Stable Lewis Base Adducts of Tetrahalodiboranes: Synthetic Methods and Structural Diversity.

A series of 22 new bis(phosphine), bis(carbene), and bis(isonitrile) tetrahalodiborane adducts has been synthesized, either by direct adduct formation with highly sensitive B2 X4 precursors (X=Cl, Br, I) or by ligand exchange at stable B2 X4 (SMe2 )2 precursors (X=Cl, Br) with labile dimethylsulfide ligands. The isolated compounds have been fully characterized using NMR spectroscopy, elemental analysis, and, for 20 of these compounds, single-crystal X-ray diffraction, revealing an unexpected variation in the bonding motifs. In addition to the classical B2 X4 L2 diborane(4) bis-adducts, certain more sterically demanding carbene ligands induce a halide displacement which led to the first halide-bridged monocationic diboron species, [B2 X3 L2 ]A (A=BCl4 , Br, I). Furthermore, low-temperature 1:1 reactions of B2 Cl4 with sterically demanding N-heterocyclic carbenes led to the formation of kinetically unstable mono-adducts, one of which was structurally characterized. A comparison of the NMR spectra and structural data of new and literature-known bis-adducts shows several trends pertaining to the nature of the halides and the stereoelectronic properties of the Lewis bases employed.

Scope of the Thermal Ring-Expansion Reaction of Boroles with Organoazides.

Electronic and steric factors have been investigated in the thermal ring expansion of boroles with organic azides, a reaction that provides access to highly arylated 1,2-azaborinines, BN analogues of benzene. Reactions of a variety of boroles and organic azides demonstrate that the synthetic method is quite general in furnishing 1,2-azaborinines, but the respective reaction rates reveal a strong dependence on the substituents on the two reactants. The products have been characterized by UV/Vis, electrochemical, NMR, and X-ray diffraction methods, clarifying their constitutions and highlighting substituent effects on the electronic structure of the 1,2-azaborinines. Furthermore, analysis of several possible mechanistic pathways for 1,2-azaborinine formation, as studied by DFT, revealed that a two-step mechanism involving azide-borole adduct formation and nitrene insertion is favored.

Formation of a Trifluorophosphane Platinum(II) Complex by P-F Bond Activation of Phosphorus Pentafluoride with a Pt0 Complex.

The reaction of PF5 with [(Cy3 P)2 Pt] gave the PF3 complex trans-[(Cy3 P)2 PtF(PF3 )][PF6 ], which was characterized by single-crystal X-ray diffraction, multinuclear NMR spectroscopy, and elemental analysis. To the best of our knowledge, this reaction is the first example of the oxidative addition of a P-F bond to a transition metal and is a rare example of an activation of a main-group-element-fluorine bond by a metal. Relativistic DFT calculations showed that the formation of the Lewis pair [(Cy3 P)2 Pt→PF5 ], which was not observed even at low temperatures, represents the initial step of the reaction. From this key intermediate, the cation trans-[(Cy3 P)2 PtF(PF3 )]+ was furnished by a two-step mechanism involving, successively, a second and a third PF5 molecule.

Desymmetrizing Electron-Deficient Diboranes(4): Diverse Products and Their Reactivity.

A comprehensive study of the reactivity of Lewis bases with dihalodiboranes(4) is presented. Diaryldihalodiboranes provide rearranged monoadducts when treated with cyclic (alkyl)(amino)carbenes, but halide-bridged adducts when treated with a range of pyridyl bases. Alternatively, the combination of diaminodihalodiboranes with strong carbene donors leads to boraborenium salts. The reduction and halide-abstraction reactivity of these adducts was also explored, leading to intramolecular C-H activation and the first 1,2-bis(borenium) dication.

Ring Expansions of Boroles with Diazo Compounds: Steric Control of C or N Insertion and Aromatic/Nonaromatic Products.

Access to novel imine-substituted 1,2-azaborinines, as well as highly arylated boracyclohexa-3,5-dienes has been developed by ring expansion of boroles with diazoalkanes with varying degrees of steric bulk. The formation of a diazoalkane intermediate is also discussed for the reaction of ortho-brominated p-tolyl-azide with 1,2,3,4,5-pentaphenylborole. Structural details as well as UV/Vis spectroscopic and cyclic voltammetric data are provided. These boron-containing heterocycles have the potential to serve as building blocks for boron-containing materials.

