Oxidation state variation in bis-calix[4]arene supported decametallic Mn clusters

The reaction of MnCl2·4H2O, H8L (2,2’-bis-p-Bu-calix[4]arene) and NEt3 in a dmf/MeOH solvent mixture results in the formation of a mixed valent decametallic cluster of formula [Mn6Mn4(L)2(μ3OH)4(μ-OH)4(MeOH)4(dmf)4(MeCN)2]·MeCN (3). Complex 3 crystallises in the monoclinic space group P21/n with the asymmetric unit comprising half of the compound. Structure solution reveals that the bis-calix[4]arene ligands are arranged such that one TBC[4] moiety in each has undergone inversion in order to accommodate a [Mn4Mn6] metallic skeleton that describes three vertex-sharing [Mn2Mn2] butterflies. The structure is closely related to the species [Mn6Mn4(L)2(μ3-O)2(μ3-OH)2(μOMe)4(H2O)4(dmf)8]·4dmf (4), the major difference being the oxidation level of the Mn ions in the core of the compound. DFT calculations on the full structures reveal that replacing the Mn ions in 4 for Mn ions in 3 results in a significant decrease in the magnitude of some antiferromagnetic exchange contributions, a switch from ferromagnetic to antiferromagnetic in others, and the loss of significant spin frustration. EastCHEM School of Chemistry, The University of Edinburgh, David Brewster Road, Edinburgh, Scotland, EH9 3FJ, UK. E-mail: ebrechin@ed.ac.uk Department of Chemistry, Indian Institute of Technology Bombay, Mumbai, 400076, India. E-mail: rajaraman@chem.iitb.ac.in Institute of Chemical Sciences, Heriot-Watt University, Riccarton, Edinburgh, Scotland, EH14 4AS, UK. E-mail: S.J.Dalgarno@hw.ac.uk


Introduction
Coordination compounds of Mn maintain long standing interest across a breadth of topics from bioinorganic chemistry 1 to molecular magnetism. 2 Key to manipulating and enhancing physicochemical behaviour for any application is structural control, and for polymetallic compounds the self-assembly process and identity of the final product is largely dominated by choice of bridging ligand. 3 We, and others, [4][5][6][7] have employed p-t Bu-calix [4]arene (H4TBC [4], Fig. 1A) for the synthesis of a range of transition metal (TM) and lanthanide metal (LnM) species in which the TM/LnM-TBC [4] moiety acts as a metalloligand that encapsulates an oxo/hydroxo-bridged polymetallic core. Structurally, this means it acts as a capping vertex in the resulting metallic skeleton. 8 The calix [4]arene tetraphenolic pocket is particularly well suited to bonding Jahn-Teller (JT) distorted ions such as Cu II and Mn III because it will preferentially coordinate metals possessing four short equatorial and two long axial bonds. 9,10 Illustrative examples include the complexes [Cu II 9(OH)3(TBC [4])3Cl2(DMSO)5.5(EtOH)0.5][Cu I Cl2] (1, Fig. 1B) and [Mn III 2Mn II 2(OH)2(TBC [4])2(dmf)6] (2, Fig. 1C). In the former the TM-TBC [4] metalloligand encapsulates a [Cu II 6(OH)3] trigonal prism, and in the latter a [Mn II 2(OH)2] dimer. This general bonding motif has been observed in the vast majority of TM/LnM complexes we have isolated under ambient conditions and this has allowed us to develop a specific set of empirical metal ion binding rules for TBC [4]. 11 (A) TBC [4] preferentially binds TM III ions; (B) TBC [4] will bind TM II ions in the absence of TM III ions; (C) TBC [4] will bind LnM III ions in the absence of TM II or TM III ions. 12

Figure 2.
Single crystal X-ray structure of H8L. Colour code C -grey, O -red, H -white.

