The synthesis, characterization, cyclovoltammetric and photophysical properties of 11 new d 8-configured Pt(II) complexes with N*N<^>C coordinated ligands, alternatively involving N*N six-ring and N<^>C five-ring chelates, are presented. By using various boronic acids, variation of the cyclometalating aryl units was achieved. The DFT-calculated HOMOs are localized on the metal with contributions from the Cl- coligand and either the phenyl/thiophenyl unit or the thiazolyl moiety, depending on the substitution pattern. The LUMOs have phenyl-pyridine pi*-character. Both calculated orbital sets agree well with the redox potentials from cyclic voltammetry. The TD-DFT calculated absorption spectra are in agreement with experimental data showing long-wavelength bands in the range from 400 to 500 nm, which matches the yellow color of the complexes. The ligand variation enabled a fine-tuning of the emissive properties related to the resulting complexes, going from greenish-blue (471 nm) to red (617 nm) phosphorescence. The position of the substituent affects the excited state properties, which is attributed to mesomeric and inductive effects on the Pt-C bond and the adjacent pyridine ring. In general, modulation of the excited state character can be achieved by variation of the cyclometalating unit, thus affecting the excited state energy as well as the radiative and radiationless deactivation rates.

Six N^C^N cyclometalated Ni(II) complexes [Ni(N^C^N)Cl] or [Ni(N^C^N’)Br] with symmetric N^C^N or non-symmetric N^C^N’ ligands in which the peripheral N-groups were varied with pyridine (Py), 4-thiazole (4Tz), 2-thiazole (2Tz), and 2-benzothiazole (2Btz) complementing the previously reported complexes with di(2-pyridyl)phenide ligands [Ni(Py(Ph)Py)X] X = Cl or Br. The non-symmetric [Ni(N^C^N’)Br] complexes were synthesized from NiBr2 and N^CH^N’ protoligands through base-assisted nickelation, while the symmetric [Ni(N^C^N)Cl] complexes were received from the N^C(Cl)^N protoligands and [Ni(COD)2] (COD = 1,5-cyclooctadiene). Introduction of 4Tz on both sides shifted the electrochemical gap ΔEexp = Eox–Ered and the long wavelength UV-vis absorption maxima of the complexes to higher energies, while 2Tz leads to a shift to lower energies. When introducing only one 4Tz or 2Tz as peripheral groups, the remaining PhPy moiety dominates the electronic properties and electrochemistry and photophysics are very similar to the Py(Ph)Py derivatives. In contrast to this, introduction of 2Btz shifts both values to lower energies, regardless of one or two 2Btz groups and the 2Btz moiety dominates the character of the frontier molecular orbitals of the complexes, as DFT calculations show. Long-wavelength UV-vis absorptions vary from 416 to 443 nm, and their energies correlate well with the first reduction potentials. Negishi-type C–C cross-coupling reactions gave total yields ranging from 1 to 60% and cross-coupling yields from 1 to 44%. The reactivities correlate roughly with the first reduction potentials. Facilitated reduction (E around –2 or higher) goes generally along with improved performance, making the thiazole-containing complexes interesting candidates for such catalysis.

Linking two cyclometalated Pt( ii ) fragments via cyanido bridge improves the luminescence and dramatically enhances the aggregation ability resulting in distinct sensitivity to the properties of the surrounding medium.

A strategy of "greening up" Pt(II) luminophores is presented. For this purpose, caffeine, as a renewable alkaloid and precursor for an N-heterocyclic carbene (NHC), is incorporated into a tridentate, pincer-type ligand framework. A series of luminescent Pt(II) complexes, containing the tridentate bis-NHC ligand and an arylacetylide ligand is reported. The complexes' electrochemistry and photophysics strongly depend on the electronic nature of the substituent on arylacetylide ligand. The use of bio-derived ligands represents one important approach to make emitters or sensitizers eco-friendlier by coming back to cheap, abundant and non-harmful staring materials for their synthesis. A series of Pt(II) complexes, which contain a tridentate, caffeine-based bis-NHC ligand as well as a phenylacetylide co-ligand, are reported. The electrochemical and photophysical properties of these complexes are discussed.image

