Giant Heterometallic [Mn36Ni4]0/2− and [Mn32Co8] “Loops-of-Loops-and-Supertetrahedra” Molecular Aggregates

Introduction

High nuclearity Mn carboxylate clusters continue to attract significant attention mainly because of their structural characteristics and physical properties (Bagai and Christou, 2009; Kostakis et al., 2010; Escuer et al., 2014). In particular, such compounds often exhibit interesting magnetic properties including high spin ground state values (Ako et al., 2006; Moushi et al., 2009) and single—molecule magnetism behavior (Sessoli et al., 1993; Bagai and Christou, 2009; Inglis et al., 2012; Milios and Winpenny, 2015). The latter appears in molecules exhibiting a large spin ground state (S_T) value and significant easy axis magnetoanisotropy (Christou et al., 2000; Nakano and Oshio, 2011; Ferrando-Soria et al., 2017). In addition, Mn complexes have attracted significant attention since they are involved in the search for structural and functional analogs of the tetranuclear Mn complex that is present in the active site of photosystem II and is responsible for the photosynthetic oxidation of H₂O to molecular O₂ (Mukherjee et al., 2012; Yano and Yachandra, 2014; Gerey et al., 2016). Thus, Mn clusters have been proposed for various applications in diverse areas including magnetic refrigeration (Zheng et al., 2014), molecular spintronics (Bogani, 2015), quantum computation (Aromí et al., 2012), and catalysis (Maayan et al., 2018).

This interest has resulted in the development of several synthetic methods to Mn carboxylate clusters and the isolation of numerous high nuclearity complexes possessing a wide variety of shapes (wheels, disks, icosahedra, cuboctahedra, spheres, rods, etc.) and nuclearities (Kostakis et al., 2010). Some of these compounds exhibit very high nuclearities and dimensions with the list of giant, nanosized Mn clusters containing [Mn₂₉] (Alexandropoulos et al., 2016), [Mn₃₀] (Soler et al., 2004), [Mn₃₁] (Abbasi et al., 2017), [Mn₃₂] (Scott et al., 2005; Langley et al., 2011; Manoli et al., 2011), [Mn₄₀Na₄] (Moushi et al., 2007, 2010a), [Mn₄₄] (Moushi et al., 2010a), [Mn₄₉] (Manoli et al., 2016), [Mn₇₀] (Vinslava et al., 2016), and [Mn₈₄] (Tasiopoulos et al., 2004) aggregates. Although there are several homometallic nanosized Mn clusters reported, a [Mn₂₈Cu₁₇] aggregate is the only example of giant heterometallic Mn/M (M = any paramagnetic metal ion) compounds (Wang et al., 2007). Apart from their exciting crystal structures some of these nanosized aggregates exhibit interesting magnetic properties. For example, compounds [Mn₃₀], [Mn₃₁], [Mn₃₂], [Mn₇₀], and [Mn₈₄] possess SMM behavior with an appreciable energy barrier to magnetization reorientation and represent a meeting of the bottom-up and top-down approaches to nanomagnetism (Papatriantafyllopoulou et al., 2016). In addition, compounds [Mn₄₉] and [Mn₂₈Cu₁₇] display dominant ferromagnetic exchange interactions leading to giant S_T values S = 61/2 and 51/2, respectively (Wang et al., 2007; Manoli et al., 2016). Note that the record S_T values have appeared in other giant homometallic Fe₄₂ (Kang et al., 2015) and heterometallic [Ni₂₁Gd₂₀] (Chen et al., 2018) clusters and are S_T = 45 and 91, respectively. Interestingly although Mn cluster chemistry has proven to be the most fruitful source of giant metal clusters among other 3d metal ions there is only one heterometallic Mn/M (M = any metal ion) reported in contrast to the situation with other metal ions. For example, in Ni²⁺ or Cu²⁺ chemistry there are only a few giant homometallic clusters but there are several hetemetallic ones and especially Ni²⁺/4f and Cu²⁺/4f aggregates (Papatriantafyllopoulou et al., 2016).

Our group has been exploring reactions of diols with Mn—containing precursor compounds targeting to new high nuclearity Mn clusters and SMMs (Tasiopoulos and Perlepes, 2008). These investigations have afforded a series of giant Mn carboxylate clusters including [Mn₂₅Na₄] and [Mn₄₉] aggregates consisting of eight and four decametallic supertetrahedral repeating sub-units (Manoli et al., 2016). Note that discrete metal clusters exhibiting a [${\text{Mn}}_6^{\text{III}}$${\text{Mn}}_4^{\text{II}}$(μ₄-O)₄]¹⁸⁺ supertetrahedral core analogous to that appeared in [Mn₂₅Na₄] and [Mn₄₉] aggregates have been stabilized in several cases, especially with polyol-type ligands and in most cases these compounds exhibited entirely ferromagnetic exchange interactions and S_T = 22 (Stamatatos et al., 2006; Manoli et al., 2007, 2008; Wu et al., 2011). The use of 1, 3-propanediol (pdH₂) and its derivatives in Mn carboxylate chemistry afforded a family of [Mn₄₄] and [Mn₄₀Na₄] loops consisting of four [Mn₁₀M(μ₃- O)₂(O₂CCH₃)₁₃(pd)₆(py)₂]${}_4^{\text{x}+}$ ([Mn₄₀M₄]; M = Na⁺, x = 0; M = Mn²⁺, x = 1) loops linked through Na⁺ or Mn²⁺ ions (called “loops-of-loops”) and have a saddle-like topology. The [Mn₄₄] analog of this family displays a spin S_T = 6 ground state and SMM behavior (Moushi et al., 2007, 2010a).

Further investigation of the reactions that afforded the [Mn₄₀M₄] loops-of-loops aggregates involved the use of various 3d paramagnetic metal ions in an attempt to isolate a series of heterometallic Mn/3d analogs and/or other large aggregates composed of smaller clusters. These investigations afforded compounds [Mn₂Ni₆(μ₄-O)₂(μ₃-OH)₃(μ₃-Cl)₃(O₂CCH₃)₆(py)₈]²⁺[Mn₃₆Ni₄(μ₄-O)₈(μ₃-O)₄(μ₃-Cl)₈Cl₄(O₂CCH₃)₂₆(pd)₂₄(py)₄]²⁻ (1) [Mn₃₆Ni₄(μ₄-O)₈(μ₃-O)₄(μ₃-Cl)₈Cl₂(O₂CCH₃)₂₆(pd)₂₄(py)₄(H₂O)₂] (2) and [Mn₃₂Co₈(μ₄-O)₈(μ₃-O)₄(μ₃-Cl)₈Cl₂(μ₂-OCH₂CH₃)₂(O₂CCH₃)₂₈(pd)₂₂(py)₆] (3). The last compound discussed herein is the discrete [Mn₂Ni₆]²⁺ cation [Mn₂Ni₆(μ₄-O)₂(μ₃-OH)₄(μ₃-Cl)₂(O₂CCH₃)₆(py)₈](ClO₄)(OH) (4), i.e., an analog of the complex co-crystallizing with the [Mn₃₆Ni₄]²⁻ anion of 1. Compounds 1–3 are rare examples of giant heterometallic Mn/M clusters and possess an unprecedented “loop-of-loops-and-supertetrahedra” structural topology. Complexes 2 and 3 exhibit dominant ferromagnetic exchange interactions and large S_T values which in the case of 2 is 26 ± 1. The [Mn₃₆Ni₄] aggregates (compounds 1 and 2) do not display SMM behavior as is also the case for their [${\text{Mn}}_6^{\text{III}}$${\text{Mn}}_4^{\text{II}}$(μ₄-O)₄]¹⁸⁺ supertetrahedral sub-unit, whereas for their [Mn₃₂Co₈] analog (complex 3) the existence of an out-of-phase tail is an indication of SMM behavior, however, further studies are required to confirm this conclusion. Part of this work, involving the synthesis and characterization of compound 2 has been communicated previously (Charalambous et al., 2012).

