Gold(III) Bis(di-N-heterocyclic carbene) Square Planar Trication with

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Gold(III) Bis(di‑N‑heterocyclic carbene) Square Planar Trication with Axial Ligand Interactions with Bromides from Ag/Br Counteranion Assemblies Marco Baron,† Cristina Tubaro,*,† Maddalena L. C. Cairoli,† Laura Orian,† Sara Bogialli,† Marino Basato,† Marta M. Natile,‡ and Claudia Graiff§ †

Dipartimento di Scienze Chimiche and ‡ICMATE-CNR, Dipartimento di Scienze Chimiche, Università degli Studi di Padova, Via Marzolo 1, 35131 Padova, Italy § Dipartimento di Scienze Chimiche, della Vita e della Sostenibilità Ambientale, Università di Parma, Parco Area delle Scienze 17/A, 43124 Parma, Italy S Supporting Information *

ABSTRACT: The mononuclear tricationic bis(di-N-heterocyclic carbene) gold(III) complex 13+ of formula [Au(MeImCH2ImMe)2]3+ (Im = imidazol-2-ylidene) was successfully synthesized by transmetalation of the di(N-heterocyclic carbene) ligand from the corresponding silver(I) complex to KAuBr4. The counteranion of the gold(III) cationic complex depends on the synthetic procedure. The crude product, isolated by the transmetalation reaction, consists of infinite (Ag4Br73−)n ribbons. By adding a stoichiometric amount of AgPF6, the complete precipitation of AgBr is achieved, and the counteranions are PF6−. If substoichiometric amounts of silver salts are added, then the (Ag4Br73−)n ribbon breaks, and silverbromides anionic aggregates of lower nuclearity are obtained, for example, Ag2Br64−. The X-ray crystal structures of 1-Ag4Br7, 1-PF6, and 1-Br,(Ag2Br6)0.5 were determined. The complexes with any type of bromide present interactions between the Au center and two bromides of the counteranions so that the geometry around gold is pseudo-octahedral. The gold-bromide interaction has been investigated via DFT calculations and is mainly electrostatic.

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INTRODUCTION

In particular, for several years we have been interested in the coordination properties of di(N-heterocyclic carbene) ligands (diNHC) toward several transition metal centers.4,18 While the examples of diNHC coordinated to gold(I) centers are numerous, the number of gold(III) complexes still remains limited.4,19 In this frame, the initial effort of our research was to find a more straightforward synthesis for gold(III) complexes with diNHC, as an alternative to the classical and welldeveloped one based on halogen oxidative addition to the corresponding gold(I) complexes. One intrinsic limitation of the latest two-stage procedure is, inter alia, the exclusive formation of bridged complexes, chelation being almost impossible on the gold(I) precursor, especially with short linkers between the carbene fragments. We report here on the synthesis of the square planar cationic complex [Au(MeImCH2ImMe)2]3+ (Im = imidazol-2-ylidene) with different counteranions. It can be anticipated that one of these consists of infinite (Ag4Br73−)n ribbons, where the repeating Ag4Br73− unit formally results from the autoassembling of 3 AgBr2− anions and 1 AgBr molecule. The obtained

Gold is no longer mainly a seductive noble metal used in jewelry but has been revealed to have an extremely rich chemistry. For example, complexes are formed in which the gold center not only exhibits the usual oxidation numbers I and III but also shows that the II one is easily stabilized by an Au− Au bond in dinuclear structures.1,2 Novel and tunable behavior is observed for example in halogen oxidative additions,3−6 and carbon−carbon reductive eliminations,7 while unprecedented gold(I, III) hydride complexes can be obtained using Nheterocyclic carbenes or the doubly metalated pincer 2,6-bis(4′tert-butylphenyl)pyridine as ancillary ligands.8−10 Gold complexes are efficiently used in catalysis due to their selective electrophilic activation of coordinated double or triple bonds.11−14 Gold complexes as well as nanoclusters, due the low toxicity, have important applications in medicinal chemistry.15,16 Part of this tumultuous development of gold chemistry parallels the relatively recent employment of Nheterocyclic carbenes (NHC) as ancillary ligands for this metal. The gold(I, III) carbene complexes are particularly stable, and this characteristic can be exploited to further expand their chemistry.17 © XXXX American Chemical Society

Received: March 2, 2017

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DOI: 10.1021/acs.organomet.7b00163 Organometallics XXXX, XXX, XXX−XXX

