M═Nα Cycloaddition and Nα−Nβ Insertion in the Reactions of Titanium Hydrazido Compounds with Alkynes: A Combined Experimental and Computational Study

Carbene-Like Reactivity in an Iron Azametallacyclobutene Complex: Insights from Electronic Structure

Herein we describe our investigation into the electronic structure of the first isolated monometallic iron azametallacyclobutene complex. Computational analysis through density functional theory calculations reveals electron delocalization throughout the four atoms of the ring system, in line with experimental observations and supporting the classification of this complex as a conjugated metallacycle. The results of this study also point to significant contribution from an imine-substituted iron carbene resonance structure to the overall bonding picture for the azametallacyclobutene. Accordingly, this complex participates in carbene-like reactivity in the presence of an isocyanide substrate to generate a ketenimine product. The related reaction with carbon monoxide leads to the isolation of a five-membered metallacycle that is analogous to the proposed intermediate in ketenimine formation, and confirms the α-carbon as the site of reactivity.

N═N Bond Cleavage by Tantalum Hydride Complexes: Mechanistic Insights and Reactivity

The reaction of [TaCp^RX₄] (Cp^R = η⁵-C₅Me₅, η⁵-C₅H₄SiMe₃, η⁵-C₅HMe₄; X = Cl, Br) with SiH₃Ph resulted in the formation of the dinuclear hydride tantalum(IV) compounds [(TaCp^RX₂)₂(μ-H)₂], structurally identified by single-crystal X-ray analyses. These species react with azobenzene to give the mononuclear imide complex [TaCp^RX₂(NPh)] along with the release of molecular hydrogen. Analogous reactions between the [{Ta(η⁵-C₅Me₅)X₂}₂(μ-H)₂] derivatives and the cyclic diazo reagent benzo[c]cinnoline afford the biphenyl-bridged (phenylimido)tantalum complexes [{Ta(η⁵-C₅Me₅)X₂}₂(μ-NC₆H₄C₆H₄N)] along with the release of molecular hydrogen. When the compounds [(TaCp^RX₂)₂(μ-H)₂] (Cp^R = η⁵-C₅H₄SiMe₃, η⁵-C₅HMe₄; X = Cl, Br) were employed, we were able to trap the side-on-bound diazo derivatives [(TaCp^RX)₂{μ-(η²,η²-NC₆H₄C₆H₄N)}] (Cp^R = η⁵-C₅H₄SiMe₃, η⁵-C₅HMe₄; X = Cl, Br) as intermediates in the N=N bond cleavage process. DFT calculations provide insights into the N=N cleavage mechanism, in which the ditantalum(IV) fragment can promote two-electron reductions of the N=N bond at two different metal–metal bond splitting stages.

The series of dinuclear tantalum(IV) hydrides [{TaCp^RX₂}₂(μ-H)₂] (Cp^R = η⁵-C₅Me₅, η⁵-C₅H₄SiMe₃, η⁵-C₅HMe₄; X = Cl, Br) show the ability to promote N=N bond cleavage in their reactions with azobenzene and benzo[c]cinnoline in absence of reducing reagents. Both the characterization of intermediate species and DFT studies point to a mechanism in two stages, in which the Ta−Ta bond splitting is key for the reduction of the N=N bond and its complete scission.

Synthesis of Pyridylimido Complexes of Tantalum and Niobium by Reductive Cleavage of the N═N Bond of 2,2'-Azopyridine: Precursors for Early-Late Heterobimetallic Complexes

