Sven Krieck

An Efficient General Synthesis of Halide‐Free Diarylcalcium

Contradictory reports on the synthesis and stability of arylcalcium compounds can be found in the literature, which reflects the difficulties that has hampered the broad acceptance of these heavy Grignard reagents. First reports date back to 1905, when Beckmann first published the synthesis of phenylcalcium halides. Doubts concerning these results were raised by Gilman and Schulze, who also noticed that the obtained yields were usually far from satisfactory. Whereas some research groups heat the reaction solutions to complete the turnover, other groups recommend performing the direct synthesis at low temperatures and using the formed reagents immediately. The main reasons for contradictory reports on the preparative procedures and on the stability of arylcalcium derivatives in ether solutions are rooted in the large discrepancy between the rather unreactive metal itself and the high reactivity of the organocalcium derivative, the necessity to activate the metal prior to use, and, last but not least, the tendency for the ether solvent to be cleaved during the direct synthesis of arylcalcium halides. Re-investigation of the synthesis of arylcalcium compounds led to the isolation of oxygen-centered arylcalcium cages, calcium vinylate derivatives, and after prolonged heating of phenylcalcium iodide in THF even to aryl-free [(CaO)4{(thf)3Ca(m-I)2}4]. [8] For these reasons, the direct synthesis of arylcalcium iodides from activated calcium and aryliodides has to be performed at low temperatures. A solvent-dependent equilibrium leads to the formation of calcium diiodide and diarylcalcium. A separation was, however, only successful (in a rather low yield) for [(thf)3CaMes2] (Mes = 2,4,6-trimethylphenyl), but this derivative is extremely reactive because of the rather low coordination number of the calcium and its tendency to form more stable benzyl compounds. Metathesis reactions of [(thf)4Ca(aryl)I] with potassium compounds allow exchange of the halide by phosphanides and amides. An efficient high-yielding synthesis of halide-free diarylcalcium is necessary to expedite the development of organocalcium chemistry. In principle, two routes seem to be appropriate. Transmetalation of readily accessible diarylmercury allows the preparation of diarylcalcium. However, the reactions of calcium with other arylmetal compounds often yield metalates such as [(thf)3Ca(m-Ph)3Ca(thf)3] + [Ph-CuPh] . A quantitative shift of the Schlenk equilibrium toward diarylcalcium and calcium diiodide offers another preparative method. However, the 1,4-dioxane precipitation method, which was applied successfully in organomagnesium reactions, failed for the heavier congeners. Halide-free diphenylmanganese reacts readily with activated calcium powder to give an aryl-rich heterobimetallic ion pair [(thf)3Ca(m-Ph)3Ca(thf)3] + [(thf)2PhCa(m-Ph)3MnPh] (1) according to Equation (1). A complete substitution of manganese by calcium was impossible by this procedure. The complex cation of 1 is isostructural to the cation of the aforementioned cuprate. The anion has a remarkable structure since it contains a calcium as well as a manganese atom, which are bridged by three phenyl groups. Both metal atoms also bind to another terminal phenyl substituent. The coordination spheres of the calcium atoms are saturated by THF molecules. In heterobimetallic compounds, the cation contains the more electropositive metal whereas the anion contains the less electropositive ones. Here we report one of the very rare examples of a heterodimetallic manganate anion that contains the transition metal as well as the electropositive calcium.

Mechanistic Elucidation of the Formation of the Inverse Ca(I) Sandwich Complex [(thf)3Ca(μ-C6H3-1,3,5-Ph3)Ca(thf)3] and Stability of Aryl-Substituted Phenylcalcium Complexes

