Complexes of (EtO)2P(O)CH2P(O)(OEt)2 with lanthanide nitrates

Article abstract of DOI:10.1039/b408536a

The preparation of complexes of (EtO)₂P(O)CH₂P(O) (OEt)₂ = L, with lanthanide nitrates is described. Stable complexes with composition LnL₂(NO₃)S can be isolated for Ln = La-Eu and fully characterised. For Ln = Gd-Lu solid compounds could not be isolated. Conductivity and ³¹P NMR spectroscopy indicate structural changes in solution between the lighter and heavier lanthanides and, whilst electrospray mass spectrometry confirms a dramatic difference in behaviour with complexes of the heavier lanthanides readily decomposing via loss of EtNO₃, other experiments show that this does not occur under the conditions of complex formation. The single crystal X-ray structures for Ln = La and Sm show the nitrates and OEt groups to be in close proximity. The changes in spectroscopic properties correlate well with the difficulties in isolating the complexes of heavier metals, and are possibly due to the formation of dimeric complexes rather than loss of ethyl nitrate.

Full text of DOI:10.1039/b408536a

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P A P E R
Complexes of (EtO)2P(O)CH2P(O)(OEt)2 with lanthanide nitrates
Anthony M. J. Lees,a Roman A. Kresinskib and Andrew W. G. Platt*a
a School of Sciences, Staffordshire University, College Road, Stoke-on-Trent, UK ST4 2DE
b School of Chemical and Pharmaceutical Sciences, Kingston University, Penrhyn Road,
Kingston upon Thames, UK KT1 2EE
Received (in Durham, UK) 4th June 2004, Accepted 16th September 2004
First published as an Advance Article on the web 3rd November 2004
The preparation of complexes of (EtO)2P(O)CH2P(O)(OEt)2 ¼ L, with lanthanide nitrates is described.
Stable complexes with composition LnL2(NO3)3 can be isolated for Ln ¼ La–Eu and fully characterised.
For Ln ¼ Gd–Lu solid compounds could not be isolated. Conductivity and 31P NMR spectroscopy indicate
structural changes in solution between the lighter and heavier lanthanides and, whilst electrospray mass
spectrometry confirms a dramatic difference in behaviour with complexes of the heavier lanthanides readily
decomposing via loss of EtNO3, other experiments show that this does not occur under the conditions of
complex formation. The single crystal X-ray structures for Ln ¼ La and Sm show the nitrates and OEt
groups to be in close proximity. The changes in spectroscopic properties correlate well with the difficulties
in isolating the complexes of heavier metals, and are possibly due to the formation of dimeric complexes
rather than loss of ethyl nitrate.
to the influence of primary coordination sphere effects. Addi-
tionally, smaller alkyl chains should lead to more crystalline
products, perhaps allowing single crystal X-ray diffraction
studies to be carried out.
1
Introduction
Complexes of lanthanide nitrates continue to attract attention as
mimics for less accessible actinide complexes1 of potential
interest in nuclear waste treatment, and for their inherent
interest in exploring structural variations amongst the 4f metals.
Many multifunctional PO based ligands have been examined
and the structures of their lanthanide2–4 and actinide5,6 com-
plexes reported. Solvent extraction properties of a variety of
organophosphorus compounds with f-block metals have also
been examined.7–13 Ligands in which a central methylene group
is bonded to two electron withdrawing phosphoryl groups have
been previously explored as extractants. Solvent extraction
studies of lanthanides with tetra(p-tolyl)methylene diphosphine
dioxide,10 where no complexes were isolated, indicate that
compounds with 4 : 1 ligand to metal ratios for lighter
lanthanides and 3 : 1 ratios for heavier lanthanides are formed.
