N,N′-bis(2,2′-bipyridine-6-ylmethyl)-2,2′- biphenylenediamines: A tuneable ligand scaffold for room temperature Fe 2+ SCO complexes

Article abstract of DOI:10.1002/ejic.201001139

Condensation and subsequent reduction of 2,2′-diaminobiphenyls 5 with 6′- and 5′-substituted 6-carbaldehyde-2,2′-bipyridines 4 yielded N,N′-bis(2,2′-bipyridine-6-ylmethyl)-2,2′-biphenyl- enediamines 7, which were employed as hexadendate ligands with N6 donor sets in the synthesis of dicationic [Fe²⁺(7-κ⁶N)] complexes 8. Dependent on the substitution pattern the respective complexes are found in the HS state (8b and 8c) or show SCO behaviour. By means of temperature- dependent susceptibility measurements, usingEvans' method, the thermodynamic parameters ΔH, ΔS and T_1/2 for the SCO have been determined. T_1/2 as well as ΔH are remarkably susceptible to substitution next to the central C-C bond of the biphenyl bridge. A new series of ligands with an AB₂ structure were synthesized and employed in Fe²⁺ SCO complexes. ΔH, ΔS and T_1/2 for the SCO were determined using Evans' method. The results clearly show a dependency of the spin state on the substitution pattern. The steric effects of substituents placed at the 6 and 6′ positions in the biphenyl moiety are effectively transported through the biphenyl bridge. Copyright

Full text of DOI:10.1002/ejic.201001139

FULL PAPER
DOI: 10.1002/ejic.201001139
N,NЈ-Bis(2,2Ј-bipyridine-6-ylmethyl)-2,2Ј-biphenylenediamines: A Tuneable
Ligand Scaffold for Room Temperature Fe2+ SCO Complexes
Holm Petzold*[a] and Silvio Heider[a]
Keywords: Iron / Spin crossover / N ligands / Steric effects / Solution studies
Condensation and subsequent reduction of 2,2Ј-diaminobi-
phenyls 5 with 6Ј- and 5Ј-substituted 6-carbaldehyde-2,2Ј-
bipyridines 4 yielded N,NЈ-bis(2,2Ј-bipyridine-6-ylmethyl)-
2,2Ј-biphenyl-enediamines 7, which were employed as hexa-
state (8b and 8c) or show SCO behaviour. By means of tem-
perature-dependent susceptibility measurements, using
Evans’ method, the thermodynamic parameters ΔH, ΔS and
T1/2 for the SCO have been determined. T1/2 as well as ΔH
dendate ligands with N6 donor sets in the synthesis of dicat- are remarkably susceptible to substitution next to the central
ionic [Fe2+(7-κ6N)] complexes 8. Dependent on the substitu-
C–C bond of the biphenyl bridge.
tion pattern the respective complexes are found in the HS
and even more sophisticated tuning is necessary to obtain
SCO in a defined temperature range. Synthesis of multiden-
tate ligand systems that allow this fine tuning is not trivial.
Here we wish to report on our strategy to design Fe2+ SCO
complexes with N6 chelate ligands that show spin crossover
near room temperature in solution.
Introduction
Since the first discovery and interpretation of spin cross-
over (SCO) systems in the last century,[1] this class of com-
plexes has attracted much interest because of its possible
applications, for example, in data storage, molecular
switches or sensors.[2] Promising candidates are iron(II)
complexes as they show the most significant changes in
their properties upon SCO from a high-spin state (HS),
which is paramagnetic, to a low-spin state (LS), resulting in
a diamagnetic complex. Pairing of four electrons and subse-
quent depopulation of all antibonding eg orbitals in an oc-
tahedral Fe2+ complex goes along with substantial shorten-
ing of the metal ligand bonds, when comparing the LS with
the HS.[3,4] These large changes can be transported to
neighbouring complexes in the crystal lattice. Therefore,
most research on Fe2+ SCO complexes is focused on SCO
in the solid state, mainly aimed to find hysteresis.[5] Com-
pared with the number of investigations in the solid state,
the spin crossover behaviour of complexes in solution is sel-
dom investigated.[6–9] This is because most SCO systems
lack the required stability in diluted solutions, especially in
the presence of air and/or water. As SCO complexes can
only have a well-defined ligand field and consequently sta-
bilisation energy, they are substitution labile. To prevent
these systems from ligand substitution and oxidation, com-
plexes are often designed using chelating ligands.[4,10–12] To
obtain SCO complexes the ligand has to be tuned precisely
Results and Discussion
In order to have a useful flexibility for tuning the ligand
field of the resulting complexes we decided to build up the
potential ligands with an AB2 structure, where A is a 2,2Ј-
diaminobiphenyl (5) and B a 2,2Ј-bipyridine-6-carbal-
dehyde (4) that was first used by Constable et al.[13] to ob-
tain N6 chelating ligands for transition metal complexes.
