Showing papers by "Martin Lutz published in 2004"
TL;DR: Both the (195)Pt chemical shift and the calculated natural population charge on platinum correlate linearly with the sigma(p) Hammett substituent constants, and thus the electronic properties of predesigned pincer complexes can be predicted.
Abstract: A variety of para-substituted NCN-pincer palladium(II) and platinum(II) complexes [MX(NCN-Z)] (M=Pd(II), Pt(II); X=Cl, Br, I; NCN-Z=[2,6-(CH(2)NMe(2))(2)C(6)H(2)-4-Z](-); Z=NO(2), COOH, SO(3)H, PO(OEt)(2), PO(OH)(OEt), PO(OH)(2), CH(2)OH, SMe, NH(2)) were synthesised by routes involving substitution reactions, either prior to or, notably, after metalation of the ligand. The solubility of the pincer complexes is dominated by the nature of the para substituent Z, which renders several complexes water-soluble. The influence of the para substituent on the electronic properties of the metal centre was studied by (195)Pt NMR spectroscopy and DFT calculations. Both the (195)Pt chemical shift and the calculated natural population charge on platinum correlate linearly with the sigma(p) Hammett substituent constants, and thus the electronic properties of predesigned pincer complexes can be predicted. The sigma(p) value for the para-PtI group itself was determined to be -1.18 in methanol and -0.72 in water/methanol (1/1). Complexes substituted with protic functional groups (CH(2)OH, COOH) exist as dimers in the solid state due to intermolecular hydrogen-bonding interactions.
102 citations
TL;DR: In this paper, a series of bulky monodentate phosphoramidite ligands, based on biphenol, BINOL and TADDOL backbones, have been employed in the Pd-catalysed allylic alkylation reaction.
Abstract: A series of bulky monodentate phosphoramidite ligands, based on biphenol, BINOL and TADDOL backbones, have been employed in the Pd-catalysed allylic alkylation reaction. Reaction of disodium diethyl 2-methyl malonate with monosubstituted allylic substrates in the presence of palladium complexes of the phosphoramidite ligands proceeds smoothly at room temperature. The regioselectivities observed depend strongly on the leaving group and the geometry of the allylic starting compounds. Mono-coordination occurs when these ligands are ligated in [Pd(allyl)(X)] complexes (allyl=C3H5, 1-CH3C3H4, 1-C6H5C3H4, 1,3-(C6H5)2C3H3; X=Cl, OAc). The solid-state structure determined by X-ray diffraction of [Pd(C3H5)(1)(Cl)] reveals a non-symmetric coordination of the allyl moiety, caused by the stronger trans influence of the phosphoramidite ligand relative to X-. In all of these complexes, the syn,trans isomer is the major species present in solution. Because of fast isomerisation and high reactivity of the syn,cis complex, the major product formed upon alkylation is the linear product, especially for monosubstituted phenylallyl substrates in the presence of halide counterions. In the case of biphenol- and BINOL-based phosphoramidites, however, a strong memory effect is observed when 1-phenyl-2-propenyl acetate is employed as the substrate. In this case, nucleophilic attack competes effectively with the isomerisation of the transient cinnamylpalladium complexes. The asymmetric allylic alkylation of 1,3-diphenyl-2-propenyl acetate afforded the chiral product in up to 93 % ee. Substrates with smaller substituents gave lower enantioselectivities. The observed stereoselectivity is explained in terms of a preferential rotation mechanism, in which the product is formed by attack on one of the isomers of the intermediate [Pd[1,3-(C6H5)2C3H3](L)(OAc)] complex.
98 citations
TL;DR: In this article, a novel class of sterically demanding diphosphonites based on rigid back-bones is described, and the starting materials are all commercially available and the methodology allows for a modular approach.
