Synthesis and structures of acyclic monoanionic tetradentate aza β-diketiminate complexes of magnesium, zinc, and cadmium

Article abstract of DOI:10.1039/b609616f

An acyclic monoanionic tetradentate nitrogen ligand was prepared through the condensation of 2-(4-tolyl)-malondialdehyde and 8-aminoquinoline to give (BDI^QQ)H where (BDI^QQ)H = (8-quinolyl)-NCHC(4-tolyl)CHNH- (8-quinolyl). Metal compl

Full text of DOI:10.1039/b609616f

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis and structures of acyclic monoanionic tetradentate aza
b-diketiminate complexes of magnesium, zinc, and cadmium†
Joseph M. Fritsch,a,b Kristen A. Thoresona and Kristopher McNeill*a
Received 6th July 2006, Accepted 14th August 2006
First published as an Advance Article on the web 30th August 2006
DOI: 10.1039/b609616f
An acyclic monoanionic tetradentate nitrogen ligand was prepared through the condensation of
2-(4-tolyl)-malondialdehyde and 8-aminoquinoline to give (BDIQQ)H where (BDIQQ)H =
(8-quinolyl)-NCHC(4-tolyl)CHNH-(8-quinolyl). Metal complexes, (BDIQQ)MX, were prepared where
MX = MgBr 2, ZnCl 3, and CdOAc 4. The spectroscopic and crystallographic properties of
compounds 2, 3, and 4 were explored. Structures of complexes 2, 3, 4, and the tridentate ligand,
(BDIQ)OH, 5, are reported.
Introduction
Acyclic multidentate ligands containing nitrogen donors have
received increased attention as analogues to Cp ligands1 and
for the preparation of olefin polymerization catalysts.2 Several
monoanionic tridentate ligands and corresponding metal com-
plexes have been prepared.1,3–5 Recently, dianionic tetradentate
benzimidinate,6 bisamidinate,7–9 aminotroponiminates,10 isoindo-
line pincer11 ligands and complexes have been prepared and
characterized. Yet, monoanionic tetradentate nitrogen donor
ligands remain relatively uncommon.
b-Diketiminate ligands represent an important class of chelating
ligands that have been used across many different applications.12,13
Fig. 1 Structure of tetradentate, monoanionic b-diketiminate ligand,
(BDIQQ)−.
The bidentate structure of b-diketiminates has been expanded
to include higher coordination numbers. Monoanionic acyclic
tetradentate b-diketiminate ligands and complexes have been
prepared with pendant donor groups of oxygen (Li, Ca, Mg)14–16
and nitrogen (La, Sc, Yb, Pr).2,17–19 A pendant arm b-diketimine
iron complex, containing pyridyl donors has been synthesized but
the ligand is reportedly neutral.20,21
In 1971, Zatka et al. reported the preparation of a tetradentate
b-diketiminate ligand containing quinoline pendant arms.22 Based
on UV-vis spectroscopy, this ligand was found to form 1 : 1 com-
plexes with Zn and Cd. Subsequent work has further established
the affinity of quinoline ligands for Zn, giving complexes of high
stability and high molar absorptivity values.23–26
In this work we describe the synthesis and characteriza-
tion of a new tetradentate, monoanionic b-diketiminate ligand
containing quinoline pendant donor groups that was prepared
through the condensation of 2-(4-tolyl)-malondialdehyde and 8-
aminoquinoline. This ligand differs from that reported by Zatka
et al. in that it bears a substituent on the central carbon of the
BDI backbone. The new ligand, (BDIQQ)−, is shown in Fig. 1.
Experimental
Chemicals that were purchased commercially and used without
further purification include 2-(4-tolyl)malondialdehyde (Aldrich),
8-aminoquinoline (Aldrich), ethylmagnesium bromide (3 M so-
lution in Et2O, Aldrich), p-toluene sulfonic acid (Aldrich), and
zinc(II) chloride (Mallinckrodt). Cadmium(II) acetate dihydrate
(Aldrich) was dried with azeotropic distillation.
1H NMR spectra were acquired at 500 and 300 MHz, and
13C NMR spectra were acquired at 75 MHz on Varian Inova
Instruments. Positive polarity, electrospray ionization-time of
flight (ESI-TOF) mass spectra were collected on a Bruker BioTOF
II instrument. UV-vis absorbance spectra were collected with a
Jasco V530 spectrophotometer (300–900 nm) with a 4 mL septum-
sealed quartz cuvette.
