Photoluminescent properties of three lanthanide compounds of formulae LnCl3(diphenyl((5-phenyl-4H-1λ4-pyrazol-3-yl)methyl)phosphine oxide)2, Ln?=?Sm, Eu and Tb: X-ray structural, emission and vibrational spectroscopies, DF

Article abstract of DOI:10.1016/j.ica.2017.11.049

The synthesis and properties of a new sensitizing bidentate ligand, diphenyl((5-phenyl-4H-1λ⁴-pyrazol-3-yl)methyl)phosphine oxide, 2, capable of demonstrating the emissive properties of lanthanide elements, is described. Two ligands are attache

Full text of DOI:10.1016/j.ica.2017.11.049

Inorganica Chimica Acta 471 (2018) 481–492
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Photoluminescent properties of three lanthanide compounds of
4
formulae LnCl (diphenyl((5-phenyl-4H-1k -pyrazol-3-yl)methyl)
3
phosphine oxide) , Ln = Sm, Eu and Tb: X-ray structural, emission and
2
vibrational spectroscopies, DFT and thermogravimetric studies
John S. Maass , Randall K. Wilharm , Rudy L. Luck , Matthias Zeller b
a
a
a,
⇑
a
Department of Chemistry, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA
Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907-2084, USA
b
a r t i c l e i n f o
a b s t r a c t
4
Article history:
Received 14 October 2017
Received in revised form 22 November 2017
Accepted 23 November 2017
Available online 26 November 2017
The synthesis and properties of a new sensitizing bidentate ligand, diphenyl((5-phenyl-4H-1k -pyrazol-
-yl)methyl)phosphine oxide, 2, capable of demonstrating the emissive properties of lanthanide
elements, is described. Two ligands are attached to LnCl moieties, Ln = Sm, Eu and Tb, resulting in com-
plexes of formulae LnCl (2) . These complexes were structurally characterized by single-crystal X-ray
3
3
3
2
crystallography and have geometries that are pentagonal bipyramidal with the two bidentate ligands
in a cisoid conformation on the equatorial plane and the three chloride ligands in a mer-configuration.
The coordination bond distances decrease from Sm to Tb paralleling the decrease in lanthanide ionic
radii. The photoluminescent properties of these complexes were examined and, one of them, TbCl
was assessed a of ca. 1.00. DFT calculations were conducted to understand the mechanism of proton
Keywords:
Lanthanide compounds
Photoluminescent
TGA
3 2
(2) ,
U
F
transfer in ligand 2 for a hydrogen atom hopping between nitrogen atoms on the pyrazolyl moiety and, to
assess aspects of the coordination sphere of the lanthanide complexes.
Ó 2017 Elsevier B.V. All rights reserved.
Crystal structures
Pyrazole
Phosphine oxide
1
. Introduction
We had reported on the utilization of tertiary phosphine oxide
the syntheses of phosphorus containing ligands pertaining mainly
to the air-sensitive nature of the P-atom, may be behind the few
studies on visible/emitting phosphine ligands on transition metal
complexes [16]. However, as lanthanides are hard Lewis acids
[17], perhaps better stability is afforded from binding to an O-atom
attached to the P-atom in the desired ligand, i.e., a phosphine oxide
ligand.
Nitrogen atom, ring-based molecules such as pyridine [18],
phenanthroline [19], phenylpyrazole [20], bipyridine [21], dipicol-
inates [22], and azaxanthone [23] have been reported to form com-
plexes with lanthanides with interesting photoluminescent
properties. Of these N-atom based ligands, pyrazoles bonded to
palladium [24] and zinc [25] have also displayed luminescent
properties. The importance of lanthanide complexes have been
reviewed recently as they pertain to the fields of luminescent
hybrid materials [26], biolabelling [27], nanoprobes with optical
spectroscopy and bioapplications [28], and imaging and sensing
[29].
ligands in the preparation of catalysts capable of the oxidation of
olefins with some of these employing hydrogen peroxide as the
oxygen source [1–4]. We also demonstrated the utilization of phos-
phinate ligands in the synthesis of dimeric [5], trimeric [6–8] and
tetrameric [9,10] metal clusters. Phosphine oxide ligands have also
been bonded to lanthanide elements for the production of photolu-
minescent materials. In some of these examples, the phosphine
oxide ligand completes the coordination sphere of the lanthanide
element and does not absorb radiation; some other separate
N-atom based ligand provides this function [11–14]. In others,
organic ligands that can absorb light are bonded directly to the
phosphine oxide ligand [15]. The absorbance of light leading to
1
an excited state, S, then intersystem crossing (ISC) leading to a
triplet excited state, T, followed by energy transfer into one of
several excited states of the lanthanide ion followed by the emis-
sion of light has been previously described [15]. Difficulties in
3
We decided to synthesize a bidentate ligand that would take
advantage of the coordinating capabilities of N- and O-donor atom
ligands (by combining methyldiphenylphosphine oxide and
phenylpyrazole)andthelightcapturingandtransferringcapabilities
⇑
Corresponding author.
E-mail address: rluck@mtu.edu (R.L. Luck).
https://doi.org/10.1016/j.ica.2017.11.049
020-1693/Ó 2017 Elsevier B.V. All rights reserved.
0

4
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J.S. Maass et al. / Inorganica Chimica Acta 471 (2018) 481–492
ofthephenylpyrazole moiety. Inthis report, weoutlinethe synthesis
of this new ligand, 5-phenyl-4H-1k -pyrazol-3-yl)methyl)phos-
in the form of a white precipitate, 0.639 g, was obtained by filtra-
tion in a 37.5% yield. An H NMR spectrum of pure sample at this
4
1
phine oxide, illustrate how isomers of this ligand are produced based
on the synthetic strategy, and analyze the interconversion of these
isomers by theoretical and variable temperature NMR methods.
The ligating ability of the ligand is also reported in the form of com-
plexation of the ligand to three lanthanide elements (i.e., Sm, Eu and
Tb) and the luminescent properties of these complexes are
described.
stage suggested that two isomers with a H atom attached to either
of the two N atoms in the pyrazole moiety were produced. H NMR
1
of the isomers (400 MHz, CD
d = 3.95 (d, JHP = 12 Hz, 2H, CH
1H, CH ), d = 7.20–7.90 (m, 15H, (C
be isolated as it is soluble in acetone. Anal. Calc. for C22
3
SOCD
), d = 6.10 (s, 1H, CH
Fig. S1. One isomer could
OP: C,
): d =
3
): d = 3.85 (d, JHP = 12 Hz, CH
2
),
2
1
), d = 6.32 (s,
1
6 5 3
H )
19 2
H N
Cl
2 2
3
1
73.73; H, 5.34. Found: C, 73.82; H, 5.44%. P NMR (CH
0.25 ppm; (CDCl ): d = 30.70 ppm; (THF): d = 28.65 ppm.
