摘要:
J . CHEM. SOC. DALTON TRANS. 1992 329 1Macrocyclic Ligands with Pendant Phosphonic Acid Groups *William Clegg, Peter B. lveson and Joyce C. LockhartDepartment of Chemistry, University of Newcastle upon Tyne, NE7 7RU, UKSmall aza- and oxaaza-macrocycles bearing two aliphatic nitrogens have been derivatised with pendantmethylenephosphonates. Multinuclear NMR studies (31P and 'H) have elicited the protonation behaviourof one of these macrocycles. Improved methods of separation and purification of the ligands are discussed.The crystal structure of the parent 1,4,7-triazacyclononane-l,4,7-triyltris(methylenephosphonic acid)has been determined, indicating that two of the nitrogens are protonated under the conditions ofcrystallisation. Extensive hydrogen- bonding networks are evident in the structure.The CambridgeStructural Database has been used t o survey structures based on 1,4,7-triazacycIononane and otherami no (a I ky I p hosp honates) , am i no (a1 ky I p hosp h i nates), and amino (a1 ka necar boxy lates) and their metalcomplexes, for comparison.Amines and ammonia may be converted into aminomethyl-phosphonic acids using the reaction with formaldehyde andphosphorous acid [see equation (l), the Moedritzer-IraniRNH, + 2H3PO3 + 2 C H 2 O eRN(CH2P03H2), + 2H20 (1)reaction '1. The ligands thus obtained provide a plentiful seriesof new co-ordination compounds, interesting stereochemistryand a wide range of commercial applications.2 The importanceof the ligands as phosphorus analogues of the amino acids canscarcely be exaggerated.They have also been extensively used aspseudo-substrates in biological studies, a recent example beingthe use of a set with planned variation, to probe the ditopicanion-binding site implicated in the iron uptake of tran~ferrin.~The ligands are moderately acidic and have been widely used indescaling. and to prevent the build-up of scale. Characterisationhas usually been in solution, by titrimetric and by NMRmethods, while few crystal structures of free acids or of saltshave been ~ b t a i n e d . ~ A natural extension of this range ofaminophosphonate ligands is to the smaller macrocyclic aminessuch as 1.4,7-triazacyclononane la, the special co-ordinatingproperties of which have been widely exploited in complexationof d-block metals which prefer nitrogen donors.Thetriphosphonic acid ligand l b derived from la was first made byPolikarpov ut ul.' who obtained crystal structures of the iron(1rr)and copper(1r) ~omplexes.~.~ A number of workers interested inco-ordinating the larger lanthanide ions with a view to theproduction of NMR imaging reagents, have similarlysynthesised the larger tetraazacyclododecane derivative 2a andrelated tetraaza macro cycle^.^ Parker and co-workers havealso made a range of phosphinic acid derivatives of smallpolyaza macrocycles, many with potential applications inbioimaging and nuclear medicine. We report the derivatisationof two small macrocycles 3b and 4b containing two aliphaticnitrogens, together with some ether links.The ligands havebeen investigated mostly by NMR methods, to elucidate theprotonation behaviour, and by the production of a leadcomplex of 4c. The crystal structure obtained in the presentwork for l b has been compared with information on thegeneral ligand type, extracted from the Cambridge StructuralDatabase.* Suppk~nwti~cir?, dutu ucuiluhle: see Instructions for Authors, J. Chem.SOL.., Dulfon Truns., 1992, Issue 1, pp. xx-xxv.Rl a Hl b CH2P03H2R2a H2b CH2P03H2RExperimentalPreparation of Macrocyclic Amino(methy1phosphonic acids).-General method. The di tosylated glycols were synt hesised usinga modification ' of an established method.' Ditosylatedethylenediamine was synthesised using a published procedure.'The purity of all tosylates was checked by TLC analysis (onKieselgel 60 F254 plates, run in MeOH-CH,C12 mixtures) andby comparison of the melting points with literature values.Thecyclisation reactions were carried out according to the originalmethod of Richman and Atkins. l 4 The 'H NMR data for th3292 J. CHEM. SOC. DALTON TRANS. 1992Table I Proton NMR of macrocyclic tosylates, amines and methylphosphonates"Ligand OCH,CH,O OCH,CH2Nb OCH,CH,N* NCH,CH,N CH,P3a 3.89 (4 H, N 10.0) 3.25 (4 H) 3.45 (4 H, s)4a 3.50 (4 H, s) 3.63 (4 H, N 8.5) 3.26 (4 H) 3.50 (4 H, s)3b 3.89 (4 H, N 10.2) 3.03 (4 H) 3.07 (4 H, s)4b 3.95 (4 H, s) 4.07 (4 H, N 10.0) 3.64 (4 H) 3.84 (4 H, s)3c 3.97 (4 H, N 12.0) 3.57 (4 H) 3.73 (4 H, s) 3.41 [4 H, d, J(PH) 12.014c 3.87 (8 H, m) 3.39 [4 H, d, J( PH) 12.633" 200 MHz spectra run in CDCI, (3a, 4a), CD,OD (3b, 4b) or D,O (pD zz 1.6) (k, 4c).These signals for an AA'BB' system are apparent triplets fromwhich only N = J + J' (the sum of the averaged vicinaf couplings) can be extracted, J , N in Hz.3.6 (8 H, br)tosylated macrocycles, derived amines and correspondingmethylphosphonates are shown in Table 1.Prepurutions. - 4 7 - Ditosyf- 1 -oxa-4,7-diazacyclononane 3a.The ditosylate was obtained in 42% yield after recrystallisationfrom hot ethanol, m.p. 196198 "C (from EtOH) 199-201,194-195 "C (Found:C, 54.9; H, 5.9; N,6.3%;(M + H+)439[fast atom bombardment (FAB)]. Calc. for C20H24N205S2:C, 54.8; H, 6.0; N, 6.4%; M 438). GC(CDC1,) 21.2 (m,C&4)48.0, 48.7 (OCH2CH2N, NCH2CH,N), 73.8 (OCH2CH2N)and 124.3, 129.0 (CH,C,H,) (TLC: single spot, 1.5%MeOH-CH2C12).Mass spectrum (FAB): m/z 283 ( M - CH,-7,lO- Ditosyf- 1,4-dio.uu-7,lO-diazac~clododecane 4a. The pro-duct was isolated in a similar manner to the N 2 0 derivative 3a.Yield = 23%, m.p. 2 16.5-21 7.5 "C [from dimethylformamide(dmf)-water] (lit.," 223-224 "C).Tosyl groups were removed using a modification of apublished method,' detailed for the