Field-Induced Slow Magnetic Relaxation in the Ni(I) Complexes [NiCl(PPh3)2]·C4H8O and [Ni(N(SiMe3)2)(PPh3)2]

Article abstract of DOI:10.1021/acs.inorgchem.5b02497

Direct current (dc) and alternating current (ac) magnetic measurements have been performed on the three Ni(I) complexes: [NiCl(PPh₃)₃], [NiCl(PPh₃)₂]·C₄H₈O, and [Ni(N(SiMe₃)

Full text of DOI:10.1021/acs.inorgchem.5b02497

Article
pubs.acs.org/IC
Field-Induced Slow Magnetic Relaxation in the Ni(I) Complexes
[NiCl(PPh3)2]·C4H8O and [Ni(N(SiMe3)2)(PPh3)2]
Weiquan Lin,†,‡ Tilmann Bodenstein,‡ Valeriu Mereacre,§ Karin Fink,*,‡ and Andreas Eichhofer*,†,‡,∥
̈
†Lehn Institute of Functional Materials, Sun Yat-Sen University, Guangzhou 510275, China
‡Institut fur Nanotechnologie, Karlsruher Institut fur Technologie (KIT), Campus Nord, Hermann-von-Helmholtz-Platz 1, 76344
̈
̈
Eggenstein-Leopoldshafen, Germany
§Institut fur Anorganische Chemie, Karlsruher Institut fur Technologie (KIT), Campus Sud, Engesserstrasse 15, 76131 Karlsruhe,
̈
̈
̈
Germany
∥Karlsruhe Nano Micro Facility (KNMF), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
S
* Supporting Information
ABSTRACT: Direct current (dc) and alternating current (ac) magnetic
measurements have been performed on the three Ni(I) complexes:
[NiCl(PPh3)3], [NiCl(PPh3)2]·C4H8O, and [Ni(N(SiMe3)2)(PPh3)2]. Fits
of the dc magnetic data suggest an almost similar behavior of the three
compounds, which display only moderate deviations from the spin-only
values. The ac magnetic investigations reveal that the two complexes with
trigonal planar coordination[NiCl(PPh3)2]·C4H8O and [Ni(N(SiMe3)2)-
(PPh3)2]display slow magnetic relaxation at low temperatures under
applied dc fields, whereas tetrahedral [NiCl(PPh3)3] does not. Ground and
excited states as well as magnetic data were calculated by ab initio wave
function based multi-configurational methods, including dynamic correlation
as well as spin−orbit coupling. The two trigonal planar complexes comprise
well-isolated S = 1/2 ground states, whereas two S = 1/2 states with a splitting
of less than 100 cm−1 were found in the tetrahedral compound.
[(IPr)Ni(NHAr)] (Ar = 2,6-di-isopropylphenyl).13 Most
recently it was reported that the cationic, two-coordinate
INTRODUCTION
■
Complexes of nickel(I) are far less numerous than those of
Ni(I) NHC complex [Ni(6-Mes)2]Br (6-Mes = 1,3-bis(2,4,6-
either nickel(0) or nickel(II).1 Nonetheless, a number of
trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene) exhib-
examples are meanwhile known that represent thermodynami-
its single molecule magnet (SMM) behavior that the authors
cally stable species with respect to a possible disproportionation
to the more ‘normal’ 0 and +2 oxidation states. In 1964,
assign to orbital degeneracy.14
In most cases, SMM behavior is observed for systems with a
Heimbach reported the syntheses of [NiCl(PPh3)3] and
1
ground spin state larger than S = /2 in combination with a
[NiCl(PPh3)2] as the products of nickel(0) and nickel(II)
reactions for the first time.2 The proposed molecular structures
negative axial zero-field splitting, which is the origin of a barrier
to the magnetization relaxation, a typical Orbach process.15
were confirmed later by analysis of single-crystal XRD data of
They have promising applicability in molecular electronics.16−20
[NiBr(PPh3)3],3 [NiCl(PPh3)2]·C4H8O, [NiCl(PPh3)3],4
[NiCl(PPh3)3]·C7H8,5 and [NiCl(PPh3)2].6 Interestingly, it
When observed in mononuclear complexes, such compounds
are called single-ion molecule magnets (SIMs), a behavior that
was found that the trigonal complexes display a distorted planar
was first observed for lanthanide-containing complexes.21,22
geometry, which was also observed early on for the amide
complex [Ni(N(SiMe3)2)(PPh3)2].7 Related complexes with
Meanwhile, also several first-row transition-metal complexes,
especially of iron23−29 and cobalt,30−32 are known to display
other phosphine ligands have been prepared with tristriethyl-
phosphine8 and with 1,1,1-tris(diphenylphosphinomethyl)-
slow relaxation of the magnetization at low temperatures (for
ethane = tripod,9 whereas single-crystal X-ray data have been
reported for [{PhCH2C(CH2PPh2)3}NiCl]10 and [(tripod)-
recent reviews, see refs 33−35). Among them, coordinatively
unsaturated compounds of iron have been found to display the
largest energy barriers, like Ueff = 186 cm−1 in [Fe{N(SiMe3)-
11
t
NiSR] (R = Ph, Bu) and [(tripod)NiSePh].
