First report on N,N¢-diisoalkylisonicotinamide 1D coordination networkcontaining linear trinuclear [Co3L4Cl6] units with mixedCoII(Td)–CoII(Oh)–CoII(Td) geometries: structure and magnetic properties†
Nuria Aliaga-Alcalde*d,e and Maninder Singh Hundal*b
Received 1st April 2010, Accepted 2nd June 2010First published as an Advance Article on the web 26th July 2010DOI: 10.1039/c0dt00245c
Reaction of anhydrous CoCl2 with N,N¢-diisopropylisonicotinamide (L) has yielded a coordinationpolymer containing linear trinuclear [Co3L4Cl6] units with a rare, mixed Co(Td)–Co(Oh)–Co(Td)assembly (compound 1). The central CoII ion, of each trinuclear entity, exhibits a distorted octahedralgeometry, with two ligand molecules coordinating through their carbonyl oxygen atoms along with twobridging Cl- ions and two pyridine N atoms from the neighboring molecules. Also, in each unit, twoouter CoII ions display distorted tetrahedral geometry, coordinating to one ligand molecule through thepyridine N atom and to three Cl- ions (one of them bridged to the central CoII and the two acting as aterminal ligands). The magnetic properties of this compound were investigated in the temperaturerange of 2.0 to 300.0 K. Owing to the complexity of the system and the weak interactions amongtrinuclear aggregates, the magnetic response has been analyzed using a model which considers theseunits as isolated systems. In addition, magnetic data has been examined in two separated blocks, aboveand below 50 K, applying programs VPMAG FORTRAN and MAGPACK-fit, respectively. This way,only the most significant effects at each interval of temperature were considered: spin–orbit coupling ofthe Co(Oh), at high temperatures and zero-field splitting parameters of the Co(Td) at the low. Spin–spinmagnetic interaction has been taken into account for the whole range of temperatures. As a result, theanalysis of the magnetic data shows that, within every trinuclear unit, the central position matches wellwith a high-spin CoII (S = 3/2) and also reveals weak ferromagnetic interactions between the Co(Oh)and the two terminal Co(Td) ions (J = +0.34 cm-1).
Introduction
Over a period of a few years, magnetic metal–organic frameworks(MOFs) have been included and well-described in the field ofmolecular magnetism although these systems are engaged as wellin other multidisciplinary fields as for example, supramolecularchemistry, molecular recognition and biology, among others.1 Theapproaches to attain magnetic MOFs vary depending on the goals,but in general all of them rely on the design of crystalline specieswith paramagnetic centers and are based on covalent bonds, inter-molecular hydrogen bonds or a combination of both.2–11 There areseveral reviews covering synthetic methodologies, structures andphysical properties of these materials, where synthetic design hasalways been the key to achieve functionality of the new species.1,12,13
aDepartment of Chemistry, Panjab University, Chandigarh, 160014, India.E-mail: [email protected] of Chemistry, Guru Nanak Dev University, Amritsar, 143005,India. E-mail: [email protected] of Chemistry, Indian Institute of Science Education andResearch, Mohali, Chandigarh, 160 019, IndiadDepartment de Quimica inorganica, Universitat de Barcelona (UB), Martii Franques 1-11, 08028, Barcelona, Spain. E-mail: [email protected] Junior Researcher at the UB (ICREA: Institucio Catalana deRecerca i Estudis Avancats), Spain. E-mail: [email protected]† Electronic supplementary information (ESI) available: Fig. S1. CCDCreference number 768478. For ESI and crystallographic data in CIF orother electronic format see DOI: 10.1039/c0dt00245c
Among all magnetic MOFs containing first row transition metalions, CoII frameworks are usually pursued due to the wide varietyof geometries that the use of this metal can permit.14 Besides, theyshow a wide range of colors which provide valuable informationon phases recognition and overall arrangements.15 Magnetically,there is extensive work performed with CoII ions, where dependingon structural parameters (e.g.: binding modes, ligand featuresetc.), can exhibit ferromagnetic, antiferromagnetic or even bothtypes of couplings in the same structure.16 In addition, someCoII species exhibit peculiar magnetic responses and form partof families as the SMMs and SCMs.16b,17 Systems containingsix-coordinated CoII ions display orbital angular momentum,a feature that challenges correlations and theoretical analyses,although, complicated CoII systems are nowadays suitable formagnetic analysis due to the assistance of specialized programsaffording accurate results.18,19
