Two-coordinate, quasi-two-coordinate, and distorted three coordinate, T-shaped chromium(II) amido complexes: Unusual effects of coordination geometry on the lowering of ground state magnetic moments

Article abstract of DOI:10.1021/ic202661n

The synthesis and characterization of the mononuclear chromium(II) terphenyl substituted primary amido-complexes Cr{N(H)ArPrⁱ ₆}₂ (ArPrⁱ₆ = C₆H ₃-2,6-(C₆H₂-

Full text of DOI:10.1021/ic202661n

Article
pubs.acs.org/IC
Two-Coordinate, Quasi-Two-Coordinate, and Distorted Three
Coordinate, T-Shaped Chromium(II) Amido Complexes: Unusual
Effects of Coordination Geometry on the Lowering of Ground State
Magnetic Moments
Jessica N. Boynton,† W. Alexander Merrill,† William M. Reiff,‡ James C. Fettinger,† and Philip P. Power*,†
†Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616, United States
‡Department of Chemistry and Chemical Biology, Northeastern University, Boston, Massachusetts 02115, United States
S
* Supporting Information
ABSTRACT: The synthesis and characterization of the
mononuclear chromium(II) terphenyl substituted primary
Pri6
Pri4
Pri6
amido-complexes Cr{N(H)Ar }2 (Ar = C6H3-2,6-(C6H2-
i
4
2,4,6-iPr3)2 (1), Cr{N(H)Ar }2 (ArPr = C6H3-2,6-(C6H3-
2,6-iPr2)2 (2), Cr{N(H)ArMe }2 (Ar = C6H3-2,6-(C6H2-2,4,6-
Me6
6
Me3)2 (4), and the Lewis base adduct Cr{N(H)ArMe }2(THF)
6
(3) are described. Reaction of the terphenyl primary amido
Pri6
Pri4
lithium derivatives Li{N(H)Ar } and Li{N(H)Ar } with
CrCl2(THF)2 in a 2:1 ratio afforded complexes 1 and 2, which
are extremely rare examples of two coordinate chromium and the first stable chromium amides to have linear coordinated high-
spin Cr2+. The reaction of the less crowded terphenyl primary amido lithium salt Li{N(H)ArMe } with CrCl2(THF)2 gave the
6
tetrahydrofuran (THF) complex 3, which has a distorted T-shaped metal coordination. Desolvation of 3 at about 70 °C gave 4
which has a formally two-coordinate chromous ion with a very strongly bent core geometry (N−Cr−N= 121.49(13)°) with
secondary Cr--C(aryl ring) interactions of 2.338(4) Å to the ligand. Magnetometry studies showed that the two linear chromium
species 1 and 2 have ambient temperature magnetic moments of about 4.20 μB and 4.33 μB which are lower than the spin-only
value of 4.90 μB typically observed for six coordinate Cr2+. The bent complex 4 has a similar room temperature magnetic moment
of about 4.36 μB. These studies suggest that the two-coordinate chromium complexes have significant spin−orbit coupling effects
which lead to moments lower than the spin only value of 4.90 μB because λ (the spin orbit coupling parameter) is positive. The
three-coordinated complex 3 had a magnetic moment of 3.79 μB.
