Synthesis and characterization of three-coordinate and related β-diketiminate derivatives of manganese, iron, and cobalt

Article abstract of DOI:10.1021/ic025552s

Treatment of M{N(SiMe₃)₂}₂ (M = Mn, Fe, Co) with various bulky β-diketimines afforded a variety of new three-coordinate complexes which were characterized by UV-vis, ¹H NMR and IR spectroscopy, magnetic measurements, and x-ray crystallography. Reaction of the β-diketimine H(Dipp)NC(Me)CHC(Me)N(Dipp) (Dipp₂N^NH; Dipp = C₆H₃-2,6-Prⁱ₂) with M{N(SiMe₃)₂}₂ (M = Mn or Co) gave Dipp₂N^NMN(SiMe₃)₂ (M = Mn, 1; Co, 3) while the reaction of Fe{N(SiMe₃)₂}₂ with Ar₂N^NH (Ar = Dipp, C₆F₅, Mes, C₆H₃-2,6-Me₂, or C₆H₃-2,6-Cl₂) afforded the series of iron complexes Ar₂N^NFe{N(SiMe₃)₂} (Ar = Dipp, 2a; C₆F₅, 2b; Mes, 2c; C₆H₃-2,6-Me₂, 2d; C₆H₃-2,6-Cl₂, 2e). This represents a new synthetic route to β-diketiminate complexes of these metals. The four-coordinate bis-β-diketiminate complex Fe{N^N(C₆F₅)₂}₂, 4, was also isolated as a byproduct from the synthesis of 2b. Direct reaction of the Dipp₂N^NLi with CoCl₂ gave the ate salt Dipp₂N^NCoCl₂Li(THF)₂, 5, in which the lithium chloride has formed a complex with Dipp₂N^NCoCl through chloride bridging. The Fe(III) species Dipp₂N^NFeCl₂, 6, was obtained cleanly from the reaction of FeCl₃ with Dipp₂N^NLi. Magnetic measurements showed that all the complexes have a high spin configuration. The different substituents in the series of iron complexes 2a-e allowed assignment of their paramagnetically shifted ¹H NMR spectra. The x-ray crystal structures 1-2d and 3 showed that they have a distorted three-coordinate planar configuration at the metals whereas complexes 4-6 have highly distorted four- coordinate geometries.

Full text of DOI:10.1021/ic025552s

Inorg. Chem. 2002, 41, 3909−3916
Synthesis and Characterization of Three-Coordinate and Related
â-Diketiminate Derivatives of Manganese, Iron, and Cobalt
Arunashree Panda,† Matthias Stender,† Robert J. Wright,† Marilyn M. Olmstead,† Peter Klavins,‡ and
,†
Philip P. Power*
Departments of Chemistry and Physics, UniVersity of California, DaVis, One Shields AVenue,
DaVis, California 95616
Received February 20, 2002
Treatment of M{N(SiMe3)2}2 (M ) Mn, Fe, Co) with various bulky â-diketimines afforded a variety of new three-
coordinate complexes which were characterized by UV−vis, 1H NMR and IR spectroscopy, magnetic measurements,
and X-ray crystallography. Reaction of the â-diketimine H(Dipp)NC(Me)CHC(Me)N(Dipp) (Dipp N∧NH; Dipp ) C H -
2
6 3
i
2,6-Pr2) with M{N(SiMe3)2}2 (M ) Mn or Co) gave Dipp2N∧NMN(SiMe3)2 (M ) Mn, 1; Co, 3) while the reaction
of Fe{N(SiMe3)2}2 with Ar2N∧NH (Ar ) Dipp, C F , Mes, C H -2,6-Me2, or C H -2,6-Cl2) afforded the series of iron
6
5
6
3
6 3
complexes Ar2N∧NFe{N(SiMe3)2} (Ar ) Dipp, 2a; C F , 2b; Mes, 2c; C H -2,6-Me2, 2d; C H -2,6-Cl2, 2e). This
6
5
6
3
6 3
represents a new synthetic route to â-diketiminate complexes of these metals. The four-coordinate bis-â-diketiminate
complex Fe{N∧N(C F ) }2, 4, was also isolated as a byproduct from the synthesis of 2b. Direct reaction of the
6
5 2
Dipp2N∧NLi with CoCl2 gave the “ate” salt Dipp2N∧NCoCl2Li(THF)2, 5, in which the lithium chloride has formed a
complex with Dipp2N∧NCoCl through chloride bridging. The Fe(III) species Dipp2N∧NFeCl2, 6, was obtained cleanly
from the reaction of FeCl3 with Dipp2N∧NLi. Magnetic measurements showed that all the complexes have a high
spin configuration. The different substituents in the series of iron complexes 2a−e allowed assignment of their
1
paramagnetically shifted H NMR spectra. The X-ray crystal structures 1−2d and 3 showed that they have a
distorted three-coordinate planar configuration at the metals whereas complexes 4−6 have highly distorted four-
coordinate geometries.
Introduction
librium.6,7 Their monomeric, coordinatively unsaturated
structures confer high reactivity, and their chemistry has been
dominated by reactions with Lewis bases such as THF,6,8
pyridine, or phosphines, or with protic reagents such as
alcohols, phenols, thiols, selenols, tellurols,9 calixarenes,10
boronous acids,11 secondary phosphines,12 or silanols.13
Transamination reactions involving treatment of M{N-
(SiMe3)2}2 with primary or secondary amines have not been
extensively studied. Rare examples include the reaction of
H2NDipp (Dipp ) C6H3-2,6-Pri2) with Mn{N(SiMe3)2}2 to
The divalent transition metal silylamides M{N(SiMe3)2}2
(M ) Mn,1 Fe,2 or Co3) are useful hydrocarbon soluble
sources of M2+ that have semipolar and reactive M-N bonds.
They possess two-coordinate monomeric structures in the
vapor phase2 but exist as amide bridged dimers in the
crystalline state.4-6 In solution, at room temperature, they
are essentially dissociated to monomers but are weakly
associated at low temperatures in an entropy driven equi-
* To whom correspondence should be addressed. E-mail: pppower@
ucdavis.edu.
