Dual switchable molecular tweezers incorporating anisotropic MnIII–salphen complexes

Benjamin Doistau a, Lorien Benda a, Jean-Louis Cantin b, Olivier Cador c, Fabrice Pointillart c, Wolfgang Wernsdorfer d, Lise-Marie Chamoreau a, Valérie Marvaud a, Bernold Hasenknopf a and Guillaume Vives *a
aSorbonne Université, UMR CNRS 8232, Institut Parisien de Chimie Moléculaire, 4 place Jussieu, 75005, Paris, France. E-mail: guillaume.vives@sorbonne-universite.fr
bSorbonne Université, INSP, 4 place Jussieu, 75005, Paris, France
cUniv Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) – UMR 6226, F-35000 Rennes, France
dPhysikalisches Institut and Institute of Nanotechnology, Karlsruhe Institute of Technology 76131 Karlsruhe, Germany CNRS and Université Grenoble Alpes, Institut Néel, 38042 Grenoble, France

Received 21st April 2020 , Accepted 25th May 2020

First published on 29th May 2020


An alternative strategy for the synthesis of terpyridine based switchable molecular tweezers has been developed to incorporate anisotropic Mn(III)–salphen complexes. The free ligand was synthesized using a building block strategy based on Sonogashira coupling reactions and was then selectively metalated with manganese in a last step. The conformation of the tweezers was switched from an open ‘W’ shaped form to a closed ‘U’ form by Zn(II) coordination to the terpyridine unit bringing the two Mn–salphen moieties in close spatial proximity as confirmed by X-ray crystallography. An alternate switching mechanism was observed by the intercalation of a bridging cyanide ligand between the two Mn–salphen moieties that resulted in the closing of the tweezers. These dual stimuli are attractive for achieving multiple controls of the mechanical motion of the tweezers. A crystallographic structure of unexpected partially oxidized closed tweezers was also obtained. One of the two Mn–salphen moieties underwent a ligand-centered oxidation of an imino to an amido group allowing an intramolecular Mn–Oamide–Mn linkage. The magnetic properties of the manganese(III) dimers were investigated to evaluate the magnetic exchange interaction and analyze the single molecule magnet behavior.


Introduction

Inspired by Nature or by her macroscopic analogues, chemists have developed a large variety of molecular machines1 since the pioneering work of J.-P. Sauvage, F. Stoddart and B. Feringa who were awarded the Nobel Prize in Chemistry in 2016. While molecular machines allow a fine control of mechanical motion, exploiting their operation to modulate physical or chemical properties at the molecular level remains a challenge.2 Among the large variety of molecular machines developed such as molecular switches,3 shuttles,4 motors,5 or nanovehicles,6 switchable molecular tweezers7 are particularly interesting mechanical switches. Such systems exploit stimuli-responsive switching units to control the distance between functional units enabling reversible guest recognition. Thus, a multi-responsive system by orthogonal stimuli (i.e. that can be applied independently) is potentially accessible due to (i) the switching stimulus that can be redox,8 photochemical9 or coordination driven10 and (ii) the guest interaction.

We have developed switchable molecular tweezers by metal coordination that present the advantage of stability and full conversion between the two states. Our system is composed of a terpyridine ligand substituted in 6 and 6′′ positions by two arms bearing M–salphen11 complexes as functional moieties (Fig. 1). Upon metal coordination terpyridine can switch from a “W” shaped open form to a “U” shaped closed form,10a–f bringing the two functional units in close proximity. This controlled and large modification of the distance between the two functional units has been successfully applied to modulate luminescence,12 redox13 and catalytic14 properties with Pt(II), Ni(II) and Zn(II) salphen complexes respectively. As a proof of concept with Cu(II) tweezers, we have obtained a switch from a paramagnetic to a weakly antiferromagnetically coupled system upon closing.15 Since this innovative approach to modulate magnetic properties remains largely unexplored,16 we wish to incorporate Mn(III)–salphen complexes that present attractive magnetic properties. Indeed, Mn(III)–salen complexes are versatile complexes that have been widely used in catalysis for oxidation17 or epoxidation18 reactions, as biomimetic models of manganese oxidase or catalase19 and extensively for their magnetic properties.20 In particular, the strong magnetic anisotropy of Mn(III)–salen originating from the zero field splitting (ZFS) imposed by the Jahn–Teller elongation has attracted great interest to generate Single Molecule Magnets (SMMs).21 Such an effect has been evidenced not only in Mn12[thin space (1/6-em)]22 and Mn6[thin space (1/6-em)]23 compounds but also in lower-nuclearity clusters, such as Mn(III) dimers,24 or even in mononuclear Mn(III) complexes.25 By modulating the magnetic properties of high spin (S = 2) Mn(III)–salen complexes with the mechanical motion of tweezers, we were interested in exploring two main aspects: (i) the potential additional intercalation of a bridging cyanide ligand to modulate the magnetic properties, (ii) a single molecule magnet behavior that might occur in Mn(III) dimers. Indeed, cyanide ligands have been widely used to obtain bridged polynuclear complexes with controlled geometries and predictable exchange coupling interactions resulting in attractive magnetic properties.26 Herein the synthesis of Mn-based molecular tweezers as well as their switching and stimuli responsive magnetic properties are presented.


image file: d0dt01465f-f1.tif
Fig. 1 (a) Schematic principle of switchable molecular tweezers with two stimuli. (b) Molecular representation of synthesized Mn(III) tweezers 1-Cl2.

