Intramolecular charge transfer with 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6) and other aminobenzonitriles. A comparison of experimental vapor phase spectra and crystal structures with calculations.

Calculations of molecular structures in the electronic ground state S(0) and of excited state and fluorescence energies generally refer to the gas phase. This complicates a comparison with experimental data, which often are only available for molecules in solution. Therefore, experimental absorption and fluorescence spectra in the vapor phase are presented for 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6), 1-methyl-6-cyano-1,2,3,4-tetrahydroquinoline (NMC6), 4-(dimethylamino)benzonitrile (DMABN), and 4-(diisopropylamino)benzonitrile (DIABN). NTC6 and DIABN show a dual fluorescence in the gas phase, with emissions from an intramolecular charge transfer (ICT) and a locally excited (LE) state, whereas with NMC6 and DMABN only LE emission is observed. For a comparison of the experimental molecular structure in S(0) with the results of recent computations, X-ray crystal structures of NTC6, NMC6, and several analogues are presented. For DMABN, NMC6, and NTC6, LE/ICT energy diagrams are constructed, in which the experimental energies of the Franck-Condon singlet excited states S(1) and S(2), and the LE and ICT states together with their emissions, are compared with the calculations. The LE and ICT dipole moments are also discussed. This comparison reveals substantial differences, in particular for the ICT energies, but even for the structure of the S(0) ground states. It is concluded that the computed ICT states of NTC6 and DMABN, in which the full conjugation of the phenyl ring is interrupted, is different from the ICT states measured in the experiments.

Counterintuitive absence of an excited-state intramolecular charge transfer reaction with 2,4,6-tricyanoanilines. Experimental and computational results.

The fluorescence spectra of 2,4,6-tricyano-N,N-dimethylaniline (TCDMA), 2,4,6-tricyano-N-methylaniline (TCMA), and 2,4,6-tricyanoaniline (TCA) consist of a single emission band, even in the polar solvent acetonitrile (MeCN). This indicates that an intramolecular charge transfer (ICT) reaction from the initially prepared locally excited (LE) state does not take place with these molecules, in contrast to 4-(dimethylamino)benzonitrile (DMABN), although the electron accepting capability of the tricyanobenzene moiety in TCDMA, TCMA, and TCA is substantially larger than that of the benzonitrile group in DMABN. In support of this conclusion, the picosecond fluorescence decays of the tricyanoanilines are single-exponential. Only with TCDMA in MeCN at the highest time resolution, double-exponential decays are observed. On the basis of a similar temporal evolution of around 2 ps in the femtosecond excited-state absorption (ESA) spectra of TCDMA in this solvent, the time development is attributed to the presence of two rapidly interconverting S(1) conformers. The same conclusion is reached from CASPT2/CASSCF computations on TCDMA, in which two S(1) minima are identified. The ESA spectra of TCDMA, TCMA, and TCA resemble that of the LE state of DMABN, but are different from its ICT ESA spectrum, likewise showing that an ICT reaction does not occur with the tricyanoanilines. From the luminescence spectrum of TCDMA in n-propyl cyanide at -160 degrees C, it follows that intersystem crossing and not internal conversion is the main S(1) deactivation channel. The radiative rate constant of TCDMA in MeCN is smaller than that of TCMA and TCA, which indicates that the S(1) state of TCDMA has a larger ICT contribution than in the case of TCMA and TCA, in accordance with the results of the calculations, which show that the S(1) state displays ICT valence bond character. Extrapolated gas-phase data for TCDMA and TCA are compared with the results of the computations, revealing a good agreement. The calculations on TCDMA and TCA also lead to the conclusion that the lowest excited singlet state S(1) determines its photophysical behavior, without the occurrence of an LE --> ICT reaction, in the sense that the initially excited LE state has already a strong ICT character and there is no equilibrium between two electronic states with strongly different electronic structures (i.e., LE and ICT with very different dipole moments) leading to dual (LE + ICT) fluorescence.

Synthesis, molecular, and electronic structure of (eta(8)-C8H8)Ln(scorpionate) half-sandwich complexes: an experimental key to a better understanding of f-element-cyclooctatetraenyl bonding.

