Effects of the Counteranion on the Pyrazole−Nitrile Coupling Reaction

Aug 5, 2009 - Patricia Gómez-Iglesias , Marta Arroyo , Sonia Bajo , Carsten Strohmann ... Marta Arroyo , Patricia Gómez-Iglesias , Jose Miguel Martín-...
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Organometallics 2009, 28, 4923–4928 DOI: 10.1021/om9004753

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Effects of the Counteranion on the Pyrazole-Nitrile Coupling Reaction Mediated by Nickel(II) Ions Chang-Chih Hsieh, Chia-Jung Lee, and Yih-Chern Horng* Department of Chemistry, National Changhua University of Education, 1 Jin-De Road, Changhua 50058, Taiwan Received June 4, 2009

The reaction of Ni(ClO4)2 with 4 equiv of pyrazole (pzH) in MeCN leads to [Ni(pzH)2(HNd C(Me)pz)2](ClO4)2 via the intermediate [Ni(MeCN)2(pzH)4](ClO4)2, which is the first pyrazole-nitrile coupling reaction activated by a Ni(II) complex. The reaction is also successfully conducted with other nitriles RCN bearing an electron-donating group (R=Et, Bz) and is counteranion-specific. The reaction proceeds more quickly with NO3 as counteranion than with ClO4 , and BF4 is found to be unsatisfactory for the coupling reaction. As for the X-ray crystal structures, [Ni(MeCN)2(pzH)4](ClO4)2 possesses stronger pyrazolyl NH 3 3 3 anion hydrogen-bonding interactions than does [Ni(MeCN)2(pzH)4](BF4)2. The same trend is also observed by solution IR (pyrazolyl NH) analysis. The supramolecular structure of the complex [Ni(pzH)2(HNdC(Me)pz)2](ClO4)2 displays a two-dimensional network dominated by cooperative face-toface π-π stacking and edge-to-face CH 3 3 3 π bonding interactions. In addition, a mechanism based on anion-mediated/hydrogen-bonding-driven proton transfer for the coupling reaction is proposed.

Introduction The activation of organonitriles (RCN) by transitionmetal centers toward nucleophilic addition of pyrazole (pzH) was first reported in 1986.1 Since then, only a limited number of works dealing with the reaction have been published.2-14 From these reports, only the transition-metal ions Ru2+, Ir3+, Mo2+, Cu2+, Pt2+, Pt4+, Mn+, and Re+ have been proven to be effective in promoting such a pyrazolenitrile coupling reaction. Although these research groups *To whom correspondence should be addressed. E-mail: ychorng@ cc.ncue.edu.tw. (1) Jones, C. J.; McCleverty, J. A.; Rothin, A. S. J. Chem. Soc., Dalton Trans. 1986, 109. (2) Romero, A.; Vegas, A.; Santos, A. J. Organomet. Chem. 1986, 310, C8. (3) Albers, M. O.; Crosby, S. F. A.; Liles, D. C.; Robinson, D. J.; Shaver, A.; Singleton, E. Organometallics 1987, 6, 2014. (4) Gracey, G. D.; Rettig, S. J.; Storr, A.; Trotter, J. Can. J. Chem. 1987, 65, 2469. (5) Cinellu, M. A.; Stoccoro, S.; Minghetti, G.; Bandini, A. L.; Banditelli, G.; Bovio, B. J. Organomet. Chem. 1989, 372, 311. (6) Lopez, J.; Santos, A.; Romero, A.; Echavarren, A. M. J. Organomet. Chem. 1993, 443, 221. (7) Carmona, D.; Ferrer, J.; Lahoz, F. J.; Oro, L. A.; Lamata, M. P. Organometallics 1996, 15, 5175. (8) Tong, M.-L.; Wu, Y.-M.; Tong, Y.-X.; Chen, X.-M.; Chang, H.-C.; Kitagawa, S. Eur. J. Inorg. Chem. 2003, 2385. (9) Kollipara, M. R.; Sarkhel, P.; Chakraborty, S.; Lalrempuia, R. J. Coord. Chem. 2003, 56, 1085. (10) Govindaswamy, P.; Mozharivskyj, Y. A.; Kollipara, M. R. J. Organomet. Chem. 2004, 689, 3265.  Miguel, D.; Villafa~ (11) Arroyo, M.; L opez-Sanvicente, A.; ne, F. Eur. J. Inorg. Chem. 2005, 4430. (12) Khripun, A. V.; Kukushkin, V. Y.; Selivanov, S. I.; Haukka, M.; Pombeiro, A. J. Inorg. Chem. 2006, 45, 5073. (13) Arroyo, M.; Miguel, D.; Villafane, F.; Nieto, S.; Perez, J.; Riera, L. Inorg. Chem. 2006, 45, 7018. (14) Anton, N.; Arroyo, M.; Gomez-Iglesias, P.; Miguel, D.; Villafane, F. J. Organomet. Chem. 2008, 693, 3074. r 2009 American Chemical Society

