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.

Reduction of previously reported (ArL)FeCl with potassium graphite furnished a low-spin (S = 1/2) iron complex (ArL)Fe which features an intramolecular η6-arene interaction and can be utilized as an FeI synthon (ArL = 5-mesityl-1,9-(2,4,6-Ph3C6H2)dipyrrin). Treatment of (ArL)Fe with adamantyl azide or mesityl azide led to the formation of the high-spin (S = 5/2), three-coordinate imidos (ArL)Fe(NAd) and (ArL)Fe(NMes), respectively, as determined by EPR, zero-field 57Fe Mössbauer, magnetometry, and single crystal X-ray diffraction. The high-spin iron imidos are reactive with a variety of substrates: (ArL)Fe(NAd) reacts with azide yielding a ferrous tetrazido (ArL)Fe(κ2-N4Ad2), undergoes intermolecular nitrene transfer to phosphine, abstracts H atoms from weak C–H bonds (1,4-cyclohexadiene, 2,4,6-tBu3C6H2OH) to afford ferrous amido product (ArL)Fe(NHAd), and can mediate intermolecular C–H amination of toluene [PhCH3/PhCD3 kH/kD: 15.5(3); PhCH2D kH/kD: 11(1)]. The C–H bond functionalization reactivity is rationalized from a two-step mechanism wherein each step occurs via maximal energy and orbital overlap between the imido fragment and the C–H bond containing substrate.

We report the isolation of a room temperature stable dipyrromethene Cu(O2) complex featuring a side-on O2 coordination. Reactivity studies highlight the unique ability of the dioxygen adduct for both hydrogen-atom abstraction and acid/base chemistry towards phenols, demonstrating that side-on superoxide species can be reactive entities.

AbstractThe reaction of nitroxyl radicals TEMPO (2,2′,6,6′-tetramethylpiperidinyloxyl) and AZADO (2-azaadamantane-N-oxyl) with an iron(I) synthon affords iron(II)-nitroxido complexes (ArL)Fe(κ1-TEMPO) and (ArL)Fe(κ2-N,O-AZADO) (ArL=1,9-(2,4,6-Ph3C6H2)2-5-mesityldipyrromethene). Both high-spin iron(II)-nitroxido species are stable in the absence of weak C−H bonds, but decay via N−O bond homolysis to ferrous or ferric iron hydroxides in the presence of 1,4-cyclohexadiene. Whereas (ArL)Fe(κ1-TEMPO) reacts to give a diferrous hydroxide [(ArL)Fe]2(μ-OH)2, the reaction of four-coordinate (ArL)Fe(κ2-N,O-AZADO) with hydrogen atom donors yields ferric hydroxide (ArL)Fe(OH)(AZAD). Mechanistic experiments reveal saturation behavior in C−H substrate and are consistent with rate-determining hydrogen atom transfer.

AbstractThe reaction of nitroxyl radicals TEMPO (2,2′,6,6′-tetramethylpiperidinyloxyl) and AZADO (2-azaadamantane-N-oxyl) with an iron(I) synthon affords iron(II)-nitroxido complexes (ArL)Fe(κ1-TEMPO) and (ArL)Fe(κ2-N,O-AZADO) (ArL=1,9-(2,4,6-Ph3C6H2)2-5-mesityldipyrromethene). Both high-spin iron(II)-nitroxido species are stable in the absence of weak C−H bonds, but decay via N−O bond homolysis to ferrous or ferric iron hydroxides in the presence of 1,4-cyclohexadiene. Whereas (ArL)Fe(κ1-TEMPO) reacts to give a diferrous hydroxide [(ArL)Fe]2(μ-OH)2, the reaction of four-coordinate (ArL)Fe(κ2-N,O-AZADO) with hydrogen atom donors yields ferric hydroxide (ArL)Fe(OH)(AZAD). Mechanistic experiments reveal saturation behavior in C−H substrate and are consistent with rate-determining hydrogen atom transfer.

AbstractWe report herein the improved diastereoselective synthesis of 2,5-disubstituted pyrrolidines from aliphatic azides. Experimental and theoretical studies of the C−H amination reaction mediated by the iron dipyrrinato complex (AdL)FeCl(OEt2) provided a model for diastereoinduction and allowed for systematic variation of the catalyst to enhance selectivity. Among the iron alkoxide and aryloxide catalysts evaluated, the iron phenoxide complex exhibited superior performance towards the generation of syn 2,5-disubstituted pyrrolidines with high diastereoselectivity.

AbstractWe report herein the improved diastereoselective synthesis of 2,5-disubstituted pyrrolidines from aliphatic azides. Experimental and theoretical studies of the C−H amination reaction mediated by the iron dipyrrinato complex (AdL)FeCl(OEt2) provided a model for diastereoinduction and allowed for systematic variation of the catalyst to enhance selectivity. Among the iron alkoxide and aryloxide catalysts evaluated, the iron phenoxide complex exhibited superior performance towards the generation of syn 2,5-disubstituted pyrrolidines with high diastereoselectivity.

