ZIRCONIUM IN ORGANIC CHEMISTRY

covalent bond between zirconium and carbon. To this extent, it might be stated that there is really no organozirconium chemistry, but at most some odd...
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RROUNDED E

KYGEN

EACH OXYGEN CAN THEN BE BOUND TO LN ORGANIC RADICAL. T H I S STRUCTURE HAS

W O EQUAL SQUARE FACES, JOINED BY EIGHT DENTICAL EQUl LATERAL TRIANGLES

,&onium

f o m my important organometallic

compounds by bonding through electronegative atoms such as oxygen and nitrogen WARREN B. B L U M E N T H A L

o compound is known in which there is a stable

N covalent bond between zirconium and carbon.

To this extent, it might be stated that there is really no organozirconium chemistry, but at most some oddities. Such materials as the cyclopentadienyl compounds of zirconium serve to accentuate the tangential relationship between carbon chemistry and zirconium chemistry, rather than to expose an area of chemistry dealing with Even if diligent effort carbon-to-zirconium bonds. finally leads to synthesis of compounds in which there is a reasonably stable carbon-zirconium bond, it seems likely from our present point of view that the synthesis will be another oddity rather than a n establishment of a substantial area of organozirconium chemistry. Such synthetic products will be more significant as intermediates for further reaction than for their value as isolated substances. There are, however, large and interesting groups of organic compounds which contain zirc mium bound to some atom of high electronegativity such as oxygen or nitrogen. These compounds have been studied extensively and have been shown to occupy important places in both theoretical and applied chemistry. Moreover, the role of zirconium compounds in a surprising number and diversity of organic chemical reactions has attracted considerable attention to its examination and utilization. I t is the purpose of the present paper to offer a broad sketch of the behavior of the zirconium atom toward organic molecules and radicals, and to characterize the’ place of zirconium in organic chemistry. The chemistry of zirconium and its place in inorganic chemistry has been described (5). It is worth noting at the outset that there is a greater chemical similarity between hafnium and zirconium than between any other two elements in the periodic table. Virtually anything that is said of the chemistry of zirconium applies almost identically to the chemistry of hafnium. Zirconium and hafnium always occur together in nature, and hafnium is generally not separated from zirconium in the production of the zirconium compounds of c o p e m e , because its presence makes no appraciable difference in the behavior of the zirconium, except for the difference in equivalent weight. There is considerable justification for treating hafnium as though it were a giant isotope of zirconium. Hafnium a t atomic weight 178.50 is nearly twice as heavy as zirconium, atomic weight 91.22. The relationship of the two elements is best compared to the relationship between deuterium and hydrogen, isotopes in which the ratio of

atomic weights is also approximately 2 : l . We might note, for example, that the ratios between the stability constants of zirconium p-bromomandelates and hafnium g-bromomandelates are of the same order of magnitude as the ratios of the dissociation constants of water and heavy water ( I , 27) indicating the near relationship of zirconium and hafnium over the whole of their respective chemistries. Why Am Carbon-Zirconium Bonds Unlibiy?

If “explaining a phenomenon” is understood as demonstrating that it is a logical consequence of something more basic that is already known, then we can supply at least a partial explanation of the absence from the scientific literature of compounds containing stable carbon-to-zirconium bonds. O n efamining any number of approaches to the preparation of such compounds, we find that in each case there is some reason why it is not likely to give rise to carbon-to-zirconium bonds. First, the bonding forces in metallic zirconium are too great to permit the formation of organozirconium compounds by reaction of metallic zirconium with organic substances, comparable to such formation of alkyls and aryls of alkalies, alkaline earths, aluminum, mercury, tin, or lead. The resistance of the metal lattice to disruption by formation of organic compounds is indicated to some extent by the magnitude of the melting point and the difference between the free energy content, A F f O , in the crystalline and in the gaseous state. The following listing shows how zirconium compares with a number of metals which are known to react to form organometallic compounds.

