Langmuir 1995,11, 1817-1821
1817
The First Cathodically Generated Tetraalkylammonium-Tin Compounds Essie Kariv-Miller, Paul D. Christian, and Vesna SvetliEib" Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 Received October 21, 1994. I n Final Form: January 18, 1 9 9 9 The reduction products of tetramethylammonium (TMA+)and dimethylpyrrolidinium (DMP+)at a tin cathode were isolated and examined in the solid-state for the first time. The empirical formula of R4Nmetals derived from tin is R4N(Snx),wherex is equal or close t o 5 . Existence of R4N+cations and negatively charged tin particles within the structure was demonstrated. Isolated TMA(Sn5)is a deep red solid while DMP(Sn5) is dark gray. They show a weak electron spin resonance signal, but the bulk material is diamagnetic as in the case of DMP(Pb5). X-ray powder diffraction patterns of DMP(Pb5),TMA(Sn5),and DMP(Sn5) were generated. We find that the lead compound shows the elemental lead diffraction pattern, suggesting charged metal particles, and the tin compounds show no diffraction lines, consistent with charged metal clusters, of size less than 50 A, arranged in a noncrystalline morphology. We concluded that these materials should not be considered molecular; i.e., they are not composed of discreet metal clusters, (M5)- or (MI#-, but oflarger aggregates (nanoclusters or nanocrystals) approaching the structure of the bulk metal.
Introduction Tetraalkylammonium-metals (W-metals) are formed by the reduction of tetraalkylammonium (R4N+)salts at a variety of post-transition-metal ~ a t h o d e s . l - ~They comprise ofthe cation (R4N+),metal (MI from the cathode, and electrons. Reductions at Hg and Pb have been studied in more detai1,4-7 and the kinetics has been elucidated. The products were determined to be ordered solids of the stoichiometric composition lR4N+/le-/5M, and the general empirical formula R4N(M5)was proposed. The reduction of R4N+ at Sn is similar, but the information available is limited. It was shown2 that the product is a strongly reducing ordered solid which maintains the ratio of 1R4N+/ le-. The empirical formula R4N(Sn5)was proposed, based on the similarity to the Hg and Pb derivatives. Very recently the reduction product of dimethylpyrrolidinium (DMP+) at a lead cathode was isolated and the first properties of a pure R4N-metal were reported.8 The proposed empirical formula, DMP(Pb5), was confirmed by elemental analysis, and the existence of a cation, DMP+, and a polyatomic Pb anion within the structure was demonstrated. Isolated DMP(Pb5) is a strong reducing agent, it transfers electrons with regeneration of DMP+ and metallic Pb. It reacts readily with fluorenone, benzophenone, and protic solvents (water, DzO, and alcohols). It is a black solid which is insoluble in common organic solvents. It decomposes (without melting) around 210 "C,liberating a n amine and metallic Pb. DMP(Pb5) is conducting and its measured conductivity of 1640 (Q cm)-l is in the range expected for metals. It shows a weak ESR signal, but the bulk material is diamagnetic.
* Permanent address of corresponding author: Rudjer BogkoviC Institute, P.O. Box 1016, 41001 Zagreb, Croatia. Abstract Dublished in Advance A C S Abstracts. A ~ r i 1. l 1995. @
(1)Kariv-Miller, E.; Lawin, P. B.; Vajtner, Z. J . Electroanal.'Chem. 1985,195,435. (2)SvetliEiC,V.; Lawin, P. B.; Kariv-Miller, E. J.Electroanal. Chem. 1990,284,185. (3)Fidler, M. M.; SvetliEiC,V.; Kariv-Miller, E. J.Electroanal. Chem. 1993,360,221. (4)SvetliEiC, V.:Kariv-Miller, E. J . Electroanal. Chem. 1986,209, 91. ( 5 ) Ryan, C. M.; SvetliEiC,V.; Kariv-Miller, E. J.Chem.Soc.,Faraday Trans. 1 1988,84,4023. ( 6 )Kariv-Miller, E.; Lawin, P. B. J.Electroanal. Chem. 1988,247, 345. (7) Lawin, P.B.; SvetliEiC,V.; Kariv-Miller, E. J.Electroanal. Chem. 1989,258,357.
