om901032v_si_002.cif (37.03 kB)

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(CN)(μ-Cl)}2] as Precursor of Luminescent Platinum(II) Compounds

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posted on 22.03.2010, 00:00 by Juan Forniés, Violeta Sicilia, Carmen Larraz, José A. Camerano, Antonio Martín, José M. Casas, Athanassios C. Tsipis
The activation of a CPh−H bond in the phenyl ring of 2-(4-bromophenyl)imidazol[1,2-a]pyridine (HCN) by [{Pt(η3-C4H7)(μ-Cl)}2] (η3-C4H7 = η3-2-methylallyl) renders the new cyclometalated complex [{Pt(CN)(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(η3-C4H7)Cl(HCN-κN)] (1). Compound 1 could be isolated and fully characterized. The X-ray structure shows the coordination of HCN through only the N and the existence of a weak Pt···H−C hydrogen bridging bond (Pt···H1 = 2.78 Å, Pt···C1 = 3.365(3) Å, 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(CN)L] (L = tht (3), PPh3 (4), CN-Xyl (5), CN-tBu (6)) were obtained by cleavage of the bridging system in [{Pt(CN)(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(CN)/L(Cl)] < T[C(CN)/L(S, P, C)]. Complexes [PtCl(CN)L] (L = CN-Xyl (5), CN-tBu (6)) containing two strong-field ligands, a CCN σ-bonded and an isocyanide ligand, are luminescent. TD-DFT calculations were performed for the singlet ground state, S0, 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 (CN) mixed with a small contribution of 1MLCT/1L′LCT (L = CN; 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 CN group of monomer species, while at 298 K an orange-red emission is observed, being tentatively assigned to excited states of emissive aggregates (3MMLCT/3π−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.