Lithiation of ferrocene and preparation of
bis-diphenylphosphino ferrocene
Dominated by electrophilic aromatic substitution reactions, ferrocene (Fc) can be treated as behaving like an aromatic organic compound with respect to electrophiles. Ferrocene is however ca. 100,000 times more reactive(!) than benzene. Thus, Fc is useful for the design of bidentate ligands capable of chelating to a metal center. In this experiment you will functionalize Fc by generation of dilithioferrocene in the form of its TMEDA adduct. You will then prepare bis-diphenylphosphino ferrocene (dppf) from this intermediate. Finally, we will make a heterobimetallic complex, using the dppf as a bidentate chelating ligand. We will experimentally determine the geometry of the metal center M, where M = Ni or Pd, using methods below.
Minimum requirements: We will not be isolating the dilithioferrocene. The dppf is to be characterized by H-1 NMR and P-31 NMR. The M(dppf)Cl2 is to be characterized by H-1 NMR, P-31 NMR, and Evans Method (if you determine your complex is paramagnetic). If your compound is not paramagnetic, you should take Evans method of the other M(dppf)Cl2 complex. You should compare your data for your heterobimetallic complex to literature data of the other M(dppf)Cl2 complex.
New methods: Proper use and handling of lithium reagents, P-31 NMR, Evans Method
References to get you started:
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J. Organomet. Chem. 1967, 10, 127.
J. Organomet. Chem. 1973, 51, 1.
Organomet. 1985, 4, 2196.
Organomet. 1983, 2, 714.
Inorg. Chim. Acta 1989, 157, 259.
Inorg. Chem. 1978, 17, 2859.
Questions asked during the prelab orals:
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What is hapticity? What symbol is used to describe the hapticity of a ligand?
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What is the electron count of Fc and [Fc]+?
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The two cyclopentadienyl (Cp) rings of ferrocene may be oriented indifferent ways (shown below) giving rise to an eclipsed or staggered conformation. What is the point group of each conformation?
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What is the purpose of the TMEDA (N,N,N’,N’-tetramethylethylenediamine) in the lithiation reaction?
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What is the reactivity of n-BuLi with air? With water? (Give/know reactions) Why don’t we want to isolate the dilithioferrocene?
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From a chemical standpoint, t-butyllithium could have also been used in the lithiation reaction of Fc. Explain why t-butyllithium is not a good choice from a safety point of view.
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The preparation of dppf results in substitution of each cyclopentadienyl ring. Why doesn’t disubstition of onering not occur?
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We will be characterizing the dppf by H-1 NMR and P-31 NMR. Which method will be easiest and quickest to assess the purity of your product? Why?
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For heterobimetallic complex M(dppf)Cl2, what are the possible geometries of metal center M? Draw the d-orbital splitting diagrams for those geometries and fill them with electrons. What difference do you observe that can be used to distinguish between the two geometries without using X-ray crystallography?