Redox chemistry of tellurium bis(tert-butylamido)cyclodiphosph(V)azane disulfide and diselenide systems: a spectroscopic and structural study

The redox chemistry of tellurium-chalcogenide systems is examined via reactions of tellurium(IV) tetrachloride with Li[(t)()BuN(E)P(mu-N(t)Bu)(2)P(E)N(H)(t)Bu] (3a, E = S; 3b, E = Se). Reaction of TeCl(4) with 2 equiv of 3a in THF generates the tellurium(IV) species TeCl(3)[HcddS(2)][H(2)cddS(2)] 4a [cddS(2) = (t)BuN(S)P(mu-N(t)Bu)(2)P(S)N(t)Bu] at short reaction times, while reduction to the tellurium(II) complex TeCl(2)[H(2)cddS(2)](2) 5a is observed at longer reaction times. The analogous reaction of TeCl(4) and 3b yields only the tellurium(II) complex TeCl(2)[H(2)cddSe(2)](2) 5b. The use of 4 equiv of 3a or 3b produces Te[HcddE(2)](2) (6a (E = S) or 6b (E = Se)). NMR and EPR studies of the 5:1 reaction of 3a and TeCl(4) in THF or C(6)D(6) indicate that the formation of the Te(II) complex 6a via decomposition of a Te(IV) precursor occurs via a radical process to generate H(2)cddS(2). Abstraction of hydrogen from THF solvent is proposed to account for the formation of 2a. These results are discussed in the context of known tellurium-sulfur and tellurium-nitrogen redox systems. The X-ray crystal structures of 4a.[C(7)H(8)](0.5), 5a, 5b, 6a.[C(6)H(14)](0.5), and 6b.[C(6)H(14)](0.5) have been determined. The cyclodiphosph(V)azane dichalcogenide ligand chelates the tellurium center in an E,N (E = S, Se) manner in 4a.[C(7)H(8)](0.5), 6a.[C(6)H(14)](0.5), and 6b.[C(6)H(14)](0.5) with long Te-N bond distances in each case. Further, a neutral H(2)cddS(2) ligand weakly coordinates the tellurium center in 4a small middle dot[C(7)H(8)](0.5) via a single chalcogen atom. A similar monodentate interaction of two neutral ligands with a TeCl(2) unit is observed in the case of 5a and 5b, giving a trans square planar arrangement at tellurium.     

Facially coordinating triamine ligands with a cyclic backbone: some structure-stability correlations

Metal complex formation of the two cyclic triamines 6-methyl-1,4-diazepan-6-amine (MeL(a)) and all-cis-2,4,6-trimethylcyclohexane-1,3,5-triamine (Me(3)tach) was studied. The structure of the free ligands (H(x)MeL(a))(x+) and H(x)Me(3)tach(x+) (0 ≤ x ≤ 3) was investigated by pH-dependent NMR spectroscopy and X-ray diffraction experiments. The crystal structure of (H(2)Me(3)tach)(p-O(3)S-C(6)H(4)-CH(3))(2) showed a chair conformation with axial nitrogen atoms for the doubly protonated species. In contrast to a previous report, Me(3)tach was found to be a stronger base than the parent cis-cyclohexane-1,3,5-triamine (tach); pK(a)-values of H(3)Me(3)tach(3+) (25 °C, 0.1 M KCl): 5.2, 7.4, 11.2. The crystal structures of (H(3)MeL(a))(BiCl(6))·2H(2)O and (H(3)MeL(a))(ClO(4))Cl(2) exhibited two distinct twisted chair conformations of the seven membered diazepane ring. [Co(MeL(a))(2)](3+) (cis: 1(3+), trans: 2(3+)), trans-[Fe(MeL(a))(2)](3+) (3(3+)), [(MeL(a))ClCd(μ(2)-Cl)](2) (4), trans-[Cu(MeL(a))(2)](2+) (5(2+)), and [Cu(HMeL(a))Br(3)] (6) were characterized by single crystal X-ray analysis of 1(ClO(4))(3)·H(2)O, 2Br(3)·H(2)O, 3(ClO(4))(3)·0.8MeCN·0.2MeOH, 4, 5Br(2)·0.5MeOH, and 6·H(2)O. Formation constants and redox potentials of MeL(a) complexes were determined by potentiometric, spectrophotometric, and cyclovoltammetric measurements. The stability of [M(II)(MeL(a))](2+)-complexes is low. In comparison to the parent 1,4-diazepan-6-amine (L(a)), it is only slightly enhanced. In analogy to L(a), MeL(a) exhibited a pronounced tendency for forming protonated species such as [M(II)(HMeL(a))](3+) or [M(II)(MeL(a))(HMeL(a))](3+) (see 6 as an example). In contrast to MeL(a), Me(3)tach forms [M(II)L](2+) complexes (M = Cu, Zn) of very high stability, and the coordination behavior corresponds mainly to an "all-or-nothing" process. Molecular mechanics calculations showed that the low stability of L(a) and MeL(a) complexes is mainly due to a large amount of torsional strain within the pure chair conformation of the diazepane ring, required for tridentate coordination. This behavior is quite contrary to Me(3)tach and tacn (tacn =1,4,7-triazacyclononane), where the main portion of strain is already preformed in the free ligand, and the amount, generated upon complex formation, is comparably low.     

