Science

IR instrument combination gives reflectance spectrum of spot area of acrylic polymer % Transmittance 100 I

90

80

70

60

50

40

30

20

10

Spectrum of a 4Q-;ftm spot of pojymethyl methacrylate on silicon wafer, coliected with 4 crrr1 resolution using an IBM in­struments IR microscope on an IBM IB/ 38 FHft spectrophotometer

Ι ι ι J_ J I t L—L 4U00 3500 3000 2500 2000

Wave number, cm"1 1500 1000

Phοtomicrograph (magnification 300X, 1 cm = 33μπ\) of polymethyl methacrylate on silicon wafer corresponds to spectrum above

tungsten needle. IR microscopy showed the substance to be sodium carboxymethylcellulose. This find­ing helped to identify the coating as a cosmetic that the suspect used.

To identify specks of spray paint in a hit-and-run case, Paleneik used a pyrolysis technique that McCrone Associates developed. Using this technique, the IR spectrum of a paint speck pyrolyzed in a melting-point capillary tube, extracted, and dried

showed the presence of bisphenol A. This helped in tracing the paint to the car in the case.

Detective Kubic described sever­al cases he had investigated with police officers. In one case, red fi­bers were found near the body of a man shot to death in his driveway. IR microscopy showed that the fi­bers came from a rug laid in the back seat of a car parked nearby. On taking up the rug and a ply­wood floor from the back seat, offi­cers found more red fibers beneath. They proved to come not from the rug but from a red ski cap that a suspect wore. These findings linked the suspect to the car and the car to the scene of the slaying.

In a hit-and-run case, Kubic and his partners had to trace the make and model of the car from a speck of paint. Light microscopy showed that the car had been repainted. Us­ing a tungsten needle, the investi­gators pried a layer of original black paint from between the white top­coat and a brown primer. IR mi­croscopy of the black paint identi­fied it and pointed to the kind of car involved.

In another case, a child riding a

bicycle was hit by a car, and the Mineola criminologists looked for evidence to implicate a particular car. IR microscopy showed that a smear of blue paint on one of the car's rubber bumper guards had come from the bicycle, and that a bit of resin on the front of the car hood had come from a cap on one of the bicycle handlebar grips.

The IR microscope attachments in­t roduced at the symposium by Analect and IBM have binocular, stereoscopic eyepieces to view sam­ples in visible light. Moving an out­side lever selects for viewing under visible light or for recording of transmission or reflectance IR spec­tra. A user may move metal plates with pinholes of different diame­ters over samples to mask areas whose IR spectra are to be deter­mined. For such samples as fibers, rectangular slits are adjustable for widths and lengths.

The Analect attachment, not in­cluding an FTIR spectrometer, is priced at $23,000 and the IBM at $35,000. Also, for $17,000, Analect offers an attachment for transmis­sion spectroscopy only. D

Limits of phosphorus hypervalency explored A quirky molecule first made about 10 years ago at Iowa State Universi­ty is helping chemists there ex­plore the limits of hypervalency in nonmetallic elements such as phosphorus. The molecule is an amine phosphate in which three of the phosphate oxygens each are con­nected to the same nitrogen via a two-carbon bridge. Nomenclature fans would call it 2,8,9-trioxa-5-aza-l-phosphabicyclo[3.3.3]undecane 1-oxide.

When graduate student Dean S. Milbrath prepared it as part of his doctoral thesis research for chemis­try professor John G. Verkade, nei­ther one realized what he had stum­bled onto. But molecular orbital cal­culations done on this molecule in collaboration with professor Henk M. Buck and coworkers at Eindhoven University of Technology in the Netherlands revealed an intriguing possibility: The nitrogen atom might

16 December 9, 1985 C&EN

be able to donate some electron den­sity to the four-coordinate phos­phorus atom, forming an axial or transannular bond; in the process, the phosphorus would become hy-pervalent.

With a little prodding, says Ver-kade, that's what the molecule seems to do. For example, when former graduate student Leslie E. Carpen­ter II treated the amine phosphate with boron trifluoride, he discov­ered that the electron-deficient bo­ron binds to the phosphoryl oxy­gen—not, as one might expect, to the nitrogen. In addition, a dative bond from the nitrogen to the phos­phorus forms, creating a neutral, tricyclic paddle-wheel structure called a phosphatrane [/. Am. Chem. Soc, 107, 7084 (1985)]. In the pro­cess, the geometry around the phos­phorus shifts from four-coordinate tetrahedral to five-coordinate trigo­nal bipyramidal. The phosphorus, by strict definition, has become hypervalent because it is surrounded by 10 electrons, rather than its usu­al octet, and it coordinates to five groups, rather than the usual three or four.

Carpenter and Verkade have dem­onstrated that other Lewis acids, by virtue of their electron-with­drawing ability, also can trigger the collapse of the amine phosphate to a phosphatrane. This happens, for instance, when a triethylsilyl group is attached to the phosphoryl oxy­gen. Even more unexpected, Car­penter found that by using an ex­cess of the silylating reagent, he could hitch a second triethylsilyl moiety to the oxygen, forming a dication (positive charges on oxy­gen and nitrogen).

