Helicates with Ether‐Substituted Catechol Esters as Ligands

Monoor biscatechol esters with ether-type substituents or spacers form either triple lithium bridged dimeric helicates or triple stranded helicates with the ability to bind three lithium cations in their interior. Hierarchical helicates with ether or thioether substituents show in solution a monomerdimer equilibrium which is independent of the heteroatom in


Introduction
Self-assembly provides a facile way to synthesise complex supramolecular structures starting from easily available building blocks. Dynamic behavior of the obtained supramolecular aggregates, which ideally can be controlled by some external stimuli, leads into the world of molecular devices like machines or switches. [1] Many chemical devices have been prepared e.g. based on rotaxane and catenane motifs, but other structural moieties have been successfully used as well. [2] In 2005 we introduced hierarchically [3] formed triple lithium bridged helicates [4] based on dinuclear titanium catecholates, [5] which in solution represent a unique class of lithium dependent molecular switches (Figure 1a). NMR spectroscopy allows to observe the equilibrium between the monomeric and dimeric titanium(IV)triscatecholates. [6] The equilibrium mainly depends on the kind of carbonyl substituent (aldehyde, ketone, thioester, the ester substituent. However, dimerization constants are significantly lower than for corresponding alkyl derivatives. Dinuclear helicates with oligoether spacers are well obtained in the presence of lithium cations. Upon removal of the cations the helicates expand and successive addition of LiCl results in compression again. Li [Li 3 (1a) 6 Ti 2 ] are observed at δ = 6.96, 6.50, and 6.42, and for the dominating monomer Li 2 [(1a) 3 Ti] at δ = 6.77, 6.26, and 6.13. In the dimer the methylene group adjacent to the ester splits into two signals of diastereotopic protons at δ = 3.67 and 3.02 ppm (6H each) while in the monomer one signal at δ = 4.18 ppm (12 H) is observed for this group.
From the NMR spectra dimerization constants can be easily extracted (Table 1) at ambient temperature. [6,7] It is observed that the dimerization constants steadily increase with the chain length. However, the observed constants of the glycol and thioglycol derivatives are by one magnitude lower as observed for the corresponding alkyl derivatives. It is assumed that the higher polarity of the heteroatom derivatives results in repulsion between the oxygen or sulfur lone pairs with the hetero atoms and π systems of neighboring catechol units. This effect is strong despite a related high solvophobicity of ethers and alkanes in DMSO which would rather stabilize the dimer. The similarity of the dimerization constants of the oxygen and sulfur derivatives is remarkable, showing the similarity of the two atoms in their solvophobic as well as electronic features in [D 6 ]DMSO.  (1)(2)(3) 6 Ti 2 ] in [D 6 ]DMSO at r.t. determined at a concentration of 10 -2 mol/ L. Li[Li 3 (1)(2)(3) 6 Ti 2 ] X=O X=S X=CH 2 Li[Li 3 (1) 6 Ti 2 ] Li[Li 3 (2) 6 Ti 2 ] Li[Li 3 (3) 6  [a] Ref. [7] In addition to the solution studies it was possible to obtain crystal structures of Li[Li 3 (1a) 6 Ti 2 ] and Li[Li 3 (2a) 6 Ti 2 ] (Figure 2). The overall features of the complex structures are similar to the ones observed before for corresponding dimeric helicates. [6,7] The structures seem to indicate, that, due to the repulsion of Figure 2. The molecular structures of the anions [Li 3 (1a) 6 Ti 2 ] -(a) and [Li 3 (2a) 6 Ti 2 ] -(b) in the crystal. the electron pairs, the heteroatoms in the side chains adopt positions which are located far away from each other. The Xray structural results can be correlated with NMR, ESI-MS based structural assignments. However, in solution side chains possess some high flexibility at the side chains.
Dinuclear helicates with ether type spacers. Connecting two complex units of the hierarchical helicates leads to compounds which act as lithium dependent expandable/compressable switches. A series of oligoglycol bridged ligands 4,5-H 4 have been made by Steglich type esterification [13] of appropriate diols with dibenzyl protected dihydroxybenzoic acid followed by removal of the benzyl protecting groups (Scheme 2). The coordination chemistry of the ligands with titanium(IV) in the presence of alkali metal cations has been tested. The complexes Li[Li 3 (4b) 3 Ti 2 ] and Na 2 [Na 2 (5a) 3 Ti 2 ] were communicated recently and their crystal structures were presented ( Figure 3). Both compounds show a compressed structure with the expected geometry for Li[Li 3 (4b) 3 Ti 2 ] while Na 2 [Na 2 (5a) 3 Ti 2 ] represents a topological isomer to the "classical" helicates with two of the spacers attached to one of the catechols from the inside and to the other from the outside. [14] Figure 3. The structures of Li(DMF) 2 [Li 3 (4b) 3 Ti 2 ] and [Na 2 (5a) 3 Ti 2 ] 2-.
