Synthesis and Charactorization of Mixed Ring Sandwich Complexes

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Synthesis and Charactorization of Mixed Ring
Sandwich Complexes
By Stephen Bell, Gina Roberts, Dylan Houghton,
Gordon Carlson, and R. Chris Schnabel
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Transition Metals have s, p and d orbitals
available for bonding
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Coordination Complexes consist of a Lewis Acid
(the transition metal) and a Lewis Base (called
the Ligand)
Lewis Acid Electron Pair Acceptor Lewis Base
Electron Pair Donor
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Aromatic Rings consists of a ring of carbon atoms
that are planar and have 4N 2? electrons
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The normal bonding mode for Cp is ?5
(pentahapto), for which several different
resonance structures can be drawn for the bonding
of an ?5-Cp ligand to a transition metal complex.
The one on the right makes it easy to remember
that a Cp ligand donates 6 electrons to a
transition metal complex as it looks like one
alkyl ligand two alkene ligands
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The cyclopentadienyl (Cp) ligand is a monoanionic
ligand with the formula C5H5. The first
characterized example of a cyclopentadienyl
complex was ferrocene, Cp2Fe, which has an iron
atom "sandwiched" between two planar Cp rings as
shown below. For this reason, bis(cyclopentadienyl
) complexes are sometimes called "sandwich
compounds" or metallocenes.
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The discovery of ferrocene in 1951 and its
structural elucidation by two separate research
groups the following year marked the birth of
contemporary organometallic chemistry. This
revolutionary advance in organometallic chemistry
was recognized with a Nobel Prize in Chemistry in
1973.
Press Release The 1973 Nobel Prize in
Chemistry 23 October 1973 The Royal Swedish
Academy of Sciences has decided to award the 1973
Nobel Prize in Chemistry half each to Professor
Ernst Otto Fischer, Technical University of
Munich, Munich, Federal Republic of Germany
and Professor Geoffrey Wilkinson, Imperial
College, London, Great Britain for their
pioneering work performed independently on the
chemistry of the organometallic, so called
sandwich compounds.
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In the 1H NMR spectrum, Cp rings appear as
singlets in the range of 4.0 to 5.5 ppm. The
barrier to rotation around the M-ring centroid
vector is quite low (approximately 1 kcal/mol),
so even though the molecule may possess low
symmetry, all five ring protons are equivalent on
the NMR timescale. In the 13C NMR, the carbons
are also equivalent and typically appear between
80 and 95 ppm. The methyl groups of a Cp ligand
typically appear at 1-2 ppm in the 1H NMR and
20-30 ppm in the 13C NMR.
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Making the Cyclopentadienyl Anion
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Making Metallocenes
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Counting Electrons Neutral verses Ionic
Method 18e- and 16e- most stable
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Counting Electrons Neutral verses Ionic
Method 18e- and 16e- most stable
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Target Molecules, Mixed Ring Complexes
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Synthesis of the Iron(II) Mixed Ring Complex
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Synthesis of the Cobalt(III) Mixed Ring Complex
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Proton NMR of the cobalt complex
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Ligand Exchange Reactions with the Cobalt Complex
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Carbon NMR of the p-Cymene Cp Cobalt(III) Complex
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Synthesis of the Manganese(I) Mixed Ring Complex
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Synthesis of the Manganese(I) Mixed Ring Complex
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Reactivity of These Mixed Ringed Systems
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Crystal Structure of the Iron Complex
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Acknowledgements
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References
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