NMR of a pure sample (400 MHz, CDCl ): d = 3.74 (d, JHP = 12 Hz,
H, CH ), d = 6.36 (s, 1H, CH ), d = 7.20–7.90 (m, 15H, (C ),
Fig. S2. IR (cm ): 3184–2892 multiple small ridges, 1573sh,
1
3
3
H
3
2
. Experimental
2
2
1
6 5 3
H )
ꢀ1
2.1. Materials
1
1
7
564m, 1463s, 1436s, 1234m, 1170vs (mP@O), 1118s, 1103s,
069m, 1027m, 998m, 955s, 917m, 837s, 806m, 791m, 766s,
47m, 731m, 695s, 684vs.
Chemicals were purchased from Sigma-Aldrich Chemicals and
solvents were purified as needed. Elemental analyses were con-
ducted by Galbraith Laboratories, Knoxville, TN. IR spectra were
1
3.3. Preparation of SmCl
3 6
[(C H
5 2 3
) PO(C H
2 2 6 5 2
N -C H )] , 3
recorded on a PerkinElmer Spectrum One (neat) spectrometer. H
3
1
and P NMR data were recorded on a Varian XL-400 spectrometer
referenced to CDCl or acetone d and 85% H PO respectively. The
0
.025 g (0.069 mmol) of SmCl
3
2
ꢁ6H O was dissolved in 2.5 ml of
3
6
3
4
THF and added to 0.047 g (0.131 mmol) of 2 which was dissolved
in 10 ml of boiling THF in a 50 ml Erlenmeyer flask exposed to
the atmosphere. The solution went from cloudy to clear at which
stage heating was turned off and the mixture allowed to stir over-
night. A white precipitate formed which was obtained by filtration
resulting in 0.037 g (0.033 mmol) of 3, 48.3% yield. The crude pro-
duct was dissolved in methylene chloride, precipitated by the addi-
tion of hexanes, obtained by filtration and dried. Anal. Calc. for
TGA analyses were conducted on a Shimadzu TGA-50 analyzer.
Absorbance spectra were obtained using quartz cells in a Perkin
Elmer Lambda 35 UV/Vis Spectrometer. Fluorescence spectra were
obtained using quartz cells (solution) and glass capillaries (solid)
on a Horiba (Jobin Yvon) Fluoromax-4 Spectrofluorometer. A com-
parative method, involving the use of well characterized samples
with known
. Synthesis of ligand
.1. Preparation of (C
Methyldiphenylphosphine oxide, 2.824 g. (0.013 mol), was dis-
F F
U values was utilized to assess the U [30].
C
44
H
38
N
4
O
2
P
2
Cl
3
Smꢁ0.3CH
2 2
Cl : C, 53.26; H, 3.89. Found: C, 53.29;
3
ꢀ1
H, 3.89%. IR (cm ): 3220br, 3058br, 2957br, 1589m, 1565m,
1
1
489s, 1470s, 1431vs, 1397m, 1301m, 1263m, 1154vs, 1121vs,
092vs, 1068s, 1011vs, 959m, 907br, 849s, 753s, 719s, 686vs.
3
6 5 2 2 3
H ) PO(CH COCH ), 1
3 6 5 2 3 2 2 6 5 2
3.4. Preparation of EuCl [(C H ) PO(C H N -C H )] , 4
solved in 90 ml dried THF and cooled to 0 °C with an ice bath under
n
nitrogen. Butyl lithium in hexane, 1.5 M, 9.1 ml (0.014 mol) was
This complex was prepared in a similar manner to that for com-
plex 3 from 0.049 g (0.137 mmol) of 2 with 0.025 g (0.068 mmol)
of EuCl O) and resulted in 0.057 g (0.051 mmol) of 4, 74.6%
added and the solution was cooled to ꢀ77 °C with an ethanol/liq-
uid nitrogen bath. 1.5 ml (0.015 mol) of dried ethyl acetate was
added dropwise and the solution allowed to warm to room tem-
perature and then stirred for 3 h. The mixture was quenched with
saturated ammonium chloride solution, the solvent evaporated
and the crude product recrystallized from ethyl acetate. 0.780 g
3
ꢁ6H
2
yield. The crude produce was dissolved in methylene chloride, pre-
cipitated by the addition of hexanes, obtained by filtration and
dried. Anal. Calc. for C44
H
38
N
4
O
2
P
2
Cl
3
Euꢁ0.3CH
2 2
Cl : C, 53.18; H,
ꢀ1
3
2
1
9
.90. Found: C, 53.34; H, 3.90%. IR (cm ): 3224br, 3055m,
(
0.003 mol) of (C
6
H
5
)
2
PO(CH
): d = 25.70 ppm. H NMR (400 MHz, CDCl
), d = 3.58 (d, JHP = 16 Hz, 2H, CH ), d = 7.40–7.80
). IR (cm ): 3316br, 3057w, 2947br, 1707vs
C@O), 1438m, 1359m, 1223s, 1179vs ( P@O), 1120s, 1102sh,
2 3
COCH ), 1, was produced in a 23.2%
951m, 2869m, 1585sh, 1563s, 1497s, 1459s, 1431vs, 1387m,
299m, 1271m, 1145vs, 1123vs, 1112vs, 1090vs, 1062m, 1013vs,
59m, 905br, 838s, 748m, 715s, 688vs.
3
1
1
yield. P NMR (CH
d = 2.30 (s, 3H, CH
2
Cl
2
3
):
3
2
ꢀ
1
(
(
6 5 2
m, 10H, (C H )
m
m
3 6 5 2 3 2 2 6 5 2
3.5. Preparation of TbCl [(C H ) PO(C H N -C H )] , 5
7
33s, 692s.
