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

Si-H Bond Activation and Dehydrogenative Coupling of Silanes across the Iron-Amide Bond of a Bis(amido)bis(phosphine) Iron(II) Complex

Despite the utility of Si-Si bonds, there are relatively few examples of Si-Si bond formation by base metals. In this work, a four-coordinate iron complex, (PNNP)Fe^II, is shown to strongly activate the Si-H bonds in primary silanes across the Fe-amide bonds in a metal-ligand cooperative fashion. Upon treatment with excess silane, Si-Si dehydrogenative homocoupling is shown to occur across the Fe-N_amide bond without concomitant oxidation and spin state changes at the Fe center.

Heterolytic cleavage of a Si-H bond by a metal-ligand cooperation of a cationic iridium amido complex and hydrosilylation of aldehydes

Heterolytic cleavage of a Si-H bond was achieved mediated by a metal-ligand cooperation of a cationic iridium amido complex. The reaction was applied to the catalytic hydrosilylation of benzaldehyde and its derivatives, affording the corresponding hydrosilylated products in moderate to good yields.

Redox-Controlled Reactivity at Boron: Parallels to Frustrated Lewis/Radical Pair Chemistry

We report the synthesis of new Lewis-acidic boranes tethered to redox-active vanadium centers, (Ph₂N)₃V(μ-N)B(C₆F₅)₂ (1a) and (N(CH₂CH₂N(C₆F₅))₃)V(μ-N)B(C₆F₅)₂ (1b). Redox control of the V^IV/V couple resulted in switchable borane versus "hidden" boron radical reactivity, mimicking frustrated Lewis versus frustrated radical pair (FLP/FRP) chemistry, respectively. Whereas heterolytic FLP-type addition reactions were observed with the V^V complex (1b) in the presence of a bulky phosphine, homolytic peroxide, or Sn-hydride bond cleavage reactions were observed with the V^IV complex, [CoCp₂^*][(N(CH₂CH₂N(C₆F₅))₃)V(μ-N)B(C₆F₅)₂] (3b), indicative of boron radical anion character. The extent of radical character was probed by spectroscopic and computational means. Together, these results demonstrate that control of the V^IV/V oxidation states allows these compounds to access reactivity observed in both FLP and FRP chemistry.

Cleavage of Si-H, B-H, and C-H Bonds by Metal-Ligand Cooperation

Metal-ligand cooperation, in which metal and ligand participate in bond cleavage and formation, is gathering great attention in recent years. In contrast to the classical bond cleavage by active metal centers with spectator ligands, metal-ligand cooperation has enabled unprecedented reactivities. Especially, metal-ligand cooperative H-H bond cleavage has been extensively studied and applied to various catalysts. On the other hand, there are substantial efforts to expand the scope of the bond to be cleaved other than the H-H bond. This review summarizes the recent progress in the metal-ligand cooperative cleavages of Si-H, B-H, and C-H bonds and their catalytic applications.

A silica-supported titanium catalyst for heterogeneous hydroamination and multicomponent coupling reactions

Highly dehydrated silica gel, SiO₂⁷⁰⁰, gave a material with a total surface hydroxyl density of 0.31 ± 0.05 mmol g^-1, 0.9 ± 0.1 Si-OH sites per nm². Treatment of this material with Ti(NMe₂)₄ gave Ti(NMe₂)₃/SiO₂⁷⁰⁰, which is 1.50% ± 0.07 Ti, where the titanium is bound to the surface, on average, through a single O-Si-Ti linkage. This material was tested for its properties as a catalyst for C-N bond forming reactions and was found to be a competent alkyne hydroamination and iminoamination catalyst. For iminoamination, which is the 3-component coupling of an alkyne, primary amine, and isonitrile, this heterogeneous catalyst was able to carry out some catalyses faster than previously reported homogeneous catalysts with lower catalyst loadings. The material is also a catalyst for the addition of aniline to dicyclohexylcarbodiimide to form a substituted guanidine. In addition, a known quinoline with biological activity was prepared using the heterogeneous catalyst in a one-pot procedure using half the catalyst loading of the previously reported synthesis.

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.

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.