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Inorganics 2013, 1(1), 70-84; doi:10.3390/inorganics1010070
Abstract: The synthesis of diazonium tetrachloroaurate(III) complexes [R-4-C6H4N≡N]AuCl4 involves protonation of anilines CN-4-C6H4NH2, C8F17-4-C6H4NH2, and C6H13-4-C6H4NH2 with tetrachloroauric acid H[AuCl4] 3H2O in acetonitrile followed by one-electron oxidation using [NO]PF6. FT-IR shows the diazonium stretching frequency at 2277 cm−1 (CN), 2305 cm−1 (C8F17), and 2253 cm−1 (C6H13). Thermogravimetric Analysis (TGA) shows the high stabilities of the electron-withdrawing substituents C8F17 and CN compared with the electron-donating substituent C6H13. Residual Gas Analysis (RGA) shows the release of molecular nitrogen as the main gas residue among other small molecular weight chlorinated hydrocarbons and chlorobenzene. Temperature-Dependent X-Ray Powder Diffraction (TD-XRD) shows the thermal decomposition in C6H13 diffraction patterns at low temperature of 80 °C which supports the TGA and RGA (TGA-MS) conclusions. X-ray structure shows N≡N bond distance of approximately 1.10 Å and N≡N-C bond angle of 178°.
Modification of surfaces by redox grafting of diazonium salts is a progressing area of materials chemistry [1,2,3,4,5,6,7,8,9,10,11,12]. Reduction of aryl diazonium salts has been studied on carbon [3,4,6], semiconductors , and nanoparticles surfaces [13,14,15,16,17,18,19]. Grafted organic layers demonstrated distinctive performance in the formation of corrosion inhibitor film on iron [10,20], modification of graphene [2,11,12] and diamond [5,21], preparation of diazonium-modified enzyme electrodes , immobilization of proteins [5,23], grafting polymers , and printing gold via soft lithography [25,26]. In addition to surface modification, aryl diazonium salts have been utilized in palladium-catalyzed cross-coupling reactions. In particular, Heck reactions involving aryl diazonium salts have been widely used in the synthesis of natural products. These electrophiles can also be used in Suzuki-Miyaura, carbonylative, Stille, and carbon-heteroatom cross-couplings .
Electrochemical reductive grafting of diazonium has proven to be an effective, versatile, and simple method for producing organic monolayers (Figure 1) . Despite the wide applications of diazonium salts in fundamental and materials chemistry there are many serious drawbacks: their intrinsic instability at room temperature, difficulty to isolate and purify, and their explosive nature [28,29,30]. In order to avoid some of their limitations diazonium salts must undergo the targeted application in the same medium without isolation. This can be achieved by the in situ diazotization of aryl amines followed by reductive grafting .
The electron density model described the bonding in diazonium by synergistic N→C σ-bonding and C→N π-back-bonding [32,33,34]. A stabilizing effect for the C-N bond includes the formation of charge-transfer complexes, metal-based anion, and coordination [35,36,37]. Based on the relative size of diazonium and the void of the polyether ring, basicity of the oxygen atoms, and steric hindrance in the ring, it is possible to stabilize the diazonium by complexation with macrocyclic polyethers [34,35]. The formation of aryl diazonium ion-crown ether complexes increased the thermal and photochemical stabilities of aryl diazonium ions in solution and in the solid state. Metal-based anion approach was also utilized in order to stabilize the C-N bond . A complex of aryl diazonium with the anion [ZnCl4]2− was synthesized and X-ray structure was determined . Coordination of the in situ generated diazonium demonstrated to be a plausible route for the stabilization of aliphatic diazonium salts .
The synthesis of gold complexes of potential applications in materials chemistry is a rising field of interest [38,39]. We describe a facile procedure for the synthesis of stable diazonium tetrachloroaurate(III) complexes. Preliminary results on the synthesis and grafting have been reported in a communication to Inorganic Chemistry .
