Three different types of bridging ligands in a 3d–3d′–3d′′ heterotrimetallic chain

Maria-Gabriela Alexandru a, Diana Visinescu b, Sergiu Shova c, Marius Andruh *d, Francesc Lloret e, Joan Cano e and Miguel Julve *e
aDepartment of Inorganic Chemistry, Physical Chemistry and Electrochemistry, Faculty of Applied Chemistry and Materials Science, University Politehnica of Bucharest, 1-7 Gh. Polizu Street, 011061, Bucharest, Romania
bCoordination and Supramolecular Chemistry Laboratory, “Ilie Murgulescu” Institute of Physical Chemistry, Romanian Academy, Splaiul Independentei 202, Bucharest-060021, Romania
c“Petru Poni” Institute of Macromolecular Chemistry of the Romanian Academy, Aleea Grigore Ghica Vodă 41-A, RO-700487 Iasi, Romania
dInorganic Chemistry Laboratory, Faculty of Chemistry, University of Bucharest, Str. Dumbrava Rosie 23, 020464-Bucharest, Romania. E-mail: marius.andruh@dnt.ro
eDepartament de Química Inorgànica/Instituto de Ciencia Molecular (ICMol), Facultat de Química de la Universitat de València, C/Catedrático José Beltrán 2, 46980 Paterna, València, Spain. E-mail: miguel.julve@uv.es

Received 5th December 2017 , Accepted 14th December 2017

First published on 14th December 2017


A one-pot synthesis of a 3d–3d′–3d′′ heterotrimetallic coordination polymer with double diphenoxido, single cyanido and bis-bidentate oxalate as alternating bridges which exhibits an overall antiferromagnetic behaviour has been developed.


Examples of heterotrimetallic complexes are still rare.1–4 There are several polynuclear species made up through one-pot spontaneous self-assembly processes, some of them showing thermal bistability.1 However, most of them were built using a bottom-up rational strategy by means of cyanide-based metalloligands,2 and others from oxalato3 and binuclear 3d–3d′ metalloligands.4 Much attention have been paid to 3d–4f–3(4,5)d coordination polymers whereas, to the best of our knowledge, there are only four examples of heterotrimetallic complexes constructed from double phenoxido-bridged 3d–3d′ building blocks connected through cyanido spacers to the third paramagnetic metal ion: two chains of the formula [CuIILMnII(μ-NC)2MIII(bpb)]n [M = Fe, Cr; H2L is a macrocycle derived from 2,6-diformyl-4-methylphenol, ethylenediamine and diethylenetriamine; bpb = 1,2-bis(pyridine-2-carboxamido)benzenate],2c the neutral octanuclear square [{CuII(valpn)MnII(μ-NC)2CrIII(phen)(CN)2}2 {(μ-NC)CrIII(phen)(CN)3}2]·2CH3CN [H2valpn = 1,3-propanedyilbis(2-iminomethylene-6-methoxyphenol); phen = 1,10-phenanthroline], and the 2D coordination polymer, {[CuII(valpn)MnII(μ-NC)2CrIII(ampy)(CN)2]2·2CH3CN}n (ampy = 2-aminomethylpyridine).2g

Herein we propose a node and spacer rational approach to prepare heterotribridged heterotrimetallic coordination polymers where the cyanide-bearing chromium(III) unit is the spacer and the {CuII(H2O)(valpn)MnII(μ-ox)MnII(valpn)(H2O)CuII} and {CuII(H2O)(valpn)MnII} entities act as nodes. Because of the possibility of the manganese(II) ion to exhibit a coordination number greater than six, the oxalate anion was introduced to connect the {CuII(H2O)(valpn)MnII} moieties resulting into the chain formation. This anion has been used as a linker between the paramagnetic metal ions due to its ability to adopt a bis-bidentate ligand and also to mediate magnetic interactions between them.3,5,6 Additionally, the oxalate anion can act as a hydrogen bond acceptor favouring the formation of high-dimensional supramolecular networks.6 Finally, it deserves to be noted that L-ascorbic acid decomposition into oxalate ions is at the origin of a simple synthetic route towards heterobimetallic 3d–4f dodeca- and hexanuclear compounds.7

The new heterotrimetallic chain of the formula {[CuII(H2O)(valpn)MnII(μ-ox)MnII(valpn)(H2O)CuII]{[(μ-NC)3Cr(CN)3][CuII(H2O)(valpn)MnII]}2·3H2O}n (1) was obtained by mixing the [Cr(CN)6]3− metalloligand with the [CuII(H2O)(valpn)MnII]2+ complex cation [formed in situ by reacting [Cu(H2O)(vapn)]2+ and Mn(NO3)2] and oxalate anions in a 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 molar ratio, the solvent being a water/acetonitrile mixture (1[thin space (1/6-em)]:[thin space (1/6-em)]1 v/v). X-ray quality green prisms of 1 were grown after a week. Weak absorptions at 2154 and 2126 cm−1 are attributed to the stretching vibrations of the bridging and terminal cyanide ligands, respectively, whereas the bands at 1649s and 1620m cm−1 would correspond to the oxalate and imine groups (Fig. S1). The agreement between the powder X-ray diffraction of 1 and the simulated diffraction pattern confirms the purity of the bulk (see Fig. S3).