(Dipp)}2] (Dipp = C6H3-2,6-iPr)24 and Ueff = 226 cm−1 in
Recently, there has been also some interest in N-heterocyclic
carbene (NHC) complexes of Ni(I) that resulted in the
synthesis of Sigman’s d9−d9 dimer [(IPr)Ni(μ-Cl)]2 (IPr = 1,3-
di(2,6-di-isopropylphenyl)imidazolin-2-ylidene)12 and the two
monomeric d9 amide complexes [(IPr)Ni{N(SiMe3)2}] and
23
[Fe(C(SiMe3)3)2]−
that originated from high orbital
Received: October 31, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b02497
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Inorganic Chemistry
Article
momenta in the ground state, which give rise to large first-order
spin−orbit coupling. But also low-lying excited states can give
rise to distinct magnetic anisotropy in these compounds
through second-order spin−orbit coupling.29,36 Apart from the
two-coordinate nickel(I) NHC complex [Ni(6-Mes)2]Br
mentioned above,14 a few mononuclear SIMs based on
transition metals other than iron and cobalt have been reported
The bistrimethylsilylamido complex [Ni(N(SiMe3)2)-
(PPh3)2] (3) can be isolated from reactions of [NiCl2(PPh3)2]
with 2.1 equiv of LiN(SiMe3)2 if reaction solutions are
refluxed.7 In addition, we found that complex 3 can also be
synthesized by reactions of 1 with 1.1 equiv of LiN(SiMe3)2.
The crystal structures of tetrahedral 1 and trigonal planar 2
(Scheme 2, Table 1) have been discussed in detail before.4−6
so far, including complexes of nickel(II) {[Ni-
1
Scheme 2
(MDABCO)2Cl3]ClO4 (MDABCO +
/ 1-methyl-4-aza-1-
4
azoniabicyclo[2.2.2]octanium cation)37 and [Ni(pydc)-
(pydm)]·H2O (pydc = pyridine-2,6-dicarboxylate, pydm =
2,6-bis(hydroxymethyl)pyridine)38}, manganese(III) {Na5[Mn-
(L-tart)2]·12H2O (L-tart = L-tartrate),39 Ph4P[Mn(opbaCl2)-
(py)2] (H4opbaCl2 = N,N′-3,4-dichloro-o-phenylenebis(oxamic
acid),40 [Mn(5-TMAM(R)-salmen)(H2O)Co(CN)6]·7H2O·
MeCN (5-TMAM-(R)-salmen = (R)-N,N′-(1-methylethylene)-
bis(5-trimethylammoniomethylsalicylideneiminate)),41 and
[Mn((OPPh2)2N)3]42}, rhenium(IV) {(NBu)4[ReX4(ox)] (X
= Cl, Br)43 and [ReF6]2− 44}, and chromium(II) {[Cr(N-
(SiMe3)2)2(L)2] (L = py, thf)}.45
The nickel(I) NHC complex is particularly interesting,
because it possess an isolated S = 1/2 ground state. Thus, the
SIM behavior cannot be related to an Orbach process in this
case. A similar conclusion was recently drawn for the Yb3+
compound [Yb(trensal)3] (H3trensal = 2,2′,2″-tris-
(salicylideneimino)triethylamine).46
Table 1. Crystallographic Data for [Ni(N(SiMe3)2)(PPh3)2]
(3)
molecular formula
fw (g/mol)
crystal system
space group
unit cell
a (Å)
C42H48NNiP2Si2
743.64
triclinic
P1
̅
9.2652(4)
12.1614(6)
18.5275(8)
98.082(4)
91.505(3)
106.332(4)
1978.74(16)
2
b (Å)
c (Å)
In this paper, we present a detailed analysis of the magnetic
behavior and the electronic structure of three different Ni(I)
complexes.
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
SYNTHESES AND STRUCTURE
■
T (K)
dc (g cm−3
150(2)
1.248
The Ni(I) bis- or trisphosphine complexes [NiCl(PPh3)n] (n =
2, 3) have been isolated as products of nickel(0) or nickel(II)
reactions several times.2,8,47−54 For the synthesis of [NiCl-
(PPh3)3] (1), we used the reduction of NiCl2(PPh3)2 with
LiBH4 in the presence of excess PPh3 in ethanol,52 which had
been used before also for the synthesis of Ni(I) η3-tripod
complexes (Scheme 1).10 In accordance with findings that 1 is
extensively dissociated in solution, recrystallization of 1 in
tetrahydrofuran (thf) afforded pale yellow crystals of [NiCl-
(PPh3)2]·C4H8O (2) in a reasonable yield (45%).
)
μ(λ) (mm−1
F[000]
)
0.661
786
2θmax (deg)
meas reflns
53
22011
unique reflns
Rint
8377
0.0272
6968
reflns with I > 2σ(I)
refined params
439
a
R1 (I > 2σ(I))
0.0257
b
wR2 (all data)
0.0678
a
b
2
2
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = {∑[w(Fo − Fc )2]/
Scheme 1
2
∑[w(Fo )2]}1/2
.
The crystal structure of 3 (Figure 1) was determined again, as
no coordinates were supplied with the original paper (for
crystallographic data and selected bond lengths and angles, see
Tables 1 and 2).7 A lower measurement temperature (180 K)
results in slightly smaller lattice constants accompanied by
slightly different bond lengths and bond angles. However, the
characteristics of the trigonal planar structure remain the same.
The two P−Ni−N angles are different (122.98°, 130.20°) and
surprisingly larger than the P−Ni−P angle (106.67°) in view of
almost similar Ni−P bond lengths. A similar situation is found
in 2, where the P−Ni−P angle of 111.52° is significantly
smaller than the corresponding Cl−Ni−P angles of 126.98°
and 121.33°.4 Although isoelectronic Cu(II) is known to
display strong Jahn−Teller effects, we expect those to be at
least weak in 2 and 3 because the heteroleptic ligand sphere lifts
2
2
the degeneracy of the E representations (dxz/dyz and dx −y /dxy).
B
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Inorganic Chemistry
Article
structure optimization was started from the structure
determined by single-crystal XRD. For the analysis of the
structural changes (for full coordinates see the SI), we focus
primarily on the distances between the nickel and the
coordinating phosphorus, nitrogen, and chlorine atoms; the
angle θ, ∠(P−Ni−P); the deviation of the trigonal coordination
of the nickel atom from a plane defined by the dihedral angle γ,
∠(P−Ni−L−P); and on the distances between the 1,1′ and 4,4′
carbon atoms of two adjacent phenyl rings (Figure 2),
respectively. The latter parameters serve as indicators for π−π
interactions.
