Solid-state spin crossover of Ni(II) in a bioinspired N3S 2 ligand field

Article abstract of DOI:10.1021/ja110890v

The complex Tp^Ph,MeNiS₂CNMe₂ [Tp ^Ph,Me = hydrotris(3-phenyl-5-methyl-1-pyrazolyl)borate] features a bioinspired N₃S₂ ligand set supporting a five-coordinate, trigonally distorted square-pyramidal geometry in the solid state. Spin crossover of Ni(II) was demonstrated by temperature-dependent X-ray crystallography and magnetic susceptibility measurements. The crystal lattice contains two independent molecules (i.e., Ni1 and Ni2). At 293 K, the observed bond lengths and susceptibility are consistent with high-spin (S = 1) Ni(II), and both molecules exhibit relatively short axial Ni-N bonds and long Ni-N and Ni-S equatorial bonds. At 123 K, the Ni1 complex remains high-spin, but the Ni2 molecule substantially crosses to a structurally distinct diamagnetic (S = 0) state with significant elongation of the axial Ni-N bond and offsetting contraction of the equatorial bonds. The temperature-dependent susceptibility data were fit to a spin equilibrium at Ni2 [ΔH° = 1.13(2) kcal/mol and ΔS° = +7.3(1) cal mol^-1 K^-1] consistent with weak coupling to lattice effects. Cooling below 100 K results in crossover of the Ni1 complex.

Full text of DOI:10.1021/ja110890v

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pubs.acs.org/JACS
Solid-State Spin Crossover of Ni(II) in a Bioinspired N S Ligand Field
3
2
†
‡
§
||
,†
Huaibo Ma, Jeffrey L. Petersen, Victor G. Young, Jr., Gordon T. Yee, and Michael P. Jensen*
†
Department of Chemistry and Biochemistry, Ohio University, Athens, Ohio 45701, United States
‡
C. Eugene Bennett Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506, United States
§
X-ray Crystallographic Facility, Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States
Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, United States
S Supporting Information
b
5
,14ꢀ21
crossover,
but the relevant spin isomers have not been
Ph,Me
Ph,Me
11
ABSTRACT: The complex Tp
hydrotris(3-phenyl-5-methyl-1-pyrazolyl)borate] features a
bioinspired N S ligand set supporting a five-coordinate,
NiS CNMe [Tp
=
2
2
structurally characterized. In the present work, we obtained
magnetic and structural evidence for solid-state spin crossover of
pentacoordinate Ni(II) in a bioinspired N S ligand field.
3
2
3
2
trigonally distorted square-pyramidal geometry in the solid
state. Spin crossover of Ni(II) was demonstrated by tem-
perature-dependent X-ray crystallography and magnetic
susceptibility measurements. The crystal lattice contains
two independent molecules (i.e., Ni1 and Ni2). At 293 K,
the observed bond lengths and susceptibility are consistent
with high-spin (S = 1) Ni(II), and both molecules exhibit
relatively short axial NiꢀN bonds and long NiꢀN and
NiꢀS equatorial bonds. At 123 K, the Ni1 complex remains
high-spin, but the Ni2 molecule substantially crosses to a
structurally distinct diamagnetic (S = 0) state with signifi-
cant elongation of the axial NiꢀN bond and offsetting
contraction of the equatorial bonds. The temperature-
dependent susceptibility data were fit to a spin equilibrium
at Ni2 [ΔH° = 1.13(2) kcal/mol and ΔS° = þ7.3(1) cal
The inspiration for our work was the nickel-dependent super-
oxide dismutase, which recently was structurally characterized as
a mixture of Ni(III) in a square-pyramidal N S ligand field
3
2
(
including the N-terminal amine, the adjacent backbone amide,
Cys-2 and -6 thiolates, and an axial His-1 imidazole) and Ni(II) in
a square-planar N S ligand field (with a detached axial
2
2
22,23
imidazole).
Retention of the latter ligand in a reduced
catalytic intermediate would give square-pyramidal spin isomers
that are either elongated and diamagnetic (S = 0) or compressed
and paramagnetic (S = 1), depending on the strength of the
24,25
interaction.
