Reductions by titanium(ii) as catalyzed by titanium(iv)

Article abstract of DOI:10.1039/b510212j

The cobalt(iii) complexes, [(NH₃)₅CoBr]²⁺ and [(NH₃)₅CoI]²⁺ are reduced by Ti(ii) solutions containing Ti(iv), generating nearly linear (zero-order) profiles that become curved only during

Full text of DOI:10.1039/b510212j

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/dalton | Dalton Transactions
Reductions by titanium(II) as catalyzed by titanium(IV)†‡
Ritam Mukherjee, Zhiyong Yang and Edwin S. Gould*
Received 18th July 2005, Accepted 18th October 2005
First published as an Advance Article on the web 18th November 2005
DOI: 10.1039/b510212j
2
+
2+
The cobalt(III) complexes, [(NH
3
)
5
CoBr] and [(NH
3
) CoI] are reduced by Ti(II) solutions containing
5
Ti(IV), generating nearly linear (zero-order) profiles that become curved only during the last few percent
of reaction. Other Co(III)–Ti(II) systems exhibit the usual exponential traces with rates proportional to
[Co(III)]. Observed kinetics of the biphasic catalyzed Ti(II)–Co(III)Br and Ti(II)–Co(III)I reactions
support the reaction sequence:
k1
2
+
−
2+
−
[
Ti(II)(H
2
O)n] + [Ti(IV)F
5
]
ꢀ [Ti(II)(H
2
O)
]
+ [(H
2
O)Ti(IV)F ]
5
n−1
k−1
k2
2
+
[
Ti(II)(H
2
O)
]
+ Co(III) → Ti(III) + Co(II)
n−1
3
−1 −1
◦
with rates determined mainly by the slow Ti(IV)–Ti(II) ligand exchange (k
1
= 9 × 10−
M
s
at 22 C).
Computer simulations of the catalyzed Ti(II)–Co(III) reaction in perchlorate–triflate media yield relative
2
+
rates for reduction by the proposed active [Ti(II)(H
2
O)n−1
]
intermediate; kBr/k
I
= 8.
Introduction
for two more hours to remove dissolved oxygen. Trifluoromethane-
sulfonic acid (triflic acid), as well as inorganic reagents (Aldrich or
Fisher products) were used as received. Pentaamminecobalt(III)
Kinetic examinations of the reactions of two-electron reductants
have disclosed instances in which rates are independent of the
concentration of oxidant, even when the latter is in deficiency. The
8
complexes, bromopentaamminecobalt(III) perchlorate, and its
9
10
11
iodo, binoxalato, and azido complexes were prepared as
described. Titanium(II) solutions in 1.0–2.0 M triflic acid were
−
2
3−
3
4
oxidations of Ge(II) by I
3
, by [W(CN)
8
] , and by Cr(VI) in HCl,
5
as well as oxidation of In(I) by vitamin B12a in HCl, behave in this
manner, indicating that, in each case, the reaction is initiated by a
slow step not involving the oxidant. Kinetic profiles are consistent
with a rate-determining heterolysis of the reductants (e.g., Ge(II)–
7
prepared under argon by a modification of the method of Kolle.
These green solutions were kept in sealed containers and used
within 10 h of preparation. Titanium(IV) solutions were prepared
by air oxidation of titanium(II) solutions; after oxidation was
complete, as indicated bycomplete loss of color, argonwas bubbled
through the solution for 2 h to remove traces of dissolved oxygen.
Sodium triflate was prepared by neutralization of concentrated
triflic acid, boiled to remove carbon dioxide, and the salt obtained
OH
2
) to a more reactive form (Ge(II):) which then reacts rapidly
with the oxidant:
Ge(II)–OH
2
→ Ge(II): + H
2
O (slow)
(1)
(2)
12
−
−
by cooling.
Ge(II): + I
3
→ Ge(IV)–I + 2I (rapid)
If sequence (1)–(2) is correct, it represents one of the rare cases
where a sluggish bond breaking at a simple metal complex in
aqueous solution occurs without a substantial loss in ligand field
Kinetic studies
Reactions were performed under argon. Rates were obtained from
measurements of absorbance decreases associated with the oxi-
dants using a Shimadzu 1601 UV-visible spectrophotometer. Tem-
6
stabilization.
It has recently been reported that [Br(NH
analog are reduced by Ti(II)aq at rates independent of oxidant.
2
+
3
)
5
Co] and its iodo
1
◦
peratures were kept at 22.0 ± 0.5 C. Ionic strengths were main-
7
Since this reductant, as prepared by the method of Kolle, contains
tained with NaClO /HClO /CF SO H or CF SO H/CF SO Na.
