Transition Met Chem (2013) 38:843–847 DOI 10.1007/s11243-013-9757-x
Synthesis and electrocatalytic properties of a dinuclear palladium(I) 1-[(2-bromo)benzene]-3[(2-carboxymethyl)benzene]triazenide complex Jing Chu • Qi-ying Lv • Xiao-hua Xie Wei Li • Shuzhong Zhan
•
Received: 24 July 2013 / Accepted: 27 August 2013 / Published online: 5 September 2013 Ó Springer Science+Business Media Dordrecht 2013
Abstract The reaction of 1-[(2-bromo)benzene]-3-[(2carboxymethyl)benzene]triazene and [Pd(CH3CN)4]Cl2 gave a new dinuclear triazenido complex [Pd2L2] 1, which has been characterized by NMR spectra and X-ray crystallography. Electrochemical studies showed that complex 1 is capable of generating hydrogen from acetic acid in CH2Cl2.
Introduction The molecular hydrogen derived from non-carbon sources has emerged as a potential fuel for sustainable energy cycles that minimize carbon dioxide emissions [1, 2]. Hydrogenase enzymes [3, 4] efficiently catalyze both the production and the oxidation of hydrogen using earth-abundant metals. The electrocatalytic mechanism of enzymes in hydrogen-producing systems has been investigated [5, 6], but an important fundamental question for the enzymes that remains to be answered is the precise role that the proposed pendant base would play in hydrogen oxidation and production. These
Electronic supplementary material The online version of this article (doi:10.1007/s11243-013-9757-x) contains supplementary material, which is available to authorized users. J. Chu Q. Lv S. Zhan (&) College of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China e-mail:
[email protected] X. Xie College of Chinese Language and Culture, Jinan University, Guangzhou 510610, China W. Li Department of Chemistry, Guizhou Minzu University, Guiyang 550000, China
considerations have led to the development of molecular catalysts employing more abundant metals, and several complexes that contain nickel [7–10], cobalt [11–16], iron [17–20], or molybdenum [21, 22] have been developed as electrocatalysts for the production of hydrogen. However, to the best of our knowledge, there is as yet no report on the use of palladium complexes to electrocatalyze hydrogen evolution. With this in mind, our group is trying to probe the possible mechanism through the synthesis of dinuclear palladium complexes with triazenido ligands, which have been successfully used in transition metal chemistry [23, 24], and to explore the possibility of electrocatalysis for hydrogen evolution. The central nitrogen of the triazenido moiety imparts greater basicity on the [NNN]- relative to the neutral nitrogen, making the triazenido group more electron donating, amenable to binding to H?, and hydrogen production. Here, we present the synthesis, structure, and properties of a new dinuclear triazenido-palladium(I) complex [Pd2L2] 1, as well as its electrocatalytic properties for the production of hydrogen from acetic acid in CH2Cl2.
Experimental All manipulations were carried out under air. Solvents and chemicals were of analytical grade purity from commercial sources and were used without further purification. Physical measurements 1
H NMR spectroscopy was recorded on a Bruker AM 500 instrument at room temperature and was measured in chloroform-d using tetramethylsilane (TMS) as an internal reference. UV–vis absorption was measured on a Hitachi U-3010. All electrochemical measurements were undertaken
123
844
