Relationship between structures and stabilities of molybdate complexes of alditols: A potentiometric, 13C and 95Mo nmr study

Polyhedron Vol. 9, No. 9, pp. 12254234. Printed in Great Britain

0277-5387/YO $3.00+ .OO Q 1990 Pergamon Press plc

1990

RELATIONSHIP BETWEEN STRUCTURES AND STABILITIES OF MOLYBDATE COMPLEXES OF ALDJTOLS: A POTENTIOMETRIC, 13C AND %Mo NMR STUDY S. CHAPELLE” Laboratoire de RMN de l’Universite de Rouen, URA 464 du CNRS, Faculte des Sciences, B.P. 11876134 Mont-Sent-Aignan Cedex, France

J. F. VERCHERE

and J. P. SAUVAGE

Laboratoire de Chimie Macromol~ulaire, URA 500 du CNRS, Universite de Rouen, Faculti: des Sciences, B.P. 118,76134 Mont-Saint-A&ran Cedex, France (Received 9 November 1989 ; accepted 15 January 1990) Abstract-Dinuclear molybdate complexes of alditols have been studied in aqueous acidic solution by 95Mo and ’ 3C NMR. Their formation constants were determined by potentiometry. In all complexes, the dimolybdate group was chelated by four vicinal hydroxyls. Two types of complexes were characterized by NMR, de~nding on the erythro oi three configuration of the central diol group. Their structures were precised using literature crystal data. A single asymmetrical complex of the first type was formed by symmetrical erythro ligands (erythritol, galactitol), while the asymmetrical D-arabinitol and D-mannitol formed pairs of isomeric species of this type. Ligands with complexing sites involving a central three group (Dr.-threitol, xylitol) were complexed in a symmetrical manner. D-Glucitol formed four complexes as pairs of each type. Rules relating the stabilities of the complexes to the structures of the ligands are presented.

In aqueous acidic solution, molybdate ions MOO:- react with many polyhydroxy organic compounds to form soluble complexes. Sugars and alditols form a particular class of dinuclear anionic complexes, as demonstrated by a series of polarimetric and potentiometric studies. I-4 The enhancement of optical rotation of alditols by molybdate has been used for their identification1 and determination, ’ and sugars were separated by paper electrophoresis in molybdate solution. 2 The crystal structures of D-mannitol complexes have been described.6 Monosaccharide complexes in solution have been studied by ‘H and 13C NMR. Some aldoses have been shown’-I0 to complex with molybdate in cyclic structures as tridentate donors. 95Mo NMR has been used to demonstrate the presence of two non-equivalent MO atoms’ * in

*Author to whom correspondence should be addressed.

complexes of aldoses of the lyxo-manno series, which chelated molybdate as tetradentate donors. While our work on homologous complexes of alditols, inclu~ng 95Mo NMR spectroscopy, was in progress, a paper was published describing a ‘H and r3C NMR investigation on such compounds. I2 It was stated that “alditols form four types of dinuclear molybdate complexes in which they serve as tetradentate donors with four vicinal hydroxyl groups”. Although our results were mainly in agreement, important differences are presented in this paper. Several alditols were found to form small proportions of complexes unreported or poorly characterized up to now, allowing us to compare the stabilities of complexes involving various chelating sites. A specificity of our approach was the simultaneous determination of both the structures and the formation constants of the complexes, in order to establish correlations between their stabilities and the configurations of the ligands.

