RESEARCH
PF3 Replaces CO in Metal Carbonyls Monodentates produce all possible substitution compounds with iron and molybdenum carbonyls When carbon monoxide in either nickel carbonyl or iron carbonyl is replaced by phosphorus trifluoride, a mixture of compounds results that represents every possible degree of substitution. A combination of analytical techniques has yielded information about the bonding of phosphorus trifluoride in complexes with metal carbonyls. The substituted compounds were isolated and identified by vapor-phase chromatography, together with infrared and nuclear magnetic resonance analyses. In describing the work at the recent ACS Southeastern Regional Meeting (in Charleston, W.Va.), Dr. R. J. Clark, of Florida State University's department of chemistry, said that work just completed by P. I. Hoberman (also at Florida State) shows the same result when molybdenum carbonyl undergoes substitution with phosphorus trifluoride. These are thought to be the first instances of a monodentate ligand (which has only one point of bonding to the rest of the molecule) producing all possible substitution compounds with iron carbonyl [Inorg. Chem. 3, 1395 (1964)] and molybdenum carbonyl. Random. In one series of tests, nickel tetracarbonyl was mixed with its fully substituted equivalent, tetrakis (trifluorophosphine) nickel, and heated at 75° C. for 24 hours. The resulting mixture was separated by vapor-phase chromatography, which revealed an almost statistically random distribution among the two compounds and their three possible intermediate substitution products. This distribution depends only upon the molar ratio of carbon monoxide to phosphorus trifluoride present. Similar mixtures occur when phosphorus trifluoride reacts with nickel tetracarbonyl, and when nickel, carbon monoxide, and phosphorus trifluoride react together. The FSU chemists also obtained random mixtures of substitution products when phosphorus trifluoride 52
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reacted with iron pentacarbonyl. Here, substitution was carried out at about 150° C. and from 200 to 500 p.s.i. The substitution of molybdenum hexacarbonyl by phosphorus trifluoride also requires high pressures and temperatures above 125° C. However, substitution occurs at atmospheric pressure under ultraviolet irradiation. AH nine possible substitution compounds, including three pairs of cis and trans isomers, were detected. Thus, phosphorus trifluoride and carbon monoxide seem to replace one another freely and randomly. This points to their similarity as ligands toward metal carbonyls. This similarity was given further support when the physical properties of intermediate substitution compounds were measured. In general, these properties are
intermediate between those of the unsubstituted carbonyl and the fully substituted carbonyl. For example, vapor pressures and densities of the substituted nickel carbonyls vary linearly through the series of substituted carbonyls. Organometallics. Several organometallic carbonyls were reacted with phosphorus trifluoride to determine whether the organic group might also be replaced by a phosphine group. Reactions with tricarbonylmethylcyclopentadienylmanganese (either under UV or above 125° C. and more than 200 p.s.i.) resulted in mixtures of the three possible substitution products. No evidence for replacement of the cyclopentadienyl group was found, however. Tricarbonylbenzenechromium and tricarbonylmesitylenechromium also were studied. Since the arene groups are assigned no formal charge, they might be more easily substituted than is the cyclopentadienyl group in the organomanganese carbonyl. Even so, the arene groups form the equivalent of three bonds with the rest of the molecule, compared to the one-point bond of a carbonyl group. This apparently makes carbonyl replacement the more likely possibility. This was the result observed when
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Phosphorus Trifluoride and Carbon Monoxide Substitute Freely and Randomly NiCC0) 4 ;
Ni(PF 3 ) 4 v NiCC0)2CPF3)2 ^iCCOKPF3)3
E
NiCCO)3PF
"o *o "c o CD
0%Ni(CO) 4 IOO%Ni(PF 3 ) 4
25 75
50 50
75 25
100 0
Initial composition
INTERMEDIATES. When tetrakis(trifluorophosphine)nickel and nickel tetracarbonyl are mixed, the distribution of intermediate substitution compounds after 24 hours at 75° C. is almost statistically random. Distribution depends only on the molar ratio of the two ligands