Figure 1. Molecular structure of [Ni(N(SiMe3)2)(PPh3)2] (3) (H
atoms omitted for clarity). For crystallographic data and selected bond
length and angles, see Tables 1 and 2
Table 2. Selected Bond Lengths (pm) and Angles (deg) of
[NiCl(PPh3)3] (1), [NiCl(PPh3)2]·C4H8O (2),4 and
[Ni(N(SiMe3)2)(PPh3)2] (3)
a
1
2
3
Ni−Cl
229.33(17), 228.58(16), 214.81(6)
229.82(16)
Ni−N
Ni−P
187.42(12)
230.82(9), 228.87(9),
229.66(9)
220.12(6),
220.19(6)
220.65(4),
220.85(1)
P−Ni−Cl
P−Ni−N
P−Ni−P
110.71(3), 111.79(3),
110.88(3)
126.98(2),
121.33(2)
Figure 2. Schematic definition of the structural parameters of
[NiCl(PPh3)2]·C4H8O (2) and [Ni(N(SiMe3)2)(PPh3)2] (3) (d1 =
Ni−L (L = Cl, N(SiMe3)2), d2 and d3 = Ni−P, θ = ∠(P−Ni−P). The
parameters d4 and d5 denote the distances between the 1,1′ and 4,4′
carbon atoms of two adjacent phenyl rings of different phosphine
ligands.
122.98(4),
130.20(4)
108.21(3), 107.06(3),
108.03(3)
111.52(2)
106.67(2)
a
Three independent molecules per asymmetric unit.
In addition, we note that distorted coordination spheres are
also observed in Cu(I) complexes with an identical ligand
sphere, namely, [CuCl(PPh3)2·thf55 and [CuCl(PPh3)2]·
C6H656 with P(1)−Cu−P(2) ≠ P(1)−Cu−Cl ≠ Cl−Cu−P(2).
The measured powder patterns of 1−3 show a good
agreement with the calculated ones based on the single-crystal
data [Figures S1−S3, Supporting Information (SI)], which
proves the crystalline purity of the compounds.
In order to investigate the origin for the distorted
coordination sphere in 2 and 3, geometry optimizations were
performed by employing density functional theory (DFT) with
the hybrid functional B3-LYP.57 (For details of the calculations,
see the Experimental Section.) For each complex, three
different optimization procedures were applied: (1) only the
positions of the hydrogen atoms were optimized; the resulting
structures are used in the calculations on magnetic and
spectroscopic data; (2) all atoms were relaxed; and (3) all
atoms were relaxed, but in contrast to procedure 2, an empirical
dispersion correction term (D3-BJ)58,59 was added to the
functional. The differences in the structures between
optimizations 2 and 3 allow one to analyze the influence of
van der Waals interactions on the structural parameters,
whereas the differences between optimizations 1 and 3 display
crystal-packing effects. In those cases where all atoms were
relaxed, we verified that we obtained local minima by
calculating harmonic force constants (for vibrational frequen-
cies obtained for optimization procedure 3, see the SI). The
The results are summarized in Table 3. In all calculations, a
distorted first coordination sphere has been found, represented
by the angle θ. In general, the calculations including dispersion
Table 3. Comparison of Experimental and Calculated
Structural Parameters of [NiCl(PPh3)2]·C4H8O (2) and
a
[Ni(N(SiMe3)2)(PPh3)2] (3)
b
c
expl
calcd
calcd
2
d1 (pm)
d2 (pm)
d3 (pm)
d4 (pm)
d5 (pm)
θ (deg)
γ (deg)
d1 (pm)
d2 (pm)
d3 (pm)
d4 (pm)
d5 (pm)
θ (deg)
γ (deg)
214.81
220.12
220.91
379
219.76
228.44
230.55
457
217.67
222.26
224.27
348
445
537
406
111.52
175.34
187.42
220.65
220.85
399
114.65
178.01
192.90
231.78
232.19
425
104.88
174.91
189.41
222.80
223.04
407
3
448
573
509
106.67
175.49
108.63
175.92
107.21
171.40
a
θ depicts the P−Ni−P angle and γ the dihedral angle on the nickel
atom (P−Ni−L−P). For definition of the parameters d1−d5, see
Figure 2. By DFT. By DFT with additional dispersion correction.
b
c
C
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Inorganic Chemistry
Article
corrections agree better with the X-ray structures than those
without dispersion corrections. In particular, the distances of
the first coordination sphere of the nickel atoms and between
the phenyl rings are much better represented. From the
differences of the structures with and without dispersion
correction it can be concluded that the dispersion interaction
between the π-systems of the phenyl rings is the major source
for the distorted coordination sphere.
The influence of dynamic correlation can be seen by
comparison of the CASSCF and the MCCEPA data and the
importance of spin−orbit interaction from the changes in
excitation energies between CASSCF and SOCI. Both effects
can change the peak positions dramatically.
Electronic spectra of 1−3 have been measured in a region
from 250 to 2380 nm (40 000 to 4200 cm−1) as grindings of
polycrystalline powders in mineral oil between two quartz
plates (Figure S4, SI). All spectra can be roughly divided into a
region above and below 620 nm (16 129 cm−1), the former
ones being assigned to charge transfer bands and the latter ones
belonging to d−d transitions. For a free ion Ni+, the ground
state is 2D5/2, which in a trigonal planar crystal field of D3h point
ELECTRONIC STRUCTURE AND EXCITED STATES
■
UV−vis−NIR data (λ > 800 nm) have so far only been
reported for a limited number of Ni1+ complexes like the
distorted tetrahedral complexes [NiCl(tripod)] (tripod = 1,1,1-
tris(diphenylphosphinomethyl)ethane)9 and [Ni(EPh)-
(tripod)] (E = S, Se).11 There exists no such investigations
on trigonal planar Ni1+ compounds. Therefore, we performed a
more detailed study of the electronic structure and the excited
states of 1−3, including calculations and experiment.
