Download The strict determination of the term volatility is based on the

2.23. Volatile compounds of lanthanides (for Comprehensive Inorganic chemistry)
A. Drozdov, N. Kuzmina, Moscow State University
The term volatility, first suggested by Lewis in 1901, characterizes the tendency of a
substance to vaporise. The strict determination precise definition of the term volatility is based
on determination of vapor pressure at different temperatures. Here we define volatile compounds
as those that can evaporate and condense without changes at moderate temperatures.
During sublimation and condensation, the composition of volatile substance remains
unchanged. Besides, in some cases we call compounds, that during sublimation dissociate loose
neutral ligands, but during condensation the initial composition forms, volatile as well. The
preliminary experimental criterion of volatility is the compound’s ability to sublime. Sublimation
in vacuo with detectible rate at fixed temperature means that vapor pressure of a substance
exceeds 0.01-1Pa. It should be noted that the sublimation temperature is indicative of volatility.
It is the temperature at which a substance sublimes with detectible rate under given conditions,
i.e. in a given reactor and at fixed pressure. In literature, the sublimation temperature (that in
reality changes within some limits even during the experiment) is widely used as an approximate
characteristic of volatility. More strictly, the volatility can be proved using mass-spectrometry by
the presence of the complete molecules of the substance or the metal containing fragmentized
ions formed from the ionization of the molecules directly in the vapor phase, in mass spectra.
What fully charaterizes a volatile compound is the dependence of its vapor pressure on
temperature. However, only some lanthanide compounds are have been characterized in this
way. For most volatile compounds of lanthanides only the sublimation temperature range is
known.
The volatile compounds of lanthanides can be divided into several groups. The first one is
complexes with covalent sigma-donor ligands such as halides, amides, alkoxides, including
covalent organometallic compounds without pi-bonding. The second group is represented by
metal chelates, among which the derivatives of beta-diketones being the most important. This
group also includes dialkyldithiocarbamates, carboxylates and others. Pi-complexes with Cp and
others ligands should be discussed separately. This classification fails to sign mixed ligand
complexes, containing, for example, both diketonate and alkoxide ligands, to a separate group.
Volatility significantly depends on the structure of a compound. It is logical that it favors
molecular packing with weak intermolecular interactions, for example, complexes with bulky
groups. On the other hand, large substituents (such as tertial tret-butyl) facilitate the
intermolecular interactions that increase significantly with the increase in molecular weight, thus
suppressing the predicted increase in volatility partially or completely.
The lanthanide cations are hard Pyrson acids that have affinity to fluoride and oxygen
rather then to chlorine, sulfur and phosphorus donor atoms. Most lanthanide compounds
including complexes are predominantly ionic with high coordination numbers (from six to eight
or even higher) which is in accordance with their large ionic radii. Volatility can also be
expected in the case of weak intermolecular interactions that occur between molecules with fully
occupied, coordination sphere. Taking into account high coordination numbers and oxidation
state of the metal (generally +3, in some cases +2 or +4) one can predict that the saturation of
coordination sphere requires, besides two to four negatively single-charged anionic ligands
(usually bidentate such as nitrate or diketonate), some (from one to three) monodentate neutral
ligands from the solvent or of other nature, added to a reaction mixture. Such compounds are
called adducts, i.e. neutral ligand weakly linked to a metal center. While heating or even storing
this neutral ligand can become free and leave the coordination sphere of a metal, which causes
polymerization of the species. To prevent it, in the absence of additional neutral ligands
chelating ligands with bulky groups that restrain polymerization by steric hindrances can be
used.
Yttrium, lanthanum and lanthanides (also called rare-earth elements, REE) have similar
properties due to the proximity of their ionic radii. Because of lanthanide сontraction, the metalligand bonding strengthens in the REE row from La to Lu, yttrium being similar to the heavy
lanthanides (of the so called yttrium family). This results in the weakening of the intermolecular
interactions and increase in volatility. Calculated molecular contractions for molecular trihalides
and hydrated cations show the contribution of the relativistic effects by participation of a 4f-shell
in bonding only in lutetium fluoride [1]. The augmentation of the oxidation state, for example, in
cerium compounds diminishes ionic radius and consequently leads to the increase in volatility.
The volatile compounds must be stable up to the temperature range of sublimation.
Nevertheless, a large number of compounds dissociate while heating, usually losing additional
ligands or solvent molecules. It results in polymerization of the remaining part of the molecule.
The resulting polymer can be often sublimed without decomposition that allows to use such
complexes as precursors of volatile compounds and to include them in further discussion [2].
1. Halides
The fluorides of REE are high-melting ionic compounds, pour soluble in water and
hydrofluoric acid [3].
