Generation and Identification of the Trifluorosilylarsinidene F3SiAs and Isomeric Perfluorinated Arsasilene FAsSiF2.

The trifluorosilylarsinidene F3SiAs in the triplet ground state has been generated through the reaction of laser-ablated silicon atoms with AsF3 in cryogenic Ne- and Ar-matrices. The reactions proceed with the initial formation of perfluorinated arsasilene FAsSiF2 in the singlet ground state by two As-F bonds insertion reaction on annealing. The trifluorosilylarsinidene F3SiAs was formed via F-migration reactions of FAsSiF2 under irradiation at UV light (λ = 275 nm). The characterization of FAsSiF2 and F3SiAs by IR matrix-isolation spectroscopy is supported by computations at CCSD(T)-F12/aug-cc-pVTZ and B3LYP/aug-cc-pVTZ levels of theory.

Photoinduced Dual C-F Bond Activation of Hexafluorobenzene Mediated by Boron Atom.

The reaction of laser-ablated boron atoms with hexafluorobenzene (C6 F6 ) was investigated in neon and argon matrices, and the products are identified by matrix isolation infrared spectroscopy and quantum-chemical calculations. The reaction is triggered by a boron atom insertion into one C-F bond of hexafluorobenzene on annealing, forming a fluoropentafluorophenyl boryl radical (A). UV-Vis light irradiation of fluoropentafluorophenyl boryl radical causes generation of a 2-difluoroboryl-tetrafluorophenyl radical (B) via a second C-F bond activation. A perfluoroborepinyl radical (C) is also observed upon deposition and under UV-Vis light irradiation. This finding reveals the new example of a dual C-F bond activation of hexafluorobenzene mediated by a nonmetal and provides a possible route for synthesis of new perfluorinated organo-boron compounds.

Spectroscopic Identification of Trifluorosilylphosphinidene and Isomeric Phosphasilene and Silicon Trifluorophosphine Complex.

The perfluorinated silylphosphinidene, F3SiP, in the triplet ground state is generated by the reaction of laser-ablated silicon atoms with PF3 in solid neon and argon matrices. The reactions proceed with the initial formation of a silicon trifluorophosphine complex, F3PSi, in the triplet ground state, and a more stable inserted phosphasilene, FPSiF2, in the singlet ground state upon deposition. The trifluorosilylphosphinidene was formed through F-migration reactions of FPSiF2 and F3PSi following a two-state mechanism under irradiation with visible light (λ = 470 nm) and full arc light (λ > 220 nm), respectively. High-level quantum-chemical methods support the identification of F3PSi, FPSiF2, and F3SiP by matrix-isolation IR spectroscopy.

Infrared Spectroscopic and Theoretical Investigations of Group 13 Oxyfluorides OMF2 and OMF (M=B, Al, Ga, In).

Group 13 oxyfluorides OMF2 were produced by the reactions of laser-ablated group 13 atoms M (M=B, Al, Ga and In) with OF2 and isolated in excess neon or argon matrices at 5 K. These molecules were characterized by matrix-isolation infrared spectroscopy and isotopic substitution experiments in conjunction with quantum-chemical calculations. The calculations indicate that the OMF2 molecules have a 2 B2 ground state with C2v symmetry. The computed molecular orbitals and spin densities show that the unpaired electron is mainly located at the terminal oxygen atom. Oxo monofluorides OMF were only observed in solid argon matrices and exhibit a linear structure in the singlet ground state. The M-O bonding in the OMF molecules can be rationalized as highly polar multiple bonds based on the calculated bond lengths and natural resonance theory (NRT) analyses. In particular, the molecular orbitals of OBF exhibit the character of a triple bond B-O resulting from two degenerate electron-sharing π bonds and an O→B dative σ bond formed by the oxygen 2p lone pair which donates electron density to the boron empty 2p orbital.

CO-Induced Dinitrogen Fixation and Cleavage Mediated by Boron.

