Highly Reduced Ruthenium Carbide Carbonyl Clusters: Synthesis, Molecular Structure, Reactivity, Electrochemistry, and Computational Investigation of [Ru6C(CO)15]4–

The reaction of [Ru6C(CO)16]2– (1) with NaOH in DMSO resulted in the formation of a highly reduced [Ru6C(CO)15]4– (2), which was readily protonated by acids, such as HBF4·Et2O, to [HRu6C(CO)15]3– (3). Oxidation of 2 with [Cp2Fe][PF6] or [C7H7][BF4] in CH3CN resulted in [Ru6C(CO)15(CH3CN)]2– (5), which was quantitatively converted into 1 after exposure to CO atmosphere. The reaction of 2 with a mild methylating agent such as CH3,I afforded the purported [Ru6C(CO)14(COCH3)]3– (6). By employing a stronger reagent, that is, CF3SO3CH3, a mixture of [HRu6C(CO)16]− (4), [H3Ru6C(CO)15]− (7), and [Ru6C(CO)15(CH3CNCH3)]− (8) was obtained. The molecular structures of 2–5, 7, and 8 were determined by single-crystal X-ray diffraction as their [NEt4]4[2]·CH3CN, [NEt4]3[3], [NEt4][4], [NEt4]2[5], [NEt4][7], and [NEt4][8]·solv salts. The carbyne–carbide cluster 6 was partially characterized by IR spectroscopy and ESI-MS, and its structure was computationally predicted using DFT methods. The redox behavior of 2 and 3 was investigated by electrochemical and IR spectroelectrochemical methods. Computational studies were performed in order to unravel structural and thermodynamic aspects of these octahedral Ru–carbide carbonyl clusters displaying miscellaneous ligands and charges in comparison with related iron derivatives.

It was recently reported that treatment of [Fe 6 C(CO) 16 ] 2− with NaOH in DMSO or Na/naphthalene in THF afforded a highly reduced [Fe 6 C(CO) 15 ] 4− anion. 56Herein, we report the synthesis and structural characterization of the Ru analogue [Ru 6 C(CO) 15 ] 4− , as well as the study of its reactivity that resulted in further new Ru 6 C carbide carbonyl clusters.All the new compounds have been spectroscopically characterized and their structures determined by single-crystal X-ray diffraction (SC-XRD).The redox chemistry of the highly reduced [Ru 6 C(CO) 15 ] 4− cluster was investigated by electrochemical and IR spectroelectrochemical methods.All the new findings were further supported by computational studies employing DFT methods.2), which was isolated as [NEt 4 ] 4 [2]• CH 3 CN crystals suitable for SC-XRD after working up of the reaction mixture (see Section 4 for details).As previously reported for the synthesis of [Fe 6 C(CO) 15 ] 4− , 56 formation of 2 is likely to proceed via nucleophilic attack of OH − to a coordinated CO of 1, followed by reduction of the cluster and elimination of CO 2 and H 2 O according to eq 1.This is a welldocumented reaction in the chemistry of metal carbonyl clusters. 7,8,16This mechanism has been further supported by DFT studies (see Section 2.4).(1)

RESULTS AND DISCUSSION
The reaction was easily monitored by IR spectroscopy (Scheme 1), since the ν CO bands of 1 [ν CO 1981(vs), 1763(m) cm −1 , Figure S1 in the Supporting Information] were significantly red shifted after reduction to 2 [ν CO 1890(vs), 1708(m) cm −1 ; Figure S2 in the Supporting Information].Alternatively, 2 could be obtained by treating 1 with Na/ naphthalene in THF.The tetra-anion 2 was readily protonated to [HRu 6 C(CO) 15 ] 3− (3) (see below for its characterization and Figure S3 in the Supporting Information) and, often, the two compounds were obtained in mixture.They could be separated owing to their different solubility in organic solvents.Indeed, as [NEt 4 ] + salts, 3 was soluble in acetone and 2 in CH 3 CN.It must be noted that improved yields of 2 could be obtained by adding dropwise the crude DMSO reaction mixture to a solution of [NEt 4 ]Br in H 2 O/ i PrOH.In this way, [NEt 4 ] 4 [2] precipitated immediately and the formation of 3 was limited.Conversely, addition of a saturated water solution of [NEt 4 ]Br to the crude DMSO reaction mixture caused the precipitation of [NEt 4 ] 4 [2] in a mixture with a significant amount of [NEt 4 ] 3 [3].
