Nickel complexes with two types of noninnocent ligands: α-Diimine and phenazine

Article abstract of DOI:10.1021/om400130m

The α-diimine nickel dibromo complex LNiBr₂ (L = [(2,6-iPr₂C₆H₃)NC(Me)]₂) reacts with various amounts of potassium metal and phenazine (Phz) to afford the complexes [LNi(η⁴-C₄-Ph

Full text of DOI:10.1021/om400130m

Note
pubs.acs.org/Organometallics
Nickel Complexes with Two Types of Noninnocent Ligands:
α‑Diimine and Phenazine
Qingsong Dong,†,§ Ji-Hu Su,‡ Shida Gong,# Qian-Shu Li,# Yanxia Zhao,†,§ Biao Wu,¶
and Xiao-Juan Yang*,†,¶
†State Key Laboratory for Oxo Synthesis & Selective Oxidation, Lanzhou Institute of Chemical Physics, CAS, Lanzhou 730000, China
‡Hefei National Laboratory for Physical Sciences at Microscale and Department of Modern Physics, University of Science and
Technology of China, Hefei 230026, China
#Center for Computational Quantum Chemistry, South China Normal University, Guangzhou 510631, China
¶College of Chemistry and Materials Science, Northwest University, Xi’an 710069, China
§Graduate University of Chinese Academy of Sciences, Beijing 100049, China
S
* Supporting Information
ABSTRACT: The α-diimine nickel dibromo complex LNiBr2 (L = [(2,6-
iPr2C6H3)NC(Me)]2) reacts with various amounts of potassium metal and
phenazine (Phz) to afford the complexes [LNi(η4-C4-Phz)] (1) and
[K(DME)]2[(LNi)2(μ-η1:η1-N,N-Phz)] (2), in which the Phz molecule
adopts significantly different coordination modes with different oxidation
states of Phz, Ni, and L.
henazine (Phz) is involved in many electron transfer
processes due to its easily reducible (−0.364 V vs SCE)
carried out, which display interesting redox properties and
result in two nickel complexes with a variety of oxidation states
of Phz, Ni, and α-diimine and significantly different
coordination modes of Phz.
P
nature.1,2 It has been recognized as a noninnocent ligand, and
the reduced Phz (radical anion or dianion) ligands can form
complexes with main group metals,3,4 transition metals,5 and
lanthanides and actinides.6 In these complexes, the redox
transformations are modulated by the Phz ligand serving as an
“electron reservoir” that can reversibly accept and donate
electrons.4 On the other hand, as a weak bridging ligand,
phenazine can act as the “organic linker” in the construction of
1D and 2D metal coordination networks as well as layered
complexes by the noncovalent interactions (hydrogen bonding
or π−π stacking).7 However, nickel complexes with reduced
Phz ligands have not been reported.
RESULTS AND DISCUSSION
■
The dibromo nickel(II) precursor LNiBr2 (L = [(2,6-
iPr2C6H3)NC(Me)]2) is inert to Phz in the absence of
reducing agents. However, as illustrated in Scheme 1, when
LNiBr2 was first reduced by 2.0 equiv of potassium metal and
then reacted with 1.0 equiv of Phz, the mononuclear complex
[LNi(η4-C4-Phz)] (1) was isolated in 60% yield as black-purple
crystals. In addition, treatment of LNiBr2 with 3.0 equiv of K
metal followed by reaction with 0.5 equiv of Phz afforded the
dinuclear, three-coordinate nickel compound, [K-
(DME)]2[(LNi)2(μ-η1:η1-N,N-Phz)] (2), as dark-green crys-
tals. The complexes are thermally stable under argon but are
sensitive to air and moisture. Very interestingly, the molecular
structures of 1 and 2 reveal two coordination modes for Phz.
Complex 1 contains a Phz ligand that is η4-coordinating to the
Ni atom by using only one carbocyclic ring, while complex 2 is
a centrosymmetric dimer wherein the Phz ligand bridges two
Since Brookhart reported that nickel(II) α-diimine com-
plexes are highly active catalysts for olefin polymerization,8 this
class of complexes stirred up great interest.9 Moreover, the α-
diimines are also well-known noninnocent ligands that can
form a wide range of complexes in the neutral and singly or
doubly reduced states. Our recent research has focused on
metal complexes containing low-valent, low-coordinate metal
ions stabilized by α-diimines.10 These complexes show
interesting reactivity toward organic molecules through various
electron transfer processes involving the metal centers and
ligands.10e,f In the current work, an investigation of the
noninnocence of both the α-diimine and Phz ligands was
Received: February 14, 2013
Published: April 16, 2013
© 2013 American Chemical Society
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Organometallics
Note
the coordinating ring. The Ni−C distances of the outer carbon
atoms (av. 2.039 Å of C31 and C32) are shorter than those to
the central carbons (av. 2.242 Å of C30 and C33). This is
similar to the reported Mo complex (η4-C4-Phz)2Mo(PMe3)2
that has two folded η4-C4-Phz molecules.5 The average C−N
bond length of the Phz molecule retains 1.35 Å which can be
found in free Phz.12 The C−C distances of the coordinated
carbocyclic ring (from 1.400(3) to 1.453(3) Å) are obviously
longer than those (from 1.371(3) to 1.419(3) Å) of the other
(uncoordinated) carbocyclic ring in complex 1.
