Heteroleptic [n]chromoarenophanes: ansa complexes derived from [Cr(η(5)-C5H5)(η(6)-C6H6)].

The synthesis of ansa complexes has been studied intensively owing to their importance as homogeneous catalysts and as precursors of metal-containing polymers. However, paramagnetic non-metallocene derivatives are rare and have been limited to examples with vanadium and titanium. Herein, we report an efficient procedure for the selective dilithiation of paramagnetic sandwich complex [Cr(η(5)-C(5)H(5))(η(6)-C(6)H(6))], which allows the preparation of a series of [n]chromoarenophanes (n = 1, 2, 3) that feature silicon, germanium, and tin atoms at the bridging positions. The electronic and structural properties of these complexes were probed by X-ray diffraction analysis, cyclic voltammetry, and by UV/Vis and EPR spectroscopy. The spectroscopic parameters for the strained and less strained complexes (i.e., with multiple-atom linkers) indicate that the unpaired electron resides primarily in a d(z)2 orbital on chromium(I); this result was also supported by density functional theory (DFT) calculations. We did not observe a correlation between the experimental UV/Vis and EPR data and the degree of molecular distortion in these ansa complexes. The treatment of tin-bridged complex [Cr(η(5)-C(5)H(4))(η(6)-C(6)H(5))SntBu(2)] with [Pt(PEt(3))(3)] results in the non-regioselective insertion of the low-valent Pt(0) fragment into the C(ipso)-Sn bonds in both the five- and six-membered rings, thereby furnishing a bimetallic complex. This observed reactivity suggests that ansa complexes of this type are promising starting materials for the synthesis of bimetallic complexes in general and also underline their potential to undergo ring-opening processes to yield new metal-containing polymers.

Alkynyl-functionalised and linked bicyclo[1.1.1]pentanes of group 14.

We report the synthesis and properties of alkynyl-functionalised and -bridged bicyclo[1.1.1]pentane derivatives consisting of the heavier group 14 elements silicon and tin.

New perspectives for "non-classical" molecules: heavy [1.1.1]propellanes of group 14.

Heavy analogues of hydrocarbons intrigue chemists for a number of reasons, not least because they are often fundamentally different from their carbon counterparts and have remained a challenge for both experimentalists and theoreticians for a long time. The appealing properties of [1.1.1]propellanes of group 14 consisting of inverted tetrahedral bridgehead atoms can mainly be attributed to the particular bonding between the latter. More than 20 years after the first member of this family has been published, several contributions to this area have impressively extended the spectrum of these so-called main-group biradicaloids. Still in its infancy, further perspectives for these "non-classical" molecules are now arising. In this tutorial review, early findings and recent developments in this area are presented. Particular attention is drawn on the relationship of unusual structures and unusual reactivities of main-group element compounds in general and in particular of heavy propellane scaffolds of group 14.

Transfer of a disilenyl moiety to aromatic substrates and lateral functional group transformation in aryl disilenes.

The reaction of 1 equiv of the disilenide Tip2Si═Si(Tip)Li (5; Tip = 2,4,6-(i)Pr3C6H2) with para-substituted phenyl iodides, 4-X-PhI, transfers the Tip2Si═Si(Tip) moiety with elimination of lithium iodide to yield the laterally functionalized disilenes Tip2Si═Si(Tip)(4-X-Ph) [X = H (6a), F (6b), Cl (6c), Br (6d), I (6e)]. The UV-vis absorptions of 6a-d suggest a linear correlation with electronic Hammett parameters. In addition, X-ray structural analyses of 6a-d verified the theoretically predicted linear dependence of the Si═Si bond length and trans-bent angles. The p-bromophenyl-substituted disilene 6d undergoes a metal-halogen exchange reaction to give 6f (X = Li), which was trapped with Me3SiCl to afford 6g (X = SiMe3). In the case of simple phenyl halides PhX without additional functionality, the reaction with 5 proceeded smoothly for X = Br, but phenyl chlorides and fluorides did not react at room temperature even after one week, hinting at an S(N)2-type aromatic substitution mechanism. Reactions of p- and m-diiodobenzene with 5 afford the corresponding phenylene-bridged tetrasiladienes p-7 and m-7. While red p-7 (λ(max) = 508 nm) exhibits efficient conjugation of the two Si═Si bonds with the phenylene linker, the conjugation in yellow m-7 (λ(max) = 449 nm) is much less effective. Electrochemical studies of m-7 and p-7 as well as density functional theory calculations and electron paramagnetic resonance studies of their respective radical anions provided further support for the notion of conjugation.

Synthesis of a pentasilapropellane. Exploring the nature of a stretched silicon-silicon bond in a nonclassical molecule.

We report on the successful synthesis of Si(5)Mes(6) (Mes = 2,4,6-trimethylphenyl), which consists of an archetypal [1.1.1] cluster core featuring two ligand-free, "inverted tetrahedral" bridgehead silicon atoms. The separation between the bridgehead Si atoms is much longer, and the bond strength much weaker, than usually observed for a regular Si-Si single bond. A detailed analysis of the electronic characteristics of Si(5)Mes(6) reveals a low-lying excited triplet state, indicative of some biradical(oid) character. Reactivity studies provide evidence for both closed-shell and radical-type reactivity, confirming the unusual nature of the stretched silicon-silicon bond in this "nonclassical" molecule.

Pentagerma[1.1.1]propellane: a combined experimental and quantum chemical study on the nature of the interactions between the bridgehead atoms.

Ge, whiz! A detailed study of the synthesis, structure, redox chemistry, and bonding properties of pentagerma[1.1.1]propellane (1, see picture) examines fundamental aspects of the interactions between the bridgehead germanium atoms. DFT and CASSCF calculations unravel the biradicaloid characteristics of 1, and preliminary reactivity studies indicate that 1 features some radical-type behavior.