New structural motifs in Mn cluster chemistry from the ketone/gem-diol and bis(gem-diol) forms of 2,6-di-(2-pyridylcarbonyl)pyridine: {Mn^II₄Mn^III₂} and {Mn^II₄Mn^III₆} complexes

Abstract

The employment of the tripyridyl/diketone ligand 2,6-di-(2-pyridylcarbonyl)pyridine [(py)CO(py)CO(py)], in conjunction with azides (N₃⁻), in Mn cluster chemistry has afforded the mixed-valence (II/III) complexes [Mn^II₄Mn^III₂(N₃)₆Cl₄(L1)₂(DMF)₄] (1) and [Mn^II₄Mn^III₆O₂(N₃)₁₂(L1)₂(L2H)₂(DMF)₆] (2) in good yields. The resulting ligands L1²⁻ and L2H³⁻ are the dianion and trianion of the ketone/gem-diol (L1H₂) and bis(gem-diol) (L2H₄) forms of (py)CO(py)CO(py), respectively, as derived from the metal-assisted hydrolysis of the parent dicarbonyl organic compound. Under the same synthetic conditions (i.e., reaction solvents, temperature and stirring time), the chemical identity of the two complexes was found to depend on the Mn^II starting material; in the presence of MnCl₂, complex 1 is the only isolated product, while complex 2 can be only obtained if Mn(ClO₄)₂ is used. Complexes 1 and 2 are the second and third highest nuclearity products reported to date from any different form of coordinated ligands derived from (py)CO(py)CO(py). Magnetic susceptibility studies on both 1 and 2 revealed the presence of predominant antiferromagnetic exchange interactions between the metal centers. The combined results demonstrate the rich chemical reactivity of carbonyl groups and the ability of poly-ketone ligands to stabilize cluster compounds with unprecedented structural motifs and interesting molecular architectures.

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Introduction

There continues to be an intense interest by many groups around the world in the synthesis and study of polynuclear 3d-metal complexes (or 3d-metal clusters), not least for their intrinsic architectural beauty and aesthetically pleasing structures.¹ Other reasons for this interest are varied. For manganese chemistry, for example, this interest derives from their relevance to three fields. First, the ability of Mn to exist in a number of oxidation states (II–IV) under normal conditions has resulted in this metal being at the active sites of several redox enzymes, the most important of which is the water-oxidizing complex (WOC) on the donor side of photosystem II in green plants and cyanobacteria.² Second, polynuclear Mn compounds containing Mn^III have been found to often have large ground-state spin values (S),³ which combined with a large and negative magnetoanisotropy have led to some of these species being able to function as single-molecule magnets (SMMs).⁴ These are individual molecules that behave as magnets below a certain (“blocking”) temperature and they display frequency-dependent out-of-phase ac (alternating current) signals and magnetization hysteresis loops.⁵ Thus, they represent a molecular, ‘bottom-up’ approach to nanomagnetism.⁶ Last, but not least, it was recently shown that Mn^III-containing clusters can also act as emissive SMMs when combined with appropriate fluorescent ligands, a new route to multifunctional (or ‘hybrid’) molecular magnetic materials.⁷

Organic and inorganic bridging ligands of various types have been utilized over the last three decades for the synthesis of both high-spin clusters and SMMs. Two families of such ligands are the groups based on di-2-pyridyl ketone [(py)₂CO, Scheme 1]⁸ and the pseudohalides (i.e., azides and cyanates).⁹ The ‘ligand blends’ of (py)₂CO and N₃⁻ have yielded a large number of nanoscale molecular compounds with large S values and SMM behaviors with appreciable energy barriers for the magnetization reversal.¹⁰ The particular interest in the (py)₂CO-based ligands arises from the reactivity of the carbonyl group(s), which can undergo metal-assisted hydrolysis or alcoholysis (ROH; R = Me, Et) forming the first class of anionic (py)₂CO₂²⁻ and (py)₂C(OR)O⁻ ligands derived from (py)₂CO.¹¹ These anions have shown a rich coordination affinity to 3d-metal ions leading to a multitude number of clusters with beautiful topologies, unusual nuclearities and fascinating physical properties.^10–12 Recent progress in the reactivity chemistry of (py)₂CO involves the attack by nucleophiles other than H₂O and alcohols (i.e., MeCN, Me₂CO, etc.) on its carbonyl C atom in the presence of metal ions, which introduces a second class of ligands derived from (py)₂CO.¹³ These new derivatives of (py)₂CO have led to molecular species with different structural and physicochemical properties, further illustrating the flexibility of pyridyl ketones in adopting a wide variety of binding modes. In addition, the bridging azido ligand is very popular in the field of molecular magnetism.^9,14 Its rich bridging modes and the ability to propagate a variety of exchange interactions have led to abundant magnetic behaviors, such as ferromagnetism, antiferromagnetism, ferrimagnetism, canted weak ferromagnetism, spin-flop, and SMM.¹⁵

Scheme 1 Structural formulae and abbreviations of the organic ligands discussed in this work. Note that L1H₂, L2H₄, and their anions do not exist as free species but exist only in their respective metal complexes.

A reasonable following step in the synthesis of novel 3d-metal clusters with carbonyl-based functionalities was the exploration of the coordination and reactivity chemistry of 2,6-di-(2-pyridylcarbonyl)pyridine [(py)CO(py)CO(py), Scheme 1], which contains two carbonyl groups directly bonded to two 2-pyridyl groups, each in a way similar to that found in (py)₂CO. (py)CO(py)CO(py) has been mainly used for the formation of new coordination compounds via its hemiketal forms in alcoholic solvate media.¹⁶ However, an attractive form of (py)CO(py)CO(py), which still remains relatively scarce in coordination chemistry,¹⁷ is that of the bis(gem-diol), possessing seven available sites for binding to the metal centers. We here report our results from the systematic study of the ligand (py)CO(py)CO(py) in Mn cluster chemistry, targeting the in situ formation and coordination of the bis(gem-diol) form (L2H₄, Scheme 1) under the additional presence of azido bridging groups. To that end, we have avoided any solvents other than MeCN and DMF, and we have indeed managed to isolate two new mixed-valence {Mn^II/III₆} and {Mn^II/III₁₀} clusters with both the ketone/gem-diol (L1H₂) and bis(gem-diol) (L2H₄) forms of (py)CO(py)CO(py).

