The effect of mixed-metal ruthenium(II)-platinum(II) and cobalt(II)-platinum(II) complexes of tetra-2-pyridyl-1,4-pyrazine chelate ligands on the substitution of chlorides from platinum metal

Four heterometallic Ru(II)-Pt(II) and Co(II)-Pt(II) complexes with tetra-2-pyridyl-1,4-pyrazine (tppz) and 2,2′,6′,2″-terpyridine (tpy) chelate ligands, viz. , [(tpy)Ru(tppz)PtCl](PF 6 ) 3 ( RuPt ), [ClPt(tppz)Ru(tppz)PtCl](PF 6 ) 4 ( PtRuPt ), [ClPtRu 2 (tppz) 3 PtCl](PF 6 ) 6 ( PtRuRuPt ) and [(tpy)Co(tppz)PtCl](PF 6 ) 3 ( CoPt ) were synthesized and characterized by 1 H and 195 Pt NMR spectroscopy, mass spectrometry and elemental analysis. The substitution kinetics of the complexes with sulfur-donor nucleophiles and ionic nucleophiles viz. ; thiourea (TU), 1-methythiourea (MTU), 1,3-dimethyl-2-thiourea (DMTU), 1,1,3,3-tetramethyl-2-thiourea (TMTU), iodide (Iˉ) and thiocynide (SCNˉ) were investigated under pseudo first-order conditions as a function of nucleophile concentration and temperature by using UV/Visible or Stopped-Flow spectrophotometry. The reactions proceed via two substitution steps for neutral S-donor nucleophiles and one reaction step for the ionic nucleophiles. The first step is due to the substitution of the chloride ligand(s) while the second step is due to the displacement of the octahedrally coordinated Co(II)/Ru(II)-tppz linker from the Pt metal to form Pt(TU) 4 . The reactivity of the complexes is influenced by the increased overall charge of the multinuclear compounds, strong π-back donation by the tppz ligand and the type of metal ion onto which the tppz ligands are octahedrally coordinated. The reactivity of the complexes is influenced by the intrinsic nucleophilicity, steric demands and polarizability of the nucleophiles. The observed low enthalpy of activation, ( ∆ H # ) and the negative entropy of activation, ( ∆ S # ) for both substitution steps support an associative mode of activation. The trends in the substitution rate constants correlated well with data from the density functional theory (DFT) calculations. • Chloride substitution from Pt(II) metal center in mixed metal complexes of Ru-Pt and Co-Pt. • Dechelation of the terpyridine chelate headgroup of the tetra-2-pyridyl-1,4-pyrazine ligand from the Pt(II) metal centre. • Reactivity of the complexes depends on the type of metal octahedrally coordinated to the tetra-2-pyridyl-1,4-pyrazine. • Further insights of the reactivity elucidated using DFT calculations.