78003-82-4Relevant articles and documents
Five-coordinate [PtIICl2(η2-olefin)L2] (L2 = σ,σN,N′-bonded α-diimine or N,N′-disubstituted 1,2-diaminoethane) complexes. Solution conformation, η2-olefin rotational barriers (NMR), and the X-ray molecular structure of [PtCl2(η2-styrene) (t-Bu-N=CHCH=N-t-Bu)]
Van Der Poel, Henk,Van Koten, Gerard,Kokkes, Maarten,Stam, Casper H.
, p. 2941 - 2950 (2008/10/08)
Five-coordinate platinum-olefin complexes [PtX2(η2-olefin)L2], in which L2 is the bidentate ligand RN=CHCR′=NR (α-diimine) or RN(H)CH2CH2N(H)R (N,N′-disubstituted 1,2-diaminoethane), were synthesized in almost quantitative yield by reaction of K[PtCl3(η2-olefin)] with the L2 ligand. The solid-state structure of [PtCl2(η2-styrene)(t-BuNCHCHN-t-Bu)], a representative of this class of compounds, was established by X-ray diffraction analysis. In the trigonal-bipyramidal structure, the diimine ligand and the olefin occupy the equatorial plane with the Cl atoms in the axial sites. The phenyl ring of the η2-bonded styrene is bent back from C=C skeleton (C-C = 1.53 (5) A?) by 27°. Extensive 1H and 13C NMR measurements confirmed that the five-coordinate structure found in the solid (IR, Raman, and 35,37Cl NQR spectroscopic data) is retained in solution. Study of the solution dynamic behavior of the [PtX2(η2-olefin)L2] compounds revealed that (i) the Pt-N interaction of the σ,σN,N′-bonded L2 ligand is inert on the NMR (15N and 195Pt) time scale, (ii) no platinum-olefin bond dissociation occurs in the range -60 to +34°C (concluded from 13C and 1H NMR data for compounds containing both a prochiral R group (e.g. CMe2Et) in L2 and a prochiral olefin (e.g., styrene)), (iii) the ligand L2 does not take part in stereoisomerization processes, and (iv) pure rotation of the olefin around the platinum-olefin bond axis can fully account for the observed temperature dependence of the NMR spectra. The barrier to olefin rotation in these five-coordinate platinum compounds studied ranges from 13.2 (styrene) to 15.9 (methylacrylate) kcal/mol. Our data suggest (i) in [PtCl2(η2-olefin)L2] the barrier to rotation is determined by steric factors, i.e., by the axial halide ligands which are cis to the olefin (cf. square-planar trans-[PtCl2(η2-olefin)L]) and (ii) the equatorial diimine ligand by virtue of its flat conformation represents a ligand with a minimal steric influence on the axial ligands. As a result of rapid N-H bond dissociation/association processes (established by H/D exchange experiments), [PtCl2(η2-styrene)(EtMe2C-diam)] does not exist in solution as a mixture of the expected diastereomers.