427

Lead Magnesium Mercury Tin ZIRCONIUM 0 W i q u i d

651 39 232 1850

-

40.4 38.5 27.6 7.5P 6.4 115

Also, if zirconium is heated with carbon to a high temperature (e.g., 1200’ C.), carbon enters into solid solution with the zirconium, forming a new face-centered cubic phase, with the zirconium and carbon atoms arranged after the pattern of sodium and chlorine in the sodium chloride lattice. However, the zirconinmcarbon structure is not ionic, but metallic. The zirconium atoms are spread only about 9% further apart than they had been in the pure metal, and the metallic appearance, metallic conductance, and numerous other metallic attributes are retained. The largest proportion of carbon that will enter into a single phase with zirconium is atom for atom. But solid solutions can be prepared with considerably less than this maximum amount of carbon, with the same face-centered cubic morphology. When the solution is saturated or nearly saturated with carbon, it has a dull metallic grey appearance and is technically designated “zirconium carbide.” VOL 55

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It has been observed that boron, carbon, nitrogen, and oxygen can replace one another in the solid solution, and the selective dissolution of these elements is related to their atomic diameters, all of which are nearly the same. They fit into the interstices between the zirconium atoms in the face-centered cubic crystal. Larger atoms, such as silicon, phosphorus, or sulfur cannot replace tlie small atoms. The formation of zirconium carbide, therefore, is not so much a reflection of the bonding properties and the chemistry of the carbon atom as it is of its size. h/loreover, the carbide does not have the chemical properties of the molecular and ionic metal carbides. It does not dissolve in water and reacts only with difficulty with strong mineral acids. It dissolves in hydrofluoric acid with the evolution of hydrogen and the freeing of elementary carbon. The carbides of aluminum, calcium, magnesium, etc., react with water or acids with evolution of hydrocarbons. All of these properties of zirconium carbide show that even in the intimate relationship of the zirconium and carbon atoms in this solid, the typical behavior of a molecular compound is missing, and ordinary chemical bonding has not occurred. I n carbon compounds in general, the carbon atom almost never has a lone pair of electrons nor exhibits Lewis base properties. Therefore, unlike nitrogen and oxygen compounds, carbon compounds are not likely to form adducts with zirconium by a process of electron donation and coordinate bond formation. While much study still remains to be done before the electrochemistry of zirconium is adequately defined, there is no strong evidence that a discrete monatomic zirconium ion exists or serves as a reacting unit in a chemical process. Hence, there is no known method of preparing organic compounds in which a carbon-tozirconium bond is formed through bringing together a zirconium cation and an organic anion (as in the formation of silver cyanide from the respective ions).

polymerize. In solid zirconium compounds, thc component molecules are oftcn cross-linked throughout the crystal or crystallite, forming giant molecules and giving rise to extraordinaril!- stable substances of high melting points, low vapor pressures, resistance to dissolution, great hardness, and sluggish chemical reactivity. The zirconium atom at coordination number eight has an arrangement of oxygen atoms in the spacc roundabout the zirconium nucleus which forms the points of an Archimedean antiprism ( 14). Fluorine atoms form a similar arrangement in solid zirconium tetrafluoride (13). If we orient an Archimedian antiprism so that one square face presents itself to our vision, we have a structure as shown in Figure 1 . Two of these can be drawn so as to have a point in common (Figure 2), in which case the)- correspond to the graphical formula 0

OllJO

0 O\I/O

0-Zr-0-Zr-0 fly0 O f P O O O 0

There are no geometrical isomers of this. In organic compounds, such as the alkoxides, edch of tlie o n g e n atom3 holds an alkyl group by a covalent bond, and may have a coordination number of two or three, but not greater than three. Hence, not more than two such figures can share a point in common. Two points of the two antiprisms can be drawn to coincide, Figures 3 and 4, corresponding to the graphic formula

The geometry of this f i p r e can be varied and depends on whether we superimpose dA, AD, or Dd on Bb or superimpose AB on BC, in either of two ways. Five isomeric arrangements are possible. Likewise. we might superimpose face dAD on bBC, Figures 5 and 6, and obtain a figure corresponding to

