Dispersion force stabilized two-coordinate transition metal-amido complexes of the -N(SiMe3)Dipp (Dipp = C6H3-2,6-Pr i2) ligand: Structural, spectroscopic, magnetic, and computational studies

Article abstract of DOI:10.1021/ic402105m

A series of high spin, two-coordinate first row transition metal-amido complexes, M{N(SiMe₃)Dipp}₂ {M = Fe (1), Co (2), or Ni (3); Dipp = C₆H₃-2,6-Prⁱ₂} and a tetranuclear C-H activated chromium amide, [Cr{N(SiMe₂CH ₂)Dipp}₂Cr]₂(THF) (4), were synthesized by reaction of their respective metal dihalides with 2 equiv of the lithium amide salt. They were characterized by X-ray crystallography, electronic and infrared spectroscopy, SQUID magnetic measurements, and computational methods. Contrary to steric considerations, the structures of 1-3 display planar eclipsed M{NSiC(ipso)}₂ arrays and short M-N distances. DFT calculations, corrected for dispersion effects, show that dispersion interactions involving C-H-H-C moieties likely stabilize the structures by 21.1-29.4 kcal mol ^-1, depending on the level of the calculations employed. SQUID measurements confirm high spin electron configurations for all the complexes and substantial orbital contributions for 1 and 2.

Full text of DOI:10.1021/ic402105m

Article
pubs.acs.org/IC
Dispersion Force Stabilized Two-Coordinate Transition Metal−Amido
Complexes of the −N(SiMe3)Dipp (Dipp = C6H3‑2,6-Pri2) Ligand:
Structural, Spectroscopic, Magnetic, and Computational Studies
Chun-Yi Lin,† Jing-Dong Guo,‡ James C. Fettinger,† Shigeru Nagase,*,‡ Fernande Grandjean,§
Gary J. Long,*,§ Nicholas F. Chilton,⊥ and Philip P. Power*,†
†Department of Chemistry, University of California, Davis, California 95616, United States
‡Fukui Institute for Fundamental Chemistry, Kyoto University, 34-4 Takano-Nishihiraki-cho, Sakyou-ku, Kyoto 606-8103, Japan
§Department of Chemistry, Missouri University of Science and Technology, University of Missouri, Rolla, Missouri 65409-0010,
United States
⊥School of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom
S
* Supporting Information
ABSTRACT: A series of high spin, two-coordinate first row transition metal−amido
complexes, M{N(SiMe3)Dipp}2 {M = Fe (1), Co (2), or Ni (3); Dipp = C6H3-2,6-
Pri2} and a tetranuclear C−H activated chromium amide, [Cr{N(SiMe2CH2)-
Dipp}2Cr]2(THF) (4), were synthesized by reaction of their respective metal dihalides
with 2 equiv of the lithium amide salt. They were characterized by X-ray
crystallography, electronic and infrared spectroscopy, SQUID magnetic measurements,
and computational methods. Contrary to steric considerations, the structures of 1−3
display planar eclipsed M{NSiC(ipso)}2 arrays and short M−N distances. DFT
calculations, corrected for dispersion effects, show that dispersion interactions involving
C−H−H−C moieties likely stabilize the structures by 21.1−29.4 kcal mol−1, depending
on the level of the calculations employed. SQUID measurements confirm high spin
electron configurations for all the complexes and substantial orbital contributions for 1
and 2.
also had a magnetic moment (2.79μB) near that of the spin only
INTRODUCTION
■
i
value.6b Their linear chromium analogue Cr{N(H)ArPr }2 (d ,
4
Several recent publications on two-coordinate, open-shell (d1−
d9) transition metal complexes have shown that they have
unusual magnetic properties. For example, two-coordinate
iron(II) complexes such as Fe{C(SiMe3)3}2,1−3 Fe(NBut2)2,4
5
6
S = 2)7 had μeff = 4.22μB, which is lower than the expected spin-
only value of 4.9μB. This is consistent with the positive value of
the spin−orbit coupling constant for the less than half-filled
valence shell. Further experiments with closely related, less-
Pri6
Pri6
or Fe{N(H)Ar }2 (Ar = C6H3-2,6(C6H2-2,4,6-Pri3)2) have
5,6a,7
crowded complexes of formula M{N(H)ArMe }2,
which
6
magnetic moments far in excess of the spin-only value μSO
=
generally have bent coordination geometries, have shown that
the bending can result in drastic changes to their magnetic
moments, owing to a lifting of orbital degeneracy and
consequent quenching of the first-order OAM.
4.9μB, and can approach the free ion magnetic moment of
6.7μB. In essence, the first-order orbital angular momentum
(OAM) in these iron(II) (S = 2) complexes remains
unquenched because both ligands are bound along only one
axis (z), so that electron circulation in the unequally occupied,
These experiments have demonstrated that the geometries of
the two-coordinate species are of key importance and that
linear or near-linear geometries are highly desirable to maximize
orbital magnetic moment effects. Furthermore, linear coordi-
nation provides a highly anisotropic metal environment that
may generate highly negative axial zero-field splittings,8,9 as well
as increased barriers to spin reversal.10,11 Yet, two-coordinate
complexes with linear geometries in the crystalline state form a
relatively small minority (ca. 12) of the ca. 80 two-coordinate
open-shell (d1−d9) transition metal complexes that are
2
2
degenerate dx −y and dxy orbitals, which cause the orbital
moment, remains unimpaired. Very high magnetic moments
have also been observed in the linear coordinated cobalt
complex Co{N(H)ArPr }2 (S = 3/2, μeff = 6.3μB),6a which is
thought to be due to out of state spin orbit coupling. In
i
6
i
6
contrast, in their nickel analogue Ni{N(H)ArPr }2,6a where first-
order effects are also expected due to unequally occupied dxz
and dyz orbitals, a magnetic moment of only 2.92μB was
measured, which is just slightly higher than the expected spin-
only value of 2.83μB. The closely related, linear nickel(II)
Received: August 16, 2013
i
4
Pri4
derivative Ni{N(H)ArPr }2 (Ar = C6H3-2,6(C6H3-2,6-Pri2)2)
© XXXX American Chemical Society
A
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Table 1. Selected Crystallographic Data and Collection Parameters for 1−4
1
2
3
4
formula
C30H52FeN2Si2
552.77
C30H52CoN2Si2
555.84
C30H52NiN2Si2
555.63
C64H108Cr4N4OSi4
1269.90
block
fw, g mol−1
habit
block
block
block
color
orange
dark red
triclinic
purple
green
cryst syst
space group
a, Å
triclinic
triclinic
monoclinic
P21/c
P1
̅
P1
P1
̅
̅
8.8101(5)
9.1759(5)
11.1624(6)
102.4670(10)
92.5030(10)
113.9320(10)
796.59(8)
1
8.8518(10)
9.1815(10)
11.0926(11)
102.5194(14)
92.4082(14)
114.0167(13)
795.28(15)
1
8.9104(13)
9.1955(13)
10.9978(16)
102.065(2)
92.547(2)
114.364(2)
794.0(2)
1
10.2421(3)
16.2208(5)
40.7752 (13)
90
b, Å
c, Å
α (deg)
β (deg)
94.5098(13)
90
γ (deg)
V, Å3
6753.2(4)
4
Z
dc, Mg m−3
θ range (deg)
μ, mm−1
obs data, I > 2σ(I)
R1 (obs data)
wR2 (all data)
1.152
1.161
1.162
1.249
1.89−27.51
0.568
1.90−32.40
0.635
1.91−29.21
0.706
3.486−70.077
6.154
3336
3520
3824
12 369
0.0378
0.0357
0.0265
0.0426
0.1073
0.1065
0.0753
0.1073
currently known.12a,b It is clear that exceptionally large ligand
substituents are required to ensure linear coordination at the
metals in the crystalline phase.13 The use of the −C(SiMe3)3
alkyl group allowed the isolation of the first two-coordinate
open-shell transition metal species Mn{C(SiMe3)3}2 in the
solid state, but until now this ligand has been demonstrated to
that attractive dispersion forces between the ligand across the
metal likely play a key role in stabilizing their structures with
energies in the range ca. 21.1−29.4 kcal mol−1 being calculated.
