Unprecedented Large Temperature Dependence of Silver(I)−Silver(I)Distances in Some N‑Heterocyclic Carbene Silver(I) Complex SaltsMargit Kriechbaum,† Johanna Holbling,† Hans-Georg Stammler,‡ Manuela List,§ Raphael J. F. Berger,*,∥

and Uwe Monkowius*,†

†Institut fur Anorganische Chemie, Johannes Kepler Universitat Linz, Altenbergerstraße 69, 4040 Linz, Austria‡Lehrstuhl fur Anorganische Chemie und Strukturchemie, Universitat Bielefeld, Universitatsstraße 15, 33615 Bielefeld, Germany§Institut fur Chemische Technologie Organischer Stoffe, Johannes Kepler Universitat Linz, Altenbergerstraße 69, 4040 Linz, Austria∥Materialwissenschaften und Physik, Abteilung Materialchemie, Paris-Lodron Universitat Salzburg, Hellbrunner Straße 34, 5020Salzburg, Austria

*S Supporting Information

ABSTRACT: Six examples from a series of complex salts containingbis(1,3-dialkylimidazol-2-ylidene)silver(I) cations (with dialkyl = dimethyl,diethyl, methyl, ethyl, diisopropyl) with [PF6]

−, [SbF6]−, [ClO4]

−, or[AgBr2]

− anions, respectively, were prepared in high yields and characterizedby elemental analysis, 1H and 13C NMR spectroscopy, and massspectrometry. Single-crystal X-ray diffraction experiments reveal unprece-dentedly large contractions of the metallophilic Ag(I)−Ag(I) distances inthe solid-state structures upon cooling. In the salt containing diisopropyl-substituted ligands and [PF6]

− anions, a contraction from 3.498(8) to3.180(2) Å was observed on cooling from 293(1) to 100(1) K.Photochemical measurements show strong hypsochromic shifts of theemission maxima upon cooling, underlining the metallophilic-based natureof the emission bands. Ab initio calculations show that the strongtemperature dependence of the observed Ag−Ag distances can be attributed to some extent to both the shallowness and theanharmonicity of the intermetallic interaction potential. The Ag−Ag interaction potentials are found to be attractive only whenrelativity is accounted for in the calculations.

■ INTRODUCTION

Metallophilic closed-shell interactions are an establishedconcept in the coordination chemistry of coinage metals withformal electronic nd10 configurations.1 They are mostprominent for linear two-coordinate Au(I) compounds withbinding energies on the order of those of hydrogen bonds andless pronounced for the lighter congeners Ag(I) and Cu(I).Metallophilic interactions generally arise primarily fromelectronic dispersion interactions which can be enhanced byrelativistic effects. Whereas examples for aurophilicity inmolecular compounds are reported frequently, analogous butpresumably weaker so-called argentophilic and cuprophilicinteractions have remained relatively hidden in molecularinorganic chemistry. In recent years, several experimental andtheoretical studies have proven that metallophilic interactionsmight also be a major organizing force in the formation ofsupramolecular aggregates of copper and silver.2

In the majority of studies published, the metal−metaldistances obtained from structural data are used as the primaryindicator for the presence of metallophilic interactions, despitethe difficulty in defining a precise metal−metal contact asbonded or nonbonded. The shortest metal−metal distancesmeasured in metallic silver and gold are 2.89 and 2.88 Å,3

respectively, whereas the sum of van der Waals (vdW) radii oftwo metal(I) cations is 3.44 and 3.32 Å4 (note that r[Ag(I)] >r[Au(I)] due to both the relativistic and the lanthanidecontraction).5 Whereas in the case of silver the accepted upperlimit for argentophilic attractions is 3.30 Å, a figure just belowthe sum of the vdW radii (3.44 Å),6 the rule of thumb upperlimit for aurophilic interactions is, at 3.5 Å, slightly above thesum of the vdW radii (3.32 Å). Distances with slightly largerthan the sum of the vdW interactions are nevertheless at timesconsidered as weak metallophilic interactions.7

A very large temperature dependence of the structuralparameters of silver and gold compounds has been reportedrecently. Goodwin et al.8,9 described a “colossal thermalexpansion” of lattice parameters,10 together with a significanttemperature dependence of intermetallic distances in thePrussian blue analogue Ag3[Co(CN)6] in the temperaturerange between 8 and 300 K. This behavior was ascribed to thepresence of argentophilic interactions, with a similar behavioralso being found for isostructural dicyanoargentates and-aurates.11 Balch and co-workers reported the temperature-

Received: October 5, 2012

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dependent reversible phase transition of the sulfur-centeredgold(I) cluster [μ3-S(AuCNC7H13)3]SbF6, which was associ-ated with a considerable variation of the gold−gold distances.12

Yang and co-workers reported on a huge thermal expansion ofthe metal−organic framework FMOF-1 due to the highflexibility of the coordinative bonds; however, in this case aconsiderable change in Ag−Ag distances was not observed.13

Recently a large (negative) thermal expansion has also beenreported for BiNO3.

14 Indications for a large thermaldependence of the metallophilic interaction can also be foundin the luminescence spectra. Emissions based on metallophilic,in particular on aurophilic, interactions are strongly affected bytemperature changes and are frequently found in theliterature.15

In this contribution we present examples of the extremetemperature dependences of the metal−metal distances inhomoleptic silver complex salts of the type [(NHC)2Ag]Y (3;see Scheme 1 for the definitions of abbreviations).

