Different metallophilic attitudes revealed by com- pression

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Different metallophilic attitudes revealed by com- pression

Stefano Racioppi,

^†,‡

Michał Andrzejewski,

^†

Valentina Colombo,

^‡

Angelo Sironi,

^*,‡

Piero Macchi*

^§

†

Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, Bern CH-3012, Switzerland

‡

Dipartimento di Chimica, Università degli Studi di Milano, Via Golgi, 19 – 20133 Milano, Italy

§

Dipartimento di Chimica, Materiali e Ingegneria Chimica, Politecnico di Milano, Via Mancinelli, 7 – 20131 Milano, Italy

Supporting Information Placeholder

Abstract: Two isostructural coordination polymers with coinage metal(I) cations were compressed with the purpose of testing interactions between chains, that may trigger metallophilic interactions or otherwise expand the metal coordination. DFT calculations and X-ray diffraction studies reveal an extraordinary difference between Ag(I) and Cu(I) in homologous compounds. Argentophilic interactions are favored by a mild compression and, at P = 7.94 GPa, Ag---Ag distance matches the value of metallic silver. On the contrary, no cuprophilic interaction is activated even by compression up to 8 GPa and Cu---Cu distances remain outside the van der Waals spheres.

Aurophilic¹, argentophilic² and cuprophilic³ interactions, where M(I)···M(I) distances are shorter than the sum of the van der Waals radii, have always stimulated the curiosity of scientists for both the intriguing nature of this bond and its frequent occurrence in solids. Recent studies demonstrate that these interactions play a major role for luminescence^4–6, and are relevant for catalysis⁷ and life science⁸.

The attraction between two cations is counterintu- itive, especially when the two atoms involved are closed-shell and cannot share d-electrons to form a covalent bond. However, closed-shell d¹⁰---d¹⁰ interaction energies are calculated as stabilizing (7-11 kcal/mol), depending on the nature of the metals and their environment.^9,10 This stabilization arises from a combination of electron correlation¹⁰, relativistic contraction¹¹ and other secondary contribu- tions like charge transfer¹². These effects not always stabilize a geometry with shorter metal-metal distance; for example, relativistic effect could also elongate it¹³. When compounds featuring metallophilic interactions at ambient conditions were brought to extreme conditions of pressure or tem-

perature, unusual mechanical phenomena were observed, like negative linear compressibility in Ag3[Co(CN)6]¹⁴ and KMn[Ag(CN)2]3,¹⁵ negative area compressibility in Au(C2H5)2 (S2CN),¹⁶ and negative thermal expansion in In[Ag(CN)2]3·xH2O.¹⁷ While several cases of metallophilic interactions induced by lowering the temperature have been reported in the past years^18–20, only few examples of supported metallophilic interactions have been recently observed at high pressure in copper iodine cubane clusters⁵ and in silver-copper tetranuclear com- plexes.⁶

Our study was undertaken to single out the different metallophilic attitude of Cu(I) and Ag(I) in two unprecedented isostructural M(I) 5-(2-fluoro-4- pyridyl)tetrazolate coordination polymers (CuFPT and AgFPT, respectively, see Tables 1 and S1-S3).

Upon compression, the packing of these species may favor metallophilic interactions, which are ab- sent at ambient conditions. An external pressure probes the forces acting on atoms in a crystal and reveals the strength of bonds and the ‘hidden’ mild attractions of electronic nature that are over- whelmed by repulsions at ambient conditions. We synthetized AgFPT and CuFPT in solvothermal con- ditions, exploiting the in situ generation of the tetrazolate.²¹ They precipitate as single crystals from a water/methanol solution, starting from the 4-cyano-2-fluoropyridine and sodium azide in the presence of the corresponding Ag(I) or Cu(I) salts.

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Figure 1. Fragment of the coordination polymers M(I)FPT, dashes lines correspond to closest M-(M, N) contacts.

Table 1. Selected crystallographic data of AgFPT and CuFPT at T = 293K.

compound AgFPT AgFPT AgFPT CuFPT CuFPT

P(GPa) 0.0001 6.15 7.94 0.0001 8.0

space group P21/n P21/n P21/n P21/n P21/n

a (Å) 6.2014(14) 6.0170(7) 6.0049(5) 5.7705(4) 5.6617(5)

b (Å) 19.212(5) 18.668(2) 18.7256(17) 18.8183(12) 17.920(4) c (Å) 12.844(3) 10.643(11) 10.474(7) 13.0141(8) 10.970(4)

β (°) 93.654(4) 97.35(3) 98.12(2) 93.668(1) 98.857(17)

V (Å3) 1527.2(6) 1185.6(1) 1166.0(8) 1410.32(16) 1099.8(5) R1/wR2

(I>2σ(I))

