- Email: [email protected]
Infrared studies of binding and structure in monobasic organophosphorus acids and their salts*
Acta
Vol. 34A. pp. 51 10 56. Pergamon
Press 1978. Printed in Great Brltain
Infrared studies of binding and structure in monobasic organophosphorus acids and their salts* LEONARD
Contribution
I.
KATZIN,
GEORGE
W. MACONand DONALDP. PEPPARD
from the Chemistry Division, Argonne National Laboratory, Argonne, IL 60439, U.S.A.
(Received 28 May 1976; revised 5 April 1977) Abstract-The P=O vibrations of a number of monobasic acids, which are alkyl and aryl derivatives of P(V), are followed through a series of their salts. Trends are pointed out, and relations to crystal bonding, including possible resonance equivalence of the phosphoryl oxygens, are discussed. Certain acids are reported in which no P=O absorption band has been detected. The relation of this phenom-
enon to crystal structure, phosphoryl base strength, and oxygen resonance is discussed. Some unusual characteristics of cations in the salts are also pointed out.
INTRODUCTION
Phosphoric acid, (HO),PO has a strong P=O vibration [ 131 at 1165 cm- ‘. Triesters show this vibration in the 126&13OOcm-’ region, the frequency for diesters generally comes at 1225124Ocm-r, and the bis-organic phosphinic acids fall generally [l, 2,9] between 1140 and 1195 cm-‘. The fully dissociated PO:- ion in aqueous solution [13], and the solid sodium salt of this ion [13,14], absorb at about 1000 cm- ‘. The accepted explanation of the last is that resonance mixes the bond orders of the oxygens in PO:-, and makes them equivalent [15], with a single P-O distance of 1.54A. In phosphoric anhydisdride, P40ro, there are two distinct P-O tances [16], a short 1.39 A for the P=O relation, and 1.62 A for the P-CL(P) configuration. Triesters such as triphenyl phosphate [ 171 show similar differentiation between P=O (1.43 A) and P-O-(C) (1.63 A). Differences even between P==O and PaH of a similar nature are seen with a free acid group, as in dibenzyl phosphate [ 1l] (1.469 and 1.545 A), phospholanic acid [l l] (1.473 and 1.567 A) and another phosphinic acid, dimethyl phosphinic [ 123 acid (1.50 and 1.56A). Hydrogen bonding exists between the acid (HO) group of one molecule and the basic P=O of a second. The crystalline salt, tetragonal KHlPOe, form [18,19] at low temperature shows two hydrogenoxygen distances, 1.05 and 1.43 A, in a hydrogen bond relation. This implies two kinds of oxygen in the crystal. At room temperature, the hydrogen apparently oscillates between the two oxygens in the hydrogen bond [18], occupying a statistical midway position. This behavior defines tautomerism, and indicates absence of electronic resonance. Electronic resonance, which makes the oxygens equivalent in the PO:- anion, would imply a fixed midpoint position for the counterion. We thus have two criteria for recognizing this resonance in the organophosphorus acids. One is the structural onelength equivalence of the free oxygens. A second, less definitive one, but experimen-
The strong absorption band of the P=O group in organic derivatives of P(V) is a characteristic feature of the infrared spectrum. The frequency often reveals the nature of the compound, being affected by the attached groups (R-, RO-, X, HO-, etc.) in fairly regular ways. The voluminous literature on these compounds, is summarized to some extent in book form [l, 21. The acidic phosphorus compounds have been investigated a great deal as complexing agents for solvent-extraction and separation of cations, in relation to the separations required in nuclear energy processing in particular [3]. It quickly became apparent that the potential for hydrogen bonding, implicit with the presence of an acidic OH group in conjunction with the basic P=O, was realized in the actuality. The compounds dimerized and polymerized strongly in solution [4]. This hydrogen bonding behavior has its infrared vibrational effects--in addition to the P=O vibration, a complex of very broad bands, three or four in number [S-S], extends from about 1500 to 2900 cm- r. We have found it in both liquid and solid compounds [9]. The crystal structures of the solid acids [l&12] in fact show the hydrogen bonding between the P-OH of one molecule and the P=O of the next, to yield an indefinitely long screw axis built around the -P-O-H..
O=P-O-H
O=P-
In this the hydrogens are asymmetric-i.e. not equally distant from the oxygens. In a recent reinvestigation of some aspects of the P=O frequency-structure relationship [9], we paid
continuity.
attention to the P=O vibrations of acidic compounds. In addition to their other features, these compounds bear on the general problem of oxygen resonance in phosphates and other oxy-anions. This may be illustrated as follows. * Work performed under the auspices of the U.S. Energy Research and Development Administration. 51
5.2
LEONARD
I. KATZIN,
GEORGE
W
tally easier and more universally applicable, is the infrared absorption frequency of the P=O group: does it approach a limiting value related to that of the PO:- ion, or, altematively, transform into an absorption not recognizably that attributable to P=o? We have explored the circumstances under which the organophosphorus acids approach or achieve resonance similar to that of the phosphate ion. In essentially all phosphate structures hitherto worked out the phosphate group has been clearly of a type with varying P-O distances, or the group is described as “distorted,” often by an “axial compression” (e.g., [ 191). As the trisodium phosphate crystal [14] shows essentially the same infrared absorption as does its aqueous solution [13,14], it may be that when this crystal structure is worked out it will show the ideal tetrahedral structure. In the balance of this paper we shall pursue the infrared criterion, and use it to scout for possible resonance situations and manifestations. 1.r. spectra of phosphorus acid salts and O---P-Oresonance The salts of the acids constitute an obvious venue to seek a resonating O===P-O- structure, as they are free of the covalent perturbation of the proton. If the simple salts fail to behave like Na,PO,, then the transition metal ions which are capable of forming chelates might influence equalization of the oxygen% Furthering this search, we have prepared and investigated salts of a number of organophosphorus acids. Table 1 illustrates data for both aryl and alkyl phosphoric acid and phosphinic acid salts, the representatives chosen being those for which we have a reasonable series of salts. A feature of the data is the occurrence of multiple
MASON
and DONALD
F.