4
4
3
3
3
3
3
3
equivalent quantities of Ti(IV), it appeared that the possibility of
catalysis by that tetrapositive state should be considered.
In most cases, the bromo- and iodo-substituted oxidants gave
very nearly linear traces (rates independent of oxidant) with
perceptible appearance of curvature during the last few percent
reaction. The oxalate and azido complexes, included here for
comparison, yielded exponential absorbance traces, with rate
constants proportional to [Ti(II)].
Experimental
Materials
All solutions were prepared from Millipore-Q system deionized
water that had been boiled for 2 h and then purged with pure argon
Results and discussion
1
A previous study emphasized differences between the kinetic
Department of Chemistry, Kent State University, Kent, Ohio, 44242, USA
behavior of the reductants Ti(II) and Ti(III). Dissimilarities are
particularly marked for the familiar homologous series of oxi-
dants, fluoro-, chloro-, bromo- and iodo-pentaamminecobalt(III).
†
Electron transfer. Part 161. For Part 160, see ref. 1.
‡
Electronic supplementary information (ESI) available: Tables S1–S3:
detailed kinetic data for redox reactions. See DOI: 10.1039/b510212j
7
72 | Dalton Trans., 2006, 772–774
This journal is © The Royal Society of Chemistry 2006

View Article Online
Reductions, using Ti(III), of each halogen-substituted complex
yields the expected exponential profiles, as were observed also for
the Ti(II)–fluoro and Ti(II)–chloro systems. Curves for the Ti(II)–
bromo and Ti(II)–iodo reactions were, however, clearly biphasic,
featuring a predominant linear (zero order) component and a small
curved portion in the late stages (Fig. 1(A), (B)). Moreover, initial
rates in these biphasic conversions appeared to be second order in
Ti(II).
Examination of the action of Ti(IV), which is present in our
Ti(II) preparations, helps to clarify the Ti(II) reductions of the
bromo and iodo oxidant. As shown in Tables S2 and S3 (ESI‡),
initial rates for both oxidants are proportional to [Ti(IV)], as well
as to [Ti(II)]. Thus, these rates conform to rate law (5)
0
rate = −d[Co(III)]/dt = k[Ti(II)][Ti(IV)][Co(III)]
(5)
Treatment of data for initial rates yields k = (8.4 ± 0.4) ×
for the bromo oxidant and (9.0 ± 0.6) × 10
for the iodo. Observed and calculated rates (in parentheses) are
−
3
−1 −1
−3
−1 −1
10
M
s
M s
14
compared at the right of these tables. Note that [Ti(II)] and
7
[
Ti(IV)] are very nearly the same in the Ti(II) master solutions used,
2
thus accounting for the apparent [Ti(II)] dependence erroneously
assigned when no additional Ti(IV) was added.
The biphasic profiles observed, in conjunction with the depen-
dence on Ti(IV) and the observed increase in absorbance at 502 nm
(
(
a Ti(III) maximum), are consistent with mechanistic sequence (6)–
7) for the bromo- and iodo-substituted Co(III) oxidants.
O)n] + [Ti(IV)F ]− ꢀ [Ti(II)(H
2
+
k1
O)
]
n−1
2+
[
Ti(III)(H
2
2
5
k−1
−
+
[(H
2
O)Ti(IV)F5]
(6)
(7)
k2
2
+
[
Ti(II)(H
2
O)n−1
]
+ Co(III) → Ti(III) + Co(II)
Here the active reducing species is taken as the coordinatively
2
+
unsaturated Ti(II) center (probably [Ti(II)(H
2
O)
5
] ), designated
2
+
[
Ti(II)(H
KINSIM treatment yielded calculated absorbance curves which
were compared to the observed kinetic profiles. Values of k and
the ratio k /k−1 giving optimal agreement with runs in which the
reactants are varied are listed in Tables 1 and 2. Note that this
treatment does not yield individual values of k−1 and k but only
their ratio. The k values for the two systems, designating the aqua
2
O)n−1] . Incorporation of sequence (6)–(7) into the
15
1
2
2
1
−
1
−1
transfer from Ti(II) to Ti(IV), are in agreement (0.008–0.010 M
at 22 C). This rate is very nearly independent of acidity.
