Peculiarities of pπ?pπ conjugation in aminosubstituted phosphaalkenes

Effets of p_π p_π conjugation in phosphaalkenes have been considered by taking into account the results of X-ray structural studies and ab initio quantum-chemical calculations. The structural consequences of the conjugation depend to a certain extent on the character of the substituent and the place of its attachment to the double PC bond. Depending on the location of the substituent, bond polarization may strengthen or weaken (compensate for) the conjugation effects. A high contribution of the s-character of the lone electron pair of the P atom is an essential feature of the electron structure of a phosphaalkene.     

Theoretical Studies on the Halogen Bond between Propargyl Radical and Dihalogen Molecules

MP2/aug‐cc‐pVDZ calculations are carried out on the geometries, vibrational frequencies, interaction energies and topological properties for the π‐type halogen‐bonded complexes between propargyl radical and dihalogen molecules ClF, BrF and BrCl. There are two kinds of geometries: complex ( a ) involves the interaction between the X (X=Cl, Br) atom and the midpoint of C(1) –C(2) bond, complex ( b ) involves the interaction between the X atom and C(3) atom. The lengths of the halogen bond, the frequencies of the halogen bond, the elongation extent of the X–Y (XY=ClF, BrF, BrCl) bond, topological parameters at the BCPs of the halogen bond and X–Y bond are all well consistent with the interaction energies. The interaction of complex ( a ) is stronger than that of complex ( b ); the interaction of propargyl···BrF is stronger than that of propargyl···ClF and propargyl···BrCl. For the complexes ( a ) and ( b ), the charge transfer is observed from propargyl radical to XY, the atomic energy, the dipolar polarization, and the volume of the halogen atom X decrease upon complex formation.     

A new unconventional halogen bond C?X···H?M between HCCX (X = Cl and Br) and HMH (M = Be and Mg): An ab initio study

In this article, a new type of halogen‐bonded complex YCCX···HMY (X = Cl, Br; M = Be, Mg; Y = H, F, CH₃) has been predicted and characterized at the MP2/aug‐cc‐pVTZ level. We named it as halogen‐hydride halogen bonding. In each YCCX···HMY complex, a halogen bond is formed between the positively charged X atom and the negatively charged H atom. This new kind of halogen bond has similar characteristics to the conventional halogen bond, such as the elongation of the C? X bond and the red shift of the C? X stretch frequency upon complexation. The interaction strength of this type of halogen bond is in a range of 3.34–10.52 kJ/mol, which is smaller than that of dihydrogen bond and conventional halogen bond. The nature of the electrostatic interaction in this type of halogen bond has also been unveiled by means of the natural bond orbital, atoms in molecules, and energy decomposition analyses. © 2009 Wiley Periodicals, Inc. J Comput Chem 2010     

Unusual Changes of C−H Bond Lengths in Chiral Zinc Complexes Induced by Noncovalent Interactions

In order to better understand the effect of non-covalent weak interactions on molecules, we have explored a variety of weak interactions, such as improper H-bonding (HB), tetrel bonds (TBs) and halogen bonds, in fluorinated chiral zinc complexes. High resolution neutron diffraction studies revealed a methylene carbon-hydrogen bond elongation and shortening due to TB and improper HB interactions, respectively. To show the accumulative effects of multiple weak interactions on the C−H bond, three types of tetrel bonds have been carefully examined. We have also shown how C−H bond elongation can be easily offset by forming an improper HB with the H atom from this C−H bond. Non-covalent interaction and electrostatic potential analysis investigations have been used to affirm the nature of the interactions based on density functional theory (DFT) and other related calculations.     

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.     

The Electronic Nature of the P=C Bond

Abstract

The influence of substituents at the carbon of the P=C bond of about 60 phosphaalkenes was investigated by ³¹P NMR spectroscopy. The P=C system is characterized as electronwithdrawing. All compounds were found to be configurationally stable.     

