Preparation of phosphabenzene compounds

Phosphabenzene compounds of the formulae I and II ##STR1## where R.sup.1 to R.sup.6 are identical or different and are, for example, hydrogen, are prepared by reacting corresponding pyrylium salts with PH.sub.3 in the presence or absence of a solvent or diluent. The pyrylium salts are combined with PH.sub.3 at above 0.degree. C. and reacted at a temperature in the range from 0.degree. C. to 200.degree. C. and a pressure above 1 bar without addition of an acid catalyst or a base.

Phosphabenzene compounds can be used as ligands in transition metal
 complexes which are used in the hydroformylation of olefins.
 DE-A 196 21 967 describes such complexes and processes for preparing them.
 In one process variant,
 3,3'-bis(2,4,6-triphenylphosphabenzene)-1,1'-biphenyl can be prepared by
 reaction of the corresponding pyrylium salt with phosphine.
 DE-A-16 18 668 describes a process for preparing substituted
 phosphabenzenes in which pyrylium salts are reacted with
 trishydroxymethylphosphine, tetrahydroxymethylphosphine chloride or
 tetrahydroxymethylphosphine hydroxide. The phosphine compounds are
 difficult to prepare and the process is not economical.
 F. Lieb "Synthesen und Reaktionen von Phosphorinen, Untersuchungen zur
 Darstellung von Systemen mit Arsen-Kohlenstoff-Doppelbindungen", Inaugural
 Thesis, Wurzburg, 1969, pages 106 and 107, describes a process for
 preparing 2,4,6-triphenylpyrylium tetrafluoroborate in n-butanol at
 -78.degree. C., in which the reaction is carried out using PH.sub.3 in the
 presence of a catalytic amount of acid (acetic anhydride/HBr) or of
 CaCO.sub.3 which dissolves in the diluent, as base. After closing the
 reaction vessel, the reaction is carried out for 41 hours at 110.degree.
 C. under autogenous pressure. The use of bases leads to formation of salts
 which necessitates further separation steps.
 DE-A-16 68 416 describes phosphabenzenes and processes for preparing them
 by reaction of substituted pyrylium salts with phosphine. The reaction
 takes place in a glass autoclave in which phosphine is formed from
 phosphonium iodide only after closing the autoclave. Alternatively,
 phosphine is condensed in the reactor, which requires a temperature below
 the boiling point of phosphine, viz. -87.degree. C. The reaction is then
 again carried out in an autoclave.
 Carrying out the reaction under autogenous pressure has the disadvantage
 that the concentration of the reactants changes constantly and long
 reaction times are necessary. The introduction of PH.sub.3 at -78.degree.
 C. requires the use of appropriate cooling equipment. The process in a
 closed reaction vessel is difficult or impossible to control.
 In DE-A-197 43 197, which has an earlier priority date but was not
 published before the priority date of the present application, some of the
 disadvantages described are avoided by a process in which pyrylium salts
 are reacted with phosphine in the presence of a catalytic amount of acid
 and in the presence or absence of a solvent or diluent. Here, the pyrylium
 salts are combined with phosphine at above 0.degree. C. and reacted at
 from .gtoreq.0.degree. C. to 200.degree. C. and a pressure above 1 bar.
 The pyrylium salts are preferably combined with phosphine at ambient
 temperature and the resulting mixture is heated to a temperature in the
 range from 110 to 130.degree. C. for the reaction. In the process
 described, catalytic amounts of acid are used. As acid catalysts, use is
 made of mineral acids such as HCl, HBr or HI. However, such free mineral
 acids can cause corrosion damage to metal autoclaves, pressure vessels or
 other reaction vessels. Furthermore, there is the risk of the desired
 product being contaminated with halide ions. When phosphabenzenes are used
 as cocatalysts, even traces of such contaminants can have an adverse
 effect on the catalyst properties.
 It is an object of the present invention to provide a process for preparing
 phosphabenzene compounds which can be carried out economically and avoids
 the disadvantages of the known processes.
