One-pot and step-by-step N-assisted CPh-H activation in 2-(4-bromophenyl)imidazol[1,2-a]pyridine: Synthesis of a new C,N-cyclometalated compound [{Pt(C^N)(μ-CI)}2] as precursor of luminescent platinum(II) compounds

Abstract

The activation of a. Cph-H bond in the phenyl ring of 2-(4-bromophenyl)imidazol[1,2-a]pyridine (HĈN)by[{Pt(n3-C 4H7)(μ-Cl)}2](n3-C 4H7 = n3-2-methylallyl) renders the new cyclometalated complex [{Pt(ĈN)(μ-Cl)}2] (2) with high yield and selectivity. Complex 2 can be achieved directly in a one-pot reaction or step by step through the intermediate [Pt(n3-C4H 7)Cl(HĈN-kN)] (1). Compound 1 could be isolated and fully characterized. The X-ray structure shows the coordination of HC ̂N through only the N and the existence of a weak Pt⋯-H-C hydrogen bridging bond (Pt⋯H1 =2.78 Å, Pt⋯Cl = 3.365(3) A, Pt-H1-C1. = 120.9°). Hence, the formation of this intermediate could be considered the first step in the cyclometalation process. The mononuclear complexes [PtCl(ĈN)L] (L = tht (3), PPh3 (4), CN-Xy1 (5), CN-tBu (6)) were obtained by cleavage of the bridging system in [{Pt(ĈN)(μ-Cl)}2] (2) by the neutral ligands, L. The resulting geometry (trans C, Cl) is that, expected, from the electronic preferences, taking into account the degree of transphobia (T) of pairs of trans ligands, T[C(ĈN)/L(C1)] < T[C(ĈN)/L(S, P, C)]. Complexes [PtCl(C^circ;N)L] (L = CN-XyI (5), CN- tBu (6)) containing two strong-field ligands, a CĉN σbonded and an isocyanide ligand, are luminescent. TD-DFT calculations were performed for the singlet ground state, So, as well as for the first triplet excited state of 6 in both the gas phase and solution. Calculations indicate that the lowest-lying absorption involves mainly 1IL (ĈN) mixed with, a small contribution of 1MLCT/1L'LCT (L = CAN; L' = Cl) transitions. Complex 5 exhibits >luminescent thermochromism> in the solid state; at. 77 K it shows a green phosphorescence band assigned to 3IL transitions located on the ĈN group of monomer species, while at 298 K an orange-red emission is observed, being tentatively assigned, to excited states of emissive aggregates (3MMLCT/3 π-π). However complex 6 shows phosphorescence only at 77 K both, in solution and in the solid state with the emissions arising from 3IL and 3L'MLCT excited states of monomer species. © 2010 American Chemical Society.