Neutral bis(1,4-diaza-1,3-butadiene)nickel complexes and their corresponding monocations: Molecular and electronic structures. A combined experimental and density functional theoretical study

Article abstract of DOI:10.1039/b709390j

The reaction of 2 equivalents of 2-methyl-1,4-bis(2,6-dimethylphenyl)-1,4- diaza-1,3-butadiene, (¹L^Ox)⁰, or 2-methyl-1,4-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene, ( ²L^Ox)⁰, in diethyl ether or n-hexane with 1 equivalent of Ni(cdt) where (cdt)⁰ is the ligand cyclododecatriene affords dark red, diamagnetic precipitates of [Ni^II( ¹L)₂] (1) and of [Ni^II(²L) ₂] (3). The ligands (¹L)^1- and ( ²L)^1- are the one-electron reduced, monoanionic π radicals of the above neutral 1,4-diaza-1,3-butadienes. 1 and 3 have been structurally characterized by X-ray crystallography; both possess a distorted tetrahedral geometry where the dihedral angle between the two metallacycles Ni-N-C-C-N is 47.9° and 53°, respectively, ( = 0° for square planar and 90° for a regular tetrahedral geometry). The reaction of 1 and 3 with 1 equivalent of ferrocenium hexafluorophosphate gives the paramagnetic (S _t = 1/2) complexes 2 and 4, respectively: [Ni^I( ¹L^Ox)₂](PF₆) (2), [Ni ^I(²L^Ox)₂](PF₆) (4). Their EPR spectra indicate the presence of a central Ni(i) ion (d⁹; S _Ni = 1/2). Thus, the one-electron oxidation of 1 and 3 by [Fc]PF ₆ induces an intramolecular one-electron reduction of the central Ni ion and a concomitant one-electron oxidation of the second π radical (L) ^1- → (L^Ox)⁰ + e. Broken symmetry DFT calculations (B3LYP) corroborate the correctness of the electronic structure descriptions of 1-4. The reaction of (¹L^Ox)⁰ with NiI₂ (1: 1) in tetrahydrofuran yields tetrahedral [Ni ^II(¹L^Ox)I₂] (5) with an S _t = 1 ground state. The Royal Society of Chemistry.

Full text of DOI:10.1039/b709390j

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Neutral bis(1,4-diaza-1,3-butadiene)nickel complexes and their corresponding
monocations: molecular and electronic structures. A combined experimental
and density functional theoretical study†‡
Nicoleta Muresan, Thomas Weyhermu¨ller and Karl Wieghardt*
Received 20th June 2007, Accepted 23rd August 2007
First published as an Advance Article on the web 10th September 2007
DOI: 10.1039/b709390j
The reaction of 2 equivalents of 2-methyl-1,4-bis(2,6-dimethylphenyl)-1,4-diaza-1,3-butadiene, (1LOx)0,
or 2-methyl-1,4-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene, (2LOx)0, in diethyl ether or n-hexane
with 1 equivalent of Ni(cdt) where (cdt)0 is the ligand cyclododecatriene affords dark red, diamagnetic
precipitates of [NiII(1L•)2] (1) and of [NiII(2L•)2] (3). The ligands (1L•)1− and (2L•)1− are the one-electron
reduced, monoanionic p radicals of the above neutral 1,4-diaza-1,3-butadienes. 1 and 3 have been
structurally characterized by X-ray crystallography; both possess a distorted tetrahedral geometry
where the dihedral angle h between the two metallacycles Ni–N–C–C–N is 47.9◦ and 53◦, respectively,
(h = 0◦ for square planar and 90◦ for a regular tetrahedral geometry). The reaction of 1 and 3 with 1
equivalent of ferrocenium hexafluorophosphate gives the paramagnetic (St = 1/2) complexes 2 and 4,
respectively: [NiI(1LOx)2](PF6) (2), [NiI(2LOx)2](PF6) (4). Their EPR spectra indicate the presence of a
central Ni(I) ion (d9; SNi = 1/2). Thus, the one-electron oxidation of 1 and 3 by [Fc]PF6 induces an
intramolecular one-electron reduction of the central Ni ion and a concomitant one-electron oxidation of
the second p radical (L•)1− → (LOx)0 + e. Broken symmetry DFT calculations (B3LYP) corroborate the
correctness of the electronic structure descriptions of 1–4. The reaction of (1LOx)0 with NiI2 (1 : 1) in
tetrahydrofuran yields tetrahedral [NiII(1LOx)I2] (5) with an St = 1 ground state.
