Stabilisation of a paramagnetic BH4--bridged dinickel(ii) complex by a macrodinucleating hexaaza-dithiophenolate ligand

Article abstract of DOI:10.1039/b512744k

The first paramagnetic borohydrido-bridged dinuclear nickel(ii) complex, [(L)Niⁱⁱ₂(μ_1,3-BH₄)] ⁺, stabilised by a sterically demanding hexaaza-dithiophenolate macrocycle, has been obtained by the reaction of [(L)Niⁱⁱ ₂(μ-ClO₄)]⁺ with NⁿBu ₄BH₄. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b512744k

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2
Stabilisation of a paramagnetic BH4 -bridged dinickel(II) complex by a
macrodinucleating hexaaza-dithiophenolate ligand{
Yves Journaux,*a Vasile Lozan,b Julia Klingele{b and Berthold Kersting*b
Received (in Cambridge, UK) 9th September 2005, Accepted 10th November 2005
First published as an Advance Article on the web 23rd November 2005
DOI: 10.1039/b512744k
The first paramagnetic borohydrido-bridged dinuclear
nickel(II) complex, [(L)NiII (m1,3-BH4)]+, stabilised by a sterically
2
demanding hexaaza-dithiophenolate macrocycle, has been
obtained by the reaction of [(L)NiII (m-ClO4)]+ with NnBu4BH4.
2
The search for dinuclear dithiolato-bridged complexes which
model key features of the active site of hydrogenase enzymes is an
active research area.1,2 Two main strategies exist to access such
compounds. One involves the addition of an electrophilic metal–
carbonyl fragment to
a nucleophilic metal complex with
cis-oriented thiolate functions.3 The resulting [NiFeS2], or
[Fe2S2], assemblies are co-ligated with CO and CN2 and represent
good structural analogues of the proposed active site structures, as
demonstrated recently by a number of research groups.4–7 In the
other strategy, macrocyclic dinucleating polyaza-dithiolate ligands
are used for the cluster assembly.8 Until now this strategy has only
allowed for the production of homodinuclear nickel complexes,
and it is unclear at present whether these more classical Werner-
type coordination compounds will ever be able to bind the
biologically relevant co-ligands CO, CN2 and H2. Herein we
provide the first evidence for nickel–hydrogen interactions in such
compounds.
Scheme 1 Complexes 1–5.
compound is stable for weeks, both in the solid state and in
solution. This stability is quite remarkable given that nickel(II)
complexes of sterically less demanding ligands are readily reduced
to nickel boride.11
IR measurements of solid 3?BPh4 showed intense absorption
bands at 2390, 2360, 2153 and 2071 cm21 indicative of terminal
12
…
B–H and bridging B–H Ni functions. The UV-Vis spectrum
recorded in acetonitrile suggested the presence of octahedral Ni(II)
ions [l = 650 (n2) and 1074 nm (n1)].13 Final confirmation came
from an X-ray crystal structure determination of 3?BPh4?2MeCN
Our study was initiated by the recent discovery of Desrochers
et al.9 who demonstrated that the sterically encumbered hydrotris-
(3,5-dimethyl-pyrazolyl)borate ligand (Tp*2) can stabilize
a
hydrogen-rich nickel environment in [Tp*NiII(m1,3-BH4)]. In order
to test whether similar dinickel(II) complexes with a bridging
borohydride co-ligand are supported by the dinucleating hexaaza-
dithiophenolate ligand (L)22 (Scheme 1),10 an acetonitrile solution
2
(Fig. 1).§ As can be seen, the BH4 ion bridges the two Ni(II)
of the chloro-bridged complex [(L)NiII (m-Cl)]ClO4 (1?ClO4) was
2
treated with NnBu4BH4 under an argon atmosphere at ambient
temperature. Unlike [Tp*NiIICl],9 however, no reaction occurred.
