CYCLOPENTADIENYLMOLYBDENUM(II) AND CYCLOPENTADIENYLMOLYBDENUM(III) COMPLEXES CONTAINING DIENE AND ALLYL LIGANDS - 2 - COMPARATIVE REACTIVITY OF THE ISOMERIC COMPLEXES CPMO(ETA-C3H5)(ETA-C4H6) WITH EITHER SUPINE OR PRONE ALLYL AND EITHER S-CIS (SUPINE) OR S-TRANS BUTADIENE LIGANDS TOWARD PROTONS

The electron-rich isomeric complexes CpMo(eta(3)-C3H5)(eta(4)-C4H6) (1 a, prone-C3H5; supine-C4H6; 1b, supine-C3H5; supine-C4H6; 1c, supine-C 3H5; s-trans-C4H6) do not react with neutral ligands under mild condit ions. They are, however, easily protonated by a variety of different a cids. Protonation of 1a and 1b involves attack at the terminal positio n of the allyl ligand and elimination of propene. Protonations with ac etic acid show rates in the order 1a > 1b and afford the same product, CpMo(O2CCH3)(eta(4)-C4H6), 2, which can be oxidized to the 17-electro n cation [2](+). HBF4 protonation of 1a in the absence of trapping don or molecules affords [CpMo(eta(4)-supine-C4H6)(mu-F2BF2)](n), 3. The l atter readily reacts with donor molecules to afford [CpMo(eta(4)-supin e-C4H6)L-2][BF4] products (L = MeCN, 4; (BuNC)-N-t, 5; or L-2 = 1,3-bu tadiene, 6), which are also directly and selectively obtained by proto nation of 1a in the presence of the appropriate ligand. Compound 6 has a (supine-C4H6)(s-trans-C4H6) configuration and converts into compoun d 4 when dissolved in MeCN. Protonation of 1c is much slower relative to the isomers 1a and 1b. The observed products depend on the nature o f the solvent. Protonation by HBF4 . Et2O in MeCN affords unstable ine -eta-C3H5)(syn-CH3-prone-eta-C3H4)(NCCH3)][BF4] (7), which rapidly exc hanges the MeCN ligand. Decomposition of the latter involves a regiose lective reductive coupling of the two allyl ligands to generate 3-meth yl-1,5-hexadiene quantitatively. In C6D6, the HBF4 protonation of 1c p roduces small amounts of propene and a violet precipitate which gives a mixture of 4 and 7 upon treatment in MeCN. In the presence of 1,3-bu tadiene, protonation of 1c in THF followed by extraction into acetone affords a mixture of 6 and [CpMo(eta-C3H4-CH2CH2-eta-C3H4)(Me2CO)][BF4 ] (8). Compound 8 converts into [CpMo(eta-C3H4-CH2CH2-eta-C3H4)(L)][BF 4] (L = MeCN, 9; PMe3, 10) when treated with the appropriate L. Proton ation of 1c in MeCN in the presence of butadiene affords 7 which slowl y decomposes, under these conditions, to a mixture of 4 and [CpMo(eta( 4)-s-trans-C4H6)(MeCN)(2)](+), 11. The collective results for the prot onation of 1c indicate that the proton attacks the s-trans diene ligan d in MeCN. The preferred position of attack in nonpolar solvents, on t he other hand, is the allyl. The difference of electronic distribution for isomers 1a-c has been investigated by DFT methods. The calculatio ns indicate that the allyl ligand is a stronger donor in the supine co nfiguration, while the diene ligand is both a weaker donor and a weake r acceptor when it is coordinated in the s-trans mode.