Ansa-bridged bis(benzene) titanium complexes.

Taking advantage of an improved synthesis of [Ti(η(6) -C6 H6 )2 ], we report here the first examples of ansa-bridged bis(benzene) titanium complexes. Deprotonation of [Ti(η(6) -C6 H6 )2 ] with nBuLi in the presence of N,N,N',N'',N''-pentamethyldiethylenetriamine (pmdta) leads to the corresponding 1,1'-dilithio salt [Ti(η(6) -C6 H5 Li)2 ]⋅pmdta that enables the preparation of the first one- and two-atom-bridged complexes by simple salt metathesis. The ansa complexes were fully characterized (NMR spectroscopy, UV/Vis spectroscopy, elemental analysis, and X-ray crystallography) and further studied electrochemically and computationally. Moreover, [Ti(η(6) -C6 H6 )2 ] is found to react with the Lewis base 1,3-dimethylimidazole-2-ylidene (IMe) to give the bent sandwich complex [Ti(η(6) -C6 H6 )2 (IMe)].

Multiple complexation of CO and related ligands to a main-group element.

The ability of an atom or molecular fragment to bind multiple carbon monoxide (CO) molecules to form multicarbonyl adducts is a fundamental trait of transition metals. Transition-metal carbonyl complexes are vital to industry, appear naturally in the active sites of a number of enzymes (such as hydrogenases), are promising therapeutic agents, and have even been observed in interstellar dust clouds. Despite the wealth of established transition-metal multicarbonyl complexes, no elements outside groups 4 to 12 of the periodic table have yet been shown to react directly with two or more CO units to form stable multicarbonyl adducts. Here we present the synthesis of a borylene dicarbonyl complex, the first multicarbonyl complex of a main-group element prepared using CO. The compound is additionally stable towards ambient air and moisture. The synthetic strategy used--liberation of a borylene ligand from a transition metal using donor ligands--is broadly applicable, leading to a number of unprecedented monovalent boron species with different Lewis basic groups. The similarity of these compounds to conventional transition-metal carbonyl complexes is demonstrated by photolytic liberation of CO and subsequent intramolecular carbon-carbon bond activation.

f-Block Ansa Complexes in the Solid State: [3]Thoro- and [3]Uranocenophanes.

The preparation of [3]thoro- and [3]uranocenophanes, the first structurally authenticated ansa-bridged complexes of actinocenes, is reported. Following a flytrap route, 1,2-bis(cyclooctatetraenyldimethylsilyl)methane was synthesized, reduced to its tetraanion, and subsequently converted into bridged uranocene and thorocene complexes by salt metathesis with the corresponding actinide tetrachlorides. In addition, their electronic structures have been investigated by experimental (UV/Vis spectroscopy, cyclic voltammetry) and theoretical (DFT) methods.

Formation of BN Isosteres of Azo Dyes by Ring Expansion of Boroles with Azides.

Herein, we present the results of our investigations on the effect of ortho substitution of aryl azides on the ring-expansion reaction of boroles, five-membered unsaturated boron heterocycles. These studies led to the isolation of the first 1,2-azaborinine-substituted azo dyes, which are bright yellow solids. One of the derivatives, (E)-2-mesityl-1-(mesityldiazenyl)-3,4,5,6-tetraphenyl-1,2-azaborinine, was found to be unstable in solution and to transform through a Jacobsen-like reaction into an indazole and 1-hydro-1,2-azaborinine. DFT calculations were performed to shed light on possible mechanisms to rationalize the unexpected azo-azaborinine formation and to draw conclusions about the role played by the ortho substituents in the reaction.

Platinum oxoboryl complexes as substrates for the formation of 1:1, 1:2, and 2:1 Lewis acid-base adducts and 1,2-dipolar additions.

