Haptotropic Metal Migrations in hexahapto-coordinated Arene Chromium Tricarbonyl Complexes

 

We are also interested in the haptotropic migration of hexahapto-coordinated bi- or oligocyclic fused arene metal complexes. Here, the metal fragment is considered as a moveable functional group that is shifted between two non-equivalent rings of the oligocyclic ligand: 1

We use hydroquinoid tricarbonylchromium complexes that are conveniently accessible by benzannulation of alpha,beta-unsaturated chromium carbene complexes: 2

In the starting material for the rearrangement (the kinetically stable complex), the organometallic moiety is attached to the higher substituted (hydroquinoid) terminal ring. Upon heating to temperatures typically between 70 and 90 °C this complex reacts to the product (thermodynamically stable complex) in which the chromium tricarbonyl moiety is coordinated to the less substituted terminal ring. The metal migration can be monitored by both IR and 1H-nmr spectroscopy. For IR spectroscopy dibutyl ether is used as solvent while for nmr spectroscopy non-coordinating C6F6 and C6F5CF3 are applied. The 1H-nmr spectroscopy allows the acquisition of kinetic data of the metal shifts of both tricarbonyl chromium and phosphine dicarbonyl chromium arenes 3a,3i,3j. The metal migration investigated by us so far all exhibit first order kinetics.

Substituted naphthalenes, 3a phenanthrenes 3b and benzonaphthofurans and thiophenes 3c have been submitted to haptotropic metal migration. Theoretical studies for the naphthalene ligand 4 indicate that a metal migration along the periphery of the naphthalene ligand with a h4-coordinated trimethylenemethane complex as transition state is favored over the least motion pathway across the carbon-carbon bond common to both six-membered rings.

For phenanthrene ligands 3b DFT calculations disfavor least motion pathways between adjacent rings and suggest alternative pathways along the inner and/or outer pi-periphery with a preference for the former occurring along the central biphenyl axis. These shifts are characterized by a dihapto-coordinated complex intermediate or a “central ring complex” intermediate, respectively, and proceed via tetrahapto-coordinated-trimethylenemethane-like complex transition states.

Using naphthalene complexes 1 and 2 we recently succeeded in transforming the thermodynamically stable complex (2) back into the kinetically stable complex (1). This is accomplished after substituting a carbonyl by a cyclooctene ligand: 5

Further means for switching the haptotropic rearrangement of chromium tricarbonyl arene complexes are currently being investigated.

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5 H. C. Jahr, M. Nieger, K. H. Dötz; Chem. Commun. 2003, 2866-2867.