Experimental section

Syntheses

All manipulations were performed under aerobic conditions using chemicals and solvents as received. The ligand (py)CO(py)CO(py) was synthesized and adequately characterized as previously reported.¹⁸ Safety note: perchlorate and azide salts are potentially explosive; such compounds should be synthesized and used in small quantities, and treated with utmost care at all times.

[Mn₆(N₃)₆Cl₄(L1)₂(DMF)₄] (1). To a stirred, pale yellow solution of (py)CO(py)CO(py) (0.08 g, 0.25 mmol) and NEt₃ (35 μL, 0.25 mmol) in MeCN/DMF (20 mL, 3 : 1 v/v) were added solid MnCl₂·4H₂O (0.10 g, 0.5 mmol) and NaN₃ (0.03 g, 0.5 mmol). The resulting orange solution was stirred for 1 h, during which time all the solids dissolved and the color of the solution changed to dark brown. The solution was filtered, and the filtrate was layered with Et₂O (40 mL, 1 : 1 v/v). After three weeks, X-ray quality dark red plate-like crystals of 1 had appeared and were collected by filtration, washed with cold MeCN (1 × 3 mL) and Et₂O (2 × 5 mL), and dried under vacuum. The yield was 20% (based on the total available Mn). Elemental analysis (%) calcd for 1: C, 33.97; H, 3.10; N, 24.11; found: C, 33.75; H, 3.02; N, 24.25. Selected IR data (ATR): 2073 (vs), 1652 (s), 1599 (w), 1437 (m), 1386 (w), 1322 (m), 1281 (m), 1234 (w), 1103 (w), 1051 (m), 829 (w), 758 (w), 681 (s), 620 (s), 511 (w), 482 (w), 415 (m).

[Mn₁₀O₂(N₃)₁₂(L1)₂(L2H)₂(DMF)₆] (2). To a stirred, pale yellow solution of (py)CO(py)CO(py) (0.08 g, 0.25 mmol) and NEt₃ (70 μL, 0.5 mmol) in MeCN/DMF (20 mL, 3 : 1 v/v) were added solid Mn(ClO₄)₂·6H₂O (0.18 g, 0.5 mmol) and NaN₃ (0.03 g, 0.5 mmol). The resulting orange solution was stirred for 1 h, during which time all the solids dissolved and the color of the solution changed to dark brown. The solution was filtered, and the filtrate was allowed to slowly evaporate at room temperature. After one day, X-ray quality dark red rod-like crystals of 2·2DMF·MeCN had appeared and were collected by filtration, washed with cold MeCN (1 × 3 mL), and dried under vacuum. The yield was 50% (based on the total available Mn). Elemental analysis (%) calcd for dried 2 (solvent-free): C, 37.16; H, 3.19; N, 27.21; found C, 37.24; H, 3.32; N, 27.03. Selected IR data (ATR): 2089 (vs), 2050 (vs), 1653 (vs), 1599 (s), 1471 (w), 1437 (s), 1386 (s), 1336 (m), 1282 (m), 1234 (m), 1157 (w), 1103 (vs), 1047 (vs), 829 (w), 758 (m), 679 (s), 619 (s), 515 (w), 413 (m).

X-ray crystallography

Crystals of the complexes 1 and 2 were selected and mounted on cryoloops using inert oil.¹⁹ Diffraction data were collected at 150.0(2) K on a Bruker X8 Kappa APEX II Charge-Coupled Device (CCD) area detector diffractometer controlled by the APEX2 software package²⁰ (MoKα graphite-monochromated radiation, λ = 0.71073 Å), and equipped with an Oxford Cryosystems Series 700 cryostream monitored remotely with the software interface Cryopad.²¹ Images were processed with the software SAINT+,²² and absorption effects corrected with the multiscan method implemented in SADABS.²³ The two structures were solved using the algorithm implemented in SHELXT-2014,^24,25 and refined by successive full-matrix least-squares cycles on F² using the latest SHELXL-v.2014.^24,26 The non-hydrogen atoms were successfully refined using anisotropic displacement parameters, except from the atoms of one of the lattice solvent molecules in 2, which were only refined with isotropic parameters. Hydrogen atoms bonded to carbon were placed at their idealized positions using the appropriate HFIX instructions in SHELXL and included in subsequent refinement cycles in riding-motion approximation with isotropic thermal displacements parameters (U_iso) fixed at 1.2 or 1.5 × U_eq of the relative atom.

Two DMF and one MeCN lattice solvate molecules were located and modelled in the crystal structure of compound 2. Additional electron density was also found in the diffraction data, almost certainly due to other non-coordinated and disordered solvent molecules occupying the spaces created by the packing arrangement of the complexes. Several attempts to locate, model and refine properly these residues turned out to be unsuccessful. The search for the total potential solvent area using the software package PLATON^27,28 confirmed the existence of cavities with potential solvent accessible void volume. Consequently, the original data set was treated with the program SQUEEZE.²⁹ The programs used for molecular graphics were MERCURY³⁰ and DIAMOND.³¹

Unit cell parameters and structure solution and refinement data for complexes 1 and 2 are listed in Table 1. Crystallographic data for the reported structures have been deposited with the Cambridge Crystallographic Data Centre (CCDC) as supplementary publication numbers: CCDC – 1503291 and 1503292 for complexes 1 and 2, respectively.

