N-anchored tripodal N-heterocyclic carbene ligands, satTIMMNMes and Me2TIMMNMes, were synthesized and used to stabilize a series of mid-valent Fe(IV) and high-valent Fe(V) and Fe(VI) nitrides. The Fe(IV) and Fe(V) species adopt trigonal-pyramidal and trigonal-bipyramidal geometries, respectively, whereas the Fe(VI) nitrides exhibit octahedral coordination, representing rare examples of structurally characterized iron(VI) nitrido complexes. All complexes were characterized by single-crystal X-ray diffraction, multinuclear (1H, 13C, 15N, and 19F) NMR, zero- and applied-field 57Fe Mössbauer, electron paramagnetic resonance, as well as vibrational and electronic absorption spectroscopy. Combined spectroscopic, electrochemical, and computational studies examined how systematic variation in the NHC backbone modulates the electronic structures of the [Fe≡N]n+ and Fe-CNHC moieties. Structural and spectroscopic parameters of Me2TIMMNMes-based Fe(IV-VI) nitrides closely resemble those of parent TIMMNMes analogues, while more pronounced deviations are observed for satTIMMNMes derivatives. 57Fe Mössbauer spectroscopy at 80 K revealed an unusually negative isomer shift for the tetravalent (d4, S = 0) [(satTIMMNMes)FeIV≡N]+ (δ = -0.45 mm s-1), distinct from Me2TIMMNMes and TIMMNMes analogues (δ = -0.36 and -0.35 mm s-1), attributable to enhanced Fe-CNHC covalency in satTIMMNMes ligand. Cyclic voltammetry, supported by theoretical calculations, quantifies relative ligand π-donation in the Fe(IV) nitrido complexes, complementary to established NHC σ-donor descriptors, yielding the trend satTIMMNMes < TIMMNMes < Me2TIMMNMes. Consistent with this trend, structural and EPR data indicate differing degrees of Jahn-Teller distortion among the paramagnetic Fe(V) nitrides (d3, S = 1/2). In contrast, the structural and spectroscopic differences are relatively diminished in highly covalent Fe(VI) nitrides (d2, S = 0).

We report the synthesis and comprehensive characterization of a closed-shell iron(IV) oxo (ferryl) complex, S = 0, which is accessible through multiple methods and stabilized by a tripodal tris-N-heterocyclic carbene (NHC) ligand. Reaction of the FeI precursor [(Me2TIMMNMes)FeI(η1-N2)](PF6) (1) (Me2TIMMNMes = tris-[2-(3-mesityl-4,5-dimethyl-imidazolin-2-ylidene)methyl]amine) with an excess of N2O at -78 °C in THF yields the reactive FeIII oxo complex [(Me2TIMMNMes)FeIII(O)](PF6) (3). One-electron oxidation of the in situ generated 3 with [Cp2Fe][PF6] provides access to a rare, nonmagnetic (d 4, S = 0) FeIV oxo complex, namely [(Me2TIMMNMes)FeIV(O)](PF6)2 (4). Alternatively, tetravalent 4 could also be obtained by treatment of the divalent iron complex [(Me2TIMMNMes)FeII(THF)](PF6)2 (2-THF) with 1 equiv of trimethylamine N-oxide (TMAO) or by the photolysis reaction of the ferrous sulfoxide isotopomers [(Me2TIMMNMes)FeII(MeS(16/18O)Ph)](PF6)2 (2-16/18OSR2). Although the fleeting nature of trivalent oxide 3 precludes extensive characterization and isolation in the solid state, the one-electron oxidation product [(Me2TIMMNMes)FeIV(O)](PF6)2 (4) is isolable and was reproducibly synthesized as well as fully characterized, including CHN elemental analysis, multinuclear NMR, IR vibrational, UV/vis electronic absorption, zero- and applied-field 57Fe Mössbauer spectroscopy, and single-crystal X-ray crystallography studies. Diamagnetic 4 features a remarkably short Fe-O bond (d(Fe-O) = 1.576(1) Å) and a notably negative isomer shift (δ = -0.38 mm s-1) in the 57Fe Mössbauer spectrum. Computational analyses corroborate the + IV oxidation state and advocate an iron-oxygen triple bond, FeIV≡O. Preliminary reactivity studies show that the closed-shell iron(IV) oxo complex can mediate intermolecular oxygen-atom transfer chemistry. A reversible redox event at a half-wave potential, E1/2, of 1.25 V vs Fe(Cp)2/Fe(Cp)2+ in the cyclic voltammogram of 4 suggests the existence of an Fe(V) oxo species.

