103240-45-5Relevant articles and documents
Influence of steric factors on the coordination mode (η1 or η2) of phosphaalkenes to zerovalent Pt0L2 centers. X-ray structure of bis(triphenylphosphine){(2,6-dimethylphenyl)-9-fluorenylidenephosphine} platinum(0)-toluene
Kraaijkamp, Johanna G.,Van Koten, Gerard,Van Der Knaap, Theo A.,Bickelhaupt, Friedrich,Stam, Casper H.
, p. 2014 - 2020 (2008/10/08)
New phosphaalkene-platinum(0) complexes, Pt(PCy3)2(XylP=CPh2) (4) (Xyl = 2,6-dimethylphenyl) and Pt(PPh3)2(C12H8C=PXyl) (6), have been prepared by reacting the respective phosphaalkenes with the corresponding Pt(PCy3)2 and Pt(PPh3)2 precursors. 1H and 31P NMR spectra showed that the phosphaalkene ligand is η1-P-coordinated in 4 (δ(31Pfree ligand) - δ(31Pcomplex = Δδ(31P) = 12.7 ppm; 1J(31P,195Pt) = 4475 Hz) but η2-P=C-bonded in 6 (Δδ(31P) = 277 ppm; 1J(31P,195Pt) = 319 Hz). 31P NMR spectra of 4 and 6 at different temperatures (range -60 to +40°C) revealed that the η1-P ? η2-P=C equilibrium established for the phosphaalkene bonding mode in Pt(PPh3)2(MesP=CPh2) (1) (Mes = mesityl) does not exist in the case of 4 and 6; the latter complexes are examples of the extreme situations in this equilibrium, i.e., either η1-P (4) or η2-P=C (6) coordination. The structure of 6 has also been studied by X-ray diffraction (see Figure 1): monoclinic, space group P21/n, Z = 4 with cell constants a = 13.228 (2) A?, b = 17.086 (1) A?, c = 23.353 (2) A?, β = 102.92 (1)°, V = 5144.5 (7) A?3, dcalcd = 1.45 g cm-3, and μ(Cu Kα) = 63.5 cm-1. The structure was refined to a final R of 0.031 for 6113 observed reflections above the 2.5σ(I) level. Characteristic structural features of 6 are the η2-P=C bonding mode of the phosphaalkene with an elongated P=C bond of 1.814 (4) A?, as well as the small value of the P(1)-Pt-P(2) angle of 98.92 (5)°. The Pt-P distances are 2.302 (1) and 2.318 (1) A? with the PPh3 ligands and 2.356 (1) A? with the phosphaalkene. On the basis of the nature of the frontier orbitals involved when combining the ML2 and the P=C fragments in 4 and 6, respectively, the specific coordination mode encountered in these complexes is discussed. It appears that the actual coordination mode is connected to the interligand bond angles that can be realized in a L-M-L fragment. For steric reasons the PCy3-Pt-PCy3 angle in 4 will be large (?170°) which makes the σ-donation component in the PtL2-P=C bonding more important and hence stabilizes the η1-P bonding mode. In contrast, the PPh3-Pt-PPh3 angle can be as small as about 99°; in this situation extensive electron transfer from the Pt dxz into the π* P=C can take place which stabilizes the η2-P=C bonding as found in 6.
