A Spin Crossover Transition in a Mn(ii) Chain Compound

Three new compounds have been synthesized and characterized with Fe(II), Co(II) and Mn(II), the polynitrile anionic ligand 1,1,3,3-tetracyano-2-(3-hydroxypropoxy)-propenide (tcnopr3OH´) and the co-ligand 4,4 1-bipyridine (4,4 1-bpy). The Fe(II) compound, formulated as [Fe II (tcnoprOH) 2 (H 2 O) 2 (4,4 1-bpy) 2 ] (1), contains monomeric complexes where the Fe(II) ion is coordinated to two trans polynitrile ligands, two trans 4,4 1-bpy ligands and two trans water molecules.

In this context, in previous works with polynitrile anions, we have synthesized different ligands with up to four cyano groups oriented in two different directions and an additional potentially coordinating alcohol or thiol function (Scheme 1) [27].
In this context, in previous works with polynitrile anions, we have synthesized different ligands with up to four cyano groups oriented in two different directions and an additional potentially coordinating alcohol or thiol function (Scheme 1) [27].
In the present work, we will combine both strategies in order to prepare new CP with magnetic properties.Thus, we will explore the effect of extending the alkyl chain in the polynitrile ligand by including an extra carbon atom (i.e., with a -prOH arm instead of -etOH) and the effect of the presence of a bridging co-ligand as 4,4 1 -bpy.Thus, here, we present the results obtained with the polynitrile anion 1,1,3,3-tetracyano-2-(3-hydroxypropoxy)-propenide (tcnopr3OH ´) and 4,4 1 -bipyridine (4,4 1 -bpy) as a bridging co-ligand combined with different transition metal ions as Mn(II), Fe(II) and Co(II).This study shows that both the additional carbon atom in the side chain of the polynitrile ligand and the transition metal ion play a key role in the final structure obtained.Thus, Fe(II) forms a centro-symmetric monomer formulated as [Fe II (tcnopr3OH) 2 (H 2 O) 2 (4,4 1 -bpy) 2 ] (1) with two trans (tcnopr3OH) ´ligands, two trans 4,4 1 -bpy ligands and two trans water molecules.In contrast, Mn(II) and Co(II) form two isostructural coordination polymers formulated as [M II (H 2 O) 4 (µ-4,4 1 -bpy)][M II (tcnoprOH) 4 (µ-4,4 1 -bpy)]¨3H 2 O with M = Mn (2) and Co (3).These CP present segregated cationic and anionic chains formulated as [M II (H 2 O) 4 (µ-4,4 1 -bpy)] 2+ and [M II (tcnoprOH) 4 (µ-4,4 1 -bpy)] 2´.We also present here the magnetic characterization of the three compounds.Compounds 1 and 3 show the expected paramagnetic behavior for high spin Fe(II) and Co(II) ions, whereas the Mn(II) compound presents a very unusual partial spin crossover transition in the anionic Mn(II) chain coexisting with the high spin Mn(II) cationic chain.Compound 2 is, therefore, one of the very scarce examples of spin crossover transitions in a Mn(II) complex and the first one with the coexistence of smooth spin crossover (SCO) and high spin Mn(II) ions.

Syntheses and Spectroscopic Characterizations of the Complexes
The synthesis of the three compounds was performed under similar conditions and using a method similar to that used to prepare the closely-related compounds with the ligand tcnoetOH (Scheme 1).Unexpectedly, the Fe(II) ion crystallized as a monomer surrounded by two trans tcnopr3OH ´ligands, two trans water molecules and two trans terminal 4,4 1 -bpy molecules.In contrast, Mn(II) and Co(II) ions crystallize forming segregated linear cationic and anionic chains with trans bridging 4,4 1 -bpy ligands and four tcnopr3OH ´ligands (anionic chains) or four water molecules (cationic chains) completing the octahedral coordination of the metal ions.Of course, we cannot attribute these different behaviors to the metal ion size, since the ionic radius of Fe(II) is intermediate between Mn(II) and Co(II).Furthermore, when using the closely-related tcnoetOH ´ligand with Fe(II), Co(II) and Ni(II), the obtained compounds present linear neutral chains containing the metal ions coordinated to two trans tcnoetOH ´ligands, two trans water molecules and two trans 4,4 1 -bpy ligands [29] (i.e., the metal ions have the same coordination environmentthan Fe(II) in Compound 1, but with bridging 4,4 1 -bpy molecules instead of terminal ones, as in (1).
The IR spectra of the three complexes are very similar: they show doublets at 2245/2210, 2237/2206 and 2238/2207 cm ´1 in 1-3, respectively, corresponding to the coordinated and non-coordinated cyano groups, respectively.A broad band at ca. 3400 cm ´1 confirms the presence of water molecules in the three compounds.