angular momentum remains essentially unquenched since ligation
INTRODUCTION
■
occurs solely on the z-axis.4,6,7 Thus, there is a doubly degenerate
Strictly linear coordination is known only for a small minority of
two-coordinate open shell (d1−d9) transition metal complexes in
the solid state.1,2 Bending of their coordination geometry is
generally favored because of (a) ionic intramolecular interactions
between the metal ion and electron density at regions of the
ligand other than the primary coordination site, (b) packing
forces in the crystal structure, or (c) electronic or hybridization
effects. However, the use of highly sterically demanding ligands is
proving effective for the imposition of rigorously linear
coordination in two-coordinate first row transition metal
complexes.1−7 Ligands based on terphenyl groups are particularly
useful because they can be readily functionalized, thereby
allowing for the comparison of complexes with electronically
similar but sterically different ligands.3
Recent investigations of the magnetic properties of two-
coordinate high-spin iron(II) amido complexes show that
bending their coordination geometries has a drastic effect on
the ground state magnetic moments. In rigorously linear
coordination the, d6 Fe2+ ions exhibit essentially free ion magnetic
behavior and unprecedentedly large internal hyperfine fields in
nuclear gamma resonance spectra because first order orbital
orbital ground state, which is associated with the unequally
3
2
2
occupied degenerate (dx −y , dxy) orbital configuration of high-
spin d6. Alternatively, one can say that this set of d orbitals and
configuration is nonbonding with respect to the axial σ bonding
ligands so that the in plane electron circulation is unfettered and
the associated orbital angular momentum is unquenched.4,6−8
Upon bending the coordination geometry, however, this
degeneracy is expected to be lifted which results in the loss
of the first order angular momentum contribution.4,6−8 In the
Pri6
bis(amido) iron(II) complexes Fe{N(H)Ar }2 and Fe{N(H)-
Pri6
ArMe }2, the linear Fe{N(H)Ar }2 displayed a much higher
6
magnetic moment (7−7.5 μB) than the bent species Fe{N(H)-
4
ArMe }2 (5.25−5.8 μB). Parallel investigations of the analogous
6
Mn2+ amides show that their magnetic moments correspond to
spin-only values as expected for their high-spin d5 orbitally
5
6
nondegenerate(6A or Σg+) ground states whether in bent or
linear geometry.5
Received: December 15, 2011
Published: February 22, 2012
© 2012 American Chemical Society
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solution was added dropwise to a stirred suspension of CrCl2(THF)2
(0.713 g, 2.65 mmol) in about 30 mL of hexane cooled in a dry ice/
acetone bath. The mixture was stirred for 3 days at room temperature,
by which time the solution had become a dull orange/red color and a
white precipitate had formed. This was allowed to settle and was
separated by careful decanting of the supernatant solution by cannula.
The resultant red/orange solution was concentrated to about 10 mL,
which, after storage for 1 day at −18 °C, afforded X-ray quality air-
sensitive red crystals of 2. Exposure to air led to a blue solution. Yield
0.267 g (12%), mp 288−292 °C. Calcd. for C60H96N2Cr: C, 82.15; H,
8.73; N, 3.19. Found: C, 81.7; H, 8.77; N, 3.02. UV−vis, nm (ε,
M−1 cm−1), 340 (3500) and 402 (900). IR in Nujol mull (cm−1) in
In contrast to the iron and manganese amides, monomeric
two-coordinate chromium(II) amides are almost wholly
unknown (likely the result of the extreme instability owing to
ready oxidation and of course coordinative unsaturation) and are
confined to two borylamide derivatives Cr{N(Ph)BMes2}29a and
Cr{N(Mes)BMes2}29b which exhibit strongly bent geometries
(N−Cr−N = 110.8(1) and 112.3(3)°) as well as close
interactions (2.32−2.41 Å) with ipso carbons of ligand aromatic
substituents.9 Other well-characterized Cr(II) amides are either
dimerized or complexed with Lewis bases as in Cr{N-
(SiMe3)2}2(THF)210 or the dimeric species {Cr(NPri2)2}2,11
{Cr(NPh2)2}2,12 {Cr(NCy2)2}2,12 and (Cr{N(1-Ad)C6H3-3,5-
Me2}2)2 (Cy = cyclohexyl, 1-Ad = 1-adamantyl).13 The magnetic
moments of the mononuclear complexes Cr{N(Ph)BMes2}2,9a
KBr: νN−H 3354 (w).
Me6
Cr{N(H)ArMe }2(THF) (3). To a solution of Ar NH2 (1.0 g, 3 mmol)
6
in about 30 mL of diethyl ether was added LiBun (2.5 M in C6H14)
(1.3 mL, 3.3 mmol) at about −78 °C. After stirring for 24 h, the pale
yellow solution was added dropwise to a stirred suspension of
CrCl2(THF)2 (0.403 g, 1.5 mmol) in about 30 mL of diethyl ether
cooled to about −78 °C in a dry ice/acetone bath. Upon addition, an
immediate change to a green colored suspension was observed. The
mixture was stirred for 4 days at room temperature, by which time it
had become a brown/orange color and a white precipitate had formed.
