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

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 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).