RESEARCH
Iron complex fixes atmospheric oxygen Compound abstracts oxygen atom from molecules in air; work continues on complexes imitating hemoglobin At Brookhaven National Laboratory, a second look at a particular iron complex already noted for its unusual magnetic properties has added the molecule to a growing array of oxygen-activating compounds. These metalcontaining complexes trap oxygen from the air and incorporate the gas into their molecular structure, often as a first step before subsequent oxygentransfer to another reactant. At Brookhaven, Dr. Earl F. Epstein and Dr. Ivan Bernai have found oxygen activation by a triphenylphosphineiron dithiolate compound, discovered two years ago by Dr. A. L. Balch at University of California, Los Angeles. The publication of the work of the three scientists is currently in press in Chemical Communications in London. The oxygen pickup occurs in a reaction of triphenylphosphine with (Bu 4 N) 2 {Fe 2 [S 2 C 2 (CF 3 ) 2 ] 4 }. Molecular oxygen slips in between iron and phosphorus to give an oxide adduct, (Bu 4 N) {(C e H 5 ) 3 P0Fe[S 2 C 2 ( C F 3 ) 2 ] 2} · Before the present Brookhaven work, the adduct had been thought to be a non-oxide phosphine complex. Evidence for the oxide adduct is from x-ray crystallographic data. Dr. Epstein and Dr. Bernai collected two forms of diffraction data on the dark red adduct, obtaining about 4000 reflections which were above background. By standard methods, initial phases were obtained from positions of the iron and the four sulfur atoms. A final, refined map of the molecule shows presence of the phosphorus atom, two complete phenyl rings, and four carbon atoms of the third phenyl ring. All these atoms are farther from the iron atom than normally expected for a triphenylphosphine group bonded directly to an iron atom. The map also shows an atom between iron and phosphorus which the Brookhaven investigators assume to be oxygen. The oxide adduct shows a magnetic susceptibility of 4.00 Bohr magnetons at 298° K. This indicates a quartet ground state and a rare intermediate spin quantum number of 3/2 for the trivalent, five-coordinate 30 C&EN JAN. 5, 1970
iron. Normally, this oxidation state of iron would have a low spin of 1/2 or a high spin of 5/2. Such a property makes the compound all the more suggestive as a possible model for biological systems. Thirty years ago, Dr. Charles D. Coryell, Dr. Fred Stitt, and Dr. Linus Pauling made magnetic susceptibility measurements on various compounds of hemoglobin, and discovered an apparent intermediate magnetic state of hydroxy derivatives. These measurements have been debated since that time, and evidence exists that the intermediate value actually resulted from combined high and low spin states present in the sample. However, there are now at least two iron complexes which seem unambiguously to have the middle spin number. Besides the compound found by Dr. Balch, another has been reported by R. L. Martin and A. H.
White at University of Melbourne, Australia. This second compound has the general formula FeX(S 2 CNR 2 ) 2 , in which X is a halogen and R is an alkyl group. Obviously, there was much for the Brookhaven scientists to check in the new oxide adduct, since alternative explanations for the oxygen-activating and magnetic properties are possible. Dr. Bernai states that the compound is magnetically "clean" in that it is not a case of an apparently intermediate magnetic moment caused by an antiferromagnetic interaction (between high and low spin states) between molecules in the solid state. As for oxygen activation, Dr. Balch, Dr. Epstein and Dr. Bemal have devised a synthesis for the new iron complex in which the only possible source of oxygen is the atmosphere. Previously, the complex has been
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• His synthesized in a ketone solvent, which could have been the oxygen source. The present preparation for the iron complexes by Dr. Balch's original method, is done by slow evaporation of an acetone-toluene or dichloromethane-toluene solution of (Bu 4 N) 2 {Fe 2 [S 2 C 2 (CF 3 )2]4} and triphenylphosphine, or directly from triphenylphosphine oxide and {Fe.>[S2C2(CF3)2]4}-Mon. In the original synthesis, Dr. Balch adds a solution of triphenylphosphine in acetone to a solution of (Bu 4 N) 2 {Fe2[S2C2(CF3)4]2j in acetone. Toluene is added, acetone distilled off, and prisms form when the solution stands at room temperature for two days. The yield is 70%. The new iron complex joins a group of compounds characterized by intense activity of research in the last few years. Oxygen-activating complexes first came to light in the 1930's in research on cobalt complexes in Japan and Germany. The cobalt compounds received much attention during World War II as possible oxygen-carriers, in work by groups under Dr. Melvin Calvin at University of California, Berkeley, and Dr. Harvey Diehl of Iowa State University, Ames, Iowa. An inherent difficulty in this work was that the cobalt complexes were unstable, and this hampered structural determinations. Hence, the field stayed fairly quiet until the discovery of a stable, reversibly reacting, oxygen-carrying iridium compound in 1962 by Dr. Lauri Vaska