14
EXPERIMENTAL SECTION
■
General Methods. All manipulations were carried out by using
modified Schlenk techniques under an atmosphere of N2 or in a
Vacuum Atmospheres HE-43 drybox. All solvents were dried over an
alumina column, followed by storage over 3 Å molecular sieves
overnight, and degassed three times (freeze−pump−thaw) prior to
use. Anhydrous FeCl2 was obtained by heating FeCl2·4H2O to 150 °C
under vacuum for 18 h. CoCl2 was purchased from Strem Chemicals
Inc. and was used as received. The compounds LiN(SiMe3)Dipp18
(Dipp = C6H3-2,6-Pri2), CrCl2(THF)2,19 and NiCl2(DME)20 (DME =
1, 2-dimethoxyethane) were prepared according to literature
procedures. Melting points were measured in glass capillaries sealed
under N2 by using a Mel-Temp II apparatus and are uncorrected. UV−
vis spectra were collected using an Ocean Optics DH2000 light source
and HR2000 CG-UVNIR spectrometer.
stabilize two coordination for one other metal: in the iron
1−3
species Fe{C(SiMe3)3}2
and in the recently described salt
[K(crypt-222)][Fe{C(SiMe3)3}2].11 Most of the other exam-
ples of strictly linearly coordinated Cr−Ni metal complexes
feature the terphenyl-substituted amido and thiolato ligands
i65−7
i
6
i
−N(H)ArPr
and −SArPr (ArPr = C6H3-2,6(C6H2-2,4,6-
6
Pri3)2).15 However, these ligands, although effective, have the
disadvantage that they require time-consuming syntheses. We
turned to the more easily synthesized −N(SiMe3)Dipp (Dipp =
C6H3-2,6-Pri2) ligand of Wigley16 (which has also been shown
to stabilize two-coordination in the magnesium, zinc, and
mercury derivatives M′{N(SiMe3)Dipp}2,16b,c M′ = Mg, Zn,
Hg), to isolate a series of two-coordinate complexes, M{N-
(SiMe3)Dipp}2 (M = Fe, 1; Co, 2; Ni, 3), which we describe in
full here. We show that these complexes have rigorously linear
coordination in the solid state, with unusual extended planar
M{NSi(C(ipso))}2 core arrays and short M−N distances. We
also show that attempts to synthesize the corresponding Cr2+
derivative afforded a further reaction involving a C−H moiety
from a SiMe3 group of the ligands, which yielded the unusual
tetrametallic complex [Cr{N(SiMe2CH2)Dipp}2Cr]2(THF)
(4). We had earlier given the M−N distances and N−M−N
bond angles for 1−3 in a review,12a and while their
characterization was still in progress, the synthesis, structure,
Fe{N(SiMe3)Dipp}2 (1). LiN(SiMe3)Dipp (3.0078 g, 11.78 mmol)
was dissolved in 50 mL of Et2O, and the solution was added dropwise
to a suspension of FeCl2 (0.712 g, 5.62 mmol) in Et2O (10 mL) with
cooling to ca. 0 °C. The dark brown reaction mixture was allowed to
warm to room temperature and was stirred for 18 h. All volatile
materials were removed under vacuum, and the residue was extracted
with hexanes (40 mL). The solution was filtered, and the dark brown
filtrate was concentrated to incipient crystallization. Storage at ca. −18
°C overnight afforded large, X-ray quality orange crystals of 1. Yield:
1.7836 g (57.5%). Mp: 209−211 °C. UV−vis (hexane) λmax (ε): 344
nm (6200 mol−1 L cm−1).
Co{N(SiMe3)Dipp}2 (2). LiN(SiMe3)Dipp (1.4669 g, 5.74 mmol)
was dissolved in Et2O (50 mL) and added dropwise to CoCl2 (0.3425
g, 2.64 mmol) at ca. 0 °C with rapid stirring. The dark red reaction
mixture was allowed to warm to room temperature and stirred for 18
h. All volatile materials were removed under vacuum, and the residue
was extracted with hexanes (40 mL). The solution was filtered, and the
dark red filtrate was concentrated to incipient crystallization. Storage at
ca. −18 °C overnight afforded large, X-ray quality dark red crystals.
Yield: 1.1252 g (76.8%). Mp: 177−180 °C. UV−vis (hexane) λmax (ε):
504 nm (470 mol−1 L cm−1). Anal. Calcd for C30H52CoN2Si2: C,
64.82; H, 9.43; N, 5.04. Found: C, 65.08; H, 9.51; N, 4.96.
17
and several reactions of Ni{N(SiMe3)Dipp}2 were reported
by Tilley and co-workers. As a part of a collaborative effort, we
also described some magnetic properties of Fe{N(SiMe3)-
Dipp}2 as part of a larger study involving the magnetic
relaxation of two coordinate iron(II) species.10 Here, we
describe a more detailed study of the spectroscopic and
magnetic properties of compounds 1−4 and describe their
electronic structure and bonding through a combination of
spectroscopic and computational methods. Moreover, we show
Ni{N(SiMe3)Dipp}2 (3). LiN(SiMe3)Dipp (1.5131 g, 5.92 mmol)
was dissolved in 50 mL of Et2O and slowly added to 0.6182 g (2.81
B
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Inorganic Chemistry
Article
mmol) of NiCl2(DME) at ca. 0 °C. The purple reaction mixture was
allowed to warm to room temperature and stirred for 18 h. All volatile
material was removed under vacuum, and the residue was extracted
with hexanes (40 mL). The solution was filtered, and the dark purple
filtrate was concentrated to incipient crystallization. Storage at ca. −18
°C for several days afforded X-ray quality dark purple crystals. Yield:
0.3899 g (24.9%). Mp: 163−165 °C. UV−vis (hexane) λmax (ε): 508
nm (5600 mol−1 L cm−1). Anal. Calcd for C30H52NiN2Si2: C, 64.85; H,
9.43; N, 5.04. Found: C, 65.19; H, 9.30; N, 4.99.
