DALTON J. Chem. Soc., Dalton Trans., 1997, Pages 2321–2326 2321 Polymeric silver(I) complexes of the multinucleating ligand 4,7-dihydro-5-methyl-7-oxo[1,2,4]triazolo[1,5-a]pyrimidine. Analogous hydrogen-bonded structures in the crystal and vapour phases of the ligand Jorge A. R. Navarro,a M. Angustias Romero,a Juan M. Salas,*,†,a René Faure b and Xavier Solans c a Departamento de Química Inorgánica, Universidad de Granada, Av. de Fuentenueva S/N, 18071 Granada, Spain b Laboratoire de Chimie Analytique II, LICAS, Université Claude Bernard, Lyon-1, 69622 Villeurbanne, France c Departamento de Cristalografía, Mineralogía y Depositos Minerales, Universidad de Barcelona, Barcelona, Spain X-Ray crystallography showed that the crystal structure of Hmtpo (4,7-dihydro-5-methyl-7-oxo[1,2,4]triazolo- [1,5-a]pyrimidine) is built up of hydrogen-bonded polymeric chains, whereas mass spectroscopy and solution 15N NMR spectroscopy suggested the existence of a dimeric structure comprising eight-membered ? ? ?H]N]C]N ? ? ?H]N]C]N rings in less condensed phases.On the other hand, the reaction between the silver(I) salts AgX (X = NO3, ClO4 or ��� SO4) and Hmtpo yielded polymeric complexes. The compounds have been structurally characterised by IR spectroscopy and X-ray crystallography. The X-ray results show that the compounds containing the Hmtpo derivative in its neutral form comprise infinite one-dimensional chains, in which the ligand bridges the metal centres through N(1) and N(3) donor atoms.The presence of an additional soft and bulky ligand such as triphenylphosphine in [{AgX(m-Hmtpo-k2N1,N3)(PPh3)}n] (X = NO3 or ClO4) partially changes the tendency to form polymeric complexes, lengthening the Ag]N(1) bond distance by 0.35 Å. Finally, deprotonation of Hmtpo results in an additional available coordination position at N(4), the ligand bridging three metal centres in [{Ag3(m3-mtpo-k4N1,N3,N4,O7)2(HSO4)(H2O)2]?H2O}n].The new co-ordination mode results in the formation of eight-membered Ag]N]C]N]Ag]N]C]N rings, which give rise to short intermetallic contacts of 3.078(1) Å. The structures of free Hmtpo in the solid state and the postulated vapour-phase structure are compared to those of linear co-ordinated silver(I) complexes. Hydrogen bonding plays a fundamental role in several chemical and biochemical processes. Its highly selective and directional nature makes it ideal for the construction and stabilisation of large non-covalently linked molecular and supramolecular architectures.1 Indeed, the biological world is replete with examples of such systems: nucleic acids and protein tertiary structures.2 Some authors have extensively studied the replacement of hydrogen bonds by metal entities of appropriate geometry [e.g.AgI, trans-(H3N)2PtII, AuI, etc.] in models of biological interest, such as nucleobase Watson–Crick and Hoogstein pairs,3,4 triplets 5 and quartets,6 or materials of technological interest,7 concluding that the basic structures are preserved, but the strength of interaction is changed.At the same time, the tendency of AgI to build polymeric complexes with the help of ligands containing multiple donor positions makes this ion suitable for building supramolecular co-ordination compounds.8,9 We are currently involved in a detailed study of the coordination chemistry of the polydentate 4,7-dihydro-5-methyl- 7-oxo[1,2,4]triazolo[1,5-a]pyrimidine (Hmtpo), which has a great versatility as a ligand forming either mono-10 or polynuclear metal complexes.11,12 At the same