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Coordination aggregates of bifunctional β-diphosphoryl compounds as studied by 31P and 113Cd NMR in solution
ELSEVIER
Colloids and Surfaces A: Physicochemicatand Engineering Aspects 115 11996) 149 155
COLLOIDS AND SURFACES
A
Coordination aggregates of bifunctional fi-diphosphoryl compounds as studied by 31p and l 3Cd NMR in solution N. Travillot, G. Diou, L. RodehOser * Laboratoire d'Etudes des Syst~mes Organiques et Collofdaux ( LESOC ), {:RA CARS 406, University; Henri Poim'ar~!. BP 239, F-54506 Vandoeuvre-les-nancy Cedex, France
Received 12 September 1995; accepted 26 February 1996
Abstract Bifunctional fl-diphosphorylated ligands of general formula [ R 2 P ( O ) ] 2 N - X - N [ P ( O ) R 2 ] 2 with R = O C H 3 or form stable complexes with a variety of metal cations. Under certain conditions, coordination aggregates can be obtained as shown by multinuclear NMR spectroscopy, UV/vis spectrometry, and electrochemical techniques. We have studied systems formed by the above-mentioned compounds with the divalent metal cations Mg 2+ and Cd 2+ by 31p and 113Cd NMR spectroscopy. The spectra reveal the presence of different types of coordinating groups for the ligands and different environments for the metal ions in solution. A detailed analysis of the data in combination with conductivity and viscosity measurements allowed us to conclude that polyionic chainlike structures are formed in these systems. N(CH3) 2
Kcywords: Coordination aggregates; Metal ion complexes; NMR; Phosphoryl compounds
1. Introduction
2. Experimental
In previous publications we have given evidence for the strong complexing ability of fl-diphosphoryl groups in compounds such as imidodiphosphoramides [1] and pyrophosphoramides [2]. Molecules derived from these compounds which show a pronounced surfactant character form micelles and more extended organized phases in aqueous solutions [3]. On the basis of these results it seemed worthwhile to synthesize linear molecules bearing fldiphosphoryl groups at both ends with the aim of obtaining di- or tridimensional supramolecular aggregates in the presence of metal cations.
2.1. Materials
* Corresponding author. 0927-7757/96"$15.00 ~) 1996 ElsevierScience B.V. All rights reserved Pll S0927-7757(96)03613-8
The complexes M(OMPA)3(X)2 ( M = C d 2+, Mg 2+, Zn z - , CoZ+; X = C 1 0 4 , BF4) were prepared in the solid state from the hydrated metal perchlorates or tetrafluoroborates (Alfa, 99% purity) and the ligands in the presence of methylorthoformiate (Fluka) in a glove box under an atmosphere of purified argon. Caution: all metal
perchlorates are potentially explosive and should be prepared and handled in small quantities only! Anhydrous CD3NO 2 and CD3CN were purchased from CEA, Service des Molecules Marquees; the solvents C H 3 N O a and CH3CN for conductivity measurements were from Aldrich and SDS respectively.
150
N. Tr~villot et al./Colloids Surfaces A: Physicochem. Eng. Aspects 115 (1996) 149-155
2.2. Sample preparation Solutions of constant metal concentration but different ligand/metal ratios were prepared by weighing the corresponding amounts of ligand into a volumetric flask and completing with a solution of M(OMPA)3(X)2 of known concentration in CD3NO z. The ligand-to-metal ratios were calculated from the masses of the bifunctional ligand and of the added solution. All samples were prepared in a glove box.
2.3. NMR spectroscopy 113Cd and 31p spectra were recorded on a Bruker DRX 400 spectrometer at 88.74 and 161.93 MHz respectively; the temperature was held constant using a high precision temperature control unit of the spectrometer. The 3~p spectra were proton-decoupled by a WALTZ pulse sequence. Chemical shifts are referenced to a 0.1 M solution of Cd(C104) 2 in D 2 0 for cadmium and to 85% aqueous H3PO 4 for phosphorus. Lower shift values indicate increased shielding. The solvents CD3NO 2 and CD3CN served as internal lock substances. Samples were placed in 10 mm (H3Cd) or 5 mm (31p) o.d. tubes. Typical acquisition parameters for lX3Cd spectra are: 5000 scans over 4k points, zero-filled to 16k, sweep width 2500 Hz, 45 ° pulse, relaxation delay 2 s; and for 31p: 200 scans over 8k points, zerofilled to 16k, sweep width 4000 Hz, 45 ° pulse, relaxation delay 5 s, quadrature detection. 7"1relaxation times for cadmium and phosphorus are significantly less than 1 s in these systems so that the indicated relaxation delays and pulse angles ensure the quantitative determination of the species studied.
