Half-Sandwich Zirconium and Hafnium Amidoborane Complexes: Precursors of Hydride Derivatives.

Half-sandwich zirconium(IV) and hafnium(IV) complexes with amidoborane and hydride ligands have been isolated in the stoichiometric reactions of mono(pentamethylcyclopentadienyl)metal alkyl and amido derivatives with the amine-boranes NHR2BH3 (R2 = H2, Me2, HtBu). Treatment of the tris(trimethylsilylmethyl) complexes [M(η5-C5Me5)(CH2SiMe3)3] with NH3BH3 (3 equiv) gives the seven-coordinate species [M(η5-C5Me5)(NH2BH3)3] (M = Zr (1), Hf (2)) with three κ2N,H-NH2BH3 ligands. The tris(neophyl) [M(η5-C5Me5)(CH2CMe2Ph)3] or tris(dimethylamido) [M(η5-C5Me5)(NMe2)3] derivatives react with NHMe2BH3 (≥3 equiv) to afford bis(dimethylamidoborane) hydride complexes [M(η5-C5Me5)H(NMe2BH3)2] (M = Zr (3), Hf (4)) via thermally unstable [M(η5-C5Me5)(NMe2BH3)3] species. The reaction of [M(η5-C5Me5)(NMe2)3] and NH2tBuBH3 (≥4 equiv) affords analogous mixed amidoborane hydride derivatives [M(η5-C5Me5)H(NHtBuBH3)(NMe2BH3)] (M = Zr (5), Hf (6)) with κ2N,H-NHtBuBH3 and κ3N,H,H-NMe2BH3 ligands. The addition of NHR2BH3 (≥1 equiv) on the mono(dimethylamido) complexes [M(η5-C5Me5)Cl2(NMe2)] in hexane leads to the precipitation of the ionic compounds [(NHR2)2BH2][{M(η5-C5Me5)Cl2}2(µ-H)3] (R2 = Me2, M = Zr (7), Hf (8); R2 = HtBu, M = Zr (9), Hf (10)). Molecular hydride species [Cl2(η5-C5Me5)M(µ-Cl)(µ-H)2M(η5-C5Me5)Cl(NH2tBu)] (M = Zr (11), Hf (12)) could be isolated from mixtures of complexes [M(η5-C5Me5)Cl2(NMe2)] and lower ratios of NH2tBuBH3. The zirconium complex 11 decomposes in solution to give the mononuclear tert-butylamido derivative [Zr(η5-C5Me5)Cl2(NHtBu)] (13) along with other byproducts.

Studies on the chemical reduction of polynuclear titanium(IV) nitrido complexes.

The redox chemistry of cube-type titanium(IV) nitrido complexes [{Ti4(η5-C5Me5)3(R)}(µ3-N)4] (R = η5-C5Me5 (1), N(SiMe3)2 (2), η5-C5H4SiMe3 (3), and η5-C5H5 (4)) was investigated by electrochemical methods and chemical reactions. Cyclic voltammetry studies indicate that 1-4 undergo a reversible one-electron reduction at ca. -1.8 V vs. ferrocenium/ferrocene. Thus, complex 1 reacts with sodium sand in tetrahydrofuran to produce the highly reactive ionic compound [Na(thf)6][{Ti(η5-C5Me5)}4(µ3-N)4] (5). The treatment of complexes 1-4 in toluene with one equivalent of [K(C5Me5)] in the presence of macrocycles (L) leads to C10Me10 and the formation of more stable derivatives [K(L)][{Ti4(η5-C5Me5)3(R)}(µ3-N)4] (R = η5-C5Me5, L = 18-crown-6 (6), crypt-222 (7); R = N(SiMe3)2, L = 18-crown-6 (8), crypt-222 (9); R = η5-C5H4SiMe3, L = 18-crown-6 (10), crypt-222 (11); R = η5-C5H5, L = crypt-222 (12)). However, the analogous reaction of 4 with [K(C5Me5)] and 18-crown-6 affords [{(18-crown-6)K}2(µ-η5:η5-C5H5)][{Ti4(η5-C5Me5)3(η5-C5H5)}(µ3-N)4] (13) via abstraction of one cyclopentadienide group from a putative intermediate [(18-crown-6)K(µ-η5:η5-C5H5)Ti4(η5-C5Me5)3(µ3-N)4]. In contrast to the cube-type nitrido systems 1-4, the cyclic voltammogram of the trinuclear imido-nitrido titanium(IV) complex [{Ti(η5-C5Me5)(µ-NH)}3(µ3-N)] (14) does not reveal any reversible redox event and 14 readily reacts with [K(C5Me5)] to afford C5Me5H and the diamagnetic derivative [{K(µ4-N)(µ3-NH)2Ti3(η5-C5Me5)3(µ3-N)}2] (15). The treatment of 15 with two equiv. of 18-crown-6 polyethers produces the molecular species [(L)K{(µ3-N)(µ3-NH)2Ti3(η5-C5Me5)3(µ3-N)}] (L = 18-crown-6 (16), dibenzo-18-crown-6 (17)). Complex 17 further reacts with one equiv. of dibenzo-18-crown-6 to yield the ion-separated compound [K(dibenzo-18-crown-6)2][Ti3(η5-C5Me5)3(µ3-N)(µ-N)(µ-NH)2] (18) similar to the ion pair [K(crypt-222)][Ti3(η5-C5Me5)3(µ3-N)(µ-N)(µ-NH)2] (19) obtained in the treatment of 15 with cryptand-222.