 We have found that this object is achieved by a process for preparing
 phosphabenzene compounds of the formulae I and II
 ##STR2##
 where R.sup.1 to R.sup.6 are, independently of one another, hydrogen, COOM,
 SO.sub.3 M, NR.sub.3 X, NR.sub.2, OR, COOR or SR (where M=hydrogen,
 NH.sub.4 or alkali metal, X=an anion, R=hydrogen or C.sub.1-6 -alkyl) or
 C.sub.1-12 -alkyl, C.sub.6-12 -aryl, C.sub.7-12 -aralkyl, C.sub.7-12
 -alkaryl or C.sub.3-6 -heterocycloalkyl having from 1 to 3 heteroatoms
 which may be substituted by the above radicals, where two or more of the
 radicals may also be joined to form aliphatic or fused-on rings,
 and
 W is a bridge comprising a covalent bond, an oxo group, a sulfur group, an
 ammo group, a di-C.sub.1-6 -alkylsilicon group or a C.sub.1-16 -radical
 which may be a constituent of one or more linked cyclic or aromatic rings
 and may be interrupted by from 1 to 3 heteroatoms, where the o or m
 position of the phosphabenzene ring which is not bound to the bridge may
 in each case bear one of the radicals R.sup.1 to R.sup.6, by reacting
 corresponding pyrylium salts with PH.sub.3 in the presence or absence of a
 solvent or diluent. In the process of the invention, the pyrylium salts
 are combined with PH.sub.3 at above 0.degree. C. and are reacted at a
 temperature in the range from 0.degree. C. to 200.degree. C. and a
 pressure above 1 bar without addition of an acid catalyst or a base. Not
 adding an acid catalyst or a base enables the process to be simplified and
 made more economical.
 In one embodiment, 3,3'-bis(2,4,6-triphenylphosphabenzene)-1,1'-biphenyl is
 excepted.
 It has surprisingly been found that phosphabenzene compounds of the above
 formulae can be obtained by reacting the corresponding pyrylium salts,
 i.e. compounds in which the phosphorus in the formulae I and II is
 replaced by O.sup.+ and an appropriate counterion is present, with
 PH.sub.3 if certain process conditions are adhered to. The pyrylium salts
 are readily available industrially or can be prepared by simple means.
 PH.sub.3 is available industrially.
 Even when the addition of an acid catalyst, in particular a mineral acid,
 is omitted, the process can be carried out in yields which are as high as
 before or even higher.
 The free acid corresponding to the anion of the pyrylium salt can be
 separated from the product mixture obtained after the reaction by aqueous
 extraction and returned to the preparation of the pyrylium salt. The
 aqueous solution of, for example, tetrafluoroboric acid obtained in the
 extraction with water can, if desired, be isolated prior to recycling. The
 aqueous solution contains no mineral acid contamination.
 The reaction is preferably carried out at a PH.sub.3 partial pressure in
 the range from 0.1 to 100 bar, particularly preferably from 5 to 35 bar,
 in particular from 20 to 30 bar. The total pressure in the system depends
 on the solvent employed. The total pressure can be increased by injection
 of PH.sub.3 or inert gas.
 During the reaction, PH.sub.3 is preferably passed into the reaction
 mixture in order to keep the PH.sub.3 partial pressure essentially
 constant. This procedure allows a particularly economical and rapid
 reaction to form the desired phosphabenzene compounds. High product
 purities and conversions are achieved. The process of the present
 invention can be used reliably for many products. It can be carried out
 continuously or batchwise, preferably batchwise.
 In a particularly advantageous process variant, the pyrylium salts are
 combined with PH.sub.3 at ambient temperature, and the resulting mixture
 is reacted by heating to a temperature in the range from 60 to 140.degree.
 C., preferably from 80 to 130.degree. C.
 The temperature of the reaction is particularly preferably from 100 to
 120.degree. C. The reaction is preferably carried out in an autoclave. In
 addition to PH.sub.3, it is possible to use an inert gas by means of which
 the desired total pressure is set. However, preference is given to using
 only PH.sub.3.
 The reaction can be carried out in the presence or absence of a solvent or
 diluent. It is preferably carried out in the presence of a solvent or
 diluent. Suitable solvents and diluents are, for example, lower aliphatic
 alcohols such as methanol, ethanol, n-propanol, n-butanol, i-butanol,
 tert-butanol or pentanol isomers, preferably ethanol, propanol or
 butanols, in particular n-butanol.
 After the reaction, the reaction mixture is preferably depressurized and,
 if desired, purged with an inert gas. The gases carried from the reaction
 mixture are cooled and passed through a separator to separate off
 unreacted PH.sub.3 in liquid form and the PH.sub.3 separated off is
 returned to the reaction.