2
2
2
group symmetry splits into A1′ + E′ + E″, whereas in a
65
2
2
tetrahedral field of Td symmetry it would split into E + T2.
In the ideal symmetric complexes, one would therefore expect
two d−d transitions for the trigonal planar and one for the
tetrahedral complex, respectively. Complete removal of
symmetry would lift the degeneracy of the T and E
representations, leading to four transitions in 1−3 (for
calculated orbital diagrams of 1−3, see Figure S5, SI). The
observed transitions, together with the calculated energies, are
given in Table 4. Below 620 nm (16 129 cm−1), we observed
for tetrahedral 1 one broad band around 1540 nm (6494 cm−1),
whereas for the trigonal planar complexes we observed two
bands at 1380 nm (7246 cm−1)/1755 nm (5698 cm−1) in 2 and
at 1040 nm (9615 cm−1)/1330 nm (7519 cm−1) in 3.
For 1, the lowest energy transition obtained from the SOCI*
calculations is close to zero due to the almost degenerate
ground state, but also the second excited states in 1 are much
lower in energy than in 2 and 3. The other two transitions are
close in energy, probably due to the fact that the symmetry in 1
is still close to C3v. Therefore, we assume that the broad band
around 6494 cm−1 covers the two transitions at higher energies,
whereas the two lowest energy peaks are located well below
4200 cm−1 (above 2380 nm) and cannot be detected.
The calculated energy values of the four excited states of 2
and 3 can be divided into two pairs of states, each leading to
one of the two broad bands in the spectra of 2 and 3. The
energetic position of the bands is in good agreement with the
mean values of the calculated SOCI* transition energies. In
addition, higher transition energies in 2 in comparison to 3 are
in agreement with the fact that the −N(SiMe3)2 ligand is a
stronger σ-donor ligand than chloride.
Magnetic Behavior. For all complexes 1−3 dc and ac
magnetic properties have been studied on crystalline powders.
In addition, the dc magnetic properties have been investigated
using the SOCI* method.
To take into account the open shell character of the ground
state and the low-lying d−d excitations, the electronic
structures of 1−3 were investigated using multireference wave
function based methods employing the Bochum package of ab
initio programs.60−63 For all three complexes, CAS(9,5)SCF
calculations were performed. (For details of the calculations see
the Experimental Section.) The calculations reveal well-isolated
1
1
S = /2 ground states for 2 and 3, whereas for 1, two S = /2
states are almost degenerate (Table 4).
Table 4. Calculated and Observed Electronic Excitation
Energies (cm−1) of [NiCl(PPh3)3] (1), [NiCl(PPh3)2]·
a
C4H8O (2), and [Ni(N(SiMe3)2)(PPh3)2] (3)
theory
CASSCF
MCCEPA
SOCI
SOCI*
experiment
1
2
3
8.41
1428.98
3927.81
3929.08
3119.73
3449.85
4901.09
5257.30
4693.42
5053.84
6382.23
6719.45
93.70
1727.58
5003.20
5007.28
4581.94
5242.35
6363.35
6625.20
7260.97
8134.91
8708.58
9326.21
40.19
1719.30
3944.87
4547.38
2776.56
3826.80
4869.50
5767.86
4343.62
5406.59
6253.44
7242.41
94.56
1985.93
4943.96
5562.07
4308.45
5441.63
6312.42
7140.81
7016.62
8196.96
8739.14
9758.69
6494
5698
7246
7519
9615
a
For details of the theoretical methods, see the text.
The static magnetic behavior of complexes 1−3 was studied
between 2 and 300 K in a field of 0.1 T and by magnetization
measurements from 0 to 7 T at 2, 3, 4, 6, 10, and 25 K. The χT
vs T and M vs H curves at different temperatures were
simultaneously fitted using the PHI program66 by means of an
isotropic Zeeman Hamiltonian (eq 1).
The corresponding spin densities are depicted in Figures
S6−S8 of the SI. For 1, the ground-state spin density arises
from a linear combination of dxz and dyz orbitals (assuming the
Ni−Cl bond is collinear to the z-axis) and the spin density of
2
the first excited state from a dz orbital. For 2 and 3, the orbital
that lies in the trigonal plane is singly occupied. We accounted
for dynamic correlation by multi-configurational coupled
electron-pair approximation (MCCEPA) calculations.63 Spin−
orbit coupling was included either by spin−orbit configuration
interaction (SOCI) based on the CASSCF data or by the
SOCI* method, where the energies of the CASSCF states were
shifted by the MCCEPA results in the SOCI calculations.64 The
excitation energies of the d−d excited states are summarized in
Table 4. The highest theoretical level is the SOCI* calculations.
⃗
⃗
H = gμBSB
(1)
⃗
⃗
where S denotes the spin operator, B the magnetic induction, g
the Lande factor, and μB Bohr’s magneton.
The graphs of χT vs T of 1−3 decrease almost linearly with
decreasing temperature from room temperature down to 10 K
by approximately 0.07 cm3 mol−1 (Figure 3) before a stronger
downturn indicates intermolecular interactions at low temper-
́
D
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Table 5. Results of the Fittings of the Direct Current
Magnetic Data (simultaneous treatment of χT vs T and M vs
H plots at different temperatures) (Figures 3 and S9, SI) by
the PHI Program66 of [NiCl(PPh3)3] (1), [NiCl(PPh3)2]·
a
C4H8O (2), and [Ni(N(SiMe3)2)(PPh3)2] (3)
giso
TIP (cm3 mol−1
)
ΘW (K)
R
1
2
3
2.283
2.238
2.139
3.79 × 10−4
3.62 × 10−4
2.37 × 10−4
0.999 99
0.999 99
0.999 99
−0.014
−0.026
a
Parameters: g, temperature-independent paramagnetism (TIP),
intermolecular interaction parameter ΘW, goodness-of-fit factor R
(least square approach).
found to be larger than the spin-only value of 2.002 32. An
anisotropic refinement of the g-factors did in all cases yield
almost similar values of gx, gy and gz, although solid-state EPR
measurements on 2 in a host lattice of [CuCl(PPh3)2]·0.5C6H6
indicated g-factor anisotropy (gx = 2.111, gy = 2.167, gz =
2.446). However, the average g factor determined in this way
(gav = 2.241) agrees well with the one found in this work (g =
2.232).67 For 3, a magnetic moment of 1.91 μB is reported,7
which is equal to a χT value of 0.456 cm3 mol−1 and a g factor
of 2.206, in reasonable agreement with the values determined in
this work (Table 5).