The volatile materials were removed under reduced pressure and
hexane (ca. 50 mL) was added, which resulted in a bright orange
colored mixture. The precipitate was allowed to settle and after careful
decanting by cannula, the dark brown/orange supernatant liquid was
concentrated to about 10 mL. Storage for 1 week at −18 °C, afforded
X-ray quality dichroic orange/green crystals of 3. Exposure to air led to
a blue-black solution. Yield 0.064 g (5.5%), mp 199−206 °C,
decomposition at 170 °C. UV−vis, nm (ε, M−1 cm−1), 334 (3600) and
10
Cr{N(Mes)BMes2}2,9b and Cr{N(SiMe3)2}2(THF)2 were
reported to have μeff values 4.91, 4.92, and 4.93 μB by the
Evans’ method. In contrast, the moments for the amido dimers
were studied as polycrystalline solids over a range of temper-
atures using a Faraday balance. These were found to have μeff
values varying from 2.30−2.67 μB, which were interpreted in
terms of antiferromagnetic interactions between two low-spin
three-coordinate Cr2+ ions. The variety of magnetic behavior
suggests that further investigation of the magnetic properties of
low coordinate chromium complexes is warranted. We now
describe the synthesis, characterization, and magnetic properties
of four mononuclear Cr(II) amides. These are two derivatives of
i
Pri4
the very bulky N(H)ArPr and N(H)Ar primary amido ligands
6
which possess very rare linear coordination for chromium. In
contrast, strongly bent metal coordination is observed in the less
402 (1100). IR in Nujol mull (cm−1) in KBr: νN−H 3359 (w).
Me6
Cr{N(H)ArMe }2 (4). To a solution of Ar NH2 (1.0 g, 3 mmol) in
6
crowded Cr{N(H)ArMe }2 which in turn readily complexes with
6
about 30 mL of diethyl ether cooled to about −78 °C was added LiBun
(2.5 M in C6H14) (1.3 mL, 3.3 mmol) . After stirring for 24 h, the pale
yellow solution was added dropwise to a stirred suspension of
CrCl2(THF)2 (0.401 g, 1.5 mmol) in about 30 mL of diethyl ether
cooled to about −78 °C. Upon addition, an immediate color change to
green was observed. The mixture was stirred for 4 days at room
temperature, by which time the solution had become a brown/orange
color and a white precipitate had formed. Diethyl ether solvent was
removed under reduced pressure and the residue was extracted with
three 20 mL aliquots of hot hexane to give a dark orange colored
solution. The precipitate was allowed to settle and after careful decanting
by cannula, the dark brown/orange solution was concentrated to about
10 mL. Storage for 1 week at −18 °C, afforded X-ray quality dark orange
air-sensitive crystals of 4. Exposure to air led to a blue-black solution.
Yield 0.079 g (7.4%), mp 199−206 °C. Calcd. for C48H52N2Cr: C,
81.32; H, 7.39; N, 3.95. Found: C, 81.67; H, 7.58; N, 3.62. UV−vis, nm
(ε, M−1 cm−1), 336 (1100) and 398 (300). IR in Nujol mull (cm−1) in
KBr: νN−H 3359 (w).
tetrahydrofuran (THF) to give the three coordinate Cr{N(H)-
ArMe }2(THF).
6
EXPERIMENTAL SECTION
■
General Procedures. All manipulations were carried out by using
modified Schlenk line techniques under a dinitrogen atmosphere or in a
Vacuum Atmospheres HE-43 drybox. All of the solvents were first dried
by the method of Grubbs et al. and then stored over potassium.14 All
physical measurements were obtained under strictly anaerobic and
anhydrous conditions. IR spectra were recorded as Nujol mulls between
KBr plates on a Bruker Alpha spectrophotometer. UV−visible spectra
were recorded as dilute hexane solutions in 3.5 mL quartz cuvettes
using a HP 8452 diode array spectrophotometer. Melting points were
determined on a Meltemp II apparatus using glass capillaries sealed
with vacuum grease, and are uncorrected. Unless otherwise stated, all
materials were obtained from commercial sources and used as received.
i
6 16
6 17
i418
CrCl2(THF)2,15 H2NArMe
,
LiN(H)ArPr
,
and LiN(H)ArPr were
X-ray Crystallography. Deep orange-red X-ray quality crystals of
1−4 were obtained from concentrated hexane solutions after storage
at −18 °C for 2 days. Suitable crystals were selected and covered with
a layer of hydrocarbon oil under a rapid flow of dinitrogen. They were
mounted on a glass fiber attached to a copper pin and placed in a cold
N2 stream on a diffractometer. X-ray data for 1 and 2 were collected at
90(2) K with 0.71073 Å Mo Kα radiation using a Bruker SMART
Apex II diffractometer. Data for 3 and 4 were collected at 90(2) K with
1.5418 Å Cu Kα1 radiation with a Bruker DUO diffractometer in
conjunction with a CCD detector.19 The collected reflections were
corrected for Lorentz and polarization effects and for absorption by
use of Blessing’s method as incorporated into the program SADABS.20
The structures were solved by direct methods and refined with the
SHELXTL v.6.1 software package.21 Refinement was by full-matrix
least-squares procedures with all carbon-bound hydrogen atoms
included in calculated positions and treated as riding atoms.
