Polycentric binding in complexes of trimethylamine- N-oxide with dihalogens

Abstract

Dihalogens readily interact with trimethylamine-N-oxide under ambient conditions. Accordingly, herein, stable 1 : 1 adducts were obtained in the case of iodine chloride and iodine bromide. The crystal and molecular structure of the trimethylamine-N-oxide-iodine chloride adduct was solved. Furthermore, the geometry and electronic structure of the trimethylamine-N-oxide-dihalogen complexes were studied computationally. Only molecular ensembles were found in the global minimum for the 1 : 1 stoichiometry. The O⋯X-Y halogen bond is the main factor for the thermodynamic stability of these complexes. Arguments for electrostatic interactions as the driving force for this noncovalent interaction were discussed. Also, the equilibrium structures are additionally stabilised by weak C-H⋯X hydrogen bonds. Consequently, formally monodentate ligands are bound in a polycentric manner.

Scheme 1. Formation of molecular complexes upon the interaction of dihalogens with trimethylamine-N-oxide (1). X = I, Y = Cl for 2a and 3a; X = I, Y = Br for 2b and 3b.

Fig. 1. Computed (top) and experimentally solved (bottom) structures of the TMAO⋯ICl complex and adduct 3a.

Fig. 2. Hirshfeld surfaces for ICl (top) and TMAO (bottom) fragments of adduct 3a.

Fig. 3. Electrostatic potential distribution on the molecular surface of trimethylamine-N-oxide (top) and chlorine (bottom).

Fig. 4. Electron density in BCP of O⋯X vs. interatomic distance of O⋯X (top) and Laplacian of electron density in BCP of O⋯X vs. interatomic distance (bottom). Linear approximations were made for the iodine-centred electrophiles.

Fig. 5. Interaction energy ΔE (BSSE corrected) for TMAO⋯X–Y vs. potential energy density V(r) in the BCP of the X⋯O bond (top) and vs. kinetic energy density G(r) in the BCP of the X⋯O bond (bottom).

Fig. 6. Complex TMAO⋯X–Y formation energy ΔE (BSSE corrected) vs. halogen X–Y maximum surface electrostatic potential V_S,max (top) and Gibbs energy vs. halogen X–Y maximum surface electrostatic potential V_S,max (bottom).

Fig. 7. Full energy changes under enlargement of X–O–N angle for the TMAO⋯Cl–Cl complex (top) and for the TMAO⋯I–I complex (bottom).

Fig. 8. Structural evolutions of TMAO⋯Cl–Cl under forced enlargement of the Cl–O–N angle: global minimum Cl2-globmin (top); maximum Cl2-Max (centre); and local minimum Cl2-locmin (bottom).

Fig. 9. Structure of TMAO⋯Cl–Cl complex in the staggered conformation resulting after 60° degree turn around the N–O bond from the global minimum (top) and energy profile for the Cl–Cl fragment “slipping” around the N–O bond continuation (bottom).

Fig. 10. Short contacts in the crystal structure of adduct 3a (top), in TMAO (mid), and in trimethylamine-N-oxide–trimethylaluminum adduct (bottom).