s
◦
2+
Table 1 Rate constants pertaining to the reduction of [(NH
3
)
5
CoBr] by
a
Ti(II) as catalyzed by Ti(IV)
+
3
−1 −1
10−2k
/k−1
[
Ti(II)]/mM [Ti(IV)]/mM [H ]/M 10 k
1
/M
s
2
2+
3 5
Fig. 1 Kinetic profiles for the reduction of [Co(NH ) Br] (A, 551 nm)
2
+
and [Co(NH
3
)
5
I] (B, 580 nm) with Ti(II)aq (12.0 mM) in the presence of
40
25
40
25
20
12
24
48
72
25
25
25
25
25
25
25
0.70
0.70
0.70
0.70
0.70
0.70
0.70
0.40
0.50
0.60
0.90
0.70
0.70
0.70
8.0
9.0
10.0
11.0
9.5
9.0
10.0
9.8
9.5
8.2
8.6
8.6
1.6
1.4
1.6
1.4
1.5
1.6
1.6
3.0
2.4
2.1
1.3
1.3
1.6
2.0
+
added Ti(IV) (72 mM); [H ] = 0.7 M (CF
3
SO
3
H/HClO
4
); l = 1.0 M. Circles
2
1
1
1
0
2
2
2
represent the experimental data, whereas the solid line was obtained by
KINSIM simulation of the proposed mechanism, (6)–(7), using parameters
−
3
−1 −1
from Tables 1 and 2 (bromo: k
1
= 10.0 × 10
M
s
, k
2
/k−1 = 160; iodo:
−
3
−1 −1
−1 −1
k
1
= 9.5 × 10
M
s
, k
2
/k−1 = 22). eCo(III) was taken as 53 M cm for
12
25
−
1
−1
bromo and 79 M cm for iodo. Optical path length = 1.00 cm.
2
2
5
5
No other Co(III) complex examined here mimics the bromo
and iodo reduction patterns. The azido derivative, [(NH
follows a straightforward monomial rate law (3) whereas the
oxalato complex (Table S1, ESI‡) exhibits a 1/[H ] -dependence
4) reminiscent of the analogous
2
+
25
3
)
5
CoN
3
]
b
c
d
2
2
5
5
8.2
8.2
+
2
25
(
a
◦
Parameters pertain to sequence (6)–(7) in text. T = 22 C; [Co(III)] =
1.1–1.8 mM; l = 1.0 M (NaClO /HClO /CF SO H) unless otherwise
indicated; k = 551 nm. Values of k listed are those giving optimal
agreement between observed and calculated absorbances. Extinction
][Ti(II)]; k = (6.9 ± 0.2) M−1
−1
4
4
3
3
Rate = k[CoN
Rate = k[Co(C
reduction of [(NH
3
s
(3)
(4)
1
+
−2
−1
2+
−1
−1
2
O
4
)][Ti(II)][H ] ; k = (0.45 ± 0.01) M s
3 5
coefficient of [(NH ) CoBr] was taken as 53 M cm . Optical path
b c d
length = 1.0 cm. l = 0.80 M. l = 1.40 M. l = 1.80 M.
+
13
3
)
5
Co(C
2
O
4
)] by Ti(III).
This journal is © The Royal Society of Chemistry 2006
Dalton Trans., 2006, 772–774 | 773

View Article Online
2
+
Table 2 Rate constants pertaining to the reduction of [(NH
3
)
5
CoI] by
matches that pertaining to Ti(II) in the absence of catalysis by
Ti(IV), but the selectivities of the two centers is likely to be
quantitatively different.
In sum, our thinking ascribes the zero order disappearance of
these Co(III) oxidants to a preliminary rate-determining transfer
a
Ti(II) as catalyzed by Ti(IV)
+
3
−1 −1
[
Ti(II)]/mM
[Ti(IV)]/mM
[H ]/M
10 k
1
/M
s
k /k−1
2
3
2
1
1
1
1
3
3
3
3
2
5
6
2
2
2
2
2
2
2
32
25
16
24
48
72
32
32
32
32
0.70
0.70
0.70
0.70
0.70
0.70
0.20
0.30
0.40
0.50
9.5
10.0
10.0
11.6
9.5
9.5
9.7
9.5
8.4
8.5
20
20
18
22
20
22
60
42
35
27
of a Ti(II)-bound aqua ligand to an electrophilic Ti(IV) center. As
−
such, it is closely related to the zero-order disappearance of I
3
−
2
in the Ge(II)–I
3
reaction. In both instances initiation involves
slow heterolysis of a cation–OH linkage of the reductant, and
2
both systems feature electrophilic catalysis, in the present case by
+
Ti(IV), in the Ge(II) reaction, by H .
One puzzling point emerges. Why is the preliminary heterolysis
at the Ti(II) center (which involves no appreciable alteration of
crystal field stabilization energy) so slow? In the absence of further
evidence, we suspect that this sluggishness simply reflects the
relative reluctance of the fluorophilic Ti(IV) center to take on an
additional (softer and O-donor) ligand.
a
◦
Parameters pertain to sequence (6)–(7) in text. T = 22 C; [Co(III)] =
0
.7–2.5 mM; l = 1.0 M (NaClO
4 4 3 3
/HClO /CF SO H). Values listed
are those giving optimal agreement between observed and calculated
2+
absorbances; k = 580 nm. Extinction coefficient of [(NH
3 5
) CoI] was
−
1
−1
taken as 79 M cm . Optical path length = 1.00 cm.
