3 research outputs found
Ru(II) Complexes with a Chemical and Redox-Active S<sub>2</sub>N<sub>2</sub> Ligand: Structures, Electrochemistry, and Metal–Ligand Cooperativity
Here we describe
the synthesis, structures, and reactivity of Ru
complexes containing a triaryl, redox-active S<sub>2</sub>N<sub>2</sub> ligand derived from <i>o</i>-phenylenediamine and thioanisole
subunits. The coordination chemistry of <i>N</i>,<i>N′</i>-bisÂ[2-(methylthio)Âphenyl]-1,2-diaminobenzene [H<sub>2</sub>(<sup>Me</sup>SNNS<sup>Me</sup>)] was established by treating
RuCl<sub>2</sub>(PPh<sub>3</sub>)<sub>3</sub> with H<sub>2</sub>(<sup>Me</sup>SNNS<sup>Me</sup>) to yield {RuÂ[H<sub>2</sub>(<sup>Me</sup>SNNS<sup>Me</sup>)]ÂClÂ(PPh<sub>3</sub>)}Cl (<b>1</b>). Coordinated
H<sub>2</sub>(<sup>Me</sup>SNNS<sup>Me</sup>) was sequentially deprotonated
to form RuÂ[HÂ(<sup>Me</sup>SNNS<sup>Me</sup>)]ÂClÂ(PPh<sub>3</sub>) (<b>2</b>) followed by the five-coordinate, square pyramidal complex
RuÂ(<sup>Me</sup>SNNS<sup>Me</sup>)Â(PPh<sub>3</sub>) (<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
NaBH<sub>4</sub> or treatment of <b>3</b> with BH<sub>3</sub>·tetrahydrofuran (THF) yielded RuÂ[(μ-H)ÂBH<sub>2</sub>]Â(<sup>Me</sup>SNNS<sup>Me</sup>)Â(PPh<sub>3</sub>) (<b>5</b>) with
BH<sub>3</sub> 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 (Ru<sup>3+</sup>/Ru<sup>2+</sup>). Reactions of <b>3</b> with CO, HBF<sub>4</sub>, and benzoic acid yielded the new
complexes RuÂ(<sup>Me</sup>SNNS<sup>Me</sup>)Â(CO)Â(PPh<sub>3</sub>),
{RuÂ[HÂ(<sup>Me</sup>SNNS<sup>Me</sup>)]Â(PPh<sub>3</sub>)Â(THF)}ÂBF<sub>4</sub>, and RuÂ[HÂ(<sup>Me</sup>SNNS<sup>Me</sup>)]Â(PPh<sub>3</sub>)Â(PhCO<sub>2</sub>), 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 d<sup>0</sup> 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 [Ph<sub>2</sub>PÂ(CH<sub>2</sub>)<sub><i>n</i></sub>PPh<sub>2</sub>]ÂTiCl<sub>4</sub> complexes, where <i>n</i> = 1,
2, or 3. The d<sup>0</sup> metal complexes (Ti<sup>4+</sup>) 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
[Ph<sub>2</sub>PÂ(CH<sub>2</sub>)<sub>2</sub>PPh<sub>2</sub>]ÂTiCl<sub>4</sub> were subsequently compared to electronic structure calculations
and simulated spectra for [R<sub>2</sub>PÂ(CH<sub>2</sub>)<sub>2</sub>PR<sub>2</sub>]ÂTiCl<sub>4</sub>, where R = cyclohexyl or CF<sub>3</sub>, 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 d<sup>0</sup> 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
NiCl<sub>2</sub> and PdCl<sub>2</sub> complexes containing PPh<sub>3</sub> or Ph<sub>2</sub>PÂ(CH<sub>2</sub>)<sub><i>n</i></sub>PPh<sub>2</sub>, 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 NiÂ(PPh<sub>3</sub>)<sub>2</sub>Cl<sub>2</sub> (<b>1</b>) to square planar in NiÂ(dppe)ÂCl<sub>2</sub> (<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 PdÂ(dppm)ÂCl<sub>2</sub> (<b>4</b>), PdÂ(dppp)ÂCl<sub>2</sub> (<b>6</b>),
and PdÂ(dppb)ÂCl<sub>2</sub> (<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 PdÂ(dppe)ÂCl<sub>2</sub> (<b>5</b>). A similar observation was made when comparing
NiÂ(dppe)ÂCl<sub>2</sub> (<b>2</b>) to NiÂ(dppp)ÂCl<sub>2</sub> (<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>-PdÂ(PPh<sub>3</sub>)<sub>2</sub>Cl<sub>2</sub> (<b>8</b>) for comparison to the cis diphosphine complex PdÂ(dppe)ÂCl<sub>2</sub> (<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