Neutron Diffraction and Spectroscopic Studies of Intramolecular Tetrel Bonds in Three Fluorinated Zinc Complexes: Significant Redshift in the sp ³ C–H Stretch Confirmed by Experiments and Theory

Three fluorinated Zn complexes were synthesized and characterized. Their single crystals were grown and studied via X-ray and neutron diffraction methods. These fluorinated metal complexes tend to form two special noncovalent interactions: an intramolecular tetrel bond (TB) and an intramolecular blueshifting hydrogen bond (HB). In 4FH-ZnCl₂ [2,6-(HCF₂CF₂CH₂OCH₂)₂-py-ZnCl₂], an intramolecular TB induces significant elongation of a C-H bond in a methylene group, H1A-C1-H1B, to 1.108(2) Å, whereas the intramolecular C1-H1A···F2 blueshifting HB shortens the C1-H1A bond to 1.092(1) Å. Thus, C1-H1B is longer than C1-H1A by 16(2) mÅ such that either one of the C1-H1 stretches becomes a methine-like stretch with a peak appearing at 3020 or 2688 cm^-1. A peak corresponding to a lengthened sp³ C1-H1B bond is greatly redshifted and appears at 2688 cm^-1. When this 2688 cm^-1 peak is compared with its other methylene C-H peak (3020 cm^-1), their C-H vibrational stretches then differ by 332 cm^-1, which is likely the largest ever sp³ C-H bond stretching difference reported between two methylene C-H bonds. Additionally, two C-D bonds of deuterated 4FH-ZnCl₂, which has an intramolecular C···F TB that induces very different lengths of the two C-D bonds in a D-C-D group, become a local mode, as proven by Hooke's law. Thus, Badge's rule has been extended by using both neutron and spectroscopic studies.