0743-7463/95/2411-1817$09.00/0
The properties of isolated DMP(Pb5) support the hypothesis developed from earlier in situ (without isolation) studies. I n this hypothesis the general structure of R4Nmetals is R,N(M,), where x is equal or close to 5, and the metal moiety bears a negative charge counteracted by R4N+. A major question that could not be addressed by studies in situ concerns the structural unit of R4N-metals. Is R4N(Mx)the true unit, or is it only the expression of the stoichiometry of a larger entity? In the present investigation, R4N-metals derived from Sn were isolated and studied and some additional studies with DMP(Pb5) were performed. The goal was to expand our knowledge of isolated R4N-metals and to learn about the structure, beyond the simplest empirical formula, of these unique materials.
Experimental Section Chemicals. Dimethylformamide (DMF,American Burdick and Jackson, 0.007% HzO) was distilled at 30 mmHg and 65 "C over activated 4 A molecular sieves and stored under argon in the absence of light followingappropriate degassingprocedures. Tetrahydrofuran (THF) was distilled from sodium and benzophenone and used fresh. The preparation of DMP(BF4) has been described previously.s The following tetramethylammonium (TMA+)salts were purchased from Aldrich: TMA(BF4) (97%),TMA(1)(99%),and TMA(Br)(98%). They were recrystallized from acetonitrile/ethanol, dried in a vacuum, and stored under nitrogen. Tetramethylammonium tetraphenylborate, TMA(BPh4), was synthesized by adding Na(BPh4) (Aldrich, 99.5%) to an aqueous solution of TMA(Br). The product was washed with water and ethanol, dried in vacuum, and recrystallized from a 3/1 mixture of acetonitrile/ethanol. The redox indicator 9-fluorenone(Aldrich)was recrystallized from ethanoy acetonitrile and dried before dissolving in DMF. Chemical synthesis of the Zintal salt (TMA)4Sn9was performed according to the procedure reported by Haushalter and co-w~rkers.~ Preparative Electrolysis. Constant current preparative electrolyses were carried out using an Electronic Instruments power source together with an Electrosynthesis 640 Digital Coulometer. The cell, the reaction conditions, and the isolation procedure in the inert atmospherewere those developeds for the preparation and isolation of DMP(Pb5). The 50 cm3electrochemicalcell was (8)Kariv-Miller, E.; Christian, P. D.; SvetliEiC, V. Langmuir 1994, 10, 3338. (9) Teller, R.G.; Krause, L. J.;Haushalter, R. C. Inorg. Chem. 1983,
22, 1809.
0 1995 American Chemical Society
Kariv-Miller et al.
1818 Langmuir, Vol. 11, No. 5, 1995 a four-port divided cell designed for anaerobic use on a modified inert gas-vacuum line while performing electrolysis. The cathode was a 10 mm x 40 mm x 0.5 mm tin foil (Alfa, 99+%). Solutions of 0.1 M tetramethylammonium (TMA+)and dimethylpyrrolidinium (DMP+)tetrafluoroborate salts were cathodically reduced using a constant current (4-5 mA/cm2)at 25 "C. The amount of charge transferred was equivalent to 0.25 e-/mol &N+. Following electrolysis, the cathode was covered with a solid product. The solution was removed and the remaining solid was washed with DMF followed by THF and dried in vacuum. The entire cell was then sealed and transferred to the glove box for further handling in an argon atmosphere. The product stability was tested at various stages of isolation and storage with the redox indicator 9-fluorenone.lo For elemental analysis samples were flame-sealedunder argon in glass ampules. Analyses of C, H, N, F, and Sn were performed byhalytical Laboratories,P.O. Box 1315,D-5250Engelskirchen, Germany. 