Synthesis and reactivity of metal complexes supported by the tetradentate monoanionic ligand bis(2-picolyl)(2-hydroxy-3,5-di-tert-butylbenzyl)amide (BPPA)

Article abstract of DOI:10.1021/ic700775k

Metal-halide complexes of Ti, V, Y, Zr, Al, Ga, and U supported by the tetradentate monoanionic (TDMA) ligand bis(2-picolyl)(2-hydroxy-3,5-di-tert- butylbenzyl)amine, H(BPPA), were synthesized and spectroscopically characterized. In addition, the complexes (BPPA)TiCl₂, (BPPA)VBr ₂, [(BPPA)YCl₂]₂, (BPPA)AlCl₂, (BPPA)GaCl₂, and (BPPA)UI₃ were characterized by single-crystal X-ray crystallography. In all cases the ligand is bound κ⁴ to the metal center. All structurally characterized compounds are monomeric in the solid-state with the exception of [(BPPA)YCl ₂]₂, which exists as a dimer in the solid-state. The metal-alkyl complexes (BPPA)AlMe₂ and (BPPA)Zr(CH₂Ph) ₃ were also synthesized and characterized, and an X-ray structure of (BPPA)Zr(CH₂Ph)₃ was obtained. The transformation of BPPA from a monoanionic to a dianionic ligand via proton abstraction was observed and monitored by NMR spectroscopy.

Full text of DOI:10.1021/ic700775k

Inorg. Chem. 2007, 46, 7199−7209
Synthesis and Reactivity of Metal Complexes Supported by the
Tetradentate Monoanionic Ligand
Bis(2-picolyl)(2-hydroxy-3,5-di-tert-butylbenzyl)amide (BPPA)
Wayne A. Chomitz, Stefan G. Minasian, Andrew D. Sutton, and John Arnold*
Department of Chemistry, UniVersity of California, Berkeley, California 94720-1460
Received April 23, 2007
Metal-halide complexes of Ti, V, Y, Zr, Al, Ga, and U supported by the tetradentate monoanionic (TDMA) ligand
bis(2-picolyl)(2-hydroxy-3,5-di-tert-butylbenzyl)amine, H(BPPA), were synthesized and spectroscopically characterized.
In addition, the complexes (BPPA)TiCl2, (BPPA)VBr2, [(BPPA)YCl2]2, (BPPA)AlCl2, (BPPA)GaCl2, and (BPPA)UI3
were characterized by single-crystal X-ray crystallography. In all cases the ligand is bound κ4 to the metal center.
All structurally characterized compounds are monomeric in the solid-state with the exception of [(BPPA)YCl2]2,
which exists as a dimer in the solid-state. The metal−alkyl complexes (BPPA)AlMe2 and (BPPA)Zr(CH2Ph)3 were
also synthesized and characterized, and an X-ray structure of (BPPA)Zr(CH2Ph)3 was obtained. The transformation
of BPPA from a monoanionic to a dianionic ligand via proton abstraction was observed and monitored by NMR
spectroscopy.
Introduction
prepared, where LX is a bidentate monoanionic ligand and
R is an alkoxy, alkyl, or amido group. A nonexhaustive list
of examples of such bidentate ligands include benzamidi-
nate,11,12 guanidinate,13 and mixed N, O donor sets such as
bis(alkoxydimethylsilyl)amido frameworks14 and bis(oxazo-
linates).15 Alternatively, complexes of the general form
(L2X2)MR, where L2X2 is a dianionic tetradentate ligand have
also been studied. Many examples include bis(phenolates),16-22
linked amidinates,23 and porphyrin rings.24-27
Group 3 and lanthanide complexes supported by cyclo-
pentadienyl (Cp) ligands have been demonstrated as effective
precatalysts for a variety of homogeneous processes.1-5 These
metals typically adopt the +3 oxidation state and are readily
complexed by ligands incorporating hard σ donors such as
oxygen and nitrogen. In addition to having suitable donor
groups, an ideal ligand system from our perspective must
also possess sufficient steric bulk to prevent unwanted
aggregation and solvent ligation. Recent efforts by numerous
research groups focusing on ligand design have sought to
supplant the formerly ubiquitous Cp fragment.6-9 A number
of complexes of the general form (LX)2MR10 have been
Our interest lies in the comparatively understudied com-
plexes of the general form (L3X)MR2, where L3X is a
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* To whom correspondence should be addressed. E-mail: arnold@
berkeley.edu.
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10.1021/ic700775k CCC: $37.00
Published on Web 07/26/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 17, 2007 7199

Chomitz et al.
tetradentate monoanionic (TDMA) ligand.28 Recently we
reported the use of TDMA phenoxytriamine ligands sup-
porting Group 3 metals in lactide polymerization.29 The use
of TDMA ligands opens up the possibility to study com-
plexes of the general form (L3X)MR2. These coordinately
saturated starting materials could provide a synthetic route
to relatively rare trivalent (L3X)MR+ compounds,30-32 thus
allowing the opportunity to compare reactivity to related
(LX)2MR and (L2X2)MR systems. In addition, TDMA ligand
systems could allow for the synthesis of a range of other
trivalent main group and actinide metal complexes.
Our efforts to expand TDMA chemistry using phenoxy-
triamine ligands to other metals was hampered by the
observation of low yields and intractable mixtures. The
modular nature of the TDMA motif allows us to carry out
simple modifications in order to test the resulting steric and
electronic consequences on complex structure and reactivity.
Here we examine the effects of replacing the two diethy-
lamine donors with pyridines. This general ligand framework
has previously been shown to bind to middle and late
transition metals,33-52 although its application to main group
and early transition metals was unknown until recently, when
Bercaw et al. described the synthesis and reactivity of a
number of Group 3 complexes with BPPA.53 Here we report
the synthesis, coordination chemistry, and reactivity of a
series of main group, transition metal, and actinide metal
complexes supported by bis(2-picolyl)(2-hydroxy-3,5-di-tert-
butylbenzyl)amine, H(BPPA).
Experimental Section
Unless otherwise noted, all reactions were performed using
standard Schlenk-line techniques or in an MBraun dry box under
an atmosphere of nitrogen (<1 ppm O2/H2O). All glassware,
cannulae, and Celite were stored in an oven at ca. 425 K. Pentane,
toluene, methylene chloride, diethyl ether, DME, and THF were
purified by passage through a column of activated alumina and
degassed prior to use.54 Pyridine was distilled from CaH2. Deuter-
ated solvents were vacuum-transferred from sodium/benzophenone
(benzene, toluene, and THF) or calcium hydride (chloroform and
pyridine). NMR spectra were recorded at ambient temperature on
Bruker AV-300, AVQ-400, AVB-400, and DRX-500 spectrometers.
1H and 13C{1H} chemical shifts are given relative to residual solvent
peaks, and coupling constants (J) are given in Hz. 27Al NMR
chemical shifts are referenced to an external standard of 1 M Al-
(NO3)3 in H2O/D2O (0.0 ppm). Proton and carbon NMR assign-
1
1
ments were routinely confirmed by H-1H (COSY) or H-13C
(HMQC and HMBC) experiments. Infrared (IR) samples were
prepared as Nujol mulls and were taken between KBr disks.
Magnetic susceptibility (µeff) values were determined using the
solution Evans’ method.55 Melting points were determined using
sealed capillaries prepared under nitrogen and are uncorrected. The
following chemicals were purified prior to use: AlI3 (95%, Aldrich)
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57
was recrystallized from CS2. H[B(C6F5)4‚2Et2O],56 Zr(CH2Ph)4,
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60
prepared using the literature procedures, and unless otherwise noted,
all reagents were acquired from commercial sources and used as
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mined at the College of Chemistry, University of California,
Berkeley. The X-ray structural determination was performed at
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7200 Inorganic Chemistry, Vol. 46, No. 17, 2007

Synthesis and ReactiWity of Metal Complexes Supported by BPPA
(22 °C) under N2. A three-electrode setup was implemented with
a glassy carbon working electrode, platinum auxiliary electrode,
and a Ag/AgCl reference electrode. All potentials are reported
relative to the Fc/Fc+ couple as an internal standard.
then added until a pH of 11 was reached. The mixture was extracted
with methylene chloride (6 × 100 mL), dried over sodium sulfate,
and then filtered. Removal of volatiles afforded 17.1 g (89% yield)
of analytically pure di-(2-picolyl)amine (DPA) as a brown oil.
A flask containing 17.1 g (86.0 mmol) of DPA and 2.71 g
(86.0 mmol) of paraformaldehyde was heated at 80 °C for 1 h. To
the resulting paste was added 2,4-di-tert-butylphenol (17.7 g,
86.0 mmol) and 60 mL of methanol, and the reaction mixture was
heated at reflux for 24 h. The solvent was removed under vacuum,
leaving a brown oil. Analytically pure product was collected as a
yellow solid (27.9 g, 77.6% yield) after crystallization from pentane
at -40 °C. 1H NMR (300 MHz, C6D6): δ 1.36 (s, 9 H, t-Bu); 1.77
(s, 9 H, t-Bu); 3.67 (s, 2 H, -CH2-); 3.81 (s, 4 H, -CH2-); 6.53
(m, 2 H, py-H); 6.97 (d, JHH ) 6.3 Hz, 2 H, py-H); 7.01 (m, 2 H,
py-H); 8.42 (d, JHH ) 4.8 Hz, 2 H, py-H).
