Metal imidazolato polymers: synthesis, characterization and crystal structure of new silver(I) triphenylphosphine derivatives

Polyhedron 23 (2004) 3063–3068 www.elsevier.com/locate/poly

Metal imidazolato polymers: synthesis, characterization and crystal structure of new silver(I) triphenylphosphine derivatives G. Attilio Ardizzoia, Stefano Brenna, Fulvio Castelli, Simona Galli *, Girolamo LaMonica, Norberto Masciocchi, Angelo Maspero * Dipartimento di Scienze Chimiche e Ambientali, Universita` degli Studi, dell
Insubria, Via Valleggio 11, 22100 Como, Italy Received 26 May 2004; accepted 6 September 2004 Available online 11 November 2004

Abstract When the polymeric complex [Ag(im)]n (Him = imidazole) is reacted with PPh3 (PPh3 = triphenylphosphine), it yields the [Ag2(l2-im)2(PPh3)3]n and [Ag(l2-im)(PPh3)2]n species, shown to contain wavy chains of metal ions, singly bridged by N,N 0 -exobidentate imidazolate ligands. The former, crystallised as the CH2Cl2 solvate, contains two non-equivalent silver(I) ions, differing in the number of coordinated phosphines (one, in trigonal planar stereochemistry, or two, having tetrahedral geometry). The latter has a unique independent silver(I) ion in a tetrahedral environment, with two coordinated PPh3 ligands. The reactivity of known silver(I) azolates with PPh3, as well as the solution behaviour and (when available) the crystal structures of the corresponding derivatives are taken into consideration for a due comparison.
2004 Elsevier Ltd. All rights reserved. Keywords: Coordination polymers; Silver; Imidazolate; X-ray structure

1. Introduction In the recent past, a non negligible number of homoleptic transition metal azolates have been synthesised and shown to possess valuable properties, such as high thermal stability [1], magnetic hysteresis [2] and luminescence [3], as well as anticorrosive [4], sorption [5] and protein staining [6] activities. To highlight both the origin of such properties and their possible optimisation by means of a complete structural insight, we have been actively working to prepare and fully characterise by X-ray powder diffraction (XRPD) manifold 3d-transition metal coordination complexes bearing simple diazaheterocycles: Fe(III), Co(III), Co(II), Ni(II) [7], Cu(I), Ag(I) [8], Zn(II), Cd(II) and Hg(II) [9] pyrazolates, *

Corresponding author. Tel.: +39 31 326227/25 314458; fax: +39 31 326230/25 314454. E-mail address: [email protected] (S. Galli). 0277-5387/$ - see front matter
2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2004.09.023

Cu(I), Ag(I) [10] and Cu(II) [11] imidazolates, Co(II), Ni(II), Zn(II) [12], Cu(I) [13] and Ag(I) [14] pyrimidin2-olates and Co(II), Ni(II) and Zn(II) pyrimidin-4-olates [15]. In the pharmaceutical field, an important role is played by certain silver(I) and gold(I) azolates. Actually, it has been recently discovered that the simple [Ag(l2im)]n polymer (Him = imidazole) shows a wide spectrum of antimicrobial activities against bacteria, yeast and even mould [16]. A comparable spectral range against bacteria and yeast, yet not against mould, has been demonstrated by Ag(1,2,4-triz) [17] and Ag(tetz) [18], while a more modest one, involving a few bacteria, has been observed with Ag(1,2,3-triz) [17] (Htriz = triazole, Htetz = tetrazole). The easy rupture of the Ag(I)–N bond upon dissolution has been invoked as one of the key factors of these performances. To better characterise this and other key factors in the quoted (and related) pharmaceutically active silver(I) complexes and to tune