■ RESULTS AND DISCUSSIONSynthesis. Interest in NHC−Ag(I) complexes has grown

significantly since Wang and Lin proved their capability asversatile carbene transfer agents.16 Since then, a great variety ofdifferent NHC−metal complexes have been synthesized by thisso-called “silver-salt” method. Due to this versatility, as well astheir ease of preparation, a huge number of NHC−Ag(I)complexes have been reported,17 albeit not always isolated andaccurately characterized but rather used as intermediates in areaction sequence. Such complexes are air and moisture stableand can be conveniently prepared via the deprotonation ofimidazolium salts (2) using Ag2O in polar organic solvents.These reactions only work well if halides are present in thereaction mixture, as in case of the formation of 3b (Scheme 1).If imidazolium salts with weakly coordinating anions such as[PF6]

−, [ClO4]−, and [SbF6]

− are used, additional halidesources such as tetraalkylammonium chlorides are required. Wefound that a variation of this procedure using an excess of AgClin the presence of KOH in dichloromethane (DCM) underaerobic conditions at room temperature gave the respectivechloride-free NHC−Ag complexes 3a,c,d,d′,d″ in good yieldsand high purity. Conversely, the hexafluorophosphate salt 2dyields no complex upon addition of Ag2O. Also, as is indicatedby 1H NMR spectroscopic data, only 2d can be recovered fromthese reaction mixtures. Therefore, we propose the initialformation of chloro species such as (NHC)AgCl and[(NHC)2Ag][AgCl2] according to eq 1. It should be noted

that a detailed study on such equilibrium reactions betweenionic and neutral forms of “(NHC)AgX” (X = halide) waspublished recently.18 Owing to the low solubility of AgCl andKCl (K+ stems from the KOH) in DCM, the equilibrium isshifted to the right side, eventually yielding [(NHC)2Ag]Y asthe sole product.

⇋

→ + + ↓+ −

NHC NHC

NHC

2( )AgCl [( ) Ag][AgCl ]

[( ) Ag] Cl AgCl

2 2

2 (1)

All products have been characterized by elemental analysis,ESI-mass spectrometry, and 1H and 13C NMR spectroscopy(see the Experimental Section). The ipso carbon atoms insolutions of complex salts 3a−3d″ show signals in the typicalrange between 175 and 180 ppm in their 13C NMR spectra.19,20

This corresponds to a downfield shift of approximately 45 ppmin comparison to the signals found in the correspondingimidazolium salts 2a−2d″.

Structural Studies. In each case (3a−3d″) crystals suitablefor single-crystal X-ray diffraction were obtained by slow gas-phase diffusion of diethyl ether into a solution of the respectivesalt in DCM. Although the anions are weakly coordinating andsimilar in size, they have a considerable influence on the crystalstructures of the compounds containing the cation[(iPrNHC)2Ag]

+. Only 3d and 3d′ were found to beisostructural at room temperature and can be solved in theorthorhombic space group Fddd (Tables S1 − S3, SupportingInformation).21 Neglecting the disorder of the Ag position(vide infra), the [(iPrNHC)2Ag]

+ cations are arranged intocolumns parallel to the a axis with a longer and a shorter Ag−Ag distance. The silver atom is not, as is usually found in“undistorted” [(NHC)2Ag]

+ fragments, (almost) linearlycoordinated, but the C−Ag−C angle deviates significantlyfrom 180° (e.g., 165.3(4)° at room temperature for 3d),whereas the convex sides of the complexes face each other(Figure 1b). This arrangement results in dimers of cations. Themolecules within these dimers have a crossed-sword con-formation with dihedral angles C−Ag−Ag−C of 46−48°. Sinceone isopropyl group of each NHC moiety lies directly above theNHC ring of the neighboring complex cation, they function asrigid spacers between complexes within the dimers. A furthertwisting of the [(NHC)2Ag]

+ fragments in dimers relative toeach other is blocked by [PF6]

− anions, which are positionedbetween the columns and are almost completely “wrapped” byisopropyl groups (Figure S2b, Supporting Information). Thesteric repulsion of isopropyl groups of neighboring cations

Scheme 1. Abbreviations Defining the NHC species (Left) and Synthesis of Complex Salts 3a−d″ and Definitions ofAbbreviations for the Reactants (Right)

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would also increase with increasing torsion angles. In contrastto the other NHC−Ag complexes 3a−c, where smaller N-alkylsubstituents enable a closer contact of the cations, thearrangement in 3d,d′ leaves the Ag atom in a position wherea closer approach of the complexes is locked by (i) theisopropyl spacer and (ii) the impossibility of further twistingthe fragments to avoid steric repulsion of the isopropyl groups.The Ag atoms in the dimers are thus positioned at a distancewhereby argentophilic interactions are expected to be weak butoperative. These special structural features seem to be theprecondition for the exceptional thermal responsiveness of theAg−Ag distances discussed below.For 3d, the structure refinement is somewhat complicated by

the typical tendency of a [PF6]− anion to disorder in the lattice.

Furthermore, the large thermal displacement parameters of thesilver atom suggest a disorder of the whole linear[(iPrNHC)2Ag]

+ unit and at least a model with two splitpositions for the silver atom could be properly refined (Figure1a). The site occupancy for the predominant position Ag1aincreases with decreasing temperature from 73(2)% (293(1) K)to 88(2)% (200(1) K). In this temperature range, theintermonomer distances between these Ag positions decreasealmost linearly by ∼5% from 3.498(4) to 3.325(10) Å. Thedistances between the two split positions Ag1a and Ag1b at 293and 200 K are 0.441(4) and 0.433(4) Å, respectively. A visualinspection of the diffraction pattern at different temperaturesreveals a phase transition at around 180 K (Figure S5,Supporting Information). Indeed, at 100(1) K the structure issolved in C2221 (a subgroup of Fddd) with a doubled c axis. Inthe asymmetric unit 3 + 2 × 3/2 of a formula unit is present,leading to two different and short Ag−Ag distances of 3.180(2)and 3.188(2) Å (Figure S3, Supporting Information), whichcorresponds to a mean shrinkage of more than 10% compared

to structure at room temperature. In general, the crystallo-graphic unit cell dimensions are only affected to a minor extentby the temperature change in 3d,d′.For the perchlorate salt 3d′, a phase transition from the space

group Fddd (293 K) to Fdd2 (200 K) is observed at around 250K, which is a considerably higher transition temperature thanthat observed for 3d. Again, in the high-temperaturemodification a disorder of the silver position (site Ag1a andsite Ag1b) was detected with a site occupancy of Ag1a of about91(2)%. The shrinkage of the Ag−Ag distances on going fromthe high- to the low-temperature form in 3d′ (3.345(1) Å(Ag1a)/3.845(1) Å (site Ag1b) at 293(1) K and 3.245(1) Å at200(1) K) is not as pronounced as in 3d, but still significant. Itcan be expected that this distance will further increase byelevating the temperature above room temperature. A plot ofthe Ag−Ag distances in dependence of the crystal temperature,as was found in the XRD experiment of the relevant complexsalts 3a−d′, is given in Figure 2. It is interesting to note that an

extrapolation of the Ag−Ag distances of the structurally closelyrelated 3d,d′ leads to an intersection of the lines at around 0 Kand at an Ag−Ag distance of 2.95 Å.The [SbF6]

