Iron(II) complexes of 2,6-di(1-alkylpyrazol-3-yl)pyridine derivatives - The influence of distal substituents on the spin state of the iron centre

Article abstract of DOI:10.1016/j.poly.2013.01.057

2,6-Di(1-methyl-pyrazol-3-yl)pyridine (L^Me), 2,6-di(1-allyl-pyrazol-3-yl)pyridine (L^All), 2,6-di(1-benzyl-pyrazol- 3-yl)pyridine (L^Bz) and di(1-isopropyl-pyrazol-3-yl)pyridine (L ^iPr) have been synthesized by alkylation of deprotonated di{1H-pyrazol-3-yl}pyridine (3-bpp), and converted to salts of the corresponding [Fe(L^R)₂]²⁺ complexes (R = Me, All, Bz and iPr). Crystal structures of [Fe(L^Me)₂]X₂ (X^- = BF₄^-, ClO₄^- and PF₆^-), [Fe(L^All)₂][BF ₄]₂, [Fe(L^Bz)₂][BF₄] ₂ and [Fe(L^iPr)₂][PF ₆]₂ have been determined at 150 K. All of these contain high-spin iron centres except [Fe(L^Me)₂][BF ₄]₂·xH₂O, which is predominantly low-spin at that temperature. All the complexes are high-spin between 5 and 300 K as solvent-free bulk powders, and are also high-spin in (CD₃) ₂CO solution between 193 and 293 K. This was unexpected, since the parent complex [Fe(3-bpp)₂]²⁺ undergoes spin-crossover in the same solvent with T = 247 K [40]. The high-spin nature of the [Fe(L ^R)₂]²⁺ complexes in solution must reflect a subtle balance of steric and electronic factors involving the ligand 'R' substituents.

Full text of DOI:10.1016/j.poly.2013.01.057

Polyhedron xxx (2013) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Iron(II) complexes of 2,6-di(1-alkylpyrazol-3-yl)pyridine derivatives – The influence
of distal substituents on the spin state of the iron centre
Thomas D. Roberts a, Marc A. Little a,b, Laurence J. Kershaw Cook a, Simon A. Barrett a, Floriana Tuna c,
Malcolm A. Halcrow a,
⇑
a School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK
b Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, UK
c School of Chemistry and Photon Science Institute, University of Manchester, Oxford Road, Manchester M13 9PL, UK
a r t i c l e i n f o
a b s t r a c t
2,6-Di(1-methyl-pyrazol-3-yl)pyridine (LMe), 2,6-di(1-allyl-pyrazol-3-yl)pyridine (LAll), 2,6-di(1-benzyl-
Article history:
Available online xxxx
pyrazol-3-yl)pyridine (LBz) and di(1-isopropyl-pyrazol-3-yl)pyridine (LiPr) have been synthesized by
alkylation of deprotonated di{1H-pyrazol-3-yl}pyridine (3-bpp), and converted to salts of the corre-
Dedicated to George Christou on the
occasion of his 60th birthday.
sponꢀding [Fe(LR)2]2+ complexes (R = Me, All, Bz and iPr). Crystal structures of [Fe(LMe)2]X2 (Xꢀ = BF4
,
ꢀ
ClO4 and PF6ꢀ), [Fe(LAll)2][BF4]2, [Fe(LBz)2][BF4]2 and [Fe(LiPr)2][PF6]2 have been determined at 150 K.
All of these contain high-spin iron centres except [Fe(LMe)2][BF4]2ꢁxH2O, which is predominantly low-spin
at that temperature. All the complexes are high-spin between 5 and 300 K as solvent-free bulk powders,
and are also high-spin in (CD3)2CO solution between 193 and 293 K. This was unexpected, since the par-
Keywords:
Iron
N-donor ligands
Crystal structure
Magnetic measurements
Spin-crossover
ent complex [Fe(3-bpp)2]2+ undergoes spin-crossover in the same solvent with T = 247 K [40]. The high-
½
spin nature of the [Fe(LR)2]2+ complexes in solution must reflect a subtle balance of steric and electronic
factors involving the ligand ‘R’ substituents.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
The coordination chemistry of substituted 3-bpp derivatives is
less developed by comparison, because of the poorer availability
The chemistry of spin-crossover complexes [1–3] continues to
be heavily studied, because of their potential applications as
switchable components in memory and display devices [4], in
nanoscience [2] and in MRI contrast agents [5]. A class of com-
pound that has been heavily used in spin-crossover research
during the past ten years are iron(II) complexes of the isomeric
2,6-di(pyrazolyl)pyridine ligands, 1-bpp and 3-bpp (Scheme 1)
[6,7]. The 1-bpp ligand framework can be substituted at every po-
sition of its pyrazole and pyridine rings [7]. Substitution at the pyr-
idine ring allows functional groups to be included at the periphery
of the [Fe(1-bpp)2]2+ centre without significantly perturbing the
iron centre. This approach has afforded multifunctional spin-
crossover complexes [8], coordination polymers of [Fe(1-bpp)2]2+
centres [9], and complexes with tether groups for deposition on
surfaces [10]. In contrast, substituents at the pyrazole groups allow
for steric and electronic control of the spin-state properties of a
[Fe(1-bpp)2]2+ complex, so its spin-crossover properties can be
of suitable synthetic precursors. 3-bpp ligands derivatised at N1
and C5 of the pyrazole rings are well-established, and have been
employed in luminescent complexes [11–13], in catalysis [14–
16], in hydrometallurgical applications [17] and in self-assembly
reactions [18,19]. However, although [Fe(3-bpp)2]2+ itself is a ver-
satile spin-crossover compound [6], the application of substituted
3-bpp ligands to spin-crossover chemistry has only recently been
investigated, by us [20] and by Aromí co-workers [21].
We describe here the first investigation of iron complexes of 3-
bpp derivatives that are disubstituted at the pyrazole N1 positions.
