{Co^IIIMn^III}_n corrugated chains based on heteroleptic cyanido metalloligands

Abstract

The use of the cyanide-bearing complexes PPh₄[Co^III(4,4′-dmbipy)(CN)₄] and PPh₄[Co^II(dmphen)(CN)₃] as metalloligands towards [Mn(salen)(H₂O)]ClO₄ affords one-dimensional coordination polymers with the formulas {[Mn^III(salen)(μ-NC)₂Co^III(4,4-dmbipy)(CN)₂]·H₂O}_n (1) and {[Mn^III(salen)(μ-NC)₂Co^III(dmphen)(CN)₂]}_n (2) [PPh₄⁺ = tetraphenylphosphonium cation, 4,4′-dmbipy = 4,4′-dimethyl-2,2′-bipyridine, dmphen = 2,9-dimethyl-1,10-phenanthroline and H₂salen = N,N′-ethylenebis(salicylideneimine)]. Compounds 1 and 2 were structurally characterized. Their structures consist of neutral chains with regular alternating [Mn(salen)]⁺ and [Co^III(4,4′-dmbipy)(CN)₄]⁻ (1)/[Co^III(dmphen)(CN)₄]⁻ (2) moieties, the latter ones acting as bis-monodentate ligands towards the Mn(III) units through two of their four cyanide groups. During the synthesis, the cobalt(II) ion of the starting [Co^II(dmphen)(CN)₃]⁻ metalloligand is oxidized to Co(III) and it takes an additional cyanide ligand to transform into {Co^III(dmphen)(CN)₄} in 2. Magnetic studies have been carried out on 1 and 2 in the temperature range 1.9–300 K which yielded local negative zero-field splitting parameters of −3.26 (1) and −4.38 cm⁻¹ (2). Frequency-dependent alternating current susceptibility signals under an external applied magnetic field (dc) were clearly observed for 1 and 2 indicating slow magnetic relaxation, that is, Single Ion Magnet (SIM) behaviour. The energy barriers (E_a) to reverse the magnetization direction under an applied dc magnetic field of 2000 Oe were 12.0(2) (1) and 9.4(3) cm⁻¹ (2), whereas the values of the pre-exponential factor (τ_o) were 1.40(2) × 10⁻⁸ (1) and 2.5(2) × 10⁻⁸ s (2).

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Introduction

Cyanide complexes have proven to be very efficient metalloligands for the construction of coordination polymers with different dimensionalities and topologies whose magnetic properties have been thoroughly investigated.¹ Since the discovery of Prussian blue and its analogues, a rich chemistry has emerged from 3d-type cyanide metalloligands such as the homoleptic [M^III(CN)₆]³⁻ and heteroleptic [M^III(AA)(CN)₄]⁻ and [Fe^IIIL(CN)₃]⁻ paramagnetic complexes (M = Fe and Cr, AA = bidentate ligand and L = pyrazolylborate derivative).^2,3 Moreover, the extension of these studies to heavier metal ions with the [M′^V(CN)₈]³⁻ (M′ = Mo and W) and [W^V(bipy)(CN)₆]⁻ (bipy = 2,2′-bipyridine) paramagnetic metalloligands has produced a plethora of heterometallic compounds which exhibit interesting magnetic phenomena mediated by the cyanido bridges.^4–6 Without being exhaustive, ferrimagnetic Fe^III(Cr^III)-(μ-CN)-Mn^II/III chains,⁷ Single Molecule Magnets (SMMs),^1e,3b,8 Single Chain Magnets (SCMs),^2k,3e,9 nanocages,¹⁰ and photomagnetic molecular squares¹¹ have been reported.

Although the diamagnetic cyanido building blocks have been largely used in crystal engineering, they are less explored in molecular magnetism in spite of the fact that several examples of heterometallic complexes constructed using diamagnetic cyanido metalloligands show interesting magnetic properties. Two selected examples are: (i) a 2D network of 3d–4f SMMs obtained by reacting the [W^IV(bipy)(CN)₆]⁻ metalloligand with the [Ni^IIDy^III(valpn)]³⁺ heterobimetallic unit [H₂valpn = 1.3-propanediyl-bis(2-iminomethylene-6-methoxyphenol)],¹² and (ii) a heterotrimetallic decanuclear metal-capped square resulting from the assembly of the cyanido-bearing [Mo^IV(CN)₈]⁴⁻ species and the [Cu^IITb^III(valen)]³⁺ heterodinuclear complex [H₂valen = N,N′-bis(3-methoxysalycilidene)ethylenediamine], where the SMM behaviour of the starting {Cu^IILn^III} unit is preserved.¹³