Table 1 Crystallographic data for complexes 1 and 2

Parameter	1	2
a R1 = ∑(||Fo| − |Fc||)/∑|Fo|.b wR2 = [∑[w(Fo^2 − Fc^2)^2]/∑[w(Fo^2)^2]]^1/2, w = 1/[σ^2(Fo^2) + [(ap)^2 + bp], where p = [max(Fo^2, 0) + 2Fc^2]/3.
Formula	C46H50Mn6Cl4N28O10	C94H105Mn10N57O24
Fw/g mol^−1	1626.58	2966.74
Crystal type	Red block	Red block
Crystal size/mm^3	0.07 × 0.04 × 0.03	0.07 × 0.06 × 0.05
Crystal system	Triclinic	Monoclinic
Space group	P1̄	P21/c
a/Å	10.6867(9)	29.294(6)
b/Å	12.6297(12)	28.311(6)
c/Å	14.2706(13)	16.462(3)
α/°	113.964(4)	90
β/°	104.709(4)	100.953(7)
γ/°	97.420(4)	90
V/Å^3	1642.6(3)	13 404(5)
Z	1	4
T/K	150(2)	150(2)
Dc/g cm^−3	1.644	1.470
μ/mm^−1	1.356	0.992
θ range	3.646–24.692	3.657–23.323
Index ranges	−12 ≤ h ≤ 12	−31 ≤ h ≤ 32
	−13 ≤ k ≤ 14	−31 ≤ k ≤ 31
	−14 ≤ l ≤ 16	−18 ≤ l ≤ 18
Reflections collected	25 410	163 590
Independent reflections	5448 (Rint = 0.0529)	19 153 (Rint = 0.1516)
Data completeness	To θ = 24.69°, 97.2%	To θ = 23.32°, 98.6%
Final R indices [I > 2σ(I)]^a^,^b	R1 = 0.0584	R1 = 0.0978
	wR2 = 0.1242	wR2 = 0.2210
Final R indices (all data)	R1 = 0.0833	R1 = 0.1555
	wR2 = 0.1372	wR2 = 0.2596
(Δρ)max,min/e Å^−3	0.953 and −0.508	0.920 and −0.760

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Physical measurements

Infrared spectra were recorded in the solid state on a Bruker's FT-IR spectrometer (ALPHA's Platinum ATR single reflection) in the 4000–400 cm⁻¹ range. Elemental analyses (C, H, and N) were performed on a Perkin-Elmer 2400 Series II Analyzer. Electrochemical studies were performed under argon using a BASi EC-epsilon Autoanalyzer and a standard three-electrode assembly (glassy carbon working, Pt wire auxiliary, and Ag/AgNO₃ reference) with 0.1 M NⁿBu₄PF₆ as supporting electrolyte. Quoted potentials are versus the ferrocene/ferrocenium couple, used as an internal standard. The scan rates for cyclic voltammetry were 50 mV s⁻¹. Distilled solvents were employed, and the concentrations of the complexes were approximately 1 mM. Magnetic susceptibility studies were performed at the Chemistry Department of the University of Barcelona on a MPMS5 Quantum Design magnetometer. Pascal's constants were used to estimate the diamagnetic correction, which was subtracted from the experimental susceptibility to give the molar paramagnetic susceptibility (χ_M).³² The magnetic data of complex 1 were fitted to the appropriate spin Hamiltonian (vide infra) using the PHI software.³³ The quality of the fit was parameterized as the factor R = (χ_MT_exp − χ_MT_calc)²/(χ_MT_exp)².

Results and discussion

Synthetic comments

Given the similarities in the chemical reactivity of the ligands (py)₂CO and (py)CO(py)CO(py), we targeted the stabilization of the bis(gem-diol) form of (py)CO(py)CO(py), namely the ligand L2H₄ (Scheme 1), and the subsequent isolation of high-nuclearity Mn complexes with unique structures and interesting magnetic properties. The ligand L2H₄ possesses, upon deprotonation, seven available sites for coordination (3 pyridine N and 4 alkoxo-type O atoms) and its employment in metal cluster chemistry presages a fruitful route to nanoscale molecular materials. This has been demonstrated by the aesthetically beautiful {Co₂₀} cage-like cluster reported by Boudalis and coworkers, which in addition exhibited superparamagnetic relaxation.^17a In contrast to the extensive and successful use of (py)₂CO in Mn chemistry, as confirmed by the large {Mn₂₄} and {Mn₂₆} clusters,^10a the Mn/(py)CO(py)CO(py) system has only been explored in solvent CH₃COCH₃, yielding a [Mn^II₂Mn^III₂(OH)₂(L)₂(H₂O)₂](ClO₄)₄ complex, where L²⁻ is the dianion of (py)C(CH₂COCH₃)(O⁻)(py)C(CH₂COCH₃)(O⁻)(py).³⁴ We have thus decided to avoid solvents such as acetone, as well as alcoholic media which could in principle facilitate the stabilization of the hemiketal forms of (py)CO(py)CO(py), resulting from the nucleophilic attack of the carbonyl C atoms by the MeO⁻ or EtO⁻ nucleophiles. Simple MnX₂ salts were used as metal sources, with the coordination ability (i.e., Cl⁻) or inability (i.e., ClO₄⁻) of the X⁻ anion being a key factor for the crystallization of the reported {Mn₆} and {Mn₁₀} compounds (vide infra). In addition, NaN₃ was used as the azido precursor, since “ligand blends” containing polyalcohols and azides were known to lead to polynuclear compounds with exciting magnetic properties.³⁵

