Diverse modes of reactivity of dialkyl azodicarboxylates with P(III) compounds: synthesis, structure, and reactivity of products other than the Morrison-Brunn-Huisgen intermediate in a Mitsunobu-type reaction

The reactivity of diethyl azodicarboxylate (DEAD)/diisopropyl azodicarboxylate (DIAD) with P(III) compounds bearing oxygen or nitrogen substituents is explored. Compounds with structures quite different from that of Morrison-Brunn-Huisgen intermediate R'(3)P(+)N(CO(2)R)N(-)(CO(2)R) (1), observed in the Mitsunobu reaction, have been established by using X-ray crystallography and NMR spectroscopy. Thus reactions with X(6-t-Bu-4-Me-C(6)H(2)O)(2)P-NH-t-Bu [X = S (8), CH(2) (9)] or XP(mu-N-t-Bu)(2)P-NH-t-Bu [X = Cl (14) or NH-t-Bu (15)] and DEAD/DIAD lead to phosphinimine-carbamate-type of products X[6-t-Bu-4-Me-C(6)H(2)O](2)P[N-t-Bu][N(CO(2)R)NH(CO(2)R)] [X = S, R = Et (16); X = CH(2), R = Et (17); X = CH(2), R = i-Pr (18)] or XP(mu-N-t-Bu)(2)P(N-t-Bu)[N-(CO(2)-i-Pr)-N(H)(CO(2)-i-Pr) [X = Cl (19), NH-t-Bu (20)]. Treatment of 19 with 2,2,2-trifluoroethanol afforded the product [(CF(3)CH(2)O)P(mu-N-t-Bu)(2)P(+)(NH-t-Bu)[N(CO(2)-i-Pr)(HNCO(2)-i-Pr)]][Cl(-)] (21) whose structure is close to one of the intermediates proposed in the Mitsunobu reaction. The isocyanate CH(2)(6-t-Bu-4-Me-C(6)H(2)O)(2)P-NCO (10) underwent 1,3-(P,C) cycloaddition with DEAD/DIAD to lead to CH(2)(6-t-Bu-4-Me-C(6)H(2)O)(2)P[N(CO(2)R)N(CO(2)R)-C(O)-N] [R = Et (22), i-Pr (23)]. Reaction of 22-23 with 1,1'-bi-2-naphthol or catechol leads to novel tetracoordinate CH(2)(6-t-Bu-4-Me-C(6)H(2)O)(2)P(2,2'-OC(10)H(6)-C(10)H(6)-OH)[NC(O)-(CO(2)R)NH(CO(2)R)] [R = Et (24), i-Pr (25)] or pentacoordinate CH(2)(6-t-Bu-4-Me-C(6)H(2)O)(2)P(1,2-O(2)C(6)H(4))[NHC(O)-N(CO(2)R)NH(CO(2)R)] [R = Et (26), i-Pr (27)] compounds in which the original NCO residue is retained; this mode of reactivity is quite different from that observed for the MBH betaine 1. In 27, the nitrogen, rather than the oxygen, occupies an apical position of the trigonal bipyramidal phosphorus violating the commonly assumed preference rules for apicophilicity. It is shown that the previously reported azide derivative 3, obtained from the reaction of 11 with DIAD, undergoes a Curtius-type rearrangement to lead to the fused cyclodiphosphazane [(CH(2)(6-t-Bu-4-Me-C(6)H(2)O)(2))P(OC(O-i-Pr)NN(CO(2)-i-Pr)N)](2) (28); this compound is in equilibrium with its monomeric form in solution at >300 K. Finally, reaction of S(6-t-Bu-4-Me-C(6)H(2)O)(2)P(OPh) (13) with DIAD gave the hexacoordinate compound S[6-t-Bu-4-Me-C(6)H(2)O](2)P(OPh)[N(CO(2)-i-Pr)NC(O-i-Pr)O] (30) with an intramolecular S-->P bond. X-ray crystallographic evidence for compounds 16, 19, 21, 22, 25, 27, 28, and 30 has been provided.     