hexane solutions of the arene complexes were irradiated by UV in the presence of phosphorus trifluoride. Lack of stability of intermediate substitution compounds at the temperatures required for vapor-phase chromatography prevented their isolation. Infrared spectra of samples taken at various intervals during reaction, however, indicated successive replacement of the carbonyl groups by phosphine groups. Benzenetris (trifluorophosphine) chromium, the fully substituted product, was isolated. Contrary data on mesitylene replacement were recently reported by others [Chem. Ber., 97, 2018 (1964)]. At 100° C. and high pressures, phosphorus trifluoride replaces the organo group quantitatively in tricarbonylmesitylenechromium. The apparent difference between UV and pressure conditions remains to be determined. Bonds. Clearly defined coordination compounds of phosphorus trifluoride with metals were first prepared more than 10 years ago. These compounds had much in common with the corresponding carbonyl compounds. Accordingly, chemists have generally thought that phosphorus trifluoride and carbon monoxide have very similar bonding properties. Dr. Clark's IR and NMR spectra of the intermediate substitution compounds appear to bear this out. In most substituted carbonyl compounds, the CO stretching frequencies shift to lower and lower values as the degree of substitution increases. This doesn't happen when the substituent is phosphorus trifluoride. Furthermore, NMR spectra show only a very slight chemical shift for the resonance of both phosphorus and fluorine nuclei with varying carbonyl content. One hypothesis is that both carbon monoxide and phosphorus trifluoride form their fairly strong bonds with transition metals by the back-donation of electron pairs from the metal to available orbitals on the ligand. The behavior of phosphorus trifluoride in substituted carbonyls suggests that it has a pi-bonding ability about equal to that of carbon monoxide. The mechanism, however, would be different. The pi bonding of a carbonyl group, according to this hypothesis, is by donation of an electron pair from the metal to antibonding pi orbitals on the carbon atom. Phosphorus trifluoride would bond by donation from the metal to empty 3d orbitals on the phosphorus atom.
Proteins May Have Hexagonal Structure Arrangement could account for key role of water in structure and activity of proteins The key to many polypeptide and protein molecular structures is a hexagonal arrangement of the carbonyl oxygen atoms in these compounds, according to Dr. Donald T. Warner of Upjohn's research laboratory. This arrangement places the oxygen atoms in a pattern corresponding to the oxygen lattice of the ordered water structure. Such a structure helps to explain how water plays such an important role in the structure and activity of proteins and polypeptides, Dr. Warner told the recent conference of the New York Academy of Sciences. The Upjohn scientist also has examined the structures of other biologically active compounds to see how they fit into the water lattice. This fit—or lack of it—gives clues to the activities of these compounds. Protein. Peptide and protein molecules can be pictured as either a helixes or pleated sheets. Neither of these approaches provides for the structural arrangement of the roughly 30% by weight of bound water in protein, according to Dr. Warner. Also, these models must be severely distorted to represent some of the smaller peptides. With these thoughts in mind, Dr. Warner made up space-filling molecular models of some simple peptide rings. He found that the carbonyl oxygen can be laid out in the form of regular hexagons—a single hexagon for a hexapeptide, a pair of rings (similar to naphthalene) for a decapeptide. All the oxygens are in a single plane on one side of the models with the side chains neatly making up most of the other side. There is no strain in these models. The spacing of these oxygen atoms is almost identical to the spacing of the alternate (second neighbor) oxygens in a water lattice. Both are hexagonal, both are in one plane, and the distance between the oxygen atoms is nearly 4.8 A. in both cases. With this similarity of spacing of the oxygen atoms, a water layer could be linked to a peptide layer by collinear hydrogen bonds. This would provide maximum bond strength. It also suggests a way of bringing water into a protein molecule in a structurally
useful way. The same hexagonal structure in both water and protein could theoretically stabilize both of them. Carrying this theory a step further, Dr. Warner made a space-filling model of the B chain of insulin. This chain has 30 peptide residues, thus 30 carbonyl oxygens. Starting with the decapeptide structure, he spiraled the chain around it, building up new hexagons on its edges. This way, the 30 carbonyl oxygens are located at the corners of nine contiguous regular hexagons arranged in a honeycomb.
There is no distortion in this model. This shows "that the hexagonal approach can be applied to large as well as small peptides, Dr. Warner says. TMV. After devising the model for insulin, Dr. Warner considered the well-studied tobacco mosaic virus (TMV) protein. This protein contains 158 amino acids in a straight chain with an N-acetyl terminal group and a total of 159 carbonyl oxygens. Dr. Warner did not make molecular models to represent TMV; he used a graphical method. He explains that this molecule is too large to make practically with a spacefilling model. The model of insulin had shown that the hexagonal principle works with larger peptides. By continuing the spiral of hexagons on paper, Dr. Warner showed that the 159 carbonyl oxygens of TMV can be fitted at the corners of a honeycomb of 65 hexagons. This theoretical flat, disk-like TMV unit is about 79 A. across, about 3 to 4 A. thick, and hexagonal in shape. Like the smaller peptides, it has a hydrophilic side containing the OCT. 26, 1964 C&EN
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