Boris Tumanskii

Radical Reactions of a Stable N-Heterocyclic Germylene: EPR Study and DFT Calculation

The first radical adducts of a stable N-heterocyclic germylene were investigated. Novel radical species were produced from a variety of precursors and studied by EPR spectroscopy. DFT (B3LYP) calculations of radical adducts of different (C, Si, Ge) unsaturated N-heterocyclic divalent species with phenoxyl radical show that in the radicals studied the unpaired electron is delocalized over the five-membered ring and the spin density on the central atoms decreases in the following order: C > Si > Ge. These trends can be understood in terms of zwitterionic structure of radical adducts.

Bis(1,3-di-tert-butylimidazolin-2-iminato) Titanium Complexes as Effective Catalysts for the Monodisperse Polymerization of Propylene

The use of bis(1,3-di-tert-butylimidazolin-2-iminato) titanium dichloride (1) and dimethyl (2) complexes in the polymerization of propylene is presented. The complexes were activated using different amounts of methylalumoxane (MAO), giving in each case a very active catalytic mixture and producing polymers with a narrow molecular weight distribution (polydispersity = 1.10). The use of the cocatalyst triphenylcarbenium (trityl) tetra(pentafluorophenyl)borate totally inhibits the reaction, producing the corresponding bis(1,3-di-tert-butylimidazolin-2-iminato) titanium(III) methyl complex, the trityl radical ((•)CPh(3)), the anionic MeB(C(6)F(5))(4)(-), B(C(6)F(5))(3), and the bis(1,3-di-tert-butylimidazolin-2-iminato) titanium(IV) dimethyl·B(C(6)F(5))(3) complex. The use of a combination of physical methods such as NMR, ESR-C(60), and MALDI-TOF analyses enabled us to propose a plausible mechanism for the polymerization of propylene, presenting that the polymerization is mainly carried out in a living fashion. In addition, we present a slow equilibrium toward a small amount of a dormant species responsible for 2,1-misinsertions and chain transfer processes.

Effect of bromination on the electrochemistry, frontier orbitals, and spectroscopy of metallocorroles

A series of fully β-pyrrole brominated triarylcorrole metal complexes has been prepared for investigating the changes in visible spectra and redox potentials relative to the non-brominated derivatives, as well as for comparing the effect of bromination in corroles and porphyrins. The results reveal that bromination has a much larger effect on the electrochemistry of metallocorroles relative to metalloporphyrins, for both macrocycle- and metal-centered redox processes. The HOMO–LUMO gap energy of the triarylcorrole post-transition metal complexes decreases upon bromination because the effect on the LUMO is about twice as large of as on the HOMO; and both the HOMO and the LUMO are more affected in corroles than in porphyrins. Spectroscopic examinations of the transition metal complexes reveal that the synthetic access to divalent metallocorroles becomes feasible for the brominated derivatives.

Different electronic structure of phosphonyl radical adducts of N-heterocyclic carbenes, silylenes and germylenes: EPR spectroscopic study and DFT calculations

Stable N-heterocyclic carbenes and germylenes were allowed to react with a phosphonyl radical, (i-PrO)2(O)P˙ (7), generated by photolysis of [(i-PrO)2(O)P]2Hg. The products were identified by EPR spectroscopy. An unsaturated carbene (1) and germylene (3) react with 7 at the divalent atom to give unstable radical products (τ½ = 0.2 s). A benzo-annulated carbene (4) and a saturated germylene (6) react with 7 to give more active radicals. An unsaturated (2) and a saturated silylene (5) undergo rapid reaction (in the dark) with [(i-PrO)2(O)P]2Hg to yield unusual silyl phosphites. In these cases only secondary radicals were observed. DFT (PBE0/TZVP//B3LYP/6-31+G(d)) calculations of the radical adducts of the different (C, Si, Ge) unsaturated N-heterocyclic divalent species with the phosphonyl radical show that the unpaired electron is delocalized over the five-membered ring; the spin density on the central atoms decreases in the order C, 39% > Si, 14% > Ge, 2%. These trends can be understood in terms of a zwitterionic structure of the radical adducts. The calculations of the radical adducts of 4, 5 and 6 with 7 indicate larger spin density on the central atom, 47%, 58% and 42% on C, Si, Ge, respectively.

