Jeremy M. Smith

The stabilization of low-coordinate and high-spin iron complexes plays a central role in enabling novel pathways for C–H functionalization and catalysis of organic reactions. Previous work from our group demonstrated that the bulky bis(carbene)borate ligand Ph2B(tBuIm)2– supports the three-coordinate iron(II) imido complex [Ph2B(tBuIm)2Fe=NDipp]−. In this study, we extend this platform by employing an even larger, more electron-withdrawing imido ligand to generate Ph2B(tBuIm)2Fe(N(terph)) (terph = 2,6-C6H5(C6H3)). This iron(III) imido complex was isolated and fully characterized, including by single-crystal X-ray diffraction. The complex reacts with 2-picoline to afford C–N bond functionalization at the benzylic position of the substrate. Mechanistic investigations by density functional theory calculations support a pathway in which intramolecular hydrogen atom transfer from the coordinated substrate to the imido ligand provides a carbon-centered radical. Subsequent intramolecular radical rebound forms a new C–N bond. Together, these two steps define a mechanistic sequence reminiscent of enzymatic C–H functionalization pathways at a well-defined, low-coordinate iron complex. In addition, DFT provides insight into the directing effect of the picoline ligand.

High‐spin ( S = 2) iron(II) imido complexes [Ph ₂ B( ^t BuIm) ₂ Fe═NDipp]M (M = Li ⁺ , K ⁺ , K(18‐c‐6) ⁺ ) are catalysts for the hydrogen isotope exchange (HIE) reaction with pyridine as the substrate. As dictated by the counter‐cation, these complexes catalyze site‐selective α ‐, α , β , γ ‐, and β , γ ‐deuteration of pyridine. Experimental and computational mechanistic investigations reveal the critical role of the counter‐cation in catalysis, which activates the substrate, facilitates deuteration, and dictates HIE regioselectivity. The stoichiometric reaction of pyridine with [Ph ₂ B( ^t BuIm) ₂ Fe═NDipp]Li affords the catalytically active bis(2‐pyridyl) complex [Ph ₂ B( ^t BuIm) ₂ Fe(2‐Py) ₂ Li(THF) ₂ ]. By maintaining coordination to the substrate during the catalytic cycle, Li ⁺ preorganizes pyridine for regioselective α ‐deuteration by this catalyst. On the other hand, [Ph ₂ B( ^t BuIm) ₂ Fe═NDipp]K reacts with pyridine to afford the 2‐pyridyl amido complex [Ph ₂ B( ^t BuIm) ₂ Fe(2‐Py)NHDipp] ⁻ , which has been structurally characterized with [K(18‐c‐6)(THF) ₂ ] ⁺ and [K(dibenzo‐18‐c‐6)(THF) ₂ ] ⁺ counterions. As dictated by the size of the counter‐cation, [Ph ₂ B( ^t BuIm) ₂ Fe═NDipp] ⁻ catalyzes regioselective α , β , γ ‐ and β , γ ‐deuteration of pyridine. Here, the counter‐cation stabilizes the appropriate pyridyl regioisomer for the selectivity‐determining deuteration step.

High‐spin ( S = 2) iron(II) imido complexes [Ph ₂ B( ^t BuIm) ₂ Fe═NDipp]M (M = Li ⁺ , K ⁺ , K(18‐c‐6) ⁺ ) are catalysts for the hydrogen isotope exchange (HIE) reaction with pyridine as the substrate. As dictated by the counter‐cation, these complexes catalyze site‐selective α ‐, α , β , γ ‐, and β , γ ‐deuteration of pyridine. Experimental and computational mechanistic investigations reveal the critical role of the counter‐cation in catalysis, which activates the substrate, facilitates deuteration, and dictates HIE regioselectivity. The stoichiometric reaction of pyridine with [Ph ₂ B( ^t BuIm) ₂ Fe═NDipp]Li affords the catalytically active bis(2‐pyridyl) complex [Ph ₂ B( ^t BuIm) ₂ Fe(2‐Py) ₂ Li(THF) ₂ ]. By maintaining coordination to the substrate during the catalytic cycle, Li ⁺ preorganizes pyridine for regioselective α ‐deuteration by this catalyst. On the other hand, [Ph ₂ B( ^t BuIm) ₂ Fe═NDipp]K reacts with pyridine to afford the 2‐pyridyl amido complex [Ph ₂ B( ^t BuIm) ₂ Fe(2‐Py)NHDipp] ⁻ , which has been structurally characterized with [K(18‐c‐6)(THF) ₂ ] ⁺ and [K(dibenzo‐18‐c‐6)(THF) ₂ ] ⁺ counterions. As dictated by the size of the counter‐cation, [Ph ₂ B( ^t BuIm) ₂ Fe═NDipp] ⁻ catalyzes regioselective α , β , γ ‐ and β , γ ‐deuteration of pyridine. Here, the counter‐cation stabilizes the appropriate pyridyl regioisomer for the selectivity‐determining deuteration step.

The MEH ₂ salts (M = Na, K; E = P, As) react with high spin ( S = 2) PhB(AdIm) ₃ FeCl (PhB(AdIm) ₃ ^– = tris(3‐adamantylimidazol‐2‐ylidene)phenylborate) to afford the corresponding PhB(AdIm) ₃ FePH ₂ and PhB(AdIm) ₃ FeAsH ₂ complexes. Remarkably, these complexes can be deprotonated by benzyl potassium in the presence of equimolar 18‐crown–6 or 2,2,2‐crypt to provide the parent phosphinidene and arsinidene complexes, [PhB(AdIm) ₃ FePH] ^– and [PhB(AdIm) ₃ FeAsH] ^– . Structural and spectroscopic characterization, combined with computational investigations, provide evidence for multiconfigurational electronic structures in which the metal–ligand multiple bond character is attenuated by the high spin iron(II) center. Initial reactivity studies reveal the nucleophilic character of the pnictogenidene ligands, which provide phosphinophosphinidene and phosphinoarsinidene products with a chlorodiazaphospholidine electrophile.

