Variable-temperature 1H NMR Studies Research Articles

Electron-Rich PNP- and PNN-Type Ruthenium(II) Hydrido Borohydride Pincer Complexes. Synthesis, Structure, and Catalytic Dehydrogenation of Alcohols and Hydrogenation of Esters

Electron-rich PNP- and PNN-type ruthenium(II) hydrido borohydride pincer complexes, [RuH(BH4)(tBu-PNP)] (tBu-PNP = (2,6-bis(di-tert-butylphosphinomethyl)pyridine) (5) and [RuH(BH4)(tBu-PNN)] (tBu-PNN = 2-di-tert-butylphosphinomethyl-6-diethylaminomethylpyridine) (6), were prepared from their corresponding N2-bridged dinuclear Ru(II) complexes [(tBu-PNP)RuCl2]2(μ-N2) (3) and [(tBu-PNN)RuCl2]2(μ-N2) (4), respectively. The X-ray structure of 5 reveals a BH4– anion η2 coordinated to ruthenium through two bridging hydrides. A variable-temperature 1H NMR study of 6 exhibits interesting fluxional behavior of the BH4– ligand. Similarly, the Ru(II) hydrido borohydride complex 9, in which the BH4– moiety is coordinated in a η1 bonding mode, was obtained by reaction of [RuCl2(PPh3)(iPr-PNP)] (iPr-PNP = 2,6-bis(diisopropylphosphinomethyl)pyridine) (8) with two equivalents of NaBH4 at room temperature. The hydrido borohydride pincer complexes 5, 6, and 9 catalyze the acceptorless dehydrogenative coupling of primary al...

Carbazole-bis(oxazolines) as Monoanionic, Tridentate Chelates in Lanthanide Chemistry: Synthesis and Structural Studies of Thermally Robust and Kinetically Stable Dialkyl and Dichloride Complexes.

The carbazole-bis(oxazoline) ligand 1,8-bis(4′,4′-dimethyloxazolin-2′-yl)-3,6-di-tert-butylcarbazole (H-Czx, 14) was prepared and reacted with Ln(CH2SiMe3)3(THF)2 to afford the five-coordinate dialkyl complexes (Czx)Ln(CH2SiMe3)2 (Ln = Y, 15; Er, 16; Yb, 17) in high yield. Deprotonation of 14 with NaN(SiMe3)2, followed by reaction with LnCl3(THF)x, affords the six-coordinate dichloride complexes cis,mer-(Czx)Ln(Cl)2(THF) (Ln = Y, 18; Er, 19; Yb, 20). Dialkyl complexes 15–17 were also prepared by reaction of the dichloride complexes 18–20, respectively, with 2 equiv of LiCH2SiMe3. Complexes 16–20 were characterized by X-ray crystallography. The dialkyl complexes 16 and 17 display a five-coordinate trigonal-bipyramidal geometry with the Czx ligand spanning both axial and one equatorial site with the CH2SiMe3 rotated such that the SiMe3 groups sit over the carbazole ring system. The solution structure for 15–20 was found to be consistent with that in the solid state by NMR spectroscopy. However, paramagnetic dichloride 20 was shown by variable-temperature 1H NMR studies to undergo rapid THF exchange by a dissociative process in toluene-d8 solution (ΔH‡ = 98 ± 10 kJ mol–1, ΔS‡ = +185 ± 25 J mol–1 K–1). Treatment of dialkyl 18 with [Ph3C]+[B(C6F5)4]− cleanly produces the alkyl cation [(Czx)Y(CH2SiMe3)]+[B(C6F5)4]− (21) by 1H NMR spectroscopy, but this species does not insert 2,3-dimethylbutadiene.