Results and discussion

Synthesis

Two main routes were envisioned for the synthesis of tweezers 1-Cl2: (i) a “chemistry on complex” route where the Mn–salphen moieties are connected by a Sonogashira reaction to the terpyridine central unit (Scheme 1) and (ii) the synthesis of the free ligand followed by a double coordination in the last step (Scheme 2). The former approach avoids the challenge to coordinate selectively metal cations to the salphen and terpy parts but requires inert complexes as demonstrated with Pt(II), Cu(II) and Ni(II) analogues. Even if Mn(III)–salphen complexes are more labile, this route was first investigated (Scheme 1). Free Br–salphen ligand 212a was coordinated to MnCl2 in the presence of Et3N to deprotonate the phenol groups, followed by air oxidation27 to obtain MnIII–salphen complex 3 in a quantitative yield. Complex 3 was further subjected to a Sonogashira coupling reaction with trimethylsilylacetylene (TMSA). High catalyst loading (Pd(PPh3)2Cl2 15% mol, CuI 30% mol) and an excess of alkyne (10 equiv.) were required for the reaction to proceed. The coupling product 4 was isolated after purification by column chromatography in 59% yield. This indicates the compatibility of Mn(III)–salphen complexes with Sonogashira coupling conditions. The deprotection of the TMS group was attempted with K2CO3 in MeOH/THF, but a mixture of the product with partial decoordination was obtained probability due to the formation of manganese hydroxide under basic deprotection conditions. Alternative conditions using tetrabutylammonium fluoride (TBAF) were also attempted without success, leading only to the degradation of the complex.
image file: d0dt01465f-s1.tif
Scheme 1 First strategy based on a chemistry on complex with Mn–salphen.

image file: d0dt01465f-s2.tif
Scheme 2 Second strategy based on convergent free tweezers’ ligand synthesis.

In order to circumvent this problem, the coordination of manganese on the alkyne substituted salphen ligand 7 was envisioned (Scheme 2). By a Sonogashira coupling of 2 with TMSA followed by deprotection, free salphen 7 was obtained. The yield of the Sonogashira coupling is moderate (56%) due to a partial coordination of the product by the copper catalyst and hydrolysis of the imine bond during the purification by column chromatography. Coordination with MnCl2 yielded complex 5 in 71% yield. The final double Sonogashira coupling was then attempted using the standard PdCl2(PPh3)2 catalyst and more reactive electron-rich tri-tert-butyl phosphine (PdCl2(PhCN)2, [(tBu)3PH]BF4)28 but in both cases only the Glaser homo-coupling product from two molecules of 5 was observed. The redox potential of Mn(III) being higher than that of Cu(I), a partial oxidation of Cu(I) to Cu(II) favoring Glaser coupling is probably the reason for this failure. Since it was difficult to use a large excess of alkyne like in the synthesis of salphen 4, the “chemistry on complex” synthetic strategy with Mn–salphen coupling in the last step appeared not suitable for the incorporation of such sensitive complexes in the tweezers.

An alternate route was designed by introducing Mn(III) in an ultimate coordination step to metal-free tweezers ligand 9-H4. Following this new route, 7 was coupled to dibromoterpyridine 8 (Scheme 2). Free tweezers’ ligand 9-H4 was obtained in 51% yield after purification by column chromatography and recrystallization. In a last step, manganese was coordinated to 9-H4 using the usual conditions. An extraction with pentamethyldiethylenetriamine (PMDETA) was performed in order to remove the excess manganese ions potentially coordinated to the terpyridine ligand. Due to the higher binding constant of the salphen moiety compared to the terpyridine tweezers, [MnIII2(9)Cl2] (noted 1-Cl2) was obtained with 65% yield after a final recrystallization in acetonitrile. The HRMS spectrum is in agreement with an uncoordinated terpyridine moiety with signals at m/z 1499.62 and 731.84 corresponding to [1-Cl]+ and [1]2+ and no signal corresponding to a species with three Mn(III) ions (see Fig. S1). Due to the repulsion between the nitrogen lone pairs, we can assume that 1-Cl2 adopts a ‘W’ shaped open conformation as observed for diamagnetic analogs. Compared to the chemistry on complex route for the analog Cu(II) tweezers,15 the overall yield of this route is lower (43 and 15% respectively) but still acceptable with a 63% average yield. This difference is principally due to the acid lability of the imine bonds of the free salphen ligands which results in some degradation during the purifications by column chromatography. Nevertheless, this new synthetic strategy opens up perspectives for the synthesis of tweezers incorporating complexes sensitive to cross-coupling reactions conditions, or labile first row transition metals. All organic compounds have been fully characterized by 1H, 13C and 2D NMR and mass spectrometry (ESI-HSMS). Full details are given in the Experimental section and the ESI.

Switching studies by zinc coordination

The closing of tweezers 1-Cl2 was monitored by UV-Vis spectroscopy. Titration of a solution of 1-Cl2 in chloroform by the addition of a small aliquot of a concentrated solution of ZnCl2 in acetonitrile (Fig. 2) showed a single evolution up to 1.0 equiv. of Zn2+. Isosbestic points were observed at the two curve crossings (λ = 358 and 485 nm), which are consistent with an equilibrium solely between the open and closed forms. The formation of a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 complex was confirmed by mass spectrometry with signals at m/z 1634.49 corresponding to mono-cationic [Zn(1-Cl2)Cl]+ and 799.72 di-cationic [Zn(1-Cl2)]2+ complexes (see Fig. S2). Fitting of the titration curve with a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 binding model revealed a strong association constant (log[thin space (1/6-em)]K > 7) comparable to previous terpy-based tweezers.12a When the titration was performed in methanol, a similar behavior was observed with a single evolution, leading to the formation of [Zn(1-Cl2)Cl2] (see Fig. S3). However, the apparent binding constant is much lower (log[thin space (1/6-em)]K = 4.7) due to the presence of methanol as a competing coordinating solvent for Zn(II).
image file: d0dt01465f-f2.tif
Fig. 2 (a) UV-Vis titration of 1-Cl2 (5.0 × 10−6 mol L−1 in CHCl3) by ZnCl2 (1.0 × 10−3 mol L−1 in CH3CN). (b) Absorption at 274 nm and fitting with a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 binding model.

