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Preparation of a novel thallium compound of core formula, TIMo6S88+
Pohhedron Vol. 15, No. I 1, pp. 1b;87 1892, 1996 Copyright c 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0277 s387'96 $15.00+ll.00
~ r j ~ Pergamon 0277-5387(95)00052-6
PREPARATION
OF
A NOVEL
CORE
THALLIUM
FORMULA,
COMPOUND
OF
TIMo6SSs +
J. E. VAREY and A. G. SYKES* Chemistry Department, Bedson Building, University of Newcastle upon Tyne. NEI 7RU, U.K.
(Received 13 December 1994 ; accepted 9 January 1995) Abstract--A new mixed metal cluster of molybdenum, sulfur and thallium has been prepared. The complex can be prepared by the reaction of [Mo3S4(H20)9] 4+ with thallium metal, but preparation is more successful using sodium borohydride reduction of a solution of [Mo3S4(H20)9]4+ in the presence of solid T1CI. The new compound occurs as a turquoise solution when eluted in 4 M HCI or Hpts (p-toluenesulfonic acid) from a Dowex chromatography column. ICP analysis on the pure solution indicates a ratio of 5.92 : 1 for the ratio Mo : TI, confirming the proposed core formula of TlMo6S~ +. The charge on the core has been confirmed as 8 + using stoichiometric experiments with ferric solutions. The compound has proven to be thermally unstable, eluting only from ice-cooled columns. On warming a solution from 10 to 25-C, a gas can be seen to evolve, presumably hydrogen. The thermal instability has been investigated further to yield values of AH + = 79 kJ mol- ~, AS~ = - 39 mol-l.
The possibility of preparing a heterometallic derivative of [Mo3S4(H20)9]4+ using thallium was of interest since the latter is a representative of the group 13 metals. After this work began, Sakane and Shibahara reported the incorporation of indium to give a single cubane-type cluster, [InMo3S4(pts)2 (H20)m] ~+. Another cuboidal cluster, Ga4S4(But)4, has been prepared by Barron and colleagues.: Thallium compounds are also of interest because of their potential use in medicine. At present, 2
of the metal, followed by excretion of the complex formed, and the use of antidotes. If one considers the former, clearly the ligand needs to strongly chelate the metal and be selective, as well as forming water-soluble compounds to enable excretion via the kidneys. The use of sodium diethyldithiocarbamate 3 (dithiocarbamate = S2CNR2 ) as a chelator of thallium indicates the need for sulfur ligation, and studies by SchrOder and co-workers s'~ have shown complexation of macrocyclic thioether ligands to thallium. In the latter (I), only one face of the thallium is complexed, e.g. [Tl([9]aneSs)]+," although Kahwa et all have shown that coor-
* Author to whom correspondence should be addressed. 1887
.c c
c/\~c...c'l
c
s \s ~
\J/ TI
I (No hydrogenatomsshown)
1888
J. E. VAREY and A. G. SYKES
dination of the [CuBr4] 2- anion is possible to the opposite face of the thallium in [(Tll8-crown6)4CuBr4][T1Br4]2. The proposal of [Mo354(H20)9] 4+ as a chelator would appear reasonable since there are three/L2-S atoms forming a basal plane (II). Thallium(I) has been incorporated into the structurally analogous [3Fe-4S] cluster in ferredoxin III, from Desulfovibrio africanus. 8
. ./~-S
. .m~°
/
H (No H20 ligands shown on the Mo's)
It was therefore of interest to determine whether thallium could be incorporated into [Mo3S4(H20)9] 4+ and what solution properties any resultant (cuboidal) cluster might display. EXPERIMENTAL
All procedures were performed under dinitrogen unless stated otherwise, using standard air-free techniques• Preparation of the thallium cluster Two preparative routes for this species, referred to as T1Mo6S~ ÷, have been identified: (i) direct reaction of thallium metal with [Mo384(H20)9] 4+ in 2•00 M HC104; (ii) sodium borohydride reduction of a solution • ' containing M03S 44+ (in 0.5 M HCI) in the presence of excess solid T1C1, followed by column purification. In method (i), only perchloric acid was used; with hydrochloric acid no reaction was observed and a precipitate (TIC1) formed with time. Reacting [Mo354(H20)9] 4+ (~>1 mM) and thallium metal in perchloric acid (2 M), with a vigorous nitrogen stream through the solution, produced a colour change of green to blue-green in 1-2 h. However, the species only remained for 10-20 rain unless icecooling was used, in which case the lifetime increased to ca 1 h. Complete conversion of [Mo3S4(H20)9] 4+ to the thallium cluster did not occur.
Method (ii) proved to be the most successful, although the reduction must be performed in hydrochloric acid. Sodium borohydride was dis-
solved in distilled water and added, dropwise, to a flat-bottomed conical flask (50 cm 3) containing Mo3S~ + (in ~<0.5 M HC1) and solid T1C1 (sufficient to cover the base of the flask) until a turquoise colour appeared• The solution was then loaded on to an ice-cooled, air-free Dowex 50W-X2 chromatography column. Only two products appeared on the column: the green, unreacted Mo3 S4+ which eluted in 1-2 M H + ; and the turquoise TIMo6S88+ which eluted in 2-4 M H +. Elution with 4 M HCI and 4 M Hpts (Hpts = p-toluenesulfonic acid) was performed. Figure 1 shows the UV-vis spectra obtained for these turquoise elutants. When solutions did not need to be pure, the reduction procedure [method (ii)] could be performed in 2 M HCI and used without further purification. Attempts were made to repeat this procedure in 2 M Hpts and 2 M HCIO4, but the product only remained for 5-10 min when stored in ice, before the colour of the solution reverted to green• ICP analysis 1CP analysis was performed at Albright and Wilson, Whitehaven, Cumbria. Thallium standards were prepared by dissolving T1203 in 4 M Hpts or 4 M HC1. Solubility/detection problems were observed for the standard in Hpts, and hence the samples, and so only results in HC1 are available. The samples were purified by column chromatography to remove unreacted Mo3 S4+. A value of 5.92 : 1 was obtained for the ratio Mo : TI, confirming the proposed formula•
15i o
~d I0 t ~ 0
300
""1.................. ,. I 500
700
k(nm) Fig. 1. UV-vis spectra of T1Mo6S~+ [( ) 4 M Hpts, (- -) 4 M HC1] and Mo354+ [(" ') 4 M Hpts] at 10.ff'C.