Intermolecular Halogen Bond Detected in Racemic and Optically Pure N-C Axially Chiral 3-(2-Halophenyl)quinazoline-4-thione Derivatives

The halogen bond has been widely used as an important supramolecular tool in various research areas. However, there are relatively few studies on halogen bonding related to molecular chirality. 3-(2-Halophenyl)quinazoline-4-thione derivatives have stable atropisomeric structures due to the rotational restriction around an N-C single bond. In X-ray single crystal structures of the racemic and optically pure N-C axially chiral quinazoline-4-thiones, we found that different types of intermolecular halogen bonds (C=S⋯X) are formed. That is, in the racemic crystals, the intermolecular halogen bond between the ortho-halogen atom and sulfur atom was found to be oriented in a periplanar conformation toward the thiocarbonyl plane, leading to a syndiotactic zig-zag array. On the other hand, the halogen bond in the enantiomerically pure crystals was oriented orthogonally toward the thiocarbonyl plane, resulting in the formation of a homochiral dimer. These results indicate that the corresponding racemic and optically pure forms in chiral molecules are expected to display different halogen bonding properties, respectively, and should be separately studied as different chemical entities.     

N,N''''-bis(3-Chlorophenyl)urea

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Effects of solvent on weak halogen bonds: Density functional theory calculations

In recent years, many applications of solution‐phase halogen bonding in anion recognition, catalysis, and pseudorotaxane formation have been reported. Moreover, a number of thermodynamic data of halogen bonding interactions in organic solution are now available. To obtain detailed information of the influence of the surrounding medium on weak halogen bonds, a series of dimeric complexes of halobenzene (PhX) with three electron donors (H₂O, HCHO, and NH₃) were investigated by means of DFT/PBE calculations in this work. The PCM implicit solvation approach was utilized to include the effects of three solvents (cyclohexane, chloroform, and water) as representatives for a wide range of dielectric constant. In some cases, halogen‐bond distances are shown to shorten in solution, accompanied by concomitant elongation of the C? X bonds. For the remaining systems, the intermolecular distances tend to increase or remain almost unchanged under solvent effects. In general, the solvent has a slight destabilizing effect on weak halogen bonds; the strength order of halogen bonds observed in vacuum remains unchanged in liquid phases. Particularly, the interaction strength attenuates in the order I > Br > Cl in solution, consistent with the experimental measurements of weak halogen bond door abilities. The similarities between halogen and hydrogen bonding in solution were also elucidated. The results presented herein would be very useful in future applications of halogen bonding in molecular recognition and medicinal chemistry. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Cooperative and substitution effects in enhancing strengths of halogen bonds in FCl⋯CNX complexes

In this paper, the cooperative effect of halogen bond with hydrogen bond has been used to make a halogen bond in FCl-CNH dimer vary from a chlorine-shared one to an ion-pair one. The halogen bond is strengthened in FCl-CNH-CNH trimer and its maximal interaction energy equals to -76 kJ∕mol when the number of CNH in FCl-CNH-(CNH)(n) polymer approaches infinity. Once the free H atom in FCl-CNH-CNH trimer is replaced with alkali metals, the halogen bond becomes strong enough to be an ion-pair one in FCl-CNH-CNLi and FCl-CNH-CNNa trimers. An introduction of a Lewis acid in FCl-CNH dimer has a more prominent effect on the type of halogen bond. A prominent cooperative effect is found for the halogen bond and hydrogen bond in the trimers. FH-FCl-CNH-CNH and FH-FCl-CNH-CNLi tetramers have also been studied and the interaction energy of halogen bonding in FH-FCl-CNH-CNLi tetramer is about 12 times as much as that in the FCl-CNH dimer. The atoms in molecules and natural bond orbital analyses have been carried out for these complexes to understand the nature of halogen bond and the origin of the cooperativity.     