Complete conversion of 2 into 3 could be achieved after treating the former with a stoichiometric amount of HBF 4 • Et 2 O as monitored by IR spectroscopy (Figure 1).The molecular structure of 3 was fully unraveled by SC-XRD as [NEt 4 ] 3 [3]  The molecular structures of 2 and 3 are both based on a carbide-centered Ru 6 C octahedron (Figure 2).There are 12 terminal carbonyls and, in addition, 3 μ-CO ligands are bridging 3 nonconsecutive Ru−Ru edges.Thus, each Ru atom is bonded to two terminal and one μ-CO ligand.The μ-H hydride ligand of 3 is located on a further Ru−Ru edge.The Ru−Ru and Ru−C carbide distances of 2 and 3 are very similar (Table 1).−53 This is probably the result of the compensation of two opposite effects: increasing the negative charge in the series 4, 1, 3, 2 should lead to increased bonding distances, whereas decreasing the number of ligands in the same series should reduce steric hindrance and, thus, the bonding parameters.The new clusters 2 and 3, as well as the previously reported 1 and 4, possess 86 CVE, as expected for an octahedral cluster. 55The reaction of 2 with CH 3 I led to a purported methylated species [Ru 6 C(CO) 14 (COCH 3 )] 3− (6) as evidenced by IR, 1 H NMR, and ESI-MS analyses (Figures S7, S8, S15, and S20 in the Supporting Information).Indeed, the IR spectrum of 6 [ν CO 1939(vs), 1748 cm −1 ] is very similar to that of 3 [ν CO 1935(s), 1746(m) cm −1 ], in view of the fact that both possess a 3− charge.Nonetheless, 6 does not show any hydride resonance in the 1 H NMR spectrum, but reveals the presence of a singlet at δ H 3.64 ppm, attributable to the −OCH 3 group.Moreover, the ESI-MS spectrum of 6 recorded in CH 3 CN displays a negative ion at m/z 526, corresponding to [Ru 6 C(CO) 14 (COCH 3 )] 2− that results from monoelectron oxidation of 6 during ESI-MS analysis.The 2− charge of the ion in the mass spectrum is corroborated by a secondary peak at m/z 512, corresponding to the loss of a CO ligand from the oxidized 6.Unfortunately, all attempts to crystallize 6 failed, hampering its complete structural characterization.Indeed, during crystallization, 6 partially decomposes and, among the decomposition products, it has been possible to identify 5, as mentioned above.Thus, the structure of 6 has been computationally determined using DFT methods (Section 2.4).In all cases, the formulation of 6 must be done with care, since the present formulation is based solely on indirect and not very well resolved data.4).In all cases, by carefully choosing the experimental conditions, it was possible to minimize the formation of 4, resulting in almost pure 7 after work-up of the reaction mixture (see Section 4 for details, and Figures S6 and S9 in the Supporting Information for the IR spectra).

Oxidation
Cluster 4 was previously obtained from the reaction of 1 with acids. 51The structure of 4 was previously reported and, thus, it will be only briefly described herein and compared to 7. 51 They are both based on a common Ru 6 C-carbide-centered octahedral core with similar bonding parameters (Figure 5 and Table 1).There are 12 terminal carbonyls, 2 per each Ru atom, in both clusters and, in addition, 7 contains 3 μ-CO ligands, whereas 4 μ-CO ligands are present in 4. The unique hydride of 4 is in an edge-bridging position.All the three hydride  Inorganic Chemistry ligands of 7 are also edge bridging, two on the same face of the octahedron, whereas the third hydride is located on an adjacent edge.The trihydride nature of 7 was further corroborated by VT 1 H NMR studies (Figure 6).The lower field resonance (2H) is broader than the higher field one (1H), suggesting that a further splitting could occur at even lower temperatures, in accordance with the SC-XRD structure where all three hydrides are not equivalent.Unfortunately, it was not possible to reach such a temperature.The structure of 7 can be also compared to 3. They both possess 12 terminal CO (2 per each Ru atom) and 3 edgebridging CO, but their stereochemistry is slightly different (Figures 2 and 5), likely because of their different anionic  charges and different number of hydride ligands.Thus, the three μ-CO ligands of trianionic 3 are located on three Ru−Ru edges without any Ru atom in common (not connected edges), whereas two μ-CO ligands of monoanionic 7 are located on two Ru−Ru edges with a common Ru atom, and the third μ-CO ligand on a not connected edge.