Single crystals of the closed tweezers suitable for X-ray diffraction were obtained by slow evaporation of a methanol solution (Fig. 3). [Zn(1-Cl2)]Cl2 crystalizes in a triclinic space group P[1 with combining macron] (no. 2) with a unit cell of 5137.24(13) Å3 (a = 14.1647(2) Å; b = 15.3020(2) Å; c = 25.9901(4) Å; α = 88.142(1)°; β = 84.223(1)°; γ = 66.438(1)°) bearing two molecules in a tight packing (see Fig. S4). The molecule adopts helicoidal geometry with the two enantiomers present in the crystal. The alkyne spacers are distorted from linearity with average angles of 175.6(1)° and 169.1(1)° to enable the two salphen moieties to point outward and avoid steric hindrance between the two pseudo-octahedral Mn ions. This results in quite a large Mn–Mn distance of 6.2 Å compared to similar tweezers where the intramolecular M–M distance was between 3.8 and 4.8 Å for Pt(II)12a and Ni(II)13 respectively. Surprisingly the two Mn–salphen complexes present a different coordination sphere for Mn; the first one adopts a distorted Jahn Teller octahedral geometry with average Mn–N and Mn–O distances of 1.98 Å and 1.87 Å respectively, with a chloride ligand in an apical position (Mn–Cl: 2.4827(8) Å) pointing inward the cavity and a methanol ligand (Mn–OMe: 2.508(3) Å) pointing to the outside. The second Mn ion adopts pseudo-octahedral geometry which is less distorted with methanol and water ligands in apical positions (Mn–OMe: 2.292(3) Å; Mn–OH2: 2.284(2) Å). The chloride ligand on this Mn was replaced by MeOH and H2O ligands during the crystallization but is still present as a counter ion in the asymmetric unit confirming the +III oxidation state for both Mn ions. The coordination of these apical ligands on manganese prevents their stacking and results in an open cavity despite the coordination of zinc on the terpyridine ligand that forces the salphen moieties to be in proximity.


image file: d0dt01465f-f3.tif
Fig. 3 Single crystal X-ray diffraction structure of closed tweezers [Zn(1-Cl2)]Cl2 (a) side and (b) front view; solvent molecules, counter anions and hydrogen atoms (except on H2O and CH3OH) are omitted for clarity.

The effect of the conformation change on the magnetic properties was first studied by EPR spectroscopy. High spin Mn(III) is an integer spin (S = 2) system which is considered as EPR silent for standard perpendicular mode EPR spectroscopy.29 However parallel mode EPR can be used and a characteristic sextuplet was observed at a low field for open tweezers 1-Cl2 (Fig. 4a). Due to the large zero field splitting value (2.1 to 2.5 cm−1 for Mn(III)30) of the ground state, the transitions observed in the X-band arise only from MS = –2 → 2 levels with an hyperfine coupling with the nuclear spin (I = 5/2) of Mn as described in the literature.30 The spectrum was accurately simulated (Fig. 4) with D = –2.5 cm−1, E = 0.046 cm−1, A = 190 MHz, A = 118 MHz, g = 2.0, and g = 1.98 parameters. Since the EPR simulation is not sensitive to the perpendicular g and A values, typical values for axially elongated six-coordinate Mn(III) ions were selected. Upon addition of ZnCl2 the signal is broadened (Fig. 4b) probably due to dipolar interactions between the two Mn(III) centers that are in spatial proximity in the closed conformation.


image file: d0dt01465f-f4.tif
Fig. 4 X-band EPR spectra of (a) open tweezers 1-Cl2, (b) closed tweezers [Zn(1-Cl2)Cl2] and (c) tweezers 1-Cl2 with 50 equiv. of CN (in MeOH at 5 K) in parallel mode. Closed tweezers were prepared in situ by the addition of a concentrated solution of ZnCl2 or TBACN in MeOH.

Since anisotropic Mn(III) complexes have been reported to display SMM behavior, the effect of the conformation change was further investigated by SQUID magnetometry. The alternating current (ac) magnetic susceptibilities were measured in frozen solution to avoid intermolecular interactions for different applied static fields. No out-of-phase component of the magnetic susceptibility was observed for the open and closed tweezers at any magnetic field, indicating the absence of the slow relaxation of magnetization, i.e. no SMM behavior. The lack of significant magnetic interactions between the two Mn–salphen units in the closed state as expected from the large Mn–Mn distance in the solid state should be responsible for this absence of SMM behavior.

Intercalation studies with the cyanide ligand

In order to increase the magnetic interaction between the two Mn(III) centers in the closed form, the effect of a bridging ligand was investigated. Since cyanide bridged Mn(III)–salen complexes display attractive properties such as spin-crossover, photomagnetic or single molecule magnet behavior,20d,21 the binding of a cyanide ligand to tweezers 1-Cl2 was explored by UV-Vis titration. Upon addition of tetrabutylammonium cyanide (TBACN) to a solution of open tweezers 1-Cl2 in chloroform, a bathochromic shift was observed for the MLCT transition at 510 nm with a broader band appearing around 550 nm (Fig. 5). The transitions in the UV regions are also affected by the addition of a cyanide ligand with a hypochromic shift. This behavior was also observed for model Mn–salphen complex 10 (see Fig. S5), indicating the coordination of cyanide to the Mn(III) center. A single evolution with isosbestic points at all curve crossings (λ = 398, 428 and 540 nm) was observed from 0 to around 1 equiv. of CN, indicating a strong binding of one cyanide ligand to tweezers 1-Cl2 (log[thin space (1/6-em)]K = 7.2). In contrast, the titration with model Mn–salphen presented a smooth evolution up to 4 equivalents corresponding to the successive coordination of two CN ligands (log[thin space (1/6-em)]K1 = 4.5; log[thin space (1/6-em)]K2 = 3.3) to form the [Mn–salphen(CN)2] complex as previously reported.31 This significant difference between tweezers 1-Cl2 and the model Mn–salphen complex is probably due to the formation of an intramolecular cyanide bridged Mn dimer that forces the closing of the tweezers instead of forming the expected open [1(CN)4]2− species. An intermolecular cyanide bridging to yield a cyclic dimer [(1-Cl2)2(CN)2] is less plausible to explain this difference. Furthermore, no signal corresponding to such a dimer was observed by mass spectrometry.
image file: d0dt01465f-f5.tif
Fig. 5 UV-Vis titration of 1-Cl2 (5.0 × 10−6 mol L−1) by TBACN in CHCl3. (b) Absorption at 338 nm and fitting with a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 binding model.

