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On the Mobility of Trivalent Ions Pr3+ in Pr3+--Al2O3.

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metrical position relative to the aromatic ring (“structured”
complex) and retain the position until substitution takes place,
for example according to a concerted mechanism, pronounced
stereoselectivity would be expected in contrast to the experimental results. Instead, the picture emerging from the present study
is consistent with a model in which the reagents confined in INC
2 b are not constrained in a fixed geometrical arrangement
(“loose” complex).
Despite its preliminary nature, this study shows that the use
of chiral reagents can probe gaseous INCs, in particular with
regard to the identity of the reacting species and the structure of
the complex (“loose” vs “structured”). Further work along
these lines is in progress, and the results will be reported in a full
paper.
Experimental Procedure
All reagents were obtained from commercial sources or prepared according to
standard procedurea. CD, (99 atom% D) and J2C,H, (99.9 a t o m % ”C) were
purchased from Cambridge Isotope Laboratories. R-( -)-s-butyl chloride was prepared from S-( )-s-butanol (Aldrich) [16, 171 and its enantiomeric purity was
checked by GC with the same column employed for the analysis of the radiolytic
products (25-m long, 0.25 mm i d fused-silica capillary column coated with
DACTBS-Beta-CDX chiral phase from Mega). The radiolysis was performed at
25 c in a commercial ;-irradiation facility t o a total dose of 2.5 x 10‘ Gy.
+
Received: July 18,1996 [29349IEj
German version : AnRew. Chem. 1997, 109, 116 - 117
-
Keywords: aromatic substitution chiral reagents * gas-phase
chemistry ion-molecule complexes stereochemistry
-
-
[I] P Longevialle. Mass Specfrom. Rev 1992, 11, 157.
[Z] D. J. McAdoo. T. H. Morton, Acc. Chem Rer. 1993, 26. 295. and references
therein.
[3] D. Berthomiru. V. Brenner, G. Ohanessian. J. P. Denhez, P. Millie, H. E. Audier. J Am. Chcn?. SOL..1993, llS, 2505.
[4] D. Berthomieu, V. Brenner. G. Ohanessian, J. P. Denhez, P. Millie, H E. Audier. J PI~J..~.
C1ic.m. 1995, 9Y. 712
[5] M. Aschi. M. Attini. F. Cacace. Angen,. Chem. 1995, 107,1719; Angew. Chem.
Inr. Ed. &I.
1995. 34, 1589.
[6] M. Aschi. M. Attini, F. Cacace. J Am. Chem. So(. 1995, 117, 12832.
[7] M Aschi, M . Attini, F. Cacace, Res. Chem. Infermed 1996, 22, 645.
[XI The usc of “C‘,H, facilitates the analysis of traces ofdeuterated compounds by
reducing the ”(. interference.
[91 Review of the radiolytic technique: F. Cacace, Acc. Chem. Res. 1988, 21, 215.
(101 M. Saunders. M. R Kates. J. Am. Chem. Soc. 1978, 100, 7082.
[Ill P. C Myhrc. C S. Yannoni. J. Am. Chem. Soc. 1981, 103, 230.
[12] S. A Johnson. D. T. Clark, J Am. Chem Soc. 1988. 110,4112.
11 31 M. Saunders. personal communication, G. Walker, Disserlation, Yale University. cited in ref. [tS]. See also G. A. Olah, D. J. Donovan, J Am. Chem. Soc.
1977. YY. 5026.
[141 J J. Dannenberg. D. H. Weinwurzel. K. Dill, B. J. Goldberg, TefruhedronLett.
1972. 1241
[I51 S. Sieber. I? Buzek, P. von R. Schleyer, W. Koch, J. W. de M Carneiro, J Am.
Chrw. Soc 1993. 115, 259, and references therein.
(161 D. (3. Coodwin, H. R . Hudson, J Chem Soc. B 1968, 1333.
[I71 H. R. Hudson, Sni.fhe.sis1969, 112.