The molecular structure of 8 is based on an octahedral Ru 6 C core displaying one μ-CO ligand and one μ-imidoyl CH 3 CNCH 3 ligand on two not-connected edges (Figure 7).

Inorganic Chemistry
The two Ru atoms not bonded to any edge bridging ligand bear three terminal carbonyls each, whereas the other four Ru atoms bear only two.The μ-imidoyl CH 3 CNCH 3 ligand can be described as a three-electron donor if considered as a neutral ligand.Thus, overall 8 possesses 86 CVE as expected for an octahedron.The same μ-imidoyl CH 3 CNCH 3 ligand was previously found in HOs 3 (CO) 10 (CH 3 CNCH 3 ). 57The Ru− Ru and Ru−C carbide bonding contacts are comparable to those found in other clusters containing the same Ru 6 C core.The electrochemical conversion of Ru 6 C(CO) 17 into 1 and the back oxidation of 1, in the presence of CO, to give Ru 6 C(CO) 17 was previously reported. 58Controlled bulk electrolysis of [Ru 6 C(CO) 16 ] 2− (1) performed under a steady stream of carbon monoxide was reported to consume 1.9 electrons and to produce Ru 6 C(CO) 17 in near quantitative yield; on the other hand, the reduction of the neutral cluster at the suitable potential, under a constant stream of argon gas, was reported to consume 1.9 electrons and to give back 1 in quantitative yield.
We voltammetrically observed one irreversible reduction of 1 at the potential of −1.77V vs Ag/AgCl and IR-SEC experiments confirmed the presence of the reduced tetraanion 2 [ν CO 1890(vs) and 1708(m) cm −1 ] in the mixture of the reaction products (Figure S22 in the Supporting Information).The reduction of a CH 3 CN solution of 1 by controlled bulk electrolysis at a platinum gauze electrode at −1.8 V under a steady stream of argon required 1.73 mol of electrons per mol of cluster and an IR spectrum of the solution showed the presence of a mixture of products in which 2 constituted only a minor component, according to results of the spectroelecrochemical analysis.
The cyclovoltammetric profiles of the new clusters 2 and 3 in CH 3 CN/[NBu 4 ][PF 6 ] 0.1 M solution, under an inert atmosphere, are reported in Figures 8 and 9, respectively.
For the tetraanion 2, one electrochemically and chemically reversible oxidation and one oxidation complicated by subsequent chemical reactions are observed at −0.60 and −0.42 V formal potentials, respectively, and these are followed by three irreversible processes at higher potentials (E pa = +0.25,+0.52, and +0.83 V, respectively).A controlled potential coulometric measurement was performed at the potential E w = −0.2V in correspondence of both the first two very near oxidations of Figure 8 and proved that 1.8 electrons per mol of cluster were involved.An IR spectrum of the solution at the end of the electrolysis confirmed the quantitative formation of 5 (see also in situ IR SEC experiments).
Six oxidation processes characterize the cyclic voltammetry response of the hydride tri-anionic derivative 3; relatively fast chemical reactions accompany all the electron removals.The first two oxidations recall in the shape those of the parent 2 and, accordingly to the decreased negative charge, are shifted toward more anodic potentials (−0.41 and −0.29 V); further oxidations occur at +0.04, +0.22, +0.46, and +0.68 V, respectively (half-wave potential values for all the processes obtained by the differential pulse voltammetry, DPV).By the comparison of the peak currents of cyclic voltammetries of 2 and 3, we concluded that the same electron number, namely two, is involved in the first two unresolved oxidation redox steps.
The oxidation processes of 2 and 3 were investigated by in situ IR-SEC in an optically transparent thin-layer electrochemical (OTTLE) cell. 59igure 10    wavenumbers points out an oxidation of the hydride accompanied by a partial decomposition of the oxidized cluster and the initial spectrum was not completely restored in the reverse reduction scan (Figure 11b).
