Herein, we analyse the catalytic boron-boron dehydrocoupling reaction that leads from the base-stabilised diborane(6) [H2 B(hpp)]2 (hpp=1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate) to the base-stabilised diborane(4) [H2 B(hpp)]2 . A number of potential transition-metal precatalysts was studied, including transition-metal complexes of the product diborane(4). The synthesis and structural characterisation of two further examples of such complexes is presented. The best results for the dehydrocoupling reactions were obtained with precatalysts of Group 9 metals in the oxidation state of +I. The active catalyst is formed in situ through a multistep process that involves reduction of the precatalyst by the substrate [H2 B(hpp)]2 , and mechanistic investigations indicate that both heterogeneous and (slower) homogeneous reaction pathways play a role in the dehydrocoupling reaction. In addition, hydride abstraction from [H2 B(hpp)]2 and related diboranes is analysed and the possibility for subsequent deprotonation is discussed by probing the protic character of the cationic boron-hydrogen compounds with NMR spectroscopic analysis.

A new boron–boron dehydrocoupling strategy was established, providing convenient access to some diborane(4) compounds starting from simple borane adducts under mild conditions. In contrast to the traditional pathway using a reducing reagent, the reduction from BIII to BII was paradoxically initiated by the addition of the oxidation‐reagent iodine. A reaction pathway for this unusual reaction was proposed based on quantum‐chemical calculations.

The condensation of imidazole moieties with boranes leading to molecular units with N–BR2–N bridges was accomplished by attaching three imidazole units to a single phosphorus atom. In this way, boron bridged trisimidazolylphosphines 3–7 were synthesized in good yields. The chiral derivatives 5a, 5d, 6, and 7 are C3 symmetric, whereas 5b and 5c carry no symmetry element (C1). The steric bulk of the ligands can easily be tuned by introduction of different substituents at the three bridging boron atoms. All phosphines feature small CPC angles between 91–93° leading to a s‐type lone pair donor at the phosphorus atom combined with reasonable π‐accepting properties. Ni (8) and Au metal complexes (9, 10) with very short metal phosphorus bonds were synthesized. The short metal–phosphorus distance is a consequence of the pronounced s‐character of the phosphine lone pair.

The cover picture shows a new class of chiral phosphine ligands bearing three imidazole units bridged by boranes. The bowl shaped, C3 symmetric molecules are obtained by a simple condensation procedure. The steric bulk of these monodentate ligands can easily be adjusted over a large range by the use of different chiral or achiral boranes. The ligand character is determined by the almost pure s‐character of the phosphorous lone‐pair, leading to very short metal‐P distances. Details are discussed in the article by Markus Enders, Helene Schall, and Olaf Fritz, et al. on page http://onlinelibrary.wiley.com/doi/10.1002/zaac.201800016/abstract.

The rational formation of element–element bonds is one of the key concerns in synthetic chemistry. Boron–boron bonds are usually generated by reaction of a halo‐borane with a reducing agent. This work reports on the formation of boron–boron bonds by reaction of a hydrido‐borane with the oxidizing agent iodine, being a seemingly paradoxical approach. The “magic effect” of iodine (I2) is highlighted in the artwork. A combined experimental and theoretical analysis shows that the boron–boron bond results from a dehydrocoupling step. More information can be found in the Full Paper by H.‐J. Himmel et al. on page 6553.