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Steric Hindrance in Substitution Reactions on Arsenic Acid by Pentaam(m)ine Complexes of Co III and Cr III
Author(s) -
González Gabriel,
Martinez Manuel,
Rodríguez Elisa
Publication year - 2000
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/(sici)1099-0682(200006)2000:6<1333::aid-ejic1333>3.0.co;2-g
Subject(s) - chemistry , steric effects , arsenic , substitution (logic) , substitution reaction , medicinal chemistry , stereochemistry , organic chemistry , computer science , programming language
The kinetics of the substitution reaction of water by H 2 AsO 4 – in [M(N) 5 (H 2 O)] 3+ (M = Co III , Cr III ) complexes have been studied at 25 °C for a series of (N) 5 groups, from monodentate (NH 3 ) 5 and (CH 3 NH 2 ) 5 to macrocyclic 12‐methyl‐1,4,7,10‐tetraazacyclotridecane‐12‐amine (L 13 ), 6‐methyl‐1,4,8,11‐tetraazacyclotetradecane‐6‐amine (L 14 ), and 10‐methyl‐1,4,8,12‐tetraazacyclopentadecane‐10‐amine (L 15 ) in their cis (L 13 and L 14 , Co III and Cr; L 15 , Co III ) and trans (L 15 , Co III and Cr III ) water‐amino geometrical arrangements around the metal ion. Equilibration rate constants were found to be too large to be interpreted as occurring through the M–OH 2 bond scission that takes place in standard substitution processes on these t 2g 3 and t 2g 6 pentaamine complexes, and were also found to be highly dependent on the acidity of the medium. The results are interpreted in view of previously studied, similar reactions, where protonation played a very important role. This role was expected for reactions occurring via As–O bond scission; that is, the data fits well with that expected for oxygen exchange reactions of the labile tetrahedral {AsO 4 } moiety. The values found are a clear indication of the differences in steric requirements of the metal‐bound water attacking the tetrahedral As V centre [i.e. for M = Co III , k + non acid catalysed = 93, 1.9, 0.76, and 0.44 M –1 s –1 for the (NH 3 ) 5 , (CH 3 NH 2 ) 5 , cis ‐L 13 , and trans ‐L 15 , respectively], as well as of electronic influences of the amines of the cobalt complex on the As V centre when attacked by the bulk water [i.e. for M = Co III , 10 3 k – non acid catalysed = 280, 5.7, 4.4, and 4.5 s –1 for the (NH 3 ) 5 , (CH 3 NH 2 ) 5 , cis ‐L 13 , and trans ‐L 15 respectively].