1225

1226

S. CHAPELLE

EXPERIMENTAL Materials A stock solution of disodium molybdate (Prolabo, 0.1 mol dm- ‘) was prepared and kept in polyethylene bottles. All products were of analytical grade. Commercial alditols (Fluka or Aldrich) were used as received. Water was de-ionized in a Millipore apparatus. Potentiometric measurements were made at constant ionic strength (KCI, 0.1 mol dmm3). The acid used either in potentiometric or NMR experiments was HCl. Determination offormation constants They were measured by potentiometry according to a procedure described previously, ’ ’ based on the determination of the half-equivalence pH 1,2values in acidimetric titrations of molybdate-alditol mixtures. NMR experiments All 1-D and 2-D NMR spectra were recorded on a Bruker AM 400 spectrometer equipped with a 5 mm multinuclear probe. Solution samples were prepared by dissolving weighed amounts of alditol (0.5 mmol) and disodium molybdate dihydrate (1.5 mmol) in 0.5 cm3 of D20. Then HCl (0.75 mmol) was added. The samples were stable for several weeks at 4°C in the dark. The 95Mo NMR experiments were made as before. ’ ’ For the 13C NMR experiments, the proton coupled and decoupled spectra were obtained with nuclear overhauser enhancement. The chemical shifts were determined by the substitution method ’ 3 using the TMPS reference signal in D20. 2-D heteronuclear experiments were classically performed’ 4 with polarization transfer from ‘H to 13C, the number of experiments being 64 x 1 K. Initial assignment of ‘H spectra in the case of mixtures of complexes, made necessary 2-D homonuclear experiments (COSY-45).‘5,‘6

et al.

Table 1. 26.076 MHz “MO NMR chemical shifts and linewidths in molybdate-alditol complexes ; n is the number of complexes” Alditol

6 (ppm)

Av (Hz)

n

Erythritol DL-Threitol Ribitol Xylitol D-Arabinitol D-Mannitol D-Glucitol Galactitol

33.5 22.5 34 21 29.5 31 33 34

290 270 290 350 450 530 730 550

1 1 1 1 2 2 4

1

Reference : Na*MoO, in D20. Accuracy “From 13C NMR data.

: 6 f 1 ppm.

spectra, contrary to those of complexes of some aldoses which displayed two sharper signals ’ ’ attributed to the dinuclear MO core. The broader signals were given by hexitol complexes, which could be a consequence of their larger molecular radii and/or of the formation of a mixture of species giving unresolved spectra. It is shown below that while galactitol formed a single complex, D-mannitol and Dglucitol formed at least two complexes. Similarly, the broad signal of the D-arabinitol complex, compared to other pentitols, can result from the formation of two different complexes. The range of chemical shifts for all the complexes was narrow, 6 = 21-34 ppm, in agreement with molybdenum being in the +VI oxidation state. I7 Flscher

formulae

of olditots

Tetnfols

Erythratol

HOCH,-

CH,OH

o-Thrwtol

HOCH,-+-

CH20H

Pentltols xylltol

HOCH,+

Rib&to1

HOCH,

o-Arobln~tol

HOCH2d

CH20H

-

CHPOH

CH20H

RESULTS Hexltols

95Mo NMR Using an excess of molybdate, the spectra of alditol-molybdate-HCl mixtures were recorded for a series of tetritols, pentitols and hexitols, showing in every case the sharp signal of free molybdate at 6 N 0 ppm and a new broad signal (Av b 270 Hz) attributed to the complex(es) (Table 1). No individual signals could be characterized in these

D- Glucltol

HOCH,

o - Mann&o1

HOCH,+

Goloctltol

HOCH2

Carbon

numbering

C-I

+

CHzOH

CHZOH

+

CH20H

is skuoted

Scheme

on the

1.

right-

hand

side.

In fact, two polyols, namely DL-threitol and xylitol, were characterized by 6 values lower than other alditols (22.5 and 21 ppm respectively) and probably formed complexes of a distinct structural type. Besides, the complexes of all other alditols had 6 va+i!iLps ?EY$i~~%Wel?l~ 3.3 a& X &nV, &?& >S tke same range as one signal @en by the complexes of aldoses of the lyxo-manno series” (two signals at 30 f 4 ppm and 42 + 1 ppm). The aldose 42 ppm signal had been attributed to the MO atom bound to the anomeric HO-l and neighbouring hydroxyls, which obviously was not possible with alditols. It was remarked that DL-threitol and xylitol, which gave MO signals at 6 N 22 ppm, did not posX?SSer~%J‘%&&, g~?Y%~,V&;‘ti SYS,&%~E%ati pr lyxose, which gave signals at 6 N 30 ppm, possessed an erythro diol group at C-2,3. This suggested that alditols giving rise to signals at 6 N 30 ppm had a related complexing site involving also an erythro diol group.

