| Summary: | The coordination chemistry of several Rh and Ir precursors, containing
monodentate tertiary phosphines (PRâ, 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 Hâ-hydrogenation of imines, were investigated. The
reactivity of the Rh(I) and Ir(I) precursors [M(diene)(PRâ)â]PFâ (1) under 1 atm Hâ was
examined and several of the corresponding M(III)-bis(hydrido) complexes [M(H)â (PRâ)â-
(solv)â]PFâ (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 Hâ, when M = Ir the corresponding species
2 display no tendency to lose Hâ neither in the solid state nor in solution. Consequently,
upon loss of Hâ in solution ready formation of the corresponding cis-[M(PRâ)â (solv)â]PFâ
(3), characterized in situ for solv = MeOH, acetone, was observed only when M = Rh.
[diagram]
The new Rh(I) dimeric complexes [Rhâ(PRâ)â][PFâ]â (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 CDClâ and CDâClâ solutions; however, evidence for a change
in hapticity of the bridging arenes from n6 in the solid state to ηⴠin solution is presented.
When R = benzyl (Bz), the 1:1 (Rh:phosphine) dimer Rhâ(PBzâ)â-PFâ
(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){RâN=C(R")(o-CâHâ)}-
(PRâ)â(solv)]PFâ, 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(PRâ)â(diq)â]PFâ (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 Hâ0 was the imine, although trace moisture in the solvent is not
excluded. As a result, the new complexes [Ir(H){PhCHâN=CH(o-CâHâ)}(PPhâ)â (NHâ-
CHâPh)]PFâ (12*a), [Rh(H){PhCHâN=C(Me)(o-CâHâ)}(PPhâ)â(NHâCHâPh)]PFâ (18a)
and cw-[Rh(PRâ)â(PhCHâN=CHPh)(NHâCHâPh)]PFâ (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(PPhâ)â (PhCHâNHâ)â]PFâ (15a) and cis,trans,cis-[Rh(R)â (PPhâ)â-NHâCHâPh) â]PFâ (17a) were isolated and characterized, their structure and stability being solvent-dependent; for example, in acetone solution, both 15a and 17a convert into
[Rh(PPhâ)â{NHâCHâ (r,2-CâHâ
)}]PFâ (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 PhCHâN=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)â-(CâHâ
)NHCHâPh}(PPhâ)â]PFâ (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 Hâ and in sequestration of the active site in the form of neutral, insoluble complexes such as [Ir(H)â{PhCHâN=C(Ph)(o-CâHâ)}(PPhâ)â](22*a).
[diagram]
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