Preparation of TIMo6S~ ÷
1889
Determination of the end-products of the air-oxidation reaction To determine the end-products of the air-oxidation reaction of TIM06S~ +, a sample was allowed to decompose in air. To one portion, dilute sodium hydroxide (0.1 M) was added (brown precipitate if TIm present) ; to a second, dilute hydrochloric acid (1 M) was added (white precipitate if T1~present)/ T1~was determined to be the end-product.
Stoichiometrv
~--c
I I I/-I~I-- c
1.2
The charge on the compound was determined by the stoichiometry of the ferric oxidation reaction. Aliquots of [Fe(H,O)6] 3+ in 0.5 M Hpts were added to solutions of T1M06S~ + in 2 M HC1, at 10.(FC. The latter contained some impurity of M03S] +, but this is known to be unreactive towards oxidation by [Fe(H20)6] 3+. By adding aliquots of[Fe(H20)6] 3~ to samples of the thallium complex, in 2 M HCI at 10.0 C, a value of 1.01 4-0.10:1 was obtained for the ratio [Fe3+] : [TIMo6Ss], indicating a charge of 8 + on the cluster. TIMo,,S~+ + Fe 3+
2.4
,2M03S]+ + T I + + F e :+
0.0
3O0
600
900
Mnm) Fig. 2. UV vis spectra of TIMo~S~ + in 2 M HCI, demonstrating the thermal instability at 25.0C. (a) Initial sample at 10.0'C; (b) sample raised to 25.0 C : (c) after 30 min at 25,0"C : (d) sample lowered to 10.0 C : (e) after 25 min at 10.0' C.
Thermal stability If no ice-cooling was used during the column purification of the borohydride-reduced solution, incomplete separation of the green (MosS] +) and turquoise (TIMo6S~ +) bands occurred, and pure samples of the latter were unobtainable. In a separate experiment, an ice-cooled, air-free column was first loaded (in 0.5 M HCI) with the air-sensitive ,tCoMo3S4}28+ compound m followed by the thallium complex. The column was then washed with 0.5-1.0 M Hpts and two bands were seen: the first (lower band) was attributable to unreacted [Mo3S4(H20)9] 4+ ; the second, to the thallium and cobalt cuboidal clusters. This latter band contained a colour gradient, the lower section containing the blue colour of the thallium complex, whilst the upper contained the brown colour of the cobalt species. Since both species are sensitive to oxygen, any trace amounts of the latter should cause their decomposition, with the cobalt complex decomposing first since it is nearest to the resin surface at which contact with the reaction atmosphere occurs. On removing the ice-cooling column the blue colour could be seen to disappear with time, and elution of this band showed a mixture of [Mo384(H20)9] 4+ and [{CoMo3S4}2(H20)~d s+, indicating that none of the thallium complex had survived.
The thermal (in)stability was therefore investigated further. A sample ofTlMo~S~ +, in 2 M HC1, was placed in a 1 m m cell thermostatted at 10.0 'C. A flow of nitrogen gas into the cell and cell compartment was maintained throughout the experiment. The UV vis spectrum was recorded at 10.0C (Fig. 2a), after which the cell temperature was raised to 25.0~C. A spectrum was recorded after 10 min (to allow temperature equilibration', Fig. 2b) and the solution then thermostatted at 25.0 C for a further 30 min before a repeat spectrum (Fig. 2c) was recorded. The temperature was then lowered to 10.0 C, the spectrum recorded after 10 min equilibration (Fig. 2d), and the cell then thermostatted at 10.0"C for 25 rain before a final spectrum (Fig. 2e) was recorded. A quantitative study was also pertk)rmed. The rate of decay, after 10 min equilibration, was measured at various temperatures (10 25'C) by following the changes in the UV-vis spectra. These experiments were designed only to give approximate values of the activation parameters, since the beginning of the decay was not observable due to the time (10 rain) needed to equilibrate the cell temperature.
1890
J. E. VAREY and A. G. SYKES (precipitate of PbSO4, etc.) and then extracted by the addition of HC1 to give the white precipitate, T1CI/2
RESULTS
UV-visible spectrum The spectra of T1Mo6S~ + in 4 M HC1 and 4 M Hpts were recorded as pure fractions obtained from a Dowex chromatography column. Spectra were recorded at 10.0°C due to the thermal instability of the solutions. Values of e (M -~ cm ~ per TIM06S~ +) were determined by decomposition to Mo3S 4+ by air oxidation or warming. The spectra of T1Mo6S~ + (4 M Hpts and 4 M HC1) and Mo3S44+ (4 M Hpts) are shown in Fig. 1. The spectra obtained for T1Mo6S 8+ were similar in both Hpts and HC1 except for a shift in peak positions (Table 1). Beer's law was obeyed at 10.0°C for a columnpurified sample of T1Mo6S~ + eluted, and subsequently diluted, with 4 M Hpts but not at 25.0°C, due to thermal instability.