A sterically accessible tert-butyl-substituted dipyrrinato di-iron(II) complex [(tBuL)FeCl]2 possessing two bridging chloride atoms was synthesized from the previously reported solvento adduct. Upon treatment with aryl azides, the formation of high-spin FeIII species was confirmed by 57Fe Mössbauer spectroscopy. Crystallographic characterization revealed two possible oxidation products: (1) a terminal iron iminyl from aryl azides bearing ortho isopropyl substituents, (tBuL)FeCl(•NC6H3-2,6-iPr2); or (2) a bridging di-iron imido arising from reaction with 3,5-bis(trifluoromethyl)aryl azide, [(tBuL)FeCl]2(μ-NC6H3-3,5-(CF3)2). Similar to the previously reported (ArL)FeCl(•NC6H4-4-tBu), the monomeric iron imido is best described as a high-spin FeIII antiferromagnetically coupled to an iminyl radical, affording an S = 2 spin state as confirmed by SQUID magnetometry. The di-iron imido possesses an S = 0 ground state, arising from two high-spin FeIII centers weakly antiferromagnetically coupled through the bridging imido ligand. The terminal iron iminyl complex undergoes facile decomposition via intra- or intermolecular hydrogen-atom abstraction (HAA) from an imido aryl ortho isopropyl group, or from 1,4-cyclohexadiene, respectively. The bridging di-iron imido is a competent N-group transfer reagent to cyclic internal olefins as well as styrene. Although solid-state magnetometry indicates an antiferromagnetic interaction between the two iron centers (J = −108.7 cm–1) in [(tBuL)FeCl]2(μ-NC6H3-3,5-(CF3)2), we demonstrate that in solution the bridging imido can facilitate HAA as well as dissociate into a terminal iminyl species, which then can promote HAA. In situ monitoring reveals the di-iron bridging imido is a catalytically competent intermediate, one of several iron complexes observed in the amination of C–H bond substrates or styrene aziridination.

The one-electron reduction of (tbsL)Fe3(thf)1 furnishes [M][(tbsL)Fe3] ([M]+ = [(18-C-6)K(thf)2]+ (1, 76%) or [(crypt-222)K]+ (2, 54%)). Upon reduction, the ligand tbsL6– rearranges around the triiron core to adopt an almost ideal C3-symmetry. Accompanying the (tbsL) ligand rearrangement, the THF bound to the neutral starting material is expelled, and the Fe–Fe distances within the trinuclear cluster contract by ∼0.13 Å in 1. Variable-temperature magnetic susceptibility data indicates a well-isolated S = 11/2 spin ground state that persists to room temperature. Slow magnetic relaxation is observed at low temperature as evidenced by the out-of-phase (χM″) component of the alternating current (ac) magnetic susceptibility data and by the appearance of hyperfine splitting in the zero-field 57Fe Mössbauer spectra at 4.2 K. Analysis of the ac magnetic susceptibility yields an effective spin reversal barrier (Ueff) of 22.6(2) cm–1, nearly matching the theoretical barrier of 38.7 cm–1 calculated from the axial zero-field splitting parameter (D = −1.29 cm–1) extracted from the reduced magnetization data. A polycrystalline sample of 1 displays three sextets in the Mössbauer spectrum at 4.2 K (Hext = 0) which converge to a single six-line pattern in a frozen 2-MeTHF glass sample, indicating a unique iron environment and thus strong electron delocalization. The spin ground state and ligand rearrangement are discussed within the framework of a fully delocalized cluster exhibiting strong double and direct exchange interactions.

To assess the impact of terminal ligand binding on a variety of cluster properties (redox delocalization, ground-state stabilization, and breadth of redox state accessibility), we prepared three electron-transfer series based on the hexanuclear iron cluster [(HL)2Fe6(L′)m]n+ in which the terminal ligand field strength was modulated from weak to strong (L′ = DMF, MeCN, CN). The extent of intracore M–M interactions is gauged by M–M distances, spin ground state persistence, and preference for mixed-valence states as determined by electrochemical comproportionation constants. Coordination of DMF to the [(HL)2Fe6] core leads to weaker Fe–Fe interactions, as manifested by the observation of ground states populated only at lower temperatures (<100 K) and by the greater evidence of valence trapping within the mixed-valence states. Comproportionation constants determined electrochemically (Kc = 104–108) indicate that the redox series exhibits electronic delocalization (class II–III), yet no intervalence charge transfer (IVCT) bands are observable in the near-IR spectra. Ligation of the stronger σ donor acetonitrile results in stabilization of spin ground states to higher temperatures (∼300 K) and a high degree of valence delocalization (Kc = 102–108) with observable IVCT bands. Finally, the anionic cyanide-bound series reveals the highest degree of valence delocalization with the most intense IVCT bands (Kc = 1012–1020) and spin ground state population beyond room temperature. Across the series, at a given formal oxidation level, the capping ligand on the hexairon cluster dictates the overall properties of the aggregate, modulating the redox delocalization and the persistence of the intracore coupling of the metal sites.

The field of single molecule magnetism remains predicated on super- and double exchange mechanisms to engender large spin ground states. An alternative approach to achieving high-spin architectures involves synthesizing weak-field clusters featuring close M–M interactions to produce a single valence orbital manifold. Population of this orbital manifold in accordance with Hund’s rules could potentially yield thermally persistent high-spin ground states under which the valence electrons remain coupled. We now demonstrate this effect with a reduced hexanuclear iron cluster that achieves an S = 19/2 (χMT ≈ 53 cm3 K/mol) ground state that persists to 300 K, representing the largest spin ground state persistent to room temperature reported to date. The reduced cluster displays single molecule magnet behavior manifest in both variable-temperature zero-field 57Fe Mössbauer and magnetometry with a spin reversal barrier of 42.5(8) cm–1 and a magnetic blocking temperature of 2.9 K (0.059 K/min).