Preparation of tetradentate b-diketimine ligand
2-(4-Tolyl)-1,3-bis(quinolyl)malondiimine,
(BDIQQ)H,
(1).
aDepartment of Chemistry, University of Minnesota, 207 Pleasant St SE,
Minneapolis, MN, USA. E-mail: mcneill@chem.umn.edu; Fax: +1 612-
626-7541; Tel: +1 612-625-0781
Compound 1 was prepared with two different methods for use as
isolated material (Method A) and for reaction in situ (Method
B) with transition metals. Method A: 8-aminoquinoline (400 mg,
2.81 mmol) and p-toluene sulfonic acid (33 mg, 0.20 mmol)
were dissolved in 10 mL CHCl3, diluted into 125 mL toluene,
and brought to reflux in an azeotropic distillation apparatus.
bNatural Science Division, Pepperdine University, 24255 Pacific Coast High-
way, Malibu, CA, USA. E-mail: joseph.fritsch@pepperdine.edu; Fax: +1-
310-506-4785; Tel: +1–310–506–6705
† Electronic supplementary information (ESI) available: Crystallographic
information for compounds 2, 3, 4, and 5. See DOI: 10.1039/b609616f
4814 | Dalton Trans., 2006, 4814–4820
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
2-(4-Tolyl)malondialdehyde (200 mg, 1.23 mmol) was dissolved in
10 mL CHCl3 and added to the refluxing solution. The reaction
solution was held at reflux for 1 h while H2O and CHCl3 were
removed from the system. The reaction solution was concentrated
at elevated temperature to 60 mL. The reaction mixture was
slowly cooled to −35 ◦C. Orange solids were isolated from the
orange liquid with vacuum filtration. The ligand was neutralized
and extracted with saturated NaHCO3 solution and CHCl3 at
was added to the metal over 1 h with stirring. An immediate
color change from the orange ligand and colorless zinc chloride
solutions to a red metal complex solution was observed. The
solution was stirred overnight. Gold-colored solids were isolated
by vacuum filtration with a glass fiber filter. Compound 3 was
purified with recrystallization from CH2Cl2 and hexanes, 53%
yield. UV-Vis (THF): kmax 559 nm (e = 8600 M−1 cm−1). 1H NMR
(CD2Cl2) d 9.09 (d, J = 4.0 Hz, 2H); 8.79.50 (s, 2H); 8.45 (dd,
J = 0.5, 8.0 Hz, 2H); 7.80 (d, J = 8.0 Hz, 2H); 7.72 (dd, J =
2.0, 5.0 Hz, 2H); 7.68 (t, J = 8.0 Hz, 2H); 7.61 (d, J = 8.0 Hz,
2H); 7.46 (d, J = 8.0 Hz, 2H); 7.25 (d, J = 8.0 Hz, 2H); 2.40 (s,
3H) 13C NMR (CDCl3) d 154.78, 148.04, 144.25, 139.86, 135.47,
130.34, 129.88, 129.14, 127.05, 122.63, 121.74, 112.90, 112.39,
21.26. ESI-TOF MS (MeOH): m/z observed: C28H21N4Zn, M+ =
477.2 m/z. ESI-TOF High resolution mass spectrometry (HRMS)
with polypropylene glycol standard: C28H21N4Zn, M+ = 477.0161
m/z, −1.81 ppm error. X-Ray quality crystals were obtained by
slow evaporation from a CH2Cl2 : IPA (1 : 1) solution.
◦
0 C, and the solvent was removed in vacuo, yield 46%. Method
B: A similar reaction sequence was utilized with the following
changes including 8-aminoquinoline as the limiting reagent.
8-Aminoquinoline (340 mg, 2.40 mmol) and p-toluene sulfonic
acid (33 mg, 0.20 mmol) were dissolved in 10 mL CHCl3, diluted
into 125 mL toluene, and brought to reflux in an azeotropic
distillation apparatus. Then, 2-(4-tolyl)malondialdehyde (200 mg,
1.23 mmol) was dissolved in 10 mL CHCl3 and added to the
refluxing solution. The reaction solution was held at reflux for
1 h while H2O and CHCl3 were removed from the system. The
reaction solution was cooled to room temperature and used
directly to prepare metal complexes. UV-Vis (THF): kmax 429,
2-(4-Tolyl)-1,3-bis(quinolyl)malondiiminate cadmium(II) acetate,
(BDIQQ)Cd(OAc), (4). Under inert atmosphere, (BDIQQ)H (pre-
pared with Method A, 40 mg, 0.10 mmol) was dissolved in toluene.
