Functionalised phosphonate ester supported lanthanide (Ln = La, Nd, Dy, Er) complexes

Article abstract of DOI:10.1039/d0dt03047c

A series of phosphonate ester supported lanthanide complexes bearing functionalities for subsequent immobilisation on semiconductor surfaces are prepared. Six phosphonate ester ligands (L1-L6) with varying aromatic residues are synthesised. Subsequent com

Full text of DOI:10.1039/d0dt03047c

Dalton
Transactions
View Article Online
View Journal
PAPER
Functionalised phosphonate ester supported
lanthanide (Ln = La, Nd, Dy, Er) complexes†
Cite this: DOI: 10.1039/d0dt03047c
Ingo Koehne, a Artur Lik,a Miriam Gerstel,b Clemens Bruhn,a
a
Johann Peter Reithmaier, b Mohamed Benyoucef b and Rudolf Pietschnig
*
A series of phosphonate ester supported lanthanide complexes bearing functionalities for subsequent
immobilisation on semiconductor surfaces are prepared. Six phosphonate ester ligands (L1–L6) with
varying aromatic residues are synthesised. Subsequent complexation with lanthanide chloride or -nitrate
precursors (Ln = La, Nd, Dy, Er) affords the corresponding mono- or dimeric lanthanide model complexes
[LnX3(L1–L3 or L5–L6)3]n (X = NO3, Cl; n = 1 (Nd, Dy, Er), 2 (La, Nd)) or [LnCl2Br(L4-Br)2(L4-Cl)]n (n = 1
(Nd, Dy, Er), 2 (La, Nd)) (1–32). All compounds are thoroughly characterised, and their luminescence pro-
perties are investigated in the visible and NIR spectral regions, where applicable.
Received 31st August 2020,
Accepted 13th October 2020
DOI: 10.1039/d0dt03047c
rsc.li/dalton
stituted mono phosphonate ester ligands. A reactive function-
ality at the para-position with respect to the phosphonate sub-
Introduction
Lanthanide ions already find widespread application in light- stituent shall give additional access to future substitution reac-
ing, sensing and display technologies, due to their outstanding tions. Selected related aryl-bound mono-phosphonate esters
photoluminescence properties.1–3
are summarised in Chart 1. These range from differently para-
i
Bis-phosphonic acid ester ligands have already been suc- substituted ligands A and B (X = H, Cl, Br; R = Me, Et, Pr,
cessfully shown to be versatile building blocks in transition tBu),14–16 over polyfluorinated species C,17 to anthracene D (X
metal organic frameworks (MOFs).4 In contrast to their car- = H, Cl) and 1,1′-biphenyl derivates E (X = H, Me).14,18
boxylic acid congeners, the key advantage of phosphonate
esters originates from their lower vibrational frequencies
resulting in reduced non-emissive excited state quenching and
Results and discussion
Synthesis of ligands L1–L6
enhanced quantum yields.5 In case of lanthanide-based
coordination polymers also complexes with improved lumine-
scence properties have been obtained.6 Our group has Ligands L1, L2 and L4 are prepared by straight forward
focussed on this topic during the last years preparing a Michaelis–Arbuzov19 reactions from the corresponding benzyl-
variety of efficiently luminescent lanthanide based MOFs.7–9 bromide starting materials in excellent yields of 90% and 99%
Moreover, different poly-fluoroaryl substituted mono-10,11 and
bis-phosphanes,12 and phosphinic acids13 with enhanced
rigidity have been prepared and investigated in terms of reac-
tivity and coordination behaviour. Now, to obtain distinct
molecular lanthanide complexes with enhanced excited state
lifetimes in the context of a future attachment to semiconductor
surfaces, our research focusses on preparation of aryl-sub-
aInstitute of Chemistry and Center for Interdisciplinary Nanostructure Science and
Technology (CINSaT), University of Kassel, Heinrich-Plett-Str. 40, 34132 Kassel,
Germany. E-mail: pietschnig@uni-kassel.de
bInstitute of Nanostructure Technologies and Analytics (INA) and CINSaT, University
of Kassel, Heinrich-Plett-Str. 40, 34132 Kassel, Germany
†Electronic supplementary information (ESI) available: Synthetic procedures &
detailed spectroscopic data, NMR spectra, X-ray data, PL spectra. CCDC
2021050–2021063. For ESI and crystallographic data in CIF or other electronic
Chart 1 Selected known aryl-substituted phosphonate ester ligands
A–E.
format see DOI: 10.1039/d0dt03047c
This journal is © The Royal Society of Chemistry 2020
Dalton Trans.

View Article Online
Paper
Dalton Transactions
Fig. 2 Molecular structure of L3. Anisotropic displacement parameters
are depicted at the 50% probability level. Hydrogen atoms are omitted
for clarity. Structural data are given in Table 1 (vide infra).
from pentafluoro benzaldehyde, a bromide substituent is
introduced at the para position via a nucleophilic aromatic
substitution to yield S5 followed by reduction to alcohol S6.