Stereoselective synthesis of oxime containing Pd(II) compounds: highly effective, selective and stereo-regulated cytotoxicity against carcinogenic PC-3 cells.

New palladium compounds [Pd{(1S,4R)-NOH^NH(R)}Cl2] (R = Ph 1a or Bn 1b), [Pd{(1S,4R)-NOH^NH(R)}{(1S,4R)-NO^NH(R)}][Cl] (R = Ph 2a or Bn 2b) and corresponding [Pd{(1R,4S)-NOH^NH(R)}Cl2] (R = Ph 1a' or Bn 1b') and [Pd{(1R,4S)-NOH^NH(R)}{(1R,4S)-NO^NH(R)}][Cl] (R = Ph 2a' or Bn 2b') have been synthesized. Novel compounds 1a, 1b, and 2b (and 1a', 1b', and 2b') were obtained in solution as a mixture of diastereomers whose relative ratios depend on the solvent and the nature of the amino substituent. In contrast, the synthetic reactions of derivatives 2a and 2a' were stereospecific, and afforded single enantiomers of absolute configuration (SN,1SC,4RC)-(RN,1SC,4RC) and (RN,1RC,4SC)-(SN,1RC,4SC), respectively. All compounds have been fully characterized by NMR and IR spectroscopy, time-dependent UV-spectroscopy, ESI-HR-MS in water, and CHN elemental analysis. Absolute configurations of the major epimers of 1a and 1a', both epimers of 1b and enantiomer 2a', were determined by single crystal X-ray crystallography, and confirmed by 2D NOESY NMR experiments in solution. Additionally, the pH-dependent stability of 2b in water was assessed by 1H-NMR spectroscopy. Metal derivatives have been tested in vitro against three human cancer (prostate PC-3, cervical HeLa, and breast MCF-7) cell lines. The highest anticancer activities were shown by palladium compound 2a' in all cancer cells, with IC50 values up to 80 times lower than those found for cisplatin. The cytotoxicity of 2a and 2a'' is stereo-dependent, with IC50 values that differ significantly for each enantiomer in all the cell lines tested. The cytotoxic activity of 2a and 2a' was further evaluated against the non-tumorigenic human prostate RWPE-1 cell line, revealing a selectivity index (SI) of ca. 30 for derivative 2a'. DNA interactions have been investigated by equilibrium dialysis, fluorescence resonance energy transfer (FRET) DNA melting assays, and viscometric titrations, pointing to groove and/or external binding. Cell cycle assay on PC-3 cells after treatment with 2a or 2a' shows cell cycle arrest in the S and G2/M phases, especially when the cells are treated with compound 2a'.

Dinitrogen Binding at a Trititanium Chloride Complex and Its Conversion to Ammonia under Ambient Conditions.