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typical of a carbene carbon coordinated to gold(III).4,5,19,20 In the 1H NMR spectrum, information on the chelating mode of the diNHC ligand is gained from the AB system (two doublets at 6.76 and 7.15 ppm) relative to the diastereotopic protons of the methylene bridge. This indicates a restrained metallacyclic boat-shaped conformation of the complex, which has been already observed in other dicarbene metal complexes21,22 or in complexes with different bidentate ligands with bridging methylene group.23 The energy barrier for the fluxional behavior of the metallacycle, via an equilibration of the CH2 protons passing through a planar arrangement, is expected to be very high. The fluxionality of the complex is therefore observed neither at room temperature nor at high temperature. The 1H NMR spectra of the analogous trication in 1-PF6 remains unchanged even at 150 °C (see further in the text). High-resolution ESI-MS spectra in DMSO-MeCN mixtures show the higher signals at 274.0867 and 183.0604 m/z for [C18H23N8Au]2+ and [Au(MeImCH2ImMe)2]3+, respectively, and other signals relative to [Au(MeImCH2ImMe)2Br]2+ (base peak 314.0508 m/z), [Au(MeImCH2ImMe)2Br2]+ (base peak 709.0195 m/z), [Au(MeImCH2ImMe)2AgBr3]+ (base peak 896.8431 m/z), and [Au(MeImCH2ImMe)2Ag2Br4]+ (base peak 1084.6646 m/z) with mass error < 8 mDa (Figure S1). These data suggest that the counteranion should contain in organic solution both silver and bromine atoms, for example, AgBr2− or related polymeric species.24 Further information on the gold oxidation state and on the nature of the counteranion is given by an XPS analysis of the solid sample. The shape and peak positions of Au 4f7/2 and 4f5/2 (87.8 and 91.5 eV are characteristic of gold(III) centers (Figure 1).4b,d,25 Moving to Ag 3d core level, the observed binding energies (BEs) (368.2 and 374.2 eV, respectively, for Ag 3d5/2 and 3d3/2) are consistent with those reported in literature for silver(I).26 Concerning Br 3d, the BEs observed in 1-Ag4Br7 for Br 3d5/2 peak (68.5 eV) is significantly lower than the one observed for bromide ions coordinated to an gold(III) center (69.7 eV) (Figure 1).4b This shift of Br peaks toward lower BEs indicates that the bromide ions are bonded to a metal ion with lower oxidation state. Moreover, these results allow to exclude the coordination of bromide ions to gold(III). The XPS atomic composition shows a good confidence with the stoichiometry. The atomic ratios between Au/Ag/Br/N are, in fact, 1.0:4.2:6.8:8.0 (expected ones 1:4:7:8). The bis-dicarbene square planar structure of the complex has been determined by an X-ray analysis on the few crystals obtained after months by slow diffusion of a dichloromethane/ n-hexane mixture (1/5) into a DMSO solution of 1-Ag4Br7.

complex, analyzed by X-ray diffraction studies, presents an interaction between the gold center and the bromide atoms of the anion, thus providing a pseudo-octahedral coordination sphere on the gold atom. The energetics of the gold-bromide interaction is clarified also by means of density functional theory (DFT) scalar relativistic calculations.

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RESULTS AND DISCUSSION The synthetic procedure adopted to obtain gold(III) homoleptic diNHC complexes involves ligand transfer from the silver diNHC complex [Ag2(MeImCH2ImMe)2](PF6)2 to gold(III) precursor KAuBr4. This simple procedure has been employed with several metals, but only once for a gold(III) center: Jenkins et al. report in fact the only example of a Ag(I)− gold(III) transmetalation of a macrocyclic tetracarbene (tetraNHC) ligand, giving a mononuclear complex in limited yield, in which the chelation of the ligand is forced by its macrocyclic structure.20 Reaction of [Ag2(MeImCH2ImMe)2](PF6)2 with KAuBr4. The reaction between [Ag2(MeImCH2ImMe)2](PF6)2 and KAuBr4 has been performed in acetonitrile at room temperature using a 2:1, 1:1, or 1:2 [Ag2]:[Au] molar ratios; in all cases from the red solution precipitated a rather insoluble white-yellowish bis-diNHC gold(III) complex, which analyses identified as [Au(MeImCH2ImMe)2](Ag4Br7) (1-Ag4Br7) (Scheme 1). Scheme 1. Synthesis of [Au(MeImCH2ImMe)2](Ag4Br7) (1Ag4Br7) via Carbene Transfer Processa

a

Reagents and reaction conditions: KAuBr4, (2:1, 1:1 or 1:2 [Ag2]: [Au] molar ratios), acetonitrile, r.t., 5 h.

Considering the composition of the complex, a 1:1 ratio of the silver and gold reagents is a reasonable compromise, which reflects the content of gold and dicarbene ligand in the product. The isolated solid is sparingly soluble in DMSO-d6, and its NMR spectra show the presence of only one set of NMR signals attributable to a complex in which the gold metal center is symmetrically coordinated to two dicarbene ligands. Particularly diagnostic is the 13C resonance at 146.6 ppm,

Figure 1. Au 4f, Ag 3d, and Br 3d XPS peaks of 1-Ag4Br7 (black solid line) and [Au2Br4(MeImCH2ImMe)2](PF6)2 (black dotted line). Spectra are normalized with respect to their maximum value. B

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Organometallics Alternatively, the same crystals can be obtained by DMSO/ acetonitrile stratification. The charge of the gold trication is formally balanced by an Ag4Br73− anion, which is present in the crystal not as discrete unit, but as an extended polymeric framework. The tricationic square planar mononuclear gold(III) units with two chelating dicarbene ligands are present in two very similar forms, A (Figure 2) and B, which are crystallographically

Figure 3. Representation in ORTEP view of the polysilverbromide ribbons developing along the b axis, together with the atomic numbering scheme. Ellipsoids are drawn at their 50% probability level. Bond distances (Å) and angles (deg) for the asymmetric unit: Br1− Ag1 2.6570(8), Br1−Ag3 2.7152(7), Br2−Ag1 2.6148(11), Br3−Ag3 2.6641(8), Br3−Ag2 2.7008(8), Br4−Ag4 2.6873(7), Br5−Ag4 2.6051(7), Br5−Ag2 2.6380(8), Br6−Ag1 2.6677(7), Br6−Ag2 2.7184(10), Br7−Ag3 2.7176(8), Br7−Ag1 2.8540(7), Ag1−Ag3 3.0421(13); Ag1−Br1−Ag3 68.97(3), Ag3−Br3−Ag2 101.46(3), Ag4−Br5−Ag2 118.30(3), Ag1−Br6−Ag2 111.13(2), Ag3−Br7−Ag1 66.13(3), Br2−Ag1−Br1 116.67(3), Br2−Ag1−Br6 105.31(2), Br1− Ag1−Br6 126.46(2), Br2−Ag1−Br7 111.44(2), Br1Ag1−Br7 100.33(2), Br6−Ag1−Br7 93.25(3), Br2−Ag1−Ag3 157.30(2), Br1− Ag1−Ag3 56.42(2), Br6−Ag1−Ag3 94.127(16), Br7−Ag1−Ag3 54.779(14), Br5−Ag2−Br3 139.65(2), Br5−Ag2−Br6 92.53(3), Br3−Ag2−Br6 110.58(3), Br3−Ag3−Br1 109.08(2), Br3−Ag3−Br7 129.45(2), Br1−Ag3−Br7 102.40(3), Br3−Ag3−Ag1 111.35(2), Br1− Ag3−Ag1 54.611(16), Br7−Ag3−Ag1 59.09(2), Br5−Ag4−Br4 126.89(2). Symmetry code to generate atoms: ‘ = x, 1 + y, z; ‘‘ = −x, −y, 1 − z.

