Person:

Betley, Theodore

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.

A modular hexadentate polyamide ligand directs the assembly of triiron complexes (see structure). Crystallographic studies reveal significant metal–metal bonding interactions, which are enhanced upon oxidation.

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.

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.

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)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.

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.

Utilizing a hexadentate ligand platform, a series of trinuclear iron clusters (PhL)Fe3L3 (PhLH6 1⁄4 MeC (CH2NPh-o-NPh)3; L 1⁄4 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 1⁄4 6 or S 1⁄4 5 ground state, as ascertained through variable-temperature dc magnetic susceptibility and low- temperature magnetization measurements. Replacing the tetrahydrofuran ligands with stronger s-donating pyridine or tertiary phosphine ligands reduces the ground state to S 1⁄4 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 1⁄4 2 and S 1⁄4 4. Fits to these data provide thermodynamic parameters of DH 1⁄4 406 cm 1 and Tc 1⁄4 187 K for 2 and DH 1⁄4 604 cm 1 and Tc 1⁄4 375 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 s orbitals of the electronic structure are perturbed by substitution of the ancillary ligands.

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.

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–.

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.