Rhenium tricarbonyl chloro of di-2-pyridylketone 4-aminobenzoyl hydrazone (dpk4abh), fac-[Re(CO) 3(κ 2-N,N-dpk4abh)Cl]: Synthesis, spectroscopic and electrochemical properties

The reaction between di-2-pyridyl ketone 4-aminobenzoylhydrazone hydrate (dpk4abh.H 2O) and [Re(CO) 5Cl] in refluxing toluene gave fac-[Re(CO) 3(κ 2-N,N-dpk4abh)Cl]. Hydrolysis of fac-[Re(CO) 3(κ 2-N,N-dpk4abh)Cl] to form fac-[Re(CO) 3(κ 3-N,O,N-dpkO,OH)] occurs in wet solvents. The identity of fac-[Re(CO) 3(κ 2-N,N-dpk4abh)Cl] was established from the results of its elemental analysis, spectroscopic measurements, and electrochemical properties. The infrared spectra of fac-[Re(CO) 3(κ 2-N,N-dpk4abh)Cl] show the facial coordination of the carbonyl (C O) groups, the coordination of dpkabh and none coordination of the carbonyl (C O) group. 1H-NMR measurements on fac-[Re(CO) 3(κ 2-N,N-dpk4abh)Cl] in d 6-dmso and d 7-dmf confirmed the coordination of dpkabh, and demonstrate sensitivity of fac-[Re(CO) 3(κ 2-N,N-dpk4abh)Cl] to its surroundings as manifested by the chemical shift variations observed in different solvents. Variable temperature 1H-NMR studies showed stronger interaction between d 7-dmf and fac-[Re(CO) 3(κ 2-N,N-dpk4abh)Cl] compared to d 6-dmso and the amine protons of coordinated dpk4abh undergo faster exchanges with solvated water protons compared to the amide proton. The electronic absorption spectra of fac-[Re(CO) 3(κ 2-N,N-dpk4abh)Cl] showed significant solvent dependence. In CH 3CN, a single intra-ligand charge transfer (ILCT) transition appeared at 332 nm and assigned to π → π * of dpk followed by dpk → hydrazone charge transfer. In a protophilic solvent, such as dmf, two intra-ligand charge transfer (ILCT) transitions assigned to fac-[Re(CO) 3(κ 2-N,N-dpk4abh-H)Cl] and fac-[Re(CO) 3(κ 2-N,N-dpk4abh)Cl] appeared at 464 and 350 nm, respectively. Reversible interconversion between fac-[Re(CO) 3(κ 2-N,N-dpk4abh)Cl] and its conjugate base fac-[Re(CO) 3(κ 2-N,N-dpk4abh-H)Cl] was established in protophilic solvents by using a base to shift the equilibrium to the conjugate base and an acid to shift the equilibrium or conjugate base to the neutral form. Acid–base titrations of fac-[Re(CO) 3(κ 2-N,N-dpk4abh-H)Cl] or fac-[Re(CO) 3(κ 2-N,N-dpk4abh)Cl] show acids or bases in concentrations as low as 1x10 −6 M can be detected and determined using protophilic solutions of fac-[Re(CO) 3(κ 2-N,N-dpk4abh)Cl]. Electrochemical measurements on fac-[Re(CO) 3(κ 2-N,N-dpk4abh)Cl] in CH 3CN and CH 2Cl 2 showed sequential irreversible redox processes consistent with the electrochemical reduction or oxidation of the hydrazone backbone.

Intramolecular hydroamination of unactivated alkenes with secondary alkylamines catalyzed by iridium phosphino–phenolate complexes

The phosphino–phenolate complex (κ2-{2-i-Pr2PC6H4}O)Ir(COD) (COD = η4-1,5-cyclooctadiene; 1) is shown to be an effective precatalyst for the intramolecular hydroamination of unactivated alkenes with pendant secondary alkylamines, providing either pyrrolidine or piperidine heterocycles in high isolated yields. While monitoring the progress of a selected test reaction of this type, a significant induction period (~3 h) was observed; variable-temperature 1H and 31P NMR studies (25–100 °C) under catalytically relevant conditions revealed no new metal-containing species. In an effort to develop an enantiopure variant of 1, a synthetic route to the chiral (κ2-P,O)Ir(COD) complex (7), which features a 1-aryl-2,5-dialkylphospholane structure derived from (2S,5S)-2,5-hexanediol cyclic sulfate, was developed. The structure of 7 was confirmed by use of single-crystal X-ray diffraction techniques. While 7 failed to provide enantioselectivity in the intramolecular hydroamination of unactivated alkenes with pendant secondary alkylamines, the activity of 7 was found to be comparable to that of 1.

