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Formation and Dissociation of Tetrahedral Nickel(II) and Cobalt(II) Complexes of ‘Dithioimidodiphosphato’ Ligands, Measurement of Kinetically Determined Stability Constants, and Kinetic Investigations of the Rapid Formation of Tetrahedral Bis(ligand)copper(II) Complexes and Their Rates of Reduction to Trinuclear Copper(I) Species
Author(s) -
Moore Peter,
Errington William,
Sangha Satindra P.
Publication year - 2005
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.200590055
Subject(s) - chemistry , denticity , cobalt , nickel , ligand (biochemistry) , dissociation (chemistry) , stability constants of complexes , medicinal chemistry , chelation , copper , crystallography , stereochemistry , ion , inorganic chemistry , crystal structure , organic chemistry , biochemistry , receptor
Abstract The stopped‐flow technique was used to measure the rates of formation and dissociation of tetrahedral [ML 2 ] complexes (M 2+ =Ni 2+ or Co 2+ ) of four bidentate S 2 ‐donor ‘dithioimidodiphosphato’ ligands L − (HL=[R 1 R 2 P(S)]NH[P(S)R 3 R 4 ], R 1 to R 4 =alkyl) at 25.0° in MeOH/H 2 O 95 : 5 ( v / v ) solution and in the presence of either MOPS (=3‐(morpholin‐4‐yl)propane‐1‐sulfonic acid) or 2,6‐lutidine (=2,6‐dimethylpyridine) buffers. The kinetically determined equilibrium formation constants for [ML] + ions (M=Ni or Co) are 10 −5 K =0.50±0.01 or 1.64±0.07 l mol −1 for L=L 3 (R 1 =R 2 =Me(CH 2 ) 2 CH(Me), R 3 =R 4 =Me 2 CH), 1.27±0.02 or 7.93±0.09 l mol −1 for L=L 7 (R 1 to R 4 =Me 2 CHCH 2 ), 0.88±0.04 or 3.84±0.13 l mol −1 for L=L 8 (R 1 to R 4 =Me 2 CH), and in case of Ni 2+ 1.88±0.04 l mol −1 for L=L 6 (R 1 =R 3 =Bu, R 2 =R 4 = t Bu) (see Table 3 ; for L 3 and L 6 –L 8 , see Table 1 ). Whereas the tetrahedral Ni 2+ complexes dissociate more slowly than the analogous Co 2+ species, in all cases, the Co 2+ complexes are more stable than those of Ni 2+ due to their larger formation rate constants ( Table 3 ). Reactions of Cu 2+ with eight ligands HL (R 1 to R 4 =alkyl, alkoxy, aryl, and aryloxy) show that formation of intensely colored tetrahedral [Cu II L 2 ] species is too fast be measured with the available stopped‐flow apparatus ( t 1/2 <2 ms), but the subsequent rates of reduction of [Cu II L 2 ] to give trinuclear products [Cu I 3 L 3 ] are measurable. An X‐ray analysis establishes the structure of one of the [Cu 3 L 3 ] complexes, where R 1 =R 2 =Me 2 CHO and R 3 =R 4 =2‐( tert ‐butyl)phenyl (L=L 5 ), and a multiwavelength stopped‐flow kinetic experiment establishes the spectrum of a tetrahedral [Cu II L 2 ] species prior to the reduction reactions. The redox reactions proceed at 25.0° with first‐order rate constants in the range 0.285 s −1 (R 1 to R 4 =PhO; L=L 11 ) to 2.58⋅10 −4 s −1 (R 1 to R 4 =Me 2 CHCH 2 ; L=L 7 ) ( Table 4 ).