Hexacoordinated Silicon Gives Polymers Thermally stable phthalocyaninosiloxane polymer seems to have nearly straight backbone
POLYMER. Dr. Malcolm E. Kenney (left) and Dr. Ralph Joyner of Case study a proposed model of the phthalocyaninosiloxane polymer. The polymer apparently has a nearly straight backbone, they say, and is bridged through the central silicon atoms. It has very great stability
A thermally stable phthalocyaninosil oxane polymer that apparently has a nearly straight backbone and is bridged through the central silicon atoms has been prepared by chemists at Case Institute of Technology, Cleve land, Ohio. Based on a hexacoordi nated organosilicon dihydroxide, the polymer is the forerunner of other organometallic polymers now being studied, Case's Dr. Malcolm E. Ken ney says. The phthalocyaninosiloxane polymer is a result of work which Dr. Kenney's group is doing on hexacoordi nated organosilicon compounds. One of the group, Dr. Ralph Joyner, found that heating P c S i ( O H ) 2 (Pc = phthalocyanino ring) in a vacuum at 400° C. gives a blue powder that's evidently the siloxane HO(PcSiO) ; r H. The amount of water liberated in the dehydration reaction suggests that the average chain is reasonably long. In frared measurements show that the 42
C&EN
APRIL
23,
1962
compound no longer has bands that can be attributed to silanol groups. However, a new band, probably due to the siloxane linkage, is present. The siloxane has some interesting properties, Dr. Kenney says. It differs from ordinary siloxanes in that only one group (the phthalocyanino ring) encircles each silicon atom. The phthalocyanino ring replaces the sep arate, noninterlocking groups usually found in siloxanes. The backbone of the polymer is ap parently almost linear because the Si— O—Si bond angle is greater than the usual 130° to 150° found in ordinary siloxanes. This is due to the steric effects of the phthalocyanino rings. Small rings aren't possible because of the size and shape of the phthalo cyanino ring. Branched chains are ruled out as well, as there's a limit of two silanol groups to each silicon atom, Dr. Kenney explains. As its stereochemistry indicates, the
polymer is stable. The Si—O—Si back bone is buried in the center of the phthalocyanino ring and is well pro tected from attack. The polymer doesn't decompose when treated with concentrated sulfuric acid, and it holds up at 520° C. in a vacuum. It isn't liquid or flexible as are ordinary sil oxanes. Organoaluminum Derivatives. Us ing end-blocking groups, Dr. James Owen at Case made short-chain com pounds directly related to the siloxane. These compounds can be considered short-chain, end-blocked polymers. The compounds are made by heat ing P c A 1 0 H H 2 0 and P c S i ( O H ) 2 to give aluminosiloxanes of the type PcAlOiSiPcO^AlPc, where χ can be 1 or 2. The compound in which χ is 2 is interesting because it shows that two different trans groups can be at tached to the same hexacoordinate sili con. Acid hydrolysis of this com pound breaks the Al—O—Si bonds but not the Si—O—Si bonds, and produces the interesting "two-layer" compound HOSiPcOSiPcOH. This compound gives the siloxane when heated. The Case scientists are continuing their studies of the phthalocyaninosilicon and aluminum series. They're also extending this work to other tetravalent metals. Hexacoordinate Silicon. The initial aim of Dr. Kenney's group was to pre pare a new series of compounds in which the silicon's coordination num ber is 6 (usually, organosilicons have four groups around each silicon). They had hoped that these hexacoor dinated silicon compounds would have new and interesting properties. The Case chemists decided to use the phthalocyanino ring as the basic ligand for the preparations. This ligand is planar, bivalent, and tetradentate, and it appeared as though it might form an extensive series of hex acoordinate compounds with silicon. Using readily available materials, Dr. Joyner prepared PcSiCl 2 . The dichloride, although less reactive than an ordinary dichlorosilane, opened the way for making other hexacoordinated phthalocyaninosilicon compounds. For example, Dr. Joyner used the di chloride to prepare PcSi(OH) 2 ; this dihydroxide is a versatile intermedi ate. In fact, it's the main building block for most of the other compounds in the series. The hexacoordinate compounds have silicon at the center of the phthalocyanino ring. The ring is such