The spectrum of an alditol possessing n carbon atoms showed at most n signals, depending on the molecular symmetry. The assignments were readily made from literature. I8 The addition of molybdate and acid gave rise to new signals, the number and intensity of which indicated the number and relative proportions of complexes formed. Depending on the stabilities of the complexes, signals of the uncomplexed ligand could also be observed in variable proportions. When the molybdate/alditol ratio varied from 3 to 10, the complex/free ligand ratio increasd, but the chemical shifts and relative intensities of the signals remained unchanged, indicating that the ratio of the complex species and their nature was not modified. In agreement with a previous work, I2 it was ffound that e@hirto]. t&to\. &It01 andga’lactitol gave only one complex each. The same result was cD~~~~n~hyj)thDL-~f~~~~,Be~~~es.D-ara~~Tj)~~anb m-man&$ wereTormir’co_~ve’cwo com_&exes eadn. in unequal proportions, and up to four complexes were identified in the case of D-glucitol. This contrasted with the same referenceI in which only one cVZZ-+Zi‘SVEi rt-ffi-ii& %X“y-~&ii&~&a& “ttlr% GX Io&udico’r. The principle of the 13C NMR identification of the chelating sites of the ligands lies on the important deshielding effect on carbons bearing the hydroxyls bound to molybdate.‘~” These carbons also show increased direct coupling constants ‘JcH. (cDnsequent)y, ow- stuby reqti& fi.e assgnmEDi CoF the 66erent carbonsin tie com$exes, w%ch was a&iiLgJLti?&6X?& 2-~~~“GXXisZEi%L?&&-X X%FF?

experiments. In every case, the comparison of the spectra of the free and complexed alditol demonstrated that four vicinal carbons were deshielded. The variations of chemical shifts due to complexation (A6) were calculated and are given in T&&F.. AL!+.T& ~SS~~ZTZZT~~ ??Lfe ~“?&iYX& b] the CorresDonding variations of tie direct coupling constants ‘Jc-. Unless contrary mention, the results were in agreement with literature data. ’ 2 Galactitol. The spectra (Table 2) of the single complex showed that the characteristic signals of the CH,OH groups were almost unchanged, proving that complexation involved the four central secondary hydroxyls, which were in a sym~~~&4++%%%?$ ~~~.~~~~f~~.~~~~~~~~~,f~~~~~~~~ symmetry was destroyed in the complex, since the chemical shift for C-4 (+ 19.7 ppm) was much higher than those for C-2,3,5 (+7.9-11.8 ppm). This result was unexpected, because the dinuclear MO core also had a symmetrical structure. Possible reasons for this finding are presented in the Discussion. Erythritol. This tetritol formed a single asymmetrical complex involving all four hydroxyls. A comparison (Table 2) with the A6 values calculated for the CHOH groups of galactitol showed a strong analogy, which was not unexpected because both alditols have a central erythro diol group. It was remarked that the deshielding effect of complexation on a carbon of the chelating site was nearly the same, whether it bore a central secondary or a terminal primary hydroxyl. Ribjtol. The formation of an asymmetrical camplex was expected for this pentitol, because one of the terminal hydroxyls must be involved in complex formation. Accordingly, carbons C-2,3,4,5 were deshielded (Table 2) and displayed increased coupling constants. As in the galactitol and erythritol complexes, one carbon was more strongly deshielded than the o thers,G 2: 17~nmfor C-+)._swgesting the ribitol complex to belong to the same @pX, ‘&v%t~L Xirce ii&si[. &s _~vmme&c&r meso pentitol formed an asymmetrical complex involving one terminal hydroxyl (Table 3). The chelating site involved carbons C-1,2,3,4. Contrary to that tisl& +i‘+.R ?l+Vvt ~-V.iZ,-@XZ3, VifeVZirElSG&iXfi G$?j were equilp &Stiu+Xte~ {Gi ‘v VJ.3 ,~m@,l but less than the lateral carbons C-1,4, which were also equally deshielded (AS 1: 12.5 ppm). DL-fireirof. This compound formed a single complex which has not been described heretofore. Data in Table 3 showed a strong analogy with the ..r$t~1 CLVL@.., E&Z? LZ&VLS C-2,2,3,9 hating J&&d PDSjtDLW WH.9 CDJ.J2~232& %X ~XJ.?BYX3~c-4

1228

S. CHAPELLE

et al.