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† Department of Chemistry.
‡ Department of Physics.
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F.; Leung, W.-P.; Rypdal, K. Inorg. Chem. 1988, 27, 1782.
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10.1021/ic025552s CCC: $22.00 © 2002 American Chemical Society
Published on Web 06/22/2002
Inorganic Chemistry, Vol. 41, No. 15, 2002 3909

Panda et al.
give Mn3{µ-N(H)Dipp}4{N(SiMe3)2}214 and the reaction of
Mn{N(SiMe3)2}2 with the 1,2-diamine H(Dipp)NCH2CH2N-
(Dipp)H to afford Mn{N(Dipp)CH2CH2N(Dipp)H}2.15 The
steric properties of the diprotic bidentate ligand H(Dipp)-
NCH2CH2N(Dipp)H bear a resemblance to those of the
monoprotic bidentate ligand H(Dipp)NC(Me)CHC(Me)N-
(Dipp) which may be abbreviated as Dipp2N∧NH.16 This
ligand and closely related ones have enjoyed considerable
recent use in a variety of transition metal16-37 and main group
complexes38-62 which have displayed a variety of stoichi-
ometries, low-coordination numbers, and bonding. In addi-
tion, they have been used as model species for the active
sites in metalloenzymes28,29,31 and as catalysts for olefin
polymerization.16,19,22-25,32,33,40,44 Very recent work has dis-
closed the preparation and characterization of the iron salts
(Dipp2N∧N)Fe(µ-Cl)2Li(THF)2 and {(Dipp2N∧N)Fe(Cl)(µ-
Cl)}2Mg(THF)2 and the neutral species (Dipp2N∧N)′FeCl in
which three coordination has been achieved by increasing
the steric properties of the â-diketiminate ligand via replace-
ment of the methyl groups on the central C3N2 moiety by
tert-butyl groups.63 Moreover, reduction of the latter under
N2 has resulted in very interesting N2 complexes of the type
(Dipp2N∧N)′FeNNFe(N∧NDipp2)′ and K2(Dipp2N∧N)′Fe-
NNFe(N∧N′Dipp2)′.64 It is now shown that reaction of
Dipp2N∧NH and related â-diketiminate ligands, as repre-
sented by the general formula
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with the amides M{N(SiMe3)2}2 (M ) Mn, Fe or Co)
smoothly affords new â-diketiminates of the type (Ar2N∧N)-
MN(SiMe3)2 where three coordination has been obtained with
use of the bulky coligand N(SiMe3)2. In addition, it is shown
that, in contrast to the reaction of LiN∧NDipp2 with FeCl2
or CoCl2 which afford “ate” complexes of the type Dipp2-
N∧NMCl2Li(THF)2 (M ) Fe63 or Co), reaction of
LiN∧NDipp2 with FeCl3 affords the neutral derivative
Dipp2N∧NFeCl2.
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Experimental Section
General Procedures. All work was performed by using Schlenk
techniques under an atmosphere of N2 or in a Vacuum Atomspheres
HE-43 drybox. All solvents were freshly distilled from Na/K and
degassed three times immediately before use.
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Cowley, A. H. Chem. Commun. 2001, 1866.
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3910 Inorganic Chemistry, Vol. 41, No. 15, 2002

â-Diketiminate DeriWatiWes of Mn, Fe, and Co
The compounds M{N(SiMe3)2}2 {M ) Mn,1 Fe,2 Co3}, Dipp2-
N∧NH,16,61 Mes2N∧NH,22 (2,6-Me2C6H3)2N∧NH,23 and (2,6-
Cl2C6H3)2N∧NH23 were synthesized by literature methods. Anhy-
drous CoCl2 (Aldrich), FeCl3 (Aldrich), and 1.6 M n-BuLi in
hexanes (Acros) were purchased from the commercial suppliers
and were used without further purification. 1H NMR spectra were
recorded in C6D6 or CDCl3 at 300 MHz on a QE-300 spectrometer.
Infrared spectra were obtained as Nujol mulls with use of a Perkin-
Elmer-1430 spectrometer. Electronic absorption spectra were
obtained on a Hitachi U-2000 UV-vis spectrometer. Melting points
are uncorrected and were determined for samples in capillaries
sealed with grease. For magnetic measurements, the samples were
sealed under vacuum in 3.2 × 2 mm2 quartz tubing. The sample
holder was designed to minimize the background signal. The sample
magnetization was measured using a Quantum Design MPMSXL7
superconducting quantum interference device (SQUID) magnetom-
eter. For each measurement, the sample was zero-field cooled to 5
K, and the magnetization was measured as a function of field to 2
T. The field was then reduced to 1 T, and the magnetization of the
sample was measured in 5 K increments to 300 K.
mmol) to afford the product as orange crystals. Yield: 0.51 g (46%),
mp 135-137 °C. IR (Nujol, cm-1): 2920, 2860, 1610, 1460, 1380,
1260, 1100, 800, and 350. UV-vis (hexane, λmax, nm (ꢀ, M-1
cm-1): 378 (12 400), 406 (11 100), 482 (3400), 779 (130), and
972 (250).
(2,6-Me2C6H3)2N∧NFe(SiMe3)2 (2d). This was prepared in a
manner similar to 2a as orange crystals from (2,6-Me2C6H3)2N∧-
NH (0.60 g, 2 mmol) and Fe{N(SiMe3)2}2 (0.75 g, 2 mmol).
Yield: 0.49 g (47%), mp 88-90 °C. IR (Nujol, cm-1): 2920, 2850,
1520, 1460, 1365, 1260, 1245, 1180, 1095, 1020, 970, 870, 760,
670, 350. UV-vis (hexane; λmax, nm (ꢀ, M-1 cm-1)): 376 (12 000),
404 (10 900), 480 (3 200), 776 (130), and 970 (230).
