Photochemical pump and NMR probe : Chemically created NMR coherence on a microsecond time scale

We report pump-probe experiments employing laser-synchronized reactions of para-hydrogen (para-H2) with transition metal dihydride complexes in conjunction with nuclear magnetic resonance (NMR) detection. The pump-probe experiment consists of a single nanosecond laser pump pulse followed, after a precisely defined delay, by a single radio frequency (rf) probe pulse. Laser irradiation eliminates H2 from either Ru(PPh3) 3(CO)(H)2 1 or cis-Ru(dppe)2(H)2 2 in C6D6 solution. Reaction with para-H2 then regenerates 1 and 2 in a well-defined nuclear spin state. The rf probe pulse produces a high-resolution, single-scan 1H NMR spectrum that can be recorded after a pump-probe delay of just 10 μs. The evolution of the spectra can be followed as the pump-probe delay is increased by micro- or millisecond increments. Due to the sensitivity of this para-H2 experiment, the resulting NMR spectra can have hydride signal-to-noise ratios exceeding 750:1. The spectra of 1 oscillate in amplitude with frequency 1101 ± 3 Hz, the chemical shift difference between the chemically inequivalent hydrides. The corresponding hydride signals of 2 oscillate with frequency 83 ± 5 Hz, which matches the difference between couplings of the hydrides to the equatorial 31P nuclei. We use the product operator formalism to show that this oscillatory behavior arises from a magnetic coherence in the plane orthogonal to the magnetic field that is generated by use of the laser pulse without rf initialization. In addition, we demonstrate how chemical shift imaging can differentiate the region of laser irradiation thereby distinguishing between thermal and photochemical reactivity within the NMR tube

Preparation, X-ray structures, and NMR spectra of elongated dihydrogen complexes with four- and five-coordinate tin centers

4 pages, 4 figures, 1 scheme.-- Supporting Information Available: http://pubs.acs.org.Complex OsH3(SnPh2Cl){[è2-CH2dC(CH3)]PiPr2}(Pi- Pr3) (1) undergoes protonation-addition of benzoic acid to giVe the elongated dihydrogen deriVatiVe Os(SnPh2Cl)( 2-O2CPh)- (è2-H2)(PiPr3)2 (2). In solution the tin atom exchanges with the osmium on the chloride ligand by one of the oxygen atoms of the carboxylate group to afford OsCl{OC(Ph)OSnPh2}(è2- H2)(PiPr3)2 (3). Treatment of the latter with 1.0 equiV of benzoic acid leads to OsCl{OC(Ph)OSn( 2-O2CPh)Ph}(è2-H2)(PiPr3)2 (4). The elongated dihydrogen ligands of 2 and 3 show blocked rotation on the NMR time scale, whereas the hydrogen atoms undergo quantum exchange coupling. The X-ray structures of 3 and 4 are also reported.Financial support from the MEC of Spain (Project CTQ2005-00656) is acknowledged. B.E. thanks the Ministerio de Educación y Ciencia for her grant.Peer reviewe

Abnormal and normal N-heterocyclic carbene osmium polyhydride complexes obtained by direct metalation of imidazolium salts

6 pages, 4 figures, 1 table, 2 schemes.The complex OsH6(PiPr3)2 (1) is a basic precursor which promotes the direct metalation of imidazolium salts. Thus, it reacts with 1-mesityl-3-methylimidazolium tetraphenylborate and 1-mesityl-3-ethylimidazolium tetraphenylborate to give the abnormal NHC derivatives [OsH5(1-mesityl-3-methylimidazol-4-ylidene)(PiPr3)2]BPh4 (2) and [OsH5(1-mesityl-3-ethylimidazol-4-ylidene)(PiPr3)2]BPh4 (3), respectively. Treatment of 2 and 3 with NaH produces the deprotonation of the metal center and the formation of the tetrahydride derivatives OsH4(1-mesityl-3-methylimidazol-4-ylidene)(PiPr3)2 (4) and OsH4(1-mesityl-3-ethylimidazol-4-ylidene)(PiPr3)2 (5). In contrast to the case for the mesityl-substituted imidazolium salts, the reaction with 1-benzyl-3-methylimidazolium tetraphenylborate leads to the normal NHC complex [OsH5(1-benzyl-3-methylimidazol-2-ylidene)(PiPr3)2]BPh4 (6). The deprotonation of 6 with NaH affords OsH4(1-benzyl-3-methylimidazol-2-ylidene)(PiPr3)2 (7). Complexes 2 and 4 have been characterized by X-ray diffraction analysis.Financial support from the MEC (Projects CTQ2005-00656 and Consolider Ingenio 2010 CDS2007-00006) and the Diputación General de Aragón (E35) are acknowledged. B.E. thanks the Spanish MEC for her grant.Peer reviewe

