Janus-like squaramide-based hosts: dual mode of binding and conformational transitions driven by ion-pair recognition

New tripodal squaramide-based hosts have been synthesised and structurally characterised by spectroscopic methods. In 2.5?% (v/v) [D(6)]DMSO in CDCl(3), compound 4 formed dimeric assemblies [log?K(dim)=3.68(8)] as demonstrated by (1)H?NMR spectroscopy and UV dilution experiments. AFM and SEM analyses revealed the formation of a network of bundled fibres, which indicates a preferential mechanism for aggregation. These C(3)-symmetric tripodal hosts exhibited two different and mutually exclusive modes of binding, each one easily accessible by simultaneous reorientation of the squaramide groups. In the first, a convergent disposition of the NH squaramide protons allowed the formation of an array of N-H???X(-) hydrogen bonds with anions. In the second mode, reorientation of carbonyl squaramide groups allowed multiple C=O???H interactions with ammonium cations. The titration of 4 with different tetraalkylammonium iodides persistently showed the formation of 1:1 complexes, as well as 1:2 and 1:3 complexes. The corresponding stoichiometries and binding affinities of the complexes were evaluated by multi-regression analysis. The formation of high-order complexes, supported by ROESY, NOESY and mass spectrometry experiments, has been attributed to the insertion of NR(4)I ion pairs between the carbonyl and NH protons of the squaramide groups located in adjacent arms of 4. The observed effects reflect the induction of significant conformational changes in the hosts, mainly in relation to the relative orientation of the squaramide groups adapting their geometries to incoming ion-pair complementary substrates. The results presented herein identify and fully describe two different modes of ion-pair recognition aimed at directing conformational transitions in the host, therefore establishing a base for controlling more elaborate movements of molecular devices through ion-pair recognition.     

Recognition of N‐Alkyl and N‐Aryl Acetamides by N‐Alkyl Ammonium Resorcinarene Chlorides

N‐Alkyl ammonium resorcinarene chlorides are stabilized by an intricate array of intra‐ and intermolecular hydrogen bonds that leads to cavitand‐like structures. Depending on the upper‐rim substituents, self‐inclusion was observed in solution and in the solid state. The self‐inclusion can be disrupted at higher temperatures, whereas in the presence of small guests the self‐included dimers spontaneously reorganize to 1:1 host–guest complexes. These host compounds show an interesting ability to bind a series of N‐alkyl acetamide guests through intermolecular hydrogen bonds involving the carbonyl oxygen (C?O) atoms and the amide (NH) groups of the guests, the chloride anions (Cl^?) and ammonium (NH₂⁺) cations of the hosts, and also through CH ??? π interactions between the hosts and guests. The self‐included and host–guest complexes were studied by single‐crystal X‐ray diffraction, NMR titration, and mass spectrometry.     

A Two-dimensional Lanthanide Coordination Framework with a New Amide-type Tripodal Ligand, 2,2′,2＂-Nitrilotris{[（2′- benzylaminoformyl）phenoxy]ethyl}amine

The La(III) complex with a new amide‐type tripodal ligand,2,2′.2″‐nitrilotris{[(2′‐benzylaminofomyl)‐phenoxylethyl)amine (L), was synthesized and characterized by X‐ray crystallographic analysis. Crystal data: C₄₈H_55.50LaN₇O_18.75, M_r=1169.40, monoclinic. space group, P2₁/n, a= 1.0644(3) nm. b=2.3889(5) nm, c= 2.1917(5) nm, β=90.65°, V=5.573(2) nm³, Z=1, D_c=1.394 g°cm^?3, R₁=0.0487, wR [1>2s?(I)]=0.1266. The results reveal that each La(III) ion binds to 9 oxygen atoms, three of which belong to carbonyl groups from three tripodal ligands and six to three bidentate nitrate groups and a two‐dimensional sheet of 4.8² networks is assembled by metal‐ligand coordination interaction. L, a heptadentate compound, merely acts as a tridentate bridging spacer due to its steric hindrance and links the La(III) ions as three‐connected nodes.     