Materials and Methods

Materials and Physical Measurements

All manipulations were performed under aerobic conditions using chemicals and solvents as received, unless otherwise stated. [Mn₃O(O₂CCH₃)₆(py)₃]·py was prepared as previously described (Vincent et al., 1987).

IR spectra were recorded in the solid state (KBr pellets) in the 4,000–400 cm⁻¹ range using a Shimadzu Prestige−21 spectrometer. Elemental analysis (C, H, and N) were performed by the in-house facilities of the Chemistry Department at the University of Florida.

Variable-temperature dc magnetic susceptibility data down to 1.80 K were collected on a Quantum Design MPMS-XL SQUID magnetometer equipped with a 70 kG (7 T) dc magnet at the University of Florida. Diamagnetic corrections were applied to the observed paramagnetic susceptibilities using Pascal's constants. Samples were embedded in solid eicosane to prevent torquing. AC magnetic susceptibility data were collected on the same instrument employing a 3.5 G AC field oscillating at frequencies up to 1,500 Hz. Magnetization vs. field and temperature data were fit using the program MAGNET (Davidson, E. R.)¹.

Experimental

[Mn₂Ni₆(μ₄-O)₂(μ₃-OH)₃(μ₃-Cl)₃(O₂CCH₃)₆(py)₈]²⁺ [Mn₃₆ Ni₄(μ₄-O)₈(μ₃-O)₄(μ₃-Cl)₈Cl₄(O₂CCH₃)₂₆ (pd)₂₄(py)₄]²⁻·6H₂O, (1)·6H₂O

To a stirred brown solution of [Mn₃O(O₂CCH₃)₆(py)₃]·py (0.23 g, 0.27 mmol) in 15 ml CH₃CN were added pdH₂ (100 μL, 0.105 g, 1.38 mmol) and solid NiCl₂·6H₂O (0.066 g, 0.27 mmol). The reaction mixture was left under magnetic stirring for 10 min, filtered off and the filtrate was left undisturbed at room temperature. After 1 week brown X-ray quality crystals of 1 suitable for X-ray structural determination were formed. The crystals were isolated by filtration, washed with CH₃CN and dried in vacuo; the yield was ~36%. The crystals for X-ray studies were maintained in contact with mother liquor to prevent solvent loss. % C H N Anal. for C₁₉₆H₃₁₅N₁₂O₁₃₅Cl₁₅Mn₃₈Ni₁₀ [(1)·6H₂O]: calcd: C 28.69, H 3.87, N 2.05; found: C 29.02, H 3.75, N 2.35. Metal analysis was performed via ICP-OES. Anal. Calc. for (1)·6H₂O: Mn 25.44, Ni 7.15; found: Mn 25.64, Ni 7.38%. Selected IR data (cm⁻¹, KBr pellet): 3,420 (s, br), 2,931 (m), 2,854 (m), 1,560 (s, br), 1,419 (s, br), 1,341 (w), 1,087 (s), 951 (m), 835 (w), 637 (s, br).

[Mn₃₆Ni₄(μ₄-O)₈(μ₃-O)₄(μ₃-Cl)₈Cl₂(O₂CCH₃)₂₆(pd)₂₄ (py)₄(H₂O)₂]·10H₂O, (2)·10H₂O

To a brown solution of [Mn₃O(O₂CCH₃)₆(py)₃]·py (0.23 g, 0.27 mmol) in 15 mL CH₃CN were added under magnetic stirring pdH₂ (200 μL, 0.211 g, 2.77 mmol) and NiCl₂·6H₂O (0.066 g, 0.27 mmol). The reaction mixture was left under magnetic stirring at room temperature for ~1 h and then undisturbed for ~3 h. The resulting dark brown slurry was filtered off and the filtrate was left undisturbed at room temperature. After a few days X-ray quality crystals of 2·2CH₃CN·12.30H₂O were formed, isolated by filtration, washed with CH₃CN and dried under vacuum. The crystals for X-ray studies were maintained in contact with mother liquor to prevent solvent loss. The yield was 35% based on total Mn content. % C H N Anal. for C₁₄₄H₂₆₆N₄O₁₂₄Cl₁₀Mn₃₆Ni₄ [(2)·10H₂O]: calcd: C 26.19, H 4.06, N 0.85; found: C 26.32, H 4.09, N 0.94. Metal analysis was performed via ICP-OES. Anal. Calc. for (2)·10H₂O: Mn 29.94, Ni 3.55; found: Mn 30.09, Ni 3.68%. Selected IR data (cm⁻¹, KBr pellet): 3,426 (s, br), 2,934 (m), 2,849 (m), 1,593 (s, br), 1,553 (w), 1,404 (m), 1,085 (s), 945 (w), 629 (s, br).

[Mn₃₂Co₈(μ₄-O)₈(μ₃-O)₄(μ₃-Cl)₈Cl₂(μ₂-OCH₂CH₃)₂ (O₂CCH₃)₂₈(pd)₂₂(py)₆]·20H₂O, (3)·20H₂O

To a stirred brown solution of [Mn₃O(O₂CCH₃)₆(py)₃]·py (0.23 g, 0.27 mmol) in 12 ml of EtOH were added pdH₂ (300 μL, 0.316 g, 4.15 mmol) and solid CoCl₂·6H₂O (0.066 g, 0.27 mmol) and the reaction mixture was left under magnetic stirring for 2 h. The resulting red-brown slurry was filtered off and the filtrate was layered with Et₂O (1:3 v/v). After 2 weeks brown crystals of (3)·3.84 EtOH·6H₂O were formed suitable for X-ray structural determination. The crystals were isolated by filtration, washed with EtOH and dried in vacuo; the yield was 29%. The crystals for X-ray studies were maintained in contact with mother liquor to prevent solvent loss. % C H N Anal. for C₁₅₆H₂₉₆N₆O₁₃₄Cl₁₀Mn₃₂Co₈ [(3)·20H₂O]: calcd: C 26.83, H 4.27, N 1.20; found: C 26.55, H 4.01, N 0.97. Metal analysis was performed via ICP-OES. Anal. Calc. for (3)·20H₂O: Mn 25.17, Co 6.75; found: Mn 25.38, Co 6.88%. Selected IR data (cm⁻¹, KBr pellet): 3,414 (s, br), 3,110 (s, br), 2,843 (w), 2,770 (w), 1,610 cm⁻¹ (m), 1,556 (w), 1396 (s), 1,078 (m), 621(m).

