Pushing the Limits of Delta Bonding in Metal-Chromium Complexes with Redox Changes and Metal Swapping

Into the metalloligand Cr[N(o-(NCH₂P(ⁱPr)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 ¹H NMR spectra, and the diamagnetic anisotropy of the metal-metal bond was calculated as -3500 × 10^-36, -3900 × 10^-36, and -5800 × 10^-36 m³ 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 δ⁴ → δ³δ∗ 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).