0.04/0.07 0.10/0.24 0.06/0.19 0.06/0.11 0.05/0.11

GooF/Rint 1.015/0.0 4

1.125/0.14 1.105/0.03 1.038/0.02 1.037/0.039

completeness for d ≤ 0.83 Å

99.9 % 35.3 % 45.1 % 100 % 28.3 %

Figure 2. Left: compressibility of unit-cell parameters and volume of (a) AgFPT (synchrotron data) and (b) CuFPT (diffractometer data); Right: Crystal structures of (c,d) AgFPT and (e,f) CuFPT projected along [100] at ambient and high pressure. Red zig-zags show ribbons of molecules, which approach to form Ag···Ag and Ag···N for AgFPT and Cu···N for CuFPT, respectively. Yellow spheres are voids in structures calculated in Mer- cury (probe radius 0.2 Å, grid spacing 0.1 Å).

Figure 3. Evolution of the most significant experimental (bold symbols) and theoretical (empty symbols) inter- atomic distances.

Single crystal X-ray diffraction confirms that both coordination polymers crystallize in the monoclinic P21/n space group, with two independent cations in the asymmetric unit (Figure 1,2 and S1).

Both are three-coordinated to tetrazolate rings but differ in their interaction to a fourth nitrogen of the adjacent ribbon: M1 is close, but not yet bound, to

a side-on tetrazolate (N(9)), whereas M2 is gen- uinely bound to fluoropyridine (N(1)). For both species, the hydrostatic compression produces a significant contraction of the axis c, whereas a and b are almost unaffected (Figure 2 and S1). This contraction is slightly more pronounced in AgFPT (18.5% at 7.94 GPa) than in CuFPT (16% at 8 GPa).

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This response entails a significant approach of adjacent zig-zag chains inherent to the shortening of inter-ribbon distances (Figures 1, 2 and S2-S4).

There are two kinds of M---M inter-ribbons distances (Figure 1): a) M(1)---M(2), bridged by the tetrazolate (through N(9)); b) M(2)----M(2), unsupported by any ligand and rather long at ambient conditions. In addition, there are intra-ribbon M(1)----M(2) contacts, η²-bridged by two tetrazolate ligands forming a six-membered ring. Both types of M(1)---M(2) contacts undergo significant changes upon compression (as determined from single crystal X-ray diffraction in diamond anvil cells), but one cannot recognize any incipient metallophilic interactions, for both species. In fact, instead of metallophilic interactions, M(1) centers show a much stronger affinity for the tetrazolate rings (Figure 3, Table S5 and S6). On the other hand, metallophilic interactions may occur between ribbons involving the M(2) cations. Noteworthy, the Ag(2)---Ag(2) distance becomes shorter than the sum of van der Waals radii already at 3.25 GPa²² and it shrinks down to 2.944(5) Å at 7.94 GPa, i.e. just 0.05 Å longer than in metallic silver². At variance, Cu(2)--- Cu(2) contacts shorten by only 13% at 8 GPa, re- maining well above the van der Waals limit (Figure 3). As anticipated, the inter-ribbon M(1)---N(9) contacts are sensitive to compression (Figure 3, S2- S4). Their shortening competes with the M(2)--- M(2) one (Figure S2-S4). At the highest P, M(1)--- N(9) is definitely stronger in CuFPT than in AgFPT.

This correlates with an anisotropic stiffness: the cell axis b shrinks twice as much compared with a in CuFPT, whereas they are equally compressible in AgFPT (Figure 2 and S1). In both species, the compression implies an approaching of the ribbons, al- though not identical: in CuFPT the M(1)---N(9) shortening dominates, whereas in AgFPT M(2)--- M(2) is the leading interaction. In order to test the precision, we repeated all in-house experiments on AgFPT using synchrotron X-ray diffraction and found good reproducibility. The experimental pic- ture is fully coherent with periodic DFT calculations (Figures 3, S1 and Table S4).

In addition, DFT calculations enable a detailed analysis of the chemical bonding through the topological analysis of the electron density²³. In the op- timized structures, both M(1) and M(2) form four bond paths. While this is expected for M(2), it should not surprise also for the substantially trigonal M(1) (Figure 1), since weak interactions may originate bond paths. Indeed, M(1)---N(9) is associated to a much smaller electron density at the bond critical point (bcp) and a smaller electron delocalization index. The ambiguous stereochemistry of M(1) emerges more clearly from the calculated atomic graph (Figure 4), which addresses three va- lence charge concentrations²⁴ (CCs) for M(1) (as expected for a trigonal metal) and four ligand op- posed concentrations for M(2), in keeping with a distorted trigonal pyramid. N(9) orients one of its three charge concentrations mainly toward M(2), though slightly shifted in direction of M(1) (Figure 4), typical of asymmetric semi-bridging ligands.²⁵

Figure 4. Atomic graph of M(1), N(9) and M(2).