Novel reactions of phosphorus(III) azides and isocyanates: unusual modes of cycloaddition with dipolarophiles and an unexpected case of ring expansion

New modes of 1,3-dipolar cycloaddition are uncovered by the isolation of [CH2(6-t-Bu-4-Me-C6H2O)2]P(C(CO2Me)C(CO2Me)N[NP(N3)(OC6H2-6-t-Bu-4-Me)2CH2]N) (3) and [CH2(6-t-Bu-4-Me-C6H2O)2]P(C(CO2Me)C(CO2Me)C(O)N) (4) on treating [CH2(6-t-Bu-4-Me-C6H2O)2]P-X [X = N3 (1) and NCO (2)] with the dipolarophile MeO2CC identical to CCO2Me; compound 4 undergoes an unprecedented ring expansion upon addition of 2-(methylamino)ethanol to afford the spirocycle [CH2(6-t-Bu-4-Me-C6H2O)2]P(OCH2CH2N(Me)CH(CO2Me)CH(CO2Me)C(O)N) (5).     

Addition products of a P(III)-isothiocyanate to dialkyl acetylenedicarboxylates: a spirocyclic phosphinimine and a triphosphorus heterocycle with tetra- and penta-coordinate phosphorus

Treatment of the P(III) isothiocyanate CH2[6-t-Bu-4-Me-C6H2O]2PNCS (1) with dimethyl acetylenedicarboxylate (DMAD) or diethyl acetylenedicarboxylate (DEAD) yields the spirocyclic phosphinimines CH2[6-t-Bu-4-Me-C6H2O]2P[NC(S)C(CO2R)C(CO2R)][R=Me (2), Et (3)], in a reaction unlike those of organic isocyanates. From the reaction of 1 with DEAD, a second product, the triphosphorus compound 5, with the composition [2x1+3] but with a completely reorganized structure {CH2[6-t-Bu-4-Me-C6H2O]2P=C(CO2Et)C(CO2Et)=CN-}{CH2[6-t-Bu-4-Me-C6H2O]2P(NCS)}-SC=N-P(S)[(OC6H2-6-t-Bu-4-Me)2CH2] with tetra- and penta-coordinate phosphorus, is also isolated. Structure and reactivity of these compounds are discussed. Addition of 2,2,2-trifluoroethanol to 2 or 3 leads to the pentacoordinate phosphorus compounds [CH2(6-t-Bu-4-Me-C6H2O)2P(OCH2CF3){C(CO2R)C(CO2R)-C(S)-NH-}][R=Me (6), Et (7)]. The phosphonate [CH2(6-t-Bu-4-Me-C6H2O)2P(O)C(CO2Et)=C(CO2Et)-C(S)-NH2] (8) is obtained by evaporating a solution of 7 in open air.     

Does a sterically bulky group occupy the equatorial site in trigonal bipyramidal phosphorus?

[structure: see text] The sterically bulky tert-butyl group occupies an apical position in trigonal bipyramidal phosphorus in the compound [CH2(6-t-Bu-4-Me-C6H2O)2]P(t-Bu)(1,2-O2C6Cl4) in contrast to the occupation of an equatorial position by the small methyl group in [CH2(6-t-Bu-4-Me-C6H2O)2]P(Me)(1,2-O2C6Cl4); this observation contradicts the familiar "apicophilicity rules" for trigonal bipyramidal phosphorus. Low-temperature solution 31P NMR spectra of [CH2(6-t-Bu-4-Me-C6H2O)2]P(R)(1,2-O2C6Cl4) (R = Me, Et, and n-Bu) show the presence of more than two isomers.     