Scheme 1. Synthesis of Complexes 1 and 2
Complex 1 contains two redox active ligands, the α-diimine L
and Phz, as well as the transition metal nickel, and thus an
investigation of the electron transfer during the reduction by
the alkali metal is necessary. It is known that the oxidation state
of α-diimine ligands can be conveniently judged by the C−N
and C−C bond lengths of the central C2N2 core.10,13 These
values (C−N: 1.316(2) and 1.314(2) Å and C−C: 1.456(2) Å)
in complex 1 correspond to a neutral ligand L0. Considering the
analogy of the η4-coordination mode of Phz to butadiene, the
complex appears to have the formal oxidation state Ni(0) and
neutral Phz. Moreover, the NMR spectrum of 1 shows sharp
signals, and it is EPR silent in frozen Et2O solution at 77 K,
demonstrating the diamagnetism of complex 1. The electronic
structure of 1 was further proven by DFT computations at the
B3LYP/6-31G level with the broken symmetry (BS) formalism
in the singlet state. The optimized structure agrees well with the
X-ray data (Table S1, Supporting Information). A Mulliken
spin density analysis reveals zero spin density on the Ni centers,
which supports the assignment of a Ni(0) center coordinated
by the neutral α-diimine and Phz ligands. The HOMO−2
demonstrates the metal-to-ligand backbonding (Figure S4).
The dinuclear complex 2 (Figure 2) shows a centrosym-
metric structure with a Phz molecule bridging two [K-
(DME)]·[LNi] moieties. The Ni centers are 3-fold coordinated
by a ligand L and one N atom of the bridging Phz in a trigonal-
planar geometry; thus it can be considered as a nickel amido
complex. The Ni−N bond length to the Phz ligand is 1.934(2)
Å, while those to the α-diimine ligand L are 1.955(2) and
nickel centers via the nitrogen atoms. In fact, since in complex 1
the other carbocyclic ring of Phz is free from interaction with
metals, the parallel reaction with 0.5 equiv of Phz was also
carried out in an attempt to synthesize the 2:1 (nickel to Phz)
complex. A color change of the reaction solution from deep
green to dark blue was observed; unfortunately, the product
could not be isolated.
In the mononuclear complex [LNi(η4-C4-Phz)] (1) (Figure
1), the Ni atom is chelated by an α-diimine ligand L (Ni−N
Figure 1. Molecular structure of 1 (thermal ellipsoids are set at the
20% probability level; hydrogen atoms have been omitted for clarity).
Selected bond lengths (Å) and angles (°): Ni1−N1 1.9153(14), Ni1−
N2 1.9379(14), Ni1−C31 2.0299(18), Ni1−C33 2.2432(19), N1−C1
1.316(2), C1−C2 1.456(2), C2−N2 1.314(2), C30−C31 1.405(3),
C31−C32 1.413(3), C32−C33 1.400(3), C29−C34 1.453(2), C29−
N3 1.327(2), C34−N4 1.320(2), C40−N3 1.377(2), C35−N4
1.372(3), N1−Ni1−N2 81.92(6).
distances: 1.9379(14) and 1.9153(14) Å) and interacts with
one of the carbocyclic rings of the Phz molecule in an η4
manner, which is similar to the interaction of α-diimine-nickel
complexes with butadiene.11 This coordination mode for Phz is
very rare; the only examples were reported very recently in two
molybdenum complexes.5 The η4-coordination mode is
1
reflected by the H NMR spectrum, in which the signals of
Figure 2. Molecular structure of 2 (thermal ellipsoids are set at the
20% probability level; hydrogen atoms have been omitted for clarity).
Selected bond lengths (Å) and angles (°): Ni1−N1 1.928(2), Ni1−N2
1.955(2), Ni1−N3 1.934(2), N1−C1 1.351(4), C1−C2 1.401(4),
C2−N2 1.353(4), N3−C29 1.401(4), N3−C34 1.411(4), C29−C34
1.435(4), K1−N3 2.861(2), K1−O1 2.653(3), K1−O2 2.725(3), N1−
Ni1−N2 81.33(9). Symmetry code A): 1−x, 1−y, 1−z.
the protons on the coordinating carbocyclic ring (at δ = 5.61
ppm for H31 and H32, and δ = 5.16 ppm for H30 and H33)
are shifted to higher field by Δδ = 2.04 and 2.08 ppm,
respectively, compared to the uncoordinated carbocyclic ring.