Formation of a Thymine‐HgII‐Thymine Metal‐Mediated DNA Base Pair: Proposal and Theoretical Calculation of the Reaction Pathway

A reaction mechanism that describes the substitution of two imino protons in a thymine:thymine (T:T) mismatched DNA base pair with a Hg^II ion, which results in the formation of a (T)N3‐Hg^II‐N3(T) metal‐mediated base pair was proposed and calculated. The mechanism assumes two key steps: The formation of the first Hg^II? N3(T) bond is triggered by deprotonation of the imino N3 atom in thymine with a hydroxo ligand on the Hg^II ion. The formation of the second Hg^II? N3(T) bond proceeds through water‐assisted tautomerization of the remaining, metal‐nonbonded thymine base or through thymine deprotonation with a hydroxo ligand of the Hg^II ion already coordinated to the thymine base. The thermodynamic parameters ΔG_R=?9.5 kcal mol^?1, ΔH_R=?4.7 kcal mol^?1, and ΔS_R=16.0 cal mol^?1 K^?1 calculated with the ONIOM (B3LYP:BP86) method for the reaction agreed well with the isothermal titration calorimetric (ITC) measurements by Torigoe et al. [H. Torigoe, A. Ono, T. Kozasa, Chem. Eur. J. 2010 , 16, 13218–13225]. The peculiar positive reaction entropy measured previously was due to both dehydration of the metal and the change in chemical bonding. The mercury reactant in the theoretical model contained one hydroxo ligand in accord with the experimental pK_a value of 3.6 known for an aqua ligand of a Hg^II center. The chemical modification of T:T mismatched to the T‐Hg^II‐T metal‐mediated base pair was modeled for the middle base pair within a trinucleotide B‐DNA duplex, which ensured complete dehydration of the Hg^II ion during the reaction.     

Calix[4]arene‐Phosphine Dimers: Precursors of Flexible Metallo‐Capsules and Self‐Compacting Molecules

The first diphosphines based on a double calixarene, namely 1,4 (or 1,3)‐bis‐(5‐diphenylphosphino‐25,26,27,28‐tetrapropoxycalix[4]aren‐17‐yl)benzene ( L² , L³ ) were each prepared in four steps starting from 5,17‐dibromo‐25,26,27,28‐tetrapropoxycalix[4]arene. Upon reaction of L² with [Au(tht)(thf)]BF₄, (tht=C₄H₈S) a rigid metallo‐capsule was quantitatively formed, which adopts an oblique form owing to the distinct nature of the spacers linking the two calixarene half‐spheres. In the solid state, the 1,4‐substituted phenylene linker is turned towards the gold ion, suggesting the existence of weak bonding interactions between two aromatic CH protons of this ring and the metal centre (Au???H=2.67 Å). In contrast to this gold complex, the related silver complex shows dynamic behaviour in solution, the exchange between two enantiomeric oblique forms being facilitated by the greater stereochemical flexibility of Ag^I vs. Au^I. A heteronuclear ¹⁰⁹Ag{¹H} HMQC experiment established strong correlations between the CH protons of the phenylene linker and the ¹⁰⁹Ag ion. Dynamic behaviour similar to that observed for the silver complex was further observed in trans‐[PtCl₂? L² ], a chelate complex that could be obtained quantitatively from L² and [PtCl₂(PhCN)₂]. The intended formation of a chelate complex leading to a capsule with an endo‐oriented metal centre was achieved by reacting L³ with [Pd(allyl)(thf)₂]BF₄. The complex thus formed constitutes the first organometallic transition metal complex embedded in a cavity with large portals. Binding of [RuCl₂(p‐cymene)] to L² and L³ resulted in self‐compacting bimetallic complexes in which each calixarene basket entraps a Ru(p‐cymene) unit, thereby forming molecules occupying a minimal volume.     

d–d Dative Bonding Between Iron and the Alkaline‐Earth Metals Calcium,Strontium, and Barium