Abstract: The synthesis of a novel class of sterically demanding diphosphonites 1 ± 8, based on rigid back- bones, is described. The starting materials are all com- mercially available and the methodology allows for a modular approach. All ligands have been fully charac- terized, including an X-ray crystal structure for com- pound 1, 4,5-bis{di((2-tert-butyl)phenyl)phosphoni- to}-9,9-dimethylxanthene. The coordination of these diphosphonite ligands towards Ni(II) and Ni(0) pre- cursors is investigated, bothby NMR spectroscopy as well as X-ray crystallography and compared with the behaviour of diphosphine ligands such as Xant- phos. The molecular structure for complex 9, trans- (NiBr2(1)) is described in detail. The nickel-catalyzed isomerization of 2-methyl-3-butenenitrile to 3-pen- tenenitrile is studied, a relevant step in the industrial- ly important hydrocyanation of butadiene (the Du- Pont adiponitrile process). Good activities and selec- tivities to the desired 3-pentenenitrile are obtained in this reversible CC bond activation reaction.
77 citations
TL;DR: A new, stable pentaorganosilicate of which different configurational isomers coexist in solution, and present thermodynamic and kinetic data on their interconversion is reported.
Abstract: Stable silicates with five carbon substituents are extremely rare. These pentacoordinate anions often have a trigonalbipyramidal geometry with three equatorial and two apical sites that should make them prone to configurational isomerism in which the substituent sites interchange. Such a process is known as the Berry pseudorotation mechanism. However, pseudorotamers other than the energetically preferred species have seldom been observed, even among the more accessible neutral phosphoranes. Insight into the thermodynamics of this type of isomerism is even more scarce, and nonexistent for the silicates. Here we report a new, stable pentaorganosilicate of which different configurational isomers coexist in solution, and present thermodynamic and kinetic data on their interconversion. The starting material bis(1-phenylpyrrole-2,2’-diyl)silane (1) was synthesized in 53% yield (white crystals from ethyl acetate; m.p. 276 8C (decomp)) from 2’-bromo-1-phenylpyrrole in diethyl ether by treatment with 2 equivalents of butyllithium (0 8C) and 0.5 equivalents SiCl4 (reflux). [5] Reaction of 1 with methyllithium in THFat 78 8C afforded a pale yellow solution of lithium silicate 2a (Scheme 1), as indicated by the upfield shift of the Si NMR signal from d= 35 to 131 ppm. The NMR spectra were recorded at 50 8C to minimize signal broadening (see below). Compound 2a was fully characterized by H NMR,C NMR, HMQC, and HMBC spectroscopic measurements. The silicate anion can potentially adopt three configurations I–III, which differ in the orientation of the bidentate substituents. The H and C NMR spectra revealed twofold symmetry, while the 2D NOESY spectrum showed correlations between H3’ and H3 as well as between H3’ and the methyl group. Only geometry I is compatible with these observations (Table 1). Furthermore, the JC2,Si value of 86 Hz is similar to that of a typical Si(sp ) C(sp) bond, while the JC2’,Si value of 30 Hz is much smaller than that of a Si(sp) C(sp) bond (64–70 Hz) and reflects the small silicon s character of the apical bonds.
54 citations
TL;DR: In this paper, the synthesis and X-ray crystal structure of a number of aryl−rhodium dihalide compounds with bis(imino)aryl type tridentate NCN ligands are reported.
42 citations
TL;DR: In this article, the reaction of the phenol-based ligand 4-methyl-2,6-bis{[(2-methylpyridyl)( 2-methylthiophenyl)amino]methyl}phenol(Hpy2th2s), containing pyridine and thiophene substituents, with copper(II) chloride and bromide yields two new dinuclear complexes with the composition [Cu2(py 2th 2s)X3], where X = Cl or Br.
Abstract: The reaction of the phenol-based ligand 4-methyl-2,6-bis{[(2-methylpyridyl)(2-methylthiophenyl)amino]methyl}phenol(Hpy2th2s), containing pyridine and thiophene substituents, with copper(II) chloride and bromide yields two new dinuclear complexes with the composition [Cu2(py2th2s)X3], where X = Cl or Br. In both complexes, the copper(II) ions are pentacoordinate and bridged by the deprotonated phenolate anion and by one halogen anion. Both complexes exhibit geometric asymmetry, as the coordination environment around one of the two copper ions is square-pyramidal, whereas the geometry around the other can be best described as a distorted trigonal bipyramid. The complexes were characterized by means of X-ray single-crystal diffraction, ligand field and EPR spectroscopy, mass spectrometry, and electrochemically. Magnetic susceptibility measurements indicate an antiferromagnetic coupling between the two metal centers (2J ≈ −200 cm−1). The interaction of the complexes with model substrates 3,5-di-tert-butylcatechol and tetrachlorocatechol is reported. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)
42 citations
TL;DR: The phenol-based compartmental ligand Hpy2ald as mentioned in this paper contains a tridentate amino arm and a weak donor aldehyde group at the 2 0 and 6 0 positions of the phenol ring, respectively.