In the search of new cobalt(II) frameworks, the nature of theligands that complement the CoII ions should also be stressed.In this regard, it is well-known that pyridine based ligands,such as nicotinic and isonicotinic acids, and their correspondingamides have structural adaptability for both, the metal–ligandcoordination as well as the H-bonding, presenting themselves asappealing and versatile ligands from a crystal engineering pointof view.3–11 On this matter, we aimed to extend our previous stud-ies based on the coordination of N,N,N¢,N¢-tetraalkylpyridine-2,6-dicarboxamide derivatives20a–d with first row transition
metals to N,N¢-dialkylisonicotinamides ligands. In the past,some complexes containing nicotinamide (1(a), R = H) andN,N¢-dialkylnicotinamides (1(a), R = alkyl) as chelating ligands(coordination to the metal centers through the amide oxygen andthe pyridine nitrogen) have been already reported.9,10 The ligandisonicotinamide (1(b), R = H) has also been described and ithas been found to act as a bridging ligand leading to polymericstructures.10a,11 However, to the best of our knowledge, there hasnot been any structure of a coordination compound containingdialkylisonicotinamide (1(b), R = alkyl) prior to this work.
Here, we report the structure and magnetic propertiesof a new coordination polymer of cobalt(II) with N,N¢-diisopropylisonicotinamide, which is the first example of a coordi-nation compound involving such a ligand. Compound 1 has beencharacterized by single crystal X-ray and variable temperature(2.0 to 300.0 K) magnetic susceptibility together with additionalspectroscopic techniques. The crystallographic data shows theexistence of trinuclear units that assemble together forming 1Dsystems and finally extend into 2D networks through H-bondinginteractions. Most trinuclear CoII and CoII/CoIII systems known inthe literature present hexacoordinate high-spin Co centers.21 Thesecompounds have been classified in the past based on the nature ofthe bridging ligands: dipyridylamine,22 carboxylates23 and azide.24
Now, we present a fourth group by introducing trinuclear CoII
units connected by chloride ions. Moreover, with reference to thepeculiar Td–Oh–Td arrangement, this complex is the second inthe literature that displays this linear trinuclear moiety; a closelyrelated 2D framework [Co3(OH)2(pa)2(ina)2] synthesized by Zeng,Liang and coworkers was the only previously reported.13g Never-theless, magnetically these two compounds behave differently. Thereported compound exhibits antiferromagnetic behaviour whereascompound 1 shows weak ferromagnetic interactions within theCo(Oh) and Co(Td) centres (see below). These results have beenachieved from the study of the magnetization and molar magneticsusceptibility data of the compound using VPMAG FORTRAN19
and MAGPACK18 programs.
Experimental
All additions and manipulations were carried out under drynitrogen environment unless otherwise stated. All solvents weredistilled prior to use (benzene, toluene over P4O10 and acetonitrilefrom calcium hydride). Anhydrous CoCl2 was prepared by treatingpowdered sample of hydrated CoCl2 with thionyl chloride andrefluxing the contents for a few hours till evolution of SO2
ceased. The anhydrous product was washed with petroleum etherand dried under vacuo. Elemental analyses (C, H, N) wereperformed on a Perkin-Elmer model 2400 CHN analyzer. IRspectra were recorded as KBr pellets on a Perkin-Elmer RX-1FTIR spectrophotometer. Thermal analysis was carried out ona Shimadzu-DTG 60 analyser. 1H NMR spectra of ligandswas recorded on 300 MHz JEOL FT NMR spectrometer withTMS as the reference compound. UV-vis spectra were recordedon Shimadzu Pharmaspec UV-1700 UV-vis spectrophotometer.Molar Conductance of millimolar solution of the complex wasmeasured on a conductivity bridge-Digital Conductivity MeterCC 601. Magnetic susceptibility measurements between 2–300 Kand magnetization measurements, at 2 K, between 0–5 T, werecarried out in a SQUID magnetometer Quantum Design Magne-
tometer, model MPMP at the “Unitat de Mesures Magnetiques(Universitat de Barcelona)” on polycrystalline samples (30 mg).Two different magnetic fields were used for the susceptibilitymeasurements, 0.3 T (2–30 K) and 0.5 T (2–300 K), with superim-posable graphs. The diamagnetic corrections were evaluated fromPascal’s constants. R is the agreement factor defined as
∑[(cM)exptl
- (cM)calcd]2/∑
[(cM)exptl]2.