This complex was prepared in a similar manner to that for com-
plex 3 from 0.048 g (0.134 mmol) of 2 with 0.025 g (0.067 mmol)
of TbCl O) and resulted in 0.041 g (0.036 mmol) of 4, 54.36%
yield. The crude produce was dissolved in methylene chloride pre-
cipitated by the addition of hexanes, obtained by filtration and
3
6 5 2 3 2 2 6 5
.2. Preparation of (C H ) PO(C H N -C H ), 2
3
ꢁ6H
2
Compound 1, 1.292 g (0.005 mol) was dissolved in 50 ml of
dried THF under nitrogen and cooled to 0 °C with an ice bath. In
a separate flask, 10 ml of 1.5 M BuLi in hexane was added to 1.5
ml of diisopropylamine in 25 ml of dry THF under nitrogen and
stirred for 20 min. The BuLi mixture was then added to the flask
n
dried. Anal. Calc. for C44
H
38
N
4
O
2
P
2
Cl
3
Tbꢁ0.3CH
2 2
Cl : C, 52.81; H,
ꢀ
1
3.86. Found: C, 52.66; H, 4.00%. IR (cm ): 3230br, 3054br,
2963br, 2901bmp, 2867br, 1591s, 1562s, 1490vs, 1461s, 1434vs,
1388br, 1316sh, 1297m, 1268m, 1149vs, 1120vs, 1091s, 1076s,
1014vs, 962m, 915m, 843s, 795sh, 751s, 727s, 689vs.
n
containing 1 slowly via a canula and allowed to stir for 30 min.
The color of the solution changed from clear to dark red. The solu-
tion was cooled to ꢀ77 °C with an ethanol/liquid nitrogen bath and
0
.625 ml of methyl benzoate added dropwise. The color of the
3.6. X-ray crystallography
solution turned yellow-red. This solution was allowed to warm
to RT and left to stir for 1.75 h. The reaction mixture was then
quenched with 5 ml of a saturated ammonium chloride solution.
The solution was removed to give a yellowish oil which was then
X-ray quality crystals for compound 3 were obtained by diffu-
sion of hexanes into a THF solution of 3 and those for 4 and 5 were
obtained by the slow diffusion of hexanes into methylene chloride
solutions of compounds 3–5. Diffraction data for all compounds
were collected using a Bruker AXS SMART APEX CCD diffractometer
dissolved in 30 ml of ethanol. 1.28 g of (50%–60% in H
2
O) hydrazine
was added and the solution was allowed to stir overnight. Ligand 2

J.S. Maass et al. / Inorganica Chimica Acta 471 (2018) 481–492
483
using monochromatic Mo K
a
radiation with the omega scan tech-
3–5 was performed with atom positions derived from the crystal
structure of 5. The program GaussSum [39] was utilized in check-
ing the progression of the various refinements.
nique. Single crystals of compounds containing 3, 4 and 5 were
mounted on Mitegen micromesh supports using viscous oil flash-
cooled to 100 K. Data were collected, unit cells determined, and
the data integrated and corrected for absorption and other system-
atic errors using the Apex2 suite of programs [31]. The structures
were solved by direct methods and refined by full matrix least
4
. Discussion
2
4.1. Ligand synthesis
squares against F with all reflections using SHELXL [32]. In each
structure, solvent molecules were present with varying degrees of
disorder which were all satisfactorily accounted for. H atoms were
refined mixed with the ones attached to the pyrazole moiety dis-
covered in difference maps and refined freely and the rest at calcu-
lated positions. Final figures of merit for the three structures are
listed in Table 1.
The syntheses of the ligands 1 and 2 are outlined in Eqs. (1)–(3)
and extreme care in the preparation of solvents and reactants is
required in order to obtain reasonable yields of the compounds.
Ligand 1 contained absorptions frequencies at 1707 and 1179
ꢀ1
cm which were assigned to C@O and P@O stretches based on a
theoretical calculation of the FTIR spectrum, Fig. S3 [33]. The
NMR spectra for 1 were also unambiguous as signals for the methyl
1
3.7. DFT calculations
(s, d 2.30), the methylene (d, d 3.58) and the H NMR and one signal
3
1
at d 25.70 for the P atom in the P NMR attest. Evidence for the for-
1
Superior, a high-performance computing infrastructure at
mation of the two isomers of 2 (Eq. (3)) is in the H NMR spectra of
Michigan Technological University, was used for the theoretical
calculations presented in this publication. All calculations were
carried out using Gaussian 09 [33]. Geometry optimizations were
the crude product where two doublets at d 3.85 and 3.95 were
observed for the methyl group and two singlets for the pyrazole
CAH atom at d 6.10 and 6.32, Fig. S1.
performed using the B3LYP [34,35] functional with
a
While a single pyrazole results from the reaction between
hydrazine and a symmetrical b-dicarbonyl, with other reactants
(i.e., unsymmetrical b-dicarbonyl) two isomeric pyrazoles are often
isolated from the reaction mixture [40,41]. Interestingly calcula-
tions of ground state energy levels indicate that 2b, with the H
atom on the pyrazole ligand involved in an H-bond to the O atom
attached to the P atom, is slightly more stable than 2a by 4.24 and
1.98 kcal/mol based on calculations of ground state and THF solva-
tion energies respectively at 298.15 K, see Table 2. It is possible to
isolate one isomer as one, presumably 2b, is more soluble in non-
polar solvents than 2a which may contain intermolecular H-bonds.
Only one set of peaks are observed if acetone or dichloromethane is
used to dissolve the mixture, see experimental details and Fig. S2.
As the ground state energy levels indicate that 2b was more stable
than 2a, it was of interest to see if a conversion of 2b to 2a could be
accomplished that would allow for 2a to coordinate in a bidentate
manner to transition metals. Variable temperature NMR spectra
6
-311+G(d,p) basis set for the ligand 2, and the B3LYP [34,35]
⁄⁄
functional with D95V ++ and LanL2DZ (La) basis sets for the
non-lanthanide and La elements, respectively, for calculations of
generalized lanthanide complexes. This method was reported
recently in a calculation with La substituting for Tb [36]. Alterna-
tively, complexes 3–5 were handled computationally using the
⁄
PBEPBE functional with 6-31G basis sets for the non-lanthanide
elements and the Stuttgart RSC 1997 ECP basis set for each individ-
ual lanthanide element, i.e., Sm, Eu and Tb. The basis sets were
downloaded from the Basis Set Exchange [37]. The computations
for the isomerization of ligand 2 were conducted in vacuum and
also including appropriate solvent fields with the PCM equilibrium
solvation. Frequency calculations were performed at the B3LYP
level of theory. Geometry optimization for the ligand 2 was per-
formed with atom positions drawn initially and then optimized
using Avogadro [38]. The geometry optimization for complexes

4
84
J.S. Maass et al. / Inorganica Chimica Acta 471 (2018) 481–492
à
(
25–130 °C) of 2 dissolved in DMSO d
tious H O) show that coalescence between the two isomers, i.e.,
a and 2b, is attained at a temperature of approximately 70 °C
6
(which contained adventi-
activation energy barrier (DG ) of 14.9 kcal/mol for the methylene
2
peaks and 14.4 kcal/mol for the methine H atom [42,43]. This
method of calculation is only an estimation as there are difficulties
in accurately determining coalescence temperature and peak
separation [42,43].
