First-row transition metal complexes of sterically-hindered amidinates

Article abstract of DOI:10.1039/b202737m

Complexes of sterically-hindered amidinate ligands with first-row transition metals are described. The amidinate ligands feature bulky terphenyl substituents [2,6-(2,4,6-Me₃Ph)₂Ph or 2,6-(4-^tBuPh)₂Ph] attached t

Full text of DOI:10.1039/b202737m

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First-row transition metal complexes of sterically-hindered
amidinates
Joseph A. R. Schmidt and John Arnold*
Department of Chemistry, University of California at Berkeley and Chemical Sciences Division,
Lawrence Berkeley National Laboratory, Berkeley, California 94720-1460, USA.
E-mail: arnold@socs.berkeley.edu
Received 18th March 2002, Accepted 5th July 2002
First published as an Advance Article on the web 15th August 2002
Complexes of sterically-hindered amidinate ligands with first-row transition metals are described. The amidinate
ligands feature bulky terphenyl substituents [2,6-(2,4,6-Me3Ph)2Ph or 2,6-(4-tBuPh)2Ph] attached to the backbone
carbon atoms, providing bowl-shaped ligand environments. When employing divalent transition metal halides,
bis-amidinate metal complexes are formed exclusively, whereas the use of Ni(acac)2 or CuCl allows access to
t
mono-amidinate species. Additionally, the solid-state structure of one mono-amidinate [(LBu )Ni(acac)] and three
bis-amidinate complexes [(LMe)2M; M = Mn, Co, Ni] are presented.
Introduction
Amidines have been known for many years, being first
synthesized in 1858 by the reaction of N-phenylbenzimidyl
chloride with aniline.1 With the recent surge in the use of
nitrogen-donor ligands for organometallic systems, amidines
have seen wide investigation. The amidinate ligand is a
monoanionic nitrogen-donor ligand that is characterized by an
N–C–N backbone in which π-bonds allow delocalization of the
negative charge associated with this ligand (Fig. 1). Each of the
Fig. 2 Common structural motifs found in first-row transition metal
amidinate complexes.
structural motifs are prominent: bis-amidinate coordination to
a single metal center, and combined bidentate and bridging
coordination to a pair of transition metal atoms. Additionally,
for trivalent metals, compounds of the form (amidinate)3M and
(amidinate)2MX are frequently observed, where X = halide. A
few known mono-amidinate complexes of first-row transition
metals have been formed using monovalent ions, such as Mn()
and Cu() (Fig. 3), by reaction of lithium amidinate with
Mn2(CO)8Cl2 and [LCuCl]2 starting materials.37,38
Fig. 1 General form of the amidinate ligand.
three atoms of this backbone has one substituent attached,
denoted here by R, RЈ, and RЉ and the ability to modify these
three groups allows for tuning of the ligand’s electronic and
steric environment.
Throughout the course of many studies, coordination of the
amidine ligand to virtually every metal in the periodic table has
been observed, ranging across the main-group and transition
metal series and into the lanthanides.2 The bis(trimethylsilyl)-
benzamidinate ligand (R = Ph; RЈ = RЉ = SiMe3) has been used
extensively because of its easy synthesis, strong binding
properties, and the high crystallinity of its compounds.3–15 More
recent work in this area has focused on the use of a wider range
of substituents on the amidinate backbone in attempts to
tune the electronic and steric properties of this ligand.16–32
Numerous binding modes have been observed, including simple
bidentate coordination to a single metal, monodentate coord-
ination to a single metal center, and coordination to bridge two
metal centers. In general, most amidinates are found in the first
mode.
A number of studies have focused on the coordination of
amidinate ligands to first-row transition metals, with the goal
of creating new catalysts having industrial utility.30–36 Mono-
amidinate complexes of di- or tri-valent first-row transition
metals appear to be very difficult to synthesize. Rather, bis-
amidinate complexes are prevalent in these reactions, regardless
of the synthetic pathways employed. As shown in Fig. 2, two
Fig. 3 Mono-amidinate complexes of Mn() and Cu().
We have previously reported the synthesis and structural
properties of a series of amidinate ligands incorporating
terphenyl substituents bound to the amidinate carbon atom.39,40
The terphenyl moieties in these ligands provide steric shielding
above and below the plane of the amidinate ligand, creating a
bowl-shaped ligand. Through investigation of the free-base and
lithium salts of these ligands, it was found that functional
groups located at the ortho position of the external phenyls of
the terphenyl substituents played a strong role in the steric
3454
J. Chem. Soc., Dalton Trans., 2002, 3454–3461
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b202737m

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effect of these ligands.39 The large steric bulk of these
terphenyl-substituted ligands allowed for the formation of
mono-amidinate yttrium halide and amide species, in contrast
to the bis-amidinate complexes observed when using less
sterically-hindered ligands.40
Here, the coordination chemistry of these sterically-hindered
amidinates with first-row transition metals is presented. Specifi-
cally, reactions forming mono- and bis-amidinate transition
metal complexes are detailed. The reactions of the lithium
amidinates with divalent metal halides lead exclusively to bis-
amidinate metal complexes. By using acetylacetonato (acac)
metal complexes or monovalent metal halides, we have been
able to prepare mono-amidinate complexes using these
sterically-bulky ligands.
stirred and slowly warmed to room temperature overnight with
a color change to a tan-brown hue. The solvent was removed
under vacuum, pentane (30 ml) was added, and the volatile
fraction was again removed in vacuo, yielding a sticky tan-
brown solid. The solid was extracted with pentane (40 ml), and
the extract was filtered and concentrated to saturation. Cooling
to Ϫ30 ЊC yielded the product as a tan microcrystalline solid.
