Elsevier

Polyhedron

Volume 76, 7 July 2014, Pages 36-44
Polyhedron

Magnetostructural effects of changing spin unit structure and molecular connectivity on 1H-benzimidazole functionalized radicals

Dedicated to George Christou in celebration of his 60th birthday
https://doi.org/10.1016/j.poly.2014.03.047Get rights and content

Abstract

A single hydrogen bonding molecular assembly unit (benzimidazole = BIm) was used for solid state assembly of different types of radicals connected at different sites on the assembly unit. The effects of changing radical spin unit and molecular connectivity were tested by comparing tetramethylnitronylnitroxide (NN), dimethyloxoverdazyl (mV), and diisopropyloxoverdazyl (iV) spin units attached to BIm at either the 2-position or 5/6-position. The mV systems were not isolated with sufficient purity and stability for crystallographic or magnetic analysis, but chain-type hydrogen bonding was found in the crystal structures of all of the other, stable compounds. Close contacts are induced between radical groups in some cases by the hydrogen bonded chains, giving exchange behavior ranging from essentially isolated spin paramagnetism (BIm2iV) to antiferromagnetic interactions of (−)1–3 K.

Graphical abstract

1H-Benzimidazole substituted with nitronylnitroxide (5-position, NN) or oxo-verdazyl (2- or 6-positions, iV) radicals form 1-D hydrogen bonded chains in crystalline solids; the NN variant incorporates H2O in the chains. The chain motifs bring the radical spin units into different degrees of proximity to give 1-D antiferromagnetic exchange interactions up to J/k  (−)2–3 K where 6-iV  5-NN > 2-iV in order of magnitude.

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Introduction

Hydrogen bonding is extremely important in strategies to engineering molecular crystal packing motifs with some degree of control. Use of hydrogen bonding substituents in crafting molecule based magnetic materials, for example, has yielded a variety different kinds of interactions between unpaired spins on organic radicals, and even given materials that show bulk magnetic ordering [1]. For example, Yoshioka et al. showed [2] that benzimidazole could be used to assembly nitronylnitroxide-incorporating BIm2NN radicals (Scheme 1) into a crystalline array that gives unusually strong ferromagnetically exchange coupled chain structures. Blundell and coworkers later showed [3] by μSR studies that BIm2NN undergoes bulk ordering at about 1 K. Murata et al. showed that the structurally related, but highly fluorinated F4BImNN system also gives strong 1-D ferromagnetic exchange coupling, with ordering at 0.7 K [4]. Lahti, Palacio, Sorai, and coworkers described magnetic and calorimetric studies showing that BABI forms hydrogen bonded chains in a crystal lattice that gives rise to bulk antiferromagnetic ordering at a relatively high (for second row element based radicals) temperature of 1.7 K [5]. A system analogous to BABI, but with a 1,4-phenylene unit interposed between the benzimidazole and the aminoxyl radical, has also been reported [6].

These results encouraged further exploration of the role of benzimidazole to assemble radicals in molecule based magnetic materials. Accordingly, we decided to test effects of changing the radical connection point from the azole to the phenylene ring of the benzimidazole, and of using a verdazyl radical spin unit. This article reports the synthesis of BIm5NN, BIm2iV, BIm6iV, BIm2mV, and BIm6mV, as well as the crystallography and magnetic behaviors of BIm5NN, BIm2iV, BIm6iV.

Section snippets

General

All melting points are uncorrected. 1H NMR spectra were obtained on a Bruker AVANCE-400 spectrometer. Single crystal X-ray diffraction analysis was performed at the University of Massachusetts Amherst (UMAss) X-ray Structural Characterization Facility. Mass spectral measurements were obtained at the UMass Mass Spectrometer Facility. Electron spin resonance (ESR) spectra were obtained using a Bruker Biospin Elexsys E-500 spectrometer at X-band frequencies (9.4–9.8 GHz) in toluene; samples were

Results and discussion

The synthesis, crystallography, and magnetic properties of BIm2NN have been previously reported [2], [15]. Comparisons to some of those results will be made below. To simplify discussion in this article, the informal, non-IUPAC terms “dimethyloxoverdazyl” (mV) and “diisopropyloxoverdazyl” (iV) will be used to refer collectively to the verdazyl-type radicals. Similarly, the non-IUPAC term “nitronylnitroxide” (NN) will be used to refer to the 4,4,5,5-tetramethyl-4,5-dihydro-1H

Conclusions

Benzimidazole tends to form 1-D hydrogen bonded chains with multiple radical units attached at different positions on this bicyclic ring system. Even BIm5NN – in which water intervenes as part of hydrogen bonding – adopts a 1-D crystal structure. Despite the constancy of this crystallization motif, attached radical units are not held in particularly close proximity, at least not enough to induce the strong inter-radical exchange effects seen in some benzimidazole-functionalized radicals. The

Acknowledgements

This work was supported in part by the National Science Foundation under Grants CHE 0415716 (PST, PML) and 0809791 (GS, PML). RAAC thanks CNPq (Brazil) for financial support.

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