The unhydrous chlorides can be prepared from hydrated products by treatment with
desiccating agents such as oxochloride of sulphur(IV) or by heating with ammonia chloride, or in
the atmosphere of dry chlorine, hydrogen chloride or phosgene. Crude products containing
oxochlorides can be prepared by chlorination of oxides using carbon tetrachloride, hydrogen
chloride, phosgene, phosphorus pentachloride, sulphur monochloride together with chlorine or
by chlorination of oxide mixed with carbon. They can be purified by distillation. The REE
chlorides are solids with high melting points (from 500 to 957C) that can be sublimed in vacuo
at high temperatures. They form infinite frameworks of predominantly ionic character, in which
REE metal cations are linked by bridging chlorides. The boiling points, temperatures of vapor at
different pressures and enthalpies of evaporation are given in Table 1.
Table 1 The vapor pressare, entahlpies of vaporization and bowling points of REE chlorides
[3]
Compound
B.p., C
Hv, kJ/mol
Vapor temp at pressure, mm Hg
4
2
1
0,1
YCl3
1510
1050
975
909
735
129,2
LaCl3
1750
1027
997
969
886
329,8
CeCl3
1730
1195
1125
1065
888
170,5
PrCl3
1710
1144
1085
1031
878
187,3
NdCl3
1690
1166
1106
1048
892
185,6
SmCl2
2030
1400
1229
1087
764
83,2
EuCl3
dec
998
930
869
703
129,2
GdCl3
1580
1048
995
947
808
183,9
TbCl3
1550
1121
1068
1010
854
175,6
DyCl3
1530
979
939
899
779
201,5
HoCl3
1510
986
953
919
827
262,1
ErCl3
1500
1155
1076
1000
809
137,5
TmCl3
1490
951
925
899
824
324,0
YbCl3
dec
1091
1039
994
(856)
199,4
LuCl3
1480
998
959
926
(819) 
239,1
 extrapolated values
To summarize, the halides of REE are low volatile at moderate temperatures. For example,
to sublime neodymium trichloride at reduced pressure we need ca 700C. One of the possible
ways to diminish the temperature range of sublimation is to use mixed complexes.
In the molten KCl medium the vaporization of LnCl3 occurs better that in the single phase.
The investigation of LnCl3-KCl equimolar molten mixtures at 1018-1273 K by means of
Knudsen effusion mass spectrometry shows the presence of vapor species KLnCl 4 over the melt.
The volatility enhancement of NdCl3 by the formation of the vapor complex KNdCl4 decreases
with increase in temperature. A relatively small enthalpy change, -10 ± 21 kJ/mol, of the gas
phase exchange KNdCl4(g) + KCl(g) = NdCl3(g) + K2Cl2(g) allows to suggest that the structural
change during the reaction is not drastic and that the KNdCl4(g) complex has two bridging and
two terminal chlorine atoms [4].
It is well known that aluminum trichloride (as well as FeCl3, GaCl3) due to the strong
chloride ion affinity forms gaseous complexes with different metal chlorides including REE. The
solution of REE chlorides in molten aluminum trichloride doesn’t contain any products of
interaction between these compounds, but in the vapor phase the mixed metal complexes form:
LnCl3(s) + n/2Al2Cl6(g) = LnCl3nAlCl3(g)
The thermodynamic parameters of such reactions have been calculated. For example, for
neodymium compounds H is 157,0  13 kJ/mol, S is 110,3  13 J/molK for n = 1 and 45,2 
1 kJ/mol, 8,4  1 J/molK, respectively, for n = 3 [5]. The vapor pressure of neodymium
compound determined from electron spectra by the intensity of Nd3+ band at 17030 cm-1 showed
the significant increase in volatility (ca 1013 times at 600K and 3107 times at 800K, p(AlCl3) =
105 Pa. This increase is due to the depolymerization of solid REE chlorides as a result of
interaction between chloride-group of the former with aluminum chloride giving [AlCl4] unit
that prevents polymerization by means of Cl-bridges. From the band at 17030 cm-1 in the
electron spectra of NdCl3-AlCl3 vapor the octahedral coordination of neodymium [NdCl6] can be
proposed. Partial pressure of such compounds with general formula LnCl3nAlCl3 has a very
complicated dependence on temperature and AlCl3 vapor pressure. At fixed p(AlCl3) the vapor
pressure of LnCl3nAlCl3 with temperature has a maximum that shifts at high temperatures with
increase of p(AlCl3). The similar effects of gas-phase transport are also known for anhydrous
ferric chloride and gallium chloride, the complexes of former being less stable in comparison
with aluminum derivatives discussed above. The enhancement of REE halide vapor pressure in
the melts containing alkali metal halides or aluminum halides has been used in the
manufacturing of REE sulphides. The process comprises heat-treating a mixure of a rare earth
halide and an alkali metal halide or aluminum halide in the presence of H2S at a temperature
high enough to volatilize the halides [6].
