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(u-1,3-N-3)}(2) (2) ([PhBP3] = [PhB(CH2PPh2)(3)](-)), yields the diamagnetic bridging nitride species [{[PhBP3]Fe}(2)(mu-N)][Na(THF)(5)] (3). The Fe-N-Fe linkage featured in the anion of 3 exhibits an unusually bent angle of approximately 135degrees, and the short Fe-N bond distances (Fe-N-av approximate to 1.70 Angstrom) suggest substantial Fe-N multiple bond character. The diamagnetic imide complex {[PhBP3]Fe(parallel to)dropN(1-Ad)}{(Bu4N)-Bu-n} (4) has been prepared by sodium amalgam reduction of its low-spin iron(III) precursor, [PhBP3]Fe(lll)dropN(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.