32 citations
TL;DR: The reaction of zinc(II) acetate with 4-methyl-2-N-(2-pyridylmethyl)aminophenol (HPyramol) in methanol leads to a solution stable linear trinuclear zinc cluster with intramolecular ligand oxidation.
Abstract: The reaction of zincII acetate with 4-methyl-2-N-(2-pyridylmethyl)aminophenol (HPyramol) in methanol leads to a solution stable linear trinuclear zinc cluster with intramolecular ligand oxidation.
31 citations
TL;DR: These multi-ligand site compounds could be selectively metalated at their peripheries, which was shown by X-ray crystallography.
30 citations
TL;DR: In this paper, the structure of the [Ru(PCP)(tpy)]Cl complexes was determined by X-ray crystallography and the spectroscopic and electrochemical properties of these complexes were compared with those obtained for [Ru{C 6 H 3 (CH 2 NMe 2 ) 2 -2,6}(tpy](Cl) (3) containing the monoanionic bisaminoaryl ligand (NCN).
29 citations
TL;DR: In this article, the phenol-based ligand Hpy2ald containing formyl, amine and pyridine functions reacts with copper(II) nitrate, copper( II) bromide and manganese(II), chloride to yield complexes with unexpected structures.
Abstract: The phenol-based ligand Hpy2ald containing formyl, amine and pyridine functions reacts with copper(II) nitrate, copper(II) bromide and manganese(II) chloride to yield complexes with unexpected structures. In contrast to previously reported coordination compounds with similar ligands, the carbonyl group of the ligand in all complexes does not coordinate to the metal ions. The metal-to-ligand ratio is 2:1 in the case of the copper(II) nitrate complex and 1:1 in the case of the copper(II) bromide and manganese(II) chloride complexes. The copper(II) nitrate complex exhibits both donor-atom and coordination number asymmetry, as one copper ion is pentacoordinate with an N3O2 donor set, and the other one is hexacoordinate with an O6 donor set. In the latter two complexes, the phenol group of the ligand remains protonated and fails to bridge the two metal ions; instead it is semi-coordinated to only one metal ion. This results in the formation of mononuclear complexes in which copper(II) or manganese(II) ions have a distorted square-pyramidal surrounding. All complexes were structurally, spectroscopically and magnetically investigated. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)
TL;DR: Reaction of an in situ generated phosphinidene complex with the aromatic azulene and guaiazulene leads to unexpected 1,4-adducts of the seven-membered ring and to C--H bond insertion of the five- Membered ring.
Abstract: Reaction of an in situ generated phosphinidene complex [PhPW(CO)(5)] with the aromatic azulene and guaiazulene leads to unexpected 1,4-adducts of the seven-membered ring and to C--H bond insertion of the five-membered ring. A DFT analysis suggests that the reaction is initiated by formation of a eta(1)-complex between the phosphinidene and the five-membered ring of the aromatic substrate. Four conformations of this complex were identified. Two convert without barrier to the slightly more stable syn- and anti-1,2-adducts. These undergo pericyclic 1,7-sigmatropic rearrangements with remarkably low barriers to give 1,4-adducts, with an inverted configuration at the phosphorus center. An X-ray crystal structure is presented for one of the 1,4-adducts of guaiazulene. The other two eta(1)-complexes insert with modest barriers into a C--H bond of the five-membered ring.
TL;DR: In this article, the first X-ray crystal structure of stable trans-arylplatinum methyl complexes with imine type N ligands was reported, and the synthesis and the structure of the first stable transarylpl platinum methyl complexes [Pt(CH3)NCN] with n ligands were reported.
TL;DR: Walking rings: At room temperature, the P-R entity of a bicyclic phosphirane "walks" along the perimeter of the larger ring.
Abstract: Walking rings: At room temperature, the P-R entity of a bicyclic phosphirane (see picture) "walks" along the perimeter of the larger ring. Although this concerted rearrangement is similar to that of its hydrocarbon analogue, the additional 1,5-sigmatropic shift follows a different mechanism.
