Variable-temperature 1H NMR Studies Research Articles

Synthesis and Dynamic Behavior of Ce(IV) Double-Decker Complexes of Sterically Hindered Phthalocyanines.

Phthalocyanines and their double-decker complexes are interesting in designing rotative molecular machines, which are crucial for the development of molecular motors and gears. This study explores the design and synthesis of three bulky phthalocyanine ligands functionalized at the α-positions with phenothiazine or carbazole fragments, aiming to investigate dynamic rotational motions in these sterically hindered molecular complexes. Homoleptic and heteroleptic double-decker complexes were synthesized through the complexation of these ligands with Ce(IV). Notably, CeIV(Pc2)2 and CeIV(Pc3)2, both homoleptic complexes, exhibited blocked rotational motions even at high temperatures. The heteroleptic CeIV(Pc)(Pc3) complex, designed to lower symmetry, demonstrated switchable rotation along the pseudo-C4 symmetry axis upon heating the solution. Variable-temperature 1H-NMR studies revealed distinct dynamic behaviors in these complexes. This study provides insights into the rotational dynamics of sterically hindered double-decker complexes, paving the way for their use in the field of rotative molecular machines.

Synthesis and Studies of Pyridoneimine-Functionalized PETIM Dendrimers.

Pyridinoimine-functionalized poly(ether imine) (PETIM) dendrimers of 1-3 generations, possessing 4-16 moieties at the peripheries, are synthesized. Chloride-functionalized dendrimers are reacted with N-methylamino pyridine, under basic conditions, which led to functionalization of the peripheries of a dendrimer with pyridoneimine moieties. Variable-temperature 1H NMR studies are performed to assess the contributing resonance forms of pyridoneimine in the dendrimers. Solvatochromism and 15N NMR studies aid further the assessment of the contributing resonance forms. Comparison with derivatives that possess 1 and 2 pyridoneimines illustrates the contributing resonance forms between nonaromatic pyridoneimine and zwitter ionic aromatic imidopyridinium species.

Thermoresponsive Poly(N,N'-dimethylacrylamide)-Based Diblock Copolymer Worm Gels via RAFT Solution Polymerization: Synthesis, Characterization, and Cell Biology Applications.

RAFT solution polymerization is used to polymerize 2-hydroxypropyl methacrylate (HPMA). The resulting PHPMA precursor is then chain-extended using N,N'-dimethylacrylamide (DMAC) to produce a series of thermoresponsive PHPMA-PDMAC diblock copolymers. Such amphiphilic copolymers can be directly dispersed in ice-cold water and self-assembled at 20 °C to form spheres, worms, or vesicles depending on their copolymer composition. Construction of a pseudo-phase diagram is required to identify the pure worm phase, which corresponds to a rather narrow range of PDMAC DPs. Such worms form soft, free-standing gels in aqueous solution at around ambient temperature. Rheology studies confirm the thermoresponsive nature of such worms, which undergo a reversible worm-to-sphere on cooling below ambient temperature. This morphological transition leads to in situ degelation, and variable temperature 1H NMR studies indicate a higher degree of (partial) hydration for the weakly hydrophobic PHPMA chains at lower temperatures. The trithiocarbonate end-group located at the end of each PDMAC chain can be removed by treatment with excess hydrazine. The resulting terminal secondary thiol group can form disulfide bonds via coupling, which produces PHPMA-PDMAC-PHPMA triblock copolymer chains. Alternatively, this reactive thiol group can be used for conjugation reactions. A PHPMA141-PDMAC36 worm gel was used to store human mesenchymal stem cells (MSCs) for up to three weeks at 37 °C. MSCs retrieved from this gel subsequently underwent proliferation and maintained their ability to differentiate into osteoblastic cells.

Synthesis of Thermoresponsive Diblock Copolymer Nano-Objects via RAFT Aqueous Emulsion Polymerization of Hydroxybutyl Methacrylate

We recently reported that the reversible addition–fragmentation chain transfer (RAFT) aqueous emulsion polymerization of hydroxybutyl methacrylate (HBMA) using a relatively short non-ionic poly(glycerol monomethacrylate) (PGMA) precursor enables convenient preparation of diblock copolymer nano-objects with spherical, worm-like, or vesicular morphologies. We postulated that the relatively high aqueous solubility of HBMA (∼25 g dm–3 at 50 °C) was likely to be a key parameter for overcoming the problem of kinetically trapped spheres that is observed for many RAFT aqueous emulsion polymerization formulations. In this study, we revisit the RAFT aqueous emulsion polymerization of HBMA using a poly(ethylene glycol) (PEG) precursor as a steric stabilizer block. Remarkably, the resulting PEG45–PHBMA20 diblock copolymer nanoparticles exhibit thermoreversible morphological transitions in aqueous solution. More specifically, transmission electron microscopy and small-angle X-ray scattering studies confirmed that spheres are formed at 25 °C, worms at 58 °C, and vesicles at 65 °C. This is the first time that such behavior has been reported for nano-objects prepared by RAFT aqueous emulsion polymerization. Moreover, variable temperature dynamic light scattering and oscillatory rheology studies confirmed that these transitions are highly reversible at 0.1 and 10% w/w, respectively. Variable temperature 1H NMR studies indicated that (i) the PEG stabilizer block undergoes dehydration on heating and (ii) the apparent degree of hydration of the hydrophobic PHBMA block increases on heating from 25 to 65 °C. This suggests that the change in copolymer morphology is best explained in terms of a uniform plasticization mechanism.