Green spheres are charge concentrations, i.e. max- ima of–²ρ(r); the angle M(1)-N(9)-CC is 66.7 ° at 0.1 MPa and 65.7 ° at 8 GPa for M = Ag, and 68.7 ° at 0.1 MPa and 73.2 ° at 8 GPa for M = Cu.

In Figure 5a, the electron density along M(2)---M(2) is shown. Once again, the two compounds differ:

AgFPT features a bcp, whereas CuFPT displays only a ring critical point (rcp) and much lower electron density. The compression induces in AgFPT an even larger electron density at the bcp. At 8 GPa, (rbcp)

= 0.14 eÅ^-3, a value very close to typical electron density associated with covalent M-M bonds (ca.

0.2-0.3 eÅ^-3)^25,26,27. On the other hand, Cu(2)---Cu(2) remains longer and the geometrical mid-point re- mains a rcp with (rrcp) << 0.1 eÅ^-3 (Figure 5a and Table S6). The Laplacian of the electron density,

²ρ(r)^28,29, does not differentiate the two metals upon compression (Figure 6), whereas the delocalization indexes δ(M,M)³⁰ clearly address the Ag(2)---Ag(2) bond formation, associated with the sharing of 0.1 electron pairs at 8 GPa (Figures 5b and 6), one order of magnitude larger than Cu(2)--- Cu(2) or Cu(1)---Cu(2). The topological analysis of the M---N contacts agrees with the geometrical features described above. The inter-ribbons M(1)--- N(9) is significantly strengthened for M=Cu (Figure 5b, Table S6-S7), confirming the affinity of this metal for nitrogen atoms. Instead, the electron delocalization of Ag(1)---N(9) only slightly increases.

Notably, even if the rotation of the CC (Figure 4) could suggest that M(1)---N(9) interaction becomes stronger for Ag than for Cu, the delocalization in- dices behave oppositely. Indeed, such rotation is induced by the formation of the metallophilic interaction.

As anticipated in previous studies,^31,32,33 a pressure in the range 0-10 GPa is an extraordinary tool to learn more on the nature of some elusive bonding, like the metallophilic.

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Figure 5. Theoretical ρ(r) (a) and delocalization indexes (b) at significant intra- and inter-layer critical points at different pressure.

In this study, we reported the first unsupported argentophilic interaction induced by compression

and an unambiguous analysis on the different metallophilic attitude between two d¹⁰ metals, namely Cu(I) and Ag(I). In fact, in spite of the isostructural relationship at ambient conditions, they behave differently at high pressure. In AgFTP, we observe both the establishing of a metallophilic interaction and the strengthening of a looser Ag---N coordination, whereas in CuFTP only the latter occurs (to a larger extent). This trend reveals not only a lower repulsion between Ag(I) cations, compared to Cu(I), but also a favorable polarization of Ag(I), which en- ables electron sharing with another Ag(I).

In future studies, we will investigate transport properties correlated to metallophilicity that may tune electrical conductivity. This was not possible for the compounds here investigated, due to the size and shape of the crystal samples and to the absence of M----M extended networks, which topo- logically hampers a conductance.

Figure 6. Maps of the theoretically calculated Laplacian of the electron density in the plane M(2)-M(1)-M(2) and delocalization indexes for M---M interactions; δ M(2)---M(2) is highlighted in red. Contours value of the Laplacian (in a.u.) are in blue, on a logarithmic scale (negative values are not shown).

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI:

Synthesis, high-pressure X-ray diffraction; Theoreti- cal Calculations; tables and supplementary figures (PDF). CIF files are deposited in CCDC 1896824- 1896843

AUTHOR INFORMATION Corresponding Authors

piero.macchi@polimi.it; angelo.sironi@unimi.it

ORCID

Piero Macchi: 0000-0001-6292-982 Angelo Sironi: 0000-0001-6902-6987 Valentina Colombo: 0000-0003-0263-4456 Michał Andrzejewski: 0000-0001-8997-8301 Stefano Racioppi: 0000-0002-4174-1732 Notes

The authors declare no competing financial inter- ests.

ACKNOWLEDGMENT

Dr. M. Airoldi is kindly acknowledged for his support during the synthesis of the materials. This re-

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search was in part supported by the NCCR MAR- VEL, funded by the Swiss National Science Founda- tion. We thank the Swiss National Science Founda- tion for financial support (project 162861). VC and AS thank the University of Milan for partial funding through the Development Plan of Athenaeum grant 2017. We acknowledge Paul Scherrer Institute, Villi- gen, Switzerland for provision of synchrotron radia- tion beamtime at beamline X04SA of the SLS.