[Mn₂Ni₆(μ₄-O)₂(μ₃-OH)₄(μ₃-Cl)₂(O₂CCH₃)₆(py)₈] (ClO₄)(OH)·2H₂O, (4)·2H₂O

To a stirred brown solution of [Mn₃O(O₂CCH₃)₆(py)₃]·py (0.20 g, 0.24 mmol) in 15 mL EtOH were added solids NiCl₂·6H₂O (0.06 g, 0.24 mmol) and NaClO₄(0.03 g, 0.24 mmol) and the reaction mixture was left under magnetic stirring at room temperature for 2 h. The resulting brown slurry was filtered off and the dark brown filtrate was layered with Et₂O (1:3 v/v) and left undisturbed at room temperature for a period of 1 week, upon which yellow-brown crystals of 4 suitable for X-ray structural determination were formed. The crystals were isolated by filtration, washed with EtOH and dried in vacuo; the yield was ~21%. C H N Anal. for C₅₂H₆₇N₈O₂₅Cl₃Mn₂Ni₆ [(4)·2H₂O]: calcd: C 35.24, H 3.81, N 6.32; found: C 35.54, H 3.55, N 6.74. Metal analysis was performed via ICP-OES. Anal. Calc. for (4)·2H₂O: Mn 6.20, Ni 19.87; found: Mn 6.52, Ni 20.28 %. Selected IR data (cm⁻¹, KBr pellet): 3,549 (w, br), 3,078 (w, br), 1,584 (s), 1,443 (s), 1,413 (s), 1,223 (m), 1,105 (s), 698 (s), 627 (m).

Single Crystal X-Ray Crystallography

Single crystal X-ray diffraction data for (1), (2)·2CH₃CN·12.30H₂O, (3)·3.84EtOH·6H₂O, and (4) were collected on an Oxford-Diffraction Supernova diffractometer, equipped with a CCD detector utilizing Mo Ka (λ = 0.71073 Å) radiation. A suitable crystal was mounted on a Hampton cryoloop with Paratone-N oil and transferred to a goniostat where it was cooled for data collection. Empirical absorption corrections (multiscan based on symmetry-related measurements) were applied using CrysAlis RED software (Oxford Diffraction, 2008). The structures were solved by direct methods using SIR2004 (Burla et al., 2005) and refined on F² using full-matrix least-squares with SHELXL-2014/7 (Sheldrick, 2014) Software packages used were as follows: CrysAlis CCD for data collection (Oxford Diffraction 2008). CrysAlis RED for cell refinement and data reduction (Oxford Diffraction 2008). WINGX for geometric calculations (Farrugia, 1999), and DIAMOND (Brandenburg, 2006) for molecular graphics. The non-H atoms were treated anisotropically, whereas the aromatic H atoms were placed in calculated, ideal positions and refined as riding on their respective carbon atoms. Electron density contributions from disordered guest molecules were handled using the SQUEEZE procedure from the PLATON software suit (Van der Sluis and Spek, 1990; Spek, 2003). Selected crystal data for (1), (2)·2CH₃CN·12.30H₂O, (3)·3.84 EtOH·6H₂O, and (4) are summarized in Table S1, whereas selected bond lengths and angles are given in Tables S2–S5.

CCDC 1859793, CCDC 862029, CCDC 1859811, and CCDC 1859814 contain the supplementary crystallographic data for (1), (2)·2CH₃CN·12.30H₂O, (3)·3.84 EtOH·6H₂O, and (4), respectively. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Results

Syntheses

We have been systematically studying reactions of Mn salts and preformed clusters with diols as a route to new polynuclear clusters with novel structural characteristics and interesting magnetism (Tasiopoulos and Perlepes, 2008; Moushi et al., 2009, 2010b; Skordi et al., 2018). These studies have focused on the use of simple aliphatic diols such as pdH₂ and its derivatives which due to their alkoxide arms exhibit a high bridging capability and a fruitful coordination chemistry. We recently reported a family of large molecular aggregates consisting of four smaller clusters linked through Na⁺ or Mn²⁺ ions (Moushi et al., 2010a). These large tetrameric {[Mn₁₀M(μ₃- O)₂(O₂CCH₃)₁₃(pd)₆(py)₂]₄}^x+ ([Mn₄₀M₄]; M = Na⁺, x = 0; M = Mn²⁺, x = 1), clusters contain four Mn₁₀ loops linked through Na⁺ or Mn²⁺ ions and have a saddle-like topology. They were prepared from reactions of [Mn₃O(O₂CMe)₆(py)₃]·py (py = pyridine) with H₂pd in the presence of NaN₃ [Mn₄₀Na₄] or Mn(ClO₄)₂·6H₂O [Mn₄₄]. We were interested to extend this study by preparing analogous heterometallic Mn_xM_y (M = a 3d metal ion) complexes. These studies involved the investigation of similar reactions to those led to the [Mn₄₄] or [Mn₄₀Na₄] aggregates which however, in the place of Mn(ClO₄)₂·6H₂O or NaN₃ contained a 3d metal ion salt. The initial result of these studies was compound [Mn₂Ni₆(μ₄-O)₂(μ₃-OH)₃(μ₃-Cl)₃(O₂CCH₃)₆(py)₈]²⁺[Mn₃₆Ni₄(μ₄-O)₈(μ₃-O)₄(μ₃-Cl)₈Cl₄(O₂CCH₃)₂₆(pd)₂₄(py)₄]²⁻ (1) that was obtained in ~36% yield from the reaction of [Mn₃O(O₂CCH₃)₆(py)₃]·py with 1,3-propanediol (pdH₂) in the presence of NiCl₂·6H₂O in 1:5:1 molar ratio in CH₃CN. The formation of compound 1 is summarized in Equation (1):