Remotely controllable supramolecular rotor mounted inside a porphyrinic cage

•Design and synthesis of a supramolecular rotor within a porous Zn–porphyrinic cage•High-yielding IEDDA reaction enables facile construction of the rotor inside the cage•Initiation of both rotary and tumbling motions of the rotor with chemical stimuli•Reversible control of the dual mechanical motions by a photo-dissociable ligand The confinement of biological molecular machines, which are controlled by various chemical or physical stimuli within the cytoskeleton matrix, endows them with remarkable precision and programmable mechanical motions. Biological molecular machines are far more complex than any artificial molecular machine ever built. To mimic the intricate behavior of biological machinery, strategies need to be developed for embedding and confining artificial molecular machines within nanoscopic domains in which their functions can be specifically controlled by external stimuli. One such approach is the confinement of molecular machines inside porous organic cages. The high porosity of the organic cages provides ample room for uninterrupted movement, and its shape-persistent and interactive framework provides a high degree of control over the mechanical motion triggered by external stimuli. Our strategy may lay the foundation for the development of tunable molecular devices using porous organic cages. The confinement of molecular machines into nanostructured cages and controlling their functions by external stimuli holds great potential for the creation of smart functional materials that imitate the embodied intelligence of biological processes. Herein, we report the construction of a supramolecular rotor in a porous Zn-metallated porphyrinic cage (1) by encapsulation of a tetrazine-based linear axle (LA) via metal-ligand coordination bond, followed by post-assembly modification to append a controllable side arm to LA via inverse electron demand Diels-Alder (IEDDA) reaction. While the rotor alone shows nearly no motion, the addition of pyridine derivatives as a zinc coordinating ligand results in both 90° jump-like rotary motion of the rotor and slow tumbling motion of the rotor axle in a stochastic manner. Interestingly, the dual motions of the rotor can be reversibly controlled by the UV and visible light-induced coordination and dissociation of an azopyridine-based ligand with Zn centers as a signal transducer. The confinement of molecular machines into nanostructured cages and controlling their functions by external stimuli holds great potential for the creation of smart functional materials that imitate the embodied intelligence of biological processes. Herein, we report the construction of a supramolecular rotor in a porous Zn-metallated porphyrinic cage (1) by encapsulation of a tetrazine-based linear axle (LA) via metal-ligand coordination bond, followed by post-assembly modification to append a controllable side arm to LA via inverse electron demand Diels-Alder (IEDDA) reaction. While the rotor alone shows nearly no motion, the addition of pyridine derivatives as a zinc coordinating ligand results in both 90° jump-like rotary motion of the rotor and slow tumbling motion of the rotor axle in a stochastic manner. Interestingly, the dual motions of the rotor can be reversibly controlled by the UV and visible light-induced coordination and dissociation of an azopyridine-based ligand with Zn centers as a signal transducer. “Confinement effect” at various length scales is believed to play an indispensable role in evolution of life,1Szostak J.W. Bartel D.P. Luisi P.L. 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However, using a light source to control a molecular machine that is devoid of any photoresponsive component can be very interesting considering its resemblance with the biological machines, e.g., light-driven ion pumps. We have recently reported cube-shaped porous organic cages self-assembled from six square-shaped porphyrins and eight triangular linkers by dynamic covalent chemistry.52Mukhopadhyay R.D. Kim Y. Koo J. Kim K. Porphyrin boxes.Acc. Chem. Res. 2018; 51: 2730-2738Google Scholar, 53Benke B.P. Aich P. Kim Y. Kim K.L. Rohman M.R. Hong S. Hwang I.-C. Lee E.H. Roh J.H. Kim K. Iodide-selective synthetic ion channels based on shape-persistent organic cages.J. Am. Chem. Soc. 2017; 139: 7432-7435Google Scholar, 54Hong S. Rohman M.R. Jia J. Kim Y. Moon D. Kim Y. Ko Y.H. Lee E. Kim K. Porphyrin boxes: rationally designed porous organic cages.Angew. Chem. Int. Ed. Engl. 2015; 54: 13241-13244Google Scholar Shape-persistent porphyrinic cages, due to their well-defined receptor cavities, are ideal platforms to install molecular machines in a precise and predictable manner. Previously, a number of Zn–porphyrins-based dynamic molecular machines have been reported.55Goswami A. Schmittel M. Double rotors with fluxional axles: domino rotation and azide–alkyne Huisgen cycloaddition catalysis.Angew. Chem. Int. Ed. Engl. 2020; 59: 12362-12366Google Scholar, 56Gaikwad S. Goswami A. De S. Schmittel M. A Metalloregulated four-state nanoswitch controls two-step sequential catalysis in an eleven-component system.Angew. Chem. Int. Ed. Engl. 2016; 55: 10512-10517Google Scholar, 57Liu S. Kondratuk D.V. Rousseaux S.A.L. Gil-Ramírez G. O'Sullivan M.C. Cremers J. Claridge T.D.W. Anderson H.L. Caterpillar track complexes in template-directed synthesis and correlated molecular motion.Angew. Chem. Int. Ed. Engl. 2015; 54: 5355-5359Google Scholar, 58Muraoka T. Kinbara K. Aida T. Mechanical twisting of a guest by a photoresponsive host.Nature. 