VT 1H NMR Research Articles

Organomagnesia: Reversibly High Carbon Dioxide Uptake by Magnesium Pyrazolates.

A series of new pyrazolate and mixed pyrazolate/pyrazole magnesium complexes is described and their reactivity toward carbon dioxide is examined. The dimeric complex [Mg(pzt Bu, t Bu)2]2 inserts CO2 instantly and quantitatively forming the tetrameric complex [Mg(CO2·pzt Bu, t Bu)2]4 and monomeric donor-stabilized [Mg(CO2·pzt Bu, t Bu)2(thf)2]. Complexes of the type [Mgx(pzR,R)2 x(HpzR,R)y]n (R = iPr, tBu) engage in similar insertion reactions involving dissociation of the carbamic acid HOOCpzR,R. Even solid polymeric derivatives [Mg(pzR,R)2]n (R = Me, H) react instantaneously and exhaustively with CO2, the resulting [Mg(CO2·pz)2]m featuring a CO2 capacity of 35.7wt% (8.2mmol g-1). All described magnesium pyrazolates display completely reversible CO2 uptake in solution and in the solid state, respectively, as monitored via VT 1H NMR and in situ FTIR spectroscopy as well as thermogravimetric analysis. Fluorinated [Mg2(pzCF3,CF3)4(thf)3] does not yield any isolable CO2 insertion product but exhibits the highest activity in the catalytic transformation of epoxides and CO2 to cyclic carbonates.

Flexible NHC-aryloxido aluminum complex and its zwitterionic imidazolium aluminate precursor in ring-opening polymerization of ε-caprolactone.

Anionic donor-functionalized NHC (N-heterocyclic carbene) complexes of Al are rare. We report one such case here, an NHC-aryloxido AlMe2 complex [Al(L)Me2] (2), following a stepwise synthesis from the proligand [HO-4,6-tBu2-C6H2-2-CH2{CH(NCHCHNAr)}]Br [LH2Br; Ar = 2,6-iPr2-C6H3 (Dipp)] and AlMe3via the zwitterionic intermediate [Al(LH)Me2Br] (1). The ligand's flexibility in 2 is evident from the conformational fluxionality revealed by VT-1H NMR spectroscopic analysis. The ∠O-Al-C (ca. 100.5°) bite angle is also wider than the ∠O-Ti-C (ca. 80.6°) as seen in our recently reported Ti complex [Ti(L)(NMe2)2Br]. DFT analysis showed that the CNHC-Al bond is significantly ionic, as is the CNHC-Ti bond. Both 1 and 2 are active in the ring-opening polymerization (ROP) of ε-caprolactone (CL). 2, similar to [Ti(L)(NMe2)2Br], exhibits bifunctional MLC-type monomer activation, but only at an elevated temperature. However, the 2/BnOH combination is catalytically active at room temperature, likely through a zwitterionic [Al(LH)Me2(OBn)]. The 1/BnOH combination follows a similar mechanism but surprisingly at a faster rate.

Synthesis, Characterization, and Spectroscopic Studies of 2,6-Dimethylpyridyl-Linked Cu(I)-CNC Complexes: The Electronic Influence of Aryl Wingtips on Copper Centers.