Anionic Roussin's red esters (RREs) syn-/anti-[Fe(mu-SEt)(NO)2]2(-): the critical role of thiolate ligands in regulating the transformation of RREs into dinitrosyl iron complexes and the anionic RREs

The anionic syn-/ anti-[Fe(mu-SEt)(NO) 2] 2 (-) ( 2a) were synthesized and characterized by IR, UV-vis, EPR, and X-ray diffraction. The geometry of the [Fe(mu-S) 2Fe] core is rearranged in going from [{Fe(NO) 2} (9)-{Fe(NO) 2} (9)] Roussin's red ester [Fe(mu-SEt)(NO) 2] 2 ( 1a) (Fe...Fe distance of 2.7080(5) A) to the [{Fe(NO) 2} (9)-{Fe(NO) 2} (10)] complex 2a (Fe...Fe distance of 2.8413(6) A) to minimize the degree of Fe...Fe interaction to stabilize complex 2a. On the basis of X-ray absorption (Fe K- and L-edge), EPR and SQUID, complex 2a is best described as the anionic [{Fe(NO) 2} (9)-{Fe(NO) 2} (10)] Roussin's red ester with the fully delocalized mixed-valence core. The complete bridged-thiolate cleavage yielded DNIC [(EtS) 2Fe(NO) 2] (-) ( 3a) in the reaction of 2 equiv of [EtS] (-) and complex 1a, whereas reaction of 2 equiv of [(t)BuS] (-) with [Fe(micro-S (t)Bu)(NO) 2] 2 (1b) gave DNIC [((t)BuS) 2Fe(NO) 2] (-) (3b) and the anionic Roussin's red ester [Fe(mu-S (t)Bu)(NO) 2] 2 (-) (2b) through bridged-thiolate cleavage in combination with reduction. In contrast to the inertness of DNIC 3b toward complex 1b, nucleophile DNIC 3a induces the reduction of complex 1a to produce the anionic Roussin's red ester 2a. Interestingly, dissolution of complex 3a in MeOH at 298 K finally led to the formation of a mixture of complexes 2a and 3a, in contrast to the dynamic equilibrium of complexes 3b and 1b observed in dissolution of complex 3b in MeOH. These results illustrate the aspect of how the steric structures of nucleophiles ([EtS] (-) vs [ (t)BuS] (-) and [(EtS) 2Fe(NO)2](-) vs [((t)BuS) 2Fe(NO)2] (-)) function to determine the reaction products.     

X-ray Crystal Structure, ab Initio Calculations, and Reactivity of 1,3,2lambda(5)-Diazaphosphetes: A New Type of 4-pi-Electron 4-Membered Heterocycle

The structure of P,P-bis(diisopropylamino)-4-phenyl-1,3,2lambda(5)-diazaphosphete, 1a, has been determined by a single-crystal X-ray diffraction study (C(19)H(33)N(4)P, monoclinic system, space group P2(1), a = 9.482(1) ?, b = 11.374(3) ?, c = 9.668(2) ?, beta = 97.16(1) degrees, Z = 2). According to quantum chemical calculations at an RHF level of optimization utilizing the 6-31g(d,p) basis set, 1a has a zwitterionic structure with the negative charge delocalized on the NCN allylic fragment and the positive charge localized at the phosphorus. Heterocycle 1a reacts with water and benzaldehyde affording N-phosphoranylbenzamidine 3 (95% yield) and the expected aza-Wittig adduct 4 (85% yield), respectively. Addition of 1 equiv of methyl trifluoromethanesulfonate and of 2 equiv of BH(3).THF to 1a affords cyclic phosphonium salt 5 (94% yield) and the bis(borane) adduct 6a (90% yield), respectively. Dimethyl acetylenedicarboxylate slowly reacts with 1a giving rise to 1,3,4lambda(5)-diazaphosphinine, 9, in 70% yield. The X-ray crystal structures of products 2,3, and 6a are reported (2: C(26)H(38)N(5)P, monoclinic system, space group C2/c, a = 16.337(8) ?, b = 19.810(2) ?, c = 8.800(2) ?, beta = 117.68(2) degrees, Z = 4. 3: C(19)H(35)N(4)OP, orthorhombic system, space group P2(1)2(1)2(1), a = 9.090(1) ?, b = 12.955(2) ?, c = 17.860(3) ?, Z = 4. 6a: C(19)H(39)B(2)N(4)P, orthorhombic system, space group P2(1)2(1)2(1), a = 10.340(1) ?, b = 13.247(1) ?, c = 16.996(1) ?, Z = 4).     

Amide-silyl ligand exchanges and equilibria among group 4 amide and silyl complexes

M(NMe2)4 (M = Zr, 1a; Hf, 1b) and the silyl anion (SiButPh2)- (2) in Li(THF)2SiButPh2 (2-Li) were found to undergo a ligand exchange to give [M(NMe2)3(SiButPh2)2]- (M = Zr, 3a; Hf, 3b) and [M(NMe2)5]- (M = Zr, 4a; Hf, 4b) in THF. The reaction is reversible, leading to equilibria: 2 1a (or 1b) + 2 2 <--> 3a (or 3b) + 4a (or 4b). In toluene, the reaction of 1a with 2 yields [(Me2N)3Zr(SiButPh2)2]-[Zr(NMe2)5Li2(THF)4]+ (5) as an ionic pair. The silyl anion 2 selectively attacks the -N(SiMe3)2 ligand in (Me2N)3Zr-N(SiMe3)2 (6a) to give 3a and [N(SiMe3)2]- (7) in reversible reaction: 6a + 2 2 <--> 3a + 7. The following equilibria have also been observed and studied: 2M(NMe2)4 (1a; 1b) + [Si(SiMe3)3]- (8) <--> (Me2N)3M-Si(SiMe3)3 (M = Zr, 9a; Hf, 9b) + [M(NMe2)5]- (M = Zr, 4a; Hf, 4b); 6a (or 6b) + 8 <--> 9a (or 9b) + [N(SiMe3)2]- (7). The current study represents rare, direct observations of reversible amide-silyl exchanges and their equilibria. Crystal structures of 5, (Me2N)3Hf-Si(SiMe3)3 (9b), and [Hf(NMe2)4]2 (dimer of 1b), as well as the preparation of (Me2N)3M-N(SiMe3)2 (6a; 6b) are also reported.     

