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]− (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).