PEPPARD
P===Ovibrations, or vibrations not otherwise accountable. One has the possibilities that (a) there is splitting of the P==O vibrations; that (b) the P=O group finds itself in more than one environment, hence multiple vibrational resonances; or that (c) there is more than one chemical substance present. The probable resolution of the question is furnished by the lithium salt of diphenyl phosphinic acid. A small sample, (a), was set aside for a source of spectroscopic sampling from a freshly prepared batch, (b) of lithium diphenyl phosphinate. At this time. the preparation showed the P==O vibration to be at 1164cm-‘, and the P-4 vibration[9] at 1136cm-‘. Sample (a) was remeasured two months later. and the spectrum now showed the P==O at 12OOcm-’ and the P-+ at 1129cm-‘. At a time some 9-t months following initial preparation, (a) showed no further change in spectrum. At this time (b) was-also investigated, and was found apparently to have retained the initial spectrum, with the P=O at 1164cm-‘. This large stock (b) was still essentially anhydrous, while (a) now showed some water bands. It was. therefore, possible that the difference was one between anhydrous and hydrated chemical species. However, lf hours of pumping under vacuum removed all of the water without altering the rest of the spectrum, so this was not the difference. We conclude that there are two crystalline forms, CI, showing the P=O at 1164 and P#J at 1136 cm- I, and, p, showing the P=O at 1200 and P4 at 1129 cm-‘. There are also characteristic differences between the two forms of the lithium salt in the phenyl vibration pattern at 1000-1075 cm-‘, which reflect the difference in the crystal structure. Thus, the probable resolution of the case of the lithium salt is possibility (b). multiple environments of the P=O group due to multiple crystal forms of the same chemical species. The rate of change of crystal structure which must
Table 1. P=O Vibration wavenumber for salts of representative P(V) monobasic acids* Acid
Cation
(C,H,O),P(O)OH
H' LI' Na' I(+ Rb' CS' NH;
k’
Mg*'
ca++ St” lb”
cu++ zn++ La”
Ce'+ Yb’+ ce+
Et.
I271 I259 1233
p& 1263 1284. I266 1285 1289.1276.1241 1225 1271:1227/; 1277 (12561@. 1228 1255.1240,I229
(C,H&p(OIOH
(n-C,H,O)>P(O)OH
1181 1200.1163\\ IIYO.~ =.(I 170)f. Il531: 1196.ll75,@11 1178,~l; ll6i(llS4) II54 1179.II40 a. Il54/( 2180.II70 11541~ (1141) 1155 113x 1140 ll67.1137(?J 1130
*cm-‘.
t Stronger component underlined. ~Parenth~tical values are shoulders rather than overt peaks. $From spectrum of sample (a’) of text. Ij Hydrated salt.
1235 1237.E 1219~1 1247 (l234), 12251, 1228 1228,
(+C,H,l,P(O)OH I144
1167 Il56.1144l~ 1133” II34 1146, 1102
124l.~lJ II77 (11X5).ll671/
(n-C,H , ,I>P(O)OH
I144 II46 I MO,1 1139’1 II261 1127 1124
Ll45.~lI (I 123)li ,114 1138 III9
il2XI III2 II35 IIZZI,
II351
II31 (
Monobasic organophosphorus
0
0.K
I.C 2.C IOC
BC
2c
C
I
I
1200
1000
I 600
3
cm-l
Fig. 1. 1.r. spectra of mineral-oil mulls, in the 13OCMOO cm-’ region, of (A) di-n-octyl phosphinic acid. and (B) its sodium salt (P=O at 1146cm-I). underlie the spectral difference [20] might be catalytically affected by moisture content. A second batch of the lithium salt was prepared 2imonths after the first, and showed the same initial spectrum (1164cm-‘). After seven months both samples (a’) and (b’) were still very dry, and both showed mixtures of the spectra of a and /I (rather than completed change), with alteration having proceeded slightly further in the smaller portion, (a’). The sodium salt is also anhydrous. Like (a’) and (b’), it has the spectrum of the two mixed forms, including the 1138 cm- ’ vibration of the shifted P-4. Unlike the lithium salt, however, neither form has
acids and their salts
53
been found essentially free of the other, and there seems to be no significant spectral change with time. The potassium salt as first formed comes closest to duplicating the 1200 cm- ’ spectrum of the /I form of the lithium salt, but it is hydrated, as are also the rubidium and cesium salts. The appearance of an absorption near 1175 cm -i in these three salts, and its gradual emphasis with increasing Z at the expense of the 1200 cm-’ vibration, may be a function of varying hydration as well as of increasing cationic radius. Tentatively, we extend to the alkaline-earth salts, and to the salts of diphenyl phosphoric acid which show multiple peaks, the assumption that these multiple peaks represent different bonding and crystallization forms (i.e., our possibility (b) above). The theoretical possibility exists of a single structure with two O=P-Oenvironments. Detailed structure determinations are needed to clarify the situation. There is a tendency in each acid series for an approximately terminal low-frequency for P=O vibration to be reached, as cationic charge and complexing strength increase. This is in the direction expected for the resonance equalization of the oxygens. Zinc dibutylphosphinate (Table 1) is reported [21] to consist of linear polymeric molecules, in which single and multiple &P-O bridges alternate in connecting Zn *+ ions along the chain. Quite a few of the salts we have handled have physical characteristics more like organic polymers than crystalline salts, and the drifts of the P=O frequency could be related to detailed differences in how the polymer chains are set up in the solids--e.g., relative frequencies of single and multiple bridgings, number of phosphorus groups in the multiple bridge, cross-bridging, etc. These factors do not necessarily bear directly, however, on the equivalence of the two phosphorusoxygen bonds. It is thought-provoking that in the molecular compound di-p-diphenylphosphinato acetylacetonato chromium(II1). in which two C&-P-O bridges link a pair of Cr(II1) to form a ring [22], the two P-O distances are nor symmetric, and are 1.487 and 1.515 A respectively. We must conclude that though the salts might be expected to favor formation of the resonating O=P-Ostructure in the phosphoorganic acids, we have not been able to identify a case of this configuration. The accessory evidence makes it improbable that the metastable lithium diphenylphosphinate represents this case. and the complexities of the transition element complex compounds make it unlikely that they are the exemplars either. Acids
lacking
P=O
absorption
Most monobasic phosphorus acids, both liquid and solid, show both the absorptions of the P=O vibration, and the i.r. bands of asymmetric hydrogen bonds (see [ 191 for examples). In particular, di-n-butyl
54
LEONARD
I.
KATZIN.
GEORGE
W.
