Abstract
A family of polydentate pyridine-substituted pyridylidene amide (PYA) complexes bound to iron(ii) was developed. The variation of the coordination set from NN-bidentate PYA to tridentate pincer-type pyPYA2 systems (pyPYA2 = 2,6-bis(PYA)pyridine) had a large influence on the binding mode to iron(ii), including a change from the N- to rare O-coordination of the PYA site and a concomitant shift of the predominant ligand resonance structure. These binding mode variations invoke changes in the reactivity of the complexes, which were probed in the peroxide-mediated oxidation of 1-phenylethanol to acetophenone. A comparison with uncomplexed FeCl2 indicated that bidentate NN coordination is unstable and presumably leads to the dissociation of FeCl2. In contrast, the tridentate ligand binding is robust. Remarkably, the tridentate PYA pincer coordination inhibits catalytic activity in the NNN binding mode, while the ONO coordination greatly enhances catalytic performance. Under optimized conditions, the bis-ligated ONO pincer iron complex [Fe(pyPYA2)2][2PF6] reaches full conversion within one hour (0.5 mol% catalyst loading) and under dilute conditions turnover numbers over 20 000 (0.005 mol% catalyst loading).
Highlights
Neutral N-donor ligands are ubiquitous in homogeneous catalysis, including imines, amines, pyridines, and as a more recent addition, pyridylidene amide (PYA) ligands.[1,2]
As a direct result of these multiple resonance structures, the ligand is electronically flexible in its donor properties to metal centers: it can alter between L-type bonding through resonance form A and X-type metal coordination through resonance form B, and adapt to the influence and requirements of the metal center
A catalyst loading as low as 0.005 mol% gave a 92% yield after 24 h and full conversion after 48 h, resulting in 20 000 TONs.[43]. Such high TONs are unusual for iron complexes and are assumed to be a direct consequence of the integrity of the catalytic species due to the rigid tridentate coordination of the PYA pincer ligand combined with the beneficial electronic impact of the ONO donor motif
Summary
Neutral N-donor ligands are ubiquitous in homogeneous catalysis, including imines, amines, pyridines, and as a more recent addition, pyridylidene amide (PYA) ligands.[1,2] This latter scaffold has been emerging as a promising class of ligands with strong σ-donating properties. Single crystals suitable for X-ray diffraction analysis were grown by the slow evaporation of a MeCN solution under inert conditions, and identified complex 2 as a dimeric structure (Fig. 2).[23] Each asymmetric unit in the crystal consists of an iron center bound to two nitrogen atoms of the bidentate chelating pyridyl-PYA ligand, and a terminal as well as two bridging chlorido ligands.
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