Search

The cationic platinum complexes [Pt(η²-alkene)X(tmeda)](ClO4) (1:  X = Cl, alkene = propene, 1-butene, 1-hexene; X = NO2, alkene = (E)-2-butene, (Z)-2-butene; tmeda = N,N,N‘,N‘-tetramethylethylenediamine) smoothly react with triethylamine to undergo loss of one allylic proton of the alkene ligand and formation of the η¹-allyl species [Pt(η¹-allyl)X(tmeda)] (2). In the case of 1-butene and 1-hexene, the η¹-allyl species containing a nonterminal olefinic function (X(tmeda)PtCH2CH&dbd;CHR, R = Me, Pr) undergoes release of the X ligand and η¹ → η³ rearrangement of the allyl to form [Pt(η³-allyl)(tmeda)]⁺ (3). The reaction is stereoselective, and the R substituent is always found syn to the central hydrogen of the allyl group; however, in solution, a partial isomerization of the syn into the anti form slowly takes place. In the case of propene and (E)- and (Z)-2-butene the terminal olefinic function of 2, [X(tmeda)PtCHRCH&dbd;CH2], can displace the η²-alkene from a second molecule of 1 to form the η¹:η²-allyl-bridged dinuclear complex [{PtX(tmeda)}2(μ-η¹:η²-CH2−CH&dbd;CHR)]⁺ (4). Further deprotonation of the η¹-bonded methylene of 4 and simultaneous release of the X ligand by one platinum unit leads to formation of the η¹:η³-allyl-bridged platinum dimer [{PtX(tmeda)}(μ-η¹:η³-CHCHCHR){Pt(tmeda)}]⁺ (5). In the case of (E)- and (Z)-2-butene 3 and 5 are formed in comparable yield, while in the case of propene 5 is by far the major product. The structure of 5a (R = H) has been confirmed by single-crystal X-ray diffraction. The two platinum subunits are nearly perpendicular (dihedral angle between the two PtN2 planes of 77.0°). The allyl plane (C(7)C(8)C(9)) forms a dihedral angle of 60.6° with the coordination plane of the η³-bonded platinum (Pt(2)N(3)N(4)). The Pt−C distances are shorter for the η¹-bound platinum (Pt(1)−C(7) = 2.00(1) Å) than for the η³-bound platinum (Pt(2)−C(7) = 2.24, Pt(2)−C(8) = 2.15, and Pt(2)−C(9) = 2.07 Å).

The conversion of a η²-bonded propene (complex 1) into a bridging η¹,η³-allyl moiety (complex 4) is reported. The formation of 4 occurs through the following sequence of reactions:  deprotonation of the methyl group of the coordinated propene in 1 and formation of the η¹-allyl complex 2, displacement of the η²-olefin of a second molecule of 1 by the η¹-allyl moiety of 2 and formation of the η¹,η²-allyl-bridged dinuclear complex 3, and finally further deprotonation of the η¹-bonded methylene group of 3 and simultaneous release of a chloride ion by one platinum unit leading to formation of the η¹,η³-allyl-bridged platinum dimer 4.