A Brucite-Like Mixed-Valent Cluster Capped by [Mn III p - t Bu-calix[4]arene] − Moieties

: p - t Bu-calix[4]arene (H 4 TBC[4]) has proven to be an incredibly versatile ligand for the synthesis of 3 d - and 3 d / 4 f - clusters, in particular those containing mixed-valent Mn ions. These are of interest to the magnetochemist for the diversity of magnetic behaviours that can be shown, along with a huge variety of nuclearities and topologies accessible, which allow one to outline magneto-structural correlations and a quantitative understanding of their properties. This contribution reports the synthesis, analysis and magnetic properties of a Brucite-like Mn-oxo / hydroxo octanuclear fragment encapsulated within / capped by four [Mn III -TBC[4]] − moieties. A diol coligand in the reaction mixture plays a seemingly important role in determining the outcome, though it is not incorporated in the ﬁnal structure.


Introduction
Manganese continues to play a prominent role in the chemistry of 3d transition metals, owing to its significance across a breadth of research areas, including bioinorganic [1] and biomedicinal chemistry [2], catalysis [3], nanomaterials [4], spectroscopy [5], and molecular magnetism [6]. In the latter category, the ability of the Mn ion to exist in a variety of stable oxidation states (II-IV) allows for the construction of polymetallic cluster compounds exhibiting a variety of interesting magnetic behaviours, including the stabilization of large spin ground states [7], the slow relaxation of magnetization [8], spin frustration [9], vibrational coherences [10], and enhanced magnetocaloric effects [11]. A key component in understanding the physical properties of all Mn-based molecular magnets is the construction of large families of related compounds so that structure-magnetism relationships can be quantitatively rationalised, and this requires the careful design and exploitation of specific organic bridging ligands.
We have been exploring the coordination chemistry of calix[n]arenes (C[n]s) with Mn (as well as other metals), as these molecules hold the potential to isolate coordination clusters in the solid state in various different ways, for example, by exploiting the wedge shape of p-t Bu-calix [4]arene (H 4 TBC [4]). As can be seen from the acetonitrile (CH 3 CN) solvate of H 4 TBC [4] (Figure 1a) [12], the shape of the building block exerts strong influence over assembly and typically results in antiparallel bilayer formation in the solid state. The hydrophobic cavities are offset in this case and are occupied by acetonitrile of crystallization, though the same phenomenon is also observed for other solvates such Chemistry 2020, 2 254 as dmf [13]. With respect to cluster formation, p-t Bu-calix [4]arene (H 4 TBC [4]) has proven to be a particularly versatile platform for the synthesis of a wide range of different topologies, with nuclearities reaching up to fourteen [13][14][15][16]. A recurring structural theme we have noticed in this work is that the [Mn III TBC [4]] − moieties act as bridges to metal ions within the cluster through their phenolate groups (Figure 1b), but also as polyhedral capping units (Figure 1c) [17]. The latter suggests that these can be used to encapsulate small metal-oxo-hyroxo fragments growing in 2-or 3-dimensions. Thia, sulfonyl and sulfinyl calix [4]arenes have also been employed in this way, though they give access to markedly different topologies due to the presence of donor atoms at the bridge positions [18][19][20][21][22][23]. In all cases, new species isolated with methylene-or heteroatom-bridged calix [4]arenes would be of particular interest to magnetochemists researching topological spin frustration [24]. Herein, we discuss the synthesis, structure and magnetic behaviour of a mixed-valent [Mn III 8 Mn II 4 ] species built with TBC [4], the core metallic skeleton of which is related to the hydroxide-based mineral Brucite.
Chemistry 2020, 2, x 2 a particularly versatile platform for the synthesis of a wide range of different topologies, with nuclearities reaching up to fourteen [13][14][15][16]. A recurring structural theme we have noticed in this work is that the [Mn III TBC [4]] − moieties act as bridges to metal ions within the cluster through their phenolate groups (Figure 1b), but also as polyhedral capping units (Figure 1c) [17]. The latter suggests that these can be used to encapsulate small metal-oxo-hyroxo fragments growing in 2-or 3dimensions. Thia, sulfonyl and sulfinyl calix [4]arenes have also been employed in this way, though they give access to markedly different topologies due to the presence of donor atoms at the bridge positions [18][19][20][21][22][23]. In all cases, new species isolated with methylene-or heteroatom-bridged calix [4]arenes would be of particular interest to magnetochemists researching topological spin frustration [24]. Herein, we discuss the synthesis, structure and magnetic behaviour of a mixed-valent [Mn III 8Mn II 4] species built with TBC [4], the core metallic skeleton of which is related to the hydroxidebased mineral Brucite. (a) Section of the extended structure found in the CH3CN solvate of H4TBC [4], showing the antiparallel bi-layer assembly and offset head-to-head arrangement of the host cavities [12]. (b) Partial single crystal X-ray structure of a mixed-valence TBC[4]-supported manganese cluster containing a butterfly-like [Mn III 2Mn II 2(OH)2] core [13]. Mn III ions occupy the tetraphenolato pocket of the TBC [4] and act as capping units (shown as larger spheres in the inset diagram). (c) Partial single crystal X-ray structure of a C[4]-supported 3d-4f cluster topology that can be isolated with a range of lanthanides [14]. Analogous capping behaviour to that found in (b) is also observed here and is represented by the larger spheres in the inset diagram. Colour code: Mn III -purple; Ln III -green; O-red; C-grey; N-blue; ligated solvent-orange. t Bu groups, hydrogen atoms, and solvent of crystallization omitted for clarity.