Synthetic routes leading to two series of (eta(8)-cyclooctatetraenyl)lanthanide(III) scorpionate "mixed sandwich" complexes are reported. The early lanthanide derivatives (COT)Ln(Tp) (Ln = Ce (1), Pr (2), Nd (3), Sm (4)) and (COT)Ln(Tp(Me2)) (Ln = Ce (5), Pr (6), Nd (7), Sm (8)) (COT = eta(8)-cyclooctatetraenyl, Tp = hydrotris(pyrazolyl)borate, Tp(Me2) = hydrotris(3,5-dimethylpyrazolyl)borate) were obtained by reacting the dimeric halide precursors [(COT)Ln(mu-Cl)(THF)]2 with K[Tp] or K[Tp(Me2)], respectively For the late lanthanide elements a different synthetic route was developed. The complexes (COT)Ln(Tp) (Ln = Er (9), Lu (10)) were made by the reaction of (Tp)LnCl2(THF)1.5 with equivalent amounts of K2C8H8. All new compounds were isolated as intensely colored crystalline materials and fully characterized by elemental analyses and spectroscopic methods. The molecular structures of 4, 5, and 8 were elucidated by X-ray diffraction. The optical spectra of compounds 2 and 4-8 were run at room and low temperatures. From the spectra obtained, the underlying crystal field splitting patterns of complexes 2, 4, 6, and 7 were derived and simulated by fitting the free parameters of a phenomenological Hamiltonian. The parameters used allow the estimation of the crystal field strengths experienced by the Ln3+ central ions and the insertion of complexes 2, 4, 6, and 7 into empiric nephelauxetic and relativistic nephelauxetic series. Besides, the experimentally oriented non-relativistic and relativistic molecular orbital schemes of compound 6 were set up and compared with the results of previous model calculations on [Ln(COT)2]-, Pa(COT)2, and U(COT)2.

Intramolecular charge transfer with the planarized 4-cyanofluorazene and its flexible counterpart 4-cyano-N-phenylpyrrole. Picosecond fluorescence decays and femtosecond excited-state absorption.

The fluorescence spectrum of the rigidified 4-cyanofluorazene (FPP4C) in n-hexane consists of a dual emission from a locally excited (LE) and an intramolecular charge-transfer (ICT) state, with an ICT/LE fluorescence quantum yield ratio of Phi'(ICT)/Phi(LE) = 3.3 at 25 degrees C. With the flexible 4-cyano- N-phenylpyrrole (PP4C) in n-hexane, such an ICT reaction also takes place, with Phi'(ICT)/Phi(LE) = 1.5, indicating that for this reaction, a perpendicular twist of the pyrrole and benzonitrile moieties is not required. The ICT emission band of FPP4C and PP4C in n-hexane has vibrational structure, but a structureless band is observed in all other solvents more polar than the alkanes. The enthalpy difference Delta H of the LE --> ICT reaction in n-hexane, -11 kJ/mol for FPP4C and -7 kJ/mol for PP4C, is determined by analyzing the temperature dependence of Phi'(ICT)/Phi(LE). Using these data, the energy E(FC,ICT) of the Franck-Condon ground state populated by the ICT emission is calculated, 41 (FPP4C) and 40 kJ/mol (PP4C). These large values for E(FC,ICT) lead to the conclusion that with FPP4C and PP4C, direct ICT excitation, bypassing LE, does not take place. FPP4C has an ICT dipole moment of 15 D, similar to that of PP4C (16 D). Picosecond fluorescence decays allow the determination of the ICT lifetime, from which the radiative rate constant k'(f)(ICT) is derived, with comparable values for FPP4C and PP4C. This shows that an argument for a twisted ICT state of PP4C cannot come from k'(f)(ICT). After correction for the solvent refractive index and the energy of the emission maximum nu(max)(ICT), it appears that k'(f)(ICT) is solvent-polarity-independent. Femtosecond transient absorption with FPP4C and PP4C in n-hexane reveals that the ICT state is already nearly fully present at 100 fs after excitation, in rapid equilibrium with LE. In MeCN, the ICT state of FPP4C and PP4C is likewise largely developed at this delay time, and the reaction is limited by dielectric solvent relaxation, which shows that the ICT reaction is ultrafast, at the experimental time limit of 50 fs. PP4C and FPP4C have a similar planar ICT structure, without an appreciable twist of the pyrrole and benzonitrile subgroups. Their crystal structure is compared with calculations for the S0 ground state.

