Polymerization Isomerism in [{MFe (CO) 4} n] n−(M= Cu, Ag, Au; n= 3

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Polymerization Isomerism in [{MFe(CO)4}n]n− (M = Cu, Ag, Au; n = 3, 4) Molecular Clusters Supported by Metallophilic Interactions Beatrice Berti,† Marco Bortoluzzi,‡ Cristiana Cesari,† Cristina Femoni,† Maria Carmela Iapalucci,† Rita Mazzoni,† Federico Vacca,† and Stefano Zacchini*,† †

Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy Dipartimento di Scienze Molecolari e Nanosistemi, Ca’ Foscari University of Venice, Via Torino 155, 30175 Mestre (Ve), Italy

‡

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S Supporting Information *

isomerism in molecular clusters is a very rich topic and further exciting advancement may be envisioned. To widen the scope of isomerism in molecular clusters, we report herein an unique case of polymerization isomerism in [{MFe(CO)4}n]n− (M = Cu, Ag, Au; n = 3, 4) molecular clusters supported by metallophilic interactions. Polymerization isomers are described in inorganic textbooks as two compounds having the same elemental compositions but different molecular weights.11 Nonetheless, even for coordination compounds is rather rare, and to the best of our knowledge, polymerization isomerism has not been reported before for molecular clusters. The reaction of Collman’s reagent Na2[Fe(CO)4]·2thf with two equivalents of M(IMes)Cl (IMes = C3N2H2(C6H2Me3)2; M = Cu, Ag, Au) in thf resulted in the bimetallic Fe(CO)4(MIMes)2 (M = Cu, 1; Ag, 2; Au, 3) clusters (Scheme 1). The gold species 3 was previously described.12 The synthetic details and crystal structures of the new clusters 1 and 2 are reported in the Supporting Information. Complexes 1-3 rapidly decomposed after heating in dmso at 130 °C affording the higher nuclearity [{MFe(CO)4}3]3− (M = Cu, 4; Ag, 5; Au, 6) clusters (Scheme 1). Formation of 4−6 from 1−3 may be accounted for by the decompositionionization reaction depicted in eq 1:

ABSTRACT: Triangular clusters [{MFe(CO)4}3]3− (M = Cu, 4; Ag, 5; Au, 6) were selectively obtained by heating Fe(CO)4(MIMes)2 (M = Cu, 1; Ag, 2; Au, 3; IMes = C3N2H2(C6H2Me3)2). 1−3 were synthesized by reacting Na2[Fe(CO)4]·2thf with 2 equiv of M(IMes)Cl. As previously described, the direct reactions of Na2[Fe(CO)4]·2thf with one equivalent of M(I) salts resulted in the triangular cluster [{CuFe(CO)4}3]3− for Cu, whereas the square clusters [{MFe(CO)4}4]4− were formed for Ag and Au. Thus, depending on the synthetic protocol adopted, both the triangular [{MFe(CO)4}3]3− and square [{MFe(CO)4}4]4− polymerization isomers can be selectively obtained, at least for Ag and Au. Polymerization isomerism, that is two compounds having the same elemental compositions but different molecular weights, was investigated in [{MFe(CO)4}n]n− (n = 3, 4; M = Cu, Ag, Au) by means of structural and theoretical methods and the role of metallophilic interactions was computationally studied by means of the atoms-in-molecules (AIM) approach.

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tructural isomerism in molecular clusters has recently received a great interest because of its relevance to metal nanoclusters, nanoparticles and nanomaterials.1,2 The intimate understanding of isomerism in metal aggregates of increasing size may, indeed, contribute to the design and synthesis of metal nanoparticles, nanoclusters and nanomaterials with atomic precision. This goal has been impeded for a longtime because of the intrinsic difficulty of unraveling the structure of nanomaterials at the atomic level. A great advancement in this direction has been recently represented by the synthesis of atomically precise thiolated gold nanoclusters and the determination of their total structures by single-crystal X-ray crystallography (SCXC).3−6 In addition to enantiomerism, three further isomerism types have been identified for thiolated protected gold nanoclusters, that is core isomerism, staple isomerism, and complex isomerism.1 These may be viewed as analogous to the chain isomerism, point isomerism, and functional isomerism in organic molecules. Cluster core isomerism was, actually, documented for the first time in the case of the [Pt3(μ-PPh2)3Ph(PPh3)2] organometallic cluster.7 Moreover, surface ligand isomerism and dynamic permutational isomerism were subsequently discovered for molecular organometallic clusters.8−10 These discoveries reveal that © XXXX American Chemical Society

3Fe(CO)4 (MIMes)2 → [{MFe(CO)4 }3 ]3 − + 3[M(IMes)2 ]+

(1)