Subsequent bromination with PBr3 and Michaelis–Arbuzov
reaction gives S7 and L6, respectively. After workup, L6 is
Scheme 1 Syntheses of ligands L1, L2, L4 (top) and L3 (bottom).
for L1 and L2, respectively (Scheme 1, top). L4 is pre- obtained in a good overall yield of 45% as a colourless oil
pared according to a modified literature procedure starting which solidifies upon standing. L6 crystallises in the triclinic
ˉ
from
a
threefold excess of 1,4-bis(bromomethyl)benzene space group P1 hosting one molecule in the asymmetric unit.
treated with 20 h of reflux in dry toluene to promote formation Moreover, a short intermolecular Br⋯O distance of only 2.77 Å
of the mono-phosphorylated product.20 L4 is obtained in a is observed between the sp2 hybridised O-atom of the phos-
good yield of 58% after workup. All three ligand species are phonate moieties and the para Br substituents of neighbour-
obtained as colourless or pale-yellow oils. To obtain a ligand ing molecules which remains 0.58 Å under the sum of the van
with increased bulkiness and antenna effect, anthracene- der Waals radii of 3.35 Å (ref. 27) of both atoms (Fig. 1). This
based L3 is prepared in a two-step synthesis starting with a can only be rationalised by the formation of a so-called
modified double bromination reaction of 9-(hydroxymethyl) halogen bond for which first crystallographic evidence was
anthracene.21 Subsequent Michaelis–Arbuzov reaction of the provided by Hassel as early as 1954.28,29 Here, the solid-state
obtained 9-bromo-10-(bromomethyl) anthracene (S1) gives L3 structure of bromine-1,4-dioxanate revealed a comparable
as a yellow powder in an excellent overall yield of 80% strong intermolecular Br⋯O interaction with a distance of
(Scheme 1, bottom). Crystals suitable for SCXRD experiments 2.71 Å. Halogen bonds are strong, specific and directional
were grown from vapor diffusion of pentanes into a saturated interactions including a significant charge transfer.30 Hence,
solution of L3 in THF. L3 crystallises in the monoclinic space due to the short Br⋯O distance in L6 the present halogen
group P21/c containing one molecule in the asymmetric unit bonds can be assigned to the inner-sphere complexes of the
(Fig. 2). Phosphonate ester L5 is synthesised in a good overall type [BX]+⋯Y− according to Mulliken.31–33 Despite that, also
yield of 36% via a four-step procedure starting from 4,4′- many analogies have been drawn between halogen- and the
dibromo-1,1′-biphenyl (Scheme 2, top; vide infra). The starting much weaker hydrogen bonds,30 and many applications have
materials S2, S3 and S4 are prepared according to modified lit- been found in polymer science.34 An intrinsic feature of the
erature protocols.22,23 S4 is then phosphorylated with P(OiPr)3 halogen bond is the [BX]+⋯Y− bond vector being nearly
i
to give L5 under release of PrBr as a colourless oil which soli- linear.29 The corresponding C–Br⋯O and PvO⋯Br angles in
difies upon standing. Unfortunately, only crystals of poor L6 of 167.4° and 166.5° deviate significantly from 180° but are
quality were obtained from this solid material. Electron with- in a similar range like in comparable literature structures.35
drawing substituents like fluorides are known to improve the This deviation is most likely due to a crystallographic packing
photoluminescence properties and quantum yields of polycon- effect causing the formation of an advantageous ladder-struc-
jugated organic ligands and derived metal complexes due to ture between adjacent molecules. Nonetheless, both angles are
lowered HOMO as well as LUMO energy levels and thus elec- nearly identical classifying this interaction as a type I halogen
tron injection.24,25 Hence, also polyfluorinated phosphonate bond.35 In comparison to related bis-phosphonate esters
ester ligand L6 was prepared for subsequent lanthanide com- (anthracene-based I,9 and tetra-fluorophenyl-based II 36),
plexation (Scheme 2, bottom, vide infra). Compounds S5 and ligands L3 and L6 show comparable structural features
S6 are synthesised according to modified protocols.26 Starting (Table 1, vide infra). The P1–C1 and P1–O1 distances differ
Dalton Trans.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Dalton Transactions
Paper
Scheme 2 Four-step preparations of phosphonate ester ligands L5 (top) and L6 (bottom).
Table 2 31P-NMR chemical shifts [ppm] of ligands L1–L6 and the
derived lanthanum(III) complexes 1, 5, 9, 13, 17, 25, 29 [LaX3(L1–L3 or
L5–L6)3]2 (X = NO3, Cl) and 21 [LaCl2Br(L4-Br)2(L4-Cl)]2
3 3
3
Ligand
[LaCl3(L1–L3 or L5–L6)3]2 or
[LaCl2Br(L4-Br)2(L4-Cl)]2
[La(NO ) (L1–L6) ]
L1: 24.3
L2: 23.6
L3: 23.0
L4: 24.0
L5: 24.4
L6: 19.0
24.2
23.2
23.0
23.73, 23.67
24.0
23.2
22.3
—
—
—
Fig. 1 Molecular structure of L6 with schematic halogen-bond.
Anisotropic displacement parameters are depicted at the 50% probability
level. Hydrogen atoms are omitted for clarity. Structural data are given in
Table 1.