under nitrogen in CH2Cl2 solution with 0.1 M [n-Bu4N][ClO4] as a supporting electrolyte. Cyclic voltammograms (CV) were recorded on an Ingsens1030 Electrochemical Workstation [Ingsens Instruments (Guangzhou) Co. Ltd., China] using a standard three-electrode cell with an auxiliary platinum wire electrode (CE), a glassy carbon electrode (GCE, 3 mm in diameter), and an Ag/Ag? (a certain concentration of AgClO4 in CH2Cl2) reference electrode. Acetic acid was added by syringe. Synthesis of ligand, HL A solution of 2-bromoaniline (1.72 g, 10 mmol) in water (5 mL) was mixed with 1 mol L-1 HCl 30 mL (30 mmol) at 0 °C. An aqueous solution (15 %) of sodium nitrite (15 mmol) was added dropwise with stirring. Once the amine was dissolved, a solution of methyl anthranilate in ethanol (1.52 g, 10 mmol) was added at 0 °C, and the mixture was stirred for 2 h. The reaction mixture was neutralized with 15 % aqueous NaCH3CO2 to give a yellow precipitate. The reaction mixture was filtered, and the solid was purified by crystallization at -4 °C from ethyl acetate to obtain yellow crystals, which were collected and dried in vacuo (2.6 g, 78 %). Calcd for C14H12N3O2Br:C, 50.3; H, 3.6; N, 12.6. Found: C, 50.4; H, 3.7; N, 12.5. 1 H NMR (400 MHz, CDCl3) d 12.57 (s, 1H), 7.99 (d, J = 7.2 Hz, 1H), 7.90 (d, J = 8.4 Hz, 1H), 7.63 (d, J = 8.0 Hz, 1H), 7.53 (d, J = 8.0 Hz, 1H), 7.50 (t, J = 7.8 Hz, 1H), 7.31 (t, J = 7.6 Hz, 1H), 7.10 (t, J = 8.2 Hz, 1H), 7.03 (t, J = 7.6 Hz, 1H), 3.92 (s, 3H). 13 C NMR (101 MHz, CDCl3) d 167.63, 147.28, 143.48, 134.52, 133.54, 131.14, 128.72, 127.96, 121.70, 120.92, 119.56, 114.62, 112.25, 52.29. UV–vis [CH2Cl2, kmax/nm (e/L mol-1 cm-1)]: 226 (1.52 9 104), 366 (1.75 9 104). Synthesis of complex 1 To a solution containing 1-[(2-bromo)benzene]-3-[(2-carboxymethyl)benzene]triazene (HL) (0.33 g, 1 mmol) and triethylamine (0.10 g, 1 mmol) in acetonitrile (20 mL), [Pd(CH3CN)4]Cl2 (0.34 g, 1 mmol) was added and the mixture was stirred for 2 days. The solution was allowed to slowly evaporate, affording deep red crystals, which were collected and dried in vacuo (0.23 g, 52.0 %). Calcd for C28H22N6O4Br2Pd2: C, 38.3; H, 2.5; N, 9.6. Found: C, 36.3; H, 2.6; N, 9.5. 1H NMR (400 MHz, CDCl3) d 7.83–7.65 (m, 2H), 7.51 (t, J = 9.7 Hz, 2H), 7.42–7.35 (m, 4H), 7.19–7.10 (m, 6H), 6.91–6.87 (m, 2H), 3.76 (d, J = 5.6 Hz, 6H). 13C NMR (101 MHz, CDCl3) d 168.82, 152.23, 148.52, 132.44, 131.19, 130.85, 127.86, 125.36, 124.66, 122.99, 122.96, 121.92, 114.38, 52.72. UV–vis [CH2Cl2, kmax/nm (e/L mol-1 cm-1)]: 302 (1.72 9 104), 387 (1.70 9 104), 483 (8.69 9 103).
123
Transition Met Chem (2013) 38:843–847 Table 1 Crystal data and structure refinements for complex 1 Parameter
Complex 1
Empirical formula
C28H22Br2Pd2N6O4
Formula weight ˚) k (A
879.14
Crystal system
Triclinic
Space group ˚ a/A
8.1283(4)
0.71073 P-1
˚ b/A ˚ c/A
11.3780(6)
a/°
81.6460(10)
b/°
84.1320(10)
c/° ˚° V/A
75.0330(10) 1441.47(13)
16.3440(8)
Z
2
Dc/Mgm-3
2.025
F(000)
850
h range for data collection
1.66°–27.51°
Reflections collected/unique
8874/3240
Data/restraints/parameters
2860/0/163
Goodness-of-fit on F2
1.017
Final R indices [I [ 2sigma(I)]
R1 = 0.0396 wR2 = 0.0997
R indices (all data)
R1 = 0.0697 wR2 = 0.1173
X-ray crystallography Data were collected with a Bruker SMART CCD area detector using graphite monochromated Mo Ka radiation ˚ ) at room temperature. All empirical absorption (0.71073 A corrections were applied by using the SADABS program [25]. The structures were solved using direct methods, and the corresponding non-hydrogen atoms were refined anisotropically. All the hydrogen atoms of the ligands were placed in calculated positions with fixed isotropic thermal parameters and included in the structure factor calculations in the final stage of full-matrix least-squares refinement. All calculations were performed using the SHELXTL computer program [26]. Table 1 listed details of the crystal parameters, data collection, and refinement for complex 1. The selected bond distances and angles are listed in Table 2.
