Synthesis, characterization and X-ray structural studies of novel dinuclear silver(I) complexes of poly(azolyl)borate ligands

www.elsevier.nl/locate/ica Inorganica Chimica Acta 308 (2000) 65 – 72

Synthesis, characterization and X-ray structural studies of novel dinuclear silver(I) complexes of poly(azolyl)borate ligands Effendy a,b, Giancarlo Gioia Lobbia a,c, Claudio Pettinari c,*, Carlo Santini c, Brian W. Skelton b, Allan H. White b a

Department of Chemistry, The Uni6ersity of Western Australia, Nedlands, Western Australia 6907, Australia b Jurusan Kimia, Uni6ersitas Negeri Malang, Jalan Surabaya 6, Malang 65145, Indonesia c Dipartimento di Scienze Chimiche, Uni6ersita` degli Studi di Camerino, 6ia S. Agostino 1, 62032 Camerino MC, Italy Received 11 February 2000; accepted 5 May 2000

Abstract A series of Ag(I) complexes of tris- and tetrakis-(pyrazolyl)hydroborates, tetrakis(imidazol-1-yl)borate, and hydrotris(3-methyl1-imidazolyl-2-thione)borate, namely the previously recorded [Ag{HB(pz)3}]2, [Ag{HB(3,5-Me2pz)3}]2, [Ag{B(pz)4}]n, together with [Ag{HB(4-Brpz)3}]2, [Ag{B(3-Mepz)4}]n, [Ag{B(im)4}]n (Him =imidazole) and [Ag{Tm}]2 (Tm=hydrotris(3-methyl-2-thioxo-1-imidazolyl)borate) have been synthesized and further characterized by elemental analysis, IR, Far– IR, 1H, 13C NMR spectroscopy, and in the case of [Ag{Tm}]2, also by a single-crystal X-ray study. Variable-temperature 1H NMR spectra indicate that [Ag{HB(3,5-Me2pz)3}]2, [Ag{HB(4-Brpz)3}]2, and [Ag{B(pz)4}]n are fluxional, with a pyrazolyl ring exchange process occurring rapidly at 293 but not at 193 K, whereas [Ag{HB(pz)3}]2 and the crystalline form of [Ag{HB(3,5-Me2pz)3}]2 are not fluxional, even at room temperature. The reactions between K[HB(pz)3], K[HB(3,5-Me2pz)3], K[B(pz)4] or K[Tm] and AgNO3 in presence of N-and S-donor, unidentate or bidentate ligands such as pyrazole, imidazole, 1-10-phenanthroline and 1-methylimidazoline(2,3H)thione (Hmimt) were investigated. We also report the results of positive ion FAB MS studies carried out for selected derivatives. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Crystal structures; Silver complexes; Poly(azolyl)borate complexes

1. Introduction The coordination chemistry of poly(pyrazolyl)borate ligands [Hx B(pz)4 − x ] has been extensively developed over the last few decades [1]. This family of N-donors is able not only to control the steric and electronic environment about a metal center by modification of the pyrazolyl substituents, but also to give multimetallic transition metal complexes maintaining the metal centers in close proximity. This is a very interesting feature because of the potential role of this kind of derivative in multimetal-centered catalysis in both biological and industrial reactions [2]. For example the dimers [Cu{HB(pz)3}]2 have been found to be useful as starting materials for bioinorganic modeling studies [3 – 5]. It is * Corresponding author. Tel.: + 39-0737-402 217; fax: +39-0737637 345. E-mail address: [email protected] (C. Pettinari).

interesting to view the crystal structures of the [Cu{HB(pz)3}]2 complexes as representatives of intermediate structures that lie along the potential energy surface of the pyrazolyl ring exchange pathway [6]. It has been reported that coordination number, geometry and CuN distances within this class of compounds are strongly dependent on the size of the ligand substituents. For example in [Cu{HB(3-Butpz)3}]2 [3] the copper is two-coordinate with a linear geometry whereas in [Cu{HB(pz)3}]2 [4,5] one pyrazolyl ring adopts a bridging binding mode with the copper ion four-coordinated. Very recently it has also been observed that in the sterically encumbered [Cu{HB(3-But5-Mepz)3}]2 [7] an inverted disposition of the hydrotris(pyrazolyl)borate ligand permits close contact between the Cu (coordinated exclusively by two pyrazolyl moieties) and the HB center. The crystal structure determination of the binary silver complex [Ag2{HB(3,5-Me2pz)3}2] was first reported in 1999 [8].

