Borylene-Based Direct Functionalization of Organic Substrates: Synthesis, Characterization, and Photophysical Properties of Novel π-Conjugated Borirenes

Room temperature photolysis of aminoborylene complexes, [(CO)5MBN(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 = C6H4-4-OMe and 4: R = C6H4-4-CF3); [{(μ-BN(SiMe3)2(RCC−)}2], (5: R = C6H5 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 ((C6H4NMe2)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 pz orbital of the boron atom, and TD-DFT calculations were carried out to examine the nature of the electronic transitions.