Working with the Eindhoven group, the Iowa State researchers also generated cationic phospha­tranes simply by protonating or di-protonating the phosphoryl oxygen.

More surprises were in store when Carpenter treated the amine phos­phate with anhydrous phosphoric acid. A white solid immediately pre­cipitated out of solution. A 31P nu­clear magnetic resonance spectrum of the solid product, as well as other evidence, suggests a novel "double-cage" adduct in which an H2P04~ moiety is hydrogen-bonded to a hy­droxy phosphatrane cation.

This solid-state NMR spectrum is the only concrete evidence Verkade has so far to support the proposed structure of the adduct. That's be­cause the adduct is so insoluble that Carpenter, now a postdoctoral re­search associate at Rensselaer Poly­technic Institute, never was able to crystallize it and get an x-ray struc­ture. The one solvent that does dis­solve the adduct—dimethylsulfox-ide—also dissociates it into the re-actants used to make it, Verkade says.

The Iowa State professor readily admits that the pleasing symmetry of the proposed structure is specu­lation on his part, but, he adds, "it's not bad speculation because it ex­plains why the stuff is so insolu­ble." In the proposed structure, the bond dipole moments all point in the same direct ion, and, w h e n summed, give a molecular dipole moment of about 10.7 debye units. Such an extraordinarily large di­pole moment could explain the ad-

duct's extreme insolubility, Verkade notes.

Although the trigonal bipyramid­al structure of other phosphatranes already has been established by x-ray methods, these new deriva­tives created by Carpenter cry out for crystal structures. But the pros­pects for some of them don't look very promising. The trifluorobo-ratooxy derivative, which hasn't been isolated, is "probably a lost cause," in Verkade's words, "because the BF3 group dissociates too easi­ly." The phosphoric acid adduct ap­pears to be too insoluble for crystal­lization. The silylated phospha­tranes, on the other hand, offer some hope because they have been iso­lated as an apparently microcrystal-line mixture. Verkade thinks other silyl derivatives may be easier to crystallize in pure form. One of his new coworkers is now tackling the challenge, with continuing support from the National Science Founda­tion.

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HOOCCH2-N-CH2COOH IMINODIACETIC ACID (IDA)

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December 9, 1985 C&EN 17

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Science

Bicyclic molecule collapses to hypervalent tricyclics

Amine phosphate

BF3-(CH3CH2)20 (CH3CH2)3SiCI04 (1 equivalent)

.Si(CH2CH3)3

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P-p'Qr

f"* H^J

p-K

Trifluoroboratooxy phosphatrane

(CH3CH2)3SiCl04

(CH3CH2)3Si + Si(CH2CH3)3

Ο

Verkade's explorations in this area are aimed at finding out to what extent phosphorus "wants to be hypervalent/ ' he tells C&EN. The collapse of the amine phosphate to the phosphatrane cage seems to be driven by two stabilizing forces, he says. One is the effect of chelating the phosphorus as part of a tricyclic skeleton. The other is the forma­tion of the axial four-electron three-center (N-P-O) bond. In this bond, the two pairs of electrons occupy two sigma molecular orbitals—one bonding, the other nonbonding. The electron density in the nonbonding orbital is concentrated mainly on the apical atoms (ni trogen and phosphoryl oxygen), which also car­ry lone pairs. When the oxygen's lone pair is donated to a Lewis acid such as the triethylsilyl cation, the buildup of positive charge on the oxygen is believed to cause the nonbond ing orbital to contract, Verkade explains. This lowers the orbital's energy and stabilizes the phosphatrane.

In an effort to find out how gen­eral these stabilizing factors are, Verkade and his coworkers also are

Hydroxy phosphatrane cation/dihydrogen phosphate adduct

studying other hypervalent systems. They find, for example, that alkoxy silatranes (the silicon analogs of phosphatranes) appear to behave much like their phosphorus cous­ins in that their alkoxy oxygens can be protonated and silylated. Other researchers in Verkade's labs have used tetra-alcohols to chelate a hypervalent phosphorus atom. In these compounds, the oxygen tips of the molecule's four "arms" form a rectangular array in whose midst sits the chelated atom, which car­ries a fifth group perpendicular to the plane. The Iowa State workers have shown that such rigid molec­ular frameworks can stabilize phos­phorus and perhaps other hyper­valent atoms in a rectangular py­ramidal environment.

"Whether these particular com­pounds end up being dinosaurs that sit on a shelf remains to be seen," Verkade comments. At the very least, they make fascinating, and research-provoking, signposts on the road to a clearer understanding of hypervalency and chemical bond­ing in general.

Ron Dagani, Washington CIRCLE 34 ON READER SERVICE CARD

18 December 9, 1985 C&EN