− salt 3d″ is not isostructural with 3d or 3d′. Thecomplex 3d″ could only be crystallized as the diethyl ethersolvate, and no solvent-free crystals could be grown in othersolvent combinations. The shortest Ag−Ag separation is morethan 7.4 Å, well beyond the limit of argentophilic interactions(Figure S4, Supporting Information), and so this complex willnot be discussed further here.The solid-state structures of 3a−c also show different forms

of aggregation via argentophilic interactions in comparison to3d,d′. For the sake of completeness, their solid-state structureswill also be described briefly in the following. The methylderivative 3a crystallizes in the monoclinic space group C2c.The cations are aggregated to infinite columns parallel to the caxis with rather long and equidistant Ag−Ag separations of3.554(1) Å at 293(1) K and 3.515(1) Å at 205(1) K (Figure3a), which are each exactly the half of the lattice parameter c atthese temperatures. Therefore, the c axis is affected by thetemperature change to the same extent as the Ag−Ag distances.3b crystallizes in a not-uncommon tetranuclear aggregate,22

with the dimeric dianion [Ag2Br4]2− bridging two adjacent

[(EtMeNHC)2Ag]+ cations by a short Ag1−Ag2 contact of

Figure 1. (a) Cation in crystals of 3d (ORTEP; displacementellipsoids at the 50% probability level). The site occupancy for thepredominant silver positions Ag1a increases from 73(2)% at roomtemperature to 88(2)% at 200 K. (b) View of the cationic columnillustrating the alternating convex/concave orientation of the complexcations at 200(1) K (iPr groups are omitted for clarity). Selected bondlengths (Å) and angles (deg): at 293 K, Ag1−Ag1i = 3.498(4), C1−Ag1 = 2.098(4), C1−Ag1−C1i = 165.3(4); at 200 K, Ag1−Ag1i =3.325(10), C1−Ag1 = 2.090(4), C1−Ag1−C1i = 168.9(12). Selectedbond lengths (Å) and angles (deg) for the isostructural perchloratesalt: at 293 K, Ag1−Ag1i = 3.345(14), C1−Ag1 = 2.093(5), C1−Ag1−C1i = 167.6(5); at 200 K, Ag1−Ag1i = 3.2448(4), C1−Ag1 =2.091(3), C1−Ag1−C1i = 168.18(15).

Figure 2. Temperature dependence of the Ag−Ag distance in thereported [(NHC)2Ag]Y type complex salts. The red line is the linearfit of the Ag−Ag distances in 3d. For 3b, both the longer and shorterdistances are shown.

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3.124(2) Å at 293(1) K and 3.091(1) Å at 205(1) K (Figure 4).The distance between Ag2 and Ag2i is significantly above what

would be expected for argentophilically interacting Ag(I)atoms. Nevertheless, it increases slightly from 3.649(2) Å atroom temperature to 3.689(1) Å at 205(1) K, resulting in ashortening of the shorter Ag−Ag distance and a lengthening ofthe longer length upon cooling in a push−pull manner. In 3bthe C−Ag−C units in the cations deviate significantly fromlinearity (C1−Ag1−C7 = 167.6(3)°) as a consequence of thedifferent aggregation mode described previously.In the ethyl derivative 3c, the cations again form pairs with

very long Ag−Ag distances of 3.637(1) Å (293(1) K) and3.575(1) Å (205(1) K) in the solid state. Judging from thelength of these distances, argentophilicity cannot play asignificant role. In 3a−c the Ag−Ag distances are altered bynot more than a maximum of 1.9% in the temperature rangebetween 200 and 293 K.

The temperature behavior of the isopropyl derivatives isparticularly remarkable. When an “expansion coefficient” αM···Mis calculated for the metallophilic interactions, an analogue ofthe linear thermal expansion coefficient αd = δ(ln d)/δT, with dbeing a characteristic length (here the Ag−Ag distance), thevalue for 3d accounts for 545 × 10−6 K−1 in the temperaturerange 100−293 K. This is several times the size of theexpansion of 144 × 10−6 K−1 observed in Ag3[Co(CN)6], avalue previously reported as a “colossal” expansion.8,9 For theother complex salts, the αM···M values are not as extreme but stilllarge: 157 × 10−6 (3d′), 125 × 10−6 (3a), 195 × 10−6 (3c), and120/−124 × 10−6 K−1 (3b).

Luminescence Behavior. The temperature dependence ofthe Ag−Ag distances is also reflected in the emission spectra ofthe compounds. The [ClO4]

− salt 3d′ exemplifies the emissionbehavior of the compounds (Figure 5): the salt features a broademission peak at 484 nm under ambient conditions and at 358

Figure 3. (a) Excerpt from the cell plot depicting the linear chain of the cations in crystals of 3a along the c axis. The lattice parameter c is exactlydouble the Ag−Ag distance. Selected bond lengths (Å) and angles (deg): at 293 K, Ag−Agii = 3.554(3), Ag1−C1 = 2.073(6), C1i−Ag1−C1 =180.0(2); at 205 K, Ag−Agii = 3.515(2), Ag1−C1 = 2.085(5), C1i−Ag1−C1 = 179.999(1). (b) Parallel arrangement of pairs of cations in the solid-state structure of 3c. Selected bond lengths (Å) and angles (deg): at 293 K, Ag−Agi = 3.637(2), Ag1−C1 = 2.083(4), Ag1−C8 = 2.082(4), C8−Ag1−C1 = 176.7(1); at 205 K, Ag−Agi = 3.575(2), Ag1−C1 = 2.092(4), Ag1−C8 = 2.080(4), C8−Ag1−C1 = 176.6(2).