TL;DR: A highly strained, thermally stable (up to 150 degrees C) branched phospha[7]triangulane was synthesized from the second-generation bicyclopropylidene and transient phosphinidene, followed by demetalation in refluxing xylene.
Abstract: A highly strained, thermally stable (up to 150 °C) branched phospha[7]triangulane was synthesized from the second-generation bicyclopropylidene and transient phosphinidene [Ph−PW(CO)5], followed by demetalation in refluxing xylene. Bulkier transient CuCl−alkene-complexed phosphinidene gave 2-phosphabicyclo-[3.2.0]hept-1(5)-ene as an additional product. The “outer sphere” spirocyclopropanes provide a stabilizing factor for both of these novel compounds.
TL;DR: The 2,21:6',2"-terpyridine (tpy), 4'-chloro-2,2':6', 2"-tpy (4'-Cl-tpy) and 4'-tert-butyldimethylsilyl-ortho-carboranyl)-2, 2':6, 6, 2"-ter pyridine ligands were used for successful syntheses and characterisation of the corresponding Rh-I complexes with halide coligands as discussed by the authors.
Abstract: Rh-I-terpyridine complexes have been unambiguously formed for the first time. The 2,21:6',2"-terpyridine (tpy), 4'-chloro-2,2':6',2"-terpyridine (4'-Cl-tpy) and 4'-(tert-butyldimethylsilyl-ortho-carboranyl)-2,2':6',2"-terpyridine (carboranyl-tpy) ligands were used for successful syntheses and characterisation of the corresponding Rh-I complexes with halide coligands, [Rh(X)(4'-Y-terpyridine)] (X = Cl, Y = H, Cl, carboranyl; X = Br, Y = H). All four neutral Rh-tpy complexes are square planar, with Rh-X bonds in the plane of the 4'-Y-terpyridine ligands. Full characterisation of these dark blue, highly air-sensitive compounds was hampered by their poor solubility in various organic solvents. This is mainly due to the formation of pi-stacked aggregates, as evidenced by the crystal structure of [Rh(Cl)(tpy)]; in addition, [Rh(Cl)(carboranyl-tpy)] merely forms discrete dimers. The (bonding) properties of the novel Rh-I-terpyridine complexes have been studied with single-crystal X-ray diffraction, (time-dependent) density functional theoretical (DFT) calculations, far-infrared spectroscopy, electronic absorption spectroscopy and cyclic voltammetry. From DFT calculations, the HOMO of the studied Rh-I-terpyridine complexes involves predominantly the metal centre, while the LUMO resides on the terpyridine ligand. Absorption bands of the studied complexes in the visible region (400-900 nm) can be assigned to MLCT and MLCT/XLCT transitions. The relatively low oxidation potentials of [Rh(X)(tpy)] (X = Cl, Br) point to a high electron density on the metal centre. This makes the Rh-I-terpyridine complexes strongly nucleophilic and (potentially) highly reactive towards various (small) substrate molecules containing carbon-halide bonds.
TL;DR: Density functional theory calculations show a preferred bent geometry for the model complex [CpIr=PH], in contrast to the linear structure of [C pIr=NH], which arises because of the large steric congestion in [[Cp*Ir=PAr]2], which forces it toward a more reactive planar structure that is apt to rearrange.
Abstract: The properties of the 16-electron phosphinidene complex [CpRIr=PR] were investigated experimentally and theoretically. Density functional theory calculations show a preferred bent geometry for the model complex [CpIr=PH], in contrast to the linear structure of [CpIr=NH]. Dimerization to give [[CpIr=PH]2] and ligand addition to afford [Cp(L)Ir=PH] (L=PH3, CO) were calculated to give compounds that were energetically highly favorable, but which differed from the related imido complexes. Transient 16-electron phosphinidene complex [Cp*Ir=PAr] could not be detected experimentally. Dehydrohalogenation of [Cp*IrCl2(PH2Ar)] in CH2Cl2 at low temperatures resulted in the novel fused-ring systems 17 (Ar=Mes*) and 20 (Ar=Mes), with dimeric [[Cp*Ir=PAr]2] being the likely intermediate. Intramolecular C-H bond activation induced by steric factors is considered to be the driving force for the irreversible formation of 17 and 20. ONIOM calculations suggest this arises because of the large steric congestion in [[Cp*Ir=PAr]2], which forces it toward a more reactive planar structure that is apt to rearrange.
