The characteristics of several new metal phosphinate complexes

J. inorg,nucl.Chem.,1968,Vol.30. pp. 1299to 1308. PergamonPress. Printedin Great Britain

THE

CHARACTERISTICS OF SEVERAL NEW PHOSPHINATE COMPLEXES*

METAL

JAMES J. PITTS, MARTIN A. ROBINSON A N D S A M U E L I. TROTZ Olin Mathieson Research Center, Chemicals Division, New Haven, Connecticut (Received 18 September 1967) Abstract-- Four series of metal phosphinate complexes based on the phosphinic acids, (C6H~)zP(O)OH, CnHs(H)P(O)OH, (CnHs)2P(S)OH, and (C6Hs)2P(S)SH were synthesized and characterized. Although the primary interest was in the complexes of aluminum(l II), nickel(II) and cadmium(I I), several other metal complexes were synthesized for comparison purposes. Structural features are discussed, relating the formation of chelated monomer or ligand bridged polymer to the size of the metal cation and to the sulfur or oxygen moieties of the ligands. Hydrolytic stabilities were successfully correlated to the "hard-soft" concept of acid-base theory. The predicted general relationship seems to exist between the thermal stability and the ionic character of the metal to ligand bonding.

INTRODUCTION

IN THE interest of evaluating various ligands as potential bridging links in polymers with inorganic backbones, we have synthesized a number of metal complexes of four phosphinic acids, (C6Hs)zP(O)OH, C6H~(H)P(O)OH, (C~H~)2P(S)OH, and (CnHs)2P(S)SH, and attempted to correlate their bonding and structures to chemical and thermal stability. The complexes of primary interest are the twelve derived from aluminum(Ill), nickel(II) and cadmium(II), metals representing "hard", "borderline", and "soft" acids, respectively, on the basis of the now familiar acid-base theory of Pearson [ 1]. Several sodium, chromium, magnesium and calcium derivatives of the aforementioned ligands have also been prepared for comparison purposes. The ligands may. also be considered to cover the range of "hard" to "soft" bases from the "dioxygen" to the "dithio", respectively, with the "monothio" being borderline. Current literature represents considerable interest in various metal phosphinate complexes, attention centering around the high thermal stability and probable polymeric nature of these materials. There are several references to metal diphenylphosphinates [2-7], monophenylphosphinates *Presented in part at the 153rd National ACS Meeting, Miami Beach, Fla., April 1967; Abstracts, L 80. 1. R. G. Pearson, J. Am. chem. Soc. 85, 3533 (1963); Science 151, 172 (1966); Chemistry in Britain 3, 103 (1967). 2. W.C. Drinkard and G, M. Kosolapoff, J. Am. chem. Soc. 74, 5520 (1952). 3. G . E . Coates and D. S. Golightly,J. chem. Soc. 2523 (1962). 4. B. P. Block, E. Roth, C. Schaumann and L. Ocone, inorg. Chem. 1,860 (1962). 5. B. P. Block, S. H. Rose, C. W. Schaumann, E. S. Roth and'J. Simkin, J. Am. chem. Soc. 84, 3200 (1962). 6. S.H. Rose and B. P. Block, J. Poly. Sci. A, 4, 573 (1966); 4, 583 (1966). 7. R.A. Sutton and J. Wood, Brit. Pat. No. 1,018,456; Chem. Abstr. 64, 17748f(1966). 1299

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J A M E S J. PITTS, M A R T I N A. R O B I N S O N and S A M U E L I. T R O T Z