A thorough analysis of the IR spectra and the profile of the i/E curve made it possible to separate the complete sequence of the IR spectra in two groups, each belonging to a different redox step.The sequence of spectra showed in Figure S24a in the Supporting Information was collected by increasing the WE potential from −0.1 to + 0.4 V (vs Ag pseudoreference electrode).The IR spectrum of the starting cluster was reobtained in the back-scan (Figure S24c in the Supporting Information) when the WE potential returned to a reducing value (Figure S24b in the Supporting Information), indicating a full chemical reversibility of the first redox process.
The spectra of 3 and of the products of its first and second oxidation are reported in Figure S25a in the Supporting Information.Upon oxidation, the profiles appear to be complicated by the presence of multiple overlapping bands, presumably due to deprotonation equilibria following the decrease of the negative charge in the CH 3 CN polar solvent.In an attempt to gain more information, we obtained the differential absorbance spectra (Figure S25b in the Supporting Information) where the initial spectrum of 3 (red line) and that of its first oxidation product (green line) are used to calculate, respectively, the difference spectra shown in black and red of Figure S25b in the Supporting Information.The absorbance maximum at 1980 cm −1 appears to be related to a monooxidation product of 3, stable in the time scale of the spectroelectrochemical experiment, while the band at 2018 cm −1 , that emerges in the difference spectrum after the second oxidation, can be tentatively attributed to a relatively instable monoanionic hydride that, as 5, attains the 86 CVE through CH 3 CN coordination, and, eventually, to its decomposition products.
The electrochemistry of [Ru 6 C(CO) 17−x ] 2x− (x = 0−2) is dominated by the stability of the 86 CVE species, so the twoelectron reduction of the neutral Ru 6 C(CO) 17 is accompanied by the dissociation of a CO ligand to give 1 58 that, in turn, can be further reduced losing CO to produce 2. The reverse oxidation process of 1 yields a stable cluster only in the presence of CO whose coordination accompanies the twoelectron removal.However, the oxidation of 2 in CH 3 CN solution appears to be a chemically reversible process in the absence of CO and the coordination of the solvent ensuring the attainment of 86 CVE.Moreover, bulk electrolysis and IR SEC experiments proved the electrochemical oxidation of 2 to be an improved synthesis of 5 with respect to the chemical one, typified by the absence of byproducts.

Computational Investigations.
The DFT study on the ruthenium carbide carbonyl clusters, carried out at C-PCM/PBEh-3c level (DMSO as implicit solvent), studied the Gibbs energy variations related to the formation of 3 and 2 from 1 and OH − .For the sake of comparison, the calculations were also conducted on the analogous iron species at the same theoretical level. 56The relative Gibbs energy values are reported in Figure 12.The hydride position on a Ru−Ru edge in 3 was confirmed by the calculations (computed Ru−H distances 1.813 and 1.828 Å).The shift of the ν CO stretching at lower wavenumbers moving toward more reduced species was confirmed by the unscaled simulated IR spectra and is shown in Figure S26 in the Supporting Information.For these compounds and the others computationally investigated (vide infra), the reduction of the computed wavenumbers by about 6% on applying suitable scaling factors allowed a good superposition with the experimental data.
The    56 and therefore, it is probably related to its faster decomposition with respect to the analogous ruthenium species and not to its acidity.
Computational studies were then addressed to selected products derived from the oxidation of 2. The simple removal of two electrons should lead to the unsaturated species [Ru 6 C(CO) 15 ] 2− (9), whose structure was simulated at C-PCM/PBEh-3c for the sake of clarity, despite the fact that only its CH 3 CN adduct 5 was experimentally isolated and structurally characterized.The DFT-optimized structure of 9 is comparable to that of 2 at the same theoretical level, the root-mean-square deviation (RMSD) between the two geo-metries being 0.453 Å.The similarity between the two clusters can be also observed from the superposition reported in Figure S27 in the Supporting Information.The simulated IR spectra of the two compounds, shown in Figure S27 in the Supporting Information, highlight the expected shift of the carbonyl stretching at higher wavenumbers for the less reduced 9.The comparisons of the electron density (ρ) and potential energy density (V) values at Ru−carbide (3,−1) bond critical points (BCPs) indicate a slight weakening of the Ru−carbide bonds for the less reduced species.The Hirshfeld charges of the central cores are 0.187 and −0.097 a.u., respectively, for 9 and 2; therefore, most of the charge variation between the two clusters is accounted by the carbonyl ligands.The different core charges are related to the ruthenium centers, while the partial charge of the central carbide remains almost constant (Table S1 in the Supporting Information).