Crystallographic Analysis. Single crystals of 4-6 and 12-14
were coated in Paratone-N oil, mounted on a Kaptan loop,
transferred to a Siemens SMART diffractometer or a Bruker APEX
CCD area detector,61 centered in the beam, and cooled by a nitrogen
flow low-temperature apparatus that has been previously calibrated
by a thermocouple placed at the same position as the crystal.
Preliminary orientation matrices and cell constants were determined
by collection of 60 30 s frames, followed by spot integration and
least-squares refinement. An arbitrary hemisphere of data was
collected, and the raw data were integrated using SAINT.62 Cell
dimensions reported were calculated from all reflections with I >
10σ. The data were corrected for Lorentz and polarization effects,
but no correction for crystal decay was applied. Data were analyzed
for agreement and possible absorption using XPREP.63 An empirical
absorption correction based on comparison of redundant and
equivalent reflections was applied using SADABS.64 The structure
was solved and refined with the teXsan software package.65 Thermal
parameters for all non-hydrogen atoms were refined anisotropically,
except in solvent molecules disordered over multiple partially
occupied positions. ORTEP diagrams were created using the
ORTEP-3 software package.66
K(BPPA) (1). A solution of H(BPPA) (7.0 g, 17 mmol) in
50 mL of THF was added to a suspension of potassium hydride
(1.35 g, 33.6 mmol) in 70 mL of THF at 0 °C. The reaction mixture
was allowed to warm to room temperature and was stirred for 12
h. After removal of volatiles under vacuum, the crude material was
extracted with toluene and filtered through a bed of Celite. The
solvent was removed under vacuum, and the crude product was
washed with cold pentane (2 × 30 mL) leaving analytically pure
product as an off-white solid (7.1 g, 93%). Colorless crystals were
1
grown from toluene at -40 °C overnight. H NMR (400 MHz,
C6D6): δ 1.63 (s, 9 H, t-Bu); 1.82 (s, 9 H, t-Bu); 3.58 (s, 4 H,
Single-crystal diffraction data for 10 were collected using
synchrotron radiation at the Advanced Light Source at Lawrence
Berkeley National Laboratory. A crystal was coated in Paratone-N
oil and mounted on a Kaptan loop, transferred to a Bruker D8
controller, and cooled in a dinitrogen stream. Data were collected
using a Bruker Platinum 200 with an APEX2 v1.0-27 detector.
Initial lattice parameters were obtained from a least-squares analysis
of more than 30 centered reflections; these parameters were later
refined against all data. A full hemisphere of data was collected.
Data were integrated using Bruker SAINT version 7.06 and
corrected for Lorentz and polarization effects using SADABS. Space
group assignments were made on the basis of systematic absences,
E statistics, and successful refinement of the structures. Structures
were solved by direct methods with the aid of successive difference
Fourier maps and were refined against all data using the SHELXTL
5.0 software package. Thermal parameters for all non-hydrogen
atoms were refined anisotropically, except in solvent molecules
disordered over multiple partially occupied positions. A summary
of the X-ray diffraction data is presented in the Supporting
Information.
H(BPPA). A modified literature procedure was used.37 To a
suspension of 2-pyridinecarboxaldehyde (10.4 g, 96.7 mmol) in 200
mL of absolute ethanol was added a solution of 2-(aminomethyl)-
pyridine (10.5 g, 97.1 mmol) in 200 mL of absolute ethanol
dropwise at 0 °C. After 4 h, the solution was cooled to 0 °C and
sodium borohydride (7.26 g, 192 mmol) was added in small
portions. After the reaction was stirred for 12 h at room temperature,
240 mL of aqueous hydrochloric acid (5 M) was added slowly and
stirred for 1 h. A 2 M aqueous solution of sodium hydroxide was
-CH2-); 3.93 (s, 2 H, -CH2-); 6.30 (m, 2 H, py-H); 6.55 (d, JHH
7.8 Hz, 2 H, py-H); 6.78 (m, 2 H, py-H); 7.35 (d, JHH ) 2.4 Hz,
1 H, Ar-H); 7.62 (d, JHH ) 2.4 Hz, 1 H, Ar-H); 8.26 (d, JHH
)
)
4.2 Hz, 1 H, py-H). IR (cm-1): 1593 (s); 1570 (m); 1323 (s); 1260
(w); 1234 (w); 1200 (w); 1155 (w); 1125 (w); 1073 (s); 1000 (m);
967 (w); 952 (w); 905 (w); 875 (m); 828 (m); 791 (w); 765 (s);
694 (w); 672 (w); 624 (m); 500 (m). Anal. Calcd for C27H34N3-
OK: C, 71.15; H, 7.53; N, 9.22. Found: C, 71.60; H, 7.92; N,
8.63. mp ) 150-151 °C.
Li(BPPA) (2). To a solution of H(BPPA) (1.0 g, 2.4 mmol) in
40 mL of pentane at 0 °C was added a solution of lithium
hexamethyldisilazide (0.39 g, 2.4 mmol) in 20 mL of pentane. The
reaction mixture was allowed to warm to room temperature and
was stirred for 4 h. The solvent was removed under vacuum, and
the crude product was washed with pentane. Colorless crystals were
grown from toluene at -40 °C overnight (0.53 mg, 52% yield). 1H
NMR (400 MHz, C6D6): δ 1.43 (s, 9 H, t-Bu); 1.81 (s, 9 H, t-Bu);
3.17 (s, 1 H, -CH2-); 3.18 (s, 1 H, -CH2-); 3.77 (s, 2 H, -CH2-
); 4.41 (s, 1 H, -CH2-); 4.71 (s, 1 H, -CH2-); 6.15 (s, 1 H,
py-H); 6.36 (s, 2 H, py-H); 6.72 (s, 1 H, py-H); 6.84 (s, 1 H, py-
H); 6.92 (d, JHH ) 2.4 Hz, 1 H, Ar-H); 7.26 (d, JHH ) 2.4 Hz, 1
H, Ar-H); 8.30 (s, 1 H, py-H); 8.59 (s, 1 H, py-H). IR (cm-1):
1601 (m); 1318 (m); 1299 (w); 1262 (w); 1235 (w); 1199 (w);
1050 (w); 1008 (w); 949 (w); 870 (m); 834 (m); 806 (w); 749 (m);
576 (m); 542 (m). Anal. calcd for C27H34N3OLi: C, 76.55; H, 8.11;
N, 9.92. Found: C, 76.24; H, 8.39; N, 9.71. mp (dec) ) 210 °C.
Na(BPPA) (3). A solution of H(BPPA) (1.0 g, 2.4 mmol) in
10 mL of THF was added to a suspension of sodium hydride
(0.15 g, 6.0 mmol) in 30 mL of THF at 0 °C. The reaction mixture
was allowed to stir for 4 h at room temperature. The solvent was
removed under vacuum, and the crude material was washed with
pentane. Colorless crystals were grown from toluene at -40 °C
(61) SMART: Area-Detector Software Package; Bruker Analytical X-ray
Systems, Inc.: Madison, WI, 2001-2003.
(62) SAINT: SAX Area-Detector Integration Program, V6.40; Bruker
Analytical X-ray Systems Inc.: Madison, WI, 2003.
(63) XPREP; Bruker Analytical X-ray Systems Inc.: Madison, WI, 2003.
(64) SADABS: Bruker-Nonius Area Detector Scaling and Absorption V.
2.05; Bruker Analytical X-ray Systems, Inc.: Madison, WI, 2003.
(65) teXsan: Cystal Structure Analysis Package; Molecular Structure
Corporation: The Woodlands, TX, 1992.
1
overnight (460 mg, 47% yield). H NMR (400 MHz, C6D6): δ
1.55 (s, 9 H, t-Bu); 1.90 (s, 9 H, t-Bu); 3.21 (s, 3 H, -CH2-);
3.60 (s, 2 H, -CH2-); 4.72 (s, 1 H, -CH2-); 6.35 (s, 2 H, py-H);
6.41 (s, 2 H, py-H); 6.79 (m, 2 H, py-H); 7.11 (d, JHH ) 2.4 Hz,
1 H, Ar-H); 7.41 (d, JHH ) 2.4 Hz, 1 H, Ar-H); 8.32 (br s, 2 H,
(66) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.
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Chomitz et al.
py-H). IR (cm-1): 1594 (s); 1571 (m); 1358 (w); 1328 (m); 1300
(m); 1258 (w); 1232 (w); 1197 (w); 1148 (m); 1131 (m); 1049
(w); 1003 (m); 988 (w); 937 (w); 871 (m); 830 (m); 804 (w); 766
(s); 745 (s); 636 (m); 502 (m). Anal. Calcd for C27H34N3ONa: C,
73.76; H, 7.81; N, 9.56. Found: C, 73.30; H, 7.82; N, 9.39. mp )
235-237 °C.