0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 0 0 ) 0 0 2 1 1 - 5

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As part of a continuing study on the coordination chemistry of poly(azolyl)borate ligands we have recently initiated an investigation into the coordination chemistry of anionic poly(pyrazolyl)borate ligands together with analogous systems obtained by attempting to replace the hard N-donor pyrazole ring, with the softer S-donor 1-methylimidazoline-2(3H)-thione ring, with silver(I) and a variety of phosphine coligands. The compounds obtained show an interesting, and in some cases unpredictable, structural variety, both in the local coordination environment and in the overall geometry. We were especially interested in exploring the factors causing these variations, detailed investigation of which suggests that they are determined primarily by the steric demands of the poly(pyrazolyl)borate and only marginally by the steric demands of the phosphine ligands [9]. Recently, Dias et al. [10] have reported the synthesis of several poly(pyrazolyl)borate systems containing perfluorinated substituents, and their copper(I) and silver(I) chemistry. These studies indicate that fluoro substituents on the pyrazole rings can exert a significant effect on the electronic properties of the metal center. Silver(I) complexes of tris(triazolyl)borate [11] and tris(pyrazolyl)methane [12] ligands have also attracted some current interest [13]. Despite the importance and the wide utility of poly(azolyl)borate complexes of group 11 metals(I) the chemistry of silver(I) complexes with this family of donors, in the absence of stabilizing coligands such as PR3 or CNR, is largely unexplored except for the pioneering studies of [14]. Here we report further synthetic and spectroscopic studies, extending our knowledge of those previously recorded in [14], encompassing as well [Ag{HB(4Brpz)3}]2, [Ag{B(3-Mepz)4}]n, [Ag{B(im)4}]n, and, finally, of hydrotris(3-methyl-1-imidazolyl-2-thione)borate, [Ag{Tm}]2 with its crystal structure determination. Here we also report on the interaction between M[Hx B(pz)4 − x ] (M= Na or K) and K[Tm] with AgNO3 in the presence of equimolar quantities of unidentate (pyrazole or imidazole) or bidentate (1,10phenanthroline) N-donor or unidentate S-donor (1methylimidazoline(2,3H)thione) ligands.

2. Experimental

2.1. General procedures All reactions were carried out under an atmosphere of dry oxygen-free dinitrogen, using standard Schlenk techniques. Solvents were freshly distilled over an appropriate drying agent and further degassed before use where necessary. In some cases the reactions were protected from light by covering reaction vessels with

aluminum foil. Concentration was always carried out in vacuo (water aspirator). The samples for microanalysis were dried in vacuo to constant weight (20°C, ca. 0.1 Torr). Elemental analyses (C,H,N) were performed in house with a Carlo-Erba model 1106 instrument. IR spectra were recorded from 4000 to 100 cm − 1 with a Perkin –Elmer System 2000 FT –IR instrument. The electrical conductance of the dichloromethane solutions was measured with a Crison CDTM 522 conductimeter at room temperature (r.t.). 1H and 13C spectra were recorded on a VXR-300 Varian spectrometer operating at r.t. (300 MHz for 1H and 75 MHz for 13C). FAB mass spectra were obtained on a Finnigan-MAT TSQ70 triple stage quadrupole instrument equipped with an Ion Tech (Teddington, UK) atom gun using Xe as bombarding gas. The emission current was typically set at 2 mA with an accelerating voltage of 8 keV. For all experiments, the source was kept at r.t., using CsI for mass calibration. Samples were directly dissolved in a 3-nitrobenzyl alcohol matrix.

2.2. Syntheses The proligands K[HB(pz)3], K[HB(4-Brpz)3] [15], K[HB(3,5-Me2pz)3], K[B(pz)4] [16], K[Tm] [9a], Na[B(im)4] [17] and K[B(3-Mepz)4] [18] were prepared in accordance with procedures reported in the literature. KBH4, NaBH4, Hpz (pyrazole), Him (imidazole), 4-BrpzH (4-Br-pyrazole), 3,5-Me2pzH (3,5-Me2-pyrazole), Hmimt (1-methylimidazolyl-2-thione) were purchased (Aldrich) and used as received.