preparation of 3b below.1 -O.ua-4,7-~iarac:,~cf~nonune 3b. A sodium amalgam wasprepared by carefully adding Na (1.8 g, in small pieces) to Hg( 1 10 g) under nitrogen. A slurry containing the ditosylate 3a(3.97 g, 9.06 mmol) in absolute ethanol (500 cm3) was thenadded to the amalgam. The stirred reaction mixture was heatedto reflux and left for 24 h.After this time, a TLC in 1.5%MeOH-CH2C12 indicated that there was only a small amountof ditosylate starting material left. The product was isolated byadding water (100 cm3) and then extracting with CHCl,(3 x 500 cm3). The yellow oil obtained after removal of theCHCl, was then dissolved in propan-2-01. Most of theremaining tosylated starting material precipitated immediatelyfrom the alcoholic solution. A small amount of concentratedHCl was added to the solution but the dihydrochloride couldnot be precipitated as a solid. The propan-2-01 was stripped offand the brown oil was then used without further purification forthe preparation of 3c. The NMR data for the free amine 3b inTable 1 refer to a pure sample of the amine which was isolatedas an oil using a Kugelrohr (Buchi GKR-51) at 150°C(lo-' mmHg, 2 13.3 Pa).1,4- Dioxct-7,lO-diazacycfononane 4b.The reaction conditionswere similar to those used for 3b. No further purification wasnecessary after extraction with CHCI, (yield = 66%).phonic acid) 3c. Phosphorous acid (0.49 g, 6 mmol) and thedihydrochloride of 3b (0.73 g, 3.596 mmol) were initiallydissolved in distilled water (5 cm3). After slow addition of37% wjv HCI ( 5 cm3) the temperature was raised to reflux(=llO"C) and 37% wjv aqueous formaldehyde (1 cm3,0.012 mol) was added dropwise to the stirred solution over30 min. The reaction was then continued for a further 3 h.The HCI-H20 solvent mixture was concentrated almost todryness and then a small amount of ethanol was layered overthe oil.The product crystallised overnight and was eventuallypurified in very low yield after a further crystallisation fromethanol-water ( ~ 9 : 1) and two recrystallisations from water-propan-2-01 (= 1 : l), m.p. 258-260 "C (from water-propan-2-01)(Found: C, 27.2; H, 6.5; N, 8.0. C8H20N207P2-2H20 requiresC6H,SO2)+.1 - O.un-4,7-diazacyclononane-4,7-diylbis(methylenephos-C, 27.1; H, 6.8; N, 7.9%). 6c (50 MHz, solvent DzO, pD = 1.6)53.66 (OCH,CH,N), 55.0 (d, 'Jcp 138.6 Hz, CH,P), 55.82(NCH2CH2N) and 66.66 (OCH,CH,N). 6p (121.495 MHz,solvent D20, pD z 1.6) 16.25 (t, 2JpH 11.3 Hz*).1,4- Dioxa- 7,1O-diazacycfododecane-7,1O-diylbis(methylene-phosphonic acid) 4c. The product made by the general methoddescribed was found to be soluble in water, methanol andethanol and insoluble in propan-2-01, but could not becrystallised from mixtures of these solvents.A lead salt wasprepared from the crude product in the following manner: 0.81 gof oil from the crude reaction mixture was dissolved in water(25 cm3). It was estimated that %0.5 g of the oil was in the formof the disubstituted product (60% by weight). The pH of thesolution was raised to 6.5 with NaOH ( 5 rnol dm-,). A solutionof Pb(N03)2 (0.91 g, 2.76 mmol) in distilled water (25 cm3) wasthen added dropwise to the stirred ligand solution. The lead saltof 4c precipitated at ca. 5 "C after the volume of the solution hadbeen reduced to 10 cm3. The precipitate was filtered off andwashed with ethanol, affording a white solid (0.65 g, 29%).Thebest fit for the elemental analysis was obtained for the presenceof half a nitrate ion and three Pb atoms per disubstituted ligand(Found: C, 11.6; H, 1.9; N, 3.2. CloH22N208P2Pb3-0.5N03requires C, 11.9; H, 2.2; N, 3.5%). The lead was removed usingthe hydrogen sulfide technique of Rajan et af.,' and the product4c was isolated in low yield after three recrystallisations fromwater-propan-2-01 (z 1 : 2), m.p. 210-214 "C (from waterpropan-2-01) (Found: C, 29.5; H, 6.4; N, 6.6. CIoH24N208P2*2.5H20 requires C, 29.5; H, 7.2; N, 6.9%). 6,(121.495 MHz,D20, pD = 1.6) 8.52 (t). See Table 1 for 'H NMR data.NMR Studies.-NMR spectra were run on Briiker WB300and AC200 instruments with 'H, I3C and ,'P nuclei.Table 1contains the 'H NMR data. For studies of the protonationbehaviour of ligands, standard solutions at different pD valueswere prepared containing the ligand ( ~ 0 . 0 3 4 rnol dm-,),KCI (1 rnol drn-,) and varying amounts of KOH (0.1 mol drn-,)in D20. The sample at pD 0.1 was obtained using dilute DCIinstead of KOH in D20. The final pD values were correctedfor a deuterium isotope effect by using the equation pD =pH + 0.4.19Structure of 1,4,7- Triazacyclononane- 1,4,7-triyftris(methyl-enephosphonic acid) Monohydrate 1 b.-A crystal suitable forthe structure determination was obtained on slow cooling of asaturated aqueous solution of the ligand at 70 "C and pH = 1.2.Crystal data. C9H2,N,O9P,-H,O, M , = 429.2, ortho-rhombic, s ace group Pccn, a = 10.537(1), b = 27.464(3), c =11.989(1) 1, U = 3469.5 A3, Z = 8, D, = 1.643 g cm-,, h(Mo-Ka) = 0.710 73 A, p = 0.39 mm-', F(000) = 1808, T = 240 K.Data collection and treatment.Colourless crystal, size0.40 x 0.40 x 0.25 mm, on glass fibre, Stoe-Siemens diffracto-meter, unit-cell parameters from 20 values of 32 reflections(20-25") measured at +a. Data collection in 6 ~ 0 scan modewith on-line profile fitting2' 20,,, 50°, index ranges, h 0-12,* J could not be determined at pD 1.6; this value refers to pD 3.0J. CHEM. SOC. DALTON TRANS. 