have reported the metal-based coupling reaction between RCN and pzH, only a few of them have attempted to provide a plausible mechanism for this unusual conversion. On the basis of the fact that most of the documented pyrazolenitrile coupling reactions give chelated complexes (Scheme 1, C), the assumption of an intramolecular mechanism was made. The coupling reaction proceeds through an intermediate (Scheme 1, A), in which the bound pyrazole (via pyridine N atom) and RCN are in the cis positions, followed by nucleophilic addition of pzH (NH pyrrole site) to electrophilically activated RCN. To explain the appearance of the nucleophilic site of pzH, the deprotonation of the pyrrole group (Scheme 1, B) to advance the coupling was suggested.15 Although an intermolecular mechanism, in which the unbound pzH directly attacks at the coordinated nitrile, is also proposed,12 how the pyrrole NH of pzH is deprotonated and what the nature of the final proton-transfer step is have not been discerned so far. We demonstrate here the first example of the pyrazolenitrile coupling reaction activated by less expensive and commercial available nickel(II) ions. In addition, a plausible mechanism to explain the mysterious deprotonation of metal-bound pyrazole and the final proton transfer during the coupling reaction is provided.

Experimental Section General Considerations. Commercially available chemicals were purchased from Aldrich or Acros and used as received. The complexes [Ni(MeCN)6](ClO4)2 (1)16 and [Ni(MeCN)6](BF4)2 (4)17 (15) Reisner, E.; Arion, V. B.; Rufinska, A.; Chiorescu, I.; Schmid, W. F.; Keppler, B. K. Dalton Trans. 2005, 2355. (16) Wickenden, A. E.; Krause, R. A. Inorg. Chem. 1965, 4, 404. (17) Heintz, R. A.; Smith, J. A.; Szalay, P. S.; Weisgerber, A.; Dunbar, K. R.; Beck, K.; Coucouvanis, D. Inorg. Synth. 2002, 33, 75. Published on Web 08/05/2009

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Scheme 1. Proposed Catalytic Mechanism of the Metal-Activated Pyrazole-Nitrile Coupling Reaction

Table 1. Summary of Crystallographic Data for Complexes 2, 3, and 5 2 formula C16H22Cl2N10NiO8 fw 612.05 temp, K 150(2) cryst syst monoclinic space group P21/c a, A˚ 10.2512(7) b, A˚ 15.4555(11) c, A˚ 7.8203(6) β, deg 96.330(5) 1231.47(15)/2 V, A˚3/Z 3 1.651 calcd density, Mg/m -1 1.069 abs coeff, mm cryst size, mm 0.15  0.15  0.12 θ range, deg 2.00-28.57 no. of rflns collected 12 753 no. of indep rflns 3148 max, min transmissn 0.882, 0.841 no. of data/restraints /params 3148/0/178 1.125 goodness of fit on F2 a 0.0526 R1 (I > 2σ(I)) 0.1704 wR2b (all data) 1.154, -0.666 largest diff peak, hole, e A˚-3 P P P P a R1 = |Fo| - |Fc|/ |Fo|. b wR2 = [ [w(F2o - F2c )2]/ [w(F2o)2]1/2.

3

5

C16H22Cl2N10NiO8 612.05 150(2) monoclinic P21/n 8.4875(5) 8.5181(5) 34.6576(19) 90.3410(10) 2505.6(2)/4 1.622 1.050 0.12  0.11  0.04 1.18-28.78 23 329 6528 0.959, 0.888 6528/0/336 1.196 0.0675 0.2076 1.011, -0.627

C16H22B2F8N10Ni 586.77 150(2) monoclinic P21/c 10.1198(9) 15.3981(14) 7.7713(7) 97.092(2) 1201.70(19)/2 1.622 0.897 0.24  0.23  0.16 2.03-28.64 12 557 3097 0.8698, 0.8135 3097/0/198 0.945 0.0336 0.1218 0.625, -0.377

Scheme 2. Syntheses of 2, 3, and 5-7a

a

Reaction conditions: MeCN, room temperature.

