Bulky and Modular 3,3′‐Bipyrazoles as Ligands: Synthesis, Characterization, and Catalytic Activity of Pd Complexes

Abstract

In the present study, the properties of a new bidentate N,N′-chelating ligand class that bears an electron-excessive 3,3′-bipyrazole core have been investigated. The ligands are easily accessible in a three-step procedure by condensation with diethyl oxalate followed by tandem condensation with hydrazine hydrate and finally by aryl- or alkylation exclusively at the N-1,1′-pyrazole positions to furnish overall eleven new ligands with different electronic properties. After structural analysis of the ligands, their coordination to palladium, copper, and cobalt has been studied. These ligands coordinate the 2,2′-pyrazolyl nitrogen atoms in a bidentate fashion to the metals to realize complexes with an (L)MX₂ motif. We present two crystal structures of Pd and Cu complexes, which to the best of our knowledge represent the first d⁸ and d⁹ 2,2′-bipyrazole compounds coordinated through bidentate complexation. Initial catalytic experiments have been performed with palladium complexes with three bipyrazole ligands of this new class; the palladium-catalyzed copper-free Wacker oxidation of different alkenes showed superior activity compared to 2,2′-bipyridines. We attribute this to a higher redox potential of the 3,3′-bipyrazoles, which are ― besides electronic effects ― also strongly influenced by steric effects. These might be enforced by the extended ligand backbone, the choice of the wingtip substitution, and the smaller coordination cavity within the N²,N^2′ atoms compared to 2,2′-bipyridine ligands

Synthesis and Crystal Structure of C1-Symmetric 3,3′-Bi(1,1′-dinaphthyl-camphopyrazole)

“…Optical rotations were measured on a Perkin-Elmer 341 polarimeter. (1,1 -dinaphthyl-camphopyrazole) [Systematic Name: (1,7 ,8,8,8 ,4 ,5,5 ,6,6 ,7,…”

“…However, chiral pyrazole ligands derived from camphor coordinated to transition metals and the catalytic applications of the latter have been less explored [4]. Recently, chiral 3,3 -bipyrazole compounds have been proposed as important ligands that exert electronic and steric stronger influence to the metal center [5,6]. These ligands were prepared by condensation reaction between a bis-1.3-diketone previously reported [7,8] and hydrazine hydrate, followed by aryl-or alkylation.…”

C2 and C1‐Symmetric Camphopyrazole Dioxomolybdenum(VI) Complexes catalyze the Epoxidation of Cyclic Olefins

“…In this sense, the development of new chiral ligands and molybdenum complexes is of great interest. In particular, ligand designs with C 1 and C 2 symmetry have appeared that use (+)‐camphor as an inexpensive, readily available, and easily modified building block from the chiral pool [37,38,39,40,41] . We therefore set out to prepare new chiral dioxomolybdenum(VI) complexes bearing (+)‐camphor‐derived bis‐pyrazole ligands [42] with different symmetries and to evaluate their catalytic performance in the epoxidation of cyclohexene and cis ‐cyclooctene with cumyl hydroperoxide or aqueous hydrogen peroxide.…”

“…2CH 2 ), 1.89-1.93 (m, 2H, 2CH), 2.22-2.27 (m, 2H, 2CH), 3.00-3.01 (d, J = 4.0 Hz, 2H, 2CH), 7 41. 7.43 (m, 6H, 6CH), 7.59-7.61 (m, 4H, 4CH).…”