A variety of reactions differing in the Mn : (py)CO(py)CO(py) : N₃⁻ ratio, the presence or absence of organic base, and/or the reaction solvent or solvent mixture were explored in identifying the following successful systems. The one-pot reaction of MnCl₂·4H₂O, (py)CO(py)CO(py), NEt₃ and NaN₃ in a 2 : 1 : 1 : 2 molar ratio in a mixture of MeCN/DMF led to a dark brown solution, which upon layering with Et₂O afforded dark red plate-like crystals of the hexanuclear complex [Mn^II₄Mn^III₂(N₃)₆Cl₄(L1)₂(DMF)₄] (1) in 20% yield, where L1²⁻ is the dianion of the ketone/gem-diol form of (py)CO(py)CO(py) (Scheme 1). The synthesis of complex 1 involves Mn oxidation by atmospheric O₂, under the prevailing basic conditions. The ligand L1H₂ results from the metal-assisted, nucleophilic attack of H₂O at one of the carbonyl C atoms of (py)CO(py)CO(py), a mechanism already reported for the transformation of (py)₂CO to (py)₂C(OH)₂.¹⁰ The base NEt₃ is necessary to ensure basic conditions and to act as a proton acceptor for the deprotonation of the L1H₂ groups, which are crucial for the stabilization of 1. In addition, the presence of NEt₃ facilitates the metal-assisted deprotonation of H₂O to OH⁻ ions and the subsequent nucleophilic attack of the latter at the (py)CO(py)CO(py). In the absence of base, yellow or pale-orange solutions were observed, indicative of Mn^II products. The use of larger NEt₃ : (py)CO(py)CO(py) ratios led to the formation of dark brown amorphous solids which we were unable to crystallize. The presence of chloride ions is also important, since they act as both bridging and terminal ligands (vide infra). Finally, the solvent mixture of MeCN/DMF was found to be important for the formation and crystallization of complex 1; this was eventually confirmed by the presence of DMF molecules as terminal ligands in the structure of 1. Performance of the exact same reaction but solely in MeCN led to the precipitation of brown non-crystalline solids, while the reaction in DMF only led to a dark brown solution which remained clear for more than six months without affording any solid-state material, crystalline or not.

In the next step of our synthetic endeavours to isolate higher nuclearity Mn/(py)CO(py)CO(py) compounds, we decided to replace MnCl₂·4H₂O with starting materials that do not contain anions with coordination affinity. To that end, the one-pot reaction of Mn(ClO₄)₂·6H₂O, (py)CO(py)CO(py), NEt₃ and NaN₃ in a 2 : 1 : 2 : 2 molar ratio in a mixture of MeCN/DMF led to a dark brown solution, which was allowed to slowly evaporate at room temperature, yielding dark red rod-like crystals of the decanuclear complex [Mn^II₄Mn^III₆O₂(N₃)₁₂(L1)₂(L2H)₂(DMF)₆] (2) in 50% yield. The coordinated ligand L2H³⁻ is the trianion of the bis(gem-diol) form of (py)CO(py)CO(py) (Scheme 1), and it was probably resulted from the metal-assisted nucleophilic attack of H₂O at both carbonyl C atoms of (py)CO(py)CO(py). The oxidation of Mn ions is facilitated by the atmospheric O₂ and the presence of base is of vital importance for the formation of the O²⁻, L1²⁻ and L2H³⁻ binding species. Finally, the performance of the same reaction with Mn(NO₃)₂·4H₂O in place of Mn(ClO₄)₂·6H₂O led to the same compound 2 albeit in much lower yields (∼5%).

Description of structures

Selected interatomic distances and angles for complexes 1 and 2 are listed in Tables 2 and 3, respectively. The crystallographically established coordination modes of L1²⁻ and L2H³⁻ ligands present in complexes 1 and 2 are shown in Scheme 2.

Table 2 Selected interatomic distances (Å) and angles (°) for complex 1^a

a Symmetry code: (′) = −x + 1, −y + 1, −z + 1.
Mn(1)–O(3)	1.915(3)	Mn(2)–N(4′)	2.257(4)
Mn(1)–N(7)	1.948(5)	Mn(2)–N(10)	2.265(4)
Mn(1)–N(10)	2.036(4)	Mn(2)–Cl(1)	2.645(2)
Mn(1)–N(4)	2.069(4)	Mn(3)–O(2)	2.071(3)
Mn(1)–O(3′)	2.176(3)	Mn(3)–N(3)	2.210(4)
Mn(1)–N(1)	2.220(4)	Mn(3)–N(2)	2.247(4)
Mn(2)–O(2)	2.100(3)	Mn(3)–Cl(2)	2.432(2)
Mn(2)–O(4)	2.169(4)	Mn(3)–Cl(1)	2.436(2)
Mn(2)–O(5)	2.183(4)	C(12)–O(1)	1.219(6)
Mn(1)–O(3)–Mn(1′)	104.3(2)	Mn(2)–O(2)–Mn(3)	111.3(2)
Mn(1)–N(10)–Mn(2)	120.2(2)	Mn(2)–Cl(1)–Mn(3)	85.2(5)
Mn(1)–N(4)–Mn(2′)	119.9(2)

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Table 3 Selected interatomic distances (Å) and angles (°) for complex 2^a

a Symmetry code: (′) = −x, −y, −z + 1.
Mn(1)–O(1)	1.876(6)	Mn(3)–N(8)	2.270(8)
Mn(1)–O(1′)	1.887(6)	Mn(3)–O(3)	2.377(7)
Mn(1)–O(8)	1.938(6)	Mn(4)–O(7)	1.874(7)
Mn(1)–O(3′)	1.954(6)	Mn(4)–N(20)	1.973(1)
Mn(1)–N(1′)	2.254(8)	Mn(4)–N(17)	2.022(1)
Mn(1)–N(5′)	2.372(8)	Mn(4)–N(11)	2.069(9)
Mn(2)–O(4)	1.871(6)	Mn(4)–O(11)	2.215(9)
Mn(2)–O(1)	1.924(6)	Mn(4)–N(15)	2.228(9)
Mn(2)–N(23)	1.989(8)	Mn(5)–O(8)	2.172(6)
Mn(2)–N(12)	2.090(8)	Mn(5)–O(9)	2.204(9)
Mn(2)–O(2)	2.091(7)	Mn(5)–N(8′)	2.206(9)
Mn(2)–O(10)	2.429(6)	Mn(5)–N(2′)	2.286(8)
Mn(3)–O(2)	2.107(6)	Mn(5)–N(14)	2.315(8)
Mn(3)–O(6)	2.185(7)	Mn(5)–O(5′)	2.322(7)
Mn(3)–N(5)	2.230(8)	Mn(5)–O(3′)	2.365(6)
Mn(3)–N(11)	2.252(9)	C(9)–O(5)	1.224(1)
Mn(1)–O(8)–Mn(5)	106.5(3)	Mn(1)–O(1)–Mn(2)	137.7(3)
Mn(2)–O(2)–Mn(3)	117.7(3)	Mn(1′)–O(1)–Mn(2)	126.6(3)
Mn(1′)–O(3)–Mn(5′)	99.1(3)	Mn(5′)–N(8)–Mn(3)	104.8(3)
Mn(1′)–O(3)–Mn(3)	106.0(3)	Mn(4)–N(11)–Mn(3)	116.7(4)
Mn(5′)–O(3)–Mn(3)	96.8(2)	Mn(3)–N(5)–Mn(1′)	97.7(3)
Mn(1)–O(1)–Mn(1′)	95.7(3)

