Dirhodium Paddlewheel with Functionalized Carboxylate Bridges: New Building Block for Self-Assembly and Immobilization on Solid Support

A new dirhodium­(II,II) paddlewheel complex, [Rh2(O2CC6H4COOC2H5)4] (1), has been synthesized using a predesigned
functionalized carboxylate, namely, 4-(ethoxycarbonyl)­benzoate. The
target product has been crystallized from the acetone solution and
structurally characterized as a bis-acetone adduct, [Rh2(O2CC6H4COOC2H5)4(OCMe2)2]·C6H14 (2). By utilizing the ability of dangling ester
groups to coordinate to open axial ends of neighboring dirhodium units, 1 can self-assemble to form 2D networks upon crystallization
from solutions of noncoordinating solvents such as chlorobenzene and
chloroform. The resulting [Rh2(O2CC6H4COOC2H5)4]·2C6H5Cl (3) and [Rh2(O2CC6H4COOC2H5)4]·2CHCl3 (4) products have microporous
solid state structures with the pores filled with the corresponding
disordered solvent molecules. Notably, 3 and 4 represent unique examples of 2D extended frameworks based on dirhodium
tetracarboxylate paddlewheel units devoid of any exogenous ligands.
In solution, the dangling ends of carboxylate bridges of 1 have been successfully utilized for condensation reaction with the
selected solid support, benzylamine-functionalized polystyrene, allowing
successful heterogenization of dirhodium units through the equatorial
covalent attachment to the substrate. The resulting solid product
was tested as a catalyst in the cyclopropanation reaction of styrene
with methyl phenyldiazoacetate to show good yields and diastereoselectivity.