1H NMR Spectra were recorded on an IBM NR2OOAF instrument. Solution NMR samples were obtained by reacting a small portion of solid R4N-tin with D2O and extracting the liquid away from the residual metal. Differential scanning calorimetry (DSC) experiments were performed on a Perkin-Elmer 7 Series Thermal Analysis System. Preweighed samples were sealed under argon into aluminum pellets. The reference was an aluminum pellet. Scans were run under a nitrogen atmosphere. The temperature scan rate for TMA(Sn5)was 20 "C/min between 25 and 280 "C. Temperature programmed mass spectral analysis was performed on a Finnigan 4000 Instrument at a temperature profile from 20 to 200 "C at 30 "C/min. The m / e peak of 58 (100%)was the predominant fragment. Solid-statemagic angle spinning 13C NMR spectra were obtained on an IBM NRlOOAF spectrometer equipped with a solids accessory and a Doty solid-state probe. The operating frequency for carbon was 25.17 MHz. Samples were contained in 7 mm Kel-F rotors packed in the glovebox. Cross-polarization contact times were 4 ms and recycle times 8 s. Spinning speeds ranged from 2.5 to 3.8 kHz. Spectra were taken at 25 "C under was an external a stream of nitrogen. 1,4-Di-tert-butylbenzene standard for instrumental calibration with a high field peak at 31.0 ppm. For most compounds the total number of transients measured was between 1000 and 3000. X-band microwave electron spin resonance (ESR)experiments were done at 100 K on a Bruker Instruments ER300 Spectrometer employingan ESP 1600data system and equipped with a Bruker liquid nitrogen variable temperature controller. A preweighed sample of TMA(Sn5)was reacted with 0.4 mL of 1.5 M benzophenone. The ESR spectra of TMA(Sn5)and the resulting benzophenone radical anion were recorded. The concentration of unpaired electrons in the solid TMA(Sn6)sample and in the benzophenone radical anion solution were calculated by integration of the spectra and by comparison to a known concentration(l.8 x 10-4M)ofthe radical standard2,2-diphenyl1-picrylhydrazyl (DPPH). X-ray diffraction analyses were carried out on a Siemens D-500 wide-angle X-ray diffractometer equipped with a Radix Data Box integra$ed to an IBM PS/2 computer using Cu Ka radiation (1.5405A). All patterns were generated at a scan speed of 0.5"/s, counting time of 1-2 s, and a step size of 0.05" at 25 "C and 1atm. Peak 20 values, d-spacings, and relative intensity data were generated using RigakuAJSA,Inc., powder diffraction software. Diffraction lines were identified by comparison to computer-matched standard diffraction files (JCDPS). Samples of R1N-metals were placed in containers and sealed under argon using a quartz capillary tube (d = 1mm) or a glass slide holder covered by an adhesive cellophane tape. The samples are airsensitive and the diffraction patterns were recorded both before and after exposure to air. The glass plate, glass capillaries, and cellophane tape all exhibit a broad scattering (20 between 10 and 30").
Results and Discussion The tetrafluoroborate salts of tetrarmethylammonium (TMA+) and dimethylpyrrolidinium (DMP+) were ca(10)Dehl, R.; Fraenkel, G. K. J. Chem. Phys. 1963,39,1793.
Table 1. Elemental Analyses of Isolated DMP(Sn,) and TMA(Sn,)
composition %n TMA(Sn2 DMP(Sn,Y C 7.04 12.58 H 1.65 2.50 N 1.99 2.44 Sn 86.60 80.31 Total 99.28 97.83 a Calculated empirical formula: C4.1H11.6N1.0Sn5.2. Calculated empirical formula: C~H14.3N1.0Sn3.9. element
thodically reduced a t tin electrodes. Solutions of 0.1 M reactant salt in DMF were electrolyzed using a constant current (4-5 mA/cm2)a t 25 "C. The products, TMA(Sn,) and DMP(Sn,), were isolated. The amount of product prepared in each experiment was 200 mg. Both tin derivatives are solid and were isolated as fine powders. TMA(Sn,) is deep red, while DMP(Sn,) is dark gray. The first falls off the cathode while the latter adheres to it. Under an inert atmosphere, the materials were stable for a long time. Powder TMA(Sn,) and DMP(Sn,) were sealed in ampules and analyzed under a n inert atmosphere for C, H, N, F, and Sn. The results and the empirical formulae derived from them are presented in Table 1. Taken a t face value, the formula indicate the stoichiometries TMA(SnE) and DMP(Sn4). However, R4N-metals are very reactive, air-sensitive materials and it is reasonable to expect that their elemental analysis will be less than perfect. Therefore, elemental