R. Pettit

Diene-Iron Carbonyl Complexes and Related Species

Publisher Summary This chapter presents an introduction to diene–iron carbonyl complexes and related species and discusses the classification, behavior, and bonding of chemical compounds and iron metals. Olefin–iron carbonyl complexes were introduced by two entirely different synthetic methods obtaining butadiene–iron tricarbonyl by a reaction of butadiene with iron pentacarbonyl and by the reaction of acetylene with iron carbonyls. A brief study on the structure of diene–iron carbonyl complexes is comprised of various studies of X-ray and other physical, electrical, and chemical properties, such as infrared absorption and nuclear magnetic resonance absorption. The chapter also presents a brief study on the preparation and properties of diene–iron tricarbonyl complexes, such as conjugated diene–Fe(CO) 3 complexes, nonconjugated diene–iron tricarbonyl complexes, and triene–iron carbonyl complexes. Iron carbonyl complexes can be derived from iron carbonyl anions, substituted acetylenes and iron carbonyls, and acetylene and iron carbonyls.

Tetracarbonyldi-µ-2,2,5,5-tetramethylhex-3-yne-di-iron. A novel complex containing an iron–iron double bond

The reaction of Fe3(CO)12 with 2,2,5,5-tetra-methylhex-3-yne gives a complex which appears to contain an iron–iron double bond.

Synthesis of cyclic hydrocarbons via intramolecular coupling of bis-pentadienyl iron tricarbonyl cations

Abstract A general synthesis for the preparation of medium sized cycloalkanes having 1,2-butadienyl substituents is described. The reaction sequence involved acylation of butadiene-iron tricarbonyl with a diacid chloride, reduction of the resultant diketone to a diol derivative and conversion with HBF 4 to an acyclic bis-pentadienyl Fe(C0) 3 dicationic complex. Upon treatment with zinc the dication undergoes intramolecular ring closure to afford the bis-Fe(C0) 3 complex of the 1,2-dibutadienyl cycloalkane. Five-, six- and ten-membered cyclolalkene derivatives have been prepared in this manner.

REDUCTION OF ORGANIC COMPOUNDS BY USE OF CARBON MONOXIDE AND WATER INSTEAD OF HYDROGEN

In 1953, Reppe and Vetter reported an interesting variation of the hydroformylation reaction, or “0x0” process, in which they were able to catalytically effect the reductive addition of carbon monoxide to an olefin to yield a higher alcohol, not with the use of hydrogen, as is normally done, but through the use of an extra mole of carbon monoxide plus water.’ The overall stoichiometries of the two reactions are shown in Equation 1.

THE CHEMICAL PROPERTIES OF OLEFIN-IRON PI COMPLEXES

As has been stated many times the area of organometallic chemistry is currently expanding at a rapid rate and the number of organometallic compounds is now very great. This is true for compounds having ionic and covalent type bonding, but particularly so for those organometallic complexes in which an organic moeity is attached to a metal atom by means of a pi bond. The reason for such an active interest in this latter area is due largely to the fact that such compounds are believed to be intermediates in several important industrial processes such as Ziegler polymerization, the 0x0 process, the Wacker process and the cyclic trimerization of butadiene. However, despite the obvious importance of the role of organometallic pi complexes in industrial chemistry, with the possible exception of ferrocene very little is known about the chemical reactivity of these organometallic systems. The current emphasis is more towards the isolation of new types of organometallic complexes rather than investigation of the chemical properties of the various systems to any searching degree. The present paper discusses some studies made on the chemical structure and reactivity of organometallic pi complexes and is mainly restricted to a discussion of the olefin-iron carbonyl complexes. Even with this restriction the paper treats only a few selected topics and in no way attempts to give a comprehensive discussion of organoiron carbonyl complexes.

Concerted Reactions of Organic Ligands on Transition Metals

The development of the Woodward-Hoffman Rules1 has provided a basis for the understanding of concerted reactions occurring in purely organic systems. Within the framework of this theory concerted reactions may be classified into two groups i.e. “allowed” and “forbidden” processes; and associated with the latter group there is an added energy barrier in the reaction coordinate of the process which generally renders these processes to be of such a high energy that alternative non concerted processes of lower energy are favored if reaction occurs at all. However, when such “forbidden” reactions are conducted with the organic species simultaneously coordinated to a transition metal then it appears that the concerted reaction may then become “allowed”. Theoretical arguments, based on the correlation or orbital symmetries of the reactants and products, have been developed to substantiate the idea of the changeover of the process from “forbidden” to “allowed”;2 these will not be reproduced here, but rather I shall concentrate on the experimental results of three separate reactions which illustrate the phenomenon.

SOME ASPECTS OF CONCERTED REACTIONS IN ORGANOMETALLIC SYSTEMS

After a considerable amount of effort, there now exists a good understanding of concerted reactions in organic chemistry; however this is not yet the case in the area of organometallic chemistry, where the problem is greatly complicated by the presence of the metal, even more so by the widely differing natures of the various metals. The present report deals with results from our laboratory in which we have tried to bring some understanding to several reactions in which transition metals are involved. In organic chemistry, concerted reactions can be placed into two large categories: (a) reactions involving internal reorganization and (b) intermolecular reaction. These can be depicted as follows:

Evidence for the existence of the dianion of cyclobutadiene

Evidence is presented for the generation of cyclobutadiene dianion, a Huckel aromatic, from cis-dichlorocyclobutene and sodium naphthalide; although highly reactive, the dianion can be deuteriated to afford 3,4-dideuteriocyclobutene.