2
for the methylene H atoms and 85 °C for the methine H atoms,
see Fig. 1. These temperatures allow for an estimation of the
Table 1
Crystal Data and Refinement Details of complexes 3–5.
Compound reference
Chemical formula
Formula Mass
Crystal system
a/Å
3ꢁ2.5THF
(C44 38Cl
1153.695
Monoclinic
36.331(2)
15.4655(7)
25.7740(10)
90
131.3718(5)
90
4ꢁ2CH
38Cl
1144.90
Monoclinic
25.667(1)
15.497(1)
26.530(1)
90
114.412(1)
90
9609.2(8)
100(2)
2
Cl
2
5ꢁ1.5CH
2
Cl
2
H
3
N
4
O
2
P
2 4
Sm)ꢁ2.5(C H
8
O)
C
44
H
3
EuN
4
O
2
P
2 2
ꢁ2(CH Cl
2
)
(C44 38Cl
H
3
N
4
O
2
P
2
Tb)ꢁ1.5(CH
2 2
Cl )
1109.395
Triclinic
12.5225(7)
13.3637(8)
15.2391(9)
101.8140(9)
93.6495(9)
109.2976(9)
2332.0(2)
b/Å
c/Å
a
/°
b/°
/°
c
3
Unit cell volume/Å
Temperature/K
Space group
10867.7(9)
100(2)
C2/c
100(2)
P1ꢀ
1
P2 /n
No. of formula units per unit cell, Z
8
8
2
ꢀ
1
Absorption coefficient,
No. of reflections measured
No. of independent reflections
l/mm
1.334
1.804
1.971
34,332
11,469
0.0256
35,578
13,301
0.0296
119,246
23,819
0.0341
R
int
a
a
b,c
a
b,d
a
a
b,e
Final R indices (I > 2
r
(I))
R
R
1
= 0.0369,
= 0.0462,
R
R
2
= 0.0998
= 0.1077
R
R
1
= 0.0254,
= 0.032,
R
2
= 0 0592
= 0.0633
R
R
1
= 0.0221,
= 0.0237,
R
R
2
= 0.0548
= 0.056
b,c
a
b,d
b,e
R indices (all data)
1
2
1
R
2
1
2
Goodness of fit on F2
0.986
1.05
1.013
CCDC number
1528931
1528932
1528933
P
P
a
b
c
R
R
1
=
||F
o
| ꢀ |Fc||/ |F
o
|.
P
P
2
2
2
2
)2]]1/2.
2
= [ [w(F
o
ꢀ F
c
) ]/ [w(F
o
2
2
2
2
2
w = 1/[
w = 1/[
w = 1/[
r
r
r
(Fo ) + (0.0603P) + 28.2500P] where P = (Fo + 2(Fc) )/3.
d
e
2
2
2
2
(Fo ) + (0.0232P) + 12.5065P].
(Fo ) + (0.0260P) + 1.7927P].
2
2
Table 2
Internal energies in Hartrees for the various forms of ligand 2. For 2a and 2b numbers within parentheses are the differences in kcal/mol between that molecule and the transition
ꢀ
1
state. The numbers listed within square brackets for the transition state energies are for the one imaginary frequency in cm obtained from the calculations.
2
a
Transition state
2b
Ligand 2
Gas Phase
SCRF-THF (298.15 °C)
SCRF-THF (339.15 °C)
ꢀ1375.79404300 (46.61)
ꢀ1375.81201400 (47.34)
ꢀ1375.80600695 (47.33)
ꢀ1375.71976800 [ꢀ1740.34]
ꢀ1375.73656700 [ꢀ1749.46]
ꢀ1375.73058546
ꢀ1375.80080500 (50.85)
ꢀ1375.81517400 (49.33)
ꢀ1375.80918140 (49.32)
2
2
+ H O
Gas Phase
SCRF-THF (298.15 °C)
SCRF-THF (339.15 °C)
ꢀ1452.23807900 (24.51)
ꢀ1452.26279200 (29.53)
ꢀ1452.25617096 (29.46)
ꢀ1452.19902300 [ꢀ1640.42]
ꢀ1452.21572500 [ꢀ492.28]
ꢀ1452.20922466
ꢀ1452.24238900 (27.21)
ꢀ1452.26089600 (28.35)
ꢀ1452.25424690 (28.25)
2
2
+ 2H O
Gas Phase
SCRF-THF (298.15 °C)
SCRF-THF (339.15 °C)
ꢀ1528.69454700 (13.63)
ꢀ1528.71037800 (13.17)
ꢀ1528.70313383 (13.01)
ꢀ1528.67282800 [ꢀ1292.16]
ꢀ1528.68939200 [ꢀ1024.47]
ꢀ1528.68240537
ꢀ1528.69526000 (14.08)
ꢀ1528.71031900 (13.13)
ꢀ1528.70307417 (12.97)

J.S. Maass et al. / Inorganica Chimica Acta 471 (2018) 481–492
485
Fig. 1. 1H NMR variable temperature of 2 (the ‘‘fleeting” transition state is depicted at 85 °C) in DMSO d
s)) and the CH (d 3.85(d) and 3.95(d)) resonate at 25 °C with the traces obtained in sequence from top to bottom.
for the region where the pyrazole methine CAH (d 6.10(s) and 6.32
6
(
2
Table 3
This transition state, Table 3, contained the transient H atom at
bond distances and angles consistent with that reported in the lit-
erature for pyrazole [44]. Interestingly, the transfer of the H atom
is facilitated by the presence of water molecules judging from the
decrease in the transition state energies in the gas phase for 2a(2b)
from 46.61(50.85) to 24.51(27.21) for one water molecule and
Geometry of the transition state of Ligand 2.
1
3.63(14.08) kcal/mol for two water molecules, see Table 2. The
calculated values for the two water molecules are close to those
estimated (14.4–14.9 kcal/mol) from the variable temperature
NMR experiments. This would suggest that this is the pathway uti-
lized. All of the transition states listed in Table 2 contained one
negative frequency corresponding to motions of the H atom(s) in
motion. Calculations in THF at 298.15 and 339.15 K, the boiling
point of THF (the temperature where reactions were preformed),
were also conducted and contained values not very different from
those obtained in the gas phase. These calculations were important
to demonstrate that even if mixtures of the ligand were obtained,
subsequent chemistry that would utilize 2 in a bidentate manner
is possible due to the H atom transfer should the more stable 2b
be produced. Surprisingly, there was a great difference in the tran-
sition state frequency for 2 with one water molecule, i.e., ꢀ1640.42
à
TS
R
NN/Å
Distance Hꢁ ꢁ ꢁX/Å
1.011
1.010
HXY/°
NHN/°
Pyrazole [44]
1.464
1.442
63.4
62.56
71.8
71.04
2
It has been previously demonstrated that while [1,5] hydrogen
atom shifts are symmetry allowed for thermal reactions, the barri-
ers for pyrazole, tetrazole and hydroxypyrazole are very high and
on the order of 50 kcal/mol [44]. Similar values were obtained for
the calculated transition barrier between 2a and 2b as shown in
Table 2 for the ligand 2 with differences in energy levels of 50.85
and 46.61 between 2a and 2b respectively and the transition state.
