A low-coordinate nickel(ii) hydride complex and its reactivity

Article abstract of DOI:10.1039/b816136b

The preparation of a novel dinuclear nickel(ii) hydride complex and its reactivity that often leads to nickel(i) compounds is described.

Full text of DOI:10.1039/b816136b

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www.rsc.org/dalton | Dalton Transactions
A low-coordinate nickel(II) hydride complex and its reactivity†
Stefan Pfirrmann, Christian Limberg* and Burkhard Ziemer
Received 15th September 2008, Accepted 21st October 2008
First published as an Advance Article on the web 24th October 2008
DOI: 10.1039/b816136b
The preparation of a novel dinuclear nickel(II) hydride com-
plex and its reactivity that often leads to nickel(I) compounds
is described.
There is a lot of interest in low-coordinate transition metal
complexes as they are often highly reactive: their flexibility
in exogenous ligand binding accounts for a high diversity of
conversions within the coordination sphere of the metal, if this
contains a reactive unit to start from. In this respect metal-hydride
entities are particularly interesting, first, because their behaviour
depends a lot on the residual ligand sphere and secondly as they
are capable of initiating the most different transformations. A lot
of fascinating chemistry is found for the combination of hydride
ligands with nickel: this ranges from the activation of H by
2
1
2,3
Raney nickel, via elementary steps within the SHOP process to
hydrogenase enzymes where hydride bridges between nickel and
4
iron are proposed to be central to the reactivity.
Here we describe a low-coordinate dinuclear nickel(II) complex
that contains hydride bridges.
The b-diketiminato ligand has proved a very versatile ligand in
5
the past, both in main group and transition metal chemistry, as
Scheme 1 Synthesis and reactivity of 1; D = donor.
(
(
i) it can be used to prepare coordinatively unsaturated complexes,
ii) it stabilises high as well as low oxidation states, and (iii) often
6
its complexes show interesting reactivities. Hence we chose this
ligand system—that had already allowed for the isolation of low-
7
valent iron-hydride complexes —for our investigation concerning
low-valent nickel hydrides.
For this purpose we have employed LNi(m-Br)
L is the bulky b-diketiminato ligand shown in Scheme 1—for a
reaction with KBEt H. Appropriate work-up led to the isolation
of dark green crystals in 36% yield, and the X-ray‡ crystal structure
revealed the product to have a dimeric constitution, [LNiH] , 1, in
2
Li(thf)
2
—where
3
2
the solid state. The nickel atoms are bridged by two hydride ligands
which were located in the Fourier difference map (Scheme 1,
Fig. 1). The distance between the nickel centers was found to be
˚
2
.3939(6) A. Similarly as in other LNi complexes the N–Ni bond
˚
lengths are between 1.912(2) and 1.935(2) A and the N–Ni–N
◦
8
chelate bite angles amount to 94.93(8) and 93.90(9) . As described
for the analogous iron hydride complex the b-diketiminato-Ni
rings in 1 twist into a distorted boat form caused by steric
hindrance within the dimer. Therefore the N–Ni(1)–N plane is
◦
located at an angle of 30.9(3) with respect to the C–C–C plane of
Fig. 1 Molecular structure of [LNi(m-H)]2 1. Solvent molecules and
the b-diketiminato ligand backbone.
hydrogen atoms apart from the bridging hydrides are omitted for clarity. Se-
lected bond lengths ( A˚ ) and angels ( ): Ni(1)–Ni(2) 2.3939(6), Ni(1)–N(1)
◦
1
1
.912(2), Ni(1)–N(2) 1.9125(18), Ni(2)–N(3) 1.922(2), Ni(2)–N(4)
.935(2); N(1)–Ni(1)–N(2) 94.93(8), N(3)–Ni(2)–N(4) 93.90(9).
Humboldt-Universit a¨ t zu Berlin, Institut f u¨ r Chemie, Brook-Taylor-Str. 2,
D-12489, Berlin, Germany. E-mail: christian.limberg@chemie.hu-berlin.de;
Fax: +49 30 2093 6966; Tel: +49 30 2093 7370
†
Electronic supplementary information (ESI) available: Experimental
The magnetic moment of 1 in the solid state at r.t. amounts
details and analytical data. CCDC reference numbers 694380–694382. For
ESI and crystallographic data in CIF or other electronic format see DOI:
1
to 3.05 m
B
per complex which is lower than the spin-only value
for
0.1039/b816136b
expected for two uncoupled high-spin Ni(II) ions (4.00 m
B
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g = 2.00), which points to an antiferromagnetic coupling. 1 is
EPR silent in frozen toluene solutions at 77 K.
instance it has been observed for a nickel hydride complex bearing
diphosphane ligands, too. Remarkably, 1 exhibits this reactivity
11
The results of spectroscopic studies did not provide any evidence
even in contact with B–H units, as we suspected noticing that
