(sulfito)cobalt(III) complexes: effect of chelate-ring size
Abstract
The kinetics of formation of
cis-[Co(tn)
2
(OH
2
)(OSO
2
-O
)]
+
(tn = 1,3-diaminopropane), its
acid-catalysed aquation to the parent diaqua complex, anation of
trans-[Co(tn)
2
(OH
2
)(SO
3
-
S
)]
+
by N
3
-
,
NCS
-
,
SO
3
2-
–HSO
3
-
, anation of
trans-[Co(tn)
2
(OH)(SO
3
-S
)] by SO
3
2-
and acid-catalysed
aquation of
trans-[Co(tn)
2
(SO
3
-S)
2
]
-
to the corresponding
(aqua)(sulfito-S) complex were investigated and the
results compared with analogous data for the corresponding
1,2-diaminoethane (en) complexes. Expanding the chelate-ring size from
five to six had virtually no effect on the rate of formation of the
sulfito-O complex, but retarded its acid-catalysed aquation.
The latter effect was attributed to a pK perturbation;
intramolecular hydrogen bonding between the co-ordinated H
2
O
and sulfite in
cis-[Co(tn)
2
(OH
2
)(OSO
2
-O
)]
+
hindered the protonation pre-equilibrium of the
sulfito-O complex involved in the acid-catalysed aquation. This
is further supported by the fact that there was no ring-size effect on
the acid-catalysed aquation of
trans-[Co(L–L)
2
(OH
2
)(OSO
2
)
]
+
(L–L = tn or en). The strong
labilising action due to chelate-ring expansion is remarkably attenuated
by the trans effect of S-bonded sulfite as observed in the
anation of
trans-[Co(L–L)
2
(OH
2
/OH)(SO
3
-S)]
+/0
. However,
trans-[Co(tn)
2
(OH
2
)(SO
3
-
S)]
+
was found to be prone to intramolecular
electron transfer between Co
III
and S
IV
under
thermal conditions unlike its en analogue, further reflecting the
ring-size effect. Flash photolysis of
trans-[Co(L–L)
2
(OH
2
)(SO
3
-S)]
+
(L–L = en
or tn) generated the transient
trans-[Co(L–L)
2
(OH
2
)(OSO
2
)]
+
. The photochemical ligand isomerisation of both
complexes
(Co
III
–SO
3
+
→
Co
III
–OSO
2
+
) also
occurred at comparable rates
[k
iso
= (4.1 ±
0.8) × 10
4
and
(3.2 ± 1.3) × 10
4
s
-1
at 25 °C for the en and tn complexes
respectively]. Steady-state photolysis at 254 nm indicated that
trans-[Co(en)
2
(OH
2
)(SO
3
-S
)]
+
underwent photoaquation and photoreduction.
Strikingly photoreduction could not be detected for this complex at
pH > 8.
)]
+
by N
3
-
,
NCS
-
,
SO
3
2-
–HSO
3
-
, anation of
trans-[Co(tn)
2
(OH)(SO
3
-S
)] by SO
3
2-
and acid-catalysed
aquation of
trans-[Co(tn)
2
(SO
3
-S)
2
]
-
to the corresponding
(aqua)(sulfito-S) complex were investigated and the
results compared with analogous data for the corresponding
1,2-diaminoethane (en) complexes. Expanding the chelate-ring size from
five to six had virtually no effect on the rate of formation of the
sulfito-O complex, but retarded its acid-catalysed aquation.
The latter effect was attributed to a pK perturbation;
intramolecular hydrogen bonding between the co-ordinated H
2
O
and sulfite in
cis-[Co(tn)
2
(OH
2
)(OSO
2
-O
)]
+
hindered the protonation pre-equilibrium of the
sulfito-O complex involved in the acid-catalysed aquation. This
is further supported by the fact that there was no ring-size effect on
the acid-catalysed aquation of
trans-[Co(L–L)
2
(OH
2
)(OSO
2
)
]
+
(L–L = tn or en). The strong
labilising action due to chelate-ring expansion is remarkably attenuated
by the trans effect of S-bonded sulfite as observed in the
anation of
trans-[Co(L–L)
2
(OH
2
/OH)(SO
3
-S)]
+/0
. However,
trans-[Co(tn)
2
(OH
2
)(SO
3
-
S)]
+
was found to be prone to intramolecular
electron transfer between Co
III
and S
IV
under
thermal conditions unlike its en analogue, further reflecting the
ring-size effect. Flash photolysis of
trans-[Co(L–L)
2
(OH
2
)(SO
3
-S)]
+
(L–L = en
or tn) generated the transient
trans-[Co(L–L)
2
(OH
2
)(OSO
2
)]
+
. The photochemical ligand isomerisation of both
complexes
(Co
III
–SO
3
+
→
Co
III
–OSO
2
+
) also
occurred at comparable rates
[k
iso
= (4.1 ±
0.8) × 10
4
and
(3.2 ± 1.3) × 10
4
s
-1
at 25 °C for the en and tn complexes
respectively]. Steady-state photolysis at 254 nm indicated that
trans-[Co(en)
2
(OH
2
)(SO
3
-S
)]
+
underwent photoaquation and photoreduction.
Strikingly photoreduction could not be detected for this complex at
pH > 8.
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