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 NOPF(6) and NOBF(4) give the ionic compounds [Mo(2)(TiPB)(4)]PF(6) and [Mo(2)(TiPB)(4)]BF(4), 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) A). Each complex displays a characteristic EPR signal, showing hyperfine coupling to the spin active isotopes (95)Mo and (97)Mo, with g( parallel) = g( perpendicular) = 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)(O(2)CC(4)H(9))(4)]PF(6) from the reaction of Mo(2)(O(2)CC(4)H(9))(4) with AgPF(6). This product crystallizes in the space group C2/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)(O(2)CR)(4)](+) cationic species.