Osmium(II)–Bis(dihydrogen) Complexes Containing C_aryl,C_NHC–Chelate Ligands: Preparation, Bonding Situation, and Acidity

Abstract

ABSTRACT:The hexahydride complex OsH 6 (P i Pr 3 ) 2 (1) reacts with the BF 4 -salts of 1-phenyl-3-methyl-1-H-benzimidazolium, 1-phenyl-3-methyl-1-H-5,6-dimethyl-benzimidazolium, and 1-phenyl-3-methyl-1-H-imidazolium to give the respective trihydrideosmium(IV) derivatives OsH 3 ( 2 -C aryl ,C NHC )(P i Pr 3 ) 2 (2-4). The protonation of these compounds with HBF 4 ·OEt 2 produces the reduction of the metal center and the formation of the bis(dihydrogen)-osmium(II) complexes [Os( 2 -C aryl ,C NHC )(η 2 -H 2 ) 2 (P i Pr 3 ) 2 ]BF 4 (5-7). DFT calculations using AIM and NBO methods reveal that the Os-NHC bond of the Os-chelate link tolerates a significant π-backdonation from a doubly occupied d π (Os) atomic orbital to the p z atomic orbital of the carbene carbon atom. The π-accepting capacity of the NHC unit of the C aryl ,C NHC -chelate ligand, which is higher than those of the coordinated aryl group and phosphine ligands, enhances the electrophilicity of the metal center activating one of the coordinated hydrogen molecules of 5-7 towards the heterolysis. As a result, these compounds are strong Brønsted acid with pK a water values between 2.5 and 2.8. In acetonitrile the hydrogen molecules of 5 and 6 are displaced by the solvent, the resulting bis(solvento) compounds [Os( 2 -C aryl ,C NHC )(CH 3 CN) 2 (P i Pr 3 ) 2 ]BF 4 (8, 9) react with acetylacetonate (acac) and cis-1,2-bis(diphenylsphosphino)ethylene (bdppe) to give Os( 2 -C aryl ,C NHC )(acac)(bdppe) (10, 11) as a mixture of the two possible isomers, namely with P trans to the aryl group or to the NHC moiety.

Preparation of Tris-Heteroleptic Iridium(III) Complexes Containing a Cyclometalated Aryl-N-Heterocyclic Carbene Ligand

“…17 Remarkable M-NHC π-backbonding has been found in complexes with electron rich metal centers, in particular for those based on third row metals. 18 Thus, the cyclometalated arylpyridines, widely used to prepare emitters for OLEDs, are being replaced by cyclometalated aryl-NHCs because of the latter effectively raise the LUMO energy, which gives rise to a wider band gap and therefore more energetic emissions. 19 In the search for an efficient tuning of the color and stable systems towards ligand redistribution issues, we decided to prepare trisheteroleptic iridium(III) complexes in which one of the cyclometalated ligands is an aryl-NHC group.…”

Visible-Light-Induced Decarboxylative C–H Adamantylation of Azoles at Ambient Temperature

“…While a variety of bases could be utilized, the photoinduced C-H adamantylation was most effective in the presence of K 2 HPO 4 . The key importance of the photocatalyst, base, and light irradiation in the decarboxylative C-H adamantylation manifold was verified by probing the transformation in the absence of each component under otherwise identical reaction conditions (entries [17][18][19]. Notably, the use of blue light was found beneficial to realize satisfactory yields (entries 20 and 21).…”

“…In cases wherein air-or moisture-sensitive reagents were used, reactions were performed under N 2 atmosphere using standard Schlenk techniques. The following substrates were prepared according to previously described procedures: Benzothiazoles 1b-h, 17 benzoxales 1l-p, 18 benzimidazole 1q, 19 [Co(dmgH)(dmgH 2 )Cl 2 ], 20 and tetrabutylammonium adamantane carboxylate. 21 Other chemicals were obtained from commercial sources and were used without further purification, unless otherwise noted.…”

Cyclometalated Osmium Compounds and beyond: Synthesis, Properties, Applications

“…Stable compounds 185 and 187 , like possible intermediates in catalytic hydrogenation, were isolated (with yields from 83 to 89%) through the osmium reduction in the presence of HBF 4 . The authors also explored the substitution of the η 2 -bonded molecular hydrogen by a coordinating molecule like acetonitrile [ 100 ].…”