TL;DR: Density functional theory calculations indicate 6 c to result from a concerted disrotatory ring opening of an undetected tricyclic exo-syn phosphirane intermediate and the stabilizing effect of transition-metal coordination is discussed.
Abstract: Reaction of electrophilic phosphinidene complex [MePW(CO) 5 ] with 1,6-methano-[10]annulene results in the sole formation of the isomeric C-C insertion products 6c (main) and 6d (minor). The single-crystal X-ray structure of the complexed 1,7-methano-3-phospha[ll]annulene (6c) shows a syn-W(CO) 5 group at the exo bent phosphorus. The structure displays C-C bond alternation without bonding between the bridgehead carbon atoms. Density functional theory calculations indicate 6c to result from a concerted disrotatory ring opening of an undetected tricyclic exo-syn phosphirane intermediate. The endo-anti phosphirane cannot undergo ring expansion, due to the high barrier that is associated with an intramolecular antara-antara retro Diels-Alder reaction. The stabilizing effect of transition-metal coordination is discussed.
TL;DR: X-ray crystallographic studies and molecular weight determinations by cryoscopy in benzene showed that, both in the solid state and in apolar, noncoordinating solvents such as benzene, these compounds exist as discrete dimeric aggregates.
Abstract: Aryllithium compounds [LiC6H4(CH2N(Et)CH2CH2NEt2)-2]2 (2b), [LiC6H4(CH(Me)N(Me)CH2CH2NMe2-(R))-2]2 ((R)-3b), and [LiC6H4(CH(Me)N(Me)CH2CH2NMe2-(rac))-2]2 ((rac)-3b) were synthesized and characterized in the solid state and in solution. X-ray crystallographic studies of 2b and (R)-3b and molecular weight determinations of 2b, (R)-3b, and (rac)-3b by cryoscopy in benzene showed that, both in the solid state and in apolar, noncoordinating solvents such as benzene, these compounds exist as discrete dimeric aggregates. For (R)-3b and (rac)-3b the aggregation process of two monomeric aryllithium units to one dimer is highly diastereoselective.
Journal Article•
TL;DR: The structure, spectroscopy, and magnetism of a century-old copper salt, Cu(OAc)(OMe), is reported, which contains two independent Cu(II) ions, which are both five-coordinated and which are bridged by methoxo and acetate anions to form an infinite 2D network.
TL;DR: The three-component condensation between an amine, an aldehyde, and an alpha-acidic isocyanide efficiently provides substituted 2-imidazolines in a one-pot reaction under mild conditions as discussed by the authors.
Abstract: [reaction: see text] The three-component condensation between an amine, an aldehyde, and an alpha-acidic isocyanide efficiently provides substituted 2-imidazolines in a one-pot reaction under mild conditions.
TL;DR: Re-investigation of the crystal structure as twinned by merohedry in the trigonal centrosymmetric space group P-3, with a twofold rotation about [001] as twin law, resulted in a well ordered structure and low R values.
Abstract: The crystal structure of cyclohexanone oxime, C6H11NO, was reported as severely disordered in the trigonal non-centrosymmetric space group P3 [Olivato, Ribeiro, Zukerman-Schpector & Bombieri (2001). Acta Cryst. B57, 705–713]. Re-investigation of the crystal structure as twinned by merohedry in the trigonal centrosymmetric space group P\overline3, with a twofold rotation about [001] as twin law, resulted in a well ordered structure and low R values. The asymmetric unit contains three independent molecules, existing as a hydrogen-bonded trimer, having an R_3^3(9) graph set.
Journal Article•
TL;DR: The observed stereoselectivity is explained in terms of a preferential rotation mechanism, in which the product is formed by attack on one of the isomers of the intermediate [Pd[1,3-(C6H5)2C3H3](L)(OAc)] complex.
