ic5b01163_si_002.cif (1.16 MB)

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

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posted on 03.08.2015, 00:00 by Reed J. Eisenhart, P. Alex Rudd, Nora Planas, David W. Boyce, Rebecca K. Carlson, William B. Tolman, Eckhard Bill, Laura Gagliardi, Connie C. Lu
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] (2red), MnCrL (3), and [FeCrL]+ (4ox) 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 2red, 3, and 4ox 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 2red 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 2red was further investigated by resonance Raman spectroscopy, and a band at 434 cm–1 was assigned as the CrCr bond vibration. Finally, 4ox exhibited a Mössbauer doublet with an isomer shift of 0.18 mm/s that suggests a primarily Fe-based oxidation to Fe­(I).