Acknowledgements
An alternative sequence, (8)–(9), features electron transfer from
Ti(III) to Co(III) and initiation by a Ti(IV)–Ti(II) comproportiona-
tion:
We are grateful to the National Science Foundation for partial
support of this work and to Mrs Arla Dee McPherson for technical
assistance.
k8
Ti(II) + Ti(IV) ꢀ 2Ti(III)
(8)
(9)
k−8
k9
2
Ti(III) + 2Co(III) → 2Ti(IV) + 2Co(II)
References
Although (8) + (9) appears to be very nearly consistent with our
1
Z. Yang and E. S. Gould, Dalton Trans., 2005, 1781.
16
profiles, this alternative is ruled out by the reported very slow
2 O. A. Babich and E. S. Gould, Inorg. Chem., 2000, 39, 4119.
3 Z. Yang and E. S. Gould, Dalton Trans., 2004, 1858.
4
5
6
+
Ti(III) reductions (in 0.44 M H ) of both the bromo (k = 4 ×
O. A. Babich and E. S. Gould, Inorg. Chem., 2001, 40, 5708.
O. A. Babich and E. S. Gould, Res. Chem. Intermed., 2002, 28, 79.
(a) For additional examples, see: J. P. Birk, Inorg. Chem., 1975, 14, 1724
−
3
−1 −1
−2
−1 −1
1
0
M
s ) and the iodo (k = 5.4 × 10
M
s ) complexes.
The final curved portions of the profiles in Fig. 1 indicate half-
life periods approximately 25 s for the iodo and <10 s for the
bromo, whereas the reported kTi(III) values correspond to t1/2 = 3.9 h
III
(Ti ); (b) G. Paquette and M. Zador, Inorg. Chim. Acta, 1978, 26, 123
II
(
Zn ).
7
8
U. Kolle and P. Kolle, Angew. Chem., Int. Ed., 2003, 42, 4540.
F. Basolo and R. K. Murmann, Inorg. Synth., 1953, 4, 171.
(
bromo) and 0.28 h (iodo) when the reductant is 12 mM. Neither
of these Ti(III) reactions is catalyzed by Ti(IV).
9 A. Haim and H. Taube, J. Am. Chem. Soc., 1963, 85, 495.
10 J. Price and H. Taube, Inorg. Chem., 1968, 7, 1.
+
The ratio k
2
/k−1 is seen to vary as 1/[H ] for both oxidants. Since
1
1
1
1 M. Linhard and H. Flygare, Z. Anorg. Allg. Chem., 1950, 262, 328.
2 B. Saha and D. M. Stanbury, Inorg. Chem., 2000, 39, 1294.
3 A. H. Martin and E. S. Gould, Inorg. Chem., 1975, 14, 875.
14 Observed rates deviate slightly from those calculated by (5) at Ti(II)
0.03 M, suggesting association between titanium reagents. Our data
in this range are not sufficient to estimate Keq for that association.
5 B. A. Barshop, R. F. Wrenn and C. Frieden, Anal. Biochem., 1983, 130,
134.
16 G. A. K. Thompson and A. G. Sykes, Inorg. Chem., 1976, 15, 638.
17 F. Basolo and R. G. Pearson, Mechanisms of Inorganic Reactions, Wiley,
New York, 2nd edn, 1968, ch. 1 and 6.
measured rates for faster Ti(II)–Co(III) reactions have been shown
1
to be independent of acidity, values of k−1 must be proportional
+
to [H ]. It is reasonable that protonation of Ti(IV)-bound water in
>
(
6) eases its departure from the Ti(IV) center.
Comparison of k /k−1 ratios indicates that the activated Ti(II)
2
1
center reduces the bromo oxidant about eight times as rapidly as
the iodo. The order of reactivity corresponds to that for reductions
1
7
17
by Eu(II), but is the reverse of that for reductions by Fe(II),
18
19
Cr(II), and In(I), suggesting that the present reducing species,
like Eu(II), is a hard acid in the Pearson sense. This conclusion
1
8 J. P. Candlin and J. Halpern, Inorg. Chem., 1965, 4, 766.
19 S. K. Chandra and E. S. Gould, Inorg. Chem., 1996, 35, 3159.
17
7
74 | Dalton Trans., 2006, 772–774
This journal is © The Royal Society of Chemistry 2006