Investigating Silver Coordination to Mixed Chalcogen Ligands

Six silver(I) coordination complexes have been prepared and structurally characterised. Mixed chalcogen-donor acenaphthene ligands L1–L3 [Acenap(EPh)(E'Ph)] (Acenap = acenaphthene-5,6-diyl; E/E' = S, Se, Te) were independently treated with silver(I) salts (AgBF4/AgOTf). In order to keep the number of variables to a minimum, all reactions were carried out using a 1:1 ratio of Ag/L and run in dichloromethane. The nature of the donor atoms, the coordinating ability of the respective counter-anion and the type of solvent used in recrystallisation, all affect the structural architecture of the final silver(I) complex, generating monomeric, silver(I) complexes {[AgBF4(L)2] (1 L = L1; 2 L = L2; 3 L = L3), [AgOTf(L)3] (4 L = L1; 5 L = L3), [AgBF4(L)3] (2a L = L1; 3a L = L3)} and a 1D polymeric chain {[AgOTf(L3)]n 6}. The organic acenaphthene ligands L1-L3 adopt a number of ligation modes (bis-monodentate μ2-η2-bridging, quasi-chelating combining monodentate and η6-E(phenyl)-Ag(I) and classical monodentate coordination) with the central silver atom at the centre of a tetrahedral or trigonal planar coordination geometry in each case. The importance of weak interactions in the formation of metal-organic structures is also highlighted by the number of short non-covalent contacts present within each complex.

Crystal engineering utilises the metal-ligand coordination bond to construct coordination networks, generally through the self-assembly of tuneable building blocks [4][5][6][7][8][9][10][11][12][13][14].Bridging organic ligands acting as rigid supports are linked in an ordered lattice, building extended and often multidimensional networks with central metal ions.Modification of the functional groups within the ligand shell can control the properties, topology and geometry of the extended network and lead to potential applications as new functional materials [10][11][12][13][14].
Nevertheless, the unpredictability of the polymeric architecture is a major challenge when designing supramolecular complexes.Self-assembly, which dictates the structural motif of the final complex is controlled by experimental conditions [10][11][12][13][14]. Factors such as the central metal ion oxidation state, the coordination geometry, the metal-to-ligand ratio, the nature and spacer length of the bridging ligand, the presence of solvents and the type of counter-anions, all play a significant role [10][11][12][13][14].A subtle variation to any one of these parameters can influence the geometry of the final solid state structure, generating for example extended three-dimensional networks, linear chain polymers or simple monomeric species [10][11][12][13][14][15].
Naphthalenes [18][19][20][21] and related 1,2-dihydroacenaphthylenes (acenaphthenes) [22] provide the perfect framework from which to design tunable donor ligands for thc preparation of metal complexes [23,24].The rigidity of the organic backbone and the geometric constraints unique to these compounds, imposed by a double substitution at the close peri-positions, ensures metal coordination is favoured in order to achieve a relaxed geometry [24].We have previously utilised the naphthalene backbone to prepare a variety of chalcogen and phosphorus compounds and associated metal complexes .

Results and Discussion
The three mixed acenaphthene derivatives Acenap[EPh][E'Ph] (Acenap = acenaphthene-5,6-diyl; EE' = L1 SeS, L2 TeS, L3 TeSe [49][50][51], were each independently treated with silver tetrafluoroborate [AgBF 4 ] and silver trifluoromethanesulfonate [AgOTf].In order to keep the number of variables to a minimum, the reactions were carried out using a 1:1 ratio of Ag/L and run in dichloromethane under an oxygen-and moisture-free nitrogen atmosphere.The complexes 1-6 obtained were characterised by multinuclear NMR and IR spectroscopy and mass spectrometry and the homogeneity of the new compounds was where possible confirmed by microanalysis; 77 Se and 125 Te-NMR data can be found in Table 1.Crystal structures were determined for 1-6 and 2a and 3a (recrystallisation products of 2 and 3, respectively).A number of the silver(I) complexes were found to be unstable towards light whilst in solution.Selected interatomic distances, angles and torsion angles are listed in Tables 2 and 3. Hydrogen-bond and other non-conventioanl weak inter-and intra-moleular interaction data can be found in Table S1 in the Electronic Supporting Information (ESI).Further crystallographic information can be found in Tables 4-6 and in Figures S1-S4 and Tables S2 and S3 in the ESI.Table 1. 77Se and 125 Te-NMR spectroscopic data [a] .  a] van der Waals radii used for calculations: r vdW (Br) 1.85Å, r vdW (S) 1.80Å, r vdW (Se) 1.90Å, r vdW (Te) 2.06Å [55]; [b] Splay angle: Σ of the three bay region angles-360.
TeSe [a] In order to keep the number of variables to a minimum, the reactions were carried out using a 1:1 ratio of Ag/L and run in dichloromethane.

