ABSTRACT
The classical route to the PMe3-stabilised polycyclic aromatic hydrocarbon (PAH)-substituted diborenes B2Ar2(PMe3)2 (Ar = 9-phenanthryl 7-Phen; Ar = 1-pyrenyl 7-Pyr) via the corresponding 1,2-diaryl-1,2-dimethoxydiborane(4) precursors, B2Ar2(OMe)2, is marred by the systematic decomposition of the latter to BAr(OMe)2 during reaction workup. Calculations suggest this results from the absence of a second ortho-substituent on the boron-bound aryl rings, which enables their free rotation and exposes the B-B bond to nucleophilic attack. 7-Phen and 7-Pyr are obtained by the reduction of the corresponding 1,2-diaryl-1,2-dichlorodiborane precursors, B2Ar2Cl2(PMe3)2, obtained from the SMe2 adducts, which are synthesised by direct NMe2-Cl exchange at B2Ar2(NMe2)2 with (Me2S)BCl3. The low-lying π* molecular orbitals (MOs) located on the PAH substituents of 7-Phen and 7-Pyr intercalate between the B-B-based π and π* MOs, leading to a relatively small HOMO-LUMO gap of 3.20 and 2.72 eV, respectively. Under vacuum or at high temperature 7-Phen and 7-Pyr undergo intramolecular hydroarylation of the B[double bond, length as m-dash]B bond to yield 1,2-dihydronaphtho[1,8-cd][1,2]diborole derivatives. Hydrogenation of 7-Phen, 7-Pyr and their 9-anthryl and mesityl analogues III and II, respectively, results in all cases in splitting of the B-B bond and isolation of the monoboranes (Me3P)BArH2. NMR-spectroscopic monitoring of the reactions, solid-state structures of isolated reaction intermediates and computational mechanistic analyses show that the hydrogenation of the three PAH-substituted diborenes proceeds via a different pathway to that of the dimesityldiborene. Rather than occurring exclusively at the B-B bond, hydrogenation of 7-Ar and III proceeds via a hydroarylated intermediate, which undergoes one B-B bond-centered H2 addition, followed by hydrogenation of the endocyclic B-C bond resulting from hydroarylation, making the latter effectively reversible.
ABSTRACT
We report that the use of a diaminoalkyne in the azide-alkyne cycloaddition with aryl azides leads to 3H-indoles under mild, uncatalysed conditions. Computations reveal that N2 extrusion from, in one case, isolable triazoles is facile, generating imino carbenes, which undergo intramolecular aryl C-H bond activation and give 3H-indoles as products.
ABSTRACT
The 1:2 reaction of [µ-(dmpm)Pt(nbe)]2 (dmpm=bis(dimethylphosphino)methane, nbe=norbornene) with Cl2 BNR(SiMe3 ) (R=tBu, SiMe3 ) yields unsymmetrical (N-aminoboryl)aminoboryl PtI 2 complexes by B-N coupling via ClSiMe3 elimination. A subsequent intramolecular ClSiMe3 elimination from the tBu-derivative leads to cyclization of the BNBN unit, forming a unique 1,3,2,4-diazadiboretidin-2-yl ligand. In contrast, the analogous reaction with Br2 BN(SiMe3 )2 leads, via a twofold BrSiMe3 elimination, to a PtII 2 A-frame complex bridged by a linear BNBN isostere of butatriene. Structural and computational data confirm πâ electron delocalization over the entire BNBN unit.
ABSTRACT
A series of boranediyl-bridged diplatinum A-frame complexes, [Pt2X2(µ-BY)(µ-dmpm)2] (X = Cl, Br, I; Y = aryl, alkyl, amino, halo; dmpm = bis(dimethylphosphino)methane), were synthesised by the twofold oxidative addition of BX2Y to [Pt2(nbe)2(µ-dmpm)2] (nbe = norbornene) or to the paddlewheel complex [Pt2(µ-dmpm)3]. Similarly, the addition of B2X2(NMe2)2 (X = Cl, Br) to [Pt2(nbe)2(µ-dmpm)2] provided access to the diborane-1,2-diyl-bridged A-frame complexes [Pt2X2(µ-1,2-B2(NMe2)2)(µ-dmpm)2]. X-ray crystallographic studies of these (BY)n-bridged complexes show structural trends depending on the steric demands of Y and the nature of X. Analysis of higher-order 31P NMR satellites provided information on JP-Pt and JPt-Pt coupling constants, the latter correlating with the PtPt distance. All (di)boranediyl complexes also proved unstable towards (successive) loss of the bridging "BY" unit(s), resulting in the formation of [Pt2X2(µ-dmpm)2].
ABSTRACT
Diplatinum A-frame complexes with a bridging (di)boron unit in the apex position were synthesized in a single step by the double oxidative addition of dihalo(di)borane precursors at a bis(diphosphine)-bridged Pt0 2 complex. While structurally analogous to well-known µ-borylene complexes, in which delocalized dative three-center-two-electron M-B-M bonding prevails, theoretical investigations into the nature of Pt-B bonding in these A-frame complexes show them to be rare dimetalla(di)boranes displaying two electron-sharing Pt-B σ-bonds. This is experimentally reflected in the low kinetic stability of these compounds, which are prone to loss of the (di)boron bridgehead unit.
ABSTRACT
The reaction of the tetrahalodiboranes(4) B2F4, B2Cl4, and B2Br4 with a Lewis basic platinum(0) complex led to the isolation of the cis-bis(difluoroboryl) complex cis-[(Cy3P)2Pt(BF2)2] (1) and the novel borylborato complexes trans-[(Cy3P)2Pt{B(X)-BX3}] (2, X = Cl; 3, X = Br), respectively. The trans influence of the borylborato group was found to be one of the strongest ever observed experimentally. Furthermore, the reactivity of little-explored diaryldifluorodiboranes(4) F2B-BMes2 and the new derivative F2B-BAn2 (An = 9-anthryl) toward a range of platinum(0) complexes was investigated. Reactions with relatively nonbulky platinum(0) complexes led to the formation of unsymmetrical cis-bis(boryl) complexes cis-[(R3P)2Pt(BF2)(BMes2)] (6, R = Me; 7, R = Et) as well as the first example of a fourfold-unsymmetrical bis(boryl) complex, [(Me3P)(Cy3P)Pt(BF2)(BMes2)] (12). The use of a more bulky Pt complex provided access to the unprecedented dinuclear bis(boryl) complexes [{( iPr3P)Pt}2(µ-BF2)(µ-BAr2)] (8, Ar = Mes; 9, Ar = An), which feature two different µ2-bridging boryl ligands.
ABSTRACT
A number of zerovalent ruthenium tri- and tetracarbonyl complexes of the form [Ru(CO)5-n Ln] (n=1, 2) with neutral phosphine or N-heterocyclic carbene donor ligands have been treated with the Lewis acids GaCl3 and Ag(+) to form a range of metal-only Lewis pairs (MOLPs). The spectroscopic and structural parameters of the adducts are compared to each other and to related iron carbonyl based MOLPs. The Lewis basicity of the original Ru(0) complexes is gauged by transfer experiments, as well as through the degree of pyramidization of the bound GaCl3 units and the Ru-M bond lengths. The work shows the benefits of the MOLP concept as one of the few direct experimental gauges of metal basicity, and one that can allow comparisons between metal complexes with different metal centers and ligand sets.