3.68 (s, 2 H, -CH2-); 4.26 (d, JHH ) 15 Hz, 2 H, -CH2-); 5.50
(d, JHH ) 15.3 Hz, 2 H, -CH2-); 6.71 (d, JHH ) 2.1 Hz, 1 H,
Ar-H); 7.14 (d, JHH ) 2.4 Hz, 1 H, Ar-H); 7.17 (m, 2 H, py-H);
7.35 (d, JHH ) 7.8 Hz, 2 H, py-H); 7.68 (m, 2 H, py-H); 10.27 (d,
JHH ) 5.1 Hz, 2 H, py-H). 13C{1H} NMR (125 MHz, C5D5N): δ
17.06, 31.07, 31.99, 34.62, 35.56, 62.98, 65.76, 104.23, 117.02,
125.16, 140.58, 142.94, 153.75, 158.06, 158.39. IR (cm-1): 1608
(s); 1572 (m); 1533 (w); 1360 (m); 1293 (m); 1255 (s); 1204 (w);
1158 (m); 1129 (w); 1092 (m); 1060 (m); 1013 (s); 978 (w); 915
(m); 847 (s); 812 (w); 760 (s); 732 (s); 682 (w); 639 (m); 603 (m);
552 (m); 471 (m). Anal. Calcd for C27H34N3OZrCl3: C, 52.79; H,
5.59; N, 6.84. Found: C, 52.80; H, 5.48; N, 6.97. mp (dec) )
170 °C
(BPPA)AlMe2 (8). To a solution of H(BPPA) (1.0 g, 2.4 mmol)
in 5 mL of toluene at -78 °C was added AlMe3 (1.2 mL, 2.0 M in
hexanes, 2.4 mmol) by syringe, and the reaction mixture was stirred
for 6 h at 0 °C. After evaporation of volatiles under vacuum and
trituration with pentane, the product was obtained as an off-white
solid (2.1 g, 92%). The crude product was of sufficient purity for
subsequent reactions; however, it must be stored as a solid below
0 °C to avoid thermal decomposition. 1H NMR (500 MHz, C6D6):
δ -0.11 (s, 6 H, AlMe2), 1.44 (s, 9 H, t-Bu), 1.91 (s, 9 H, t-Bu),
(BPPA)TiCl2 (4). To a suspension of TiCl3 (0.51 g, 3.3 mmol)
in 30 mL of THF was added a solution of K(BPPA) (1.5 g,
3.3 mmol) in 20 mL of THF at room temperature. The dark red
reaction mixture was allowed to stir for 12 h. The solvent was
removed under vacuum, and the crude product was washed with
diethyl ether. The resultant solid was extracted with methylene
chloride (50 mL) and filtered through a bed of Celite. Concentration
and cooling to -40 °C led to the formation of red, blade-like
crystals overnight (0.97 g, 55% yield). IR (cm-1): 1607 (s); 1570
(w); 1338 (w); 1304 (m); 1269 (s); 1239 (m); 1204 (w); 1171 (m);
1131 (w); 1096 (m); 1054 (m); 1023 (m); 981 (w); 916 (w); 856
(s); 762 (s); 648 (m); 606 (m); 576 (s); 517 (w); 482 (m). Anal.
Calcd for C27H34N3OTiCl2: C, 60.56; H, 6.41; N, 7.85. Found: C,
60.28; H, 6.47; N, 7.65. mp ) 304-306 °C. µeff ) 1.71 µB.
(BPPA)VBr2 (5). To a slurry of VBr3(THF)3 (0.60 g, 1.2 mmol)
in 20 mL of THF was added a solution of K(BPPA) (0.54 g,
1.2 mmol) in 10 mL of THF at room temperature. The dark purple-
red reaction mixture was stirred for 12 h. The volatiles were
removed under vacuum, and the crude product was washed with
diethyl ether and subsequently extracted with methylene chloride
(50 mL). The solution was filtered through a bed of Celite and
concentrated. Red blade-like crystals were obtained after cooling
to -40 °C overnight (0.53 g, 71% yield). IR (cm-1): 1608 (s);
1358 (m); 1308 (w); 1259 (s); 1238 (m); 1206 (w); 1169 (m); 1110
(m); 1086 (m); 1055 (w); 1022 (s); 934 (w); 837 (m); 809 (w);
762 (m); 723 (w); 650 (m); 611 (m); 575 (w). Anal. Calcd for
C27H34N3OVBr2: C, 51.69; H, 5.47; N, 6.70. Found: C, 51.30; H,
5.36; N, 6.59. mp (dec) ) 190 °C. µeff ) 2.67 µB.
[(BPPA)YCl2]2 (6). To a suspension of YCl3 (0.37 g, 1.9 mmol)
in 40 mL of THF was added a solution of K(BPPA) (0.87 g,
1.9 mmol) in 35 mL of THF at room temperature. After stirring
the yellow reaction mixture for 24 h, the solvent was removed under
vacuum. The resultant solid was washed with diethyl ether before
extraction with methylene chloride and filtration through Celite.
The product was isolated as colorless crystals from THF after
storage at -40 °C overnight (0.67 g, 61% yield). 1H NMR
(400 MHz, CDCl3): δ 1.17 (s, 9 H, t-Bu); 1.41 (s, 9 H, t-Bu); 3.71
(s, 2 H, -CH2-); 4.02 (d, JHH ) 13.5 Hz, 2 H, -CH2-); 4.88 (d,
JHH ) 13.5 Hz, 2 H, -CH2-); 6.61 (s, 1 H, Ar-H); 6.70 (s, 1 H,
Ar-H); 7.07 (m, 2 H, py-H); 7.19 (m, 2 H, py-H); 7.58 (m, 2 H,
py-H); 9.50 (s, 2 H, py-H). 13C{1H} NMR (125 MHz, C5D5N): δ
25.10, 29.57, 31.41, 33.26, 33.99, 61.89, 67.12, 125.01, 127.96,
128.70, 136.08, 138.10, 157.43, 160.87. IR (cm-1): 1605 (s); 1570
(m); 1299 (s); 1238 (w); 1203 (w); 1167 (w); 1131 (w); 1099 (m);
1056 (w); 1013 (m); 838 (m); 805 (w); 758 (m); 636 (w); 533 (w).
Anal. Calcd for C27H34N3OYCl2: C, 56.25; H, 5.96; N, 7.29.
Found: C, 55.93; H, 5.84; N, 7.17. mp (dec) ) 278-279 °C.
(BPPA)ZrCl3 (7). To a suspension of ZrCl4 (1.2 g, 3.3 mmol)
in 40 mL of THF was added a solution of K(BPPA) (1.5 g,
3.3 mmol) in 20 mL of THF at -70 °C. The yellow reaction mixture
was stirred for 12 h at room temperature. After removal of volatiles
under vacuum, the crude product was washed with diethyl ether,
extracted with methylene chloride, and filtered through a bed of
Celite. Removal of solvent under vacuum left analytically pure
compound (1.3 g, 64% yield). Colorless crystals of 7 were grown
from a concentrated pyridine solution at -15 °C overnight. 1H NMR
(400 MHz, C5D5N): δ 1.18 (s, 9 H, t-Bu); 1.68 (s, 9 H, t-Bu);
3.62 (s, 2 H, -CH2-), 3.90 (br s, 4 H, -CH2-), 6.54 (d, JHH
7.5 Hz, 2 H, py-H), 6.54 (m, 3 H, Ar-H + py-H), 6.90 (dt, JHH
)
)
7.6 Hz, JHH ) 1.5 Hz, 2 H, py-H), 7.63 (d, JHH ) 2.5 Hz, 1 H,
Ar-H), 8.34 (d, JHH ) 4.5 Hz, 2 H, py-H). 13C{1H} NMR
(125 MHz, C6D6): δ 30.37, 32.26, 34.18, 35.58, 54.88, 59.46,
119.60, 122.95, 124.04, 124.57, 125.12, 135.74, 136.66, 137.61,
147.89, 153.69, 160.16. 27Al NMR (500 MHz, CDCl3): no signal
detected.
(BPPA)AlI2 (9). Method A. To a solution of freshly prepared 8
(17.0 g, 35.9 mmol) in 70 mL of toluene was added a solution of
I2 (18.2 g, 71.8 mmol) in 150 mL of toluene dropwise Via cannula
at -78 °C. The bright purple solution was stirred for 16 h.
Following filtration and crystallization from methylene chloride
(200 mL), the product was obtained as light orange plates (8.07 g,
32.2%).
Method B. To a suspenstion of AlI3 (0.893 g, 0.219 mmol) in
30 mL of toluene was added a solution of K(BPPA) (0.100 g,
0.219 mmol) in 20 mL of toluene at -78 °C. The reaction mixture
was stirred for 16 h, forming a thick white suspension. Following
removal of solvent under vacuum, the product was extracted with
methylene chloride (75 mL) and the solvent removed under vacuum
to yield the crude product. (0.106 g, 69.6%). 1H NMR (500 MHz,
CDCl3): δ 1.18 (s, 9 H, t-Bu), 1.32 (s, 9 H, t-Bu), 4.07 (s, 2 H,
-CH2-), 4.74 (d, JHH ) 16.5 Hz, 2 H, -CH2-), 5.24 (br s, 2 H,
-CH2-), 6.87 (s, 1 H, Ar-H), 7.11 (s, 1 H, Ar-H), 7.69 (t, JHH
) 6.5 Hz, 2 H, py-H), 7.81 (s, 2 H, Ar-H), 8.18 (s, 2 H, py-H),
9.78 (s, 2 H, py-H). 13C{1H} NMR (125 MHz, CDCl3): δ 29.76.