MAXIN
and DONALD
F. PEPPARD
Table 2. Acids lacking both P=O
and hydrogen-bonding absorptions, with frequency wavenumbers for P=O absorption of their sodium salts Acid
Di-n-heptyl phosphinic acid DI-n-octyl phosphinic acid Pentamethyl phosphetane DC(p-bromphenyl) phosphoric acid Di+chlorphenyl) phosphoric aud DC@-naphthyl) phosphoric acid Di-@-tolyl) phosphoric acid Octadecyl hydrogen phosphinic acid Di-(a-t-amylphenyl) phosphoric acid D&-tolylj phosphikc &id Di-(o-hinhenvl) phosphoric acid Di-i2,6_him~;h;iphen) phosphoric acid Di-(2,4,6-trichlorphenyl) phosphoric and
phosphinic acid has the P=O absorption at 1144cm-‘, and in di-n-hexyl phosphinic acid it is at 1142 cm-’ [19]. The spectrum of di-n-octyl phosphinic acid revealed no P==O absorption in our range of observation (above 6OOcm-i), and had only a broad 1550 cm- ’ absorption of the usual hydrogen bond vibration complex. The sodium salt and other salts (last column, Table 1) did show the customary P=O vibration absorption (1146 cm-’ for the Naf salt) (Fig. 1). Di-n-heptyl phosphinic acid gave the duplicate of these observations. This break occurs at the six-carbon and seven-carbon straight-chain acids only. Di-2-ethylhexyl phosphinic acid and di-cyclooctyl phosphinic acids show all the normal vibration absorptions [93. The disappearance of the P=O absorption, and alteration of the typical hydrogen-bond complex, fit our criteria for presumption of oxygen equivalence and resonance. Inasmuch as the acids whose crystal structures demonstrate the customary spiral hydrogen-bond chain and two P-O distances (e.g., [lO-12)) also show both the P=O and hydrogen bond absorptions, the absorption absence indicates a different structure for the heptyl and octyl phosphinic acids, in which the oxygens and hydrogen are presumably symmetrically disposed. The reversion to P=O absorption in the salts would signify greater polarization by the cation and unsymmetrical bonding of the oxygens in those crystals. The literature revealed a unique structure, that of di-p-chlorphenyl phosphoric acid, in which a two-fold axis [23] makes the two oxygens equivalent, even though hydrogen bonding is present. The structure of this compound was actually worked out on instigation of the authors of an earlier infrared spectral study [24] in which the absence of the usual hydrogen bonding absorption was noted and taken to connote symmetric hydrogen bonding. The work [24] identified the 1194cm-’ (P-G-)~ band [9,23] and its weak 1210 cm-’ satellite as “new bands.. . which are probably due to the stretching vibrations of hydrogen-bonded PO groups.. . .” replacing the usual P=O band. We verified the relevant spectra for di-pchlorophenyl phosphoric acid, with complete absence of the
P=O
absorption, Na’ salt (cm-‘) ll4B(LI’) II46 1162. 1150 1278 1279 1267 1278 1169(L1+) 1285 1191.~ 1252 1249. I201 1267
P=O band, and also found that di-p-bromophenyl phosphoric acid completely lacks the P=O band ‘as well as the bands of the symmetric hydrogen bond. The sodium salts show the customary strong P=O absorption, in the same spectral range as the alkali salts of diphenyl phosphoric acid (cf. Table 1). Some acid spectra in which we had hesitated to designate a P=O vibration could now be recognized as having none. We also investigated a number of analogs and relatives of the para-substituted diphenyl phosphates, in addition to the alkyl phosphinic acids which had previously engaged our attention. Table 2 lists acids we have identified so far in whose spectra the P=O stretching vibration and at least the higher-frequency hydrogen bond absorptions are absent. We list also the P=O frequency for an alkali metal salt. generally the sodium salt. As can be seen, the vibrations for the salts of the phosphinic acids are at frequencies comparable to other phosphinic acids, and values for the salts of the substituted-phenyl phosphoric acids fall in the range of the corresponding parent substances (cf. Table 2). The sodium salt of di-p-tolyl phosphinic acid like that of diphenyl phosphinic acid (Table 1) shows two P=O peaks, indicative of similar mixture of crystal types (see above). The salts of the multi-ring acids, di-/I-naphthyl phosphoric acid and di-o-biphenyl to have characteristic phosphoric acid, seem wavenumber values slightly lower than the simpler compounds. Air-dried sodium di-(2,6_dimethylphenyl) phosphate is heavily hydrated, and the P=O absorp Following dehydration by tion is at 1201cm-‘. pumping at high vacuum for a number of hours, the P=O absorption was at 1249 cm-‘. The lithium salt of di-(2,6_dimethylphenyl) phosphoric acid, anhydrous as isolated, has a P=O absorption at 1244 cm-‘. We have stated that the P=O absorption is not seen in the above acids above 600 cm- ‘. It is conceivable that (as might be argued) the absorption may be shifted, broadened and weakened so that it exists in the designated range but has not been recognized. This is an argument to which there is never a satisfactory solution, but certainly any such absorption is not a characteristic P=O absorption such as is seen
Monobasic organophosphorus in the widest variety of P(V) compounds, and even under conditions of metal salt coordination to neutral compounds [30]. The great base strength of the P=O group must be coupled with tremendous stability to be responsible for the difference in behavior from the carboxylic acids, with their M--OH groups, and from certain inorganic oxyacids. Thus, with these, acid double salts of the type MHAz are used to search out symmetric hydrogen bonds (cf. [26]). In the case of the P(V) acids, the strength of the P=O group leads to the usual asymmetric spiral chain hydrogen bonding configuration. Salt formation keeps the O=Padifferentiation. Even with salts of acids which show no P=O absorption, and which presumably (e.g. dioctyl phosphinic acid) or demonstrably (di-(p-chlorphenyl) phosphoric acid) have equivalence of the oxygens, the two oxygens become spectrally differentiated-i.e., the P=O absorption is seen. Also, when we crystallized a mixture of di-(p-chlorphenyl) phosphoric acid and its sodium salt, the absorptions accompanying the usual asymmetric hydrogen bonding and spiral chain formation were seen, whereas they were not apparent in the spectrum of the parent acid, with its equivalent oxygens. This is attributable to the differentiation of P=O and P-Oon salt formation. The acid is then hydrogen-bonded to the basic P=O to give an asymmetric bond with the attendant vibrations. There is therefore good reason to attribute the final influence in producing the symmetric bonding of the pure acid, when it occurs (i.e. di-(p-chlorphenyl) phosphoric acid), to crystal forces. The published spectrum of the fused acid [24] shows both asymmetric hydrogen bonding absorptions and what is probably the P=O. Further, none of the acids in Table 2 is a liquid, and all liquid acids which have been tested [lo] show the “normal” behavior, if not indeed exaggeratedly strong hydrogen bonding absorption. It is only in the two phalophenyl acids that no signs of any of the usual hydrogen bonding transitions can be seen. Both di-n-heptyl and di-n-octyl phosphinic acids have a very broad and strong absorption at about 1550 cm- ’ which is like that seen about 100 cm- ’ higher in the asymmetric hydrogenbonded cases. This is probably some sort of OH distortion vibration in both cases, akin to the water 164Ocm-’ absorption. The peak comes at about 1660 cm- ’ for the octadecyl hydrogen phosphinic acid. It is not entirely missing from the /Snaphthyl or o-biphenyl compounds, the two p-tolyl acids, the p(t-amyl) phenyl compound (at about 17OOcm-‘), the 2,6_dimethylphenyl acid (where it is fairly strong at 16OOcm-‘) or the trichlorphenyl acid. The relation of P=O absorption, coordination modes of the oxygens, and crystal structure, seen in the acids, undoubtedly bears on the salts also. The two forms of lithium diphenyl phosphinate, say, with demonstrated difference in infrared absorption and in crystal structure, can be expected to differ in oxygen coordination environments also. There is not suffi-
acids and their salts
55
cient basis yet to presume that the oxygens in the 1163 cm- ’ absorbing a form necessarily are equivalent, but they are certainly of a different relation than in the 1200 cm-’ fi form. We can see certain frequency shifts of the P=O vibration within a series, as ions get larger, more highly hydrated and more highly charged. The effect of varying hydration of sodium di-(2,6-dimethylphenyl) phosphate on the P=O is further illustration of the same trend (see also sodium dibutylphosphate in Table 1). One can visualize, qualitatively, that increased ionic size might decrease polarization (which would favor differentiation of P=O and P-Oin the anion) and make the environment more uniform, favoring possible resonance equalization. We do not know, however, the essential relations between this parameter and, for example, P==G bond length, which would enable finer analysis to be made from the infrared frequencies. We also know that in some cases the operative factor is alteration in structure of polymeric chains which we have discussed above. Qualitatively, concentration of electrons in the bond between the phosphorus and the oxygen should give a higher-frequency vibration and a less strongly basic oxygen, and presumably a shorter bond length. Dispersion of more of the electron cloud to the oxygen should make it more basic, lower the vibration frequency and lengthen the bond. It is well known that phosphoric acid triesters, which have higher-frequency P=O vibrations [9] are not as strong bases as phosphine oxides, which have lowerfrequency absorptions[9] of the P==O. The P=O length for triphenyl phosphate [ 171 (1294 cm- ‘) is 1.43 A; that for triphenyl phosphine oxide [27] (1201 cm-‘) is 1.46& Diphenyl phosphinic acid [28] (1181cm-‘) has a reported P=O of 1.49& while dibenzyl phosphoric acid, which probably absorbs like dialkyl phosphoric acids, has [lo] 1.47 8, for the P=O distance. The substitution of one pchlorphenyl in triphenyl phosphine oxide [29] raises the P=O to 1.485 A, and a corresponding pbromphenyl substitution, to 1.497 A, while the lengthening to 1.5OA in di-(p-chlorphenyl) phosphoric acid [23] makes the P=O slightly longer than even in diphenyl phosphinic acid, and presumably still closer to the P-OH length. This could facilitate the crystallographic equalization of the O=P-Ooxygens in this solid.