Description of the Structures
Structure of [Fe II (tcnoprOH) 2 (4,4 1 -bpy) 2 (H 2 O) 2 ] (1): Compound 1 crystallizes in the triclinic space group P-1 and is formed by isolated monomeric Fe(II) complexes.The asymmetric unit of 1 contains a Fe(II) ion located in a special position (1/2, 1/2, 1/2), one tcnopr3OH ´anion, one 4,4 1 -bpy ligand and one water molecule, all located in general positions.This asymmetric unit generates an isolated centro-symmetric complex with two terminal tcnopr3OH ´ligands, two terminal 4,4 1 -bpy ligands and two water molecules (Figure 1).The inversion center located on the Fe(II) ion generates a trans disposition of each couple of ligands.The bond lengths and angles (Table 1) show that the coordination geometry around the Fe(II) ion is a quite regular octahedron of the type of N 4 O 2 containing two N atoms from two 4,4 1 -bpy molecules (N5 and N5*), two N atoms from the tcnopr3OH ´ligands (N1 and N1*) and two oxygen atoms from two water molecules (O1 and O1*) (Figure 1).The molecules are isolated and are only connected by π-π interactions established between the aromatic rings of the 4,4 1 -bpy molecules.These interactions give rise to stacks of molecules along the a direction (Figure 2) with two different short average inter-plane distances of 3.37 and 3.48 Å.These π-π interactions may be the reason for the longer Fe-N bpy bond distance compared to the Fe-N tcnoprOH one (2.212(16) vs.

