Reaction of Cp*(CO)₂ReRe(CO)₂Cp* with Dimethyl Acetylenedicarboxylate Produces a 3,4-Dimetallacyclobutene Which Undergoes Photochemical Isomerization to a 2,4-Dimetallabicyclo[1.1.0]butane

The reaction of Cp*(CO)2ReRe(CO)2Cp* (1) with dimethyl acetylenedicarboxylate (DMAD) produced the 3,4-dimetallacyclobutene Cp*(CO)2Re(μ-η1,η1-CH3O2CCCCO2CH3)Re(CO)2Cp* (2). Photochemical rearrangement of 2 produced Cp*(CO)2Re(μ-η2,η2-CH3O2CC⋮CCO2CH3)Re(CO)2Cp* (3), in which both Cp*(CO)2Re units are coordinated to the alkyne. Upon heating at 72 °C, 3 was converted (t1/2 ≈ 120 min) to a 78:22 equilibrium mixture of 2:3. Photolysis of 3 in a Rayonet photoreactor (maximum emission 300 nm) resulted in CO loss and formation of Cp*(CO)Re(μ-η2,η2-CH3O2CC⋮CCO2CH3)(μ-CO)Re(CO)Cp* (4). Compounds 2, 3, and 4 were characterized by X-ray crystallography: 2, orthorhombic, Pcba, a = 14.480(3) Å, b = 16.246(3) Å, c = 26.027(3) Å, Z = 8, Dc = 1.946 Mg/m3; 3, monoclinic, C2/c, a = 12.854(2) Å, b = 14.729(3) Å, c = 16.650(3) Å, β = 108.188(12)°, Z = 4, Dc = 1.989 Mg/m3; 4, trigonal, P3(2), a = 16.277(2) Å, b = 16.277(2) Å, c = 9.408(3) Å, α = 90°, β = 90°, γ = 120°, Z = 3, Dc = 2.005 Mg/m3.