Adjacent Nitrogen Atoms Research Articles

Electron energy-loss spectroscopy study of the electronic structure of boron nitride nanotubes

Electron energy-loss spectra were obtained from single boron-nitride nanotubes (BNTs), which were synthesized by the arc-discharge method. The π and π + σ plasmon energies of the BNTs were smaller than those of hexagonal boron-nitride (h-BN). The π + σ plasmon energy is explained by the surface plasmon excitation. Dielectric functions of the BNTs were derived from the loss functions by Kramers-Kronig analysis. The bandgap energy was obtained to be ∼5 eV, which is smaller than that of h-BN of 5.8 eV, from the imaginary part of the dielectric function. Full width at half maximum (FWHM) value of the 1s → π* transition peak in boron K-shell excitation spectra was nearly the same as that of h-BN, whereas the FWHM value of the peak of carbon nanotubes was greater than that of graphite. No additional broadening of the peaks of the BNTs may be attributed to a weak interaction between the pz orbitals of adjacent boron and nitrogen atoms.

Hydroxyethylation and its effect on the rate of metal ion ingress into macrocyclic ligands

The hydroxyethylation effect, by which the rate of metal ion uptake by tetraaza macrocyclic ligands is enhanced, was previously known to occur when all four nitrogen atoms are hydroxyethylated or when two, on adjacent nitrogens, are so treated. In this work two mono-hydroxyethylated tetraaza macrocycles, 1-(2-hydroxyethyl)-1,4,8,11-tetraazacyclotetradecane and 1-(2-hydroxyethyl)-4,8,11-trimethyl-1,4,8,11-tetraazacyclotetradecane have been synthesised as well as one tri-hydroxyethylated macrocycle, 1,4,8-tris(2-hydroxyethyl)-11-methyl-1,4,8,11-tetraazacyclotetradecane. The stability constants for the monometallic complexes formed from these ligands with Co 2+, Ni 2+, Cu 2+, Zn 2+, Cd 2+, Hg 2+ and Pb 2+ have been determined in aqueous solution at 298.2 K by potentiometric titration. During the course of these titrations it was noted that the tri-hydroxyethylated macrocycle complexes establish equilibrium at each titration point within a maximum of 5 min of the addition of each aliquot of base. The mono-hydroxyethylated ligands, on the other hand, equilibrate very much more slowly. This indicates that the facilitation of metal ion ingress into tetraaza macrocycles, afforded by pendant hydroxyethyl groups, requires a minimum of two hydroxyethyl groups located on adjacent nitrogen atoms.

Structural requirements for rapid metal ion ingress into hydroxyethylated tetraaza macrocycles

Synthesis of two new bis(2-hydroxyethyl)_substituted cyclam (1,4,8,11-tetraazacyclotetradecane) 1,4-bis(2-hydroxyethyl)-8,11-dimethylcyclam (1,4-DMHEC-14) and 1,11-bis(2-hydroxyethyl)-4,8-dimethylcyclam (1,11-DMHEC-14), has enabled a study of the effects of partial hydroxyethylation of the nitrogen atoms in a macrocyclic ligand framework to be made. The ligands were synthesised through the reaction of ethylene oxide with the appropriately methylated cyclam precursor. Stability constants for complexes of these ligands with Co 2+, Ni 2+, Cu 2+, Zn 2+ Cd 2+ Hg 2+ and Pb 2+ have been measured by potentiometric titration and are generally intermediate between those shown by tetramethylcyclam (TMC-14) and tetrakis(2-hydroxyethyl)cyclam (THEC-14), consistent with a lower level of ligand stabilisation through internal hydrogen bond formation than in THEC-14. Equilibration times noted during the potentiometric titrations indicate that these bis(2-hydroxyethyl) ligands, like THEC-14 and unlike TMC-14, facilitate rapid entry of the metal ion into the macrocyclic annulus demonstrating that two pendant 2-hydroxyethyl arms, when located on adjacent nitrogen atoms, are sufficient to generate facilitated entry. The crystal structures of both [Ni(1,4-DMHEC-14)](ClO 4) 2 and [Ni(l,II-DMHEC14)](ClO 4) 2 have been determined by a combined molecular mechanics and X-ray crystallography study and show that in each isomer both pendant donors are coordinated in a trans-relationship, in addition to the four nitrogen atoms, giving rise to octahedral stereochemistry. In the former case the ligand is in the trans-III configuration whereas the latter complex has a trans-II geometry.