that on dissolution 1 enters into an equilibrium with a monomeric
employing excessive KBEt
decreased yields: Subsequently, independent experiments proved
that indeed treatment of 1 with KBEt H in toluene leads to the
known complex I and H
The reactions of 1 with donors D must proceed through a
3
H for its synthesis leads to significantly
1
form: a H-NMR spectrum of 1 dissolved in toluene-d
at 80 C did not show a further set of signals as compared to the
r.t. spectrum. However, it turned out that on heating the complex
8
recorded
◦
3
2
.
slowly decomposes to give the known nickel(I) complex (LNi)
2
(m-
Me), I, which could unambiguously be identified
by H NMR spectroscopy, as well as H (see Scheme 1). To
clarify how the LNi(I) species formed in course of the reductive
elimination of H behaves, if no stabilising ligand (like toluene)
3
3
9
h :h -C
6
H
5
mechanism where D binds to the metal first before H loss occurs,
2
1
2
as 1 is stable at r.t. in solution and under vacuum.
1 is stable in contact with potential p-ligands like dipheny-
◦
2
lacetylene. However on warming to 80 C it transfers hydrogen
is present in solution, 1 was also heated in hexane solution.
Formation of a nickel mirror was observed then, and the solution
obtained contains protonated ligand LH beside other unidentified
onto the triple bond to give cis- and trans- diphenylethylene as
well as diphenylethane (in 35%, 7% and 12% yield, respectively) as
1
revealed by GC-MS and H NMR spectroscopy. A further product
(
paramagnetic) products.
is (in toluene) I. As PhCCPh does not react with H
2
, we assume
In order to see whether the dimer 1 can be broken up
that H is transferred directly from 1 to this substrate. There was
2
into monomers via the addition of external potential ligands,
it was treated with nitrogen donors, D. Reaction with both 4-
dimethylaminopyridine and propionitrile led to the reductive
no indication of an intermediate insertion of PhCCPh into one of
the Ni–H bonds that would lead to a Ni–C(Ph)=C(Ph)H species
7a,d
as observed for corresponding iron complexes.
In conclusion we have shown that the first isolable low-
coordinate nickel hydride complex readily eliminates H in contact
with external donors (or on heating) which leads to nickel(I)
complexes. Alternatively, H can be transferred to unsaturated
elimination of H
2
to give the T-shaped Ni(I) complexes LNiD (D =
4
-dimethylaminopyridine (4-DMAP), 2, and propionitrile, 3), that
2
3
3
can also be obtained by treating (LNi)
2
(m-h :h -C
6
H
5
Me) with
D (see Scheme 1, for H
information, ESI†).
2
detection see electronic supplementary
2
substrates. Future research will now for instance address its
reactivity towards oxidants (which might reveal interesting insights
The molecular structure of 3 as revealed by single crystal
X-ray crystallography‡ is shown in Fig. 2. Due to the asymmetric
coordination of the propionitrile ligand the distances between the
N atoms belonging to the b-diketiminato ligand and the Ni ion
concerning the behaviour of [NiFe] hydrogenases that are O
2
sensitive) or towards iron hydrides in order to prepare [NiFe]
hydrogenase mimics.
˚
Ni(1)–N(1) 1.861(2) and Ni(1)–N(2) 1.928(2) A) and also the
(
N(1)–Ni(1)–N(3) and N(2)–Ni(1)–N(3) angles (157.86(10) and
◦
Acknowledgements
1
04.50(10) ) differ significantly. Strong s donation of the nitrile
˚
ligand expresses itself by a Ni–N bond length of 1.857(3) A.
Altogether the geometry of 3 is in good agreement with other
three coordinate Ni(I) b-diketiminato complexes described in the
We would like to thank the Cluster of Excellence “Unifying
Concepts in Catalysis” (EXC 314/1) funded by the Deutsche
Forschungsgemeinschaft for financial support. Furthermore we
are grateful to Prof. Dr. R. St o¨ ßer for recording the EPR data and
to Marcel Ahijado Salomon and Ann-Katrin Jungton for support
9,10
literature.
with respect to H detection.
2
Notes and references
‡
1
5
0
Crystal data for 1·OEt
2
: C62
H
94
N
4
Ni
2
O, M = 1028.83, monoclinic, a =
◦
1.5933(13), b = 18.187(2), c = 28.265(4) A˚ , b = 98.742(15) , V =
890.4(12) A˚ , T = 180(2) K, space group P2
3
/c, Z = 4, m(Mo-K ) =
1
a
-1
.681 mm , 31426 reflections measured, 10624 unique (Rint = 0.0486),
R
1
= 0.0380 (I > 2s(I)), wR = 0.0788 (all data). For 3: C32 Ni,
2
46 3
H N
M = 531.43, triclinic, a = 8.812(2), b = 9.1249(16), c = 19.980(4) A˚ , a =
◦
3
8
7.59(2), b = 80.83(3), g = 73.63(3) ; V = 1521.7(5) A˚ , T = 180(2) K,
-1
a
space group P-1, Z = 2, m(Mo-K
) = 0. 661 mm , 15879 reflections
= 0.0523 (I > 2s(I)), wR =
measured, 5185 unique (Rint = 0.0601), R
1
2
0
.1365 (all data).
1
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