analysis can serve to deduced only a n approximate stoichiometric composition, and the safe conclusion is that the empirical formula of R4N-metals derived from Sn is R4N(Sn,), where x is equal or close to 5, is correct. Nevertheless these elemental analyses are the first quantitative evidence for the metal content of R4N(SnX).For simplicity we will refer to them as R4N(Sn5). The elemental analyses provide additional, crucial information. The total of the elements C, H, N, and Sn is nearly 100% thus, the isolation procedure successfully removes solvent and reactant and no counteranion from the parent salt (BF4-) is incorporated in the product. The carbon to hydrogen to nitrogen ratio is of particular importance. It is 4.1/11.6/1 for TMA+ (expected C/WN = 4/12/11 and 6/14.3/1 for DMP+ (expected C/WN = 6/14/11, demonstrating the presence of intact cations in the product. Electroneutrality requires a negative charge on the metal moiety. Isolated TMA(Sn5) and DMP(Sn5) were examined to determine some of their properties. When exposed to air, they combust instantaneously and release smoke, leaving behind small, shiny droplets of metal in a brown oil. They are insoluble in ethyl acetate, acetonitrile, chloroform, and hexane. They react with acids (HN03, HC1) and protic solvents (HzO and alcohols), yielding metal and a clear solution. To understand the reaction with proton sources, TMA(Sn5) was reacted with D20 and the 'H NMR of the resulting solution was recorded. The only detectable peak was that of TMA+. The logical conclusion is that R4N(Sn5) reduces acids and protic solvents, causes hydrogen evolution, and regenerates TMA+. Isolation and handling of R4N(Sn5) is tedious and time consuming and most of the experiments that follow were performed with the TMA+ derivative only. Thermal Behavior. The thermal behavior of TMA(SnS)was first studied with a melting point apparatus, by heating it in a capillary sealed under argon. No change occurred up to 100 "C; between 100 and 130 "C the color changed from deep red to light gray and brown liquid
Cathodically Generated Tetraalkylammonium-Tins Table 2.
lacSolid-state NMR of TMA+ Compounds
compound
chemical shift (ppm)=
TMA-BF4 Tm-B(CsHd4 TMA-I
54.6 53.7 55.0
a
compound
TMA-Br TMA-(Sn,) Tm-Sng
chemical shift (ppmP
55.4 59.0 58.9
Relative to the external standard 1,4-di-tert-butylbenzene.
condensed on the capillary walls. At 235 "C the gray residue melted, forming metallic droplets. The melting point of tin is 231.7 "C. No further changes took place upon heating to 280 "C. When cooled, the metallic droplets turned to a gray powder. The conclusion was that TMA(Sn5) decomposes to liberate organic components and metallic Sn which consequently melts. It is noteworthy that TMABF4 melts above 330 "C. The same behavior was observed using differential scanning calorimetry (DSC). In the range of 25-280 "C, the decomposition manifests itself as a large, symmetrical exotherm (114125 "C) and the melting of Sn as a sharp endotherm (229 "C). To learn about the organic thermal decomposition products, TMA(Sn5)was heated in the mass spectrometer from 25 to 200 "C a t a rate of 30 "C/min. At 120 "C the total ionic current increased and fragmentation was detected. The most intense peak was observed a t m I e = 58, which corresponds to (CH&N+=CH2, a typicalll fragment of dimethylalkylamines. An identical peak, which was detected in a similar mass spectrometry experiment with DMP(Pb5),was rationalized by reductive cleavage of DMP+. Assuming a pathway similar to t h a t for DMP(Pb51, we propose that upon heating of TMA(Sn5) electron transfer takes place from the negatively charged metal to the cation. Metallic Sn is regenerated and TMA+ cleaves to trimethylamine, which shows the m l e = 58 fragment. 