Herein, the first report on the isolated and unambiguously proven benzene radical trianion is presented. This unprecedented radical oxidation state of benzene is stabilized through two trivalent rare earth (RE) metal cations each supported by a bis(guanidinate) scaffold. Specifically, the one‐electron chemical reduction of the neutral inverse‐sandwich yttrium complex [[{(Me 3 Si) 2 NC(N i Pr) 2 } 2 Y] 2 ( μ – ƞ 6 : ƞ 6 –C 6 H 6 )] 1 , containing a benzene dianion, with potassium graphite (KC 8 ) in the presence of [2.2.2]‐cryptand yielded the title complex [K([2.2.2]‐cryptand)][[{(Me 3 Si) 2 NC(N i Pr) 2 } 2 Y] 2 ( μ – ƞ 6 : ƞ 6 –C 6 H 6 • )] 2 , featuring a benzene radical trianion. Analyses through single‐crystal X‐ray diffraction, EPR and UV–vis spectroscopy, elucidated its molecular structure and revealed strong [Y III –(C 6 H 6 ) 3–• –Y III ] metal–radical interactions. Although the Y centers remain in the +3 oxidation state, the spin density of the unpaired electron resides primarily on the benzene trianion moiety and extends toward the Y III ions. Density functional theory (DFT) calculations on 2 corroborate this assignment and further suggest weak aromaticity for the benzene radical trianion.

Complexes of iron in high oxidation states are captivating research subjects due to their pivotal role as active intermediates in numerous catalytic processes. Structural and spectroscopic studies of well-defined model complexes often provide evidence of these intermediates. In addition to the fundamental molecular and electronic structure insights gained by these complexes, their reactivity also affects our understanding of catalytic reaction mechanisms for small molecule and bond-activation chemistry. Here, we report the synthesis, structural and spectroscopic characterization of a stable, octahedral Fe(VI) nitrido complex and an authenticated, unique Fe(VII) species, prepared by one-electron oxidation. The super-oxidized Fe(VII) nitride rearranges to an Fe(V) imide through an intramolecular amination mechanism and ligand exchange, which is characterized spectroscopically and computationally. This enables combined reactivity and stability studies on a single molecular system of a rare high-valent complex redox pair. Quantum chemical calculations complement the spectroscopic parameters and provide evidence for a diamagnetic ( S = 0) d 2 Fe(VI) and a genuine S = 1/2, d 1 Fe(VII) configuration of these super-oxidized nitrido complexes.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

As key intermediates in metal-catalyzed nitrogen-transfer chemistry, terminal imido complexes of iron have attracted significant attention for a long time. In search of versatile model compounds, the recently developed second-generation N-anchored tris-NHC chelating ligand tris-[2-(3-mesityl-imidazole-2-ylidene)-methyl]amine (TIMMNMes) was utilized to synthesize and compare two series of mid- to high-valent iron alkyl imido complexes, including a reactive Fe(V) adamantyl imido intermediate en route to an isolable Fe(V) nitrido complex. The chemistry toward the iron adamantyl imides was achieved by reacting the Fe(I) precursor [(TIMMNMes)FeI(N2)]+ (1) with 1-adamantyl azide to yield the corresponding trivalent iron imide. Stepwise chemical reduction and oxidation lead to the isostructural series of low-spin [(TIMMNMes)Fe(NAd)]0,1+,2+,3+ (2Ad-5Ad) in oxidation states II to V. The Fe(V) imide [(TIMMNMes)Fe(NAd)]3+ (5Ad) is unstable under ambient conditions and converts to the air-stable nitride [(TIMMNMes)FeV(N)]2+ (6) via N-C bond cleavage. The stability of the pentavalent imide can be increased by derivatizing the nitride [(TIMMNMes)FeIV(N)]+ (7) with an ethyl group using the triethyloxonium salt Et3OPF6. This gives access to the analogous series of ethyl imides [(TIMMNMes)Fe(NEt)]0,1+,2+,3+ (2Et-5Et), including the stable Fe(V) ethyl imide. Iron imido complexes exist in a manifold of different electronic structures, ultimately controlling their diverse reactivities. Accordingly, these complexes were characterized by single-crystal X-ray diffraction analyses, SQUID magnetization, and electrochemical methods, as well as 57Fe Mössbauer, IR vibrational, UV/vis electronic absorption, multinuclear NMR, X-band EPR, and X-ray absorption spectroscopy. Our studies are complemented with quantum chemical calculations, thus providing further insight into the electronic structures of all complexes.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