We calculated the G-tensors using the SOCI* wave functions
following the procedure described by Bolvin.68 The g-factors
are found to be slightly anisotropic with a magnetic mean axis
orthogonal to the trigonal plane for 2 and 3, respectively, and
along a Ni−P bond for 1 (Table 6). As the isotropic Zeeman
Table 6. Calculated g-Factors from G-Tensors Constructed
from SOCI* Wave Functions
a
1
2
3
g1
2.80
0.55
0.52
1.29
3.00
2.28
2.04
2.44
2.58
2.18
2.14
2.30
2.43
2.12
2.10
2.22
g2
g3
giso
a
For first and second Kramers doublet.
Figure 3. Temperature dependence of χT of (a) [NiCl(PPh3)3] (1),
(b) [NiCl(PPh3)2]·C4H8O (2), and (c) [Ni(N(SiMe3)2)(PPh3)2] (3)
(solid gray lines, calculated from SOCI* energies; filled squares,
measured at Hdc = 1000 Oe). Solid green lines represent the results of
the simultaneous fittings with the temperature-dependent magnet-
ization (Table 5, Figure S9, SI) according to a spin Hamiltonian (eqs 1
and S1, SI) by the PHI program.33
Hamiltonian (eq 1) is not suited for the description of the
quasi-degenerate situation in 1, we simulated the χT and M vs
H data for all three complexes directly from Boltzmann-
weighted numerical derivatives of the SOCI* energies with and
without field using finite perturbation theory (see Experimental
Section). All curves are in good agreement with the
experimental data, as can be seen in Figure 3 and Figure S10
in the SI. However, there is one difference. While in experiment
TIP was observed for all three compounds, the calculations
only indicate TIP for complex 1. Here, the first excited state is
less than 100 cm−1 above the ground state. In the calculations,
contributions to the TIP are only included within the active
space of the SOCI. All other excited states are higher in energy.
The origin of the TIP in complexes 2 and 3 is so far not clear.
Dynamical magnetic properties of 1−3 were first probed in
field-dependent ac measurements performed at 2 K using a 3.0
Oe oscillating ac field at frequencies between 1 and 1500 Hz.
(Figures S11 and S14, SI). In the absence of an external dc
field, the out-of-phase component of the ac susceptibility (χ″)
of 1−3 has a much lower intensity than the in-phase
component (χ′), which indicates that the spin−lattice
relaxation is faster than the time scale of the experiment.
atures. The former deviation from the ideal Curie behavior is
indicative of a temperature-independent paramagnetism (TIP),
whereas the origin of the weak discontinuity in the
susceptibility of 1 below 7 K is presently unclear. Least-square
fits of the dc magnetic data to eq 1, which include corrections
for the TIP and intermolecular interactions (eq S1, SI),
reproduce the experimental data quite well. The best sets of
parameters are listed in Table 5 and shown as green solid lines
in Figure 3 and Figure S9 in the SI. The values of χT at room
temperature corrected for TIP (1, 0.489 cm3 mol−1; 2, 0.468
cm3 mol−1; and 3, 0.430 cm3 mol−1) are all larger than the spin-
1
only value of χT = 0.375 cm3 mol−1 for one Ni+ ion (S = /2),
which indicates the influence of spin−orbit coupling in these
complexes. Hence, in the fitting procedure, all g-values are
E
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With the application of a static dc field, the intensity of χ″ is
significantly enhanced for the two trigonal planar complexes 2
and 3, whereas for tetrahedral 1 it is not. The temperature
dependence of the relaxation time provides an experimental
probe of the processes responsible for spin reversal in a
magnetic system. The magnetic relaxation times for 2 and 3
were therefore evaluated by measuring the frequency depend-
ences (Figures 4 and 5). From the frequency dependencies of
Figure 5. Frequency dependence of the in-phase χ′ (a) and out-of-
phase χ″ (b) component of the ac magnetic susceptibility at 2000 Oe
at different temperatures for [Ni(N(SiMe3)2)(PPh3)2] (3). Solid lines
represent the simultaneous fits according to eqs S2 and S3 in the SI.
intermediate state. The latter one seems to be improbable for 2
1
and 3, because they have well-isolated S = /2 states as ground
state. The excited states calculated for 2 and 3 by SOCI* are
too high in energy to serve as intermediate state. In the case of
́
Kramers ions with an isolated doublet, the temperature and
field dependence are described by eq 2.69
1/τ = AH4T + BT9
(2)
This expression has been successfully applied to describe the
temperature behavior of electron spin−lattice relaxation rates in
another d9 system, namely, the copper Tutton salt (NH4)2Cu-
(SO4)2·6H2O.71
Figure 4. Frequency dependence of the in-phase χ′ (a) and out-of-
phase χ″ (b) component of the ac magnetic susceptibility at 2000 Oe
at different temperatures (2−2.8 K, 0.2 K steps; 2.8−10 K, 0.4 K steps)
for [NiCl(PPh3)2]·C4H8O (2). Solid lines represent the simultaneous
fits according to eqs S2 and S3 in the SI.