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Scheme 2 Crystallographically established coordination modes of L1²⁻ and L2H³⁻ ligands present in complexes 1 and 2.

Complex 1 crystallizes in the triclinic space group P1̄ and displays crystallographic C_i symmetry. The structure of 1 (Fig. 1, top) consists of six Mn atoms linked through the alkoxido arms (O2, O3, O2′, O3′) of two η¹:η¹:η²:η²:η¹:μ₄ L1²⁻ ligands, four μ-1,1 (end-on) N₃⁻ and two μ-Cl⁻ ligands. The core of 1 (Fig. 1, bottom) can be described as a central Mn₄ rectangle (Mn1, Mn1′, Mn2, and Mn2′) that is further linked to two extrinsic Mn ions (Mn3 and Mn3′) via the bridging chlorido and alkoxido bridges. The four Mn ions within the rectangle are bridged by the four end-on azido ligands, while each L1²⁻ ligand bridges four metal centers in total (Mn1, Mn1′, Mn2, and Mn3) through its two alkoxido arms. Thus, complex 1 contains an overall [Mn₆(μ-N₃)₄(μ-Cl)₂(μ-OR)₄]⁴⁺ core, with peripheral ligation about it provided by the pyridine groups of the L1²⁻ ligands, two terminally bound azides at Mn1 and Mn1′, two terminally bound chlorides at Mn3 and Mn3′, and four coordinated DMF molecules, two at Mn2 and two at Mn2′. Mn3 and Mn3′ are five-coordinate with distorted square pyramidal geometries (τ = 0.35, where τ is 0 and 1 for perfect square-pyramidal and trigonal bipyramidal geometries,³⁶ respectively). The remaining four Mn ions are six-coordinate with distorted octahedral geometries. The Mn^II (Mn2, Mn2′, Mn3, and Mn3′) and Mn^III (Mn1, Mn1′) oxidation states were determined by metric parameters and charge balance considerations, and confirmed by bond valence sum (BVS)³⁷ calculations (Table 4). The two Mn^III atoms also show the Jahn–Teller (JT) distortion expected for a high-spin d⁴ ion in near-octahedral geometry, taking the form of an axial elongation with a pyridyl nitrogen N(1,1′) and an alkoxide oxygen O(3′,3) atoms from two different L1²⁻ groups occupying the axial positions of the Mn1 and Mn1′ distorted octahedra, respectively. The Mn–Cl bond distances (Table 2) fall into the expected range for similar μ-Cl⁻-bridged Mn^II⋯Mn^II pairs.³⁸ Finally, there are some weak but noticeable intermolecular hydrogen bonding interactions between the methyl H atoms of the coordinated DMF molecules and the Cl⁻ ligands of adjacent molecules; these interactions serve to weakly link the {Mn₆} clusters into 1-D chains along the crystallographic a axis.

Fig. 1 (top) Partially labeled representation of the structure of 1 and (bottom) its complete [Mn₆(μ-N₃)₄(μ-Cl)₂(μ-OR)₄]⁴⁺ core. All H atoms are omitted for clarity. Primed and unprimed atoms are related by the crystallographic inversion center. Color scheme: Mn^II, yellow; Mn^III, blue; O, red; N, green; Cl, cyan; C, gray.

Table 4 Bond Valence Sum (BVS)^a calculations for Mn atoms in complex 1

Atom	Mn^II	Mn^III	Mn^IV
a The value in bold is the one closest to the charge for which it was calculated. The oxidation state is the nearest whole number to the value in bold.
Mn1	3.20	3.03	3.02
Mn2	1.97	1.86	1.88
Mn3	1.89	1.81	1.85

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The crystal of 2 contains two co-crystallized {Mn₁₀} clusters with the exact same formulas, topologies and oxidation state descriptions. Therefore, only the structure of one of these molecules (the one containing Mn(1,2,3,4,5) and their symmetry equivalents) will be described in detail. The structure of centrosymmetric 2 (Fig. 2, top) consists of 10 Mn atoms linked through the alkoxido arms (O2, O3, O8, O2′, O3′, O8′) of two L1²⁻ and two L2H³⁻ ligands, six μ-1,1 (end-on)-N₃⁻ and two μ₃-O²⁻ groups (O1, O1′). The coordination modes of the ligands L1²⁻ and L2H³⁻ are shown in Scheme 2; the former bridges four metal ions while the latter brings together as many as five Mn ions, clearly demonstrating the bridging capacity of the bis(gem-diol) form of (py)CO(py)CO(py) and its ability to facilitate the formation of polynuclear metal complexes. The core of 2 (Fig. 2, bottom) can be described as a central [Mn₄O₂] planar-butterfly (Mn1, Mn2, Mn1′, Mn2′) in which the metal centers are bridged by the two μ₃-O²⁻ ligands. The central Mn₄ subunit is further linked to two nearly linear Mn₃ subunits (Mn3⋯Mn4⋯Mn5′ and Mn3′⋯Mn4′⋯Mn5 = 153.7°) through the alkoxido arms of L1²⁻ and L2H³⁻ ligands, as well as by two end-on N₃⁻ (N5 and N5′). The three metal ions in each Mn₃ subunit are bridged through an alkoxido group (O3 and O3′) and two end-on N₃⁻ (N8/N11 and N8′/N11′) groups. Thus, complex 2 contains a complete [Mn₁₀(μ₃-O)₂(μ-N₃)₆(μ₃-OR)₂(μ-OR)₄]¹⁰⁺ core, with peripheral ligation around it provided by (i) the non-bridging parts of L1²⁻ and L2H³⁻ ligands, except from a pyridine group of each L1²⁻ ligand which remains unbound, (ii) six terminally ligated azido ligands (on Mn2, Mn4, Mn2′ and Mn4′), and (iii) and six terminal DMF solvate molecules (on Mn3, Mn4, Mn5, and their symmetry equivalents).