Figure 4. Molecular structure of 3b depicting the tetranuclearaggregate [(EtMeNHC)2Ag]2[AgBr2]2. Selected bond lengths (Å) andangles (deg) at 205 K: Ag1−C1 = 2.116(8), Ag1−C7 = 2.093(8),Ag1−Ag2 = 3.091(1), Ag2−Ag2i = 3.689(1), Ag2−Br1 = 2.5926(15),Ag2−Br2 = 2.5086(13), Ag2−Br1i = 2.7078(15), Ag2i−Br1 =2.7078(15), C1−Ag1−C7 = 167.6(3). For more structural parametersat 205 and 293 K, see the Supporting Information.

Figure 5. Excitation (left) and emission spectra (right) of a solidsample of 3d′ at 77 K (blue; emission, λexc 300 nm; excitation, λdet 360nm) and 293 K (red; emission, λexc 310 nm; excitation, λdet 475 nm).The inset depicts the spectral changes upon thawing of the samplefrom 77 K (blue) to 293 K (red; λexc 300 nm).

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nm at 77 K, an energy difference of 7270 cm−1. Warming fromliquid nitrogen to room temperature recovers the low-energy(LE) emission with a progressively decreasing intensity of thehigh-energy (HE) band and an “isosbestic” point at 420 nm.Both emission bands exhibit essentially identical excitationprofiles. We have recently reported the metallophilicity-basedemissions of NHC−M complexes (M = Ag, Au) featuring M−M distances at the edge of or even above the metallophilicdistances of the sum of the vdW radii23 and explained theirluminescence with an excimer model as suggested by Omary etal.24 Similarly we ascribe here the luminescence of 3d′ to anexcited state based on metallophilic interactions. As expectedfor a gradual thermal contraction of the Ag−Ag distances, themaximum of the LE emission band is gradually shifted uponcooling (from 484 to 466 nm). The assignment to ametallophilicity-based luminescence is further supported bythe shift of the band in the excitation spectrum in the solid statein comparison to the absorbance spectrum in solution. Nosignificant absorbance above 260 nm in dilute solutions isobserved for the [ClO4]

− salt (Figure S9, SupportingInformation), but due to the presence of the dimers in theneat compound, the excitation band extends up to ∼320 nm.Although strong hypsochromic shifts have been observed for avariety of Au and Ag complexes, different explanations wereassigned (e.g., excimers of varying nuclearity,24 spin−orbitstates of the same triplet state,14 etc.) and this does not accountfor the [ClO4]

− (and [PF6]−) salt. Due to the structure of the

HE emission band, a participation of a π → π* transition of theimidazol-2-ylidene moiety might be possible. Indeed, for dilutesolutions of 3d, we found a structured emission band withmaxima at 374 and 393 nm and an extreme broadening athigher concentrations, due to aggregation by Ag−Aginteraction and/or π−π stacking (Figure S8, SupportingInformation). Nevertheless, we could not detect an analogousemission in solutions of 3d′. If π → π* transitions of the NHCligand are the origin of a strong, structured emission band inthe solid state, they should be detectable also for similarcomplexes. We could not observe such emissions. The latterargument also makes the participation of an MLCT excitedstate unlikely. Furthermore, the emission behavior of theimidazolium salt is very different, supporting the interpretationthat a π → π* transition of the imidazol-2-ylidene moiety is nota sufficient explanation for the structure of the emission band(Figures S6−S12, Supporting Information). We therefore inferthat the phase transition from Fddd to Fdd2, as a result of thestrong alterations of the Ag−Ag distances, might be the reasonfor the strong hypsochromic shift of the band leading to the HEluminescence. A neat sample of 3d behaves in very similarfashion (Figure S7, Supporting Information).Of all the compounds engaged in argentophilic bonding in

this series, 3c is the only one that is exclusively emissive in thesolid state at 77 K, but not at room temperature, where a verybroad emission band around 390 nm is observed (Figure S10,Supporting Information). In contrast to the usually very broadbands of argentophilicity-based emissions, compounds 3a,bfeature relatively sharp peaks at 346 nm (3a) and 354 nm (3b)at 77 K, which are similar to the luminescence of imidazoliumsalts25 (e.g., 2d, λmax 346 nm at 77 K; Figure S6, SupportingInformation). Therefore, we tentatively assign these emissionsto an excited state located on the imidazol-2-ylidene moiety.For 3a,b low-energy emission peaks evolve at room temper-ature, again a typical feature for emissions originating from

metallophilic interactions (Figure 6 and Figure S12 (Support-ing Information)).

These results illustrate the delicate balance of differentemissive electronic states in silver complexes containing[(NHC)2Ag]

+ cations involved in (weak) argentophilicbonding. Further detailed studies are underway to develop acomplete picture of the photophysical properties and under-stand the multifaceted luminescence behavior of this class ofcomplexes.