2.182(2) Å).
Structure of [M(H 2 O) 4 (µ-4,4 1 -bpy)][M(tcnoprOH) 4 (µ-4,4 1 -bpy)].3H 2 O, M II = Mn (2) and Co (3): Compounds 2 and 3 are isostructural and crystallize in the monoclinic space group P2/n.The asymmetric unit contains two independent metal ions, two tcnopr3OH ´ligands, two water molecules and two 4,4 1 -bpt bridging ligands.Each independent metal ion (M1 and M2) forms a different linear chain, although with identical bridges.Thus, in both chains, the metal ions are connected through two trans 4,4 1 -bpy bridging ligands (Figure 3).The differences between both chains consist in the ligands located perpendicular to the chain direction: four tcnopr3OH ´ligands for M1 and four H 2 O molecules for M2.This different ligand distribution gives rise to different charges in the chains.Thus, the chain        Structure of [M(H2O)4(μ-4,4′-bpy)][M(tcnoprOH)4(μ-4,4′-bpy)].3H2O,M II = Mn (2) and Co (3): Compounds 2 and 3 are isostructural and crystallize in the monoclinic space group P2/n.The asymmetric unit contains two independent metal ions, two tcnopr3OH − ligands, two water molecules and two 4,4′-bpt bridging ligands.Each independent metal ion (M1 and M2) forms a different linear chain, although with identical bridges.Thus, in both chains, the metal ions are connected through two trans 4,4′-bpy bridging ligands (Figure 3).The differences between both chains consist in the ligands located perpendicular to the chain direction: four tcnopr3OH − ligands for M1 and four H2O molecules for M2.This different ligand distribution gives rise to different charges in the chains.Thus, the chain formulated as [M(H2O)4(μ-4,4′-bpy)] 2+ is cationic and bears a +2 charge per metal ion, whereas the chain formulated [M(tcnoprOH)4(μ-4,4′-bpy)] 2− bears a charge of −2 per metal ion.In the cationic chain, the asymmetric unit contains one metal ion (M2) (Mn in 2 and Co in 3), a 4,4′-bpy molecule, both located in a special position (3/4,−y,3/4) corresponding to a C2 axis that runs along the chain axis.The asymmetric unit also contains two coordinated water molecules (O5 and In the cationic chain, the asymmetric unit contains one metal ion (M2) (Mn in 2 and Co in 3), a 4,4 1 -bpy molecule, both located in a special position (3/4,´y,3/4) corresponding to a C 2 axis that runs along the chain axis.The asymmetric unit also contains two coordinated water molecules (O5 and O6) that become four when the C 2 axis is applied, to generate the cationic chain formulated as [M(H 2 O) 4 (µ-4,4 1 -bpy)] 2+ .The asymmetric unit of the anionic chain is similar to that of the cationic one, but the water molecules are now replaced by (tcnopr3OH) ´ligands, and the metal ions are labelled as M1.Both chains are parallel, run along the b axis and show a chessboard-like disposition, where each cationic chain is surrounded by four anionic chains and vice versa (Figure 4).O6) that become four when the C2 axis is applied, to generate the cationic chain formulated as [M(H2O)4(μ-4,4′-bpy)] 2+ .The asymmetric unit of the anionic chain is similar to that of the cationic one, but the water molecules are now replaced by (tcnopr3OH) − ligands, and the metal ions are labelled as M1.Both chains are parallel, run along the b axis and show a chessboard-like disposition, where each cationic chain is surrounded by four anionic chains and vice versa (Figure 4).The bond distances and angles (Table 1) show that the coordination geometry of both metal atoms is octahedral, of the type N6 for M1 and N2O4 for M2.M1 is surrounded by two N atoms from two equivalent 4,4′-bpy bridges (N11 and N12*) and four oxygen atoms from four water molecules (O5, O6 and their symmetry-related O5* and O6*; Figure 3).M2 is also surrounded by two N atoms from two equivalent 4,4′-bpy bridges (N9 and N1O*) and four nitrogen atoms from four tcnopr3OH − ligands (N1, N5 and their symmetry-related N1* and N5*; Figure 3).
As can be seen in Table 1, the M-O bond distances are shorter than the M-N ones, in agreement with the smaller size of the oxygen atom.In both compounds, the M-N bond distances for the tcnopr3OH − ligands are longer than for the 4,4′-bpy ligands, probably due to the larger steric effect of the bulky tcnopr3OH − ligands.The Mn-O and Mn-N bond distances are larger than the corresponding Co-O and Co-N, in agreement with the larger size of the Mn(II) ion.
A deeper analysis of the M-O and M-N bond distances shows that the average Mn-O bond distances are only 0.4 pm longer than the corresponding average Co-O bond distances.The same difference is observed between the average Mn-Nbpy and Co-Nbpy bond distances.Finally, the difference between the average Mn-NtcnoprOH bond distances and the corresponding Co-NtcnoprOH ones is 0.75 pm.Even if the standard deviations are ca.0.4-0.5 pm, we can conclude that Mn(II) is only ca.0.5-1.0pm larger than Co(II).This low value contrasts with the calculated one (8.5 pm) from the ionic radii of both divalent high spin ions in an octahedral environment (Co(II) = 88.5, Mn(II) = 97 pm) [30].Since the magnetic measurements show that in Compound 3, the Co(II) ions are in the high spin configuration (see below), these bond differences suggest that one of the two Mn(II) ions may be in the low spin configuration (whose ionic radius is only 81 pm).If we assume that one of the Mn(II) ions is in a low spin and the other is in a high spin configuration, then the calculated difference between the average Mn-O/N and Co-O/N bond distances is 0.5 pm, in agreement with the observed value.This observation fully agrees with the magnetic measurements (see below).
Note that the Mn1-Nbpy bond distances are slightly longer than those of Mn2-Nbpy, but this fact must be attributed to the huge steric effect of the polynitrile ligand in the anionic chain.This effect prevents a clear comparison between both Mn(II) centers from the Mn-Nbpy bond lengths.The bond distances and angles (Table 1) show that the coordination geometry of both metal atoms is octahedral, of the type N 6 for M1 and N 2 O 4 for M2.M1 is surrounded by two N atoms from two equivalent 4,4 1 -bpy bridges (N11 and N12*) and four oxygen atoms from four water molecules (O5, O6 and their symmetry-related O5* and O6*; Figure 3).M2 is also surrounded by two N atoms from two equivalent 4,4 1 -bpy bridges (N9 and N1O*) and four nitrogen atoms from four tcnopr3OH ´ligands (N1, N5 and their symmetry-related N1* and N5*; Figure 3).
As can be seen in Table 1, the M-O bond distances are shorter than the M-N ones, in agreement with the smaller size of the oxygen atom.In both compounds, the M-N bond distances for the tcnopr3OH ´ligands are longer than for the 4,4 1 -bpy ligands, probably due to the larger steric effect of the bulky tcnopr3OH ´ligands.The Mn-O and Mn-N bond distances are larger than the corresponding Co-O and Co-N, in agreement with the larger size of the Mn(II) ion.
A deeper analysis of the M-O and M-N bond distances shows that the average Mn-O bond distances are only 0.4 pm longer than the corresponding average Co-O bond distances.The same difference is observed between the average Mn-N bpy and Co-N bpy bond distances.Finally, the difference between the average Mn-N tcnoprOH bond distances and the corresponding Co-N tcnoprOH ones is 0.75 pm.Even if the standard deviations are ca.0.4-0.5 pm, we can conclude that Mn(II) is only ca.0.5-1.0pm larger than Co(II).This low value contrasts with the calculated one (8.5 pm) from the ionic radii of both divalent high spin ions in an octahedral environment (Co(II) = 88.5, Mn(II) = 97 pm) [30].Since the magnetic measurements show that in Compound 3, the Co(II) ions are in the high spin configuration (see below), these bond differences suggest that one of the two Mn(II) ions may be in the low spin configuration (whose ionic radius is only 81 pm).If we assume that one of the Mn(II) ions is in a low spin and the other is in a high spin configuration, then the calculated difference between the average Mn-O/N and Co-O/N bond distances is 0.5 pm, in agreement with the observed value.This observation fully agrees with the magnetic measurements (see below).
Note that the Mn1-N bpy bond distances are slightly longer than those of Mn 2 -N bpy , but this fact must be attributed to the huge steric effect of the polynitrile ligand in the anionic chain.This effect prevents a clear comparison between both Mn(II) centers from the Mn-N bpy bond lengths.

Magnetic Properties
The product of the magnetic susceptibility times the temperature (χ m T) per Fe(II) atom for Compound 1 shows a value at room temperature of ca.3.1 cm 3 ¨K¨mol ´1, close to the expected value for an isolated S = 2 high spin Fe(II) with g « 2. When the sample is cooled, χ m T remains constant down to ca. 30 K and shows a progressive decrease at lower temperatures to reach a value of ca.1.7 cm 3 ¨K¨mol ´1 at 2 K (Figure 5).Since Compound 1 presents isolated Fe(II) monomers, we have fit the magnetic data to a simple model for an S = 2 monomer with a zero field splitting [31], responsible for the decrease in χ m T at low temperatures.This model reproduces very satisfactorily the magnetic properties in the whole temperature range with g = 2.033(1) and |D| = 5.7(1), K = 4.0(1) cm ´1 (solid line in Figure 5).This value is within the normal range found for other Fe(II) complexes [32] and may include a weak antiferromagnetic interaction through the π-π interactions observed between the Fe(II) monomers.Note that the sign of D cannot be determined from powder susceptibility measurements.