These are analogues of 1-bpp ligands bearing substituents at the
pyrazole C3 sites, where the pyrazole substitutents are known to
have a strong bearing on the spin-state properties of a coordinated
iron centre [6]. Four 3-bpp derivatives have been investigated in
this work: 2,6-di(1-methylpyrazol-3-yl)pyridine (LMe), 2,6-di(1-
allylpyrazol-3-yl)pyridine (LAll), 2,6-di(1-benzylpyrazol-3-yl)pyri-
dine (LBz
)
and 2,6-di(1-isopropylpyrazol-3-yl)pyridine (LiPr
)
modified in
a
rational way [7]. The synthetic versatility of
(Scheme 1). Some noble metal complexes of LMe [13] and LAll
[16] have been reported before but their iron complexes have
not yet been investigated, while LBz and LiPr are new ligands to
our knowledge. We were particularly interested in salts of
[Fe(LMe)2]2+ since the Fe[BF4]2 complex of the equivalent 1-bpp
derivative, 2,6-di(3-methylpyrazol-1-yl)pyridine (Me2-1-bpp),
[Fe(1-bpp)2]2+ is unique among the commonly used compounds
in the field of spin-crossover.
⇑
Corresponding author. Fax: +44 113 343 6565.
E-mail address: M.A.Halcrow@leeds.ac.uk (M.A. Halcrow).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.01.057
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2
T.D. Roberts et al. / Polyhedron xxx (2013) xxx–xxx
ence of benzyl bromide (4.86 g, 28.4 mmol). The mixture was then
heated at reflux for 44 h under a nitrogen atmosphere. The resul-
tant white precipitate was removed via filtration and washed with
water. The solid was then recrystallised from chloroform and dried
in vacuo. Yield 1.67 g, 45%. EI HR mass spectrum: m/z 391.1795
([LBz]+; calcd for C25H21N5 m/z 391.1797). 1H NMR (CDCl3) d5.31
(br s, 4H, CH2), 7.01 (br s, 2H, Pz H4), 7.18–7.38 (br m, 10H,
C6H5), 7.40 (d, 2.3 Hz, 2H, Pz H5‘), 7.75 (t, 7.5 Hz, 1H, Py H4), 7.86
(br s, 2H, Py H3/5). 13C{1H} NMR (CDCl3): d56.2 (2C, CH2), 105.2
(2C, Pz C4), 118.6 (2C, Py C3/5), 127.5 (Ph C2/6), 128.0 (Ph C3/5),
128.8 (Ph C4), 130.9 (2C, Pz C5), 136.4 (2C, Ph C1), 137.3 (1C, Py
C4), 151.7 and 152.0 (both 2C, Py C2/6 and Pz C3).
2.2. Synthesis of 2,6-di(1-isopropylpyrazol-3-yl)pyridine (LiPr
)
The same method as described for LBz was followed, using 2-
iodopropane (4.83 g, 28.4 mmol). After 72 h at reflux under a nitro-
gen atmosphere, the resultant white precipitate was collected,
washed with water and dried in vacuo. The product was employed
without further purification. Yield 2.67 g, 95%. EI HR mass spec-
trum: m/z 296.1874 ([HLiPr]+; calcd for C17H22N5 m/z 296.1870).
1H NMR ({CD3}SO) d1.47 (d, 6.6 Hz, 12H, CH{CH3}2), 4.57 (sept,
6.6 Hz, 2H, CH{CH3}2), 6.92 (d, 2.1 Hz, 2H, Pz H4), 7.83 (s, 2H, Pz
H5), 7.84 (s, 3H, Py H3–5). 13C{1H} NMR ({CD3}SO): d22.2 (4C,
CH{CH3}2), 54.3 (2C, CH{CH3}2), 104.0 (2C, Pz C4), 119.3 (2C, Py
Scheme 1.
exhibits an unusually low thermal spin-transition temperature for
a complex of this type which leads to unique light-induced spin-
crossover properties [22]. We were therefore keen to see whether
salts of [Fe(LMe)2]2+ exhibit comparable effects.
C
3/5), 128.9 (2C, Pz C5), 139.0 (1C, Py C4), 149.6 and 151.0 (both
2C, Py C2/6 and Pz C3).
2.3. Synthesis of the complexes
2. Experimental
The same basic method, as described here for 1[BF4]2, was fol-
lowed for all the complexes in this study. Iron(II) tetrafluoroborate
hexahydrate (0.14 g, 0.4 mmol) was added to a stirred solution of
LMe (0.20 g, 0.8 mmol) in nitromethane (15 mL) and the resulting
yellow solution was stirred for a further 30 min. Diethyl ether
was then added until a yellow precipitate formed which was col-
lected via filtration. The product was then recrystallised from
methanol/diethyl ether to give a yellow crystalline solid. The same
method, using the equivalent quantities of the appropriate ligand
and metal salt, yielded the other complexes. Recrystallised yields
ranged from 38% to 70%. Caution! Although we have experienced
Unless otherwise stated, all manipulations were carried out in
air using reagent-grade solvents. 2,6-Di(pyrazol-3-yl)pyridine (3-
bpp) [23], 2,6-di(1-methylpyrazol-3-yl)pyridine (LMe) [13] and
2,6-di(1-allylpyrazol-3-yl)pyridine (LAll) [16] were prepared by lit-
erature methods, while all other reagents and solvents were used
as supplied.
2.1. Synthesis of 2,6-di(1-benzylpyrazol-3-yl)pyridine (LBz
)
2,6-Bis(pyrazol-3-yl)pyridine (2.00 g, 9.5 mmol) and lithium hy-
dride (0.22 g, 28.4 mmol) were suspended in dry THF, in the pres-
Table 1
Experimental details for the single crystal structure determinations in this work.
