NH-Tautomerization of Quinolines and 2-Methylpyridine Promoted by a Hydride-Iridium(III) Complex: Importance of the Hydride Ligand

Metal-Ligand Proton Tautomerism, Electron Transfer, and C(sp3)-H Activation by a 4-Pyridinyl-Pincer Iridium Hydride Complex

The para-N-pyridyl-based PCP pincer proligand 3,5-bis(di-tert-butylphosphinomethyl)-2,6-dimethylpyridine (pN-tBuPCP-H) was synthesized and metalated to give the iridium complex (pN-tBuPCP)IrHCl (2-H). In marked contrast with its phenyl-based congeners, e.g., (tBuPCP)IrHCl and derivatives, 2-H is highly air-sensitive and reacts with oxidants such as ferrocenium, trityl cation, and benzoquinone. These oxidations ultimately lead to intramolecular activation of a phosphino-t-butyl C(sp3)-H bond and cyclometalation. Considering the greater electronegativity of N than C, 2-H is expected to be less easily oxidized than simple PCP derivatives; cyclic voltammetry and DFT calculations support this expectation. However, 2-H is calculated to undergo metal-ligand-proton tautomerism (MLPT) to give an N-protonated complex that can be described with resonance forms representing a zwitterionic complex (with a negative charge on Ir) and a p-N-pyridylidene (a remote N-heterocyclic carbene) Ir(I) complex. One-electron oxidation of this tautomer is calculated to be dramatically more favorable than direct oxidation of 2-H (ΔΔG° = -31.3 kcal/mol). The resulting Ir(II) oxidation product is easily deprotonated to give metalloradical 2• which is observed by NMR spectroscopy. 2• can be further oxidized to give cationic Ir(III) complex, 2+, which can oxidatively add a phosphino-t-butyl C-H bond and undergo deprotonation to give the observed cyclometalated product. DFT calculations indicate that less sterically hindered analogues of 2+ would preferentially undergo intermolecular addition of C(sp3)-H bonds, for example, of n-alkanes. The resulting iridium alkyl complexes could undergo facile β-H elimination to afford olefin, thereby completing a catalytic cycle for alkane dehydrogenation driven by one-electron oxidation and deprotonation, enabled by MLPT.

Organocatalytic Atroposelective Construction of Axially Chiral Compounds Containing Benzimidazole and Quinoline Rings

An organocatalytic atroposelective strategy for the construction of axially chiral compounds containing benzimidazole and quinoline rings is described. The enantioselective heteroannulation reaction of 2-alkynylbenzimidazoles with ortho-aminophenylketones proceeded smoothly in the presence of chiral phosphoric acid to provide axially chiral heterobiaryls with good yields and enantioselectivities. This is the first example of the combination of benzimidazole and quinoline rings at the 2- and 3-positions, respectively, into axially chiral heterobiaryls by this new strategy.

Rhodium-Promoted C-H Bond Activation of Quinoline, Methylquinolines, and Related Mono-Substituted Quinolines

The C-H bond activation of methylquinolines, quinoline, 3-methoxyquinoline, and 3-(trifluoromethyl)quinoline promoted by the square-planar rhodium(I) complex RhH{κ³-P,O,P-[xant(PⁱPr₂)₂]} [1; xant(PⁱPr₂)₂ = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene] has been systematically studied. Results reveal that the activation of the heteroring is preferred over the activation of the carbocycle, and the activated position depends upon the position of the substituent in the substrate. Thus, 3-, 4-, and 5-methylquinoline reacts with 1 to quantitatively form square-planar rhodium(I)-(2-quinolinyl) derivatives, whereas 2-, 6-, and 7-methylquinoline quantitatively leads to rhodium(I)-(4-quinolinyl) species. By contrast, quinoline and 8-methylquinoline afford mixtures of the respective rhodium(I)-(2-quinolinyl) and -(4-quinolinyl) complexes. 3-Methoxyquinoline displays the same behavior as that of 3-methylquinoline, while 3-(trifluoromethyl)quinoline yields a mixture of rhodium(I)-(2-quinolinyl), -(4-quinolinyl), -(6-quinolinyl), and -(7-quinolinyl) isomers.