Introduction
It has now been clearly established by spectroscopy and X-
ray crystallography as well as density functional theoretical
calculations that neutral, distorted tetrahedral bis(1,4-diaza-1,3-
butadiene)nickel complexes1–6 possess an electronic structure
which is best described as nickel(II) species (d8; SNi = 1) containing
two monoanionic p radical ligands.5–7 The two singly occupied
metal d-orbitals (t2 in Td symmetry) are coupled intramolecularly
Scheme 1
antiferromagnetically to two ligand p radicals (Srad = 1/2)
yielding a diamagnetic ground state (St = 0):5–7 [NiII(L•)2]. It has
been shown by DFT calculations that these neutral 1,4-diaza-
1,3-butadiene ligands possess an empty energetically low-lying,
antibonding p* orbital8–10 (with respect to the C–N bond but
bonding with respect to the C–C bond) which readily accepts
[NiI(3LOx)2]+ (S = /2) with a central nickel(I) ion (d9) and two
1
neutral, closed-shell (3LOx)0 ligands.6
One might envisage reaction (1) as an oxidatively induced
reduction of the metal center or, conversely, a reductively induced
oxidation of the central metal ion.
one-electron generating a monoanionic p radical, (L•)1−, or two
ꢀ
ꢁ
ꢂ ꢃ
ꢀ
ꢁ
ꢂ ꢃ+
−e
NiII 3L
NiI 3LOx
(1)
᭹
−−−→
←−−−
+e
2−
electrons affording a closed-shell enediamide (LRed
Scheme 1.
)
as shown in
2
2
In order to test the generality of this reaction we have
synthesized two new neutral complexes, 1 and 3, and oxidized
them with one equivalent of ferrocenium hexafluorophosphate to
the corresponding monocations in 2 and 4 (Chart 1). We have also
synthesized and characterized the neutral complex [NiII(1LOx)I2]
(5) which possesses an St = 1 ground state.
Balch and Holm1 have demonstrated as early as 1966 that these
neutral nickel complexes, [Ni(L)2]z, are a member of one respective
electron transfer series where z = 0, 1−, 2−, and 1+, 2+. We have
recently discovered for the species [NiII(3L•)2] (S = 0) (Chart I) that
its one-electron oxidation leads to the tetrahedral monocation
Complexes 1 and 3 differ in the steric bulkiness of the N,Nꢀ-
substituents when N,Nꢀ-coordinated in the bis(ligand)nickel com-
plexes. Consequently, the dihedral angle h between the two five-
membered chelate rings Ni–N–C–C–N varies from ∼40◦ to 90◦ (h
is 0◦ in a square planar complex and 90◦ in a regular tetrahedral
species). Thus, the present study was initiated to explore the
influence of h on the electronic structure of these complexes.
Max-Planck-Institut fu¨r Bioanorganische Chemie, Stiftstrasse 34-36, D-
45470, Mu¨lheim an der Ruhr, Germany
† CCDC reference numbers 650249–650251. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b709390j
‡ Electronic supplementary information (ESI) available: Optimized ge-
ometries, spin density maps and qualitative MO schemes for the complexes.
See DOI: 10.1039/b709390j
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1,3-butadiene ligands (1LOx)0 and (2LOx)0 shown in Chart I in
diethyl ether or an n-hexane–diethyl ether mixture (1 : 1 v/v) at
ambient temperature affords dark red precipitates of the neutral
complexes [NiII(1L•)2] (1) and [NiII(2L•)2]·hexane (3) in ∼80% yield,
respectively. Both species are quite air-sensitive.
◦
Oxidation of 1 and 3 in CH2Cl2 at 20 C with 1 equivalent of
ferrocenium hexafluorophosphate, [Fc](PF6), affords dark violet
microcrystals of [NiI(1LOx)2](PF6) (2) and [NiI(2LOx)2](PF6) (4) in
∼90% yield.
Finally, the reaction of NiI2 in tetrahydrofuran with 1 equivalent
of the ligand (1LOx)0 gives red [NiII(1LOx)I2] (5) in ∼90% yield.
Complexes 1 and 3 are diamagnetic as was judged from their
“normal” 1H NMR spectra (see below and Experimental section);
they possess an St = 0 ground state. In contrast, complexes 2
and 4 are paramagnetic. From temperature-dependent magnetic
susceptibility measurements (4–300 K) temperature-independent
magnetic moments of 1.9 lB and 1.88 lB, respectively, have been
measured. Thus, both 2 and 4 possess an S = 1/2 ground state.
As expected, a magnetic moment of 2.9 lB for 5 indicates an S =
1 ground state.
The cyclic voltammogram of 1 in tetrahydrofuran solution
(0.10 M [N(n-Bu)4]PF6) measured at 20 ◦C at a glassy carbon
working electrode is shown in Fig. 1. Ferrocene was used as inter-
nal standard, and potentials are referenced vs. the ferrocenium–
ferrocene couple (Fc+/Fc). The cyclic voltammogram of 2 is
identical to that of 1 shown in Fig. 1.
Chart 1
Note that the ligands (1LOx)0 and (2LOx)0 carry a single methyl
group at the 1,4-diaza-1,3-butadiene backbone in the 2-position.
This, in principle, induces an asymmetry which leads to the
existence of two geometrical isomers6,11 of distorted tetrahedral
complexes containing two such bidentate ligands provided that
the dihedral angle h between the two five-membered chelate rings
is = 90◦. In this case the two diastereomers A and B prevail.
Fig.
1
Cyclic voltammogram of 1 in tetrahydrofuran (0.10 M
[N(n-Bu)4]PF6 supporting electrolyte) at 20 ◦C at a glassy carbon working
electrode.
