Reductive deprotonation and dehydrogenation of phenylhydrazine at a nickel center to give a nickel diazenido complex

Article abstract of DOI:10.1021/ic301066x

The reaction of [L^tBuNi(OEt₂)] (where L^tBu = [HC(C^tBuNC₆H₃(ⁱPr) ₂)₂]^-) with phenylhydrazine leads to the phenylhydrazido(1-) complex [L^tBuNi(η²-NPhNH ₂)] (1) with concomitant formation of H₂. Treatment of 1 with potassium graphite in the presence of crown ether again leads to H ₂ evolution and affords the heterobimetallic complex [L ^tBuNi(μ-η²:η²-NPhNH)]K(18-crown-6) containing the doubly deprotonated phenylhydrazido(2-) ligand. 1 can be converted into a phenyldiazenido complex [L^tBuNi(η¹- NNPh)] in the course of a dehydrogenation reaction employing 1,2-diisopropylazo dicarboxylate (DIAD) as the oxidant.

Full text of DOI:10.1021/ic301066x

Article
pubs.acs.org/IC
Reductive Deprotonation and Dehydrogenation of Phenylhydrazine
at a Nickel Center To Give a Nickel Diazenido Complex
Claudia Kothe, Ramona Metzinger, Christian Herwig, and Christian Limberg*
̈
Humboldt-Universitat zu Berlin, Institut fur Chemie, Brook-Taylor-Str. 2, 12489 Berlin, Germany
̈
̈
S
* Supporting Information
ABSTRACT: The reaction of [LtBuNi(OEt2)] (where LtBu
=
[HC(CtBuNC6H3(iPr)2)2]−) with phenylhydrazine leads to the
phenylhydrazido(1−) complex [LtBuNi(η2-NPhNH2)] (1) with
concomitant formation of H2. Treatment of 1 with potassium graphite
in the presence of crown ether again leads to H2 evolution and affords
the heterobimetallic complex [LtBuNi(μ-η2:η2-NPhNH)]K(18-crown-
6) containing the doubly deprotonated phenylhydrazido(2−) ligand. 1
can be converted into a phenyldiazenido complex [LtBuNi(η1-NNPh)]
in the course of a dehydrogenation reaction employing 1,2-
diisopropylazo dicarboxylate (DIAD) as the oxidant.
unsubstituted diazene, hydrazine, or hydrazido ligands have
been reported in the last two decades.11−18
INTRODUCTION
■
After the pioneering work by Holland,1 Warren,2 and Stephan3
on β-diketiminate complexes featuring mononuclear, three-
coordinate NiI centers representatives with weakly bound
coligands have proved to be versatile precursors for the
Hydrazine represents a 1,2-σ donor ligand and can thus
coordinate in various different modes and protonation states,
whereas the assessment of the latter can pose problems, when a
single-crystal XRD analysis does not unequivocally reveal the
position of the H atoms. Because of the low electron density of
H atoms, this is quite often the case and then determining how
many protons are located at a certain N atom can be difficult.
Only few nickel hydrazine complexes are known. The
dinuclear complex [L*Ni2(μ-N2H4)]2+ (L* = hexaazodithio-
phenolate) contains a bridging N2H4 ligand.17 Furthermore,
complexes of the type [Ni(NH2NHR)4Cl2] (R = CH3CH2,
CF3−CH2) are known, where the coordination modes of the
organo hydrazine ligands vary with the residues R.18
Turning the focus onto β-diketiminato metal hydrazine/
hydrazido systems, in particular, work on iron compounds
should be mentioned. Treatment of an FeII dihydridoborate
complex ([LMeFe(μ-H)2BEt2], LMe = [HC(C(Me)-
NC6H3(iPr)2)2)2]−, compare Scheme 1 showing LR) with
hydrazine led to the formation of [LMeFe(η1-N2H4)(μ-
H)2BEt2]. At slightly elevated temperatures this complex
eliminates H2 to give the diamido compound [LMeFe(μ-
NH2)2BEt2].19 A 1,2-diphenylhydrazido complex [LtBuFeN-
(Ph)NHPh] has been obtained during the course of a
hydrometallation of azobenzene with [(LtBuFeH)2].20 Further-
more, treatment of the sulfide-bridged diiron(II) compound
LMeFe(μ-S)FeLMe with 1.5 equiv of phenylhydrazine resulted in
the formation of a mixed-valency FeIIFeIII complex LMeFe(μ-
S)(μ-NPhNH2)FeLMe containing an anionic, bridging phenyl-
hydrazido ligand; aniline and ammonia were found to be
activation of small molecules,4 including H2 and N2.6,7 The
5
latter represent the reactants for the synthesis of ammonia
during the course of the Haber−Bosch process8 and in nature
nitrogenases9 catalyze the same conversion utilizing protons
and electrons instead of intact H2; in both the corresponding
reaction mechanisms proceed via NxHy intermediates in various
reduction and protonation states.
The finding that, after coordination of N2 at LNiI moieties, it
could be further reduced6,7 (Scheme 1) encouraged an
extension of these studies to important intermediates en
route from N2 to ammonia.
