In Situ Generation of Magnesium- and Calcium-Based Grignard Reagents for Amide Synthesis.

The alkaline-earth metals Mg and Ca are too inert for the direct metalation of primary and secondary amines. Consequently, activation prior to use is required. Alternatively, the Grignard reagents RMgX (R=alkyl, aryl, X=halide) can be applied in metalation of amines. However, such a straightforward procedure for the synthesis of alkylcalcium reagents is disadvantageous due to diverse side reactions, including Wurtz-type C-C coupling and ether degradation reactions. Therefore, suspensions of magnesium or calcium with amine can be treated in a smooth reaction with ethyl bromide in an ethereal solvent at room temperature. Intermediately formed RAeX (Ae=alkaline-earth metal, i. e., Mg, Ca) either metalates amines yielding the corresponding amides in an in situ Grignard metalation method (iGMM) or adds across C=N bonds of imines in an in situ Grignard addition method (iGAM). The amides R'2 N-AeX (Ae=Mg: Hauser bases) undergo Schlenk-type ligand exchange reactions yielding homoleptic Ae(NR'2 )2 and potentially sparingly soluble AeX2 .

In situ Grignard Metalation Method, Part II: Scope of the One-Pot Synthesis of Organocalcium Compounds.

The in situ Grignard Metalation Method (iGMM) is a straightforward one-pot strategy to synthesize alkaline-earth metal amides in multi-gram scale with high yields via addition of bromoethane to an ethereal suspension of a primary or secondary amine and magnesium (Part I) or calcium (Part II). This method is highly advantageous because no activation of calcium is required prior to the reaction. Contrary to the magnesium-based iGMM, there are some limitations, the most conspicuous one is the large influence of steric factors. The preparation of Ca(hmds)2 succeeds smoothly within a few hours with excellent yields opening the opportunity to prepare large amounts of this reagent. Side reactions and transfer of the iGMM to substituted anilines and N-heterocycles as well as other H-acidic substrates such as cyclopentadienes are studied. Bulky amidines cannot be converted directly to calcium amidinates via the iGMM but stoichiometric calciation with Ca(hmds)2 enables their preparation.

In Situ Grignard Metalation Method for the Synthesis of Hauser Bases.

The in situ Grignard Metalation Method (iGMM) is a straightforward one-pot procedure to quickly produce multigram amounts of Hauser bases R2 N-MgBr which are valuable and vastly used metalation reagents and novel electrolytes for magnesium batteries. During addition of bromoethane to a suspension of Mg metal and secondary amine at room temperature in an ethereal solvent, a smooth reaction yields R2 N-MgBr under evolution of ethane within a few hours. A Schlenk equilibrium is operative, interconverting the Hauser bases into their solvated homoleptic congeners Mg(NR2 )2 and MgBr2 depending on the solvent. Scope and preconditions are studied, and side reactions limiting the yield have been investigated. DOSY NMR experiments and X-ray crystal structures of characteristic examples clarify aggregation in solution and the solid state.

One-Step Synthesis and Schlenk-Type Equilibrium of Cyclopentadienylmagnesium Bromides.

In the in situ Grignard metalation method (iGMM), the addition of bromoethane to a suspension of magnesium turnings and cyclopentadienes [C5 H6 (HCp), C5 H5 -Si(iPr)3 (HCpTIPS )] in diethyl ether smoothly yields heteroleptic [(Et2 O)Mg(CpR )(µ-Br)]2 (CpR =Cp (1), CpTIPS (2)). The Schlenk equilibrium of 2 in toluene leads to ligand exchange and formation of homoleptic [Mg(CpR )2 ] (3) and [(Et2 O)MgBr(µ-Br)]2 (4). Interfering solvation and aggregation as well as ligand redistribution equilibria hamper a quantitative elucidation of thermodynamic data for the Schlenk equilibrium of 2 in toluene. In ethereal solvents, mononuclear species [(Et2 O)2 Mg(CpTIPS )Br] (2'), [(Et2 O)n Mg(CpTIPS )2 ] (3'), and [(Et2 O)2 MgBr2 ] (4') coexist. Larger coordination numbers can be realized with cyclic ethers like tetrahydropyran allowing crystallization of [(thp)4 MgBr2 ] (5). The interpretation of the temperature-dependency of the Schlenk equilibrium constant in diethyl ether gives a reaction enthalpy ΔH and reaction entropy ΔS of -11.5 kJ mol-1 and 60 J mol-1 , respectively.