Cooperativity between OH...O and CH...O hydrogen bonds involving dimethyl sulfoxide-H2O-H2O complex

The cooperativity between the O-H...O and C-H...O hydrogen bonds has been studied by quantum chemical calculations at the MP2/6-311++G(d,p) level in gaseous phase and at the B3LYP/6-311++G(d,p) level in solution. The interaction energies of the O-H...O and C-H...O H-bonds are increased by 53 and 58%, respectively, demonstrating that there is a large cooperativity. Analysis of hydrogen-bonding lengths, OH bond lengths, and OH stretching frequencies also supports such a conclusion. By NBO analysis, it is found that orbital interaction plays a great role in enhancing their cooperativity. The strength increase of the C-H...O H-bond is larger than that of the O-H...O H-bond due to the cooperativity. The solvent has a weakening effect on the cooperativity.     

Crystal structures and 77Se NMR spectra of molybdenum(IV) areneselenolates having intramolecular NH...Se hydrogen bonds

Salts of the monooxomolybdenum(IV,V) areneselenolates having intramolecular NH...Se hydrogen bonds, [Mo(IV)O(Se-2-RCONHC6H4)4]2- (R = t-Bu, CH3, CF3) and [Mo(V)O(Se-2-t-BuCONHC6H4)4]-, were synthesized and characterized by 1H nuclear magnetic resonance (NMR), 77Se NMR, electron spin resonance (ESR), UV-visible spectra, X-ray analysis, and electrochemical measurements. 77Se-1H correlated spectroscopy (COSY) indicated a significant correlation between amide 1H and selenolate 77Se atoms through an NH...Se hydrogen bond with 1J(77Se-1H) = 5.4 Hz coupling. The hydrogen bonds contribute to the positive shift in the Mo(V)/Mo(IV) redox potential. In the crystal structure of (PPh4)2[Mo(IV)O(Se-2-CH3CONHC6H4)4], an NH...O=Mo hydrogen bond was found. Ab inito calculations support the presence of intramolecular NH...O=Mo and NH...Se hydrogen bonds.     

Geometric isotope effect of various intermolecular and intramolecular C-H...O hydrogen bonds, using the multicomponent molecular orbital method

The geometric isotope effect (GIE) of sp- (acetylene-water), sp(2)- (ethylene-water), and sp(3)- (methane-water) hybridized intermolecular C-H...O and C-D...O hydrogen bonds has been analyzed at the HF/6-31++G level by using the multicomponent molecular orbital method, which directly takes account of the quantum effect of proton/deuteron. In the acetylene-water case, the elongation of C-H length due to the formation of the hydrogen bond is found to be greater than that of C-D. In contrast to sp-type, the contraction of C-H length in methane-water is smaller than that of C-D. After the formation of hydrogen bonds, the C-H length itself in all complexes is longer than C-D and the H...O distance is shorter than D...O, similar to the GIE of conventional hydrogen bonds. Furthermore, the exponent (alpha) value is decreased with the formation of the hydrogen bond, which indicates the stabilization of intermolecular C-H...O hydrogen bonds as well as conventional hydrogen bonds. In addition, the geometric difference induced by the H/D isotope effect of the intramolecular C-H...O hydrogen bond shows the same tendency as that of intermolecular C-H...O. Our study clearly demonstrates that C-H...O hydrogen bonds can be categorized as typical hydrogen bonds from the viewpoint of GIE, irrespective of the hybridizing state of carbon and inter- or intramolecular hydrogen bond.     

Coplanar recognition by a Rebek cleft is provided by cooperative structural effects involving a combination of O-H...O, O-H...N, and C-H...O forces

Reaction of 2,7-di-tert-butyl-9,9-dimethyl-4,5-xanthenedicarboxylic acid (1), a Rebek cleft, with 1,2-trans-bis(2-pyridyl)ethylene (2) yields a three-component organic assembly, 2(1).2 (3), of nanoscale dimensions that is held together by 10 cooperative O-H.N, O-H.O, and C-H.O hydrogen bonds. The cleft adopts a planar conformation, by forming an intramolecular O-H.O hydrogen bond, which enables the host to recognize the guest in a coplanar orientation that facilitates the cooperativity displayed by the multiple forces.     