1992 3293Table 2 Atomic coordinates for ligand l bAtom Y Y ZN(1) 0.007 29( 14) 0.442 10( 5) 0.677 3 1 (1 1)N(2) 0.165 02(15) 0.365 55(5) 0.619 86(11)N(3) -0.086 92(15) 0.346 16(6) 0.556 15(13)C(11) -0.041 OO(17) 0.493 52(6) 0.677 97(14)C( 12) 0.126 86( 17) 0.435 34(7) 0.744 38( 15)C(13) 0.176 OO(18) 0.383 22(6) 0.734 22(14)C(21) 0.263 57( 18) 0.382 7q7) 0.542 78( 15)C(22) 0.127 09(19) 0.314 70(7) 0.608 lO(15)C(23) -0.015 12(20) 0.309 02(6) 0.625 89(15)C(31) -0.170 90(18) 0.324 23(7) 0.467 86(17)C(32) -0.166 21(17) 0.380 92(7) 0.624 63(15)C(33) -0.095 99(18) 0.407 51(6) 0.716 29(15)P(1) 0.069 34(4) 0.539 81(2) 0.622 87(4)P(2) 0.424 42(4) 0.359 97(2) 0.561 64(4)P(3) - 0.090 22(5) 0.285 63(2) 0.365 17(4)O(11) -0.009 92(14) 0.574 00(5) 0.554 77(11)O(12) 0.123 37(13) 0.565 96(5) 0.73003(12)O(13) 0.177 47(14) 0.514 83(5) 0.565 56(11)O(21)’ 0.469 36(15) 0.371 96(6) 0.677 81(12)O(22)” 0.507 20(15) 0.382 61(5) 0.469 78(12)O(23)‘ 0.415 16(16) 0.304 61(6) 0.543 98(16)O(24j” 0.482 9(14) 0.345 7(7) 0.457 3(10)O(25j” 0.4106(13) 0.311 3(4) 0.643 5(9)0(26)* 0.482 9(16) 0.401 4(5) 0.620 5(13)O(31 1 0.034 88(13) 0.312 54(6) 0.338 18(13)O(32) -0.178 Ol(14) 0.285 40(4) 0.263 73(10)O(33) -0.067 49(15) 0.236 70(5) 0.414 41(12)O(4) 0.420 69(25) 0.530 89(10) 0.532 25(29)Minor disordercomponent, occupancy 0.082(2), ignored in the discussion of thestructure.Major disorder component, occupancy 0.908(2).k 0-32, 1 0-14, together with some equivalent reflections,correction for approximately 6% decay in intensities of threestandard reflections, no absorption or extinction corrections,3831 reflections measured, 3065 unique, 2596 with F > 4o,(F)(oc from counting statistics only), Rint = 0.026.Structure determination.Solution by direct methods anddifference syntheses, blocked-cascade least-squares refine-ment on F, weighting MY-’ = 0 2 ( F ) = oC2(F) + 59 - 71G +227G2 - 155H + IOOH2 + 70GH [G = Fo/Fm,,, H = sine/sin€JmaXJ,’ anisotropic thermal parameters for all non-H atoms,H atoms constrained [C-H 0.96 A, H-C-H 109.5”, N-H and0-H soft-restrained to 0.9 A, U(H) = 1.2Ueq(C), H atomsbonded to 0 and N with freely refined thermal parameters].Two-fold disorder was resolved and refined for one of thephosphonic acid groups. R = 0.0397, R’ = (Z:wA’/CwFo2)* =0.0393, S = 1.34 for 284 parameters, mean A/o = 0.14, max.Ajo = 0.93, ( A P ) , , , ~ ~ = 0.75, (AP),,,~,, = -0.87 e k3. Scatteringfactors were taken from ref.22. SHELXTL23 and localcomputer programs were used. The final fractional atomiccoordinates are given in Table 2. The structure of ligand l b isshown in Fig. 1 , with its numbering system.Modelling Cak-ulutions.-The CHARMm/QUANTA pro-grams, version 3.2.3, were used for geometric calculationsas implemented on Silicon Graphics Personal Iris 4D/20G.Crystal structure coordinates were obtained from the SERCCDS System.Results and DiscussionLigands 3c and 4c are the first amino(alky1phosphonic acids) tobe described for small-ring oxaza macrocycles; after extensiveresearch they have been obtained analytically pure, and theknown ligand 1 b, prepared as part of the work, was obtained insuitable crystalline form for X-ray structure analysis.The protonation sequence of ligand 3c was followed byFig.1 Structure of ligand lb, with the labelling of the non-hydrogenatoms. Bonds in the macrocycle ring are shown filled, others hollow.The minor component of disorder is not shownstudying the variation of 31P and ‘H NMR chemical shifts withpD. At pD 0.1 a broad signal appeared in the 31P spectrum. Asthe pD was raised, the resolution increased and a sharp tripletwas observed at pD 2.50.* The sharp signal was observed for theremainder of the titration (up to pD 13.1). It is not clear why theligand signals are broader at low pD: possibly this indicates apD-dependent relaxation of the NCH, protons, consequent onN deuteron exchange. A similar phenomenon has been observedin a series of triaza macrocyclic tricarboxylate~.~~ In this casethe broadening, which was only present in the asymmetricligands studied, was attributed to a slow rate of nitrogeninversion which reduced the rate of interconversion between thevarious conformers.Geraldes et ~ 1 . ~ ~ suggested that the slowrate of nitrogen inversion may have been due to intramolecularhydrogen bonding between protonated nitrogens and non-protonated carboxylates or vice versa. The line broadeningobserved by Geraldes et al. was observed in the ‘H and the I3Cspectra while in this study, the line broadening was onlyobserved in the 31P spectra.Fig. 2 shows a plot of chemical shift (6,) us. pD, for this ligand.There are two large upfield shifts in the 31P NMR titration.The first of these, from pD 13-10, is probably due to thedeuteriation of the first nitrogen atom in the completelydeprotonated ligand. This large shift (x 5 ppm) may in part bedue to complexation of K f (the counter ion present) within themacrocyclic cavity.A similarly large shift ( x 3 ppm) wasobserved for the first protonation of 1,4,7-triazacyclononane-1,4,7-triyltris(methylenephosphonic acid) 1 b using sodiumhydroxide as titrant, but when the titration was repeatedusing tetramethylammonium hydroxide instead the change inchemical shift (6,) was reduced to 0.7 The upfield shift(which has been found to decrease as the number of CH, groupsbetween the phosphorus and nitrogen atoms was increased 26)has recently been interpreted in terms of the formation ofan intramolecular hydrogen bond between the protonatednitrogen atom and an unprotonated phosphonateA strong hydrogen bond is hypothesised when there is onemethylene group between the phosphorus and nitrogen atomsand a five-membered ring is formed.The strength of theintramolecular hydrogen bond is proposed to decrease as thenumber of CH, groups increases and the size of the ring is alsoincreased.The 31P NMR titration curve shown in Fig. 2 is verysimilar to that obtained for ligand l b and its symmetrical*The presence of only one sharp triplet, from the CH,P coupling,indicates that there is a rapid exchange between the various protonatedspecies in solution3294 J. CHEM. SOC. DALTON TRANS. 