Relationship of Zirconium to Oxygen-Containing Organics

Compounds in which zirconium is bound through oxygen to an organic radical comprise a large, fascinating, and instructive group. Of these, the alkoxides have been studied most extensively. An excellent summary of their properties and uses has been yiven in D. C. Bradley’s paper, “Metal Alkoxides” (6). The characteristics of the zirconium atom which are most significant to its role in organic chemistry are: its atomic (covalent) radius (1.454 A.), its tetravalence, its ability to form compounds in which it manifests coordination numbers 4, .5, 6, 7 , and 8, and its tendencv to attain to the highest coordination number that is sterically possible. The first feature often determines the number of ligands that will fit around the zirconium atoms. and hence the extent to which zirconium will react with various molecules; the second gives rise to a wide diversity of compounds with different ligands bound to the same zirconium atom; the third and fourth are responsible for the great tendency of zirconium compounds to form sol~atesand other complexes, and to 52

INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY

Three different geometric arrangements corresponding to three different isomers are possible. Further, it is conceivable that the square face ABCD be superimposed on face abcd of another antiprism (Figure 7 ) . Structure and Behavior of Alkoxide Polymers

Based on the foregoing patterns, two molecules of zirconium alkoxide, such a s Zr(OCHa)4, can be conceived as dimerizing (and subsequently pol)-merizing) by sharing 1, 2, 3, or 4 oxygen atoms at positions corresponding to the points of antiprisms, or alternatively, they might share points of octahedra rather than of antiprisms. A large number of space arrangements is thus possible. If it be assumed that a particular space arrangement corresponds to the lowest energy state, all other arrangements will be metastable or unstable kvith respect to the lowest energy arrangement. Any number of intermediate arrangements or rearrangements might

DIMERS OF ZIRCONIUM ALKOXIDES a

b

d

C

occur as the molecules polymerize and isomerize. T h e precise condition in which we measure the properties of a zirconium alkoxide will be a result-possibly not strictly reproducible-of the entire history of the alkoxide; its age, its temperature, the presence or absence of solvents or other substances capable of forming adducts. and all other environmental influences. When organic compounds or radicals are precipitated by zirconium from aqueous solution, the previous arrangement of the zirconium atoms into polycations or polyanions may have signifiH / o \ Zr cant effects on the nature of Z=.p the precipitate. In zirconyl / \ HO OH HOf \OH chloride solutions, the prevailing species, at moder\ / \ I ate concentrations, is a tetZr Zr ramer of which the chief ‘ 0 ’ H structural feature is a square, in which one oxygen atom of each pair is above and the other below the plane of the zirconium atoms (74). It has not bren ascertained to what extent the tetramer unit present in the original zirconyl chloride remains intact during the coordination of oxygencontainidg ligands, nor to what extent it separates into its constituent units ( Z r 0 0 H m H 2 0 + ) or rearranges into other polymers (ZrOOH .rH20+)p The tetralkoxide of zirconium forms if an excess of ammonia is present:

/z.

Figure 2.

Tam molcdes can dimniu by shoring an oaygen atom

ZrClr

Figwe 3. Tux oxygen atoms can be shared

F i g m 5. Superimposing two triangular / o m rcprcserzis fheshoring of

Zr(OR),

+ 4NH4CI

T h e tetralkoxides of zirconium are often polymerized or solvated or both, due to the strong tendency of the zirconium atom to attain to a coordination number higher than 4. Because of this same tendency, when sodium alkoxide, rather than ammonia, is used as the base, complex alkoxyzirconates are formed. Meerwein and Bersin (25) deduced from their experiments a relatively simple equation; ZrCl,

Figure 4. Another isomw:of Figurt 3-fioe i s o w s areporsible

+ 4ROH + 4NHs -+

+ 5NaOEt + EtOH-

NaHZr(OEt)6

+ 4NaCI

but Bradley and Wardlaw (72) found that the reaction is more complex and sodium ethoxyzirconates of a variety of compositions are obtained. Condensed species are formed. The analysis of one of their products corresponds quite closely to NarZr6(OH),(OEt)rt. In accordance with principles discussed abovr, the polymerization of the alkoxides will proceed until no valency sites remain unoccupied or until steric hindrance intervenes against further reaction. The combining of alkoxide monomers to form chains might conceivably proceed through sharing of triangular faces of the Archimedran antiprisms. In such a case a circle could form, the growth of which would terminate when the circle was closed or when the ends of the growing are approach one another and the projecting alkoxide radicals act as shields against further approach or further build-up. It has been observed experimentally that if the alkyl radical of the alkoxide has a bsaashed chain, the screening effect of the chain can prevent the approach of another molecule and inhibit polymerization al-

three ofom VOL 5 5

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T A B L E I.

