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Posts Tagged ‘directing groups

Epic Ligand Survey: Dative Ligands of N, O, and S

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Epic Ligand Survey: Dative Ligands of N, O, and SIn this post, we’ll take a quick look at L-type ligands of nitrogen, oxygen, and sulfur. Ligands of this type are important for at least two reasons: (1) coordination to a metal can modify the reactivity of the bound functional group, and (2) dative coordination is a critical element of organometallic reactions that depend on intramolecular directing group effects. “Long-term” ligands containing two-connected nitrogens, such as pyridines and oxazolines, are now among the most commonly used for organometallic reactions. The behavior of coordinated dinitrogen is also a hot research area right now. Although they look boring on the surface, dative ligands of N, O, and S are rich in chemistry!

General Properties

This might be the first class of ligand for which we can reliably say that backbonding is rarely important. Dative coordination of amines and alcohols involves a straightforward n → dσ orbital interaction. Intuitively, we should expect the acidity of amines, alcohols, and thiols to increase upon coordination, because removal of electron density from nitrogen and oxygen through coordination makes these atoms more electrophilic. Consider the charged model of dative bonding at left in the figure below.

In the absence of backbonding, coordination increases acidity. Write that down.

Coordination increases acidity.

Transfer of the lost proton to an organic substrate is an important aspect of hydrogenation reactions employing amine ligands (see below).

Food for thought: why aren’t (cheaper) amines found in place of phosphines in organometallic catalysts? History has ruled against tertiary amines, but are there any good reasons why? Yes—for one thing, amine nitrogens are more sterically hindered than analogous phosphorus atoms, because N–C bonds are shorter than P–C bonds. Plus, the cone angles of amines are generally wider than those of phosphines. Getting amines to play nice with hindered metal centers can thus be very difficult. Read the rest of this entry »

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Written by Michael Evans

February 19, 2012 at 11:46 pm