2.2.1. [Ag(Tm)]2 (1) This complex was prepared in methanol following literature methods [9a]. Its analytical and spectral data are in accordance with those reported in the literature [9a]. X-ray quality crystals of 1 were obtained from CHCl3 –CH3CN at 0°C as the bis(chloroform) solvate. \M (CH2Cl2)= 1.0 V − 1 cm2 mol − 1. MS, FAB: m/z 1027 [M+ Ag]+ (60%) 917 [M+H]+ (3%) 788 [M’2pzH+ Ag]+ (70%) 567 [M−Tm]+ (100%) (all peaks are at the center of an isotopic cluster). 2.2.2. [Ag{HB(pz)3}]2 (2) Compound 2 was prepared following the method of [14]. To a stirred methanol solution (30 ml) of Na[HB(pz)3] (0.236 g, 1 mmol) was added AgNO3 (0.170 g, 1 mmol) at r.t. The mixture was stirred for 2 h and the solid obtained was then filtered off, washed with methanol, and shown to be compound 2 (yield 94%). m.p. (dec.): 163°C (lit. 162− 165°C [14]). 1H NMR (DMSO, 293 K): l 6.22 (s br, 3H, 4-CH), 7.53 (s br, 3H, 5-CH), 7.80 (s br, 3H, 3-CH). 1H NMR (CDCl3, 293 K): l 6.25 (s br, 2H, 4-CH), 6.38 (pt, 1H, 5-CH), 7.42 (s br, 2H, 5-CH), 7.62 (pd, 1H, 3-CH), 7.80 (s br, 2H, 3-CH). \M (CH2Cl2)= 0.5 V − 1 cm2 mol − 1.

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IR (Nujol, cm − 1): 3124 w (CH), 2429 m, 2397 m, 2379 sh (BH), 1544 w, 1502 m (C···C, C···N), 355 m, 321 mbr, 279 m. Anal. Calc. for C9H10AgBN6; C, 33.8; H, 3.1; N, 26.3; Found: C, 33.4, H, 3.3; N, 26.1%. MS, FAB: m/z 747 [M + Ag]+ (80%), 641 [M+ H]+ (3%), 429 [M − HB(pz)3]+ (100%) (all peaks are at the center of an isotopic cluster).

2.2.3. [Ag{HB(3,5 -Me2pz)3}]2 (3) 3 was prepared following the methods of [14], and as for 2 above, by using AgNO3 (0.170 g, 1 mmol) and K[HB(3,5-Me2pz)3] (0.336 g, 1 mmol) in methanol (50 ml) at 298 K. Compound 3 was recrystallized from CHCl3 –Et2O (1:1) (yield 90%). X-ray quality crystals of this molecule were obtained from ethyl acetate – CHCl3 at r.t. m.p. (dec.): 200°C (lit. dec. \190° [14]). 1H NMR (CDCl3, 293 K): l 1.92 (s, 9H, 5-CH3 ), 2.31 (s, 9H, 3-CH3 ), 5.77 (s, 3H, 4-CH). \M (CH2Cl2) = 0.1 V − 1 cm2 mol − 1. 1H NMR of solution of crystals (CDCl3, 293 K): l 1.91 (s, 6H, 5-CH3 ), 2.24 (s, 6H, 3-CH3 ), 2.27 (s, 2H, 3- or 5-CH3 ), 2.30 (s, 2H, 3- or 5-CH3 ), 5.76 (s, 2H, 4-CH), 5.83 (s, 1H, 4-CH). 1H NMR (CDCl3, 193 K): l 1.87 (s, 18H, 5-CH3 ), 2.30 (s, 18H, 3-CH3 ), 5.72 (s, 6H, 4-CH). 13C NMR (CDCl3, 293 K): l 13.9 (s, CH3), 14.8 (s, CH3), 105.3 (s, 4-CH), 146.8 (s, 5-CCH3), 150.0 (s, 3-CCH3). IR (Nujol, cm − 1): 3118 vw (CH), 2509 w (BH), 1538 m (C···C,C···N), 553 w, 473 m, 456 s, 359 m, 349 w, 327 w, 303 w, 255 s. Anal. Calc. for C15H22AgBN6; C, 44.6; H, 5.5; N, 20.8; Found: C, 44.2; H, 5.6; N, 20.6%. MS, FAB: m/z 917 [M + Ag]+ (100%), 809 [M +H]+ (3%), 714 [M+H − pzH]+ (10%), 606 [M +H −pzH +Ag]+ (30%), 512 [M+ 2H−2pzH+Ag]+ (10%), 403 [M −HB(3,5-Me2pz)3]+ (15%) (all peaks are at the center of an isotopic cluster). 2.2.4. [Ag{HB(4 -Brpz)3}]2 (4) Compound 4 was prepared similarly to compound 2, by using AgNO3 (0.170 g, 1 mmol) and K[HB(4-Brpz)3] (0.480 g, 1 mmol) in methanol (50 ml) at 298 K. Compound 4 was recrystallized from CHCl3/Et2O (1:1) (yield 91%). m.p. (dec.): 170°C. 1H NMR (CDCl3, 293 K): l 6.90 (s, 6H, 3-CH), 6.60 (s, 6H, 5-CH). 1H NMR (CDCl3, 193 K): l 5.18 (s, 1H), 5.30 (s, 2H), 6.83 (s, 3H, 3-CH), 6.58 (s, 3H, 5-CH), 7.51 (s, 6H), 7.53 (s, 3H), 7.68 (s, 6H). 13C NMR (CDCl3, 293 K): l 94.1 (s, 4-CBr), 134.7 (s, 5-CH), 142.6 (s, 3-CH). \M (CH2Cl2)= 0.8 V − 1 cm2 mol − 1. IR (Nujol, cm − 1): 3125 vw (CH), 2374 m (BH), 1518 w (C···C,C···N), 419 w, 331 m, 285 w. Anal. Calc. for C9H7AgBBr3N6; C, 19.4; H, 1.3; N, 15.1; Found: C, 19.2; H, 1.2; N, 15.2%. 2.2.5. [Ag{B(pz)4}]n (5) Compound 5 was prepared following the method for compound 2 and [14] by using AgNO3 (0.170 g, 1 mmol) and K[B(pz)4] (0.318 g, 1 mmol) in methanol (50 ml) at 298 K. Compound 5 was recrystallized from CHCl3/

