Novel heterobimetallic ruthenium(iii)-cobalt(ii) compounds constructed from trans-[RuIII(Q)2(CN)2]- (Q = 8-quinolinolato): Synthesis, structures and magnetic properties

Article abstract of DOI:10.1039/c1cc12446c

Reaction of [Ru^II(PPh₃)₃Cl₂] with HQ and KCN produces a new dicyanoruthenium(iii) building block, [Ru ^III(Q)₂(CN)₂]^-. It reacts with hydrated CoCl₂ in MeOH or DMF to produce a trinuclear compound 2 or a 1-D zigzag chain 3. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1cc12446c

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COMMUNICATION
Novel heterobimetallic ruthenium(III)–cobalt(II) compounds constructed
from trans-[RuIII(Q)2(CN)2]ꢀ (Q = 8-quinolinolato): synthesis,
structures and magnetic propertiesw
Jing Xiang,a Li-Hui Jia,b Wai-Lun Man,a Kang Qian,b Shek-Man Yiu,a Gene-Hsiang Lee,c
Shie-Ming Peng,c Song Gao*b and Tai-Chu Lau*a
Received 27th April 2011, Accepted 21st June 2011
DOI: 10.1039/c1cc12446c
Reaction of [RuII(PPh3)3Cl2] with HQ and KCN produces a new
dicyanoruthenium(III) building block, [RuIII(Q)2(CN)2]ꢀ. It
reacts with hydrated CoCl2 in MeOH or DMF to produce a
trinuclear compound 2 or a 1-D zigzag chain 3.
The use of cyanometallates to design molecule-based magnetic
materials continues to be of great interest to chemists.1,2 We
have recently made use of the building blocks trans-
[RuIII(salen)(CN)2]ꢀ, trans-[RuIII(acac)2(CN)2]ꢀ and mer-
[RuIII(CN-sap)(CN)3]2ꢀ (Fig. S1, ESIw) to construct a series
of 3d-RuIII and 4f-RuIII polynuclear complexes that exhibit a
variety of novel structural and magnetic properties.3 The
design of new ruthenium(III) building blocks continues to be
of interest to us in order to construct 3d-RuIII and 4f-RuIII
polynuclear complexes with stronger magnetic coupling
between the paramagnetic metal centres. Herein we report the
synthesis and characterization of a new dicyanoruthenium(III)
complex, PPh4[RuIII(Q)2(CN)2] (1, Q = 8-quinolinolato).4
Reaction of 1 with hydrated CoCl2 in MeOH or DMF
produces {[RuIII(Q)2(CN)]2(m-CN)2[CoII(MeOH)4]}ꢁ8MeOH (2)
or {[RuIII(Q)2(CN)](m-CN)2[CoII(DMF)2(Cl)]}n (3). 2 is a tri-
nuclear compound that exhibits 3-D long range ferromagnetic
ordering below 4.5 K, while 3 is a 1-D zigzag material that
contains trigonal bipyramidal Co(II) centres and there is weak
ferromagnetic coupling between Ru(III) and Co(II).
Fig. 1 ORTEP diagram of the anion of 1 and thermal ellipsoids are
drawn at 30% probability. (Symmetry codes: (i) ꢀx + 1, ꢀy + 1, ꢀz).
shows a predominant peak at m/z 442, which is assigned to the
parent ion (Fig. S2, ESIw). The cyclic voltammogram of 1 in
CH3CN shows a quasi-reversible reduction wave at ꢀ1.01 V
and an irreversible oxidation wave at Epa = +0.51 V vs.
Cp2Fe+/0 (Fig. S3, ESIw). The IR spectrum shows a single
v(CRN) stretch at 2095 cmꢀ1. The structure of 1 has been
determined by X-ray crystallography.z Fig. 1 shows the structure
of the anion. The ruthenium atom is octahedrally coordinated by
the N2O2 donors from the two Q ligands and the two carbon
atoms of the cyanide ions in a trans configuration. There are
extensive face-to-face p–p interactions of the quinolinolato
ligands along the a-axis, with an average centroid-to-centroid
distance of 3.832 A, resulting in a 1-D ribbon structure (Fig. S4,
+
ESIw). These ribbons are well separated by the counter-ion PPh4
,
thus no direct interaction between ribbons could be observed. The
closest RuꢁꢁꢁRu separation along the ribbon is 9.034 A.