2.4. Conductivity measurements The conductance of solutions of different ligandto-metal ratios R is measured in a cell thermostated to 0.05°C under a slow stream of ultra-pure, solvent-saturated nitrogen, using a Wayne-Kerr B331 autobalance precision bridge.
3. Results and discussion The molecular linking units chosen for this study are compounds of general formula [ R z P ( O ) ] z N - C , H z , - N [P(O)R212, with R= O C H 3 or N(CH3) 2 and n=6. Their synthesis follows essentially a three-step procedure described elsewhere [4], starting from alkyl-c~, m-diamines and phosphortrichloride. The metal salts are introduced in the form of anhydrous trisoctamethylpyrophosphate (OMPA) complexes, M(OMPA)3(X)2, where X = C104 or BF4. This is to avoid the formation of partially hydrated species in the nonaqueous solvents acetonitrile and nitromethane used as non-coordinating polar media. It is known from previous experiments [5] that imido-diphosphoryl compounds, [RzP(O)]zN-R' ( R ' = a l k y l group), readily substitute pyrophosphates in complexes of di- and trivalent metal ions in these media. When increasing amounts of bifunctional ligands are added to solutions of the abovementioned OMPA complexes, we may therefore expect that they replace one or more OMPA molecules at different metal sites simultaneously, and thereby form linking bridges between two or more metal centers. The continuation of this process would lead to extended two- or threedimensional aggregates. When this experiment is carried out starting from the complex salt [Cd(OMPA)3](C104) 2 and the bifunctional ligand [(Me2N)2P(O)]2N-C6Hlz N[P(O)(NMe2)2]z (I), characteristic 113Cd N M R spectra are obtained. Cadmium has two magnetically-active isotopes, lllCd and 113Cd, of 12.86 and 12.34% natural abundance respectively, both of nuclear spin 1/2, which can be observed rather easily with modern multinuclear N M R equipment. The sensitivity of the latter isotope is slightly higher so that most N M R studies of this element are carried out at the resonance frequency of 113Cd (88.47 MHz at 9.4 T). One of the most outstanding features of cadmium N M R is the very extended chemical shift dispersion (>800 ppm) of this nucleus [6], depending on its local environment, which makes it a valuable probe in the elucidation of the structure and composition of metal-ligand coordination compounds.
N. Tr&,illot et al./Colloids Surfaces A: Physicochem. End,,. Asl,ects 115 ( 1996~ 149 155
Moreover, scalar couplings with other spin //2 nuclei such as ~ P often allow one to deduce directly the number of ligands bound to the cadmium ions and the mode of binding, e.g. monodentate or bidentate coordination [7]. As shown in Fig. la, the spectrum obtained for a 5 × 10 2M solution of the complex salt in nitromethane in the absence of ligand I is a heptuplet centered at - 3 4 . 7 ppm, due to a coupling of 29.5 Hz with the six magnetically equivalent 3~p nuclei of the three O M P A molecules in the initial monomeric complex Cd(OMPA)3+. At a molar ratio R (R = [ I ] / [ C d ] ) of 0.25, a second heptuplet is observed at - 4 5 . 4 p p m (Fig. lb). The ratio of the areas, I /I', of the two multiplets is close to unity (0.97). On increasing R to a value of 0.5, the signal at higher field is
a) I
b)
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practically the only one observed, the initial signal of Cd(OMPA),~ + having disappeared ( Fig. lcl. For R>0.5, a third heptuplet at still higher field ( - 4 8 . 6 ppm) appears (Fig. ld), with an intensity of 0.91 at R = 0 . 7 5 with respect to the second heptuplet. Finally, for a wdue of R equal to 0.9, the ratio of intensities increases to 3.7. For still higher R values ( > 1 ) the integral ratio levels off" at a value of v 10. The metal concentration is kept constant at 5 × 10 2 M throughout the series of experiments. The shift values encountered in these spectra are characteristic for cadmium nuclei surrounded by six oxygen atoms [8]. The constant value of the coupling constant 2ycd v- of 29.5 Hz for the difl'erent multiplets, indicates the absence of anion or solvent coordination to the metal center [9]. The observed spectral changes can be explained satisfactorily if we assume that the ligand I substitutes in successive steps one or two O M P A molecules in each initial complex C d ( O M P A E ~ as indicated in Scheme I. It is clear from Scheme 1 that the cadmium nuclei m this system can be found in three different environments: (i) surrounded by three pyrophosphate ligands in the initial complex: (ii) bound to one iminodiphosphoryl and two pyrophosphate moieties in the terminal groups of the chains: or (iii) coordinated to two iminodiphosphoryl groups and one O M P A molecule in the inner links of the chains. These different environmcnts give rise. in the ~3Cd N M R spectra, to the three multiplets observed at -34.7, 45.4 and 48.6 ppm respectively. If we assume that for all these steps the substitution of OM PA by I is quantitative, i.e. the equilibrium is shifted completely to the right-hand side, then we can calculate the relative intensities of the different N M R signals as a function of the ligand l-to-metal ratio. R, from the relationships I I'
1 2R
1
and
1 I'
2\ 1 - R
)
-30
-35
-40
ppm
-45
-50
-55
Fig. I. ~t'~Cd spectra at 20 C and 88.74MHz for different ratios R of ligand I to Cd: 0 (complex Cd(OMPA)2+ only}(a); .,) 0..5 {bl: 0.5 (c}:0.75 (d).