Results and discussion Synthesis and characterization In the presence of Et3N, reaction of the ligand (HL) and [Pd(CH3CN)4]Cl2 affords red crystals of complex 1 in
Transition Met Chem (2013) 38:843–847
845
˚ ) and bond angles (°) for comTable 2 Selected bond distances (A plex 1 ˚) Bond distances (A N(1)–N(2)
1.300(5)
N(2)–N(3)
1.304(5)
N(4)–N(5)
1.291(5)
N(5)–N(6)
1.290(6)
Pd(1)–N(1)
2.024(4)
Pd(1)–N(4)
2.035(4)
Pd(1)–Pd(2)
2.4277(5)
Pd(2)–N(3)
2.020(4)
Pd(2)–N(6)
2.027(4)
Pd(2)–Br(1)
2.5920(6)
Pd(1)–O(1)
2.141(4)
Bond angles (°) N(1)–N(2)–N(3)
116.6(4)
N(4)–N(5)–N(6)
114.9(4)
N(1)–Pd(1)–N(4)
174.39(15)
N(3)–Pd(2)–N(6)
172.23(15)
Pd(1)–Pd(2)–Br(1) N(1)–Pd(1)–Pd(2)
169.27(2) 88.20(10)
O(1)–Pd(1)–Pd(2) N(4)–Pd(1)–Pd(2)
177.37(11) 86.32(12)
N(6)–Pd(2)–Pd(1)
86.51(11)
N(3)–Pd(2)–Pd(1)
85.74(11)
N(5)–N(6)–Pd(2)
126.2(3)
N(5)–N(4)–Pd(1)
126.0(3)
N(2)–N(1)–Pd(1)
123.3(3)
N(2)–N(3)–Pd(2)
126.1(3)
N(6)–Pd(2)–Br(1)
103.42(11)
N(3)–Pd(2)–Br(1)
84.27(11)
N(4)–Pd(1)–O(1)
94.75(15)
N(1)–Pd(1)–O(1)
90.77(14)
52 % yield, which has been characterized by NMR spectra (Figs. S1–S4) and X-ray crystallography. Palladium(I), which has the d9 configuration, is expected to exhibit paramagnetic behavior. Complex 1, however, is diamagnetic and amenable to NMR analysis (Fig. S3). It is assumed that the d9 Pd(I) centers are spin-coupled through a Pd–Pd bond, as observed in other dinuclear Pd(I) complexes [27]. Crystal structures Single crystal X-ray diffraction analysis reveals the solid state structure of the complex 1 to be a dipalladium complex [Pd2(L)2]. The molecular structure of the complex [Pd2(L)2] is depicted in Fig. 1. The palladium complex [Pd2(L)2] consists of two metal centers bridged by two triazenido ligands in an l, g1, g1-fashion, with approximately linear two coordinate geometries [N(1)–Pd(1)– N(4): 174.39(15)°, N(3)–Pd(2)–N(6): 172.23(15)°] with the deviation caused by the metal atoms being displaced away from each other. Complex 1 is formed by the two N3ligands and the {M2} unit (M = Pd). The Pd–N bond distances in [Pd2(L)2] are within the expected ranges for M(I)–N bonds [Pd–N: 2.035(4), 2.020(4), 2.021(3), and ˚ ] [27]. Similarly, the N–N bond lengths within 2.027(5) A ˚ , N(2)–N(3): the triazenido ligands [N(1)–N(2): 1.300(5) A ˚ ˚ 1.304(5) A, N(4)–N(5): 1.291(5) A, N(5)–N(6): 1.290(6) ˚ ] are also comparable to those observed in related sysA ˚ in complex tems [27]. The Pd–Pd distances of 2.4277(5) A 1 are consistent with a Pd–Pd bond. The distance is similar to those Pd–Pd bonds reported for dinuclear Pd(I)–
Fig. 1 Molecular structure of complex 1. Hydrogen atoms have been omitted
Pd(I) complexes with triazenido ligands [2.4309(3), ˚ ] [27]. 2.4202(3), and 2.4158(4) A Electronic spectra The electronic spectra of HL and complex 1 were recorded in dichloromethane solution (Fig. S5). HL exhibits two bands at 226 and 366 nm, respectively. Complex 1 shows three bands at 302, 387, and 483 nm. Compared with that of HL (366 nm), the absorption maximum of the lowenergy band of complex 1 (387 nm) is red-shifted. This is assigned to spin–orbit coupling from the heavy atom effect of the palladium(I) ion, as well as a Pd ? L MLCT transition upon deprotonated L-. The band at 483 nm is assignable to a r ? r* transition. It is known