Encapsulation of the BeII cation: Spectroscopic and computational study

Article abstract of DOI:10.1021/ic302770t

The structures of a series of tetracoordinate beryllium(II) complexes with ligands derived from tertiary-substituted amines have been computationally modeled and their ⁹Be magnetic shielding values determined using the gauge-including atomic orbital (GIAO) method at the 6-311++g(2d,p) level. A good correlation was observed between calculated ⁹Be NMR chemical shifts when compared to experimental values in polar protic solvents, less so for the values recorded in polar aprotic solvents. A number of alternative complex structures were modeled, resulting in an improvement in experimental versus computational ⁹Be NMR chemical shifts, suggesting that in some cases full encapsulation on the beryllium atom was not occurring. Several of the synthesized complexes gave rise to unexpected fluorescence, and inspection of the calculated molecular orbital diagrams associated with the electronic transitions suggested that the rigidity imparted by the locking of certain conformations upon Be^II coordination allowed delocalization across adjacent aligned aromatic rings bridged by Be^II.

Full text of DOI:10.1021/ic302770t

Article
pubs.acs.org/IC
Encapsulation of the BeII Cation: Spectroscopic and Computational
Study
Karl J. Shaffer,† Ross J. Davidson,† Anthony K. Burrell,‡ T. Mark McCleskey,‡ and Paul G. Plieger*,†
†Chemistry Institute of Fundamental Sciences, Massey University, Turitea Campus, Private Bag 11 222, Palmerston North, New
Zealand 4442
‡Chemistry Division MS J582, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States
S
* Supporting Information
ABSTRACT: The structures of a series of tetracoordinate beryllium(II)
complexes with ligands derived from tertiary-substituted amines have been
9
computationally modeled and their Be magnetic shielding values determined
using the gauge-including atomic orbital (GIAO) method at the 6-311++g(2d,p)
9
level. A good correlation was observed between calculated Be NMR chemical
shifts when compared to experimental values in polar protic solvents, less so for
the values recorded in polar aprotic solvents. A number of alternative complex
structures were modeled, resulting in an improvement in experimental versus
9
computational Be NMR chemical shifts, suggesting that in some cases full
encapsulation on the beryllium atom was not occurring. Several of the synthesized
complexes gave rise to unexpected fluorescence, and inspection of the calculated
molecular orbital diagrams associated with the electronic transitions suggested
that the rigidity imparted by the locking of certain conformations upon BeII
coordination allowed delocalization across adjacent aligned aromatic rings bridged by BeII.
relatively few examples of ligands which provide this mode of
coordination. One rare example is the coordination of BeII to
nitrilotripropionate (NTP), a tertiary-substituted amine with
three appended carboxylic acid donors which provided a
tetrahedral binding environment.23 We observed that bidentate
ligands with mixed N/O donors such as pyridine−phenol24 and
10-hydroxybenzoquinoline18 show promise as selective coor-
dinators of beryllium and have in the past extended these
platforms using a binaphthyldiimine ligand, which has the
ability to completely encapsulate BeII via tetradentate
tetrahedral coordination using both phenol and imine donors.17
In the present study we sought to use a series of related
tetradentate ligands capable of tetrahedral coordination akin to
the earlier NTP-type ligands. These new ligands contain a mix
of neutral and charged N-pyridine and O-phenolate donor
atoms which due to their decreased conformational flexibility
may lead to enhanced selectivity of BeII. Using our established
9Be NMR characterization techniques we then evaluated these
ligands for their ability to coordinate to BeII and explored their
photophysical properties.
INTRODUCTION
■
Beryllium is a particularly important metal for a number of
applications that are vital to the aerospace, automotive, and
electronics industries, particularly when alloyed with metals
such as copper.1,2 Beryllium has been described as one of the
most toxic nonradioactive metals on the periodic table,3
primarily due to the latent toxicity associated with beryllium
sensitization,4 which can lead to chronic beryllium disease
(CBD), an incurable sometimes fatal lung disease.5−8 The
mode of delivery of the toxicity, for example, whether as a
carcinogen, immune response, or sensitization, and the extent
to which each play a role is currently under renewed
debate.9−15
Our groups have been engaged in beryllium studies focused
on the fundamental understanding of the nature of selective
binding sites for BeII. To this end we explored the development
of applications to detect the presence of or sequester BeII in the
environment16−18 and for tracking BeII within biological
systems.19−21 In the course of our studies, we also developed
9
theoretical modeling methods based on measurement of Be
NMR to predict BeII speciation in solution with the aim of
predicting the structure of beryllium complexes. This method is
both convenient and avoids potential exposure to beryllium
particulates.22
EXPERIMENTAL SECTION
■
Unless otherwise stated, all reagents and solvents were purchased from
commercial sources and used without further purification. NMR
spectra were collected on Bruker Avance 300, 400, and 500 MHz
spectrometers; the particular instrument is specified for each
In the present work we are interested in investigating ligands
which have the potential to fully encapsulate the BeII cation. To
do this we employed tetradentate ligands containing mixed N/
O donors capable of providing the beryllium atom with
tetrahedral coordination geometry upon chelation. There are
Received: December 17, 2012
Published: March 11, 2013
© 2013 American Chemical Society
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Inorganic Chemistry
Article
compound. All chemical shifts are reported relative to residual solvent
CDCl3): δ 3.89 (s, 2H; CH2), 3.91 (s, 2H; CH2), 3.99 (s, 2H; CH2),
6.80−6.97 (m, 3H; ArH), 7.07−7.22 (m, 3H; ArH), 7.35 (t, J = 6.5,
1H; ArH), 7.79 (dt, J = 1.6, 7.7, 1H; ArH), 8.06 (d, J = 2.8, 1H; ArH),
8.12 (dd, J = 2.8, 8.9, 1H; ArH), 8.68 (d, J = 4.9, 1H; ArH) 9.97 (brs,
2H; OH). 13C NMR (125 MHz, CDCl3): δ 56.0 (d), 56.3 (d), 56.8
(d), 117.0 (s), 117.6 (s), 119.5 (s), 120.8 (q), 122.0 (q), 123.0 (s),
123.4 (s), 126.0 (s), 126.6 (s), 129.8 (s), 130.3 (s), 138.2 (s), 140.0
(q), 148.0 (s), 155.5 (q), 157.0 (q), 163.9 (q). IR: ν (cm−1) 1585,
1478, 1453, 1436, 1330, 1271, 1151, 1088, 750. LRMS for
C20H19N3O4(MH+): 366.72. Anal. Calcd for C20H19N3O4 (379.41):
C, 65.74; H, 5.24; N, 11.50. Found: C, 65.79; H, 5.20; N, 11.49. MP =
102−104 °C.
