Easy access to nucleophilic boron through diborane to magnesium boryl metathesis

Organoboranes are some of the most synthetically valuable and widely used intermediates in organic and pharmaceutical chemistry. Their synthesis, however, is limited by the behaviour of common boron starting materials as archetypal Lewis acids such that common routes to organoboranes rely on the reactivity of boron as an electrophile. While the realization of convenient sources of nucleophilic boryl anions would open up a wealth of opportunity for the development of new routes to organoboranes, the synthesis of current candidates is generally limited by a need for highly reducing reaction conditions. Here, we report a simple synthesis of a magnesium boryl through the heterolytic activation of the B–B bond of bis(pinacolato)diboron, which is achieved by treatment of an easily generated magnesium diboranate complex with 4-dimethylaminopyridine. The magnesium boryl is shown to act as an unambiguous nucleophile through its reactions with iodomethane, benzophenone and N,N′-di-isopropyl carbodiimide and by density functional theory

The coordination chemistry of the neutral tris-2-pyridyl silicon ligand [PhSi(6-Me-2-py)3]

Producción CientíficaDifficulties in the preparation of neutral ligands of the type [RSi(2-py)3] (where 2-py is an unfunctionalised 2-pyridyl ring unit) have thwarted efforts to expand the coordination chemistry of ligands of this type. However, simply switching the pyridyl substituents to 6-methyl-pyridyl groups (6-Me-2-py) in the current paper has allowed smooth, high-yielding access to the [PhSi(6-Me-2-py)3] ligand (1), and the first exploration of its coordination chemistry with transition metals. The synthesis, single-crystal X-ray structures and solution dynamics of the new complexes [{PhSi(6-Me-2-py)3}CuCH3CN][PF6], [{PhSi(6-Me-2-py)3}CuCH3CN][CuCl2], [{PhSi(6-Me-2-py)3}FeCl2], [{PhSi(6-Me-2-py)3}Mo(CO)3] and [{PhSi(6-Me-2-py)3}CoCl2] are reported. The paramagnetic Fe2+ and Co2+ complexes show strongly shifted NMR resonances for the coordinated pyridyl units due to large Fermi-contact shifts. However, magnetic anisotropy also leads to considerable pseudo-contact shifts so that both contributions have to be included in the paramagnetic NMR analysis.The Leverhulme Trust (Grant for DSW and RG-R, postdoctoral funding for ALC, RGP-2017-146Ministerio de Economía, Industria y Competitividad - Agencia Estatal de Investigación (AEI)European Social Fund (ESF)Ramón y Cajal contract (RG-R, RYC-2015–19035

A General, Rhodium-Catalyzed, Synthesis of Deuterated Boranes and N-Methyl Polyaminoboranes

The rhodium complex [Rh(Ph2PCH2CH2CH2PPh2)(η6‐FC6H5)][BArF4], 2, catalyzes BH/BD exchange between D2 and the boranes H3B⋅NMe3, H3B⋅SMe2 and HBpin, facilitating the expedient isolation of a variety of deuterated analogues in high isotopic purities, and in particular the isotopologues of N‐methylamine‐borane: R3B⋅NMeR2 1‐dx (R=H, D; x=0, 2, 3 or 5). It also acts to catalyze the dehydropolymerization of 1‐dx to give deuterated polyaminoboranes. Mechanistic studies suggest a metal‐based polymerization involving an unusual hybrid coordination insertion chain‐growth/step‐growth mechanism

Beyond dehydrocoupling:group 2 mediated boron-nitrogen desilacoupling

The alkaline earth bis(trimethylsilyl)amides, [Ae{N(SiMe3)2}2(THF)2] [Ae = Mg, Ca, Sr], are effective pre-catalysts for boron-nitrogen bond formation through the desilacoupling of amines, RR’NH (R = alkyl, aryl; R′ = H, alkyl, aryl), and pinBSiMe2Ph. This reactivity also yields a stoichiometric quantity of Me2PhSiH and provides the first example of a catalytic main group element-element coupling that is not dependent on the concurrent elimination of H2