2. Results and Discussion
2.1. Synthesis and Characterization
In diazotization reactions, a considerably greater amount of HCl, H2SO4, or HClO4 acid should be used even if strong basic amines are used as reagents. The tetrachloroauric(III) acid based procedure can be utilized in preparation of a wide range of diazonium compounds in organic solvents without the need for mineral acids (Scheme 1) . Protonation of CN-4-C6H4NH2 with H[AuCl4]•3H2O in water resulted in the formation of CN-4-C6H4NH2AuCl3, 1 (Scheme 2). However, the protonation in acetonitrile formed the anilinium tetrachloroaurate(III) complex [CN-4-C6H4NH3]AuCl4, 2 (Scheme 3), which was easily oxidized by [NO]PF6 to form the yellow diazonium tetrachloroaurate(III) complex [CN-4-C6H4N≡N]AuCl4, 3, in a high yield. Similarly, the diazonium tetrachloroaurate(III) complexes [C8F17-4-C6H4N≡N]AuCl4, 4, and [C6H13-4-C6H4N≡N]AuCl4, 5, were synthesized in high yields. The diazonium cation was verified in solution by azo coupling with 2-naphthol to give the dark orange color of diazine. The diazonium tetrachloroaurate(III) complexes are soluble in chloroform, acetonitrile, dimethylformamide, and dimethylsulfoxide. The good solubility is an important advantage over diazonium salts of BF4−, Cl−, and PF6−. Although caution should be exercised when handling all diazonium salts, there were no indications that gold diazonium compounds are particularly hazardous.
FT-IR spectrum of 3 in the solid state showed the νN≡N stretching frequency at 2277 cm−1 and 1H NMR spectrum in CDCl3/DMSO-d6 displayed two doublets assigned to the phenyl protons at δ = 7.71 and 8.33 ppm, remarkably shifted from 6.65 and 7.45 ppm in the 4-aminobenzonitrile ligand (Figure 2). FT-IR spectrum of 4 displayed the νN≡N stretching frequency at 2305 cm−1 and 1H NMR spectrum in CDCl3/DMSO-d6 displayed the phenyl protons at δ = 7.67 and 8.59 ppm. FT-IR spectrum of 5 displayed the νN≡N stretching frequency at 2253 cm−1 and 1H NMR spectrum in CDCl3/DMSO-d6 displayed the phenyl protons at δ = 7.75 and 8.60 ppm.
2.2. X-ray Structure
From acetonitrile solvent at room temperature well-shaped yellow crystals of diazonium tetrachloroaurate(III) complexes were obtained. Structures 1–3 have been reported in reference . X-ray structure of 5 is shown in Figure 3 and X-ray data is presented in Table 1. The crystals are quite stable and no sign of decomposition was seen in the X-ray frames. The N≡N distance is typical of diazonium, approximately 1.10 Å. The unit cell shows the arrangement of the tetrachloroaurate anions around the diazonium. Each diazonium group is surrounded by four [AuCl4]− normal to the N≡N axis. The N(outer)…Cl distances are 3.217, 3.267, 3.300, and 3.468 Å (Figure 4). The outer nitrogen is connected to four chloride centers in a square-planar arrangement. The close proximity to the N≡N group, Cl…N≡N interaction, and the fact that the chloride is less nucleophilic in the tetrachloroaurate anion than the free chloride are presumably contributing factors for the increased stability of the diazonium salts.
Several diazonium salts have been isolated in the last few decades and their X-ray structures have been reported [40,41,42,43,44,45]. The structure of benzene diazonium chloride showed N≡N…Cl distance 3.22–3.56 Å. The first example of diazonium stabilization by multiple and close contacts with tosylate anion shows the shortest distances between the diazonium nitrogen and the tosylate oxygen of 2.673–2.770 Å . shorter than the sum of van der Waals radii (2.90 Å) . The ionic structures of metal-based diazonium complexes [R-4-C6H4N≡N]X (X = SbCl6, FeCl4, Cu2Br2) and [R-4-C6H4N≡N]2ZnCl4 show the shortest interionic distances between the diazonium and the metal anion occur between the halide and the outer nitrogen than to the ammonium nitrogen [42,43]. Metal-based diazonium complexes structurally characterized so far indicate that the bond order in diazonium nitrogen-nitrogen is close to a triple bond distance of approximately 1.1 Å.
|a (Å); ( ° )||15.5144(3)|
|b (Å); ( ° )||38.4270(8) 90.0250(10)|
|c (Å); ( ° )||8.7773(2)|
|Density (cal.), Mg/m3||2.011|
|Abs. coefficient, mm−1||21.395|
|GOF on F2||1.061|
|R1, wR2 [I > 2σ(I)]||0.0568, 0.1530|
2.3. Thermal Stability and Residual Gas Analysis Studies
The enormous literature on the thermal stability of diazonium salts covers the greatest variety with inorganic stabilizing anions. Study clearly indicates an important influence of the stabilizing anion, substituent, and water in the unit cell. A correlation between the rate of diazonium nitrogen evolution and the electronic configuration and the electronegativity of the metal ion has been studied . It cannot be concluded from the correlation study if the type of metal has a direct effect on diazonium stability due to the failure to stabilize diazonium salts with some of the metal chlorides used.
