Synthesis, Structural Characterization, and DFT Investigations of [MxM'5- xFe4(CO)16]3- (M, M' = Cu, Ag, Au; M ≠ M') 2-D Molecular Alloy Clusters

Abstract

Miscellaneous 2-D molecular alloy clusters of the type [M_xM'_5-xFe₄(CO)₁₆]^3- (M, M' = Cu, Ag, Au; M ≠ M') have been prepared through the reactions of [Cu₃Fe₃(CO)₁₂]^3-, [Ag₄Fe₄(CO)₁₆]^4- or [M₅Fe₄(CO)₁₆]^3- (M = Cu, Ag) with M'(I) salts (M' = Cu, Ag, Au). Their formation involves a combination of oxidation, condensation, and substitution reactions. The total structures of several [M_xM'_5-xFe₄(CO)₁₆]^3- clusters with different compositions have been determined by means of single crystal X-ray diffraction (SC-XRD) and their nature in solution elucidated by electron spray ionization mass spectrometry (ESI-MS) and IR and UV-visible spectroscopy. Substitutional and compositional disorder is present in the solid state structures, and ESI-MS analyses point out that mixtures of isostructural clusters differing by a few M/M' coinage metals are present. SC-XRD studies indicate some site preferences of the coinage metals within the metal cores of these clusters, with Au preferentially in corner sites and Cu in the central site. DFT studies give theoretical support to the experimental structural evidence. The site preference is mainly dictated by the strength of the Fe-M bonds that decreases in the order Fe-Au > Fe-Ag > Fe-Cu, and the preferred structure is the one that maximizes the number of stronger Fe-M interactions. Overall, the molecular nature of these clusters allows their structures to be fully revealed with atomic precision, resulting in the elucidation of the bonding parameters that determine the distribution of the different metals within their metal cores.

Figure 1

Molecular structures of some representative (a–c) 2-D clusters and (d, e) 3-D clusters. (a) [M₃Fe₃(CO)₁₂]^3– (M = Cu, Ag, Au),^, (b) [M₄Fe₄(CO)₁₆]^4– (M = Ag, Au),^, (c) [M₅Fe₄(CO)₁₆]^3– (M = Cu, Ag, Au),^,, (d) [Ag₁₃Fe₈(CO)₃₂]^3–,^, (e) [Au₂₈{Fe(CO)₃}₄{Fe(CO)₄}₁₀]^8–.

Scheme 1. Schematic Structures of the Clusters Discussed in the Paper

(a) [Cu₃Fe₃(CO)₁₂]^3–. (b) [M₄Fe₄(CO)₁₆]^4– (M = Ag, Au). (c) [M₅Fe₄(CO)₁₆]^3– (M = Cu, Ag, Au). (d) [Ag₁₃Fe₈(CO)₃₂]^3– (only one Fe(CO)₄ is represented). (e) [AuFe₄(CO)₁₆]⁻. (f) Fe(CO)₄(AuPPh₃)₂. Carbonyl ligands are represented as lines.

Scheme 2. Synthesis of [Ag_xCu_5–xFe₄(CO)₁₆]^3–

All of the reactions have been carried out in CH₃CN solution at room temperature. The reagents (AgNO₃, [Cu(CH₃CN)₄][BF₄], Cu(IMes)Cl) have been slowly added to the starting cluster solutions and the reactions monitored through IR spectroscopy. The stoichiometric ratios employed are summarized in Table 1. Complete details are given in the Experimental Section. The structure of “[Ag₆Fe₄(CO)₁₆]^2–” is represented in Scheme 3. Only one Fe(CO)₄ group is included in the schematic representation of [Ag₁₃Fe₈(CO)₃₂]^3–. The complete structure is reported in Figure 1. Carbonyl ligands are represented as lines.

Scheme 3. Growth Scheme of the [M₆Fe₄(CO)₁₆]^2– (M = Cu, Ag, Au) Cluster in the Oligomeric Form: (A) [M₅Fe₄(CO)₁₆]^3– Unit; (B) [M{M₅Fe₄(CO)₁₆}₂]^5– Dimer; (C) [M₂{M₅Fe₄(CO)₁₆}₃]^7– Trimer

Scheme 4. Synthesis of [Au_xCu_5–xFe₄(CO)₁₆]^3–

All of the reactions have been carried out in CH₃CN solution at room temperature. The reagents (Au(PPh₃)Cl or Au(Et₂S)Cl) have been slowly added to the starting cluster solutions and the reactions monitored through IR spectroscopy. The stoichiometric ratios employed are summarized in Table 2. Complete details are given in the Experimental Section. The structure of “[Au₆Fe₄(CO)₁₆]^2–” is represented in Scheme 3. Carbonyl ligands are represented as lines.