Synthesis, Structures, and Characterizations of [ArTi(O-i-Pr)3]2and Efficient Room-Temperature Aryl−Aryl Coupling of Aryl Bromides with [ArTi(O-i-Pr)3]2Catalyzed by the Economic Pd(OAc)2/PCy3System

The series of aryltris(2-propoxo)titanium reagents [ArTi(O-i-Pr)3]2 (Ar = Ph (1a), 2-MeC6H4 (1b), 4-MeC6H4 (1c), 4-ClC6H4 (1d), 4-TMSC6H4 (1e), 4-CF3C6H4 (1f), 3,5-Me2C6H3 (1g)) was synthesized from reactions of the in situ prepared ClTi(O-i-Pr)3 with ArMgBr in THF. All compounds were characterized by NMR spectroscopy and elemental analyses. A variable-temperature 1H NMR study of 1c suggested that compounds 1 are dimeric species in solution. The structures of complexes 1b,g were further determined by single-crystal X-ray analyses. X-ray diffraction studies confirmed that the aryltris(2-propoxo)titanium complexes have a dimeric structure with a Ti2O2 core bridging through the oxygen atom of two 2-propoxide groups. The application of aryltris(2-propoxo)titanium reagents to cross-coupling with aryl bromides catalyzed by the catalytic system of 1 mol % Pd(OAc)2 and 2 mol % PCy3 was studied. Results show that aryltris(2-propoxo)titanium compounds are excellent reagents for aryl−aryl coupling reactions at room ...

Encapsulated-guest rotation in a self-assembled heterocapsule directed toward a supramolecular gyroscope

The self-assembled heterocapsule 1.2, which is formed by the hydrogen bonds of tetra(4-pyridyl)-cavitand 1 and tetrakis(4-hydroxyphenyl)-cavitand 2, encapsulates 1 molecule of guests such as 1,4-diacetoxybenzene 3a, 1,4-diacetoxy-2,5-dimethylbenzene 3b, 1,4-diacetoxy-2,5-dialkoxybenzenes (3c, OCH(3); 3d, OC(2)H(5); 3e, OC(3)H(7); 3f, OC(4)H(9); 3g, OC(5)H(11); 3h, OC(6)H(13); 3i, OC(8)H(17)), 1,4-diacetoxy-2,5-difluorobenzene 4a, and 1,4-diacetoxy-2,3-difluorobenzene 4b. The X-ray crystallographic analysis of 3c@(1.2) showed that the acetoxy groups at the 1,4-positions of 3c are oriented toward the 2 aromatic cavity ends of 1.2 and that 3c can rotate along the long axis of 1.2. Thus, the 1.2 (stator) with the encapsulation guest (rotator) behaves as a supramolecular gyroscope. A variable temperature (VT) (1)H NMR study in CDCl(3) showed that 3a, 3b, 4a, and 4b within 1.2 rotate rapidly even at 218 K, whereas guest rotation is almost inhibited for 3h and 3i even at 323 K. In this respect, 4b with a large dipole moment is a good candidate for the rotator of 1.2. For 3c-3g, the enthalpic (DeltaH(double dagger)) and entropic (DeltaS(double dagger)) contributions to the free energy of activation (DeltaG(double dagger)) for the guest-rotational steric barriers within 1.2 were obtained from Eyring plots based on line-shape analysis of the VT (1)H NMR spectra. The value of DeltaG(double dagger) increased in the order 3c < 3d < 3e < 3f < 3g. Thus, the elongation of the alkoxy chains at the 2,5-positions of 3 puts the brakes on guest rotation within 1.2.