The infrared spectrum is in agreement with a bridging CN ligand as the observed ν(CN) band at 2205 cm−1 for [1-CN] is shifted to a higher energy compared to model [Mn–salphen(CN)2] at 2196 cm−1 (Fig. S23 and S24). Such an increase in the ν(CN) values is typical of a bridged M–CN–M system compared with a terminal M–CN as the donation of σ-electron density from the antibonding lone pair on the nitrogen depopulates the antibonding CN orbital and strengthens the triple bond.32 EPR spectroscopy provided further evidence on the cyanide binding mode. Upon addition of a cyanide ligand to the model Mn–salphen complex, a disappearance of the characteristic sextuplet was observed in EPR (see Fig. S6) as expected for the low spin (S = 1) EPR silent [Mn–salphen(CN)2] complex.31 In contrast, upon addition of a CN ligand to open tweezers 1-Cl2, an EPR signal is still observed with only a slight change in the hyperfine structure from 42.5 G to 40 G (Fig. 4c), indicating a modification in the coordination sphere of the Mn and a high spin (HS) Mn–salphen moiety. One HS Mn(III) is expected in the case of an intramolecular Mn–CN–Mn bridge as the coordination by the nitrogen atom is not an enough sigma donor to induce a spin transition of Mn(III) to a LS configuration.20a The formation of a mixed HS/LS configuration for the cyanide-closed tweezers was confirmed by magnetometry experiments performed in frozen solution (see Fig. S9). The high temperature χT saturation value for the cyanide closed tweezers is lower than that for the zinc closed tweezers [Zn(1-Cl2)]Cl2 as expected from the resulting total spin system S = 3 compared to S = 4. AC magnetic susceptibilities were also measured in frozen solution, but no out-of-phase component of the magnetic susceptibility was observed at any magnetic field, indicating no SMM behavior of this cyanide bridged Mn dimer. Nevertheless, the cyanide ligand by bridging the two functional Mn–salphen complexes can be considered as an alternate stimulus that combines mechanical motion and a spin crossover mechanism by switching from two HS Mn(III) to a HS/LS mixed configuration (Fig. 6). Even if the binding of CN is very strong and is not proved reversible, this orthogonal mechanism compared to Zn(II) coordination to the terpy unit offers attractive conceptual perspectives to achieve multiple controls of tweezers’ mechanical motion.


image file: d0dt01465f-f6.tif
Fig. 6 Schematic representation of the dual closing mechanism induced by Zn(II) or CN stimuli and its effect on the spin state of the system.

The effect of the addition of a cyanide ligand to the already zinc-closed tweezers was also investigated. The UV-Vis titration in chloroform (see Fig. S7) presents a first evolution up to 2 equiv. of CN that can be attributed to the exchange of the chloride ligands on Zn(II) and a second evolution corresponding to the exchange of chloride on the Mn(III) centers. Indeed, at the end of the titration upon addition of an excess of cyanide (∼15 equiv.), a [Zn(1)(CN)5] species was observed by mass spectrometry with a signal at m/z 1659.6 (see Fig. S11). This species corresponds to the exchange of the chloride ligands by cyanide on both Zn(II) and Mn(III) centers and the coordination of additional cyanide that should act as a bridging ligand between the two Mn–salphen moieties. Upon addition of cyanide to closed tweezers [Zn(1-Cl2)Cl2], the EPR spectra first exhibit a broadening and then a decrease in the signal intensity that might correspond to the formation of the LS Mn(III) species. Then upon addition of a large excess (∼50 equiv.) of cyanide, a signal similar to the one obtained with the open tweezers in the presence of cyanide but with a lower intensity is observed (see Fig. S8) which might be explained by cyanide-induced zinc demetallation, as [Zn(CN)4]2− species was observed by mass spectrometry.

Unexpected tweezers’ oxidation

In an attempt to gain a better insight into the structure of the tweezers with cyanide ions, single crystals suitable for XRD were grown by slow evaporation of a solution of closed tweezers [Zn(1-Cl2)]Cl2 in methanol with TBACN. The tweezers crystalize in the triclinic space group P[1 with combining macron] (no. 2) with a unit cell of 5254.72 Å3 (a = 15.1358(12) Å; b = 16.521(1) Å; c = 22.9401(16) Å; α = 89.629(5)°; β = 72.981(5)°; γ = 73.972(5)°) (Fig. 7).
image file: d0dt01465f-f7.tif
Fig. 7 Single crystal X-ray diffraction structure of an oxidized closed tweezers’ dimer with cyanide [ZnIIMnIII2(9ox)(CN)1.5(OH)(CH3OH)]2+; (a) side view (b) packing showing the dimer with a cyanide bridging ligand. Solvent molecules, counter anions and hydrogen atoms are omitted for clarity.