N,N¢-Diisopropylisonicotinamide (L)20e
Isonicotinic acid (0.15 mol) was reacted with thionyl chlorideto obtain isonicotinyl chloride hydrochloride. It was added toa mixture of about 100 mL toluene, 0.30 mol of pyridine anddiisopropylamine (0.46 mol). The mixture was refluxed for 15–20 min and was then kept at room temperature overnight. Thesolid obtained on cooling was removed by filtration and thefiltrate was treated with 50% aqueous NaOH solution and chilled.The toluene layer was separated and the alkaline solution wasextracted with ether. The combined toluene and ether layers weredried over sodium sulfate and excess solvent was removed bydistillation. The yellow colored distillate solidified on cooling. Thefinal product was obtained after recrystallization from n-heptanes.Color: yellowish brown. Anal. Found: C, 69.23; H, 8.94; N, 13.14.Calc. for C12H18N2O : C, 69.90; H, 8.74; N, 13.59%. M.p., 97 ◦C.IR: uCO 1660 cm-1. 1H NMR :d 8.580 (2H, dd, J = 6, 1.5 Hz, Py),7.130 (2H, dd, J = 6, 1.5 Hz, Py), 3.621, 3.471 (broad singlets, 1Heach, -CH); 1.465, 1.083 (broad singlets, 6H each, -CH3).
Compound, [Co3(L)4)Cl6]n (1)
This complex was prepared from a reaction mixture containingequimolar amounts of anhydrous CoCl2 and diisipropylisonicoti-namide dissolved in minimum amount of isopropylalcohol. Thecontents on keeping at room temperature for 2–3 days yielded ablue crystalline solid which was filtered and dried in vacuo. Anal.Found: C, 47.21; H, 6.13; N, 9.14. Calc. for C48H72Cl6Co3N8O4: C,47.46; H, 5.93; N, 9.22%. M.p.: >300 ◦C. Molar conductance (X-1
Intensity data were collected with a Siemens P4 single crystalX-ray diffractometer using a graphite monochromatized Mo-Ka(l = 0.71069 A). Table 1 shows the unit cell parameters anddata measurement details. The lattice parameters and standarddeviations were obtained by least squares fit to 40 reflections 20◦
< q < 25◦. The data were collected by the q–2q scan mode witha variable scan speed ranging from 2.0 to a maximum of 60◦
per minute. Three reflections were used to monitor the stabilityand orientation of the crystal and were measured after every 97reflections. Their intensities showed only statistical fluctuationsduring exposure time. The data were corrected for Lorentz andpolarization factors and an absorption correction was appliedusing psi scan. The structure was solved by direct methods usingSIR9725a and refined on F 2 using SHELX-97.25b All non-hydrogenatoms were refined using anisotropic thermal parameters. Thehydrogen atoms were included in the ideal positions with fixedisotropic U value and were riding on their respective non-hydrogenatoms. Experimental details of the X-ray analyses are provided
Table 1 Crystal data and structure refinement for 1
Empirical formula C48H72Cl6Co3N8O4
Formula weight 1214.63Temperature/K 295(2)Wavelength/A 0.71069Crystal system MonoclinicSpace group P21/ca/A 7.842(5)b/A 11.799(5)c/A 32.729(5)a/◦ 90.0b/◦ 90.930(5)g /◦ 90.0Volume/A3 3028(2)Z 2Calculated density/Mg m-3 1.332Absorption coefficient/mm-1 1.122F(000) 1262q range for data collection/◦ 1.24 to 25.51Limiting indices 0 ≤ h ≤ 9
0 ≤ k ≤ 14-39 ≤ l ≤ 39
Reflections collected/unique 6409/5442 [R(int) = 0.0899]Completeness to q = 25.51◦ 96.3%Refinement method Full-matrix least-squares on F 2
Data/restraints/parameters 5442/0/313Goodness-of-fit on F 2 0.869Final R indices [I > 2s(I)] R1 = 0.0818, wR2 = 0.2149R indices (all data) R1 = 0.2174, wR2 = 0.2876Largest diff. peak and hole 0.574 and -1.857 e. A-3
CCDC no. 768478
Table 2 Selected bond lengths (A) and angles (◦) for 1
Symmetry transformations used to generate equivalent atoms: #1 -x + 2,-y, -z + 1.