Reaction of [TiCp*Cl3 ] (Cp*=η5 -C5 Me5 ) with one equivalent of magnesium in tetrahydrofuran at room temperature affords the paramagnetic trinuclear complex [{TiCp*(µ-Cl)}3 (µ3 -Cl)], which reacts with dinitrogen under ambient conditions to give the diamagnetic derivative [{TiCp*(µ-Cl)}3 (µ3 -η1 : η2 : η2 -N2 )] and the titanium(III) dimer [{TiCp*Cl(µ-Cl)}2 ]. The structure of the trinuclear mixed-valence complexes has been studied by experimental and theoretical methods and the latter compound represents the first well-defined example of the µ3 -η1 : η2 : η2 coordination mode of the dinitrogen molecule. The reaction of [{TiCp*(µ-Cl)}3 (µ3 -η1 : η2 : η2 -N2 )] with excess HCl in tetrahydrofuran results in clean NH4 Cl formation with regeneration of the starting material [TiCp*Cl3 ]. Therefore, a cyclic ammonia synthesis under ambient conditions can be envisioned by alternating N2 /HCl atmospheres in a [TiCp*Cl3 ]/Mg(excess) reaction mixture in tetrahydrofuran.

Low-Valent Titanium Species Stabilized with Aluminum/Boron Hydride Fragments.

Low-valent titanium species were prepared by reaction of [TiCp*X3 ] (Cp*=η5 -C5 Me5 ; X=Cl, Br, Me) with LiEH4 (E=Al, B) or BH3 (thf), and their structures elucidated by experimental and theoretical methods. The treatment of trihalides [TiCp*X3 ] with LiAlH4 in ethereal solvents (L) leads to the hydride-bridged heterometallic complexes [{TiCp*(µ-H)}2 {(µ-H)2 AlX(L)}2 ] (L=thf, X=Cl, Br; L=OEt2 , X=Cl). Density functional theory (DFT) calculations for those compounds reveal an open-shell singlet ground state with a Ti-Ti bond and can be described as titanium(II) species. The theoretical analyses also show strong interactions between the Ti-Ti bond and the empty s orbitals of the Al atom of the AlH2 XL fragments, which behave as σ-accepting (Z-type) ligands. Analogous reactions of [TiCp*X3 ] with LiBH4 (2 and 3 equiv.) in tetrahydrofuran at room temperature and at 85 °C lead to the titanium(III) compounds [{TiCp*(BH4 )(µ-X)}2 ] (X=Cl, Br) and [{TiCp*(BH4 )(µ-BH4 )}2 ], respectively. The treatment of [TiCp*Me3 ] with 4 and 5 equiv. of BH3 (thf) produces the diamagnetic [{TiCp*(BH3 Me)}2 (µ-B2 H6 )] and paramagnetic [{TiCp*(µ-B2 H6 )}2 ] complexes, respectively.

PhenQE8, a Novel Ligand of the Human Telomeric Quadruplex.

A novel quadruplex ligand based on 1,10-phenanthroline and incorporating two guanyl hydrazone functionalities, PhenQE8, is reported herein. Synthetic access was gained in a two-step procedure with an overall yield of 61%. X-ray diffraction studies revealed that PhenQE8 can adopt an extended conformation that may be optimal to favor recognition of quadruplex DNA. DNA interactions with polymorphic G-quadruplex telomeric structures were studied by different techniques, such as Fluorescence resonance energy transfer (FRET) DNA melting assays, circular dichroism and equilibrium dialysis. Our results reveal that the novel ligand PhenQE8 can efficiently recognize the hybrid quadruplex structures of the human telomeric DNA, with high binding affinity and quadruplex/duplex selectivity. Moreover, the compound shows significant cytotoxic activity against a selected panel of cultured tumor cells (PC-3, HeLa and MCF-7), whereas its cytotoxicity is considerably lower in healthy human cells (HFF-1 and RPWE-1).

Selective Synthesis of Phenanthrenes and Dihydrophenanthrenes via Gold-Catalyzed Cycloisomerization of Biphenyl Embedded Trienynes.

Readily available o'-alkenyl-o-alkynylbiaryls, a particular type of 1,7-enynes, undergo a selective cycloisomerization reaction in the presence of a gold(I) catalyst to give interesting phenanthrene and dihydrophenanthrene derivatives in high yields. The solvent used provokes a switch in the evolution of the gold intermediate and plays a key role in the reaction outcome.