Halogen as halogen-bonding donor and hydrogen-bonding acceptor simultaneously in ring-shaped H3N·X(Y)·HF (X = Cl, Br and Y = F, Cl, Br) complexes

A series of ring-shaped molecular complexes formed by H(3)N, HF and XY (X = Cl, Br and Y = F, Cl, Br) have been investigated at the MP2/aug-cc-pVTZ level of theory. Their optimized geometry, stretching mode, and interaction energy have been obtained. We found that each complex possesses two red-shifted hydrogen bonds and one red-shifted halogen bond, and the two hydrogen bonds exhibit strong cooperative effects on the halogen bond. The cooperativity among the NH(3)···FH, FH···XY and H(3)N···XY interactions leads to the formations of these complexes. The AIM analysis has been performed at the CCSD(T)/aug-cc-pVQZ level of theory to examine the topological characteristics at the bond critical point and at the ring critical point, confirming the coexistence of the two hydrogen bonds and one halogen bond for each complex. The NBO analysis carried out at the B3LYP/aug-cc-pVTZ level of theory demonstrates the effects of hyperconjugation, hybridization, and polarization coming into play during the hydrogen and halogen bonding formations processes, based on which a clockwise loop of charge transfer was discovered. The molecular electrostatic potential has been employed to explore the formation mechanisms of these molecular complexes.     

Chemistry of Several Sterically Bulky Molecules with P=P,P=C,and C≡P Bond

Several sterically protected, low-coordinate organophosphorus compounds with P=P, P=C, and C≡P bond are described in this study. Molecules such as diphosphenes, phosphaalkenes, 1-phosphaallenes, 1,3-diphosphaallenes, 3,4-diphosphinidenecyclobutenes, and phosphaalkynes are stabilized with an extremely bulky 2,4,6-tri-t-butylphenyl (Mes*) group. The synthesis, structures, physical, and chemical properties of these molecules are discussed, together with some successful applications in catalytic organic reactions.     

Interplay between halogen bond and lithium bond in MCN-LiCN-XCCH (M = H, Li, and Na; X = Cl, Br, and I) complex: the enhancement of halogen bond by a lithium bond

Quantum chemical calculations have been performed to study the complex of MCN-LiCN-XCCH (M = H, Li, and Na; X = Cl, Br, and I). The aim is to study the cooperative effect between halogen bond and lithium bond. The alkali metal has an enhancing effect on the lithium bond, making it increased by 77 and 94% for the Li and Na, respectively. There is the cooperativity between the lithium bond and halogen bond. The former has a larger enhancing effect on the latter, being in a range of 11.7-29.4%. The effect of cooperativity on the halogen bond is dependent on the type of metal and halogen atoms. The enhancing mechanism has been analyzed in views with the orbital interaction, charge transfer, dipole moment, polarizability, atom charges, and electrostatic potentials. The results show that the electrostatic interaction plays an important role in the enhancement of halogen bond.     

Alternative Synthesis and Structures of C‐monoacetylenic Phosphaalkenes

An alternative synthesis of C‐monoacetylenic phosphaalkenes trans‐Mes*P=C(Me)(C≡CR) (Mes* = 2, 4, 6‐^tBu₃Ph, R = Ph, SiMe₃) from C‐bromophosphaalkenes cis‐Mes*P=C(Me)Br using standard Sonogashira coupling conditions is described. Crystallographic studies confirm cis‐trans isomerization of the P=C double bond during Pd‐catalyzed cross coupling, leading exclusively to trans‐acetylenic phosphaalkenes. Crystallographic studies of all synthesized compounds reveal the extend of π‐conjugation over the acetylene and P=C π‐systems.     

Darstellung,Charakterisierung und Struktur funktionalisierter Fluorphosphaalkene des Typs R3E–P=C(F)NEt2 (R/E = Me/Si,Me/Ge,CF3/Ge,Me/Sn)