In further studies diphosphine dioxides, R2P(O)CH2P(O)R2 and
related ligands were examined in solution by EXAFS.14 With
R ¼ p-tolyl holmium nitrate forms 2 : 1 complexes, whilst for
R ¼ octyl 1 : 1 complexes were formed. Powder X-ray diffrac-
tion studies of complexes of lanthanide nitrates with (RO)2-
P(O)CH2P(O)(OR)2 (R ¼ iPr) indicated that complexes are
formed in three structural classes La - Nd, Sm - Tb and
Dy - Yb, although no single crystal data were reported.15 The
recent structural characterisation of 1 : 1 and 2 : 1 complexes of
La and Pr nitrate complexes with (iPrO)2P(O)CH2P(O)(OiPr)16
and the fact that complexes with (iPrO)2P(O)(CH2)nP(O)(OiPr)2
ligands with n ¼ 1 were found to be much better extractants for
lanthanide metals than with higher values of n,17 prompted us to
carry out a structural study on related complexes in order to
characterise the transition points between the structures formed
along the lanthanide series. Our past studies with bis-phospho-
nate lanthanide complexes have indicated a marked decrease in
their stability for the heavier lanthanides,18 and it is thus of
interest from the point of view of reusability of the extractant to
determine whether similar effects occur here. We chose to
examine complexes of (EtO)2P(O)CH2P(O)(OEt)2 ¼ L, as this
offers a less sterically congested outer coordination sphere,
meaning any structural changes observed would be mainly due
2
Results and discussion
2.1 Synthesis
The reactions of lanthanide nitrates with L were carried out in
trimethyl orthoformate solution as an in situ dehydration agent
and afford
a series of compounds of general formula
LnL2(NO3)3 for Ln ¼ La – Eu. Much like the complexes of
the hydroxydiphosphonate [(MeO)2P(O)]2C(OH)R (¼ L0) from
acetonitrile solution,18 the yield is seen to decrease markedly
with the increasing atomic mass of the lanthanide. Characteris-
ing data are given in Table 1. Analytically pure samples of the
complexes of the lighter lanthanides (La–Sm) crystallized spon-
taneously from a solution of the reagents. The heavier lantha-
nides (Eu–Lu) behaved differently. They did not crystallise
spontaneously from trimethyl orthoformate solution, but on
addition of diethyl ether, oils having the characteristic colour of
the lanthanide ion were precipitated. A solid material was
obtained on trituration of the europium containing oil with
diethyl ether, and the gadolinium complex reached a waxy semi-
solid consistency with similar treatment, but satisfactory ele-
mental analyses could be obtained for the La–Eu complexes
only. The oils obtained for the heavier lanthanides could be
crystallised neither from any common organic solvents, nor on
prolonged drying. The infrared spectrum of the Yb containing
oil is very similar to those obtained from the crystalline com-
plexes discussed below. The trend in the melting points of the
crystalline solids is linear with an approximately 5 1C decrease
per lanthanide, with the formation of much lower melting point
materials after Eu/Gd implying a significant structural change.
2.2 Solid state structures
2.2.1 Infrared spectroscopy. The infrared spectra of all of
the solid complexes (as KBr discs) show a shift in the P
O
Q
stretching frequency from 1257 cmꢀ1 in the free ligand, to
T h i s j o u r n a l i s & T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w J . C h e m . , 2 0 0 4 , 2 8 , 1 4 5 7 – 1 4 6 3
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Table 1 Characterising data for LnL2(NO3)3
Elemental Analysis: Found (calc)/%
31P NMRa
Ln
Typical Yield/%
C
H
N
Mp/1C
d/ppm
Dn1/2/Hzb
La
Ce
Pr
93
84
76
72
62
48
23.91(23.99)
23.90(23.95)
23.92(23.93)
23.84(23.84)
23.61(23.68)
23.45(23.64)
4.96(4.92)
4.92(4.91)
4.92(4.91)
4.89(4.89)
4.86(4.86)
4.82(4.85)
4.60(4.66)
4.62(4.66)
4.55(4.65)
4.55(4.63)
4.59(4.60)
4.58(4.60)
b
120.5
114.2
109.7
103.4
90.6
21.3
42.0
o 5
10
68.7
77.7
25
35
Nd
Sm
Eu
a
21.4
ꢀ71.3
o 5
195
Proton decoupled spectra. 50 mg/3 g CDCl3. All solutions showed a single peak. Linewidth at half height.
B1210 cmꢀ1 in the complexes, indicating coordination via
both phosphoryl oxygen atoms. The observed decrease in
frequency is similar to that reported for the tetraisopropyl
analogues.15 The spectra all show the nitrate n1(A1) and n5(B2)
bands at B1305 cmꢀ1 and B1465 cmꢀ1 consistent with
nitrates being bound in a bidentate chelating manner. The
splitting of the nitrate bands, Dn, shows a steady decrease form
La to Eu. The n4 band of the nitrate ion also splits on adoption
of C2v symmetry: the combination band n1 þ n4 at 1700–1800
cmꢀ1 reflects this splitting and the separation of these two
bands has previously been used to establish the mode of nitrate
coordination.19 The splitting of these bands in the spectra of
the solid complexes obtained (29–44 cmꢀ1) is again consistent
with bidentate coordination. In all of the spectra the band at
B1465 cmꢀ1 appears as a sharp doublet. Similar effects have
been observed previously and have been attributed to non-
equivalence of nitrate groups within either the crystal or the
molecule.15
Infrared spectra obtained on the crystalline complexes under
higher resolution (ꢁ2 cmꢀ1) and with no sample pretreatment
resolve the PO absorption into 3 (La–Nd) and 2 (Sm) bands.