Six new Cu(I) complexes containing pincer ligands of the type 2,6-bis(3-alkyl/arylimidazol-2-ylidene) methylpyridine I(R/R'Ar) ĈN̂C, where R = trifluoroethyl (TFE) and R' = 4-CF3, 4-NO2, 4-CN, 4-H, and 4-CH3, have been synthesized. These complexes, namely, [Cu(I(TFE)ĈN̂C)]PF6, 1-TFE; [Cu(ICF3Ar ĈN̂C]PF6, 2-CF3; [Cu(INO2Ar ĈN̂C)]PF6, 3-NO2; [Cu(ICNAr ĈN̂C]PF6, 4-CN; [Cu(IHAr ĈN̂C)]2(PF6)2, 5-H; and [Cu(ICH3Ar ĈN̂C)]2(PF6)2, 6-CH3, were fully characterized by 1H, 13C, and HMBC NMR spectroscopy, elemental analysis, electrochemical studies, and single-crystal X-ray crystallography. The crystallographic data revealed different structures and copper nuclearities for the complexes bearing aryl wingtips with electron-withdrawing (2-CF3, 3-NO2, and 4-CN) and electron-donating (5-H and 6-CH3) substituents. The solution-phase conductivity measurements in acetonitrile revealed a mix-electrolyte behavior for these complexes, supporting the presence of both mono- and binuclear forms of each complex. The fast monomer-dimer equilibrium of the Cu-CNC complexes at room temperature is reflected in their simple 1H NMR spectra in acetonitrile. However, both mono- and binuclear forms were identifiable in 1H diffusion-ordered spectroscopy (DOSY) at low temperatures. The dynamic behavior of these complexes in solution was further examined by variable-temperature 1H NMR (VT 1H NMR) experiments, and the relevant thermodynamic parameters were determined. The process was also probed by one-dimensional rotating-frame Overhauser enhancement spectroscopy (1D ROESY) experiments to elucidate the coexisting species in solution. The 2,6-dimethylpyridyl-linked Cu-CNC complexes also presented a quasi-reversible Cu(II)/Cu(I) couple in cyclic voltammetry studies, wherein a clear influence of the aryl wingtips on the E1/2 values was observed. Furthermore, the percent buried volumes (% Vbur) of the complexes were calculated, showing a similar steric hindrance around copper in all complexes. These findings support the importance of electronic effects, induced by the aryl wingtips, on the preferred coordination geometry, copper nuclearity, and redox properties of the Cu-CNC complexes.

Isoselective Polymerization of rac-Lactide by Magnesium Initiators Bearing Achiral Di(2-pyridyl)methyl Substituted Aminophenolate Ligands.

Reactions of achiral di(2-pyridyl)methyl substituted aminophenols L1-6H (2-{N-R3-N-[di(2-pyridyl)methyl]aminomethyl}-4-R1-6-R2-C6H2OH: R1 = R2 = tBu, R3 = nBu (L1H), R3 = nhexyl (L2H), R3 = cyclohexyl (L3H); R1 = R2 = cumyl, R3 = nBu (L4H), R3 = nhexyl (L5H), R3 = cyclohexyl (L6H)) with {Mg[N(SiMe3)2]2}2 ([L1-6H]:[Mg] = 1:1) afforded a series of magnesium silylamido complexes 1-6. In the solid state, the magnesium center of 3, 4, and 6 is penta-coordinated by the tetradentate aminophenloate ligand and one silylamido ligand to form a seriously distorted square-pyramidal geometry as confirmed by X-ray crystallography diffraction analysis. VT 1H NMR and ROESY experiments further indicate that these magnesium complexes are also five-coordinated in solutions where the coordination of either of the two pyridyl pendants to the magnesium center is maintained. Complexes 1-6 are highly active toward the ring-opening polymerization of rac-lactide (rac-LA) at r.t. both in toluene and in tetrahydrofuran, capable of polymerizing 500 equiv of monomer to high conversions just within minutes. Among them, complex 3 exhibited the highest iso-stereoselectivity, affording moderately isotactic polylactide in toluene (Pm = 0.75). It is found that the isoselectivities and activities of these magnesium complexes toward the polymerization of rac-LA are closely associated with the substituents at the ortho-position of the phenoxide unit and on the skeleton nitrogen atom of the ligand. On the basis of NMR spectroscopic studies, the formation of isotactic PLAs with dominant stereoblock sequences was witnessed by using these magnesium complexes as initiators, and the inequivalent coordination of two pyridyl pendant arms in these magnesium complexes might be the source of exerting isoselective control.

Cleavage of an Aromatic C–C Bond in Ferrocene by Insertion of an Iridium Nitrido Nitrogen Atom

The intermolecular cleavage of C-C bonds is a rare event. Herein, we report on a late transition-metal terminal nitrido complex, which upon oxidation undergoes insertion of the nitrido nitrogen atom into the aromatic C-C bond of ferrocene. This reaction path was confirmed through 15N and deuterium isotope labeling experiments of the nitrido complex and ferrocenium, respectively. Cyclic voltammetry and UV/vis spectroscopy monitoring of the reaction revealed that oxidation is the initial step, yielding the tentative radical cationic nitrido complex, which is experimentally supported by extended X and Q-band electron paramagnetic resonance (EPR) and ENDOR, UV/vis, vT 1H NMR, and vibrational spectroscopic data. Density functional theory (DFT) and multireference calculations of this highly reactive intermediate revealed an S = 1/2 ground state. The high reactivity can be traced to the increased electrophilicity in the oxidized complex. Based on high-level PNO-UCCSD(T) calculations and UV/vis kinetic measurements, it is proposed that the reaction proceeds by initial electrophilic exo attack of the nitrido nitrogen atom at the cyclopentadienyl ring and consecutive ring expansion to a pyridine ring.