Cation behavior in salts On the inorganic chemical side, the hydration of the salts in these series is an interesting feature. In inorganic salts (chlorides, nitrates, etc.) the general tendency is for lithium salts to be hydrated, even hygroscopic, and for larger alkali metal cations to be of lower hydration or generally anhydrous. The reverse is seen in the organophosphorus salts-lithium forms are generally anhydrous, sodium forms variable, and larger cations increasingly hydrated.
LEONARDI. KATZIN,GEORGEW. MASONand DONALDF. PEPPARD
56
EXPERIMENTAL Spectral measurements were performed on freshly prepared mineral oil mulls, as in our prior report [9]. Preparation of the organic acids was also described there. The alkali, alkaline earth and ammonium salts (except for Mg salts) were prepared by neutralization of a methanol solution of the organophosphorus acids to pH 7 with an aqueous solution of the respective metal or ammonium hydroxide, followed by evaporation to dryness at room temperature and pressure. Due to the low solubility of Mg(OH),, aqueous MgCl, solution was added to a methanol solution of the sodium salts of the organophosphorus acids. The resulting precipitate was filtered, washed and dried at room temperature and pressure. The Agf, Cu’+, Zn’+, La3+, Ce3+, Yb’+ and Ce“+ salts were prepared by adding a methanol solution of the metal chlorides (nitrate of Ag+) to the sodium salts of the organophosphorus acids dissolved in 50% methanol-50% water solution. The resulting precipitates were filtered, washed with 50” methanol-50% water solution and air dried at room temperature and pressure. REFERENCES [l] L. J. BELLAMY, The Infrared Spectra of Complex Molecules, 2nd edn. (1958); Advances in Infrared Group Frequencies. Methuen, London (1968). [Z] L. C. THOMAS.Interpretation of the Infrared Spectra of Organophosphorus Compounds. Heyden, London (1974). [3] D. F. PEPPARDand G. W. MASON,Nucl. Sci. Eng. 16, 382 (1963). [4] G. M. KOSOLAP~FF and J. S. POWELL,J. Chem. Sot. 3535, (1950).; D. F. PEPPARD,J. R. FERRARO andG. W. MASON, J. Inorg. Nucl. Chem. 4, 371 (1957; D. DYRSSEN,Acta Chem. Stand. 11, 1771 (1958). [S] L. W. DAASCHand D. C. SMITH, Anal. Chem. 23, 853 (1951). [6] L. J. BELLAMYand L. BEECHER,J. Chem. Sot. 1701 (1952). [7] D. F. PEPPARDand J. R. FERRARO,J. Inorg. Nucl. Chem. 10, 275 (1959). [8] S. DETONIand D. HADZI, Spectrochim. Acta 20, 949 (1964).
[9] L. I. KATZIN,G. W. MASON and D. F. PEPPARD,in press. [lo] J. D. DUNITZ and J. S. ROLLETT,Acta Cryst. 9, 327 (1956). [ll] E. ALVERand H. M. KJBJE, Actu Chem. Stand. 23, 1101 (1969). [12] F. GIORDANOand A. RIPAMONTI,Acta Cryst. 22, 678 (1967). [13] A. C. CHAPMAN and L. E. THIRLWELL,Spectrochim. Acta 20, 937 (1964). [14] F. A. MILLER and C. H. WILKINS,Anal. Chem. 24, 1253 (1952). [IS] L. PAULING,The Nature of the Chemical Bond 3rd edn. Cornell University Press, Ithaca, New York (1960). [16] H. C. J. DE DECKERand C. H. MACGILLAVRY,Rec. Trau. Chim. 60, 153 (1941). [17] W. 0. DAVIESand E. STANLEY,Acta Cryst. 15, 1092 (1962). [18] G. E. BACONand R. S. PEASE,Proc. Roy. Sot. (Lendon) A220, 397 (1953). [I93 G. E. BACON and R. S. PEASE,Proc. Roy. Sot. (Lendon) A 230, 359 (1955). [20] X-ray powder patterns of the two forms have been investigated with the cooperation of Mrs. Elizabeth Sherry, and have been shown to be different. [21] V. GIANCOTTI,F. GIORDANO,L. RANDACCIOand A. RIPAMONTI, J. Chem. Sot. A 757 (1968). [22] C. E. WILKESand R. A. JACOBSEN,Inorg. Chem. 4, 99 (1965). [23] M. CALLERIand J. C. SPEAKMAN, Acta Cryst. 17, 1097 (1964). [24] D. HADZI and A. NOVAK,Proc. Chem. Sot. (London) 241 (1960). [25] L. J. BELLAMYand L. BEECHER,J. Chem. SOC. 475. 1701 (1952). [26] J. C. SPEAKMAN,Struct. Bond. 12, 141 (1972). [27] G. BANWLI, G. BARTOMZZO,D. A. CLEMENTE,V. CROATTUand C. PANAT~ONI,J. Chem. Sot. A 2778 (1970). [28] T. -T. LIANGand K. -C. CHIAO, Hua Hsueh Hsueh Poa 31, 155 (1965). [29] W. DREI~SIGand K. PLIETH, Acta Cryst. 27B, 1140, 1146 (1971). [30] L. I. KATZIN, J. Inorg. Nucl. Chem. 20, 300 (1961).
