Interactions of Planar Organic Radicals: Stacking and Bonding

Examination of the stacking interactions and bonding in planar organic radicals reveals a variety of non-covalent and weak covalent interactions such as hydrogen bonding, halogen bonding, and pancake bonding. This study highlights the significance of multicentric two-electron bonding and explores the in-depth analysis of pancake bonding through X-ray charge density, variable-temperature, and high-pressure crystallography methods.

• Planar Organics
• Stacking Interactions
• Bonding Analysis
• Non-covalent
• Pancake Bonding

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1. Stacking of planar organic radicals: strong non-covalent or weak covalent interactions? Kre imir Mol anov,1Valentina Mila inovi ,1Nikita Bogdanov,2 Elena V. Boldyreva2,3 1Ru er Bo kovi Institute, Bijeni ka 54, 10000 Zagreb, Croatia 2Novosibirsk State University, 630090 Novosibirsk, Russia 3Boreskov Institute of Catalysis, 5 Ac. Lavrentieva avenue, 630090 Novosibirsk, Russia

2. weak < 5 dispersion medium 5 - 15 electrostatic strong 15 covalent E / kcal mol-1 Dominant interaction Hydrogen bonding C-H O, C-H S, C-H Cl O-H O, N-H N, O-H N RAHB, CAHB, LBHB, HF FH, OH2 H+ H2O (Zundel ion) Halogen bonding C-Cl O N-Br N, N-I N N-bs lut, halonium ions -stacking aromatics, heteroaromatics ... quinones, metal- chelates planar radicals: dimers, trimers, equidistant stacks IUCrJ, 2019, 6, 156-166. Angew. Chem., Int. Ed., DOI: 10.1002/anie.201908875 Cryst. Growth Des., DOI: 10.1021/acs.cgd.9b00540

3. Multicentric two-electron bonding: tetracyanoethylene, the prototype Miller & Novoa, Acc. Chem. Res., 2007, 40, 189-196. Cui, Lischka, Mueller, Plasser, Kertesz, ChemPhysChem, 2014, 15, 165-176. IUCrJ, 2019, 6, 156-166.

4. Pancake bonding: multicentric two-electron bonding between planar radicals R. S. Mulliken, 1960s pancake bonds -bonds IUCrJ, 2019, 6, 156-166. Cryst. Growth Des., DOI: 10.1021/acs.cgd.9b00540

5. In-depth study of pancake bonding 1. X-ray charge density/quantum crystallography 2. Variable-temperature crystallography (80 400 K) 3. High-pressure crystallography

6. Pancake bonding: three types of stacks equidistant radicals trimers dimers 2.84 2.86 2.84 3.17 3.60 3.59 B A IUCrJ, 2019, 6, 156-166. Cryst. Growth Des., 2016, 16, 4777-4782. Chem. Eur. J., 2018, 24, 8292-8297.

7. Pancake bonded dimers: Anything that can go wrong will go wrong, and at the worst possible time. Edward A. Murphy

8. Pancake bonded dimers: N-MePy Cl4Q (triclinic) N-Me-4-CNPy DDQ CrystEngComm, 2018, 20, 1862-1873. Cryst. Growth Des., 2016, 16, 4777-4782.

9. Pancake bonded dimers: N-MePy Cl4Q N-Me-4-CNPy DDQ 2.92 2.86 3.49 3.60 2.92 2.86 CrystEngComm, 2018, 20, 1862-1873. Cryst. Growth Des., 2016, 16, 4777-4782.

10. Pancake bonded dimers (N-MePy Cl4Q): critical points max> 0.095 e -3 2.86 max< 0.040 e -3 3.60 (3, -1) bond (3, +1) ring cenroid (3, +3) cage centroid Cryst. Growth Des., 2019, 19, 391-402.

11. Pancake bonded dimers (N-MePy Cl4Q): HOMO orbitals Intermolecular bond order all atoms 0.80 only C atoms 0.27 inter-dimer 0.04 Covalent contribution: 9.4 kcal mol-1 Cryst. Growth Des., 2019, 19, 391-402.

12. Pancake bonded dimers (N-Me-4-CNPy DDQ): variable temperature 3.6 3.5 Interplanar distance / 3.4 3.3 Linear expansion coefficient: Cg plane(Cg2) / 3.2 ?? ? 10-4 K-1 ? = ?? ?? 3.1 3 2.9 2.8 2.7 90 120 150 180 210 T / K 240 270 300 330 360 T / K

13. Pancake bonded dimers (N-Me-4-CNPy DDQ): high pressure 3.6 3.49 3.5 Interplanar distance / 3.4 3.3 3.2 3.1 3.00 2.93 3 2.9 2.96 2.92 2.8 2.7 2.75 2.6 2.5 0.0 1.0 2.0 3.0 4.0 5.0 6.0 p / GPa

14. Pancake bonded dimers (N-Me-4-CNPy DDQ): phase transition? 0 GPa 6 GPa 0 0 2.93 b b 2.92 c c 2.75 3.49 a a 2.93 2.92

15. Pancake bonded trimers: [N-Me-4-(Me)2NPy]2(Cl4Q)3 0,58 Charges from Pval: +0.96 0,76 1.92 0,58 +0.96 2 1+ 1+ Chem. Eur. J., 2018, 24, 8292-8297.

