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

Azolium Control of the Osmium-Promoted Aromatic C-H Bond Activation in 1,3-Disubstituted Substrates

The hexahydride complex OsH₆(PⁱPr₃)₂ promotes the C-H bond activation of the 1,3-disubstituted phenyl group of the [BF₄]^- and [BPh₄]^- salts of the cations 1-(3-(isoquinolin-1-yl)phenyl)-3-methylimidazolium and 1-(3-(isoquinolin-1-yl)phenyl)-3-methylbenzimidazolium. The reactions selectively afford neutral and cationic trihydride-osmium(IV) derivatives bearing κ²-C,N- or κ²-C,C-chelating ligands, a cationic dihydride-osmium(IV) complex stabilized by a κ³-C,C,N-pincer group, and a bimetallic hexahydride formed by two trihydride-osmium(IV) fragments. The metal centers of the hexahydride are separated by a bridging ligand, composed of κ²-C,N- and κ²-C,C-chelating moieties, which allows electronic communication between the metal centers. The wide variety of obtained compounds and the high selectivity observed in their formation is a consequence of the main role of the azolium group during the activation and of the existence of significant differences in behavior between the azolium groups. The azolium role is governed by the anion of the salt, whereas the azolium behavior depends upon its imidazolium or benzimidazolium nature. While [BF₄]^- inhibits the azolium reactions, [BPh₄]^- favors the azolium participation in the activation process. In contrast to benzimidazolylidene, the imidazolylidene resulting from the deprotonation of the imidazolium substituent coordinates in an abnormal fashion to direct the phenyl C-H bond activation to the 2-position. The hydride ligands of the cationic dihydride-osmium(IV) pincer complex display intense quantum mechanical exchange coupling. Furthermore, this salt is a red phosphorescent emitter upon photoexcitation and displays a noticeable catalytic activity for the dehydrogenation of 1-phenylethanol to acetophenone and of 1,2-phenylenedimethanol to 1-isobenzofuranone. The bimetallic hexahydride shows catalytic synergism between the metals, in the dehydrogenation of 1,2,3,4-tetrahydroisoquinoline and alcohols.

Water-soluble transition metal complexes of ruthenium(ii), osmium(ii), rhodium(iii) and iridium(iii) with chelating N-heterocyclic carbene ligands in hydrogenation and transfer hydrogenation catalysis

The synthesis of novel Ru(ii), Os(ii), Rh(iii) and Ir(iii) mono-N-heterocyclic carbene (NHC) complexes and their application in hydrogenation catalysis is reported.

A comparative study on the N-heterocyclic carbene adducts of Ih-C60, D5h-C70 and Sc3N@Ih-C80

The exclusive formations of the normal nNHC-C_60/70 and abnormal aNHC-Sc₃N@I_h-C₈₀ complexes in two recent experiments were suggested to be thermodynamically and kinetically controlled, respectively.

A Capped Octahedral MHC6 Compound of a Platinum Group Metal

A MHC6 complex of a platinum group metal with a capped octahedral arrangement of donor atoms around the metal center has been characterized. This osmium compound OsH{κ(2) -C,C-(PhBIm-C6 H4 )}3 , which reacts with HBF4 to afford the 14 e(-) species [Os{κ(2) -C,C-(PhBIm-C6 H4 )}(Ph2 BIm)2 ]BF4 stabilized by two agostic interactions, has been obtained by reaction of OsH6 (PiPr3 )2 with N,N'-diphenylbenzimidazolium chloride ([Ph2 BImH]Cl) in the presence of NEt3 . Its formation takes place through the C,C,C-pincer compound OsH2 {κ(3) -C,C,C-(C6 H4 -BIm-C6 H4 )}(PiPr3 )2 , the dihydrogen derivative OsCl{κ(2) -C,C-(PhBIm-C6 H4 )}(η(2) -H2 )(PiPr3 )2 , and the five-coordinate osmium(II) species OsCl{κ(2) -C,C-(PhBIm-C6 H4 )}(PiPr3 )2 .