The boron atoms react with carbon monoxide and dinitrogen forming the end-on bonded NNBCO complex in solid neon or in nitrogen matrices. The NNBCO complex rearranges to the (η2 -N2 )BCO isomer with a more activated side-on bonded dinitrogen ligand upon visible light excitation. (η2 -N2 )BCO and its weakly CO-coordinated complexes further isomerize to the NBNCO and B(NCO)2 molecules with N-N bond being completely cleaved under UV light irradiation. The geometries, energies and vibrational spectra of the molecules are calculated with quantum chemical methods and the electronic structures are analyzed with charge- and energy-partitioning methods.

Generation and Identification of the Linear OCBNO and OBNCO Molecules with 24 Valence Electrons.

Two structural isomers containing five second-row element atoms with 24 valence electrons were generated and identified by matrix-isolation IR spectroscopy and quantum chemical calculations. The OCBNO complex, which is produced by the reaction of boron atoms with mixtures of carbon monoxide and nitric oxide in solid neon, rearranges to the more stable OBNCO isomer on UV excitation. Bonding analysis indicates that the OCBNO complex is best described by the bonding interactions between a triplet-state boron cation with an electron configuration of (2s)0 (2pσ )0 (2pπ )2 and the CO/NO- ligands in the triplet state forming two degenerate electron-sharing π bonds and two ligand-to-boron dative σ bonds.

Generation and Characterization of the C3 O2 - Anion with an Unexpected Unsymmetrical Structure.

The carbon suboxide anion C3 O2 - is generated in solid neon matrix. It is characterized by infrared absorption spectroscopy as well as quantum chemical calculations to have a planar Cs structure where two CO groups with significantly different bond lengths and angles are attached in a zigzag fashion to the central carbon atom. Bonding analysis indicates that it is best described by the bonding interactions between a neutral CO in a triplet excited state and a doublet excited state of CCO- .

Filling a Gap: The Coordinatively Saturated Group†4 Carbonyl Complexes TM(CO)8 (TM=Zr, Hf) and Ti(CO)7.

Homoleptic Group 4 metal carbonyl cation and neutral complexes were prepared in the gas phase and/or in solid neon matrix. Infrared spectroscopy studies reveal that both zirconium and hafnium form eight-coordinate carbonyl neutral and cation complexes. In contrast, titanium forms only the six-coordinate Ti(CO)6 + and seven-coordinate Ti(CO)7 . Titanium octacarbonyl Ti(CO)8 is unstable as a result of steric repulsion between the CO ligands. The 20-electron Zr(CO)8 and Hf(CO)8 complexes represent the first experimentally observed homoleptic octacarbonyl neutral complexes of transition metals. The molecules still fulfill the 18-electron rule, because one doubly occupied valence orbital does not mix with any of the metal valence atomic orbitals. Zr(CO)8 and Hf(CO)8 are stable against the loss of one CO because the CO ligands encounter less steric repulsion than Zr(CO)7 and Hf(CO)7 . The heptacarbonyl complexes have shorter metal-CO bonds than that of the octacarbonyl complexes due to stronger electrostatic and covalent bonding, but the significantly smaller repulsive Pauli term makes the octacarbonyl complexes stable.

Side-On Bonded Beryllium Dinitrogen Complexes.

The preparation and spectroscopic identification of the complexes NNBe(η2 -N2 ) and (NN)2 Be(η2 -N2 ) and the energetically higher lying isomers Be(NN)2 and Be(NN)3 are reported. NNBe(η2 -N2 ) and (NN)2 Be(η2 -N2 ) are the first examples of covalently side-on bonded N2 adducts of a main-group element. The analysis of the electronic structure using modern methods of quantum chemistry suggests that NNBe(η2 -N2 ) and (NN)2 Be(η2 -N2 ) should be classified as π complexes rather than metalladiazirines.

Beryllium Atom Mediated Dinitrogen Activation via Coupling with Carbon Monoxide.

The reactions of laser-ablated beryllium atoms with dinitrogen and carbon monoxide mixtures form the end-on bonded NNBeCO and side-on bonded (η2 -N2 )BeCO isomers in solid argon, which are predicted by quantum chemical calculations to be almost isoenergetic. The end-on bonded complex has a triplet ground state while the side-on bonded isomer has a singlet electronic ground state. The complexes rearrange to the energetically lowest lying NBeNCO isomer upon visible light excitation, which is characterized to be an isocyanate complex of a nitrene derivative with a triplet electronic ground state. A bonding analysis using a charge- and energy decomposition procedure reveals that the electronic reference state of Be in the NNBeCO isomers has an 2s0 2p2 excited configuration and that the metal-ligand bonds can be described in terms of N2 →Be←CO σ donation and concomitant N2 ←Be→CO π backdonation. The results demonstrate that the activation of N2 with the N-N bond being completely cleaved can be achieved via coupling with carbon monoxide mediated by a main group atom.