Identification of the 1,3,2,4-Dithiadiazolyl RCNSNS(*) Radical Dimers in Solution, Their Dimeric Concerted Photochemically Symmetry-Allowed Rearrangement to 1,2,3,5-Dithiadiazolyl RCNSSN(*) by the Net Exchange of Adjacent Cyclic Sulfur and Nitrogen Atoms, and the Photolysis of RCNSNS(*).

The 1,3,2,4-dithiadiazolyl RCNSNS(*) radicals undergo an unprecedented concerted rearrangement to the thermodynamically more stable 1,2,3,5-dithiadiazolyl RCNSSN(*) radicals by the net exchange of adjacent cyclic sulfur and nitrogen atoms. The UV-visible spectra of RCNSNS(*) (R = Ph, p-O(2)NC(6)H(4), 3,5-(O(2)N)(2)C(6)H(3), CF(3)) in solution show bands at 250 nm (strong) and 680 nm (very weak) attributable to monomer and two dimer bands at 376 and 480 nm, the positions of which are independent of the substituents, providing direct identification of the radical dimers in solution. The dimerization equilibrium constant (K(298) approximately 0.7 for R = Ph) at room temperature was derived from the enthalpy and entropy changes for the dimerization of PhCNSNS(*) (DeltaH(d) degrees = -19.0 kJ/mol, DeltaS degrees = -66.5 J/mol) estimated by a variable-temperature ESR spectroscopic study. In addition, RCNSNS(*) (R = Bu(t), Ph) undergo an apparent unimolecular photolysis to RCN and possibly SNS(*) (analogue of ONO(*)). The photochemical rearrangement and dissociation (for R = Ph and 3,5-(O(2)N)(2)C(6)H(3)) were shown to proceed by irradiation of the radical dimer (376 and 480 nm) and monomer (250 nm), respectively. Thus, the radical rearrangement reasonably occurs via a concerted dimeric pathway shown by molecular orbital calculations (CNDO) to be photochemically symmetry-allowed. In addition, we propose that the radical dissociation proceeds via a concerted unimolecular photochemically symmetry-allowed process.

New Metal Complexes Derived from 1-N-(6-Nitrobenzimidazole-1-carbonylmethyl)-4-(α-naphthyl)-3-thiosemicarbazide

Abstract The synthesis and characterization of Mn(II), Cu(II), Co(II), Ni(II), Pb(II) and UO2 (VI) complexes of the new ligand 1-N-(6-nitrobenzimidazole-1-carbonylmethyl)-4-(α-naphthyl)-3-thiosemicarbazide (NBNMS) is reported. Infrared spectral data show that NBNMS behaves as a tetradentate ligand involving the carbonyl and thiocarbonyl groups in conjunction with the adjacent nitrogen atoms (C = N groups). Each ligand bridges to two metal centers. This result in polymers that grow through consecutive ligand-metal linkages for all but UO2 (VI). In contrast, NBNMS acts as a monobasic tridentate ligand with UO2(VI).

Comparative study of the interaction of ‘lop-sided’ 14- to 17-membered tetraaza macrocycles containing fused dibenzo substituents with nickel(II)