This is consistent with the crystallography (discussed below)
which shows 3 distinct PO distances for the La complex and 2
for the more symmetrical Sm complex. Interestingly the spec-
trum of the oil obtained from the Yb(NO3)3 reaction resembles
that of the La complex in the PO region with 3 absorptions at
similar frequencies and relative intensities. The NO absorp-
tions are, however, significantly different from those of the La–
Sm complexes and show two intense bands centred at 1491 and
1297 cmꢀ1 suggesting a change in the nitrate environment. The
magnitude of the splitting is larger than that observed for the
La–Sm complexes (171–158 cmꢀ1) yet is still indicative of
bidentate nitrate.
changes in properties along the lanthanide series. Although
both complexes have similar overall molecular geometries, the
samarium complex has a C2 axis of symmetry passing through
the N(1) nitrate ligand along the Smꢂ ꢂ ꢂN(1)–O(4) line. One half
of the N(1) nitrate, and the other two complete pairs of ligands
are related by symmetry with equivalent atoms generated by
the transformation 1 ꢀ x, y, 3/2 ꢀ z. This symmetry is not
apparent in the lanthanum complex. The molecular structure
of LaL2(NO3)3 is shown in Fig. 1, details of the data collection
and refinement are given in Table 2 and selected bond lengths
and angles in Table 3.
The complexes adopt structures which are very similar to the
tetraethylmalondiamide complex, La{{(Et2N)C(O)}2CH2}2
20
(NO3)3 and to the recently published La and Pr tetraisopro-
pylmethylene diphosphonates.16 The La–O(P) distances are
slightly smaller, ranging from 2.464(3)–2.515(3) A for the
lanthanum complex compared with a range of 2.511(2)–
2.546(2) A, these differences reflecting the reduced steric de-
mands of the ethyl relative to the isopropyl groups. The metals
are ten-coordinate with the nitrate and the diphosphonate
ligands each chelating through two oxygen atoms. The coordi-
nation polyhedron around the metal in each complex can be
considered to be a distorted bi-capped square antiprism as
shown in Fig. 2. The distortion here can be attributed solely to
the small bite of the nitrate ligands. The La and Sm complexes
can also be considered as pseudo-pentagonal bipyramids,
defined by the N-atoms of the nitrates and the phosphoryl
oxygens. Two nitrates are axial and one equatorial with the PO
oxygen atoms forming the other 4 positions in the equatorial
plane. The inter-ligand repulsions, particularly between the
positively charged N and P centres generally increase between
La and Sm. With the exception of the shortest Pꢂ ꢂ ꢂP distances
which increase from 5.3990(17) A (La) to 5.7002(29) A (Sm),
the repulsions between the N and P atoms increase from La
Infrared spectra of the Yb complex in CDCl3 were examined
and show a smaller splitting of the N–O peaks compared with
the neat oil, at 168 compared with 194 cmꢀ1, whilst the PO
region shows no significant difference. Interestingly there is no
peak which could be assigned to a pendant PO group from a
monodentate ligand or from an ionic nitrate.