19.0
—
31P-NMR shifts of L1–L6 are in-between 19.0 ppm (L6) and
24.4 ppm (L5) (Table 2) and are located within the range of
related compounds found in the literature.7–9
Table 1 Selected bond lengths [Å] and angles [°] for L3 and L6 and
related literature compounds I and II. I and II contain two molecules in
the asymmetric unit thus averaged values are given
Lanthanide complexes 1–32
Compound
P1–C1
P1–O1
P1–C1–C2
O1–P1–C1
With the desired phosphonate ester ligands L1–L6 in hand,
synthesis of derived lanthanide complexes (Ln = La, Nd, Dy,
Er) is performed in EtOH starting from [LnX3(H2O)6] precur-
sors (Scheme 3, vide infra). To have recourse to NMR spec-
troscopy for a proper characterisation of the products, all prep-
arations have first been carried out with the diamagnetic La3+
derivative. For X = NO3 or the use of the steric demanding
L3
L6
I
1.805(2)
1.803(3)
1.7978(2)
1.7900(2)
1.4679(2)
1.4653(2)
1.4633(1)
1.4690(2)
114.22(2)
113.60(2)
113.64(1)
113.67(1)
116.07(1)
112.97(1)
115.37(1)
113.29(8)
II
only marginally between 1.79–1.81 Å and 1.46–147 Å, respect- anthracene-based L3, only three equivalents of the phospho-
ively. A similar tendency can be found for the respective P1– nate esters are needed to replace all aqua ligands in the start-
C1–C2 and O1–P1–C1 angles. Interestingly, L3 shows in both ing material and to obtain product IR spectra without residual
cases the widest ((114.22(2)° and 116.07(1)°) and L6 the nar- OH-bands. In contrast, in the case of X = Cl or the less bulky
rowest angles ((113.60(2)° and 112.97(1)°) within the row. The L1, L2, L4–L6, four equivalents of the ligands are needed to
This journal is © The Royal Society of Chemistry 2020
Dalton Trans.

View Article Online
Paper
Dalton Transactions
plexation with the lanthanum nitrates causing a high-field
shift of 1.1 ppm (1) and 1.3 ppm (9), respectively. Due to a
Br–Cl exchange between one ligand and the lanthanum chlor-
ide, two 31P signals at 23.73 and 23.67 ppm in a ratio of 1 : 2
are observed for the L4-based La3+ complex 21. Two sets of
signals in the same ratio are also found in the corresponding
1H- and 13C-NMR spectra and are assigned to the two ligand
modifications. Even in the MALDI-MS spectrum, aggregates
with bromide as well as chloride substituted ligands are
detected (see ESI†). In general, the nitrate supported com-
plexes 1–4 and 9–12 easily solidify while most of the chloride
substituted complexes tend to form viscous waxes that do not
solidify or only solidify after some time. Because of that, no
crystal structure determination could be carried out for the L4-
(21–24) and L5-based complexes 25–28.
Scheme 3 Synthesis of the L1–L6 supported lanthanide complexes
1–32.
Crystals of L1- and L2-based compounds 2, 3, 5, 10, 13
and 14 are grown from slow evaporation of saturated
DCM solutions or from obtained wax-like material that crystal-
lised after some time. The lanthanide nitrate complexes 2
(see ESI†), 3 (Fig. 3) and 10 (Fig. 4) form monomers and
show a nine-fold coordination in the solid-state. Just like
oust the H2O substituents and to obtain products featuring no
OH-bands in the IR spectra. For a ligand overview, see Chart 2.
After reaction, the excess of the phosphonate esters is
removed by thorough extraction with pentanes so that in
almost all cases mono- or dimeric complexes of the form
[LnX3(L)3]n (X = NO3, Cl, n = 1 (Nd, Dy, Er), 2 (La, Nd)) are
obtained in a quantitative yield. For L4, complexes of the form
[LnCl2Br(L4-Br)2(L4-Cl)]n (n = 1 (Nd, Dy, Er), 2 (La, Nd)) are
found due to an Br–Cl exchange between one ligand and the
lanthanide chloride precursor.
−
In comparison to the chlorides, the NO3 anions are
bulkier and act as two-dentate chelate ligands. This might
cause the [LnCl3(H2O)6] precursors to be prone to take up
more than six H2O molecules into their coordination sphere
over time resulting in an aggravated permanent replacement
of the water molecules. Despite of the phosphonate esters
L1–L6 being potential three-dentate chelate ligands, all
obtained lanthanide complexes exhibit only a mono-dentate
coordination by the oxygen atom of the P+–O− moieties like it
is observed in related structures.7–9 Except for the L4- and
L5-supported complexes 21 and 25, the 31P-NMR signal of the
free ligands experiences a 0.1–0.4 ppm shift to higher field
upon complex formation with the lanthanum chloride precur-
sors (Table 2). This effect is even more pronounced upon com-
Fig. 3 Molecular structure of [Dy(NO3)3(L1)3] (3). Anisotropic displace-
ment parameters are depicted at the 50% probability level. Hydrogen
atoms are omitted for clarity. Structural data are given in Table 3
(vide infra).
Chart 2 Overview of the phosphonate ester ligands L1–L6 used within this work.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Dalton Transactions
Paper
Fig. 6 Molecular structure of [LaCl3(L2)3]2 (13). Anisotropic displace-
ment parameters are depicted at the 50% probability level. Hydrogen
atoms are omitted for clarity. Structural data are given in Table 3.