2006; 440: 512-515Google Scholar However, the establishment of a rational and high-yielding synthetic strategy to precisely organize molecular machines inside Zn–porphyrin-based porous organic cages with a 3D encompassed intrinsic nanoscopic cavity and to control their functions with multiple external stimuli requires further exploration. Herein, we report the synthesis of a supramolecular rotor built inside a porous Zn–porphyrinic cage (1) by insertion of a tetrazine-based LA followed by post-assembly modification to append a controllable side arm to the axle. The 1H and 2D EXSY NMR studies reveal a 90° jump-like random rotary motion of the rotor arm and a slow tumbling motion of the rotor axle upon the addition of pyridine derivatives, which compete with the rotor for zinc–porphyrin coordination. Computational calculations confirm the stochastic nature of the molecular motions. Moreover, the rotation rates of these two motions can be adjusted by varying the amounts of pyridine derivatives. The reversible control over the dual motions of the rotor is achieved by UV and visible light-induced coordination and dissociation of azopyridine-based photo-dissociable ligand (PDL), which acts as a signal transducer. The nanoscale hydrophobic cavity of the Zn–porphyrinic cage and the remotely controllable functions of the rotor may collectively lead to the development of tunable molecular devices with potential applications in reversible on-off ion channels, switchable catalysis, and drug delivery. At the outset of this work, we synthesized a pyridine terminated LA (length, ∼1.9 nm, see synthesis in Scheme S1), which can coordinate to two Zn metal centers of 1 (distance, ∼2.3 nm), across the cavity (Figure S1). LA consists of a central tetrazine moiety that can react efficiently with alkenes or alkynes dienophiles via inverse electron demand Diels-Alder (IEDDA) reaction to attach a new functionality on LA. By using the remarkably high-yielding IEDDA reactions for post-assembly modification, a number of different rotor moieties can be precisely incorporated into the cage. We envisaged that post-assembly modification of LA inside 1 with (1R,8S,9s)-bicyclo[6.1.0]-non-4-yn-9-ylmethanol (L1) may provide a controllable side arm for the freely rotating axle as L1 contains a hydroxy group (–OH) at the terminal position, which can coordinate with the “equatorial” Zn–porphyrins of 1 (Figure 1). The mechanical motion of the resulting assembly L1-LA⊂1 can be triggered in the presence of chemical stimuli, such as pyridine derivatives, which can coordinate with the Zn–porphyrinic faces of L1-LA⊂1 from outside the hydrophobic cavity, thus initiating the motion by inhibiting the formation of coordination bonds between the Zn centers of 1 and the rotor L1-LA (Figure 1B). The axle insertion experiment was performed by adding a stoichiometric amount of LA to the chloroform solution of 1, yielding LA⊂1 (Figure 1A). The binding affinity (Ka) of 1 for LA was determined to be 3.8 ± 0.1 × 107 M−1 by UV-vis titration (Figure S2), confirming a strong host-guest interaction between 1 and LA. In the 1H NMR spectrum of LA⊂1 in toluene-d8, LA protons showed a large upfield shift compared with their free state, due to the strong ring current of the porphyrin units. In particular, the α-py protons that are closest to the porphyrin ring showed the largest upfield shift (8.84 to 1.88 ppm), indicating that LA is precisely positioned inside 1 (Figure S3). Moreover, the insertion of LA into the cavity of 1 splits the identical pyrrolic protons (a) of all six porphyrins of 1 into three sets: a1 protons for two “axial” porphyrins (connecting to the axle) and a2 and a3 protons for four “equatorial” porphyrins (parallel to the axle). The a2 and a3 protons show different chemical shifts as they are directed toward the axial and equatorial porphyrins, respectively (Figure 2). The NMR splitting pattern of the cage protons suggests the O → D4 desymmetrization of the cage structure. All peaks were assigned by several 2D NMR experiments (1H–13C HSQC [heteronuclear single quantum coherence spectroscopy], 1H–1H COSY [homonuclear correlation spectroscopy], and ROESY [rotating-frame nuclear overhauser effect spectroscopy]; Figures S4–S7). A single trace in the DOSY (diffusion-ordered spectroscopy) spectrum confirmed the presence of a single species (LA⊂1) in the solution (Figure S8). Finally, X-ray structure analysis of LA⊂1 (for crystallization conditions, see supplemental information) confirmed the coordination of the py moieties of LA to the Zn metal centers of the opposite faces of 1 (Zn–Npy = 2.081(5) Å). In the crystal structure, LA⊂1 maintained an overall cube-shape with the same opposite Zn⋅⋅⋅Zn distance (23.8 Å) in all three directions despite the insertion of LA (Figure 2A). To demonstrate facile post-assembly modification of LA⊂1, norbornadiene was chosen as a model compound for IEDDA reaction that quantitatively converts the tetrazine-based axle (LA) to a pyridazine-based axle (PLA) inside 1 (PLA⊂1) within 4 h at room temperature, which is manifested by the disappearance of 1H NMR signals of LA and the appearance of PLA signals (Figures S9 and S10). The formation of PLA⊂1 was confirmed by 2D NMR and DOSY analysis (Figures S11–S13). The post-assembly modification of LA⊂1 with L1 substrate (1 equiv per tetrazine) by IEDDA reaction proceeded almost instantly at room temperature to furnish a ring-fused pyridazine-based axle inside 1 (L1-LA⊂1), as verified by 1H NMR, MALDI-TOF, and elemental analysis (EA) (Figures S14 and S15). Thermal stability