Double deprotonation of the diamine 1,1′‐(tBuCH₂NH)‐ferrocene ( 1 ‐H₂) by alkaline‐earth (Ae) or Eu^II metal reagents gave the complexes 1 ‐Ae (Ae=Mg, Ca, Sr, Ba) and 1 ‐Eu. 1 ‐Mg crystallized as a monomer while the heavier complexes crystallized as dimers. The Fe???Mg distance in 1 ‐Mg is too long for a bonding interaction, but short Fe???Ae distances in 1 ‐Ca, 1 ‐Sr, and 1 ‐Ba clearly support intramolecular Fe???Ae bonding. Further evidence for interactions is provided by a tilting of the Cp rings and the related ¹H NMR chemical‐shift difference between the Cp α and β protons. While electrochemical studies are complicated by complex decomposition, UV/Vis spectral features of the complexes support Fe→Ae dative bonding. A comprehensive bonding analysis of all 1 ‐Ae complexes shows that the heavier species 1 ‐Ca, 1 ‐Sr, and 1 ‐Ba possess genuine Fe→Ae bonds which involve vacant d‐orbitals of the alkaline‐earth atoms and partially filled d‐orbitals on Fe. In 1 ‐Mg, a weak Fe→Mg donation into vacant p‐orbitals of the Mg atom is observed.     

The Counterion Effect on the Assembly of Coordination Networks of Cationic (η3‐Allyl)palladium Complexes Containing 3,5‐Dimethyl‐4‐nitro‐1H‐pyrazole

Two new (η³‐allyl)palladium complexes containing the ligand 3,5‐dimethyl‐4‐nitro‐1H‐pyrazole (Hdmnpz) were synthesized and characterized as [Pd(η³‐C₃H₅)(Hdmnpz)₂]BF₄ ( 1 ) and [Pd(η³‐C₃H₅)(Hdmnpz)₂]NO₃ ( 2 ). The structures of these compounds were determined by single‐crystal X‐ray diffraction to evaluate the intermolecular assembly. Each complex exhibits similar coordination behavior consistent with cationic entities comprised of two pyrazole ligands coordinated with the [Pd(η³‐C₃H₅)]⁺ fragment in an almost square‐planar coordination geometry. In 1 , the cationic entities are propagated through strong intermolecular H‐bonds formed between the pyrazole NH groups and BF ions in one‐dimensional polymer chains along the a axis. These chains are extended into two‐dimensional sheet networks via bifurcated H‐bonds. New intermolecular interactions established between NO₂ and Me substituents at the pyrazole ligand of neighboring sheets give rise to a three‐dimensional network. By contrast, compound 2 presents molecular cyclic dimers formed through N? H???O H‐bonds between two NO counterions and the pyrazole NH groups of two cationic entities. The dimers are also connected to each other through C? H???O H‐bonds between the remaining O‐atom of each NO ion and the allyl CH₂ H‐atom. Those interactions expand in a layer which lies parallel to the face (101).     

Using (FH)2 and (FH)3 to Bridge the σ‐Hole and the Lone Pair at P in Complexes with H2XP,for X=CH3, OH,H, CCH,F, Cl,NC, and CN

Ab initio MP2/aug′‐cc‐pVTZ calculations are used to investigate the binary complexes H₂XP:HF, the ternary complexes H₂XP:(FH)₂, and the quaternary complexes H₂XP:(FH)₃, for X=CH₃, OH, H, CCH, F, Cl, NC, and CN. Hydrogen‐bonded (HB) binary complexes are formed between all H₂XP molecules and FH, but only H₂FP, H₂ClP, and H₂(NC)P form pnicogen‐bonded (ZB) complexes with FH. Ternary complexes with (FH)₂ are stabilized by F?H???P and F?H???F hydrogen bonds and F???P pnicogen bonds, except for H₂(CH₃)P:(FH)₂ and H₃P:(FH)₂, which do not have pnicogen bonds. All quaternary complexes H₂XP:(FH)₃ are stabilized by both F?H???P and F?H???F hydrogen bonds and P???F pnicogen bonds. Thus, (FH)₂ with two exceptions, and (FH)₃ can bridge the σ‐hole and the lone pair at P in these complexes. The binding energies of H₂XP:(FH)₃ complexes are significantly greater than the binding energies of H₂XP:(FH)₂ complexes, and nonadditivities are synergistic in both series. Charge transfer occurs across all intermolecular bonds from the lone‐pair donor atom to an antibonding σ* orbital of the acceptor molecule, and stabilizes these complexes. Charge‐transfer energies across the pnicogen bond correlate with the intermolecular P?F distance, while charge‐transfer energies across F?H???P and F?H???F hydrogen bonds correlate with the distance between the lone‐pair donor atom and the hydrogen‐bonded H atom. In binary and quaternary complexes, charge transfer energies also correlate with the distance between the electron‐donor atom and the hydrogen‐bonded F atom. EOM‐CCSD spin‐spin coupling constants ^2hJ(F–P) across F?H???P hydrogen bonds, and ^1pJ(P–F) across pnicogen bonds in binary, ternary, and quaternary complexes exhibit strong correlations with the corresponding intermolecular distances. Hydrogen bonds are better transmitters of F–P coupling data than pnicogen bonds, despite the longer F???P distances in F?H???P hydrogen bonds compared to P???F pnicogen bonds. There is a correlation between the two bond coupling constants ^2hJ(F–F) in the quaternary complexes and the corresponding intermolecular distances, but not in the ternary complexes, a reflection of the distorted geometries of the bridging dimers in ternary complexes.     