[Cr{N(SiMe2CH2)Dipp}2Cr]2(THF) (4). LiN(SiMe3)Dipp (2.1048
g, 8.24 mmol) was dissolved in 50 mL of Et2O and slowly added to
1.1058 g (3.80 mmol) of CrCl2(THF)2 at ca. 0 °C. The dark green
reaction mixture was allowed to warm to room temperature. After 2
days of stirring, all volatile material was removed under reduced
pressure, and the residue was extracted with pentane. The solution was
filtered, and the deep green filtrate was concentrated to incipient
crystallization. Storage at ca. − 18 °C for several days afforded X-ray
quality deep green crystals. Yield: 0.3968 g (31.6%). UV−vis (hexane)
λmax (ε): 366 (3700), 468 (1000) nm (mol−1 L cm−1). Anal. Calcd for
C64H108Cr4N4OSi4: C, 60.53; H, 8.57; N, 4.41. Found: C, 60.01; H,
8.19; N, 4.16.
X-ray Crystallography. Crystals of 1−4 were removed from a
Schlenk flask under a stream of nitrogen and immediately covered with
a layer of hydrocarbon oil. A suitable crystal was selected, attached to a
glass fiber on a copper pin, and quickly placed in the cold N2 stream
on the diffractometer. Data for compound 2 were collected at ca. 90 K
on a Bruker SMART 1000 diffractometer with Mo Kα radiation (λ =
0.710 73 Å). Data for compounds 1, 3, and 4 were collected at 90 K on
a Bruker APEX DUO diffractometer with Mo Kα radiation (λ = 0.710
73 Å). Absorption corrections were applied using SADABS.21 The
crystal structures were solved by direct methods and refined by full-
matrix least-squares procedures in SHELXTL.22 All non-H atoms were
refined anisotropically. All H atoms were placed at calculated positions
and included in the refinement using a riding model.
RESULTS AND DISCUSSION
■
Synthesis and Structure. The complexes 1−3 were
synthesized in moderate yields by a transmetalation approach
that involved the treatment of FeCl2, CoCl2, and NiCl2(DME)
with 2 equiv of LiN(SiMe3)Dipp in Et2O at ca. 0 °C (Scheme
1).
Scheme 1. Synthesis of 1, 2, and 3
Workup was straightforward and involved extraction of the
reaction mixture residue with hexanes and filtration to remove
lithium chloride. Recrystallization of the deeply colored
solutions of 1−3 afforded crystals that were suitable for X-ray
crystallography.
The thermal stability of compounds 1−3 is noteworthy,
especially that of 3 where the related −N(SiMe3)2 derivative,
Ni{N(SiMe3)2}2,26 was found to decompose at room temper-
ature. In contrast, the corresponding manganese, iron, and
cobalt M{N(SiMe3)2}2 (M = Mn,26 Fe,27 or Co28) species are
thermally stable and can be distilled under reduced pressure at
temperatures >100 °C. It appears that the use of aryl
substituents at nitrogen in nickel amides confers increased
stability, as evidenced by the stable homoleptic Ni(II)
monomers Ni{N(H)ArMe
}
(ArMe = C6H3-2,6(C6H2-2,4,6-
6
6
2
6b
i
i
6
Data collection parameters and a summary of the structural
refinements are given in Table 1.
Me3)2),6a Ni{N(H)ArPr }2, Ni{N(H)ArPr }2, Ni{N(Ph)B-
(Mes)2}2 (Mes = C6H2-2,4,6-Me3),29 Ni{N(Mes)B(Mes)2}2,30
the dimeric {Ni(NPh2)2}2,31 or the Ni(I) complex {CHN-
(Dipp)}2Ni{N(H)Dipp}.32 Within limits, therefore, the lack of
stability does not appear to be due to insufficient steric
protection, since bulkier silylamide ligands such as −N-
(SiMe2Ph)2 and −N(SiMePh2)2 can form stable two-
coordinate manganese, iron, or cobalt complexes (see below,
Table 3), but have not been shown to afford stable nickel(II)
derivatives to date.31,32
6a
4
Magnetic Studies. The powdered samples of 1−4 used for
magnetic measurements were sealed under N2 in 3 mm diameter
quartz tubing. The sample magnetization was measured using a
Quantum Design MPMSXL7 superconducting quantum interference
magnetometer. For each compound, the sample was zero-field cooled
to 2 or 5 K and the long moment was measured upon warming to 320
K in an applied field of 0.01 T. To ensure thermal equilibrium between
the powdered sample in the quartz tube and the temperature sensor,
the long moment at each temperature was measured after 50, 36, 28,
20, and 12 min intervals over the temperature ranges of 2−6, 6−10,
10−25, 25−70, and 70−320 K, respectively; the measurements
required ca. 20 h for each sample. Diamagnetic corrections of
−0.000353, −0.000352, −0.000351, and −0.000851 emu/mol,
obtained from tables of Pascal’s constants,23 were applied to the
measured susceptibility of 1−4, respectively.
The X-ray data showed that crystals of 1−3 are isomorphous,
with very similar cell parameters (Table 1). Their structures
consist of well-separated two-coordinate centrosymmetric
monomers, as illustrated by the thermal ellipsoid plot of the
cobalt derivative 2 in Figure 1. Selected structural parameters
are given in Table 2.
Computational Methods. All calculations were carried out using
the Gaussian program.24 Geometry optimization was performed with
hybrid density functional theory (DFT) at the spin-unrestricted
B3PW91 level, by using the 6-311+G* basis set for Fe, Co, and Ni and
the 6-31G(d) basis set for other atoms. The harmonic vibrational
frequencies and infrared intensities of optimized geometries were also
calculated. The UV/vis absorption spectra of optimized geometries
were calculated using the time dependent (TD) B3PW91 method. In
order to estimate the dispersion effects, these complexes were
reoptimized with the dispersion-corrected B3PW91-D3 functional.25a
The performance of another dispersion-corrected GGA functional,
B97D, was also explored and compared with that of the higher-level
B3PW91-D3 treatment.