Moreover, it was reported that the hydrazones derived from
4 react with Fe2+ salts to yield SCO complexes.[14] The two
building blocks A and B can be combined together by con-
densation and subsequent reduction of the formed Schiff
bases 6 (Scheme 2), which is a commonly employed strategy
to prepare substituted 2,2Ј-diaminobiphenyls.[15]
The flexibility of this ligand system depends on the flexi-
bility of the substitution pattern of the building blocks A
and B, respectively. We found that Negishi cross-coupling[16]
of 1 with a 2-bromopyridine derivative gives efficient access
to a broad range of different building blocks B (4)
(Scheme 1). The advantages of our synthetic strategy, over
commonly employed methods,[17] are that the formed zinc
complexes[18] can be isolated in high purity since they are
insoluble in tetrahydrofuran (THF). Therefore column
chromatography for means of purification is not necessary.
Secondly, with our strategy it is possible to introduce dif-
ferent 2-bromopyridine derivatives in a late step of the syn-
thesis, which enables one to convert expensive pyridine de-
[a] Institut für Chemie, Anorganische Chemie, Technische
Universität Chemnitz,
Strasse der Nationen 62, 09111 Chemnitz, Germany
Fax: +49-371-53137463
E-mail: holm.petzold@chemie.tu-chemnitz.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201001139.
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H. Petzold, S. Heider
FULL PAPER
rivatives very economically (Scheme 1). Moreover, 1 can be
prepared on a large scale and in good yield. Suitable bi-
phenyl building blocks A are commercially available or can
be easily prepared by Ullmann coupling.[19,20]
Scheme 2. Synthesis of the hexadentate molecules 7 with AB2 struc-
ture by condensation of 2,2Ј-diaminobiphenyls (5) with 2,2Ј-bipyr-
idine-6-carbaldehydes (4) and subsequent reduction with NaBH4.
Scheme 1. Synthesis of 4a–d by a Negishi cross-coupling reaction
(DMF = dimethylformamide).
Diimines 6 can simply be prepared by stirring 4 and 5 in
a 2:1 ratio in anhydrous ethanol (or diethyl ether) without
any addition of water binding additives or catalytic
amounts of acids. In contrast to the starting materials the
formed Schiff bases 6 (Scheme 2) are insoluble in ethanol
and are best isolated by centrifugation. The imines are gen-
erally well soluble in chlorinated solvents like dichlorometh-
ane but tend to hydrolyse rapidly if any traces of water are
present. The obtained colourless solids have been used
without further purification. Treatment of the Schiff bases
with NaBH4 in ethanol/NMP(N-methyl-2-pyrrolidone) af-
forded amines 7 (Scheme 2). These compounds are isolated
in 60–80% yield in good purity as oily substances that are
difficult to crystallize. The presented derivatives 7 crystallize
Figure 1. ORTEP diagram of the molecular structure of 7c in the
solid state and the applied numbering scheme for amines 7 and
complexes 8.
In order to obtain the complexes 8a–c the amines 7a–c
upon treatment with a suitable cocrystallizing solvent have been treated with [FeCl2(THF)1.5] or [Fe(H2O)7(SO4)]
[MeOH (7a, 7e), EtOH (7b–7d, 7g), CH2Cl2 (7f)] as crystal- in ethanol (Scheme 3). After appropriate workup and ex-
2–
–
–
line materials with excellent purity making further purifica- change of the counterions Cl– or SO4 by BF4 or PF6 ,
tion unnecessary. Crystallization of 7c from hot [D6]DMSO complexes 8 could be isolated as deep red to purple solids.