After filtration, the product was dried under vacuum; (251 mg,
50%); mp 205–207 ЊC; µeff = 5.70 µB; MS: m/z 933 (Mϩ); IR:
2360 (w), 2342 (w), 1609 (w), 1561 (w), 1484 (w), 1426 (s), 1308
(m), 1224 (w), 1135 (w), 1101 (w), 1029 (w), 1001 (w), 849 (m),
772 (m), 738 (w), 590 (w), 580 (w), 499 (w), 458 (w) cmϪ1. Anal.
calc. for C62H78N4Mn: C, 79.71; H, 8.41; N, 6.00. Found: C,
79.92; H, 8.33; N, 6.05%.
Bis(N,NЈ-diisopropyl(2,6-bis-mesityl)benzamidinato)iron(II)
[(LMe)2Fe] (3). A flask was charged with FeCl2 (56 mg, 0.44
mmol) and THF (20 ml) was added, forming a suspension. The
flask was cooled to Ϫ78 ЊC and a solution of LMe–Li(TMEDA)
(500 mg, 0.89 mmol) in THF (50 ml) was added slowly via
cannula. The pale yellow solution was stirred overnight while
slowly warming to room temperature. After removal of solvent
under vacuum, pentane (30 ml) was added to the sticky solid
and the reaction mixture was again pumped down to a solid.
The solid was extracted with pentane (50 ml), resulting in a pale
yellow solution. This was concentrated slightly under vacuum
and then cooled to Ϫ30 ЊC. Large yellow crystals grew over a
period of one week at this temperature. The yellow crystals
were isolated by filtration and dried under vacuum. The super-
natant solution yielded a second crop of crystalline material
similarly. The crystals were found to desolvate, forming a tan
powder over a few days; (197 mg combined, 47%); mp 178 ЊC
(dec.); µeff = 4.92 µB; MS: m/z 935 (Mϩ); IR: 1610 (m), 1563 (w),
1485 (m), 1422 (s), 1309 (s), 1261 (w), 1222 (m), 1183 (w), 1165
(w), 1135 (m), 1101 (m), 1028 (m), 999 (m), 931 (w), 850 (s), 771
(s), 739 (m) cmϪ1. Anal. calc. for C62H78N4Fe: C, 79.63; H, 8.41;
N, 5.99. Found: C, 79.65; H, 8.35; N, 6.12%.
Experimental
General considerations
Standard Schlenk-line and glove box techniques were used
throughout. Pentane, diethyl ether, and toluene were passed
through a column of activated alumina and degassed with
argon. THF was passed through a column of activated molec-
ular sieves and degassed with argon. MnCl2, FeCl2, CoCl2,
NiCl2, Ni(acac)2, and CuCl were purchased as anhydrous solids,
and used as received. CrCl2(THF)2,41 LMe–Li(TMEDA),40 and
39
t
LBu –Li(TMEDA) were synthesized as detailed previously.
C6D6 was vacuum transferred from a sodium/benzophenone
ketyl. Melting points were determined in sealed capillary tubes
under nitrogen and are uncorrected. H and 13C{1H} NMR
1
spectra were recorded in C6D6 at ambient temperature on a
Bruker DRX-500 spectrometer, unless otherwise specified.
1H NMR chemical shifts are given relative to C6D5H (δ 7.16).
13C NMR chemical shifts are relative to C6D6 (δ 128.39). IR
samples were prepared as Nujol mulls and taken between KBr
plates. Magnetic susceptibilities were determined in C6D6 using
Evans’ NMR method.42 Elemental analyses were determined by
the Microanalytical Laboratory of the College of Chemistry,
University of California, Berkeley. Mass spectra are from the
Mass Spectrometry Laboratory of the College of Chemistry,
University of California, Berkeley, and in all cases employed
electron impact conditions. Single crystal X-ray structure
determinations were performed at CHEXRAY, University of
California, Berkeley.