Abstract: A series of bulky monodentate phosphoramidite ligands, based on biphenol, BINOL and TADDOL backbones, have been employed in the Pd-catalysed allylic alkylation reaction. Reaction of disodium diethyl 2-methyl malonate with monosubstituted allylic substrates in the presence of palladium complexes of the phosphoramidite ligands proceeds smoothly at room temperature. The regioselectivities observed depend strongly on the leaving group and the geometry of the allylic starting compounds. Mono-coordination occurs when these ligands are ligated in [Pd(allyl)(X)] complexes (allyl=C3H5, 1-CH3C3H4, 1-C6H5C3H4, 1,3-(C6H5)2C3H3; X=Cl, OAc). The solid-state structure determined by X-ray diffraction of [Pd(C3H5)(1)(Cl)] reveals a non-symmetric coordination of the allyl moiety, caused by the stronger trans influence of the phosphoramidite ligand relative to X-. In all of these complexes, the syn,trans isomer is the major species present in solution. Because of fast isomerisation and high reactivity of the syn,cis complex, the major product formed upon alkylation is the linear product, especially for monosubstituted phenylallyl substrates in the presence of halide counterions. In the case of biphenol- and BINOL-based phosphoramidites, however, a strong memory effect is observed when 1-phenyl-2-propenyl acetate is employed as the substrate. In this case, nucleophilic attack competes effectively with the isomerisation of the transient cinnamylpalladium complexes. The asymmetric allylic alkylation of 1,3-diphenyl-2-propenyl acetate afforded the chiral product in up to 93 % ee. Substrates with smaller substituents gave lower enantioselectivities. The observed stereoselectivity is explained in terms of a preferential rotation mechanism, in which the product is formed by attack on one of the isomers of the intermediate [Pd[1,3-(C6H5)2C3H3](L)(OAc)] complex.
TL;DR: In this paper, the interaction between the title compound bis(coronand) 8 and diarylmagnesium compounds Ph2Mg and (p-tBuC6H4) 2Mg in diethyl ether leads to the formation of 1:2 complexes, irrespective of the initial ratio of the components.
Abstract: The interaction between the title compound, bis(coronand) 8,and the diarylmagnesium compounds Ph2Mg and (p-tBuC6H4)2Mg in diethyl ether leads to the formation of 1:2 complexes (9 and 10, respectively), irrespective of the initial ratio of the components. In [D8]toluene, the three complex types can be discerned by 1H NMR spectroscopy: double side-on (9a, 10a), side-on/rotaxane (9b, 10b), and, to a very minor extent, double rotaxane (9c, 10c). In the case of 10a and 10b, the temperature dependence showed that rotaxane formation is enthalpically favored at the expense of a more negative entropy. As crystals suitable for structure determination could not be obtained from 9 and 10, the interaction of 8 with the sterically less demanding Hg(SCN)2 was studied. In this case, analogous pseudorotaxanes are formed exclusively, and both the 1:2 complex [8·{Hg(SCN)2}2] (11) and its 1:1 analogue 12 were observed. The double rotaxane structure of 11 was confirmed by X-ray crystal structure determination. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)
TL;DR: In this paper, the effect of steric bulk on the vanadium coordination geometry in O,N-chelated vanadium oxo (bis)phenol- ates was studied.
Abstract: In order to study the effect of steric bulk on the vanadium coordination geometry in O,N-chelated vanadium oxo (bis)phenol- ates, six different ortho-aminophenolate ligands have been used. The ortho-aminophenolate system was changed at three different places, i.e. 1) the second ortho position (C 6 ) of the arene ring (R), 2) the substituents at the amino nitrogen (R and R), and 3) the benzylic carbon atom (R * ). The phenols were used in the pre- paration of the vanadium oxo (bis)phenolate complexes. In order to study whether it is possible to predict geometrical features of
TL;DR: The title compound, rac-(R,R)-N,N'-bis(1-hydroxy-3-methyl-2-butyl)oxalamide, crystallizes as a non-merohedral twin in the triclinic space group P1.
Abstract: The title compound, rac-(R,R)-N,N'-bis(1-hydroxy-3-methyl-2-butyl)oxalamide, C 12 H 24 N 2 O 4 , crystallizes as a non-merohedral twin in the triclinic space group P1. The twin is generated by a twofold rotation about c*. The terminal hydroxy groups of molecules related by an inversion center form hydrogen-bonded dimers. This hydrogen-bonding pattern is further extended into a one-dimensional chain by N-H...O hydrogen bonds.