ꢀ1
and ꢀ492.28 cm , whereas the data for the other two, namely 2

4
86
J.S. Maass et al. / Inorganica Chimica Acta 471 (2018) 481–492
ꢀ
1
with two water molecules, were not that different, see Table 2.
Lower transition state frequencies are indicative of plateau regions
for the transition state due to a more stepwise nature in the trans-
formation [45]. The conclusion from these calculations in Table 2 is
that it is not necessary to isolate 2a in order to synthesize com-
plexes as the isomerization of 2b to 2a is facile in the presence of
water molecules.
absorptions at 1154, 1145 and 1149 cm
for 3–5 respectively
can be assigned to the P@O stretches in the compounds reflecting
a slight decrease upon coordination. Attempts to obtain H NMR
1
spectra of these compounds were not successful due to the param-
agnetic nature of the samples. Analyses of TGA spectra on bulk
sample, Figs. S4–S6, revealed that each of samples 3–5 decom-
posed over the temperature range of 300–600 °C in a step-wise
manner with the Sm and Eu samples initially showing slight loss
of solvent. The samples likely lost two of ligand 2 perhaps resulting
4.2. Synthesis and structures of the lanthanide complexes 3–5
in the formation of LnCl
3
, Ln = Sm, 3 (calcd. 73.6%, expt 65.5%), Eu,
4
(calcd. 73.5%, expt 69%) or Tb, 5 (calcd. 73.0%, expt. 66%), respec-
Reacting two equivalents of ligand 2 with one equivalent of
tively. There have been reports of differences in weights (albeit
much smaller) in TGA between calculated and experimental data
on lanthanide-based samples [46] of much simpler composition
LnCl
Ln(2)
in CH
3
ꢁ6H
Cl
Cl
2
O in boiling THF resulted in the formation of complexes
, Ln = Sm 3, Eu 4 and Tb 5. These complexes were soluble
and CH CN but not stable for prolonged periods. On the
2
3
2
2
3
basis of comparing the IR spectra of 2 to that of 3–5, the
Fig. 2. Mercury [47] representation of the major form of 3. Thermal ellipsoids are
drawn at the 50% probability level and spheres of arbitrary radii represent hydrogen
atoms. Solvent molecules are not drawn.
Fig. 4. Mercury [47] representation of 5. Thermal ellipsoids are drawn at the 50%
probability level and spheres of arbitrary radii represent hydrogen atoms. Solvent
molecules are not drawn.
0
00
Fig. 3. Mercury [47] representation of the two molecules (4 on the right and 4 on the left) constituting the asymmetric unit of 4. Thermal ellipsoids are drawn at the 50%
probability level and spheres of arbitrary radii represent hydrogen atoms. Solvent molecules are not drawn.

J.S. Maass et al. / Inorganica Chimica Acta 471 (2018) 481–492
487
0
00
such as LaI
be precise. If the final compounds were the oxyhalide or an oxide
of formulae LnOCl and Ln , this would lead to lower residual
mass remaining on the order of 18 and 21% respectively in contrast
to the ꢂ33% that was obtained on average for the three samples.
Additionally if extra uncoordinated ligand was present, we would
expect this to decompose and thus lower final weight percentages
3
(MeCN)1.25 suggesting that the decomposition may not
in 4 relate to Cl(5) and Cl(4) in 4 respectively. The asymmetric
unit of complex 3 was refined to reveal 2.5 THF molecules some
of which were disordered. One phenyl group attached to the pyra-
zole moiety on the ligand on the complex was also disordered.
2 3
O
There were two molecules of 4 and four CH
ing the asymmetric unit and of these one CH
Complex 5 contained two CH Cl molecules in the asymmetric unit
and one CH Cl was situated around an inversion point. In all cases,
2
Cl
2
molecules compris-
2
2
Cl was disordered.
2
2
(
i.e., perhaps <25%) would be obtained. Therefore, while the TGA
2
2
results are not conclusive regarding purity of the samples (as the
final composition is unknown), they do suggest that uncoordinated
ligand 2 was not present in the samples.
disorder was accounted for with refinement [48].
All complexes have distorted pentagonal bipyramidal geome-
tries. Two chloride ligands on apical positions with the remaining
chloride and two ligand 2’s (both bonded though the O atom and
the N atoms on the pyrazole moiety proximal to the P@O bond) sit-
uated on the equatorial plane complete the coordination sphere to
the central lanthanide element as evident in Figs. 2–4. Interestingly
the O atoms on the ligands are cis to one another and this facili-
tates an H-bonded interaction between the H atoms on both pyra-
zole moieties distal to their respective P@O bonds and the
equatorial chloride ligand. The data listed in Table 4 reveal that
there are significant decreases in the coordination sphere bond
lengths of these complexes going from 3 to 5 which is most likely
related to the fact that the respective lanthanide element’s radii
X-ray suitable crystals of the complexes were obtained as
described in the Experimental Section and crystal data and final
structural parameters are listed in Table 1. Mercury [47] con-
structed thermal ellipsoid plots of the molecules are in Figs. 2–4
of 3–5 respectively and selected bond distances and angles are
listed in Table 4. There is an asymmetry in the shape of these mole-
cules to the extent that two phenyl groups on adjacent ligands
attached to the P atom are roughly perpendicular to the pseudo
plane produced by the equatorial (pentaganol coordination) and
the data in Table 4 are presented such that these planes are in sim-
0
ilar orientations. While the labels in Figs. 2 and 4, and, for 4 in
0
0
3+
3+
3+
Fig. 3, are meaningful as indicated in Table 4, that for complex 4
decreases in the order Sm
(1.140 Å) > Eu
(1.120 Å) > Tb
follows the same numbering scheme except that Cl(1) and Cl(2)
(1.090 Å) as estimated in aqueous solutions [49].