31.72, 34.25, 34.91, 53.62, 58.02, 60.69, 118.94, 124.30, 125.23,
125.29, 138.59, 141.13, 144.05, 150.10, 154.43. 27Al NMR (500
MHz, CDCl3): no signal detected. IR (cm-1): 1615 (m), 1569 (m),
1390 (m), 1360 (m), 1320 (m), 1300 (m), 1285 (m), 1267 (m),
1239 (m), 1204 (w), 1172 (w), 1158 (w), 1131 (w), 1118 (w), 1103
(w), 1059 (s), 1029 (m), 959 (w), 861 (s), 836 (s), 813 (w), 767
(s), 749 (m), 727 (m), 661 (m), 618 (m), 577 (m), 566 (m), 489
(w). Anal. Calcd for C27H34ON3AlI2‚CH2Cl2: C, 42.99; H, 4.64;
N, 5.37. Found: C, 43.33; H, 4.58; N, 5.38. mp (dec) ) 145 °C.
(BPPA)AlCl2 (10). To a suspension of AlCl3 (1.46 g, 11.0 mmol)
in 50 mL of toluene was added a solution of K(BPPA) (5.00 g,
11.0 mmol) in 50 mL of toluene at -78 °C. The reaction mixture
was stirred for 16 h, forming a thick white suspension. Following
removal of solvent under vacuum, the product was extracted with
7202 Inorganic Chemistry, Vol. 46, No. 17, 2007

Synthesis and ReactiWity of Metal Complexes Supported by BPPA
methylene chloride (200 mL), filtered through Celite, and stored
resultant dark green solution was filtered and concentrated affording
emerald green, X-ray quality crystals of 13 after storage for 24 h
at -40 °C (0.181 g, 18%). 1H NMR (300 MHz, CDCl3): δ -22.1
(br, s); -14.9 (br, s); -9.5 (br, s); -4.4 (br, s); 12.8 (br, s); 24.3
(br, s); 32.1 (br, s); 46.8 (br, s); 50.7 (br, s). IR (cm-1): 1603 (s),
1323 (m), 1302 (m), 1261 (m), 1224 (m), 1201 (m), 1167 (m),
1126 (m), 1100 (m), 1084 (m), 1056 (m), 1012 (s), 967 (w) 947
(w), 915 (m), 847 (s), 831 (s), 809 (s), 751 (m), 724 (s), 669 (m),
638 (m), 595 (s), 464 (w). Calc for C27H34I3N3O1U1: C, 31.32; H,
3.31; N, 4.06. Found: C, 31.20; H, 3.64; N, 3.83%. mp ) 176-
179 °C.
at -10 °C for 3 days, resulting in the formation of colorless needles
1
(2.35 g, 41.6%). H NMR (500 MHz, CDCl3): δ 1.14 (s, 9 H,
t-Bu), 1.28 (s, 9 H, t-Bu), 3.90 (br s, 2 H, -CH2-), 4.46 (br s, 2
H, -CH2-), 4.75 (d, 2 H, -CH2-), 6.69 (s, 1 H, Ar-H), 6.91 (d,
JHH ) 2.0 Hz, 1 H, Ar-H), 7.18 (br s, 2 H, py-H), 7.32 (m, 2 H,
py-H), 7.71 (s, 2 H, py-H), 9.36 (d, JHH ) 5 Hz, 2 H, py-H). 13C-
{1H} NMR (125 MHz, CDCl3): δ 30.19, 31.91, 33.98, 34.92, 53.64,
63.51, 120.07, 124.33, 137.59, 140.13, 147.72, 152.73. 27Al NMR
(500 MHz, CDCl3): δ 31.5. IR (cm-1): 1612 (m), 1572 (w), 1481
(s), 1459 (m), 1423 (w), 1306 (m), 1284 (m), 1239 (w), 1203 (w),
1155 (w), 1090 (w), 1057 (w), 1027 (w), 930 (w), 878 (m), 842
(m), 777 (m), 760 (m), 737 (m), 647 (m), 612 (w), 576 (w), 559
(w), 521 (w), 494 (w). Anal. Calcd for C27H34ON3AlCl2‚CH2Cl2:
C, 56.11; H, 6.05; N, 7.01. Found: C, 57.03; H, 6.11; N, 7.06. mp
(dec) ) 221 °C.
(BPPA)Zr(CH2Ph)3 (14). To a solution of Zr(CH2Ph)4 (1.51 g,
3.32 mmol) in 50 mL of diethyl ether was added a solution of
H(BPPA) (1.39 g, 3.32 mmol) in 50 mL of diethyl ether at
-40 °C. After 3 h, the cold bath was removed, and the solvent
was removed under vacuum leaving a bright orange powder that
was extracted with DME ensuring the temperature was maintained
below 0 °C. The solution was quickly filtered and concentrated,
yielding large orange parallelograms (0.96 g, 36%) after cooling
(BPPA)AlBr2 (11). 11 was prepared in an analogous manner to
10 with AlBr3 (2.93 g, 11.0 mmol). The product was obtained as
colorless needles after crystallization from methylene chloride
1
1
overnight at -40 °C. H NMR (300 MHz, THF-d8): δ 1.25 (s, 9
(2.34 g, 35.4%). H NMR (500 MHz, CDCl3): δ 1.12 (s, 9 H,
H, t-Bu); 1.55 (s, 9 H, t-Bu); 2.60 (br s, 6 H, CH2Ph); 3.11 (s, 2 H,
t-Bu), 1.30 (s, 9 H, t-Bu), 3.94 (s, 2 H, -CH2-), 4.51 (d, JHH
)
CH2); 3.15 (d, JHH ) 15.9 Hz, 2 H, -CH2-); 3.35 (d, JHH
)
15.5 Hz, 2 H, -CH2-), 5.06 (d, JHH ) 15.5 Hz, 2 H, -CH2-),
6.72 (d, JHH ) 2.5 Hz, 1 H, Ar-H), 6.92 (d, JHH ) 2.5 Hz, 1 H,
16.2 Hz, 2 H, -CH2-); 6.66 (d, JHH ) 2.4 Hz, 1 H, Ar-H); 6.92
(d, JHH ) 7.8 Hz, 2 H, py-H); 7.06 (d, JHH ) 2.4 Hz, 1 H, Ar-H);
7.41 (m, 2 H, py-H); 7.67 (m, 2 H, py-H); 9.00 (d, JHH ) 5.1 Hz,
2 H, py-H). 13C{1H} NMR (125 MHz, THF-d8): δ 29.43, 29.54,
30.13, 30.97, 31.11, 33.51, 34.56, 12208, 123.02, 123.35, 123.59,
124.36, 124.99, 126.26, 126.54, 127.34, 127.86, 128.08, 128.63,
134.84, 136.60, 138.16, 149.11, 156.68. IR (cm-1): 1590 (s); 1414
(w); 1304 (m); 1273 (m); 1239 (w); 1205 (s); 1173 (m); 1090 (m);
1009 (m); 976 (s); 842 (m); 800 (m); 745 (s); 699 (s); 612 (w);
543 (m). mp (dec) ) 180-185 °C.
Ar-H), 7.31 (m, 4 H, py-H), 7.74 (m, 2 H, py-H), 9.48 (d, JHH
)
5.0 Hz, 2 H, py-H). 13C{1H} NMR (125 MHz, CDCl3): δ 30.24,
31.83, 33.98, 34.84, 53.63, 60.94, 63.11, 120.09, 122.73, 123.38,
124.26, 124.50, 128.39, 129.19, 137.66, 138.41, 140.79, 147.61,
152.97, 158.43. 27Al NMR (500 MHz, CDCl3): δ 29.2. IR (cm-1):
1613 (s), 1571 (m), 1377 (m), 1361 (m), 1295 (m), 1275 (m),
1238 (m), 1204 (w), 1169 (w), 1157 (m), 1133 (m), 1088 (m), 1057
(m), 1027 (m), 981 (w), 929 (w), 913 (w), 879 (m), 840 (s), 810
(w), 777 (s), 735 (s), 657 (m), 612 (m), 577 (m), 561 (m), 521 (w),
495 (w), 468 (w), 449 (w). Anal. Calcd for C27H34ON3AlBr2‚0.5CH2-
Cl2: C, 51.14; H, 5.46; N, 6.51. Found: C, 50.68; H, 5.29; N,
6.59. mp (dec) ) 220 °C.
[(BPPA)Zr(CH2Ph)2][B(C6F5)4] (16). To an NMR tube contain-
ing (BPPA)Zr(CH2Ph)3 (0.010 g, 0.013 mmol) and H[B(C6F5)4‚
2Et2O] (0.011 g, 0.013 mmol) was added C6D5Cl. The resulting
1
yellow-orange solution was analyzed by NMR spectroscopy. H
(BPPA)GaCl2 (12). To a suspension of GaCl3 (2.96 g,
16.8 mmol) in 100 mL of toluene (100 mL) at -78 °C was added
a solution of K(BPPA) (7.65 g, 16.8 mmol) in 200 mL of toluene.