The effect of macrocyclic and chelate-ring size on the complexation behaviour of a series of dibenzo tetraaza macrocycles incorporating 14- to 17-membered inner rings has been investigated. Change in ring size along this ‘lop-sided%’ series has been achieved solely by varying the number of methylene carbons that link adjacent benzyl nitrogen atoms. Several solid complexes of type NiLX2·xH2O (L = macrocyclic ligand; x= 0 or 1; X = Cl, NCS or NO3) with these rings have been isolated. The results of five X-ray crystallographic analyses, as well as molecular mechanics studies, have allowed comparison of the effect of change of ring size on the structures of the 14- to 17-membered ring complexes with X = NCS. All complexes have similar six-co-ordinate trans-isothiocyanato geometries in which the four donors of the macrocycle occupy the equatorial plane. For this series an inverse correlation between in-plane and axial bond lengths around the nickel atoms occurs for the 14- to 16-membered ring complexes; the correlation is less apparent in the case of the 17-membered species. The X-ray and molecular mechanics studies indicate that, as the number of methylene carbons linking adjacent benzylamino nitrogens increases, the corresponding bite angle at the metal increases, with the opposite bite angle decreasing in a (partial) compensatory manner. As a consequence, there is only a minor variation in the planarity of the macrocyclic donor plane along the series. The crystal structure of the 16-membered ring complex of nickel(II) nitrate is also reported, as is that of the 17-membered ring complex of nickel(II) chloride. The former contains an equal mixture of two (trans-octahedral) complexes of type [NiL(NO3)2] and [NiL(NO3)(H2O)]NO3. Cyclic voltammetry indicates that formation of the respective nickel(III) species is facilitated by the smaller-ring ligands; the oxidation of the 17-membered ring complex is non-reversible and probably ligand based.

Hexakis(trifluoromethyl)tetrazane

A new synthesis provides the title compound (CF3)2NN(CF3)N(CF3)2 (1) smoothly, and its gas-phase structure was determined. This tetrazane has a twisted N4 chain with a dihedral angle of 95.2(11)°; the lone pairs of electrons on adjacent nitrogen atoms are perpendicular.

The coordination chemistry of mixed pyridine-phenol and phenanthroline-phenol ligands; The crystal structure of 2-(2-hydroxyphenyl)-1,10-phenanthroline (HL) and the crystal structure and properties of [FeL 2][PF 6

The crystal structures of the N,N,O-terdentate ligand 2-(2-hydroxyphenyl)-1,10-phenanthroline (HL) and its Fe(III) complex [FeL 2][PF 6] ( 1) have been determined. In HL, an intramolecular OH … N hydrogen bond between the phenolic OH and the adjacent nitrogen atom of the phenanthroline fragment constrains these two donor atoms to be cisoid, with the proton ‘chelated’. The molecules of HL are associated into pairs in the crystal via a face-to-face aromatic π-stacking interaction. Complex 1 has a conventional distorted octahedral structure; sections of the aromatic ligands overlap between adjacent molecules to form an interleaved stack. It is high-spin down to 83 K, and its EPR spectrum (frozen glass at 77 K) is entirely typical of a rhombically distorted high-spin Fe III site.

Synthesis of Some Transition Metal Complexes Derived from (1-Carbonylmethyl-1H-benzotriazole)-4-phenyl-3-thiosemicarbazide

Abstract Complexes of the type (ML · X H2O)n where M = Mn(II), Ni(II), Zn(II), Cd(II), Pb(II), L = (1-carbonylmethyl-1H-benzotriazole)-1-phenyl-3-thiosemicarbazide (BPTSH2), X = 0, 1, 2 and n = degree of polymerization have been synthesized and characterized by elemental and thermal analyses along with IR and 1H NMR spectra. Infrared spectra are consistent with tetradentate coordination involving the carbonyl and thiocarbonyl groups in conjunction with the adjacent nitrogen atoms (C = N groups) with each ligand binding to two metal centers. The result is proposed to be polymers that grow through consecutive ligand-metal linkages.

A computational study of azine, azoethene, and diimine linkages in the poly/oligoazine system

AbstractHartree‐Fock (HF) and molecular mechanics calculations were performed on linear azine oligomers and model compounds. The rotational energy curves for the model compounds formaldazine, H2C = NN = CH2, ethenyl diazene, H2C = CHN = NH, and ethanediimine, HN = CHCH = NH were calculated for a variety of basis sets at the HF and MP2 level. In all of these cases the rotational energy barriers are quite different from butadienes or aza‐substituted butadienes because of the lone pair–lone pair interaction of the adjacent nitrogen atoms. The results on the model compounds were used to generate a set of molecular mechanics (MM) parameters that are appropriate for linear oligo‐ and polyazines. Comparison of the geometries of the HF results and MM results for the oligoazines showed that the two methods gave comparable results. © 1994 by John Wiley & Sons, Inc.