The cyanide ligands are coordinated to the zinc of the terpyridine unit as expected from the UV-Vis titration since the sample that crystallized contained only 2 equiv. of TBACN. One cyanide acts as a bridge between two tweezers by a Zn–CN–Zn linkage (with a disordered orientation of CN), with the zinc coordination sphere being completed by a second terminal cyanide. The tweezers’ dimer being cationic, a [ZnCl4]2− counter anion is also present in the unit cell. The C–N distance of the bridging cyanide (1.18(1) Å) is slightly longer than that of the non-bridging one (1.12(1) Å). Interestingly one Mn–salphen moiety of each tweezer undergoes a ligand localized oxidation of an imino to an amido group. The oxygen of the so-formed amido group is coordinated to the other Mn of the tweezers resulting in an intramolecular bridge with a Mn–Mn distance of 5.408 Å much shorter than in [Zn(1-Cl2)]Cl2. While the oxidized Mn–salphen displays a distorted pyramidal geometry with an hydroxyl in the apical position (Mn–OH: 2.336(5) Å), the non-oxidized Mn–salphen adopts a Jahn Teller distorted octahedral geometry with a methanol molecule in the apical position (Mn–OMe: 2.274(7) Å), with the amide oxygen of the oxidized Mn–salphen completing the coordination sphere (Mn–Oamide: 2.210(6) Å). This type of selective ligand oxidation of monometallic Mn–salphen complexes by dioxygen has already been described by Floriani as a self-catalyzed Mn oxidation with the intermediate formation of manganese dimers with the Mn–OamideMn linkage (Fig. 8).33 The ligand oxidation seems to be herein promoted by the spatial proximity of the Mn–salphen moieties in the closed tweezers enabling room temperature air oxidation in methanol during the crystallization time. The formation of such oxidized tweezers species was also observed by mass spectrometry on a solution of tweezers in MeOH left standing for a few weeks with signals at m/z 1479.6 and 1596.6 corresponding to [MnIII2(9Ox)]+ and [ZnMnIII2(9Ox)(CN)2]+ (see Fig. S12). Thus, the intramolecular linkage between the Mn and salphen moieties corroborates the oxidized product obtained by Floriani involving manganese salphen dimers.


image file: d0dt01465f-f8.tif
Fig. 8 Schematic representation of the ligand 9 oxidation to form a manganese dimer.

Preliminary micro-SQUID experiments were performed on single crystals of the zinc-closed oxidized tweezers. At a low temperature the sample displayed only a little magnetic response up to 1.4 T, indicative of an S = 0 ground state due to an antiferromagnetic exchange interaction between the two Mn centers. Even if this low spin ground state precludes any SMM behavior, the mechanical motion of the tweezers’ closing enabled a switch from a paramagnetic high spin system to a low spin antiferromagnetically coupled one.

Conclusion

An alternative to the “chemistry on complex” strategy was successfully conducted for the synthesis of manganese based molecular tweezers, thus enabling the incorporation of labile cations, not compatible with post-functionalization steps, into such switchable tweezers. The closing mechanical motion of the tweezers was achieved by the coordination of Zn(II) to the terpyridine unit bringing in spatial proximity the two Mn–salphen units as shown by XRD and EPR. Interestingly, the binding of a cyanide guest ligand to the Mn–salphen units also resulted in the closing of the tweezers via an intramolecular Mn–CN–Mn bridge. This bridge is stable enough to trigger the closing without Zn-coordination to the terpyridine unit offering an orthogonal closing stimulus combining mechanical motion and spin switch. The magnetic properties of the closed tweezers with Zn(II) or the cyanide ligand were investigated, which indicated an increase of the magnetic interaction between the spin carriers compared to the open form. However, no single molecule magnet behavior was detected in these manganese dimers. During crystallization studies, a closed tweezer presenting a salphen ligand with one imine oxidized to amide was unexpectedly obtained and characterized by XRD in the solid state. An intramolecular bridge by the apical coordination of the so formed amido group to the second Mn–salphen unit resulted in an antiferromagnetic coupling between the two Mn(III) centers. To conclude, Mn(III) based molecular tweezers offer stimulating bridged closed conformation with dual stimuli that were not accessible in previous systems. In particular, the cyanide stimulus offers attractive perspectives to achieve multiple controls of the mechanical motion of tweezers even if further optimizations are required to achieve reversibility with this stimulus. Optimization of the tweezers considering μ-oxo bridges or modified salphen ligands will be achieved in future work to exploit Mn(III) anisotropy and combine mechanical motion and SMM properties.

Experimental section

General analytical and synthetic methods

1H NMR and 13C NMR spectra were recorded at 400 or 600 MHz on Bruker Avance III spectrometers. Chemical shifts (δ) were reported in ppm from tetramethylsilane using residual solvent peaks for calibration. Electrospray ionisation (ESI) mass spectrometry was performed on a Bruker microTOF spectrometer. Reagent grade tetrahydrofuran was distilled from sodium and benzophenone. Tetrahydrofuran and triethylamine were degassed by three freeze–pump–thaw cycles before being used in the Sonogashira coupling reactions. Chloroform and methanol solvents were EPR grade. Chloroform was previously neutralized on Al2O3. All other chemicals were purchased from commercial suppliers and used without further purification. Flash column chromatography was performed using silica gel from Merck (40–63 μm) or GraceResolv High Resolution Flash Cartridges (particle size 40 μm). Thin layer chromatography was performed using aluminium plates pre-coated with silica gel 60 F254 0.20 mm layer thickness purchased from VWR. Absorption spectra were recorded on a JASCO V-670 spectrophotometer. All closed tweezer complexes were prepared from a solution of 1-Cl2 in CHCl3 or MeOH and studied directly in solution without prior isolation. Details for each analysis are provided in their respective sections.

Caution! Cyanides (TBACN) are very toxic and must be handled with care.

