The complex [Ag(Py)3][Au(C6F5)2]·Py (1) (Py = pyridine) has been prepared by the reaction of NBu4[Au(C6F5)2] with AgClO4 in the presence of pyridine. The crystallographic measurements indicate the presence of an extended unsupported one-dimensional chain of alternating gold and silver atoms. This arrangement is due to the formation of molecular Au-Ag ion pairs, to the -stacking of C6F5 and pyridine arene ligands and to the packing effects that lead to molecular ion pairs at short distance. Theoretical calculations based on approximate experimental distances and angles reveal the presence of both metallophilic Au1-Ag1 and aromatic C6F5-pyridine interactions. From a theoretical point of view the nature of the intermetallic attraction arises from both an ionic contribution and from dispersion-type correlation effects; the aromatic interaction is mainly due to dispersion-type correlation effects

Fernández, E.J.

Laguna, A.

López-de-Luzuriaga, J.M.

Monge, M.

Pyykkö, P.

Runeberg, N.

Repositorio de Universidad de La Rioja

FULL PAPERA Study of the Interactions in an Extended Unsupported Gold-Silver ChainEduardo J. Fernández,[a] Antonio Laguna,*[b] José M. López-de-Luzuriaga,[a]Miguel Monge,[a] Pekka Pyykkö,*[c] and Nino Runeberg[c]Keywords: Gold / Silver / Ab initio calculations / N ligandsThe complex [Ag(Py)3][Au(C6F5)2]·Py (1) (Py = pyridine) hasbeen prepared by the reaction of NBu4[Au(C6F5)2] withAgClO4 in the presence of pyridine. The crystallographicmeasurements indicate the presence of an extended unsup-ported one-dimensional chain of alternating gold and silveratoms. This arrangement is due to the formation of molecularAu−Ag ion pairs, to the π-stacking of C6F5 and pyridine ar-ene ligands and to the packing effects that lead to molecularIntroductionThe tendency of gold centers to aggregate has beentermed ‘‘aurophilicity’’ by Schmidbaur.[1] Many researchgroups have been involved in an exhaustive search for struc-tural information concerning such systems. The results haveencouraged the theorists to construct theoretical modelswhose results could be compared with the experimentaldata. These models lead one to conclude that these interac-tions arise from correlation and relativistic effects.[2,3] Theattraction due to electron correlation is dominated by dis-persion, with a significant contribution from the recentlyidentified ionic-type excitations.[4] These studies are now-adays being extended to other metals with closed shell con-figurations in theoretical models of systems with the sameor different centers. Previous studies carried out in ourgroup of heteropolynuclear metallacycles of the type[AuM(HSCH2PH2)2]21 (M 5 Ag, Cu) concluded that thepresence of only one gold atom is enough to induce metal-lophilic attractions in the group congeners and that this ef-fect could be modulated depending on the gold ligand.[5]We also performed a detailed analysis of the d8-d10 interac-tion between PdII and AuI in the model complex[AuPdCl3(HSCH2PH2)2] showing a dominant dispersion ef-fect, some ionic contributions and no net charge transfer[a] Departamento de Quı́mica. Universidad de la Rioja-UA-CSICMadre de Dios, 51, 26004 Logroño, Spain[b] Departamento de Quı́mica Inorgánica, Instituto de Ciencia deMateriales de Aragón, Universidad de Zaragoza-CSIC50009 Zaragoza, SpainE-mail: alaguna@posta.unizar.es[c] Department of Chemistry. University of Helsinki,P. O. B. 55 (A. I. Virtasen aukio 1), 00014 Helsinki, FinlandE-mail: pekka.pyykko@helsinki.fi750  WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 143421948/02/030320750 $ 17.501.50/0 Eur. J. Inorg. Chem. 2002, 7502753ion pairs at short distance. Theoretical calculations based onapproximate experimental distances and angles reveal thepresence of both metallophilic AuI−AgI and aromatic C6F5-pyridine interactions. From a theoretical point of view thenature of the intermetallic attraction arises from both an ioniccontribution and from dispersion-type correlation effects; thearomatic interaction is mainly due to dispersion-type correla-tion effects.between the metals.[6] In both cases a bidentate ligand actsas a bridge between the metallic centers and its contributionto the close approach of the metal atoms cannot be neg-lected.In contrast, the anionic complex [Au(C6F5)2]2 has beenfound to be a very valuable agent in the preparation of het-eropolynuclear complexes in which the metal-metal interac-tions are not imposed by the ligand architecture.