19920 2 4 6 8 1 0 1 2 1 4NMR titration curve.Plot of 31P chemical shift of a solution ofPDFig. 2compound 3c in D,O as pD is variedtriazacyclododecane analogue 5, for which it was concludedthat two nitrogen atoms were deuteriated at high pD.25 There issome discrepancy in the numerical values of the pK, obtained byGeraldes er af.25 and those reported by Delgado er aLZ7 Thegeneral interpretation is not in doubt, and is applicable here.As the pD is lowered, the NMR titration curve of ligand 3cindicates that the phosphorus atoms are deshielded in twodistinct stages. These shifts are much smaller (0.7 and 1 ppm)and in the opposite direction to that which was observed forthe deuteriation of the first nitrogen atom. It is likely thatthese shifts are associated with the deuteriation of one oxygenatom in each of the phosphonate groups.The final largeupfield shift (3.6 ppm) is consistent with the deuteriation ofthe second nitrogen atom. Substantiation of these assignmentsis given by the ‘H NMR titration curves of the macrocyclicand methylphosphonate CH2 protons which are shown inFig. 3.Deuteriation of the nitrogen atoms results in large downfieldshifts of the methylene protons in the macrocyclic ring which areadjacent to the ring nitrogen atoms (see labelling of 3c in Fig. 3).During the remainder of the titration (pD 10-3), these protonshifts are relatively unaffected while the shifts of the CH2Pgroups continue to increase as the protons are deshielded by thedeuteriation of the phosphonate oxygen atoms.The methyleneprotons adjacent to the ether oxygen atom in the macrocyclicring exhibit much smaller shifts on deuteriation of the nitrogens,and appear to be unaffected by deuteriation of the phosphonateoxygen atoms.Although the deuteriation sequence seems relatively straight-forward in this case, it must be stressed that the aboveassignments should remain tentative in the absence of amore thorough study involving the determination of accurateprotonation/deuteriation constants and the degree of deuteri-ation of the nitrogen and oxygen sites. The tentative nature ofthe assignments can be illustrated by studying the last threedeuteriations in the 31P titration curve of ligand 3c. Thephosphonate and nitrogen deuteriations would be expected todeshield and shield respectively the phosphorus nucleus.Although part of the assignment was based on this ‘rule’, it doesnot necessarily always hold true.Several possible electroniceffects, potentially of opposite signs, might contribute to the 31Pshifts. Although the protonation of a phosphonate groupis likely to deshield the phosphorus atom in the samephosphonate group, it may have the opposite effect on thephosphorus atom in the other phosphonate group. When theprotonation equilibria are fast, the observed chemical shift is aweighted average for the two phosphorus atoms and as a resultthe phosphorus signal may appear to be shielded or deshieldedin the corresponding 3’ P spectrum.I cg3.63.43.23.02.8I i 0 2 4 6 8 1 0 1 2 1 4PDFig.3 Plot of identifiable ‘H NMR chemical shifts of a solution ofcompound 3c in D,O as pD is varied: labels on graph correspond tolabelling scheme shownThe method of Sudmeier and Reilley 28 was originally used tocalculate the degree of protonation of oxygen and nitrogen sitesin open-chain polyamine and aminocarboxylate ligands and inmodifications has since been applied to macrocyclic a m i n e ~ , ~ ~aminocarbo~ylates,~~ and aminoalkylphosphonates.25 In thismethod, the changes in chemical shift of the methylene protonsconsequent on the basic centres becoming fully protonated arereferred to as shielding constants. The Sudmeier and Reilleymethod requires protonation constants, chemical shift valuesfor the non-labile methylene protons and accurate shieldingconstants for the calculation of the degree of protonationof phosphonate oxygen and nitrogen sites.In linear amino-carboxylates the shift effects of protonation at the basic sites areassumed to be additive and a pH-independent set of shieldingconstants may be obtained.28 The chemical shift effects are notadditive in macrocyclic ligands. This is the result of pH-dependent conformational changes in the macrocycles whichvary the average relative orientations throughout the pH range.The shielding ‘constants’ derived have been found to be verydependent on the protonation stage, macrocyclic ring size andtype of proton considered within each molecule. In this study,the ‘H and 31P NMR chemical shift data obtained during theprotonation of ligand 3c indicated that the first protonation wasalso associated exclusively with the nitrogen sites.Thesymmetrical nature of the ligand suggests that both nitrogensites are protonated equally (on average) during the firstprotonation.X-Ray Structure of Ligand 1 b.--The crystal structure analysisof ligand l b is shown in Fig. 1. Bond lengths and angles aregiven in Table 3. A sufficient number of related structures isavailable for a detailed analysis to be made of certain generalfeatures of the amino(alky1phosphonic acid) ligands in thecrystalline state, which are also compared with the details of thecurrent structure lb. An outstanding feature of the structurepresented in Fig. 1, and one which correlates well with the NMRtitration data just discussed, is that two of the three nitrogens“(1) and N(3)] are protonated (c-6 the solution NMR data 2 5 )partly balanced by the loss of a proton from the phosphonicacid group connected to the unprotonated N(2) atom.Database Studies.