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Dinuclear Metal Complexes Based on all‐cis ‐2,4,6‐Triaminocyclohexane‐1,3,5‐triol as Catalysts for Cleavage of Phosphate Esters
Author(s) -
Mancin Fabrizio,
Rampazzo Enrico,
Tecilla Paolo,
Tonellato Umberto
Publication year - 2004
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.200300381
Subject(s) - chemistry , catalysis , cooperativity , medicinal chemistry , cleavage (geology) , hydrolysis , metal ions in aqueous solution , metal , phosphate , transesterification , stereochemistry , polymer chemistry , organic chemistry , biochemistry , geotechnical engineering , fracture (geology) , engineering
Two dimetallic ligands 2 and 3 for transition metal ions were obtained by connecting two all ‐ cis ‐2,4,6‐triamino‐cyclohexane‐1,3,5‐triol (TACI, 1 ) subunits via 1,3‐ or 1,4‐xylyl linkers. Their dimetallic Cu II and Zn II complexes were investigated as catalysts for the cleavage of the phosphate diesters 2,4‐dinitrophenyl ethyl phosphate (DNPEP) and 2‐hydroxypropyl p ‐nitrophenyl phosphate (HPNP) and the triester 2,4‐dinitrophenyl diethyl phosphate (DNPDEP). The results of a comparative kinetic study using the monometallic complexes of TACI as a reference indicate that the Cu II complexes of 2 and 3 are virtually inert; this finding is ascribed to the formation of intra‐complex μ‐hydroxo bridges that prevent the required interactions with the substrate. On the other hand, the dimetallic Zn II complexes produce remarkable accelerations, particularly in the case of the HPNP transesterification. The dimetallic systems are more efficient than the TACI·Zn II complex in promoting the hydrolysis of HPNP at pH values close to neutrality (7.0−7.8). In this case, the effects of cooperativity between the two metal centers were highlighted in a detailed kinetic study; the catalytic efficiency seems to be related to the stronger binding of the substrate to the dimetallic Zn II complexes. Additionally, in this case, although to a much lesser extent than in the case of the Cu II counterparts, the formation of μ‐hydroxo bridges apparently hampers the catalytic efficiency, as indicated also by the observation that the activity of the dimetallic complexes increases as the distance between the two metal centers increases, which, thus, thwarts the formation of intermetallic bridges. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)