Some tropylium compounds containing planar metal-complex anions: tropylium tetrabromopalladium (II) tropylium tetrabromoplatinum (II)

588

Notes

J. inorg, nucl. Chem., 1967, Vol. 29, pp. 588 to 591, Pergamon Press Ltd. Printed in Nortl~rn Ireland

Some tropylium compounds containing planar metal-complex anions: tropylium tetrabromopalladium (H) tropylium tetrabromoplatinum (I1) (First received 13 June 1966; in revisedform 8 August i966) ONLY a few tropylium (cycloheptatrienylium ion) metal-complex-anion compounds have been reported, m Most of the "planar-tropylium" complexes~ have essentially ionic binding of the metal containing anion to the tropylium ion, e.g. ditropylium hexabromostanate and tropylium tetrachloroiron (III). ~1.~ In these tropylium compounds (salts) the tropylium ion remains an ionic aromatic species as in the tropylium halides. ~ One might presuppose that complex anions with square-planar configuration (e.g. tetrahaloplatinum (II) ions) would form compounds with tropylium ion which would have a sandwich structure (Fig. 1) because this should have a lower potential energy than other possible ionic forms and, possibly, bisarene covalent bonding. We attempted to prepare such compounds by mixing aqueous tropylium bromide with aqueous solution of certain known square-planar anions (see Table 1). Of these, only the solutions of tetrabromo- and tetrachloroplatinum (II) anions (and the palladium (I.I) analogs) formed precipitates. Although a rather specific structural assignment was made for the tropylium-iron-tricarbonyl cation essentially on the basis of i.r. data (mull method), ~2~we do not feel that we should apply a comparable interpretation to our data. Besides the variety of reasonable structural interpretations of even solution i.r. data, the spectra of some tropylium-ion-containingsolid salts ~1c~are much more complex than that of simple tropylium salts in solution ~Sb~and this variability (in number of bands, band positions and intensities) is most likely due to solid state interactions. Analogous effects may account for the variations in the i.r. spectra (mulls) of the ~--tropenium-Group VIB-tricarbonyl and tropylium--iron-tricarbonylcationic cases. ~1~(a) D. BRYCE-SMITHand N. A. P~RKINS, Chemy lnd. 1022 (1959); idem.J, chem. Soc. 2320(1961). (b) H. VOLZ, Angew. Chem. 75, 921 (1963); (c) C. R. KISTNER,J. R. DOYLE, N. C. BAENZIGER, J. H. HLrTCmNSONand P. KASPER,Inorg. Chem. 3, 1525 (1964). ~ ~--Tropenium-molybdenum-tricarbonyl fluoroborate [H. J. DAtraEr~, JR. and L. R. HONN~N, J. Am. chem. Soc. 80, 5570 (1958)] and chromium and tungsten analogs [L. R. HONNEN, Dis. Abstr. 24, 972 (1963)] are reported to have tropylium-to-metal ~r-bonding. However, the tungsten compound has been reported to have a non-planar seven-carbon ring. lB. R. KIN~ and A. FRONZAOHA, Paper 39, Division of Inorganic Chemistry, 151st Meeting of American Chemical Society, Pittsburgh, Pa., March 28-31, 1966.] Specifically, the ring is believed to be contorted so that a set of less than seven (3 or 5) carbons, acting as a carbonium ion set, bond ionically, at least to the metal. We think that this may also be the situation in the molybdenum compound (the reported i.r. spectrum is probably in error because it was taken by the KBr pellet method. The spectrum of this compound and that of the other two analogs have been taken by the mull method and reported by these authors [H. J. DAUBEN,L. R. HONNERand D. J. BERTELLI,Abstracts, 15th Southwest Regional Meeting, American Chemical Society, Baton Rouge, Louisiana, p. 89, Dec. 3, 1959]. The reported C - H stretch bands, for each of the species, are mentioned in the paper, below, by MAHLER,JONES and PETTIT,and these bands do not match any of those shown for the spectrum as taken by the KBr pellet method). The single proton resonance band reported for these compounds could be the result of rapid valence tautomerism ("exchange process") as is thought to be the case in the corresponding tropylium-iron-tricarbonyl cation [J. E. MAHLER, D. A. K. JONES and R. PETTIT, J. Am. chem. Soc. 86, 3590 (1964)] where it is believed that a fivecarbon portion binds directly to the metal atom. If these materials contained ~r-bound planartropylium ion the i.r. spectrum should show a pattern similar to that of ionic tropylium ion. Consider that the i.r. spectrum of the covalently bound rr-complexed dicyclopentadienyliron (II) is not much different from that of the essentially ionic dicyclopentadienyl magnesium [F. A. COTTONin Modern Co-ordination Chemistry (Edited by J. LEWISand R. G. WmKINS),pp. 355--360 Interscience, New York (1960)]. ts~ (a) W. voN E. DOERI~G and L. H. KNOX, J. Am. chem. Soc. 76, 3205 (1954); (b) W. G. FATELY and E. R. LIPPmCOTT,J. Am. chem. Soc. 77, 249 (1955); idem. J. chem. Phys. 26, 1471 (1957).