Figure 2. ORTEP drawing of one of the two independent cationic complexes in 1-Ag4Br7 (A). Ellipsoids are drawn at their 50% probability level. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (deg) (data for cationic complex B are reported in the square parentheses): C1−N1 1.349(5) [1.353(5)], C1−N2 1.353(5) [1.353(5)], C1−Au1 2.046(4) [2.044(4)], C9−N3 1.341(5) [1.351(5)], C9−N4 1.345(5) [1.348(5)], C9−Au1 2.041(4) [2.049(4)]; C9−Au1−C1′ 94.71(15) [96.20(15)], C9−Au1−C1 85.29(15) [83.80(15)]. Symmetry code to generate atoms ‘= −x, −y, −z.

independent and lie on two inversion centers. The relevant geometric parameters (bond angles and distances) nicely compare to those of a mononuclear tetracarbene20 and of dinuclear bridged dicarbene gold(III) complexes.4 The values of Au−C distances are indicative of a single bond as expected for a mainly σ-donor ligand. The ligands form six-member chelating rings on the gold atom showing a boat conformation, and the C1−Au−C9 bond angle is narrower than 90°. Moreover, in order to minimize the steric hindrance of the methyl substituents groups, the boat conformations of the two ligands are in trans disposition, as imposed by the inversion crystallographic center. Similar conformation has been observed in other bis(1,1′-methylene-3,3′-dialkyl-diimidazol-2,2′-diylidene) metal chelate complexes of palladium,21 platinum22 and nickel.27 The crystal packing shows an alternation of “layers” defined by the cationic complexes and the polysilverbromide chains. The Ag4Br73− anion represents the repetitive unit of an infinite chain which develops as a ribbon along the b axis passing through the symmetry centers (Figures 3 and 4). In the literature, only one example of a Ag4Br73− anion structurally determined is reported, which is arranged as a 1D chain formed by μ2-Br bridges which is extended along a crystallographic axis.28 In our case, the asymmetric unit Ag4Br73− is arranged in a different fashion, forming a new silverbromide polymer. The silver atoms show a distorted tetrahedral coordination geometry with Br−Ag−Br angles spanning from 88.80(3) to 139.65(2)° and Br−Ag bond distances spanning from 2.6148(11) to 2.8704(8) Å. In some cases, the Ag···Ag

Figure 4. Crystal packing of 1-Ag4Br7 evidencing the alternation of layers constituted by the cationic gold complexes and the polysilverbromide ribbons, together with selected Au···Br interactions (red dashed lines).

separation is very short, 3.0421(13) and 3.0735(12) Å for Ag1−Ag3 and Ag4−Ag4′, respectively. Those distances are longer than that in metallic silver (2.88 Å) but shorter than the sum of the van der Waals radii of silver (3.44 Å),29 indicating Ag···Ag interaction.30 There are terminal (Br2), μ2-bridging (Br1, Br5, and Br6), and μ3-bridging (Br3, Br4, and Br7) halogens. Considering the crystal packing, Br4 is pointing toward Au11 at a distance of 3.479 Å, and Br6 is pointing toward Au21 at a distance of 3.499 Å (Figures 4, S2, and S3). In order to gain insight on the nature of gold−bromide interaction, explaining the spatial arrangement of the gold(III) complexes with respect to the anionic ribbon revealed by the crystallographic analyses, we have carried out DFT calculations C

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Organometallics initially on both isolated species, i.e., the (Ag4Br73−)n ribbon and the gold(III) complex. Single point energies have been computed for Ag4Br7 and Ag16Br28 (as a model for the extended polymeric structure) using the atomic coordinates taken from the crystallographic structure. The potential plots show the distribution of the negative and positive charges on the Br and Ag nuclei, respectively (Figure 5). The pattern

Figure 6. Crystal packing of the complex 1-PF6. View along the b axis. Square planar cations are represented in wireframe; PF6− anions and disordered acetonitrile molecules are represented in ball and stick. Hydrogen atoms have been omitted. Selected bond distances (Å) and angles (deg) for the gold complex: C1−N1 1.339(3), C1−N2 1.342(3), C1−Au 2.041(3), C9−N4 1.337(3), C9−N3 1.342(3), C9− Au 2.043(3); C1−Au−C9 83.32(10), C1−Au−C9′ 96.68(10).

Figure 5. Electron potential plots; isosurface value: 0.02; color code: from red (negative values) to blue (positive values).