allowed the isolation of single crystals suitable for X-ray The parent complex 8a is found to be a low spin Fe2+ com-
structure analysis. Part a of Figure 1 shows the molecular plex. Marginal broadening of proton resonances of the pro-
structure of 7c in the solid state, interestingly the crystals tons near to the Fe2+ ion indicates a small paramagnetic
–
–
include no solvent molecules even though the crystallization contribution (vide infra). The PF6 and BF4 salts of 8 are
requires a cocrystallizing solvent. The packing is dominated very soluble in acetonitrile and DMF but only sparingly
by π stacking of the bipyridyl moieties that are completely soluble in diethyl ether, dichloromethane and THF; they
flattened and adopt a typical trans arrangement of the pyr- partially decompose by decomplexation in dmso solution.
idine nitrogen atoms. One of the two amino groups of the Upon exposure to air all complexes 8 are oxidized to the
2,2Ј-biphenylenediamine bridge forms a weak hydrogen corresponding imine compounds over a period of several
bond to the adjacent amino nitrogen [N1–H···N4: days, this process is much slower for the HS than for the
2.9670(17) Å], while the other one forms a shorter hydrogen LS complexes.[21] This oxidation is even more severe if an
bond to the nearby pyridine nitrogen [N4–H···N5: excess of Fe2+ or Fe3+ ions are present, for example, during
2.6205(15) Å].
the synthesis. We used a large excess of ascorbic acid and a
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Tuneable Ligand Scaffold for Iron SCO Complexes
small amount of sodium borohydride to establish a reduc-
ing environment as well as NH4Cl to prevent deprotonation
of the amino groups – initial step for oxidation – by apply-
ing slightly acidic conditions. Precipitation of the complexes
by the addition of an NH4PF6 solution in water allowed an
easy and efficient separation of 8a,d–g from an excess of
Fe2+/3+ remaining in solution. Once “free Fe2+/3+” has been
removed the complexes are stable against oxidation in solu-
tion and in the solid state but should best be stored as solids
at low temperature to prevent oxidation.[21]
Figure 2. Molecular structures of complexes 8a and 8b in the solid
state. Counter ions and solvent molecules are omitted for clarity;
probability level is 50%.
angle from the ideal angle of 90°. The angles N3–Fe–N5
[8b: 109.89(7)°; 8c: 118.43(6)°] and N2–Fe–N6 [8b:
118.69(7)°; 8c: 117.19(6)°] are remarkably large. The in-
creased angles are caused by the steric influence of substitu-
ents at the 6Ј position bound to C23 and C34 of the bipyr-
idyl moiety.[4] The Fe–N bond lengths agree well with the
determined spin states, i.e. high-spin in the cases of 8b and
8c, while low-spin for the other Fe2+ complexes at 153 K
(also see Table 1). As expected from known SCO systems
the differences in the metal–ligand distances in HS vs. LS
complexes are 0.18–0.25 Å.[4,11–13]
Table 1. Magnetism at room temperature and thermodynamic data
of the SCO of selected complexes.
Complex
μeff (25 °C)[a]
[μB]
ΔH
ΔS
T1/2
Δ(α – β)[b]
[kJmol–1] [JK–1 mol–1] [K]
[°]
Scheme 3. Synthesis of complexes 8 by treatment of the amines 7
8a (SCO)
8b (HS)
8c (HS)
8d (SCO)
8e (SCO)
8f (SCO)
8g (SCO)
0.45
4.90
5.10
0.50
0.82
1.43
1.20
22.5
–
–
56
–
–
n.d.
55
56
n.d.
403
–
–
n.d.
375
341
n.d.
2.8(10)
1.1(4)
1.6(3)
n.d.
5.0(4)
7.8(4)
n.d.
with [FeCl2(THF)1.5].
n.d.[c]
20.6
19.1
n.d.
By diffusion of diethyl ether vapour to a solution of the
chloride salt in methanol dark crystals of 8a suitable for X-
ray structure analysis were obtained. Figure 2 (a) shows the
molecular structure of dicationic 8a. In the solid state 8a
forms layers that are connected by π stacking of the bipyr-
idyl moieties and that are separated by the chloride ions
and several disordered solvent molecules. The arrangement
of the donor atom set around the Fe2+ ion is slightly dis-
torted octahedral with a mer arrangement of the two arms.
[a] Determined by Evans’ Method[22] as described in the Support-
ing Information. [b] Difference of torsion angles in biphenyl moi-
ety, expressing steric strain. Determined by X-ray crystallography
at 153 K. [c] n.d.: not determined.