However, fits of the graphs of τ−1 against T for 2 and 3
according to eq 2 result in poor agreement, especially at low
temperatures (not shown). Separate fits of the low-temperature
data to a direct process suggest that these cannot be described
by a simple τ−1 ∝ T dependence and require the use of an
exponent n > 1. Accordingly, reasonable fits for the whole
temperature range were obtained by using eq 3 with n = 7 for 2
and n = 4 in the case of 3 (Table 7).
χ′ and χ″ over a range of temperatures in an applied dc field,
the relaxation time τ was extracted by fits to eqs S2 and S3 in
the SI (Tables S1−S4, SI). The inverse relaxation times of 2
and 3 vs the temperature are plotted on a double-logarithmic
scale in Figure 6. Both show two different regions of
temperature dependence with a larger effect at higher
temperatures. In order to describe the transfer of energy
between the magnetic spin system and the lattice vibrations,
one needs to consider three possible processes:69,70 a direct
process under emission or absorption of a phonon, a
nonresonant Raman process, and an Orbach process, which
describes a direct, resonant two-phonon process via a real
1/τ = AT2 + BTn
(3)
Exponents n = 2 for a direct process at temperatures below 4
K are indicative for a so-called phonon bottleneck effect.72
Smaller exponents than 9 occur when both acoustic and optical
phonons are considered in the evaluation of Raman processes.
It was found that depending on the structure of energy levels,
the relaxation rate at low temperatures depends on Tn with n =
1, 2, 3, 4, 5, or 6.73 Reduced exponents in the temperature
F
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Article
relaxation behavior as systematically investigated before on
samples of (NH4)2Cu(SO4)2·6H2O.78
For 2 and 3, a negative field dependence of the inverse
relaxation time was found at 2 K (Figures S11 and S14, SI).
Such a behavior was also observed in the low-field region
(<3000 Oe) for dilute samples of Cu2+ in (NH4)2Zn(SO4)2·
6H2O71 and in recent investigations on linear coordinated
molecular Fe2+ complexes24 and is commonly attributed to
1
quantum tunneling effects (QTM). However, for an S = /2
system, the existence of QTM is still under debate.79 Even at 7
K the field dependence of the inverse relaxation time of 2 and 3
was found to be negative (Figures S12 and S15, SI).
The Argand plots (χ″ vs χ′) (see Figures S13 and S16, SI)
show semicircular isotherms. For 2 and 3, values of the
distribution coefficient α were found to increase distinctly upon
cooling (2, 0.247 01 at 2 K to 0.035 66 at 9.6 K; 3, 0.227 71 at 2
K to 0.043 79 at 14 K). This also supports the occurrence of a
phonon-bottlenecked direct process that becomes dominant at
lower temperatures because the process of transferring heat
from the spin system to the outside becomes less well-defined.
Furthermore, the broadening of the χ″ peaks at high fields at T
= 2 K seen for both 2 and 3 (Figures S11 and S14, SI) is a
result of increasing α-values, which again is indicative of the
presence of phonon-bottleneck effects.80 The fact that this
broadening is not observed for the T = 7 K data supports this
point even further (Figures S12 and S15, Tables S2 and S5, SI).
The question remains why 2 and 3 display a slow relaxation
of the magnetization under applied dc fields and 1 does not. As
outlined above, the direct and Raman relaxation processes,
under an applied dc field, are highly dependent upon the
interactions between the spins and the vibrations of the crystal.
Such interactions are mediated by the coupling between the
spin moment of the nickel atoms and vibrations that modulate
the ligand field.81 In view of the significantly shorter Ni−P and
Ni−Cl bond lengths in 2 and 3 compared to 1 (Table 2), one
can expect differences in the phonon densities. We assume a
higher phonon density in 1 because of the weaker force
constants for the interaction of the nickel atom with the first
coordination shell and therefore faster relaxation.
Figure 6. Temperature dependence of the relaxation rate τ−1 of (a)
[NiCl(PPh3)2]·C4H8O (2) and (b) [Ni(N(SiMe3)2)(PPh3)2] (3) at
2000 Oe dc fields. Solid lines represent fits according to eq 3. For the
respective fit parameters, see Table 7.
Table 7. Results of the Fittings of the Inverse Relaxation
Time against the Temperature According to eq 3 (Figure 5)
for [NiCl(PPh3)2]·C4H8O (2) and [Ni(N(SiMe3)2)(PPh3)2]
a
(3)
n
A (s−1 K−2
)
B (s−1 K−n
)
R
Due to the long intermolecular distances between the
paramagnetic Ni(I) ions [shortest Ni−Ni distances 794 pm
(2), 928 pm (3)], we exclude an influence on the relaxation
rates by interactions between paramagnetic ions, such as
dipole−dipole and exchange interactions.
2
3
7
4
12.02
2.62
0.001 61
0.212 47
0.999 48
0.998 92
a
Parameters: external magnetic field Hdc = 2000 Oe, exponent n in eq
3, goodness-of-fit factor R (least-square approach).
CONCLUSION
■
range above 4 K have recently also been observed for molecular
complexes of Kramers ions like a CoII−YIII single molecular
magnet74 and a six-coordinate YbN3O3 complex.75
The structural, electronic, and magnetic properties of three
Ni(I) complexes[NiCl(PPh3)3], [NiCl(PPh3)2]·C4H8O, and
[Ni(N(SiMe3)2)(PPh3)2]have been investigated experimen-
tally and by quantum chemical calculations. DFT calculations
(structure optimization) indicate that the distortion of the
trigonal planar coordination in [NiCl(PPh3)2]·C4H8O and
[Ni(N(SiMe3)2)(PPh3)2] is not primarily induced by a Jahn−
Teller effect but by π−π interactions of the phenyl rings of the
phosphine ligands.
In the quantum chemical calculations, an energetically well
separated S = 1/2 ground state was obtained for [NiCl(PPh3)2]·
C4H8O and [Ni(N(SiMe3)2)(PPh3)2]. The calculated g values
show a small anisotropy and are, with average values of 2.30
and 2.22, a bit larger than the spin-only values. Similar results
were obtained from the fits of the dc magnetic data.
