Molecular and Electronic Structures of Mononuclear and Dinuclear Titanium Complexes Containing π-Radical Anions of 2,2'-Bipyridine and 1,10-Phenanthroline: An Experimental and DFT Computational Study.

Whereas reaction of [(η(5)-Cp*)Ti(IV)Cl3](0) (1) with 2 equiv of neutral 2,2'-bipyridine (bpy) and 1.5 equiv of magnesium in tetrahydrofuran affords the mononuclear complex [(η(5)-Cp*)Ti(III)(bpy(•))2](0) (2), performing the same reaction with only 1 equiv each of magnesium and bpy provides the dinuclear complex [{(η(5)-Cp*)Ti(µ-Cl)(bpy(•))}2](0) (3). Conducting the latter reaction using 1,10-phenanthroline (phen) in place of bpy resulted in formation of dinuclear [{(η(5)-Cp*)Ti(µ-Cl)(phen(•))}2](0) (4). The structures of 2, 3, and 4 have all been determined by high-resolution X-ray crystallography at 153 K; the Cpy-Cpy distances of 1.420(3) and 1.431(4) Å in the N,N'-coordinated bpy ligands of 2 and 3, respectively, are indicative of the presence of (bpy(•))(1-) ligands, rather than neutral (bpy(0)). The electronic spectra (300-1600 nm) of these two complexes are similar in form, and contain intense π → π* transitions associated with the (bpy(•))(1-) radical anion. Temperature dependent magnetic susceptibility measurements (4-300 K) show that mononuclear 2 possesses a temperature independent magnetic moment of 1.73 µB, which is indicative of an S = (1)/2 ground state. Broken symmetry density functional theory (BS-DFT) calculations yield a picture consistent with the experimental findings, in which the central Ti atom possesses a +3 oxidation state and is coordinated by a η(5)-Cp* ligand and two (bpy(•))(1-). Strong intramolecular antiferromagnetic coupling of these three unpaired spins, one each on the Ti(III) center and on the two (bpy(•))(1-) ligands, affords the experimentally observed doublet ground state. The magnetic susceptibility measurements for dinuclear 3 and 4 display weak but significant ferromagnetic coupling, and indicate that these complexes possess S = 1 ground states. The mechanism of the spin coupling phenomenon that yields the observed behavior was analyzed using BS-DFT calculations, and it was discovered that the tight π-stacking of the N,N'-coordinated (bpy(•))(1-)/(phen(•))(1-) ligands in these two complexes results from direct overlap of their SOMOs and formation of a two-electron multicentered bond. This yields a diamagnetic {(bpy)2}(2-)/{(phen)2}(2-) bridging unit whose doubly occupied HOMO is spread equally over both ligands. The two remaining unpaired electrons, one at each Ti(III) center, couple weakly in a ferromagnetic fashion to yield the experimentally observed S = 1 ground states.

Efficient access to titanaaziridines by C-H activation of N-methylanilines at ambient temperature.

Titanaaziridines or η(2)-imine titanium complexes are considered key intermediates of the titanium-catalyzed hydroaminoalkylation of alkenes. Herein, we present an efficient synthetic route to this class of compounds, starting from N-methylanilines and a bis(η(5):η(1)-pentafulvene)titanium complex. Consecutive reactions on the η(2)-methyleneaniline complexes, characterized for the first time, prove a high chemical versatility. In particular, hydroaminoalkylation products were found in reactions of the three-membered titanacycles with alkenes. For the first time, all the intermediates of the hydroaminoalkylation of alkenes were isolated and characterized.

Dibenzosilanorbornadienyl cations and their fragmentation into silyliumylidenes.