Qualitative study of the thermal instability T1Mo6 $8+ (in 2 M HC1) was shown to be stable at 10.0°C, with no decomposition observed over 25 rain. At 25.0°C, a significant amount of decomposition (ca 13%) was observed over 30 min, Fig. 2. Any decomposition due to oxygen would be observed by the presence of a green colour at the neck of the cell. Since such a change in colour was not evident, the decomposition was assigned to thermal instability of TIMo6S~ +. A gas was seen to evolve from these solutions during the decomposition suggesting the formation of TP and hydrogen. No precipitation of T1C1 was observed; however, at the concentrations involved it is unlikely that the amount of precipitate occurring in the cell would be detectable. One possible mechanism is as shown. T1M06S~ + TI+H +
~Tl+2Mo3S44+ 1
~Tl++sH2
The reduction potential of T1 +/° ( - 0 . 3 3 6 V) ~ indicates that the above is thermodynamically favourable. This proposal is supported by an industrial process in the purification of thallium in which the impure metal is dissolved in warm dilute acid
Table 1. Values of J-max (nm) and gmax (M-~ cm-~ per TIMo6$8+) for TIMo6S~ + (all values ± 10%) Acid
/~max
~max
~'max
F'max
(4 M)
(nm)
(M -1 cm -~)
(nm)
(M i cm J)
HC1 Hpts
667 660
11,982 14,252
386 380
10,413 12,918
Quantitative study of the thermal instability A kinetic study of the decay at various temperatures was performed by placing an air-free sample of the thallium species, in 2 M HC1, in a 1 m m narrow-necked cell and studying the decay under a dinitrogen atmosphere. The sample was ice-cooled before use and allowed to equilibrate for 10 min at each temperature before the decomposition was studied. Rate constants are given in Table 2. An Arrhenius plot was then performed, Fig. 3a and values of E~ = 8 2 + 9 kJ mol-1 and A = (1.39+0.15)× 10 ~ M -t s -~ obtained. Plotting In (k/T) vs lIT (Eyring plot), Fig. 3b, gave values ofAH~=79±8 k J m o l 1, A S ~ = - 3 9 _ + 1 1 J K -1 mo1-1, where errors were calculated using a percentage error method for each data point. A linear fit was assumed based on the relatively small number of experimental values. The latter were all obtained from samples of a stock solution to allow a consistent level of impurities (TI+,Mo3S4+). The values obtained for the activation parameters are worthy of some discussion. The number of rate-controlling processes in the decay of the thallium complex are unknown, hence, since it is possible that kobs = klKj q-k2Kig2q- .... etc., and so kHob~ + = A H ~ { + A H 2 { + .... etc., then the activation parameters observed here could consist of the sum of a number of separate reaction values. However, if the values refer to only one process, the activation parameters must refer to the formation of an activated complex. I f one assumes that such a complex may be an excited state of the double cuboidal cluster, then clearly excitation of the vibrational energy levels must occur, a process clearly having unfavourable parameters. The enthalpy and entropy values on the breakdown of this c o m p o u n d (presumably too fast to observe) to M03S 4+ and TI °, would be expected to be large and favourable, since six T I - - S bonds will be broken,
Table 2. First-order decay rate constants for TIMo6S88+ in 2.00 M HCI Temperature (°C)
104 k (s ')
25.0 20.0 15.0 10.0
5.35+0.25 4.12 + 0.07 1.98___0.20 0.98 + 0.07
Preparation of T1Mo6S~ ' (a)
8.0
into the effect of adding Hpts to solutions of TIMo6S~ + in 2 M HC1 indicated an increase in the decay rate constant approximating to exponential. Problems were encountered during these studies due to an equilibration time of 10 rain being necessary before measurements could begin, and hence only the later stages of the decay of the cluster could be recorded.
7.5
Stability towards air oxidation
9.5-
/
9.08.5- ~ "7
1891
7.O
3.35
a
I
3.40
L
I 3.45 3.50 103(I/T)(K"i)
ll~'
I
3.55
(b)
°I 14'5I
Solutions of T1Mo6S~ + eluted from a column in 4 M Hpts were stable towards air oxidation for a minimum of several hours under nitrogen (in ice). Preparations in 2 M HCI prepared by the addition of borohydride with no further purification remained stable, under nitrogen (in ice) with negligible decomposition for 1-2 days if stored frozen overnight. Air oxidation yielded the products TI + and Mo~S~ ÷ and would, therefore, seem to follow the equation below. 4T1Mo6S~+ +O~ + 4 H +
14"0I
8Mo3S 4+ + 4 T l + + 2 H 2 0
13"5I / 13.01 3.35/
I 3.40
I L 3.45 3.50 103(I/T)(K-1)
I 3.55
Fig. 3. (a) An Arrhenius plot of the decay of T1Mo6S~+ at varying temperatures. (b) An Eyring plot of the decay of T1Mo6S~~ at varying temperatures.
and would compensate for the unfavourable activation parameters observed.
DISCUSSION Thallium is only the fourth main-group element to be incorporated into the incomplete cuboidal cluster, [Mo3S4(H20)9]4+, indium, ~ antimony t~ and tin 14 being the others. It is also only the fourth instance of a MM06S~ + core occurring. Both antimony, 13 mercury ~ and tin ~4 produce compounds with this core on reaction of the metal with [Mo3S4(H20)9]4+. Since these three structures have been determined, by Shibahara and co-workers, ~ ~ as corner-shared double cuboidal clusters (Ill), a similar structure is proposed for the thallium species.