Dynamic Properties of a Hexadecamolybdenum Wheel: Studies in Solution and Density Functional Theory Calculations

Variable temperature (1)H NMR studies of the host-guest complex [Mo(16)O(16)S(16)(OH)(16)(H(2)O)(4)(PDA)(2)](4-) (1 ; PDA(2-) = phenylenediacetate) previously carried out in D(2)O have revealed a complex behavior in solution, involving a gliding motion of both parallel phenyl rings of the PDA(2-) ligands. In the present work, we present new NMR spectra carried out in the aprotic solvent CD(3)CN, which allow the observation of the proton signals associated with the bridging hydroxo groups of the inorganic host. The new spectra provide detailed information about the concerted reorganization of the guest components, that is, PDA(2-) and water molecules. The existence of an equilibrium between two distinct isomers differing in the linking mode between the inorganic host and the two equivalent PDA(2-) ligands is evidenced. This equilibrium appears strongly dependent upon the temperature, leading to a complete inversion of the distribution between 300 and 226 K. The thermodynamic data related to the isomerization reaction have been determined (Delta(r)H = -50.5 kJ mol(-1) and Delta(r)S = -215 J mol(-1) K(-1)). Furthermore, at low temperature, one of the isomers exists in two conformations, only differing in the H-bond network involving the inner water molecules. Density functional theory calculations were carried out to push ahead the interpretations obtained from experiment, identify the isomers of 1, and specify the role and the positions of the guest water molecules. Among the various structures that have been calculated for 1, three fall in a narrow energy range and should correspond to the species characterized by variable-temperature (1)H NMR experiments in CD(3)CN. Finally, this study shows how the internal disposition of the ligands affects the ellipticity of the Mo(16) ring which varies from one isomer to the other in the 0.73-1 range and highlights solvation of the ring as one of the key parameters for the conformational design of these flexible host-guest systems.

Iron(II) Tris-[N4-substituted-3,5-di(2-pyridyl)-1,2,4-triazole] Complexes: Structural, Magnetic, NMR, and Density Functional Theory Studies

Eight mononuclear iron(II) complexes of N(4)-3,5-di(2-pyridyl)-1,2,4-triazole (Rdpt) ligands have been prepared and characterized. In all cases the iron(II)/ligand ratio used is 1:3, giving red complexes of the general formula [Fe(II)(Rdpt)(3)](BF(4))(2) x solvents, in 55-89% yield. The ligands differ only in the nature of the N(4)-substituent (amino, pyrrolyl, iso-butyl, methyl, phenyl, para-tolyl, 3,5-dichlorophenyl, and 4-pyridyl; for ligands adpt, pldpt, ibdpt, medpt, phdpt, ptdpt, Cldpt, and pydpt, respectively) allowing substituent effects on the properties of the resulting iron(II) complexes to be probed. The low temperature crystal structures of seven of the complexes reveal low spin iron(II) environments. Packing analyses reveal anion-pi and acetonitrile-pi interactions involving the tetrafluoroborate counteranions and interstitial acetonitrile molecules, respectively. Both "pi-pockets" and "pi-sandwiches" are observed. Solid state magnetic susceptibility measurements (4-300 K) indicate the iron(II) is low spin (LS) in all complexes at all temperatures studied, except for [Fe(II)(pldpt)(3)](BF(4))(2) x 1 1/2 H(2)O which has the beginnings of spin crossover (SCO) at elevated temperatures. Downfield shifts and peak broadening observed in the variable temperature (1)H NMR studies indicate that in d(3)-nitromethane solution the LS [Fe(II)(Rdpt)(3)](2+) complexes are in equilibrium with a trace of a high spin (HS) species. (15)N NMR spectra (measured and calculated) of the ligands reveal that altering the N(4)-substituent changes the chemical shift of the N(1) triazole and pyridine nitrogen atoms, allowing probing of the relationship between ligand substituent and the nature of the coordinating nitrogen atoms.

A conformational study of [3.3](3,5)pyridinophane

A variable temperature 1H NMR study of the [3.3](3,5)pyridinophane indicated that the syn(chair/chair) is more stable than syn(chair/boat) by 0.2 kcal/mol in solution, whereas the trimethylene bridges show disorder even at −150 °C in the solid state. A transition state search by ab initio MO calculations suggested two competitive conformational changes for syn(chair/chair)– syn(chair/boat) conversion via a bridge wobble or a ring inversion in the [3.3](3,5)pyridinophane.

