Michael T. Gamer

The syntheses and structural characterization of Cu(I)–Rh(I) and Cu(I)–Ir(I) complexes are reported. These complexes are synthesized via the reaction of a previously reported copper amidinate complex, Cu ₂ L ₂ (L= {Ph ₂ PC≡CC(NDipp) ₂ }, Dipp = N , N ′‐2,6‐diisopropylphenyl), with [M(cod)Cl] ₂ , (M= Rh, Ir; cod = 1,5‐cyclooctadiene) yielding complexes [{(M(cod)Cl)Ph ₂ PC≡CC(NDipp) ₂ } ₂ Cu ₂ ] (M= Rh, Ir). Additionally, an alternative one‐pot synthetic route is explored by the reaction of lithium salt of the ligand, [M(cod)Cl] ₂ , (M= Rh, Ir), and CuCl, resulting in improved yields of [{(M(cod)Cl)Ph ₂ PC≡CC(NDipp) ₂ } ₂ Cu ₂ ]. Further, reaction of lithium salt of the ligand with [M(cod)Cl] ₂ , (M= Rh, Ir) leads to the formation of intermediate complexes [{(M(cod)Cl)Ph ₂ PC≡CC(NDipp) ₂ }Li(thf) ₃ ] (M= Rh, Ir), which upon hydrolysis, yielded protonated amidinate derivatives [(M(cod)Cl)Ph ₂ PC≡CC(NDipp)(NHDipp)] (M= Rh, Ir). In the second pathway, lithium salt of the ligand is hydrolyzed to obtain [Ph ₂ PC≡CC(NDipp)(NHDipp)] ₂ and further reaction with [M(cod)Cl] ₂ , (M= Rh, Ir) yields [(M(cod)Cl)Ph ₂ PC≡CC(NDipp)(NHDipp)] via an alternative reaction pathway. The catalytic potential of Cu(I)–Ir(I) complex [{(Ir(cod)Cl)Ph ₂ PC≡CC(NDipp) ₂ } ₂ Cu ₂ ] is evaluated for transfer hydrogenation (TH) of aryl aldehydes using isopropanol as a hydrogen transfer source. Furthermore, the Ir complexes [{(Ir(cod)Cl)Ph ₂ PC≡CC(NDipp) ₂ } ₂ Cu ₂ ] and [(Ir(cod)Cl)Ph ₂ PC≡CC(NDipp)(NHDipp] are found to be luminescent in the solid state at 77 K with an emission maximum around 580 nm.

Cyclic nanometre-scale sandwich complexes assembled from individual building blocks were synthesized. Sandwich complexes, in which a metal ion is π-coordinated by two planar aromatic organic rings belong to the foundations of organometallic chemistry. They have been successfully used in a wide variety of applications ranging from catalysis, synthesis and electrochemistry to nanotechnology, materials science and medicine^1,2. Extending the sandwich structural motif leads to linear multidecker compounds, in which aromatic organic rings and metal atoms are arranged in an alternating fashion. However, the extension to a cyclic multidecker scaffold is unprecedented. Here we show the design, synthesis and characterization of an isomorphous series of circular sandwich compounds, for which the term ‘cyclocenes’ is suggested. These cyclocenes consist of 18 repeating units, forming almost ideally circular, closed rings in the solid state, that can be described by the general formula [cyclo-M^II(μ-η⁸:η⁸-Cot^TIPS)]₁₈ (M = Sr, Sm, Eu; Cot^TIPS = 1,4-(ⁱPr₃Si)₂C₈H₆²⁻). Quantum chemical calculations lead to the conclusion that a unique interplay between the ionic metal-to-ligand bonds, the bulkiness of the ligand system and the energy gain on ring closure, which is crucially influenced by dispersion interactions, facilitate the formation of these cyclic systems. Up to now, only linear one-dimensional multidecker sandwich compounds have been investigated for possible applications such as nanowires^{3,4,5,6,7,8,9,10}. This textbook example of cyclic sandwich compounds is expected to open the door for further innovations towards new functional organometallic materials.

The first half-sandwich complexes of the cyclononatetraenyl are reported. Coordination and deccordination of THF leads to the formation of either the ionic [Ln^III(η³-BH₄)₂(thf)₅][Cnt] or the polymeric compound [La^III(η⁹-Cnt)(η³-BH₄)₂(thf)].