Table 2. 100.13 MHz ’3C NMR chemical shifts and ‘Jcu direct coupling constants of alditols and of their molybdate complexes (erythro type) Alditol

Carbon position

Galactitol 6 (ppm) 6 (ppm)” ‘Jw (H#’ A6 (ppm)

1 65.5 65.5 143 0

Erythritol 6 (ppm) 6 (ppm)b ‘JcH (Hz)’ Aa (ppm) Ribitol 6 (ppm) 6 (ppm)” ‘JCH(Hz)* Aa (ppm)

1 64.6 63.9 141 -0.7

2 71.5 83.3 146 11.8

3 72.1 83.7 147 11.6

4 72.1 91.8 149 19.7

5 71.5 79.4 144 7.9

1 64.7 74.3 144 9.6

2 74.3 83.5 147 9.2

3 74.3 92.8 147 18.5

4 64.7 71.3 144 6.6

2 74.1 87.9 147 13.8

3 74.4 83.7 147 9.3

4 74.1 91.0 147 16.9

5 64.6 72.1 142 7.5

6 65.5 64.7 143 -0.8

LIUncomplexed alditols. ‘Jc- = 141 Hz for all carbons. 6 assigned from literature. I8 “Molybdate complexes. Carbons in the same column have related positions. Accuracy : 6 &-0.1 ppm ; .I_+1 Hz.

Table 3. 100.13 MHz 13C NMR chemical shifts and ‘Jcu direct coupling constants of alditols and of their molybdate complexes (three type) Alditol

Carbon position

Xylitol 6 (ppm)” 6 (ppm)” ‘JCH(Hz)~ Aa (ppm)

1 64.8 77.1 146 12.3

2 74.0 84.3 148 10.3

3 72.8 83.4 149 10.6

4 74.0 86.6 149 12.6

Dr.-Threitol 6 (ppm>” 6 (ppm)b ‘Jc” (Hz)’ A6 (ppm)

1 64.8 77.0 144 12.2

2 73.6 83.4 147 9.8

3 73.6 83.4 147 9.8

4 64.8 77.0 144 12.2

5 64.8 64.1 142 -0.7

“Uncomplexed alditols. ‘1,-n = 141 Hz for all carbons. 6 assigned from literature. I8 “Molybdate complexes. Carbons in the same column have related positions. Accuracy : 6 + 0.1 ppm, J+ 1 Hz.

of the terminal C-4 of threitol, i.e. higher than the typical value (N 10 ppm) for other CHOH groups. Thus the deshielding effects were similar for a CHOH or CH,OH group in related position, as in galactitol and erythritol complexes. Moreover, the similar deshielding effects at C-1,4 (AB = 12.2 ppm) and C-2,3 (As = 9.8 ppm) indicated that the ligand

was complexed in a symmetrical fashion, contrary to galactitol and erythritol. The same was probably true for the above xylitol complex. These results indicated the existence of at least two types of molybdate complexes, depending on the alditol configurations. One type corresponded to three compounds (xylitol and DL-threitol). The second type was obtained with erythro compounds, typically erythritol and galactitol, and probably included ribitol. D-Arabinitol. Two complexes in 60 : 40 ratio were detected in the ‘%ZNMR spectrum, in accordance with literature results. The first one, symbolized as A, was that previously identified, ‘* involving C-2,3,4,5, since the C-l signal remained almost unchanged (Table 4). A comparison with the chemical shifts in the galactitol complex showed a striking similarity, indicating an analogous structure of both ligands, as expected because of the identical configurations of the three carbons C-2,3,4. The second minor complex, noted AZ, which had not been characterized in literature, was formed by the same tetritol group, C-2,3,4,5 (the C-l signal was slightly modified) but the chemical shifts were different from those of the major A, complex. The most striking feature was that the more deshielded carbon (A6 1: 20 ppm) was C-3 in A2 instead of C4 in A, and that the overall deshielding patterns were reversed, the variations of chemical shifts

A potentiometric,

1229

13C and 95Mo NMR study

Table 4. 100.13 MHz ’3C NMR chemical shifts and ‘Jcu direct coupling constants of D-arabinitot, D-mannitol and of their molybdate complexes Alditol

