“Rollover” cyclometalation – early history, recent developments, mechanistic insights and application aspects

Abstract

''Rollover'' cyclometalation constitutes a special case among the well-known class of cyclometalation reactions. An overview is given that covers the very first description of this reaction type, as well as recent developments. In addition, not only condensed-phase experiments are reviewed, but also investigations based on mass spectrometric techniques, together with ''in silico'' studies using DFTbased calculations are considered. While the latter two methods allow for a detailed analysis of the intrinsic factors that affect the reaction mechanisms, consideration of all three regimes permits to develop a coherent mechanistic picture and to address the often noted gap between condensed-and gasphase studies. Moreover, the quite unexpected reactivity of ''rollover'' cyclometalated complexes in gasphase experiments, as well as potential applications, e.g. in synthetic procedures, are discussed in some detail.

Electrospray mass spectrometric studies of nickel(II)-thiosemicarbazones complexes: Intra-complex proton transfer in the gas phase ligand exchange reactions

“…Thiosemicarbazones were also reported to undergo C sp2 -H activation in the presence of Ru(II) or Pd(II) metal ion in the condensed phase, and the formed cyclometallated complexes have particular structure and antimalarial activities [9][10][11]. There have been quite a few reports on ligand C H bond activation in the gas phase to give cyclometallated products [12]. But no report on the formation of the gas-phase cyclometallated thiosemicarbazones was found to our knowledge.…”

True and masked three-coordinate T-shaped platinum(II) intermediates

“…A recent review on this topic collects the most important features of this reaction [122]. Early work by Young and co-workers [123] proposes the formation of coordinately unsaturated species 39 and 39’ as intermediates in the “rollover” reactions of 38 , leading to polymeric species (Scheme 19).…”

Chiral cyclometalation of 6-(1-phenylbenzyl)-2,2′-bipyridine

“…Under certain conditions, activation of the pyridine C 3 -H bond may occur, following the so-called "rollover" cyclometalation, which is an unusual behaviour rarely shown by bidentate heterocyclic ligands. 3,24 In the case of LH, reaction with the electron-rich Pt(II) complex [PtMe 2 (DMSO) 2 ] in refluxing acetone gave the rollover complex 9, [Pt(L-H)(Me)(DMSO)], as the only product, in high yield (ca. 90%).…”

“…The driving force of these reactions is not fully understood, and subtle factors can drive the reaction towards unexpected results. As an example, Pd(II) and Pt(II) complexes can activate 6-dimethylbenzyl-2,2'-bipyidine in three different positions to give a terdentate complex with [5,6] fused rings by C(sp 2 )-H bond activation or with [5,5] fused rings by C(sp 3 )-H bond activation, and a cyclometalated rollover complex by C(sp 2 )-H bond on a pyridine ring (Chart 1, species A, B and C, respectively).…”

“…In particular, cyclometalated complexes of d 8 metal ions, such as Pt(II), Pd(II) and Au(III) have been studied as sensors, catalysts, anticancer agents, light-emitting diodes, molecular switches, photovoltaic cells, liquid crystalline materials. 1,2 This class of compounds include classical bidentate complexes (such as those derived from 2-phenylpyridine), 1,2 non-classical species (such as those derived from rollover cyclometalation), 3 symmetrical terdentate complexes (the so-called pincer complexes, e.g. N,C,N), 4 and un-symmetrical terdentate complexes, such as those derived from substituted 2,2'-bipyridines 5 or di-imines.…”