The tribromides and triiodides of REE are similar to trichlorides discussed above. The
synthesis of tribromides can be carried out by the ways discussed above for trichlorides or by
heating of hydrated salts in vacuo at 70C followed by gradually increasing the temperature up to
120C [7]. The triiodides are generally prepared by heating of anhydrous trichlorides in the
stream of hydrogen iodide and hydrogen. This method does not work for europium, and in the
case of samarium and ytterbium, the temperature control should be very accurate. All the
compounds except unstable iodides of europium(III) and samarium(III) are volatile at high
temperatures. Aluminum bromide was used to accelerate the volatility of anhydrous LnBr3 in a
way already discussed above for trichlorides.
Among the binary tetrahalides only fluorides of cerium, praseodymium and terbium are
known, all of them are easily decomposed upon heating.
The dihalides that are stable for Nd, Sm, Eu, Dy and Yb are high-melting poorly-volatile
ionic solids prepared by reduction of trihalides with hydrogen or ammonia. The diiodides can be
made by thermal decomposition of triiodides, EuI2 forms as olive-green precipitate by the
addition of sodium iodide to the solution of europium(III) salts. The dichlorides of samarium and
europium can be transferred into a vapor phase at lower temperatures by the addition of
aluminum chloride.
2. Complexes with amides
Among the amide derivatives of REE only complexes with donor-functionalised amido
ligands contating bulky groups are stable. Dialkylamides such as lithium diisopropylamide give
homoleptic tetracoordinate anionic complexes with REE, e.g. [Li(THF)4][Ln(NR2)4] are stable,
but non volatile due to ionic character. The steric saturation of the metal center in such
compounds is achieved by interaction between REE and carbon atoms of the amide [8]. The
complexes with di-isopropylamide, iPr2N−, (iPr = (CH3)2CH) can be sublimed only with
decomposition, e.g., [Ln(NiPr2)3] decomposes at 80C and 10−4 Torr [9]. Complexes with the
dimethylamide ligand can be stabilized only in the presence of MMe3 (M=Ga or Al) to form
[Ln(NMe2)3(MMe3)3].
To make the compound more volatile, sterically hindered bulky ligands such as
silylamides can be introduced [10]. The large size of the silylamide ligands is effective in filling
the coordination sphere of the Ln3+ ion. The (trimethylsilyl)amide group due to the trivalent
nitrogen directly linked to a metal center, has a unique ability to shield REE from attacks of
other ligands. As a matter of fact, in homoleptic silylamido-Ln(III) compounds the coordination
number of REE is three even in solid state. For instance, in Nd(N(SiMe3)2)3 the angle N-Nd-N is
117,8, that is close to planar triangular geometry of the coordination sphere [11]. Such
compounds, first reported in 1973 [12], show the unique example of three-coordinate lanthanide
complexes. To prepare such compounds the reaction of unhydrous chloride with lithium
hexamethyldisilylamide is used:
LnCl3 + 3LiN(SiMe3)2 = Ln(N(SiMe3)2)3 + 3LiCl
The products are moisture sensitive and easily hydrolize.
Fig. 1. Crystal structure of Nd(N(SiMe3)2)3 [11]
The compounds without solvent molecules are volatile due to non-polar nature of the
exterior of a complex and weak intermolecular interactions, and can be sublimed in vacuo 10 -2
Torr at temperature range 353 – 373 K; the sublimation temperature poorly dependent on the
atomic number of REE and being close to that for analogous complexes of d-metals. If the
synthesis or recrystallization is performed in solvent which molecules contain electron-dnor
atoms such as THF, the adducts, like Ln(N(Me3Si)2)3(THF) form. The TPPO adducts are also
reported [13]. The adducts are more stable in the case of less sterically hindered amide ligands
such as (Me2HSi)2N− [14]. The monomeric nature of these compounds provides corresponds to
their volatility that changes from 100 - 105C/ 10-4 Torr for La (and Y) to 75-80 C/ 10-4 Torr for
Lu [12].
Lithium hexamethyldisilylamide reacts with Eu(II) and Yb(II) derivatives forming
anionic tricoordinate complexes MLn(N(SiMe3)2)3 or molecular adducts with ethers. The latter
are volatile according to their mass-spectra [15].
Bulky alkyl ligand −CH(SiMe3)2, similar to silylamide one, also gives monomeric threecoordinate complexes, similar to those of silylamides [16], but no volatility data are available.
3. Alkoxides and siloxides
The interest in volatile alkoxides, including the derivatives of alkylsubstituted phenols
and heterobimetallic compounds, is linked to the search for soluble and volatile precursors for
laser and superconducting materials, sensors, catalysts etc. The results of such studes are
summarized in numerous books and reviews by D. Bradley [17], Mehrotra et al [18-21],
Bochkarev et al. [22], Turova and Turevskaya [23], Hubert-Pfalzgraf [24]. Derivatives with a
general formula “Ln(OR)3” in realty have more complex composition, containing oxo-groups.
For instance, for a family of crystalline RE isopropoxides the crystal structure determination
shows Ln5O(OiPr)13 composition, where Ln= Sc, Y, Er, Yb. The oxo-group forms due to the
desolvation of the very unstable [Ln(OiPr)3(iPrOH)]2 solvates (perfectly soluble and rather
reactive), characterized for Nd. Desolvation of Ln(OBut)3•2L (Ln=Y, La; L=tBuOH, THF, Py)
also leads to the formation of oxocomplexes.
