Hexamethylphosphoramide complexes of niobium(V) and tantalum(V) chlorides and oxychlorides

J. inorg,nud. Chem., 1972,Vol. 34, pp, 2665-2668. PergamonPress. Printedin Great Britain

NOTES Hexamethylphosphoramide complexes of niobium(V) and tantalum(V) chlorides and o x y c h l o r i d e s (Received 1 July 1971)

IN A PREVIOUS study, Brown et al. prepared a hexamethylphosphoramide (HMPA), OP(N(CH3~2)3, complex of niobium pentachloride [ 1] from the reaction of equimolar amounts of these compounds in a mixture of methyl cyanide-methylene dichloride. Brown was able to show that in a CHaCN-CH2C12 mixture, even in the presence of a large excess of hexamethylphosphoramide, only the 1 : l adduct NbCls, OP(N(CH3)2)3 is formed. It has now been shown that H M P A complexes of both M Ci5 and MOC13, (M = Nb, Ta) exist and the synthesis, i.r. spectra, X-ray powder diffraction data and some physical properties of these compounds are reported. EXPERIMENTAL Reagents

Commercially available niobium and tantalum pentachlorides (Fluka, puriss, grade) were vacuumsublimed prior to use. Niobium oxytrichloride was prepared by the action of oxygen on the pentachloride as described previously [2, 3]. Samples of hexamethylphosphoramide and octamethylpyrophosphoramide (OMPA) were kindly supplied by the Compagnie Fran~:aise de l'Azote and the Murphy Chemical Company Ltd. A nalysis Niobium(V) and tantalum(V) were weighed as the pentoxides after ignition of the hydrous oxide formed by hydrolysis of the compounds. Chloride was determined by potentiometric titration against standard silver nitrate. Carbon, hydrogen, nitrogen and phosphorus analyses were performed by microanalytical methods. Physical measurements X-ray powder diffraction photographs were obtained using a Debye Scherrer 229, 2 mm camera and Cu Kc~ radiation. I.R. spectra in the range 200-4000 cm -~ were recorded with a Beckman I.R. 12 spectrometer using Nujol mulls mounted between caesium iodide plates. Ebullioscopic measurements were carried out using two Cottrell-Washburn cells [4] and a double electronic thermometer (Hewlett-Packard, precision 1/100°C). Tonometric data were obtained with a Hewlett-Packard Mechrolab osmometer using chloroform solutions at 37°C. Synthesis

TaCIs, HMPA. Yellow crystals of TaCIs, H M P A are obtained by the addition of 2,60 g of hexamethylphosphoramide to 2,55 g of tantalum pentachloride in 30 ml of a 50% CH~CN-CH2CL2 mixture at room temperature. The crystals are insoluble in chloroform, carbon tetrachloride and benzene, and soluble in acetonitrile and dichloromethane from which the compound can be recrystallized. Amdysis: Found (%) Ta: 33,9; CI: 32,7; P: 5,7; C: 13,2; H: 3,4; N: 7,7; Calc. (%)Ta: 33,7; CI: 33: P: 5,8; C: 13,4; H: 3,4;N: 7,8. 1. 2. 3. 4.

D. Brown, J. F. Easey and J. G. H. du Preez, J. chem. Soc. (A), 258 (1966). F. Fairbrother, A. H. Cowley and N. Scott, J. less-common Metals 1,206 (1959). F. Fairbrother, The Chemistry o f Niobium and Tantalum. Elsevier. New York (1967). J. M. Wilson, R. J. Newcombe, /',. R. Denaro and R. M. W. Rickett, Experiments in Physical Chemistry, p. 17, Pergamon Press, Oxford (1962). 2665