The role of neutral Rh(PONOP)H, free NMe2H, boronium and ammonium salts in the dehydrocoupling of dimethylamine-borane using the cationic pincer [Rh(PONOP)(η2-H2)]+ catalyst

The σ-amine-borane pincer complex [Rh(PONOP)(η1-H3B·NMe3)][BArF4] [2, PONOP = κ3-NC5H3-2,6-(OPtBu2)2] is prepared by addition of H3B·NMe3 to the dihydrogen precursor [Rh(PONOP)(η2-H2)][BArF4], 1. In a similar way the related H3B·NMe2H complex [Rh(PONOP)(η1-H3B·NMe2H)][BArF4], 3, can be made in situ, but this undergoes dehydrocoupling to reform 1 and give the aminoborane dimer [H2BNMe2]2. NMR studies on this system reveal an intermediate neutral hydride forms, Rh(PONOP)H, 4, that has been prepared independently. 1 is a competent catalyst (2 mol%, ∼30 min) for the dehydrocoupling of H3B·Me2H. Kinetic, mechanistic and computational studies point to the role of NMe2H in both forming the neutral hydride, via deprotonation of a σ-amine-borane complex and formation of aminoborane, and closing the catalytic cycle by reprotonation of the hydride by the thus-formed dimethyl ammonium [NMe2H2]+. Competitive processes involving the generation of boronium [H2B(NMe2H)2]+ are also discussed, but shown to be higher in energy. Off-cycle adducts between [NMe2H2]+ or [H2B(NMe2H)2]+ and amine-boranes are also discussed that act to modify the kinetics of dehydrocoupling

Secondary Phosphinocarbyne and Phosphaisonitrile Complexes

The palladium-mediated reaction of [W­(CBr)­(CO)₂(Tp*)] (Tp* = hydrotris­(3,5-dimethylpyrazol-1-yl)­borate) with primary phosphines PH₂R (R = Ph, Cy) affords the secondary phosphinocarbyne complexes [W­(CPHR)­(CO)₂(Tp*)], deprotonation of which provides the anionic phosphaisonitrile complexes [W­(CPR)­(CO)₂(Tp*)]⁻, including the structurally characterized salt [W­(CPPh)­(CO)₂(Tp*)]­[K­(kryptofix)]

Synthesis and Reactivity of Phosphinocarbyne Complexes

The successive treatment of [W­(CBr)­(CO)₂(Tp*)] (Tp* = hydrotris­(3,5-dimethylpyrazol-1-yl)­borate) with ^<i>n</i>BuLi and ClPPh₂ affords the phosphinocarbyne complex [W­(CPPh₂)­(CO)₂(Tp*)] (<b>1</b>), DFT interrogation of which suggests that reactions with electrophiles may involve both the phosphorus atom and/or the metal–carbon multiple bond. This is borne out in reactions of <b>1</b> with a variety of electrophilic reagents. With iodomethane or dimethylsulfide borane, electrophilic attack occurs exclusively at phosphorus to afford the compounds [W­(CPMePh₂)­(CO)₂(Tp*)]I ([<b>2</b>]­I) and [W­{CP­(BH₃)­Ph₂}­(CO)₂(Tp*)] (<b>3</b>). The reaction of <b>1</b> with sulfur affords both the thiophosphorylcarbyne complex [W­{CP­(S)­Ph₂}­(CO)₂(Tp*)] (<b>4</b>) and the thioacyl complex [W­{η²-SCP­(S)­Ph₂}­(CO)₂(Tp*)] (<b>5</b>), though <b>4</b> fails to react with sulfur to provide <b>5</b>. In a similar manner, the complexes <b>2</b> and <b>3</b> also fail to react with sulfur, indicating that increasing the valance of the phosphorus center of <b>1</b> deactivates the WC bond toward further attack. Addition of selenium to <b>1</b> occurs exclusively at phosphorus to afford [W­{CP­(Se)­Ph₂}­(CO)₂(Tp*)] (<b>6</b>) with no indication of selenoacyl formation. Reversible protonation of <b>1</b> with HBF₄ in diethyl ether precipitates the phosphoniocarbyne salt [W­(CPHPh₂)­(CO)₂(Tp*)]­BF₄, [<b>8</b>]­BF₄, which, however, on dissolution in dichloromethane rearranges irreversibly to the thermodynamic (Δ<i>G</i>_cacld = 22.4 kJmol^–1) phosphinocarbene isomer [W­(η²CHPPh₂)­(CO)₂(Tp*)]­BF₄, [<b>9</b>]­BF₄