We report the syntheses of 2-pyridylimido complexes of tantalum and niobium by N═N bond cleavage of 2,2'-azopyridine. Reaction of MCl5 (M = Ta and Nb) with 2,2'-azopyridine in the presence of 0.5 equiv of 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (abbreviated Si-Me-CHD) afforded a dark red solution (for Ta) and a dark blue solution (for Nb) with some insoluble precipitates. After removing the solids, another 0.5 equiv of Si-Me-CHD was added to each solution, giving [M(═Npy)Cl3]n (1a: M = Ta; 1b: M = Nb) through reductive cleavage of the N═N bond of 2,2'-azopyridine. The initial products of the above reactions were determined to be 2,2'-azopyridine-bridged dinuclear complexes, [(MCl4)2(μ-pyNNpy)] (2a: M = Ta; 2b: M = Nb), which were isolated by treating MCl5 with 2,2'-azopyridine and Si-Me-CHD in a 2:1:1 molar ratio. In 2a and 2b, the N═N bond was reduced to a single bond via two-electron reduction. Further reduction of complexes 2a and 2b with 1 equiv of Si-Me-CHD afforded complexes 1a and 1b. An anionic doubly μ-imido-bridged ditantalum complex, [nBu4N][Ta2(μ-Npy)2Cl7] (3a), was generated upon addition of nBu4NCl to complex 1a, while addition of nBu4NCl to niobium complex 1b gave a polymeric terminal imido complex, [nBu4N]n/2[{Nb(═Npy)Cl3}2(μ-Cl)]n/2 (3b). Complexations of 1a and 1b with 1 equiv of 2,2'-bipyridine resulted in the formation of mononuclear 2-pyridylimido complexes, M(═Npy)Cl3(bipy) (4a: M = Ta; 4b: M = Nb), whose main structural feature is intramolecular hydrogen bonding between the ortho hydrogen atom of 2,2'-bipyridine and the nitrogen atom of the pyridyl group on the imido ligand. Isolated 2-pyridylimido complexes 4a and 4b reacted with [RhCl(cod)]2 to produce the corresponding early-late heterobimetallic complexes, (bipy)MCl3(μ-Npy)RhCl(cod) (5a: M = Ta; 5b: M = Nb).

Room-Temperature Ring-Opening of Quinoline, Isoquinoline, and Pyridine with Low-Valent Titanium

The complex (PNP)Ti═CH^tBu(CH₂^tBu) (PNP = N[2-PⁱPr₂-4-methylphenyl]₂^-) dehydrogenates cyclohexane to cyclohexene by forming a transient low-valent titanium-alkyl species, [(PNP)Ti(CH₂^tBu)], which reacts with 2 equiv of quinoline (Q) at room temperature to form H₃C^tBu and a Ti(IV) species where the less hindered C₂═N₁ bond of Q is ruptured and coupled to another equivalent of Q. The product isolated from this reaction is an imide with a tethered cycloamide group, (PNP)Ti═N[C₁₈H₁₃N] (1). Under photolytic conditions, intramolecular C-H bond activation across the imide moiety in 1 occurs to form 2, and thermolysis reverses this process. The reaction of 2 equiv of isoquinoline (Iq) with intermediate [(PNP)Ti(CH₂^tBu)] results in regioselective cleavage of the C₁═N₂ and C₁-H bonds, which eventually couple to form complex 3, a constitutional isomer of 1. Akin to 1, the transient [(PNP)Ti(CH₂^tBu)] complex can ring-open and couple two pyridine molecules, to produce a close analogue of 1, complex (PNP)Ti═N[C₁₀H₉N] (4). Multinuclear and multidimensional NMR spectra confirm structures for complexes 1-4, whereas solid-state structural analysis reveals the structures of 2, 3, and 4. DFT calculations suggest an unprecedented mechanism for ring-opening of Q where the reactive intermediate in the low-spin manifold crosses over to the high-spin surface to access a low-energy transition state but returns to the low-spin surface immediately. This double spin-crossover constitutes a rare example of a two-state reactivity, which is key for enabling the reaction at room temperature. The regioselective behavior of Iq ring-opening is found to be due to electronic effects, where the aromatic resonance of the bicycle is maintained during the key C-C coupling event.