The interaction of tetramethylammonium acetate with [(MeCN)2Ni(CF3)2], [(MeCN)2Ni(C2F5)2], and [NMe4][(MeCN)Ni(CF3)3] was explored by 19F NMR spectroscopy. We show that, depending on the nature of the nickel complex, one or two acetate ligands can add to the metal center and replace the nickel-bound acetonitriles, depending on the acetate concentration. The number of acetates that could bind to nickel, and whether the resulting complex exists as a monomer or dimer, was determined to be dependent on the nature of the fluoroalkyl ligand. Moreover, we observe that oxidation of the nickel center of [(MeCN)2Ni(CF3)2] in the presence of two equivalents of acetate leads cleanly to the octahedral, paramagnetic, and anionic nickel(III) complex [NMe4][(OAc)2Ni(CF3)2.

Fromthe two organonickel(II) scaffolds [Ni(Phbpy)](+) and [Ni(PyPhPy)](+) (HPhbpy = 6-phenyl-2,2 & PRIME;-bipyridine (HCNN)-N-& BOTTOM;-N-& BOTTOM; and Py(HPh)Py = 2,6-di-2-pyridyl-benzene N-& BOTTOM;(HC)N-& BOTTOM;) the hydrido complexes [Ni((CNN)-N-& BOTTOM;-N-& BOTTOM;)H] and [Ni((NCN)-C-& BOTTOM;-N-& BOTTOM;)H] were studied in a combined experimental/theoreticalapproach. The hydrido complexes were prepared via the reaction ofthe halido derivatives [Ni((CNN)-N-& BOTTOM;-N-& BOTTOM;)X] and [Ni((NCN)-C-& BOTTOM;-N-& BOTTOM;)X] (X = Cl,Br) with Li(Et3BH) (superhydride). The (CNN)-N-& BOTTOM;-N-& BOTTOM; complex undergoes rapid reductive elimination(RE) yielding HPhbpy and Ni particles even at the lowest temperatures,while the PyPhPy derivative is more stable. Low-temperature H-1 nuclear magnetic resonance (NMR) spectroscopy allowed detectionof a signal at & delta; = -2.86 ppm assignable to the hydridoligand. The different stabilities can be directly correlated to the cis (Phbpy) and trans (PyPhPy) orientationsof the carbanionic phen-2-ide group with the hydrido ligand, and rapidRE occurs from the cis position which is also supportedby the density functional theory (DFT) calculations which are presented.A further TD-DFT/UV-vis absorption study is also reported torationalize and confirm the fleeting existence of the Ni-Hmoiety, and proposals are made on the route of its decomposition.

Ni(II), Pd(II), and Pt(II) complexes [M(Y-terpy)X] (X = Cl or Br) containing the tridentate N^C^N-cyclometalating 2,3′:5′,2″and 2,2′:4′,2″ stereoisomers of the well-known tridentate N^N^N ligand 2,2′:6′,2″-terpyridine (terpy) were synthesised in moderate to good yields through C–H activation. For the Pt complexes, the phenyl ethynide derivatives [Pt(Y-terpy)(C≡CPh)] were also obtained under Sonogashira conditions. In contrast to this, C^N^N cyclometalated complexes using the 2,2′:6′,3″- and 2,2′:6′4″-terpy isomers were not obtained. Comparison of the N^C^N complexes of the cyclometalated 2,3′:5′,2″- and 2,2′:4′,2″-terpy ligands with complexes [M(dpb)Cl] of the prototypical N^C^N cyclometalating ligand dpb− (Hdpb = 2,6-diphenyl-pyridine) showed higher potentials for the terpy complexes for the ligand-centred reductions in line with the superior π-accepting properties of the terpy ligands compared with dpb. Metal-centred oxidations were facilitated by the dpb ligand carrying a central σ-donating phenyl group instead of a metalated pyridine moiety. The same trends were found for the long-wavelength absorptions and the derived electrochemical and optical band gaps. The lower σ-donating capacities of the cyclometalated terpy derivatives is also confirmed by a reduced trans influence in the structure of [Ni(2,3′:5′,2″-terpy)Br0.14/OAc0.86]. Attempts to re-crystallise some poorly soluble Pd(II) and Pt(II) complexes of this series under solvothermal conditions (HOAc) gave two structures with N-protonated cyclometalated pyridine moieties, [Pt(2,3′:5′,2″-terpyH)Cl].Cl and [Pd(2,3′:5′,2″-terpyH)Cl2].

