Modulating the Steric, Electronic, and Catalytic Properties of Cp* Ruthenium Half-Sandwich Complexes with β-Diketiminato Ligands

Abstract

Five different types of β-diketiminate ligands, bearing electrondonating to strongly electron-withdrawing substituents, were synthesized and used in the synthesis of Cp* ruthenium complexes (Cp* = η 5 -C 5 Me 5 ). One series consists of complexes with a covalent Ru III −Cl bond, and the other series features a reduced Ru II center, where the chloride is abstracted by treatment of the corresponding Ru III compounds with Zn or Mg. All compounds were characterized by single-crystal X-ray diffraction, UV−visible spectroscopy, and cyclic voltammetry. In the case of Ru II complexes, solution NMR techniques provided key information regarding the electronic and structural differences induced by the different β-diketiminate ligands employed. Capitalizing on the facile reduction−oxidation cycle of the Cp* ruthenium β-diketiminato complexes, catalytic atom transfer radical addition (ATRA) and cyclization (ATRC) reactions were performed on relevant substrates. The turnover rates are strongly dependent on the type of β-diketiminate used, where ligands with electron-withdrawing substituents, i.e., trifluoromethyl groups, provided complexes that efficiently catalyze the addition of CCl 4 or toluenesulfonyl chloride to styrene. In contrast, complexes with electron-donating substituents on the β-diketiminate promoted efficient ATR cyclization of N-allyl-N-phenyltrichloroacetamide and 2,2,2-trichloroethyl ether. Thus, the overall product conversion and yield are dependent on matching the ligand substitution pattern of the catalyst to the type of substrate.

Synthesis, structural characterization, and reactivity of Cp*Ru(N-phosphinoamidinate) complexes

“…Both complexes 2a (67%) and 2b (using n BuLi, 82%) were readily isolated and appear to be stable at room temperature under a nitrogen atmosphere both in solution and in the solid state. The highly colored nature of 2a and 2b is consistent with previously reported 16-electron Cp*Ru complexes, including Cp*RuL(X), 1,6-8 Cp*Ru(amidinate), 1,13 and Cp*Ru (diketiminate), 14,15 which are typically blue or purple in color. The solution NMR ( 1 H, 13 C, 31 P) spectra of 2a and 2b (benzene-d 6 ) are consistent with their formulation as C s -symmetric, diamagnetic, N-phosphinoamidinate complexes.…”

“…The Ru−N(2) distance of 2.0734(16) Å in 2b is somewhat elongated relative to that of 2.0458(13) Å in 2a, which can most likely be ascribed to the greater steric demands of the isopropylfunctionalized ligand in 2b. Nonetheless, these Ru−N distances fall within the range observed for crystallographically characterized Cp*Ru(amidinate) (2.073(3) Å) 13 and Cp*Ru(diketiminate) (2.025(6)−2.071(2) Å) 14,15 complexes. The solid-state structures of 2a and 2b are consistent with the C s -symmetric species observed in solution, as indicated by the perpendicular orientation of the P−Ru−N(2) plane relative to the plane of the Cp* ligand.…”

“…In both cases, the complexes adopt a monomeric twolegged piano stool structure that is analogous to that previously observed for related crystallographically characterized Cp*RuL(X) complexes 1,6-8 as well as complexes of the type Cp*Ru(amidinate) 1,13 and Cp*Ru(diketiminate). 14,15 The Ru−P distances of 2.3679(5) and 2.3599(5) Å for 2a and 2b, respectively, are slightly shorter than that of 2.395(2) Å reported for Cp*Ru(P i Pr 3 )Cl, 6 possibly due to the rigid chelating structure of the N-phosphinoamidinate complexes. The Ru−N(2) distance of 2.0734(16) Å in 2b is somewhat elongated relative to that of 2.0458(13) Å in 2a, which can most likely be ascribed to the greater steric demands of the isopropylfunctionalized ligand in 2b.…”

“…1), 1,[9][10][11][12] reports documenting the preparation of isolable, coordinatively unsaturated, and formally 16-electron Cp*Ru( 2 -LϳX) chelate complexes supported by monoanionic, LX-type ligands are rare and involve primarily synthetic investigations of 2 -N,N amidinate (B) 1,2,13 or ␤-diketiminate (C) 14,15 complexes. It is worthy of mention that analogous Cp*Ru( 2 -PϳN) species, 16 derived from 2-(diphenylphosphino)ethylamine and related ligands (D), have been implicated by Ikariya and co-workers as key intermediates in a diversity of synthetically useful metal-catalyzed transformations including the chemoselective hydrogenolysis of epoxides, 17 the isomerization of allylic alcohols, 18 the oxidative lactonization of 1,4-diols, 19 and the hydrogenation of imides.…”

Ruthenium-catalyzed Atom Transfer Radical Addition Reactions

“…Similar to AIBN, the additive Mg allows the catalyst loading to be reduced dramatically. [9] Recently we have shown that manganese powder can be an interesting alternative to Mg. [10] It should be pointed out that AIBN and Mg have also been used with great success in Cu-catalyzed AT RA reactions. [11] The combination of Cp*RuCl 2 (PPh 3 ) with Mg represents one of the most powerful catalytic systems for AT RA/ATRC reactions described so far.…”