Determination of the intrinsic site pK(a) value and cooperativity of the symmetrical hexadentate chelator N,N',N'-tris[2-(3-hydroxy-2-oxo-1,2-dihydropyridin-1-yl)acetamido]ethylamine

The three overlapping pK(a) values of N,N',N'-tris[2-(3-hydroxy-2-oxo-1,2-dihydropyridin-1-yl)acetamido]ethylamine, a tripodal hexadentate chelator formed from three 3-hydroxy-2(1H)-pyridinone moieties amide linked to tris-(2-aminoethyl)amine, were determined by simultaneous spectrophotometric and potentiometric titration. The data was analysed by non-linear regression with constraints to deal with (a) the highly correlated absorptivities and (b) the highly correlated pK(a) values. The three pK(a) values were optimized first from the spectrophotometric data (absorbance vs. pH) by non-linear regression to a model in which the molar absorptivity of the ith species ((i)) was constrained by the correlation equation (i) = epsilon (0) + (epsilon (3) - epsilon (0))i 3 with i = 0, 1, 2, 3, where (3) and (0) represent the molar absorptivities of the most protonated and least protonated species, respectively. The molar absorbitivity of the four species defined by three pK(a) values is, therefore, linearly related to proton stoichiometry. The pK(a) values were then optimized from the potentiometric data (pH vs. titrant volume) by non-linear regression to a model in which the three pK(a) values were constrained by the correlation equation pK(a(i)) = pK(a(int)) + b(i - 1) + (i - 2)log(3) where i = 1, 2 or 3. This expresses the three pK(a) values in terms of only two optimizable parameters, the intrinsic site pK(a) (pK(a(int))) and the interaction energy between sites (b). The fixed term (i - 2)log(3) accounts for the statistical effect on the pK(a) values of three equivalent ionizable sites. The modified analytical derivatives required for optimization of these parameters by the Gauss-Newton-Marquardt algorithm and the merits of optimizing pK(a) values with these two correlation equations are discussed. The optimized pK(a) values were 9.31 +/- 0.01, 8.75 +/- 0.01 and 8.19 +/- 0.01. The separation between pK(a) values is 0.58 comprising 0.477 for the statistical effect and 0.081 for the interaction energy while the intrinsic site pK(a) is 8.672 +/- 0.005. The tertiary amine at the centre of the tripodal backbone has a pK(a) of 5.88 +/- 0.03.     

O2(a1Δg) + Mg, Fe, and Ca: experimental kinetics and formulation of a weak collision, multiwell master equation with spin-hopping

The first excited electronic state of molecular oxygen, O(2)(a(1)Δ(g)), is formed in the upper atmosphere by the photolysis of O(3). Its lifetime is over 70 min above 75 km, so that during the day its concentration is about 30 times greater than that of O(3). In order to explore its potential reactivity with atmospheric constituents produced by meteoric ablation, the reactions of Mg, Fe, and Ca with O(2)(a) were studied in a fast flow tube, where the metal atoms were produced either by thermal evaporation (Ca and Mg) or by pulsed laser ablation of a metal target (Fe), and detected by laser induced fluorescence spectroscopy. O(2)(a) was produced by bubbling a flow of Cl(2) through chilled alkaline H(2)O(2), and its absolute concentration determined from its optical emission at 1270 nm (O(2)(a(1)Δ(g) - X(3)Σ(g) (-)). The following results were obtained at 296 K: k(Mg + O(2)(a) + N(2) → MgO(2) + N(2)) = (1.8 ± 0.2) × 10(-30) cm(6) molecule(-2) s(-1); k(Fe + O(2)(a) → FeO + O) = (1.1 ± 0.1) × 10(-13) cm(3) molecule(-1) s(-1); k(Ca + O(2)(a) + N(2) → CaO(2) + N(2)) = (2.9 ± 0.2) × 10(-28) cm(6) molecule(-2) s(-1); and k(Ca + O(2)(a) → CaO + O) = (2.7 ± 1.0) × 10(-12) cm(3) molecule(-1) s(-1). The total uncertainty in these rate coefficients, which mostly arises from the systematic uncertainty in the O(2)(a) concentration, is estimated to be ±40%. Mg + O(2)(a) occurs exclusively by association on the singlet surface, producing MgO(2)((1)A(1)), with a pressure dependent rate coefficient. Fe + O(2)(a), on the other hand, shows pressure independent kinetics. FeO + O is produced with a probability of only ～0.1%. There is no evidence for an association complex, suggesting that this reaction proceeds mostly by near-resonant electronic energy transfer to Fe(a(5)F) + O(2)(X). The reaction of Ca + O(2)(a) occurs in an intermediate regime with two competing pressure dependent channels: (1) a recombination to produce CaO(2)((1)A(1)), and (2) a singlet∕triplet non-adiabatic hopping channel leading to CaO + O((3)P). In order to interpret the Ca + O(2)(a) results, we utilized density functional theory along with multireference and explicitly correlated CCSD(T)-F12 electronic structure calculations to examine the lowest lying singlet and triplet surfaces. In addition to mapping stationary points, we used a genetic algorithm to locate minimum energy crossing points between the two surfaces. Simulations of the Ca + O(2)(a) kinetics were then carried out using a combination of both standard and non-adiabatic Rice-Ramsperger-Kassel-Marcus (RRKM) theory implemented within a weak collision, multiwell master equation model. In terms of atmospheric significance, only in the case of Ca does reaction with O(2)(a) compete with O(3) during the daytime between 85 and 110 km.     