Pseudohelical and helical primary structures of 1,2-spiroannelated four- and five-membered rings: syntheses and chiroptical properties.

The pseudohelical hydrocarbons (R)-6, (S)-7, and (R)-8 and the helical hydrocarbon (P)-9, formally derived from the helical hydrocarbon (P)-4 by stepwise replacement of each of the four-membered rings by a five-membered ring, have been prepared. Their optical rotations vary systematically, both in magnitude and sign. Of the extremes, (P)-4 represents the usual case of a right-handed dextrorotatory helix, while (P)-9 represents the unusual case of a right-handed levorotatory helix. To rationalize these facts, DFT calculations of the rotatory power of (P)-helices of three-, four-, and five-membered rings have been performed. The results show a very good agreement with the experimental data for the rigid helices of three-membered rings and always show the correct sign and order of magnitude for the flexible helices of four- and five-membered rings for which Boltzmann-averaged optical rotations of up to six conformers had to be used. Within the conformers of the latter, a set of large dihedral angles for the bonds of the inner sphere correspond to a high specific rotation, and a set of small dihedral angles correspond to a low specific rotation. As a consequence, the Boltzmann-averaged values markedly depend on the geometry and weight of the conformers involved.

Structural, electrochemical and oxygen atom transfer properties of a molybdenum selenoether complex [Mo2O4(OC3H6SeC3H6O)2] and its thioether analogue [Mo2O4(OC3H6SC3H6O)2].

The first crystallographically characterized molybdenum(vi) selenoether complex [Mo(2)O(4)(OC(3)H(6)SeC(3)H(6)O)(2)] and its thioether analogue [Mo(2)O(4)(OC(3)H(6)SC(3)H(6)O)(2)] were synthesised. Their structural, electrochemical and oxygen atom transfer properties are compared. This is relevant for the molybdenum cofactors of the DMSO reductase family where the coordination of the active site metal occurs through O (serine/aspartate), S (cysteine) or Se (selenocysteine). Both structures are almost identical except for those parameters that are directly derived from the different sizes of the varied ligand atoms (Se and S). No trans influence was observed. The metal centered redox process (Mo(V)<-->Mo(VI)) is at slightly lower voltage for the sulfur than for the selenium complex. The selenium compound catalyses the oxygen atom transfer from DMSO to PPh(3) by a different mechanism and at a higher rate than the sulfur compound, which is an indication that cysteine and selenocysteine might be used for a purpose in the different molybdenum and tungsten cofactors.

Synthesis, structural characterization, catalytic properties, and theoretical study of compounds containing an Al-O-M (M = Ti, Hf) core.

Two single oxygen-bridged heterobimetallic oxides of Al(III) with group 4 metals (Ti, Hf) have been prepared. The reaction of LAlMeOH (1) [L = CH(N(Ar)(CMe))2, Ar = 2,6-iPr2C6H3] with dimethylmetallocenes of Ti and Hf in toluene (80 degrees C) and ether (room temperature), respectively, resulted in the formation of LAl(Me)(mu-O)M(Me)Cp2 [M = Ti (2), Hf (3)] in moderate to good yield. Compounds 2 and 3 were characterized by elemental analysis, IR, NMR (1H and 13C), EI-MS, and single-crystal X-ray structural analysis. Furthermore, compound 2 showed good catalytic activity in ethylene and styrene homopolymerization, while compound 3 is less active in ethylene polymerization. The styrene polymerization yields atactic polystyrene.

Soluble molecular compounds with the Mg-O-Al structural motif: a model approach for the fixation of organometallics on a MgO surface.

We report a facile route to the molecular compounds with the Mg-O-Al structural motif. The reaction of Mg[N(SiMe3)2]2 (1) with a stoichiometric amount of LAlOH(Me) (2) [L = CH{(CMe)(2,6-iPr2C6H3N)}2] in THF/n-hexane at 0 degrees C results in the formation of the heterobimetallic compound (Me3Si)2NMg(THF)2-O-Al(Me)L (3) in high yield. The similar reaction of 1 equiv of Mg[N(SiMe3)2]2 and 2 equiv of LAlOH(Me) results in the formation of trimetallic compound L(Me)Al-O-Mg(THF)2-O-Al(Me)L (4). Structural analyses of 3 and 4 have been carried out, revealing the presence of the Mg-O-Al motif. A tentative assignment of the Mg-O-Al vibrations has been made and was supported by calculations.