In a separate vial, Cd(OAc)2 (dried by azeotropic distillation,
30 mg, 0.13 mmol) was dissolved in THF. The ligand was added
dropwise to the metal over 10 min with stirring. An immediate
color change from the orange ligand and colorless cadmium
solutions to a red metal complex solution was observed. A THF
extraction was performed, and the solids were isolated by glass
fiber filter filtration and washed with toluene, yield 11%. UV-Vis
(THF): kmax 382, 455, 556 nm. 1H NMR (CD2Cl2) d 9.18 (1H–1H d,
J = 4.0, 111/113Cd–1H, d, 3J = 1.0 Hz, 2H), 8.80 (111/113Cd–1H, t, 3J =
12.0 Hz, 2H, Cd coupling), 8.39 (dd, J = 8.0, 1.0 Hz, 2H); 7.69 (dd,
J = 8.0, 4.0 Hz, 2H); 7.66 (d, J = 8.0 Hz, 2H); 7.65 (d, J = 5.0 Hz,
2H); 7.56 (d, J = 8.0, 2H); 7.49 (d, J = 8.0, 2H); 7.26 (d, J = 8.0,
2H); 2.42 (s, 3H); 1.79 (s, 3H). 13C NMR (CD2Cl2) d179.72, 156.76,
150.06, 145.36, 140.65, 140.50, 139.53, 135.11, 130.33, 129.76,
128.85, 127.21, 122.42, 121.46, 113.26, 22.22, 21.24. ESI-TOF MS
(MeOH): m/z observed: C28H21N4Cd, M+ = 527.2. ESI-TOF High
resolution mass spectrometry (HRMS) with polypropylene glycol
standard: C28H21N4Cd, M+ = 527.0803 m/z, −1.6 ppm error. X-
Ray quality crystals were obtained by diffusion of hexanes into a
saturated CH2Cl2 solution (hexane : CH2Cl2 (5 : 1)).
1
741 nm. H NMR (d6-DMSO) d 14.00 (t, J = 6.0 Hz, 1H); 8.83
(dd, J = 4.0, 2.0 Hz, 2H); 8.60 (d, J = 6.0 Hz, 2H); 8.38 (dd, J =
8.0, 1.5 Hz, 2H); 7.84 (dd, J = 6.0, 3.0 Hz, 2H); 7,65 (m, 4H);
7.56 (dd, J = 4.0 Hz, 4H); 7.51 (d, J = 8.0 Hz, 2H); 7.22 (d, J =
8.0 Hz, 2H); 2.33 (s, 3H). 13C NMR (CDCl3) d 156.85, 149.98,
138.77, 136.95, 132.15, 130.13, 129.15, 128.81, 128.02, 126.47,
125.49, 122.94, 115.23, 115.10, 21.91. ESI-TOF MS (MeOH):
m/z observed: C28H23N4, [M + H]+ = 415.3 m/z. ESI-TOF High
resolution mass spectrometry (HRMS) with polypropylene glycol
standard: C28H23N4, [M + H]+ = 415.1916 m/z, 0.3 ppm error.
Preparation of tetradentate b-diketiminate metal complexes
2-(4-Tolyl)-1,3-bis(quinolyl)malondiiminate magnesium bromide,
(BDIQQ)MgBr, (2). Under inert atmosphere, (BDIQQ)H (pre-
pared with Method A, 40 mg, 0.10 mmol) was dissolved in toluene.
In a separate vial, EtMgBr (100 lL of a 3 M solution in Et2O,
0.30 mmol) was diluted in toluene. The ligand was added dropwise
to the metal over 10 min with stirring. An immediate color change
from the orange ligand and colorless magnesium solutions to a red
metal complex solution was observed. The solids were isolated by
glass fiber filter filtration, yield 24%. UV-Vis (THF): kmax 443, 549,
738 nm. 1H NMR (CD2Cl2) d 9.11 (dd, J = 5.0, 1.5 Hz, 2H); 8.78
(s, 2H); 8.48 (dd, J = 8.5, 1.5 Hz, 2H); 7.84 (d, J = 8.0 Hz, 2H);
7.73 (dd, J = 8.5, 5.0 Hz, 2H); 7.69 (t, J = 8.0 Hz, 2H); 7.61 (d,
J = 8.0, 2H); 7.46 (d, J = 8.0, 2H); 7.25 (d, J = 8.0, 2H); 2.40 (s,
3H). ESI-TOF MS (MeOH): m/z observed: C28H21N4Mg, M+ =
437.5. ESI-TOF High resolution mass spectrometry (HRMS) with
polypropylene glycol standard: C28H21N4Mg, M+ = 437.1628 m/z,
−3.9 ppm error. X-Ray quality crystals were obtained by diffusion
of hexanes into a saturated CH2Cl2 solution (hexane : CH2Cl2 (5 :
1)) under inert atmosphere.