in Table 1 while selected bond distances and angles are listed inTable 2.
Results and discussion
N,N-Diisopropylisonicotinamide possesses two potential donorsites in the form of pyridine nitrogen atom and carbonyl oxygenatoms. As a monodentate ligand, the bonding may involveonly one of these donor atoms. From studies carried out onnicotinamide10a,11 it has been observed that coordination prefer-ably takes place through the pyridine nitrogen in preference tothe carbonyl oxygen. In the present work owing to the stericrequirement, the ligand acts in a bridging bidentate fashionbetween two separate metal atoms using both: the pyridinenitrogen and carbonyl oxygen atoms. The reaction of CoCl2
with N,N-diisopropylisonicotinamide yielded a polymeric solidcompound containing linear trinuclear CoII units, [Co3L4Cl6].The IR spectrum of compound 1 displays interesting features,
as for example the downward shift of uCO indicating coordinationthrough carboxamide O atom. Vibrations attributed to pyridine(C–C and C–N) ring also show positive shifts, implying theparticipation of both pyridine N and carboxamide O atoms inthe bonding.
Absorption spectral data
The absorption spectrum of the blue complex (1) in acetonitrilewas measured in the spectral range from 200 nm to 1100 nm. Itshows a multiple band (Fig. 1(a)) in the visible region. The threecomponents of which appear, in the decreasing order of energy,at lmax. 15 060, 15 898 and 16 778 cm-1 and have reasonably highintensities (emax l mol-1 cm-1) 712, 614 and 611, respectively. Thereis no other low energy band seen in the available range of theinstrument (~9090 cm-1). A very intense band is found on the highenergy side (Fig. 1(b)) at lmax cm-1(emax l mol-1 cm-1) 39 682 (3600).The multiple band in the visible region has been assigned as n3,4A2 → 4T1(P) transition and the latter as a p → 4t2, ligand tometal charge transfer band in the tetrahedral environment aroundCoII. The tetrahedral and pseudo-tetrahedral complexes of CoII
usually show a multiple absorption band26a,b having emax close to103 l mol-1 cm-1 in the visible region whereas in the octahedralcomplexes the corresponding n3 band, 4T1g(F) → 4T1g(P) usuallylies at ~ 20 000 cm-1 and has much lower intensity (5–20 l mol-1
cm-1). The latter also shows multiple structure but the lmax valuesof all the components are more than 17 000 cm-1. Similarly in thetetragonal complexes the splitting of the n3, 4T1g(F) → 4T1g(P) intotwo components 4T1g(F) → 4Eg and 4T1g(F) → 4A2g are observedat higher energy i.e. from 17 500 to 23 000 cm-1 in the complexeswith similar ligands.26c Therefore the multiple band (Fig. 1(a))in complex 1 is highly characteristic of a tetrahedral CoII ion26a
and offers an unambiguous distinction between octahedral andtetrahedral CoII complexes. The lmax. values found here fall inthe range observed in other chloride containing, mixed ligand
Fig. 1 Electronic spectra in acetonitrile of compound 1, showing (a) d–dabsorption band in the visible region (b) L–M charge transfer band.
tetrahedral complexes of CoII.26d,e The bandwidth is too largeto be caused by spin–orbit coupling and has been attributed tothe low symmetry field and vibrational fine structure26a in similarcomplexes. The high energy band has been assigned as a ligand tometal charge transfer band tentatively by comparison with datafrom other tetra-halide complexes26f–h CoII.
