Ground State and Excited State Tuning in Ferric Dipyrrin Complexes Promoted by Ancillary Ligand Exchange

Abstract

Three ferric dipyrromethene complexes featuring different ancillary ligands were synthesized by one electron oxidation of ferrous precursors. Four-coordinate iron complexes of the type (ArL)FeX2 [ArL = 1,9-(2,4,6-Ph3C6H2)2-5-mesityldipyrromethene] with X = Cl or tBuO were prepared and found to be high-spin (S = 5/2), as determined by superconducting quantum interference device magnetometry, electron paramagnetic resonance, and 57Fe Mössbauer spectroscopy. The ancillary ligand substitution was found to affect both ground state and excited properties of the ferric complexes examined. While each ferric complex displays reversible reduction and oxidation events, each alkoxide for chloride substitution results in a nearly 600 mV cathodic shift of the FeIII/II couple. The oxidation event remains largely unaffected by the ancillary ligand substitution and is likely dipyrrin-centered. While the alkoxide substituted ferric species largely retain the color of their ferrous precursors, characteristic of dipyrrin-based ligand-to-ligand charge transfer (LLCT), the dichloride ferric complex loses the prominent dipyrrin chromophore, taking on a deep green color. Time-dependent density functional theory analyses indicate the weaker-field chloride ligands allow substantial configuration mixing of ligand-to-metal charge transfer into the LLCT bands, giving rise to the color changes observed. Furthermore, the higher degree of covalency between the alkoxide ferric centers is manifest in the observed reactivity. Delocalization of spin density onto the tert-butoxide ligand in (ArL)FeCl(OtBu) is evidenced by hydrogen atom abstraction to yield (ArL)FeCl and HOtBu in the presence of substrates containing weak C–H bonds, whereas the chloride (ArL)FeCl2 analogue does not react under these conditions.

C−H Activation from Iron(II)‐Nitroxido Complexes

“…[7,11] The 57 Fe Mössbauer spectrum for ( Ar L)Fe(AZADO) ( 3 ) features a single quadrupole doublet with an isomer shift as well as a quadrupole splitting that are significantly increased as compared to the values observed for 2 (δ = 0.74 mm / s , |ΔE Q | = 0.59 mm / s for 2 , δ = 0.95 mm / s , |ΔE Q | = 1.51 mm / s for 3 ) (Figure 2c). The parameters for 3 are in line with other four-coordinate high-spin iron(II) dipyrrin complexes and we therefore assign 3 as a four-coordinate species with a κ 2 - N,O -AZADO ligand bound to the central iron, [12] a binding mode that has been observed previously upon treatment of an Fe(I) precursor with 9-azanoradamantane N -oxyl. [13] The possibility for a bidentate binding mode is further supported by crystallographic characterization of independently synthesized ( Ar L)FeCl(AZADO) ( 4 ) (Figure 2b).…”

C−H Activation from Iron(II)‐Nitroxido Complexes

“…[7,11] The 57 Fe Mçssbauer spectrum for ( Ar L)Fe-(AZADO) (3)f eatures as ingle quadrupole doublet with an isomer shift as well as aq uadrupole splitting that are significantly increased as compared to the values observed for 2 (d = 0.74 mm s À1 , j DE Q j= 0.59 mm s À1 for 2, d = 0.95 mm s À1 , j DE Q j= 1.51 mm s À1 for 3) ( Figure 2c). The parameters for 3 are in line with other four-coordinate highspin iron(II) dipyrrin complexes and we therefore assign 3 as af our-coordinate species with a k 2 -N,O-AZADO ligand bound to the central iron, [12] ab inding mode that has been observed previously upon treatment of an Fe I precursor with 9-azanoradamantane N-oxyl. [13] Thepossibility for abidentate binding mode is further supported by crystallographic characterization of independently synthesized ( Ar L)FeCl-(AZADO) (4)( Figure 2b).…”

“…[8] Reaction of the iron(I) precursor ( Ar L)Fe [9] (1)w ith TEMPO or AZADO radical (TEMPO = 2,2',6,6'-tetramethylpiperidinyloxyl radical;A ZADO = 2-azaadamantane-Noxyl radical) in benzene at room temperature leads to an immediate color change from dark purple to bright pink and Single-crystal X-ray diffraction data and 57 Fe Mçssbauer spectroscopy support the formation of ( Ar L)Fe(k 1 -TEMPO) (2)f eaturing at hree-coordinate iron center (Figure 2a,c). The parameters for 3 are in line with other four-coordinate highspin iron(II) dipyrrin complexes and we therefore assign 3 as af our-coordinate species with a k 2 -N,O-AZADO ligand bound to the central iron, [12] ab inding mode that has been observed previously upon treatment of an Fe I precursor with 9-azanoradamantane N-oxyl. [7,11] The 57 Fe Mçssbauer spectrum for ( Ar L)Fe-(AZADO) (3)f eatures as ingle quadrupole doublet with an isomer shift as well as aq uadrupole splitting that are significantly increased as compared to the values observed for 2 (d = 0.74 mm s À1 , j DE Q j= 0.59 mm s À1 for 2, d = 0.95 mm s À1 , j DE Q j= 1.51 mm s À1 for 3) ( Figure 2c).…”

Diastereoselective C−H Bond Amination for Disubstituted Pyrrolidines

“…Unlike the alkoxide variants, the phenoxide unit can engage in π interactions with the iron center; these interactions may lock the phenoxide ligand within the Fe–dipyrrin plane and induce more steric clashes with the substrate. Furthermore, given our recent study on the role of ancillary ligands in ferric dipyrrin complexes, [13] we propose that the enhanced Fe–OPh covalency stabilizes the Fe–iminyl intermediate, improving cyclization yields. The only exceptions for this substrate class are entries 2n (d.r.…”