Synthesis and Structural Studies
Reaction of H4TBC [4] with MnCl2·6H2O, H2bd (1,4-butanediol), [NH4]ClO4, and NEt3 in a mixture of CH3CN and dmf, followed by vapour diffusion of diethyl ether into the mother liquor, afforded single crystals suitable for X-ray diffraction of formula [Mn III 8Mn II 4(µ4-O)2(µ3-OH)6(µ-OH)4(µ-Cl)2(TBC [4])4(dmf)8(H2O)4(CH3CN)2]·2dmf·6CH3CN (1, Figure 2, please also see Appendix A). The crystals were found to be in a triclinic cell, and structure solution was carried out in the space group P-1, with the asymmetric unit (ASU) comprising half of the cluster (Mn1-Mn6, Figure 3). Before moving to the structural description of 1, it is interesting to note that inspection of Figure 2 shows that 1,4-butanediol is not incorporated in the prevailing structure. Our original intention was to form a heteroleptic TBC [4]-diol cage, and although 1,4-butanediol is not incorporated, its presence in the reaction mixture is required for 1 to form. Indeed, analogous experiments in the absence of diol result in the formation of the well-known TBC [4]-supported [Mn III 2Mn II 2] cluster topology shown in Figure 1B; this particular cluster can be isolated quantitatively in under 1 h, further indicating that the diol plays a crucial role in the formation of 1.
The central metal-oxygen core of 1 describes a near-planar, mixed-valent [Mn III 6Mn II 4(µ4-O)2(µ3-OH)6(µ-OH)4(Cl)2] sheet-like structure (Figures 2 and 4). The Mn III ions in 1 (Mn1, Mn2, Mn4, and symmetry equivalent, s.e.) are easily distinguished through the presence of coparallel Jahn-Teller (JT) Figure 1. (a) Section of the extended structure found in the CH 3 CN solvate of H 4 TBC [4], showing the antiparallel bi-layer assembly and offset head-to-head arrangement of the host cavities [12]. (b) Partial single crystal X-ray structure of a mixed-valence TBC[4]-supported manganese cluster containing a butterfly-like [Mn III 2 Mn II 2 (OH) 2 ] core [13]. Mn III ions occupy the tetraphenolato pocket of the TBC [4] and act as capping units (shown as larger spheres in the inset diagram). (c) Partial single crystal X-ray structure of a C[4]-supported 3d-4f cluster topology that can be isolated with a range of lanthanides [14]. Analogous capping behaviour to that found in (b) is also observed here and is represented by the larger spheres in the inset diagram. Colour code: Mn III -purple; Ln III -green; O-red; C-grey; N-blue; ligated solvent-orange. t Bu groups, hydrogen atoms, and solvent of crystallization omitted for clarity.