16. Pancake bonded trimers: critical points max> 0.077 e -3 2.84 3.59 max< 0.045 e -3 (3, -1) bond (3, +1) ring cenroid (3, +3) cage centroid Chem. Eur. J., 2018, 24, 8292-8297.

17. Pancake bonded trimers: HOMO orbitals Intermolecular bond order all atoms ~ 0.5 Covalent contribution: 4.11 kcal mol-1 Chem. Eur. J., 2018, 24, 8292-8297.

18. Pancake bonded trimers: variable temperature 3.7 Interplanar distance / 3.5 Linear expansion coefficients: ?? ? 2.4 10-4 K-1 ? = 3.3 ?? ?? 7.7 10-5 K-1 3.1 2.9 2.7 100 120 140 160 180 200 220 240 260 280 300 T / K

19. Pancake bonded trimers: high pressure 3.8 3.59 Interplanar distance / 3.6 3.4 3.2 3.09 3 2.84 2.8 2.84 2.70 2.6 0 1 2 3 4 5 6 p / GPa

20. Pancake bonded trimers: pressure- induced phase transition? 0 GPa 5.19 GPa

21. Pancake bonded trimers: pressure- induced phase transition? 0 GPa 5.19 GPa 2.84 2.70 2.84 2.84 3.59 B 3.09 A 2.70 2.84

22. Pancake bonding in stacks of equidistant radicals N-MePy Cl4Q (orthorhombic) N-MePy Br4Q Cryst. Growth Des., 2016, 16, 4777-4782.

23. Pancake bonding in stacks of equidistant radicals N-MePy Cl4Q (orthorhombic) N-MePy Br4Q 3.28 3.17 3.28 3.17 3.28 3.17 Cryst. Growth Des., 2016, 16, 4777-4782.

24. Pancake bonding in stacks of equidistant radicals (N-MePy Cl4Q): critical points max< 0.050 e -3 3.17 (3, -1) bond (3, +1) ring cenroid (3, +3) cage centroid Cryst. Growth Des., 2019, 19, 391-402.

25. Pancake bonding in stacks of equidistant radicals (N-MePy Cl4Q): HOMO orbitals Intermolecular bond order all atoms 0.26 only C atoms 0.09 Covalent contribution: 2.9 kcal mol-1 Cryst. Growth Des., 2019, 19, 391-402.

26. Pancake bonding in stacks of equidistant radicals (N-MePy Br4Q): variable temperature 3.48 Interplanar distance / 3.44 3.4 3.36 Linear expansion coefficient: 3.32 ?? ? 1.9 10-4 K-1 ? = ?? ?? 3.28 100 150 200 250 300 350 400 T / K

27. Conclusions Linear expansion coefficients for pancake bonds and weak stacking contacts are ca. 10-5 10-4 K-1 Phase transitions at high pressure The shortest pancake bonds at high pressure: 2.07 (trimers) and 2.75 (dimers) The long contact between dimers: 2.93 At high pressure: pancake-bonded polymers Possible long-range ordering and conductivity In progress: high-pressure conductivity and magnetism

28. CAD4 The 80 s never ended.

29. Acknowledgements RBI, Zagreb Marijana Juri Lidija Andro Dubraja Nadica Maltar-Strme ki Ana anti Dijana ili Darko Babi Valentina Mila inovi KI, Ljubljana Jernej Stare Gregor Mali Bruno Landeros Jes s Hern ndez-Trujillo University of Zagreb Vladimir Stilinovi Damir Paji Nikolina Novosel Kre o Zadro Universit t G ttingen Dietmar Stalke Serhiy Demeshko Universit de Lorraine Christian Jelsch Claude Lecomte Emmanuel Wenger UNAM, Mexico City University of Ljubljana Anton Meden Georgetown Univ., USA Miklosz Kertesz Zhongyu Mou Rigaku Oxford Diffraction Ltd. Marcus Winter Dyanne Cruikshank Croatian Science Foundation, grant no. IP-2014-09-4079 Croatian Academy of Sciences and Arts French-Croatian bilateral, Slovenian-Croatian bilateral, German-Croatian bilateral