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Et2O (1:1) (yield 98%). m.p. (dec.): 258°C. 1H NMR (CDCl3, 293 K): l 6.21 (s br, 4H, 4-CH), 7.04 (s br, 4H, 5-CH), 7.42 (s br, 4H, 3-CH). 1H NMR (CDCl3, 193 K): l 5.16 (s, 2H, 4-CH), 5.30 (s, 4H, 4-CH), 7.51 (s, 6H, 5-CH), 7.68 (s, 6H, 3-CH). 13C NMR (CDCl3, 293 K): l 105.5 (s, 4-CH), 135.5 (s, 5-CH), 143.0 (s, 3-CH). \M (CH2Cl2)= 0.4 V − 1 cm2 mol − 1. IR (Nujol, cm − 1): 3132 vw, 3093 vw (CH), 1504 w (C···C,C···N), 368 m, 350 m, 331 w, 311 w, 261 m. Anal. Calc. for C12H12AgBN8; C, 37.3, H; 3.1; N, 29.0; Found: C, 37.0; H, 3.1; N, 28.9%. MS, FAB: m/z 1651 [4M +Ag]+ (30%), 1545 [4M + H]+ (25%), 1275 [3M + Ag]+ (45%), 883 [2M + Ag]+ (20%), 495 [2M − B(pz)4]+ (100%), 426 [2M + H−pzH − B(pz)4]+ (60%), 319 [M− Ag −pz − B(pz)4]+ (90%), (all peaks are at the center of an isotopic cluster).