Among those are hydrazine and diazene,10 and accordingly, a
large variety of metal complexes containing substituted and
Scheme 1. Reaction of a LRNiI Complex with N2 and
Subsequent Reduction with Potassium Graphite
Received: May 22, 2012
Published: September 5, 2012
© 2012 American Chemical Society
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Inorganic Chemistry
Article
(CH(CH3)2), 24.8 (CH(CH3)2), 24.4 (CH(CH3)2), 23.8 (CH-
(CH3)2), 23.4 (CH(CH3)2). IR (KBr): 3329(w), 3249(vw),
3163(vw), 2957(vs), 2867(s), 1592(m), 1533(m), 1513(s),
1483(m), 1461(m), 1432(m), 1404(vs), 1363(s), 1319(s), 1251(s),
1218(m), 1186(m), 1161(m), 1098(w), 1054(w), 1035(m), 933(w),
803(w), 783(m), 754(m), 692(m), 526(w) cm−1.
produced concomitantly, suggesting that the N−N single bond
can be cleaved by the diiron(II) sulfide complex.21 A dinuclear
vanadium(V) complex is the only precedence for an arrange-
ment featuring a bridging and side-on bound (μ−η2:η2)
phenylhydrazine(2−) ligand.22
In contrast to hydrazine, free diazene and its derivates are
unstable species that readily decompose.23 However, complex-
ation to transition metals increases the stability of substituted
diazenes, and it was shown that they can coordinate to
transition metals in a number of different binding
modes.13,15,24,25 The most common motif found for aryldia-
zenes is the end-on (η1) mode, including singly and doubly
bent versions.15 Only a few examples of side-on (η2)13,24 or
bridging (μ−η1:η1)25 aryl-substituted diazene complexes have
been reported.
Synthesis of [LtBuNi(μ−η2:η2-NPhNH)]K([18]-crown-6) (2).
[LtBuNi(η2-NPhNH2)] (1) (100 mg, 0.15 mmol) and potassium
graphite (25 mg, 0.19 mmol, 1.25 equiv) were suspended in 20 mL of
diethyl ether in the presence of 18-crown-6 (40 mg, 0.15 mmol) at
ambient temperature. Intense gas development was observed, and a
color change from brown-red to orange occurred. The mixture was
stirred overnight, solid components were filtered off, and the solvent
was removed in vacuo. The residue was washed with hexane and
dissolved in diethyl ether again. Slow evaporation of the solvent led to
red crystals of [LtBuNi(μ−η2:η2-NHNPh)]K([18]-crown-6) (2) (97
mg, 0.10 mmol, 67%). Elemental analysis (%) calc. for
C53H83KN4NiO6 (970.04 g mol−1): C 65.62, H 8.62, N 5.78; found:
C 65.60, H 8.72, N 4.83. As mentioned in the text, 2 is very sensitive
to air, especially O2, which replaces the PhN2H unit. This posed severe
problems during sample preparation and may explain the deviation in
the N content. UV−vis (Et2O) (λmax (ε in mM−1 cm−1)): 301 (25),
Here, we report the reaction of a LtBuNiI precursor (LtBu
=
[HC(C(tBu)NC6H3(iPr)2)2]−) with phenylhydrazine, yielding
a NiII phenylhydrazido(1−) complex (1), which can be
converted to a heterobimetallic NiII phenylhydrazido(2−)
complex (2) and oxidized to a diazenido complex (3).
1
395 (7), 470 (2); H NMR (benzene-d6, 300 MHz): δ 7.21 (2H, m,
Ph m-H), 7.06 (4H, m, Ar m-H), 6.94 (2H, m, Ar o-H), 6.83 (2H, m,
Ph o-H), 6.96 (1H, t, JH,H = 7.6 Hz, Ph p-H), 5.43 (1H, s,
EXPERIMENTAL SECTION
General Procedures. All manipulations were carried out in a
■
2
CHCC(CH3)3), 4.78 (2H, m, CH(CH3)2), 4.30 (1H, m, CH(CH3)2),
3.80 (1H, m, CH(CH3)2), 2.88 (24H, s, O-(CH2)2−O), 2.21 (3H, d,
2JH,H = 6.8 Hz, CH(CH3)2), 2.07 (3H, d, 2JH,H = 6.8 Hz, CH(CH3)2),
glovebox, or else by means of Schlenk-type techniques involving the
use of a dry argon atmosphere. The H and 13C NMR spectra were
1
recorded on a Bruker DPX 300 NMR spectrometer (1H 300.1 MHz,
2
2
2.02 (3H, d, JH,H = 6.8 Hz, CH(CH3)2), 1.70 (3H, d, JH,H = 6.8 Hz,
13C 75.5 MHz, MHz) with C6D6 as solvent at 20 °C. The H NMR
1
CH(CH3)2), 1.60 (6H, d×d, CH(CH3)2), 1.55 (3H, d, 2JH,H = 6.8 Hz,
CH(CH3)2), 1.47 (1H, s, NH), 1.40 (3H, d, JH,H = 6.8 Hz,
spectra were calibrated against the residual proton, the 13C NMR
spectra against natural abundance 13C resonances of the deuterated
solvents (1H 300.13 MHz; 2H 61.42 MHz; 13C 100.63 MHz).
