Electrochemical oxidation of diaquaruthenium(II) complexes of quaterpyridines and crystal structure of [RuL1(PPh3)2][ClO4]2(L1= 3″,5,5′,5‴-tetramethyl-2,2′: 6′,2″: 6″,2‴-quaterpyridine)

Abstract

The perchlorate salts of [RuL²(OH₂)₂]²⁺1 and [RuL¹(OH₂)₂]²⁺2(L²= 2,2′:6′,2″:6″,2‴-quaterpyridine, L¹= 3″,5,5′,5‴-tetramethyl-2,2′:6′,2″:6″,2‴-quaterpyridine) have been isolated. In acidic solutions both 1 and 2 exhibit reversible Ru^VI–Ru^IV and Ru^III–Ru^II couples, though the Ru^IV–Ru^III couples are only quasireversible. The effect of pH on the E_½ of these couples has been investigated. At pH 1, the E° values of the [RuL²O₂]²⁺–[RuL²(O)(OH₂)]²⁺ and [RuL¹O₂]²⁺–[RuL¹(O)(OH₂)]²⁺ couples are 1.12 and 1.05 V vs. saturated calomel electrode respectively. The crystal structure of [RuL¹(PPh₃)₂][ClO₄]₂·MeCN·H₂O has been determined: monoclinic space group P2₁/n, a= 13.210(4), b= 29.711(7), c= 14.878(3)Å, β= 98.89(2)°, Z= 4. In this complex the quaterpyridine ligand adopts a non-planar conformation with a dihedral angle of 28.3° between the two central pyridyl rings. The two triphenylphosphine ligands are in a trans configuration [P(1)–Ru–P(2) 177.6°]. The complex [RuL²(OH₂)₂]²⁺ is an active catalyst for the electrochemical oxidation of propan-2-ol.