Photodecomposition of Thienylsulfonyl Azides: Generation and Spectroscopic Characterization of Triplet Thienylsulfonyl Nitrenes and 3-Thienylnitrene.

The photochemistry of 2-thienylsulfonyl azide (1) and 3-thienylsulfonyl azide (6) has been disclosed by combining matrix-isolation spectroscopy with quantum chemical calculations. Two novel heteroaryl sulfonyl nitrenes, 2-thienylsulfonyl nitrene (2) and 3-thienylsulfonyl nitrene (7), have been generated during the 266 nm laser photolysis of 1 and 6, respectively. Both nitrenes in the triplet ground state have been characterized with matrix-isolation IR (15N-labeling) in solid Ar (10.0 K), N2 (10.0 K), and Ne (2.8 K) matrixes and EPR spectroscopy (2, |D/hc| = 1.452 cm-1 and |E/hc| = 0.0058 cm-1; 7, |D/hc| = 1.492 cm-1 and |E/hc| = 0.0060 cm-1) in solid toluene at 5.0 K. Upon subsequent UV-light irradiation (365 nm), no Curtius rearrangement but decomposition of 2 occurs by SO2-elimination and the concurrent formation of ring-opening product (s-Z)-4-thioxo-2-butenenitrile (3) via the intermediacy of the putative 2-thienylnitrene (4). In contrast, violet-light irradiation (400 ± 20 nm) of 7 causes SO2-elimination to yield triplet 3-thienylnitrene (8), for which the IR spectroscopic identification is supported by quantum chemical calculations at the B3LYP/6-311++G(3df,3pd) level. 3-Thienylnitrene is highly reactive, since it not only combines with SO2 to furnish 3-thienyl-N-sulfonylamine (9) but also undergoes ring-opening to (s-E)-4-thioxo-2-butenenitrile (10) under the UV-light irradiation (365 nm).

Transition-Metal Chemistry of Alkaline-Earth Elements: The Trisbenzene Complexes M(Bz)3 (M=Sr, Ba).

We report the synthesis and spectroscopic identification of the trisbenzene complexes of strontium and barium M(Bz)3 (M=Sr, Ba) in low-temperature Ne matrix. Both complexes are characterized by a D3 symmetric structure involving three equivalent η6 -bound benzene ligands and a closed-shell singlet electronic ground state. The analysis of the electronic structure shows that the complexes exhibit metal-ligand bonds that are typical for transition metal compounds. The chemical bonds can be explained in terms of weak donation from the π MOs of benzene ligands into the vacant (n-1)d AOs of M and strong backdonation from the occupied (n-1)d AO of M into vacant π* MOs of benzene ligands. The metals in these 20-electron complexes have 18 effective valence electrons, and, thus, fulfill the 18-electron rule if only the metal-ligand bonding electrons are counted. The results suggest that the heavier alkaline earth atoms exhibit the full bonding scenario of transition metals.

Magnetically Bistable Nitrenes: Matrix Isolation of Furoylnitrenes in Both Singlet and Triplet States and Triplet 3-Furylnitrene.

Two simple acylnitrenes, 2-furoylnitrene (2) and 3-furoylnitrene (6), were generated through 266 nm laser photolysis of the corresponding azides. Both are magnetically bistable in cryogenic matrices, as evidenced by the direct observation of the closed-shell singlet state with IR spectroscopy in solid Ne, Ar, Kr, Xe, and N2 matrices (3-40 K) and the triplet state in toluene (10 K) with EPR spectroscopy (32: |D/hc| = 1.48 cm-1 and |E/hc| = 0.029 cm-1; 36: |D/hc| = 1.39 cm-1 and |E/hc|c = 0.039 cm-1). Subsequent visible-light and UV laser irradiations led to the formation of furyl isocyanates (3 and 7) and ring-opening product 3-cyanoacrolein (9-E and 9-Z), respectively, in which the elusive 3-furylnitrene (38) was also identified by IR and EPR spectroscopy (|D/hc| = 1.12 cm-1 and |E/hc| = 0.005 cm-1).

