Theoretical investigation of the scope of sequential ligand tuning using a bifunctional scorpionate tris(1,2,4-triazolyl)borate-based architecture

The donor properties of a series of tripodal mixed N-donor/carbene ligands derived through sequential alkylation of hydrotris(1,2,4-triazolyl)borate have been investigated by density functional theory (DFT) methods. The structures of complexes of the form [Mo(L)(CO)₃]^- were optimized (L = [HB(1,2,4-triazolyl)_n(1,2,4-triazol-5-ylidene)_3-n] ^- (n = 0-3), hydrotris(pyrazolyl)borate, hydrotris(3,5- dimethylpyrazolyl)borate and hydrotris(imidazol-2-ylidene)borate) and ν_CO frequencies for these complexes and partial charges of their Mo(CO)₃ fragments were determined. Results show that ligand donation is highly tunable when compared to similar experimentally known ligands with a shift in the symmetric ν_CO stretching mode of -39 cm^-1 on going from the tris(1,2,4-triazolyl)borate complexes to that of the triscarbene hydrotris(1,2,4-triazol-5-ylidene) and an increase in partial charge (distributed multipole analysis) of the Mo(CO)₃ fragment from -0.23 to -0.48.