The synthesis, characterization, and reactivity of a series of cobalt terminal imido complexes supported by an N-anchored tripodal tris(carbene) chelate is described, including a Co-supported singlet nitrene. Reaction of the CoI precursor [(TIMMNmes)CoI](PF6) (TIMMNmes = tris-[2-(3-mesityl-imidazolin-2-ylidene)-methyl]amine) with p-methoxyphenyl azide yields a CoIII imide [(TIMMNmes)CoIII(NAnisole)](PF6) (1). Treatment of 1 with 1 equiv of [FeCp2](PF6) at -35 °C affords a formal CoIV imido complex [(TIMMNmes)Co(NAnisole)](PF6)2 (2), which features a bent Co-N(imido)-C(Anisole) linkage. Subsequent one-electron oxidation of 2 with 1 equiv of AgPF6 provides access to the tricationic cobalt imido complex [(TIMMNmes)Co(NAnisole)](PF6)3 (3). All complexes were fully characterized, including single-crystal X-ray diffraction (SC-XRD) analyses, infrared (IR) vibrational, ultraviolet/visible (UV/vis) electronic absorption, multinuclear NMR, X-band electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and high-energy-resolution fluorescence-detected X-ray absorption spectroscopy (HERFD XAS). Quantum chemical calculations provide additional insight into the electronic structures of all compounds. The dicationic CoIV imido complex 2 exhibits a doublet ground state with considerable imidyl character as a result of covalent Co-NAnisole bonding. At room temperature, 2 readily converts to a CoII amine complex involving intramolecular C-H bond amination. Electronically, tricationic complex 3 can be understood as a singlet nitrene bound to CoIII with significant CoIV imidyl radical character. Verifying the pronounced electrophilicity, nucleophiles such as H2O and tBuNH2 add to 3─analogous to the parent free nitrene─in the para position of the aromatic substituent, thus, clearly corroborating singlet nitrene-type reactivity.

Reaction of the CoI complex [(TIMMNmes )CoI ](PF6 ) (1) (TIMMNmes =tris-[2-(3-mesityl-imidazolin-2-ylidene)-methyl]amine) with mesityl azide yields the CoIII imide [(TIMMNmes )CoIII (NMes)](PF6 ) (2). Oxidation of 2 with [FeCp2 ](PF6 ) provides access to a rare CoIII imidyl [(TIMMNmes )Co(NMes)](PF6 )2 (3). Single-crystal X-ray diffractometry and EPR spectroscopy confirm the molecular structure of 3 and its S= 1/2 ground state. ENDOR, X-ray absorption spectroscopy and computational analyses indicate a ligand-based oxidation; thus, an imidyl-radical electronic structure for 3. Migratory insertion of one ancillary NHC to the imido ligand in 2 gives the CoI N-heterocyclic imine (4) within 12 h. Conversely, it takes merely 0.5 h for 3 to transform to the CoII congener (5). The migratory insertion in 2 occurs via a nucleophilic attack of the imido ligand at the NHC to give 4, whereas in 3, a nucleophilic attack of the NHC at the electrophilic imidyl ligand yields 5. The reactivity shunt upon oxidation of 2 to 3 confirms an umpolung of the imido ligand.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