Bis(N,NЈ-diisopropyl(2,6-bis-mesityl)benzamidinato)cobalt(II)
[(LMe)2Co] (4). A suspension of CoCl2 (150 mg, 1.16 mmol) in
THF (20 ml) was cooled to Ϫ78 ЊC, and a solution of LMe
–
Li(TMEDA) (1.3 g, 2.3 mmol) in THF (60 ml) was added via
cannula. After rapidly changing to a forest green solution, the
mixture was stirred while slowly warming to ambient temper-
ature over a period of 14 h. The solvent was removed under
vacuum, and pentane (50 ml) was added; this was stirred for
15 min and the solvent was again removed in vacuo. The result-
ing green solid was extracted with two portions of pentane
(50, 25 ml), and the combined extracts were concentrated to
15 ml. The solution was cooled to Ϫ40 ЊC, producing green
crystals, which were filtered and dried under vacuum. A second
crop could be isolated similarly from the supernatant solution;
(655 mg combined, 60%); mp 178 ЊC (dec.); µeff = 4.73 µB; MS:
m/z 937 (Mϩ); IR: 1609 (w), 1563 (w), 1426 (s), 1311 (m), 1261
(w), 1228 (m), 1183 (w), 1165 (w), 1137 (w), 1103 (m), 1029 (m),
1002 (m), 850 (m), 800 (w), 771 (m), 738 (w), 589 (w), 581 (w),
508 (w), 455 (w) cmϪ1. Anal. calc. for C62H78N4Co: C, 79.37; H,
8.38; N, 5.97. Found: C, 79.39; H, 8.47; N, 5.86%.
Syntheses
Bis(N,NЈ-diisopropyl(2,6-bis-mesityl)benzamidinato)-
chromium(II) [(LMe)2Cr] (1). A flask containing CrCl2(THF)2
(300 mg, 1.12 mmol) in THF (30 ml) was cooled to Ϫ78 ЊC, and
a solution of LMe–Li(TMEDA) (1.26 g, 2.24 mmol) in THF (40
ml) was added via cannula. After a rapid color change to green,
the solution was stirred and slowly warmed to room temper-
ature overnight. The solvent was removed in vacuo, pentane
(40 ml) was added, and the solvent was again removed under
vacuum. The solid was extracted with two portions of pentane
(40, 20 ml) and the combined extracts were concentrated to
25 ml and cooled to Ϫ40 ЊC. The resulting red crystals were
isolated by filtration and dried under vacuum; (298 mg, 29%);
mp 242–243 ЊC; µeff = 4.65 µB; MS: m/z 931 (Mϩ); IR: 1611 (m),
1570 (w), 1420 (s), 1356 (s), 1344 (s), 1301 (w), 1255 (w), 1212
(m), 1185 (w), 1163 (w), 1146 (m), 1106 (w), 1030 (w), 995 (m),
849 (s), 815 (w), 776 (s), 752 (w), 721 (w), 656 (w) cmϪ1. Anal.
calc. for C62H78N4Cr: C, 79.96; H, 8.44; N, 6.02. Found: C,
79.57; H, 8.69; N, 5.70%.
Bis(N,NЈ-diisopropyl(2,6-bis-mesityl)benzamidinato)nickel(II)
[(LMe)2Ni] (5). A reaction flask was charged with NiCl2 (60 mg,
0.46 mmol) and THF (20 ml) and cooled to Ϫ78 ЊC. A solution
of LMe–Li(TMEDA) (521 mg, 0.93 mmol) was added via
cannula and the reaction mixture was stirred overnight while
warming to ambient temperature. The solvent was removed
under vacuum to yield a red oil. After treatment with pentane
(20 ml), the solvent was again removed under vacuum to yield a
red solid. This solid was extracted with pentane (30 ml), and
the extract was concentrated to 7 ml. Cooling this solution to
Ϫ30 ЊC yielded red crystals; (79 mg, 19%); mp 170 ЊC (dec.);
Bis(N,NЈ-diisopropyl(2,6-bis-mesityl)benzamidinato)-
manganese(II) [(LMe)2Mn] (2). A reaction flask charged with
MnCl2 (67 mg, 0.53 mmol) and THF (20 ml) was cooled to
Ϫ78 ЊC and a solution of LMe–Li(TMEDA) (600 mg, 1.07
mmol) in THF (30 ml) was added via cannula. The solution was
J. Chem. Soc., Dalton Trans., 2002, 3454–3461
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Table 1 Crystal data and collection parameters
Compound
2
4
5
7ؒ1/2C5H12
Formula
C62H78N4Mn
934.26
C2/c (no. 15)
Monoclinic
Ϫ106
C62H78N4Co
938.26
P21/m (no. 11)
Monoclinic
Ϫ131
C62H78N4Ni
938.03
P21/m (no. 11)
Monoclinic
Ϫ80
C40.5H56N2NiO2
661.60
P21/c (no. 14)
Monoclinic
Ϫ133
Formula weight
Space group
Crystal system
T /ЊC
a/Å
b/Å
c/Å
15.9920(5)
15.6524(2)
25.7702(7)
98.003(2)
12.4890(5)
15.8629(7)
14.0309(6)
105.818(1)
2674.4(2)
12.459(1)
15.941(1)
14.070(1)
105.396(1)