The physical significance of the coefficients A and B in Table
7 is not easy to discuss, as the values depend on variables such
as the crystal density, the speed of sound in the solid, the
strength of the interaction of the spin system with the phonons,
and the crystal field splitting.24,76,77 In addition, different
exponents have to be used in order to describe the temperature
dependence of the Raman processes in 2 and 3, making a
comparison of the obtained values difficult. According to our
results of the investigations of the electronic structure, the
crystal field splitting is expected to be only 2000 cm−1 larger in
3 than in 2. The crystal densities (2, 1.341 g cm−3; 3, 1.248 g
cm−3) differ only slightly. However, A and B differ distinctly for
2 and 3. In this respect, we note that measurements on different
samples indicate that also crystal size and quality influence the
Although the experimental dc susceptibilities of [NiCl-
(PPh3)3] could also be described by a an isotropic Zeeman
G
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Inorganic Chemistry
Article
C42H48NP2Si2Ni (743.65) calcd: C 67.8, H 6.5, N 1.9. Found: C
67.2, H 6.2, N 1.9.
Hamiltonian with a g value of 2.283, this fit is physically not
justified. For this complex, two almost degenerate S = 1/2 states
are obtained because of the almost tetrahedral coordination
sphere. On the basis of the calculations, the g values of both
states are significantly anisotropic with an average value of 1.29
and 2.44, respectively. Temperature-dependent susceptibility
data could be successfully modeled for all three complexes
directly from the quantum chemical data.
Ac magnetic investigations reveal that the two complexes
with trigonal planar coordination, [NiCl(PPh3)2]·C4H8O and
[Ni(N(SiMe3)2)(PPh3)2], display slow magnetic relaxation at
low temperatures under applied dc fields, whereas tetrahedral
[NiCl(PPh3)3] does not. The temperature-dependence of the
relaxation times for the two trigonal planar complexes could be
fitted successfully using a direct process, including a phonon-
bottleneck effect, and a Raman process, including optical as well
as acoustic phonons. We assume that the fast relaxation for the
tetrahedral complex is caused by higher phonon density of
states in the resonant region.
(b) LiN(SiMe3)2 (92 mg 0.55 mmol) dissolved in 10 mL of toluene
was dropwise added over a period of 15 min to a suspension of
[NiCl(PPh3)3] (440 mg, 0.5 mmol) and PPh3 (131 mg, 0.5 mmol) in
10 mL of toluene at −40 °C to give a light red solution. After warming
up to room temperature and additional stirring for 4 h, LiCl was
separated by filtering or centrifugation. Then the solution was reduced
to 3−5 mL, and 5 mL of pentane added. Formation of orange needles
starts overnight and proceeds further upon standing for one or two
nights at 2 °C. The supernatant solution was then decanted and the
crystals of 3 washed two times with 5 mL of cold n-pentane at −70 °C
before drying in vacuum to yield 245 mg, 63%.
C42H48NP2Si2Ni (743.65) calcd: C 67.8, H 6.5, N 1.9. Found: C
67.2, H 6.6, N 1.9.
Crystallography. Crystals suitable for single-crystal X-ray
diffraction were selected in perfluoroalkyl ether oil and mounted on
a diffractometer equipped with an Oxford Cryosystem. Single-crystal
X-ray diffraction data of 3 were collected using Mo radiation (λ =
0.710 73 Å) generated by a multilayer optic on a STOE STADI Vari
(Pilatus Hybrid Pixel Detector 300K). Raw intensity data were
collected and treated with the STOE X-Area software (version 1.64).
Interframe scaling was done with the implemented program LANA.
Data were corrected for Lorentz and polarization effects. On the basis
of the crystal description, a numerical absorption correction was
applied.82
The structures were solved with the direct methods program
SHELXS of the SHELXTL PC suite of programs83 and were refined
with the use of the full-matrix least-squares program SHELXL.
Molecular diagrams were prepared using Diamond.84
In 3, all Ni, N, P, Si, and C atoms were refined with anisotropic
displacement parameters, while H atoms were either localized in the
difference Fourier map and refined with isotropic displacement
parameters or computed and refined, using a riding model, with an
isotropic temperature factor equal to 1.2, the equivalent temperature
factor of the atom to which they are linked.
Supplementary crystallographic data for compound 3 of this paper
have been deposited at the CCDC. These data can be obtained free of
charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax (int.), +44-1223/336-033; e-mail,deposit@ccdc.
cam.ac.uk].
X-ray powder diffraction patterns (XRD) for 1, 2, and 3 were
measured on a STOE STADI P diffractometer (Cu Kα1 radiation,
Germanium monochromator, Debye−Scherrer geometry, Mythen 1K
detector) in sealed glass capillaries. The experimental patterns were
fitted with the FullProf program85 using the atom coordinates from
single crystal data. Main refinement parameters were cell constants,
scale factor, background, overall temperature factor, and coordinates of
the Ni, P, Cl, and N atoms.
EXPERIMENTAL SECTION
■
Synthesis. Standard Schlenk techniques were employed through-
out the syntheses using a double manifold vacuum line with high-
purity dry nitrogen (99.9994%) and a MBraun glovebox with high-
purity dry argon (99.9990%). The solvents thf (tetrahydofuran) and
toluene were dried over sodium−benzophenone and heptane and
pentane were dried over LiAlH4, and all were distilled under nitrogen.
LiN(SiMe3)2, PPh3 (Ph = phenyl, C6H5), and NiCl2·6H2O were
purchased from Aldrich. LiN(SiMe3)2 was distilled prior to use.