Table 4
Selected bond distances and angles for 3–5 together with data from theoretical models. Data for La are based on the B3LYP functional and a LanL2DZ (La) basis set [35]. Data for
Sm, Eu and Tb on the B3LYP PBEPBE functional with 6-31G* basis sets for the non-lanthanide elements and the Stuttgart RSC 1997 ECP basis set for each individual lanthanide
[
37].
0
00
Complex
3
4
4
5
La
Sm
Eu
Tb
Bond distances (Å)
Ln(1)–O(1)
Ln(1)–O(2)
Ln(1)–N(1)
Ln(1)–N(3)
Ln(1)–Cl(1)
Ln(1)–Cl(2)
Ln(1)–Cl(3)
O(1)–P(1)
2.3515(19)
2.360(2)
2.592(3)
2.615(3)
2.6450(8)
2.6997(8)
2.8182(7)
1.507(2)
1.505(2)
1.357(3)
1.356(3)
0.83(5)
2.2882(15)
2.3293(15)
2.6685(18)
2.6969(19)
2.6393(5)
2.6306(6)
2.7778(5)
1.5060(16)
1.4998(16)
1.360(2)
2.3315(15)
2.3342(15)
2.6707(19)
2.6383(19)
2.6495(6)
2.7018(5)
2.7654(5)
1.5037(16))
1.5063(16
1.358(2)
2.2937(13)
2.2862(13)
2.5788(15)
2.5726(15)
2.6262(5)
2.6725(5)
2.7372(5)
1.5028(14)
1.5020(13)
1.354(2)
2.485
2.487
2.809
2.809
2.869
2.775
2.918
1.542
1.538
1.352
1.352
1.024
1.024
2.231
2.226
2.405
2.384
2.697
2.692
2.684
2.738
2.863
1.535
1.534
1.349
1.348
1.035
1.036
2.162
2.142
2.392
2.442
2.718
2.639
2.707
2.756
2.849
1.535
1.533
1.347
1.347
1.035
1.035
2.161
2.202
2.352
2.320
2.609
2.666
2.607
2.672
2.783
1.535
1.534
1.349
1.348
1.033
1.035
2.188
2.134
O(2)–P(2)
N(1)–N(2)
N(3)–N(4)
N(2)–H(2)
N(4)–H(4)
Cl(3)–H(2)
Cl(3)–H(4)
1.362(2)
0.84(3)
0.83(3)
2.26(3)
1.363(2)
0.86(3)
0.83(3)
2.25(3)
1.355(2)
0.83(3)
0.80(3)
2.52(3)
0.85(3)
2.30(4)
2.33(3)
2.22(3)
2.34(3)
2.46(3)
Bond Angles (°)
O(1)–Ln(1)–O(2)
O(1)–Ln(1)–N(1)
O(2)–Ln(1)–N(1)
O(1)–Ln(1)–N(3)
O(2)–Ln(1)–N(3)
N(1)–Ln(1)–N(3)
O(1)–Ln(1)–Cl(1)
O(2)–Ln(1)–Cl(1)
N(1)–Ln(1)–Cl(1)
N(3)–Ln(1)–Cl(1)
O(1)–Ln(1)–Cl(2)
O(2)–Ln(1)–Cl(2)
N(1)–Ln(1)–Cl(2)
N(3)–Ln(1)–Cl(2)
Cl(1)–Ln(1)–Cl(2)
O(1)–Ln(1)–Cl(3)
O(2)–Ln(1)–Cl(3)
N(1)–Ln(1)–Cl(3)
N(3)–Ln(1)–Cl(3)
Cl(1)–Ln(1)–Cl(3)
Cl(2)–Ln(1)–Cl(3)
P(1)–O(1)–Ln(1)
P(2)–O(2)–Ln(1)
H(2)–Cl(3)–H(4)
Equatorial sum
76.01(7)
71.59(7)
147.14(7)
145.82(7)
69.92(8)
141.81(8)
86.06(5)
91.48(6)
91.65(6)
97.24(6)
89.83(5)
92.21(6)
82.47(6)
88.86(6)
173.67(3)
142.98(5)
139.55(5)
73.29(5)
70.96(6)
83.59(3)
96.89(3)
139.52(12)
138.28(13)
155(1)
72.29(5)
71.81(5)
142.77(5)
142.15(5)
69.88(5)
144.66(5)
89.26(4)
92.30(4)
96.82(4)
93.65(4)
94.04(4)
87.65(4)
85.29(4)
83.07(4)
176.519(18)
143.09(4)
144.12(4)
72.90(4)
74.67(4)
84.467(19)
93.53(2)
143.31(9)
141.90(9)
167(1)
78.33(5)
72.27(5)
146.54(5)
150.87(5)
72.93(5)
133.15(6)
83.10(4)
85.93(4)
105.68(4)
99.30(4)
88.50(4)
86.07(4)
77.65(4)
84.95(4)
169.415(18)
141.40(4)
135.54(4)
72.01(4)
73.13(4)
81.91(2)
108.640(19)
139.80(9)
139.96(9)
154(1)
74.17(5)
72.02(5)
139.49(5)
139.70(5)
72.11(5)
146.97(5)
98.03(4)
83.59(3)
79.73(4)
99.48(4)
83.98(4)
95.95(3)
101.99(4)
78.25(4)
177.709(15)
143.06(3)
142.33(3)
72.29(4)
74.76(4)
84.667(17)
94.385(16)
144.16(8)
140.06(8)
136.3(8)
365.4
81.5
68.9
78.5
72.4
78.9
75.2
73.4
70.57
141.3
153.1
74.3
134.1
86.3
80.4
119.7
87.6
91.7
79.5
78.3
85.0
159.7
131.6
145.5
72.6
73.9
85.5
110.4
138.9
136.3
165.1
370.3
149.8
149.2
69.3
138.0
81.5
81.8
87.6
84.8
83.5
150.2
146.2
72.3
132.8
81.8
81.6
99.7
109.6
81.8
86.3
83.9
73.8
161.2
140.46
137.0
72.5
148.6
141.9
71.8
137.3
83.8
83.4
92.9
110.4
80.9
89.4
86.0
80.3
164.4
144.7
137.1
73.3
83.7
98.4
102.3
160.5
139.0
138.5
71.7
71.6
108.0
91.5
142.7
141.2
172.1
363.0
73.1
86.8
73.0
86.5
111.7
138.8
138.8
162.6
368.8
108.0
138.6
140.9
173.9
366.7
361.8
361.6
368.7

4
88
J.S. Maass et al. / Inorganica Chimica Acta 471 (2018) 481–492
make the calculations computationally more manageable. An
investigation from 2006 based on 52 lanthanide complexes sug-
gested the RHF/STO-3G/ECP method and basis sets would be most
capable of reproducing the coordination polyhedron geometries
[
54,55]. More sophisticated methods have since been developed,
with most based on various Density Functional methods. Chal-
lenges [56] and difficulties [57,58] in the DFT method have been
noted recently. We decided to test several of the DFT basis sets
for conducting theoretical studies on complexes 3–5, using the
Gaussian [33] platform and data from the crystal structures as
the starting model. Recently Parker and coworkers used a LanL2DZ
(
3
La) basis for calculations on a TbL compound [36] replacing the
Tb by La for the purpose of calculations. It is clear given the
decrease in bond lengths from complexes 3 to 4 and then 5, that
this would not result in an accurate geometry determination for
compounds, and Parker and coworkers restrained the distances
between the La and N/O atoms to the values found by X-ray diffrac-
tion. The inadequacy of the LanL2DZ (La) basis for use with the
other lanthanides without use of distance restraints was confirmed
by our calculations. The bond lengths obtained were much longer
than those obtained via crystallography for 3 and may indeed be
meaningful for the element La, but not for the other lanthanides.
