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

Here we describe the synthesis, structures, and reactivity of Ru complexes containing a triaryl, redox-active S₂N₂ ligand derived from <i>o</i>-phenylenediamine and thioanisole subunits. The coordination chemistry of <i>N</i>,<i>N′</i>-bis­[2-(methylthio)­phenyl]-1,2-diaminobenzene [H₂(^MeSNNS^Me)] was established by treating RuCl₂(PPh₃)₃ with H₂(^MeSNNS^Me) to yield {Ru­[H₂(^MeSNNS^Me)]­Cl­(PPh₃)}Cl (<b>1</b>). Coordinated H₂(^MeSNNS^Me) was sequentially deprotonated to form Ru­[H­(^MeSNNS^Me)]­Cl­(PPh₃) (<b>2</b>) followed by the five-coordinate, square pyramidal complex Ru­(^MeSNNS^Me)­(PPh₃) (<b>3</b>). 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 <i>o</i>-phenylenediamine backbone. Deprotonation of <b>2</b> with NaBH₄ or treatment of <b>3</b> with BH₃·tetrahydrofuran (THF) yielded Ru­[(μ-H)­BH₂]­(^MeSNNS^Me)­(PPh₃) (<b>5</b>) with BH₃ bound across a Ru–N bond in a metal–ligand cooperative fashion. The cyclic voltammogram of <b>3</b> in THF revealed three redox events consistent with one-electron oxidations and reductions of the <i>o</i>-phenylenediamine backbone and the metal (Ru³⁺/Ru²⁺). Reactions of <b>3</b> with CO, HBF₄, and benzoic acid yielded the new complexes Ru­(^MeSNNS^Me)­(CO)­(PPh₃), {Ru­[H­(^MeSNNS^Me)]­(PPh₃)­(THF)}­BF₄, and Ru­[H­(^MeSNNS^Me)]­(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

Measurement of Diphosphine σ‑Donor and π-Acceptor Properties in d⁰ Titanium Complexes Using Ligand K‑Edge XAS and TDDFT

Diphosphines are highly versatile ancillary ligands in coordination chemistry and catalysis because their structures and donor–acceptor properties can vary widely depending on the substituents attached to phosphorus. Experimental and theoretical methods have been developed to quantify differences in phosphine and diphosphine ligand field strength, but experimentally measuring individual σ-donor and π-acceptor contributions to metal–phosphorus bonding remains a formidable challenge. Here we report P and Cl K-edge X-ray absorption spectroscopy (XAS), density functional theory (DFT), and time-dependent density functional theory (TDDFT) studies of a series of [Ph₂P­(CH₂)_<i>n</i>PPh₂]­TiCl₄ complexes, where <i>n</i> = 1, 2, or 3. The d⁰ metal complexes (Ti⁴⁺) revealed both P 1s → Ti–P π and P 1s → Ti–P σ* transitions in the P K-edge XAS spectra, which allowed spectral changes associated with Ti–P σ-bonding and π-backbonding to be evaluated as a function of diphosphine alkane length. DFT and TDDFT calculations were used to assign and quantify changes in Ti–P σ-bonding and π-backbonding. The calculated results for [Ph₂P­(CH₂)₂PPh₂]­TiCl₄ were subsequently compared to electronic structure calculations and simulated spectra for [R₂P­(CH₂)₂PR₂]­TiCl₄, where R = cyclohexyl or CF₃, to evaluate spectral changes as a function of diphosphine ligand field strength. Collectively, our results demonstrate how P K-edge XAS can be used to experimentally measure M-P π-backbonding with a d⁰ metal and corroborate earlier studies showing that relative changes in covalent M-P σ bonding do not depend solely on changes in diphosphine bite angle

Impact of Coordination Geometry, Bite Angle, and Trans Influence on Metal–Ligand Covalency in Phenyl-Substituted Phosphine Complexes of Ni and Pd

Despite the long-standing use of phosphine and diphosphine ligands in coordination chemistry and catalysis, questions remain as to their effects on metal–ligand bonding in transition metal complexes. Here we report ligand K-edge XAS, DFT, and TDDFT studies aimed at quantifying the impact of coordination geometry, diphosphine bite angle, and phosphine trans influence on covalency in M–P and M–Cl bonds. A series of four-coordinate NiCl₂ and PdCl₂ complexes containing PPh₃ or Ph₂P­(CH₂)_<i>n</i>PPh₂, where <i>n</i> = 1 (dppm), 2 (dppe), 3 (dppp), and 4 (dppb), was analyzed. The XAS data revealed that changing the coordination geometry from tetrahedral in Ni­(PPh₃)₂Cl₂ (<b>1</b>) to square planar in Ni­(dppe)­Cl₂ (<b>2</b>) more than doubles the intensity of pre-edge features assigned to Ni–P and Ni–Cl 1s → σ* transitions. By way of comparison, varying the diphosphine in Pd­(dppm)­Cl₂ (<b>4</b>), Pd­(dppp)­Cl₂ (<b>6</b>), and Pd­(dppb)­Cl₂ (<b>7</b>) yielded Pd–P 1s → σ* transitions with identical intensities, but a 10% increase was observed in the P K-edge XAS spectrum of Pd­(dppe)­Cl₂ (<b>5</b>). A similar observation was made when comparing Ni­(dppe)­Cl₂ (<b>2</b>) to Ni­(dppp)­Cl₂ (<b>3</b>), and DFT and TDDFT calculations corroborated XAS results obtained for both series. Comparison of the spectroscopic and theoretical results to the diphosphine structures revealed that changes in M–P covalency were not correlated to changes in bite angles or coordination geometry. As a final measure, P and Cl K-edge XAS data were collected on <i>trans</i>-Pd­(PPh₃)₂Cl₂ (<b>8</b>) for comparison to the cis diphosphine complex Pd­(dppe)­Cl₂ (<b>5</b>). Consistent with phosphine’s stronger trans influence compared to chloride, a 35% decrease in the intensity of the Pd–P 1s → σ* pre-edge feature and a complementary 34% increase in Pd–Cl 1s → σ* feature was observed for <b>8</b> (trans) compared to <b>5</b> (cis). Overall, the results reveal how coordination geometry, ligand arrangement, and diphosphine structure affect covalent metal–phosphorus and metal–chloride bonding in these late transition metal complexes