9
(1H, 13C). Be NMR spectra were recorded at a concentration of 28
mmol L−1 and a temperature of 298 K on a Bruker Avance 300 at
2+
42.17 MHz and referenced to an internal standard of Be(H2O)4 (as
the sulfate salt). All NMR samples containing beryllium were
contained within Teflon sleeves in addition to the standard NMR
glass tube. Microanalyses were performed at the Campbell Micro-
analytical Laboratory at the University of Otago. Electrospray mass
spectra were recorded on a Micromass ZMD spectrometer run in
positive-ion mode. High-resolution mass spectra were recorded on a
microTOF-Q mass spectrometer at the University of Auckland
operating at a nominal voltage of 3500 V. IR spectra were recorded
on a Bruker Alpha-P diamond anvil system. UV−vis spectra were
recorded on a Shimadzu UV-3101PC spectrophotometer using UV
Probe v1.1. Fluorescence measurements were made on a Perkin-Elmer
LS50B luminescence spectrometer using FL Winlab v4.00.02.
Reactions were followed by TLC on aluminum-backed silica gel 60
F254 sheets from E-Merck, visualized by UV light. Flash column
chromatography was performed using Scharlau silica gel 60, 0.04−0.06
mm, 230−400 mesh. The length of silica was typically 20 cm, and the
diameter was varied according to reaction scale. The silica gel slurry
was compacted with the specified solvent system of hexanes/EtOAc or
CH2Cl2/MeOH. The compound was then loaded onto the column
and eluted with the specified solvent under positive pressure.
Synthesis. General Procedure for Secondary Amines 1a−f. The
appropriate starting primary amine (4.10 mmol) and 2-hydroxyben-
zaldehyde (4.10 mmol) in MeOH (10 mL) were stirred for 30 min at
RT. NaBH4 (6.48 mmol) and NaOH (0.82 mmol) in H2O (2 mL)
were added to the reaction mixture, and the resulting solution was
stirred for 1 h. The reaction was diluted with CH2Cl2 (200 mL) and
washed with water (200 mL). The organic layer was filtered and dried
over MgSO4, and the solvent was removed. The secondary amine was
purified by silica gel column chromatography using 95:5 CH2Cl2/
MeOH. Characterization data for all new compounds are presented
below.
2-(((2-Hydroxybenzyl)(pyridin-2-ylethyl)amino)methyl)-4-nitro-
1
phenol (2b). Rf = 0.29 (95:5 CH2Cl2/MeOH). H NMR (500 MHz,
CDCl3): δ 2.94 (t, J = 6.7, 2H; CH2), 3.14 (t, J = 6.7, 2H; CH2), 3.49
(brs, 2H; OH), 3.85 (s, 4H; CH2), 6.69−6.82 (m, 3H; ArH), 7.05−
7.24 (m, 4H; ArH), 7.64 (dt, J = 1.7, 7.8, 1H; ArH), 7.94 (d, J = 2.2,
1H; ArH), 8.00 (dd, J = 2.8, 8.9, 1H; ArH), 8.54 (d, J = 5.0, 1H; ArH).
13C NMR (125 MHz, CDCl3): δ 33.9 (d), 53.7 (d), 55.2 (d), 55.7 (d),
116.5 (s), 116.6 (s), 119.7 (s), 121.6 (q), 122.1 (s), 122.4 (s), 123.7
(s), 125.5 (s), 126.2 (q), 129.6 (s), 130.9 (s), 137.6 (s), 139.7 (q),
148.8 (s), 156.1 (q), 158.6 (q), 163.9 (q). IR: ν (cm−1) 1589, 1478,
1456, 1438, 1332, 1272, 1150, 1086, 751. LRMS for C21H21N3O4
(MH+): 380.84. Anal. Calcd for C21H21N3O4·0.35H2O (385.72): C,
65.39; H, 5.67; N, 10.89. Found: C, 65.40; H, 5.48; N, 10.84. MP =
108−110 °C.