0
00
Fig. 5. Mercury [47] overlap drawing of the molecules 4 and 4 which constitute
the asymmetric unit of 4.
There is considerable variation in the two molecules that con-
stitute the asymmetric unit of 4 as is evident in an overlap presen-
0
00
tation of the two O’s and Eu atoms in 4 and 4 , Fig. 5. The unit cell
did contain eight CH Cl molecules and the difference may be a
⁄
2
2
We decided to instead use the PBEPBE functional with 6-31G basis
result of sterics resulting from the packing arrangement. One note-
worthy difference is in the N(1)–Ln(1)–N(3) angles with 144.66(5)
sets for the non-lanthanide elements and the Stuttgart RSC 1997
ECP basis set for each individual lanthanide element, i.e., Sm, Eu
and Tb. These are available from the Basis Set Exchange [37] Using
these specific basis set combinations for 3–5 did result in closer
agreement to the X-ray determined data, with a similar decrease
in the distances. The results of these calculations are summarized
in Table 4, and details for all calculations are given in the SI.
Relationships between the bond distance and angle data from
calculated and experimental are not easily drawn. The data from
the crystal structures may depict geometric differences purely
based on the packing effects of the interstitial solvent molecules
and these would be difficult to calculate. Additionally, complex 4
had two molecules in the asymmetric unit and these had signifi-
cant differences in bond lengths and angles, see Table 4. One could
conclude that there was a poor relationship in this case. In partic-
ular, the angles in the line labelled as ‘‘equatorial sum” in Table 4
are much larger for the calculated structures than those listed for
the specific complex, i.e., 3–5, had more twists in them on average
than the X-ray determined structures as the larger related equato-
rial sum of angles illustrates.
0
00
and 133.15(6)° for 4 and 4 respectively, see Table 4. This corre-
00
sponds to a larger O(1)–Ln(1)–O(2) of 78.33(5) for 4 compared
0
to 72.29(5)° for 4 suggesting that the two ligands, i.e., 2, have
00
twisted towards the equatorial Cl atom in 4 . However, the dis-
tance between the two H atoms involved in the H-bond to this
00
equatorial Cl ligand is longer in 4 (4.480 compared to 4.452 Å in
0
4
). These effects can be rationalized as the deviation away from
pentagonal planarity as evident in the deviations away from 360°
0
0
in the angles labelled as ‘‘equatorial sum” in Table 4. Complex 4
at 368.7° exhibits the largest deviation but complex 5 at 365.4°
is also not planar. This illustrates the flexibility of the coordination
sphere around these lanthanide elements.
Currently there are only two structural studies of mononuclear
3 2 2
complexes of formulae TbCl O N (none with Sm and Eu) depicting
a meridonal pentagonal bipyramidal coordination geometry listed
in the Cambridge Crystallographic Data Base [50]. The structures
consist of O-atom donors from two DMF molecules and N-atom
0
0
donors in the form of either bidentate dipyrido[3,2-d:2 ,3 -f]
0
0
quinoxaline or dipyrido[3,2-a:2 ,3 -c]phenazine ligands binding to
the mer-TbCl moieties [51]. Two related Sm derivatives were also
3
4.3. Luminescent data
reported with these ligands but these contained an additional
water molecule resulting in a distorted dodecahedral coordination
The lanthanide complexes absorb at 256 nm for 3, 257 for 4 and
geometry [52]. A mononuclear Eu complex, EuCl
3
(tptz)(MeOH)
2
,
263 nm for 5 in dichloromethane, a region which corresponds to
⁄
tptz = 2,4,6,-tris-2-pyridyl-1,3,5-triazine was reported but this also
featured a dodecahedral coordination geometry as there were
three N-atoms binding from the tptz ligand [53]. These different
coordination geometries do not allow for bond angles to be com-
pared in the case of the Sm and Eu complexes and the different
ligands used also render meaningless comparisons between the
bond angles in 5 and the two Tb complexes [53]. However the bond
distances in 5 specifically the Tb–O, Tb–N and Tb–Cl are within the
range of distances reported for the two Tb complexes with the N,N-
bidentate ligands previously reported [53] except for the Tb–Cl(eq)
distance in 5 (2.7372(5) Å) which is significantly longer than the
equivalent ones reported (2.678, 2.673 and 2.670 Å), Table S1. This
difference may be due to the fact that this Cl(eq) ligand is engaged
in H-bonding to the H atoms attached to the pyrazole N atoms as
shown in Figs. 2–4.
the
p-
p
transition in the pyrazole moiety of ligand 2. Complexes
3–5 display visible fluorescence in the solid and solution states
as the image in Fig. S7 attests. The luminescent properties of ligand
2 and complexes 3–5 were assessed in acetonitrile (Figs. S8–S11)
and dichloromethane solutions, see Figs. S12 and 6–8. The notable
difference between the spectra of complexes 3–5 in these two sol-
vents was the presence of more dissociated ligand 2 in acetonitrile,
judging by the increase in the peaks at 309 and 611 nm ascribed to
the ligand, Fig. S12. Solutions in acetonitrile lose lanthanide lumi-
nescent properties after one day suggesting complete decomposi-
tion occurs. Interestingly, ligand 2 absorbs at 249 (253 nm) in
acetonitrile (dichloromethane) solutions and emits at 311 (309)
and 611 (611) nm. The ligand 2 could be thought of as consisting
of the addition of phenylpyrazole (phpz) and methyldiphenylphos-
phine oxide. The spectra for these molecules, presented as Fig. S13,
reveal that the emission spectra are mostly due to the phenylpyra-
zole moiety on ligand 2, judging from the much larger emission
Quantum mechanical calculations on lanthanide compounds
are challenging due to the size of the lanthanide ions, and ab initio
effective core potential calculations have traditionally been used to
spectrum for phpz compared to MePh PO. The band at lower
2

J.S. Maass et al. / Inorganica Chimica Acta 471 (2018) 481–492
489
Fig. 6. Absorption (left) and emission (right) spectra for complex 3 dissolved in dichloromethane. The kmax is 258 nm. Inset is the solid state spectrum of 3 in a glass capillary.