Cβ-H activation of aldehydes promoted by an osmium complex

Evidence for the Cβ-H bond activation of aldehydes is shown. Complex OsH3(SnPh2Cl)-{η2-CH 2=C(CH3)PiPr2}(PiPr 3) (1) reacts with benzaldehyde and p- and m-substituted benzaldehydes to give OsH2(SnPh2Cl){C6H 3RC(O)H}(PiPr3)2 (R = H (2), OCH3 (3, 5, 6), CF3 (4, 7)), which result from the C-H activation of one ortho-C-H bond. Reactions of 1 with 3-furaldehyde and 1-cyclohexene-1-carboxaldehyde lead to OsH2(SnPh 2Cl){C2(O)H2C(O)H}-(PiPr 3)2 (8) and OsH2(SnPh2Cl){C 6H8C(O)H}(PiPr3)2 (9), respectively. 1H NMR spectroscopic studies indicate that the OsH 2 unit of 2-9 forms elongated dihydrogen systems with H-H separations ranging between 1.41 and 1.50 Å. Furthermore, the hydrogen atoms undergo thermally activated site exchange processes with ΔG‡ between 9 and 12 kcal·mol1 and quantum exchange coupling. Complexes 2, 5, and 8 have been characterized by X-ray diffraction analysis.Financial support from the MCYT of Spain (Projects PPQ2000-0488-P4-02, BQU-2002-00606, and BQU2002-04110-CO2-02) is acknowledged. B.E. thanks the Spanish MCYT for her grant. M.O. thanks the Spanish MCYT/Universidad de Zaragoza for funding through the “Ramón y Cajal” program.Peer Reviewe

Compuestos de osmio ciclometalados con propiedades luminiscentes

Resumen del póster presentado a la XXXV Reunión Bienal de la Real Sociedad Española de Química celebrada en A Coruña del 19 al 23 de julio de 2015.El derivado hexahidruro OsH6(PiPr3)2 promueve la ruptura de enlaces C-H de moléculas orgánicas diversas. La estrategia de asistencia por quelatación es crucial en estas reacciones, debido al elevado número de hidruros presentes en este compuesto. De entre los auxiliares de coordinación usados, el átomo de nitrógeno de heterociclos ha sido particularmente útil, y ha permitido la activación de enlaces C-H, no sólo aromáticos y olefínicos, sino también la de enlaces C(sp3)–H. Recientemente, partiendo de este derivado hexahidruro, hemos preparado nuevos compuestos homolépticos Os(CNHCCariloCNHC)2 y heterolépticos Os(CNHCCariloCNHC)(CNHCCarilo’CNHC) y Os(CNHC’CariloCNHC’)(CNHCCarilo’CNHC) que son fotoluminiscentes y emiten en el verde/azul con rendimientos cuánticos elevados. Siguiendo con esta línea de investigación, en este trabajo presentamos la síntesis de nuevos compuestos de osmio ciclometalados y el estudio de sus propiedades luminiscentes.Peer Reviewe

Influence of the Anion of the Salt Used on the Coordination Mode of an N-Heterocyclic Carbene Ligand to Osmium