Computational Section. In the single-crystal XRD experi-ment we found a strong temperature dependence of the Ag−Agdistances. As was pointed out earlier, for similar cases of gold(I)and silver(I) compounds, a large thermal variation of crystalstructure parameters can be caused by both a very shallow andan anharmonic interatomic potential.8,11 To help understandthe degree of shallowness and anharmonicity of theintermetallic potential function in the investigated compounds,we undertook a series of ab initio calculations suited todescribing closed-shell Ag(I)−Ag(I) interactions in a quantita-tive manner. Appropriate molecular model systems for thesubstructures of the solid state had to be chosen. A principalproblem in this respect is the cationic nature of the monomer[(HNHC)2Ag]

+, leading to a superimposed repulsive Coulombpotential in the dimer, which is compensated in the solid stateby the anionic counterparts [PF6]

−, [ClO4]−, etc. For this

reason we have investigated the three model systems[(HNHC)2Ag]2

2+ (42+), {[(HNHC)AgH][(HNHC)2Ag]+} (5+),

and [(HNHC)AgH]2 (6), which vary in total charge andchemical composition (see Figure 7).The monomer geometries were (partially) optimized, and

the monomer structures were fixed for potential scans of thecomplexes. In the [(HNHC)2Ag]

+ monomer the torsion anglebetween the two HNHC ligands was fixed to 0°, as is found inthe solid state, resulting in D2h symmetry (full optimizationresults in a D2 “kayak paddle” structure). In the calculations ofthe intermolecular interaction potential functions, the mono-mer planes were set parallel, the C−Ag−Ag′−C′ dihedralangles were fixed to 45° as is found in the solid state, and the

Figure 6. Excitation (left) and emission (right) spectra of a solidsample of 3a at 77 K (blue; emission, λexc 300 nm; excitation, λdet 380nm) and 293 K (red; emission, λexc 300 nm; excitation, λdet 350 nm).The inset shows the spectral changes upon warming of a neat sample3a from 77 K (blue) to 293 K (red; λexc 300 nm).

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C−Ag−Ag′ angles were fixed to 90°. The internal parameter r(monomer−monomer distance) was scanned from 2.2 to 4.2 Åin steps of 0.05 Å. All other parameters were fixed at theirequilibrium structure values in the monomers.The model system 42+ resembles the chemical composition

of the systems investigated herein most closely; this is, however,doubly charged and thus suffers from uncompensated Coulombcharges. The model system 6 is electroneutral and will give apotential which is not biased by Coulomb repulsion but whichis also more dissimilar, in a chemical sense, to the synthesizedcompounds. The model system 5+ offers a compromisebetween 42+ and 6.Some issues have to be taken into account in order to

prepare a well-balanced quantitative quantum chemicaldescription of these systems. Dynamic correlation is essentialfor metallophilic interactions.26 For reliable potential functionsthis must be neither under- nor overestimated, for that and forreasons of computational costs SCS-MP2 is the method ofchoice.27 For silver, relativity can be important. To test this, weused all-electron basis sets in connection with explicitlyrelativistic calculations in the “zero order regular approxima-tion”.28 The basis sets have to be balanced, must be sufficientlyflexible, and contain enough diffuse functions.26 For thesereasons we chose the SARC-TZVPP basis set from Buhl et al.on silver29 in connection with Ahlrich’s def2-TZVPP basis setsfor the light elements.30 Finally, in general for perturbation-theoretical treatment of intermolecular potentials below thebasis set limit, the so-called basis-set superposition errors(BSSE) have to be compensated for.31 All energy values were,therefore, corrected using a simple counterpoise (CP)correction scheme.32 We denote the described level of theorywith the shorthand ZORA/SCS-MP2/TZVPP/CP. All calcu-lations have been performed with the ORCA program, version2.8.33

Interestingly, at this level of computation the only potentialwhich appears to be bound (having a local minimum) is that ofmodel system 5+. Omitting the CP correction (ZORA/SCS-MP2/TZVPP) results in bound potentials for all three systems42+, 5+, and 6. Using MP2 instead of SCS-MP2 in connectionwith the CP scheme (ZORA/MP2/TZVPP/CP), on the otherhand, results again in bound potentials for all three cases. Lastbut not least, at the nonrelativistic level of theory (SCS-MP2/

TZVPP/CP) model system 5+ is also unbound. We note that,in this case of argentophilic interactions, the details in theemployed computational method have to be validated withgreat care and that relativity plays a small but decisive role.The resulting potential energy values were fitted to the

Morse potential

= − − −M r D a r D e( ; , , ) (1 )a r ree

( ) 2(2)

with well depth D, anharmonicity parameter a, and minimum reusing a least-squares refinement procedure. The calculatedrelative potential energy values and the fitted Morse potentialfunctions for the three model systems are given in Figure 8 (the

respective fitted parameters D, a, and re are given in the legend,and the standard deviations resulting from the least-squaresfitting procedure are in all cases smaller than 0.01%; since theerrors are intrinsically systematic, these values do not reflect thequality of the fit and hence the standard deviations are notreported here).As already mentioned, the monocationic dimer 5+ is the only

one of the three model systems which appears to be bound atthe ZORA/SCS-MP2/TZVPP/CP level, with a well depth D of3.5 kcal/mol, a potential minimum re of 3.29 Å, and ananharmonicity constant a of 1.58 Å−2. To test the influence ofthe computing scheme and the used models on theanharmonicity parameter, a was determined also for thepotential energy curves which were not corrected for BSSE(ZORA/SCS-MP2/TZVPP) for 42+, 5+, and 6, yielding avalues of 1.67, 1.52, and 1.65 Å−2. Thus, we note that, on theone hand, the CP correction influences at least theanharmonicity constant a only insignificantly (1.58 vs 1.52Å−2) and, on the other hand, the variation of a between themodel systems is also only small. For comparison, a is used forinstance in gas-phase electron diffraction for structure refine-ment purposes and according to a rule of thumb for covalentlybound atoms a value of 2.0 Å−2 is used and 0.0 Å−2 fornonbonded atoms.34 For many diatomic molecules ratheraccurate values have been determined spectroscopically (e.g.,2.422 Å−2 for HgH and 2.475 Å−2 for I2).

35

Using the anharmonicity constant the effect of thermalvibrations onto the difference of the mean (expectation) value⟨r⟩ and equilibrium distance re of the displacement can be

Figure 7. Structures used for the calculations of the intermolecularpotential functions of the model systems [(HNHC)2Ag]2

2+ (42+),{[(HNHC)AgH][(HNHC)2Ag]

+} (5+), and [(HNHC)AgH]2 (6). Themonomer planes were set parallel, the C−Ag−Ag′−C′ dihedral angleswere fixed to 45°, and the C−Ag−Ag angles were fixed to 90°. Theinternal parameter r was scanned from 2.2 to 4.2 Å; all otherparameters were fixed at their monomer equilibrium structure values.