Chemical shifts are reported in ppm, relative to residual proton signals
and natural-abundance 13C resonances of C6D6 at 7.15 ppm and
128.02 ppm, respectively. Microanalyses were performed on a Leco
CHNS-932 elemental analyzer. Infrared (IR) spectra were recorded
using samples prepared as KBr pellets with a Shimadzu FTIR-8400S
spectrometer. UV−vis data were recorded on a Cary 100 (Varian)
spectrometer using quartz cuvettes.
2
CH(CH3)2), 1.32 (9H, s, C(CH3)3), 1.30 (9H, s, C(CH3)3). 13C
NMR (benzene-d6, 300 MHz): δ 164.0 (NCC(CH3)3), 154.7 (Ph i-
C), 152.2 (Ar o-C), 151.3 (Ar o-C), 142.5 (Ar i-C), 141.7 (Ar i-C),
141.1 (Ar i-C), 140.6 (Ar i-C), 130.8 (Ph p-C), 123.3 (Ar p-C), 122.7
(Ar p-C), 122.3 (Ar m-C),121.9 (Ph m-C), 121.3 (Ar m-C), 114.5
(Ph o-C), 98.00 (CHC(CH3)3), 69.8 (O−(CH2)2−O), 43.0
(C(CH3)3), 42.7 (C(CH3)3), 33.6 (C(CH3)3), 28.8 (CH(CH3)2),
28.5 (CH(CH3)2), 28.4 (CH(CH3)2), 27.8 (CH(CH3)2), 25.5
(CH(CH3)2), 25.6 (CH(CH3)2), 25.1 (CH(CH3)2), 24.6 (CH-
(CH3)2), 24.5 (CH(CH3)2), 24.0 (CH(CH3)2), 23.3 (CH(CH3)2),
22.7 (CH(CH3)2). IR (KBr): 3309(vw), 3148(vw), 3013(w),
2953(vs), 2886(vs), 2864(vs), 1603(m), 1585(m), 1513(m),
1501(m), 1470(m), 1460(m), 1445(m), 1412(vs), 1352(m),
1321(s), 1251(m), 1220(w), 1194(w), 1113(vs), 1057(w), 1028(w),
963(s), 756(m), 703(w) cm−1.
Materials. Solvents were purified, dried, degassed, and stored over
molecular sieves prior to use.
[LtBuNi(Et2O)] (I) was prepared by reduction of [LtBuNiBr]6 with
KC8 in a diethyl ether solution. The analytical data were identical to
those published previously.6
Synthesis of [LtBuNi(η2-NPhNH2)] (1). Phenylhydrazine (47 μL,
0.47 mmol) was added to a solution of [LtBuNi(OEt2)] (I) (300 mg,
0.47 mmol) in 20 mL of diethyl ether at room temperature. During the
reaction, gas development was observed, and a color change to red-
brown occurred immediately. The mixture was stirred for 16 h and the
solvent was removed in vacuo. The residue was extracted with hexane
(15 mL). After filtration, hexane was removed in vacuo to yield dark
red [LtBuNi(η2-NPhNH2)] (1) (246 mg, 0.37 mmol, 78%).
Recrystallization of the crude product from hexane at −30 °C yielded
analytically pure red crystals.
Synthesis of [LtBuNi(μ−η2:η2-O2)]K(18-crown-6) (2a). [LtBuNi-
(μ−η2:η2-NPhNH)]K([18]-crown-6) (2) (30 mg, 0.03 mmol) was
dissolved in 20 mL of diethyl ether. The argon atmosphere in the flask
was exchanged by dioxygen. The color of the solution immediately
changed from red to orange. After stirring for 15 min, all volatiles were
removed in vacuo and an orange residue of [LtBuNi(μ−η2:η2−
O2)]K(18-crown-6) (2a) (26 mg, 0.028 mmol, 93%) could be
isolated. Elemental analysis (%) calc. for C47H77KN2NiO8 (895.91 g
1
mol−1): C 63.01, H 8.66, N 3.13; found: C 63.00, H 8.46, N 3.09. H
Elemental analysis (%) calc. for C41H60N4Ni (667.64 g mol−1): C
73.76, H 9.06, N 8.39; found: C 74.10, H 9.23, N 8.16; UV−vis (Et2O)
NMR (benzene-d6, 300 MHz): δ 6.84 (4H, m, Ar m-H), 6.79 (2H, m,
Ar o-H), 5.24 (1H, s, CHCC(CH3)3), 4.47 (5H, sept., CH(CH3)2),
(λmax (ε in mM−1 cm−1)): 278 (21), 390 (18), 531 (2); H NMR
1
2
3.03 (24H, s, O−(CH2)2−O), 2.15 (12H, d, JH,H = 6.6 Hz,
(benzene-d6, 300 MHz): δ 7.04 (1H, m, Ph p-H), 6.96 (2H, m, Ar p-
2
CH(CH3)2), 1.55 (12H, d, JH,H = 6.9 Hz, CH(CH3)2), 1.30 (18H,
H), 6.86 (4H, m, Ar m-H), 6.72 (2H, m, Ph m-H) 5.38 (1H, s,
s, C(CH3)3). 13C NMR (benzene-d6, 300 MHz): δ 164.3 (NCC-
(CH3)3), 152.4 (Ar o-C), 141.8 (Ar i-C), 141.6 (Ar i-C), 123.1 (Ar p-
C), 122.5 (Ar p-C), 122.0 (Ar m-C), 121.5 (Ar m-C), 98.1
(CHC(CH3)3), 69.9 (O−(CH2)2−O), 42.8 (C(CH3)3), 42.7
(C(CH3)3), 33.4 (C(CH3)3), 28.6 (CH(CH3)2), 28.3 (CH(CH3)2),
25.5 (CH(CH3)2), 24.1 (CH(CH3)2), 23.9 (CH(CH3)2), 23.8
(CH(CH3)2). IR (KBr): 2954(vs), 2900(vs), 2866(s), 1527(w),
1513(m), 1460(m), 1445(m), 1412(vs), 1365(w), 1381(w), 1352(m),
1321(m), 1251(m), 1220(w), 1196(vw), 1161(w), 1112(vs),
1056(vw), 1031(w), 961(m), 828(m), 778(w), 768(w), 757(w) cm−1.