New copper(I) complexes of bulky 5-substituted-2-iminopyrrolyl ligands as catalysts for azide-alkyne cycloaddition.

Five dinuclear copper(I) complexes of the type [Cu{κN,κN'-5-R-NC4H2-2-C(H)N(2,6-iPr2C6H3)}]2 (1a-e; R = 2,4,6-iPr3C6H2 (a), R = 2,6-Me2C6H3 (b), R = 3,5-(CF3)2C6H3 (c), R = 2,6-(OMe)2C6H2 (d), R = CPh3 (e)) were prepared by the reaction of the respective 5-R-2-iminopyrrolyl potassium salts KLa-e and [Cu(NCMe)4]BF4 in moderate yields. These new copper(I) complexes were characterized by NMR spectroscopy, elemental analysis and, in selected cases, by single crystal X-ray diffraction and their structural and electronic features further analyzed by DFT calculations and cyclic voltammetry, respectively. X-ray diffraction studies reveal dimeric Cu structures assembled by 2-iminopyrrolyl bridging ligands adopting a transoid conformation (complexes 1a and 1d), while complexes 1c and 1e displayed a cisoid conformation of those moieties, with respect to the Cu(I) centers. Additionally, VT-1H NMR and 1H-1H NOESY NMR experiments on complexes 1a-e exhibited complex fluxional processes in solution, assigned to a conformational inversion of the respective Cu2N4C4 metallacycles in all complexes but 1c, accompanied by a cisoid-transoid isomerization in the cases of complexes 1d,e. The Cu(I) complexes were also analyzed by cyclic voltammetry, where all complexes exhibit two oxidation processes, where the first oxidation is reversible, with the exception of 1b and 1c, which also show the highest oxidation potentials. The oxidation potentials follow clear trends related to the structural parameters of the complexes, in particular the Cu⋯Cu distance and the Cu2N4C4 macrocycles torsion angles. All new 5-substituted-2-iminopyrrolyl Cu(I) complexes 1a-e served as catalysts for azide-alkyne cycloaddition (CuAAC) reactions, being able to generate the respective 1,2,3-triazole products in yields as high as 82% and turnover frequencies (TOFs) as high as 859 h-1, after optimizing the conditions. The activity, as measured by the TOF, is in accordance with the oxidation potential of the corresponding complexes, the easier the oxidation, the higher the TOF value. Complex 1-H, where R = H, proved to be a poor catalyst for the same reactions, indicating that the 5-substitution in the ligand framework is crucial in stabilizing any potential catalytic species.

Palladium(II) Pincer Complexes Derived From N,N’,N’’‐Tris(2‐thioanisyl)guanidine. Syntheses, Structural Aspects, Solution Behaviour and Suzuki‐Miyaura Coupling Reactions

AbstractGuanidine, [(ArNH)2C=NAr] (Ar=2‐(MeS)C6H4) was prepared in 70 % (method 1)/92 % (method 2) yields. Cyclopalladation of the guanidine with Pd(OC(O)R)2 (R=Me and tBu) afforded the CNS pincer complexes, [Pd{κ3(C,N,S)}(OC(O)R)] in 82 % and 75 % yields, respectively. [Pd{κ3(C,N,S)}(OC(O)R)] (R=Me and tBu) upon metathesis reaction with LiBr gave [Pd{κ3(C,N,S)}Br] in 67 % and 72 % yields, respectively. The reaction of [Pd{κ3(C,N,S)}Br] with AgBF4 in MeCN gave [Pd{κ3(C,N,S)}(MeCN)]BF4 in 82 % yield. The new compounds were characterised by elemental analysis, IR and NMR spectroscopies and mass spectrometry. Molecular structures of [(ArNH)2C=NAr] (Ar=2‐(MeS)C6H4) and [Pd{κ3(C,N,S)}(OC(O)tBu)] were determined by single crystal X‐ray diffraction while the solution behaviour of the former compound studied by VT 1H NMR spectroscopy. [Pd{κ3(C,N,S)}(OAc)] was used as a catalyst in Suzuki‐Miyaura coupling reactions of aryl bromides with aryl boronic acids.