Reaction of Er(NO3)3 with L in a 1 : 1 ratio produced an oil
which solidified on standing. The product was partially soluble
on addition of CDCl3 and the transmittance infrared spectrum
of the resulting suspension was similar to that of the Yb
complex. On addition of L to the suspension a clear solution
was formed whose infrared spectrum was identical to that of
the original suspension. It seems that the heavier lanthanides
also form 2 : 1 complexes and that the material isolated from
the 1 : 1 reaction is a mixture of Er(NO3)3 and ErL2(NO3)3
rather than a discrete 1 : 1 complex.16
2.2.2 Single crystal X-ray crystallography. Crystals of the
lanthanum and samarium complexes were chosen for structur-
al analysis, since they represent the complete span of the series
for which crystalline materials could be obtained and, thus,
their structures may indicate underlying reasons for the
Fig. 1 The structure of LaL2(NO3)3 with 50% probability ellipsoids,
showing the atom-labelling scheme. H atoms have been omitted for
clarity. Generated using SNOOPI.38
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Table 2 Crystal data, data collection and refinement parameters
Table 3 Bond lengths, selected non-bonding distances and bond
angles of LnL2(NO3)3 (A, 1)
[La(CH2
{EtO2PO}2)2
(NO3)3]
[Sm(CH2
{EtO2PO}2)2
(NO3)3]
La
Sm
Compound
Ln–O(1)
Ln–O(2)
Ln–O(3)
Ln–O(5)
Ln–O(6)
La–O(12)
La–O(13)
La–O(14)
La–O(15)
La–O(16)
2.501(3)
2.431(4)
Empirical formula
Molecular weight, Mr
Crystal system
Space group
Unit cell dimensions
a (A)
C18H44LaN3O21P4 C18H44N3O21P4Sm
901.35
2.464(3)
2.566(3)
2.683(3)
2.602(3)
2.601(3)
2.502(3)
2.515(3)
2.638(3)
2.617(3)
2.432(3)
2.516(4)
2.507(4)
2.708(4)
2.516(4)
2.431(4)
2.432(3)
2.507(4)
2.708(4)
912.79
monoclinic
C2/c
monoclinic
P21/c
Sm–O(3)i
Sm–O(1)i
Sm–O(2)i
Sm–O(5)i
Sm–O(6)i
8.8205(11)
20.0885(6)
20.6300(14)
91.716(16)
3653.8(5)
293(2)
9.6295(2)
21.7274(4)
17.2755(3)
96.6898(10)
3589.8(12)
120(2)
b (A)
c (A)
b (1)
Cell volume, U (A3)
T (K)
Formula units/unit cell, Z
Lnꢂ ꢂ ꢂN(1)
Lnꢂ ꢂ ꢂN(2)
Laꢂ ꢂ ꢂN(3)
3.016(4)
3.060(4)
3.042(4)
2.935(7)
3.039(4)
3.039(4)
4
1.425
4
1.896
m (mmꢀ1
)
Smꢂ ꢂ ꢂN(2)i
Measured reflections
Independent reflections
Rint
15032
5518
0.0846
18874
4117
0.0530
Lnꢂ ꢂ ꢂP(1)
Lnꢂ ꢂ ꢂP(2)
Laꢂ ꢂ ꢂP(3)
Laꢂ ꢂ ꢂP(4)
3.762(1)
3.717(1)
3.752(1)
3.748(1)
3.701(2)
3.712(1)
3.701(2)
3.712(1)
Final R indicesa
(Fo > 4s (Fo))
R (F)
Smꢂ ꢂ ꢂP(1)i
Smꢂ ꢂ ꢂP(2)i
0.0365
0.0833
0.0448
0.1159
wR2 (F2)
P(1)–O(1)
P(1)–O(8)
P(1)–O(9)
P(1)–C(1)
P(2)–O(2)
P(2)–O(10)
P(2)–O(11)
P(2)–C(1)
P(3)–O(13)
P(4)–O(14)
1.480(3)
1.553(3)
1.567(3)
1.796(4)
1.481(3)
1.551(3)
1.560(3)
1.788(4)
1.479(3)
1.473(3)
1.478(4)
1.564(4)
1.565(4)
1.797(6)
1.475(4)
1.554(5)
1.538(6)
1.789(6)
1.478(4)
1.475(4)
Final R indicesa
(all data)
R (F)
0.0464
0.0853
0.0429
0.1153
wR2 (F2)
a
2
R ¼ S||Fo| ꢀ |Fc||/S|Fo; wR2 ¼ [S [w(Fo ꢀ Fc2)2]/S [w(Fo2)2]]0.5
.
P(1)i–O(1)i
P(2)i–O(2)i
to Sm, with Pꢂ ꢂ ꢂN decreasing from 4.2635(36) A (La) to
4.0378(47) A (Sm) and Nꢂ ꢂ ꢂN from 4.6918(54) A (La) to
4.4872(70) A (Sm). Similarly the Pꢂ ꢂ ꢂLn and Nꢂ ꢂ ꢂLn distances
show significant changes, decreasing from 3.744(1) (La) to
3.707(2) (Sm) and 3.039(4) (La) to 2.987(6) (Sm), respectively.