Four reversible one-electron transfer waves have been observed
The coordination chemistry of N,Nꢀ-disubstituted a-diimines
with main group metal ions (Li, Mg, Al, Ga, In) and zinc(II) ions
has been investigated in detail experimentally and theoretically.12–25
It has been shown—mainly by X-ray crystallography—that these
ligands can adopt three different oxidation levels, namely the
neutral a-diimines, (LOx)0, monoanionic p radicals, (L•)1−, and the
at E11/2 = −0.16 V, E12/2 = −0.70 V, E13/2 = −2.25 V, and E41/2
=
−2.80 V vs. Fc+/Fc; the first two correspond to two successive
one-electron oxidations of 1 whereas the latter two are one-
electron reductions as shown by coulometry at appropriately fixed
potentials, eqn (2).
ꢀ
ꢁ
ꢂ ꢃ
+e
ꢀ
ꢁ
ꢂ ꢃ
+e
ꢀ
ꢁ
ꢂ ꢃ
2+
1+
0
reduced a-diamides (LRed 2− as displayed in Scheme 1. They clearly
)
II
NiII 1LOx
←−−− NiI 1LOx
←−−− Ni 1L
᭹
−−−→
−−−→
−e
−e
2
2
2
1
2
E
E
differ in their respective C–N and C–C bond lengths.
½
½
+e
ꢀ
ꢁ
ꢂ ꢁ
ꢂꢃ
+e
ꢀ
ꢁ
ꢂꢃ
1−
2−
←−−− Ni 1L 1LRed
←−−− Ni 1LRed
(2)
᭹
−−−→
−−−→
−e
−e
3
4
E
E
Results and discussion
½
½
Interestingly, the cyclic voltammogram of 3 (or 4) in THF at
20 ◦C displays only two oxidation waves: E1p,c = −1.4, Ep,a = −0.8
(E1/2 ca. −1.1 V) and E21/2 = −0.5 V (reversible) Fc+/Fc in the
range −1.6 V to 0.0 V.
Syntheses and spectroscopic characterizations
The reaction of 1 equivalent of [Ni(cdt)]0 (cdt = cyclododeca-
triene) with 2 equivalents of the N,Nꢀ-disubstituted 1,4-diaza-
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The electronic spectra of the neutral complexes 1 and of the
monocation in 2 in CH2Cl2 solution at 20 ◦C are displayed in
Fig. 2 (top); those of the corresponding species 3 and 4 are also
shown (bottom). The spectrum of 5 in CH2Cl2 is shown in Fig. 3.
Table 1 summarizes the data.
Fig. 3 Electronic spectrum of 5 in CH2Cl2 solution at 20 ◦C.
Table 1 Electronic spectra of complexes in CH2Cl2 solution
Complex
k/nm (104 e/M−1 cm−1
)
1
497 (0.8), 751 (0.6)
404 (0.4), 567 (0.3), 1100 (0.2)
531 (0.3), 789 (0.2)
360 (0.5), 478 (0.2), 610 (0.2)
388 (0.6), 472 (0.3) sh, 578 (0.22)
495 (1.5), 700 (0.09)
2
3
4
5
[NiII(3L•)2]a
[NiI(3LOx)2]+a
559 (0.9), 610 sh, 757 (0.4) sh, 1700 (0.2)
a Ref. 6.
100 nm. The intensity of this transition appears to correlate with
the dihedral angle h (between the two five-membered chelate rings)
of these neutral complexes as shown in Table 2 and Fig. 4 where
a plot of log e vs. h is linear (R = 0.982). Interestingly, in square
planar complexes9 (h = 0◦) this transition exhibits an intensity
of e ∼ 5 × 104 M−1◦ cm−1 whereas in nearly regular tetrahedral
complexes (h = 88.3 )2b e is small at ∼350 M−1 cm−1.
In ref.9 this transition has been identified for square planar
complexes as ligand-to-ligand charge transfer (LLCT) rather than
a simple HOMO–LUMO transition. In square planar complexes
this transition is dipole- and spin-allowed. It carries a significant
amount of information about the very large ground state exchange
coupling.9 In tetrahedral complexes of this type the LLCT
transition is spin-forbidden and, thus, its intensity is significantly
decreased (Scheme 2).
Fig. 2 Electronic spectra of 1 and 2 (top) and 3 and 4 (bottom) in CH2Cl2
solution at 20 ◦C.
The electronic spectra of all neutral complexes [NiII(L•)2] are
very similar.2 They consist of an intense absorption maximum at
∼500
50 nm with an extinction coefficient e ∼ 104 M−1 cm−1
The X-band EPR spectra of 2 and 4 have been recorded in a
CH2Cl2–toluene mixture and CH2Cl2 at 30 and 10 K, respectively;
and a second absorption maximum in the near infrared at 750
Table 2 Correlation between the dihedral angle h and log e of the lowest-energy transition in the near infrared spectruma
Complex
No. b
Ref.