The solution was then allowed to warm to room temperature and
was stirred for 16 h, forming a thick white suspension. Following
evaporation of volatiles under vacuum, the residue was triturated
with pentane, extracted with methylene chloride (200 mL), and
filtered through Celite. Colorless needles formed after storage at
-10 °C for 7 days (1.59 g, 17.2%). 1H NMR (500 MHz, CDCl3):
δ 1.11 (s, 9 H, t-Bu), 1.32 (s, 9 H, t-Bu), 3.83 (br s, 2 H, -CH2-),
4.36 (br s, 2 H, -CH2-), 4.84 (br s, 2 H, -CH2-), 6.64 (s, 1 H,
Ar-H), 6.87 (s, 1 H, Ar-H), 7.19 (br s, 2 H, py-H), 7.40 (br s,
2 H, py-H), 7.77 (s, 2 H, py-H), 9.22 (s, 2 H, py-H). 13C{1H} NMR
(125 MHz, CDCl3): δ 30.31, 31.92, 33.96, 35.04, 53.64, 59.77,
66.46, 119.85, 122.77, 123.77, 124.55, 124.74, 137.70, 138.21,
140.47, 146.75, 151.26, 160.86. IR (cm-1): 1608 (m), 1574 (w),
1422 (s), 1377 (m), 1361 (m), 1394 (m), 1279 (s), 1238 (m), 1203
(w), 1167 (w), 1155 (w), 1096 (w), 1054 (m), 1023 (w), 981 (w),
929 (m), 878 (w), 833 (m), 776 (s), 759 (m), 736 (s), 646 (w), 536
(w), 512 (w), 430 (w). Anal. Calcd for C27H34ON3GaCl2‚CH2Cl2:
C, 52.37; H, 5.65; N, 6.54. Found: C, 53.04; H, 5.53; N, 6.57. mp
(dec) ) 259 °C.
NMR (400 MHz, C6D5Cl): δ 1.21 (s, 9 H, t-Bu); 1.33 (s, 9 H,
t-Bu); 3.07 (br s, 2 H, -CH2-); 3.42 (d, JHH ) 16.4 Hz, 2 H,
-CH2-); 3.90 (d, JHH ) 16.4 Hz, 2 H, -CH2-); 6.59 (d, JHH
)
7.6 Hz, 2 H, py-H); 6.68 (br s, 2 H, py-H); 6.76 (m, 2 H, py-H);
7.25 (d, JHH ) 2.4 Hz, 1 H, Ar-H); 8.26 (d, JHH ) 5.2 Hz, 2 H,
py-H). 19F NMR (400 MHz, C6D5Cl): δ -130.92 (s); -161.18 (t,
JFF ) 24 Hz); -165.12 (m).
Results and Discussion
Ligand Synthesis and Characterization. H(BPPA) was
synthesized by a two-step reaction. A solution of 2-(ami-
nomethyl)pyridine was treated with 2-pyridinecarboxalde-
hyde in ethanol followed by reduction with NaBH4. This
simple procedure afforded DPA in high yield. Subsequent
reaction of DPA with paraformaldehyde, followed by 2,4-
di-tert-butylphenol in refluxing methanol for 24 h yielded
H(BPPA), which was obtained in high yield as a crystalline
solid.
The reaction of H(BPPA) with excess KH resulted in the
isolation of the alkali metal salt 1 in high yield after filtration
and evaporation of solvent under vacuum. The salt can be
used without further purification or can be crystallized from
toluene at -40 °C. The Li (2) and Na (3) analogues were
synthesized through the reaction of H(BPPA) with LiN-
(SiMe3)2 and NaH, respectively. Both were crystallized from
toluene at -40 °C as colorless crystals in good yield.
(BPPA)UI3 (13). To a solution of UI3(THF)4 (0.903 g, 1.0 mmol)
in 25 mL of diethyl ether at 0 °C was added a solution of K(BPPA)
(0.455 g, 1.0 mmol) in 25 mL of diethyl ether. The resulting dark
brown solution was allowed to warm to room temperature and was
stirred overnight. The solvent was removed under vacuum, and the
resulting dark solid was extracted with methylene chloride. The
Inorganic Chemistry, Vol. 46, No. 17, 2007 7203

Chomitz et al.
Figure 2. Thermal ellipsoid (50%) plot of 5. Hydrogen atoms and solvent
molecules have been omitted for clarity.
Figure 1. Thermal ellipsoid (50%) plot of 4. Hydrogen atoms and solvent
molecules have been omitted for clarity.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for (BPPA)VBr2
(5)
Scheme 1. Synthesis of 4
V(1)-Br(1)
V(1)-O(1)
V(1)-N(2)
2.549(1)
1.851(4)
2.165(4)
92.18(3)
87.8(1)
170.3(1)
96.3(1)
93.0(1)
V(1)-Br(2)
V(1)-N(1)
V(1)-N(3)
Br(1)-V(1)-O(1)
Br(1)-V(1)-N(2)
Br(2)-V(1)-O(1)
Br(2)-V(1)-N(2)
O(1)-V(1)-N(1)
O(1)-V(1)-N(3)
N(1)-V(1)-N(3)
2.543(1)
2.176(4)
2.131(5)
95.5(1)
94.1(1)
96.4(1)
170.8(1)
166.7(2)
92.0(2)
Br(1)-V(1)-Br(2)
Br(1)-V(1)-N(1)
Br(1)-V(1)-N(3)
Br(2)-V(1)-N(1)
Br(2)-V(1)-N(3)
O(1)-V(1)-N(2)
N(1)-V(1)-N(2)
N(2)-V(1)-N(3)
Table 1. Selected Bond Lengths (Å) and Angles (deg) for
(BPPA)TiCl2 (4)
Ti(1)-Cl(1)
Ti(1)-O(1)
Ti(1)-N(2)
2.385(1)
1.862(2)
2.208(3)
93.85(4)
96.38(8)
167.86(7)
87.82(8)
92.58(7)
87.46(10)
81.40(10)
77.30(10)
Ti(1)-Cl(2)
Ti(1)-N(1)
Ti(1)-N(3)
Cl(1)-Ti(1)-O(1)
Cl(1)-Ti(1)-N(2)
Cl(2)-Ti(1)-O(1)
Cl(2)-Ti(1)-N(2)
O(1)-Ti(1)-N(1)
O(1)-Ti(1)-N(3)
N(1)-Ti(1)-N(3)
2.417(1)
2.230(3)
2.235(3)
102.44(7)
94.71(8)
100.34(7)
166.91(8)
158.8(1)
86.49(9)
73.57(10)
89.7(2)
77.2(2)
79.8(2)
83.5(2)
Cl(1)-Ti(1)-Cl(2)
Cl(1)-Ti(1)-N(1)
Cl(1)-Ti(1)-N(3)
Cl(2)-Ti(1)-N(1)
Cl(2)-Ti(1)-N(3)
O(1)-Ti(1)-N(2)
N(1)-Ti(1)-N(2)
N(2)-Ti(1)-N(3)
magnetic susceptibility (µeff ) 1.71 µB) is close to the spin-
only value of 1.73 µB for a d1 complex.68
5 was synthesized through the metathesis reaction of
K(BPPA) with VBr3(THF)3 in THF and was isolated as dark
red crystals in 71% yield (Scheme 2). X-ray quality crystals
of 5 were grown by vapor diffusion of diethyl ether into a
methylene chloride solution of 5 at room temperature. The
structure is shown in Figure 2, with selected bond lengths
and angles given in Table 2. As with 4, the vanadium atom
is six-coordinate with a κ4-bound ligand in addition to two
terminal bromides. The V-Br bond lengths of 2.549(1) and
2.543(1) Å are slightly longer than the bond distance found
in [VBr(NS2S2′)] (2.5029(3) Å).69 The V-O bond length of
1.851(4) Å is slightly shorter than the V-O bond lengths in
[V(salophen)(py)2][ZnCl3py] (1.915(9) and 1.88(1) Å);70
however, the V-Npy bond lengths (2.176(4) and 2.131(5)
Å) agree well with those observed in [V(salophen)(py)2]-
[ZnCl3py]. The structure of 4 is more distorted from
octahedral with N(3)-Ti(1)-Cl(1), N(2)-Ti(1)-Cl(2), and
O(1)-Ti(1)-N(1) angles of 167.86(7)°, 166.91(8)°, and
158.8(1)° than 5 with N(2)-V(1)-Br(2), N(3)-V(1)-Br-
(1), and O(1)-V(1)-N(1) angles of 170.8(1)°, 170.3(1)°,
and 166.7(1)°. Complex 5 gives a µeff ) 2.7 µB, which falls
in the range of expected spin-only values of 2.6-2.8 µB for
a V3+ d2 complex.68
Scheme 2. Synthesis of 5
Synthesis and Characterization of BPPA Metal-Halide
Complexes. Early Metals. The reaction of K(BPPA) and
TiCl3 in THF proceeded cleanly giving 4, which was isolated
as dark red crystals in 55% yield (Scheme 1). X-ray quality
crystals were grown from methylene chloride at -15 °C;
the solid-state X-ray structure is shown in Figure 1, with
selected bond lengths and angles provided in Table 1. The
titanium atom in compound 4 is six-coordinate with the
ligand bound in a tetradentate manner and two terminal
chlorides completing the octahedron. The Ti-Cl bond
lengths of 2.417(1) and 2.385(1) Å are similar to those in
the related Ti(IV) compound Ti(O2tBuNN′)Cl2 (2.3929(12)
and 2.2840(12) Å).67 The Ti-O (1.862(2) Å) and Ti-Npy
(2.230(3) and 2.208(3) Å) bond distances also correlate well
with the literature values.67 The room-temperature, solution
The redox properties of compounds 4 and 5 were
investigated using cyclic voltammetry, and the results are
(68) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry; Wiley:
New York, 1980; pp 627-628.
(69) Nekola, H.; Wang, D.; Gruning, C.; Gatjens, J.; Behrens, A.; Rehder,
D. Inorg. Chem. 2002, 41, 2379-2384.
(67) Boyd, C. L.; Toupance, T.; Tyrrell, B. R.; Ward, B. D.; Wilson, C.