The oxoboryl complex trans-[(Cy3 P)2 BrPt(B≡O)] (2) reacts with the Group 13 Lewis acids EBr3 (E=Al, Ga, In) to form the 1:1 Lewis acid-base adducts trans-[(Cy3 P)2 BrPt(B≡OEBr3 )] (6-8). This reactivity can be extended by using two equivalents of the respective Lewis acid EBr3 (E=Al, Ga) to form the 2:1 Lewis acid-base adducts trans-[(Cy3 P)2 (Br3 Al-Br)Pt(B≡OAlBr3 )] (18) and trans-[(Cy3 P)2 (Br3 Ga-Br)Pt(B≡OGaBr3 )] (15). Another reactivity pattern was demonstrated by coordinating two oxoboryl complexes 2 to InBr3 , forming the 1:2 Lewis acid-base adduct trans-[{(Cy3 P)2 BrPt(B≡O)}2 InBr3 ] (20). It was also possible to functionalize the B≡O triple bond itself. Trimethylsilylisothiocyanate reacts with 2 in a 1,2-dipolar addition to form the boryl complex trans-[(Cy3 P)2 BrPt{B(NCS)(OSiMe3 )}] (27).

Experimental assessment of the strengths of B-B triple bonds.

Diborynes, molecules containing homoatomic boron-boron triple bonds, have been investigated by Raman spectroscopy in order to determine the stretching frequencies of their central B≡B units as an experimental measure of homoatomic bond strengths. The observed frequencies between 1600 and 1750 cm(-1) were assigned on the basis of DFT modeling and the characteristic pattern produced by the isotopic distribution of boron. This frequency completes the series of known stretches of homoatomic triple bonds, fitting into the trend established by the long-known stretching frequencies of C≡C and N≡N triple bonds in alkynes and dinitrogen, respectively. A quantitative analysis was carried out using the concept of relaxed force constants. The results support the classification of the diboryne as a true triple bond and speak to the similarities of molecules constructed from first-row elements of the p block. Also reported are the relaxed force constants of a recently reported diborabutatriene, which again fit into the trend established by the vibrational spectroscopy of organic cumulenes. As part of these studies, a new diboryne with decreased steric bulk was synthesized, and a computational study of the rotation of the stabilizing ligands indicated alkyne-like electronic isolation of the central B2 unit.

Stepwise isolation of low-valent, low-coordinate Sn and Pb mono- and dications in the coordination sphere of platinum.

Synthetic access to low-coordinate Pb mono- and dications is in general impeded due to their poor solubility and highly electrophilic nature. However, the electrophilicity of these cations can be tamed by attaching them to electron-rich transition metals. Following this principle we have isolated low-valent Pb mono- ([(Cy3P)2Pt-PbCl]2[AlCl4]2, 8a) and dications ([(Cy3P)2Pt(Pb)][AlCl4]2, 11) in the coordination sphere of platinum. The same approach then has been implemented for the isolation of analogous low-valent Sn mono- (7a) and dications (10). An energy decomposition analysis (EDA-NOCV) was performed to investigate the nature of Pt-Pb and Pb-Cl bonding in [(Cy3P)2Pt(PbCl2)] (2), 8a and 11. The results show that the Pt-Pb bonds in 8a and 11 are electron-sharing in nature, whereas that of the precursor 2 is a dative bond. The breakdown of attractive interactions in 2, 8a and 11 reveals that the ionic interactions in the analyzed Pt-Pb and Pb-Cl bonds are always stronger than the covalent interactions, except for the Pb-Cl bond in 8a. The calculated D3 dispersion energies show that dispersion interactions play a key role in the thermodynamic stability of 2, 8a and 11.

Unprecedented oxidative addition and metal-only Lewis pair chemistry of antimony trihalides.

Reaction of the zero-valent platinum complex [Pt(PCy3 )2 ] with SbF3 generates the cationic diplatinum stibenium complex [{(Cy3 P)2 Pt}2 (µ-SbF2 )](+) , the first unsupported metal-only Lewis pair containing an antimony-centered Lewis acid. In contrast, SbCl3 undergoes oxidative addition to [Pt(PCy3 )2 ], resulting in the dihalostibanyl complex trans-[PtCl(SbCl2 )(PCy3 )2 ], the first example of oxidative addition of an antimony-halide bond to a transition metal.

Silver(I) and thallium(I) cations as unsupported bridges between two metal bases.

