Synthesis and Structural Characterization of p-Carboranylamidine Derivatives.

In this contribution, the first amidinate and amidine derivatives of p-carborane are described. Double lithiation of p-carborane (1) with n-butyllithium followed by treatment with 1,3-diorganocarbodiimides, R-N=C=N-R (R = iPr, Cy (= cyclohexyl)), in DME or THF afforded the new p-carboranylamidinate salts p-C2H10B10[C(NiPr)2Li(DME)]2 (2) and p-C2H10B10[C(NCy)2Li(THF)2]2 (3). Subsequent treatment of 2 and 3 with 2 equiv. of chlorotrimethylsilane (Me3SiCl) provided the silylated neutral bis(amidine) derivatives p-C2H10B10[C{iPrN(SiMe3)}(=NiPr)]2 (4) and p-C2H10B10[C{CyN(SiMe3)}(=NCy)]2 (5). The new compounds 3 and 4 have been structurally characterized by single-crystal X-ray diffraction. The lithium carboranylamidinate 3 comprises a rare trigonal planar coordination geometry around the lithium ions.

Synthesis and Structure of Alkaline Earth Bis{hydrido-tris(3,5-diisopropyl-pyrazol-1-yl)borate} Complexes: Ae(TpiPr2)2 (Ae = Mg, Ca, Sr, Ba).

The synthesis and structural characterization of Ae(TpiPr2)2 (Ae = Mg, Ca, Sr, Ba; TpiPr2 = hydrido-tris(3,5-diisopropyl-pyrazol-1-yl)borate) are reported. In the crystalline state, the alkaline earth metal centers are six-coordinate, even the small Mg2+ ion, with two κ3-N,N',N''-TpiPr2 ligands, disposed in a bent arrangement (B···Ae···B < 180°). However, contrary to the analogous Ln(TpiPr2)2 (Ln = Sm, Eu, Tm, Yb) compounds, which all exhibit a bent-metallocene structure close to Cs symmetry, the Ae(TpiPr2)2 compounds exhibit a greater structural variation. The smallest Mg(TpiPr2)2 has crystallographically imposed C2 symmetry, requiring both bending and twisting of the two TpiPr2 ligands, while with the similarly sized Ca2+ and Sr2+, the structures are back toward the bent-metallocene Cs symmetry. Despite the structural variations, the B···M···B bending angle follows a linear size-dependence for all divalent metal ions going from Mg2+ to Sm2+, decreasing with increasing metal ion size. The complex of the largest metal ion, Ba2+, forms an almost linear structure, B···Ba···B 167.5°. However, the "linearity" is not due to the compound approaching the linear metallocene-like geometry, but is the result of the pyrazolyl groups significantly tipping toward the metal center, approaching "side-on" coordination. An attempt to rationalize the observed structural variations is made.

The difficult search for organocerium(iv) compounds.

This Tutorial Review provides an overview of the historic and current development of the organometallic chemistry of cerium in its oxidation state 4+. Among the tetravalent lanthanide ions, only Ce4+ forms stable coordination compounds (e.g. (NH4)2[Ce(NO3)6]). Important fields of applications for cerium(iv) compounds include organic synthesis, bioinorganic chemistry, materials science, and industrial catalysis. In sharp contrast, organometallic cerium(iv) compounds are still exceedingly rare. The history of organocerium(iv) compounds is an exciting story of ups and downs. The so-called cerocene (= bis(η8-cyclooctatetraenyl) cerium) has been known since 1976. Other early reports e.g. about Cp4Ce (Cp = η5-cyclopentadienyl), were later disproven. However, significant progress in this field has been made in recent years through the use of carefully designed ligands and more sophisticated synthesis protocols. Taking the case of organocerium(iv) chemistry, this Tutorial Review also tries to exemplarily show how difficult synthetic and theoretical problems can eventually be solved through newly designed synthesis strategies (e.g. as accomplished for cyclopentadienyl and carbene derivatives) and a rewarding collaboration between synthetic and theoretical chemists (cf. the cerocene problem).