$\begin{array}{l}38[{\text{Mn}}_3\text{O}{({\text{O}}_2{\text{CCH}}_3)}_6{(\text{py})}_3]\cdot \text{py}+72{\text{H}}_2\text{L}+30{\text{NiCl}}_2\cdot 6{\text{H}}_2\text{O}\\ +7/2{\text{O}}_2\stackrel{{\text{CH}}_3\text{CN}}{\to }3{[M{n}_{36}N{i}_4{O}_{12}C{l}_{12}{(O_{2}CC{H}_3)}_{26}{(pd)}_{24}{(py)}_4]}^{2-}\\ {[M{n}_{2}N{i}_6{O}_2{(OH)}_3{(Cl)}_3{(O_{2}CC{H}_3)}_6{(py)}_8]}^{2+}\\ +15\text{HCl}+116\text{py}+132{\text{CH}}_3\text{COOH}+174{\text{H}}_2\text{O}& (1)\end{array}$

Since compound 1 crystallized as a mixture of a cationic and an anionic complexes, we targeted the isolation of both of its components, and especially of [Mn₃₆Ni₄] aggregate, in a discrete form. From the molecular formula of 1 it was observed that in the [Mn₂Ni₆]²⁺ cation of 1 there are no pd²⁻ ligands in contrast to the [Mn₃₆Ni₄]²⁻ anion of 1 which contains 24 pd²⁻ groups. We thus decided to increase the ratio of pd²⁻ in the reaction mixture in order to facilitate the formation of the complex that contains pd²⁻ groups and this modification resulted in the isolation of the discrete [Mn₃₆Ni₄] analog. In particular, the reaction of [Mn₃O(O₂CCH₃)₆(py)₃]·py with pdH₂ and NiCl₂·6H₂O in a 1:10:1 molar ratio in CH₃CN afforded the discrete [Mn₃₆Ni₄] complex [Mn₃₆Ni₄O₁₂Cl₁₀(O₂CCH₃)₂₆(pd)₂₄(py)₄(H₂O)₂]·10H₂O in 35% yield. The formation of compound 2 is summarized in Equation (2):

$\begin{array}{l}12[{\text{Mn}}_3\text{O}{({\text{O}}_2\text{CMe})}_6{(\text{py})}_3]\cdot \text{py}+24{\text{H}}_2\text{pd}+5{\text{NiCl}}_2\cdot 6{\text{H}}_2\text{O}\\ +{\text{O}}_2\stackrel{{\text{CH}}_3\text{CN}}{\to }[M{n}_{36}N{i}_4{O}_{12}C{l}_{10}{(O_{2}CC{H}_3)}_{26}{(pd)}_{24}{(py)}_4\\ {(H_{2}O)}_2]+44\text{py}+44{\text{CH}}_3\text{COOH}+\text{Ni}{({\text{CH}}_3\text{COO})}_2+30{\text{H}}_2\text{O}& (2)\end{array}$

After the isolation of compound 2 was realized, our efforts were focused on the synthesis of other Mn_40−xM_x (M = a 3d metal ion) heterometallic “loops-of-loops-and-supertetrahedra” molecular aggregates and also of the [Mn₂Ni₆] cation of 1 in a discrete form. The preparation of a Mn_40−xCo_x analog was our initial target since the incorporation in this structure of the highly anisotropic Co²⁺ ions could result in the appearance of a different magnetic behavior in the resulting compound than those shown in 1 and 2. The formation of the Mn/Co species was achieved from the reaction of [Mn₃O(O₂CCH₃)₆(py)₃]·py with pdH₂ and CoCl₂·6H₂O in a 1:15:1 molar ratio in C₂H₅OH in ~29% yield. The formation of compound 3 is summarized in Equation (3):

$\begin{array}{l}32[{\text{Mn}}_3\text{O}{({\text{O}}_2{\text{CCH}}_3)}_6{(\text{py})}_3]\cdot \text{py}+66{\text{H}}_2\text{pd}+24{\text{CoCl}}_2\cdot 4{\text{H}}_2\text{O}\\ +6{\text{CH}}_3{\text{CH}}_2\text{OH}+5{\text{O}}_2\stackrel{{\text{CH}}_3{\text{CH}}_2\text{OH}}{\to }3[M{n}_{32}C{o}_8{O}_{12}C{l}_{10}\\ {(OC{H}_{2}C{H}_3)}_2{(O_{2}CC{H}_3)}_{28}{(pd)}_{22}{(py)}_6]+18\text{HCl}+110\text{py}\\ +108{\text{CH}}_3\text{COOH}+102{\text{H}}_2\text{O}& (3)\end{array}$

The discrete [Mn₂Ni₆] cluster was also isolated by following a similar synthetic procedure to the one that led to the discrete [Mn₃₆Ni₄] aggregate which however, did not involve the use of pdH₂ which is not present in this compound. Thus, the reaction of [Mn₃O(O₂CCH₃)₆(py)₃]·py with NiCl₂·6H₂O in the presence of NaClO₄ in a 1:1:1 molar ratio in C₂H₅OH afforded complex 4 in ~21% yield. The formation of compound 4 is summarized in Equation (4):

$\begin{array}{l}[{\text{Mn}}_3\text{O}{({\text{O}}_2{\text{CCH}}_3)}_6{(\text{py})}_3]\cdot \text{py}+6{\text{NiCl}}_2\cdot 6{\text{H}}_2\text{O}+3\text{Na}({\text{ClO}}_4)\\ +4\text{py}\stackrel{{\text{CH}}_3{\text{CH}}_2\text{OH}}{\to }[M{n}_{2}N{i}_6{O}_2{(OH)}_4{(Cl)}_2-\\ {(O_{2}CC{H}_3)}_6{(py)}_8](Cl{O}_4)(OH)+7\text{HCl}+3\text{NaCl}\\ +\text{Mn}{({\text{ClO}}_4)}_2+30{\text{H}}_2\text{O}& (4)\end{array}$

Description of the Structures

Complexes (1) and (3)·3.84 EtOH·6H₂O crystallize in the triclinic space group P$\overline{1}$ and (2)·2CH₃CN·12.30H₂O and (4) in the monoclinic I 2/a one. The [Mn₃₆Ni₄]²⁻, [Mn₃₆Ni₄], and [Mn₃₂Co₈] aggregates of 1, 2, and 3, respectively exhibit related structures. Similarly, the molecular structures of the [Mn₂Ni₆]²⁺ cation of 1 and 4 are also related. Thus, the structures of the [Mn₃₆Ni₄]²⁻ anion and the [Mn₂Ni₆]²⁺ cation will be discussed in detail and compared to those of their related analogs 2–4. The molecular structures of the cation and the anion of 1, and the [Mn₃₂Co₈] aggregate and the sub-units of 3 are shown in Figures 1, 2, respectively. Structural figures and tables for 1–4 (bond lengths and angles, Mn/Ni/Co BVS calculations are reported in the Supplementary Material).

FIGURE 1

Figure 1. Representations of the molecular structures of the [Mn₃₆Ni₄]²⁻ anion (top) and the [Mn₂Ni₆]²⁺ cation (bottom) of 1 Color code: Mn^II purple, Mn^III blue, Ni^II orange, O red, N light green, Cl green, C gray. H atoms are omitted for clarity.