(2,6-Cl2C6H3)2N∧NFe(SiMe3)2 (2e). This was synthesized in a
manner similar to 2a as orange crystals from (2,6-Cl2C6H3)2N∧NH
(0.78 g, 2 mmol) and Fe{N(SiMe3)2}2 (0.75 g, 2 mmol). Yield:
0.72 g (73%), mp 94-96 °C. IR (Nujol, cm-1): 2920, 2840, 1525,
1435, 1370, 1255, 1195, 1150, 1020, 980, 930, 825, 675, and 365.
UV-vis (hexane; λmax, nm (ꢀ, M-1 cm-1)): 362 (10 000), 411
(8100), 490 (2100), and 780 (110).
Dipp2N∧NCoN(SiMe3)2 (3). This was prepared in a manner
similar to the manganese derivative 1 and isolated as red needles
from hexane. Yield: 0.35 g (30%), mp 192-194 °C. IR (Nujol,
cm-1): 2920, 2840, 1525, 1315, 1175, 1095, 1020, 930, 870, 720,
660, 385, and 360. UV-vis (hexane; λmax (ꢀ, M-1 cm-1)): 235
(14 300), 316 (14 300), 437 (1230), 470 (670). µeff: 4.81(3) µB.
Fe{N∧N(C6F5)2}2 (4). This was obtained as red crystals from
the mother liquor of 2b. Yield: 0.70 g (38%), mp 206-208 °C.
IR (Nujol, cm-1): 2900(b), 2640, 2440, 1630, 1500, 1360, 1160,
1000, 765, 710, 700, 640, 530, 475, 330, and 300. UV-vis (hexane;
λmax (ꢀ, M-1 cm-1)): 223 (8400), 338 (12 300), 437 (2900), 868
(230), 976 (140).
Dipp2N∧NCoCl2Li(THF)2 (5). n-BuLi (6.2 mL of a 1.6 M
solution in n-hexane) was added dropwise to a stirred solution of
Dipp2NNH (4.0 g, 9.5 mmol) in ca. 30 mL of diethyl ether with
cooling in an ice bath. After overnight stirring, the solution of the
Dipp2N∧NLi was added dropwise to a suspension of CoCl2 (1.24
g, 9.5 mmol) in THF (ca. 15 mL) with cooling in an ice bath. After
overnight stirring, the color had changed to dark green. The solvents
were removed under reduced pressure, and the residue was
recrystallized from toluene (30 mL) as red crystals. Yield: 4.4 g
(55%) IR (Nujol, cm-1): 1520, 1460, 1315, 1260, 1175, 1100, 1040,
935, 890, 790, 755, 400, and 305. UV-vis (hexane; λmax (ꢀ, M-1
cm-1)): 250 (9200), 322 (9300), 432 (1900), 532 (1250), 615
(1100), and 868 (1150).
Dipp2N∧NFeCl2 (6). The synthesis was accomplished with use
of Dipp2N∧NLi (9.5 mmol) and FeCl3 (1.54 g, 9.5 mmol) in ca. 15
mL of toluene. Filtration and cooling in ca. -20 °C freezer afford
green crystals of 6 that were suitable for X-ray crystallography.
Yield: 2.75 g (53%), mp 202-204 °C. IR (Nujol, cm-1): 1520,
1460, 1375, 1335, 1315, 1260, 1096, 1020, 930, 795, 755, 400,
and 365. UV-vis (hexane; λmax (ꢀ, M-1 cm-1)): 319 (8200), 382
(8200), 553 (4600), and 813 (1800).
X-ray Crystallographic Studies. Crystals of 1-2d and 3-6
were removed from the Schlenk tube under a stream of N2 and
immediately covered with a layer of hydrocarbon oil. A suitable
crystal was selected, attached to a glass fiber, and quickly placed
in the low-temperature nitrogen stream.65 The data were recorded
near 90 K (293 K for 6) for a Bruker SMART 1000 (Mo KR
radiation and a CCD area detector). The SHELXTL version 5.03
program package was used for the structure solutions and refine-
ments.66 Absorption corrections were applied using the SADABS
program.67 The crystal structures were solved by direct methods
and refined by full-matrix least-squares procedures. All non-
(C6F5)2N∧NH. A mixture of 2,3,4,5,6-pentafluoroaniline (8.0 g,
43.7 mmol), 2,4-pentanedione (1.9 g, 19.0 mmol), and p-toluene-
sulfonic acid (3.4 g, 18 mmol) in toluene (150 mL) was refluxed
in a Dean-Stark apparatus for 24 h. The slurry was filtered to give
a brown solid that was treated with 50 g of Na2CO3 in water (300
mL) and 300 mL of diethyl ether. The mixture was stirred for 45
min. The organic layer was isolated, dried with MgSO4, and filtered.
The solvent was removed, and the product was washed with 50
mL of cold methanol. Yield: 4.8 g (59%), mp 66-68 °C. 1H NMR
(CDCl3, 300 MHz, 25 °C): δ 1.95 (s, 6H, CH3), 5.20 (s, H, g-CH),
12.08 (s, H, NH). 13C{1H} NMR (CDCl3, 75 MHz, 25 °C): δ 21.0
(CH3), 99.2 (γ-C), 119.9 (m-C), 136.5 (p-C), 139.5 (o-C), 142.8
(i-C), 164.2 (CN). 19F{1H} NMR (CDCl3, 376.12 MHz, 25 °C): δ
3
4
-146.6 (m, 4F, o-F, J ) 22.9 Hz, J ) 6.1 Hz), -158.32 (t, 2F,
p-F, 3J ) 21.3 Hz), -160.78 (m, 4F, m-F, 3J ) 21.3 Hz, 4J ) 6.1
Hz).
