Steric Demand Research Articles

Combined Experimental and Theoretical Study on Hampered Phosphine Dissociation in Heteroleptic Ni/Zn Complexes.

Heterometallic Ni/Zn complexes can serve as molecular models for the semihydrogenation of acetylene catalyzed by heterogeneous Ni/Zn phases. Pursuing this target, we present the synthesis of the series [Ni(ZnCp*)n(ZnMe)n(PEt3)4-n] (n = 1-3; 1, 2, 3) which is obtained via E/Zn exchange from [Ni(ECp*)n(PEt3)4-n] (n = 1-3, E = Al, Ga; P1, P2, P3). The isolation of the intermediate compound [Ni(GaCp*)(ZnCp*)(ZnMe)(PEt3)2] (2a) supports the assumption of a stepwise Ga/Zn exchange in the formation of 3. The dissociation behavior of PEt3 in 2 and 3 was investigated experimentally using variable temperature (VT) UV-vis spectroscopy indicating suppressed phosphine dissociation in both cases. For comparison, the absorption spectra of the saturated and unsaturated compounds were calculated using time dependent DFT calculations (TDDFT). Energy decomposition analysis with the natural orbital for chemical valence extension (EDA NOCV) calculations shows a bond strengthening of the Ni-P bond by successive substitution of the phosphines with (ZnR)2 units. The influence of different phosphines (PMe3, PEt3, PPh3, P(OEt)3) on Ni-P bond length and on Zn-Zn interactions in [Ni(ZnR)2n(PR')4-n] (R = Cp*, Me; R' = Me, Et, Ph, OEt) was also studied by DFT calculations. A correlation of increasing sterical demand of the phosphine ligand and a shortening of the Zn-Zn distances is observed.

Copper(II) as a Platform for Probing the Steric Demand of Bulky β-Diketonates.

The synthesis and characterization of a series of copper bis(β-diketonate) complexes, functionalized with sterically hindered o-biphenyl and m-terphenyl functional groups, are reported. X-ray structural analysis reveals that the ligands exhibit several modes of flexibility in order to accommodate the steric demand. Increased steric bulk of the ligands influences the CuII/I electrochemical reduction, which is likely due to inhibited ligand rotation. Chemical reduction of CuII forms CuI, which disproportionates to Cu0 and CuII. The CuI species could be quantitatively trapped using triphenylphosphine to form Cu(β-diketonate)(PPh3)2 (7), which is also characterized. The catalytic ability of these complexes, along with several common precatalysts, was determined for the reaction of bromobenzene and 2-naphthol, an Ullmann-type C-O bond coupling reaction. Control experiments in toluene show no catalytic ability in the absence of β-diketonates, suggesting involvement of the ligand in catalytic turnover.

Z-Enolate Geometry in the Thioamide Aldol Reaction Illuminated by the 7-Azaindoline Auxiliary.

Z or E enolate geometry is the primary determinant of diastereoselectivity in the aldol reaction. Although amide and thioamide enolates are anticipated to have predominantly the E geometry because of the intrinsic steric demand, spectroscopic confirmation of the geometry in solution has remained elusive, particularly in the realm of highly stereoselective catalytic asymmetric aldol reactions. Herein we demonstrate that the 7-azaindoline auxiliary enables direct observation of the exclusive formation of the Z-enolate of the thioamide en route to a highly syn-selective aldol reaction.

Palladium Catalysed Acid‐Free Carboxytelomerisation of 1,3‐Butadiene with Alcohols Accessing Pelargonic Acid Derivatives Including Triglycerides under Selectivity Control

AbstractPalladium catalysed carboxytelomerisation of 1,3‐butadiene with alcohols yields unsaturated C9‐pelargonic esters in an atom‐economic and straightforward way. Although carboxytelomerisation is of current interest, limitations of applicable alcohol substrates have not been fully investigated. In here, we present the transfer of the catalytic system comprising palladium acetate and tri‐n‐butyl phosphine in pyridine to a broad variety of 20 alcohols with vast difference in nucleophilicity and sterical demands yielding the corresponding esters in a 100% atom economic manner. Effects of nucleophilicity and sterical demands were revealed for monoalcohols, di‐ and polyols. Whilst yields, chemoselectivity and E/Z‐selectivity of the pelargonic acid derivatives were excellent with up to 99%, selectivity towards mono‐, di‐ and triesters could be controlled successfully. Finally, the reaction profile of the carboxytelomerisation glycerol was uncovered, yielding up to 97% of industrially relevant unsaturated C9‐short chain fats.magnified image