The volatility of RE alkoxides depends on the nature of the substituent in the alkoxygroup. The simplest alkoxides – methoxide and ethoxide – give unvolatile derivatives due to the
formation of the stable unsoluble polymeric species. The better results show the derivatives with
branched or fluorinated groups such as isopropyl, tret-butyl, neo-pentyl, triethylmethyl and their
fluorinated analogues including phenoxides (Scheme 1).
Scheme 1. The most important alkoxy-groups providing volatility of RE derivatives
O
O
O
i-PrO-
t-BuO-
MeEt(i- Pr)O-
OEt
F3C
O
O
O
t-Bu
CF 3
EtOCH2(t-Bu) 2CO-
(CF3)2MeCO-
Et 3CO-
t-Bu
t-Bu
t-Bu
O
O
t-Bu
t-Bu
(t-Bu)2-2,6-C6H3O-
(t-Bu)2-2,6-Me-4-C6H2O-
The REE alkoxides can be prepared by exchange reactions between REE chlorides and alkali
metal alkoxides in molar ratio 1:3. As a by-product in such reaction, mixed-ligand alkoxide
chloride can be formed. Only few of such compounds have been characterized
crystallographically (e.g. Nd6Cl(OiPr)17 [25], Y3Cl(OtBu)8•2THF [26, 27] ). The incorporation of
chloride in a multinuclear complex has been also reported for alkaline-earth metals, such as
barium [28]. The fact is, that after being incorporated, chloride cannot be easily removed even by
the
excess
of
alkoxide.
In
such
cases,
bimetallic
alkoxide
complexes
such
as
[LiNdCl(OCtBu3)3(THF)3] [29] form. Instead of REE halide, REE carboxylates can be used. The
synthesis of isopropoxides was performed by reaction of acetates or bensoates with LiOPr, in
order to obtain t-buthoxides KOtBu, oxalates of REE have been used [30]. Another synthetic
route is to start from easily accessible anhydrous Ln(OCOCCl3)3 [31, 32], or to use the adducts
of Ln(NO3)3 with glycols or polyethers [33, 34] or alcoholysis of Ln[N(SiR 3)2]3 [35]. The latter
reaction in the presence of excess fluoroalcohol can give ammonia derivatives due to the
reaction of released silylamine (Me3Si)2NH [36]:
Ln[N(SiMe3)2]3 + 3ROH = Ln(OR)3 + 3(SiMe3)2NH
(SiMe3)2NH + 2ROH = NH3 + 2Me3SiOR
The common method for the preparation of REE isopropoxides is the electrochemical (anodic)
dissolution of a metal. Outside of the electrolytic cell, REE metals react with alcohols very
slowly even under reflux and in the presence of the mercury [37, 38]. In the case of primary
alcohols, unsoluble products that are formed, prevent the reaction.
Fig. 2. The crystal structures of REE alkoxides with bulky ligands
Volatile RE alkoxides are crystalline molecular products, consisting of oligonuclear
species, or polymers. The products obtained from solvents with electron donor atoms, usually
contain coordinated solvent molecules, that can be easily removed by heating before
sublimation. The low temperatures of their transition into the gas phase and their solubility in
nonpolar organic solvents correlate with the small size of their molecules. The coordination
numbers in these compounds are rather low for RE and usually do not exceed six. That is why
the structure of RE alkoxides depends on the nature of substituent rather than the radius of the
central atom; the derivatives of different REE with the same alkoxide are usually isostructural
for the whole RE family. Complexes with sterically hindered ligands such as tret-butyl or
substituted aryl-groups contain tri- or tetracoordinated metal atoms. Such species are monomeric
or dimeric. The compounds with solvent molecules such as THF, MeCN are usually monomeric
with CN = 5. Good examples of octaherdal coordination are isopropoxides and t-buthyl-oxides.
In Ln5O(OiPr)13 that is Ln5(5-O)(3-OiPr)4(-OiPr)4(OiPr)5, Ln atoms form tetragonal pyramid
linked by four 3-OiPr-groups located over the side faces and four -OiPr groups located in the
basal plane. The remaining alkoxy-groups are monodentate. All the metal atoms are in
octahedral [LnO6] environment. It is interesting to note that in the [Ln5] core one of the atoms
can be substituted by another RE, forming bimetallic species such as [Y4Pr], [Nd4Ti], [Sm4Ti]
[23, 39].
A family of t-butylates consist of trimers [Ln3(OtBu)9L2], where L = THF, py, tBuOH etc.
The [Ln3] triangles are linked by two 3-OtBu groups and three 2-OtBu groups. The four
remaining alkoxy-groups are monodentate, saturating the coordination sphere of Ln together
with L molecules up to CN = 6. The solvate [Nd(OiPr)3(iPrOH)]4 has a unique structure that
belongs to the [Ti(OMe)4]4 type. Some examples of heptacoordination are known, among them
the monomer [La(OC6H3iPr2-2,6)3(NH3)4] containing seven monodentate ligands, four of which
are rather small.