[8, 9] and diphenyldithiophosphinates [ 10-14]. This paper describes the synthesis and characterization of several previously unreported metal phosphinates, presents the basis for hydrolytic stability predictions as related to the "hard"-"soft" theory of acids and bases, and confirms the expected relationship between thermal stability and ionic character of the metal-ligand bond. EXPERIMENTAL Reagents Diphenylphosphinic acid. K and K Laboratories, Inc. (Used as received.) Phenylphosphinic acid. Aldrich Chemical Co., Inc. (Used as received.) Diphenylphosphinodithoic acid. The Lubrizol Corporation. (Technical grade, recrystallized from isopropyi alcohol.) Methyl diphenylphosphinodithioate. Prepared by the method of Hopkins and Vogel [ 15]. Diphenylphosphinothioic acid. This ligand was prepared by refluxing methyl diphenylphosphinodithioate in excess 10 per cent sodium hydroxide for 16-19 hr. The resulting solution was then cooled in an ice bath and acidified with excess concentrated hydrochloric acid to effect the precipitation of the white monothio acid. Crude yields on air dried material ranged from 67-96 per cent theory. The acid was purified by recrystalhzation from toluene. The melting point and analysis of this acid is listed in Table 1 as are all the other reagents mentioned above. All metal salts used were standard reagent grade chemicals. Synthesis Since many of these metal phosphinate complexes were prepared by the same procedures, only one example of each will be included. Procedure A. The sodium salt of diphenylphosphinodithioic acid was obtained by neutralization with 10 per cent sodium hydroxide. A white crystalline solid was isolated upon evaporation of the water and was dried in vacuo over P~O5 overnight yielding the hemihydrate. Further drying at 120° overnight gave the anhydrous salt. Procedure B. Diphenylphosphinothioic acid (3.02 g, 12-9 mmole) in 150 ml water was neutralized with 4.7 ml (12.9 mmole) 10% NaOH. The resulting solution was slowly added with stirring to AIz(SO4)a.18H20 (1.43 g, 2.15 mmole) dissolved in 100 ml of water; the immediate formation of a white precipitate was observed. The white solid was filtered, washed with small portions of water and air dried, yielding 1.46 g (47 per cent yield) of product which was further dried in vacuo over P2Os overnight. The infrared spectrum of the aluminum complex showed that it was hydrated (OH bands at 3400 cm -~ and 3175 cm-~), and on heating above 100 °, water vapor was evolved. Procedure C. Cadmium acetate tetrahydrate (1.94g, 7.28 mmole) was added to diphenylphosphinodithioic acid (3.66 g, 14-58 mmole) dissolved in 400 ml of benzene. The mixture was refluxed for 1 hr; a white precipitate formed. The resulting mixture, after cooling to room temperature, was filtered and the white solid washed with fresh benzene and petroleum ether; yield, 3.83 g(86 per cent) of air dried product. The i.r. spectrum of the cadmium complex showed no unreacted acetate or free acid. The metal complex was purified by recrystallization from DMF. Procedure D. Triethyl aluminum (0.55 g, 4.82 mmole) was weighed out and dissolved in 100 ml of benzene (dried over LiAIH4) in a nitrogen-flushed dry box. The solution was then slowly added with stirring to diphenylphosphinic acid (3.15 g, 14.43 mmole) dissolved in 300 ml of dry benzene, under a 8. J.E. Banks and D. A. Skoog, Analyt. Chem. 29, 109 (1957). 9. A.K. Mukherji, Analytica. chim. Acta 30, 591 (1964). 10. L. Malatesta and R. Pizzotti, Gazz. chim. ital. 76, 167 (1946); 77, 509 (1947); Chem. Abstr. 41, 2012a (1947); 42,541 li (1948). 1 I. W . G . Craig, U.S. Pat. No. 2,809,979; Chem.Abstr. 52, 3859f(1958). 12. W. Kuchen, J. Metten and A. Judat, Chem. Bet. 97, 2306 (1964). 13. R. N. Mukherjee, V. V. Krishna Rao andJ. Gupta, lndianJ. Chem. 4, 209 (1966); Chem. Abstr. 65, 8315c (1966). 14. R. N. Mukherjee, A. Y. Sonsale andJ. Gupta, lndianJ. Chem. 4, 500 (1966). 15. T. R. Hopkins and P. W. Vogel, J. Am. chem. Soc. 78, 4447 (I 956).

