Abstract
In this work, the covalent immobilization of two ruthenium(II) complexes, i.e., [RuIICl (bpea){(S)(-)(BINAP)}](BF4), 1, and [RuIICl(bpea)(DPPE)](BF4), 2, where BINAP = 2,2’-bis(diphenylphosphino)-1,1’-binaphthyl and DPPE = 1,2-bis(diphenylphosphino)ethane, have been obtained (AlPO4-Sepiolite@1 and AlPO4-Sepiolite@2) by using a N-tridentate ligand N,N-bis-(2-pyridylmethyl)ethylamine (bpea), linked to an amorphous AlPO4-Sepiolite (20/80) inorganic support. This AlPO4-sepiolite support is able to immobilize the double amount of ruthenium complex (1.65%) than the amorphous AlPO4 (0.89%). Both heterogenized complexes have been assessed as catalysts in the liquid phase hydrogenation of several substrates with carbonyl and/or olefinic double bonds using methanol as solvent, attaining good catalytic activity and high enantioselectivity (99%). The highest Turn Over Number (TON) value (748.6) was obtained over the [RuII Cl (bpea)(DPPE)](BF4) 2 catalyst, although the [RuIICl(bpea){(S)(-)(BINAP)}](BF4) 1 exhibits better reusability. In fact, the [RuIICl(bpea){(S)(-)(BINAP)}](BF4) immobilized on AlPO4-Sepiolite maintained the activity throughout 14 successive runs. Furthermore, some findings on hydrogenation mechanisms of the α,β-unsaturated carbonyl compounds over Ru catalysts have been also obtained.
Highlights
The results obtained show that the AlPO4 -Sepiolite support was successfully employed for the covalent immobilization of ruthenium complexes
This phosphamide bond anchors the organic chains through the formation of an organic-inorganic hybrid bond
(1.65 and 1.50% of Ru in Ru-diphenylphosphine ethane (DPPE) and Ru-BINAP complexes) is almost the double than that obtained on pure AlPO4 (0.89% for Ru-BINAP complex)
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. These are obtained by the covalent attachment of the ruthenium complex, corresponding to the atropoisomeric chelate phosphine, 2,20 -bis(diphenylphosphino)1,10 -binaphthyl (BINAP) [7,22] or the PPh3 ligand [8,22] In these cases, the activation of the inorganic support to obtain the initial organic linker is achieved by reacting the surface acid –OH groups of AlPO4 with another diamine, in this case ethylenediamine, on which the Noyori-type complex, or the Wilkinson-type complex, are obtained in successive steps by the chemical modification of the pendant amine group. By employing the PPh3 ligand, instead of the atropoisomeric chelate phosphine BINAP, the immobilized Wilkinson type complex was obtained This heterogenized catalyst exhibited very good activities and excellent reusability (up to 25 runs) in the hydrogenation reaction of several alkenes in methanol as solvent, as well as in the liquid phase enantioselective hydrogenation of prochiral substrates such as dimethyl itaconate, methyl. The ruthenium complexes were employed as homogeneous catalysts in the hydrogenation reactions for comparison
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