Fig. 7. Absorption (left) and emission (right) spectra for complex 4 dissolved in dichloromethane. The kmax is 257 nm. Inset is the solid state spectrum of 4 in a glass capillary.
energies occurring at roughly twice the wavelength may be
assigned as Rayleigh second order scattering [59].
555 and 685 nm contains the second order spectrum of the ligand
emission. Figs. 6–8 also contains what is believed to be Rayleigh
scattering from the solvent dichloromethane centered at 548 nm
together with a very large anti-Stokes shift at 492 nm and a Stokes
absorption overlapping with the large band at 603 nm. The inten-
sity of the solvent scatter is strongest in the spectrum for complex
3, Fig. 6, much reduced for complex 4, Fig. 7, and would appear to
be overlapping in the spectrum for complex 5, Fig. 8. In these cases,
assignment of the emissions for 4 and 5 can be obtained by exam-
ining the solid state profile insets which are free of the solvent
contributions.
The state symbol designations for the emissions in Figs. 6–8 are
based on literature assignments [15,19,22,60,61]. The intensity of
the emission spectrum of the samarium based complex 3 is much
weaker than that for 4 (Eu) and 5 (Tb), in particular in the solid
state, Fig. 6 (see inset). This has been ascribed to a smaller energy
4
difference between the first excited, i.e., G5/2, and the level imme-
6
diately below, i.e., F11/2, which increases the likelihood of nonra-
diative decay of the excited states [62], which tend to occur at a
much faster time scale than radiative transitions. Transitions for
4
6
4
6
5
5
7
complex 3 consists of G5/2 ? H11/2 (702 nm), G5/2 ? H9/2 (648
The spectrum for complex 4 contains transitions
D
D
0
? F
4
4
5/2 ? H7/2 (603 nm) and 4G5/2 ? H5/2 (570 nm) can be
6
6
5
7
5
7
7
nm),
G
(702 nm),
D
0
? F
5
3
(655 nm),
D
0
? F
2
(618 nm),
0
? F
1
7
identified in the solution spectrum and the last three in the solid
state inset spectrum. Large emissions presumably from dissociated
ligand at 308 and a bump around 600 nm also appear in the spec-
trum together with an absorption at 511 nm corresponding to dou-
ble the excitation wavelength. The transition to J = 7/2 is larger
than the one to 9/2. This is presumably because the band between
(596 nm) and D
0
? F
0
(585 nm) which are all clearly present in
the solid state spectrum, Fig. 7. In this case there appears to be less
dissociation of the ligand 2 from the complex judging from the
reduced intensity of the emission associated with the free ligand.
The spectrum for complex 5, Fig. 8, contains the least dissociated
5
4 0 4
D ? F D ?
(683 nm), 5
7
ligand and the following transitions

4
90
J.S. Maass et al. / Inorganica Chimica Acta 471 (2018) 481–492
Fig. 8. Absorption and emission spectra for complex 5 dissolved in dichloromethane. The kmax is 257 nm. Inset is the solid state spectrum of 5 in a glass capillary.
7F
(672 nm), 5D
? 7F
(650 nm), 5
D
? F
7
(626 nm), 5
(494 nm) in solution.
D
4
? 7F
dichloromethane solutions. The photoluminescent properties of
the complexes were illustrated in both solid and solution forms
and the emission spectra were found to be identical to relaxation
pathways previously assigned to the specific lanthanide ion. This
report demonstrates the feasibility of ligand 2 as a sensitizing
agent for emissions with metal ions with the terbium complex, 5,
having a very high quantum efficiency for the transfer.
1
4
7
2
4
3
4
5
5
7
(
590 nm), D
4
? F
5
(552 nm) and D
4
? F
6
These emissions are also observed in the solid state with fine struc-
ture discernible in some cases, see inset Fig. 8.
The efficiency of the transduction of light from the ligand to the
lanthanide element was assessed using a comparative method [30]
with tyrosine which has a known
F
U of 0.14 [63]. Due to the insta-
bility of the complexes in solution as noted above, this measure-
ment was only obtained for complex 5 in dichloromethane. The
quantum yield of ligand 2 was found to be similar to that of tyro-
sine at 0.15 but that for complex 5 was assessed close to 1.00. It is
believed that this method has a large error of up to 20% [64]. How-
ever, the fact remains that the energy transfer was qualitatively
very efficient. Interestingly, the graphs in Figs. 6–8 do show that
the intensity of the emission spectra increased from complex 3
to 5. One reason for this could be better energy transfer in complex
Acknowledgements
The diffractometer used to solve the structures was funded by
NSF Grant 0087210, by Ohio Board of Regents Grant CAP-491,
and by Youngstown State University. Support by Michigan Techno-
logical University is acknowledged. Helpful comments by the
reviewers, in particular, comments on the emission spectra, are
gratefully acknowledged.
5. Interestingly the distances between the lanthanide element and
the O- and N-atoms that bind from 2 to the lanthanide element
also decrease from complex 3 to 5, see Table 4. This shorter dis-
tance in complex 5 may result in a greater stability in solution
Appendix A. Supplementary data
CCDC’s 1528931–1528933 contain the supplementary crystal-
lographic data for complexes 3–5 respectively. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Center via www.ccdc.cam.ac.uk/data_request/cif. Supplementary
data associated with this article can be found, in the online version,
at https://doi.org/10.1016/j.ica.2017.11.049.
(
in acetonitrile) and may also be the reason for the highest inten-
sity in the emission spectrum compared to complexes 3 and 4.
5
. Conclusions
The bidentate ligand, diphenyl((5-phenyl-4H-1k -pyrazol-3-yl)
4
methyl)phosphine oxide, 2, was synthesized and shown to undergo
proton transfer between the N-atoms on the pyrazolyl moiety.
Variable temperature NMR measurements estimated an activation
barrier for this process of ca. 14.4–14.9 kcal/mol in close agree-
ment with calculated values of 13.63 and 14.08 kcal/mol for a
mechanism involving two water molecules. Three lanthanide com-
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