The dipyrrinato iron catalyst reacts with organic azides to generate a reactive, high-spin imido radical intermediate, distinct from nitrenoid or imido species commonly observed with low-spin transition metal complexes. The unique electronic structure of the putative group-transfer intermediate dictates the chemoselectivity for intermolecular nitrene transfer. The mechanism of nitrene group transfer was probed via amination and aziridination of para-substituted toluene and styrene substrates, respectively. The Hammett analysis of both catalytic amination and aziridination reactions indicate the rate of nitrene transfer is enhanced with functional groups capable of delocalizing spin. Intermolecular amination reactions with olefinic substrates bearing allylic C–H bonds give rise to exclusive allylic amination with no apparent aziridination products. Amination of substrates containing terminal olefins give rise exclusively to allylic C–H bond abstraction, C–N recombination occurring at the terminal C with transposition of the double bond. A similar reaction is observed with cis-β-methylstyrene where exclusive amination of the allylic position is observed with isomerization of the olefin to the trans-configuration. The high levels of chemoselectivity are attributed to the high-spin electronic configuration of the reactive imido radical intermediate, while the steric demands of the ligand enforce regioselective amination at the terminal position of linear α-olefins.

Iron(III) complexes of the tris(pyrrolide)ethane trianion have been synthesized by reaction of one- and two-electron oxidants with [(tpe)Fe(THF)][Li(THF)4] (tpe = tris(5-mesitylpyrrolyl)ethane). X-ray crystallography, 57Fe Mössbauer, 1H NMR and EPR spectroscopy, SQUID magnetometry, and density functional theory calculations were employed to rigorously establish the iron 3+ oxidation state. All oxidants employed are proposed to operate via an inner-sphere electron transfer mechanism. Dialkyl peroxides and dibenzyldisulfide served to oxidize iron by one electron, and group transfer of an aryl nitrene unit to the Fe2+ starting material resulted in formation of Fe3+ amido species following H-atom abstraction by a presumed nitrenoid intermediate. Single electron transfer to and from diphenyldiazoalkane was also observed to yield a diphenyldiazomethanyl radical anion antiferromagnetically coupled to the S = 5/2 Fe3+. Isolation of Fe3+ complexes of tpe, in comparison with previous results wherein the tpe ligand was the redox active moiety, presents an unusual juxtaposition of two noncommunicating redox reservoirs, each accessible via different reaction pathways (namely, inner- and outer-sphere electron transfer).

Concomitant deprotonation and metalation of hexadentate ligand platform tbsLH6 (tbsLH6 = 1,3,5-C6H9(NHC6H4-o-NHSiMe2tBu)3) with divalent transition metal starting materials Fe2(Mes)4 (Mes = mesityl) or Mn3(Mes)6 in the presence of tetrahydrofuran (THF) resulted in isolation of homotrinuclear complexes (tbsL)Fe3(THF) and (tbsL)Mn3(THF), respectively. In the absence of coordinating solvent (THF), the deprotonation and metalation exclusively afforded dinuclear complexes of the type (tbsLH2)M2 (M = Fe or Mn). The resulting dinuclear species were utilized as synthons to prepare bimetallic trinuclear clusters. Treatment of (tbsLH2)Fe2 complex with divalent Mn source (Mn2(N(SiMe3)2)4) afforded the bimetallic complex (tbsL)Fe2Mn(THF), which established the ability of hexamine ligand tbsLH6 to support mixed metal clusters. The substitutional homogeneity of (tbsL)Fe2Mn(THF) was determined by 1H NMR, 57Fe Mössbauer, and X-ray fluorescence. Anomalous scattering measurements were critical for the unambiguous assignment of the trinuclear core composition. Heating a solution of (tbsLH2)Mn2 with a stoichiometric amount of Fe2(Mes)4 (0.5 mol equiv) affords a mixture of both (tbsL)Mn2Fe(THF) and (tbsL)Fe2Mn(THF) as a result of the thermodynamic preference for heavier metal substitution within the hexa-anilido ligand framework. These results demonstrate for the first time the assembly of mixed metal cluster synthesis in an unbiased ligand platform.

High-spin trinuclear iron complex (tbsL)Fe3(thf) ([tbsL]6– = [1,3,5-C6H9(NC6H4-o-NSitBuMe2)3]6–) (S = 6) facilitates 2 and 4e– reduction of NxHy type substrates to yield imido and nitrido products. Reaction of hydrazine or phenylhydrazine with (tbsL)Fe3(thf) yields triiron μ3-imido cluster (tbsL)Fe3(μ3-NH) and ammonia or aniline, respectively. (tbsL)Fe3(μ3-NH) has a similar zero-field 57Fe Mössbauer spectrum compared to previously reported [(tbsL)Fe3(μ3-N)]NBu4, and can be directly synthesized by protonation of the anionic triiron nitrido with lutidinium tetraphenylborate. Deprotonation of the triiron parent imido (tbsL)Fe3(μ3-NH) with lithium bis(trimethylsilyl)amide results in regeneration of the triiron nitrido complex capped with a thf-solvated Li cation [(tbsL)Fe3(μ3-N)]Li(thf)3. The lithium capped nitrido, structurally similar to the pseudo C3-symmetric triiron nitride with a tetrabutylammonium countercation, is rigorously C3-symmetric featuring intracore distances of Fe–Fe 2.4802(5) Å, Fe–N(nitride) 1.877(2) Å, and N(nitride)–Li 1.990(6) Å. A similar 2e– reduction of 1,2-diphenylhydrazine by (tbsL)Fe3(thf) affords (tbsL)Fe3(μ3-NPh) and aniline. The solid state structure of (tbsL)Fe3(μ3-NPh) is similar to the series of μ3-nitrido and -imido triiron complexes synthesized in this work with average Fe–Nimido and Fe–Fe bond lengths of 1.941(6) and 2.530(1) Å, respectively. Reductive N═N bond cleavage of azobenzene is also achieved in the presence of (tbsL)Fe3(thf) to yield triiron bis-imido complex (tbsL)Fe3(μ3-NPh)(μ2-NPh), which has been structurally characterized. Ligand redox participation has been ruled out, and therefore, charge balance indicates that the bis-imido cluster has undergone a 4e– metal based oxidation resulting in an (FeIV)(FeIII)2 formulation. Cyclic voltammograms of the series of triiron clusters presented herein demonstrate that oxidation states up to (FeIV)(FeIII)2 (in the case of [(tbsL)Fe3(μ3-N)]NBu4) are electrochemically accessible. These results highlight the efficacy of high-spin, polynuclear reaction sites to cooperatively mediate small molecule activation.

