Structure and Magnetic Properties of a 1D Alternating Cu(II) Monomer―Paddlewheel Chain

One-dimensional metal–organic coordination polymers make up a class of compounds with potential towards the development of practical, new magnetic materials. Herein, a rare example of an ABBABB coupled linear chain comprised of alternating dicopper(II) tetraacetate units bridged to copper(II) acetate monomer units via axial η:η:μ2 coordinated acetate is reported. Examination of the structure, determined by small molecule X-ray crystallography, shows that each Cu(II) ion is in a dx2–y2 magnetic ground state. Magnetic susceptibility and magnetization data were collected and, consistent with the structural interpretation, demonstrate that the Cu(II) dimer (paddlewheel) exhibits classic antiferromagnetic exchange, while the S = 1/2 Cu(II) monomer is uncompensated in the ground state (low temperature regime.) Data were therefore fitted to a modified Bleaney-Bowers model, and results were consistent with the only other reported chain in this class for which magnetic data are available.


Introduction
Coordination polymers and metal-organic frameworks (MOFs) have been reported from a large variety of metal nodes and organic linker combinations, and range in size from micro [1,2]/nano [3] to mesoporous [4] materials. Leong and Vittal's review [5] on 1D coordination polymers illustrates many examples reported from polydentate ligands with suitably disposed donor atoms for bridging two metals, but that still leave the metal coordination sphere unsaturated. As a result, further bonding to an additional donor ligand can take place, and result in an infinite repetition of one simple metal-organic building unit. Another recent review includes synthetic strategies for achieving alternating 1D coordination polymers via block-co-polymerization [6]. Further structural complexity and dimensionality for simple chains can be achieved through a variety of supramolecular means, including H-bonding [7][8][9], halogen bonding [10], and π-stacking [11].
These coordination polymers can have a variety of applications, and our interest is in the development of new magnetic materials. One technique to achieve magnetically active 1D coordination polymers is to include a magnetic sub-unit as a building block in the chain [5]. One subunit, for which exists a large body of literature, is the dicopper(II) tetracarboxylate-paddlewheel. Substitution at the axial water sites with ligands that possess multiple donor sites is amenable to the formation of 1D chains, which has led to a variety of bridged paddlewheel motifs. Perec and colleagues [12] performed a survey of the Cambridge Structural Database, and in 2010 reported three different motifs by which Cu(II) paddlewheels bridge themselves (without the requirement for any additional co-ligands) to generate 1D chains: an in-plane flat ribbon motif, and less commonly, two motifs of alternating in-plane and perpendicular bridging to adjacent carboxylates. Alternatively, the water could be replaced by bridging heteroligands as reported, for example, for α-naphthoic acid [13] and 3-thiopheneacetate [14] paddlewheels with bridging 4,4 -bipyridine. These materials have a variety of uses, and one report has even demonstrated the potential use of pyridazine-bridged dicopper(II) tetracarboxylate 1D chains for gas adsorption [15].
Herein we report a new 1D coordination polymer consisting of alternating dicopper(II) tetraacetate units bridged to copper(II) acetate monomer units via axial η 2 :η 1 :µ 2 coordinated acetate (Figure 1). The structural and magnetic properties of this uncommon repeating ABBABB motif are discussed. motifs of alternating in-plane and perpendicular bridging to adjacent carboxylates. Alternatively, the water could be replaced by bridging heteroligands as reported, for example, for α-naphthoic acid [13] and 3-thiopheneacetate [14] paddlewheels with bridging 4,4′-bipyridine. These materials have a variety of uses, and one report has even demonstrated the potential use of pyridazine-bridged dicopper(II) tetracarboxylate 1D chains for gas adsorption [15]. Herein we report a new 1D coordination polymer consisting of alternating dicopper(II) tetraacetate units bridged to copper(II) acetate monomer units via axial η 2 :η 1 :µ2 coordinated acetate ( Figure 1). The structural and magnetic properties of this uncommon repeating ABBABB motif are discussed.

Physical Measurements
Microanalyses were carried out by Midwest Microlab, Inc., Indianapolis, IN, USA. Infrared spectra were collected on a Perkin Elmer Spectrum BX, FT-IR System (Norwalk, CT, USA). Samples were prepared as KBr disks. Variable temperature magnetic data were obtained using a Quantum Design MPMS-XL SQUID magnetometer (San Diego, CA, USA). Background corrections for the sample holder assembly, measured independently, and diamagnetic components of the complexes were applied. Susceptibility data were taken over the temperature range from 1.8 to 310 K in an applied field of 1 kOe. Magnetization data as a function of applied field were collected from 0 to 50 kOe at 1.8 K. Several data points were collected as the field was returned to zero; no hysteresis was observed. Magnetic data were fitted using MAGMUN4.1 software [16].