New metal phosphinate complexes

1301

stream of nitrogen. A white precipitate immediately appeared, and the resulting solution was heated to reflux to insure complete evolution of ethane. After cooling, the solution was filtered, the solid washed with fresh benzene and chloroform, and dried at I l0 ° for 1 hr. The yield of aluminum diphenylphosphinate was 3.12 g (96 per cent yield). The i.r. spectrum of the aluminum complex showed no unreacted free acid. Procedure E. N ickel(ous) carbonate [16,17] (1-70 g, 4.52 mmole) was ground in a mortar with diphenylphosphinic acid (5.94 g, 27.2 mmole) to a fine homogeneous powder and the mixture was heated to 250 ° for 2 hr. After cooling, the fused, lavender-colored mass was broken up and again ground to a fine powder. After reheating to 250 ° for another 2 hr period, the slightly caked mass was cooled and ground to a fine homogeneous lavender powder. A 6-48 g yield (97 per cent) of product was collected. The i.r. spectrum of the nickel diphenylphosphinate complex showed no unreacted carbonate or free acid. The nickel complex was found to be hygroscopic, changing from lavender to pale green in moist air. The complex was readily soluble in water, and on evaporation a pale green crystalline solid was obtained. The i.r. spectrum of this material showed the presence of OH at 3630 cm-l(s) and 3225 cm-l(vs, broad). On heating, the pale green solid evolved water vapor above 100°, and reverted to the lavender color above 140°. The properties, analysis and preparatory procedures for all the metal phosphinate complexes are listed in Table I. Conductivity measurements. Molar conductances were measured using an Industrial Instruments, Inc., Model RC-16B2 conductivity bridge and a cell with a constant of 0-500 cm -1. The measurements were made at 25 ° using 10-aM solutions and a bridge frequency of 1000 cycles/sec (See Table 1). Magnetic susceptibility measurements. The magnetic susceptibilities were obtained at room temperature (22°) by the Gouy method using ferrous ammonium sulfate hexahydrate and nickel(ll) chloride hexahydrate as standards. Diamagnetic corrections were made for the ligands and anions [22, 23], so the reported values are the moments attributable to the metal ions. (See Table 2). Spectra. The i.r. spectra on the solids were run as potassium bromide pellets on the Perkin-Elmer Model 521 spectrophotometer. (See Table 3). All spectra in the visible region were obtained on a Cary Model 14 recording spectrophotometer as Nujol mulls. (See Table 2). Thermal stability measurements. The thermal stabilities of most of the metal phosphinate complexes were obtained on a Perkin-Elmer Model DSC-1 Differential Scanning Calorimeter equipped with a Texas Instruments, Inc. Recti/riter chart recorder. The samples were scanned from 40 ° to 490° at 10 deg/min in a nitrogen atmosphere. At intervals of 100 degrees the samples were removed and weighed in order that percentage wt. loss at 100°, 200°, 300°, 400° and 490° could be measured. As comparison, several complexes were run under similar conditions on a duPont Model 950 Thermogravimetric Analyzer. The thermal stabilities at 490° of the various complexes are listed in Table 4. Molecular weight determinations. The molecular weights of several of the chelated complexes were obtained on a Mechrolab Model 302 Osmometer and the results are shown in Table I. Solvents used included benzene, o-dichlorobenzene and dimethylformamide, the latter yielding non-conducting solutions.

DISCUSSION

There are a number of bonding possibilities to consider for these phosphinate complexes as represented by Structures (i), (ii), (iii) and (iv), and it is important to be aware of structural differences and similarities when comparing chemical and physical properties. It is obvious that all four ligands have the capacity to complex in a bidentate manner. When co-ordination does occur through the oxygen and/or sulfur atoms attached to the phosphorus site, Structure (i) or (ii) results. If 16. R. E. Kirk and D. F. Othmer, Encyclopedia of Chemical Technology, Vol. 9, p. 290. Interscience, New York (1952). 17. The Merck Index of Chemicals and Drugs, 7th Edn, p. 717. Merck, Rahway, N. J. (1960).

1302

JAMES J. PI'ITS, MARTIN A. ROBINSON and S A M U E L I. TROTZ

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I0a × M (cor.)*

Complex

Magnetic moment B.M.

[(CtHs)2P(O)O]zNi

5665

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5179

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Band position (cm -~) 8230 13100 9000 13900 15400 25000 -7250 13300 23000 8530 13600 14900 25000 7800 13600 13700 180OO

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Table 3. Infra-red absorbance

M+"--[O(O)P(CsHs)dn M+'L--[O(O)P(H)CsHs)]n v(P = O) v(P = O) cm-I cm-i Na + AI +3 Mg +2 H+

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658 648 642 639 635 635 629

Reference standards: (C~HD3P ~ Oa,(P = O) = 1190-95 cm -1 [39, 40] (CtHs)2P(O)CI, u(P = O) = 1236 cm -1 [40]. (CsHD~P(S)CI, v(P = S) = 661 cm-~[25]. (CtH~)2P(S)O(S)P(CtHs)2, v(P = S) = 658 cm -1 [41 ].

the ligand chelates, the resulting complex will be a monomer containing fourmembered rings (i). In contrast, bridging will give rise to a polymeric species derived from eight-membered cycles linked through the metal atoms (ii). The former is favoured when A and B are sulphur atoms (due to their large size and 22. B. N . F i g g i s a n d J. L e w i s in Modern Coordination Chemistry ( E d i t e d b y J. L e w i s a n d R. G . W i l k i n s ) , p p . 3,03, 4 1 5 . I n t e r s c i e n c e , N e w Y o r k ( 1 9 6 0 ) . 2 3 . P . W . S e l w o o d , Magnetochemistry, p p . 9 2 - 9 3 . l n t e r s e i e n c e , N e w Y o r k ( 1 9 5 6 ) .