that Pd(I) complexes can easily cleave their metal–metal bond. In order to confirm this assignment, a photochemical reaction was carried out in the system containing complex 1 and CCl4. Complex 1 and carbon tetrachloride were mixed in CH2Cl2 and irradiated by a 100 W high-pressure mercury lamp, resulting in cleavage to two PdIL moieties, which can abstract a chlorine atom from CCl4. The dynamic changes were monitored by UV–vis absorption spectroscopy. The band at 483 nm decreased during the photolysis (Fig. S6), indicating the cleavage of the metal–metal bond [28]. Electrochemical studies The cyclic voltammogram of complex 1 was measured at a glassy carbon electrode in CH2Cl2 in the presence of [n-Bu4N][ClO4] as supporting electrolyte (Fig. 2). All potentials in this study were referenced to the Fc?/0 couple internal standard (E1/2 = 0.54 V). The cyclic voltammogram
123
846
Fig. 2 Cyclic voltammogram of a 0.50 mM solution of complex 1. Conditions: r.t, 0.1 M [n-Bu4N][ClO4] as supporting electrolyte, scan rate = 100 mV/s, GC working electrode (1 mm diameter), Pt counter electrode, Ag/Ag? reference electrode, Fc?/0 internal standard (0.00 V)
Fig. 3 Scan rate dependence of precatalytic wave for a 0.50 mM solution of complex 1 in CH2Cl2 with 0.1 M [n-Bu4N][ClO4], at scan rates from 0 to 500 mV/s on a glassy carbon electrode (1 mm)
of complex 1 in CH2Cl2 exhibits two reversible couples at -1.58 and 0.39 V. We assign the reversible couples to the metal-centered PdI/0 and PdI/II processes, respectively. As observed in Fig. 3, the current response of the redox events at -1.62 V varies linearly with the square root of the scan rate, which is indicative of a diffusion-controlled process. Such distinctive potential suggests possible usage of this complex as an electrocatalyst for hydrogen evolution. Therefore, to determine possible electrocatalytic activity of this complex, cyclic voltammograms of complex 1 were recorded in the presence of acetic acid in CH2Cl2. To test the catalytic ability of complex 1, we obtained CVs at
123
Transition Met Chem (2013) 38:843–847
Fig. 4 Cyclic voltammogram of a 0.50 mM solution of complex 1, at increasing concentrations of acetic acid. Conditions: r.t, 0.1 M [n-Bu4N][ClO4] as supporting electrolyte, scan rate = 100 mV/s, GC working electrode(1 mm diameter), Pt counter electrode, Ag/Ag? reference electrode, Fc?/0 internal standard (0.00 V)
Fig. 5 Charge buildup versus time at applied potentials from -1.36 V to -1.48 V in a 0.1 M [n-Bu4N][ClO4] solution in CH2Cl2
different acetic acid concentrations. Figure 4 shows a systematic trend in icat observed near -1.62 V with increasing acid concentration from 0 to 9.06 mM. The CVs in the presence of increasing acid indicate hydrogen evolution, with catalytic onset shift to more positive potentials. In order to further confirm the electrocatalytic activity of complex 1, bulk electrolysis of complex 1 was carried out. The onset potential -1.56 V of acetic acid in CH2Cl2 was obtained according to the CVs, which are in the presence and absence of acetic acid (Fig. S7). Bulk electrolysis of complex 1 (0.75 mM) with acetic acid (25 mM) at potential from -1.36 V to -1.48 V was conducted using a glassy carbon plate electrode in a double compartment cell.