 A particularly economically and ecologically acceptable variant is thus a
 process in which PH.sub.3 is pased into a reactor in which the reaction is
 carried out and the gas stream is passed via a further line to a cooler of
 any construction type in which the PH.sub.3 is condensed out. In a
 downstream separator of any construction type, the PH.sub.3 is then
 separated off and returned to the reaction, for example by means of a
 pump. In order to obtain a waste gas which is particularly low in
 PH.sub.3, the use of a downstream second cooler and separator is
 advantageous. To free the reactor gas space and the equipment used
 completely of PH.sub.3, which is advantageous owing to the toxicity of
 PH.sub.3, a purge line for purging with an inert gas such as nitrogen
 should be provided. The purged gas should be passed through the
 combination of cooler and separator.
 The time required for the reaction depends on the type of pyrylium salt.
 Depending on the pyrylium salt, the reaction is preferably carried out for
 from 1 to 4 hours. In the reaction in a solvent, the concentration of
 PH.sub.3 in the solvent depends on the PH.sub.3 partial pressure and the
 type of solvent; particularly when the reaction is carried out
 continuously, a high concentration of PH.sub.3 in the solvent should be
 maintained.
 To achieve high conversions in a short reaction time, the reaction is
 preferably carried out using high PH.sub.3 pressures and continuous
 injection of further PH.sub.3.
 Many different pyrylium salts can be used in the process of the present
 invention. The process is generally not restricted to particular classes
 of compound. For example, the pyrylium salts can be ferrates, zincates,
 chlorides, borates, if desired substituted by a C.sub.1-6 -alkyl radical,
 triflates, trifluoroacetates or preferably tetrafluoroborates,
 perchlorates, hydrogensulfates, bromides, iodides or mixtures thereof.
 Preference is given to using tetrafluoroborates. The organic radical of
 the pyrylium salts used according to the present invention is described in
 more detail below by means of the phosphabenzene compounds prepared
 therefrom.
 In the above compounds of the formula I, the radicals R.sup.1 to R.sup.5
 are, independently of one another, hydrogen, COOM, SO.sub.3 M, NR.sub.3 X,
 NR.sub.2, OR, COOR or SR (where M=hydrogen, NH.sub.4 or an alkali metal,
 X=an anion, R=hydrogen or C.sub.1-16 -alkyl), or C.sub.1-12 -alkyl,
 C.sub.6-12 -aryl, C.sub.7-12 -aralkyl, C.sub.7-12 -alkaryl or C.sub.3-6
 -heterocycloalkyl, where the alkyl, aryl, alkaryl and aralkyl radicals may
 be substituted by the abovementioned radicals and two or more of the
 radicals may be joined to form aliphatic or fused-on rings. The radicals
 R.sup.1 to R.sup.5 can be identical or different. R.sup.1 to R.sup.5 are
 preferably alkyl, aryl, alkaryl or aralkyl radicals which may be
 substituted.
 If two or more of the radicals are joined to form aliphatic or fused-on
 rings, the compounds can be, for example, phosphanaphthalene or higher
 aromatic compounds. The radicals are preferably C.sub.6-12 -aryl radicals,
 particularly preferably phenyl radicals, or C.sub.7-12 -aralkyl radicals,
 particularly preferably benzyl radicals, or C.sub.7-12 -alkaryl radicals,
 particularly preferably 2- or 2,6-dialkylaryl radicals in the case of
 R.sup.1 and R.sup.5.
 Particular preference is given to compounds of the formula I in which the
 2, 6 and possibly 4 positions are substituted by substituted or
 unsubstituted phenyl radicals. In particular, phenyl radicals are present
 in all three positions. These radicals preferably have at most one further
 substituent. The substituent in the 4 position is preferably an acid or
 amine radical, possibly in salt form. Furthermore, a benzyl radical or
 2-alkyl-substituted phenyl radical can be present in one or two or the 2,6
 positions.
 In the compounds of the formula II, R.sup.1 to R.sup.6 have the above
 meanings. W is a bridge comprising a covalent bond, an oxo group, a sulfur
 group, an amino group, a di-C.sub.1-6 -alkylsilicon group or a C.sub.1-16
 -, preferably C.sub.1-6 -radical which may be a constituent of one or more
 linked cyclic or aromatic rings and may be interrupted by from 1 to 3
 heteroatoms, where the o or m position of the phosphabenzene ring which is
 not bound to the bridge may in each case bear one of the radicals R.sup.1
 to R.sup.6.