The asymmetric oxidation product [(PhL)Fe3(μ-Cl)]2 [PhLH6 = MeC(CH2NHPh-o-NHPh)3], where each trinuclear core is comprised of an oxidized diiron unit [Fe2]5+ and an isolated trigonal pyramidal ferrous site, reacts with MCl2 salts to afford heptanuclear bridged structures of the type (PhL)2Fe6M(μ-Cl)4(thf)2, where M = Fe or Co. Zero-field, 57Fe Mössbauer analysis revealed the Co resides within the trinuclear core subunits, not at the octahedral, halide-bridged MCl4(thf)2 position indicating Co migration into the trinuclear subunits has occurred. Reaction of [(PhL)Fe3(μ-Cl)]2 with CoCl2 (2 or 5 equivalents) followed by precipitation via addition of acetonitrile afforded trinuclear products where one or two irons, respectively, can be substituted within the trinuclear core. Metal atom substitution was verified by 1H NMR, 57Fe Mossbauer, single crystal X-ray diffraction, X-ray fluorescence, and magnetometry analysis. Spectroscopic analysis revealed that the Co atom(s) substitute(s) into the oxidized dimetal unit ([M2]5+), while the M2+ site remains iron-substituted. Magnetic data acquired for the series are consistent with this analysis revealing the oxidized dimetal unit comprises a strongly coupled S = 1 unit ([FeCo]5+) or S = 1/2 ([Co2]5+) that is weakly antiferromagnetically coupled to the high spin (S = 2) ferrous site. The kinetic pathway for metal substitution was probed via reaction of [(PhL)Fe3(μ-Cl)]2 with isotopically enriched 57FeCl2(thf)2, the results of which suggest rapid equilibration of 57Fe into both the M2+ site and oxidized diiron site, achieving a 1:1 mixture.

The reaction of (ArL)Co(py) with tBuN3 afforded the isolable three-coordinate Co–imido complex (ArL)Co(NtBu), which is paramagnetic at room temperature. Variable-temperature (VT) 1H NMR spectroscopy, VT crystallography, and magnetic susceptibility measurements revealed that (ArL)Co(NtBu) undergoes a thermally induced spin crossover from an S = 0 ground state to a quintet (S = 2) state. The reaction of (ArL)Co(py) with mesityl azide yielded an isolable S = 1 terminal imido complex that was converted into the metallacycloindoline (ArL)Co(κ2-NHC6H2-2,4-Me2-6-CH2) via benzylic C–H activation.

Utilizing a hexadentate ligand platform, a series of trinuclear iron clusters (PhL)Fe3L*3 (PhLH6 = MeC(CH2NPh-o-NPh)3; L* = tetrahydrofuran (1), pyridine (2), PMePh2 (3)) has been prepared. The phenyl substituents on the ligand sterically prohibit strong iron–iron bonding from occurring but maintain a sufficiently close proximity between iron centers to permit direct interactions. Coordination of the weak-field tetrahydrofuran ligand to the iron centers results in a well-isolated, high-spin S = 6 or S = 5 ground state, as ascertained through variable-temperature dc magnetic susceptibility and low-temperature magnetization measurements. Replacing the tetrahydrofuran ligands with stronger σ-donating pyridine or tertiary phosphine ligands reduces the ground state to S = 2 and gives rise to temperature-dependent magnetic susceptibility. In these cases, the magnetic susceptibility cannot be explained as arising simply from superexchange interactions between metal centers through the bridging amide ligands. Rather, the experimental data are best modelled by considering a thermally-induced variation in molecular spin state between S = 2 and S = 4. Fits to these data provide thermodynamic parameters of ΔH = 406 cm−1 and Tc = 187 K for 2 and ΔH = 604 cm−1 and Tc = 375 K for 3. The difference in these parameters is consistent with ligand field strength differences between pyridine and phosphine ligands. To rationalize the spin state variation across the series of clusters, we first propose a qualitative model of the Fe3 core electronic structure that considers direct Fe–Fe interactions, arising from direct orbital overlap. We then present a scenario, consistent with the observed magnetic behaviour, in which the σ orbitals of the electronic structure are perturbed by substitution of the ancillary ligands.