The reaction of the cobalt(I) complex [(TIMMNmes)Co-I](BPh4) (2) (TIMMNmes=tris-[2-(3-mesitylimidazolin-2-ylidene)methyl]amine) with 1-adamantylazide yields the cobalt(III) imido complex [(TIMMNmes)Co-III(NAd)](BPh4) (3) with concomitant release of dinitrogen. The N-anchor in diamagnetic 3 features an unusual, planar tertiary amine, which results from repulsive electrostatic interaction with the filled d(z(2))-orbital of the cobalt ion and negative hyperconjugation with the neighboring methylene groups. One-electron oxidation of 3 with [FeCp2](OTf) provides access to the rare, high-valent cobalt(IV) imido complex [(TIMMNmes)Co-IV(NAd)](OTf)(2) (4). Despite a half-life of less than 1 h at room temperature, 4 could be isolated at low temperatures in analytically pure form. Single-crystal X-ray diffractometry and EPR spectroscopy corroborate the molecular structure and the d(5) low-spin, S=1/2 , electron configuration. A computational analysis of 4 suggests high covalency within the Co-IV=NAd bond with non-negligible spin density located at the imido moiety, which translates into substantial triplet nitrene character.

A new supporting ligand, tris-[2-(3-mesityl-imidazol-2-ylidene)methyl]amine (TIMMNMes), was developed and utilized to isolate an air-stable iron(V) complex bearing a terminal nitrido ligand, which was synthesized by one-electron oxidation from the iron(IV) precursor. Single-crystal X-ray diffraction analyses of both complexes reveal that the metal-centered oxidation is escorted by iron nitride (Fe≡N) bond elongation, which in turn is accompanied by the accommodation of the high-valence iron center closer to the equatorial plane of a trigonal bipyramid. This contrasts with the previous observation of the only other literature-known Fe(IV)≡N/Fe(V)≡N redox pair, namely, [PhB(tBuIm)3FeN]0/+. On the basis of 57Fe Mössbauer, EPR, and UV/vis electronic absorption spectroscopy as well as quantum chemical calculations, we identified the lesser degree of pyramidalization around the iron atom, the Jahn-Teller distortion, and the resulting nature of the SOMO to be the decisive factors at play.

Reaction of a scandium phosphoniomethylidene with carbon monoxide provides the first scandium phosphonioketene (1). X-ray diffraction analysis shows that the complex has a very short Sc-C bond (2.138(2) Å), and DFT calculations indicate that this unusual short bond length is due to the significant contribution of ionic coulomb interaction between carbon and scandium and the η2 -O,C coordination fashion. Complex 1 is thermally stable, albeit shows high reactivity towards a series of unsaturated substrates, including carbon dioxide, ketone, imine, nitrile and isocyanide. In the reaction with tert-butyl isocyanide, not only an insertion of tert-butyl isocyanide into the Sc-C bond occur, but also a C-H activation on the phenyl ring. DFT calculations show that the reactivity of 1 operated by nucleophilic properties, and therefore the reaction mechanism favors the nucleophilic attack to isocyanide as a rate-determining step, followed by the stepwise C-H activation through an interesting C-H deprotonation.

A series of monomeric rare-earth metal silyl-thiophosphinoyl-alkylidene complexes [LLn{C(SiR3 )PPh2 S}] (5: Ln=Lu, R=Me; 6: Ln=Lu, R=Ph; 7: Ln=Y, R=Me; 8: Ln=Y, R=Ph; 9: Ln=Sm, R=Ph; 10: Ln=Sm, R=Me; 11: Ln=La, R=Ph; L=[MeC(NDIPP)CHC(Me)(NCH2 CH2 N(Me)2 )]- , DIPP=2,6-(iPr)2 C6 H3 ) have been synthesized and structurally characterized. The influences of rare-earth metal ions, ancillary ligands, and alkylidene groups on the reactivity of complexes 5-11 and the related scandium complexes [LSc{C(SiR3 )PPh2 S}] (1: R=Me; 2: R=Ph) and [L'Sc{C(SiR3 )PPh2 S}] (3: R=Me; 4: R=Ph; L'=[MeC(NDIPP)CHC(Me)(NCH2 CH2 N(iPr)2 )]- ) have been studied. Reactions of these rare-earth metal alkylidene complexes with PhCN give four kinds of products, the formation of which is dependent on the rare-earth metal ions, ancillary ligands, and alkylidene groups of the complexes. In the reactions with tBuNC, unusual C-P bond cleavage of the alkylidene group and C≡C triple bond formation occur. Complexes 10 and 11 also react with PhSiH3 to form hydrides, which subsequently undergo Ln-H addition to the C=N bond of the ancillary ligand L. DFT calculations have been used to analyze the bonding in complex 10, which exhibits a polarized three centers Sm-C-P π interaction, and to rationalize the reactivity by computing reaction mechanisms. The difference in reactivity of PhCN and tBuNC is due to the electron density delocalization that is enabled by the phenyl group rather than the tBu group.