The terphenyl-substituted dibenzosilanorbornadienyl cation 11 was synthesized and isolated in the form of its [B(C6F5)4](-) salt. The salt was characterized by NMR spectroscopy supported by quantum mechanical computations and by an XRD analysis of a corresponding acetonitrilium salt. The thermal fragmentation of 11[B(C6F5)4] in benzene results in the high-yield formation of diphenylterphenylsilylium borate 17[B(C6F5)4]. High-lying intermediates in this process are solvent-complexed terphenylsilyliumylidene 8 and the hydrogen- and phenyl-substituted silylium ion 20. The formation of silylium ion 20 by reaction of silyliumylidene 8 with the solvent benzene demonstrates the high potential of this four valence electron species in C-H bond activation reactions. In addition, the instability of the hydrogen-substituted silylium ion 20 in benzene opens new mechanistic perspectives particular for dihydrogen activation by silyl cationic frustrated Lewis pairs and in general for the dihydrogen activation by strong Lewis acids.

Titanium-catalyzed intermolecular hydroaminoalkylation of conjugated dienes.

Ti me kangaroo down: Conjugated dienes undergo intermolecular hydroaminoalkylation in the presence of Ti catalyst [Ind2 TiMe2 ] (Ind=η(5) -indenyl). This new reaction offers a highly atom-efficient approach to homoallylic amines from 1,3-butadienes.

Persistent dialkyl(silyl)stannylium ions.

The synthesis of dialkyl(silyl)stannylium borates 2[B(C(6)F(5))(4)] by reaction of a stable ß-silyl-substituted stannylene 3 with silylarenium borates 4[B(C(6)F(5))(4)] is reported. The stannylium borates 2[B(C(6)F(5))(4)] are characterized by NMR spectroscopy and single-crystal X-ray diffraction, supported by the results of quantum mechanical calculations. The accumulated experimental and theoretical data indicate that the stannylium ions 2 are not stabilized by ß-silyl hyperconjugation and that intramolecular C-H/Sn(+) interactions are not important.

Stannylium ions, a tin(II) arene complex, and a tin dication stabilized by weakly coordinating anions.

The reactivity of aryl-substituted stannylenes, Ar(2)Sn (4), towards silylarenium borates, [R(3)SiArH][B(C(6)F(5))(4)] (3), was investigated. The reaction with 2,3,4-trimethyl-6-tert-butylphenyl (mebp)-substituted stannylene gave silyl-substituted stannylium ions 2a,b, which were characterized by NMR spectroscopy supported by the results of quantum-mechanical computations of molecular structures and magnetic properties. The tri-iso-propylphenyl-substituted stannylium ions 2c,d undergo a decomposition reaction in toluene to give the dicationic tin-arene complex [Sn(C(7)H(8))(3)](2+) (5) in the form of the [B(C(6)F(5))(4)] salt in high yields. The 5[B(C(6)F(5))(4)](2) salt was identified by single crystal X-ray diffraction analysis and by Mössbauer spectroscopy. The bonding situation was investigated by using natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) calculations. The substitution of the weakly coordinating borate anion by the carboranate [CB(11)H(6)Br(6)](-) results in replacement of the toluene ligands and formation of tin(II) carboranate with only weak Sn(2+)-anion interactions as suggested by the solid-state structure of the isolated salt.

A new synthesis of triarylsilylium ions and their application in dihydrogen activation.

Well-shuffled: An unexpected substituent distribution reaction via alkyldiarylsilylium ions leads to a distribution of substituents. Starting from alkyldiaryl silanes, this reaction provides a facile synthetic approach to sterically highly hindered triarylsilylium ions. These silylium ions can be applied in dihydrogen activation reactions.

A germanium(II) cation with [1+1] coordination.

Breaking the molecular symmetry by protonation of germylene 1 is the key step in the synthesis of the germyliumylidene 2, which is stabilized by an intramolecular interaction with a distant imido group.

Molecular structure of a cyclopropyl substituted vinyl cation.

The experimentally determined molecular structure of vinyl cation 1 provides clear evidence for the occurrence of extended sigma-conjugation involving the cyclopropyl ring and the beta-C-Si bonds.

