Key words
self-assembly - helicates - molecular switches - stereochemistry
Introduction
The development of molecular switches is one important aspect in the transition of
“classical” supramolecular chemistry to “functional” nanotechnology. Hereby, supramolecular
chemistry closes the gap between the natural science chemistry and the technology-orientated
nanosciences. Basic concepts of molecular switches have been described for several
decades and were based on photoswitchable entities as well as catenanes and rotaxanes.[1]
For some years, we have been studying the expansion and compression of helicate-type
coordination complexes based on the binding of internal cations which can be released
upon an external stimulus (e.g. addition of a cryptand).[2] This kind of research is based on some preliminary concepts published by Yashima,
who switched the length of a boran(III)-based double-stranded helicate by addition
of sodium cations.[3] As an important finding, we could show that with chiral biscatechol ligands 1-H4, a three-state molecular switch is obtained, which allows control of the extended
and compressed state of the complex, as well as of the helical twist.[2] In the present report, we describe a related system, 2-H4, in which the chiral information is not located close to the complex units, but rather
far away in the center of the ligand spacer ([Figure 1]).
Figure 1 Biscatechol ligands for the stereoselective self-assembly of expandable and compressible
helicates.
Results and Discussion
The preparation of ligand 2-H4 follows the sequence shown in [Scheme 1]. The tartaric acid ester 3 is benzyl-protected to obtain 4, which is reduced with LiAlH4 to obtain diol 5. This is elongated to the benzyl-protected 6. Removal of the THP groups affords again a diol 7, which finally is coupled with the benzyl-protected catechol 8 to afford, after removal of the protecting groups, the ligand 2-H4.
Scheme 1 Preparation of ligand 2-H4.
In a coordination study, ligand 2-H4 (3 equiv.) and TiO(acac)2 (2 equiv.) are reacted in methanol in the presence of appropriate bases (2 equiv.,
M2CO3, M = Li, Na, K).[4] For the lithium salt Li4[Ti2
2
3], a well-resolved NMR spectrum is observed in DMSO-d6. The ESI MS reveals in the negative mode the signals of the monoanionic helicate
at m/z = 1551.2829 (calcd. m/z = 1551.2778).[5] The corresponding sodium or potassium salts, M4[Ti2
2
3], show rather broad signals by NMR as observed in earlier cases, but they again show
in the ESI MS the presence of helicate-type complexes M3[Ti2
2
3]− (M = Na: 1599.1966, calcd.: 1599.1991; M = K: 1647.1262, calcd. 1647.1209). The different
spectroscopic behavior already shows the difference of the compressed lithium compound
in comparison to the expanded sodium and potassium salts ([Figure 2]).
Figure 2 The compressed complex Li4[Ti2
2
3] (A) and the expanded helicate-type complexes M4[Ti2
2
3] (B, M = Na, K).
The lithium salt KLi3[Ti2
2
3] was crystallized from DMF by diffusion of diethyl ether into the solution. In order
to facilitate crystallization, a trace amount of KPF6 was added.[6]
As observed in other cases,[2] a dinuclear titanium(IV) helicate is formed, which encapsulates three lithium cations
by binding to the internal ester and catechol oxygen atoms. Hereby, the three lithium
atoms adopt a tetrahedral coordination geometry. The titanium ions coordinate with
three catecholates, each resulting in a distorted octahedral geometry. A special feature
of Li3[Ti2
2
3]− is the presence of the diol unit located in the center of the ligand spacer possessing
an SS-configuration while the complex units show a ΔΔ configuration (right-handed twist).
In Li3[Ti2
1
3]−, the opposite twist (ΛΛ) was observed. In the former complex, the stereochemical
control was due to the spatial requirements of the phenyl substituents close to the
catecholates,[2] while in the new complex Li3[Ti2
2
3]−, it was due to the position of the OH substituents in the plane of the three lithium
cations (“pseudo-equatorial”) ([Figure 3]).
Figure 3 The structure of the compressed complex Li3[Ti2
2
3]− in the crystal. a) Side view, b) top view; grey: C, white: H, blue: Li, red: O, yellow:
Ti.
Upon addition of [2.1.1]cryptand to the complex Li4[Ti2
2
3], lithium cations are removed, switching the structure to its expanded form. Addition
of LiCl reverses this process. This can be nicely observed by following the NMR signals
of the catecholate units ([Figure 4]).
Figure 4 Expansion and compression of the helicate by removing and addition of lithium cations.
In order to evaluate the stereochemistry of the compressed and expanded helicate in
solution, CD spectra of the complex Li4[Ti2
2
3] were measured ([Figure 5]). The observed Cotton effects between 300 and 500 nm represent the chirality of
the titanium catecholates. Hereby, the ΔΔ configuration is confirmed for the metal
complex units in solution (blue line) as has been found by X-ray diffraction.[7] Upon addition of [2.1.1]-cryptand, the chiral induction of the diol on the complex
units nearly vanishes (black line) but is in part recovered upon addition of LiCl
(red line). The sodium and potassium salts M4[Ti2
2
3] (M = Na, K) show CD spectra similar to the lithium compound with much lower intensity
of the Cotton effects. This indicates a less effective induction of the stereochemistry
at the complex units and explains the observation of broad signals by NMR spectroscopy.
Figure 5 CD spectra of Li4[Ti2
2
3] upon successive addition of [2.1.1]-cryptand and LiCl in DMSO solution. The CD spectrum
after successive addition of cryptand and LiCl does not correspond to the initial
spectrum due to the addition of cryptand and LiCl dissolved in DMSO.
Conclusions
In here we presented a new length-switchable helicate with central diol units in the
spacer. The chirality of the spacer is transferred to the metal complex units and
in the expanded form of the helicate it strongly controls the helicity of the complex.
This stereocontrol is lost upon expansion.
The introduction of the diol unit at the ligand enables further reactivity of the
helicate, e.g., for metal coordination or for the generation of switchable catalysts.[8]
Funding Information
This study was supported by the Deutsche Forschungsgemeinschaft (Al 410/37 – 1).
