Ground-State Singlet L₃Fe-(μ-N)-FeL₃ and L₃Fe(NR) Complexes Featuring Pseudotetrahedral Fe(II) Centers

Pseudotetrahedral iron(II) coordination complexes that contain bridged nitride and terminal imide linkages, and exhibit singlet ground-state electronic configurations, are described. Sodium amalgam reduction of the ferromagnetically coupled dimer, {[PhBP3]Fe(μ-1,3-N3)}2 (2) ([PhBP3] = [PhB(CH2PPh2)3]-), yields the diamagnetic bridging nitride species [{[PhBP3]Fe}2(μ-N)][Na(THF)5] (3). The Fe−N−Fe linkage featured in the anion of 3 exhibits an unusually bent angle of approximately 135°, and the short Fe−N bond distances (Fe−Nav ≈ 1.70 Å) suggest substantial Fe−N multiple bond character. The diamagnetic imide complex {[PhBP3]FeII⋮N(1-Ad)}{nBu4N} (4) has been prepared by sodium amalgam reduction of its low-spin iron(III) precursor, [PhBP3]FeIII⋮N(1-Ad) (5). Complexes 4 and 5 have been structurally characterized, and their respective electronic structures are discussed in the context of a supporting DFT calculation. Diamagnetic 4 provides a bona fide example of a pseudotetrahedral iron(II) center in a low-spin ground-state configuration. Comparative optical data strongly suggest that dinuclear 3 is best described as containing two high-spin iron(II) centers that are strongly antiferromagnetically coupled to give rise to a singlet ground-state at room temperature.