C(sp3 )-H Oxidative Addition and Transfer Hydrogenation Chemistry of a Titanium(II) Synthon: Mimicry of Late-Metal Type Reactivity

From Coordination to Noncoordination: Syntheses and Substitution Lability Studies of Titanium Triflato Complexes

A new concept for obtaining cationic complexes with triflate counteranions from coordinating triflato ligands was developed. Various routes are leading to titanium(IV) and titanium(III) triflato complexes efficiently. The reactions of pentafulvene titanium complexes with either triflic acid or silver triflate give the corresponding titanium(IV) triflato complexes in excellent yields. Hydrolysis of the titanium(IV) bistriflato complexes leads to cationic aqua complexes via displacement of the triflato ligand, which consequently acts as a noncoordinating anion. A functionalized titanium(IV) monotriflato complex was synthesized by insertion of a nitrile into the Ti-C bond and the triflato ligand was displaced by an NHC. While the titanium(IV) complexes are mostly inert toward substrates, the donor-free titanium(III) triflato complex is a strong Lewis acid and forms various adducts with monodentate Lewis bases. The titanium(III) complex was oxidized by reaction with TEMPO, resulting in a diamagnetic titanium(IV) complex. The reaction with bidentate ligands results in cationic titanium(III) complexes due to displacement of the triflato ligand by the bidentate ligands. Treatment with acetone leads to an aldol reaction of two acetone molecules and the formation of a cationic diacetone alcohol complex.

A Mixed-Valence Ti(II)/Ti(III) Inverted Sandwich Compound as a Regioselective Catalyst for the Uncommon 1,3,5-Alkyne Cyclotrimerization

The synthesis, structure, and catalytic activity of a Ti(II)/Ti(III) inverted sandwich compound are presented in this study. Synthesis of the arene-bridged dititanium compound begins with the preparation of the titanium(IV) precursor [TiCl2(MesPDA)(thf)2] (MesPDA = N,N'-bis(2,4,6-trimethylphenyl)-o-phenylenediamide) (2). The reduction of 2 with sodium metal results in species [{Ti(MesPDA)(thf)}2(μ-Cl)3{Na}] (3) in oxidation state III. To achieve the lower oxidation state II, 2 undergoes reduction through alkylation with lithium cyclopentyl. This alkylation approach triggers a cascade of reactions, including β-hydride abstraction/elimination, hydrogen evolution, and chemical reduction, to generate the Ti(II)/Ti(III) compound [Li(thf)4][(TiMesPDA)2(μ-η6: η6-C6H6)] (4). X-ray and EPR characterization confirms the mixed-valence states of the titanium species. Compound 4 catalyzes a mild, efficient, and regiospecific cyclotrimerization of alkynes to form 1,3,5-substituted arenes. Kinetic data support a mechanism involving a binuclear titanium arene compound, similar to compound 4, as the resting state. The active catalyst promotes the oxidative coupling of two alkynes in the rate-limiting step, followed by a rapid [4 + 2] cycloaddition to form the arene product. Computational analysis of the resting state for the cycloaddition of trimethylsilylacetylene indicates a thermodynamic preference for stabilizing the 1,3,5-arene within the space between the two [TiMesPDA] fragments, consistent with the observed regioselectivity.

Catalytic utility of PNN-based MnI pincer complexes in the synthesis of quinolines and transfer hydrogenation of carbonyl derivatives

This manuscript describes the synthesis of a triazolyl-pyridine-based phosphine, N-((diphenylphosphaneyl)methyl)-N-methyl-6-(1-phenyl-1H-1,2,3-triazol-4-yl)pyridin-2-amine, [2,6-{(PPh2)CH2N(Me)(C5H3N)(C2HN3C6H5)}] (1) (here onwards referred to as PNN) and its cationic and neutral MnI complexes and catalytic applications. The reaction of 1 with Mn(CO)5Br afforded a cationic complex [Mn(CO)3(PNN)]Br (2), which is highly stable in solid state, but in solution it gradually loses one of the CO groups to form a neutral complex [Mn(CO)2(PNN)Br] (3). Complex 2 on treatment with AgBF4 also yielded a cationic complex [Mn(CO)3(PNN)]BF4 (4). These complexes efficiently promoted the synthesis of quinoline derivatives via acceptor-less dehydrogenative coupling of 2-aminobenzyl alcohol and ketones, with complex 3 showing the highest activity with a very low catalyst loading (0.03 mol%) at 110 °C. Complex 3 (0.5 mol%) also showed excellent catalytic activity in the transfer hydrogenation of ketones and aldehydes to form respective secondary and primary alcohols.

