Ligand Rearrangement Research Articles

Artificial magnetosomes: Molecularly restructured SPIONs with enhanced potential for magnetic imaging

Magnetosomes represent an elegant example of ultrastable nanomaterials that nature produces by encapsulating magnetic nanoparticles in liposomal sub-compartments of magnetotactic bacteria. Materials chemists continue to strive for ways to mimic the performance of natural systems, but the artificial synthesis of magnetosomes remains unrealized. Here, we report molecular restructuring of the surface of oleic-acid-capped superparamagnetic iron oxide nanoparticles (SPIONs) to produce magnetosomes. Our strategy involves the use of triethylamine as an amphiphilic surfactant that through a combination of ligand exchange and rearrangement mechanisms, facilitates an organic-to-aqueous phase transfer of SPIONs while turning them into magnetosomes. These liposome-encapsulated SPIONs showed dynamic aqueous stability alongside noticeably improved magnetic properties. This improved their performance as contrast agents for magnetic resonance imaging (MRI) and magnetic particle imaging (MPI), the latter revealing a 33.5 % improvement in signal sensitivity over the original SPIONs. The presented strategy is extendable to prepare liposomal dispersions of other biomedically important nanomaterials.

Cycloheptatrienyl-Bridged Triple-Decker Complexes.

The first structurally characterized organometallic multidecker sandwich complexes featuring a cycloheptatrienyl ring (Cht, C7H73-) in the coordination sphere are presented. The synthesis of inverse sandwich complexes of the rare earth elements YIII and ErIII with a bridging cycloheptatrienyl ligand of the type [(thf)(BH4)2LnIII(μ-η7:η7-Cht)LnIII(BH4)(thf)2] is described first. The subsequent introduction of the CotTIPS ligand (CotTIPS = 1,4-(iPr3Si)2C8H62-) into the coordination sphere of the rare earth cations resulted in the isolation of unprecedented triple-decker compounds with the formula [(thf)3K{(η8-CotTIPS)LnIII}2(μ-η7:η7-Cht)], bearing a seven-membered aromatic carbon ring as a middle deck. These compounds are also the first examples of rare earth triple-decker complexes not bridged by a Cot derivative, based on purely carbon-based ligands. The magnetic properties of the respective ErIII congeners were investigated in detail, leading to the observation of antiferromagnetic coupling of the ErIII cations and a blocking temperature of 13.5 K. The conversion of the YIII compound [(thf)3K{(η8-CotTIPS)YIII}2(μ-η7:η7-Cht)] with [YIII(Cot)I(thf)2] resulted in ligand rearrangement and the selective formation of the first triple-decker complex ([(η8-CotTIPSYIII)2(μ-η8:η8-Cot)]) featuring two Cot ligands with different substituents in its coordination sphere.

On Haptotropic Rearrangements of Diphosphene and Diarsene Ligands in Titanium Complexes.

Dipnictenes of the type RE=ER (E=P, As, Sb, Bi) are the isovalence electronic heavier analogs of alkenes. Although diphosphenes and dipnictenes in general show a variety of binding modes in metal complexes, little is known about haptotropic shift reactions involving these ligands. Herein, we report an unprecedented η2 to η1 rearrangement of the dipnictene ligands in titanocene complexes of the type Cp2Ti(Pn2Ar2) (Pn=P, As; Ar=2,4,6-Me3-C6H2, Mes; 2,6-iPr2-C6H3, Dip; 2,4,6-iPr3-C6H2, Tip), initiated by Lewis basic ligands (L=MeCN, PMe3, AdNC, CO). In the presence of L the dipnictene ligand changes its hapticity from η2 to η1 and complexes of the general form Cp2Ti(L)(Pn2Ar2) with a succinctly different electronic structure are obtained. Electronically, the new complexes are best described as biradicaloids with antiferromagnetically coupled (via a π-bond) [Cp2TiIII]⋅+ and [Pn2Ar2]⋅- fragments. However, the biradical character of these systems is affected by the electronic features of the co-ligand and significantly decreases moving from PMe3/MeCN (σ-donors) to CNAd/CO (σ-donors/π-acceptors).