FIGURE 2

Figure 2. Representations of the molecular structure of 3 (top) and its [${\text{Mn}}_8^{\text{III}}$Co₂] loop (bottom, left) and [${\text{Mn}}_6^{\text{III}}$${\text{Mn}}_2^{\text{II}}$${\text{Co}}_2^{\text{II}}$] supertetrahedral (bottom, right) sub-units. Color code: Mn^II purple, Mn^III blue, Co^II yellow, O red; N light green, Cl green, C gray. H atoms are omitted for clarity.

The molecular structure of 1 consists of a [Mn₃₆Ni₄]²⁻ anionic aggregate (Figure 1, top) the charge of which is balanced from a cationic [Mn₂Ni₆]²⁺ cluster (Figure 1, bottom). Note that the oxidation states of the Mn/Ni ions and the protonation levels of the ligands were determined by bond valence sum calculations (Brown and Altermatt, 1985; Liu and Thorp, 1993), charge considerations and inspection of metric parameters. The giant [Mn₃₆Ni₄]²⁻ aggregate contains two [${\text{Mn}}_8^{\text{III}}$Ni₂(μ₃-O)₂(O₂CCH₃)₁₂(pd)₆(py)₂] loops (Figure S1, top) and two [${\text{Mn}}_6^{\text{III}}$${\text{Mn}}_4^{\text{II}}$(μ₄-O)₄(μ₃-Cl)₄(O₂CCH₃)Cl₂(pd)₆(H₂O)] supertetrahedral sub-units (Figure S1, bottom) which are related to compounds either appeared as fragments in high nuclearity clusters or in a discrete form. In particular the [${\text{Mn}}_8^{\text{III}}$Ni₂] loop exhibits analogous structure to the [${\text{Mn}}_8^{\text{III}}$${\text{Mn}}_2^{\text{II}}$] sub-unit of the tetrameric [Mn₄₄] or [Mn₄₀Na₄] “loops-of-loops” aggregates (vide supra) with their main difference being the presence of two Ni²⁺ ions in the place of two Mn²⁺ ions (Moushi et al., 2010a). In addition, the [${\text{Mn}}_6^{\text{III}}$${\text{Mn}}_4^{\text{II}}$(μ₄-O)₄)]¹⁸⁺ supertetrahedral core that is present in 1 has appeared in several compounds either in discrete form (Stamatatos et al., 2006; Manoli et al., 2007, 2008; Wu et al., 2011) or as fragment of higher nuclearity clusters (Manoli et al., 2016). The Mn^III and Ni^II ions are hexacoordinated exhibiting a distorted octahedral coordination geometry whereas the Mn^II ions adopt various coordination numbers and geometries. As expected, the Mn^III ions display the expected Jahn-Teller (JT) elongations although the JT axes are not co-parallel.

Thus, each [${\text{Mn}}_8^{\text{III}}$Ni₂] unit consists of two [${\text{Mn}}_3^{\text{III}}$(μ₃-O)]⁷⁺ triangles and two dinuclear Mn^IIINi^II moieties linked by pd²⁻ RO⁻ groups, and bridging CH₃${\text{CO}}_2^{-}$ ligands. The Mn and Ni ions are held together through 12 acetate and six pd²⁻ bridging ligands. The acetate groups adopt syn, syn- η¹:η¹:μ (six CH₃${\text{CO}}_2^{-}$ ligands), η¹:η²:μ₃ (four CH₃${\text{CO}}_2^{-}$ ligands) and η²:η²:μ₄ (two CH₃${\text{CO}}_2^{-}$ ligands) bridging modes whereas the pd²⁻ ligands link metal ions in a η²:η²:μ₃ fashion. The peripheral ligation of the [${\text{Mn}}_8^{\text{III}}$Ni₂] loop is completed by two terminal py molecules. The [${\text{Mn}}_8^{\text{III}}$Ni₂] loops are connected to the [${\text{Mn}}_6^{\text{III}}$${\text{Mn}}_4^{\text{II}}$] supertetrahedral sub-units through two μ₃- and one μ- CH₃${\text{CO}}_2^{-}$ ligands bridging the Mn ions of each [${\text{Mn}}_3^{\text{III}}$O]⁷⁺ triangle to a Mn^II ion of a supertetrahedral sub-unit (Figure S3) constructing the nearly—planar [${\text{Mn}}_{28}^{\text{III}}$${\text{Mn}}_8^{\text{II}}$Ni₄] “loop-of-loops-and-supertetrahedra” aggregate. The Mn^II ions of the [${\text{Mn}}_6^{\text{III}}$${\text{Mn}}_4^{\text{II}}$] sub-unit occupy the apex positions of a tetrahedron and the Mn^III ions are located on its edges. The metal ions are connected by four μ₄-O²⁻ ions forming the [${\text{Mn}}_6^{\text{III}}$${\text{Mn}}_4^{\text{II}}$(μ₄-O)₄]¹⁸⁺ core which consists of four [${\text{Mn}}_3^{\text{III}}$Mn^II(μ₄-O)]⁹⁺ vertex-sharing tetrahedra. The Mn^III ions are bridged through four μ₃- Cl⁻ ions which occupy their JT axes. The two Mn²⁺ and one Mn³⁺ ions located in each edge of the tetrahedron are connected through six pd²⁻ ligands bridging in a η²:η²:μ₃ mode. The peripheral ligation of the [${\text{Mn}}_6^{\text{III}}$${\text{Mn}}_4^{\text{II}}$(μ₄-O)₄(μ₃-Cl)₄(pd)₆(O₂CCH₃)Cl₂] subunit (Figure S1) is completed by two terminal Cl⁻ anions.

The molecular structure of the [${\text{Mn}}_2^{\text{III}}$${\text{Ni}}_6^{\text{II}}$] cation consists of two mixed metal [Mn^III${\text{Ni}}_3^{\text{II}}$(μ₄-O)(μ₃-OH)_1.5(μ₃-Cl)_1.5(O₂CMe)₃(py)₄]⁺ cubanes linked through oxide and carboxylate ligands. In particular, the three Ni^II and one Mn^III ions are connected through one μ₄-O²⁻, one μ₃-OH⁻, one μ₃-Cl⁻ and a mixed 0.5 Cl⁻/0.5 OH⁻ site. The μ₃-OH⁻ and μ₃-Cl⁻ bridge two 2 Ni^II and one Mn^III ions, the mixed 0.5 Cl⁻/0.5 OH⁻ connects 3 Ni^II ions of the cubane whereas the μ₄-O²⁻ links 2 Ni^II and one Mn^III ions of the cubane thus forming the [Mn^III${\text{Ni}}_3^{\text{II}}$(μ₄-O)(μ₃-OH)_1.5(μ₃-Cl)_1.5]⁴⁺ cubane core and one additional Mn^III ion of the second cubane of the cation of 1. The Mn/Ni ions of each cubane are also bridged by three carboxylate ligands two of which bridge in the common syn, syn- η¹:η¹:μ fashion and the third one in a η¹:η²:μ₃ one. Their peripheral ligation is completed by four terminal pyridine molecules linked to the Ni^II ions. The two cubanes of the cation of 1 are linked apart from the μ₄-O²⁻ ions from two syn, syn- η¹:η¹:μ and two η¹:η²:μ₃ carboxylate ligands constructing the [Mn₂Ni₆(μ₄-O)₂(μ₃-OH)₃(μ₃-Cl)₃(O₂CCH₃)₆(py)₈]²⁺ cation of 1.