The coordination of the nitrate groups is of interest: the N(1)
nitrates in both the structures in the present study bond to the
lanthanide such that the metal and the four atoms of the nitrate
ion lie in essentially the same plane, the deviation being at most
0.050 A for O(4) in the La structure, equating to a bending
along the O(3)ꢂ ꢂ ꢂO(12) line of 4.32(13)1. The bonding of the
other nitrate groups in LaL2(NO3)3 show greater deviation
from their mean plane, due to a greater degree of bending
along the line connecting their ligating O-atoms; this angle is
16.91(11)1 for the N(2) group and 15.32(11)1 for the N(3)
group. Such deviations are not unprecedented; all of the similar
L0 complexes18 show some degree of nitrate group bending
(10.96(20)1 along O(3)ꢂ ꢂ ꢂO(4) of the lanthanum complex) with
an angle of 14.21 for the nitrate groups of Dy(phen)2(acac)
(NO3)2.21 The nitrate groups of La[{Et2NC(O)}2CHCH2-
O(1)–Ln–O(2)
O(1)–Ln–O(3)
O(1)–Ln–O(5)
O(1)–Ln–O(6)
O(2)–Ln–O(3)
O(2)–Ln–O(5)
O(2)–Ln–O(6)
O(3)–Ln–O(5)
O(3)–Ln–O(6)
O(5)–Ln–O(6)
O(1)–P(1)–C(1)
O(2)–P(2)–C(1)
P(1)–O(1)–Ln
P(2)–O(2)–Ln
P(1)–C(1)–P(2)
74.87(9)
130.18(9)
67.38(9)
74.13(12)
145.57(12)
73.74(13)
112.86(13)
74.54(13)
70.55(13)
107.97(12)
82.69(15)
64.62(15)
48.50(12)
113.3(3)
75.70(9)
76.99(9)
72.38(9)
120.11(9)
65.27(10)
84.25(10)
48.28(9)
112.71(18)
112.31(18)
140.41(16)
139.54(16)
113.4(2)
112.7(2)
141.2(2)
142.4(2)
112.7(3)
Symmetry Code: (i) 1 ꢀ x, y, 3/2 ꢀ z.
22
CH2OCH2CH3]2(NO3)3 similarly display a marked inclina-
tion, the greatest bending in this case being 18.21. In addition
to the bending along the Oꢂ ꢂ ꢂO line, the nitrate groups show
varying degrees of asymmetry in their bidentate Ln–O(N)
bonds. The N(1) nitrate group in the Sm complex is symme-
trically coordinated, whereas all the other nitrate groups
exhibit unequal Ln–O(N) bond lengths. Differences in the
lengths of the two Ln–O(N) bonds are much greater in the
Sm complex (about 0.2 A in the bonding of the N(2) nitrate)
compared with values ranging from 0.02 to 0.08 A in the La
complex. Asymmetry in the N–O(Ln) distances is likewise
much less pronounced with only small differences (0.02 and
0.004 A) being observed in the La complex.
There are weak intermolecular interactions between the oxy-
gen atoms of the nitrates and methylene protons of the ethoxy
groups. These interactions are stronger for the Sm complex with
the O(3)ꢂ ꢂ ꢂH(6A) and O(7)ꢂ ꢂ ꢂH(802) distances being 2.386(4) A
and 2.388(5) A compared with the closest contact in the La
complex being between O(17)ꢂ ꢂ ꢂH(6B) at 2.476(3) A.
Whilst the absolute strength of these contacts is doubtless
not great, their existence coincides with the difficulties in
isolating complexes for the elements following samarium and
the tendency for similar complexes to decompose via loss of
alkyl nitrate, and we conclude that the intermolecular hydro-
gen bonding between the coordinated nitrate and OCH2 con-
tributes to the ease of decomposition. On heating the solid Sm
complex to 175 1C the solid firstly melts and then resolidifies
producing ethyl nitrate during the course of these changes. This
was detected by the gas phase infrared spectrum which showed
absorptions at 3004(w), 1663(vs), 1652(vs), 1294(s), 1285(s),
1025(m), 912(m) and 853(s) in excellent agreement with the
literature values.23
2.2.3 Solution structures. Most techniques for studying
solution properties of lanthanide complexes give only indirect
information. Here the lability and paramagnetism of many of
N e w J . C h e m . , 2 0 0 4 , 2 8 , 1 4 5 7 – 1 4 6 3
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Fig. 2 Coordination geometry about the lanthanum atom of LaL2
(NO3)3.
the complexes make interpretations more difficult. We have
used a combination of 31P NMR spectroscopy, conductivity
and electrospray mass spectrometry in an attempt to gain
insight into the changes in solution structures which may
accompany the transition from crystalline complexes of the
lighter lanthanides to non-crystalline materials for their heavier
counterparts.
Fig. 3 Lanthanide Induced Shift plots for LnL2(NO3)3 in CDCl3
solution.