Magnetic Properties
The product of the magnetic susceptibility times the temperature (χmT) per Fe(II) atom for Compound 1 shows a value at room temperature of ca.3.1 cm 3 •K•mol −1 , close to the expected value for an isolated S = 2 high spin Fe(II) with g ≈ 2. When the sample is cooled, χmT remains constant down to ca. 30 K and shows a progressive decrease at lower temperatures to reach a value of ca.1.7 cm 3 •K•mol −1 at 2 K (Figure 5).Since Compound 1 presents isolated Fe(II) monomers, we have fit the magnetic data to a simple model for an S = 2 monomer with a zero field splitting [31], responsible for the decrease in χmT at low temperatures.This model reproduces very satisfactorily the magnetic properties in the whole temperature range with g = 2.033(1) and |D| = 5.7(1), K = 4.0(1) cm −1 (solid line in Figure 5).This value is within the normal range found for other Fe(II) complexes [32] and may include a weak antiferromagnetic interaction through the π-π interactions observed between the Fe(II) monomers.Note that the sign of D cannot be determined from powder susceptibility measurements.The isothermal magnetization at 2 K shows a saturation value of ca. 3 B, below the expected one for an isolated S = 2 Fe(II) ion (4 mB) due to the presence of the zero field splitting ZFS (Figure 6).In fact, the magnetization can be well reproduced with a Brillouin function for an S = 2, although with a lower g value due to the ZFS (solid line in Figure 6).The χmT product for Compound 2 shows a room temperature value of ca.6.2 cm 3 •K•mol −1 (Figure 7).This value is well below the expected one for two high spin (HS) S = 5/2 Mn(II) ions with g = 2 (8.75 cm 3 •K•mol −1 ), suggesting that there is an important fraction of Mn(II) ions in the low spin (LS) configuration (in agreement with the Mn-O and Mn-N bond distances; see above).When the sample is cooled from 300 to 2 K (blue Line 1 in Figure 7), χmT smoothly decreases to reach a value of The isothermal magnetization at 2 K shows a saturation value of ca. 3 µ B , below the expected one for an isolated S = 2 Fe(II) ion (4 m B ) due to the presence of the zero field splitting ZFS (Figure 6).In fact, the magnetization can be well reproduced with a Brillouin function for an S = 2, although with a lower g value due to the ZFS (solid line in Figure 6).