Mn–Salphen 3. In a round bottom flask Salphen 212a (1.2 g, 1.94 mmol, 1 equiv.) was dissolved in CH2Cl2 (20 mL). After addition of NEt3 (0.78 mL, 588 mg, 5.81 mmol, 3 equiv.), a solution of MnCl2, 4H2O (1.15 g, 5.81 mmol, 3 equiv.) in MeOH (20 mL) was slowly added. The mixture was stirred at room temperature for 6 h. The solvent was evaporated under reduced pressure and the crude product was purified by plug filtration (SiO2: CH2Cl2/MeOH (100/0; 95/5)). Complex 3 was isolated after evaporation of the filtrate obtained by elution with a 95/5 solvent ratio, in a yield of 98% (1.34 g). ESI-HRMS m/z (%): [M − Cl]+ calc (C36H45BrMnN2O2): 673.2021 (100), found: 673.2017; elemental analysis calc (%) for C36H45BrClMnN2O2: C 61.07, H 6.41, N 3.96; found: C 60.83, H 6.45, N 3.77.
Mn–Salphen 4. In a Schlenk tube under argon were introduced 3 (1.0 g, 1.41 mmol, 1 equiv.), PdCl2(PPh3)2 (148 mg, 0.212 mmol, 15 mol%), and CuI (81 mg, 0.424 mmol, 30 mol%). A mixture of NEt3 (10 mL) and THF (20 mL) previously distilled and degased was then added. The mixture was heated to 70 °C, and TMSA (2.00 mL, 1.39 g, 14.1 mmol, 10 equiv.) was added. The mixture was heated under argon at 70 °C for 15 h. The solvent was evaporated under reduced pressure, and the crude product was purified by column chromatography (SiO2: cyclohexane/CH2Cl2/MeOH (100/0/0)–(0/100/0)–(0/90/10)) yielding pure product 4 as a brown solid (59%, 603 mg). ESI-HRMS m/z (%): [M − Cl]+ calc (C41H54MnN2O2Si): 689.3330 (100), found: 689.3346 (100). Elemental analysis calc (%) for (C41H54ClMnN2O2Si + 2 CH3OH): C 65.42, H 7.92, N 3.55; found: C 64.98, H 7.18, N 3.48.
Salphen 6. In a Schlenk tube under argon were introduced 2 (2.0 g, 3.23 mmol, 1 equiv.), PdCl2(PPh3)2 (340 mg, 0.485 mmol, 15 mol%), and CuI (185 mg, 0.970 mmol, 30 mol%). A mixture of NEt3 (10 mL) and THF (20 mL) previously distilled and degased was then added. The mixture was heated at 70 °C, and TMSA (3.65 mL, 2.54 g, 25.8 mmol, 8 equiv.) was added. The mixture was heated under argon at 70 °C for 16 h. The solvent was evaporated under reduced pressure, and the crude product was purified by column chromatography (SiO2: cyclohexane/CH2Cl2 (100/0)–(50/50)). After solvent evaporation, the product was recrystallized in MeOH yielding 56% (1.159 g) of product 6. 1H NMR (400 MHz, CDCl3): δ 13.39 (s, 1H), 13.37 (s, 1H), 8.66 (s, 1H), 8.65 (s, 1H), 7.44 (d, J = 2.4 Hz, 2H), 7.41 (dd, J = 8.2, 1.8 Hz, 1H), 7.35 (d, J = 1.7 Hz, 1H), 7.22 (d, J = 2.5 Hz, 1H), 7.20 (d, J = 2.4 Hz, 1H), 7.17 (d, J = 8.2 Hz, 1H), 1.43 (s, 18H), 1.32 (s, 9H), 1.31 (s, 9H), 0,28 (s, 9H); 13C NMR (101 MHz, CDCl3) δ 165.30, 165.07, 158.81, 158.76, 143.07, 142.81, 140.60, 140.57, 140.53, 137.42, 137.38, 131.00, 128.65, 128.58, 127.08, 127.04, 123.27, 122.15, 119.92, 118.44, 104.43, 95.52, 35.27, 34.33, 31.61, 29.58, 0.13; ESI-HRMS m/z (%): [M + Na]+ calc (C41H56N2O2SiNa): 659.4003 (100), found: 659.4027 (100); elemental analysis calc (%) for C41H56N2O2Si: C 77.31, H 8.86, N 4.40; found: C 77.15, H 9.02, N 4.35.
Salphen 7. In a round bottom flask, 6 (1.30 g, 2.04 mmol, 1 equiv.) was dissolved in a mixture of THF (10 mL) and MeOH (10 mL) and K2CO3 (564 mg, 4.08 mmol, 2 equiv.) was added. The mixture was stirred at room temperature for 3 h. The solvent was evaporated under reduced pressure, and the yellow crude product was purified by plug filtration (SiO2, CH2Cl2). After solvent evaporation, the solid was recrystallized from MeOH. Pure product 7 was obtained after filtration and drying as an orange solid (714 mg, 81%). 1H NMR (400 MHz, CDCl3): δ 13.36 (s, 1H), 13.33 (s, 1H), 8.66 (s, 1H), 8.65 (s, 1H), 7.45 (d, J = 2.4 Hz, 2H), 7.43 (dd, J = 8.2 Hz, 1.8 Hz, 1H), 7.37 (d, J = 1.8 Hz, 1H), 7.24–7.16 (m, 3H), 3.15 (s, 1H), 1.43 (s, 18H), 1.32 (s, 18H); 13C NMR (101 MHz, CDCl3) δ 165.47, 165.25, 158.82, 158.75, 143.37, 142.95, 140.64, 137.45, 137.41, 131.09, 128.73, 128.67, 127.10, 127.06, 123.54, 121.07, 120.04, 118.42, 83.11, 78.26, 35.28, 34.32, 31.60, 29.58; ESI-HRMS m/z (%): [M + Na]+ calc (C38H48N2O2Na): 587.3608 (100), found: 587.3619 (100)