Preparation, Characterization, and Structure of Functionalized Fluorophosphaalkenes of the Type R₃E–P=C(F)NEt₂ (R/E = Me/Si, Me/Ge, CF₃/Ge, Me/Sn) P‐functionalized 1‐diethylamino‐1‐fluoro‐2‐phosphaalkenes of the type R₃E–P=C(F)NEt₂ [R/E = Me/Si ( 2 ), Me/Ge ( 3 ), CF₃/Ge ( 4 ), Me/Sn ( 5 )] are prepared by reaction of HP=C(F)NEt₂ ( 1 , E/Z = 18/82) with R₃EX (X = I, Cl) in the presence of triethylamine as base, exclusively as Z‐Isomers. 2–5 are thermolabile, so that only the more stable representatives 2 and 4 can be isolated in pure form and fully characterized. 3 and 5 decompose already at temperatures above –10 °C, but are clearly identified by ¹⁹F and ³¹P NMR‐measurements. The Z configuration is established on the basis of typical NMR data, an X‐ray diffraction analysis of 4 and ab initio calculations for E and Z configurations of the model compound Me₃Si–P=C(F)NMe₂. The relatively stable derivative 2 is used as an educt for reactions with pivaloyl‐, adamantoyl‐, and benzoylchloride, respectively, which by cleavage of the Si–P bond yield the push/pull phosphaalkenes RC(O)–P=C(F)NEt₂ [R = tBu ( 6 ), Ad ( 7 ), Ph ( 8 )], in which π‐delocalization with the P=C double bond occurs both with the lone pair on nitrogen and with the carbonyl group.     

Oxygen‐Doped Zig‐Zag Molecular Ribbons

The synthesis of a zig‐zag oxygen‐doped molecular rhombic ribbon has been achieved. This includes oxidative C?C and C?O bond formations that allowed the stepwise elongation and planarization of an oxa‐congener of 2,7‐periacenoacene. X‐ray diffraction analysis corroborated the flat structure and the zig‐zag topology of the O‐doped edges. Photophysical and electrochemical investigations showed that the extension of the peri‐xanthenoxanthene (PXX) into the molecular ribbon induces a noticeable shrinking of the molecular band gap devised by a rising of the HOMO energy level, a desirable property for p‐type organic semiconductors.     

The many flavours of halogen bonds – message from experimental electron density and Raman spectroscopy

Experimental electron‐density studies based on high‐resolution diffraction experiments allow halogen bonds between heavy halogens to be classified. The topological properties of the electron density in Cl…Cl contacts vary smoothly as a function of the interaction distance. The situation is less straightforward for halogen bonds between iodine and small electronegative nucleophiles, such as nitrogen or oxygen, where the electron density in the bond critical point does not simply increase for shorter distances. The number of successful charge–density studies involving iodine is small, but at least individual examples for three cases have been observed. (a) Very short halogen bonds between electron‐rich nucleophiles and heavy halogen atoms resemble three‐centre–four‐electron bonds, with a rather symmetric heavy halogen and without an appreciable σ hole. (b) For a narrow intermediate range of halogen bonds, the asymmetric electronic situation for the heavy halogen with a pronounced σ hole leads to rather low electron density in the (3,?1) critical point of the halogen bond; the properties of this bond critical point cannot fully describe the nature of the associated interaction. (c) For longer and presumably weaker contacts, the electron density in the halogen bond critical point is only to a minor extent reduced by the presence of the σ hole and hence may be higher than in the aforementioned case. In addition to the electron density and its derived properties, the halogen–carbon bond distance opposite to the σ hole and the Raman frequency for the associated vibration emerge as alternative criteria to gauge the halogen‐bond strength. We find exceptionally long C—I distances for tetrafluorodiiodobenzene molecules in cocrystals with short halogen bonds and a significant red shift for their Raman vibrations.     