Restricting ourselves to the discipline of molecular magnetism, most of the heterometallic complexes based on the use of the diamagnetic homoleptic [Co^III(CN)₆]³⁻ mononuclear complex as a metalloligand were studied as models for their congeners (with a paramagnetic ion instead of Co^III).^2a,k,14 Interestingly, there are examples of {Co^IIIMn^III} complexes for which slow relaxation of the magnetization occurs due to the {Mn^III(SB)} entity (SB = Schiff base ligand), thus being examples of Single Ion Magnets (SIMs).¹⁵ Heteroleptic cyanido-containing cobalt(III) complexes have only scarcely been used as metalloligands, leading essentially to three systems: the pentanuclear compound {K{[Mn(valen)(CH₃OH)][Co(bpb)]}₂}ClO₄·H₂O (bpd = 1,2-bis(pyridine-2-carboxamido)benzenate),^16a and the trinuclear species {[Ni(cyclam)[Co(bpb)(CN)₂]₂}·CH₃OH·2H₂O (cyclam = 1,4,8,11-tetraazacyclotetradecane)^16b and {Co(salen)(CN)₂]₂[Mn(bipy/phen)₂]}¹⁷ [H₂salen = N,N′-ethylenebis(salicylideneimine) and phen = 1,10-phenanthroline]. Regarding the heteroleptic cyanido precursors of the type {Co^III(AA)(CN)₄}⁻, as far as we are aware, there is no report where they are involved in a rational molecular assembly. Only four structures containing this type of fragment, the crystals being grown by hydrothermal methods, are known to date: two ionic salts,¹⁸ and two trinuclear complexes with the formulas [{Co^III(phen)(CN)₃(μ-CN)}₂{Co^II(phen)₂}]¹⁹ and [{Co^III(phen)(CN)₃(μ-CN)}₂{Fe^II(phen)₂}].²⁰

In this paper we present the structures and variable-temperature magnetic study of two new 1D coordination polymers: {[Mn^III(salen)(μ-NC)₂Co^III(4,4′-dmbipy)(CN)₂]·H₂O}_n (1) and {[Mn^III(salen)(μ-NC)₂Co^III(dmphen)(CN)₂]}_n (2) together with the synthesis and spectroscopic characterization of the PPh₄[Co^III(4,4′-dmbipy)(CN)₄] metalloligand which is used to prepare compound 1 (4,4′-dmbipy = 4,4′-dimethyl-2,2′-bipyridine, dmphen = 2,9-dimethyl-1,10-phenanthroline and PPh₄⁺ = tetraphenylphosphonium cation). The two chains were obtained from the reaction of [Mn^III(salen)(H₂O)]ClO₄ ²¹ with the PPh₄[Co^III(4,4′-dmbipy)(CN)₄] and PPh₄[Co^II(dmphen)(CN)₃]²² metalloligands for 1 and 2, respectively. The cobalt(II) metal ion from the starting [Co^II(dmphen)(CN)₃]⁻ complex underwent oxidation in the synthesis of 2 and it turned into the {Co^III(dmphen)(CN)₄}⁻ moiety which is identified as a component in 2.

Results and discussion

Synthesis and IR spectroscopy

The synthesis and X-ray structures of the new diamagnetic cobalt(III) complexes with the formulas PPh₄[Co^III(en)(CN)₄], PPh₄[Co^III(ampy)(CN)₄]·1.5H₂O and PPh₄[Co^III(phen)(CN)₄]·CH₃OH·0.25H₂O [en = 1,2-ethylenediamine and ampy = 2-aminomethylpyridine] together with that of the paramagnetic cobalt(II) compound PPh₄[Co^II(dmphen)(CN)₃] were the subject of a recent report.²² The latter species is used here for the synthesis of 2, whereas a new member of this cobalt(III) series with the formula PPh₄[Co^III(4,4′-dmbipy)(CN)₄] whose preparation is described herein, is employed as a metalloligand to obtain compound 1.

The synthesis of 1 and 2 followed the node and spacer approach. By employing Co^III/II cyanido metallates in combination with a Mn^III complex with a Schiff base type ligand, two similar 1D coordination polymers were obtained. In the first case, the self-assembly of [Co^III(4,4′-dmbipy)(CN)₄]⁻ anions with [Mn^III(salen)(H₂O)]⁺ cations acting as spacers and nodes respectively, afforded a corrugated chain with the formula {[Mn^III(salen)(μ-NC)₂Co^III(4,4′-dmbipy)(CN)₂]·H₂O}_n (1). In the second case, by reacting the PPh₄[Co^II(dmphen)(CN)₃] metalloligand with the same [Mn^III(salen)(H₂O)]ClO₄ complex, a neutral chain of similar topology and the formula {[Mn^III(salen)(μ-NC)₂Co^III(dmphen)(CN)₂]}_n (2) was obtained. This second structure was less expected because of the oxidation of the Co(II) to Co(III) during the synthesis, which is most likely due to the oxidizing power of the [Mn^III(salen)(H₂O)]⁺ complex, being followed by the uptake of an additional cyanide ligand to form the diamagnetic [Co^III(dmphen)(CN)₄]⁻ unit. The strong field ligand character of the dmphen and cyanido ligands is at the origin of the stabilization of the trivalent oxidation state of the cobalt centre in this entity. The aerial oxidation of the [Mn^II(salen)(H₂O)] product would regenerate the [Mn^III(salen)(H₂O)]⁺ species²³ that would lead to 2 through its reaction with [Co^III(dmphen)(CN)₄]⁻.