N-bound hydrogens were located directly from the Fourier difference
map. A summary of crystallographic and data collection parameters for
1−4 is given in Table 1.
prepared according to literature procedures.
Pri6
Pri6
Cr{N(H)Ar }2 (1). To a solution of Ar NH2 (1.0 g, 2 mmol) in
about 30 mL of hexane was added LiBun (2.5 M in C6H14) (0.9 mL,
2.2 mmol) at about −78 °C using a dry/acetone bath. After stirring for
24 h, the pale yellow solution was added dropwise to a stirred suspension
of CrCl2(THF)2 (0.269 g, 1 mmol) in about 30 mL of hexane at about
−78 °C. The mixture was stirred for 3 days at room temperature, by
which time the solution had become orange and a white precipitate
had formed. The precipitate was allowed to settle and after careful
decanting by cannula, the orange solution was concentrated to about
10 mL, which, upon storage for 1 day at −18 °C, afforded X-ray
quality bright orange highly air-sensitive crystals of 1. Exposure to air
immediately led to a blue solution. Yield 0.392 g (18%), mp 185−
189 °C. Calcd. for C72H100N2Cr: C, 82.71; H, 9.64; N, 2.68. Found: C,
82.01; H, 9.21; N, 2.54. UV−vis, nm (ε, M−1 cm−1), 342 (5100) and
410 (1300). IR in Nujol mull (cm−1) in KBr: νN−H 3350 (w).
i
Pri4
Cr{N(H)ArPr }2 (2). To a solution of Ar NH2 (2.2 g, 5.3 mmol) in
4
about 30 mL of hexane was added LiBun (2.5 M in C6H14) (2 mL,
6 mmol) at about −78 °C. After stirring for 24 h, the pale yellow
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Table 1. Selected Crystallographic and Data Collection Parameters for the Complexes 1−4
i
Pri4
Me6
Cr{N(H)ArPr }2 (1)
Cr{N(H)Ar }2 (2)
Cr{N(H)ArMe }2(THF) (3)
Cr{N(H)Ar }2 (4)
6
6
formula
Fw. g/mol
color, habit
crystal system
space group
a, Å
C72H100N2Cr
1045.54
C60H76N2Cr
877.23
C52H60N2OCr
781.02
C48H52N2Cr
708.92
orange, rod
triclinic
red, block
monoclinic
P21/n
dichroic, block
monoclinic
P21/c
dark orange, block
triclinic
P1
̅
P1
̅
10.341(2)
11.393(2)
14.585(3)
70.389(3)
81.988(3)
80.301(3)
1589.0(6)
1
10.7222(3)
20.3204(6)
11.2743(3)
90
10.520(2)
16.081(2)
25.496(4)
90
13.1525(5)
17.6782(8)
18.5516(7)
77.851(3)
82.704(2)
86.023(2)
4178.7(3)
2
b, Å
c, Å
α, deg
β, deg
90.531(1)
90
90.413(2)
90
γ, deg
V, Å3
2456.33(12)
2
4313.0(11)
4
Z
crystal dims, mm
T, K
0.41 × 0.34 × 0.29
90(2)
0.199 × 0.193 × 0.243
90(2)
0.098 × 0.366 × 0.645
90(2)
0.148 × 0.246 × 0.455
90(2)
d
calc, g/cm3
1.093
1.186
1.203
1.195
abs. coefficient μ, mm−1
0.221
0.273
0.305
0.314
θ range, deg
1.49−27.50
6140
2.61−27.50
4782
1.94−27.48
8352
2.45−68.25
12198
obs reflections [I > 2σ(I)]
data/restraints/parameters
R1, observed reflections
wR2, all
7278/0/356
0.064
5639/0/438
0.0366
9881/21/534
0.0448
14018/25/105
0.0935
0.1932
0.0462
0.0519
0.1039
Magnetic Measurements. Polycrystalline samples of complexes
1−4 were sealed under vacuum in 3 mm diameter quartz tubes for
magnetic studies. The samples’ magnetizations were measured using a
Quantum Design MPMSXL7 Superconducting Quantum Interference
Device (SQUID). In each case the sample was zero-field cooled to
4 K. The magnetization was measured upon warming to 380 K in an
applied field of 0.01 T (100 Oe). Diamagnetic corrections of −766 ×
10−6, −624 × 10−6, −535 × 10−6, and −482 × 10−6 emu/mol,
obtained from tables of Pascal’s constants,22 were applied to the
measured molar magnetic susceptibilities of complexes 1−4,
respectively.