1[BF4]2ꢁxH2O
26H28B2F8FeN10
726.05
monoclinic
C2/c
17.1632(16)
20.9906(19)
19.1771(17)
96.605(5)
6863.0(11)
8
1[ClO4]2
1[PF6]2
2[BF4]2
3[BF4]2
4[PF6]2ꢁ2CH3CN
Formula
Mr
Crystal system
Space group
a (Å)
C
O
C26H26Cl2FeN10O8
733.32
trigonal
R32
18.6378(12)
–
24.3461(14)
–
7324.0(8)
9
C26H26F12FeN10P2
824.36
monoclinic
C2/c
34.124(3)
12.3128(11)
17.6817(17)
114.441(6)
6763.4(11)
8
C34H34B2F8FeN10
812.18
cubic
C50H42B2F8FeN10
1012.41
orthorhombic
Pbca
C38H48F12FeN12P2
1018.67
monoclinic
P21/n
20.494(2)
23.257(3)
20.609(2)
101.271(6)
9633.3(19)
8
ꢀ
I43d
22.8650(18)
–
–
15.4334(17)
14.0656(17)
43.884(5)
–
9526.3(19)
8
b (Å)
c (Å)
b (°)
–
V (Å3)
Z
11954.0(16)
12
T (K)
150(2)
1.405
0.520
150(2)
1.496
0.690
150(2)
1.619
0.643
150(2)
1.354
0.454
150(2)
1.412
0.396
150(2)
1.405
0.468
qcalc (g cmꢀ3
)
l
(mmꢀ1
)
Measured reflections
Independent reflections
Rint
37795
6723
0.056
23056
4972
0.030
62406
10895
0.082
87527
2127
0.041
162266
11794
0.049
524427
23521
0.047
Observed reflections [I > 2
r
(I)]
4552
4218
7634
1847
9529
17909
Data, restraints, parameters
6723, 40, 460
0.085, 0.297
1.041
4972, 14, 216
0.061, 0.171
1.058
10895, 0, 464
0.048, 0.132
1.023
2127, 25, 162
0.048, 0.136
1.108
11794, 0, 640
0.045, 0.127
1.024
23521, 102, 1282
0.063, 0.188
1.100
R1(I > 2r
(I))a, wR2(all data)b
GOF
D
qmin
,
D
qmax (e Åꢀ3
)
ꢀ0.58, 1.05
ꢀ0.55, 0.68
ꢀ0.62, 0.78
ꢀ0.21, 0.36
ꢀ0.76, 1.00
ꢀ0.74, 0.94
Flack parameter
–
0.00(3)
–
ꢀ0.02(3)
–
–
a
R =
wR = [
R
[|Fo| ꢀ |Fc|]/
R
|Fo.
R
b
2
4 1/2
R
w(Fo ꢀ Fc2)2/
wFo
]
.
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T.D. Roberts et al. / Polyhedron xxx (2013) xxx–xxx
3
no problems with 1[ClO4]2, metal–organic perchlorates are poten-
tially explosive and should be handled with due care in small
quanitities.
For [Fe(LBz)2][BF4]2 (3[BF4]2): Anal. Calc. for C50H42B2F8FeN10: C,
59.3; H, 4.18; N, 13.8. Found: C, 58.8, H, 4.10; N, 13.8%. Electro-
spray mass spectrum: m/z 419.2 ([Fe(LBz)2]2+). 1H NMR (CD3CN):
d ꢀ0.5 (8H, CH2), 2.1 (8H, Ph H2/6), 5.5 (8H, Ph H3/5), 6.5 (4H, Ph
H4), 5.5 (8H, CH2), 20.8 (2H, Py H4), 46.0 (4H, Pz H4), 59.5 and
60.9 (both 4H, Py H3/5 and Pz H5).
2.3.1. Analytical data
For [Fe(LMe)2][BF4]2 (1[BF4]2): Anal. Calc. for C26H26B2F8FeN10: C,
44.1; H, 3.70; N, 19.8. Found: C, 44.0; H, 3.65; N, 19.7%. Electro-
spray mass spectrum: m/z 267.1 ([Fe(LMe)2]2+). 1H NMR (CD3OD):
d 2.2 (12H, CH3), 22.6 (2H, Py H4), 48.5 (4H, Pz H4), 57.1 and 62.0
(both 4H, Py H3/5 and Pz H5).
For [Fe(LiPr)2][BF4]2ꢁH2O (4[BF4]2ꢁH2O): Anal. Calc. for C34H42B2
F8FeN10ꢁH2O: C, 48.7; H, 5.29; N, 16.7. Found: C, 48.9; H, 4.95; N,
16.7%. Electrospray mass spectrum: m/z 323.2 ([Fe(LiPr)2]2+). 1H
NMR (CD3CN): d 2.0 and 2.1 (both 6H, CH{CH3}2), 4.7 (4H,
CH{CH3}2), 21.1 (2H, Py H4), 45.8 (4H, Pz H4), 60.8 and 61.6 (both
4H, Py H3/5 and Pz H5).
For [Fe(LMe)2][ClO4]2 (1[ClO4]2): Anal. Calc. for C26H26Cl2FeN10
O8: C, 42.6; H, 3.57; N, 19.1. Found: C, 42.6; H, 3.50; N, 19.1% Elec-
trospray mass spectrum: m/z 267.1 ([Fe(LMe)2]2+).
For [Fe(LiPr)2][PF6]2 (4[PF6]2): Anal. Calc. for C34H42F12FeN10P2:
C, 43.6; H, 4.52; N, 15.0. Found: C, 43.2; H, 4.40; N, 14.9%. Electro-
spray mass spectrum: m/z 323.2 ([Fe(LiPr)2]2+).
For [Fe(LMe)2][PF6]2 (1[PF6]2): Anal. Calc. for C26H26F12FeN10P2:
C, 37.9; H, 3.18; N, 17.0. Found: C, 37.8; H, 3.10; N, 17.1% Electro-
spray mass spectrum: m/z 267.1 ([Fe(LMe)2]2+).