Photocatalytic [2+2]-cycloadditions between cyclic enones and electron-rich cyclic enol ethers are initiated by triplet-triplet energy transfer from an excited iridium photocatalyst to the enone acceptor. The composition of the resulting cycloadduct mixture shows a surprising time dependency of the cyclobutane stereoisomeric ratio which indicates that the products are not photostable under the reaction conditions. The isomerisation of the cycloadducts 3 by a photoinduced redox process points to a switch from triplet energy transfer (PenT) to photoinduced electron transfer (PET) catalysis.

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 (λem = 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 (λem = 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 (λem = 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.

The coligand X was varied in the organonickel complexes [Ni(Phbpy)X] (X = F, Cl, Br, I, C6F5) carrying the anionic tridentate C boolean AND N boolean AND N ligand 6-(phen-2-ide)-2,2'-bipyridine (Phbpy(-)) to study its effect on electronic structures of these complexes and their activity in Negishi-like C-C cross-coupling catalysis. The complexes were synthesized from the precursor [Ni(COD)(2)] (COD = 1,5-cyclooctadiene) by chelate-assisted oxidative addition into the phenyl C-X bond of the protoligand 6-(2-halidophenyl)-2,2'-bipyridine) and were obtained as red powders. Protoligands X-Phbpy carrying the halide surrogates X = OMe, OTf (triflate) failed in this reaction. Single-crystal XRD allowed us to add the structures of [Ni(Phbpy)Cl] and [Ni(Phbpy)I] to the previously reported Br derivative. Cyclic voltammetry showed reversible reductions for X = C6F5, F, Cl, while for Br and I the reversibility is reduced through rapid splitting of X - after reduction (EC mechanism). UV-vis spectroelectrochemistry confirmed the decreasing degree of reversibility along the series C6F5 > F > CI >> Br > I, which parallels the "leaving group character" of the X coligands. This method also revealed mainly bpy centered reduction and essentially Ni(II)/Ni(III) oxidations, as corroborated by DFT calculations. The rather X-invariant long-wavelength UV-vis absorptions and excited states were analyzed in detail using TD-DFT and were consistent with predominant metal to ligand charge transfer (MLCT) character. Initial catalytic tests under Negishi-like conditions showed the complexes to be active as catalysts in C-C cross-coupling reactions but did not display marked differences along the series from Ni-F to Ni-I.

The three complexes [M(Me2dpb)Cl] (M = Ni, Pd, Pt) containing the tridentate N,C,N-cyclometalating 3,5-dimethyl-1,5-dipyridyl-phenide ligand (Me2dpb−) were synthesised using a base-assisted C‒H activation method. Oxidation potentials from cyclic voltammetry increased along the series Pt < Ni < Pd from 0.15 to 0.74 V. DFT calculations confirmed the essentially ligand-centred π*-type character of the lowest unoccupied molecular orbital (LUMO) for all three complexes in agreement with the invariant reduction processes. For the highest occupied molecular orbitals (HOMO), contributions from metal dyz, phenyl C4, C2, C1, and C6, and Cl pz orbitals were found. As expected, the dz2 (HOMO-1 for Ni) is stabilised for the Pd and Pt derivatives, while the antibonding dx2−y2 orbital is de-stabilised for Pt and Pd compared with Ni. The long-wavelength UV-vis absorption band energies increase along the series Ni < Pt < Pd. The lowest-energy TD-DFT-calculated state for the Ni complex has a pronounced dz2-type contribution to the overall metal-to-ligand charge transfer (MLCT) character. For Pt and Pd, the dz2 orbital is energetically not available and a strongly mixed Cl-to-π*/phenyl-to-π*/M(dyz)-to-π* (XLCT/ILCT/MLCT) character is found. The complex [Pd(Me2dpb)Cl] showed a structured emission band in a frozen glassy matrix at 77 K, peaking at 468 nm with a quantum yield of almost unity as observed for the previously reported Pt derivative. No emission was observed from the Ni complex at 77 or 298 K. The TD-DFT-calculated states using the TPSSh functional were in excellent agreement with the observed absorption energies and also clearly assessed the nature of the so-called “dark”, i.e., d‒d*, excited configurations to lie low for the Ni complex (≥3.18 eV), promoting rapid radiationless relaxation. For the Pd(II) and Pt(II) derivatives, the “dark” states are markedly higher in energy with ≥4.41 eV (Pd) and ≥4.86 eV (Pt), which is in perfect agreement with the similar photophysical behaviour of the two complexes at low temperatures.