2694.2(3)
11.344(2)
32.816(7)
10.441(2)
99.938(3)
β/Њ
V/Å3
6387.8(2)
3828(1)
Z
4
2
2
4
Diffractometer
Radiation
Monochromator
Detector
Scan type, width
Scan speed
Reflections measured
µ/cmϪ1
Siemens SMART
Mo-Kα (λ = 0.71069 Å)
Graphite
CCD area detector
ω, 0.3Њ
30.0 s/frame
Hemisphere
2.42
Siemens SMART
Mo-Kα (λ = 0.71069 Å)
Graphite
CCD area detector
ω, 0.3Њ
10.0 s/frame
Hemisphere
3.63
Siemens SMART
Mo-Kα (λ = 0.71069 Å)
Graphite
CCD area detector
ω, 0.3Њ
10.0 s/frame
Hemisphere
4.02
Siemens SMART
Mo-Kα (λ = 0.71069 Å)
Graphite
CCD area detector
ω, 0.3Њ
20.0 s/frame
Hemisphere
5.41
Crystal dimensions/mm
Reflections measured
Unique reflections
Observations
Parameters
R, Rw, Rall
0.31 × 0.14 × 0.07
13465
4783
2506
255
0.34 × 0.24 × 0.13
12040
4641
2534
343
0.44 × 0.35 × 0.32
13053
4960
3086
334
0.26 × 0.21 × 0.13
17406
6551
2987
421
0.102, 0.137, 0.156
0.040, 0.042, 0.081
0.035, 0.041, 0.062
0.035, 0.040, 0.095
µeff = 2.98 µB; MS: m/z 937 (Mϩ); IR: 1609 (m), 1562 (w), 1423
(s), 1311 (s), 1261 (w), 1229 (m), 1183 (w), 1166 (w), 1137 (m),
1103 (m), 1029 (m), 1002 (m), 932 (w), 850 (s), 820 (w), 772 (s),
737 (w), 588 (w), 580 (w) cmϪ1. Anal. calc. for C62H78N4Ni: C,
79.39; H, 8.38; N, 5.97. Found: C, 79.07; H, 8.51; N, 5.86%.
624 (Mϩ); IR: 1577 (s), 1523 (s), 1476 (s), 1452 (s), 1391 (s),
1360 (m), 1334 (w), 1307 (w), 1290 (w), 1262 (w), 1240 (m), 1201
(w), 1191 (w), 1167 (w), 1153 (w), 1131 (w), 1113 (m), 1100 (m),
1020 (m), 1009 (m), 844 (w), 830 (w), 807 (m), 777 (m) cmϪ1
.
Anal. calc. for C38H50N2O2Ni: C, 72.97; H, 8.06; N, 4.48.
Found: C, 72.62; H, 8.03; N, 4.47%.
(N,NЈ-Diisopropyl(2,6-bis-mesityl)benzamidinato)(acetyl-
acetonato)nickel(II) [(LMe)Ni(acac)] (6). THF (20 ml) was added
to a flask containing Ni(acac)2 (230 mg, 0.90 mmol), forming a
dark green suspension. A solution of LMe–Li(TMEDA) (0.50 g,
0.89 mmol) in THF (50 ml) was added slowly via cannula,
resulting in the immediate formation of pale brown precipitate.
The solution was stirred for 14 h at ambient temperature, and
then the volatile materials were removed under vacuum to yield
a brown material. Pentane (35 ml) was added, the mixture was
stirred, and the volatiles were again removed in vacuo. Extrac-
tion with pentane (3 × 25 ml) yielded a pale green solution,
which was pumped down to a green oil that thickened over a
period of 1 week. This oil, though contaminated with the free-
base ligand LMeH,40 was mostly the desired product, 6; (253 mg,
47%); MS: m/z 597 (Mϩ); IR: 1665 (m), 1646 (w), 1613 (s), 1598
(s), 1516 (s), 1411 (w), 1399 (s), 1358 (w), 1313 (w), 1259 (m),
1181 (w), 1169 (w), 1127 (w), 1116 (w), 1029 (w), 1015 (m), 957
(N,NЈ-Diisopropyl(2,6-bis(4-tert-butylphenyl))benzamidinato)-
copper(I) [(LBu )Cu]x (8). A reaction flask was charged with
t
CuCl (750 mg, 7.58 mmol) and THF (25 ml) was added,
t
forming a suspension. A solution of LBu –Li(TMEDA) (3.0 g,
5.1 mmol) in THF (75 ml) was added via cannula. After stirring
for 14 h, the solvent was removed from the reaction mixture
under vacuum, giving a cream-colored solid. The solid was
washed with diethyl ether (70 ml) and was then extracted with
toluene (150 ml). After filtration through Celite, the solvent was
removed in vacuo and the product collected as a white solid;
1
(1.234 g, 46%); mp 281 ЊC (dec.); H NMR δ 8.034 (d, 4H,
8.4 Hz), 7.537 (d, 2H, 7.6 Hz), 7.465 (d, 4H, 8.4 Hz), 7.253 (t,
1H, 7.6 Hz), 3.327 (sept, 2H, 6.0 Hz), 1.314 (s, 18H), 0.818 (d,
12H, 6.0 Hz); 13C NMR δ 171.4, 150.5, 139.9, 138.2, 134.0,
130.1, 129.4, 128.5, 125.2, 49.4, 34.5, 31.5, 27.2; IR: 1611 (w),
1576 (w), 1513 (m), 1487 (s), 1341 (m), 1271 (m), 1167 (w), 1127
(m), 1099 (w), 1020 (w), 1007 (w), 837 (w), 804 (m), 777 (m), 727
(w), 685 (w), 576 (m) cmϪ1. Anal. calc. for C33H43N2Cu: C,
74.61; H, 8.16; N, 5.27. Found: C, 74.76; H, 7.98; N, 5.01%.
