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Synthesis and structure of PdII and PtII complexes containing chiral diphosphazane ligands

1998; Elsevier BV; Volume: 554; Issue: 2 Linguagem: Inglês

10.1016/s0022-328x(97)00665-7

1872-8561

Angel Badı́a, Rafael Navarro, Esteban P. Urriolabeitia,

Organometallic Complex Synthesis and Catalysis

Abstract The reaction of the dichloro derivatives cis-[Cl2M(S-peap)] [M=Pd 1, Pt 2; S-peap=S-(Ph2P)2NC(H)(Me)(Ph)] with AgClO4 (1:1 molar ratio) results in the formation of the corresponding cationic dinuclear chlorine-bridge complexes [M(μ-Cl)(S-peap)]2(ClO4)2 (M=Pd 5, Pt 6) which, by further treatment with Tl(acac) give the acac-O,O′ derivatives [M(acac-O,O′)(S-peap)](ClO4) (M=Pd 7, Pt 8). Complex 8 reacts in refluxing acetone with an excess of α-amino acids, such as glycine, l -alanine and d -alanine, to give the α-amino acidato complexes [Pt(Aa)(S-peap)](ClO4) (Aa=gly 9, l -ala 10, d -ala 11). When an equimolecular mixture of l - and d -alanine was allowed to react with 8, an equimolecular mixture of 10 and 11 was obtained, showing that the chiral centre present in the diphosphazane ligand does not promote an stereoselective induction in the coordination of the amino acids. On the other hand, the reaction of cis-[Cl2Pt(S-diphos)] [S-diphos=S-peap 2, S-plap 4; S-plap=S-(Ph2P)2NC(H)(CH2Ph)(CO2Me)] with an excess of N2CH2 results in the formation of the neutral bis(chloromethyl) complexes [Pt(CH2Cl)2(S-diphos)] (S-diphos=S-peap 12, S-plap 13). However, the reaction of 2 or 4 with N2C(H)CO2Et (1:1.5 molar ratio) results in the insertion of the C(H)CO2Et fragment in only one of the two Pt–Cl bonds, giving the corresponding chloro(chloroalkyl) complexes [PtCl(CHClCO2Et)(S-diphos)] (S-diphos=S-peap 14, S-plap 15) as a mixture of the two diastereoisomers (RS/SS). Once again, the chiral centre present in the diphosphazane ligand does not promote an stereoselective insertion reaction. Several concomitant factors could probably account for this lack of stereoselectivity: the remote location of the chiral centre from the reaction site, the free rotation of the C(H)(R)(R′) fragment around the C–N bond and the exo position of the chiral centre relative to the four-membered ring of the coordinated diphosphazane.