Treatment of [RuII(PPh3)3Cl2] with 8-hydroxyquinoline
(HQ) and KCN in MeOH afforded [RuIII(Q)2(CN)2]ꢀ, isolated
as the PPh4+ salt (1).4 The ESI-MS of 1 (ꢀve mode) in MeOH
Compounds 2 and 3 are obtained by treatment of 1 with
hydrated CoCl2 in MeOH and DMF, respectively.4 The IR
shows v(CRN) stretch at 2126 and 2108 cmꢀ1 for 2 and at
2124 cmꢀ1 for 3. 2 crystallizes in the P21/c group system with
an inversion centre on the CoII atom. The CoII in 2 is octa-
hedrally coordinated by two [RuIII(Q)2(CN)2]ꢀ units through
the nitrogen atoms and by four MeOH molecules in a trans
configuration (Fig. 2). The bond lengths and angles in the
[Ru(Q)2(CN)2]ꢀ units are essentially identical to those of 1.
Extensive face-to-face p–p stacking among the quinolinolato
ligands is retained among the [RuIII(Q)2(CN)2]ꢀ units with a
centroid-to-centroid distance of 3.683 A, which connect the
discrete trinuclear units into 1-D supramolecular double chains
(Fig. S5, ESIw). The closest intermolecular Ruꢁ ꢁ ꢁRu and
Ruꢁ ꢁ ꢁCo separations in 2 are 8.845 and 5.245 A, respectively.
a Department of Biology and Chemistry, City University of Hong Kong,
Tat Chee Avenue, Kowloon Tong, Hong Kong (China).
E-mail: bhtclau@cityu.edu.hk; Fax: (+852)34420522
b Beijing National Laboratory of Molecular Science,
State Key Laboratory of Rare Earth Materials Chemistry and
Applications, College of Chemistry and Molecular Engineering,
Peking University, Beijing 100871, China.
E-mail: gaosong@pku.edu.cn; Fax: (+86) 10-62751708
c Department of Chemistry, National Taiwan University, Taipei 106,
Taiwan
w Electronic supplementary information (ESI) available: Synthesis,
spectroscopic data, magnetic data and crystallographic data. CCDC
821916–821918. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c1cc12446c
c
8694 Chem. Commun., 2011, 47, 8694–8696
This journal is The Royal Society of Chemistry 2011

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Fig. 4 Temperature dependence of wMT (J) and wꢀ1 (&) for 2 (left)
and hysteresis loop at 2 K (right) (Inset: ZFCM/FCM at 10 Oe).
Fig. 2 The perspective view of 2. (Symmetry codes: (a) ꢀx, ꢀy + 1, ꢀz).
consists of one high-spin Co(II) and two low-spin Ru(III) that
are magnetically non-interacting, suggesting the occurrence of
an intratrimer ferromagnetic interaction between Ru(III) and
Co(II) ions through the cyanide bridges. The smooth decrease
of wMT in the high temperature range is mainly due to spin–
orbit coupling effects of Co(II) ions. Below 4.5 K the wMT value
decreases sharply again and reaches a value of 8.13 cm3 molꢀ1
K at 2 K, which might be attributed to low temperature
magnetization saturation. The molar magnetic susceptibility
above 40 K obeys the Curie–Weiss law [wM = C/(T ꢀ y)]
with a Weiss constant y = ꢀ5.7 K and a Curie constant
C = 3.98 cm3 molꢀ1. The small negative value of the Weiss
constant also suggests possible ferromagnetic coupling in 2, as
observed in previous Ru(III)–Co(II) complexes.
Fig. 3 The perspective view of 3. (Symmetry codes: (a) ꢀx + 1,
ꢀy + 1, ꢀz + 1).
Fig. 3 shows the 1-D zigzag chain of 3 consisting of alter-
nating Ru(III) and Co(II) centres that are bridged by CNꢀ. The
Co(II) in 3 has a distorted trigonal bipyramidal geometry,
which is unusual in polymeric Co(II) structures.5 The Co(II) is
bonded to two nitrogen atoms from bridging CNꢀ ligands,
two DMF molecules and a Clꢀ ligand. The Co–O, Co–N and
Co–Cl distances are 2.109(2), 2.041(3) and 2.290(1) A, respec-
tively. The intrachain Coꢁ ꢁ ꢁRu and Ruꢁ ꢁ ꢁRu separations are
5.249 and 8.913 A, respectively. There are again extensive
p–p stackings of the quinolinolato ligands along the a-axis
with an average centroid-to-centroid distance of 3.682 A
(Fig. S6, ESIw).