P O L Y M E R I Z A T I O N N U M B E R S OF 2 I RCONI U M AL KOX I DES, [ 2 r (0R4)] Polymerization Boiling Point, Composition of R Arumber C , / M m . Hg

MeEtCHCH2MeaCCHZMePrCHMeiso-PrCHMe2EtC-

256/0,01 247/0.1 238/0.1 188/0.2 178/0.05 175/0.05 95/0.1

3.7 2.4

2.0 2.0 1. o

Figure 6. A second a a j in which zirconium alkoxides can dimerize by sharing three atoms. Three isomers may exist together. Table I, which appears above, lists the observed polymerization numbers, n, for alkoxides of the generic formula [Zr(OR)d], (6). The molecular weights of zirconium alkoxides are satisfactorily determined with an ebulliometer, using benzene as solvent. The process of zirconium alkoxide formation can be reversed by the action of acidic halides and of water. Hydrolysis tends toward the complete dealkoxylation of the alkoxides, with formation of hydrous zirconia. Zr(OR)r

+ n HzO

+ ZrOz.xHeO

+ 4ROH +

( n - x - 2) H 2 0 . Controlled hydrolysis may be presumed to allow the formation of polymers of the type (6, 9 ) : OR

OR

I I -0-Zr-O-Zr-O-Zr-OI I OR

OR

OR

i I

OR

with interesting implications for the preparation of organic-inorganic resins and plastics. However, relatively little has been accomplished experimentally with controlled hydrolysis, and it is premature to judge the practical possibilities of partial hydrolysis of zirconium alkoxides. Somewhat analogous to the hydrolysis of the zirconium alkoxides are their reactions with carboxylic acids. When zirconium alkoxides, e.g., zirconium tetraethoxide or tetraisopropoxide, are heated with fatty acids in a solvent such as heptane, alkoxyzirconium carboxylates are obtained (2, 26). They, too, are polymerized. A molecular weight of 2700-2800 has been reported for ethoxyzirconium stearate (2). Typical products of this process are R C O z Z r ( 0 isoPr)3 (from palmitic acid) and (RCO*)zZr(O iso-Pr) 2 (from stearic acid) (22). I t is also possible to obtain products of the type (RCO2)3Zr--O--Zr(RCOz) 3 by refluxing isopropoxyzirconium tristearate with zirconium tetrastearate. The siloxy analogs of the zirconium alkoxides have been prepared, of generic formula Zr(OSiR1R2R3)4. Thus, the reaction of trialkylsilyl acetates with zirconium 54

INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y

alkoxides was found to give trialkylsiloxyzirconium compounds such as Zr(OSiMe3)r and Zr(OSiEt3)d (8, 70). Analogs of the alkoxides in which nitrogen ( 7 7 ) or sulfur (7) takes the place of oxygen are known-i.e., Zr(NR1R2)4 and Zr(SR)d. Ketonates and Carboxylates

Aldehydes and ketones are alcohols when in their enol forms, and show a tendency to form alkoxides. Thus, trichlorozirconiumisopropenoxide was isolated as a reaction product of zirconium tetrachloride with acetone : ZrC14

+ CH3COCH3+ C13ZrOC : CH2CH3 + HC1

A similar reaction occurred between zirconium tetrachloride and methylisopropyl ketone (27), Substitution products in which there was replacement of more than one chlorine were not obtained, apparently due to a catalytic effect of zirconium whereby condensation of the ligands occurred with liberation of water and hydrolysis of the alkoxide. The P-diketonates of zirconium are more stable than the isopropenoxide, and can be crystallized from aqueous solution under suitable circumstances. The first precipitate obtained in the preparation of the acetylacetonate contains a component whose analysis corresponds to (CH3COCHCOCH3)3ZrOZr(CH3COCHCOCH3)a but zirconium tetraacetylacetonate is easily recovered from the mixed precipitate by recrystallization from benzene. Other P-diketonates of zirconium are prepared in a similar fashion. While the diketonates may be prepared in nonaqueous as well as aqueous media, it is often difficult or impossible to replace all four chlorine atoms of zirconium tetrachloride with P-diketonate ligands, in anhydrous environments (75). The hydrolysis constants of a number of zirconium P-diketonates have been determined (30). I t has been found that zirconium tetraacetylacetonate AUTHOR Warren B. Blumenthal is the Chief of Chemical Research f o r the Titanium Alloy Manufacturing Division of the National Lead Co., :Viapara Falls, N . Y .