2-(((2-Hydroxybenzyl)(quinolin-8-yl)amino)methyl)-4-nitrophe-
nol (2c). Rf = 0.82 (95:5 CH2Cl2/MeOH). 1H NMR (500 MHz,
CDCl3): δ 4.48 (s, 4H; CH2), 6.80−6.85 (m, 2H; ArH), 6.90 (d, J =
9.1, 1H; ArH), 7.12−7.22 (m, 2H; ArH), 7.47 (d, J = 5.0, 2H; ArH),
7.59 (q, J = 4.1, 2H; ArH), 8.05 (dd, J = 3.2, 8.6, 1H; ArH), 8.17 (d, J
= 2.7, 1H; ArH), 8.31 (dd, J = 1.4, 8.6, 1H; ArH), 9.01 (dd, J = 1.4, 4.6,
1H; ArH), 10.46 (brs, 1H; OH), 12.59 (brs, 1H; OH). 13C NMR (125
MHz, CDCl3) δ 56.6 (d), 56.7 (d), 117.1 (s), 117.7 (s), 119.5 (s),
120.9 (q), 121.4 (s), 121.8 (s), 122.2 (q), 124.4 (s), 125.9 (s), 126.8
(s), 127.0 (s), 129.7 (s), 130.1 (q), 130.6 (s), 138.9 (s), 140.0 (q),
141.8 (q), 143.6 (q), 148.3 (s), 156.9 (q), 163.7 (q). IR: ν (cm−1)
1580, 1514, 1486, 1390, 1340, 1281, 1262, 1250, 1233, 1088, 792, 759.
LRMS for C23H19N3O4 (MH+): 402.82. Anal. Calcd for C23H19N3O4
(401.41): C, 68.82; H, 4.77; N, 10.47. Found: C, 69.03; H, 4.69; N,
10.50. MP = 184−185 °C.
2-((2-Phenylquinolin-8-ylamino)methyl)phenol (1d). Rf = 0.80
1
(95:5 CH2Cl2/MeOH). H NMR (500 MHz, CDCl3): δ 4.66 (d, J =
3.8, 2H; CH2), 6.67 (brs, 1H; NH), 6.90−6.98 (m, 3H; ArH), 7.21−
7.26 (m, 2H; ArH), 7.34 (t, J = 7.8, 1H; ArH), 7.42−7.52 (m, 3H;
ArH), 7.88 (d, J = 8.5, 1H; ArH), 7.11−7.16 (m, 3H; ArH), 8.31 (brs,
1H; OH). 13C NMR (125 MHz, CDCl3): δ 48.2 (d), 109.7 (s), 116.6
(s), 117.4 (s), 119.3 (s), 120.1 (s), 123.4 (q), 127.1 (s), 127.2 (q),
127.4 (s), 128.5 (s), 128.8 (s), 128.9 (s), 129.3 (s), 137.0 (s), 138.8
(q), 139.3 (q), 144.6 (q), 154.7 (q), 156.7 (q). IR: ν (cm−1) 1585,
1515, 1490, 1460, 1346, 1329, 1292, 1249, 1215, 767, 752, 740. HRMS
calcd for C29H23N3O4 (MH+): 478.1761. Found: 478.1756 (ESI+).
MP = 136−137 °C.
2-(((2-Hydroxybenzyl)(2-phenylquinolin-8-yl)amino)methyl)-4-ni-
trophenol (2d). Rf = 0.82 (95:5 CH2Cl2/MeOH). 1H NMR (500
MHz, CDCl3): δ 4.48 (s, 2H; CH2), 4.51 (s, 2H; CH2), 6.77−6.85 (m,
3H; ArH), 7.12−7.22 (m, 2H; ArH), 7.46−7.64 (m, 6H; ArH), 7.80
(d, J = 8.7, 1H; ArH), 7.86 (m, 2H; ArH), 8.03 (dd, J = 2.4, 9.0, 1H;
ArH), 8.17 (d, J = 2.4, 1H; ArH), 8.32 (d, J = 8.7, 1H; ArH), 10.59
(brs, 2H, OH). 13C NMR (125 MHz, CDCl3): δ 56.6 (d), 57.4 (d),
117.2 (s), 117.7 (s), 119.6 (s), 121.2 (s), 121.7 (s), 122.4 (s), 124.5
(s), 125.7 (s), 126.4 (s), 126.5 (s), 128.5 (s), 128.5 (s), 128.9 (q),
129.2 (s), 129.2 (s), 129.6 (s), 129.7 (s), 130.5 (s), 138.8 (q), 138.9
(q), 140.2 (q), 142.6 (q), 144.1 (q), 156.5 (q), 156.8 (q), 159.8 (q),
163.1 (q). IR: ν (cm−1) 1584, 1515, 1490, 1459, 1342, 1327, 1290,
1246, 1217, 765, 752, 740. LRMS for C29H23N3O4 (MH+): 478.88.
Anal. Calcd for C29H23N3O4 (477.52): C, 72.94; H, 4.85; N, 8.80.
Found: C, 71.76; H, 4.55; N, 8.67. MP = 175−176 °C.