Zinc(II) Complexes with 1H‐Imidazol‐4‐yl‐Containing Polyamine Ligand

The protonation and Zn^II/Cu^II complexation constants of tripodal polyamine ligand N¹‐(2‐aminoethyl)‐N¹‐(1H‐imidazol‐4‐ylmethyl)‐ethane‐1,2‐diamine (HL) were determined by potentiometric titration. Three new compounds, i.e. [H₃(HL)](ClO₄)₃ ( 5 ), [Zn(HL)Cl](ClO₄) ( 6 ) and {[Zn(L)](ClO₄)}_n ( 7 ) were obtained by reactions of HL · 4HCl with Zn(ClO₄)₂ · 6H₂O under different reaction pH, and they were compared with the corresponding Cu^II complexes reported previously. The results indicate that the reaction pH and metal ions have remarkable influence on the formation and structure of the complexes.     

The cooperative effect in ion-pair recognition by a ditopic hemicryptophane host

The heteroditopic hemicryptophane 1 , which bears a tripodal anion binding site and a cation recognition site in the molecular cavity, proved to be an efficient ion‐pair receptor. The hemicryptophane host binds anions selectively depending on shape and hydrogen‐bond‐accepting ability. It forms an inclusion complex with the Me₄N⁺ ion, which can simultaneously bind anionic species to provide anion@[ 1? Me₄N⁺] complexes. The increased affinity of [ 1? Me₄N⁺] for anionic species is attributed to a strong cooperative effect that arises from the properly positioned binding sites in the hemicryptophane cavity, thus allowing the formation of the contact ion pair. Density functional theory calculations were performed to analyze the Coulomb interactions of the ion pairs, which compete with the ion‐dipole ones, that originate in the ion–hemicryptophane contacts.     

Conjugated Polyelectrolyte‐Induced Self‐Assembly of Alkynylplatinum(II) 2,6‐Bis(benzimidazol‐2′‐yl)pyridine Complexes

Water‐soluble cationic alkynylplatinum(II) 2,6‐bis(benzimidazol‐2′‐yl)pyridine (bzimpy) complexes have been demonstrated to undergo supramolecular assembly with anionic polyelectrolytes in aqueous buffer solution. Metal–metal‐to‐ligand charge transfer (MMLCT) absorptions and triplet MMLCT (³MMLCT) emissions have been found in UV/Vis absorption and emission spectra of the electrostatic assembly of the complexes with non‐conjugated polyelectrolytes, driven by Pt???Pt and π–π interactions among the complex molecules. Interestingly, the two‐component ensemble formed by [Pt(bzimpy‐Et){C?CC₆H₄(CH₂NMe₃‐4)}]Cl₂ ( 1 ) with para‐linked conjugated polyelectrolyte (CPE), PPE‐SO₃^?, shows significantly different photophysical properties from that of the ensemble formed by 1 with meta‐linked CPE, mPPE‐Ala. The helical conformation of mPPE‐Ala allows the formation of strong mPPE‐Ala– 1 aggregates with Pt???Pt, electrostatic, and π–π interactions, as revealed by the large Stern–Volmer constant at low concentrations of 1 . Together with the reasonably large Förster radius, large HOMO–LUMO gap and high triplet state energy of mPPE‐Ala to minimize both photo‐induced charge transfer (PCT) and Dexter triplet energy back‐transfer (TEBT) quenching of the emission of 1 , efficient Förster resonance energy transfer (FRET) from mPPE‐Ala to aggregated 1 molecules and strong ³MMLCT emission have been found, while the less strong PPE‐SO₃^?– 1 aggregates and probably more efficient PCT and Dexter TEBT quenching would account for the lack of ³MMLCT emission in the PPE‐SO₃^?– 1 ensemble.     