The M−N distances decrease across the series M = Fe, Co,
and Ni from 1.853(1) (Fe) to 1.818(1) (Co) to 1.8029(9) (Ni)
Å (cf. Ni−N = 1.7987(11) Å in ref 17), consistent with
decreasing element size. The closest metal−metal approaches
are 8.8101(5) Å for 1, 8.8493(7) Å for 2, and 8.910.4(13) Å for
3. The two most important features of the structures are the
rigorously linear coordination of the metals, and the extended
planar structure of the M{NSi(Cipso)}2 (Cipso = C(1)) core
arrays, which have local C2h symmetry. Strictly linear two
coordination for open-shell transition metal complexes, which
is important because of its effect on magnetic properties, is
relatively rare in the solid state, and a total of nine such
examples3−7,11,14,15,35,36 had been characterized prior to the
three −N(SiMe3)Dipp derivatives. The extended planarity of
the M{NSi(Cipso)}2 core is noteworthy because it may not
represent a steric minimum (see below) for interactions
To account for basis set superposition error (BSSE),25b the
correction was estimated with the complex 3 as an example. It was
found that this error is smaller than 5 kcal/mol in the bond
dissociation energy (BDE) but only 0.7 kcal/mol in the dispersion
energy given in Table 6, suggesting that the correction can be
neglected to the dispersion term.
C
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Inorganic Chemistry
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Table 3. Comparison of M−N Bond Lengths of Two-
Coordinate Fe(II), Co(II), and Ni(II) Diamido Complexes
complex
M−N (Å)
N−M−N (deg)
ref
Fe{N(CH2But)Dipp}2
1.842(2)
1.8532(13)
1.880(2)
1.896(2)
1.902(1)
1.916(2)
1.909(3)
1.938(2)
1.8179(14)
1.836(11)
1.865(2)
1.898(3)
1.910(3)
1.909(5)
1.8029(9)
1.818(3)
1.8284(15)
1.866(2)
1.885(4)
168.8(2)
180.0
40
1
this work
Fe{N(But)2}2
Fe{N(SiMe2Ph)2}2
179.45(8)
172.1(1)
180.0
4
33, 34
i
Fe{NHArPr
}
2
5
6
Fe{N(SiMePh2)2}2
169.0(1)
140.94(16)
166.6(1)
180.0
33
Fe{NHArMe
}
2
5
6
Fe{N(Mes)B(Mes)2}2
30
2
this work
Co(NHArMe
)
144.1(4)
180.0
6a
6
2
i
Co(NHArPr
)
6b
6
2
Co{N(SiMePh2)2}2
Co{N(Mes)B(Mes)2}2
Co{N(Ph)B(Mes)2}2
3
147.0(1)
168.4(1)
127.1(2)
180.0
33
Figure 1. Solid state molecular structure of 2. Thermal ellipsoids for
non-H atoms are shown at 30% probability. See Table 2 for important
bond lengths and angles.
30
30
this work
i
Ni{NHArPr
Ni{NHArPr
}
}
180.0(3)
180.0
6b
6a
29
30
4
2
Table 2. Important Interatomic Distances (Å) and Angles
(deg) for Compounds 1−3
i
6
2
Ni{N(Mes)B(Mes)2}2
Ni{N(Ph)B(Mes)2}2
167.9(1)
135.7(1)
1
2
3
M(1)−N(1)
N(1)−C(1)
N(1)−Si(1)
M(1)−C(1)
N(1)−M(1)−N(1A)
M(1)−N(1)−C(1)
M(1)−N(1)−Si(1)
C(1)−N(1)−S(1)
1.8532(13)
1.4289(19)
1.7261(14)
2.686(1)
180
1.8179(14)
1.4288(16)
1.7305(11)
2.650(1)
180
1.8029(9)
1.4325(13)
1.7417(10)
2.612(1)
180
Fe,42 Co,31 or Ni31) and an increase in the metal coordination
number from 2 to 3.
The chromium species 4 was obtained as dark green crystals
from hexanes in moderate yield by treatment of CrCl2(THF)2
with 2 equiv of LiN(SiMe3)Dipp in an analogous manner to
that used for 1−3. The structure of 4 was solved by X-ray
crystallography and shown to be the tetrametallic chromium-
amido/alkyl complex, [Cr{N(SiMe2CH2)Dipp}2Cr]2(THF)
(Figure 2), in which the amido ligands have each been
109.18(10)
125.08(8)
125.74()
108.71(8)
125.46(6)
125.83()
107.13(7)
126.02(5)
126.85()
between SiMe3 and Dipp substituents of the two amido groups
across the metal. An examination of the structural data show
that the SiMe3−Pri H···H separations are only 2.43(1) Å (1,
Fe), 2.37(1) Å (2, Co), and 2.36(1) Å (3, Ni), and each
complex is stabilized by attractive dispersion forces (see below)
between these moieties. Such forces have been shown by
computational data to be of high importance in stabilizing a
variety of structures with large substituents.37−39 The M−N
distances measured for each of the complexes are among the
shortest known and are the shortest, in the case of 2 and 3, for
two-coordinate cobalt(II) or nickel(II) amido complexes (cf.
Table 3). Prior to the M{N(SiMe3)Dipp}2 series, the transition
metal compound most closely related stoichiometrically to 1−3
was the iron species Fe{N(CH2But)Dipp}2.40 However, it does
not have linear iron coordination, N−Fe−N = 168.8(2)°,
although its Fe−N bond length, 1.842(2) Å, is marginally
shorter than the 1.8532(13) Å in 1. The closest nonbonded
approaches of ligand atoms to the metal in 1−3 involve the
ipso-carbon (C(1)) of the Dipp aryl ring at a distance of 2.686
Å (1), 2.650 Å (2), and 2.612 Å (3). These distances are
Figure 2. Solid state molecular structure of 4 (cf. Table 4 for details).
H atoms are not shown for clarity, and thermal ellipsoids are shown at
30% probability.
dehydrogenated at one of the methyls attached to the silicon
such that the ligand is in effect bidendate and carries a 2−
charge. The dehydrogenation of methyl substituents in
sterically crowded trimethylsilylamido derivatives of transi-
tion43−47 (and other48) metals is well-known but is rare for
chromium.49
Selected bond angles and distances for 4 are given in Table 4.
Compound 4 exhibits an unusual tetrametallic structure, for
which only a few related examples have been reported.50,51 Of
the four metals in the structure, the two outer chromiums,
Cr(1) and Cr(4), are coordinated to two nitrogens, each from
an amido ligand. One of these amido ligands bridges through its
nitrogen to an inner chromium atom, while the other is
terminally bound. Each outer chromium is also bonded to a
comparable to the secondary M···C interactions observed in
i
5,6a
2
M{N(H)ArPr
}
complexes of these metals, where Mossba-
6
̈
i
5
uer studies of the Fe{N(H)ArPr }2 indicate that the spectral
parameters are consistent with two-coordination, implying that
the M···C interactions are weak.
6
The use of less bulky −N(SiMe3)2 and −NPh2 ligands on Fe,
Co, and Ni results in dimerization through bridging by the
amido ligands, [(Me3Si)2NM{μ-N(SiMe3)2}2MN(SiMe3)2] (M
= Mn,41 Fe,42 or Co41b) and [Ph2NM{μ-NPh2}2MNPh2] (M =
D
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CrCl2(THF)2 and LiN(Dipp)SiMe3 did not result in the
isolation of any crystalline product.
