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Journal of Organometallic Chemistry 728 (2013) 23e29

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Journal of Organometallic Chemistry

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Cyclic dimers of variable size, formed from FeCu carbide clusters:Synthesis, structure and electrochemical behaviour of[{Fe4C(CO)12Cu2(m-X)}2]n�, (X ¼ phenylthiolate, pyrazolate, (n ¼ 2) ordiphenolate (n ¼ 4))

Roberto Della Pergola a,*, Annalisa Sironi a, Massimo Moret b,*, Stefano Bergantin b, Patrizia R. Mussini c,Monica Panigati c

aUniversità di Milano-Bicocca, Dipartimento di Scienze Ambientali e del Territorio, piazza della Scienza 1, 20126 Milano, ItalybUniversità di Milano Bicocca, Dipartimento di Scienza dei Materiali, Via Cozzi 5, 20126 Milano, ItalycUniversità degli Studi di Milano, Dipartimento di Chimica, Via Golgi 19, 20133 Milano, Italy

a r t i c l e i n f o

Article history:Received 19 November 2012Received in revised form8 January 2013Accepted 9 January 2013

Keywords:IronCopperInterstitial carbideSubstitution reactionsElectrochemistryHeterometallic clusters

* Corresponding authors.E-mail addresses: roberto.dellapergola@unim

massimo.moret@mater.unimib.it (M. Moret).

0022-328X/$ e see front matter � 2013 Elsevier B.V.http://dx.doi.org/10.1016/j.jorganchem.2013.01.006

a b s t r a c t

The reaction of [Fe4C(CO)12(CuNCMe)2] with anionic units in 1:1 molar ratio systematically yields cyclicdimers of composition [{Fe4C(CO)12Cu2(m-X)}2]n�, where X are bridging ligands with different size, donoratom and charge: {X ¼ phenylthiolate, PhS�, (n ¼ 2 (1)), pyrazolate, C3H3N2

�, (n ¼ 2 (2)), diphenolate, (OeC6H4eC6H4eO2�, n ¼ 4 (3)}. All anions have been characterized by elemental analysis, IR and 1H NMRspectroscopy and by single crystal X-ray diffraction. They share the same structure with the known anion[{Fe4C(CO)12Cu2(m-Cl)}2]2� (X ¼ Cl, (4)), with the obvious difference in the size of the ring, depending onthe number of atoms in the spacer. All these macrocycles display rich electrochemical behaviour, un-dergoing two reversible reductions and two irreversible monoelectronic oxidations, poorly affected bythe substituent and therefore assigned to the Fe4C core. Electronic communication increases in the order3 z 4 > 1 > 2.

� 2013 Elsevier B.V. All rights reserved.

Aggregation of metal clusters with polytopic ligands may rep-resent a chemical approach to the bottom-up preparation ofnanostructures with well-defined composition, size and shape. Inthe field of carbonyl clusters progresses are limited by their scarcepropensity to carbonyl substitution and by the lack of suitable softligands [1]. These difficulties can be overcome by exploiting otherkinds of linkers, including metal cations [2] or functionalizing theclusters with electrophilic M-Lþ fragments [3]. Potentially, thesestudies would lead to the obtainment of coordination polymers ormolecular wires that, owing to the well known sink behaviour ofmetal clusters, could find application for nanoelectronic devices [4].As an example, Fe-Te-Cu [5] and RueTeeCu [6] clusters have beenused for the formation of 1D-polymers with dipyridine or halogenbridges, that were found to be semiconductors with dc conduc-tivities on the order of 10�2 U�1 cm�1.

ib.it (R. Della Pergola),

All rights reserved.

FeeCu heterometallic carbido clusters, [Fe4C(CO)12(CuNCMe)]�

and [Fe4C(CO)12(CuNCMe)2] [7], were found suitable for thesepurposes, since they can be assembled with ditopic donors (such aspyrazine or 4,40-dipyridine) in dimeric, oligomeric or 1D-polymericstructures [8].

In this paper are described the properties of some cyclic struc-tures of increasing size prepared from the neutral cluster andbridging anionic ligands with N, S, O donor atoms. The presentwork definitely assesses communication between FeCu clusterunits and gives further support to the possibility of electronicconduction along cluster-based 1D-chains, which was originallysupposed but could not be measured [8].