Carbon position 1 65.3

2 72.4

3 72.6

4 73.1

65.1

65.1 143 -0.2

84.1 146 11.7

84.0 146 11.4

92.9 148 19.8

71.7 146 6.6

66.1 142 0.8

79.9 143 7.5

92.4 148 19.8

83.7 148 10.6

74.1 148 9.0

1 65.8

2 74.2

3 71.6

4 71.6

5 74.2

6 65.8

65.4 142 -0.4

73.1 142 -0.9

83.3 145 11.7

83.8 149 12.2

93.1 148 18.9

71.7 145 5.9

65.9 142 0.1

74.1 142 -0.1

80.3 143 8.7

92.3 148 20.7

83.8 149 9.6

74.1 145 8.3

o-Arabinitol 6 (ppm) Complex A, 6 h-M ‘Jm 0-W 86 (ppm) Complex A, 6 (ppm) ‘JCH(Hz) Aa (ppm) D-Mannitot S (ppm) Complex MI 6 h-v-4 ‘JcH 0-W AS (pw) Complex M, 6 tppm) ‘Jcr, (Hz) A6 (ppm)

5

‘JCH= 141 Hz for all carbons of uncomplexed alditols. 6 assigned from literature. I8 Carbons in the same column have related positions. Accuracy : 6 _CO.1 ppm ; .I& 1 Hz.

Table 5. 100.13 MHz ’3C NMR chemical shifts and ‘Jcu direct coupling constants of D-&CitOl and of its molybdate complexes Alditol D-Glucitol 6 @pm) Complex G I 6 kv4 ‘&I (Hz) AS @pm> Complex G, 6 (ppm)

‘JCH@W A6 (ppmf

Complex G, 6 (ppm)

‘Jm (Hz) As (wm>

Complex G, 6 (ppm)

‘JCHO-W As (m-0

Carbon position 1 64.7

2 75.0

3 71.7

4 73.2

73.2

6 65.0

77.2 143 12.5

84.2 149 9.2

83.1 146 11.4

85.7 146 12.5

72.1 147 -1.1

65.3 144 0.3

64.8 142 0.1

75.2 143 0.2

83.8 146 12.1

83.7 146 10.5

92.9 148 19.7

71.3 148 6.3

63.5 ND -1.2

86.6 147 11.6

80.4 143 8.7

83.7 146 10.5

86.4 148 13.2

64.1 144 -0.9

64.5

76.1 143 1.1

78.9 146 7.2

92.2 147 19.0

84.7 145 11.5

74.1 148 9.1

ND -0.2

5

o ‘Jc. = 141 Hz for all carbons of uncomplexed D-ghCitOi. 6 assigned from literature. ” ND : not determined. Accuracy : 6 t_ 0.1 ppm ; Jk 1 Hz.

1230

S. CHAPELLE

being related in the sequences C-2,3,4,5 in Ai and C-5,4,3,2 in AZ. D-Mannitol. Two complexes in 70: 30 ratio, which were called M , (major species) and Ml, could be identified (Table 4). Both species involved the tetritol group at C-3,4,5,6 and were analogous to those described for D-arabinitol, in accordance to the related configurations of C-2,3,4 of arabinitol and C-3,4,5 of mannitol. This makes surprising the report that only one complex (the M 1 species) was formed by D-mannitol, while both A, and A2 species had been detected for D-arabinitol in the same study. I2 Such discrepancies may be due to the difference in techniques for preparing the complexes, since we used sodium molybdate acidified with varying amounts of HCl instead of ammonium molybdate. D-Gkitol. The spectrum of this hexitol in acidified molybdate showed 24 new signals in addition to those of uncomplexed ligand, due to the formation of four complexes referred as G, (32%), G2 (26%) G3 and G, (21% each) (Table 5). The spectra of species G, (molybdate bound to C- 1,2,3,4) and G2 (molybdate bound to C-3,4,5,6) have been already reported in literature, a third complex being believed to involve all secondary hydroxyl groups, from the products of its periodate oxidation. I2 The patterns of variations of chemical shifts were typical of the “xylitol type” for G, and of the “arabinitol type” for G2 Unambiguous assignments for the G, and G4 complexes were not possible because homonuclear ‘H 2-D experiments were precluded, owing to the large number of overlapping signals. However, using only 13C NMR data, we found that the G3 species was characterized by two unchanged signals for the CH20H groups, and therefore involved C2,3,4,5 as the chelating system. The G4 species had one unchanged CH20H signal and since only one complex could be imagined when C- 1 was involved (the “xylitol type” G J, the complexed hydroxyl was assigned to C-6. Thus G, had the same complexing site as G, at C-3,4,5,6 (arabinitol-like configuration) and the G2-G4 couple corresponded to the homologous couples of complexes observed with D-arabinitol and D-mannitol. Tentative assignments for G4 were made by considering that the C-3,4,5 systems had the same configurations in D-glucitol and in D-mannitol. Thus the variations of chemical shifts were assumed to be similar in complexes M2 and Gq, as they were in the M ,-G, pair. Possible assignments for G3 were proposed by analogy with the xylitol-type complex G,, since the chelating sites of both species involved the same xylo group at C-2,3,4.