Geometrical isomers of [TEAH][Co(LSe)2]·xH2O: synthesis, structural, spectroscopic and computational studies

Trans-1, [HNEt3][CoIII(LSe)2].H2O and cis-1, [HNEt3][CoIII(LSe)2].3H2O have been synthesized and characterized by single-crystal X-ray studies. The counter ion Et3NH+ plays a crucial role in the crystal packing leading to the formation of two distinctly different supramolecular assemblies in the two complexes. In trans-1, Co-bisphenolate units and triethylamine molecules are arranged in a linear fashion leading to a supramolecular columnar assembly along the crystallographic a-axis. In this assembly, triethylammonium ions are sandwiched between successive Co-bisphenolate units and act as gluing agents joining Co-bisphenolate units on either side through C-Hpi interactions. In sharp contrast to trans-1, Co-bisphenolate units and triethylammonium ions in cis-1 are arranged in a helical supramolecular assembly through similar C-Hpi interactions along the crystallographic b-axis. The SeSe van der Waals interactions may be responsible for the predominant occurrence of the cis- isomer. The cyclic voltammetric studies showed quasi-reversible waves for the cobalt(III)-->cobalt(II) reductions with E1/2=0.635 and 0.628 V vs. Ag/AgCl for cis-1 (at approximately 5 degrees C) and trans-1 (at approximately 25 degrees C), respectively. DFT calculations show that the trans-form is the thermodynamic product with higher stability than the cis- one, which is consistent with the variable temperature 1H NMR studies.

Group 4 bis(chelate) metal complexes of monoanionic bidentate [E,O−] ligands (E = O, S): synthesis and application as α-olefin polymerization catalysts

Monoanionic bidentate phenoxy-ether [O(-),O] and phenoxy-thioether [O(-),S] ligands were synthesized and used to prepare octahedral group 4 metal complexes with general formula [O(-),E](2)ML(2) (complex 1: E = S, M = Hf, L = Bn; complex 2: E = O, M = Ti, L = NMe(2); complex 3: E = S, M = Ti, L = NMe(2); complex 4: E = S, M = Ti, L = Cl). Variable-temperature 1H NMR studies were performed on all the complexes 1-4 in the range of +20 to -70 degrees C. For the hafnium complex 1 a dynamic interchange of the Delta and Lambda enantiomers for a C2-symmetric isomer was revealed. Eyring analysis indicated that rearrangement of the ligands occurs via a non-dissociative mechanism. In the range of the explored temperatures, a broadening of the signals was observed for complexes 2-3. Finally, a reversible fluxional process between several isomers was revealed for complex 4. Complexes 1-4 were tested as pre-catalysts for ethylene and propene polymerization in combination with different activators and under variable conditions. Complex 1 was found to be inactive whereas complexes 2-4 showed moderate activities in the polymerization of ethylene and propene. A mixture of polymers with different microstructure was obtained in all cases, coherently with the fluxional behaviour of the pre-catalysts observed in solution.

Stereochemically non-rigid helical mercury(II) complexes

Reactions of the 1:2 condensate (L) of benzil dihydrazone and 2-acetylpyridine with Hg(ClO 4) 2 · xH 2O and HgI 2 yield yellow [HgL 2](ClO 4) 2 ( 1) and HgLI 2 ( 2), respectively. Homoleptic 1 is a 8-coordinate double helical complex with a Hg(II)N 8 core crystallising in the space group Pbca with cell dimensions: a = 16.2250(3), b = 20.9563(7), c = 31.9886(11) Å. Complex 2 is a 4-coordinate single helical complex having a Hg(II)N 2I 2 core crystallising in the space group P2 1/ n with cell dimensions a = 9.8011(3), b = 17.6736(6), c = 16.7123(6) Å and β = 95.760(3) o. In complex 1, the N-donor ligand L uses all of its binding sites to act as tetradentate. On the other hand, it acts as a bidentate N-donor ligand in 2 giving rise to a dangling part. From variable temperature 1H NMR studies both the complexes are found to be stereochemically non-rigid in solution. In the case of 2, the solution process involves wrapping up of the dangling part of L around the metal.