The combination of iron carbonyl clusters and Cu(I) complexeswas recently exploited for the photocatalytic water splitting [9]. Atthe same time, phenoxo-copper complexes are investigated forpolymerization of phenol [10], for catalytic oxidations [11] and fortheir magnetic properties [12]. Coordination of copper with thiolateand phenolate is extremely relevant to bioinorganic chemistry: theredox interplay between copper and ligand is the base for the

Table 1Reagents and yields for the syntheses of 1e3.

Ligand precursor Base Solvent Yield

1 Ph-SH NEt4OH THF 82%2 PzH K2CO3 THF 42%3 HOeC6H4eC6H4eOH NEt4OH THF 47%

1 Electronic Supplementary Information (ESI) available: Fig. S1 (the solid statestructure of 3, with relevant crystal data) and Fig. S2 (the composition of thefrontier orbitals of 2). CCDC905377 (1) and CCDC905378 (2) contain the supple-mentary crystallographic data for compounds 1 and 2. These data can be obtainedfree of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

R. Della Pergola et al. / Journal of Organometallic Chemistry 728 (2013) 23e2924

activity of many metalloproteins [13,14] and for the syntheses ofbiomimetic complexes [15].

1. Results

1.1. Synthesis and reactivity

The preparations of 1e3 proceed all by the same way, that is bymixing a THF solution of the cluster [Fe4C(CO)12(CuNCMe)2] witha solution containing the neutral ligand precursor (PhSH, pzH,dihydroxodiphenyl), and a strong base (NaH, NEt4OH, K2CO3). NaH,nominally the strongest deprotonating agent, was initially used, butits main drawbacks are the formation of Naþ salts. Therefore, thepreparation was followed by a metathesis step with bulky ammo-nium or phosphonium cations, (usually as BF4� salts, to avoid halideexchange). Afterwards, to introduce at once the deprotonatingagent and the counterion, ammonium hydroxide was preferred andNaH abandoned. The following Table 1 summarizes the best com-binations of ligand, solvent and base which allowed the highestyields for the three products, Scheme 1 collects the three substi-tution reactions.

It should also be remarked that [{Fe4C(CO)12Cu3(m-pz)2}2]2� (5),a product similar to 2, was already obtained and characterized [16].A mixture of the two products is always obtained, but the yield of 2is maximized if additional Cuþ ions and excess pzH are avoided.However, owing to the limited solubility of 2 in THF, the separationis straightforward.

1.2. Spectroscopic characterization

Clusters 1e3 were all characterized by IR and NMR spectro-scopy. The IR spectra of the three anions in solution are all verysimilar, in agreement with the same disposition of the carbonylligands, and consist of four sharp bands, the internal ones muchstronger than the external ones. The bands of 1 and 2 are almostcoincident, whereas they are definitely shifted to lower wave-numbers in 3, owing to the increased negative charge. The 1H NMRspectrum of 1 is composed by two multiplets, centered at 7.45 and7.00 ppm of relative intensity 2:3. The total area of these two peaksintegrate 5:8 with the methylenic signal of the NEt4þ cation, inagreement with the formula. The spectrum of 2 consists of twosinglets at 7.67 and 6.21 ppm, of relative intensity 2:1. This spec-trum is quite informative since 5, in the same conditions, showsthree equally intense singlets (at 7.66, 7.44, 6.28 ppm), in agree-ment with the lower symmetry and the non-equivalence of the two“outer” protons in the 3,5 positions of the pyrazolate ring. There-fore, NMR is a useful tool to quantify the 2:5 ratio in solution.Cluster 3 shows, in the zone of aromatic hydrogens, two sharpdoublets at 7.27 and 6.80 ppm. This spectrum suggests that the twoligands are able to rotate around their main axis, making equivalentthe internal and the external protons of the rings.

1.3. Solid state structures

The three octahedral dimers 1e3 are structurally related to theknown [{Fe4C(CO)12Cu2(m-Cl)}2]2�(4) [7].

All three anions possess the same crystallographic symmetry,sitting on crystallographic inversion centres, so that all individualstructural parameters can be easily compared.

