Rehan Basta

Bonding in ‘closed,’ open, and half-open ferrocenes: new insight from structural and Mössbauer spectroscopic studies

Abstract A 17 electron edge-bridged open metallocene, bis(6,6-dimethylcyclohexadienyl)iron cation, which had previously been observed electrochemically, has been isolated and characterized. Mossbauer spectroscopic data have been obtained for this compound, and for various neutral 18 electron open, half-open, and ‘closed’ ferrocenes, and are in accord with the previous indications that much greater metal–ligand orbital mixing occurs for pentadienyl, as opposed to cyclopentadienyl, ligands. Pertinent structural data have also been obtained for Fe(C5H5)(2,4-C7H11) and Fe(c-C7H9)2 (C7H11=dimethylpentadienyl; c-C7H9=cycloheptadienyl) in order to aid in comparisons between metal–pentadienyl and metal–cyclopentadienyl bonding.

Synthesis and structure of the edge-bridged open zirconocene, Zr(6,6-dmch)2(PMe3)2 (dmch=dimethylcyclohexadienyl), and its imine coupling product

Abstract The reaction of ZrCl4 with four equivalents of the 6,6-dimethylcyclohexadienyl anion (6,6-dmch−) in the presence of PMe3 leads to the 18 electron Zr(6,6-dmch)2(PMe3)2. This complex was found to undergo a coupling reaction with two equivalents of PhCHNPh, such that the couplings involved the two termini of the same dienyl ligand, yielding a formal Zr(η5-dienyl)(η3-allyl)(π-amide)2 complex. Both metal complexes have been structurally characterized.

Crystal structure of bis(η5-6,6-dimethylcyclohexadienyl)chromium, Cr(C8H11)2

C16H22Cr, orthorhombic, P212121 (no. 19), a = 7.6112(2) Å, b = 9.2521(4) Å, c = 20.2174(9) Å, V = 1423.7 Å, Z = 4, Rgt(F) = 0.046, wRref(F) = 0.085, T = 200 K. Source of material The compoundwas prepared as described in the literature [1], and single crystals were obtained by slow sublimation at 50 °C under vacuum. Discussion Cr(6,6-dmch)2 (dmch = dimethylcyclohexadienyl) adopts a conformation consistent with expectations [2].The conformation angle, defined as the relative twist between the two dienyl ligands away from a syn-eclipsed orientation, is found to be 83.8°, as compared to very similar values of 82.2° and 84.0° for its 2,4dimethylpentadienyl [3] and cyclooctadienyl analogues [4]. The value is intermediate between those found for Fe(6,6-dmch)2 (47.5°) andV(6,6-dmch)2 (96.2°) [5,6]. The averageM—C bond distances (M = transition metal) for these three M(6,6-dmch)2 complexes decrease from V to Fe, being 2.194 Å, 2.166 Å, and 2.068 Å. These values may be compared with those for the respective metallocenes, at 2.280(5)Å, 2.169(4)Å, and 2.064(3)Å [7]. Consistent with the intermediate size of chromium, the average C1—C5 separation for Cr(6,6-dmch)2 is intermediate between those of its vanadium and iron analogues, at 2.380 Å vs. 2.437 Å and 2.305 Å, respectively. Z. Kristallogr. NCS 222 (2007) 335-336 / DOI 10.1524/ncrs.2007.0141 335

Crystal structure of bis(6,6-dimethylcyclohexadienyl)vanadium, V(C8H11)2

Abstract C16H22V, monoclinic, P121/a1 (no. 14), a = 12.1600(3) Å, b = 9.0848(3) Å, c = 13.2123(4) Å, β = 104.822(2)°, V = 1411.0 Å3, Z = 4, Rgt(F) = 0.046, wRref(F2) = 0.110, T = 200 K.