Ti(III) Catalysts for CO2/Epoxide Copolymerization at Unusual Ambient Pressure Conditions

Titanium compounds in low oxidation states are highly reducing species and hence powerful tools for the functionalization of small molecules. However, their potential has not yet been fully realized because harnessing these highly reactive complexes for productive reactivity is generally challenging. Advancing this field, herein we provide a detailed route for the formation of titanium(III) orthophenylendiamido (PDA) species using [LiBHEt3] as a reducing agent. Initially, the corresponding lithium PDA compounds [Li2(ArPDA)(thf)3] (Ar = 2,4,6-trimethylphenyl (MesPDA), 2,6-diisopropylphenyl (iPrPDA)) are combined with [TiCl4(thf)2] to form the heterobimetallic complexes [{TiCl(ArPDA)}(μ-ArPDA){Li(thf)n}] (n = 1, Ar = iPr 3 and n = 2, Ar = Mes 4). Compound 4 evolves to species [Ti(MesPDA)2] (6) via thermal treatment. In contrast, the transformation of 3 into [Ti(iPrPDA)2] (5) only occurs in the presence of [LiNMe2], through a lithium-assisted process, as revealed by density functional theory (DFT). Finally, the Ti(IV) compounds 3-6 react with [LiBHEt3] to give rise to the Ti(III) species [Li(thf)4][Ti(ArPDA)2] (Ar = iPr 8, Mes 9). These low-valent compounds in combination with [PPN]Cl (PPN = bis(triphenylphosphine)iminium) are proved to be highly selective catalysts for the copolymerization of CO2 and cyclohexene epoxide. Reactions occur at 1 bar pressure with activity/selectivity levels similar to Salen-Cr(III) compounds.

Reassessment of N2 activation by low-valent Ti-amide complexes: a remarkable side-on bridged bis-N2 adduct is actually an arene adduct

The complex {(TMEDA)2Li}{[Ti(N(TMS)2)2]2(μ-η2:η2-N2)2} (5-Li) is the only transition metal N2 complex ever reported with two side-on N2 adducts. In this report, the similarity of 5-Li to a new inverse sandwich toluene adduct {(PhMe)K}{[Ti(N(TMS)2)2]2(μ-PhMe)} (6-K) necessitated a re-examination of the structure of 5-Li. Through a reassessment of the original disordered crystal data of 5-Li and new independent syntheses brought about through revisitation of the original reaction conditions, 5-Li has been re-assigned as an inverse sandwich toluene adduct, {(TMEDA)2Li}{[Ti(N(TMS)2)2]2(μ-PhMe)} (6-Li). The original crystal data could be fitted almost equally well to structural solutions as either 5-Li or 6-Li, and this study highlights the importance of a holistic examination of modeled data and the need for secondary/complementary analytical methods in paramagnetic inorganic syntheses, especially when presenting unique and unexpected results. In addition, further examination of reduction reactions of Ti[N(TMS)2]3 and [(TMS)2N]2TiCl(THF) in the presence of KC8 revealed rich solvent- and counterion-dependent chemistry, including several degrees of N2 activation (bridging nitride complexes, terminal bridging N2 complexes) as well as ligand C-H activation.

Molecular Capacitors: Accessible 6- and 8-Electron Redox Chemistry from Dimeric "Ti(I)" and "Ti(0)" Synthons Supported by Imidazolin-2-Iminato Ligands