Complexation el ects Pure fractio,ls of TIMo6S~ + eluted in 4 M Hpts proved stable over several hours when stored in ice. However, sat ~ples prepared in 2 M Hpts, using the borohydrid: reduction procedure without purification, did not remain for > 10 rain. (Purification was not possible due to the short lifetime.) A possible reas, n for this would appear to be greater complexatio, by the 4 M acid, stabilizing the cuboidal cluster. 'guch a proposal is supported by the apparently much greater stability of the cubane in HC1 than in either Hpts or HC104. Dowex chromatography presumably stabilizes the cluster during washings with < 4 M Hpts by binding of the compound to the resin. A preliminary investigation
Mo s
III
If one compares the UV vis spectra of the mercury, thallium and tin corner-shared double cuboidal species, a surprising similarity is observed,
1892
J. E. VAREY and A. G. SYKES
indicating perhaps metals in similar oxidation states 16 (the antimony cluster's spectra also have similar features), i3 The observation of gas evolution during the thermal instability studies indicates the initial release of Tl ° and so would indicate perhaps a formal oxidation state of zero for the thallium in the compound. However, it is possible that thallium enters as TI 1 and delocalization of the electrons within the structure lowers the formal oxidation state at the thallium. Interestingly, Sakane and Shibahara have proposed an InMo6Ss(aq) s+ compound as an intermediate in the formation of the [InMo3S4(pts)2(HzO)jo] 3+ complex, with this intermediate being oxidized by H + to the monomeric species and [Mo3S4(H20)9] 4+. It is possible such a mechanism occurs in the thallium system with the oxidation not being that of T1° --* TI ~, as proposed, but rather of the entire compound, possibly to a 9 + intermediate, or perhaps via a T1M03S45+ species. No evidence for any intermediate(s) has yet been obtained in the thallium system, however. A more interesting fact is that chloride ions do not appear to break down the structure. In chloride preparations of SnM06S~ +, an attack by chloride ion at the tin has been proposed, to explain the formation of (C1)3SnMo3S4(aq) 3+ on the addition of chloride ion to solutions of the double cuboidal cluster, t6 This has not been observed in studies on the thallium analogue. The shifts in peak positions between pts and chloride media for the thallium cluster are similar to those observed for Mo 3 S 44+ 17 and may indicate complexation at the molybdenum atoms so shielding the thallium from attack. Since similar ideas would apply to the tin system a logical factor must be the temperature. At 10.0°C, the rates of C I - / H 2 0 exchange at each molybdenum and the rate of chloride attack at the thallium will be far slower than at 25.ff'C. Hence, similar chemistry to that observed for the tin cuboidal cluster may well occur if the thallium complex was stable at 25.0°C and a monomeric TIMo3S~ + core obtainable. A method of stabilizing both the double cuboidal structure, and obtaining any monomeric species, may be possible using other ligands, e.g. 1,4,7-triazacyclononane, to yield crystal structures in future studies. This technique was used by Hidai and coworkers to isolate the monomeric PdMo3S~ + core. TM
Acknowledgements--J.E.V. would like to thank SERC for financial support and also SCI for the Messel Scholarship.
REFERENCES 1. G. Sakane and T. Shibahara, Inorg. Chem. 1993, 32, 777. 2. M. B. Power, J. W. Ziller, A. N. Tyler and A. R. Barron, Organometallics 1992, 11, 1055. 3. A. Korolkovas, Essentials of Medicinal Chemistry, 2nd edn. John Wiley and Sons, New York (1988). 4. F. A. Cotton and G. Wilkinson, Advancedlnorganic Chemistry, 5th edn. John Wiley and Sons, New York (1980). 5. A. J. Blake, J. A. Grieg and M. Schr6der, J. Chem. Soc., Dalton Trans. 1991, 529. 6. A.J. Blake, G. Reid and M. Schr6der, J. Chem. Soc., Dalton Trans. 1992, 2987. 7. 1. A. Kahwa, D. Miller, F. R. Fronczek and M. Mitchel, Acta Cryst. 1993, C49, 320. 8. J. N. Butt, A. Sucheta, F. A. Armstrong, J. Breton, A. J. Thomson and E. C. Hatchikian, J. Am. Chem. Soc. 1991, 113, 8948. 9. Vogel's Textbook of Macro and Semimicro Qualitative Inorganic Analysis, (Revised by G. Svehla), 5th edn. Longman, London (1979). 10. P. W. Dimmock, D. M. Saysell and A. G. Sykes, lnorg. Chim. Acta 1994, 225, 157. 11. A. J. Bard, R. Parsons and J. Jordan (Ed.). Standard Potentials in Aqueous Solution. Marcel Dekker Inc., New York (1985). 12. N. N. Greenwood and A. Earnshaw, Chemistry ~?/' the Elements, p. 250. Pergamon Press Ltd, Oxford (1989). 13. T. Shibahara, K. Hashimoto and G. Sakane, Poster abstract D047, J. Inorg. Biochem. 1991, 43; No. 23, 280, Abstracts for the Fifth International Conference on Bioinorganic Chemistry, August 1991, Oxford. 14. H. Akashi and T. Shibahara, lnorg. Chem. 1989, 28, 2906. 15. T. Shibahara, H. Akashi, M. Yamasaki and K. Hashimoto, Chem. Lett. 1991, 689. 16. J. E. Varey, PhD Thesis, University of Newcastle upon Tyne (1993). 17. B.-L. Ooi, PhD Thesis, University of Newcastle upon Tyne (1988). 18. T. Murata, H. Gao, Y. Mizobe, F. Nakano, S. Motomura, T. Tanase, S. Yano and M. Hidai, J. Am. Chem. Soc. 1992, 114, 8287.