8 pages, 6 figures, 1 table.The reactions of the hexahydride OsH6(PiPr3)2 (1) with 1.0 equiv of the BPh4−, BF4−, and Br− salts of 1-(2-pyridylmethyl)-3-methylimidazolium, in tetrahydrofuran under reflux, have been studied. In the three cases, mixtures of the abnormal [OsH(η2-H2){κC5,N-[1-(2-pyridylmethyl)-3-methylimidazol-5-ylidene]}(PiPr3)2]A (A = BPh4 (2a), BF4 (2b), Br (2c)) and normal [OsH(η2-H2){κC2,N-[1-(2-pyridylmethyl)-3-methylimidazol-2-ylidene]}(PiPr3)2]A (A = BPh4 (3a), BF4 (3b), Br (3c)) isomers are obtained. The formation rate of the abnormal isomer and the abnormal to normal ratio decrease as the coordinating power of the anion of the used salt increases. Treatment of 2b with either HBF4·OEt2 or LiBF4 gives rise to its isomerization to 3b. The X-ray structure of 2a and T1(min) values of the OsH3 resonances of the cations support the hydride-elongated dihydrogen nature of these compounds. The nonclassical interaction between the hydrogen atoms of the OsH3 unit is more important in the normal isomer than in the abnormal. Treatment of 1 with 2.0 equiv of 1-(2-pyridylmethyl)-3-methylimidazolium bromide yields the bis(normal-NHC) complex [OsH{κC2,N-[1-(2-pyridylmethyl)-3-methylimidazol-2-ylidene]}2(PiPr3)]Br (4), which has been also characterized by X-ray diffraction analysis.Financial support from the Spanish MEC (Projects CTQ2005-00656 and Consolider Ingenio 2010 CSD2007-00006) and the Diputación General de Aragón (E35) is gratefully acknowledged. B.E. thanks the Spanish MEC for her grant. M.B. thanks the Spanish MEC/ Universidad de Zaragoza for funding through the “Ramón y Cajal” program.Peer reviewe

Stoichiometric and catalytic deuteration of pyridine and methylpyridines by H/D exchange with benzene-d6 promoted by an unsaturated osmium tetrahydride species

Treatment of toluene solutions of the hexahydride complex OsH 6(PiPr3)2 (1) with pyridine, 3-methylpyridine, and 4-methylpyridine affords the tetrahydride derivatives OsH4(Rpy)(PiPr3)2 (Rpy = py (3), 3-methylpyridine (4), and 4-methylpyridine (5)). In benzene-d6, these compounds release the heterocycles, and the resulting unsaturated tetrahydride OsH4(PiPr3)2 (2) promotes the stoichiometric and catalytic deuteration of pyridine, 3-methylpyridine, and 4-methylpyridine by means of H/D exchanges between the heterocycles and the solvent. The deuteration rates of the pyridinic C. - H bonds depend upon their positions in the heterocycles. For pyridine, they increase as the C. - H bonds are separated from the heteroatom. A methyl substituent has a marked negative effect on the deuteration of its adjacent C. - H bonds. The kinetic analysis of the deuteration reveals that the rate-determining step for the H/D exchanges is the C. - H activation of the bond that is deuterated. DFT calculations show that this step is formed by two elemental stages: the direct coordination of the C. - H bond and its subsequent rupture. © 2009 American Chemical Society.Financial support from the MICINN(Projects CTQ2008-00810 and Consolider Ingenio 2010 CSD2007-00006) and Diputación General de Aragón (E35) is acknowledged.Peer Reviewe

Osmium NHC complexes from alcohol-functionalized imidazoles and imidazolium salts