mixtures slowly deepened to orange upon warming to room
temperature. Stirring had to be continued for extended periods
(ca. 2−4 days) to obtain significant yields of the products 1−4,
and crystals were grown from the reaction solution (after
separation for the LiCl precipitate) by concentration and cooling
to about −18 °C of the hexane extracts of the dry reaction
mixtures. Solutions of 1−4 were highly sensitive such that
filtration through Celite padded glass frit invariably resulted in
significant decomposition even when extreme precautions were
taken to exclude air and moisture. We found that separation of
the solutions from the precipitate by decanting afforded the
products 1−4 in relatively low but reproducible yield.
RESULTS AND DISCUSSION
■
The synthesis of 1−4 differs from the approach used for the
Synthesis. Compounds 1−4 were synthesized via salt
i
Me6
iron complexes Fe{N(H)ArPr }2 and Fe{N(H)Ar }2 which
6
metathesis routes as shown in Scheme 1. While the synthetic
were obtained by transamination involving the treatment of
23
Fe{N(SiMe3)2}2 with 2 equiv of the respective primary
Scheme 1. Synthetic Routes to 1−4
amines. This approach was used because the alkali metal salt
elimination route analogous to that used for 1−4 (Scheme 1)
proved unsatisfactory owing to sluggish reactions and the
formation of anionic products. In contrast, the greater solubility
of CrCl2(THF)2 permitted reactions to occur, albeit slowly,
to afford 1−4 in low but acceptable yields. The reasons for
the low-yields in these reactions are unclear at present. How-
ever, the slow rate of reaction, which probably involves an
unfavorable nucleophilic attack on the square planar geometry
CrCl2(THF)2 complex, together with the possible formation of
salt-like products, may be factors in reducing the yields.
Structures. The structures of 1 and 2 are shown below in
Figures 1 and 2. Selected bond lengths and angles are presented
in Table 2 along with data from their manganese5 and iron4
analogues. The structure of the bent geometry bis(amido)
complex 4 is shown in Figure 3. Selected structural data for 4
and its manganese and iron analogues are given in Table 3.
Complexes 1 and 2 are the first reported examples of linear
coordinated amido chromium species. Linear coordination for
chromium is very rare and has precedent only in the related
routes appear straightforward, they required considerable
investigation of the reaction conditions to obtain reliable
product yields. A variety of solvents such as diethyl ether,
tetrahydrofuran, toluene, and so forth, as well as reaction
temperatures, were tested for these reactions. It was found that
a synthetic procedure involving the slow addition of lithium
primary aryl amide Li{N(H)Ar} to a hexane (1 and 2) or
diethyl ether (3 and 4) suspension of the metal halide-
tetrahydrofuran complex cooled to about −78 °C afforded the
most consistent results. The initial green color of the reaction
i
24
Cr2+ thiolato complex Cr(SArPr )2. The ipso carbons of the
6
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Figure 3. Thermal ellipsoid drawing (30%) of the X-ray crystal
i
Figure 1. X-ray crystal structure of linearly coordinated Cr{N(H)ArPr }2,
(1). (Non-nitrogen H atoms are not shown for clarity, thermal ellipsoids
are shown at 30% probability.) Select bond distances and angles are given
in Table 2.
6
structure of nonlinear coordinated Cr{N(H)ArMe }2, (4). (Non-
6
nitrogen H atoms are not shown for clarity.) Select bond distances
and angles are given in Table 3.
Table 3. Selected Interatomic Distances (Å) and Angles
(deg) for the Bent Complex 4 and the Corresponding Mn5
a
and Fe4 Derivatives
Cr{N(H)ArMe
}
2
Mn{N(H)
Fe{N(H)
4
6
6
ArMe
}
ArMe
}
2
5
6
(4)
2
M−N(1) (Å)
M−N(2) (Å)
1.977(3)
1.943(3)
2.338(4)
121.49(13)
119.1
1.976(2)
1.982(3)
2.63
1.909(3)
1.913(3)
2.64
M---(i-MesC) (Å)
N(1)−M−N(2) (deg)
M−N(1)−H(1) (deg)
M−N(2)−H(2) (deg)
C(1)−N(1)−M (deg)
C(25)−N(2)−M (deg)
138.19(9)
141.94(16)
116
111.7
114
121.8(2)
136.7(2)
128.5(3)
127.2(3)
i
4
a
Figure 2. X-ray crystal structure of linearly coordinated Cr{N(H)ArPr }2,
The average distance of M---(i-MesC) interactions are also given.