et

al.

Formation constants of the complexes The potentiometric determination of the formation constants was made according to the method described previously. ’ ’ Two equilibria were observed in molybdate-alditol(L)H+ systems : 2MoO:-

+L+2H+

e(2,1,2)2-

2MoO;-

+L+3H+

1

(2,1X.

The corresponding equilibrium constants K2 12 constants of the and K213 defined the formation complexes in the classical way. ” The equilibrium between the (2,1,2) and (2,1,3) species was characterized by the pK, value relative to reaction : (2~3~

e(2,1,2)2-

+H+.

The values of the formation constants are presented in Table 6. Good agreement was observed with literature values’9-2’ when available.

DISCUSSION This part will be devoted to the examination of factors governing the relative stabilities of the molybdate complexes, with emphasis on those related to the structures of the ligands. Only the (2,1,2) species will be considered, since the acidic (2,1,3) complexes were not characterized by NMR. However, the stabilities of both series of complexes should be strongly related, because the corresponding pK, values show little differences. The first problem to be considered was the origin of the different behaviours of the homologous tetritols, erythritol and threitol, which differed only by the configuration of one CHOH group. It resulted in two important effects, first the existence of a symmetrical complex with threitol, while the erythritol one was asymmetrical, and second the higher stability of the erythritol complex. The former result was surprising because both ligands have symmetrical structures. The relationship between stabilities and structures is not simple and is discussed below. Besides, the problem of symmetry, which concerned also pentitols and hexitols, was discussed using available literature data6,23-26 on the structures of related molybdate complexes. Molybdate complexes of D-lyxose23 and poly01s~~~~possess a dinuclear core bridged by three oxygen atoms, two of them being triply-bonded. Each molybdenum atom is also bound to one oxygen atom of the ligand. This unusual dimolybdate group is not typical of carbohydrate complexes, as similar structures have been reported for some complexes of quinones24 and catechol25 derivatives. The geometry of the acceptor site can be

A potentiometric,

’ 'C and 95M~ NMR study

1231

Table 6. Formation constants of molybdatealditol complexes (KC1 0.1 M, 25°C) determined by potentiometry. log Kxyzrefers to the formation of the (x,y,z) species from x MOO:- y alditol and z H+. K, is the acidity constant of the (2,1,3) complex

Alditol

logK,,,

logKZ,3

PK,

Erythritol m_-Threitol Ribitol Xylitol D-Arabinitol D-Mannitol

15.20 14.60 15.55 16.25 16.35 16.70

19.50 18.20 19.45 19.65 20.45 20.80

4.30 3.60 3.90 3.40 4.10 4.10

D-Glucitol Galactitol

16.60 17.30

20.50 20.90

3.90 3.60

logK,,, (lit.)

logK,,, (lit.)

