Effects of Supramolecular Architecture on Halogen Bonding between Diiodine and Heteroaromatic N-Oxides

Abstract

Cocrystallization of diiodine with a series of heteroaromatic Noxides produced three types of halogen-bonded associates: (a) alternating chains comprising I 2 molecules bridged by oxygen atoms of N-oxides of pyridine, quinoline, or 4-methylpyridine; (b) discrete 2:1 complexes, in which diiodine links a pair of acridine N-oxide molecules; and (c) amphoteric 1:1 adducts, in which one end of each diiodine molecule is halogen-bonded to Noxide of 4-methoxypyridine or 4-chloroquinoline, and another diiodine's end forms contacts with the other I 2 molecules. In all cases, halogen bonds between diiodine and N-oxides are characterized by nearly linear I−I•••O angles, close to perpendicular dihedral C−N−O•••I angles and N−O•••I angles of about 110°.The halogen bond length in the 1:1 adducts is about 0.3 Å shorter than those in the 2:1 associates and in the infinite chains. Computational analyses confirmed that the variations in I•••O separations are related predominantly to the distinct effects of halogen bond competition in different supramolecular associates. Experimental and computational data also indicated that coordination of the second diiodine to the same oxygen atom of N-oxide has a smaller effect on the halogen bond length and energy than coordination of the second N-oxide to another iodine atom in the I 2 molecule.

Strong N−X⋅⋅⋅O−N Halogen Bonds: A Comprehensive Study on N‐Halosaccharin Pyridine N‐Oxide Complexes

“…The calculated Δ E int vary, from −84.3 to −120.3 kJ mol −1 for NISac‐ Z , and from −47.5 to −81.5 kJ mol −1 for NBSac‐ Z complexes. The largest Δ E int in both series for NISac‐ 13 (−120.3 kJ mol −1 ) and NBSac‐ 12 (−81.5 kJ mol −1 ) are weaker than the corresponding 3‐center‐4‐electron (3c‐4e) XBs, [N−I−N] + (−159.2 kJ mol −1 ) and [N−Br−N] + (−151.0 kJ mol −1 ), respectively, yet stronger than the other reported similar XBs, for example, I−I⋅⋅⋅ − O−N + (−42.0 kJ mol −1 ), CF 3 −I⋅⋅⋅ − O−N + (−33.7 kJ mol −1 ), NISac⋅⋅⋅N py (py=pyridine; −61.4 kJ mol −1 ), NBSac⋅⋅⋅N py (−43.5 kJ mol −1 ), NBS⋅⋅⋅N py (−38.6 kJ mol −1 ), and C 6 F 4 Br 2 ⋅⋅⋅N bpe (bpe=1,2‐bis(4‐pyridyl)ethylene, −14.5 kJ mol −1 ) complexes. This comparison evidence the strong binding that can be achieved by N−X⋅⋅⋅ − O−N + (X=Br, I) XBs.…”

“…have shown using N‐iodosaccharin (NISac) and highly nucleophilic 4‐ N , N ‐dimethylaminopyridine (DMAP) as the XB acceptor, that the N−I bond of the N‐haloimide can be dissociated forming an iodo‐pyridinium cation and N‐saccharinate anion, that is, forming a salt . While strong XBs with N‐heterocycles are common, the development of (strong) halogen bonds using other Lewis bases for example, ( N ‐oxide) oxygen, has remained largely unexplored …”

In Situ Assessment of Intrinsic Strength of X-I⋯OA-Type Halogen Bonds in Molecular Crystals with Periodic Local Vibrational Mode Theory

“…with N-oxide acceptors via crystallography and theoretical calculations [96,97]. Their analysis based on molecular dimer models suggests that the charge transfer is a key factor in the I• • • O halogen bonding besides electrostatic attraction.…”

“…The substituent effect in acceptor molecules L, M and P on their N + -O − bonds is more similar to that in acceptor molecule O than in acceptor molecule N, as revealed by corresponding local stretching force constants being larger but close to the force constant values for acceptor molecule O (see Table 3). This explains why the local stretching force constants of I [96,97] including P-1(iQnO), L-1(2MePyO), O-1(Me2NPyO), N-1(PyrazO) and Q(ClQnO) (abbreviations in parentheses are taken from Reference [97]). The ordering for intrinsic strength of the first four halogen bonds (see Figure 4) is consistent with the ordering for binding energies of dimer complexes, except for P-1 and L-1.…”

“…By checking the crystal structure containing halogen bond Q, we found there exists a highly ordered I 2 network between two rows of acceptor molecules Q (see Figure 8). For each halogen bonding Q, the other side of donor I 2 forms two I• • • I halogen bonds of equal distance, and the resulting zigzag pattern is repeated as an infinitely long one-dimensional network [96]. These peculiar interactions between diiodine molecules are the major factor which greatly weakens the I• • • O bonds.…”