The volatility of RE alkoxides does not correlate exactly with the nature of REE that
makes it impossible to map out any correlations with the change of the ionic radius of a metal.
The literature data even for the most common isopropoxide precursor Ln5O(OiPr)13 are rather
scarce and can not be compared “as is”, taking into account different experimental conditions.
As an example, the sublimation and decomposition temperatures obtained in [23] are presented
in (Table 2).
Table 2. The sublimation and decomposition temperatures for Ln5O(OiPr)13 [23]
Metal Y
La
Pr
Nd
Sm Eu
Gd
Tb
Temp 200 170 175 250 200 200 200 190
Dy
Ho
Er
Tm Yb
Lu
190 195 180 185 195
198
0,01 0,1
0,17 0,18 0,01 0,06 0,2
0,1
Temp. 280 260 200 250 200 200 200 200
250 250 260 250 250
250
subl,
C
P,
0,1
0,01 0,04 0,1
0,1
0,1
Torr
Dec.,
C
The mononuclear and oligonuclear alkoxides without additional ligands regardless of
their nuclearity sublime without decomposition. The Ln3(OtBu)9 and Ln5O(OiPr)13 molecules are
stable even in the gas phase. The polymeric species partly decompose during the sublimation.
The compounds containing additional ligands transform into gas phase with dissociation. For
instance, La3(hftb)9Et2O, where htfb is anion of (CF3)2(CH3)COH sublimes at 90 - 130C in
vacuo 0,01 Torr losing ether molecules. In mass-spectra the La2F3(hftb)2+, La2F4(hftb)+, La2F5+
and LaF3O+ ions have been found [40]. Neopentyl derivatives under heating usually decompose
according to equation:
Ln(OCtBu)3  CH2=CH(CH3)2 + Ln(OCHtBu2)3.
For Nd decomposition occurs at 160C (10-3 Torr).
The branching of a substituent in the alkyl or aryl group results in the increase in
volatility. For this reason the derivatives of 3-ethylpentanol-3 are among the most volatile ones.
For example, Y(OCEt3)3 sublimes with partial decomposition at 224C at atmospheric pressure.
The derivatives of fluorinated ligands have better volatility in comparison with nonfluorinated ones. The most studied are derivatives of hexafluoroisoproponanol and
perluorotertbutanol. The complex La(OCH(CF3)2)3 sublimes at 130C in vacuo 0,01 Torr that is
40 degrees lower than non-fluorinated product. It should be taken into account that fluorination
also decreases the decomposition temperature: polymeric La(OC(CH3)(CF3)2)3 sublimes at
130C/0,01 Torr with decomposition.
The fluorinated alkoxides of RE form stable adducts with donor molecules [41]. In some
cases such adducts transform into vapor without decomposition. It is well known that
hexafluoroisopropoxides form complexes with two molecules of ammonia, such compounds can
be sublimed at 120 - 140C/11 Torr. On the contrary, La(OC(CH3)(CF3)2)3(NH3)2 loses ammonia
while subliming at 85-105C/0,01 Torr. In the gas phase, the peaks corresponding to
La2F3(OR)3+, La2F4(OR)+, La2F5+ have been detected. The sublimation product in some cases
depends on the way of heating. Under slow heating at moderate temperatures,
Ln(OCH(CF3)2)3(NH3)2 loses ammonia and its volatility diminishes. The complex with diglyme
upon sublimation loses the neutral ligand partly:
La2(OCMe(CF3)2)6Diglyme3(s)  2La(OCMe(CF3)2)3Diglyme(g) + Diglyme(g)
In the mass-spectra, La2(OCMe(CF3)2)6Diglyme+ peak is detected.
Studies of heterometallic alkoxides have been initiated by their potential use as a volatile
single-source heterobimetallic precursor for different solid-state materials (HTSC, perovskites,
LnAlO3 etc). It is well known that alkoxide ligands can easily act as bridging between different
metal centers.
All bimetallic alkoxides can be divided into three groups. The first one represents the
complexes with alkoxides acting as efficient assembling ligands between different metallic
centers, the second one includes the derivatives of functionalized (e.g., ether or amino) alkoxides
and the third one – the mixed-ligand heterobimetallic complexes such as alkoxydiketonates. The
complexes of the second group are non volatile [42].
The complexes of the first group in some cases can be sublimed without decomposition.