The [M(IMes)2]+ cation may be partially exchanged with tetraalkylammonium cations by metathesis with [NR4]X (R = Me, Et; X = Cl, Br). Thus, the anions 4−6 have been structurally characterized by means of SCXC as their [NEt4]2[Cu(IMes)2][{CuFe(CO)4}3], [NEt4]2[Ag(IMes)2] [{AgFe(CO)4}3]·solv, [NEt4]2[Au(IMes)2][{AuFe(CO)4}3]· CH3COCH3 and [NMe4]2[Au(IMes)2][{AuFe(CO)4}3] salts (Figures 1 and S16−S18 in the Supporting Information). The molecular structures of 4−6 are based on a M3Fe3 core which consists of a M3-triangle with edge-bridging Fe(CO)4 groups. Such a triangular structure is unprecedented for Ag and Au, whereas the hexanuclear copper cluster [{CuFe(CO)4}3]3− was previously obtained by reaction of Na2[Fe(CO)4]·2thf with Cu(I) salts.13 Conversely, analogous reactions of Collman’s reagent with Ag(I) and Au(I) salts afforded the Received: November 30, 2018

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DOI: 10.1021/acs.inorgchem.8b03334 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Syntheses of Complexes 1−8

Ag, Au (square) clusters might be related to the intrinsic differences of such metals and/or effects of the organometallic stabilizing fragments employed. Indeed, by using [Co(CO)4]− as organometallic ligand, square-in-a-square clusters [{MCo(CO)4}4] were obtained both for Cu and Ag.18 It was, thus, exciting to find out that, by changing the synthetic protocol, it was possible to selectively obtain the square-type clusters [{MFe(CO)4}4]4− (direct synthesis from Collman’s reagent and M(I) salts) or the triangular clusters [{MFe(CO)4}3]3− (thermal decomposition of 2, 3) at least in the case of Ag and Au. Conversely, for Cu only the [{CuFe(CO)4}3]3− triangular cluster was obtained independently of the synthetic strategy adopted. [{MFe(CO)4}4]4− and [{MFe(CO)4}3]3− (M = Ag, Au) represent the first examples of polymerization isomers for molecular clusters, having the same elemental compositions but different molecular weights. The compounds 4−6 contain short M−Fe [2.3942(5)− 2.4502(5) Å for 4; 2.6105(9)−2.6632(9) Å for 5; 2.5724(8)− 2.6155(9) Å for 6] and Fe−CO [1.754(3)−1.787(3) Å for 4; 1.743(7)−1.783(7) Å for 5; 1.752(6)−1.796(6) Å for 6] bonds as well as long M···M [2.5945(5)−2.6524(4) Å for 4; 2.8515(7)−2.8801(7) Å for 5; 2.8867(5)−2.9494(5) Å for 6] and M···C(O) [2.210(3)−2.791(3) Å for 4; 2.509(8)− 2.736(9) Å for 5; 2.328(6)−3.027(7) Å for 6] interactions. Similar interactions are present also in the square-type clusters 7 and 8.14,15 Both the polymerization isomers [{MFe(CO)4}4]4− and [{MFe(CO)4}3]3− are accessible in the case of Ag and Au, suggesting that they possess very similar energies. Thus, formation of triangular or square-type clusters in the case of Ag and Au seems to be dictated by the synthetic protocol adopted. Conversely, the triangular isomer seems to be the only stable compound in the case of Cu. The M(I) centers displays the conventional linear twocoordination involving two M−Fe bonds. The M3 and M4 cores of the clusters are further supported by metallophilic interactions.16,19 Their softer and flexible character allows (at least for Ag and Au) the formation of both triangular and square isomers without altering the Fe−M−Fe frameworks. AIM analyses have been carried out on DFT-optimized geometries (ωB97X-V/def2-SVPD calculations) of [{MFe(CO)4}3]3− (M3) and [{MFe(CO)4}4]4− (M4) clusters to shed

Figure 1. Molecular structure of (a) [{AgFe(CO)4}3]3− compared to the previously reported (b) [{AgFe(CO)4}4]4−.14 Ag−C(O) contacts are represented as fragmented lines (orange, Ag; green, Fe; gray, C; red, O).

square-in-a-square-type clusters [{MFe(CO)4}4]4− (M = Ag, 7; Au, 8) as previously reported in the literature (Scheme 1).14,15 For sake of comparison, their molecular structures are reported in Figures S19 and S20. In a similar way, the reactions of [MoCp(CO)3]− with M(I) salts resulted in the triangular [{CuMoCp(CO)3}3] cluster in the case of copper, whereas the square-in-a-square clusters [{MMoCp(CO)3}4] were formed with Ag and Au, respectively.16,17 It was somehow speculated that the different structures adopted by Cu (triangular) and B