2
CHCC(CH3)3), 5.05 (2H, d, JH,H = 7.6 Hz, Ph o-H), 4.14 (2H, m,
2
CH(CH3)2), 3.82 (2H, m, CH(CH3)2), 2.01 (6H, d, JH,H = 6.4 Hz,
2
CH(CH3)2), 1.68 (2H, s, NH2), 1.55 (6H, d, JH,H = 6.8 Hz,
2
CH(CH3)2), 1.38 (6H, d, JH,H = 6.8 Hz, CH(CH3)2), 1.27 (6H, d,
2JH,H = 6.8 Hz, CH(CH3)2), 1.08 (18H, s, C(CH3)3) ppm; 13C NMR
(benzene-d6, 300 MHz): δ 167.8 (NCC(CH3)3), 165.8 (NCC-
(CH3)3), 154.1 (Ph i-C), 152.8 (Ar o-C), 149.3 (Ar o-C), 142.6 (Ar i-
C), 140.1 (Ar i-C), 129.5 (Ph p-C), 124.6 (Ar p-C), 124.4 (Ar p-C),
123.7 (Ar m-C), 123.1 (Ar m-C), 118.4 (Ph m-C), 114.3 (Ph o-C),
98.6 (CHCC(CH3)3), 42.2 (C(CH3)3), 33.3 (C(CH3)3), 28.5
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Synthesis of [LtBuNi(η1-NNPh)] (3). 1,2-Diisopropyl azodicarbox-
ylate (DIAD) (45 μL, 0.22 mmol) was added to a solution of
[LtBuNi(η2-NPhNH2)] (1) (150 mg, 0.22 mmol) in 20 mL acetonitrile
at room temperature. After stirring for 16 h a violet solid precipitated
which was separated from the solvent by filtration. The residue was
extracted with hexane twice (10 mL). Subsequent filtration and
removing the solvent from the filtrate in vacuo yielded violet
[LtBuNi(η1-NNPh)], 3, (94 mg, 0.14 mmol, 64%). Recrystallization
of the crude product from diethyl ether afforded dark violet crystals.
Elemental analysis (%) calc. for C41H58N4Ni (665.64 g mol−1): C
73.98, H 8.78, N 8.42; found: C 73.87, H 8.91, N 7.96; UV−vis (Et2O)
(λmax (ε in mM−1 cm−1)): 274 (14), 349 (11); 1H NMR (benzene-d6,
300 MHz): δ 7.09 (6H, m, Ar−H), 6.80 (1H, m, Ph p-H), 6.41 (2H,
sealed tube generator with a graphite monochromator. The
structures were solved by direct methods (SHELXS-97)38 and
refined by full-matrix least-squares procedures based on F2 with
all measured reflections (SHELXL-97).38 Numerical absorption
correction was applied for complexes 1 and 3. All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms
were introduced in their idealized positions and refined as
riding. Crystallographic data (excluding structure factors) for
the structures reported in this paper have been deposited at the
Cambridge Crystallographic Data Centre as Supplementary
Publication Nos. CCDC 882129 (for 1) and CCDC 882130
(for 3). These data can be obtained free of charge from The
Cambridge Crystallographic Data Center via www.ccdc.cam.ac.
uk/data_request/cif. Residual electron density for 1 belonging
to the solvent of crystallization could not be modeled; squeeze
refinement was performed.