Radically Enhanced Dual Recognition

AbstractComplexation between a viologen radical cation (V.+) and cyclobis(paraquat‐p‐phenylene) diradical dication (CBPQT2(.+)) has been investigated and utilized extensively in the construction of mechanically interlocked molecules (MIMs) and artificial molecular machines (AMMs). The selective recognition of a pair of V.+ using radical‐pairing interactions, however, remains a formidable challenge. Herein, we report the efficient encapsulation of two methyl viologen radical cations (MV.+) in a size‐matched bisradical dicationic host — namely, cyclobis(paraquat‐2,6‐naphthalene)2(.+), i.e., CBPQN2(.+). Central to this dual recognition process was the choice of 2,6‐bismethylenenaphthalene linkers for incorporation into the bisradical dicationic host. They provide the space between the two bipyridinium radical cations in CBPQN2(.+) suitable for binding two MV.+ with relatively short (3.05–3.25 Å) radical‐pairing distances. The size‐matched bisradical dicationic host was found to exhibit highly selective and cooperative association with the two MV.+ in MeCN at room temperature. The formation of the tetrakisradical tetracationic inclusion complex — namely, [(MV)2⊂CBPQN]4(.+) – in MeCN was confirmed by VT 1H NMR, as well as by EPR spectroscopy. The solid‐state superstructure of [(MV)2⊂CBPQN]4(.+) reveals an uneven distribution of the binding distances (3.05, 3.24, 3.05 Å) between the three different V.+, suggesting that localization of the radical‐pairing interactions has a strong influence on the packing of the two MV.+ inside the bisradical dicationic host. Our findings constitute a rare example of binding two radical guests with high affinity and cooperativity using host‐guest radical‐pairing interactions. Moreover, they open up possibilities of harnessing the tetrakisradical tetracationic inclusion complex as a new, orthogonal and redox‐switchable recognition motif for the construction of MIMs and AMMs.

Radically Enhanced Dual Recognition.

Complexation between a viologen radical cation (V.+ ) and cyclobis(paraquat-p-phenylene) diradical dication (CBPQT2(.+) ) has been investigated and utilized extensively in the construction of mechanically interlocked molecules (MIMs) and artificial molecular machines (AMMs). The selective recognition of a pair of V.+ using radical-pairing interactions, however, remains a formidable challenge. Herein, we report the efficient encapsulation of two methyl viologen radical cations (MV.+ ) in a size-matched bisradical dicationic host - namely, cyclobis(paraquat-2,6-naphthalene)2(.+) , i.e., CBPQN2(.+) . Central to this dual recognition process was the choice of 2,6-bismethylenenaphthalene linkers for incorporation into the bisradical dicationic host. They provide the space between the two bipyridinium radical cations in CBPQN2(.+) suitable for binding two MV.+ with relatively short (3.05-3.25 Å) radical-pairing distances. The size-matched bisradical dicationic host was found to exhibit highly selective and cooperative association with the two MV.+ in MeCN at room temperature. The formation of the tetrakisradical tetracationic inclusion complex - namely, [(MV)2 ⊂CBPQN]4(.+) - in MeCN was confirmed by VT 1 H NMR, as well as by EPR spectroscopy. The solid-state superstructure of [(MV)2 ⊂CBPQN]4(.+) reveals an uneven distribution of the binding distances (3.05, 3.24, 3.05 Å) between the three different V.+ , suggesting that localization of the radical-pairing interactions has a strong influence on the packing of the two MV.+ inside the bisradical dicationic host. Our findings constitute a rare example of binding two radical guests with high affinity and cooperativity using host-guest radical-pairing interactions. Moreover, they open up possibilities of harnessing the tetrakisradical tetracationic inclusion complex as a new, orthogonal and redox-switchable recognition motif for the construction of MIMs and AMMs.

Iridium(i) complexes bearing hemilabile coumarin-functionalised N-heterocyclic carbene ligands with application as alkyne hydrosilylation catalysts.