R.; Cowley, A. R.; Mountford, P. Organometallics 2005, 24, 309-
330.
(70) Mazzanti, M.; Gambarotta, S.; Floriani, C.; Chiesivilla, A.; Guastini,
C. Inorg. Chem. 1986, 25, 2308-2314.
7204 Inorganic Chemistry, Vol. 46, No. 17, 2007

Synthesis and ReactiWity of Metal Complexes Supported by BPPA
Table 3. Electrochemical Data for 4 and 5a
oxidation
reduction
(BPPA)TiCl2
(BPPA)VBr2
-0.588 Vb
+0.100 Vb
-2.55 V
-1.84 V
a Potentials are referenced to Fc/Fc+. Cyclic voltammograms recorded
at a scan rate of 25 mV/s in CH2Cl2 with [nBu4N][PF6] (0.3 M) as the
supporting electrolyte. b Reversible process.
Figure 4. Thermal ellipsoid (50%) plot of 6. Hydrogen atoms, modeled
disorder in tert-butyl groups, and solvent molecules have been omitted for
clarity.
Table 4. Selected Bond Lengths (Å) and Angles (deg) for
[(BPPA)YCl2]2 (6)
Y(1)-Cl(1)
2.671(1)
2.586(1)
2.572(4)
2.521(3)
73.62(4)
128.77(8)
78.37(8)
100.95(8)
157.07(9)
143.3(1)
78.8(1)
Y(1)-Cl(1*)
Y(1)-O(1)
Y(1)-N(2)
2.794(1)
2.127(3)
2.501(3)
Y(1)-Cl(2)
Y(1)-N(1)
Y(1)-N(3)
Cl(1)-Y(1)-Cl(1*)
Cl(1)-Y(1)-O(1)
Cl(1)-Y(1)-N(2)
Cl(2)-Y(1)-O(1)
Cl(2)-Y(1)-N(2)
O(1)-Y(1)-N(1)
O(1)-Y(1)-N(3)
N(1)-Y(1)-N(3)
Y(1)-Cl(1)-Y(1*)
Cl(1)-Y(1)-Cl(2)
Cl(1)-Y(1)-N(1)
Cl(1)-Y(1)-N(3)
Cl(2)-Y(1)-N(1)
Cl(2)-Y(1)-N(3)
O(1)-Y(1)-N(2)
N(1)-Y(1)-N(2)
N(2)-Y(1)-N(3)
116.06(4)
80.62(9)
131.91(8)
79.07(9)
89.83(8)
80.3(1)
Figure 3. Cyclic voltammograms of 4 (a) and 5 (b).
Scheme 3. Synthesis of 6
86.4(1)
67.8(1)
64.6(1)
106.38(4)
Scheme 4. Synthesis of 7
summarized in Table 3. Representative cyclic voltammo-
grams (CV) are shown in Figure 3. The CV of 4 shows a
reversible oxidation at -0.588 V for the Ti(III)/Ti(IV) couple
and an irreversible reduction at -2.55 V. Similarly, the CV
of 5 also shows a reversible oxidation at +0.100 V and an
irreversible reduction at -1.35 V.
1
2.789(2) Å).71 In solution, the H NMR spectrum of 6
indicates the presence of equivalent pyridyl groups. Also of
note are the two equivalent diastereotopic methylene reso-
nances corresponding to the two 2-picolyl fragments, indicat-
ing the ligand remains fully coordinated in solution.
The reaction of K(BPPA) with YCl3 in THF led cleanly
to the formation of 6 (Scheme 3). Large colorless blocks
suitable for X-ray crystallography were grown from a
solution of methylene chloride layered with pentane at room
temperature and isolated in 61% yield. The solid-state X-ray
structure of 6 is shown in Figure 4, with selected bond
lengths and angles given in Table 4. Dimeric 6 is seven-
coordinate in the solid state with each yttrium atom bound
to the BPPA ligand in addition to one terminal and two
bridging chlorides. The two halves of the dimer are related
by a center of inversion between the two yttrium atoms, with
the asymmetric unit being the (BPPA)YCl2 fragment. The
Y-Npy bond distances of 2.572(4) and 2.501(3) Å are similar
to the value found in the related compound Y2(O2tBuNN′)2-
(-Cl)2(py)2 (2.520(7) Å),67 as is the Y-O bond distance of
2.127(3) Å (2.155(6) and 2.139(5) Å). The bridging Y-Cl
bond lengths of 2.671(1) and 2.794(1) Å are also comparable
to those found in Y2(O2tBuNN′)2(µ-Cl)2(py)2 (2.744(2) and
The metathesis reaction between K(BPPA) and ZrCl4 in
THF led to the formation of 7 in 64% yield (Scheme 4).
Compound 7 is sparingly soluble in THF and methylene
chloride but can be crystallized from pyridine at -15 °C.
Though numerous attempts were made, crystals suitable for
1
X-ray crystallography were not obtained. The H NMR
spectrum of 7 in pyridine-d5 is nearly identical to that of 6
in the same solvent. This similarity suggests the zirconium
atom may be seven-coordinate with three bound chlorides
and a κ4-bound ligand, as was observed in the case of 6.
Group 13 Metals. The synthesis of Group 13 metal-
BPPA complexes can be achieved via two routes: proto-
nolysis and metathesis. Compound 8 was prepared by
reaction of AlMe3 with H(BPPA) in toluene at -78 °C.
Following removal of solvent under vacuum, 8 was obtained
1
as an off-white solid and was characterized by H and 13C-
(71) Long, D. P.; Chandrasekaran, A.; Day, R. O.; Bianconi, P. A.;
Rheingold, A. L. Inorg. Chem. 2000, 39, 4476-4487.
Inorganic Chemistry, Vol. 46, No. 17, 2007 7205

Chomitz et al.
Scheme 5. Synthesis of 8 and 9
Scheme 6. Synthesis of Group 13 BPPA Halide Complexes (E ) Al,
X ) Cl, Br, I; E ) Ga, X ) Cl)
Table 5. Selected Bond Lengths (Å) and Angles (deg) for
(BPPA)AlCl2 (10)
Al(1)-Cl(1)
2.306(2)
1.802(3)
2.081(4)
92.7(1)
91.4(1)
88.5(1)
171.1(1)
94.4(1)
89.8(2)
81.1(2)
84.6(2)
Al(1)-Cl(2)
2.259(2)
2.098(4)
2.084(4)
95.9(1)
170.1(1)
95.3(1)
94.1(1)
92.2(2)
169.2(2)
77.8(2)
Al(1)-O(1)
Al(1)-N(1)
Al(1)-N(2)
Al(1)-N(3)
Cl(1)-Al(1)-Cl(2)
Cl(1)-Al(1)-N(1)
Cl(1)-Al(1)-N(3)
Cl(2)-Al(1)-N(1)
Cl(2)-Al(1)-N(3)
O(1)-Al(1)-N(2)
N(1)-Al(1)-N(2)
N(2)-Al(1)-N(3)
Cl(1)-Al(1)-O(1)
Cl(1)-Al(1)-N(2)
Cl(2)-Al(1)-O(1)
Cl(2)-Al(1)-N(2)
O(1)-Al(1)-N(1)
O(1)-Al(1)-N(3)
N(1)-Al(1)-N(3)
{1H} NMR spectroscopy. Attempts to crystallize 8 from a
variety of solvent systems were unsuccessful. Compound 8
is stable below 0 °C and can be stored cold as the solid or
in solution for several months without visible decomposition.
Upon warming to room temperature, solutions and solid
samples of 8 decomposed, as evidenced by a color change
from colorless to magenta. Upon mild heating of a sealed
NMR tube of 8 in C6D6, decomposition was observed with
concomitant release of CH4 (see below).
(1)-Cl(1) bond length is slightly longer than that of Al(1)-
Cl(2) due to the greater trans influence of pyridine than
amine. The bond lengths about the aluminum atom are
slightly shorter when compared with the related compound
L2Al2Cl2(py) (L ) MeC(O)CHC{NCH2CH(Me)O}Me) (Al-
Halogenation of 8 with 2 equiv of I2 in toluene at -78 °C
afforded 9 as orange plates following crystallization from
Cl ) 2.3936(5) Å, Al-O ) 1.8823(10) Å, Al-Npy
)
1
methylene chloride (Scheme 5). The H NMR spectrum of
2.1373(12) Å) but remain consistent with those of other
crystallographically characterized six-coordinate Al(III) com-
pounds.72
9 shows two sharp diastereotopic methylene resonances,
assigned to the 2-picolyl groups, as well as equivalent pyridyl
groups suggesting full coordination of the ligand in solution.
No 27Al NMR shift was observed, presumably as a result of
the large quadrupole moment for aluminum. Crystals of 9
were of twinned and were not suitable for X-ray structural
analysis.