Figure 8. Calculated relative potential energy values and the fittedMorse potential function M(r;D,a,re) for model system 5+. re is thepotential minimum, and ⟨r⟩ is the expectation value of the interatomicdistance at room temperature estimated from eq 3 and assuming l2 =Uaa (see text). In the legend and throughout the text r is given in unitsof Å, D in units of kcal/mol, and a in units of Å−2.

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estimated. For the ground state vibration in the quantummechanical Morse potential oscillator it is, to a first-orderapproximation

− =r r al3 /2e2

(3)

with the mean square amplitude of vibration l2.36 Since ourobserved mean interatomic distances are linearly dependent onthe temperature, we also expect a linear dependence betweenthe temperature and the observed mean square amplitudes inthe relative thermal displacement of the involved Ag atoms withrespect to each other. This is in agreement with the observedtemperature dependence of the refined anisotropic atomicdisplacement parameters Uij (see the Supporting Information)of the Ag atoms. To obtain a rough estimation for the “thermalelongation” ⟨r⟩ − re of the Ag−Ag distance caused byanharmonicity of the potential in the model system 5+, weassume that l2 = Uaa (the Ag atoms are stacked along thecrystallographic a axis). With these values we obtain thermalelongations of 0.08−0.17 Å in the temperature range 100−293K for the [PF6]

− salt. This is a large effect; nevertheless, it onlycan explain a part of the effect we observe experimentally in the[PF6]

− salt (elongation of 0.32 Å when going from 100 to 293K). Other effects, such as the occupational disorder of theanions, might contribute synergistically to this phenomenon.

■ CONCLUSIONSAn extraordinary temperature dependence of the Ag−Agdistances in bis(1,3-diisopropylimidazol-2-ylidene)silver(I)salts with [PF6]

− and [ClO4]− anions has been discovered.

Other related NHC−Ag complexes also feature still significant,if somewhat less pronounced, thermal responses in their Ag−Ag separations. Ab initio calculations were shown to underlinethe observation that a large fraction of the large thermalexpansion in the Ag−Ag distances can be attributed to theanharmonic character of the intermetallic interaction potential.However, the computationally predicted elongation accountsfor only about one-third of what is experimentally observed inthe [PF6]

− salt. It is possible that such extreme temperaturedependences are very common, even in well-known silver andgold compounds. On the basis of these results we wouldstrongly encourage temperature-dependent structural inves-tigations of oligonuclear transition-metal compounds of formald10 configuration, particularly if metallophilic interactionsappear to be operative.

■ EXPERIMENTAL SECTIONGeneral Considerations. All solvents and reagents were

commercially available and were used as received. Elemental analyseswere carried out by the Institut fur Technologie Organischer Stoffe atthe Universitat Linz. NMR spectra were recorded on a Bruker DigitalAvance DPX 200 (200 MHz), Avance DRX 300 (300 MHz), orAvance DRX 500 (500 MHz) spectrometer, and 1H and 13C shifts arereported in ppm relative to Si(CH3)4 and were referenced internallywith respect to the residual signal of the deuterated solvent. Massspectra were taken on a Finnigan LCQ DecaXPplus ion trap massspectrometer with ESI ion source. Single crystals suitable for X-raydetermination were obtained by gas-phase diffusion of diethyl etherinto a solution of the silver complex in DCM. Single-crystal structureanalyses were carried out on a Bruker Smart X2S diffractometeroperating with Mo Kα radiation (λ = 0.71073 Å). The 100 Kmeasurement of 3d was carried out on a Bruker AXS X8 ProspectorUltra with APEX II diffractometer operating with Cu Kα radiation (λ= 1.54178 Å). The structures were solved by direct methods(SHELXS-97)37 and refined by full-matrix least squares on F2

(SHELXL-97).38 The H atoms were calculated geometrically, and ariding model was applied during the refinement process. The [PF6]

−

anions in 3d were found to be disordered but could not be resolvedsatisfactorily in the refinement. Therefore, the P−F distances of thedisordered [PF6]

− anions (in all data sets from 200 to 293 K) wererestrained to be the same, as were the F−F distances. The occupanciesof both positions of the silver atoms of 3d,d′ (200−293 K) wererefined, and the anisotropic displacement parameters of both silveratoms were set to be equal. The 200 K structure of 3d′ was solved as aracemic twin. In 3d″ the C104−C105 distance (carbon atoms of oneethyl group of the diethyl ether molecule) was restrained to 1.51 Å.Further crystallographic details can be found in Tables S1−S3 in theSupporting Information. CCDC 901702−901717 and 903638 containsupplementary crystallographic data for all NHC−Ag complexes atdifferent temperatures. These data can be obtained free of charge fromthe Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. For electronic absorption and emission spectroscopyspectroscopic grade solvents were used throughout all measurementsin solution. Absorption spectra were recorded with a Varian Cary 300double-beam spectrometer. Emission spectra at 300 and at 77 K weremeasured with a steady-state fluorescence spectrometer (Jobin YvonFluorolog 3).