2666

Notes

NbOC13, 2HMPA. A threefold excess (7,35 g) of OP(N(CHa)~)a was added to niobium pentachloride (3,70 g) in 50 ml carbon tetrachloride with vigorous stirring and the resulting suspension was refiuxed for 6 hr under dry nitrogen. The orange-red crystals were filtered, washed with CC14 and dried in vacuo at room temperature. The X-ray powder pattern of this product showed that another compound was present in addition to NbCIs, HMPA. In contrast to NbC15, HMPA, the new compound proved to be readily soluble in chloroform, thus permitting its separation by continuous extraction with this solvent in a Soxhlet apparatus. The chloroform solution thus obtained was concentrated in vacuum until crystals began to appear; further crystallisation was induced by cooling for several hours at - 5°C or by adding dry benzene. Several recrystallisations are necessary in order to eliminate traces of NbCIs, HMPA in the white crystalline NbOC13, 2HMPA adduct. The yield is 70 per cent. This compound is not air-stable, and must be handled and stored in a dry-box. It was also prepared by direct action of H M P A (1,55 g) on NbOC13 (1,80 g) in carbon tetrachloride (or chloroform) at room temperature. Analysis: Found (%): Nb: 16,1; CI: 18,3; P: 10,6; C: 25,3; H: 6,2; N: 14,5; Calc. (%): Nb: 16,2; CI: 18,5; P: 10,8; C: 25,1 ; H: 6,3; N: 14,6. TaOCI3, 2HMPA. The same procedure is employed as in the case of NbOCI3, 2HMPA using 2,85 g of TaCI5 and 4,20 g of hexamethylphosphoramide. In contrast to NbOCI3, 2HMPA, extraction with hot chloroform yielded only 10% of TaOCI3, 2HMPA. Even after addition of benzene, the chloroform solution had to be stored for several weeks at - 10°C to induce crystallisation. The pale yellow crystals are very sensitive to moisture and must be kept in sealed vessels. Analysis: Found (%): Ta: 27,5; CI: 16,0; P: 9,2; C: 21,6; H: 5,5; N: 12,5; Calc. (%): Ta: 27,3; CI: 16,1 ; P: 9,4; C: 21,8; H: 5,5; N: 12,7. NbOC13, OMPA. 1,35 g of NbOCl3 were suspended in 20 ml of dry carbon tetrachloride and 3,60 g of octamethylpyrophosphoramide OP(N(CH3)~)2-O-(N(CH3)2)~PO were slowly added with stirring at room temperature under dry nitrogen. Stirring was stopped when the white suspension became homogeneous. The fine powder was then filtered, washed with dry CC14 and dried in vacuo. NbOCI3, OMPA can be recrystallized from acetonitrile. In contrast to the other oxychloro-complexes, it is insoluble in chloroform. Analysis: Found (%): Nb: 18,6; CI: 19,3; P: 12,1; C: 19,3; H: 4,9; N: 11,3. Calc. (%): Nb: 18,5; CI: 21,2; P: 12,3: C: 19,2; H: 4,8; N: 11,2. RESULTS Formation of only a 1 : 1 adduct NbCIs, H M P A in the mixed solvent CHaCN/CH2CI2 may be explained by initial formation of the stable monomeric six-coordinate NbCIs, CH3CN complex. This nitrile adduct undergoes exchange with one molecule of hexamethylphosphoramide and leads to the stable NbCIs, H M P A complex. This compound did not react further with H M P A (even in excess). However, if the reaction is carried out in an inert solvent not containing methyl cyanide (e.g. CHCI3 or CC14), direct action of H M P A on the dimeric pentachloride leads to substitution of two chlorine atoms by one atom of oxygen, accompanied by coordination of two molecules of ligand. This oxygen atom is evidently supplied by a molecule of HMPA, since the white compound C12P(NMe2)3 was recrystallized from the mother liquor. Analysis: Found (%): CI: 29,9; C: 30,1; H: 7,2; N: 17,2; Calc. (%): CI: 30,3; C: 30,7; H: 7,7; N: 17,9. X-RAY DATA Partial X-ray diffraction data of NbCIs, HMPA have been reported previously[l]. Comparison of the powder patterns of the niobium and the tantalum H M P A 1 : 1 adducts showed that these compounds are isostructural. The oxychlorocomplexes NbOCI3, 2HMPA and TaOCI3, 2HMPA gave satisfactory powder photographs. They are isostructural and partial data are given in Table 1.

i.R. spectra I.R. spectral data for the phosphoramide complexes are given in Table 2.

Notes

2667

Table 1. Partial X-ray diffraction data for hexamethylphosphoramide-oxychlorocomplexes NbOCI3, 2HMPA Sin2 0 obs. I est. -

0,0041

W

0,0060 0,0086 0,0094 0,0098 0,0109 0,0125 0,0130 0,0154 0,0217 0,0235 0,0265 0,0277 0,0289 0,0322

