Filling a Void: Isolation and Characterization of Tetracarboxylato Dimolybdenum Cations

Dimolybdenum tetracarboxylato cations have been prepared and structurally characterized for the first time. The
reactions of the new, quadruply bonded compound, Mo2(TiPB)4, where TiPB = 2,4,6-triisopropylphenyl carboxylate,
with NOPF6 and NOBF4 give the ionic compounds [Mo2(TiPB)4]PF6 and [Mo2(TiPB)4]BF4, respectively. Each product
crystallizes in space group P21/n and displays an elongation of the Mo−Mo bond of 0.060 and 0.068 Å, respectively,
over that of the parent compound (2.076(1) Å). Each complex displays a characteristic EPR signal, showing hyperfine
coupling to the spin active isotopes 95Mo and 97Mo, with g∥ = g⊥ = 1.936, that is consistent with the presence of
an unpaired electron. Electronic spectroscopy indicates the expected red shift in the δ → δ* transition for the
cations, due to the loss of exchange energy in going from the two-electron to one-electron system. We have also
obtained a small amount of crystalline [Mo2(O2CC4H9)4]PF6 from the reaction of Mo2(O2CC4H9)4 with AgPF6. This
product crystallizes in the space group C2/c, and the Mo−Mo bond is elongated by 0.063 Å over that of the neutral
parent compound. These ionic compounds have the first isolated and characterized [Mo2(O2CR)4]+ cationic species.