An instructive experiment in the formation of insoluble hydroxyzirconium acetate and its conversion to soluble diacetatozirconic acid is performed as follows :

Dissoloe 0.2 mole of sodium acetate in 50 nil. of water, and to this add an aged solution (about 24 hours or more at room temperature) of zirconyl chloride contazning 0.1 mole of the salt in 50 ml. Stir while mixing and f o r a f e w seconds after mzxing. I n a minute or so the mixture sets to a clear jelly. As the jelly stands, it rapidly changes from transparent to opaque, and in a f e w days it becomer a limpzd liquid ( 2 1 , p p . 316-17).

Figure 7.

Zirconium alkoxides can share four atoms.

Only one

structure of this tybe is Possible

T h e reason for this behavior is that the zirconyl chloride is hydrolyzed in its aqueous solution to hydroxyzirconyl chloride (“basic zirconyl chloride”), the empirical equation being ZrOClz

will react with alcohols to form zirconium alkoxides (18). Much interest has been aroused by the implications of this reaction for the cross-linking of polymers which contain hydroxyl groups or other groups which will react in a similar fashion with zirconium tetraacetylacetonate. A number of workers in the field have informed the writer that the physical properties of plastics which have been permitted to react with zirconium tetraacetylacetonate confirm the supposition that cross-linking would occur. Tetracarboxylates (tetraacylates) of zirconium can be prepared by heating zirconium tetrachloride with an excess of anhydrous carboxylic acid, in the presence or absence of an unreactive solvent, such as benzene (32). The zirconium tetracarboxylates hydrolyze in water. The relatively low molecular weight tetracarboxylates, tetraformate and tetraacetate, break down to the watersoluble complex acids, HZrOOH(HC02) 2 and HZrOOH(CH3C02) 2, diformatozirconic acid and diacetatozirconic acid, and the higher molecular weight tetracarboxylates decompose in water to form water-insoluble zirconium soaps, probably of generic formula (ZrOOHX),, where X is a carboxylate radical. Problems in determining true values for free and combined fatty acid cause some uncertainty as to the exact composition of these soaps. They are generally insoluble, or nearly so, in alcohols, and soluble in nonpolar solvents. The true solubilities are often obscured by peptization of the undissolved solid and by interference by contamination with hydrolyzates. Greases can be prepared by dissolving the zirconium soaps in hot mineral oil and then cooling the solutions to room temperature. Diformatozirconic acid is unstable in aqueous solution, tending to thicken into a gel which is probably composed of hydroxyzirconyl formate : HZrOOH(HC02)2

HzO -+

ZrOOH(HC02) f H C 0 2 H

The hydrolytic action of water in this case consists essentially of the displacement of formic acid by water. Diacetatozirconic acid appears to undergo a similar decomposition when its aqueous solution is heated, but the reaction is reversed when the solution cools and clear diacetatozirconic acid solution forms again.

+

H20

+ .

ZrOOHCl

+ HC1

T h e basic species, Z r O O H f (polymeric) forms the basic zirconium acetate, ZrOOH(CH&O2), and the hydrochloric acid of the solution reacts with the second acetate ion to form free acetic acid. O n standing, the free acetic acid slowly combines with the basic acetate to form soluble diacetatozirconic acid. When diacetatozirconic acid solution is evaporated below the hydrolysis temperature, no solid crystallizes from the solution. Instead, the solution becomes progressively more viscous until it sets to a taffy. When the removal of water is complete, an amorphous solid has formed. T h e powdered solid will redissolve in water. This behavior suggests that the highly polymerized diacetatozirconic acid is made up of long chain molecules that cannot be oriented to fit into a crystal lattice. Diacetatozirconic acid dissolves in methanol, and the solution sets to a rigid jelly on standing. T h e molecules of polymer must be of colloidal size, and the sol particles become oriented with formation of jelly. The aryl carboxylic acids, of which benzoic acid is the prototype, precipitate cationic zirconium from aqueous solution almost quantitatively. Long chain aliphatic dicarboxylic acidt react with soluble zirconium salts to form water-insoluble soap-like compounds, while oxalic acid forms a number of relatively soluble complex oxalatozirconic acids. Numerous alkali salts of these acids have been prepared and described. Tri- and tetraoxalatozirconate salts of the alkali metals are particularly well known, and both hydrated and anhydrous salts have been isolated ( 4 ) . Hydroxy carboxylates