The FTIR spectrum of the mononuclear PPh₄[Co^III(4,4′-dmbipy)(CN)₄] complex shows a medium intensity band at 2130 cm⁻¹ which is characteristic of the terminal cyanido groups. As expected, this band is split and shifted to higher wavenumbers in the FTIR spectrum of 1 (Fig. S1a†), the peaks at 2136 and 2154 cm⁻¹ being assigned to the terminal and bridging cyanido groups. Also, the out-of-phase bending vibrations of the phenyl rings from the tetraphenylphosphonium cation at 760, 725, 690 and 528 cm⁻¹ in the IR spectrum of the mononuclear precursor are absent in the spectrum of 1, supporting the lack of this organic cation in this last compound. The most relevant feature in the infrared spectrum of 2 (Fig. S1b†) is due to the stretching vibrations of the cyanido ligands: a peak at 2137 cm⁻¹ (terminal cyanido groups) and a shoulder at 2150 cm⁻¹ (assigned to the bridging cyanido ligands). The peaks at 1622 and 1628 cm⁻¹ would correspond to the imine C=N bond vibration belonging to the salen ligand in both complexes 1 and 2. The bands assigned to the ring-stretching modes of the 4,4′-dmbipy and dmphen ligands overlap in the 1630–1560 cm⁻¹ region. Finally, the medium intensity absorption at ca. 765 cm⁻¹ in the infrared spectra of 1 and 2 is attributed to the chelating 4,4′-dmbipy and dmphen molecules. All of these spectroscopic features have been confirmed using the crystal structures of 1 and 2 (see below).

Crystal structures

Compounds 1 and 2 crystallize in the monoclinic system, space groups P2₁/c (1) or P2₁/n (2), respectively. Their structures consist of neutral, similarly corrugated chains with the general formula {[Mn^III(salen)(μ-NC)₂Co^III(AA)(CN)₂]}_n [AA = 4,4′-dmbipy (1) or dmphen (2)] running along the crystallographic b axis, in both cases. One disordered water molecule of crystallization per asymmetric unit is also present in 1. The asymmetric units of the two structures comprise one [Mn^III(salen)(H₂O)]⁺ cationic moiety connected to one [Co^III(AA)(CN)₄]⁻ anionic fragment through a cyanido bridge [C(1)–N(1)] (see Fig. 1).

Fig. 1 Perspective view of the asymmetric units of 1 (a) and 2 (b) showing the atom numbering. Thermal ellipsoids are drawn at the 30% probability level. The water molecule of crystallization in the asymmetric unit of 1 is omitted. Symmetry codes: (a) = −x + 1, y + 1/2, −z + 1/2 and (b) −x + 1, y − 1/2, −z + 1/2 for 1; (a) = −x + 3/2, y − 1/2, −z + 3/2 and (b) = −x + 3/2, y + 1/2, −z + 3/2 for 2.

A second cyanido group [C(2)–N(2)] belonging to the Co^III metalloligand contributes to the chain propagation by coordinating to an adjacent Mn^III metal ion. A fragment of each of the two heterobimetallic alternating Co^III–Mn^III chains 1 and 2 is shown in Fig. 2, while selected bond distances and angles for both compounds are listed in Table 1.

Fig. 2 Perspective view along the crystallographic [001] (a) or [101] (b) directions of a fragment of the corrugated chain structure of 1 (a) and 2 (b).