13C Solid-state NMR. The cross-polarization magic angle spinning 13C NMR spectrum of TMA(Sn5) shows only a singlet a t 59.0 ppm. The peak corresponds to the four equivalent carbons of TMA+. It demonstrates the presence of TMA+and the absence of other carbon sources in the solid. For comparison, the spectra of several TMA+ salts were recorded and the chemical shifts are listed in Table 2. It is noted that the absorption of TMA(Sn5)is at a significantly lower field than the absorptions of regular TMA+ salts. The anions of the salts affect the chemical shift of the associated cations.12 The chemical shift of TMA+ in TMA(Sn5)may indicate a high degree of charge separation within the solid. Alternatively, the larger chemical shift may arise from the conduction electrons of the metal moiety. To obtain a pertinent model for comparison the Zintl salt (TMA)4(Sn9), in which TMA+is associated with (Sr19)~-, was synthesizedg and its spectrum was recorded (Table 2). The chemical shifts of TMA+, 58.9 ppm for (TMA)4(Snd and 59.0 ppm TMA(Sn5), are very close and present evidence for the similarity of the associated anions. It is emphasized that (TMA)d(Sng)is similar, but not identical, to TMA(Sn5). (TMA)4(Sng)is brown, not red, and ofcourse has a different elemental composition. We further note that, in our hands, the Zintl salt was significantly less stable than TMA(Sn5);for this reason it was not considered as a standard for other experiments. Electron SpinResonance. The ESR spectrum of solid TMA(Sn5) (100 K and microwave frequency of 9.43 GHz) (11)Tello, M. J.; Bocanegra, E. H.; Arrandiaga, M. A. Thermochim. Acta 1975,11, 96. (12) Pretsch, E.; Clerc, T.; Seibl, J.; Simon, W. Tables of Spectral Data for Structure Determination of Organic Compounds; SpringerVerlag: Heidelberg, Germany, 1983.
Langmuir, Vol. 11, No. 5, 1995 1819 is similar to that of DMP(Pb5). It shows a single derivative peak with a Dysonian line shape and a g factor of 2.0021. The ESR spectrum of DMP(Pb5) was discussed in detail6 elsewhere, and in a similar manner the absorption ofTMA(Sn5)can be attributed to conduction electrons of the metal moiety. It was expected that, like DMP(Pbb), the reducing electrons of TMA(Sn5) are mostly paired and, like DMP(Pbd, reaction with benzophenone8 was used to validate the hypothesis. The ESR spectrum of a TMA(Sn5)sample was recorded, the sample was then reacted with a known amount of benzophenone in THF and the spectrum of the resulting blue solution of benzophenone radical anion was recorded. The concentration of free electrons before and after reaction with benzophenone was estimated by integration of the respective ESR peaks and using a calibration curve constructed from known concentrations of DPPH in THF. The concentration of unpaired electrons was 5.3 x M in TMA(Sn5) and 2.0 x lop3M in the benzophenone radical anion solution. Thus, the amount of electrons transferred from TMA(Sn5)to benzophenone is 3600 times larger than that of its unpaired electrons. It is concluded that most of the reducing electrons of TMA(Sn5) are paired and the bulk solid is diamagnetic. X-ray Powder Diffraction. The next step was to attempt understanding the structure of the solid R4Nmetals and if possible to identify the structural unit. X-ray analysis of single crystals is the obvious method for this purpose. It also seemed practicable because nucleation and growth have been e ~ t a b l i s h e d ~as, ~a n integral, important part of the electrogeneration of R4N-metals. The three solids that have been isolated, DMP(Pb5), TMA(Sn5),and DMP(Sn5)were used in this investigation. Numerous efforts for crystallization were made. A large variety of conditions that have been developed13 for electrochemical crystal growth, such as low current density, long generation times (2 days to 2 weeks), high and low electrolyte concentrations, potential pulse methods and different electrode arrangements, were tested. All attempts to produce a crystal of a suitable size failed. A study of the X-ray powder diffraction patterns was a natural resort and indeed provided useful structural information. A series of experiments was conducted for each of the three materials, DMP(Pb5), TMA(Sn51, and DMP(Sn5). Each