In here the ligands 4a,c,d are additionally introduced in order to study the influence of different spacer length on the complex formation. Furthermore, the new ligand 5b represents a chiral version of the earlier investigated 5a, allowing the study of the stereochemical influence of the remote chiral group.
However, 1 H NMR spectroscopy reveals some surprises. For Li[Li 3 (4a) 3 Ti 2 ] and Li[Li 3 (4b) 3 Ti 2 ] the expected spectra are observed which show three resonances for the protons of the catechol units in the aromatic region. In case of the complexes with very long spacers Li[Li 3 (4c) 3 Ti 2 ] and Li[Li 3 (4d) 3 Ti 2 ] two sets of signals are observed for the catechol units ( Figure 4). The ESI MS results as well as the NMR spectra indicate that in case of Li[Li 3 (4c,d) 3 Ti 2 ] two different isomers with high symmetry are present. We assume that this only can be due to different orientations of the spacers. The structure of Li[Li 3 (4b) 3 Ti 2 ] reveals that in this case the spacer bridges the ester units in front of the lithium cations (Type I). This is the only possible arrangement in case of short chain length. With longer spacers the alternative Type II structure with the spacers bridging "over" the aromatics becomes also possible. This is schematically illustrated in Figure 5 for the Type I and Type II structures of [Li 3 (4d) 3 Ti 2 ] -.
It was possible to obtain crystals of Na[Li 3 (4c) 3 Ti 2 ] which were sufficient for crystal structure analysis. In the crystal the complex adopts a Type I structure with the spacers bridging in front of one lithium cation. Hereby crown ether-type loops are found and in one of those the sodium cation is bound (Figure 6). [15] The lithium as well as the sodium complexes M 4 [(5a) 3 Ti 2 ] (M = Li, Na) have already been described. [14] The potassium salt shows two different sets of signals relating to two complexes which structurally could not be assigned. Based on the ESI MS observations (m/z = 1552.1572 [K 3 (5a) 3 Ti 2 ] -, 1629.0768 K 5 [(5a) 3 Ti 2 ] + ) it is assumed that at least one isomer should adopt the triple stranded helicate structure.  Ligand 5b-H 4 [16] represents a chiral version of 5a-H 4 . The coordination compounds M 4 [(5b) 3 Ti 2 ] (M = Li, Na, K) can be obtained from this ligand.
The potassium salts K 4 [(5a,b) 3 Ti 2 ] show two major isomers which cannot be structurally assigned.
In the case of the lithium complex Li[Li 3 (5b) 3 Ti 2 ] similar shifts as found for Li[Li 3 (5a) 3 Ti 2 ] are observed by proton NMR spectroscopy. [14] However, the dominating signals split into two sets. This is tentatively assigned to the inefficient stereocontrol at Li[Li 3 (5b) 3 Ti 2 ] by the remote chiral units of the spacer resulting in two different diastereoisomers (SSΛΛ and SSΔΔ).
The spectrum of the sodium complex Na 2 [Na 2 (5b) 3 Ti 2 ] correlates with the one of the achiral complex Na 2 [Na 2 (5a) 3 Ti 2 ] showing that again the "topological" helicate isomer is formed (Figure 7). [14]  CD spectra were measured for the chiral complexes M 4 [(5b) 3 Ti 2 ] (M = Li, Na, K). The transitions at the titanium(IV) catecholate moieties provide information on the chirality (Δ vs. Λ). [17] The results show that in the lithium salt, the complexes preferably adopt Δ configuration which is also favored in the sodium complex. However, in the latter case much lower ellipticity is observed. For the potassium salt, the favored stereochemistry at the catecholate complexes is inverted compared to the lithium or sodium salt ( Figure 8). A similar stereochemical inversion effect has been already observed earlier with ester catecholate based titanium(IV) complexes and has been discussed in detail at this time. [11,18]  In order to obtain crystals of the coordination compounds of ligand 5b, different salts were added to lead to better crystallization properties. Thus, the crystal structure of [AsPh 4 ] 2 -[Na 2 (5b) 2 Ti 2 O 2 ]·2MeOH has been obtained. A central bis-μ-oxo bis titanium(IV) moiety is formed. The two titanium centers are bridged by two oxygen atoms as well as two ligands 5b. Two sodium cations are included in the complex, binding to the catecholesters and additionally to one molecule of methanol each ( Figure 9). The structure of [Na 2 (5b) 2 Ti 2 O 2 ] 2is related to the ones observed earlier for dinuclear titanium(IV) complexes with amino acid bridged dicatechol ligands. [19]  However, due to the absence of resonances of the free ligand 5b in the crude product spectra of Na 4 [(5b) 3 Ti 2 ] formed from three equivalents of ligand with two equivalents of titanium(IV) ions it is expected that [Na 2 (5b) 2 Ti 2 O 2 ] 2is only formed under the crystallization conditions.
Expansion and compression of helicates with ether type spacers. Due to the expanded and compressed structures of the helicates M 4 [(4/5) 3 Ti 2 ], switching in a spring-type fashion is feasible depending on the cations. The corresponding switching of M 4 [(5a) 3 Ti 2 ] has been already reported. [14] In here consecutive expansion and compression experiments have been performed in one NMR test tube starting with the compressed forms Li[Li 3 (4a-d) 3