Results and discussion
Reaction of H8L with MnCl2·4H2O in a basic dmf/MeOH mixture affords single crystals of [Mn III 4Mn II 6(L)2(µ3-OH)4(µ-OH)4(MeOH)4(dmf)4(MeCN)2]·MeCN (3, Fig. 3), following vapour diffusion of MeCN into the mother liquor. The crystals were found to be in a monoclinic cell and structure solution was carried out in the space group P21/n. The asymmetric unit (ASU) comprises half of the cluster, with an inversion centre located in the middle of the Mn5-O14-Mn5'-O14' rhomb. Pertinent bond lengths and angles (Table S1) and BVS calculations (Table S2) are given in the supplementary information.
Both L ligands are arranged such that one TBC [4] moiety in each has undergone inversion in order to accommodate a [Mn III 4Mn II 6] metallic skeleton that describes three vertex-sharing [Mn III 2Mn II 2] butterflies (Fig. 3). These are of two types. The central butterfly (Mn III 2, Mn II  Examination of the extended structure (Fig. S2) reveals that the closest intermolecular interactions are between coordinated dmf molecules and the t Bu C-atoms at C···C distances ≥ 3.2 Å, and between neighbouring t Bu groups, C···C ≥ 3.8 Å. The closest M···M distance is ~12.3 Å between the Mn1 ions of distinct molecules, meaning they are structurally isolated thanks to the framework of the L ligands and overall shape of the assembly.  Figure 4 shows that the structure of 3 is also related to the compounds [Mn III 6Mn II 2Gd III 2(L)2(μ4-O)2(μ3-OH)2(μ-OMe)2(μ-OH)2(MeOH)4(dmf)8](NO3)2(H2O)2] (5) 18 and [Mn III 4Mn II 4(L)2(μ3-OH)2(μ-OH)(μ-Cl)(H2O)(MeOH)(dmf)4] (6), 16 which was the very first compound isolated with H8L. The metallic skeleton of 5 describes three vertex-sharing [Mn III 2Mn II Ln III ] butterflies, and that of 6 two vertexsharing [Mn III 2Mn II 2] butterflies. Indeed, from an inspection of the metallic skeletons of 3-6 ( Fig. 3-4) it is clear to see that it is the Mn III L metalloligand that directs structure formation, with the additional Mn II /Ln III ions encapsulated within this framework, connected via bridging hydroxides/alkoxides. What is also clear is that these encapsulated metal ions can be replaced, whilst maintaining the general structure. This is illustrated by complex 6 undergoing selective Mn II /Ln III substitution to form [Mn III 4Mn II 2Gd III 2(L)2(Cl)2(μ3-OH)4(MeOH)2(dmf)8], 7 (Fig. 4). 16 This mirrors the behaviour observed for TBC [4]. For example, the Mn II ions in 2 can be replaced in a stepwise fashion with Ln III ions, from [Mn III 2Mn II 2] to [Mn III 2Mn II Ln III ] and [Mn III 2Ln III 2] . 20 The isolation of multiple, structurally-related compounds is reflective of the versatility of the bis-calix [4]arene ligand for the construction of polymetallic clusters where subtle changes in reactants/conditions can be exploited to direct the nature of the metallic core and associated physical properties.

Magnetic measurements
Direct current (dc) magnetic susceptibility studies were performed on a polycrystalline sample of 3 over the temperature range T = 2-298 K, in an applied magnetic field B = 0.1 T (Fig. 5), where χ = M/B and M is the magnetisation. At 298 K, the χMT value of 37.4 cm 3 mol -1 K is in agreement with the expected value for spin-only contributions to the susceptibility for a [Mn III 4Mn II 6] unit (38.25 cm 3 mol -1 K, g = 2.0). Upon cooling, the χMT product decreases slowly until approximately T = 100 K, wherefrom it decreases more rapidly, reaching a value of 8.6 cm 3 mol -1 K at 2 K. Variable-temperature-variablefield (VTVB) magnetisation data (inset of Fig. 5, Fig. S3) shows M rising slowly with increasing B, reaching a maximum value of M = 17.6 µB at 7 T but without saturating. Both are suggestive of the presence of weak, competing exchange interactions dominated by the antiferromagnetic contributions. The large nuclearity of 3 and the presence of six different exchange interactions prevents a quantitative analysis of the susceptibility and magnetisation data. Previously published Mn complexes of TBC [4] and H8L show that exchange interactions tend to be relatively weak, with JMn(III)-Mn(II) being weakly ferromagnetic, JMn(II)-Mn(II) being weakly antiferromagnetic and JMn(III)-Mn(III) being borderline anti/ferromagnetic. 16,20 In order to investigate this in more detail, we now turn to theory.