New Titanium Borylimido Compounds: Synthesis, Structure, and Bonding

We report a combined experimental and computational study of the synthesis and electronic structure of titanium borylimido compounds. Three new synthetic routes to this hitherto almost unknown class of Group 4 imide are presented. The double-deprotonation reaction of the borylamine H₂NB(NAr'CH)₂ (Ar' = 2,6-C₆H₃ⁱPr₂) with Ti(NMe₂)₂Cl₂ gave Ti{NB(NAr'CH)₂}Cl₂(NHMe₂)₂, which was easily converted to Ti{NB(NAr'CH)₂}Cl₂(py)₃. This compound is an entry point to other borylimides, for example, reacting with Li₂N₂^pyrN^Me to form Ti(N₂^pyrN^Me){NB(NAr'CH)₂}(py)₂ and with 2 equiv of NaCp to give Cp₂Ti{NB(NAr'CH)₂}(py) (23). Borylamine-tert-butylimide exchange between H₂NB(NAr'CH)₂ and Cp*Ti(N^tBu)Cl(py) under forcing conditions afforded Cp*Ti{NB(NAr'CH)₂}Cl(py), which could be further substituted with guanidinate or pyrrolide-amine ligands to give Cp*Ti(hpp){NB(NAr'CH)₂} (16) and Cp*Ti(N^pyrN^Me₂){NB(NAr'CH)₂} (17). The Ti-N_im distances in compounds with the NB(NAr'CH)₂ ligand were comparable to those of the corresponding arylimides. Dialkyl- or diaryl-substituted borylamines do not undergo the analogous double-deprotonation or imide-amine exchange reactions. Reaction of (Cp″₂Ti)₂(μ₂:η¹,η¹-N₂) with N₃BMes₂ gave the base-free, diarylborylimide Cp″₂Ti(NBMes₂) (26) by an oxidative route; this compound has a relatively long Ti-N_im bond and large Cp″-Ti-Cp″ angle. Reaction of 16 with H₂N^tBu formed equilibrium mixtures with H₂NB(NAr'CH)₂ and Cp*Ti(hpp)(N^tBu) (Δ_rG = -1.0 kcal mol^-1). In contrast, the dialkylborylimide Cp*Ti{MeC(NⁱPr)₂}(NBC₈H₁₄) (2) reacted quantitatively with H₂N^tBu to give the corresponding tert-butylimide and borylamine. The electronic structures and imide-amine exchange reactions of half-sandwich and sandwich titanium borylimides have been evaluated using density functional theory (DFT), supported by quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analysis, and placed more generally in context with the well-established alkyl- and arylimides and hydrazides. The calculations find that Ti-N_im bonds for borylimides are stronger and more covalent than in their organoimido or hydrazido analogues, and are strongest for alkyl- and arylborylimides. Borylamine-tert-butylimide exchange reactions fail for H₂NBR₂ (R = hydrocarbyl) but not for H₂NB(NAr'CH)₂ because the increased strength of the new Ti-N_im bond for the former is outweighed by the increased net H-N bond strengths in the borylamine. Variation of the Ti-N_im bond length over short distances is dominated by π-interactions with any appropriate orbital on the N_im atom organic substituent. However, over the full range of imides and hydrazides studied, overall bond energies do not correlate with bond length but with the Ti-N_im σ-bond character and the orthogonal π-interaction.

Molecular titanium nitrides: nucleophiles unleashed

In this contribution we present reactivity studies of a rare example of a titanium salt, in the form of [μ2-K(OEt2)]2[(PN)2Ti[triple bond, length as m-dash]N]2 (1) (PN- = N-(2-(diisopropylphosphino)-4-methylphenyl)-2,4,6-trimethylanilide) to produce a series of imide moieties including rare examples such as methylimido, borylimido, phosphonylimido, and a parent imido. For the latter, using various weak acids allowed us to narrow the pK a range of the NH group in (PN)2Ti[triple bond, length as m-dash]NH to be between 26-36. Complex 1 could be produced by a reductively promoted elimination of N2 from the azide precursor (PN)2TiN3, whereas reductive splitting of N2 could not be achieved using the complex (PN)2Ti[double bond, length as m-dash]N[double bond, length as m-dash]N[double bond, length as m-dash]Ti(PN)2 (2) and a strong reductant. Complete N-atom transfer reactions could also be observed when 1 was treated with ClC(O)tBu and OCCPh2 to form NCtBu and KNCCPh2, respectively, along with the terminal oxo complex (PN)2Ti[triple bond, length as m-dash]O, which was also characterized. A combination of solid state 15N NMR (MAS) and theoretical studies allowed us to understand the shielding effect of the counter cation in dimer 1, the monomer [K(18-crown-6)][(PN)2Ti[triple bond, length as m-dash]N], and the discrete salt [K(2,2,2-Kryptofix)][(PN)2Ti[triple bond, length as m-dash]N] as well as the origin of the highly downfield 15N NMR resonance when shifting from dimer to monomer to a terminal nitride (discrete salt). The upfield shift of 15Nnitride resonance in the 15N NMR spectrum was found to be linked to the K+ induced electronic structural change of the titanium-nitride functionality by using a combination of MO analysis and quantum chemical analysis of the corresponding shielding tensors.

Redox Non-Innocent Behavior of a Terminal Iridium Hydrazido(2-) Triple Bond

The synthesis of the first terminal Group 9 hydrazido(2-) complex, Cp*IrN(TMP) (6) (TMP=2,2,6,6-tetramethylpiperidine) is reported. Electronic structure and X-ray diffraction analysis indicate that this complex contains an Ir-N triple bond, similar to Bergman's seminal Cp*Ir(Nt Bu) imido complex. However, in sharp contrast to Bergman's imido, 6 displays remarkable redox non-innocent reactivity owing to the presence of the Nβ lone pair. Treatment of 6 with MeI results in electron transfer from Nβ to Ir prior to oxidative addition of MeI to the iridium center. This behavior opens the possibility of carrying out facile oxidative reactions at a formally IrIII metal center through a hydrazido(2-)/isodiazene valence tautomerization.