Magnetic Properties. For Fe{N(SiMe3)Dipp}2, 1, the
temperature dependence of χMT and 1/χM obtained between 5
and 300 K in an applied field of 0.01 T is shown in Figure 3;
Table 4. Selected Bond Distances (Å) and Angles (deg) for 4
Cr(1)−N(1)
Cr(1)−N(2)
Cr(1)−O(1)
Cr(1)−C(13)
Cr(2)−N(2)
Cr(2)−C(13)
Cr(2)−C(32)
Cr(2)−C(47)
Cr(3)−N(3)
Cr(3)−C(32)
Cr(3)−C(47)
Cr(3)−C(62)
Cr(4)−N(3)
Cr(4)−N(4)
Cr(4)−C(62)
1.9894(14)
2.1236(19)
2.0848(16)
2.184(2)
N(1)−Cr(1)−O(1)
N(1)−Cr(1)−N(2)
O(1)−Cr(1)−N(2)
N(1)−Cr(1)−C(13)
O(1)−Cr(1)−C(13)
N(2)−Cr(1)−C(13)
N(2)−Cr(2)−C(32)
N(2)−Cr(2)−C(47)
C(32)−Cr(2)−C(47)
N(2)−Cr(2)−C(13)
C(32)−Cr(2)−C(13)
C(47)−Cr(2)−C(13)
N(3)−Cr(3)−C(47)
N(3)−Cr(3)−C(32)
C(47)−Cr(3)−C(32)
N(3)−Cr(3)−C(62)
C(47)−Cr(3)−C(62)
C(32)−Cr(3)−C(62)
N(4)−Cr(4)−N(3)
N(4)−Cr(4)−C(62)
N(3)−Cr(4)−C(62)
93.50(7)
158.13(8)
100.32(7)
81.07(8)
167.98(8)
88.17(8)
79.94(8)
175.57(9)
99.23(9)
87.33(8)
157.38(8)
94.82(9)
81.16(8)
161.06(8)
97.53(9)
95.96(8)
159.67(10)
91.56(8)
167.09(8)
82.21(9)
100.9(8)
2.062(19)
2.279(3)
2.210(3)
2.231 (3)
2.0805(18)
2.267(2)
2.231(3)
2.275(2)
2.0531(18)
1.9703(18)
2.159(2)
methylene moiety, which bridges to an inner chromium. In
addition, a THF molecule binds to Cr(1) to afford three four-
coordinate chromiums {Cr(1), Cr(2), and Cr(3)} and one
three-coordinate (Cr(4)) metal. For the latter, the interligand
angles are 167.14(8)°, 100.45(8)°, and 82.27(8)°, consistent
with a distorted T-shaped geometry. Such a coordination mode
is unusual for chromium and has only been found in a handful
of complexes.52,53 The two inner chromium atoms, Cr(2) and
Cr(3), are each coordinated to one bridging amide ligand and
three methylene moieties that bridge to the outer metals. The
coordination modes of Cr(2) and Cr(3) are consistent with
distorted square planar geometries (the interligand angles for
Cr(2) are N(2)−Cr(2)−C(13) 87.33(8)°, N(2)−Cr(2)−
C(32) 79.99(8)°, C(13)−Cr(2)−C(47) 94.82(9)°, and
C(32)−Cr(2)−C(47) 99.23(9)°; for Cr(3) they are N(3)−
Cr(3)−C(47) 81.16(8)°, N(3)−Cr(3)−C(62) 95.96(8)°,
C(32)−Cr(3)−C(62) 91.56(8), and C(32)−Cr(3)−C(47)
97.53(9)°). The terminal metal amido distances to Cr(1)
(1.990(2) Å) and Cr(2) (1.971(2) Å) lie between those
observed in Cr{N(SiMe3)2}2(THF)2 (Cr−N = 2.09(1) Å)54
and in the dimers R2NCr(μ-NR2)2CrNR2 (R = Pri and Cy)
Figure 3. The temperature dependence of χMT obtained between 2
and 300 K in an applied field of 0.01 T, in black, and 0.1 T, in red, for
1; the red data has been obtained from Figure 2 of ref 10. Right inset:
The temperature dependence of 1/χM obtained for 1 at 0.01 T. Left
inset: The field dependence of the magnetization of 1 obtained at 5 K
at 0.01 and 0.1 T. All the solid curves correspond to the fits obtained
with PHI and discussed in the text.
the χMT obtained at 0.1 T of a separate preparation and study
of 1 reported earlier10 is also included in Figure 3 for
comparison. Similar magnetic properties have been reported3−5
for several somewhat similar linear two-coordinate iron(II)
complexes.
It is apparent from Figure 3 either that χMT of 1 exhibits a
small dependence on the applied field or that there are small
differences in the samples used in this study and the earlier
study.61 At 0.01 T, χMT of 1 is observed to increase from 3.84
emu K/mol at 2 K to a maximum of 4.56 emu K/mol at 110 K
and then to decrease to 4.34 emu K/mol at 300 K; the
corresponding effective magnetic moment, μeff, increases from
5.41μB at 2 K to a maximum of 6.04μB at 110 K and then
decreases to 5.89μB at 300 K. This behavior is typical3−5,10,61 of
a paramagnetic S = 2 iron(II) ion in a low-coordination
environment that possesses a substantial spin−orbit coupling.
The paramagnetic behavior is confirmed by the observed 1/χM
temperature dependence of 1, a temperature dependence that is
linear between 100 and 300 K.
The magnetization of 1 obtained at 5 K between 0 and 7 T is
shown in black in the left inset to Figure 3. As expected, no
hysteresis is observed at 5 K between 7 T, and the slope of
the initial magnetization agrees very well with the observed 5 K
molar magnetic susceptibility. The magnetization, which is 2.83
Nβ at 7 T, is not saturated and is lower than the 4 Nβ that
would be expected for a high-spin S = 2 iron(II) complex in the
absence of any orbital contribution and any substantial single-
ion magnetic anisotropy.
(Cr−N = 1.927(3) and 1.942(7) Å)55 and are close to the
i
Pri6
distance of 1.996(1) Å found in Cr{N(H)ArPr }2 (Ar
=
6
C6H3-2,6(C6H2-2,4,6-Pri3)2).7 The bridging Cr−N metal amido
distances resemble those found in other Cr(II) bridged amido
complexes.55,56 The Cr−C bond lengths (2.159(2)−2.275(2)
Å) are longer than the 2.08(1) Å observed in Cr(Mes)2(THF)2
(Mes = C6H2-2,4,6-Me3), Cr(Mes)2(bipyridyl) (Cr−C =
2.168(4) Å),57 Cr(CH2SiMe3)2(dippe)58 (Cr−C = 2.131(2)
Å), or CrMe2(dmpe)2,59 perhaps as a result of their bridging
nature in 4. The Cr(2)···Cr(3) and Cr(3)···Cr(4) distances,
2.727 and 2.662 Å, respectively, are longer than twice the single
bond radius of Cr (2.44 Å).60 The Cr(1)···Cr(2) distance
(2.371 Å) is about 0.3 Å shorter than the Cr(2)···Cr(3) and
Cr(3)···Cr(4) distances, presumably because of the coordina-
tion of THF, resulting in the increase of steric effect, “pushing”
Cr(1) inward. Interestingly, the stoichiometry of 4 shows that
the ratio of Cr to amido ligand is 1:1. However, the reaction of
E
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As noted above, the various experimental results shown in
Figure 3 are typical of high-spin S = 2 iron(II) in a two
coordinate environment in the presence of spin−orbit coupling.
