Molybdenum and copper catalysis of reductions by titanium(II) and titanium(III)

Article abstract of DOI:10.1039/b508453a

Reductions of vanadium(iv), benzoquinone, and tri-iodide, both by titanium(iii) and by titanium(ii), are catalyzed by molybdenum(vi). The VO ²⁺-Ti(ii) reaction is catalyzed by copper(ii) as well. Reactions of Ti(ii) with the oxidant in excess y

Full text of DOI:10.1039/b508453a

View Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/dalton | Dalton Transactions
Molybdenum and copper catalysis of reductions by titanium(II) and
titanium(III)†‡
Zhiyong Yang and Edwin S. Gould*
Received 14th June 2005, Accepted 28th October 2005
First published as an Advance Article on the web 29th November 2005
DOI: 10.1039/b508453a
Reductions of vanadium(IV), benzoquinone, and tri-iodide, both by titanium(III) and by titanium(II),
are catalyzed by molybdenum(VI). The VO2+–Ti(II) reaction is catalyzed by copper(II) as well. Reactions
of Ti(II) with the oxidant in excess yield Ti(IV), as do reductions by Ti(III). Reactions proceed via
competing uncatalyzed and catalyzed paths, with the latter components first order in catalyst. Kinetic
patterns indicate that monomeric Mo(V) is the active species, but the dimeric Mo(V) species, [Mo2O4]2+,
is without catalytic action. Catalytic constants pertaining to Ti(III) are remarkably similar to those for
Ti(II), despite the 0.47 V difference in the standard potentials of the two reductants.
Stoichiometric studies. Stoichiometric determinations were
Introduction
carried out under argon, generally with the oxidant in excess,
and were monitored at or near the absorbance maximum of
that oxidant. Measured deficient quantities of the reductant were
added, and the decreases in absorbance were compared to those
resulting from addition of excess reductant.
Convenient generations of aqueous titanium(II) solutions were
documented in 2003 by Ko¨lle and Ko¨lle,2 thus adding an addi-
tional strongly reducing species to the roster of available reagents.
These solutions, which contain equimolar quantities of Ti(IV), are
unexpectedly robust. In contrast to earlier preparations described
by Forbes and Hall3 and Olver and Ross,4 0.1 M Ti(II) solutions in
triflic acid–HF media survive for 12 h at room temperature when
kept under argon.
Although Ti(II) is an active reductant, its action is not notably
rapid. The difference in standard potentials (0.47 V)3,5 for TiII
and TiIII corresponds, in the Marcus treatment for outer sphere
reductions,6 to a rate ratio of >104. The rate differences observed
for the two titanium reductants, tend, however, to be more modest,
and in some instances, Ti(III) reacts more rapidly.7 In many cases,
we have examined these reactions for evidence of metal catalysis.
The present contribution considers catalysis by Mo(VI) and Cu(II).
Results are summarized in Table 1.
Kinetic Studies. Reactions were performed under argon. Rates
were obtained from absorbance decreases associated with the
oxidants, using a Durrum–Gibson stopped-flow spectropho-
tometer interfaced with an OLIS computer system or a Shi-
madzu 1601 UV-visible spectrophotometer. Temperatures were
kept at 22.0
0.5 ◦C. Ionic strength was maintained with
NaCl/LiCl/HCl or NaClO4/HClO4/CF3SO3H. Concentrations
were generally adjusted so that no more than 10% of the
reagent in excess was consumed in a given run (pseudo-first
order conditions). All reductions yielded exponential decay curves.
Reactions were followed for at least five half-life periods, and rate
constants were evaluated by nonlinear least squares fittings to the
relationship corresponding to first order decay.
Experimental
Attention was centered on the reactions of the three oxidants,
vanadium(IV) (VO2+), 1,4-benzoquinone, and tri-iodide (I3−) with
Materials
All solutions were prepared from Millipore-Q System deionized
water that had been boiled for 2 h and then purged with pure
argon for 2 h more to remove dissolved oxygen. Inorganic reagents
(Aldrich or Fisher), benzoquinone (Aldrich), and TiCl3 (10% in
25% HCl) (Aldrich) were used as received. Ti(II) solutions were
prepared under argon by a modification of the method of Ko¨lle.1,2
Ti wire was dissolved in a mixture of 2 M triflic acid and HF. These
green solutions, containing slightly less than 2% HF, were kept in
sealed containers and used within 10 h of preparation.8 Solutions
of dimeric Mo(V), [Mo2O42+(aq)], were prepared by the method of
Linn.9
Table 1 Catalyzed and uncatalyzed reductions by Ti(II) and Ti(III).