The molecular structure of the [Mn₃₆Ni₄] cluster of 2·2CH₃CN·12.30H₂O exhibits a striking similarity to the anion of 1. The main difference between the two complexes is their overall charge since compound 1 is anionic with a 2- charge and 2 is neutral. This difference in the charges appears because a terminal Cl⁻ ligand of 1 linked to a Mn^II ion of the supertetrahedral sub-units has been replaced in 2 by a terminal H₂O molecule. As a result, compound 2 contains two less Cl⁻ anions and is neutral and thus, the [Mn₃₆Ni₄] aggregate is the only metal cluster appearing in the crystal structure.

The molecular structure of (3)·3.84 EtOH·6H₂O (Figure 2, top) is also related to the anion of 1 and to 2·2CH₃CN·12.30H₂O with the main difference obviously being the presence in 3 of 32 Mn/8 Co ions instead of 36 Mn/4 Ni ions that appear in the anion of 1 and in 2. These eight Co^II ions are located in the decametallic [${\text{Mn}}_8^{\text{III}}$${\text{Co}}_2^{\text{II}}$] loops (Figure 2, bottom left) in the same positions that the Ni^II ions of the [Mn₃₆Ni₄] aggregates are found and also in the [${\text{Mn}}_6^{\text{III}}$${\text{Mn}}_2^{\text{II}}$${\text{Co}}_2^{\text{II}}$] supertetrahedral sub-units (Figure 2, bottom right). In the latter the Co^II ions occupy two apex positions in which Mn^II ions are located in the known [${\text{Mn}}_6^{\text{III}}$${\text{Mn}}_4^{\text{II}}$] supertetrahedra including the sub-units of the anion of 1 and in 2. In fact, this is a major difference between the [Mn₃₆Ni₄]^0/2− and [Mn₃₂Co₈] since the former ones consist of one heterometallic [${\text{Mn}}_8^{\text{III}}$${\text{Ni}}_2^{\text{II}}$] and one homometallic [${\text{Mn}}_6^{\text{III}}$${\text{Mn}}_4^{\text{II}}$] sub-units, whereas the latter is based on two different types of heterometallic sub-units. Apart from these major differences in the structures of the [Mn₃₆Ni₄]^0/2− and [Mn₃₂Co₈] compounds there are also some minor ones. These include the presence in the structure of [Mn₃₂Co₈] aggregate of two less pd²⁻ ligands and two additional CH₃${\text{CO}}_2^{-}$ and C₂H₅O⁻ groups compared to the structures of the [Mn₃₆Ni₄]^0/2− complexes. This is because one pd²⁻ group bridging 2 Mn^II/1Mn^III ions of one edge of each supertetrahedron in [Mn₃₆Ni₄]^0/2− complexes has been replaced by a syn, syn- η¹:η¹:μ- CH₃${\text{CO}}_2^{-}$ and a μ-EtO ligands in [Mn₃₂Co₈] aggregate. In addition, [Mn₃₂Co₈] aggregate contains a terminal py ligand bound to a Co^II ion located in the [${\text{Mn}}_6^{\text{III}}$${\text{Mn}}_2^{\text{II}}$${\text{Co}}_2^{\text{II}}$] supertetrahedra in the place of a Cl⁻ ligand or a H₂O molecule in the anion of 1 or 2, respectively.

The molecular structure of compound 4 is related to that of the cation of 1. In fact, there are only very minor differences between the two compounds. These include the replacement in 4 of the mixed 0.5 Cl⁻/0.5 OH⁻ site that bridges 3 Ni^II ions of each cubane by a μ₃-OH⁻ anion. In addition, in complex 4 the positive charge of the [${\text{Mn}}_2^{\text{III}}$${\text{Ni}}_6^{\text{II}}$]²⁺ cation is balanced by a Cl${\text{O}}_4^{-}$ and a OH⁻ lattice anions instead of the [Mn₃₆Ni₄]²⁻ anionic aggregate.

Magnetic Properties

Solid-state dc magnetic susceptibility measurements were performed on polycrystalline samples of complexes 1·6 H₂O, 2·10 H₂O, 3·20 H₂O, and 4·2 H₂O under a magnetic field of 0.1 T in the temperature range 5-300 K. The obtained data are shown as χ_MT vs. T plot in Figure 3. For complexes 1·6 H₂O, 2·10 H₂O, and 3·20 H₂O the χ_MT value increases continuously from 172.1, 118.6, and 130.5 cm³ mol⁻¹ K at 300 K to a maximum value of 492.0 (at 20 K), 325.6 (at 15 K), and 273.7 cm³ mol⁻¹ K (at 25 K) and then decreases at low T to 465.1, 304.3, and 226.1 cm³ mol⁻¹ K at 5 K, respectively. For 4·2 H₂O, the χ_MT value at 300 K is 18.03 cm³ mol⁻¹ K and decreases continuously with decreasing temperature reaching a value of ~15.03 cm³ mol⁻¹ K at 100 K and then rapidly to 2.94 cm³ mol⁻¹ K at 5.0K (Figure 3 and Figure S6). The increase of the χ_MT values with decreasing T in 1·6 H₂O—3·20 H₂O indicates the presence of dominant ferromagnetic exchange interactions. The maximum χ_MT values of 2·10 H₂O and 3·20 H₂O suggest S_T values of ~ 26 ± 1 and 22 ± 1, respectively. However, we note that in the case of 3·20 H₂O it may not be safe to exclude any conclusions for the spin ground state using the spin-only formula due to the presence of Co^II ions which is well known that exhibit strong spin-orbit coupling. The decrease in the χ_MT value at the lowest temperatures is attributed to zero-field splitting (ZFS), Zeeman effects from the applied field, and/or any weak intermolecular antiferromagnetic exchange interactions. In the case of 4·2 H₂O, the continuous decrease of χ_MT with decreasing temperature, the small χ_MT value at 5 K and the fact that the curve heads to 0 at 0 K suggests the presence of antiferromagnetic exchange interactions possibly leading to a diamagnetic ground state. This may be rationalized assuming that the two [Mn^III${\text{Ni}}_3^{\text{II}}$] are antiferromagnetically coupled leading to a diamagnetic ground state.