A set of iridium(i) complexes of formula IrCl(κC,η2-IRCouR')(cod) or IrCl(κC, η2-BzIRCouR')(cod) (cod = 1,5-cyclooctadiene; Cou = coumarin; I = imidazolin-2-carbene; BzI = benzimidazolin-2-carbene) have beeen prepared from the corresponding azolium salt and [Ir(μ-OMe)(cod)]2 in THF at room temperature. The crystalline structures of 4b and 5b show a distorted trigonal bipyramidal configuration in the solid state with a coordinated coumarin moiety. In contrast, an equilibrium between this pentacoordinated structure and the related square planar isomer is observed in solution as a consequence of the hemilability of the pyrone ring. Characterization of both species by NMR was achieved at the low and high temperature limits, respectively. In addition, the thermodynamic parameters of the equilibrium, ΔHR and ΔSR, were obtained by VT 1H NMR spectroscopy and fall in the range 22-33 kJ mol-1 and 72-113 J mol-1 K-1, respectively. Carbonylation of IrCl(κC,η2-BzITolCou7,8-Me2)(cod) resulted in the formation of a bis-CO derivative showing no hemilabile behaviour. The newly synthesised complexes efficiently catalyze the hydrosilylation of alkynes at room temperature with a preference for the β-(Z) vinylsilane isomer.

1,4‐Diethynylbenzene‐Bridged [Cp*(dppe)Fe]n+ Units: Effect of 2,5‐Ethynyl Groups on the Chemical and Electronic Properties

The bis(ironvinylidene) complex [1,4‐{Cp*(dppe)Fe=C=CH}2‐2,5‐(C≡CH)2‐C6H2](PF6)2 ([2–2H](PF6)2) was prepared from 1,2,4,5‐tetraethynylbenzene and two equiv. of Cp*(dppe)FeCl in 87 % yield. The reaction is very selective and [2–2H](PF6)2 is the unique organoiron species to be formed. Deprotonation of [2–2H](PF6)2 provided the target complex 2 (66 % yield) and subsequent oxidation gave the monocationic and dicationic complexes 2(PF6) and 2(PF6)2. The new complexes were characterized by ESI‐mass spectrometry, IR, multinuclear NMR, ESR and Mössbauer spectroscopy, and XRD analyses on single crystals for [2–2H](PF6)2 and 2(PF6)2. The magnetic properties of 2(PF6)2 were investigated by VT 1H NMR. The data were analyzed with the support of quantum chemistry calculations. The terminal ethynyl groups on the aromatic ring of the bridge modify the population of the bridge‐oxidized state, weaken the electronic communication (Hab ≈ 601 cm–1) and strengthen magnetic coupling interactions (J = –530 cm–1) compared to the unsubstituted species (Hab = 1700 cm–1 and J = –340 cm–1, respectively).

Self‐assembly of sulfur‐containing heterocyclic compounds constructed by thianthrene units and their sulfonium salts

AbstractThe self‐assembly of sulfur‐containing heterocyclic compounds, 5,6,11,12,17,18‐hexathia‐5,6,11,12,17,18‐hexahydrotrinaphthylene and 5,7,12,14‐tetrathiapentacene having octyloxy groups 1 and 2, was examined. Sulfides 1 and 2 self‐assembled to form fibrous materials. The methylation reactions of 1 and 2 proceeded to give monomethylated sulfonium salts 3 and 4, and the self‐assembly of 3 and 4 was also studied. Fibrous materials of sulfonium salts 3 and 4 were formed in hexane/CHCl3 solution. The structure of the fibrous material of 3 might be similar to that of the fibrous material of 1. The aggregation behavior of 1 and 3 in cyclohexane‐d12 was studied by VT 1H NMR measurements at various concentrations. VT 1H NMR measurements revealed that the stacking structure of 1 was different from that of 3.

Cooperative Binding of SO2 under M–O and C–S Bond Formation in a Rhenium(I) Complex with Activated Amino- or Iminopyridine Ligand

Metal–ligand cooperative activation of C=O and CN multiple bonds in transition metal complexes with pyridine-based ligands is a currently active field of research. We herein expand the substrate scope to S=O bonds. The anionic complex K [Re(amidopy)(CO)3] readily reacts with a SO2 source to give the sulfinate fac-K[Re(amidopy-OSO)(CO)3] under Re–O and C–S bond formation as a diastereomeric mixture, which shows mutual dynamic interconversion even at ambient temperature. The chemical exchange was studied by 2D 1H 1H EXSY NMR spectroscopy. Activation parameters are obtained via VT 1H NMR and spectral line shape analysis.

A Stimuli-Responsive Molecular Capsule with Switchable Dynamics, Chirality, and Encapsulation Characteristics.