Successive Protonation and Methylation of Bridging Imido and Nitrido Ligands at Titanium Complexes.

The reactions of nitrido complexes [{Ti(η5-C5Me5)(µ-NH)}3(µ3-N)] (1) and [{Ti(η5-C5Me5)}4(µ3-N)4] (2) with electrophilic reagents ROTf (R = H, Me; OTf = OSO2CF3) in different molar ratios have allowed the structural characterization of a series of titanium intermediates en route to the formation of the ammonium salts [NR4]OTf and [NR4][Ti(η5-C5Me5)(OTf)4]. The treatment of the trinuclear imido-nitrido complex 1 with 5.5 equiv of triflic acid in toluene at room temperature led to the dinuclear complex [Ti2(η5-C5Me5)2(µ-N)(NH3)(µ-O2SOCF3)2(OTf)] (3) and [NH4]OTf. Compound 3, along with the ammonium salts [NMe4]OTf and [NMe4][Ti(η5-C5Me5)(OTf)4] (5), was also obtained in the reaction of 1 with 8 equiv of methyl triflate in toluene at 100 °C. The trinuclear complex [Ti3(η5-C5Me5)3(µ-N)(µ-NH)2(µ-O2SOCF3)(OTf)] (4), an intermediate in the formation of 3, was isolated in the treatment of 1 with 4 equiv of MeOTf, although compound 4 was prepared in better yield by treatment of 1 with Me3SiOTf (2 equiv). Addition of a large excess of MeOTf or HOTf reagents to solutions of 3 resulted in the clean formation of ammonium salts [NR4][Ti(η5-C5Me5)(OTf)4] (R = Me (5), H (6)). Treatment of the tetranuclear nitrido complex [{Ti(η5-C5Me5)}4(µ3-N)4] (2) with 1 equiv of ROTf in toluene afforded the precipitation of the ionic compounds [{Ti(η5-C5Me5)}4(µ3-N)3(µ3-NR)][OTf] (R = H (8), Me (9)), while a large excess of HOTf led to the formation of [{Ti(η5-C5Me5)}4(µ3-N)3(µ3-NH)][Ti(η5-C5Me5)(OTf)4(NH3)] (10) by rupture of a fraction of tetranuclear molecules. Complex 2 reacted with 1 equiv of [M(η5-C5H5)(CO)3H] (M = Mo, Cr) via hydrogenation of one nitrido ligand to give the molecular derivative [{Ti(η5-C5Me5)}4(µ3-N)3(µ3-NH)] (11) and [{M(η5-C5H5)(CO)3}2], while a second 1 equiv of [M(η5-C5H5)(CO)3H] produced the ionic compounds [{Ti(η5-C5Me5)}4(µ3-N)2(µ3-NH)2][M(η5-C5H5)(CO)3] (M = Mo (12), Cr (13)) by protonation of another nitrido group. The X-ray crystal structures of 3-5, 9, 10, and 13 were determined.

Expanding the BN-embedded PAH family: 4a-aza-12a-borachrysene.

Previously unknown 4a-aza-12a-borachrysene has been synthesized in only four steps. The reactions of this BN-embedded PAH with bromine and organolithium compounds proceed with complete regioselectivity, resulting in the formation of nine derivatives. One of these, a phenylalkynyl-substituted derivative, exhibits a remarkably high fluorescence quantum yield (ΦF = 0.68).

Preparation of Dimeric Monopentamethylcyclopentadienyltitanium(III) Dihalides and Related Derivatives.

