Investigating cubane formation and effect of co-crystallization agents in oxo-bridged Co complexes using X-ray absorption spectroscopy

Abstract

• Cubane core in Co-oxo complexes studied using XAS to probe its spectroscopic nature. • XANES shows presence of octahedrally coordinated Co 3+ ions in cubane structure. • EXAFS confirm octahedral Co with 6 O/N, 3 Co-Co backscattering forming cubane core. • XANES simulation indicate interaction with co-crystallizing salts NaNO 3 and NaClO 4 . • Formation of defect dicubane with Co 2+ /Co 3+ ions predicted by XANES simulation. The formation of cubane core and effect of co-crystallization agent have been studied in the oxo-bridged Co complexes using X-ray absorption spectroscopy. XAFS at Co K-edge have been measured in the oxo-bridged Co complexes [Co 4 O 4 (O 2 CCH 3 ) 4 (C 5 H 5 N) 4 ]( 1 ), 1 .0.5NaNO 3 ·8H 2 O( 2 ), 1 .NaClO 4 ·3·5H 2 O( 3 ) and [Co 4 O 4 (O 2 CCH 3 ) 4 (NH 3 ) 4 ]( 4 ). Complexes 2 and 3 actually contain the same complex 1 co-crystalized by NaNO 3 and NaClO 4 moieties. While 1 and 4 are in powder form, 2 and 3 are in crystalline form. Different XANES features have been correlated to the coordination around the absorbing Co atom in all the complexes. EXAFS analysis has been performed to determine coordination environment around the Co atoms to confirm the formation of the oxo-cubane structure in these complexes. For each of the Co centers present at the four different positions of the cubane core in complexes 2 and 3, XANES spectra have been theoretically simulated and simultaneously p-DOS calculated to examine the effect of co-crystallizing moieties interactions with the individual Co centres present in these two complexes. After preliminary indication by EXAFS analysis, XANES simulations were also performed to examine the presence of defect dicubane structure in 4. The study can be helpful in the detection of these structures during in-situ/operando studies of these catalytically active complexes.