Thermally Stable Half-Sandwich Benzhydryl Ln(II) (Ln = Sm, Yb) Complexes Supported by Sterically Demanding Carbazolyl and Fluorenyl Ligands

A series of new isolable and thermally stable half-sandwich Ln(II) benzhydryl complexes coordinated by the sterically demanding ligands tert-butylcarbazol-9-yl [tBu4Carb]Ln[(p-tBu-C6H4)2CH](L) (Ln = Sm, L = DME (4); Ln = Yb, L = DME (5); Ln = Yb, L = TMEDA (6)) and 2,7-di-tert-butyl-fluoren-9-trimethylsilylyl [2,7-tBu2-9-Me3Si-C13H6]Yb[(p-tBu-C6H4)2CH](DME) (7) were synthesized by the alkane elimination reaction of [(p-tBu-C6H4)2CH]2Ln(Ln) (Ln = Sm, Yb) with tBu4CarbH and 2,7-tBu2-9-Me3Si-C13H7. X-ray analysis revealed that in 4, 5, and 7 the benzhydryl ligand is coordinated to the metal ion in an η3 coordination mode, while in 6 it is η1-bound. The type of coordination of the benzhydryl ligands in diamagnetic 5–7 is retained in their C6D6 solutions. Complexes 4–7 demonstrated unprecedented thermal stability and do not undergo decomposition after heating their solutions in C6D6 or toluene at 100 °C for 72 h. The reactions of [tBu4Carb]Ln[(p-tBu-C6H4)2CH](DME) (Ln = Sm (4), Ln = Yb (5)) with an excess of DME led to the formation of the symmetrical bis(carbazolyl) complex products [tBu4Carb]2Ln(DME)4 (Ln = Sm (8), Yb (9)) isolated in the form of separated ion pairs.

All Sugar Based Cellulose Derivatives Synthesized by Azide–Alkyne Click Chemistry

AbstractCellulose is functionalized with different monosaccharides by the azide–alkyne click chemistry approach. Either azide or alkyne moieties are attached to the cellulose backbone and allowed to react with sugar moieties bearing the opposite clickable groups. Between 16% and 100% of the reactive sites at the polymer can be functionalized with sugar molecules and a clear correlation between steric demand and DSSugar is observed. The polymers remain soluble in aprotic dipolar media like dimethyl sulfoxide. Products with a sufficient hydrophilicity dissolve in water as well. FTIR‐ and NMR spectroscopy reveal the triazole formation and indicate an incomplete conversion of the reactive sites as usually observed in the field of click chemistry with polymers.

Formal Aniline Synthesis from Phenols through Deoxygenative N-Centered Radical Substitution.

Phenolic, lignin-derived substrates have emerged as desirable biorenewable chemical feedstocks for coupling reactions. A radical-mediated conversion of phenol derivatives to anilines is reported, using unfunctionalized hydroxamic acids as the N-centered radical source. The applicability of this triethyl phosphite mediated O-atom transfer approach, which tolerates a range of steric and electronic demands to naturally occurring phenols and lignin models, has been demonstrated in this work to access the corresponding aniline derivatives.

High-Throughput Discovery and Evaluation of a General Catalytic Method for N-Arylation of Weakly Nucleophilic Sulfonamides.

Through targeted high-throughput experimentation (HTE), we have identified the Pd/AdBippyPhos catalyst system as an effective and general method to construct densely functionalized N,N-diaryl sulfonamide motifs relevant to medicinal chemistry. AdBippyPhos is particularly effective for the installation of heteroaromatic groups. Computational steric parametrization of the investigated ligands reveals the potential importance of remote steric demand, where a large cone angle combined with an accessible Pd center is correlated to successful catalysts for C-N coupling reactions.