(2). (Non-nitrogen H atoms are not shown for clarity, thermal ellipsoids
are shown at 30% probability.) Select bond distances and angles are listed
in Table 2.
in the dimers R2NCr(μ-NR2)2CrNR2 (R = Pri and Cy)12 which
have three-coordinate Cr(II).
There are also relatively close interactions between the metal
and ipso carbon from one of the flanking aryl rings of the
terphenyl ligand of 2.48 Å for complex 1 and 2.63 Å for complex 2.
These structural data support the view that terphenyl based
ligands protect space surrounding the metal primarily via the
shielding action of their flanking aryl rings. In this respect,
inspection of the structures of 1 and 2 shows that 1 is the more
crowded molecule. The presence of para-isopropyl groups on
the flanking aryl rings cause the C6H2-2,4,6-Pri3 ring closest to
the chromium to bend away from the metal as indicated by an
angle of 19.56° between the C(2)−C(7) bond and the plane of
Table 2. Selected Interatomic Distances (Å) and Angles
(deg) for the Linear Complexes 1 and 2 and the Mn5 and Fe4
i
6
Derivatives of N(H)ArPr
Cr{N(H)
Cr{N(H)
Mn{N(H)
Fe{N(H)
i
i
i
i
6
ArPr }2 [1]
ArPr }2 [2]
ArPr
}
2
ArPr
}
6
4
6
2
M−N (Å)
M---(C7,7A) (Å) 2.481(2)
1.9966(14)
1.9775(12) 1.952(2)
1.907(14)
2.79
2.630
2.73
N−M−N (deg)
M−N−H (deg)
C−N−M (deg)
180.0
180.0
176.09(12) 180.0
117.5(16)
130.06(11)
121.3(18)
123.33(11)
121.3(13)
125.70(9)
the C(7) ring. This bending is caused by the interaction of
i
the para Pri group and the flanking ring of the opposite ArPr
6
ligand. However, the shorter Cr---C interactions in 1 are
probably caused by the presence of the para −Pri groups on the
flanking aryl rings that cause trans-metallic steric repulsion (i.e.,
between the para −Pri substituents on the C(23) and C(7A)
flanking tri-iso-propyl phenyl rings) which cause the Cr−N−
C(ipso) angles to close (cf. Table 2, C−N−M angles =
123.33(11) in 1 and 125.70(9)° in 2 and give a shorter Cr---C
approach in 1.
central aryl rings of the terphenyl group, the nitrogens, the
hydrogens on the two nitrogens, and chromium form a plane
with the terphenyls in a trans-fashion resulting in local C2h
symmetry for the M{N(H)C(ipso)}2 arrays. The key structural
data for 1 and 2 and related manganese and iron species are
summarized in Table 2. The M−N distances in 1, 1.9966(14) Å,
and 2, 1.9775 (12) Å, are very similar. The bond lengths are
significantly longer than those in the corresponding man-
ganese5 and iron4 complexes consistent with the decreasing size
(Cr, 1.22 Å; Mn, 1.19 Å; Fe, 1.16 Å) of the metal radius on
proceeding to the right across the d-block.25 They are about
0.1 Å shorter than the 2.09(1) Å Cr−N bond length in
Cr{N(SiMe3)2}2(THF)2 which has four-coordinate Cr(II) in
square planar complexation. However, the bonds are longer
than the terminal Cr−N distances of 1.927(3) and 1942(7) Å
The structure of the complex Cr{N(H)ArMe }2, (4) is shown
6
in Figure 3. Key structural data for this complex as well as
data for its manganese and iron congeners are given in Table 3.
These show that 4 has a very strongly bent coordination with a
N−Cr−N angle of 121.49(13)° which resembles the N−Cr−N
angles of 110.8(1)° in the borylamido complexes Cr{N(Ph)-
BMes2}2 and 112.3(3)° in Cr{N(Mes)BMes2}2. It is also
noteworthy that the degree of bending in 4 is considerably
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Article
(DFT) calculations, led them to the conclusion that σ-donor
coligands favor T-coordination whereas π-donor ligands prefer Y-
coordination.27 To classify the geometries they introduced a “T”
parameter which was defined as the difference between the two
N−Cr−X angles divided by 90°. The T values can range from 0
(perfectly Y-shaped) to 1 (a perfect T-shape). Because of the large
size of the β-diketiminate ligand that was employed, an ideal T value
of 1 was deemed unlikely. However, the geometry can be regarded
as distorted T-shaped when the T values become significantly larger
than 0, and they chose a value that was larger than 0.1 as
the characteristic criterion for a distorted T-shaped geometry.