PF6, the PF6− anions do not interact with the gold centers, but they rather favor the cohesion in the crystals via multiple H···F interactions. In fact, in the crystal packing, the PF6− anions and acetonitrile molecules keep separate the tricationic complexes. The addition of AgPF6 in a KAuBr4/AgPF6 1:2 ratio, i.e., in the stoichiometric amount required for the precipitation of all the bromide ions as AgBr, is necessary to completely breakup the polysilverbromide ribbon and prevents the formation of different aggregates containing Ag and Br in variable ratios. For example, in the process for optimizing both the experimental procedure for the synthesis of 1-PF6 and the growing of polyhalides, very few crystals of the species 1-Br,(Ag2Br6)0.5 have been isolated (Figure 7). In particular, this compound has

changes significantly when going from the isolated Ag4Br7 unit to the assembled Ag16Br28, where the negative charge is concentrated in the core and the Br nuclei on the termini are almost neutral. This aggregate is stabilized by the interaction with the gold centers. It is expected that gold(III) complexes will assembly in the central region with the metal pointing to the most negatively charged halogen centers, as confirmed by the X-ray structure. Also the isolated gold(III) complex has been considered and has been optimized under C2h symmetry constraint. The overall computed molecular structure and the relevant interatomic distances are in nice agreement with the crystallographic parameters. The HOMO and the LUMO, which have Bg and Bu character, respectively, are strongly ligand-centered with a small contribution of Au dyz, while in the latter, a large Au py lobe dominates the composition (Figure S4). The calculated positive charge on the gold center is +0.409. The conversion of KAuBr4 to complex 1-Ag4Br7 is ca. 45%, with [Ag2(MeImCH2ImMe)2](PF6)2 as the limiting reagent. The remaining gold reagent is mainly converted to 1-PF6. In fact, the 1H NMR analysis of the fraction soluble in acetonitrile (spectra in DMSO-d6) presents a set of signals attributable to the [Au(MeImCH2ImMe)2]3+ cation, with the CH2 resonances at the much closer values 6.75 and 6.86 ppm, coincident with those observed for a pure sample of [Au(MeImCH2ImMe)2](PF 6 ) 3 (1-PF 6 ) (vide inf ra). Also in this case, the interconversion between the exo and endo hydrogens of the CH2 bridge is hindered, and it was not possible to reach the coalescence even upon heating the solution up to 150 °C. A more straightforward synthesis of 1-PF6 involves addition of AgPF6 to the reacting mixture of [Ag2(MeImCH2ImMe)2](PF6)2 and KAuBr4. Under these experimental conditions, the dicarbene transfer from silver to gold was followed by anion metathesis to afford 1-PF6 as main product. This compound is soluble in acetonitrile, and it allows to confirm the scarce coordinating properties of the PF6− counteranion. Crystals of 1-PF6 (Figure 6) suitable for X-ray analysis have been obtained by slow diffusion of diethyl ether on an acetonitrile solution of the complex. Acetonitrile molecules are also present, disordered over two positions with occupancy factor of 0.5. Bond distances and angles of the square planar cation [Au(MeImCH2ImMe)2]3+ in 1-PF6 are very close to those found in 1-Ag4Br7. The complex is centrosymmetric with the gold atom lying on the inversion center. In compound 1-

Figure 7. ORTEP crystal packing of the complex 1-Br,(Ag2Br6)0.5. View along the a axis. Square planar cations are represented in wireframe; Au atoms, Br− anions, Ag2Br64− anions, and acetonitrile molecules are represented in ball and stick. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (deg) for the square planar gold complexes (data for molecule B are reported in parentheses): Au1−C1 2.034(14) [2.016(17)], Au1−C9 2.051(13) [2.068(15)], N1−C1 1.374(17) [1.367(19)], N2−C1 1.320(18) [1.369(18)], N3−C9 1.346(17) [1.319(17)], N4−C9 1.337(19) [1.315(18)]; C1−Au1−C9 84.7(5) [83.3(6)], C1−Au1−C9′ 95.3(4) [96.4(6)].

been isolated starting from a noncrystallized sample of 1-PF6, thus containing small amounts of silver bromides, by adding Br−/Br2 in acetonitrile. In the crystals of 1-Br,(Ag2Br6)0.5, two crystallographically independent but very similar cationic square planar gold complexes, bromine anions, Ag2Br64− anions, and acetonitrile D

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interaction term is quite large, i.e., −231.11 kcal mol−1 and is largely dominated by the electrostatic contribution. By inspecting the frontier MOs of 1Br2+, one finds that HOMO−2, HOMO−1 and HOMO are close in energy (being in a range of approximately 3 kcal mol−1, the HOMO−LUMO gap is 53.06 kcal mol−1), spanning the irreducible representations A′A″ and A′, respectively; LUMO and LUMO+1 are also in a range smaller than 2 kcal mol−1 and span the irreducible representations A″ and A′. In HOMO−2 of 1Br2+ the MOs of adapt symmetry (A′) of the two fragments mix: It involves essentially a Br p occupied orbital and the LUMO of 13+ complex, which is strongly centered on the p orbitals of correct symmetry of gold and of its four surrounding carbon atoms (Figure S4). In HOMO−1 and HOMO, mixing is definitively less pronounced, and they are mainly nonbonding Br p orbitals spanning A″ and A′, respectively. The dispersion contribution to the interaction term is −11.06 kcal mol−1. In 1Br2+, a methylene proton and one proton of each methyl groups of the ligand arranged on the same side of bromide point toward the halogen nucleus and are at a distance of 2.42 and 2.56 Å, respectively. In HOMO−2 and HOMO−1, a contribution from s orbitals of the methylene (6.4 and 2.4%) and two methyl protons (16.4 and 3.1%) is found, arranged in antibonding fashion with respect to the Br p lobes. In HOMO only an s contribution of one methyl proton is found (17.6%). We come to the conclusions that the gold(III) complexes arranged in proximity of the Ag/Br ribbon are only slightly deformed and that the nature of the interaction between gold and the negatively charged Br atoms is mainly electrostatic.