Mn–Salphen 5. In a round bottom flask compound 7 (530 mg, 0.938 mmol, 1 equiv.) was dissolved in CH2Cl2 (15 mL). After addition of NEt3 (0.38 mL, 285 mg, 2.81 mmol, 3 equiv.), a solution of MnCl2, 4H2O (223 mg, 1.13 mmol, 1.2 equiv.) in MeOH (15 mL) was slowly added. The mixture was stirred at room temperature for 3 h. The solvent was evaporated under reduced pressure, and the crude product was purified by plug filtration (SiO2: CH2Cl2/MeOH (100/0; 95/5)). Product 5 (71%, 437 mg) was isolated after evaporation of the filtrate obtained by elution with a 95/5 solvent ratio. ESI-HRMS m/z (%): [M − Cl]+ calc (C38H46MnN2O2): 617.2934 (100), found: 617.2950 (100); elemental analysis calc (%) for (C38H46ClMnN2O2 + CH3OH): C 68.36, H 7.36, N 4.09; found: C 68.97, H 7.14, N 4.11.
Tweezers’ ligand 9-H4. In a Schlenk tube under argon 6,6′′-dibromo-2,2′:6′,2′′-terpyridine 8 (35 mg, 0.089 mmol, 1 equiv.), Salphen 7 (200 mg, 0.354 mmol, 4 equiv.), PdCl2(PPh3)2 (9 mg, 0.013 mmol, 15 mol%) and CuI (2.5 mg, 0.013 mmol, 15 mol%) were introduced. A mixture of NEt3 (3 mL)/THF (6 mL) previously distilled and degased by freeze pump thaw was then added. The mixture was heated at 70 °C under argon for 15 h. After solvent evaporation, the crude product was purified by column chromatography (SiO2: cyclohexane/CH2Cl2/AcOEt (100/0/0)–(0/100/0)–(0/50/50)) and then recrystallized from EtOH yielding 9-H4 as an orange solid (44 mg, 51%). 1H NMR (400 MHz, CDCl3): δ 13.39 (s, 2H), 13.36 (s, 2H), 8.73 (s, 2H), 8.70 (s, 2H), 8.61 (dd, J = 8.1, 1.1 Hz, 2H), 8.60 (d, J = 7.9 Hz, 2H), 8.00 (t, J = 7.8 Hz, 1H), 7.89 (t, J = 7.8 Hz, 2H), 7.63–7.59 (m, 4H), 7.55 (d, J = 1.7 Hz, 2H), 7.47 (d, J = 2.4 Hz, 4H), 7.28–7.23 (m, 6H), 1.44 (s, 36H), 1.34 (s, 18H), 1.33 (s, 18H); 13C NMR (101 MHz, CDCl3) δ 165.52, 165.25, 158.88, 158.78, 156.86, 154.87, 143.53, 143.08, 142.84, 140.67, 138.13, 137.47, 137.43, 137.20, 131.08, 128.78, 128.70, 127.51, 127.15, 127.09, 123.50, 122.00, 121.33, 120.68, 120.18, 118.46, 90.10, 88.48, 35.30, 34.34, 31.61, 29.59; ESI-HRMS m/z (%): [M + Na]+ calc (C91H103N7O4Na): 1381.7997 (100), found: 1381.7984 (100).
Tweezers 1-Cl2. In a round bottom flask 9-H4 (30 mg, 0.049 mmol, 1 equiv.) was dissolved in CH2Cl2 (20 mL). After addition of NEt3 (78 μL, 60 mg, 0.58 mmol, 12 equiv.), a solution of MnCl2, 4H2O (58 mg, 0.292 mmol, 6 equiv.) in MeOH (15 mL) was slowly added. The mixture was stirred at room temperature for 4 h. After solvent evaporation under reduced pressure, CH2Cl2 and 10 drops of N,N,N′,N′′,N′′-pentamethyldiethylenetriamine were added. The organic layer was then washed with water and evaporated. A final recrystallization in acetonitrile yielded tweezers 1-Cl2 as a brown solid (22 mg, 65%). ESI-HRMS m/z (%): [M − 2Cl]2+ calc (C91H99Mn2N7O4): 731.8254 (100), found: 731.8392 (100); [M – Cl]+ calc (C91H99ClMn2N7O4): 1499.6242 (100), found: 1499.6203 (100). Elemental analysis calc (%) for (C91H99Cl2Mn2N7O4 + H2O) C 70.72, H 6.55, N 6.31; found: C 70.81, H 6.51, N 6.45.
Mn–Salphen 10. In a round bottom flask N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-phenylenediamine34 (80 mg, 0.15 mmol, 1 equiv.) was dissolved in CH2Cl2 (5 mL). After addition of 3 drops of NEt3, a solution of MnCl2, 4H2O (87 mg, 0.44 mmol, 3 equiv.) in MeOH (5 mL) was slowly added. The mixture was stirred at room temperature for 6 h. The solvent was evaporated under reduced pressure. The final product was obtained by precipitation in acetonitrile as a brown solid (59 mg, 67%). ESI-HRMS m/z (%): [M − Cl]+ calc (C36H46MnN2O2): 593.2940 (100), found: 593.2945 (100); elemental analysis calc (%) for (C36H46ClMnN2O2 + H2O) C 66.81, H 7.48, N 4.33; found: C 66.64, H 7.39, N 4.25.