Dipp2N∧NMnN(SiMe3)2 (1). A mixture of Mn{N(SiMe3)2}2
(0.75 g, 2 mmol) and Dipp2N∧NH (0.84 g, 2 mmol) in a Schlenk
tube was heated at 110-120 °C under reduced pressure (ca. 0.03
mm) until bubbling had ceased (ca. 5 min). Upon cooling to room
temperature, a dark yellow solid was obtained. This was dissolved
in hexane (15 mL) and cooled in a ca. -20 °C freezer overnight to
afford the product as yellow crystals. Yield: 0.55 g (43%), mp
235-237 °C. IR (Nujol, cm-1): 2950, 2920, 2840, 1520, 1310,
1170, 1095, 1015, 930, 870, 750, 660, 400, and 350. UV-vis
(hexane, λmax, nm (ꢀ, M-1 cm-1)): 231 (11 000), 376 (11 000),
443 (1040), and 466 (690). µeff: 5.65(3) µB.
Dipp2N∧NFeN(SiMe3)2 (2a). This was synthesized in a manner
similar to the manganese derivative 1 with use of Dipp2N∧NH (0.84
g, 2 mmol) and Fe{N(SiMe3)2}2 (0.75 g, 2 mmol) and recrystallized
as orange plates from hexane. Yield: 0.47 g (37%), mp 212-214
°C. IR (Nujol, cm-1): 2920, 2850, 1520, 1315, 1170, 1100, 1020,
980, 875, 760, 670, and 355. UV-vis (hexane, λmax, nm (ꢀ, M-1
cm-1)): 318 (12 500), 371 (12 700), 483 (600), and 799 (600).
µeff: 4.94(2) µB.
(C6F5)2N∧NFeN(SiMe3)2 (2b). The synthesis was accomplished
in the same manner with use of (C6F5)2N∧NH (0.81 g, 1.88 mmol)
and Fe{N(SiMe3)2}2 (0.75 g, 2 mmol) to afford the product as
orange-yellow crystals. Yield: 0.75 g (62%), mp 108-110 °C. IR
(Nujol, cm-1): 2920, 2840, 1510, 1500, 1445, 1370, 1310, 1250,
1010, 990, 825, 670, 630, and 350.
Mes2N∧NFe(SiMe3)2 (2c). This was prepared in the same manner
from Mes2N∧NH (0.66 g, 2 mmol) and Fe[N(SiMe3)2]2 (0.75 g, 2
Inorganic Chemistry, Vol. 41, No. 15, 2002 3911

Panda et al.
Table 1. Data Collection Parameters for Compounds 1, 2a-d, 3-6
1
2a
2b
2c
2d
3
4
5
6
formula
C35H59Mn-
N3Si2
632.97
C35H59Fe-
N3Si2
633.88
C23H25F10Fe-
N3Si2
645.49
C29H47Fe-
N3Si2
549.73
C27H43Fe-
N3Si2
521.67
C35H59Co-
N3Si2
636.96
C34H14F20-
FeN4
914.34
C47.5H67.5Cl2-
CoLiN2O2
835.31
C29H41Cl2-
FeN2
544.39
fw
cryst syst
a (Å)
b (Å)
monoclinic
9.0944(6)
20.232(1)
20.278(1)
monoclinic
9.0729(5)
20.186(1)
20.215(1)
triclinic
monoclinic
17.091(3)
15.102(3)
12.501(3)
monoclinic
20.321(2)
14.173(1)
20.577(2)
monoclinic
9.0909(5)
20.199(1)
20.105(1)
triclinic
monoclinic
16.455(1)
17.827(1)
15.830(1)
monoclinic
12.5392(6)
19.5799(9)
13.1323(6)
10.1239(5)
13.8608(6)
21.041(1)
104.737(1)
90.050(1)
100.889(2)
2800.4(2)
4
10.1625(4)
11.3316(4)
15.9244(6)
100.059(1)
95.239(1)
111.038(1)
1673.5(1)
2
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
90.053(2)
90.503(1)
101.248(9)
96.404(5)
91.090(1)
91.486(1)
117.136(1)
3731.3(4)
4
3702.1(3)
4
3164.5(11)
4
5927(1)
8
3691.4(1)
4
4642.2(5)
4
2869.3(2)
4
µ (Mo KR)
0.444
0.498
0.709
0.573
0.608
0.556
0.597
1.195
1.260
(mm-1
)
T/ K
R1
wR2
90(2)
0.0473
0.114
90(2)
0.0349
0.0933
90(2)
0.0341
0.0928
90(2)
0.0302
0.0867
91(2)
0.0493
0.1463
90(2)
0.0342
0.0864
90(2)
0.0342
0.0881
90(2)
0.0394
0.1047
293(2)
0.0481
0.1048
Dipp2N∧NMCl (M ) Fe or Co) is insufficiently crowding
to prevent association with another chloride. However, it has
been shown that when Dipp2N∧N ligand was modified by
replacement of the methyls on the ligand backbone with tert-
butyl groups (designated (Dipp2N∧N)′) the neutral species
(Dipp2N∧N)′-
hydrogen atoms were refined anisotropically. Hydrogen atoms were
included in the refinement at calculated positions using a riding
model included in the SHELXTL program. Some details of the
data collection and refinement are given in Table 1. Further details
are provided in the Supporting Information.
Results and Discussion
FeCl was obtained in which lithium chloride was eliminated
and the iron is bound only to the â-diketiminate ligand and
a chloride to yield a three-coordinate metal environment.63
Direct reaction of Dipp2N∧NLi with FeCl3 in toluene afforded
the neutral Fe3+ compound Dipp2N∧NFeCl2, 6, as green
crystals in ca. 50% yield. No problems were encountered
with the separation of LiCl, probably as a result of the more
crowded, higher coordinate, character of the iron.