Stoichiometriesa
Reductant
Ti(II)
Oxidant
Catalyst
D[ox]/D[red]
Bzqn
V(IV)
Mo(VI)b
none
0.92 0.02
2.0 0.1
2.0 0.1
Mo(VI)c
Cu(II)d
Mo(VI)e
Mo(VI)b
none
2.0 0.1
−
I3
0.99 0.03
0.46 0.01
0.97 0.01
1.03 0.05
0.50 0.01
Ti(III)
Bzqn
V(IV)
Mo(VI)c
Mo(VI)e
−
I3
a Reactions were carried out in 0.50 M HClO4, 0.50 M HCl, or
0.5 M CF3SO3H. D[Ox] was determined spectrophotometrically. [Bzqn] =
0.050 mM; [V(IV)] = 7–18 mM; [I3−] = 10 mM; [Ti(II)] = 0.01–5.0 mM;
[Ti(III)] = 0.01–6.0 mM. b [Mo(VI)] = 0.10 mM. c [Mo(VI)] = 0.50 mM.
d [Cu(II)] = 0.05–0.30 mM. e [Mo(VI)] = 0.050 mM.
Department of Chemistry, Kent State University, Kent, OH 44242, USA
† Electron transfer. Part 162. For Part 161, see ref. 1.
‡ Electronic supplementary information (ESI) available: Tables S1–S7,
detailed kinetics data for redox reactions. See DOI: 10.1039/b508453a
396 | Dalton Trans., 2006, 396–398
This journal is
The Royal Society of Chemistry 2006
©

View Online
−
both Ti(II) and Ti(III). Five of these systems exhibit measurable
catalysis by Na2MoO4 and one by Cu(ClO4)2. No catalysis
was observed with Cr(ClO4)3, Mn(ClO4)2, Co(ClO4)2, Ni(ClO4)2,
Na2WO4, or the octacyano complexes of Mo(V), Mo(IV), W(V), or
W(IV).
conditions, I3 rates with both Ti(III) and Ti(II) closely followed
expression (4).
Discussion
Although the importance of molybdenum centers in redox
catalysis is widely recognized, it is centered about oxygen atom
transfers from peroxy and oxo compounds to alkenes, amines,
and sulfides. Such reactions are generally considered to proceed
through complexes of Mo(VI) and the oxidant.13 Catalysis by
molybdenum of electron transfer in aqueous media is much less
usual.
Each of the catalyzed paths (Table 2) is first order in catalyst,
suggesting that each is initiated by reduction of Mo(VI) to a
more reactive lower oxidation state. The latter cannot be the
dimeric species, [Mo2O4]2+ (which is known to persist in our
media in the absence of oxygen),9a,14 for this is found to be
devoid of catalytic activity. Instead, we suspect the intervention
of a metastable monomeric Mo(V) species proposed by Sykes15
and generated electrolytically by Chalipoyil and Anson.16 This
monomer is observable in 10 M HCl but decomposes quickly
in 2 M CF3SO3H.17 The proposed sequence, indicated for the
catalyzed Ti(II)–benzoquinone reaction, is indicated as eqn (6)–
(8):
Results
Stoichiometric experiments (Table 1), both in the presence and
absence of catalyst, point to very nearly complete conversion to
Ti(IV) with the three oxidants employed:
Ti(II) + 2V(IV) → Ti(IV) + 2V(III)
(1)
(2)
Ti(II) + I3 → Ti(IV) + 3I−
−
(3)
Each of the oxidants exhibited MoVI catalysis in reactions with
both Ti(III) and Ti(II), but catalysis by Cu(II) was observed only
for the VO2+–Ti(II) system.10 Solutions containing pentapositive
molybdenum in its dimeric form, Mo2O4
action.
,
9a were without catalytic
2+
Kinetic acidity effects or medium effects reported for these
systems in the absence of catalysis,1,11 were, with one exception,
negligible in comparison to the much more rapid catalyzed paths.
Thus, rates for all but one of the catalyzed systems follow the
straightforward binomial expression (4)
k
1
−−−→
←−−−
Ti (II) + Mo (VI)
Ti (III) + Mo (V)
(6)
k
−1
k
2
−
᭹
Bzqn + Mo(V) −→ Mo(V) + [Bzqn ]
(7)
If the steady state approximation is applied to monomeric Mo(V)
rate = −d[Ox]/dt = [Red][Ox](k0 + kcat[cat]),
(4)
where ko and kcat denote rate constants for the uncatalyzed and
catalyzed components, whereas the Mo(VI)-catalyzed Ti(III)–bzq
rates generate rate law (5):
(8)
under our conditions, and if reaction (8), the reduction of a
semiquinone radical, is taken to be kinetically silent, the overall
rate constant, (kcat), approaches k1k2/k−1. Rate constants for the
individual steps cannot be evaluated from our experiments.
rate = [TiIII][bzqn](ko + kꢀ[MoVI]/[H+])
(5)
Rates for individual runs are assembled in Tables S-1 to S-7. Rate
constants resulting from refinements of rate laws (4) and (5) are
summarized in Table 2. Observed rates are compared with those
calculated from (4) and (5) on the right hand side of the data tables.
