Homogeneous catalysis with polyhydride complexes

This review analyzes the role of transition metal polyhydrides as homogeneous catalysts for organic reactions. Discussed reactions involve nearly every main organic functional group.

H/D exchange in N-heterocycles catalysed by an NHC-supported ruthenium complex

NHC-supported trihydrides Cp(NHC)RuH₃show excellent catalytic activity in the H/D exchange of pyridine and some other N-heterocycles under mild conditions and low catalyst loading.

Tandem transfer hydrogenation–epoxidation of ketone substrates catalysed by alkene-tethered Ru(ii)–NHC complexes

Eight novel alkene-tethered Ru(ii)–NHC complexes were employed as catalysts in tandem transfer hydrogenation–epoxidation reactions using phenacyl bromide derivatives as substrates.

Cyclopentadienyl-Ru(II)-Pyridylamine Complexes: Synthesis, X-ray Structure, and Application in Catalytic Transformation of Bio-Derived Furans to Levulinic Acid and Diketones in Water

A series of cationic half-sandwich cyclopentadienyl-ruthenium(II)-pyridylamine complexes, [(η⁵-C₅H₅)Ru(κ²-L)(PPh₃)]⁺ (L = N_amine-substituted pyridylamine ligands) ([Ru]-1-[Ru]-6), along with the analogous cyclopentadienyl-ruthenium(II)- N-isopropylpyridylimine complex [(η⁵-C₅H₅)Ru(κ²-L)(PPh₃)]⁺ (L = N-isopropylpyridylimine) ([Ru]-7), have been synthesized in good yields. Structural identities of all the complexes have been authenticated by ¹H, ¹³C, and ³¹P NMR, mass spectrometry, and X-ray crystallography. The synthesized complexes exhibited high catalytic activity for the transformation of the bio-derived furans, 2-furfural (furfural), 5-methyl-2-furfural (5-MF), and 5-hydroxymethyl-2-furfural (5-HMF) to levulinic acid (LA) and the diketones, 3-hydroxyhexane-2,5-dione (3-HHD), 1-hydroxyhexane-2,5-dione (1-HHD), and hexane-2,5-dione (HD) in water. Efficient transformation of furfural to LA over a range of η⁵-Cp-Ru-pyridylamine complexes is substantially affected by the N_amine-substituents, where a η⁵-Cp-Ru- N-propylpyridylamine complex ([Ru]-2) exhibited higher catalytic activity in comparison to other η⁵-Cp-Ru-pyridylamine and η⁵-Cp-Ru-pyridylimine complexes. The relative catalytic activity of the studied complexes demonstrated a substantial structure-activity relationship which is governed by the basicity of N_amine, steric hindrance at N_amine, and the hemilabile nature of the coordinated pyridylamine ligands.

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.