Dimers 1e3 all contain two Cu sites in cis, with the organicspacers bridging two octahedral metal cores by linking them. Thestructures of 1 and 2 are shown in Figs. 1 and 2 with selected bondparameters, respectively (the crystal data of 3 are listed in the ESI1

and its structure is sketched in Fig. S1). The metal core is a dis-torted octahedron, with one edge much longer than the others.Interestingly, the long edge is not always the same. In 1 it is theFe2eCu2 bond, in the equatorial plane, which is elongated(2.9132(4) �A), a feature also observed in the dimer bridged bychloride [7].

On the contrary, in 2 the long interaction involves the Cu1eCu2edge (2.9731(5) �A). However, it has been repeatedly noticed thatCueCu and CueFe bonds in these structures must be very weak, sothat their lengthening is relatively easy. In general the octahedral[Fe4Cu2] cores as observed in 1e3 are somewhat expanded withrespect to the butterfly anion [Fe4C(CO)12]2� [17] by about 0.05 �A.The nature of the bridging ligands can be considered as the mainresponsible for the relevant differences found for the copperecopper distances. In 1 the m�SPh groups together with the inter-stitial carbide atom define a linear coordination around the copperatoms. The resulting Cu4S2 six-membered cycle exhibits a very flatchair conformationwith the sulphur atoms deviated by 0.2784(6)�Afrom the Cu4 plane. In anion 2, the exo-bidentate pyrazolate ligand,trans to the carbide C atom, forces a much larger intra-octahedralCueCu distance (2.9731(5) �A) and also between different clusters(3.5897(5) �A). The eight-membered Cu4N4 cycle is again very flatwith N1 and N2 lying 0.062(2) and 0.153(2)�A out of the Cu4 plane,respectively.

1.4. Electrochemistry

The redox behaviour of many substituted FeeCu clusters wasdetermined by cyclic voltammetry.

Originally, the cyclic voltammetry of [Fe4C(CO)12(CuCl)2]2� and[{Fe4C(CO)12Cu2(m-Cl)}2]2� were recorded in MeCN, and wereinterpreted as a reduction of the Cu(I) ions, with formation of ele-mental copper [7]. Since monomer and dimer showed exactly thesame features we deduced that halides do not allow electroniccommunication between the two cluster units. Afterwards, havingaccumulated more experience in the substitution reactivity of FeeCu carbide clusters, we realized that acetonitrile is not suitable forthis kind of experiments, since it can promptly add to copper andbreak the chloro bridges. Indeed the IR monitoring of the MeCNsolution of cluster 4 showed a slow change of the pattern of thecarbonyl stretching bands in agreement with the formation ofmonomeric species. This feature is moreover confirmed by thecomparison of the CV pattern of cluster 4 recorded in acetone so-lution. Indeed from two to three reduction peaks are perceivable inthe available potential window instead of a single one observed inMeCN [7], indicating how a significant change of the coordinatingcapability of the solvent can lead to significant differences in thepotentials and morphology of the CV peaks.

Other non cyclic dimers, based on [Fe4CCu2] cores, also dis-played poorly reversible processes. Conversely, the substitutedcluster 5 in acetone showed a greater redox flexibility, but the full

Scheme 1.

R. Della Pergola et al. / Journal of Organometallic Chemistry 728 (2013) 23e29 25

interpretation of its redox behaviour (even with the use of the IRspectroelectrochemistry technique) was ambiguous because ofsome uncertainness about its real nature in solution, either mon-omeric or dimeric [16].

Fig. 1. The solid state structure of anion 1, [{Fe4C(CO)12Cu2(m�SPh)}2]2� (primed atoms are gat 30% probability level. Selected bond parameters: Cu1eCu2 2.7476(4), Cu1...Cu20 3.2122(4),Fe3 2.7070(4), Cu2eFe4 2.6445(4), Fe1eFe2 2.5807(4), Fe1eFe3 2.7158(4), Fe1eFe4 2.6689(4S 174.51(5), CeCu2eS0 168.89(5)� .