It is worth noting that the magnetic behaviour below 4.5 K
exhibits a long-range magnetic ordering, which is clearly
evidenced by the low-field magnetization. The temperature
dependence of the zero-field-cooled magnetization and the
field-cooled magnetization (ZFCM and FCM) measured in a
low field of 20 Oe shows a divergence at ca. 4.5 K (Fig. 4 right
(inset)). This behaviour is consistent with a spontaneous
magnetization under a long range magnetic phase transition.
The presence of 3-D long range magnetic ordering is also
evidenced by the ac magnetic data, which exhibit a sharp peak
in the in-phase magnetic susceptibility, w0ac, at 4.5 K. This is in
excellent agreement with the ZFC–FC bifurcation data above.
The nonzero out-of-phase susceptibility, w00ac(T), also supports
the 3-D ferromagnetic-like magnetic ordering nature of 2
(Fig. 5). No frequency dependence was observed and this rules
out the presence of glassy behaviour. The field dependence of
the magnetization at 2 K was also investigated (Fig. S9, ESIw).
The magnetization per [RuIII2CoII] unit displays a steady
increase as the field is raised, reaching a value of 3.30 Nb at
50 kOe, which is close to the expected value for ferromagnetic
coupling between CoII and RuIII (gCoSCo + 2gRuSRu). This can
The temperature dependence of the molar magnetic suscepti-
bility for 1 measured in the temperature range 2–300 K under
an external magnetic field of 1 kOe is illustrated in Fig. S7
(ESIw). The molar magnetic susceptibility above 10 K obeys
the Curie–Weiss law [wM = C/(T ꢀ y)] with Curie constant
C = 0.41 cm3 molꢀ1 K and Weiss constant y = ꢀ2.18 K. The
C value is comparable to those of trans-(Bu4N)[Ru(salen)(CN)2]
and trans-(PPh4)[Ru(acac)2(CN)2].3 The field dependence
magnetization of 1 at 2.0 K is shown in Fig. S8 (ESIw), the
experimental M–H curve is consistent with the Brillouin
function curve and the M value at 50 KOe is 0.67 Nb molꢀ1
,
which is much smaller than the expected saturation value of
1.0 for a S = 1/2 state, assuming g = 2.0.
The temperature dependence of the molar magnetic suscepti-
bility of 2 was measured in the range of 2–300 K under an
external magnetic field of 1 kOe (Fig. 4). The wMT value at
room temperature is 3.92 cm3 molꢀ1 K, which is expected for
uncoupled, one high-spin Co(II) (S = 3/2) and two low-spin
4
2
Ru(III) (S = 1/2) with an orbitally degenerate T1g and T2g
single ion ground state, respectively. On lowering the tempera-
ture, the wMT value decreases slowly and reaches a minimum
value of 3.62 cm3 molꢀ1 K at ca. 40 K. Upon further cooling,
the wMT value increases abruptly and reaches a maximum
value of 15.40 cm3 molꢀ1 K at 4.5 K. The wMT value at the
minimum is well above that calculated for a trimer that
Fig. 5 Temperature dependence of real (w0) and imaginary (w0 0) parts
of the ac susceptibility for 2 measured under various oscillating
frequencies (10, 110, 1000 Hz).
c
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Chem. Commun., 2011, 47, 8694–8696 8695

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[Ru(Q)2(CN)2]ꢀ reacts with Co(II) in MeOH to produce
{[RuIII(Q)2(CN)]2(m-CN)2[CoII(MeOH)4]}ꢁ8MeOH, which, as
far as we are aware, is the first example of a trinuclear
compound that exhibits 3-D long range ferromagnetic order-
ing. This is made possible by p–p stacking of the quinolinolato
ligands, which connect the trinuclear units into 1-D supra-
molecular double chains. [Ru(Q)2(CN)2]ꢀ also reacts with
Co(II) in DMF to produce a 1D zigzag chain, in which the
Co(II) has a trigonal bipyramidal geometry. Co(II) usually has
octahedral or tetrahedral geometry in coordination polymers.
Again, as far as we are aware, this is the first example of a
trigonal bipyramidal Co(II) in a coordination polymer. The
work described in this paper was supported by the Research
Grants Council of Hong Kong (N_CityU 107/08), the City
University of Hong Kong (7002446), and the National
Natural Science Foundation of China (NSFC 20831160505).