Reduction of the diamagnetic Ti(III)/Ti(III) dimer [Cl₂Ti(μ-NIm^Dipp)]₂ (1) (NIm^Dipp = [1,3-bis(Dipp)imidazolin-2-iminato]^-, Dipp = C₆H₃-2,6-ⁱPr₂) with 4 and 6 equiv of KC₈ generates the intramolecularly arene-masked, dinuclear titanium compounds [(μ-N-η⁶-Im^Dipp)Ti]₂ (2) and {[(Et₂O)₂K](μ-N-μ-η⁶:η⁶-Im^Dipp)Ti}₂ (3), respectively, in modest yields. The compounds have been structurally characterized by X-ray crystallographic analysis, and inspection of the bond metrics within the η⁶-coordinated aryl substituent of the bridging imidazolin-2-iminato ligand shows perturbation of the aromatic system most consistent with two-electron reduction of the ring. As such, 2 and 3 can be assigned respectively as possessing metal centers in formal Ti(III)/Ti(III) and Ti(II)/Ti(II) oxidation states. Exploration of their redox chemistry reveal the ability to reduce several substrate equivalents. For instance, treatment of 2 with excess C₈H₈ (COT) forms the novel COT-bridged complex [(Im^DippN)(η⁸-COT)Ti](μ-η²:η³-COT)[Ti(η⁴-COT)(NIm^Dipp)] (4) that dissociates in THF solutions to give mononuclear (Im^DippN)Ti(η⁸-COT)(THF) (5). Addition of COT to 3 yields heterometallic [(Im^DippN)(η⁴-COT)Ti(μ-η⁴:η⁵-COT)K(THF)(μ-η⁶:η⁴-COT)Ti(NIm^Dipp)(μ-η⁴:η⁴-COT)K(THF)₂]_n (6). Compounds 4 and 5 are the products of the 4-electron oxidation of 2, while 6 stands as the 8-electron oxidation product of 3. Reduction of organozides was also explored. Low temperature reaction of 2 with 4 equiv of AdN₃ gives the terminal and bridged imido complex [(Im^DippN)Ti(═NAd)](μ-NAd)₂[Ti(NIm^Dipp)(N₃Ad)] (7) that undergoes intermolecular C-H activation of toluene at room temperature to afford the amido compound [(Im^DippN)Ti(NHAd)](μ-NAd)₂[Ti(C₆H₄Me)(NIm^Dipp)] (8-tol). These complexes are the 6-electron oxidation products of the reaction of 2 with AdN₃. Furthermore, treatment of 3 with 4 equiv of AdN₃ produces the thermally stable Ti(III)/Ti(III) terminal and bridged imido [K(18-crown-6)(THF)₂]{[(Im^DippN)Ti(NAd)](μ-NAd)₂K[Ti(NIm^Dipp)]} (10). Altogether, these reactions firmly establish 2 and 3 as unprecedented Ti(I)/Ti(I) and Ti(0)/Ti(0) synthons with the clear capacity to effect multielectron reductions ranging from 4 to 8 electrons.

Titanium complexes with unsymmetrically substituted imidazolin-2-iminato ligands

The reaction of the unsymmetrical N-heterocyclic carbenes 1-(2,4,6-trimethylphenyl)-3-(adamantyl)imidazolin-2-ylidene (IAdMes, 1a) and 1-(2,6-diisopropylphenyl)-3-(adamantyl)imidazolin-2-ylidene (IAdDipp, 1b) with trimethylsilyl azide furnished the 2-(trimethylsilylimino)imidazolines 2a (Im^AdMesNSiMe₃) and 2b (Im^AdDippNSiMe₃). Desilylation by stirring in methanol gave the corresponding imidazolin-2-imines 3a (Im^AdMesNH) and 3b (Im^AdDippNH). 2a and 2b were treated with [TiCl₄(THF)₂] (THF = tetrahydrofuran) and [CpTiCl₃] (Cp = η⁵-C₅H₅) to form the mono- and bis(imidazolin-2-iminato) titanium(IV) complexes [(Im^AdRN)TiCl₃] (4, R = Mes, Dipp), [Cp(Im^AdRN)TiCl₂] (5, R = Mes, Dipp), and [(Im^AdRN)₂TiCl₂] (6, R = Mes, Dipp). The crystal structures of all compounds except 2b were determined by X-ray diffraction analysis.

An Isolable Azide Adduct of Titanium(II) Follows Bifurcated Deazotation Pathways to an Imide