Molecular Encapsulation via Metal-to-Ligand Coordination in a Cu(I)-Folded Molecular Basket

A molecular basket, composed of a semirigid C3v symmetric tris-norbornadiene framework and three pyridine flaps at the rim, has been shown to coordinate to a Cu(I) cation and thereby fold in a multivalent fashion. The assembly was effective (Ka = 1.73 +/- 0.08 x 10(5) M(-1)) and driven by enthalpy (DeltaH(o) = -7.2 +/- 0.1 kcal/mol, DeltaS(o) = -0.25 eu). Variable temperature (1)H NMR studies, assisted with 2D COSY and ROESY investigations, revealed the existence of Cu(I)-folded basket 10b with a molecule of acetonitrile occupying its interior and coordinated to the metal. Interestingly, 10b is in equilibrium with Cu(I)-folded 10a , whose inner space is solvated by acetone or chloroform. The incorporation of a molecule of acetonitrile inside 10a was found to be driven by enthalpy (DeltaH(o) = -3.3 +/- 0.1 kcal/mol), with an apparent loss in entropy (DeltaS(o) = -9.4 +/- 0.4 eu); this is congruent with a complete immobilization of acetonitrile and release of a "loosely" encapsulated solvent molecule during 10a/b interconversion. From an Eyring plot, the activation enthalpy for incorporating acetonitrile into 10a was found to be positive (DeltaH(double dagger) = 6.5 +/- 0.5 kcal/mol), while the activation entropy was negative (DeltaS(double dagger) = -20 +/- 2 eu). The results are in agreement with an exchange mechanism whereby acetonitrile "slips" into an "empty" basket through its side aperture. In fact, DFT (BP86) calculations are in favor of such a mechanistic scenario; the calculations suggest that opening of the basket's rim to exchange guests is energetically demanding and therefore less feasible.

Reactions of the (2-Pyridyl) Pyrrolide Platinum(II) Complex Driven by Sterically Encumbered Chelation: A Model for the Reversible Attack of Alcohol at the Coordinated Carbon Monoxide

Treatment of 3,5-bis(trifluoromethyl)-2-(2'-pyridyl)pyrrole (fpyroH) with Pt(DMSO)2Cl2 and Na2CO3 in THF solution gave a light-yellow complex denoted as [Pt(fpyro)2] (1). A single-crystal X-ray diffraction study on 1 revealed a large conformational distortion around the platinum(II) center, which is attributed to interligand repulsion between the pyridyl groups and the CF3 substituents of the nearby pyrrolides. Reaction of 1 with N- and C-donor ligands such as acetonitrile, pyridine, isocyanide, and CO affords the adducts [Pt(fpyro)2(L)], L = NCMe (2), pyridine (3), CNBut (4), and CO (5), showing formation of one monodentate fpyro ligand by release of the strain energy. The variable-temperature 1H NMR studies showed a static structure for the N-substituted adducts 2 and 3, whereas the C-adducts 4 and 5 are shown to be more labile, displaying a pairwise exchange of bidentate and monodentate fpyro ligands in solution. Addition of ethanol to the coordinated CO in 5 during recrystallization is also established, affording an ethoxycarbonyl complex [Pt(fpyro)(fpyroH)(CO2Et)] (6), which was isolated as a crystalline solid and can be readily converted back to 5 and free ethanol upon dissolution at room temperature.

