Conversion of a Fleeting Open-Shell Iron Nitride into an Iron Nitrosyl

Photoinduced Cleavage of a Strained N-C Bond in an Iron Complex Supported by Super-Bulky Amidinate and Guanidinate Ligands

The reaction of Fe2(mes)4 with the super-bulky amidines and guanidines HLAr*-R (LAr*-R = [(Ar*N)2C(R)]-, Ar* = 2,6-bis(diphenylmethyl)-4-tert-butylphenyl), R = Me (LAr*-Me), tBu (LAr*-tBu), Ph (LAr*-Ph), NiPr2 (LAr*-iPr2N), and Pip (LAr*-Pip)) gives access to the three-coordinate iron-mesityl complexes (LAr*-R)Fe(mes) only where LAr*-R = LAr*-Me, LAr*-Ph, or LAr*-Pip. Subsequent protonolysis with the N-atom transfer reagent Hdbabh (Hdbabh = 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene) is limited in success, providing in one instance a few crystals of four-coordinate (LAr*-Me)Fe(dbabh)(Hdbabh), while three-coordinate (LAr*-Pip)Fe(dbabh) is synthesized reproducibly. Complexes (LAr*-Me)Fe(dbabh)(Hdbabh) and (LAr*-Pip)Fe(dbabh) are thermally insensitive in solution to temperatures of up to 100 °C. On the other hand, both (LAr*-Me)Fe(dbabh)(Hdbabh) and (LAr*-Pip)Fe(dbabh) show sensitivity to blue LED light (395 nm), undergoing photochemical transformations. For instance, the photolysis of (LAr*-Me)Fe(dbabh)(Hdbabh) leads to N-C bond scission and C-C bond coupling across the -dbabh moieties to give four-coordinate (LAr*-Me)Fe(N=dbabh-dbabhNH2). Photolyzing pyridine-d5 (py-d5) solutions of (LAr*-Pip)Fe(dbabh) at -5 °C produces a new paramagnetic photoproduct, [P]. Due to the thermal sensitivity of compound [P], it has eluded structural characterization; yet, Evans' method measurements suggest that the iron(II) oxidation state is maintained, thereby pointing to the -dbabh moiety as the locus of chemical change. In line with this assessment, addition of excess Me3SiCl to solutions of [P] produces the iron(II) complex (LAr*-Pip)FeCl(py-d5) as shown by 1H NMR spectroscopy. Gas chromatography/mass spectrometry analysis of the solutions of [P] shows a peak in the chromatogram with a molecular mass corresponding to a formulation of C14H11N that cannot be attributed to Hdbabh. This provides evidence for the photochemical-induced isomerization of the -dbabh ligand, revealing a heretofore unknown photochemical sensitivity of this N atom transfer reagent.

Terminal, Open-Shell Mo Carbide and Carbyne Complexes: Spin Delocalization and Ligand Noninnocence

Open-shell compounds bearing metal-carbon triple bonds, such as carbides and carbynes, are of significant interest as plausible intermediates in the reductive catenation of C₁ oxygenates. Despite the abundance of closed-shell carbynes reported, open-shell variants are very limited, and an open-shell carbide has yet to be reported. Herein, we report the synthesis of the first terminal, open-shell carbide complexes, [K][1] and [1][BAr^F₄] (1 = P2Mo(≡C:)(CO), P2 = a terphenyl diphosphine ligand), which differ by two redox states, as well as a series of related open-shell carbyne complexes. The complexes are characterized by single-crystal X-ray diffraction and NMR, EPR, and IR spectroscopies, while the electronic structures are probed by EPR studies and DFT calculations to assess spin delocalization. In the d₁ complexes, the spin is primarily localized on the metal (∼55-77% Mo d_xy) with delocalization on the triply bonded carbon of ∼0.05-0.09 e^-. In the reduced carbide [K][1], a direct metal-arene interaction enables ancillary ligand reduction, resulting in reduced radical character on the terminal carbide (⩽0.02 e^-). Reactivity studies with [K][1] reveal the formation of mixed-valent C-C coupled products at -40 °C, illustrating how productive reactivity manifolds can be engendered through the manipulation of redox states. Combined, the results inform on the electronic structure and reactivity of a new and underrepresented class of compounds with potential significance to a wide array of reactions involving open-shell species.