The structure of 1 consists of neutral heterotrimetallic chains and water molecules of crystallization. The chains comprise regular alternating single cyanide-bridged tetranuclear {CuII(H2O)(valpn)MnII(μ-ox)MnII(valpn)(H2O)CuII} nodes and hexanuclear {CuII(H2O)(valpn)MnII(μ-NC)3CrIII(CN)3}2 squares where the hexacyanidochromate(III) entity acts as a tris-monodentate ligand toward three manganese(II) ions through three mer-positioned cyanide groups (Fig. 1).


image file: c7dt04586g-f1.tif
Fig. 1 (Left) Asymmetric unit of 1. (Right) View of a fragment of the chain structure of 1. The water molecules of crystallization are omitted for clarity.

Each chromium(III) ion in 1 (Cr1) is six-coordinated in a slightly distorted octahedral surrounding [Cr–C bond lengths and Cr–C–N bond angles in the range 2.052(5)–2.0995(5) Å and 175.9(4)–177.5(5)°]. Two crystallographically independent manganese(II) ions occur in 1 which are six- (Mn1) and seven-coordinate (Mn2) with two phenoxido and two methoxo oxygen atoms from the valpn2− ligand (Mn1 and Mn2) plus two- (Mn1) or one cyanide–nitrogen (Mn2) and two oxalate-oxygen atoms (Mn2) building distorted pentagonal pyramidal (Mn1) and monocapped trigonal prismatic (Mn2) surroundings (Fig. S4 and Table S2). Also, two crystallographically independent copper(II) ions exist in 1 (Cu1 and Cu2). They are five-coordinate in a square pyramidal surrounding with two phenoxido-oxygen and two imino-nitrogen atoms from the Schiff-base in the basal plane and a water molecule in the apical position. The values of the trigonality parameter τ are 0.05 (Cu1) and 0.004 (Cu2) [τ is equal to 0 or 1 for ideal square pyramidal or trigonal bipyramidal geometries, respectively].8

The values of the distances at the manganese(II) atoms involving the bridging cyanido ligands are 2.218(5) (Mn1a–N5), 2.309(5) (Mn1–N9) and 2.309(5) Å [Mn2–N10] and those of the angles at the cyanide side are 166.8(4) (Mn1a–N5–C40), 163.6(4) (Mn1–N9–C44) and 169.7(4)° (Mn2–N10–C45) [symmetry code: (a) = 1 − x, −y, 2 − z]. The chromium⋯manganese distances through the cyanido bridges are 5.305(1) (Cr1⋯Mn1), 5.326(1) (Cr1⋯Mn1a) and 5.329(1) Å (Cr1⋯Mn2), values that are somewhat shorter than the manganese⋯manganese separation across the bis-bidentate oxalate [5.664(2) Å for Mn2⋯Mn2b; (b) = x, −1 − y, 1 − z]. The two crystallographically independent diphenoxido-bridged {CuII(valpn)MnII} fragments have a roof shape, the values of the dihedral angles at the O1/O2 and O5/O6 hinges (ϕ) are 6.10(15)° and 9.21(15)°, respectively. The copper⋯manganese separation within these units is 3.2782(9) (Cu1⋯Mn1) and 3.3072(9) Å (Cu2⋯Mn2) and the average values of the angle at the bridgehead phenoxido-oxygen atoms (γ) are 106.1 (O1/O2) and 105.9° (O5/O6).

The chains are interlinked by hydrogen bonds leading to a supramolecular 3D network (see Fig. S5). Two of the three non-coordinated water molecules (O3Wc and O5W) act as linkers of three adjacent chains being involved in a supramolecular pentagon with the O2Wb, O4W and N6 atoms [O5W⋯O3Wc = 2.823(6), O3Wc⋯O2Wb = 2.822(5), O2Wb⋯O4W = 2.769(5), O4W⋯N6 = 2.951(6) and O5W⋯N6 = 3.016(7) Å; (c) = x, −1 + y, z] (see Fig. S6). Remarkably, the oxalate-oxygens O9 and O10 are attached to an edge of the pentagon by hydrogen bonds [2.842(6) and 2.840(6) Å for O2Wb⋯O10 and O4W⋯O9, respectively].

The magnetic properties of 1 in the form of χMT vs. T plot (χM is the magnetic susceptibility per CuII4MnII4CrIII2 unit) are shown in Fig. 2. At room temperature, χMT is equal to 18.34 cm3 mol−1 K, a value that is somewhat smaller than the expected one for a set of four copper(II), four manganese(II) and two chromium(III) ions magnetically non-interacting (χMT = 22.75 cm3 mol−1 K with gCu = gMn = gCr = 2.0). Upon cooling, χMT continuously decreases to reach a value of 3.32 cm3 mol−1 K at 1.9 K. This shape is characteristic of an overall antiferromagnetic behaviour in 1.


image file: c7dt04586g-f2.tif
Fig. 2 χ M T against T plot (○) for 1 showing the simulated curves through the parameters indicated in the inset and text.