Synthesis. The compounds that have been synthesized and
characterized in this paper are indicated by the formulas
Dipp2N∧NMnN(SiMe3)2, 1; Dipp2N∧NFeN(SiMe3)2, 2a;
(C6F5)2N∧NFeN(SiMe3)2, 2b; Mes2N∧NFeN(SiMe3)2, 2c;
(2,6-Me2C6H3)2N∧NFeN(SiMe3)2, 2d; (2,6-Cl2C6H3)2-
N∧NFeN(SiMe3)2, 2e; Dipp2N∧NCoN(SiMe3)2, 3; Fe{N∧N-
(C6F5)2}2, 4; Dipp2N∧NCoCl2Li(THF)2, 5; Dipp2N∧NFeCl2,
6, where the abbreviation N∧N has been used to represent
the central ring of the â-diketiminate ligand, as illustrated
in the Introduction. The series of eight compounds 1-4 were
obtained in a facile manner by the direct reaction of the
Ar2N∧NH ligand with the metal amides M{N(SiMe3)2}2 at
ca. 100 °C in the absence of solvent. Purification was
accomplished by recrystallization from hexane to give the
products as air-sensitive crystals with nonoptimized yields
in the range 40-70%. It was found during the preparation
of pentafluorphenyl compound 2b that both N(SiMe3)2
groups could be displaced from iron to give bis-â-diketimi-
nate species 4. The double substitution probably occurs as a
result of the higher acidity of (C6F5)2N∧NH and the fact that
it is the least sterically crowding of the â-diketiminate ligands
used in this work. There was no evidence for a bis-
(diketiminate) product in any of the reactions with the bulkier
Dipp2N∧NH ligand. The lack of a doubly substituted product
may be contrasted with the reaction between the sterically
related diamine H(Dipp)NCH2CH2(Dipp)H and Mn-
{N(SiMe3)2}2 which afforded the unusual disubstituted
species Mn{N(Dipp)CH2CH2N(Dipp)H}2.15 The reaction of
LiN∧NDipp2 with CoCl2 in the hexane/ether/THF solvent
mixture afforded the “ate” product 5 as red crystals. This
result is similar to the previously reported synthesis of the
iron compound Dipp2N∧NFeCl2Li(THF)2 which also crystal-
lizes as an “ate” complex.63 Seemingly, the combination of
Dipp2N∧N and a single chloride ligand in the putative species
Spectroscopic and Magnetic Studies
The magnetic moments of the compounds were measured
in C6D6 solution by the Evans’ method. These measurements
afforded µeff(C6D6) values that were consistent with high spin
d5 (1, µB ) 5.9(1); 6, µB ) 5.9(1)), d6 (2a-e and 4, µB )
4.9-5.1(1)), and d7 (3, µB ) 4.9(1); 5, µB ) 5.0(1))
configurations that have five, four, and three unpaired
electrons, respectively. Magnetic studies of crystalline samples
of 1, 2a, and 3 were also undertaken. Plots of 1/ø versus
temperature in the range 5-300 K afforded linear or near
linear relationships. The compounds displayed the expected
Curie-Weiss behavior and afforded µeff values of 5.65(3)
µB (1), 4.94(2) µB (2a), and 4.81(3) µB (3). These values are
slightly lower than the solution values in the case of 1 and
3. The reasons for these differences are unclear.
The presence of unpaired electrons in all the compounds
gives rise to paramagnetically shifted 1H NMR spectra. The
spectra of the iron compounds 2a-e (Table 2), which differ
only in their nitrogen substituents, can be assigned as a result
of the effects of the differences in substitution on the signals.
The simplest 1H NMR spectrum arises from C6F5 substituted
species 2b which has only three types of magnetically
inequivalent hydrogens. In C6D6 at 25 °C, three major
broadened signals were observed at δ -9.58, 50.84, and
93.45. In addition, a minor signal at -66.50 ppm was
observed. The three major signals can be assigned to â-Me
(δ -9.58), N(SiMe3)2 (δ 50.84), and γ-H (δ 93.45). Their
(65) Hope, H. Prog. Inorg. Chem. 1995, 41, 1.
3912 Inorganic Chemistry, Vol. 41, No. 15, 2002

â-Diketiminate DeriWatiWes of Mn, Fe, and Co
Figure 1. 1H NMR spectrum of (2,6-Me2H3C6)2N∧NFeN(SiMe3)2 (2d) in C6D6 at 25 °C. Solvent and free ligand peaks are marked x.
Table 2. 1H NMR Shifts (ppm) of the Iron Compounds
those of 2e. The p-H signal (δ -65.92) is very close to that
in 2e but the m-H signal is observed at δ 46.24. The new
Ar2N∧NFeN(SiMe3)2, 2a-e in C6D6 at ca. 298 K
Ar
γ-H â-Me N(SiMe3)2
o-R
m-H
p-H(R)
signal at δ -55.91 is attributable to the o-Me hydrogens.
No signal readily assignable to γ-H could be observed. The
mesityl substituted 2c differs from 2d at the para positions
only. Thus, the chemical shifts of â-Me, N(SiMe3)2, o-Me,
and m-H of 2c are very similar to those observed in 2d. The
p-H signal seen at δ -65.92 in 2d is not observed in 2c.
Instead, a new, more intense, narrow signal at δ 27.2,
attributable to the p-Me of the mesityl groups, is observed.
Finally, in 2a, the resonances at δ -9.2 and δ 13.6 are
assignable to the â-Me hydrogens and N(SiMe3)2 groups,
and the signal at δ 51.6 is due to the m-Hs. These values
are similar to those observed in 2c and 2d. Assignments of
further peaks at δ -52.8 and δ 83.2 to the p-H and γ-H
signals seem reasonable in view of their similarity to the
corresponding signals in 2b, 2d, and 2e. The assignment of
the signals associated with the Pri substituents is more
complicated because the methyl groups are diastereotopic,
and two equally intense resonances which integrate in the
ratio 12H each (when their intensity is compared to the
N(SiMe3)2 and â-Me signals) were observed at δ 3.2 and
-44.1.
The electronic spectra of 1-6 are characterized by intense
absorptions (ꢀ values as high as 14 000) at wavelengths
generally shorter than 380 nm that are attributable to π-π*
transitions of the â-diketiminate and aryl substituents or
metal-â-diketiminate charge transfer transitions. These in-
tense peaks tail into the visible region, and this feature is
probably responsible for the prevalence of the yellow to red
color of the majority of the complexes. Less intense
absorptions are observed at longer wavelengths, and those
that occur at ca. <500 nm are usually shoulder features on
the more intense π-π* absorption at shorter wavelengths.
