Trianionic NCN^3– Pincer Complexes of Chromium in Four Oxidation States (Cr^II, Cr^III, Cr^IV, Cr^V): Determination of the Active Catalyst in Selective 1-Alkene to 2-Alkene Isomerization

Abstract

The synthesis and characterization of a series of four Cr complexes in the +2, +3, +4, and +5 oxidation states supported by an NCN trianionic pincer ligand are reported. Treating CrMeCl2(THF)3 with the dilithio salt pincer ligand precursor {[2,6- i PrNCHN]Li2}2 provides the CrIII complex [2,6- i PrNCN]CrIII(THF)3 (1), CrIV complex [2,6- i PrNCN]CrIVMe(THF) (2), and CrII complex [2,6- i PrNHCN]CrII(THF)2 (3). Complexes 2 and 3 are the result of disproportionation. Treating 1 with 1 equiv of styrene oxide in THF converts the CrIII complex to the CrV(O) species [2,6- i PrNCN]CrV(O)(THF) (4). Complex 2, characterized by single-crystal X-ray diffraction, is a rare CrIV methyl complex that is kinetically stable at 25 °C; at 85 °C, Cr–Me bond homolysis occurs. The homolytic cleavage results in CH4 formation and biphenyl via a radical mechanism. The metal-containing product from thermolysis is the same CrII species formed during metalation, except one of the protons is substituted for a deuterium from C6D6 (3- d ). Complex 2 is a precatalyst for the selective isomerization of 1-hexene and 1-octene to the corresponding 2-alkenes. An induction period occurs during the catalytic isomerization, and the active catalyst was determined to be the CrII complex 3, not 2.

Alkene Isomerisation Catalysed by a Ruthenium PNN Pincer Complex

“…A reductive elimination can take place in two directions and will provide either the olefin isomer or the starting olefin. [42] Among the catalysts developed for this reaction, only a few belong to the family of the pincer complexes [43][44][45][46][47] and, to our surprise, the reactivity of PNN (PNN: 2-di-tert-butylphosphinomethyl-6-diethylaminomethylpyridine) ruthenium complexes with alkenes has not been explored. In particular, the pyridinebased PNN ruthenium pincer complexes are of great interest, due to the unique participation of the ligand in the reaction (non-innocent ligand).…”

A neutral trianionic pincer [NCN]CrIV–Me complex as a highly active ethylene polymerization precatalyst

“…The catalytic activity increases significantly with an increase in reaction temperature from 25 C to 75 C, but decreases from 75 C to 100 C, indicating the system becomes unstable at some temperature above 75 C (Table 1, correlates to the known thermal instability of 1 above 85 C [48]. Varying the TIBA:1 M ratio in polymerization experiments reveals a surprising optimal activity (4.00 Â 10 6 g PE(molCr) À1 h À1 ) at 10 equiv.…”

“…The trianionic pincer ligand stabilizes the electronically deficient Cr IV d 2 metal center through strong s-donation from the pincer arylide and two pdonating anilides. The long CreTHF bond length of 2.1878(15) Å [48] in 1 suggests the THF is labile and permits rapid coordination of ethylene. The high activity (Gibson's scale [88]; 351 g PE(mmolCr) À1 h À1 barr À1 ) is consistent with theoretical studies by Ziegler et al that indicate Cr IV model complexes should be efficient catalysts because fewer d-electrons open a lower metalbased LUMO acceptor orbital [89].…”

“…B(C 6 F 5 ) 3 was purchased from Strem Chemicals and used as received. [2,6-i PrNCN]Cr IV Me(THF) [48] was prepared according to literature procedure. NMR spectra were obtained on Varian Mercury 300 MHz, Varian Mercury Broad Band 300 MHz, or Varian INOVA2 500 MHz spectrometers.…”

“…Herein, we report the novel trianionic pincer supported Cr IV eMe precatalyst, [2,6- [48], that catalyzes ethylene polymerization with high activity when activated with minimal triisobutylaluminum (TIBA). Trianionic pincer ligands [48e61] constrain three anionic donors in a meridional plane that permit substrate attack at unhindered vacant coordination sites.…”