The largest T value they observed for their β-diketiminate
derivatives was 0.447 for a σ-donating coligand. It can be seen
from the data provided for 3 in Table 5, that 3 has a very similar
Table 5. Comparison of T-Values of Selected Three-
Coordinate Cr(II) Complexes
Figure 4. Thermal ellipsoid drawing (30%) of the X-ray crystal
structure of the three-coordinated Cr{N(H)ArMe }2(THF), (3). (Non-
6
Cr{(μ-NH2)
Cr{N(H)
(THF)
i
a
b
,
28
c26
,
ArMe }2(THF) (3)
CrArPr
}
2
LiClCr(OCBut3)2
6
4
nitrogen H atoms are not shown for clarity.) Select bond distances and
angles are given in Table 4.
L−Cr−X
L−Cr−X
difference
134.36(5)
93.44(6)
40.92
110.11(7)
86.21(8)
23.90
110.9(1)
91.1(1)
19.8
Table 4. Selected Interatomic Distances (Å) and Angles
a
(deg) for 3 and the Related Mn5 Derivative
d
T value
0.455
0.266
0.22
Pri4
t
Cr{N(H)ArMe }2 (THF)
Mn{N(H)
6
a
b
c
L = N(H)ArMe , X = O. L = NH2, X =Ar . L = OCBu 3, X = Cl.
6
5
[3]
ArMe }2(THF)
d
6
T = angular difference divided by 90°.
M−N(1) (Å)
M−N(2) (Å)
M−O (Å)
M---(i-MesC) (Å)
N(1)−M−N(2) (deg)
N(1)−M−O(1) (deg)
N(2)−M−O(1) (deg)
1.9779(14)
1.9546(14)
2.0682(13)
2.948
1.986(2)
1.988(2)
2.175(2)
2.907
three-coordinate T value of 0.455. Table 5 also includes structural
data for two Cr(II) complexes with all monodentate ligands
which meet the T value for distorted T-shaped geometry. This is
very much in agreement with the σ-donor character of the THF
132.08(6)
134.36(5)
93.44(6)
143.83(5)
117.10(7)
99.05(7)
i
28
ligand. The remaining two complexes, Cr{(μ-NH2)CrArPr
}
2
4
and (THF)LiClCr(OCBut3)2,26 also feature T values well above
the 0.1 criterion for distorted T-shaped geometry. It is of course
possible to argue that our analysis of the geometry of the three
complexes is unjustified since the X−Cr−X angles are much
wider than the about 90° bite angle of the β-diketiminates.
Nonetheless, the essential message is that the complexes involved
all have very severe distortions from regular trigonal geometries.
Electronic Spectroscopy. The UV−visible absorption
spectra of the intensely colored complexes 1−4 in dilute hexane
solution revealed moderately intense electronic transitions for
the series of the linear complexes. UV−vis absorption peaks
(λmax, nm (ε, M−1 cm−1)) were observed at 342 (5100) and 410
(1300) for linear complex 1. The other linear complex 2
displayed two similar absorptions at 340 (3500) and 402 (900).
The three-coordinate complex 3 also displayed two absorption
maxima at 334 (3600) and 402 (1100). The bent two-
coordinate complex 4 had absorption maxima at 336 (1100)
and 398 (300). Crystalline samples and solutions of the linear
compound 1 were intensely orange while those of 2 were a
darker red/orange. Both nonlinear chromium compounds 3 and
4 had a darker orange/brown color. The observation of similar
spectra for 1−4 suggest that their structures are broadly similar
in solution. If it is assumed that the 5D0 ground state of Cr2+ has
a
The average distance of M---(i-MesC) interactions are given.
greater (by ca. 17°) than that in its manganese analogue. The
Cr−N distances in 4, 1.977(3) and 1.943(3) Å, are slightly
shorter than those in 1 and 2. Thus the data in Tables 2 and 3,
show that as the bulk of the ligand decreases, so do the Cr−N
bond lengths, supporting the view that the steric crowding causes
the slight lengthening of the bond in more hindered complexes 1
and 2. The structure 4 also has the closest interaction between
the metal and ipso carbon from one of the flanking aryl rings of
the terphenyl ligand at only 2.338(4) Å.