The spectra of the iron complexes 2a-e feature two
2a C6H3-2,6-Pri2
83.2
-9.2
13.6
3.2,
51.6 -52.8
-44.1
2b C6F5
93.45 -9.58
-12.41
50.84
19.44
20.35
30.34
2c C6H2-2,4,6-Me3
2d C6H3-2,6-Me2
2e C6H3-2,6-Cl2
-55.41 48.01 27.2 (Me)
-55.91 46.24 -65.92
9.13 -67.99
-12.51
93.73 -9.46
integration corresponded to the ratio of â-Me (6), N(SiMe3)2
(18), and γ-H (1) hydrogens present in the complex. The
minor signal at δ -66.50 may be due to a small amount of
bis-â-diketiminate complex 4 which is a byproduct of the
reaction. The assignment of the δ 50.84 signal to the
N(SiMe3)2 hydrogens is also supported by the similarity of
the shift value to the δ 63.5 signal observed for monomeric
7
Fe{N(SiMe3)2}2 and the methyl signals in the related
monomeric complexes Fe{N(SiMe2Ph)2}2 (δ 49.9) and
Fe{N(SiMePh2)2}2 (δ 57.6).68 A change in the aryl substit-
uents at nitrogen from C6F5 (2b) to C6H3-2,6-Cl2 (2e) should
result in two new hydrogen signals (i.e., of the meta and
para hydrogens), and this is what is observed. The signals
in the spectrum of 2e due to N(SiMe3)2 (δ 30.34), â-Me (δ
-9.46), and γ-H (δ 93.73) are readily assignable. The new
signals appear at δ 9.13 and δ -67.99. These are assignable
to the meta and para hydrogens. They integrate in a 2:1 ratio
and in the correct intensity relative to the N(SiMe3)2 and
â-Me signals. Replacement of the chlorines in 2e with methyl
1
groups affords 2d whose H NMR spectrum is shown in
Figure 1. The â-Me (δ -12.51) and N(SiMe3)2 (δ 20.35)
signals of 2d are shifted slightly upfield in comparison with
(66) SHELXL, version 5.1; Bruker AXS: Madison, WI.
(67) SADABS an empirical absorption correction program, part of the
SAINTPlus NT version 5.0 package; Bruker AXS: Madison, WI,
1998.
(68) Chen, H.; Bartlett, R. A.; Dias, H. v. R.; Olmstead, M. M.; Power, P.
P. J. Am. Chem. Soc. 1989, 111, 4338.
Inorganic Chemistry, Vol. 41, No. 15, 2002 3913

Panda et al.
Figure 2. Illustrations (30% thermal ellipsoids) of the structures of Dipp2N∧NMnN(SiMe3)2 (1), (C6F5)2N∧NFeN(SiMe3)2 (2b), Dipp2N∧NCoN(SiMe3)2
(3), Fe{N∧N(C6F5)2}2 (4), Dipp2N∧NCoCl2Li(THF)2 (5), and Dipp2N∧NFeCl2 (6). H atoms are not shown. Selected bond distances and angles are in Table
3.
prominent absorptions in the ranges 480-490 nm and 775-
790 nm with a shoulder feature at ca. 400 nm visible for 2c,
2d, and 2e. A weak band at ca. 970 nm in 2c and 2d is also
apparent. The ground state of the free ion Fe2+(d6) term is
5D, and this is expected to split into 5A′ + 5E′ + 5E′′ terms
in a trigonal planar (D3h) field so that at least two d-d
transitions are expected. However, the geometric distortions
due to the â-diketiminate ligand lower the local symmetry
at the metal further to C2V. Thus, the E bands could be further
split which, if great enough, could give rise to further
absorptions. However, the spectra of 2a suggest that this
splitting is not large.
ever, there are very marked deviations of the interligand
angles from 120°. The narrowest angle in each complex is
associated with the â-diketiminate ligand where the N(1)-
M-N(2) angles are in the range 92.57(6)-96.60(4)°. The
remaining interligand angles are wider and span the range
126.98(5)-139.76(5)°. Another notable structural distortion
concerns the MN2C3 ring geometries which display folding
along the N(1)‚‚‚N(2) line in the case of 1, 2a, and 3 (fold
angles ) ca. 165°) but are much closer to planarity for 2b,
2c, and 2d. It is noteworthy that the greatest folding is
observed in the derivatives of the bulkiest Dipp2N∧N ligand
(i.e., complexes 1, 2a, and 3) and this finding is consistent
with previous structural studies of other metal complexes of
this ligand where folding was attributed to the steric
requirements of the Dipp substituents.62 Within the â-diketim-
inate rings, the N2C3 moiety is planar, and the N-C and
C-C distances lie in the narrow ranges 1.328(2)-1.347(2)
Å and 1.396(3)-1.416(2) Å consistent with the multiple
character of these bonds. The internal angles within the rings
display little variation across the series. In each complex,
the widest angle is observed at C(2) which has an average
value of 129.3(4)°. The angle in each complex is within ca.
0.6° of this value. The M-N bonds to the â-diketiminate
ligands are ca. 0.05-0.10 Å longer than those to the
N(SiMe3)2 groups. This is consistent with their semidative
character. The compounds 1, 2a, and 3 have the same ligand
Structures
All compounds except 2e were characterized by X-ray
crystallography. Six of these nine structures, that is, 1, 2a-
d, and 3, are related in that they feature the metal bound to
one â-diketiminate and one N(SiMe3)2 ligand. Representative
structures are shown in Figure 2. Selected bond lengths and
angles are given in Table 3. They display an essentially
planar, three-coordinate69-72 geometry at the metals. How-
(69) Bradley, D. C. Chem. Ber. 1975, 11, 393.
(70) Eller, P. G.; Bradley, D. C.; Hursthouse, M. B.; Meek, D. W. Coord.