In this study, we report the preparation, conformational dynamics, and recognition characteristics of novel molecular capsule 1 comprising a bowl-shaped framework conjugated to a tris(2-pyridylmethyl)amine (TPA) lid. With the assistance of experiment (1H NMR spectroscopy) and theory (MM and DFT) we found that C3 symmetric 1 is poorly preorganized with three pyridines at the rim adopting a propeller-like orientation and undergoing P-to- M (or vice versa) stereoisomerization (Δ G⧧ < 8 kcal/mol, VT 1H NMR). Capsule 1 binds CH4, CH3Cl, CH2Cl2, CHCl3, and CCl4 with Ka < 7 M-1. Protonation of 1 with HCl, however, gives [1·H]-Cl, with the solid-state structure showing the TPA lid being "flattened" and the +N-H---Cl hydrogen-bonded group residing outside. Importantly, the P-to- M stereoisomerization would for [1·H]-Cl occur with Δ G⧧ = 11 kcal/mol (VT 1H NMR). Less dynamic and more preorganized [1·H]-Cl binds CH4, CH3Cl, CH2Cl2, CHCl3, and CCl4 guests with a greater affinity ( Ka = 100-400 M-1) than 1. The results of our studies suggest that the complexation of increasingly larger guests takes place in an induced-fit fashion, with [1·H]-Cl (a) elongating along its vertical axis and concurrently potentially (b) twisting pyridines from P into M (and vice versa) orientation. The addition of Et3N to [1·H]-Cl⊂CH2Cl2 causes deprotonation of the capsule and the release of CH2Cl2 with the process being fully reversed after the addition of HCl. Allosteric capsule 1 with unique structural and dynamic characteristics is expected to, in the future, assist the construction of complex molecular machines and smart functional materials.

Syntheses, characterization, solution behavior and catalytic activity of trans-[(guanidine)2PdX2] (X = Cl and OC(O)R; R = Me, Ph and tBu) in Heck–Mizoroki coupling reactions involving chloroarenes/methyl acrylate

Trans-[{(ArNH)2CNAr}2PdX2] (Ar = 2,5-Me2C6H3; X = Cl (1) and OC(O)R; R = Me (2), Ph (3), and tBu (4)) were isolated in high yields and characterized by elemental analysis, IR, and NMR (1H and 13C) spectroscopy. The molecular structures of the aforementioned complexes were determined by single crystal X-ray diffraction which revealed trans syn anti-anti (1·CHCl3), trans anti anti-syn (2·CHCl3 and 3·C7H8) and trans anti anti-anti (4) stereochemistry in solid state. Complexes 2·CHCl3, 3·C7H8 and 4 contain a pair of R11(8) rings stabilized by an intramolecular N–H⋯O hydrogen bond between the guanidine ligand and the carboxylate moiety. The influence of shape and size of the anion upon the stereochemistry of the complexes in the solid state and in solution are discussed. VT 1H NMR spectroscopic studies carried out on samples of 1 and 3 revealed the presence of a mixture of two rotamers in solution which arise due to the restricted CN(H)Ar single bond rotation of the guanidine ligand in both complexes. Complexes 1–4 were shown to be active catalysts even when used in 0.001 mol% in Heck–Mizoroki coupling reactions involving chlorobenzene and methyl acrylate. The scope of 1 in Heck–Mizoroki coupling reactions involving methyl acrylate and nine distinct chloroarenes were explored at 0.01 mol% which afforded the coupling products in 81−96% yields.