~ r j ~ Pergamon 0277-5387(95)00052-6
PREPARATION
OF
A NOVEL
CORE
THALLIUM
FORMULA,
COMPOUND
OF
TIMo6SSs +
J. E. VAREY and A. G. SYKES* Chemistry Department, Bedson Building, University of Newcastle upon Tyne. NEI 7RU, U.K.
(Received 13 December 1994 ; accepted 9 January 1995) Abstract--A new mixed metal cluster of molybdenum, sulfur and thallium has been prepared. The complex can be prepared by the reaction of [Mo3S4(H20)9] 4+ with thallium metal, but preparation is more successful using sodium borohydride reduction of a solution of [Mo3S4(H20)9]4+ in the presence of solid T1CI. The new compound occurs as a turquoise solution when eluted in 4 M HCI or Hpts (p-toluenesulfonic acid) from a Dowex chromatography column. ICP analysis on the pure solution indicates a ratio of 5.92 : 1 for the ratio Mo : TI, confirming the proposed core formula of TlMo6S~ +. The charge on the core has been confirmed as 8 + using stoichiometric experiments with ferric solutions. The compound has proven to be thermally unstable, eluting only from ice-cooled columns. On warming a solution from 10 to 25-C, a gas can be seen to evolve, presumably hydrogen. The thermal instability has been investigated further to yield values of AH + = 79 kJ mol- ~, AS~ = - 39 mol-l.
The possibility of preparing a heterometallic derivative of [Mo3S4(H20)9]4+ using thallium was of interest since the latter is a representative of the group 13 metals. After this work began, Sakane and Shibahara reported the incorporation of indium to give a single cubane-type cluster, [InMo3S4(pts)2 (H20)m] ~+. Another cuboidal cluster, Ga4S4(But)4, has been prepared by Barron and colleagues.: Thallium compounds are also of interest because of their potential use in medicine. At present, 2
of the metal, followed by excretion of the complex formed, and the use of antidotes. If one considers the former, clearly the ligand needs to strongly chelate the metal and be selective, as well as forming water-soluble compounds to enable excretion via the kidneys. The use of sodium diethyldithiocarbamate 3 (dithiocarbamate = S2CNR2 ) as a chelator of thallium indicates the need for sulfur ligation, and studies by SchrOder and co-workers s'~ have shown complexation of macrocyclic thioether ligands to thallium. In the latter (I), only one face of the thallium is complexed, e.g. [Tl([9]aneSs)]+," although Kahwa et all have shown that coor-
* Author to whom correspondence should be addressed. 1887
.c c
c/\~c...c'l
c
s \s ~
\J/ TI
I (No hydrogenatomsshown)
1888
J. E. VAREY and A. G. SYKES
dination of the [CuBr4] 2- anion is possible to the opposite face of the thallium in [(Tll8-crown6)4CuBr4][T1Br4]2. The proposal of [Mo354(H20)9] 4+ as a chelator would appear reasonable since there are three/L2-S atoms forming a basal plane (II). Thallium(I) has been incorporated into the structurally analogous [3Fe-4S] cluster in ferredoxin III, from Desulfovibrio africanus. 8
. ./~-S
. .m~°
/
H (No H20 ligands shown on the Mo's)
It was therefore of interest to determine whether thallium could be incorporated into [Mo3S4(H20)9] 4+ and what solution properties any resultant (cuboidal) cluster might display. EXPERIMENTAL
All procedures were performed under dinitrogen unless stated otherwise, using standard air-free techniques• Preparation of the thallium cluster Two preparative routes for this species, referred to as T1Mo6S~ ÷, have been identified: (i) direct reaction of thallium metal with [Mo384(H20)9] 4+ in 2•00 M HC104; (ii) sodium borohydride reduction of a solution • ' containing M03S 44+ (in 0.5 M HCI) in the presence of excess solid T1C1, followed by column purification. In method (i), only perchloric acid was used; with hydrochloric acid no reaction was observed and a precipitate (TIC1) formed with time. Reacting [Mo354(H20)9] 4+ (~>1 mM) and thallium metal in perchloric acid (2 M), with a vigorous nitrogen stream through the solution, produced a colour change of green to blue-green in 1-2 h. However, the species only remained for 10-20 rain unless icecooling was used, in which case the lifetime increased to ca 1 h. Complete conversion of [Mo3S4(H20)9] 4+ to the thallium cluster did not occur.
Method (ii) proved to be the most successful, although the reduction must be performed in hydrochloric acid. Sodium borohydride was dis-
solved in distilled water and added, dropwise, to a flat-bottomed conical flask (50 cm 3) containing Mo3S~ + (in ~<0.5 M HC1) and solid T1C1 (sufficient to cover the base of the flask) until a turquoise colour appeared• The solution was then loaded on to an ice-cooled, air-free Dowex 50W-X2 chromatography column. Only two products appeared on the column: the green, unreacted Mo3 S4+ which eluted in 1-2 M H + ; and the turquoise TIMo6S88+ which eluted in 2-4 M H +. Elution with 4 M HCI and 4 M Hpts (Hpts = p-toluenesulfonic acid) was performed. Figure 1 shows the UV-vis spectra obtained for these turquoise elutants. When solutions did not need to be pure, the reduction procedure [method (ii)] could be performed in 2 M HCI and used without further purification. Attempts were made to repeat this procedure in 2 M Hpts and 2 M HCIO4, but the product only remained for 5-10 min when stored in ice, before the colour of the solution reverted to green• ICP analysis 1CP analysis was performed at Albright and Wilson, Whitehaven, Cumbria. Thallium standards were prepared by dissolving T1203 in 4 M Hpts or 4 M HC1. Solubility/detection problems were observed for the standard in Hpts, and hence the samples, and so only results in HC1 are available. The samples were purified by column chromatography to remove unreacted Mo3 S4+. A value of 5.92 : 1 was obtained for the ratio Mo : TI, confirming the proposed formula•
15i o
~d I0 t ~ 0
300
""1.................. ,. I 500
700
k(nm) Fig. 1. UV-vis spectra of T1Mo6S~+ [( ) 4 M Hpts, (- -) 4 M HC1] and Mo354+ [(" ') 4 M Hpts] at 10.ff'C.