solvent molecules, are present. The gold complexes are centrosymmetric and lie on two inversion centers. Their geometrical parameters are comparable with those observed in 1-PF6 and 1-Ag4Br7. The Ag2X64− anion (X = Br, I) was already described in the literature.31 Interestingly, the analogous Au2Br64− anion has never been reported. A Au2Br6 unit has been observed only in 3,5-di-t-butylpyrazolium bromide bis(μ2bromo)-tetrabromo-digold(III), but the poor quality of the crystallographic data prevented the fully structural characterization of the compound.32 In the crystal packing, the Br7 bromine anion is involved in an interaction with Au2 atom (Br7···Au2 3.452 Å) and the Br4 atom of the Ag2Br64− anion is involved in a similar manner with Au1 atom (Br4···Au1 3.467 Å). These distances nicely match to those found for Au···Br in 1-Ag4Br7. Although the observed axial Au−Br distances (ca. 3.46 Å) are significantly longer than an Au−Br bond (2.38−2.45 Å),4b they are shorter than the sum of the van der Waals radii of Au and Br (4.18 Å).33 Therefore, the coordination geometry around the gold center can be considered distorted octahedral. These octahedral gold complexes characterized by Au···Br or Au··· Br(Ag) interactions, are rather uncommon. A powder diffraction study on Cs2Au2Cl6 suggests interaction between the gold center of AuCl4− unit and the chlorine in AuCl2−.34 Afterward, this type of six-coordinate complexes was postulated in late 1950 by Nyholm et al. on the basis of conductivity and spectrophotometric measurements in nonaqueous solutions involving [Au(diars) 2 ] 3+ /X − (diars = o-phenylenebis(dimethylarsine))35 or AuBr4−/Br− systems.36 The crystallographic evidence of a discrete six-coordinate [Au(diars)2I2]I complex has been obtained only in 1969 by Stephenson et al.37 Other few examples of tetragonally distorted octahedral gold(III) complexes are reported in the literature.20,38 More common are five-coordinate gold(III) complexes in which the fifth position is occupied by one arm of a bidentate ligand such as phenanthroline, 2-(2′-pyridyl)quinolone or nitrogen-substituted NHC.39 Carrying on our DFT analyses, the Au−Br complex (1Br2+) was optimized under Cs symmetry constraint. In the optimized species 1Br2+ the Au−Br distance is 2.98 Å and the positive charge on the gold center is reduced to +0.389. The Au-Br bond (Table 1) was analyzed partitioning the molecule into two fragments, i.e., the 13+ complex and the bromide ion. The coordination of the halide ion only slight deforms the gold complex structure, resulting in a small strain contribution to the total energy of approximatively 2 kcal mol−1. In contrast, the

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CONCLUSIONS We have synthesized a new mononuclear gold(III) complex with two chelating di(N-heterocyclic carbene) ligands, by transfer of the diNHC ligand from the corresponding silver complex. The positive charge of the tricationic complex is balanced by infinite ribbons of the repeating Ag4Br73− unit, which are stabilized by an interaction between the bromide atoms with the positively charged gold center (Au···Br 3.479 and 3.499 Å). This autoassembly of high-nuclearity silver bromide counteranions has important synthetic consequences and as confirmed by the results at different [Ag2(MeImCH2ImMe)2](PF6)2/[KAuBr4] ratios may limit the selectivity of the transmetalation reaction. In the case of low silver content, the formation of low nuclearity Br− and Ag2Br64− counteranions is observed, and also in this case, bromides interact with the positive gold center (Au···Br 3.452 and 3.467 Å). This sort of interaction is not observed with the poorly coordinating PF6− anions in 1-PF6. The crystallographic data and the related DFT calculations give general information on the silver bromide aggregates resulting on the transmetalation from silver NHC complexes, which should also be useful with metals different from gold. The Au···Br and Au···Br(Ag) interactions appear also interesting. The characteristic of the gold(III) cationic complex, in particular the easy regular stacking of the square planar structures, closely resembles the behavior of platinum(II) complexes. However, for example, the analogue bis-diNHC platinum complex [Pt(MeImCH2ImMe)2](I)2, with iodide instead of bromide counteranions, shows no interaction of the iodide anions with the platinum center (Pt···I > 5 Å).22 The higher positive charge in the gold NHC complex (3+ versus 2+ for Pt) should be responsible for this different behavior, an hypothesis corroborated by a theoretical analysis of the Br···Au

Table 1. Energy Decomposition Analysis of the Cs Optimized 1Br2+ Complexa 1Br2+ ΔVelstat ΔEPauli ΔEoi A′ A″ ΔEdisp ΔEint ΔEstrain ΔE

−253.78 84.09 −50.35 −39.21 −11.14 −11.06 −231.11 2.03 −229.08

a All values are in kcal mol−1; level of theory: ZORA-BLYP-D3(BJ)/ TZ2P sc.