h/◦
kmax/nm
ec/L mol−1 cm−1
log e
1
1
2
3
4
5
6
7
8
9
This work
This work
6
5
9
9
4(b)
2(b)
2(c)
47.9
53
78.9
55.3
0
0
51
751
789
700
756
790
839
768
682
732
0.58 × 104
0.20 × 104
0.9 × 103
3.76
3.30
2.95
3.50
4.73
4.60
3.70
2.54
3.82
3
[NiII(3L•)2]0
[NiII(5L•)2]0
[NiII(6L•)2]0
[NiII(7L•)2]0
[NiII(8L•)2]0
[NiII(9L•)2]0
[NiII(10L•)2]0
3.20 × 103
5.43 × 104
4.0 × 104
0.5 × 104
88.3
44.5
0.035 × 104
0.66 × 104
a Ligand abbreviations: (6LRed
)
= 3,5-di-tert-butyl-o-phenylenediimide(2−); (7LRed
)
= N-phenyl-o-phenylenediimide(2−); (8LOx
)
= N,Nꢀ-bis(2,6-
2−
2−
0
isopropylphenyl)-1,4-diaza-1,3-butadiene; (9LOx
)
= N,Nꢀ-bis(cyclohexyl)-1,4-diaza-1,3-butadiene; (10LOx
)
= N,Nꢀ-bis(2,6-dimethylphenyl)-1,4-diaza-
0
0
1,3-butadiene. b Number of complex in Fig. 4. c Extinction coefficient.
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they are shown in Fig. 5. In contrast to the two EPR spectra6
of the two geometrical isomers of [NiI(3LOx)2]+ the spectra of
[NiI(1LOx)2]+ and [NiI(2LOx)2]+ each consist only of one component.
Fig. 4 Plot of the intensity (log e) of the lowest energy transition of
[NiII(L•)2]0 complexes vs. the dihedral angle h of the two metallacycles
Ni–N–C–C–N. The numbers refer to the complex numbers listed in Table 2.
Fig. 5 X-Band EPR spectra of 2 (CH2Cl2–toluene 1 : 1 v/v) at 30 K (top)
and 4 (CH2Cl2) at 10 K (bottom). Conditions for 2 (and 4): frequency:
9.43 (9.43) GHz; power 1.26 (25.2) lW; modulation 7 (10) G. See Table 3
for simulation parameters.
Table 3 summarizes the observed and calculated values. In
all three cases the observed g anisotropy of the paramagnetic
monocations is quite large indicating that the unpaired electron
resides predominantly in a metal d-orbital. Given that the structure
of tetrahedral [NiI(3LOx)2](PF6) has been determined6 and the
oxidation level of the two ligands has been clearly identified as
(1LOx)0, we conclude that the monocations 2 and 4 have a very
Table 3 X-Band EPR spectral data of complexes
gx
gy
gz
Complex
Exp.
Calcd Exp.
Calcd Exp.
Calcd
2.370
[NiI(3LOx)2]+
2.066
2.070
2.038
2.109
2.085
2.210
2.198
2.105
2.171
2.244
2.444
2.359
2.259
2.258
a
[NiI(1LOx)2]+
[NiI(2LOx)2]+
2.067
2.095
2.182
2.181
2.271
2.307
Scheme 2 LLCT transitions of square planar and tetrahedral [Ni(L•)2]
a Ref. 6.
complexes.
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Table 4 Crystallographic data for 1, 3·hexane, and 5
1
3·hexane
5
Formula
M
C38H44N4Ni
615.48
P1, No. 2
C60H90N4Ni
926.07
P1, No. 2
C19H22I2N2Ni
590.90
P1, No. 2
¯
¯
¯
Space group
˚
a/A
11.3674(8)
11.8340(8)
13.8385(8)
65.052(5)
80.316(5)
77.999(5)
1644.2(2)
2
13.1901(6)
13.8920(6)
14.9786(6)
93.853(3)
93.563(3)
94.205(3)
2724.8(2)
2
7.0556(6)
8.2305(6)
18.0170(12)
94.370(4)
98.977(4)
98.200(4)
1017.69(13)
2
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
˚
V/A
Z
T/K
100(2)
1.243
100(2)
1.129
100(2)
1.928
qcalcd/g cm−3
Refl. collected, 2hmax
◦
/
42057, 60.00
9582, 0.045, 7568
398, 0
0.71073, 6.22
0.0427, 1.022
0.0926
35076, 50.00
9590, 0.064, 7841
626, 18
0.71073, /3.96
0.0607, 1.095
0.1183
7093/, 44.98
2161, 0.117, 1577
112, 0
0.71073, 39.90
0.0806, 1.083
0.1688
Unique refl., R(int), I > 2r(I)
No. of params, restr.
−1
˚
k/A, l(Ka)/cm
R1a, goodness of fitb
wR2c (I > 2r(I))
−3
˚
Residual density/e A
+0.44, −0.23
+0.53, −0.34
+1.21, −1.59
ꢄ
ꢄ
ꢄ
ꢄ
a Observation criterion: I > 2r(I). R1 = ꢂFo| − |Fcꢂ/R|Fo|. b GooF = [ [w(Fo − Fc2)2]/(n − p)]1/2
.
c wR2 = [ [w(Fo − Fc2)2]/ [w(Fo2)2]]1/2
2
2
where w = 1/r2(Fo2) + (aP)2 + bP, P = (Fo + 2Fc2)/3.