Table 1 Selected bond distances [Å] and angles [°] for 1 and 2^ab

Cobalt(III) environment			Manganese(III) environment
	1	2		1	2
a N(1*) denotes either atom N(1B) in 1 or N(1P) in 2; N(2*) denotes either atom N(2B) in 1 or N(2P) in 2.b Symmetry operation used to generate equivalent atoms: (a) = −x + 1, y + 1/2, −z + 1/2 (1) and (a) = −x + 3/2, y − 1/2, −z + 3/2 (2).
Co(1)–C(1)	1.867(3)	1.900(3)	Mn(1)–O(1L)	1.895(2)	1.898(2)
Co(1)–C(2)	1.928(3)	1.936(3)	Mn(1)–O(2L)	1.876(2)	1.905(2)
Co(1)–C(3)	1.913(4)	1.931(3)	Mn(1)–N(1L)	2.003(2)	2.011(2)
Co(1)–C(4)	1.879(4)	1.900(3)	Mn(1)–N(2L)	1.992(2)	2.004(2)
Co(1)–N(1*)	1.980(2)	2.032(2)	Mn(1)–N(1)	2.282(3)	2.353(3)
Co(1)–N(2*)	1.981(3)	2.050(2)	Mn(1)–N(2a)	2.412(3)	2.326(3)
C(1)–Co(1)–C(2)	88.2(2)	85.9(1)	O(1L)–Mn(1)–O(2L)	95.01(9)	93.3(1)
C(1)–Co(1)–C(3)	88.2(2)	90.2(1)	O(1L)–Mn(1)–N(1L)	92.2(1)	92.5(1)
C(1)–Co(1)–C(4)	86.9(2)	80.2(1)	O(1L)–Mn(1)–N(2L)	173.0(1)	174.0(1)
C(1)–Co(1)–N(1*)	96.7(1)	99.0(1)	O(1L)–Mn(1)–N(1)	95.9(1)	96.5(1)
C(1)–Co(1)–N(2*)	178.2(1)	174.5(1)	O(1L)–Mn(1)–N(2a)	91.0(1)	92.3(1)
C(2)–Co(1)–C(3)	175.5(1)	175.6(1)	O(2L)–Mn(1)–N(1L)	172.6(1)	173.6(1)
C(2)–Co(1)–C(4)	88.6(2)	91.2(1)	O(2L)–Mn(1)–N(2L)	90.9(1)	92.5(1)
C(2)–Co(1)–N(1*)	92.2(1)	86.9(1)	O(2L)–Mn(1)–N(1)	89.7(1)	89.3(1)
C(2)–Co(1)–N(2*)	92.6(1)	88.9(1)	O(2L)–Mn(1)–N(2a)	95.9(1)	93.2(1)
C(3)–Co(1)–C(4)	88.6(2)	90.2(1)	N(1L)–Mn(1)–N(2L)	82.0(1)	81.8(1)
C(3)–Co(1)–N(1*)	90.8(1)	91.7(1)	N(1L)–Mn(1)–N(1)	88.0(1)	87.3(1)
C(3)–Co(1)–N(2*)	91.2(1)	95.0(1)	N(1L)–Mn(1)–N(2a)	85.6(1)	89.3(1)
C(4)–Co(1)–N(1*)	176.4(2)	178.0(1)	N(2L)–Mn(1)–N(1)	87.9(1)	84.9(1)
C(4)–Co(1)–N(2*)	94.8(2)	98.2(1)	N(2L)–Mn(1)–N(2a)	84.7 (1)	86.1(1)
N(1*)–Co(1)–N(2*)	81.6(1)	82.5(1)	N(1)–Mn(1)–N(2a)	170.8(1)	170.7(1)

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In both cases the Mn^III ion, confined within the tetradentate Schiff base ligand, is six-coordinate by the two imine-nitrogen and the two phenoxo-oxygen atoms from the tetradentate salen ligand, together with two cyanido nitrogen atoms belonging to the Co^III metalloligand (see Fig. 1 and 2). The donor atoms are arranged in a distorted octahedral environment, with the equatorial positions filled by the almost coplanar Schiff base heteroatoms [the values of the O1L–N1L–N2L–O2L torsion angle are about 4.1 (1) and 3.1° (2)] and the axial positions occupied by the two cyanido nitrogen atoms. The two cyanido bridges are bent with respect to the central Mn^III ion, with the values of the Mn–N–C angles varying in the ranges 149.5(2)–151.6(2) (1) and 147.6(2)–148.7(2)° (2). As expected for octahedral Mn^III complexes, the Jahn–Teller distortion leads to elongated axial bond lengths (see Table 1). Therefore, the Mn–N_cyanido bond lengths [Mn1–N1 = 2.282(3) (1)/2.353(2) Å (2) and Mn1–N2a = 2.412(3) (1)/2.326(3) Å (2)] are longer than Mn–N_imine [Mn1–N1L = 2.003(2) (1)/2.011(2) Å (2) and Mn1–N2L = 1.992(2) (2)/2.004(2) Å (2)] and Mn–O_phenoxo [Mn1–O1L = 1.895(2) (1)/1.8988(19) Å (2) and Mn1–O2L = 1.8762(19) (1)/1.905(2) Å (2)]. These structural data for the Mn(III) environment agree with those observed in the related zigzag chains of the general formula {[Mn^III(salhd)(μ-NC)₂M^III(bpym)(CN)₂]·H₂O}_n (M = Fe and Cr)²⁴ which result from the reaction of PPh₄[M^III(bpym)(CN)₄]^9c with [Mn^III(salhd)]ClO₄ [bpym = 2,2′-bipyrimidine and H₂salhd = 1,2-bis(salicylideneimine)cyclohexane].²⁵

Concerning the anionic fragment, the Co^III ion is also six-coordinate with four cyanido ligands and two nitrogen atoms belonging to the chelating 4,4′-dmbipy (1) or dmphen (2) molecules. The coordination environment of the metal ion in this case is again distorted octahedral, the distortion being mainly due to the reduced bite angle of the chelating ligand [81.6(1) (1) or 82.5(1)° (2)]. The values are very close to the corresponding one in the structure of the complex [Co(phen)(CN)₄]⁻ [82.9(1)°],²² which considerably resembles the {Co(dmphen)(CN)₄} and {Co(dmbipy)(CN)₄} moieties in 2 and 1, respectively. The Co–C_cyanido bond lengths have values in the range 1.867(3)–1.928(3) Å for 1 and 1.900(3)–1.935(3) Å for 2, with the longest bond, in each case, involving the bridging cyanide carbon (C2). The Co–N distances are slightly longer than the Co–C bonds: 1.980(2)/1.981(3) Å for Co1–N1B/Co1–N2B (1) and 2.032(2)/2.049(2) Å for Co1–N1P/Co1–N2P (2). The four Co–C–N sets of atoms including the cyanido groups are close to linearity with the values of the Co–C–N angle varying in the range 176.1(3)–179.6(6)° for 1 and 173.3(3)–176.6(3)° for 2. The bond distances and angles within the two tetracyanido-bearing Co(III) fragments in 1 and 2 (see Table 1) are in agreement with similar reported examples.^22,26