Fig. 4 Molecular structure of [Nd(NO3)3(L2)3] (10). Anisotropic displace-
ment parameters are depicted at the 50% probability level. Hydrogen
atoms are omitted for clarity. Structural data are given in Table 3
(vide infra).
L2-based Nd3+ nitrate complexes 2 and 10 and the La3+ chlor-
ide compounds 5 and 13 shows a shortening of the M–O dis-
tances in both cases when switching from L1 to L2 (2.381(6)
(2) to 2.372(2) Å (10) and 2.433(2) (5) to 2.409(4) Å (13)). The
same trend can be seen for the M–Cl distances of the non-brid-
ging chlorides in 5 and 13 (2.821(6) to 2.750(2) Å) while the
bond length of the bridging chlorides (2.924(6) to 2.914(2) Å)
seems to be less effected by an increasing steric demand of
the ligand periphery in L2. Apparently contrary trends are
obtained for the P+–O− distances and the M–O–P angles. The
P+–O− bond length in 2 and 10 is unaffected while the M–O–P
angle is slightly narrowed from 154.6(4)° in 2 to 152.4(2)° in
complex 10. In contrast, the P+–O− distance decreases from
1.486(2) Å in 5 to 1.463(5) Å in 10 while the M–O–P angle is sig-
nificantly increased from 151.3(1)° to 159.4(3)°.
For the L3-based complexes, crystals suitable for X-ray diffr-
action experiments could only be obtained from saturated
EtOH solutions at −20 °C (17–19) or from vapor diffusion of
pentanes into a saturated THF solution (20). Just like com-
plexes 5, 13 and 14, compound 17 shows a seven-fold coordi-
nation forming a chloride bridged dimer in the solid-state
(Fig. 7, vide infra). Owing to the increased steric demand of
L3, it is most likely that the preferred dimer formation only
can occur upon exchange of an equivalent L3 with an EtOH
molecule per La3+ ion. Complex 18 also shows a seven-fold
pentagonal-bipyramidal coordination but forms a monomer
because of three attached bulky molecules of L3 (Fig. 8,
vide infra). Since all obtained products show no residual OH-
bands in the IR spectrum, the additionally attached H2O mole-
cule to suit the coordination sphere of the Nd3+ cation has to
origin from airborne water during the crystallisation process.
Just like in 17, a phosphonate ester L3 is substituted by an
EtOH molecule in complex 19 (Fig. 9). A monomeric octahedral
coordination is most likely preferred due to the smaller ionic
radius of the Dy3+ ion. The same coordination mode can be
observed for complex 20 (Fig. 10). The coordination sphere is
made up by three chloride anions and three L3 oxygen atoms.
in L6, halogen bonds are formed in complex 10. In this case
between the bromides (Br2) and the outwards titled NvO (O9)
oxygen atoms of a NO3 anion of adjacent molecules. In con-
−
trast to L6, there is less lack of electron density at the bromides
thus a weaker halogen bond is formed. The O⋯Br distance of
3.06 Å is elongated but still shorter than the sum of the van
der Waals radii of both atoms (3.35 Å (ref. 27)). The present C–
Br⋯O angle of 169.1° is in a similar range like in L6. However,
the NvO⋯Br angle of 108.3° indicates a certain perpendicular
arrangement of the nitrate moiety with respect to the bromide
substituent assigning this interaction rather as a type II
halogen bond.35 The L1- and L2-based lanthanide chloride
compounds 5 (Fig. 5), 13 (Fig. 6) and 14 (see ESI†) show
seven-fold, pentagonal-bipyramidal coordination forming
chloride bridged dimers in the solid-state with an averaged
M–M distance of 4.78 Å. A comparison of the L1- and
Fig. 5 Molecular structure of [LaCl3(L1)3]2 (5). Anisotropic displacement
parameters are depicted at the 50% probability level. Hydrogen atoms
are omitted for clarity. Structural data are given in Table 3 (vide infra).
This journal is © The Royal Society of Chemistry 2020
Dalton Trans.

View Article Online
Paper
Dalton Transactions
Table 3 Selected bond lengths [Å] and angles [°] of L1-based complexes 2, 3, 5, L2-based 10, 13, 14, L3-based 17–20 and L6-based 31 and 32.
