Abstract
Crystal structures are reported for bicyclic 3-CF3C6H4CN5S3 and monocyclic 3-CF3C6H4CN3S2, the latter of which is strongly dimerized in a cis-cofacial geometry [3-CF3C6H4CN3S2]2. The title compounds have previously been characterized in solution by NMR, displaying spectra that are consistent with the structure of [3-CF3C6H4CN3S2]2 in the crystal with anti-oriented CF3 substituents. The interannular binding was investigated using density functional theory (DFT) methods. However, the DFT-optimized geometry spreads the aryl rings too far apart (centroid–centroid distances of ≥4.353 Å versus experimental distance of 3.850 Å). Significant improvements are obtained with dispersion-corrected DFT functionals B3LYP-D3, B3LYP-D3BJ, M062X, and APFD using the 6-311+G(2d,p) basis set. However, all of these overbind the aryl rings with centroid–centroid distances of 3.612, 3.570, 3.526, and 3.511 Å, respectively. After selecting B3LYP-D3BJ/6-311+G(2d,p) as the best method, five alternative dimer geometries were tested, and all were found to be binding; however, anti cofacial-4 (matching the structure in the solid state) is the most stable. Computed energies of the remainder are as follows: +7.0 kJ mol–1 (syn-cofacial-5), +26.7 kJ mol–1 (anti-cofacial-64), +27.0 kJ mol–1 (syn-cofacial-150), +102.0 kJ mol–1 (S,S-antarafacial), and +103.7 kJ mol–1 (S,N-antarafacial), where the suffixes are torsional angles around the CN3S2 thiazyl ring centroids. The binding in the four most stable cofacial dimers may be described by “double pancake bonding”.
Highlights
Pancake bonding (PB), is an evocative name for two-electron/ multicenter (2e/mc) bonding between π-stacked dimers of organic and light-atom radicals. This terminology was apparently introduced quite casually by Mulliken and Person back in 1969,1 was revived in this century by Suzuki et al.,[2] and has since become popular.[3−23] Several reviews are available on the PB concept,[24,25] which should be distinguished from dispersion interactions in conventional “π-stacking” between diamagnetic aromatic rings, which is strictly due to London forces
Article corresponding thiazyl RCN5S3 cage compound 4 suspended in CCl4 is reacted with gaseous Cl2, which leads in high yield to the thermally unstable S,S-dichloride 5.38 Reduction of 5 is best undertaken in rigorously freeze−thaw (3×) degassed CHCl3 using the effective reducing agent Ph3Sb under conditions where the byproduct Ph3SbCl2 remains in solution
When essentially flat rings associate as π-dimers in such a way as to maximize diffuse π-orbital overlap, there is strong computational evidence for an orbital-overlap contribution to the bond energy, often of comparable magnitude to contributions from dispersion, electrostatics, or “charge flipping”. This concept is summarized in the term pancake bonding and, “double” pancake bonding in the case of DTTAs.[24,25,35]
Summary
Pancake bonding (PB), is an evocative name for two-electron/ multicenter (2e/mc) bonding between π-stacked dimers of organic and light-atom radicals. After selecting B3LYP-D3BJ/6-311+G(2d,p) as the best method, five alternative dimer geometries were tested, and all were found to be binding; anti cofacial-4 (matching the structure in the solid state) is the most stable.
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