title: Pushing the Limits of Delta Bonding in Metal-Chromium Complexes with Redox Changes and Metal Swapping.
abstract: Into the metalloligand Cr[N(o-(NCH2P((i)Pr)2)C6H4)3] (1, CrL) was inserted a second chromium atom to generate the dichromium complex Cr2L (2), which is a homobimetallic analogue of the known MCrL complexes, where M is manganese (3) or iron (4). The cationic and anionic counterparts, [MCrL](+) and [MCrL](-), respectively, were targeted, and each MCr pair was isolated in at least one other redox state. The solid-state structures of the [MCrL](+,0,-) redox members are essentially the same, with ultrashort metal-metal bonds between 1.96 and 1.74 Å. The formal shortness ratios (r) of these interactions are between 0.84 and 0.74 and are interpreted as triple to quintuple metal-metal bonds with the aid of theory. The trio of (d-d)(10) species [Cr2L](-) (2(red)), MnCrL (3), and [FeCrL](+) (4(ox)) are S = 0 diamagnets. On the basis of M-Cr bond distances and theoretical calculations, the strength of the metal-metal bond across the (d-d)(10) series increases in the order Fe < Mn < Cr. The methylene protons in the ligand are shifted downfield in the (1)H NMR spectra, and the diamagnetic anisotropy of the metal-metal bond was calculated as -3500 × 10(-36), -3900 × 10(-36), and -5800 × 10(-36) m(3) molecule(-1) for 2(red), 3, and 4(ox) respectively. The magnitude of diamagnetic anisotropy is, thus, affected more by bond polarity than by bond order. A comparative vis-NIR study of quintuply bonded 2(red) and 3 revealed a large red shift in the δ(4) → δ(3)δ* transition energy upon swapping from the (Cr2)(2+) to the (MnCr)(3+) core. Complex 2(red) was further investigated by resonance Raman spectroscopy, and a band at 434 cm(-1) was assigned as the Cr-Cr bond vibration. Finally, 4(ox) exhibited a Mössbauer doublet with an isomer shift of 0.18 mm/s that suggests a primarily Fe-based oxidation to Fe(I).
authors.forename: Reed J
authors.initials: RJ
authors.lastname: Eisenhart
authors.forename: P Alex
authors.initials: PA
authors.lastname: Rudd
authors.forename: Nora
authors.initials: N
authors.lastname: Planas
authors.forename: David W
authors.initials: DW
authors.lastname: Boyce
authors.forename: Rebecca K
authors.initials: RK
authors.lastname: Carlson
authors.forename: William B
authors.initials: WB
authors.lastname: Tolman
authors.affiliation: ∥Max Planck Institut für Chemische Energiekonversion (MPI-CEC), Stiftstraße 34-36, 45470 Mülheim an der Ruhr, Germany.
authors.forename: Eckhard
authors.initials: E
authors.lastname: Bill
authors.forename: Laura
authors.initials: L
authors.lastname: Gagliardi
authors.forename: Connie C
authors.initials: CC
authors.lastname: Lu
mesh_terms: D002857:Chromium / Q000737:chemistry*; D056831:Coordination Complexes / Q000737:chemistry*; D004563:Electrochemistry; D008958:Models, Molecular; D008968:Molecular Conformation; D010084:Oxidation-Reduction
chemical_list: D056831:Coordination Complexes; D002857:Chromium