Coordination chemistry and X-ray studies with novel sterically constrained diphosphonite ligands

Abstract

The coordination of two novel, sterically constrained diphosphonite ligands towards different palladium, platinum and rhodium precursors has been studied. Complexes of this hitherto unexplored class of diphosphonites, based on rigid xanthene backbones, have been characterized by X-ray crystallography as well as by NMR and IR spectroscopy. A short and concise overview is given on the scarce literature on phosphonite related chemistry. Structural differences in the complexes obtained due to steric influences of the ligands are discussed. Ligand 1, bearing 2-tert-butylphenolate groups, led to orthometallated complexes cis-[MCl{1-κ 3 C,P,P}] (M = Pd, Pt) and to trans- [RhCl(CO) (1)]. These complexes were compared with those formed with ligand 2, bearing 2,2Ј-dioxo-3,3Ј-di-tert-butyl-5,5Ј-dimethoxy-1,1Ј-biphenyl groups, preventing orthometallation. The complexes trans-{PdCl 2 (2)], cis-[PtCl 2 (2)] and trans-[RhCl(CO)(2)] were structurally characterized. Valuable data were collected on chemical shifts, Pt-P coupling constants, as well as CO stretch frequencies of Rh-CO complexes as important tools for structural characterization of such complexes. IntroductionProgress and development of homogeneous catalysis is very much related and dependent on the synthesis and availability of ligands. In recent years a number of powerful new ligand classes have emerged, most of which are chelating phosphorusbased compounds 1 such as phosphines 2 or phosphites.3 Phosphonites however have been widely neglected and only very recently a few examples were reported. Reetz et al. used several backbones, amongst others ferrocene, to prepare chelating diphosphonites and used them in asymmetric Rh-catalyzed hydrogenations of the typical benchmark substrates 4 as well as in conjugate addition of aryl boronic acids.5 ZanottiGerosa et al. also described diphosphonites but they used the para-cyclophane backbone and performed hydrogenation reactions.6 Pringle andco-workers used binaphthol-derived mono-and bidentate phosphonites for conjugate additions of diethyl zinc to enones. 7 The same monophosphonites were used by Claver and Pringle in asymmetric hydrogenations.8 Scharf and co-workers used the monodentate phosphonites based on TADDOL as the chiral auxiliary for the rhodum-catalyzed hydrosilylation of ketones.9 Börner and co-workers have published on the use of both monodentate and bidentate phosphonite ligands in the hydroformylation of alkenes, with varying results.10 Finally the Reetz group, in line with their earlier work, prepared phosphite-phosphonite bidentate ligands but no applications in catalysis were reported so far. 11During recent years xanthene-based chelating ligands have been introduced and extensively used in transition metal catalyzed C-C bond forming reactions.12 These backbones provide a unique combination of rigidity and P-P distance in such a way, that specific, large bite-angle coordination modes are supported. Additionally interaction of the central oxygen atom in the backbone with a metal atom coordinated is possible...

Versatile Phosphite Ligands Based on Silsesquioxane Backbones

“…We were therefore interested to study the coordination with a third metal species, the rhodium dimer [Rh(m-Cl)(CO) 2 ] 2 . As we [16,34,35] and others [36] have previously reported, the corresponding RhCl(CO)(P) 2 -complexes normally exhibit trans-coordination of two phosphorus moieties, although some cis complexes have also been described.…”

Sterically Demanding Diphosphonite Ligands –Synthesis and Application in Nickel‐Catalyzed Isomerization of 2‐Methyl‐3‐Butenenitrile

“…This coordination mode has been observed before for Pd and Rh complexes. [21] The observed bite angle P1 ± Ni À P2, is 151.81(3)8, while the angle Br1 ± Ni À Br2 is 163.073(18)8. The xanthene backbone is bent, as indicated by the interplanar angle of 27.65(12)8 between the two aromatic rings.…”

Coordination Chemistry and Asymmetric Catalysis with a Chiral Diphosphonite.

“…Illustration of the geometry around the square-planar palladium atom in complex 2, showing the dihedral angle between the metal plane and the ligand plane metallated Pd-diphosphonite species described previously by our group. [16] The structure of complex 2 also bears close resemblance to the molecular structures for the related platinum complex of ligand 1, which will be communicated elsewhere, [23] as well as for cis-[PtCl 2 (Xantphos)]. [24] This indicates that such dispositioning of the metal atom is highly favored in the crystal packing regardless of the metal atom involved.…”

“…[19] Compared with crystal structures of Pd complexes known for xanthene diphosphane ligands, the bite angle of 100°for ligand 1 is unusually small. For instance, we previously reported on trans-coordinated diphosphonites [16] and both van Leeuwen et al [20] and Buchwald et al [21] reported on trans-Pd(Xantphos) complexes. Only in the case of Pd 0 complexes or with cationic palladium species has cis-coordination of Xantphos been ob- (1)]; displacement ellipsoids are drawn at the 50% probability level; all hydrogen atoms and solvent molecules are omitted for clarity; PdϪP 1 2.2449(9); PdϪP 2 2.2532(9); PdϪCl 1 2.3412(10); PdϪCl 2 2.3442 (9); P 1 ϪO served by X-ray crystallography.…”

“…[14] We recently published on the synthesis and use of sterically constrained achiral diphosphonites L 4 in the rhodium-catalyzed hydroformylation of alkenes and showed that these systems are wellsuited for the conversion of 2-butene into n-pentanal. [15] Together with our studies on the coordination chemistry of these xanthene-derived diphosphonites towards various transition metals, [16] this prompted us to explore the potential of chiral diphosphonite 1 in various asymmetric catalytic reactions. In this paper we report on the improved synthesis of compound 1, its coordination with palladium and …”