FIGURE 3

Figure 3. χ_MT vs. T plots for complexes (1)·6 H₂O (•), (2)·10 H₂O (◦), (3)·20 H₂O (▾), and (4)·2 H₂O (Δ) in the temperature range 5–300 K in a 0.1 T applied dc field.

Given the size and the complexity of the structures of 1–3, it is not possible to apply the Kambe method to determine the individual pairwise Mn-Mn, Mn-Ni, and Mn-Co exchange interaction parameters. In addition, the existence of two complexes co-crystallizing together in the structure of 1·6 H₂O does not allow to obtain information for the D and S_T values of these complexes. Furthermore, in 3·20 H₂O, the presence of Co^II ions exhibiting strong spin-orbit coupling and of many low lying excited states appearing due to the complexity of the giant [Mn₃₂Co₈] aggregate did not allow to obtain reliable S and D values from reduced magnetization fitting.

In the case of compound 2·10 H₂O magnetization vs. dc field measurements at applied magnetic fields and temperatures in the 1–10 kG and 1.8–4.0 K ranges, respectively were performed. Low field data (≤ 1.0 T) were used, as we have previously done for many Mn clusters containing Mn^II atoms, to avoid problems from low-lying excited states. The data for complex 2·10 H₂O are shown in Figure S7 as reduced magnetization (M/Nμ_B) vs. H/T plot, where M is the magnetization, N is Avogadro's number, μ_B is the Bohr magneton, and H is the magnetic field.

The M/Nμ_B vs. H/T data were fit by assuming that only the ground state is populated and by including axial zero-field splitting (D${Ŝ}_{\text{z}}^2$) and isotropic Zeeman interactions. The corresponding spin Hamiltonian is given by Equation (5),

$\begin{array}{ll}\text{H}=\mathit{\text{D}}{Ŝ}_{\text{z}}^2+\mathit{\text{g}}{\mu }_{\text{B}}{\mu }_0Ŝ·\mathit{\text{H}}& (5)\end{array}$

where D is the axial ZFS parameter, Ŝ_z is the easy-axis spin operator, μ₀ is the vacuum permeability, and H is the applied field. Equal quality fits, shown as the solid lines in Figure S7, were obtained for S = 25, 26, and 27 with parameters g = 2.03(1)/D = −0.007(1) cm⁻¹, g = 1.96(1)/D = −0.004(1) cm⁻¹, and g = 1.91(1)/D = −0.004(1) cm⁻¹, respectively. Based on the obtained fits, we conclude that 2·10 H₂O has a ground state of S_T = 26 ± 1, and a very small D value.

Alternating current (ac) magnetic susceptibility data were collected for compounds 1–3 to obtain additional information about their S_T values and the possibility to exhibit slow relaxation of the magnetization phenomena indicative of SMM behavior. The temperature dependence of the in-phase (${\chi }_{\text{M}}^{\prime }$), shown as ${\chi }_{\text{M}}^{\prime }$T and out-of-phase (${\chi }_{\text{M}}^{\prime \prime }$) ac signals for 1–3 is shown in Figures S8–S10). These studies revealed that there are not any out-of-phase ac signals in 1 and 2 suggesting that these compounds are not new SMMs. In the case of 3, there is a barely visible beginning in the 1.8 K data of a frequency dependence in the in-phase plot and a concomitant very weak tail of an out-of-phase signal (${\chi }_{\text{M}}^{\prime \prime }$/${\chi }_{\text{M}}^{\prime }$ ~ 1.5% at 1.8 K) representing the beginning of a ${\chi }_{\text{M}}^{\prime \prime }$ signal whose peak is clearly far below the 1.8 K limit of our SQUID instrument, i.e., the anisotropy barrier is extremely small. In addition, ac data are in line with the conclusions obtained from dc studies concerning the S_T values of 2 and 3. In particular, extrapolation of the χ_M'T signal of compounds 2 and 3 to 0 K from above ~8 K (to avoid the effects of intermolecular interactions at lower temperatures) gives values of ~340 and 255 cm³mol⁻¹ K, respectively. These values are consistent with S_T in the range 25–27 (S_T = 25, g = 2.05; S_T = 26, g = 1.97; S_T = 27, g = 1.90) for 2 and 21–23 (S_T = 21, g = 2.10; S_T = 22, g = 2.01; S_T = 23, g = 1.92) for 3.

Discussion

We described herein the synthesis, crystal structures and magnetic properties of a series of high nuclearity mixed metal Mn/Ni and Mn/Co clusters. The first member of this family of complexes, compound 1, was prepared by targeted modifications in the reaction procedures that afforded the family of [Mn₄₄] and [Mn₄₀Na₄] “loops-of-loops” aggregates reported by our group previously (Moushi et al., 2010a). Since complex 1 consists of a [Mn₃₆Ni₄]²⁻ anion which co-crystallized together with a [Mn₂Ni₆]²⁺ cation, we targeted and achieved, following synthetic procedures containing elements of rational design, the isolation of the discrete [Mn₃₆Ni₄] (compound 2) and [Mn₂Ni₆]²⁺ (the cation of 4) clusters and also of another Mn/3d analog the [Mn₃₂Co₈] aggregate of 3. Arguably most of the known giant molecular aggregates have been afforded from serendipitous assembly synthetic procedures although recently there have been a few reports about elegant, rationally designed synthetic strategies that led to such high nuclearity clusters (Papatriantafyllopoulou et al., 2016 and references therein). The isolation of 2–4 thus, represents a rare example of targeted synthesis of high nuclearity metal—organic complexes from procedures that contain elements of rational design.

