A new insight into the chemistry of iridium(iii) complexes bearing phenyl phenylphosphonite cyclometalate and chelating pyridyl triazolate: the excited-state proton transfer tautomerism via an inter-ligand PO–H⋯N hydrogen bond

Abstract

Treatment of [IrCl3(tht)3], where tht = tetrahydrothiophene, with two equiv. of phenyl diphenylphosphinite (pdpitH) gave [Ir(pdpitH)(pdpit)(tht)Cl2] (1), which on further reaction with 3-t-butyl-5-(2-pyridyl)-1,2,4-triazole (bptzH) and NaOAc using a one-pot reaction afforded [Ir(pdpit)2(bptz)] (2). In sharp contrast, the reaction of [IrCl3(tht)3], pdpitH, and bptzH in the presence of a stronger base, Na2CO3, afforded a phenyl phenylphosphonite (pppo)-containing Ir(III) complex [Ir(pdpit)(pppo)(bptz)] (3) that reveals a strong PO-H-N inter-ligand hydrogen bond (H-bond), as evidenced by the single crystal X-ray structural analysis. For confirmation, addition of diazomethane to a diethylether solution of 3 led to the isolation of two methylated Ir(III) isomeric complexes, i.e. [Ir(pdpit)(pppoMe)(bptz)] (4) and [Ir(pdpit)(pppo)(bptzMe)] (5), possessing either a PO-Me or N-Me bonding fragment, respectively. The absorption spectrum of 3 in CH2Cl2 resembles that of 4, implying the dominant PO-H character in solution. Despite the prevailing PO-H character both in the solid crystal and in solution, its corresponding emission resembles that of 5, leading us to propose a mechanism incorporating the excited-state inter-ligand proton transfer (ESILPT) from PO-H to N-H isomeric form via the pre-existing PO···H···N hydrogen bond. The thermodynamics of proton transfer tautomerism are discussed on the basis of absorption/emission spectroscopy in combination with computational approaches; additional support is given by the relationship between emission pattern versus the position of protons and methyl substituents. The results demonstrate for the first time a paradigm of excited-state proton transfer for the transition metal complexes in the triplet manifold.

A density functional theory‐time‐dependent density functional theory investigation of photo‐induced hydrogen bond and proton transfer for 2‐(3,5‐dichloro‐2,6‐dihydroxy‐phenyl)‐benzoxazole‐6‐carboxylicacid

“…Generally, the ESIPT reaction begins from the ground‐state enol form and returns back to the same state after the four‐level reaction cycle . The absorption of light could result in the initial normal excited‐state enol*.…”

Emissive Iridium(III) Complexes with Phosphorous‐Containing Ancillary

“…This result thus leads to the proposed involvement of excited‐state inter‐ligand proton transfer (ESILPT) from the PO−H to N−H isomer via the existing H‐bond between O−H of phosphonite fragment and the N atom of the triazolate ligand. More detailed discussion on ESILPT can be found in the original manuscript . However, the observation of H‐bonding tautomerism, especially the inter‐ligand proton transfer in the framework of transition metal complex is apparently unprecedented, which adds a new chapter to those reported for the excited‐state intramolecular proton transfer (ESIPT) process mostly taking place in the fluorescent organic molecules…”

“…Studies have also been made using phenyl diphenylphosphinite (pdpitH) as the starting material for dual P-based ancillaries. [63] As showed in Scheme 24, treatment of [IrCl 3 (tht) 3 ] with two equiv. of pdpitH gave [Ir(pdpitH)(pdpit)(tht)Cl 2 ] (76), for which it may be formed en route to a speculated intermediate species [Ir(pdpit)(tht) 2 Cl 2 ] (A), for which its structure is analogous to the previous characterized [Ir(dpn)(tht) 2 Cl 2 ] (72).…”

“…More detailed discussion on ESILPT can be found in the original manuscript. [63] However, the observation of Hbonding tautomerism, especially the inter-ligand proton transfer in the framework of transition metal complex is apparently unprecedented, which adds a new chapter to those reported for the excited-state intramolecular proton transfer (ESIPT) process mostly taking place in the fluorescent organic molecules. [64]…”

“…Studies have also been made using phenyl diphenylphosphinite (pdpitH) as the starting material for dual P‐based ancillaries . As showed in Scheme , treatment of [IrCl 3 (tht) 3 ] with two equiv.…”