title: Oxidative stretching of metal-metal bonds to their limits.
abstract: Oxidation of quadruply bonded Cr2(dpa)4, Mo2(dpa)4, MoW(dpa)4, and W2(dpa)4 (dpa = 2,2'-dipyridylamido) with 2 equiv of silver(I) triflate or ferrocenium triflate results in the formation of the two-electron-oxidized products [Cr2(dpa)4](2+) (1), [Mo2(dpa)4](2+) (2), [MoW(dpa)4](2+) (3), and [W2(dpa)4](2+) (4). Additional two-electron oxidation and oxygen atom transfer by m-chloroperoxybenzoic acid results in the formation of the corresponding metal-oxo compounds [Mo2O(dpa)4](2+) (5), [WMoO(dpa)4](2+) (6), and [W2O(dpa)4](2+) (7), which feature an unusual linear M···M≡O structure. Crystallographic studies of the two-electron-oxidized products 2, 3, and 4, which have the appropriate number of orbitals and electrons to form metal-metal triple bonds, show bond distances much longer (by >0.5 Å) than those in established triply bonded compounds, but these compounds are nonetheless diamagnetic. In contrast, the Cr-Cr bond is completely severed in 1, and the resulting two isolated Cr(3+) magnetic centers couple antiferromagnetically with J/kB= -108(3) K [-75(2) cm(-1)], as determined by modeling of the temperature dependence of the magnetic susceptibility. Density functional theory (DFT) and multiconfigurational methods (CASSCF/CASPT2) provide support for "stretched" and weak metal-metal triple bonds in 2, 3, and 4. The metal-metal distances in the metal-oxo compounds 5, 6, and 7 are elongated beyond the single-bond covalent radii of the metal atoms. DFT and CASSCF/CASPT2 calculations suggest that the metal atoms have minimal interaction; the electronic structure of these complexes is used to rationalize their multielectron redox reactivity.
authors.affiliation: Department of Chemistry, University of Wisconsin-Madison , 1101 University Avenue, Madison, Wisconsin 53706, United States.
authors.forename: David W
authors.initials: DW
authors.lastname: Brogden
authors.forename: Yevgeniya
authors.initials: Y
authors.lastname: Turov
authors.forename: Michael
authors.initials: M
authors.lastname: Nippe
authors.forename: Giovanni
authors.initials: G
authors.lastname: Li Manni
authors.forename: Elizabeth A
authors.initials: EA
authors.lastname: Hillard
authors.forename: Rodolphe
authors.initials: R
authors.lastname: Clérac
authors.forename: Laura
authors.initials: L
authors.lastname: Gagliardi
authors.forename: John F
authors.initials: JF
authors.lastname: Berry