One-Handed Nickel Helicates of Long Chain Oligopyrroles

Open chain oligopyrroles are regarded as a unique scaffold for helices because of their intense absorptions in the visible region and redox activity due to their π-conjugated system. They can provide hydrogen bonding network and strong metal coordinating site that limit conformational flexibility leading to a unique stereochemical structure. A number of linear tetrapyrroles including bile pigments have been known to generate a helicate that was originally coined for helical metal complexes of oligopyridines.1) Proceeding to study on long chain oligopyrroles with more than five pyrrolic units is a logical extension aimed at developing new functional materials, but such compounds are difficult to prepare and their handling under ambient conditions is frequently problematic.2) Since only a few tetrapyrrolic helicates have so far been shown to induce helical sense bias,3) we are focusing on the generation of one-handed helicates of long chain oligopyrroles without using optical resolution. We have developed an efficient synthetic method for stable hexapyrroles by reacting bis(azafulvene) with two parts of gem-dimethyldipyrrylmethane-1-carboxaldehyde. These hexapyrrole-α,ω-dicarboxaldehydes are readily metallated with divalent metal ions to give dinuclear single helicates of two turns and a pair of stereogenic centers are easily introduced by converting terminal aldehyde groups into the imine groups with a variety of optically active amines.4) We have demonstrated that very high helical sense bias can be induced in this way. Here, we describe stereochemical feature and chiroptical properties of dinickel helicates of benzenehexapyrrole as longer homologues of hexapyrrole. BisNi complexes of benzenehexapyrrole-α,ω-dicarboximines 1b, 1c, and 1d were prepared in good yields by reacting the corresponding α,ω-dicarboxaldehyde 1a with benzylamine, (S)-(–)-1-phenylethylamine, and (R)-(–)-1-cyclohexylethylamine, respectively. In these complexes, a pair of 4N metal coordination planes are helically distorted and the π-conjugated plane in the central part is twisted. These three components of local helicity designated as P or M specify their stereochemical feature. It was found that fast rotation around the C(pyrrole)-C(benzene) bond and slow flipping of the terminal formimidoylpyrrole are taking place to cause some conformational isomers in equilibrium at ambient temperature. For example, a single diastereomer of C 2-symmetry was observed in the 1H NMR spectrum of 1d at 20 ˚C in CDCl3 but three diastereomers of 1c in a ratio of 85(C 2) : 13(C 1) : 2(C 2) were discerned. VT 1H NMR and CD study indicated that 1d undergoes rapid interchange between a major (P, P, P)-closed C 2 form and a minor (P, M, P)-open C 2 form. Two pyrroles in the 1,4-dipyrrylbenzene unit take a syn conformation in a closed form and anti in an open form. Either of both forms was exclusively taken in the case of 3d or 2d having 2,3-dimethoxy or 2,5-dimethoxy substituents, respectively, at the 1,4-phenylene spacer. The positive or negative CD first Cotton effect observed for 1d (3d) or 2d, respectively, was indicative of the handedness of the twisted π-conjugated central unit. In any case, it is noteworthy that the (R)-(–)-1-cyclohexylethylimine group exclusively induces the P-helical sense of the Ni coordination sites. If a pair of Ni coordination sites take opposite helical sense, it is asymmetric in the presence of stereogenic centers at the termini of the oligopyrrole chain and favors an open form. Such a (P, –, M)-open C 1 form appeared as a substantial contribution (>98%) of 2c and as a minor component accompanying a major (M, M, M)-closed C 2 form of 1c. These results indicate that combination of the aromatic spacer of the central part and the N-substituent of the terminal imine part is crucial in obtaining one-handed helicates of the oligopyrroles. References Bröring, in Handbook of Porphyrin Science, Vol. 8 (Eds: K. M. Kadish, K. M. Smith, R. Guilard), World Scientific, Singapore, 2010, pp. 343–501, and references therein.Zhang, M. Savage, X. Li, Y. Jiang, M. Ishida, K. Mitsuno, S. Karasawa, T. Kato, W. Zhu, S. Yang, H. Furuta, Y. Xie, Chem. Commun. 2016, 52, 5148–5151.a) S. Yagi, R. Yamada, T. Takagishi, N. Sakai, H. Takahashi, T. Mizutani, S. Kitagawa, H. Ogoshi, Commun. 1999, 911–912. b) A. Al-Sheikh-Ali, R. E. Benson, S. Blumentritt, T. S. Cameron, A. Linden, D. Wolstenholme, A. Thompson, J. Org. Chem. 2007, 72, 4947–4952.a) C. Eerdun, S. Hisanaga, J. Setsune, Chem. Int. Ed. 2013, 52, 929–932. b) C. Eerdun, S. Hisanaga, J. Setsune, Chem. Eur. J. 2015, 21, 239–246. Figure 1

Solution, Solid-State, and Computational Analysis of Agostic Interactions in a Coherent Set of Low-Coordinate Rhodium(III) and Iridium(III) Complexes.

A homologous family of low‐coordinate complexes of the formulation trans‐[M(2,2′‐biphenyl)(PR3)2][BArF 4] (M=Rh, Ir; R=Ph, Cy, iPr, iBu) has been prepared and extensively structurally characterised. Enabled through a comprehensive set of solution phase (VT 1H and 31P NMR spectroscopy) and solid‐state (single crystal X‐ray diffraction) data, and analysis in silico (DFT‐based NBO and QTAIM analysis), the structural features of the constituent agostic interactions have been systematically interrogated. The combined data substantiates the adoption of stronger agostic interactions for the IrIII compared to RhIII complexes and, with respect to the phosphine ligands, in the order PiBu3>PCy3>PiPr3>PPh3. In addition to these structure–property relationships, the effect of crystal packing on the agostic interactions was investigated in the tricyclohexylphosphine complexes. Compression of the associated cations, through inclusion of a more bulky solvent molecule (1,2‐difluorobenzene vs. CH2Cl2) in the lattice or collection of data at very low temperature (25 vs. 150 K), lead to small but statistically significant shortening of the M−H−C distances.