−
) and eight additional dichloromethane molecules.The asymmetric unit of 3 contains four silver(I) centres, eight L3 ligands and four non-coordinating counter-anions (BF 4
Within the structural architecture of complexes 1 and 2, two crystallographically unique molecules of the unsymmetrical mixed-chalcogen acenaphthene donor (L1/L2) act as monodentate ligands, binding in each case via the least electronegative chalcogen atom (Se/Te; Figure 3).The two-coordinate central silver atom adopts a distorted bent coordination geometry, with E(1)-Ag(1)-E(2) angles of 135.In all three structures the geometry around the silver centre is governed by the conformation of the rigid acenaphthene supports.The axial-equatorial conformation of the aromatic rings in both acenaphthene fragments of each complex (type AB) [56][57][58][59][60][61][62][63][64][65][66][67][68], positions the E-C Ph bonds close to the acenaphthene plane with the secondary (E'-C Ph ) bond aligned perpendicular to it [in each case χ(E) < χ(E'); E is the monodentate coordinating chalcogen donor].The two facially bound axial E'(phenyl) rings are orientated parallel to their respective C Acenap -E'-C Ph plane and subsequently linked to the adjacent silver centre via a η 6 -E'(phenyl)•••silver type interaction to complete a quasi-chelate ring in each case (Figure 3).Coordination to silver has no significant effect on the conformation of the acenaphthene components or the degree of molecular distortion occurring within the organic frameworks of 1-3 compared with parent ligands L1-L3 [49][50][51].The degree of distortion is related to the size of the atoms residing in the bay-region, with an expected lengthening of the peri-gap observed as the heavier congeners are located at the 5,6-positions along the series [

Reactions of Silver(I) Trifluoromethanesulfonate
In contrast to the reactions with AgBF 4 , treatment of L1 and L2 with one molar equivalent of AgOTf afforded two isomorphous three-coordinate, monomeric, silver(I) complexes [Ag(OTf){Acenap(L)} 3 ] (4 (L1); 5 (L2); Figures 2 and 6).Crystals suitable for X-ray diffraction were obtained by slow diffusion of hexane into a saturated dichloromethane (4), dichloromethane/methanol (5) solution of the respective product.Recrystallisations of both products were performed at room temperature, in the absence of light to prevent the complexes from decomposing.The two nearly identical asymmetric units contain six silver(I) centres, eighteen mixed-donor ligands (L1/L2) and interestingly six non-coordinating triflate counter-anions.

[AgBF 4 (L2) 3 ] 2a & [AgBF 4 (L3) 3 ] 3a
Experimental conditions such as central metal ion oxidation state, the metal-to-ligand ratio, the nature and spacer length of the bridging ligand, the presence of solvents and the type of counter-anions can have a profound influence on the structural architecture of the final complex and adds unpredictability to the self-assembly process [10][11][12][13][14]. Techniques and solvents used in the recrystallisation process can also affect the outcome of the final product.A subtle adjustment to the recrystallisation solvent systems for complexes 2 and 3 afforded two nearly identical three-coordinate, mononuclear, monomeric silver(I) complexes [Ag(BF 4 ){Acenap(TePh)(EPh)} 3 ] (2a E = S, 3a E = Se) with structures analogous to complexes 4 and 5. Crystals suitable for X-ray diffraction were obtained by slow diffusion of hexane into saturated dichloromethane/methanol (2) and tetrahydrofuran (3) solutions of the respective product.Further information on the crystal structures of 2a and 3a can be found in the ESI.

