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, Mo-2(TiPB)(4), where TiPB = 2,4,6-triisopropylphenyl carboxylate, with NOPF6 and NOBF4 give the Ionic compounds [Mo-2(TiPB)(4)]PF6 and [Mo-2(TiPB)(4)]BF4, respectively. Each product crystallizes in space group P2(1)/n and displays an elongation of the Mo-Mo bond of 0.060 and 0,068 A, respectively, over that of the parent compound (2.076(1) Angstrom). Each complex displays a characteristic EPR signal, showing hyperfine coupling to the spin active isotopes Mo-95 and Mo-97, with g(parallel to) = g(perpendicular to) = 1.936, that is consistent with the presence of an unpaired electron. Electronic spectroscopy indicates the expected red shift In the delta --> delta* 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 [Mo-2(O2CC4H9)(4)]PF6 from the reaction of Mo-2(O2CC4H9)(4) with AgPF6. This product crystallizes in the space group C-2/c, and the Mo-Mo bond Is elongated by 0.063 A over that of the neutral parent compound. These ionic compounds have the first isolated and characterized [Mo-2(O2CR)(4)](+) cationic species.