We report the first isolation of phosphine copper boryl complexesspecies pivotal to numerous copper-catalyzed borylation reactions. The reaction of diboron(4) derivatives with copper tert-butoxide complexes of phosphine ligands allows the isolation of the dimeric μ-boryl-bridged Cu­(I) complexes [(iPr3P)­Cu–Bdmab]2 (4) and [(C6H4(Ph2P)2)­Cu–Bpin]2 (6) with Cu···Cu distances of 2.24–2.27 Å (dmab = (NMe)2C6H4, pin = (OCMe2)2)). A slightly more sterically demanding boryl ligand furnishes the unprecedented multinuclear copper boryl complex [(iPr3P)2Cu8(B­(iPrEn))3(OtBu)3] (5), a potential intermediate of the decomposition of an initial Cu­(I) boryl complex (iPrEn = (NiPr)2C2H4). All complexes were characterized by single-crystal X-ray diffraction, NMR spectroscopy, and elemental analysis. DFT computations support the nature of these unique complexes and give insight into their electronic structures.

Diaminoboryl ligands are currently intensively investigated because of their unique coordination chemical properties, for example, as part of pincer ligands. Owing to synthetic restrictions, however, access to diaminoboryl complexes is limited. Unsymmetrical diborane(4) derivatives comprising a dialkoxy-and a diaminoboron moiety provide efficient access to diaminoboryl complexes either by oxidative addition or by σ-

Silylboranes are important reagents in a variety of catalytic silylation and silaboration reactions. While transition‐metal‐catalyzed reactions are well established, organo‐/Lewis base‐catalyzed reactions of silylboranes have only recently emerged. For both catalytic processes the reactivity of silylboranes toward Lewis bases is of relevance. While for organo‐catalyzed reactions Lewis base activation of the silylborane has been proposed, transition‐metal‐ and especially copper‐catalyzed reactions also frequently require the presence of Lewis basic alkali metal alkoxides. In the present study we explore the reaction of K(18‐crown‐6) tert‐butoxide and the NHC 1,3‐diisopropyl‐4,5‐dimethyl‐imidazol‐2‐ylidene as exemplary Lewis bases with the two silylboranes pinB‐SiMe2Ph and pinB‐SiPh3 (pin = OCMe2CMe2O). The reaction with K(18‐crown‐6) tert‐butoxide results in activation of the boron–silicon bond. The isolated product of this activation is either the potassium silyl complex [K(18‐crown‐6)SiPh3] or [K(18‐crown‐6)(thf)2][pinB(SiMe2Ph)2], the formal Lewis acid/base adduct of [K(18‐crown‐6)SiMe2Ph] with pinB‐SiMe2Ph. Both complexes react essentially as sources of nucleophilic silyl moieties in reactions with exemplary electrophiles. In contrast, usage of the carbene leads to the formation of isolable Lewis acid/base adducts of the type (NHC)pinB‐SiR3, which do not react as sources of nucleophilic silyl moieties. The identification and characterization of these species appears of relevance for the mechanistic understanding and further development of Lewis base/organo‐ as well as transition‐metal‐catalyzed silyl transfer reactions.

Synthesis and characterisation of NHC copper boryl complexes [(NHC)Cu–Bneop] and their decomposition to low-valent copper clusters.

A novel, versatile and modular route to unsymmetrical diborane(4) derivatives bearing either two different diol moieties or one diol and one diamine moiety is presented. Utilising the very basic approach of reacting a boron nucleophile with a boron electrophile to establish the B-B bond allows a simple variation of the two individual boron moieties. The copper(i) boryl complexes used as nucleophilic boron sources are readily accessible from commercially available symmetrical diborane(4) derivatives, whilst equally readily available boron halide derivatives are used as electrophiles. Seven previously inaccessible diborane(4) derivatives were obtained and fully characterised, including single crystal X-ray structure determinations, illustrating the broad scope of the method.

Five novel NHC copper(I) boryl complexes were synthesized by B−B activation via σ-bond metathesis of symmetrical tetraalkoxy and unsymmetrical dialkoxy diamino diborane(4) derivatives. Despite their low stability, the NHC copper boryl complexes were thoroughly characterized spectroscopically and structurally. Variation of the NHC ligand (ItBu or Me 2 IiPr) as well as of the boryl ligand (Bpin, Bdmab, or BiPrEn) allowed, for the first time systematically, a study in such complexes of the dependence on steric encumbrance. For sterically more demanding ligand combinations, mononuclear linear complexes were obtained, while with less demanding ligand combinations, dimeric dinuclear complexes with two bridging μ-boryl ligands were obtained, exhibiting extremely short Cu•••Cu distances (<2.26 Å). The decomposition of all these complexes was found to proceed via a common pathway, leading ultimately to elemental copper, the free NHC ligand, and the respective symmetrical diborane(4) derivative. The rate of decomposition depended strongly on the steric encumbrance of the individual complex. Two apparently low-valent copper clusters were observed and suggested to be relevant species with respect to the reductive decomposition of the copper(I) boryl complexes.

The synthesis of the digermyl and germylsilyl hydronium borates 7[B(C 6 F 5 ) 4 ] and 8[B(C 6 F 5 ) 4 ] is reported. Spectroscopic (IR, NMR) and structural data supported by the results of density functional calculations indicate in both cases a symmetric or almost symmetric E−H−E′ three-center−two-electron linkage (7, E = E′ = Ge; 8, E = Si, E′ = Ge). The [B(C 6 F 5 ) 4 ] − and the [HCB 11 H 5 Br 6 ] − salts of both cations are active in catalytic hydrodefluorination reactions of alkyl and benzyl fluorides. No significant effect of the element atom E on the determined turnover numbers was found.

The unsymmetrical diborane(4) pinB-B((RN)2(C6H4)) (R = Me, Bn) reacts with [(Me3P)4Rh-X] (X = Me, OtBu) giving predominantly either [(Me3P)4Rh-Bpin] or [(Me3P)3Rh-B((RN)2(C6H4))] depending on X. At low temperatures in the presence of excess PMe3 the unprecedented equatorial boryl complex [(Me3P)4Rh-B((MeN)2(C6H4))] is formed.