Ti(IV)-centered dynamic interconversion between Pd(II), Ti(IV)-containing ring and cage molecules

Article abstract of DOI:10.1021/ja803115j

Heteronuclear, supramolecular ring and cage complexes have been constructed from a pyridyl catechol ligand, TiO(acac)₂, and PdCl₂(CH₃CN)₂. These two complexes are quantitatively interconvertible, in which Ti⁴⁺-centered coordination changes take place between a well-known Ti(catecholato)₃ and a newly established TiH(catecholato)₂(acetylacetonato) structures. The Ti⁴⁺-centered structural changes arise from the changes in the component fraction and basicity condition. Copyright

Full text of DOI:10.1021/ja803115j

Published on Web 07/10/2008
Ti(IV)-Centered Dynamic Interconversion between Pd(II), Ti(IV)-Containing
Ring and Cage Molecules
Shuichi Hiraoka,*,†,‡ Yoko Sakata,† and Mitsuhiko Shionoya*,†
Department of Chemistry, Graduate School of Science, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-0033, Japan, and Precursory Research for Embryonic Science and Technology (PRESTO),
Japan Science Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
Received April 28, 2008; E-mail: shionoya@chem.s.u-tokyo.ac.jp; hiraoka@chem.s.u-tokyo.ac.jp
Supramolecular coordination compounds with dynamic func-
tions1 have provided a series of mechanical devices1a and containers
with an inclusion/release control function.1h Such unique functions
are based on their structural changes between multistable states
arising from the intrinsic, dynamic natures of metals, which are
strongly influenced by the structures of metal ligands,1d,e the
component fractions,1b,c,g–i photoexcitation,1f and the oxidation state
of metals.1a So far, most of dynamic supramolecules have been
achieved using soft metal ions with a normal coordination geometry.
To construct multifunctional molecular devices, heterometal systems
containing both hard and soft metal ions would have the potential
to exhibit high site-selectivity and cooperativity in their structural
changes and thereby novel molecular functions that are not seen in
homonuclear metal complexes. Although a number of supermol-
ecules with hard metal ions and their unique functions2 have been
reported so far, examples of dynamic supramolecular systems that
allow an interconversion of their multistable states by a hard metal
Figure 1. Formation of Ti4+ and heteronuclear Pd2+-Ti4+ complexes.
ion-centered coordination change are scarce.3,4 Herein, we report
a novel Ti4+-catecholato complex, TiH12(acac) (acac ) acetyl-
acetonato), formed from H21 and TiO(acac)2 in the presence of a
weak base. This Ti4+ complex was successfully converted into
[Ti13]2- with a stronger base. In addition, this conversion was found
to take place reversibly by changing the basicity condition and the
component fraction in solution. On the basis of this finding and
the HSAB theory, we established an interconvertible system with
multinuclear Pd(II), Ti(IV)-containing ring- and cage-shaped
complexes (Figure 1).
In order to investigate dynamic natures of Ti4+ centers in detail,
1
a H NMR titration study of H21 and TiO(acac)2 in the presence
of n-Bu4NOH was conducted (Figure 1). Upon addition of
TiO(acac)2 to a mixture of H21 and n-Bu4NOH in DMF-d7, a
reaction mixture turned dark red immediately, and its 1H NMR
spectrum showed the formation of two distinct species (Figure 2b).
The color of the mixture gradually changed to orange in 20 h, and
finally, the signals for one species completely disappeared and only
one set of signals for the other species remained (Figure 2d). In
either case, the proton signals of the catechol moiety in 1, He-g
,
Figure 2. 1H NMR spectra (500 MHz, DMF-d7, 293 K, [H21] ) 15 mM);
(a) H21; (b-d) a 1:0.43:0.67 mixture of H21, TiO(acac)2, and n-Bu4NOH:
(b) 5 min; (c) 1 h; (d) 20 h ([Ti13]2-); (e) a 4:4:3 mixture of H21, TiO(acac)2,
and N-methylmorpholine (TiH12(acac)); (f) Pd-Ti ring ([Pd2Ti2(H12)2-
(acac)2Cl4]2+); (g) Pd-Ti cage ([Pd3Ti216Cl6]4-). Signals with a + mark
were shifted significantly upfield (∆δ ) -0.26 to -0.79 ppm),
but those of the pyridyl moiety did not. This result indicates that
both of the two species include Ti4+-catecholato complexes. In
ESI-TOF mass measurements of the mixture at specified time
intervals, two prominent signals were observed at m/z ) 845.4 and
516.8 for [Ti13 ·n-Bu4N]- and [Ti12(acac)]-, respectively, im-
mediately after mixing all components, whereas only a signal for
[Ti13 ·n-Bu4N]- was observed after 20 h (Figure S3). These results
exhibited that a novel Ti4+ species bound by one acac and two
in (f) are assigned to those of
a cyclic 3:3 complex [Pd3Ti3-
(H12)3(acac)3Cl6]3+ (see Figures S13 and S14).
catecholato ligands was thus generated in a kinetically controlled
way prior to the formation of a [Ti13]2- species.
The Ti4+ complex as a kinetic intermediate was also detected
in its ESI-TOF mass measurement as [TiH212(acac)]+ at m/z )
519.1 (Figure S4). On the basis of this observation, we deduced
that the intermediate species would be a neutral TiH12(acac) with
† The University of Tokyo.
‡ Japan Science Technology Agency.
9
10058 J. AM. CHEM. SOC. 2008, 130, 10058–10059