The synthesis, crystal structure, and reactivity of a series of half-sandwich titanium(III) dihalide complexes [Ti(η5-C5Me5)X2] (X = Cl, Br, I) and several of its Lewis base derivatives were investigated. The reaction of the trihalides [Ti(η5-C5Me5)X3] (X = Cl (1), Br (2), I (3)) with LiAlH4 (≥1 equiv) in toluene at room temperature results in the formation of the halide-bridged dimers [{Ti(η5-C5Me5)X(µ-X)}2] (X = Cl (4), Br (5), I (6)). The treatment of 4 with [Li{N(SiMe3)2}] (≥2 equiv) at room temperature affords the precipitation of the amido titanium(III) complex [{Ti(η5-C5Me5)(µ-Cl){N(SiMe3)2}}2] (7), but analogous reactions of 4 with other lithium reagents [LiR] (R = Me, CH2SiMe3, NMe2) lead to disproportionation into titanium(IV) [Ti(η5-C5Me5)R3] and presumably titanium(II) derivatives. Similarly, complex 4 in solution at temperatures higher than 100 °C undergoes disproportionation as demonstrated by its reactions with cobaltocene and N-(4-methylbenzylidene)aniline yielding the ionic paramagnetic compound [Co(η5-C5H5)2][Ti(η5-C5Me5)Cl3] (8) and the diamagnetic diazatitanacyclopentane [Ti(η5-C5Me5)Cl{N(Ph)CH(p-tolyl)}2], respectively. Treatment of complex 4 with 2 equiv of 2,6-dimethylphenylisocyanide or tert-butylisocyanide in toluene at room temperature affords the paramagnetic titanium(III) dinuclear adducts [{Ti(η5-C5Me5)Cl(µ-Cl)(CNR)}2] (R = 2,6-Me2C6H3 (9), tBu (10)). Magnetic studies for polycrystalline 9 show that it displays a weak intramolecular antiferromagnetic coupling between the Ti ions, which is consistent with the long Ti-Ti distance of 3.857(1) Å determined by X-ray diffraction. The isocyanide ligands in complex 10 undergo a reductive coupling reaction in toluene to give the titanium(IV) iminoacyl derivative [{Ti(η5-C5Me5)Cl2}2(µ-η2:η2-tBuN═C-C═NtBu)] (11). Whereas an analogous dinuclear structure was found in the aqua titanium(III) complex [{Ti(η5-C5Me5)Cl(µ-Cl)(OH2)}2] (12), resulting from the reaction of 4 with adventitious amounts of water, compound 4 reacts with excess ammonia to give a mononuclear adduct [Ti(η5-C5Me5)Cl2(NH3)2] (13) with a robust layered pattern in the solid state.

Studies on the preparation of aminobipyridines and bipyridine sultams via an intramolecular free radical pathway.

A variety of aminated bipyridines and bipyridine sultams are prepared by intramolecular radical [1,5]-ipso and [1,6]-ortho substitutions, using a sulfonamide as a linker to connect the pyridyl radical to the pyridine under attack. For the cases studied, different regiochemistries are observed depending on the initial position of the sulfonamide linker.

A New Member of the BN-Phenanthrene Family: Understanding the Role of the B-N Bond Position.

3,4-Dihydro-4-aza-3-boraphenanthrene, which shows the highest fluorescence quantum yield of all nonsubstituted BN-phenanthrenes reported to date (ϕF = 0.61), has been synthesized in only three steps (76% overall yield) from easily accessible 1-bromo-2-vinylnaphthalene, along with several substituted derivatives. The reactivity of these previously unknown BN-aromatic compounds toward organolithium compounds and bromine has been studied. This latter reaction affords bromo-substituted compounds that are suitable for further functionalization via Suzuki and Sonogashira couplings, with complete regioselectivity. The optical properties and excited state deactivation mechanisms of selected compounds were studied using computational methods.

Synthesis, Functionalization, and Optical Properties of 1,2-Dihydro-1-aza-2-boraphenanthrene and Several Highly Fluorescent Derivatives.

Previously unknown 1,2-dihydro-1-aza-2-boraphenanthrene has been synthesized in only three steps from 2-bromo-1-vinylnaphthalene. The reactivity of this new BN-phenanthrene, and of several substituted derivatives, has been tested against bromine and organolithium compounds. Bromination proceeded with complete regioselectivity, affording bromo-substituted compounds suitable for further functionalization via cross-coupling reactions. This new family of BN-phenanthrenes exhibits a substantial increase in the quantum yield (up to ϕF = 0.93) with respect to phenanthrene.

The Puzzling Monopentamethylcyclopentadienyltitanium(III) Dichloride Reagent: Structure and Properties.

