Sequential deprotonation of olefins in the coordination sphere of platinum(II): Occurrence and synthetic aspects

The cationic platinum complexes [pt(eta(2) -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 triethylamineto undergo loss of one allylic proton of the alkene ligand and formation of the eta(1)-allyl species [Pt(eta(1)-allyl)X-(tmeda)] (2). In the case of 1-butene and 1-hexene,. the eta(1)-allyl species containing a nonterminal olefinic function (X(tmeda)PtCH2CH=CHR, R = Me, Pr)undergoes release of the X ligand and eta(1) --> eta(3) rearrangement of the allyl to form [Pt(eta(3)-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=CH2], can displace the eta(2)-alkene from a second molecule of 1 to form the eta(1):eta(2)-allyl-bridged dinuclear complex [{PtX(tmeda)}(2)(mu-eta(1):eta(2)-CH2-CHdropCHR)](+) (4). Further deprotonation of the eta(1)-bonded methylene of 4 and simultaneous release of the X ligand by one platinum unit leads to formation of the eta(1):eta(3)-allyl-bridged platinum dimer [{PtX(tmeda)}(mu-eta(1):eta(3)-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 77.0degrees). The allyl plane (C(7)C(8)C(9)) forms a dihedral angle of 60.6degrees with the coordination plane of the eta(3)-bonded platinum (Pt(2)N(3)N(4)). The Pt-C distances are shorter for the yl-bound platinum (Pt(1)-C(7) = 2.00(1) Angstrom) than for the eta(3)-bound platinum (Pt(2)-C(7) = 2.24, Pt(2)-C(8) = 2.15, and Pt(2)-C(9) = 2.07 Angstrom).