Synthesis of an aluminum spirocyclic hybrid with an inorganic B2O3 and an organic C3N2 core.

The reaction of LAl (L = HC(CMeNAr)2, Ar = 2,6-iPr2C6H3) or LAlH2 with PhB(OH)2 yields the unprecedented spirocyclic LAl[(OBPh)2O] compound. The former reaction proceeds under hydrogen formation and simultaneous oxidation of the aluminum(I).

Aluminacyclopropene: syntheses, characterization, and reactivity toward terminal alkynes.

Reactions of LAl with ethyne, mono- and disubstituted alkynes, and diyne to aluminacyclopropene LAl[eta2-C2(R1)(R2)] ((L = HC[(CMe)(NAr)]2, Ar = 2,6-iPr2C6H3); R1 = R2 = H, (1); R1 = H, R2 = Ph, (2); R1 = R2 = Me, (3); R1 = SiMe3, R2 = C[triple bond]CSiMe3, (4)) are reported. Compounds 1 and 2 were obtained in equimolar quantities of the starting materials at low temperature. The amount of C2H2 was controlled by removing an excess of C2H2 in the range from -78 to -50 degrees C. Compound 4 can be alternatively prepared by the substitution reaction of LAl[eta2-C2(SiMe3)2] with Me3SiC[triple bond]CC[triple bond]CSiMe3 or by the reductive coupling reaction of LAlI2 with potassium in the presence of Me3SiC[triple bond]CC[triple bond]CSiMe3. The reaction of LAl with excess C2H2 and PhC[triple bond]CH (<1:2) afforded the respective alkenylalkynylaluminum compounds LAl(CH=CH2)(C[triple bond]CH) (5) and LAl(CH=CHPh)(C[triple bond]CPh) (6). The reaction of LAl(eta2-C2Ph2) with C2H2 and PhC[triple bond]CH yielded LAl(CPh=CHPh)(C[triple bond]CH) (7) and LAl(CPh=CHPh)(C[triple bond]CPh) (8), respectively. Rationally, the formation of 5 (or 6) may proceed through the corresponding precursor 1 (or 2). The theoretical studies based on DFT calculations show that an interaction between the Al(I) center and the C[triple bond]C unit needs almost no activation energy. Within the AlC2 ring the computational Al-C bond order of ca. 1 suggests an Al-C sigma bond and therefore less pi electron delocalization over the AlC2 ring. The computed Al-eta2-C2 bond dissociation energies (155-82.6 kJ/mol) indicate a remarkable reactivity of aluminacyclopropene species. Finally, the 1H NMR spectroscopy monitored reaction of LAl(eta2-C2Ph2) and PhC[triple bond]CH in toluene-d8 may reveal an acetylenic hydrogen migration process.

Synthesis and characterization of aluminum-containing tin(IV) heterobimetallic sulfides.

Three novel aluminum-containing tin(IV) heterobimetallic sulfides are reported. The reaction of [LAl(SLi)2(THF)2]2 (1) [L = HC(CMeNAr)2, Ar = 2,6-iPr2C6H3] with Ph2SnCl2, Me2SnCl2, and SnCl4 in THF respectively afforded LAl(mu-S)2SnPh2 (2), LAl(mu-S)2SnMe2 (3), and LAl(mu-S)2Sn(mu-S)2AlL (4) in moderate yields. Compounds 2, 3, and 4 were characterized by elemental analysis, NMR, electron-impact mass spectrometry, and single-crystal X-ray structural analysis.

Synthesis, characterization, and hydrolysis of aluminum(III) compounds bearing the C6F5-substituted beta-diketiminate HC[(CMe)(NC6F5)]2 (L) ligand.