2
m, Ph m-H), 5.97 (2H, d, JH,H = 7.5 Hz, Ph o-H), 5.02 (1H, s,
CHCC(CH3)3), 4.31 (4H, sept, CH(CH3)2), 1.97 (12H, d, 2JH,H = 6.6
2
Hz, CH(CH3)2), 1.45 (12H, d, JH,H = 6.9 Hz, CH(CH3)2), 1.10
(18H, s, C(CH3)3) ppm; 13C NMR (benzene-d6, 300 MHz): δ 186.4
(Ph i-C), 167.2 (NCC(CH3)3), 142.2 (Ar i-C), 130.3(Ph m-C), 127.0
(Ph p-C), 125.6 (Ph o-C), 125.1 (Ar p-C), 122.9 (Ar m-C), 118.9 (Ar
o-C), 94.6 (CHCC(CH3)3), 43.0 (C(CH3)3), 33.0 (C(CH3)3), 28.8
(CH(CH3)2), 25.9 (CH(CH3)2), 23.4 (CH(CH3)2). IR (KBr):
3063(w), 2957(vs), 2925(s), 2904(s), 2867(m), 1689(vs), 1585(m),
1576(s), 1543(m) 1532(m), 1511(s), 1482(w), 1471(w), 1458(m),
1444(w), 1433 (m), 1388(vs), 1365(vs), 1317(s), 1253(m), 1220(m),
1194(w), 1180(w), 1159(w), 1135(w), 1098(m), 1030(w), 1022(w),
970(w), 933(w), 801(m), 780(s), 768(m), 754(s), 685(m), 613(w),
583(w), 529(w) cm−1.
The solution of the X-ray crystal structure of 2 did not allow
reliable structural metrics to be obtained.
RESULTS AND DISCUSSION
■
Treatment of [LtBuNi(OEt2)]6 with hydrazine dissolved in
tetrahydrofuran (THF) led to intractable products, so that
phenylhydrazine was employed instead of the parent N2H4.
During the reaction of [LtBuNi(OEt2)] with H2NNHPh in a
diethyl ether solution, gas evolution was observed and a red-
brown solution was obtained. Dark red crystals could be
isolated upon slow evaporation of the solvent from a hexane
solution. Their characterization by infrared (IR) and multi-
nuclear nuclear magnetic resonance (NMR) spectroscopy,
elemental analysis, and single-crystal X-ray diffraction (XRD)
analysis identified the product as [LtBuNi(η2-NPhNH2)] (1; see
Scheme 2).
CRYSTAL STRUCTURE DETERMINATIONS
■
All data collections were performed at 100 K with a STOE
IPDS 2T diffractometer (see Table 1). In all cases, Mo Kα
radiation (λ = 0.71073 Å) was used; the radiation source was a
Table 1. Crystal Data and Experimental Parameters for the
Crystal Structure Analyses of Compounds 1, 2 and 3
Scheme 2. Synthesis of [LtBuNi(η2-NPhNH2)] (1)
1
2
3
formula
C41H60N4Ni
667.64
C106H166N8Ni2K2O12
1940.08
C41H58N4Ni
665.62
weight (g
mol−1
crystal
)
monoclinic
triclinic
triclinic
system
space group
a (Å)
P21/n
P1
P1
̅
̅
18.6151(6)
12.2221(3)
19.5038(7)Å
90
13.2242(17)
21.3214(4)
24.3297(4)
80.963(15)
76.447(12)
78.705(13)
6495.15
9.8640(10)
12.3300(13)
16.8483(16)
95.819(8)
96.008(8)
110.731(8)
1884.7(3)
2
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
90
90
The molecular structure of 1 is shown in Figure 1. 1 contains
an anionic phenylhydrazido(1−) ligand binding in a side-on
mode, and thus the Ni ion is surrounded by four N donor
atoms in a distorted square planar fashion. The N3−N4 bond
length of the phenylhydrazido(1−) ligand amounts to
1.405(2) Å and therefore lies within the range that is typical
for η2-organohydrazido(1−) ligands;11,13 it corresponds to a
N−N single bond.
4055.9(2)
4
4
density (g
1.093
0.954
1.173
cm−3
)
μ(Mo Kα)
0.509
0.402
0.547
(mm−1
)
F(000)
1448
720
goodness of
fit, GoF
0.839
1.040
The phenyl ring is bent out of the Ni−N−N plane, as
observed previously for the side-on bound phenylhydrazi-
R
ind[I >
2σ(I)]
R1 = 0.0370
R1 = 0.0875
26
do(1−) ligands in [W(Cp)2(η2-NH2NPh)]BF4 and [Ru(η2-
wR2 = 0.0818
R1 = 0.0593
wR2 = 0.2143
R1 = 0.1082
NH2NPh)(dmpe)2]BPh4,13 which points to a sp3 hybridization
of the NPh unit. Contrary to the expectation, the Ni−NPh
bond (1.9332(15) Å) is slightly longer than the Ni−NH2 bond
(1.8691(16) Å). In most of the other known phenylhydrazide
complexes, the M−NPh bonds are shorter; however, the
situation in 1 resembles that recently reported for [Ru(η2−
Rind (all
data)
wR2 = 0.0855
−0.651/0.694
wR2 = 0.2307
−0.819/2.408
Δρmin
Δρmax
(eÅ−3
/
)
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Article
inversion motion is still fast at that temperature. However,
rotation of the phenyl ring was slowed sufficiently, so that all
CH units became inequivalent.
In order to examine whether it is possible to further activate
the N−N bond of the phenylhydrazido(1−) ligand, 1 was
treated with 1 equiv of KC8 in the presence of 18-crown-6 (see
Scheme 3), which led to a color change from red-brown to dark
red.
Scheme 3. Synthesis of [LtBuNi(μ-η2:η2-NPhNH)]K(18-
crown-6) (2)
Figure 1. Molecular structure of 1 using 50% thermal ellipsoids.