2.2.3.1 31P NMR spectroscopy. The proton decoupled 31P
NMR spectra of the complexes all show single peaks. Since
solid complexes were only isolated for the lanthanides La -
Eu, 31P NMR data were collected for the remainder of the
series by dissolving stoichiometric amounts of nitrate and
diphosphonate in deuterochloroform in the manner used pre-
viously for the analogous L0 complexes.18
For Eu and possibly onwards, the decrease could be explained
by a different mechanism for the reduction in steric crowding
involving the phosphoryl groups bridging to form a less
congested dimer, competing with the above and reducing the
overall conductivity:
2 Ln(k2L)2(NO3)3 $ Ln(k2L)(NO3)3(mL)2Ln(k2L)(NO3)3.
Lanthanide induced shift analysis applied to this data in-
dicates that there is a structural change in solution between the
lighter lanthanides (La–Sm) and the heavier ones (Eu–Lu).
Plots of di/hSzii versus Di/hSzii and di/Di versus hSzii/Di (where
the paramagnetic shift di ¼ dLn ꢀ 0.5(dLa þ dLu) and where dLn
is the observed shift for the Ln complex; hSzii are the spin
expectation values and Di a parameter related to the variation
in di which should occur if the crystal field coefficients are
independent of the lanthanide ion respectively24), have two
linear regions with breaks between Sm and Eu, indicating a
major change in solution structure. These are shown in Fig. 3.
These results are from the analysis of one nucleus in the
compounds, the analysis of two nuclei25 has been shown to
give more reliable results and the conclusions based on the
current NMR analysis must be considered as tentative.
Precedent for this type of bridging has recently been reported
in UO2 complexes with [Ph2P(O)]2CH2.27
21
The alternative means of reducing in the crowding of the first
coordination sphere achieved by dissociation of L, is felt to be
less likely as the infrared spectra of Yb(NO3)3L2 either as the
neat oil or in CDCl3 solution, give no indication of the
presence of a pendant phosphoryl group or free nitrate ion,
the absorption profile in the PO region being very similar
to that observed for the La complex. The possibility of
forming nitrate bridged dimers, Ln(k2L)2(NO3)2(mNO3)2Ln
(k2L)2(NO3)2 analogous to the solid state structures of the
1 : 1 complexes of La(NO3)3 and Pr(NO3)3 with (iPrO)2-
i
16
2
P(O)CH P(O)(O Pr)2 seems less likely to be the reason here,
2.2.3.2 Conductivity. Although limited to the first half of the
series, the trend in molar conductance (at a concentration of
approximately 1 mM in acetonitrile) as the series is traversed
follows the same pattern previously observed for the analogous
L0 complexes:18 a steady increase to samarium followed by a
decrease at europium as shown in Fig. 4. The values of molar
conductance, 12–25 Oꢀ1 cm2 molꢀ1 are low compared to those
expected for 1 : 1 electrolytes in acetonitrile (120–160 Oꢀ1 cm2
molꢀ1 26) and suggest that there is little dissociation of the
molecular species LnL2(NO3)3. The increasing conductivity
from La to Sm could be due to increasing steric crowding
of the complexes being relieved by ionisation of nitrate in
solution:
Ln(k2L)2(NO3)3 $ [Ln(k2L)2(NO3)2]1 þ [NO3]ꢀ.
Fig. 4 Molar conductance of LnL2(NO3)3 in acetonitrile @ 0.001 M.
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Table 4 Positive ion electrospray mass spectral dataa (þ20 V) for LnL2(NO3)3 in methanol
La
Pr
Nd
Sm
Yb
[LnL3]31
334.5(30)
335.1(35)
336.1(30)
346.0(5)
442.1(15)
[LnL4]31
430.6(65)
388.7(15)
532.7(100)
431.2(55)
389.5(10)
533.7(100)
432.2(50)
535.2(100)
844.3(30)
434.9(5)
[Ln(NO3)L2]21
[Ln(NO3)L3]21
[Ln(NO3)L3–EtNO3]21
[Ln(NO3)2L2]1
[Ln(NO3)2L2–EtNO3]1
539.1(30)
493.7(100)
852.2(20)
761.2(10)
550.2(35)
504.7(100)
839.2(20)
841.2(25)
783.3(5)
[Ln(NO3)2L3]1
1127.3(15)
1129.2(10)
1132.4(5)
1140.3(o 5)
1049.2(15)
958.3(20)
858.5(10)
813.3(15)
[Ln(NO3)2L3–EtNO3]1
[Ln(NO3)2L3–2EtNO3]1
[Ln2(NO3)4L5–3EtNO3]21
[Ln2(NO3)4L5–4EtNO3]21
a
1071.3(5)
980.3(10)
881.0(o 5)
834.8(o 5)
m/z values quoted for the most intense peak in the isotope profile and are within ꢁ Da of the theoretical values. The intensity relative to most
intense ion is given in parentheses.
as this would also lead to an increase in the coordination
number about the metal, thus increasing the steric crowding in
2 : 1 complexes.