Aluminyl derived ethene functionalization with heteroallenes, leading to an intramolecular ligand rearrangement.

The aluminacyclopropane K[Al(NON)(η-C2H4)] ([NON]2- = [O(SiMe2NDipp)2]2-, Dipp = 2,6-iPr2C6H3) reacts with CO2 and iPrNCNiPr to afford ring-expanded products of C-C bond formation. The latter system undergoes a 1,3-silyl retro-Brook rearrangement of the NON-group, to afford the [NNO]2- ligand ([NNO]2- = [N(Dipp)SiMe2N(Dipp)SiMe2O]2-). The mechanism of transformation was examined by density functional theory (DFT).

Siloxide tripodal ligands as a scaffold for stabilizing lanthanides in the +4 oxidation state.

Synthetic strategies to isolate molecular complexes of lanthanides, other than cerium, in the +4 oxidation state remain elusive, with only four complexes of Tb(iv) isolated so far. Herein, we present a new approach for the stabilization of Tb(iv) using a siloxide tripodal trianionic ligand, which allows the control of unwanted ligand rearrangements, while tuning the Ln(iii)/Ln(iv) redox-couple. The Ln(iii) complexes, [LnIII((OSiPh2Ar)3-arene)(THF)3] (1-LnPh) and [K(toluene){LnIII((OSiPh2Ar)3-arene)(OSiPh3)}] (2-LnPh) (Ln = Ce, Tb, Pr), of the (HOSiPh2Ar)3-arene ligand were prepared. The redox properties of these complexes were compared to those of the Ln(iii) analogue complexes, [LnIII((OSi(OtBu)2Ar)3-arene)(THF)] (1-LnOtBu) and [K(THF)6][LnIII((OSi(OtBu)2Ar)3-arene)(OSiPh3)] (2-LnOtBu) (Ln = Ce, Tb), of the less electron-donating siloxide trianionic ligand, (HOSi(OtBu)2Ar)3-arene. The cyclic voltammetry studies showed a cathodic shift in the oxidation potential for the cerium and terbium complexes of the more electron-donating phenyl substituted scaffold (1-LnPh) compared to those of the tert-butoxy (1-LnOtBu) ligand. Furthermore, the addition of the -OSiPh3 ligand further shifts the potential cathodically, making the Ln(iv) ion even more accessible. Notably, the Ce(iv) complexes, [CeIV((OSi(OtBu)2Ar)3-arene)(OSiPh3)] (3-CeOtBu) and [CeIV((OSiPh2Ar)3-arene)(OSiPh3)(THF)2] (3-CePh), were prepared by chemical oxidation of the Ce(iii) analogues. Chemical oxidation of the Tb(iii) and Pr(iii) complexes (2-LnPh) was also possible, in which the Tb(iv) complex, [TbIV((OSiPh2Ar)3-arene)(OSiPh3)(MeCN)2] (3-TbPh), was isolated and crystallographically characterized, yielding the first example of a Tb(iv) supported by a polydentate ligand. The versatility and robustness of these siloxide arene-anchored platforms will allow further development in the isolation of more oxidizing Ln(iv) ions, widening the breadth of high-valent Ln chemistry.

Versatility of a diamidosilylether ligand supporting yttrium complexes: Synthesis, structure and reactivity

Yttrium alkyl complex [NONDippY(CH2SiMe3)(THF)], 1 (NONDipp = (DippNSiMe2)2O, Dipp = 2,6-iPr-C6H3) can be prepared in high yields by a σ-bond metathesis reaction between [Y(CH2SiMe3)3(THF)2] and the free bis(aminosilyl)ether. The reaction of 1 with one equivalent of PhSiH3 generates the hydride bridged yttrium dimer [μ-NONDippY(μ-H)(THF)]22, however, each diamidosilylether ligand does not chelate each yttrium centre but instead bridges to both yttrium atoms in a “flyover” fashion. This ligand rearrangement maybe reversed by the addition of a strong Lewis base to give the dimeric yttrium hydride [NONDippY(μ-H)(DMAP)]23, where each diamidosilylether ligand now chelate to each yttrium center respectively. The effect of the different diamidosilylether bonding modes on the reactivity of the yttrium complexes is examined with heterocumulenes.