Remarkable stability of tetrabenzo[a,c,g,i]fluorenyl ammonium salts in water: syntheses, reactions, and crystal structures.

Deprotonation of 8bH-tetrabenzo[a,c,g,i]fluorene (8bH-Tbf), an extremely large benzannulated cyclopentadienyl derivative, by [NR3R'][OH] (R = (n)Bu, Et; R' = (n)Bu, Et, Bn) leads to a series of Tbf ammonium salts of the type [NR3R'][Tbf]. These cyclopentadienide analogs were smoothly isolated from a hydrous medium and structurally characterized by single X-ray diffraction. Subsequent reactions demonstrate the high potential of derivatization by converting the presented [Tbf] anions into functionalized Tbf compounds.

Synthesis and Characterization of Neutral Hexanuclear Iron Sulfur Clusters Containing Stair-like [Fe(6)(&mgr;(3)-S)(4)(&mgr;(2)-SR)(4)] and Nest-like [Fe(6)(&mgr;(3)-S)(2)(&mgr;(2)-S)(2)(&mgr;(4)-S)(&mgr;(2)-SR)(4)] Core Structures.

The binuclear iron complex [{Fe("EtN(2)S(2)")}(2)] (1b, "EtN(2)S(2)" = N,N'-diethyl-3,7-diazanonane-1,9-dithiolate) was prepared from the free ligand and ferrous bis[bis(trimethylsilyl)amide] in toluene. In dichloromethane 1b reacts with the [Fe(4)S(4)I(4)](2-) cubane cluster to displace two iodo ligands and to form the neutral hexanuclear cluster [{Fe("EtN(2)S(2)")}(2)Fe(4)S(4)I(2)] (2), which is isolated as black crystals in 87% yield. As elucidated by an X-ray structure analysis, 2 contains the novel hexanuclear stair-like [Fe(6)(&mgr;(3)-S)(4)(&mgr;(2)-SR)(4)] core, which exhibits crystallographic inversion symmetry. The compound crystallizes as a solvate with two molecules of CH(2)Cl(2) per formula unit in the monoclinic space group P2(1)/n with a = 1570.5(2) pm, b = 1060.2(1) pm, c = 1604.0(2) pm, beta = 114.93(1) degrees, and Z = 2. In the aprotic polar solvents DMF, 1,2-propylenecarbonate, and DMSO, 2 dissolves with decomposition and formation of the cluster [{Fe("EtN(2)S(2)")}(2)Fe(4)S(5)] (3), which is isolated as black needles from DMF. 3.2DMF crystallizes in the triclinic space group P&onemacr; with a = 950.9(1) pm, b = 1086.0(1) pm, c = 2381.5(2) pm, alpha = 101.81(1) degrees, beta = 91.94(1) degrees, gamma = 97.01(1) degrees, and Z = 2. The neutral compound contains a nest-like [Fe(6)(&mgr;(4)-S)(&mgr;(3)-S)(2)(&mgr;(2)-S)(2)(&mgr;(2)-SR)(4)] core of idealized C(2)(v)() symmetry that is closely related to that of other well-known clusters, e.g., the cluster anion [Fe(6)S(9)(SR)(2)](4-). The zero-field (57)Fe Mössbauer spectrum of 3 is in accordance with four Fe(2.5+)S(4) centers (delta = 0.46 mm/s; DeltaE(Q) = 1.14 mm/s) and two N(2)S(3)-bound high-spin Fe(2+) sites (delta = 0.83 mm/s; DeltaE(Q) = 3.64 mm/s). A total cluster spin of 0 is deduced from the Mössbauer spectrum at 4.2 K and 5.3 T, which yields magnetic splitting from the applied field only. For 2, three subspectra are observed in the Mössbauer spectrum (a, delta = 0.45 mm/s, DeltaE(Q) = 1.05 mm/s; b, delta = 0.55 mm/s, DeltaE(Q) = 1.61 mm/s, c, delta = 0.80 mm/s; DeltaE(Q) = 3.83 mm/s) reflecting different coordination environments of the iron atoms rather than different oxidation states. The electrochemical properties of 1b, 2, and 3 were determined by cyclic voltammetry. 1b can be quasi-reversibly oxidized in dichloromethane solution at -75 mV (vs SCE). Whereas 2 shows only an irreversible redox behavior in N,N'-dimethylimidazolidin-2-one solution, 3 in the same solvent can be quasi-reversibly reduced in two consecutive steps at -830 and -1630 mV (vs SCE) to the dianion, which consists entirely of Fe(II).