−
For the Mo(VI)-catalyzed Ti(II)–I3 reaction, steps (7) and (8)
may be replaced by (9) and (10),
•
I3 + Mo(V) → Mo(VI) + I + 2I−
(9)
−
−
Since the equilibrium involving I3 and the more reactive
oxidant I2 is strongly dependent on added iodide, catalytic
experiments were carried out with [I−] kept constant at 0.050 M,
thus holding the ratio [I3−]/[I2−] at 35.12 Under these (simplified)
I• + Ti(II) → Ti(III) + I− (rapid)
(10)
and the catalyzed VO2+–Ti(II) reaction may be taken to utilize
propagation via (11)
Table 2 Reductions by Ti(III) and Ti(II) as catalyzed by Mo(VI) and Cu(II);
kinetic parametersa rate = −d[Ox]/dt = [Red][Ox](k0 + kcat[cat])
V(IV) + Mo(V) → V(III) + Mo(VI)
(11)
An analogous sequence is reasonable for the catalyzed Ti(III)–
I3 reaction, but in this instance initiation would be expected to
proceed via (12):
Ox
Red
Cat
ko, M−1 s−1
k
cat, M−2 s−1
−
VO2+
VO2+
Bzqn
Ti(III)b
Ti(II)c
Mo(VI)
Cu(II)
Mo(VI)
Cu(II)
Mo(VI)
Mo(VI)
Mo(VI)
Mo(VI)
1.70 0.09
No catalysis
3.3 0.1
3.15 0.06
d
(2.33 0.07) × 104
(9.1 0.4) × 103
(9.2 0.3) × 103
d
Ti(III) + Mo(VI) → Ti(IV) + Mo(V)
(12)
The inverse-[H+] dependence appearing in the catalytic term for
the Ti(III)–bzqn reaction [eqn (5)] points to initiation involving
Ti(OH)2+ rather than Ti3+(aq), a feature common to many
reductions by this d1 center.18 Reversibility of the initiation step
(12) appears to be negligible under our conditions, for the overall
reaction is not appreciably inhibited by addition of Ti(IV) at
concentrations as high as 0.60 mM.
Ti(III)
Ti(II)c
Ti(III)b
Ti(II)c
218
6
(4.5 0.1) × 107
(1.50 0.03) × 106
(7.1 0.2) × 105
−
I3
0.33 0.02
0.32 0.03
a Reactions at 22.0
0.5 ◦C, l = 0.50–0.55 M. b HCl/LiCl. c 0.5 M
CF3SO3H. d Rate = [TiIII][bzqn](ko + kꢀ [MoVI]/[H+]): ko = 103 7 M−1s−1
;
kꢀ = (1.12 0.04) × 107 M−1 s−1
.
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 396–398 | 397
©

View Online
Rate constants pertaining to the catalytic components for the
VO2+–Ti(II) reaction, as catalyzed by Cu(II) and by Mo(VI), are
seen to be remarkably similar, suggesting the involvement of a
common intermediate utilizing parallel paths. Catalysis by Cu(II)
may be taken to be initiated by reduction (13)
With both Ti(II) and Ti(III), the growth and decay of the binuclear
successor complex can be monitored at 430 nm.
We suspect that this multistep path provides a more facile inner-
sphere route for electron transfer, supplementing the more sluggish
uncatalyzed outer-sphere transfer from the Jahn–Teller-distorted
hypervalent titanium center.
Ti(II) + Cu(II) → Ti(III) + Cu(I)
(13)
In analogy with (10) which applies to Mo(VI) catalysis. The species
common to steps (12) and (13) is Ti(III), but there is no reasonable
way that this can be recycled between initiation and propagation
steps. We suspect, therefore, that the near correspondence between
rates for the two catalyzed VO2+–Ti(II) systems is coincidental.
Note that the catalytic constant associated with the Ti(II)–
Acknowledgements
We are grateful to the National Science Foundation for support of
this work and to Mrs Arla Dee McPherson for technical assistance.