Synthesis,Structure, and Electrochemical Properties of Sterically Protected Molybdenum Trihydride Redox Pairs: A Paramagnetic “Stretched” Dihydrogen Complex?

Complexes [MoCp(#)(PMe(3))(2)H(3)] (Cp(#)=1,2,4-C(5)H(2)tBu(3), 2 a; C(5)HiPr(4), 2 b) have been synthesized from the corresponding compounds [MoCp(#)Cl(4)] (1 a, 1 b) and fully characterized, including by X-ray crystallography and by a neutron diffraction study for 2 a. Protonation of 2 a led to complex [Mo(1,2,4-C(5)H(2)tBu(3))(PMe(3))(2)H(4)](+) (3 a) in THF and to [Mo(1,2,4-C(5)H(2)tBu(3))(PMe(3))(2)(MeCN)H(2)](+) (4 a) in MeCN. Complex 4 b analogously derives from protonation of 2 b in MeCN, whereas the tetrahydride complex 3 b is unstable. One-electron oxidation of 2 a and 2 b by [FeCp(2)]PF(6) produces the EPR-active 17-electron complexes 2 a(+) and 2 b(+). The former is thermally more stable than the latter and could be crystallographically characterized as the PF(6) (-) salt by X-ray diffraction, providing evidence for the presence of a stretched dihydrogen ligand (H...H=1.36(6) angstroms). Controlled thermal decomposition of 2 a(+) yielded the product of H(2) elimination, the 15-electron monohydride complex [Mo(1,2,4-C(5)H(2)tBu(3))(PMe(3))(2)H]PF(6) (5 a), which was characterized by X-ray crystallography and by EPR spectroscopy at liquid He temperature. The compound establishes an equilibrium with the solvent adduct in THF. An electrochemical study by cyclic voltammetry provides further evidence for a rapid H(2) elimination process from the 17-electron complexes. In contrast to the previously investigated [MoCp*(dppe)H(3)](+) system (dppe=1,2-bis(diphenylphosphino)ethane; Cp*=pentamethylcyclopentadienyl), the decomposition of 2 a(+) by H(2) substitution with a solvent molecule appears to follow a dissociative pathway in MeCN.     

Hf3 cluster is triply (sigma-, pi-, and delta-) aromatic in the lowest D3h, 1A1' state

The extensive search for the global minimum structure of Hf3 at the B3LYP/LANL2DZ level of theory revealed that D3h 3A2' (1a1'(2)1a2'(2)1e'(4)2a1'(2)1e'2) and D3h 1A1' (1a1'(2)2a1'(2)1e'(4)1a2'(2)3a1'2) are the lowest triplet and singlet states, respectively, with the triplet state being the lowest one. However, at the CASSCF(10,14)/Stuttgart+2f1g level of theory these two states are degenerate, indicating that at the higher level of theory the singlet state could be in fact the global minimum structure. The triplet D3h 3A2' (1a1'21a2'(2)1e'(4)2a1'(2)1e'2) structure is doubly (sigma- and pi-) aromatic and the singlet D3h 1A1' (1a1'(2)2a1'(2)1e'(4)1a2'(2)3a1'2) structure is the first reported triply (sigma-, pi-, and delta-) aromatic system.     

Adsorption and dissociation of NH3 on clean and hydroxylated TiO2 rutile (110) surfaces: a computational study

The adsorption and dissociation of NH(3) on the clean and hydroxylated TiO(2) rutile (110) surfaces have been investigated by the first-principles calculations. The monodentate adsorbates such as H(3)N-Ti(a), H(2)N-Ti(a), N-Ti(a), H(2)N-O(a), HN-O(a), N-O(a) and H-O(a), as well as the bidentate adsorbate, Ti-N-Ti(a) can be formed on the clean surface. It is found that the hydroxyl group enhances the adsorption of certain adsorbates on the five-fold-coordinated Ti atoms (5c-Ti), namely H(2)N-Ti(a), HN-Ti(a), N-Ti(a) and Ti-N-Ti(a). In addition, the adsorption energy increases as the number of hydroxyl groups increases. On the contrary, the opposite effect is found for those on the two-fold-coordinated O atoms (2c-O). The enhanced adsorption of NH(x) (x = 1-2) on the 5c-Ti is due to the large electronegativity of the OH group, increasing the acidity of the Ti center. This also contributes to diminish the adsorption of NH(x) (x = 1-2) on the two-fold-coordinated O atoms (2c-O) decreasing its basicity. According to potential energy profile, the NH(3) dissociation on the TiO(2) surface is endothermic and the hydroxyl group is found to lower the energetics of H(2)N-Ti(a)+H-O(a) and HN-Ti(a)+2{H-O(a)}, but slightly raise the energetic of Ti-N-Ti(a)+3{H-O(a)} compare to those on the clean surface. However, the dissociation of NH(3) is found to occur on the hydroxylated surface with an overall endothermic by 31.8 kcal/mol and requires a barrier of 37.5 kcal/mol. A comparison of NH(3) on anatase surface has been discussed. The detailed electronic analysis is also carried out to gain insights into the interaction nature between adsorbate and surface.     