On the Mobility of Trivalent Ions:
Pr3 in Pr3+ - ~ - A l 2 o 3 * *
+
Joachim Kohler and Werner Urland*
A growing number of reports on ion-conducting systems with
trivalent ions as mobile charge carriers were published recently.[’ - 3l Indications for charge transport processes of that kind
were found particularly in the lanthanide ion exchanged N a + p”-aluminas Na+/Ln3+-p-A120, (Ln = La,[41 Pr,[’. 5 3 6 . ’I
Gd,I31 Hor2]),as well as in rare earth tungstates, for example
SC2(WO4),.I81
From electrostatic considerations ions with a low charge are
especially suitable for the transport of ionic current. Accordingly, numerous cationic ion-conducting systems with monovalent
ions exist (for example a-AgI, Na+-p-A120,, and Li,N). In
contrast, only few compounds containing mobile divalent
cations (for example M2+-,B”-Al,03where M = Ca, Sr, Pb) or
trivalent charge carriers are known. Within these systems the
ion transport is hindered by strong coulombic interactions between the polyvalent ions and the surrounding host lattice. In
this report we present temperature-dependent, single-crystal Xray diffractionrg1that confirms and substantiates the results
concerning ion conduction by mobile trivalent cations obtained
from impedance spectroscopic measurements on the Na+/Pr3
P”-AI,O,[‘, 6] system.
Generally, the composition of the Mg2 +-stabilized superionic
conductor N a + - ~ - A I 2 O is
, given by the formula Na, +xMgxAIll-,O,, (x = 0.24-0.70). The layered crystal structure[”] consists of alternating sequences of close-packed spinel
block layers and the “conduction planes” with loosely packed
oxygen layers (three-quarters of the oxygen positions remain
unoccupied). The spinel blocks are interconnected by covalent
AI-0-A1 bonds as well as by coulombic attractive forces between
the N a + ions (which are located exclusively within the conduction planes) to adjacent oxygen ions. The lowered oxygen occupation facilitates a comparatively easy Na migration generating the two-dimensional ionic conductivity. Due to the high
mobility the N a + ions can be replaced by other mono-, di-, and
trivalent cations, particularly by lanthanide ions (for example
I2]Thesealumina compounds
Na, +x-yLn,,,MgxAll -xO17).[11~
offer an easy opportunity to vary the conducting ion within an
ion-conducting system and to investigate the dependence of the
ionic conductivity on the type and concentration of the charge
carrier.
An idealized representation of a conduction plane in Na+-,B”A1,0, (ab-plane) is given in Figure 1 . Two different crystallographic positions within the planes are available to the cations:
the Beevers-Ross (BR) position (C,,, symmetry) with a triply
capped tetrahedron (4 + 3 coordination) and the eight-coordinate “mid-oxygen’’ (mO) position (Czh symmetry, doubly
capped octahedron). The height N, of the conduction layers
(extension in c direction) is determined through coulombic interactions by the type (size and charge) and concentration of the
cations located within these planes. H L decreases with increasing
cation concentration as demonstrated recently by X-ray diffraction studies on Na+-/3’-A1,03[6.131 and Pr3+-/j”-A1203.1141
In
turn H , influences the cation distribution within the planes as
+ -
+
[*] Prof. Dr. W Urland, Dr. J. Kohler
Institut fur Anorganische Chemie und Sonderforschungsbereich 173
Callinstrasse 9, D-30167 Hannover (Germany)
Fax: Int. code +(511)762-3006
e-mail: urland@mbox.acc.uni-hannover.de
[**I
AnEPn. Chem. I n f . Ed Engl. 1997,36, No. 1/2
The financial support by the Deutsche Forschungsgemelnschaft
acknowledged.
VCH Verlagsgesellschyfr mhH. 0-69451 Wetnheim. 1997
0570-0833/Y7~3601-0OS5$ 15.00 +- .25/0
IS
gratefully
85
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occupation ratio (xBR:xmO)
amounts to 17:83 for the two crystallographic positions.
These results indicate considerable Pr3 mobility at elevated
temperatures. The activation energy E, required for every site
exchange process differs for the two sites: it is significantly
larger for departure from the BR sites than for departure from
the mO sites (E,(BR) > E,(mO)). The Pr3+ ions are able to leave
the mO sites at comparatively low temperatures ( T <180 "C). In
contrast, the thermal energy is not sufficient in the temperature
range 25<T<250°C to facilitate the migration from the BR
into mO sites. Consequently, the Pr3+ ions accumulate in the
BR positions with lower coordination number. Only above
250 "C (the so-called transition temperature 1;)are the Pr3+ions
in BR sites sufficiently thermally activated to overcome the potential barriers to migrate from BR into mO positions. Therefore, the percentage BR occupation xBR(Pr3+)
is lowered in the
temperature range 250 < T <400 "C from 50 YOto 17 YOat temperatures above 400 "C. This behavior differs significantly from
temperature-induced site exchange processes in some spinels in
which no ionic conductivity is observed.