2
similar electronic structure with a central Ni(I) (d9; S = 1/2) ion. As
˚
Table 6 Selected experimental and calculated bond distances (A) in 5
discussed previously6 a [NiII(LOx)(L•)]+ ↔ [NiII(L•)(LOx)]+ charge
Expt.
Calcd
distribution has been ruled out (see below).
Ni–N1
Ni–N4
Ni–I2
1.996(16)
2.008(14)
2.471(3)
2.542(3)
1.28(2)
1.46(3)
1.30(2)
2.077
2.080
2.564
2.578
1.281
1.470
1.290
Crystal structures
Ni–I1
The crystal structures of the neutral complexes 1, 3·hexane, and
5 have been determined by single crystal X-ray crystallography
at 100(2) K. Table 4 gives crystallographic data, and Tables 5
and 6 summarize important bond lengths in 1, 3·hexane, and 5,
respectively.
N1–C1
C2–C3
C3–N4
Fig. 6 displays the structure of a neutral, tetrahedral molecule
As noted before5,6 and shown in Scheme 1, the oxidation
level of a given, N,Nꢀ-coordinated a-diimine derivative such as
the enediimide(2−), (LRed )2−, the p radical monoanion, (L•)1−,
or the neutral a-diimine, (LOx)0, can be clearly identified from
the observed C–N and C–C bond lengths. The average C–NR
in 5. The average C–NR bond length at 1.29 A and the C–C bond
˚
1
Ox 0
˚
distance at 1.46 A are typical for a neutral ( L ) oxidation level.
Fig. 7 and 8 exhibit the structure of the neutral molecules in
crystals of 1 and 3·hexane, respectively. In both structures the two
ligands are equivalent and in both cases the average C–N and C–C
bond lengths are compatible only with the (L•)1− oxidation level.
Thus, 1 and 3·hexane each contain a central Ni(II) ion and two
ligand p radical monoanions. It is noted that the average Ni–N
Ox
˚
=
distance at ∼1.29 A is indicative of C N double bonds in M(L )
fragments, or of a C–N bond order of ∼1.5 in M(L•) fragments at
Red
˚
˚
1.34 A, or of a C–N single bond in M(L ) fragments at ∼1.40 A.
˚
Table 5 Selected experimental bond distances (A) in 1 and 3. Calculated values are from BS(2,2) MS = 0 B3LYP DFT calculations
Expt. (1)
Calcd (1)
Calcd (2)
Expt. (3)
Calcd (3)
Calcd (4)
Ni–N1
Ni–N4
1.948(1)
1.921(1)
1.948(1)
1.928(1)
1.337(2)
1.405(3)
1.330(2)
1.338(2)
1.397(2)
1.330(2)
47.9
2.060
2.032
2.046
2.043
1.325
1.420
1.324
1.336
1.420
1.324
47.0
2.095
2.086
2.088
2.093
1.297
1.474
1.288
1.297
1.474
1.288
38
1.963(2)
1.999(2)
1.967(2)
1.978(2)
1.327(2)
1.402(4)
1.326(2)
1.340(2)
1.396(2)
1.326(3)
53
2.046
2.140
2.046
2.141
1.339
1.419
1.322
1.340
1.418
1.323
52
2.144
2.212
2.159
2.191
1.292
1.476
1.280
1.294
1.474
1.281
46
Ni–N31
Ni–N34
N1–C2
C2–C3
C3–N4
N31–C32
C32–C33
C33–N34
ha/◦
a Dihedral angle between two chelate rings.
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The dihedral angle h between the two five-membered chelate
rings Ni–N–C–C–N in 1 at 48◦ and in 3·hexane at 53◦ indicate
that both structures are intermediate between square planar (h =
0) and regular tetrahedral (h = 90◦).
It is now interesting that solid 3·hexane consists of two isomers
namely the transoid form A (∼70%) and the cisoid form B (∼30%).
Both forms co-crystallize and lead to an apparent statistical
disorder of the 2-methyl group of the ligand backbone. The
same ratio has been observed to prevail in solution by 1H NMR
spectroscopy. The proton signal of the 2-methyl groups of the
transoid form A is observed at −3.62 ppm (∼70%) and at −3.27
(∼30%) for the cisoid form B. From the fact that these signals
(and their ratio) is independent of the temperature we conclude
that the A and B forms are not in a dynamic equilibrium in
solution. The steric bulky isopropyl groups obviously prevent the
interconversion of these two forms.
Fig. 6 Structure of the neutral complex in crystals of 5.
DFT Calculations
a. Optimized geometries for 1–5. Here, a detailed picture of
the molecular and electronic structures of the neutral complexes
1, 3, and 5 and of the monocations in 2 and 4 will be developed.
Assuming that tetrahedral 1 and 3 feature a high-spin Ni(II) ion
and two ligand radicals, we optimized the ground state geometry
for (a) a closed-shell S = 0, (b) a high-spin, open-shell S = 2, and
(c) a BS(2,2) MS = 0 state for 1 and 3.