Procedure for the Synthesis of the Imidazolium Salts.EtMeNHC·HBr (2b) was prepared by following a literature method,with the variation that no dry solvents and no inert conditions wereused.39 The imidazolium chloride iPrNHC·HCl was prepared accordingto a literature method from formaldehyde, isopropylamine, and glyoxalin concentrated HCl.40 MeNHC·HCl and EtNHC·HCl (2c) wereprepared analogously to a published procedure from sodiumimidazolide and methyl iodide and ethyl bromide, respectively.20

2a,c were precipitated by mixing of aqueous solutions of thecorresponding imidazolium halides and KPF6 as colorless solids.41

The analytical data of 2a,b are in agreement with previously publisheddata.42

EtNHC·HPF6 (2c). Complementary NMR data: 1H NMR (DMSO,room temperature) δ 9.12 (s, 1 H, C2H), 7.74 (s, 2 H, C4/5H), 4.18(quart, 4 H, J = 7.3 Hz, CH2), 1.43 (t, 6 H, J = 7.3 Hz, CH3);

13CNMR (DMSO, room temperature) δ 135.7 (C2), 122.4 (C3/4), 44.6(CH2), 12.3 (CH3).

iPrNHC·HPF6 (2d) . This compound was obtained analogously to2c.41 Complementary NMR data: 1H NMR (DMSO, room temper-ature) δ 9.24 (s, 1 H, C2H), 7.91 (s, 2 H, C4/5H), 4.60 (sept, JHH =6.6 Hz, CHMe2, 2 H), 1.48 (d, JHH = 6.6 Hz, 24 H, CH3);

13C NMR(DMSO, room temperature) δ 133.5 (C2), 120.6 (C4/5), 52.2 (CH),22.3 (CH3).

iPrNHC·HClO4 (2d′). The imidazolium perchlorate was preparedanalogously to the PF6 salt from 2d by metathesis with LiClO4 inwater.41 Complementary NMR data: 1H NMR (DMSO, roomtemperature) δ 9.29 (t, 1 H, JHH = 1.6 Hz, C2H), 7.98 (d, 2 H, JHH= 1.6 Hz, C4/5H), 4.67 (sept, JHH = 6.7 Hz, CHMe2, 2 H), 1.54 (d,JHH = 6.7 Hz, 24 H, CH3);

13C NMR (DMSO, room temperature) δ133.5 (C2), 120.6 (C4/5), 52.2 (CH), 22.3 (CH3). MS (ESI, CHCl3/CH3OH): m/z 156.13 [C9H17N2]

+. MS (ESI neg, CHCl3/CH3OH):m/z 99.07/101.01 [ClO4]

−. Anal. Calcd for C9H17N2ClO4; (252.70 g/mol): C, 42.78; H, 6.78; N, 11.03. Found: C, 42.97; H, 7.07; N, 11.10.

iPrNHC·HSbF6 (2d″). The SbF6 salt is obtained from 2d bymetathesis with NaSbF6 in water.41 Complementary NMR data: 1HNMR (DMSO, room temperature) δ 9.23 (t, JHH = 1.7 Hz, 1 H,C2H), 7.91 (d, JHH = 1.7 Hz, 2 H, C4/5H,), 4.60 (sept, JHH = 6.7 Hz,CHMe2, 2 H), 1.48 (d, JHH = 6.7 Hz, 24 H, CH3);

13C NMR (DMSO,room temperature) δ 133.4 (C2), 120.6 (C4/5), 52.2 (CH), 22.3(CH3). Anal. Calcd for C9H17N2SbF6 (388.99 g/mol): C, 28.79; H,4.41; N, 7.20. Found: C, 28.70; H, 4.67; N, 7.43.

Procedures for the Synthesis of the NHC−Silver Complexes.[(iPrNHC)2Ag]PF6 (3d). To finely powdered AgCl (0.30 g, 2.1 mmol)and KOH (0.30 g, 5.3 mmol) was added a solution of 2d (0.50 g, 1.67mmol) in 50 mL of DCM. After 3 h of stirring the solution was filteredand concentrated and the product was precipitated with diethyl ether.The off-white solid was filtered and dried under vacuum. Yield: 0.77 g,82% (based on the imidazolium salt). 1H NMR (DMSO, room

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dx.doi.org/10.1021/om300932r | Organometallics XXXX, XXX, XXX−XXXG

temperature): δ 7.62 (s, 4 H, C4/5H), 4.64 (sept, JHH = 6.7 Hz,CHMe2, 4 H), 1.46 (d, JHH = 6.7 Hz, 24 H, CH3).

13C NMR (DMSO,room temperature): δ 180.2 (C2), 118.9 (C4/5), 53.2 (CH), 23.2(CH3). MS (ESI, MeCN/H2O/tfa): m/z (%) 411.2 [M]+ (39), 299.9[(iPrNHC)Ag·MeCN]+ (40), 259.0 [(iPrNHC)Ag]+ (23), 153.1[iPrNHCH]+ (100). Anal. Calcd for C18H32N4AgPF6 (557.31 g/mol):C, 38.59; H, 5.59; N, 8.45. Found: C, 38.41; H, 5.56; N, 8.51.[(iPrNHC)2Ag]ClO4 (3d′). To finely powdered AgCl (0.30 g, 2.1

mmol) and KOH (0.30 g, 5.3 mmol) was added a solution of 2d′(0.51 g, 2.0 mmol) in 50 mL of DCM. After 3 h of stirring the solutionwas filtered and concentrated and the product was precipitated withdiethyl ether. The off-white solid was filtered and dried under vacuum.Yield: 0.80 g, 79% (based on the imidazolium salt). 1H NMR (DMSO,room temperature): δ 7.60 (s, 4 H, C4/5H), 4.62 (sept, JHH = 6.7 Hz,CHMe2, 4 H), 1.43 (d, JHH = 6.7 Hz, 24 H, CH3);

13C NMR (DMSO,room temperature): δ 175.2 (C2), 118.6 (C4/5), 53.6 (CH), 23.4(CH3). MS (ESI, CHCl3/CH3OH): m/z 413.2 [(

iPrNHC)2Ag]+, 259.1

[(iPrNHC)Ag]+, 153.1 [iPrNHCH]+. MS (ESI neg, CHCl3/CH3OH):m/z 99.07/101.01 [ClO4]