Highly Efficient Copper‐Mediated Atom‐Transfer Radical Addition (ATRA) in the Presence of Reducing Agent

AbstractThe synthesis, characterization and exceptional activity of CuI(TPMA)Br [TPMA = tris(2‐pyridylmethyl)amine] and [CuII(TPMA)Br][Br] complexes in ATRA reactions of polybrominated compounds to alkenes in the presence of reducing agent (AIBN) was reported. [CuII(TPMA)Br][Br], in conjunction with AIBN, effectively catalyzed ATRA reactions of CBr4 and CHBr3 to alkenes with concentrations between 5 and 100 ppm, which is the lowest number achieved in copper‐mediated ATRA. The molecular structure of CuI(TPMA)Br indicated that the complex was pseudo‐pentacoordinate in the solid state due to the coordination of TPMA [CuI–N: 2.1024(15), 2.0753(15), 2.0709(15) and 2.4397(14) Å] and bromide anion to the copper(I) center [CuI–Br 2.5088(3) Å]. Variable temperature 1H NMR and cyclic voltammetry studies confirmed the equilibrium between CuI(TPMA)Br and [CuI(TPMA)(CH3CN)][Br], indicating some degree of halide anion dissociation in solution. The coordination of the bromide anion to the [CuI(TPMA)]+ cation resulted in a formation of much more reducing CuI(TPMA)Br complex (E1/2 = –720 mV vs. Fc/Fc+) than the corresponding ClO4– (E1/2 = –422 mV vs. Fc/Fc+) and PF6– (E1/2 = –421 mV vs. Fc/Fc+) analogues. In [CuII(TPMA)Br][Br], the CuII atom was coordinated by four nitrogen atoms [CuII–Neq 2.073(2) Å and CuII–Nax 2.040(3) Å] from TPMA ligand and a bromine atom [CuII–Br 2.3836(6) Å]. The overall geometry of the complex was distorted trigonal bipyramidal. CuI(TPMA)Br and [CuII(TPMA)Br][Br] complexes showed similar structural features from the point of view of TPMA coordination. The only more pronounced difference in the TPMA coordination to the copper center was observed in the shortening of Cu–Nax bond length by approximately 0.400 Å on going from CuI(TPMA)Br to [CuII(TPMA)Br][Br]. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2008)

Variable temperature 1H NMR studies on Grubbs catalysts

Variable temperature 1H NMR studies were conducted to investigate whether steric congestion is influencing the structural rigidity of (IMesH 2)(PCy 3)(Cl) 2Ru CHPh (IMesH 2 = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene) in solution. It was shown that both mesityl ligands rotate at about the same rate around the N-Mesityl bonds in the IMesH 2 ligand and that changing the solvent does not significantly alter this rotation. It was found that the increased steric congestion in (IMesH 2)(PCy 3)(Cl) 2Ru CHPh compared to (PCy 3) 2(Cl) 2Ru CHPh does affect the rates of rotation around the C alkylidene–Ph bonds. Unusual chemical shift positions were also observed in the low temperature 1H NMR spectrum for the aromatic proton signals for (IMesH 2)(PCy 3)(Cl) 2Ru CHPh and (PCy 3) 2(Cl) 2Ru CHPh.

Coupling of CpCr(CO)3and Heterocyclic Dithiadiazolyl Radicals. Synthetic, X-ray Diffraction, Dynamic NMR, EPR, CV, and DFT Studies

The reaction of the 1,2,3,5-dithiadiazolyls (4-R-C(6)H(4)CN(2)S(2))(2) (R = Me, 2a; Cl, 2b; OMe, 2c; and CF3, 2d) and (3-NC-5-tBu-C(6)H(3)CN(2)S(2))(2) (2e) with [CpCr(CO)(3)](2) (Cp = eta(5)-C(5)H(5)) (1) at ambient temperature respectively yielded the complexes CpCr(CO)(2)(eta(2)-S(2)N(2)CC(6)H(4)R) (R = 4-Me, 3a; 4-Cl, 3b; 4-OMe, 3c; and 4-CF(3), 3d) and CpCr(CO)(2)(eta(2)-S(2)N(2)CC(6)H(3)-3-(CN)-5-(tBu)) (3e) in 35-72% yields. The complexes 3c and 3d were also synthesized via a salt metathesis method from the reaction of NaCpCr(CO)(3) (1B) and the 1,2,3,5-dithiadiazolium chlorides 4-R-C(60H(4)CN(2)S(2)Cl (R = OMe, 8c; CF(3), 8d) with much lower yields of 6 and 20%, respectively. The complexes were characterized spectroscopically and also by single-crystal X-ray diffraction analysis. Cyclic voltammetry experiments were conducted on 3a-e, EPR spectra were obtained of one-electron-reduced forms of 3a-e, and variable temperature 1H NMR studies were carried out on complex 3d. Hybrid DFT calculations were performed on the model system [CpCr(CO)(2)S(2)N(2)CH] and comparisons are made with the reported CpCr(CO)(2)(pi-allyl) complexes.