Fig. 2 (top) Partially labeled representation of the structure of 2 and (bottom) its complete [Mn₁₀(μ₃-O)₂(μ-N₃)₆(μ₃-OR)₂(μ-OR)₄]¹⁰⁺ core. All H atoms are omitted for clarity. Primed and unprimed atoms are related by the crystallographic inversion center. Color scheme as in Fig. 1.

Charge considerations and an inspection of the metric parameters indicated a 4Mn^II, 6Mn^III description for 2. This was confirmed quantitatively by bond valence sum (BVS) calculations (Table 5), which identified Mn3 and Mn5 (and their symmetry equivalents) as the Mn^II ions, and the others as Mn^III. The latter was also consistent with the Jahn–Teller (JT) axial elongations at Mn1, Mn2, Mn4, and their symmetry equivalents, as expected for high-spin d⁴ ions in near-octahedral geometry. Atoms Mn5 and Mn5′ atoms are seven-coordinate with distorted pentagonal bipyramidal geometries, whereas the remaining Mn^II ions (Mn3 and Mn3′) are six-coordinate with distorted octahedral geometries. The protonation levels of the O²⁻ groups and the non-bridging OH groups of L2H³⁻ were also confirmed by BVS calculations (Table 5). Finally, there are no significant intermolecular interactions in the crystal structure of complex 2, other than some weak π–π stacking interactions between the pyridine groups of L1²⁻ and L2H³⁻ in neighboring {Mn₁₀} compounds.

Table 5 Bond Valence Sum (BVS)^a calculations for Mn and selected O^b atoms in complex 2

Atom	Mn^II	Mn^III	Mn^IV
a The value in bold is the one closest to the charge for which it was calculated. The oxidation state is the nearest whole number to the value in bold.b A BVS in the ∼1.8–2.0, ∼1.0–1.2, and ∼0.2–0.4 ranges for an O atom is indicative of non-, single- and double-protonation, respectively.
Mn1	3.26	3.02	3.12
Mn2	3.19	2.98	3.02
Mn3	1.92	1.81	1.82
Mn4	3.07	2.92	2.89
Mn5	2.03	1.91	1.92

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	BVS	Assignment
O1	1.83	O^2−
O10	1.06	ROH

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Finally, complexes 1 and 2 are the second and third highest nuclearity products reported to date from any form of coordinated ligands derived from (py)CO(py)CO(py). Although there are numerous {Mn₆} and {Mn₁₀} clusters presented in the literature, the reported Mn/(py)CO(py)CO(py) reaction scheme has led to compounds 1 and 2 exhibiting new topologies and unprecedented core motifs.

Electrochemistry

The poor solubility of complex 1 in all common organic solvents did not allow us to investigate its electrochemical response. However, this was not the case for complex 2. The electrochemical properties of complex 2 were studied in MeCN and its cyclic voltammogram (CV) is shown in Fig. 3. The CV spectrum displays a well-defined reversible oxidation at ∼0.45 V and a less-defined reversible reduction at approximately −1.50 V. For the reversible oxidation couple, the forward and reverse waves are well formed with a peak separation of ∼100 mV comparable to that of ferrocene under the same conditions; this is indicative of a one-electron process.³⁹ Thus, the electrochemical results may be illustrated as {Mn₁₀}⁺ ↔ {Mn₁₀} ↔ {Mn₁₀}⁻. Synthetic efforts to isolate and structurally characterize the cation and anion were all turned to be unsuccessful.

Fig. 3 Cyclic voltammogram at 50 mV s⁻¹ for complex 2 in MeCN containing 0.1 M NⁿBu₄PF₆ as supporting electrolyte. The indicated potentials are vs. Fc/Fc⁺.

Solid-state magnetic susceptibility studies

Variable-temperature (2.0–300 K range), direct-current (dc) magnetic susceptibility measurements were performed on freshly-prepared microcrystalline solids of 1 and 2; a dc field of 0.3 T was applied from 30 to 300 K and a weak dc field of 0.03 T was used from 2 to 30 K to avoid saturation effects.

The obtained data for complex 1 are plotted as χ_MT vs. T in Fig. 4. Field-dependent magnetization studies were also performed and the obtained data are plotted as M vs. H in Fig. 5. The χ_MT value at 300 K is 20.47 cm³ Kmol⁻¹, less than the theoretical value of 23.5 cm³ Kmol⁻¹ for four Mn^II and two Mn^III non-interacting ions (g = 2.00). The χ_MT product steadily decreases with decreasing temperature to a value of 10.71 cm³ Kmol⁻¹ at 12 K, and then decreases more rapidly to a value of 7.53 cm³ Kmol⁻¹ at 2 K. The overall decrease of the χ_MT product and the shape of the curve indicate the presence of predominant antiferromagnetic exchange interactions between the metal centers. The very low temperature decrease of the χ_MT product may be attributed to the presence of zero-field splitting, Zeeman effects and/or weak intermolecular antiferromagnetic exchange interactions between the {Mn₆} clusters. The χ_MT value at 12 K is suggestive of an S = 4 spin ground state; the spin-only value for an S = 4 is 10 cm³ Kmol⁻¹ (with g = 2.00). The magnetization of 1 reaches a value of ∼8Nμ_B at the highest fields (5.0 T) and lowest possible temperature (2 K), in agreement with an S = 4 ground state. The lack of true saturation of magnetization indicates the presence of magnetic anisotropy and/or the population of low-lying excited states, as expected for a mixed-valence Mn^II/III complex with weak-to-moderate exchange interactions.⁴⁰ As a result, the magnetization vs. field data could not be fit to give reasonable values of the zero-field splitting parameter, D, for a well-isolated S = 4 spin ground state.