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that each silicon is bonded to four planar nitrogens in the ring, and to two other functional groups on either side of the ring in trans-octahedral positions (C&EN, April 9, page 47). The basic phthalocyaninosilicon portion of these compounds is stable and resists attack by a variety of chemicals. Although acids such as nitric acid can destroy it, most of the common bases and nonoxidizing acids tried do not rupture it. This stability is important and has enabled the chemists to develop reactions for substituting various groups in the trans position. For instance, the difluoride, dibromide, and dibenzyloxide have been prepared and studied by Paul Krueger. And two other Case chemists, Joseph Cekada and Robert Linck, succeeded in preparing the diphenoxide. All these derivatives are relatively stable. And because of the ring structure, they are in the blue-to-green color range. The hexacoordinate silicon compounds are unique in a number of ways. For instance, PcSi(OH) 2 doesn't readily yield protons to form salts, in contrast to the relatively easy formation of salts by R3SiOH compounds. And the dibromide is unusual because generally only relatively small atoms such as fluorine and oxygen form hexacoordinate compounds with silicon—for instance in SiF e -2 and the acetylacetonates. And compared to other members of the series, the dibenzyloxide is quite soluble. This solubility is apparently due to the bulky trans groups. Carrying the work a step further, Dr. Joyner made siloxy derivatives that have both tetra- and hexacoordinated silicon in the same compound. This was done by reacting PcSi(OH) 2 with (CeH5)3SiOH to make PcSi[OSi(C e H 5 ) 3 ] 2 . He next succeeded in preparing a more complex but related compound, PcSi [ OSi ( CeH5 ) 2(OCH 2 C e H 5 )] 2 . The central silicon in these compounds has four nitrogens and two oxygens, and the outer silicons have three carbons and one oxygen. These compounds are of interest because they show that bulky groups can be added to the basic phthalocyaninosilicon ligand. They also support the structural picture that shows that the functional groups are in the trans position. According to Dr. Kenney, the bulk of these groups rules out anything but a trans stereochemical arrangement. 44
C & E N A P R I L 23. 1962
Theory Explains Gas Kinetics Problem Research on diatomic gas dissociation points to failure in theory of chemical kinetics; Boltzmann formula fails at high temperatures An English chemist has proposed a theory that could bear directly on such space-age problems as the burning rate of rocket fuels and the in-flight heating of missiles. Dr. H. O. Pritchard of Manchester University claims his theory explains the observed anomalies in the rates of some chemical reactions under the wide variations of temperature and stress that they would be subjected to in missile use. He proposes that the number of transitions which a gas molecule undergoes when changing from one energy state to another has a certain minimum value, and that the Boltzmann distribution formula among higher energy levels does not apply. Dr. Pritchard's work stems from the poor correlation between the activa-
tion energy measurements for dissociation of simple diatomic molecules such as I2 and Br2 under shock-wave conditions, and the actual dissociation energy values measured at equilibrium. For simple diatomic molecules, the minimum (activation) energy values measured under shock-wave conditions are considerably less than the dissociation energy values. The activation energy for dissociation is equal to the dissociation energy when measured at equilibrium. Other diatomic molecules such as H2, F2, and 0 2 , and simple substances such as 0 3 , H 2 0 2 , C2N2, and possibly N2H4—most of which have application in rocketry—would very likely act the same way. The energy of activation is the en-
Classical Gas Concepts Don't Apply
These Arrhenius plots show the failure of the classical concepts for gas dissociation equations measured at nonequilibrium (Non-Eq) and equilibrium (Eq) conditions, if the Boltzmann distribution constant applied to gases at nonequilibrium, the two curves would overlap
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