On cooling the system slowly from 250 "C to room temperature the original cation distribution (xBR:xmO
=7:93) is not restored. The fraction of Pr3' ions occupying BR sites are rather
trapped within these positions. The required activation energy
for leaving the BR sites is no longer provided thermally. About
50% of all Pr3+ ions are found within the BR positions after
cooling (not shown in Figure 2).
All of the results presented here agree very well with data
obtained from impedance spectroscopic measurements on N a + /
Pr3+-/?"-AI,O, crystals.['. 6 , 71 Figure 3 shows the conductivity
+
AL
Figure 1. Idealized representation of a conductlon plane in Na+-p"-AI,O, (R3m,
2 = 3 ) with the adjoining oxygen layers of the adjacent spinel blocks. Corresponding to the hypothetical composition Na,MgAl,,O,, the BR sites are fully occupied
by N a + ions. The coordination spheres of a BR and an mO site are displayed as
black circles. The dashed lines indicate possible pathways of the N a + Ions. The
hexagonal unit cell is represented by solid lines.
well as the ionic conduction behavior.114, The ion transport
in the conduction layers is achieved by migration of mobile
charge carriers between nonequivalent sites (see conduction
pathways in Figure 1). In the fully exchanged Na+/Pr3+/?"-AI,O,
with the composition Nao.o,Pro.53Mgo~,2A1i,~,30,,(degree of exchange [ = 99%)[17]Pr3+ ions,
nearly exclusively, are available for the current transport.
Raising the temperature is expected to increase the mobility of
these cations. The migration should be detectable from the difference in site occupations in X-ray crystal structure investigations.
The most important result of these measurements is given in
Figure 2.[lS1 The dependence of the fraction of the Pr3+ ions
occupying BR sites (xBR(Pr3+))
on temperature is depicted. At
60.00j
50.00
I
I
I
1
30.00
I
t
i
t1
I
1
I
40.00
A
I
I
85 %
97 Yo
I,o
0.0
Ig (0 T ) [R-lcm -IK]
.
8
8
8
XBR(PC+)[O/.I
1
20.00
l
I
_ . ,i
1
V
i
A
-3.0
-2.0
I
I
0.5
1.0
1.5
20
IOOOlT [K-I]
10.00
2.5
30
L
I
3.5
Figure 3. Arrhenius plot of Na+/Pr3+-r-AI,O, crystals differing in degree of
exchange.
0.00
0
100
200
300 400
T Y C ] __t
500
Figure 2. Dependence ofthe percentage BR site occupation (xBR)
with Pr3+ ions on
temperature in Na, ,,Pro saMgo72A1,0330,7.
room temperature 7-10% of the cations are in BR and 9093 Yoin mO positions. This ratio agrees well with the results of
calculations based on the crystal composition.[6, 14] Increased
temperature leads to elevated BR occupations. Even at 180°C
about 25% of the Pr3+ ions occupy BR sites, whereas at 250 "C
the two positions are occupied nearly equally. However, a further temperature increase leads to a redistribution of the Pr3
ions into the mO sites again. At temperatures above 400°C the
+
86
J
3.0
Q VCH Verlagsgesellxhafl mbH. 0-69451 Weinherm, 1997
data in form of an Arrhenius diagram for selected Na'/Pr3+-PA1,03 crystals with different degrees of exchange."] Two temperature regions are distinguishable and can be interpreted as
follows: At low temperatures the conductivity is generated by
the mobility of unexchanged N a + ions, whereas the steeper
conductivity curve in the higher temperature region is due to the
enhanced mobility of the Pr3+
The start of the increased Pr3+ mobility is deduced from the Arrhenius plots of all
investigated Na+/Pr3+-P"-AI,0, crystals to be 290_+40"C
(by determining the intersection of the two straight lines obtained by the extrapolation of the experimental data within the
upper and lower temperature range).L6, 15] Exactly in this tem-
0570-0833i9713601-0086$15.00+ ZSj0
Angex, Chem. In! Ed. Engl. 1997, 36, N o 112
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perature region the migration of the Pr3+ions from BR into mO
sites sets in, as mentioned above.