The effects of substituent position and orientation on the structures and dipole moments of the cyanocyclohexanes using density functional theory calculations

The effects of position and orientation (axial/equatorial) on the structures and dipole moments of all possible cyano-substituted cyclohexanes were examined using B3LYP/6-311++G**. All optimized molecules exhibited the chair conformation; molecules exhibiting dipole moments ≥6.00 D were examined to determine the effect of substituent position and orientation on the dipole moment. Axial substituents had a greater effect than did equatorial on the dipole moment. For conformer pairs, axial substituents produced higher energy conformers, indicating that equatorial substituents lowered steric interference. Intramolecular non-bonded distances between co-facial axial/axial and vicinal axial/equatorial cyano-groups did not support the idea of greater axial steric demand. The ranges, but not the magnitudes, of ring C-C-C angles were influenced the atoms vicinal to the central carbon atom of the angle. Patterns in the highest occupied and lowest unoccupied molecular orbital energies may provide a basis from which specific cyanocyclohexanes may be “tuned” for electron-acceptor properties.

A Strategy for Trapping Molecular Guests in MOF-5 Utilizing Surface-Capping Groups

We have developed a simple strategy allowing molecular guests to be trapped within MOF-5 using carboxylic acids bearing sterically demanding substituents as capping reagents. We demonstrate that introducing triphenylacetic acid or diphenylacetic acid onto the surface of crystals of MOF-5 loaded with crystal violet (CV) prevents CV from escaping by blocking the openings of pores. In this study, MOF-5 was capped with four carboxylic acids and two 3° amines, and diffusion of CV out of capped MOFs was monitored using UV–vis spectroscopy to assess how variation in the steric demand of substituents affected retention of CV.

Synthesis, Structural Characterization, and Coordination Chemistry of (Trineopentylphosphine)palladium(aryl)bromide Dimer Complexes ([(Np3P)Pd(Ar)Br]2).

A series of [(PNp3)Pd(Ar)Br]2 complexes (PNp3 = trineopentylphosphine, Ar = 4-tolyl, 4-tert-butylphenyl, 2-tolyl, 4-methoxy-2-methylphenyl, 2-isopropylphenyl, and 2,6-dimethylphenyl) were synthesized and structurally characterized by X-ray crystallography and density functional theory optimized structures. The trineopentylphosphine ligand is able to accommodate coordination of other sterically demanding ligands through changes in its conformation. These conformational changes can be seen in changes in percent buried volume of the PNp3 ligand. The binding equilibria of the [(PNp3)Pd(Ar)Br]2 complexes with pyridine derivatives were determined experimentally and analyzed computationally. The binding equilibria are sensitive to the steric demand of the pyridine ligand and less sensitive to the steric demand of the aryl ligand on palladium. In contrast to previous studies, the binding equilibria do not correlate with pyridine basicity. The binding equilibria results are relevant to fundamental ligand coordination steps in cross-coupling reactions, such as Buchwald-Hartwig aminations.

Controlling Charge-Transport in Metal-Organic Frameworks: Contribution of Topological and Spin-State Variation on the Iron-Porphyrin Centered Redox Hopping Rate.

Metal-organic frameworks (MOFs) are an emerging class of compositions for electro- and photoelectrocatalytic energy conversion processes. Understanding and improving the charge-transport processes within these high surface area molecular redox catalyst assemblies are critical since the charge carrier conductivity is inherently limited in MOFs. Here, we examine a series of four chemically identical but structurally different hydrolytically robust ZrIV-MOFs constructed from tetrakis(4-carboxyphenyl)porphyrinato iron(III), TCPP(FeIII) to understand how their topological construction affects redox hopping conductivity. While a structural variation fixes center-to-center distances to define the hopping rate, we probe that altering the central metal spin-state can further tune the TCPP(FeIII/II) reorganization energy of the self-exchange process. Significant increase in the hopping rate was observed upon axial coordination of 1-methyl imidazole (MIM), which converts a weakly halide bound high-spin (HS) TCPP(FeIII/II) to the six-coordinated low-spin (LS) complex. Our electrochemical and resonance Raman data reveal that pore geometry that defines the Fe-Fe distance in these frameworks dictate the steric demand to accommodate two MIM-molecules, and thus, the population of LS vs HS species is a function of topology in the presence of an excess ligand.

Modulation of Gas-Phase Lithium Cation Basicities by Microsolvation.