where I . I', and I" are the integrals of the three heptuplets in the order of increasing field strength. The results of these model calculations are shown in Fig. 2, The experimentally determined values have been
N. Trkvillot et al./Colloids Surfaces A." Physicochem. Eng. Aspects 115 (1996) 149-155
152
g 1
2Cd(OMPA)3 + 1
~'
q
Cd(OMPA)2 I-Cd(OMPA), + 2OMPA
(Ao)
(A,) g 2
A~+ I + Cd(OMPA)3
Cd(OMPA)2-I-Cd(OMPA~I-Cd(OMPA)2
4
(A2)
+ 2OMPA
K n
A~I + I + Cd(OMPA)3
Cd(OMPA)z[-I-Cd(OMPA)],
41
(A.)
+ 2OMPA
Scheme 1.
12
10
•
Integral ratio
'
j
Ii
8 6
i io/r
4 \\,\
2
\ J
0 0
0,2
0,4
0,6
0,8
1
1,2
FI 1,4
Fig. 2. Calculated and experimental integral ratios I ° / I ' and I"/I' as a function of the ligand-to-metal ratio R.
added to Fig. 2 for comparison. The fact that practically no species other than A0 and A~ are observed below R=0.5 indicates that K1 should be at least 103 times higher than K2, i.e. the first step of Scheme 1 is favoured with respect to the following steps. It is evident that for higher values of R the experimental points deviate significantly from the calculated values. This indicates that the maximum chain length which can be obtained in this system is limited to ~20 units and that for greater values of R the assumption of quantitative substitution of OMPA is therefore no longer valid, i.e. the Ki values decrease significantly for higher i. No evidence can be found, neither from the H3Cd nor from the corresponding 3~p spectra (e.g.
Fig. 3c), for the existence of cyclic or tridimensional aggregates in this system. When we replace the initial cadmium complex by its magnesium analogue, it is no longer possible to observe the metal nucleus directly, due to the low sensitivity of magnetically active magnesium isotopes and their generally fast nuclear quadrupolar relaxation [ 10]. In this case, however, pertinent information can be obtained from phosphorus NMR. The initial Mg(OMPA)32+ complex yields a sharp singlet at 12.6 ppm. Upon addition of ligand I, new lines at 23.65, 12.65 and 10.45 ppm appear as well as two small signals at 23.75 and 12.4 ppm for a ligand-to-metal ratio of 0.5 (Fig. 3a). On the basis of their relative intensities and the spectra of previously studied mixed ligand complexes of Mg 2+, we can assign the major signals 2 and 2' to a species where the cation is surrounded by two OMPA ligands and one imidodiphosphoryl group, and line 3 to free OMPA. This is in agreement with the predominant formation of a dimer, (OMPA)2Mg-LMg(OMPA)2, according to Scheme i, accompanied by the liberation of one OMPA ligand per initial complex. When R increases, the smaller signals 1 and 1' gain in intensity and finally outweigh the initial lines 2 and 2' by a factor of ~ 4 when R reaches 1 (Fig. 3b). These signals correspond to complex species where one OMPA ligand and two imidophosphoryl
N. Trg'villot et al./Colloids Surfaces A: Physicochem. Eng. Aspects 115 (1996) 149 155
L53
2'
a) 2
3
5 b)
3
4
2' 1
25
20
1'
15
lO
ppm
5
2'
c) 2
25
20
15
ppm
10
Fig. 3. ~IP{~H} spectra at 161.98 M H z of the system Mg 2*/'OMPA,'ligand I at 21) C for R =0.5 (at and R = 1 (bl and of the system Cd2 +/OMPA/ligand l at - 2 7 ° C for R = 0 . 5 (c).