C-H...N and C-H...S hydrogen bonds--influence of hybridization on their strength

Simple complexes connected through C-H...S and C-H...N interactions are investigated: CH4...NH3, C2H4...NH3, C2H2...NH3, CH4...SH2, C2H4...SH2, and C2H2...SH2. Ab initio and DFT calculations are performed (SCF, MP2, B3LYP) using different basis sets up to the MP2/aug-cc-pVQZ//MP2/aug-cc-pVDZ level of approximation. The Bader theory is applied since MP2/6-311++G(d,p) wave functions are used to find and to characterize bond critical points in terms of electron densities and their Laplacians. The influence of hybridization on the properties of C-H...S and C-H...N systems is also studied showing that the strength of such interactions increases in the following order: C(sp3)-H...Y, C(sp2)-H...Y, C(sp)-H...Y, where Y = S, N--it is in line with the previous findings on C-H...O hydrogen bonds. The results also show that CH4...SH2 and C2H4...SH2 complexes should be rather classified as van der Waals interactions and not as hydrogen bonds. The frequency associated with the C-H stretch of C(sp3)-H...S is blue-shifted.     

Modeling of hydrogen bonds in monohydrated 2,4-dithiothymine: an ab initio and AIM study

Twelve tautomers of 2,4-dithiothymine are calculated at the MP2/6-31+G(d) level, and the most stable one is referred to the di-keto form (P12). Then four H-bonded complexes between P12 and water are optimized at the MP2/6-31+G(d) level of theory. The calculation of vibrational frequencies and natural bond orbital analysis are also carried out at the same level to investigate the hydrogen bonds involved in all the systems. Within all the four complexes, three types of hydrogen bonds are formed, in which the O-H...S and N-H...O bonds are the normal bonds with the X-H bond elongation and red shift of the corresponding stretch frequencies, while the C-H...O interaction is an improper, blue-shifting hydrogen bond accompanied with the contraction of the C-H bond and a blue shift of the C-H stretch frequency. The topological properties are investigated with the atoms-in-molecules (AIM) theory. The NMR chemical shielding for the isolated and the four monohydrated 2,4-dithiothymine are calculated using the "gauge-including atomic orbital" (GIAO) method. The 1H chemical shifts are influenced by the formation of hydrogen bonds.     

How to determine whether intramolecular H...H interactions can be classified as dihydrogen bonds

Different types of intramolecular H...H interactions have been analyzed using the MP2/6-311++G(d,p) level of approximation. These are C-H...H-B, C-H...H-Al, C-H...H-C, C-H...H-O, O-H...H-Al and O-H...H-B contacts. Quantum theory of atoms in molecules and natural bond orbitals methods were applied to better understand the nature of these interactions. It was found that some of the species analyzed possess the characteristics of typical hydrogen bonds, such as the O-H...O ones. The electron charge transfer from the Lewis base to the antibonding X-H (for example O-H) orbital of the Lewis acid is one such characteristic. The NBO method may be considered decisive to classify any system as dihydrogen bonded.     

Limitations of the molecular multipole expansion treatment of electrostatic interactions for C-H...O and O-H...O hydrogen bonds and application of a general charge density approach

A molecular multipole expansion treatment (up to hexadecapole) is examined for its accuracy in describing hydrogen-bond electrostatic interactions, with particular reference to explaining the differences between blue-shifted C-H...O and red-shifted O-H...O bonds. In interactions of H2O and CH4 with point charges at hydrogen-bonding distances, we find that the molecular multipole treatment not only fails to reproduce ab initio energies but also forces on OH or CH bonds, and therefore cannot properly account for the electrostatic component of the interaction. A treatment based on a molecule's permanent charge density and its derivatives and the charge density and its derivatives induced by an external multipole distribution is in full accord with ab initio results, as shown by application to models of the H2O-H2O and CH4-FH systems. Such a charge density approach provides a fundamental basis for understanding the importance of interaction forces in initiating structural change and thereby altering molecular properties.     