was confirmed by thermogravimetric analysis (TGA) (Figures S16 and S17). In the 1H NMR spectrum of L1-LA⊂1 in toluene-d8, the L1 protons are shifted in the upfield region (appeared between 0 to −5 ppm), which may be due to their close proximity with one of the “equatorial” Zn–porphyrin from inside the cavity. In addition, the pyrrolic protons a3 of the equatorial Zn–porphyrins of LA⊂1 are divided into two sets a3a and a3b, indicating further desymmetrization of the cage structure (Figure 2C). The splitting of phenyl and imine protons also suggests the lowering of cage symmetry. All peaks were assigned by several 2D NMR experiments (Figures S18–S21). Moreover, DOSY experiment confirmed the single species in solution (Figure S22). Unexpectedly, the pyrrolic protons a1 and a2 of L1-LA⊂1 showed no splitting, the reason for which is currently not fully understood. On the other hand, upon directly mixing the 1:1 ratio of L1-LA and 1, a non-symmetrical pattern of LA protons was observed in the 1H NMR spectrum, indicating that L1-LA coordinates to 1 from outside the cage in a monodentate fashion (Figure S23), probably due to small-sized windows of the cage and long alkyl chains blocking the cage windows. After numerous attempts, the structure of L1-LA⊂1 has been finally confirmed by single-crystal X-ray crystallography (Figure 2B). The molecular structure revealed that L1-LA resides inside 1 by the coordination of the py groups of L1-LA to the “axial” Zn–porphyrins (Zn–Npy = 2.129(5) Å) and the hydroxy group (–OH) to the “equatorial” Zn–porphyrin (Zn–OH = 2.379(5) Å). As shown in Figure 2B, the slightly bent axle LA (bent angle, ∼167°) and curved shape of the side arm L1 assist in the coordination of the hydroxy group (–OH) with one of the “equatorial” Zn–porphyrins. The opposite Zn⋅⋅⋅Zn distances in the a and b axes are both 23.7 Å, whereas that in the c axis is 20.1 Å. The shorter Zn⋅⋅⋅Zn distance may be ascribed to the coordination of the –OH group of L1-LA to the equatorial Zn–porphyrin. The bent shape of LA and the curved shape of L1 were also observed in the X-ray crystal structure of L1-LA (Figure S24). Next, we investigated the operation of the supramolecular rotor L1-LA⊂1. In the absence of external stimulus, upon cooling the solution of L1-LA⊂1 (in toluene-d8) down to 243 K, no splitting of pyrrolic protons peaks was observed in the 1H NMR spectrum (Figure S25), which suggests that L1-LA is either rotating too slowly on the NMR timescale or no rotation at all, possibly due to the Zn–OHL1-LA coordination bond. It is known that Zn–porphyrins bind more strongly to pyridine than to alcohols.59Olsson S. Dahlstrand C. Gogoll A. Design of oxophilic metalloporphyrins: an experimental and DFT study of methanol binding.Dalton Trans. 2018; 47: 11572-11585Google Scholar In fact, coordination of pyridine to the Zn center can replace the Zn–OH coordination bond for maintaining a five-coordinate binding mode. The coordination of pyridine to L1-LA⊂1 is borne out by the upfield shift of the pyridine protons peaks in the 1H NMR spectrum. However, in the presence of excess pyridine, the peaks shifted back to the downfield region due to the establishment of a fast equilibrium between coordinated and free pyridine molecules (Figure S26). Thereafter, the effect of the addition of the chemical stimulant (pyridine) to L1-LA⊂1 was studied in detail by 1H NMR spectroscopy. Upon stepwise addition of pyridine (up to 6 equiv) to L1-LA⊂1, the pyrrolic protons a3a and a3b of the four equatorial porphyrins merged into a sharp singlet at 8.98 ppm (Figure 3A). The imine protons and the phenyl protons that correspond to the equatorial porphyrins also merged, indicating the symmetrization of the cage framework (Figure S26A). Additionally, the side arm (L1) protons that appear between 0 to −5 ppm due to their close proximity with one of the equatorial Zn–porphyrins broadened and shifted slightly downfield (Figure S26B). The symmetrical structure of the cage framework as derived from the 1H NMR spectroscopic studies and broadened resonances of the side arm protons suggested that the coordination of pyridine molecules to the Zn centers dissociated the Zn–OH coordination bond, and, as a result, the side arm started jumping rapidly around all four equatorial Zn–porphyrin stations at room temperature by using the rotor axle as a hinge (Figure 1B). To confirm that the mixing of the peaks is due to the rotary motion and not to pyridine coordination, 1H NMR titration between pyridine (0–6 equiv) and 1 was performed, which showed that the coordination of pyridine does not affect the protons signals of cage 1 due to its relatively rapid association/dissociation equilibrium with the Zn–porphyrins of 1 on the NMR timescale (Figure S27). Further insight into the rotor dynamics was obtained from variable-temperature (VT) 1H NMR studies. While the a3a and a3b protons of L1-LA⊂1 appeared as a single peak in the presence of 6 equiv of pyridine at 298 K, the peak was broadened at 263 K and emerged as two distinct peaks at 9.05 and 9.09 ppm at 243 K (Figure 3B). The 1H NMR could not be recorded below 243 K due to poor solubility of L1-LA⊂1 in toluene-d8 at the low temperatures. Upon raising the temperature back to 298 K, the peaks again merged into a single peak at 8.99 ppm (Figure S28). These experiments further corroborated the notion that the rotor exhibits a fast rotary motion in the presence of the chemical stimulant pyridine, whereas the rotation decelerates at low temperatures. Line shape analysis of the VT 1H NMR data in the presence of 6 equi