For [Fe(LAll)2][BF4]2 (2[BF4]2): Anal. Calc. for C34H34B2F8FeN10: C,
50.3; H, 4.22; N, 17.2. Found: C, 50.1; H, 4.15; N, 17.0% Electrospray
mass spectrum: m/z 319.12 ([Fe(LAll)2]2+). 1H NMR (CD3CN): d ꢀ0.6
(8H, CH2), 2.1 (8H, CH = CH2), 2.2 (4H, CH = CH2), 21.7 (2H, Py H4),
46.8 (4H, Pz H4), 58.9 and 61.8 (both 4H, Py H3/5 and Pz H5).
2.4. Single crystal X-ray structure determinations
All the single crystals in this work were grown by slow diffusion
of diethyl ether vapour into nitromethane or acetonitrile solutions
of the compounds except for 1[BF4]2ꢁxH2O which was crystallised
Fig. 1. Views of the [Fe(LMe)2]2+ dication in 1[PF6]2 (top left); [Fe(LAll)2]2+ in 2[BF4]2 (top right); [Fe(LBz)2]2+ in 3[BF4]2 (bottom left); and [Fe(LiPr)2]2+ in 4[PF6]2ꢁ2CH3CN
(molecule A, bottom right). The allyl substitutents in [Fe(LAll)2]2+ are plotted in different disorder orientations, while only the major orientation of the disordered isopropyl
group in the molecule of [Fe(LiPr)2]2+ is shown. Displacement ellipsoids are at the 50% probability level, and H atoms have been omitted for clarity. Colour code: C (ligand
backbone), dark gray; C (substituents), white; N, blue; Fe, green. (Colour online.)
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4
T.D. Roberts et al. / Polyhedron xxx (2013) xxx–xxx
ꢀ
from methanol/diethyl ether, and 2[BF4]2 which was crystallised
from acetonitrile/di-isopropyl ether. Diffraction data were mea-
sured using a Bruker X8 Apex diffractometer fitted with an Oxford
Cryostream low temperature device, using graphite-monochro-
axis x, x, 0; and, one-sixth of a ClO4 ion disordered about the
1
2
1
32 site
,
, 6, with the Cl atom lying on this position and one
3
3
1
2
O atom on the axis
,
3, z. The pyrazole ring C(14)–C(19) was
3
modelled as disordered, over two equally occupied sites. The
following restraints were applied to that residue: intra-ring C–
C = 1.40(2), C–N = N–N = 1.34(2), N–C{methyl} = 1.48(2) and C–
C{pyridyl} = 1.41(2) Å. Attempts to extend the disorder model to
the complete complex half-cation did not afford a chemically rea-
sonable refinement. No othꢀer disorder was included in the model,
although some of the ClO4 O atoms also have slightly high dis-
placement ellipsoids. All non-H atoms except for the disordered li-
gand atoms were refined anisotropically, and H atoms were placed
in calculated positions and refined using a riding model.
mated Mo Ka radiation (k = 0.71073 Å) generated by a rotating an-
ode. Experimental details of the structure determinations in this
study are given in Table 1. All the structures were solved by direct
methods (SHELXS97 [24]), and developed by full least-squares refine-
ment on F2
(SHELXL97 [24]). Crystallographic figures were prepared
using XSEED [25] which incorporates POVRAY [26].
2.4.1. X-ray structure determination of [Fe(LMe)2][BF4]2ꢁxH2O
(1[BF4]2ꢁxH2O; x ꢂ 1)
The asymmetric unit contains two half-molecules of the com-
1
plex, labelled ‘A’ and ‘B’. Fe(1A) spans the C2 axis ½, y, 4, while
2.4.3. X-ray structure determinations of [Fe(LMe)2][PF6]2 (1[PF6]2) and
[Fe(LBz)2][BF4]2 (3[BF4]2)
1
Fe(1B), N(2B), C(5B), N(12B) and C(15B) lie on 0, y, 4. There are
also two unique anions and a disordered solvent site lying on gen-
eral crystallographic positions. Both anions are disordered over
two sites, with independently refined occupancy ratios close to
0.6:0.4. The refined restraints B–F = 1.40(2) and Fꢁ ꢁ ꢁF = 2.29(2) Å
were applied to these residues. The disordered solvent site con-
tained three Fourier peaks separated by 1.2 Å, that lie within
hydrogen-bonding distance of one of the BF4ꢀ anion sites, and also
with its own symmetry equivalent related by the crystallographic
inversion centre ꢀx, ꢀy, 1 ꢀ z. These were refined as three partial
water sites, whose occupancies summed to 1. All non-H atoms
with occupancy >0.5 were refined anisotropically, while C-bound
H atoms were placed in calculated positions and refined using a
riding model. The partial water H atoms could not be located and
are not included in the model, but are accounted for in the density
and F(000) calculations. The highest residual Fourier peak of
+1.0 e Åꢀ3 lies one of the disordered anions.
The asymmetric units of these crystals contain one complete
formula unit, with all the molecules lying on general crystallo-
graphic sites. No disorder was detected during the refinements,
and no restraints were applied. All non-H atoms were refined
anisotropically, and H atoms were placed in calculated positions
and refined using a riding model.
2.4.4. X-ray structure determinations of [Fe(LAll)2][BF4]2 (2[BF4]2)
The asymmetric unit contains one-quarter of
a
complex
3
7
dication, with Fe(1) lying on the 4 site
, 8, ½, and N(2) and
4
3
C(5) on the C2 axis 4, yꢀ, ½. There is also one-third of a crystallo-
graphically ordered BF4 ion, with B(14) and F(16) lying on the
C3 axis x, x, x; and, a second anion site with occupancy 1/6 lying
3
5
on a general crystallographic position close to the 4 site 1,
,
.