In contrast to the extensive knowledge of lithium cation affinities and basicities, the thermochemistry of microsolvated lithium cations is much less explored. Here, we determine the relative stabilities of Li(A,B)n+ complexes, n = 2 and 3, by monitoring their gas-phase reactions with A and B substrate molecules, A/B = Me2O, Et2O, tetrahydrofuran, and MeCN, in a three-dimensional quadrupole-ion trap mass spectrometer. Kinetic analysis of the observed ligand displacement reactions affords equilibrium constants, which are then converted into Gibbs reaction energies. In addition, we use high-level quantum chemical calculations to predict the structures and sequential ligand dissociation energies of the homoleptic Li(A)n+ complexes, n = 1-3. As expected, the ligands dissociate more easily from complexes in higher coordination states. However, the very nature of the ligand also matters. Ligands with different steric demands can, thus, invert their relative Li+ affinities depending on the coordination state of the metal center. This finding shows that microsolvation of Li+ can result in specific effects, which are not recognized if the analysis takes into account only simple lithium cation affinities and basicities.

Enhancing the Variability of [Ge9 ] Cluster Chemistry through Phosphine Functionalization.

The synthetic approach towards molecules that contain Ge atoms with oxidation state 0, and which are exclusively connected to other Ge atoms, is explored by using anionic clusters extracted from binary solids. Besides providing a novel variable method for the introduction of alkenyl moieties to [Ge9 ] cluster compounds, this work expands the spectrum of mixed-functionalized [Ge9 ] cluster anions, which are suitable for the straightforward synthesis of zwitterionic compounds upon coordination to metal cations. In detail, the synthesis of a series of mixed-functionalized [Ge9 ] clusters is reported, including [Ge9 {Si(TMS)3 }3 PRRI ] (R=tBu, RI =(CH2 )3 CH=CH2 ; 2) and [Ge9 {Si(TMS)3 }2 PRRI ]- (R and RI : alkyl, alkenyl, aryl, aminoalkyl; 3 a to 11 a, TMS: (trimethyl)silyl). In 2 and 3 a, pentenyl functionalization of the [Ge9 ] clusters was achieved by reaction of the novel chlorophosphine tBu{(CH2 )3 CH=CH2 }PCl (1) with silylated [Ge9 ] clusters. Furthermore, the reactivity of the cluster anions 3 a to 11 a towards NHCDipp MCl (NHCDipp =1,3-di(2,6-diisopropylphenyl)imidazolylidine; M=Cu, Ag) showed a dependency on the steric demand of the phosphine either zwitterions (3-MNHCDipp to 7-MNHCDipp ) featuring P-M interactions are formed, or Ge-M coordination (8-MNHCDipp to 11-MNHCDipp ) occurs. For M=Ag, the formation of zwitterionic complexes was unequivocally proven by NMR investigations showing 1 J(31 P-107 Ag/109 Ag) spin-spin coupling.

Modulating Guest Uptake in Core-Shell MOFs with Visible Light.

A two-component core-shell UiO-68 type metal-organic framework (MOF) with a nonfunctionalized interior for efficient guest uptake and storage and a thin light-responsive outer shell was prepared by initial solvothermal MOF synthesis followed by solvent-assisted linker exchange. The bulky shell linker features two tetra-ortho-fluorinated azobenzene moieties to exploit their advantageous photoisomerization properties. The obtained perfect octahedral MOF single crystals can be switched repeatedly and with an unprecedented efficiency between E- and Z-rich states using visible light only. Due to the high photoswitch density per pore of the shell layer, its steric demand and thus molecular uptake (and release) can be conveniently modulated upon green and blue light irradiation. Therefore, the "smart" shell acts as a light-controlled kinetic barrier or "gate" for the diffusion of cargo molecules in and out of the MOF crystals.

High-Yielding Syntheses of Multifunctionalized RuII Polypyridyl-Type Sensitizer: Experimental and Computational Insights into Coordination.

RuII complexes based on functionalized 2,6-di(quinolin-8-yl)pyridine (dqp) ligands feature excellent photophysical and geometrical properties, thus suggesting dqp ligands as ideal surrogates for 2,2'-bipyridine (bpy) or 2,2':6',2″-terpyridine (tpy). However, the synthesis of multifunctionalized [Ru(dqp)2]2+-based complexes is often low-yielding, which has hampered their practical value to date. In this study, a universal high-yielding route was explored and corroborated by a mechanistic investigation based on 1H NMR, MS, and density functional theory. With application of high-boiling but less-coordinating solvents (i.e., DMF) during the coordination of dqp by the precursor [Ru(dqp)(MeCN)3]2+, the required reaction temperature is lowered considerably (by 30 °C). In comparison to tpy, the reaction rate for dqp is further reduced which is assigned to the higher steric demand upon the coordination process. Namely, the onset of coordination of a tpy derivative at 60 °C and of dqp at 90 °C is significantly milder than in previous protocols. The versatility of the procedure is demonstrated by the high-yielding syntheses of multifunctionalized RuII complexes reaching up to 90%, whereby the presence of hydroxyl groups and losses during purification may lower the isolated yields substantially. In addition, the same strategy of high-boiling but less-coordinating solvents enabled a milder one-pot protocol to prepare [Ru(dqp)2]2+ from a [Ru(MeCN)6]2+ source, i.e., without the need for in situ reduction or halide abstraction as typical for RuIIICl3 hydrate. Hence, the developed protocol benefits from an improved thermal tolerance of sensitive functional groups, which may be applicable also to related polypyridyl-type ligands.