The experimental observation of the CH 3 CN derivative 5 is justified by the very negative Gibbs energy variation for the reaction 9 + CH 3 CN → 5, ΔG = −25.3kcal mol −1 .The DFToptimized structure of 5 is in line with the X-ray outcomes, with RMSD value of 0.311 Å.The carbonyl regions of the unscaled simulated IR spectrum and the ground-state computed geometry are shown in Figure S28 in the Supporting Information.The computed ν CO wavenumbers are quite similar to those calculated for 9, despite the presence of the additional ligand CH 3 CN.Good agreement was observed from the comparison of the experimental and simulated IR spectra after proper scaling of the computed data (best scaling factor for 5 = 0.94).
As expected, CH 3 CN is a weaker ligand with respect to CO, given the Gibbs energy reaction for the reaction 5 + CO → 1 + CH 3 CN, −21.8 kcal mol −1 .
Another cluster formally derived from the intermediate 9 is the trihydride [H 3 Ru 6 C(CO) 15 ] − (7) that can be considered as  S2) on the hydrogen atoms of 7 and 3 are quite similar (−0.102 and −0.110 a.u., respectively), whereas the charge on the hydride of 4 is slightly less negative (−0.087 a.u.).This is probably due to the presence of a further carbonyl ligand in 4 compared to 3 and 7.
Another product of the reaction between 2 and CF 3 SO 3 CH 3 is the cluster 8, with the [CH 3 CNCH 3 ] + ligand in the coordination sphere.The DFT-optimized structure is in line with the X-ray outcomes, having an RMSD value of 0.310 Å.The carbonyl region of the simulated IR spectrum and the ground-state computed geometry are shown in Figure S30 in the Supporting Information.The less negative charge of 8 with respect to 5 probably accounts for the slight increase of computed terminal ν CO frequencies (compare Figure S30 and S28 in the Supporting Information).The unscaled ν CN stretching is predicted (with low intensity) at 1757 cm −1 .For comparison, the calculated ν CN stretching of 5 is 2490 cm −1 , in line with the change of CN bond order.The Hirshfeld charges of the {Ru 6 C} core in 8 is 0.309 a.u., meaningfully more positive with respect to 5, which is 0.178 a.u., an effect attributable to the coordination of a formally positively charged ligand in 8 (Table S3 in the Supporting Information).Charge decomposition analysis (CDA) calculations on 8 revealed a noticeable Lewis acidity of [CH 3 CNCH 3 ] + .The computed donation from the ligand to the ruthenium carbide carbonyl fragment is 0.189 electrons, while the reverse process accounts for 0.169 electrons.According to the AIM data collected in Table S3 in the Supporting Information, the Ru−C(carbide) interactions are stronger in 8 with respect to 5.
The reaction of 2 with CH 3 I led to the isolation of the methylated species [Ru 6 C(CO) 14 (COCH 3 )] 3− (6).Several attempts to optimize the geometry starting from that of 2 with the addition of a [CH 3 ] + fragment on the oxygen atoms were carried out, and the lowest energy stationary point found is depicted in Figure 13.The carbyne ligand formed by the methylation of CO asymmetrically bridges two Ru centers with computed Ru−C bond lengths of 1.845 and 2.055 Å.The asymmetry of the bonds is also evidenced by the computed values at the respective (3,−1) BCPs summarized in Table 2.The unscaled carbonyl region of the simulated IR spectrum is slightly shifted toward higher wavenumbers with respect to the parent cluster 2 (Figure 13; best scaling factor for 2 and 6 =  The C(carbyne)-O stretching is combined with C−H bending vibrations, and it is thus related to two bands simulated at 1379 and 1543 cm −1 .