The aluminum dichloride (10) and dibromide (11) com-
plexes of BPPA were prepared Via reaction of K(BPPA) with
AlCl3 and AlBr3, respectively (Scheme 6). Both 10 and 11
were crystallized from methylene chloride, yielding fine
colorless needles. X-ray quality crystals of 10 were obtained
by vapor diffusion of pentane into a methylene chloride
solution. The structure of 10 is shown in Figure 5, with
selected bonds and angles provided in Table 5. The alumi-
num atom is six-coordinate with a κ4-bound BPPA ligand
and two terminal chlorides. The octahedral coordination
geometry around the aluminum center is distorted as a result
of the chelating ligand, with N(1)-Al(1)-Cl(1), N(2)-Al-
(1)-Cl(1), and N(3)-Al(1)-O(1) angles all ∼170°. The Al-
1
In contrast to 9, the solution H NMR spectrum of 10
reveals two broad diastereotopic 2-picolyl methylene reso-
nances, indicating that the complex may exhibit some
fluxional behavior in solution; however, the 27Al NMR shift
of 31.5 ppm is within the range commonly encountered for
six-coordinate aluminum complexes.73 No crystallographic
1
data were obtained for 11, but the H NMR spectrum has
similar resonances to the spectrum of 9. Furthermore, the
27Al NMR shift of 29.2 ppm also suggests a six-coordinate
environment.73
The reaction of K(BPPA) and GaCl3 afforded 12 in modest
yield (Scheme 6). Large colorless needles of 12 suitable for
X-ray diffraction were grown by vapor diffusion of pentane
into a methylene chloride solution of 12 at 5 °C. The solid-
state structure of 12 is isostructural with that of 10, as shown
in Figure 6 with selected bond lengths and angles provided
in Table 6. The bond lengths about the gallium atom are
slightly shorter when compared with the related compound
GaCl(1,2-O2C6Cl4)(py)(phen) (py ) C5H5N, phen ) 1,10-
phenanthroline) (Ga-O ) 1.952(4) Å, Ga-Npy ) 2.155(4)
Å, Ga-Cl ) 2.329(2) Å) but remain consistent with those
of other crystallographically characterized six-coordinate Ga-
(III) compounds.74
Uranium. Reaction of UI3(THF)4 with K(BPPA) led to
the subsequent isolation of the product 13 in relatively low
yield, presumably Via disproportionation of the metal.
(72) Doherty, S.; Errington, R. J.; Housley, N.; Clegg, W. Organometallics
2004, 23, 2382-2388.
(73) Atwood, D. A.; Harvey, M. J. Chem. ReV. 2001, 101, 37-52.
(74) Lawson, Y. G.; Norman, N. C.; Orpen, A. G.; Quayle, M. J. Acta
Crystallogr., Sect. C 1997, 53, 1805-1809.
Figure 5. Thermal ellipsoid (50%) plot of 10. Hydrogen atoms and solvent
molecules have been omitted for clarity.
7206 Inorganic Chemistry, Vol. 46, No. 17, 2007

Synthesis and ReactiWity of Metal Complexes Supported by BPPA
Table 7. Selected Bond Lengths (Å) and Angles (deg) for (BPPA)UI3
(13)
U(1)-I(1)
3.067(3)
3.057(6)
3.057(4)
2.616(2)
90.26(2)
84.25(6)
41.58(5)
90.32(3)
158.03(4)
80.42(3)
136.42(4)
73.84(4)
80.12(5)
106.66(4)
174.01(2)
U(1)-N(2)
U(1)-N(3)
U(1)-O(1)
2.627(3)
2.591(2)
2.050(4)
U(1)-I(2)
U(1)-I(3)
U(1)-N(1)
I(1)-U(1)-I(2)
I(1)-U(1)-I(3)
I(2)-U(1)-I(3)
I(1)-U(1)-O(1)
I(1)-U(1)-N(1)
I(1)-U(1)-N(2)
I(1)-U(1)-N(3)
I(2)-U(1)-N(1)
I(2)-U(1)-N(2)
I(2)-U(1)-N(3)
I(2)-U(1)-O(1)
I(3)-U(1)-N(1)
I(3)-U(1)-N(2)
I(3)-U(1)-N(3)
I(3)-U(1)-O(1)
N(1)-U(1)-N(2)
N(1)-U(1)-N(3)
N(1)-U(1)-O(1)
N(2)-U(1)-N(3)
N(2)-U(1)-O(1)
N(3)-U(1)-O(1)
82.76(3)
164.34(5)
130.98(4)
89.13(3)
110.75(2)
64.33(5)
107.10(4)
64.11(3)
94.09(4)
69.07(3)
Figure 6. Thermal ellipsoid (50%) plot of 12. Hydrogen atoms and solvent
molecules have been omitted for clarity.
Table 6. Selected Bond Lengths (Å) and Angles (deg) for
(BPPA)GaCl2 (12)
Å),75 though comparable with the shortest U-Npy bond in
[UI4(py)3] (2.586(4) Å).76 The U-O bond length (2.050(4)
Å) is shorter than those in [U(OPh)4(dmpe)2] (U-O ) 2.158-
(3)-2.182(4) Å),77 [U(O(2,6-tBu(C6H3))3(NEt2)] (U-O )
2.140(3)-2.146(5) Å)78 and in [U(O(2,6-tBu(C6H3))4] (U-O
) 2.136(5) Å).79,80 The three U-I bonds are within error
(3.067(3), 3.057(6), and 3.057(4) Å) and are slightly shorter
than the U-I bond lengths in [U(tpa)(OMe)I3] (tpa ) tris-
[(2-pyridyl)methyl]amine) (3.1776(2), 3.1678(2), and 3.1931-
(2) Å).75
Ga(1)-Cl(1)
2.332(2) Ga(1)-Cl(2)
1.891(5) Ga(1)-N(1)
2.138(7) Ga(1)-N(3)
2.278(2)
2.144(7)
2.150(7)
96.1(2)
Ga(1)-O(1)
Ga(1)-N(2)
Cl(1)-Ga(1)-Cl(2)
Cl(1)-Ga(1)-N(1)
Cl(1)-Ga(1)-N(3)
Cl(2)-Ga(1)-N(1)
Cl(2)-Ga(1)-N(3)
O(1)-Ga(1)-N(2)
N(1)-Ga(1)-N(2)
N(2)-Ga(1)-N(3)
94.0(1)
90.8(2)
87.9(2)
171.2(2)
96.5(2)
89.8(2)
80.8(3)
84.6(3)
Cl(1)-Ga(1)-O(1)
Cl(1)-Ga(1)-N(2) 169.9(2)
Cl(2)-Ga(1)-O(1)
Cl(2)-Ga(1)-N(2)
O(1)-Ga(1)-N(1)
O(1)-Ga(1)-N(3)
N(1)-Ga(1)-N(3)
94.9(2)
93.7(2)
91.9(2)
167.6(3)
76.3(3)
BPPA Metal-Alkyl Complexes. Numerous reactions
with 6 and alkylating reagents such as LiR, MgR2, RMgCl,
and ZnR2 were attempted. In a typical reaction, for example,
the metal complex was suspended in THF at -70 °C before
the slow addition of a THF solution of alkylating reagent,
and then the reaction mixture was warmed to room temper-
ature. The colorless starting materials changed color to dark
red upon warming under all conditions examined. The
solution 1H NMR spectrum of the crude intractable material
displayed a multitude of resonances between 1.5 and 2.0
ppm.
Although this was unexpected, the isolation of 13 was
reproducible, albeit in low crystalline yield. Attempts to
isolate the U(IV) chloride analogue directly from interaction
of UCl4 and K(BPPA) failed to yield any isolable crystalline
material. X-ray quality crystals of 13 were grown from a
concentrated solution of methylene chloride at -40 °C. The
X-ray structure is shown in Figure 7, with selected bond
lengths and angles provided in Table 7. The solid-state
structure shows a monomeric U(IV) coordinated κ4 by the
BPPA ligand and to three terminal iodides. The seven-
coordinate uranium is best described as distorted pentagonal
bipyrimidal. The two U-Npy bond lengths (2.616(2) and
2.627(2) Å) are comparable to those in [U(tpa)(OMe)I3]
(tpa ) tris[(2-pyridyl)methyl]amine) (U-Npyridyl ) 2.61(3)
Å av).75 The basal U-N bond length (2.591(2) Å) is
significantly shorter than in [U(tpa)(OMe)I3] (2.6672(18)
The protonolysis of Y-C bonds has been shown to be
effective for preparing ligated metal complexes,30,31,81 and
with this in mind, the protonolysis reaction of H(BPPA) and
Y(CH2SiMe3)3(THF)2 was explored in pentane. As the
reaction proceeded at -70 °C, a white solid precipitated from
the colorless reaction mixture. Upon removal of the cold bath,
the solution quickly changed color to forest green and finally
to dark red before reaching room temperature.53 No isolable
product could be obtained from the intractable mixture that
was formed.
(75) Karmazin, L.; Mazzanti, M.; Pecaut, J. Inorg. Chem. 2003, 42, 5900-
5908.
(76) Berthet, J. C.; Thuery, P.; Ephritikhine, M. Inorg. Chem. 2005, 44,
1142-1146.
(77) Edwards, P. G.; Andersen, R. A.; Zalkin, A. J. Am. Chem. Soc. 1981,
103, 7792-7794.
(78) Hitchcock, P. B.; Lappert, M. F.; Singh, A.; Taylor, R. G.; Brown, D.
J. Chem. Soc., Chem. Commun. 1983, 561-3.
(79) Berg, J. M.; Clark, D. L.; Huffman, J. C.; Morris, D. E.; Sattelberger,
A. P.; Streib, W. E.; Van der Sluys, W. G.; Watkin, J. G. J. Am. Chem.
Soc. 1992, 114, 10811-21.
(80) Vandersluys, W. G.; Sattelberger, A. P. Polyhedron 1989, 8, 1247-
1249.