Mesoionic and Related Less Heteroatom-Stabilized N-Heterocyclic Carbene Complexes: Synthesis, Catalysis, and Other Applications

Mesoionic carbenes are a subclass of the family of N-heterocyclic carbenes that generally feature less heteroatom stabilization of the carbenic carbon and hence impart specific donor properties and reactivity schemes when coordinated to a transition metal. Therefore, mesoionic carbenes and their complexes have attracted considerable attention both from a fundamental point of view as well as for application in catalysis and beyond. As a follow-up of an earlier Chemical Reviews overview from 2009, the organometallic chemistry of N-heterocyclic carbenes with reduced heteroatom stabilization is compiled for the 2008-2017 period, including specifically the chemistry of complexes containing 1,2,3-triazolylidenes, 4-imidazolylidenes, and related 5-membered N-heterocyclic carbenes with reduced heteratom stabilization such as (is)oxazolylidenes, pyrrazolylidenes, and thiazolylidenes, as well as pyridylidenes as 6-membered N-heterocyclic carbenes with reduced heteroatom stabilization. For each ligand subclass, metalation strategies, electronic and steric properties, and applications, in particular, in metal-mediated catalysis, are compiled. Mesoionic carbenes demonstrate particularly high activity in (water) oxidation, hydrogen transfer reactions, and cyclization reactions. Unique features of these ligands are identified such as their dipolar structure, their specific donor properties, as well as stability aspects of the ligand and the complexes, which provides opportunities for further research.

Selective formation of formamidines, carbodiimides and formimidates from isocyanide complexes of Mn(i) mediated by Ag2O

The easily accessible cation [Mn(bipy)(CO)₃]⁺ induces transformations of isocyanides into a variety of organic functionalities with the mediation of Ag₂O.

σ-Aromaticity in polyhydride complexes of Ru, Ir, Os, and Pt

Transition-metal hydrides represent a unique class of compounds, which are essential for catalysis, organic synthesis, and hydrogen storage. In this work we study IrH5(PPh3)2, (RuH5(P(i)Pr3)2)(-), (OsH5(P(i)Pr3)2)(-), and OsH4(PPhMe2)3 polyhydride complexes, inspired by the recent discovery of the σ-aromatic PtZnH5(-) cluster anion. The distinctive feature of these molecules is that, like in the PtZnH5(-) cluster, the metal is five-fold coordinated in-plane, and holds additional ligands at the axial positions. This work shows that the unusual coordination in these compounds indeed can be explained by σ-aromaticity in the pentagonal arrangement, stabilized by the atomic orbitals on the metal. Based on this newly elucidated bonding principle, we additionally propose a new family of polyhydrides that display a uniquely high coordination. We also report the first indications of how aromaticity may impact the reactivity of these molecules.

Metal–ligand bifunctional reactivity and catalysis of protic N-heterocyclic carbene and pyrazole complexes featuring β-NH units

The metal–ligand bifunctional cooperation of protic N-heterocyclic carbene and pyrazole complexes bearing an NH unit at the position β to the metal is surveyed.

Steric parameters in the Ir-catalyzed regio- and diastereoselective isomerization of primary allylic alcohols

The iridium-catalyzed diastereo- and regioselective isomerization of primary allylic alcohols using Crabtree's catalyst or sterically modified analogs is reported. The importance of the size of the substituents on either the substrates or the catalysts has been rationalized by linear free energy relationships.

Photophysical properties of cyclometalated Pt(II) complexes: counterintuitive blue shift in emission with an expanded ligand π system

A detailed examination was performed on photophysical properties of phosphorescent cyclometalated (C(^)N)Pt(O(^)O) complexes (ppy)Pt(dpm) (1), (ppy)Pt(acac) (1'), and (bzq)Pt(dpm) (2) and newly synthesized (dbq)Pt(dpm) (3) (C(^)N = 2-phenylpyridine (ppy), benzo[h]quinoline (bzq), dibenzo[f,h]quinoline (dbq); O(^)O = dipivolylmethanoate (dpm), acetylacetonate (acac)). Compounds 1, 1', 2, and 3 were further characterized by single crystal X-ray diffraction. Structural changes brought about by cyclometalation were determined by comparison with X-ray data from model C(^)N ligand precursors. The compounds emit from metal-perturbed, ligand-centered triplet states (E(0-0) = 479 nm, 1; E(0-0) = 495 nm, 2; E(0-0) = 470 nm, 3) with disparate radiative rate constants (kr = 1.4 × 10(5) s(-1), 1; kr = 0.10 × 10(5) s(-1), 2; kr = 2.6 × 10(5) s(-1), 3). Zero-field splittings of the triplet states (ΔE(III-I) = 11.5 cm(-1), 1'; ΔE(III-I) < 2 cm(-1), 2; ΔE(III-I) = 46.5 cm(-1), 3) were determined using high resolution spectra recorded in Shpol'skii matrices. The fact that the E0-0 energies do not correspond to the extent of π-conjugation in the aromatic C(^)N ligand is rationalized on the basis of structural distortions that occur upon cyclometalation using data from single crystal X-ray analyses of the complexes and ligand precursors along with the triplet state properties evaluated using theoretical calculations. The wide variation in the radiative rate constants and zero-field splittings is also explained on the basis of how changes in the electronic spin density in the C(^)N ligands in the triplet state alter the spin-orbit coupling in the complexes.

