Influence of the N-N Coligand: C-C coupling instead of formation of imidazol-2-yl complexes at {Mo(η(3)-allyl)(CO)2} fragments. Theoretical and experimental studies

New N-methylimidazole (N-MeIm) complexes of the {Mo(η3-allyl)(CO)2(N–N)} fragment have been prepared, in which the N,N-bidentate chelate ligand is a 2-pyridylimine. The addition of a strong base to the new compounds deprotonates the central CH group of the imidazole ligand and subsequently forms the C–C coupling product that results from the nucleophilic attack to the imine C atom. This reactivity contrasts with that previously found for the analogous 2,2′-bipyridine compounds [Mo(η3-allyl)(CO)2(bipy)(N-RIm)]OTf [N-RIm = N-MeIm, N-mesitylimidazole (N-MesIm, Mes= 2,4,6-trimethylphenyl); OTf = trifluoromethanesulfonate) which afforded imidazol-2-yl complexes upon deprotonation. Density Functional Theory (DFT) computations uncover that the reactivity of the imine C atom along with its ability to delocalize electron density are responsible for the new reactivity pattern found for the kind of molybdenum complexes reported herein.Financial support from the Ministerio de Economia y Competitividad (Projects CTQ2010-18231, CTQ2012-37370-C02-01, and CTQ2012-37370-C02-02) is gratefully acknowledged.Peer Reviewe

Hybrid xerogels: study of the sol-gel process and local structure by vibrational spectroscopy

The properties of hybrid silica xerogels obtained by the sol-gel method are highly dependent on the precursor and the synthesis conditions. This study examines the influence of organic substituents of the precursor on the sol-gel process and determines the structure of the final materials in xerogels containing tetraethyl orthosilicate (TEOS) and alkyltriethoxysilane or chloroalkyltri-ethoxysilane at different molar percentages (RTEOS and ClRTEOS, R = methyl [M], ethyl [E], or propyl [P]). The intermolecular forces exerted by the organic moiety and the chlorine atom of the precursors were elucidated by comparing the sol-gel process between alkyl and chloroalkyl series. The microstructure of the resulting xerogels was explored in a structural theoretical study using Fourier transformed infrared spectroscopy and deconvolution methods, revealing the distribution of (SiO)4 and (SiO)6 rings in the silicon matrix of the hybrid xerogels. The results demonstrate that the alkyl chain and the chlorine atom of the precursor in these materials determines their inductive and steric effects on the sol-gel process and, therefore, their gelation times. Furthermore, the distribution of (SiO)4 and (SiO)6 rings was found to be consistent with the data from the X-ray diffraction spectra, which confirm that the local periodicity associated with four-fold rings increases with higher percentage of precursor. Both the sol-gel process and the ordered domains formed determine the final structure of these hybrid materials and, therefore, their properties and potential applications.The authors gratefully acknowledge the financial support received from the Ministerio de Economia y Competitividad of Spain (Project MAT2016-78155-C2-2-R). G.C. thanks MINECO and the 'Ministerio de Ciencia, Investigación y Universidades' of Spain for his 'FPU' grant (FPU18/03467). The authors also acknowledge the use of the 'Centro de Instrumentación Científico-Técnica' at the University of Jaén and UCTAI at the Public University of Navarre

Room temperature iron-catalyzed transfer hydrogenation and regioselective deuteration of carbon-carbon double bonds

An iron catalyst has been developed for the transfer hydrogenation of carbon-carbon multiple bonds. Using a well-defined beta-diketiminate iron(II) precatalyst, a sacrificial amine and a borane, even simple, unactivated alkenes such as 1-hexene undergo hydrogenation within 1 h at room temperature. Tuning the reagent stoichiometry allows for semi-and complete hydrogenation of terminal alkynes. It is also possible to hydrogenate aminoalkenes and aminoalkynes without poisoning the catalyst through competitive amine ligation. Furthermore, by exploiting the separate protic and hydridic nature of the reagents, it is possible to regioselectively prepare monoisotopically labeled products. DFT calculations define a mechanism for the transfer hydrogenation of propene with (BuNH2)-Bu-n and HBpin that involves the initial formation of an iron(II)-hydride active species, 1,2-insertion of propene, and rate-limiting protonolysis of the resultant alkyl by the amine N-H bond. This mechanism is fully consistent with the selective deuteration studies, although the calculations also highlight alkene hydroboration and amine borane dehydrocoupling as competitive processes. This was resolved by reassessing the nature of the active transfer hydrogenation agent: experimentally, a gel is observed in catalysis, and calculations suggest this can be formulated as an oligomeric species comprising H-bonded amine borane adducts. Gel formation serves to reduce the effective concentrations of free HBpin and (BuNH2)-Bu-n and so disfavors both hydroboration and dehydrocoupling while allowing alkene migratory insertion (and hence transfer hydrogenation) to dominate

Heterobimetallic ruthenium–zinc complexes with bulky N-heterocyclic carbenes: syntheses, structures and reactivity

