In organometallic chemistry, (diene)iron tricarbonyl describes a diverse family of related coordination complexes consisting of a diene ligand coordinated to a Fe(CO)3 center. Often the diene is conjugated, e.g., butadiene, but the family includes nonconjugated dienes as well. The compounds are yellow, air-stable, often low-melting, and soluble in hydrocarbon solvents. The motif is so robust that even unstable dienes form easily characterized derivatives, such as norbornadienone and cyclobutadiene.
Scope
The inventory of complexes is large. Receiving particular attention are complexes of linear conjugated 1,3-dienes and cyclic dienes with various spacings of the two alkene units around the ring.
+ Selected (diene)Fe(CO)3 Complexes Diene CAS RN Physical propertiesNotes (Butadiene)Fe(CO)3 12078-32-9 yellow-orange, m.p. 19 °Cmajor prototype(Cyclobutadiene)Fe(CO)3 12078-17-0 orange solid, b.p. 47 °C (3 mmHg)antiaromatic ligand (1,3-Cyclohexadiene)Fe(CO)312252-72-6 yellow, m.p. 8–9 °Cmajor prototype for steroidal and terpenoid derivatives (1,3-Cyclooctadiene)Fe(CO)3 33270-50-7 yellow, m.p. 36 °Cisomeric with 1,5-cyclooctadiene derivative (1,5-Cyclooctadiene)Fe(CO)3 12093-20-8 yellow, m.p. 76 °Cnon-conjugated diene (Isoprene)Fe(CO)3 32731-93-4 yellow liquidchiral(Methoxy-1,3-cyclohexadiene)Fe(CO)3 yellow oil, b.p. 66–68 °C (0.1 mmHg) Mixture of two isomers, from Birch reduction of anisole (Norbornadiene)Fe(CO)3 12307-07-2 yellow, m.p. −2 °Cnon-conjugated diene(Norbornadienone)Fe(CO)312307-01-6 yellow, m.p. 93–95 °Cfree diene-one is unstable(Thebaine)iron tricarbonyl (η4-Thiepine)Fe(CO)3-yellow, , m.p. 54.5–55 °C chiral; stable, even though free thiepine is not (Xylylene)Fe(CO)3, m.p. 39 °Cyellow free diene is unstable
Preparation and uses Many of diene complexes were originally prepared by reaction of iron pentacarbonyl with the diene under UV-radiation:
In some cases, isolated yields are modest because the complexes, which are often liquids, volatilize during workup. Some derivatives are prepared by displacement of bda from (benzylideneacetone)iron tricarbonyl (). Addition of nucleophiles to (pentadienyl) iron tricarbonyl cations gives the neutral diene complex. Diene complexes can also be produced by dehalogenation of 1,4-dihalobutene derivatives:
The Fe(CO)3 unit serves as a protecting group for the diene, preventing the diene from participating in Diels-Alder reactions and hydrogenation. The diene is usually deprotected with ceric ammonium nitrate.
Reactions These complexes participate in several reactions of potential value in organic synthesis. The dienes are susceptible to acylation using fairly standard Friedel-Crafts conditions. Once transformed, the diene can be removed with ceric ammonium nitrate as well as trimethylamine N-oxide.
Some iron tricarbon complexes of cyclopentadienones catalyze hydrogenation, see Knolker complex.
Characterization
IR spectra of these complexes show νCO bands near 2040 and 1969 cm−1. At low temperatures, the lower energy band splits, which has been interpreted as evidence for fluxionality on the IR timescale.
References