Then, we took advantage of the isolation of the series of newdefinitely dimeric 1e3 clusters, whose ligands are potentially ableto support electronic communication, to reinvestigate thoroughlythe subject, including cluster 4, in a non-coordinating solvent.

enerated through the symmetry operation (�x, 1 � y, 1 � z). ADP ellipsoids were drawnCu1eFe1 2.6477(4), Cu1eFe3 2.6912(4), Cu1eFe4 2.6948(4), Cu2eFe2 2.9132(4), Cu2e), Fe2eFe3 2.6821(4), Fe2 Fe4 2.6826(5), Cu1eS 2.1927(5), Cu2eS 2.1823(5)�A; CeCu1e

Fig. 2. The solid state structure of anion 2, [{Fe4C(CO)12Cu2(m-pz)}2]2� (primed atoms are generated through the symmetry operation (�x, 1 � y, 1 � z). ADP ellipsoids were drawn at30% probability level. Selected bond parameters: Cu1eCu2 2.9727(4), Cu1...Cu20 3.5897(5), Cu1eFe1 2.7836(5), Cu1eFe3 2.6224(5), Cu1eFe4 2.6533(5), Cu2eFe2 2.6018(5), Cu2eFe3 2.7697(5), Cu2eFe4 2.6281(5), Fe1eFe2 2.5444(5), Fe1eFe3 2.6875(6), Fe1eFe4 2.6777(5), Fe2eFe3 2.6728(6), Fe2eFe4 2.7705(5), Cu1eN1 1.895(2), Cu2eN2 1.906(2) �A; CeCu1eN1 176.37(11), CeCu2eN2 174.01(10)� .

R. Della Pergola et al. / Journal of Organometallic Chemistry 728 (2013) 23e2926

Fig. 3 shows the redox activity of the anionic clusters 1e3 inacetone solution and the corresponding oxidation and reductionpeak potentials are reported in Table 2. In the cathodic region(Fig. 4) all of the investigated clusters display three successive re-ductions having some common features.

The first two reduction peaks appear to be much more chemi-cally reversible than those observed for the cluster 4 in MeCN so-lution [7] (the ratio ipa/ipc progressively increases from about 0.6 tothe unit value with increasing scan rate); moreover, their half-peakwidth (w57 mV) together with the scan rate dependency of thepeak potential (nearly zero Ep vs log v slopes) are consistent with anelectrochemically reversible monoelectronic transfer. Concurrently,a one-electron stoichiometry can be assigned to the same peaks bycomparing their normalized limiting current density after peakconvolution [18] with the normalized limiting current densityobserved for the Cp*2Feþ/Cp*2Fe couple adopted as one-electronredox standard.

Both of these two reduction peaks are not connected withcopper deposition, since the corresponding dissolution peak isabsent.

Fig. 3. Normalized CV features of the clusters 1, 2 and 3 on GC electrode inacetone þ (NBu4)PF6 0.1 M solution, scan rate 0.2 V s�1 recorded with ohmic dropcompensation (full range).

This latter peak might instead be perceivable (at the potential atwhich it is perceived in THF, and therefore not at the same potentialobserved for 4 in MeCN solution) [7] including in the scan the thirdpeak (for 1) or the background onset (for 3), in case of cluster 2 itmight happen, too, but the position of the redissolution peak issignificantly different probably due to some coordination effect bya suitable species possibly present or locally formed (see Fig. 5).Moreover the extent of the chemical irreversibility is associatedwith the intensity of the anodic stripping peak (indicated with anasterisk) observed in the backscan, which is therefore much moreintense for the cluster 3.

Accordingly, we could assume that:

1. More than the molecular structure, the solvent is determiningon the redox features of this family of clusters, so that complex[Fe5C(CO)14(CuCl)]2�, albeit having a different Fe5CCu metalcore [7], shows a CV pattern much more similar to our dimersthan that observed for [{Fe4Cu2C(CO)12(m-Cl)}2]2� (4) recordedin MeCN solution.

2. The first two reduction processes are to be attributed to theFe4C core(s) since they do not generate, in the backscan, theanodic stripping peak, usually observed for the oxidation of theelectrodeposited copper metal. This is also confirmed by thefact that the nature of the p conjugated linkers only slightlyaffects the first and second reduction peak potential andmorphology.

On the other hand, the presence of two separated mono-electronic reduction peaks, one for each Fe4C group, confirms theoccurrence of the electronic communication between the two

Table 2Reduction and oxidation peak potentials (V vs. SCE) of the complexes 1e4 in acetonesolution and the corresponding log Kcom values.