were synthesized by following the published procedures. UV-vis spectra were collected on a HP 8453 spectrophotometer with a 1 cm path length quartz sample compartment, while infrared spectra were recorded on a Bio-Rad FTS-185 instrument using KBr disks. Elemental analyses were performed on a Heraeus CHN-OS Rapid elemental analyzer at the Instruments Center of National Chung Hsing University, Taiwan. Preparation of [Ni(MeCN)2(pzH)4](ClO4)2 (2). A solution of [Ni(MeCN)6](ClO4)2 (1; 0.49 g, 0.97 mmol) and pyrazole (0.30 g, 4.30 mmol) in MeCN (25 mL) was stirred at room temperature for 10 min. After the resultant blue solution was filtered and concentrated to 5 mL under vacuum, the concentrated filtrate was layered with diethyl ether (5-fold portion) and then kept at room temperature for 3 days. The air-stable blue crystals of 2 (0.57 g, 96%) obtained were suitable for X-ray crystallographic analysis. Alternatively, 2 can be readily prepared by simply mixing Ni(ClO4)2 3 6H2O (0.36 g, 0.97 mmol) and 4 equiv of pyrazole in MeCN. IR (KBr, νmax/cm-1): 3113 w (NH), 2311 m (CtN), 2284m (CtN). UV-vis in MeCN (λmax, nm (ε, M-1 cm-1)): 356 (13.5), 567 (8.1), 741 (1.95), 920 nm (6.2). Anal.

Calcd for C16H22Cl2N10NiO8: C, 31.40; H, 3.62; N, 22.89. Found: C, 31.50; H, 3.72; N, 22.89. Preparation of [Ni(pzH)2(HNdC(Me)pz)2](ClO4)2 (3). A solution of 2 (0.59 g, 0.97 mmol) in MeCN (20 mL) was stirred at room temperature for 36 h. The resulting solution was then concentrated in vacuo to a volume of 5 mL, and slow diffusion of diethyl ether at 4 °C afforded pink crystals of 3 (0.59 g, 99%). IR (KBr, νmax/cm-1): 3312 w (NimineH), 3144 w (NpzHH), 1661 vs (CdN). UV-vis in MeCN (λmax, nm (ε, M-1 cm-1)): 240 (16 600), 543 (13.1), 761 (7.3), 874 (11.6). Anal. Calcd for C16H22Cl2N10NiO8: C, 31.40; H, 3.62; N, 22.89. Found: C, 31.47; H, 3.81; N, 22.89. Preparation of [Ni(MeCN)2(pzH)4](BF4)2 (5). This complex was obtained in 98% yield in a way similar to that for 2 using [Ni(MeCN)6](BF4)2 or Ni(BF4)2 3 6H2O as the source of nickel ions. IR (KBr, νmax/cm-1): 3119 w (NH), 2318 m (CtN), 2291 m (CtN). UV-vis in MeCN (λmax, nm (ε, M-1 cm-1)): 356 (16), 567 (10.0), 741 (3.0), 916 nm (7.0). Anal. Calcd for C16H22B2F8N10Ni: C, 32.75; H, 3.85; N, 23.87. Found: C, 32.44; H, 3.57; N, 23.76.

