Square‐Planar Carbonylchlororhodium(I) Complexes Containing trans‐Spanning Diphosphine Ligands as Catalysts for the Carbonylation of Methanol

Abstract

Dedicated to Professor Andre Merbach on the occasion of this 65th birthdayThe rhodium(I) complexes trans-[Rh(diphos)(CO)Cl] 7 (diphos pbpb), 8 (diphos nbpb), and 9 (diphos cbpb) were synthesized (Scheme 4) and used as catalysts for the carbonylation of MeOH to AcOH (Scheme 1). The trans coordination imposed by the rigid C-spacer framework of the diphos ligands pbpb, nbpb, and cbpb, demonstrated by 31 P-NMR and IR spectroscopy of 7 ± 9 and unambiguously confirmed by singlecrystal X-ray structure analysis of 7, improved the thermal stability of the rhodium(I) system under carbonylation conditions and, hence, the catalytic performance of the complexes. For the catalytic carbonylation of MeOH, the active catalyst could be prepared in situ from the mixture of [Rh(CO) 2 Cl] 2 and the corresponding diphos ligand pbpb, nbpb, or cbpb, giving the same results as carbonylation in the presence of the isolated complexes 7, 8 or 9 (see Table). The highest activity was observed for complex 7 (or the mixture [Rh(CO) 2 Cl] 2 / pbpb, the catalytic turnover number (TON) being 950 after 15 min (1708, 22 bar). The anion cis-[Rh(CO) 2 I 2 ] À was found to be the initial catalytically active species [3]. As the rate-determining step of the catalytic cycle is the oxidative addition of MeI to cis-[Rh(CO) 2 I 2 ] À [4], electron enrichment at the metal center is expected to facilitate this step and to improve the rate of AcOH formation. Consequently, square-planar rhodium(I) complexes with ligands that increase the electron density at the Rh-atom have been developed and studied as catalysts; they give better or at least comparable activities than the original Monsanto catalyst [5 ± 8]. Introduction. ± The carbonylation of MeOH to generate AcOH is one of the most important homogeneously catalyzed industrial processes [1] (Scheme 1). The production of AcOH by the

Trans-chelating diphosphines, the elusive ligands!

“…2.5 times higher than that obtained with the phosphine-free Monsanto system under the same catalytic conditions [97,99,159]. The stability of the systems is evident by the ability to perform several runs without any noticeable loss in activity.…”

“…Exploiting the methodology developed by Trost et al [95,96], Süss-Fink applied his modular approach to generate a family of trans-diphosphine ligands 9-11 by using different spacers and combining amide and ester linkages [97][98][99]. Ligands 9-11 are (as Trost's ligands) 2-diphenylphosphino benzoic acid derivatives formed by condensation with different diamines (9), diols (10) and aminoalcohols (11).…”

“…Many square-planar trans-complexes as IrL(CO)Cl, RhL(CO)Cl, PtLCl 2 , PtLI 2 or PdLCl 2 have been characterized by means of spectroscopic techniques and several X-ray structures confirmed the ligand arrangement (see Table 6) [97][98][99]101,102].…”

“…Presumably, the isolated dinuclear compounds act as resting state for the acetyl complex [Rh(Ph-11)I 2 (COMe)], generated by two of such molecules by loss of one diphosphine [97,99,159].…”

Carbon Monoxide as a Chemical Feedstock: Carbonylation Catalysis

“…In an effort to develop new systems able to combine both stability and activity, Süss-Fink and coworkers reported a family of trans-spanning, but flexible diphosphines [115][116][117]. These ligands form stable complexes due to the chelate effect which resemble the ones obtained with PEt 3 (trans-[RhI(CO)(PEt 3 ) 2 ]) with the two phosphorus atoms in a trans disposition.…”