4
8
The latter residue has very high displacement ellipsoids and is
certainly disordered, although attempts to model this using two
or three partial anion sites led to unstable refinements. It was
therefore refined as a single site in the final least squares cycles,
subject to the fixed restraints B–F = 1.35(2) and Fꢁ ꢁ ꢁF = 2.20(2) Å.
The unique allyl substituent is also disordered, and was refined
over three orientations labelled ‘A’ (occupancy 0.60), ‘B’ (0.25)
and ‘C’ (0.15). The fixed restraints C–N = 1.47(2), C–C = 1.51(2),
C = C = 1.34(2), 1,3-C...N = 2.43(2) and 1,3-Cꢁ ꢁ ꢁC = 2.46(2) Å were
applied to this residue. All crystallographically ordered non-H
2.4.2. X-ray structure determination of [Fe(LMe)2][ClO4]2 (1[ClO4]2)
The structure was originally solved in the monoclinic space
group C2, then transformed up to R32 following an initial refine-
ment using the ADSYMM routine in PLATON [27]. The asymmetric unit
contains: half a complex cation, with Fe(1) spanning the C2 axis x,
x + 1
,
6; one-third of a ClO4ꢀ ionꢀ, with the Cl and one O atom lying
1
3
on the C3 axis 0, 0, z; half a ClO4 ion whose Cl atom spans the C2
Table 2
Selected bond distances and angles for the crystal structures of [Fe(LMe)2]2+ (12+) salts (Å, °).
a, R and H are indices showing the spin state of the complex [7,34], while h and / are
measures of the angular Jahn–Teller distortion sometimes shown by [Fe(bpp)2]2+ centres in their high-spin state (see the text for details) [35,36]. Typical values of these
parameters in [Fe(bpp)2]2+-type derivatives are given in Ref. [7].
1[BF4]2ꢁxH2O (half-molecule A)
1[BF4]2ꢁxH2O (half-molecule B)
1[ClO4]2
1[PF6]2
FeꢀN{pyridyl}
1.958(5)
2.019(4), 2.020(4)
78.8(3)
98.3(6)
317
2.034(7)
2.048(6), 2.079(5)
77.2(2)
113.7(5)
360
2.079(6)
2.1503(16), 2.1539(19)
2.1981(17)–2.2843(17)
74.21(14)
147.8(2)
456
FeꢀN{pyrazolyl}
2.140(3)–2.329(6)a
a
a
73.7(2)–75.0(2)
a
R
H
/
h
146.1(7)–153.9(7)
413–467a
178.6(3)
86.28(5)
180
89.28(5)
178.78(16)
83.33(6)–88.13(5)
168.92(7)
78.93(2)
a
a
Range of values given for all the ligand disorder sites in the molecule.
Table 3
Selected bond distances and angles for the crystal structures of salts of [Fe(LAll)2]2+ (22+), [Fe(LBz)2]2+ (32+) and [Fe(LiPr)2]2+ (42+) (Å, °). See Table 2 and Refs. [7] and [34–36] for the
definitions of the distortion parameters , h and /.
a, R, H
2[BF4]2
3[BF4]2
4[PF6]2ꢁ2CH3CN (molecule A)
4[PF6]2ꢁ2CH3CN (molecule B)
FeꢀN{pyridyl}
2.129(3)
2.217(3)
74.53(6)
140.1(2)
440
2.1357(15), 2.1436(14)
2.2000(15)–2.2497(15)
74.43(12)
141.8(2)
447
2.147(2), 2.156(2)
2.203(2)–2.255(2)
73.91(17)
146.7(3)
457
2.144(2), 2.145(2)
2.192(2)–2.275(2)
74.08(17)
145.7(3)
456
FeꢀN{pyrazolyl}
a
R
H
/
h
180
90
171.00(6)
88.11(2)
175.94(8)
87.80(3)
173.50(9)
89.51(3)
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T.D. Roberts et al. / Polyhedron xxx (2013) xxx–xxx
5
Fig. 2. Partial packing diagram of 1[ClO4]2, showing the intermolecular steric clash
that leads to the ligand disorder. The intermolecular Cꢁ ꢁ ꢁC contacts of 2.8 Å shown
in red, between the ‘B’ disorder site on adjacent molecules related by –x, –x + y, –z,
mean that this site cannot be simultaneously occupied in both molecules in each
pair. All atoms have arbitrary radii, and the view is along the [110] vector, with the c
axis vertical. Colour code: C (ligand backbone), dark gray; C (substituents), white; H,
pale gray; N, blue; Fe, green. (Colour online.)
atoms, plus the major allyl group disorder site, were refined aniso-
tropically while H atoms were placed in calculated positions and
refined using a riding model.
Fig. 3. Comparison of the molecular structures of the [Fe(LMe)2]2+ dications in
1[BF4]2ꢁxH2O (molecule B; top), 1[ClO4]2 (centre) and 1[PF6]2 (bottom). The view in
each case is along the Fe(1)–N(2) bond. All atoms have arbitrary radii, H atoms are
omitted for clarity, and both disorder orientations of the LMe ligand in 1[ClO4]2 are
shown. Colour code: C, white; N, blue; Fe, green. (Colour online.)
2.4.5. X-ray structure determination of [Fe(LiPr)2][PF6]2ꢁ2MeCN
(4[PF6]2ꢁ2MeCN)
There are two formula units in the asymmetric unit (Z0 = 2),
whose complex cations are labelled ‘A’ and ‘B’. Two of the four un-
ꢀ
ique PF6 ions are disordered, one of them over three sites with a
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6
T.D. Roberts et al. / Polyhedron xxx (2013) xxx–xxx
also applied. Magnetochemical calculations and graph preparation
were carried out using SIGMAPLOT [29]. Susceptibility measurements
in solution were obtained by Evans method using a Bruker DRX500
spectrometer operating at 500.13 MHz [30]. A diamagnetic correc-
tion for the sample [28], and a correction for the variation of the
density of the (CD3)2CO solvent with temperature [31], were ap-
plied to these data.