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their molecular design, the syntheses, solution behaviour investigation and structural determination of their PPh3 derivatives (PPh3 = triphenylphosphine) have been carried out. With this aim, the Ag(im)(PPh3)3 [19], [Ag(l2-1,2,3-triz)(PPh3)3]n, [Ag(l2-1,2,4-triz)(PPh3)2]n [17] and [Ag(l2-tetz)(PPh3)2]n [18] species have been prepared and their solution behaviour has been characterised (especially) by variable temperature 31P NMR; as in the case of the previously studied [Ag2(l2-pz)2(PPh3)3] (Hpz = pyrazole) [20], the concomitant presence of mononuclear species of different Ag(az)(PPh3)n formulation (Haz = azole), due to fast PPh3 dissociation equilibria, has been proved. The pharmaceutical activities of Ag(im)(PPh3)3 [19], [Ag(l2-1,2,3-triz)(PPh3)]n, [Ag(l21,2,4-triz)(PPh3)]n [17] and [Ag(l2-tetz)(PPh3)2]n [18] have been properly tested, while a full X-ray structural characterisation has been obtained for the tri- and tetra-azolates only. Within this context, we report hereafter on the synthesis and the single crystal X-ray structural characterisation of two new derivatives of [Ag(l2-im)]n with triphenylphosphine, the polymeric [Ag2(l2-im)2(PPh3)3]n and [Ag(l2-im)(PPh3)2]n species. A detailed comparison is drawn to the reactivity of Ag(pz), Ag(1,2,3-triz), Ag(1,2,4-triz) and Ag(tetz) with PPh3, as well as to the solution behaviour and the crystal structure of the corresponding derivatives.

2. Experimental 2.1. General procedures

2.3. Synthesis of [Ag(im)(PPh3)2]n (2) Complex 2 was obtained in the form of well shaped crystals suitable for X-ray analysis by slow diffusion of a saturated diethyl ether solution of PPh3 into a dichloromethane solution of 1 (71% yield). Anal. Calc. for C39H33N2P2Ag: C, 66.96; H, 4.72; N, 4.00. Found: C, 70.10; H, 4.68; N, 3.97%. Complexes 1 and 2 are not significantly light sensitive and no particular precautions were taken in order to avoid light exposure. However, direct exposure to intense sunlight for prolonged times (>3 h) causes a superficial darkening of the crystals. 2.4. X-ray crystallography Suitable colourless flat (0.25 · 0.20 · 0.05 mm, 1 Æ CH2Cl2) and prismatic (0.13 · 0.09 · 0.07 mm, 2) crystals were mounted in air on a goniometer head. Data were collected on a Nonius CAD4 diffractometer, equipped with graphite-monochromatised Mo Ka radi˚ ), by the x-scan method, acquiring ation (k = 0.71073 A in both cases all unique reflections in the 5 < 2h < 50 range. Empirical corrections for absorption and Lorentz polarisation effects were applied to the integrated reflections (w-scans method [21]). The structures were solved by direct methods [22] and refined with full-matrix least-square calculations on F2 [23]. Anisotropic temperature factors were assigned to all atoms except hydrogen atoms which were refined riding on their parent atoms with a common isotropic displacement parameter chosen as 1.2 times that of the pertinent parent.

2.2. Synthesis of [Ag2(im)2(PPh3)3]n (1)

2.4.1. [Ag2(im)2(PPh3)3]n Æ (CH2Cl2), (1 Æ CH2Cl2) Ag2(C3H3N2)2(PPh3)3 Æ (CH2Cl2), C61H53Cl2N4P3Ag2, M = 1221.68 g mol
1; monoclinic, P21/c, a = 10.678(1), ˚ , b = 91.31(1), V = b = 39.858(12), c = 13.310(2) A ˚ 3, Z = 4; T = 298(2) K, Dc = 1.43 g cm
3, 5663(2) A l(Mo Ka) = 0.91 mm
1, F(0 0 0) = 2480; 644 parameters, no restraints, 9926 unique reflections measured, of which 3633 observed [I > 2r(I)]; Robs = 0.0433, wR2 all = 0.0787, ˚
3, highest peak and deepest hole = 0.40 and
0.48 e A GOF = 0.791.

To a suspension of [Ag(im)]n [10] (0.50 g, 2.9 mmol) in 20 mL of CH2Cl2, PPh3 (1.3 g, 5.0 mmol) was added under stirring. In a few minutes [Ag(im)]n dissolved, giving a clear solution. The solution was stirred for an additional hour and then evaporated to dryness. The residue was treated with 10 mL of diethyl ether giving a white solid, which was filtered, washed with diethyl ether and dried under vacuum (90% yield). Anal. Calc. for C60H51N4P3Ag2: C, 63.40; H, 4.49; N, 4.93. Found: C, 63.54; H, 4.50; N, 4.89%. Crystals suitable for X-ray analysis were obtained by slow diffusion of diethyl ether into a dichloromethane solution of 1 containing PPh3 (Ag:PPh3 ratio = 1:1).

