[3] whose combination of nucleophilic and electrophilic sites are well-suited to combine with the carbon and oxygen atoms of CO2, respectively ( Figure 1A). Another common mechanism of main-group-based CO2 activation is the (initial) 2+2 cycloaddition of one C=O bond of CO2 with another E-E multiple bond, e.g. P=N (i.e. the Aza-Wittig reaction), Si=O, Si=N, Ge=O, Sn=O, and B=N bonds ( Figure 1B). [4] To our knowledge, no CO2 fixation or activation has been observed by solely utilizing a nonpolar multiple bond, despite the fact that a range of highly reactive compounds with E-E multiple bonds are known.[5] However, in 2011 Kato and Baceiredo reported [6] the reaction of CO2 with a disilyne bisphosphine adduct, a compound thought to possess some multiple bonding character between its two silicon atoms despite the clearly non-planar geometry around the silicon atoms. Herein we present fixation and splitting reactions of CO2 through its interaction with distinctly non-polar multiple bonds of two significantly different diboron species:[5c,d,e] a doubly basestabilized diborene [5,7] with tricoordinate boron atoms and a B=B double bond, and a linear diboryne species bearing strongly π-acidic cyclic (alkyl)(amino)carbene (CAACs) donors, [8] effectively a diboracumulene [9] species with a B-B bond order between two and three. Interestingly, in the reaction of CO2, we were also able to isolate the thermally unstable 2+2 (C=O + B=B) cycloaddition product, which slowly undergoes cleavage of one C=O bond. The apparently facile reaction of CO2 with B-B multiply bound species is attributed to the high reactivity of the latter, which is able to overcome the lack of polarity in the bond and effect the initial cycloaddition step. Dibromodiborenes (L(Br)B=B(Br)L), very few of which exist in the literature, [10,11] were chosen as candidates for CO2 binding due to their sterically unhindered B=B bonds and thus presumed high reactivity. Upon treatment with one atmosphere of CO2 at room temperature, after 7 min the 11 B NMR spectroscopic signal of diborene 1 [10] (dB 20) was found to have completely disappeared, replaced by two broad signals (dB ca. 0, -10). Removal of the solvent from this mixture and extraction of the residue into hexane provided a solution from which orange crystals (2) were grown. The solid-state structure of 2 (Figure 2, middle) confirms the combination of the diborene 1 with CO2 to form a dibora-b-lactone structure in which the two boron atoms form a slightly puckered four-membered B-B-C-O ring with one carbon and one oxygen atom of the CO2 unit. The remaining oxygen atom is part of a carbonyl group with a short C-O distance of 1.20(1) Å but a relatively wide O-C-B angle (136.2(8)º). Interestingly, the endocyclic B-B-C angle is strongly acute (73.7(8)º). The NHC and Br groups are each oriented in a trans fashion with respect to the ring.

The linear and nonlinear optical properties of a series of oligomeric squaraine dyes were investigated by one-photon absorption spectroscopy (1PA) and two-photon absorption (2PA) induced fluorescence spectroscopy. The superchromophores are based on two indolenine squaraine dyes with transoid (SQA) and cisoid configuration (SQB). Using these monomers, linear dimers and trimers as well as star-shaped trimers and hexamers with benzene or triphenylamine cores were synthesised and investigated. The red-shifted and intensified 1PA spectra of all superchromophores could well be explained by exciton coupling theory. In the linear chromophore arrangements we also found superradiance of fluorescence but not in the branched systems. Furthermore, the 2PA showed enhanced cross sections for the linear oligomers but only additivity for the branched systems. This emphasizes that the enhancement of the 2PA cross section in the linear arrangements is probably caused by orbital interactions of higher excited configurations.

Cationization of Bi(NPh 2 ) 3 has recently been reported to allow access to single-and double-CH activation reactions, followed by selective transformation of Bi−C into C−X functional groups (X = electrophile). Here we show that this approach can successfully be transferred to a range of bismuth amides with two aryl groups at the nitrogen, Bi(NR aryl 2 ) 3 . Exchange of one nitrogen-bound aryl group for an alkyl substituent gave the first example of a homoleptic bismuth amide with a mixed aryl/ alkyl substitution pattern at the nitrogen, Bi(NPhiPr) 3 . This compound is susceptible to selective N−N radical coupling in its neutral form and also undergoes selective CH activation when transformed into a cationic species. The second CH activation is blocked due to the absence of a second aryl moiety at nitrogen. The Lewis acidity of neutral bismuth amides is compared with that of cationic species "[Bi(aryl)(amide)(L) n ] + " and "[Bi(aryl) 2 (L) n ] + " based on the (modified) Gutmann−Beckett method (L = tetrahydrofuran or pyridine). The heteroaromatic character of [Bi(C 6 H 3 R) 2 NH(triflate)] compounds, which are iso-valence-electronic with anthracene, is investigated by theoretical methods. Analytical methods used in this work include nuclear magnetic resonance spectroscopy, single-crystal X-ray diffraction, mass spectrometry, and density functional theory calculations.

A pair of trigonal imido iron complexes ([Fe(NMes)L2]0,−) in two oxidation states is reported. The anionic complex K{crypt.222}[Fe(NMes)L2] is best described as an iron(ii) imide.