The Organometallic Reader

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Posts Tagged ‘olefin metathesis

Epic Ligand Survey: Carbenes

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Epic Ligand Survey: CarbenesIn a previous post, we were introduced to the N-heterocyclic carbenes, a special class of carbene best envisioned as an L-type ligand. In this post, we’ll investigate other classes of carbenes, which are all characterized by a metal-carbon double bond. Fischer carbenes, Shrock carbenes, and vinylidenes are usually actor ligands, but they may be either nucleophilic or electrophilic, depending on the nature of the R groups and metal. In addition, these ligands present some interesting synthetic problems: because free carbenes are quite unstable, ligand substitution doesn’t cut the mustard for metal carbene synthesis. Off we go!

General Properties

Metal carbenes all possess a metal-carbon double bond. That’s kind of a given. What’s interesting for us about this double bond is that there are multiple ways to deconstruct it to determine the metal’s oxidation state and number of d electrons. We could give one pair of electrons to the metal center and one to the ligand, as we did for the NHCs. This procedure nicely illustrates why compounds containing M=C bonds are called “metal carbenoids”—the deconstructed ligand is an L-type carbenoid. Alternatively, we could give both pairs of electrons to the ligand and think of it as an X2-type ligand. The appropriate procedure depends on the ligand’s substituents and the electronic nature of the metal. The figure below summarizes the two deconstruction procedures.

The proper method of deconstruction depends on the electronic nature of the ligand and metal.

The proper method of deconstruction depends on the electronic nature of the ligand and metal.

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Epic Ligand Survey: N-heterocyclic Carbenes

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Epic Ligand Survey: N-heterocyclic CarbenesOur romp through the common dative ligands continues with the N-heterocyclic carbenes (NHCs). Although we’ll tackle carbenes in general in another post, NHCs deserve their own nod due to their unique structure, properties, and steric tunability. Unlike most metal carbenes, NHCs are typically unreactive when coordinated to a metal (with some exceptions). Like phosphines, they are commonly used to modulate the steric and electronic properties of metal complexes. In fact, the similarities between NHCs and phosphines are notable. Overall, few ligands are as effective as NHCs at ramping up the electron density on a metal center while remaining innocent.

General Properties

Free NHCs contain carbon in the rarely encountered +2 oxidation state. In general, we can classify carbenes according to the nature of the two non-bonding electrons—if they are spin paired (one up and one down), the carbene is called a singlet; if their spins are parallel, we call the carbene a triplet. Whether a carbene is in the singlet or triplet state depends primarily on the difference in energy between its frontier orbitals—when the FMOs are close in energy, single occupation of each FMO (the triplet state) is likely. As the energy difference increases, the singlet state becomes more likely because the higher-energy FMO is less likely to be occupied.

FMO pictures for singlet and triplet carbenes.

FMO pictures for singlet and triplet carbenes.

How do we tinker with the FMO energies, you ask? The nature of the R groups is key. When R is electron-donating, the energy of the LUMO is raised through a fairly straightforward n → 2pz orbital interaction. An analogous interaction is responsible for the stability of carbocations adjacent to lone-pair-bearing heteroatoms (such as the oxocarbenium ion). Thanks to this orbital interaction, electron-donating groups stabilize the singlet state…and NHCs are no exception! The figure below depicts only one of two possible n → 2pz interactions in free N-heterocyclic carbenes. The LUMOs of free NHCs are quite high in energy, relative to other kinds of carbenes. Read the rest of this entry »