A series of Al(III) compounds containing the C6F5-substituted beta-diketiminate ligands LAlMeCl (2), LAlMe2 (3), LAlMeI (4), and LAlBr2 (5) (L = HC[(CMe)(NC6F5)]2) were synthesized and characterized. The hydrolysis of 2 and 4 in the presence of 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene as the hydrogen halide acceptor both lead to (LAlMe)2(mu-O) (6), a methylalumoxane derivative, which is the first hydrolysis product with the general formula of (RAlMe)(n)O. A comparison of the hydrolysis products of 2 and 4 with that of L'AlMeCl (L' = HC[(CMe)(NAr)]2, Ar = 2,6-iPr2C6H3) shows that with the C6F5-substituted beta-diketiminate ligand, it was not possible to generate LAlMe(OH). This is obviously due to the stronger Brönsted acidity of the proton and the smaller size of the C6F5 group in this compound compared to that of the corresponding 2,6-iPr2C6H3 derivative.

Syntheses, characterization, and X-ray crystal structures of beta-diketiminate group 13 hydrides, chlorides, and fluorides.

A series of organometallic compounds of group 13 metals supported by the sterically encumbered beta-diketiminate ligand containing hydrides, fluorides, chlorides, and bromide have been synthesized and structurally characterized. The synthetic strategy applied utilizes halide metathesis and reduction of metal chlorides to the corresponding hydrides. Thus, the reaction of LLi.OEt2 with MeMCl2 affords LM(Me)Cl (M = Al (1), Ga (2), In (3)) and LGaBr2 (4) with GaBr3. Reduction of LGa(Me)Cl with LiH.BEt3 leads to the formation of LGa(Me)H (10). Synthesis of LGaH(2) (12) has been accomplished by reacting LGaI2 (8) with LiH.BEt3. LAl(Me)Cl (1) and LAlH2 (6) have been converted to LAl(Me)F (5) and LAlF2 (7), respectively. The former was obtained in a reaction of LAl(Me)Cl with Me3SnF while the latter was isolated in a reaction of LAlH2 with BF3.OEt2. Similarly reaction of LGaI2 (8) with Me3SnF affords LGaF2 (9). Compounds reported herein have been characterized by elemental analyses, IR, NMR, EI-MS, and single-crystal X-ray diffraction techniques.

Ultrafast intramolecular charge transfer and internal conversion with tetrafluoro-aminobenzonitriles.

The five 2,3,5,6-tetrafluoro-4-aminobenzonitriles XABN4F with a dimethyl-amino (DMABN4F), diethyl-amino (DEABN4F), azetidinyl (AZABN4F), methyl-amino (MABN4F) or amino (ABN4F) group undergo ultrafast intramolecular charge transfer (ICT) at room temperature, in the polar solvent acetonitrile (MeCN) as well as in the nonpolar n-hexane. ICT also takes place with the corresponding non-fluorinated aminobenzonitriles DMABN, DEABN and AZABN in MeCN, whereas for these molecules in n-hexane only minor (DMABN, DEABN) or no (AZABN) ICT fluorescence is detected. For the secondary (MABN) and primary (ABN) amines, an ICT reaction does not occur, which makes ABN4F the first electron donor/acceptor molecule with an NH(2) group for which ICT is observed. The ICT state of the XABN4Fs has a dipole moment of around 14 D, clearly smaller than that of DMABN (17 D). This difference is attributed to the electron withdrawing from the CN group to the phenyl ring, exerted by the four F-substituents. The reaction from the initially prepared locally excited (LE) to the ICT state in n-hexane proceeds in the sub-picosecond time range: 0.35 ps (DMABN4F), 0.29 ps (DEABN4F) and 0.13 ps (AZABN4F), as determined from femtosecond transient absorption measurements. In the highly polar solvent MeCN, an ICT reaction time of around 90 fs is observed for all five XABN4Fs, irrespective of the nature of their amino group. This shows that with these molecules in MeCN the ICT reaction rate is limited by the solvent dielectric relaxation time of MeCN, for which a value of around 90 fs has been reported. It is therefore concluded that, during this ultrashort ICT reaction, a large-amplitude motion such as a full 90 degrees twist of the amino group is unlikely to occur in the XABN4Fs. The ICT state of the XABN4Fs is strongly quenched via internal conversion (IC), with a lifetime tau'(0) (ICT) down to 3 ps, possibly by a reaction passing through a conical intersection made accessible due to a deformation of the phenyl group by out-of-plane motions induced by vibronic coupling between low-lying pisigma* and pipi* states in the XABN4Fs.