2.2.6. [Ag{B(3 -Mepz)4}]n (6) Compound 6 was prepared similarly to compound 2, by using AgNO3 (0.170 g, 1 mmol) and K[B(3-Mepz)4] (0.374 g, 1 mmol) in methanol (50 ml) at 298 K (yield 97%). m.p. (dec.): 220°C. 1H NMR (DMSO, 293 K): l 2.08 (s br, 12H, 3-CH3 ), 5.92 (d, 4H, 5-CH), 6.70 (d, 4H, 3-CH). IR (Nujol, cm − 1): 3126 vw, 3107 w (CH), 1519 m (C···C,C···N), 423 m, 413 m, 405 s, 393 m, 253 s. Anal. Calc. for C16H20AgBN8; C, 43.5; H, 4.6; N, 25.3; Found: C, 43.4; H, 4.6; N, 25.0%. MS, FAB: m/z 993 [M+ Ag]+ (10%), 912 [M+ Ag +H-pzH]+ (10%), 739 [M+ H+Ag − B(3-Mepz)3]+ (40%), 550 [M−B(3-Mepz)4]+ (50%), 468 [M+H− 3-MepzH−B(3Mepz)4]+ (38%), 363 [M− Ag − 3-Mepz−B(3Mepz)4]+ (100%), (all peaks are at the center of an isotopic cluster). 2.2.7. [Ag{B(im)4}n] (7) Compound 7 was prepared similarly to compound 2, by using AgNO3 (0.170 g, 1 mmol) and Na[B(im)4] (0.302 g, 1 mmol) in methanol (50 ml) at 298 K (yield 93%); 2 is insoluble in all common organic solvents. m.p. (dec.): 250°C. IR (Nujol, cm − 1): 3140 w, 3112 w (CH), 1559 w (C···C, C···N), 374 w, 332 w, 240 w. Anal. Calc. for C12H12AgBN8; C, 37.3; H, 3.1; N, 29.0; Found: C, 37.0; H, 3.2; N, 28.7%. 2.2.8. [(Hmimt)Ag{B(3 -Mepz)4}] (8) To a stirred diethyl ether suspension (20 ml) of 6 (0.442 g, 1 mmol) was added Hmimt (0.114 g, 1 mmol) at r.t. The mixture was stirred for 4 h and the solid obtained was then filtered off, washed with diethyl ether, and shown to be compound 8 (yield 90%). m.p. (dec.): 230°C. 1H NMR (CDCl3, 293 K): l 2.19 (s br, 12H, 3-CH3 ), 3.62 (s, 3H, N-CH3 ), 6.00 (d, 4H, 5-CH), 6.59 (d, 1H, 4- or 5-CH), 6.68 (d, 4- or 5-CH), 7.11 (d, 4H, 3-CH). 13C NMR (CDCl3, 293 K): l 13.9 (s, 3-CH3), 34.7 (s, N-CH3), 104.4 (s, 4-CH), 117.7 (br, C= S), 119.8 (s, 3-CCH3), 134.8 (s, 4- or 5-CH), 136.5

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(s, 5-CH), 143.0 (s, 3-CH), 150.1 (s, 4- or 5-CH). \M (CH2Cl2)= 0.1 V − 1 cm2 mol − 1. IR (Nujol, cm − 1): 3110 vw (C− H), 1581 w, 1517 m (C···C,C···N), 518 m, 419 m, 411 m, 397 s, 352 w, 308 w, 279 m, 247 m. Anal. Calc. for C20H26AgBN10S; C, 41.8; H, 4.9; N, 24.3; Found: C, 41.9; H; 4.8, N, 24.6%.

2.2.9. [(phen)Ag{B(3 -Mepz)4}] (9) To a stirred diethyl ether suspension (20 ml) of 6 (0.442 g, 1 mmol) was added 1,10-phenanthroline (phen) (0.180 g, 1 mmol) at r.t. The mixture was stirred for 4 h and the yellow solid obtained was then filtered off, washed with diethyl ether, and shown to be compound 9 (yield 95%). m.p. (dec.): 280°C. 1H NMR (CDCl3, 293 K): l 2.08 (s br, 12H, 3-CH3 ), 5.90 (br, 4H, 5-CH), 6.90 (br, 4H, 3-CH), 7.67 (m, 2H, CH), 7.86 (s, 2H, CH), 8.34 (dd, 2H, CH), 9.02 (d, 2H, CH). 13 C NMR (CDCl3, 293 K): l 14.3 (s, 3-CH3), 103.9 (s, 4-CH), 123.2 (s, CH), 126.6 (s, CH), 128.7 (s, 3-CCH3), 136.1 (s, CH), 136.4 (s, 5-CH), 150.4 (s, CH). \M (CH2Cl2)= 1.0 V − 1 cm2 mol − 1. IR (Nujol, cm − 1): 1588 w, 1565 w, 1505 m (C···C,C···N), 417 m, 410 m, 396 m, 388 m, 341 w, 324 w, 304 w, 279 w, 256 m, 247 m. Anal. Calc. for C28H28AgBN10; C, 54.0; H, 4.5; N, 22.5; Found: C, 54.5; H, 4.5; N, 22.7%. 2.3. Attempted reactions The reactions between complexes 1, 2, 3 or 5 and Hpz, Him, Hmimt and phen have been carried out similarly, but no products were obtained, the starting material being always recovered. The reaction between compound 6 and Hpz or Him was always unsuccessful, the starting material being recovered.