Figure 7. Thermal ellipsoid (30%) plot of 13. Hydrogen atoms and solvent
molecules have been omitted for clarity.
(81) Bambirra, S.; Boot, S. J.; van Leusen, D.; Meetsma, A.; Hessen, B.
Organometallics 2004, 23, 1891-1898.
Inorganic Chemistry, Vol. 46, No. 17, 2007 7207

Chomitz et al.
Scheme 7. Synthesis of 14
All attempts to alkylate 7 with standard alkylating reagents
were thwarted by decomposition. As with yttrium, the
colorless reaction mixtures turned dark red upon warming
to room temperature, and intractable material was obtained.
It was found, nonetheless, that the reaction of H(BPPA) and
Zr(CH2Ph)4 in diethyl ether at -40 °C led cleanly to the
formation of 14 (Scheme 7). Isolation of 14 was ac-
complished by removal of solvent under vacuum at 0 °C,
leaving a bright orange powder. The desired product was
extracted with DME, quickly filtered, and concentrated.
Cooling to -40 °C led to the isolation of thermally unstable,
orange parallelograms of 14 in 36% yield. X-ray quality
crystals grown from DME at -40 °C provided the solid-
state X-ray structure shown in Figure 8 (selected bond lengths
and angles in Table 8). The zirconium atom is seven-
coordinate with a κ4-bound BPPA ligand and three coordi-
nated benzyl groups. The Zr-O bond distance of 2.044(3)
Å is similar to those found in a related bis(phenolate)
zirconium complex with Zr-O distances of 2.000(4) and
2.008(4) Å.82 The Zr-Npy bond lengths of 2.372(4) and
2.506(4) Å and the Zr-C bond distances of 2.349(5), 2.358-
(4), and 2.394(4) Å are also similar to those found in the
Figure 8. Thermal ellipsoid (50%) plot of 14. Hydrogen atoms, modeled
disorder of tert-butyl group, and solvent molecules have been omitted for
clarity.
Table 8. Selected Bond Lengths (Å) and Angles (deg) for
(BPPA)Zr(CH2Ph)3 (14)
Zr(1)-O(1)
2.044(3)
2.506(4)
2.349(5)
2.394(4)
Zr(1)-N(1)
Zr(1)-N(3)
Zr(1)-C(8)
2.372(4)
2.467(4)
2.358(4)
Zr(1)-N(2)
Zr(1)-C(1)
Zr(1)-C(15)
O(1)-Zr(1)-N(1)
O(1)-Zr(1)-N(3)
O(1)-Zr(1)-C(8)
N(1)-Zr(1)-N(2)
N(1)-Zr(1)-C(1)
N(1)-Zr(1)-C(15)
N(2)-Zr(1)-C(1)
N(2)-Zr(1)-C(15)
N(3)-Zr(1)-C(8)
121.7(1)
O(1)-Zr(1)-N(2)
O(1)-Zr(1)-C(1)
O(1)-Zr(1)-C(15)
N(1)-Zr(1)-N(3)
N(1)-Zr(1)-C(8)
N(2)-Zr(1)-N(3)
N(2)-Zr(1)-C(8)
N(3)-Zr(1)-C(1)
N(3)-Zr(1)-C(15)
74.1(1)
98.7(1)
154.2(1)
69.3(1)
78.9(1)
69.7(1)
148.2(1)
150.4(1)
88.4(1)
80.0(1)
82.1(1)
131.6(1)
131.5(1)
74.0(1)
81.5(1)
80.3(1)
126.7(1)
Scheme 8. Decomposition of 14
1
literature.82 The H NMR spectrum of 14 in C6D6 shows
equivalent pyridyl groups, as well as a pair of equivalent
diastereotopic 2-picolyl protons. The three sets of benzyl
protons appear to be in rapid exchange on the NMR time
scale at room temperature, as evidenced by the presence of
one broad peak integrating to six protons centered at 2.60
ppm. As an NMR sample of 14 in toluene-d8 is cooled to
-20 °C, the broad resonance corresponding to the benzyl
protons separates to three sets of peaks. Two sets of benzyl
protons remain in the region between 2 and 3 ppm, and one
set shifts downfield to 4.98 ppm, suggestive of a µ2-benzyl
group.83 Although not observed in the crystal structure, in
solution this interaction may provide additional electron
density, as well as steric protection to the zirconium center.
Reactivity of 14. It was observed that a solution of 14
underwent a color change from bright orange to dark red
upon standing at room temperature overnight. To determine
the identity of the decomposition product, the thermal
of one set of pyridyl protons with the ortho proton resonance
appearing in the vinyl region at 5.14 ppm (see below). In
addition to the loss of symmetry, 1 equiv of toluene is
observed to be present at 2.31 ppm, as well as a new singlet
resonance integrating to one proton at 3.9 ppm in THF-d8.
With the aid of a 1H-1H COSY experiment, all eight pyridyl
protons were assigned, and the presence of five of the six
sets of diastereotopic methylene protons corresponding to
the six bridging and six benzyl protons were also identified.
1
The JCH of the carbon adjacent to the singlet proton at 3.9
ppm is 175 Hz, indicative of an sp2-hybridized carbon. To
provide further evidence for the structure of the thermolysis
product, a 1H-15N HMBC experiment was carried out. The
proton resonance at 3.9 ppm is split into a doublet with a
2JNH of 4.1 Hz. These data are consistent with the assigned
thermolysis product 15 (Scheme 8) with BPPA transformed
to a new tetradentate dianionic ligand with the loss of
aromaticity in one of the pyridyl groups. A related decom-
position reaction was reported recently by Bercaw et al.53
The loss of aromaticity of a pyridyl group as a result of
1
decomposition of 14 was monitored by H NMR spectros-
copy. A NMR tube sample of 14 in either THF-d8 or C6D6
was heated to 35 °C for 2 h, leading to full conversion to a
new species. The 1H NMR spectrum of the product displays
an asymmetric molecule as indicated by the presence of two
different pyridyl donor groups. Of note is the upfield shift
(82) Tshuva, E. Y.; Goldberg, I.; Kol, M.; Goldschmidt, Z. Organometallics
2001, 20, 3017-3028.
(83) Giesbrecht, G. R.; Whitener, G. D.; Arnold, J. Organometallics 2000,
19, 2809-2812.
7208 Inorganic Chemistry, Vol. 46, No. 17, 2007

Synthesis and ReactiWity of Metal Complexes Supported by BPPA
deprotonation of a 2-picolyl proton has been observed
We believe these BPPA derivatives may function as useful
starting materials for the preparation of a range of coordina-
tion complexes, although the preliminary signs are that their
organometallic chemistry may be somewhat limited. Despite
numerous attempts to synthesize a (BPPA)YR2 complex
through metathesis and protonolysis reactions, only intrac-
table mixtures were obtained. Aluminum (8) and zirconium
(14) alkyl complexes were successfully prepared and isolated,
but both are prone to thermal decomposition via deproto-
nation of the BPPA ligand. In the case of 14, the transforma-
tion of the BPPA ligand from a monoanionic to a dianionic
ligand was monitored by NMR spectroscopy. A more stable
alkyl complex was obtained following the protonation of 14
by H[B(C5F5)4‚2Et2O]. The cationic species 16 was observed
1
previously by H NMR spectroscopy and by X-ray crystal-
lography.53,84 Attempts to obtain crystalline material of 15
were unsuccessful.
Reaction of 14 and H[B(C5F5)4‚2Et2O] in C6D5Cl resulted
in an immediate color change from bright orange to yellow-
orange and formation of the cationic species 16. The 1H and
19F NMR spectra show a clean conversion to a new
compound with concomitant loss of 1 equiv of toluene.
According to 1H NMR, the pyridyl groups remain equivalent,
as do the diastereotopic 2-picolyl and benzyl protons. In
contrast to the parent complex, 16 appears to be thermally
robust with no decomposition observed in solution after
sitting for several days at room temperature in a sealed NMR
tube. Attempts to crystallize 16 were unsuccessful.
1
by H and 19F NMR spectroscopy.
Summary
Acknowledgment. We are grateful to the National
Science Foundation for financial support of this work and
Dr. Fred Hollander and Dr. Allen Oliver for crystallographic
assistance. The Advanced Light Source is supported by the
Director, Office of Science, Office of Basic Energy Sciences,
of the U.S. Department of Energy under Contract No. DE-
AC02-05CH11231.
A broad examination of the usefulness of the TDMA
ligand BPPA has been undertaken. Complexes 4-7 and
9-13 were synthesized and isolated in good yields and were
characterized by NMR, IR, EA, magnetic susceptibility, and
when possible, X-ray crystallography. In all structurally
characterized cases, the BPPA ligand coordinates to the metal
center in a tetradentate fashion. Additionally, the ligand
shows a mild degree of flexibility, as demonstrated by the
ability of the pyridine groups to be either cis or trans to one
another in the solid state.
Supporting Information Available: Crystallographic informa-
tion files (CIF) and a detailed summary of the X-ray diffraction
data for compounds 4-6, 10, and 12-14. This material is available
free of charge via the Internet at http://pubs.acs.org.
(84) Ben-Ari, E.; Leitus, G.; Shimon, L. J. W.; Milstein, D. J. Am. Chem.
Soc. 2006, 128, 15390-15391.
IC700775K
Inorganic Chemistry, Vol. 46, No. 17, 2007 7209