Mn(II), Fe(II), and Co(II) Aryloxides: Steric and Dispersion Effects and the Thermal Rearrangement of a Cobalt Aryloxide to a Co(II) Semiquinone Complex.

A series of Mn(II), Fe(II), and Co(II) bisaryloxide dimers ([M(OC6H2-2,4,6-Cy3)2]2 {M = Mn (1), Fe (2), and Co (3)} were synthesized by the addition of 2,4,6-tricyclohexylphenol (HOC6H2-2,4,6-Cy3) to the silyl amido dimers [M(N(SiMe3)2)2]2 (M = Mn, Fe, Co; Cy = cyclohexyl). An unexpected and unique Co(II) phenoxide derivative (4), [Co(OC6H2-2,4,6-Cy3)(O2C6H-3,5,6-Cy3)]2, was obtained via ligand rearrangement of 3 at ca. 180 °C. This yielded 4 in which there are two unchanged, bridging phenoxide ligands as well as a terminal bidentate semiquinone ligand bound to each cobalt. Complexes 1 and 2 did not undergo such a rearrangement under the same conditions; both are thermally stable to temperatures exceeding 250 °C and feature numerous short-contact (<2.5 Å) H···H interactions consistent with the presence of dispersion stabilization. Use of the aryloxide ligand -OC6H3-2,6-Pri2 (Pri = isopropyl), which is sterically similar to -OC6H2-2,4,6-Cy3 but produces fewer close H···H interactions, gave the trimeric species [M(OC6H3-2,6-Pri2)2]3 {M = Fe (5) or Co (6)} which feature a linear array of three metal atoms bridged by aryloxides. The higher association number in 5 and 6 in comparison to that of 1-3 is due to the lower dispersion energy donor properties of the -OC6H3-2,6-Pri2 ligand and the lower stabilization it produces.

Nanoindentation creep of supercrystalline nanocomposites

Supercrystalline nanocomposites (SCNCs) are inorganic-organic hybrid materials with a unique periodic nanostructure, and thus they have been gaining growing attention for their intriguing functional properties and parallelisms with hierarchical biomaterials. Their mechanical behavior remains, however, poorly understood, even though its understanding and control are important to allow SCNCs’ implementation into devices. An important aspect that has not been tackled yet is their time-dependent deformation behavior, which is nevertheless expected to play an important role in materials containing such a distribution of organic phase. Hereby, we report on the creep of ceramic-organic SCNCs with varying degrees of organic crosslinking, as assessed via nanoindentation. Creep strains and their partial recoverability are observed, hinting at the co-presence of viscoelasticity and viscoplasticity, and a clear effect of crosslinking in decreasing the overall material deformability emerges. We rationalize our experimental observations with the analysis of stress exponent and activation volume, resulting in a power-law breakdown behavior and governing deformation mechanisms occurring at the organic sub-nm interfaces scale, as rearrangement of organic ligands. The set of results is reinforced by the evaluation of the strain rate sensitivity via strain rate jump tests, and the assessment of the effect of oscillations during continuous stiffness measurement mode.

Intracluster ligand rearrangement: an NMR-based thermodynamic study.

Ligand and metal exchange reactions are powerful methods to tailor the properties of atomically precise metal nanoclusters. Hence, a deep understanding of the mechanisms behind the dynamics that rule the ligand monolayer is crucial for its specific functionalization. Combining variable-temperature NMR experiments and dynamic-NMR simulations, we extract the thermodynamic activation parameters of a new exchange reaction: the intracluster ligand rearrangement between the two symmetry-unique positions in [Ag25(DMBT)18]- and [Ag24Au(DMBT)18]- clusters. We report for the first time that this peculiar intracluster modification does not seem to proceed via metal-sulphur bond breaking and follows a first-order rate law, being therefore a process independent from the well-described collisional ligand exchange.

