Substitution of aqua ligands from alkyldiamine-bridged dinuclear Pt(II) complexes using azole nucleophiles

Abstract

The rate of substitution of aqua ligands in dinuclear Pt(II) complexes, which are bridged by alkyldiamine linkers of variable chain lengths, and their mononuclear Pt(II) analogues were studied under pseudo-first order conditions as a function of concentration and temperature using azoles. The results indicate that substitution of aqua ligands of the dinuclear Pt(II) complexes occurs simultaneously and increases as the alkyl chain length of the diamine bridge increases. Steric hindrance due to the C_2h conformational symmetry, whose influence decreases as the length of alkylamine linker increases, appears to be the dominant factor controlling the reactivity of the dinuclear Pt(II) complexes. Prop, a homologue which has a 1,3-propanediamine bridge and a C_2v conformation, shows an unusual high reactivity. Weak sigma donicity due to the α,ω-alkyldiamine bridge is evident when the reactivity of dinuclear species is compared to their mononuclear analogues. Mononuclear Pt(II) complexes are more reactive than the dinuclear Pt(II) complexes with their reactivity increasing with increasing chain length of the alkylamine tail. The nucleophilicity of the azoles decreases in the order Pz > Tz > mPz. This is in accord with the basicity of the coordinating nitrogen donor in the case of Pz and Tz, while a steric hindrance to the approach of 1-methylpyrazole due to the ortho N-methyl substituent on the ring is evident for mPz. The substitution of aqua ligands by azoles remains associatively-activated.

Keywords:

• Aqua substitution
• Dinuclear Pt(II) complexes
• Azoles
• α,ω-Alkyldiamine bridges

Acknowledgements

The authors gratefully acknowledge financial support and bursary for Mr. G. Enos from the University of KwaZulu-Natal and a study leave from the University of Dodoma (UDOM).