Studies on the Atropisomerism of Fe(II) 2,6-Bis(N-arylimino)pyridine Complexes

Abstract

NMR spectra of free 2,6-bis(N-arylimino)pyridine (PDI) ligands displaying different substituents at the ortho and ortho' positions of the two N-aryl rings indicate that they can exist in syn (meso) and anti (chiral) configurations. These interconvert in solution at room temperature, via rotation of the aryl group. The corresponding paramagnetic FeX(2)(PDI) complexes exhibit the same kind of isomerism, a property that is thought to be important for their activity as alpha-olefin polymerization catalysts. For the first time, this has been detected by (1)H NMR and studied in solution. Although the conformational stability of the diastereoisomeric complexes varies widely (depending on the size of the substituents at the imine and the aromatic rings), a moderate degree of steric hindrance suffices to allow their chemical separation. A simple procedure is developed for the preparation of these complexes in diastereoisomerically pure form. In addition, introduction of a prochiral substituent in the pyridine ring enables positive assignment of the stereoisomers. Isomerization rate measurements of the Fe(II) complexes in solution suggest that isomerization very likely involves the dissociation of the corresponding Fe-N(imino) bond prior to the rotation of N-aryl groups. DFT calculations provide additional support to the conformational assignment as well as the dissociative isomerization mechanism.

Conformational analysis via calculations and NMR spectroscopy for isomers of the mono(imino)pyridine ligand, 2-{(2,6-Me2-C6H3)NC(i-Pr)}C5H4N

“…14 In other cases, atropisomerism is not invoked, presumably since no experimental evidence exists to suggest its relevance. 15,17,19 Atropisomerism was implied by Small and Brookhart 20 and Ca ´mpora et al 21 for free asymmetric bis(imino)pyridine ligands and for select corresponding Fe(II) complexes; 13 C NMR spectroscopy provided evidence of hindered rotation about the imino N-aryl C bonds. It is notable that these authors do not invoke E/Z isomerization as an explanation for the fluxional behavior observed by NMR spectroscopy; presumably, the primary alkyl substituents on the imino carbon positions (methyl 20,21 or butyl 21 ) render the E,E-isomer energetically favorable.…”

“…15,17,19 Atropisomerism was implied by Small and Brookhart 20 and Ca ´mpora et al 21 for free asymmetric bis(imino)pyridine ligands and for select corresponding Fe(II) complexes; 13 C NMR spectroscopy provided evidence of hindered rotation about the imino N-aryl C bonds. It is notable that these authors do not invoke E/Z isomerization as an explanation for the fluxional behavior observed by NMR spectroscopy; presumably, the primary alkyl substituents on the imino carbon positions (methyl 20,21 or butyl 21 ) render the E,E-isomer energetically favorable. This is in contrast to studies by Orrell et al where a symmetric bis(imino)pyridine ligand serves a bidentate ligand for Pd(II) or Pt(II), 22 Re(I) 23,24 and Pt(IV), 25 and E/Z isomerization of the non-coordinated imine arm is invoked in combination with atropisomerism (again, with methyl substituents on the imino carbon positions).…”

“…13 Identification of both the E and Z isomers for N-aryl imines has been made by X-ray crystallography 14 and by 1 H NMR spectroscopy, with NOE experiments used to assist with conformational assignments, 15 and variable temperature NMR used to measure the free energies of activation for the E/Z isomerization. 15,16 Calculations suggest that CLN rotation would have a free energy of activation that is twice the magnitude of that for inversion at nitrogen, with the latter typically falling in a range from 12-29 kcal mol 21 . 16,17 and cited references For select imines, atropisomerism 18 is invoked in combination with E/Z isomerism with experimental evidence derived from NMR spectroscopy 16 and X-ray crystallography.…”

New Bis(imino)pyridine Complexes of Iron(II) and Iron(III), and Their Catalytic­ Activity in the Mukaiyama Aldol Reaction

“…Accordingly, we describe herein the synthesis and characterization of bis(imino)pyridine complexes of iron(II) and iron(III). The bis(imino)pyridine ligand class is known 37 and has previously been employed in the synthesis of iron complexes. 38 Iron complexes of bis(imino)pyridines are widely utilized as ethylene polymerization catalysts, 39 and also have been used to catalyze cyclization or cycloisomerization reactions; 40 however, they have not been employed in the Mukaiyama aldol reaction to date.…”

Atropisomerism in a thermally switchable, cyclometallated iridium complex

“…This approach offers the possibility of interconversion between atropisomers, which may be induced chemically, thermally, photochemically or electrochemically. 8 Currently, there are very few examples of this type; these involve diimine 9 or bisiminopyridine 10,11 ligands with di-orthosubstituted phenyl substituents on the imine N atom and some metal oligomers of 3-pyridyl porphyrin. 12 In all these cases the presence of two rotational axes leads to syn and anti isomers.…”