Complex Ep,cI [V] Ep,cII [V] Ep,aI [V] Ep,aII [V] log Kcom

1 �1.30 �1.50 þ 0.56 þ0.71 3.42 �1.37 �1.54 þ0.73 þ0.90 2.93 �1.45 �1.68 þ0.71 þ0.90 3.94 �1.25 �1.47 3.7

Fig. 4. Normalized CV features of the cluster 1e4 on GC electrode in acetone þ (NBu4)PF6 0.1 M solution, scan rate 0.2 V s�1 recorded with ohmic drop compensation(cathodic region).

R. Della Pergola et al. / Journal of Organometallic Chemistry 728 (2013) 23e29 27

different units in the dimeric clusters. In order to understand thenature of these mixed-valence compounds and therefore the de-gree of the delocalization of the charge, directly related to theextent of the electronic communication, the comproportionationconstant Kcom has been calculated. Taking into account that for one-electron transfers at 25 �C it holds that:

ln Kcom ¼ FDE�=RT

or

Kcom ¼ 10ð16:90DE� Þ at 298 K

where DE� is the separation between the two reduction peaksexpressed in volts [19]. Therefore, on the base of the data reportedin Table 2, the calculated values of Kcom are ca. 103e104. Using theso-called Robin-Day classification, these values of Kcom clearlyindicate that the clusters 1e4 belong to the Class II in which thecharge is slightly delocalized between the two redox centres [20].

Even if all the clusters belong to the same Class, taking intoaccount the difference between the two reductive peak potentials(Ep,cIeEp,cII), the extent of this electronic communication increasesin the order 3z 4>1>2. This series clearly does not correlatewiththe number of atoms in the cycles, nor with the presence of

Fig. 5. Normalized CV features of the cluster 1e3 on GC electrode in acetone þ (NBu4)PF6 0.1 M solution, scan rate 0.2 V s�1 recorded with ohmic drop compensation(extended cathodic range).

aromatic rings in the spacer. It can be however related tothe number of non-bonding electron pairs in the donor atom (O]Cl > S > N), and possibly with its p-donor properties.

The anodic behaviour of the three anions is displayed in Fig. 6.Apart from two oxidation peaks at very low potential observed forthe anion 3 and attributed to the partial oxidation of the aromaticbridging ligand, all the clusters exhibit two chemically irreversiblemonoelectronic oxidation peaks which are attributed to the oxi-dation of the Fe4C cages. As already observed for the reduction, theoxidation peaks occur at less positive potentials (0.1e0.2 V) for 1than for the other two clusters. The presence of two separatedpeaks further confirms the electronic communication between thetwo metal-carbonyl units.

To support the assignment of electrochemical processes, EHMOcalculations were performed with CACAO [21] on anion 2, underthe symmetry constraint of crystallographic Ci symmetry. The re-gion of the frontier orbitals is constituted by two couples of almostdegenerate molecular orbitals, with a large HOMOeLUMO gap(1.85 eV). All the MOs in this range are iron-centred, as shown bythe Reduced Charge Matrix, shown in Fig. 7: the collective con-tribution of the d orbitals of iron was calculated much higher(>60%) than the copper contribution (<15%). The atomic orbitalsinvolved in these MOs are schematically represented in Fig. S2 ofthe ESI.1 These results give a coherent picture on thewhole series ofFe4CCu2, Fe5CCu and Fe4CCu clusters, whose HOMOeLUMO levelswere obtained from more accurate DFT calculations [16].

2. Experimental

All the solvents were purified and dried by conventionalmethods and stored under nitrogen. All the reactions were carriedout under oxygen-free nitrogen atmosphere using the Schlenk-tubetechnique [22]. [Fe4C(CO)12(CuNCMe)2], was prepared by literaturemethods [7]. Infrared spectra in solutionwere recorded on a NicoletiS10 spectrophotometer, using calcium fluoride cells previouslypurged with N2. Elemental analyses were carried out by the staff ofLaboratorio di Analisi of the Dipartimento di Scienze Ambientali.

2.1. Synthesis of (NEt4)2[{Fe4C(CO)12Cu2(m-SPh)2}] ([NEt4]21)

[Fe4C(CO)12(CuNCMe)2] (0.14 g; 0.177 mmol) was dissolved in10 mL of THF. In a different vessel 18 ml of PhSH and 180 ml ofa solution of NEt4OH in methanol (1.5 M, corresponding to0.27 mmol of hydroxide) were mixed. The first solutionwas layeredwith the second, and allowed to diffuse slowly, obtaining black

Fig. 6. Normalized CV features of the cluster 1 (bold line), 2 (thin line) and 3 (brokenline) on GC electrode in acetone þ (NBu4)PF6 0.1 M solution, scan rate 0.2 V s�1

recorded with ohmic drop compensation (anodic range).