Methyl 3-(2-Hydroxybenzylamino)propanoate (1f). Rf = 0.29
1
(95:5 CH2Cl2/MeOH). H NMR (500 MHz, CDCl3): δ 2.59 (t, J =
6.1, 2H; CH2), 2.96 (t, J = 6.1, 2H; CH2), 3.72 (s, 3H; CH3), 4.02 (s,
2H; CH2), 6.80 (td, J = 1.0, 7.3, 1H; ArH), 6.85 (dd, J = 1.0, 8.1, 1H;
ArH), 7.01 (d, J = 6.7, 1H; ArH), 7.18 (td, J = 1.0, 7.4, 1H; ArH). 13C
NMR (125 MHz, CDCl3): δ 33.6 (d), 43.6 (d), 51.8 (t), 52.4 (d),
116.3 (s), 119.0 (s), 122.2 (q), 128.4 (s), 128.8 (s), 158.1 (q), 172.8
(q). IR: ν (cm−1) 1593, 1492, 1460, 1335, 1091, 751. LRMS for
C11H15NO3 (MH+): 210.61. Anal. Calcd for C11H15NO3·0.4H2O
(216.45): C, 61.04; H, 7.36; N, 6.47. Found: C, 60.96; H, 7.10; N,
6.69.
2-(((2-Hydroxybenzyl)(2-hydroxyphenyl)amino)methyl)-4-nitro-
1
phenol (2e). Rf = 0.54 (95:5 CH2Cl2/MeOH). H NMR (500 MHz,
General Procedure for Tertiary Amines 2a−f. The appropriate
secondary amine 1a−g (2.95 mmol), 2-(chloromethyl)-4-nitrophenol
(2.95 mmol), and triethylamine (23.6 mmol) were refluxed together in
CHCl3 (20 mL) for 2 h. The reaction was diluted with CH2Cl2 (200
mL) and washed with water (200 mL). The organic layer was filtered
and dried over MgSO4. The product was purified by silica gel column
chromatography using 98:2 CH2Cl2/MeOH. The sticky residue was
isolated as a powder by dissolution in a minimum volume of CH2Cl2,
precipitating with hexane addition, decanting, and drying in vacuo.
Characterization data for all new compounds are presented below.
2-(((2-Hydroxybenzyl)(pyridin-2-ylmethyl)amino)methyl)-4-nitro-
CDCl3): δ 4.22 (s, 2H; CH2), 4.36 (s, 2H; CH2), 6.77 (d, J = 9.0, 1H;
ArH), 6.87−6.95 (m, 4H; ArH), 7.03−7.08 (m, 1H; ArH), 7.11 (brs,
3H; OH) 7.20−7.26 (m, 3H; ArH), 8.00 (dd, J = 2.8, 9.0, 1H; ArH),
8.05 (d, J = 2.8, 1H; ArH). 13C NMR (125 MHz, CDCl3): δ 54.0 (d),
57.9 (d), 116.1 (s), 116.5 (s), 116.9 (s), 120.9 (s), 121.2 (s), 121.6
(q), 121.6 (s), 122.4 (q), 125.7 (s), 126.2 (s), 127.1 (s), 130.3 (s),
131.6 (s), 133.8 (q), 140.5 (q), 151.0 (q), 154.5 (q), 163.4 (q). IR: ν
(cm−1) 1587, 1490, 1458, 1334, 1271, 1227, 1173, 1086, 747. LRMS
for C20H18N2O5 (MH+): 367.72. Anal. Calcd for C20H18N2O5
(366.37): C, 65.57; H, 4.95; N, 7.65. Found: C, 65.61; H, 4.97; N,
7.50. MP = 158−159 °C.
1
phenol (2a). Rf = 0.29 (95:5 CH2Cl2/MeOH). H NMR (500 MHz,
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Inorganic Chemistry
Article
a
Scheme 1. Synthesis of Tertiary Amine Ligands, 2a−f
a
(i) (a) 2-Hydroxybenzaldehyde, MeOH, RT, 30 min or 6 h or 7days. (b) NaBH4, NaOH, H2O, RT, 1h (yields 1a = 68%, 1b = 72%, 1c = 83%, 1d =
89%, 1e = 87%, 1f = 84%). (ii) (a) 2-Chloromethyl-4-nitrophenol, CHCl3, triethylamine, reflux, 2 h. (b) 2f only 6 M HCl, 110 °C, 4 h (yields 2a =
69%, 2b = 53%, 2c = 51%, 2d = 39%, 2e = 68%, 2f = 46%). Isolated yields are after chromatography.