The formation of the stable inverse Ca(I) sandwich complex [(thf)(3)Ca(mu-C(6)H(3)-1,3,5-Ph(3))Ca(thf)(3)] (1) has been investigated mechanistically by the reaction of bromo-2,4,6-triphenylbenzene with calcium in varying stoichiometric ratios. The key intermediate consists of a solvent-separated ion pair consisting of a dinuclear calcium cation with a bridging doubly deprotonated triphenylbenzene and a triphenylbenzene radical counteranion [(thf)(3)Ca(mu-C(6)H(2)-C(6)H(4)Ph(2))(mu-O-CH=CH(2))Ca(thf)(3)][C(6)H(3)Ph(3)] (4). A precondition of the formation of 1 is the lability of the heavy Grignard reagent [{2,4,6-Ph(3)C(6)H(2)}Ca(thf)(3)Br] (2), which has been studied along with the role of ether degradation reactions. The strong reducing reagent 1 is stable in THF solution, and ether cleavage does not occur. However, toluene is metalated in good yields, and the dibenzylcalcium complex [(tmta)(2)Ca(CH(2)C(6)H(5))(2)] (5) is generated after addition of 1,3,5-trimethyl-1,3,5-triazinane (tmta). The substitution pattern of arylcalcium halides was modified, and it was found that phenyl substituents at the para position induce lability, leading to an enhanced tendency to cleave ethers. Kinetic stabilization of the Ca-C(ipso) bond can be achieved by ortho substitution using m-terphenyl-based ligands. Direct reaction of iodo-2,6-di(4-tolyl)benzene (6) with activated calcium in THF at low temperatures yielded the first example of a stable m-terphenylcalcium halide, namely, [{2,6-(4-tol)(2)C(6)H(3)}Ca(thf)(3)I] (8). The latter reacts via insertion of carbon dioxide to form the dimeric benzoate [{2,6-(4-tol)(2)C(6)H(3)CO(2)}Ca(thf)(3)I](2) (9).

Resonance-Raman spectro-electrochemistry of intermediates in molecular artificial photosynthesis of bimetallic complexes

The sequential order of photoinduced charge transfer processes and accompanying structure changes were analyzed by UV-vis and resonance-Raman spectroscopy of intermediates of a Ru(ii) based photocatalytic hydrogen evolving system obtained by electrochemical reduction.

Heavy Grignard Reagents: Synthesis, Physical and Structural Properties, Chemical Behavior, and Reactivity

The Grignard reaction offers a straight forward atom-economic synthesis of organomagnesium halides, which undergo redistribution reactions (Schlenk equilibrium) yielding diorganylmagnesium and magnesium dihalides. The homologous organocalcium complexes (heavy Grignard reagents) gained interest only quite recently owing to several reasons. The discrepancy between the inertness of this heavy alkaline earth metal and the enormous reactivity of its organometallics hampered a vast and timely development after the first investigation more than 100 years ago. In this overview the synthesis of organocalcium reagents is described as is the durability in ethereal solvents. Aryl-, alkenyl-, and alkylcalcium halides are prepared by direct synthesis. Characteristic structural features and NMR parameters are discussed. Ligand redistribution reactions can be performed by addition of potassium tert-butanolate to ethereal solutions of arylcalcium iodides yielding soluble diarylcalcium, whereas sparingly soluble potassium iodide and calcium bis(tert-butanolate) precipitate. Furthermore, reactivity studies with respect to metalation and addition to unsaturated organic compounds and metal-based Lewis acids, leading to the formation of heterobimetallic complexes, are presented.

Organic heterobimetallic complexes of the alkaline earth metals (Ae = Ca, Sr, Ba) with tetrahedral metallate anions of three-valent metals (M = B, Al, Ga, and V)

Heterobimetallic compounds with complex cations of the very electropositive alkaline earth metals (Ae) and organic tetrahedral anions of trivalent elements (M) form solvent-separated ions. Depending on the metals, they can be prepared by addition of carbanions or amides to MR3, via reduction of VMes4 with the alkaline earth metals and by transmetallation of VMes3. For comparison reasons selected alkali metal derivatives are also included in this study. Average structural parameters of the boranates [Ca(thf)6][BPh4]2 (1) and [CaI(thf)5][BPh4] (2), the alanates [Li(thf)2(tmeda)][AlPh3(tmp)] (3), [(thf)2K(N-carbazolyl)2AlMes2] (4), and [Sr(thf)7][AlPh4]2 (5), of the gallate [Ca(dme)4][GaEt3(N-carbazolyl)]2 (6), and of the vanadates [Li(thf)4][VMes4] (7), [CaI(thf)5][VMes4] (8), [Ca(thf)6][VMes4]2 (9), and [Sr(thf)6][VMes4]2 (10), are discussed. All these complexes contain complex cations of the type [(L)nM]+, and tetrahedral anions of the type [ER4]−. Only 4 crystallizes as a contact ion pair because the soft K+ cation shows no preference for hard bases such as ethers or for soft arene π-systems.