2.4. Single crystal X-ray study For [Ag(Tm)]2·2CHCl3 (1) a unique room-temperature single counter diffractometer data set was measured (T ca. 295 K; monochromatic Mo Ka radiation, u=0.71073 A, ; 2qmax =50°) yielding 3884 independent reflections, 2744 with I \3|(I) being used in the refinement after gaussian absorption correction. Full-matrix least-squares refinement was employed, anisotropic thermal parameter forms being refined for the non-hydrogen atoms, together with (x,y,z,Uiso)H. Conventional residuals R, Rw (statistical weights) were 0.051, 0.054 at convergence; neutral atom complex scattering factors were employed within the context of the Xtal 3.4 program system [19].

2.4.1. Crystal/refinement data [Ag(Tm)]2·2CHCl3 (1). C26H34Ag2B2Cl6N12S6, M= 1157.1. Triclinic, space group P1( (C 1i , No.2) a = 11.067(5) b= 10.338(5) c =9.729(5) A, , h =86.72(4) i = 86.53(3) k= 85.66(4)°, V =1106 A, 3. Dcalc (Z= 1

dimer)= 1.737 g cm − 3; F(000)=576. vMo =15.7 cm − 1; specimen: 0.10× 0.32×0.29 mm; Tmin, max = 0.70, 0.86. ny = 313, Dzmax = 1.09 e A, − 3. Comment — ‘Thermal motion’ on the solvent chlorine atoms was high, possibly a foil for unresolved disorder. Site occupancies were set at unity after trial refinement.

3. Results and discussion

3.1. Synthesis The interaction between silver(I) nitrate and potassium or sodium salts of tris- and tetrakis-(azol-1yl)borates [Hx B(Az)4 − x ] (HAz = 1-methylimidazoline(3H)-thione, pyrazole, 3-methylpyrazole, 3,5-dimethylpyrazole, 4-bromopyrazole or imidazole) in methanol at r.t., readily gives the complexes 1–7 in high yield, in accordance with the following equation: CH3OH/r.t.

AgNO3 + M[Hx B(Az)4 − x ] [Ag{Hx B(Az)4 − x }]n + MNO3 (x=0 or 1, M=K or Na)

(1)

On the basis of literature data [8], X-ray structural studies and MS FAB (see below) a dinuclear structure can be plausibly assigned to compounds 1 –4, three different coordination modes being possible for the tris(pyrazolyl)borates (Fig. 1), whereas poly- or oligonuclear structures, containing bridging poly(pyrazolyl)borates [20], appear more likely for the poorly soluble derivatives 5–7, it not being possible to obtain good quality crystals suitable for X-ray studies for 5– 7, so that we are unable to confirm this hypothesis. All the colorless compounds 1–7 decompose without melting. They are stable to air and somewhat light-sensitive, poorly soluble in chlorinated solvents, acetone and DMSO. They are generally non-electrolytes in CH2Cl2 (\m in the range 0.1 –1.0 V − 1 mol2 cm − 1) and insoluble in acetonitrile, water, ethyl acetate, methanol, ethanol, and benzene. In chloroform solution the silver(I) tris(pyrazol-1-yl)borate complexes 2–4 often darken, even with strict exclusion of oxygen and light: this is due to the strong reducing power of the borate anion retaining one hydrogen which immediately converts the complex into metallic silver, in accordance with the following equation: [AgHB(pz)3]2 “ H2 + 2Ag + [B(pz)3]2

(2)

A similar decomposition has been previously reported for mercury derivatives containing HB(pz)3 [21]. The reaction between compounds 1, 2, 3 or 5 and uni- or bidentate, N- or S-donor ligands was always unsuccessful, the starting material being always recov-

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ered, even when the reaction was carried out under forcing conditions, i.e. large excess of the neutral ligand and refluxing solvent, whereas the reaction between compound 6 and 1-methylimidazoline(3H)-thione (Hmimt) or 1,10-phenanthroline (phen) yielded the 1:1 adducts [(Hmimt)Ag{B(3-Mepz)4}] (8) and [(phen)Ag{B(3-Mepz)4}] (9) respectively. It is worth to noting that to date this is the only procedure that allows the synthesis of this kind of complex containing both anionic and neutral azole-type ligands.