(w), 920 (w), 849 (m), 801 (m), 762 (m), 753 (w), 742 (w) cmϪ1
.
(N,NЈ-Diisopropyl(2,6-bis(4-tert-butylphenyl))benzamid-
t
inato)(acetylacetonato)nickel(II) [(LBu )Ni(acac)] (7). A solution
t
of LBu –Li(TMEDA) (2.5 g, 4.2 mmol) in THF (75 ml) was
X-Ray crystallography
added to a slurry of Ni(acac)2 (1.09 g, 4.24 mmol) in THF
(200 ml), resulting in a dark purple solution. After stirring for
3 h, the volatile materials were removed under vacuum. The
resulting solid was extracted with three portions of pentane
(200, 50, 50 ml), which were filtered through Celite and com-
bined. The resulting solution was concentrated to saturation
and cooled to Ϫ30 ЊC, and large purple crystals grew on stand-
ing at this temperature. The crystals were isolated by filtration
and dried under vacuum. A second crop could be isolated from
the supernatant solution similarly; (1.21 g combined, 46%); mp
A summary of crystal data and collection parameters for the
crystal structures of 2, 4, 5, and 7 are given in Table 1. Details
of individual data collection and solution are given below.
ORTEP diagrams were created using the ORTEP-3 software
package.43 For each sample, a crystal was mounted on a glass
capillary using Paratone-N hydrocarbon oil. The crystal was
transferred to a Siemens SMART44 diffractometer/CCD area
detector, centered in the X-ray beam, and cooled using a
nitrogen-flow low-temperature apparatus that had been
previously calibrated by a thermocouple placed at the same
position as the crystal. A least-squares refinement on data from
60 sample frames allowed determination of cell constants and
the orientation matrix. An arbitrary hemisphere of data was
collected using 0.3Њ ω-scans, and the data were integrated by
1
235 ЊC (dec.); H NMR δ 8.058 (d, 4H, 8 Hz), 7.709 (d, 4H, 8
Hz), 7.346 (d, 2H, 8 Hz), 7.200 (t, 1H, 8 Hz), 4.647 (s, 1H),
2.338 (sept, 2H, 6 Hz), 1.310 (s, 18H), 1.274 (s, 6H), 0.748 (d,
12H, 6 Hz); 13C NMR δ 185.5, 171.1, 151.3, 142.5, 139.1, 130.1,
130.0, 129.9, 126.4, 101.0, 45.1, 35.0, 31.8, 25.3, 23.8; MS: m/z
3456
J. Chem. Soc., Dalton Trans., 2002, 3454–3461

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Scheme 1
the program SAINT.45 The final unit cell parameters were
yielding final residuals:52 R = 0.035, Rw = 0.040, and GOF =
1.17.
CCDC reference numbers 182078–182081.
See http://www.rsc.org/suppdata/dt/b2/b202737m/ for crys-
tallographic data in CIF or other electronic format.
determined by least-squares analysis of the reflections with I >
10σ(I ). Data analysis using Siemens XPREP46 determined the
space group. The data were corrected for Lorentz and polariz-
ation effects, but no correction for crystal decay was applied.
Equivalent reflections were averaged, and the structure was
solved by direct methods47 and expanded using Fourier tech-
niques,48 all within the teXsan49 software package. The non-
hydrogen atoms were refined anisotropically. Hydrogen atoms
were included as fixed atoms but not refined. The weighting
schemes were based on counting statistics and included a factor
(p = 0.030) to reduce the weight of intense reflections. The ana-
lytical forms of the scattering factor tables for the neutral atoms
were used,50 and all scattering factors were corrected for both
the real and imaginary components of anomalous dispersion.51
Results and discussion
The reactions detailed here are focused on the mesityl-
substituted amidinate (LMeH) for two main reasons: first, its
unique free-base and lithium amidinate geometry indicated
the presence of a strong steric effect produced by this ligand;40
second, the mesityl substituents provided favorable solubility
properties. The iPr-phenyl substituted amidinate (LPrH) proved
to be too soluble, hampering isolation of clean metal complexes
in most cases.
(2). X-Ray quality crystals were grown from a saturated
pentane solution that was cooled to Ϫ40 ЊC. The crystal struc-
ture of this compound is isomorphic to that found for
(LMe)2Mg, and has the same two-fold ligand disorder (see
previous report).39 Similarly to (LMe)2Mg, the core of the
compound is ordered and non-hydrogen atoms in this region
were refined anisotropically, while the carbon atoms of the
disordered terphenyl moiety were refined isotropically. Many of
the hydrogen atoms were included as fixed atoms and not
refined. The final cycle of full-matrix least squares refinement
(minimizing the quantity Σw(|Fo| Ϫ |Fc|)2, where w is the weight
of a given observation) was based on 2506 observed reflections
[I > 3.00σ(I )] and 255 variable parameters and converged yield-
ing final residuals:52 R = 0.102, Rw = 0.137, and GOF = 4.41.