Reactions of Palladium Chloride with Amino Spirocyclic Cyclotriphosphazenes: Isolation and Crystal Structures of Novel Mono- and Bimetallic Complexes Formed by the Hydrolysis of a .lambda.5-Diazaphospholane Ring

The reaction of the amino spirocyclic cyclotriphosphazene N3P3(NMe2)4(NHCH2CH2CH2NH) (2) with palladium chloride gives the stable chelate complex [PdCl2.2] (4). An X-ray crystallographic study reveals that one of the nitrogen atoms of the diaminoalkane moiety and an adjacent phosphazene ring nitrogen atom are bonded to the metal. An analogous reaction with the phosphazene N3P3(NMe2)4(NHCH2CH2NH) (1) gives initially a similar complex which undergoes facile hydrolysis to give the novel monometallic and bimetallic complexes [PdCl2.HN3P3(O)(NMe2)4(NHCH2CH2NH2)] (5) and [PdCl{N3P3(NMe2)4(NCH2CH2NH2)}]2(O) (6), which have been structurally characterized; in the former, an (oxophosphazadienyl)ethylenediamine is chelated to the metal whereas, in the latter, an oxobridged bis(cyclotriphosphazene) acts as a hexadentate nitrogen donor ligand in its dianionic form. Crystal data for 4 : a = 14.137(1) angstrom, b = 8.3332(5) angstrom, c = 19.205(2) angstrom, beta = 96.108(7)degrees, P2(1)/c, Z = 4, R = 0.027 with 3090 reflections (F > 5sigma(F)). Crystal data for 5 : a = 8.368(2) angstrom, b = 16.841(4) A, c = 16.092(5) angstrom, beta = 98.31(2)degrees, P2(1)/n, Z = 4, R = 0.049 with 3519 reflections (F > 5sigma(F)). Crystal data for 6 : a = 22.455(6) angstrom, b = 14.882(3) angstrom, c = 13.026(5) angstrom, 6 = 98.55(2)degrees, C2/c, Z = 4, R = 0.038 with 3023 reflections (F > 5sigma(F)).

Solid state structure of [HB(pz) 3] 2Sn

The solid-state structure of [HB(pz) 3] 2Sn has been determined by X-ray crystallography. The complex is pseudo octahedral, counting the stereochemically active lone pair as occupying the sixth coordination site. One [HB(pz) 3] − ligand is bidentate and the other is tridentate. The coordination sphere about the tin atom is very asymmetric with Sn N bond distances varying from 2.263(3) to 2.732(4)A˚. There is also a significant distortion in the square arrangement of the nitrogen donor atoms cis to the lone pair. Three of these N Sn N angles from adjacent nitrogen atoms range from 72.3(1) to 79.4(1)°, but the fourth is much larger at 124.4(1)°. The asymmetric structure explains the fact that the 119Sn NMR chemical shift of this complex is centered between four-coordinate [B(pz) 4] 2Sn and the more symmetric, five-coordinate [HB(3,5-Me 2pz) 3] 2Sn.

Mono- and bi-nuclear copper(II) complexes of azaparacyclophanes with a single aromatic spacer. Crystal structure of [Cu2L2Cl4]·1.5H2O (L2= 2,5,8, 11-tetraaza[12]paracyclophane)

The protonation behaviour of the azaparacyclophanes 2, 6, 9, 13-tetraaza[14]paracyclophane (L1), 2, 5, 8, 11-tetraaza[12]paracyclophane (L2) and 2, 5, 8, 11, 14-pentaaza[15]paracyclophane (L3) has been studied at 298.1 K in 0.1 5 mol dm–3 NaClO4 by potentiometry, direct microcalorimetry, and 1H and 13C NMR spectroscopy. The data obtained by the different techniques suggest that first protonation of L1 and L2 occurs mainly on the benzylic nitrogens. All three ligands form mononuclear copper(II) complexes in aqueous solution characterized by an incomplete participation of the nitrogen donors in the co-ordination to the metal ion. The ligands L2 and L3 also form the binuclear species [Cu2L2(OH)2]2+ and [Cu2L3]4+, [Cu2L3(OH)]3+ and [Cu2L3(OH)2]2+ in solution. The crystal structure of the complex [Cu2L2Cl4]·1.5H2O has been solved by X-ray crystal diffraction analysis [space group P21/n, a= 10.625 (3), b= 14.014(3), c= 14.952(7)A, β= 109.55(3)°β= 109.55(3)°, Z = 4, R = 0.066 and R′= 0.06]. Both metal ions show a distorted square-pyramidal co-ordination geometry. The basal plane is defined for each metal centre by a benzylic nitrogen atom, the adjacent nitrogen atom in the chain and two chloride ions. The apical position is occupied by another chloride from a symmetry-related molecule with a much weaker bond than those in the plane. The presence of the para-phenylene subunit imposes enough strain to divide the macrocyclic cavity of L2 into two separate ethylenediamine subunits.