time, this derivative is well known as a stabilising agent for photographic silver halide emulsions.13,14 Its behaviour has been related to the stability and high insolubility of the compounds formed in its reactions with silver salts as a result of its multidentate co-ordination to the metal centres,15 e.g.pKsp = 9.7 for [{Ag(m3-mtpo-N1,N3, N4,O7)}2n].16 † E-Mail: jsalas@goliat.ugr.es In this paper we present results concerning the preparation and structural characterisation by IR spectroscopy and X-ray crystallography of a series of silver(I) complexes formed upon reaction of AgI and Hmtpo in acidic aqueous or ethanolic media.The hydrogen-bonded polymeric crystal structure of free Hmtpo, as well as its postulated vapour-phase dimer structure, from mass spectroscopy, comprising eight-membered H]N]C]N? ? ?H]N]C]N? ? ? rings, are also reported and compared to the similar structures found in linearly coordinated silver(I) complexes.Experimental Reactants and methods The compound Hmtpo was obtained from Aldrich Chem. Co. and used as received. Crystals suitable for X-ray analysis were obtained by recrystallisation from MeOH. The other chemical reagents and solvents were supplied by commercial sources. All experiments were performed in air. Preparations [{Ag(NO3)(Ï-Hmtpo-Í2N1,N3)}n] 1.The addition of an acidic solution of Hmtpo (1 mmol in ca. 20 cm3 of HNO3, 4.5 mol N N N N H Me O 1 2 3 4 5 6 7 8 3a2322 J. Chem. Soc., Dalton Trans., 1997, Pages 2321–2326 dm23) to another one of AgNO3 (1 mmol in ca. 20 cm3 of HNO3, 4.5 mol dm23) gave a colourless solution that yielded, after 1 week, a precipitate of colourless crystals of 1. The crystals were rinsed with water and air dried. Yield 18% [Found (Calc. for C6H6AgN5O4): C, 22.7 (22.5); H, 1.9 (1.9); Ag, 31.7 (32.9); N, 21.8 (21.9)%].IR data (selected bands, cm21): 650m, 820m, 1250s, 1380vs, 1570vs, 1625s, 1700vs, 2890s and 3000s. [{Ag(ClO4)(Ï-Hmtpo-Í2N1,N3)}n] 2. A similar procedure to the previous one resulted in the immediate formation of a white microcrystalline precipitate of complex 2. Yield 70% [Found (Calc. for C6H6AgClN4O5): C, 20.2 (20.1); H, 1.7 (1.7); N, 16.1 (15.7)%]. IR data (selected bands, cm21): 625s, 920m, 1110vs, 1190s, 1570s, 1635vs, 1700vs and 3080m.[{Ag(NO3)(Ï-Hmtpo-Í2N1,N3)(PPh3)}n] 3. An ethanol solution of Hmtpo (2 mmol in ca. 50 cm3) was added to another one containing PPh3 (2 mmol) and AgNO3 (2 mmol) in ethanol (80 cm3), forming a turbid solution. The subsequent careful addition of concentrated HNO3 (2 cm3) resulted in the formation of a clear solution; after some hours plate-like crystals of complex 3 were formed. Yield 81% [Found (Calc. for C24H21Ag- N5O4P): C, 49.1 (49.5); H, 3.6 (3.6); Ag, 19.1 (18.5); N, 12.5 (12.0)%].IR data (selected bands, cm21): 830w, 1270s, 1420s, 1570vs, 1640s, 1710vs and 3080m. [{Ag(ClO4)(Ï-Hmtpo-Í2N1,N3)(PPh3)}n] 4. Following the same procedure as for compound 3, but using AgClO4 instead of AgNO3 and without addition of acid, crystals suitable for X-ray diffraction were obtained. Yield 71% [Found (Calc. for C24H21AgClN4O5P): C, 46.1 (46.5); H, 3.4 (3.4); N, 9.0 (9.0)%]. IR data (selected bands, cm21): 620s, 925m, 1040s, 1100vs, 1130s, 1570vs, 1640s, 1710vs and 3080m.[{[H2mtpo][Ag(SO4)(Ï-Hmtpo-Í2N1,N3)]?H2O}n] 5. This compound was obtained by diffusion, through a sinteredglass membrane, of an acidic solution of Ag2SO4 (1 mmol in ca. 120 cm3 H2SO4, 6 mol dm23) into another one containing Hmtpo (2 mmol in ca. 120 cm3 H2SO4, 6 mol dm23). After some days, plate crystals of 5 were obtained. Yield 18% [Found (Calc. for C12H15AgN8O7S): C, 27.3 (27.5); H, 2.8 (2.9); Ag, 20.5 (20.6); N, 21.4 (21.4)%]. IR data (selected bands, cm21): 420w, 450w, 600s, 900m, 1190vs, 1410s, 1575vs, 1620vs, 1665vs, 1700vs, 1725vs, 2890s, 3160s and 3480m.