Hydrogen atoms are omitted for clarity, except those binding to N4.
Selected bond lengths (Å) and angles (°): Ni−N1, 1.8798(15) Å; Ni−
N2, 1.8837(14) Å; Ni−N3, 1.9332(15) Å; Ni−N4, 1.8691(16) Å;
N3−N4, 1.405(2) Å; N3−C36, 1.406(2) Å; N1−Ni−N2, 98.27(6)°;
N1−Ni−N3, 114.82(6)°; N2−Ni−N4, 103.63(7)°; N1−Ni−N4,
158.08(7)°; N3−Ni−N4, 43.34(7)°; N4−N3−C36, 114.68(15)°.
Workup and crystallization from diethylether led to red
crystals, which were suitable for single-crystal XRD but always
decomposed during the data collection, so that the latter could
never be completed. The solution of the X-ray crystal structure
did not allow reliable structural metrics to be obtained, but the
basic framework and structural arrangement of the system was
revealed, as displayed in Figure 2.
NH2NPh)(dmpe)2]BPh4,13 where the Ru−NPh bond is
significantly longer than the Ru−NH2 bond.
The diamagnetic complex 1 was formed by oxidation of the
Ni center during the course of a reductive deprotonation of
phenylhydrazine with a concomitant release of dihydrogen; the
formation of H2 could be detected by hydrogen sensors based
on metal insulator semiconductor structures.27 The resulting
phenylhydrazido(1−) ligand undoubtedly is bound as the
NH2−NPh− tautomer: The two protons were located in the
Fourier map and further support came from multinuclear NMR
experiments. For the NH protons of the NH2−NPh unit, a
single resonance with an integral of 2 was detected at 1.71 ppm
1
in the H NMR spectrum. The assignment was also confirmed
using a 2D 1H−15N HMQC experiment. Only one 15N
correlation with these two protons became evident, suggesting
equivalent NH protons, residing at the same N atom. The
occurrence of a single NH2 resonance results from a dynamic
behavior of the N−Ph unit.
1
Apart from the NH2 resonance, the H NMR spectrum of 1
showed a set of sharp signals for the β-diketiminato unit LtBu
.
Based on the structure depicted in Figure 1 with the N3−C36
bond sticking out (42.29(12)°) of the plane defined by Ni, N4,
and N3, eight doublet signals should be expected for the methyl
residues of the isopropyl groups, since, in the absence of a plane
of symmetry within 1, all methyl groups are chemically
inequivalent. However, apparently N3 rapidly changes config-
uration, presumably through an inversion process involving an
up−down motion of the aryl ring. On the NMR time scale, this
leads to a more symmetric average structure where C36 is
located within the N4, N3, Ni plane, and, hence, only four
doublets are observed, as the methyl groups above and below
this plane become pairwise equivalent. Consistently two of the
four methine groups also are different and lead to two
multiplets. Remarkably, the ortho protons of the phenyl group
of the phenylhydrazido(1−) unit resonate at a relatively high
field (5.06 ppm), which is unusual for protons belonging to an
aromatic systems. This may be attributed to the ring current
effect of the aryl rings at the β-diketiminato ligand. To see,
whether the dynamic process described above can be frozen
out, a sample in d8-toluene was cooled to −80 °C. However,
this did not alter the number of methyl signals; that is, the
Figure 2. Structural framework of 2, as revealed by a single crystal X-
ray diffraction (XRD) analysis. All H atoms are omitted for the sake of
clarity.
It became obvious that, similar to the case of 1, the product is
composed of a LtBuNi unit, binding a PhN2Hx entity, whose
protonation state did not become clear directly from the
diffraction analysis. However, interaction of the [(18-crown-
6)K]+ complex cation with the N2 unit already suggested that
the starting material 1 had been singly deprotonated by KC8
(with formation of H2 as a byproduct), leading to the product
[LtBuNi(μ−η2:η2-NPhNH)K](18-crown-6) (2). As mentioned
above, there is, hithero, only one report on a complex
containing a μ−η2:η2-HNNPh(2-) ligand (namely, a divana-
dium complex),22 and to our knowledge, 2 is the first
heterobimetallic complex binding a phenylhydrazido(2−)
ligand in a μ−η2:η2 geometry.
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Scheme 5. Synthesis of [LtBuNi(η1-NNPh)] (3)
To further confirm the identity, an NMR spectroscopic
analysis has been performed. As expected, the complex is
1
diamagnetic, and the H and 13C NMR spectroscopic data
revealed a highly unsymmetric structure. Apparently, the
interaction with the K+ ion freezes the N−Ph motion observed
for 1 on the NMR time scale, so that now all eight methyl
groups at the aryl rings of LtBu were found to be chemically
inequivalent, as one should expect, based on the structure
shown in Figure 2. They lead to eight doublets in the 1H NMR
spectrum, and, consistently, the four methine groups are
inequivalent, also giving rise to four multiplets. Furthermore,
the 1H NMR spectrum confirmed deprotonation of the
phenylhydrazide(1−) ligand: Only one signal with an integral
of 1 is observed for the NH proton of the NPhNH unit.