Transamination of divalent transition metal starting materials (M2(N(SiMe3)2)4, M = Mn, Co) with hexadentate ligand platforms RLH6 (RLH6 = MeC(CH2NPh-o-NR)3 where R = H, Ph, Mes (Mes = Mesityl)) or H,CyLH6 = 1,3,5-C6H9(NHPh-o-NH2)3 with added pyridine or tertiary phosphine coligands afforded trinuclear complexes of the type (RL)Mn3(py)3 and (RL)Co3(PMe2R′)3 (R′ = Me, Ph). While the sterically less encumbered ligand varieties, HL or PhL, give rise to local square-pyramidal geometries at each of the bound metal atoms, with four anilides forming an equatorial plane and an exogenous pyridine or phosphine in the apical site, the mesityl-substituted ligand (MesL) engenders local tetrahedral coordination. Both the neutral Mn3 and Co3 clusters feature S = 1/2 ground states, as determined by direct current (dc) magnetometry, 1H NMR spectroscopy, and low-temperature electron paramagnetic resonance (EPR) spectroscopy. Within the Mn3 clusters, the long internuclear Mn–Mn separations suggest minimal direct metal–metal orbital overlap. Accordingly, fits to variable-temperature magnetic susceptibility data reveal the presence of weak antiferromagnetic superexchange interactions through the bridging anilide ligands with exchange couplings ranging from J = −16.8 to −42 cm–1. Conversely, the short Co–Co interatomic distances suggest a significant degree of direct metal–metal orbital overlap, akin to the related Fe3 clusters. With the Co3 series, the S = 1/2 ground state can be attributed to population of a single molecular orbital manifold that arises from mixing of the metal- and o-phenylenediamide (OPDA) ligand-based frontier orbitals. Chemical oxidation of the neutral Co3 clusters affords diamagnetic cationic clusters of the type [(RL)Co3(PMe2R)3]+. Density functional theory (DFT) calculations on the neutral (S = 1/2) and cationic (S = 0) Co3 clusters reveal that oxidation occurs at an orbital with contributions from both the Co3 core and OPDA subunits. The predicted bond elongations within the ligand OPDA units are corroborated by the ligand bond perturbations observed by X-ray crystallography.

The symmetric, high-spin triiron complex (PhL)Fe3(THF)3 reacts with mild chemical oxidants (e.g., Ph3C-X, I2) to afford an asymmetric core, where one iron bears the halide ligand (PhL)Fe3X(L) and the hexadentate (PhL = MeC(CH2NPh-o-NPh)3) ligand has undergone significant rearrangement. In the absence of a suitable trapping ligand, the chlorine and bromine complexes form (μ-X)2-bridged structures of the type [(PhL)Fe3(μ-X)]2. In the trinuclear complexes, the halide-bearing iron site sits in approximate trigonal-bipyramidal (tbp) geometry, formed by two (PhL) anilides and an exogenous solvent molecule. The two distal iron atoms reside in distorted square-planar sites featuring a short Fe–Fe separation at 2.301 Å, whereas the distance to the tbp site is substantially elongated (2.6–2.7 Å). Zero-field, 57Fe Mössbauer analysis reveals the diiron unit as the locus of oxidation, while the tbp site bearing the halide ligand remains divalent. Magnetic data acquired for the series reveal that the oxidized diiron unit comprises a strongly coupled S = 3/2 unit that is weakly ferromagnetically coupled to the high-spin (S = 2) ferrous site, giving an overall S = 7/2 ground state for the trinuclear units.

Reaction of NO2– with the octahedral cluster (HL)2Fe6 in the presence of a proton source affords the hexanitrosyl cluster (HL)2Fe6(NO)6. This species forms via a proton-induced reduction of six nitrite molecules per cluster, utilizing each site available on the polynuclear core. Formation of the hexanitrosyl cluster is accompanied by a near 2-fold expansion of the (HL)2Fe6 core volume, where intracore Fe–Fe interactions are overcome by strong π-bonding between Fe centers and NO ligands. A core volume of this magnitude is rare in octahedral metal clusters not supported by interstitial atoms. Moreover, the structural flexibility afforded by the (HL)2Fe6 platform highlights the potential for other reaction chemistry involving species with metal–ligand multiple bonds. Carrying out the reaction of the cluster [(HL)2Fe6(NCMe)6]4+ with nitrite in the absence of a proton source serves to forestall the nitrite reduction and enables clean isolation of the intermediate hexanitro cluster [(HL)2Fe6(NO2)6]2–.

Utilizing a hexadentate ligand platform, a high-spin trinuclear iron complex of the type (tbsL)Fe3(thf) was synthesized and characterized ([tbsL]6− = [1,3,5-C6H9(NPh-o-NSitBuMe2)3]6−). The silyl-amide groups only permit ligation of one solvent molecule to the tri-iron core, resulting in an asymmetric core wherein each iron ion exhibits a distinct local coordination environment. The triiron complex (tbsL)Fe3(thf) rapidly consumes inorganic azide ([N3]NBu4) to afford an anionic, trinuclear nitride complex [(tbsL)Fe3(μ3-N)]NBu4. The nearly C3-symmetric complex exhibits a highly pyramidalized nitride ligand that resides 1.205(3) Å above the mean triiron plane with short Fe−N (1.871(3) Å) distances and Fe−Fe separation (2.480(1) Å). The nucleophilic nitride can be readily alkylated via reaction with methyl iodide to afford the neutral, trinuclear methylimide complex (tbsL)Fe3(μ3-NCH3). Alkylation of the nitride maintains the approximate C3-symmetry in the imide complex, where the imide ligand resides 1.265(9) Å above the mean triiron plane featuring lengthened Fe−Nimide bond distances (1.892(3) Å) with nearly equal Fe−Fe separation (2.483(1) Å).