Methyl 3-((2-Hydroxy-5-nitrobenzyl)(2-hydroxybenzyl)amino)-
propanoate (2f-Me). Rf = 0.50 (95:5 CH2Cl2/MeOH). 1H NMR
(500 MHz, CDCl3): δ 2.73 (t, J = 7.4, 2H; CH2), 3.02 (t, J = 7.4, 2H;
CH2), 3.69 (s, 3H; CH3), 3.85 (s, 2H; CH2), 3.89 (s, 2H; CH2), 6.77−
6.81 (m, 2H; ArH), 6.88 (t, J = 7.4, 1H; ArH), 7.13−7.20 (m, 2H;
ArH), 8.00 (d, J = 2.3, 1H; ArH), 8.05 (dd, J = 2.7, 8.9, 1H; ArH), 8.20
(brs, 2H; OH). 13C NMR (125 MHz, CDCl3): δ 31.2 (d), 49.2 (d),
52.1 (t), 55.0 (d), 56.2 (d), 116.0 (s), 116.5 (s), 120.5 (s), 121.0 (q),
121.7 (q), 125.9 (s), 130.0 (s), 131.1 (s), 139.7 (q), 155.4 (s), 157.0
(q), 164.4 (q), 172.3 (q). IR: ν (cm−1) 1593, 1496, 1458, 1336, 1284,
1090, 753. LRMS for C18H20N2O6 (MH+): 361.73. Anal. Calcd for
C18H20N2O6 (360.36): C, 59.99; H, 5.59; N, 7.77. Found: C, 59.84; H,
5.53; N, 7.70.
were used to obtain the optimized geometries. NMR shieldings for
beryllium were evaluated at the minima using B3LYP with 6-311G+
+(2d,p) as the basis set and the GIAO (gauge including atomic
orbital) NMR method27,28 as implemented by Cheeseman et al.29 in
2+
the Gaussian09 package. Complex Be(H2O)4 is the standard
reference for 9Be NMR spectroscopy and defined as 0.00 ppm.
Structure determination for this complex 6-311++G(2d,p) followed by
calculation of the magnetic shielding 6-311++G(2d,p) gave a value of
109.10 ppm. The calculated chemical shift reported in this study is in
2+
relation to Be(H2O)4 using the expression δcomplex = σreference
−
σcomplex. Time-dependent DFT (TD-DFT) calculations were carried
out using B3LYP/6-311G++(2d,p) in a N,N-dimethylformaldehyde
(DMF) solvent field using the SCRF-PCM method,30 which creates
the solvent cavity via a set of overlapping spheres. Geometry
optimizations were not carried out in a solvent field as this displayed
little increase in NMR accuracy despite the significant increase in
computational expense. However, TD-DFT calculations were found to
be more accurate with solvent contributions included than with-
out.31,32
3-((2-Hydroxy-5-nitrobenzyl)(2-hydroxybenzyl)amino)propanoic
Acid (2f). 2f-Me (0.100 g, 0.417 mmol) in 6 M HCl (2 mL) was
refluxed for 4 h at 110 °C. The reaction mixture was diluted with water
(200 mL) and washed with CH2Cl2 (200 mL). The aqueous layer was
separated and neutralized with 1 M NaOH to pH 7, and 2f was
transferred into an organic layer of 9:1 CH2Cl2/MeOH (200 mL).
The organic layer was separated, filtered, and dried over MgSO4, and
the solvent was removed. The pale yellow solid 2f was dissolved in
CH2Cl2 and precipitated upon addition of hexane and then filtered
RESULTS AND DISCUSSION
■
1
Synthesis. The secondary amines 1a−f were synthesized in
68−89% yields by a Schiff base condensation between the
appropriate amine and 2-hydroxybenzaldehyde in methanol
followed by an in situ reduction with sodium borohydride,
sodium hydroxide, and water (Scheme 1i).33 Condensations for
the alkyl-linked 1a, 1b, and 1f required 30 min stirring at RT,
while the aryl-linked 1e required 6 h, and 1c−d were stirred at
RT for 7 days. The secondary amines 1a,341b,351c,36and 1e37
are known compounds, and characterization data agreed with
the literature, while 1d and 1f had not been previously reported
and were therefore fully characterized. The final tertiary amines
2a−f were synthesized in 39−69% yields by refluxing the
appropriate secondary amine 1a−f, 2-chloromethyl-4-nitro-
phenol, and triethylamine in chloroform for 2 h (Scheme
1(ii)(a)), the carboxylic acid on 1f was isolated by heating the
methyl ester in hydrochloric acid (Scheme 1(ii)(b)). We chose
the substituted 2-chloromethyl-4-nitrophenol as it was
commercially available which meant tertiary amines 2a−f
were all new compounds (owing to the nitro substituent) and
hence fully characterized; several structurally analogous ligands
are known which do not have this nitro substituent.38−40 In
addition to the six new ligands 2a−f, 2-(bis(2-hydroxy-3,5-
dimethylbenzyl)amino)acetic acid, 2g, was synthesized via a
one-pot Mannich reaction41 which had a structurally analogous
donor set to 2f with the exception that one of the donor arms
(0.047 g, 49%). Rf = 0.36 (90:10 CH2Cl2/MeOH). H NMR (500
MHz, DMSO-d6): δ 2.53 (t, J = 7.4 Hz, 2H; CH2), 2.74 (t, J = 7.4 Hz,
2H, CH2), 3.50 (brs, 1H; COOH), 3.70 (s, 2H; CH2), 3.78 (s, 2H;
CH2), 6.77 (m, 2H; ArH), 6.89 (t, J = 9.0, 1H; ArH), 7.10 (td, J = 1.5,
7.4, 1H; ArH), 7.17 (dd, J = 1.5, 7.4, 1H; ArH), 8.03 (dd, J = 2.8, 9.0,
1H; ArH), 8.14 (d, J = 2.8, 1H; ArH). 13C NMR (125 MHz, DMSO-
d6): δ 31.0 (d), 48.6 (d), 53.2 (d), 53.6 (d), 115.7 (s), 116.2 (q), 119.3
(s), 123.1 (s), 124.7 (q), 125.3 (q), 126.3 (s), 129.0 (s), 130.7 (s),
139.5 (q), 156.8 (s), 164.2 (q), 173.6 (q). IR: ν (cm−1) 1596, 1495,
1460, 1335, 1283, 1089, 752. HRMS calcd for C17H18N4O6 (MH+):
