René T. Boeré

Preparation of N,N,N′-tris(trimethylsilyl)amidines; a convenient route to unsubstituted amidines

The tris(trimethylsilyl)amidines RC(NSiMe3)N(SiMe3)2 (R  C6H5, p-CH3C6-H4, p-ClC6H4, p-MeOC6H4, p-Me2NC6H4, p-CF3C6H4, p-C6H5C6H4 and CF3) are prepared by the reaction of the respective nitriles with (Me3Si)2NLi·OEt2 in ether to give intermediates RC(NLi)N(SiMe3)2. Heating these intermediates with ClSiMe3 in toluene affords the products, which are isolated by vacuum distillation, in high yield. With 1,4-dicyanobenzene, two equivalents of reagents affords the per(trimethylsilyl)-1,4-diamidine. Hydrolysis of the intermediates with 6N ethanolic HCl affords the unsubstituted amidine hydrochlorides RC(NH)NH2·HCl (R  C6-H5, p-MeOCh6H4, p-ClC6H4, p-O2NC6H4) in high yield.

Electrochemistry of redox-active Group 15/16 heterocyles

Abstract This review is a critical overview of the literature on electrochemistry of unsaturated ring-compounds containing bonds between Group 15 and Group 16 elements (principally S and N), including metallacycles. It contains a detailed compilation of measured redox potentials and the conditions under which these were obtained for over 200 compounds. An introduction to the application of solution electrochemistry to this class of compounds is provided, and the relevance of such measurements to materials design is discussed.

Rotational Correlation Times in Nuclear Magnetic Relaxation

Publisher Summary This chapter focuses on rotational correlation times in nuclear magnetic relaxation. The rotational motion of molecules in the gas phase is well represented by the equipartition of energy theorem and the laws of statistical mechanics, both of which are premised on the view that, except during instantaneous collisions, the molecules move independently of one another. Between collisions, the angular velocity is uniform and is determined solely by the molecules moment of inertia and the temperature. For most molecules in the liquid phase, the rotation rate depends upon the viscosity and is independent of the moment of inertia, indicating that intermolecular frictional forces rather than inertial factors are paramount. During its brief lifespan, nuclear magnetic resonance (NMR) spectroscopy has quickly become the most powerful method for investigating the intimate details of this relationship.

The syntheses and structures of some main group complexes of the sterically hindered N,N′-bis(2,6-diisopropylphenyl)-4-toluamidinate ligand

The stoichiometric reaction of the bulky benzamidine N,N′-bis(2,6-diisopropylphenyl)-4-toluamidine (HDippAm) with the metal alkyls BunLi (1∶1 in THF), Bu2Mg (2∶1 in THF) and Me3Al (1∶1 in Et2O) is presented. This provides the mononuclear dihapto benzamidinate compounds [Li(DippAm)(THF)2] (1), [Mg(DippAm)2] (2) and [Al(DippAm)Me2] (3), respectively. Compound 3 was also obtained by salt elimination using dimethylaluminium chloride and 1. All three compounds exhibit sterically strained geometries that are maintained in solution at increased temperatures. Compound 3 displays exceptional thermal and aerobic stability, while 2 constitutes a rare example of non-porphyrin supported square planar magnesium.

On the Oxidation of the Three‐Dimensional Aromatics [B12X12]2− (X=F, Cl, Br, I)