The MEH ₂ salts (M = Na, K; E = P, As) react with high spin ( S = 2) PhB(AdIm) ₃ FeCl (PhB(AdIm) ₃ ^– = tris(3‐adamantylimidazol‐2‐ylidene)phenylborate) to afford the corresponding PhB(AdIm) ₃ FePH ₂ and PhB(AdIm) ₃ FeAsH ₂ complexes. Remarkably, these complexes can be deprotonated by benzyl potassium in the presence of equimolar 18‐crown–6 or 2,2,2‐crypt to provide the parent phosphinidene and arsinidene complexes, [PhB(AdIm) ₃ FePH] ^– and [PhB(AdIm) ₃ FeAsH] ^– . Structural and spectroscopic characterization, combined with computational investigations, provide evidence for multiconfigurational electronic structures in which the metal–ligand multiple bond character is attenuated by the high spin iron(II) center. Initial reactivity studies reveal the nucleophilic character of the pnictogenidene ligands, which provide phosphinophosphinidene and phosphinoarsinidene products with a chlorodiazaphospholidine electrophile.

Herein, we report the reactions of the Fe(II) alkyl and aryl complexes (Ph₂B(^tBuIm)₂Fe(^tBuImH)R, R = CH₂^tBu and Mes) with N₂O and N-oxides (Me₃NO and C₅H₅NO), affording oxygen-inserted Fe(II) alkoxide and aryloxide complexes (Ph₂B(^tBuIm)₂Fe(^tBuImH)OR). Experimentally calibrated density functional theory (DFT) calculations implicate a mechanism in which the C-O bond is formed by oxygen atom insertion from an iron(IV) oxo intermediate. This stoichiometric oxygen insertion reaction forms the basis of a catalytic cycle to convert Grignard reagents to the corresponding arylphenols. This type of oxygen insertion enriches the family of iron-catalyzed oxidation mechanisms and complements the well-known oxygen rebound mechanism in oxygenases.

The iron(I) dinitrogen complex PhB(AdIm)₃FeN₂, which is supported by a very bulky 1-adamantyl-substituted tris(carbene)borate ligand, reacts with equimolar CO at low temperature to afford the high spin (S = 3/2) complex PhB(AdIm)₃Fe(CO). This monocarbonyl complex reacts with additional CO to afford the low spin (S = 1/2) dicarbonyl complex PhB(AdIm)₃Fe(CO)₂. By contrast, the high spin iron(I) tris(pyrazolyl)borate complex Tp^tBu,MeFe(CO) does not react with additional CO. Experimental and computational investigations reveal that coordination of CO to Tp^tBu,MeFe(CO) is spin blocked, whereas the stronger donor tris(carbene)borate ligand in PhB(AdIm)₃Fe(CO) sufficiently stabilizes the low spin state to enable a thermally accessible barrier for CO coordination. Kinetic measurements reveal that the rate of CO coordination reaction is many orders of magnitude slower than for a related cobalt complex, which is attributed in large part to the relative extent of geometric reorganization required to achieve the low spin state for iron(I) as opposed to cobalt(I). The barrier to the spin state change has allowed the mixed isotope complex PhB(AdIm)₃Fe(¹²CO)(¹³CO) to be prepared.

With the advent of the Haber-Bosch process over a century ago, the world's population experienced tremendous growth due to the million tons of fixed nitrogen used in fertilizer each year. However, the majority of the fixed nitrogen is metabolized to various nitrogen oxyanions by soil organisms that are lost to waterways, leading to eutrophication and hypoxia. Therefore, there is a need to convert the anthropogenic nitrogen content to more useful forms. In this work, the synthesis and characterization of iron nitrate (PhB(ⁱPr₂Im)₃FeNO₃) complex (2) is reported. Subsequent stepwise deoxygenation via a bis(boryl) reduced N-heterocycle (BPin₂pz) to its downstream counterparts is thermodynamically favorable by DFT. Reaction of iron nitrate with (BPin₂)pz led to the formation of iron (pinacolato)boralanolate complex (3) and a proposed diiron dinitrosyl complex having bridged hydroxide ligands ([PhB(ⁱPr₂Im)₃Fe(NO)(OH)]₂) (4). The formation of these complexes demonstrates the oxophilicity of both iron and boron.

While many mononuclear complexes for electrocatalytic NO_x^- reduction have been reported, there are few examples of dinuclear electrocatalysts. Here, we report on the electroreduction of NO₂^- by the dinuclear complex [Ni₂(tpmc)(NO₃)₂]²⁺ (tpmc = 1,4,8,11-tetrakis-(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane) and a mononuclear analogue [Ni(cyclam)]²⁺ (cyclam = 1,4,8,11-tetraazacyclotetradecane). Notably, the presence of the second metal ion in [Ni₂(tpmc)(NO₃)₂]²⁺ shifts the onset of catalysis anodically by ca. 0.3 V compared to [Ni(cyclam)]²⁺, while maintaining a similar electrocatalytic rate and Faradaic efficiency. Both complexes produce NH₄⁺ as the primary reduction product. Interestingly, in some respects, Ni²⁺(aq) is a better electrocatalyst for NO₂^- than [Ni(cyclam)]²⁺, with faster electrocatalysis occurring at a more anodic onset potential, although the Faradaic efficiency for NH₄⁺ is lower.