2-(4-Tolyl)-1-(quinolyl)malonimine-3-aldehyde, (BDIQ)OH, (5).
Compound 5 was isolated from two different recrystallizations
of compound 1 in hexanes : CH2Cl2 solutions. Yellow solids
were isolated from the orange liquid with vacuum filtration. ESI-
TOF MS (MeOH): m/z observed: C19H17N2O1, [M + H]+
=
289.3 m/z. ESI-TOF HRMS with polypropylene glycol standard:
C19H17N2O1, [M + H]+ = 289.1345 m/z, −3.20 ppm error. X-
Ray quality crystals were obtained by diffusion of hexanes into
a saturated CH2Cl2 solution (hexane : CH2Cl2 (7 : 1)) and by
diffusion of hexanes into a saturated CH2Cl2 solution (hexane :
CH2Cl2 (4 : 1)).
2-(4-Tolyl)-1,3-bis(quinolyl)malondiiminate zinc(II) chloride,
(BDIQQ)ZnCl, (3). Compound 3 was prepared in air by adding
(BDIQQ)H (prepared with Method B, 8-aminoquinoline (360 mg,
2.53 mmol), p-toluene sulfonic acid (100 mg, 0.60 mmol), 2-
(4-tolyl)malondialdehyde (215 mg, 1.33 mmol)) to a methanol
solution of ZnCl2 (200 mg, 1.47 mmol). The ligand solution
Crystal structure determinations
Crystal data for compounds 2, 3, 4, 5, and 5ꢀ were collected at
172(2) K using a Siemens SMART Platform CCD diffractometer.
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 4814–4820 | 4815
©

View Article Online
Table 1 X-Ray crystallographic data for (BDIQQ)MgBr, (BDIQQ)ZnCl, (BDIQQ)Cd(OAc)
(BDIQQ)MgBr, 2
(BDIQQ)ZnCl, 3
(BDIQQ)Cd(OAc), 4
(BDIQ)OH, 5
(BDIQ)OH, 5ꢀ
Formula
MW
C28 H21 Br Mg N4
517.71
Monoclinic
P21
6.6796(12)
15.052(3)
11.681(2)
90
91.041(3)
90
1174.2(4)
2
C28 H21 Cl N4 Zn
514.31
Triclinic
P-1
9.1946(13)
10.9583(16)
12.4820(18)
96.338(2)
96.555(2)
113.305(2)
1130.7(3)
2
C30 H24 Cd N4 O2
586.09
Triclinic
P-1
9.032(3)
12.744(4)
14.414(4)
113.725(3)
104.610(3)
95.018(3)
1435.9(7)
2
C19 H16 N2 O
288.34
Monoclinic
P21
11.4241(18)
6.1370(10)
11.8007(18)
90
116.708(2)
90
739.1(2)
2
C19 H16 N2 O
288.34
Triclinic
P-1
6.3158(10)
8.6984(14)
13.841(2)
101.945(2)
91.645(3)
98.646(3)
734.1(2)
2
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Z
Dc/g cm−3
1.464
1.802
1.511
1.230
1.549
0.982
1.296
0.081
1.304
0.082
l/mm−1
F(000)
528
red
4134
3277
0.0513
1.098
0.0809
0.2126
528
Green–gold
3984
3530
0.0264
1.128
0.0375
0.0750
676
Red
5006
4188
0.0311
1.053
0.0466
0.1106
304
Orange
2613
2242
0.0259
1.052
304
Orange
3229
2623
0.0217
0.938
Crystal colour
Ind. Reflns
Obs. Reflns
Rint
GOOF on F2
R1 [I > 2r(I)]
wR2 [I > 2r(I)]
0.0326
0.0740
0.0432
0.1095
Data collection was carried out with Mo Ka radiation (graphite
monochromator) with a frame time of 30 s, 20 s, 90 s, 30 s, and
15 s for 2, 3, 4, 5, and 5ꢀ respectively. Frames were collected
with 0.30◦ steps in x at four different φ settings and a detector
position of −28◦ in 2h. The intensity data were corrected for
adsorption and decay (SADABS).27 The data was integrated
with (SAINT).28 The structures were solved using SHELXS-
97 and refined using SHELXL-97.29 Compound 2, modeled
as an inversion twin, contained a bisecting mirror plane of
pseudosymmetry, requiring the use of 43 restraints to model the
structure. Crystallographic data for compounds 2–5 are given in
Table 1. Additional crystallographic data including tables of bond
lengths and angles are presented in the ESI.†
HRMS, and absorbance spectroscopy. The composition of 1
was confirmed by HRMS ESI-MS with polypropylene glycol
exact mass internal standard. Compound 1 bonded irreversibly
to chromatographic stationary phases and hydrolyzed during
recrystallization. The semi-condensed compound, (BDIQ)OH, was
characterized crystallographically in two different crystal systems,
5 and 5ꢀ. Crystallographic data for these compounds is included
in Table 1. ORTEP drawings are available in ESI.†
CCDC reference numbers 612332–612336
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b609616f
Results and discussion
Compound 1 was prepared by the condensation of 2-(4-
tolyl)malondialdehyde with 8-aminoquinoline and was isolated
in 46% yield (eqn (1)).