[7210] Ac-cordingly, we report here the synthesis, characterization andtheoretical study of the complex [Ag(Py)3][Au(C6F5)2]·Py(1) (Py 5 pyridine), which shows an extended unsupportedone-dimensional chain formed by repetition of gold-silverion pairs that, in addition, displays π-stackinginteractions[11214] between C6F5 and pyridine arene ligandssupporting the formation of the infinite chains.Results and DiscussionCompound 1 was synthesized according to Equation (1).An ionic formulation could be proposed in solution basedon the molar conductivity measurement in acetone solution(ΛM 5 110 Ω21 cm2 mol21, 5·1024 ) assigned to a 1:1electrolyte. The IR spectrum shows, amongst others, the ab-sorptions of the C6F5 ligands at 783, 963 and 1500 cm21[15] and pyridine ligands in the range 6172651 cm21.[16]Moreover, the 19F NMR chemical shifts of δ 5 2114.60,2162.07 and2163.87 are similar to the values observed forthe [Au(C6F5)2]2 fragment in solution. The separation ofcompound 1 into ionic components in solution is also con-firmed by the mass spectrometry measurements (elec-trospray) in which we observe, depending on the experi-ment, the [Au(C6F5)2]2 anion at m/z 5 541 (ES2) and theA Study of the Interactions in an Extended Unsupported Gold-Silver Chain FULL PAPER[Ag(Py)2]1 cation at m/z 5 266 or the [Ag(Py)]1 cation atm/z 5 187 (ES1).(1)A single-crystal X-ray diffraction study has been carriedout for compound 1. The silver atom is found to be disor-dered over two positions (50% each one), which makes theresults of this study not completely valid. Despite this, themolecular structure can be described as being formed by[Au(C6F5)2]2 units and tricoordinate [Ag(Py)3]1 cationsgiving rise to a molecular ion-pair model (Figure 1). Themolecular Au2Ag ion pairs are repeated in one dimensionforming a chain due to the presence of π-stacking interac-tions between C6F5 and pyridine rings and packing effects,.Because of the disorder, any comment about bond lengthsand angles is inappropriate, but nevertheless, the singularstructural disposition could be used as a basis for a theoret-ical study.Figure 1. Structure of complex 1; two molecular Au2Ag ion pairsand C6F5-pyridine π-stacking are shownThe structural arrangement of complex 1 shows some in-teresting features from a theoretical point of view. First, ashort distance between the AuI and AgI atoms is observed.Secondly, two C6F5···Py aromatic interactions could be pro-posed in the formula unit [Ag(Py)3][Au(C6F5)2]·Py, one be-tween a free pyridine molecule and one of the C6F5 ringsand the other between a pyridine ligand bonded to silverand the other C6F5 ring. A theoretical study of these sec-ondary interactions has been carried out separately on threedifferent model systems (Figure 2) using two levels of the-ory based on Hartree2Fock (HF) and Local second orderMøller2Plesset perturbation (LMP2) theory. The LMP2approach, which is the simplest of the local correlationmethods,[17219] is analogous to the traditional MP2 but isperformed using localized occupied and local virtual or-bitals. The virtual space is spanned by nonorthogonalatomic orbitals (AOs), which are projected against the occu-Eur. J. Inorg. Chem. 2002, 7502753 751pied space in order to ensure orthogonality between the oc-cupied and virtual spaces. These nonorthogonal projectedorbitals are inherently localized. The orbital domain [i] isthat part of the virtual space assigned to an individualLMO (i), consisting only of those basis functions that arecentered on the atoms involved in that particular LMO. Theorbital domains [i] are kept orthogonal to the occupiedspace by projecting out the LMOs. Double excitations froman orbital pair (ij) are only allowed into the pair domain [ij],which is spanned by the union of the corresponding orbitaldomains [i] and [j]. The introduction of this physically ap-pealing restriction in which electron pairs are only inter-acting in the vicinity of either electron, gives rise to thesignificant reduction of the computational cost as well as adiminished basis-set supposition error (BSSE). On the otherhand the use of localized orbitals makes it possible to de-compose the correlation contribution into the interactionenergy of a complex according to different classes of doubleexcitations. The two most important contributions arisefrom the dispersion class, in which one electron is excitedfrom each subsystem into its own virtual space, and theionic class, in which one of the electrons is excited from oneof the subsystems to the virtual space of the other subsys-tem. In addition to the partition of the correlation contribu-tion of the interaction energy, we can assume that the majorpart of the electrostatic interaction is already described atthe uncorrelated HF level. Hence, we can theoreticallystudy in great detail the forces that are responsible for theone dimensional disposition of complex 1 in the