Cycloheptatrienyl−Cyclopentadienyl−Zirconium Sandwich Complexes:  Structure and Bonding

Abstract

The reactions of the 16-electron cycloheptatrienyl−cyclopentadienyl−zirconium sandwich complex [(η7-C7H7)Zr(η5-C5H5)] (1) with tert-butyl isocyanide (tBuNC) and 2,6-dimethylphenyl isocyanide (o-XyNC) have been studied. The isocyanide complexes [(η7-C7H7)(η5-C5H5)Zr(CNR)] (R = tBu, 2a; R = o-Xy, 2b) can be isolated in quantitative yield as stable orange (2a) or red (2b) crystalline solids. The CN stretching vibrations are observed at 2156 (2a) and 2134 cm-1 (2b) in KBr, indicating a relatively weak zirconium−isocyanide interaction. Solution NMR studies consequently reveal that the isocyanides quickly exchange on the NMR time scale at room temperature. The equilibrium reaction of 1 with tBuNC has been examined in further detail by dynamic NMR spectroscopy, allowing an enthalpy for the formation of 2a of ΔH° = −39.6 ± 6.6 kJ mol-1 to be established. DFT calculations reveal that the weakness of the zirconium−isocyanide bonds in 2a and 2b can be attributed to the strong and appreciably covalent zirconium−cycloheptatrienyl interaction, leading to highly stabilized frontier orbitals and consequently to a diminishing π-electron release capability and a small propensity to efficiently interact with σ-donor/π-acceptor ligands. The X-ray crystal structures of 1 and 2a are also reported.

Cycloheptatrienyl Zirconium Sandwich Complexes with Lewis Basic Phospholyl Ligands (Phosphatrozircenes): Synthesis, Structure, Bonding and Coordination Chemistry

“…[40] Furthermore, we have used the symmetric complex 2 as a representative example to identify pertinent frontier molecular orbitals; Figure 3 shows the contour plots of the LUMO and the seven HOMOs based on the M05-2X calculation. Similarly to the bonding in trozircene [19] and halfopen trozircenes,…”

“…Our group has especially focused on Group 4 Cht-Cp sandwich complexes, [18] in which-based on experimental and theoretical results-a formal + 4 oxidation state can be assigned to the metal atom, implying that the C 7 H 7 ligand is best considered as a trianion. [19,20] Consequently, the metal atom in these 16-electron complexes is Lewis acidic, and thus able to coordinate additional two- (4). The presence of Lewis basic phosphorus atoms and Lewis acidic zirconium atoms allows ambiphilic behaviour to be observed, and X-ray diffraction analysis reveals dimeric arrangements for 2 and 3 with long intermolecular ZrÀP bonds, whereas 4 remains monomeric in the solid state.…”

“…[19,21] This behaviour can be put to good use to observe interesting secondary interactions in phosphane-functionalised complexes of the type [(h 7 -C 7 H 7 )M(h 5 -C 5 H 4 PR 2 )] (M = Zr, Hf) (Scheme 1). For instance, a complex with a weak ZrÀPd interaction was obtained, which was one of only a few earlylate heterobimetallic complexes consisting of neutral complex fragments.…”

Adduct formation of [(η7-C7H7)Hf(η5-C5H5)] with isocyanides, phosphines and N-heterocyclic carbenes: An experimental and theoretical study

“…For M = Ti, we were able to demonstrate that the introduction of a silylene bridge makes the coordinating rings significantly non-parallel and opens a potential vacancy at the titanium centre; thus adducts with carbon monoxide and isocyanides have been observed [5a], whereas the parent 16-electron complex [(g 7 -C 7 H 7 )Ti(g 5 -C 5 H 5 )] (troticene) does not form adducts with simple two-electron ligands. In contrast, the corresponding zirconium complex [(g 7 -C 7 H 7 )Zr(g 5 -C 5 H 5 )] (trozircene, 1a) has been shown to react with isocyanides [8], phosphines and N-heterocyclic carbenes [9] without the need to resort to the introduction of a bridging unit (Scheme 1).…”

Bis(cycloheptatrienyl) Derivatives of the First‐Row Transition Metals: Variable Hapticity of the Cycloheptatrienyl Ring

“…However, early work by Wilkinson and co-workers [17] suggested that reactions of tropylium salts with transitionmetal derivatives were rarely suitable for the preparation of η 7 -C 7 H 7 metal complexes. More recent studies on η 7 -C 7 H 7 -metal complexes suggest preferred formulations of such complexes as derivatives of the 10 π-electron trianion [18][19][20][21] C 7 H 7 3-, which has been generated in a metal-free form by the treatment of 1,4-cycloheptadiene with nBuLi. [22,23] Thus the metal formal oxidation states in the known mixed sandwich complexes (η 5 -C 5 H 5 )M(η 7 -C 7 H 7 ) (M = Ti, V, Cr) may be considered to be +4 in contrast to the formal zero metal oxidation states in the isoelectronic and isomeric dibenzenemetal complexes (η 6 -C 6 H 6 ) 2 M (M = Ti, V, Cr).…”