347.1238. Found: 347.1228 (ESI+).
General Procedure for Beryllium Complexes 3a−g. All BeII
complexes were prepared by adding ligands 2a−g (30 μmol), BeSO4
(30 μmol, as a 1 M solution in H2O), and triethylamine (300 μmol) to
1 mL of four different solvents (DMF, DMSO-d6, MeOH, or D2O)
and mixed at 50 °C for 16 h to aid complexation. Resulting solutions
9
(28 mmol L−1) were cooled to 298 K and analyzed via Be NMR
spectroscopy. Caution! Beryllium salts are extremely toxic and should
be handled with appropriate care. All beryllium solid manipulations
were handled in a Plas-Laboratories 818 Series, HEPA filtered (0.4 lm
prefilter), reversed-pressure glove box. All beryllium solution work was
performed in a dedicated HEPA-filtered chemical hood.
Computational Details. Density functional theory (DFT)
calculations were carried out using the Gaussian09 package of
programs.25 Becke’s three-parameter hybrid exchange correlation
function containing the nonlocal gradient correction of Lee, Yang,
and Parr (B3LYP)26 in conjunction with the 6-311G++(2d,p) basis set
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on 2g had a methyl-linked carboxylic acid rather than the
analogous ethyl-linked carboxylic acid of 2f.
the shifts. The line width is related to the symmetry of a
complex, and for highly symmetrical four-coordinate species,
line widths of less than 50 Hz have been observed.22 Typical
line widths are generally greater than this, especially when
beryllium is coordinated to form unsymmetrical four-
coordinate species, such as those reported by Niemeyer and
Power.42 Ligands 3a−g each provide four different donor arms
and are therefore expected to exhibit broad line widths, yet
given the method of sample preparation (nonisolated products)
the potential existence of fluxional minor BeII products
contributing (at least partially) to the line width broadening
cannot be excluded. Evidence for tetracoordinate chelation to
BeII was therefore further investigated via computational
methods.
BeII complexes were prepared according to Scheme 2. To
avoid potential exposure to hazardous beryllium solids the
Scheme 2. Generalized Structure for the BeII
a
Tetracoordinate Complexes 3a−g
9Be NMR Computational. Structures of the six potential
BeII complexes 3a−c and 3e−g (3d was excluded from further
9
study) were computationally optimized and their Be NMR
shifts calculated (Table 2). Complexes 3a−c contained two
ionizable donor atoms and were optimized as neutral BeII
species, while 3e−g contained three ionizable donor atoms and
were optimized as anionic BeII species. It was possible to
optimize two different conformers which were very close in
energy for the complexes for 3a, 3c, 3e, and 3g, as all contained
one five-membered chelate which “lock” these conformations.
The conformers will be herein be designated “syn” and “anti”
conformers according to the orientation of aligned aromatic
rings and equatorial protons on the CH2 directly adjacent the
central amine upon BeII chelation; an example is given for 3g in
Figure 2. The syn and anti conformer for 3a, 3c, 3e, and 3g
typically differed in energy by 10 kJ mol−1 or less. These energy
differences are equivalent to the torsional energy barrier in
simple bond rotations, which suggest that upon coordination to
BeII both conformations exist in solution. Complexes 3b and 3f
only had six-membered chelates within their structures, and this
lead to only one possible minimized structure. For simplicity,
enanantiomers brought about by having four different donor
groups around the BeII core in all complexes (except 3g) were
not considered as they would given energetically identical
models. As shown in Table 1, the experimental shift data shows
some variation when measured in different solvents; therefore,
a
(i) 1 M BeSO4(aq), triethylamine, 50 °C, 16 h.
resulting complexes 3a−g were left in solution and not isolated.
Instead, 9Be NMR spectra of the reaction mixtures were
directly recorded, and the results are summarized in Table 1.
