Ryan J. Schwamm

Isolation and Characterization of a Bismuth(II) Radical

More than 80 years after Paneths report of dimethyl bismuth, the first monomeric Bi(II) radical that is stable in the solid state has been isolated and characterized. Reduction of the diamidobismuth(III) chloride Bi(NON(Ar))Cl (NON(Ar)=[O(SiMe2NAr)2](2-); Ar=2,6-iPr2C6H3) with magnesium affords the Bi(II) radical ˙Bi(NON(Ar)). X-ray crystallographic measurements are consistent with a two-coordinate bismuth in the +2 oxidation state with no short intermolecular contacts, and solid-state SQUID magnetic measurements indicate a paramagnetic compound with a single unpaired electron. EPR and density functional calculations show a metal-centered radical with >90% spin density in a p-type orbital on bismuth.

Low-Coordinate Bismuth Cations

Chloride abstraction from the diamido-bismuth compound Bi(Me2Si{NAr}2)Cl (1, Ar = 2,6-i-Pr2C6H3) using MCl3 (M = Al, Ga) is a facile route to cationic species. Stoichiometric reactions afford the tetrachlorometallate salts [Bi(Me2Si{NAr}2)][MCl4] (2a, M = Al; 3a, M = Ga), whereas reaction with 0.5 equiv of the group 13 reagent gives the μ-chlorido bridged cations [{Bi(Me2Si{NAr}2)}2(μ-Cl)][MCl4] (2b, M = Al; 3b, M = Ga). The crystal structure of 2a shows a formally two-coordinate bismuth cation, with a Bi···Cl contact to the [AlCl4](-) anion, whereas the structure of 3b shows a total of three Bi···Cl contacts to [GaCl4](-). Both species associate as {1:1}2 dimers in the solid state through additional Bi···Cl interactions. Attempted preparation of cationic complexes using either NaBR4 (R = Ph, Et) or [HNEt3][BPh4] were unsuccessful. Instead of forming the borate salts, the neutral compounds Bi(Me2Si{NAr}2)R (4, R = Et; 5, R = Ph) were isolated as a result of aryl/alkyl transfer from boron to bismuth.

15N NMR Spectroscopy, X-ray and Neutron Diffraction, Quantum-Chemical Calculations, and UV/vis-Spectrophotometric Titrations as Complementary Techniques for the Analysis of Pyridine-Supported Bicyclic Guanidine Superbases

Pyridine substituted with one and two bicyclic guanidine groups has been studied as a potential source of superbases. 2-{hpp}C5H4N (I) and 2,6-{hpp}2C5H3N (II) (hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) were protonated using [HNEt3][BPh4] to afford [I-H][BPh4] (1a), [II-H][BPh4] (2), and [II-H2][BPh4]2 (3). Solution-state (1)H and (15)N NMR spectroscopy shows a symmetrical cation in 2, indicating a facile proton-exchange process in solution. Solid-state (15)N NMR data differentiates between the two groups, indicating a mixed guanidine/guanidinium. X-ray diffraction data are consistent with protonation at the imine nitrogen, confirmed for 1a by single-crystal neutron diffraction. The crystal structure of 1a shows association of two [I-H](+) cations within a cage of [BPh4](-) anions. Computational analysis performed in the gas phase and in MeCN solution shows that the free energy barrier to transfer a proton between imino centers in [II-H](+) is 1 order of magnitude lower in MeCN than in the gas phase. The results provide evidence that linking hpp groups with the pyridyl group stabilizes the protonation center, thereby increasing the intrinsic basicity in the gas phase, while the bulk prevents efficient cation solvation, resulting in diminished pKa(MeCN) values. Spectrophotometrically measured pKa values are in excellent agreement with calculated values and confirm that I and II are superbases in solution.

Bi-P Bond Homolysis as a Route to Reduced Bismuth Compounds and Reversible Activation of P4.

Bismuth diphenylphosphanides Bi(NONR )(PPh2 ) (NONR =[O(SiMe2 NR)2 ], R=tBu, 2,6-iPr2 C6 H3 , Aryl) undergo facile decomposition via single-electron processes to form reduced Bi and P species. The corresponding derivatives Bi(NONR )(PCy2 ) are stable. Reaction of the isolated BiII radical . Bi(NONAr ) with white phosphorus (P4 ) proceeds with the reversible and selective activation of a single P-P bond to afford the bimetallic μ,η1:1 -bicyclo[1.1.0]tetraphosphabutane compound.

Indyllithium and the Indyl Anion [InL]−: Heavy Analogues of N‐Heterocyclic Carbenes

Reduction of the indate complex In(NONAr )(μ-Cl)2 Li(OEt2 )2 (NONAr =[O(SiMe2 NAr)2 ]2- ; Ar=2,6-iPr2 C6 H3 ) with sodium generates the InII diindane species [In(NONAr )]2 . Further reduction with a mixture of potassium and [2.2.2]crypt affords the InI N-heterocyclic indyl anion [In(NONAr )]- , which crystallizes with a non-contacted [K([2.2.2]crypt)]+ cation. The indyl anion can also be isolated as the indyllithium compound In(NONAr )(Li{THF}3 ), which contains an In-Li bond. Density functional theory calculations show that the HOMO of the indyl anion is a metal-centred lone pair, and preliminary reactivity studies confirm its nucleophilic behaviour.

Bismuth(III) Complex of the [S4]•– Radical Anion: Dimer Formation via Pancake Bonds

Reaction of bismuth(II) compounds with sulfur gives mixtures of [Bi(NONR)]2(μ2-Sn) (NONR = [O(SiMe2NR)2]2-). Examples for n = 1 and 3 have been crystallographically verified for R = 2,6-iPr2C6H3 (Dipp) and R = tBu, and the pentasulfide (n = 5) for R = Dipp. The corresponding product from reaction with the new Bi(II) radical Bi(NONAr‡)• (Ar‡ = C6H2(CHPh2)2-tBu-2,6,4) exists as the dimer [Bi(NONAr‡)(S4)]2, with π*(SOMO)-π*(SOMO) interactions linking the sulfur chains through trans-antarafacial pancake bonds.

CCDC 1588419: Experimental Crystal Structure Determination

Related Article: Ryan J. Schwamm, Matthias Lein, Martyn P. Coles, Christopher M. Fitchett|2017|J.Am.Chem.Soc.|139|16490|doi:10.1021/jacs.7b10454

CCDC 1588414: Experimental Crystal Structure Determination

Related Article: Ryan J. Schwamm, Matthias Lein, Martyn P. Coles, Christopher M. Fitchett|2017|J.Am.Chem.Soc.|139|16490|doi:10.1021/jacs.7b10454

CCDC 1588418: Experimental Crystal Structure Determination

Related Article: Ryan J. Schwamm, Matthias Lein, Martyn P. Coles, Christopher M. Fitchett|2017|J.Am.Chem.Soc.|139|16490|doi:10.1021/jacs.7b10454

CCDC 1588411: Experimental Crystal Structure Determination

Related Article: Ryan J. Schwamm, Matthias Lein, Martyn P. Coles, Christopher M. Fitchett|2017|J.Am.Chem.Soc.|139|16490|doi:10.1021/jacs.7b10454