The peculiar electronic structure of scandium phosphinoalkylidene complex [LSc{C(SiMe3)PPh2}THF] (L=[MeC(NDIPP)CHC(NDIPP)Me](-)), DIPP=2,6-(Pr-i)(2)C6H3) leads to an interesting versatile reactivity, which is demonstrated both experimentally and computationally. The complex undergoes [2+2] cycloaddition reactions with alkynes, and easily activates various XO bonds such as CO of propylene oxide, NO of 3,5-dimethylisoxazole, BO of pinacolborane and SiO of triethoxysilane. These reactions occur on the ScC bond of the phosphinoalkylidene complex. Interestingly, the ScP bond can also be activated as the presence of a ScCP three center interaction in the complex allows performing CF activation of 2,6-difluoropyridine and 1,2 addition with imine or ketone. The complex also reacts with metal complexes, [(COD)RhCl](2) and (Ph3P)AuCl, to form structural intriguing heterobimetallic complexes.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Dihydrogen is easily activated by a scandium terminal imido complex containing the weakly coordinated THF. The reaction proceeds through a 1,2-addition mechanism, which is distinct from the σ-bond metathesis mechanism reported to date for rare-earth metal-mediated H2 activation. This reaction yields a scandium terminal hydride, which is structurally well-characterized, being the first one to date. The reactivity of this hydride is reported with unsaturated substrates, further shedding light on the existence of the terminal hydride complex. Interestingly, the H2 activation can be reversible. DFT investigations further eludciate the mechanistic aspects of the reactivity of the scandium anilido-terminal hydride complex with PhNCS but also on the reversible H2 activation process.

The first phosphoniomethylidene complexes of scandium and lutetium, [LLn(CHPPh3)X] (L = [MeC(NDIPP)CHC(NDIPP)Me]-; Ln = Sc, X = Me, I, TfO; Ln = Lu, X = CH2SiMe3), have been synthesized and fully characterized. DFT calculations clearly demonstrate the presence of an allylic Ln, C, P π-type interaction in these complexes. X-ray diffraction indicates that the scandium iodide complex has the shortest Sc-C bond length to date (2.044(5) Å). These phosphoniomethylidene complexes readily convert into the ylide complexes, and the reactivity is affected by both X- anion and Ln3+ ion. The reaction of lutetium complex with imine shows a rapid insertion of imine into the Lu-C(alkylidene) bond. DFT calculations indicate that, although the bonding situation seems similar to that of the scandium analog, the strong negative charge at the alkylidene carbon is not sufficiently screened by one hydrogen in the lutetium complex because of a more ionic bonding, and therefore, the reactivity of the lutetium complex is much higher.

The first scandium phosphinoalkylidene complex was synthesized and structurally characterized. The complex has the shortest Sc-C bond lengths reported to date (2.089(3) Å). DFT calculations reveal the presence of a three center π interaction in the complex. This scandium phosphinoalkylidene complex undergoes intermolecular C-H bond activation of pyridine, 4-dimethylamino pyridine and 1,3-dimethylpyrazole at room temperature. Furthermore, the complex rapidly activates H2 under mild conditions. DFT calculations also demonstrate that the C-H activation of 1,3-dimethylpyrazole is selective for thermodynamic reasons and the relatively slow reaction is due to the need of fully breaking the chelating effect of the phosphino group to undergo the reaction whereas this is not the case for H2.