Dinuclear Pd^II complexes bearing mixed NHC/Py/PPh₃ donor set ligands: Catalytic applications and electrochemical investigations

Abstract

Bimetallic palladium(II) complexes containing classical NHC donor ligands are becoming increasingly popular owing to their various catalytic applications. However, examples of the aforementioned complexes with mixed NHC/PPh3 ligands are still rare. Bimetallic palladium(II) complexes possessing these mixed ligands are described starting from a C2‐symmetric bis‐imidazolium salt containing 4,4′‐substituted central biphenyl ring. All the palladium(II) complexes have been tested as precatalysts in α‐arylation of oxindole and Suzuki–Miyaura coupling reactions. The complex composed of mixed NHC/PPh3 donor ligands shows superior catalytic activity compared with the corresponding PEPPSI type complexes when applied in α‐arylation of oxindole. The dinuclear complexes display better activity compared with the mononuclear complexes. The preliminary electrochemical measurements show the facile oxidation of PdII in the presence of combined NHC/PPh3 ligands compared with a combination of NHC/Py ligands.

Tetranuclear PdII Complexes Containing an Adamantane-Based Tetra-MIC Ligand: Ancillary-Ligand-Dependent Catalytic and Redox Activities

“…Excellent yields of the C–C coupling products were achieved with precatalyst [ 4 ], which were distinctly higher than the yields achieved with Pd II complexes [ 2 ] and [ 3 ] (Table : entry 2 vs 4/5; entry 6 vs 7/8; entry 9 vs 10/11). The better activity of the palladium(II) MIC complexes in the presence of PPh 3 as an ancillary ligand over pyridine-coordinated complexes has also been observed in earlier studies with analogous complexes. , The triphenylphosphine ligand supplies more electron density as compared to the pyridine ligand around the Pd 0 in the active Pd 0 (PPh 3 )(MIC) catalyst, which may facilitate the subsequent oxidative addition of aryl bromide. This type of observation has also been validated previously by electrochemical measurements .…”

“…This type of observation has also been validated previously by electrochemical measurements . The Sonogashira coupling reaction was sluggish using tetranuclear complexes [ 2 ]–[ 4 ] when compared to the other palladium(II) MIC complexes described previously. , Therefore, a higher palladium(II) loading was used for these tetranuclear complexes to give better yields. The mercury poisoning test was negative, and therefore, the Sonogashira coupling reaction proceeds via a homogeneous pathway under the mentioned reaction conditions (Table , entry 3).…”

Dinuclear platinum(ii) complexes with biphenyl-based bis-carbene ligands

“…The bis-imidazolium salt 1 15 gave dinuclear platinum( ii ) complexes [ 2 ] or [ 3 ] when treated with K 2 CO 3 and K 2 [PtCl 4 ] in the presence of pyridine or 4-chloropyridine (Scheme 1). Both the complexes remained unchanged when stored in air and moisture.…”

“…The corresponding dinuclear palladium( ii ) complexes possessing a bis-imidazolylidene donor ligand have been synthesized successfully without the sign of any C sp 2 –C sp 2 decoupling reaction. 15 Here, we report the synthesis of a rare version of dinuclear platinum( ii ) complexes comprising bis-NHC donor ligands featuring the 4,4′-di-(NHC)-substitution pattern of the central biphenyl ring without any sign of decoupling or demethylation reactions.…”

Trinuclear PdII Complexes with a C3‐Symmetric Triethynylbenzene‐Based Tris‐NHC Ligand: Catalytic Benefits

“…The slightly lower negative reduction potential of the trinuclear complex might be helpful for the favourable reduction of the pre‐catalyst (Pd II ) 3 to active (Pd 0 ) 3 catalyst in trinuclear system over the reduction of pre‐catalyst Pd II to active Pd 0 catalyst in the mononuclear system [20c] . The admirable catalytic outcomes of complexes ([ 6 ] and [ 9 ]) having a combination of PPh 3 and NHC donor ligands over the PEPPSI type complexes ([ 5 ] and [ 8 ]) bearing the NHC and Py combination, possibly due to greater assembled electron density at active catalyst Pd 0 (NHC)(PPh 3 ) compared to the active catalyst Pd 0 (NHC)(Py), which is expected to favor the subsequent oxidative addition of Pd 0 (NHC)(PPh 3 ) with Ar−X in the catalytic cycle [20c,23] . This observation is also well supported with different donor strength of Py and PPh 3 ligands (Py<PPh 3 ) as reffered from their HEPs (Huynh's Electronic Parameters) [24]…”