Cr(II) and Cr(I) PCP Pincer Complexes: Synthesis, Structure, and Catalytic Reactivity

Abstract

In the current investigation, the reaction of [Cr(CO)6] with the ligand precursor PO(C-Br)OP-tBu (1) was investigated. When a suspension of [Cr(CO)6] and 1 in toluene was transferred into a sealed microwave glass vial and stirred for 3 h at 180 o C the square-planar Cr(II) complex [Cr(POCOP-tBu)Br] (2) was obtained. Treatment of 2 with 1 equiv of Li[HBEt3] in THF led to the formation of the borohydride complex [Cr(POCOP-tBu)(κ 2-BH4)] (3). Exposure of a toluene solution of 3 to NO gas (1 bar) at room temperature affords the Cr(I) complex [Cr(POCOP-tBu)(NO)(κ 2-BH4)] (4). Alternatively, 4 was also obtained by reacting [Cr(POCOP-tBu)(NO)(Br)] (5) with Li[HBEt3]. Based on magnetic and EPR measurements as well as DFT calculations, compounds 4 and 5 adopt a low-spin d 5 configuration and feature a nearly linear bound NO ligand suggesting Cr I NO + rather than Cr II NO • character. The reaction of 2 with 1 equiv of LiCH2SiMe3 in toluene afforded the square planar alkyl complex [Cr(POCOP-tBu)(CH2SiMe3] (6) in 57% yield. This compound is catalytically active for the hydrosilylation of ketones at room temperature with a catalyst loading of 0.5 mol%. X-ray structures of all complexes are presented.

Azanone (HNO): generation, stabilization and detection

“…126 NOstabilization by complexes with pincer scaffolds is far better known. Several structural analyses of this kind of species exist in bibliography, 124,[127][128][129][130][131][132][133] along with reports of their catalytic activity. [134][135][136] Within our group, several rhodium-based pincer ligand systems have been studied (the most relevant are highlighted in Figure 3b).…”

Bringing Homogeneous Iron Catalysts on the Heterogeneous Side: Solutions for Immobilization

“…), where the second symbol always identifies the central coordinating atom. By virtue of such a strong interaction, the resulting complexes are highly thermally stable and have revealed interesting catalytic properties in various applications: hydrogenation reactions of olefins, amides, nitriles, esters and dehydrogenation, cross-coupling, hydroboration, and hydrosilylation [ 87 , 88 , 89 , 90 , 91 ].…”

Structure and Reactivity of NO/NO+/NO−Pincer and Porphyrin Complexes

“…The latter allows them to deduce the number of d electrons, thus obtaining the metallic center oxidation state and the overall MNO configuration. [22,[30][31][32] However, it must be noted that the reliability of this strategy depends strongly on the quality of the calculations performed, especially since transition metals can be difficult to model. Therefore, this tool is best applied when complemented by some experimental characterizations.…”

“…Most used is the direct method of nitrosylation by injection of gaseous nitric oxide into a solution of the pincer precursor complex. [22,26,27,30,[33][34][35] This route was first employed by Milstein's group, who proved Rh(PNP)(NO)(Cl) (1 + ) obtention could be achieved both by reaction of a Rh(II) precursor and NO * and by reaction of Rh(I) and NO + , as is depicted in Scheme 1. [33] This last alternative has been scarcely reported thereafter.…”

“…[37] Complex ( 16) was shown to be catallytically active for the coupling of primary alcohols to amines, whereas complex (15) catalyzed the hydrosilylation of ketones. [30,31] Pecak and coworkers' have also studied the use of a cobalt hydride complex derived from (18 + ) as a catalyst for the hydroboration of alkenes, and have used DFT calculations to elucidate the mechanism involved. Interestingly, they found nitrosyl to play an important part, since strong bending of the CoNO angle and its reversion were found to be involved, thus revealing the ligand to act as a reservoir of electrons (Scheme 5).…”