Scheme 5. Synthesis of [Au_xAg_5–xFe₄(CO)₁₆]^3–

All of the reactions have been carried out in CH₃CN solution at room temperature. The reagent (Au(Et₂S)Cl) has been slowly added to the starting cluster solutions and the reactions monitored through IR spectroscopy. The stoichiometric ratios employed are summarized in Table 3. Complete details are given in the Experimental Section. Carbonyl ligands are represented as lines.

Figure 2

Molecular structure of the [M_xM′_5–xFe₄(CO)₁₆]^3– (x = 0–5; M, M′ = Cu, Ag, Au; M ≠ M′) clusters (purple, M in the center; orange, M in the corner positions; green, Fe; gray, C; red, O). M–C(O) contacts are represented as fragmented lines. Two different views are reported (a, b), as well as the metal core (c).

Figure 3

M–M and M–Fe distances (Å) of (A) [NEt₄]₃[Ag_xCu_5–xFe₄(CO)₁₆]; (B) [NEt₄]₃[Au_xCu_5–xFe₄(CO)₁₆]; (C) [NEt₄]₃[Au_xAg_5–xFe₄(CO)₁₆]. Entries are reported in parentheses. M(1)–M(2), black; M(2)–M(2), red; M(1)–Fe(1), blue; M(2)–Fe(1), magenta; M(2)–Fe(2), green. See the scheme in the inset for the numbering.

Figure 4

UV–visible absorption spectra of [NEt₄]₃[Au_xCu_5–xFe₄(CO)₁₆] in CH₃CN at 298 K (concentration 1.25 × 10^–5 M). Cu5 = [Cu₅Fe₄(CO)₁₆]^3–; Au1.09Cu3.91 = [Au_1.09Cu_3.91Fe₄(CO)₁₆]^3–; Au1.15Cu3.85 = [Au_1.15Cu_3.85Fe₄(CO)₁₆]^3–; Au1.31Cu3.69 = [Au_1.31Cu_3.69Fe₄(CO)₁₆]^3–; Au1.67Cu3.33 = [Au_1.67Cu_3.33Fe₄(CO)₁₆]^3–; Au2.18Cu2.82 = [Au_2.18Cu_2.82Fe₄(CO)₁₆]^3–; Au2.48Cu2.52 = [Au_2.48Cu_2.52Fe₄(CO)₁₆]^3–; Au4.62Cu0.38 = [Au_4.62Cu_0.38Fe₄(CO)₁₆]^3–.

Figure 5

UV–visible absorption spectra of [NEt₄]₃[Ag_xCu_5–xFe₄(CO)₁₆] in CH₃CN at 298 K (concentration 1.25 × 10^–5 M). Ag5 = [Ag₅Fe₄(CO)₁₆]^3–; Ag5Cu0 = [Ag₅Cu₀Fe₄(CO)₁₆]^3– (see entry 2 in Table 1); Ag4.37Cu0.63 = [Ag_4.37Cu_0.63Fe₄(CO)₁₆]^3–; Ag4.25Cu0.75 = [Ag_4.25Cu_0.753Fe₄(CO)₁₆]^3–; Ag3.45Cu1.55 = [Ag_3.45Cu_1.55Fe₄(CO)₁₆]^3–; Ag3.31Cu1.69 = [Ag_3.31Cu_1.69Fe₄(CO)₁₆]^3–; Ag1.02Cu3.98 = [Ag_1.02Cu_3.98Fe₄(CO)₁₆]^3–; Cu5 = [Cu₅Fe₄(CO)₁₆]^3–.

Figure 6

UV–visible absorption spectra of [NEt₄]₃[Au_xAg_5–xFe₄(CO)₁₆] in CH₃CN at 298 K (concentration 1.25 × 10^–5 M). Ag5 = [Ag₅Fe₄(CO)₁₆]^3–; Au0.64Ag4.36 = [Au_0.64Ag_4.36Fe₄(CO)₁₆]^3–; Au0.82Ag4.18 = [Au_0.82Ag_4.18Fe₄(CO)₁₆]^3–.

Scheme 6. General Representation of the M–M and M–Fe Bonds Obtained from the Analysis of the (3, –1) Bond Critical Points

Types of interactions: (a) M_centre–M_corner; (b) M_corner–M_corner; (c) M_centre–Fe; (d and e) M_corner–Fe.

Scheme 7. Isomers of the [M_xM′_5–xFe₄(CO)₁₆]^3– Clusters with Acronyms

Only the metal centers are sketched for clarity.

Figure 7

Relative energy values for the isomers of [M_xM′_5–xFe₄(CO)₁₆]^3– clusters. Dotted lines are drawn for clarity purposes.