Pushing steric limits in osmium(IV) tetraaryl complexes.

Investigations into the reactivity, properties, and applications of osmium(IV) tetraaryl complexes have been hampered by their low yielding syntheses from volatile and toxic OsO4 (typically ≤34%). Here we show that known air-stable M(aryl)4 compounds (M = Os, Ru; aryl = 2-tolyl, 2,5-xylyl) can be prepared in ≤73% yields using new, less hazardous (Oct4N)2[MX6] precursors (M = Os, Ru; X = Cl, Br). This approach also facilitates the preparation of Os(mesityl)4 (Os3) for the first time, a complex comprising bulky 2,6-dimethyl substituted aryl ligands, albeit in low yield (5%). To better understand these yield extremes, we track, by synthesizing two additional new complexes with different 2-substituted σ-aryl ligands, a clear relationship between the yields of Os(aryl)4 and ligand steric bulk. Single-crystal X-ray structures of these compounds indicate that the observed yield trend reflects the ease of accommodating aryl substituents into an open pocket that lies directly opposite each M-aryl coordination site. We perform variable-temperature 1H NMR studies of Os3, utilize a "tetrahedricity" metric to assess geometric distortion in Ru(aryl)4 and Os(aryl)4 materials, and calculate cone angle and percentage buried volume metrics to further illustrate and help quantify σ-aryl ligand steric properties. Solution cyclic voltammograms of Os(aryl)4 show that the potentials of their reversible 1-/0 and 0/1+ redox features can be fine-tuned by varying aryl substituents, and that Os3 exhibits an additional 1+/2+ redox event not previously observed in this class of compounds. Taken together, this work helps to advance the potential application of these relatively underexplored organometallic complexes in established and emerging areas of molecular materials science, such as extended molecular frameworks and self-assembled monolayers, where analogous tetraphenylmethane and silane species (M = C, Si) have been frequently targeted.

Temperature Sensing of Thiolate Addition by Phenolate Merocyanine Dyes: Importance of the Quinone Methide Resonance Structure.

Merocyanine (MC) dyes containing an aromatic donor vinyl linked to a cationic acceptor serve as chemosensors for analyte detection. Their electrophilicity permits anion detection through addition reactions that disrupt dye conjugation. Herein, we demonstrate the temperature influence on thiolate addition to MCs containing the N-methylbenzothiazolium (Btz) acceptor. The zwitterionic phenolate dye (PhOBtz) displays impressive temperature sensitivity to thiolate addition, with the brightly colored phenolate favored upon heating and the colorless thiolate adduct favored upon cooling. In contrast, MC dyes containing neutral donors (PhOMeBtz and PhNMe2Btz) display only moderate temperature sensitivity to thiolate capture and release. Extraction of thermodynamic parameters demonstrates a strong enthalpic driving force for thiolate addition to PhOBtz that is absent for PhOMeBtz and PhNMe2Btz. Variable temperature 1H NMR studies demonstrate that PhOBtz adopts the para-quinone methide (p-QM) resonance structure. Thus, thiolate addition to PhOBtz resembles 1,6-conjugate addition to p-QMs which is accompanied by a large increase in the π-stabilization energy upon adduct formation. Manipulation of PhOBtz electrophilicity by attaching chlorine substituents to the phenolate caused the thiolate adducts to dissipate over time for p-QM regeneration. Our work provides new design ideas for the utility of phenolate MC dyes, given that they are carriers of the p-QM electrophile.

Unexpected Coordination Complexes of the Metal Tris-silylamides M{N(SiMe3)2}3 (M = Ti, V).

The synthesis, molecular structures, and spectroscopic details of a series of isocyanide and nitrile complexes of the early first-row transition-metal tris(silyl)amides M{N(SiMe3)2}3 (M = Ti, V) are reported. Previously, first-row transition-metal tris(silyl)amides were generally thought to be incapable of forming complexes with Lewis bases due to their excessive steric crowding. However, it is now shown that simple treatment of the base-free trisamides with 2 equiv of an isocyanide or nitrile base at room temperature results in the formation of the trigonal bipyramidal complexes Ti{N(SiMe3)2}3(1-AdNC)2 (1), Ti{N(SiMe3)2}3(CyNC)2 (2), Ti{N(SiMe3)2}3(ButNC)2 (3), Ti{N(SiMe3)2}3(PhCN)2 (4), V{N(SiMe3)2}3(1-AdNC)2 (5), V{N(SiMe3)2}3(CyNC)2 (6), V{N(SiMe3)2}3(ButNC)2 (7), and V{N(SiMe3)2}3(PhCN)2 (8), which incorporate two donor ligands (1-AdNC = 1-adamantyl isocyanide, CyNC = cyclohexyl isocyanide, ButNC = tert-butyl isocyanide, PhCN = benzonitrile). All complexes display a characteristic increase in the frequency of the multiple bonded C-N stretching mode which is observed to be in the range of 2170-2190 cm-1 for the isocyanide complexes 1-3 and 5-7 and at 2250 cm-1 for the nitrile complex 8. This effect was not observed for the titanium nitrile complex 4, suggesting weak binding of the donor to titanium. Paramagnetic 1H NMR studies showed these complexes to have detectable, though extremely broadened, signals attributable to the trimethylsilyl groups of the amide ligands (δ = ca. 2.8 ppm for titanium isocyanide complexes, ca. 4.5-4.7 ppm for vanadium isocyanide complexes). A variable-temperature 1H NMR study showed that in solution these complexes exist as mixtures of the five-coordinate species and a putative four-coordinate species coordinating a single Lewis basic ligand. Electronic spectroscopy indicated that the vanadium complexes 5-8 bind the Lewis bases more strongly than the corresponding titanium complexes, where the spectra of complexes 1-4 are essentially identical to the base-free Ti{N(SiMe3)2}3 at the temperatures and concentrations studied. In contrast to these results, no corresponding complexes were detected for the metal silylamides M{N(SiMe3)2}3 (M = Cr, Mn, Fe, or Co) when treated with the isocyanide or nitrile bases.