Transfer of amido groups from isolated rhodium(I) amides to alkenes and vinylarenes

The reaction of monomeric and dimeric rhodium(I) amido complexes with unactivated olefins to generate imines is reported. Transamination of {(PEt(3))(2)RhN(SiMePh(2))(2)} (1a) or its -N(SiMe(3))(2) analogue 1b with p-toluidine gave the dimeric [(PEt(3))(2)Rh(mu-NHAr)](2) (Ar = p-tolyl) (2a) in 80% isolated yield. Reaction of 2a with PEt(3) generated the monomeric (PEt(3))(3)Rh(NHAr) (Ar = p-tolyl) (3a). PEt(3)-ligated arylamides 2a and 3a reacted with styrene to transfer the amido group to the olefin and to form the ketimine Ph(Me)C=N(p-tol) (4a) in 48-95% yields. The dinuclear amido hydride (PEt(3))(4)Rh(2)(mu-NHAr)(mu-H) (Ar = p-tolyl) (5a) was formed from reaction of 2a in 95% yield, and a mixture of this dimeric species and the (PEt(3))(n)RhH complexes with n = 3 and 4 was formed from reaction of 3a in a combined 75% yield. Propene reacted with 2a to give Me(2)C=N(p-tol) (4b) and 5a in 90 and 57% yields. Propene also reacted with 3a to give 4b and 5a in 65 and 94% yields. Analogues of 2a and 3a with varied electronic properties also reacted with styrene to form the corresponding imines, and moderately faster rates were observed for reactions of electron-rich arylamides. Kinetic studies of the reaction of 3a with styrene were most consistent with formation of the imine by migratory insertion of olefin into the rhodium-amide bond to generate an aminoalkyl intermediate that undergoes beta-hydrogen elimination to generate a rhodium hydride and an enamine that tautomerizes to the imine.     

Supramolecular structural variations with changes in anion and solvent in silver(I) complexes of a semirigid, bitopic tris(pyrazolyl)methane ligand

The bitopic ligand p-C(6)H(4)[CH(2)OCH(2)C(pz)(3)](2) (pz = pyrazolyl ring) that contains two tris(pyrazolyl)methane units connected by a semirigid organic spacer reacts with silver(I) salts to yield [p-C(6)H(4)[CH(2)OCH(2)C(pz)(3)](2)(AgX)(2)]( infinity ), where X = CF(3)SO(3)(-) (1), SbF(6)(-) (2), PF(6)(-) (3), BF(4)(-) (4), and NO(3)(-) (5). Crystallization of the first three compounds from acetone yields [p-C(6)H(4)[CH(2)OCH(2)C(pz)(3)](2)(AgCF(3)SO(3))(2)]( infinity ) (1a), [p-C(6)H(4)[CH(2)OCH(2)C(pz)(3)](2)(AgSbF(6))(2)[(CH(3))(2)CO](2)]( infinity ) (2b), and [p-C(6)H(4)[CH(2)OCH(2)C(pz)(3)](2)AgPF(6)]( infinity ) (3a), where the stoichiometry for the latter compound has changed from a metal:ligand ratio of 2:1 to 1:1. The structure of 1a is based on helical argentachains constructed by a kappa(2)-kappa(1) coordination to silver of the tris(pyrazolyl)methane units. These chains are organized into a tubular 3D structure by cylindrical [(CF(3)SO(3))(6)](6)(-) clusters that form weak C-H...O hydrogen bonds with the bitopic ligand. The same kappa(2)-kappa(1) coordination is present in the structure of 2a, but the structure is organized by six different tris(pyrazolyl)methane units from six ligands bonding with six silvers to form a 36-member argentamacrocycle core. The cores are organized in a tubular array by the organic spacers where each pair of macrocycles sandwich six acetone molecules and one SbF(6)(-) counterion. The structure of 3a is based on a kappa(2)-kappa(0) coordination mode of each tris(pyrazolyl)methane unit forming a helical coordination polymer, with two strands organized in a double stranded helical structure by a series of C-H...pi interactions between the central arene rings. Crystallization of 2-4 from acetonitrile yields complexes of the formula [p-C(6)H(4)[CH(2)OCH(2)C(pz)(3)](2)[(AgX)(2)(CH(3)CN)(n)]]( infinity ) where n = 2 for X = SbF(6)(-) (2b), X = PF(6)(-) (3b) and n = 1 for X = BF(4)(-) (4b). All three structures contain argentachains formed by a kappa(2)-kappa(1) coordination mode of the tris(pyrazolyl)methane units linked by the organic spacer and arranged in a 2D sheet structure with the anions sandwiched between the sheets. Crystallization of 5 from acetonitrile yields crystals of the formula [p-C(6)H(4)[CH(2)OCH(2)C(pz)(3)](2)(AgNO(3))(2)(CH(3)CN)(4)]( infinity ), where the nitrate is bonded to the silver. The argentachains, again formed by kappa(2)-kappa(1) coordination, are arranged in W-shaped sheets that have an overall configuration very different from 2b-4b. Treating [p-C(6)H(4)[CH(2)OCH(2)C(pz)(3)](2)(AgSbF(6))(2)]( infinity ) with a saturated aqueous solution of KPF(6) or KO(3)SCF(3) slowly leads to complete exchange of the anion. Crystallization of a sample that contains an approximately equal mixture of SbF(6)(-)/PF(6)(-) from acetonitrile yields [p-C(6)H(4)[CH(2)OCH(2)C(pz)(3)](2)[Ag(2)(PF(6))(0.78(1))(SbF(6))(1.22(1))(CH(3)CN)(2)][(CH(3)CN)(0.25) (C(4)H(10)O)(0.25)]]( infinity ), a compound with a sheet structure analogous to 2b-4b. Crystallization of the same mixture from acetone yields [p-C(6)H(4)[CH(2)OCH(2)C(pz)(3)](2)(AgSbF(6))[(CH(3))(2)CO](1.5)]( infinity ), where the metal-to-ligand ratio is 1:1 and the [C(pz)(3)] units are kappa(2)-kappa(0) bonded forming a coordination polymer. The supramolecular structures of all species are organized by a combination of C-H...pi, pi-pi, or weak C-H-F(O) hydrogen bonding interactions.     