Phenylsulfinyl Radical: Gas-Phase Generation, Photoisomerization, and Oxidation.

Arylsulfinyl radicals are key intermediates in sulfoxide chemistry. The parent molecule, phenylsulfinyl radical PhSO•, has been generated for the first time in the gas phase through high-vacuum flash pyrolysis of PhS(O)R (R = CF3 and Cl) at about 1000 K. Upon UV light irradiation (365 nm), PhSO• isomerizes to novel oxathiyl radical PhOS• in cryogenic matrices (2.8 K). Prolonged irradiation causes further isomerization of PhOS• to 2-hydroxyphenylthiyl radical, the formation of which has been also observed in the 193 nm laser photolysis of matrix-isolated 2-hydroxybenzenethiol. Concomitantly, ring-opening occurs during the UV photolysis of PhOS• and 2-hydroxybenzenethiol and forms an acyclic thioketoketene radical. Phenylsulfinyl radical reacts partially with molecular oxygen in the gas phase and yields phenyl radical Ph• and OSOO. Upon irradiation (365 nm), the isomeric oxathiyl radical also combines O2 with immediate dissociation to phenoxy radical PhO• and SO2. The identification of the intermediates with IR and UV-vis spectroscopy is supported by quantum chemical computations at the B3LYP/def2-TZVPP and UCCSD(T)/aug-cc-pV(D+d)Z levels of theory. The isomerization of PhSO• has been discussed based on the computed potential energy profile and the comparison with the intensively explored photochemistry of phenylperoxy radical PhOO•.

The Trifluoromethyl Sulfinyl and Oxathiyl Radicals.

Two hitherto unreported sulfur-centered radicals CF3 SO. and CF3 OS. were generated in the gas phase through high-vacuum flash pyrolyses of sulfoxide CF3 S(O)X (X=CF3 , Cl, PhO) precursors. The CF3 OS. molecule is the first experimental example that constitutes an oxathiyl radical. It was isolated and characterized by combining matrix-isolation IR and UV/Vis spectroscopy with quantum chemical computations up to the UCCSD(T)-F12/cc-pVTZ-F12 level of theory. Upon UV light irradiation (254 or 266 nm), sulfinyl radical (CF3 SO. ) isomerizes to oxathiyl radical (CF3 OS. ) in cryogenic noble gas matrices (Ar and Ne). Natural population analyses at the BP86/def2-TZVPP//UCCSD(T)-F12/cc-pVTZ-F12 level suggest that the spin density in CF3 OS. is mainly localized on the sulfur atom (0.86), whereas, in CF3 SO. the spin density is almost equally distributed on the sulfur (0.55) and oxygen (0.43) atoms.

Decomposition of Sulfonyl Azide Isocyanate and Sulfonyl Diazide: The Oxygen-Shifted Curtius Rearrangement via Sulfonyl Nitrenes.

Sulfonyl azide isocyanate, (OCN)S(O)2N3, was prepared and characterized by IR (gas, matrix-isolation), Raman (liquid), and UV-vis spectroscopy. Upon flash vacuum pyrolysis (FVP) at ca. 1000 K, gaseous (OCN)S(O)2N3 decomposes completely and yields fragments N2, SO2, SO3, NCN, N3, NCO, CO, CN, and NO. In contrast, the azide splits off N2 and furnishes a transient triplet sulfonyl nitrene intermediate (OCN)S(O)2N upon a 266 nm laser irradiation in solid Ne-matrix at 2.8 K. Subsequent photolysis of the nitrene with visible light (λ = 380-450 nm) results in oxygen-shifted Curtius rearrangement to a novel nitroso sulfoxide (OCN)S(O)NO. For comparison, the photodecomposition of the closely related sulfonyl diazide O2S(N3)2 in a solid Ar matrix was also studied. Upon an ArF excimer laser (193 nm) photolysis, O2S(N3)2 decomposes and yields N2, SO2, and OSNNO via the intermediacy of an elusive sufonyl nitrene N3S(O)2N. Further visible light irradiation (λ > 395 nm) leads to depletion of N3S(O)2N and OSNNO and concomitant formation of SO2 and N2. The identification of the intermediates in cryogenic matrixes by IR spectroscopy was supported by 15N-labeling experiments and quantum chemical calculations. The mechanism for the decomposition of both sulfonyl azides (OCN)S(O)2N3 and O2S(N3)2 was discussed on the basis of the observed intermediates and the calculated potential energy profiles.