C-H Activation by an Iron-Nitrido Bis-Pocket Porphyrin Species

High-valent iron-nitrido species are nitrogen analogues of iron-oxo species which are versatile reagents for C-H oxidation. Nonetheless, C-H activation by iron-nitrido species has been scarcely explored, as this is often hampered by their instability and short lifetime in solutions. Herein, the hydrogen atom transfer (HAT) reactivity of an Fe porphyrin nitrido species (2 c) toward C-H substrates was studied in solutions at room temperature, which was achieved by nanosecond laser flash photolysis (LFP) of its Fe^III -azido precursor (1 c) supported by a bulky bis-pocket porphyrin ligand. C-H bonds with bond dissociation enthalpies (BDEs) of up to ≈84 kcal mol^-1 could be activated, and the second-order rate constants (k₂ ) are on the order of 10² -10⁴  s^-1  m^-1 . The Fe-amido product formed after HAT could further release ammonia upon protonation.

Conversion of a Fleeting Open-Shell Iron Nitride into an Iron Nitrosyl

Terminal metal nitrides have been proposed as key intermediates in a series of pivotal chemical transformations. However, exploring the chemical activity of transient tetragonal iron(V) nitrides is largely impeded by their facile dimerization in fluid solutions. Herein, in situ EPR and Mössbauer investigations are presented of unprecedented oxygenation of a paramagnetic iron(V) nitrido intermediate, [Fe^VN(cyclam‐ac)]⁺ (2, cyclam‐ac⁻=1,4,8,11‐tetraazacyclotetradecane‐1‐acetate anion), yielding an iron nitrosyl complex, [Fe(NO)(cyclam‐ac)]⁺ (3). Further theoretical studies suggest that during the reaction a closed‐shell singlet O atom is transferred to 2. Consequently, the N−O bond formation does not follow a radical coupling mechanism proposed for the N−N bond formation but is accomplished by three mutual electron‐transfer pathways between 2 and the O atom donor, thanks to the ambiphilic nature of 2.

A Series of Iron Nitrosyl Complexes {Fe-NO}6-9 and a Fleeting {Fe-NO}10 Intermediate en Route to a Metalacyclic Iron Nitrosoalkane

Iron-nitrosyls have fascinated chemists for a long time due to the noninnocent nature of the NO ligand that can exist in up to five different oxidation and spin states. Coordination to an open-shell iron center leads to complex electronic structures, which is the reason Enemark-Feltham introduced the {Fe-NO}ⁿ notation. In this work, we succeeded in characterizing a series of {Fe-NO}^6-9 complexes, including a reactive {Fe-NO}¹⁰ intermediate. All complexes were synthesized with the tris-N-heterocyclic carbene ligand tris[2-(3-mesitylimidazol-2-ylidene)ethyl]amine (TIMEN^Mes), which is known to support iron in high and low oxidation states. Reaction of NOBF₄ with [(TIMEN^Mes)Fe]²⁺ resulted in formation of the {Fe-NO}⁶ compound [(TIMEN^Mes)Fe(NO)(CH₃CN)](BF₄)₃ (1). Stepwise chemical reduction with Zn, Mg, and Na/Hg leads to the isostructural series of high-spin iron nitrosyl complexes {Fe-NO}^7,8,9 (2-4). Reduction of {Fe-NO}⁹ with Cs electride finally yields the highly reduced {Fe-NO}¹⁰ intermediate, key to formation of [Cs(crypt-222)][(TIMEN^Mes)Fe(NO)], (5) featuring a metalacyclic [Fe-(NO-NHC)^3-] nitrosoalkane unit. All complexes were characterized by single-crystal XRD analyses, temperature and field-dependent SQUID magnetization methods, as well as ⁵⁷Fe Mössbauer, IR, UV/vis, multinuclear NMR, and dual-mode EPR spectroscopy. Spectroscopy-based DFT analyses provide insight into the electronic structures of all compounds and allowed assignments of oxidation states to iron and NO ligands. An alternative synthesis to the {Fe-NO}⁸ complex was found via oxygenation of the nitride complex [(TIMEN^Mes)Fe(N)](BF₄). Surprisingly, the resulting {Fe-NO}⁸ species is electronically and structural similar to the [(TIMEN^Mes)Fe(N)]⁺ precursor. Based on the structural and electronic similarities between this nitrosyl/nitride complex couple, we adopted the strategy, developed by Wieghardt et al., of extending the Enemark-Feltham nomenclature to nitrido complexes, rendering [(TIMEN^Mes)Fe(N)]⁺ as a {Fe-N}⁸ species.