Notes

589

\\',,__,"/I

FIG. I .

TABLE 1 . - - T E S T SOLUTIONS AND CONCENTRATIONS USED IN THE ATTEMPTED PREPARATIONS OF TROPYLIUM COMPOUNDS

Complex anion NiCI~2Ni(CN)42CuCI2.2KC1 (soln) Cu(CN)2-2KCN (soln) PdC14~Pd(CN)a 2PdBr42PtCI4~PtBr42NiBr2 (Methanol soln)

Concentration of anion (M) 0.10, 0.10, 0.10, 0.10, 0-10 0-10,

1.0 1.0 1.0 1.0 1-0

Concentration of tropylium bromide (M) 0-20, 0.20, 0.20, 0-20, 0.20 0-20,

2.0 2-0 2.0 2.0 2.0

0-10

0'20

0-10 0' 10 0.10

0"20 0-20 0"20 (Methanol soln)

Yields (~) 0 0 0 0 ?a 0 81

?~ 60 0

a The materials isolated from solution in these reactions are apparently mixtures. See the experimental section for more detailed discussion. The similarity of the i.r. spectra of these tropylium tetrabromometal (II) complexes to that of tropyliurn bromide tsb~in solution, suggests that in these the tropylium ion is effectively planar (Table 2). The indications in the spectra of nonplanarity of most other tropylium compounds (i.e. the complexity) is an indication of the easy contortion of the tropylium ring. These tropyliurn compounds have only four or five bands with sufficient strength to be classified as fundamentals. If the tropylium rings were covalently linked there should be a greater number ( ~ 10) of fundamental modes. ~ The precipitates from the tetrachloro-anion-containing solutions were impure and apparently contained a central anion which had almost completely exchanged with the bromide ion of the solution. However, the tetrabromo-anion-containing solutions yielded pure precipitates which are evidently not rr-bound in type, but could be predecessors of similar sandwich compounds having the spacial facility for such covalent binding. While the u.v. spectra of tropylium complexes is sometimes cited as evidence for rr-bonding in tropylium compounds, ~2~the changes noted in the spectra could just as well arise from ionic effects as is most likely the case in these salts reported here--the structural effects and attendant spectral changes could be caused by pronounced ion-ion or ion-dipole interactions.

Notes

590

TABLE 2.--Trm

I . g . AND U.V. SPECTRA OF THE TROPYLIUM COMPOUNDS

Compound

I.r. Vibrational modes (cm -1)

Tropylium tetrabromopalladium (II)

3005(w) 991 (vw) 820--702(vs) 1476 (s) 862 (vw) 663--654 (w)~ 1276 (vw) 806 (vw) 629 (s) b

Tropylium tetrabromoplatinum (II)

3005 (w) 990 (vw) 827-720 (vs) 1474 (s) 860 (vw) 663-654 (w) ~ 1276 (vw) 629-633 (s)b

U.v. Bands in mt~ (~.max/') 242-254 (broad) 280 290 313 (sh) 216-232 (broad) 280 (broad)

1100 200 190 100~ 465 100~

Here there is a probable superimposition of a tetrabromometal (II) band [usually 663-654 cm -x (w)] and a tropylium band [usually 654 cm -x (w)]. b Presumably these bands are due to the tetrabromo-anion portion of the compound (PtBr,'-). ' Since concentrations in the mull situation are likely to be inexact (for various reasons) these are relative intensities based on the weakest band being set as a standard of intensity, i.e. at 1 = 100 where 1 = e max. × constant. P M R data (difficult to obtain for solids) is usually of little help in structural studies on tropylium compounds because the rapid valence tautomerism results in a single sharp peak. t~) Even in cyclooctatetraene rr-complexes where certainly most of the time there are at least two different hydrogen atoms, only one N M R peak is observed. (~) EXPERIMENTAL

Preparation of the i.r. and u.v. spectra. The spectra (Table 2) were taken by means of suspension in Nujol and in CC14, with high resolution technique, using a Beckman IR-4 spectrophotometer with sodium chloride optics. The u.v. spectra were taken on a recording Beckman model DB spectrophotometer using carbon tetrachloride mulls. The sample (0.005 g) was placed in a screwcapped Wig-l-bug capsule and dispersed in the carbon tetrachloride by shaking for I min on the Wig-l-bug. Reagents. The potassium tetrachloropalladium (II) compound was prepared by warming palladium (II) chloride (1'77 g) and potassium chloride (1.49 g) with 100 ml of water. The resulting solution (0.01 M) was filtered before use. Hydrogen tetrachloropalladium (II) was obtained by dissolving palladium(lI) chloride (3"54g) in 100ml of 1.0N hydrochloric acid. (Potassium tetrachloroplatinum (II), potassium tetrabromoplatinum 0I) and potassium tetrabromopalladium (II) (purity: 95-99 ~ ) were obtained from K and K Laboratories Inc., 177-10 93rd Avenue, Jamaica, New York.) The tropylium bromide was prepared essentially according to DOEmNG(2a) with slight modifications to produce purer and higher yields. (5)