Following the track of the useful titanocene [Ti(η5-C5H5)2Cl] reagent in organic synthesis, the related half-sandwich titanium(III) derivatives [Ti(η5-C5R5)Cl2] are receiving increasing attention in radical chemistry of many catalyzed transformations. However, the structure of the active titanium(III) species remains unknown in the literature. Herein, we describe the synthesis, crystal structure, and electronic structure of titanium(III) aggregates of composition [{Ti(η5-C5Me5)Cl2} n]. The thermolysis of [Ti(η5-C5Me5)Cl2Me] (1) in benzene or hexane at 180 °C results in the clean formation of [{Ti(η5-C5Me5)Cl(µ-Cl)}2] (2), methane, and ethene. The treatment of 1 with excess pinacolborane in hexane at 65 °C leads to a mixture of 2 and the paramagnetic trimer [{Ti(η5-C5Me5)(µ-Cl)2}3] (3). The X-ray crystal structures of compounds 2 and 3 show Ti-Ti distances of 3.267(1) and 3.219(12) Å, respectively. Computational studies (CASPT2//CASSCF and BS DFT methods) for dimer 2 reveal a singlet ground state and a relatively large singlet-triplet energy gap. Nuclear magnetic resonance spectroscopy of 2 in aromatic hydrocarbon solutions and DFT calculations for several [{Ti(η5-C5Me5)Cl2} n] aggregates are consistent with the existence of an equilibrium between the diamagnetic dimer [{Ti(η5-C5Me5)Cl(µ-Cl)}2] and a paramagnetic tetramer [{Ti(η5-C5Me5)(µ-Cl)2}4] in solution. In contrast, complex 2 readily dissolves in tetrahydrofuran to give a green-blue solution from which blue crystals of the mononuclear adduct [Ti(η5-C5Me5)Cl2(thf)] (4) were grown.

Ammonia-Borane Derived BN Fragments Trapped on Bi- and Trimetallic Titanium(III) Systems.

Titanium(III) complexes containing unprecedented (NH2 BH2 NHBH3 )2- and {N(BH3 )3 }3- ligands have been isolated, and their structures elucidated by a combination of experimental and theoretical methods. The treatment of the trimethyl derivative [TiCp*Me3 ] (Cp*=η5 -C5 Me5 ) with NH3 BH3 (3 equiv) at room temperature gives the paramagnetic dinuclear complex [{TiCp*(NH2 BH3 )}2 (µ-NH2 BH2 NHBH3 )], which at 80 °C leads to the trinuclear hydride derivative [{TiCp*(µ-H)}3 {µ3 -N(BH3 )3 }]. The bonding modes of the anionic BN fragments in those complexes, as well as the dimethylaminoborane group trapped on the analogous trinuclear [{TiCp*(µ-H)}3 (µ3 -H)(µ3 -NMe2 BH2 )], have been studied by X-ray crystallography and density functional theory (DFT) calculations.

Biological evaluation of water soluble arene Ru(II) enantiomers with amino-oxime ligands.

New water soluble, enantiopure arene ruthenium compound SRuSN-(1R,4S)-[(η6-p-cymene)Ru{ĸNH(Bn),ĸNOH}Cl]Cl (Bn = benzyl, 1a') has been synthesized. The novel compound along with that previously described RRuRN-(1S,4R)-[(η6-p-cymene)Ru{ĸNH(Bn),ĸNOH}Cl]Cl (1a) was evaluated by polarimetry, ultra-violet and circular dichroism spectroscopy. The structure of novel ruthenium derivative 1a' was determined by single crystal X-ray crystallography. Both enantiomers have been tested against several cancer cell lines in vitro: prostate PC-3, lung A-549, pancreas MIA PaCa-2, colorectal HCT-116, leukemia Jurkat and cervical HeLa. Both enantiomers are active and versatile cytotoxic agents, showing IC50 values from 2 to 12 times lower than those found for cisplatin in the different cell lines evaluated. The mechanism of cell death induced by the metal compounds was analyzed in A-549 and Jurkat cell lines. Derivatives 1a and 1a' induced apoptotic cell death of A-549 cells while dose-dependent cell death mechanisms have been found in the Jurkat cell line. Compound-DNA interactions have been investigated by equilibrium dialysis, Fluorescence Resonance Energy Transfer (FRET) melting assays and viscometric titrations, revealing moderate binding affinity of 1a and 1a' towards duplex DNA. Finally, the efficacy of 1a in a preliminary in vivo assay of PC-3 xenografts in nude mice has been evaluated, resulting in a promising inhibition of tumor growth by 45%. Analysis of tumor tissue also showed a significant decrease of levels of crucial molecules in the invasive phenotype of PC-3 cells.