Comparative estimation of the energies of intramolecular C-H…O,N-H…O,and O-H…O hydrogen bonds according to the QTAIM analysis and NMR spectroscopy data

The energies of intramolecular C-H…O, N-H…O, and O-H…O hydrogen bonds in model compounds are empirically estimated based on the values of the hydrogen bond induced weak-field shift of the bridging hydrogen atom signal in the ¹H NMR spectrum. It is supported by a theoretical estimation of these energies based on the electron density value at the hydrogen bond critical point calculated within the QTAIM method. Good agreement between the empirical and theoretical estimates is found, which gives evidence of their reliability. It is shown that from the standpoint of their strength the intramolecular N-H…O and O-H…O hydrogen bonds can be classified as moderate whereas the intramolecular C-H…O hydrogen bonds must be classified as very weak interactions similar in their energy significance to van der Waals interactions.     

Cocrystallization of N-donor type compounds with 5-sulfosalicylic acid: The effect of hydrogen-bonding supramolecular architectures

Benzotriazole,N,N’-dimethylpiperazine and N-methylpiperazine were applied to crystallize with 5-sulfosalicylic acid(5-H2SSA),affording three new binary molecular cocrystals [(C6H6N3+).(C7H5O6S-)].H2O(1),[(C6H16N22+)1/2.(C7H5O6S-)].H2O(2) and [(C5H14N22+).(C7H5O6S-)2].3H2O(3) under general conditions.Proton-transferring occurs from acid to nitrogen of N-donor compounds in all compounds 1,2 and 3.Analysis of the hydrogen-bonding synthons and their effects on crystal packing were also presented in the context of crystal engineering and host-guest chemistry.In compound 1,1-D infinite chains are extended to a 2-D layered architecture via strong O-H...O hydrogen bonds and then to a 3-D network by N-H...O interactions.Compound 2 and 3 both have the 1-D chain which is formed by O-H...O bonds and weak C-H...O hydrogen bonds.A common intramolecular S(6) [synthon I] ring is formed by the hydroxyl with the carboxyl group in all three compounds.     

Titanium, zinc and alkaline-earth metal complexes supported by bulky O,N,N,O-multidentate ligands: syntheses, characterisation and activity in cyclic ester polymerisation

The reactions of the bulky amino-bis(phenol) ligand Me(2)NCH(2)CH(2)N[CH(2)-3,5-Bu(t)(2)-C(6)H(2)OH-2](2)(1-H(2)) with Zn[N(SiMe(3))(2)](2)(4), [Mg[N(SiMe(3))(2)](2)](2)(5) and Ca[N(SiMe(3))(2)](2)(THF)(2)(6) yield the complexes 1-Zn, 1-Mg and 1-Ca in good yields. The X-ray structure of 1-Ca showed the complex to be dimeric, with calcium in a distorted octahedral coordination geometry. Five of the positions are occupied by an N(2)O(3) donor set, while the sixth is taken up by an intramolecular close contact to an o-Bu(t) substituent, a rare case of a Ca...H-C agostic interaction (Ca...H distances of 2.37 and 2.41 Angstroms). Another sterically hindered calcium complex, Ca[2-Bu(t)-6-(C(6)F(5)N=CH)C(6)H(3)O](2)(THF)(2).(C(7)H(8))(2/3)(7), was prepared by reaction of 6 with the iminophenol 2-Bu(t)-6-(C(6)F(5)N=CH)C(6)H(3)OH (3-H). According to the crystal structure 7 is monomeric and octahedral, with trans THF ligands. The complex Ti[N[CH(2)-3-Bu(t)-5-Me-C(6)H(2)O-2](2)[CH(2)CH(2)NMe(2)]](OPr(i))(2)(2-Ti) was prepared by treatment of Ti(OPr(i)(4)) with the new amino-bis(phenol) Me(2)NCH(2)CH(2)N[CH(2)-3-Bu(t)-5-Me-C(6)H(2)OH-2](2)(2-H(2)). The reduction of 2-Ti with sodium amalgam gave the titanium(III) salt Ti[N[CH(2)-3-Bu(t)-5-Me-C(6)H(2)O-2](2)[CH(2)CH(2)NMe(2)]](OPr(i))(2).Na(THF)(2)(8). A comparison of the X-ray structures of 2-Ti and 8 showed that the additional electron in 8 significantly reduced the intensity of the pi-bonding from the oxygen atoms of the isopropoxide groups to titanium. 1-Ca and 8 were active initiators for the ring-opening polymerisation of epsilon-caprolactone (up to 97% conversion of 200 equivalents in 2 hours) and yielded polymers with narrow molecular weight distributions.     