2.4.2. [Ag(im)(PPh3)2]n, (2) Ag(C3H3N2)(PPh3)2, C39H33N2P2Ag, M = 699.52 g mol
1; monoclinic, P2/1n, a = 14.561(5), b = 9.674(5), ˚ , b = 91.56(2), V = 3445(1) A ˚ 3, Z = 4; c = 24.463(5) A
3 T = 298(2) K, Dc = 1.349 g cm , l(Mo Ka) = 0.71 mm
1, F(0 0 0) = 1432; 397 parameters, no restraints, 6037 unique reflections measured, of which 4222 observed [I > 2r(I)]; Robs = 0.0263, wR2 all = 0.0685, high˚
3, est peak and deepest hole = 0.32 and
0.51 e A GOF = 0.971. Relevant bond distances and angles, with estimated standard deviations, are reported throughout the text and are collected in the captions to Figs. 1 and 2.

All reactions were carried out under an atmosphere of dry nitrogen in order to avoid moisture. All chemicals (Aldrich Chemical Co.) were used as supplied. Solvents were purified and dried by standard methods. Elemental analyses were carried out at the Microanalytical Laboratory of this University.

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Fig. 1. ORTEP drawing, at 20% probability level, of the asymmetric unit in [Ag2(im)3(PPh3)3] Æ CH2Cl2 (1 Æ CH2Cl2). Hydrogen and solvent atoms ˚ ) and angles () (e.s.d.
s in parentheses): Ag1–N11 = 2.290(4), Ag1–N21 = 2.301(4), Ag2–N13 = 2.249(5), are omitted for clarity. Significant distances (A Ag2–N23 = 2.196(4), Ag1–P1 = 2.450(2), Ag1–P2 = 2.595(2), Ag2–P3 = 2.395(2); N11–Ag1–N21 = 109.25(16), P1–Ag1–P2 = 118.40(5), N11–Ag1– N11–Ag1–P2 = 102.14(12), N21–Ag1–P1 = 113.47(13), N21–Ag1–P2 = 93.21(12), N13–Ag2–N23 = 109.47(17), N13–Ag2– P1 = 117.36(12), P3 = 118.43(13), N23–Ag2–P3 = 131.09(13); Ag1  Ag2 = 6.433(1) and 6.575(1).

On layering diethyl ether over a CH2Cl2 solution of 1, colourless single crystals suitable for X-ray diffraction (1 Æ CH2Cl2) are obtained by slow diffusion. At variance, upon layering an excess of PPh3 dissolved in diethyl ether, the formation of white crystals of a different formulation, [Ag(im)(PPh3)2, 2], is observed (Eq. (2)) PPh3

½Ag2 ðimÞ2 PPh3 n ! 2½AgðimÞðPPh3 Þ2 n

Fig. 2. ORTEP drawing, at 20% probability level, of the asymmetric unit in [Ag(im)2(PPh3)2] (2). Hydrogen atoms are omitted for clarity. ˚ ) and angles () (e.s.d.
s in parentheses): Ag– Significant distances (A N1 = 2.296(2), Ag–N3 = 2.294(2), Ag–P1 = 2.471(1), Ag–P2 = 2.456(1); N1–Ag–N3 = 97.92(8), P1–Ag–P2 = 124.22(3), N1–Ag–P1 = 105.22(6), N1–Ag–P2 = 114.01(6), N3–Ag–P1 = 105.78(6), N3–Ag–P2 = 106.25(6); Ag  Ag = 6.555(1).

3. Results 3.1. Synthesis and characterization On reacting the [Ag(l2-im)]n polymer [10] with PPh3 in CH2Cl2 at room temperature (with an Ag:PPh3 ratio of 1:1.7) a white product of Ag2(im)2(PPh3)3 formulation, 1, can be isolated (Eq. (1)) 2=n½AgðimÞn þ 3PPh3 ! ½Ag2 ðimÞ2 ðPPh3 Þ3 n