-In view of the importance of the amino-(alkylphosphonic acid) ligands as substitutes for amino acids,as enzyme substrates, and as a coherent set of anions withplannable variability in biochemical investigation^,^ a study ofsignificant similarities and differences in the structures of eachwas undertaken.A search indicated that 38 aminoalkyl-phosphonic/phosphinic acid structures (with an N-C-P linkJ. CHEM. SOC. DALTON TRANS. 1992 3295[amino(alkylphosphonic acid)] (A), 4 with an N-C-C-P link(B), and 2 with an N-C-C-C-P link (C) could be accessed in theliterature.Of type A, 22 are free phosphonate ligands, 13 aresalts or metal coixplexes of phosphonates, there is onephosphinate ligand and two phosphinate complexes. All oftypes B and C are free phosphonate ligands. Many structureshave been solved of aminoalkyl carboxylates and their metalcomplexes, but only the relevant few will be discussed in thispaper. The search was conducted using the SERC ChemicalDatabank System (CDS) (January 1992 update), which accessesthe Cambridge Structural Databaseof organic and organometal-lic crystal structure^.^^-^' The CSSR program within CDS 30was used for retrieval of the structures, with the FPROBEcommand. Structures with the e.s.d. flag set to 3 (indicatingrelatively poor precision) or with R > 0.08 were rejected innumerical averaging to be described in this paper.Of theselected structures, 23% have R values between 0.05 and 0.08,and 3404 have the e.s.d. flag set to 2. Mean bond lengths forC-N, C-P, P-0, P-O(H), P=O and anglcs C-P-0, C-P-O(H),Table 3 Bond lengths (A) and angles (") in ligand IbN( 1)-C( I I )N(2)-C(33)N( 2)-C( 2 I )N(3)-C( 23)N (3)-C( 3 2)C( 12)-C( 13)C( 22)-C( 23 )C( 3 2)-C( 3 3 )P( I )-O( 12)P( 2)-O( 2 1 )P(2)-O( 23)P(2)-O(25)P( 3)-O( 3 1 )P( 3)-O( 33 )C( I 1 )-N( I )-C( 12)C( I2 j N ( 1 )-C( 33)C( 13)-N( 2)-C( 22)C(23)-N(3)-C(3 1)C( 3 1 )-N( 3)-C(32)N( 1 )-C( I3)-C( 13)N(2)-C(2 1 )-P( 2)N( 3)-C(23)-C( 22)N( 3)-C( 32)-C( 33)C( 1 1)-P( l)-O( 11)O( 1 1 )-P( 1 )-O( 12)O( 1 1 )-P( 1 )-O( 1 3)C(2I)-P(2)-0(21)O(2 1)-P(2)-0(22)O(2 l)-P(2)-0(23)C(2 1 )-P( 2)-O( 24)0(24)-P( 2)-O( 25)0(24)-P(3)-0(26)C( 3 1 )-P( 3 )-O( 3 1 )O( 3 1 )-P( 3)-O( 32)0 ( 3 1 )-P( 3)-O( 33)1 S O 1 (2)1.5 18(2)1.468(2)1.520(2)1.51 l(2)1.527(3)1.522(3)1.5 1 3(3)1.578(2)1.507(2)1.538(2)1.664( 1 I )1.546(2)1.487(2)1 13.4( 1)11 5.5( 1)114.3( 1)108.0( 1)110.9(1)11 1.0( 1)118.1(1)110.1(1)11 1.1(1)I 15.4(2)1 06.0( 1 )117.5( 1)109.4(1)113.3( 1)11 1.3( 1)1 12.4(6)109.2(8)1 16.4(9)105.0( 1)I 10.6( 1)1 12.2( 1)N( 1)-C( 12)N(2)-C( 13)N( 2)-C( 22)N(3)-C(31)C( 1 1 )-P( 1 )C( 2 1 )-P( 2)C(3 1 )-P( 3)P( 1)-O( 1 1)P(1)-O(13)P(2)-O(22)P( 2)-O( 24)P(2)-O( 26)P(3)-O(32)C( 1 1)-N( 1)-C(33)C( 13)-N(2)-C(21)C(2 1 )-N( 2)-C( 22)C(23)-N(3)-C(32)N( 1)-C( 1 1)-P( 1)N(2)-C( 1 3)-C( 12)N(2)-C(22)-C(23)N(3)-C(3 1 )-P(3)C(1 l)-P(1)-0(12)C(2l)-P(2)-0(22)N( 1 )-C( 33)-C( 32)C( 1 1 )-P( 1)-O( 13)O( 12)-P( 1)-O( 13)C(Zl)-P(2)-0(23)0(22)-P(2)-0(23)C(2 1)-P(2)-0(25)C(21)-P(2)-0(26)0(25)-P(2)-0(26)C( 3 1)-P(3)-0(32)C(31)-P(3)-0(33)0(32)-P(3)-0(33)1.506( 2)1.459(2)1.460( 2)1.505(3)1.845(2)1.820(2)1.834(2)1.497(2)1.536( 2)1.448(13)1.474( 14)1.528(1)1.499( 1)110.1(1)1 15.4( 1)116.1(1)11 3.6( 1)1 15.6( 1)111.1(1)1 10.7( 1)11 5.5( 1)1 15.4( 1)104.4(1)109.2( 1)107.9(1)1 0 7 3 1)105.2(1)109.7( 1)105.5(5)100.6(6)1 12.0(7)104.8(1)109.3( 1)114.2(1)0-P-0 and O-P-OH have been obtained for type A; these areshown in Table 4.Amino(ulkanecarboxy/ic acid) and Amino(alky1phosphonicacid) Crystal Structures.-Both types of ligand exist as zwitter-ions in the solid state.This involves deprotonation of aphosphonic (or carboxylic) acid oxygen and protonation of anitrogen atom. Studies have shown that the more acidicphosphonic acid groups are deprotonated in preference tocarboxylic acid group^.^ The overall molecular conformationsare determined by extensive intra- and inter-molecular hydrogenbonding.* This usually involves N-H 0 or O-H 0 bondsalthough weak C-H ... 0 bonds have been observed in thecrystal structure of ethylenediamine-N,N,N',N'-tetramethyl-enephosphonic acid 6.32 Weak C-H 0 bonding has also beenobserved in amino Intramolecular hydrogen bonding ismuch stronger in amino(alkanecarboxy1ic acids) than inamino(alky1phosphonic acids), many examples of which containno IAHB.It has been argued that non-bonded interactionsbetween the tetrahedral methylene and phosphonic acid groupsmake such bonding less favourable because the most eclipsedconformation is necessary for approach of the phosphonateoxygen (0) atom to the protonated N atom.4 The weakness oftheIAHB means that the conformation of the molecules can changeunder the influence of the stronger IEHB.Structure of Ligand 1 b and Comparisons.-The macrocyclering. It is interesting to compare the ring geometries of thefree ligand and its corresponding copper(r1) and iron(m)complexes.7*8 Mean values for the macrocyclic bond lengths,angles and dihedral angles have been calculated from a group of1,4,7-triazacycIononane complex structures. The Fe3 + andCu2+ atoms are both connected to all three ring-nitrogen atomsand three and two oxygen atoms from different phosphonategroups respectively. The C-N bond lengths in ligand l binvolving N(l) and N(3) are longer than those involving N(2),the longest C-N length occurring in the bond between the twoprotonated N atoms. Thus, the protonation of N(3) must alsohave the effect of increasing the length of the adjacentC(33)-N( 1) bond. Both sets of lengthened C-N bonds in ligandl b are longer than the calculated mean value (1.490 A), while thebonds around N(2) are much shorter than this mean value.Thering C-N bond lengths in the iron(m) and copper(r1) complexesin which the N atoms are co-ordinated to the metal ion are closeto the calculated mean value. The C-N bond