We previously reported Ni(II) complexes of facially tridentate
hydrotris(pyrazolyl)borate chelates [hydrotris(3-R-5-methyl-1-
Me,Me
Ph,Me 26
pyrazolyl)borate: R = Me (Tp
), Ph (Tp
)] with
0
ꢀ
0
zwitterionic dithiocarbamate (R NCS : R = Et, Ph) coligands
2
2
ꢀ
1
ꢀ1
2
7,28
mol
K ] consistent with weak coupling to lattice effects.
as models for the biologically unique NiSOD active site.
These
scorpionate equilib-
rium in solution between a green paramagnetic N S isomer and a
3
2
R,Me
Cooling below 100 K results in crossover of the Ni1
complex.
synthetic complexes exhibit a κ -/κ -Tp
3
2
red diamagnetic N S isomer with a detached axial pyrazolyl
2
2
donor. An analogous spin equilibrium driven by eneꢀallyl
cyclopentadienyl ring slippage was observed in solution for
2
9
Me,Me
C Me Ni(acac). Sterically unhindered Tp
NiS CNEt
2 2
27
5
5
pin crossover is a means to obtain bistable molecular switches
for nanoscale devices. Cooperative intermolecular behavior
crystallized as a green square pyramid in the solid state with
1,2
S
3
30
a κ -Tp ligand and slight trigonal distortion (τ = 0.31), while
in the solid state can lead to abrupt spin transitions with thermal
Ph,Me
the bulkier Tp
NiS CNR (R = Et, Ph) complexes both
2 2
2 28
3
hysteresis, enabling bistability and optical switching. This phe-
crystallized as red square planes (τ = 0.01) with κ -Tp ligands.
In the present work, we prepared Tp
4
7
nomenon is usually defined to include octahedral 3d ꢀ3d metal
Ph,Me
NiS CNMe , which
2 2
6
ions, and a majority of examples involve crossing of d Fe(II)
has intermediate bulkiness, and isolated a green crystalline solid
that exhibits spin crossover (for synthesis and characterization,
see the Supporting Information).
1
5
from a low-spin state to a high-spin state (i.e., A1 f T ) in
g
2g
which isotropic ligand breathing is coupled to t2 f e electron
g
g
4
unpairing.
In contrast, “anomalous magnetism” of d Ni(II) complexes
Two structure determinations were performed on separate
8
Ph,Me
crystals of Tp
NiS CNMe at 293 and 123 K. The crystal
2 2
4,5
typically involves ligand addition or a conformational change.
data are summarized in Table 1, and coordinate bond lengths are
given in Table 2. Essentially identical triclinic P1 lattices were
observed at both temperatures (Table 1 and Figure 1), with
1.0ꢀ1.5% contractions in the unit cell axes and a 3.1% reduction
in unit cell volume at the lower temperature. The lattices contain
two independent molecules (i.e., Ni1 and Ni2; Figure 2), both of
For example, (Ph BnP) NiBr crystallizes as a mixture of square-
2
2
2
planar diamagnetic (S = 0) and tetrahedral paramagnetic (S = 1)
6
“
allogons”. Nonallogonic spin crossover of Ni(II) is also possi-
3
ble through axial modulation of a tetragonal ligand field; B
A1(g) spin crossover can be attained for a d metal ion under axial
C (D ) distortion.
are difficult to achieve in the solid state. Nevertheless, the solid-
state anomalous magnetism of a few penta- and hexacoordi-
nate Ni(II) complexes has long been attributed to spin
T
1(g)
1
8
5,7ꢀ15
Dynamic ligand field rearrangements
4
v
4h
13
Received: December 3, 2010
Published: March 28, 2011
r 2011 American Chemical Society
5644
dx.doi.org/10.1021/ja110890v
|J. Am. Chem. Soc. 2011, 133, 5644–5647

Journal of the American Chemical Society
COMMUNICATION
which are pentacoordinate and approximately square-pyramidal
This is consistent with spin crossover, suggesting that the Ni2 site
represents a temperature-dependent superposition of spin iso-
mers. Elongated square-pyramidal geometries were observed for
(
τ = 0.28 and 0.32 for Ni1 and Ni2, respectively, at 293 K).