Compound 2, (BDIQQ)MgBr, was prepared through the reaction
of 1 and EtMgBr under inert atmosphere and isolated with vacuum
filtration in 24% yield (eqn (2)).
(1)
(2)
Water generated in the reaction was removed by azeotropic
distillation driving the equilibrium towards the products and
avoiding ready hydrolysis of the imines generated in this reaction.
Compound 1 was fully characterized with 1H and 13C NMR,
Compound 2 was characterized by 1H NMR spectroscopy, HRMS
ESI-MS, absorbance spectroscopy, and X-ray crystallography. In
the NMR spectroscopy, the spectrum showed highly resolved
4816 | Dalton Trans., 2006, 4814–4820
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
proton resonances with those closest to the metal appearing
furthest downfield and the tolyl group appearing furthest upfield
among the aromatic protons. Coupling constants were consistent
with the ligand spectrum and the other metal complexes. Dur-
ing ESI-MS characterization, a signal for (C28H21N4)Mg+ was
observed at 437.5 m/z, and its composition was confirmed by
HRMS ESI-MS with polypropylene glycol exact mass internal
standard.
Compound 3, (BDIQQ)ZnCl, was prepared with 1 prepared by
Method B and ZnCl2 in air and was isolated in 53% yield (eqn (3)).
(3)
X-Ray quality crystals were isolated by slow diffusion of hexanes
into a CH2Cl2 solution (Fig. 2). The structure shows tetradentate
equatorial coordination of the Mg center by the ligand with a fifth
apical coordination site occupied by bromine, yielding a distorted
square pyramidal geometry. Unfortunately, the structure is not
of high enough quality to be useful for rigorous bond length
and bond angle comparisons with other structures because the
molecule is bisected by a mirror plane of pseudosymmetry. As a
result, the canted quinolyl rings are reflected by pseudosymmetry
from actual atom positions on one quinolyl ring to locations
where the corresponding mirror image quinolyl atoms would be.
This effect is also observed for the phenyl ring that is neither
perpendicular nor parallel to the mirror of pseudosymmetry. As
a result, the pseudosymmetry contributes to poor refinement
values, large thermal parameters, and the use of many restraints
to achieve planarity for the quinolyl and phenyl rings. We believe
this contributes to the apparent “wagging” of the phenyl ring as
well. See Table 2 for selected bond lengths of interest.
Compound 3 was characterized with 1H and 13C NMR spec-
troscopy, HRMS ESI-MS, absorbance spectroscopy, and X-ray
crystallography. Both 1H and 13C NMR spectroscopy show highly
resolved quinolyl signals consistent with average mirror symmetry
in the complex (Fig. 3). The coupling constants were similar to
those observed in 1. During ESI-MS characterization, a signal
for (C28H21N4)Zn+ was observed at 477.2 m/z and had an isotope
pattern consistent with Zn incorporation. The composition was
confirmed by HRMS ESI-MS with polypropylene glycol internal
exact mass standard. The absorbance spectrum shows an intense
transition at 559 nm (Fig. 4).
Fig. 3 Downfield region of the 1H NMR spectrum of (BDIQQ)ZnCl.
The X-ray crystal structure shows tetradentate equatorial
coordination of the Zn center by (BDIQQ) with a fifth apical
coordination site occupied by chlorine, yielding a distorted square
Fig. 2 ORTEP diagram of (BDIQQ)MgBr, 2, with thermal ellipsoids
drawn at the 50% probability level.
Table 2 Bond lengths, distances, and angles of interest for (BDIQQ)MgBr, (BDIQQ)ZnCl, (BDIQQ)Cd(OAc)
(BDIQQ)MgBr, 2a
(BDIQQ)ZnCl, 3
(BDIQQ)Cd(OAc), 4
b
˚
M ionic radius/A
M–N1/M–N4 (quinolyl)/A
0.71
0.74
0.92
2.143(4)c
2.089(6)c
3.314
2.154(2)/2.161(2)
2.045(2)/2.071(2)
3.335
2.883
0.5202
26.9
2.293(4)/2.315(4)
2.304(4)/2.281(4)
3.808
3.017
0.6761
16.3
˚
˚
M–N2/M–N3 (iminate)/A
˚
N1–N4/A
˚
N2–N3/A
M–N4 plane/A
2.907
0.5445
29.7
˚
Q,Q plane twist/◦
N2–M–N3 angle/◦
88.2
88.9
82.3
a Data for 2 are from a lower quality structure and are shown only for trend comparison purposes. b Reference 30. c Equivalent bonds constrained to be
the same length.