Influence of N-H...O and C-H...O hydrogen bonds on the 17O NMR tensors in crystalline uracil: computational study

We report a computational study for the 17O NMR tensors (electric field gradient and chemical shielding tensors) in crystalline uracil. We found that N-H...O and C-H...O hydrogen bonds around the uracil molecule in the crystal lattice have quite different influences on the 17O NMR tensors for the two C=O groups. The computed 17O NMR tensors on O4, which is involved in two strong N-H...O hydrogen bonds, show remarkable sensitivity toward the choice of cluster model, whereas the 17O NMR tensors on O2, which is involved in two weak C-H...O hydrogen bonds, show much smaller improvement when the cluster model includes the C-H...O hydrogen bonds. Our results demonstrate that it is important to have accurate hydrogen atom positions in the molecular models used for 17O NMR tensor calculations. In the absence of low-temperature neutron diffraction data, an effective way to generate reliable hydrogen atom positions in the molecular cluster model is to employ partial geometry optimization for hydrogen atom positions using a cluster model that includes all neighboring hydrogen-bonded molecules. Using an optimized seven-molecule model (a total of 84 atoms), we were able to reproduce the experimental 17O NMR tensors to a reasonably good degree of accuracy. However, we also found that the accuracy for the calculated 17O NMR tensors at O2 is not as good as that found for the corresponding tensors at O4. In particular, at the B3LYP/6-311++G(d,p) level of theory, the individual 17O chemical shielding tensor components differ by less than 10 and 30 ppm from the experimental values for O4 and O2, respectively. For the 17O quadrupole coupling constant, the calculated values differ by 0.30 and 0.87 MHz from the experimental values for O4 and O2, respectively.     

Very strong centrosymmetric [O...H...O]+ and asymmetric [O-H...O]+ hydrogen bonds in a new POM-Based hybrid material

A specific assembly process, driven by coexisting X-H...O hydrogen bonding and X...O short intermolecular contacts (X = C, N, O) is described, in which the pseudo-Keggin polyoxoanion and two types of molecule...cation pairs (with C1 and C_i symmetries) were assembled to the programmed supramolecular architecture. Cooperation of the positive-charge, resonance effect and the O=C...O_terminal intermolecular contact led to the short and strong symmetrical [O...H...O]⁺ hydrogen bond (O...O = 2.449(13) Å) in one of the molecule...cation pairs [C₄H₉NO...H...ONC₄H₉]⁺ with the H-bonded proton in the center of inversion. The other [C₄H₉NOH...ONC₄H₉]+ molecule...cation pair (non-centrosymmetric) was formed through a very strong asymmetric [O.H.O]+ hydrogen bond of 2.431(13) Å in length which was created via the synergistic effect between the minor N-H...O secondary interaction, +CAHB (positive-charge-assisted hydrogen bond) and RAHB (resonance assisted hydrogen bond) mechanisms.     

