Synthesis, Characterization, and Catalytic Reactivity of {CoNO}8 PCP Pincer Complexes.

Jan Pecak,Wolfgang Linert,Wolfgang Eder,Berthold Stöger,Maria José Calhorda,Sara Realista,Karl Kirchner,Paulo N Martinho

doi:10.1021/acs.organomet.0c00167

Abstract

The reaction of coordinatively unsaturated Co(II) PCP pincer complexes with nitric oxide leads to the formation of new, air-stable, diamagnetic mono nitrosyl compounds. The synthesis and characterization of five- and four-coordinate Co(III) and Co(I) nitrosyl pincer complexes based on three different ligand scaffolds is described. Passing NO through a solution of [Co(PCPNMe-iPr)Cl], [Co(PCPO-iPr)Cl] or [Co(PCPCH2-iPr)Br] led to the formation of the low-spin complex [Co(PCP-iPr)(NO)X] with a strongly bent NO ligand. Treatment of the latter species with (X = Cl, Br) AgBF4 led to chloride abstraction and formation of cationic square-planar Co(I) complexes of the type [Co(PCP-iPr)(NO)]+ featuring a linear NO group. This reaction could be viewed as a formal two electron reduction of the metal center by the NO radical from Co(III) to Co(I), if NO is counted as NO+. Hence, these systems can be described as {CoNO}8 according to the Enemark–Feltham convention. X-ray structures, spectroscopic and electrochemical data of all nitrosyl complexes are presented. Preliminary studies show that [Co(PCPNMe-iPr)(NO)]+ catalyzes efficiently the reductive hydroboration of nitriles with pinacolborane (HBpin) forming an intermediate {CoNO}8 hydride species.

Highlights

The investigation of pincer ligands and their transition metal complexes has received significant attention during the past decades.[1]
An overview of existing group 9 nitrosyl complexes being supported by pincer type ligands is depicted in Chart 1.5−7 Recently our group reported a series of chromium, manganese, and iron nitrosyl complexes supported by aromatic PCP and PNP pincer ligands.[8]
We describe a series of well-defined four- and five-coordinate low-spin {CoNO}8 nitrosyl PCP pincer

Summary

■ INTRODUCTION

The investigation of pincer ligands and their transition metal complexes has received significant attention during the past decades.[1] Their adjustable steric and electronic properties enable the stabilization of various chemical systems and allow for the investigation of fundamental reactivity and for the design of innovative catalysts Since their introduction by Moulton and Shaw in 1976, no pincer ligand has retained as much attention as the PCP scaffold based on the phosphinomethylated benzene.[2] Related pincer ligands that feature an aromatic anionic benzene backbone connected to phosphine donors via CH2, O, or NR (R = H, alkyl or aryl) linkers provide for a very attractive class of ligands that are simple to prepare and modify. With respect to first-row transition metals, the chemistry of nickel PCP complexes is already quite comprehensive, but studies of cobalt, iron, manganese and chromium PCP pincer complexes are exceedingly rare This may be attributed to the failure of many simple metal salts to cleave the C−H bonds of the arene moiety of the pincer ligands and/or the thermodynamic instability of the resulting complexes.

■ RESULTS AND DISCUSSION

■ CONCLUSION

■ REFERENCES