groups compose the coordination sphere of Mg 2+ e.g. the inner links of polynuclear chains formed in this system. The relatively low ratio of the integrals of lines 1 + 1' with respect to signals 2 + 2' indicates, however, that the aggregates are rather small in this case, the maximum chain length being about 10 units. This may be due to a stronger solvation of magnesium, an ion of relatively high charge density, by the solvent acetonitrile used as medium in these experiments. The ligand 1 present in excess in the solution gives rise, to signal 4 (at 19.3 ppm in Fig. 3b), An interesting feature of this
spectrum is the appearance of a second signal (4'i in the region characteristic of uncoordinated imidophosphoryl groups, the chemical shift of which varies slightly with the concentration of I. Possibly a fraction of the bifunctional ligand is coordinated to the metal by only one of its complexing groups, the second being surrounded by solvent and therefore in a similar chemical environment as the free ligand (see Fig. 3c). As in the case of cadmium, no evidence exists either for the formation of threedimensional aggregates by interconnection of the polynuclear chains or for mononuclear species in
154
N. Trbvillot et al./Colloids Surfaces A. Physicochem. Eng. Aspects 115 (1996) 149-155
which the metal centers are triply coordinated to the bifunctional ligands. In order to c o r r o b o r a t e the conclusions drawn from the above N M R experiments we have carried out conductivity measurements for all the systems studied. The results are shown in Fig. 4. In addition to c a d m i u m and magnesium, we have also measured the conductivity changes of solutions of cobalt(lI) and zinc(II), ions of similar ionic radii. The general observation is that for all ions studied, the conductivity decreases rapidly with increasing R. This would be expected for systems where initially m o n o n u c l e a r complex ions self-assemble into more or less extended polyionic aggregates, the mobilities of which are largely reduced when c o m p a r e d to the monomers. A m o n g the systems studied, those based on Cd 2+, Co 2+ and Zn z+ behave quite similarly, whereas the curve obtained for magnesium is clearly flatter than for the other ions, thereby confirming the conclusion from the N M R experiments that shorter chains are formed in this case. Viscosity measurements have shown that the variations of the conductivity of the different metal ions is not due to i m p o r t a n t changes in viscosity. At low concentrations of ligands (5 x 10 3 mol 1-1 < C I < 4 x 10 2 m o l l -1) and metals ( C u = 1 x 10 -2 tool I x t h r o u g h o u t ) viscosity changes do not exceed 5%, whereas changes in conductivity are of the order of 20%. The ability of ligand lI, in which the N(CHg)E groups are replaced by OCH3 substituents, to form polyionic chains has proved to be largely inferior to that of c o m p o u n d I. Rapid ligand exchange reactions lead to line-broadening in N M R spectro-
400
350
A(S.cm2/mol
)
r.
M
'~~-,~_ -
300
\B-
200 *- - 0
~--I
~
- ---~ _ __
c> -Co O- iJ
.~
÷-
-~---
2
3
-~ R
4
Fig. 4. Variation of molar conductivity for systems M2+/OMPA/ligand I (M = Mg, Co, Zn, Cd) in acetonitrile as a function of the ligand-to-metal ratio R at 20°C.
scopy even at lower temperatures, and conductivity as well as spectrophotometric measurements show that practically no polyionic aggregates are formed with this c o m p o u n d . To summarize, the results from N M R experiments and c o m p l e m e n t a r y techniques show that neutral, non-ionic bifunctional e,co-bis(imidodiphosphoramides) induce the formation, in the presence of metal salts, of linear polyionic aggregates in solution which can be considered as coordination oligomers [ 1 1 ] . Studies are under way to verify whether they yield coordination polymers of higher molecular weight under more favourable conditions.
Acknowledgements
The authors are indebted to E. Eppiger for help with N M R measurements; they thank Professor C. Selve for fruitful discussions concerning the synthesis of the ligands. They acknowledge financial support from the CNRS.
References
[1] (a) P. Rubini, L. Rodehaser and J.-J. Delpuech, Inorg. Chem., 18 (1979) 2962. (b) C. Ben Nasr, L. Rodehaser, P. Rubini and J.-J. Delpuech, J. Chim. Phys., Phys.-Chim. Biol. 83 (1986) 499. [2] G. Doucet Ladeveze, L. Rodehiiser, P. Rubirli and J.-J. Delpuech, Nouv. J. Chim., 8 (1984) 93. [3] N. Laakel, P. Rubini, L. Rodehaser and J.-J. Delpuech, New J. Chem., 16 (1992) 809. [4] (a) G. Doucet Ladeveze, L. Rodehtiser, P. Rubini, C. Selve and J.-J. Delpuech, Tetrahedron Lett., 23 (1982) 643. (b) G. Doucet Ladeveze, Y. Jabbari Azad, L. Rodeh~iser, P. Rubini, C. Selve and J.-J. Delpuech, Tetrahedron, 42 (1986) 371. (c) L, Rodeh~lser, D. Chouchi, N. Tr6villot and C. Selve, unpublished work. [5] (a) T. Chniber, Thesis, University of Nancy I, 1986. (b) L. Rodeht~ser, T. Chniber and P. Rubini, Quim. Anal., 14 (1995) (in press). [6] C. Brevard and P, Granger, Handbook of High Resolution Multinuclear NMR, John Wiley, New York, 1981, pp. 164-165. [7] L. Rodehaser, T. Chniber, P. Rubini and J.-J. Delpuech, Inorg. Chim. Acta, 148(2) (1988) 227.