Five intrachain exchange pathways are involved in 1 (Scheme 1a): (i) the double phenoxido bridge in the Cu1⋯Mn1 and Cu2⋯Mn2 fragments (J1); the single cyanido bridges concerning the (ii) Cr1⋯Mn1 (J2), (ii) Cr1⋯Mn1a (J3) and (iv) Cr1⋯Mn2 (J4) units; and (v) the oxalate-bridged dimanganese(II) entity (J5). Although there is no theoretical law to simulate this complicated pattern of magnetic interactions in 1, tentative values can be assigned to the Ji parameters in light of previous magneto-structural studies. So, the strongest magnetic interaction would correspond to J1. Given that the values of −63.1 (106 and 13.28° for γ and ϕ) and −71.6 cm−1 (106 and 4.11° for γ and ϕ) were reported for the exchange coupling of the diphenoxido-bridged CuIIMnII unit in the compounds [{CuII(valpn)MnII(μ-NC)2CrIII(phen)(CN)2}2{(μ-NC)-CrIII(phen)(CN)3}2]·2CH3CN2g and {[CuII(H2O)(valpn)MnII(μ-ox)]·3H2O}n[thin space (1/6-em)]9, respectively, they would define the lower and upper limits for J1 in 1 (mean values of 106 and 7.7° for γ and ϕ). Dealing with the (ii)–(iv) exchange pathways, the magneto-structural correlation established between the value of the Mn–N–C(cyanide) angle (α) and the magnetic coupling (J in cm−1) [JCuMn = −40 + 0.2α]10 leads to the calculated values of −7.28, −6.64 and −6.06 cm−1 for Mn1–N9–C44 (J2), Mn1a–N5–C40 (J3) and Mn2–N10–C45 (J4), respectively. Finally, values between −0.8 and −3.0 cm−1 were reported for J5.11 A uniform chain of S = 5 spins which interact with AF through the bridging oxalate (J5) would result from the coupling pattern (Scheme 1b). This is in agreement with the magnetization (M) versus H plot of 1 at 2.0 K where M at 5 T tends to a value above 8 BM (Fig. S7). In summary J2, J3 and J4 can be considered equivalent (JCrMn) and J1 and J5 are now noted as JCuMn and JMnMn, respectively.


image file: c7dt04586g-s1.tif
Scheme 1 (a) Exchange pathways and the (b) resulting uniform chain of interacting spin undecets in 1.

Having in mind these conclusions, a theoretical simulation of the magnetic data can be carried out through the effective Hamiltonian methodology.12 In such a context a fragment of the 1D system, including the stronger magnetic coupling (Cu2Mn2Cr1Mna1aCu1aMn1Cu1Cr1aMn2aCu2a unit), is solved exactly. The parameters used for that were JCuMn = −60 cm−1, JCrMn = −7.0 cm−1, gCu = 2.05 and gMn = gCr = 2.0 (see above). Each unit can be magnetically visualized as an effective spin (Seff) and an effective g-factor (geff) which are temperature-dependent and are obtained from the energies and wavefunctions of the real spin states. These units are organized in a chain interacting with the adjacent ones through a temperature-dependent effective magnetic coupling (Jeff) related to the real JMnMn constant. A classical spin approach is useful to magnetically couple these units because of the high Seff and weak Jeff values. Theoretical simulations for different values of JMnMn are shown in Fig. 2. A reasonable match of the magnetic data of 1 was obtained (black solid line in Fig. 2) with a JMnMn = −2.0 cm−1, a value which is within the expected range (see above).

In summary, the present synthetic route afforded a {CuIIMnIICrIII} heterotrispin chain containing three different bridging ligands which mediate antiferromagnetic interactions between the metal ions: diphenoxido- (CuIIMnII), single cyanido- (CrIIIMnII), and oxalato- (MnIIMnII) spacers. Its knowledge opens a route to the rational preparation of other 3d–3d–3(4,5)d or 3d–4f–3(4,5)d compounds in the future.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Thanks are due to the CNCS-UEFISCDI (Projects PN-II-RU-TE-2014-4-1556 and PN-II-ID-PCCE-2011-2-0050), the Spanish MICINN (Projects CTQ2016-75068P, CTQ2016-75671P and Unidad de Excelencia María de Maetzu MD2015-0538) and the Generalitat Valenciana (PROMETEOII/2014/070) for financial support.

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Footnote

Electronic supplementary information (ESI) available: FTIR spectrum, Fig. S1; UV-Vis spectrum, Fig. S2; PXRD pattern, Fig. S3; coordination geometry for Mn(II) ions, Fig. S4, packing drawings, Fig. S5 and S6, M vs. H at 2.0 K, Fig. S7; crystallographic data, SHAPE analysis and H-bond parameters, Tables S1–S3. CCDC 1582340. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7dt04586g

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