The formation of dimeric complexes is consistent with the
NMR analysis which indicates major structural changes in the
region of europium and is also in accord with the difficulties in
isolation of pure compounds for the heavier lanthanides.
to lutetium. The mass spectra of the samarium and ytterbium
complexes are very different from those of the lighter lantha-
nides. The spectra show peaks due to ions which have lost
EtNO3 as the main features in the spectra at þ20 V, corre-
spondingly, the base peak is due to [LnL3(NO3)–EtNO3]21
.
Sequential loss of ethyl nitrate is seen in several series of peaks
due to ions such as [LnL3(NO3)2–nEtNO3]1 (n ¼ 1, 2). The
assignments are based on the mass to charge ratio and on
comparisons of the theoretical and observed isotope profiles
which are all in excellent agreement, one example of
[SmL3(NO3)2–2EtNO3]1 being shown in Fig. 5. Dimeric ions
such as [Ln2L4(NO3)5–nEtNO3]1 (n ¼ 2–4), [Ln2L3(NO3)5–
nEtNO3]1 (n ¼ 1–4) , [Ln2L5(NO3)4–nEtNO3]21 (n ¼ 2–4),
[Ln2L4(NO3)4–nEtNO3]21 (n ¼ 2, 3) are also observed as peaks
of moderate intensity in some cases. The isotope patterns are
consistent with Ln2 based ions and a comparison of the
observed and theoretical isotope profiles of [Sm2L4(NO3)5–
3EtNO3]1 is shown in Fig. 6 as a representative example.
Attempts to mimic the loss of ethyl nitrate observed in the
mass spectra were made by heating either solutions of lutetium
nitrate and ligand in triethyl orthoformate or a neat mixture of
the ligand and lutetium nitrate. The infrared spectra of the gas
above the reaction mixture held at about 140 1C showed only
triethyl orthoformate vapour or no spectrum respectively, with
no absorptions due to ethyl nitrate. The stability at these
temperatures is consistent with the decomposition tempera-
tures observed in the solid state. Whilst we have unambigu-
ously identified dimeric species in the gas phase decomposition
of the complexes of the heavier lanthanides, it is unlikely that
the same complexes are formed in solution under ordinary
conditions. The dimers formed during the electrospray process
2.2.3.3 Electrospray mass spectrometry. Electrospray mass
spectra were obtained from methanol solutions of the com-
plexes and the results are presented in Table 4. The low cone
voltage spectra are most likely to be representative of the
species present in solution and are discussed first. For all the
complexes ligand redistribution is apparent with the base peak
in the spectra of the complexes La–Nd due to [LnL3(NO3)]21
.
Ions derived from the solid state structure of the complexes
such as [LnL2(NO3)2]1 are present at moderate intensity.
Interestingly, for the complexes of the lighter lanthanides there
are no ions which might arise by loss of ethyl nitrate. Raising
the cone voltage simplifies the mass spectra with
[LnL2(NO3)2]1 being the only metal based ion observable. In
all the spectra peaks due to [L þ H]1 and [L þ Na]1 are
observed, and at higher cone voltage decomposition of the
ligand by sequential loss of C2H4 is observed. The same effect is
seen in the spectrum of L itself at higher cone voltages and is
not dependent on lanthanide coordination.
As the Yb complex could not be isolated as a pure solid, a
solution of Yb(NO3)3 and L in methanol was used to study the
behaviour of the heavier lanthanides. Ytterbium has a char-
acteristic isotope signature and was thus chosen in preference
Fig. 5 Theoretical Isotope Profile of [SmL3(NO3)2–2EtNO3]1 and
that observed at þ20 V cone in the Electrospray Mass Spectrum.
Fig. 6 Theoretical Isotope Profile of [Sm2L4(NO3)5–3EtNO3]1 and
that observed at þ20 V cone in the Electrospray Mass Spectrum.
N e w J . C h e m . , 2 0 0 4 , 2 8 , 1 4 5 7 – 1 4 6 3
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probably contain bridging phosphonate anions rather than
neutral bridging bisphosphonate ligands.
4.2.2 Europium - Gadolinium. No product crystallised
spontaneously from trimethyl orthoformate solution. The
addition of diethyl ether however precipitated a viscous liquid,
which solidified on trituration with diethyl ether. The gadoli-
nium containing material was of a waxy semi-solid consistency.