Vol. 34A. pp. 51 10 56. Pergamon
Press 1978. Printed in Great Brltain
Infrared studies of binding and structure in monobasic organophosphorus acids and their salts* LEONARD
Contribution
I.
KATZIN,
GEORGE
W. MACONand DONALDP. PEPPARD
from the Chemistry Division, Argonne National Laboratory, Argonne, IL 60439, U.S.A.
(Received 28 May 1976; revised 5 April 1977) Abstract-The P=O vibrations of a number of monobasic acids, which are alkyl and aryl derivatives of P(V), are followed through a series of their salts. Trends are pointed out, and relations to crystal bonding, including possible resonance equivalence of the phosphoryl oxygens, are discussed. Certain acids are reported in which no P=O absorption band has been detected. The relation of this phenom-
enon to crystal structure, phosphoryl base strength, and oxygen resonance is discussed. Some unusual characteristics of cations in the salts are also pointed out.
INTRODUCTION
Phosphoric acid, (HO),PO has a strong P=O vibration [ 131 at 1165 cm- ‘. Triesters show this vibration in the 126&13OOcm-’ region, the frequency for diesters generally comes at 1225124Ocm-r, and the bis-organic phosphinic acids fall generally [l, 2,9] between 1140 and 1195 cm-‘. The fully dissociated PO:- ion in aqueous solution [13], and the solid sodium salt of this ion [13,14], absorb at about 1000 cm- ‘. The accepted explanation of the last is that resonance mixes the bond orders of the oxygens in PO:-, and makes them equivalent [15], with a single P-O distance of 1.54A. In phosphoric anhydisdride, P40ro, there are two distinct P-O tances [16], a short 1.39 A for the P=O relation, and 1.62 A for the P-CL(P) configuration. Triesters such as triphenyl phosphate [ 171 show similar differentiation between P=O (1.43 A) and P-O-(C) (1.63 A). Differences even between P==O and PaH of a similar nature are seen with a free acid group, as in dibenzyl phosphate [ 1l] (1.469 and 1.545 A), phospholanic acid [l l] (1.473 and 1.567 A) and another phosphinic acid, dimethyl phosphinic [ 123 acid (1.50 and 1.56A). Hydrogen bonding exists between the acid (HO) group of one molecule and the basic P=O of a second. The crystalline salt, tetragonal KHlPOe, form [18,19] at low temperature shows two hydrogenoxygen distances, 1.05 and 1.43 A, in a hydrogen bond relation. This implies two kinds of oxygen in the crystal. At room temperature, the hydrogen apparently oscillates between the two oxygens in the hydrogen bond [18], occupying a statistical midway position. This behavior defines tautomerism, and indicates absence of electronic resonance. Electronic resonance, which makes the oxygens equivalent in the PO:- anion, would imply a fixed midpoint position for the counterion. We thus have two criteria for recognizing this resonance in the organophosphorus acids. One is the structural onelength equivalence of the free oxygens. A second, less definitive one, but experimen-
The strong absorption band of the P=O group in organic derivatives of P(V) is a characteristic feature of the infrared spectrum. The frequency often reveals the nature of the compound, being affected by the attached groups (R-, RO-, X, HO-, etc.) in fairly regular ways. The voluminous literature on these compounds, is summarized to some extent in book form [l, 21. The acidic phosphorus compounds have been investigated a great deal as complexing agents for solvent-extraction and separation of cations, in relation to the separations required in nuclear energy processing in particular [3]. It quickly became apparent that the potential for hydrogen bonding, implicit with the presence of an acidic OH group in conjunction with the basic P=O, was realized in the actuality. The compounds dimerized and polymerized strongly in solution [4]. This hydrogen bonding behavior has its infrared vibrational effects--in addition to the P=O vibration, a complex of very broad bands, three or four in number [S-S], extends from about 1500 to 2900 cm- r. We have found it in both liquid and solid compounds [9]. The crystal structures of the solid acids [l&12] in fact show the hydrogen bonding between the P-OH of one molecule and the P=O of the next, to yield an indefinitely long screw axis built around the -P-O-H..
O=P-O-H
O=P-
In this the hydrogens are asymmetric-i.e. not equally distant from the oxygens. In a recent reinvestigation of some aspects of the P=O frequency-structure relationship [9], we paid
continuity.
attention to the P=O vibrations of acidic compounds. In addition to their other features, these compounds bear on the general problem of oxygen resonance in phosphates and other oxy-anions. This may be illustrated as follows. * Work performed under the auspices of the U.S. Energy Research and Development Administration. 51
5.2
LEONARD
I. KATZIN,
GEORGE
W
tally easier and more universally applicable, is the infrared absorption frequency of the P=O group: does it approach a limiting value related to that of the PO:- ion, or, altematively, transform into an absorption not recognizably that attributable to P=o? We have explored the circumstances under which the organophosphorus acids approach or achieve resonance similar to that of the phosphate ion. In essentially all phosphate structures hitherto worked out the phosphate group has been clearly of a type with varying P-O distances, or the group is described as “distorted,” often by an “axial compression” (e.g., [ 191). As the trisodium phosphate crystal [14] shows essentially the same infrared absorption as does its aqueous solution [13,14], it may be that when this crystal structure is worked out it will show the ideal tetrahedral structure. In the balance of this paper we shall pursue the infrared criterion, and use it to scout for possible resonance situations and manifestations. 1.r. spectra of phosphorus acid salts and O---P-Oresonance The salts of the acids constitute an obvious venue to seek a resonating O===P-O- structure, as they are free of the covalent perturbation of the proton. If the simple salts fail to behave like Na,PO,, then the transition metal ions which are capable of forming chelates might influence equalization of the oxygen% Furthering this search, we have prepared and investigated salts of a number of organophosphorus acids. Table 1 illustrates data for both aryl and alkyl phosphoric acid and phosphinic acid salts, the representatives chosen being those for which we have a reasonable series of salts. A feature of the data is the occurrence of multiple
MASON
and DONALD
F.