1305

N e w metal phosphinate complexes Table 4. Thermal stability Total % wt. loss at 490°C in N.2 Atm. Complex

DSC

TGA

[(C6Hs)zP(O)O]2Mg [(CsHs)2P(O)O]zAI (C6Hs)2P(O)ONa [(C6Hs)zP(O)O]2Ni [(C6Hs)zP(O)O]2Ca [(CnHs)zP(O)O]3Cr [(CsHs)zP(O)O]2Cd

0"8 4"3, 6"3 7'6 13'1 13'3 16'5 26"2

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18"1 21 '6 23 "3

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41 "5

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66'0 69"5

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JAMES J. PITTS, M A R T I N A. R O B I N S O N and S A M U E L I. T R O T Z

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Several authors [3-7, 24-28] have postulated that the phosphinate complexes involve double ligand bridges, similar to the proposed structures for the complex metal dialkylphosphates [29, 30]. Crystallographic proof of such eight-membered, phosphinate rings has now been firmly established [3 l, 32]. X-ray work has also recently revealed single-triple alternating ligand bridges as illustrated in structure IV[33]. In the case of metal dithiophosphinates and dithiophosphates, there is evidence that chelation occurs to form monomers with four-membered rings [25, 34, 35] and in some cases, dimers featuring both bridged and chelated ligands [12, 36] (Structure (iii)). This has been shown to be the case also with monothiophosphinates [37]. In this study, the dithiophosphinates of aluminum, nickel and cadmium were all found to be monomeric, (See Table 1) and are presumed to be chelated. Characteristic infra-red absorptions to be discussed later substantiates the coordination. The "dioxygen"-containing ligands contrast with the dithio-, in as much as they behave as three atom bridging groups. Diphenylphosphinic acid has been regarded as a three atom bridging group in a lengthy series of co-ordination polymers [3-7, 24-28]. The new metal phosphinates reported herein all appear to conform to this bridged structure. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37.

H. E. Podall and T. L. Iapalucci, J. Polym. Sci. B, 1,457 (1963). G. E. Coates and R. N. Mukherjee, J. chem. Soc. 1295 (1964). I . G . M . Campbell, G. W. A. Fowles and L. A. Nixon, J. chem. Soc. 1389 (1964). V. Crescenzi, V. Giancotti and A. Ripamonti,J.Am. chem. Soc. 87, 39I (1965). D. Delman, J. Kelley, J. Mironov and B. B. Simms, J. Polym. Sci. A, 4, 1277 (1966). C. F. Bags, R. A. Zingaro and C. F. Coleman, d. phys. Chem. 129 (1958). C.J. Hardy and D. Scargill, J. inorg, nucl. Chem. 17, 337 (1961). J. Danielsen and S. E. Rasmussen, Acta chem. scand. 17, 1971 (1963). C. E. Wilkes and R. A. Jacobson, lnorg. Chem. 4, 99 (1965). F. Giordano, L. Randaccio and A. Ripamonti, Chem. Comm. 19 (1967). W. Kuchen and A. Judat, Angew. Chem. Intern. Edn, 2,554 (1963). C. K. Jorgensen, Inorganic Complexes, pp. 133-135. Academic Press, New York (1963). W. Kuchen andJ. Metten, Angew. Chem. 72, 584 (1960). W. Kuchen and H. Hertel, Angew. Chem. Intern. Edn. 6, 175 (1967).

New metal phosphinate complexes

1307

Monothiophosphinic acid presents an intermediate situation between the chelating dithio ligands and the bridging "dioxygen" ligands. Coates and Mukherjee[25] report that the diphenylmonothiophosphinate derivative of dimethyl aluminum is a ligand bridged dimer. The aluminum trisdiphenylmonothiophosphinate, as well as the nickel analog, both of which are reported herein, appear to be polymeric, bridged structures. The cadmium analog, on the other hand, was found to be monomeric and presumably chelated in the fashion described for the dithiophosphinates. The large size of the cadmium cation undoubtedly accounts for the formation of the four-membered rings. The series of nickel complexes lent itself to stereochemical elucidation primarily through the interpretation of magnetic susceptibility data and visible spectra. As shown in Table 2, the diphenyl and mono-phenylphosphinate complexes of nickel, as well as a monothiophosphinate analog, exhibit magnetic and spectral properties characteristic of tetrahedral stereochemistry. In contrast, the dithiophosphinate complex of nickel is diamagnetic and therefore square planar, as substantiated by the visible spectrum. Its behavior is that which would be predicted, since the square planar and tetrahedral structures of a complex differ chiefly in the angles subtended by the ligand atoms, and this in turn is dependent upon size and orbital arrangement. The combination of two large sulphur atoms in the last ligand is sufficient to permit square planar configuration about the nickel.

