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Homogeneous H2-hydrogenation of imines catalyzed by Rh- and Ir-bis (tertiary phosphine) complexes Marcazzan, Paolo

Abstract

The coordination chemistry of several Rh and Ir precursors, containing monodentate tertiary phosphines (PR₃, R = aryl), in the presence of N-donor ligands (imines and amines), and aspects of the resulting mechanistic implications in their use as catalysts for the homogeneous H₂-hydrogenation of imines, were investigated. The reactivity of the Rh(I) and Ir(I) precursors [M(diene)(PR₃)₂]PF₆ (1) under 1 atm H₂ was examined and several of the corresponding M(III)-bis(hydrido) complexes [M(H)₂ (PR₃)₂- (solv)₂]PF₆ (2) formed in MeOH and acetone were isolated and characterized. While for M = Rh, complexes 2 exhibit moderate and low stability in the solid state and in solution, respectively, toward reductive elimination of H₂, when M = Ir the corresponding species 2 display no tendency to lose H₂ neither in the solid state nor in solution. Consequently, upon loss of H₂ in solution ready formation of the corresponding cis-[M(PR₃)₂ (solv)₂]PF₆ (3), characterized in situ for solv = MeOH, acetone, was observed only when M = Rh. [diagram] The new Rh(I) dimeric complexes [Rh₂(PR₃)₄][PF₆]₂ (R = Ph, 4a; p-tolyl, 4b), obtained by in vacuo removal of the coordinated solvent and hydrides from the corresponding 2, were isolated and characterized in both the solid state and in solution. The dimeric assembly observed in the solid state for complexes 4a and 4b is retained and as such observable only in CDCl₃ and CD₂Cl₂ solutions; however, evidence for a change in hapticity of the bridging arenes from n6 in the solid state to η⁴ in solution is presented. When R = benzyl (Bz), the 1:1 (Rh:phosphine) dimer Rh₂(PBz₃)₂-PF₅ (4c) was obtained. In coordinating media, species 4a and 4b generate 2 equivalents of the corresponding monomeric 3, and in either form provide useful precursors for an array of Rh(I) complexes. Several of the aryl imines investigated undergo ortho-metallation at both Rh and Ir centers. Several new M(III)-o-metallated complexes [M(H){R’N=C(R")(o-C₆H₄)}- (PR₃)₂(solv)]PF₆, with different combinations of R, R' and R", were isolated and/or characterized in situ for M = Ir and Rh (5-11). The Ir species display an inherent stability in all solvents, whereas some of the Rh analogues are fluxional in MeOH solution: solvent-exchange, discussed on the basis of variable temperature NMR studies, is proposed, and the implications of the kinetic lability of MeOH in catalytic hydrogenation reactions are discussed. The lack of suitable ortho-positions in the cyclic ketimine 6,7- dimethoxy-l-methyl-3,4-dihydro-isoquinoline (diq) results in the formation of cis- [Rh(PR₃)₂(diq)₂]PF₆ (R = Ph, 20a; p-tolyl, 20b), the only bis-imine species observed in this work. Metal-catalyzed hydrolytic cleavage of some (liquid) imines with formation of the corresponding coordinated amine and free aldehyde or ketone was observed suggesting that the source of H₂0 was the imine, although trace moisture in the solvent is not excluded. As a result, the new complexes [Ir(H){PhCH₂N=CH(o-C₆H₄)}(PPh₃)₂ (NH₂- CH₂Ph)]PF₆ (12*a), [Rh(H){PhCH₂N=C(Me)(o-C₆H₄)}(PPh₃)₂(NH₂CH₂Ph)]PF₆ (18a) and cw-[Rh(PR₃)₂(PhCH₂N=CHPh)(NH₂CH₂Ph)]PF₆ (R = Ph, 14a; p-tolyl, 14b) were isolated (~ 15-30 % yield) and characterized. [diagram] Observation of the hydrolytic cleavage of the imines led to investigations on the reactivity of Rh precursors toward benzylamine. Depending on the conditions, the new complexes cw-[Rh(PPh₃)₂ (PhCH₂NH₂)₂]PF₆ (15a) and cis,trans,cis-[Rh(R)₂ (PPh₃)₂-NH₂CH₂Ph) ₂]PF₆ (17a) were isolated and characterized, their structure and stability being solvent-dependent; for example, in acetone solution, both 15a and 17a convert into [Rh(PPh₃)₂{NH₂CH₂ (r,2-C₆H₅)}]PF₆ (16a). Homogeneous catalytic hydrogenation of several imines was achieved when using The Rh systems in MeOH. A mechanistic proposal for the catalysis is formulated based on studies conducted mostly on the imine PhCH₂N=CHPh: occurrence of hydrolysis and formation of a mono-amine-solvento species and of species 1a, and their involvement and role in the catalytic cycle, are discussed in particular. Variable temperature monitoring of stoichiometric hydrogenations was also carried out, and the implications of fluxionality of the corresponding o-metallated systems are discussed. Catalyst poisoning by the hydrogen ation product, examined and excluded for (PhCH2)2NH and PhCH2NHMe, was conclusively established for PhCH2NHPh: the new complex [Rh{r)₄-(C₆H₅)NHCH₂Ph}(PPh₃)₂]PF₆ (21a), isolated and fully characterized, contains the amine coordinated through a phenyl ring, the hapticity of which varies from the solid to solution state. Only one substrate (PhCH=NPh) was hydrogenated when using the Ir systems. The higher stability of the Irvs. Rh o-metallated complexes even in MeOH results in heterolytic cleavage of H₂ and in sequestration of the active site in the form of neutral, insoluble complexes such as [Ir(H)₂{PhCH₂N=C(Ph)(o-C₆H₄)}(PPh₃)₂](22*a). [diagram]

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