Role of Magnetic Exchange Interactions in the Magnetization Relaxation of {3d–4f} Single‐Molecule Magnets: A Theoretical Perspective

Combined density functional and ab initio calculations are performed on two isomorphous tetranuclear {Ni₃^IIILn^III} star‐type complexes [Ln=Gd ( 1 ), Dy ( 2 )] to shed light on the mechanism of magnetic exchange in 1 and the origin of the slow magnetization relaxation in complex 2 . DFT calculations correctly reproduce the sign and magnitude of the J values compared to the experiments for complex 1 . Acute ?Ni?O?Gd bond angles present in 1 instigate a significant interaction between the 4f_xyz orbital of the Gd^III ion and 3d orbital of the Ni^II ions, leading to rare and strong antiferromagnetic Ni???Gd interactions. Calculations reveal the presence of a strong next‐nearest‐neighbour Ni???Ni antiferromagnetic interaction in complex 1 leading to spin frustration behavior. CASSCF+RASSI‐SO calculations performed on complex 2 suggest that the octahedral environment around the Dy^III ion is neither strong enough to stabilize the m_J |±15/2〉 as the ground state nor able to achieve a large ground‐state–first‐excited‐state gap. The ground‐state Kramers doublet for the Dy^III ion is found to be the m_J |±13/2〉 state with a significant transverse anisotropy, leading to very strong quantum tunneling of magnetization (QTM). Using the POLY_ANISO program, we have extracted the J_NiDy interaction as ?1.45 cm^?1. The strong Ni???Dy and next‐nearest‐neighbour Ni???Ni interactions are found to quench the QTM to a certain extent, resulting in zero‐field SMM behavior for complex 2 . The absence of any ac signals at zero field for the structurally similar [Dy(AlMe₄)₃] highlights the importance of both the Ni???Dy and the Ni???Ni interactions in the magnetization relaxation of complex 2 . To the best of our knowledge, this is the first time that the roles of both the Ni???Dy and Ni???Ni interactions in magnetization relaxation of a {3d–4f} molecular magnet have been established.     

Dynamic Properties of Molecular Tweezers with a Bis(2‐hydroxyphenyl)pyrimidine Backbone

4,6‐Bis(2‐hydroxyphenyl)‐2‐alkylpyrimidines with two anthryl or 9‐ethylnylanthryl substituents at the positions para to the OH groups prefer a U‐shaped conformation supported by two intramolecular OH ??? N hydrogen bonds in the solid state and in CDCl₃ solution. The compound with a hexyl substituent on the pyrimidine group and two 9‐ethynylanthryl arms at the hydroxyphenyl groups forms a 1:1 complex with 2,4,7‐trinitrofluorenone. Its association constant K_a was estimated to be 2100 M ^?1 at 298 K, which is larger than those of other molecular tweezers (K_a<1000 M ^?1). DFT calculations suggested that the complex adopts a stable conformation supported by intramolecular hydrogen bonds among the OH groups and the pyrimidine ring as well as by intermolecular π–π interaction between the anthryl groups and 2,4,7‐trinitrofluorenone. Addition of nBu₄NF to a solution of the molecular tweezers or their complexes causes the cleavage of one or two OH ??? N hydrogen bonds, formation of new O ??? HF hydrogen bonds, and changes in the molecular conformation. The resulting structure of the molecular tweezers contains nonparallel anthryl groups, which do not bind the guest molecule. Photochemical measurements on 4,6‐bis(2‐hydroxyphenyl)‐2‐methylpyrimidine with two anthryl substituents showed negligible luminescence (quantum yield ?<0.01), owing to photoinduced electron transfer of the molecule with a U‐shaped structure. However, the O‐hexylated compound exhibits emission from the anthryl groups with ?=0.39.     

