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 νCO 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 νCO 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.

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► Tris(1,2,4-triazolyl)borate provides a highly tunable ligand system.
► Successive carbene derivatization expected to yield increased donor capacity.
► Molybdenum tricarbonyl complexes of these ligands modelled by DFT methods.
► Increased carbene content leads to reduced νCO and increased negative charge on Mo(CO)3 fragment.
► νCO tuned over range greater than that covered by experimentally known analogues.