Crystal data for closed tweezers

Single crystals were grown by slow evaporation of a solution of [Zn(1-Cl2)]Cl2 in MeOH. Brown plate-like crystals were obtained: C98H129Cl4Mn2N7O12Zn, triclinic, P[1 with combining macron], a = 14.1647(2), b = 15.3020(2), c = 25.9901(4) Å, α = 88.142(1), β = 84.223(1), γ = 66.438(1)°, V = 5137.24(13) Å3, Z = 2, T = 100(2) K. 34[thin space (1/6-em)]853 reflections measured, 25[thin space (1/6-em)]877 observed [I ≥ 2σ(I)], 1246 parameters, final R indices R1 [I ≥ 2σ(I)] = 0.0654 and wR2 (all data) = 0.2087, GOF = 1.043.

Crystal data for oxidized tweezers’ dimer [ZnIIMnIII2(9ox)(CN)1.5(OH)(CH3OH)]2

Single crystals were grown by slow evaporation of a solution [Zn(1-Cl2)]Cl2 in the presence of TBACN (2 equiv.) in MeOH. Brown crystals were obtained: C194H240Cl4Mn4N17O27Zn3, triclinic, P[1 with combining macron], a = 15.1358(12), b = 16.5212(10), c = 22.9401(16) Å, α = 89.629(5), β = 72.981(5), γ = 73.972(5)°, V = Å3, Z = 1, T = 200(2) K. 18[thin space (1/6-em)]995 reflections measured, 6549 observed [I ≥ 2σ(I)], 1123 parameters, final R indices R1 [I ≥ 2σ(I)] = 0.0843 and wR2 (all data) = 0.2657, GOF = 1.047.

Crystallography

A single crystal of [Zn(1-Cl2)]Cl2 was selected in a drop of glue, mounted onto a glass fiber, and transferred to a cold nitrogen gas stream. The data collection for [Zn(1-Cl2)]Cl2 was carried out on the 4-circle diffractometer at the CRISTAL beamline (SOLEIL synchrotron, Paris) using the synchrotron radiation source (λ = 0.66825 Å) up to a maximum resolution of 0.8 Å−1, achieving 97% of completeness. Data collection strategies were generated with the CrysAlisPro CCD package. Unit-cell parameters refinement, data reduction, scaling and absorption correction were carried out with CrysAlisPro. The sample was treated as a two domain twin for data reduction. In the WinGX suite of programs,35 the structure was solved with SHELXS-97 program36 and refined by full-matrix least-squares methods using SHELXL-2013 and a HKLF5 type hkl file. All non-hydrogen atoms were refined anisotropically while one acetonitrile solvent molecule was refined isotropically. Hydrogen atoms were placed at calculated positions. Geometrical restraints were introduced for t-butyl groups and solvent molecules. Restraints on ADPs were also used for the latter parts of the structure.

A single crystal of the oxidized tweezers was mounted onto a cryoloop and transferred to a cold nitrogen gas stream. Intensity data were collected with a BRUKER Kappa-APEXII diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). Data collection was performed with APEX2 suite (BRUKER). Unit-cell parameters refinement, integration and data reduction were carried out with SAINT program (BRUKER). SADABS (BRUKER) was used for scaling and multi-scan absorption corrections. Similarly, WinGX and the SHELXS/L software were used for structure solution and refinement.

CCDC 1846793, 1846794 contain the supplementary crystallographic data for this paper.

UV-Vis titrations

UV-Vis titrations were performed on a 3.0 mL of open tweezers (5.0 × 10−6 M) dissolved in CHCl3 (previously neutralized on Al2O3) in a quartz cell (10 mm), and 0.2 equiv. (3 μL of solution 1.0 × 10−3 M) of ZnCl2 dissolved in CH3CN were added to them. After each addition, a UV-Visible absorption spectrum (250–700 nm, 400 nm min−1, 25 °C) was recorded immediately and after 5 minutes to check that the equilibrium was reached. The same procedure was performed for titration cyanide binding with TBACN (1.0 × 10−3 M in CH3CN). Binding constants were obtained by a nonlinear least-squares fit of the absorbance versus the concentration of guest added using the Matlab program developed by P. Thordarson.37

EPR experiment

EPR experiments were performed on 150 μL of open tweezers (1.0 × 10−4 M) dissolved in MeOH in a quartz tube (4 mm). To the open form ZnCl2 dissolved in MeOH (1.0 × 10−1 M) was added. To the open/closed form was added a solution of TBACN dissolved in MeOH (5.0 × 10−2 M).

SQUID experiment

Liquid squid experiment was performed on 200 μL of open tweezers (1.30 × 10−3 M) dissolved in chloroform (previously neutralized on Al2O3) in a quartz tube with a Quantum Design MPMS-XL SQUID magnetometer. 1 equiv. of a solution of ZnCl2 in MeCN was added (0.1). A solution of TBACN was prepared in MeCN (5 × 10−2 M) and 1.2 equivalents were added.

The background signal of the sample holder (closed quartz tube, see Fig. S10) was recorded on an empty tube with an integrated procedure in MPMS multi view software. The correction of intrinsic diamagnetism of the solvent was estimated from the volume of solvent injected. The volume was converted into mass and the diamagnetic corrections calculated from tables in Handbook of Chemistry and Physics. Measurements were performed in the solid phase only and the tube and solution system were trapped at 110 K for 3 minutes directly into the magnetometer before evacuating the sample chamber.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

B. D. thanks the Ecole Normale Supérieure of Cachan for a PhD Fellowship. We are grateful to Dr P. Fertey (CRISTAL beamline team, synchrotron SOLEIL) and Dr S. Pillet (CRM2, Nancy) for their kind help in the collection of SC-XRD data. This work benefited from the support of the French National Research Agency (ANR) for the project JCJC SMARTEES (15-CE07-0006-01).

Notes and references

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Footnotes

Electronic supplementary information (ESI) available: 1H, 13C NMR, UV-Vis titration data. CCDC 1846793 and 1846794. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/D0DT01465F
These authors have contributed equally to the work.

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