Fig. 4 χ_MT vs. T plot for complex 1. The solid red line is the fit of the data; see the text for the spin Hamiltonian and the corresponding fit parameters.

Fig. 5 Plot of magnetization (M) vs. field (H) for complex 1 at 2 K.

Given the topology and symmetry of complex 1, and considering the bridging ligation within its {Mn₆} core, there are three different exchange pathways and thus three different J coupling constants (Scheme 3). J₁ is associated with the interaction between Mn1 (S₁) and Mn1′ (S₁′), bridged by the O atoms (O3 and O3′) of two alkoxido bridges from L1²⁻; the Mn1–O3–Mn1′ angle is 104.3°. J₂ is associated with the interactions between Mn1⋯Mn2 (S₂), Mn2⋯Mn1′, Mn1′⋯Mn2′ (S₂′) and Mn1⋯Mn2′. These pairs are directly bridged by end-on azido ligands, with the Mn1–N10–Mn2 and Mn1′–N4′–Mn2 angles being 120.2° and 119.9°, respectively. Finally, J₃ is related to the interactions between Mn2⋯Mn3 (S₃) and Mn2′⋯Mn3′ (S₃′). Each of these pairs is bridged by a μ-Cl⁻ group and an alkoxido O atom (O2 and O2′) from the organic chelate; the Mn2–Cl1–Mn3 and Mn2–O2–Mn3 angles are 85.2° and 111.3°, respectively.

Scheme 3 J-coupling scheme employed for the elucidation of the magnetic exchange interactions in complex 1.

Therefore, the spin Hamiltonian employed for this system is given by eqn (1). An excellent fit of the experimental data (red solid line in Fig. 4) in the temperature range 300–12 K was obtained using the program PHI (H = −2J_ijŜ_iŜ_j) convention. The best-fit parameters for an overall 3-J model were: J₁ = −3.9 cm⁻¹, J₂ = −3.4 cm⁻¹, J₃ ∼ 0 cm⁻¹, and g = 1.96 (R = 5.8 × 10⁻⁵).

H = −2J₁(Ŝ₁Ŝ_1′) − 2J₂(Ŝ₁Ŝ₂ + Ŝ₁Ŝ_2′ + Ŝ_1′Ŝ₂ + Ŝ_1′Ŝ_2′) − 2J₃(Ŝ₂Ŝ₃ + Ŝ_2′Ŝ_3′) (1)

The simulation of the data was performed in the 300–10 K range because at lower temperatures the effect of D would become apparent and this could not be modeled due to computer limitations. The obtained J₁ value corresponds to a moderate antiferromagnetic exchange interaction between the Mn^III atoms. This value is in agreement with previously reported J values that correspond to magnetic exchange interactions between Mn^III centers bridged by alkoxido bridges with similar Mn^III–O–Mn^III angles and subcore topologies.⁴¹ The obtained J₂ value also corresponds to antiferromagnetic exchange interactions promoted by the end-on azido ligands. Although the presence of end-on bridging azides in the structure of 1 was promising with respect to the anticipated ferromagnetic exchange interactions, this turned out not to be the case. This is not surprising given the fact that end-on azides are known to promote antiferromagnetic coupling when the bridging angles between the metal ions are larger than 110°.^9,13–15 The Mn–N_azide–Mn angles in complex 1 are 120°, thus the coupling between the Mn^II⋯Mn^III atoms is expected to be antiferromagnetic.⁴² Finally, in all of our fitting attempts the obtained J₃ coupling constant was always very small (approximately zero) and negative, as expected for magnetic interactions involving Mn^II atoms.⁴⁰ The sign and magnitude of this interaction is mainly determined by the Mn–O–Mn angle; for Mn^II–O–Mn^II angles of ∼110° the exchange interaction is expected to be weakly antiferromagnetic.⁴³ The antiferromagnetic contribution of the alkoxido bridge to the magnetic coupling of the two metal ions can be further supported or – in few cases – overcome by the presence of the bridging chloride group.^38,44

Tong and coworkers have recently reported the structural and magnetic characterization of the highly symmetric complex [Mn^II₆(μ₆-Cl)(phenda)₆]⁻ (phenda²⁻ = 1,10-phenanthroline-2,9-dicarboxylate) with an octahedral topology.⁴⁴ The magnetic susceptibility studies have revealed predominant antiferromagnetic exchange interactions between the metal centers. Each Mn^II⋯Mn^II pair was bridged by an O atom from the carboxylate group of phenda²⁻ and the Cl⁻ group. The Mn^II–O–Mn^II and Mn^II–Cl–Mn^II angles were 116° and 90°, respectively, very close to the corresponding values of complex 1. Finally, with the obtained J-coupling constants it becomes apparent that the dominant antiferromagnetic interactions in 1 are these between the Mn^II⋯Mn^III and Mn^III⋯Mn^III atoms. The four interactions which correspond to the J₂ coupling constant appear to force the spin vectors of Mn^II (S = 5/2) and Mn^III (S = 2) to align antiparallel to each other, thus leaving the single interaction (J₁) between the Mn^III⋯Mn^III atoms frustrated (competing exchange interactions). Furthermore, the very weak antiferromagnetic interaction between the Mn^II atoms completes the ‘spin-up’/‘spin-down’ vector scheme which rationalizes the proposed S = 4 spin ground state value (Scheme 3).