Preparation of TIMo6S~ ÷
1889
Determination of the end-products of the air-oxidation reaction To determine the end-products of the air-oxidation reaction of TIM06S~ +, a sample was allowed to decompose in air. To one portion, dilute sodium hydroxide (0.1 M) was added (brown precipitate if TIm present) ; to a second, dilute hydrochloric acid (1 M) was added (white precipitate if T1~present)/ T1~was determined to be the end-product.
Stoichiometrv
~--c
I I I/-I~I-- c
1.2
The charge on the compound was determined by the stoichiometry of the ferric oxidation reaction. Aliquots of [Fe(H,O)6] 3+ in 0.5 M Hpts were added to solutions of T1M06S~ + in 2 M HC1, at 10.(FC. The latter contained some impurity of M03S] +, but this is known to be unreactive towards oxidation by [Fe(H20)6] 3+. By adding aliquots of[Fe(H20)6] 3~ to samples of the thallium complex, in 2 M HCI at 10.0 C, a value of 1.01 4-0.10:1 was obtained for the ratio [Fe3+] : [TIMo6Ss], indicating a charge of 8 + on the cluster. TIMo,,S~+ + Fe 3+
2.4
,2M03S]+ + T I + + F e :+
0.0
3O0
600
900
Mnm) Fig. 2. UV vis spectra of TIMo~S~ + in 2 M HCI, demonstrating the thermal instability at 25.0C. (a) Initial sample at 10.0'C; (b) sample raised to 25.0 C : (c) after 30 min at 25,0"C : (d) sample lowered to 10.0 C : (e) after 25 min at 10.0' C.
Thermal stability If no ice-cooling was used during the column purification of the borohydride-reduced solution, incomplete separation of the green (MosS] +) and turquoise (TIMo6S~ +) bands occurred, and pure samples of the latter were unobtainable. In a separate experiment, an ice-cooled, air-free column was first loaded (in 0.5 M HCI) with the air-sensitive ,tCoMo3S4}28+ compound m followed by the thallium complex. The column was then washed with 0.5-1.0 M Hpts and two bands were seen: the first (lower band) was attributable to unreacted [Mo3S4(H20)9] 4+ ; the second, to the thallium and cobalt cuboidal clusters. This latter band contained a colour gradient, the lower section containing the blue colour of the thallium complex, whilst the upper contained the brown colour of the cobalt species. Since both species are sensitive to oxygen, any trace amounts of the latter should cause their decomposition, with the cobalt complex decomposing first since it is nearest to the resin surface at which contact with the reaction atmosphere occurs. On removing the ice-cooling column the blue colour could be seen to disappear with time, and elution of this band showed a mixture of [Mo384(H20)9] 4+ and [{CoMo3S4}2(H20)~d s+, indicating that none of the thallium complex had survived.
The thermal (in)stability was therefore investigated further. A sample ofTlMo~S~ +, in 2 M HC1, was placed in a 1 m m cell thermostatted at 10.0 'C. A flow of nitrogen gas into the cell and cell compartment was maintained throughout the experiment. The UV vis spectrum was recorded at 10.0C (Fig. 2a), after which the cell temperature was raised to 25.0~C. A spectrum was recorded after 10 min (to allow temperature equilibration', Fig. 2b) and the solution then thermostatted at 25.0 C for a further 30 min before a repeat spectrum (Fig. 2c) was recorded. The temperature was then lowered to 10.0 C, the spectrum recorded after 10 min equilibration (Fig. 2d), and the cell then thermostatted at 10.0"C for 25 rain before a final spectrum (Fig. 2e) was recorded. A quantitative study was also pertk)rmed. The rate of decay, after 10 min equilibration, was measured at various temperatures (10 25'C) by following the changes in the UV-vis spectra. These experiments were designed only to give approximate values of the activation parameters, since the beginning of the decay was not observable due to the time (10 rain) needed to equilibrate the cell temperature.
1890
J. E. VAREY and A. G. SYKES (precipitate of PbSO4, etc.) and then extracted by the addition of HC1 to give the white precipitate, T1CI/2
RESULTS
UV-visible spectrum The spectra of T1Mo6S~ + in 4 M HC1 and 4 M Hpts were recorded as pure fractions obtained from a Dowex chromatography column. Spectra were recorded at 10.0°C due to the thermal instability of the solutions. Values of e (M -~ cm ~ per TIM06S~ +) were determined by decomposition to Mo3S 4+ by air oxidation or warming. The spectra of T1Mo6S~ + (4 M Hpts and 4 M HC1) and Mo3S44+ (4 M Hpts) are shown in Fig. 1. The spectra obtained for T1Mo6S 8+ were similar in both Hpts and HC1 except for a shift in peak positions (Table 1). Beer's law was obeyed at 10.0°C for a columnpurified sample of T1Mo6S~ + eluted, and subsequently diluted, with 4 M Hpts but not at 25.0°C, due to thermal instability.