3. Results and discussion
Alkylation of 3-bpp is achieved by deprotonating preformed 3-
bpp with lithium hydride, and reacting the resultant dianion with
appropriate alkyl halides [15,16]. The desired N1,N10-disubstituted
products LMe [13], LAll [16], LBz and LiPr were cleanly obtained in
each case, with no evidence for competitive alkylation at the pyr-
azole N2 sites [32]. This presumably reflects protection of the N2
sites of the doubly deprotonated [3-bpp–2H]2ꢀ intermediate by
lithium ion chelation [33]. The salts [Fe(LMe)2]X2 (1X2; Xꢀ = BF4
,
ꢀ
ClO4ꢀ and PF6ꢀ) were prepared by treatment of the appropriate ir-
on(II) salt with 2 equiv of LMe in nitromethane at room tempera-
ture. Similar reactions with the other ligands afforded
[Fe(LAll)2][BF4]2 (2[BF4]2) [Fe(LBz)2]ꢀ[BF4]2 (3[BF4]2) and [Fe(LiPr)2]Y2
(4Y2, Yꢀ = BF4 and PF6ꢀ; the PF6 salt of this complex was also
ꢀ
ꢀ
studied because it afforded better single crystals than the BF4
salt).
Single crystal X-ray analyses at 150 K were obtained of all the
complex salts in this work, except for 4[BF4]2 (Fig. 1, Tables 2
and 3). The crystallography of the compounds is quite varied, in
that only 1[PF6]2 and 3[BF4]2 contain one complete formula unit
with no internal symmetry in their asymmetric units. 1[BF4]2 crys-
tallises as a monohydrate phase from undried MeOH/Et2O, with
two half-molecules of the complex spanning crystallographic C2
axes. The asymmetric unit of 1[ClO4]2 contains one C2-symmetric
half-molecule, and is complicated by disorder in the unique LMe li-
gand. This disorder is a consequence of a close intermolecular con-
tact between one of the two unique pyrazole rings and its
symmetry equivalent related by ꢀx, ꢀx + y, ꢀz. Only the ‘A’ disor-
der site in one molecule of this pair, and the ‘B’ disorder site of
the other, can be occupied at the same time, with a random distri-
bution of ‘A’ and ‘B’ occupancies in each pair of half-cations
throughout the crystal (Fig. 2). Crystalline 2[BF4]2 adopts a cubic
space group with one-quarter of a formula unit in the asymmetric
unit. The unique allyl substituent is extensively disordered, reflect-
ing the presence of a neighbouring BF4ꢀ anion site that is only part-
occupied on charge neutrality grounds. Finally, the solvate 4[PF6]2
ꢁ2MeCN contains two formula units in its asymmetric unit (i.e.
Z0 = 2), with only minor structural differences between the two un-
ique complex dications.
Fig. 4. Top: solid state variable temperature magnetic susceptibility data for
1[BF4]2 (d), 1[ClO4]2
( ) and 1[PF6]2 (h). Variable temperature susceptibility
measurements for the other complexes in this study are essentially identical to
these data. Bottom: Magnetic susceptibility data in (CD3)2CO solution for 1[BF4]2
(d), 2[BF4]2
(
), 3[BF4]2 (e) and 4[BF4]2ꢁH2O (s).
0.4:0.4:0.2 occupancy ratio, and the other over two equally occu-
pied orientations. The refined restraints P–F = 1.60(2) and cis-
Fꢁ ꢁ ꢁF = 2.61(2) Å were applied to those residues. One isopropyl
group [C(13A)–C(15A)] was also disordered over two sites with re-
fined occupancies of 0.8 and 0.2, and was modelled with the fixed
restraints C–N = 1.48(2), C–C = 1.52(2), 1,3-Cꢁ ꢁ ꢁN = 2.44(2) and 1,3-
Cꢁ ꢁ ꢁC = 2.48(2) Å. Finally one solvent molecule was modelled over
two half-occupied sites, that were refined without restraints. All
non-H atoms with occupancy P0.5 were refined anisotropically,
except for the disordered solvent sites. All H atoms were placed
in calculated positions and refined using a riding model.
The iron centres in 1[PF6]2, 2[BF4]2, 3[BF4]2 and 4[PF6]2 are all
clearly high-spin at this temperature (Tables 2 and 3), based on
their Fe–N distances and the distortion parameters
R and H (these
measure the deviation of the FeN6 polyhedron from an ideal octa-
hedral geometry, which is always significantly larger in the high-
spin state [34]). Consideration of 1[ClO4]2 is complicated by its li-
2.5. Other measurements
Electrospray mass spectra were obtained using a Waters Micro-
mass ZQ4000 spectrometer from MeCN solution. CHN microanaly-
ses were performed by the University of Leeds Department of
Chemistry microanalytical service. Infra-red spectra were obtained
as Nujol mulls pressed between NaCl windows between 600 and
4000 cmꢀ1, using a Nicolet Avatar 360 spectrophotometer. TGA
measurements employed a TA Instruments TGA 2050 analyser.
Magnetic susceptibility measurements were obtained using a
Quantum Design SQUID magnetometer in an applied field of
1000 G. Diamagnetic corrections were estimated from Pascal’s con-
stants [28], and a diamagnetic correction for the sample holder was
gand disorder (Fig. 2), but R and H imply that this complex is also
high-spin at 150 K (Table 2). The only exception to this trend is
1[BF4]2ꢁxH2O, whose asymmetric unit contains two unique half-
molecules that are predominantly low-spin at 150 K on the basis
of their metric parameters. While molecule A is apparently fully
low-spin, however, molecule B has a detectable residual high-spin
fraction at that temperature according to its larger Fe–N bond
lengths and distortion parameteres (Table 2). Interestingly, like
1[BF4]2ꢁxH2O, [Fe(Me2-1-bpp)2][BF4]2 also crystallises as a hydrate
phase from undried methanol, although the two compounds are
not isostructural [22].