Synthesis and Structural Studies
Reaction of H 4 TBC [4] with MnCl 2 ·6H 2 O, H 2 bd (1,4-butanediol), [NH 4 ]ClO 4 , and NEt 3 in a mixture of CH 3 CN and dmf, followed by vapour diffusion of diethyl ether into the mother liquor, afforded single crystals suitable for X-ray diffraction of formula Figure 2, please also see Appendix A). The crystals were found to be in a triclinic cell, and structure solution was carried out in the space group P-1, with the asymmetric unit (ASU) comprising half of the cluster (Mn1-Mn6, Figure 3). Before moving to the structural description of 1, it is interesting to note that inspection of Figure 2 shows that 1,4-butanediol is not incorporated in the prevailing structure. Our original intention was to form a heteroleptic TBC[4]-diol cage, and although 1,4-butanediol is not incorporated, its presence in the reaction mixture is required for 1 to form. Indeed, analogous experiments in the absence of diol result in the formation of the well-known TBC [4]-supported [Mn III 2 Mn II 2 ] cluster topology shown in Figure 1B; this particular cluster can be isolated quantitatively in under 1 h, further indicating that the diol plays a crucial role in the formation of 1.
.   Interestingly, Mn6 (and s.e.) is formally five-coordinate and in square-pyramidal geometry ( Figure 3), with the CH3CN molecule occupying the TBC [4] cavity and interacting with the Ph rings through CH···π interactions. We have observed such behaviour in other TBC [4]-supported Mn cages [25], and this is also reminiscent of the host-guest chemistry found in the CH3CN solvate of TBC [4] ( Figure 1a) [12]. The preference of the TBC [4] ligands to host Mn III ions over Mn II ions in 1 is entirely consistent with our previously published empirical binding rules for this ligand [17]. . Figure 3. Partial single crystal X-ray structure of 1 showing the asymmetric unit with selected labels shown. Colour code: Mn III -purple; Mn II -pale blue; O-red; C-grey; Cl-green; dmf-orange. t Bu groups, hydrogen atoms, and solvent of crystallization omitted for clarity. Two s.e. atoms (O12′ and O13′) are included in order to show the octahedral coordination sphere around Mn4.
Interestingly, Mn6 (and s.e.) is formally five-coordinate and in square-pyramidal geometry (Figure 3), with the CH3CN molecule occupying the TBC [4] cavity and interacting with the Ph rings through CH···π interactions. We have observed such behaviour in other TBC [4]-supported Mn cages [25], and this is also reminiscent of the host-guest chemistry found in the CH3CN solvate of TBC [4] ( Figure 1a) [12]. The preference of the TBC [4] ligands to host Mn III ions over Mn II ions in 1 is entirely consistent with our previously published empirical binding rules for this ligand [17].  Interestingly, Mn6 (and s.e.) is formally five-coordinate and in square-pyramidal geometry ( Figure 3), with the CH 3 CN molecule occupying the TBC [4] cavity and interacting with the Ph rings through CH···π interactions. We have observed such behaviour in other TBC [4]-supported Mn cages [25], and this is also reminiscent of the host-guest chemistry found in the CH 3 CN solvate of TBC [4] (Figure 1a) [12]. The preference of the TBC [4] ligands to host Mn III ions over Mn II ions in 1 is entirely consistent with our previously published empirical binding rules for this ligand [17].

Magnetic Behaviour
Magnetic susceptibility (χM) data were measured on a powdered, restrained crystalline sample of 1 under a 0.1 T dc field in the T = 5-300 K temperature range. The temperature dependence of the χMT product is shown in Figure 6. At room temperature, the χMT value of 40.82 cm 3 K mol -1 is close to the Curie constant expected for eight Mn III and four Mn II ions, assuming a g-value of 2.00 (41.5 cm 3 K mol -1 ). The χMT value remains approximately constant as the temperature is decreased until ~150 K, where it starts to decrease, reaching a minimum value of 13.27 cm 3 K mol -1 at 5 K. This behaviour Figure 5. Extended structure of 1 looking down the a-axis. Packing of neighbouring clusters is dominated by TBC [4]s, which mimic head-to-head dimer assembly akin to that shown for the CH 3 CN solvate of H 4 TBC [4] in Figure 1a [12]. Colour code: Mn III -purple polyhedra; Mn II -pale blue polyhedra; O-red; C-grey; hydrogen atoms, solvent of crystallization and ligated solvent omitted for clarity.