Thermal analysis data
The thermogravimetric analyses performed on 1 is shown inFig. 2. The percentage weight loss observed (% TGA) along withthe negative values of the first derivative are plotted as functions oftemperature between 100 and 800 ◦C. The graph clearly shows thethermal stability of compound 1 which begins losing weight onlyabove 275 ◦C. The high thermal stability is attributable to the 2D
Fig. 2 Thermogravimetric analysis (TGA) data of 1 as a function oftemperature between 100 and 800 ◦C. The negative of the first derivativeis also plotted as a function of temperature.
extended network and the polymeric nature of the compound. Thissystem undergoes weight loss in two distinct steps. First, it loses[Co2L2Cl4] between 281–392 ◦C (observed weight loss = 56.6%and expected weight loss = 55.4%) that matches well with two[CoLCl2] fragments, leaving the central chain, formed exclusivelyby the octahedral cobalt centers. This also may explain the highstability of the remaining product, [CoL2Cl2]n (up to 500 ◦C);beyond this temperature it experiences a slow weight loss up to720 ◦C which corresponds to 19.8%. This weight loss correspondsto half a ligand molecule (19.0%).
X-Ray crystal structure†
Single-crystal X-ray diffraction shows that compound 1 consistsof a 1D system constructed by linear trinuclear CoII ions subunits.Fig. 3 shows the coordination environments around the cobaltatoms (Co(1) and Co(2)) in the trinuclear moieties. The centralcobalt atom Co(1) lies on crystallographic inversion centre andcoordinates to two m-chloride ions (Co(1)–Cl(1) 2.732(3) A) andtwo carbonyl oxygen atoms (Co(1)–O(2) 2.278(7) A) providing adistorted square planar arrangement around Co(1). Two pyridinenitrogen atoms N(3) from adjacent symmetry related molecules(-1 + x, y, z) complete the final tetragonally distorted octahedralarrangement around Co(1) with Co(1)–N(3) distances of 2.528(2)A by occupying the trans positions. Each of the two terminalcobalt(II) ions, named Co(2), is tetrahedrally coordinated througha m-chloride ion (Co(2)–Cl(1) 2.268(4) A), two terminal chlorideions (Co(2)–Cl(2) 2.204(4) A and Co(2)–Cl(3) 2.217(3) A) and bythe pyridine nitrogen atom (Co(2)–N(1) 2.064 A) from the ligand.The central Co(1) is linked to the two terminal cobalt atoms byunsymmetrical m-chloride atom bridges (Co(1)–Cl(1) 2.732(3) Aand (Co(2)–Cl(1) 2.268(4) A) with Co(1) ◊ ◊ ◊ Co(2) distance being4.768(3) A and ∠ Co(1)–Cl(1)–Co(2) 143.86(2)◦, thus forming a
Fig. 3 Showing (a) the ORTEP view of the asymmetric unit of the complex (b) the coordination environment around the cobalt atoms in the trinuclearmoiety in the repeating unit of the polymer.
Fig. 4 Each polyhedron in a trinuclear unit shares a corner with its neighbour. The p ◊ ◊ ◊ p interactions between antiparallel pyridine rings facilitatingcoordination polymerization are also shown.
linear trinuclear unit with Td–Oh–Td mixed geometries with cornersharing polyhedra (Fig. 4).
These trinuclear subunits form 1D tapes running parallel to thea-axis (Fig. 4 and ESI, Fig. S1†) with the coordinating ligandacting in a bridging fashion using pyridine nitrogen and carbonyloxygen for coordination between two Co(1) centers within thepolymeric chain. Both the tetrahedra in a trinuclear subunit aswell as all the pyridine rings are trans with respect to each otherowing to the steric bulk on the fully substituted amide nitrogenatoms N(2) and N(4) of the coordinating ligands. This not onlyensures efficient packing of the molecules along the a-axis butalso gives rise to p ◊ ◊ ◊ p interactions between the two antiparallelpyridine rings (C13–C17, N3) having centroid to centroid distance3.612 A as shown in Fig. 4.