Ceric Cyclopentadienides Bearing Alkoxy, Aryloxy, Chlorido, or Iodido Co-Ligands.

A series of heteroleptic tris(cyclopentadienyl) CeIV complexes has been isolated and crystallographically characterized, revealing the broad accessibility of such organocerium(IV) compounds. The oxidation of complexes CpMe3 Ce(thf) and Cp'3 Ce(thf) (CpMe =C5 H4 Me, Cp'=C5 H4 SiMe3 ), bearing monosubstituted electron-poor cyclopentadienyl ligands, with 0.5 equivalents of 1,4-benzoquinone or C2 Cl6 gave the cerium(IV) hydroquinolate complexes CpMe3 Ce(OC6 H4 O)CeCpMe3 and Cp'3 Ce(OC6 H4 O)CeCp'3 , or the chloride complexes CpMe3 CeCl and Cp'3 CeCl, respectively; the iodide complex Cp'3 CeI was obtained from the reaction of Cp'3 Ce(thf) with elemental iodine. The behavior of Cp'3 CeCl in salt metathesis protocols employing alkali metal amides or alkyl, alkoxide, and aryloxide reagents was investigated, which gave rise to the robust and isolable cerium(IV) alkoxide Cp'3 Ce(OtBu). Trivalent [Cp'2 CeCl]2 was synthesized by AlMe4 →Cl exchange utilizing Cp'2 Ce(AlMe4 ) and AlMe2 Cl. The reactivity of [Cp'2 CeCl]2 towards the oxidants Ph3 CCl, C2 Cl6 , 1,4-benzoquinone, and I2 has been assessed, and has provided useful information on CeIII /CeIV redox deactivation pathways. In addition to X-ray structure analysis, all the complexes were characterized by NMR, DRIFT (diffuse reflectance IR Fourier transform), and UV/Vis spectroscopy as well as elemental analysis. The tetravalent compounds were further analyzed for their magnetic susceptibility by using Evans' method.

Synthesis and Crystal Structures of the First Antimony(III) Aziridinides.

The first antimony(III) aziridinyl derivatives are reported. Treatment of anhydrous SbCl3 with N-lithioaziridine Li(Azn) (Azn = NC2H4) afforded the structurally unique heterobimetallic lithium/antimony(III) amide complex [Li3Sb(µ3-Cl)2(µ-Azn)4(THF)2]∞ (1). Homoleptic Sb2(Azn)6 (2) has become available for the first time through an amide group exchange reaction between Sb(NMe2)3 and 3 equiv of aziridine. The low-melting Sb2(Azn)6 exhibits a "weak dimer" structure in the crystal.

A Structurally Characterized Organometallic Plutonium(IV) Complex.

The blood-red plutonocene complex Pu(1,3-COT'')(1,4-COT'') (4; COT''=η8 -bis(trimethylsilyl)cyclooctatetraenyl) has been synthesized by oxidation of the anionic sandwich complex Li[Pu(1,4-COT'')2 ] (3) with anhydrous cobalt(II) chloride. The first crystal structure determination of an organoplutonium(IV) complex revealed an asymmetric sandwich structure for 4 where one COT'' ring is 1,3-substituted while the other retains the original 1,4-substitution pattern. The electronic structure of 4 has been elucidated by a computational study, revealing a probable cause for the unexpected silyl group migration.

Scandium-Mediated Formation of a Bis(tetrahydropentalene).

The reactivity of Li[Sc(COT'')2 ] (1; COT''=1,4-bis(trimethylsilyl)cyclooctatetraenyl) towards CoCl2 is considerably different from that of related lanthanide triple-decker sandwich complexes. In addition to the expected triple-decker complex Sc2 (COT'')3 (2), the complex Sc2 {µ-BTHP}(COT'')2 (3) is formed, which comprises the novel BTHP2- ligand (BTHP2- =bis(3,5-bis(trimethylsilyl)-1,3a,6,6a-tetrahydropentalene-1-yl)diide or bis(2,7-bis(trimethylsilyl)bicyclo[3.3.0]octa-2,7-dien-4-yl)diide, C16 H10 (SiMe3 )42- ). The formation of 3 is likely facilitated by the fact that scandium prefers η8 ,η3  coordination rather than highly symmetric η8 ,η8  coordination, and the η3 -coordinated COT'' ligand in 1 is activated owing to a loss of aromaticity. Acid hydrolysis of 3 leads to air-stable H2 BTHP (4).