Synthesis and Structural Characterization of Magnesium Ketiminate Complexes: Efficient Initiators for the Ring-Opening Polymerization of l-Lactide

A series of dinuclear magnesium complexes of the type [LMg(μ-OBn)]2 (where L = NNO-tridentate ketiminate ligand) have been prepared. Their activities toward the ring-opening polymerization of l-lactide have been investigated. Experimental results indicate that the reactivity of [LMg(μ -OBn)]2 is dramatically affected by the steric and electronic effect of the substituents on the ketiminate ligand. On the basis of the melting point depression and variable-temperature 1H NMR studies, it is believed that these magnesium complexes exist as an equilibrium between a monomeric and a dimeric species in solution in which the monomeric species is the most active species during polymerization. The polymerization probably proceeds with the coordination of the lactide to the monomeric species giving a five-coordinated magnesium complex as an intermediate.

1,4,5,8,9,12-Hexamethyltriphenylene. A Molecule with a Flipping Twist

The synthesis and characterization of 1,4,5,8,9,12-hexamethyltriphenylene (5) is described. In the solid state, X-ray crystallographic studies reveal that compound 5 presents a highly distorted C2 geometry with a 53 degrees end-to-end twist. In solution, variable-temperature 1H NMR studies and molecular modeling present a story of rapid dynamic conformational interconversions between two C2 enantiomers (with a low activation barrier) and a slower C2-D3 interconversion (with a relatively high barrier)--the first time clear evidence of conformational interchange for these hindered triphenylenes has been provided. Further studies have established that 5 is a fluorescent stable blue emitter, and that the compound undergoes an irreversible one-electron electrochemical oxidation. Calculations have predicted this to be a radical cation of C2 geometry with 60 degrees end-to-end twist.

Ruthenium Polyhydrido Clusters Having a Bridging Alkylzinc Group, [(η5-C5Me5)Ru(μ-H)]3(μ3-ZnR)n(μ3-H)2-n and [(η5-C5Me5)Ru]2(μ-ZnR)n(μ-H)4-n (R = Me and Et; n = 1 and 2)

Novel triruthenium polyhydrido complexes having triply bridging alkylzinc ligands, {Cp‘Ru(μ-H)}3(μ3-ZnR)(μ3-H) (3a: R = Me, 3b: R = Et) and {Cp‘Ru(μ-H)}3(μ3-ZnEt)2 (4a: R = Me, 4b: R = Et), are obtained by the reaction of {Cp‘Ru(μ-H)}3(μ3-H)2 (2) with a stoichiometric amount of the corresponding dialkylzinc ZnR2. X-ray diffraction studies of 3b and 4a display the structural identity of those that have a triply bridging alkylzinc ligand. The X-ray studies as well as 1H NMR spectroscopy reveal the existence of bonding interactions between the μ3-zinc and the Ru-bound hydrido ligands in 3 and 4. In the variable-temperature 1H NMR study of 3b, activation parameters, ΔS⧧ and ΔH⧧, for the coordination-site exchange process among the hydrido ligands were estimated at −1.5 ± 1.3 cal mol-1 K-1 and 15.0 ± 0.4 kcal mol-1, respectively. The reaction of a diruthenium tetrahydrido complex, (Cp‘Ru)2(μ-H)4 (1), with an equimolar amount of dialkylzinc yields (Cp‘Ru)2(μ-ZnR)(μ-H)3 (5a: R = Me, 5b: R = Et). Di-μ-alkyl...