An Entry to Stable Mixed Phosphine-Osmium-NHC Polyhydrides

An entry to mixed phosphine-osmium-NHC polyhydride complexes is described, starting from the five-coordinate hydrido-alkylidyne compounds [OsHCl(≡CPh)(IPr)(PR3)]OTf (IPr = 1,3-bis(2,6-disopropylphenyl)imidazolylidene; OTf = CF3SO3; PR3 = P(i)Pr3 (1), PPh3 (2)). The experimental procedure involves the borylation of the Os-C triple bond of 1 and 2 with NaBH4 and the subsequent alcoholysis of the borylation products OsH2Cl(η(2)-H-BCH2Ph)(IPr)(PR3) (PR3 = P(i)Pr3 (3), PPh3 (4)) or OsH2(η(2):η(2):H2BCH2Ph)(IPr)(P(i)Pr3) (5). Stirring of 3 in 2-propanol affords the five coordinate chloride-trihydride OsH3Cl(IPr)(P(i)Pr3)2 (6), which reacts with NaBH4 to give OsH3(κ(2)-H2BH2)(IPr)(P(i)Pr3) (7). This trihydride-tetrahydrideborate derivative and its PPh3 counterpart OsH3(κ(2)-H2BH2)(IPr)(PPh3) (8) can be also obtained in a one-pot procedure, starting from 1 and 2 and using methanol at -60 °C instead of 2-propanol as alcoholysis agent. The bonding situation in 7 and 8, analyzed by DFT calculations using AIM and NBO methods, resembles that found in B2H6 and contrasts with the bonding situation in the bis-σ-borane derivative 5. Stirring of 7 and 8 in 2-propanol leads to the corresponding d(2)-hexahydride derivatives OsH6(IPr)(PR3) (PR3 = P(i)Pr3 (9), PPh3 (10)), which reduce the C≡N triple bond of benzonitrile and promote the subsequent chelate-assisted ortho-CH bond activation of the resulting phenylmethanimine, to form the trihydride compounds OsH3{κ(2)-N,C-(NH═CH-C6H4)}(IPr)(PR3)2 (PR3 = P(i)Pr3 (11), PPh3 (12)), containing a stabilized orthometalated aldimine.

Room-temperature hydrohydrazination of terminal alkynes catalyzed by saturated abnormal N-heterocyclic carbene-gold(I) complexes

A number of saturated abnormal N-heterocyclic carbene (NHC) complexes of gold, in combination with KBAr(F) 4 as activator, were successfully applied in the chemoselective addition of hydrazine to alkynes. The reaction proceeds even at room temperature, which was not possible to date with gold catalysts. The reaction can be applied to a number of substituted arylalkynes. With alkylalkynes the yields are low. The saturated abnormal NHC ligands are resistant to isomerization to the saturated normal NHC coordination mode under basic reaction conditions. Under acidic conditions, a simple protonation at the nitrogen atom not neighboring the carbene center was observed and unambiguously characterized by an X-ray crystal-structure analysis. Computational studies confirm that such an isomerization would be highly exothermic, the observed kinetic stability probably results from the need to shift two protons in such a process.

Picolyl-NHC hydrotris(pyrazolyl)borate ruthenium(II) complexes: synthesis, characterization, and reactivity with small molecules