A good example is the isopropoxides Ln[Al(OiPr)4]3 that forms in good yield by the interaction
between aluminium and RE alkoxides [43 - 45]. The molecular weight determination in boiling
benzene shows the monomeric structure of complexes with aluminium atoms occupying the
tetrahedrally coordinated sites. Such molecules have been detected in mass-spectra. All the REE
including cerium (for which a simple alkoxide is not air-stable due to oxidation) in the form of
heterobimetallic alkoxide have been prepared and studied. They sublime at 180 - 200C in vacuo
0,1 Torr
Upon coordination with aluminium the volatility of the alkoxide rises, but not
dramatically, as it has been demonstrated for mixed metal halides. In contrast to the latter,
alkoxides are rather stable in solid state, solution and vapor. They can be distilled without
decomposition. Similar complexes have been prepared for gallium [46] respectively.
A series of bimetallic isopropoxides Ln2Al2(OiPr)12(iPrOH)2, Ln = Y, La, Nd, Pr, Er
prepared by the interaction of [47 - 52] of REE and aluminium alkoxides in the isopropanoltoluene mixture or by dissolving rare-earth metal and aluminium isopropoxide in the same
medium (using mercury(II) chloride or iodine as initiator), are volatile, but they dissociate into
LnAl3(OiPr)12 and Ln5O(OiPr)13 upon evaporation
15Ln2Al2(OiPr)12(iPrOH)2  4Ln5O(OiPr)13 + 10LnAl3(OiPr)12 + 4iPr2O + 30 iPrOH
In solution, however, these molecules remain intact according to NMR data, that allows their
application as single-source precursors in the synthesis of Ln.
Complexes containing alkali metals have different structures and stechiometry, e.g.
[Na2Gd4(μ6-O)(OtBu)12] is an octahedron [53, 54], Li5Sm(OtBu)8 has a structure Li5Sm(4OR)2(3-OR)4(OR)2 consisting of two cubes with a common Li2O2 face [55]. The latter complex
sublimes at 135C/0,5 Torr. Synthesis of a group of bimetallic alkoxides containing additional
anionic ligands such as chloride or hydroxide has been reported. Such compounds, e.g.
[Na8EuX(OtBu)10]], X = Cl, OH sublime without decomposition at 125C/10-3 Torr [56 - 58].
The use of fluorinated ligands increases stability of the heterobimetallic complex due to
the possible M...F interactions that prevent the dissociation of molecule in the vapor phase [59].
The third group of heterobimetallic complexes is based on alkoxides and b-diketonates.
The first volatile Y -Ba species Y2Ba[OCH(CF3)2]4(thd) has been reported in 1993 [60].
The donor-functionalized alkoxide ligands HOC(R1)(R2)CH2X, where R1 is H or alkyl group, R2
is an optionally substituted alkyl group and X is OR or NR2 (R ia alkyl) give highly volatile and
n-hexane-soluble complexes with lanthanides [61]. The volatility of these complexes is the
highest achieved for RE alkoxides (sublimation < 100C/10-3 Torr) [62]. The neodimium
complexes with Et2NCH2C(H)(tBu)O−, EtOCH2CtBu2O− and EtOCH2CiPr2O− sublime at
150◦C, 125◦C and 115◦C respectively [63].
Potentially tridentate alcohols HOCtBu(CH2OiPr2)2 and HOCiPr2CH2OCH2OMe afford
tris-alkoxides of reduced volatility. The most common of this family of ligands is 1-methoxy-2methylpropan-2-ol (Hmmp), giving dinuclear tris-complexes [Ln2(mmp)6] with significantly
polarized metal centers originating from unsymmetrical ligand association (triple-bridging) [64].
A common synthetic route to them is the reaction of alkylamid or silylamide with stechiometric
quantity of (Hmmp) ligand in toluene in the presence of tetraglyme. In this reaction tetraglyme
acts as a stabilizing Lewis base that prevents the formation of condensed unvolatile oxoalkoxides [{Ln(mmp)3-n}2On]. Salt metathesis reactions using M(mmp) (M = Li or Na) and
Ln(NO3)3(tetraglyme) is also used by it can give some amount of heterobimetallic species. [65].
Unintentional employment of water-contaminated reagents give poorly-volatile oxo-species,
such as Lu4(O)(OH)(OCMe2CH2OMe)9.
Fig. 3. Crystal structure of [Lu(mmp)3]2
The RE complexes with bulky siloxide ligands such as HOSi(CMe3)3-n[(CH2)3NMe2]n (n
= 1, 2) are analogous to those donor-functionalized alkoxides. The complexes with silanols are
more thermally stable compared with alcohols as the decomposition of siloxide complexes is not
autocatalytic [63].The tris(siloxide) complexes are made by the reaction between the silanol
(prepared starting from chlorosilanes) and silylamide of REE. They are viscous slowly
crystallizing oils of high solubility in hexane. Y{OSi(CMe3)2[(CH2)3NMe2]} complex can be
sublimed at 115 /10-4 Torr without decomposition [66]
Among the RE in oxidation states higher than three, only for cerium(IV) such volatile
alkoxides are known. Due to the oxidizing power of Ce4+ the complexes with aryloxides do not
form. Due to the higher charge and smaller radius of Ce4+ compared with Ce3+ one can expect
the monomeric complexes to be more common. Meanwhile, the higher acidity of Ce4+ causes
hydrolysis and oligomerization. Thus, monomeric [Ce(OtBu)4(THF)2] condenses on standing in
solution at room temperature for 3 days giving the oxo-bridged cluster [Ce3(OtBu)10O] that
sublimes at 140C/0,1 Torr [67], isopropoxide Ce(OiPr)4•iPrOH gives Ce4(μ4-O)(OiPr)14 [68]. As
in the case of Ln3+, the volatility of Ce(IV) alkoxides increase in the case of branched
(Ce(OCMeEt2)4 140C/0,06 Torr, boil.; Ce(OCMe2Et)4 120C/0,1 Torr, subl.; Ce(OCMe2nPr)4
146C/0,05 Torr, boil., Ce(OCEt3)4 154C/0,05 Torr, boil.) or fluorinated (Ce(OCH(CF3)2)4
70C/10-4 Torr, subl., 200C dec.) ligands .[69, 70].
