science
Metallocycie chemistry broadens role Organometallic reagents permit synthesis and study of increasing number of heterocycles
Dilithiobutadiene derivative is key intermediate in synthesis H 5 C 6 -C = C - C 6 H 5 2
"- C < H > L i
/=\
inhexane
26 C&EN NOV. 8, 1971
C4H
(C.HJ.O/THF^M* V Li®
containing metals In just 10 years, metallocycie chemistry has moved from a single novel reaction involving formation of a metallocyclopentadiene ring to a myriad of new processes involving basic fields such as homogeneous catalysis and organometallic variations of heterocyclic chemistry. For example, as the many paths in studies of metallocycles developed, some chemists have found evidence that metallocycles may be important intermediates in the cyclotrimerization of acetylenes at transition metal centers. Others have determined that some resonance stabilization occurs in metallocycles such as phospholes and their arsenic analogs, and that metallocycles can serve as useful intermediates in making derivatives of carbenes containing silicon and germanium. Still other chemists, notably Dr. Marvin D. Rausch and his associates at the University of Massachusetts, Amherst, are carving out new areas of metallocycie chemistry in studies using organometallic reagents to synthesize metal-containing heterocycles, and in studies of their subsequent reactions and spectral properties. Dr. Rausch recently presented research findings in these areas at the Fifth International Conference on Organometallic Chemistry, held in August in Moscow, U.S.S.R. In their work, which has been supported by the National Science Foundation, Dr. Rausch and his associates have concentrated on organometallic reagents, especially dilithiobutadiene derivatives, as basic intermediates in a general route to metallocycles. They find that certain arylacetylenes, in addition to diphenylacetylene, will dimerize in the presence of lithium to serve as intermediates in forming other metallocycles. But the di-
f=S=\-^'C*H9 lj~\
2TMEDA H
»
,,®
L
H.C.-< M >-\ >—(/ 7 H,C4 \==/ Spirocycles of Group IV metals
•C 4 H,
( f f -C i H,)JiCl 1
^C6H5
Titana-indene
C 6 H 5 PCI 2
A
M = Si, Ge, Sn
(CH 3 ) 2 SnCI 2
/
C4H9
•C 4 H 9
x
/
Sn'
r
i
s
Stanna-indene
(TT-C5H5)CO(CO)2
Phospha-indene
TMEDA = N,N,N\N'-Tetramethylethylenediamine THF = Tetrahydrofuran
lithium reagent derived from a reaction between diphenylacetylene and n-butyllithium is among the most important organometallic reagents used by the group at Amherst. To increase yields of the dilithium reagent—first made by Dr. J. E. Mulvaney and his associates at the University of Arizona several years ago— Dr. Rausch replaced the rc-butyllithium with "chelated" compounds such as n-butyllithium- A^A^Af'-tetramethylethylenediamine (TMEDA) or used a solvent system of ethyl ether and tetrahydrofuran. With these two techniques, yields (80 to more than 90%) of metallo-indene products such as sila-indenes made using the dilithium reagent as an intermediate are more than double those of the products made without chelating or solvent techniques. Indenes. Using either method of making the dilithium reagent, Dr. Rausch and his associates—Dr. Larry Klemann, now at the Corporate Research Laboratories of Esso Research and Engineering Co., Dr. Edmond Samuel, currently on leave from the Laboratoire de Chimie de Coordination, Paris, France, Dr. Henry Gordon,
" H*C< / C ° - | | oc Coordination complex of a phospha-indene
Dr. Alan Siegel, and Sally Gardner— can synthesize a wide range of metallo-indenes containing silicon, tin, or phosphorus, as well as corresponding spirocycles of the Group IV-B metals. The phospha-indene, 3-n-butyl-l,2-diphenylbenzophosphole, has proved to be a very interesting new ligand in coordination chemistry, Dr. Rausch says. It reacts with metal carbonyls to form various organometallic complexes. Transition metal analogs of the metallo-indenes have also been obtained. For example, reaction of titanocene dichloride with the reagent yields a titana-indene derivative which is a remarkably air-stable, deep-red metallocycie with an unusual nuclear magnetic resonance spectrum. The resonances representing the methylenic protons adjacent to the metallocyclic ring system as well as one of the aromatic protons fall at significantly higher fields than in any other such metallo-indenes yet studied by Dr. Rausch and his associates. Dimers. Reductive dimerization of acetylenes with lithium metal also can be used to make metallocyclic compounds. In a typical reaction, phenyl-