Scope and Limitations of the s-Block Metal-Mediated Pudovik Reaction.

The hydrophosphorylation of phenylacetylene with di(aryl)phosphane oxides Ar2 P(O)H (Pudovik reaction) yields E/Z-isomer mixtures of phenylethenyl-di(aryl)phosphane oxides (1). Alkali and alkaline-earth metal di(aryl)phosphinites have been studied as catalysts for this reaction with increasing activity for the heavier s-block metals. The Pudovik reaction can only be mediated for di(aryl)phosphane oxides whereas P-bound alkyl and alcoholate substituents impede the P-H addition across alkynes. The demanding mesityl group favors the single-hydrophosphorylated products 1-Ar whereas smaller aryl substituents lead to the double-hydrophosphorylated products 2-Ar. Polar solvents are beneficial for an effective addition. Increasing concentration of the reactants and the catalyst accelerates the Pudovik reaction. Whereas Mes2 P(O)H does not form the bis-phosphorylated product 2-Mes, activation of an ortho-methyl group and cyclization occurs yielding 2-benzyl-1-mesityl-5,7-dimethyl-2,3-dihydrophosphindole 1-oxide (3).

Resonance Raman Spectro-Electrochemistry to Illuminate Photo-Induced Molecular Reaction Pathways.

Electron transfer reactions play a key role for artificial solar energy conversion, however, the underlying reaction mechanisms and the interplay with the molecular structure are still poorly understood due to the complexity of the reaction pathways and ultrafast timescales. In order to investigate such light-induced reaction pathways, a new spectroscopic tool has been applied, which combines UV-vis and resonance Raman spectroscopy at multiple excitation wavelengths with electrochemistry in a thin-layer electrochemical cell to study [RuII(tbtpy)2]2+ (tbtpy = tri-tert-butyl-2,2':6',2''-terpyridine) as a model compound for the photo-activated electron donor in structurally related molecular and supramolecular assemblies. The new spectroscopic method substantiates previous suggestions regarding the reduction mechanism of this complex by localizing photo-electrons and identifying structural changes of metastable intermediates along the reaction cascade. This has been realized by monitoring selective enhancement of Raman-active vibrations associated with structural changes upon electronic absorption when tuning the excitation wavelength into new UV-vis absorption bands of intermediate structures. Additional interpretation of shifts in Raman band positions upon reduction with the help of quantum chemical calculations provides a consistent picture of the sequential reduction of the individual terpyridine ligands, i.e., the first reduction results in the monocation [(tbtpy)Ru(tbtpy•)]⁺, while the second reduction generates [(tbtpy•)Ru(tbtpy•)]0 of triplet multiplicity. Therefore, the combination of this versatile spectro-electrochemical tool allows us to deepen the fundamental understanding of light-induced charge transfer processes in more relevant and complex systems.

Direct Synthesis of Heavy Grignard Reagents: Challenges, Limitations, and Derivatization.