Magnetic Properties
The product of the magnetic susceptibility times the temperature (χmT) per Fe(II) atom for Compound 1 shows a value at room temperature of ca.3.1 cm 3 •K•mol −1 , close to the expected value for an isolated S = 2 high spin Fe(II) with g ≈ 2. When the sample is cooled, χmT remains constant down to ca. 30 K and shows a progressive decrease at lower temperatures to reach a value of ca.1.7 cm 3 •K•mol −1 at 2 K (Figure 5).Since Compound 1 presents isolated Fe(II) monomers, we have fit the magnetic data to a simple model for an S = 2 monomer with a zero field splitting [31], responsible for the decrease in χmT at low temperatures.This model reproduces very satisfactorily the magnetic properties in the whole temperature range with g = 2.033(1) and |D| = 5.7(1), K = 4.0(1) cm −1 (solid line in Figure 5).This value is within the normal range found for other Fe(II) complexes [32] and may include a weak antiferromagnetic interaction through the π-π interactions observed between the Fe(II) monomers.Note that the sign of D cannot be determined from powder susceptibility measurements.The isothermal magnetization at 2 K shows a saturation value of ca. 3 B, below the expected one for an isolated S = 2 Fe(II) ion (4 mB) due to the presence of the zero field splitting ZFS (Figure 6).In fact, the magnetization can be well reproduced with a Brillouin function for an S = 2, although with a lower g value due to the ZFS (solid line in Figure 6).The χmT product for Compound 2 shows a room temperature value of ca.6.2 cm 3 •K•mol −1 (Figure 7).This value is well below the expected one for two high spin (HS) S = 5/2 Mn(II) ions with g = 2 (8.75 cm 3 •K•mol −1 ), suggesting that there is an important fraction of Mn(II) ions in the low spin (LS) configuration (in agreement with the Mn-O and Mn-N bond distances; see above).When the sample is cooled from 300 to 2 K (blue Line 1 in Figure 7), χmT smoothly decreases to reach a value of The χ m T product for Compound 2 shows a room temperature value of ca.6.2 cm 3 ¨K¨mol ´1 (Figure 7).This value is well below the expected one for two high spin (HS) S = 5/2 Mn(II) ions with g = 2 (8.75 cm 3 ¨K¨mol ´1), suggesting that there is an important fraction of Mn(II) ions in the low spin (LS) configuration (in agreement with the Mn-O and Mn-N bond distances; see above).When the sample is cooled from 300 to 2 K (blue Line 1 in Figure 7), χ m T smoothly decreases to reach a value of ca.3.6 cm 3 ¨K¨mol ´1 at 2 K.When the sample is heated to 400 K (red Line 2 in Figure 7), the χ m T value increases to ca. 7.0 cm 3 ¨K¨mol ´1.The second cooling scan from 400 to 2 K (light blue Line 3 in Figure 7) shows a smooth decrease of χ m T, although with values well above the observed ones in the heating scan.At ca. 70 K, both scans converge, and at lower temperatures (ca.5-10 K), χ m T reaches a plateau at a value of ca.4.05 cm 3 ¨K¨mol ´1, close to the expected value for an HS Mn(II) ion.Finally, at very low temperatures, χ m T shows a more abrupt decrease, suggesting the presence of a ZFS in the HS Mn(II) ion, reaching a value of ca.3.6 cm 3 ¨K¨mol ´1 at 2 K.The last heating scan (orange Line 4 in Figure 7) is identical to the first one.This behavior indicates that at 300 K, we have one Mn(II) with the HS configuration (whose χ m T contribution is 4.375 cm 3 ¨K¨mol ´1 for g = 2), whereas the second Mn(II) ion is mainly in the LS configuration, although there is a non-negligible fraction in the HS one.Since low spin S = 1/2 Mn(II) ions have a 2 T 2g ground level and present an orbital contribution, the expected χ m T value is ca.0.4-0.6 cm 3 ¨K¨mol ´1.With this value, we can estimate that at 300 K, Mn2 (in the cationic chain) is in the HS configuration, whereas Mn1 (in the anionic chain) presents a mixture with ca.75% in the LS configuration and ca.25% in the HS one.The increase in χ m T observed when heating the sample from 300-400 K has to be attributed to a spin crossover of the LS fraction.Although the increase of χ m T is very smooth, the inflexion point could be located at ca. 370 K. From the χ m T value at 400 K, we can estimate an HS fraction of ca.60%, i.e., from 300-400 K, the HS fraction has increased from ca. 25%-ca.60% (Figure 7).The continuous decrease of χ m T observed when cooling the sample may be attributed to: (i) the spin-orbit coupling in the LS Mn(II) fraction; (ii) to the ZFS present in the HS Mn(II) centers; and/or (iii) to a continuous and smooth SCO of the HS fraction present in the anionic chain.In fact, the thermal variation of the derivative of χ m T for the cooling scan from 400 to 2 K shows a rounded maximum at ca. 50 K, suggesting that the HS to LS SCO transition is quite smooth and takes place at around 50 K (inset in Figure 7).ca.3.6 cm 3 •K•mol −1 at 2 K.When the sample is heated to 400 K (red Line 2 in Figure 7), the χmT value increases to ca. 7.0 cm 3 •K•mol −1 .The second cooling scan from 400 to 2 K (light blue Line 3 in Figure 7) shows a smooth decrease of χmT, although with values well above the observed ones in the heating scan.At ca. 70 K, both scans converge, and at lower temperatures (ca.5-10 K), χmT reaches a plateau at a value of ca.4.05 cm 3 •K•mol −1 , close to the expected value for an HS Mn(II) ion.Finally, at very low temperatures, χmT shows a more abrupt decrease, suggesting the presence of a ZFS in the HS Mn(II) ion, reaching a value of ca.3.6 cm 3 •K•mol −1 at 2 K.The last heating scan (orange Line 4 in Figure 7) is identical to the first one.
This behavior indicates that at 300 K, we have one Mn(II) with the HS configuration (whose χmT contribution is 4.375 cm 3 •K•mol −1 for g = 2), whereas the second Mn(II) ion is mainly in the LS configuration, although there is a non-negligible fraction in the HS one.Since low spin S = 1/2 Mn(II) ions have a 2 T2g ground level and present an orbital contribution, the expected χmT value is ca.0.4-0.6 cm 3 •K•mol −1 .With this value, we can estimate that at 300 K, Mn2 (in the cationic chain) is in the HS configuration, whereas Mn1 (in the anionic chain) presents a mixture with ca.75% in the LS configuration and ca.25% in the HS one.The increase in χmT observed when heating the sample from 300-400 K has to be attributed to a spin crossover of the LS fraction.Although the increase of χmT is very smooth, the inflexion point could be located at ca. 370 K. From the χmT value at 400 K, we can estimate an HS fraction of ca.60%, i.e., from 300-400 K, the HS fraction has increased from ca. 25%-ca.60% (Figure 7).The continuous decrease of χmT observed when cooling the sample may be attributed to: (i) the spin-orbit coupling in the LS Mn(II) fraction; (ii) to the ZFS present in the HS Mn(II) centers; and/or (iii) to a continuous and smooth SCO of the HS fraction present in the anionic chain.In fact, the thermal variation of the derivative of χmT for the cooling scan from 400 to 2 K shows a rounded maximum at ca. 50 K, suggesting that the HS to LS SCO transition is quite smooth and takes place at around 50 K (inset in figure 7).The assignation of the LS centers to the anionic chain (where Mn1 is surrounded by four -CN groups from four tcnopr3OH − ligands) is straightforward, since the ligand field of these -CN groups is expected to be much larger than the one created by the four water molecules coordinated to Mn2 in the cationic chain.The presence of Mn(II) in the LS configuration is very unusual.In fact, there are very few LS Mn(II) complexes, and as far as we know, this is the first time that it has been observed in a chain.In these very few examples of LS Mn(II) complexes, the metal is surrounded by a N-rich chromophore with N atoms mainly arising from cyano or oxime groups.Examples of chromophores include N3O3 [33], N4S2 [34], N4O2 [35] and N6 [36][37][38][39], including six -CN groups [40,41].The assignation of the LS centers to the anionic chain (where Mn1 is surrounded by four -CN groups from four tcnopr3OH ´ligands) is straightforward, since the ligand field of these -CN groups is expected to be much larger than the one created by the four water molecules coordinated to Mn2 in the cationic chain.The presence of Mn(II) in the LS configuration is very unusual.In fact, there are very few LS Mn(II) complexes, and as far as we know, this is the first time that it has been observed in a chain.In these very few examples of LS Mn(II) complexes, the metal is surrounded by a N-rich chromophore with N atoms mainly arising from cyano or oxime groups.Examples of chromophores include N 3 O 3 [33], N 4 S 2 [34], N 4 O 2 [35] and N 6 [36][37][38][39], including six -CN groups [40,41].
Since the crystal field must be close to the thermal energy of the LS Mn(II) ions at room temperature, it is not surprising that heating the sample to 400 K leads to a partial spin crossover of the LS fraction, resulting in an increase of the HS fraction from ca. 25%-ca.60% between 300 and 400 K. Again, this behavior is very rare, since there are very few Mn(II) complexes showing SCO [42][43][44], and as far as we know, although partial, this is the first time it has been observed in a Mn(II) chain.
The presence of a smooth transition with a large hysteresis suggests that the SCO transition in 2 is a kinetic process.This kind of incomplete and smooth SCO transition is typical of chain compounds, since when the metallic centers are connected, the transition of one of them exerts a chemical pressure on the bridging ligands that hampers the transition of the neighboring metal centers [45][46][47].
The isothermal magnetization at 2 K shows a saturation value slightly below 5 µ B , the expected value for an HS Mn(II) ion, confirming the presence of one HS and one LS Mn(II) ion per formula unit at 2 K (Figure 6).In fact, the magnetization can be fit to a Brillouin function for an S = 5/2 ion, although with a reduced g value due to the presence of ZFS in the HS Mn(II) ion (solid line in Figure 6).Note that at very low temperatures, the contribution of the LS Mn(II) ions of the anionic chain must be almost negligible given the expected decrease of the magnetic moment due to the spin-orbit coupling.
Compound 3 shows a room temperature χ m T value of ca.3.2 cm 3 ¨K¨mol ´1 per Co(II) ion, a value within the normal range (2.8-3.4 cm 3 ¨K¨mol ´1) observed in octahedral monomeric isolated Co(II) complexes [22,[48][49][50][51][52][53] These values are higher than the spin-only contribution of an S = 3/2 ion (1.875 cm 3 ¨K¨mol ´1) due to the orbital contribution of the 4 T 1g ground state of octahedral high spin Co(II) ions.When the sample is cooled, χ m T shows a smooth decrease due to the first-order spin orbit coupling arising from the 4 T 1g ground state.Below ca. 5 K, the sample shows a more abrupt decrease due to a ZFS of the Co(II) ion (Figure 8).This behavior shows that the Co(II) ions are well isolated in Compound 3, in agreement with the structural data that show that the only bridge connecting the Co(II) is the long 4,4 1 -bpy.