X-ray Crystallography
Data for a single crystal of 1, measuring 0.233 × 0.079 × 0.54 mm, were collected on a Bruker APEX-II CCD diffractometer (Madison, WI, USA). The crystal was kept at 110(2) K during data collection. SADABS-2014/5 [17] was used for absorption correction; wR2(int) was 0.0634 before and 0.0489 after correction. Using Olex2 [18], the structure was solved with the ShelXT [19] structure solution program using direct methods and refined with the ShelXL [20] refinement package using least squares minimization. H-atoms were introduced in calculated positions and refined on a riding

Physical Measurements
Microanalyses were carried out by Midwest Microlab, Inc., Indianapolis, IN, USA. Infrared spectra were collected on a Perkin Elmer Spectrum BX, FT-IR System (Norwalk, CT, USA). Samples were prepared as KBr disks. Variable temperature magnetic data were obtained using a Quantum Design MPMS-XL SQUID magnetometer (San Diego, CA, USA). Background corrections for the sample holder assembly, measured independently, and diamagnetic components of the complexes were applied. Susceptibility data were taken over the temperature range from 1.8 to 310 K in an applied field of 1 kOe. Magnetization data as a function of applied field were collected from 0 to 50 kOe at 1.8 K. Several data points were collected as the field was returned to zero; no hysteresis was observed. Magnetic data were fitted using MAGMUN4.1 software [16].

X-ray Crystallography
Data for a single crystal of 1, measuring 0.233 × 0.079 × 0.54 mm, were collected on a Bruker APEX-II CCD diffractometer (Madison, WI, USA). The crystal was kept at 110(2) K during data collection. SADABS-2014/5 [17] was used for absorption correction; wR 2 (int) was 0.0634 before and 0.0489 after correction. Using Olex2 [18], the structure was solved with the ShelXT [19] structure solution program using direct methods and refined with the ShelXL [20] refinement package using least squares minimization. H-atoms were introduced in calculated positions and refined on a riding model, except for H2, which was introduced in its difference map position, and refined positionally with a distance restraint. All non-hydrogen atoms were introduced in difference map positions and refined anisotropically.

Structural Description
1 crystallized in the monoclinic space group P2 1 /c. Cu1, and the symmetry related Cu1 i (i = 1−x, 1−y, 1−z) define a dicopper(II) tetracarboxylate unit (Figure 2.) Cu1 is in a square pyramidal geometry, with four short contacts to O1, O2 i , O3 and O4 i , and one long contact to O5 (Table 1), defining the magnetic ground state as d x 2y 2. Cu2 is in a distorted octahedral environment, with two bidentate acetate ions, and two neutral trans monodentate imidazoles in its coordination sphere. It exhibits four short contacts to ligands which define the equatorial plane (Cu2-O6, Cu2-N1, and those related by symmetry; Table 1), and longer axial bonds to O5 and O5 ii ((ii) 2−x,1−y,1−z) The acetate ligand containing O5 therefore bridges the Cu1 dimer and the Cu2 monomer in an η 2 :η 1 :µ 2 fashion (Figure 1), leading to an infinite one-dimensional chain parallel to the crystallographic a-axis ( Figure 3). Further, the geometry about Cu2, with four "short" and two "long" bonds also defines a d x 2y 2 magnetic ground state, and the two "long"-bridged bonds (Cu1-O5 and Cu2-O5), with a large bond angle of 141.98(6) • , suggests poor magnetic orbital overlap between the monomer and dimer units (vide infra.) IR spectra are consistent with the presence of both acetate and imidazole ligands, with characteristic peaks for acetate observed at 2931 cm −1 (alkyl C-H stretch) and 1637 cm −1 (C=O stretch; shifted from 1605 cm −1 in the IR of the starting material, copper(II) acetate hydrate), and for imidazole at 3121 cm −1 (=C-H stretch) and 1560 cm −1 (C=N stretch) (See Figure S1 in Supplementary Material).

