Heterobimetallic Carbene Complexes Bearing Cyclometalated Ir^III/Rh^III and Mixed NHCPy/PPh₃ Coordinated Pd^II Centers: Structures and Tandem Catalysis

Abstract

Dedicated to Professor F. Ekkehardt Hahn on the occasion of his 65th birthday Heterobimetallic complexes bearing NHC donor ligands are gaining immense popularity in organometallic chemistry and tandem catalysis. It is known that the NHC reacts with Pd II in the presence of pyridine to yield PEPPSI type complexes and the NHC ligands having ortho-CÀ H proton easily orthometalate to Ir III or Rh III centers. Combining these two methodologies in a stepwise fashion, we present here a series of heterobimetallic Ir III À Pd II and Rh III À Pd II complexes from a dicarbene donor ligand featuring cyclometalated Ir III or Rh III and mixed NHC^Py/PPh 3 coordinated Pd II centers. All the heterobimetallic complexes have been structurally characterized by X-ray crystallographic analysis. The heterobimetallic complexes featuring mixed NHC^PPh 3 coordinated Pd II centers show better activity in tandem Suzuki-Miyaura/transfer hydrogenation reactions compared to both, the heterobimetallic complexes possessing PEPPSI type Pd II centers, and the equimolar mixture of their mononuclear Pd II and Rh III or Ir III counterparts. The heterobimetallic complex featuring cyclometalated Ir III and mixed NHC^PPh 3 coordinated Pd II center shows excellent selectivity for 4-biphenylmethanol (isolated yield: 92 %) in tandem catalysis.

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

“…This reduction process of Pd II to Pd 0 along with the ligand loss makes the step irreversible one. The complex trans- [5] indicated the reduction peak at À1.54 V, which is also irreversible, and the re-oxidation peak associated with this reduction appeared at +0.11 V. The potential differences observed for [2] versus trans- [5] are attributed to the different σ-donor properties of pyridine and PPh 3 ligands (PPh 3 > Py). Donor strength of the ligands is also reflected in irreversible oxidation process.…”

“…The formation of cis ‐[ 8 ] was also supported with the 31 P{ 1 H} NMR spectroscopy, which showed a singlet at δ = 22.68 ppm (see Supporting Information). [ 5a,7a,11 ] A very small amount of trans ‐[ 8 ] was also obtained along with the cis ‐[ 8 ] complex. However, the trans ‐[ 8 ] could not be isolated in pure form as a spontaneous conversion of trans ‐[ 8 ] to cis ‐[ 8 ] was detected in 31 P{ 1 H} NMR spectroscopy (see Supporting Information).…”

“…[9,15] The Pd II complex with 4-chloropyridine ligands ( [3]) shows very little activity (10% conversion) for the 4-chlorobenzaldehyde substrate. The complex trans- [5] consisting of NHC/PPh 3 donor units showed the best activity (87% conversion) among all complexes in the Suzuki-Miyaura reaction using 4-chlorobenzaldehyde (Table 2). A positive Hg poison test was observed for the Suzuki-Miyaura cross-coupling reaction, possibly indicating the involvement of Pd-NPs as an active catalyst under the specified reaction conditions.…”

“…[ 1g,5 ] The poly‐NHC ligands have also been used for the synthesis and catalytic applications of Pd II PEPPSI type complexes (pyridine enhanced precatalyst preparation, stabilization, and initiation). [ 5b,d,6 ] Most of these complexes have been synthesized utilizing monodentate NHC ligands, [ 7 ] though some of the rare version of polynuclear complexes containing poly‐NHC ligands have received great attention owing to the cooperative action of metal centers in catalysis for the polynuclear complexes compared with the corresponding mononuclear complexes. [ 5b,8 ] Bimetallic PEPPSI complexes possessing fluorene‐derived di‐NHC ligands (type a ; Figure 1) have been prepared and employed for the synthesis of disubstituted oxindole derivatives using α‐arylation reaction.…”

“…[ 5b,d,6 ] Most of these complexes have been synthesized utilizing monodentate NHC ligands, [ 7 ] though some of the rare version of polynuclear complexes containing poly‐NHC ligands have received great attention owing to the cooperative action of metal centers in catalysis for the polynuclear complexes compared with the corresponding mononuclear complexes. [ 5b,8 ] Bimetallic PEPPSI complexes possessing fluorene‐derived di‐NHC ligands (type a ; Figure 1) have been prepared and employed for the synthesis of disubstituted oxindole derivatives using α‐arylation reaction. [ 6a ] Dinuclear complexes of types b and c (Figure 1) containing classical or mesoionic NHC (MIC) donor ligands possessing 1,4‐substituted phenyl ring have also been synthesized.…”