Building Square Planar Cobalt (II) Complexes via Sodium Mediated Cobaltation of Fluoroarenes

AbstractWhile cobalt complexes have already shown their potential for C−H and C−F bond activation of fluoroarenes, their reactivity as metalating agents via Co−H exchange towards these substrates has not been explored. Herein, we report a Co(HMDS)2 [HMDS=N(SiMe3)2] system which, when synergistically enhanced via sodium amide Na(HMDS) mediation, can render chemo‐ and regioselective cobaltation of a series of fluoroarenes to produce a new class of homoleptic square planar [Na2CoAr4] complexes. Density functional theory calculations elucidate the key roles of the Na/Co counterparts in a stepwise sodiation/cobalt transmetalation process, leading to this novel C−H metalation. Depending on the reaction stoichiometry, this process can occur inter‐ or intramolecularly, furnishing transient [NaCo(HMDS)2Ar] intermediates which can undergo ligand rearrangement to afford [Na2CoAr4] with concomitant formation of Co(HMDS)2 and [NaCo(HMDS)3].

Building Square Planar Cobalt (II) Complexes via Sodium Mediated Cobaltation of Fluoroarenes.

While cobalt complexes have already shown their potential for C-H and C-F bond activation of fluoroarenes, their reactivity as metalating agents via Co-H exchange towards these substrates has not been explored. Herein, we report a Co(HMDS)2 [HMDS=N(SiMe3 )2 ] system which, when synergistically enhanced via sodium amide Na(HMDS) mediation, can render chemo- and regioselective cobaltation of a series of fluoroarenes to produce a new class of homoleptic square planar [Na2 CoAr4 ] complexes. Density functional theory calculations elucidate the key roles of the Na/Co counterparts in a stepwise sodiation/cobalt transmetalation process, leading to this novel C-H metalation. Depending on the reaction stoichiometry, this process can occur inter- or intramolecularly, furnishing transient [NaCo(HMDS)2 Ar] intermediates which can undergo ligand rearrangement to afford [Na2 CoAr4 ] with concomitant formation of Co(HMDS)2 and [NaCo(HMDS)3 ].

Line Shape Analysis of Dynamic NMR Spectra for Characterizing Coordination Sphere Rearrangements at a Chiral Rhenium Polyhydride Complex.

Dynamic solution nuclear magnetic resonance (NMR) spectroscopy is the typical method of characterizing the dynamic rearrangements of atoms within the coordination sphere for transition metal polyhydride complexes. Line shape fitting of the dynamic NMR spectra can lead to estimates for the activation parameters of the dynamic rearrangement processes. A combination of dynamic 31P-{1H} NMR spectroscopy of metal-bound phosphorus atoms with dynamic 1H-{31P} NMR spectroscopy of hydride ligands may identify hydride ligand rearrangements that occur in conjunction with a phosphorus atom rearrangement. For molecules that exhibit such a coupled pair of rearrangements, dynamic NMR spectroscopy can be used to test theoretical models for the ligand rearrangements. Dynamic 1H-{31P} NMR spectroscopy and line shape fitting can also identify the presence of an exchange process that moves a specific hydride ligand beyond the metal's inner coordination sphere through a proton exchange with a solvent molecule such as adventitious water. The preparation of a new compound, ReH5(PPh3)2(sec-butyl amine), that exemplifies multiple dynamic rearrangement processes is presented along with line shape fitting of dynamic NMR spectra of the complex. Line shape fitting results can be analyzed by the Eyring equation to estimate the activation parameters for the identified dynamic processes.

Photo-induced ligand substitution of Cr(CO)6 in 1-pentanol probed by time resolved X-ray absorption spectroscopy.