Estimation of magnetic exchange interactions using DFT
For full details of the computational methodology, see the SI. Analysis of the structures of 3 and 4 reveals a total of six different exchange interactions (J1-6), as shown in Figure 6. The magnitude and sign of the DFT calculated exchange constants is provided in Table 1, alongside pertinent structural information describing the bond distances and angles in each pairwise interaction. In each case the sign and magnitude of the J values obtained can be easily explained via magneto-structural correlations previously published for O-bridged Mn II/III complexes, 21 23 Based on these J values complex 3 has an S = 2 ground state (Fig. S4).  For complex 4, the J values are found to be in a much larger range, with both ferro-and antiferromagnetic exchange being observed (+4.1 ≤ J ≤ -40.4 cm -1 ). J1, between two central Mn III ions connected by two µ3-OH bridges, is the largest antiferromagnetic exchange present due to the small Mn-O-Mn angles (97.6°) and short Mn-O (1.92 Å) and Mn-Mn (2.88 Å) distances. The large J value has been confirmed through calculations on a dinuclear model complex adapted from the X-ray structure (Fig. S5). We note that the JT axes of the two Mn III ions are oriented parallel to the bridging ligands, which can be classified as a type I structure based on detailed studies of the dinuclear {Mn III 2(OR)2} motif. In this class of interactions, a large JAF contribution and a negligible JF contribution are expected, leading to strong antiferromagnetic coupling, as observed here. 21,24 J2 is ferromagnetic in nature, mediated between Mn II and Mn III  From these calculations we can conclude that that a change in oxidation state levels from [Mn III 6Mn II 4] in compound 4 to [Mn III 4Mn II 6] in compound 3 results in a significant decrease in the magnitude of some antiferromagnetic exchange contributions (J1 = -4.8 cm -1 (3), -40.4 cm -1 (4)), a switch from ferromagnetic to antiferromagnetic in others (J2 = -1.1 cm -1 (3), +3.1 cm -1 (4); J5 = -0.2 cm -1 (3), +4.1 cm -1 (4)), and the loss of significant spin frustration. The computed spin density plots for the high spin state of complexes 3 and 4 are given in Fig. S6, with the spin density plots for all the broken symmetries computed given in Figure S7-8.

Conclusions
Altering reaction stoichiometry results in a change in oxidation state distribution in bis-calix [4]arene supported [Mn10] cages, without significantly altering structural topology. DFT calculations reveal that the shift in oxidation state levels from [Mn III 6Mn II 4] in compound 4 to [Mn III 4Mn II 6] in compound 3 results in a significant decrease in the magnitude of some antiferromagnetic exchange contributions, a switch from ferromagnetic to antiferromagnetic in others, and the loss of significant spin frustration. Indeed, the change in oxidation state from Mn III -Mn III to Mn II -Mn II in the central J1 interaction causes a 10-fold change in the magnitude of the exchange interaction.
These results, alongside those we and others have already reported in the chemistry of p-t Bucalix [4]arene and 2,2'-bis-p-t Bu-calix [4]arene, suggest that the Mn III L metalloligand can be employed as a structure-directing unit capable of encapsulating additional TM (and LnM) moieties in its core. The ability to construct families of structurally analogous species, but whose oxidation state distribution, or metal identity, differs allows for more detailed investigation of the magneto-structural relationship. This ability underpins the design of magnetic molecules whose properties can be tuned toward a specific application. It also has wider design implications for scientists interested in molecules that can robustly cycle through multiple oxidation levels in, for example, bioinorganic chemistry and catalysis.

Author Contributions
LRBW and MC performed the synthetic chemistry and collected the magnetic data. SJD collected the single crystal X-ray data and performed structure solution. RJ and GR performed the theoretical calculations. EKB and SJD conceived the concept. All authors contributed to the writing/editing of the manuscript.

Conflicts of interest
There are no conflicts to declare.

Computational details
DFT Calculations were performed on the full crystal structures of complex 3 and 4. The energy of each spin configurations were computed using hybrid UB3LYP functional with TZV basis set 1 as implemented in Gaussian 16 software. The exchange coupling interactions were estimated using broken symmetry approach, 2 which has been shown to yield good numerical estimates of J values in polymetallic systems 3 based on the extended pair-wise interaction model proposed by Ruiz and coworkers. 4 The following spin Hamiltonian was used to estimate the exchange coupling interactions.