solid state.Figure 2. The assumed structures of the theoretical models, (a):[Au(C6F5)2]···Py CASE 1; (b): [Ag(NH3)3]···[Au(C6F5)2] CASE 2;(c): [Ag(NH3)2(Py)]···[Au(C6F5)2] CASE 3Finally, we emphasize that the role of dynamic correla-tion effects in transition-metal chemistry is well-known.The particular correlation effects leading to the R26 disper-sion forces involve dipole-allowed virtual excitations at thetwo centers involved (A R A9, B R B9). The ‘‘ionic correla-tion’’ mentioned above means (A R A9, B R A9). Theseinteraction can only be reproduced by explicitly correlatedmethods.One advantage of the LMP2 method is that it has almostno BSSE. Thus, no counterpoise corrections are needed.In principle one should simulate the entire crystal struc-ture but we have only considered particular contributionsusing cluster models.A. Laguna, P. Pyykkö et al.FULL PAPERIn case 1 (Figure 2a) we have studied the interaction be-tween [Au(C6F5)2]2 and a free pyridine molecule present inthe crystal structure. The results of this interaction are givenin Table 1 and the calculated interaction energy curves areshown in Figure 3. The HF curve is repulsive, whereas theLMP2 curve is attractive. The calculated [Au(C6F5)2]···Pydistance, defined as the distance between the virtually paral-lel Py and C6F5 units, is 344.7 pm for our simplified modelsystem and the interaction energy is 36.5 kJ/mol at the equi-librium distance. Thus, we can assume that the aromaticinteraction between a C6F5 group from the [Au(C6F5)2]2unit and the free pyridine molecule is due to dispersion-type correlation effects, since a stabilization energy is ob-served when these effects are included at the LMP2 levelof theory and a repulsion between the rings at HF levelis found.Table 1. Optimized distance (Re) and interaction energy (Ee) forthe three model systems at HF and LMP2 levels (distances in pmand energy in kJ/mol)Re (SCF) Ee (SCF) Re (LMP2) Ee (LMP2)CASE 1 REPULSIVE 20.44[a] 317.8[b] 36.5CASE 2 312.3 160 284.3 234CASE 3 323.2 143 289.4 239[a] At the LMP2 optimized distance. [b] Referred to the Au2Ag dis-tance.Figure 3. Interaction energy curves for the[Au(C6F5)2]···[XAg(NH3)2] (X 5 NH3, Py) interaction calculatedat the SCF and LMP2 levelsMany earlier examples of coplanar ‘‘π-stacks’’ betweenaromatic rings are known. A typical ring-ring distance is350 pm.[13] If the two rings have opposite electric quadru-pole moments, like C6H6 and C6F6, the electric quadrupole-quadrupole interactions already yield an attraction of about6 kJ/mol.[20] Examples of π-stacks in organometallic chem-istry are also known.[14] For further information on thissubject see ref.[21]Eur. J. Inorg. Chem. 2002, 7502753752In case 2 (Figure 2b) we have proposed a model system[Ag(NH3)3]···[Au(C6F5)2] in order to study only the inter-metallic Au2Ag interaction. The results of the interactionenergies at the theoretically predicted equilibrium distanceare given in Table 1 and the calculated interaction energycurves are shown in Figure 3. In this case, the HF curve isattractive with a minimum corresponding to the equilib-rium distance at 312.3 pm and an attractive interaction en-ergy of 160 kJ/mol compared to the ionic dissociation limit.Regarding the LMP2 curve we also obtain an attractive be-havior with an interaction energy at the equilibrium dis-tance (284.3 pm) of 234 kJ/mol. In this case, in which onlythe Au···Ag interaction is studied, both electrostatic attrac-tion and dispersion-type correlation effects are responsiblefor this interaction. The electrostatic interaction is respons-ible for the attractive behavior obtained at the HF levelsince dispersion-type correlation effects are not included.The additional stabilization obtained at the LMP2 level isdue to the introduction of these correlation effects, whichinclude dispersion.Going from case 2 to case 3, we exchange one of theNH3 ligands bonded to silver for one pyridine ligand thatis parallel and at a short distance from one of the C6F5rings bonded to gold (Figure 2c). In this case we study theinfluence of the presence of two interactions, the intermetal-lic Au2Ag and the aromatic C6F5···Py (Ag-bonded), at theequilibrium distances and at the interaction energy values.For this case we obtain an AuI···AgI equilibrium distanceat the HF level of 323.2 pm and of 289.4 pm at the LMP2level. The equilibrium distance at both levels of theory isaccompanied by interaction energy values of 143 kJ/mol atthe HF level and 239 kJ/mol at the LMP2 level (see Table 2and Figure 3). Thus, this comparison allows us, if we makethe simplified conclusion that HF covers electrostatic inter-actions and the correlation energy (the difference betweenLMP2 