 The bridge W may be, for example, a substituted or unsubstituted methylene
 group of the formula CR.sup.7 R.sup.8, where R.sup.7 and R.sup.8 can be
 identical or different and are hydrogen or alkyl radicals having a total
 of from 1 to 15 carbon atoms or phenyl radicals, or else alkaryl or
 aralkyl radicals, as long as the radical has no more than 16 carbon atoms.
 W can also be an oxa group (--O--), sulfur group (--S--), di-C.sub.1-6
 -alkylsilicon group (--Si(alkyl).sub.2) or an amino group --NR.sup.9 --,
 where R.sup.9 is C.sub.1-6 -alkyl, C.sub.6-12 -aryl, C.sub.7-15 -aralkyl,
 in particular benzyl.
 W can comprise one or more linked cyclic or aromatic rings, particularly
 preferably one or two cyclic or aromatic rings. For example, W can be a o-
 or p-phenylene radical. Other suitable radicals are cycloalkyldiyl
 radicals, in particular cyclopentyldiyl and cyclohexyldiyl. Suitable
 aryldiyl groups are, for example, 1,1'-diphenyidiyl radicals, naphthyldiyl
 radicals, (diphenyl ether)diyl radicals, diphenylmethanediyl radicals,
 diphenylethanediyl radicals, diphenylpropanediyl radicals and
 ferrocenediyl radicals. Examples of suitable phosphabenzene compounds are
 listed in the abovementioned DE-A 196 21 97:
 ##STR3##
 The pyrylium salts used in the direct reaction with PH.sub.3 can be
 obtained, for example, as described in Houben-Weyl, Hetarene II, Part 2,
 editor R. Kreher, Volume B7b, page 755 ff, Thieme Verlag, Stuttgart.
 Further suitable phosphabenzene compounds are:
 ##STR4##
 The compounds prepared according to the present invention can be used for
 the preparation of complexes of metals of transition group VIII of the
 Periodic Table of the Elements. Such complexes can be used as cocatalyst
 in hydroformylations. Suitable reaction conditions are described in DE-A
 196 21 967 and in DE-A-197 43 197.
 The invention is illustrated by the examples below:

EXAMPLES
 Example 1
 An autoclave (capacity: 300 ml; material: HC) was charged with
 2,6-bis(2,4-dimethylphenyl)-4-phenylpyrylium tetafluoroborate (20 g) and
 n-butanol (150 ml) and pressurized with 5 bar of nitrogen. The gas space
 was subsequently flushed with PH.sub.3. 5 bar of PH.sub.3 were injected at
 room temperture and further PH.sub.3 was injected until the pressure
 remained constant at 5 bar. The reaction mixture was heated to 110.degree.
 C., and the solution was stirred vigorously using a sparging stirrer.
 Further PH.sub.3 was injected to a pressure of 30 bar. During the
 reaction, the pressure in the reactor was kept at the desired pressure
 level by injection of further PH.sub.3 via a pressure regulator. After a
 reaction time of 4 hours, the autoclave was cooled, vented, thoroughly
 purged with nitrogen while stirring and the reaction mixture was taken
 out. The autoclave product was evaporated to half its volume under reduced
 pressure The solid which precipitated was filtered off with suction,
 washed with n-butanol and subsequently dissolved in toluene. The toluene
 solution was then washed with water until the aqueous phase was neutral.
 After removal of the solvent and washing with a little n-pentane, the
 residue was dissolved in 250 ml of diethyl ether/methanol (3:2). The
 solution obtained was evaporated at 30.degree. C. and 170 mbar until a
 white solid precipitated. The solid was filtered off with suction, washed
 with a little methanol and n-pentane and subsequently dried in a high
 vacuum. Yield: 8.9 g (53%).
 General Experimental Description for Preparing Phosphabenzenes
 All the following experiments (batchwise) were carried out in a 300 ml
 autoclave (material: HC). The autoclave was charged with pyrylium salt and
 a suitable solvent and pressurized with 5 bar of nitrogen. The gas space
 was subsequently flushed with PH.sub.3. 5 bar of PH.sub.3 were injected at
 room temperature and further PH.sub.3 was injected until the pressure
 remained constant at 5 bar. The reaction mixture was heated to 110.degree.
 C., and the solution was stirred vigorously using a sparging stirrer.
 Further PH.sub.3 was injected to a pressure of 30 bar. During the
 reaction, the pressure in the reactor was kept at the desired pressure
 level by injection of further PH.sub.3 via a pressure regulator. After a
 reaction time of 4 hours, the autoclave was cooled, vented, thoroughly
 purged with nitrogen while stirring and the reaction mixture was taken
 out. The autoclave product was then worked up as described below.
