The investigation of photoaquation mechanisms of CR(III) am(m)ine complexes
The photoaquation mechanisms for a series of Cr(III) am(m)ine complexes have been investigated using laser flash photolysis with conductivity detection. The observation of transient increases in solution conductivity at pH > 4 and a conductivity decay lifetime longer than the doublet emission dec...
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| Format: | Others |
| Language: | English en |
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2017
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| Online Access: | https://dspace.library.uvic.ca//handle/1828/8883 |
| Summary: | The photoaquation mechanisms for a series of Cr(III) am(m)ine complexes have been investigated using laser flash photolysis with conductivity detection. The observation of transient increases in solution conductivity at pH > 4 and a conductivity decay lifetime longer than the doublet emission decay lifetime at pH < 3, have confirmed an intermediate in the photoaquation of cis-Cr(cyclam)(NH₃)₂³⁺. Transient increases in solution conductivity characteristic of an intermediate species were also observed for Cr(en)₃³⁺, Cr(tn)₃³⁺ and Cr(sen)³⁺ at pH > 4.
The conductivity changes occurring in solution have been modelled for possible photoaquation mechanisms for the am(m)ine complexes, based on numerical integration of the rate expressions for all relevant mechanistic species. The comparison of these results with experimental data indicates that the intermediates observed for Cr(en)₃³⁺, Cr(tn)₃³⁺ and Cr(sen)³⁺ are the initially formed photoproducts, Cr(NN)₂(N-N)(OH₂)³⁺, where NN = a bidentate ligand or a sen arm. These species can undergo two processes, protonation of the dangling amine arm, or deprotonation of the aquo group. At low pH the first process dominates and conductivity decays due to proton uptake are observed. As the pH of the solution increases, the second process becomes competitive, and transient increases in solution conductivity are observed when this becomes the faster process.
The modelled results for cis-Cr(cyclam)(NH₃)₂³⁺ indicate that the photoaquation occurs via two modes; (i) direct loss of ammonia, and (ii) loss and recoordination of a cyclam to displace ammonia, with both modes generating the observed photoproduct, cis-Cr(cyclam)(NH₃)(OH₂)³⁺. The modelling indicates that 0.67 of the overall photochemistry occurs via the cyclam loss mode. The intermediate has been identified as the initial product of the cyclam loss mode. Cr(cyc-N)(NH₃)₂(OH₂)³⁺. The rate of reaction observed via this mode is limited by the recoordination of the cyclam amine. As this is slower than the rate of doublet decay, the conductivity lifetimes observed at pH < 3 are longer than the doublet lifetime. The slow rate of recoordination also delays the release and subsequent protonation of ammonia, allowing for competitive deprotonation of the aquo group at pH > 4, and generating the observed transient increases in solution conductivity. The relevance of these results to Cr(III) chemistry in general, including possible ²E state reaction mechanisms is discussed.
The photoaquation of Cr(CN)₆³⁻ was investigated using laser flash photolysis with conductivity detection. Theory predicts that the signal magnitudes observed for this complex should be constant throughout the pH range 2.7 - 5.3. Experimental results showed that the observed signals dropped from a maximum of 120 mV at pH 2.75 to 45 mV at pH 5.25. Possible explanations for this pH dependence are presented.
The stereochemistry of the thermal and photoaquation products of rac- & Λ-Cr(sen)³⁺ has been investigated using capillary electrophoresis. Two products were found in the photoaquation reactions, trans-Cr(sen-NH)(OH₂)⁴⁺ and a product resulting from loss of a secondary amine. The thermal reaction produced trans-Cr(sen-NH)(OH₂)⁴⁺ as the major product with virtually no cis-Cr(sen-NH)(OH₂)⁴⁺ enantiomers being observed. Efficient racemization of Λ-Cr(sen)³⁺ to Δ-Cr(sen)³⁺was also observed in the thermal reaction, consistent with racemization occurring via bond rupture and recoordination. The photoaquation results are discussed in terms of VC theory and a reinterpretation of conflicting literature results for the thermal and photochemical aquation of Cr(sen)³⁺ is presented. === Graduate |
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