A common preparation route to Ce(IV) alkoxides is the interaction between nitrato(NH4)2Ce(NO3)6 [71] or chlorocerrates (IV) ((PyH)2CeCl6 [72]), with alcohol in the alkaline
media. In some cases the products of incomplete substitution for the nitrate-ligands, such as
Ce(NO3)2(OtBu)2(tBuOH)2] can be isolated.
A number of volatile mixed ligand Ce(IV) complexes contatining alkoxide and different
neutral ligands such as 2,2’-bipyridine, N,N,N’,N’-tetramethylethane-1,2-diamine, iso-propanol
and N,N,N’,N’’,N’’-pentamethyldiethylenetriamine (pmdien) have been prepared [73]. A
peculiar ionic product [Hpmdien]2[Ce(hfip)6] has been reported, in which the counter-ions are
associated by a short F … C contacts. The compound can be sublimed at 70 C/10-4 Torr in 85%
yield [73].
4. Borohydrides
The tetrahedral borohydride-ion acts as di-, tri- and bridging tetradentate ligand towards
different metal centers [74]. A possibility to occupy three or even four positions in the
coordination sphere of metal together with small weight and small size leads to the volatility of
metal hydrides. However, the lanthanide complexes of BH4 - and B3H8- are poorly volatile. As
an example, borohydrides LnCl(BH4)2 of heavy REE can be sublimed in vacuo with low yield
only, for other REE such hydrides are unvolatile [74]. It was shown that the adduct
Y(BH4)3(THF)2 obtained by the reaction between yttrium chloride and LiBH4 in THF can be
sublimed in vacuo at 90C. The compound is ionic [Y(BH4)2(THF)4][Y(BH4)4] with all BH4groups being tri-coordinated. The analogous La compound with three molecules of THF is
unvolatile [75].
One of the ways to increase the volatility is to use substituted hydrides. The
methylborohydrides isolated as adducts with diethyl ether and pyridine M(BH3CH3)3·O(C2H5)2
and M(BH3CH3)3·2C5H5N can be sublimed in vacuo at 100C with the loss of the solvent [76].
A new class of borohydrides is the aminodiboranates - multidentate (often chelating)
borohydride
ligands
binding
to
metal
centers
via
M-H-B
bridges.
The
N,N-
dimethylaminodiboranate H3BNMe2BH3 (DMADB) is involved in synthesis as a sodium salt
Na(DMADB). Its interaction with anhydrous RE chloride in THF or in solid gives triscomplexes Ln(H3BNMe2BH3)3, that can be sublimed under a dynamic vacuum sublime at low
temperatures in vacuum with yields greater than 90%, despite the fact that some of them are
polymeric [77]. The sublimation temperatures at 10-2 Torr decrease steadily across the period,
from 120˚C (La) to 65˚C (Lu). Treatment of the trichlorides EuCl3 and YbCl3 with Na(DMADB
results in a reduction of the RE to a divalent state. The resulting complexes can be separated
from trivalent byproducts by extraction and crystallization from pentane. In contrast with the
trivalent ones, they are thermally unstable and decompose at 107 - 115C without sublimation
[78].
Fig. 4. Crystal structures of Er(H3BNMe2BH3)3 and Pr(H3BNMe2BH3)3 [77]
5. Organolanthanide Complexes
Lanthanides are capable of forming stable organometallic complexes predominantly with
-bonding [79 - 81]. The first compounds of this type were tris-cyclopentadienyls [LnCp3]
prepared in 1956 [82] by the interaction between RE chloride and cyclopentadienyl derivative of
alkaline metal in an aromatic hydrocarbon solvent or THF. The structures of these compounds
differ from polymeric such as PrCp3 to tetrameric (Nd4(Cp*)12) and monomeric for Ln around
the middle of the 4f-series, such as Sm [83]. These compounds are stable indefinitely in inert
atmosphere, but are readily decomposed with protolytic solvents and are highly reactive towards
oxygen .They also react with CS2, halogenated hydrocarbons, ketones and even ferrous chloride,
instantly giving ferrocene. All the compounds are thermally robust and can be heated without
decomposition up to 300C except europium, poor stability of which is caused by reduction of
metal to divalent state. In vacuo 10−4 Torr they can be sublimed without decomposition at 200–
250˚C; volatility being increased along the 4f-series from :La to Eu, i.e., with decreasing ionic
radius. Bulky groups in the Cp ring increase the volatility of tris-complexes as shown below
(temperatures of sublimation for Nd tris-complexes at 7,510-4 Torr) [63, 84].
