Ru(II) Complexes with a Chemical and Redox-Active S₂N₂ Ligand: Structures, Electrochemistry, and Metal–Ligand Cooperativity

Abstract

Here we describe the synthesis, structures, and reactivity of Ru complexes containing a triaryl, redox-active S2N2 ligand derived from o-phenylenediamine and thioanisole subunits. The coordination chemistry of N,N′-bis­[2-(methylthio)­phenyl]-1,2-diaminobenzene [H2(MeSNNSMe)] was established by treating RuCl2(PPh3)3 with H2(MeSNNSMe) to yield {Ru­[H2(MeSNNSMe)]­Cl­(PPh3)}Cl (1). Coordinated H2(MeSNNSMe) was sequentially deprotonated to form Ru­[H­(MeSNNSMe)]­Cl­(PPh3) (2) followed by the five-coordinate, square pyramidal complex Ru­(MeSNNSMe)­(PPh3) (3). Single-crystal X-ray diffraction (XRD) studies revealed that the ligand structurally rearranged around the metal at each deprotonation step to conjugate the adjacent aryl groups with the o-phenylenediamine backbone. Deprotonation of 2 with NaBH4 or treatment of 3 with BH3·tetrahydrofuran (THF) yielded Ru­[(μ-H)­BH2]­(MeSNNSMe)­(PPh3) (5) with BH3 bound across a Ru–N bond in a metal–ligand cooperative fashion. The cyclic voltammogram of 3 in THF revealed three redox events consistent with one-electron oxidations and reductions of the o-phenylenediamine backbone and the metal (Ru3+/Ru2+). Reactions of 3 with CO, HBF4, and benzoic acid yielded the new complexes Ru­(MeSNNSMe)­(CO)­(PPh3), {Ru­[H­(MeSNNSMe)]­(PPh3)­(THF)}­BF4, and Ru­[H­(MeSNNSMe)]­(PPh3)­(PhCO2), indicating broader suitability for small molecule binding and reactivity studies. Subsequent nuclear magnetic resonance spectroscopy, infrared spectroscopy, and mass spectrometry data are reported in addition to molecular structures obtained from single-crystal XRD studies.

Influence of Multisite Metal–Ligand Cooperativity on the Redox Activity of Noninnocent N2S2 Ligands

“…The terminal B− H resonances yielded a very broad resonance with a full width at half-maximum (fwhm) of 470 Hz centered at δ 1.22 ppm. The 11 B NMR spectrum in CD 2 Cl 2 also revealed a broad resonance at δ −18.3 ppm with no clear 11 B− 1 H coupling, but the solid-state IR spectra of 2-(BH 3 ) showed distinct bridging and terminal B−H stretches at 2346, 2417, and 2432 cm −1 , respectively. These are similar to B−H stretches reported for 1-(BH 3 ) at 2338 cm −1 (B−H−Ru) and 2404 and 2430 cm −1 (terminal B−H).…”

“…These are similar to B−H stretches reported for 1-(BH 3 ) at 2338 cm −1 (B−H−Ru) and 2404 and 2430 cm −1 (terminal B−H). 11 Dual-MLC compound 1-(BH 3 ) 2 was prepared by refluxing RuCl 3 •xH 2 O with L1 and EtOH in air to form 1-Cl 2 , followed by reduction of 1-Cl 2 with 2 equiv of NaBH 4 (Figure 2a). Structures of 1-Cl 2 and 1-(BH 3 ) 2 obtained from single-crystal XRD studies revealed C s and C 2 symmetry, respectively, with chlorides and hydrides bound in the axial coordination sites.…”

“…Similar to 2-(BH 3 ), a broad 1 H resonance assigned to the terminal B−H bonds was observed at δ 1.72 ppm (fwhm = 280 Hz) and became more resolved at lower temperature. The room-temperature 11 B NMR spectrum revealed a single resonance at δ −9.3 ppm, and this peak broadened as the sample was cooled (Figure S12). Similar to 1-(BH 3 ) and 2-(BH 3 ), the IR spectrum of 1-(BH 3 ) 2 revealed bridging B−H−Ru stretches at 2336 cm −1 and terminal B−H stretches at 2411 and 2435 cm −1 .…”

“…The equilibrium observed by CV between 1 and 1-(BH 3 ) was initially unexpected because NMR data collected on crystals of 1-(BH 3 ) immediately isolated from their mother liquor showed no obvious evidence of 1 and free BH 3 •THF in C 6 D 6 and THF-d 8 . 11 However, these spectroscopic and electrochemical differences can likely be attributed to changes in BH 3 lability when 1-(BH 3 ) is oxidized under an applied potential. As shown in Scheme 4, BH 3 can be lost from 1-(BH 3 ) in THF to form BH 3 •THF before or after oxidation.…”

Insertion of BH3 into a Cobalt–Aryl Bond: Synthetic Routes to Arylborohydride and Borane-Amino Hydride Complexes

“…Surprisingly, complex 2 undergoes reversible oxidation to afford the corresponding cationic borohydride (Figure ). A cyclic voltammogram of 2 recorded in THF- n Bu 4 NPF 6 solution exhibits a redox event at −0.423 V ( i pa / i pc = 0.99), which is assigned to be a one-electron process for [ 2 ] +/0 . Oxidation of 2 by FcBAr F 4 on a preparative scale gave the cationic complex ( 2 + ) in 97% yield.…”