We describe the unusual reactivity of a highly labile diethyl ether adduct of an asymmetric niobium(V) bis(imide) 2.OEt2 containing the monoazabutadiene (MAD) ligand. This species undergoes clean nitrene transfer on treatment with tert-butyl- or di-isopropylphenyl azide resulting in the unprecedented reformation of nacnac ligands bound to the metal center. Corresponding reactions with trimethylsilyl- or tert-butyl azide allowed the isolation of two rare intermediates prior to N2 loss; mechanistic studies support the involvement of two different niobium species.
Red light has the advantages of low energy, less health risk and high penetration depth through various media. Herein, a helical carbenium ion (N,N'-din -propyl-1,13-dimethoxyquinacridinium (nPr-DMQA+) tetrafluoroborate) has been used as an organic photoredox catalyst for photoreductions and photooxidations in the presence of red ligh (max = 640 nm). It has catalyzed red-lightmediated dual transition-metal/photoredox-catalyzed C-H arylation and intermolecular atom transfer radical addition through oxidative quenching, affording products in 57-93% yields. Moreover, its potential in photooxidation catalysis has also been demonstrated by successful applications in red-light-induced aerobic oxidative hydroxylation of arylboronic acids and benzylic C(sp3)-H oxygenation through reductive quenching, delivering products in up to 92% yield. Thus, a versatile organic photoredox catalyst (helical carbenium ion) for red-light-mediated photoredox reactions has been developed. In summary, we have disclosed a helical carbenium ion [nPr-DMQA+][BF4-] (3), which catalyzes photoreductions and photooxidations in the presence of low-energy red light. The role of 3 as an efficient PC in oxidative and reductive quenching were evaluated by transition-metal/nPr-DMQA+-catalyzed C-H arylations and intermolecular ATRA (oxidative quenching), as well as aerobic oxidative hydroxylation of arylboronic acids and benzylic C(sp3)-H oxygenation (reductive quenching). Eight diverse substrates were well-tolerated for red-light-mediated dual Pd/nPr-DMQA+-catalyzed C(sp2)-H arylation. Moreover, with twelve different arylboronic acids as the substrates, red-lightinduced nPr-DMQA+-catalyzed aerobic oxidative hydroxylation proceeded smoothly as well. The successful applications of 3 in these red-light-mediated reactions have established its role as a versatile organic PC, which can serve as a complementary option for current white, blue or green-light-mediated photocatalysis. Further investigations on the applications of 3 towards more challenging photoredox catalysis are in progress.
Monometallic niobium arene complexes [Nb(BDI)(N(t)Bu)(R-C(6)H(5))] (2a: R = H and 2b: R = Me, BDI = N,N'-diisopropylbenzene-β-diketiminate) were synthesized and found to undergo slow conversion into the diniobium inverted arene sandwich complexes [[(BDI)Nb(N(t)Bu)](2)(μ-RC(6)H(5))] (7a: R = H and 7b: R = Me) in solution. The kinetics of this reaction were followed by (1)H NMR spectroscopy and are in agreement with a dissociative mechanism. Compounds 7a-b showed a lack of reactivity toward small molecules, even at elevated temperatures, which is unusual in the chemistry of inverted sandwich complexes. However, protonation of the BDI ligands occurred readily on treatment with [H(OEt(2))][B(C(6)F(5))(4)], resulting in the monoprotonated cationic inverted sandwich complex 8 [[(BDI(#))Nb(N(t)Bu)][(BDI)Nb(N(t)Bu)](μ-C(6)H(5))][B(C(6)F(5))(4)] and the dicationic complex 9 [[(BDI(#))Nb(N(t)Bu)](2)(μ-RC(6)H(5))][B(C(6)F(5))(4)](2) (BDI(#) = (ArNC(Me))(2)CH(2)). NMR, UV-vis, and X-ray absorption near-edge structure (XANES) spectroscopies were used to characterize this unique series of diamagnetic molecules as a means of determining how best to describe the Nb-arene interactions. The X-ray crystal structures, UV-vis spectra, arene (1)H NMR chemical shifts, and large J(CH) coupling constants provide evidence for donation of electron density from the Nb d-orbitals into the antibonding π system of the arene ligands. However, Nb L(3,2)-edge XANES spectra and the lack of sp(3) hybridization of the arene carbons indicate that the Nb → arene donation is not accompanied by an increase in Nb formal oxidation state and suggests that 4d(2) electronic configurations are appropriate to describe the Nb atoms in all four complexes.
The discovery of a Nb(III)-mediated catalytic hydrogenation of internal alkynes to (Z)-alkenes that proceeds through an unprecedented mechanism is reported. The mechanistic proposal involves initial reduction of the alkyne by the Nb(III) complex (BDI)Nb(N(t)Bu)(CO)(2) to provide a Nb(V) metallacyclopropene, itself capable of σ-bond metathesis reactivity with H(2). The resulting alkenyl hydride species then undergoes reductive elimination to provide the (Z)-alkene product and regenerate a metal complex in the Nb(III) oxidation state. Support for the proposed mechanism is derived from (i) the dependence of the product selectivity on the relative concentrations of CO and H(2), (ii) the isolation of complexes closely related to those proposed to be part of the catalytic cycle, (iii) H/D crossover experiments, and (iv) DFT studies of multiple possible reaction pathways.
The oxidation of alcohols with N2O as the hydrogen acceptor was achieved with low catalyst loadings of a rhodium complex that features a cooperative bis(olefin)amido ligand under mild conditions. Two different methods enable the formation of either the corresponding carboxylic acid or the ester. N2 and water are the only by-products. Mechanistic studies supported by DFT calculations suggest that the oxygen atom of N2O is transferred to the metal center by insertion into the Rh-H bond of a rhodium amino hydride species, generating a rhodium hydroxy complex as a key intermediate.
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