Cr(CO)6 was investigated by X-ray absorption spectroscopy. The spectral signature at the metal edge provides information about the back-bonding of the metal in this class of complexes. Among the processes it participates in is ligand substitution in which a carbonyl ligand is ejected through excitation to a metal to ligand charge transfer (MLCT) band. The unsaturated carbonyl Cr(CO)5 is stabilized by solution media in square pyramidal geometry and further reacts with the solvent. Multi-site-specific probing after photoexcitation was used to investigate the ligand substitution photoreaction process which is a common first step in catalytic processes involving metal carbonyls. The data were analysed with the aid of TD-DFT computations for different models of photoproducts and signatures for ligand rearrangement after substitution were found. The rearrangement was found to occur in about 790 ps in agreement with former studies of the photoreaction.

Syntheses, rearrangements, and structural analyses of unsaturated nitrogen donor ligands derived from diphenyldiazomethane and the chiral rhenium Lewis acid [(η5-C5H5)Re(NO)(PPh3)

Diphenyldiazomethane and a labile chlorobenzene complex of [(η5-C5H5)Re(NO)(PPh3)]+ BF4- react to give the η1 adduct [(η5-C5H5)Re(NO)(PPh3)(NNCPh2)]+ BF4- (73%). When this is conducted in the presence of copper powder, a 3-phenyl-1H-indazole complex derived from carbon-hydrogen bond activation, [(η5-C5H5)Re(NO)(PPh3)(NC(Ph)CCHCHCHCHCNH)]+ BF4-, is obtained (65%). Subsequent reaction with NaOCH3 gives indazolyl complex (η5-C5H5)Re(NO)(PPh3)(NCCHCHCHCHCC(Ph)N) (85%), derived from NH deprotonation and a 1,2-rhenium shift. Crystal structures of the three new complexes are determined. DFT calculations are used to probe the mechanism of the 1,2-shift and energetics of alternative Re-N rotamers and linkage isomers, and assign bond orders and dominant resonance formulations.

Hybrid Liquid-Crystalline Electrolytes with High-Temperature-Stable Channels for Anhydrous Proton Conduction.

Modern electrochemical and electronic devices require advanced electrolytes. Liquid crystals have emerged as promising electrolyte candidates due to their good fluidity and long-range order. However, the mesophase of liquid crystals is variable upon heating, which limits their applications as high-temperature electrolytes, e.g., implementing anhydrous proton conduction above 100 °C. Here, we report a highly stable thermotropic liquid-crystalline electrolyte based on the electrostatic self-assembly of polyoxometalate (POM) clusters and zwitterionic polymer ligands. These electrolytes can form a well-ordered mesophase with sub-10 nm POM-based columnar domains, attributed to the dynamic rearrangement of polymer ligands on POM surfaces. Notably, POMs can serve as both electrostatic cross-linkers and high proton conductors, which enable the columnar domains to be high-temperature-stable channels for anhydrous proton conduction. These nanochannels can maintain constant columnar structures in a wide temperature range from 90 to 160 °C. This work demonstrates the unique role of POMs in developing high-performance liquid-crystalline electrolytes, which can provide a new route to design advanced ion transport systems for energy and electronic applications.

Bis(phenyl-β-diketonato)titanium(IV) ethoxide complexes: Ring-opening polymerization of l-lactide by solvent-free microwave irradiation

A range of functionalized bis(phenyl-β-diketonato)titanium(IV) ethoxide complexes of the type [Ti(L)2(OEt)2], containing two asymmetric or symmetric phenyl-β-diketonate ligands (L), have been synthesized and fully characterized. Single crystal X-ray diffraction has been used to confirm that all structures crystallize with the ethoxide ancillary ligands in a cis arrangement and NMR spectra show characteristic ligand rearrangement in solution. The complexes have been screened as catalysts in the ring-opening polymerization (ROP) of l-lactide (L-LA) to yield polylactide (PLA), by using microwave (MW) and conventional heating. The resulting polymers have high molecular weights but a high polydispersity index (PDI) which is suggestive of transesterification. However, unlike conventional heating methods, the polymerization by MW reaction leads to a retention in stereochemistry of the resulting polymer in significantly shorter time periods.