Dipyrromethene ligand scaffolds were synthesized bearing large aryl (2,4,6-Ph3C6H2, abbreviated Ar) or alkyl (tBu, adamantyl) flanking groups to afford three new disubstituted ligands (RL, 1,9-R2-5-mesityldipyrromethene, R = aryl, alkyl). While high-spin (S = 2), four-coordinate iron complexes of the type (RL)FeCl(solv) were obtained with the alkyl-substituted ligand varieties (for R = tBu, Ad and solv = THF, OEt2), use of the sterically encumbered aryl-substituted ligand precluded binding of solvent and cleanly afforded a high-spin (S = 2), three-coordinate complex of the type (ArL)FeCl. Reaction of (AdL)FeCl(OEt2) with alkyl azides resulted in the catalytic amination of C−H bonds or olefin aziridination at room temperature. Using a 5% catalyst loading, 12 turnovers were obtained for the amination of toluene as a substrate, while greater than 85% of alkyl azide was converted to the corresponding aziridine employing styrene as a substrate. A primary kinetic isotope effect of 12.8(5) was obtained for the reaction of (AdL)FeCl(OEt2) with adamantyl azide in an equimolar toluene/toluene-d8 mixture, consistent with the amination proceeding through a hydrogen atom abstraction, radical rebound type mechanism. Reaction of p-tBuC6H4N3 with (ArL)FeCl permitted isolation of a high-spin (S = 2) iron complex featuring a terminal imido ligand, (ArL)FeCl(N(p-tBuC6H4)), as determined by 1H NMR, X-ray crystallography, and 57Fe Mössbauer spectroscopy. The measured Fe−Nimide bond distance (1.768(2) Å) is the longest reported for Fe(imido) complexes in any geometry or spin state, and the disruption of the bond metrics within the imido aryl substituent suggests delocalization of a radical throughout the aryl ring. Zero-field 57Fe Mössbauer parameters obtained for (ArL)FeCl(N(p-tBuC6H4)) suggest a FeIII formulation and are nearly identical with those observed for a structurally similar, high-spin FeIII complex bearing the same dipyrromethene framework. Theoretical analyses of (ArL)FeCl(N(p-tBuC6H4)) suggest a formulation for this reactive species to be a high-spin FeIII center antiferromagnetically coupled to an imido-based radical (J = −673 cm−1). The terminal imido complex was effective for delivering the nitrene moiety to both C−H bond substrates (42% yield) as well as styrene (76% yield). Furthermore, a primary kinetic isotope effect of 24(3) was obtained for the reaction of (ArL)FeCl(N(p-tBuC6H4)) with an equimolar toluene/toluene-d8 mixture, consistent with the values obtained in the catalytic reaction. This commonality suggests the isolated high-spin FeIII imido radical is a viable intermediate in the catalytic reaction pathway. Given the breadth of iron imido complexes spanning several oxidation states (FeII−FeV) and several spin states (S = 0 → 3/2), we propose the unusual electronic structure of the described high-spin iron imido complexes contributes to the observed catalytic reactivity.

Using a trinucleating hexaamide ligand platform, the all-ferrous hexanuclear cluster (HL)2Fe6 (1) is obtained from reaction of 3 equiv of Fe2(Mes)4 (Mes = 2,4,6-Me3C6H2) with 2 equiv of the ligand (HL)H6. Compound 1 was characterized by X-ray diffraction analysis, 57Fe Mössbauer, SQUID magnetometry, mass spectrometry, and combustion analysis, providing evidence for an S = 6 ground state and delocalized electronic structure. The cyclic voltammogram of [(HL)2Fe6]n+ in acetonitrile reveals a rich redox chemistry, featuring five fully reversible redox events that span six oxidation states ([(HL)2Fe6]n+, where n = −1 → 4) within a 1.3 V potential range. Accordingly, each of these species is readily accessed chemically to provide the electron-transfer series [(HL)2Fe6(NCMe)m][PF6]n (m = 0, n = −1 (2); m = 2, n = 1 (3); m = 4, n = 2 (4); m = 6, n = 3 (5); m = 6, n = 4 (6)). Compounds 2–6 were isolated and characterized by X-ray diffraction, 57Fe Mössbauer and multinuclear NMR spectroscopy, and combustion analysis. Two-electron oxidation of the tetracationic cluster in 6 by 2 equiv of [NO]+ generates the thermally unstable hexacationic cluster [(HL)2Fe6(NCMe)m]6+, which is characterized by NMR and 57Fe Mössbauer spectroscopy. Importantly, several stepwise systematic metrical changes accompany oxidation state changes to the [Fe6] core, namely trans ligation of solvent molecules and variation in Mössbauer spectra, spin ground state, and intracluster Fe–Fe separation. The observed metrical changes are rationalized by considering a qualitative, delocalized molecular orbital description, which provides a set of frontier orbitals populated by Fe 3d electrons.

