Ferro- or antiferromagnetism? Heisenberg chains in the crystal structures of verdazyl radicals

Abstract

In this study, we address the question of the origin of ferromagnetic or antiferromagnetic interactions in alkynyl-substituted 1,5-diphenyl-6-oxo verdazyl radicals. While a TMS-alkynyl derivative (3) shows antiferromagnetic ordering at low temperatures, the corresponding deprotected alkynyl verdazyl (4) shows ferromagnetic interactions. For both compounds, magnetic Heisenberg chains are characteristic, which were studied systematically by means of X-ray crystallography and quantum chemical calculations. Ferromagnetic interactions are rarely found in such radicals. Therefore, uncovering such structure-property relationships is of crucial importance in order to understand and design promising ferromagnetic networks. Using this knowledge, we were able to design and crystallize diyne derivatives showing comparable solid state characteristics and therefore antiferro- and ferromagnetic Heisenberg chain structures. We show that the understanding of such property-structure relationships is adequate for the design of organic-magnetic materials with defined cooperative effects within the class of verdazyl radicals.

Halogen Bonding as a Supramolecular Modulator of Crystal Packing and Exchange Interactions in Nitronyl Nitroxides

“…An experimental study on such weak exchange interactions requires magnetometry at extremely low temperatures (<2 K), which is unjustifiably challenging. Instead, it seems reasonable to evaluate the efficiency of channels of exchange interactions by methods of quantum chemistry that have proven themselves well in research on solid phases of organic paramagnets. − For example, in 1 , the shortest distance between the nitroxide O atoms ( r O···O ) is 3.724 Å, and according to quantum chemical calculations, this state of affairs leads to the exchange interaction with J 1 = −13.4 cm –1 . There is another exchange channel with an antiparallel arrangement of radicals under each other and a moderate interaction with J 2 = −11.5 cm –1 , although the r O···O is much longer (5.889 Å).…”

Pragmatic Improvement of Magnetic Exchange Couplings from Subsystem Density-Functional Theory through Orthogonalization of Subsystem Orbitals

“…Efficient approximations for sDFT exchange-coupling calculations are particularly interesting for applications requiring a large number of couplings to be computed. One example is “heat maps” representing scans of magnetic couplings along different geometric variables, as performed in ref for a verdazyl radical system. Here, we performed a similar study in which we shifted one monomer of 1 vertically and horizontally with respect to a second one (see also Figure ).…”

“…Furthermore, the error in the spin density introduced by the potential reconstruction or Loẅdin orthogonalization is listed. Leaving aside the cases with small exchange couplings (3,6), we observe an increase of around 50% for the absolute value of the sDFT magnetic exchange coupling constant when the basis-set is changed from def2-SVP to def2-TZVP and Loẅdin orthogonalized orbitals or the NAKE functional is used. A notable exception is dimer 2, where this increase is even more pronounced.…”

“…Magnetic materials consisting of organic radical crystals have received increasing attention over the past 1 to 2 decades. − These materials are very interesting because desired physical properties may be tuned by structural modifications. One interesting application is to use these radicals as molecular-scale switches, where the magnetic behavior can be controlled by an external influence like temperature or light.…”

Self-assembly of iodoacetylenyl-substituted nitronyl nitroxides via halogen bonding

“…[23][24][25][26][27][28] It is commonly recognized that NCIs affect the magnetic properties of the materials in which seemingly insignificant crystal packing forces may change the strength and type of spin-spin interactions or even bulk magnetic properties. 26,[29][30][31] Among various classes of organic radicals, well-developed synthetic methods and easy crystallization of nitronyl nitroxide radicals make them some of the most attractive building blocks for targeted design of organic paramagnets. 23,32,33 Analysis of reported nitronyl nitroxide X-ray diffraction (XRD) data indicates the presence of C-H Me ⋯O NO hydrogen bonds in the predominant fraction of the studied crystals.…”