The perhalogenated closo-dodecaborate dianions [B12 X12 ](2-) (X=H, F, Cl, Br, I) are three-dimensional counterparts to the two-dimensional aromatics C6 X6 (X=H, F, Cl, Br, I). Whereas oxidation of the parent compounds [B12 H12 ](2-) and benzene does not lead to isolable radicals, the perhalogenated analogues can be oxidized by chemical or electrochemical methods to give stable radicals. The chemical oxidation of the closo-dodecaborate dianions [B12 X12 ](2-) with the strong oxidizer AsF5 in liquid sulfur dioxide (lSO2 ) yielded the corresponding radical anions [B12 X12 ](⋅-) (X=F, Cl, Br). The presence of radical ions was proven by EPR and UV/Vis spectroscopy and supported by quantum chemical calculations. Use of an excess amount of the oxidizing agent allowed the synthesis of the neutral perhalogenated hypercloso-boranes B12 X12 (X=Cl, Br). These compounds were characterized by single-crystal X-ray diffraction of dark blue B12 Cl12 and [Na(SO2 )6 ][B12 Br12 ]⋅B12 Br12 . Sublimation of the crude reaction products that contained B12 X12 (X=Cl, Br) resulted in pure dark blue B12 Cl12 or decomposition to red B9 Br9 , respectively. The energetics of the oxidation processes in the gas phase were calculated by DFT methods at the PBE0/def2-TZVPP level of theory. They revealed the trend of increasing ionization potentials of the [B12 X12 ](2-) dianions by going from fluorine to bromine as halogen substituent. The oxidation of all [B12 X12 ](2-) dianions was also studied in the gas phase by mass spectrometry in an ion trap. The electrochemical oxidation of the closo-dodecaborate dianions [B12 X12 ](2-) (X=F, Cl, Br, I) by cyclic and Osteryoung square-wave voltammetry in liquid sulfur dioxide or acetonitrile showed very good agreement with quantum chemical calculations in the gas phase. For [B12 X12 ](2-) (X=F, Cl, Br) the first and second oxidation processes are detected. Whereas the first process is quasi-reversible (with oxidation potentials in the range between +1.68 and +2.29 V (lSO2 , versus ferrocene/ferrocenium (Fc(0/+) ))), the second process is irreversible (with oxidation potentials ranging from +2.63 to +2.71 V (lSO2 , versus Fc(0/+) )). [B12 I12 ](2-) showed a complex oxidation behavior in cyclic voltammetry experiments, presumably owing to decomposition of the cluster anion under release of iodide, which also explains the failure to isolate the respective radical by chemical oxidation.

Synthesis, Characterization, and Electrochemical Studies of PPh3–n(dipp)n (dipp = 2,6-Diisopropylphenyl): Steric and Electronic Effects on the Chemical and Electrochemical Oxidation of a Homologous Series of Triarylphosphines and the Reactivities of the Corresponding Phosphoniumyl Radical Cations

Activation barriers to the electrochemical oxidation for the series PPh3-n(dipp)n (dipp = 2,6-diisopropylphenyl) in CH2Cl2/Bu4NPF6 were measured using large amplitude FT ac voltammetry. Increasing substitution across this series, which offers the widest range of steric requirements across any analogous series of triarylphosphines reported to date, increases the energetic barrier to electron transfer; values of 18, 24, and 25 kJ mol(-1) were found for compounds with n = 1, 2, and 3, respectively. These values are significantly greater than those calculated for outer sphere activation barriers, with deviations between observed and calculated values increasing with the number of dipp ligands. This suggests that the steric congestion afforded by these bulky substituents imposes significant reorganizational energy on the electron transfer processes. This is the first investigation of the effect of sterics on the kinetics of heterogeneous electron transfer across a structurally homologous series. Increased alkyl substitution across the series also increases the chemical reversibility of the oxidations and decreases the oxidation peak potentials. As the compounds for which n = 1 and 2 are novel, the synthetic strategies employed in their preparation are described, along with their full spectroscopic, physical, and crystallographic characterization. Optimal synthesis when n = 1 is via a Grignard reagent, whereas when n = 2 an aryl copper reagent must be employed, as use of a Grignard results in reductive coupling. Chemical oxidation studies were performed to augment the electrochemical work; the O, S, and Se oxidation products for the parent triarylphosphines for which n = 1 and 2 were isolated and characterized.