Unsaturated Hetero Chains, II. 1‐Oxa‐3,5,7‐triaza‐1,3,5,7‐octatetraenes – Syntheses and Structures

AbstractThree independent routes for the synthesis of the hitherto unknown substituted 1‐oxa‐3,5,7‐triazaoctatetraenes 4 have been developed. Method 1 utilizes N‐lithiated imines 1 and 1,3,5‐oxadiazinium salts 2 for the synthesis of 4b–e. Method 2 takes advantage of the new 6‐alkoxy‐1‐oxa‐3,5‐diazahexatrienes 5, which are allowed to react with imino lithium compounds to produce 4f. Method 3 is based on the combination of the lithiated 1,3‐diazabutadiene 7 with the N‐acylimidate 8 and gives access to 4g–j. Depending on the substitution pattern, the title compounds show ring‐chain tautomerism; in the case of the pentaphenyl system, only the triazine derivative 3a is observed. An X‐ray analysis of 4f reveals a non‐planar structure with (+)‐gauche‐Z‐(–)‐gauche‐Z‐(+)‐gauche conformation, resulting from amide‐like n/π interactions between C=N–π systems and the lone pair of the adjacent nitrogen atoms. Detailed spectroscopic data for all open‐chain and cyclic compounds are given. Quantum mechanical calculations (MP2/6‐31G*//6‐31G*) of model compounds 13 predict that non‐planar isomers with gauche‐configurated subunits are favored by more than 8 kcal/mol over a planar all‐trans structure as in polyenes. Most planar isomers correspond to low‐lying transition states for the rotation around the C–N bonds, but not to local minima, indicating high molecular flexibility of these oligonitrile chains.

Rigorous interpretation of electronic wavefunctions. IV. Origins of the unusual stability of the 1,3-dimethylimidazol-2-ylidene carbene

Ab initio electronic structure calculations, carried out in conjunction with rigorous analysis of the resulting wavefunctions, demonstrate that the electronic factors responsible for the unusual stability of the 1,3-dimethylimidazol-2-ylidene carbene are directly related to the substantial σ-backdonation from the carbenic carbon to the adjacent nitrogen atoms. The π-donation is found to play only a minor role, ruling out the presence of significant stabilization due to ylidic resonance structures. The unusually large proton affinity of the carbene is explained by the extra π-electron stabilization of the corresponding 1,3-dimethylimidazolium cation. The MP2/6-311++G** level of theory is found to be sufficient for accurate predictions of the geometries of the imidazol-2-ylidene carbenes. © 1993 John Wiley & Sons, Inc.

Luminescence and picosecond photoinduced absorption of polyaniline

The photoluminescence and photoinduced absorption are reported for leucoemeraldine and emeraldine forms of polyaniline in the picosecond to nanosecond time domain. The importance of bond vibration, charge tunneling and ring rotations in varying relaxation processes is determined. The temperature dependence of the photoluminescence decay time of the leucoemeraldine base dissolved in N-methyl pyrrolidinone is reported. The decay time decreases as the temperature increases. From the Arrhenius plot, the activation energy is estimated to be ∼0.1 eV. These data suggest that backbone vibrations dominate this photoluminescence decay mechanism. From the comparison of the result to that of the monomer, aniline, it is concluded that a polaron-exciton confined in the basic unit including a ring and adjacent nitrogen atoms is responsible for the photoluminescence. The picosecond photoinduced absorption of emeraldine base films optically pumped at 2.1 eV and probed at 1.5 eV and 2.9 eV is compared with the previously measured picosecond photoinduced bleaching probed at 2.0 eV. While all three of them show initial fast decay, the photoinduced absorption at 1.5 eV does not show a substantial residue on the nanosecond time scale. This is in contrast to the photoinduced bleaching at 2.0 eV and 2.9 eV photoinduced absorption, for which more than half of the signal remains even after a few nanoseconds. Preliminary measurements of emeraldine base films at a probe energy of 2.9 eV indicate that the decay slows down when the temperature is lowered. For emeraldine base in N-methyl pyrrolidinone solution, this long-lived tail in picosecond photoinduced absorption disappears. These data strongly suggest the importance of the ring-torsion or interchain processes in the excitation decay mechanism of the 2.9 eV photoinduced (polaron) absorption.