15.57”

19.52”

16.89b 17.49’ 16.90’ 17.50

20.77’ 21.55’ 20.72’ 20.93”

Accuracy: log K,,,f0.06; log K,,,+O.ll ; pK,+O.O5. “Ref. 21. *Ref. 20. ‘By potentiometry and polarimetry ” in NaClO, 3 M.

schematized by projecting, perpendicularly to the medium plane of the dimolybdate core, the four bonded oxygens which form either a stretched diamond6,23,26 or a parallelogram (naphthalenediol complex”) as shown in Fig. 1. The fitting of this quadrilateral with the figure drawn by the donor groups of a ligand was checked using molecular models. A first type of complex was characterized with alditols possessing a complexing site involving a central erythro group (erythritol, galactitol, ribitol). As established by NMR, D-mannitol and D-arabinitol belonged to this type, corresponding to types A and C in the literature. I2 The known crystal structures of the D-mannito16 and erythritol complexes26 could then be used as models for the whole series. The four carbons of the complexing site must adopt a sickle arrangement allowing their four hydroxyls to point in the same direction, forming a trapeze which did not match exactly the acceptor site. Accordingly, the molecule must twist at one internal carbon as shown in Fig. l(a). It agreed with the NMR result that one of the central carbons in the ligands was more strongly deshielded (Tables 2 and 4) than the other one. In fact, we observed that two ligands (D-mannitol, D-arabinitol) formed a pair of complexes, with reversed deshielding patterns, involving the same chelating site : C-2,3,4,5 for arabinitol complexes AI-A2 and C-3,4,5,6 for mannitol complexes M ,M2. Accordingly, the scheme in Fig. l(a) predicted that two isomers could be formed when the lateral carbons of the complexinp site hore different substituents Rand R’, depending on the site of twisting.

Their proportions were not equal, the favoured isomer being that in which the more deshielded carbon bore the shortest side chain, i.e. a CH20H group. In contrast, one isomer only could be formed by symmetrical ligands (R = R’) like erythritol and galactitol. Ribitol was remarkable among ligands forming asymmetrical complexes of the erythro type, since it gave a single complex instead of two. It was assumed that one complex was much less stable than the other, for reasons similar to those invoked in the case of D-arabinitol. Moreover, the existing complex had a low stability because in the sickle arrangement of the C-2,3,4,5 complexing site, an unfavourable steric interaction existed between Cl and C-5. The high A6 value at C-2 (Table 2) may reflect some strain due to this interaction, which did not occur in the D-arabinitol complex possessing the opposite C-2 configuration. Considering that types A and C differed only by the possible substitution of one external carbon, which was not associated with a particular deshielding effect, no reason was found to make a distinction between them. A second type of complex was formed by ligands possessing only threo diol groups (DLthreitol and xylitol) which had a carbon chain in a zig-zag arrangement. They were referred to as type B and type D in the literature. I2 Four hydroxyls

already point towards the four tops of a parallelogram, which could not match the acceptor site described above for the erythro series. Considering the symmetry of the complexation site, demonstrated by NMR, it seemed likely that the acceptor

1232

S. CHAPELLE

et al.

HO

(b)

(a)

Fig. 1. Schematic representation of the chelation of the dimolybdate core by four vicinal hydroxyls of an alditol molecule. R and R’ are either H or CH,OH groups. (a) Complexes of erythro type, showing the formation of a pair of isomers depending on the site of twisting. The dimolybdate structure is that reported in the n-mannitol,6 n-lyxosez3 and erythritolz6 complexes. (b) Complex of three type. The proposed symmetrical dimolybdate structure is that observed in the naphthalene-2,3dial*’ complex.

site had rather a parallelogram shape (in planar projection), allowing the ligands to complex molybdate without appreciable loss of symmetry. The proposed structure, which had never been characterized by X-ray studies in literature, was in agreement with NMR data. Figure l(b) shows that the triply-bonded oxygens are borne by the internal carbons of the ligand complexing sites, and that the doubly-bonded oxygens are borne by the external carbons. Accordingly, these pairs of carbons were found to be respectively equally deshielded. Finally, there is no more reason to differentiate between types Band D than between types A and C. Moreover, y5Mo NMR afforded a supplementary

criterion to distinguish between both types, since complexes of the three type (B-D) gave a MO signal at 1: 22 ppm while those of the erythro type (A-C) gave a MO signal at N 30-34 ppm. D-Glucitol had a unique behaviour, forming complexes belonging to both types in comparable proportions. All four possible complexes allowed by the structure of this hexitol were detected by 13C NMR : (1) A threo complex G 1, in which the complexing site involved C-l and the C-2,3,4 xylo system. (2) and (3) A pair of erythro complexes G2 and Gq, in which the complexing site involved C-6 and the C-3,4,5 arabino system.