The Phz ligand in 1 is significantly distorted from planar, being
folded by 14.8° at the central (C30 and C33) carbon atoms of
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Note
1.928(2) Å, the longer one possibly resulting from the
interaction of the [K(DME)]+ unit with the aryl ring. Two
DME-solvated K+ ions sit above and below the bridging Phz
ligand and being bonded by one aryl ring of the α-diimine
ligand and the Phz molecule in an η6/η4 fashion. The
crystallographic symmetry constrains the two C2N2Ni planes
to be parallel to each other, which are nearly perpendicular
(dihedral angle 86.7°) to the Phz moiety.
The Phz molecule in 2 is close to planar with longer N−C
distances (1.401(4) Å/1.411(4) Å) than those (1.35 Å) in the
neutral Phz,4,12 corresponding to its dianionic form.3,6 Different
from the η4-coordination of one carbocyclic ring in complex 1,
the Phz moiety coordinates to the nickel centers by the N
atoms (μ-η1:η1-bonding mode). The structure of 2 compares
with the complexes [Mg2Br2(η1:η1-Phz)(thf)6]·[MgBr2(thf)4],3
[(η5-C5Me5)(η8-C8H8)U]2[η1:η1-Phz],6f and [(η5-
C5Me5)2M]2[η3:η3-Phz] (M = Yb, Sm, La, or Lu)6a−d with
the dianion Phz2−. However, in the later four lanthanide
complexes, the Phz bridges the metal ions using three adjacent
atoms in an aza-allyl mode on each side rather than using only
the N atom. On the other hand, the C−N (1.351(4)/1.353(4)
Å) and C−C (1.401(4) Å) bond distances of the C2N2 moiety
of the α-diimine L in 2 indicate the monoanionic π radical
character. The total negative charges of the Phz and α-diimine
ligands (−4) are counterbalanced by two K+ ions and two Ni
centers. Thus, the nickel centers display the formal oxidation
state of +1.
DFT and EPR studies were carried out to elucidate the
electronic structure of 2. DFT geometry optimizations were
performed on a model compound [K(H2O)2]2[{(C6H3−
NCH)2Ni}2(μ-η1:η1-N,N-Phz)] (2′) in the singlet (S = 0),
triplet (S = 1), and quintet (S = 2) spin states, respectively, at
the B3LYP/6-31G level. The results demonstrate that the
quintet spin state is energetically more favored by 30.09 and
25.50 kcal mol−1, respectively, than the singlet and triplet states,
implying the existence of the paramagnetic Ni(I) centers and
ligand radicals. This is also consistent with the EPR studies.
The EPR spectrum of 2 in the solid state at 77 K shows four
components (Figure S1, Supporting Information). The g1
component appears at 2.412, g2 at 2.339, and g3 at 2.136,
which clearly establish the presence of Ni(I) cores.14 The
experimental signal is in good agreement with the simulated
one (Figure S2) and is also similar to literature reports on
related systems.14 The fourth peak at g = 2.012 is attributable to
the organic radical (L•−). In addition, the room-temperature
EPR spectrum of 2 shows the hyperfine structure and a broad
signal (Figure S3, Supporting Information). The 10-lined signal
of the hyperfine structure is well resolved and agrees with the
simulated one (Figure 3), which can be explained as originating
from the superimposition of overlapping quintets (due to two
equivalent 14N nuclei, I = 1) and septets (due to six equivalent
1H nuclei of two methyl groups, I = 1/2) of the ligand L. The g
value of 2.017 compares well with the reported compounds
containing radical diimine ligands.10d,15
Figure 3. X-band EPR spectrum of complex 2 in DME at room
temperature (black: experimental; red: simulated).
carbocyclic rings in 1, whereas it bridges two nickel atoms via
the nitrogen atoms in complex 2.
EXPERIMENTAL SECTION
■
General Considerations. All manipulations with air- and
moisture-sensitive compounds were carried out under argon with
standard Schlenk or drybox techniques. The solvents (DME and
Et2O) were dried using appropriate methods and distilled under argon
prior to use. Benzene-d6 was dried over Na/K alloy. NMR spectra
were recorded on a Mercury Plus-400 spectrometer in benzene-d6.
EPR spectra were recorded on a Bruker EMX-10/12 spectrometer. IR
spectrum was recorded (thin film on KBr plate) using a Nicolet
AVATAR 360 FT-IR spectrometer. Elemental analyses were
performed with an Elementar VarioEL III instrument. Phz (99%)
was purchased from Alfa Aesar. LNiBr2 was prepared according to a
published procedure.8
Synthesis of [LNi(η4-C4-Phz)] (1). Potassium (0.080 g, 2.0 mmol)
was added to a suspension of LNiBr2 (0.31 g, 1.0 mmol) in Et2O (30
mL), and the mixture was stirred for 2 days at room temperature, upon
which a color change from brown-red to dark green was observed.
