Concurrent Stabilization of π‐Donor and π‐Acceptor Ligands in Aromatized and Dearomatized Pincer [(PNN)Re(CO)(O)₂] Complexes

Abstract

Aromatized cationic [(PNN)Re(π acid)(O)2](+) (1) and dearomatized neutral [(PNN*)Re(π acid)(O)2] (2) complexes (where π acid=CO (a), tBuNC (b), or (2,6-Me2)PhNC (c)), possessing both π-donor and π-acceptor ligands, have been synthesized and fully characterized. Reaction of [(PNN)Re(O)2](+) (4) with lithiumhexamethyldisilazide (LiHMDS) yield the dearomatized [(PNN*)Re(O)2] (3). Complexes 1 and 2 are prepared from the reaction of 4 and 3, respectively, with CO or isocyanides. Single-crystal X-ray structures of 1 a and 1 b show the expected trans-dioxo structure, in which the oxo ligands occupy the axial positions and the π-acidic ligand occupies the equatorial plane in an overall octahedral geometry about the rhenium(V) center. DFT studies revealed the stability of complexes 1 and 2 arises from a π-backbonding interaction between the d(xy) orbital of rhenium, the π orbital of the oxo ligands, and the π* orbital of CO/isocyanide.

Oxidorhenium(V) and Rhenium(III) Complexes with m‐Terphenyl Isocyanides

“…Their IR spectra show the ν(ReO) bands at 966 cm –1 for (NBu 4 )[ReOCl 4 (CNAr Mes2 )] and 978 cm –1 for the CNAr Dipp2 complex, respectively. This is in the normal range for rhenium(V) oxido complexes, and similar absorptions have also been found for [ReOCl 3 (CNCMe 3 ) 2 ] and [ReOCl 3 {CN‐C 6 H 3 ‐(2,6‐Me) 2 } 2 ] , …”

“…Their IR spectra show the ν(ReO) bands at 966 cm -1 for (NBu 4 )[ReOCl 4 (CNAr Mes2 )] and 978 cm -1 for the CNAr Dipp2 complex, respectively. This is in the normal range for rhenium(V) oxido complexes, [24] and similar absorptions have also been found for [ [26,28] The m-terphenyl isocyanides in the (NBu 4 )[ReOCl 4 (CNAr R )] complexes are bonded in the equatorial coordination spheres cis to the oxido ligands as has been found by single-crystal X-ray diffraction. Figure 2 depicts the molecular structures of the complex anions.…”

“…Ellipsoid plots of the structures and more details of the structure determinations are contained in the Supporting Information. The Re-C1 bonds are slightly shorter than in other rhenium(V) complexes with aliphatic isocyanides, [24,25] but similar to those in the dimeric complex [Re 2 O 3 Cl 3 {CN-C 6 H 3 -(2,6-Me)} 4 ]. [26] Reactions of the siderable amounts of the corresponding m-terphenyl isocyanates, ONCAr R , were present in the final reaction mixtures.…”

“…There are only a few examples of structurally characterized oxidorhenium(V) complexes with isocyanides. Most of them possess {ReO 2 } + or {Re 2 O 3 } 4+ cores, [23][24][25][26] which is a common structural feature of such complexes with neutral ligands and is also frequently found with related amines, imines or chelating phosphines. [27] To the best of our knowledge, there are only two reports about monooxidorhenium(V) complexes with isocyanide ligands: [26,28] Herein, we report an initial survey of reactions between m-terphenyl isocyanides and oxorhenium(V) complexes.…”

Carbon Dioxide Reduction to Silyl-Protected Methanol Catalyzed by an Oxorhenium Pincer PNN Complex

“…We have recently reported the synthesis of the high-valent dioxorhenium PNN pincer complex [(PNN)­Re­(O) 2 ]­[OTf] ( 2 ; OTf = CF 3 SO 3 – ) . In this communication, we describe the catalytic conversion of CO 2 to silyl formate under mild conditions (25 °C, 100 psig) with Me 2 PhSiH as the reductant.…”