The direct synthesis of organocalcium compounds (heavy Grignard reagents) by the reduction of organyl halides with activated calcium powder succeeded in a straightforward manner for organic bromides and iodides that are bound at sp2 -hybridized carbon atoms. Extension of this strategy to alkyl halides was very limited, and only the reduction of trialkylsilylmethyl bromides and iodides with activated calcium allowed the isolation of the corresponding heavy Grignard reagents. Substitution of only one hydrogen atom of the methylene moiety by a phenyl or methyl group directed this reduction toward the Wurtz-type coupling and the formation of calcium halide and the corresponding C-C coupling product. The stability of the methylcalcium and benzylcalcium derivatives in ethereal solvents suggests an unexpected reaction behavior of the intermediate organyl halide radical anions. Quantum chemical calculations verify a dependency between the ease of preparative access to organocalcium complexes and the C-I bond lengths of the organyl iodides. The bulkiness of the trialkylsilyl group is of minor importance. Chloromethyltrimethylsilane did not react with activated calcium; however, halogen-exchange reactions allowed the isolation of [Ca(CH2 SiMe3 )(thf)3 (µ-Cl)]2 . Furthermore, the metathetical approach of reacting [Ca(CH2 SiMe3 )I(thf)4 ] with KN(SiMe3 )2 and the addition of N,N,N',N'',N''-pentamethyldiethylenetriamine (pmdeta) allowed the isolation of heteroleptic [CaCH2 SiMe3 {N(SiMe3 )2 }(pmdeta)]. In the reaction of this derivative with phenylsilane, the trimethylsilylmethyl group proved to be more reactive than the bis(trimethylsilyl)amido substituent.

Alkaline-Earth Metal Bis[bis(trimethylsilyl)amide] Complexes with Weakly Coordinating 2,2,5,5-Tetramethyltetrahydrofuran Ligands.

The reaction of [(Me3Si)2N-Ae{µ-N(SiMe3)2}]2 with 2,2,5,5-tetramethyltetrahydrofuran in pentane yields the mononuclear complexes [(Me4thf)Ae{N(SiMe3)2}2] (Ae = Mg (1), Ca (2), Sr (3), and Ba (4)) with three-coordinate alkaline-earth metal centers. With increasing radius of the alkaline-earth metal atoms, the N-Ae-N bond angles decrease. These ether adducts significantly enhance the solubility of the bis(trimethylsilyl)amides of the alkaline-earth metals in hydrocarbon solvents. Contrary to the magnesium derivative 1, the heavier congeners dissociate into mononuclear [Ae{N(SiMe3)2}2] and free Me4THF without formation of sparingly soluble dinuclear [(Me3Si)2N-Ae{µ-N(SiMe3)2}]2.

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.

Influence of 18-Crown-6 Ether Coordination on the Catalytic Activity of Potassium and Calcium Diarylphosphinites in Hydrophosphorylation Reactions.

The addition of 18-crown-6 ether (1,4,7,10,13,16-hexaoxacyclooctadecane) to tetranuclear [(thf)K(OPAryl2)]4 and [(thf)4Ca(OPAryl2)2] yields the corresponding mononuclear complexes [(18C6)K(OPAryl2)] [Aryl = Ph (1a), Mes (1b)] and [(18C6)Ca(OPAryl2)2] [Aryl = Ph (2a), Mes (2b)]. The metathesis reaction of [(thf)K(OPAryl2)]4 with CaI2 yields the calciate [K2Ca(thf)x{OPMes2}4]. The addition of dimesitylphosphane oxide and crystallization from a hexane solution gives [K2Ca{OPMes2}4{Mes2P(O)H}] (3). The complexes [(thf)K(OPMes2)]4, [(thf)4Ca(OPMes2)2], 1b, 2b, and the calciate 3 are tested as catalysts in the hydrophosphorylation of isopropylisocyanate with dimesitylphosphane oxide, quantitatively yielding N-isopropyl(dimesitylphosphoryl)formamide. The potassium complexes are more efficient catalysts than the calcium congeners, and coordination of 18-crown-6 decelerates the catalytic conversion.

Surprisingly Different Reaction Behavior of Alkali and Alkaline Earth Metal Bis(trimethylsilyl)amides toward Bulky N-(2-Pyridylethyl)-N'-(2,6-diisopropylphenyl)pivalamidine.