Then Phz (0.180 g, 1.0 mmol) was added, and the mixture was stirred
further for 1 day to generate a dark purple color. After filtration of the
mixture and concentration of the filtrate to ca. 8 mL, the solution was
stored at room temperature for several days to afford dark purple
crystals of 1 (0.39 g, 60%). 1H NMR (400 MHz, C6D6, 25 °C, TMS):
δ = 0.89 (d, 24H, J = 6.8 Hz, CH(CH3)2), 2.10 (s, 6H, N-CCH3), 2.87
(m, 4H, CH(CH3)2), 5.16 (m, 2H, Phz-H), 5.61 (m, 2H, Phz-H),
7.00−7.15 ppm (m, 6H, C6H3), 7.24 (m, 2H, Phz-H), 7.65 (m, 2H,
Phz-H). 13C{1H} NMR (100.6 MHz, C6D6): δ = 14.2 (N−CCH3),
23.5 (CH(CH3)2), 26.7 (CH(CH3)2), 147.8 (N-CCH3), 122.3−140.8
(aryl-C). IR (KBr, ν/cm−1): 1638, 1565, 1451, 1383, 1317, 1292,
1188, 1110, 1090, 978, 943, 868, 786, 747, 681. Anal. Calcd for
C40H48N4Ni (643.51): C 74.65, H 7.52, N 8.71. Found: C 74.12, H
7.21, N 8.82.
Synthesis of [K(DME)]2[(LNi)2(μ−η1:η1-N,N-Phz)] (2). LNiBr2
(0.31 g, 1.0 mmol) and potassium (0.12 g, 3.0 mmol) were suspended
in a mixed solvent (20 mL, Et2O: DME = 3:1 v/v) and stirred for 2
days at room temperature to generate a dark brown color. Then Phz
(0.09 g, 0.50 mmol) was added, and the stirring continued for another
1 day with the color changed to dark green. The solution was filtered,
and the filtrate was concentrated to ca. 8 mL and stored at room
temperature for several days to yield 2 as dark-green crystals (0.12 g,
16%). EPR (DME, room temperature), g = 2.017 and g = 2.100 (broad
signal); EPR (solid state, 77 K): g1 = 2.412, g2 = 2.339, g3 = 2.136, and
g = 2.012. IR (KBr, ν/cm−1): 1642, 1580, 1458, 1437, 1385, 1315,
1253, 1188, 1102, 1060, 985, 932, 861, 787, 7739, 690. Anal. Calcd for
C84H128N6O6K2Ni2 (1513.54): C 66.66, H 8.52, N 5.55. Found: C
66.03, H 8.63, N 5.42.
In conclusion, two nickel complexes with the noninnocent α-
diimine (L) and phenazine (Phz) ligands have been synthesized
from the reduction of the dibromo nickel(II) compound
LNiBr2 and Phz. The mononuclear complex 1 contains a Ni(0)
center coordinated by a neutral α-diimine and a Phz ligand,
while in the dinuclear complex 2 the α-diimine ligands are
radical monoanions, the Phz displays the dianion form, and the
nickel shows the formal oxidation state of +1. Accordingly, the
Phz coordinates to nickel in an η4-fashion through one of the
Crystal Data for 1 (C40H48N4Ni). M = 643.51, monoclinic, space
group P21/c, a = 10.4103(13) Å, b = 18.630(2) Å, c = 17.843(2) Å, β =
98.3530(10)°, V = 3423.9(7) Å3, Dcalc = 1.248 g cm−3, Z = 4, μ = 0.600
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Organometallics
Note
mm−1; 22 231 reflections collected, Rint = 0.0278, R1 = 0.0293, wR2 =
0.0847 (I > 2σ(I)).
Crystal Data for 2 (C84H128N6K2Ni2O6). M = 1513.54, triclinic,
space group P-1, a = 10.7791(17) Å, b = 13.479(2) Å, c = 15.832(2) Å,
α = 94.520(2)°, β = 107.634(2)°, γ = 102.176(2)°, V = 2117.5(6) Å3,
Dcalc = 1.187 gcm−3, Z = 1, μ = 0.595 mm−1; 15 195 reflections
collected, Rint = 0.0292, R1 = 0.0509, wR2 = 0.1483 (I > 2σ(I)).
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ASSOCIATED CONTENT
* Supporting Information
■
S
Computational details and crystallographic information files
(CIFs). This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: yangxj@lzb.ac.cn.
■
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
Weyhermuller, T.; Wieghardt, K. Dalton Trans. 2007, 4390.
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This work was supported by the National Natural Science
Foundation of China (20972169 and 21273170).
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