The Co^III⋯Mn^III separation through one of the two cyanido bridges is somewhat greater than the other one with values of 5.0904(6)/5.3103(7) Å in 1 and 5.180(1)/5.151(1) Å in 2 for the heterometallic separation across C(1)N(1) and C(2)N(2), respectively [Co(1)⋯Mn(1) and Co(1)⋯Mn(1b) with (b) = −x + 1, y − 1/2, −z + 1/2 (1) and (b) = −x + 3/2, y + 1/2, −z + 3/2 (2)]. The closest homometallic, Mn⋯Mn and Co⋯Co intrachain distances, i.e. Mn(1)⋯Mn(1b) and Co(1)⋯Co(1a) (see Fig. 2) are 6.5317(6) and 9.7752(9) Å in 1, and 6.766(1) and 9.581(2) Å in 2, respectively [symmetry code: (a) = −x + 1, y + 1/2, −z + 1/2 (1) and (a) = −x + 3/2, y − 1/2, −z + 3/2 (2)].

An inspection of the packing diagram of 1 reveals weak hydrogen bonds established between the crystallization water molecules and the terminal cyanido groups [O(1w)⋯N(3) = 3.171(1) Å, O(1w)⋯N(4c) = 3.184(9) Å; (c) = −x, y + 1/2, −z + 1/2]. These interactions serve to assemble adjacent chains in the direction of the crystallographic a axis, according to an AAAA trend, thus forming supramolecular layers parallel to the crystallographic ab plane (Fig. 3a). The AAAA packing arrangement of the corrugated chains within each layer in 1 is such that it generates shorter closest interchain distances for Co⋯Co than Mn⋯Mn [Co(1)⋯Co(1c) = Co(1)⋯Co(1d) = 9.8546(8) Å and Mn(1)··· Mn(1c) = Mn(1)··· Mn(1d) = 12.909(1) Å; (d) = −x, y − 1/2, −z + 1/2]. Adjacent chains in 2 are assembled in the direction of the greater ac diagonal by weak off-set π–π stacking interactions between pairs of dmphen molecules, the interplanar distance being ca. 4.0 Å. The resulting supramolecular 2D structure, although resembling that found in 1, exhibits the ABAB pattern for symmetry reasons (see Fig. 3b). The closest homometallic Co⋯Co and Mn⋯Mn interchain distances correspond in this case to Co(1)⋯Co(1c) = 7.996(1) Å and Mn(1)⋯Mn(1d) = 11.020(2) Å [symmetry code: (c) = −x + 1, −y + 1, −z + 2; (d) = –x + 2, −y + 1, −z + 1]. Additional weak aromatic π–π interactions help to stabilize a supramolecular 3D structure. In particular, the most significant interlayer interactions in 1 are established between pairs of 4,4′-dmbipy as well as pairs of salen ligands with interplanar distances of 3.60 and 3.45 Å, respectively. Weaker stacking interactions are also established between 4,4′-dmbipy and the salen ligands, with an interplanar distance of 3.34 Å but an interplanar angle of ca. 16.4° (Fig. S2†). Off-set π–π interactions between pairs of salen ligands in 2 [interplanar distance of 3.33 Å] are the main factor accounting for the 3D cohesion (Fig. S3†), and such interactions generate short interlayer Mn⋯Mn distances of 9.273(1) Å in 1 and 8.840(2) Å in 2.

Fig. 3 Perspective views of the weak hydrogen bonding and π–π stacking patterns leading to a supramolecular 2D structure in 1 (a) and 2 (b).

Magnetic properties

The magnetic properties of 1 and 2 under the form of the χ_MT versus T plot [χ_M is the magnetic susceptibility per mole of compound] are shown in Fig. 4 (1) and 5 (2). Both plots exhibit the same shape indicating a practically identical magnetic behaviour. At room temperature, χ_MT for 1 and 2 are equal to ca. 3.0 cm³ mol⁻¹ K, a value which is as expected for a magnetically isolated high-spin Mn(III) with g_Mn = 2.0, the Co(III) being diamagnetic. Upon cooling, this value remains constant until 50 K and it further decreases to attain 2.02 (1) and 1.82 cm³ mol⁻¹ K (2) at 1.9 K. No maximum of the magnetic susceptibility occurs in the temperature range explored. This behaviour is as expected for a magnetically diluted spin quintet containing complex, the decrease of χ_MT in the low temperature region being due to the zero-field splitting (zfs, D_Mn) of the Mn(III) ion and/or intra/interchain magnetic interactions.