Consistent with the green color of the crystals, the oMveer,Malel
Me,Me
31
32
structure ofthe Ni1 sitecompares well with the high-spin Tp
diamagnetic Tp
Ni(CN)2, TpNi(PMe )Ph, and TpNi-
3
27
3
33,34
NiS CNEt analogue (Table 2). The coordinate bond lengths
(η -C H ).
2
2
3 5
are consistent with single-electron occupation of both the axial
and equatorial dσ* orbitals in a high-spin (S = 1) d electron
configuration. The Ni1 complex displays a relatively short apical
The high-spin fractions at Ni2 can be estimated from the
observed equatorial bond lengths, taking the averaged Ni1 values
8
2
Ph,Me
as the high-spin limit and those for square-planar κ -Tp
NiS CNEt as the low-spin limit (Table 2). Linear interpolation
Ni1ꢀN1 bond length of 2.048(1)
Å and an axial
2
2
HꢀB1ꢀN2ꢀN1 torsion angle of 179.6(2)° at 293 K, while the
equatorial NiꢀN and NiꢀS bond lengths are significantly longer
between these values gives high-spin fractions of 38 ( 2% at 123
K and 87 ( 5% at 293 K. Back-extrapolation of a limiting NiꢀN
axial bond length for the low-spin square pyramid gives a value of
2.62 Å from the 123 K data, compared with the average value of
2.04 Å for high-spin Ni1. Corresponding square-pyramidal spin
isomers of reduced NiSOD have not been observed experimen-
tally, but axial NiꢀN(His1) bond lengths of 2.612 and 1.958 Å
were calculated for the low- and high-spin states, respectively,
2
Ph,Me
28
than those in the red κ -Tp
NiS CNEt complex. Trigonal
2 2
distortion is evident, with an axis along the N3ꢀNi1ꢀS1 bond
vector of 172.10(4)°. The Ni1 structure at 123 K is essentially
identical.
The Ni2 site displays a longer apical NiꢀN bond length of
2
.149(1) Å at 293 K, with a decreased HꢀB2ꢀN9ꢀN11 torsion
25
angle of 167.2(2)°. Shorter equatorial bond lengths are also
apparent (Table 2). While the Ni1 site is invariant between 293
and 123 K (Table 2 and Figure 2), Ni2 at 123 K exhibits a further
increase of the axial bond length to 2.401(2) Å with decreases in
the HꢀBꢀNꢀN torsion angle to 163.5(2)° and the τ value to
using density functional theory.
To confirm the spin crossover, solid-state magnetic suscept-
Ph,Me
ibility measurements were performed for Tp
NiS CNMe
2 2
from 5 to 300 K (Figure 3). The susceptibility observed at 300 K,
χT = 1.40 emu K/mol (μ = 3.35μ ), is consistent with an S = 1
B
0
.22. Contractions of the equatorial bonds are also observed.
ground state and second-order spinꢀorbit coupling; for compar-
Me,Me
ison, previous measurements for Tp
NiS COMe fit the
2
Table 1. Summary of the X-ray Structure Determinations
Curie law, with χT = 1.42 emu K/mol (μ = 3.37μ ) and Θ =
B
27
ꢀ
6.1 K. However, the susceptibilities in the present work do
T (K)
empirical formula
293(2)
34BN
123(2)
7 2
34BN NiS
not conform to the Curie law. Consistent with this observation
and the structural data, the susceptibility data were fit to
MaxwellꢀBoltzmann statistics over the 100ꢀ300 K range,
C
33
H
7
NiS
2
C
33
H
formula weight
crystal system
space group
a (Å)
662.31
triclinic
P1
662.31
triclinic
P1
12.1442(8)
12.6611(9)
25.058(2)
90.556(1)
102.357(1)
116.994(1)
3328.3(4)
4
12.004(1)
12.536(1)
24.678(2)
91.034(1)
b (Å)
c (Å)
R (deg)
β (deg)
101.353(1)
116.788(1)
3225.9(5)
4
γ (deg)
3
V (Å )
Z
3
dcalcd (g/cm )
1.322
1.364
R
1
[I > 2σ(I)]
[I > 2σ(I)]
(all data)
(all data)
goodness of fit
0.0500
0.0373
0.0849
0.0525
0.0934
1.040
wR
2
0.1373
R
1
0.0632
Ph,Me
Figure 1. Unit cell of Tp
NiS CNMe at 293 K. The Ni2 sites are
2 2
wR
2
0.1496
disposed to the outside left and right, with the Ni1 sites near the center.