CORM-EDE1: A Highly Water-Soluble and Nontoxic Manganese-Based photoCORM with a Biogenic Ligand Sphere

[Mn(CO)5Br] reacts with cysteamine and 4-amino-thiophenyl with a ratio of 2:3 in refluxing tetrahydrofuran to the complexes of the type [{(OC)3Mn}2(μ-SCH2CH2NH3)3]Br2 (1, CORM-EDE1) and [{(OC)3Mn}2(μ-SC6H4-4-NH3)3]Br2 (2, CORM-EDE2). Compound 2 precipitates during refluxing of the tetrahydrofuran solution as a yellow solid whereas 1 forms a red oil that slowly solidifies. Recrystallization of 2 from water yields the HBr-free complex [{(OC)3Mn}2(μ-S-C6H4-4-NH2)2(μ-SC6H4-4-NH3)] (3). The n-propylthiolate ligand (which is isoelectronic to the bridging thiolate of 1) leads to the formation of the di- and tetranuclear complexes [(OC)4Mn(μ-S-nPr)2]2 and [(OC)3Mn(μ-S-nPr)]4. CORM-EDE1 possesses ideal properties to administer carbon monoxide to biological and medicinal tissues upon irradiation (photoCORM). Isolated crystalline CORM-EDE1 can be handled at ambient and aerobic conditions. This complex is nontoxic, highly soluble in water, and indefinitely stable therein in the absence of air and phosphate buffer. CORM-EDE1 is stable as frozen stock in aqueous solution without any limitations, and these stock solutions maintain their CO release properties. The reducing dithionite does not interact with CORM-EDE1, and therefore, the myoglobin assay represents a valuable tool to study the release kinetics of this photoCORM. After CO liberation, the formation of MnHPO4 in aqueous buffer solution can be verified.

Oxidation products of calcium and strontium bis(diphenylphosphanide).

The tetrahydrofuran adducts [(thf)(4)M(PPh(2))(2)] (M = Ca, Sr) are air sensitive and can easily be oxidized by chalcogens. Metalation of diphenylphosphane oxide, diphenylphosphinic acid, and diphenyldithiophosphinic acid as well as salt metathetical approaches of the potassium salts with MI(2) allow the synthesis of [(thf)(4)Ca(OPPh(2))(2)] (1), [(dmso)(2)Ca(O(2)PPh(2))(2)] (2), [(thf)(3)Ca(O(2)PPh(2))I](2) (3), [(thf)(3)Ca(S(2)PPh(2))(2)] (4), [(thf)(2)Ca(Se(2)PPh(2))(2)] (5), [(thf)(3)Sr(S(2)PPh(2))(2)] (6), [(thf)(3)Sr(Se(2)PPh(2))(2)] (7), and [(thf)(2)Ca(O(2)PPh(2))(S(2)PPh(2))](2) (8), respectively. The diphenylphosphinite anion in 1 contains a phosphorus atom in a trigonal pyramidal environment and binds terminally via the oxygen atom to calcium. The diphenylphosphinate anions act as bridging ligands leading to polymeric structures of calcium bis(diphenylphosphinates). Therefore strong Lewis bases such as dimethylsulfoxide (dmso) are required to recrystallize this complex yielding chain-like 2. The chain structure can also be cut into smaller units by ligands which avoid bridging positions such as iodide and diphenyldithiophosphinate (3 and 8, respectively). In general, diphenyldithio- and -diselenophosphinate anions act as terminal ligands and allow the isolation of mononuclear complexes 4 to 7. In these molecules the alkaline earth metals show coordination numbers of six (5) and seven (4, 6, and 7).

Tris(pyrazolyl)methanides of the Alkaline Earth Metals: Influence of the Substitution Pattern on Stability and Degradation

