One-Pot and Step-by-Step N-Assisted C_Ph−H Activation in 2-(4-Bromophenyl)imidazol[1,2-a]pyridine: Synthesis of a New C,N-Cyclometalated Compound [{Pt(C^∧N)(μ-Cl)}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 (HC∧N) by [{Pt(η3-C4H7)(μ-Cl)}2] (η3-C4H7 = η3-2-methylallyl) renders the new cyclometalated complex [{Pt(C∧N)(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(HC∧N-κN)] (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···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(C∧N)L] (L = tht (3), PPh3 (4), CN-Xyl (5), CN-tBu (6)) were obtained by cleavage of the bridging system in [{Pt(C∧N)(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(C∧N)/L(Cl)] < T[C(C∧N)/L(S, P, C)]. Complexes [PtCl(C∧N)L] (L = CN-Xyl (5), CN-tBu (6)) containing two strong-field ligands, a CC∧N 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 (C∧N) mixed with a small contribution of 1MLCT/1L′LCT (L = C∧N; 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 C∧N 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.

Benzothiazole‐Based Cycloplatinated Chromophores: Synthetic, Optical, and Biological Studies

“…The presence of the negatively charged phosphine ligand made the TPPTS complexes highly soluble in polar media (i.e., D 2 O, MeOD, and DMSO) and insoluble in CDCl 3 as a difference to the corresponding PTA derivatives. The spectroscopic data discussed below indicate that only the trans ‐C,Cl isomer is present in each case, as was previously found in related compounds . Complex [Pt(Br‐PBT‐κ C )Cl(PTA) 2 ] ( 3 ) was produced during an attempted synthesis of complex 1 b by reaction of [{Pt(Br‐PBT‐κ C , N )(μ‐Cl)} 2 ] with PTA (molar ratio 1:3).…”

“…Thus, the deviations of the benzothiazole (BT) group with respect to the platinum coordination plane are 9.64 and 5.12° in 1 a and 1 b , respectively, and the angles between the Ph ring and the BT group are 1.96 and 5.61° in 1 a and 1 b , respectively, whereas these angles are 25.91 and 17.64° and 22.41 and 4.91° in complexes 2 a and 2 b , respectively. Bond lengths of the donor atoms to the platinum center (Pt‐Cl: 2.3923(14)–2.400(3); Pt‐S: 2.2119(13)–2.2133(12); Pt‐P: 2.2091(14), 2.207(3); Pt‐C: 2.016(4)–2.036(12) Å) are within the normal ranges expected for cyclometalated complexes …”

Stepwise Strategy to Cyclometallated PtII Complexes with N‐Heterocyclic Carbene Ligands: A Luminescence Study on New β‐Diketonate Complexes

“…After workup of the mixture, the neutral complex [PtCl(η 3 ‐2‐Me‐C 3 H 4 )(H$\widehat{CC}$ *‐κC*)] ( 3 ) was isolated as a pale‐yellow and air‐stable solid in very good yield (75 %). In the IR spectrum, an absorption band at 280 cm −1 was attributed to υ Pt−Cl , which is consistent with a terminal PtCl bond in trans disposition to a ligand with a large trans influence as η 3 ‐2‐Me‐C 3 H 4 18a…”

“…The methyl allyl group (η 3 ‐2‐Me‐C 3 H 4 ) gives the expected 1 H and signals, with values of J Pt−H and J Pt−C slightly smaller than those found in the η 3 ‐allyl derivatives [{Pt(μ‐Cl)(η 3 ‐2‐Me‐C 3 H 4 )} 2 ]22 and [Pt(η 3 ‐C 4 H 7 )Cl(H$\widehat{CN}$ ‐κN)] (H$\widehat{CN}$ =2‐(4‐bromophenyl)imidazol[1,2‐ a ]pyridine),18a as expected for the stronger trans influence of a C (NHC) with respect to a Cl or N, respectively. The 13 C resonance of C1 appears at 176.4 ppm and the 195 Pt{ 1 H} NMR resonance at −4457 ppm, in the same spectral region as the analogous complex [Pt(η 3 ‐C 4 H 7 )Cl(H$\widehat{CN}$ ‐κN)] 18a. All these spectroscopic features indicate the success of the transmetalation to the Pt center.…”

Critical Role of Anions in Platinum(II) Precursors upon the Structural Motifs of Six-Membered Cycloplatinated N,N′,N″-Triarylguanidines

“…The configuration of Pd(II)/Pt(II) atoms in cyclometallated N-donor ligands was explained by invoking steric factor and antisymbiosis / transphobia of two mutually trans-oriented ligands and the influence of the latter factor was independently verified in one study by theoretical calculations. 4,24,38−40 The cisoid configuration of the Pt(II) atom in 1 ·1/2CH 2 Cl 2 and 3 is unfavorable from the point of view of degree of transphobia , as hard imine N atom of both the guanidines lies trans to each other. In a hypothetical transoid configuration such as that illustrated in M , there would be greater steric repulsion between 2-tolyl substituents of the imine N atom of both platinated and nonplatinated guanidines (see Figure 9).…”