trimethylsilylacetylene and lithium lead to a dilithium intermediate which, when quenched with water, produces 1,4 - bis(trimethylsilyl) - 2,3 - diphenyltrans, £ran5-l,3-butadiene. NMR analyses suggest that reductive dimerization occurs at an acetylenic carbon bearing the phenyl group rather than the trimethylsilyl group. The 1,3-butadiene thus formed can also be heated with sulfur to produce a thiophene derivative of known structure, Dr. Rausch says. Using phenylmethylacetylene and lithium metal, the chemists discovered the first example of an acetylene that does not dimerize via carbon atoms bearing the phenyl groups. Quenching produces a single butadiene that does not have phenyl groups at the 2 and 3 positions, according to NMR analyses. Metallocycles can be formed from the bis (trimethylsilyl) -dithiobutadiene intermediate by reacting it with metallic dihalides such as organotin dihalides and phenyldichlorophosphine. Typical yields are not high, ranging from traces to 20%. Analogous reactions with dichlorosilanes have not yet been successful, possibly because of steric hindrance around the silicon atom imposed by the bulky trimethylsilyl groups. Halogens. The Massachusetts group has also successfully prepared a variety of highly halogenated metallocycles, and has studied their properties, since organometallic compounds containing o-bonded perhaloaryl groups are normally much more stable than those containing normal aryl groups. Best results have been obtained using 2,2 , -dilithiooctafluorobiphenyl, a reagent originally prepared by Dr. A. G. Massey and associates at Queen Mary College in England. Reactions between (Tr-cyclopentadienyl)-metalcarbonyl diiodides and the Massey reagent lead to very thermally stable metallofluorenes contain-
ing cobalt, rhodium, and iridium. Similar reactions occur with zirconocene and hafnocene dichlorides; the latter is among the few organohafnium compounds known, and is the first example of a halfnium-containing heterocycle. In other work on metallofluorenes, a reaction between the nonhalogenated intermediate 2,2'-dilithiobiphenyl and titanocene dichloride has produced a bright red, air-stable titanacycle. [A related nonmetallocyclic compound, di- (a-phenyl) titanocene, must be kept below 0° C. to prevent its thermal decomposition]. Similar reactions have led to nonhalogenated cobaltacycles and rhodacycles. All these compounds are remarkably stable in air, which allows their characterization and facilitates a study of their chemistry. If the metallocycles are treated with triphenylphosphine, for example, the carbonyl group is readily displaced to form the respective triphenylphosphines. Thermal reactions should also lead to many new organometallic complexes. Future. Dr. Rausch and his group are presently collaborating in x-ray crystallographic studies with Dr. Ivan Bernal of Brookhaven National Laboratory and Dr. Larry Dahl at the University of Wisconsin to detect possible electron derealization effects in various metallocycles. They also hope to synthesize metallocycles that can serve to elucidate the possible intermediacy of such systems in the homogeneous oligomerization of acetylenes. An ultimate goal is to synthesize and study the properties of unsubstituted metallocycles—metallo derivatives of pyrrole, furan, and thiophene, Dr. Rausch says. He points out that practical applications of these new heterocycles have yet to be examined.
Massachusetts team (from left): Samuel, Siegel, Gardner, Gordon, Klemann, Rausch
COBRATEC STOPS FREEDOM! Free copper ions in multimetal liquid handling systems tarnish and corrode metals. Cobratec® an aromatic triazole, stops the freedom of movement of free copper ions by tying them up in a chemical knot that checks copperinduced corrosion of iron, steel, aluminum and other metals. It builds a monomolecular shield that is chemically bonded to the copper or copper alloy surface. Cobratec can't wash off; or wipe off. It keeps copper, bronze, brass and other copper alloys bright and shiny for months. Cobratec can be used alone or in combination with other compounds for both atmospheric and solution applications. For your free sample of Cobratec (or technical literature) write SHERWIN WILLIAMS CHEMICALS, P. 0. Box 5638, Cleveland, Ohio 44101. Or call (216) 566-2960.
Williams chemicals NOV. 8, 1971 C&EN 27