NMe 2
220 C
210 C
200 C
i-Pr
t-Bu
175 C
t-Bu
MeO
135 C
115 C
95 C
80 C
For some derivatives the vapour pressure data at different temperatures are available. The
authors showed that isopropylcyclopentadienyl derivatives are more volatile in comparison with
non-substituted ones. It can be explained by weaker intermolecular interactions that make the
polymeric structure unstable.
The coordination sphere of metal in tris-cyclopentadienyl complexes of Ln is unsaturated
that explains the formation of adducts with THF, ammonia, pyrazine (Pzn), pyridines [85]. They
sublime with evolution of neutral ligand. (YbCp3)2Pzn compound sublimes in vacuo at 80 105C [74].
The “lanthanocene” derivatives LnCp2X have been studied for a number of anionic
ligands X including halides, alkoxy, phenoxy, carboxy, amino, dialkylditiocarbamato and other
substituents including “classical” chelating groups as β-diketones [86]. Lanthanide chlorides
LnCp2Cl are known for heavy REE from Sm. They can be prepared by interaction between Ln
trichloride with two equivalents of cyclopentadienylsodium in THF or by treating the LnCp3
with LnCl3 [87].The complexes have been isolated by sublimation at 150-250C/10-5 Torr. Di(methylcyclopentadieny1)-lanthanide chlorides sublime at somewhat lower temperatures. In
benzene and in solid state these compounds are dimeric with two chloride bridging group [88].
Phenoxides and carboxylates are more stable to oxidation and hydrolysis. In vapour both
dimmers and monomers are detected. Th interaction between Ln(Cp)2Cl and LiAlMe4 gives
bimetallic compounds LnCp2AlMe4 with two bridging methyl-groups. Yb complex sublimes at
100C/7Pa while Gd complex under the same conditions gives GdCp3 [89]. The borohydide
derivatives of Ln bis-cyclopentadienyls obtained as adducts with THF can be sublimed at
190C/0,13 Pa for Er and Yb [74].
The existence of CeCp4 is under question [90], while CeCp3(OiPr) prepared from
cerium(IV) isopropoxide and MgCp2 can be sublimed in high vacuo at 150C [91].
The cyclopentadienyl, cyclooctatetraenyl and cyclononatetraenyl derivatives of divalent
Yb, Sm, Eu have been studied [92]. The LnCp2 were prepared by dissolving of Ln metal in
liquid ammonia with further addition of HCp. The compounds are air and moisture sensitive.
They can be sublimed in vacuo only at 400C due to their polymeric structure. The substituted
cp-derivatives
of
divalent
RE
are
more
volatile.
The
Yb
complex
with
trimethylsilylcyclopentadiene sublimes at 300C/0,13 Pa [93], while SmCp*2 sublimes at 85C
[94].
Besides cyclopentadienyls, the alternative ligand sets which are able to satisfy the
coordination requirements of the large Ln3+ cations are amidinate and guanidinate anions [95].
R'N
R
NR'
R
N
C
NR'
amidinate
R = H, alkyl, aryl
R' = alkyl, cycloalkyl, aryl, SiMe3
R'N
R
guanidinate
R = alkyl, SiMe3
R' = alkyl, cycloalkyl, aryl, SiMe3
Two such ligands coordinate to Ln3+ giving a metallocene-like coordination environment.
Alkyl-substituted lanthanide tris(amidinates) and tris(guanidinates) with branched substituents
such as N,N’-diisopropyl-2-dimethylamidoguanidinate were found to have good thermal stability
and high volatily [96 - 98], some of the complexes sublime at 90C at low pressure [99].
6. Phtalocyanines
The bis-phalocyanine derivatives of REE LnPc2 known since 1967 [100] are prepared by
the interaction between solid REE acetate with o-phtalodinitrile in molar ratio 1 : 8 at 280 290ºC. Other synthetic approaches are described in [101]. The complexes have a sandwich-type
molecular structure and are soluble in hexane. The complexes are radical species containing Ln
in oxidation state (+3) and one of the phtalocyanine anions in unusual Pc2- form. The properties
of these compounds and their uses as sensors are discussed in [102]. The LnPc2 complexes with
different substituents are stable in vacuo up to 500ºC. Most of them can be sublimed with
decomposition at 560ºC/0,01 Torr [103], at 450ºC/0,001 Torr [104] or even at ca 300 - 400ºC in
high vacuum [102]. This procedure is used for the deposition of Langmuir-Blodgett films.