Syntheses, Crystal Structures and Fluorescence Properties of Two Triazolyl Derivatives with Biphenyl Links

Two novel triazolyl derivatives with biphenyl links, namely 4,4-bis（1,2,4-triazol-1-ylmethyl）biphenylene 1 and 4,4-bis（3,5-dimethyl-1,2,4-triazol-1-ylmethyl）biphenylene 2, have been synthesized and structurally characterized by single-crystal X-ray diffraction. Compounds 1·H2O·HCl and 2·2H2O·2HCl crystallize in the monoclinic system with space group C2/c and P21/c, respectively. The dihedral angles of the two phenyl rings are 30.26（6）° in 1·H2O·HCl, while co-planar （0.00（7）°） in 2·2H2O·2HCl. In 1·H2O·HCl, the N-H…O hydrogen bonds link 1 to form chain-like structures which are further connected by O-H…Cl, C-H…Cl, C-H…O and π-π supramolecular interactions. The hydrogen bonds of O-H…Cl in 2·2H2O·2HCl provide distinguishing P/M helical chains along the b axis, and these chains are further connected by N-H…O hydrogen bonds to generate a 2D structure. Compounds 1 and 2 in methanol solution show much stronger emission bands at 319 nm than biphenyl at 316 nm under excitation at 260 nm.     

The molecular structure of the tris(2-mercapto-1-tolylimidazolyl)hydroborato zinc(2-mercapto-1-tolylimidazole) complex, [[Tm(p-Tol)]Zn(mim(p-Tol))][ClO4]: intermolecular N-H...OClO3versus intramolecular N-H...S hydrogen bonding interactions of the mercaptoimidazole ligand

The molecular structure of the tris(2-mercapto-1-tolylimidazolyl)hydroborato complex [[Tm(p-Tol)]Zn(mim(p-Tol))][ClO(4)].3MeCN has been determined by X-ray diffraction, thereby demonstrating that the mim(p-Tol) ligand exhibits a N-H...O hydrogen bond with the [ClO(4)](-) counterion, [[Tm(p-Tol)]Zn(mim(p-Tol))...(OClO(3))], rather than hydrogen bond with a sulfur of the [Tm(p-Tol)] ligand. DFT calculations on a series of related complexes, namely [[Tm(Me)]Zn(mim(Me))](+), [[Tm(Me)]Zn(mim(Me))]...(OClO(3))], [[Tm(Me)]Zn(mim(Me))]...[O(H)Me]](+), and [[Tm(Me)]Zn(mim(Me))]...(NCMe)](+) demonstrate that an intramolecular N-H...S hydrogen bond within [[Tm(Me)]Zn(mim(Me))](+) is also less favored than the corresponding hydrogen bonding interactions with MeCN, MeOH, and [ClO(4)](-). The inability of the sulfur atoms of [Tm(R)] ligand to act as an effective hydrogen bond acceptor is in marked contrast to the ability of sulfur atoms in thiolate ligands to participate in the formation of N-H...S hydrogen bonds, an observation that reflects the "thione"versus"thiolate" nature of the [Tm(R)] ligand.     

Probing C-H...X hydrogen bonds in amide-functionalized imidazolium salts under high pressure

We have probed under high pressure the C-H hydrogen bonds formed by N,N(')-disubstituted imidazolium ions having PF(6) (-) and Br(-) counterions. High-pressure infrared spectral profiles, x-ray crystallographic analysis, and ab initio calculations allow us to make a vibrational assignment of these compounds. The appearance of a signal for the free-NH unit (or weakly bonded N-H...F unit) in the infrared spectrum of the PF(6) (-) salt indicates that conventional N-H...O and N-H...N hydrogen bonds do not fully dominate the packing. It is likely that the charge-enhanced C(2)-H...F interactions, combined with other weak hydrogen bonds, disturb the formation of N-H hydrogen bonds in the PF(6) (-) salt. This finding is consistent with the pressure-dependent results, which reveal that the C(2)-H...F interaction is enhanced upon increasing the pressure. In contrast to the PF(6) (-) salt, the imidazolium C-H bonds of the Br(-) salt have low sensitivity to high pressure. This finding suggests that the hydrogen bonding patterns are determined by the relative hydrogen bond acceptor strengths of the Br(-) and PF(6) (-) ions.