ð1Þ

ð2Þ

In both cases, to recover analytically pure products, we found it necessary to operate in a slight excess of PPh3 with respect to the stoichiometric amount required; only under these conditions is the dissociation of the already coordinated, labile phosphines inhibited. Actually, on suspending 2 in diethyl ether, loss of PPh3 with concomitant formation of 1 takes place (from IR evidence). Seemingly, to obtain 2, the required PPh3 excess lies in the 1:1.7–1:3 range. Actually, on working with a large excess of PPh3 (i.e., with an Ag:PPh3 ratio of 1:3 or higher), Ag(im)(PPh3)3 can be isolated [19]. 3.2. Crystallographic aspects Compound 1 Æ CH2Cl2 is characterised by singlestranded wavy chains of Ag2(im)2(PPh3)3 formulation and clathrated molecules of dichloromethane (Figs. 1 and 3(a)). The chains evolve along a with a transla˚ (a) and pack creating a hertion step of 10.678(5) A ringbone motif appreciable down [1 0 0]. Within each chain, two crystallographically independent metal ˚, ions [Ag1 0   Ag2 = 6.433(1) Ag1 0   Ag2 = 6.575(1) A Ag1  Ag2  Ag1 = 110.3] are singly bridged by imidazolate anions, which act in their usual N,N 0 -exobidentate coordination mode. Both in a general

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Fig. 3. Schematic representation of the single-stranded chain in 1 Æ CH2Cl2 (a) and 2 (b). Phenyl carbons and hydrogen atoms are omitted for clarity.

position, the two metal centres exploit different coordination numbers and geometries: Ag1, bonded to two phosphine ligands, is actually the centre of a distorted Ag(N)2(P)2 tetrahedral moiety; Ag2, bonded to just one PPh3 ligand, shows an almost planar trigonal ˚ from the plane of stereochemistry (emerging by 0.06 A the three atoms coordinated to it). A non-negligible distortion of the Ag1 tetrahedral stereochemistry is highlit by both the discrepancy between Ag1–P1,2 bond distances and the actual spread of the L–Ag1–L bond angles from the ideal tetrahedral one (L = N or P; see caption to Fig. 1). The observed distortion can be tentatively ascribed to the steric demands of the two PPh3 ligands, resulting in their different positions with respect to the Ag–N  N–Ag skeleton. Indeed, both ligands point outwards from the polymeric spine: yet P2–Ag1–N angles are smaller than those involving P1 and are compensated by an Ag1–P2 bond that is longer than Ag1–P1. This asymmetry is not transferred to the Ag1–N bonds, Ag1–N11 and Ag1–N21 being almost comparable. Even the Ag2 stereochemistry is somewhat distorted: its PPh3 is more proximate to the N23-imidazolate than to the N13-one, this, however, only slightly differentiates Ag2–N13 and Ag2–N23. Solvent molecules are located among the chains and al-

ways face the less hindered Ag2 ions, with a short ˚. Ag2  Cl2 non-bonding contact of 3.567(3) A Even 2 is composed of single-stranded wavy chains (though of Ag(im)(PPh3)2 formulation) winding up about the monoclinic axis as 21 degenerate helices of ˚ (see Figs. 2 and 3(b)). The skeleton of pitch 9.674(5) A each chain is formed by consecutive, non collinear, me˚ and Ag  Ag   tal ions [Ag  Ag = 6.555(1) A Ag = 95.1], singly bridged by N,N 0 -exo-bidentate imidazolate ligands. Each metal, in a generic position, is further bonded to two independent PPh3 ligands, thus showing a heavily distorted tetrahedral arrangement due to the steric hindrance of the PPh3 moieties (see caption to Fig. 2). The crystal packings of both 1 Æ CH2Cl2 and 2 show a few C  C contacts between the phenyl ˚ rings, the shortest values falling in the usual 3.3–3.4 A range. Focusing on the most significant structural aspects, a comparison can be made between 1 Æ CH2Cl2 or 2 and related triphenylphosphine derivatives of silver(I) azolates, i.e., the [Ag(l2-pz)(PPh3)]2 and [Ag2(l2-pz)2(PPh3)3]2 dimers [20] and the [Ag(l2-1,2,3-triz)(PPh3)2]n, [Ag(l2-1,2,4-triz)(PPh3)2]n [17] and [Ag(l2-tetz)(PPh3)2]n [18] polymers. The pitch characterising the 21 helices of 2 is shorter than the translation step of 1 Æ CH2Cl2, but almost comparable to those found for the [Ag(l2-1,2,3-triz)(PPh3)2]n, [Ag(l2-1,2,4-triz)(PPh3)2]n and [Ag(l2-tetz)˚, (PPh3)2]n polymers [9.119(5), 9.525(6) and 9.471(3) A respectively]. Indeed, both the Ag  Ag contacts and the N–Ag–N angles characterising these species (Table 1) confirm the presence of a more relaxed
N– Ag  Ag–N skeleton in the crystals of 1 Æ CH2Cl2. No reasonable comparison can obviously be made to the metal  metal contacts of the doubly bridged [Ag(l2-pz) (PPh3)]2 and [Ag2(l2-pz)2(PPh3)3]2 dimers. The latter can be included when considering the Ag–N bond distances. Those departing from tetrahedral centres span ˚ range (Table 1), couples of bonds on the 2.29–2.34 A the same metal ion not always being comparable within experimental error. Noteworthy, is that the quoted range partially overlaps with that spanned by the Ag– ˚ , Table N bonds of trigonal metal centres (2.20–2.31 A 1), thus evidencing, for the Ag–N vectors of the two stereochemistries, a lack of internal consistency. The situation appears somewhat different for Ag–P bond distances, the intervals for tetrahedral and trigonal silver(I) ions being clearly differentiated (2.44–2.59 and ˚ , respectively; Table 1). Once again, how2.37–2.39 A ever, couples of bonds on the same metal centre may differ more than allowed by the related e.s.d.
s. The only bond angles showing a certain regularity are N–Ag–N, insisting on three-coordinated metal ions; the other L–Ag–P angles (L = N or P) cover rather wide ranges, though, for a given compound, adhering to the N– Ag–N < N–Ag–P and N–Ag–N < N–Ag–P < P–Ag–P