Metal complexes of curcumin--synthetic strategies, structures and medicinal applications.

This Tutorial Review presents an overview on the synthesis, characterization and applications of metal complexes containing curcumin (=1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione) and its derivatives as ligands. Innovative synthetic strategies leading to soluble and crystallizable metal curcumin complexes are outlined in detail. Special emphasis is placed on the highly promising and exciting medicinal applications of metal curcumin complexes, with the three most important areas being anticancer activity and selective cytotoxicity, anti-Alzheimer's disease activity, and antioxidative/neuroprotective effects. Overall, this Tutorial Review provides the first general overview of this emerging and rapidly expanding field of interdisciplinary research.

Heterometallic europium disiloxanediolates: synthesis, structural diversity, and photoluminescence properties.

This contribution presents a full account of a structurally diverse class of heterometallic europium disiloxanediolates. The synthetic protocol involves in situ metalation of (HO)SiPh2OSiPh2(OH) (1) with either (n)BuLi or KN(SiMe3)2 followed by treatment with EuCl3 in suitable solvents such as 1,2-dimethoxyethane (DME) or tetrahydrofuran (THF). Reaction of EuCl3 with 2 equiv of (LiO)SiPh2OSiPh2(OLi) in DME afforded the Eu(III) bis(disiloxanediolate) "ate" complex [{(Ph2SiO)2O}2{Li(DME)}3]EuCl2 (2), which upon attempted reduction with Zn gave the tris(disiloxanediolate) [{(Ph2SiO)2O}3{Li(DME)}3]Eu (3). Treatment of EuCl3 with (LiO)SiPh2OSiPh2(OLi) in a molar ratio of 1:2 yielded both the ate complex [{(Ph2SiO)2O}3Li{Li(THF)2}{Li(THF)}]EuCl·Li(THF)3 (4) and the LiCl-free europium(III) complex [{(Ph2SiO)2O}2{Li(THF)2}2]EuCl (5). Compound 5 was found to exhibit a brilliant red triboluminescence. When (KO)SiPh2OSiPh2(OK) was used as starting material in a 3:1 reaction with EuCl3, the Eu(III) tris(disiloxanediolate) [{(Ph2SiO)2O}3{K(DME)}3]Eu (6) was isolated. Attempted ligand transfer between 5 and (DAD(Dipp))2Ba(DME) (DAD(Dipp) = N,N'-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene) afforded the unique mixed-valent Eu(III)/Eu(II) disiloxanediolate cluster [(Ph2SiO)2O]6Eu(II)4Eu(III)2Li4O2Cl2 (7). All new complexes were structurally characterized by X-ray diffraction. Photoluminescence studies were carried out for complex 5 showing an excellent color quality, due to the strong (5)D0→(7)F2 transition, but a weak antenna effect.

Crystal structure of the high-energy-density material guanylurea dipicryl-amide.

The title compound, 1-carbamoylguanidinium bis-(2,4,6-tri-nitro-phen-yl)amide [H2NC(=O)NHC(NH2)2](+)[N{C6H2(NO2)3-2,4,6}2](-) (= guanylurea dipicryl-amide), was prepared as dark-red block-like crystals in 70% yield by salt-metathesis reaction between guanylurea sulfate and sodium dipicryl-amide. In the solid state, the new compound builds up an array of mutually linked guanylurea cations and dipicryl-amide anions. The crystal packing is dominated by an extensive network of N-H⋯O hydrogen bonds, resulting in a high density of 1.795 Mg m(-3), which makes the title compound a potential secondary explosive.