A Single Thermoresponsive Diblock Copolymer Can Form Spheres, Worms or Vesicles in Aqueous Solution

AbstractIt is well‐known that the self‐assembly of AB diblock copolymers in solution can produce various morphologies depending on the relative volume fraction of each block. Recently, polymerization‐induced self‐assembly (PISA) has become widely recognized as a powerful platform technology for the rational design and efficient synthesis of a wide range of block copolymer nano‐objects. In this study, PISA is used to prepare a new thermoresponsive poly(N‐(2‐hydroxypropyl) methacrylamide)‐poly(2‐hydroxypropyl methacrylate) [PHPMAC‐PHPMA] diblock copolymer. Remarkably, TEM, rheology and SAXS studies indicate that a single copolymer composition can form well‐defined spheres (4 °C), worms (22 °C) or vesicles (50 °C) in aqueous solution. Given that the two monomer repeat units have almost identical chemical structures, this system is particularly well‐suited to theoretical analysis. Self‐consistent mean field theory suggests this rich self‐assembly behavior is the result of the greater degree of hydration of the PHPMA block at lower temperature, which is in agreement with variable temperature 1H NMR studies.

A Single Thermoresponsive Diblock Copolymer Can Form Spheres, Worms or Vesicles in Aqueous Solution.

It is well‐known that the self‐assembly of AB diblock copolymers in solution can produce various morphologies depending on the relative volume fraction of each block. Recently, polymerization‐induced self‐assembly (PISA) has become widely recognized as a powerful platform technology for the rational design and efficient synthesis of a wide range of block copolymer nano‐objects. In this study, PISA is used to prepare a new thermoresponsive poly(N‐(2‐hydroxypropyl) methacrylamide)‐poly(2‐hydroxypropyl methacrylate) [PHPMAC‐PHPMA] diblock copolymer. Remarkably, TEM, rheology and SAXS studies indicate that a single copolymer composition can form well‐defined spheres (4 °C), worms (22 °C) or vesicles (50 °C) in aqueous solution. Given that the two monomer repeat units have almost identical chemical structures, this system is particularly well‐suited to theoretical analysis. Self‐consistent mean field theory suggests this rich self‐assembly behavior is the result of the greater degree of hydration of the PHPMA block at lower temperature, which is in agreement with variable temperature 1H NMR studies.

Synthesis and properties of dimethylpalladium complex with new PS3-type tripodal tetradentate ligand

Dimethylpalladium complex bearing PS3-type tripodal tetradentate ligand, [PdMe2{P(C6H4-2-S-i-Pr)3}], was synthesized by the reaction of [PdCl{P(C6H4-2-S-i-Pr)3}]X (X = Cl, BF4) with methyllithium (Br free). The dimethylpalladium complex reacted with an equimolar amounts of HCl to give the corresponding chloromethylpalladium complex, [PdClMe{P(C6H4-2-S-i-Pr)3}], while the similar reaction using 2 M amounts of HCl afforded the dichloropalladium complex, [PdCl{P(C6H4-2-S-i-Pr)3}]Cl. The detailed structures of the complexes were revealed by X-ray crystallography and variable temperature 1H NMR studies.

High variety of coordination modes of π-conjugated phospholes in dinuclear rhenium carbonyls. Fluxional behavior of σ,π-complexes

A variety of dirhenium carbonyl complexes containing π-conjugated phosphole derivatives were obtained from reaction of [Re2(CO)8(CH3CN)2] with each of the following phospholes: 2,5-bis(2-thienyl)-1-phenylphosphole (btpp), 2,5-bis(2-pyridyl)-1-phenylphosphole (bpypp) and 1,2,5-triphenylphosphole (tpp). The π-conjugated phospholes were found to behave as two-, four- or six-electron donor ligands via σ or σ-π interactions with the metal centers, presenting bridging or chelating coordination modes as determined by spectroscopic methods and single crystal X-ray diffraction analysis. Metal-metal bond cleavage was evidenced when btpp was used, leading to a mono-substituted mononuclear complex. Variable-temperature 1H NMR studies for σ,π-complexes showed a fluxional behavior due to the restricted rotation around the PC and CC bonds of the 1,2,5-trisubstituted phosphole ring.

Subphthalocyanine-tetracyanobuta-1,3-diene-aniline conjugates: stereoisomerism and photophysical properties.