Quantum‐Chemical and Electrochemical Investigation of the Electrochemical Windows of Halogenated Carborate Anions

The range of electrochemical stability of a series of weakly coordinating halogenated (Hal=F, Cl, Br, I) 1-carba-closo-dodecaborate anions, [1-R-CB(11)X(5)Y(6)](-) (R=H, Me; X=H, Hal, Me; Y=Hal), has been established by using quantum chemical calculations and electrochemical methods. The structures of the neutral and dianionic radicals, as well as the anions, have been optimized by using DFT calculations at the PBE0/def2-TZVPP level. The calculated structures are in good agreement with existing experimental data and with previous calculations. Their gas-phase ionization energies and electron affinities were calculated based on their optimized structures and were compared with experimental (cyclic and square-wave) voltammetry data. Electrochemical oxidation was performed in MeCN at room temperature and in liquid sulfur dioxide at lower temperatures. All of the anions show a very high resistance to the onset of oxidation (2.15-2.85 V versus Fc(0/+)), with only a minor dependence of the oxidation potential on the different halogen substituents. In contrast, the reduction potentials in MeCN are strongly substituent dependent (-1.93 to -3.32 V versus Fc(0/+)). The calculated ionization energies and electron affinities correlate well with the experimental redox potentials, which provide important verification of the thermodynamic validity of the mostly irreversible redox processes that are observed for this series. The large electrochemical windows that are afforded by these anions indicate their suitability for electrochemical applications, for example, as supporting electrolytes.

Photophysical, dynamic and redox behavior of tris(2,6-diisopropylphenyl)phosphine

The title phosphine, Dipp3P, was synthesized using an aryl copper reagent and the structure determined by X-ray crystallography (R = 2.94%): d(P–C) = 1.852(1) A, ∠C–P–C = 111.88(5)°. In hexane solution, the electronic spectrum displays 3 bands [326 (9.3), 254 (8.7), 205 (11.4) nm (log|e|)] and the fluorescence spectrum has a Stokes shift of 129 kJ mol−1. NMR: (δ) 31P = −49.7 ppm in solution and −49.5 in the solid (CP-MAS). Room temperature 1H and 13C spectra reflect D3 symmetry, changing below −30 °C to C3. A variable temperature NMR study provided an activation enthalpy of 49(±1) kJ mol−1 and entropy of 24–27(±5) J mol−1K−1. An energy surface calculation using HF/3-21G theory discovered a single low-energy path describing pyramidal inversion through a transition state that is close to D3 geometry. The B3LYP/6-31G(d) calculated barrier to planarization is 37.5 kJ mol−1. Voltammetric studies employing cyclic, rotating disk, steady state and Fourier Transform ac methods confirm a fully chemically reversible one-electron oxidation of Dipp3P to Dipp3P+˙ at +0.18 (CH3CN–nBu4NPF6) and +0.09 (CH2Cl2–nBu4NPF6) V vs. Fc+/0 (Fc = ferrocene). The diffusion coefficient for Dipp3P is 1.0–1.2 × 105 cm2 s−1. The electrode process displays quasi-reversible electron transfer kinetics [ks ≈ 0.01 (CH2Cl2) to 0.08 (CH3CN) cm s−1]. Optically transparent thin layer electrolysis reversibly generates Dipp3P+˙ in CH2Cl2–nBu4NPF6 [UV-Vis: 498 (3.31), 456 (3.29), 373 (4.04), 357 (3.84), 341 (3.49), 296 (3.78), 385 (3.91), 251 (3.99) nm (log|e|)]. The EPR spectrum of Dipp3P+˙ in solution is a doublet (a(P) = 23.9 mT, g = 2.008), and in frozen solution is axial (a∥ = 42.6 mT, g∥ = 2.0045; a⊥ = 12.7 mT, g⊥ = 2.0085 mT).

An N,P-disubstituted-2-aminophosphaalkene and lithium and potassium complexes of the deprotonated “phosphaamidinate” anion

The reaction of DippPH2(Dipp = 2,6-iPr2C6H3) with DippN=C(p-CH3C6H4)Cl in refluxing xylenes affords DippP=C(p-CH3C6H4)N(H)Dipp; deprotonation with alkali metal reagents produces unique lithium and potassium complexes with the ligand in a different geometry to that of the free phosphaamidine.

η1 and η2 complexes of λ3-1,2,4,6-thiatriazinyls with CpCr(CO)x

The title heterocyclic radicals coordinate to either 17e CpCr(CO)3 or 15e CpCr(CO)2 moieties as one-electron or as three-electron donors, respectively; in the former the bonding is via the perpendicular p orbital of the sulfur atom, while in the latter bonding is via p(pi) orbitals on both sulfur and nitrogen.