DOI: 10.1021/acs.inorgchem.8b03334 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry light on the relative stability of these two classes of compounds on varying the coinage metal (see Supporting Information, Table S.5). Electron density (ρ) and potential energy density (V) values at Fe−M bond critical points (b.c.p.) are quite similar on comparing M3 and M4 clusters of the same metal, suggesting scarce variations of the Fe−M bond strengths, even if the M4 geometry appears slightly more favorable for Ag and Au with respect to Cu (see the M4-ρ/M3-ρ Fe−M b.c.p. electron density ratio depicted in Figure S.21). The positive values of the Laplacian of electron density (∇2ρ) and the negative energy density (E) values are in line with Bianchi’s20 characterization of metal−metal or dative bonds. Hirshfeld charges, collected in Table S.6, highlight comparable charge distributions on the metal centers between M3 and M4 clusters. The polarization of the M−Fe bonds is in agreement with the increment of the electronegativity along Group 11. In all the clusters here considered no b.c.p. for M−M interactions was found (the gradient norm of electron density is higher than zero along the M-M bonds), suggesting delocalized dispersion interactions. Ring critical points (r.c.p.) were instead localized for all the M3 and M4 clusters. Electron density at r.c.p. is in all the cases meaningfully lower in M4 clusters with respect to the corresponding M3 species, but the M4-ρ(r.c.p.)/M3-ρ(r.c.p.) ratio shows important variations on changing the metal center (Figure 2), being

Figure 3. Surface maps with projections of the Cu electron densities (a.u.) in the [{CuFe(CO)4}3]3− and [{CuFe(CO)4}4]4− clusters.

small increase of relative stability of Fe-M bonds in Ag and Au derivatives moving from M3 to M4; (II) the increase of metallophilic interactions in M4 clusters moving from Cu to Ag and Au. Because of this, polymerization isomerism in [{MFe(CO)4}n]n− (n = 3, 4) clusters may be observed for Ag and Au but not for Cu. Nonetheless, it is noteworthy that both the polymerization isomers [{MFe(CO)4}4]4− and [{MFe(CO) 4} 3]3− (M = Ag, Au) may be selectively synthesized just by choosing the appropriate synthetic protocol. The new case of polymerization isomerism herein described adds to the other isomerism types (enantiomerism, core isomerism, staple isomerism, complex isomerism, surface ligand isomerism, and dynamic permutational isomerism) previously reported for molecular metal clusters.1−11 Despite the different intrinsic nature of Au−thiolate and organometallic (carbonyl) molecular clusters, there seems to be some general analogies among these types of atomically precise clusters.21,22 Overall, as our knowledge of the molecular structures of metal clusters of increasing sizes is growing, concepts such as structural isomerism, that have been developed for molecular coordination and organometallic chemistry, find their way also in the field of metal clusters, nanoclusters, and nanoparticles.23−26

Figure 2. Average M4-ρ/M3-ρ electron density ratios (green line) and M4-V/M3-V potential energy density ratios (blue line) calculated from data at Mn r.c.p. of the clusters.

much lower for Cu with respect to Ag and Au derivatives. The M4-V(r.c.p.)/M3-V(r.c.p.) potential energy density ratio confirms the different behavior of copper derivatives (Figure 2). These trends can be explained on supposing that metallophilic interactions are lower in M4 clusters with respect to the analogous M3 species, but this lowering is more accentuated for [{CuFe(CO)4}4]4−, where cuprophilic interactions are weak. Comparable results were achieved on studying the M4-ρ/M3-ρ density ratios measured at the middle of M−M bonds (see Figure S.22). The electron densities associated to the coinage metals in the [{CuFe(CO)4}3]3− and [{CuFe(CO)4}4]4− copper clusters are shown as example in Figure 3, while those for the other clusters are depicted in Figure S.23. In conclusion, the preference of Cu toward Cu3 rather than Cu4 and the possibility of triangular and square geometries for Ag and Au can be ascribed to two different factors: (I) the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03334. Synthetic procedures, NMR spectra, igures of the molecular structures of 1−8, tables of main bond distances and angles of 1−8, X-ray crystallographic study, and computational details (PDF) C

DOI: 10.1021/acs.inorgchem.8b03334 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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Optimized coordinates in XYZ format (XYZ) Accession Codes

CCDC 1881203−1881208 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

*(S.Z.) E-mail: [email protected]. ORCID

Cristina Femoni: 0000-0003-4317-6543 Rita Mazzoni: 0000-0002-8926-9203 Stefano Zacchini: 0000-0003-0739-0518 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the University of Bologna for financial support REFERENCES

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DOI: 10.1021/acs.inorgchem.8b03334 Inorg. Chem. XXXX, XXX, XXX−XXX