Chem. ReV. 1987, 24, 1.
(71) Cummins, C. C. Prog. Inorg. Chem. 1998, 47, 685.
(72) Alvarez, S. Coord. Chem. ReV. 1999, 193-195, 13.
3914 Inorganic Chemistry, Vol. 41, No. 15, 2002

â-Diketiminate DeriWatiWes of Mn, Fe, and Co
in the neutral M(II) dimers [M{N(SiMe3)2}2]24-6 which also
feature three-coordinate metals. However, they are up to ca.
0.08 Å longer than the M-N distances in the three-
coordinate, M(II) anions [M{N(SiMe3)2}3]-.73 The longer
M-N distances in the latter are probably a result of increased
interelectronic repulsion due to the presence of negative
charge from the “extra” electron. The terminal MNSi2
moieties in 1, 2, and 3 have planar geometries (as they have
in all the structures), and they are oriented at angles of ca.
74° with respect to the MN3 coordination planes.
Table 3. Selected Bond Distances (Å) and Angles (deg) for 1-5
1
2a
3
M-N(1)
2.050(1)
2.096(1)
1.992(1)
2.027(1)
1.988(1)
1.928(1)
94.33(4)
131.07(4)
134.39(4)
116.96(7)
118.95(7)
1.328(2)
1.340(2)
1.410(2)
1.400(2)
1.716(1)
1.718(1)
111.77(5)
121.58(6)
125.50(6)
164.8/0.365
1.982(1)
1.947(1)
1.903(1)
96.60(4)
129.03(4)
132.83(4)
118.20(8)
119.39(8)
1.330(2)
1.339(2)
1.411(2)
1.396(2)
1.713(1)
1.721(1)
114.72(6)
125.49(6)
125.49(6)
165.3/0.340
M-N(2)
M-N(3)
N(1)-M-N(2)
N(1)-M-N(3)
N(2)-M-N(3)
M-N(1)-C(6)
M-N(2)-C(18)
N(1)-C(1)
92.57(5)
135.81(6)
131.54(6)
118.98(10)
116.66(10)
1.342(2)
1.325(2)
1.398(2)
1.416(2)
1.707(2)
N(2)-C(3)
C(1)-C(2)
The structures of the series of four iron derivatives 2a-d
also display interesting trends. The Fe-N (â-diketiminate)
bond lengths are all within 0.01 Å of each other and have
an average value of 2.000(9) Å. This is significantly longer
than the 1.948(2) Å reported for (Dipp2N∧N)′FeCl (in which
the Me groups on the core ring are replaced by But groups).63
Seemingly, the electronegative chloride ligand in the latter
complex decreases the effective ionic radius of iron. This,
together with the relatively small size of Cl, which relieves
steric crowding, results in shorter Fe-N bonds. The trend
for the Fe-N(SiMe3)2 bond lengths in 2a-d lends support
to the importance of steric effects. The longest distance,
1.928(1) Å, is observed for the bulkiest Dipp2N∧N derivative,
2a, whereas the shortest distance, 1.908(1) Å, is seen with
the least bulky (C6F5)2N∧N complex, 2b. The essentially
equal intermediate distances of 1.915(1) and 1.917(2) Å are
observed with the Mes2N∧N and 2,6-Me2C6H3N∧N deriva-
tives 2c and 2d which have almost identical steric properties
near the iron and sizes between the extremes of 2a and 2d.
In essence, the observed trend suggests that the bond length
variations in these complexes are mainly a result of steric
effects. However, electronic effects may also play a role
because the widest Si-N-Si angle (129.31(3)°) corresponds
to the shortest Fe-NSi2 bond in the least crowded 2b, which
suggests that greater charge separation occurs across the
Fe-NSi2 bond. In addition, the torsion angles between
FeNSi2 and FeN3 do not display a regular pattern. Thus, 2b,
the least sterically crowded molecule in the series, has an
average torsion angle of 71.8° whereas the bulkier mesityl
substituted 2c has a torsion angle of 52.3° and the sterically
very similar 2d has a torsion angle of 85.0°.
C(2)-C(3)
N(3)-Si(1)
N(3)-Si(2)
1.709(2)
M-N(3)-Si(1)
M-N(3)-Si(2)
Si(2)-N(3)-Si(3)
fold angle/metal
distance from
N2C3 plane
109.95(8)
121.88(8)
127.12(8)
164.2/0.394
2b
2c
2d
M-N(1)
2.008(1)
1.993(1)
1.908(1)
1.9969(9)
1.999(2)
1.990(2)
1.917(2)
94.23(8)
129.79(8)
135.88(8)
118.3(2)
121.2(2)
1.331(3)
1.346(3)
1.408(3)
1.396(3)
1.726(2)
1.714(2)
114.5(1)
120.2(1)
124.7(1)
157.3/0.556
177.8/0.039
M-N(2)
M-N(3)
1.915(1)
95.73(5)
132.13(3)
N(1)-M-N(2)
N(1)-M-N(3)
N(2)-M-N(3)
M-N(1)-C(2)
M-N(1)-C(2)
N(1)-C(2)
93.20(5)
126.98(5)
139.76(5)
116.20(10)
116.38(10)
1.341(2)
1.347(2)
1.399(2)
1.401(2)
1.718(1)
118.90(7)
1.338(1)
1.404(1)
1.7221(7)
118.51(4)
N(2)-C(2)
C(1)-C(2)
C(2)-C(3)
N(3)-Si(1)
N(3)-Si(2)
1.716(1)
M-N(3)-Si(1)
M-N(3)-Si(2)
Si(2)-N(3)-Si(3)
fold angle/metal
distance from
N2C3 plane
114.81(7)
115.52(7)
129.31(8)
178.6/0.028
122.98(8)
178.4/0.031
4
5
6
M(1)-N(1)
2.038(1)
2.013(1)
2.016(1)
1.999(1)
1.963(2)
1.957(2)
1.978(2)
1.951(2)
M(1)-N(2)
M(1)-N(3)
M(1)-N(4)
M(1)-Cl(1)
2.2933(6)
2.2955(6)
1.336(3)
1.33493)
2.1852(6)
2.2106(6)
1.331(3)
1.343(3)
M(2)-Cl(2)
N(1)-C(1)
1.337(2)
1.343(2)
1.334(2)
1.350(2)
The three compounds 4, 5, and 6 are also illustrated in
Figure 2. The structure of 4 shows that iron is bound to two
â-diketiminate ligands to afford an FeN4 array that ap-
proximates D2d symmetry with an average Fe-N distance
of 2.017(11) Å. However, this averaged distance obscures