The ionic conduction behavior of Na+/Pr3+-/l”-Al,03 can
therefore be described as follows: At temperatures below 250 “C
an exchange of sites (from mO into BR positions) is only possible for those Pr3+ ions occupying mO positions, but a further
migration from the BR sites with lower coordination number is
not possible. Despite this partial mobility a continous current is
not detectable. because the ions trapped within the BR positions
interrupt the current transport. The observed conductivities at
these low temperatures are caused by residual, unexchanged
Na’ ions, which are able to pass through both sites without any
hindrance. Only above 250 “C are the Pr3+ ions completely mobile. These trivalent cations are now able to overcome all potential barriers along the conduction pathways. Consequently, ion
transport is detectable, and an increase in ionic conductivity
occurs (cf. Figure 3).
The above-mentioned transition temperature (T = 250 “C)
only holds for Na+/Pr3+-/3”-AIZOS
crystals with the composibecause of the pronounced
tion Na, oiPro,53Mgo,,2Allo,330i,
dependence of the height H , of the conduction layers on the
cation concentrat~on.[’~]
For steric reasons the extension of H L
in turn influences the size of the potential barrier that must be
overcome by ions leaving BR sites. Appropriate experiments to
investigate quantitatively the correlation of the cation concentration with the transition temperature are under way.
Received: June 10, 1996 [Z9210IE]
German version: Angew Chem. 1997, 109, 150- 152
Keywords: aluminum . conducting materials
lanthanides * solid-state structures
- ion mobility -
[I] J. Kohler, W. Urkdnd, 2.Anorg. A&. Chem. 1996, 622, 191.
[2] F. Tietz. W. Urland. Solid Sture Ionics 1995, 78, 35.
[3] G. C. Farrington, B. Dunn, J. 0. Thomas, Appl. Phys. A 1983,32, 159.
141 J. Kohler. W. Urland, 2.Anorg. Allg. Chem. 1996, in press.
[5] T. Dedecke. J. Kohler, F. Tietz, W. Urland, Orr. J Solid State Inorg. Chem.
1996.33, 185
161 J Kohler, Dmerturion, Universitat Hannover, 1996.
[7] J. Kohler, W. Urland, Solid State Ionics 1996,86-88, 93.
[8] N. Imanaka, Y. Kobayashi, G. Adachi, Absr. 10th Int. Conf: Solid State lonics,
Singapore. 1995.407; N. Imanaka, Y Kobayashi, G. Adachi, Chem. Lett. 1995,
433.
[9] In these investigations the crystal structure of a selected Na+/Pr’+-/l’-Al,O,
crystal with known composition is determined on the four-circle diffractometer
at different temperatures. The temperature-dependent measurements are performed with an Enraf-Nonius-goniometer and the mountable crystal furnace
F R 559. For an exact temperature control the crystal furnace is calibrated with
an external Pt/Rh thermocouple as well as by melting point determinations of
the low melting metals In, Sn, Pb, Zn, and Al
[lo] M. Bettman. C. R Peters, J Phys. Chem. 1969, 73, 1774.
[ l l ] B. Dunn, G C . Farrington, Solid State lonics 1983, 9/10, 223.
[12] S. Sattar, B. Ghosal. M. L. Underwood, H. Mertwoy, M. A. Saltzberg, W S.
Frydrych, G. S Rohrer. G. C. Farrington, J Solid Stare Chem. 1986,65, 31 7.
I
Muter. Sci. 1983, 18, 2437
[13] F. Harbach. .
[14] J. Kohler, W. Urland, J SolidState Chem. 1996, 124, 169.
[15] J. Kohler, W. Urland, J Solid State Chem., in press.
[I61 Na+-/Y’-AI,O, crystals are grown (flux evaporation method) by slowly evaporating the Na,O flux at nearly 1700°C. The complete ion exchange with Pr3+
ions is obtained by immersing the Na+-/l’-AI,O, crystals into molten anhydrous PrCl, for 2 - 3 hours a t 790 “C (inert gas atmosphere).
[17] The crystal composition and the degree of exchange 5 ( 5 refers to the original
N a + content of the employed Na+-P”-AI,O, crystal) are determined by electron probe microanalysis
[I81 Crystal structure data of P r ’ + - ~ - A I , O , for some selected temperatures.
Siemens-AED-2-Diffractometer, Mo,,, i.~ 7 1 . 0 pm,
7 graphite monochromator, 0-20 scan. Lp and numerical absorption correction, anisotropic refinement with SHELXL-93. a) Measurement at room temperature: space
group RJm (No. 166). Z = 3, a = 563.50(28), c = 3337.7(17) pm, V =
916.99 x 10’ pm’. pGd,cd
= 3.480 gcm-’, 20,,, = 45.97”, 0-20 scan, 1713 measured reflections. 201 symmetry independent reflections, 196 reflections with
Angew Chrm Int Ed End 1997, 36. No 112
10>40(1,), 48 refined parameters, R values (all reflections) RIIFI = 0.0433,
M.RZ/F*J= 0.1096, minimax Ap = - 0.67/3.08 e ~ m x-10.