Nonchelated alkene and alkyne complexes of d(0) zirconocene pentafluorophenyl cations

This paper describes the generation and properties of nonchelated d(0) zirconocene-aryl-alkene and alkyne adducts that are stabilized by the presence of beta-SiMe(3) substituents on the substrates and the weak nucleophilicity of the -C(6)F(5) ligand. The cationic complexes [(C(5)H(4)R)(2)Zr(C(6)F(5))][B(C(6)F(5))(4)] (4a: R = H, 4b: R = Me) were generated by methide abstraction from (C(5)H(4)R)(2)Zr(C(6)F(5))Me by Ph(3)C(+). NMR studies show that 4a,b contain an o-CF...Zr dative interaction and probably coordinate a PhCl molecule in PhCl solution. Addition of allyltrimethylsilane (ATMS) to 4a,b in C(6)D(5)Cl solution at low temperature produces an equilibrium mixture of (C(5)H(4)R)(2)Zr(C(6)F(5))(H(2)C=CHCH(2)SiMe(3))(+) (7a,b), 4a,b, and free ATMS. Similarly, addition of propargyltrimethylsilane (PTMS) to 4a produces an equilibrium mixture of Cp(2)Zr(C(6)F(5))(HCCCH(2)SiMe(3))(+) (8a), 4a, and free PTMS. The NMR data for 7a,b,and 8a are consistent with highly unsymmetrical substrate coordination and substantial polarization of the substrate multiple bond with significant positive charge buildup at C(int) and negative charge buildup at C(term). PTMS binds to 4a more strongly than ATMS does. The ATMS adducts undergo nondissociative alkene face exchange ("alkene flipping"), i.e., exchange of the (C(5)H(4)R)(2)Zr(C(6)F(5))(+) unit between the two alkene enantiofaces without decomplexation of the alkene, on the NMR time scale.     

Insertion of CO2, CS2, and COS into iron(II)-hydride bonds

The reactions between cis-Fe(dmpe)2H2 (dmpe = Me2PCH2CH2PMe2) (1) or cis-Fe(PP3)H2 (PP3 = P(CH2CH2PMe2)3) (2) and carbon dioxide (CO2), carbon disulfide (CS2), and carbonyl sulfide (COS) are investigated. At 300 K, additions of CO2 (1 atm), CS2 (2 equiv), and COS (1 atm) to 1 result in the formation of a stable transformato hydride, trans-Fe(dmpe)2(OCHO)H (3a), a trans-dithioformato hydride, trans-Fe(dmpe)2(SCHS)H (4a), and a trans-thioformato hydride, trans-Fe(dmpe)2(SCHO)H (5a), respectively. When CS2 and COS are added to cis-Fe(dmpe)2H2 at 195 K, a cis-dithioformato hydride, 4b, and a cis-thioformato hydride, 5b, respectively, are observed as the initially formed products, but there is no evidence of the corresponding cis-formato hydride upon addition of CO2 to cis-Fe(dmpe)2H2. Additions of excess CO2, CS2, and COS to 1 at lower temperatures (195-240 K) result in the formation of a trans-bis(formate), trans-Fe(dmpe)2(OCHO)2 (3b), a trans-bis(dithioformate), trans-Fe(dmpe)2(SCHS)2 (4c), and a cis-bis(thioformate), cis-Fe(dmpe)2(SCHO)2 (5c), respectively. trans-Fe(dmpe)2(SCHO)2 (5d) is prepared by the addition of excess COS at 300 K. Additions of CO2 (1 atm), CS2 (0.75 equiv), and COS (1 atm) to 2 at 300 K result in the formation of a thermally stable, geometrically constrained cis-formato hydride, cis-Fe(PP3)(OCHO)H (6a), a cis-dithioformato hydride, cis-Fe(PP3)(SCHS)H (7a), and a cis-thioformato hydride, cis-Fe(PP3)(SCHO)H (8a), respectively. Additions of excess CO2 and COS to 2 yield a cis-bis(formate), cis-Fe(PP3)(OCHO)2 (6b), and a thermally stable cis-bis(thioformate), cis-Fe(PP3)(SCHO)2 (8b), respectively. All complexes are characterized by multinuclear NMR spectroscopy, with IR spectroscopy and elemental analyses confirming structures of thermally stable complexes where possible. Complexes 3b and 5a are also characterized by X-ray crystallography.     

