Double Pancake Bonds: Pushing the Limits of Strong π–π Stacking Interactions

Abstract

The concept of a double-bonded pancake bonding mechanism is introduced to explain the extremely short π–π stacking contacts in dimers of dithiatriazines. While ordinary single pancake bonds occur between radicals and already display significantly shorter interatomic distances in comparison to van der Waals (vdW) contacts, the double-bonded pancake dimer is based on diradicaloid or antiaromatic molecules and exhibits even shorter and stronger intermolecular bonds that breach into the range of extremely stretched single bonds in terms of bond distances and binding energies. These properties give rise to promising possibilities in the design of new materials with high electrical conductivity and for the field of spintronics. The analysis of the double pancake bond is based on cutting edge electron correlation theory combining multireference (nondynamical) effects and dispersion (dynamical) contributions in a balanced way providing accurate interaction energies and distributions of unpaired spins. It is also shown that the present examples do not stand isolated but that similar mechanisms operate in several analogous nonradical molecular systems to form double-bonded π-stacking pancake dimers. We report on the amazing properties of a new type of stacking interaction mechanism between π conjugated molecules in the form of a “double pancake bond” which breaks the record for short intermolecular distances and provides formidable strength for some π–π stacking interactions.

Radical–Radical Recognition: Switchable Magnetic Properties and Re-entrant Behavior

“…312 K (Figure 6). The χT value approaches 0.56 cm 3 Kmol -1 as the temperature approaches the sublimation temperature of 380 K. These observations are consistent with the observed structural phase change wherein the remaining "closed" [pyDTDA]2 dimers are ruptured ("opened") upon transitioning from Phase II to Phase IHT. At a scan rate of 0.6 K/min, no conclusive evidence for thermally induced hysteresis was observed upon cycling the temperature.…”

“…Below 80 K, the value of the χT product is very close to zero (0.036 cm 3 Kmol -1 ; Figure 6), suggesting that La(hfac)3(pyDTDA)2 is in a diamagnetic state, consistent with the formation of "closed" [pyDTDA]2 dimers throughout the supramolecular chains. The residual paramagnetism likely originates from defects in the crystal structure and is a common magnetic feature of such [DTDA]2 dimers, 48,49 and generally of materials with a singlet ground state.…”

“…K (see dχT/dT versus T in Figure 6) and, at 240 K, reaches a value of 0.33 cm 3 Kmol -1 that corresponds well to one S = ½ spin per molecule (C = 0.375 cm 3 Kmol -1 for g = 2). The magnetic properties and the sigmoidal-shape feature centered at 163 K are thus in agreement with the assessment of the crystallographic behavior, specifically that the phase transition…”

“…While the structural data shown in Figure 7 allow an estimation of u(T), the magnetic properties of La(hfac)3(pyDTDA)2 (i.e., χT vs. T data, Figure 6), give access to the temperature dependence of the energy gap, ∆(T). To employ the measured magnetic data, the non-zero value of the χT product below 75 K (χTLT = 0.036 cm 3 Kmol -1 ) must be taken into account. To simplify the subsequent calculation, the χT product has been normalized between 0 and 1 using the following expression, χnT = (χT-χTLT)/(2C½ -χTLT) with C½ = g 2 NµB 2 /4kB being the Curie constant for an S = ½ spin (with g = 2 expected for radicals, C½ = 0.375 cm 3 Kmol -1 ).…”

Halogen Bonding in the Assembly of Silver(I)-ethynide Complexes with 1-Iodo-3,5-bis(prop-2-ynyloxy)benzene

“…Hydrogen bonding [4][5][6][7][8][9] and pÁÁÁp interactions [10][11][12][13][14][15] are valuable in crystal engineering for the construction of supramolecular assemblies with organometallic architectures. In recent years, the interactions involving halogens such as halogen bonds [16][17][18][19][20] and metalÁÁÁhalogen [21][22][23][24][25][26][27] interactions have also been found to be important tools for assembling molecules in crystal lattices.…”