Construction of a Boron Chain on a Single Metal by Dehydrocoupling of Borane Ligands

Borane coordination, B-H borane bond activation, and borane catenation via metal-mediated dehydrocoupling to form electron-precise B-B bonds are reported. The reaction of trans-[M(IMes)2Cl4] (M = W, Mo) (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene) with borates Li[BH3R] (R = Mes, Dur; Mes = 2,4,6-Me3C6H2 and Dur = 2,3,5,6-Me4C6H) afforded the complexes [M(IMes)(η2-H2BR)2(η1-H2BR)] (M = W: R = Mes 1, R = Dur 3; M = Mo: R = Mes 2, R = Dur 4). Three borane ligands are coordinated in 1-4 to the group 6 metal atom via five (σ-B-H) bonds. Reaction of 1 with the phosphines PMe3 and PEt3, respectively, led to the elimination of one of the borane ligands and afforded the hydrido (σ-B-H)-boryl bis(σ-B-H)-borane complexes trans-[W(IMes)(PR3)(η1-HBMes)(η2-H2BMes)(H)] (R = Me 5, R = Et 6), in which the metal atom inserted into one of the remaining σ-B-H bonds of the borane ligands. Reaction of 1 with an additional equivalent borane BH2Mes resulted in borane dehydrocoupling and formation of complex [W(IMes)(η4-BH2Mes-BMes-BMes-BH2Mes)] 7, featuring a unique B4 chain as a ligand. Reaction of trans-[W(IMes)2Cl4] with NaBH4 also led to B-B coupling, and the metallaborane cluster [{W(IMes)(BH4)}2(B5H9)] 9 was formed, in which two tungsten atoms bridge a B5 chain.

Hetero-trimetallic complexes comprising bridging boryl and borylene ligands: an experimental and theoretical study

In an effort to explore the coordination chemistry of the coordinative sulfur centers in arachno-ruthenaborane [(Cp*Ru)2(B3H8)(CS2H)] (arachno-1), we have thermolyzed arachno-1 with group-6 metal carbonyls [M(CO)5·THF] (M = Cr, Mo and W). The reaction of arachno-1 with [Cr(CO)5·THF] resulted in the formation of hetero-trimetallic triply bridging borylene [(Cp*Ru)2(μ-CO)(μ3-CH2S2-κ2S':κ2S''){Cr(CO)3}(μ3-BH)] (2), bridging boryl-borylene [(Cp*Ru)2(μ-CO){(μ3-BH(CH2S2)-κ2B:κ2S':κ1S'')}{Cr(CO)3}(μ3-BH)] (3), and sulfido bridged hetero-trimetallic complex [(Cp*Ru)2(μ-CO)3{Cr(CO)3}(μ3-S)] (4). In 2, one side of Ru2Cr-triangle features a μ3-BH ligand while the other side is quadruply bridged by a methanedithiolato ligand in an unsymmetrical fashion. Unlike 2, in complex 3, one side of the Ru2Cr-triangle has a μ3-BH ligand while the opposite side is bridged by a boryl ligand BH(CH2S2) in an unsymmetrical way (μ3-κ2:κ2:κ1) to the metal centers. Interestingly, when the similar reactions of arachno-1 were performed with heavier group-6 metal carbonyls [M(CO)5·THF] (M = Mo and W), it led to the formation of methanedithiolato bridged hetero-trimetallic chain complexes, [{Cp*Ru(CO)}2(μ-CO)2(μ3-CH2S2-κ2S':κ2S''){M(CO)2}] (5, M = Mo; 6, M = W) and sulfido-bridged hetero-trimetallic complexes [(Cp*Ru)2(μ-CO)3{M(CO)3}(μ3-S)] (7, M = Mo; 8, M = W), analogous to 4. In complexes 5 and 6, a Ru2M-chain is symmetrically bridged by a methanedithiolato ligand. On the other hand, in complexes 4, 7, and 8, a sulfido ligand coordinates to two ruthenium and one group-6 metal atoms in μ3-fashion. All the complexes have been characterized by 1H NMR, 13C NMR, UV-vis, IR spectroscopy, and mass spectrometry and their structural architectures have been unambiguously established by single crystal X-ray diffraction studies. In addition, theoretical investigations provided valuable insights into their electronic structures and bonding properties.

Nucleophilic reactivity of the gold atom in a diarylborylgold(i) complex toward polar multiple bonds

A di(o-tolyl)borylgold complex was synthesized via the metathesis reaction of a gold alkoxide with tetra(o-tolyl)diborane(4). The resulting diarylborylgold complex exhibited a Lewis acidic boron center and a characteristic visible absorption that arises from its HOMO-LUMO excitation, which is narrower than that of a previously reported dioxyborylgold complex. The diarylborylgold complex reacted with isocyanide in a stepwise fashion to afford single- and double-insertion products and a C-C coupled product. Reactions of this diarylborylgold complex with C[double bond, length as m-dash]O/N double bond species furnished addition products under concomitant formation of Au-C and B-O/N bonds, which suggests nucleophilic reactivity of the gold metal center. DFT calculations provided details of the underlying reaction mechanism, which involves an initial coordination of the C[double bond, length as m-dash]O/N bond to the boron vacant p-orbital of the diarylboryl ligand followed by a migration of the gold atom from the tetracoordinate sp3-hybridized boron center, which is analogous to the reactivity of the conventional sp3-hybridized borate species. The DFT calculations also suggested a stepwise mechanism for the reaction of this diarylborylgold complex with isocyanide, which afforded three different reaction products depending on the applied reaction conditions.

Complexation and Release of N-Heterocyclic Carbene-Aminoborylene Ligands from Group VI and VIII Metals

The coordination chemistry and stability of aminoborylene ligands bearing different N-heterocyclic carbene (NHC) stabilizing groups has been investigated with Group VI and VIII metals. NHC-aminoborylene complexes have been accessed via reduction of NHC-dihaloaminoborane adducts with Na₂[M(CO) _x] species (M = Fe, Ru, Cr, W). Imidazol-2-ylidene-stabilized aminoborylene ligands were found to afford thermally robust metal-borylene complexes, which are inert to oxidation, hydrolysis, and insertion of unsaturated substrates. Such ligands have additionally been demonstrated to be significantly more electron releasing than NHCs and other carbon-based ligands by infrared spectroscopy, and can be regarded as unique examples of highly nucleophilic borylene ligands isolobal to classical NHCs. In contrast, cyclic alkylaminocarbene (CAAC)-bound dihaloaminoboranes were found to be reduced by one or two electrons upon reaction with Na₂[M(CO) _x] species to form either a stable borane-centered radical, or the free CAAC-aminoborylene complex, which further reacts to form a carbonyl-stabilized aminoborylene. Borylene-to-CO migration was also observed upon reaction of a ruthenium imidazol-2-ylidene aminoborylene complex with B(C₆F₅)₃, where the product borylene remains trapped by the Ru center.

Comparison of nucleophilic- and radical-based routes to the formation of manganese-main group element single bonds

One-electron activation of main-group substrates by a stable manganese metalloradical provides a facile pathway to Mn-element single bonds.

The phosphinoboration of carbodiimides, isocyanates, isothiocyanates and CO2

The transition metal-free addition of phosphinoboronate ester Ph₂PBpin (pin = 1,2-O₂C₂Me₄) to heterocumulenes including carbodiimides, isocyanates, isothiocyanates and carbon dioxide proceeds with remarkable selectivity to give products in high yield.