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T.D. Roberts et al. / Polyhedron xxx (2013) xxx–xxx
7
Fig. 5. Space-filling views of the [Fe(LMe)2]2+ dication in 1[BF4]2ꢁxH2O (molecule B; top left); [Fe(LAll)2]2+ in 2[BF4]2 (top right); [Fe(LBz)2]2+ in 3[BF4]2 (bottom left); and
[Fe(LiPr)2]2+ in 4[PF6]2ꢁ2CH3CN (molecule A, bottom right). The allyl substitutents in [Fe(LAll)2]2+ are plotted in different disorder orientations, while only the major orientation
of the disordered isopropyl group in the molecule of [Fe(LiPr)2]2+ is shown. Colour code: C (ligand backbone), dark gray; C (substituents), white; H, pale gray; N, blue; Fe, green.
(Colour online.)
Table 4
Comparison of the metric parameters determining the steric influence of the methyl substituents in crystalline salts of [Fe(LMe)2]2+ (12+) and [Fe(Me2-1-bpp)2]2+ (Me2-1-
bpp = 2,6-bis{3-methylpyrazol-1-yl}pyridine). See Fig. 6 for the definitions of the parameters a–e. All the crystal structures are of the high-spin states of the complexes, unless
otherwise stated.
[Fe(LMe)2]X2
Xꢀ = BF4
ClO4
PF6
ꢀa
ꢀb
ꢀ
a (Fe–N)
b (N–N)
c (N–C)
d (Fe–N–N)
e (N–N–C)
2.019(4)–2.101(4)
1.367(7)–1.382(8)
1.365(11)–1.468(8)
138.1(4)–139.2(4)
121.2(6)–124.0(7)
2.176(3)
1.365(4)
1.458(5)
138.3(2)
120.4(3)
2.1981(17)–2.2843(17)
1.367(2)–1.377(2)
1.473(3)–1.481(3)
137.74(14)–140.26(14)
120.09(19)–121.52(17)
Xꢀ = BF4 [22]
ClO4
[42]
SbF6 [37]
ꢀ
ꢀ
a
ꢀ
[Fe(Me2-1-bpp)2]X2
a (Fe–N)
b (N–C)
c (C–C)
d (Fe–N–C)
e (N–C–C)
2.178(2)–2.204(2)
1.327(4)–1.335(4)
1.477(5)–1.493(5)
140.1(2)–140.4(2)
120.6(3)–121.8(3)
2.164(3)–2.175(3)
1.319(5)–1.334(5)
1.466(6)–1.494(8)
140.1(3)–140.8(3)
121.0(4)–122.8(4)
2.169(3)–2.214(3)
1.331(5)–1.337(5)
1.489(5)–1.495(5)
139.7(2)–141.1(3)
119.9(3)–121.8(4)
a
This complex has a mixed high:low-spin state population at the temperature of measurement.
Only the parameters from the crystallographically ordered part of the ligand are given.
b
Comparison of the three 1X2 salts (Xꢀ = BF4ꢀ, ClO4 and PF6
)
but has rarely been seen thus far in their [Fe(3-bpp)2]2+ analogues
[7]. The distortion involves a reduction in the trans-N{pyridyl}–Fe–
N{pyridyl} angle (/) from its ideal value of 180°, and/or a twisting
of the two tridentate ligands away from the perpendicular (h < 90°,
where h is the dihedral angle between the least squares planes of
the two ligands). Distorted [Fe(bpp)2]2+ complexes in the solid
phase are trapped in their high-spin state, since the structural
ꢀ
ꢀ
sheds some light on their different behaviour in the crystal
(Fig. 3). The iron coordination geometry in 1[PF6]2 is unique in this
study, in being significantly distorted from the ideal D2d symmetry
associated with a [Fe(bpp)2]2+ centre (Table 2). This angular distor-
tion is a manifestation of the Jahn–Teller effect in a high-spin d6 ion
[35–37], which is common in high-spin [Fe(1-bpp)2]2+ derivatives
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8
T.D. Roberts et al. / Polyhedron xxx (2013) xxx–xxx
changes required to convert them to their (undistorted) low-spin
forms are too great to be accommodated by a rigid solid lattice.
These angles in 1[PF6]2 imply a significant Jahn–Teller distortion
is present in that salt, since they are significantly lower than their
ideal values and lie in the range where spin-crossover would not
normally be observed (Table 2) [7]. The Jahn–Teller distortion in
1[ClO4]2 is less clear cut because of the ligand disorder but is
clearly smaller (Table 2), with a combination of distortion angles
/ = 178.8° and h P 83.3° that would not preclude spin-crossover
based on our earlier work [7]. Rather, the twisted ligand conforma-
tion in the cation [37] and the close intermolecular contacts that
give rise to the ligand disorder [38] are more likely to contribute
to the inhibition of spin-crossover in that salt. In comparison, the
fully and predominantly low-spin iron centres in 1[BF4]2ꢁxH2O ex-
hibit almost perfect D2d molecular symmetry, with regular coordi-
nation geometries and essentially planar LMe ligands (Fig. 3).
Although some of the compounds contain occluded solvent in
the crystalline state, after drying they all afford solvent-free bulk
powders by elemental microanalysis except 4[BF4]2, which analy-
ses as a monohydrate. Variable temperature magnetic susceptibil-
ity measurements showed that bulk samples of all these solids are
high-spin between 5 and 300 K (Fig. 4, top; the reduction in
vMT
below 50 K is not caused by spin-crossover, but reflects zero-field
splitting of the high-spin iron centres [39]). In most cases that is
consistent with their crystal structures, which contain high-spin
iron centres at 150 K. The exception is 1[BF4]2, which is almost
fully low-spin in its hydrated crystal at 150 K but is high-spin as
a solvent-free powder (the anhydrous nature of the sample was
confirmed by a TGA analysis, which showed <0.2% mass loss below
500 K). Evidently, dehydration of crystalline 1[BF4]2ꢁxH2O causes a
structure change with a concomitant change in spin-state. At-
tempts to shed light on this by growing single crystals of sol-
vent-free 1[BF4]2 have been unsuccessful, however.