Infra-red spectral interpretations The degree of polymerization, as indicated by solubility and/or molecular weight measurements, is practically sufficient to indicate whether the ligand is chelating or bridging; infra-red spectra confirm the important bonding feature; namely, co-ordination to the metal of the oxygen or sulphur atom double bonded to the phosphorus. Co-ordination of the electron pair from the oxygen or sulphur to a metal results in a decrease in the P = O or P = S double-bond character; the greater the degree of co-ordination, the greater the shift of the characteristic vibration frequency to a lower value. On the basis of force constants of the M - O bonds, as described by Nakamoto [38], one might predict the following order for degree of shift of the P = O or P = S absorptions: Co > Ni > Cr > AI. The results shown in Table 3 are generally consistent with this predicted order, and the positions of the Na ÷, Mg ÷2, Ca +.' and Cd +2 complexes are not unexpected. The P = O shift in the free acid (H ÷) demonstrates hydrogen bonding in the free acid. The secondary absorptions listed for several complexes are very weak (being shoulders in some cases). These are believed to be indicative of terminal, unco-ordinated end-groups in the bridged polymers. Our assignments of the P = S absorptions are consistent with those previously assigned for comparable compounds [42] and follow the predicted order. 38. K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compounds, pp. 216-220. Wiley, New York (1963). 39. F. A. Cotton, R. D. Barnes and E. Bannister, J. chem. Soc. 2199 (1960). 40. L.W. Daasch and D. C. Smith, Analyt. Chem. 23, 853 (1951). 41. A. Schmidpeter and H. Groeger, Z. anorg, allgem. Chem. 345, 106 (1966). 42. S. Husebye,Acta chem. scan& 19, 774 (1965).

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.lAMES J. PITI'S, M A R T I N A. R O B I N S O N and S A M U E L I. T R O T Z

Hydrolytic and thermal stability The ease of ligand displacements in these complexes can be correlated to Pearson's Modern Theory of Acids and Bases. One must qualify any comparison with the reminder that we are dealing with complexes with considerably different structures; namely, crosslinked and linear polymers, and chelated monomers. Furthermore, this study deals with displacement of bidentate ligands with monodentate water. On a qualitative basis, the comparisons are nonetheless interesting. The A1+a is representative of the class (a), "Hard Acids", Cd ÷z is classified as a "Soft", class (b), acid, and Ni +2 a borderline case between the two extremes. The phosphinic acid ligands are classed as "Hard Bases", the dithiophosphinic acid is considered "Soft" and the monothio, borderline. In the series of aluminum complexes, the decreasing order of stability toward ligand replacement by water molecules was predicted to be (CaHs)2P(O)OC6Hs(H)P(O)O- > (C6Hs)2P(S)O- > (C6Hs)2P(S)S-. These expectations were realized as the [(C6Hs)2P(S)S]aAI complex was quickly decomposed in moist air to the free acid, with the three AI-S bonds being cleaved. The monothiophosphinate complex is obtained from aqueous solution as [(C6Hs)2P(S)O]3AI.3H20 with unbonded P = S as shown by its infrared spectrum, the aluminum coordination sphere thus being filled by water molecules. Finally, both the diphenyl and monophenylphosphinates are unaffected by water for prolonged contact times. As expected, the order in the cadmium series is reversed, with the nickel series closely paralleling the behavior of cadmium. The "hard-hard" Ca +2 and Mg ÷2phosphinates do not equate as well as aluminum, since they are soluble in water and act like 1 : 1 electrolytes (See Table 1). Finally, the thermal stability is related, as expected, to the polarizability of the metal-ligand bond. As shown in Table 4, the more ionic in character, the more thermally stable is the compound in general. The observed order of decreasing thermal stability is thus phosphinate > monothio > dithiophosphinate. Acknowledgements-The authors wish to thank the following members of the Olin Central Analytical Department: J. Giunta and F. Pards, microanalytical; Mrs. I. Cowern and W. H, Harple, infra-red; J. E. Schingh, D.S.C. andJ. J. Kane, T.G.A. thermal stability studies. This work was supported in part by the Office of Naval Research.