3
Conclusion
The solid state structures give an indication of a possible
reason for the nascent formation of ethyl nitrate, particularly
for the Sm complex, in which intermolecular contacts between
nitrate and ethyl groups are strongest. However, we are at
present unable to account for why, since the production of
ethyl nitrate only occurs on heating the solid complexes to
175 1C, we have difficulties in isolating crystalline materials
from solution, wherein the conditions during attempted crys-
tallisations are much less severe, the complexes being subject to
temperatures of 100 1C at most. The cut-off point for synthetic
availability correlates well with changes in NMR spectra and
conductivity; as for the electrospray mass spectra, whilst it may
be possible that decomposition by loss of ethyl nitrate may be
partly caused by the electrospray process itself, the presence of
significant intensity peaks from dimeric species is consistent
with an intermolecular reaction producing ethyl nitrate which
may operate in addition to any intramolecular one. In this
context it is interesting to speculate on whether any parallel
exists between the catalytic activity of lanthanide ions in the
hydrolysis of DNA28 and the loss of ethyl nitrate in our
systems. Of the two mechanisms thought to be responsible
for DNA cleavage P–O bond polarisation by lanthanide
coordination seems most likely and the observation of
dinuclear species in Ce(IV) induced DNA hydrolysis29 is mi-
micked by the observation of bimetallic species in the electro-
spray mass spectra.
4.2.3 Terbium - Lutetium. No solid material crystallised
from trimethyl orthoformate solution but, again, the addition
of diethyl ether precipitated a viscous oil. In each case, the oil
showed the characteristic colour of the lanthanide ion. The
material could not be solidified on trituration with diethyl
ether.
4.2.4 Mass spectrometry. Electrospray mass spectra were
obtained by the EPSRC National Mass Spectrometry Service
Centre at University of Wales, Swansea as described pre-
viously.31
4.2.5 X-ray crystallography. Data were collected by the
EPSRC National Crystallography Service at the University
of Southampton using previously described procedures.32–34
For both the La and Sm structures, the positions of the
metal atoms were estimated using Patterson methods
(SHELXS-97),35 and all remaining non-H atom positions were
obtained through subsequent Fourier syntheses (SHELXL-
97).36 Refinement was by full-matrix least-squares on F2 data
using SHELXL-9736 from within the WinGX37 suite of soft-
ware. An absorption correction34 was applied to the Sm data.
In both of the structures, all non-H atoms were refined
anisotropically. All H atoms were refined at calculated idea-
lised positions (methylene C–H ¼ 0.98 A, methyl C–H ¼ 0.99 A).
H-atoms in similar functional groups or environments were
assigned single, refined isotropic displacement parameters.
H-atom displacement parameters were, on reconvergence,
0.037(5) A2 (La, PCH2P), 0.070(5) A2 (La, OCH2C), 0.096(5)
A2 (La, CH3), 0.034(14) A2 (Sm, PCH2P), 0.052(9) A2 (Sm,
OCH2C), 0.058(8) A2 (Sm, CH3).
4
Experimental
Infrared spectra were obtained as either KBr discs on an ATI
Mattson Genesis FTIR spectrometer or without pretreatment
on a Thermo Nicolet AVATAR 370 FTIR. NMR spectra were
obtained on a JEOL FX-90Q and conductivity measurements
made using a Union Cambridge WPA CM35 conductivity
meter.
In the Sm structure, one of the ethoxy groups was disordered
and modelled in two alternative positions with a complemen-
tary site occupation factor. The factor was refined to a final
value of 0.533. The O–CH2 bonds of the disordered groups
(O(11)–C(8) and O(11)–C(80)) were restrained to an idealised
bond distance of 1.47 A consistent with the other O–CH2
bonds in the Sm and La structures.w
4.1 Synthesis of (EtO)2P(O)CH2P(O)(OEt)2
Stoichiometric amounts of the reagents 12.2 g (70 mmol)
CH2Br2 and 23.3 g (140 mmol) (EtO)3P were mixed, heated
to boiling, and were allowed to reflux under nitrogen for 1
week. The brown liquid thus produced was fractionated by
vacuum distillation. The 130–140 1C fraction (0.5 mm Hg)
Lit:30 135–137 1C (0.4 mm Hg) was collected, 2.40 g (12%).
Acknowledgements
We are grateful to the EPSRC for use of the X-ray crystal-
lography service at Southampton University and the National
Mass Spectrometry Service at Swansea University.
Infrared spectroscopy: (neat) n(P O) 1255 cmꢀ1(s), n(P–O)
Q
1030 cmꢀ1(s), 31P NMR (neat liquid) d 19.9 ppm, 19.4 ppm
(CDCl3).
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2
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þ44-1223-336033;
E-mail:
deposit@ccdc.cam.ac.uk;
http://
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