Chalcogenatetrasilacyclopentenes: Five-Membered Rings with Endocyclic Si-Si Double Bonds.

The formal [4+1] cycloaddition of the chalcogens S, Se, and Te to the tetrasilabuta-1,3-diene 1 furnishes the molecules 2-4, the first five-membered rings with endocyclic Si-Si double bonds. The new compounds are thermally stable and chemically inert. However, 4 is light-sensitive and decomposes in daylight with deposition of tellurium. R=2,4,6-iPr3C6H2.

Ga9 (CMe3 )9 , an Important New Building Block in the Structural Chemistry of the Alkylelement(I) Compounds En Rn (E=B-In).

A simple preparative method has been employed for the synthesis of the novel cluster compound Ga9 (CMe3 )9 , which contains a tricapped trigonal prism of monovalent gallium atoms. Electron transfer processes were observed similar to those of polyboranes, leading to the the reversible formation of the corresponding radical anion.

A Novel Route to Fulvene Complexes of Titanium-Diastereoselective Complexation of Pentafulvenes to Cyclopentadienyltitanium Fragments.

An alternative to the known thermally induced syntheses of fulvene complexes of early transition metals is now provided by the direct reductive complexation of pentafulvenes to [CpTiCl] fragments. These generally highly diastereoselective reactions enable a broad variation of substitution patterns and establish an extensive subsequent chemistry.

Selective oxidation and reduction of trinuclear titanium(II) hexaazatrinaphthylene complexes-synthesis, structure, and electrochemical investigations.

Titanocene complexes with chelating N-heterocyclic ligand bridges react with ferrocenium salts as selective oxidants to afford air-stable cationic complexes and allow the preparation of exceptional mixed valence hexaazatrinaphthylene complexes [(Cp2Ti)3(mu3-HATNMe6)]n+ (1n+) (n=1, 2, 3, 4). Cyclic voltammograms (CV) and differential pulse voltammograms (DPV) show that nine oxidation states of 1 are generated without decomposition. Comproportionation constants Kc have been calculated in order to determine the extent of electronic communication between the titanium centers. The Kc values of the mixed valence states are indicative of uncoupled (14+), moderately coupled (15+), and strongly coupled (1-, 1+, and 12+) systems. Small but significant structural changes occurring upon oxidation of neutral 1 are observed by X-ray structural analysis on 1+-14+. Anion-pi interactions between the electron-deficient central ring of the HATNMe6 moiety and PF6- and BF4- counterions, respectively, are found for 12+, 13+, and 14+. The short cation-anion contacts cause interesting molecular allignments in terms of molecular architecture. For 12+ the assembly of an one-dimensional (1D) polymer is observed. Electrochemical investigations on the mononuclear cationic titanocene complexes [(Cp2Ti)(L)]+ (L=2,2'-biquinoline (2+), 4,4'-dimethyl-2,2'-biquinoline (3+), and 5,8'-dimethyl-2,3'-biquinoxaline (4+)) showed similar oxidation and reduction characteristics among each other. Conversion to monoanionic, neutral, and dicationic states is enabled. As found for the trinuclear compounds 1n+, the molecular structures of 2+-4+ reveal significant differences compared to their neutral parents.