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Figure 1. Conversion of 2 to 3 at room temperature. Time-dependent changes in the UV-vis spectrum from 0 to 36 h in MeCN (left). The reaction was monitored by following the decrease in the concentration of 2 (solid circle) and the increase in the concentration of 3 (open circle) (right). The concentrations were calculated on the basis of the extinction coefficients at 545 nm (7.307 (2) and 13.10 M-1 cm-1 (3)), and the rate constants were determined by fitting the data to single-exponential equations. Preparation of [Ni(MeCN)2(pzH)4](NO3)2 (6). This complex was obtained in 74% yield (0.39 g) in a way similar to that for 2 using Ni(NO3)2 3 6H2O (0.29 g, 0.97 mmol), pyrazole (0.30 g, 4.30 mmol), and MeCN as the solvent. IR (KBr, νmax/cm-1): 3120 w (NH), 2283 m (CtN), 2210 m (CtN). UV-vis in MeCN (λmax, nm (ε, M-1 cm-1)): 370 (17.0), 596 (9.0), 719 (3.0), 995 (6.0). Anal. Calcd for C16H22N12NiO6: C, 35.78; H, 4.13; N, 31.29. Found: C, 35.32; H, 4.01; N, 31.03. Preparation of [Ni(pzH)2(HNdC(Me)pz)2](NO3)2 (7). This complex was obtained in 98% yield (0.53 g) in a way similar to that for 3 using Ni(NO3)2 3 6H2O (0.29 g, 0.97 mmol), pyrazole (0.30 g, 4.30 mmol), and MeCN as the solvent. IR (KBr, νmax/cm-1): 3393 w (NimineH), 3044 w (NpzHH), 1668 vs (CdN). UV-vis in MeCN (λmax, nm (ε, M-1 cm-1)): 239 (20 900), 570 (10.0), 770 (4.0), 920 (8.0). Anal. Calcd for C16H22N12NiO6: C, 35.78; H, 4.13; N, 31.29. Found: C, 35.25; H, 3.91; N, 31.19. Preparation of [Ni(pzH)2(HNdC(Et)pz)2](ClO4)2 (8). This complex was obtained in 98% yield (0.17 g) in a way similar to that for 3 using Ni(ClO4)2 3 6H2O (0.10 g, 0.27 mmol), pyrazole (0.08 g, 1.10 mmol), and EtCN as the solvent. IR (KBr, νmax/cm-1): 3341 w (NimineH), 3142 w (NpzHH), 1661 vs (CdN). UV-vis in MeCN (λmax, nm (ε, M-1 cm-1)): 237 (17 900), 545 (14.1), 763 (7.9), 878 (12.0). Anal. Calcd for C18H26Cl2N10NiO8: C, 33.78; H, 4.09; N, 21.88. Found: C, 33.62; H, 3.84; N, 21.41. Preparation of [Ni(pzH)2(HNdC(Bz)pz)2](ClO4)2 (9). This complex was obtained in 86% yield (0.18 g) in a way similar to that for 3 using Ni(ClO4)2 3 6H2O (0.10 g, 0.27 mmol), pyrazole (0.08 g, 1.10 mmol), and BzCN as the solvent. IR (KBr, νmax/cm-1): 3400 w (NimineH), 3152 w (NpzHH), 1665 vs (CdN). UV-vis in MeCN (λmax, nm (ε, M-1 cm-1)): 234 (12 000), 257 (8100), 545 (15.0), 759 (7.3), 879 (12.2). Anal. Calcd for C28H30Cl2N10NiO8: C, 44.01; H, 3.96; N, 18.33. Found: C, 43.89; H, 3.97; N, 18.01. UV-Visible Kinetic Studies. The experiments were carried out by using a known concentration of 2 in MeCN and measuring the UV-vis response at each time point. The concentrations of 2 and 3 at each time point were calculated on the basis of the following two equations: A = ε[2] + ε0 [3] and [2]0 = [2]+[3]. A, ε, and ε0 represent the absorbance, the extinction coefficient of 2, and the extinction coefficient of 3 at a fixed wavelength, respectively. [2] and [3] illustrate the concentrations of 2 and 3 at that time point. [2]0 denotes the initial concentration of 2. Crystal Structure Determination. Crystals suitable for structure analysis were mounted on glass fibers with silicone grease and placed in the cold stream of a Bruker APEX II with

graphite-monochromated Mo KR radiation (λ = 0.710 73 A˚) at 150(2) K. All structures were solved by direct methods using SHELXS-97 and refined by full-matrix least-squares methods against F2 with SHELXL-97.18 Tables of neutral atom scattering factors, f 0 and f 00 , and absorption coefficients are from a standard source.19 All atoms except hydrogen atoms were refined with anisotropic displacement parameters. In general, hydrogen atoms were fixed at calculated positions, and their positions were refined by a riding model. Crystallographic data collection and refinement parameters are given in Table 1. CCDC 730915 (2), CCDC 730916 (3), and CCDC 730917 (5) contain supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif.

Results and Discussion Syntheses and Physical Properties. As part of our research interest in the chemistry of nickel complexes with N-donor chelating ligands,20 we occasionally obtained a pyrazolylamidinonickel complex (3) (Scheme 2). The addition of pyrazole to RCN mediated by nickel ion has not been documented in the literature so far. Herein we report the first examples of such coupling reactions. The reaction was expected to proceed through nickel-mediated activation of nitrile molecules without the detection of intermediate A or B. However, when a 1:4 mixture of [Ni(MeCN)6](ClO4)2 (1)16 and pzH in MeCN was stirred for 10 min at room temperature, a single navy blue compound was unexpectedly obtained in almost quantitative yield and unambiguously identified as the trans-[Ni(MeCN)2(pzH)4]2+ compound (2). When the reaction was stirred at room temperature over 36 h, a pink pyrazolylamidinonickel complex (3) was obtained in high yield (Scheme 2). The absorption spectrum of 2 in MeCN showed weak spin-allowed d-d transitions at 567 and 920 nm, and the transition of 3 is blue-shifted from that of 2 (541 and 874 nm). (18) Sheldrick, G. M. SHELXTL, Version 5.1; Bruker AXS Inc., Madison, WI, 1998. (19) Sutton, L. E. Tables of Interatomic Distances and Configurations in Molecules and Ions; Chemical Society Publications: London, 1965. (20) Hsieh, C.-C.; Chao, W.-J.; Horng, Y.-C.; Lee, H. M. J. Chin. Chem. Soc. 2009, 56, 435.