Design concept for α-hydrogen-substituted nitroxides

Stable nitroxides (nitroxyl radicals) have many essential and unique applications in chemistry, biology and medicine. However, the factors influencing their stability are still under investigation, and this hinders the design and development of new nitroxides. Nitroxides with tertiary alkyl groups are generally stable but obviously highly encumbered. In contrast, α-hydrogen-substituted nitroxides are generally inherently unstable and rapidly decompose. Herein, a novel, concept for the design of stable cyclic α-hydrogen nitroxides is described, and a proof-of-concept in the form of the facile synthesis and characterization of two diverse series of stable α-hydrogen nitroxides is presented. The stability of these unique α-hydrogen nitroxides is attributed to a combination of steric and stereoelectronic effects by which disproportionation is kinetically precluded. These stabilizing effects are achieved by the use of a nitroxide co-planar substituent in the γ-position of the backbone of the nitroxide. This premise is supported by a computational study, which provides insight into the disproportionation pathways of α-hydrogen nitroxides.

Homolytic reactive mass spectrometry of fullerenes: interaction of C60 and C70 with organo- and organoelement mercurials in the electron impact ion source of a mass spectrometer; EPR, CIDEP, and MS studies of several analogous reactions of C60 performed in solution.

Interaction of C(60) with organo- and organoelement mercurials (CF(3)HgBr, PhHgBr, p-CH(3)C(6)H(4)HgBr, p-CH(3)OC(6)H(4)HgCl, CF(3)HgPh, Ph(2)Hg, (o-carborane-9-yl)(2)Hg, (m-carborane-9-yl)(2)Hg, (p-carborane-9-yl)(2)Hg, and (m-carborane-9-yl)HgCl) in the ionization chamber (IC) of the electron impact (EI) ion source of a mass spectrometer at 250-300 degrees C results in the transfer of the corresponding organic or organoelement radicals from the mercurials to the fullerene. Some of the processes are accompanied by hydrogen addition. C(70) reacts with Ph(2)Hg and (o-carborane-9-yl)(2)Hg at 300 degrees C in a similar fashion. A homolytic reaction path is considered for the reactions. It suggests both the thermal and EI initiated homolytic dissociation of the mercurials to the intermediate organic or organoelement radicals followed by their interaction with the fullerenes at the metallic walls of the IC. When EI is involved, the dissociation is supposed to occur via superexcited states (the excited states with the electronic excitation energies higher than the first ionization potentials) of the mercury reagents, with possible contribution of the process proceeding via their molecular ions. In line with the results obtained in the IC, C(60) reacts with Ph(2)Hg and (o-carborane-9-yl)(2)Hg under UV-irradiation in benzene and toluene solutions to furnish phenyl and carboranyl derivatives of the fullerene, respectively, some also containing the acquired hydrogen atoms. EPR monitoring of the processes has shown the formation of phenylfullerenyl and o-carborane-9-yl-fullerenyl radicals. g-Factors and hyperfine coupling (hfc) constants with (10)B, (11)B, and (13)C nuclei of both the latter and m-carborane-9-yl-fullerenyl radical formed in the reaction of C(60) with (m-carborane-9-yl)(2)Hg have been determined by the special EPR studies. The unusually great chemically induced dynamic electron polarization (CIDEP) of the latter radical where even the (13)C satellite lines are polarized has been observed and is discussed in terms of both radical-triplet-pair and radical-pair mechanisms. The similar CIDEP effect is also intrinsic to the o-carborane-9-yl-fullerenyl radical obtained under the same conditions. The analogous transfer of the carboranyl radicals from (o-carborane-9-yl)(2)Hg to C(60) occurs when their mixture is boiled in (t)BuPh for 10-15 h.

Iron complexes of tris(4-nitrophenyl)corrole, with emphasis on the (nitrosyl)iron complex

The iron complexes of 5,10,15-tris(4-nitrophenyl)corrole have been prepared and characterized by various spectroscopic techniques. The (nitrosyl)iron complex is diamagnetic and its X-ray structure reveals an almost perfectly linear Fe–N–O bond. EPR spectroscopy in conjunction with 15N labelling were used to deduce the redox centre of the one-electron reduction and oxidation products of the (nitrosyl)iron corrole.