Distances to solvent oxygen atoms are not considered
Complex
M–O/M–O(NO )
M–Cl/M–(µ-Cl)
P+–O−
M–O–P
3
[Nd(NO3)3(L1)3] (2)
[Dy(NO3)3(L1)3] (3)
[LaCl3(L1)3]2 (5)
[Nd(NO3)3(L2)3] (10)
[LaCl3(L2)3]2 (13)
2.381(6)/2.531(6)
2.297(2)/2.455(2)
2.433(2)
2.372(2)/2.538(2)
2.409(4)
2.378(9)
2.421(3)
2.378(4)
2.253(3)
2.230(4)
2.256(6)
—
—
1.483(7)
1.476(2)
1.486(2)
1.483(3)
1.463(5)
1.480(9)
1.483(3)
1.483(4)
1.489(3)
1.493(4)
1.477(6)
1.457(6)
154.6(4)
155.6(1)
151.3(1)
152.4(2)
159.4(3)
160.2(6)
149.2(2)
147.9(2)
153.1(2)
150.1(3)
167.0(4)
166.9(4)
2.821(6)/2.924(6)
—
2.750(2)/2.914(2)
2.725(3)/2.910(3)
2.802(1)/2.873(1)
2.764(1)
2.612(1)
2.616(1)
2.574(3)
2.521(2)
[NdCl3(L2)3]2 (14)
17 as [LaCl3(L3)2 (EtOH)]2·EtOH
18 as [NdCl3(L3)3 (H2O)]
19 as [DyCl3(L3)2 (EtOH)]·2EtOH
[ErCl3(L3)3] (20)
31 as [DyCl2(L6)3 (H2O)]Cl
32 as [ErCl2(L6)3 (H2O)]Cl
2.209(6)
Fig. 7 Molecular
structure
of
17,
crystallised
as
Fig. 9 Molecular structure of 19, crystallised as [DyCl3(L3)2(EtOH)]·2EtOH.
Anisotropic displacement parameters are depicted at the 50% probability
level. Except for freely refined hydrogens, these and two co-crystallised
EtOH molecules are omitted for clarity. Structural data are given in
Table 3.
[LaCl3(L3)2(EtOH)]2·EtOH. Anisotropic displacement parameters are
depicted at the 50% probability level. Except for freely refined hydro-
gens, these and a co-crystallised EtOH molecule are omitted for clarity.
Structural data are given in Table 3.
Fig. 10 Molecular structure of [ErCl3(L3)3] (20). Anisotropic displace-
ment parameters are depicted at the 50% probability level. Hydrogen
atoms and a co-crystallised THF molecule are omitted for clarity.
Structural data are given in Table 3.
Fig. 8 Molecular structure of 18, crystallised as [NdCl3(L3)3(H2O)].
Anisotropic displacement parameters are depicted at the 50% probability
level. Except for freely refined hydrogens, these are omitted for clarity.
Structural data are given in Table 3.
Consistent with a step-wise decrease of the effective ionic radii Å and 2.764(1) Å (18) and 2.253(3) Å and 2.612(1) Å (19) to
of the cations: La3+ (103 pm) > Nd3+ (98 pm) > Dy3+ (91 pm) > 2.230(4) Å and 2.616(1) Å in complex 20. The P+–O− bond
Er3+ (89 pm),37 the M–O and M–Cl distances are successively lengths show a contrasting trend by slightly increasing from
decreasing from 2.421(3) Å and 2.802(1) Å in 17, over 2.378(4) 1.483(3) Å in 17 to 1.493(4) Å in 20. Moreover, the acutest
Dalton Trans.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Dalton Transactions
Paper
M–O–P angles amongst all synthesised complexes are found in
17 (149.2(2)°) and 18 (147.9(2)°).
Complexes 31 and 32 are obtained as slowly crystallising
waxes. They are isostructural and crystallise as pseudo-octa-
hedrally coordinated monomers (Fig. 11). A chloride anion is
replaced by a water substituent in both molecules resulting in
the Cl− to function now as counterion forming a partially
solvent separated ion pair. Just like in complex 18, the IR
spectra of both complexes show no residual OH-bands after
workup. In contrast to the free ligand L6, no halogen bonds
are formed between the complex molecules in 31 and 32 due
to the phosphonate esters now donating electron density
towards the lanthanide cations. On the one hand, both com-
pounds show with 2.256(6), 2.574(3) and 1.477(6) Å (31) and
2.209(6), 2.521(2) and 1.457(6) Å (32) the shortest M–O, M–Cl
and P+–O− distances among the crystallised complexes. On the
other hand, this automatically causes the widest M–O–P angles
of 167.0(4)° (31) and 166.9(4)° (32), respectively.
Fig. 12 Examplary normalised absorption spectra of complexes [Nd
(NO3)3(L1)3] (2) (black), [Dy(NO3)3(L1)3] (3) (orange) and [Er(NO3)3(L1)3] (4)
(blue) at room temp. with typically sharp Nd3+, Dy3+ and Er3+ absorption
bands.
is most likely due to
a ligand-to-metal energy transfer
(Fig. 13(a)). In turn, the obtained Dy3+ chloride-based com-
plexes 7, 15, 19, 23, 27 and 31 show the expected very weak
emissions with no evidence for bands affiliated to ligand-to-
metal energy transfer or modified ligand emission. This indi-
cates an enhanced stabilisation of long living excited states
within the latter systems. The only difference between the
nitrate- and chloride-based Dy3+ complexes is the anions
themselves. Hence, this deviating luminescence behaviour can
Solid-state absorption properties
The absorption spectra of complexes 1–32 in the range of
300–850 nm essentially look the same showing expected sharp
absorption bands of the Nd3+ (λabs, max = 584 nm), Dy3+ (λabs,
= 811 nm) and Er3+ (λabs, max = 526 nm) cations, respect-
max
ively. Some exemplary spectra of complexes 2 (black), 3
(orange) and 4 (blue) are summarised in Fig. 12 indicating all
three cations to share an absorption around 800 nm (for
detailed data, see ESI†). The obtained spectra only differ by an
either more or less strong ligand absorption in the blue region
that is most pronounced for the L3-based complexes 17–20.