N-(2,6-Diisopropylphenyl)-N'-(2-pyridylethyl)pivalamidine (Dipp-N=C(tBu)-N(H)-C2 H4 -Py) (1), reacts with metalation reagents of lithium, magnesium, calcium, and strontium to give the corresponding pivalamidinates [(tmeda)Li{Dipp-N=C(tBu)-N-C2 H4 -Py}] (6), [Mg{Dipp-N=C(tBu)-N-C2 H4 -Py}2 ] (3), and heteroleptic [{(Me3 Si)2 N}Ae{Dipp-N=C(tBu)-N-C2 H4 -Py}], with Ae being Ca (2 a) and Sr (2 b). In contrast to this straightforward deprotonation of the amidine units, the reaction of 1 with the bis(trimethylsilyl)amides of sodium or potassium unexpectedly leads to a ß-metalation and an immediate deamidation reaction yielding [(thf)2 Na{Dipp-N=C(tBu)-N(H)}] (4 a) or [(thf)2 K{Dipp-N=C(tBu)-N(H)}] (4 b), respectively, as well as 2-vinylpyridine in both cases. The lithium derivative shows a similar reaction behavior to the alkaline earth metal congeners, underlining the diagonal relationship in the periodic table. Protonation of 4 a or the metathesis reaction of 4 b with CaI2 in tetrahydrofuran yields N-(2,6-diisopropylphenyl)pivalamidine (Dipp-N=C(tBu)-NH2 ) (5), or [(thf)4 Ca{Dipp-N=C(tBu)-N(H)}2 ] (7), respectively. The reaction of AN(SiMe3 )2 (A=Na, K) with less bulky formamidine Dipp-N=C(H)-N(H)-C2 H4 -Py (8) leads to deprotonation of the amidine functionality, and [(thf)Na{Dipp-N=C(H)-N-C2 H4 -Py}]2 (9 a) or [(thf)K{Dipp-N=C(H)-N-C2 H4 -Py}]2 (9 b), respectively, are isolated as dinuclear complexes. From these experiments it is obvious, that ß-metalation/deamidation of N-(2-pyridylethyl)amidines requires bases with soft metal ions and also steric pressure. The isomeric forms of all compounds are verified by single-crystal X-ray structure analysis and are maintained in solution.

Potassium-Mediated Hydrophosphorylation of Heterocumulenes with Diarylphosphane Oxide and Sulfide.

The preparation of the hydrophosphorylation catalysts succeeds via the metalation of dimesitylphosphane oxide and diphenylphosphane sulfide with potassium hydride in ethereal solvents such as tetrahydropyran (THP) and tetrahydrofuran (THF) yielding the tetramers [(thp)K(OPMes2)]4 (1a) and [(thf)3{K(OPMes2)}4] (1b) as well as [(thp)KSPPh2]∞ (2) with a strand-like structure in the crystalline state. In ethereal solution these complexes very slowly degrade into KPAr2 and KE2PAr2 (E = O, S). The catalytic conversion of iPr-N═C═E' (E' = O, S) and of R-N═C═N-R (R = iPr, cHex) to the addition products Ar2P(E)-C(=E')-NHR (Ar = Ph, Mes; E = O, S; E' = O, S, NR) was studied in the presence of catalytic amounts of Ar2PEK (Ar = Ph, Mes; E = O, S). Steric hindrance prevents the addition of dimesitylphosphane oxide to N,N'-diisopropylcarbodiimide, whereas diphenylphosphane oxide and sulfide smoothly add to iPr-N═C═N-iPr yielding Ph2P(E)-C(═N-iPr)-NHiPr (E = O, S).

Calcium-Mediated Catalytic Synthesis of 1-(Diorganylamino)-1,4-diphenyl-4-(diphenylphosphanyl)buta-1,3-dienes.

The hydroamination of diphenylbutadiyne with 1 equiv of the secondary amines HNRR' (R/R' = Ph/Ph, Ph/Me, and pTol/Me) in the presence of catalytic amounts of the tetrakis(amino)calciate K2[Ca{N(H)Dipp}4] (Dipp = 2,6-diisopropylphenyl) yields the corresponding 1-(diorganylamino)-1,4-diphenylbut-1-ene-3-ynes as a mixture of E/Z isomers. These tertiary alkenylamines react with diphenylphosphane to form RR'N-C(Ph)═CH-CH═C(Ph)-PPh2 [R/R' = Ph/Ph (1), Ph/Me (2), and pTol/Me (3)] in the presence of catalytic amounts of [(THF)4Ca(PPh2)2] or of the same calciate K2[Ca{N(H)Dipp}4]. Whereas the hydroamination is regio- (amino group in 1-position) but not stereoselective (formation of E and Z isomers), this second hydrofunctionalization step is regio- (phosphanyl group in 4-position) and stereoselective (only E isomers are formed), finally leading to mixtures of (E,E)- and (Z,E)-1-(diorganylamino)-1,4-diphenyl-4-(diphenylphosphanyl)buta-1,3-dienes.