NMR and NIR Studies of the Tautomerism of 5,10,15,20‐Tetraphenylporphyrin including Kinetic HH/HD/DD Isotope and Solid State Effects

AbstractThe liquid and solid state tautomerism of 5,10,15,20‐tetraphenylporphyrin‐15N4 (meso‐tetraphenylporphyrin, TPP) has been studied by dynamic NMR spectroscopy and by NIR spectroscopy. The kinetic HH/HD/DD isotope effects on the liquid state tautomerism were reinvestigated in the temperature region between 200 K and 300 K. It was confirmed that the kinetic HH/HD isotope effects are large and the HD/DD isotope effects small. At 298 K the values 9.7 and 1.8 were obtained. The Arrhenius curve of the HH reaction is non‐linear indicating that the proton transfer takes place by thermally activated tunneling at low temperatures. The kinetic isotope effects are consistent with a stepwise proton transfer pathway involving intermediate tautomeric states in which protons are bound to adjacent nitrogen atoms. Solid state NMR experiments were performed on triclinic TPP, where the reaction barrier is higher compared to TPP in the liquid state. In addition, the degeneracy of the tautomerism in the solid state is lost due to intermolecular interactions. The latter conclusion is supported by the finding that for tetragonal mixed crystals of [(TPP—14N4—Ni)0.1(TPP—15N4—H2)0.9] the degeneracy of the tautomerism and its barrier is equal to the values found for the liquid solution. The NIR measurements show the presence of NH‐stretching overtones in the region of about 6450 cm−1. The anharmonicity of this vibration is of the usual order, which indicates that the reaction coordinate of proton transfer is not identical with the normal modes of the NH‐stretching vibrations, but rather a combination of various vibrational modes including the stretching, bending and skeletal modes. As a consequence, there will be a large number of vibrational states from which tunneling might occur.

Synthesis of 2-ferrapyridine complexes and their use as precursors for substituted pyridinones and pyrroles

The 2-ferra-3-azetine complexes Fe{sub 2}({mu}-CHCH{double_bond}NR)(CO){sub 6}, which form from the reaction of Fe{sub 2}({mu}-CH{sub 2})(CO){sub 8} with phosphinimines, have been found to react photochemically with a variety of alkynes (R{sup 1}C{triple_bond}CR{sup 2}) by inserting the alkyne into the four-membered metallacycle to give the 2-ferrapyridine complexes Fe{sub 2}({mu}-C(R{sup 1})C(R{sup 2})CHCH{double_bond}NR)(CO){sub 6}. Two of the 2-ferrapyridine complexes have been crystallographically characterized and shown to consist of six-membered metallacycles having adjacent iron and nitrogen atoms with this metallacycle {pi}-coordinated to the second iron atom by three carbon atoms and an Fe-Fe bond. Terminal alkynes (R{sup 1} = Me, Ph, Bu{sup t}, SiMe{sub 3}) undergo the insertion regioselectively to give only the 2-ferrapyridine isomer with the substituted carbon adjacent to the iron atom. Unsymmetrical internal alkynes insert to give generally a mixture of isomers which is influenced by the steric and electronic nature of the alkyne substituents. The 2-ferrapyridine complex Fe{sub 2}({mu}-C(Ph)CHCHCH{double_bond}NBu{sup t})(CO){sub 6} was shown to undergo a ring contraction when heated at 160 {degrees}C under CO (500 psi) to five the new 2-ferra-3-azetine complex Fe{sub 2}({mu}-C[CH{double_bond}C(Ph)(H)]CH{double_bond}NR)(CO){sub 6}, which has also been crystallographically characterized and has a vinyl substituent attached to the 3-carbon of the 2-ferra-3-azetine ring. When heated, the 2-ferrapyridine complexes releasedmore » 2-pyridinones and pyrroles with the ratio of these products dependent upon the ring substituents and the conditions employed. 21 refs., 3 figs., 9 tabs.« less