Unexpected formation and crystal structure of tetra-kis-(1H-pyrazole-κN (2))-palladium(II) dichloride.

The title salt, [Pd(C3H4N2)4]Cl2, was obtained unexpectedly by the reaction of palladium(II) dichloride with equimolar amounts of 1-chloro-1-nitro-2,2,2-tris-(pyrazol-yl)ethane in methanol solution. The Pd(2+) cation is located on an inversion centre and has a square-planar coordination sphere defined by four N atoms of four neutral pyrazole ligands. The average Pd-N distance is 2.000 (2) Å. The two chloride anions are not coordinating to Pd(2+). They are connected to the complex cations through N-H⋯Cl hydrogen bonds. In addition, C-H⋯Cl hydrogen bonds are observed, leading to a three-dimensional linkage of cations and anions.

Homoleptic gadolinium amidinates as precursors for MOCVD of oriented gadolinium nitride (GdN) thin films.

Five new homoleptic gadolinium tris-amidinate complexes are reported, which were synthesized via the salt-elimination reaction of GdCl(3) with 3 equiv of lithiated symmetric and asymmetric amidinates at ambient temperature. The Gd-tris-amidinates [Gd{(N(i)Pr)(2)CR}(3)] [R = Me (1), Et (2), (t)Bu (3), (n)Bu (4)] and [Gd{(NEt)(N(t)Bu)CMe}(3)] (5) are solids at room temperature and sublime at temperatures of about 125 °C (6 × 10(-2) mbar) with the exception of compound 4, which is a viscous liquid at room temperature. According to X-ray diffraction analysis of 3 and 5 as representative examples of the series, the complexes adopt a distorted octahedral structure in the solid state. Mass spectrometric (MS) data confirmed the monomeric structure in the gas phase, and high-resolution MS allowed the identification of characteristic fragments, such as [{(N(i)Pr)(2)CR}GdCH(3)](+) and [{(N(i)Pr)(2)CR}GdNH](+). The alkyl substitution patterns of the amidinate ligands clearly show an influence on the thermal properties, and specifically, the introduction of the asymmetric carbodiimide leads to a lowering of the onset of volatilization and decomposition. Compound 5, which is the first Gd complex with an asymmetric amidinate ligand system to be reported, was, therefore, tested for the MOCVD of GdN thin films. The as-deposited GdN films were capped with Cu in a subsequent MOCVD process to prevent postdeposition oxidation of the films. Cubic GdN on Si(100) substrates with a preferred orientation in the (200) direction were grown at 750 °C under an ammonia atmosphere and exhibited a columnar morphology and low levels of C or O impurities according to scanning electron microscopy, Rutherford backscattering, and nuclear reaction analysis.

Lanthanide amidinates and guanidinates in catalysis and materials science: a continuing success story.

Today the rare-earth elements play a critical role in numerous high-tech applications. This is why various areas of rare-earth chemistry are currently thriving. In organolanthanide chemistry the search for new ligand sets which are able to satisfy the coordination requirements of the large lanthanide cations continues to be a hot topic. Among the most successful approaches in this field is the use of amidinate and guanidinate ligands of the general types [RC(NR')(2)](-) (R = H, alkyl, aryl; R' = alkyl, cycloalkyl, aryl, SiMe(3)) and [R(2)NC(NR')(2)](-) (R = alkyl, SiMe(3); R' = alkyl, cycloalkyl, aryl, SiMe(3)), which can both be regarded as steric cyclopentadienyl equivalents. Mono-, di- and trisubstituted lanthanide amidinate and guanidinate complexes are all readily available. Various rare earth amidinates and guanidinates have turned out to be very efficient homogeneous catalysts e.g. for the polymerization of olefins and dienes, the ring-opening polymerization of cyclic esters or the guanylation of amines. Moreover, certain alkyl-substituted lanthanide tris(amidinates) and tris(guanidinates) were found to be highly volatile and are thus promising precursors for ALD (= atomic layer deposition) and MOCVD (= metal-organic chemical vapor deposition) processes in materials science, e.g. for the production of lanthanide nitride thin layers. This tutorial review covers the continuing success story of lanthanide amidinates and guanidinates which have undergone an astonishing transition from mere laboratory curiosities to efficient homogeneous catalysts as well as ALD and MOCVD precursors within the past 10 years.