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Hsieh et al. Table 2. Selected Bond Lengths (A˚) and Angles (deg) for Complexes 2 and 5a

Ni(1)-N(1) Ni(1)-N(3) Ni(1)-N(5) N(5)-C(7) N(1A)-Ni(1)-N(1) N(1A)-Ni(1)-N(3) N(1)-Ni(1)-N(3) N(1A)-Ni(1)-N(3A) N(1)-Ni(1)-N(3A) N(3)-Ni(1)-N(3A) N(1A)-Ni(1)-N(5A) N(1)-Ni(1)-N(5A) N(3)-Ni(1)-N(5A) N(3A)-Ni(1)-N(5A) N(1A)-Ni(1)-N(5) N(1)-Ni(1)-N(5) N(3)-Ni(1)-N(5) N(3A)-Ni(1)-N(5) N(5A)-Ni(1)-N(5) C(7)-N(5)-Ni(1) C(8)-C(7)-N(5)

2

5

2.091(2) 2.095(3) 2.101(2) 1.135(4) 180.000 92.16(9) 87.84(10) 87.84(9) 92.16(10) 180.00(11) 89.01(10) 90.99(10) 89.89(10) 90.11(10) 90.99(10) 89.01(10) 90.12(10) 89.88(10) 180.000 177.5(3) 179.7(4)

2.0895(14) 2.0924(14) 2.0996(14) 1.133(2) 180.000 92.04(5) 87.96(5) 87.96(5) 92.04(5) 180.000 89.01(6) 90.99(6) 89.97(6) 90.03(6) 90.99(6) 89.01(6) 90.03(6) 89.97(6) 180.000 177.94(15) 179.4(2)

a Symmetry transformations used to generate equivalent atoms: for 2, (A) -x, 2 - y, -z; for 5, (A) 1 - x, 1 - y, -z.

Table 3. Structural Parameters for Weak Interactions in Complexes 2 and 5a 2

Figure 2. (a) ORTEP drawing of 2. Thermal ellipsoids are drawn at the 35% probability level. Hydrogen atoms bound to carbon atoms are omitted for clarity. (b) Packing diagram of 2, with dashed lines showing the NH 3 3 3 O hydrogen-bonding interactions.

Figure 3. ORTEP drawing of 5. Thermal ellipsoids are drawn at the 35% probability level, with dashed lines showing the NH 3 3 3 F hydrogen-bonding interactions. Three fluorine atoms of the BF4 molecule are disordered in two positions, and the minor components (F(5)-F(7)) are represented as dotted ellipsoids. Hydrogen atoms bound to carbon atoms are omitted for clarity.

The time-dependent spectra changes revealed several isosbestic points. In addition, the calculated rate constant of formation of 3 (0.087 ( 0.002 h-1) is equal to the rate constant of decay of 2 (0.087 ( 0.002 h-1) (Figure 1). Therefore, any intermediate through the conversion of 2 to 3 is excluded on the basis of the detection limit of the instrument. The formation of two pyrazolylamidino ligands upon one metal center is conceivably cooperative.

5

H(4) 3 3 3 O(2A) 2.193(42) H(4) 3 3 3 F(7A) 2.1306(97) H(8) 3 3 3 O(1C) 2.0383(33) N(2) 3 3 3 F(7A) 2.9854(98) N(2) 3 3 3 O(2A) 2.9214(45) N(2)-H(4) 3 3 3 F(7A) 172.847(269) N(4) 3 3 3 O(1C) 2.8830(46) N(2)-H(4) 3 3 3 O(2A) 149.632(3977) N(4)-H(8) 3 3 3 O(1C) 167.379(225) a Symmetry transformations used to generate equivalent atoms: for 2, (A) 1 - x, 1 - y, 1 - z and (C) x, 1 + y, -1 + z; for 5, (A) 1 - x, 1 - y, 1 - z.