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.

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.

In situ generation of organocalcium compounds for a calcium-based Grignard-type chemistry.

Organocalcium compounds are highly reactive reagents whereas the alkaline-earth metal itself is a weak reductant. This discrepancy hampered a straightforward development of an organocalcium chemistry. The in situ generation of the highly reactive organocalcium reagent and immediate metalation of a H-acidic compound (iGMM) or addition onto a polar π-system (iGAM) offers not only a loophole to organocalcium reagents but opens the entry to a rich organic chemistry of this non-toxic and globally abundant alkaline-earth metal, being competitive to the organolithium chemistry.

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).

Sterically shielded primary anilides of the alkaline-earth metals of the type (thf)nAe(NH-Ar*)2 (Ae = Mg, Ca, Sr, and Ba; Ar* = bulky aryl).

Metalation of 2,4,6-triphenylphenylamine (H2N-C6H2-2,4,6-Ph3, 1a) and 4-methyl-2,6-bis(diphenylmethyl)aniline (2,6-bis(diphenylmethyl)-p-toluidine, H2N-C6H2-4-Me-2,6-(CHPh2)2, 2a) with dibutylmagnesium and Ae[N(SiMe3)2]2 with a stoichiometric 1 : 2 ratio in THF at room temperature yields the corresponding primary anilides [(thf)nAe{N(H)-C6H2-2,4,6-Ph3}2] (Ae/n = Mg/2 (1b), Ca/2 (1c), Sr/3 (1d), and Ba/3 (1e)) and [(thf)nAe{N(H)-C6H2-4-Me-2,6-(CHPh2)2}2] (Ae/n = Mg/2 (2b), Ca/3 (2c) and Sr/2 (2d)), respectively. The 1 : 1 reaction of Mg(n/sBu)2 and MgPh2 with 2a leads to the formation of heteroleptic [(thf)2Mg(R){N(H)-C6H2-4-Me-2,6-(CHPh2)2}] (R = n/sBu (2bBu), Ph (2bPh)). At 50 °C, the strontium complex 2d liberates one equivalent of 2avia intramolecular deprotonation of the triarylmethyl functionality yielding dinuclear [(thf)2Sr{N(H)-C6H2-4-Me-2-(CPh2)-6-(CHPh2)2}]2 (2d'). The barium compound is significantly more reactive and regardless of applied stoichiometry the isotypic barium congener [(thf)2Ba{N(H)-C6H2-4-Me-2-(CPh2)-6-(CHPh2)2}]2 (2e') forms. The molecular structures of 1c, 2d, 2d', and 2e' are stabilized by metal-phenyl π-interactions.

Rubidium-mediated birch-type reduction of 1,2-diphenylbenzene in tetrahydrofuran.

The reaction of 1,2-diphenylbenzene with rubidium metal in THF yields extremely sensitive and pyrophoric [η(5)-{1,2-diphenyl-2,5-cyclohexadienyl}rubidium](∞) (1). Compound 1 characterizes a possible intermediate in a Birch-type reaction and represents a very rare example of a fully characterized organorubidium complex as well as an open main-group metal pentadienide as part of a six-membered ring. In the solid state the rubidium atoms interact with the cyclohexadienyl moiety, whereas the coordination sphere of the soft cation is additionally stabilized exclusively by several metal π-arene interactions despite the presence of strongly coordinating donors. The bonding situation was elucidated by MP2/def2-TZVPP calculations including population analysis.