T h e a-hydroxycarboxylates of zirconium present some unique features. While many metallic elements form chelates with a-hydroxycarboxylic acids, only zirconium forms insoluble or nearly insoluble chelates with this entire class of acids, (Yttrium forms sparingly soluble a-carboxylates under some conditions.) Because of this, zirconium can be precipitated from solutions of its salts in highly pure condition by the action of the a-hydroxycarboxylic acids. The precipitates are found to be complex acids of zirconium, which though insoluble in water and in hydrochloric acid solution, are readily VOL. 5 5

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Catalvtic behavior can be deduced from the properties of the zirconium atom J

soluble in alkalies, with which they form complex salts. I t is striking that the relatively low molecular weight acids, glycolic and lactic, form trih) droxycarboxylate complexes with zirconium, e.g., H3ZrOH(OCH2C02)?, while the higher molecular weight mandelic acid forms a tetramandelatozirconic acid, HdZr(OCHCeH5C02)4. Since the spatial requirements for the large mandelato group must be greater than those of the small glycolato and lactato groups, it follows that mandelic acid must be reactive enough to displace the last zirconyl or -01 oxygen atom from zirconium, while glycolic and lactic acid are not. I t has been shown experimentally that in the formation of zirconium tetramandelate, compositions with approximately 1, 2, and 3 mandelato qroups per zirconium atom form succcssively before the tetramandelate composition is achiek ed. IVhen tliree a-hpdroxy-carboxylate ligands have been chelated, the structure M ould be

H RC-

H

c=o o=c - CR

O=C -CR H

HC-C=O R

T h e Z r ( 0 H ) 2Zr structure cannot be broken by the glycolic or lactic acid, but it can be broken by mandelic acid, presumably a consequence of inductive and resonance effects of the benzene ring. Relationship of Zirconium to Nitrogen-Containing Organics

The behavior of zirconium toward organic nitrogen is similar to its behavior toward organic oxy-gen, but zirconium shows a marked preference for bonding with oxygen when both oxygen and nitrogen are available for reaction. Organic compounds in which zirconium is bonded to nitrogen will decompose in water or alcohol, usually with the formation of hydrous zirconia or alkoxides. Cupferron is an excellent agent for the analytical determination of zirconium. Its zirconium compound is stable in 2077, sulfuric acid, and can be extracted with chloroform from aqueous medium. The chelation of cupferron by zirconium is doubtless through its oxygen atoms and not through its nitrogen atoms. T h e similarity of its structure to that of mandelic acid is noteworthy :

0A-i

0f-l

cupferron

mandelic acid

HO

0

HO 0

H OH

56

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M’hen zirconium ClBrI tetrahalides are added to amines in anhydrous media, simple adducts generally form in the case of secondary and tertiary amines, but aminoly-sis occurs with primary amines with the formation of chlorozirconium amides, such as C12Zr(NHMe) .”?Me and C12Zr(NHEt)2.NH2Et(76, 77). I n order to obtain tetrakis-alkylammino-zirconium compounds, it is necessary to treat the zirconium tetrahalides with alkali alkylamides ( 7 7 ) . This field of study is in its infancy, and holds forth promise of much that is important to both theoretical and applied chemistry. Zirconium Compounds in the Catalysis of Organic Reactions