E

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Organometallics

C18H24N8Au3+ = 183.0591), 274.0867 [AuL2 − H]2+ (required for C18H23N8Au2+ = 274.0850), 314.0508 [AuL2Br]2+ (required for C18H24N8AuBr2+ = 314.0481), 709.0195 [AuL2Br2]+ (required for C18H24N8AuBr2+ = 709.0131), 896.8431 [AuL2Br3Ag]+ (required for C18H24N8AgAuBr3+ = 896.8354), 1084.6646 [AuL2Ag2Br4]+ (required for C18H24N8Ag2AuBr4+ = 1084.6576). Synthesis of [Au(MeImCH2ImMe)2](PF6)3 (1-PF6). A solution of silver complex [Ag2(MeImCH2ImMe)2](PF 6)2 (0.1 mmol) in acetonitrile (20 mL) was added slowly to a solution of KAuBr4 (0.1 mmol) in the same solvent (20 mL). The addition was immediately followed by the precipitation of a white solid. The reaction mixture was maintained under stirring and sheltered from light for 5 h. Subsequently, AgPF6 (0.2 mmol) was added, and the reaction mixture was maintained under stirring and sheltered from light for further 16 h, then filtered over Celite, and the filtrate concentrated at reduced pressure to 5 mL. Addition of Et2O (20 mL) lead to the precipitation of the product as a white solid, which was filtered, washed with Et2O (3 mL), and finally dried under vacuum. Complex 1-PF6 could be further crystallized by slow diffusion of diethyl ether into an acetonitrile solution of the complex. White solid (yield 73%). Anal. Calcd for C18H24AuF18N8P3: C, 21.96; H, 2.46; N, 11.38%. Found: C, 22.36; H, 2.31; N, 11.04%. 1H NMR (DMSO-d6, 25 °C, ppm): δ = 3.51 (s, 12H, CH3), 6.75 (d AB system, 2H, CH2), 6.86 (d AB system, 2H, CH2), 7.77 (s, 4H, CH), 8.01 (s, 4H, CH). 1H NMR (CD3CN, 25 °C, ppm): δ = 3.47 (s, 12H, CH3), 6.43 (d AB system, 2H, CH2), 6.70 (d AB system, 2H, CH2), 7.46 (s, 4H, CH), 7.76 (s, 4H, CH). 1H NMR (D2O, 25 °C, ppm): δ = 3.57 (s, 12H, CH3), 6.74 (d AB system, 2H, CH2), 6.80 (d AB system, 2H, CH2), 7.64 (s, 4H, CH), 7.93 (s, 4H, CH). 13C NMR (CD3CN, 25 °C, ppm): δ = 39.3 (CH3), 64.8 (CH2), 125.3 (CH), 126.1 (CH), 147.9 (NCN). Computational Details. All density functional theory (DFT) calculations have been done with the Amsterdam Density Functional (ADF) program.41,42 Scalar relativistic effects were accounted for using the zero-order regular approximation (ZORA),43 an approach which proved to be efficient in the presence of heavy nuclei.44 The BLYP45 density functional was chosen with an added dispersion contribution term developed by Grimme et al.46 (BLYP-D3(BJ)), combined with the TZ2P basis set for all elements. The TZ2P basis set, a large uncontracted set of Slater-type orbitals (STOs), is of triple-ζ quality and has been augmented with two sets of polarization functions on each atom: 2p and 3d in the case of H, 3d and 4f in the case of C and N, 5d and 6f in the case of Br, 5f and 6g in the case of Ag, and 6g and 7h in the case of Au. The frozen-core approximation was employed: up to 1s for C and N, up to 3p for Br, and up to 3d and 4d in the case of Ag and Au, respectively. The numerical integration was performed using the fuzzy cells integration scheme developed by Becke.47 Symmetry constraints were used and are specified in the text. Ag/Br anionic aggregates are labeled using their minimal formulas, but omitting the total charge for clarity. In order to gain insight into the nature of the bonding between the gold complex and the bromide ion, the activation strain model48 was used, which can provide meaningful description of structural and reactivity properties of chemical species.49 In the activation strain model, the energy relative to the fragments forming the system, i.e., the gold complex and the bromide ion, (ΔE), is decomposed into the strain energy ΔEstrain and the interaction energy ΔEint (eq 1):

interaction which is mainly electrostatic. Noticeably in the 1H NMR spectra, the resonance of the methylene group protons appears to depend on this interaction, being different in 1Ag4Br7 and 1-PF6. Another peculiar characteristic of this type of square planar gold(III) complexes is the possibility of constructing appropriate counteranions also with oxidizing reagents, being the metal center in the highest observed oxidation state.

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EXPERIMENTAL SECTION

All manipulations were carried out using standard Schlenk techniques under an atmosphere of argon or dinitrogen. The reagents were purchased by Aldrich as high-purity products and generally used as received; all solvents were used as received as technical grade solvents. The silver(I) complex [Ag2(MeImCH2ImMe)2](PF6)2 was prepared according to literature procedures.40 NMR spectra were recorded on a Bruker Avance 300 MHz (300.1 MHz for 1H and 75.5 for 13C); chemical shifts (δ) are reported in units of parts per million (ppm) relative to the residual solvent signals. XPS spectra were recorded using a PerkinElmer PHI 5600 ci spectrometer with a standard Al Kα source (1486.6 eV) working at 250 W. The working pressure was less than 1 × 10−8 Pa. The spectrometer was calibrated by assuming the binding energy (BE) of the Au 4f7/2 line to lie at 84.0 eV with respect to the Fermi level. Extended spectra (survey) were collected in the range of 0−1350 eV (187.85 eV pass energy, 0.5 eV step, 0.025 s step−1). Detailed spectra were recorded for the following regions: Au 4f, Ag 3d, Br 3d, N 1s, and C 1s (11.75 eV pass energy, 0.1 eV step, 0.2 s step−1). The standard deviation in the BE values of the XPS line is 0.10 eV. The peak positions were corrected for the charging effects by considering the C 1s peak at 285.0 eV and evaluating the BE differences. The powder for the XPS analysis was evacuated for 12 h at ca. 1 × 10−3 Pa before measurement. Liquid chromatography (LC)-HRMS analyses were performed with an UHPLC system (Agilent Series 1200; Agilent Technologies, Palo Alto, CA, USA), consisting of vacuum degasser, autosampler, and a binary pump coupled to a quadrupole-time-of-flight (Q-TOF) mass analyzer (Agilent Series 6520; Agilent Technologies) with an electrospray ionization-mass spectrometry (ESI-MS) source, using nitrogen as gas and operating in positive acquisition, with the following operation parameters: capillary voltage, 3500 V; nebulizer pressure, 35 psi; drying gas, 8 L/min; gas temperature, 350 °C; fragmentor voltage in the range of 80−200 V; skimmer 65 V. Analyses were performed injecting a total of 40 μL (8 × 5 μL injection volume) of samples by means of an injection program, varying the fragmentor voltage each time. The mobile phase was water/acetonitrile 50:50 (v:v) at flow rate of 0.25 mL/min. Full scan mass spectra were recorded as centroid over the 50−3000 m/z range with a scan rate of 2 spectra/s. Mass spectra acquisition and data analysis was processed with Masshunter Workstation B 04.00 software (Agilent Technologies). The purity of the compounds was determined by multinuclear NMR spectroscopy and CHN elemental analysis. Reaction of [Ag2(MeImCH2ImMe)2](PF6)2 with KAuBr4: Synthesis of 1-Ag 4 Br 7 . A solution of silver(I) complex [Ag2(MeImCH2ImMe)2](PF6)2 (0.1 mmol) in CH3CN (20 mL) was added in small portions to a solution of the gold(III) salt (0.10 mmol) in CH3CN (20 mL). The addition was immediately followed by the precipitation of an off-white solid. The reaction mixture was maintained under stirring and sheltered from light for 5 h. The offwhite solid was filtered, washed with Et2O (3 mL), and finally dried under vacuum. White solid (yield 43% calculated on the basis of the starting gold). Anal. Calcd for C18H24Ag4AuBr7N8: C, 14.04; H, 1.57; N, 7.26%. Found: C, 14.26; H, 1.53; N, 7.14%. 1H NMR (DMSO-d6, 25 °C, ppm): δ = 3.55 (s, 12H, CH3), 6.76 (d AB system, 2H, CH2), 7.15 (d AB system, 2H, CH2), 7.77 (s, 4H, CH), 8.00 (s, 4H, CH). 1H NMR (D2O, 25 °C, ppm): δ = 3.59 (s, 12H, CH3), 6.73 (d AB system, 2H, CH2), 6.91 (d AB system, 2H, CH2), 7.62 (s, 4H, CH), 7.92 (s, 4H, CH). 13C NMR (DMSO-d6, 25 °C, ppm): δ = 37.9 (CH3), 63.2 (CH2), 124.3 (CH), 125.2 (CH), 146.6 (NCN). MS (HRMS, Frag = 160.0 V, base peak, m/z) = 183.0604 [AuL2]3+ (required for