lengths in thesecomplexes are relatively short although not as short as thoseassociated with the unprotonated N(2) in ligand lb. The onlyother example of an X-ray structure of a 1,4,7-triazacyclononanetype ligand and a corresponding transition metal complex isthat described by Moore et The gallium(r1r) complexof 1,4,7-tris(2-hydroxy-3,5-dimethylbenzyI)- 1,4,7-triazacyclo-nonane has C-N bonds of similar length, which are close tothe calculated mean for other 1,4,7-triazacyclononane typecomplexes (X = 1.493 A). The C-N bonds in the free ligand* Abbreviations: IAHB = intramolecular hydrogen bonding, IEHB =intermolecular hydrogen bonding as employed by Shkol'nikova andPorai-Koshits4 are used in the following text.Table 4 Mean bond lengths and angles in amino(alky1phosphonic acid) ligands"Length Mean.F/A 0, n AngleC-N 1.495 0.017 58 N-C-Pc-P 1.826 0.012 58 0-P-0P-O(H)' 1.561 0.013 40 O-P-OHP-O( H) 1.544 0.012 14 O=P-OHP--0 1.474 0.007 8 HO-P-OHP-0 1.507 0.014 107 c-P-0C-P-OHa Data from the CDS system.Of the N-C-P link. Phosphonate. Phosphonic acid.Mean .f/'112.5114.9109.9114.8107.9107.0105.00, n3.6 582.3 652.3 772.1 111.7 62.5 1062.6 53296 J. CHEM. SOC. DALTON TRANS. 1992Table 5 Mean 1,4,7-triazacyclononane ring angles (") (n = 33)Angle Mean fIo crnC-N-C 112.1 1.36C-C-N* 111.1 0.95C-C-N* 110.1 1.07* Two types of C-C-N angle were distinguished in each N-C-C-Ndihedral angle.Table 6iron(m) complexesRing dihedral angles (") for ligand Ib and its copper(i1) andAngle lb Cuz + Fe3 +N( l)-C(12)-C(13)-N(2) 41.25 - 51.0 -44.8N(2)-C(22)-C(23)-N(3) 49.1 - 47.5 - 44.8N(lw(33tC(32)-N(3) 64.4 -43.4 -44.8Ring dihedral angle .T = 45.6", (J, = 2.74, no.of angles = 33.Table 7 NH+ - - - N lengths (A) and angles (") in IAHBN-H H...N N . . . N N-H...NIb N(1)-H(1) - * - N(2) 0.833 2.391 2.767 108lb N(3kH(3) * * * N(2) 0.856 2.386 2.813 1117 N-H...N 0.99 2.25 2.825 1168 N-H..*N 0.95 2.23 2.830 120are much shorter (2 = 1.476 A).Thus protonation or co-ordination of the N atoms appears to increase the length ofthe ring C-N bonds. Other factors may also have an effect. Boththe C-N-C and the C-C-N angles in the 1,4,7-triazacyclonon-ane complexes tend to be very similar, illustrated by the smallstandard deviations in Table 5. The two C-C-N angles aroundthe N atom attached to the unco-ordinated phosphonate groupin the copper(1r) derivative are smaller than those around the co-ordinated N atoms attached to the co-ordinated phosphonategroups.The C-C-N angles in ligand l b are very similar to those ofTable 5, except for those connecting the two protonated Natoms. Both of these angles are larger (by ~ 4 ' ) than the otherC-C-N angles in the ring, and the largest N-C-C-N dihedralangle (Table 6) is that between the two protonated N atoms[N( 1) and N(3)].The increased size of these angles relative tothe others in the ring may be the result of one or more of thefollowing factors, JAHB between the NH+ and N atoms in thering, and/or Coulombic NH + repulsions. The lengths andangles between these atoms shown in Table 7 seem toindicate relatively strong IAHB in both cases. IntramolecularN-H N bonds in other amino(alkanecarboxy1ate) andamino(alky1phosphonic acid) ligands are rare and there onlyappear to be two other examples (see Table 7) in the literat~re.~This is probably because there are only a very few ligands whichcontain both protonated and unprotonated N atoms.It is conceivable that the relatively large C-C-N andN-C-C-N dihedral angles around the protonated N atoms area result of the ring adjusting its conformation to form thesestabilising, IAHB N-H N bonds.Another possible reasonfor the size of these angles is that the conformation of themacrocyclic ring is set to keep the repulsion between the NH+groups to a minimum. Thus, although the H atoms on N( 1) andN(3) are further apart (2.290 A) than those in CH, groups, thecharge on the NH+ groups and hence the Coulombic repulsionbetween these NH+ protons is likely to be greater than thatbetween the protons in the ring and pendant-arm CH, groups.The pendant groups: bond distances. Protonation of N atomsresults in a lengthening of C-N bonds in the N-C-P link ofmethylphosphonate groups.For the 58 methylphosphonate/phosphinate C-N bond lengths compared, no significant trendsor differences were observed between aminomethylphosphonic/phosphinic ligands and complexes when the N atoms wereprotonated. The shortest distance found was for the guanidino-methylphosphonic acid (C-N 1.455 A);35 the apparentshortening of C-N bonds attached to a guanidine nitrogen (e.g.arginine residues) is common.The methylphosphonate C-Y bonds in ligand 1 b involvingthe protonated N atoms, N(1) and N(3), are slightly longerthan the mean bond length (1.495) shown in Table 4. They arealso slightly shorter than those in the 1,4,7-triazacyclononanemacrocyclic ring. The C-N bond length involving the un-protonatedN atom in ligand l b is much shorter and it compareswell with the value quoted for other 'normal' C-N bonds(1.47 in which the N atom is not quaternised.The C-N,C-P bond lengths and N-C-P angles in the unco-ordinatedphosphonate group in the corresponding copper(I1) complexare similar to those in the methylphosphonate group attachedto the unprotonated N atom in lb. The two C-P bonds inthe methylphosphonate groups attached to the protonated Natoms are the longest of the three in ligand lb. A comparisoncan be made with two aminomethyl carboxylate ligandstructures which contain both quaternised and unquaternisedN a toms. In trans-cyclohexane- 1,2-diarnine-N,N,N',N'- tetra-acetic acid 7 both methylenecarboxylate C-N bonds involvingthe quaternised N atom are longer than those involving theunquaternised