Magnetic Behaviour
Magnetic susceptibility (χ M ) data were measured on a powdered, restrained crystalline sample of 1 under a 0.1 T dc field in the T = 5-300 K temperature range. The temperature dependence of the χ M T product is shown in Figure 6. At room temperature, the χ M T value of 40.82 cm 3 K mol -1 is close to the Curie constant expected for eight Mn III and four Mn II ions, assuming a g-value of 2.00 (41.5 cm 3 K mol -1 ). The χ M T value remains approximately constant as the temperature is decreased until~150 K, where it starts to decrease, reaching a minimum value of 13.27 cm 3 K mol -1 at 5 K. This behaviour is indicative of the presence of predominantly weak antiferromagnetic interactions between the constituent metal ions. A fit of the 1/χ M versus T data to the Curie-Weiss law afforded the Weiss constant, θ = −10 K. Magnetization measurements, performed in fields between 0.5-7 T (Figure 7), are in agreement with this observation, showing M increasing in a near linear like fashion with H, indicative of the field-induced population of low-lying excited states with larger magnetic moments. Previous magneto-structural correlations in alkoxide-bridged Mn III dimers in which the JT axes are coparallel, as seen in the central [Mn 10 ] planar core (Figure 4), predict borderline and weak ferro-or antiferromagnetic exchange interactions, as observed [28]. No signals were observed in ac susceptibility measurements for data collected in the T = 1.8-10 K temperature range in frequencies up to 1500 Hz.
indicative of the field-induced population of low-lying excited states with larger magnetic moments. Previous magneto-structural correlations in alkoxide-bridged Mn III dimers in which the JT axes are coparallel, as seen in the central [Mn10] planar core (Figure 4), predict borderline and weak ferro-or antiferromagnetic exchange interactions, as observed [28]. No signals were observed in ac susceptibility measurements for data collected in the T = 1.8-10 K temperature range in frequencies up to 1500 Hz.   Previous magneto-structural correlations in alkoxide-bridged Mn III dimers in which the JT axes are coparallel, as seen in the central [Mn10] planar core (Figure 4), predict borderline and weak ferro-or antiferromagnetic exchange interactions, as observed [28]. No signals were observed in ac susceptibility measurements for data collected in the T = 1.8-10 K temperature range in frequencies up to 1500 Hz.

Conclusions
The Somewhat surprisingly, 1,4-butanediol is a crucial component required for the formation of 1 despite the fact that it is not incorporated in the resulting cluster. Absence of this key reactant affords the previously reported mixed-valence TBC [4]-supported Mn-butterfly cluster, suggesting that this chelate plays a crucial role in (possibly hindering) cluster formation and producing a significantly higher nuclearity species in the process; the TBC[4]-supported Mn-butterfly cluster (Figure 1b) can be formed rapidly in approximately 15 min, and quantitatively precipitates from solution within an hour. The development reported here suggests that the use of other potential coligands with similar chemical make-up may offer the possibility to access/isolate other large, discrete high-nuclearity species. Results from this study will be reported in due course and will ideally shed additional light on the important role of such non-innocent reactants. Funding: This research was funded by the EPSRC through grant reference numbers EP/I03255X/1 and EP/I031421/1.

Conflicts of Interest:
The authors declare no conflict of interest.

Appendix A
MnCl 2 ·4H 2 O (99 mg, 0.5 mmol, Sigma Aldrich, UK), H 4 TBC [4] (97 mg, 0.15 mmol, Sigma Aldrich, UK) and NH 4 ClO 4 (140 mg, 1.2 mmol, Sigma Aldrich, UK) were dissolved in a mixture of dmf (10 mL, Fisher Scientific, UK) and CH 3 CN (10 mL, Fisher Scientific, UK). 1,4 butanediol (90 mg, 1 mmol, Sigma Aldrich, UK) was then added with stirring. After five minutes, NEt 3 (0.42 mL, 3 mmol, Sigma Aldrich, UK) was added dropwise and the reaction mixture stirred for two hours. X-ray quality crystals were obtained after 3 days in~20% yield following filtration of the mother liquor with subsequent diffusion of diethyl ether (or hexane) into the resulting solution. , 25871 unique (R int = 0.0328, R sigma = 0.0396), which were used in all calculations. The final R 1 was 0.0564 (I > 2σ(I)) and wR 2 was 0.1810 (all data). Magnetic properties were determined using a MPMS-XL SQUID magnetometer (Quantum Design Inc., San Diego, CA, USA) for direct current (dc) and alternating current (ac) measurements. The powdered microcrystalline sample was immobilised in eicosane. Experimental dc data were recorded at 0.1 T in the temperature range 5.0-300 K and at 2.0-7.0 K in the field range 0.5-7.0 T. Experimental ac data were collected in the temperature range 1.8-10 K and frequency range 0-1500 Hz using an amplitude of B ac = 3 G. All data were corrected for the sample holder contributions and intrinsic diamagnetic contributions that were determined from Pascal's constants.