ABSTRACT
The reaction of [Ta(=CHtBu)(CH2tBu)3] or [Cp*Ta(CH3)4] with a silica partially dehydroxylated at 700 degrees C gives the corresponding monosiloxy surface complexes [([triple bond]SiO)Ta(=CHtBu)(CH2tBu)2] and [([triple bond]SiO)Ta(CH3)3Cp*] by eliminating a sigma-bonded ligand as the corresponding alkane (H-CH2tBu or H-CH3). EXAFS data show that an adjacent siloxane bridge of the surface plays the role of an extra surface ligand, which most likely stabilizes these complexes as in [([triple bond]SiO)Ta(=CHtBu)(CH2tBu)2([triple bond]SiOSi[triple bond])] (1a') and [([triple bond]SiO)Ta(CH3)3Cp*([triple bond]SiOSi[triple bond])] (2a'). In the case of [(SiO)Ta(=CHtBu)(CH2tBu)2([triple bond]SiOSi[triple bond])], the structure is further stabilized by an additional interaction: a C-H agostic bond as evidenced by the small J coupling constant for the carbenic C-H (JC-H = 80 Hz), which was measured by J-resolved 2D solid-state NMR spectroscopy. The product selectivity in propane metathesis in the presence of [([triple bond]SiO)Ta(=CHtBu)(CH2tBu)2([triple bond]SiOSi[triple bond])] (1a') as a catalyst precursor and the inactivity of the surface complex [([triple bond]SiO)Ta(CH3)3Cp*([triple bond]SiOSi[triple bond])] (2a') show that the active site is required to be highly electrophilic and probably involves a metallacyclobutane intermediate.
ABSTRACT
The reaction of CpMoH(CO)(2)L with AuPPh(3)(+)BF(4)(-) in THF at -40 degrees C proceeds directly to the MoAu(2) cluster compounds [CpMo(CO)(2)L(AuPPh(3))(2)](+)BF(4)(-) (L = PMe(3) (1), PPh(3) (2)) with release of protons. A 1:1 reaction leaves 50% of the starting hydride unreacted. At lower temperature, however, the formation of a [CpMo(CO)(2)(PMe(3))(&mgr;-H)(AuPPh(3))](+) intermediate is observed. This compound evolves to the cation of 1 and CpMoH(CO)(2)(PMe(3)) upon warming and is deprotonated by 2,6-lutidine to afford CpMo(CO)(2)(PMe(3))(AuPPh(3)). The X-ray structure of 1 can be described as a four-legged piano stool with the PMe(3) and the "eta(2)-(AuPPh(3))(2)" ligands occupying relative trans positions. [Cp(CO)(2)(PMe(3))Mo(AuPPh(3))(2)](+)[BF(4)](-) (M(r) = 1298.41): monoclinic, space group P2(1)/n, a = 18.1457(13) Å, b = 9.7811(7) Å, c = 26.096(2) Å, beta = 105.086(5) degrees, V = 4472.0(5) Å(3), Z = 4. The reaction of CpMoH(CO)(2)(PMe(3)) with 3 equiv of AuPPh(3)(+) affords a MoAu(3) cluster, [CpMo(CO)(2)(PMe(3))(AuPPh(3))(3)](2+) (3), in good yields under kinetically controlled conditions. Under thermodynamically controlled conditions, 3 dissociates extensively into 1 and free AuPPh(3)(+). It is proposed that the hydride ligand helps build higher nuclearity Mo-Au clusters. The difference in reaction pathways for the interaction of AuPPh(3)(+) with CpMoH(CO)(2)L when L = PR(3) or CO and for the interaction of CpMoH(CO)(2)(PMe(3)) with E(+) when E = H, Ph(3)C or AuPPh(3) is discussed. The lower acidity and greater aurophilicity of the [CpMo(CO)(2)L(&mgr;-H)(AuPPh(3))](+) intermediate when L = PMe(3) favor attack by AuPPh(3)(+) before deprotonation.
