[Bis(pyridine)iodine(I)]+ complexes offer controlled access to halonium ions under mild conditions. The reactivity of such stabilized halonium ions is primarily determined by their three‐center, four‐electron [N−I−N]+ halogen bond. We studied the importance of chelation, strain, steric hindrance and electrostatic interaction for the structure and reactivity of halogen bonded halonium ions by acquiring their 15N NMR coordination shifts and measuring their iodenium release rates, and interpreted the data with the support of DFT computations. A bidentate ligand stabilizes the [N−I−N]+ halogen bond, decreasing the halenium transfer rate. Strain weakens the bond and accordingly increases the release rate. Remote modifications in the backbone do not influence the stability as long as the effect is entirely steric. Incorporating an electron‐rich moiety close by the [N−I−N]+ motif increases the iodenium release rate. The analysis of the iodine(I) transfer mechanism highlights the impact of secondary interactions, and may provide a handle on the induction of stereoselectivity in electrophilic halogenations.
A model system for the investigation of intramolecular halogen bonds is introduced. Two molecules capable of intramolecular halogen bonding have been studied in comparison with eight control compounds by 15N, 13C, and 19F NMR spectroscopy. Iodine‐ and bromine‐centered halogen bonds are indicated by decreases in the 15N NMR chemical shifts of the halogen bond acceptor atom of approximately 6 and 1 ppm, respectively. 13C NMR chemical shifts of the alkynyl carbons in 2‐ethynylpyridine systems are good indicators of halogen bonding, with differences of up to 2.4 ppm between halogen‐bonded and related control compounds. Halogen bond strengths in different solvents, as indicated by 19F NMR chemical shifts, decrease in the following order: Cyclohexane > toluene > benzene > dichloromethane > acetone > pyridine. Chemical shift effects associated with the structural and electronic properties of intramolecular halogen‐bonded systems are modeled well by calculations at the B3LYP/6‐311+G(2d,p) level of theory.
A series of arylene ethynylene compounds has been generated in order to study the differences between strong and weak intramolecular halogen bonding in solution. With strong intramolecular halogen bonding, the presence of electron‐withdrawing fluorine substituents near the halogen‐bond donor has a significant impact on the halogen bond and consequently on 13C NMR chemical shifts. UV/Vis studies suggest increased conjugation of the arylene ethynylene backbone when strong halogen bonds are present. UV/Vis and NMR signatures of intramolecular halogen bonding are diminished significantly when electron‐withdrawing fluorine substituents are absent, however. This change in behavior is further illustrated by differences in crystallization tendencies with the presence or absence of electron‐withdrawing substituents. Calculations using the M06‐2X functional provide some insight into the energies associated with both strong and weak intramolecular halogen bonding.
A conjugated, pyridine-containing, phenylethynyl ligand that forms complexes with Ag I and Pd II has been developed. NMR titration studies with Pd II reveal a stoichiometric binding of the ligand to the metal atom, while similar studies with Ag I reveal a binding that is dynamic on the NMR timescale. Analysis of the NMR spectroscopic data by Job's plot analysis and non-linear curve fitting of a titration curve reveals a 1:1
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