Qualitative study of the thermal instability T1Mo6 $8+ (in 2 M HC1) was shown to be stable at 10.0°C, with no decomposition observed over 25 rain. At 25.0°C, a significant amount of decomposition (ca 13%) was observed over 30 min, Fig. 2. Any decomposition due to oxygen would be observed by the presence of a green colour at the neck of the cell. Since such a change in colour was not evident, the decomposition was assigned to thermal instability of TIMo6S~ +. A gas was seen to evolve from these solutions during the decomposition suggesting the formation of TP and hydrogen. No precipitation of T1C1 was observed; however, at the concentrations involved it is unlikely that the amount of precipitate occurring in the cell would be detectable. One possible mechanism is as shown. T1M06S~ + TI+H +
~Tl+2Mo3S44+ 1
~Tl++sH2
The reduction potential of T1 +/° ( - 0 . 3 3 6 V) ~ indicates that the above is thermodynamically favourable. This proposal is supported by an industrial process in the purification of thallium in which the impure metal is dissolved in warm dilute acid
Table 1. Values of J-max (nm) and gmax (M-~ cm-~ per TIMo6$8+) for TIMo6S~ + (all values ± 10%) Acid
/~max
~max
~'max
F'max
(4 M)
(nm)
(M -1 cm -~)
(nm)
(M i cm J)
HC1 Hpts
667 660
11,982 14,252
386 380
10,413 12,918
Quantitative study of the thermal instability A kinetic study of the decay at various temperatures was performed by placing an air-free sample of the thallium species, in 2 M HC1, in a 1 m m narrow-necked cell and studying the decay under a dinitrogen atmosphere. The sample was ice-cooled before use and allowed to equilibrate for 10 min at each temperature before the decomposition was studied. Rate constants are given in Table 2. An Arrhenius plot was then performed, Fig. 3a and values of E~ = 8 2 + 9 kJ mol-1 and A = (1.39+0.15)× 10 ~ M -t s -~ obtained. Plotting In (k/T) vs lIT (Eyring plot), Fig. 3b, gave values ofAH~=79±8 k J m o l 1, A S ~ = - 3 9 _ + 1 1 J K -1 mo1-1, where errors were calculated using a percentage error method for each data point. A linear fit was assumed based on the relatively small number of experimental values. The latter were all obtained from samples of a stock solution to allow a consistent level of impurities (TI+,Mo3S4+). The values obtained for the activation parameters are worthy of some discussion. The number of rate-controlling processes in the decay of the thallium complex are unknown, hence, since it is possible that kobs = klKj q-k2Kig2q- .... etc., and so kHob~ + = A H ~ { + A H 2 { + .... etc., then the activation parameters observed here could consist of the sum of a number of separate reaction values. However, if the values refer to only one process, the activation parameters must refer to the formation of an activated complex. I f one assumes that such a complex may be an excited state of the double cuboidal cluster, then clearly excitation of the vibrational energy levels must occur, a process clearly having unfavourable parameters. The enthalpy and entropy values on the breakdown of this c o m p o u n d (presumably too fast to observe) to M03S 4+ and TI °, would be expected to be large and favourable, since six T I - - S bonds will be broken,
Table 2. First-order decay rate constants for TIMo6S88+ in 2.00 M HCI Temperature (°C)
104 k (s ')
25.0 20.0 15.0 10.0
5.35+0.25 4.12 + 0.07 1.98___0.20 0.98 + 0.07
Preparation of T1Mo6S~ ' (a)
8.0
into the effect of adding Hpts to solutions of TIMo6S~ + in 2 M HC1 indicated an increase in the decay rate constant approximating to exponential. Problems were encountered during these studies due to an equilibration time of 10 rain being necessary before measurements could begin, and hence only the later stages of the decay of the cluster could be recorded.
7.5
Stability towards air oxidation
9.5-
/
9.08.5- ~ "7
1891
7.O
3.35
a
I
3.40
L
I 3.45 3.50 103(I/T)(K"i)
ll~'
I
3.55
(b)
°I 14'5I
Solutions of T1Mo6S~ + eluted from a column in 4 M Hpts were stable towards air oxidation for a minimum of several hours under nitrogen (in ice). Preparations in 2 M HCI prepared by the addition of borohydride with no further purification remained stable, under nitrogen (in ice) with negligible decomposition for 1-2 days if stored frozen overnight. Air oxidation yielded the products TI + and Mo~S~ ÷ and would, therefore, seem to follow the equation below. 4T1Mo6S~+ +O~ + 4 H +
14"0I
8Mo3S 4+ + 4 T l + + 2 H 2 0
13"5I / 13.01 3.35/
I 3.40
I L 3.45 3.50 103(I/T)(K-1)
I 3.55
Fig. 3. (a) An Arrhenius plot of the decay of T1Mo6S~+ at varying temperatures. (b) An Eyring plot of the decay of T1Mo6S~~ at varying temperatures.
and would compensate for the unfavourable activation parameters observed.
DISCUSSION Thallium is only the fourth main-group element to be incorporated into the incomplete cuboidal cluster, [Mo3S4(H20)9]4+, indium, ~ antimony t~ and tin 14 being the others. It is also only the fourth instance of a MM06S~ + core occurring. Both antimony, 13 mercury ~ and tin ~4 produce compounds with this core on reaction of the metal with [Mo3S4(H20)9]4+. Since these three structures have been determined, by Shibahara and co-workers, ~ ~ as corner-shared double cuboidal clusters (Ill), a similar structure is proposed for the thallium species.