N atom.The methylenecarboxylate C-C bondsare of similar length except in the deprotonated methylcarboxylate group.3 A similar situation exists in diethylene-triamine- N,N,N',N",N"-pentaacetic acid 8,3 * where the methyl-ene carboxylate C-C bond associated with the deprotonatedCH2C02 - group is longer than all the other C-C bonds in theCH2C02H groups, irrespective of whether the adjacent N atomis quaternised or unquaternised. On this evidence it appearsthat the effect is associated with the deprotonated carboxylateoxygen atom rather than the protonated N atom. A similartrend is observed in amino(alky1phosphonic acid) ligands suchas iminobis(methy1enephosphonic acid) 9 and cyclohexylimino-bis(methy1enephosphonic acid)In ligand l b the longest C-P bond length is associatedwith that methylenephosphonate group containing both theprotonated N atom and de rotonated group. Interestingly, thisbond [C(ll)-P(l) 1.845 I] is the longest out of 58 studiedC-P bonds.The next longest [C(31)-P(3) 1.834 A] is associatedwith a protonated N atom and a protonated phosphonic acidgroup. Both of these bonds are longer than the calculatedmean for such bonds (X = 1.826). The shortest C-P bond inligand l b is in the methylphosphonate group containing theunprotonated N atom and the deprotonated phosphonategroup [C(21)-P(2) 1.820 A]. Thus. in contrast to the previouslymentioned amino carboxylate structures 7 and 8 and the twoamino bis(alky1phosphonic acid) ligands, 9 and 10, the C-Pbond lengths in ligand l b seem to be lengthened more whenassociated with the protonated N atom rather than thedeprotonated phosphonate groups.Attempted Correlation of P-0 Bond Length with Hydrogen-bonding Characteristics.-The P-0 bonds can be divided intofour different groups.These are double bonds, partial doublebonds, single bonds to protonated 0 atoms in phosphonategroups and single bonds to protonated 0 atoms in phosphonicacid groups. All of the 0 atoms in ligand l b are involved inIEHB, with significant effect on the lengths of P-0 bonds. TheIEHB for l b listed in Table 8 were visualised with theQUANTA 40 molecular modelling program. Previous workershave discovered that the lengths of C-O(H), C=O and C-0bonds in aminoalkyl carboxylates are definitely related to thenature of the hydrogen bonds formed by these groups.38Twenty-three amino(alky1phosphonic acid) ligand structures(found in the database) have been studied in this project toestablish whether such a relationship exists between P-0distances and the nature of the hydrogen bonds in aminJ .CHEM. SOC. DALTON TRANS. 19922.6 -a, u(dw c5 p 2.55-6m rLu2.5 -3297Table 8 Intermolecular hydrogen bonds for ligand Ib: distances in A, angles in ODonor Acceptor Angle 0-H 0 - - 0 H - - - 0 Transformation *O( 1 2)-H( 1 2) O(2 1) 171 0.88 2.60 1.73 -f + x, 1 - y,$ - 20(23)-H(23) O(33) 178 0.90 2.51 1.61 - - ; x,+- y,--z0(31)-H(31) O(21) 177 0.84 2.52 1.68 + - x,y, - 4 + z0(4)-H(41) O(13) 158 0.66 2.63 2.0 1 x, Y1O( 4)-H (42) O( 22) 159 0.80 2.49 1.74 1 -x,1 - - , I - 2N-H N * .* O H . * * ON(I)-H(I) O(l1) 1 60 0.83 2.82 2.02 -x, 1 - y, 1 - zN(3)-H(3) O(11) 149 0.86 2.76 1.99 -x, 1 - y , 1 - z* Symmetry operation for acceptor atom.I Aaaa Da aaaaa a* A aaa a nanI I 11.52 1.54 1.56 1.58P -O(H) Length//jFig. 4 Plot of the 0. . 0 distance for intermolecular hydrogen bondsfound in amino(alky1phosphonic acid) crystal structures (taken fromthe CSD) rs. P-O(H) bond lengthalkylphosphonates/phosphinates. It has already been observedthat P-O(H) lengths in phosphonate groups are generallylonger than those in phosphonic acid It appears thatthe single IEHB formed by the O(H) atoms in these P-O(H)bonds is generally stronger in the phosphonic groups thanin the phosphonate groups, permitting the hypothesis thatstrong O-H...O TEHB bonding reduces the length of thecorresponding P-O(H) bonds.A plot of P-O(H) d' istances us.0 - 0 distances in the corresponding IEHB bonds, shown inFig. 4, indicates that generally a correlation exists between theP-O(H) bond length and the IEHB 0 0 distance. ShorterP-O(H) bonds are associated with shorter 0-H - 0 IEHBdistances for these molecules. In ligand Ib, the P-0 distancesinvolving atoms 0(12), O(23) and O(31) are 1.578, 1.538 and1.546 A while the corresponding 0 0 distances are 2.60,2.5 1and 2.52 I$.* Intermolecular hydrogen bonding involving theP-O( H) single bonds have 0 0 distances varying between2.465 and 2.613 A.It is often accepted that the differentIEHB bond lengths and hence P-O(H) distances are due toconstraints of molecular packing.41 The mean IEHB distance is2.545 A, on = 0.046, less than the contact (van der Waals)distance of two 0 atoms (3.00 A).33 The P=O bond length of1.487 I$ in ligand l b is 0.013 A longer than the calculated meanfor such bonds. It is intermediate between the mean value for adouble bond and that for a partial double bond. With thelimited amount of data available, there does not seem to be anycorrelation between the P=O lengths and the nature of thecorresponding I EHB interactions, as noted p r e v i ~ u s l y . ~ ~ The*The H atom connected to O(32) is disordered and as a result thenature of the 0(32)-H(32) - 0 IEHB could not be determined.formation of the characteristic zwitterionic structure involvesthe deprotonation of a phosphonic acid group and theprotonation of an N atom.The methylphosphonate groupsaround P(1) and P(2) are deprotonated in the structure ofligand Ib. The P(1)-0(11) and P(1)-O(13) lengths of 1.499and 1.497 A indicate that the negative charge is spread overthe 0 - P - 0 group and that the bonds are intermediatebetween single and double. In the other deprotonatedmethylphosphonate group, the P-0 distances in P(2)-0(21)and P(2)-O(22) are 1.507 and 1.536 A. This indicates that theP(2)-O(21) bond has more double-bond character thanP(2)-O(22). Although the difference in the lengths of thesebonds is almost certainly related to the nature of the IEHBinteractions involving the 0 atoms, there does not seem to beany correlation between the P - 0 distances and the strength,nature or number of the IEHB bonds formed.Hydrogen Bonding Interactions in Ligand 1 b.