Experimental and theoretical study of the intramolecular C–H···N and C–H···S hydrogen bonding effects in the 1H and 13C NMR spectra of the 2‐(alkylsulfanyl)‐5‐amino‐1‐vinylpyrroles: a particular state of amine nitrogen

In the ¹H NMR spectra of the 1‐vinylpyrroles with amino‐ and alkylsulfanyl groups in 5 and 2 positions, an extraordinarily large difference between resonance positions of the H_A and H_B terminal methylene protons of the vinyl group is discovered. Also, the one‐bond ¹J(C_β,H_B) coupling constant is surprisingly greater than the ¹J(C_β,H_A) coupling constant in pyrroles under investigation, while in all known cases, there was a reverse relationship between these coupling constants. These spectral anomalies are substantiated by quantum chemical calculations. The calculations show that the amine nitrogen lone pair is removed from the conjugation with the π‐system of the pyrrole ring so that it is directed toward the H_B hydrogen. These factors are favorable to the emergence of the intramolecular C–H_B???N hydrogen bonding in the s‐cis(N) conformation. On the other hand, the spatial proximity of the sulfur to the H_B hydrogen provides an opportunity of the intramolecular C–H_B???S hydrogen bonding in the s‐cis(S) conformation. Presence of the hydrogen bond critical points as well as ring critical point for corresponding chelate ring revealed by a quantum theory of atoms in molecules (QTAIM) approach confirms the existence of the weak intramolecular C–H???N and C–H???S hydrogen bonding. Therefore, an unusual high‐frequency shift of the H_B signal and the increase in the ¹J(C_β,H_B) coupling constant can be explained by the effects of hydrogen bonding. Copyright © 2013 John Wiley & Sons, Ltd.     

The Squaramide versus Urea Contest for Anion Recognition

The interaction of a neutral squaramide‐based receptor, equipped with two 4‐nitrophenyl substituents ( R_sq ), with halides and oxoanions has been studied in MeCN. UV/Vis and ¹H NMR spectroscopy titration experiments clearly indicated the formation of 1:1 hydrogen bonding [ R_sq ???X]⁺ complexes with all the investigated anions. X‐ray diffraction studies on the chloride and bromide complex salts confirmed the 1:1 stoichiometry and indicated the establishment of bifurcated hydrogen‐bond interactions between the squaramide‐based receptor and the halide anion that involved both 1) amide N? H and 2) aryl proximate C? H fragments, for a total of four bonds. Probably due to the contribution of C? H fragments, complexes of R_sq with halides are 1 to 2 orders of magnitude more stable than the corresponding ones with the analogous urea‐based receptor that contains two 4‐nitrophenyl substituents ( R_ur ). In the case of oxoanions, R_sq forms complexes, the stability of which decreases with the decreasing basicity of the anion (H₂PO₄^?>NO₂^?≈HSO₄^?>NO₃^?), and is comparable to that of complexes of the urea‐based receptor R_ur . Such a behaviour is ascribed to the predominance of different contributions: electrostatic interaction for halides, acid‐to‐base ‘frozen’ proton transfer for oxoanions. Finally, with the strongly basic anions F^? and CH₃COO^?, R_sq first gives genuine hydrogen‐bond complexes of 1:1 stoichiometry; then, upon addition of a second anion equivalent, it undergoes deprotonation of one N? H fragment, with the simultaneous formation of the dianion hydrogen‐bond complexes, [HF₂]^? and [CH₃COOH???CH₃COO]^?, respectively. In the case of the urea‐based derivative R_ur , deprotonation takes place with fluoride but not with acetate. The apparently higher Brønsted acidity of R_sq with respect to R_ur reflects the capability of the squaramide receptor to delocalise the negative charge formed on N? H deprotonation over the cyclobutene‐1,2‐dione ring and the entire molecular framework.     