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Table 1 ˚ ) and bond angles () in the known triphenylphosphine derivatives of silver(I) azolates (e.s.d.
s Synoptic collection of chemically relevant distances (A in parentheses) Compound

References

Ag  Ag

CN(Ag) = 3a

CN(Ag) = 4a

Ag–N

Ag–P

N–Ag–N

N–Ag–P

Ag–N

Ag–P

N–Ag–N

N–Ag–P

P–Ag–P

2.204(3) 2.313(3) 2.176(2)

2.376(1)

110.68(10)

2.370(1)

110.39(8)

124.04(8) 125.24(8) 117.28(6)

2.295(2)

2.461(1)

100.67(8)

99.38(5)– 112.03(6)

121.73(3)

[Ag(pz)(PPh3)]2

20

3.870(1)

[Ag2(pz)2 (PPh3)3]2

20

3.706(1)

[Ag(1,2,3-triz) (PPh3)2]n

17

6.265

2.323(2) 2.326(7)

2.484(1) 2.460(2)

93.1(3)

108.1(2)– 114.3(2)

116.63(8)

[Ag(1,2,4-triz) (PPh3)2]n

17

6.554

2.327(7) 2.309(5)

2.512(2) 2.441(2)

97.7(2)

101.9(2)– 114.8(1)

126.29(7)

[Ag(tetz) (PPh3)2]n

18

6.498

2.321(5) 2.316(4)

2.469(2) 2.441(1)

94.3(2)

101.5(1)– 114.3(1)

127.41(5)

[Ag2(im)2 (PPh3)3]n

b

6.433(1)

2.249(5)

2.337(3) 2.290(4)

2.456(1) 2.450(2)

109.25(16)

93.21(12)– 117.36(12)

118.40(5)

b

6.575(1) 6.555(1)

2.196(4)

[Ag(im) (PPh3)2]n

2.301(4) 2.294(2)

2.595(2) 2.471(1)

97.92(8)

105.22(6)– 114.01(6)

124.22(3)

2.296(2)

2.456(1)

2.235(2)

132.24(6)

2.395(2)

109.47(17)

118.43(13) 131.09(13)

For N–Ag–P bond angles on tetrahedral metal centres, only the spanned ranges, not the whole list of values, are reported. a CN(Ag) = coordination number of silver(I). b Present work.

orderings for trigonal and tetrahedral coordinations, respectively (Table 1).