Synthesis of functionalized helical BN-benzo[c]phenanthrenes.

A novel parent BN-benzo[c]phenanthrene, with helical chirality and remarkable structural features, has been easily obtained in three steps with a global yield of 55%. Moreover, Cl-substituted derivatives have been prepared and these have served as useful starting materials for the development of palladium-catalyzed cross-coupling reactions.

Group 4 Half-Sandwich Tris(trimethylsilylmethyl) Complexes: Thermal Decomposition and Reactivity with N,N-Dimethylamine-Borane.

The thermal decomposition of group 4 trimethylsilylmethyl derivatives [M(η5-C5Me5)(CH2SiMe3)3] (M = Ti (1), Zr (2), Hf (3)) in solution and their reactivity with N,N-dimethylamine-borane were investigated. Heating of hydrocarbon solutions of compounds 2 and 3 at 130-200 °C results in the elimination of SiMe4 and the clean formation of the singular alkylidene-alkylidyne zirconium and hafnium compounds [{M(η5-C5Me5)}3{(µ-CH)3SiMe}(µ3-CSiMe3)] (M = Zr (4), Hf (5)). The reaction of 2 and 3 with NHMe2BH3 (≥1 equiv) at room temperature affords the dialkyl(dimethylamidoborane) complexes [M(η5-C5Me5)(CH2SiMe3)2(NMe2BH3)] (M = Zr (6), Hf (7)). Compounds 6 and 7 are unstable in solution and decompose with formation of the alkyl(dimethylamino)borane [B(CH2SiMe3)H(NMe2)] (8), SiMe4, and other minor byproducts, including the tetranuclear zirconium(III) octahydride complex [{Zr(η5-C5Me5)}4(µ-H)8] (9) in the decomposition of 6. Addition of NHMe2BH3 to the titanium tris(trimethylsilylmethyl) derivative 1 gives the trinuclear mixed valence Ti(II)/Ti(III) tetrahydride complex [{Ti(η5-C5Me5)(µ-H)}3(µ3-H)(µ3-NMe2BH2)] (10) at 45-65 °C. While the complete conversion of 1 under argon atmosphere requires excess NHMe2BH3 (up to 15 equiv), complex 10 is readily prepared with 3 equiv of NHMe2BH3 under a hydrogen atmosphere indicating that the formation of 10 involves hydrogenolysis of 1 in the presence of (NMe2BH2)2. In absence of amine-borane, the reaction of 1 with H2 leads to the tetranuclear titanium(III) octahydride [{Ti(η5-C5Me5)}4(µ-H)8] (11), which upon addition of NHMe2BH3 and subsequent heating at 65 °C affords complex 10. The X-ray crystal structures of 2, 4, 5, 10, and 11 were determined.

Synthesis, Optical Properties, and Regioselective Functionalization of 4a-Aza-10a-boraphenanthrene.

4a-Aza-10a-boraphenanthrene has been synthesized in only four steps from commercially available materials with a remarkable overall yield of 62%. In contrast to other BN-isosteres of phenathrene, this isomer is weakly fluorescent, which has been explained by means of computational studies that found a low energy conical intersection for the nonradiative deactivation of the excited state. Moreover, a completely regioselective functionalization of 4a-aza-10a-boraphenanthrene at C-1 by reaction with activated electrophiles has been achieved.

Isolable zirconium hydride species in the reaction of amido complexes with amine-boranes.

Mono-, di- and trinuclear zirconium hydride species have been isolated in the treatment of amido complexes [Zr(η5-C5Me5)(NMe2)nCl3-n] (n = 3, 1) with amine-borane adducts NHR2BH3 (R2 = Me2, HtBu). The reactions involve the formation of amidoborane ligands with ZrH-B interactions which readily undergo ß-hydride elimination to give hydride functions.