Keywords: argentophilic interactions; • metal- lophilic interactions • high pressure • X-ray diffraction • DFT calculations

REFERENCES

(1) Schmidbaur, H.; Schier, A. Aurophilic Interactions as a Subject of Current Research: An up-Date.

Chem. Soc. Rev. 2012, 41 (1), 370–412.

https://doi.org/10.1039/C1CS15182G.

(2) Schmidbaur, H.; Schier, A. Argentophilic Interac- tions. Angew. Chemie - Int. Ed. 2015, 54 (3), 746–784.

https://doi.org/10.1002/anie.201405936.

(3) Carvajal, M. A.; Alvarez, S.; Novoa, J. J. The Nature of Intermolecular CuI CuI Interactions: A Com - bined Theoretical and Structural Database Analy- sis. Chem. - A Eur. J. 2004, 10 (9), 2117–2132.

https://doi.org/10.1002/chem.200305249.

(4) Jia, J.; Wang, Q. Intensely Luminescent Gold (I)-Sil- ver(I) Cluster with Hypercoordinated Carbon. J.

Am. Chem. Soc. 2009, 131, 16634–16635.

(5) Benito, Q.; Baptiste, B.; Polian, A.; Delbes, L.; Mar- tinelli, L.; Gacoin, T.; Boilot, J. P.; Perruchas, S.

Pressure Control of Cuprophilic Interactions in a Luminescent Mechanochromic Copper Cluster. In- org. Chem. 2015, 54 (20), 9821–9825.

https://doi.org/10.1021/acs.inorgchem.5b01546.

(6) Jarzembska, K. N.; Kamiński, R.; Dziubek, K. F.;

Citroni, M.; Paliwoda, D.; Durka, K.; Fanetti, S.;

Bini, R. Impact of High Pressure on Metallophilic Interactions and Its Consequences for Spectro- scopic Properties of a Model Tetranuclear Silver(I)- Copper(I) Complex in the Solid State. Inorg.

Chem. 2018, 57 (14), 8509–8520.

https://doi.org/10.1021/acs.inorgchem.8b01196.

(7) Weber, D.; Gagné, M. R. Aurophilicity in Gold(I) Catalysis: For Better or Worse?. In: Slaughter L.

(Eds) Homogeneous Gold Catalysis. Topics in Cur- rent Chemistry, Vol 357. Springer, Cham; 2014.

https://doi.org/10.1007/128.

(6)

the Transition Metal Methyl and Phenyl Com- pounds MCH3 and MC6H5 (M = Cu, Ag, Au) and M(CH3)2 and M(C6H5)2 (M = Zn, Cd, Hg).

Organometallics 1995, 14 (6), 4263–4268.

https://doi.org/10.1021/om00009a032.

(29) Macchi, P.; Donghi, D.; Sironi, A. The Electron Density of Bridging Hydrides Observed via Experi- mental and Theoretical Investigations on [Cr2(Μ2-H)(Co)10]-. J. Am. Chem. Soc. 2005, 127, 16494–16504.

(30) Bader, R. F. W.; Stephens, M. E. Spatial Localiza- tion of the Electronic Pair and Number Distribu- tions in Molecules. J. Am. Chem. Soc. 1975, 97

(26), 7391–7399.

https://doi.org/10.1021/ja00859a001.

(31) Scherer, W.; Dunbar, A. C.; Barquera-Lozada, J. E.;

Schmitz, D.; Eickerling, G.; Kratzert, D.; Stalke, D.;

Lanza, A.; Macchi, P.; Casati, N. P. M.; et al.

Anagostic Interactions under Pressure: Attractive or Repulsive? Angew. Chemie - Int. Ed. 2015, 54

(8), 2505–2509.

https://doi.org/10.1002/anie.201410532.

(32) Casati, N.; Macchi, P.; Sironi, A. Molecular Crystals under High Pressure: Theoretical and Experimen- tal Investigations of the Solid-Solid Phase Transi- tions in [Co2(CO)6(XPh3)2] (X = P, As). Chem. - A Eur. J. 2009, 15 (17), 4446–4457.

https://doi.org/10.1002/chem.200801528.

(33) Macchi, P.; Casati, N.; Evans, S. R.; Gozzo, F.; Si- moncic, P.; Tiana, D. An “Off-Axis” Mn–Mn Bond in Mn2(CO)10 at High Pressure. Chem. Commun.

2014, 50 (85), 12824–12827.

https://doi.org/10.1039/C4CC04152F.

Different metallophilic attitudes revealed by com- pression