Spectroscopic Identification of H2 NSO and syn- and anti-HNSOH Radicals.

The simplest aminosulfinyl radical H2 NSO has been generated in the gas phase through flash vacuum pyrolysis of CF3 S(O)NH2 at approximately 1000 K. Upon UV light irradiation (365 nm), 1,3-H migration occurs in H2 NSO and furnishes an elusive N-amidyl radical HNSOH in syn and anti conformations in cryogenic matrices (N2 or Ar, 15 K). Further 266 nm laser irradiation results in dissociation of HNSOH to H2 O and SN and concomitant reformation of H2 NSO in trace amount. The identification of H2 NSO, syn-HNSOH, and anti-HNSOH by matrix-isolation IR spectroscopy is supported by high-level quantum chemical computations.

The decomposition of benzenesulfonyl azide: a matrix isolation and computational study.

The thermal-decomposition and photo-decomposition of benzenesulfonyl azide, PhS(O)2N3, have been studied by combining matrix-isolation IR spectroscopy and quantum chemical calculations. Upon flash vacuum pyrolysis at 800 K, the azide splits off molecular nitrogen and exclusively furnishes phenylnitrene (PhN) and SO2 in the gas phase. In contrast, the azide favors stepwise photodecomposition in solid Ar and Ne matrices at 2.8 K. Specifically, the UV laser photolysis (193 and 266 nm) of PhS(O)2N3 results in the formation of the key nitrene intermediate PhS(O)2N in the triplet ground state, which undergoes pseudo-Curtius rearrangement into N-sulfonyl imine PhNSO2 under subsequent visible light irradiation (380-450 nm). Further fragmentation of PhNSO2 into SO2 and PhN followed by ring-expansion to didehydroazepine also occurs upon visible light irradiation. The preference of the stepwise mechanism for the decomposition of PhS(O)2N3 is supported by quantum chemical calculations using DFT B3LYP/6-311++G(3df,3pd) and CBS-QB3 methods.

Fast Heavy-Atom Tunneling in Trifluoroacetyl Nitrene.

Chemical reactions involving quantum mechanical tunneling (QMT) increasingly attract the attention of scientists. In contrast to the hydrogen-tunneling as frequently observed in chemistry and biology, tunneling solely by heavy atoms is rare. Herein, we report heavy-atom tunneling in trifluoroacetyl nitrene, CF3 C(O)N. The carbonyl nitrene CF3 C(O)N in the triplet ground state was generated in cryogenic matrices by laser (193 or 266 nm) photolysis of CF3 C(O)N3 and characterized by IR and EPR spectroscopy. In contrast to the theoretically predicted activation barriers (>10 kcal mol-1 ), CF3 C(O)N undergoes rapid rearrangement into CF3 NCO with half-life times of less than 10 min and unprecedentedly large 14 N/15 N kinetic isotope effects (1.18-1.33) in solid Ar, Ne, and N2 matrices even at 2.8 K. The tunneling disappearance of CF3 C(O)N becomes much slower in the chemically active toluene and in 2-methyltetrahydrofuran at 5 K.

Simplest N-Sulfonylamine HNSO2.

The simplest N-sulfonylamine HNSO2 has been generated in the gas phase through flash vacuum pyrolysis of methoxysulfonyl azide CH3OS(O)2N3. Its identification was accomplished by combining matrix-isolation IR spectroscopy and quantum chemical calculations. Both experimental and theoretical evidence suggest a stepwise decomposition of the azide via the methoxysulfonyl nitrene CH3OS(O)2N, observed in the 193 nm laser photolysis of the azide, with concerted fragmentation into CH2O and HNSO2. Upon the 193 nm laser irradiation, HNSO2 isomerizes into the novel N-hydroxysulfinylamine HONSO.