Amine adducts of (4-ClC6H4)3B3O3, Lewis acidity of triarylboroxines, and an XRD study on the related tetraphenylboroxinate(1-) salt, [C6H11NMe3][Ph4B3O3

The stoichiometric reactions of amines with tri(4-chlorophenyl)boroxine (CPB) led to the formation of adducts CPB·L {L = morpholine (1a), benzylamine (1b) cyclohexylamine (1c), 2-picoline (1d), 4-picoline (1e), piperidine (1f)} and (CPB)2·(4,4′-trimethylenedipiperidine) (1g). Compounds 1a-1g have been characterized by NMR (11B, 1H, 13C), IR spectroscopy and powder-XRD. VT 1H NMR spectra on 1a and 1d-1g were consistent with a ligand exchange/recombination process and activation energies of 49–58 kJ mol-1 were obtained. The synthesis of [C6H11NMe3][Ph4B3O3] (2) from Ph2BOBPh2, PhB(OH)2 and [C6H11NMe3][OH] in a 1:4:2 ratio in MeOH/H2O is also reported. Compound 2 was characterized by thermal analysis (TGA/DSC), spectroscopy (IR, NMR) and 1a, 1b and 2 were further characterized by single-crystal XRD studies.

Bis-meridional Fe2+ spincrossover complexes of phenyl and pyridyl substituted 2-(pyridin-2-yl)-1,10-phenanthrolines.

A series of Fe2+ SCO complexes of substituted 2-(pyridin-2-yl)-1,10-phenanthrolines 2 was prepared and the SCO (spincrossover) properties were characterized in the solid state (X-ray crystallography, SQUID magnetometry) and in solution (VT-1H NMR spectroscopy), augmented by theoretical modelling. Bis-meridional coordination of the tridendate 2a-c and tetradentate 2d ligands gives octahedral and distorted trigonal-dodecahedral complexes [Fe(2)2]2+, respectively, which were identified as SCO complexes with the transition temperature T1/2 below room temperature. SCO in the solid state is limited to bromo-substituted [Fe(2a)2]2+ (Dalton Trans., 2017, 46, 6218-6229) and [Fe(2b)2]2+ with a pyridine-appended phenyl group, whereas solution state NMR studies reveal SCO behaviour for all complexes, which is in agreement with DFT derived results. As anticipated from its N6(+2) coordination in the HS state, DFT structure modelling of [Fe(2d)2]2+ identified deviation from a structure-conserving SCO reaction coordinate; that is, Fe-N breathing is accompanied by a change in the coordination number. Accordingly, a remarkably slow SCO is observed in [Fe(2d)2]2+, owing to an extended coordinate. De-novo defined characteristic temperatures T(τHSLS) are introduced as structure-dependent parameters deemed to define the onset of phenomenological "slow" SCO. The rich phenomenology of the NMR spectra of [Fe(2)2]2+ is identified to be largely controlled by the dynamics of spin-state exchange and a qualitative illustration of the NMR-reporters of SCO is suggested.

Hindered C[sbnd]N bond rotation in triazinyl dithiocarbamates

The substituent and solvent effects on the rotation around a CN amide bond were studied for a series of triazine dibenzylcarbamodithioates. The Gibbs free energies (ΔG‡) were measured to be 16–18 kcal/mol in DMSO-d6 and toluene-d8 using variable-temperature nuclear magnetic resonance (VT-1H NMR) spectroscopy. Density functional theory (DFT) calculations reproduced the experimental observations with various substituents, as well as solvents. From the detailed analysis of the DFT results, we found that the electron donating dibenzyl amine group increased the electron population on the triazinyl ring, which decreased the rotational barrier of the CN bond in the dithiocarbamate group attached to the triazinyl ring. The higher electron population on the triazine moiety stabilizes the partial double bond character of the SC bond, which competitively excludes the double bond character of the CN bond. Therefore, the rotational dynamics of the CN bond in dithiocarbamates can be a sensitive probe to small differences in the electron population of substituents on sulfur.