The THF complex 3 shown in Figure 4 is a relatively rare
example of a monomeric three-coordinate chromium(II)
complex,26−28 and is analogous to the manganese Lewis base
5
adduct Mn{N(H)ArMe }2(THF). The interaction between the
6
metal and ipso carbon from one of the flanking aryl rings of the
terphenyl ligand is prevented by the coordination to the THF
(Cr−O = 2.338(4) Å) and a very long distance of 2.948 Å to
the closest flanking aryl carbon is observed. M−N distances,
1.9779(14) and 1.9546(14) Å, are only slightly lengthened
despite the higher metal coordination number.
The most interesting feature of the structure of 3 is the highly
distorted metal coordination. Although the metal coordination
is planar, the two O−Cr−N angles, 134.36(5) and 93.44(6)°,
differ by more than 40° (cf. an 18° difference in its manganese
analogue). There is growing evidence that the gross irregularities
in the interligand angles in three-coordinate Cr(II) complexes is
a preferred coordination mode. An analysis by Baik, Mindiola,
and co-workers of the structural distortions in several β-diketiminate
Cr(II) derivatives, combined with density functional theory
approximately linear coordination it will be split into Σg+, Πg,
5
5
5
and Δg states (for local D∞h symmetry) so that the higher
energy (ca. 340 nm) and lower energy transitions at about
400 nm could be assigned to Σg+→Δg and Σg+→Πg transitions.
However, definitive assignments are not possible because of the
lack of both comparison data and detailed computational work
on the energy states. It is noteworthy that these wavelengths
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Inorganic Chemistry
Article
splitting is quite large or that the absorptions involve charge
transfer. Since the former possibility seems inconsistent with the
low coordination number, the latter possibility combined with
their broadness suggests they are charge transfer transitions.
DC Magnetometry Measurements. The DC SQUID
magnetometry results for complexes 1 through 4 are
summarized in Table 6 and Figures 5 and 6. In view of the
linearity of the χ−1 versus T data for complexes 1, 3, and 4 over
the entire temperature range 5 to 300 K, it seems reasonable to
assume that these systems more or less conform to a single
Curie−Weiss Law while the plot for complex 2 deviates
somewhat from linearity. This in turn is likely the result of trace
Table 6. Curie−Weiss Law-Parameters Derived for
Complexes 1−4
a
complex
temperature range/K
C/emu K mol−1
μeff, μβ
1
2
3
4
5−300
5−300
5−300
5−300
2.20
2.34
1.80
2.38
4.20
4.33
3.79
4.36
a
The effective magnetic moment obtained from the Curie constant, C,
and uncorrected for any values of H.
when converted to cm−1 frequency units become 29,400 and
25,000 cm−1. These values suggest either that the ligand field
Figure 5. μEFF versus T plots for complexes 1−4.
Figure 6. Inverse molar susceptibilities versus T plots for complexes 1−4.
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Inorganic Chemistry
Article
impurity or possibly that 2 undergoes some type of structural
transformation or phase change at intermediate temperature.
The latter have in fact been observed for other linear systems
studied in our laboratory via X-ray crystallography.28
elements. Further studies of these complexes involving EPR,
electrochemical and computational techniques are in hand.
ASSOCIATED CONTENT
* Supporting Information
CIF files for the X-ray structures of 1−4. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
In attempting to account for the uniformly lower than
predicted magnetic moments of 1−4, we note that previous
detailed magnetic susceptibility investigations of divalent
chromium compounds are almost exclusively concerned with
octahedral or six coordinate pseudo-octahedral complexes. As
such their effective moments are generally close to the spin-
only moment expected (√24 or 4.89 μβ) for the high-spin d4
configuration. These moments are usually discussed in terms of
the equation μeff = μso(1 − αλ/10Dq), corresponding to a
second-order effect of spin−orbit coupling29,30 where for the
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: pppower@ucdavis.edu.
■
Notes
The authors declare no competing financial interest.
5
free ion D ground term of Cr2+, the spin−orbit coupling
parameter λ is +57 cm−1.30 The J states for this ground term are
ACKNOWLEDGMENTS
■
́
0, 1, 2, 3, 4 and span 10 λ (using the Lande interval rule) or
We are grateful to the National Science foundation (CHE-
0948417) for financial support and Peter Klavins for assistance
in the magnetization measurements.
some 570 cm−1. For d4 α = 2 and 10Dq has its usual meaning of
the ligand field splitting. The expected decrease in μeff implied
(by the second “negative” term) of the foregoing equation is
usually very small, and the observed moments are usually not
significantly different from μso consistent with the orbitally
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■
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6
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