A high-spin, mononuclear TiII complex, [(Tp tBu,Me)TiCl] [Tp tBu,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 [(Tp tBu,Me)TiCl2] with KC8. Complex [(Tp tBu,Me)TiCl] has a 3A2 ground state (assuming C 3v 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 [(Tp tBu,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 [(Tp tBu,Me)TiCl]2(η1,η1;μ2-N2), with a linear TiNNTi 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 N2 2– unit. A π acid such as CNAd (Ad = 1-adamantyl) coordinates to [(Tp tBu,Me)TiCl] without inducing spin pairing of the d electrons, thereby forming a unique high-spin and five-coordinate TiII complex, namely, [(Tp tBu,Me)TiCl(CNAd)]. The reducing power of the coordinatively unsaturated TiII-containing [(Τp tBu,Me)TiCl] species, quantified by electrochemistry, provides access to a family of mononuclear TiIV complexes of the type [(Tp tBu,Me)TiE(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.
X-ray absorption near-edge structure (XANES) spectroscopy is a powerful tool to reveal key structural and electronic features of isolated catalytic sites, yet insights into the molecular structure and more detailed orbital analysis through a combination of experimental and computed XANES analysis are necessary for accurate interpretation of the spectra, especially when significant heterogeneity exists among the catalytic sites. Herein, we present an integrated computational and experimental strategy to determine both primary and secondary bonding interactions within the XANES pre-edge region for organovanadium complexes, which was developed using a series of well-defined molecular vanadium complexes and then applied to the characterization of a supported organovanadium olefin hydrogenation catalyst. Timedependent density functional theory is used to predict the energy of pre-edge XANES features for a series of vanadium complexes with a variety of oxidation states and local coordination environments. A calibration scheme incorporating different density functionals and basis sets is established, resulting in an optimized scheme that accurately predicts pre-edge energies with a mean absolute error of 0.40 eV. Second-shell coordination (e.g., V•••V) effects within XANES are identified through the analysis of the computed dominant orbital contributions for multi-vanadium complexes. Orbital analysis also provided confirmation that the vanadium-hydride formation combined with the heterogeneity of the catalytic active species in olefin hydrogenation caused the energy shift and broadening of the pre-edge peak after hydrogen treatment of the silica-supported organovanadium pre-catalyst. This work further elucidates computational XANES simulations and techniques potentially guiding characterization in surface organometallic chemistry.
AdN3 (Ad = 1-adamantyl) reacts with the tetrahedral TiII 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 TiII 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].
A dinuclear hafnium complex containing the parent imido ligand [(PN)(PNC)Hf=NH{μ2‐K}]2 (2) (PN−=(N‐(2‐PiPr2‐4‐methylphenyl)‐2,4,6‐Me3C6H2; PNC2−=(N‐(2‐PiPr2‐4‐methylphenyl)‐2,4,6‐CH2Me2C6H2), was prepared by reduction of the bisazide trans‐[(PN)2Hf(N3)2] (1) with two equiv of KC8. Encapsulation of K+ in 2 with crown‐ether or cryptand affords the first discrete salt [K(encap)][(PN)(PNC)Hf≡NH] (encap=18‐crown‐6(THF)2, 3; 2,2,2‐Kryptofix, 4), featuring a terminal parent imide and possessing some of the shortest Hf−N bond lengths known to date. DFT calculations revealed formation of 2 to proceed via an extremely basic monomeric nitrido, [(PN)2Hf≡N]−(A), having a computed pKBH+ of ∼57 followed by heterolytic splitting of an inert 1,2‐CH bond of a benzylic methyl group across the Hf≡N triple bond in A. An electronic structure analysis reveals A to possess a covalent Hf≡N triple bond and of super‐basic character. We also showcase reactivity of the Hf≡NH bond with various electrophiles.
Decarbonylation along with E atom transfer from Na(OCE) (E=P, As) to an isocyanide coordinated to the tetrahedral TiII complex [(TptBu,Me)TiCl], yielded the [(TptBu,Me)Ti(η3‐ECNAd)] species (Ad=1‐adamantyl, TptBu,Me−=hydrotris(3‐tert‐butyl‐5‐methylpyrazol‐1‐yl)borate). In the case of E=P, the cyanophosphide ligand displays nucleophilic reactivity toward Al(CH3)3; moreover, its bent geometry hints to a reduced Ad−NCP3− resonance contributor. The analogous and rarer mono‐substituted cyanoarsenide ligand, Ad−NCAs3−, shows the same unprecedented coordination mode but with shortening of the N=C bond. As opposed to TiII, VII fails to promote P atom transfer to AdNC, yielding instead [(TptBu,Me)V(OCP)(CNAd)]. Theoretical studies revealed the rare ECNAd moieties to be stabilized by π‐backbonding interactions with the former TiII ion, and their assembly to most likely involve a concerted E atom transfer between Ti‐bound OCE− to AdNC ligands when studying the reaction coordinate for E=P.
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