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Nucleophilic and electrophilic promotion of ligand substitution reactions in oxo‐bridged, dinuclear chromium(III) complexes
Author(s) -
Bessire David R.,
Holwerda Robert A.
Publication year - 1999
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/(sici)1097-4601(1999)31:5<351::aid-kin4>3.0.co;2-r
Subject(s) - chemistry , thiocyanate , nucleophile , medicinal chemistry , ligand (biochemistry) , chromium , electrophile , substitution reaction , stereochemistry , reactivity (psychology) , nucleophilic substitution , reaction rate constant , kinetics , inorganic chemistry , catalysis , organic chemistry , biochemistry , receptor , medicine , physics , alternative medicine , pathology , quantum mechanics
Base hydrolysis reactions of [Cr(tmpa)(NCSe)] 2 O 2+ , [Cr(tmpa)(N 3 )] 2 O 2+ , [Cr 2 (tmpa) 2 (μ−O)(μ−PhPO 4 )] 4+ and [Cr 2 (tmpa) 2 (μ−O)(μ−CO 3 )] 2+ follow the pseudo‐first‐order relationship (excess OH − ): k obsd =k o +k b Q p [OH − ]/(1+Q p [OH − ]). For the CO 3 2− complex, k b (60°C)=(1.50±0.03)×10 −2 s −1 ; ΔH‡=61±2 kJ/mol, ΔS‡=−99±7 J/mol K; Q p (60°C)=(3.8±0.3)×10 1 M −1 ; ΔH°=67±2 kJ/mol, ΔS°=230±7 J/mol K (I=1.0 M). An isokinetic relationship among k OH (=k b Q p ) activation parameters for five (tmpa)CrOCr(tmpa) complexes shows that all follow essentially the same pathway. Activated complex formation is thought to require nucleophilic attack of coordinated OH − at the chromium‐leaving group bond in the k b step, accompanied by reattachment of a tmpa pyridyl arm displaced by OH − in the Q p preequilibrium. Abstraction of both thiocyanate ligands was observed upon mixing [Cr(tmpa)(NCS)] 2 O 2+ with [Pd(CH 3 CN) 4 ] 2+ in CH 3 CN solution. The proposed mechanism requires rapid complexation of both reactant thiocyanate ligands by Pd(II) (K p (25°C)=(4.5±0.2)×10 8 M −2 ; ΔH°=−32±6 kJ/mol, ΔS°=59±19 J/mol K) prior to rate‐limiting CrNCS bond‐breaking (k 2 (25°C)=(1.17±0.02)×10 −3 s −1 ; ΔH‡=98±2 kJ/mol, ΔS‡=27±5 J/mol K). Pd(II)‐assisted NCS − abstraction is not driven by weakening of the Cr()NCS − bond through ligation of the sulfur atom to palladium, but rather by a favorable ΔS‡ resulting from the release of Pd(NCS) + fragments and weak solvation of the activated complex in CH 3 CN solution. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 351–356, 1999