References
I3 reaction (7 × 105 M−2 s−1) is smaller than that for Ti(III)
−
(1.5 × 106 M−2 s−1), whereas with the catalyzed reductions of
benzoquinone at 1 M H+, the selectivity is reversed (4.5 ×
107 M−2 s−1 vs. 1.1 × 107 M−2 s−1. If, however, the comparison
is made at 0.22 M H+, rates for the two reductants become
virtually equal. These differences are modest and fall well below
the ratio near 104 that would be expected based on the 470
mV difference in formal potentials documented for Ti(III,II)3 and
Ti(IV,III).5 Rate similarities for uncatalyzed reductions by the two
low-valent titanium centers have earlier been noted1 and assigned
to mismatches of the Jahn–Teller distortions associated with the
two states.
Designation of the reactive intermediate as Mo(V) does not
imply a pentapositive aquated cation. Recent examination19 of
reductions of H2MoO4 with both Ti(II) and Ti(III) suggests that
these reactions proceed through a bridged MoIV–TiII (or MoVI–
TiIII) precursor complex (14), which is converted to a Mo(V)
successor complex (15). The latter, in turn, yields a monomeric
Mo(V) species (16), which, in the present case, reacts with the
oxidant (17):
1 Z. Yang and E. S. Gould, Dalton Trans., 2005, DOI: 10.1039/b510212j.
2 U. Ko¨lle and P. Ko¨lle, Angew. Chem., Int. Ed., 2003, 42, 4540.
3 G. S. Forbes and L. P. Hall, J. Am. Chem. Soc., 1924, 46, 385.
4 J. W. Olver and J. W. Ross, Jr., J. Am. Chem. Soc., 1963, 85, 8565.
5 W. J. James and J. W. Johnson, in Standard Potentials in Aqueous
Solutions, ed. A. J. Bard, R. Parsons and J. Jordan, Marcel Dekker,
New York, USA, 1985, ch. 18.
6 R. A. Marcus, Annu. Rev. Phys. Chem., 1964, 15, 155.
7 Z. Yang and E. S. Gould, Dalton Trans., 2004, 3601.
8 Titanium(II) solutions exhibited e660 = 3.5 M−1 cm−1 and e430
=
6 M−1 cm−1, in contrast to the lower reported2 values 0.5 and
0.8 M−1 cm−1
.
9 (a) D. E. Linn, Jr., S. K. Ghosh and E. S. Gould, Inorg. Chem., 1989,
28, 3225; (b) G. Brauer, Handbook of Preparative Inorganic Chemistry,
2nd edn, Academic Press, New York, USA, 1963, p. 1413.
10 Because of complications resulting from reactions between iodide and
copper(II), we did not extend Cu(II)-catalytic studies to reactions of
tri-iodide with Ti(II) or Ti(III).
11 (a) C. E. Johnson, Jr. and S. Winstein, J. Am. Chem. Soc., 1951, 73,
2601; (b) J. D. Ellis and A. G. Sykes, J. Chem. Soc., Dalton Trans.,
1974, 537; (c) A. Adegite and J. F. Iyun, Inorg. Chem., 1979, 18,
3602.
12 D. A. Palmer, R. W. Ramette and R. E. Mesmer, J. Solution Chem.,
1984, 13, 673.
13 See, for example: A. Keller, J. M. Sobczak and J. J. Ziolkowski, in
Molybdenum: An Outline of its Chemistry and Uses, ed. E. R. Braith-
waite and J. Haber, Elsevier, Amsterdam, Netherlands, 1994, ch. 11.
14 M. Ardon and A. Pernick, Inorg. Chem., 1973, 12, 2484.
15 G. R. Gayley, R. S. Taylor, R. K. Wharton and A. G. Sykes, Inorg.
Chem., 1977, 16, 1377.
(14)
(15)
(16)
16 P. Chalipolyil and F. C. Anson, Inorg. Chem., 1978, 17, 2418.
17 M. T. Paffett and F. C. Anson, Inorg. Chem., 1981, 20, 3967.
18 See, for example: (a) M. Orhanovich and J. E. Earley, Inorg. Chem.,
1975, 14, 1478; (b) A. H. Martin, E. S. Gould and J. E. Earley, Inorg.
Chem., 1976, 15, 1934; (c) S. Swavey and E. S. Gould, Inorg. Chem.,
2000, 39, 352.
MoV–OH + VIV → MoVI–OH +VIII
(17)
19 Z. Yang, Dalton Trans., 2005, submitted.
398 | Dalton Trans., 2006, 396–398
This journal is
The Royal Society of Chemistry 2006
©