Utilizing a hexadentate ligand platform, a trinuclear manganese complex of the type (HL)Mn3(thf)3 was synthesized and characterized ([HL]6– = [MeC(CH2N(C6H4-o-NH))3]6–). The pale-orange, formally divalent trimanganese complex rapidly reacts with O-atom transfer reagents to afford the μ6-oxo complex (HL)2Mn6(μ6-O)(NCMe)4, where two trinuclear subunits bind the central O-atom and the (HL) ligands cooperatively bind both trinuclear subunits. The trimanganese complex (HL)Mn3(thf)3 rapidly consumes inorganic azide ([N3]NBu4) to afford a dianionic hexanuclear nitride complex [(HL)2Mn6(μ6-N)](NBu4)2, which subsequently can be oxidized with elemental iodine to (HL)2Mn6(μ6-N)(NCMe)4. EPR and alkylation of the interstitial light atom substituent were used to distinguish the nitride from the oxo complex. The oxo and oxidized nitride complexes give rise to well-defined Mn(II) and Mn(III) sites, determined by bond valence summation, while the dianionic nitride shows a more symmetric complex, giving rise to indistinguishable ion oxidation states based on crystal structure bond metrics.

Oxidation of the nominally all-ferrous hexanuclear cluster (HL)2Fe6 with six equivalents of ferrocenium in the presence of bromide ions results in a six-electron oxidation of the Fe6 core to afford the nominally all-ferric cluster (HL)2Fe6Br6. The hexabromide cluster is also structurally characterized in a 4+ core oxidation state. A structural comparison of these two clusters provides an insight into the Fe6 core electronic structure.

Systematic electronic variations were introduced into the monoanionic dipyrrinato ligand scaffold via halogenation of the pyrrolic β-positions and/or via the use of fluorinated aryl substituents in the ligand bridgehead position in order to synthesize proligands of the type 1,9-dimesityl-β-R4-5-Ar-dipyrrin [R = H, Cl, Br, I; Ar = mesityl, 3,5-(F3C)2C6H3, C6F5 in ligand 5-position; β = 2,3,7,8 ligand substitution; abbreviated (β,ArL)H]. The electronic perturbations were probed using standard electronic absorption and electrochemical techniques on the different ligand variations and their divalent iron complexes. The free-ligand variations cause modest shifts in the electronic absorption maxima (λmax: 464–499 nm) and more pronounced shifts in the electrochemical redox potentials for one-electron proligand reductions (E1/2: −1.25 to −1.99 V) and oxidations (E1/2: +0.52 to +1.14 V vs [Cp2Fe]+/0). Installation of iron into the dipyrrinato scaffolds was effected via deprotonation of the proligands followed by treatment with FeCl2 and excess pyridine in tetrahydrofuran to afford complexes of the type (β,ArL)FeCl(py) (py = pyridine). The electrochemical and spectroscopic behavior of these complexes varies significantly across the series: the redox potential of the fully reversible FeIII/II couple spans more than 400 mV (E1/2: −0.34 to +0.50 V vs [Cp2Fe]+/0); λmax spans more than 40 nm (506–548 nm); and the 57Fe Mössbauer quadrupole splitting (|ΔEQ|) spans nearly 2.0 mm/s while the isomer shift (δ) remains essentially constant (0.86–0.89 mm/s) across the series. These effects demonstrate how peripheral variation of the dipyrrinato ligand scaffold can allow systematic variation of the chemical and physical properties of iron dipyrrinato complexes.

The reactivity of the high-spin (S = 2) [(Mestpe)Fe(THF)][Li(THF)4] (1) complex (Mestpe = tris(mesitylpyrrolide)ethane) with isocyanide and CO substrates is explored. Reaction of 1 with excess tBuNC forms a low-spin (S = 0), six-coordinate iron(II) species with three tBuNC ligands bound to iron, producing a notable tautomerization of one of the pyrrolide units from N- to C-ligation to iron. Reaction of 1 with an atmosphere of CO also produces a new diamagnetic complex, wherein two molecules of CO are consecutively reductively coupled, driven by the two-electron oxidation and fragmentation of the tris(pyrrolide)ethane ligand. The product features a six-coordinate Fe(II) species bound to a dipyrromethene ligand (resulting from oxidative fragmentation of the Mestpe ligand), an oxalyl-imino pyrrole fragment from pyrrole coupling to two molecules of CO. The reactions of 1 with tBuNC and CO provide insight into how tautomerization of the tris(pyrrolide) ligand upon substrate binding initiates the contiguous reductive coupling of CO.

First-row transition metal complexes of the tris(pyrrolyl)ethane (tpe) trianion have been prepared. The tpe ligand was found to coordinate in a uniform η1,η1,η1-coordination mode to the divalent metal series as revealed by X-ray diffraction studies. Magnetic and structural characterization for complexes of the type [(tpe)MII(py)][Li(THF)4] (M: Mn, Fe, Co, Ni) reveal each divalent ion to be high-spin and have a distorted trigonal-monopyramidal geometry in the solid state. The pyridine ligand binds significantly canted from the molecular C3 axis due to a stabilizing π-stacking interaction with a ligand mesityl substituent. Cyclic voltammetry on the [(tpe)MII(py)]− series reveals a common irreversible oxidation pathway that is entirely ligand-based, invariant to the divalent metal bound. This latter observation indicates that fully populated ligand-based orbitals from the tpe construct are energetically most accessible in the electrochemical experiments, akin to their dipyrromethane analogues. Chemical oxidation of [(tpe)FeII(py)]− yields a product in which the ligand has dissociated one pyrrole (following tpe oxidation and H-atom abstraction) and binds a second equivalent of pyridine to form the neutral, tetrahedral FeII species (κ2-tpe)Fe(py)2. Similarly, chemical oxidation of the Zn(II) analogue shows evidence for tpe oxidation by electron paramagnetic resonance spectroscopy (77 K, toluene glass) with an isotropic signal for the organic radical at g = 2.002. Density functional theory analysis on this family of complexes reveals that the highest lying molecular orbitals are completely ligand-based, corroborating our proposed electronic structure assignment.