and J. W. DiLuzio, then at the Mellon Institute, Pittsburgh. Dr. Vaska's important compound, [ IrCl ( CO ) ( P h 3 P ) 2 ] , takes up oxygen and hydrogen reversibly to yield adducts valuable for both electronic and stereochemical studies. Structures of the iridium complexes were worked out by groups under Dr. James Ibers and Dr. S. J. La Plaça at Brookhaven and Northwestern University, Ε vans ton, 111. Throughout the rest of the decade, other oxygen-activating complexes came out of the considerable research following Dr. Vaska's finding. Metal complexes with this interesting prop erty have been discovered containing iron, cobalt, iridium, nickel, and pal ladium. Work on the last two types of metal complexes has just come out of Japan [/. Amer. Chem. Soc., 9 1 , 6994 (1969)]. From Osaka University, Sei Otsuka, Akira Nakamura, and Yoshitaka Tatsuno report novel oxygen complexes made by oxygenation of metal isocyanide complexes. These complexes are intermediates in the catalytic oxygenation of alkyl isocyanides to alkyl isocyanates. The current direction of work at Brookhaven is toward synthesizing complexes related to the new iron complex, which will more closely mimic hemoglobin. The actual oxy gen-carrier in blood picks up oxygen gently, carrying it in molecular form as a ligand on the central iron atom of the large molecule's porphine active sites. By contrast, the complex found by Dr. Balch is more violent, since it
breaks up the oxygen molecule and adds a single oxygen atom into the central molecular arrangement. Dr. Bernai states that finding the right derivative won't be easy, as the iron complex shows a very subtle mechanism in taking on oxygen. For instance, trialkylphosphine analogs of the triphenylphosphine iron complex show the ordinary F e ( I I I ) spin state of V 2 and do not pick up oxygen. Further, related F e + 4 complexes which were synthesized by Dr. Balch, and whose structures have been worked out by Dr. Epstein and Dr. Bernai, also do not pick up oxygen. This seems to show that the extra electron is necessary for oxygen acti vation. As it sometimes happens in muchexplored coordination chemistry, re search from many quarters can help pinpoint the derivatives which would give the properties the Brookhaven scientists seek. For instance, in a long paper in J. Chem. Soc. (Inorg. Sect.), 2242 (1969), a group under Dr. J. A. McCleverty at University of Sheffield, England, ran through an ex tensive series of compounds related to the Balch oxygen-carrying complex. This series is one of five- and six-co ordinate Lewis base complexes of co balt and iron bisdithiolenes. The properties found will help define fu ture work on oxygen-activating species. The Brookhaven research will also further explore aspects of the present synthesis of the iron complex. The non-oxide anion of the present com pound has not been isolated, although there is evidence for its existence in solution. Spectra of the dianion {Fe 2 [S 2 C 2 ( C F 3 ) 2 ] 4 } ~ 2 in dichloromethane solu tion with varying amounts of tri phenylphosphine indicate the pres ence of two complexes. For short periods of time, the spectrum of the non-oxide anion is not affected by the presence of molecular oxygen. Dr. Balch, Dr. Epstein, and Dr. Bernai call this spectrum significantly differ ent from the spectrum of the solid phosphine oxide adduct. The phosphine adduct is oxidized polarographically to the correspond ing neutral complex at +0.32 volt in the presence of excess phosphine. Under similar conditions, the phos phine oxide adduct undergoes polarographic oxidation at about 0.7 volt. The scientists add, however, that the process may be more complicated than a one-electron oxidation. Hence, much refinement of experi ments remains before a synthetic com pound can truly model the behavior of the even more mysterious and im portant hemoglobin. JAN. 5F 1970 C&EN
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