Magnetic Studies
Magnetization as a function of field ( Figure S2), and magnetic susceptibility as a function of temperature for 1 were measured and show a ground state of just less than one unpaired electron (saturation expected to be~5700 emu/mol. It is possible that this may have been achieved if a field greater than 5 T had been available.) The magnetization data are consistent with a singlet ground state for the dicopper(II) tetracarboxylate moiety, with a residual spin from the Cu(II) monomer unit. A plot of χT vs. T ( Figure 5; triangles) reveals that the high temperature χT limit was not yet achieved at 310 K, consistent with the presence of very strong exchange within the dicopper(II) tetracarboxylate moiety, and an essentially independent single Cu(II) ion. A plateau just over 0.4 (one Cu(II)) can be seen at low temperatures for χT vs. T, and then at the lowest temperatures a further drop is observed, possibly due to very weak interchain exchange via hydrogen bonding. Finally, the effect of the 'uncoupled' Cu(II) is enough to swamp the dimer at low temperature, so there is no maximum seen in the plot of χ vs. T ( Figure 5 This alternating Cu(II)-dimer … Cu(II)-monomer carboxylate-bridged chain motif has been observed previously [21][22][23]. In the chain reported by Deka [21] and co-authors, acetate and imidazole are replaced by benzoate and 3,5-dimethylpyrazole, respectively. Therein, the Cu(II) monomer is very distorted from ideal octahedral geometry, owing at least in part to greater steric bulk from both ligands, and also from the ability of the protonated (uncoordinated) nitrogen of 3,5dimethylpyrazole, to exhibit interchain hydrogen bonding with a dimer-coordinated benzoate oxygen-atom. The Cu(II)dimer-Obenzoate-Cu(II)monomer bond lengths and angles were 2.1733(17) Å, 2.684(2) Å and 131.28(8)°. Sarma [22] reports a near identical motif to Deka, again with 3,5dimethylpyrazole, but with 4-methylbenzoate as the dimer and bridging monomer ligand. Cu(II)dimer-Obenzoate-Cu(II)monomer bond lengths and angles were 2.150(10) Å, 2.549(10) Å and 136.9(6)°. More recently, Zhang [23] has reported a structure with an acetate-based chain, however, in this case, each chain is bridged to two others via the axial monomer ligand, 2,6-di-imidazol-1-ylpyridine, which coordinates at both terminal imidazole nitrogen sites. Cu(II)dimer-Oacetate-Cu(II)monomer bond lengths and angles were

Magnetic Studies
Magnetization as a function of field ( Figure S2), and magnetic susceptibility as a function of temperature for 1 were measured and show a ground state of just less than one unpaired electron (saturation expected to be ~5700 emu/mol. It is possible that this may have been achieved if a field greater than 5 T had been available.) The magnetization data are consistent with a singlet ground state for the dicopper(II) tetracarboxylate moiety, with a residual spin from the Cu(II) monomer unit. A plot of χT vs. T ( Figure 5; triangles) reveals that the high temperature χT limit was not yet achieved at 310 K, consistent with the presence of very strong exchange within the dicopper(II) tetracarboxylate moiety, and an essentially independent single Cu(II) ion. A plateau just over 0.4 (one Cu(II)) can be seen at low temperatures for χT vs. T, and then at the lowest temperatures a further drop is observed, possibly due to very weak interchain exchange via hydrogen bonding. Finally, the effect of the 'uncoupled' Cu(II) is enough to swamp the dimer at low temperature, so there is no maximum seen in the plot of χ vs. T ( Figure 5, circles.)  The crystal structure indicates that 1 is a rare example of an ABBABB one-dimensional chain ( Figure 6), which could be treated with two exchange values; J d (exchange between the two copper(II) centers in the dimer; pathway A) and J c (exchange between the dimer units and the monomer unit; pathway B.) The crystal structure indicates that 1 is a rare example of an ABBABB one-dimensional chain ( Figure 6), which could be treated with two exchange values; Jd (exchange between the two copper(II) centers in the dimer; pathway A) and Jc (exchange between the dimer units and the monomer unit; pathway B.) Instead of fitting the data to a chain model, however, careful examination of 1 (vide supra) showed that each Cu(II) ion was in a dx2-y2 magnetic ground state, with poor magnetic orbital overlap between the dimer and monomer units. This suggests that Jc = 0, and that the magnetic coupling can be described by the spin Hamiltonian:

Conclusions
A new linear chain comprised of Cu(II) acetate dimer...Cu(II) acetate monomer units, assembled via the η 2 :η 1 :µ 2 bonding of two acetate ligands has been reported. The magnetic properties are consistent with the assignment of d x2-y2 magnetic ground states for each Cu(II) center. The Cu(II) dimer (paddlewheel) exhibits classic antiferromagnetic exchange, while the S = 1/2 Cu(II) monomer is uncompensated, and can be detected by both the remaining moment at low temperatures, as well as by the lack of a maximum in the magnetic susceptibility. Our future work in this area is focused on the possible applications of 1D chains in magnetic materials and for small molecule detection and uptake.