4. Discussion Already for the long-known [Ag(l2-pz)]n polymer, the reaction with PPh3 allows the recovery of different species, Ag(l2-pz)(PPh3) and Ag2(l2-pz)2(PPh3)3, depending on the Ag:PPh3 ratio employed (1:1.5 and >1:5, respectively) [20]. In the present case, i.e., with the imidazolate analogue, the required excess of PPh3 not only prevents the dissociation of the already coordinated ligands, but, at an earlier stage of the reaction, also promotes depolymerisation of the starting silver(I) species: actually, as confirmed by single crystal X-ray data collections, both Ag(l2-pz)(PPh3) and Ag2(l2pz)2(PPh3)3 consist of truly molecular dinuclear species (their correct formulation being thus [Ag(l2-pz)(PPh3)]2 and Ag2(l2-pz)2(PPh3)3, respectively). The different amplitude of the N,N 0 -exo-bidentate coordination bite in imidazolate and pyrazolate suggests the impossibility of forming doubly bridged dinuclear species in the case of 1 Æ CH2Cl2 and 2, thus suggesting either cyclic oligomers of higher nuclearity, or monodimensional polymers such as [Ag(l2-1,2,3-triz)(PPh3)2]n, [Ag(l21,2,4-triz)(PPh3)2]n [17] and [Ag(l2-tetz)(PPh3)2]n [18]. In the latter species, N1 and N3 (strictly paralleling the reciprocal position of nitrogens in imidazolate), N1

and N4, or N1 and N3, respectively, are involved in metal bridging, N2 (retraceable to the only other bite available in pz) being uncoordinated in all three cases. Thus, at least in the solid state, the presence of multiple N donor sites (as in 1,2,3-triz, 1,2,4-triz and tetz) results in N,N 0 -exo-bidentate (bridging) azolates, strictly avoiding the coordination of adjacent N atoms. The structural features of 1 Æ CH2Cl2 and 2, established by X-ray single crystals experiments (see above), definitely support these considerations. The reaction of Ag(az) with PPh3, depending on the azolate moiety, leads to different products, also on the basis of the Ag:PPh3 reaction ratios employed, dimeric or polymeric species being typically obtained. The only monomeric tris–triphenylphosphine silver(I) azolate so far isolated is Ag(im)(PPh3)3; however, variable temperature 31P NMR has shown that, although never recovered from Ag2(l2-pz)2(PPh3)3 solutions, Ag(pz)(PPh3)3 is stable (in solution) when a large excess of PPh3 is employed [20]. The existence of other Ag(az)(PPh3)3 species has been proven, in solution, for az = 1,2,3-triz, 1,2,4triz (variable temperature 31P NMR) and tetz (r.t. 31P NMR evidences). Noteworthy, with tetz, the Ag:PPh3 ratio required to obtain Ag(im)(PPh3)3 (1:3) leads directly to the [Ag(l2-az)(PPh3)2]n polymer [17,18]. Thus, even if ‘‘naturally’’ addressed as a homogeneous set of species, there are still some distinct peculiarities within the Ag(az)(PPh3)n family (witnessed, inter alia, by their different pharmacological activity); moreover, among

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the missing members of the pyrazolato and imidazolato systems studied in our group, we need to mention [Ag(l2-pz)(PPh3)2]2 (with two phosphines per silver(I) ion [20]) and [Ag(l2-im)(PPh3)]n [with only one phosphine per silver(I) ion], which could never be prepared, regardless of the Ag:PPh3 ratio applied.

5. Conclusions Two novel polymeric silver-imidazolate derivatives, [Ag2(l2-im)2(PPh3)3]n, (1), and [Ag(l2-im)(PPh3)2]n, (2), have been synthesised and characterized, complementing a recent report on the solution and structural chemistry of silver(I) complexes with polyimidazolyl derivatives and tertiary phosphines [24]. Although lacking pharmaceutical investigations, the results herein reported may non-negligibly contribute to the development of pharmaceutically active silver(I) azolates, throwing light on their remarkably different versatility towards phosphorous donor ligands. Actually, Nomiya et al. [17–19] have often remarked that, independently from the solid state organisation, the phosphine derivatives are, once dissolved, markedly less active than the corresponding homoleptic parents, possibly because of the higher affinity of silver(I) toward softer ligands (phosphines vs. azolates). Nevertheless, the stoichiometry, the reactivity and the structural features of the new species presented certainly add significant information on the nature of the complexes which can exist, in equilibrium with the more active species, in physiological conditions.

6. Supplementary material Crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre, supplementary publications Nos. CCDC 235858 and 235859. Copies of this data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [fax: +44 1223 336 033; e-mail: [email protected]; or on the web: http://www.ccdc.cam.ac.uk].

Acknowledgements The Italian MIUR (PRIN 2003), the University of Insubria (Progetto di Eccellenza ‘‘Sistemi Poliazotati’’) and the Fondazione Provinciale Comasca are acknowledged for funding. A special acknowledgement is due to Prof. Angelo Sironi (Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Universita` degli Stu-

di di Milano) for the generous loan of the X-ray single crystal equipment.

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