PEPPARD
P===Ovibrations, or vibrations not otherwise accountable. One has the possibilities that (a) there is splitting of the P==O vibrations; that (b) the P=O group finds itself in more than one environment, hence multiple vibrational resonances; or that (c) there is more than one chemical substance present. The probable resolution of the question is furnished by the lithium salt of diphenyl phosphinic acid. A small sample, (a), was set aside for a source of spectroscopic sampling from a freshly prepared batch, (b) of lithium diphenyl phosphinate. At this time. the preparation showed the P==O vibration to be at 1164cm-‘, and the P-4 vibration[9] at 1136cm-‘. Sample (a) was remeasured two months later. and the spectrum now showed the P==O at 12OOcm-’ and the P-+ at 1129cm-‘. At a time some 9-t months following initial preparation, (a) showed no further change in spectrum. At this time (b) was-also investigated, and was found apparently to have retained the initial spectrum, with the P=O at 1164cm-‘. This large stock (b) was still essentially anhydrous, while (a) now showed some water bands. It was. therefore, possible that the difference was one between anhydrous and hydrated chemical species. However, lf hours of pumping under vacuum removed all of the water without altering the rest of the spectrum, so this was not the difference. We conclude that there are two crystalline forms, CI, showing the P=O at 1164 and P#J at 1136 cm- I, and, p, showing the P=O at 1200 and P4 at 1129 cm-‘. There are also characteristic differences between the two forms of the lithium salt in the phenyl vibration pattern at 1000-1075 cm-‘, which reflect the difference in the crystal structure. Thus, the probable resolution of the case of the lithium salt is possibility (b). multiple environments of the P=O group due to multiple crystal forms of the same chemical species. The rate of change of crystal structure which must
Table 1. P=O Vibration wavenumber for salts of representative P(V) monobasic acids* Acid
Cation
(C,H,O),P(O)OH
H' LI' Na' I(+ Rb' CS' NH;
k’
Mg*'
ca++ St” lb”
cu++ zn++ La”
Ce'+ Yb’+ ce+
Et.
I271 I259 1233
p& 1263 1284. I266 1285 1289.1276.1241 1225 1271:1227/; 1277 (12561@. 1228 1255.1240,I229
(C,H&p(OIOH
(n-C,H,O)>P(O)OH
1181 1200.1163\\ IIYO.~ =.(I 170)f. Il531: 1196.ll75,@11 1178,~l; ll6i(llS4) II54 1179.II40 a. Il54/( 2180.II70 11541~ (1141) 1155 113x 1140 ll67.1137(?J 1130
*cm-‘.
t Stronger component underlined. ~Parenth~tical values are shoulders rather than overt peaks. $From spectrum of sample (a’) of text. Ij Hydrated salt.
1235 1237.E 1219~1 1247 (l234), 12251, 1228 1228,
(+C,H,l,P(O)OH I144
1167 Il56.1144l~ 1133” II34 1146, 1102
124l.~lJ II77 (11X5).ll671/
(n-C,H , ,I>P(O)OH
I144 II46 I MO,1 1139’1 II261 1127 1124
Ll45.~lI (I 123)li ,114 1138 III9
il2XI III2 II35 IIZZI,
II351
II31 (
Monobasic organophosphorus
0
0.K
I.C 2.C IOC
BC
2c
C
I
I
1200
1000
I 600
3
cm-l
Fig. 1. 1.r. spectra of mineral-oil mulls, in the 13OCMOO cm-’ region, of (A) di-n-octyl phosphinic acid. and (B) its sodium salt (P=O at 1146cm-I). underlie the spectral difference [20] might be catalytically affected by moisture content. A second batch of the lithium salt was prepared 2imonths after the first, and showed the same initial spectrum (1164cm-‘). After seven months both samples (a’) and (b’) were still very dry, and both showed mixtures of the spectra of a and /I (rather than completed change), with alteration having proceeded slightly further in the smaller portion, (a’). The sodium salt is also anhydrous. Like (a’) and (b’), it has the spectrum of the two mixed forms, including the 1138 cm- ’ vibration of the shifted P-4. Unlike the lithium salt, however, neither form has
acids and their salts
53
been found essentially free of the other, and there seems to be no significant spectral change with time. The potassium salt as first formed comes closest to duplicating the 1200 cm- ’ spectrum of the /I form of the lithium salt, but it is hydrated, as are also the rubidium and cesium salts. The appearance of an absorption near 1175 cm -i in these three salts, and its gradual emphasis with increasing Z at the expense of the 1200 cm-’ vibration, may be a function of varying hydration as well as of increasing cationic radius. Tentatively, we extend to the alkaline-earth salts, and to the salts of diphenyl phosphoric acid which show multiple peaks, the assumption that these multiple peaks represent different bonding and crystallization forms (i.e., our possibility (b) above). The theoretical possibility exists of a single structure with two O=P-Oenvironments. Detailed structure determinations are needed to clarify the situation. There is a tendency in each acid series for an approximately terminal low-frequency for P=O vibration to be reached, as cationic charge and complexing strength increase. This is in the direction expected for the resonance equalization of the oxygens. Zinc dibutylphosphinate (Table 1) is reported [21] to consist of linear polymeric molecules, in which single and multiple &P-O bridges alternate in connecting Zn *+ ions along the chain. Quite a few of the salts we have handled have physical characteristics more like organic polymers than crystalline salts, and the drifts of the P=O frequency could be related to detailed differences in how the polymer chains are set up in the solids--e.g., relative frequencies of single and multiple bridgings, number of phosphorus groups in the multiple bridge, cross-bridging, etc. These factors do not necessarily bear directly, however, on the equivalence of the two phosphorusoxygen bonds. It is thought-provoking that in the molecular compound di-p-diphenylphosphinato acetylacetonato chromium(II1). in which two C&-P-O bridges link a pair of Cr(II1) to form a ring [22], the two P-O distances are nor symmetric, and are 1.487 and 1.515 A respectively. We must conclude that though the salts might be expected to favor formation of the resonating O=P-Ostructure in the phosphoorganic acids, we have not been able to identify a case of this configuration. The accessory evidence makes it improbable that the metastable lithium diphenylphosphinate represents this case. and the complexities of the transition element complex compounds make it unlikely that they are the exemplars either. Acids
lacking
P=O
absorption
Most monobasic phosphorus acids, both liquid and solid, show both the absorptions of the P=O vibration, and the i.r. bands of asymmetric hydrogen bonds (see [ 191 for examples). In particular, di-n-butyl
54
LEONARD
I.
KATZIN.
GEORGE
W.