ΔE = ΔEstrain + ΔE int

(1)

ΔEstrain is the energy associated with deforming the fragments from their equilibrium geometry into the geometry they acquire at the complex of interest. It can be divided into a contribution stemming from each fragment. ΔEint is the actual interaction energy between the deformed fragments, which can also be further analyzed in the framework of the Kohn−Sham molecular orbital (MO) model using a quantitative energy decomposition analysis of the bond into electrostatic attraction, Pauli repulsion (or exchange repulsion), and stabilizing orbital interactions (eq 2);48 when dispersion is included, then it must be accounted for in the sum:

ΔE int = ΔVelstat + ΔE Pauli + ΔEoi + ΔEdisp F

(2)

DOI: 10.1021/acs.organomet.7b00163 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Crystal Data. Complex 1-Ag4Br7 was crystallized by slow diffusion of a dichloromethane/n-hexane mixture (1/5) into a DMSO solution of the complex (or by DMSO/acetonitrile stratification). Crystals of 1PF6 were obtained by slow diffusion of diethyl ether into an acetonitrile solution of the complex. Crystals of 1-Br,(Ag2Br6)0.5 were obtained by slow diffusion of diethyl ether into an acetonitrile solution of a crude batch of complex 1-PF6 (thus containing small quantities of silver bromides), KBr, and Br2 (in Au/Br−/Br2 1/3/4 ratio). Crystal data for all complexes were collected at room or low temperature on a Bruker APEX II single-crystal diffractometer, working with Mo Kα graphite monochromatic radiator (λ = 0.71073 Å) and equipped with an area detector. The raw frame data (20 s per frame scan time for a sphere of diffraction data) were processed using SAINT software;50 a correction for absorption was made using SCALE program51 to yield the reflection data file. The structures were solved by direct methods with SHELXS-97 and refined against F2 with SHELXL-2014/752 using anisotropic thermal parameters for all non-hydrogen atoms. The hydrogen atoms were placed in the ideal geometrical positions. Crystallographic data for the complexes have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications CCDC 1533228 (complex 1-Ag4Br7), 1533229 (complex 1PF6), and 1533230 (complex 1-Br,(Ag2Br6)0.5).