CONCLUSIONS
The reduction of 1 afforded the highly reduced tetraanion 2 that represented an interesting platform for the synthesis of further Ru−carbide carbonyl clusters.Indeed, protonation of 2 resulted in the monohydride 3, whereas oxidation of 2 under different experimental conditions allowed the synthesis of 4−8.Clusters 1 and 4 were previously described, 50−55 whereas 2, 3, 5, 6, 7, and 8 were reported here for the first time.Clusters 1− 8 are all based on the same octahedral Ru 6 C core and display 86 CVE.Based on the present findings, it is likely that the previously reported unsaturated 84 CVE cluster [HRu 6 C-(CO) 15 ] −53 could be better reformulated as the trihydride 7. Indeed, they show almost identical structures and, in addition, the purported [HRu 6 C(CO) 15 ] − was obtained under H 2 pressure.It must be remarked that 7 could not be obtained by further protonation of 3, since the latter species decomposed upon addition of acids.Thus, 7 could only be obtained upon treatment of 2 with CF 3 SO 3 CH 3 under controlled experimental conditions.
It is noteworthy that two-electron oxidation of 2, by chemical and electrochemical methods afforded the dianion 5 upon coordination of a CH 3 CN molecule in order to maintain 86 CVE.−29 Overall, the Ru 6 C framework appears to be very robust, may exist with charges ranging from 0 to −4, and can coordinate several different combinations of ligands on its surface, allowing for the isolation of a large variety of clusters.

EXPERIMENTAL SECTION
4.1.General Procedures.All reactions and sample manipulations were carried out using standard Schlenk techniques under nitrogen and in dried solvents.All the reagents were commercial products (Sigma-Aldrich) of the highest purity available and used as received, except [NEt 4 ][HRu 3 (CO) 11 ], which has been prepared according to the literature. 60Analyses of C, H, and N were carried out with a Thermo Quest Flash EA 1112NC instrument.IR spectra were recorded on a PerkinElmer Spectrum One interferometer in CaF 2 cells. 1 H NMR measurements were performed on a Varian Mercury Plus 400 MHz instrument.The proton chemical shifts were referenced to the nondeuterated aliquot of the solvent.Structure drawings have been performed with SCHAKAL99.[5]) suitable for X-ray analyses (yield 17%).
[NEt 4 ] 3 [6]: IR (CH  S4 in the Supporting Information.The diffraction experiments were carried out on a Bruker APEX II diffractometer equipped with a PHOTON2 detector using Mo−Kα radiation.Data were corrected for Lorentz polarization and absorption effects (empirical absorption correction SADABS). 62Structures were solved by direct methods and refined by full-matrix least-squares based on all data using F 2 . 63Hydrogen atoms were fixed at calculated positions and refined by a riding model.All non-hydrogen atoms were refined with anisotropic displacement parameters, unless otherwise stated.The crystals of [NEt 4 ] 3 [3] appeared to be non-merohedrally twinned.The TwinRotMat routine of PLATON was used to determine the twinning matrix and to write the reflection data file (.hkl) containing the twin components. 64The unit cell of [NEt 4 ][8]•solv contains an additional total potential solvent accessible void of 947 Å 3 (ca.12% of the cell volume), which is likely to be occupied by highly disordered solvent molecules.These voids have been treated using the SQUEEZE routine of PLATON.was purchased from Fluka and was used without further purification.Cyclic voltammetry was performed in a three-electrode cell, and the working and the counterelectrodes consisted of a Pt disk and a Pt gauze, respectively, both sealed in a glass tube.A leakless miniature Ag/AgCl/KCl electrode (eDAQ) was employed as a reference.The three-electrode home-built cell was predried by heating under vacuum and filled with argon.The Schlenk-type construction of the cell maintained anhydrous and anaerobic conditions.The solution of supporting electrolyte, prepared under argon, was introduced into the cell and the cyclic voltammetry of the solvent was recorded.The analyte was then introduced and voltammograms were recorded.Under the present experimental conditions, the one-electron oxidation of ferrocene occurs at E°= +0.42 V vs Ag/AgCl.
Controlled potential coulometry was performed in an H-shaped cell with anodic and cathodic compartments separated by a sinteredglass disk.The working macroelectrode and counterelectrode were platinum gauze.CH 3 CN presaturated argon was bubbled in the solution during the reduction of 1.
