Direct versus Mediated Through‐Space Magnetic Interactions: A First Principles, Bottom‐Up Reinvestigation of the Magnetism of the Pyridyl‐Verdazyl:Hydroquinone Molecular Co‐Crystal

Abstract

The mechanism of the magnetic interaction in the pyridyl-verdazyl radical:hydroquinone (pyvd:hq) molecular co-crystal is important as it has been suggested to originate by a unique "mediated through-space" magnetic interaction. This interaction was proposed to magnetically connect two nonadjacent pyridyl-verdazyl radicals within a pi stack, where adjacent radicals pile up in a head-over-tail orientation. The connection is made through a third radical sitting between the previous two mediated radicals. Given the relevance of this proposal, we decided to reinvestigate the magnetic properties of this co-crystal by using our recently proposed first-principles "bottom-up" procedure. Based on B3LYP/6-31+G(d) and CASSCF(6,6)/6-31+G(d) calculations (the results of which are identical to those provided by CASSCF(10,10)/6-31+G(d) calculations), we have computed the microscopic J(AB) values for all direct through-space magnetic interactions between nearby pyridyl-verdazyl radicals. The magnetic interactions give rise to two dominant values of similar strength: -56 and -54 cm(-1) at the B3LYP level, which are calculated as -38 and -31 cm(-1) at the CASSCF(6,6) and CAS(10,10) levels (all other interactions being smaller than |1| cm(-1)). The dominant interactions correspond to the direct through-space interaction between two adjacent radicals of a pi stack. The crystal also exhibits a radical-mediated through-space interaction of -0.31 cm(-1) between two nonadjacent radicals of a pi stack. The direct through-space magnetic interactions are two orders of magnitude larger than the mediated through-space interaction. Thus, first-principles calculations do not support a mediated through-space mechanism to explain the magnetism of the pyvd:hq co-crystal. The magnetic topology generated by the two dominant antiferromagnetic interactions in the pyvd:hq co-crystal consists of one-dimensional (1D) alternating chains (interacting very weakly along the b and c axes). By using this topology, the computed macroscopic magnetic susceptibility curve reproduces the experimental one properly. This first-principles bottom-up description of the magnetism in the pyvd:hq co-crystal differs in some fundamental aspects from that previously proposed in the literature.

Microscopic origins of the ferromagnetic exchange coupling in oxoverdazyl-based Cu(II) complex

“…8,13,14 The originality of such systems stems from the intrinsically molecular character of the magnetic orbitals, in contrast with standard paramagnetic metal ion-based systems. In this respect, thiazyl or verdazyl radical ͑vdz͒ containing ligands represent interesting targets toward strongly coupled discrete [15][16][17][18] and extended [16][17][18][19] molecule-based magnetic systems. [20][21][22] These systems are particularly stable as compared to other classes of organic radicals that exhibit a strong propensity to dimerization.…”

Beyond Kahn's Model: Substituent and Heteroatom Influence on Exchange Interaction in a Metal‐Verdazyl Complex

“…[8] The crystal packing of [Cu(hfac) 2 (Svdpy)] is rather complex (see Supporting Information). Given its crucial influence on intermolecular exchange interaction through π-π interactions, [11][12][13] its analysis deserves special attention. Such interactions appear in alternating staggered π-stacks along the a axis ( Figure 2).…”

“…Following the analysis of the crystal packing and the exponential decrease of the coupling with distances, we can estimate that the intermolecular exchange interaction J 2 (B) through the B-stacks ( Figure 2) is at most 30 % of that occurring through the A-stacks (Figure 2). [11][12][13] In consequence, a first approach consists in focusing on the main intermolecular interactions by neglecting J 2 (B). The system can then be considered as metal (S 1 = 1/2)-radical (S 2 = 1/2) coupled species (J 1 intramolecular exchange constant) further coupled through the A-stacking of the molecules (J 2 ).…”

“…Such interactions appear in alternating staggered π-stacks along the a axis ( Figure 2). To figure out the possible existence of through-space exchange interactions between the radicals, these π-π interactions are described using the parameters identified by Jornet et al: [12] for the A-type (B-type, respectively) stacking, the distance between the centroids of the verdazyl rings is 5.019(3) Å [5.268(3) Å, respectively] while for both types of stacking, the angle between the mean planes of the verdazyl rings is of 7.22(15)°. Because the stacking is more complicated than the one observed in the hydroquinone:oxoverdazyl adduct, [11] the closest contacts between atoms bearing a strong electronic density in the SOMO are also considered (see Supporting Information).…”

Verdazyls and Related Radicals Containing the Hydrazyl [R2N‐NR] Group

“…71 In contrast, calculations on 6-oxoverdazyl derivatives have been principally based on DFT or hybrid (HF/DFT) methods, 51,56,72 -75 and occasionally multiconfigurational methods (CASSCF) have been employed. 75,76 In general terms, the unpaired electron in all verdazyl types is part of a 7π -electron system delocalized over the N1-N2-C3-N4-C5 portion of the heterocycle; C6 is not formally conjugated because it is either a saturated carbon (in 6) or part of an exocyclic double bond (7, 8). The singly occupied molecular orbital (SOMO) in all cases is a π * orbital based on the four nitrogen atoms (Figure 7.4a shows the SOMO for a derivative of 6 with R = phenyl and R = R = isopropyl) 51 .…”