A coumarin functionalized aminodiphosphine has been introduced as a bidentate ligand in coinage metal chemistry. Mono‐, di‐, and trimetallic copper and silver complexes were synthesized with this ligand. The hybrid character of the ligand led to compounds with rich luminescence properties. These include coumarin‐based blue fluorescence, observed as a sole emission in solution at room temperature, and green phosphorescence, which is efficient at low temperatures and dominates the spectra of the metal complexes. In the rigid environment of frozen solutions, the green phosphorescence shows an unusually long (for metal complexes) decay on the seconds timescale in high quantum yield. In addition, a red phosphorescence, which may be assigned to the triplet state localized in the phosphine‐M₃Cl₂ (M=Cu, Ag), is observed for the trinuclear complexes at low temperature. Neither the second‐long phosphorescence nor the red emission is observed for the coumarin ligand, thus they must be a result of the coordination to coinage metal clusters. The excited states in these compounds were also investigated by femtosecond transient absorption spectroscopy and quantum chemical calculations.

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.

A bis(diphenyl)‐phosphine functionalized ß‐diketimine ligand (PNac‐H) was applied for the synthesis of a subvalent Ni(I) complex [PNac‐Ni]. Here, the Ni(I) center is stabilized by a tetradentate PNNP‐type pocket, forming a square planar coordination sphere. Subsequently, the Ni(I) complex was investigated with regard to its reactivity and the activation of small molecules. The reductive potential of Ni(I) enabled an activation of different substrate classes, such as CH₂X₂ (X=Br, I), I₂ or Ph₂E₂ (E=S, Se). The ligand's design allows a stabilization of the reactive Ni(I) species while at the same time enabling activation processes due to a hemilabile coordination behavior and accessible axial coordination sites. The activation products have been characterized by single crystal X‐ray diffraction, NMR and IR spectroscopy as well as elemental analysis.

Electromerism is a very well‐known phenomenon in transition metal chemistry. In main group chemistry, this concept has only started getting attention recently. We report stimuli responsive low‐valent silicon compounds exhibiting electromerism. A mixed‐valent silaiminyl‐silylene1, [LSi−Si(NDipp)L] (L=PhC(N^tBu)₂), was synthesized in a single step from amidinate‐chlorosilylene. Compound1has two interconnected Si atoms in formally +I and +III oxidation states. Upon treatment with Lewis acidic Cu^IX (X=mesityl, Cl, Br, I), electron redistribution occurs resulting in the formation of [{LSi(NDipp)Si(L)}−CuX], in which both silicon atoms are in the +II formal oxidation state. Removal of the copper center from [{LSi(NDipp)Si(L)}−CuX] by using a Lewis basic carbene led to reformation of the precursor [LSi−Si(NDipp)L]. Thus, the process is fully reversible. This showcases the first example of Lewis acid/base‐induced reversible electromerism in silicon chemistry.

Electromerism is a very well‐known phenomenon in transition metal chemistry. In main group chemistry, this concept has only started getting attention recently. We report stimuli responsive low‐valent silicon compounds exhibiting electromerism. A mixed‐valent silaiminyl‐silylene1, [LSi−Si(NDipp)L] (L=PhC(N^tBu)₂), was synthesized in a single step from amidinate‐chlorosilylene. Compound1has two interconnected Si atoms in formally +I and +III oxidation states. Upon treatment with Lewis acidic Cu^IX (X=mesityl, Cl, Br, I), electron redistribution occurs resulting in the formation of [{LSi(NDipp)Si(L)}−CuX], in which both silicon atoms are in the +II formal oxidation state. Removal of the copper center from [{LSi(NDipp)Si(L)}−CuX] by using a Lewis basic carbene led to reformation of the precursor [LSi−Si(NDipp)L]. Thus, the process is fully reversible. This showcases the first example of Lewis acid/base‐induced reversible electromerism in silicon chemistry.

The reaction of the dilithium salt of the enantiopure (S)‐BINOL (1,1’‐bi‐2‐naphthol) with two equivalents of the amidinate‐stabilized chlorosilylene [L^PhSiCl] (L^Ph=PhC(NtBu)₂) led to the formation of the first example of a chiral cyclic silene species comprising an (S)‐BINOL ligand. The reactivity of the Si=C bond was investigated by reaction with elemental sulfur, CO₂ and HCl. The reaction with S₈ led to a Si=C bond cleavage and concomitantly to a ring‐opened product with imine and silanethione functional groups. The reaction with CO₂ resulted in the cleavage of the CO₂ molecule into a carbonyl group and an isolated O atom, while a new stereocenter is formed in a highly selective manner. According to DFT calculations, the [2+2] cycloaddition product is the key intermediate. Further reactivity studies of the chiral cyclic silene with HCl resulted in a stereoselective addition to the Si=C bond, while the fully selective formation of two stereocenters was achieved. The quantitative stereoselective addition of CO₂ and HCl to a Si=C bond is unprecedented.