Fig. 6 Temperature dependence of wMT (J) and wꢀ1 (&) for 3 (left);
field dependence of the magnetization (right).
be attributed to the larger spin-orbit coupling of Co(II).
Careful examination of the data in the low-field region reviews
a small hysteresis loop at 2 K (Fig. 4). The coercive field is
ca. 100 Oe, which is characteristic of a soft magnet and the
remnant magnetization is 0.55 Nb. The intratrimer magnetic
coupling in 2 is difficult to be evaluated because there is no
suitable model to analyze its magnetic properties and there are
also problems associated with the octahedral high-spin
Co(II) ion. The intertrimer interaction transmitted through
p–p interactions should be held responsible for the occurrence
of 3-D long range ordering in 2.6
Notes and references
z Crystal data for 1: C44H32N4O2PRu, M = 780.78, monoclinic,
a = 16.4737(10), b = 7.4234(5), c = 28.4371(16) A, b = 93.451(5),
V = 3471.3(3) A3, T = 173(2) K, space group C2/c, Z = 4, 17 636
measured reflections, 3080 independent reflections (Rint = 0.062),
R1(obs) = 0.037, wR(all) = 0.074; for 2: C44H40CoN8O8Ru2ꢁ8MeOH,
M = 1326.25, monoclinic, a = 8.8324(2), b = 12.7196(4), c =
27.7063(8) A, b = 92.2018(14)1, V = 3110.35(15) A3, T = 150(2) K,
space group P21/c, Z = 2, 15 480 measured reflections, 5463 inde-
pendent reflections (Rint = 0.0389), R1(obs) = 0.096, wR(all) = 0.270;
for 3: C26H26ClCoN6O4Ru, M = 681.98, monoclinic, a = 14.4733(3),
b = 10.41624(18), c = 17.8260(3) A, b = 93.0942(15), V = 2683.48(8) A3,
T = 133(2) K, space group C2/c, Z = 4, 8064 measured reflections, 2408
independent reflections (Rint = 0.028), R1(obs) = 0.036, wR(all) = 0.096;
CCDC for 1–3: 821916–821918.
The magnetic properties of 3 were measured at an external
field of 1 kOe (Fig. 6). The wMT value at room temperature is
2.97 cm3 molꢀ1 K, which is larger than the uncoupled, spin-
only value of 2.25 cm3 molꢀ1 K for one high-spin d7 CoII
centre with S = 3/2 and one low-spin d5 octahedral RuIII
centre with S = 1/2, due to orbital contributions. On lowering
the temperature, the wMT value decreases smoothly and then
abruptly below 50 K, and it reaches a minimum value of
1.04 cm3 molꢀ1 K at about 2 K. The wM values above 30 K obey
the Curie–Weiss law [wM = C/(T ꢀ y)] with C = 3.00 cm3 molꢀ1
K and y = ꢀ7.1 K. The magnetization of this compound per
[CoIIRuIII] unit reaches a value of 2.42 Nb molꢀ1 at 2.0 K and
50 kOe (Fig. 6), which is not saturated, but is close to the
Ms value of uncoupled one Co(II) ion and one Ru(III) ion. The
observed Curie constant and the Ms value suggest that the five-
coordinate Co(II) ion couples with Ru(III) ferromagnetically,
since the five-coordinate Co(II) ion in 3 has smaller orbital
contributions than that of octahedral Co(II).
In the reported CoIIRuIII coordination polymers
{CoII[RuIII(acac)2(CN)2]2}n and {[CoII(MeOH)3][RuIII(CN-sap)-
(CN)3]ꢁ2MeOH}n,3 the CoII centres have tetrahedral and octa-
hedral geometry, respectively; and a relatively strong ferromagnetic
coupling between RuIII and CoII centres via the cyanide
bridges is observed in both cases. For compound 3, the CoII
centre has trigonal bipyramidal geometry, and in this case only
weak ferromagnetic coupling between RuIII and CoII centres
via the cyanide bridges is observed. These results indicate that
the magnetic coupling between CoII and RuIII can be affected
by the geometry of the metal centres.
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In summary we have designed a new dicyanoruthenium(III)
building block, [Ru(Q)2(CN)2]ꢀ. Through the incorporation
of the aromatic ligand Qꢀ, we have added p–p inter-
actions as additional attractive forces for the construction
of supramolecular structures with possibly enhanced
magnetic interactions between paramagnetic metal centres.
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c
8696 Chem. Commun., 2011, 47, 8694–8696
This journal is The Royal Society of Chemistry 2011