Large substituents in the 6Ј position (bound to C23 and
Because of the better coordination ability of the bipyridine C34) should push the other donor atoms (N1–N6) sur-
moiety the Fe–N bond lengths to the pyridyl nitrogen rounding the Fe2+ ion together. But on closer look at the
atoms are shorter than those to the amine nitrogen atoms biphenyl bridge (Part A) an elongation of about 0.34 Å of
by 0.1–0.2 Å (for details see Table S2).
the intramolecular distance N1···N4 – the two amino nitro-
In a similar manner to 8a, complexes 8b and 8c were gens of the biphenyl moiety – is found; obviously the expan-
obtained. The less intense red colour points to a HS of the sion of the Fe2+ ion upon SCO from a LS to a HS overcom-
Fe2+ ion in both transition metal complexes. Indeed, deter- pensates the steric demand of the methyl or methoxy group
mination of the susceptibility by Evans’ method[22] revealed and is large enough to allow all donor atoms to gain more
that 8b [μeff(25 °C) = 4.9 μb] and 8c [μeff(25 °C) = 5.1 μb] space. The larger distance N1···N4 (3.355 Å) is found in 8b,
are HS complexes at room temperature in solution with μeff with respect to 8c, due to the smaller size of the methoxy
close to the spin only value, resembling the lower C2 sym- group.
metry. Both complexes 8b and 8c have been investigated at
Substituents with size – based on the van der Waals ra-
153 K by single-crystal X-ray structure analysis [Figure 2 dii – between the methoxy group and hydrogen atom (1.5 Å
(b) and S1d in the Supporting Information]. The molecular vs. 1.1 Å) should result in SCO behaviour but unfortunately
structures of 8b and 8c exhibit a distorted octahedral coor- the necessary substitution patterns are not easily accessible.
dination sphere with significant deviations of the N–Fe–N Therefore, we tested a modification of the bridge part A in
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H. Petzold, S. Heider
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order to directly influence the distance N1···N4 and sub- the lack of suitable values for the isotropic shifts hampers
sequently the spin state of the Fe2+ ion. Substitution of accurate fits, the qualitative correlation of mol fraction
building block A biphenyl-2,2Ј-diamine (5a) by 6,6Ј-di- γ(HS) and extent of paramagnetic contribution to the
methylbiphenyl-2,2Ј-diamine (5b), 6,6Ј-dibromo-4,4Ј-di- chemical shift is obvious. Comparing the curves of 8a, 8d
methylbiphenyl-2,2Ј-diamine (5c) or 1,1Ј-binaphthyl-2,2Ј- and 8e underlines the different influence of additional
diamine (5d) results in the formation of complexes 8e, 8f methyl groups bound to C23 and C34 of the biphenyl moi-
and 8g, respectively, which show SCO behaviour near room ety compared to methyl groups placed at C8 and C2 of the
temperature, indicated by broad and shifted signals in the bridge A that approximately differ by a factor of 5. The two
1H NMR spectra (Figure 3).
additional methyl groups in 8d have a minor influence on
T1/2 with respect to 8a; although they are separated from
the iron atom by only four bonds in contrast to those in 8e
that are separated by five bonds.
1
Figure 3. Downfield region of H NMR spectra of complexes 8a,
8e and 8f at 25° in CD3CN solution.
In order to gain more information on the influence of
bridge A on the SCO behaviour of the Fe2+ complexes, tem-
perature-dependent measurements of the susceptibility have
been performed on 8a, 8e and 8f in acetonitrile using Evans’
method.[22] Fitting of measured μeff versus T to a regular
solution model with the assumption that μeff(HS) = 5.2 μb
and μeff(LS) = 0 μb yielded T1/2, ΔH and subsequently ΔS
for the SCO (Figure 4, Table 1). T1/2 is 341 K for 8f and is
the lowest among the investigated complexes; for 8e this
temperate is 375 K; finally for 8a T1/2 is calculated to be
403 K. Interestingly ΔS is equal for all three complexes with
calculated values of 56 JK–1 mol–1, 55 JK–1 mol–1 and
56 JK–1 mol–1 for 8f, 8e and 8a, respectively. The values for
ΔH are markedly different with 19.1 kJmol–1 (8f),
20.6 kJmol–1 (8e) and 22.5 kJmol–1 (8a). The ΔS values fall
in the lower range from 30–130 JK–1 mol–1[4,7,9,23] of re-
ported data, which is not unexpected for complexes with
rigid ligands. The successive population of the HS results
Figure 4. The μeff versus T plot of the measured (measd) values in
the range 25–75 °C with a 10 K step width and the calculated
curves using a regular solution model with best fit parameters given
in the text.