N. TrSvillot et al./Colloids SurJaces A: Physicochem. Eng. Aspects 115 (1996, 149 155 [8] G.E. Maciel and M. Borzo, J. Chem. Soc.. Chem. Commun., (1973) 394. [9] P.A.W. Dean and L. Polensek, Can. J. Chem.. 58 11980) 1627.
155
[ 10] K.J. Neurohr, T. Drakenberg and S. Forsdn, in P. Lazslo (Ed.), NMR of Newly Accessible Nuclei, Vol. 2, Academic Press, New York, 1983, pp. 229 246. [11] M. Hanack, Synth. Met., 15(1986)207.
Colloids and Surfaces A: Physicochemicatand Engineering Aspects 115 11996) 149 155
COLLOIDS AND SURFACES
A
Coordination aggregates of bifunctional fi-diphosphoryl compounds as studied by 31p and l 3Cd NMR in solution N. Travillot, G. Diou, L. RodehOser * Laboratoire d'Etudes des Syst~mes Organiques et Collofdaux ( LESOC ), {:RA CARS 406, University; Henri Poim'ar~!. BP 239, F-54506 Vandoeuvre-les-nancy Cedex, France
Received 12 September 1995; accepted 26 February 1996
Abstract Bifunctional fl-diphosphorylated ligands of general formula [ R 2 P ( O ) ] 2 N - X - N [ P ( O ) R 2 ] 2 with R = O C H 3 or form stable complexes with a variety of metal cations. Under certain conditions, coordination aggregates can be obtained as shown by multinuclear NMR spectroscopy, UV/vis spectrometry, and electrochemical techniques. We have studied systems formed by the above-mentioned compounds with the divalent metal cations Mg 2+ and Cd 2+ by 31p and 113Cd NMR spectroscopy. The spectra reveal the presence of different types of coordinating groups for the ligands and different environments for the metal ions in solution. A detailed analysis of the data in combination with conductivity and viscosity measurements allowed us to conclude that polyionic chainlike structures are formed in these systems. N(CH3) 2
Kcywords: Coordination aggregates; Metal ion complexes; NMR; Phosphoryl compounds
1. Introduction
2. Experimental
In previous publications we have given evidence for the strong complexing ability of fl-diphosphoryl groups in compounds such as imidodiphosphoramides [1] and pyrophosphoramides [2]. Molecules derived from these compounds which show a pronounced surfactant character form micelles and more extended organized phases in aqueous solutions [3]. On the basis of these results it seemed worthwhile to synthesize linear molecules bearing fldiphosphoryl groups at both ends with the aim of obtaining di- or tridimensional supramolecular aggregates in the presence of metal cations.
2.1. Materials
* Corresponding author. 0927-7757/96"$15.00 ~) 1996 ElsevierScience B.V. All rights reserved Pll S0927-7757(96)03613-8
The complexes M(OMPA)3(X)2 ( M = C d 2+, Mg 2+, Zn z - , CoZ+; X = C 1 0 4 , BF4) were prepared in the solid state from the hydrated metal perchlorates or tetrafluoroborates (Alfa, 99% purity) and the ligands in the presence of methylorthoformiate (Fluka) in a glove box under an atmosphere of purified argon. Caution: all metal
perchlorates are potentially explosive and should be prepared and handled in small quantities only! Anhydrous CD3NO 2 and CD3CN were purchased from CEA, Service des Molecules Marquees; the solvents C H 3 N O a and CH3CN for conductivity measurements were from Aldrich and SDS respectively.
150
N. Tr~villot et al./Colloids Surfaces A: Physicochem. Eng. Aspects 115 (1996) 149-155
2.2. Sample preparation Solutions of constant metal concentration but different ligand/metal ratios were prepared by weighing the corresponding amounts of ligand into a volumetric flask and completing with a solution of M(OMPA)3(X)2 of known concentration in CD3NO z. The ligand-to-metal ratios were calculated from the masses of the bifunctional ligand and of the added solution. All samples were prepared in a glove box.