[{[Ag3(Ï3-mtpo-Í4N1,N3,N4,O7)2(HSO4)(H2O)2]?H2O}n] 6. The addition with stirring of an acidic solution of Hmtpo (1 mmol in ca. 40 cm3 of H2SO4, 6 mol dm23) to another one of Ag2SO4 (1 mmol in ca. 50 cm3 H2SO4, 6 mol dm23) gives a clear solution from which crystals of complex 6 suitable for X-ray analysis separated after some days. Yield 63% [Found (Calc. for C12H17Ag3N8O9S): C, 18.8 (18.6); H, 2.1 (2.2); Ag, 41.8 (41.9); N, 14.5 (14.7)%]).IR data (selected bands, cm21): 435w, 595w, 620s, 960m, 1110vs, 1140 (sh), 1195m, 1515s, 1570s, 1630s, 1695vs, 2800m and 3440s. [H2mtpo]Cl?H2O 7. This compound was prepared by dissolving Hmtpo (2 mmol) in concentrated HCl (ca. 5 cm3). From the resulting clear solution it was pe to collect, after 1 week, chunky colourless crystals of 7 [Found (Calc. for C6H9ClN4O2): C, 35.1 (35.2); H, 4.4 (4.4); N, 27.2 (27.4)%]. IR data (selected bands, cm21): 1410s, 1600m, 1665vs, 1725vs, 3165s and 3480m. Instrumentation Microanalyses of C, H and N were performed with a Fisons- Instruments EA-1008 microanalyser at the Instrumentation Centre of the University of Granada, whereas the silver content was determined thermogravimetrically by means of Mettler TA-3000 equipment provided with a Mettler TG-50 thermobalance, at a heating rate of 20 K min-1 and using an atmosphere of pure air (100 cm3 min21).Infrared spectra were recorded in the range 4000–180 cm21 on a Perkin-Elmer 983G spectrophotometer, using KBr and polyethylene pellets. Mass spectra by chemical ionisation were performed with Fisons Platform-II equipment, using direct sample injection and a potential of 36 eV (ca. 5.76 × 10218 J) and methane as reactant gas. Crystallography Relevant crystallographic data and details of the refinement are presented in Table 1. A crystal of compound 1 was mounted on a Philips PW-1100, 4 on a Nonius CAD-4 and 6 and Hmtpo on a Stoe-Siemens AED-2 diffractometer.The intensity data were corrected for Lorentz-polarisation effects and empirically (y scans) for absorption (with the exception of Hmtpo). The structures were solved by direct methods in the case of Hmtpo, and by heavy-atom and Fourier methods in the other cases, applying the SHELXTL PLUS17 (compounds 1, 6 and Hmtpo) and SDP18 programs (4). Full-matrix least-squares refinements were performed with anisotropic thermal parameters for nonhydrogen atoms.The hydrogen atoms of the organic ligands were idealised and those of water and hydrogensulfate groups refined with fixed O]H distances (0.85 Å). In the case of compounds 6 and Hmtpo, the last sets of refinements were made on F2 applying the SHELXL 93 program package.19 Atomic coordinates, thermal parameters, and bond lengths and angles have been deposited at the Cambridge Crystallographic Data Centre (CCDC).See Instructions for Authors, J. Chem. Soc., Dalton Trans., 1997, Issue 1. Any request to the CCDC for this material should quote the full literature citation and the reference number 186/507. Results Crystal and vapour-phase structures of Hmtpo A view of the crystal structure of Hmtpo is presented in Fig. 1 and the most relevant interatomic parameters are summarised in Table 2. The structure is built of one-dimensional infinite polymers, supported by strong hydrogen bonds formed between N(4)]H