 Example 2a
 ##STR5##
 271 g (2.6 mol) of benzaldehyde and 542 g (3.7 mol) of
 2,4-dimethylacetophenone were dissolved in 400 ml of 1,2-dichloroethane
 and heated to 80.degree. C. 606 g (3.7 mol) of a 54% strength ether
 solution of tetrafluoroboric acid were slowly added dropwise while
 stirring. The mixture was subsequently stirred for another 4 hours at this
 temperature and then allowed to cool to room temperature. The volatile
 constituents of the deep red solution obtained were distilled off in a
 high vacuum with gentle heating. The residue was admixed with a
 toluene/water mixture (1:1). The orange-yellow solid which precipitated
 was filtered off, washed with water and toluene and dried in a high
 vacuum. To recrystallize the product, the solid was suspended in methanol
 and subsequently admixed with dichloromethane until a clear solution had
 been obtained. The solvent was distilled off in a high vacuum with gentle
 heating until solid again precipitated. The product was filtered off,
 washed in succession with methanol and n-pentane and subsequently dried in
 a high vacuum. Yield: 180 g (32%) of light-yellow solid.
 Example 2b
 ##STR6##
 20 g (44 mmol) of 2,6-bis(2,4-dimethylphenyl)4-phenyl- pyrylium
 tetrafluoroborate in 150 ml of n-butanol were used. The autoclave product
 obtained after the reaction with PH.sub.3 was evaporated to half its
 volume under reduced pressure. The solid which precipitated was filtered
 off with suction, washed with n-butanol and subsequently dissolved in
 toluene. The toluene solution was then washed with water until the aqueous
 phase was neutral. After removal of the solvent and washing with a little
 n-pentane, the residue was dissolved in 250 ml of diethyl ether/methanol
 (3:2). The solution obtained was evaporated at about 30.degree. C. under
 reduced pressure until a solid precipitated. The solid was filtered off
 with suction, washed with a little methanol and n-pentane and subsequently
 dried in a high vacuum. Yield: 8.9 g (53%) of white solid.
 Comparative Example 2b
 The reaction was carried out using a method analogous to Example 2b, but
 with addition of 1 ml of a 30% strength HBr solution in acetic acid in the
 reaction with PH.sub.3. The work-up was carried out using a method
 analogous to Example 2b. Yield: 8.9 g (53%).
 Example 3a
 ##STR7##
 1.sup.st step: 40 g (0.20 mol) of deoxybenzoin were dissolved in 100 ml of
 ethanol. While stirring, 10.2 g (0.10 mol) of a 30% strength aqueous
 formaldehyde solution and subsequently 12.2 g (0.22 mol) of potassium
 hydroxide dissolved in a mixture of 7 ml of water and 150 ml of ethanol
 were added to the deoxybenzoin solution. After 2 hours, the solid which
 had precipitated was filtered off, washed with a little n-pentane and then
 dissolved in 300 ml of a mixture of dichloromethane/ethanol (2:1). The
 solvent was distilled off in a high vacuum with gentle heating until solid
 again precipitated. The product was filtered off, washed with n-pentane
 and subsequently dried in a high vacuum. Yield: 38 g (90%) of white solid.
 ##STR8##
 2.sup.nd step: A solution of 0.70 g (2.7 mmol) of triphenylmethanol in 10
 ml of acetic anhydride was added while stirring to a solution of 1.0 g
 (2.4 mmol) of 1,2,4,5-tetraphenyl-1,5-pentanedione in 20 ml of acetic
 anhydride. The reaction mixture was heated to 50.degree. C. While
 stirring, 0.88 g (5.4 mol) of a 54% strength ether solution of
 tetrafluoroboric acid diluted with 5 ml of acetic anhydride was slowly
 added dropwise. The mixture was subsequently stirred for another 4 hours
 at 80.degree. C., and then allowed to cool to room temperature. The solid
 formed after addition of 10 ml of diethyl ether was filtered off. The
 mother liquor was completely evaporated under reduced pressure and then
 admixed with 40 ml of a toluene/water mixture (1:1). The precipitated
 solid was filtered off and washed with a little n-pentane. To
 recrystallize the collected solids, they were suspended in about 30 ml of
 methanol and subsequently admixed with dichloromethane (about 50 ml) until
 a clear solution had been obtained. The solvent was distilled off in a
 high vacuum with gentle heating until solid again precipitated. The
 product was filtered off, washed with a little n-pentane and subsequently
 dried in a high vacuum. Yield: 0.6 g (53%) of light-yellow solid.