M S S M S S M W W W W W W M

TaOCI.~,2HMPA Sin2 0 obs. 1 est. 0,0026 0,0035

W W

0,0052 0,0061 0,0088 0,0095

W W S S

0,0110 0,0126 0,0133 0,0155 0,0218

S S M W W

0,0266 0,0279

W W

0,0326

W

Table 2. I.R. vibrations of the phosphoramide complexes Compound

v ( P ~ O ) obs. cm -~

A v(P~-----O)cm -1

HMPA OMPA TaCIt, HMPA NbOC13, 2HMPA TaOC13, 2HMPA NbOCI:j, OMPA

1208 1238 972 1162 1180 1171

236 46 28 67

Considerable shifts in the P~-----0stretching frequency have been observed in the 1:1 HMPA adducts of niobium and tantalum pentachlorides; these shifts are due to the decrease in the phosphorusoxygen bond order as a result of coordination. The shifts are much lower in the oxychlorocomplexes since niobium(V) and tantalum(V) oxychlorides are less electrophylic than the pentachlorides. Further, the i.r. spectra of the niobium and tantalum-HMPA oxychloro-complexes have strong peaks at 940 cm -1 and 936 cm -1 respectively, which can be assigned to Nb~------Oand Ta~------Ostretching modes. The existence of this Nb~--------Obond and the absence of the N b - O - N b vibration in NbOCI:t, 2 H M P A implies that this compound is monomeric, in contrast to the polymeric structure of NbOCI3 itself. Tonometric and ebulliometric measures with various solutions of NbOCI3, 2HMPA indicated a molecular weight in chloroform corresponding to monomeric NbOCI:I, 2HMPA. Precision was of the order of 7 per cent and 5 per cent respectively. Appreciable quantities of complex were required in order to observe the boiling point elevation with sufficient precision. We were unable to carry out mol. wt. measurements with TaOCI:~, 2HMPA because of its low yield. However, the similarity with NbOCI:~, 2HMPA may suggest that this compound is also monomeric.

2668

Notes

Bidendate oxychlorocomplexes of niobium(V) The fact that we observe addition of two molecules of OP(N(CHa)~)a to NbOCla by oxygen bonding involving the P = O group, suggests that similar addition of bidendate organophosphorous ligands such as octamethylpyrophosphoramide OP(N(CHa)2)~-O-(N(CHa)~)2PO should be possible, and we have prepared the bidendate complex. Laboratoire de Chimie Min~rale et Structurale associ~ au C.N.R.S. Groupe de Spectrochimie Moleculaire 1, rue Blaise Pascal-67 Strasbourg Esplanade France

J. inorg, nucL Chem., 1972, Vol. 34, pp. 2668-2670.

Pergamon Press.

R O B E R T J. D O R S C H N E R

Printed in Great Britain

A re-investigation of the supposed potassium octacyanomanganate(IV) preparation (Received 28 October 1971) IN 1930, YAKIMACH[1] reported the preparation of an apparently eight-co-ordinate compound of Mn(IV), K4Mn(CN)s, by reacting KMnO4 and KCN in aqueous solution. This red material was not characterized other than by some solubility behavior and the preparation appears not to have been repeated. A later attempt by Goldenberg[2] to prepare this compound produced brown crystals whose stoichiometry was quite different from that reported by Yakimach. Goldenberg assigned the tentative formula K3Mnz(CN)9.4KOH to his compound. No other work on this preparation has been reported to our knowledge and no other Mn(IV) cyanides are known[3]. Eight-coordinate complexes of the first transition series are known but are not common. An octacyanide would be of interest and in view of the discrepancy between the results of Yakimach and Goldenberg, a reinvestigation of this system appeared worthwhile.

EXPERIMENTAL The method described by Yakimach in which nearly saturated KMnO4 and KCN solutions are mixed, was followed. MnO2 was filtered off during the next few days, and the brownish solution allowed to stand. After a period of weeks large octahedral crystals formed within the space of several hours. These were green by transmitted light and brown by reflected light, and yellow when ground. They were washed quickly with water and vacuum dried. Analytical results were 15-8% Mn, 23.8% N, 34.1% K. This fits an empirical formula KaMn(CN)~H20 (calculated 15.9% Mn, 24"3% N, 33.9% K). Water was shown by the i.r. spectrum. On standing for some additional weeks the filtrate finally deposited red crystals fitting Yakimach's description. Analysis gave 25"2% N, 21'9% C. K4[Mn(CN)a] requires 26.7% N, 22.9% C, while Ka[Mn(CN)6] requires 25.6% N, 21.9% C. This material is soluble in water to form a red-yellow solution which rapidly decomposes to MnOz, and has the other general properties reported for Ka[Mn(CN)8]. The brown product is stable in dry air, and dissolves in water to give yellow solutions which slowly deposit MnO2. It gives an intense wine-red solution in concentrated HC1 which fades to colorless in several hours. 1. A. Yakimach, Compt. rend. 190, 681 (1930). 2. N. Goldenberg, Trans. Faraday Soc. 36,847 (1940). 3. B. M. Chadwick and A. G. Sharpe, Advances in Inorganic Chemistry andRadiochemistry, Vol. 8, p. 109. Academic Press (1966).