and O(7) groups from adjacent Hmtpo moieties, such that the H(4) ? ? ? O(7) separation is 1.80 Å [N(4) ? ? ? O(7) is 2.630(2) Å and the N(4)]H? ? ? O(7) angle is 1638].The geometry of the Hmtpo rings, which are planar to ±0.01 Å, does not differ significantly from that of the unco-ordinated Hmtpo in [Pd(tu)4]Cl2?2Hmtpo?2H2O21 (tu = thiourea). An examination of the derived interatomic parameters reveals that each of the N(1)]C(2), N(3)]C(3A), C(5)]C(6) and C(7)]O(7) bonds has significant double-bond character, which is in accord with the presence of the N(4)]H tautomer in the solid state.The chemical ionisation mass spectrum of free Hmtpo presents a high-intensity peak with m/z 301 corresponding to 2M 1 1. This result points to the existence of a dimeric structure in the vapour phase of Hmtpo. A feasible structure may be such that it contains H]N]C]N? ? ?H]N]C]N? ? ? rings (see Fig. 2). Analogous structures are also found for similar compounds such as 8-azaxanthine 22 in the solid state.The formation of dimeric structures in the vapour phase is reasonable, since the less-condensed matter may favour interactions of lower multiplicity 23 than those of the polymer found in the solid state (see Fig. 2). On the other hand, 15N NMR studies for Hmtpo, reported by Kleinpeter et al.,24 show the N(3) and N(4) signals broadened by proton exchange. A dimeric structure in solution similar to that postulated in the vapour phase may facilitate such N(3), N(4) proton transfer. Silver complexes The reaction of AgX (X = NO3 or ClO4) with Hmtpo in 1 : 1 molar ratio in strongly acidic aqueous media yields polymericJ.Chem. Soc., Dalton Trans., 1997, Pages 2321–2326 2323 Table 1 Crystal data and details of refinement for compounds 1, 4, 6 and Hmpto studied by X-ray crystallography * Formula M Space group a/Å b/Å c/Å b/8 U/Å3 Crystal dimensions Dm/g cm23 Dc/g cm23 F(000) m(Mo-Ka)/mm21 h kl 2qmax/8 Maximum, minimum transmission Maximum feature in DF map/e Å23 No.parameters refined No. reflections measured No. reflections independent No. reflections observed RR 9 wR2(F2, all) Weighting scheme Goodness of fit 1 C6H6AgN5O4 320.0 P21/c 12.205(3) 8.269(3) 9.858(3) 109.03(3) 940.5(9) 0.3 × 0.08 × 0.08 2.33 2.26 624 2.1 215 to 15 0–7 0–13 62 83, 62 0.4 175 3156 (0.05) 3060 965 [I > 2.5s(I)] 0.054 0.054 — 1/[s2(Fo) 1 0.005Fo 2] 1.18 4 C24H21AgClN4O5P 619.7 P21/c 16.489(3) 8.064(2) 25.462(8) 132.92(2) 2479(2) 0.5 × 0.5 × 0.35 — 1.661 1248 1.0 21 to 16 21 to 18 218 to 17 60 100, 98 0.69 389 7377 (0.05) 7222 5685 [I > 3s(I)] 0.031 0.031 — 1/[s2(Fo)] 1.10 6 C12H17Ag3N8O9S 773.0 P21/c 11.972(2) 13.455(3) 13.672(3) 112.30(3) 2037.6(7) 0.8 × 0.5 × 0.5 2.52 2.517 1496 3.0 28 to 8 210 to 10 0–13 60 87, 52 1.56 322 6866 (0.07) 6613 6613 0.058 — 0.193 1/[s2(Fo 2) 1 (0.1P)2] 1.26 Hmpto C6H6N4O 150.15 P21/n 3.9119(8) 14.711(2) 11.711(3) 85.670(8) 672.0(2) 1 × 0.5 × 0.1 — 1.484 312 0.11 25 to 5 217 to 17 0–21 65 — 0.246 101 9525 (0.04) 2890 2890 0.062 — 0.178 1/[s2(Fo 2) 1 (0.1P)2] 1.05 * Details in common: monoclinic; Z = 4, colourless, 293 K, l(Mo-Ka) 0.710 73 Å, R = ||Fo| 2 |Fc||/S|Fo|; R9 = (||Fo| 2 |Fc||w)� �� /S|Fo|w� �� ; wR2 = [Sw[(Fo 2 2 Fc 2)2]/S(wFo 4)]� �� ; P = (Fo 2 1 2Fc 2)/3.complexes of formula [{AgX(m-Hmtpo-k2N1,N3)}n] (X = NO3 1 or ClO4 2) in which the Hmtpo ligand shows a bidentate bridging mode via N(1), N(3).The additional presence of PPh3 gives also rise to the formation of [{AgX(m-Hmtpo-k2N1,N3)(PPh3)}n] with Hmtpo coordinated in the same way but with weaker Ag]N(1) bonds. Fig. 1 An