AdN₃ (Ad = 1-adamantyl) reacts with the tetrahedral Ti^II complex [(Tp^tBu,Me)TiCl] (Tp^tBu,Me = hydrotris(3-tert-butyl-5-methylpyrazol-1-yl)borate) to generate a mixture of an imide complex, [(Tp^tBu,Me)TiCl(NAd)] (4), and an unusual and kinetically stable azide adduct of the group 4 metal, namely, [(Tp^tBu,Me)TiCl(γ-N₃Ad)] (3). In these conversions, the product distribution is determined by the relative concentration of reactants. In contrast, the azide adduct 3 forms selectively when a masked Ti^II complex (N₂ or AdNC adduct) reacts with AdN₃. Upon heating, 3 extrudes dinitrogen in a unimolecular process proceeding through a titanatriazete intermediate to form the imide complex 4, but the observed thermal stability of the azide adduct (t_1/2 = 61 days at 25 °C) is at odds with the large fraction of imide complex formed directly in reactions between AdN₃ and [(Tp^tBu,Me)TiCl] at room temperature (∼50% imide with a 1:1 stoichiometry). A combination of theoretical and experimental studies identified an additional deazotation pathway, proceeding through a bimetallic complex bridged by a single azide ligand. The electronic origin of this deazotation mechanism lies in the ability of azide adduct 3 to serve as a π-backbonding metallaligand toward free [(Tp^tBu,Me)TiCl]. These findings unveil a new class of azide-to-imide conversions for transition metals, highlighting that the mechanisms underlying this common synthetic methodology may be more complex than conventionally assumed, given the concentration dependence in the conversion of an azide into an imide complex. Lastly, we show how significantly different AdN₃ reacts when treated with [(Tp^tBu,Me)VCl].

Redox chemistry of discrete low-valent titanium complexes and low-valent titanium synthons

Titanium is a versatile metal that has important applications in practical synthesis, though this is typically limited to stoichiometric reactions or Lewis acid catalysis. Recently, interest has grown in using titanium and other early-metals for redox catalysis; however, notable limitations exist due to the thermodynamic preference of these metals to adopt high oxidation states. Nonetheless, discrete low-valent titanium (LVT) complexes and their synthons (titanium complexes which chemically behave as LVT sources) are known. Here, we detail the various ligand platforms that are capable of stabilizing LVT compounds and present the redox chemistry of these systems. This includes a discussion of recent developments in the use of LVT synthons for accessing fully reversible oxidative-addition/reductive-elimination reactions.

A Mononuclear and High-Spin Tetrahedral TiII Complex

A high-spin, mononuclear TiII complex, [(TptBu,Me)TiCl] [TptBu,Me- = hydridotris(3-tert-butyl-5-methylpyrazol-1-yl)borate], confined to a tetrahedral ligand-field environment, has been prepared by reduction of the precursor [(TptBu,Me)TiCl2] with KC8. Complex [(TptBu,Me)TiCl] has a 3A2 ground state (assuming C3v symmetry based on structural studies), established via a combination of high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy, solution and solid-state magnetic studies, Ti K-edge X-ray absorption spectroscopy (XAS), and both density functional theory and ab initio (complete-active-space self-consistent-field, CASSCF) calculations. The formally and physically defined TiII complex readily binds tetrahydrofuran (THF) to form the paramagnetic adduct [(TptBu,Me)TiCl(THF)], which is impervious to N2 binding. However, in the absence of THF, the TiII complex captures N2 to produce the diamagnetic complex [(TptBu,Me)TiCl]2(η1,η1;μ2-N2), with a linear Ti═N═N═Ti topology, established by single-crystal X-ray diffraction. The N2 complex was characterized using XAS as well as IR and Raman spectroscopies, thus establishing this complex to possess two TiIII centers covalently bridged by an N22- unit. A π acid such as CNAd (Ad = 1-adamantyl) coordinates to [(TptBu,Me)TiCl] without inducing spin pairing of the d electrons, thereby forming a unique high-spin and five-coordinate TiII complex, namely, [(TptBu,Me)TiCl(CNAd)]. The reducing power of the coordinatively unsaturated TiII-containing [(ΤptBu,Me)TiCl] species, quantified by electrochemistry, provides access to a family of mononuclear TiIV complexes of the type [(TptBu,Me)Ti═E(Cl)] (with E2- = NSiMe3, N2CPh2, O, and NH) by virtue of atom- or group-transfer reactions using various small molecules such as N3SiMe3, N2CPh2, N2O, and the bicyclic amine 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene.

Generation of Masked TiII Intermediates from TiIV Amides via β-H Abstraction or Alkyne Deprotonation: An Example of Ti-Catalyzed Nitrene-Coupled Transfer Hydrogenation

Simple Ti amide complexes are shown to act as sources for masked Ti^II intermediates via several pathways, as demonstrated through the investigation of a unique Ti-catalyzed nitrene-coupled transfer hydrogenation of 3-hexyne. This reaction proceeds through reduction of azobenzene by a masked Ti^II catalyst, wherein both amines and 3-hexyne can serve as the hydrogen source/reductant for Ti by forming putative titanaziridines via β-H abstraction or putative titanacyclopentynes via protonolysis, respectively.

