Achiral and Chiral PNP-Pincer Ligands with a Carbazole Backbone: Coordination Chemistry with d⁸ Transition Metals

Abstract

Two new monoanionic PNP pincer type ligands have been synthesized, the achiral 3,6-di-tert-butyl-1,8-bis((diphenyl-phosphino)methyl)-9H-carbazole CbzdiphosH (5) and the chiral 3,6-di-tert-butyl-1,8-bis(((2R,5R)-2,5-diphenylphospholan-1-yl)methyl)-9H-carbazole CbzdipholH (7), both of which were initially prepared as their borane complexes. The synthesis of CbzdiphosH is based on the reaction between the key intermediate 1,8-bis(bromomethyl)-3,6-di-tert-butyl-9H-carbazole (3) and lithium diphenylphosphide-borane complex. The chiral ligand CbzdipholH was prepared by treating 3 with lithium (2R,5R)-2,5-diphenylphospholanide-borane complex and subsequent deprotection with diethylamine. The complexation of the two ligands with nickel, palladium and rhodium was investigated, for which the conformational behavior of the ligands was found to be different. Although the arrangement of the donor atoms in all crystallographically characterized complexes is approximately square planar, the carbazole plane in Cbzdiphos complexes is inclined relative to the coordination plane. On the other hand, a helical twist is observed in Cbzdiphol complexes.

Catalytic Water Splitting with an Iridium Carbene Complex: A Theoretical Study

“…We recently reported the synthesis of the novel PNP pincer ligand Cbzdiphos and its coordination chemistry in square‐planar d 8 metal complexes 13. Due to the flexibility of the ligand and the chelate‐ring‐size, metals that exhibit a large atomic radius, such as third‐row transition metals, are readily coordinated by the ligand.…”

Iridium Complexes Containing a PNP Pincer Ligand: Syntheses, Structural Chemistry, and Bond Activations

“…The ligand was synthesized according to literature. [13] All other chemicals were used as received without further purification. NMR spectra were recorded on Bruker Avance III (600 MHz) instruments.…”

“…The 1 H NMR spectrum, however, showed only one signal for all four methylene protons (δ = 4.03 ppm) and a broad signal for the two hydrides (δ = -25.1 ppm), thus indicating a rapid exchange between the two conformers in solution as already observed in square-planar complexes with Cbzdiphos. [13] Treatment of the dihydrido complex with a solution of ammonia in THF led to the formation of 3 (Scheme 1). The 1 H NMR spectrum displayed two signals that correspond to the hydrido ligands at δ = -21.5 and -20.0 ppm, and the three protons of the ammine ligand could be assigned to the signal at δ = -0.68 ppm in accordance with an 1 H, 15 N heteronuclear multiple bond correlation (HMBC) experiment.…”

“…Apart from a hydrogen transfer from the dihydrido complex to one equivalent of dpa to selectively yield trans-stilbene, the formation of the corresponding alkyne complex 4 was observed. The X-ray diffraction analysis ( the 13 C NMR spectrum showed a signal at δ = 82.2 ppm for the two equivalent atoms C(48) and C(49). Under catalytic conditions (excess amount of H 2 and alkyne) the alkyne was completely hydrogenated to give 1,2-diphenylethane.…”

“…[12] We recently reported the synthesis of the novel PNP pincer ligand Cbzdiphos and its coordination chemistry in square-planar d 8 metal complexes. [13] Due to the flexibility of the ligand and the chelate-ring-size, metals that exhibit a large atomic radius, such as third-row transition metals, are readily coordinated by the ligand. Iridium [14] pincer complexes have been used for a wide range of organic reactions such as hydrogenations, [15] hydroaminations, [16] asymmetric allylic substitutions, [17] alkane dehydrogenations, [18][19][20] C-H bond activations, [21,22] C-E bond activations, [23] C-C bond activations, [24] and ammonia borane dehydrogenation.…”