Further insight into the spin-state properties of the complexes
was gained from their solution behaviour, which was determined
by variable temperature Evans method measurements in (CD3)2CO
[30]. The BF4ꢀ salts of all four complexes remain fully high-spin in
this solvent, between 193 and 293 K (Fig. 4, bottom). This is unex-
pected, since the parent complex [Fe(3-bpp)2][BF4]2 undergoes
Fig. 6. Comparison of the metal:ligand geometry in [Fe(LMe)2]2+ (12+, top) and
[Fe(Me2-1-bpp)2]2+ (bottom; Me2-1-bpp = 2,6-bis{3-methylpyrazol-1-yl}pyridine).
The labels a–e refer to the parameters listed in Table 4. Colour code: C, white; N,
blue; Fe, green. (Colour online.)
spin-crossover with a midpoint temperature T = 247 K under the
½
same conditions [40]. The methyl and isopropyl substituents in
1[BF4]2 and 4[BF4]2ꢁH2O are electron-donating, which should stabi-
lise the low-spin state of those complexes on inductive grounds,
steric clashes during a spin-crossover equilibrium in solution.
Moreover, there are no prohibitive intramolecular steric clashes
involving the methyl groups in the salts of 12+, while the predom-
inantly low-spin nature of 1[BF4]2ꢁxH2O at 150 K shows that
[Fe(LMe)2]2+ can indeed undergo spin-crossover in principle
(Fig. 5). Hence, the absence of spin-crossover in solution for
1[BF4]2–3[BF4]2 cannot be explained from steric effects alone,
and thus raise T . Since this is not observed, the steric influence
½
of the pyrazole substituents in 1[BF4]2-4[BF4]2ꢁH2O must be more
important in determining their spin-states. Notably the iron com-
plex of the corresponding 1-bpp derivative, 2,6-di(3-isopropyl-
pyrazol-1-yl)pyridine, is high-spin under the same conditions
which was also attributed to the steric properties of the isopropyl
groups in that compound [36,41].
and must reflect
influences.
a careful balance of steric and electronic
Space-filling plots of the crystal structures identify close intra-
molecular C–Hꢁ ꢁ ꢁ
p
interactions of 2.6–2.9 Å in 2[BF4]2, 3[BF4]2
4. Conclusion
and 4[PF6]2 between the allyl, benzyl or isopropyl ‘R’ substituents
of one LR ligand and the pyridyl ring of the other (Fig. 5). Such close
contacts will prevent spin-crossover on steric grounds, by inhibit-
ing the associated contraction of the Fe–N bonds [41]. In [Fe(Li-
Pr)2]2+ this steric repulsion is a consequence of the bulk of the
isopropyl groups. The steric influence of the ligand substituents
in [Fe(LAll)2]2+ and [Fe(LBz)2]2+ is less clear, but in both crystal struc-
tures there are individual allyl and benzyl groups oriented to form
comparably close inter-ligand contacts (Fig. 5). In the solid state
the orientations of these substituents are fixed by the surrounding
lattice, which would thus inhibit spin-crossover as observed. How-
ever the allyl group disorder in 2[BF4]2, and the different benzyl
conformations in 3[BF4]2, imply that these substituents have a de-
gree of conformational flexibility which would ameliorate these
Alkylation of the four pyrazolyl N1 sites in [Fe(3-bpp)2]2+ sup-
presses the thermal spin-crossover undergone by this complex,
regardless of the steric and electronic properties of the new sub-
stitutents. Although the crystal structure of 1[BF4]2ꢁxH2O shows
that complex at least can undergo spin-crossover under certain
conditions, all the other complex salts in this work remain high-
spin at all the temperatures examined, in solution and the solid
state. For 4X2 (Xꢀ = BF4 and PF6ꢀ) this was to be expected, owing
ꢀ
to the steric influence of the LiPr isopropyl substituents [36,41]. For
the other complexes, whose ligand substituents are less bulky, the
origin of their high-spin nature is less clear.
This result contrasts with the known chemistry of analogous
complexes from the [Fe(1-bpp)2]2+ series [7]. Although solution
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T.D. Roberts et al. / Polyhedron xxx (2013) xxx–xxx
9
phase data were not reported, some salts of [Fe(1-bpp)2]2+ deriva-
tives bearing methyl [22] or hydroxymethyl [38] substituents at
the pyrazole C3 position do undergo thermal spin-crossover in
the solid state. Moreover, the steric influence of the distal subsitu-
tents in 1,1-disubstituted-3-bpp (LR) and the corresponding 3,3-
disubstituted-1-bpp ligands is essentially identical (Table 4).
Hence it was unexpected that none of the complexes in this work
should exhibit spin-crossover as bulk materials, or in solution.
Conversely, however, for those compounds where a direct compar-
ison is available, none of the complexes in this work is isostructural
with its corresponding salt from the [Fe(1-bpp)2]2+ series
[22,36,42]. Therefore, although the differences between [Fe(LR)2]2+
and [Fe(1-bpp)2]2+ derivatives bearing the same distal ‘R’ substitu-
ents are small at the molecular level (Table 4 and Fig. 6), they are
clearly sufficient to change their solid state chemistry.
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Appendix A. Supplementary material
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CCDC 912782, 912783, 912784, 912785, 912786 and 912787,
respectively, contain the supplementary crystallographic data for
1[PF6]2, 1[BF4]2ꢁxH2O, 1[ClO4]2, 4[PF6]2ꢁ2CH3CN, 3[BF4]2 and
2[BF4]2. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.
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Please cite this article in press as: T.D. Roberts et al., Polyhedron (2013), http://dx.doi.org/10.1016/j.poly.2013.01.057