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Since the crystal field must be close to the thermal energy of the LS Mn(II) ions at room temperature, it is not surprising that heating the sample to 400 K leads to a partial spin crossover of the LS fraction, resulting in an increase of the HS fraction from ca. 25%-ca.60% between 300 and 400 K. Again, this behavior is very rare, since there are very few Mn(II) complexes showing SCO [42][43][44], and as far as we know, although partial, this is the first time it has been observed in a Mn(II) chain.
The presence of a smooth transition with a large hysteresis suggests that the SCO transition in 2 is a kinetic process.This kind of incomplete and smooth SCO transition is typical of chain compounds, since when the metallic centers are connected, the transition of one of them exerts a chemical pressure on the bridging ligands that hampers the transition of the neighboring metal centers [45][46][47].
The isothermal magnetization at 2 K shows a saturation value slightly below 5 μB, the expected value for an HS Mn(II) ion, confirming the presence of one HS and one LS Mn(II) ion per formula unit at 2 K (Figure 6).In fact, the magnetization can be fit to a Brillouin function for an S = 5/2 ion, although with a reduced g value due to the presence of ZFS in the HS Mn(II) ion (solid line in Figure 6).Note that at very low temperatures, the contribution of the LS Mn(II) ions of the anionic chain must be almost negligible given the expected decrease of the magnetic moment due to the spin-orbit coupling.
Compound 3 shows a room temperature χmT value of ca.3.2 cm 3 •K•mol −1 per Co(II) ion, a value within the normal range (2.8-3.4 cm 3 •K•mol −1 ) observed in octahedral monomeric isolated Co(II) complexes [22,[48][49][50][51][52][53] These values are higher than the spin-only contribution of an S = 3/2 ion (1.875 cm 3 •K•mol −1 ) due to the orbital contribution of the 4 T1g ground state of octahedral high spin Co(II) ions.When the sample is cooled, χmT shows a smooth decrease due to the first-order spin orbit coupling arising from the 4 T1g ground state.Below ca. 5 K, the sample shows a more abrupt decrease due to a ZFS of the Co(II) ion (Figure 8).This behavior shows that the Co(II) ions are well isolated in Compound 3, in agreement with the structural data that show that the only bridge connecting the Co(II) is the long 4,4′-bpy.

