Synthesis, structures, and electrochemistry of palladium and platinum macrocyclic complexes of [18]aneN2S4(1,4,10,13-tetrathia-7,16-diazacyclo-octadecane) and Me2[18]aneN2S4(7,16-dimethyl-1,4,10,13-tetrathia-7,16-diazacyclo-octadecane). Single crystal X-ray structures of [Pd(Me2[18]aneN2S4)][PF6]2·Me2CO, [Pd([18]aneN2S4)][BPh4]2, and [Pd2Cl2([18]aneN2S4)][PF6]2·2MeCN

Abstract

Reaction of PdCl₂ with the N₂S₄ donor macrocycles [18]aneN₂S₄(1,4,10,13-tetrathia-7,16-diazacyclo-octadecane) and Me₂[18]aneN₂S₄(7,16-dimethyl-1,4,10,13-tetrathia-7,16-diazacyclo-octadecane) affords the 1 : 1 complex cations [Pd([18]aneN₂S₄)]²⁺ and [Pd(Me₂[18]aneN₂S₄)]²⁺ respectively in high yield. The yellow complex [Pd(Me₂[18]aneN₂S₄)][PF₆]₂·Me₂CO crystallises in the orthorhombic space group Pcab, with a= 14.3369(15), b= 17.691 5(7), c= 24.295 2(11)Å, and Z= 8. The single crystal X-ray structure of [Pd(Me₂[18]aneN₂S₄)]²⁺ shows square-planar coordination at Pd^II with the four thioether donors of the macrocycle bound to the metal centre, Pd–S(1) 2.339 9(22), Pd–S(4) 2.333 1(22), Pd–S(10) 2.326 1(22), and Pd–S(13) 2.323 9(21)Å; the two N atoms are non-bonding, Pd ⋯ N(7) 3.744(7) and Pd ⋯ N(16) 3.760(6)Å. The green complex [Pd([18]aneN₂S₄)][BPh₄]₂ crystallises in the monoclinic space group P2₁/c, with a= 16.888 8(12), b= 16.553 3(15), c= 18.537 6(12)Å, β= 93.144(8)°, and Z= 4. The structure of [Pd([18]aneN₂S₄)]²⁺ is markedly different to that of [Pd(Me₂[18]aneN₂S₄)]²⁺ and shows the Pd^II ion co-ordinated to an N₂S₂ donor set in a square-planar configuration, Pd–S(1) 2.311(3), Pd–N(7) 2.123(7), Pd–S(13) 2.357(3), and Pd–N(16) 2.068(7)Å; the S(1)–N(16)–S(13) linkage binds meridionally to the metal centre, The two remaining thioether donors interact at long range with the metal centre, Pd ⋯ S(10) 2.954(4) and Pd ⋯ S(4) 3.000(3)Å with S(4)–Pd–S(10) 158.94(9)°. The overall stereochemistry at Pd^II is therefore tetragonally distorted octahedral with a formal [N₂S₂+ S₂] co-ordination sphere. The structural differences between [Pd(Me₂[18]aneN₂S₄)]²⁺ and [Pd([18]aneN₂S₄)]²⁺ are reflected in their spectroscopic and redox properties. Cyclic voltammetry of [Pd(Me₂[18]aneN₂S₄)][PF₆]₂ in MeCN (0.1 mol dm^–3 NBuⁿ₄PF₆ supporting electrolyte) at platinum electrodes shows a chemically reversible one-electron reduction at E_1/2=–0.74 V vs. ferrocene–ferrocenium. Controlled potential electrolysis affords a bright yellow paramagnetic species which has been assigned on the basis of coulometry, e.s.r. and u.v.–visible spectroelectrochemistry as a mononuclear d⁹ Pd^I species. No oxidative activity is observed for [Pd(Me₂[18]aneN₂S₄)][PF₆]₂ within the range of the solvent up to +2.0 V. In contrast, cyclic voltammetry of [Pd([18]aneN₂S₄)][PF₆]₂ shows a chemically reversible one-electron oxidation at E_1/2=+0.57 V vs. ferrocene-ferrocenium (ΔE_p= 195 mV at a scan rate of 150 mV s^–1). Controlled potential electrolysis affords a bright-red paramagnetic product which is assigned on the basis of coulometry, e.s.r. and u.v.–visible spectroelectrochemistry as a mononuclear d⁷ Pd^III species. Two irreversible reductions are also observed for [Pd([18]aneN₂S₄)][PF₆]₂ at E_pc=–1.01 and –1.54 V vs. ferrocene–ferrocenium. Reaction of PdCl₂ with Me₂[18]aneN₂S₄ or [18]aneN₂S₄ in a 2 : 1 molar ratio affords the corresponding binuclear complexes [Pd₂Cl₂(Me₂[18]aneN₂S₄)]²⁺ and [Pd₂Cl₂([18]aneN₂S₄)]²⁺. The complex [Pd₂Cl₂([18]aneN₂S₄)][PF₆]₂·2MeCN crystallises in the monoclinic space group C2/c, with a= 18.617(13), b= 15.569(11), c= 14.323(14)Å, β= 113.59(5)°, and Z= 4. The single crystal X-ray structure of [Pd₂Cl₂([18]aneN₂S₄)]²⁺ shows each Pd^II bound in a square plane to two S-donors, Pd–S(1) 2.317(4), and Pd–S(7) 2.316(4)Å, and to one N-donor, Pd–N(4) 2.049(13)Å, of the macrocycle, and a terminal Cl^– ligand, Pd–Cl 2.305(4)Å. The Cl^– ligand is displaced out of the least-squares plane S(1)–N(4)–S(7)–Pd by 0.0712 Å due to the steric influence of the central methylene groups bridging the S-donors. The closest non-bonded interaction of 3.406(2)Å is between two Pd atoms in adjacent molecules related by a crystallographic two-fold axis. The intramolecular Pd ⋯ Pd distance is 4.196(2)Å. The synthesis and structures of the analogous mono- and bi-nuclear Pt^II complexes [Pt(Me₂[18]aneN₂S₄)]²⁺, [Pt([18]aneN₂S₄)]²⁺, and [Pt₂Cl₂([18]aneN₂S₄)]²⁺ are also discussed.