Fig. 4 Thermal dependence of χ_MT for 1. The inset shows the field dependence of the magnetization for 1 at 2.0 K. The open circles are experimental data and the solid lines are the best-fit curves obtained using eqn (1) (see main text).²⁷

Fig. 5 Thermal dependence of χ_MT for 2. The inset shows the field dependence of the magnetization for 2 at 2.0 K. The open circles are experimental data and the solid lines are the best-fit curves obtained using eqn (1) (see main text).²⁷

The magnetization versus H plots at 2.0 K for 1 and 2 (see the insets of Fig. 4 and 5) also support the occurrence of significant zfs of the Mn(III) ion in these complexes, the values of the magnetization at 50 kOe being 2.90 (1) and 2.75 Nβ (2) (compared with the calculated saturation value of M_S = g_MnS_Mn = 4.0 Nβ).

Keeping in mind the above considerations, we have treated the magnetic susceptibility data of 1 and 2 using the spin Hamiltonian [eqn (1)]

Ĥ = D_Mn(Ŝ_Mn² + 2) + βHg_MnŜ_Mn (1)

where the first term deals with the local anisotropy of the high-spin Mn(III) ion and the second one is the Zeeman interaction. A Curie–Weiss term (θ) was introduced as a variable parameter in the fitting procedure in order to account for the possible intra/interchain magnetic interactions between Mn(III) ions and the simulations were performed using matrix diagonalization techniques by the use of the Irreducible Tensor Operators (ITO) implemented in the XVPMAG program.²⁷

The best-fit parameters are: D_Mn = −3.26 cm⁻¹, g_Mn = 1.99 and θ = −0.24 K for 1 and D_Mn = −4.38 cm⁻¹, g_Mn = 2.00 and θ = −0.39 K for 2. The calculated curves (see the solid lines in Fig. 4 and 5) reproduce very well the experimental data in the whole temperature range explored. From these values, we can reproduce perfectly the magnetization curve at 2.0 K (see the inset in Fig. 4 and 5). In fact, we obtained the same values when fitting both experimental curves (susceptibility and magnetization simultaneously).²⁷ It deserves to be noted that the obtained values of D_Mn are in the usual range for the Schiff base complexes with Mn(III) that can be found in the literature (values between −1 and −4 cm⁻¹),^15,28 their negative sign being consistent with the Jahn–Teller axis elongation of the octahedral geometry. As far as the very weak values of θ are concerned, they are not unexpected because of the large intra- [ca. 6.5 (1) and 6.8 Å (2) through the multiatom –N–C–Co(III)–C–N– pathway] and interchain [ca. 12.9 (1) and 11.0 Å (2)] manganese–manganese distances.

To explore the SIM characteristics of 1 and 2, the frequency-dependent alternating current (ac) magnetic susceptibility was measured at very low temperatures. These measurements show a frequency-dependence of both the in-phase (χ^′_M) and out-of-phase (χ^′′_M) signals between 5 and 2.0 K at a 2000 Oe applied dc field for 1 (Fig. 6) whereas under the same conditions, only incipient χ^′_M signals and maxima of χ^′′_M for frequencies above 3600 Hz are observed for 2 (Fig. S4†). In this latter case, the maxima of χ^′′_M could be observed in the whole frequency range under an applied dc field of 5000 Oe (Fig. S5†).

Fig. 6 Temperature dependence of the in-phase (top) and out-of-phase (bottom) ac susceptibilities for 1 under an applied static field H_dc = 2000 Oe with a ± 5.0 Oe oscillating field at frequencies in the range 1000–10 000 Hz. The solid lines are given for guidance.

The values of the relaxation time of 1, which are calculated from the maximum of χ^′′_M at a given frequency (τ = 1/2πν), follow the Arrhenius law characteristics of a thermally activated mechanism [τ = τ_o exp(E_a/k_BT)]; the solid line in Fig. 7). The calculated values of the pre-exponential factor and the activation energy (τ_o = 1.40(2) × 10⁻⁸ s and E_a = 12.0(2) cm⁻¹) are consistent with those previously reported for related Mn(III) SIMs.^15,29 Moreover, the calculated value of E_a compares reasonably well with that obtained from the fit of the magnetic data (E_a = −4D_Mn = 13.0 cm⁻¹). The Cole–Cole plots of 1 in the temperature range 2.0–2.6 K and at an applied field of 2000 Oe give almost perfect semicircles which can be fitted using the generalized Debye model (Fig. 8).^30a The calculated low values of the α parameter (0.11–0.12; Table 2) support a single relaxation process (α = 0 for a Debye model) and discard a spin glass behaviour.^30b

Fig. 7 Arrhenius plot, ln(τ) vs. 1/T, obtained from ac measurements on a polycrystalline powder sample of 1 under an applied dc magnetic field of 2000 Oe. The data (○) were derived from the maxima of χ^′′_M vs. T at different frequencies.

Fig. 8 Cole–Cole plots for 1 at the indicated temperatures. The solid lines are the best-fit curves (see the main text).

