Facile Activation of Triarylboranes by Rhenium(V) Oxo Imido Complexes

Diverse Self-Assembled Molecular Architectures Promoted by C-H···O and C-H···Cl Hydrogen Bonds in a Triad of α-Diketone, α-Ketoimine, and an Imidorhenium Complex: A Unified Analysis Based on XRD, NEDA, SAPT, QTAIM, and IBSI Studies

X-ray structural elucidation, supramolecular self-assembly, and energetics of existential noncovalent interactions for a triad comprising α-diketone, α-ketoimine, and an imidorhenium complex are highlighted in this report. Molecular packing reveals a self-assembled 2D network stabilized by the C-H···O H-bonds for the α-diketone (benzil), and the first structural report of Brown and Sadanaga stressing on the prevalence of only the van der Waals forces seems to be an oversimplified conjecture. In the α-ketoimine, the imine nitrogen atom undergoes intramolecular N···H interaction to render itself inert toward intermolecular C-H···N interaction and exhibits two types of C-H···O H-bonds in consequence to generate a self-assembled 2D molecular architecture. The imidorhenium complex features a self-aggregated 3D packing engendered by the interplay of C-H···Cl H-bonds along with the ancillary C-H···π, C···C, and C···Cl contacts. To the best of our knowledge, in rhenium chemistry, this imidorhenium complex unravels the first example of self-associated 3D molecular packing constructed by the directional hydrogen bonds of C-H···Cl type. The presence of characteristic supramolecular synthons, viz., R2 2(12), R2 2(16), and R2 2(14), in the α-diketone, α-ketoimine, and imidorhenium complex, respectively, has prompted us to delve into the energetics of noncovalent interactions. Symmetry-adapted perturbation theory analysis has authenticated a stability order: R2 2(14) > R2 2(12) > R2 2(16) based on the interaction energy values of -25.97, -9.93, and -4.98 kcal/mol, respectively. The respective average contributions of the long-range dispersion, electrostatic, and induction forces are 58.5, 32.8, and 8.7%, respectively, for the intermolecular C-H···O interactions. The C-H···Cl interactions experience comparable contribution from the dispersion force (57.9% on average), although the electrostatic and induction forces contribute much less, 28.0 and 14.1%, respectively, on average. The natural energy decomposition analysis has further attested that the short-range, interfragment charge transfer occurring via the lp(O/Cl) → σ*(C-H) routes contributes 17-25% of the total attractive force for the C-H···O and C-H···Cl interactions. Quantum theory of atoms in molecules analysis unfolds a first-order exponential decay relation (y = 8.1043e -x/0.4095) between the electron density at the bond critical point and the distance of noncovalent interactions. The distances of noncovalent interactions in the lattices are internally governed by the individual packing patterns rather than the chemical nature of the H-bond donors and acceptors. Intrinsic bond strength index analysis shows promise to correlate the electron density at BCP with the SAPT-derived interaction energy for the noncovalent interactions. Two factors: (i) nearly half the HOMO-LUMO energy difference for the imidorhenium complex (∼30 kcal/mol) compared to the organics, and (ii) ∼60% localization of HOMO over the mer-ReCl3 moiety clearly indicate an enhanced polarizability of the complex facilitating the growth of weak C-H···Cl H-bonds.

1,2-Addition and cycloaddition reactions of niobium bis(imido) and oxo imido complexes

The bis(imido) complexes (BDI)Nb(N t Bu)2 and (BDI)Nb(N t Bu)(NAr) (BDI = N,N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate; Ar = 2,6-diisopropylphenyl) were shown to engage in 1,2-addition and [2 + 2] cycloaddition reactions with a wide variety of substrates. Reaction of the bis(imido) complexes with dihydrogen, silanes, and boranes yielded hydrido-amido-imido complexes via 1,2-addition across Nb-imido π-bonds; some of these complexes were shown to further react via insertion of carbon dioxide to give formate-amido-imido products. Similarly, reaction of (BDI)Nb(N t Bu)2 with tert-butylacetylene yielded an acetylide-amido-imido complex. In contrast to these results, many related mono(imido) Nb BDI complexes do not exhibit 1,2-addition reactivity, suggesting that π-loading plays an important role in activating the Nb-N π-bonds toward addition. The same bis(imido) complexes were also shown to engage in [2 + 2] cycloaddition reactions with oxygen- and sulfur-containing heteroallenes to give carbamate- and thiocarbamate-imido complexes: some of these complexes readily dimerized to give bis-μ-sulfido, bis-μ-iminodicarboxylate, and bis-μ-carbonate complexes. The mononuclear carbamate imido complex (BDI)Nb(NAr)(N( t Bu)CO2) (12) could be induced to eject tert-butylisocyanate to generate a four-coordinate terminal oxo imido intermediate, which could be trapped as the five-coordinate pyridine or DMAP adduct. The DMAP adducted oxo imido complex (BDI)NbO(NAr)(DMAP) (16) was shown to engage in 1,2-addition of silanes across the Nb-oxo π-bond; this represents a new reaction pathway in group 5 chemistry.

Molybdenum(VI) bis-imido Complexes of Dipyrromethene Ligands

We report the synthesis of high-valent molybdenum(VI) bis-imido complexes 1-4 with dipyrromethene (DPM) supporting ligands of the general formula (DPM^R)Mo(NR')₂Cl (R, R' = mesityl (Mes) or tert-butyl (^tBu)). The electrochemical and chemical properties of 1-4 reveal unexpected ligand noninnocence and reactivity. ¹⁵N NMR spectroscopy is used to assess the electronic properties of the imido ligands in the tert-butyl complexes 1 and 3. Complex 1 is inert toward ligand (halide) exchange with bulky phenolates such as KOMes or amides (e.g., KN(SiMe₃)₂), whereas the use of the lithium alkyl LiCH₂SiMe₃ results in a rare nucleophilic β-alkylation of the DPM ligand. While the reductions of the complexes occur at molybdenum, the oxidation is centered at the DPM ligand. Quantum-chemical calculations (complete active space self-consistent field, density functional theory) suggest facile (near-infrared) interligand charge transfer to the imido ligand, which might preclude the isolation of the oxidized complex [1]⁺ in the experiment.