ORTEP20 view of the one-dimensional chain in the crystal structure of Hmtpo Table 2 Bond distances (Å) and angles (8) for Hmpto N(1)]C(2) C(2)]N(3) C(3A)]N(8) N(4)]C(5) C(5)]C(5) C(7)]O(7) N(4) ? ? ? O(7I) C(2)]N(1)]N(8) C(3A)]N(3)]C(2) N(3)]C(3A)]N(4) C(5)]N(4)]C(3A) N(4)]C(5)]C(51) C(5)]C(6)]C(7) O(7)]C(7)]C(6) C(3A)]N(8)]C(7) C(7)]N(8)]N(1) 1.243(3) 1.382(3) 1.320(2) 1.230(2) 1.524(3) 1.200(2) 2.630(2) 100.2(2) 105.3(2) 131.2(2) 121.8(2) 117.8(2) 124.8(2) 130.0(2) 120.9(2) 127.1(2) N(1)]N(8) N(3)]C(3A) C(3A)]N(4) C(5)]C(6) C(6)]C(7) C(7)]N(8) N(1)]C(2)]N(3) N(3)]C(3A)]N(8) N(8)]C(3A)]N(4) N(4)]C(5)]C(6) C(6)]C(5)]C(51) O(7)]C(7)]N(8) N(8)]C(7)]C(6) C(3A)]N(8)]N(1) 1.406(2) 1.283(2) 1.381(3) 1.323(3) 1.456(3) 1.349(2) 115.6(2) 107.0(2) 121.8(2) 116.4(2) 125.7(2) 115.7(2) 114.3(2) 112.0(2) Symmetry relation: I x 2 1, y, z 1 1.Structure of [{Ag(NO3)(Ï-Hmtpo-Í2N1,N3)}n] 1. A view of the polymeric structure of compound 1 is presented in Fig. 3 and the most relevant interatomic parameters are summarised in Table 3. The structure of the complex comprises infinite chains, as a result of the nearly linear co-ordination shown by the Ag atom and the Hmtpo ligand in a N(1), N(3) bidentate bridging mode. The weaker interactions with the oxygen atoms O(1) [2.412(9) Å] and O(7) [2.691(9) Å] from the nitrate group and Hmtpo, respectively, make the effective co-ordination number 212.The Ag]N and Ag]O bond distances are comparable to those found in other complexes with similar environments.3,25 The geometry of Hmtpo appears almost unaltered after coordination, compared to that of free Hmtpo (see above). With respect to the geometry of the nitrate group, O(1) co-ordination to Ag results in a lengthening of the N]O(1) bond by ª0.04(1) &Ar[{Ag(ClO4)(Ï-Hmtpo-Í2N1,N3)(PPh3)}n] 4. This compound is also built of infinite Hmtpo N(1), N(3)-bridged Fig. 2 Structure conversion from solid-state hydrogen-bonded polymers to vapour-phase dimers N N N N N N N N O H O Me H Me N N N N N N N N O Me O Me H H less condensed state solid-state polymer vapour-phase dimer2324 J. Chem. Soc., Dalton Trans., 1997, Pages 2321–2326 chains (see Fig. 4) but the addition of the bulky and soft ligand PPh3 nearly displaces N(1) from the co-ordination environment of Ag, lengthening the Ag]N(1) bond distance by 0.35 Å, compared to the previous structure (see Tables 3 and 4). This displacement is a consequence of the preference of the soft acid AgI for a ligand such as PPh3 which is softer than Hmtpo.The silver atoms are placed in a distorted-tetrahedral N2PO environment. Nitrogen N(3) from Hmtpo and P from PPh3 appear strongly bonded to Ag, whereas N(1) and O(4) interact weakly. The Ag]P separation is normal.26 Regarding the geometry of the perchlorate group, the lengthening of the Cl]O(1) bond, as a consequence of its involvement in a hydrogen bond [N(4) ? ? ? O(1), 2.864 Å], is noteworthy, whereas the weak O(4) ? ? ? Ag interaction does not result in an appreciable modi- fication of the C(l)]O(4) distance.Fig. 3 View of the polymeric [{Ag(NO3)(m-Hmtpo-N1,N3)}n] chains in compound 1 Table 3 Selected bond distances (Å) and angles (8) for compound 1 Ag]N(1) Ag]N(3I) N]O(1) N]O(3) N(4) ? ? ? O(1II) N(3I)]Ag]N(1) O(7)]Ag]N(1) O(7)]Ag]N(3I) 2.253(8) 2.154(8) 1.28(1) 1.25(1) 2.760 150.8(3) 70.1(3) 105.3(3) Ag]O(7) Ag]O(1) N]O(2) O(1)]Ag]N(1) O(1)]Ag]N(3I) O(1)]Ag]O(7) 2.691(9) 2.412(9) 1.23(1) 82.3(3) 126.6(3) 98.9(3) Symmetry relations: I x 2 ��� , 2y 1 ��� , z 2 ��� ; II 2x 1 1, 2y, 2z 1 1.Table 4 Selected bond