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 4814–4820 | 4817
©

View Article Online
Fig. 4 Absorbance spectrum of (BDIQQ)ZnCl, 3, in THF with absorbance
maximum at 559 nm.
pyramidal geometry (Fig. 5). There is a 27◦ twist angle between
˚
the quinolyl rings and the zinc center is located 0.52 A above the
Fig. 6 Downfield region of the 1H NMR spectra of (BDIQQ)Cd(OAc).
111/113Cd satellites are indicated with *.
N1–N2–N3–N4 plane. See Table 2 for other selected structural
metrics.
was confirmed by HRMS ESI-MS with polypropylene glycol
internal standard.
X-Ray quality crystals were obtained by slow diffusion of
hexanes into a CH2Cl2 solution of 4 (Fig. 7). The structure
shows tetradentate equatorial coordination of the Cd center
by the BDIQQ ligand in addition to an acetate binding in an
asymmetric bidentate manner. The result is a capped square
pyramidal coordination geometry. Compound 4 contains a 16◦
twist angle between the quinolyl rings and the cadmium center is
˚
located 0.67 A above the N1–N2–N3–N4 least squares plane. The
acetate is in an intermediate coordination between unidentate and
˚
symmetric bidentate coordination (Cd–O1 2.276(4) A and Cd–O2
Fig. 5 ORTEP diagram of (BDIQQ)ZnCl, 3, with thermal ellipsoids drawn
at the 50% probability level.
Compound 4, (BDIQQ)Cd(OAc), was prepared through the
reaction of 1 and Cd(OAc)2 under an inert atmosphere and isolated
by vacuum filtration in 11% yield (eqn (4)).
(4)
Compound 4 was characterized with 1H and 13C NMR spec-
troscopy, HRMS ESI-MS, absorbance spectroscopy, and X-ray
1
crystallography. Both H and 13C NMR spectra showed highly
Fig. 7 ORTEP diagram of (BDIQQ)Cd(OAc), 4, with thermal ellipsoids
drawn at the 50% probability level. One molecule of CH2Cl2 in the
asymmetric unit is omitted for clarity.
resolved resonances (Fig. 6). During ESI-MS characterization, a
signal for (C28H21N4)Cd+ was observed at 527.2 m/z and had an
isotope ratio indicative of Cd incorporation, and the composition
4818 | Dalton Trans., 2006, 4814–4820
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
(BQIQQ)ZnCl. While the ionic nitrogen bonds are 0.30 A longer in
˚
˚
2.610(4) A) revealing a weak secondary interaction for Cd–O1.
Further evidence of asymmetric coordination was revealed by
comparison of the O1–Cd–N bond angles (100.7–110.83◦) and
O2–Cd–N bond angles (quinolyl 81.1◦ and 81.2◦ and iminate
132.7◦ and 137.6◦). See Table 2 for selected additional metrics.
The structural features of the reported compounds reflect
variations between central atom ionic radii and metal hardness.
For example, Mg and Zn have very similar ionic radii, but MgII is
a hard metal center while ZnII is soft. In contrast, ZnII and CdII
have different sizes but have similar metal softness.30,31
(8AQ)2Zn(H2O).24
For five-coordinate BDI complexes with pendant donor groups
characterized here and elsewhere, the metal sits above the plane in
a distorted square pyramidal geometry.16 This stands in contrast
to octahedral structures of Sc and Fe with pendant donors. In
these examples, the metal sits in the plane of the equatorial BDI
ligand.18,20
There are several comparisons that can be made between the
Mg, Zn, and Cd structures (Table 2).
Among their similarities, all structures show the BDIQQ ligand
in an equatorial position. It appears that the BDIQQ ligand is
sufficiently rigid as to force equatorial coordination. This is
most clearly seen in the six-coordinate (BDIQQ)Cd(OAc) structure,
which adopts an unusual capped square pyramidal geometry.