For these cases, stationary states on the potential energy surfaces
were located. The closed-shell solutions and the S = 2 configura-
tions were found to be higher in energy than the BS solutions
by 11 and 14 kcal mol−1, respectively. An attempt to calculate a
BS(1,1) state corresponding to [NiI(L•)(LOx)] failed; the calculation
converged back to the BS(2,2) solution. Thus, the BS(2,2) solution
involving a high-spin Ni(II) ion antiferromagnetically coupled to
two ligand p radicals, (L•)1−, is the preferred one. The calculated
bond distances (Table 5) of the BS solutions within the two
equivalent ligands are in acceptable agreement with experiment
and support the presence of two (L•)1− p radicals in both cases.
Fig. 7 Structure of the neutral complex in crystals of 1.
˚
The experimental average Ni–N distance in 1 at 1.936 0.003 A
˚
˚
and in 3 at 1.977 0.006 A are overestimated by 0.11 and 0.12 A,
respectively; a result which is typical for the B3LYP functional.
It is noteworthy, that the BS calculated dihedral angles between
the two five-membered chelate rings in 1 and 3 are in excellent
agreement with experiment (Table 5).
For complexes 2 and 4 the open-shell S = 1/2 and the BS(2,1)
MS = 1/2 states converged to the same solution. Thus, attempts
to calculate [NiII(L•)(LOx)]+ with a high-spin Ni(II) ion and a single
antiferromagnetically coupled ligand p radical reverted to the S =
1/2 (Ni(I), d9) solution. Both ligands in 2 and 4 are identical and
the C–N and C–C bond lengths indicate the presence of two neu-
tral (LOx)0 ligands. These distances agree very well with those found
experimentally in [NiI(3LOx)2](PF6).6 Significantly, the same ligand
distances have been found experimentally and calculated for a
spin-unrestricted S = 1 state for complex 5. Thus, the ligands have
unambiguously the oxidation level (LOx)0 (Table 4) in 2, 4, and 5.
Fig. 8 Structure of the neutral, transoid majority component in crystals
of 3; that of the cisoid minority component is not shown.
˚
bond length in 1 at 1.936 0.003 A is significantly shorter than
b. Electronic structures of 1–5. For pseudo-tetrahedral 5 the
MO bonding scheme (supporting information) clearly demon-
strates the presence of a central Ni(II) ion since the five highest
energy orbitals are of predominantly metal-d character; three of
which are doubly occupied and two (originating from the t2 set)
˚
in 3·hexane at 1.977 0.006 A. This is a clear indication that the
greater steric bulk of the four isopropyl groups in 2 as compared
to four methyl groups in 1 has a significant effect on the overall
structure.
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are singly occupied as is expected for a Ni(II) ion in a tetrahedral
ligand field. The Mulliken spin population analysis corroborates
this notion where ∼1.6 electrons are located at the central Ni(II)
ion and 0.25 on both iodide ligands but only 0.08 on the ligand
(LOx)0.
A qualitative MO bonding scheme derived from the spin-
unrestricted BS(2,2) calculation for 1 is shown in Fig. 9. Five
orbitals which are predominantly of metal-d character are again
identified. Three of these are found in the spin-up and spin-
down manifolds; they are doubly occupied. The other two nickel-
based orbitals originating from the t2 set occur only in the spin-
up manifold. These two orbitals are thus singly occupied with
parallel spins. This pattern defines a high-spin Ni(II) configuration
at the metal center. In addition, two ligand centered orbitals are
identified in the spin-down manifold which are not populated in
the spin-up manifold; thus leading to the observed overall MS =
0 BS state. These orbitals correspond to two symmetry-adapted
combinations of the SOMO of the two ligand radicals. Complex
1, therefore, features a high-spin Ni(II) ion which is strongly
antiferromagnetically coupled to two ligand-centered p radicals.
As pointed out previously, the spin density arising from BS
SCF (DF or HF) calculations is unphysical. Nevertheless, it is
quite suggestive of the physical situation at hand. The spin density
plot shown in Fig. 9 (bottom) nicely shows the antiparallel spin
alignment between the high-spin Ni(II) (positive spin density in
red) and the radical ligands (negative spin density in yellow).
An approximate breakdown of the spin density into atomic
contributions via a spin population analysis supports the presence
of two unpaired electrons at the nickel ion with a positive spin and
two unpaired electrons with a negative spin localized on the two
ligand p radicals.
The corresponding orbital transformation can be used to
visualize the overlapping magnetic pairs of the system. The
spin–orbitals obtained from single-point unrestricted calculations
were transformed in such a way that for each spin-up orbital
there exists at most one spin-down partner that has non-zero
spatial overlap. Values of S close to 1 indicate a standard doubly
occupied MO with little spin-polarization, whereas S ꢃ 1 is the
signature of non-orthogonal magnetic orbital pairs. For 1, two
such magnetically interacting pairs which interact via a p pathway
have been identified. Each of these pairs consists of one metal
orbital and the corresponding ligand radical orbital. The mutual
overlap between those two orbitals are 0.35 and 0.70, as shown in
Fig. 9. This indicates fairly strong antiferromagnetic interactions
between the two metal and two ligand radicals.