Di-µ-oxido-bis-[bis-(diiso-propyl-aceta-midinato)-κN;κ(2) N,N'-germanium(IV)].

The title compound, [Ge2(C8H17N2)4O2], crystallizes with imposed twofold symmetry, which allows the monodentate amidinate ligands to be arranged in a cisoid fashion. The independent Ge-O distances within the central Ge2O2 ring, which is essentially planar (r.m.s. deviation = 0.039 Å), are 1.7797 (8) and 1.8568 (8) Å. The germanium centres adopt a distorted trigonal-bipyramidal geometry, being coordinated by the two O atoms and by one bidentate and one monodentate amidinate ligand (three N atoms). One N-isopropyl group is disordered over two positions; these are mutually exclusive because of 'collisions' between symmetry-equivalent methyl groups and thus each has 0.5 occupancy.

Bromidobis[3-(1H-pyrazol-1-yl-κN(2))propionamide-κO]copper(II) bromide methanol monosolvate.

The title copper(II) N-pyrazolylpropanamide (PPA) complex, [CuBr(PPA)(2)]Br, was obtained in 78% yield by treatment of CuBr(2) with an excess of the ligand in methanol. Crystallization from the mother liquid afforded the title compound, i.e. the methanol solvate [CuBr(C(6)H(9)N(3)O)(2)]Br·CH(3)OH or [CuBr(PPA)(2)]Br·MeOH, as bright green crystals. In the solid state, the title salt comprises isolated [CuBr(PPA)(2)](+) cations, separated bromide ions and methanol of crystallization. In the cation, the central Cu(II) ion is coordinated by two N,O-chelating PPA ligands and one Br(-) ion. The coordination geometry around the Cu(II) ion is distorted trigonal-bipyramidal with the bromide ligand and the amide O atoms occupying the equatorial positions [Cu-Br = 2.4443 (4) Å; Cu-O = 2.035 (2) and 2.179 (2) Å], while the pyrazole N atoms coordinate in the axial positions [Cu-N = 1.975 (2) and 1.976 (2) Å]. In the crystal, the three constituents are linked by N-H⋯Br, O-H⋯Br, and N-H⋯O hydrogen bonds, forming a three-dimensional network.

Small compound - big colors: synthesis and structural investigation of brightly colored alkaline earth metal 1,3-dimethylviolurates.

A series of brightly colored alkaline earth metal 1,3-dimethylviolurates M(Me2Vio)2 have been prepared and fully characterized. The title compounds AE(Me2Vio)2·nH2O (AE = Mg, n = 6 (3); AE = Ca, n = 8 (4), AE = Sr, n = 6 (5); AE = Ba, n = 4 (6)) were obtained by neutralizing 1,3-dimethylvioluric acid monohydrate (=H(Me2Vio)·H2O; 2) with 0.5 equiv. of the corresponding metal dihydroxides AE(OH)2. The hair-like appearance of the Sr derivative 5 prevented the growth of single-crystals. This problem could be solved by crystallizing the crown ether derivative Sr(Me2Vio)2(18-crown-6) (5a). The isolated salts exhibit intense colors ranging from red to purple. Various attempts to prepare the beryllium derivative Be(Me2Vio)2 failed. Instead, work-up of the reaction mixtures provided pink crystals of a new modification of 2 formulated as [H3O][Me2Vio] (2b) as shown by an X-ray diffraction study. An unexpected oxidation reaction of the barium salt Ba(Me2Vio)2 led to formation of the novel mixed-anion salt Ba(Me2Vio)(Me2NO2Barb)·2H2O (8, Me2NO2Barb- = 1,3-dimethyl-5-nitrobarbiturate anion). Compound 8 could also be synthesized deliberately by treatment of Ba(OH)2 with a 1 : 1 mixture of 2 and 1,3-dimethyl-5-nitrobarbituric acid (7, =H(Me2NO2Barb)·H2O). All new compounds were fully characterized by their IR, Raman, NMR (1H, 13C{1H}) and UV-vis spectra as well as elemental analyses. Single-crystal X-ray diffraction studies revealed that the solid-state structures of compounds 3, 4, 5a and 6 are governed by the typical coordination behavior of the alkaline-earth metals, i.e. increasing coordination numbers and a decreasing degree of hydration when going from Mg to Ba. The dimensions of the structures range from hydrogen-bonded ions (3) over monomeric, neutral complex molecules (4, 5a), to polymeric networks (6). The successful isolation of the mixed-anion barium salt 8 adds a new facet to the coordination chemistry of violurate and related ligands.