The hexahydride complex OsH6(PiPr3) 2 (1) reacts with 1-mesitylimidazole, 1-methylimidazole, 1-(2-hydroxy-2-phenylethyl)imidazole, and 1-(2-hydroxypropyl)imidazole to give the N-bound imidazole compounds OsH4(RIm)(PiPr 3)2 (R = Mes (2), Me (3), CH2CH(OH)Ph (4), CH2CH(OH)CH3 (5)) and H2. In toluene under reflux the alcohol derivatives 4 and 5 evolve into the C-bound imidazole complexes OsH3{CNHCHCHNCH=C(R)O}(PiPr3) 2 (R = Ph (6), CH3 (7)), bearing an NH wingtip. These NHC-enolate species result from the N-bound to C-bound transformation of the heterocycle and the deprotonation-dehydrogenation of the alcohol substituent. The addition of HBF4 to 6 and 7 affords the NHC-keto derivatives [OsH3{CNHCHCHNCH2C(R)=O}(PiPr3) 2]BF4 (R = Ph (8), CH3 (9)). Treatment of 1 with 3-benzyl-1-(2-hydroxy-2-phenylethyl)imidazolium tetrafluoroborate and 3-benzyl-1-(2-hydroxypropyl)imidazolium tetraphenylborate leads to the NHC-keto complexes [OsH3{CN(CH2Ph)CHCHNCH2C(R)=O}(P iPr3)2]A (A = BF4, R = Ph (10); A = BPh4, CH3 (11)), as a consequence of the direct metalation of the heterocycle and the dehydrogenation of the alcohol substituent. The deprotonation of 10 and 11 with KtBuO gives the NHC-enolate derivatives OsH3{CN(CH2Ph)CHCHNCH=C(R)O}(P iPr3)2 (R = Ph (12), CH3 (13)). The X-ray diffraction structures of 2, 6, and 10 are also reported. © 2011 American Chemical Society.Financial support from the MICINN of Spain (Project Nos. CTQ2007-65218, CTQ2008-00810, and Consolider Ingenio 2010 CSD2007-00006), the Diputación General de Aragón (E35), the Generalitat Valenciana (PROMETEO/ 2009/0349 and FEDER), and the European Social Fund is acknowledged.Peer Reviewe

Coordination and rupture of Methyl C(sp3)−H bonds in osmium−polyhydride complexes with δ agostic interaction

13 pages, 1 table, 13 figures, 4 schemes.-- Supporting Information Available: http://pubs.acs.org.The hexahydride complex OsH6(PiPr3)2 (1) reacts with 8-methylquinoline and 2-(dimethylamino)pyridine to give OsH3(CH2C9H6N)(PiPr3)2 (2) and OsH3{CH2N(CH3)-o-C5H4N}(PiPr3)2 (3), respectively, as a result of the release of two hydrogen molecules of 1 and the C(sp3)-H bond activation of a methyl group of the organic substrates. In solution the hydride ligands and the hydrogen atoms of the methylene group of 2 exchange their positions. The activation parameters for the process are ¢Hq ) 18.9 ( 0.1 kcalâmol-1 and ¢Sq ) 3 ( 2 eu. Treatment of 2 with HBF4 affords the hydride-elongated dihydrogen derivative [OsH(è2-H2)(CH3C9H6N)(PiPr3)2]BF4 (6), with the methyl group of the quinoline ligand coordinated in a è3-H2C fashion. The X-ray structure of 6 and the DFT optimization of the structure of the model cation [OsH(è2-H2)(CH4)(NH3)(PMe3)2]+ prove that the methyl coordination in the ä agostic complex is similar to the methane coordination in the model compound. The reaction of 3 with HBF4 leads to the cyclic carbene derivative [OsH3{dCHN(CH3)-o-C5H4N}(PiPr3)2]BF4 (7), as a result of the release of a hydrogen molecule and a C(sp3)-H bond activation on the methylene group of 3. The formation of 2, 3, 6, and 7 has been analyzed by DFT calculations.Financial support from the MEC of Spain (Projects CTQ2005-00656, CTQ2005-9000-C02-01, and Consolider Ingenio 2010 CSD2007-00006) is gratefully acknowledged. B.E. thanks the Spanish MEC for her grant. M.B. thanks the Spanish MEC/Universidad de Zaragoza for funding through the “Ramón y Cajal” program.Peer reviewe

Synthesis and characterisation of [6]-azaosmahelicenes: the first d 4-heterometallahelicenes

[6]-Azaosmahelicenes, the first d4-heterometallahelicenes, have been synthesised and fully characterised. Their optical properties (UV-Vis absorption and luminescence) are reported.Financial support from the Spanish MICINN Projects CTQ2010-20414-C02-01, CTQ2010-20500-C02-01, CTQ2011-23459, and Consolider Ingenio 2010 (CSD2007-00006), DGA (E35), Comunidad de Madrid (CCG07-UCM/PPQ-2596) and European Social Fund is acknowledged. I.F. is a Ramón y Cajal Fellow.Peer reviewe