The previously elusive diphosphadibenzo­[a,e]­pentalene core skeleton was assembled via a surprisingly straightforward cyclization pathway starting from R2P-substituted 2,2′-diphosphinotolanes (R = Ph, i Pr). The resulting P-protected diylidic compounds 4 (R = Ph, i Pr) were converted to the corresponding P-bridged ladder stilbenes via two consecutive oxidation steps: upon selective one-electron oxidation, the persistent radical monocations 5 (R = Ph, i Pr) were obtained and further oxidized to afford the respective fluorescent and air-stable dications 6 (R = Ph, i Pr).

One of the most attractive routes for the preparation of reactive tantalum(III) species relies on the efficient salt-free hydrogenolysis of tantalum(V) alkyls or tantalum(V) alkylidenes, a process known as reductive hydrogenation. For silica-crafted tantalum alkyls and alkylidenes, this process necessarily proceeds at well-separated tantalum centers, while related reductive hydrogenations in homogeneous solution commonly involve dimeric complexes. Herein, an NHC scaffold was coordinated to a novel tri(alkoxido)tantalum(V) alkylidene to circumvent the formation of dimers during reductive hydrogenation. Employing this new model system, a key intermediate of the process, namely a hydrido-tantalum alkyl, was isolated for the first time and shown to exhibit a bidirectional reactivity. Upon being heated, the latter complex was found to undergo either an αelimination or a reductive alkane elimination. In the (overall unproductive) α-elimination step, H 2 and the parent alkylidene were regenerated, while the sought-after transient d 2 -configured tantalum(III) derivative was produced along the reaction coordinate of the reductive alkane elimination. The reactive low-valence metal center was found to rapidly attack one of the NHC substituents via an oxidative C−H activation, which led to the formation of a cyclometalated tantalum(V) hydride. The proposed elemental steps are in line with kinetic data, deuterium labeling experiments, and density functional theory (DFT) modeling studies. DFT calculations also indicated that the S = 0 spin ground state of the Ta(III) center plays a crucial role in the cyclometalation reaction. The cyclometalated Ta(V) hydride was further investigated and reacted with several alkenes and alkynes. In addition to a rich insertion and isomerization chemistry, these studies also revealed that the former hydride may undergo a formal cycloreversion and thus serve as a tantalum(III) synthon, although the original tantalum(III) intermediate is not involved in this process. The latter reactivity was observed upon reaction with internal alkynes and led to the corresponding η 2 -alkyne derivatives via vinyl intermediates, which rearrange via a remarkable, hitherto unprecedented, hydrogen shift reaction.

An [AsCCAs] ligand featuring a central alkyne and two flanking arsenic donors was employed for the synthesis of a trihydrido rhenium complex, while the corresponding phosphorus ligand was shown to be less suited. The reactivity of the former trihydride [AsCCAs]ReH 3 (3) was examined in detail, which revealed that two alternative reaction channels may be entered in dependence of the substrate. Upon reaction of 3 with PhC�CPh, ethylene, and CS 2 , monohydrides of the general formula [AsCCAs]Re(L)H with L = η 2 -PhC�CPh (4), η 2 -H 2 C�CH 2 (5), and η 2 -CS 2 (6) were formed along with H 2 . In contrast, insertion products of the type [AsCCAs]Re(X)H 2 (7−9) were obtained upon treatment of 3 with CyN�C�NCy, PhN�C�O, and Ph 2 C�C�O, while CO 2 failed to react with 3 under identical reaction conditions. Given that several productive reactions between CO 2 and hydrido rhenium carbonyls have been reported in the literature, 3 was further derivatized by introducing CO and t BuNC coligands, respectively. This led to the isolation of trans-[AsCCAs]ReH(CO) 2 (trans-10) and trans-[AsCCAs]ReH(CN t Bu) 2 (trans-11), which were shown to thermally isomerize to the corresponding cis-configured products, cis-10 and cis-11. Interestingly, only the cis-complexes were found to react with CO 2 , which was rationalized by evaluating the relative nucleophilicities of the hydrides in cis-10, trans-10, cis-11, and trans-11 via Fukui analysis. The formates cis-[AsCCAs]Re(OCHO)(CO) 2 (12) and cis-[AsCCAs]Re(OCHO)(CN t Bu) 2 (13) were isolated and shown to contain κ 1 -O-coordinated formate moieties. Treatment of 12 with [LutH]Cl/B(C 6 F 5 ) 3 (or with Ph 3 SiCl) led to the liberation of [LutH][OCHO•••B(C 6 F 5 ) 3 ] (or triphenylsilyl formate) with concomitant formation of the expected chloro complex cis-[AsCCAs]ReCl(CO) 2 ( 14). In a closed synthetic cycle, hydride 12 was regenerated from the latter chloride using NaBEt 3 H as a hydride source.

Engineering the energetics of perovskite solar cells through the introduction of surface dipoles that assist with charge carrier extraction is a promising route to enhance the device performance without altering other device layers or fabrication parameters. In this work, we introduce four different derivatives of dicationic phosphonium-bridged ladder stilbenes (PYMC) in inverted perovskite solar cells with the device structure of ITO/Meo-2pacz/perovskite/PYMC/phenyl-C61-butyric acid methyl ester (PCBM)/bathocuproine/Ag. We show that the derivatives introduce a dipole at the perovskite/PCBM interface, which for derivatives with suitable energy levels can enhance the charge carrier extraction, leading to a quenched photoluminescence of perovskite thin films and an improved photovoltaic performance. As a result, both a higher average and maximum power conversion efficiency could be achieved and an overall better device reproducibility. This work highlights the significant potential of energetics engineering between perovskites and transport layers in perovskite solar cells for highly efficient photovoltaic devices.

The central phosphorus atom of a novel hydroxyl-functionalized triarylphosphane was shown to reversibly insert into one of the molecule’s O–H bonds, which forms the basis for a tautomeric λ3/λ5-phosphane equilibrium. For the first time, this equilibrium was detected for a λ3-triarylphosphane and the underlying dynamic process was elucidated by NMR spectroscopy. On the basis of reactivity studies, a nucleophilic character was assigned to the minor species present in solution, the λ3-phosphane. Upon methylation, for example, the λ3-form was selectively removed from the equilibrium and converted to the corresponding phosphonium salt. However, upon generation of an alkoxide via proton abstraction, the electrophilic character of the λ5-phosphane in the equilibrium became evident since the alkoxide was found to attack the molecule’s phosphorus atom. This intramolecular reaction led to the selective formation of a new anionic λ6-hydridospirophosphane.