Differential Scanning Calorimetry
In order to confirm the presence of a smooth partial SCO transition in the Mn(II) compound (3), we have performed differential scanning calorimetric measurements on Compound 3 (Figure 9).

Differential Scanning Calorimetry
In order to confirm the presence of a smooth partial SCO transition in the Mn(II) compound (3), we have performed differential scanning calorimetric measurements on Compound 3 (Figure 9).These measurements show a large exothermic peak in the heating scan at ca. 370 K and a smaller one at ca. 350 K corresponding to the smooth SCO transition of the Mn(II) ions of the anionic chain observed at the same temperatures in the magnetic measurements.In the cooling scan, the SCO transition is observed with a very large peak centered below 120 K, precluding a clear observation of the maximum.This large peak confirms the reversibility of the SCO and the large hysteresis observed in the magnetic measurements and suggests that the decrease observed in the magnetic moment on cooling the sample is due to a gradual and smooth SCO of the Mn(II) centers.Unfortunately, since the SCO transition is very smooth and is only partial, it is very hard to obtain reliable values of the transition temperatures in the hysteresis loop from DSC measurements.

Synthesis of Compound [Fe II (tcnopr3OH)2(H2O)2(4,4′-bpy)2] (1)
An aqueous solution (9 mL) of K[tcnopr3OH] (250 mg, 1 mmol) was added with stirring to an aqueous solution (2 mL) of FeCl2•4H2O (99 mg, 0.5 mmol).An ethanolic solution (15.6 mL) of 4,4′-bipyridine (78 mg, 0.5 mmol) was added with stirring to the resulting solution, and the final solution was filtered.Slow evaporation of the filtrate at room temperature afforded amber single crystals of 1 suitable for X-ray structure determination.IR data (n, cm −1 ): 3455m, 3320br, 2196s, 1610w, 1508w, 1485s, 1415m, 1347m, 1171m, 1044m.Anal.calcd.for C40H34FeN8O6 (778.These measurements show a large exothermic peak in the heating scan at ca. 370 K and a smaller one at ca. 350 K corresponding to the smooth SCO transition of the Mn(II) ions of the anionic chain observed at the same temperatures in the magnetic measurements.In the cooling scan, the SCO transition is observed with a very large peak centered below 120 K, precluding a clear observation of the maximum.This large peak confirms the reversibility of the SCO and the large hysteresis observed in the magnetic measurements and suggests that the decrease observed in the magnetic moment on cooling the sample is due to a gradual and smooth SCO of the Mn(II) centers.Unfortunately, since the SCO transition is very smooth and is only partial, it is very hard to obtain reliable values of the transition temperatures in the hysteresis loop from DSC measurements.

Synthesis of Compound
An aqueous solution (9 mL) of K[tcnopr3OH] (250 mg, 1 mmol) was added with stirring to an aqueous solution (2 mL) of FeCl 2 ¨4H 2 O (99 mg, 0.5 mmol).An ethanolic solution (15.6 mL) of 4,4 1 -bipyridine (78 mg, 0.5 mmol) was added with stirring to the resulting solution, and the final solution was filtered.Slow evaporation of the filtrate at room temperature afforded amber single crystals of 1 suitable for X-ray structure determination.IR data (n, cm

Physical Measurements
IR spectra (4000-400 cm ´1) were recorded with a Nexus Nicolet (Madison, WI, USA) FT-IR spectrophotometer in KBr pellets.DSC measurements were carried out on a Mettler Toledo (Leicester, UK) DSC 821e over the range 120-500 K with a scan rate of 5 K/min in N 2 .
The magnetic susceptibility measurements were carried out in the temperature range 2-300 K (2-400 K for 2) with an applied magnetic field of 0.1 T on polycrystalline samples of Compounds 1-3 (with masses of 37.64, 19.27 and 31.75 mg, respectively) with a Quantum Design (San Diego, CA, USA) MPMS-XL-5 SQUID susceptometer.The isothermal magnetizations were performed on the same samples at 2 K with magnetic fields up to 5 T. The susceptibility data were corrected for the sample holders previously measured using the same conditions and for the diamagnetic contributions of the salt as deduced by using Pascal's constant tables (χ dia = ´422.9ˆ10 ´6, ´734.5 ˆ10 ´6 and ´727.3 ˆ10 ´6 emu¨mol ´1 for 1-3, respectively) [54].