 Example 3b
 ##STR9##
 1.3 g (2.8 mmol) of 2,3,5,6-tetraphenylpyrylium tetrafluoroborate in 150 ml
 of n-butanol were used. The autoclave product obtained after the reaction
 with PH.sub.3 was evaporated to one-third of its volume at about
 80.degree. C. under reduced pressure. The solid which precipitated was
 filtered off with suction, washed with n-butanol and subsequently
 dissolved in toluene. The toluene solution was then washed with water
 until the aqueous phase was neutral. After removal of the solvent, the
 residue was dissolved in a mixture of dichloromethane and methanol. The
 solution obtained was evaporated at about 50.degree. C. under reduced
 pressure until a solid precipitated. The solid was filtered off with
 suction, washed with a little n-pentane and subsequently dried in a high
 vacuum. Yield: 0.8 g (71%) of white solid.
 Comparative Example 3b
 The reaction was carried out by a method analogous to Example 3b using 1.0
 g (2.1 mmol) of 2,3,5,6-tetraphenylpyrylium tetrafluoroborate as starting
 material, but with addition of 1 ml of a 30% strength HBr solution in
 acetic acid in the reaction with PH.sub.3. The work-up was carried out
 using a method analogous to Example 3b. Yield: 0.3 g (35%).
 Example 4
 ##STR10##
 The reaction was carried out twice using 15 g (35 mmol) of
 2,6-bis(2-methylphenyl)-4-phenylpyrylium tetrafluoroborate in 150 ml of
 n-butanol each time. The two autoclave products obtained after the
 reaction with PH.sub.3 were combined and evaporated to about 50 ml at
 about 80.degree. C. under reduced pressure. The solid which precipitated
 was filtered off with suction, washed with n-pentane and subsequently
 dissolved in toluene. The toluene solution was then washed with water
 until the aqueous phase was neutral. After removal of the solvent, the
 residue was suspended in methanol and then admixed with dichloromethane
 until the solid had completely dissolved. The solution obtained was
 evaporated at about 40.degree. C. under reduced pressure until a solid
 precipitated. The solid was filtered off with suction, washed with a
 little n-pentane and subsequently dried in a high vacuum. Yield: 15.6 g
 (63%) of white solid.
 Example 5
 ##STR11##
 2.1 g (4.4 mmol) of 2,6-bis(2,4,5-trimethylphenyl)-4-phenylpyrylium
 tetrafluoroborate in 150 ml of n-butanol were used. The autoclave product
 obtained after the reaction with PH.sub.3 was evaporated to about 50 ml at
 about 80.degree. C. under reduced pressure. The solid which precipitated
 was filtered off with suction, washed with n-pentane and subsequently
 dissolved in toluene. The toluene solution was then washed with water
 until the aqueous phase was neutral. After removal of the solvent and
 washing with a little n-pentane, the residue was dissolved in
 dichloromethane. The solution obtained was diluted with methanol and then
 evaporated at 30.degree. C. under reduced pressure until a solid
 precipitated. The solid was filtered off with suction, washed with a
 little n-pentane and subsequently dried in a high vacuum. Yield: 0.8 g
 (45%) of light-yellow solid.
 Example 6
 ##STR12##
 3.5 g (7.8 mmol) of
 10-phenyl-1,2,7,8-dibenzo-3,4,5,6-tetrahydro-9-oxoniaanthracene
 tetrafluoroborate in 150 ml of ethanol were used. The autoclave product
 obtained after the reaction with PH.sub.3 was freed of volatile
 constituents at about 80.degree. C. under reduced pressure. The orange
 residue was suspended in about 200 ml of warm toluene and immediately
 filtered through a frit. The toluene solution was then washed with water
 until the aqueous phase was neutral. After removal of the solvent at
 80.degree. C. under reduced pressure, the solid which remained was
 dissolved in about 30 ml of methanol and about 200 ml of dichloromethane
 and then slowly evaporated at 50.degree. C. under reduced pressure until a
 solid precipitated. The solid was filtered off with suction, washed with a
 little n-pentane and subsequently dried in a high vacuum. Yield: 0.7 g
 (24%) of light-yellow solid.