Complexation el ects Pure fractio,ls of TIMo6S~ + eluted in 4 M Hpts proved stable over several hours when stored in ice. However, sat ~ples prepared in 2 M Hpts, using the borohydrid: reduction procedure without purification, did not remain for > 10 rain. (Purification was not possible due to the short lifetime.) A possible reas, n for this would appear to be greater complexatio, by the 4 M acid, stabilizing the cuboidal cluster. 'guch a proposal is supported by the apparently much greater stability of the cubane in HC1 than in either Hpts or HC104. Dowex chromatography presumably stabilizes the cluster during washings with < 4 M Hpts by binding of the compound to the resin. A preliminary investigation
Mo s
III
If one compares the UV vis spectra of the mercury, thallium and tin corner-shared double cuboidal species, a surprising similarity is observed,
1892
J. E. VAREY and A. G. SYKES
indicating perhaps metals in similar oxidation states 16 (the antimony cluster's spectra also have similar features), i3 The observation of gas evolution during the thermal instability studies indicates the initial release of Tl ° and so would indicate perhaps a formal oxidation state of zero for the thallium in the compound. However, it is possible that thallium enters as TI 1 and delocalization of the electrons within the structure lowers the formal oxidation state at the thallium. Interestingly, Sakane and Shibahara have proposed an InMo6Ss(aq) s+ compound as an intermediate in the formation of the [InMo3S4(pts)2(HzO)jo] 3+ complex, with this intermediate being oxidized by H + to the monomeric species and [Mo3S4(H20)9] 4+. It is possible such a mechanism occurs in the thallium system with the oxidation not being that of T1° --* TI ~, as proposed, but rather of the entire compound, possibly to a 9 + intermediate, or perhaps via a T1M03S45+ species. No evidence for any intermediate(s) has yet been obtained in the thallium system, however. A more interesting fact is that chloride ions do not appear to break down the structure. In chloride preparations of SnM06S~ +, an attack by chloride ion at the tin has been proposed, to explain the formation of (C1)3SnMo3S4(aq) 3+ on the addition of chloride ion to solutions of the double cuboidal cluster, t6 This has not been observed in studies on the thallium analogue. The shifts in peak positions between pts and chloride media for the thallium cluster are similar to those observed for Mo 3 S 44+ 17 and may indicate complexation at the molybdenum atoms so shielding the thallium from attack. Since similar ideas would apply to the tin system a logical factor must be the temperature. At 10.0°C, the rates of C I - / H 2 0 exchange at each molybdenum and the rate of chloride attack at the thallium will be far slower than at 25.ff'C. Hence, similar chemistry to that observed for the tin cuboidal cluster may well occur if the thallium complex was stable at 25.0°C and a monomeric TIMo3S~ + core obtainable. A method of stabilizing both the double cuboidal structure, and obtaining any monomeric species, may be possible using other ligands, e.g. 1,4,7-triazacyclononane, to yield crystal structures in future studies. This technique was used by Hidai and coworkers to isolate the monomeric PdMo3S~ + core. TM
Acknowledgements--J.E.V. would like to thank SERC for financial support and also SCI for the Messel Scholarship.
REFERENCES 1. G. Sakane and T. Shibahara, Inorg. Chem. 1993, 32, 777. 2. M. B. Power, J. W. Ziller, A. N. Tyler and A. R. Barron, Organometallics 1992, 11, 1055. 3. A. Korolkovas, Essentials of Medicinal Chemistry, 2nd edn. John Wiley and Sons, New York (1988). 4. F. A. Cotton and G. Wilkinson, Advancedlnorganic Chemistry, 5th edn. John Wiley and Sons, New York (1980). 5. A. J. Blake, J. A. Grieg and M. Schr6der, J. Chem. Soc., Dalton Trans. 1991, 529. 6. A.J. Blake, G. Reid and M. Schr6der, J. Chem. Soc., Dalton Trans. 1992, 2987. 7. 1. A. Kahwa, D. Miller, F. R. Fronczek and M. Mitchel, Acta Cryst. 1993, C49, 320. 8. J. N. Butt, A. Sucheta, F. A. Armstrong, J. Breton, A. J. Thomson and E. C. Hatchikian, J. Am. Chem. Soc. 1991, 113, 8948. 9. Vogel's Textbook of Macro and Semimicro Qualitative Inorganic Analysis, (Revised by G. Svehla), 5th edn. Longman, London (1979). 10. P. W. Dimmock, D. M. Saysell and A. G. Sykes, lnorg. Chim. Acta 1994, 225, 157. 11. A. J. Bard, R. Parsons and J. Jordan (Ed.). Standard Potentials in Aqueous Solution. Marcel Dekker Inc., New York (1985). 12. N. N. Greenwood and A. Earnshaw, Chemistry ~?/' the Elements, p. 250. Pergamon Press Ltd, Oxford (1989). 13. T. Shibahara, K. Hashimoto and G. Sakane, Poster abstract D047, J. Inorg. Biochem. 1991, 43; No. 23, 280, Abstracts for the Fifth International Conference on Bioinorganic Chemistry, August 1991, Oxford. 14. H. Akashi and T. Shibahara, lnorg. Chem. 1989, 28, 2906. 15. T. Shibahara, H. Akashi, M. Yamasaki and K. Hashimoto, Chem. Lett. 1991, 689. 16. J. E. Varey, PhD Thesis, University of Newcastle upon Tyne (1993). 17. B.-L. Ooi, PhD Thesis, University of Newcastle upon Tyne (1988). 18. T. Murata, H. Gao, Y. Mizobe, F. Nakano, S. Motomura, T. Tanase, S. Yano and M. Hidai, J. Am. Chem. Soc. 1992, 114, 8287.