The ruthenium–zinc heterobimetallic complexes, [Ru(IPr)2(CO)ZnMe][BArF4] (7), [Ru(IBiox6)2(CO)(THF) ZnMe][BArF4] (12) and [Ru(IMes)’(PPh3)(CO)ZnMe] (15), have been prepared by reaction of ZnMe2 with the ruthenium N-heterocyclic carbene complexes [Ru(IPr)2(CO)H][BArF4] (1), [Ru(IBiox6)2(CO)(THF)H][BArF4] (11) and [Ru(IMes)(PPh3)(CO)HCl] respectively. 7 shows clean reactivity towards H2, yielding [Ru(IPr)2(CO) (¿2-H2)(H)2ZnMe][BArF4] (8), which undergoes loss of the coordinated dihydrogen ligand upon application of vacuum to form [Ru(IPr)2(CO)(H)2ZnMe][BArF4] (9). In contrast, addition of H2 to 12 gave only a mixture of products. The tetramethyl IBiox complex [Ru(IBioxMe4)2(CO)(THF)H][BArF4] (14) failed to give any isol- able Ru–Zn containing species upon reaction with ZnMe2. The cyclometallated NHC complex [Ru(IMes)’ (PPh3)(CO)ZnMe] (15) added H2 across the Ru–Zn bond both in solution and in the solid-state to afford [Ru(IMes)’(PPh3)(CO)(H)2ZnMe] (17), with retention of the cyclometallati

Activation of aromatic C−C bonds of 2,2’-bipyridine ligands

4,4’-Disubstituted-2,2′-bipyridine ligands coordinated to Mo and Re cationic fragments become dearomatized by an intramolecular nucleophilic attack from a deprotonated N-alkylimidazole ligand in cis disposition. The subsequent protonation of these neutral complexes takes place on a pyridine carbon atom rather than at nitrogen, weakening an aromatic C−C bond and affording a dihydropyridyl moiety. Computational calculations allowed for the rationalization of the formation of the experimentally obtained products over other plausible alternatives.Financialsupport from Ministerio de Economía y Competitividad/FEDER (grant CTQ2015-70231-P) and Principado de Asturias (grant GRUPIN14-103) is gratefully acknowledged. J.D.thanks COMPUTAEX for granting access to LUSITANIA supercomputing facilities.Peer Reviewe

Hydrophosphination of unactivated alkenes and alkynes using iron(II):catalysis and mechanistic insight

The catalytic addition of phosphines to alkenes and alkynes is a very attractive process that offers access to phosphines in a 100% atom-economic reaction using readily available and inexpensive materials. The products are potentially useful ligands and organocatalysts. Herein, we report the first example of intramolecular hydrophosphination of a series of nonactivated phosphinoalkenes and phosphinoalkynes with a simple iron β-diketiminate complex. Kinetic studies suggest that this transformation is first-order with respect to both the phosphine and the catalyst. A mechanistic interpretation of the iron-catalyzed hydrophosphination is presented, supported by the experimental evidence collected

Organocatalytic Michael Addition of Unactivated α-Branched Nitroalkanes to Afford Optically Active Tertiary Nitrocompounds

The direct, asymmetric conjugate addition of unactivated α-branched nitroalkanes is developed based on the combined use of chiral amine/ureidoaminal bifunctional catalysts and a tunable acrylate template to provide tertiary nitrocompounds in 55–80% isolated yields and high enantioselectivity (e.r. up to 96:4). Elaboration of the ketol moiety in thus obtained adducts allows a fast entry to not only carboxylic and aldehyde derivatives but also nitrile compounds and enantioenriched 5,5-disubstituted γ-lactams.We thank the Basque Government (EJ, grant IT1583-22) and Agencia Estatal de Investigación (grants PID2019-109633GB and PID2022-137153NB-C21/AEI/10.13039/501100011033) for financial support. A.G.-U. thanks EJ; B.L. thanks the Navarra Government, and M.E.-V. thanks UPNA (PJUPNA18-2022). Authors also thank SGIker (UPV/EHU/ERDF, EU) for providing NMR, HRMS, and X-ray resources

Activación de N-heterociclos aromáticos coordinados a fragmentos organometálicos

En la presente Memoria se estudia fundamentalmente la evolución de los productos de desprotonación de compuestos organometálicos con ligandos N-alquilimidazol (N-RIm). En primer lugar se han estudiado los compuestos que presentan un ligando 2,2¿-bipiridina (bipy) sustituido en las posiciones 4,4¿, y un ligando N-RIm en posiciones mutuamente cis, coordinados al fragmento fac-{Re(CO)3}. En función de la naturaleza de los sustituyentes de la bipy se ha encontrado que, el producto resultante de la desprotonación puede ser un complejo 2-imidazolilo, precursor de un carbeno NH N-heterocíclico (NH-NHC), o el producto de acoplamiento C-C entre el imidazol y la diimina. La extensión de estos estudios a los derivados de Re(I) con tres ligandos N-heterocíclicos monodentados da lugar, tras el tratamiento con un oxidante, a la formación de ligandos biheterocíclicos en la esfera de coordinación del metal. Finalmente se han estudiado las reacciones de desprotonación de los complejos catiónicos [Mo(¿3-alilo)(CO)2(N-N)(N-MeIm)]+, donde N-N es un ligando 2-piridilimina. Se ha obtenido así un patrón de reactividad diferente al previamente encontrado para los complejos análogos con bipy. Mientras que en aquéllos se producía la formación de ligandos imidazol-2-ilo, en los derivados con 2-piridiliminas se obtienen los productos de acoplamiento C-C. Los estudios computacionales realizados están de acuerdo con los resultados experimentales y han mostrado que, efectivamente, la formación de los productos de acoplamiento es la reacción más favorable.Peer Reviewe

Acoplamiento entre piridinas e imidazoles en la esfera de coordinación del fragmento fac-{Re(CO)3}

Resumen del trabajo presentado a la XXXIV Reunion Bienal de la Real Sociedad Española de Química: "Química para un futuro común", celebrada en Santander (España) del 15 al 18 de septiembre de 2013.Peer reviewe