Under the same reaction conditions (temperature and time) in the presence of 4 equiv of pzH and excess RCN (R = Et, Bz), Ni(ClO4)2 can also catalyze the coupling reaction to afford the pyrazolylamidinonickel complexes [Ni(pzH)2(HNd C(R)pz)2](ClO4)2 (R = Et (8), Bz (9)). However, RCN with an electron-withdrawing R group (R = CCl3, Ph) was not catalyzed under similar conditions, on the basis of IR (CdN) analyses. Interestingly, we found the reaction is anion-specific. The as the counteranion of parallel reaction using BF4 [Ni(MeCN)6]2+ (4)17 only affords [Ni(MeCN)2(pzH)4](BF4)2 (5), even after stirring for 3 days. To exclude the possibility that the purchased chemical Ni(BF4)2 3 6H2O is contaminated with some HBF4, which suppressed the deprotonation of the pyrazole, complex 5 was repeatedly washed three times with MeCN/ether. However, the coupling reaction was still not observed within 3 days. On the other hand, the coupling reaction for the complex [Ni(MeCN)2(pzH)4](NO3)2 (6) proceeded with a slightly faster rate (rate constant 0.096 ( 0.002 h-1), leading to [Ni(pzH)2(HNdC(Me)pz)2](NO3)2 (7). The different reaction rates were evaluated by X-ray crystal structures and solution IR analyses (see the following). All complexes described above were characterized by UV-vis, IR, and EA analyses. Complexes 2, 3, and 5 were further characterized by X-ray structure determination.

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Table 4. Selected Bond Lengths (A˚) and Angles (deg) and Structural Parameters for Hydrogen-Bonding Interactions in Complex 3a Ni(1)-N(1) Ni(1)-N(2) Ni(1)-N(3) Ni(1)-N(4) Ni(1)-N(5) N(3)-Ni(1)-N(1) N(3)-Ni(1)-N(6) N(1)-Ni(1)-N(6) N(3)-Ni(1)-N(5) N(1)-Ni(1)-N(5) N(6)-Ni(1)-N(5) N(3)-Ni(1)-N(4) N(1)-Ni(1)-N(4) N(6)-Ni(1)-N(4) N(5)-Ni(1)-N(4) H(7) 3 3 H(19) 3 H(15) 3

3 O(3A) 3 3 O(5A) 3 3 O(1A)

N(1)-H(7) 3 3 3 O(3A) N(9)-H(19) 3 3 3 O(5A)

2.070(4) 2.100(3) 2.067(4) 2.100(4) 2.081(3) 95.26(18) 169.03(17) 95.11(16) 90.90(14) 93.02(15) 92.14(13) 76.75(17) 167.91(16) 92.44(16) 96.12(14) 2.2173(1) 2.1951(1) 2.0817(1) 160.133(8)

Ni(1)-N(6) N(1)-C(1) N(4)-C(5) N(7)-C(1) N(8)-C(5) N(3)-Ni(1)-N(2) N(1)-Ni(1)-N(2) N(6)-Ni(1)-N(2) N(5)-Ni(1)-N(2) N(4)-Ni(1)-N(2) C(1)-N(1)-Ni(1) C(5)-N(4)-Ni(1) C(8)-C(1)-N(1) C(9)-C(5)-N(4) N(1) 3 3 N(9) 3 3 N(10) 3

3 O(3A) 3 O(5A) 3 3 O(1A)

N(10)-H(15) 3 3 3 O(1A)

2.077(4) 1.267(6) 1.251(6) 1.401(6) 1.365(7) 88.26(13) 77.06(15) 90.57(13) 169.91(14) 93.47(14) 117.9(3) 117.7(4) 127.1(5) 127.7(6) 3.0401(1) 2.9275(1) 2.8884(1) 155.948(9)

142.926(8)

a Symmetry transformations used to generate equivalent atoms: (A) x, -1 + y, 1 + z (O(3), O(5)); x, -1 + y, -z (O(1)).

Figure 4. (a) ORTEP drawing of complex 3. Thermal ellipsoids are drawn at the 35% probability level, with dashed lines showing the NH 3 3 3 O hydrogen-bonding interactions. Hydrogen atoms bound to carbon atoms are omitted for clarity. (b) Packing diagram of 3 without perchlorate anions, showing the cooperative face-to-face π-π stacking (solid lines) and edge-toface C-H 3 3 3 π bonding (dashed lines) interactions.