Table 1. Be NMR Shifts and Line Widths of BeII Chelated
to 3a−g
9
9Be NMR shift, ppm (line width, Hz)
BeII complex
DMF
DMSO-d6
MeOH
D2O
3a
3b
3c
3d
3e
3f
4.51 (67)
2.77 (62)
5.15 (89)
no complex
4.40 (59)
not measured
4.06 (66)
no complex
3.18 (123)
no complex
no complex
5.10 (106)
2.11 (174)
4.64 (130)
4.18 (188)
2.94 (150)
no complex
no complex
5.28 (159)
2.33 (151)
insoluble
insoluble
insoluble
insoluble
insoluble
Insoluble
2.60 (175)
5.15 (104)
9
3g
for convenience, the closest experimental Be NMR shift value
(regardless of solvent) was chosen for comparison with the gas-
phase-calculated values of the modeled complexes. It has
recently been shown that DFT NMR calculations tend to
overestimate the experimental 9Be values by approximately
10%, and it may be possible to obtain more accurate values
from using the CCSD(T) level of theory.43
9
With the exception of 3d, all reactions showed new Be
NMR shifts within the range expected for a four-coordinate BeII
complex (e.g., Figure 1).22 The phenyl substituent at the 2-
position on the quinoline of 2d coupled with poor solubility
likely hindered chelation to BeII. Use of several different
9
9
The calculated Be NMR shifts for the beryllium complexes
solvents allowed the Be NMR shifts and line widths (width at
with ligands 2b, 2e, 2f, and 2g correlated reasonably well with
the experimental shifts, providing good evidence that the
expected four-coordinate species formed in solution. A Δδ of
approximately 0.5 ppm or less is considered an acceptable
deviation for these types of calculations.22 These four ligands all
offered one tertiary amine donor and two phenol groups, and
each differed by one donor group (Figure 3). BeII coordinated
1/2 heights) to be measured despite the variable solubility of
this series of ligands and their corresponding BeII complexes;
unfortunately, no single solvent could be used to directly
9
compare all Be NMR shifts. Control reactions in the absence
9
of all ligands gave Be NMR shifts centered around 0 ppm (as
expected), suggesting ligand chelation rather than simple BeII
speciation with the solvent or triethylamine was the cause of
9
Figure 1. Be NMR shift of 3g coordinated to beryllium in DMF.
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a
9
Table 2. Calculated Be NMR Shifts for Tetracoordinate Ligands
complex
δref
δmodel[a]
δ(ref‑model)
δexptl
|Δexptl‑calcd|
2+
Be(H2O)4
3a-syn
3a-anti
3b
109.10
109.10
102.89
103.12
105.19
102.65
102.81
103.28
103.57
106.63
103.92
103.97
0.00
6.21
5.98
3.91
6.45
6.29
5.82
5.53
2.47
5.18
5.13
0.00
0.00
1.70
1.47
0.73
1.30
1.14
0.54
0.25
0.13
0.03
0.02
4.51 (DMF)
3.18 (DMSO-d6)
3c-syn
3c-anti
3e-syn
3e-anti
3f
5.15 (DMF)
5.28 (MeOH)
2.60 (D2O)
5.15 (D2O)
3g-syn
3g-anti
a
9
Gas-phase model utilized as inclusion of solvent models had negligible influence on the Be shielding tensors.
Figure 2. Geometry-optimized syn conformer (left) and anti
conformer (right) of 3g. BeII is light blue, O is red, N is blue, and
C is gray. Equatorial protons are shown in green for clarity.
Figure 4. Tetracoordinate ligands with poorly correlating calculated vs
experimental Be NMR shifts.
9
Figure 3. Tetracoordinate ligands with well-correlated 9Be NMR shifts
upon BeII coordination.
to the central amine and the two phenols would form two
unstrained six-membered chelate rings. The remaining arm on
2e, 2f, and 2g offered a strongly coordinating oxygen donor.
Once fully enveloped around beryllium, ligands 2e and 2g
would generate an additional five-membered chelate ring and 2f
a third six-membered chelate ring (red outline, Figure 3).
Ligand 2b with a weaker coordinating nitrogen donor would
also give an unstrained six-membered chelate ring upon
coordination to BeII.
Figure 5. Optimized models of the syn (left) and anti (right)
[Be(2a)(DMF)] complexes. BeII is yellow, O is red, N is blue. and C is
gray. Hydrogen atoms have been removed for clarity.
donor atoms of the ligand is unlikely. NMR calculations were
also performed on similar beryllium-coordinated DMF adducts
of the syn and anti conformers of the quinoline ligand 2c, again
showing an improvement in accuracy between the modeled and
experimental NMR values (syn calculated = 4.01 ppm (Δ 1.14)
and anti calculated = 4.98 ppm (Δ 0.17)). A two-coordinate
beryllium complex of [BeII(2a)(DMF)2] with the beryllium
bound as a bischelate through both phenolate groups of the
ligand and with two coordinated DMF solvent molecules was
also tested (syn conformer), but the resulting high Δexptl‑calcd
value of 3.28 ppm indicated that this species was not likely to
9
The calculated Be NMR shifts for the beryllium complexes
of 2a and 2c did not correlate as well with the experimental
shifts. This suggested full encapsulation via tetradentate
chelation was perhaps not occurring. In both ligands, the
fourth donor in both cases is neutral, weakly coordinating
nitrogen atoms which if coordinated would form a strained five-
membered chelate ring (Figure 4). Combination of both of
these factors appears to not favor tetradentate coordination.
In an effort to explore this further, the syn and anti
conformations of the beryllium complex [BeII(2a)(DMF)]
were optimized containing a DMF molecule in place of the
9
be present. Other untested possibilities for the observed Be
9
pyridine arm (Figure 5). The calculated Be NMR shift values
NMR shift present in solution may include polymeric or μ-
hydroxy cluster complexes,44−47 for example, the Be3(OH)3
core is well known.48−52
for each of these conformers were 3.30 (Δexptl‑calcd = 1.21 ppm)
and 3.40 ppm (Δexptl‑calcd = 1.11 ppm), respectively. Both values
9
are in better agreement to the experimental Be NMR shift
Electronic Spectroscopy. Electronic spectra of BeII
complexes (3b, 3e, 3f, and 3g) were recorded by making an
appropriate dilution to the DMF-solvated reaction mixtures so
as to avoid manipulation of beryllium-containing solid samples.