2 is very sensitive to air (in particular, to O2). Remarkably,
directed experiments unraveled that O2 replaces the PhN2H
unit in 2 to give the heterobimetallic NiII peroxo complex
[LtBuNi(μ−η2:η2-O2)]K(18-crown-6) (2a) (see Scheme 4). A
already been employed successfully for the dehydrogenation of
CpTiCl2(NHNHR) to yield the diazenido complexes
CpTiCl2(NNR) (R = Ph, Bu).
Reaction of 1 with DIAD led to a color change from red-
brown to violet within 1 day, and workup yielded a violet
powder that could be recrystallized from diethyl ether.
Investigation of a single crystal by X-ray diffraction revealed
the molecular structure shown in Figure 3. A NNPh ligand is
29
t
Scheme 4. Formation of [LtBuNi(μ-η2:η2−O2)]K(18-crown-
6) (2a)
corresponding complex with the methyl-substituted β-diketi-
minate ligand LMe had been described already28 and the LtBu
derivative has been identified as such by means of IR and NMR
spectroscopy, as well as elemental analysis.
Since in the formation of 2 KC8 formally “only” acted as a
deprotonation reagent, naturally experiments with nonredox
active bases offered itself. However, reactions with reagents like,
for instance, KOtBu proceeded differently and did not lead to
tractable products. This might indicate that the successful
formation of 2 requires an initial electron transfer from a
reductant to the NiII center, which then reduces the NHPh
unit.
In order to gather further information cyclo voltammetric
measurements were performed. A cyclic voltammogram of 1 in
THF did not show a reduction wave. Only after an irreversible
oxidation at −0.05 V vs Fc+/Fc a reduction event is observed.
This indicated that, for a successful reduction of 1, not only the
electron is needed but also the K+ cation produced in cause of
reduction with potassium, which stabilizes the anion formed by
coordination (formation of ion pairs). Consistently, all attempts
to separate the K+ ion from the peroxide oxygen atoms in the
isoelectronic complex [LMeNi(μ−η2:η2−O2)K(18-crown-6)]
had been unsuccessful, not even the powerful [2,2,2] cryptand
was capable of cleaving the K···O2 contact.28 Hence, cyclic
voltammetry (CV) experiments with 1 were repeated in the
presence of an excess of KBr (see the Supporting Information),
and remarkably an irreversible reduction could be observed
then at −2.82 V vs Fc+/Fc (irreversibility can be rationalized by
simultaneous dihydrogen formation).
Figure 3. Molecular structure of 3 using 50% thermal ellipsoids.
Hydrogen atoms are omitted for the sake of clarity. Selected bond
lengths and angles (°): Ni−N1, 1.874(4); Ni−N2, 1.872(4) Å; Ni−
N3, 1.631(6) Å; N3−N4, 1.273(9) Å; N4−C36, 1.489(10) Å; N1−
Ni−N2, 97.90(19)°; N1−Ni−N3, 133.6(3)°; N2−Ni−N3, 128.4(3)°;
Ni−N3−N4, 158.9(7)°; N3−N4−C36, 115.1(7)°.
coordinated to nickel through only one N atom, and the NN
bond length of 1.273(9) Å suggests a double bond between
these atoms, as it is comparable to the NN distances in diazene
(HNNH, dNN = 1.247−1.266 Å),30 dimethyldiazene
(H3CNNCH3, dNN = 1.245 Å),31 and diphenyldiazene
(PhNNPh, dNN = 1.173−1.259 Å).32 This interpretation is
further corroborated by the occurrence of a strong band at
1689 cm−1 in the IR spectrum, which can be assigned to the νN
2
vibration of the phenyldiazenido ligand, based on a comparison
with the corresponding absorptions of other aryl diazene
compounds (1730−1660 cm−1).15 These findings clearly
indicate a reaction as anticipated to give the envisaged product
[LtBuNi(η1-NNPh)] (3). Because of the local symmetry around
the Ni center in 3, all isopropyl residues are equivalent, so that
To further exploit the redox chemistry of 1, it was treated
with 1,2-diisopropyl azodicarboxylate (DIAD, see Scheme 5),
as the ethyl derivative, which is far more toxic and sensitive, had
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just one methine signal and two methyl signals are observed in
the H NMR spectrum.
Like NO, the diazenido group is a versatile ligand and it can
behave in different ways: It may be regarded as a deprotonated
diazene, which consequently carries a negative charge and, after
metal coordination, leads to doubly bent structures (Scheme 6,
involves π-overlap with a d orbital of the Ni, as one should
expect for the resonance structure C. Within the oxidation state
formalism, assigning the electrons in the π-bond purely to the
metal results in a d10 configuration, while an assignment to the
diazenido ligand produces a d8 configuration, and the fact that
the Ni−N bond lengths belonging to the β-diketiminato ligand
compare well with those within 1 and other β-diketiminato-
nickel(II) complexes suggests the latter, i.e., the presence of an
“imido-like” ligand.