When examined by Thermal Gravimetric Analysis (TGA) at a heating rate of 2.0 °C/min the diazonium tetrachloroaurate(III) complexes started the decomposition at approximately 100 °C. The TGA curve of 4 displays the loss of nitrogen and chloride in the first stage of 10% which ends at about 100 °C followed by a more abrupt weight loss until 150 °C. From 150 °C to 400 °C the sample losses ca. 40% weight (Figure 5). The poor volatility of the fluorinated ligand conceivably is the reason for the sluggish weight loss. The percentage gold residue from 5 is 23% (theoretical 23%). TGA of 3 displays a 40% gold residue (theoretical 42%) and 40% (theoretical 37%) for 5.
Temperature-Dependent X-Ray Powder Diffraction (TD-XRD) patterns at 25, 50, 100, and 150 °C display peaks associated with the decomposition of 4, however, the peaks associated with the elemental gold formation are absent within the same temperature range (Figure 6). The diazonium tetrachloroaurate(III) peaks disappeared after heating above 150 °C with the concomitant appearance of a new set of peaks. The cubic gold reflections became more intense within the 250–300 °C range. The TD-XRD associated peaks of 5 (Figure 7) show elemental gold at lower temperature than 4. The diffraction patters also confirmed the highly crystalline nature of gold residues after complete decomposition of 4 and 5. Bragg reflections at 2θ 38.1°, 44.5°, 64.8°, and 78.8° could be indexed based on the face-centered cubic gold structure.
The gases released from diazonium tetrachloroaurate(III) pyrolysis were identified by Residual Gas Analysis (RGA) studies. Figure 8 shows the partial pressure of nitrogen for 4 and 5. In addition to nitrogen, the gases released from each complex in the temperature range 100–450 °C include chlorobenzene, chlorinated small molecular weight hydrocarbons, and chlorine. The fluorous chain and phenyl groups can be clearly seen leaving as PhCl, PhF, and low molecular weight halogenated hydrocarbon fractions CH2Cl2, CCl4, CHCl3, C2HCl3, CH3F, and CF4 (Table 2). In pyrolysis gas chromatography, the major gases of [R2N-4-C6H4N≡N]2ZnCl4 decomposition between 160 °C and 250 °C were, in addition to nitrogen, the corresponding chloroaromatics due to the replacement of diazonium by chlorine .
|Gas||Partial pressure||Gas||Partial pressure|
|N2||2.60 × 10−7||Cl2||1.08 × 10−11|
|CH3F||9.10 × 10−1°||C6H5Cl||6.47 × 10−12|
|CH2Cl2||5.47 × 10−1°||C6H5F||5.21 × 10−12|
|CHCl3||2.90 × 10−11||CCl4||4.29 × 10−12|
|CF4||1.10 × 10−11|
Decomposition of the diazonium tetrachloroaurate(III) complexes, ready evolution of nitrogen, under their respective temperatures using melting point apparatus occurred without melting. It appears that for diazonium tetrachloroaurate(III) complexes there are small temperature ranges over which the gases are evolved. When a heat source was applied to 3 the crystals seemed to release gases beginning at about 129 °C and continued until about 134 °C. When a heat source was applied to 5 the gases were released at temperature range of 110-114 °C and in 4 the gases were released at lower range of 96–97 °C.
3. Experimental Section
3.1. General Procedures
Unless stated otherwise, all manipulations were carried out in either a N2-filled Vacuum Atmospheres Co. glove box or on a Schlenk line using N2. Toluene, acetonitrile, CN-4-C6H4NH2, C8F17-4-C6H4NH2, C6H13-4-C6H4NH2, CDCl3, DMSO-d6, [NO]PF6, and H[AuCl4] 3H2O were purchased from Sigma-Aldrich. All other commercially available reagents were used as received.