Spectra of the associated ligands (2b, 2e, 2f, and 2g) were
obtained in DMF (4.51 ppm) than those obtained for the two
conformers of the modeled tetracoordinated species, namely,
6.21 (Δ 1.70) and 5.98 ppm (Δ 1.47), respectively, thus
providing evidence that tetrachelation of beryllium by all four
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recorded by preparing “blank” reaction mixtures, i.e., in the
absence of BeSO4(aq) and making the appropriate dilution. The
strongest transitions are summarized in Table 3, and the first 5
Table 3. Electronic Absorption Data for Ligands and Their
Associated BeII Chelates in DMF
experimental
experimental
c/10−5
λ/
nm
BeII
cm−1 complex M L−1
c/10−5
λ/
nm
ε/L M
ε/L M
−1
−1
ligand M L−1
cm−1
2b
2e
2f
5.28
5.96
5.78
5.83
428
418
390
289
9600
24 500
8800
4000
3b
3e
3f
5.28
5.96
5.78
5.83
367
385
351
302
14 700
15 400
12 000
6200
2g
3g
Figure 7. Normalized absorption and emission spectra at 10−5 M of
ligand 2g (blue) and BeII complex 3g (red) in DMF and triethylamine.
calculated transitions with nonzero oscillator strengths can be
found in the Supporting Information (Tables S1.1−S1.4). The
similarity between the measured calculated electronic spectra
(Figure S1.1, Supporting Information) further supports the
proposed computational models for 3b, 3e, 3f, and 3g.
From the TD-DFT calculations, the strongest transitions of
2b, 2e, and 2f were assigned as being π → π* localized on the
nitrated phenol. Upon coordination this transition is blue
shifted from 33 (3e) to 61 nm (3b), which is explained by
changes in the electron density of the now inhibited resonance
delocalized form of the 4-nitrophenolate group upon
coordination to the BeII cation (Figure 6).
CONCLUSIONS
■
Studies in experimental BeII coordination chemistry have
waned in the past decade, largely due to the associated toxicity
of the element. We have shown that it is possible to explore the
coordination chemistry of this element without isolation of the
resulting complexes. As the greatest hazard when working with
beryllium is inhalation of the particulate matter, this method
minimizes the risk associated with this hazard by decreasing the
amount of exposure to beryllium-containing solids. To this end,
we studied a series of tetracoordinate BeII complexes and
gained valuable insight into the coordination of these
complexes through spectroscopic assessment, complimented
with computational modeling. Several of the complexes studied
gave rise to unexpected fluorescence, which may give rise to
applications as BeII detectors.
Figure 6. Reasonance structures of 4-nitrophenolate.
ASSOCIATED CONTENT
* Supporting Information
■
S
Tables containing a summary of the first five transitions
calculated for the cationic complexes 3b, 3e-syn, 3f, and 3g-syn.
Figures of the calculated and measured ultraviolet spectra for
3b and the molecular orbitals associated with the main
transition for 3g-syn obtained from theoretical calculations.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Ligand 2g, with only methyl groups substituted on the
phenol rings, acts as a more strongly electron-donating ligand.
As a result, upon coordination to BeII to form 3g, the π → π*
transition is red shifted by 13 nm due to the electron-
withdrawing effect of the BeII cation.
Fluorescence Spectroscopy. The fluorescence spectra of
the BeII complexes (3b, 3e, 3f, and 3g) were recorded by
making the appropriate dilution to the DMF-solvated reaction
mixtures. Spectra of the associated ligands (2b, 2e, 2f, and 2g)
were also recorded by preparing “blank” reaction mixtures
containing no BeSO4(aq) and making the appropriate dilution.
The ligands did not exhibit appreciable fluorescence; see, for
example a comparison of ligand 2g with the beryllium-
coordinated complex analogue 3g (Figure 7). Upon coordina-
tion of BeII, two of the complexes, 3e and 3g, displayed
fluorescence emission (at 425 and 335 nm respectively);
however, no fluorescence was detected for 3b and 3f. This is
consistent with the fact that 3b and 3f can only form the
anticonformer, whereas 3e and 3g are capable of forming either
syn or anti confomers. We propose that when either 3e or 3g is
locked in the syn conformer that two of the phenols of each
complex are rigidly held in a plane, allowing electron density to
be distributed by an extended π system across both of these
phenol rings and BeII (Figure S1.2, Supporting Information).
AUTHOR INFORMATION
Corresponding Author
*Phone: +64 6 356 9099 (ext 7825). Fax: +64 6 350 5682. E-
mail: P.G.Plieger@massey.ac.nz.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
K.J.S. would like to thank the New Zealand Tertiary Education
Commission for a Top Achiever Doctorial Scholarship, the
New Zealand Postgraduate Study Abroad Award, and the
Institute of Fundamental Sciences Postgraduate Travel Fund
for funding to conduct this research. Supported by the Marsden
Fund Council from Government funding, administered by the
Royal Society of New Zealand.
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