Bis-amidinate complexes of first-row transition metals
The lithium amidinate LMe–Li(TMEDA) was found to react
readily with divalent first-row transition metal (Cr, Mn, Fe, Co,
and Ni) halides. Two equivalents of LMe–Li(TMEDA) reacted
with MX2 in THF at Ϫ78 ЊC to yield bis-amidinate metal
complexes in which both halide ligands were substituted by
amidinate ligands (Scheme 1). The resulting metal complexes
[(LMe)2M; M = Cr (1), Mn (2), Fe (3), Co (4), and Ni (5)] were
highly soluble in pentane and were isolated in moderate
yields as crystalline compounds. Each of these compounds is
paramagnetic and shows broad resonances in its 1H NMR spec-
trum. In all five cases, the mass spectrum (EI mode) showed a
Mϩ peak corresponding to the (LMe)2M molecular ion. Using
Evans’ NMR method,42 the magnetic susceptibilities of this
series of complexes was determined, and all values were char-
acteristic of high-spin metal() species. Additionally, the IR
spectra of these compounds, though not readily interpretable,
were very similar across this series of five compounds. From
these data, each compound (1–5) was assigned a tetrahedral
coordination geometry at the metal center composed of the
four donor nitrogen atoms from the two amidinate ligands
coordinated to the metal center.
(4). X-Ray quality crystals were grown from a saturated
pentane solution that was cooled to Ϫ40 ЊC. The final cycle of
full-matrix least squares refinement (minimizing the quantity
Σw(|Fo| Ϫ |Fc|)2, where w is the weight of a given observation)
was based on 2534 observed reflections [I > 3.00σ(I )] and
343 variable parameters and converged yielding final
residuals:52 R = 0.040, Rw = 0.042, and GOF = 1.21.
(5). X-Ray quality crystals were grown from a saturated
pentane solution that was cooled to Ϫ30 ЊC. The final cycle of
full-matrix least squares refinement (minimizing the quantity
Σw(|Fo| Ϫ |Fc|)2, where w is the weight of a given observation)
was based on 3086 observed reflections [I > 3.00σ(I )] and 334
variable parameters and converged yielding final residuals:52
R = 0.035, Rw = 0.041, and GOF = 1.38.
Interestingly, although a wide variety of reaction conditions
were tested, the bis-amidinate complexes (1–5) were the only
isolable complexes formed. Varying the solvent and the tem-
perature of the reactions did not result in formation of other
products. Additionally, reaction of the metal halides with one
equivalent of the amidinate [LMe–Li(TMEDA)] only led to the
formation of the bis-amidinate species. Moreover, addition of
strong donor ligands, such as pyridine or phosphines, also did
not prevent formation of the bis-amidinate complexes in these
reactions.
In order to confirm the geometry of these bis-amidinate
complexes, X-ray quality crystals of some of these species were
grown. The crystals of the manganese complex, (LMe)2Mn (2),
were found to be suitable for X-ray crystallography and the
solid-state structure of this compound was determined. The
structure is analogous to that observed for (LMe)2Mg,39 exhibit-
(7). X-Ray quality crystals were grown from a saturated
pentane solution that was cooled to Ϫ30 ЊC. A molecule of
pentane is incorporated in the unit cell and resides on an
inversion center, causing it to be disordered over two positions.
The final cycle of full-matrix least squares refinement (minim-
izing the quantity Σw(|Fo| Ϫ |Fc|)2, where w is the weight
of a given observation) was based on 2987 observed reflections
[I > 3.00σ(I )] and 421 variable parameters and converged
J. Chem. Soc., Dalton Trans., 2002, 3454–3461
3457

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ing the same two-fold ligand disorder observed in that system.
The amidinate core of the molecule was ordered and the bond
lengths and angles associated with this region of the crystallo-
graphic model are well determined. On the other hand,
positions of the atoms comprising the disordered terphenyl
group have a much higher degree of uncertainty. The crystal
structure of 2 confirms the tetrahedral geometry of this bis-
amidinate complex and the ordered portion of the molecule is
shown in Fig. 4, along with selected bond lengths and angles. In
Fig. 6 ORTEP view of (LMe)2Ni (5) with 50% thermal ellipsoids.
Selected bond lengths (Å): Ni1–N1 = 1.977(2), Ni1–N2 = 1.969(3), Ni1–
N3 = 1.972(2), N1–C1 = 1.337(4), N2–C1 = 1.315(4), N3–C22 =
1.325(2); bond angles (Њ): N1–Ni1–N2 = 66.3(1), N1–Ni1–N3 =
135.40(8), N1–Ni1–N3* = 135.40(8), N2–Ni1–N3 = 133.69(8), N2–
Ni1–N3* = 133.69(8), N3–Ni1–N3* = 66.1(1), N1–C1–N2 = 109.0(3),
N3–C22–N3* = 108.5(3).