MAXIN
and DONALD
F. PEPPARD
Table 2. Acids lacking both P=O
and hydrogen-bonding absorptions, with frequency wavenumbers for P=O absorption of their sodium salts Acid
Di-n-heptyl phosphinic acid DI-n-octyl phosphinic acid Pentamethyl phosphetane DC(p-bromphenyl) phosphoric acid Di+chlorphenyl) phosphoric aud DC@-naphthyl) phosphoric acid Di-@-tolyl) phosphoric acid Octadecyl hydrogen phosphinic acid Di-(a-t-amylphenyl) phosphoric acid D&-tolylj phosphikc &id Di-(o-hinhenvl) phosphoric acid Di-i2,6_him~;h;iphen) phosphoric acid Di-(2,4,6-trichlorphenyl) phosphoric and
phosphinic acid has the P=O absorption at 1144cm-‘, and in di-n-hexyl phosphinic acid it is at 1142 cm-’ [19]. The spectrum of di-n-octyl phosphinic acid revealed no P==O absorption in our range of observation (above 6OOcm-i), and had only a broad 1550 cm- ’ absorption of the usual hydrogen bond vibration complex. The sodium salt and other salts (last column, Table 1) did show the customary P=O vibration absorption (1146 cm-’ for the Naf salt) (Fig. 1). Di-n-heptyl phosphinic acid gave the duplicate of these observations. This break occurs at the six-carbon and seven-carbon straight-chain acids only. Di-2-ethylhexyl phosphinic acid and di-cyclooctyl phosphinic acids show all the normal vibration absorptions [93. The disappearance of the P=O absorption, and alteration of the typical hydrogen-bond complex, fit our criteria for presumption of oxygen equivalence and resonance. Inasmuch as the acids whose crystal structures demonstrate the customary spiral hydrogen-bond chain and two P-O distances (e.g., [lO-12)) also show both the P=O and hydrogen bond absorptions, the absorption absence indicates a different structure for the heptyl and octyl phosphinic acids, in which the oxygens and hydrogen are presumably symmetrically disposed. The reversion to P=O absorption in the salts would signify greater polarization by the cation and unsymmetrical bonding of the oxygens in those crystals. The literature revealed a unique structure, that of di-p-chlorphenyl phosphoric acid, in which a two-fold axis [23] makes the two oxygens equivalent, even though hydrogen bonding is present. The structure of this compound was actually worked out on instigation of the authors of an earlier infrared spectral study [24] in which the absence of the usual hydrogen bonding absorption was noted and taken to connote symmetric hydrogen bonding. The work [24] identified the 1194cm-’ (P-G-)~ band [9,23] and its weak 1210 cm-’ satellite as “new bands.. . which are probably due to the stretching vibrations of hydrogen-bonded PO groups.. . .” replacing the usual P=O band. We verified the relevant spectra for di-pchlorophenyl phosphoric acid, with complete absence of the
P=O
absorption, Na’ salt (cm-‘) ll4B(LI’) II46 1162. 1150 1278 1279 1267 1278 1169(L1+) 1285 1191.~ 1252 1249. I201 1267
P=O band, and also found that di-p-bromophenyl phosphoric acid completely lacks the P=O band ‘as well as the bands of the symmetric hydrogen bond. The sodium salts show the customary strong P=O absorption, in the same spectral range as the alkali salts of diphenyl phosphoric acid (cf. Table 1). Some acid spectra in which we had hesitated to designate a P=O vibration could now be recognized as having none. We also investigated a number of analogs and relatives of the para-substituted diphenyl phosphates, in addition to the alkyl phosphinic acids which had previously engaged our attention. Table 2 lists acids we have identified so far in whose spectra the P=O stretching vibration and at least the higher-frequency hydrogen bond absorptions are absent. We list also the P=O frequency for an alkali metal salt. generally the sodium salt. As can be seen, the vibrations for the salts of the phosphinic acids are at frequencies comparable to other phosphinic acids, and values for the salts of the substituted-phenyl phosphoric acids fall in the range of the corresponding parent substances (cf. Table 2). The sodium salt of di-p-tolyl phosphinic acid like that of diphenyl phosphinic acid (Table 1) shows two P=O peaks, indicative of similar mixture of crystal types (see above). The salts of the multi-ring acids, di-/I-naphthyl phosphoric acid and di-o-biphenyl to have characteristic phosphoric acid, seem wavenumber values slightly lower than the simpler compounds. Air-dried sodium di-(2,6_dimethylphenyl) phosphate is heavily hydrated, and the P=O absorp Following dehydration by tion is at 1201cm-‘. pumping at high vacuum for a number of hours, the P=O absorption was at 1249 cm-‘. The lithium salt of di-(2,6_dimethylphenyl) phosphoric acid, anhydrous as isolated, has a P=O absorption at 1244 cm-‘. We have stated that the P=O absorption is not seen in the above acids above 600 cm- ‘. It is conceivable that (as might be argued) the absorption may be shifted, broadened and weakened so that it exists in the designated range but has not been recognized. This is an argument to which there is never a satisfactory solution, but certainly any such absorption is not a characteristic P=O absorption such as is seen
Monobasic organophosphorus in the widest variety of P(V) compounds, and even under conditions of metal salt coordination to neutral compounds [30]. The great base strength of the P=O group must be coupled with tremendous stability to be responsible for the difference in behavior from the carboxylic acids, with their M--OH groups, and from certain inorganic oxyacids. Thus, with these, acid double salts of the type MHAz are used to search out symmetric hydrogen bonds (cf. [26]). In the case of the P(V) acids, the strength of the P=O group leads to the usual asymmetric spiral chain hydrogen bonding configuration. Salt formation keeps the O=Padifferentiation. Even with salts of acids which show no P=O absorption, and which presumably (e.g. dioctyl phosphinic acid) or demonstrably (di-(p-chlorphenyl) phosphoric acid) have equivalence of the oxygens, the two oxygens become spectrally differentiated-i.e., the P=O absorption is seen. Also, when we crystallized a mixture of di-(p-chlorphenyl) phosphoric acid and its sodium salt, the absorptions accompanying the usual asymmetric hydrogen bonding and spiral chain formation were seen, whereas they were not apparent in the spectrum of the parent acid, with its equivalent oxygens. This is attributable to the differentiation of P=O and P-Oon salt formation. The acid is then hydrogen-bonded to the basic P=O to give an asymmetric bond with the attendant vibrations. There is therefore good reason to attribute the final influence in producing the symmetric bonding of the pure acid, when it occurs (i.e. di-(p-chlorphenyl) phosphoric acid), to crystal forces. The published spectrum of the fused acid [24] shows both asymmetric hydrogen bonding absorptions and what is probably the P=O. Further, none of the acids in Table 2 is a liquid, and all liquid acids which have been tested [lo] show the “normal” behavior, if not indeed exaggeratedly strong hydrogen bonding absorption. It is only in the two phalophenyl acids that no signs of any of the usual hydrogen bonding transitions can be seen. Both di-n-heptyl and di-n-octyl phosphinic acids have a very broad and strong absorption at about 1550 cm- ’ which is like that seen about 100 cm- ’ higher in the asymmetric hydrogenbonded cases. This is probably some sort of OH distortion vibration in both cases, akin to the water 164Ocm-’ absorption. The peak comes at about 1660 cm- ’ for the octadecyl hydrogen phosphinic acid. It is not entirely missing from the /Snaphthyl or o-biphenyl compounds, the two p-tolyl acids, the p(t-amyl) phenyl compound (at about 17OOcm-‘), the 2,6_dimethylphenyl acid (where it is fairly strong at 16OOcm-‘) or the trichlorphenyl acid. The relation of P=O absorption, coordination modes of the oxygens, and crystal structure, seen in the acids, undoubtedly bears on the salts also. The two forms of lithium diphenyl phosphinate, say, with demonstrated difference in infrared absorption and in crystal structure, can be expected to differ in oxygen coordination environments also. There is not suffi-
acids and their salts
55
cient basis yet to presume that the oxygens in the 1163 cm- ’ absorbing a form necessarily are equivalent, but they are certainly of a different relation than in the 1200 cm-’ fi form. We can see certain frequency shifts of the P=O vibration within a series, as ions get larger, more highly hydrated and more highly charged. The effect of varying hydration of sodium di-(2,6-dimethylphenyl) phosphate on the P=O is further illustration of the same trend (see also sodium dibutylphosphate in Table 1). One can visualize, qualitatively, that increased ionic size might decrease polarization (which would favor differentiation of P=O and P-Oin the anion) and make the environment more uniform, favoring possible resonance equalization. We do not know, however, the essential relations between this parameter and, for example, P==G bond length, which would enable finer analysis to be made from the infrared frequencies. We also know that in some cases the operative factor is alteration in structure of polymeric chains which we have discussed above. Qualitatively, concentration of electrons in the bond between the phosphorus and the oxygen should give a higher-frequency vibration and a less strongly basic oxygen, and presumably a shorter bond length. Dispersion of more of the electron cloud to the oxygen should make it more basic, lower the vibration frequency and lengthen the bond. It is well known that phosphoric acid triesters, which have higher-frequency P=O vibrations [9] are not as strong bases as phosphine oxides, which have lowerfrequency absorptions[9] of the P==O. The P=O length for triphenyl phosphate [ 171 (1294 cm- ‘) is 1.43 A; that for triphenyl phosphine oxide [27] (1201 cm-‘) is 1.46& Diphenyl phosphinic acid [28] (1181cm-‘) has a reported P=O of 1.49& while dibenzyl phosphoric acid, which probably absorbs like dialkyl phosphoric acids, has [lo] 1.47 8, for the P=O distance. The substitution of one pchlorphenyl in triphenyl phosphine oxide [29] raises the P=O to 1.485 A, and a corresponding pbromphenyl substitution, to 1.497 A, while the lengthening to 1.5OA in di-(p-chlorphenyl) phosphoric acid [23] makes the P=O slightly longer than even in diphenyl phosphinic acid, and presumably still closer to the P-OH length. This could facilitate the crystallographic equalization of the O=P-Ooxygens in this solid.