Hydrogen atoms have been omitted for clarity.
1.029
Table 2. Summary of Bond Lengths and Ligand Field Geometries
Ph,Me
Ph,Me
Ph,Me
Ph,Me
Tp
NiS
2
CNMe
2
Tp
NiS
2
CNMe
2
Tp
NiS
2
CNMe
2
Tp
NiS
2
CNMe
2
Me,Me
Ph,Me
Tp
NiS
2
CNEt
2
[Ni1]
[Ni1]
[Ni2]
[Ni2]
Tp
2 2
NiS CNEt
T (K)
293
293
123
293
123
293
a
a
NiꢀNax,eq (Å)
NiꢀNeq,ax (Å)
2.027(1) [N1]
2.065(1) [N3]
2.063(1) [N5]
2.3747(8) [S1]
2.4099(7) [S2]
2.048(1) [N1]
2.111(1) [N3]
2.058(1) [N5]
2.3435(5) [S2]
2.3929(5) [S1]
179.6(2)
2.038(2) [N1]
2.111(2) [N3]
2.048(2) [N5]
2.3420(6) [S2]
2.4006(6) [S1]
179.9(2)
2.149(1) [N8]
2.083(1) [N12]
2.046(1) [N10]
2.3234(6) [S4]
2.3614(5) [S3]
167.2(2)
2.401(2) [N8]
2.003(2) [N12]
1.972(2) [N10]
2.2567(6) [S4]
2.2721(7) [S3]
163.5(2)
2.805(1) [N6]
1.934(1) [N3]
1.929(1) [N1]
2.1990(5) [S1]
2.1929(5) [S2]
133.2(2)
a
NiꢀNeq,eq (Å)
a
NiꢀSeq,eq (Å)
a
NiꢀSeq,ax (Å)
HꢀBꢀNꢀNax (deg) 176.0(2)
b
τ
0.31
27
0.28
0.29
0.32
0.22
0.01
reference
this work
this work
this work
this work
28
a
b
Ligand positions refer to (major, minor) square pyramidal, trigonal geometries. Defined as in ref 30.
5
645
dx.doi.org/10.1021/ja110890v |J. Am. Chem. Soc. 2011, 133, 5644–5647

Journal of the American Chemical Society
COMMUNICATION
the presence of independent complexes in a lattice, which may
35
arise from a phase transition. The crystal structures herein
suggest a similar origin for sequential crossover of Ni2 followed
by Ni1. The equatorial ligand plane of Ni2 is oriented with the
long c axis, while that of Ni1 is nearly orthogonal (Figure 1); thus,
the relatively large contraction along c is coincident with equa-
torial bond contraction at Ni2. These effects may be mediated by
a van der Waals contact between a dithiocarbamate substituent
on Ni2 and an equatorial pyrazole on Ni1, and also by π-stacking
between one equatorial pyrazole ring (i.e., N10) on separate Ni2
complexes, with rigorously parallel interplanar separations of
3
.685 Å at 293 K and 3.652 Å at 123 K. Offsetting axial elongation
is accomplished by displacement of the nickel atom, rather than
the apical pyrazole donor, within the rigid pocket circumscribed
by the 3-Ph substituents; the thermal ellipsoid of Ni2 exhibits
high eccentricity at 123 K, with the major axis nearly aligned with
the axial Ni2ꢀN8 bond vector (Figure 2).
