Spectroscopy and structures of neodymium solvates in high dielectric constant solvents

.L inorg, nucl, Chem. Vol. 43, No. 10, pp. 2393-2398, 1981 Printed in Great Britain.

0022-1%2/81/102393-06502.0()/0 PergamonPress l.td.

SPECTROSCOPY AND STRUCTURES OF NEODYMIUM SOLVATES IN HIGH DIELECTRIC CONSTANT SOLVENTS J. LEGENDZIEWICZ,* K. BUKIETYIqSKA and G. OCZKO Institute of Chemistry, University of Wroc,taw, WrocJ'aw, Poland

(Received 4 October 1979; receivedfor publication 5 October 1980)

Abstract--The electronic spectra of neodymium nitrate in mono- and disubstituted amides were investigated. The transitions were identified and the oscillator strengths of the 4fi-41"transitions were determined. The ~-, parameters were evaluated from the Judd equation and good agreement between Pox, and P~¢ was found. From the spectroscopic data, ultrasonic absorption and conductivities, low-symmetry inner-sphere [NdNO3(DMF)u ~]2÷species were found to be formed in the disubstituted amides. spectrophotometer. The appropriate solvents were used as reference solutions. N,N'-dimethylformamide (DMF) and Nmethylformamide (MFA) were purified by drying over anhydrous sodium sulphate and then distilled five times under vacuum with the intermediate fraction being collected each time. Physical properties of the solvents used were as follows: refractive index --nDzS= 1.4320 for DMF and nt,2°= 1.4319 for MFA and specific conductivities--x~uv. = 2.9 × 10-7 ~ - i c m - i , XMFA25 5 X 10-6tq-1 cm-L IR spectra of the DMF purified in this way provide us with evidence that water is absent (at least in any significant amounts). NdCly6H20, Nd(C104)3'7H20 were obtained from oxides (99.99% purity) according to the preparation described earlier [14]. Nd(NO3)ySH~O was provided by Merck. The concentration of Nd3÷ was determined by gravimetric method [9] and by EDTA titration using xylenol orange as the end-point indicator[10]. Ultrasonic absorption measurements in NdCI3, Nd(NO3)3 and Nd(CIO4)3 solutions in DMF and MFA were carried out in an interferometer made in the Institute for Basic Problems of Technology, Polish Academy of Sciences. Dispersion of the velocity of ultrasonic waves in the neodymium nitrate solution was measured by using a laser-ultrasonic interferometer. Molar conductivities of NdC13, Nd(NOs)3 and Nd(CIO4)3 in MFA and DMF were measured at 25°C in a Radelkis OK-102/I conductometer over the concentration range from 0.2 to 0.0008 M. Measurements at lower concentrations are not too precise, because of low accuracy of the instrument in this concentration range.

INTRODUCTION Earlier spectroscopic studies of lanthanide chloride and perchlorate solutions in amide solvents[I--4] demonstrated considerable differences between the solvates of mono- and disubstituted amides. Good agreement between the experimental and calculated values of the oscillator strengths was reached. The intensities of the 4f--4f transitions and ~-~ parameters were found to increase for Nd, Ho, Er chloride solutions in N,Ndimethylformamide as compared to the values for the aquo-ions. This was explained as due to changes in the symmetries of these systems, i.e. to a decrease of the symmetry. In such systems we have assumed the mechanism of an induced electric dipole or heterogeneous dielectric for the phenomenon of hypersensitivity. An increase of the parameter for HoCIy6H20 was higher than that for the light lanthanide chlorides, which may also result from a decrease of the symmetry, either as a result of variations in the coordination number or due to the formation of mixed solvates [Ho CI(DMF)x]. For lanthanide perchlorate solutions no differences between the intensities of 4f--4f transitions were observed for mono- and disubstituted amide solvates. Since the perchlorates were found to contain a homogeneous solvate with both mono- and disubstituted amide, the differences in the spectroscopic properties of lanthanide chlorides in disubstituted amide solvents were explained as due to the formation of mixed solvates. According to our earlier studies of the ultrasonic wave propagation velocities in aqueous lanthanide chloride, perchlorate and nitrate solutions [5, 6], the NOC ions are more inclined to form inner-sphere complexes than the other ions. These data were confirmed in papers dealing with ultrasonic absorption [7] and dispersion of ultrasonic waves in aqueous lanthanide nitrate solutions[8]. In the present paper an attempt is made to correlate the spectroscopic results with the ultrasonic absorption and conductivity data in nonaqueous solutions. The possibility of complexation in these solutions was also considered.