The heterometallic [Mn₃₆Ni₄]^2−/0 and [Mn₃₂Co₈] “loops-of-loops-and-supertetrahedra” molecular aggregates are giant clusters exhibiting nanosized dimensions and large molecular weights approaching those of small proteins (for example the MW of compound 1 is ~8,095 g/mol and is comparable to those of small proteins which are ~10 kDa). In fact these are the second highest nuclearity heterometallic Mn_xM_y (M = any metal ion) clusters and M_xM'_y (M, M′ = any 3d metal ion) with the highest nuclearity heterometallic Mn-containing complex and mixed 3d metal/3d metal cluster being a [Mn₂₈Cu₁₇] aggregate (Wang et al., 2007). Interestingly, the [Mn₃₆Ni₄]^2−/0 and [Mn₃₂Co₈] aggregates are based on polynuclear sub-units that have appeared either as fragments and/or in a discrete form previously (Stamatatos et al., 2006; Manoli et al., 2007, 2008, 2016; Moushi et al., 2010a; Wu et al., 2011). This structural feature makes them a member of a very small family of large matal-organic clusters based on known in a discrete form polynuclear repeating units (Manoli et al., 2016; Papatriantafyllopoulou et al., 2016). Their [${\text{Mn}}_6^{\text{III}}$${\text{Mn}}_4^{\text{II}}$(μ₄-O)₄]¹⁸⁺ supertetrahedral repeating unit is very well-known in Mn cluster chemistry and has attracted significant interest due to its symmetric structure and the fact that most of the compounds containing this core exhibit entirely ferromagnetic exchange interactions and the maximum possible, for a [${\text{Mn}}_6^{\text{III}}$${\text{Mn}}_4^{\text{II}}$] complex, S_T = 22 spin ground state value. The presence of this repeating unit in the [Mn₃₆Ni₄]^2−/0 and [Mn₃₂Co₈] “loops-of-loops-and-supertetrahedra” aggregates ensures the appearance of dominant ferromagnetic exchange interactions between their metal ions and of a large S_T value. Notably there are no Ni^II ions located in this sub-unit in the [Mn₃₆Ni₄]^2−/0 clusters since all of them are found in the decametallic loops. However, in the [Mn₃₂Co₈] aggregate there are two Co^II ions in each decametallic supertetrahedron located in the apex positions replacing two Mn^II ions. In fact, there have been reported some heterometallic Mn_10−xM_x (M = any metal ion) supertetrahedra where the Mn^II ions are partially or completely replaced, however, to the best of our knowledge there are no mixed metal Mn_10−xCo_x analogs in the literature. Such compounds could be very attractive magnetically since they could possibly combine the ferromagnetic exchange interactions and large S_T values appearing in the decametallic Mn-based supertetrahedra with a significant anisotropy due to the presence of Co^II ions. The appearance of the Co^II ions in the apex positions of the supertetrahedra is not surprising, not only because they replace a metal ion being in the same oxidation state (Mn^II) and as a result there are no charge variations in the new compound but also these positions have proven to be the most labile in this family of [${\text{Mn}}_6^{\text{III}}$${\text{Mn}}_4^{\text{II}}$] complexes. This is also supported from the isolation a series of heterometallic [[${\text{Mn}}_6^{\text{III}}$${\text{Mn}}_{\text{x}}^{\text{II}}$M_4−x] (M = any metal ion) supertetrahedra, appearing in a discrete form and also as fragments of high nuclearity clusters, in which the Mn^II ions have been completely or partially replaced by other metal ions (Skordi et al., 2018 and references therein). The [Mn₂Ni₆]²⁺ cation appears also for the first time in heterometallic cluster chemistry although there are homometallic [${\text{Mn}}_6^{\text{II}}$${\text{Mn}}_2^{\text{III}}$] clusters reported exhibiting an analogous structural core (Boskovic et al., 2002).

Magnetism studies revealed that the [Mn₃₆Ni₄]^2−/0/[Mn₃₂Co₈] aggregates exhibit dominant ferromamagnetic exchange interactions and large spin ground state values S_T = 26 ± 1 ([Mn₃₆Ni₄]) and 22 ± 1 ([Mn₃₂Co₈]), although in the latter it is not safe to conclude for the S_T value due to the presence of the anisotropic Co^II ions. On the other hand, the [Mn₂Ni₆]²⁺ cation of 4 exhibits dominant antiferromagnetic exchange interactions leading to a diamagnetic S_T value. In fact, a diamagnetic ground state has also been reported for the analogous homometallic [${\text{Mn}}_6^{\text{II}}$${\text{Mn}}_2^{\text{III}}$] cluster (Boskovic et al., 2002). This behavior could result from the presence of antiferromagnetic exchange interactions between the two [Mn^III${\text{Ni}}_3^{\text{II}}$] (in 4) or [Mn^III${\text{Mn}}_3^{\text{II}}$] (in the [${\text{Mn}}_6^{\text{II}}$${\text{Mn}}_2^{\text{III}}$]) cubane sub-units leading to diamagnetic ground states. The reported S_T values for 2 and 3 are among the larger ones for heterometallic aggregates with the S_T ≈ 26 being the second highest value reported for a heterometallic cluster (Chen et al., 2018). Clearly the overall magnetic behavior of 2 exhibits remarkable analogies with that of its [${\text{Mn}}_6^{\text{III}}$${\text{Mn}}_4^{\text{II}}$] building block since they both display ferromagnetic exchange interactions, large S_T and small D values and are not SMMs. On the other hand, in the case of 3 although it is not safe to conclude about the S_T value, however, it is clear that the spin ground state value in 3 is smaller than that of 2. This can be attributed to the existence of more heterometal ions, since in 3 there are 8 Co^II ions whereas in 2 only 4 Ni^II ions and also to the presence of stronger antiferromagnetic exchange interactions as expected for Mn/Co and Mn/Ni heterometallic compounds. In addition, the out-of-phase ac signals at low T in 3 appear due to the presence of the anisotropic Co^II ions leading to the increase of the anisotropy and possibly to SMM behavior.

Summarizing, a series of heterometallic [Mn₃₆Ni₄]^2−/0 and [Mn₃₂Co₈] “loops-of-loops-and-supertetrahedra” molecular aggregates and the cationic [${\text{Mn}}_2^{\text{III}}$${\text{Ni}}_6^{\text{II}}$]²⁺ cluster were prepared by employing synthetic procedures containing elements of rational design. The “loops-of-loops-and-supertetrahedra” molecular aggregates of 1–3 are among the largest heterometallic Mn-containing clusters exhibiting dimensions and molecular weights comparable to those of small proteins. In addition, they exhibit dominant ferromagnetic exchange interactions and very large S_T values. These compounds are new additions in the very small family of giant Mn_xM_y (M = any metal ion) aggregates. Since this area is merely unexplored, further studies targeting to “loops-of-loops-and-supertetrahedra” analogs with various 3d and 4f metal ions and other high nuclearity heterometallic Mn/M clusters are in progress and the results will be reported in due course.

Author Contributions

MC was involved on the synthesis, crystallization, and characterization of the reported complexes. EM was involved on the synthesis, crystallization, and characterization of the reported complexes and on manuscript preparation. TN was involved on the investigation of the magnetic properties of compounds 3 and 4. CP was involved on the investigation of the magnetic properties of compounds 1 and 2 and on manuscript preparation. VN was involved on the refinement of the crystal structures of 1–4 and on manuscript preparation. GC was involved on the investigation of the magnetic properties of compounds 1–2 and on manuscript preparation. AT supervised the reported work and was involved on all parts of the project.

Funding

This work was supported by the Cyprus Research Promotion Foundation Grant PENEK/0311/04 which is co-funded by the Republic of Cyprus and the European Regional Development Fund. GC thanks the NSF for support (CHE-1565664). We also thank the European Union Seventh Framework Program (FP7/2007-2013) under Grant agreement number PIRSES-GA-2011-295190.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The handling editor declared a past co-authorship with several of the authors (CP, VN, GC, AT).

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2019.00096/full#supplementary-material

Footnotes

1. ^MAGNET, Indiana University.

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