Fig. 7. The frontier region of the MO’s of 2 and the corresponding Reduced Charge Matrix Diagram, showing the contribution of Cu (left), Fe (central) and all other elements (right).

R. Della Pergola et al. / Journal of Organometallic Chemistry 728 (2013) 23e2928

crystals. When the mixing was complete, the mother liquor wasdiscarded, the solid was washed with 2-propanol (2 � 5 mL) anddried. Yield 0.136 g, 82%.

Calc for C54H40S2Cu4Fe8N2O24 C 34.57; H 2.69; N 1.49%; found C34.58; H 2.82; N 1.57%.

n(CO) in acetone : 2041m, 2007s, 1977vs, 1949 m cm�1.1H NMR in d6-acetone: 7.00 (3, m) and 7.45 (2,m) ppm [C6H5S].

Table 3Crystal data and experimental details for 1 and 2.

Compound 1 2

Empirical formula C54H50Cu4Fe8N2O24S2 C48H46Cu4Fe8N6O24

Formula weight 1876.04 1791.66Temperature/K 130(2) 110(2)Crystal size/mm3 0.20 � 0.25 � 0.45 0.10 � 0.15 � 0.23Crystal system Triclinic MonoclinicSpace group P-1 I2/aa/�A 9.648(1) 19.405(1)b/�A 13.386(1) 15.634(1)c/�A 13.721(1) 20.420(1)a/� 69.901(2) 90.0b/� 77.382(2) 94.185(2)g/� 86.499(3) 90.0V/�A3 1623.7(2) 6178.4(6)Z 1 4

2.2. Synthesis of (NEt4)2[{Fe4C(CO)12Cu2(m-pz)2)] ([NEt4]22)

[Fe4C(CO)12(CuNCMe)2] (0.17 g 0.22 mmol), pyrazole (0.016 g;0.23mmol) and excess K2CO3 (0.100 g; 0.72mmol) were suspendedin 12 mL of THF. The mixture was allowed to stir until IR showedcomplete conversion (about 2 h).

The solvent was removed in vacuum and the black residue wasdissolved inmethanol. NEt4BF4 (0.50 g) was added, and the productwas precipitated by addition of water. The precipitate was collectedby filtration, washed with water, dried and dissolved in the mini-mum amount of acetone (5 mL).

Crystals have been obtained by layering with 2-propanol, andallowing slow diffusion.

Yield 0.094 g (42%).Calc for C48H46Cu4Fe8N6O24 C 32.16; H 2.57; N 4.69%; found C

32.27; H 2.68; N 4.63%.n(CO) in THF: 2043w, 2006s, 1976vs, 1942 mw cm�1.1H NMR in d6-acetone: 7.67 (2, m) and 6.21 (1,m) [C3H3N2].

Calcd. density/Mg m�3 1.919 1.926m/mm�1 3.152 3.245Index ranges �11 � h � 12 �25 � h � 25

�17 � k � 17 �20 � k � 20�17 � l � 17 �26 � l � 26

q-range/� 2.16e27.52 2.55e27.60No. of collected reflns 31,119 41,975No. of indep. reflns 7440 7085No. of obsd reflns [I > 2s(I)] 7312 6671Rint 0.0656 0.0738No. of parameters 428 404Goodness-of-fit (F2) 1.157 1.173R1, wR2 [I > 2s(I)] R1 ¼ 0.0295 R1 ¼ 0.0385

wR2 ¼ 0.0781 wR2 ¼ 0.0802R1, wR2 (all data) R1 ¼ 0.0302 R1 ¼ 0.0472

wR2 ¼ 0.0786 wR2 ¼ 0.0836Completeness to q/% 99.5 98.6Largest diff. peak and hole/e �A�3 0.572, �0.862 1.039, �0.939

2.3. Synthesis of (NEt4)4[Fe4C(CO)12(Cu2(m-C12H8O2)2)] ([NEt4]43)

[Fe4C(CO)12(CuNCMe)2] (0.20 g 0.26 mmol) was dissolved in10 mL THF. In a different vessel, 4,40-dihydroxybiphenol (0.05 g,0.25 mmol) and 350 ml of a solution of NEt4OH in methanol (1.5 M,corresponding to 0.51 mmol of hydroxide), were mixed in 8 mL of2-propanol. The first solution was layered with the second, andallowed to diffuse slowly, obtaining black crystals. When the mix-ing was complete, the mother liquor was discarded, the solid waswashed with 2-propanol (2 � 5 mL) and dried. Yield 0.318 g, 47%.