Infrared (IR) spectroelectrochemical measurements were carried out using an optically transparent thin-layer electrochemical (OTTLE) cell 59 equipped with CaF 2 windows, platinum mini-grid working and auxiliary electrodes and silver wire pseudoreference electrode.During the microelectrolysis procedures, the electrode potential was controlled by a PalmSens4 instrument interfaced to a computer employing PSTrace5 electrochemical software.Argonsaturated CH 3 CN solutions of the compound under study, containing [N n Bu 4 ][PF 6 ] 0.1 M as the supporting electrolyte, were used.The in situ spectroelectrochemical experiments have been performed by collecting spectra of the solution at constant time intervals during the oxidation or reduction obtained by continuously increasing or lowering the initial working potential at a scan rate of 1.0 mV/s.IR spectra were recorded on a PerkinElmer Spectrum 100 FT-IR spectrophotometer.
4.11.Computational Details.−72 The C-PCM implicit solvation model was added to DFT calculations, considering a dielectric constant of 46.68 and a refractive index of 1.4793. 73,74IR simulations were carried out using the harmonic approximation, from which zero-point vibrational energies and thermal corrections (T = 298.15K) were obtained. 75The software used was ORCA version 5.0.3. 76The output was elaborated using MultiWFN, version 3.8. 77Cartesian coordinates of the DFT-optimized structures are collected in a separate .xyzfile.

2. 1 .
Synthesis and Molecular Structure of [Ru 6 C-(CO) 15 ] 4− and [HRu 6 C(CO) 15 ] 3− .The reaction of [Ru 6 C-(CO) 16 ] 2− (1) with NaOH in DMSO resulted in the formation of [Ru 6 C(CO) 15 ] 4− ( crystals.The hydride nature of 3 was confirmed by the presence of a singlet at δ H −19.18 ppm in the 1 H NMR spectrum in CD 3 CN (Figures S10 and S11 in the Supporting Information).Further addition of 1 mol equiv of HBF 4 •Et 2 O to 3 resulted in the formation of 1 as demonstrated by IR spectroscopy (Figure S4 in the Supporting Information).Similar results were obtained employing HCl•Et 2 O instead of HBF 4 •Et 2 O, except that, due to the stronger coordination capability of Cl − compared to [BF 4 ] − , some crystals of [NEt 4 ][RuCl 3 (CO) 2 (CH 3 CN) 2 ] were obtained as side products (Figure S21 in the Supporting Information).

Scheme 1 .Figure 1 .
Scheme 1. Synthesis of [Ru 6 C(CO) 15 ] 4− (2) and [HRu 6 C(CO) 15 ] 3− (3) Reactions of [Ru 6 C(CO) 15 ] 4− .The reactions of 2 in CH 3 CN with miscellaneous oxidizing and alkylating agents, that is, [Cp 2 Fe][PF 6 ], [C 7 H 7 ][BF 4 ], CF 3 SO 3 CH 3 , and CH 3, I have been studied (Scheme 2).The chemical oxidation of 2 [ν CO 1890(vs), 1708(m) cm −1 ] with [Cp 2 Fe][PF 6 ] or [C 7 H 7 ][BF 4 ] in CH 3 CN resulted in a new species displaying ν CO at 1963(s) and 1760(m) cm −1 (Figure S5 in the Supporting Information) as found during the electrochemical oxidation (see Section 2.3).This new species revealed to be [Ru 6 C(CO) 15 (CH 3 CN)] 2− (5), as demonstrated by SC-XRD analysis on its [NEt 4 ] 2[5] salt (Figure3).[C 7 H 7 ][BF 4 ] revealed poor selectivity in the oxidation of 2 into 5 (yield 36%), due to the formation of side products such as 1.Better yield (61%) was obtained using [Cp 2 Fe][PF 6 ], for which the expected stoichiometric ratio, that is, 2 mol of [Cp 2 Fe][PF 6 ] per mole of 2 was employed.Indeed, formation of 5 may be viewed as a two-electron oxidation of 2 with concomitant coordination of CH 3 CN to the cluster, as was electrochemically demonstrated.Compound 5 was quantitatively converted into 1 [ν CO 1981(vs), 1763(m) cm −1 ] after exposure to CO atmosphere, following replacement of the labile CH 3 CN ligand by the stronger CO ligand.The ν CO bands of 5 absorb at slightly lower wavenumbers compared to 1, in view of the stronger σdonor ability of CH 3 CN compared to CO.The molecular structure of 5 closely resembles that of 1.The replacement of one terminal CO ligand with CH 3 CN causes a slight rearrangement of the CO ligands.Indeed, 5 displays three μ-CO ligands, and 1 contains 4 μ-CO ligands, whereas the remaining 12 carbonyls are in terminal positions in both the structures.The bonding parameters of the Ru 6 C cage are comparable to all the other clusters reported here.
