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Issue 10, 2016
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Ligand conformations and spin states in open metallocenes of the first row transition metals having U-shaped 2,4-dimethylpentadienyl ligands

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Abstract

Open metallocenes containing a metal sandwiched between two U-shaped pentadienyl or substituted pentadienyl ligands have been synthesized by Ernst and co-workers using reactions of pentadienyl or substituted pentadienyl anion reagents with metal halides. The complete series of such open metallocenes (2,4-Me2C5H5)2M of the first row transition metals (M = Ti to Ni) has now been examined using density functional theory. The experimentally known lowest energy structures of the early transition metals are open metallocene structures with two pentahapto U-shaped η5-2,4-Me2C5H5 ligands. Such structures can exist as syn-eclipsed, gauche-eclipsed, and/or anti-eclipsed conformations of similar energies. The theoretical predictions of the lowest energy conformations of the open metallocenes are in excellent agreement with available experimental data. Singlet and triplet spin state structures are preferred energetically for the Ti and Cr derivatives, respectively. For (η5-2,4-Me2C5H5)2V, doublet and quartet spin state structures are spaced in energy by 7.6 kcal mol−1. The (2,4-Me2C5H5)2Mn energy surface is complicated with doublet, quartet, and sextet spin state structures closely spaced in energy. In the sextet spin state of (2,4-Me2C5H5)2Mn only the end carbon atoms of the 2,4-Me2C5H5 ligand are within bonding distance of the manganese atom. This corresponds to tetrahedral C4Mn coordination similar to that found experimentally in the sextet ground state t-butyl derivative (2,4-tBu2C5H5)2Mn as well as the related Mn(CN)42−. The low energy structures for the “open ferrocene” (η5-2,4-Me2C5H5)2Fe are singlet spin state structures with two U-shaped pentahapto η5-2,4-Me2C5H5 ligands but otherwise analogous to ferrocene. The low energy (2,4-Me2C5H5)2M structures of the late transition metals Co and Ni have at least one trihapto η3-2,4-Me2C5H5 ligand. The lowest energy (2,4-Me2C5H5)2Ni structure is a singlet spin state structure with two trihapto η3-2,4-Me2C5H5 ligands and a 16-electron nickel environment analogous to the well-known bisallylnickel, (η3-C3H5)2Ni.

Graphical abstract: Ligand conformations and spin states in open metallocenes of the first row transition metals having U-shaped 2,4-dimethylpentadienyl ligands

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Publication details

The article was received on 03 Feb 2016, accepted on 18 Aug 2016 and first published on 19 Aug 2016


Article type: Paper
DOI: 10.1039/C6NJ00379F
Citation: New J. Chem., 2016,40, 8511-8521
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    Ligand conformations and spin states in open metallocenes of the first row transition metals having U-shaped 2,4-dimethylpentadienyl ligands

    Q. Fan, H. Feng, W. Sun, H. Li, Y. Xie, R. B. King and H. F. Schaefer, New J. Chem., 2016, 40, 8511
    DOI: 10.1039/C6NJ00379F

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