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Hydrogen and methanol activation by some tertiary phosphine ruthenium complexes

University of British Columbia

The previously known complexes, RU₂H₄Cl₂(PR₃)₄, have now been correctly reformulated as the η²-H₂ species (η²-H₂)(PR₃)₂Ru(μ-Cl)₂(μ-H)RuH(PR₃)₂ (R = Ph, p-tol), 1a and 1b, and it is confirmed that in solution they are dimeric and undergo no ligand dissociation. Also, a new analogue of complexes of type 1 is reported: the complex (η²-H₂)(isoPFA)Ru(μ-Cl)₂(μ-H)RuH(PPh₃)₂,4, is formed from the reaction of RuCl₂(PPh₃)(isoPFA), 3b, with H₂ in methanol/benzene, and a crystal structure of 4 shows the η²-H₂ ligand; isoPFA and PPFA (see below) are ferrocene based, chelating P-N ligands, with the structures: [Chemical compound diagram omitted] R = Pri and Ph for isoPFA and PPFA, respectively. Complexes 1a, 1b and 4 all react with 1-hexene to give hexane; the main ruthenium phosphine product in the case of 1 is the corresponding RuHCl(PR₃)₃ complex, while 4 reacts to give a complex mixture of ruthenium phosphine complexes, including 3b. The amount of hexane formed from the reaction of 4 with hexene is quantified as 2 mol/mol 4. The hydrogenation of 1-hexene catalyzed by 1a is re-interpreted as occurring via the mechanism: (η²-H₂)(PPh₃)₂Ru(μ-Cl)₂(μ-H)RuH(PPh₃)₂ + hexene K₁→(PPh₃)₂Ru(μ-Cl)₂(μ-H)RuH(PPh₃)₂ + hexane (1) (PPh₃)₂Ru(μ-Cl)₂(μ-H)RuH(PPh₃)₂ + H₂ K₂⇆(η²-H₂)(PPh₃)₂Ru(μ-Cl)₂(μ-H)RuH(PPh₃)₂ (2) Reactions of RuCl₂(PPh₃)(PPFA), 3a, and RuCl₂(PPh₃)(isoPFA), 3b, with H₂ have been further studied, in connection with earlier mechanistic studies on hydrogenation of organic substrates catalyzed by complex 3a. The complex 3a reacts with 2-8 atm H₂ in n-butanol to give ruthenium phosphine products including 1a. The complex 3b reacts with H₂ in methanol/benzene to give 4, as mentioned above, as well as a number of unidentified hydrides; in DMA, the reaction of 3b with H₂ gives 1a, 4, RuHCl(PPh₃)(isoPFA) (7), RuHCl(PPh₃)₃ and other unidentified ruthenium phosphine complexes. The product H₂NMe₂+Cl⁻ was also isolated from the methanol/benzene reaction mixture, and this product provides evidence that the amine functionality of the P-N ligands is involved in the promotion of the heterolytic cleavage of dihydrogen to give a proton and a hydride (H₂→ H⁺ + H⁻). Kinetic studies on the hydrogenation of 1-hexene catalyzed by 3a, and by 3b in the present work, are now interpreted according to the mechanism [Chemical compound diagram omitted] Reactions involving 3b and methanol have also been studied, and 3b is also active for the transfer hydrogenation (from methanol) of ketones and activated olefins. The reaction of 3b with methanol in the absence of base is proposed to occur with the stoichiometry: RuCl₂(PPh₃)(isoPFA) + 2MeOH→ H₂NMe₂⁺Cl⁻ + H₂ + 3b RuHCl(CO)(PPh₃)(isoPOF), 5 (5) where the ligand isoPOF is formed from isoPFA by replacement of the NMe₂ group on isoPFA by a methoxo group; reaction 6 could occur via the following steps: RuCl₂(PPh₃)(isoPFA) + MeOH→ RuHCl(CO)(PPh₃)(isoPFA), 6 3b + H₂ + HCl (6) RuHCl(CO)(PPh₃)(isoPFA) + MeOH→ RuHCl(CO)(PPh₃)(isoPOF), 5 + HNMe₂ (7) HCl + HNMe₂ H₂NMe₂⁺Cl⁻ (8) A mechanism for reaction 7 is presented and invokes reversible attack by MeOH with replacement of Cl⁻, followed by reversible deprotonation of coordinated MeOH to give successively methoxo, formaldehyde and formyl intermediates, and finally the hydrido-carbonyl, 6. The reaction of 3b with methanol in the presence of KOH is proposed to occur according to the stoichiometry: RuCl₂(PPh₃)(isoPFA) + KOH + CH₃OH→ RuHCl(CO)(PPh₃)(isoPFA) + KCI + H₂ + H₂O (9) and two pathways have been identified, one base-independent, identical to that proposed for reaction 7, and one showing a second-order dependence on KOH. The latter pathway invokes initial reversible attack on RuCl₂(PPh₃)(isoPFA), 3b, by MeO⁻, replacing Cl⁻ to give RuCl(OMe)(PPh₃)(isoPFA), and subsequent reversible replacement of PPh₃ by OH⁻, followed by concerted loss of OH⁻ and hydride transfer from coordinated OMe⁻ to give a hydrido-formaldehyde complex RuHCl(η²-CH₂O)(isoPFA). A subsequently formed formyl intermediate reacts via intramolecular hydride transfer from the formyl to the metal, H₂ loss, and phosphine coordination to give the hydrido-carbonyl 6.

Text

2010-11-02

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http://hdl.handle.net/2429/29763

Chemistry

University of British Columbia

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