Calc for C82H96Cu4Fe8N4O24 C 43.07; H 4.23; N 2.45%; found C43.01; H 4.29; N 2.39%.

n(CO) in acetone : 2038 mw, 1991 s, 1968s, 1944 mw cm�1.1H NMR in d6-acetone: 7.27 (4, d) and 6.80 (4, d) ppm [C12H8O2].

2.4. X-ray single crystal structure determinations

Data collections for 1, 2, and 3 were performed on a Rigaku R-Axis II diffractometer equipped with an SHINE monochromator(MoKa radiation l ¼ 0.71075 �A) and an image plate detector. Thecollected intensities were corrected for Lorentz, polarisation factorsand absorption using CrystalClear-SM Expert 2.0� suite of pro-grammes. The structures were solved by direct methods [23] andrefined by full-matrix least-squares against Fo2 using SHELXL97[24] (Table 3). ORTEP plots were produced with programme Mer-cury CSD 3.0. Diffraction data for 3 were of too low quality to allow

R. Della Pergola et al. / Journal of Organometallic Chemistry 728 (2013) 23e29 29

a complete structural refinement. It was however possible to con-firm the molecular structure of anion 3. Anisotropic displacementparameters were assigned to all non-H atoms in 1 and 2, whereasthe H atoms have been treated with riding models. CCDC referencenumbers: CCDC905377 (1) and CCDC905378 (2).

2.5. Electrochemical measurements

The cyclovoltammetric study of the clusters has been performedat scan rates typically ranging from 0.02 to 2 V s�1, in acetone(CHROMASOLV� Plus) solutions at 2.5 � 10�4e5 � 10�4 M concen-tration in each substrate, deareated by N2 bubbling, with 0.2 Mtetrabutylammonium hexafluorophosphate ((NBu4)PF6; Fluka) asthe supporting electrolyte, at 298 K. The ohmic drop has beencompensated by the positive feedback technique [25]. The experi-ments were carried out using an AUTOLAB PGSTAT potentiostat(EcoChemie, The Netherlands) run by a PC with GPES software. Theworkingelectrodewas a glassycarbonone (AMEL, diameter 1.5mm)cleaned by diamond powder (Aldrich, diameter 1 mm) on awet cloth(STRUERS DP-NAP); the counter electrode was a platinumwire; thereference electrode was an aqueous saturated calomel electrode,having in our working medium a difference of �0.385 V versus theFcþ/Fc couple (the intersolvental redoxpotential reference currentlyrecommended by IUPAC) [26]. The Cp*2Fe has been used as one-electron electrochemically reversible standard for the comparativecurrent analysis of the first reduction peak [27].

3. Conclusions

The overall dimensions of 1, 2 and 3, measured as the distancebetween the outmost, symmetry related iron atoms, are 0.93, 0.96and 1.76 nm, respectively. Therefore, their synthesis representa chemical way to get into the nanometric regime, controlling thesize of large hybrid (metal and organic) rings.

Comparison of the structures of 1 and 3 suggested that diphe-nolate could bind to the Fe4C(CO)12Cu2 cluster in a m4, k2O,O0 mode(bidentate-tetrabridging), with each oxygen bridging between twoclusters. This arrangement would allow a new coordination poly-mer, with the [{Fe4C(CO)12Cu2(m-X1/2)}2]2� repeating unit. All at-tempts to obtain such a material with X ¼ diphenole itself,bisthiodiphenol, 1,4-bispyrazolyl benzene and 1,4-bistetrazolylbenzene [28], failed so far.

Acknowledgements

This work was supported by University of Milano Bicocca (RDP,AS and MM).

Appendix A. Supplementary material

Supplementary material related to this article can be found athttp://dx.doi.org/10.1016/j.jorganchem.2013.01.006.

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