A series of cyclometalated Ni(II) complexes [Ni(PyPhPy)X] containing anionic (NCN-)-C-boolean AND-N-boolean AND tridentate ligand Py(Ph-)Py 2,6-di(2-pyridyl)benzene1-ide, (Py(HPh)Py = 1,3-di(2-pyridyl)benzene) and X = Cl, Br, or I as coligands, were studied. All three complexes were obtained through direct C-H base-assisted nickelation from NiX2 and Py(HPh)Py using KOAc/K2CO3 in nonpolar high-boiling point solvents. While the overall molecular structures are quite similar to those of the previously studied [Ni((CNN)-N-boolean AND-N-boolean AND)X] complexes with the anionic (CNN)-N-boolean AND-N-boolean AND tridentate -Phbpy (HPhbpy = 6-(phenyl)-2,2'-bipyridine) ligand, bond lengths in the molecular structures are slightly different. Large differences between these (NCN)-C-boolean AND-N-boolean AND and (CNN)-N-boolean AND-N-boolean AND Ni complexes which can be traced to the different orientation of the X coligand to the carbanionic phen-ide group, trans or cis, and the different ligand pattern, Py-Ph-Py versus Ph-Py-Py, were found for the UV-vis absorption spectra and the electrochemical reductions, while the oxidation potentials are very similar. Extended DFT calculations with the TPSSh functional associate the indifference of oxidation potentials with conserved energies of metal-borne HOMOs. By contrast, the diminished pi-acceptor qualities of the (NCN)-C-boolean AND-N-boolean AND ligand translate into a destabilization of the LUMO by ca. 400 mV and a blue-shift of the leading visible transition by 80 nm in very good agreement with the experimental data.

A series of cyclometalated Ni­(II)complexes [Ni­(PyPhPy)­X] containinganionic N∧C∧N– tridentate ligand Py­(Ph–)­Py 2,6-di­(2-pyridyl)­benzene-1-ide,(Py­(HPh)­Py = 1,3-di­(2-pyridyl)­benzene) and X = Cl, Br, or I as coligands,were studied. All three complexes were obtained through direct C–Hbase-assisted nickelation from NiX2 and Py­(HPh)­Py usingKOAc/K2CO3 in nonpolar high-boiling point solvents.While the overall molecular structures are quite similar to thoseof the previously studied [Ni­(C∧N∧N)­X] complexes with the anionic C∧N∧N tridentate –Phbpy (HPhbpy = 6-(phenyl)-2,2′-bipyridine)ligand, bond lengths in the molecular structures are slightly different.Large differences between these N∧C∧N and C∧N∧N Ni complexes whichcan be traced to the different orientation of the X coligand to thecarbanionic phen-ide group, trans or cis, and the different ligand pattern, Py–Ph–Py versusPh–Py–Py, were found for the UV–vis absorptionspectra and the electrochemical reductions, while the oxidation potentialsare very similar. Extended DFT calculations with the TPSSh functionalassociate the indifference of oxidation potentials with conservedenergies of metal-borne HOMOs. By contrast, the diminished π-acceptorqualities of the N∧C∧N ligandtranslate into a destabilization of the LUMO by ca. 400 mV and a blue-shiftof the leading visible transition by 80 nm in very good agreementwith the experimental data.