the fact that for each â-diketiminate ligand one of the Fe-N
distances is ca. 0.02 Å longer than the other, for example,
Fe-N(1),N(2) ) 2.038(1), 2.013(1) Å. The origin of this
difference is uncertain, and it could be due to several factors
N(2)-C(3)
N(3)-C(18)
N(4)-C(20)
C(1)-C(2)
1.405(3)
1.402(6)
98.23(8)
1.415(3)
1.401(3)
95.57(7)
C(2)-C(3)
N(1)-Fe(1)-N(2)
N(3)-Fe(1)-N(4)
N(1)-Fe(1)-N(4)
N(2)-Fe(1)-N3)
Fe(1)-N(1)-C(6)
Fe(1)-N(2)-C(12)
Fe(1)-N(2)-C(18)
Cl(1)-Fe(1)-Cl(2)
fold angle/metal
distance from
N2C3 plane
89.52(5)
92.67(5)
121.00(5)
118.54(5)
119.72(9)
120.68(9)
119.03(14)
118.601(4)
121.51(1)
124.9(1)
117.15(3)
143.5/0.797
2
2
2
among which are the unequal occupancy of dx -y and dz
100.18(2)
166.0/0.317
orbitals of a d6 electron configuration in an approximately
tetrahedral crystal field and crystal packing effects. The
Fe-N distances in 4 are on average longer than the
Fe-N(â-diketiminate) distances in 2b, 2c, and 2d but are
144.0/0.305
168.3/0.098
set (isoleptic), and the M-N bond lengths are in the sequence
Mn-N > Fe-N > Co-N which is consistent with the
decreasing size of the M2+ ions.74 The M-N(SiMe3)2
distances are very similar to the terminal M-N bond lengths
(73) Putzer, M. A.; Neumuller, B.; Dehnicke, K.; Magull, J. Chem. Ber.
1996, 129, 715.
(74) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.
Inorganic Chemistry, Vol. 41, No. 15, 2002 3915

Panda et al.
similar to those in 2a. The lengthening in 4 is consistent
with the higher coordination number of iron in this complex.
The greater steric crowding in 2a apparently eliminates the
difference so that very similar distances are observed. Both
â-diketiminate rings display folding, but the amount of
folding in each ring is quite different. For the ring associated
with N(1) and N(2), the fold angle is 154.0°, and the iron is
0.305 Å out of the N2C3 plane. For the N(3) and N(4) ring,
the angle is 168.3°, and the deviation is 0.098 Å.
Cobalt species 5 crystallizes with 1.5 toluene molecules
per asymmetric unit. The cobalt is bound to two â-diketimi-
nate nitrogens and also to two chlorides which act as bridges
to lithium. The lithium is further complexed by two THF
oxygens. Thus, both metals have distorted tetrahedral
coordination. The structure is similar to that of recently
reported iron complex Dipp2N∧NFeCl2Li(THF)2.63 The Co-N
and Co-Cl bonds, av 1.960(3) and 2.294(1) Å, are shorter
by ca. 0.04 Å than the corresponding Fe-N and Fe-Cl
distances in the iron species which is in keeping with the
smaller effective ionic radius of Co2+ relative to Fe2+.74 As
in the iron complex, the narrowest interligand angle in the
Co2+ coordination sphere involves the â-diketiminate ligand
(98.23(8)°). The Li-Cl and Li-O distances 2.340(4) and
1.90(1) Å are also similar to those seen in the iron complex.
Iron(III) complex 6 is monomeric and features the metal
in distorted tetrahedral coordination with Fe-N distances
that are ca. 0.03 Å shorter than those in compounds 2a-d.
This is unexpected on the basis of the higher iron coordina-
tion number but is in agreement with the higher oxidation
state of iron in 6. In contrast, the average Fe-Cl distance,
ca. 2.20 Å, in 6 is longer than the 2.172(7) Å observed in
(Dipp2N∧N)′FeCl which is in agreement with the higher iron
coordination number but not its higher oxidation state.63 The
structure of 6 may be compared with those recently reported
for Dipp2N∧NVCl222 and the group 13 derivatives Dipp2N∧-
NMCl2 (M ) Al, Ga, or In).62 The structural parameters for
6 and Dipp2N∧NGaCl2 are particularly close although the
compounds are not isomorphous. However, the distortions
in 6 are greater than those in the DippN∧N group 13 metal
halide derivatives; the iron lies 0.797 Å out of the averaged
core N2C3 plane, and there is an angle of 143.5° between
the FeN2 and N2C3 planes. In sharp contrast to the monomeric
formulas of 6 and its vanadium analogue,22 the recently
reported chromium derivative {Dipp2N∧NCr(Cl)(µ-Cl)}2 is
dimeric.33 The reasons for the differences in association for
the complexes are not obvious. The structure of 6 also
32
resembles those of the more crowded (Dipp2N∧N)′ScCl2
22
and (Dipp2N∧N)′TiCl2 complexes in which the methyl
substituents on the â-diketiminate ring are replaced by
tertiary butyl groups.
Acknowledgment. We are grateful to the donors of the
Petroleum Research Fund administered by the American
Chemical Society and UC MEXUS-CONACYT and the
Alexander von Humboldt Stiftung for the award of a
fellowship to P.P.P.
Supporting Information Available: X-ray file in CIF format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
IC025552S
3916 Inorganic Chemistry, Vol. 41, No. 15, 2002