Reaction of Cp*(CO)₂ReRe(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) 2 Re Re(CO) 2 Cp* ( 1 ) with dimethyl acetylenedicarboxylate (DMAD) produced the 3,4-dimetallacyclobutene Cp*(CO) 2 Re(μ-η 1,η 1 -CH 3 O 2 CC CCO 2 CH 3 )Re(CO) 2 Cp* ( 2 ). Photochemical rearrangement of 2 produced Cp*(CO) 2 Re(μ-η 2,η 2 -CH 3 O 2 CC⋮CCO 2 CH 3 )Re(CO) 2 Cp* ( 3 ), in which both Cp*(CO) 2 Re units are coordinated to the alkyne. Upon heating at 72 °C, 3 was converted ( t 1/2 ≈ 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-η 2,η 2 -CH 3 O 2 CC⋮CCO 2 CH 3 )(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, D c = 1.946 Mg/m 3; 3, monoclinic, C 2/ c, a = 12.854(2) Å, b = 14.729(3) Å, c = 16.650(3) Å, β = 108.188(12)°, Z = 4, D c = 1.989 Mg/m 3; 4, trigonal, P 3(2), a = 16.277(2) Å, b = 16.277(2) Å, c = 9.408(3) Å, α = 90°, β = 90°, γ = 120°, Z = 3, D c = 2.005 Mg/m 3 .