Changes in Coordination of Sterically Demanding Hybrid Imidazolylphosphine Ligands on Pd(0) and Pd(II)

Abstract

Low-coordinate organometallic complexes are important in structure and catalysis, and hemilability or secondary interactions such as hydrogen bonding enabled by hybrid ligands are receiving increasing attention. To study the factors controlling these phenomena, three new imidazol-2-ylphosphine ligands, L, were made. In these ligands, the bulk around P and the hindrance at the basic and potentially coordinating imidazole N-3 were varied. Remarkably, L(2)Pd(0) complexes 3a-c were shown to be two-coordinate, 12-electron species, despite the availability of imidazole N-3 to enter into eta(2)-P,N chelation. In oxidative additions of C-X bonds to the Pd(0) complexes, reaction rates and products could be controlled by the nature of the C and X groups and the R groups on the phosphine. Most significantly, whereas 4c-PhI and 4c-MeOTf from 3c are normal trans-bis(phosphine)Pd(R)(X) species, 5a-PhI, 5a-PhBr, and 5b-PhI from 3a and 3b were shown by X-ray diffraction to be a monomeric species with a single eta(2)-P,N-chelating phosphine. From 3a and methyl triflate, an ionic complex [6a-Me](+)[OTf](-) with one chelating and one nonchelating phosphine was formed, with temperature-dependent windshield-wiper exchange of the two, showing hemilability. Thus, large phosphine substituents (R = tert-butyl rather than isopropyl) favor chelation. The chelate Pd-imidazole N-3 bond is longer when the heterocyclic nitrogen is hindered by an adjacent tert-butyl group at C-4 (comparing 5a-PhI and 5b-PhI). Finally, whereas in [8b-Ph](+)[OTf](-) from 5b-PhI and isopropylamine, the amine coordinates without chelate opening or hydrogen bonding, in [10c-Me](+)[OTf](-) made from 4c-MeOTf and isopropylamine, the amine is not only coordinated at N but also donates a hydrogen bond to each phosphine imidazol-2-yl substituent.

New carboalkoxybis(triphenylphosphine)palladium(II) cationic complexes: Synthesis, characterization, reactivity and role in the catalytic hydrocarboalkoxylation of ethene. X-ray structure of trans-[Pd(COOMe)(TsO)(PPh3)2]·2CHCl3

“…The CCD exploration returned about 20 Pd mononuclear complexes with the metal showing a Pd-O-S-C moiety and a Pd environment similar to the one found in our compound [45][46][47][48][49][50][51][52][53][54][55]. [17].…”

An Atropo‐Stereogenic Diphosphane Ligand with a Proximal Cationic Charge: Specific Catalytic Properties of a Palladium Complex Thereof

“…The structure of diphosphanes 4 and 5 were finally confirmed by X-ray diffraction analysis (Table 1, Figure 1). [21] Diphosphanes 4 and 5 possessing imidazolyl [23] and imidazolium [24] substituents, respectively, were then envisioned as rigidly chelating ligands. They were thus treated with a stoichiometric amount of (CH 3 CN) 2 PdCl 2 in thf.…”

Preparation, Crystal Structures, and Spectroscopic and Redox Properties of Nickel(II) Complexes Containing Phosphane–(Amine or Quinoline)‐Type Hybrid Ligands and a Nickel(I) Complex Bearing 8‐(Diphenylphosphanyl)quinoline

“…[5] In order to develop novel functionalities and catalytic and biological activities for these complexes, it is essential to design the electronic differentiations [1b,1c] of the donor groups as well as the steric requirements of the hybrid-type ligands. In this respect, we are currently focusing our attention on P-N-type hybrid ligands bearing an amine or pyridine/quinoline donor group and the investigation of their coordination behavior.…”

“…Spectroscopic and electrochemical measurements suggest that the ligand-field strengths of these four P-N-type ligands increase in the order Ph 2 Pqn (3) Ͻ MePhPqn (4) Ͻ Ph 2 Pea (1) Ͻ MePhPea (2). The Ph 2 Pqn complex 3 is readily reduced by Zn powder to afford the corresponding nickel(I) complex [Ni(Ph 2 Pqn) 2 ]BF 4 (5). The crystal structure of complex 5 reveals that the Ni I ion adopts a distorted tetrahedral coordination geometry with slightly longer (≈ 0.05 Å) Ni-P and Ni-N bond lengths than those in the corresponding Ni II complex 3.…”