3.2. Spectroscopy In the infrared spectra of derivatives 1 – 7 (the most remarkable bands are reported in Section 2) recorded in Nujol mull w(CH) for the heterocyclic ring is found above 3100 cm − 1, the ‘breathing’ of the heterocyclic rings [22] at ca. 1540 – 1500 cm − 1 and finally the BN stretching vibration as a band of medium intensity at ca. 1400 cm − 1. In 3 the BH stretch appears as a single peak at ca. 2509 cm − 1, whereas a closer examination of the spectrum of the derivative of [HB(pz)3] shows a medium absorption at ca. 2450 cm − 1 and the presence of a second weaker absorption at ca. 2400 cm − 1. These bands are not significantly shifted upon co-ordination. The IR spectrum of the complex 1 shows w(CN) shifted to higher frequencies with respect to the analogous bands in the potassium salts of Tm. In the Far-IR region several bands due to w(CS) coupled with w(AgS) bands at ca. 280 cm − 1 are also found which suggest ligand thione S-donation. The absorptions found in the 200 – 150 cm − 1 region are analogous to those reported for dinuclear S-bridged Cu(I), Ag(I)

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and Hg(II) derivatives [23]. In particular the absorptions at 150 and 130 cm − 1 are assigned, in accordance with literature data, to the bridging w(AgS) modes [23a]. In addition to several absorptions characteristic of the azole ring system and of the starting silver(I) derivatives, there are some bands at ca. 350 –250 cm − 1, similar to those described in the literature [24] for some metal(I) azolato derivatives, tentatively assigned to w(AgN) vibrations. The 1H and 13C NMR spectra were obtained using chloroform or DMSO as a solvent. They support the formulae proposed and show that our [Hx B(Az)4 − x ] donors have not undergone any structural change upon coordination. The generally significant Z values (Z is the difference in chemical shift of a given magnetically equivalent nucleus in the anionic ligand of the alkali metal salt with respect to its silver(I) derivative) suggest the persistence of the complexes in weak donor solvents such as the chlorinated ones. In the 1H NMR spectra, as previously found in other poly(azol-1-yl)borate derivatives [9], upfield resonances in the case of H(3) and H(4), and downfield ones in the case of H(5), were detected. In the 1H NMR spectra (r.t.) of derivatives 3, 4 and 5, only one set of signals is found for the pyrazolyl groups, suggesting highly fluxional species likely due to a complete dissociation and reassociation of the pyrazolyl nitrogens throughout a ‘pinwheel’ mechanism [3] which does not occur at lower temperatures: in fact, on cooling the CDCl3 solutions of 3–5 to 193 K, additional pyrazole ring signals appeared.

Fig. 1. Molecular structures proposed for compounds 2 – 4.

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cated, it is of interest to note that frequently the disposition of a species in the crystal is often indicative of the form predominant in solution at low temperature. The positive fast atom bombardment mass spectra of silver(I) complexes 1, 3, 5, and 6 (the most relevant data are reported in the Experimental section) indicated that these derivatives undergo a sequential loss of ligands; in particular, decomposition of the silver(I) complexes followed two fragmentation routes: those in which the parent molecular ion sequentially lost pyrazole, and which included direct cleavage of the BN bonds, and those corresponding to loss of tetrakis- or tris-(azol-1yl)borate groups or their fragments, which included breaking of the AgN bond. The first route was confirmed by the detection of peaks consistent with either [M− pzH]+ or [M+ nH−npzH]+ (with n =2 or 3), whereas the second route was confirmed by the existence of fragments corresponding to the loss of [Hx B(pz)4 – x ] (x=0–2) species. However, the predominant peaks, corresponding to ions containing silver, are those which undergo addition of the Ag+ ion with the probable formation of trinuclear or polynuclear species such as that identified by single crystal X-ray diffraction methods [25]. The presence of peaks due to trinuclear and tetranuclear fragments in the spectrum of 5, further support the hypothesized oligo- or poly-nuclear structure for the tetrakis(pyrazolyl)borate derivatives.

3.3. Structure determinations

Fig. 2. Two projections showing the dimer 1; 20% thermal ellipsoids are shown for the non-hydrogen atoms, hydrogen atoms having arbitrary radii of 0.1 A, .

Compound 2 shows a different behavior: in DMSO at r.t. only one set of signals is found for the protons of the three pyrazolyl groups; in CDCl3 only the H4 resonance splits into two resonances with 2/1 integrated intensity, indicating that this compound is not fluxional at r.t. in the latter solvent. It is also of interest to note that the 1H NMR spectrum (293 K) of 3 recrystallized from chloroform and ethyl acetate, is completely different from the spectrum of the same derivative not recrystallized. In the crystalline form of 3, analogously to compound 2, the H4 resonance splits into two resonances with 2/1 integrated intensity, consistent with the X-ray crystal structure data [8]. As previously indi-

