From the reactions between the quadruply bonded complexes M2(TiPB)4, where M = Mo or W and TiPB = 2,4,6-triisopropylbenzoate, and the carboxylic acids HOOC–C6H4–4-B(mesityl)2, LH (2 equivalents) the complexes trans-M2(TiPB)2L2 have been prepared. The new compounds have been characterized by 1H NMR, MALDI-TOF MS, UV-Vis-NIR and steady-state emission spectroscopy, time-resolved transient absorption spectroscopy and cyclic voltammetry. These results are compared with the related properties of the benzoates, M2(TiPB)2(O2CPh)2 (prepared similarly) and with DFT calculations on model compounds where formate substitutes for TiPB. The new compounds M2(TiPB)2L2 are intensely colored in toluene or THF solutions: red (M = Mo) and green (M = W) and the introduction of the p-B(mesityl)2 group notably shifts these metal to ligand charge transfer transitions to lower energy in comparison to the benzoate complexes M2(TiPB)2(O2C–C6H5)2. Upon the addition of fluoride ions these intense absorptions are shifted to much higher energy in a reversible manner for M = Mo.
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