The crystal packing diagram (Fig. 5) shows that 1D chainsare held together by C10–H10 ◊ ◊ ◊ Cl3i (2.876(3) A, i = -x + 1, y- 1/2, -z + 1/2) hydrogen bonding interaction thus, extendingthe network to form a 2D sheet structure parallel to ac plane.These successive coordinating chains display a very peculiar trendin orientation with respect to each other. Every chain (with anorientation having antiparallel pyridine rings being eclipsed toeach other) is followed by a chain that has them in a staggeredconformation leading to an overall ABAB… type of chains relatedby the two fold screw axis in the sheet structure.
CoII Td–Oh–Td systems leading to 1D, 2D, or 3D networks arevery uncommon. Usually octahedral geometries are reported forall metal(II) centres in linear trinuclear arrangements, containing
mixed azide and carboxylate bridged complexes.27,28 Among these,cobalt complexes form linear trinuclear mode with Oh–Oh–Oh orOh–Td–Oh geometry, and the polyhedra are always interlinkedthrough sharing of corners or edges exhibiting chain or layeredstructures within the metal organic frameworks.28
Magnetic studies
In the present work, solid-state, variable-temperature (2.0–300.0 K) dc magnetic susceptibility data were collected onpolycrystalline samples of complex 1. The magnetic behavior ofthis compound is depicted in Fig. 6 as a plot of cMT vs. T .The cMT value at room temperature is 8.95 cm3 K mol-1. Uponcooling, the magnetic susceptibility data is roughly maintained,exhibiting a plateau that decreases smoothly until 50 K. Belowthis temperature, it drops abruptly to a value of 5.39 cm3 K mol-1
at 1.99 K. Magnetization measurements at 2 K and 5 T show aM/Nmb value of 7.81 (Fig. 6). The shape of the cMT vs. T plotwould be indicative of an antiferromagnetic coupling between theCoII ions, however, the high value of M/Nmb at 5 T could beexplained by a ferromagnetic interaction, between two terminalS = 3/2 ions and a central octahedral high-spin CoII; the lattermust have an expected M/Nmb value between 2–3, due to thespin–orbit coupling.29
To further analyze the data from a rigorous magnetic view point,this complicated structure could be simplified if one consideredthe trinuclear arrays as isolated systems and consequently the
Fig. 5 The two dimensional sheets formed by the collaborative effect of coordination bonding and intermolecular H-bonding. The arrangement ofcoordinated chains following the sequence ABAB… is also shown.
Fig. 6 Plot of cMT vs T for compound 1 from 300.0 to 2.0 K. Squaressymbolize the experimental data and the solid line the fitting of results.The inset graph shows the plot of M/Nmb vs H at 2 K for compound 1.
intermolecular exchange coupling among them may be neglected(Co(Oh) ◊ ◊ ◊ Co(Oh) magnetic interactions are indeed very weakand therefore have been ignored). The assumption of independentCo(Td)–Co(Oh)–Co(Td) units simplifies the description by limitingthe number of coupling constants to one (i.e. JCo–Co = J betweenCo(Oh) and the two Co(Td)). However, the analysis of the magneticdata still presents intrinsic difficulties due to different structuralfeatures on the CoII centers; basically, the nature of the Co(Oh orTd) should be carefully evaluated along with other factors suchas spin–orbital coupling and zero-field splitting. It needs to bestressed that some parameters will be more relevant than othersdepending on the temperature, and this forms the basis of ourpresent approach. For instance, Co(Td) ions show large g valuesand zero field splitting parameters (ZFS) being more significantat low temperatures. In addition, high-spin Co(Oh) ions exhibita ground state defined by a term 4T1g that splits into a sextet, aquartet and a Kramers’ doublet by spin–orbit coupling.30 At lowtemperatures only the Kramers’ doublet will be populated and itcould be considered as if it would have an effective spin S¢ = 1/2.However, in the high temperature range, Co(Oh) exhibits a groundstate of S = 3/2 and the Hamiltonian that describes the spin–orbitcoupling for this system is given by the following expression:
Hso = -AklLS
where l is the spin–orbit coupling constant, k is the reduction ofthe orbital angular momentum caused by the delocalization of theunpaired electrons and A is a parameter describing the degree ofmixing of the two 4T1g states arising from the ground 4F and excited4P terms of the d7 free ion. Following with the Hamiltonian, if thereis an axial distortion in the Co(Oh), the triplet orbital 4T1g groundstate would split into a singlet 4A2 and a doublet 4E levels with aD energy gap. The operator responsible for an axial distortion willbe expressed as:
Δ − +( )⎡
⎣⎢⎢
⎤
⎦⎥⎥
L L Lz2 1
31 (1)
The full Hamiltonian describing the magnetic properties foran isolated CoII ion, involving the spin–orbit coupling, axialdistortion and Zeeman interaction is given by
H A LS L L L A L g S Hz B e= − + Δ − +( )⎡
⎣⎢⎢
⎤
⎦⎥⎥+ − +( )kl m k2 1
31 (2)
In principle, no analytical expression for the magnetic sus-ceptibility as a function of A, k , l and D can be derived.However, a method to solve this problem has been developedusing the program VPMAG FORTRAN, where the values ofthese parameters can be determined through numerical matrixdiagonalization.19 This program could be a good approach forthe study of the cluster being considered in this work eventhough the octahedral environment of the central CoII metal ofcompound 1 shows a rhombic distortion. Nevertheless, these areapproximations that are performed to achieve a good fit of theexperimental data.
As it has been stated above, the theoretical studies of thistrinuclear system can be simplified depending on the temperature.Thus, some of the parameters under study will be affecting themagnetic behavior in a more significant way than others. At lowtemperatures, the effects of the zero-field splitting parameters ofthe Co(Td) [DCo(Td) and ECo(Td)] will become relevant and at hightemperatures, the spin–orbit effect characteristic of the Co(Oh)will dominate. Hence, the experimental data were divided in twoparts: above and below 50 K, and two independent models wereused to obtain the best fit. Fig. 7 shows a schematic representationof the real (Co(Td)–Co(Oh)–Co(Td)) system (top), and the modelsutilized in this analysis (bottom). The results of both fittings arerepresented together with the experimental values in Fig. 7.
The cMT vs. T data below 50 K were fitted using the programMAGPACK-fit (H = -2J(STd ·SOh
+ SOh·STd )).18 The g values
for both, Co(Oh) and Co(Td), were taken as isotropic, gCo(Td)
and gCo(Oh) and the spin ground state of Co(Oh) was consideredS¢ = 0.5. As a result, ZFS parameters at such temperatures mustderive from the Co(Td) ions (DCo(Td) and ECo(Td)). The valuesobtained from the theoretical correlation at low temperaturesare the following: JCo–Co = +0.34 cm-1, gCo(Td) = 2.92, gCo(Oh) =2.50, DCo(Td) = 11.41 cm-1 and ECo(Td) = 0.81 cm-1. Therefore, themagnetic interaction between the Co(Td) and Co(Oh) was foundto be weak and ferromagnetic, as expected from the magnetizationdata. Due to the alternating Td–Oh–Td geometries, the couplingin the trinuclear unit is quite unique and comparison with similarstructures is not feasible.31 Few compounds show m-Cl bridgesthrough Co ions but either there is a lack of information ontheir magnetic studies or coupled CoII ions present the samegeometry (e.g.: Oh–Oh/Td–Td).32 Nevertheless, the fitting of thedata provides gCo(Td), DCo(Td) and gCo(Oh) values that are in the rangefound for other complexes.33
On the other hand, the cMT vs. T data above 50 K was analyzedusing the program VPMAG FORTRAN,19 considering the spin–orbit coupling of the central Co(Oh) and the magnetic interactionJCo–Co between Co(Td)–Co(Oh) (JCo–Co and gCo(Td) obtained fromthe data at low temperatures). The parameters considered in thismodel are depicted in Fig. 7.
