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

Photophysics and Excited State Dynamics of Cyclometalated [M(Phbpy)(CN)] (M = Ni, Pd, Pt) Complexes: A Theoretical and Experimental Study

Cyclometalated complexes [M(Phbpy)(CN)] (HPhbpy = 6-phenyl-2,2'-bipyridine) of the group 10 metals (Ni, Pd, and Pt) bearing a carbanionic ^-C^∧N^∧N pincer ligand were synthesized and studied in a combined experimental and computational DFT approach. All three complexes were crystallographically characterized showing closely packed dimers with head-to-tail stacking and short metal-metal contacts in the solid state. The computational models for geometries, excited states, and electronic transitions addressed both monomeric (Ni-mono, Pd-mono, and Pt-mono) and dimeric (Ni-dim, Pd-dim, and Pt-dim) entities. Photophysical properties and excited state dynamics of all title complexes were investigated in solution and in the solid at 298 and 77 K. [Ni(Phbpy)(CN)] and [Pd(Phbpy)(CN)] are virtually nonemissive in solution at 298 K, whereas [Pt(Phbpy)(CN)] shows phosphorescence in CH₂Cl₂ (DCM) solution (λ_em = 562 nm) stemming from a mixed ³MLCT/ILCT (metal-to-ligand charge transfer/intraligand charge transfer) state. At 77 K in a glassy frozen DCM:MeOH matrix, [Pd(Phbpy)(CN)] shows a remarkable emission (λ_em = 571 nm) with a photoluminescence quantum yield reaching almost unity, whereas [Ni(Phbpy)(CN)] is again nonemissive. Calculations on the monomeric models M-mono show that low-lying metal-centered states (MC, i.e., d-d* configuration) with dissociative character quench the photoluminescence. In the solid state, the complexes [M(Phbpy)(CN)] show defined photoluminescence bands (λ_em = 561 nm for Pd and 701 nm for Pt). Calculations on the dimeric models M-dim shows that the axial M···M interactions alter the photophysical properties of Pd-dim and Pt-dim toward MMLCT (metal-metal-to-ligand charge transfer) excited states with Pd-dim showing temperature-dependent emission lifetimes, suggesting thermally activated delayed fluorescence, whereas Pt-dim displayed phosphorescence with excimeric character. The metal-metal interactions were analyzed in detail with the quantum theory of atoms in molecules approach.

Synthesis and characterization of new Pd(ii) and Pt(ii) complexes with 3-substituted 1-(2-pyridyl)imidazo[1,5-a]pyridine ligands

Several palladium(ii) and platinum(ii) complexes (1-20) of general formula [M(Lⁿ)(X)(Y)] [M = Pd, X = Y = Cl (1-Cl-4-Cl), X = Y = OAc (1-OAc-4-OAc); M = Pt: X = Y = Cl (5-8); M = Pd, X = Cl, Y = CH₃ (9-12); M = Pt, X = Cl, Y = CH₃ (13-16) or X = Y = CH₃ (17-20); n = 1-4] have been synthesized by reaction of different Pd(ii) and Pt(ii) derivatives with various 3-substituted 1-(2-pyridyl)-imidazo[1,5-a]pyridines; i.e.Lⁿ = 1-(2-pyridyl)-3-arylimidazo[1,5-a]pyridine (aryl = Phenyl, L¹; 2-o-Tolyl, L²; Mesityl, L³) and 1-(2-pyridyl)-3-benzylimidazo[1,5-a]pyridine (L⁴). Detailed spectroscopic investigation (including IR, mono- and bi-dimensional ¹H NMR) and elemental analysis has been performed for all these species, allowing their complete characterization. Lⁿ act as N,N-bidentate ligands and coordinate the metal centers in a chelate fashion through the pyridyl (N_py) and the pyridine-like nitrogen atom of the imidazo[1,5-a]pyridine group (N_im). The X-ray structural analysis performed on two of Pd(ii) and three Pt(ii) complexes, namely [Pd(L²)(CH₃)Cl] (10), [Pd(L³)(CH₃)Cl] (11) and [Pt(L¹)Cl₂] (5), [Pt(L⁴)Cl₂] (8), [Pt(L²)(CH₃)Cl] (14) confirmed the spectroscopic and analytical data. Finally DFT studies unveiled the structural reasons behind the inertia of the synthesised compounds toward metalation, identified as the higher angle steric strain in comparison with the analogous bipyridine complexes.

Rollover Cyclometalation as a Valuable Tool for Regioselective C-H Bond Activation and Functionalization

Rollover cyclometalation constitutes a particular case of cyclometallation reaction. This reaction occurs when a chelated heterocyclic ligand loses its bidentate coordination mode and undergoes an internal rotation, after which a remote C-H bond is regioselectively activated, affording an uncommon cyclometalated complex, called "rollover cyclometalated complex". The key of the process is the internal rotation of the ligand, which occurs before the C-H bond activation and releases from coordination a donor atom. The new "rollover" ligand has peculiar properties, being a ligand with multiple personalities, no more a spectator in the reactivity of the complex. The main reason of this peculiarity is the presence of an uncoordinated donor atom (the one initially involved in the chelation), able to promote a series of reactions not available for classic cyclometalated complexes. The rollover reaction is highly regioselective, because the activated C-H bond is usually in a symmetric position with respect to the donor atom which detaches from the metal stating the rollover process. Due to this novel behavior, a series of potential applications have appeared in the literature, in fields such as catalysis, organic synthesis, and advanced materials.

A direct access to heterobimetallic complexes by roll-over cyclometallation

Complexes of the type [Cp*Ir(N,N')Cl]⁺ (N,N' = 2-(2-dialkylaminopyrimidin-4-yl)pyrimidine) can undergo roll-over cyclometallation leading to a novel N,N'-donor site. Following this strategy heterobimetallic complexes including iridium(iii) and a Group X metal centre in the oxidation state +II were achieved.

Gas-phase reactivity of Cp* group IX metal complexes bearing aromatic N,N′-chelating ligands

Compounds of the type [(η⁵-Cp*)M(Cl)(N,N′)]⁺ bearing N,N′-coordinating ligands show different modes of HCl cleavage depending on the nature of the N,N′-chelate ligand and the metal center.

Synthesis, crystal structure, photophysical property and metal ion-binding behavior of a cyclometalated platinum(ii) terpyridylacetylide with efficient π-conjugation degree

The “Donor–Phosphore–Receptor” platform as a novel building block for formation of heterotrinuclear transition metal complexes.