Abstract
The palladium η3-allyl complex Pd(η3-C3H5)2 reacts with bidentate phosphanes iPr2PC2H4PiPr2 and tBu2PC2H4PtBu2 below -30-degrees-C to yield the light yellow, microcrystalline palladium(II) η1-allyl compounds (R2PC2H4PR2)Pd(η1-C3H5)2 (R = iPr (1), tBu (2)), which are stable to about -30-degrees-C. Above -30-degrees-C, the allyl substituents of 1 and 2 couple with reduction of palladium to form a mixture of (R2PC2H4PR2)Pd0 1,5-hexadiene complexes. When this reaction is carried out in 1,5-hexadiene, the complexes (R2PC2H4PR2)Pd(η2-C6H10) (R = iPr (3), tBu (4)) are obtained in pure form. According to IR and NMR spectral data, the palladium atoms in 3 and 4 are coordinated at low temperature by the chelating diphosphane and (statically) by one of the two diene double bonds in a trigonal-planar geometry. At higher temperatures, a rapid exchange of the coordinated and uncoordinated double bonds occurs, passing through an intermediate with C2 symmetry. When suspensions of 1 in pentane and 2 in THF are warmed to 20-degrees-C, dinuclear diastereomers rac-/meso-{(R2PC2H4PR2)Pd}2(µ-η2:η2-C6H10) (R = iPr (5a,b), tBu (6a,b)) are obtained, which upon treatment with 1,5-hexadiene furnish mononuclear derivatives 3 and 4. Similarly, when 1 is reacted with 1,5-cyclooctadiene at 20-degrees-C, the mono- and dinuclear interconvertible complexes (iPr2PC2H4PiPr2)Pd(η2-C8H12) (7) and {(iPr2PC2H4PiPr2)Pd}2(µ-η2:η2-C8H12) (8) are produced. From the reaction of 1 and 2 with ethene, stable complexes (R2PC2H4PR2)Pd(C2H4) (R = iPr (9), colorless; R = tBu (10), tan) result. The colorless mononuclear palladium(0) ethyne complexes (R2PC2H4PR2)Pd(C2H2) (R = iPr (11), tBu (12)) may be prepared (a) by a displacement reaction of one of the isolated alkene complexes with ethyne, (b) by a reductive elimination and concomitant displacement reaction of the η1-allyl complexes 1 and 2 with ethyne, or (c) in a one-pot synthesis from either Pd(η3-C3H5)2 or Pd(η3-2-MeC3H4)2 with iPr2PC2H4PiPr2 or tBu2PC2H4PtBu2, respectively, and ethyne in excess. When the mononuclear ethyne complexes 11 and 12 are combined with an equimolar amount of the corresponding η1-allyl complex 1 or 2, or the alkene complexes (especially the mononuclear 1,5-hexadiene (3, 4) or ethene (9, 10) derivatives), yellow dinuclear palladium(0) complexes {(R2PC2H4PR2)Pd}2(µ-C2H2) (R = iPr (13), tBu (14)) are produced, in which the ethyne ligand bridges two palladium atoms. All the (diphosphane)palladium(0) alkene (3-10) and ethyne complexes (11-14) exhibit a trigonal-planar coordination geometry about the palladium atom. Most of the compounds have been isolated in high yield.