The reactions of heteroarenes with (η-C 5H 5) 2Rh 2( μ-CO)(μ-η 2-CF 3C 2CF 3)

In the reactions of (η-C 5H 5) 2Rh 2(μ-CO)(μ-η 2CF 3C 2CF 3) ( 1) with thiophene and furan, there is ready cleavage of two CH bonds in the heteroaromatics leading to the formation of the complexes C(η-C 5H 5) 2Rh 2{μ-η 1,η 2-C(CF 3)C(CF 3)H}] 2(μ-η 1,η 1- C 4H 2E) (E = S or O). With thiophene, a pronounced preference to activate the two CH bonds adjacent to the heteroatom gives the 2,5-dimetallated product with high selectivity; this is attributed to prior coordination of the sulphur to the dirhodium centre. Steric factors are believed to govern the reaction with furan, which gives mainly the 2,4-dimetallated species. The reaction with pyrrole proceeds in a similar manner, but is less extensive. A spontaneous reaction occurs when 1 is dissolved in pyridine, and an ortho-metallated product, (η-C 5H 5) 2Rh 2{μ-η 1,η 2-C(CF 3)C(F 3)H}(μ-η 1,η 1-NC 5H 4), is formed as a result of a single CH bond cleavage reaction. The bridging pyridyl ligand is attached from adjacent carbon and nitrogen atoms. Similar products are formed with 2-fluoropyridine and 4-methylpyridine. However, the presence of a methyl substituent in the 2-position of the pyridine ring causes a change in the stereochemistry of the μ-C(CF 3(CF 3)H unit from cis to gauche. The reaction with 2-aminopyridine yields two CH bond activated products similar to the above complexes, but two NH bond activated products, (η-C 5H 5) 2Rh 2{μ-η 1, η 2-C(CF 3)C(CF 3)H}(gm-η 1,η 1-NHpy) and (η-C 5H 5) 2Rh{ 2μ-η 1,η 1- C(CF 3)C(CF 3)C(O)}(μ-η 1-N-py), are formed also.

Mechanism of acyl transfer by the class A serine β-lactamase of Streptomyces albus G

Optimization by energy minimization of stable complexes occurring along the pathway of hydrolysis of benzylpenicillin and cephalosporin C by the Streptomyces albus G beta-lactamase has highlighted a proton shuttle that may explain the catalytic mechanism of the beta-lactamases of class A. Five residues, S70, S130, N132, T235 and A237, are involved in ligand binding. The gamma-OH group of T235 and, in the case of benzylpenicillin, the gamma-OH group of S130 interact with the carboxylate group, on one side of the ligand molecule. The side-chain NH2 group of N132 and the carbonyl backbone of A237 interact with the exocyclic CONH amide bond, on the other side of the ligand. The backbone NH groups of S70 and A237 polarize the carbonyl group of the scissile beta-lactam amide bond. Four residues, S70, K73, S130 and E166, and two water molecules, W1 and W2, perform hydrolysis of the bound beta-lactam compound. E166, via W1, abstracts the proton from the gamma-OH group of S70. While losing its proton, the O-gamma atom of S70 attacks the carbonyl carbon atom of the beta-lactam ring and, concomitantly, the proton is delivered back to the adjacent nitrogen atom via W2, K73 and S130, thus achieving formation of the acyl-enzyme. Subsequently, E166 abstracts a proton from W1. While losing its proton, W1 attacks the carbonyl carbon atom of the S70 ester-linked acyl-enzyme and, concomitantly, re-entry of a water molecule W'1 replacing W1 allows E166 to deliver the proton back to the same carbonyl carbon atom, thus achieving hydrolysis of the beta-lactam compound and enzyme recovery. The model well explains the differences found in the kcat. values for hydrolysis of benzylpenicillin and cephalosporin C by the Streptomyces albus G beta-lactamase. It also explains the effects caused by site-directed mutagenesis of the Bacillus cereus beta-lactamase I [Gibson, Christensen & Waley (1990) Biochem J. 272, 613-619].