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(4) (a) Baron, M.; Tubaro, C.; Basato, M.; Isse, A. A.; Gennaro, A.; Cavallo, L.; Graiff, C.; Dolmella, A.; Falivene, L.; Caporaso, L. Chem. Eur. J. 2016, 22, 10211−20224. (b) Baron, M.; Tubaro, C.; Basato, M.; Biffis, A.; Natile, M. M.; Graiff, C. Organometallics 2011, 30, 4607− 4615. (c) Baron, M.; Tubaro, C.; Basato, M.; Biffis, A.; Graiff, C. J. Organomet. Chem. 2012, 714, 41−46. (d) Baron, M.; Tubaro, C.; Basato, M.; Natile, M. M.; Graiff, C. J. Organomet. Chem. 2013, 723, 108−114. (e) Tubaro, C.; Baron, M.; Costante, M.; Basato, M.; Biffis, A.; Gennaro, A.; Isse, A. A.; Graiff, C.; Accorsi, G. Dalton Trans. 2013, 42, 10952−10963. (5) Collado, A.; Bohnenberger, J.; Oliva-Madrid, M.-J.; Nun, P.; Cordes, D. B.; Slawin, A. M. Z.; Nolan, S. P. Eur. J. Inorg. Chem. 2016, 2016, 4111−4122. (6) (a) Schneider, D.; Schier, A.; Schmidbaur, H. Dalton Trans. 2004, 1995−2005. (b) Schneider, D.; Schuster, O.; Schmidbaur, H. Dalton Trans. 2005, 1940−1947. (7) (a) Wolf, W. J.; Winston, M. S.; Toste, F. D. Nat. Chem. 2014, 6, 159−164. (b) Komiya, S.; Albright, T. A.; Hoffmann, R.; Kochi, J. K. J. Am. Chem. Soc. 1976, 98, 7255−7265. (c) Komiya, S.; Kochi, J. K. J. Am. Chem. Soc. 1976, 98, 7599−7607. (d) Tamaki, A.; Magennis, S. A.; Kochi, J. K. J. Am. Chem. Soc. 1974, 96, 6140−6148. (8) (a) Tsui, E. Y.; Müller, P.; Sadighi, J. P. Angew. Chem., Int. Ed. 2008, 47, 8937−8940. (b) Gaillard, S.; Slawin, A. M. Z.; Nolan, S. P. Chem. Commun. 2010, 46, 2742−2744. (9) Roşca, D.-A.; Smith, D. A.; Hughes, D. L.; Bochmann, M. Angew. Chem., Int. Ed. 2012, 51, 10643−10646. (10) Hashmi, A. S. K. Angew. Chem., Int. Ed. 2012, 51, 12935−12936. (11) Hashmi, A. S. K. Chem. Rev. 2007, 107, 3180−3211. (12) Krause, N.; Winter, C. Chem. Rev. 2011, 111, 1994−2009. (13) Corma, A.; Leyva-Pérez, A.; Sabater, M. J. Chem. Rev. 2011, 111, 1657−1712. (14) Roşca, D.-A.; Wright, J. A.; Bochmann, M. Dalton Trans. 2015, 44, 20785−20807. (15) Ott, I. Coord. Chem. Rev. 2009, 253, 1670−1681. (16) See for example (a) Giljohann, D. A.; Seferos, D. S.; Daniel, W. L.; Massich, M. D.; Patel, P. C.; Mirkin, C. A. Angew. Chem., Int. Ed. 2010, 49, 3280−3294. (b) Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293−346. (c) Dykman, L.; Khlebtsov, N. Chem. Soc. Rev. 2012, 41, 2256−2282. (17) see for example (a) Nolan, S. P. Acc. Chem. Res. 2011, 44, 91− 100. (b) Gaillard, S.; Cazin, C. S. J.; Nolan, S. P. Acc. Chem. Res. 2012, 45, 778−787. (c) Lin, J. C. Y.; Huang, R. T. W.; Lee, C. S.; Bhattacharyya, A.; Hwang, W. S.; Lin, I. J. B. Chem. Rev. 2009, 109, 3561−3598. (d) Raubenheimer, H. G.; Cronje, S. Chem. Soc. Rev. 2008, 37, 1998−2011. (18) (a) Volpe, A.; Sartorel, A.; Tubaro, C.; Meneghini, L.; Di Valentin, M.; Graiff, C.; Bonchio, M. Eur. J. Inorg. Chem. 2014, 2014, 665−675. (b) Tubaro, C.; Bertinazzo, D.; Monticelli, M.; Saoncella, O.; Volpe, A.; Basato, M.; Badocco, D.; Pastore, P.; Graiff, C.; Venzo, A. Eur. J. Inorg. Chem. 2014, 2014, 1524−1532. (c) Pinter, P.; Biffis, A.; Tubaro, C.; Tenne, M.; Kaliner, M.; Strassner, T. Dalton Trans. 2015, 44, 9391−9399. (d) Biffis, A.; Cipani, M.; Bressan, E.; Tubaro, C.; Graiff, C.; Venzo, A. Organometallics 2014, 33, 2182−2188. (e) Biffis, A.; Baron, M.; Tubaro, C. Adv. Organomet. Chem. 2015, 63, 203−288. (19) (a) Gil-Rubio, J.; Cámara, V.; Bautista, D.; Vicente, J. Inorg. Chem. 2013, 52, 4071−4083. (b) Hemmert, C.; Poteau, R.; Laurent, M.; Gornitzka, H. J. Organomet. Chem. 2013, 745−746, 242−250. (c) Jean-Baptiste dit Dominique, F.; Gornitzka, H.; Sournia-Saquet, A.; Hemmert, C. Dalton Trans. 2009, 340−352. (d) Hung, F.-F.; To, W.P.; Zhang, J.-J.; Ma, C.; Wong, W.-Y.; Che, C.-M. Chem. - Eur. J. 2014, 20, 8604−8614. (20) Lu, Z.; Cramer, S. A.; Jenkins, D. M. Chem. Sci. 2012, 3, 3081− 3087. (21) (a) Fehlhammer, W. P.; Bliss, T.; Kernbach, U.; Brüdgam, I. J. Organomet. Chem. 1995, 490, 149−153. (b) Heckenroth, M.; Neels, A.; Stoeckli-Evans, H.; Albrecht, M. Inorg. Chim. Acta 2006, 359, 1929−1938. (22) Unger, Y.; Zeller, A.; Ahrens, S.; Strassner, T. Chem. Commun. 2008, 3263−3265.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00163. 1 H and 13C NMR spectra of complexes 1-Ag4Br7 and 1PF6, HRMS for 1-Ag4Br7, computational details (PDF) Cartesian coordinates and electronic energies for 13+ and 1Br2+ (XYZ) Accession Codes

CCDC 1533228−1533230 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +39 049 8275655. ORCID

Cristina Tubaro: 0000-0001-7724-735X Laura Orian: 0000-0002-1673-5111 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS University of Padova is gratefully acknowledged for financial support (CPDA20140431 and Assegno di Ricerca Senior GRIC15V47A).

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REFERENCES

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