−
only be rationalised by the NO3 anions promoting a ligand-
to-metal energy transfer and fast fluorescence processes. In
general, the emission spectrum of anthracene ligand L3-based
complex 19 is dominated by the weak but broad ligand emis-
sion between 450–650 nm (blue to green) covering most of the
metal emissions (Fig. 13(b)). The emission spectra of the Dy3+
chloride complexes 7, 15, 23 and 31 supported by ligands L1,
L2, L4 and L6, respectively are very similar exhibiting very
weak to weak ligand emissions between 400 to 550 nm and
some of the weak Dy3+ emission bands can be assigned
(Fig. 13(c), vide infra). The spectrum of complex 27 is domi-
nated by a very strong emission of the 1,1′-biphenyl-based
phosphonate L5 between 400–550 nm (blue) covering the
Emission properties of the Dy3+ complexes
Owing to Laporte-forbidden 4f–4f-transitions, all complexes
are expected to show no or only weak metal emissions. An
applied excitation at 366 nm affords only in case of the Dy3+
compounds sharp emission bands in the range of
300–850 nm. Nonetheless, the Dy3+ nitrate based complexes 3
and 11 exhibit relatively strong white light emission of the
cation with quantum yields of 9% and 13%, respectively which
4F9/2
→
6H15/2 emission band of the cation (Fig. 13(d)). After
enlargement of the area between 550–850 nm, still some of
the remaining, weak Dy3+ emissions could be visualised and
assigned.
Emission properties of the Nd3+ complexes
The luminescence properties of L2, L3; L5 and L6 supported
Nd3+ chloride-based complexes 14, 18, 26 and 30 are investi-
gated using micro-photoluminescence (µ-PL) spectroscopy.
The emission spectra feature the characteristic transitions of
the Nd3+ ion in the NIR region (Fig. 14). Three narrow emis-
sion bands are illustrated which are centred at ca. 890, 1060
and 1320 nm, that correspond to the 4F3/2 → 4I9/2, 4F3/2 → 4I11/2
and F3/2
1060 nm is predominant in all four cases. Interestingly, only
complex 14 (Fig. 14(a)) exhibits a distinct signal splitting of
Fig. 11 Molecular structure of 31, crystallised as [DyCl2(L6)3(H2O)]Cl.
Anisotropic displacement parameters are depicted at the 50% probability
level. Except for freely refined hydrogens, these are omitted for clarity.
Structural data are given in Table 3 (vide supra).
4
→
4I13/2 transitions, respectively. The band at
This journal is © The Royal Society of Chemistry 2020
Dalton Trans.

View Article Online
Paper
Dalton Transactions
Fig. 13 Emission spectra (λexc = 366 nm) of Dy3+ complexes 3, 7, 11, 15, 19, 23, 27 and 31 at room temp.: (a) spectra overlay of complexes [Dy
(NO3)3(L1)3] (3) (bright orange) and [Dy(NO3)3(L2)3] (11) (black); (b) spectrum of complex [DyCl3(L3)3] (19); (c) spectra overlay of complexes
[DyCl3(L1)3] (7) (black), [DyCl3(L2)3] (15) (red), [DyCl2Br(L4-Br)2(L4-Cl)] (23) (blue) and [DyCl3(L6)3] (31) (dark orange); (d) spectrum of complex
[DyCl3(L5)3] (27).
Fig. 14 Emission spectra (λexc = 750 nm) of Nd3+ complexes 14, 18, 26 and 30 at room temp.: (a) spectrum of complex [NdCl3(L2)3]2 (14); (b)
stacked spectra of complexes [NdCl3(L3)3] (18) (black), [NdCl3(L5)3] (26) (blue) and [NdCl3(L6)3] (30) (orange).
4
4
4
4
25 nm (4F3/2 → I9/2, F3/2 → I11/2) and 47 nm (4F3/2 → I13/2), ligand–metal interactions are known to influence lanthanide
respectively in its PL-spectrum. This feature is most likely due ion f-shell electrons,38 and as a result to affect their fluo-
to a preferential formation of dimeric structures in L2-based rescence response.39–41 In a recent study by Guillou et al., a
14 resulting in a Nd–Nd distance of 4.80 Å. Electrostatic similar splitting was observed for Eu3+ clusters and tentatively
Dalton Trans.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Dalton Transactions
Paper
assigned to electronic orbital interactions between f-orbitals of and tend to form monomers in case of the smaller Dy3+ and
the rare-earth metal centres and, in the case of this cluster, the Er3+ ions and the bulky anthracene-based ligand L3. For the
oxygen-centred orbitals of an adjacent oxo-ligand.42 Such an medium sized Nd3+, monomeric as well as dimeric species are
electrostatic ligand effect could also be observed for the elec- observed. Moreover, compound [Nd(NO3)3(L2)3] (10) is the only
tronic structure of a variety of mono-lanthanide species by complex within this work to show the formation of a two-
Furet et al.38 Hence, the less pronounced splitting in the dimensional halogen bond network in the solid-state which
monomeric complexes 18, 26 and 30 can most likely be was assigned to a type II halogen bond interaction due to a
assigned as well to such a type of interaction.
more perpendicular arrangement of the respective moiety.