Taking into account the propagation of uncertainty in the
determined values,55 the average bond lengths in 4 are 1.988(2)
Å for Co–N and 1.333(3) Å for the N–C of the amidinate back-
bone. The corresponding values in 5 are 1.972(1) and 1.324(2)
Å, respectively. Based on the difference in ionic radii for these
two metals (0.69 Å for Ni versus 0.72 Å for Co),56 one would
expect to see a corresponding difference of 0.03 Å in M–N
bonds in these two species. Though the predicted trend in bond
length is followed, the actual bond difference (0.016(2) Å
shorter for Ni–N) is less than would be expected. The slight
change in the amidinate N–C bond lengths (0.009(4) Å shorter)
in the nickel complex may indicate that more electron density
remains in the delocalized π-system of the amidinate ligand
in 4. In contrast, the bond angles (N–M–N and N–C–N) are
identical within uncertainty levels for these two compounds.
The Co–N bond lengths discussed above are somewhat
shorter than those found in the only other previous example of
Fig. 4 ORTEP view of the ordered core of (LMe)2Mn (2) with 50%
thermal ellipsoids. Selected bond lengths (Å): Mn1–N1 = 2.096(7),
Mn1–N2 = 2.075(7), N1–C7 = 1.33(1), N2–C7 = 1.34(1); bond angles
(Њ): N1–Mn1–N2 = 63.3(3), N1–Mn1–N1* = 132.5(5), N1–Mn1–N2* =
135.8(3), N2–Mn1–N2* = 141.3(5), N1–C7–N2 = 110.3(8).
general, the metrical parameters associated with 2 are similar
to those observed in previously characterized manganese
amidinate complexes, although the N–C–N bond angle of the
amidinate ligand (110.3(8)Њ) is substantially smaller than those
observed previously (113.5–120.7Њ).4,53,54 This is perhaps due
to steric interactions between the terphenyl group and the
isopropyl groups of the ligand backbone.
The structures of the cobalt (4) and nickel (5) bis-amidinate
species were also determined by X-ray crystallography. The two
structures are isomorphic, crystallizing in the monoclinic space
group P21/m. A crystallographic mirror plane cuts through the
compounds, rendering the N–C–N planes of the two amidinate
ligands perpendicular (interplanar angle = 90Њ). Also, within
each ligand, the terphenyl substituent is perpendicular to the
N–C–N binding plane, and hence parallel to the opposite lig-
and’s binding plane. ORTEP diagrams of these are shown below
(Figs. 5 (4) and 6 (5)) and selected metrical parameters are given.
a
structurally characterized bis-amidinate Co() species
{[FcN(CC6H10)2]2Co, Fc = ferrocenyl} (av. Co–N = 2.011 Å).57
Bond lengths in other related cobalt() species are also
generally longer, ranging from 2.065 to 2.259 Å.58,59 In contrast,
the Ni–N bond lengths observed for 5 fall well within the
range previously observed for related Ni() compounds (1.916–
2.141 Å).4,60
Mono-amidinate complexes of nickel and copper
As the mono-amidinate transition metal complexes proved to
be inaccessible via transition metal halide precursors, other
transition metal complexes were sought as precursors in these
reactions. Specifically, one equivalent of Ni(acac)2 was reacted
t
with the lithium amidinates [LMe–Li(TMEDA) or LBu
–
Li(TMEDA)] to produce compounds of the form (LR)Ni(acac)
for both the mesityl and the tert-butylphenyl substituted lig-
ands (Scheme 2). In both cases, the amidinate ligand displaced
one acetylacetonate ligand from the metal complex to form the
four-coordinate nickel species. For [(LMe)Ni(acac)] (6), the
product is a green oil at room temperature with a substantially
1
broadened H NMR spectrum, indicative of its paramagnetic
nature. The oil was consistently contaminated with a small
amount of free-base ligand (LMeH), preventing isolation of
analytically pure material, and consequently preventing the
determination of the magnetic moment (µeff) for 6. The best
evidence for the formation of 6 was provided by EI mass
spectrometry, which showed a molecular ion peak at m/z 597,
Fig. 5 ORTEP view of (LMe
) Co (4) with 50% thermal ellipsoids.
Selected bond lengths (Å): Co21–N1 = 1.973(4), Co1–N2 = 1.992(4),
Co1–N3 = 1.993(3), N1–C5 = 1.334(6), N2–C5 = 1.329(6), N3–C22 =
1.334(4); bond angles (Њ): N1–Co1–N2 = 66.0(1), N1–Co1–N3 =
134.6(1), N1–Co1–N3* = 134.6(1), N2–Co1–N3 = 134.8(1), N2–Co1–
N3* = 134.8(1), N3–Co1–N3* = 65.9(2), N1–C5–N2 = 108.3(4), N3–
C22–N3* = 108.8(4).
t
consistent with its generation. In contrast, (LBu )Ni(acac) (7)
was a diamagnetic purple solid, which was isolated as a pure
crystalline solid. The 1H NMR spectrum of this species
3458
J. Chem. Soc., Dalton Trans., 2002, 3454–3461

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Scheme 2
indicated the presence of one amidinate ligand and one acac
the Ni–O bond lengths in 7 (1.844(3) and 1.846(3) Å) fall well
ligand with C2v symmetry. The 13C NMR spectrum also showed
resonances in agreement with the formulation of 7, and the
mass spectrum again had the expected Mϩ ion peak (m/z 624).