General
All experiments were carried out under an oxygen-and moisture-free nitrogen atmosphere using standard Schlenk techniques and glassware.Reagents were obtained from commercial sources and used as received.Dry solvents were collected from a MBraun solvent system.Elemental analyses were performed by Stephen Boyer at the London Metropolitan University.Infra-red spectra were recorded as KBr discs in the range 4000-300 cm −1 on a Perkin-Elmer System 2000 Fourier transform spectrometer. 1H-and 13 C-NMR spectra were recorded on a Jeol GSX 270 MHz spectrometer with δ(H) and δ(C) referenced to external tetramethylsilane. 77Se and 125 Te-NMR spectra were recorded on a Jeol GSX 270 MHz spectrometer with δ(Se) and δ(Te) referenced to external Me 2 Se and Me 2 Te respectively, with a secondary reference for δ(Te) to diphenyl ditelluride [δ(Te) = 428 ppm]. 19F-NMR spectra were recorded on a Bruker Ultrashield 400 MHz spectrometer with δ(F) referenced to external trichlorofluoromethane.Assignments of 13 C and 1 H-NMR spectra were made with the help of H-H COSY and HSQC experiments.All measurements were performed at 25 °C.All values reported for NMR spectroscopy are in parts per million (ppm).Coupling constants (J) are given in Hertz (Hz).Mass spectrometry was performed by the University of St. Andrews Mass Spectrometry Service.Electrospray Mass Spectrometry (ESMS) was carried out on a Micromass LCT orthogonal accelerator time of flight mass spectrometer.

Crystal Structure Analyses
X-ray crystal structures for 1-6, 2a were collected at −180(1) °C by using a Rigaku MM007 High brilliance RA generator (Mo Kα radiation, confocal optic) and Mercury CCD system.At least a full hemisphere of data was collected using ω scans.Data were collected for 3a at −148(1) °C using a

Figure 3 .
Figure 3. Two crystallographically distinct L1 ligands bind to the silver(I) center via monodentate selenium coordination (left) to form complex 1 (right; H atoms and solvent molecules omitted for clarity).The structures of 2 and 3 (adopting similar conformations to 1) are omitted here but can be found in Figure S1 in the ESI.

Figure 4 .
Figure 4.The bent metallocene motif found at the center of complex 1, formed from two η 6 -S(phenyl)•••Ag interactions.Comparative fragments found in complexes 2 and 3 are displayed in Figure S1, ESI.

Figure 5 .
Figure 5. Complex 1 viewed down the z-axis; BF 4 − counter-anions and dichloromethane solvent molecules stack in channels between the acenaphthene fragments.The packing of complexes 2 and 3, viewed down the y-axis is displayed in Figure S2, ESI.

Figure 6 .
Figure 6.The three coordinate, mononuclear silver(I) complex 4 (H atoms omitted for clarity).The structure of 5 (adopting a similar conformation to 4) is omitted here but can be found in Figure S3 in the ESI.

Figure 7 .
Figure 7. Weak Ag1•••S1 contacts in the secondary coordination sphere affords a distorted quasi-trigonal prismatic geometry around the central silver atom in 4 and 5 (phenyl rings and H atoms omitted for clarity; complex 4 shown).

Figure 9 .
Figure 9.View of the 1D extended helical chain polymer 6 along the x-axis (H atoms and solvent molecules omitted for clarity).

Figure 10 .
Figure 10.The repeating unit of extended helical chain polymer 6 (top; H atoms and solvent molecules omitted for clarity) and the central core of the repeating unit showing the three coordinate, trigonal planar silver(I) geometry (bottom).

Figure 12 .
Figure 12.View of the 1D extended helical chain polymer 6 along the z-axis; silver atoms align in two columns with the closest non-bonding Ag•••Ag distance 5.929(1) Å.