In a second approach, the reaction was carried out using the dark-
green perchlorato-bridged complex [(L)NiII (m-ClO4)]ClO4
2
(2?ClO4) which was prepared by Cl2 abstraction from 1?ClO4
with Pb(ClO4)2.{ This gave a pale-green solution of the desired
borohydrido-bridged complex 3, which was isolated as its BPh4
2
salt in ca. 75% yield. In the absence of air and protic reagents this
aLaboratoire de Chimie Inorganique et Mate´riaux Mole´culaires Bat F
UMR CNRS 7071, case courrier 42, Universite´ Pierre et Marie Curie, 4
place Jussieu, F75252, Paris cedex 05, France.
Fig. 1 ORTEP representation of the structure of complex 3 at 50%
2
probability ellipsoids. Hydrogen atoms, except those of the BH4 co-
˚
ligand, have been omitted for clarity. Selected bond lengths (A) and angles
E-mail: jour@ccr.jussieu.fr; Tel: +33 14427 5562
bInstitut fu¨r Anorganische Chemie, Johannisallee 29, Universita¨t
Leipzig, D-04013 Leipzig, Germany. E-mail: b.kersting@uni-leipzig.de;
Fax: +49-341-9736199; Tel: +49-341-9736143
(u): Ni1–N1 2.219(4), Ni1–N2 2.152(4), Ni1–N3 2.284(4), Ni1–S1 2.437(2),
Ni1–S2 2.459(1), Ni1–H1 1.93(4), Ni2–N4 2.271(4), Ni2–N5 2.135(4),
Ni2–N6 2.260(4), Ni2–S1 2.439(2), Ni2–S2 2.457(2), Ni2–H2 1.84(4), B1–
{ Electronic supplementary information (ESI) available: Preparation and
characterisation data for complexes 2–5. See DOI: 10.1039/b512744k
{ Present address: Institut fu¨r Anorganische und Analytische Chemie,
Universita¨t Freiburg, Albertstrasse 21, D-79104 Freiburg, Germany.
…
H1 1.25(4), B1–H2 1.24(4), B1–H3 1.12(4), B1–H4 1.12(4); Ni1 Ni2
3.458; Ni1–S1–Ni2 90.33(5), Ni2–S2–Ni1 89.41(5).
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 83–84 | 83

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stretching frequencies of a m1,3-bridging formate ion. This was also
confirmed by an X-ray crystal structure determination of 5?BPh4
(see ESI{).
In conclusion, we have prepared the first stable dinuclear
nickel(II) borohydrido-bridged complex of a macrodinucleating
hexaaza-dithiophenolate ligand. Work in progress is directed
towards the synthesis of related compounds with bridging hydride
ligands by taking advantage of the steric protection offered by the
supporting ligand. Such compounds may also aid in understanding
the electronic structures and the reactivities of the dinuclear active
sites of the hydrogenase enzymes.
This work was supported by the Deutsche Forschungs-
gemeinschaft (Project KE 585/4-1,2).
Fig. 2 Plot of xmT against T for 3?BPh4. The solid line represents the
best theoretical fit of the magnetic susceptibility data by full-matrix
diagonalization of the spin Hamiltonian H = 22J S1?S2 + D(Sz12 + Sz22 2
4/3) + gb(S1 + S2)B. Experimental and calculated data are provided as
ESI.{
Notes and references
§ Crystal data for [(L)NiII2(m1,3-BH4)](BPh4)?2MeCN (3?BPh4?2MeCN):
¯
C66H94B2N8Ni2S2, M = 1202.65. Triclinic, space group P1, (no. 2), a =
˚
15.992(3), b = 16.080(3), c = 16.515(3) A, a = 63.82(3), b = 70.97(3),
c = 66.47(3)u, V = 3436(1) A , Z = 2, Dcalcd. = 1.162 g cm23, m(Mo-Ka) =
3
˚
0.651 mm21, T = 210 K. Using Mo-Ka radiation (0.71073 A), a total of
˚
31051 reflections were collected of which 16023 were independent.
Refinement converged to R1 = 0.0749, wR2 = 0.2053 [I > 2s(I)].