Polyhedral cobalt(II) and iron(II) siloxanes: synthesis and x-ray crystal structure of [(RSi(OH)O2)Co(OPMe3)]4 and [(RSiO3)2(RSi(OH)O2)4(mu3-OH)2Fe8(THF)4] (R = (2,6-iPr2C6H3)N(SiMe3)).

The cobalt(II) and iron(II) siloxane compounds were prepared by the reaction of lipophilic N-bonded silanetriol 1 with metal silylamides M[N(SiMe3)2]2 (M = Co (2), Fe (3)) in a 1:1 and 3:4 molar ratio, respectively. A plot of 1/chi versus temperature in the range of 2-300 K indicates the paramagnetic behavior of 2 and 3. The composition and molecular structures of 2 and 3 were fully determined by IR, elemental analysis, and X-ray crystal structural analyses. Compound 2 possesses a pseudo-4-fold (S4) symmetry, whereas 3 reveals an inversion center. Compound 2 represents a tetracobalt(II) drum while 3 exhibits an octairon(II) cage containing siloxane ligands.

Aluminum hydride cations stabilized by weakly coordinating carbaalanates.

The reactions of t-BuCCLi with a mixture of AlH(3).NMe(3) and ClAlH(2).NMe(3) in boiling toluene with the addition of [t-BuCH(2)(Bzl)NMe(2)]Cl, or a bulky beta-diketimine instead, and [n-Bu(4)N]Cl led to the carbaalanates [H(2)Al(NMe(3))(2)](2)[(AlH)(8)(CCH(2)t-Bu)(6)], 3, and [n-Bu(4)N](2)[(AlH)(8)(CCH(2)t-Bu)(6)], 4, respectively. The reaction of Me(3)N.Al(CCt-Bu)(3) 5 and AlH(3).NMe(3) in boiling toluene yielded [H(n-Bu)Al(NMe(3))(2)][(AlH)(7)(AlNMe(3))(CCH(2)t-Bu)(6)], 6, in trace amounts. The single-crystal X-ray structures of 3 and 6 are reported. The compounds 3, 4, and 6 consist of well-separated ion pairs introducing carbaalanates as weakly coordinating anions and stabilizing aluminum hydride cations.

Synthesis of a new class of compounds containing a Ln-O-Al arrangement and their reactions and catalytic properties.

Synthesis of a new class of compounds containing a Ln-O-Al moiety has been accomplished by the reaction of LAlOH(Me) (L = HC(CMeNAr)(2), Ar = 2,6-iPr(2)C(6)H(3)) with a series of Cp(3)Ln compounds. The terminal Al-OH group shows selective reactivity, and the complexes Cp(2)Ln(THF)-O-AlL(Me) (Ln = Yb, 1; Er, 2; Dy, 3), Cp(2)Yb-O-AlL(Me) (4), and Cp(3)Ln(mu-OH)AlL(Me) (Ln = Er, 5; Dy, 6; Sm, 7) were obtained. This allows further insight into the proton exchange process, and two different mechanisms, intermolecular and intramolecular elimination of CpH, are proposed under different conditions. Complexes 1-4, 6, and 7 have been characterized by X-ray structural analyses which reveals a Ln-O-Al or Ln(mu-OH)Al core in these complexes. The obtuse Ln-O-Al angles fall in the range 151.9-169.8 degrees . The reaction of 1 or 4 with Me(3)SnF in toluene under refluxing conditions unexpectedly yielded the compounds [Cp(2)Yb(mu-OSnMe(3))](2) (8) and LAl(Me)F (9). Reactions of LAlOH(Me) with the mono- and dicyclopentadienyl complexes LYbCp(Cl) (10) and LYbCp(2) (11) supported by the bulky beta-diketiminate ligand were unsuccessful. However, the reaction of LAl(OH)Me with LYbN(SiMe(3))(2)Cl (12) containing a labile Yb-N bond leads to the formation of LYbCl-O-AlL(Me) (13) under elimination of HN(SiMe(3))(2). Furthermore, complexes 1, 3, 4, and 6 exhibit good catalytic activity for the polymerization of epsilon-caprolactone.