For complex 2, the χ_MT vs. T plot is shown in Fig. 6. The χ_MT value at 300 K is 32.74 cm³ Kmol⁻¹, less than the theoretical value of 35.5 cm³ Kmol⁻¹ for four Mn^II and six Mn^III non-interacting ions (g = 2.00), and it steadily decreases to 7.98 cm³ Kmol⁻¹ at 2 K. The decrease of the χ_MT product and the overall shape of the curve suggest the presence of predominant antiferromagnetic interactions between the metal centers. The non-zero χ_MT value at 2 K is suggestive of an S ≠ 0 spin ground state. Given the size of the {Mn₁₀} cluster, and the resulting number of inequivalent exchange constants, it was not possible to apply the Kambe method⁴⁵ to determine the individual pairwise Mn₂ exchange interaction parameters; direct matrix diagonalization methods were also computationally unfeasible.

Fig. 6 χ_MT vs. T plot for complex 2.

With the structure of 2 being more complicated than that of 1, any attempt to rationalize the spin ground state of the system based on its structural components and parameters could have led to inaccuracies and superficial results. Many of the metal ions in complex 2 are bridged through end-on azido and μ-alkoxido groups (O atoms from L1²⁻ and L2H³⁻), which are known to promote either ferromagnetic or antiferromagnetic coupling, mainly depending on the bridging angles. In addition, there are two μ₃-O²⁻ and two μ₃-OR⁻ groups, which arrange the metal ions into fused triangular Mn₃ subunits within 2. The bridging oxido groups are known to promote antiferromagnetic exchange interactions while the alkoxido groups – for Mn–O–Mn angles below 100° – can propagate ferromagnetic coupling between the metal centers or antiferromagnetic interactions for larger Mn–O–Mn angles. In this case, spin frustration effects may arise from the presence of competing exchange interactions,⁴⁰ thus preventing a direct rationalization of the ground state S value.

Our efforts were concentrated instead on characterizing the spin ground state, S, and the zero-field splitting parameter, D, by performing magnetization (M) vs. dc field measurements at applied magnetic fields and temperature in the 0.1–5 T and 2 K ranges, respectively (Fig. 7). However, an acceptable fit using data collected over the entire field range could not be obtained, which is a common problem in high-nuclearity metal complexes caused by low-lying excited states, especially if some have an S value greater than that of the ground state.^{1,3,7,10,11,40} The magnetization increases almost linearly with increasing field without reaching any saturation; this behavior is attributed to the presence of low-lying excited states and probably some intercluster interactions.

Fig. 7 Plot of magnetization (M) vs. field (H) for complex 2 at 2 K.

Unfortunately, none of the reported compounds exhibited out-of-phase ac magnetic susceptibility signals down to 1.8 K, suggesting these are not SMMs.

Conclusions

In conclusion, we have shown that the metal-assisted hydrolysis of (py)CO(py)CO(py) in Mn cluster chemistry has afforded two new complexes, {Mn^II₄Mn^III₂} (1) and {Mn^II₄Mn^III₆} (2), bearing the anions of the ketone/gem-diol (L1H₂) and bis(gem-diol) (L2H₄) forms of the parent organic chelate. These results were obtained by the performance of one-pot reactions in the non-alcoholic solvents, MeCN and DMF. It was also shown that the anion present in the Mn^II starting material has a pronounced effect on the structural identity of the two clusters; the employment of MnCl₂ leads to 1 while Mn(ClO₄)₂ – under similar conditions – supports the formation of the decanuclear complex 2. Furthermore, both compounds are the highest nuclearity Mn clusters containing any form of (py)CO(py)CO(py). Despite the plethora of previously reported {Mn₆} and {Mn₁₀} complexes, both 1 and 2 possess unique topologies in their respective oxidation state levels. Complex 1 is the first hexanuclear Mn cluster to contain both bridging azido and alkoxido-bridges; to date all the previously reported {Mn₆}/N₃⁻ complexes are supported by bridging oximato ligands.^35,46 Finally, (py)CO(py)CO(py) was proven to be very promising in the respective reaction systems. The chemical reactivity through the carbonyl carbon atoms and the bridging capability of the resulting organic molecules show an immense potential towards the isolation of products with novel metal topologies and interesting physicochemical properties.

We are currently trying to (i) expand this research into various Mn carboxylate sources as a means of increasing the nuclearities of the products and obtaining nanoscale molecular materials with both high-spin ground states and SMM behaviors; (ii) employ different bases with different basicity in order to achieve the complete hydrolysis and deprotonation of (py)CO(py)CO(py), thus unveiling the maximum number of donor atoms available for coordination to the metal centers, and (iii) explore the chemical reactivity of the Mn/(py)CO(py)CO(py) reaction system in the presence of relatively unusual (in cluster chemistry) solvent media, such as MeNO₂, bulky organonitriles, and unsaturated and cyclic ketones.

Acknowledgements

This work was supported by Brock University (Chancellor's Chair for Research Excellence), NSERC-DG and ERA (to Th. C. S.). L. C. -S. would like to acknowledge the following agencies for funding part of this research: FEDER (Fundo Europeu de Desenvolvimento Regional) through PT2020 and the national funds through the FCT (Fundação para a Ciência e a Tecnologia) for the research centre REQUIMTE-LAQV (UID/QUI/50006/2013) and for the fellowships SFRH/BPD/111899/2015. A. E. acknowledges financial support from Ministerio de Economía y Competitividad, Project CTQ2015-63614-P. B. A. thanks the Ministerio e Innovación (Spain) (project CONSOLIDER-INGENIO SUPRAMED CSD 2010-00065) and the Generalitat Valenciana (Valencia, Spain) PROMETEO II/2015/002.

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Footnote

† Electronic supplementary information (ESI) available: CCDC 1503291 and 1503292 & crystallographic data of complexes 1 and 2 in CIF formats. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra22953k

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