and HF) covers the dispersion contribution, to es-tablish that the overall interaction is very similar for bothcases 2 and 3, in terms of the LMP2 equilibrium distanceand the interaction energy (284.3 pm and 234 kJ/mol forcase 2 and 289.4 pm and 239 kJ/mol for case 3). However,the contributions are different. The pyridine ligand in case3 introduces electrostatic repulsion since the electrostatic in-teraction diminishes from 160 kJ/mol in case 2 to 143 kJ/mol in case 3, but it also introduces attractive dispersioninteractions (74 kJ/mol in case 2 to 96 kJ/mol in case 3), asoccurs in case 1. In Table 2 we compare the results for cases2 and 3 at the LMP2 optimized distances, which are quitesimilar in both cases (284 vs. 289 pm respectively).Table 2. Electrostatic (HF) and dispersion (LMP2) contributionsto the intermetallic Au2Ag interaction (kJ/mol) at the LMP2optimized distanceElectrostatic Correlation Energy TotalCASE 2 160 74 234CASE 3 143 96 239A Study of the Interactions in an Extended Unsupported Gold-Silver Chain FULL PAPERThe partitioning of the correlation contribution to the[Ag(NH3)3]···[Au(C6F5)2] (case 2) and [Ag(NH3)2-(Py)]···[Au(C6F5)2] (case 3) interaction energies based on theobtained experimental distances are given in Table 3. Theintroduction of the pyridine ring into the model system incase 3 gives, as we have commented above, a net enhance-ment of the correlation energy and, looking at the partitionof this correlation energy, the dispersion (van der Waals)contribution increases with respect to the ionic correlationcontribution seen as the charge transfer.Table 3. LMP2 correlation energy partitioning at the experimentalgeometry (kJ/mol)Ex-dispersion Dispersion Ionic AgRAu AuRAgCASE 2 0.0 29 16 4 12CASE 3 20.1 45 20 6 14Conclusions1) An ionic, Coulomb attraction obviously exists betweenthe [Au(C6F5)2]2 anions and the [Ag(Py)3]1 cations. This isan important contribution behind the infinite chains.2) In addition, dispersion-type correlation effects lead toattractions both between AgI and AuI and between C6F5and pyridine moieties.3) Looking at the detailed correlation energy parti-tioning, the most important contributions are the normaldispersion (van der Waals) terms. Charge-transfer-type con-tributions are about one third as important and, of them,the AuRAg virtual excitation is more important than theAgRAu one.Experimental SectionGeneral: All experiments were performed under argon by usingstandard Schlenk techniques. Solvents were dried and distilled un-der argon. 1H and 19F NMR spectra were recorded on a BrukerARX 300. Elemental analyses were carried out with a C.E. instru-ment EA-1110 CHNS-O microanalyzer. Infrared spectrum was re-corded in the 40002200 cm21 range on a Perkin2Elmer FT-IRSpectrum 1000 spectrophotometer as nujol mulls between poly-ethylene sheets. Mass spectra were recorded on a HP-5989B MassSpectrometer API-Electrospray with interface 59987A.[Ag(Py)3][Au(C6F5)2]·Py (1): The reaction betweenNBu4[Au(C6F5)2] (200 mg, 0.258 mmol), AgClO4 (54 mg, 0.258mmol) and an excess of pyridine in CH2Cl2 leads, after a slow crys-tallization process (24 h) at low temperature, to compound 1 ascolorless crystals. Yield: 47%. 1H NMR [(CD3)2CO]: δ 5 7.52 (dd,2 H, -CH-), 7.96 (t, 1 H, -CH-), 8.67 (d, 2 H, -CH-).C32H20AgAuF10N4 (955.4): calcd. C 40.23, H 2.18, N 5.86; foundC 39.92, H 2.25, N 5.41.Eur. J. Inorg. Chem. 2002, 7502753 753Computational Details: All calculations were performed as imple-mented in the MOLPRO program package. For the metals, the 19valence electron pseudopotentials from Stuttgart and the corres-ponding basis sets[22a] augmented with two f polarization functionswere used.[22b] For the lighter atoms the cc-pVDZ basis sets wereused.[22c] The LMOs were obtained through a Pipek2Mezey local-ization procedure.[22d]AcknowledgmentsThis work was supported by the Spanish DGES (BQU2001-2409)and the U.R. API-01/B14. P. Pyykkö and N. Runeberg aresupported by The Academy of Finland.[1] F. Scherbaum, A. Grohmann, B. Huber, C. Krüger, H. Schmid-baur, Angew. Chem. Int. Ed. Engl. 1988, 27, 154421546.[2] P. Pyykkö, Y.-F. Zhao, Angew. Chem. Int. Ed. Engl. 1991, 30,6042605.[3] P. Pyykkö, T. Tamm, Organometallics 1998, 17, 484224852.[4] N. Runeberg, M. Schütz, H.-J. Werner, J. Chem. Phys. 1999,110, 721027215.[5] E. J. Fernández, J. M. López-de-Luzuriaga, M. Monge, M. A.Rodrı́guez, O. Crespo, M. C. Gimeno, A. Laguna, P. G. Jones,Chem. Eur. J. 2000, 6, 6362644.[6] O. Crespo, A. Laguna, E. J. Fernández, J. M. 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A Study of the Interactions in a Extended Unsupported Gold-Silver Chain.

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