10.1021/ja803115j CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
bands for these Pd2+-Ti4+ complexes were found in the same
region as those of the original Ti4+ complexes (Figure 3b),
indicating that the coordination geometries around the Ti4+ centers
in the resulting complexes remained unchanged after Pd2+
complexation.
A trans configuration of the square-planar Pd2+ center in the
complexes was suggested by the selection rule for an IR-active
vibration. In the far-IR region of the ring-shaped complex, only a
peak characteristic for asymmetric stretching vibration was observed
for Pd-Cl and Pd-N(py) at 351 and 313 cm-1, respectively (Figure
S18). A similar result was obtained for the cage-shaped complex:
353 (Pd-Cl) and 325 cm-1 (Pd-N(py)).
The molecular modeling study was performed for the ring and
cage complexes. A ∆∆ (or ΛΛ) isomer arising from the two Ti(IV)
centers was found to be more stable for both complexes. As for
the ring complex, among several possible isomers, a structure shown
in Figure 1 was found the most likely structure (Figure S21).
Finally, the interconversion between the ring and cage was
achieved by Ti4+-centered coordination changes similarly to the
interconversion between [Ti13]2- and TiH12(acac) (Figure S16).
The interconversion was complete within 12 h in either direction.
In conclusion, an interconvertible molecular system was achieved
between heteronuclear Pd(II), Ti(IV)-containing ring- and cage-
shaped complexes. This quantitative interconversion is triggered
by Ti4+-centered coordination changes between a Ti(catecholato)3
and a newly established TiH(catecholato)2(acac) structures. Studies
on their functions of structure-dependent molecular recognition and
inclusion/release control are currently underway.
Acknowledgment. We thank JASCO Co., Ltd., for measuring
the far-IR spectra of the Pd(II), Ti(IV)-containing complexes. This
work was supported by Grants-in-Aid from MEXT of Japan and
the Global COE Program for Chemistry Innovation.
Supporting Information Available: Synthetic procedures, 1H NMR,
ESI-TOF mass, and IR spectra. This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 3. (a) Plot of the molar fractions of TiH12(acac) in the mixture
against the pKa values of the conjugated acids; (b) UV-vis spectra of Ti4+
and Pd2+-Ti4+ complexes of 1 (l ) 1 mm, [H21] ) 0.5 mM, 293 K).
Inset: photographs of DMF solutions of the complexes.
a protonated pyridine ring. To confirm this, we performed com-
plexation using a weaker base, N-methylmorpholine, instead of
n-Bu4NOH (Figure 1). The 1H NMR spectrum was identical to that
of the intermediate (Figure 2e). A series of amine bases was
examined to clarify the relationship between the degree of basicity
and the molar fractions of the two species. As shown in Figure 3a,
the molar fraction of the generated kinetic intermediate is potentially
correlated with the pKa values of the conjugated acids.5 It should
be noted that the 1H NMR signals of the intermediate species
remained nearly unaffected by these bases, whereas those of anionic
[Ti13]2- complex were actually affected by the protonated amine
bases as countercations (Figure S10), suggesting that the kinetic
intermediate has a neutral TiH12(acac) structure. Such TiHL2(acac)
species were also formed from various catechol derivatives (H2L),
such as catechol and 4-tert-butylcatechol (3), using N-methylmor-
pholine as a base (Figures S1, S2, S5, and S6).6,7
Furthermore, the coordinating acac in the TiH12(acac) was also
characterized by IR spectroscopy of a solid sample prepared by
removal of volatile portions: 1525 (CdC) and 1558 cm-1 (CdO)
(Figure S9). Its UV-vis spectrum in DMF showed a characteristic
LMCT band at 424 nm (ꢀ ) 23 000 M-1 cm-1) that is red-shifted
compared with a DMF solution of [Ti13]2- (λmax ) 380 nm, ꢀ )
9700 M-1 cm-1).
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,
TiH12(acac) was produced after 12 h (Figure S15). Subsequently,
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examined with softer, square-planar-coordinating Pd2+ ions. In the
1H NMR spectrum of a 1:1 mixture of TiH12(acac) with
PdCl2(CH3CN)2, the pyridyl proton signals, Ha-d, were shifted
downfield (∆δ ) 0.15-0.35 ppm), whereas those of catecholato
protons, He-f, were shifted only slightly (Figure 2f). This indicates
that site-selective complexation with Pd2+ took place at pyridyl
nitrogens. The ESI-TOF mass spectrum of the mixture showed a
signal for [Pd2Ti2(H12)2(acac)2Cl3]+ at m/z ) 1357.4, indicating
the formation of a cyclic 2:2 complex (Figure S8). The complex-
ation of [Ti13]2- with PdCl2(CH3CN)2 also showed a highly
symmetric 1H NMR spectrum, in which pyridyl proton signals were
shifted downfield (∆δ ) 0.15-0.41 ppm; Figure 2g). The ESI-
TOF mass spectrum of its potassium salt showed a signal at m/z )
1934.5 for [Pd3Ti216Cl6 ·K5]+ consisting of two [Ti13]2- and three
PdCl2 units to form a cage-shaped complex (Figure S7). The LMCT
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(7) TiHL2(acac) complexes were also formed from Ti(OiPr)4, catechols, N-
methylmorpholine, and Hacac. For the details, see the Supporting Information.
JA803115J
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