[NiCl(PPh3)3] (1). NiCl2·6H2O (1.203 g, 5.1 mmol) and PPh3 (5.351
g, 20.4 mmol) were dissolved in 50 mL of EtOH and stirred at room
temperature. A suspension of LiBH4 (0.133 g, 6.12 mmol) in 10 mL of
EtOH was dropped into the solution over a period of 30 min. After an
additional 2 h of stirring, the reaction solution was filtered and the red
precipitate washed three times with 5 mL of EtOH before it was dried
under reduced pressure to give a total yield of 4.056 g (91%). One was
further purified by recrystallization. One gram (1.14 mmol) of the raw
product was dissolved together with 2 equiv of PPh3 (596 mg, 2.27
mmol) in 20 mL of thf. Upon addition of 100 mL of dry ethanol,
orange yellow crystals of 1 formed. Crystallization was completed by
leaving the solution for one night at 2 °C to give a final yield of 770
mg, 77%.
C54H45NiClP3 (881.0) calcd: C 73.6, H 5.2. Found: C 73.3, H 5.1.
[NiCl(PPh3)2]·C4H8O (2). Two times recrystallized [NiCl(PPh3)3]
(400 mg, 0.45 mmol) was quickly dissolved in 6 mL of dry thf upon
stirring. Soon after dissolution stirring was stopped and the stirring bar
removed. Crystals of 2 appear within a few minutes, and crystallization
was completed by leaving the solution for one night at 2 °C. The
yellow crystalline precipitate was washed three times with 5 mL of cold
thf at 0 °C before it was briefly dried in vacuum to give a final yield of
151 mg, 48.1%.
Physical Measurements. C, H, and S elemental analyses were
performed on an Elementar vario Micro cube instrument.
̀ ́
Solid state UV−vis−NIR absorption spectra were measured on a
PerkinElmer Lambda 900 in transmission as micron-sized crystalline
powders between quartz plates in front of a Labsphere integrating
sphere.
C36H30NiClP2·C4H8O (690.82) calcd: C 69.5, H 5.5. Found: C
69.4, H 5.5.
Zero-field-cooled temperature-dependent susceptibilities were re-
corded for 1−3 in dc mode using a MPMS-III (Quantum Design)
SQUID magnetometer over a temperature range from 2 to 300 K in a
homogeneous 0.1 T external magnetic field. The magnetization curves
were measured on the same instrument up to a dc field of 7 T. The ac
susceptibility measurements were performed using a MPMS-XL
(Quantum Design) SQUID magnetometer with an oscillating ac
field of 3 Oe and ac frequencies ranging from 1 to 1500 Hz. The
samples were contained in gelatin capsules filled in a glovebox under
argon atmosphere owing to the high degree of moisture and oxygen
sensitivity of the compounds. The samples were transferred in sealed
Schlenk tubes from the glovebox to the magnetometer and then
rapidly transferred to the helium-purged sample space of the
magnetometer. The data were corrected for the sample holder,
[Ni(N(SiMe3)2)(PPh3)2] (3). (a) [NiCl2(PPh3)2] (3.257 g, 4.8 mmol)
and LiN(SiMe3)2 (1.690 g, 10.1 mmol) were dissolved in 25 mL of
toluene and the solution was stirred and heated to 80 °C for 2 h before
being cooled and filtered. Then the solvent was reduced to
approximately 5 mL under reduced pressure. Upon standing at
room temperature, orange crystals of 3 appeared over 2 days. The
crystals were washed three times at −70 °C with 10 mL of cold
heptane before drying to yield 1.6 g, 45%.
3 can be purified by recrystallization. A 500 mg portion of the raw
product was dissolved together with 1 equiv of PPh3 in 5 mL of
toluene upon gentle warming. Upon standing at room temperature,
orange yellow crystals of 1 form. Crystallization is completed by
addition of small aliquots of pentane and leaving the solution for some
days at 2 °C in the refrigerator.
H
DOI: 10.1021/acs.inorgchem.5b02497
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Inorganic Chemistry
Article
including the gelatin capsule, and for diamagnetism using Pascal’s
constants.86−88 Details about the simulations are given in the SI.
Details of the Quantum Chemical Calculations. DFT
Calculations. The Turbomole89 package was used for the structure
optimizations. The B3-LYP57 functional on an integration grid of size
m3 was used throughout the calculations. Ni, P, Si, N, and Cl atoms
were equipped with a def2-TZVP basis,90 whereas for C and H atoms,
a def2-SVP basis90,91 was used.
thank A. K. Powell for generous support, C. Reitz for help with
the operation of the SQUID magnetometers, M. Knapp for
advice concerning the use of the FullProf program, and S. Stahl
for the performance of the elemental analysis. W.L. acknowl-
edges support of the China Council Scholarship. T.B.
acknowledges funding by the DFG collaborative research
center SFB/TRR 88 “3MET” Project A1.
Multi-Reference Calculations. The wave function based calcu-
lations were performed on the X-ray structures with optimized
positions of the H atoms (procedure 1 of the structure optimizations
given above). The active space in the CASSCF calculations was
spanned by the nine electrons in the five 3d-orbitals of Ni(I). A def2-
TZVPP(-g) basis90 was used for nickel, as well as the atoms of the first
coordination shell, whereas the other atoms were equipped with a
def2-SVP basis.90,91 In the CASSCF calculations, the orbitals were
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b02497.
■
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AUTHOR INFORMATION
Corresponding Authors
*K.F. e-mail: karin.fink@kit.edu.
*A.E. e-mail: andreas.eichhoefer@kit.edu.
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Am. Chem. Soc. 2007, 129, 6547−6561.
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■
Author Contributions
W.L. was responsible for synthesis and characterization, V.M.
for magnetic measurements, T.B. and K.F. for CASSCF and
DFT calculations, and A.E. for synthesis, SCXRD, and magnetic
measurements.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
This work was supported by the Karlsruhe Institut fur
Technologie (KIT, Campus Nord). The authors wish to
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DOI: 10.1021/acs.inorgchem.5b02497
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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DOI: 10.1021/acs.inorgchem.5b02497
Inorg. Chem. XXXX, XXX, XXX−XXX