----

EXPERIMENTAL

The absorption spectra of Nd(NO3)3 in MFA and DMF were measured in the I0,000-30,200cm-~ region by means of a Cary 14

RESULTS AND DISCUSSION 4f N transitions (10,000-30,200cm -~ range) in the absorption spectra of neodymium nitrate solutions in amide solvents usually correspond to transition from the ground state ~i-> eJ,. At room temperature all components of the ground state may be assumed as equally populated and the absorption bands correspond to transitions to excited state components. Particular crystal field components of the excited states are not considered. Analysis of the free ion intensity treats the transitions between the ground and excited states as if they took place between the respective centres of gravity. The band intensities correspond mainly to the contribution due to the electric dipole transitions and, for particular bands, also to magnetic dipole transitions. The measured oscillator strengths can be expressed by the relation:

p

*Author to whom correspondence should be addressed. 2393

2303mc 2 f exp = ~ J ~(,,) do"

(1/

2394

J. LEGENDZIEWICZet al.

where e(o) = molar extinction of the band at the wavenumber ~ (cm-~), other symbols have usual meaning. These values were determined for all bands in the measured spectral range using an ICH30 integrating programme. From the Judd equation P=

Y.

¢~,~(t%llU(qtt"¢;.)~l,.,+,

~I~,/~ states we have found slight changes in the root mean square deviation S = 125.79 and ~-x parameters. No direct effect of the lanthanide ion concentration on the oscillator strength values variability was found. Since the 4/.-4/transition intensities and ~', parameters were compared with those found for the aquo-ion, the ~'a parameters for the aquo-ion were determined from the same number of observed transitions as for the nitrate. The results are given in Table 2. The transition intensities were substantially greater than those observed for the aquo-ion. The value of the oscillator strength for the hypersensitive transition of Nd(NO~)~ in the absorption spectra in MFA does not differ from the intensities found for the absorption spectrum of perchlorate in this solvent. The intensities of transitions in DMF are considerably higher and the oscillator strengths for the hypersensitive transition are similar to those found for Nd(NO3h in molten salts LiNO3-KNO~ and Nd(NO~h in ethyl acetate[12]. The calculated ~-x parameters are summarized in Table 2. The ¢2 parameter for Nd(NO~h in DMF was found to increase considerably with respect to Nd(NO~h in MFA (the data for the latter are corn-

(2)

A =2,4,6

the ~', parameters were evaluated using the method of

least squares [l l] (ICH30 programme). One can determine 1~ = (1.085.10" x) -~. z~ (where x = ((n ~ + 2)2/9n)) whose values also are frequently discussed in the literature. The values of P ~ , P¢~ and S = [2(AP)2/(i - 3)] ~/2 are presented in Table 1 and the ~'~ parameters in Table 2. The absorption spectra of Nd(NO~h in MFA and DMF at ct = 0.1002 and c2 = 0.1013 concentrations are shown in Figs. 1 and 2. The figure specified the transitions and energies assumed for calculations for which the lowest root mean square (r.m.s.) deviation was found. For a different set of transition energies to the ~D~/2, 4Ds~ and

N

~= N

0.8

~'o

%

07

"4

0 06 N N

05

o~

A 0.4

c~

°;

to 03

~1"

o,

N

I'

3400

4000

5000

o[~]

~

6000

7000

800

9000

Fig. 1. Absorption spectrum of Nd(NO3h in MFA c~ = 0.1002 M/I d = 0.5 cm with 4/'--4/transitions identification.