A potentiometric, 13Cand 95M~NMR study

(4) A threo complex G3, involving the four internal CHOH (the gluco system) at C-2,3,4,5. Thus three different sites competed to complex molybdate, and formed species of comparable stabilities. The following discussion examines the relationship between the formation constants and the structural types of the various complexes. In the erythro series, the stability order was: erythritol < ribitol < arabinitol < mannitol < galactitol. The favourable effect of increasing the chain length was correlated with the replacement of CHzOH groups by CHOH groups in the chelating site. Among hexitols, the highest stability of the galactic01 complex was attributed to the favourable configurations of C-2,5, which orientated the terminal CH*OH groups far away from the complexing site. Such influence of the C-2 configuration also accounted for the relative stabilities of pentitol complexes : D-arabinitol > ribitol. The D-arabinitol complexes were almost as stable as the D-mannitol ones, probably because their complexing sites differed only by the size of the substituent on a lateral carbon. Thus two rules appeared to govern the stabilities of erythro complexes. Rule 1 : complexing sites in which the lateral carbons bore secondary hydroxyls gave more stable complexes than those using terminal CH20H groups, because of corresponding entropy gain. 22 Rule 2: alditols possessing lateral CHOH groups orientated threo with respect to the central erythro groups formed complexes of higher stabilities than those with lateral groups orientated erythro, since no steric interaction took place between the complexing site and the uncomplexed side chains. In the threo series, the order of stabilities of the complexes xylitol > threitol, was in agreement with Rule 1. The presence of a threethreo system in xylitol resulted in a high stability of the complex, indicating that Rule 2 probably applied. However, the corresponding enhancement of the formation constants (Table 6) seemed more important in this series than in the erythro one, making the xylitol complex almost as stable as the D-arabinitol ones. It was concluded that Rules 1 and 2 were also valid in the threo series. We also examined the relative stabilities of complexes belonging to both series. An essential result was that the erythritol complex was more stable than the threitol one, suggesting that an erythro diol group had a stronger affinity towards molybdate than a threo group. In this respect, the reactions of D-arabinitol and D-mannitol afforded an interesting comparison of the reactivities of erythro and threo diol groups borne by the same ligand. Both alditols

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formed two complexes, but none involved the threo group in the central position of the four chelating hydroxyls. The complexing sites in both pairs of complexes had a central erythro group (C-3,4 in arabinitol and C-4,5 in mannitol). In the case of D-mannitol, the threo type complex, involving C2,3,4,5, was not detected, although it would involve only CHOH groups and should be stabilized according to Rule 1. These results were formulated as Rule 3: complexes involving a central erythro group were generally of higher stabilities than those involving central threo groups. The combined effects of Rules 2 and 3 were unfavourable to the formation of complexes of the threo series : when a ligand bore simultaneously an erythro group and a threo group, (a) complex(es) in which the complexing site(s) involved a central erythro group and a lateral threo group was stabilized with regard to complexes with a central threo group and a lateral erythro group. Only in the case of D-glucitol, we were able to detect a complex involving an erythro-threo system with the threo group in the central position. This G3 species was expected to have a higher stability, because its complexing site C-2,3,4,5 did not involve any CH20H group (Rule 1). Therefore a complexing site ofgluco configuration (erythrethreo-threo) had probably a low affinity for molybdate. Besides, in D-mannitol, the complexing site C-2,3,4,5 of manno configuration (erythrethreo-erythro) had even lower affinity for molybdate, in agreement again with predictions made by combining Rules 2 and 3. Finally, the formation constants showed a close correlation with those reported for homologous tungstate complexes,22 molybdate complexes being about three orders of magnitude less stable. This stability difference did not depend on the nature of the ligand and was ascribed” to the easier dimerization of tungstate. Consequently, it was concluded that the structural characteristics of an alditol similarly affected the stabilities of the complexes of both elements.

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