distances (Å) and angles (8) for compound 4 Ag]P Ag]N(3) Cl]O(1) Cl]O(3) N(4) ? ? ? O(1) P]Ag]N(3) P]Ag]N(1I) P]Ag]O(4) 2.3603(7) 2.217(2) 1.451(3) 1.395(3) 2.864 154.81(6) 108.22(6) 107.82(7) Ag]N(1I) Ag]O(4) Cl]O(2) Cl]O(4) N(3)]Ag]N(1I) N(3)]Ag]O(4) O(4)]Ag]N(1I) 2.597(2) 2.728(3) 1.419(3) 1.380(2) 82.7(1) 96.8(1) 75.9(1) Symmetry relation: I 2x, y 2 ��� , 2z 1 ��� .IR spectroscopy and thermal studies for compounds [{AgX- (Ï-Hmtpo-Í2N1,N3)}n] and [{AgX(Ï-Hmtpo-Í2N1,N3)(PPh3)}n] (X 5 NO3ClO4) Thermogravimetry has been shown to be of poor structural value. The compounds are thermally stable up to 150–300 8C, when pyrolysis commences. This process is very violent in the complexes containing the ClO4 2 anion.The IR spectra of compounds 1–4 present only small changes when compared to free Hmtpo, the only appreciable change above 600 cm21 being due to the concurrence of PPh3, NO3 2 and ClO4 2 characteristic bands. The absorption bands assigned to vibrations of the nitrate group, n3 (1380, 1250), n2 (820), n4 (650 cm21) for compound 1, n3 (1420, 1270), n2 (830 cm21) for 3, can be related to the presence of monoco-ordinated nitrate groups in both complexes.In 2 the original Td symmetry of the perchlorate group is preserved with n3, n1 and n4 appearing at 1120, 920 and 625 cm21, respectively. In the case of compound 4 the clear splitting of the perchlorate vibration mode n3 (1130, 1100, 1040 cm21) and n1 and n4 appearing at 925 and 620 cm21, respectively, are in accord with the loosening of the original Td geometry (see Table 4).Reactivity of Ag2SO4 towards Hmtpo The Hmtpo compound behaves very variably towards Ag2SO4. Thus, its reaction in neutral or basic media yields [{Ag(m3-mtpok4N1, N3,N4,O7)}2n] as reported by Smith and Luss.16 In strongly acidic aqueous media and depending on the silver to Hmtpo ratio, the reaction gives alternatively [{[H2mtpo][Ag(SO4)- (m-Hmtpo-k2N1,N3)]?H2O}n] 5 and [{[Ag3(m3-mtpo-k4N1,N3, N4,O7)2(HSO4)(H2O)2]?H2O}n] 6. The structure of 5 will be discussed on the basis of the IR data, whereas in the case of 6 it was possible to solve its structure by crystallography. Structure of compound 6.A perspective view of the asymmetric unit of the complex is shown in Fig. 5 and the most relevant interatomic parameters are summarised in Table 5. The crystal structure comprises infinite cationic [{Ag3(m3-mtpok4N1, N3,N4,O7)2}n]n1 chains and hydrogensulfate anions and water molecules weakly interacting with the silver atoms. The cationic chains contain dimeric [Ag(mtpo)]2 units, displaying Ag2N4C2 rings, which give rise to short Ag ? ? ? Ag contacts of 3.078(1) Å.The polymeric structure is generated by a third silver nucleus linearly bridging the N(1) atoms of adjacent dimeric [Ag(mtpo)]2 units. The co-ordination geometry about the metal centres is nearly linear (N2) with weak Ag ? ? ? O interactions from mtpo, HSO4 2 and water molecules also being present. Fig. 4 Perspective of loosely polymeric [{Ag(ClO4)(m-Hmtpo-N1, N3)(PPh3)}n] chains in compound 4J.Chem. Soc., Dalton Trans., 1997, Pages 2321–2326 2325 In the crystal, the chains associate in pairs supported by p–p interactions between the planar mtpo rings (mean separation 3.22 Å). Also, an intercatena Ag ? ? ? Ag contact of 3.268(1) Å is noted. Finally, the double chains are associated by means of weaker stacking interactions, with an interplanar separation of 3.20 Å but with lower overlap between the p rings than in the previous association.The coexistence of two kinds of short Ag ? ? ? Ag contacts