Pathway-Dependent Coordination Networks: Crystals versus Films

We demonstrate the formation of both metallo-organic crystals and nanoscale films that have entirely different compositions and structures despite using the same set of starting materials. This difference is the result of an unexpected cation exchange process. The reaction of an iron polypyridyl complex with a copper salt by diffusion of one solution into another resulted in iron-to-copper exchange, concurrent ligand rearrangement, and the formation of metal–organic frameworks (MOFs). This observation shows that polypyridyl complexes can be used as expendable precursors for the growth of MOFs. In contrast, alternative depositions of the iron polypyridyl complex with a copper salt by automated spin coating on conductive metal oxides resulted in the formation of electrochromic coatings, and the structure and redox properties of the iron complex were retained. The possibility to form such different networks from the same set of molecular building blocks by “in solution” versus “on surface” coordination chemistry broadens the synthetic space to design functional materials.

Internal and External Influences on Stability and Ligand Exchange Reactions in Bromido[3-ethyl-4-aryl-5-(2-methoxypyridin-5-yl)-1-propyl-1,3-dihydro-2H-imidazol-2-ylidene]gold(I) Complexes.

The ligand scrambling reaction of gold(I) complexes is a phenomenon occurring primarily in L–AuI–X (L = phosphine, N-heterocyclic carbene (NHC), and thiol; X = halide and thiol) complexes and has been observed among others for e.g., the bromido[3-ethyl-4-(4-methoxyphenyl)-5-(2-methoxypyridin-5-yl)-1-propyl-1,3-dihydro-2H-imidazol-2-ylidene]gold(I) complex (7a), which underwent ligand rearrangement in aqueous solutions. In this study, we investigated the influence of substituents on the 4-aryl ring of the related (NHC)AuIBr complexes (1a–9a) in terms of the conversion to the [(NHC)2AuI]+ (1b–9b) and [(NHC)2AuIIIBr2]+ (1c–9c) species. Furthermore, the influence of external factors such as solvent, temperature, concentration, and presence of halides (Cl–, Br–, and I–) or hydroxyl ions was studied to gain a deeper understanding of the ligand rearrangement reaction. The substituent on the 4-aryl ring has a marginal impact on the scrambling reaction. Out of the investigated organic solvents (dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethanol (EtOH), methanol (MeOH), and acetonitrile (ACN)), only ACN separates single complex molecules. In all other solvents, relatively stable ((NHC)AuIBr)2 dimers are present. The addition of water to ACN solutions forces the formation of such dimeric units, starting the transformation to [(NHC)2AuI]+ and [(NHC)2AuIIIBr2]+. The rate-determining step is the release of Br– from a T-shape intermediate because an excess of KBr terminates this reaction. Furthermore, it is obvious that only single molecules react with halides. The aurophilic interactions between two (NHC)AuIBr molecules are too strong in the presence of water and largely impeded reaction with halides. As a single molecule, the reaction with Cl– (e.g., in a 0.9% NaCl solution) is notable, while I– even leads to a fast and quantitative conversion to (NHC)AuII and finally to [(NHC)2AuI]+.

Ligand Rearrangement Leads to Tetrahydrothiophene-Functionalized N,S-Heterocyclic Carbene Palladium(II) Complexes.

Tetrahydrothiophene-functionalized N,S-heterocyclic carbene palladium(II) complexes are synthesized through an unexpected rearrangement that proceeds with palladium(II) trifluoroacetate and not with palladium(II) acetate, palladium(II) bromide, or palladium(II) chloride. A series of these complexes were isolated and characterized by X-ray crystallography. The mechanism of formation of these [3.2.1]-palladabicycles was explored, and the catalytic capabilities of these complexes were demonstrated in representative C-C coupling reactions.

Substituent-Mediated Transformation of Polynuclear Gold(I)-Sulfido Complexes—From Pentanuclear to Octadecanuclear Cluster-to-Cluster Transformation

Substituent-Mediated Transformation of Polynuclear Gold(I)-Sulfido Complexes—From Pentanuclear to Octadecanuclear Cluster-to-Cluster Transformation