Two subphthalocyanines (SubPcs) decorated at their peripheral (SubPc 1) or peripheral and axial (SubPc 2) positions with tetracyanobuta-1,3-diene (TCBD)-aniline moieties have been prepared as novel electron donor-acceptor (D-A) conjugates. In 1 and 2, the multiple functionalization of C 3-symmetric SubPcs by TCBD moieties, each of them having a chiral axis, results in the formation of several stereoisomers. Variable temperature 1H-NMR studies in chlorinated solvents suggest that these latter species, which are detected at low temperatures, rapidly interconvert - on the NMR timescale - into each other at room temperature. Beside their unique structural and stereochemical features, 1 and 2 present interesting physicochemical properties. Steady-state absorption and fluorescence, as well as electrochemical studies on 1 and 2 clearly point to an important degree of electronic communication between the SubPc, the TCBD and the aniline subunits. Moreover, in both derivatives, photoexcitation of the SubPc moiety yields charge transfer products involving the electron-rich SubPc moiety and the electron-withdrawing TCBD fragment. Interestingly, such polarized excited state species evolve in 1 and 2 in different ways. While in the former compound, it directly decays to the ground state, the fourth axial TCBD moiety in 2 leads to the formation of an intermediate fully charge separated state prior to the ground state deactivation.

A variable temperature 1H NMR and DFT study of procyanidin B2 conformational interchange

Two procyanidin B2 conformers were identified in a relative abundance ratio of approximately 3:1 at 298 K by 1H NMR experiments in acetonitrile. The conformational interchange reactions between these two conformers are 1st order in both reactions, with ∆G‡ for forward and reverse of 57.12 ± 5.62 and 54.56 ± 5.48 kJ mol−1, respectively. The experimentally obtained standard thermodynamic energies for this reaction are ΔH0rxn (3.67 ± 0.22 kJ mol−1), ΔS0rxn (4.05 ± 1.57 kJ mol−1 K−1), and ΔH0rxn (2.96 ± 0.33 kJ.mol−1). Conformational search results at the DFT (PBE, PBE-D2, and B3LYP with 6-311++g**) level of theory yielded four novel conformations, named fully compact (FC), partially compact (PC), partially extended (PE), and fully extended (FE). Although the FC conformer is electronically the most stable of the four as a result of extensive intramolecular non-covalent interactions, the PC and FE conformers are thermodynamically favored in a 5:1 ratio (B3LYP), with the FC and PE conformers present in negligible amounts at equilibrium. The DFT computed standard reaction energies using the B3LYP functional for the PCmajor to FEminor conformational interchange reaction compare exceptionally well with experimental data at 298 K: ∆G0rxn (3.86 kJ mol−1), ΔH0rxn (5.34 kJ mol−1), and ∆S0rxn (4.97 kJ mol−1 K−1). It was found that inclusion of solvation energies is crucial to obtain accurate thermodynamic energies. The secondary equilibria found in chromatographic separations are predicted to be highly dependent on solvent polarity and temperature. Similar conformational diversity and hierarchies should exist for all non-rigid procyanidin oligomers and the unique chromatographic behavior of these compounds may be a result of conformational interchange.

Electronic structure and reactivity studies of a nonsymmetric one-electron oxidized CuII bis-phenoxide complex

The tetradentate mixed imino/amino phenoxide ligand (N-(3,5-di-tert-butylsalicylidene)-N′-(2-hydroxyl-3,5-di-tert-butylbenzyl))-trans-1,2-cyclohexanediamine (salalen) was complexed with CuII, and the resulting Cu complex (2) was characterized by a number of experimental techniques and theoretical calculations. Two quasi-reversible redox processes for 2, as observed by cyclic voltammetry, demonstrated the potential stability of oxidized forms, and also the increased electron-donating ability of the salalen ligand in comparison to the salen analogue. The electronic structure of the one-electron oxidized [2]+ was studied in detail, and Cu K-edge X-ray Absorption Spectroscopy (XAS) measurements confirmed a CuII-phenoxyl radical complex in solution. Resonance Raman (rR) and variable temperature 1H NMR studies, coupled with theoretical calculations, showed that [2]+ is a triplet (S=1) CuII-phenoxyl radical species, with localization of the radical on the more electron-rich aminophenoxide. Attempted isolation of X-ray quality crystals of [2]+ afforded [2H]+, with a protonated phenol bonded to CuII, and an additional H-bonding interaction with the SbF6− counterion. Stoichiometric reaction of dilute solutions of [2]+ with benzyl alcohol showed that the complex reacts in a similar manner as the oxidized CuII-salen analogue, and does not exhibit a substrate-binding pre-equilibrium as observed for the oxidized bis-aminophenoxide CuII-salan derivative.

Self-Assembly of Extended Head-to-Tail Triangular Pt3 Macrocycles into Nanotubes.

New pyridylsalicylaldimine-based ligands with extended conjugation have been synthesized. The reaction of these ligands with K2PtCl4 yielded triangular Pt3 macrocycles rather than the anticipated Pt4 macrocycles. The macrocycles have been analyzed in solution by variable-temperature and variable-concentration 1H NMR, NOESY, and DOSY spectroscopy studies. These investigations show that the macrocycles aggregate at room temperature in solution by an entropy-driven process. In the solid state, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and transmission electron microscopy studies show that the macrocycles aggregate into nanotubular structures. Triangular platinum-containing macrocycles with the expanded pyridylsalicylaldimine ligands are promising for constructing nanotubes and discotic liquid crystals.

Half-sandwich complexes of iridium and ruthenium containing cysteine-derived ligands.