Exactly the same situation has previously been described
by us for similar tetrahedral nickel complexes containing
o-diiminobenzosemiquinonate(1−)7 and S-methylisothiosemi-
carbazonate(1−)11 p radicals. Similar calculations for 3 afford
exactly the same picture (see ESI).
The calculated electronic structure of the monocation in 2 is
shown in Fig. 10. Five predominantly metal-d orbitals are clearly
identified; the first four of which are doubly occupied whereas
the fifth d-orbital is only singly occupied. This is indicative of the
presence of a Ni(I) ion (d9; SNi = 1/2). All ligand orbitals are either
doubly or not occupied at all as is expected for two closed-shell,
diamagnetic (LOx)0 ligands. The Mulliken spin population analysis
shows that the unpaired electron (+1.04) is localized at the metal-
center and no spin density is observed on the ligands. For the
monocation in 4 (see ESI) the same picture has been obtained.
Finally, we have calculated reasonable g-tensors for the mono-
cations in 2 and 4 by using the above DFT methodology (Table 3).
The experimental and calculated gx, gy, gz values are in good
agreement. The large g-anisotropy is in excellent agreement with
the notion that a paramagnetic Ni(I) ion is present in 2 and 4.
Fig. 9 Qualitative MO scheme of the corresponding orbitals of magnetic
pairs and metal-d orbitals of 1 as derived from the broken symmetry
DFT calculations (B3LYP) (top). Spin density plot of 1 together with
approximate values of the spin density of the Mulliken spin population
analysis (bottom). Hydrogen atoms are omitted for clarity.
Conclusion
The present work suggests that the electronic structures of the
diamagnetic neutral complexes 1 and 3 containing NiN4 polyhedra
which are intermediate between square planar and tetrahedral (h ≈
4396 | Dalton Trans., 2007, 4390–4398
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the tetrahedral species strong antiferromagnetic coupling to the
central nickel(II) (SNi = 1) prevails and a diamagnetic ground state
S = 0 is attained in both cases.
Experimental
Preparations of compounds
All air-sensitive materials were manipulated under argon using
standard Schlenk line procedures or a glovebox. Cyclodode-
catriene-nickel(0), Ni(cdt), and ferrocenium hexafluorophosphate
were used as starting materials. The ligands 2-methyl-1,4-
bis(2,6-dimethylphenyl)-1,4-diaza-1,3-butadiene and 2-methyl-
1,4-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene28
synthesized according to published procedures.
were
[NiII(1L•)2] (1). To
a solution of 2-methyl-1,4-bis(2,6-
dimethylphenyl)-1,4-diaza-1,3-butadiene (2.390 g, 8.58 mmol) in
diethyl ether (10 mL) under an argon blanketing atmosphere was
added a solution of Ni(cdt) (1 g, 4.29 mmol) in diethyl ether
(20 mL). The resulting red solution was stirred over night at
room temperature. The solvent was removed by evaporation under
reduced pressure to give a dark red precipitate which was washed
with acetonitrile and dried in vacuo to yield 2.10 mg of 1 (79%).
X-Ray quality crystals were obtained by slow evaporation of the
solvent from a concentrated solution of 1 in n-hexane. Anal. calcd
for C38H44N4Ni: C, 74.2; H, 7.2; N, 9.1; Ni, 9.5. Found: C, 73.8; H,
7.7; N, 8.7; Ni, 9.1%. 1H NMR (400 MHz, toluene-d8): d (ppm) =
−1.68 (s, 6H, NCCH3), 2.06 (s, 12H, C6H3CH3), 2.17 (s, 12H,
C6H3CH3), 6.78 (m, 4H, C6H3), 6.92 (m, 4H, C6H3), 7.03 (m,
4H, C6H3), 8.40 (s, 2H, NCH). 13C NMR (400 MHz, toluene-
d8): d = 18.48 (CH3, C6H3Me), 19.57 (CH3, C6H3Me), 21.49
(CH3, NCMe), 124.29 (CH, C6H3), 124.39 (CH, C6H3), 127.74
(CH, C6H3), 128.09 (CH, C6H3), 128.45 (CH, C6H3), 137.30 (CH,
NCH), 143.37 (Cq, NCMe), 157 (Cq, C6H3), 160 (Cq, C6H3).
[NiI(1LOx)2](PF6) (2). To a solution of 1 (200 mg, 0.32 mmol)
in CH2Cl2 (10 mL) was added ferrocenium hexafluorophosphate
(100 mg, 0.32 mmol) with stirring for 1.5 h at 20 ◦C. The dark
red colour of the solution immediately turned to dark violet. The
resulting solution was filtered and the filtrate was concentrated
to ∼1 mL by evaporation of the solvent under reduced pressure.
n-Hexane (10 mL) was added and the resulting suspension was
stirred for 1 h. A dark violet precipitate formed, which was isolated
by filtration, washed with n-hexane (2 × 5 mL) and dried in vacuo
to give 0.21 of 2 (87%). MS (ESI, pos. ion, CH2Cl2), m/z = 614
Fig. 10 Qualitative MO scheme of the monocation in 2 as derived from
the spin unrestricted S = 1/2 DFT calculations (B3LYP) (top). Spin
density plot of 2 together with approximate values of the spin density
of the Mulliken spin population analysis. Hydrogen atoms are omitted for
clarity (bottom).
