Due to their unique
coordination structure, dirhodium paddlewheel
complexes are of interest in several research fields, like medicinal
chemistry, catalysis, etc. Previously, these complexes were conjugated
to proteins and peptides for developing artificial metalloenzymes
as homogeneous catalysts. Fixation of dirhodium complexes into protein
crystals is interesting to develop heterogeneous catalysts. Porous
solvent channels present in protein crystals can benefit the activity
by increasing the probability of substrate collisions at the catalytic
Rh binding sites. Toward this goal, the present work describes the
use of bovine pancreatic ribonuclease (RNase A) crystals with a pore
size of 4 nm (P3221 space group) for fixing
[Rh2(OAc)4] and developing a heterogeneous catalyst
to perform reactions in an aqueous medium. The structure of the [Rh2(OAc)4]/RNase A adduct was investigated by X-ray
crystallography: the metal complex structure remains unperturbed upon
protein binding. Using a number of crystal structures, metal complex
accumulation over time, within the RNase A crystals, and structures
at variable temperatures were evaluated. We also report the large-scale
preparation of microcrystals (∼10–20 μm) of the
[Rh2(OAc)4]/RNase A adduct and cross-linking
reaction with glutaraldehyde. The catalytic olefin cyclopropanation
reaction and self-coupling of diazo compounds by these cross-linked
[Rh2(OAc)4]/RNase A crystals were demonstrated.
The results of this work reveal that these systems can be used as
heterogeneous catalysts to promote reactions in aqueous solution.
Overall, our findings demonstrate that the dirhodium paddlewheel complexes
can be fixed in porous biomolecule crystals, like those of RNase A,
to prepare biohybrid materials for catalytic applications.