Ph,Me
Figure 2. ORTEP plots of Tp
2 2
NiS CNMe at 293 K (left, 30%
ellipsoids) and 123 K (center, 50% ellipsoids) along with overlays of the
Ni1 and Ni2 structures (right: 123 K, color; 293 K, graytone) based
on least-squares alignments of B1/N4/N6 (top) and B2/N11/N13
In summary, we have obtained structural and magnetic
evidence for an unusual solid-state spin equilibrium in a biomi-
metic Ni(II) complex. Crossover to low-spin Ni(II) with de-
creasing temperature results in axial bond elongation and
equatorial bond contractions, consistent with electron pairing
into the lower axial orbital absent e orbital degeneracy. The e
(
bottom). Hydrogen atoms have been omitted for clarity.
g
g
(
i.e., dσ*) orbital splitting must be tuned to offset the electron
pairing energy and support both spin states with proximal zero-
point energies. The spin state of pentacoordinate Ni(II) is
correlated to ligand donor properties; the N S ligand field
3,5
9
3
2
herein must be nearly optimal for crossover, but whether this is
true for the particular donor set of NiSOD remains unknown.
Me,Me
27
Tp
NiS COMe is rigidly paramagnetic (see above), so the
2
heteroatom in the zwitterionic dithioacid coligand and steric
contact with the opposing Tp ligand substituents also must be
important factors. Finally, the crossover behavior seems corre-
lated with crystal packing. We are endeavoring to exploit all of
these unique properties to design new Ni(II) complexes with
abrupt spin transitions, which require enhanced coupling of the
1
ꢀ4
lattice and ligand field vibrations.
Figure 3. Solid-state temperature-dependent magnetic susceptibility
Ph,Me
data (χT, left axis; 1/χ, right axis) for Tp
2 2
NiS CNMe . Dashed lines
’ ASSOCIATED CONTENT
are maximal Curie law slopes for the inverse susceptibility (i.e., T = C/χ;
b, right axis) at high-spin Ni(II) mole fractions of 50 and 100%. Solid
curves were calculated from the fit to a spin equilibrium at Ni2 (see the
S
Supporting Information. Details of syntheses, character-
b
ization, and susceptibility data and fits; crystallographic data for
Tp
Ph,Me
text). The discontinuity near 54 K is assigned to adventitious O
2
.
NiS CNMe at 293 and 123 K (CIF). This material is
2 2
available free of charge via the Internet at http://pubs.acs.org.
assuming thermal equilibrium of half of the Ni(II) sites (i.e., Ni2)
between the S = 0, 1 states (Figure 3; for details of the fit, see the
Supporting Information). The following thermodynamic para-
meters were obtained: ΔH° = 1.13(2) kcal/mol, ΔS° = þ7.3(1)
’ AUTHOR INFORMATION
Corresponding Author
jensenm@ohio.edu
ꢀ
1
ꢀ1
ꢀ1
cal mol
K
; T = 155(5) K; ΔG° = ꢀ360 cm ; μ
1
/2
293K
HS/HS
=
3.45μ . Some correlation of residuals was evident, suggesting
B
weak coupling of the crossover to lattice effects. Combined with
ligand field rearrangement, this may lead to departure of the
crossover entropy from the inherent spin-only value (i.e., R ln 3 =
’
ACKNOWLEDGMENT
The authors acknowledge the donors of the American Che-
ꢀ
1
ꢀ1 14
mical Society Petroleum Research Fund (49296-DNI3) for
support of this research. We thank Prof. Dean Johnston of
Otterbein University for assistance with powder X-ray
diffraction.
þ2.2 cal mol
K ). The fit gave estimates of 28(3) and
5(1)% for the high-spin mole fractions of Ni2 at 123 and 293 K,
8
respectively; these are comparable to the values deduced from
the X-ray structures at these temperatures (see above).
Because of the effects of zero-field splitting and antiferromag-
netic coupling, the data obtained below 100 K were not fit.
However, these data are consistent with subsequent crossover of
Ni1. Two-step spin crossovers have been documented for a
handful of monomeric octahedral complexes and typically reflect
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