Ground-State Singlet L<sub>3</sub>Fe-(μ-N)-FeL<sub>3</sub> and L<sub>3</sub>Fe(NR) Complexes Featuring Pseudotetrahedral Fe(II) Centers

2005-02-16T00:00:00Z (GMT) by Steven D. Brown Jonas C. Peters
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, {[PhBP<sub>3</sub>]Fe(μ-1,3-N<sub>3</sub>)}<sub>2</sub> (<b>2</b>) ([PhBP<sub>3</sub>] = [PhB(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>]<sup>-</sup>), yields the diamagnetic bridging nitride species [{[PhBP<sub>3</sub>]Fe}<sub>2</sub>(μ-N)][Na(THF)<sub>5</sub>] (<b>3</b>). The Fe−N−Fe linkage featured in the anion of <b>3</b> exhibits an unusually bent angle of approximately 135°, and the short Fe−N bond distances (Fe−N<sub>av</sub> ≈ 1.70 Å) suggest substantial Fe−N multiple bond character. The diamagnetic imide complex {[PhBP<sub>3</sub>]Fe<sup>II</sup>⋮N(1-Ad)}{<i><sup>n</sup></i><sup></sup>Bu<sub>4</sub>N} (<b>4</b>) has been prepared by sodium amalgam reduction of its low-spin iron(III) precursor, [PhBP<sub>3</sub>]Fe<sup>III</sup>⋮N(1-Ad) (<b>5</b>). Complexes <b>4</b> and <b>5</b> have been structurally characterized, and their respective electronic structures are discussed in the context of a supporting DFT calculation. Diamagnetic <b>4</b> 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 <b>3</b> 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.