Highly adaptive nature of group 15 tris(quinolyl) ligands─studies with coinage metals

The substitution of heavier, more metallic atoms into classical organic ligand frameworks provides an important strategy for tuning ligand properties, such as ligand bite and donor character, and is the basis for the emerging area of main-group supramolecular chemistry. In this paper, we explore two new ligands [E(2-Me-8-qy)3] [E = Sb (1), Bi (2); qy = quinolyl], allowing a fundamental comparison of their coordination behavior with classical tris(2-pyridyl) ligands of the type [E′(2-py)3] (E = a range of bridgehead atoms and groups, py = pyridyl). A range of new coordination modes to Cu+, Ag+, and Au+ is seen for 1 and 2, in the absence of steric constraints at the bridgehead and with their more remote N-donor atoms. A particular feature is the adaptive nature of these new ligands, with the ability to adjust coordination mode in response to the hard–soft character of coordinated metal ions, influenced also by the character of the bridgehead atom (Sb or Bi). These features can be seen in a comparison between [Cu2{Sb(2-Me-8-qy)3}2](PF6)2 (1·CuPF6) and [Cu{Bi(2-Me-8-qy)3}](PF6) (2·CuPF6), the first containing a dimeric cation in which 1 adopts an unprecedented intramolecular N,N,Sb-coordination mode while in the second, 2 adopts an unusual N,N,(π-)C coordination mode. In contrast, the previously reported analogous ligands [E(6-Me-2-py)3] (E = Sb, Bi; 2-py = 2-pyridyl) show a tris-chelating mode in their complexes with CuPF6, which is typical for the extensive tris(2-pyridyl) family with a range of metals. The greater polarity of the Bi–C bond in 2 results in ligand transfer reactions with Au(I). Although this reactivity is not in itself unusual, the characterization of several products by single-crystal X-ray diffraction provides snapshots of the ligand transfer reaction involved, with one of the products (the bimetallic complex [(BiCl){ClAu2(2-Me-8-qy)3}] (8)) containing a Au2Bi core in which the shortest Au → Bi donor–acceptor bond to date is observed

Fluoroarene Complexes with Small Bite Angle Bisphosphines:Routes to Amine–Borane and Aminoborylene Complexes

Fluoroarene complexes of the small bite angle bisphosphine Cy2PCH2PCy2 (dcpm) have been prepared: [Rh(dcpm)(η6-1,2-F2C6H4)][Al{OC(CF3)3}4] and [Rh(dcpm)(η6-1,2,3-F3C6H3)][Al{OC(CF3)3}4]. These complexes act as precursors to a previously inaccessible σ-amine–borane complex [Rh(dcpm)(η2-H3B·NMe3)][Al{OC(CF3)3}4] of a small bite-angle phosphine. This complex is a poor catalyst for the dehydrocoupling of H3B·NMe2H. Instead, formation of the bridging borylene complex [{RhH(µ-dcpm)}2(µ-H)(µ-BNMe2)][Al{OC(CF3)3}4] occurs, which has been studied by NMR, mass spectrometry, crystallographic and DFT techniques. This represents a new route to bridging borylene complexes.</p