Trispyrazolylmethanides commonly act as strong tridentate bases toward metal ions. This expected coordination behavior has been observed for tris(3,4,5-trimethylpyrazolyl)methane (1a), which yields the alkaline-earth-metal bis[tris(3,4,5-trimethylpyrazolyl)methanides] of magnesium (1b), calcium (1c), strontium (1d), and barium (1e) via deprotonation of 1a with dibutylmagnesium and [Ae{N(SiMe3)2}2] (Ae = Mg, Ca, Sr, and Ba, respectively). Barium complex 1e degrades during recrystallization that was attempted from aromatic hydrocarbons and ethers. In these scorpionate complexes, the metal ions are embedded in distorted octahedral coordination spheres. Contrarily, tris(3-thienylpyrazolyl)methane (2a) exhibits a strikingly different reactivity. Dibutylmagnesium is unable to deprotonate 2a, whereas [Ae{N(SiMe3)2}2] (Ae = Ca, Sr, and Ba) smoothly metalates 2a. However, the primary alkaline-earth-metal bis[tris(3-thienylpyrazolyl)methanides] of Ca (2c), Sr (2d), and Ba (2e) represent intermediates and degrade under the formation of the alkaline-earth-metal bis(3-thienylpyrazolates) of calcium (3c), strontium (3d), and barium (3e) and the elimination of tetrakis(3-thienylpyrazolyl)ethene (4). To isolate crystalline compounds, 3-thienylpyrazole has been metalated, and the corresponding derivatives [(HPz(Tp))4Mg(Pz(Tp))2] (3b), dinuclear [(tmeda)Ca(Pz(Tp))2]2 (3c), mononuclear [(pmdeta)Sr(Pz(Tp))2] (3d), and [(hmteta)Ba(Pz(Tp))2] (3e) have been structurally characterized. Regardless of the applied stoichiometry, magnesiation of thienylpyrazole 3a with dibutylmagnesium yields [(HPz(Tp))4Mg(Pz(Tp))2] (3b), which is stabilized in the solid state by intramolecular N-H···N···H-N hydrogen bridges. The degradation of [Ae{C(Pz(R))3}2] (R = Ph and Tp) has been studied by quantum chemical methods, the results of which propose an intermediate complex of the nature [{(Pz(R))2C}2Ca{Pz(R)}2]; thereafter, the singlet carbenes ([:C(Pz(R))2]) dimerize in the vicinity of the alkaline earth metal to tetrapyrazolylethene, which is liberated from the coordination sphere as a result of it being a very poor ligand for an s-block metal ion.

Concept for enhancement of the stability of calcium-bound pyrazolyl-substituted methanides.

Metalation of bis(3-thiophen-2-ylpyrazol-1-yl)phenylmethane [2, which is accessible from the reaction of bis(3-thien-2-ylpyrazol-1-yl)methanone (1) with triphosgene] with [(thf)2Ca{N(SiMe3)2}2] in tetrahydrofuran and subsequent crystallization from a mixture of toluene and 1,2-dimethoxyethane yield [(dme)Ca{C(Pz(th))2Ph}{N(SiMe3)2}] (3). The α,α-bis(3-thiophen-2-ylpyrazol-1-yl)benzyl ligand exhibits a κ(2)N,κC-coordination mode with a Ca-C σ-bond length of 262.8(2) pm. The crystalline compound is stable if air and moisture is strictly excluded; however, in solution; this calcium complex slowly degrades.

Porous NiOx nanostructures templated by polystyrene-block-poly(2-vinylpyridine) diblock copolymer micelles

A facile synthetic route to NiOx nanostructures using various amphiphilic polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) diblock copolymers as templates was investigated. The synthesis targets NiOx nanostructures with a large surface area in order to allow an efficient functionalization, e.g., through loading with dyes to enable photo-induced hole injection for use in dye-sensitized solar cells or in (photo-)catalytic systems. The complete synthetic process to NiOx contains several steps: (i) the dissolution of the diblock copolymer, (ii) the subsequent addition of Ni2+, followed by the formation of core–corona micelles and eventually, (iii) further addition of Ni2+, resulting in the formation of a macroscopic precipitate. In all cases, (iv) deposition onto different substrates and calcination yielded NiOx films. All intermediates were thoroughly investigated using scanning or transmission electron microscopy, dynamic light scattering, and UV-vis spectroscopy. In contrast to the well-established synthetic route via the commercially available Pluronic F108 triblock copolymer, in our case a variety of different morphologies was found, i.e. spherical particles, toroid structures, or networks. Furthermore, the obtained BET area of about 50 m2 g−1 is comparable to the value for conventionally obtained NiOx surfaces. First dye sensitization experiments with coumarine 343 confirm that the dye binds to the surface, which is a prerequisite for using the material as a photo-electrode. The presented route to porous NiOx is easy and provides superior control over the morphology of the intermediates involved in nanostructure formation.