Table 2 Selected ac magnetic data for 1

Hdc/Oe	T/K	α	χS^a/cm^3 mol^−1	χT^b/cm^3 mol^−1
a Adiabatic susceptibility.b Isothermal susceptibility.
2000	2.0	0.12	0.20	0.90
2000	2.2	0.11	0.20	0.87
2000	2.4	0.11	0.18	0.84
2000	2.6	0.11	0.16	0.80

---

In the case of 2, under the assumption that the SIM has only one relaxation time (τ_o) with a characteristic activation energy (E_a) and considering that the adiabatic susceptibility is zero, it is possible to use eqn (2),³¹ in which ω is the experimental ac field exciting frequency (ω = 2πν), to roughly estimate the values of τ_o and E_a

ln(χ^′′_M/(χ^′_M) = ln(ωτ_o) + E_a/k_BT (2)

This method has already been applied to evaluate these factors in other Mn(III) SIMs in the literature.^15,29b,32 In the present case, the values obtained for E_a and τ_o are 9.4(3) cm⁻¹ and 2.5(2) × 10⁻⁸ s (H_dc = 2000 Oe; Fig. S6) and 9.0(2) cm⁻¹ and 4.1(2) × 10⁻⁸ (H_dc = 5000 Oe; Fig. S7†). These values are comparable to those obtained for 1. Cole–Cole plots for 2 were established and fitted by using the generalized Debye functions (Fig. S8 and S9†). The fitting parameters are summarized in Table S1 (ESI†). The values of the α parameter for 2 [0.071 (H_dc = 2000 Oe) and 0.073–0.074 (H_dc = 5000 Oe)] in the temperature range 2.0–2.4 K also imply a single relaxation process, as in 1.

Experimental

All reagents and solvents were purchased from commercial sources and used without further purification. [Mn^III(salen)(H₂O)]ClO₄ was synthesized according to the previously described method.²¹ The syntheses of the complexes were carried out under aerobic conditions, in a fume hood. Elemental analyses (C, H, N) were performed by the Servicio de Microanálisis from the Servicio Central de Soporte a la Investigación Experimental (SCSIE) de la Universidad de Valencia. The values of the Co : P (1 : 1) for PPh₄[Co^III(4,4′-dmbipy)(CN)₄] and Co : Mn (1 : 1 for 1 and 2) molar ratios were determined using electron probe X-ray microanalysis by using a Philips XL-30 scanning electron microscope (SEM) from the SCSIE. Infrared spectra (4000–300 cm⁻¹) were recorded on a Bruker F S55 spectrophotometer on samples of PPh₄[Co^III(4,4′-dmbipy)(CN)₄], 1 and 2 prepared as KBr pellets. Variable-temperature (1.9–300 K) magnetic susceptibility measurements on polycrystalline samples of 1 and 2 were carried out with a SQUID susceptometer using applied magnetic fields of 1 T (T ≤ 50 K) and 500 G (T < 50 K). Magnetization versus magnetic field measurements of 1 and 2 were performed at 2.0 K in the field range 0–5 T. Diamagnetic corrections for the constituent atoms were made by using Pascal’s constants. The magnetic data were also corrected for the temperature-independent paramagnetism and magnetization of the sample holder (a plastic bag). The value of the TIP for each mole of the Co(III)Mn(III) pair was 300 × 10⁻⁶ cm³ mol⁻¹.

Caution: cyanides are highly toxic and should be handled with great caution. We worked at the mmol scale and the preparations were performed in a well ventilated fume hood. Concentrated aqueous solutions of sodium hypochlorite and sodium hydroxide were used to transform the cyanide from the waste into cyanate.

Preparation of the complexes

Synthesis of PPh₄[Co(4,4′-dmbipy)(CN)₄]. This compound was synthesized according to the method described previously.²² Yield: ca. 70%. IR (KBr/cm⁻¹): 3318(m), 3253(s), 2130(s), 1620(s), 1453(m), 1100(s), 760, 725(s), 690(s), 528(s). Anal calcd. for C₃₀H₂₈CoN₆P: C 64.00; H 4.98; N 14.93. Found: C 64.08; H 4.96; N 14.94%.

Synthesis of {[Mn^III(salen)(μ-NC)₂Co^III(4,4′-dmbipy)(CN)₂]·H₂O}_n (1). In a test tube, a solution of 28 mg [Mn^III(salen)(H₂O)]ClO₄ dissolved in 20 mL of a methanol/acetonitrile mixture (1 : 1 v/v) was layered over a solution of 56 mg PPh₄[Co(4,4′-dmbipy)(CN)₄] dissolved in 20 mL of the same methanol/acetonitrile mixture. X-ray quality crystals of 1 were obtained after four weeks. Yield: ca. 70%. IR (KBr/cm⁻¹): 3518(m), 3103(s), 2154, 2136(s), 1630(s), 1453(m), 1100(s). Anal calcd. for C₃₂H₂₈CoN₈O₃Mn (1): C 55.93; H 4.08; N 16.31. Found: C 56.12; H 4.16; N 16.74%.

Synthesis of [Mn^III(salen)(μ-NC)₂Co^III(dmphen)(CN)₂]_n (2). Compound 2 was synthesized by following the procedure detailed for 1, but using PPh₄[Co^II(dmphen)(CN)₃] instead PPh₄[Co(4,4′-dmbipy)(CN)₄]. X-ray quality crystals of 2 were obtained after five weeks. Yield: ca. 60%. IR (KBr/cm⁻¹): 3120(m), 2150sh, 2137(s) 1620(s), 1453(m), 1100(s). Anal calcd. for C₃₄H₂₆CoN₈O₂Mn (2): C 58.91; H 3.75; N 16.17. Found: C 59.08; H 3.83; N 16.25%.