Absorption spectra of complexes 1–32 have been recorded
showing the expected sharp lanthanide absorption bands in
all cases. The recording of emission spectra (λexc = 366 nm) of
the obtained Dy3+ model compounds shows the respective
Experimental section
X-ray diffraction experiments were performed with either a nitrate-based derivatives 3 and 11 to exhibit a relatively strong
STOE IPDS 2 with an image plate (Ø 34 cm) using a Mo-GENIX white metal emission with quantum yields of 9% and 13%,
source (λ = 0.71073 nm) or a STOE StadiVari instrument with respectively. In contrast, the chloride-based complexes show
DECTRIS PILATUS 200 K using a Mo-GENIX source (λ = only very weak emissions at this excitation wavelength.
0.71073 nm). All structures were solved using the dual Without speculation potential reasons for this behaviour must
space method (SHELXT)43 and were refined against F2 with remain elusive until more detailed studies such as excited
SHELXL44 or OLEX2.45 Additional programs used for state lifetime measurements along with quantum chemical
structural analysis include Mercury46 and Platon.47 CCDC calculations shed more light on this issue in the future. The
2021050–2021063† contain the supplementary crystallographic recorded µ-PL-spectra (λexc = 750 nm) of the Nd3+ chloride-
data for this paper.
based complexes 14, 18, 26 and 30 exhibit a similar behaviour
For µ-PL experiments, the laser (λexc = 750 nm) is focused showing weak metal emissions in the NIR region.
on the complexes by a microscope objective (NA = 0.7) to a Nonetheless, interesting splitting features tentatively assigned
spot size of ∼1 µm. The same objective was used to collect the to electronic orbital interactions between f-orbitals of the Nd3+
emission from the complexes. The luminescence is then spec- centres and the oxygen-centred orbitals of adjacent oxo-
trally filtered by a 0.75 m focal length spectrometer equipped ligands are observed.
with liquid nitrogen-cooled CCD and InGaAs detectors. The
µ-PL spectra were taken at room temperature with a laser
excitation power of 6 mW.
Conflicts of interest
The authors declare no competing financial interest.
Conclusions
The preparation of six phosphonate ligand systems (L1–L6)
carrying differently substituted aryl residues as well as of 32
Acknowledgements
derived lanthanide complexes (1–32) has been presented. For The federal state of Hesse, Germany is kindly acknowledged
L1, L2 and L4 modified literature known procedures are intro- for financial support of the SMolBits project within the
duced while phosphonates L3, L5 and L6 represent completely LOEWE program.
new ligand systems with an additional bromo functionality for
further functionalisation. As a highlight, the tetrafluoro aryl
substituted phosphonate L6 shows a unique two-dimensional
Notes and references
type I halogen bond network in the solid-state. Substitution
−
reactions with [LnX3(H2O)6] (Ln = La, Nd, Dy, Er; X = NO3
,
1 O. Guillou, C. Daiguebonne, G. Calvez and K. Bernot, Acc.
Chem. Res., 2016, 49, 844–856.
2 J.-C. Bünzli, Nat. Chem., 2010, 2, 696–696.
3 B. Samson, A. Carter and K. Tankala, Nat. Photonics, 2011,
5, 466–467.
4 J.-W. Zhang, C.-C. Zhao, Y.-P. Zhao, H.-Q. Xu, Z.-Y. Du and
H.-L. Jiang, CrystEngComm, 2014, 16, 6635–6644.
5 Y. Hasegawa, Y. Kimura, K. Murakoshi, Y. Wada, J.-H. Kim,
N. Nakashima, T. Yamanaka and S. Yanagida, J. Phys.
Chem., 1996, 100, 10201–10205.
6 L. Zhang, W. Dou, W. Liu, C. Xu, H. Jiang, C. Chen, L. Guo,
X. Tang and W. Liu, Inorg. Chem. Commun., 2015, 59,
53–56.
Cl−) precursor compounds in EtOH yielded the derived model
complexes of the form [LnX3(L1–L3 or L5–L6)3]n (X = NO3, Cl; n
= 1 (Nd, Dy, Er), 2 (La, Nd)) or [LnCl2Br(L4-Br)2(L4-Cl)]n (n =
1 (Nd, Dy, Er), 2 (La, Nd)) which have been thoroughly charac-
terised. Unfortunately, not all the obtained crystal structures
obey to this formula due to additional solvent coordination
accompanied by partial ligand replacement during the crystal-
lisation process. The obtained crystal structures show the
lanthanide nitrate-based ones to form monomers in the solid-
state. The lanthanide chloride-based compounds preferably
form chloride bridged dimers for the bigger La3+ cation and
the less sterically demanding phosphonates L1, L2 and L6,
This journal is © The Royal Society of Chemistry 2020
Dalton Trans.

View Article Online
Paper
Dalton Transactions
7 K. Krekić, E. Käkel, D. Klintuch, D. Bloß and R. Pietschnig, 26 A. Franzke and A. Pfaltz, Synthesis, 2008, 245–252.
Z. Anorg. Allg. Chem., 2018, 644, 149–154. 27 A. Bondi, J. Phys. Chem., 1964, 68, 441–451.
8 K. Krekić, D. Klintuch and R. Pietschnig, Chem. Commun., 28 O. Hassel and J. Hvoslef, Acta Chem. Scand., 1954, 8, 873.
2017, 53, 11076–11079. 29 O. Hassel, Science, 1970, 170, 497–502.