Aryl Carbamates: Mechanisms of Orthosodiations and Snieckus-Fries Rearrangements.

Aryl carbamates are orthometalated by sodium diisopropylamide (NaDA) in tetrahydrofuran. The resulting arylsodiums undergo Snieckus-Fries rearrangement to give orthoacylated phenols in good yield. The intermediate arylsodiums and resulting orthoacylated phenolates are suggested to be monomeric. The rate-limiting step in the two-step sequence depends on the steric demands of the carbamoyl moiety and the substituents in the meta position of the arene. Rate studies reveal a dominant disolvated-monomer-based orthometalation followed by a di- or trisolvated arylsodium monomer-based rearrangement. Kinetic evidence of a NaDA-catalyzed Snieckus-Fries rearrangement suggests the intermediacy of mixed trimers. Competitive halide eliminations to form benzyne are also discussed.

N-Heterocyclic Carbene Ligand-Controlled Regioselectivity for Nickel-Catalyzed Hydroarylation of Vinylarenes with Benzothiazoles.

A facile regioselective switch for nickel-catalyzed hydroarylation of vinylarenes with benzothiazoles has been developed, which relies on the simple structural variation of novel Ni(II) complexes of the type Ni(NHC)[P(OR)3]Br2. Using magnesium turnings as the reductant, Ni(IMes)[P(OEt)3]Br2 afforded branched products, while Ni(IPr*OMe)[P(OEt)3]Br2 created steric demand to afford linear products. This work also provides a rare example of the rational design of heteroleptic Ni(II) complexes that display the required air stability, reactivity, and regioselectivity via synergism between NHC and phosphite ligands.

Diphenylaminostyryl-substituted quinolizinium derivatives as fluorescent light-up probes for duplex and quadruplex DNA

(E)-2-[1'-((Diphenylamino)styryl)quinolizinium (3a) and 2,2'-{(phenylimino)-bis[(E)-1″,1″′-styryl]}-bis[quin-olizinium] (3b) were synthesized, and their interactions with duplex DNA and quadruplex DNA were investigated with a particular focus on their ability to operate as DNA-sensitive fluorescent probes. Due to the significantly different size and steric demand of these quinolizinium derivatives they exhibit different binding modes. Thus, 3a intercalates into duplex DNA and binds through n stacking to quadruplex DNA, whereas 3b favours groove binding to both DNA forms. The emission intensity of these compounds is very low in aqueous solution, but it increases drastically upon association with duplex DNA by a factor of 11 (3a) and >100 (3b) and with quadruplex DNA by a factor of >100 (3a) and 10 (3b), with emission bands between 600 and 750 nm.

A Flexible, Extremely Sterically Demanding Triazenide Ligand: Synthesis and Coordination Chemistry

Seeking to further the investigation of extremely sterically congested main group metals, the synthesis of a new oligoaryl triazene (Dipp*2N3H) is reported. This was accessed by diazotization of Dipp*NH2 to access both Dipp*I and Dipp*N3 in high yield and with scalable preparations. These compounds are used as precursors to Dipp*2N3H, via a magnesium aryl, due to the difficulties in obtaining pure triazene via an aryl‐lithium species. Deprotonation of the triazene with both potassium and thallium tert‐butoxides gives the respective metal triazenides in high yield. The solid‐state structures of these complexes have been determined and three high hapticity metal arene‐π interactions, which sterically shield the metal centers, were observed in each case; spectroscopic C2v symmetry suggests these interactions are fluxional in solution. The variability of these interactions in the solid state as well as the changes in calculated steric demand of the ligand between metals point to an adaptive, flexible coordination mode.