Oxidation and Reduction of Bis(imino)pyridine Iron Dinitrogen Complexes: Evidence for Formation of a Chelate Trianion.

Abstract

Oxidation and reduction of the bis(imino)pyridine iron dinitrogen compound, ((iPr)PDI)FeN(2) ((iPr)PDI = 2,6-(2,6-(i)Pr(2)-C(6)H(3)-N═CMe)(2)C(5)H(3)N) has been examined to determine whether the redox events are metal or ligand based. Treatment of ((iPr)PDI)FeN(2) with [Cp(2)Fe][BAr(F)(4)] (BAr(F)(4) = B(3,5-(CF(3))(2)-C(6)H(3))(4)) in diethyl ether solution resulted in N(2) loss and isolation of [((iPr)PDI)Fe(OEt(2))][BAr(F)(4)]. The electronic structure of the compound was studied by SQUID magnetometry, X-ray diffraction, EPR and zero-field (57)Fe Mössbauer spectroscopy. These data, supported by computational studies, established that the overall quartet ground state arises from a high spin iron(II) center (S(Fe) = 2) antiferromagnetically coupled to a bis(imino)pyridine radical anion (S(PDI) = 1/2). Thus, the oxidation event is principally ligand based. The one electron reduction product, [Na(15-crown-5)][((iPr)PDI)FeN(2)], was isolated following addition of sodium naphthalenide to ((iPr)PDI)FeN(2) in THF followed by treatment with the crown ether. Magnetic, spectroscopic, and computational studies established a doublet ground state with a principally iron-centered SOMO arising from an intermediate spin iron center and a rare example of trianionic bis(imino)pyridine chelate. Reduction of the iron dinitrogen complex where the imine methyl groups have been replaced by phenyl substituents, ((iPr)BPDI)Fe(N(2))(2) resulted in isolation of both the mono- and dianionic iron dinitrogen compounds, [((iPr)BPDI)FeN(2)](-) and [((iPr)BPDI)FeN(2)](2-), highlighting the ability of this class of chelate to serve as an effective electron reservoir to support neutral ligand complexes over four redox states.

Synthesis and Ligand Non-Innocence of Thiolate-Ligated (N4S) Iron(II) and Nickel(II) Bis(imino)pyridine Complexes

“…37 In contrast, the mono-reduced [( iPr BIP)Fe(OEt 2 )](BAr F 4 ) exhibits an EPR spectrum assigned to an S = 3/2 species derived from a high-spin Fe II ion antiferromagnetically coupled to the BIP radical anion. 38 The spectrum in Figure 11 does not match the spectra for either of the former complexes. These results suggest that the LN 3 S complex 4 , which contains a covalently tethered phenylthiolate group, exhibits a new electronic configuration consisting of either ls-Fe II and a ligand-based radical or an is-Fe II center antiferromagnetically coupled to a ligand-based radical, not previously observed in other mono-reduced complexes.…”

“…An additional possibility is a high-spin (hs) iron(II) ( S = 2) antiferromagnetically coupled to a ligand-based radical, but this configuration would give rise to a total spin ground state of S = 3/2, which should exhibit g values spread over a much wider range than observed in Figure 11. 38 To make a definitive assignment, it is useful to draw comparisons with other mono-reduced BIP-iron complexes. For the mono-reduced complex [( iPr BIP)Fe(CO) 2 ](BAr F 4 ), a rhombic EPR spectrum with g = 2.111, 2.043, 1.994 is observed and attributed to a low-spin Fe I configuration with an S = ½ ground state.…”

Coordination chemistry insights into the role of alkali metal promoters in dinitrogen reduction

“…Most commonly, addition of a crown ether results in an increase in ν NN , indicating a decrease in backbonding from loss of the direct interaction (Table 6). In one of the clearest examples, a study by Tondreau et al showed that the 1e − reduction of (2,6-[2,6- i Pr 2 C 6 H 3 N=CMe] 2 C 5 H 3 N)Fe(N 2 ) ( 12 ) using sodium naphthalenide generates [(2,6-[2,6- i Pr 2 C 6 H 3 N=CMe] 2 C 5 H 3 N)Fe(N 2 )][Na(THF) x ] ( 23 ) and shifts the ν NN from 2036 to 1917 cm −1 (the authors also report a second stretch at 1966 cm −1 , attributed to different cation-coordination modes) [70]. To avoid the complicated range of products formed when reducing (PDI)Fe(N 2 ) complexes with Na-based reductants (described above, see 8 - 11 ), Tondreau et al added 15-crown-5 or 18-crown-6 to 23 to reduce cation interactions.…”

Crystal structure of {N-[(6-bromopyridin-2-yl)(phenyl)methylidene]-2,6-dimethylaniline-κ2N,N′}dichloridozinc dichloromethane hemisolvate

“…Redox-active ligands bearing an -iminopyridine core have received much attention in the literature (Bianchini et al, 2007;Lu et al, 2008). While most -iminopyridine ligands reported to date feature a methyl imine 'backbone', a small number of variants featuring more electron-withdrawing phenyl backbones have been reported (Archer et al, 2006;Tondreau et al, 2013;Yang et al, 2010). Single-crystal X-ray diffraction studies have been a critical component in the elucidation of the electronic structure of base metal complexes featuring these redox-active ligands (Bart et al, 2006;Lu et al, 2008;Tondreau et al, 2013).…”

“…While most -iminopyridine ligands reported to date feature a methyl imine 'backbone', a small number of variants featuring more electron-withdrawing phenyl backbones have been reported (Archer et al, 2006;Tondreau et al, 2013;Yang et al, 2010). Single-crystal X-ray diffraction studies have been a critical component in the elucidation of the electronic structure of base metal complexes featuring these redox-active ligands (Bart et al, 2006;Lu et al, 2008;Tondreau et al, 2013). A comparison of the N imine -C imine , C imine -C ipso , and C ipso -N pyridine bond lengths for reduced and unreduced ligands as free bases or closed-shell complexes facilitate conclusions about redox non-innocence for such ligand sets.…”