Synthesis of tert‐butylcalix[4]arenes modified with germanium‐containing side‐chains at the lower rim

Bis‐, tris‐, and tetrakis‐trimethylgermylpropanoyl esters of p‐tert‐butylcalix[4]arenes 5, 7 , and 8 were synthesized and their conformations in solution were determined by ¹H and ¹³C NMR spectroscopy. The characteristic pair of doublets from the Ar CH₂ Ar methylene protons was observed, indicating that all the prepared calixarenes were in the cone conformation. These germylated calixarenes failed to exhibit any appreciable cation‐capturing ability, indicating that further modification is required to make these compounds to be effective ditopic hosts. © 2003 Wiley Periodicals, Inc. Heteroatom Chem 14:365–373, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10164     

N‐Methyl‐2, 2'‐Dipicolylamine Complexes as Potential Models for Metal‐Mediated Base Pairs

Metal‐mediated base pairs can be used to insert metal ions into nucleic acids at precisely defined positions. As structural data on the resulting metal‐modified DNA are scarce, appropriate model complexes need to be synthesized and structurally characterized. Accordingly, the molecular structures of nine transition metal complexes of N‐methyl‐2, 2'‐dipicolylamine (dipic) are reported. In combination with an azole‐containing artificial nucleoside, this tridentate ligand had recently been used to generate metal‐mediated base pairs (Chem. Commun. 2011 , 47, 11041–11043). The Pd^II and Pt^II complexes reported here confirm that the formation of planar complexes (as required for a metal‐mediated base pair) comprising N‐methyl‐2, 2'‐dipicolylamine is possible. Two Hg^II complexes with differing stoichiometry indicate that a planar structure might also be formed with this metal ion, even though it is not favored. In the complex [Ag₂(dipic)₂](ClO₄)₂, the two Ag^I ions are located close to one another with an Ag ··· Ag distance of 2.9152(3) Å, suggesting the presence of a strong argentophilic interaction.     

Crystallization experiments with the dinuclear chelate ring complex di‐μ‐chlorido‐bis[(η2‐2‐allyl‐4‐methoxy‐5‐{[(propan‐2‐yloxy)carbonyl]methoxy}phenyl‐κC1)platinum(II)]

Crystallization experiments with the dinuclear chelate ring complex di‐μ‐chlorido‐bis[(η²‐2‐allyl‐4‐methoxy‐5‐{[(propan‐2‐yloxy)carbonyl]methoxy}phenyl‐κC¹)platinum(II)], [Pt₂(C₁₅H₁₉O₄)₂Cl₂], containing a derivative of the natural compound eugenol as ligand, have been performed. Using five different sets of crystallization conditions resulted in four different complexes which can be further used as starting compounds for the synthesis of Pt complexes with promising anticancer activities. In the case of vapour diffusion with the binary chloroform–diethyl ether or methylene chloride–diethyl ether systems, no change of the molecular structure was observed. Using evaporation from acetonitrile (at room temperature), dimethylformamide (DMF, at 313 K) or dimethyl sulfoxide (DMSO, at 313 K), however, resulted in the displacement of a chloride ligand by the solvent, giving, respectively, the mononuclear complexes (acetonitrile‐κN)(η²‐2‐allyl‐4‐methoxy‐5‐{[(propan‐2‐yloxy)carbonyl]methoxy}phenyl‐κC¹)chloridoplatinum(II) monohydrate, [Pt(C₁₅H₁₉O₄)Cl(CH₃CN)]·H₂O, (η²‐2‐allyl‐4‐methoxy‐5‐{[(propan‐2‐yloxy)carbonyl]methoxy}phenyl‐κC¹)chlorido(dimethylformamide‐κO)platinum(II), [Pt(C₁₅H₁₉O₄)Cl(C₂H₇NO)], and (η²‐2‐allyl‐4‐methoxy‐5‐{[(propan‐2‐yloxy)carbonyl]methoxy}phenyl‐κC¹)chlorido(dimethyl sulfoxide‐κS)platinum(II), determined as the analogue {η²‐2‐allyl‐4‐methoxy‐5‐[(ethoxycarbonyl)methoxy]phenyl‐κC¹}chlorido(dimethyl sulfoxide‐κS)platinum(II), [Pt(C₁₄H₁₇O₄)Cl(C₂H₆OS)]. The crystal structures confirm that acetonitrile interacts with the Pt^II atom via its N atom, while for DMSO, the S atom is the coordinating atom. For the replacement, the longest of the two Pt—Cl bonds is cleaved, leading to a cis position of the solvent ligand with respect to the allyl group. The crystal packing of the complexes is characterized by dimer formation via C—H…O and C—H…π interactions, but no π–π interactions are observed despite the presence of the aromatic ring.     