_~ 0.5

0.4

~ aO eq

e,I eq

~

~, ~

F.-

02

%

~r

~

eq,

.~

~

~k-'~

.

,,,~ ~

~,

,~

o

I

o,

3,oo

4000

r,ooo

eo'oo

rooo

eooo

900o

Fig. 2, Absorption spectrum of Nd(NO3)3 in DMF c2 = 0.1013 M/I d = 0.5 cm with identification of 4[-4/transitions.

Neodymium solvates in high dielectric constant solvents

~

"

~'~

0

2395

0

,

j

'

0

0

U OD 0

OD

',~

~

O~

I'~

~

O~ 0+,

~ E'~

p0

e'~ '.0

I~ ~-

~

~

~

0

0

II

II

~t

o

•

~0 e~

~

d ,,~

0 ,'4

0

<~

O0

O~ e'~

~

p~

P,l

ol

E ~t 0

•

~

~

~

0+,

~

~D

O~

-

r,+

~.~

Z

o

z~

c~

A

"

+..,.4.

o~,

0

co o~

~o

~o

2~ ×-~

co x 0

0

c~

C~ C) II

~l

t~

~

~

~

(%1

~

04 l%

~,-

cxl

p,,+

~

",0

J. LEGENDZIEWICZ et al.

2396

Table 2. % parameters determined by the Judd method for Nd(N03)3 in MFA and DMF and for the Nd ~+ aquo-ion Nd(NO3) 3

Nd +-3 aq

M FA

O. 1002

DMF

0.07352

0.0597

O. 1013

o'.o767

O. 13574

o.o517

~'~. x 109

5"82+0"79

5"06+0"75

6"b'2+0"89

14"02"+0"92

12"52+0"8~

13"55+-0"79 1"~6+-0"46

'C'I~X. 109

10.04+0.73

10. t8-+,0.69

8. 8.3~.0. 82

7..35+O.84

6.88~,o.78

7.29~0.72 b..92.~.O.b,~

r'~(4~ X 10 9

t5.82+1.01

15.12-+0.96

16,37+1.14

1/¢.11-+1.16

13.48.+1.08

lb,.5/¢+t.01 8.50-+0.6(]

,,~,

0.07352

-----...~

4.72+o.

0.0767 70

11.7o+o. 79

~l~x 10 9

9.5o+o. 6t~

6. ~'34"0.7.3

L~x lO 9

14.11-'o.90

12.59-+1.Ol

~x

109

parable to those determined earlier for Nd(Cl04)3 in MFA) (Table 3). This leads to the conclusion that in both cases the environments of the lanthanide ions are different. In the case of monosubstituted amides, as indicated by a comparison of the 4[-4/ transition intensities in neodymium perchlorate and neodymium chloride, a homogeneous solvate seems present, possibly with an insignificant contribution from an inner-sphere

(900 '

tsoo ,too )6oo ,5oo

in DMF

,aoo

,3oo

complex since a slight increase in Pexp is found for the

~zoo

41912-->4G512,2G712 transition in

MFA as compared with neodymium perchlorate. A considerable increase of the intensity for the absorption spectrum of Nd(NO3)s in

o

'~

I100

~)~ ,ooo

DMF would suggest the presence of mixed solvates

'~ ooo

[Nd(NO3)y(DMF)d with a resulting decrease in the symmetry of the lanthanide ion environment. In order to confirm our suggestions we have carried out ultrasonic absorption and dispersion of the velocity of ultrasonic wave studies in the systems under investigation as well as conductivity measurements. In the disubstituted amide Nd(N03)3 and NdCI3 ultrasonic absorption and dispersion in the 1-50 MHz region at 25°C were found (see Figs. 3-5). This absorption is comparable with that in aqueous lanthanide nitrate and sulphate solutions[7]. In the case of Nd(CIO4)3 in DMF and in solutions of all three salts in MFA no ultrasonic absorption was observed. This confirms our suggestions based upon spectroscopic studies that NdCl3 and Nd(NO3)~ form inner-sphere complexes in the disubstituted amide. Ultrasonic absorption resulting from the presence of ion pairs should occur at higher frequencies than 500 MHz, as suggested by Voleisiene [8] for aqueous lanthanide nitrate solutions. So it may be assumed that the following equilibrium occurs in solution:

~'- soo ~-- too 600 soo

~[k / Nd(NO33) ¢~ / NdCL 3

400'

.- l .m~

aoo 2oo ,oo

,o

~o

~ _ .Nd C (L04 ~) ~o

4:o

~,o

f{MHz]

Fig. 3. Ultrasonic absorption for NdCI3 (c=0.48034M11), Nd(NOsh (c =0.54491 M/I) and Nd(CIO4)3 (c = 0.48803 M/l) in DMF at 25°C. In low frequency ( ~ 1-6 MHz) the increase of the experimental error values was observed. [Ln(DMF),] 3+ + NO3-~[Ln(DMF)2+NO3 -]

I

II

~[Ln(NO3)(DMF)(~ ,]2÷. III

(3)

Table 3. Comparison of r~ parameters for Nd(N03)3 and Nd(ClO4h in MFA and DMF I H l" A D ,,i F ~.,,Lx 109

k.,/ix lu ')

{.%x lU 9

Nd~NO3) 3

5.g2+O.79

p+ 10.~q-O. 7"3

15.b2+1.O1

Nd[CX04) 3

/4-10-+O.03

10.33+0.5S

t5 • 4o+O.~31

I.,~.x 109 13. ?O+U.L;o + t ' :>.Ol-to.9,f

k.tlx

~

~,(x 109

7.$7+-0.79

13.9721.11

I0.9 :+-0.94

11.71+1.1"i

Neodymium solvates in high dielectric constant solvents

2397

O ml$

lt.73.00.

///

75"

MFA

50. 25: 1472 00"

c~

500

7:,

aoo

i

75' 5C

300

25

[~,T._ ~00

I,~710C

IOO

.?5 I0

20

30

50

40

5(

f[MH,]

25 1470 O0

Fig. 4. Ultrasonic absorption for NdCl3 and Nd(NO3)3in MFA at 25°C (c = 0.5 M/I).

15

1

0

3 0 40 50

70

100 150 2 0 0 { , M H z

Fig. 5. Dispersion of the velocity of ultrasonic wave for the Nd(NO3)3 in DMF at 25°C (c = 0.1 M/I). The star signed data are given for different amount of water molecules absorbed during the measurements.

Molar conductivities of the investigated systems are summarized in Tables 4 and 5. In Table 4 the conductivities of neodymium salts in N-methylformamide were compared with these for KCI and CaCI~ in this solvent. Over the concentration range investigated (0.2-0.0008) the conductivities of the neodymium salts are much higher than those for KCI or CaCI> This suggests that the Nd > compounds in MFA behave as 1:3 electrolytes. Thus, conductivity measurements indicate no differences between neodymium chloride, neodymium nitrate or neodymium perchlorate in MFA. Molar conductivities of these compounds in DMF (Table 5) indicate different behaviour for the chloride

and nitrate as compared with the perchlorate in a disubstituted amide. Over the entire concentration range (0.2-0.0008) the conductivities of Nd(C]O4).~ are considerably lower than those of NdCI3 or Nd(NOD3. We have found that both in neodymium chloride and neodymium nitrate solution of the 1:2 electrolyte is predominant but in the Nd(CIOD3 solution--the 1:3 electrolyte[13]. From the conductivity measurements one could not distinguish forms II and 1II (eqn 3) and

Table 4. Molar conductivityof neodymiumsalts, KCI and CaC%in N-methylformamideal 25°C :4a(,c] % ) :~

;~ d [ NO,3) :}

NdCi 3

.JL

.