Diamidophosphines with six-membered chelates and their coordination chemistry with group 4 metals: development of a trimethylene-methane-tethered [PN2]-type "molecular claw"

The coordination chemistry of the phosphine-tethered diamidophosphine ligands PhP(CH2CH2CH2NHPh)2 (pr[NPN]H2) and PhP(1,2-CH2-C6H4-NHSiMe3)2 (bn[NPN]H2) featuring six-membered N–C3–P chelates was explored with group 4 metals, which allowed for the consecutive development of a new trimethylene-methane-tethered [PN2] scaffold. In the case of the propylene-linked system pr[NPN]H2, access to the sparingly soluble dibenzyl derivative pr[NPN]ZrBn2 (3-Zr) was gained, while thermally sensitive zirconium and hafnium diiodo complexes bn[NPN]MI2 (5-M, M = Zr, Hf) were isolated in the case of the benzylene-linked derivative bn[NPN]H2. Despite the related phosphine-tethered backbone architectures of both of these ligands, their group 4 complexes were found to exhibit either C1-symmetric (bn[NPN]MX2) or averaged CS-symmetric (pr[NPN]MX2) structures in solution. To restrain the overall flexibility of these systems and thereby control the properties of the resulting complexes without disrupting the six-membered chelates, the new trimethylene-methane-tethered N,N′-di-(tert-butyl)-substituted [PN2]H2 protioligand was designed. This tripodal ligand system was prepared on a gram scale and its CS-symmetric dichloro complexes [PN2]MCl2 (6-M, M = Ti, Zr, Hf) were isolated subsequently. The benzene-soluble dibenzyl derivative [PN2]ZrBn2 (7-Zr) was synthesised as well and characterised by X-ray diffraction. These results are discussed not only in conjunction with the known [NPN]-coordinated group 4 complexes incorporating five-membered chelates, but also in the context of “molecular claws” that are related to the new [PN2] tripod.

Reactions of Titanium Hydrazides with Silanes and Boranes: N-N Bond Cleavage and N Atom Functionalization

Reaction of Ti(N2(iPr)N)(NNPh2)(py) with Ph(R)SiH2 (R = H, Ph) or 9-BBN gave reductive cleavage of the N(α)-N(β) bond and formation of new silyl- or boryl-amido ligands. The corresponding reactions of Cp*Ti{MeC(N(i)Pr)2}(NNR2) (R = Me or Ph) with HBPin or 9-BBN gave borylhydrazido-hydride or borylimido products, respectively. N(α) and N(β) atom transfer and dehydrogenative coupling reactions are also reported.

A computationally designed titanium-mediated amination of allylic alcohols for the synthesis of secondary allylamines

A direct amination on allylic alcohols under mild conditions was enlightened by computational investigations and implemented in secondary allylamines synthesis.

Zirconium complexes supported by an N-perfluoro-arylated diamidopyridyl ligand: synthesis of hydrazinediido complexes

The N-perfluoro-phenylated pyridyldiamine H2N2(PFP)N(py) (1) has been prepared by a palladium-catalyzed coupling of hexafluorobenzene and the diamine (H2NCH2)2C(CH3)(2-C5H4N) using the palladacycle trans-di(μ-acetato)bis[o-(di-o-tolylphosphino)benzyl]palladium(II) as catalyst. Reactions of H2N2(PFP)N(py) and Zr(NMe2)4 at room temperature or 90 °C led to the complexes [(N(PFP)N2(TFAP)N(py))ZrF(NMe2)] (2) and [(N2(TFAP)N(py))ZrF2] (3) in which one or two dimethylamido groups replaced one or two ortho fluorine atoms of the pentafluorophenyl groups in the ligand. Reaction of Me3SiX (X = Cl, I) with [(N2(TFAP)N(py))ZrF2] (3) resulted in the formation of mixed halogenated complexes [(N2(TFAP)N(py))ZrFI] (4) and [(N2(TFAP)N(py))ZrFCl] (5) in which the axially bound fluorido ligand is substituted. Reaction of [(N2(TFAP)N(py))ZrF2] (3) with LiNHNPh2 afforded the monohydrazido(1-) complex [(N2(TFAP)N(py))ZrF(NHNPh2)] (6) which was converted to the dimeric fluoro-potassium bridged hydrazinediido complex [Zr(N2(TFAP)N(py))FNNPh2K]2 (7) using KHMDS. The corresponding reaction with LiHMDS yielded the monomeric, donor free complex [Zr(N2(TFAP)N(py))NNPh2] (8).