reports the IR spectra of 2 in CH 3 CN/ [N n Bu 4 ][PF 6 ] solution recorded on increasing the working electrode (WE) potential from −0.2 to +0.2 V (vs Ag pseudoreference electrode).The blue shift of the terminal and bridging CO stretching bands, from 1890 and 1708 cm −1 to 1963 and 1760 cm −1 , points out the quantitative formation of the two-electrons oxidation product 5 isolated by the chemical oxidation of 2 in CH 3 CN and structurally characterized.The complete chemical reversibility of the electrochemical oxidation of 2 was ascertained by the restoration of its IR spectrum in the backward reduction step.Moreover, during the slow increase of the potential, in the early stage of the oxidation, two bands at 1930 and 1735 cm −1 increased their intensity up till a maximum and then decreased (Figure S23 in the Supporting Information).According to the cyclic voltammogram shown in Figure8, where two close-spaced electron removal steps are evident, these absorptions can be tentatively attributed to the product of the one-electron oxidation of 2.The sequence of IR spectra of a solution of 3 in CH 3 CN/ [N n Bu 4 ][PF 6 ] recorded in an OTTLE cell during the progressive increase of the WE potential from −0.1 to +0.7 V (vs Ag pseudo reference electrode) is reported in Figure11a.A shift of the terminal and bridging CO bands at higher

Figure 10 .
Figure 10.IR spectra of a CH 3 CN solution of [Ru 6 C(CO) 15 ] 4− (4) recorded in an OTTLE cell during the progressive increase of the potential from −0.2 to +0.2 V (vs Ag pseudoreference electrode, scan rate 1 mV s −1 ).[N n Bu 4 ][PF 6 ] (0.1 mol dm −3 ) as the supporting electrolyte.The absorptions of the solvent and supporting electrolyte have been subtracted.

Figure 11 .
Figure 11.IR spectra of a solution of [HRu 6 C(CO) 15 ] 3− (3) in CH 3 CN recorded in an OTTLE cell during (a) progressive increase of the WE potential from −0.1 to +0.7 V (vs Ag pseudoreference electrode; scan rate 1 mV s −1 ) and (b) during the reduction back-scan from +0.7 to −0.6 V (vs Ag pseudoreference electrode) [N n Bu 4 ][PF 6 ] (0.1 mol dm −3 ) as the supporting electrolyte.The absorptions of the solvent and supporting electrolyte have been subtracted.

4 . 10 .
Electrochemical and Spectroelectrochemical Measurements.Electrochemical measurements were performed with a PalmSens4 instrument interfaced to a computer employing PSTrace5 electrochemical software.Cyclic voltammetry measurements were carried out at room temperature under argon in CH 3 CN solutions containing [N n Bu 4 ][PF 6 ] (0.1 mol dm −3 ) as the supporting electrolyte.HPLC grade CH 3 CN (Sigma-Aldrich) was stored under argon over 3 Å molecular sieves.Electrochemical grade [N n Bu 4 ][PF 6 ] The reactions of 2 in CH 3 CN with the stronger methylating agent CF 3 SO 3 CH 3 resulted in mixtures of the new clusters [H 3 Ru 6 C(CO) 15 ] − (7) and [Ru 6 C(CO) 15 (CH 3 CNCH 3 )] − (8), as well as the previously reported cluster 4.

Table 2 .
AIM Data for the Ru−Carbide and Ru−Carbyne Bonds of [Ru 6 C(CO) 14 (COCH 3 )] 3− (6) , suggesting that the {Ru 6 C} core is less electron-rich.The Hirshfeld charge of the {Ru 6 C} core is in fact 0.035 a.u.(carbide charge −0.385 a.u.; Ru average charge 0.070 a.u.), while the calculated value for 2 is −0.097 a.u.Despite the formal attack of [CH 3 ] + on a CO ligand, the partial charge of the {COCH 3 } fragment is close to zero (0.001 a.u.).Concerning the Ru−C(carbide) bonds, the comparison of the AIM data collected in Table2with those in TableS1in the Supporting Information indicates a slight weakening with respect to compound 2.