Diboriranide σ-Complexes of d- and p-Block Metals.

Diboriranides are the smallest conceivable monoanionic aromatic cycles, yet only limited examples have been reported and their reactivity and complexation behavior remain completely unexplored. We report a straightforward synthesis of the first peraryl diboriranide c-(DurB)2 CPh- as its lithium salt in three steps via the corresponding non-classical diborirane from a readily available 1,2-dichlorodiborane(4) (Dur=2,3,5,6-tetramethylphenyl). With the preparation and complete characterization of representative complexes with tin, copper, gold and zinc, we demonstrate the strong preference of the diboriranide for σ-type coordination modes towards main group and transition metal centers under unperturbed retention of the three-membered B2 C-ring's 2e- π-system.

Pentamethylcyclopentadienyl-substituted hypersilylsilylene: reversible and irreversible activation of C[double bond, length as m-dash]C double bonds and dihydrogen.

Recent studies of low-valent main group species underscore their resemblance to transition metal complexes with regards to the ability to activate small molecules. Here, we report synthesis and full characterisation of the persistent (hypersilyl)(pentamethylcyclopentadienyl)silylene Cp*[(Me3Si)3Si]Si: as well as its unique reactivity. Silylene Cp*[(Me3Si)3Si]Si: activates dihydrogen to give the corresponding dihydrosilane Cp*[(Me3Si)3Si]SiH2 at particularly mild conditions as well as ethylene to afford the three-membered cyclic silirane c-Cp*[(Me3Si)3Si]Si(H2CCH2). The addition of N-heterocyclic carbene NHC (NHC = 1,3,4,5-tetramethyl-imidazol-2-ylidene) to dihydrosilane Cp*[(Me3Si)3Si]SiH2 induces the reductive elimination of Cp*H, which according to DFT calculations is thermodynamically preferred over H2 elimination. With NHC, Cp*[(Me3Si)3Si]Si: forms a typical donor-acceptor complex with concomitant change in hapticity of the Cp* ligand from η2 to η1 (σ). In contrast, the reaction with the N-heterocyclic silylene c-[(CH[double bond, length as m-dash]CH(tBuN)2]Si: leads to an unusual "masked" disilene with the former Cp* ligand bridging the two silicon centres. The heterodimer is stable in the solid state, but dissociates reversibly to the constituting silylene fragments in solution.

Atomically Precise Expansion of Unsaturated Silicon Clusters.

Small- to medium-sized clusters occur in various areas of chemistry, for example, as active species of heterogeneous catalysis or as transient intermediates during chemical vapor deposition. The manipulation of stable representatives is mostly limited to the stabilizing ligand periphery, virtually excluding the systematic variation of the property-determining cluster scaffold. We now report the deliberate expansion of a stable unsaturated silicon cluster from six to seven and finally eight vertices. The consecutive application of lithium/naphthalene as the reducing agent and decamethylsilicocene as the electrophilic source of silicon results in the expansion of the core by precisely one atom with the potential of infinite repetition.

Multiple Ether-Functionalized Phosphonium Ionic Liquids as Highly Fluid Electrolytes.

Ionic liquids (ILs) are promising electrolytes, although their often high viscosity remains a serious drawback. The latter can be addressed by the introduction of multiple ether functionalization. Based on the highly atom efficient synthesis of tris(2-ethoxyethyl) phosphine, several new phosphonium ionic liquids were prepared, which allows studying the influence of the ether side chains. Their most important physicochemical properties have been determined and will be interpreted using established approaches like ionicity, hole theory, and the Walden plot. There is striking evidence that the properties of phosphonium ionic liquids with the methanesulfonate anion are dominated by aggregation, whereas the two triple ether functionalized ILs with the highest fluidity show almost ideal behavior with other factors being dominant. It is furthermore found that the deviation from ideality is not significantly changed upon introduction of the ether side chains, although a very beneficial impact on the fluidity of ILs is observed. Multiple ether functionalization therefore proves as a powerful tool to overcome the disadvantages of phosphonium ionic liquids with large cations.

Reactivity in the periphery of functionalised multiple bonds of heavier group 14 elements.