Descriptions of Structures. The molecular structures of the complexes 2 and 5 are very similar, with each nickel(II) ion, located in the crystallographic inversion center, coordinated by four pyrazole nitrogen atoms in the equatorial positions and two nitrogen atoms of MeCN at axial positions in a nearly octahedral configuration, as shown in Figures 2 and 3 and Table 2. The MeCN molecules are quite linear in both compounds and are bound to the metal center at angles slightly deviating from linearity. However, the types of cation-anion interactions with the involvement of the N-bound hydrogen of pzH in 2 and 5 are diverse. All four pyrazolyl NH groups of 2 are involved in hydrogen bonds with the oxygen atoms of the perchlorate anions, whereas only minor parts of two NH groups of 5 are engaged in hydrogen bonding with BF4 anions. The hydrogen bonds detected in both complexes can be considered as “moderate”21,22 on the basis of the contact distances and angles (Table 3). The results indicate ClO4 anions possess higher capacity as hydrogen acceptors in this system, which may be the driving force of the coupling reaction. Complex 3 can be viewed as an isomer of 2 containing a couple of both pzH and pyrazolylamidino ligands, as shown (21) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press: New York, 1997. (22) Steiner, T. Angew. Chem., Int. Ed. 2002, 41, 48.

in Figure 4a and Table 4. Two untouched pyrazoles are in the cis positions. The dihedral angle between the two neighboring pyrazole planes is 72° in 2 and is around 88° in 3. The distortion is related to the N-C bond formation between bound pzH and nitrile molecules. Interestingly, one perchlorate anion possesses hydrogen-bonding interactions with both N-H groups of one set of bound pyrazole and pyrazolylamidino ligands. Examples exist that both N-H groups of pzH and pyrazolylamidino ligands perpendicular to each other in cationic complex can simultaneously bind a - 13,14 We speculate monoanion, such as ClO4 , Cl , and BF4 . from the results that the stabilization of the coupling reaction product may be due to the formation of a molecular cleft between neighboring pyrazole and pyrazolylamidino ligands driven by NH 3 3 3 monoanion hydrogen-bonding interactions. In addition, we found that the molecules form a twodimensional sheet dominated by cooperative face-to-face ππ stacking and edge-to-face CH 3 3 3 π bonding interactions in the supramolecular structure (Figure 4b and Table 5). Such an extensive pyrazolyl embrace23 of noncovalent contacts for supramolecular assemblies can only be found in a few pyrazole complexes.24,25 Proposed Transition State of the Coupling Reaction. Since complexes 2, 5, and 6 are paramagnetic, the measurement of the hydrogen-bonding strengths in solution from NMR becomes irrelevant. However, the analyses of IR spectra in solution may provide a clue on the strengths of NH 3 3 3 anion hydrogen bonding. The pyrazolyl N-H stretching in MeCN solution occurring at 3308 (6), 3316 (2), and 3327 cm-1 (5), lower than that of free pyrazole (3356 cm-1), suggests the hydrogenbonding strength increases in the order 6 > 2 > 5. The same (23) Reger, D. L.; Gardinier, J. R.; Semeniuc, R. F.; Smith, M. D. Dalton Trans. 2003, 1712. (24) Hong, C. S.; Yoon, J. H.; Lim, J. H.; Ko, H. H. Eur. J. Inorg. Chem. 2005, 4818. (25) Takahashi, P. M.; Melo, L. P.; Frem, R. C. G.; Netto, A. V. G.; Mauro, A. E.; Santos, R. H. A.; Ferreira, J. G. J. Mol. Struct. 2006, 783, 161.

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Hsieh et al.

Table 5. Structural Parameters for Weak Interactions in Complex 3 C(3)-H(2) 3 3 3 π C(7)-H(16) 3 3 3 π C(11)-H(9) 3 3 3 π C(15)-H(20) 3 3 3 π

d(A-H) (A˚)

d(H 3 3 3 B) (A˚)

d(A 3 3 3 B) (A˚)

AH 3 3 3 B angle (deg)

symmetry code

0.930 0.930 0.930 0.930

2.708 2.776 2.708 2.774

3.487 3.657 3.604 3.504

141.80 158.59 162.12 136.11

x, 1 + y, z x, -1 + y, z 1 + x, y, z -1 + x, y, z

centroid X

centroid Y

C(6)C(7)C(16)N(5)N(10) C(13)C(14)C(15)N(6)N(9)

C(2C)C(3C)C(4C)N(2C)N(7C) C(10D)C(11D)C(12D)N(3D)N(8D)

d(X 3 3 3 Y) (A˚)

dihedral angle (deg)

symmetry code

3.661 3.738

6.352 6.567

x, -1 + y, z -1 + x, y, z

Table 6. Potential Anion Cocatalysts for Pyrazole-Nitrile Coupling Reactions Activated by Various Metal Centersa,b metal complex 6

1 2 3 4 5 6 7

reactants 0

[{Ru(η -C6H6)Cl2}2], [{Ru(η6-p-cymene)Cl2}2] [Ru(CO)H(RCN)2(PPh3)2]+ PtCl4(RCN)2, PtCl2(RCN)2