Cation behavior in salts On the inorganic chemical side, the hydration of the salts in these series is an interesting feature. In inorganic salts (chlorides, nitrates, etc.) the general tendency is for lithium salts to be hydrated, even hygroscopic, and for larger alkali metal cations to be of lower hydration or generally anhydrous. The reverse is seen in the organophosphorus salts-lithium forms are generally anhydrous, sodium forms variable, and larger cations increasingly hydrated.
LEONARDI. KATZIN,GEORGEW. MASONand DONALDF. PEPPARD
56
EXPERIMENTAL Spectral measurements were performed on freshly prepared mineral oil mulls, as in our prior report [9]. Preparation of the organic acids was also described there. The alkali, alkaline earth and ammonium salts (except for Mg salts) were prepared by neutralization of a methanol solution of the organophosphorus acids to pH 7 with an aqueous solution of the respective metal or ammonium hydroxide, followed by evaporation to dryness at room temperature and pressure. Due to the low solubility of Mg(OH),, aqueous MgCl, solution was added to a methanol solution of the sodium salts of the organophosphorus acids. The resulting precipitate was filtered, washed and dried at room temperature and pressure. The Agf, Cu’+, Zn’+, La3+, Ce3+, Yb’+ and Ce“+ salts were prepared by adding a methanol solution of the metal chlorides (nitrate of Ag+) to the sodium salts of the organophosphorus acids dissolved in 50% methanol-50% water solution. The resulting precipitates were filtered, washed with 50” methanol-50% water solution and air dried at room temperature and pressure. REFERENCES [l] L. J. BELLAMY, The Infrared Spectra of Complex Molecules, 2nd edn. (1958); Advances in Infrared Group Frequencies. Methuen, London (1968). [Z] L. C. THOMAS.Interpretation of the Infrared Spectra of Organophosphorus Compounds. Heyden, London (1974). [3] D. F. PEPPARDand G. W. MASON,Nucl. Sci. Eng. 16, 382 (1963). [4] G. M. KOSOLAP~FF and J. S. POWELL,J. Chem. Sot. 3535, (1950).; D. F. PEPPARD,J. R. FERRARO andG. W. MASON, J. Inorg. Nucl. Chem. 4, 371 (1957; D. DYRSSEN,Acta Chem. Stand. 11, 1771 (1958). [S] L. W. DAASCHand D. C. SMITH, Anal. Chem. 23, 853 (1951). [6] L. J. BELLAMYand L. BEECHER,J. Chem. Sot. 1701 (1952). [7] D. F. PEPPARDand J. R. FERRARO,J. Inorg. Nucl. Chem. 10, 275 (1959). [8] S. DETONIand D. HADZI, Spectrochim. Acta 20, 949 (1964).
[9] L. I. KATZIN,G. W. MASON and D. F. PEPPARD,in press. [lo] J. D. DUNITZ and J. S. ROLLETT,Acta Cryst. 9, 327 (1956). [ll] E. ALVERand H. M. KJBJE, Actu Chem. Stand. 23, 1101 (1969). [12] F. GIORDANOand A. RIPAMONTI,Acta Cryst. 22, 678 (1967). [13] A. C. CHAPMAN and L. E. THIRLWELL,Spectrochim. Acta 20, 937 (1964). [14] F. A. MILLER and C. H. WILKINS,Anal. Chem. 24, 1253 (1952). [IS] L. PAULING,The Nature of the Chemical Bond 3rd edn. Cornell University Press, Ithaca, New York (1960). [16] H. C. J. DE DECKERand C. H. MACGILLAVRY,Rec. Trau. Chim. 60, 153 (1941). [17] W. 0. DAVIESand E. STANLEY,Acta Cryst. 15, 1092 (1962). [18] G. E. BACONand R. S. PEASE,Proc. Roy. Sot. (Lendon) A220, 397 (1953). [I93 G. E. BACON and R. S. PEASE,Proc. Roy. Sot. (Lendon) A 230, 359 (1955). [20] X-ray powder patterns of the two forms have been investigated with the cooperation of Mrs. Elizabeth Sherry, and have been shown to be different. [21] V. GIANCOTTI,F. GIORDANO,L. RANDACCIOand A. RIPAMONTI, J. Chem. Sot. A 757 (1968). [22] C. E. WILKESand R. A. JACOBSEN,Inorg. Chem. 4, 99 (1965). [23] M. CALLERIand J. C. SPEAKMAN, Acta Cryst. 17, 1097 (1964). [24] D. HADZI and A. NOVAK,Proc. Chem. Sot. (London) 241 (1960). [25] L. J. BELLAMYand L. BEECHER,J. Chem. SOC. 475. 1701 (1952). [26] J. C. SPEAKMAN,Struct. Bond. 12, 141 (1972). [27] G. BANWLI, G. BARTOMZZO,D. A. CLEMENTE,V. CROATTUand C. PANAT~ONI,J. Chem. Sot. A 2778 (1970). [28] T. -T. LIANGand K. -C. CHIAO, Hua Hsueh Hsueh Poa 31, 155 (1965). [29] W. DREI~SIGand K. PLIETH, Acta Cryst. 27B, 1140, 1146 (1971). [30] L. I. KATZIN, J. Inorg. Nucl. Chem. 20, 300 (1961).