As a consequence, the χMT of 1 has been fit with the PHI
code62 to yield the best fits shown as the solid lines in Figure 3.
The simultaneous fits with S = 2 and L = 2 of χMT obtained at
0.01 T between 2 and 300 K and the magnetization obtained at
5 K yields B20 = −117(1) cm−1, λ = −76(1) cm−1, gx,y = 1.89(2),
and gz = 1.78(1), where Bkq = B02 is the crystal field parameter,
Aqk⟨rk⟩, in the Steven’s notation, λ is the spin−orbit coupling
constant, and gx,y and gz are the anisotropic electronic g-factors.
The corresponding fit of χMT obtained at 0.1 T between 5 and
300 K and the magnetization obtained at 5 K yields B02 =
−140(1) cm−1, λ = −90(1) cm−1, gx,y = 1.74(2), and gz =
1.95(1). The details of the crystal field parameter and the
specific Hamiltonian used in the fits may be found in the user
manual provided with the PHI code.62
inverse molar susceptibility of 2 is linear from 70 to 300 K and
a linear fit, see the inset to Figure 4, yields a Weiss temperature
of −5.8 K, a Curie constant, C, of 3.04 emu K/mol, a value that
corresponds to a μeff of 5.93μB and g = 2.54 if S = 3/2. This
behavior is very characteristic of a paramagnetic S = 3/2
cobalt(II) complex that has an L = 2 orbital contribution to its
moment.
The χMT of 2 has been fit with the PHI code62 to yield the
best fits shown as the solid lines in Figure 4. The simultaneous
fits with S = 3/2 and L = 2 of χMT obtained at 0.01 T between
10 and 300 K and the magnetization obtained at 5 K yields B02 =
−161(1) cm−1, λ = −183(2) cm−1, gx,y = 2.15(2), and gz =
2.37(1). The sharp decrease in χMT between 2 and 10 K is
presumably the result of long-range antiferromagnetic exchange
coupling between molecules of 2 and has not been included in
the fit.
Because of the L = 2 orbital contribution to the moment
required to fit the χMT of 2 there must be a different ordering
of the five 3d orbitals in 2 compared with those of 1; the
ordering given for 1 above would not permit any orbital
contribution in the case of the 3d7 cobalt(II) electronic
configuration. Thus, it seems quite likely that the 3d orbital
In the above fit, the L = 2 orbital contribution to the moment
is required to fit the temperature dependence of χMT and is
2
2
expected on the basis of the relative energies, dxy ≈ dx −y < dxz
2
≈ dyz < dz , of the iron(II) 3d orbitals in the linear coordination
environment of 1 obtained from ab initio calculations.61 These
2
2
2
2
2
calculations yield nearly degenerate 3dxy and 3dx −y orbitals at 0
and 164 cm−1, a relatively small calculated orbital ground state
splitting that facilitates an orbital contribution to the observed
moment.
ordering in 2 is at least similar to the ordering dz < dxy ≈ dx −y
< dxz ≈ dyz that has been obtained11 by ab initio calculations for
the 3d7 electronic configuration of the iron(I) ion in
[Fe(C(SiMe3)3)2]−, anion. In this case, one of the three
2
2
For a linear iron(II) coordination environment these fits
clearly indicate the importance of both an extensive orbital
contribution10 as a consequence of spin−orbit coupling63
within the electronic ground state of a complex such as 1, and
the associated extensive g-factor anisotropy, an anisotropy that
has been predicted by Dai and Whangbo.64
The temperature dependence of χMT of Co{N(SiMe3)-
Dipp}2, 2, was measured, after zero-field cooling, in a field of
0.01 T upon warming from 2 to 300 K, see Figure 4. The
electrons in the virtually degenerate dxy ≈ dx −y orbitals will
yield an L = 2 orbital moment in 2.
The temperature dependence of χMT for Ni{N(SiMe3)-
Dipp}2, 3, was measured, after zero-field cooling, in a field of
0.01 T upon warming from 2 to 300 K, see Figure 4. The inset
to this figure shows that the inverse molar susceptibility of 3 is
linear from 50 to 300 K with a Weiss temperature of −15.8 K, a
Curie constant, C, of 1.297 emu K/mol, a value that
corresponds to an effective magnetic moment, μeff, of 3.22μB
(cf. 2.67μB by the Evans’ method in ref 17), and g = 2.28 if S =
1. This behavior is very characteristic of a paramagnetic
nickel(II) complex exhibiting zero-field splitting that lowers
χMT at low temperatures.
The χMT of 3 has been fit with the PHI code62 to yield the
best fits shown as the solid lines in Figure 4. The fit with S = 1
and L = 0 of χMT obtained at 0.01 T between 2 and 300 K
yields D = −67.5(5) cm−1, E = −9.17(10) cm−1, g = 2.13(1),
and α = 0.000372 emu/mol(1), where Nα is the second-order
Zeeman contribution to the paramagnetic molar susceptibility.
It is not possible to fit the χMT of 3 if L = 2 or 1.
In the case of 3, the lack of any orbital contribution to the
moment is consistent with the full occupation in a 3d8
2
2
2
nickel(II) complex of the virtually degenerate dxy ≈ dx −y
orbitals where the relative ordering53 of the 3d orbitals is dz
2
2
< dxy ≈ dx −y < dxz ≈ dyz, that is, essentially the same as noted
above for the cobalt(II) complex, 2.
Finally, a summary plot of the temperature dependence of
the effective moments for 1−3, under an applied field of 0.01 T,
is presented in Figure 5. This plot illustrates the large difference
in orbital moments between nickel (3) and iron (1) or cobalt
(2) species; clearly, the values for 1 and 2 are significantly larger
than the g = 2 spin-only μeff values of 4.90μB and 3.87μB
expected for high spin complexes. In contrast, the μeff for Ni(II)
in 3 also exceeds the spin-only value of 2.83μB, but the
difference, 0.32μB, is smaller than the 1.06μB and 1.91μB
observed for 1 and 2, respectively.
Figure 4. The temperature dependence of χMT obtained between 2
and 300 K in an applied field of 0.01 T, for 2, in blue, and 3, in red. For
2, the solid lines correspond to a χMT fit between 10 and 300 K and
for 3 a fit between 2 and 300 K with the parameters given in the text.
Inset: the temperature dependence of 1/χM and the corresponding fits
between 2 and 300 K for 2 and 3.