Observation of a Thermally Accessible Triplet State Resulting from Rotation around a Main‐Group π Bond

We report the first direct spectroscopic observation by electron paramagnetic resonance (EPR) spectroscopy of a triplet diradical that is formed in a thermally induced rotation around a main-group πu2005bond, that is, the Siuf8feSi double bond of tetrakis(di-tert-butylmethylsilyl)disilene (1). The highly twisted ground-state geometry of singlet 1 allows access to the perpendicular triplet diradicalu20052 at moderate temperatures of 350-410u2005K. DFT-calculated zero-field splitting (ZFS) parameters of 2 accurately reproduce the experimentally observed half-field transition. Experiment and theory suggest a thermal equilibrium between 1 and 2 with a very low singlet-triplet energy gap of only 7.3u2005kcalu2009mol(-1) .

Organoactinides in the polymerization of ethylene: is TIBA a better cocatalyst than MAO?

The synthesis of two pyridylamidinate bis(N,N-bis(trimethylsilyl)-2-pyridylamidinate)An(μ-Cl)2Li(TMEDA) (An = U (1), Th (2)) complexes is presented. For complex 1 the solid state X-ray structures were studied and compared to that of complex 2. The organoactinide complexes were studied as pre-catalysts in the polymerization of ethylene when activated by methylalumoxane (MAO). The catalytic activity was improved using a mixture of trityl tetrakispentafluorophenylborate (TTPB) and a small amount of methylalumoxane (MAO) as cocatalysts, and was amazingly improved, providing the greatest activity, using only triisobutyl aluminum (TIBA). We present a combination of ESR, C60 radical trapping, and MALDI-TOF studies describing the formation of the single-site active species, capturing some unique features of the complexes and shedding light on the polymerization mechanism.

Synthesis, Isolation, and Characterization of 1,1‐DiGrignard and 1,1‐Dizincio Silanes

Geminal dimetallosilanes have already proved their great potential for the synthesis of novel silicon compounds. However, the variety and number of isolated geminal dimetallosilanes is very small. For example, all known geminal dimetallosilanes are alkali metal or mercury derivatives. Furthermore, only one example of a geminal dimetallosilane with two different metals, that is, a mercury bridged bis(silyllithium) species, was reported recently by our research group. This contrasts with the large variety of available geminal dimetallorganic reagents, which are very useful synthons in organic synthesis. In particular, diGrignard reagents and dizinc reagents are widely used in organic chemistry. In contrast, silicon analogues, (i.e. geminal diGrignard or dizincio silanes) are yet to be reported. The vast synthetic potential of such reagents remains to be explored. Herein we report the synthesis, isolation, and X-ray molecular structure of the first cyclic 1,1-dimagnesiosilane 1, the first diGrignard silane 1,1-di(chloromagnesio)silane 2, and the analogous 1,1-di(chlorozincio)silane 3. We also report the selective redox reactions of 1 with tBu2MeSiLi and with 1,1-dilithiosilane 4, thus leading to novel metallosilane species. We initially believed that geminal dimetallosilanes could be prepared by a transmetalation reaction of dilithiosilane derivatives with metal salts such as MgX2 or ZnX2, in analogy to the synthesis of metallosilanes. However, transmetalation reactions with strong electron donors often involve redox processes that lead to nonselective reactions. This is the case also with (tBuMe2Si)2SiLi2 (4). Thus, reaction of 4 with MX2 (MX2=MgCl2, MgBr2 ZnCl2, HgCl2, HgF2), leads to a complex mixture of products and a metallic M residue. In contrast, mixing 1,1-dilithiosilane 4with 1.5 equivalents of tBuMgCl·2MgCl2 (5) in THF [10] at 0 8C gave, after stirring for 30 minutes, cyclic 1,1-dimagnesiosilane 1 [Eq. (1); THF= tetrahydrofuran]. Tetrahydrofuran was