X-ray crystallography

X-ray diffraction data on single crystals of 1 and 2 were collected with a Bruker-Nonius X8-APEXII CCD area detector system by using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The data were processed through the SAINT³³ reduction and SADABS³⁴ absorption software. A summary of the crystallographic data and structure refinement for the two compounds is given in Table 3. The two structures were solved using direct methods and subsequently completed through Fourier recycling using the SHELXTL-2013 software package,³⁵ then refined using the full-matrix least-squares refinements based on F² with all observed reflections, using established methods.³⁶ All non-hydrogen atoms were refined anisotropically.

Table 3 Crystal data and structure refinement details for 1 and 2

	1	2
a R1 = ∑(|Fo| − |Fc|)/∑|Fo|.b wR2 = {∑[w(Fo^2 − Fc^2)^2]/∑w(Fo^2)^2]}^1/2 and w = 1/[σ^2(Fo^2) + (mP)^2 + nP] with P = [Fo^2 + 2Fc^2]/3, m = 0.0816 (1), 0.0670 (2) and n = 0.5826 (1), 1.5424(3) (2).
Formula	C32H28CoMnN8O3	C34H26CoMnN8O2
Mr	686.49	692.5
Crystal system	Monoclinic	Monoclinic
Space group	P21/c	P21/n
a/Å	14.0141(12)	13.206(3)
b/Å	11.9783(10)	12.307(3)
c/Å	20.0014(18)	20.386(5)
β/°	106.989(4)	104.201(13)
V/Å^3	3211.0(5)	3212.0(14)
Z	4	4
Dc/g cm^−3	1.420	1.432
T/K	296(2)	296(2)
μ(Mo-Kα)/mm^−1	0.955	0.954
F(000)	1408	1416
Refl. collected	71 843	27 556
Refl. indep. (Rint)	6516 (0.0430)	5790 (0.0401)
Refl. obs. [I > 2σ(I)]	5055	4827
Goodness-of-fit on F^2	1.112	1.134
R1^a [I > 2σ(I)] (all)	0.0401 (0.0605)	0.0392 (0.0522)
wR2^b [I > 2σ(I)] (all)	0.1233 (0.1435)	0.1098 (0.1242)
Δρmax,min/e Å^−3	1.041, −0.471	0.515, −0.500

---

The hydrogen atoms of the salen (1 and 2), 4,4′-dmbipy (1) and dmphen (2) ligands were included at geometrically calculated positions and refined using a riding model. The hydrogen atoms of the crystallization water molecule in 1 were not located, but included in the formula sum. The final geometrical calculations and graphical manipulations were performed using the XP utility within SHELXTL;³⁵ graphical manipulations were also performed with the CrystalMaker software.³⁷

Room temperature powder X-ray diffraction methods were employed to check the purity of the bulk samples of 1 and 2, the good match between the experimental and simulated patterns supporting their purity [Fig. S10† (1) and S11 (2)].

Conclusions

The use of the heteroleptic tetracyanido-bearing cobalt(III) complexes [Co^III(4,4′-dmbipy)(CN)₄]⁻ and [Co^II(dmphen)(CN)₃]⁻ as metalloligands towards [Mn^III(salen)(H₂O)]⁺ afforded the heterobimetallic chains 1 and 2 with regular alternating diamagnetic Co(III) and high-spin Mn(III) ions which behave as SIMs. In fact, when the [Co^II(dmphen)(CN)₃]⁻ complex anion was employed as reactant, the Co^II metal ion underwent oxidation and the unit was reorganized as the {Co^III(dmphen)(CN)₄} fragment in 2. Our synthetic route can be viewed as a general successful strategy to obtain tailor-made extended systems of SIMs by using these types of metalloligands in combination with anisotropic metal ions whose coordination sphere is either fully solvated or partially blocked with peripheral ligands.

Acknowledgements

Financial support by the Spanish Ministerio de Economía y Competitividad (Projects CTQ2013-44844 and Unidad de Excelencia MDM-2015-0538), the Generalitat Valenciana (Projects PROMETEOII/2014/070 and ISIC/2012/002) and by the Romanian National Authority for Scientific Research, CNCS−UEFISCDI, Project PN-II-RU-TE-2014-4-1556 is gratefully acknowledged. N. M. also thanks the European Commission, FSE (Fondo Sociale Europeo) and Calabria Region for a postdoctoral fellowship.

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Footnote

† Electronic supplementary information (ESI) available: FTIR spectra [Fig. S1a (1) and S1b (2)], packing drawings [Fig. S2 (1) and S3 (2)], frequency and temperature dependence of the ac signals [Fig. S4 (2) and S5 (2)], ln(χ_M^′/χ_M^′′) against 1/T [Fig. S6 (2) and S7 (2)], Cole–Cole plots [Fig. S8 and S9 (2)], PXRD patterns [Fig. S10 (1) and S11 (2)] and selected ac magnetic data for 2 (Table S1). CCDC 1403662 and 1403663. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra16307b

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