F
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exchange coupling between them. As a result PHI61 has been
used to simultaneously simulate the χMT and the 5 K
magnetization of 4 yielding the solid lines shown in Figure 6.
Because of the presence of four crystallographically distinct
chromium(II) ions and the extensive correlation between the
parameters given below, it has proven impossible to do a
refinement of the parameters, and the resulting simulation has
not been completely optimized. Further, the calculated values
assume that there is no change in the structure of 4 as a
function of temperature, changes that could add a small
temperature dependence to the exchange coupling.
Based on the structure of 4 presented above, the magnetic
exchange coupling has been modeled in terms of five exchange
coupling constants, Ji, with an isotropic Heisenberg exchange
coupling Hamiltonian, H = −2JS1·S2, with g = 2 and S = 2. The
five parameters correspond to antiferromagnetic exchange
coupling of J1 = −270 cm−1 between Cr(1) and Cr(2), J2 =
−220 cm−1 between Cr(3) and Cr(4), J3 = −80 cm−1 between
Cr(2) and Cr(3), and ferromagnetic coupling of J4 = 20 cm−1
between Cr(2) and Cr(4) and J5 = 28 cm−1 between Cr(1) and
Cr(3). Thus, the five coupling constants are in the order J1 < J2
< J3 < J4 < J5, an order which is consistent with the exchange
coupling pathways and distances in 4. In addition, the
simulation required the contribution of a temperature-
independent paramagnetic susceptibility, Na, of 0.001650
emu/mol or an average of 0.000412 emu/mol of chromium(II)
ions. Finally, in order to fit the ca. 0.2 emu K/mol χMT values at
the very lowest temperatures, one must include a trace of
paramagnetic impurity. If this impurity is a chromium(II) ion
with S = 2 and g = 2, then it corresponds to ca. 7 wt % of the
sample. Finally, it should be noted that all attempted
simulations with S = 2 for Cr(1) and Cr(4) and S = 1 for
Cr(2) and Cr(3) failed.
Although the magnitudes of J1 and J2 are rather larger than
one might expect for exchange coupling, the large negative
values are required to yield the very low high temperature χMT
values observed for 4. Also it should be noted that in 4 there
are two superexchange pathways and a 2.371 Å direct exchange
pathway between Cr(1) and Cr(2) and two superexchange
pathways and a 2.662 Å direct exchange pathway between
Cr(3) and Cr(4), pathways which in combination can yield
strong antiferromagnetic exchange. In contrast, there are two
superexchange pathways but a longer 2.727 Å direct exchange
Figure 5. The temperature dependence of the effective magnetic
moments for 1−3 obtained at 0.01 T and the corresponding fits shown
in Figures 3 and 4
For [Cr{N(SiMe2CH2)Dipp}2Cr]2(THF) (4), the temper-
ature dependence of χMT, obtained after zero-field cooling
between 2 and 300 K in an applied field of 0.01 T, and the field
dependence of the 5 K magnetization are both shown in Figure
6.
It is apparent that both χMT and the 5 K magnetization
values are very small for a complex containing four chromium-
(II) ions, presumably because of extensive antiferromagnetic
pathway between Cr(2) and Cr(3). For comparison, the related
i
4
trinuclear chromium(II) complex,11 Cr{ArPr Cr(μ-NMe2)2}2,
also has two superexchange pathways between the two
equivalent terminal Cr(1) sites and the central Cr(2) site, but
a direct exchange distance of 2.9515(3) Å and, as a
consequence, a less negative Cr(1) to Cr(2) exchange coupling
constant of −47(1) cm−1.
Spectroscopy and Bonding. In order to further
investigate the bonding and electronic spectra in complexes
1−3, calculations were carried out using density functional
theory.
The optimized geometries for the gas phase stuctures of 1−3
are illustrated in Figure 7. The calculated and experimental data
display good agreement with less than a 2% difference between
them. The eclipsed configuration of the ligands and linear
geometries were calculated to be lowest in energy in all cases.
The similarity of the calculated and experimental structures
indicate that packing forces play little role in determining the
details of the structure. The calculations also show the
Figure 6. The temperature dependence of χMT obtained between 2
and 300 K at 0.01 T of [Cr{N(SiMe2CH2)Dipp}2Cr]2(THF), 4. The
black line is the calculated χMT, the green line is the contribution from
exchange coupling among the four cations in terms of the parameters
given in the text, the red line corresponds to Na, the temperature
independent paramagnetic susceptibility, and the blue line corresponds
to the presence of 7 wt % of a paramagnetic chromium(II) impurity
with S = 2 and g = 2. Inset: the magnetization of 4 obtained at 5 K and
its fit with the same parameters as used for χMT.
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Figure 7. Calculated geometries of 1−3 at B3PW91//6-311+G* (M)/6-31G(d) (other atoms) level of optimization.
Figure 8. Excitation energies and oscillator strengths for Fe{N(SiMe3)Dipp}2, 1. Calculated by the TD-B3PW91 method.
Table 5. Dispersion Effects for 1 (Fe complex), 2 (Co complex), and 3 (Ni complex) with C1 and Ci Symmetry
M = Fe
calcd
M = Co
calcd
M = Ni
calcd
B3PW91
B3PW91-D3
expt
Ci
B3PW91
C1 Ci
1.849 1.829
B3PW91-D3
expt
Ci
B3PW91
C1 Ci
1.817 1.796
B3PW91-D3
expt
Ci
C1
Ci
C1
Ci
C1
Ci
C1
Ci
M−N (Å)
N−C (Å)
N−Si (Å)
1.882
1.427
1.758
180.0
1.881
1.866
1.865
1.853
1.849
1.427
1.761
179.8
1.830
1.818
1.816
1.424
1.762
180.0
1.797
1.803
1.835
1.427 1.421
1.422
1.762 1.756
1.748
180.0 175.0
1.798
1.424 1.417
1.415
1.763 1.751
1.746
180.0 177.9
1.427
1.758
180.0
1.419
1.745
180.0
1.419
1.746
180.0
1.429
1.726
180.0
1.421
1.751
180.0
1.429
1.731
180.0
1.416
1.748
180.0
1.433
1.742
180.0
N−M−N
(deg)
absorptions in their electronic spectra are primarily due to
electron transfer from various ligand-based N(p) or π-orbitals
to metal d-orbitals or from metal d-orbitals to π*-ligand
orbitals. For example, the experimental spectrum of 1 displayed
moderately intense absorptions at 467, 349, and 280 nm. The
calculated absorptions were at 496, 350, and 283 nm, in good
agreement with the observed values. These three absorptions
250 and 700 nm, possibly as a result of spin-orbit coupling. The
spectrum for 3 reveals a shoulder absorption near 640 nm and
an absorption at 510 and 345 nm, in good agreement with
calculations.