However, the ligand is flexible enough to accommodate different
metal center radii as revealed by the N1–N4 interatomic through
Spectroscopy of (BDIQQ)M complexes
The 1H NMR spectroscopy indicates that the solution structures
of the Mg, Zn, and Cd complexes all have average symmetric metal
coordination between the two quinolyl moieties with no apparent
fluctuations (Fig. 3 and 6). These results are consistent with the
solid state structures. An interesting feature of the (BDIQQ)CdOAc
1H NMR spectrum is the 111/113Cd–1H coupling observed in the
˚
space distances for the Zn and Cd structures which is 0.473 A
greater in (BDIQQ)Cd(OAc), resulting from the larger Cd ionic
radius and minimizing quinolyl C–H steric interactions. Varying
ligand pocket size has been recently reported for complexes with
pendant quinolyl donors.4
3
imine protons at 8.8 ppm of J = 12 Hz (resonance g, Fig. 6).
Similar coupling was observed between the cadmium and the a-
protons to the quinolyl nitrogen (9.2 ppm) observed as a doublet
of doublets (resonance a, Fig. 6). The 111Cd and 113Cd satellites
are not resolved from each other in these spectra due to their
very similar gyromagnetic ratios, −5.6926 and −5.9550 × 107 rad
T−1 s−1, respectively.32
Conclusions
This work represents a significant step into the development of
acyclic monoanionic tetradentate nitrogen ligands using the b-
diketiminate platform. We have shown that the ligand can be
readily prepared and metal complexes of Mg, Zn, and Cd can
be prepared. We believe this ligand has promise in modeling
the monoanionic tetraaza core found in the biologically relevant
metal-containing cofactors, vitamin B12 and F430. Experiments
testing this idea are in progress.
The absorbance spectra of the (BDIQQ)M complexes 2, 3, and
4 show many similarities as expected for binding to closed shell
spectroscopically silent metals. This is reflected in the observation
of the absorbance maxima between 549 and 559 nm for the
Mg, Zn, and Cd complexes. The spectroscopic properties are
proposed to be ligand based p → p* transitions. Also, the molar
absorptivities of the metal complexes were quite intense on the
order of 105 M−1 cm−1. Similar molar absorptivity values for bis
and tris 8-aminoquinoline complexes have been observed in this
range.23,26
Acknowledgements
This work was funded in part by the EPA STAR Fellowship
program (U-91610701) and the National Science Foundation
(CHE-0239461). We thank Victor G. Young, Jr. and Ben Kucera
from the X-Ray Crystallographic Laboratory for help solving the
crystal structures.
Structures of (BDIQQ)M complexes
These reported structures join a rare group of acyclic tetradentate
BDI ligands and are especially uncommon among nitrogen only
donors. The structures of 2, 3, and 4 allow the opportunity to
make comparisons to reported structures. A number of bidentate
b-diketiminate structures of Zn and Mg compounds have been
reported and their bond distances can be compared to complexes 2
and 3.33–35 The M–N2 and M–N3 bond distances in (BQIQQ)ZnCl
are slightly longer than the reported bidentate compounds re-
flecting the effect of the quinolyl coordination drawing the metal
away from the iminate nitrogen donors. Accordingly the N2–M–
N3 bite angle is smaller for (BQIQQ)ZnCl (88.9◦) than for the
bidentate compounds (ranging from 94.4 to 97.0◦) due to the
increased M–iminate distance.33–35 These trends are repeated for
the (BQIQQ)Cd(OAc) and (BQI)CdI2Li(OEt2)2 structures where
the bite angles are 82.3◦ and 91.5◦, respectively.34
References
1 K. Kincaid, C. P. Gerlach, G. R. Giesbrecht, J. R. Hagadorn, G. D.
Whitener, A. Shafir and J. Arnold, Organometallics, 1999, 18, 5360.
2 D. Neculai, H. W. Roesky, A. M. Neculai, J. Magull, H.-G. Schmidt
and M. Noltemeyer, J. Organomet. Chem., 2002, 643–644, 47.
3 D. Maiti, H. Paul, N. Chanda, S. Chakraborty, B. Mondal, V. G.
Puranik and G. K. Lahiri, Polyhedron, 2004, 23, 831.
4 J. C. Peters, S. B. Harkins, S. D. Brown and M. W. Day, Inorg. Chem.,
2001, 40, 5083.
5 K. S. Min, T. Weyhermueller and K. Wieghardt, Dalton Trans., 2004,
178.
6 G. D. Whitener, J. R. Hagadorn and J. Arnold, J. Chem. Soc., Dalton
Trans., 1999, 1249.
One structure of particular interest for comparison is
the trans-bis-(8-aminoquinoline)aquo zinc(II) tetrachlorozin-
cate, [(8AQ)2Zn(H2O)][ZnCl4], structure which contains a five-
coordinate zinc ion.24 The coordinate covalent bonds of the
7 J. R. Hagadorn and J. Arnold, Angew. Chem., Int. Ed., 1998, 37, 1729.
8 P. Comba, W. Goll, B. Nuber and K. Varnagy, Eur. J. Inorg. Chem.,
1998, 2041.