Crystallographic Data Collection and Refinement
Suitable single crystals of Compounds 1-3 were mounted on glass fibers using a viscous hydrocarbon oil to coat the crystals and then transferred directly to the cold nitrogen stream for data collection.X-ray data were collected at 170 K (for 1) and 120 K (for 2) on a Supernova diffractometer equipped with a graphite-monochromated Enhance (Mo) X-ray Source (λ = 0.71073 Å).For Compound 3, X-ray crystal data were collected at 170 K on an Oxford-Diffraction Xcalibur CCD diffractometer equipped with a graphite-monochromated (Mo) X-ray source (λ = 0.71073 Å).The program CrysAlisPro, Oxford Diffraction Ltd., was used for unit cell determinations and data reduction [55].Empirical absorption correction was performed using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm.The structures were solved by direct methods and successive Fourier difference syntheses and refined on F 2 by weighted anisotropic full-matrix least-squares methods [56].All non-hydrogen atoms were refined anisotropically, and all of the hydrogen atoms were located by a difference Fourier map and then refined isotropically for all compounds.Scattering factors and corrections for anomalous dispersion were taken from the International Tables for X-ray Crystallography [57].Data collection and data reduction were done with the CRYSALIS-CCD and CRYSALIS-RED programs [58,59] All of the other calculations were done with WinGX [60].Data collection and refinement parameters are given in Table 2. CCDC-1442430 (1), CCDC-1442431 (2) and CCDC-1442432 (3) contain the supplementary crystallographic data for this paper.These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Conclusions
The use of the polynitrile ligand tcnopr3OH ´with a bridging co-ligand as 4,4 1 -bpy has given rise to three novel compounds with Fe(II), Mn(II) and Co(II).The Fe(II) ion has yielded an unexpected monomer, where the Fe(II) ion is located on an inversion center and is surrounded by two trans tcnopr3OH ´ligands, two trans 4,4 1 -bpy terminal ligands and two trans water molecules.Interestingly, for Mn(II) and Co(II), the complexes formed present segregated cationic and anionic chains.In the cationic chains, formulated as [M(4,4 1 -bpy)(H 2 O) 4 ] 2+ , the M atoms are surrounded by two trans bridging 4,4 1 -bpy ligands and four water molecules, whereas in the anionic chains, formulated as [M(4,4 1 -bpy)(tcnopr3OH) 4 ] 2´, the M atoms are surrounded by two trans bridging 4,4 1 -bpy ligands and four tcnopr3OH ligands.The Mn(II) compound is very original, since it presents HS Mn(II) centers (in the cationic chain) coexisting with LS Mn(II) centers (ca.75% of the Mn(II) centers in the anionic chain).Furthermore, these LS Mn(II) centers show an even more unusual spin crossover (SCO) transition implying ca.45% of the LS centers from 300-400 K.As far as we know, this is the first Mn(II) chain compound presenting an LS configuration, the first example of the coexistence of HS and LS Mn(II) centers in the same compound and the first example of SCO in a Mn(II) chain compound.This surprising result has been confirmed by DSC measurements, which confirm the presence of the smooth SCO transition and its reversibility.

Figure 3 .
Figure 3. Structure of the anionic (top) and cationic (bottom) chains in Compound 3 showing the labelling scheme (same as in 2).

Figure 3 .
Figure 3. Structure of the anionic (top) and cationic (bottom) chains in Compound 3 showing the labelling scheme (same as in 2).

Figure 4 .
Figure 4. Perspective view of the crystal packing of Compound 3 showing the chessboard disposition of the anionic (in red) and cationic chains (in blue).

Figure 4 .
Figure 4. Perspective view of the crystal packing of Compound 3 showing the chessboard disposition of the anionic (in red) and cationic chains (in blue).

Figure 5 .
Figure 5. Thermal variation of χmT per Fe(II) ion for Compound 1.The solid line is the best fit to the model (see the text).

Figure 6 .
Figure 6.Isothermal magnetization at 2 K for Compounds 1-3 for one Fe(II) in 1 and two metal atoms in 2 and 3. Solid lines are the best fits to the corresponding Brillouin functions (see the text).

Figure 5 .
Figure 5. Thermal variation of χ m T per Fe(II) ion for Compound 1.The solid line is the best fit to the model (see the text).

Figure 5 .
Figure 5. Thermal variation of χmT per Fe(II) ion for Compound 1.The solid line is the best fit to the model (see the text).

Figure 6 .
Figure 6.Isothermal magnetization at 2 K for Compounds 1-3 for one Fe(II) in 1 and two metal atoms in 2 and 3. Solid lines are the best fits to the corresponding Brillouin functions (see the text).

Figure 6 .
Figure 6.Isothermal magnetization at 2 K for Compounds 1-3 for one Fe(II) in 1 and two metal atoms in 2 and 3. Solid lines are the best fits to the corresponding Brillouin functions (see the text).

Figure 7 .
Figure 7. Thermal variation of χmT per formula unit for Compound 2 in two consecutive cooling and warming scans.Numbers indicate the order of the cooling and warming scans.Inset: thermal variation of the derivative of χmT.The solid line is a 10% weighted fit.

Figure 7 .
Figure 7. Thermal variation of χ m T per formula unit for Compound 2 in two consecutive cooling and warming scans.Numbers indicate the order of the cooling and warming scans.Inset: thermal variation of the derivative of χ m T. The solid line is a 10% weighted fit.

Figure 8 .
Figure 8. Thermal variation of χ m T per Co(II) ion for Compound 3.

Figure 9 .
Figure 9. Differential scanning calorimetric measurements for Compound 3 in the heating (red) and cooling (blue) scans.