2.3. NMR spectroscopy 113Cd and 31p spectra were recorded on a Bruker DRX 400 spectrometer at 88.74 and 161.93 MHz respectively; the temperature was held constant using a high precision temperature control unit of the spectrometer. The 3~p spectra were proton-decoupled by a WALTZ pulse sequence. Chemical shifts are referenced to a 0.1 M solution of Cd(C104) 2 in D 2 0 for cadmium and to 85% aqueous H3PO 4 for phosphorus. Lower shift values indicate increased shielding. The solvents CD3NO 2 and CD3CN served as internal lock substances. Samples were placed in 10 mm (H3Cd) or 5 mm (31p) o.d. tubes. Typical acquisition parameters for lX3Cd spectra are: 5000 scans over 4k points, zero-filled to 16k, sweep width 2500 Hz, 45 ° pulse, relaxation delay 2 s; and for 31p: 200 scans over 8k points, zerofilled to 16k, sweep width 4000 Hz, 45 ° pulse, relaxation delay 5 s, quadrature detection. 7"1relaxation times for cadmium and phosphorus are significantly less than 1 s in these systems so that the indicated relaxation delays and pulse angles ensure the quantitative determination of the species studied.
2.4. Conductivity measurements The conductance of solutions of different ligandto-metal ratios R is measured in a cell thermostated to 0.05°C under a slow stream of ultra-pure, solvent-saturated nitrogen, using a Wayne-Kerr B331 autobalance precision bridge.
3. Results and discussion The molecular linking units chosen for this study are compounds of general formula [ R z P ( O ) ] z N - C , H z , - N [P(O)R212, with R= O C H 3 or N(CH3) 2 and n=6. Their synthesis follows essentially a three-step procedure described elsewhere [4], starting from alkyl-c~, m-diamines and phosphortrichloride. The metal salts are introduced in the form of anhydrous trisoctamethylpyrophosphate (OMPA) complexes, M(OMPA)3(X)2, where X = C104 or BF4. This is to avoid the formation of partially hydrated species in the nonaqueous solvents acetonitrile and nitromethane used as non-coordinating polar media. It is known from previous experiments [5] that imido-diphosphoryl compounds, [RzP(O)]zN-R' ( R ' = a l k y l group), readily substitute pyrophosphates in complexes of di- and trivalent metal ions in these media. When increasing amounts of bifunctional ligands are added to solutions of the abovementioned OMPA complexes, we may therefore expect that they replace one or more OMPA molecules at different metal sites simultaneously, and thereby form linking bridges between two or more metal centers. The continuation of this process would lead to extended two- or threedimensional aggregates. When this experiment is carried out starting from the complex salt [Cd(OMPA)3](C104) 2 and the bifunctional ligand [(Me2N)2P(O)]2N-C6Hlz N[P(O)(NMe2)2]z (I), characteristic 113Cd N M R spectra are obtained. Cadmium has two magnetically-active isotopes, lllCd and 113Cd, of 12.86 and 12.34% natural abundance respectively, both of nuclear spin 1/2, which can be observed rather easily with modern multinuclear N M R equipment. The sensitivity of the latter isotope is slightly higher so that most N M R studies of this element are carried out at the resonance frequency of 113Cd (88.47 MHz at 9.4 T). One of the most outstanding features of cadmium N M R is the very extended chemical shift dispersion (>800 ppm) of this nucleus [6], depending on its local environment, which makes it a valuable probe in the elucidation of the structure and composition of metal-ligand coordination compounds.
N. Tr&,illot et al./Colloids Surfaces A: Physicochem. End,,. Asl,ects 115 ( 1996~ 149 155
Moreover, scalar couplings with other spin //2 nuclei such as ~ P often allow one to deduce directly the number of ligands bound to the cadmium ions and the mode of binding, e.g. monodentate or bidentate coordination [7]. As shown in Fig. la, the spectrum obtained for a 5 × 10 2M solution of the complex salt in nitromethane in the absence of ligand I is a heptuplet centered at - 3 4 . 7 ppm, due to a coupling of 29.5 Hz with the six magnetically equivalent 3~p nuclei of the three O M P A molecules in the initial monomeric complex Cd(OMPA)3+. At a molar ratio R (R = [ I ] / [ C d ] ) of 0.25, a second heptuplet is observed at - 4 5 . 4 p p m (Fig. lb). The ratio of the areas, I /I', of the two multiplets is close to unity (0.97). On increasing R to a value of 0.5, the signal at higher field is
a) I
b)
,lulitl
i'll, t
v
~j"
2
,?
c)
..............