Deprotonation of C-alkyl groups of cationic triruthenium clusters containing cyclometalated C-alkylpyrazinium ligands: experimental and computational studies

The C-alkyl groups of cationic triruthenium cluster complexes of the type [Ru3(μ-H)(μ-κ(2)N(1),C(2)-L)(CO)10](+) (HL represents a generic C-alkyl-N-methylpyrazium species) have been deprotonated to give kinetic products that contain unprecedented C-alkylidene derivatives and maintain the original edge-bridged decacarbonyl structure. When the starting complexes contain various C-alkyl groups, the selectivity of these deprotonation reactions is related to the atomic charges of the alkyl H atoms, as suggested by DFT/natural-bond orbital (NBO) calculations. Three additional electronic properties of the C-alkyl C-H bonds have also been found to correlate with the experimental regioselectivity because, in all cases, the deprotonated C-H bond has the smallest electron density at the bond critical point, the greatest Laplacian of the electron density at the bond critical point, and the greatest total energy density ratio at the bond critical point (computed by using the quantum theory of atoms in molecules, QTAIM). The kinetic decacarbonyl products evolve, under appropriate reaction conditions that depend upon the position of the C-alkylidene group in the heterocyclic ring, toward face-capped nonacarbonyl derivatives (thermodynamic products). The position of the C-alkylidene group in the heterocyclic ring determines the distribution of single and double bonds within the ligand ring, which strongly affects the stability of the neutral decacarbonyl complexes and the way these ligands coordinate to the metal atoms in the nonacarbonyl products. The mechanisms of these decacarbonylation processes have been investigated by DFT methods, which have rationalized the structures observed for the final products and have shed light on the different kinetic and thermodynamic stabilities of the reaction intermediates, thus explaining the reaction conditions experimentally required by each transformation.

Xantphos-type complexes of group 9: rhodium versus iridium

Treatment of the dimer [Rh(μ-Cl)(C8H14)2]2 (1a) with 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene [xant(P(i)Pr2)2] leads to the d(8) square-planar complex RhCl{xant(P(i)Pr2)2} (2), whereas reaction of the iridium counterpart [Ir(μ-Cl)(C8H14)2]2 (1b) gives the d(6) octahedral compound IrHCl{xant(P(i)Pr2)[(i)PrPCH(Me)CH2]} (3) as a result of the intramolecular C-H bond activation of one of the isopropyl substituents of the phosphine. Stirring 2 and 3 in 0.5 N KOH solutions of 2-propanol gives rise to the formation of hydrides RhH{xant(P(i)Pr2)2} (4) and IrH3{xant(P(i)Pr2)2} (5), respectively. In n-octane at 60 °C, complex 2 is stable. However, compound 3 activates the alkane to give the cis-dihydride IrH2Cl{xant(P(i)Pr2)2} (6) and a mixture of 3- and 4-octene. Complex 6 can be also obtained by the reaction of 3 with H2. Under the same conditions, 2 affords the rhodium analogue RhH2Cl{xant(P(i)Pr2)2} (7). Compounds 2-4 react with triflic acid (HOTf) to give RhHCl(OTf){xant(P(i)Pr2)2} (8), IrHCl(OTf){xant(P(i)Pr2)2} (9), and RhH2(OTf){xant(P(i)Pr2)2} (10), respectively. The related iridium derivative IrH2(OTf){xant(P(i)Pr2)2} (11) has also been prepared by the reaction of 6 with Tl(OTf). Complexes 2, 6, and 9 have been characterized by X-ray diffraction analysis. The {xant(P(i)Pr2)2}M skeleton is T-shaped with the metal center situated in the common vertex.

Deprotonation of C-alkyl groups of cationic N-heterocyclic ligands

The C-alkyl groups of C-alkylpyrazinium-derived ligands have been selectively deprotonated by K[N(SiMe(3))(2)], through charge-controlled processes, to give neutral products that contain C-alkylidenepyrazine-derived ligands.