Transition Met Chem (2013) 38:843–847
A control experiment performed at -1.56 V under identical conditions, but without complex 1, showed the accumulation of just 0.023 C charge, which is obviously smaller than the charge that conducted at -1.36 V in a 0.75 mM solution of complex 1 (Fig. 5). The charge significantly increased when the applied potential was set to more negative values (Fig. 5). In summary, we have prepared a dipalladium(I) complex that is active for proton reduction to hydrogen from weak acid in organic solvent. The electrocatalytic mechanism of complex 1 is under investigation. Acknowledgments This work was supported by the Research Foundation for Returned Chinese Scholars Overseas of Chinese Education Ministry (No. B7050170), the National Science Foundation of China (No. 20971045, 21271073), the Science Foundation of GuiZhou (No. J-[2012]2190), and the Student Research Program (SRP) of South China University of Technology.
References 1. Walter MG, Warren EL, McKone JR, Boettcher SW, Mi Q, Santori EA, Lewis NS (2010) Chem Rev 110:6446–6473 2. Song LC, Xie ZJ, Wang MM, Zhao GY, Song HB (2012) Inorg Chem 51:7466–7468 3. Fontecilla-Camps JC, Volbeda A, Cavazza C, Nicolet Y (2007) Chem Rev 107:4273–4303 4. Vignais PM, Billoud B (2007) Chem Rev 107:4206–4272 5. Pickett CJ, Tard C (2009) Chem Rev 109:2245–2274 6. Vincent KA, Parkin A, Armstrong FA (2007) Chem Rev 107:4366–4413 7. Appel AM, Pool DH, O’Hagan M, Shaw WJ, Yang JY, DuBois MR, DuBois DL, Bullock RM (2011) ACS Catal 1:777–785 8. Wiese S, Kilgore UJ, DuBois DL, Bullock RM (2012) ACS Catal 2:720–727
847 9. Helm ML, Stewart MP, Bullock RM, DuBois MR, DuBois DL (2011) Science 333:863–866 10. Kilgore UJ, Roberts JA, Pool DH, Appel AM, Stewart MP, DuBois MR, Dougherty WG, Kassel WS, Bullock RM, DuBois DL (2011) J Am Chem Soc 133:5861–5872 11. Artero V, Chavarot-Kerlidou M, Fontecave M (2011) Angew Chem Int Ed 50:7238–7266 12. Razavet M, Artero V, Fontecave M (2005) Inorg Chem 44:4786–4795 13. Baffert C, Artero V, Fontecave M (2007) Inorg Chem 46:1817–1824 14. Berben LA, Peters JC (2010) Chem Commun 46:398–400 15. Stubbert BD, Peters JC, Gray HB (2011) J Am Chem Soc 133:18070–18073 16. Chang CJ, Bigi JP, Hanna TE, Harman WH, Chang A (2010) Chem Commun 46:958–960 17. Hall MB, Surawatanawong P, Tye JW, Darensbourg MY (2010) Dalton Trans 39:3093–3104 18. Lichtenberger DL, Vannucci AK, Wang SH, Nichol GS, Evans DH, Glass RS (2010) Dalton Trans 39:3050–3056 19. Camara JM, Rauchfuss TB (2012) Nat. Chem 4:26–30 20. Liu M, Zhan C, Wang X, Wei Z, Evans DJ, Ru X, Zeng X (2010) Dalton Trans 39:11255–11262 21. Karunadasa HI, Montalvo E, Sun Y, Majda M, Long JR, Chang CJ (2012) Science 335:698–702 22. Karunadasa HI, Chang CJ, Long JR (2010) Nature 464: 1329–1333 23. Lei WJ, Tan XW, Han LJ, Zhan SZ, Li BT (2010) Inorg Chem Commun 13:1325–1328 24. Johnson AL, Willcocks AM, Richards SP (2009) Inorg Chem 48:8613–8622 25. Sheldrick GM (1996) SADABS, Program for empirical absorption correction of area detector data, University of Go¨tingen, Go¨tingen, Germany 26. Sheldrick GM (1997) SHELXS 97, Program for crystal structure refinement, University of Go¨tingen, Go¨tingen, Germany 27. Nuricumbo-Escobar JJ, Campos-Alvarado C, Rı´os-Moreno G, Morales-Morales D, Walsh PJ, Parra-Hake M (2007) Inorg Chem 46:6182–6189 28. Tanase T, Kawahara K, Ukaji H, Kobayashi K, Yamazaki H, Yamamoto Y (1993) Inorg Chem 32:3682–3688
123