Ruthenium(II) hydrotris(pyrazolyl)borate chloro complexes bearing picolyl-functionalized N-heterocyclic carbenes [TpRu(κ(2)-C,N-picolyl-(R)I)Cl] (picolyl-(Me)I = 3-methyl-1-(2-picolyl)imidazol-2-ylidene) (1a), picolyl-(iPr)I = 3-isopropyl-1-(2-picolyl)imidazol-2-ylidene (1b), picolyl-(Me)45DClI = 3-methyl-1-(2-picolyl)-4,5-dichloroimidazol-2-ylidene (1c), picolyl-(Ph)I = 3-phenyl-1-(2-picolyl)imidazol-2-ylidene (1d), picolyl-(Me)BI = 3-methyl-1-(2-picolyl)benzoimidazol-2-ylidene (1e)) have been synthesized and characterized. Furthermore, cationic carbonyl derivatives 2a-e have been prepared, characterized, and used to study the donor properties of the picolylcarbene ligands (picolyl-(R)I) via infrared spectroscopy. Also, the reactivity of the 16-electron species [TpRu(κ(2)-C,N-picolyl-(R)I)](+), in situ generated using NaBAr(F)4 (Ar(F) = 3,5-bis(trifluoromethyl)phenyl) as a halide scavenger, toward N2, CH3CN, H2, CH2CH2, S8, and O2 was studied indicating a strong influence of the NHC wingtip and backbone substituents in the product distribution. The crystal structures of [TpRu(κ(2)-C,N-picolyl-(iPr)I)Cl] (1b), [TpRu(κ(2)-C,N-picolyl-(Me)I)CO][BAr(F)4] (2a), [TpRu(κ(2)-C,N-picolyl-(Ph)I)CO][BAr(F)4] (2d), [{TpRu(κ(2)-C,N-picolyl-(Me)I}2(μ-N2)][BAr(F)4]2 (3'a), [{TpRu(κ(2)-C,N-picolyl-(Ph)I)}2(μ-N2)][BAr(F)4]2 (3'd), [TpRu(κ(2)-C,N-picolyl-(iPr)I)(η(2)-CH2CH2)][BAr(F)4] (5b), and [{TpRu(κ(2)-C,N-picolyl-(Me)I)}2(μ-S2)][BAr(F)4]2 (6) are reported.

Mono- and dinuclear osmium N,N'-di- and tetraphenylbipyridyls and extended bipyridyls. Synthesis, structure and electrochemistry

The efficient synthesis of mono- and dinuclear Os(IV) bipyridyl complexes is reported. These compounds show a two-step oxidation process leading to notable structural changes, which are reflected in their emission properties. During the second oxidation process a tetracation with a hydride-dihydrogen structure (instead of a trihydride) is formed. This results in a significant bathochromic shift of the emission band, accompanied by a moderate increase in intensity.

Emissive osmium(II) complexes supported by N-heterocyclic carbene-based C^C^C-pincer ligands and aromatic diimines

Osmium(II) complexes containing N-heterocyclic carbene (NHC)-based pincer ligand 1,3-bis(1-methylimidazolin-2-ylidene)phenyl anion (C(1)^C^C(1)) or 1,3-bis(3-methylbenzimidazolin-2-ylidene)phenyl anion (C(2)^C^C(2)) and aromatic diimine (2,2'-bipyridine (bpy), 1,10-phenanthroline (phen), or 4,4'-diphenyl-2,2'-bipyridine (Ph(2)bpy)) in the form of [Os(C^C^C)(N^N)(CO)](+) have been prepared. Crystal structures for these complexes show that the Os-C(NHC) bonds are essentially single (Os-C(NHC) distances = 2.079(5)-2.103(7) Å). Spectroscopic comparisons and time-dependent density functional theory (TD-DFT) calculations suggest that the lowest-energy electronic transition associated with these complexes (λ(max) = 493-536 nm, ε(max) = (5-10) × 10(3) dm(3) mol(-1) cm(-1), solvent = CH(3)CN) originate from a d(π)(Os(II)) → π*(N^N) metal-to-ligand charge transfer transition, where the d(π)(Os(II)) and π*(N^N) levels contain significant contribution from the C^C^C ligands. All these complexes are emissive in the red-spectral region (674-731 nm) with quantum yields of 10(-4)-10(-2) and emission lifetimes of around 1-6 μs. Transient absorption spectroscopy and spectroelectrochemical measurements have also been used to probe the nature of the emissive excited-states. Overall, this joint experimental and theoretical investigation reveals that the C^C^C ligands can be used to modulate the photophysical properties of a [Os(N^N)] core via the formation of the hybrid [Os + C^C^C] frontier orbitals.

Synthesis and characterisation of [6]-azaosmahelicenes: the first d4-heterometallahelicenes

[6]-Azaosmahelicenes, the first d(4)-heterometallahelicenes, have been synthesised and fully characterised. Their optical properties (UV-Vis absorption and luminescence) are reported.