There is a literature consisting of many hundreds of articles and patents on the catalysis of organic reactions by zirconium compounds. A comprehensive summary of this literature has been compiled and discussed by P. T. Joseph (20). We would expect, in the absence of any data, that zirconium tetrahalides would show typical Lewis acid catalysis for certain types of syntheses, such as Friedel-Crafts syntheses. The literature does, indeed, report studies of Friedel-Crafts syntheses, using zirconium compounds as catalysts, but never in sufficient detail and with a sufficiently broad experimental base to permit a conclusive judgment as to whether the zirconium tetrahalide was superior or inferior to other Lewis acids. A comparison of zirconium tetrachloride and aluminum chloride under a single set of conditions will tell very little, because a wide range of data is required for a decision as to what constitutes the best conditions for a particular catalyst to operate, and whether zirconium tetrachloride under its most favorable conditions is inferior to or superior to aluminum chloride under its most favorable conditions. Some years ago, Hubbard and the writer ran a series of experiDr. R. A4. mental cyclialkylations in which 2,5-dichlorohexane was made to add on as a second ring to benzene and to a number of benzene derivatives. Zirconium tetrachloride and aluminum chloride were used as cataylsts under the same operating conditions. We obtained higher yields with the zirconium compounds, and purer products. Other investigators, studying other Friedel-Crafts syntheses, have found similar catalytic properties for zirconium tetrachloride and aluminum chloride, but larger yields with aluminum chloride. I n no field more than in the catalytic field is it necessary to reserve judgment until a large amount of data are available. Again, zirconium dioxide has been reliably reported to show superior effectiveness as an esterification catalyst for vapor phase esterifications (23). But we have become so much more aware in recent years of the effects of the crystallinity, the specific surface, the pore size, and the thermal history of an oxide on its physical and chemical properties that we cannot accept with high confidence the conclusions on the relative merits of different oxide catalysts which were based on what were regarded as meticulous studies relatively a few years ago.

The following is a list of a few properties of the zirconium atom in zirconium compounds which are pertinent to catalysis : -The zirconium atom is a powerful electron acceptor. I t might even be spoken of as a n electron attracter or a deelectronator, under some circumstance. The acceptance of lone pairs of electrons often gives rise to loosely bound complexes that react easily with other substances. -Solid compounds of zirconium, in which the atoms are so arranged that zirconium atoms are exposed at the surfaces, have reaction sites which can sorb molecules from ambient liquid or gas phases, and in sorbing them also orient them in ways that might favor their reaction with themselves or with other substances. T h e sorbate might have a lower activation energy than the unsorbed molecules. -Zirconium and its compounds are generally diamagnetic and unpaired electrons are almost never found in zirconium orbitals. This leads to the expectation that zirconium will not serve as catalyst for oxidation-reduction reactions. -Zirconium appears to be able to form compounds in which double as well as single covalent bonds are formed with atoms such as oxygen, nitrogen, and chlorine, and resonance states which are resultants of sinqle and double bonds are common. Further reactions of the zirconium atoms which alter the electrical field around the zirconium atoms can also alter the resonance energy. T h e fluctuations of electric and magnetic fields around an atom undergoing changes in resonance might promote oxidation-reduction reactions which are catalyzed by some other metal, in close proximity. These basic characteristics and behaviors are plausibly related to known catalytic behaviors of zirconium compounds. Since the zirconium atom is a powerful electron acceptor, we might expect that if an oldin molecule were to come into contact with a zirconium atom, a pair of electrons of the double bond would be attracted to a zirconium orbital, and we would get

H H R-C=C-R’ Zr

H H + R-C-C-R’

I \ Zr

\

This would release a chain reaction in which the olefin molecules would condense to give polyolefins. I n fact a large literature (mostly patents) on this subject describes the use of a number of zirconium compounds, alone or in conjunction with compounds of another element (e.g., aluminum) as highly effective olefin condensation catalysts. Zirconium compounds that have been found effective include zirconium tetrachloride, zirconium trichloride, zirconium dichloride, zirconium tetrachloride etherate, zirconium hydride (24), zirconium phosphate, zirconium dioxide-particularly in the presence of ultraviolet radiation (37)zirconium butoxide, zirconium propionate, and zirconyl chloride. Hydrous zirconia and dried zirconia are known to sorb organic acids and alcohols. If acid and alcohol

