1,2-Bis(di-tert-butylphosphino)imidazole (dtbpi): A Versatile Imidazole-Based, Rigid, Bulky Bisphosphine Ligand for Transition Metals

Abstract

A bulky bisphosphine, 1,2-bis(di-tert-butylphosphino)imidazole (dtbpi), featuring a rigid imidazole backbone, was synthesized in a simple two-step procedure. The ligand’s coordination chemistry and electron-donating abilities were investigated in the complexes (dtbpi)Ni(FDN) (3) and [(dtbpi)Rh(CO)2]BF4 (4). Protonation of 4 at the ligand backbone’s nitrogen with strong Brønsted acids such as HBF4 and HBArF gave the dicationic dicarbonyl complexes [(H-dtbpi + )Rh(CO)2](BF4)(BArF) (5a) and [(H-dtbpi + )Rh(CO)2](BF4)2 (5b), which served as model complexes of the ligand’s imidazolium-phosphine form and permitted characterization of its donating character. In order to evaluate solely the different electronic characters of the two isosteric P donors, the complexes (dtbpi)PdCl(Me) (6) and [(dtbpi)RhCl]2 (7) were synthesized. An unexpected rearrangement of one of the formed stereoisomers of 6, with cleavage of an N–P bond, was found. A novel phosphine-phosphonium ylide ligand (8) has been made accessible via chemoselective alkylation of the carbon-bound P donor of dtbpi to yield the phosphonium salt [P-CH3-dtbpi]BF4 (2). The ylide 8 was generated in situ by deprotonation of 2 with KHMDS. Its coordination to the PdCl(Me) fragment led to the isolation of exclusively the Z isomer of [(8)PdCl(Me)] ( Z-9). X-ray structure analysis of [(dtbpi)Rh(NBD)]BF4 (10) and its dicationic analogue [(H-dtbpi + )Rh(NBD)](BF4)(BArF) (11) revealed the structural changes provoked upon ligand backbone protonation.

Asymmetric Cationic Phosphines: Synthesis, Coordination Chemistry, and Reactivity

“…The P1-N1 bond length is somewhat long reflecting little to no π-contribution and the N1-C11 bond is slightly longer than N2-C11 which is a feature common to all the structures reported herein. 3; P1-N1 1.716(4); P1-C1 1.823(4); P1-C6 1.827(4); N1-C11 1.343(6); N2-C11 1.292(6); N1-C11-N2 123.5(4); P1-N1-C11 125.0(3); C1-P1-C6 97.98 (18); N1-P1-C1 109.28 (19); N1-P1-C6 109.17 (19); N1-P1-O4 108.31 (18).…”

“…The BF4 -anions, hydrogen atoms and residual solvent have been omitted for clarity. Selected bond lengths (Å) and angles (°): Rh1-P1 2.3241(11); Rh1-P2 2.3248(11); Rh1-Cl1 2.3622(10); Rh1-C41 1.843(4); C41-O7 1.126(5); P2-N3 1.751(4); P2-C21 1.865(5); P2-C24 1.876(5); N3-C31 1.338(6); N4-C31 1.286(6); P1-Rh1-P2 176.14(4); Cl1-Rh1-C41 178.36 (16); P1-Rh1-Cl1 89.12(4); P1-Rh1-C41 91.42 (15); P2-Rh1-Cl1 88.10(4); P2-Rh1-C41 91.43 (15); N3-C31-N4 124.0(4); P2-N3-C31 115.4(3); C21-P2-C24 94.8(2); N3-P2-C21 106.69 (19); N3-P2-C24 108.61 (19); N1-P1-O4 108.35 (18).…”

“…Alcarazo and co-workers have been at the forefront of this cationic phosphine revolution through the introduction and application of various mono-, 9,10 di- 10,[11][12][13] and tri-cationic 14 ligands including very recent examples of chiral forms. 15,16 Inspired by this elegant work and that of others, 17,18 we sought to combine our interest in polycyclic phosphines and amidinium salts (as precursors to NHCs) to generate cationic phosphines where both constituent units are asymmetric and connected by an N-P bond (figure 1). This is distinct from the reported examples which, without exception, have a C-P bond between the phosphine and cationic organic subunits.…”

Amidine functionalized phosphines: tuneable ligands for transition metals

“…The electronic character of the ligand need not be pre-set and electron-deficient systems can be derived through alkylation, protonation or binding of Lewis 50 acids to appropriate substituents on the phosphorus atom as identified recently by Hofmann. 9 The reported bidentate systems can be neutral or cationic at the extremes or display additivedependent chameleonic behavior upon addition of certain (usually Lewis acidic) agents. 9 These examples show that, with appropriate 55 design, an electronically diverse range of ligands can be derived from a single framework through judicious choice of receptor group and activating agent.…”

“…9 The reported bidentate systems can be neutral or cationic at the extremes or display additivedependent chameleonic behavior upon addition of certain (usually Lewis acidic) agents. 9 These examples show that, with appropriate 55 design, an electronically diverse range of ligands can be derived from a single framework through judicious choice of receptor group and activating agent. Our combined interest in cyclic phosphines 10 and ring-expanded NHC 11 ligands prompted us to explore the possibility of using 60 phosphines with amidine substituents as latent cationic ligands.…”