9 O. Rotthaus, S. LeRoy, A. Tomas, K. M. Barkigia and I. Artaud,
Eur. J. Inorg. Chem., 2004, 1545.
10 P. W. Roesky, Chem. Soc. Rev., 2000, 29, 335.
˚
(8AQ)2Zn(H2O) quinoline rings are 0.040 A shorter than in
This journal is The Royal Society of Chemistry 2006
Dalton Trans., 2006, 4814–4820 | 4819
©

View Article Online
11 B. L. Dietrich, J. Egbert, A. M. Morris, M. Wicholas, O. P. Anderson
and S. M. Miller, Inorg. Chem., 2005, 44, 6476.
12 L. Bourget-Merle, M. F. Lappert and J. R. Severn, Chem. Rev., 2002,
102, 3031.
23 E. M. Nolan, J. Jaworski, K. Okamoto, Y. Hayashi, M. Sheng and S. J.
Lippard, J. Am. Chem. Soc., 2005, 127, 16812.
24 M. C. Kerr, H. S. Preston, H. L. Ammon, J. E. Huheey and J. M.
Stewart, J. Coord. Chem., 1981, 11, 111.
13 P. B. Hitchcock, J. Hu, M. F. Lappert, M. Layh, D.-S. Liu, J. R. Severn
and T. Shun, Anal. Quim., Int. Ed., 1996, 92, 186.
14 D. T. Carey, E. K. Cope-Eatough, E. Vilaplana-Mafe, F. S. Mair, R. G.
Pritchard, J. E. Warren and R. J. Woods, Dalton Trans., 2003, 1083.
15 M. H. Chisholm, J. C. Gallucci and K. Phomphrai, Inorg. Chem., 2004,
43, 6717.
25 D. A. Pearce, N. Jotterand, I. S. Carrico and B. Imperiali, J. Am. Chem.
Soc., 2001, 123, 5160.
26 A. Furuhashi and H. Yokota, Bull. Chem. Soc. Jpn., 1979, 52, 3753.
27 R. Blessing, Acta Crystallogr., Sect. A, 1995, A51, 33.
28 Bruker Analytical X-ray Systems, SAINT+ V6.45, Madison WI,
2003.
16 M. H. Chisholm, J. C. Gallucci and K. Phomphrai, Inorg. Chem., 2005,
44, 8004.
29 Bruker Analytical X-ray Systems, SHELXTL V6.14, Madison, WI,
2000.
17 D. V. Vitanova, F. Hampel and K. C. Hultzsch, J. Organomet. Chem.,
2005, 690, 5182.
18 A. M. Neculai, H. W. Roesky, D. Neculai and J. Magull,
Organometallics, 2001, 20, 5501.
19 G. B. Nikiforov, H. W. Roesky, D. Vidovic and J. Magull, J. Mol. Struct.,
2003, 656, 155.
20 M. Goto, Y. Ishikawa, T. Ishihara, C. Nakatake, T. Higuchi, H.
Kurosaki and V. L. Goedken, Chem. Commun., 1997, 539.
21 M. Goto, Y. Ishikawa, T. Ishihara, C. Nakatake, T. Higuchi, H.
Kurosaki and V. L. Goedken, J. Chem. Soc., Dalton Trans., 1998, 1213.
22 V. Zatka, J. Holzbecher and D. E. Ryan, Anal. Chim. Acta, 1971, 55,
273.
30 F. A. Cotton, G. Wilkinson, C. A. Murillo and M. Bochmann, Advanced
Inorganic Chemistry, Wiley & Sons, New York, 1999.
31 J. E. Huheey, E. A. Keiter and R. L. Keiter, Inorganic Chemistry: Prin-
ciples of Structure and Reactivity, Harper Collins College Publishers,
New York, 1993.
32 G. H. Fuller, J. Phys. Chem. Ref. Data, 1976, 5, 835.
33 M. H. Chisholm, J. Gallucci and K. Phomphrai, Inorg. Chem., 2002,
41, 2785.
34 J. Prust, K. Most, I. Muller, A. Stasch, H. W. Roesky and I. Uson,
Eur. J. Inorg. Chem., 2001, 1613.
35 J. Prust, A. Stasch, W. Zheng, H. W. Roesky, E. Alexopoulos, I. Uson,
D. Boehler and T. Schuchardt, Organometallics, 2001, 20, 3825.
4820 | Dalton Trans., 2006, 4814–4820
This journal is
The Royal Society of Chemistry 2006
©