Tropylium tetrabromometal (II)--general procedure Fifteen millilitres of 0.10 M solution of potassium tetrabromometal (II) were added to 15 ml of 0.40 M solution of tropylium bromide. The immediate precipitate was filtered and washed with three or four 5 ml portions of acetone. The precipitates were dried in low vacuum (1 mm Hg). Tropylium tetrabrom~palladium (II). The reddish-brown precipitate (0.74 g) resulting from the general procedure, melted with decomposition at 214--216°C and darkened slowly in the atmosphere at 175°C. Anal Calc. for C14Ht~PdBr4: C, 27'64; H, 2-32; Br, 52.55; Pd, 17.49. Found: C, 27.41; H, 2.37; Br, 52.67.* The compound is insoluble in water, dimethyl formamide (except in the hot, where decomposition * Analyses were by Spang Microanalytical Laboratory, P.O. Box 111 I, Ann Arbor, Michigan. I~l B. DICKENSand W. M. LIPSCOMB,idem. J. chem. Phys. 37, 2086 (1962). (5) A. E. KEMPPAINENand E. L. COMPERE,JR., J. chem. Enffnff. Data, accepted for publication.

Notes

591

occurs), and nonpolar solvents, but it dissolves in dimethyl sulphoxideJ s~ The material dissolves (by reaction) in aqueous sodium cyanide solution, producing a clear yellow aqueous layer and a small amount of observable oil with a pungent odour which separated out. The formation of the oil is a characteristic result of this treatment of tropylium compounds and was shown to be probably cycloheptatrienyl cyanide by the preparation of a derivative--norcaradiene carboxamide (m.p. 137-140°C). Identical treatment of a freshly prepared sample of cycloheptatrienyl cyanide also yielded norcaradiene carboxamide (m.p. 139-140°Cobs. and m.p. 140-5-142°C1,,). ~7~ Dilute solutions were used in the preparation of the compounds because the usual methods of purification could not be employed on the materials. At higher concentrations analyses showed mixtures, while at low concentrations no purification was required. When the filtered precipitates were rinsed with water, a slight discoloration occurred at the surface of the precipitate and an odour of benzaldehyde was noted. If acetone was used for rinsing, no decoloration or odour were detected and since tropylium bromide is somewhat soluble in acetone, it was felt that several rinses would remove any of this that might be adsorbed.

Tropylium tetrabromoplatinum (II) The purple compound (0"62 g) which formed from the general procedure decomposes slowly at 180°C and rapidly at 230°C without melting. Other properties (solubilities and reaction with aqueous sodium cyanide) were exactly analogous to that of the palladium analog. Anal Calc. for C~H14PtBr4: C, 24'12; H, 2"02; Br, 45.87; Pt, 27.99. Found: C, 23.96; H, 2-10; Br, 45.70.* The formation of the precipitate seemed to be catalysed by light, because on exposure to sunlight, the crystals formed much more rapidly. When large crystals were formed they were observed under a low power microscope to be square plates which were quite transparent and of similar size. Tetrachloro-derivatives. Attempted preparations of tropylium tetrachloro-analogs of the above compounds of both metals were fruitless. When the potassium tetrachloro-metal (II) starting materials were used with tropylium bromide, in dilute or concentrated solution, precipitation was immediate in the palladium case but required 15-20 rain in the platinum case. In both cases, the precipitates which resulted were shown to have a considerable bromine content, qualitatively)8~and to have carbon and hydrogen content (quantitatively measured) indicative of almost, but not quite, complete exchange of bromine for the original chlorine. When hydrogen tetrachloropalladium (II) (H2PdCI4) was used the precipitate appeared to be slightly darker brown in colour than when the potassium analog was used, and analyses indicated a slightly higher chlorine content. The use of tropylium chloride ~°~in any other attempts to prepare these materials is clearly indicated.

Department of Chemistry Michigan State University East Lansing Michigan Department of Chemistry Eastern Michigan University Ypsilanti Michigan

A. E. KEMPPA1NEN

E. L. COMPERE, JR.

~8~Materials of organic composition, particularly those having such reactive species like the tropylium ion as part of their embodiment, have been shown to react chemically with dimethyl sulphoxide, often being oxidized in the process, e.g. oxidative solvolysis of a keteneimine and a ketene in dimethyl sulphoxide [I. LmLIAN, J'. org. Chem. 29, 1631 (1964)]. Thus we fee/that conductance measurements on these solutions would not yield important data. ~7~W. VON E. DOERINC;and L. H. KNOX, J. Am. chem. Soe. 79, 355 (1957). ts~ F. FIEGL, Spot Tests (Inorganic Applications) Vol. I, pp. 246--7. Elsevier, New York (1954). ~J K. M. HARMONand S. DAVIS,J. Am. chem. Soc. 84, 4359 (1962).