•
••••;•
• ••*••,••,
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••••
• • • •
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••••••••••
•••
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practically the only one observed, the initial signal of Cd(OMPA),~ + having disappeared ( Fig. lcl. For R>0.5, a third heptuplet at still higher field ( - 4 8 . 6 ppm) appears (Fig. ld), with an intensity of 0.91 at R = 0 . 7 5 with respect to the second heptuplet. Finally, for a wdue of R equal to 0.9, the ratio of intensities increases to 3.7. For still higher R values ( > 1 ) the integral ratio levels off" at a value of v 10. The metal concentration is kept constant at 5 × 10 2 M throughout the series of experiments. The shift values encountered in these spectra are characteristic for cadmium nuclei surrounded by six oxygen atoms [8]. The constant value of the coupling constant 2ycd v- of 29.5 Hz for the difl'erent multiplets, indicates the absence of anion or solvent coordination to the metal center [9]. The observed spectral changes can be explained satisfactorily if we assume that the ligand I substitutes in successive steps one or two O M P A molecules in each initial complex C d ( O M P A E ~ as indicated in Scheme I. It is clear from Scheme 1 that the cadmium nuclei m this system can be found in three different environments: (i) surrounded by three pyrophosphate ligands in the initial complex: (ii) bound to one iminodiphosphoryl and two pyrophosphate moieties in the terminal groups of the chains: or (iii) coordinated to two iminodiphosphoryl groups and one O M P A molecule in the inner links of the chains. These different environmcnts give rise. in the ~3Cd N M R spectra, to the three multiplets observed at -34.7, 45.4 and 48.6 ppm respectively. If we assume that for all these steps the substitution of OM PA by I is quantitative, i.e. the equilibrium is shifted completely to the right-hand side, then we can calculate the relative intensities of the different N M R signals as a function of the ligand l-to-metal ratio. R, from the relationships I I'
1 2R
1
and
1 I'
2\ 1 - R
)
-30
-35
-40
ppm
-45
-50
-55
Fig. I. ~t'~Cd spectra at 20 C and 88.74MHz for different ratios R of ligand I to Cd: 0 (complex Cd(OMPA)2+ only}(a); .,) 0..5 {bl: 0.5 (c}:0.75 (d).
where I . I', and I" are the integrals of the three heptuplets in the order of increasing field strength. The results of these model calculations are shown in Fig. 2, The experimentally determined values have been
N. Trkvillot et al./Colloids Surfaces A." Physicochem. Eng. Aspects 115 (1996) 149-155
152
g 1
2Cd(OMPA)3 + 1
~'
q
Cd(OMPA)2 I-Cd(OMPA), + 2OMPA
(Ao)
(A,) g 2
A~+ I + Cd(OMPA)3
Cd(OMPA)2-I-Cd(OMPA~I-Cd(OMPA)2
4
(A2)
+ 2OMPA
K n
A~I + I + Cd(OMPA)3
Cd(OMPA)z[-I-Cd(OMPA)],
41
(A.)
+ 2OMPA
Scheme 1.
12
10
•
Integral ratio
'
j
Ii
8 6
i io/r
4 \\,\
2
\ J
0 0
0,2
0,4
0,6
0,8
1
1,2
FI 1,4
Fig. 2. Calculated and experimental integral ratios I ° / I ' and I"/I' as a function of the ligand-to-metal ratio R.
added to Fig. 2 for comparison. The fact that practically no species other than A0 and A~ are observed below R=0.5 indicates that K1 should be at least 103 times higher than K2, i.e. the first step of Scheme 1 is favoured with respect to the following steps. It is evident that for higher values of R the experimental points deviate significantly from the calculated values. This indicates that the maximum chain length which can be obtained in this system is limited to ~20 units and that for greater values of R the assumption of quantitative substitution of OMPA is therefore no longer valid, i.e. the Ki values decrease significantly for higher i. No evidence can be found, neither from the H3Cd nor from the corresponding 3~p spectra (e.g.
Fig. 3c), for the existence of cyclic or tridimensional aggregates in this system. When we replace the initial cadmium complex by its magnesium analogue, it is no longer possible to observe the metal nucleus directly, due to the low sensitivity of magnetically active magnesium isotopes and their generally fast nuclear quadrupolar relaxation [ 10]. In this case, however, pertinent information can be obtained from phosphorus NMR. The initial Mg(OMPA)32+ complex yields a sharp singlet at 12.6 ppm. Upon addition of ligand I, new lines at 23.65, 12.65 and 10.45 ppm appear as well as two small signals at 23.75 and 12.4 ppm for a ligand-to-metal ratio of 0.5 (Fig. 3a). On the basis of their relative intensities and the spectra of previously studied mixed ligand complexes of Mg 2+, we can assign the major signals 2 and 2' to a species where the cation is surrounded by two OMPA ligands and one imidodiphosphoryl group, and line 3 to free OMPA. This is in agreement with the predominant formation of a dimer, (OMPA)2Mg-LMg(OMPA)2, according to Scheme i, accompanied by the liberation of one OMPA ligand per initial complex. When R increases, the smaller signals 1 and 1' gain in intensity and finally outweigh the initial lines 2 and 2' by a factor of ~ 4 when R reaches 1 (Fig. 3b). These signals correspond to complex species where one OMPA ligand and two imidophosphoryl
N. Trg'villot et al./Colloids Surfaces A: Physicochem. Eng. Aspects 115 (1996) 149 155
L53
2'
a) 2
3
5 b)
3
4
2' 1
25
20
1'
15
lO
ppm
5
2'
c) 2
25
20
15
ppm
10
Fig. 3. ~IP{~H} spectra at 161.98 M H z of the system Mg 2*/'OMPA,'ligand I at 21) C for R =0.5 (at and R = 1 (bl and of the system Cd2 +/OMPA/ligand l at - 2 7 ° C for R = 0 . 5 (c).