Synthesis of LnII -in-Cryptand Complexes by Chemical Reduction of LnIII -in-Cryptand Precursors: Isolation of a NdII -in-Cryptand Complex

Lanthanide triflates have been used to incorporate Nd^III and Sm^III ions into the 2.2.2-cryptand ligand (crypt) to explore their reductive chemistry. The Ln(OTf)₃ complexes (Ln=Nd, Sm; OTf=SO₃ CF₃ ) react with crypt in THF to form the THF-soluble complexes [Ln^III (crypt)(OTf)₂ ][OTf] with two triflates bound to the metal encapsulated in the crypt. Reduction of these Ln^III -in-crypt complexes using KC₈ in THF forms the neutral Ln^II -in-crypt triflate complexes [Ln^II (crypt)(OTf)₂ ]. DFT calculations on [Nd^II (crypt)]²⁺ ], the first Nd^II cryptand complex, assign a 4f⁴ electron configuration to this ion.

Titanium-Mediated Catalytic Hydrogenation of Monocyclic and Polycyclic Arenes

Two electron-reduction of the Ti^IV guanidinate complex (Im^Dipp N)(^Xyket guan)TiCl₂ gives (η⁶ -Im^Dipp N)(^xyket guan)Ti (1^intra ) and (Im^Dipp N)(^Xyket guan)Ti(η⁶ -C₆ H₆ ) (1^inter ) (^Xyket guan=[(tBuC=N)C(NXylyl)₂ ]^- , Xylyl=2,5-dimethylphenyl) in the absence or presence of benzene, respectively. These complexes have been found to hydrogenate monocyclic and polycyclic arenes under relatively mild conditions (150 psi, 80 °C)-the first example of catalytic, homogeneous arene hydrogenation with TON >1 by a Group IV system.

Reversible oxidative-addition and reductive-elimination of thiophene from a titanium complex and its thermally-induced hydrodesulphurization chemistry

The masked Ti(ii) synthon (Ketguan)(η6-ImDippN)Ti (1) oxidatively adds across thiophene to give ring-opened (Ketguan)(ImDippN)Ti[κ2-S(CH)3CH] (2). Complex 2 is photosensitive, and upon exposure to light, reductively eliminates thiophene to regenerate 1 - a rare example of early-metal mediated oxidative-addition/reductive-elimination chemistry. DFT calculations indicate strong titanium π-backdonation to the thiophene π*-orbitals leads to the observed thiophene ring opening across titanium, while a proposed photoinduced LMCT promotes the reverse thiophene elimination from 2. Finally, pressurizing solutions of 2 with H2 (150 psi) at 80 °C leads to the hydrodesulphurization of thiophene to give the Ti(iv) sulphide (Ketguan)(ImDippN)Ti(S) (3) and butane.

Titanium catalysis for the synthesis of fine chemicals – development and trends

Atlas as a Titan(ium) is holding the earth-abundant chemistry world. Titanium is the second most abundant transition metal, is a key player in important industrial processes (e.g. polyethylene) and shows much promise for diverse applications in the future.

N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry

N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without the plethora of available NHCs with tailor-made electronic and steric properties. While their suitability to act as strong ligands toward transition metals has led to numerous applications of NHC complexes in homogeneous catalysis, their strong σ-donating and adaptable π-accepting abilities have also contributed to an impressive vitalization of main group chemistry with the isolation and characterization of NHC adducts of almost any element. Formally, NHC coordination to Lewis acids affords a transfer of nucleophilicity from the carbene carbon atom to the attached exocyclic moiety, and low-valent and low-coordinate adducts of the p-block elements with available lone pairs and/or polarized carbon-element π-bonds are able to act themselves as Lewis basic donor ligands toward transition metals. Accordingly, the availability of a large number of novel NHC adducts has not only produced new varieties of already existing ligand classes but has also allowed establishment of numerous complexes with unusual and often unprecedented element-metal bonds. This review aims at summarizing this development comprehensively and covers the usage of N-heterocyclic carbene adducts of the p-block elements as ligands in transition metal chemistry.

Modern applications of low-valent early transition metals in synthesis and catalysis

Low-valent early transition metals are often intrinsically highly reactive as a result of their strong propensity toward oxidation to more stable high-valent states. Harnessing these highly reducing complexes for productive reactivity is potentially powerful for C-C bond construction, organic reductions, small-molecule activation and many other reactions that offer orthogonal chemoselectivity and/or regioselectivity patterns to processes promoted by late transition metals. Recent years have seen many exciting new applications of low-valent metals through building new catalytic and/or multicomponent reaction manifolds out of classical reactivity patterns. In this Review, we survey new methods that employ early transition metals and invoke low-valent precursors or intermediates in order to identify common themes and strategies in synthesis and catalysis.