Based on these initial data, 6 and 7 were assigned tetrahedral
and square planar metal coordination environments, respect-
ively. Both environments are common for previously observed
nickel complexes,61 and the differences observed in these two
species may be another indication of the increased steric bulk
of the mesityl-substituted amidinate ligand.
The oily nature of 6 made absolute assignment of its geom-
etry difficult, whereas crystallization of 7 from pentane resulted
in high-quality crystals of this material. The X-ray structure
(Fig. 7) confirms the initially assigned square planar coordin-
within the range previously observed for square planar L2Ni-
(acac) species (1.840–1.936 Å).62–69
Synthetically, the mono-amidinate complex 7 proved to be a
rather poor starting material for the generation of reactive
nickel species. The related Cp*Ni(acac) complex has been
shown to undergo a variety of substitution reactions, allowing
for the generation of a variety of alkyl and aryl complexes by
substitution of the acetylacetonate ligand.70–76 Unfortunately,
the reaction of 7 with a wide variety of alkyl, aryl, and amido
reagents77 led only to intractable decomposition mixtures,
regardless of the reaction conditions employed.
Another mono-amidinate transition metal complex was
synthesized by reaction of the tert-butylphenyl substituted
t
amidinate [LBu –Li(TMEDA)] with CuCl in THF (Scheme 3).
Scheme 3
t
Fig. 7 Side (inset) and top (main) views (ORTEP) of (LBu )Ni(acac)
This simple salt metathesis reaction proceeds with loss of LiCl
and formation of [(LBu )Cu]x (8) in good yield. The H NMR
(7) with 50% thermal ellipsoids. Selected bond lengths (Å): Ni1–N1 =
1.889(3), Ni1–N2 = 1.882(3), Ni1–O1 = 1.846(3), Ni1–O2 = 1.844(3),
N1–C1 = 1.326(5), N2–C1 = 1.323(5); bond angles (Њ): N1–Ni1–N2 =
68.8(1), N1–Ni1–O1 = 165.5(1), N1–Ni1–O2 = 98.0(1), N2–Ni1–O1
= 96.6(1), N2–Ni1–O2 = 166.8(1), O1–Ni1–O2 = 96.6(1), N1–C1–N2 =
107.1(4).
1
t
spectrum of 8 was very simple, showing only seven resonances
(all ligand-based) and indicative of a two-fold symmetric
amidinate ligand. No coordinated solvent resonances were
observed. After thorough washing of the white powder with
diethyl ether, 8 was extracted into toluene. Removal of
toluene under vacuum yielded a white powder of high purity, as
indicated by elemental analysis. Based on these data, 8 was
ation environment (see inset) composed of both amidinate
nitrogen atoms and both oxygen atoms from the acac ligand.
Though no crystallographically characterized (amidinate)-
Ni(acac) compounds exist for comparison, the Ni–N bond
lengths of 7 (1.889(3) and 1.882(3) Å) are found to be shorter
than those found in related Ni() species.4,60 On the other hand,
t
assigned an empirical formula of (LBu )Cu. The molecular
structure of 8 remains unknown as X-ray quality crystals of
this compound proved elusive, but a polymeric or cluster-type
arrangement seems likely for this material.
J. Chem. Soc., Dalton Trans., 2002, 3454–3461
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30 B. Vendemiati, G. Prini, A. Meetsma, B. Hessen, J. H. Teuben and
O. Traverso, Eur. J. Inorg. Chem., 2001, 707–711.
31 S. R. Foley, G. P. A. Yap and D. S. Richeson, Chem. Commun., 2000,
1515–1516.
32 E. A. C. Brussee, A. Meetsma, B. Hessen and J. H. Teuben, Chem.
Commun., 2000, 497–498.
33 K. M. Carlson-Day, J. L. Eglin and L. T. Smith, Inorg. Chim. Acta,
2000, 310, 282–284.
34 F. A. Cotton, L. M. Daniels, C. A. Murillo and P. Schooler, J. Chem.
Soc., Dalton Trans., 2000, 2001–2005.
Summary and conclusions
The coordination chemistry of the new sterically-bulky
amidinate ligands described recently has been explored, with
a focus on the chemistry of the mesityl-substituted ligand.
Formation of a series of bis-amidinate complexes deriving from
divalent transition metal halides has been presented. Addition-
ally, some mono-amidinate transition metal complexes have
been synthesized and characterized. These species demon-
strated the useful coordination chemistry of this ligand, but led
to little further reaction chemistry.
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45 SAINT: SAX Area-Detector Integration Program, V4.024, Siemens
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Acknowledgements
The authors gratefully acknowledge the NSF for funding, the
Department of Defense Science and Engineering Graduate
(NDSEG) Fellowship Program for fellowship support (JARS),
as well as Drs Fred Hollander and Allen Oliver for insightful
discussions.
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3461