High-pressure synthesis, crystal structure, and properties of BiPd2O4 with Pd2+ and Pd4+ ordering and PbPd2O4

BiPd(2)O(4) and PbPd(2)O(4) were synthesized at high pressure of 6 GPa and 1500 K. Crystal structures of BiPd(2)O(4) and PbPd(2)O(4) were studied with synchrotron X-ray powder diffraction. BiPd(2)O(4) is isostructural with PbPt(2)O(4) and crystallizes in a triclinic system (space group P1, a = 5.73632(4) ?, b = 6.02532(5) ?, c = 6.41100(5) ?, α = 114.371(1)°, β = 95.910(1)°, and γ = 111.540(1)° at 293 K). PbPd(2)O(4) is isostructural with LaPd(2)O(4) and BaAu(2)O(4) and crystallizes in a tetragonal system (space group I4(1)/a, a = 5.76232(1) ?, and c = 9.98347(2) ? at 293 K). BiPd(2)O(4) shows ordering of Pd(2+) and Pd(4+) ions, and it is the third example of compounds with ordered arrangements of Pd(2+) and Pd(4+) in addition to Ba(2)Hg(3)Pd(7)O(14) and KPd(2)O(3). In PbPd(2)O(4), the following charge distribution is realized Pb(4+)Pd(2+)(2)O(4). PbPd(2)O(4) shows a structural phase transition from I4(1)/a to I2/a at about 240 K keeping basically the same structural arrangements (space group I2/a, a = 5.77326(1) ?, b = 9.95633(2) ?, c = 5.73264(1) ?, β = 90.2185(2)° at 112 K). BiPd(2)O(4) is nonmagnetic while PbPd(2)O(4) exhibits a significant temperature-dependent paramagnetic moment of 0.46μ(B)/f.u. between 2 and 350 K. PbPd(2)O(4) shows metallic conductivity, and BiPd(2)O(4) is a semiconductor between 2 and 400 K.     

Catalytic and direct oxidation of cysteine by octacyanomolybdate(V)

The oxidation of cysteine by [Mo(CN)(8)](3-) in deoxygenated aqueous solution at a moderate pH is strongly catalyzed by Cu(2+), to the degree that impurity levels of Cu(2+) are sufficient to dominate the reaction. Dipicolinic acid (dipic) is a very effective inhibitor of this catalysis, such that with 1 mM dipic, the direct oxidation can be studied. UV-vis spectra and electrochemistry show that [Mo(CN)(8)](4-) is the Mo-containing product. Cystine and cysteinesulfinate are the predominant cysteine oxidation products. The stoichiometric ratio (Deltan(Mo(V))/Deltan(cysteine)) of 1.4 at pH 10.8 is consistent with this product distribution. At pH 1.5, the reaction is quite slow and yields intractable kinetics. At pH 4.5, the rates are much faster and deviate only slightly from pseudo-first-order behavior. With 2 mM PBN (N-phenyl-tert-butyl nitrone) present at pH 4.5, the reaction rate is about 20% less and shows excellent pseudo-first-order behavior, but the stoichiometric ratio is not significantly changed. The rates also display a significant specific cation effect. In the presence of spin-trap PBN, the kinetics were studied over the pH range 3.48-12.28, with [Na(+)] maintained at 0.09-0.10 M. The rate law is -d[Mo(V)]/dt = k[cysteine](tot)[Mo(V)], with k = {2(k(b)K(a1)K(a2)[H(+)] + k(c)K(a1)K(a2)K(a3))}/([H(+)](3) + K(a1)[H(+)](2) + K(a1)K(a2)[H(+)] + K(a1)K(a2)K(a3)), where K(a1), K(a2), and K(a3) are the successive acid dissociation constants of HSCH(2)CH(NH(3)(+))CO(2)H. Least-squares fitting yields k(b) = (7.1 +/- 0.4) x 10(4) M(-1) s(-1) and k(c) = (2.3 +/-0.2) x 10(4) M(-1) s(-1) at mu = 0.1 M (NaCF(3)SO(3)) and 25 degrees C. A mechanism is inferred in which k(b) and k(c) correspond to electron transfer to Mo(V) from the thiolate forms of anionic and dianionic cysteine.     

Acid-base equilibria in gamma-butyrolactone studied by use of pH-ISFETs

pH-ISFETs were used in the study of acid-base equilibria in gamma-butyrolactone (GBL). After the spectrophotometric determination of the pK(a) value of 3,5-dichloropicric acid, the pK(a) values and homo-conjugation constants of various acids (including the conjugate acids of bases) were determined potentiometrically using a Ta(2)O(5)-type pH-ISFET. The values of pK(a) in GBL were in a linear relation with those in propylene carbonate (PC) and 1.0 units smaller on average. The difference in pK(a) between GBL and PC was mainly attributable to the difference in proton solvation. The autoprotolysis constant of GBL, roughly estimated by a rapid titration with a Si(3)N(4)-ISFET, was about 30 on the pK(SH) scale. A comparative study was made of the response speeds of the Ta(2)O(5)- and Si(3)N(4)-type pH-ISFETs and a conventional pH-glass electrode. The result was Si(3)N(4)-ISFET > Ta(2)O(5)-ISFET > glass electrode. Because GBL is not stable against acids and bases, the use of pH-ISFETs was much more convenient than the use of the conventional glass electrode.     

Mode selective tunneling dynamics observed by high resolution spectroscopy of the bending fundamentals of 14NH2D and 14ND2H