1
in large shifts of the H NMR signals. Most susceptible for
changes in the spin state are the protons in the 6Ј positions
of the bipyridyl moiety bound to C23 and C34, closest to
the paramagnetic Fe2+ centre.[8] As complexes 8a,e–g only
differ in the bridge A, far away from the protons in the 6Ј
positions and the influence of the nearby methyl group in
8d is small, the chemical shift of these protons without
paramagnetic contribution should be similar and hence this
shift can be used to compare the SCO behaviour of the
Figure 5. Plot of the chemical shift of protons bound to C23 and
C34 versus T for complexes 8a,d–g.
The obtained results raise the question which factors in-
complexes 8. Figure 5 shows a plot of the chemical shifts fluence T1/2 and ΔS. Growing single crystals by diffusion of
over the temperature range for 8a,d–g. Recently, it was diethyl ether vapour into a solution of 8e and 8f in acetoni-
shown that the use of chemical shifts instead of volume trile allowed us to perform an X-ray structure analysis on
susceptibility has some advantages in terms of errors and single crystals at 153 K, which shows Fe2+ complexes in the
necessary corrections, especially if sensitive compounds low spin state (see Figures S1e and S1f in the Supporting
have to be handled.[7] Although in this particular case where Information).
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Tuneable Ligand Scaffold for Iron SCO Complexes
thesis of aldehydes 4, amines 7 and complexes 8 and details of the
susceptibility measurements.
If the molecular structures of 8a, 8e and 8f are com-
pared, only minor changes in the Fe–N distances are found.
Even the distances N1···N4 directly bound to the biphenyl
bridge vary only slightly: 3.0068(62) Å in 8a, 3.0304(32) Å
in 8e and 3.0607(31) Å in 8f. Even though the differences
in N1···N4 are on the verge of significance they correlate
well with the order found for T1/2 and ΔH. A similar corre-
lation is found for other structure parameters, for example,
strains in the biphenyl bridge can be estimated by differ-
ences among the torsion angles C6–C1–C7–C12 and C2–
C1–C7–C8, which should be zero in unstrained biphenyls
but differ by 2.8(10)° in 8a, 5.0(4)° in 8e and 7.8(4)° in 8f.
This shows that repulsion of the substituents placed on C8
and C2, separated by four bonds to the donating amine
nitrogens N1 and N4, influence the spin state of the Fe2+
ion. One would expect that the methyl groups and bromine
atoms in 8e and 8f, respectively, are very close; indeed with
a distance of 3.24 Å the two methyl substituents in 8e are
closer then the sum of van der Waals radii (2ϫ2.0 Å), while
in contrast the distance Br1···Br2 (3.71 Å) in 8f is larger
than the sum of the van der Waals radii (2ϫ1.8 Å). The
electronic influence on T1/2 of different substitution pat-
terns in the bridges A in 8e–g is expected to be rather low
as bromine and methyl groups show only small inductive
and mesomeric effects; moreover there is no correlation be-
tween electronic properties with respect to +/–M nor +/–I
Acknowledgments
We would like to acknowledge the Fonds der Chemischen Industrie
for financial support (Liebig Grant to H. P.). We would also like
to thank Prof. H. Lang and Dr. T. Rüffer for valuable discussions.
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the bipyridyl moieties, or in both of them. With the help of
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through the spatial arrangement of the donor atoms N1
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CCDC-786979 (for 7c), -786980 (for 8a), -786981 (for 8b), -786982
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cle): Details of the X-ray structure analyses (Table S1) on 7c, 8a–
c,e,f, ORTEP diagrams (Figure S1), table of selected bond lengths
and angles (Table S2), as well as experimental details for the syn-
Eur. J. Inorg. Chem. 2011, 1249–1254
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Received: October 25, 2010
Published Online: January 21, 2011
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Eur. J. Inorg. Chem. 2011, 1249–1254