(Diene)iron tricarbonyl
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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.[1]
Scope
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The inventory of complexes is large. Receiving particular attention are complexes of the cyclohexadienes, the parent organic 1,4-dienes being available through the Birch reductions.[3]
| diene | CAS RN | physical properties | notes |
|---|---|---|---|
| (cyclobutadiene)Fe(CO)3 | 12078-17-0 | orange solid, b.p. 47 °C (3 mm) | antiaromatic ligand[4] |
| (butadiene)Fe(CO)3 | 12078-32-9 | yellow-orange, m.p. 19 °C | major prototype |
| (Isoprene)Fe(CO)3 | 32731-93-4 | yellow liquid | chiral[5] |
| (1,3-Cyclohexadiene)Fe(CO)3 | 12252-72-6 | yellow, m.p. 8â9 °C | major prototype for steroidal and terpenoid derivatives[6] |
| (norbornadiene)Fe(CO)3 | 12307-07-2 | yellow, m.p. â2 °C | non-conjugated diene[7] |
| (norbornadienone)Fe(CO)3 | 12307-01-6 | yellow, m.p. 93â95 °C | free diene-one is unstable[8] |
| (xylylene)Fe(CO)3 | , m.p. 39 °C | yellow | free diene is unstable[9] |
| (1,5-Cyclooctadiene)Fe(CO)3 | 12093-20-8 | yellow, m.p. 76 °C | non-conjugated diene[10][11] |
| (1,3-Cyclooctadiene)Fe(CO)3 | 33270-50-7 | yellow, m.p. 36 °C | isomeric with 1,5-cyclooctadiene derivative[10][12] |
| (η4-Thiepine)Fe(CO)3 | - | yellow, , m.p. 54.5â55 °C | chiral, thiepine = cyclo-C6H6S, which is antiaromatic[13] |
Preparation and uses
Many of diene complexes were originally prepared by reaction of iron pentacarbonyl with the diene under UV-radiation:
- Fe(CO)5 + diene â (diene)Fe(CO)3 + 2 CO
In some cases, isolated yields are modest because the complexes, which are often liquids, volatilize during workup.[7] Some derivatives are prepared by displacement of bda from (benzylideneacetone)iron tricarbonyl (Fe(bda)(CO)3).[14] Addition of nucleophiles to (pentadienyl) iron tricarbonyl cations gives the neutral diene complex.[15] Diene complexes can also be produced by dehalogenation of 1,4-dihalobutene derivatives:[16]
- Fe2(CO)9 + C6H4(CH2Br)2 â FeBr2 + (C6H4(=CH2)2)Fe(CO)3 + 6 CO
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.[17]
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.[1]
Some iron tricarbon complexes of cyclopentadienones catalyze hydrogenation, see Knolker complex.
Characterization
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IR spectra of these complexes show νCO bands near 2040 and 1969 cmâ1. At low temperatures, the lower energy band splits, which has been interpreted as evidence for fluxionality on the IR timescale.[18]