3.2. Physical Measurements
Infrared spectra were recorded in the 4000–400 cm−1 range using a Perkin-Elmer FT-IR Spectrum 1000 spectrophotometer. C, H, and N analyses were carried out by Columbia Analytical Services, Tucson, Arizona. 1H and 13C NMR spectra were recorded on a JEOL 400 MHz. Chemical shifts are reported relative to the chloroform solvent peak.
3.3. Thermogravimetric Analysis and Residual Gas Analysis
Thermal stability was studied using a Perkin-Elmer TG/DTA Thermogravimetric analyzer. The scan rate was set at a 2 °C/min from room temperature to 500 °C. The TGA analysis was carried out in air and in argon atmosphere. Residual gas analyses were carried out using RGA Pro 2000 from Stanford Research Systems. Non-ambient (Temperature Program) X-ray diffraction analysis was carried out on a Panalytical X’Pert Pro X-ray Diffractometer (Model PW3040 Pro) using copper K-alpha radiation (λ = 0.154178 nm). The instrument was operated at 40 KV and 20 mA on an HTK 1200 oven stage.
3.4. X-ray Diffraction
Data for complexes CN-4-C6H4NH2AuCl3, 1, [CN-4-C6H4NH3]AuCl4, 2, and [CN-4-C6H4N≡N]AuCl4, 3, were collected using a Bruker D8 tricycles diffractometer with APEX II detector CCD based equipped with an LT-2 low-temperature apparatus operating at 110 K. Data for 5 was collected at 213 K with a Siemens SMART CCD diffractometer equipped with an LT-2 low-temperature apparatus. A suitable crystal was chosen and mounted on a glass fiber using grease. Cell parameters were determined and refined using APEX II software on all observed reflections which corrects for Lp and decay [50,51]. Absorption corrections were applied using SADABS supplied by George Sheldrick . The structures are solved by the direct method using the SHELXS-97 program and refined by least squares method on F2, SHELXL-97, incorporated in SHELXTL-PC V 5.03 [53,54].
Synthesis of CN-4-C6H4NH2AuCl3, 1: To a 67 mg (0.5 mmol) of CN-4-C6H4NH2 suspended in 15 of mL distilled water was added 190 mg (0.5 mmol) H[AuCl4] 3H2O at room temperature. The water insoluble yellow complex was stirred for 2 h. The yellow precipitate was filtered, washed twice with 5 mL of water, and dried under vacuum to give 153 mg. Yield 64.01%. 1H NMR (CDCl3/DMSO-d6): δ 7.20 (d, 9.7 Hz, 2H), 7.58 (d, 9.7 Hz, 2H). 13C NMR (CDCl3/DMSO-d6): δ 119.62 (C-1), 125.0 (C-2, C-3), 133.63 (C-4), 143.0 (CN). Elemental analysis calculated for C7H6AuCl3N2: %C = 19.95, %H = 1.43, %N = 6.65. Exp: %C = 19.89, %H = 1.40, %N = 6.75.
Synthesis of [CN-4-C6H4NH3]AuCl4, 2: To a 67 mg (0.5 mmol) of CN-4-C6H4NH2 dissolved in 10 mL of CH3CN was added 197 mg (0.5 mmol) H[AuCl4] 3H2O at room temperature. The yellow solution was stirred for 2 h and 30 mL of cold ether was added. The reaction mixture was stirred for additional 2 h and the precipitate was filtered, washed twice with 5 mL cold ether, and dried under vacuum to give 228 mg. Yield 87.69%. 1H NMR (CDCl3/DMSO-d6): δ 6.70 (d, 9.3 Hz, 2H), 7.17 (d, 9.3 Hz, 2H). 13C NMR (CDCl3/DMSO-d6): δ 104.5 (C-1), 118.9 (C-2), 133.39 (C-3, C-4), 144.24 (CN). Elemental analysis calculated for C7H7AuCl4N2: %C = 18.36, %H = 1.54, %N = 6.12. Exp: %C = 18.28 , %H = 1.56, %N = 6.09.
