Oxidative Stretching of Metal–Metal Bonds to Their Limits

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David W. Brogden †, Yevgeniya Turov †, Michael Nippe †, Giovanni Li Manni ‡, Elizabeth A. Hillard §, Rodolphe Clérac §, Laura Gagliardi ‡, and John F. Berry †*

† Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States

‡ Department of Chemistry, Supercomputing Institute, and Chemical Theory Center, University of Minnesota, Minneapolis, Minnesota 55455, United States

§ CNRS, CRPP, UPR 8641, F-33600 Pessac, France

 Univ. Bordeaux, CRPP, UPR 8641, F-33600 Pessac, France

Inorg. Chem., Article ASAP

DOI: 10.1021/ic5007204

Publication Date (Web): April 21, 2014

Copyright © 2014 American Chemical Society

*E-mail: berry@chem.wisc.edu.

Synopsis

The compounds [Cr2(dpa)4]2+ (1), [Mo2(dpa)4]2+ (2), [MoW(dpa)4]2+ (3), and [W2(dpa)4]2+ (4) react with oxygen atom donors to yield [Mo2O(dpa)4]2+ (5), [WMoO(dpa)4]2+ (6), and [W2O(dpa)4]2+ (7) featuring a linear M···M≡O structure. DFT and CASSCF/CASPT2 calculations indicate “stretched” triple bonds in 2, 3, and 4. The longer metal−metal distances in 5, 6, and 7 suggest minimal interaction between the metal atoms. The electronic structure of these complexes is used to rationalize their multielectron redox reactivity.

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 Cr3+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.