Borylene-based direct functionalization of organic substrates: Synthesis, characterization, and photophysical properties of novel π-conjugated borirenes

Room temperature photolysis of aminoborylene complexes, [(CO) ₅MdBdN(SiMe₃)₂] (1: M = Cr, 2: Mo) in the presence of a series of alkynes and diynes, 1,2-bis(4-methoxyphenyl)ethyne, 1,2-bis(4-(trifluoromethyl)phenyl)ethyne, 1,4-diphenylbuta-1,3-diyne, 1,4-bis(4-methoxyphenyl)buta-1,3-diyne, 1,4-bis(trimethylsilylethynyl)benzene and 2,5-bis(4-N,N-dimethylaminophenylethynyl)thiophene led to the isolation of novel mono and bis-bis-(trimethylsilyl)aminoborirenes in high yields, that is [(RC=CR)(μ-BN(SiMe₃)₂], (3: R = C₆H ₄-4-OMe and 4: R = C₆H₄-4-CF₃); [{(μ-BN(SiMe₃)₂(RC=C-)}₂], (5: R = C ₆H₅ and 6: R = C₆H₄-4-OMe); [1,4-bis-{(μ-BN(SiMe₃)₂(SiMe₃C=C)}benzene], 7 and [2,5-bis-{(μ-BN(SiMe₃)₂ ((C₆H ₄NMe₂)-C=C)}-thiophene], 8. All borirenes were isolated as light yellow, air and moisture sensitive solids. The new borirenes have been characterized in solution by ¹H, ¹¹B, ¹³C NMR spectroscopy and elemental analysis and the structural types were unequivocally established by crystallographic analysis of compounds 6 and 7. DFT calculations were performed to evaluate the extent of π-conjugation between the electrons of the carbon backbone and the empty p_z orbital of the boron atom, and TD-DFT calculations were carried out to examine the nature of the electronic transitions.