Theoretical and experimental evidence for a thermally induced .eta.4-diene .dblarw. .eta.3-allyl ligand interconversion in (s-trans-.eta.4-conjugated diene)zirconocenes

Abstract

412.046652, M2L3+ observed by LRMS11 only-mje 566 (<1% intensity)].Synthesis of Bis(i|6-diphenylmethane)chromium(0) (VI).Chromium metal (0.496 g, 14.0 mmol) was evaporated into diphenylmethane (15.1 g, 89.9 mmol) by using a time-averaged 350 W at an average pressure of 9.3 X 10-6 torr during a period of 1.0 h. After transfer and workup, compound VI [mp 132-133 °C (lit.12 132-135 °C)] was collected by sublimation [140 °C (10™3 torr)].

Zirconium−Butadiene Bonded over a Planar Set of Oxygens Derived from Calix[4]arene and Its Behavior as a Source of Zirconium(II)

“…In the solid state, as shown by the X-ray analysis performed on single crystals of 3 and 4 , the bonding mode of the butadiene ligands in both complexes can be described as π 2 ,η 4 . A variety of bonding modes of butadiene to zirconium ranging from π,η 4 to σ 2 ,π,η 4 can be found in complexes having subunits other than Cp 2 Zr. ,3a, The structural and spectroscopic characteristics of the Zr−diene fragment in 3 and 4 are more closely related to the [(tmtaa)Zr] 3a derivatives (tmtaa = dibenzotetramethyltetraaza[14]annulene dianion) than the corresponding [Cp 2 Zr] ,,, species. In particular, we can say that the s - cis form is the only butadiene conformer detected in the solid state; heating of 3 or 4 did not result in any cis − trans isomerization. , …”

“…In particular, we can say that the s-cis form is the only butadiene conformer detected in the solid state; heating of 3 or 4 did not result in any cis-trans isomerization. 24,25 The labeling scheme adopted for calixarene is given in Scheme 2. Selected bond distances and angles are quoted in Tables 2, 3 8, and 9, respectively.…”

Synthesis of conjugated diene complexes of ruthenium(0) derived from Ru(η6-naphthalene)(η4-1,5-COD): Z to E isomerisation of coordinated 1,3-pentadiene

“…EXSY measurements [34] ruled out the "envelope shift" mechanism that appears to be widely accepted to account for the E/Z isomerisation in other 1,3-diene complexes [30e32] but were consistent with an initial h 4À -diene to h 3 -allylic transformation induced by metal-carbon bond rupture (either heterolytic or homolytic) in the ene-diyl form of the h 4 -diene; rotation about the CeC bond in the resulting carbanion or radical would then lead to Z/ E isomerisation of the diene, as illustrated in Scheme 3. A mechanism of this type was first proposed by Erker, Berke et al [26] to account for the stepwise E/Z isomerisation of the 1,4-substituents in the 1,3-diene unit of [ZrCp 2 (h 4 -transoid-1,4-disubstituted-1,3-butadiene)], and we suggest that it may operate also in [Ru(h 4 -cisoid-1,3-pentadiene)(h 4 -1,5-COD)(L)] (L ¼ MeCN, PR 3 ). For the reversible E/Z isomerisation in [Ru(h 4 -cisoid-1,3-pentadiene)(h 2 -transoid-1,3-pentadiene)(h 2 -1,5-COD)], we could not completely rule out the possibility of a 1,1-hydrogen shift mechanism (Scheme 2).…”

“…However, since the 1 H NMR spectrum of 8a at room temperature showed only one A 2 B 2 X pattern for the ortho-, meta-and para-phenyl protons, the phenyl groups must be rotating in solution. Erker et al have reported formation of the h 4 -transoid-1,4-diphenylbuta-1,3-diene complex of Zr(II) [26]. In both 2a and 8a the coordinated acetonitrile ligands are almost linear.…”

Synthesis of (s-trans-η4-Butadiene)tantalocene Cation and Its CC-Coupling Reactions with Ketones, Nitriles, and Alkynes

“…Bonding of all four carbon atoms of the C 4 H 6 ligand to a single transition metal center requires some distortion of the butadiene framework from planarity, but interaction of the conjugated diene π-system with the group 4 bent metallocene valence orbitals has provided a more than ample compensation for the strain introduced by constructing the Cp 2 M(1,3-diene) framework. Theoretical work has shown that the very special bonding features of the bent metallocenes result in the ability to form stable s - trans -η 4 -conjugated diene complexes so easily at their frameworks . All three valence orbitals of, for example, the Cp 2 Zr unit are oriented in the major plane bisecting the Cp−Zr−Cp angle.…”