-The majorhydrogen-bonding interactions in ligand 1 b are intermolecularin nature; of the seven IEHB bonds two are N - H .- - Ointeractions involving protonated N atoms in the macrocyclicring and three are 0 - H 0 interactions involving phosphon-ate and/or phosphonic 0 and OH atoms. The remaining twoare 0 - H 0 interactions, found between the water molecule[H(41)-0(4)-H(42)] and phosphonate 0 atoms, and showlarge deviations from linearity (158 and 159"), whilst thoseinvolving the phosphonate 0 and OH atoms are almost linear(171-178').The 0 0 distances in ligand l b vary between2.49 and 2.63 A. The lengths in the two N-H 0 IEHB bondsare similar but the angles around the H atoms are very differentand show significant deviations away from linearity (160 and149"). Although the QUANTA program did not find it in itssearch, there is also a possible weak intramolecular hydrogenbonding interaction between N( 1) and O( 13): N-H . O 97",N-H 0.83, N 0 3.00, 0 H 2.80 A. This would result inthe formation of a five-membered ring. The correspondingdihedral angle of N(l)-C(Il )-P(t)-0(3) 11.8" is close to theeclipsed conformation which is necessary for the formation ofsuch bonds. Although the formation of similar IAHB bonds inaqueous solution has also been suggested by Geraldes et u I .~ ~ instudies on the protonation of ligand Ib, the general inferencefrom the crystal structure of I b is that IAHB would bediminished. In aqueous solution, the effect of hydrogen bondingto solvent may swamp any IAHB.Angles of the Pendant Group.-The N-C-P angles in ligandlb, involving the protonated N atoms [N( 1 ) and N(3)], are verysimilar, while those around N(2) in ligand Ib and the 'same'atom in the corresponding copper(r1) complex are the highestout of 58 studied N-C-P angles. The C-P-0 and 0-P-0angles fall into different groups depending on the nature ofthe P-0 bonds. Mean angles and their standard deviationscalculated using the data from 38 amino alkylphosphonic/phosphinic ligand X-ray structures are shown in Table 4.Onlya small number of HO-P-OH and C-P=O angles could b3298 J. CHEM. SOC. DALTON TRANS. 1992included in the survey because they are only found in amino-(alkylphosphonic acid) ligands containing unquaternised Natoms (ligand lb) or in ligands containing more than onemethylphosphonate group attached to an N atom. As a resultthe mean values for these angles must remain tentative in theabsence of more examples of their type. Two of the largest0-P-0 angles in ligand l b occur between the two P-0 partialdouble bonds [O(ll)-P(1)-0(13) and 0(21)-P(2)-0(22)].Modified neglect differential overlap (MNDO) calculationshows that the 0 atoms involved in these angles are the mostnegatively charged. It therefore seems reasonable to suggest thatthese angles are the largest because of the increased Coulombicrepulsion between the two 0 atoms, and the calculated meanvalue for this type of angle is larger than that for any of the other0-P-0 or C-P-0 angles ( I 14.9’) in Table 4.It would appear,however, that IEHB and the subsequent packing involvedduring the crystallisation must have an effect on these angles asthere is a difference of 4” between the two examples of this typeof angle in ligand lb. Since the amino(alky1phosphonic acids)are analogues of the biologically important x amino acids, theanalysis of the differences in IEHB and IAHB tendencies can bevery important in the design of experiments using amino(alky1-phosphonic acids) as biological substrate^.^ Recent work hasdemonstrated that hydrogen bonding of itself may be a majormeans by which anions are recognised in biological systems.42Packing ojligand 1 b.-Symmetry codes for IEHB are shownin Table 8.A single ligand molecule is involved in IEHBbonding interactions with five other ligand molecules. AtomsN(l), N(3) and O(11) have direct interactions with a secondligand molecule (ligand 2), while atoms O(13) and O(22) areindirectly ‘attached’ to ligand 2 via hydrogen-bondinginteractions with the water molecule of crystallisation. ThusH(41)-0(4)-H(42) can be considered as a linking moleculewhich forms chains in the overall structure. Finally, atomsO(12) and O(21) are hydrogen-bonded to ligand 3, O(23) andO(33) to ligand 4,0(21) to ligand 5 and O(31) to ligand 6. Thiscomplicated set of strong intermolecular hydrogen bonds musthave a significant stabilising effect on the overall structure ofligand lb.AcknowledgementsWe wish to thank SERC and Courtaulds Coatings for financialsupport, and acknowledge the use of the SERC fundedChemical Databank Service at Daresbury for part of the workdescribed in this paper.References1 K.Moedritzer and R. R. Irani, J. Org. Chem., 1966,31, 1603.2 M. 1. Kabachnik, T. Ya. Medved’, N. M. Dyatlova and M. V.3 W. R. Harris and D. Nesset-Tollefson, Biochemistry, 1991,30, 6930.4 L. M. Shkol’nikova and M. A. Porai-Koshits, Lisp. Khim., 1990, 59,5 P. Chaudhuri and K. Wieghardt, Prog. Inorg. Chem., 1987,35,329.6 Yu. M. Polikarpov, B. K. Shcherbakov, F. I. Bel’skii, T. Ya. Medved’and M.1. Kabachnik, Izc. Akad. Nauk SSSR, Ser. Khim., 1982,7,1669.7 M. I. Kabachnik, M. Yu. Antipin, B. K. 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ISSN:1477-9226
DOI:10.1039/DT9920003291
出版商:RSC
年代:1992
数据来源: RSC