Unprecedented bending and rearrangement of f-element sandwich complexes induced by superbulky cyclooctatetraenide ligands.

The use of the superbulky cyclooctatetraenide dianion ligand [C(8)H(6)(SiPh(3))(2)](2-) (= COT(BIG)) in organo-f-element chemistry leads to unprecedented effects such as the formation of a significantly bent anionic Ce(III) sandwich complex, a novel cerocene formed by sterically induced SiPh(3) group migration, as well as the first example of a bent uranocene.

Carboranylamidinates.

Carboranylamidinate anions are readily accessible via addition of N,N'-dialkylcarbodiimides to lithio-ortho-carborane. They represent a novel difunctional boron-rich ligand system for main group and transition elements (Li, Sn, Cr). Initial structural investigations revealed an unexpected N,C-coordination mode instead of N,N'-chelation.

Facile access to tetravalent cerium compounds: one-electron oxidation using iodine(III) reagents.

Readily accessible and easy-to-use phenyliodine(III) dichloride, PhICl(2), has been established as an innovative and superior reagent for the one-electron oxidation of cerium(III) complexes, comprising amide, amidinate, and cyclopentadienyl derivatives. Its use allowed the successful synthesis and structural characterization of the first members of three new classes of chloro-functionalized (organo)cerium(IV) compounds, including the long sought-after Cp(3)CeCl.

Lanthanide amidinates and guanidinates: from laboratory curiosities to efficient homogeneous catalysts and precursors for rare-earth oxide thin films.

For decades, the organometallic chemistry of the rare earth elements was largely dominated by the cyclopentadienyl ligand and its ring-substituted derivatives. A hot topic in current organolanthanide chemistry is the search for alternative ligand sets which are able to satisfy the coordination requirements of the large lanthanide cations. Among the most successful approaches in this field is the use of amidinate ligands of the general type [RC(NR')(2)](-) (R = H, alkyl, aryl; R' = alkyl, cycloalkyl, aryl, SiMe(3)) which can be regarded as steric cyclopentadienyl equivalents. Closely related are the guanidinate anions of the general type [R(2)NC(NR')(2)](-) (R = alkyl, SiMe(3); R' = alkyl, cycloalkyl, aryl, SiMe(3)). Two amidinate or guanidinate ligands can coordinate to a lanthanide ion to form a metallocene-like coordination environment which allows the isolation and characterization of stable though very reactive amide, alkyl, and hydride species. Mono- and trisubstituted lanthanide amidinate and guanidinate complexes are also readily available. Various rare earth amidinates and guanidinates have turned out to be very efficient homogeneous catalysts e.g. for ring-opening polymerization reactions. Moreover, certain alkyl-substituted lanthanide tris(amidinates) and tris(guanidinates) were found to be highly volatile and could thus be promising precursors for ALD (= Atomic Layer Deposition) and MOCVD (= Metal-Organic Chemical Vapor Deposition) processes in materials science and nanotechnology. This tutorial review covers the success story of lanthanide amidinates and guanidinates and their transition from mere laboratory curiosities to efficient homogeneous catalysts as well as ALD and MOCVD precursors.