9 K. Krekić, D. Klintuch, C. Lescop, G. Calvez and 30 A. C. Legon, Angew. Chem., Int. Ed., 1999, 38, 2686–2714.
R. Pietschnig, Inorg. Chem., 2019, 58, 382–390. 31 R. S. Mulliken, J. Phys. Chem., 1952, 56, 801–822.
10 A. Orthaber, M. Fuchs, F. Belaj, G. N. Rechberger, 32 R. S. Mulliken, J. Am. Chem. Soc., 1952, 74, 811–824.
C. O. Kappe and R. Pietschnig, Eur. J. Inorg. Chem., 2011, 33 R. S. Mulliken, J. Am. Chem. Soc., 1950, 72, 600–608.
2011, 2588–2596.
11 A. Orthaber, F. Belaj and R. Pietschnig, J. Organomet.
Chem., 2010, 695, 974–980.
12 A. Orthaber, F. Belaj, J. H. Albering and R. Pietschnig,
Eur. J. Inorg. Chem., 2010, 2010, 34–37.
34 G. Berger, J. Soubhye and F. Meyer, Polym. Chem., 2015, 6,
3559–3580.
35 L. Brammer, G. Mínguez Espallargas and S. Libri,
CrystEngComm, 2008, 10, 1712–1727.
36 F. C. Krebs and T. Jensen, J. Fluor. Chem., 2003, 120, 77–84.
13 A. Orthaber, J. H. Albering, F. Belaj and R. Pietschnig, 37 R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr.,
J. Fluor. Chem., 2010, 131, 1025–1031. Theor. Gen. Crystallogr., 1976, 32, 751–767.
14 T. Huang, T. Chen and L.-B. Han, J. Org. Chem., 2018, 83, 38 E. Furet, K. Costuas, P. Rabiller and O. Maury, J. Am. Chem.
2959–2965. Soc., 2008, 130, 2180–2183.
15 K. Baczko, W.-Q. Liu, B. P. Roques and C. Garbay- 39 D. Van der Voort, G. J. Dirkens and G. Blasse, J. Phys.
Jaureguiberry, Tetrahedron, 1996, 52, 2021–2030. Chem. Solids, 1992, 53, 219–225.
16 P. Li, M. Zhang, M. L. Peach, H. Liu, D. Yang and 40 D. Parker, R. S. Dickins, H. Puschmann, C. Crossland and
P. P. Roller, Org. Lett., 2003, 5, 3095–3098. J. A. K. Howard, Chem. Rev., 2002, 102, 1977–2010.
17 H. C. Li, Y. P. Lin, P. T. Chou, Y. M. Cheng and R. S. Liu, 41 S. K. Mahesh, P. P. Rao, M. Thomas, T. L. Francis and
Adv. Funct. Mater., 2007, 17, 520–530. P. Koshy, Inorg. Chem., 2013, 52, 13304–13313.
18 B. A. Arbuzov, V. M. Zoroastrova and G. A. Tudril, Izvestiya 42 F. Le Natur, G. Calvez, J.-P. Guégan, L. Le Pollès, X. Trivelli,
Akademii Nauk SSSR, Seriya Khimicheskaya, 1970, pp.
90–95.
19 A. E. Arbuzov, J. Russ. Phys.-Chem. Soc., 1906, 38, 687–671.
K. Bernot, C. Daiguebonne, Ch. Neaime, K. Costuas,
F. Grasset and O. Guillou, Inorg. Chem., 2015, 54, 6043–
6054.
20 B. Moreau, P.-A. Jaffrès and D. Villemin, Org. Prep. Proced. 43 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Adv., 2015,
Int., 2002, 34, 549–552. 71, 3–8.
21 J. H. Clements and S. E. Webber, J. Phys. Chem. B, 1999, 44 G. Sheldrick, Acta Crystallogr., Sect. C: Struct. Chem., 2015,
103, 9366–9377. 71, 3–8.
22 F. Leroux, T. U. Hutschenreuter, C. Charrière, R. Scopelliti 45 O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard
and R. W. Hartmann, Helv. Chim. Acta, 2003, 86, 2671–2686. and H. Puschmann, J. Appl. Crystallogr., 2009, 42, 339–341.
23 H. Sato, J. A. Bender, S. T. Roberts and M. J. Krische, J. Am. 46 C. F. Macrae, I. Sovago, S. J. Cottrell, P. T. A. Galek,
Chem. Soc., 2018, 140, 2455–2459.
24 F. Babudri, G. M. Farinola, F. Naso and R. Ragni, Chem.
Commun., 2007, 1003–1022.
P. McCabe, E. Pidcock, M. Platings, G. P. Shields,
J. S. Stevens, M. Towler and P. A. Wood, J. Appl. Crystallogr.,
2020, 53, 226–235.
25 Y. Hattori, T. Kusamoto, T. Sato and H. Nishihara, Chem. 47 A. Spek, Acta Crystallogr., Sect. D: Biol. Crystallogr., 2009,
Commun., 2016, 52, 13393–13396. 65, 148–155.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2020