Stirring Lewis-basic transition metal complexes with the salts M[BAr(Cl)4] (M = Ag, Tl; Ar(Cl) = 3,5-C6H3Cl2) resulted in the formation of diplatinum complexes [{(Cy3P)2Pt}2(µ-Ag)][BAr(Cl)4] (3) and [{(Cy3P)2Pt}2(µ-Tl)][BAr(Cl)4] (4) and the diiron compound [{(OC)3(Me3P)2Fe}2(µ-Ag)] (5) featuring bridging, unsupported metal cations. Their properties were investigated by multinuclear NMR spectroscopy, as well as X-ray diffraction and infrared measurements.

Platinum complexes containing pyramidalized germanium and tin dihalide ligands bound through σ,σ M=E multiple bonds.

We present the isolation of the first mononuclear dihalogermylene, and mono- and dinuclear stannylene complexes of transition metals. These exhibit exceptionally pyramidalized Group 14 centers. Additionally, removal of the halide substituents from the Ge/Sn atom was successfully performed in two ways, halide abstraction and reduction, leading to a variety of unusual structural motifs.

Complete and partial 1,2-additions across transition metal-boron double bonds.

The first 1,2-additions across a metal-boron double bond are reported, one a definitive chlorogallation of a Fe═B bond, the other a partial chlorogallation of a Mo═B bond that leads to a highly unusual, planar Mo-B-Ga-Cl rhombus. The two reactions occur with opposite regiochemistry, with the Ga atom bound to the Fe atom in the former and to both the Mo and B atoms in the latter. The bonding in the Mo adduct and the reasons for the differing regiochemistry of the reaction are explored computationally.

A B-C double bond unit coordinated to platinum: an alkylideneboryl ligand that is isoelectronic to neutral aminoborylene Ligands.

The reaction of [Pt(PCy3)2] with Br2B-CH(SiMe3)2 resulted in generation of the first alkylideneboryl complex, trans-[Br(Cy3P)2Pt{B=CH(SiMe3)}], with concomitant elimination of Me3 SiBr. The trans bromide ligand of the alkylideneboryl complex was readily substituted by a methyl group upon treatment with methyllithium, leading to another alkylideneboryl complex, trans-[Me(Cy3P)2Pt{B=CH(SiMe3)}]. Various spectrochemical techniques, single-crystal X-ray crystallography, and quantum chemical calculations confirmed the formulation of a double bond between the boron and the carbon atom. The theoretical studies also provided evidence for the stronger trans influence of the alkylideneboryl ligand over iminoboryl and oxoboryl ligands.

Structure and reactivity of distanna[2]metallocenophanes of ruthenium and osmium.

We report the molecular structures of 1,1'-dilithiometallocenes of ruthenium and osmium. These compounds served as precursors for the synthesis and subsequent structural characterization of the first [2]osmocenophanes with disilane and distannane bridges, as well as of a distanna[2]ruthenocenophane. In addition, the insertion of sulfur and selenium into the Sn-Sn bridges was studied and it was observed that the presence of the Lewis base pmdta (N,N,N',N″,N″-pentamethyldiethylenetriamine) dramatically accelerates the reaction.

σ-Donor-σ-acceptor plumbylene ligands: synergic σ-donation between ambiphilic Pt(0) and Pb(II) fragments.

Mono- and diplatinum plumbylene complexes are prepared in which the bonding comprises synergic σ donation between the Pt and Pb atoms. The bonding distinguishes the complexes from both the classical Fischer carbene bonding model and M→Pb dative-only bonding. The Pt-Pb bond of the monoplatinum complex is stable, while the diplatinum complex reversibly loses one Pt fragment.

New metal-only Lewis pairs: elucidating the electronic influence of N-heterocyclic carbenes and phosphines on the dative Pt-Al bond.

The synthesis and full characterization of a new heteroleptic N-heterocyclic carbene (NHC)-phosphine platinum(0) complex and formation of its corresponding alane adduct is reported. The influence of the ligands on the Lewis basic properties was studied via multinuclear NMR-spectroscopy, X-ray analyses, and density functional theory (DFT) calculations. Consistently, the effect of changing the halogens upon the Lewis acid properties of aluminum halides was studied by X-ray analysis and DFT calculations.