+
{2 − PF6} . Anal. calcd for C38H44N4F6NiP: C, 60.0; H, 5.8; N,
7.4; F, 15.0; Ni, 7.7; P, 4.1. Found: C, 60.2; H, 5.8; N, 7.3; F, 15.2;
Ni, 7.7; P, 4.0%.
[NiII(2L•)2]·n-hexane (3·n-hexane). To
a pale yellow so-
45◦) and other more regular tetrahedral species6 (h ≈ 90◦) differ
slightly (see the respective electronic spectra and the intensities of
the LLCT bands). On the other hand, the intriguing redox chem-
istry remains the same: the monocations are Ni(I) species (S =
1/2) with two neutral (LOx)0 ligands irrespective of the dihedral
angle between the two five-membered chelate rings. It is gratifying
that broken symmetry DFT calculations of the neutral species
clearly indicate that two monoanionic ligand p radicals are present
irrespective of h. In square planar complexes of this type spins
strongly couple antiferromagnetically with each other whereas in
lution of 2-methyl-1,4-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-
butadiene (3.53 g, 9.06 mmol) in n-hexane (30 mL) under an
argon blanketing atmosphere was added a solution of Ni(cdt)
(1 g, 4.53 mmol) in diethyl ether (20 mL). The resulting violet
solution was stirred over night at room temperature. The solvent
was removed by evaporation under reduced pressure to give a
dark violet precipitate which was washed with acetonitrile and
dried in vacuo to yield 3.10 g of 3 (81%). X-Ray quality crystals
of 3·n-hexane were obtained by cooling a concentrated n-hexane
solution of 3 to −40 ◦C. Anal. calcd for crystalline C60H90N4Ni: C,
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77.8; H, 9.8; N, 6.0; Ni, 6.3. Found: C, 77.8; H, 9.4; N, 6.1;
Crystals of 5 were found to be extremely thin and were weakly
diffracting. Due to problems during data integration, reflection
data are only complete to about 81%. Only heavy atoms were
anisotropically refined since the data/parameter ratio and data
quality did not allow full anisotropic refinement.
1
Ni, 6.3%. H NMR (400 MHz, toluene-d8): d (ppm) = −3.62,
−3.27 (s, NCCH3), 0.4–1.67 (m, C6H3(CH(CH3)2)2, 2.50–5.80 (m,
C6H3(CH(CH3)2)2, 6.83–7.16 (m, C6H3), 9.56, 9.61 (s, NCH).
[NiI(2LOx)2](PF6) (4). To a solution of 3 (400 mg, 0.476 mmol)
in CH2Cl2 (10 mL) was added ferrocenium hexafluorophosphate
(150 mg, 0.476 mmol) with stirring for 1.5 h at 20 ◦C. The dark red
colour of the solution immediately turned to green. The resulting
solution was filtered and the filtrate was concentrated to ∼1 mL
by evaporation of the solvent under reduced pressure. n-Hexane
(10 mL) was added and the resulting suspension was stirred for
1 h. A dark violet precipitate was formed, washed with n-hexane
(2 × 5 mL) and dried in vacuo to give 0.40 g of 4 (87%). Anal.
calcd for C54H76N4F6NiP: C, 65.9; H, 7.8; N, 5.7; F, 11.6; Ni, 6.0;
P, 3.1. Found: C, 65.6; H, 7.7; N, 5.6; F, 11.6; Ni, 5.8; P, 3.2%.
Acknowledgements
We thank the Fonds der Chemischen Industrie for financial support
of this work. Heike Schucht is thanked for skilful help with the
X-ray data collection.
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X-Ray crystallographic data collection and refinement of the
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˚
graphite monochromator (Mo-Ka, k = 0.71073 A) was used.
Final cell constants were obtained from least squares fits of all
measured reflections. Intensity data were corrected for absorption
using intensities of redundant reflections.
The structures were readily solved by Patterson methods and
subsequent difference Fourier techniques. The Siemens ShelXTL26
software package was used for solution and artwork of the struc-
ture, ShelXL9727 was used for the refinement. All non-hydrogen
atoms in 1 and 3·hexane were refined anisotropically but due to
the low data quality of 5 only the nickel and iodine atoms were
anisotropically refined. Hydrogen atoms were placed at calculated
positions and refined as riding atoms with isotropic displacement
parameters. Crystallographic data for the compounds are listed in
Table 4.
Due to co-crystallization of the two geometrical isomers in
3·hexane, the methyl group (C23) was found to be statically
disordered over two sites. A split atom model with equal thermal
displacement parameters of corresponding atoms and restrained
bond distances using the EADP and SAME instruction of
ShelXL97,27 refined to an occupation ratio of about 72 : 28. The
hexane solvent molecule was found to be disordered and was split
on two positions in a 68 : 32 ratio using EADP and SAME.27
4398 | Dalton Trans., 2007, 4390–4398
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©