−. Anal. Calcd for C18H32N4AgClO4 (511.80g/mol): C, 42.24; H, 6.30; N, 10.95. Found: C, 42.21; H, 6.37; N,10.76.[(iPrNHC)2Ag]SbF6 (3d″). To finely powdered AgCl (0.30 g, 2.1

mmol) and KOH (0.30 g, 5.3 mmol) was added a solution of 2d″(0.78 g, 2.00 mmol) in 50 mL of DCM. After 3 h of stirring thesolution was filtered and concentrated and the product wasprecipitated with diethyl ether. The off-white solid was filtered anddried under vacuum. Yield: 1.06 g, 82% (based on the imidazoliumsalt). 1H NMR (DMSO, room temperature): δ 7.61 (s, 4 H, C4/5H),4.66 (sept, JHH = 6.7 Hz, CHMe2, 4 H), 1.46 (d, JHH = 6.7 Hz, 24 H,CH3);

13C NMR (DMSO, room temperature): δ 176.2 (C2), 118.6(C4/5), 53.6 (CH), 23.4 (CH3). MS (ESI, CHCl3/CH3OH): m/z413.2/411.3 [(iPrNHC)2Ag]

+, 259.1/261 [(iPrNHC)Ag]+, 153.1[iPrNHCH]+. Anal. Calcd for C18H32N4AgSbF6 (648.09 g/mol): C,33.36; H, 4.98; N, 8.64. Found: C, 33.22; H, 4.56; N, 8.54.[(MeNHC)2Ag]PF6 (3a). This compound was prepared analogously to

3d from AgCl (0.30 g, 2.1 mmol), KOH (0.30 g, 5.3 mmol), and 2a(0.40 g, 1.65 mmol) in DCM. Yield: 0.57 g (78% based on theimidazolium salt). 1H NMR (d6-DMSO, room temperature): δ 7.51(s,4 H, C4/5H), 3.90 (s, 12 H, CH3).

13C NMR (d6-DMSO, roomtemperature): δ 181.0 (C2), 123.6 (C4/5), 38.5 (CH3). MS (ESI,CHCl3/CH3OH): m/z 299.2/301.13 [(MeNHC)2Ag]

+, 97.13[MeNHCH]+. MS (ESI neg, CHCl3/CH3OH): m/z 145.13 [PF6]

−.Anal. Calcd for C10H16N4AgPF6 (445.10 g/mol): C, 26.99; H, 3.62; N,12.59. Found: C, 27.12; H, 3.71; N, 12.63.[(EtNHC)2Ag]PF6 (3c). This compound was prepared analogously to

3d from AgCl (0.30 g, 2.1 mmol), KOH (0.30 g, 5.3 mmol), and 2c(0.40 g, 1.66 mmol) in DCM. Yield: 0.61 g (73% based on theimidazolium salt). 1H NMR (d6-DMSO, room temperature): δ 7.57 (s,C4/5H, 4 H), 4.25 (q, JHH = 7.3 Hz, CH2, 8 H), 1.74 (t, JHH = 7.3 Hz,CH3, 12H).

13C NMR (d6-DMSO, room temperature): δ 178.2 (C2),121.4 (C4/5), 46.1 (CH2), 17.0 (CH3). MS (ESI pos, CHCl3/CH3OH): m/z 355.27/357.27 [(MeNHC)2Ag]

+, 231.1/233.1[(MeNHC)Ag]+. MS (ESI neg, CHCl3/CH3OH): m/z 145.13[PF6]

−. Anal. Calcd for C10H18N4AgPF6 (501.21 g/mol): C, 33.55;H, 4.83; N, 11.18. Found: C, 33.63; H, 4.86;, N, 11.20.[(EtMeNHC)2Ag][AgBr2] (3b). 2b (1.00 g, 5.20 mmol) and Ag2O

(0.60 g, 2.60 mmol) were dissolved in 50 mL of DCM and stirredovernight. After filtration of the reaction mixture the product wasprecipitated in n-pentane as an off-white solid. Yield: 1.33 g, 86% (withrespect to the imidazolium salt). 1H NMR (DMSO, room temper-ature): δ 7.49 (d, JHH = 1.7 Hz, 1 H, C2/3H), 7.41 (d, JHH = 1.7 Hz, 1H, C4/5H), 4.12 (q, JHH = 7.2 Hz, 3 H, CH2CH3), 3.78 (s, 1H, CH3),1.32 (t, JHH = 7.3 Hz, 3 H, CH2CH3).

13C NMR (DMSO, roomtemperature): δ 178.7 (C2), 122.9, 121.2 (C4/5), 54.9 (CH2), 38.0(CH3), 16.9 (CH2CH3). MS (ESI, MeCN/H2O): m/z (%) 327.1 [M]+

(100), 257.9 [EtMeNHCAg·MeCN]+ (42), 219.1 [EtMeNHCAg]+ (9),111.0 [EtMeNHCH]+ (7). Anal. Calcd for C6H10N2AgBr (297.93 g/mol): C, 24.19; H, 3.38; N, 9.40. Found: C, 24.10; H, 3.35; N, 9.32.

■ ASSOCIATED CONTENT*S Supporting InformationFigures, tables, and CIF files giving crystallographic data for allcomplexes at different temperatures and electronic absorption,emission, and excitation spectra. This material is available freeof charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*R.J.F.B.: tel, +43 662 8044 5466; e-mail, raphael.berger@sbg.ac.at U.M.: fax, +43 732 2468 9681; tel, +43 732 2468 8814; e-mail, uwe.monkowius@jku.at.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe thank Professor Gunther Knor for his generous support ofthe experimental work. The preparative support of ElisabethSteingruber during the advanced laboratory course is gratefullyacknowledged. We acknowledge Ian Teasdale for valuablesuggestions. The NMR spectrometers were acquired incollaboration with the University of South Bohemia (CZ)with financial support from the European Union through theEFRE INTERREG IV ETC-AT-CZ program (project M00146,“RERI-uasb”).

■ ABBREVIATIONSBSSE, basis-set superposition error; CP, counterpoise; DCM,dichloromethane; MLCT, metal-to-ligand charge transfer;NHC, N-heterocyclic carbene; SCS-MP2, spin-componentscaled Møller−Plesset perturbation theory of second order;tfa, trifluoroacetic acid; vdW, van der Waals; XRD, X-raydiffraction; ZORA, zero-order relativistic approximation

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