Thomas J. Feuerstein

The pyridyl functionalized amidinate [{PyC≡CC(NDipp)₂}Li(thf)₂]_n was used to synthesize a series of bis‐amidinate complexes [{PyC≡CC(NDipp)₂}₂M₂] (M=Cu, Ag, Au) with fully supported metallophilic interactions. These metalloligands were then used as building blocks for the synthesis of one‐dimensional heterobimetallic coordination polymers using Zn(hfac)₂ (hfac=hexaflouroacetylacetonate) for self‐assembly. Interestingly, the three coordination polymers [{PyC≡CC(NDipp)₂}₂M₂][Zn(hfac)₂] (M=Cu, Ag, Au), exhibit a zig zag shape in the solid state. To achieve linear coordination geometry other connectors such as M’(acac) (M’=Ni, Co) (acac=acetylacetonate) were investigated. The thus obtained compounds [{PyC≡CC(NDipp)₂}₂Cu₂][M’(acac)₂] (M’=Ni, Co) are indeed linear heterobimetallic coordination polymers featuring a metalloligand backbone with fully supported metallophilic interactions.

Reaction of the ethynyl compound [{HCCC(NDipp)₂}₂Au₂] with n-BuLi followed by Cp₂MCl₂ (M = Ti, Zr) leads to two group IV – gold(i) heterobimetallic complexes with different photoluminescence properties.

A novel bis(diphenylphosphino)methane (DPPM) functionalized amidine ligand (DPPM−C(N‐Dipp)₂H) (Dipp=2,6‐diisopropylphenyl) was synthesized. Subsequent deprotonation with suitable alkali metal bases resulted in the corresponding complexes [M{DPPM−C(N‐Dipp)₂}(L_n)] (M=Li, Na, K, Rb, Cs; L=thf, Et₂O). The alkali metal complexes form monomeric species in the solid state, exhibiting intramolecular metal‐π‐interactions. In addition, a caesium derivative [Cs{PPh₂CH₂‐C(N‐Dipp)₂}]₆ was obtained by cleavage of a diphenylphosphino moiety, forming an unusual six‐membered ring structure in the solid state. All complexes were fully characterized by single crystal X‐ray diffraction, NMR spectroscopy, IR spectroscopy as well as elemental analysis. Furthermore, the photoluminescent properties of the complexes were thoroughly investigated, revealing differences in emission with regards to the respective alkali metal. Interestingly, the hexanuclear [Cs{PPh₂CH₂‐C(N‐Dipp)₂}]₆ metallocycle exhibits a blue emission in the solid state, which is significantly red‐shifted at low temperatures. The bifunctional design of the ligand, featuring orthogonal donor atoms (N vs. P) and a high steric demand, is highly promising for the construction of advanced metal and main group complexes.

The synthesis of copper and zinc complexes of four variably substituted iminophosphonamide ligands is presented. While the copper complexes form ligand‐bridged dimers, the zinc compounds are monomeric. Due to different steric demand of the ligand the arrangement of the ligands within the dimeric complexes varies. Similar to the structurally related iminophosphonamide complexes of alkali metals and calcium, the steady‐state and time‐resolved photoluminescence (PL) of four of the seven compounds studied here as solids in a temperature range of 5–295 K can be described within the scheme of thermally activated delayed fluorescence (TADF). Accordingly, they exhibit bright blue‐green phosphorescence at low temperatures (<100 K), which turns into delayed fluorescence by increasing the temperature. However, unusually, the fluorescence is practically absent in two copper complexes which otherwise still conform to the TADF scheme. In these cases, the excited singlet states decay essentially non‐radiatively and their thermal population from the corresponding low‐lying triplet states efficiently quenches PL (phosphorescence). Three other copper and zinc complexes only exhibit prompt fluorescence, evidencing a wide variation of photophysical properties in this class of compounds. The excited states of the copper complex with especially pronounced phosphorescence quenching were also investigated by low‐temperature time‐resolved infrared spectroscopy and quantum chemical calculations.