A convenient synthesis of new isolable phosphaalkenes using the base-induced rearrangement of secondary vinylphosphines

The secondary vinylphosphines Ar(F)P(H)C(R)[double bond]CH(2) [2a, Ar(F) = 2,6-(CF(3))(2)C(6)H(3), R = CH(3); 2b, Ar(F) = 2,6-(CF(3))(2)C(6)H(3), R = C(6)H(5); 2c, Ar(F) = 2,4,6-(CF(3))(3)C(6)H(2), R = CH(3)] were prepared by treating the corresponding dichlorophosphine Ar(F)PCl(2) (1) with H(2)C[double bond]C(R)MgBr. In the presence of catalytic base (DBU or DABCO) the vinylphosphines (2a-c) undergo quantitative 1,3-hydrogen migration over 3 d to give stable and isolable phosphaalkenes Ar(F)P=C(R)CH(3) (3a, Ar(F) = 2,6-(CF(3))(2)C(6)H(3), R = CH(3); 3b, Ar(F) = 2,6-(CF(3))(2)C(6)H(3), R = C(6)H(5); 3c, Ar(F) = 2,4,6-(CF(3))(3)C(6)H(2), R = CH(3)). Under analogous conditions, only 90% conversion is observed in the base-catalyzed rearrangement of MesP(H)C(CH(3))[double bond]CH(2) to MesP[double bond]C(CH(3))(2). Presumably, the increase in acidity of the P-H group when electron-withdrawing groups are employed (i.e. 2a-c) favors quantitative rearrangement to the phosphaalkene tautomer (3a-c). Thus, the double-bond migration reaction is a convenient and practical method of preparing new phosphaalkenes with C-methyl substituents.     

Substituent Effects on Hydrogen Bonding in Watson–Crick Base Pairs. A Theoretical Study

We have theoretically analyzed Watson–Crick AT and GC base pairs in which purine C8 and/or pyrimidine C6 positions carry a substituent X = H, F, Cl or Br, using the generalized gradient approximation (GGA) of density functional theory at BP86/TZ2P. The purpose is to study the effects on structure and hydrogen bond strength if X = H is substituted by a halogen atom. Furthermore, we wish to explore the relative importance of electrostatic attraction versus orbital interaction in the above multiply hydrogen-bonded systems, using a quantitative bond energy decomposition scheme. We find that replacing X = H by a halogen atom has relatively small yet characteristic effects on hydrogen bond lengths, strengths and bonding mechanism. In general, it reduces the hydrogen-bond-accepting- and increases the hydrogen-bond-donating capabilities of a DNA base. The orbital interaction component in these hydrogen bonds is found for all substituents (X = H, F, Cl, and Br) to contribute about 41% of the attractive interactions and is thus of the same order of magnitude as the electrostatic component, which provides the remaining 59% of the attraction.     

Sterically encumbered systems for two low-coordinate phosphorus centers

Tetraarylphenyls of the form 2,3,5,6-Ar4C6 (Ar = p-tert-butylphenyl) are investigated as sterically demanding ligands for the syntheses of compounds having two p-phenylene-bridged phosphorus centers. The precursor to such materials, 1,4-diiodo-2,3,5,6-tetrakis(p-tert-butylphenyl)benzene (1), is readily obtained via a one-pot procedure in 68% yield. Compound 1 is then used to provide the bis(dichlorophosphine) 1,4-bis(dichlorophosphino)-2,3,5,6-tetrakis(p-tert-butylphenyl)benzene (2) and the derived bis(phosphine) 1,4-bis(phosphino)-2,3,5,6-tetrakis(p-tert-butylphenyl)benzene (3) in yields of 56 and 94% respectively. These materials provide access to novel materials containing two low-coordinate phosphorus centers bridged by a sterically encumbered phenylene unit. Compound 2 reacts with benzaldehyde and 2,6-dichlorobenzaldehyde in the presence of excess trimethylphosphine and zinc to produce the new pale yellow crystalline bis(phosphaalkenes) (E,E)-PhC(H)=PAr4C6P=C(H)Ph (4a; 42%) and (E,E)-Ar'C(H)=PAr4C6P=C(H)Ar' (4b; 46%; Ar' = 2,6-dichlorophenyl). The crystal structure of 4a shows a P=C bond length of 1.676(5) A. Compound 2 is also used to provide the unusual red-orange bis(diphosphene) DmpP=PAr4C6P=PDmp (5; 55%; Dmp = 2,6-Mes2C6H3). Compound 5 is structurally characterized, and a P=P bond length of 2.008(2) A is ascertained.