1
Scheme 6. Valence Structures of (A) Doubly Bent, (B)
Linear, and (C) Singly Bent M−N−N−R Structural
Skeletons
To further analyze the electronic structure of 3, DFT
calculations were performed (B3LYP Def2-TZVPD/Def2-
SVP).37 A geometry optimization was carried out, setting out
with the structure shown in Figure 3. The ground state turned
out to be a closed-shell singlet state. Considering the above-
mentioned results obtained in the analysis of the complex
Tp*Ni(NO),34 also a thorough search for wave functions with
broken symmetry (e.g., Ni2+ (S = 1) antiferromagnetically
coupled to PhNN− (S = 1)) was performed. However, no
stable broken symmetry solution was found, optimizations
resulted only in an additional singlet solution with some spin
polarization and almost the same geometry and energy as the
closed-shell singlet state (energy difference of 0.90 kJ/mol; see
the Supporting Information). The calculated ground-state
structure compares well with the experimental structure (see
the Supporting Information); only the Ni−NN-Ph unit is
somewhat closer to linearity in the calculated structure, which
probably indicates that the corresponding angles in 3 are
indeed influenced by packing forces. A subsequent NBO
analysis revealed two π bonds, Ni−N3 and N3−N4, which yet
again strongly supports bonding situation C. A N3−N4 σ bond
was found, too, but instead of a Ni−N3 σ bond, a strong
stabilizing donor−acceptor interaction of the N3 lone pair with
the Ni 4s orbital was revealed (see Figure S2 in the Supporting
Information). The NBO charge distribution provides further
evidence for bonding situation C, since the N3 and N4 atoms
are much less negative (−0.3 and −0.2) than N1 and N2 (both
−0.7).
A). However, a two-electron transfer to the metal center leads
to a RNN+ ligand resembling NO+ (B) and back-donation from
the metal leads to the resonance structure C (corresponding
complexes show a singly bent MNNR geometry). In the
context of NO comparison, it is noteworthy that a β-
diketiminato-nickel-nitrosyl complex has already been re-
ported;33 however, the nature of the NiNO unit has not been
discussed. It is linear but this should not be taken as evidence
for an NO+ ligand in the compound as outlined by Bergman,
DeBeer, Toste, and Wieghardt et al.34 Applying spectroscopic
and correlated ab initio computational investigations for a
Tp*NiNO complex (Tp* = hydrotris(3,5-dimethylpyrazolyl)-
borate), they have deduced an electronic situation, where a
high-spin nickel(II) center is antiferromagnetically coupled to a
triplet (NO)− ligand (both S = 1 → singlet ground state for the
molecule). This discussion is also interesting with regard to the
electronic structure of 3, because it points out the limitations of
valence bond descriptions in certain cases and problems that
may be encountered in such compounds trying to apply
commonly established oxidation state formalisms. Nevertheless,
analyzing the bonding situation in 3, we set out with the
formulas shown in Scheme 6.
Considering these valence bond structures, in principle, it
should be possible to elucidate the nature of 3 and the most
relevant mesomeric structure through the analysis of the M−
N−N angle: In case A, it should amount to ∼120°; in cases B
and C, the unit should be linear. In turn, cases B and C can be
distinguished by the NNR angle. However, note that such
considerations must be treated with caution, as crystal packing
forces could have a serious impact, too. Within the structure of
3, the Ni−N3−N4 angle amounts to 158.9(7)° and the N3−
N4−C36 angle is found to be 115.1(7)°, suggesting an
electronic situation where the resonance structure C outweighs
the other ones. Remarkably, corresponding Ni−N3−N4 angles
observed for most other phenyldiazendio complexes amount to
172°−179°,15 so that they can be assigned to case C more
clearly. However, an angle comparable to that of 3 has been
found for an iron compound [Fe(ArN2){P(OEt)3}4]+ (Ar = 4−
CH3C6H4) (166.6(9)°),35 whose diazenido unit has been
discussed as ArNN+. The attribution of a bonding situation
within 3 that can mainly be described as C is supported by the
Ni−N3 bond length (1.631(6) Å), which is much shorter than
expected for a Ni−N single bond (e.g., 0.238 Å shorter than the
Ni−N4 bond length in 1) and is more comparable to Ni−N
distances observed for nickel imides (1.657(5)−1.703(4)
Å).2,36 It clearly indicates that binding of the diazenido ligand
CONCLUSIONS
■
LtBuNiI species react with H2NNHPh via electron transfer
followed by H2 evolution to yield a H2NNPh− ligand.
Reduction of the resulting NiII complex again leads to H2
formation and deprotonation, so that a phenylhydrazido(2−)
complex is produced. On the other hand, the NiII(NH2NPh)
unit can be dehydrogenated by means of 1,2-diisopropyl
azodicarboxylate (DIAD), which results in a nickel diazenido
moiety. Further work will explore the redox chemistry of
hydrazido complexes including bimetallic activation.
ASSOCIATED CONTENT
■
S
* Supporting Information
Cyclic voltammogram of 1, DFT calculations of 3, and X-ray
crystallographic data in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: Christian.limberg@chemie.hu-berlin.de.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We are grateful to the Deutsche Forschungsgemeinschaft, the
Fonds der Chemischen Industrie, the BMBF, and the
Humboldt-Universitat zu Berlin for financial support. We also
̈
would like to thank Dr. Werner Moritz and his group, especially
Agnieszka Siewek and Kai Northemann for hydrogen detection
̈
measurements.
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