were simultaneously sorbed on a zirconia surface, they might be expected to react readily with one another. The expectation agrees with fact, for example, zirconium dioxide strongly catalyzes the vapor phase esterification of acetic acid with ethanol ( 2 3 ) . As might have been predicted, repeated efforts have failed to adduce evidence for oxidation-reduction catalysis brought about by zirconium compounds when used alone. For example, zirconium soaps have been compared to the metal drier soaps that are made from cobalt, manganese, and lead for use as paint driers. The zirconium soaps showed no drying properties while the soaps of the other metals were powerful driers. But it was eventually found that when zirconium soaps are used in conjunction with soaps of the other metals, there is a marked enhancement of the drying action of the paramagnetic metal (28, 29). A similar enhancement by a zirconium compound of the action of nickel as a hydrogenation catalyst for unsaturated oils has been observed (79). LITERATURE CITED (1) Alimarin, I. P., Shen, H. H., Zhur. Neorg. Khim. 6 , 2062-8 (1961). (2) Balthis, J., Brit. Patent 755,558 (August 22, 1956). (3) Blumenthal, W. B., “The Chemical Behavior of Zirconium,’’ Van Nostrand, 1958. (4) Zbid., 326. (5) Blumenthal, W. B., IND.ENG.CHEM.46, 528-39 (1954). (6) Bradley, D. C., “Metal Alkoxides,” Advances in Chemistry Series 23, 10-36 (1959). (7) Bradley, D. C., personal communication. (8) Bradley, D. C., Record of Chemical Progress 21, No. 3, 179-87 (1960). (9) Bradley, D . C., Carter, D. G., Can. J . Chem. 39, 1434-43 (1961). (10) Bradley, D. C., Thomas, I. M., Chem. and Znd. 1958, pp. 1231-2. (11) Bradley, D. C., Thomas, I. M., J . Chem. Sac. 1960, pp. 385761. (12) Bradley, D. C., Wardlaw, W., J . Chem. Sac. 1951, pp. 280-5. (13) Burbank, R. D., Bensey, F. N., Jr., U. S . Atomic Energy Comm., K-1280, 19 pp. (1956). (14) Clearfield, A., Vaughan, P. A., Acta Cryst. 9, 555-8 (1956). ( I S ) Dilthey, W. J., Prakt. Chem. 111, 147-52 (1925). (16) Drake, J. E., Fowles, G. W. A., J . Chem. Sac. 1960, pp. 1498-1502. (17) Drake, J. E., Fowles, G. W. A., J . Lesr Common Metal5 2, 149-54 (1960). (18) Freidlina, R. Kh., Brainina, E. M., Nesmeyanov, A . N., Iruest. Akad. Nauk S.S.S.R., Otdel. Khim. Nauk 1957, pp. 43-7. (19) Hawley,,A. K., U. S. Patent 2,564,331 (Aug. 14, 1951). (20) Joseph, P. T., T h e Titanium Alloy Mfg. Div. of the National Lead Co., “Zirconium Compounds as Catalysts and Promoters,” 31 PP. (21) Joseph, P. T., Blumenthal, W. B., J . Org. Chem. 24, 1371-2 (1959). (22) Kapoor, R. N., Mehrotra, R. C., J . Chem. Sac. 1959, pp. 422-6, (23) Maihle, A., deGodon, F., Bull. sac. chim. 29, 101-6 (1921). (24) Mattlack, A. S., U. S . Patent 2,891,044 (June 16, 1959). (25) Meerwein, H., Bersin, T., Annalen 476, 113-50 (1929). (26) Mehrota, R. C., Nature 172, 74 (1953). (27) Moeller, T., “Inorganic Chemistry,” p. 393, Chapman and Hall, 1952. (28) Parker, E., Mack, G. P., U. S . Patent 2,739,902 (Mar. 27, 1956). (29) Zbid., U. S . Patent 2,739,905 (Mar. 27, 1956). (30) Peshkova, V. M., Mel’chakova, N. V., Zhemchuzhin, S. G., Zhur. Neorg. Khim. 6, 1233-9 (1961). (31) Schmerling, L., U. S. Patent 2,924,561 (Feb. 9, 1960). (32) T h . Goldschmidt Akt.-Ges., British Patent 800,160 (Aug. 20, 1958). VOL. 5 5

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