Heavier group 14 multiple bonds have intrigued chemists since more than a century. The synthesis of stable compounds with double and triple bonds with silicon, germanium, tin and lead had considerable impact on modern ideas of chemical bonding. These developments were made possible by the use of bulky substituents that provide kinetic and thermodynamic protection. Since about a decade the compatibility of heavier multiple bonds with various functional groups has moved into focus. This review covers multiply bonded group 14 species with at least one additional reactive site. The vinylic functionalities of groups 1 and 17, resulting in nucleophilic and electrophilic disila vinyl groups, respectively, are the most prevalent and well-studied. They have been employed repeatedly for the transfer of heavier multiple bonds to yield low-valent group 14 compounds with novel structural motifs. Vinylic functionalities of groups 2 to 16 and a few σ-bonded transition metal complexes are experimentally known, but their reactivity has been studied to a lesser extent. Donor-coordinated multiple bonds are a relatively new field of research, but the large degree of unsaturation as isomers of alkynes (as well as residual functionality in some cases) offers considerable possibility for further manipulation, e.g. for the incorporation into more extended systems. Heavier allyl halides constitute the major part of heavier multiple bonds with a functional group in allylic position and some examples of successful transformations are given. At present, remote functionalities are basically limited to para-phenylene functionalised disilenes. The reported use of the latter for further derivatisation might encourage investigations in this direction. In summary, the study of peripherally functionalised multiple bonds with heavier group 14 elements is already well beyond its infancy and may be an instrumental factor in awakening the potential of group 14 chemistry for applications in polymers and other materials.

Halogen-bonded liquid crystals of 4-alkoxystilbazoles with molecular iodine: a very short halogen bond and unusual mesophase stability.

Complexes of molecular iodine with alkoxystilbazoles are liquid crystals with unusually high mesophase stability, predicated on an intermolecular I···I contact. Attempts to prepare analogous complexes with bromine led to an unexpected electrophilic substitution product.

Hydrosilylation of alkynes by Ni(CO)3-stabilized silicon(II) hydride.

Not copy and paste: Although ß-diketiminato ligands have been employed for the stabilization of Ge(II) and Sn(II) hydrides, the corresponding Si(II) hydride is not accessible. However, coordination of silicon(II) to a {Ni(CO)(3)} fragment allowed the isolation of the first Si(II) hydride metal complex 1. This complex was used for the first silicon(II)-based and Ni(0)-mediated, stereoselective hydrosilylation of alkynes. R = phenyl, tolyl.

Formation of N-heterocyclic, donor-stabilized borenium ions.

Cationic and zwitterionic boryl bromide species and a borenium-boryl bromide cation have been synthesised which represent new N-donor stabilised cationic boron compounds with ß-diketiminate ligands. The unexpected borenium-boryl bromide results from a head-to-tail dimerisation of the corresponding zwitterionic boryl bromide accompanied by proton migration. The electronic nature of these new species was studied by DFT calculations.

An ylide-like phosphasilene and striking formation of a 4π-electron, resonance-stabilized 2,4-disila-1,3-diphosphacyclobutadiene.

The first N-donor-stabilized phosphasilene LSi(SiMe(3))═PSiMe(3) (L = PhC(NtBu)(2)) has been synthesized in 87% yield through 1,2-silyl migration of the (Me(3)Si)(2)P-substituted, N-heterocyclic silylene [LSi-P(SiMe(3))(2)]. Remarkably, the latter reacts with dichlorotriphenylphosphorane Ph(3)PCl(2) to give the unprecedented 4π-electron Si(2)P(2)-cycloheterobutadiene [(LSi)(2)P(2)] with two-coordinate phosphorus atoms. The striking molecular structures as well as the (29)Si and (31)P NMR spectroscopic features of both products indicate the presence of zwitterionic Si═P bonds which is also in accordance with results by DFT calculations.

Structure-function relationships in liquid-crystalline halogen-bonded complexes.

New liquid-crystalline materials were prepared by self-assembly driven by halogen bonding between a range of 4-alkoxystilbazoles, 4-alkyl-, and 4-alkoxy-substituted pyridines as halogen-bonding acceptors, and substituted derivatives of 4-iodotetrafluorophenyl as halogen-bonding donors. Despite the fact that the starting materials are not mesomorphic, the dimeric, halogen-bonded complexes obtained exhibited nematic and SmA phases, depending on the length of the alkyl chains present on the components. The modularity of this approach also led to new chiral mesogens starting from non-mesomorphic chiral compounds.

Addition of [(eta(5)-C(5)Me(5))IrH(4)] to a zwitterionic silylene: stepwise formation of iridium(V)-silyl and iridium(III)-silylene complexes.

The first zwitterionic silyl-iridium(v) complex is generated by insertion of silylene into an iridium-hydrogen bond of the iridium(v) hydride , [(eta(5)-C(5)Me(5))IrH(4)]. Complex undergoes proton migration from an IrH moiety to the terminal CH(2) group of the silyl ligand to furnish the N-donor stabilised Ir(iii)-silylene complex .

