On the behaviour of Ru(I) and Ru(II) carbonyl acetates in the presence of H2 and/or acetic acid and their role in the catalytic hydrogenation of acetic acid

Abstract

The reactivity of phosphine substituted ruthenium carbonyl carboxylates Ru(CO) 2 (MeCOO) 2 (PBu 3 ) 2 , Ru 2 (CO) 4 (lMeCOO) 2 (PBu 3 ) 2 , Ru 4 (CO) 8 (l-MeCOO) 4 (PBu 3 ) 2 with H 2 and/or acetic acid was investigated by IR and NMR spectroscopy to clarify their role in the catalytic hydrogenation of acetic acid. Evidences were collected to suggest hydride ruthenium complexes as the catalytically active species. Equilibria among ruthenium hydrides and carboxylato complexes take place in the presence of hydrogen and acetic acid, that is in the conditions of the catalytic reaction. Nevertheless the presence of acetic acid reduces the rate of the formation of hydrides. Working at a very high temperature (180°C) polynuclear phosphido hydrides such as [Ru 6 (l-H) 6 (CO) 10 (l-PHBu)(l-PBu 2 ) 2 (PBu 3 ) 2 (l 6 -P)] were formed. These phosphido clusters are suggested as the resting state of the catalytic system.Furthermore the bi-or tetranuclear Ru(I) carboxylato complexes react with acetic acid giving a mononuclear ruthenium complex Ru(CO) 2 (MeCOO)(l-MeCOO)(PBu 3 ), containing a monodentate and a chelato acetato ligands. This complex was spectroscopically characterised. Its identity and structure were confirmed by its reactivity with stoichiometric amount of PPh 3 to give Ru(CO) 2 (Me-COO) 2 (PBu 3 )(PPh 3 ), a new mononuclear ruthenium carbonyl carboxylate containing two different phosphines, that was fully characterised.

Mononuclear ruthenium complexes containing two different phosphines in trans position: I. Synthesis and spectroscopic characterization

“…Synthesis of Ru(CO) 2 (CH 3 COO) 2 (P n Bu 3 )-[P(p-XC 6 H 4 ) 3 ] (1a-1e) Ru(CO) 2 (CH 3 COO) 2 (P n Bu 3 )[P(p-XC 6 H 4 ) 3 ] (1a-1e) have been synthesized from a tributylphosphine substituted binuclear ruthenium carbonylcarboxylato complex. Ru 2 (CO) 4 (CH 3 COO) 2 (P n Bu 3 ) 2 reacts at 40°C with acetic acid [1] giving the unstable specie Ru-(CO) 2 (j 1 -O-CH 3 COO)(j 2 -O,O 0 -CH 3 COO)(P n Bu 3 ), containing one j 1 and one j 2 acetato ligand (Scheme 1). The conversion is 100% after 48 h.…”

“…The new specie [1] was unstable in absence of acetic acid, even under nitrogen, so the n-heptane solution was used immediately for the subsequent synthesis of the mixed complexes.…”

“…This complex has been used as intermediate for the synthesis of Ru(CO) 2 (CH 3 COO) 2 -(P n Bu 3 )(PPh 3 ), containing two different phosphinic ligands in trans position [1] (Fig. 1).…”

Sawhorse-type diruthenium tetracarbonyl complexes

“…In the first case (no solvent indicated, catalyst/substrate ratio 1:2300, 130 bar H 2 , 60 • C), cyclohexane was formed in 22% yield after 22 h [97]; in the second case (methanol/water, catalyst/substrate 1:100, 100 bar, 100 • C), ethyl benzene was obtained in 91% yield after 6 h [98]. Acetone is converted into isopropanol under hydrogen pressure (100 • C, 100 bar) in the presence of [Ru 2 (CO) 4 (OOCMe) 2 (bipy) 2 ][MeCOO] with catalytic turnover frequencies up to 142 h −1 [99].…”

“…Ru 2 (CO) 4 (OOCMe) 2 (PBu 3 ) 2 also catalyzes the hydrogenation of acetophenone to (racemic) 1-phenylethanol according to Eq. (21) at 120 • C (no solvent indicated, catalyst/substrate ratio 1:2300, 130 bar H 2 ), the conversion being 66% after 22 h [97].…”