Synthesis of [CN-4-C6H4N≡N]AuCl4, 3: To a 118 mg (1.0 mmol) of CN-4-C6H4NH2 dissolved in 10 mL of CH3CN was added 391 mg (1.0 mmol) H[AuCl4] 3H2O at 0 °C. A 184 mg (1.0 mmol) [NO]PF6 was added and stirring continued at 0 °C. The yellow solution was stirred for 2 h and 30 mL of cold ether was added. The reaction mixture was stirred for additional 2 h and the precipitate was filtered, washed twice with 5 mL of cold ether, and dried under vacuum to give 420 mg. Yield 91.90%. 1H NMR (CDCl3/DMSO-d6): δ 7.71 (d, 9.7 Hz, 2H), 8.33 (d, 9.7 Hz, 2H). 13C NMR (CDCl3/DMSO-d6): δ 115.4 (C1), 120.3 (C2), 122.4 (C3), 133.3.24 (C4), 134.3 (CN). Elemental analysis calculated for C7H4AuCl4N3: %C = 17.93, %H = 0.86, %N = 8.96. Exp: %C = 17.95, %H = 0.79, %N = 8.84. FT-IR (solid): νN≡N stretching frequency at 2277 cm−1. UV-Vis (2.13 × 10−4 M in CH3CN): 318 nm (10,938 M−1 cm−1).
Synthesis of [C8F17-4-C6H4N≡N]AuCl4, 4: To a 512 mg (1.0 mmol) of C8F17-4-C6H4NH2 dissolved in 20 mL of toluene at 0 °C was added 400 mg (1.0 mmol) H[AuCl4] 3H2O dissolved in 3 mL of acetonitrile. The solution was stirred for 2 h before a 173 mg (1.0 mmol) [NO]PF6 dissolved in 3 mL of acetonitrile was added. The dark red solution was stirred for an additional 2 h at 0 °C under nitrogen during which canary yellow microcrystals precipitated. The microcrystals were filtered, washed twice with 5 mL of cold toluene, and dried under vacuum to give 835 mg. Yield 96.70%. 1H NMR (CDCl3/DMSO-d6): δ 7.67 (d, 9.7 Hz, 2H), 8.59 (d, 9.7 Hz, 2H). 13C NMR (CDCl3/DMSO-d6): δ 120.4 (C-1), 129.5 (C-2), 133.6 (C-3), 138.0 (C-4). Elemental analysis calculated for C14F17H4AuCl4N2: %C = 19.51, %H = 0.47, %N = 3.25. Exp: %C = 19.33, %H = 0.66, %N = 3.11. FT-IR (solid): νN≡N stretching frequency at 2305 cm−1. UV-Vis (1.89 × 10−4 M in CH3CN): 305 nm (2983 M−1 cm−1).
Synthesis of [C6H13-4-C6H4N≡N]AuCl4, 5: To a 178 mg (1.0 mmol) of C6H13-4-C6H4NH2 dissolved in 20 mL of toluene at 0 °C was added 400 mg (1.0 mmol) H[AuCl4] 3H2O dissolved in 3 mL of acetonitrile. The solution was stirred for 2 h before a 173 mg (1.0 mmol) [NO]PF6 dissolved in 3 mL of acetonitrile was added. The solution was stirred for an additional 2 h at 0 °C under nitrogen and was kept in the fridge overnight to form yellow microcrystals out of olive green solution. The precipitate was filtered, washed twice with 5 mL of cold toluene, and dried under vacuum to give 425 mg. Yield 80.49%. 1H NMR (CDCl3/DMSO-d6): δ 7.75 (d, 8.1 Hz, 2H), 8.60 (d, 8.1 Hz, H). Elemental analysis calculated for C12H17AuCl4N2: %C = 27.31, %H = 3.14, %N = 5.25. Exp: %C = 27.29, %H = 3.24, %N = 5.31. FT-IR (solid): νN≡N stretching frequency at 2253 cm−1. UV-Vis (5.8 × 10−4 in CH3CN): 279 nm (14,884 M−1 cm−1).
We described the synthesis of stable diazonium tetrachloroaurate(III) complexes of potential applications in surface grafting. The structures are similar to inorganic based anions diazonium salts. The synthesized compounds are soluble in several organic solvents and stable at room temperature in the solid state and solution. Research continues on the diazonium tetrachloroaurate(III) complexes to synthesize robust gold-carbon nanoparticles and films.
Andrew Goudy of Delaware State University is acknowledged for the TGA, RGA, and TD-XRD measurements.
Conflicts of Interest
The authors declare no conflict of interest.
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