series consisted of recording the X-ray powder diffraction patterns of a solid sample, sealed under argon, and ofthe same sample after exposure to air. The patterns of the cathodes used for preparation were recorded, for comparison. They showed face centered cubic (fcc) Pb (Figure 1)and tetragonal P-white Sn (Figure 2) and were verified by computer match with the appropriate standard diffraction files.14 All patterns exhibited a broad feature (10"-30" = 28) due to amorphous scattering by the apparatus (glass plate, glass capillary, and cellophane tape). This did not interfere with the detection of crystalline diffractions. The pattern of DMP(Pb5) (Figure 3) showed intense crystalline lines. The only change in the pattern caused by exposure to air was a n increased intensity of the lines. The pattern of DMP(Pb5) and the decomposition product perfectly matched that of metallic Pb. To reiterate, the Pb cathode, the dark gray powder (that analyzes as DMP(Pbs)), and the gray clumps (regenerated lead) in brown (13)Williams, J. M.; Wang, H. H.; Emge, T. J.;Geiser, V.; Beno, M. A,; Leung, P. C. W.; Carlson, D. K.; Thorn, R. J.;Schultz, A. J.;Whangbo, M. H. Progress in Inorganic Chemistry; Lippard, S. J., Ed.; Wiley: New York, 1987; Vol. 35, pp 77-82. (14)JCPDS Powder Diffraction Files; McClune, W. F., Ed.; JCPDS International Center for Diffraction Data: Swarthmore, PA, 1986.
Kariv-Miller et al.
1820 Langmuir, Vol. 11, No. 5, 1995
Pb cathode
10
10
20
30
40
50
70
60
80
90
100
26 ( d e g r e e s )
20
30
40
SO
60
70
80
90
100
26 ( d e g r e e s ) Figure 4. X-ray powder diffraction pattern ofTMA(Sn5)sealed
by an adhesive cellophane tape on a glass plate holder: count time 2 s.
Figure 1. X-ray diffraction pattern of a lead foil cathode: glass plate holder, count time 1 s.
Sn cathode
10
20
30
40
50
60
70
80
90
100
28 (degrees)
Figure 6. X-ray powder diffraction pattern of TMA(SN5) exposed to air: glass plate holder, count time 2 s.
20 ( d e g r e e s ) Figure 2. X-ray diffraction pattern of a tin foil cathode: glass plate holder, count time 1 s.
DMP( Pb5)
,, 3
, ,
(
20
,,,,
, , , , , , ... , , ,, 30
40
I.
(
50
.I
, , ,
1.1 1
,
60
,
,,
. , . , . !. , , , . , ,
,
I
70
n.
80
90
,
, ,
100
26 ( d e g r e e s ) Figure 3. X-ray powder diffraction pattern ofDMP(Pb5) sealed in a quartz capillary tube: count time 1.5 s.
oil (amines from cleavage of DMP+), formed by the decomposition in air, all exhibit the X-ray powder pattern of fcc Pb. In view of the many proofs for the different composition and properties of the three phases it was concluded that the diffraction lines in the pattern of DMP-
(Pb5) are characteristic of the material, and crystallites of metallic Pb are a part of its structure. Interesting results were obtained by periodically recording the diffraction patterns of "air-decomposed DMP(Pb5) samples, left in the air for several hours. The initial pattern, characteristic to Pb, changed to that of PbO. The yellow, lead(I1) oxide is normally formed only a t very drastic conditions, for example by blowing molten lead through a n atmosphere of pure oxygen.15J6 This demonstrates that the Pb regenerates from DMP(Pb5) in a very reactive form. It is possible that the high reactivity is associated with the small size of the particles." Samples of TMA(Sn5) (shown in Figure 4)and DMP(Sn5) exhibited no defined diffraction lines that could be attributed to a crystalline phase. After exposure to air, both samples displayed the pattern of metallic Sn. The pattern of"air-treated" TMA(Sn6)is shown as an example in Figure 5. The absence of crystalline lines, in the patterns of TMA(Sn5) and DMP(Snd, suggests that the materials are amorphous, or that they consist of crystallites that are too small to produce well-defined X-ray patterns. In view of the Pb diffraction pattern displayed by DMP(Pb5) and the similarity of this material to the Sn derivatives, it is likely that Sn crystallites are present in the structure of R4N(Snd, but their size is insufficient (
