BN-Analogues of Vinylidene Transition Metal Complexes: The Borylnitrene Isomer

Access to adducts of parent iminoborane isomers, HBNH and NBH2, using frustrated Lewis pair chelation

Adducts of the parent iminoborane isomers, HBNH and NBH₂, have been prepared, each stabilized by the frustrated Lewis pair (FLP) chelate ⁱPr₂P(C₆H₄)BCy₂ (PB). PB{HBNH} was accessed via dehydrohalogenation, while the corresponding isomer PB{NBH₂} was obtained from the borylation of the formal nitrene-FLP complex PB{NH}.

The Reaction of CO2 with a Borylnitrene: Formation of an 3-Oxaziridinone

The reaction of a borylnitrene with carbon dioxide is studied under cryogenic matrix isolation conditions. Photogenerated CatBN (Cat=catecholato) reacts with CO2 under formation of the cycloaddition product CatBNCO2 , a 3-oxaziridinone derivative, after photoexcitation (>550 nm). The product shows Fermi resonances between the CO stretching and ring deformation modes that cause unusual 13 C and 18 O isotopic shifts. A computational analysis of the 3-oxaziridinone shows this cyclic carbamate to be less strained than an α-lactone or an α-lactame.

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.

Isomerization and fragmentation pathways of 1,2-azaborine

The generation of 1,2-azaborine (4), the BN-analogue of ortho-benzyne, was recently achieved by elimination of tert-butyldimethylchlorosilane under the conditions of flash vacuum pyrolysis. The present investigation identifies by computational means pathways for the thermal isomerization and fragmentation of 1,2-azaborine. The computations were performed using single reference (hybrid/density functional, second order Møller-Plesset perturbation, and coupled cluster theories) as well as multiconfiguration methods (complete active space SCF based second order perturbation theory, multireference configuration interaction, and multiconfiguration coupled electron pair approximation) with basis sets up to polarized triple-ζ quality. The 1,2-azaborine is, despite the distortion of its molecular structure, the most stable C4H4BN isomer investigated. The formation of BN-endiyne isomers is highly unfavorable as the identified pathways involve barriers close to 80 kcal mol(-1). The concerted fragmentation to ethyne and 2-aza-3-bora-butadiyne even has a barrier close to 120 kcal mol(-1). The fragmentation of BN-enediynes has energetic requirements similar to enediynes.

Reductive borylene-CO coupling with a bulky arylborylene complex

Partial metal-boron bond cleavage and coupling of a borylene with two CO ligands was observed upon reduction of a new bulky arylborylene complex. Both the borylene precursor and dianionic product were structurally and spectroscopically characterized. In contrast, reduction of an aminoborylene complex led to complete loss of the borylene ligand and classical Hieber reduction. A rationale for these differences based on DFT methods is presented.

Borylnitrenes: electrophilic reactive intermediates with high reactivity towards C-H bonds

Borylnitrenes (catBN 3a and pinBN 3b; cat = catecholato, pin = pinacolato) are reactive intermediates that show high tendency towards insertion into the C-H bonds of unactivated hydrocarbons. The present article summarizes the matrix isolation investigations that were aimed at identifying, characterizing and investigating the chemical behaviour of 3a by spectroscopic means, and of the experiments in solution and in the gas phase that were performed with 3b. Comparison with the reactivity reported for difluorovinylidene 1a in solid argon indicates that 3a shows by and large similar reactivity, but only after photochemical excitation. The derivative 3b inserts into the C-H bonds of hydrocarbon solvents in high yields and thus allows the formation of primary amines, secondary amines, or amides from "unreactive" hydrocarbons. It can also be used for generation of methylamine or methylamide from methane in the gas phase at room temperature. Remaining challenges in the chemistry of borylnitrenes are briefly summarized.