Conclusions
In here a series of hierarchical and expandable/compressible helicates with ether-type ester substituents is presented. In many respects those complexes behave as observed for the hydrocarbon analogs. However, some differences are found and some new observations are made: -Hierarchical helicates with ether or thioether-type substituents dissociate into the monomers more easily compared to the analogous alkyl derivatives. Hereby, the dimer stability is independent on the heteroatom oxygen vs. sulfur in the side chain.
-Helicate based expandable and compressible molecular switches are obtained with ether containing spacers and can easily be switched. However, for the first time two different isomers are observed for the lithium complexes Li[Li 3 (4c,d) 3 Ti 2 ] which are assigned to Type I and Type II isomers with different positions of the connecting units.
Thus, the coordination chemistry of the ether substituted catechol and dicatechol ester ligands is well explored and in the future will be used for host-guest chemistry with cations which may be bound in the loops [15] of M 4 [(4a-d) 3 Ti 2 ] in the presence of lithium but not of other cations.

Experimental Section
General notes. Unless stated otherwise, all commercial reagents were used without further purification. Substances and chemicals used in this research were purchased from ABCR, Acros Organics, Alfa Aesar or Sigma Aldrich. 2,3-Bis(benzyloxy)benzoic acid was prepared according to a literature procedure. [1] Moisture or oxygen intrinsic phasing (SHELXT). The structures were refined using SHELXL-201813 with a least-squares procedure against F 2 . Na[Li 3 (4c) 3 Ti 2 ] was refined as a two-component twin (x = -0.017 (15)). Due to inadequate data-parameter ratio, the disorder of the crown ether-type moieties could not be refined. The sodium ion was modelled as a disorder over two positions (0.58:0.42). The residual electron density of 1.88 (-0.0699, 0.4020, 0.4446) indicates another likely position for the disordered sodium cation. Disordered solvent in Na 2 [(5b) 2 Ti 2 ] was treated by applying a solvent mask in OLEX2. Eight molecules of methanol can be estimated per formula unit. Hydrogen atoms were refined using riding models with Ueq(H) of 1.5 Ueq(C) for terminal methyl groups, and 1.2 Ueq(C) for other groups.

General Procedure for the Preparation of ligands 1,2-H 2 .
Ligands 1a-c-H 2 and 2a-c-H 2 were synthesized by esterification, starting with the conversion of 2,3-Dihydroxybenzoic acid (1 equiv.) into its corresponding acid chloride via refluxing in thionyl chloride (30 equiv.) for 1 hour. The excess thionyl chloride was removed under reduced pressure and the remaining acid chloride used in the next step without further purification. The acid chloride was then reacted with a mixture of the corresponding alcohol (3 equiv.) and triethylamine (6 equiv.) in chloroform to afford the desired product after purification via column chromatography.