Steering S-H and N-H bond activation by a stable n-heterocyclic silylene: different addition of H(2)S, NH(3), and organoamines on a silicon(II) ligand versus its Si(II)-->Ni(CO)(3) complex.

The strikingly different behavior of the ylide-like, N-heterocyclic silylene LSi: (5: L = CH[(C horizontal lineCH(2))CMe(NAr)(2)]; Ar = 2,6-(i)PrC(6)H(3)) versus its LSi-->Ni(CO)(3) complex 13 to activate E-H bonds (E = S, N) of small molecules is reported. Remarkably, conversion of 5 with hydrogen sulfide leads exclusively to the first isolable silathioformamide, L'Si( horizontal lineS)H (16: L' = CH[C(Me)NAr](2); Ar = 2,6-(i)PrC(6)H(3)) with a donor-supported Si horizontal lineS double bond and four-coordinate silicon. The latter result demonstrates the unusual ambivalent reactivity of 5 by combining two modes of reactivity involving S-H bond activation and subsequent 1,4- and 1,1-addition, respectively. In addition, 5 can serve as a ligand with well-balanced sigma-donor and pi-acceptor capabilities toward transition metals. This has been demonstrated by the isolable [Ni(0)(arene)] complexes 12a-e (arene = Me(n)C(6)H(6-n), n = 0-3), which are ideal precursors for the formation of the corresponding Ni(CO)(3) complex 13. The latter activates a S-H bond in hydrogen sulfide, too, but the presence of the Ni(CO)(3) moiety governs the formation of the complex 17, bearing an unprecedented beta-diketiminate silicon(II) thiol ligand: L'Si(SH): (L' = CH[C(Me)NAr](2); Ar = 2,6-(i)PrC(6)H(3)). Likewise, the Si(II)-->Ni(CO)(3) coordination in 13 steers exclusively 1,4-addition of ammonia, isopropylamine, and phenylhydrazine onto the silylene ligand 5, leading to the corresponding beta-diketiminate silicon(II) amide or hydrazide complexes L'Si(NHR)-->Ni(CO)(3) (23a-c: R = H, (i)Pr, N(H)Ph). IR measurements reveal that the carbonyl stretching frequencies of the Ni(CO)(3) moiety in 23a-c are shifted to even lower wavenumbers in comparison to those of NHCs or phosphines. In other words, the beta-diketiminate silicon(II) amide ligands in 23a-c represent the strongest donors in the series of N-heterocyclic silylenes reported as yet.

Diketiminate silicon(II) and related NHSi ligands generated in the coordination sphere of nickel(0).

The first silicon(II) hydroxide and related electronically tunable NHSi ligands have been synthesized in the coordination sphere of a [Ni(CO)(3)] moiety through facile addition of water and other electrophiles to the corresponding NHSi-tricarbonylnickel complex. The latter modified silicon (II) ligands are unique and exhibit striking donor/acceptor capabilities depending on their substitution pattern. While strong Lewis acids [H(+) or B(C(6)F(5))(3)] lead to modified silicon(II) ligands in the coordination sphere of nickel with an increased pi-acceptor and decreased sigma-donor character (PF(3)-like), by contrast, addition of water or trifluoromethanesulfonic acid furnishes the corresponding donor-stabilized silicon(II)-nickel complexes with a inverse donor/acceptor strength similar to those of triorganophosphines and NHCs.

The striking stabilization of Ni0(eta6-arene) complexes by an ylide-like silylene ligand.

The right mix does the trick: Elusive {Ni(0)(eta(6)-arene)} moieties can be dramatically stabilized by the N-heterocyclic silylene ligand 1, which has a zwitterionic mesomeric structure. The sigma, pi-acid-base synergism between nickel and 1 explains the unexpectedly high stability of the new silylene complexes 2, which enables arene exchange studies at a Ni(0) center. Addition of B(C(6)F(5))(3) to 2 affords the zwitterionic silylene complex 3 (see scheme, R=2,6-iPr(2)C(6)H(3)).

Trimeric liquid crystals assembled using both hydrogen and halogen bonding.

New liquid crystals are reported that are formed from non-mesomorphic components assembled using both halogen and hydrogen bonding.

Spontaneous symmetry-breaking in halogen-bonded, bent-core liquid crystals: observation of a chemically driven Iso-N-N* phase sequence.

A chiral nematic phase is observed in a halogen-bonded, bent-core mesogen, which is itself achiral; the phase sequence arises from the lability of the halogen-bonded complex.

Fluorinated liquid crystals formed by halogen bonding.

New, halogen-bonded fluorinated mesogens are reported; the expected microphase separation associated with perfluoroalkyl chains is surprisingly absent in the mesophase.