The IR spectra of 1, 2, and 3 were calculated to yield several
bands in the range ca. 100−900 cm−1 that are associated with
the vibrations of the MN2 core units. The experimental IR
spectra of 1−3 are very similar (see Supporting Information).
The bands observed below 650 cm−1 are relatively weak, and
this is in agreement with the calculated absorptions (see
Supporting Information). Thus, the symmetric M−N vibrations
for 1, 2, and 3 were calculated to be in the range 374−382
cm−1. Experimentally, a weak band near 410 cm−1 was observed
2
originate in N(p)/aryl π* → dz , N(p)/aryl π* → dxz, and dxz,
dyz → π*, aryl π → dxz excitations, shown in Figure 8. Similar
calculations for 2 and 3 afforded bands at 541, 504, and 284 nm
for 2, and at 652, 501, and 347 nm for 3. The experimental
spectrum for the cobalt species 2 reveals a very broad
unresolved absorption (see Supporting Information) between
H
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Table 6. Calculations of the Bond Dissociation Energies of 1−3 (Ci symmetry) at the B97D and B3PW91 Levels with and
without Dispersion Corrections
BDE/Edisp (kcal mol1)
real-system model
isomer (bent)
M = Ni
M = Fe
M = Co
M = Ni
a
a
a
B3PW91
B3PW91-D3
B97D
68.3
63.8
59.1
55.1
52.6
a
a
a
89.9/21.6
87.0/26.2
84.9/21.1
80.2/28.5
83.7/24.6
77.5/29.4
79.0 /23.9
-
76.3 /23.7
74.1/29.2
a
Energies are BSSE-corrected.
for each compound (see Supporting Information for spectra).
The asymmetric M−N stretching bands were calculated to be
more intense and more energetic, with the most intense being
calculated to be near 900 cm−1. Complexes 1, 2, and 3
exhibited relatively strong absorptions at 910, 910, and 905
cm−1 respectively.
a simple model system in which the SiMe3 and isopropyl
groups are replaced by SiH3 and H in Figure S12, Supporting
Information. In marked contrast to the real-system model,
which is the global minimum energy conformation, the
analogous and simple model with C2h symmetry gives only
ca. 5 kcal/mol of dispersion energy at B97D level and is less
stable than that with the planar core atoms, suggesting that
electronic and steric effects from bulky substituents also play an
important role in the real-system model. The results show that
the NMN angle is always nearly 180°, whether or not the two
amido substituents are coplanar.
Dispersion Effects. To estimate the extent of dispersion
effects, the geometries of 1, 2, and 3 were reoptimized at the
B3PW91-D3 level. The corresponding data are included in
Tables 5 and 6. We found that the M−N bond lengths become
slightly shorter after the dispersion correction, due to dispersive
attractive forces involving the bulky substituents on the N
atoms, which can strongly stabilize the complex by transmetallic
C−H···H−C interactions that afford a dispersion stabilization
in the range 21.1−29.6 kcal mol−1. This perhaps surprising
range of energies is, however, consistent with those calculated
CONCLUDING REMARKS
■
To summarize, the synthesis of three transition metal
complexes containing a rigorously linear N−M−N (M = Fe−
Ni) array was effected by treatment of the corresponding metal
chloride with 2 equiv of LiN(SiMe3)Dipp. In addition, the
tetranuclear species [Cr{N(SiMe2CH2)Dipp}2Cr]2(THF) was
obtained by a similar reaction. The two-coordinate iron and
cobalt derivatives exhibit high effective magnetic moments due
to in-state or out-of-state orbital magnetism. DFT calculations
modified to include dispersion effects show that these effects
play a key role in stabilizing the structure. They also suggest
that dispersion effects are present in many other sterically
crowded transition metal complexes, but are generally unrecog-
nized. The investigation of the use of the −N(SiMe3)Dipp and
related ligands to stabilize other low coordinate transition metal
species is in hand.
i
i
Pri4
by Ziegler for the species ArPr CrCrAr
(ArPr = C6H3-
4
4
2,6(C6H3-2,6-Pri2)2).38
The extent of the dispersive force contribution to the M−N
bond energy was obtained by calculating the energy of the
optimized structure and subtracting the calculated energies with
and without the inclusion of dispersion correction of the
molecular fragments A ({(Me3Si)Dipp}N−M) and B ({−N-
(SiMe3)Dipp}) resulting from cleavage of the M−N bond, as
illustrated in Table 6. The B97D data afford stabilizations that
differ only by ca. 6 kcal/mol from the B3PW91-D3 data,
suggesting that the B97D functional can efficiently predict the
trends in the dispersion energy, even if it tends to under-
estimate such energies slightly. A similar treatment for
M{N(SiMe)2}2 in Figure S13, Supporting Information or
M{N(SiH3)2}2 in Figure S14, Supporting Information, show
lower dispersion stabilization energies of ca. 12 or ca. 5 kcal
mol−1 respectively, consistent with decreasing numbers of
interactions.
To explore the feature of coplanarity of the two NSiC planes
in 1−3, we took the Ni complex 3 as an example to draw a plot
of total energy vs the angle of these two planes at the B97D
level (Figure S11, Supporting Information). The plot exhibits
two minima: a global minimum for the real-system model and a
local minimum with an angle of 120° between the planes. The
fully optimized isomer with a bent NNiN angle in Table 6
corresponds to the local minimum. It is noteworthy that the
isomer with the dihedral angle around 50° between the planes
of the two amido substituents is less stable by 2.5 kcal/mol
(B3PW91 with BSSE) or 2.7 kcal/mol (B3PW91-D3 with
BSSE) than the Ni real-system model, but their dispersion
energies are similar. Therefore, it can be concluded that
dispersion effects are not the key determinant for this angular
feature.
ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files for 1−4, electronic spectra of 1−3, calculated
electronic transitions for 1−3, IR spectra for 1−3 in the
200−1500 cm−1 region, calculated M−N vibrational bands,
calculated BDEs and ΔEdisp for 3, with changes in the rotation
angle between the nitrogen coordination planes, calculated
structures of the model species M{N(SiH3)Ph}2 (M = Fe, Co,
or Ni), calculations of the BDEs of M{N(SiMe3)2}2 and
M{N(SiH3)2}2, and a complete citation for ref 24. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: pppower@ucdavis.edu.
*E-mail: nagase@ims.ac.jp.
*E-mail: glong@mst.edu.
Notes
The linear geometry has also been confirmed by the density
functional theory at B3PW91 and B97D levels carried out with
The authors declare no competing financial interest.
I
dx.doi.org/10.1021/ic402105m | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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ACKNOWLEDGMENTS
■
We thank the National Science Foundation (Grant No. CHE-
1263760) and a Grant-in-Aid for Specially Promoted Research
(Grant No. 22000009) from MEXT of Japan for support of this
work. The authors thank Dr. J. Zadrozny for providing the data
on complex 1 obtained at 0.10 T.
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dx.doi.org/10.1021/ic402105m | Inorg. Chem. XXXX, XXX, XXX−XXX