High resolution (0.004 and 0.01 cm(-1) instrumental bandwidth) interferometric Fourier transform infrared spectra of (14)NH2D and (14)ND2H were measured on a Bomem DA002 spectrometer in a supersonic jet expansion and at room temperature. We report the analysis of the bending fundamentals of (14)NH2D with term values Tv(s)=1389.9063(2) cm(-1) and Tv(a)=1390.4953(2) cm(-1) for the nu(4b) fundamental and Tv(s)=1605.6404(7) cm(-1) and Tv(a)=1591.0019(7) cm(-1) for the nu(4a) fundamental, and of (14)ND2H with term values of Tv(s)=1233.3740(2) cm(-1) and Tv(a)=1235.8904(2) cm(-1) for the nu(4a) fundamental and Tv(s)=1461.7941(9) cm(-1) and Tv(a)=1461.9918(19) cm(-1) for the nu(4b) fundamental. In all cases Tv(s) gives the position of the symmetric inversion sublevel (with positive parity) and Tv(a) the position of the antisymmetric inversion sublevel (with negative parity). The notation for the fundamentals nu(4a) and nu(4b) is chosen by correlation with the degenerate nu(4) mode in the C(3v) symmetric molecules NH3 and ND3. The degeneracy is lifted in Cs symmetry and a indicates the symmetric, b the antisymmetric normal mode with respect to the Cs symmetry plane in NH2D and ND2H. Assignments were established with certainty by means of ground state combination differences. About 20 molecular parameters of the effective S-reduced Hamiltonian could be determined accurately for each fundamental. In particular, the effect of Fermi resonances of the 2nu(2) overtone with the nu(4a) bending mode was observed, leading to an increased inversion splitting in the case of ND2H and to a strongly increased inversion splitting and an inverted order of the two inversion levels in NH2D. Rotational perturbations observed with the nu(4b) bending fundamentals are probably due to Coriolis interactions with the inversion overtone 2nu(2). The results are important for understanding isotope effects on the inversion in ammonia as well as its selective catalysis and inhibition by excitation of different vibrational modes, as treated by quantum dynamics on high dimensional potential hypersurfaces of this molecule.     

Royal crown-shaped electride Li3-N3-Be containing two superatoms: new knowledge on aromaticity

The structure and aromaticity of a royal crown-shaped molecule Li(3)-N(3)-Be are studied at the CCSD(T)/aug-cc-pVDZ level. This molecule is a charge-separated system and can be denoted as Li(3) (2+)N(3) (3-)Be(+). It is found that the Li(3) (2+) ring exhibits aromaticity mainly because the Li(3) (2+) ring can share the pi-electron with the N(3) (-3) ring. The 4n+2 electron counter rule can be satisfied for the Li(3) (2+) subunit if the shared pi valence electron of N(3) (3-) subunit is also taken into account. This new knowledge on aromaticity of a ring from the interactions between subunits is revealed first time in this paper. Li(3)-N(3)-Be can be also regarded as a molecule containing two superatoms (Li(3) and N(3)), which may be named as a "superomolecule." Li(3)-N(3)-Be is a new metal-nonmetal-metal type sandwich complex. The N(3) (3-) trianion in the middle repulses the electron clouds of the two metal subunits (mainly to the Li(3) superatom) to generate an excess electron, and thus Li(3)-N(3)-Be is also an electride. This phenomenon of the repulsion results in: (a) the HOMO energy level increased, (b) the electron cloud in HOMO distended, (c) the area of the negative NICS value extended, and (d) the VIE value lowered. So the superomolecule Li(3)-N(3)-Be is not only a new metal-nonmetal-metal type sandwich complex but also a new type electride, which comes from the interaction between the alkali superatom (Li(3)) and the nonmetal superatom (N(3)).     

Synthesis of 6aλ4-thia-1,6-diselena-3,4-diazapentalenes, 1,6,6aλ4-triselena-3,4-diazapentalenes, 6aλ4-thia-1,3,4,6-tetraazapentalenes,and 6aλ4-Selena-1,3,4,6-tetraazapentalenes from cyclic thioureas and selenoureas

Imidazolidine-2-thione (7a) and the corresponding 2-selone (7b), hexahydropyrimidine-2-thione (7c) and 2-selone (7d), and hexahydro-1H-1,3-diazepine-2-thione (7e) and 2-selone (7f) reacted with 2,4-dinitrobenzyl chloride to give the 2-(2,4-dinitrobenzylthio) and 2-(2,4-dinitrobenzylseleno) derivatives (8a)-(8f) of 4,5-dihydroimidazolium chloride, 1,4,5,6-tetrahydropyr-imidinium chloride, and 4,5,6,7-tetrahydro-1H-1,3-diazepinium chloride. Deprotonation of the chlorides (8a)-(8f) gave, respectively, 2-(2,4-dinitrobenzylthio)-and 2-(2,4-dinitrobenzylseleno)-4,5-dihydroimidazole (9a) and (9b), 2-(2,4-dinitrobenzylthio)- and 2-(2,4-dinitrobenzylseleno)-1,4,5,6-tetrahydropyrimidine (9c) and (9d), and 2-(2,4-dinitrobenzylthio)- and 2-(2,4-dinitrobenzylseleno)-4,5,6,7-tetrahydro-1H-1,3-diazepine (9e) and (9f). The bases (9a)-(9f) reacted with isoselenocyanates with elimination of 2,4-dinitrotoluene and concomitant addition of two molecules of the isoselenocyanate to give 1,6,6aλ⁴-triheterapentalenes of two structural types, depending on the size of the heteroring in the bases (9a)-(9f). The imidazoles (9a) and (9b) gave 6aλ⁴-thia-1,6-diselena-3,4-diazapentalenes (10a)-(10j) and 1,6,6aλ⁴-triselena-3,4-diazapentalenes (11a)-(11h), respectively. The sulfur-containing bases (9c) and (9e) gave 6aλ⁴-thia-1,3,4,6-tetraazapentalenes (12a)-(12j) and (14a)-(14d), respectively, and the selenium-containing bases (9d) and (9f) gave 6aλ⁴-selena-1,3,4,6-tetraazapentalenes (13a)-(13j) and (15a)-(15d). Heteroatom-heteroatom covalent bond energies have been estimated for representative members of the series (10)-(14) by using the Huggins equation and experimentally determined bond lengths. © 1997 John Wiley & Sons, Inc.