Hydrogen Bonding of Fluorinated Saccharides in Solution: F Acting as H‐Bond Acceptor in a Bifurcated H‐Bond of 4‐Fluorinated Levoglucosans

4‐Fluorinated levoglucosans were synthesised to test if OH???F H‐bonds are feasible even when the O???F distance is increased. The fluorinated 1,6‐anhydro‐β‐D ‐glucopyranoses were synthesised from 1,6 : 3,4‐dianhydro‐β‐D ‐galactopyranose ( 8 ). Treatment of 8 with KHF₂ and KF gave 43% of 4‐deoxy‐4‐fluorolevoglucosan ( 9 ), which was transformed into the 3‐O‐protected derivatives 13 by silylation and 15 by silylation, acetylation, and desilylation. 4‐Deoxy‐4‐methyllevoglucosan ( 19 ) and 4‐deoxylevoglucosan ( 21 ) were prepared as reference compounds that can only form a bivalent H‐bond from HO? C(2) to O? C(5). They were synthesised from the ⁱPr₃Si‐protected derivative of 8 . Intramolecular bifurcated H‐bonds from HO? C(2) to F? C(4) and O? C(5) of the 4‐fluorinated levoglucosans in CDCl₃ solution are evidenced by the ¹H‐NMR scalar couplings ^h1J(F,OH) and ³J(H,OH). The OH???F H‐bond over an O???F distance of ca. 3.0 Å is thus formed in apolar solvents, at least when favoured by the simultaneous formation of an OH???O H‐bond.     

Theoretical study on the interactions of halogen‐bonds and pnicogen‐bonds in phosphine derivatives with Br2, BrCl,and BrF

The MP2 ab initio quantum chemistry methods were utilized to study the halogen‐bond and pnicogen‐bond system formed between PH₂X (X = Br, CH₃, OH, CN, NO₂, CF₃) and BrY (Y = Br, Cl, F). Calculated results show that all substituent can form halogen‐bond complexes while part substituent can form pnicogen‐bond complexes. Traditional, chlorine‐shared and ion‐pair halogen‐bonds complexes have been found with the different substituent X and Y. The halogen‐bonds are stronger than the related pnicogen‐bonds. For halogen‐bonds, strongly electronegative substituents which are connected to the Lewis acid can strengthen the bonds and significantly influenced the structures and properties of the compounds. In contrast, the substituents which connected to the Lewis bases can produce opposite effects. The interaction energies of halogen‐bonds are 2.56 to 32.06 kcal·mol^?1; The strongest halogen‐bond was found in the complex of PH₂OH???BrF. The interaction energies of pnicogen‐bonds are in the range 1.20 to 2.28 kcal·mol^?1; the strongest pnicogen‐bond was found in PH₂Br???Br₂ complex. The charge transfer of lp(P) ? σ*(Br? Y), lp(F) ? σ*(Br? P), and lp(Br) ? σ*(X? P) play important roles in the formation of the halogen‐bonds and pnicogen‐bonds, which lead to polarization of the monomers. The polarization caused by the halogen‐bond is more obvious than that by the pnicogen‐bond, resulting in that some halogen‐bonds having little covalent character. The symmetry adapted perturbation theory (SAPT) energy decomposition analysis showes that the halogen‐bond and pnicogen‐bond interactions are predominantly electrostatic and dispersion, respectively.