Transition metal complexes (Mn → Zn) of the dipyrromethane ligand, 1,9-dimesityl-5,5-dimethyldipyrromethane (dpm), have been prepared. Arylation of the dpm ligand α to the pyrrolic nitrogen donors limits the accessibility of the pyrrole π-electrons for transition metal coordination, instead forcing η1,η1 coordination to the divalent metal series as revealed by X-ray diffraction studies. Structural and magnetic characterization (SQUID, EPR) of the bis-pyridine adducts of (dpm)MnII(py)2, (dpm)FeII(py)2, and (dpm)CoII(py)2 reveal each divalent ion to be high-spin and pseudotetrahedral in the solid state, whereas the (dpm)NiII(py)2 is low-spin and adopts a square-planar geometry. Differential pulse voltammetry on the (dpm)MII(py)2 series reveals a common two-electron oxidation pathway that is entirely ligand-based, invariant to the divalent metal-bound, its geometry or spin state within the dpm framework. This latter observation indicates that fully populated ligand-based orbitals from the dpm construct lie above partially filled metal 3d orbitals without intramolecular redox chemistry or spin-state tautomerism occurring. DFT analysis on this family of complexes corroborates this electronic structure assignment, revealing that the highest lying molecular orbitals are completely ligand-based. Chemical oxidation of the deprotonated dpm framework results in the four-electron oxidation of the dipyrrolide framework, although this oxidation product was not observed either in the electrochemical or chemical oxidation of the (dpm)MII(py)2 complexes.

In this Communication, we report an intramolecular C−H bond amination reaction of a dipyrromethene ferrous complex with organic azides. Monitoring of the spectral changes (variable-temperature NMR and UV−vis) of the FeII complex reveals no buildup of an intermediate during conversion of the starting material into the nitrene-inserted product. The rate-determining step appears to be azide addition to the 14-electron FeII complex, hinting at the potential that these and related platforms may have to effect atom- and group-transfer processes.

Oxidation of the nominally all-ferrous hexanuclear cluster (HL)2Fe6 with six equivalents of ferrocenium in the presence of bromide ions results in a six-electron oxidation of the Fe6 core to afford the nominally all-ferric cluster (HL)2Fe6Br6. The hexabromide cluster is also structurally characterized in a 4+ core oxidation state. A structural comparison of these two clusters provides an insight into the Fe6 core electronic structure.

The dipyrrinato iron catalyst reacts with organic azides to generate a reactive, high-spin imido radical intermediate, distinct from nitrenoid or imido species commonly observed with low-spin transition metal complexes. The unique electronic structure of the putative group-transfer intermediate dictates the chemoselectivity for intermolecular nitrene transfer. The mechanism of nitrene group transfer was probed via amination and aziridination of para-substituted toluene and styrene substrates, respectively. The Hammett analysis of both catalytic amination and aziridination reactions indicate the rate of nitrene transfer is enhanced with functional groups capable of delocalizing spin. Intermolecular amination reactions with olefinic substrates bearing allylic C–H bonds give rise to exclusive allylic amination with no apparent aziridination products. Amination of substrates containing terminal olefins give rise exclusively to allylic C–H bond abstraction, C–N recombination occurring at the terminal C with transposition of the double bond. A similar reaction is observed with cis-β-methylstyrene where exclusive amination of the allylic position is observed with isomerization of the olefin to the trans-configuration. The high levels of chemoselectivity are attributed to the high-spin electronic configuration of the reactive imido radical intermediate, while the steric demands of the ligand enforce regioselective amination at the terminal position of linear α-olefins.

Utilizing a hexadentate ligand platform, a series of trinuclear iron clusters (PhL)Fe3L*3 (PhLH6 = MeC(CH2NPh-o-NPh)3; L* = tetrahydrofuran (1), pyridine (2), PMePh2 (3)) has been prepared. The phenyl substituents on the ligand sterically prohibit strong iron–iron bonding from occurring but maintain a sufficiently close proximity between iron centers to permit direct interactions. Coordination of the weak-field tetrahydrofuran ligand to the iron centers results in a well-isolated, high-spin S = 6 or S = 5 ground state, as ascertained through variable-temperature dc magnetic susceptibility and low-temperature magnetization measurements. Replacing the tetrahydrofuran ligands with stronger σ-donating pyridine or tertiary phosphine ligands reduces the ground state to S = 2 and gives rise to temperature-dependent magnetic susceptibility. In these cases, the magnetic susceptibility cannot be explained as arising simply from superexchange interactions between metal centers through the bridging amide ligands. Rather, the experimental data are best modelled by considering a thermally-induced variation in molecular spin state between S = 2 and S = 4. Fits to these data provide thermodynamic parameters of ΔH = 406 cm−1 and Tc = 187 K for 2 and ΔH = 604 cm−1 and Tc = 375 K for 3. The difference in these parameters is consistent with ligand field strength differences between pyridine and phosphine ligands. To rationalize the spin state variation across the series of clusters, we first propose a qualitative model of the Fe3 core electronic structure that considers direct Fe–Fe interactions, arising from direct orbital overlap. We then present a scenario, consistent with the observed magnetic behaviour, in which the σ orbitals of the electronic structure are perturbed by substitution of the ancillary ligands.