groups compose the coordination sphere of Mg 2+ e.g. the inner links of polynuclear chains formed in this system. The relatively low ratio of the integrals of lines 1 + 1' with respect to signals 2 + 2' indicates, however, that the aggregates are rather small in this case, the maximum chain length being about 10 units. This may be due to a stronger solvation of magnesium, an ion of relatively high charge density, by the solvent acetonitrile used as medium in these experiments. The ligand 1 present in excess in the solution gives rise, to signal 4 (at 19.3 ppm in Fig. 3b), An interesting feature of this
spectrum is the appearance of a second signal (4'i in the region characteristic of uncoordinated imidophosphoryl groups, the chemical shift of which varies slightly with the concentration of I. Possibly a fraction of the bifunctional ligand is coordinated to the metal by only one of its complexing groups, the second being surrounded by solvent and therefore in a similar chemical environment as the free ligand (see Fig. 3c). As in the case of cadmium, no evidence exists either for the formation of threedimensional aggregates by interconnection of the polynuclear chains or for mononuclear species in
154
N. Trbvillot et al./Colloids Surfaces A. Physicochem. Eng. Aspects 115 (1996) 149-155
which the metal centers are triply coordinated to the bifunctional ligands. In order to c o r r o b o r a t e the conclusions drawn from the above N M R experiments we have carried out conductivity measurements for all the systems studied. The results are shown in Fig. 4. In addition to c a d m i u m and magnesium, we have also measured the conductivity changes of solutions of cobalt(lI) and zinc(II), ions of similar ionic radii. The general observation is that for all ions studied, the conductivity decreases rapidly with increasing R. This would be expected for systems where initially m o n o n u c l e a r complex ions self-assemble into more or less extended polyionic aggregates, the mobilities of which are largely reduced when c o m p a r e d to the monomers. A m o n g the systems studied, those based on Cd 2+, Co 2+ and Zn z+ behave quite similarly, whereas the curve obtained for magnesium is clearly flatter than for the other ions, thereby confirming the conclusion from the N M R experiments that shorter chains are formed in this case. Viscosity measurements have shown that the variations of the conductivity of the different metal ions is not due to i m p o r t a n t changes in viscosity. At low concentrations of ligands (5 x 10 3 mol 1-1 < C I < 4 x 10 2 m o l l -1) and metals ( C u = 1 x 10 -2 tool I x t h r o u g h o u t ) viscosity changes do not exceed 5%, whereas changes in conductivity are of the order of 20%. The ability of ligand lI, in which the N(CHg)E groups are replaced by OCH3 substituents, to form polyionic chains has proved to be largely inferior to that of c o m p o u n d I. Rapid ligand exchange reactions lead to line-broadening in N M R spectro-
400
350
A(S.cm2/mol
)
r.
M
'~~-,~_ -
300
\B-
200 *- - 0
~--I
~
- ---~ _ __
c> -Co O- iJ
.~
÷-
-~---
2
3
-~ R
4
Fig. 4. Variation of molar conductivity for systems M2+/OMPA/ligand I (M = Mg, Co, Zn, Cd) in acetonitrile as a function of the ligand-to-metal ratio R at 20°C.
scopy even at lower temperatures, and conductivity as well as spectrophotometric measurements show that practically no polyionic aggregates are formed with this c o m p o u n d . To summarize, the results from N M R experiments and c o m p l e m e n t a r y techniques show that neutral, non-ionic bifunctional e,co-bis(imidodiphosphoramides) induce the formation, in the presence of metal salts, of linear polyionic aggregates in solution which can be considered as coordination oligomers [ 1 1 ] . Studies are under way to verify whether they yield coordination polymers of higher molecular weight under more favourable conditions.
Acknowledgements
The authors are indebted to E. Eppiger for help with N M R measurements; they thank Professor C. Selve for fruitful discussions concerning the synthesis of the ligands. They acknowledge financial support from the CNRS.
References
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155
[ 10] K.J. Neurohr, T. Drakenberg and S. Forsdn, in P. Lazslo (Ed.), NMR of Newly Accessible Nuclei, Vol. 2, Academic Press, New York, 1983, pp. 229 246. [11] M. Hanack, Synth. Met., 15(1986)207.