Many organometallic iridium(III) complexes have photoactive excited states with mixed metal-to-ligand and intraligand charge transfer (MLCT/ILCT) character, which form the basis for numerous applications in photophysics and photochemistry. Cobalt(III) complexes with analogous MLCT excited-state properties seem to be unknown yet, despite the fact that iridium(III) and cobalt(III) can adopt identical low-spin d6 valence electron configurations due to their close chemical relationship. Using a rigid tridentate chelate ligand (LCNC), in which a central amido π-donor is flanked by two σ-donating N-heterocyclic carbene subunits, we obtained a robust homoleptic complex [Co(LCNC)2](PF6), featuring a photoactive excited state with substantial MLCT character. Compared to the vast majority of isoelectronic iron(II) complexes, the MLCT state of [Co(LCNC)2](PF6) is long-lived because it does not deactivate as efficiently into lower-lying metal-centered excited states; furthermore, it engages directly in photoinduced electron transfer reactions. The comparison with [Fe(LCNC)2](PF6), as well as structural, electrochemical, and UV–vis transient absorption studies, provides insight into new ligand design principles for first-row transition-metal complexes with photophysical and photochemical properties reminiscent of those known from the platinum group metals.

/pdf/cobalt-iii-carbene-complex-with-an-electronic-excited-state-1rmpcwtw.pdf

Cobalt(III) Carbene Complex with an Electronic Excited-State Structure Similar to Cyclometalated Iridium(III) Compounds

Self-assembly and molecular recognition are critical processes both in life and material sciences. They usually depend on strong, directional non-covalent interactions to gain specificity and to make long-range organization possible. Most supramolecular constructs are also at least partially governed by topography, whose role is hard to disentangle. This makes it nearly impossible to discern the potential of shape and motion in the creation of complexity. Here, we demonstrate that long-range order in supramolecular constructs can be assisted by the topography of the individual units even in the absence of highly directional interactions. Molecular units of remarkable simplicity self-assemble in solution to give single-molecule thin two-dimensional supramolecular polymers of defined boundaries. This dramatic example spotlights the critical function that topography can have in molecular assembly and paves the path to rationally designed systems of increasing sophistication. 

/pdf/shape-assisted-self-assembly-2aadl49z.pdf

Shape-assisted self-assembly

A triphosphine-supported Pt2Cu heterotrinuclear complex (1) constrained with 3,6-di-tert-butyl-1,8-diethynyl-9H-carbazole exhibits two-step phosphorescent mechanochromism due to intramolecular deformation. The stepwise phosphorescent mechanochromism following green → yellow → orange-red corresponds exactly to the phosphorescent emission observed in crystal morphs 1·4THF (green), 1·3acetone (yellow), and 1·4CH2Cl2 (orange-red), respectively, with different degrees of intramolecular deformation, as demonstrated by X-ray crystallography.

Two-step phosphorescent mechanochromism due to intramolecular deformation

We report on the convenient synthesis of a CNC pincer ligand composed of carbazole and two mesoionic carbenes, as well as the corresponding lithium- and magnesium complexes. Mono-deprotonation affords a rare “naked” amide anion. In contrast to the proligand and its mono-deprotonated form, tri-deprotonated s-block complexes show bright luminescence, and their photophysical properties were therefore investigated by absorption- and luminescence spectroscopy. They reveal a quantum yield of 16% in solution at ambient temperature. Detailed quantum-chemical calculations assist in rationalizing the emissive properties based on an Intra-Ligand-Charge-Transfer (ILCT) between the carbazolido- and mesoionic carbene ligands. (Earth-)alkali metals prevent the distortion of the ligand following excitation and, thus, by avoiding non-radiative deactivation support bright luminescence.

/pdf/bright-luminescent-lithium-and-magnesium-carbene-complexes-3igbdfadfv.pdf

Bright luminescent lithium and magnesium carbene complexes

The syntheses of bis(triazolium)carbazole precursors and their corresponding coinage metal (Au, Ag) complexes are reported. For alkylated triazolium salts, di- or tetranuclear complexes with bridging ligands were isolated, while the bis(aryl) analogue afforded a bis(carbene) AuI -CNC pincer complex suitable for oxidation to the redox-stable [AuIII (CNC)Cl]+ cation. Although the ligand salt and the [AuIII (CNC)Cl]+ complex were both notably cytotoxic toward the breast cancer cell line MDA-MB-231, the AuIII complex was somewhat more selective. Electrophoresis, viscometry, UV-vis, CD and LD spectroscopy suggest the cytotoxic [AuIII (CNC)Cl]+ complex behaves as a partial DNA intercalator. In silico screening indicated that the [AuIII (CNC)Cl]+ complex can target DNA three-way junctions with good specificity, several other regular B-DNA forms, and Z-DNA. Multiple hydrophobic π-type interactions involving T and A bases appear to be important for B-form DNA binding, while phosphate O⋅⋅⋅Au interactions evidently underpin Z-DNA binding. The CNC ligand effectively stabilizes the AuIII ion, preventing reduction in the presence of glutathione. Both the redox stability and DNA affinity of the hit compound might be key factors underpinning its cytotoxicity in vitro.

/pdf/a-cytotoxic-bis-1-2-3-triazol-5-ylidene-carbazolide-gold-iii-1hkb1ejr21.pdf

A Cytotoxic Bis(1,2,3-triazol-5-ylidene)carbazolide Gold(III) Complex Targets DNA by Partial Intercalation.

Four tridentate carbazole-based ligands, HLtBu/H {3,6-di(tert-butyl)-1,8-bis[5-(3-benzyl-1,2,3-triazole)]-9H-carbazole}, HLtBu/tBu {3,6-di(tert-butyl)-1,8-bis[5-(3-(4-tert-butyl)benzyl-1,2,3-triazole)]-9H-carbazole}, HLH/H {1,8-bis[5-(3-benzyl-1,2,3-triazole)]-9H-carbazole} and HLH/tBu {1,8-bis[5-(3-(4-tert-butyl)benzyl-1,2,3-triazole)]-9H-carbazole}, were prepared and complexed with cobalt(II) tetrafluoroborate. In situ air oxidation resulted in cobalt(III) complexes 1–4 with the general formula [CoIII(L)2]BF4·xH2O (1: L = LtBu/H, x = 2; 2: L = LtBu/tBu, x = 1; 3: L = LH/H, x = 0.5; 4: L = LH/tBu, x = 2). X-ray structural characterisation confirmed that the four complexes are isostructural, with two orthogonally coordinated deprotonated tridentate ligands providing an octahedral N6-donor set to the cobalt(III) ion. 1H NMR studies show that this structure is maintained in CDCl3 and DMSO-d6 solution. Cyclic voltammetry on 1–4 in MeCN showed that all of the complexes exhibit two reversible, one-electron oxidation processes (probably due to ligand oxidations), and an irreversible or quasi-reversible reduction process (probably due to reduction of Co(III) to Co(II)). As expected, the oxidations move 120–140 mV to lower potentials on adding tert-butyl substituents to the 3 and 6 positions of the carbazole rings, and unsurprisingly the potentials are far less sensitive to the nature of the benzyl ring substituents.

Tuneable reversible redox of cobalt(III) carbazole complexes

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Tuneable reversible redox of cobalt(III) carbazole complexes