If one includes the spin–orbit coupling parameters into theHeissenberg Hamiltonian described in eqn (2), then the first termof this equation would be interpreted as the exchange couplingbetween two local spin moments, with L and S values of 1 and3/2, respectively and the constant JL–S will lead this magneticinteraction whose value will be given by Akl. In a similar way,the second term will represent the zero-field splitting of the triplet
Fig. 7 Schematic representation of the trinuclear system under study: CoII(Td)–CoII(Oh)–CoII(Td) and the considered models to fit the experimentalmagnetic data.
S(L) = 1, due to geometrical distortions. Finally, the Zeemanterm would be given in the last term, where two different g-factorscorresponding to the S(L) =1 and S = 3/2 involved will be takeninto account (g¢ = -Ak and ge = 2.00, respectively). Values fromthe fitting at low temperatures, gCo(Td) and JCo–Co, were taken asfixed in this fitting.
In this manner, above 50 K the best fit provided the followingparameters: Ak = 1.32, D = 250 cm-1 and JL–S = -182 cm-1,from where l was calculated with a final value of -138 cm-1
(JL–S = Akl). The limiting value of A in a weak crystal-field is1.5, while in strong crystal-fields is equal to 1. On the other hand,the reduction orbital k fluctuates between 1 and 0.6 depending oncovalency, from where it can be extrapolated that the Ak coefficientis always between 0.6 to 1.5. The resulting Ak parameter (= 1.32)is within this range, indicating weak crystal-field and reasonablecovalency due to the nature of ligands, L and Cl-, as well astheir relation with the CoII(Oh) ion. Related to this matter, l wasfound to be lower than the free ion value (-176 cm-1); decrease ofthis parameter is expected after coordination and depends on thecovalent character of the M–L bond.15 These results are similarto other Co systems described in the literature.34 The fitting valueobtained for D indicate small distortion on the octahedral CoII
geometry and is within the range found in previous works for a six-coordinated CoII complexes where |D|values have been reportedbetween 0 and 1000 cm-1.15
Conclusions
In this work, we have presented the structural and magneticproperties of the first polymeric cobalt(II) complex that containsN,N-diisopropylisonicotinamide as a ligand. This study hasshown that the fully substituted amide derivatives of pyridinecarboxylic acids provide the necessary rigidity, binding sites andintermolecular interactions to allow the creation of sequencesof arrangements that evolve into the final structure. This way,
compound 1 can be considered as a linear trinuclear com-plex containing two side CoII(Td) and a central CoII(Oh) ions(0D). These trinuclear moieties connect to others through N,N¢-diisopropylisonicotinamide ligands coordinated to the Co(Oh)centers forming chains (1D). Finally, intermolecular interactionsamong the polymeric structures made feasible the formationof more complex network (2D). Each of these levels containsremarkable features that have been discussed through the paper,making compound 1 unique from a structural point of view.
On the other hand, analysis of the magnetic data has concludedthat the exchange coupling between Co(Td) and Co(Oh) centersin each trinuclear unit is small and ferromagnetic. To attainthese values, the experimental data were divided in two blocks,above and below 50.0 K, taking into account the most relevantparameters in each range. This way, at the lowest temperatures,the exchange coupling constant and zero-field splitting parametersfor the Co(Td) were considered for the fit and the resulting valueswhere included in the correlation of the magnetic susceptibilitydata at higher temperatures to calculate the typical parameters fora Co(Oh) center. Both fits matched well with the experimental dataand provide a comprehensible explanation of the experimentalmagnetic behavior of the trinuclear unit.
Acknowledgements
MSH thanks UGC-SAP for providing research fellowship for AjayPal Singh Pannu. Dr Pratibha Kapoor thanks CSIR, New Delhifor financial support. The Spanish authors thank the Ministeriode Educacion y Ciencia (CTQ2009-07264/BQU and CTQ2009-06959/BQU), the Comissio Interdepartamental de Recerca iInnovacio Tecnologica de la Generalitat de Catalunya (CIRIT)(2009SGR1454) and ICREA (Institucio Catalana de Recerca iEstudis Avancats) for the financial support and especially toProfessor J. Ribas for the discussion of the magnetic properties.
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