Synthesis, Characterization, and Electrochemistry of the Bis-Bridged Complexes [formula-ommited] and [formula-ommited]. Molecular Structures of [formula-ommited] Where n = 1 and 3

Photolysis of [formula-ommited] (1) with a series of bis(phosphines) (L = Ph₂P(CH₂)nPPh₂, where n = 1,2, and 3) leads to the formation of substituted dinuclear and tetranuclear compounds of the form [formula-ommited] and [formula-ommited], respectively. The ratio of these two types of compounds is dependent upon the ligand size (number of methylene units), stoichiometry, and the overall concentration employed. With the small ligands and dilute conditions, formation of the dinuclear product is favored. Under more concentrated reaction conditions and when an excess of 1 is used, the tetranuclear type of product becomes dominant. The electrochemistry of the dinuclear compounds indicate that both the silyl and the bis(phosphine) bridges are capable of stabilizing a two-electron oxidation product. This is demonstrated in the case of L = dppm where we observe, by fast-scan cyclic voltammetry, two, one-electron reversible oxidation steps. The structure of [formula-ommited] (3a) is determined by X-ray diffraction. It crystallizes in the space group Pnma (No. 62) with a = 13.754 (4) Å, b = 17.707 (4) A, c = 13.771 (3) Å and α, β, and γ = 90.00° with Z = 4. The structure was refined to R₁ = 0.041 and R₂ = 0.045 for 2226 independent reflections having I > 3σ(I). The structure of [formula-ommited] (3c) is also determined by X-ray diffraction. It crystallizes in the space group P1 (No. 2) with a = 11.638 (11) Å, b = 11.833 (11) Å, c = 16.289 (14) Å, α = 111.22 (6)°, β = 72.67 (7)°, and γ = 107.02 (7)° with Z = 2. The structure is refined to R1 = 0.067 and R₂ = 0.076 for 4115 independent reflections having I > 3σ(I). The overall geometry of 3a compared to 3c implies that some distortion occurs from the steric requirements of the larger dppp ligand. This includes a significant lengthening of the Fe-Fe bond in complex 3c by 0.024 (3) Å relative to 3a.