The synthesis of calcium complexes ligated by three different chiral iminophosphonamide ligands, L‐H (L=[Ph₂P{N(R)CH(CH₃)Ph}₂]), L′‐H (L′=[Ph₂P{NDipp}{N(R)CH(CH₃)Ph}]), (Dipp=2,6‐ⁱPr₂C₆H₃), and L′′‐H (L′′=[Ph₂P{N(R)CH(CH₃)naph}₂]), (naph=naphthyl) is presented. The resulting structures [L₂Ca], [L′₂Ca], and [L′′₂Ca] represent the first examples of enantiopure homoleptic calcium complexes based on this type of ligands. The calcium complexes show blue–green photoluminescence (PL) in the solid state, which is especially bright at low temperatures. Whereas the emission of [L′′₂Ca] is assigned to the fluorescence of naphthyl groups, the PL of [L₂Ca] and [L′₂Ca] is contributed by long‐lived phosphorescence and thermally activated delayed fluorescence (TADF), with a strong variation of the PL lifetimes over the temperature range of 5–295 K. Furthermore, an excellent catalytic activity was found for these complexes in hydroboration of ketones at room temperature, although no enantioselectivity was achieved.

This review covers the compound class of one-dimensional gold strings. These compounds feature a formally infinite repetition of gold complexes as monomers/repeating units that are held together by aurophilic interactions, i.e. direct gold–gold contacts. Their molecular structures are primarily determined in the solid state using single crystal X-ray diffraction. The chemical composition of the employed gold complexes is diverse and furthermore plays a key role in terms of structure characteristics and the resulting properties. One of the most common features of gold strings is their photoluminescence upon UV excitation. The emission energy is often dependent on the distance of adjacent gold ions and the electronic structure of the whole string. In terms of gold strings, these parameters can be fine-tuned by external stimuli such as solvent, pH value, pressure or mechanical stress. This leads to direct structure–property correlations, not only with regard to the photophysical properties, but also electric conductivity for potential application in nanoelectronics. Concerning these correlations, gold strings, consisting of self-assembled individual complexes as building blocks, are the ideal compound class to look at, as perturbations by an inhomogeneity in the ligand sphere (such as the end of a molecule) can be neglected. Therefore, the aim of this review is to shed light on the past achievements and current developments in this area.

The synthesis of linear symmetric ethynyl‐ and acetylide‐amidinates of the coinage metals is presented. Starting with the desilylation of the complexes [{Me₃SiC≡CC(NDipp)₂}₂M₂] (Dipp=2,6‐diisopropylphenyl) (M=Cu, Au) it is demonstrated that this compound class is suitable to serve as a versatile metalloligand. Deprotonation with n‐butyllithium and subsequent salt metathesis reactions yield symmetric tetranuclear gold(I) acetylide complexes of the form [{(PPh₃)AuC≡CC(NDipp)₂}₂M₂] (M=Cu, Au). The corresponding Ag complex [{(PPh₃)AuC≡CC(NDipp)₂}₂Ag₂] was obtained by a different route via metal rearrangement. All compounds show bright blue or blue‐green microsecond long phosphorescence in the solid state, hence their photophysical properties were thoroughly investigated in a temperature range of 20–295 K. Emission quantum yields of up to 41 % at room temperature were determined. Furthermore, similar emissions with quantum yields of 15 % were observed for the two most brightly luminescent complexes in thf solution.

As one of the best studied photoswitches, spiropyrans (SPs) have attracted significant interest in the scientific community. Among the many stimuli to alter the isomerization into the merocyanine (MC) isomer, such as temperature, pH, solvent polarity, redox potential, or mechanical force, the ability of the MC form to act as a ligand site for metal complexation has recently raised new attention. We herein synthesize hitherto undescribed coordination compounds of 8-methoxy-1′,3′,3′-trimethyl-6-nitrospiro-[chromene-2,2′-indoline] with s-block ([Ca(MC)₄](ClO₄)₂), d-block ([Zn(MC)₂(MeCN)₂](ClO₄)₂, [Ni(MC)₂(MeCN)₂](ClO₄)₂), as well as f-block ([La(NO₃)₃(MC)₂]) metals. All complexes are structurally described by X-ray crystallography and systematically investigated in solution via Job’s method of continuous variations (Job plots), as well as online NMR spectroscopic kinetic experiments with in situ irradiation of the analyte solution inside the NMR spectrometer (LED-NMR). We can unambiguously identify the photoresponsive nature of the complexes in solution, which is a crucial step toward the application of these promising molecules in material science.