σ–π Continuum in Indole–Palladium(II) Complexes

Abstract

The intrinsic features of (hetero-arene)-metal interactions have been elusive mainly because the systematic structure analysis of non-anchored hetero-arene-metal complexes has been hampered by their labile nature.W er eport successful isolation and systematic structure analysis of aseries of non-anchored indole-palladium(II) complexes.I tw as revealed that there is a s-p continuum for the indole-metal interaction, while it has been thought that the dominant coordination mode of indole to am etal center is the Wheland-intermediate-type s-mode in light of the seemingly strong electron-donating ability of indole.Several factors which affect the s-o rp-character of indole-metal interactions are discussed.The coordination behavior of hetero-arenes to at ransition metal center has attracted considerable attention since their coordination through low-hapticity modes have been thought to be involved in metal-mediated or metal-catalyzed transformations of hetero-arenes. [1,2] However,ithas been difficult to address the intrinsic features of (hetero-arene)-metal interactions,m ainly owing to the labile nature of nonanchored hetero-arene-metal complexes.I nf act, it is quite rare for non-anchored hetero-arene transition metal complexes to be isolated, and their structures has not been characterized by X-ray crystallography.[3] Nevertheless,ithas been assumed that Wheland-intermediate-type s-complexes are preferentially formed upon coordination of hetero-arenes to am etal center, in view of the seemingly greater electrondonating ability of hetero-arenes.F urthermore,s ynthetically useful palladium-catalyzed direct transformations of indoles have been proposed to be initiated by the formation of a sindole-palladium(II) complex, in which ametal center is s(h 1 -C)-bonded to the nucleophilic C3 atom of the indole ring, giving an indolium-type structure (A;Scheme 1).[1] However, the dominance of s-mode has not been verified by systematic structure analysis of non-anchored indole-metal complexes. We considered the possibility that the pure s-complexes are not always formed in (hetero-arene)-metal complexes;t hat is,f or the indole complexes, s/p-intermediate-(B)a nd pmodes (C or D)( Scheme 1) [4] may be taken into consideration, when the electron-donating property of ahetero-arene, or the electron-accepting property of am etal center is insufficient to induce strong intramolecular charge transfer. Herein, we confirmed the existence of a s-p continuum in (hetero-arene)-metal complexes through isolation and systematic structure analysis of non-anchored indole-metal complexes.To examine the (hetero-arene)-metal interaction, we chose non-anchored indole-Pd II complexes as model compounds because of their relevance to catalysis.I no ur study, the anchored indole complexes,ofwhich the isolation is much easier than that of non-anchored ones, [5] were not used because the chelating structures give rise to geometric constraints,w hich could affect the coordination modes of indole.[

Absolute Estimates of PdII(η2-Arene) C–H Acidity

“…2,14 Bonding between aromatic πorbitals and metals is electronically flexible, and the twoelectron M−arene interactions relevant to M−C aryl bond formation can be described on a continuum between η 2 -arene and η 1 -arenium extremes (Figure 1). 15,16 These intermediates are implicated in electrophilic activation of aromatic molecules, wherein nucleophiles deprotonate the acidified C−H bond with concomitant metalation. The driving force for this metalation, which is ultimately a proton transfer reaction, can be expressed as a difference in acidity (ΔpK a ) between the η 2 -arene complex and protonated base product (Figure 1, right).…”