~
b) Measurement at 322 ”C: u = 564.26(35), c = 3345.7(17) pm. V = 922.52 x 10‘ pm’,
pcalod
= 3.462 gcm-’, 2R,,
= 45.88”, w-2R scan, 1708 measured reflections,
201 symmetry independent reflections, 201 reflections with I, >4u(lo), 45 refined parameters, R values (all reflections): R1 IF1 = 0.0756. 12.R21FZI=
0.1947, min/max Ap = -1.17:1.27epm-’x lo-‘. c) Measurement at
=
471 ‘C: u = 565.87(36), c = 3350.1(20) pm. V = 929.01 x 10‘ pm’. prdlrd
3.438 gcm-’, 20,,, = 59.87”, 0)-20 scan, 3400 measured reflections. 388 symmetry independent, 365 reflections with I0>4u(lO), 50 refined parameters,
R values (all reflections): R l l F = 0.0704, wR21F’I = 0.1938, minimax
A p = - 1.06/1.06 epm-’ x
Further details of the crystal structure investigations may be obtained from the Fachinformationszentrum Karlsruhe,
D-76344 Eggenstein-Leopoldshafen (Germany), on quoting the depository
numbers CSD-405335 (crystal a), CSD-405334 (crystal b) and CSD-405333
(crystal c)
Atom-Bridged Intermediates in N- and P-Atom
Transfer Reactions**
Marc J. A. Johnson, P. Mae Lee, Aaron L. Odom,
William M. Davis, and Christopher C. Cummins*
Complete intermetal N-atom transfer reactions constitute
highly economical three-electron redox processes.[” A seminal
reaction of this type is the reduction of [NMn(TTP)]
(TTP = meso-tetrakis(4-toly1)porphyrin) by [Cr(TTP)] to give
[Mn(TTP)] and [NCr(TTP)] .[‘I Certain quasi-degenerate intermetal N-atom transfer reactions have been subjected to kinetic
studies to delineate energetic parameters innate to the atomtransfer event.‘21Evidence has been amassed regarding the role
of N-atom-bridged species as intermediates in N-atom transfer
reactions, but until now such intermediates have eluded characterization.”]
It was found recently[31that the three-coordinate molybdenum(rI1) complex 1[3-’1 effects N-atom abstraction from the ni71 and (under argon) 0.5 equivtrido complex 2[’] to give 3
[Mo(NRAr),]
1, R
[NMo(O-fBu),]
2
[NMo(NRAr),]
3
[Mo,(O-rBu),]
“[Mo(O-tBu),]”
=
C(CD,),CH,, A r
=
3,5-C,H3Me,
4
5
[(tBuO),Mo(g-N)Mo(NRAr),]
6
alents 4[9*’01or (under 1 atm dinitrogen) predominantly 2.
Products 4 and 2 appear to result from dimerization and dinitrogen cleavageJ5.’1 respectively, stemming from transient 5,which
was not observed. During N-atom abstraction from 2 by 1,
a blue color was observed that was tentatively attributed to
the intermediate complex 6.‘” Here we report that in a closely
related reaction system, the teal N-atom bridged species 7
(R = C(CD,),CH,, Ar, = 4-C6H,F, [Eq. (I)]) produced upon
[*] Prof. C. C. Cummins, M. J. A. Johnson, P. M. Lee, A. L. Odom,
Dr. W. M. Davis
Room 2-227, MIT Department of Chemistry
Cambridge, MA 02139-4307
Fax: Int. code +(617)253-7030
e-mail: ccummins@,mit.edu
[**I
For funding C. C. C. thanks the National Science Foundation (CAREER
Award CHE-9501992), DuPont (Young Professor Award), the Packard Foundation (Packard Foundation Fellowship), Union Carbide (Innovation Recognition Award), and 3M (Innovation Fund Award). M. J. A. J isgrateful for an
NSERC graduate research fellowship.
G VCH Verlagsgesellschaft mbH, 0-69451 Weinhem, 1997
0570-0833i9713601-0087~15 0 0 i 25 0
87
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