R 2Hpz, RCN

[Ru(CO)(CHdCHCMe3) (RCN)2(PPh3)2]+ [Ir(η5-C5Me5)(pz)2(Hpz)] Cu(ClO4)2 3 6H2O MBr(CO)3(RCN)2, [M(CO)3(RCN)2]+ (M = Mn, Re)

R0 2Hpz

0

R 2Hpz R0 2Hpz

RCN, AgBF4 Na(dca)c, Hpz R0 2Hpz, RCN

product 6

0

anion

R,R0 2+

[Ru(η -C6H6)(R 2Hpz)L ] , 0 [Ru(η6-p-cymene)(R002Hpz)LR,R ]2+ R,R + [Ru(CO)H(PPh ] 3)2L 0 0 R,R0 PtCl4(LR,R )2, PtCl , PtCl(R0 2pz)LR,R , 2L 0 R,R 0 Pt(R 2pz)2L 0 [Ru(CO)(CHdCHCMe3)(PPh3)2LR,R ]+ 0

[{Ir(η5-C5Me5)(μ-pz)(μ-LR,R )Ag}2]2+ 4+ [Cu3(dcadpz)2(pz) 2] 0 R,R0 + MBr(CO)3LR,R , [M(RCN)(CO) ] , 3L 0 [M(CO)3(R0 2pz)LR,R ]+

Cl

-

ref 1,9,10

ClO4 Cl

2 5,12

PF6

6

BF4 ClO4 Br , ClO4 , BF4

7 8 11,13,14

0

Abbreviations: LR,R = {HNdC(R)(R0 2pz)}; Hpz = pyrazole; R = Me, Et, Ph; R0 = H, Me. b Two references are not included in the table because the formation of [Mo(CO)2Br(η3-C3H5)LMe,H] and [Ru2(η4-C8H12)2H(μ-LMe,H)(μ-pz)(μ-H)] complexes3,4 involves the fragmentation of [Me2Ga(pz)(OC6H4NH2)]- and pyrazolylborate ligand, respectively. c Abbreviations: DCA = dicyanamide; dcadpz = di(pyrazolecarbimido)aminato. a

Scheme 3. Proposed Transition State for the Metal-Activated Pyrazole-Nitrile Coupling Reactiona

anion should match the requirement of the dual nature to finish the catalysis.

Conclusions

a

The catalytic anion is represented by X.

trend was also observed from the rate constants of the coupling reactions using different counteranions. In a search of the reported metal-activated pyrazole-nitrile coupling reactions, all the starting metal complexes contain counteranions, potentially acting as hydrogen-bond acceptors, or metal-bound halogen atom(s) (Table 6). It is known that a metal-bound halogen atom is strongly polar and is a good hydrogen bond acceptor via its more basic p-type lone pair.26 Thus, the pyrazole-nitrile coupling reaction activated by metal ions may proceed via a hydrogen-bonding-assisted pathway. On the basis of the fact that stronger pyrazolyl NH 3 3 3 anion hydrogen bonding induced the coupling reaction with a faster rate, a transition state of the reaction may be proposed (Scheme 3). The catalytic anion, acting as both base and acid, mediates the proton transfer from pyrazole to nitrile. Thus, the electronic and steric properties of the catalytic (26) Yap, G. P. A.; Rheingold, A. L.; Das, P.; Crabtree, R. H. Inorg. Chem. 1995, 34, 3474.

We report here the first example of a pyrazole-nitrile coupling reaction activated by less expensive nickel(II) ions. The reaction is counteranion-specific. The reaction proceeds more quickly with NO3 as counteranion than with ClO4 , and BF4 is found to be unsatisfactory for the coupling reaction. The solid-state structures of 2 and 5 show distinct noncovalent interactions between the cation and anion. The differences in the crystal structures and hydrogen-bonding strengths of pyrazolyl NH 3 3 3 anion measured by solution IR are correlated with the different rates of the coupling reaction assisted by various counteranions. On the basis of the correlation, a plausible mechanism of anion-mediated/hydrogen-bonding-driven proton transfer is proposed. The positively charged inorganic complex [Ni(RCN)2(pzH)4]2þ (R = Me, Et, Bz) synthesized in situ from readily available chemicals is the substrate, and the pyrazolylamidinonickel compound is the product catalyzed by specific anions.

Acknowledgment. We are grateful to the National Science Council (Taiwan) for their financial support of this work. Supporting Information Available: CIF files giving X-ray crystallographic data for the structure determinations of 2, 3, and 5 and Figure S1, showing the electronic spectra of 2 and 3. This material is available free of charge via the Internet at http://pubs.acs.org.