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

Abstract

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 CH_3,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.

Scheme 1. Synthesis of [Ru₆C(CO)₁₅]^4– (2) and [HRu₆C(CO)₁₅]^3– (3)

Figure 1

Reaction of 2 with HBF₄·Et₂O in CH₃CN monitored by IR spectroscopy. IR spectra were recorded after the addition of 0.30 equiv each time.

Figure 2

Molecular structures of (a) [Ru₆C(CO)₁₅]^4– (2) and (b) [HRu₆C(CO)₁₅]^3– (3) (orange Ru; red O; gray C; white H).

Scheme 2. Oxidation Reactions of [Ru₆C(CO)₁₅]^4– (2) in CH₃CN.

All the species have been structurally characterized by SC-XRD except [Ru₆C(CO)₁₄(COCH₃)]^3– (6), which was identified by spectroscopic methods (IR and ESI-MS) and its structure computationally determined.

Figure 3

Molecular structure of [Ru₆C(CO)₁₅(CH₃CN)]^2– (5) (orange Ru; red O; blue N; gray C; white H). Main bond distances (Å): Ru–Ru 2.7901(7)–2.9702(7), average 2.888(2); Ru–C_carbide 2.019(6)–2.056(6), average 2.042(15); Ru–N_CH3CN 2.075(6); C–N_CH3CN 1.126(9).

Figure 4

Hydride region of the ¹H NMR spectrum of [H₃Ru₆(CO)₁₅]⁻ (7) in CD₂Cl₂ at 298 K in the presence of minor traces of [HRu₆(CO)₁₆]⁻ (4).

Figure 5

Molecular structures of (a) [H₃Ru₆C(CO)₁₅]⁻ (7) and (b) [HRu₆C(CO)₁₆]⁻ (4) (orange Ru; red O; gray C; white H).

Figure 6

Hydride region of the VT ¹H NMR spectra of [H₃Ru₆(CO)₁₅]⁻ (7) in CD₂Cl₂. The sharp resonance at ca. δ_H −19.0 ppm is due to traces of 4.

Figure 7

Molecular structure of [Ru₆C(CO)₁₅(CH₃CNCH₃)]⁻ (8) (orange Ru; red O; blue N; gray C; white H). Main bond distances (Å): Ru–Ru 2.831(2)–2.948(2), average 2.873(7); Ru–C_carbide 2.017(19)–2.042(18), average 2.03(4); Ru–N_imidoyl 2.067(18); Ru–C_imidoyl 2.07(2); C–N_imidoyl 1.23(3).

Figure 8

Cyclic voltammetry response of [Ru₆C(CO)₁₅]^4– (2) at a Pt electrode in CH₃CN solution between −2.3 and +1.2 V, black line; between −0.8 and −0.2 V, red line; between −0.8 and −0.5 V blue line. [NⁿBu₄][PF₆] (0.1 mol dm^–3) supporting electrolyte, scan rate: 0.1 V s^–1.

Figure 9

Differential pulse voltammetry (blue line) and cyclic voltammetry response of [HRu₆C(CO)₁₅]^3– (3) at a Pt electrode in CH₃CN solution of [NⁿBu₄][PF₆] (0.1 mol dm^–3) supporting electrolyte: between −0.7 and +0.8 V (black line), scan rate: 0.1 V s^–1; between −0.7 and −0.2 V (red line), scan rate: 0.2 V s^–1.

Figure 10

IR spectra of a CH₃CN solution of [Ru₆C(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). [NⁿBu₄][PF₆] (0.1 mol dm^–3) as the supporting electrolyte. The absorptions of the solvent and supporting electrolyte have been subtracted.

Figure 11

IR spectra of a solution of [HRu₆C(CO)₁₅]^3– (3) in CH₃CN 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) [NⁿBu₄][PF₆] (0.1 mol dm^–3) as the supporting electrolyte. The absorptions of the solvent and supporting electrolyte have been subtracted.

Figure 12

DFT-optimized geometries and relative Gibbs energy values of (from top to bottom) [M₆C(CO)₁₆]^2–, [M₆C(COOH)(CO)₁₅]^3–, [HM₆C(CO)₁₅]^3– and [M₆C(CO)₁₅]^4– (M = Fe, Ru). Color map: orange Ru; green Fe; red O; gray C; white H. Carbonyl ligands are omitted for clarity.

Figure 13

DFT-optimized structure of [Ru₆C(CO)₁₄(COCH₃)]^3– (6) and simulated IR spectra of 6 and 2 (Lorentzian-broadening functions, fwhm = 8 cm^–1). Color map: orange Ru; red O; gray C; white H.