Hydrogen atoms for the g2-BH4 ligand were located from the final
difference map and were refined isotropically with Ueq(H) 1.2 times that of
the boron to which they are attached. The MeCN solvate molecules were
found to be disordered over two positions. CCDC 280004. Crystal data for
[(L)NiII2(m1,3-O2CH)](BPh4) (5?BPh4): C63H85BN6Ni2O2S2, M =
1150.72. Monoclinic, space group P21/n, a = 18.452(4), b = 34.531(7), c =
centres in a symmetrical fashion to generate a bioctahedral
N3NiII(m-S)2(m-BH4)NiIIN3 core structure that has never been
observed before in nickel–thiolate chemistry. There are no
interactions between the MeCN of solvent of crystallization and
the [(L)NiII (m-BH4)]+ cations. The average Ni–H distance at
2
˚
1.89(4) A compares well with that in the mononuclear NiS4H2
3
23
complex [NiII(bmp)2] (bmp = bis(2-mercapto-1-methylimidazolyl)-
borate),14 the only other sulfur-rich Ni(II) complex with B–H Ni
…
18.452(4) A, b = 92.22(3)u, V = 11748(4) A , Z = 8, Dc = 1.301 g cm
,
˚
˚
m(Mo-Ka) = 0.761 mm21, T = 210 K. Using Mo-Ka radiation (0.71073 A),
a total of 74914 reflections were collected of which 28479 were independent.
Refinement converged to R1 = 0.0778, wR2 = 0.1651 [I > 2s(I)]. CCDC
280005. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b512744k
˚
interactions that has been structurally characterised.
Magnetic susceptibility measurements have been carried out to
see whether magnetic exchange interactions are present in 3?BPh4.
As can be seen from Fig. 2, the product xmT (per dinuclear
complex) gradually increases from 2.69 cm3 K mol21 at 295 K to a
maximum of 3.29 cm3 K mol21 at 28 K, and then decreases
rapidly to 2.74 mB at 2 K. This behaviour indicates an
intramolecular ferromagnetic exchange interaction between the
two Ni2+ ions in 3.15
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Least-squares fit of the magnetic susceptibility data by full-
matrix diagonalization of the appropriate spin Hamiltonian H =
22J S1?S2 + D(Sz12 + Sz22 2 4/3) + gb(S1 + S2)B gave J = 27 cm21
,
D = +4.3 cm21, and g = 2.09 (or alternatively J = 27.5 cm21
,
D = 23.9 cm21, and g = 2.09). Interestingly, the J value of 27 cm21
is by far the largest value that has ever been observed in dinuclear
nickel(II) complexes supported by (L)22 16
It indicates that the
.
ferromagnetic magnetic exchange interactions are not only
propagated via the bridging thiolate functions but also through
the m1,3-bridging borohydride ion. To the best of our knowledge,
this property of the BH42 ion has not been documented previously
in the literature. This suggests its use as a building block in the
construction of molecular based magnetic materials.
12 T. J. Marks and J. R. Kolb, Chem. Rev., 1977, 77, 263.
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D. Rabinovich, Inorg. Chem., 2001, 40, 5736.
15 In the solid state the dinuclear nickel complexes are well separated by the
bulky tetraphenylborate anions. The abrupt decrease in xmT below 28 K
is therefore most likely due to zero field splitting of NiII and not to
intermolecular exchange interactions.
Preliminary results show that 3 reacts with protic reagents HA,
such as HCl, H2O, or HCO2H with liberation of H2 and formation
of the respective [(L)NiII (A)]+ species (A = Cl2 1, OH2 4 and
2
HCO22 5). Complexes 1 and 4 have been reported earlier. The new
complex 5 is also readily produced by the reaction of 3 with CO2.
IR measurements of 5?BPh4 showed two absorption bands at 1602
and 1424 cm21, attributable to the symmetric and antisymmetric
16 J. Hausmann, M. H. Klingele, V. Lozan, G. Steinfeld, D. Siebert,
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84 | Chem. Commun., 2006, 83–84
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