The results of the single crystal X-ray structure study shows [Ag(Tm)]2·2CHCl3 (1) to be binuclear accompanied by a pair of chloroform solvent molecules. One half of the dimer, (together with one solvent molecule) comprises the asymmetric unit of the structure of 1, the dimer being disposed about a crystallographic center of symmetry. The dimer 1 is depicted in two projections in Fig. 2. The core of the dimer is a four-membered Ag(m-S)2Ag ring, the sulfur atom of ring 1 bridging the two silver atoms somewhat unsymmetrically (AgS 2.857(2) 2.552(2); Ag···Ag 3.215(1), S···S 4.361(3) A, ; SAgS, 107.30(6) AgSAg 72.70(5)°; deviations of Ag from the C3N2S plane ( 2 64.7) − 2.46(1), 0.55(2) A, , with CSAg 96.4(2), 115.6(2)°). A concomitant of this mode of interaction, together with the disposition of the rest of the ligand, is that the BH bridgehead is disposed with the hydrogen atom projecting into the Ag2S2 ring, with H···Ag 2.83(5), 2.45(5), H···S 2.88(5), 3.17(5) A, and H lying only 2.07(5) A, out of the plane. The Ag2S2/ring 1 interplanar dihedral angle is 80.3(1)°. The sulfur atoms of the two remaining rings of each ligand complete four-coordinate arrays about the silver atoms, that from ring 2 coordinating to the original

Effendy et al. / Inorganica Chimica Acta 308 (2000) 65–72

silver and that from ring 3 to its inversion image, so that the ligand straddles the pair of metal atoms (Ag···S 2.553(2), 2.524(2) A, , SAgS 106.99(7)°) with associated silver atom deviations from the ring planes being rather large (1.545(6), 1.00(2) A, ) suggesting some strain in the array. This perception is reinforced by rather large associated AgSC angles (116.2(2), 111.7(2)°) but there are few other indicators to strongly support this contention: for example NBN are closely arrayed between 107.7(5) –110.5(5)°, while the boron atom deviates from the ligand planes by a maximum of 0.21(1) A, , and the interplanar dihedral angles for the ligand rings range between 58.3(2) – 82.1(2)°. There is a strong interaction between the chloroform hydrogen atom and the sulfur of ring 2 (H···S 2.69(8) A, ). Adducts of the form M[HB(Az)3] (M= Cu, Ag and Az = azolate) are now well known. For copper(I) the structure of the parent Cu(HBpz3) has long been established [4], its structure being binuclear, [Cu(HBpz3)]2, with ligands bridging the two copper atoms in a very similar manner to that found in [Ag(Tm)]2 1 above except that the donor atoms are now ring nitrogens so that the resulting structure, with four-coordinate [N2Cu(m-N)2] metal atoms, is rather more compact (Scheme 1). By contrast, the copper complex of the ligand containing 3,5-dimethyl-substituted rings, although still binuclear (Scheme 2) presumably in consequence of greater steric crowding, has lost the bridging functionality of that pyrazolate ring, so that the copper atoms are now three coordinate, although Cu···Cu becomes rather shorter (2.507(1) cf. 2.660(1) A, ) in the process; both dimers are centrosymmetric. The bridging function may also be supplanted in the presence of other appropriate donors, notably CO [10d], wherein the complex reverts to a mononuclear array of the form [(OC)M(HBpz3)] containing four-coordinate copper(I) in an environment of C36 symmetry. The latter form appears to be more readily achievable with M= Ag, such complexes having being described and structurally characterized not only for unidentate ligands such as

Scheme 1.

Scheme 2.

71

CO and tBuNC [10f,g], but also for a number of the more common crystallization solvents such as tetrahydrofuran (thf) and toluene [10f,g]. To date, no crystallographic characterization of Ag(HBpz3) 2 has been reported; however, the form of [Ag{HB(3,5-Me2pz)3}]2 3 [8], as recorded elsewhere recently, and substantiated by our own work, despite substitution, is not that of its copper(I) homologue, with non-bridging pyrazolate units and three-coordinate metal atoms, but, rather, that of the unsubstituted parent [Cu(HBpz3)]2 with N2M(m-N)2 metal atom environments, presumably a consequence of greater flexibility contingent upon the incorporation of the larger (silver) metal atom.

4. Supplementary material Crystallographic data for the structural analysis has been deposited with the Cambridge Crystallographic Data Centre, CCDC 143505. Copies of this information may be obtained free of charge from: The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44-1223-336033; e-mail: [email protected]. uk or www: http://www.ccdc.cam.ac.uk).

Acknowledgements We thank the MURST and the University of Camerino for financial help.

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