in this structure is of note. The shorter one, with separation 3.078(1) Å, is supported by two bridging mtpo, acting in a N(3), N(4) bidentate mode. The unsupported one shows an intermetallic separation of 3.268(1) Å. The existence of direct Ag ? ? ? Ag bonding interactions, in compounds displaying intermetallic distances in the range 2.655–3.377 Å, is a matter of current controversy.27 However, our ab initio molecular orbital studies 27a on the dimer [{Ag(NO3)L}2] (L = 5,7- dimethyl[1,2,4]triazolo [1,5a]pyrimidine), which displays an intermetallic separation of 3.058(1) Å, show the stabilising nature of these interactions.The fact that compound 6 displays also a short Ag ? ? ? Ag contact without bridging ligands points to the existence of a direct intermetallic interaction. A similar conclusion has been made in the case of other unsupported silver polynuclear compounds.9 However, it should be noted that even if there is not a bridging ligand, other interactions contribute to the stabilisation of these associations such as hydrogen bonding in [{[Ag(NH3)2]2(SO4)}n],28 stacking inter- Fig. 5 An ORTEP view of the asymmetric unit of compound 6. The thermal ellipsoids are drawn at the 25% probability level Table 5 Selected bond distances (Å) and angles (8) for compound 6 Ag(2)]Ag(1I) Ag(1)]Ag(3) Ag(1)]N(3) Ag(1)]N(4b) Ag(1)]O(1) Ag(1)]O(1w) Ag(2)]O(7) S]O(1) S]O(2) O(1w) ? ? ? O(3) O(1w) ? ? ? O(4IV) O(2w) ? ? ? O(7I) O(2w) ? ? ? O(3wV) Ag(1)]Ag(3)]Ag(2I) N(3)]Ag(1)]N(4b) N(1)]Ag(2)]N(1bII) N(4)]Ag(3)]N(3b) Ag(1)]N(3)]C(3a) 3.268(1) 3.078(1) 2.176(5) 2.159(5) 2.689(5) 2.718(6) 2.698(5) 1.444(6) 1.455(8) 2.831 2.976 2.755 2.837 76.8(1) 162.8(2) 175.7(2) 161.6(2) 129.3(4) Ag(2)]O(7bII) Ag(2)]N(1) Ag(2)]N(1bII) Ag(3)]N(4) Ag(3)]N(3b) Ag(3)]O(3w) Ag(3)]O(1III) S]O(3) S]O(4) O(4) ? ? ? O(2w) O(3w) ? ? ? O(2III) O(3w) ? ? ? O(7bV) Ag(1)]N(4b)]C(3ab) Ag(3)]N(4)]C(3a) Ag(3)]N(3b)]C(3ab) N(4)]C(3a)]N(3) N(4b)]C(3ab)]N(3b) 2.628(5) 2.178(5) 2.202(5) 2.186(6) 2.154(5) 2.882(6) 2.746(5) 1.446(5) 1.555(7) 2.528 2.858 2.725 119.0(4) 118.5(4) 129.8(4) 129.2(6) 128.3(5) Symmetry relations: I 2x, 12y, 2z; II x, 1 1 y, z; III 2x, 2y, 2z; IV x, ��� 2 y, ��� 1 z; V 2x, ��� 1 y, ��� 2 z.actions between aromatic rings in our case, etc. The final result may be the sum of the whole set of interactions. Another interesting feature of compound 6 is that it represents one of the few examples of X-ray characterised hydrogensulfate groups.27b,29 The location of the acidic proton at oxygen O(4) is responsible for the lengthening of the S]O bond by 0.1 Å, which results in a C3v geometry for the anion (see Table 5).Hydrogen bonding appears to be especially important in thvery strong hydrogen-bond interaction between the hydrogensulfate group and solvation water O(2w) is noted, such that H? ? ? O(2w) is 1.71 Å [O(4) ? ? ? O(2w) 2.528 Å and O(4)]H? ? ? O(2w) is 1438].Thermal studies and IR spectroscopy. Thermogravimetric curves reveal that the dehydration processes take place in the temperature ranges 110–140 and 45–130 8C for compounds 5 and 6, respectively. A thermal stability zone follows up to 200 and 300 oC, respectively, temperatures at which a pyrolitic process starts which is finished at 700 oC, leaving metallic silver as final residue.In order to establish the nature of the Hmtpo ligands in compound 5 the chlorohydrate of Hmtpo was prepared, giving [H2mtpo]Cl?H2O 7. The IR spectrum 5, apart from the anions bands, can be considered the sum of those of 1 1 7. From this result a similar polymeric