1,2-Bis(2-bromophenyl)ethynes: The influence of 5′-methoxy and 5′-difluoromethoxy groups on the electronic properties and crucial bond lengths

The effect of methoxy and difluoromethoxy groups para to the bromo substituent of 1,2-bis(2-bromophenyl)ethynes on the conformation of the molecule and the properties of the CC and CBr bonds was investigated by single crystal X-ray diffractometry and Nuclear Magnetic Resonance spectroscopy. The difluoromethoxy and methoxy groups both activated the ortho positions, as was evident from the shielding of both the proton and carbon atoms at these positions in NMR experiments. The C₆-C≡C-C₆ system in 1,2-bis(2-bromophenyl)ethyne, 1,2-bis(2-bromo-5-(difluoromethoxy)phenyl)ethyne, 1,2-bis(2-bromo-5-methoxyphenyl)ethyne and 1-bromo-2-((2-bromophenyl)ethynyl)-4-methoxybenzene is in effect coplanar, with the dihedral angle between the two ring planes ranging from 0° to 5.8°. Substitution has little effect on the electronic properties of the triple bond, as was evident from the similar ¹³C NMR spectroscopy chemical shifts and crystallographic bond distances. This is also reflected in the similar angles at the sp- centra, which deviate only slightly from linearity (177.3(2) – 179.6(1)°). The OCF₂H substituent resulted in a small contraction of the CBr bond and OCH₃ in a slight elongation. The bromo-bearing carbon was shielded with 3.8 ppm by the OCF₂H group and by more than 9 ppm relative to an H atom by the OCH₃ group in ¹³C NMR spectroscopy experiments.