L~. 2 0 8 0

54.P

0.2o32

¢) . 0 S 3 2

79 •'~

0 ,t~l

3

KCI

1 L,.

79.3

0. 200

S O . ~U

0 . L)()O2

100 .'t 11 -;

0.192

3o

0 . I 8o

1

5o

0.07o~

3q

0. U ~ 2

I '

01

b . u 3~) ;

37

0.028~;

77

0.0115

73

(:' . 0 3 2 D

1U 1 • o

0

U.ol:3t~

115.3

G.012

I I~.I

[

0.0122

L; . 0 0 3 2

122.G

O.UUSU

119

I

O . OL,:;, ~

%1

l/. uO~l)

} 7

u .0020

U.L:()2 I

122.1

0.u02;~

11'~.4

O.CUI~;

'~1

0 . u O 1 ::,

7"'

, . OO,o:~

. (~c,,J:',

12(,.9

O. ,OU( ~,

12P:

O.Lt)L~;

~,. 0 O O :

: :;

0,,.01

1 17

~2

0.0032

'!

1 I';.2

,

U"

~3

CaCl 2

i

:ii

Table 5. Molar conductivityof neodymiumsalts in N,N'-dimethylformamideat 25°C XdC£:j

hd(:,O)]

3

:,d I L I a ' %) )

(~

O. 1 4 3 4

22.~

0.05";4

33.3

U.2UI2

I~; .!)

~1 . u

0.~o'#2

u').u

u.J|2'i

0.0037

(~0.9

(;. OU , 2

o.0015

102.4

U . ,JU2 I

iO;l. i

~.9. OUUo

~2} .L~

O.OUoo

IZ,,.

I N C VoL 43, N o . I ¢ - N

]1 ~ . !

O. 03;~2

0.0229

,2. t,A.;2 ]

t

I.;

i

;.

I

J. LEGENDZIEWICZ et al.

2398

therefore, lower conductivities of NdCI~ and Nd(NO3h in DMF, compared with Nd(CIO4)3, might result both from the occurrence of an inner-sphere complex and/or an ion pair in these solutions. Since, however, spectroscopic properties and ultrasonic absorption are, as discussed above, sufficient evidence for the formation of inner-sphere complexes, form III should be assumed as predominant for the Nd solvates with disubstituted amides. Acknowledgements--We are indebted to Prof. R. P~owiec from IPPT, Polish Academy of Sciences in Warsaw, for his help in performing ultrasonic absorption measurements. REfeRENCES

1. K. Bukietyfiska, J. Legendziewicz, Rocz. Chem. 47, 1809 (1973). 2. J. Legendziewicz, K. Bukietyfiska and B. Je~owska-Trzebiatowska, Acta. Phys. Hung. 35, 167 (1974).

3. J. Legendziewicz, K. Bukietyfiska, Z. Bajsarowicz and B. .le~owska-Trzebiatowska, Acta Phys. Hung 39, 149 (1975). 4. J. Legendziewicz, K. Bukietyfiska, Z. Kopaczyfiska and B. Jezowska-Trzebiatowska, Optika i Spektroskopia 45, 288 (1978). 5. B. Je~owska-Trzebiatowska, S. Ernst, J. Legendziewicz and G. Oczko, Bull. Acad. Polon. Sci. Set. Sci. Chim. 24, 997 (1976); lbid 25, 649 (1977); lbid 26, 805 (1978). 6. B. Je~owska-Trzebiatowska, S. Ernst, J. Legendziewicz and G. Oczko, Chem. Phys. Left. 60, 19 (1978). 7. N. Purdie and M. M. Farrow, Coord. Chem. Rev. 11, 189 (1973). 8. B. Voleisiene and E. Jaronis, Ul'trazvuk 6, 33 (1974). 9. W. F. Hillebrandt and G. E. Lundell, Applied Inorganic Analysis. New York (1953). 10. S. J. Lyle and M. Rahman, Talanta 10, 1177 0963). 11. W. T. Carnall, P. R. Fields and K. Rajnak, J. Chem. Phys. 49, 4412, 4424, 4443, 4447, 4450 (1968). 12. W. T. Carnall, P. R. Fields and B. G. Wybourne, J. Chem. Phys. 42, 3797 (1965). 13. W. J. Geary, Coord. Chem. Rev. 7, 110 (1971).