The dimers [{(ηn-ring)MCl}2(μ-Cl)2] ((ηn-ring)M = (η5-C5Me5)Ir, (η6-p-MeC6H4iPr)Ru) react with the modified cysteines S-benzyl-l-cysteine (HL1) or S-benzyl-α-methyl-l-cysteine (HL2) affording cationic complexes of the formula [(ηn-ring)MCl(κ2N,S-HL)]Cl (1, 2) in good yield. Addition of NaHCO3 to complexes 1 and 2 gave equilibrium mixtures of neutral [(ηn-ring)MCl(κ2N,O-L)] (3, 4) and cationic [(ηn-ring)M(κ3N,O,S-L)]Cl (6Cl, 7Cl) complexes. Similar mixtures were obtained in one-pot reaction by successive addition of the modified cysteine and NaHCO3 to the above formulated dimers. Addition of the N-Boc substituted cysteine derivative S-benzyl-N-Boc-l-cysteine (HL3) and NaHCO3 to the dimers [{(ηn-ring)MCl}2(μ-Cl)2] affords the neutral compounds [(ηn-ring)MCl(κ2O,S-L3)] ((ηn-ring)M = (η5-C5Me5)Ir (5a), (η6-p-MeC6H4iPr)Ru (5b)). Complexes of the formula [(ηn-ring)MCl(κ3N,O,S-L)][SbF6] (6Sb-8Sb), in which the cysteine derivative acts as a tridentate chelate ligand, can be prepared by adding one equivalent of AgSbF6 to the solutions of compounds 5 or to the mixtures of complexes 3/6Cl and 4/7Cl. The amide proton of compounds 8aSb and 8bSb can be removed by addition of NaHCO3 affording the neutral complexes [(ηn-ring)M(κ3N,O,S-L3-H)] ((ηn-ring)M = (η5-C5Me5)Ir (9a), (η6-p-MeC6H4iPr)Ru (9b)). Complexes 9a and 9b can also be prepared by reacting the dimers [{(ηn-ring)MCl}2(μ-Cl)2] with HL3 and two equivalents of NaHCO3. The absolute configuration of the complexes has been established by spectroscopic and diffractometric means including the crystal structure determination of (RIr,RC,RS)-[(η5-C5Me5)Ir(κ3N,O,S-L1)][SbF6] (6aSb). The thermodynamic parameters associated with the epimerization at sulphur that the iridium compound [(η5-C5Me5)Ir(κ3N,O,S-L3-H)] (9a) undergoes have been determined through variable temperature 1H NMR studies.

Structure, absolute configuration, and variable-temperature1H-NMR study of (±)-versiorcinols A–C, three racemates of diorcinol monoethers from the sponge-associated fungus Aspergillus versicolor 16F-11

Structure, absolute configuration, and variable-temperature1H-NMR study of three racemates of diorcinol monoethers, (±)-versiorcinols A–C.

Self-Assembled Molecular Gear: A 4:1 Complex of Rh(III)Cl Tetraarylporphyrin and Tetra(p-pyridyl)cavitand.

The components of a 4:1 mixture of Rh(III)Cl tetrakis(4-methylphenyl)porphyrin 1 and a bowl-shaped tetra(4-pyridyl)cavitand 4 self-assemble into a 4:1 complex 14•4 via Rh-pyridyl axial coordination bonds. The single-crystal X-ray diffraction analysis and variable-temperature (VT) (1)H NMR study of 14•4 indicated that 14•4 behaves as a quadruple interlocking gear with an inner space, wherein (i) four subunits-1 are gear wheels and four p-pyridyl groups in subunit-4 are axes of gear wheels, (ii) one subunit-1 and two adjacent subunits-1 interlock with one another cooperatively, and (iii) four subunits-1 in 14•4 rotate quickly at 298 K on the NMR time scale. Together, the extremely strong porphyrin-Rh-pyridyl axial coordination bond, the rigidity of the methylene-bridge cavitand as a scaffold of the pyridyl axes, and the cruciform arrangement of the interdigitating p-tolyl groups as the teeth moiety of the gear wheels in the assembling 14-unit make 14•4 function as a quadruple interlocking gear in solution. The gear function of 14•4 was also supported by the rotation behaviors of other 4:1 complexes: 24•4 and 34•4 obtained from Rh(III)Cl tetrakis[4-(4-methylphenyl)phenyl]porphyrin 2 or Rh(III)Cl tetrakis(3,5-dialkoxyphenyl)porphyrin 3 and 4 also served as quadruple interlocking gears, whereas 14•5 obtained from 1 and tetrakis[4-(4-pyridyl)phenyl]cavitand 5 did not behave as a gear. The results of activation parameters (ΔH(⧧), ΔS(⧧), and ΔG(⧧)) obtained from Eyring plots based on line-shape analysis of the VT (1)H NMR spectra of 14•4, 24•4, and 34•4 also support the interlocking rotation (geared coupled rotation) mechanism.

Bowl-to-Bowl Inversion Accelerated by the Introduction of Bulky 1,1,4,4-Tetramethylbutane-1,4-diyl Groups on Sumanene

A variable-temperature 1H NMR study revealed that the bowl-to-bowl inversion of sumanene is accelerated by the introduction of bulky 1,1,4,4-tetramethylbutane-1,4-diyl groups on the peripheral aromatic ring. This is likely to be induced by a bowl flattening, as shown in the DFT-optimized structures.