structure to that found in 1 is proposed for 5 in which the ligand acts in bidentate N(1), N(3) bridging mode. The SO4 22 bands (assigned Td symmetry) are as follows: n3 at 1190, n1 900, n4 600 cm21 and n2 450, 420 cm21.The splitting of the n2 band suggests a C2v geometry for the sulfate group, consistent with a possible bidentate behaviour,30 although such a geometry would require a splitting of n3 and n4. The IR spectrum of compound 6 shows a broad band in the 2900–2000 cm21 range, corresponding to the presence of the acidic hydrogensulfate proton, this region being similar to that of KHSO4. With respect to the characteristic bands of the Hmtpo ligand, they differ considerably from those found for [{Ag(m3-mtpo-k4N1,N3,N4,O7)]}2n]16 and other complexes containing Hmtpo in its monoanionic form;11 these are very similar, however, to those containing the ligand in its neutral form.10 It appears that the higher strength of the silver–ligand interactions in 6, compared to the similarly co-ordinated [{Ag(m3- mtpo-k4N1,N3,N4,O7)}2n],16 is responsible for a major charge localisation in the molecule, resulting in a shift of the bands to higher frequency, similar to the protonation effect.The sulfate bands are consistent with the C3v symmetry found in the solid state. The activation of the n1 vibration mode centred at 960 cm21, together with the splitting of n4 into two bands at 620 and 595 cm21 and n3 , n2 appearing, respectively, at 1110 and 435 cm21, is in accord with the X-ray results. Discussion The hydrogen-bonded structures, found in the solid and vapour phases of free Hmpto are similar to those of the nearly linear co-ordinated silver(I) complexes.However, there are some significant differences: in the case of the silver polymeric systems, the involvement of the exocyclic O(7) donor atom in the coordination is minimal, in agreement with the soft nature of the AgI ion. On the other hand, the tendency of the ligand to display eight-membered A]N]C]N]A]N]C]N rings (A = H or Ag) is shown to occur both in the vapour phase of free Hmpto and in the silver complexes containing Hmtpo in its monoanionic form, namely 6 and [{Ag(m3-mtpo-k4N1,N3, N4,O7)}2n].16 These kind of structures may be considered the result of a formal replacement of the hydrogen atom of a hydrogen bond by a metal atom with a linear co-ordination geometry.This replacement appears to have a marked effect on the strength of the association but not on the geometry of the2326 J. Chem. Soc., Dalton Trans., 1997, Pages 2321–2326 system.6 The formation of eight-membered Ag]N]C]N]Ag] N]C]N rings is also interesting from a theoretical point of view, since they give rise to short intermetallic contacts of 3.078(1) Å for 6 and 3.187 Å for [{Ag(m3-mtpo-k4N1,N3,N4, O7)}2n] 16 which suggest the possibility of direct M]M bonding interactions.6 Similar eight-membered rings are also found in the binuclear compound [Pt2(m-mtpo-k2N3,N4)4]11a which displays a very short M]M separation of 2.744(2) Å, the M]M interaction being suggested, from ab initio molecular orbital calculations, to be of bonding nature.Acknowledgements The authors thank Dirección General de Investigación Científica y Técnica for financial support (Grant No. PB94-0807- CO2–01), also Dr. M. Quirós for his helpful discussions and suggestions. References 1 D. S. Lawrence, T. Jiang and M. Levett, Chem. Rev., 1995, 95, 2229; D. Philp and J. F. Stoddart, Angew. Chem., Int. Ed. Engl., 1996, 35, 1154. 2 W. 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