Metal-catalysed reactions enabled by guanidine-type ligands

A review of metal–guanidine complexes, which are selective and powerful catalysts for organic transformations, asymmetric synthesis, and polymerisation.

Strong Preference of the Redox-Neutral Mechanism over the Redox Mechanism for the TiIV Catalysis Involved in the Carboamination of Alkyne with Alkene and Diazene

Titanium catalysis generally prefers redox-neutral mechanisms. Yet it has been reported that titanium could promote bond formations in a way similar to reductive elimination. Accordingly, redox catalytic cycles involving Ti^IV /Ti^II cycling have been considered. By studying, as an example, the carboamination of alkynes with alkenes and azobenzene catalyzed by the [Ti^IV ]=NPh imido complex, we performed DFT computations to gain an understanding of how the "abnormal" catalysis takes place, thereby allowing us to clarify whether the catalysis really follows Ti^IV /Ti^II redox mechanisms. The reaction first forms an azatitanacyclohexene by alkyne addition to the [Ti^IV ]=NPh bond, followed by alkene insertion. The azatitanacyclohexene can either undergo C^α -C^γ coupling, to afford bicyclo[3.1.0]imine, or β-H elimination, to yield a [Ti^IV ]-H hydride, which then undergoes C^α =C^β or C^γ =C^δ insertion to give an α,β- or β,γ-unsaturated imine, respectively. Both the geometric and electronic structures indicate that the catalytic cycles proceed through redox-neutral mechanisms. The alternative redox mechanisms (e.g., by N-H or C-H reductive elimination) are substantially less favorable. We concluded that electronically, the Ti^IV catalysis intrinsically favors the redox-neutral mechanism, because a redox pathway would involve Ti^II structures either in the triplet ground state or in the high-lying open-shell singlet state, but the involvement of triplet Ti^II structures is spin-forbidden and that of singlet Ti^II structures is energetically disadvantageous.

Thorium Metallacycle Facilitates Catalytic Alkyne Hydrophosphination

The bis(NHC)borate-supported thorium-bis(mesitylphosphido) complex (1) undergoes reversible intramolecular C-H bond activation enabling the catalytic hydrophosphination of unactivated internal alkynes. Catalytic and stoichiometric experiments support a mechanism involving reactive Th-NHC metallacycle intermediates (Int and 2).

Differences between the elimination of early and late transition metals: DFT mechanistic insights into the titanium-catalyzed synthesis of pyrroles from alkynes and diazenes

Early transition metals (TMs), such as titanium, generally resist undergoing reductive elimination to form C-X bonds due to their weak electronegativity. By analyzing the mechanism of the titanium-catalyzed synthesis of pyrroles from alkynes and diazenes, the present study revealed that titanium is able to promote C-N bond formation via an unconventional elimination pathway, passing through a comparatively stable masked Ti^II complex (i.e., IM4) rather than pyrrole directly. The formation of IM4 originates from the bilateral donation and back-donation between Ti and the pyrrole ligand. Formally, it could be considered that the two electrons resulting from the unconventional reductive elimination are temporarily buffered by back-donation to a symmetry-allowed unoccupied π-orbital of the pyrrole ring in IM4 rather than becoming a lone pair on a Ti center as adopted in the catalysis of late TMs. Because of its stability, IM4 requires additional oxidation by diazene to liberate pyrrole. The triplet counterpart (IM4^T ) of IM4 is more stable than IM4, but the elimination is unlikely to reach IM4^T , because the process is spin-forbidden and the spin-orbit coupling is weak. Alternatively, one may consider the forming pyrrole in IM4 as a redox-active ligand, reserving the two electrons resulting from the formal reductive elimination and then releasing the electrons when IM4 is oxidized by diazene. These insights allow us to propose the conditions for early TMs to undergo a similar elimination, whereby the forming product will have symmetry-allowed frontier molecular orbitals to form donation and back-donation bonding with a TM center and a substrate possessing a comparatively strong oxidizing ability to oxidize an IM4-like intermediate for product release. These insights may provide another way of constructing C-X bonds through a similar reductive elimination pathway, using early TM catalysts.