Three-coordinate nickel carbene complexes and their one-electron oxidation products

Article abstract of DOI:10.1021/ja501900j

The synthesis and characterization of two new carbene complexes, (dtbpe)Ni-CH(dmp) (1; dtbpe = 1,2-bis(di-tert-butylphosphino)ethane; dmp = 2,6-dimesitylphenyl) and (dippn)Ni-CH(dmp) (2; dippn = 1,8-bis(di-iso- propylphosphino)naphthalene), are described. Complexes 1 and 2 were isolated by photolysis of the corresponding side-bound diazoalkane complexes, exemplified by (dtbpe)Ni{η²-N₂CH(dmp)} (3). The carbene complexes feature Ni-C distances that are short and Ni-C-C angles at the carbene carbon that are intermediate between 120° and 180° (155.7(3)° and 152.3(3)°, respectively). The difference between the two carbenes became obvious when their reactivity toward 1-electron oxidizing agents was studied: the oxidation of 1 led to an internal rearrangement and the formation of a nickel(I) alkyl [{κ²-P,C-di-tert-butylphosphino-di-tert-butyl- PCH(dmp)ethane}Ni][BAr^F₄] (4), while the oxidation of 2 allowed the isolation of an unrearranged product, formulated as the cationic nickel(III) carbene complex[(dippn)Ni-CH(dmp)][BAr^F₄] (6). Both oxidations are chemically reversible and the respective reductions lead to the neutral carbene complexes, 1 and 2.

Full text of DOI:10.1021/ja501900j

Article
pubs.acs.org/JACS
Three-Coordinate Nickel Carbene Complexes and Their One-Electron
Oxidation Products
Vlad M. Iluc* and Gregory L. Hillhouse
Gordon Center for Integrative Science, Department of Chemistry, The University of Chicago, Chicago, Illinois 60637, United States
S
* Supporting Information
ABSTRACT: The synthesis and characterization of two new
carbene complexes, (dtbpe)NiCH(dmp) (1; dtbpe = 1,2-
bis(di-tert-butylphosphino)ethane; dmp = 2,6-dimesitylphen-
yl) and (dippn)NiCH(dmp) (2; dippn = 1,8-bis(di-iso-
propylphosphino)naphthalene), are described. Complexes 1
and 2 were isolated by photolysis of the corresponding side-
bound diazoalkane complexes, exemplified by (dtbpe)Ni{η2-
N2CH(dmp)} (3). The carbene complexes feature Ni−C distances that are short and Ni−C−C angles at the carbene carbon that
are intermediate between 120° and 180° (155.7(3)° and 152.3(3)°, respectively). The difference between the two carbenes
became obvious when their reactivity toward 1-electron oxidizing agents was studied: the oxidation of 1 led to an internal
rearrangement and the formation of a nickel(I) alkyl [{κ2-P,C-di-tert-butylphosphino-di-tert-butyl-PCH(dmp)ethane}Ni][BArF ]
4
(4), while the oxidation of 2 allowed the isolation of an unrearranged product, formulated as the cationic nickel(III) carbene
complex[(dippn)NiCH(dmp)][BArF ] (6). Both oxidations are chemically reversible and the respective reductions lead to the
4
neutral carbene complexes, 1 and 2.
(6). These oxidations are chemically reversible and the
respective reductions lead to the neutral carbene complexes 1
and 2.
INTRODUCTION
■
Low-coordinate, late-transition metal complexes are important
targets in synthetic organometallic chemistry because these
species show unique reactivity in small molecule activation.1,2
Although terminal nickel imides are well established, there are
few reported examples of nickel carbenes: (dtbpe)NiCPh2
(dtbpe = 1,2-bis(di-tert-butylphosphino)ethane)3 and the
RESULTS AND DISCUSSION
■
The reaction of the Ni(0)-benzene adduct {(dtbpe)Ni}2(μ-
C6H6) with (dmp)CHN212 (Scheme 1) led to the formation of
the side-bound diazo complex (dtbpe)Ni{η2-N2CH(dmp)} (3).
The reaction took place rapidly at room temperature in hexanes
or pentanes, and the isolation of 3 was facilitated by its low
solubility in hydrocarbon solvents. Its IR spectrum showed a
supporting ligand-based carbene (PCcarbeneP)NiPR3 (PCcarbene
P
= o-iPr2P-C6H4-Ccarbene-C6H4-PiPr2-o).4 This situation extends
to other late-transition metals as well, with only a handful of
non-Fisher carbenes being known.5−8
3
The previously isolated nickel carbene (dtbpe)NiCPh2
took advantage of the special characteristics of the dtbpe ligand,
which increases the steric congestion around the metal center
and supports multiple bonds between nickel and π donors.3,9,10
We became interested in exploring whether a species with a
noninteger Ni−C bond order in the (dtbpe)NiCPh2
manifold was accessible. Herein, we report the synthesis of
two new nickel(II) complexes, (dtbpe)NiCH(dmp) (1, dmp
= 2,6-dimesitylphenyl) and (dippn)NiCH(dmp) (2, dippn =
1,8-bis(di-iso-propylphosphino)naphthalene), which feature
hydrogen and a bulky aryl group as substituents at the carbene
carbon. The ancillary ligand dippn is introduced to engender
rigidity and to prevent the dissociation of a phosphine donor, a
process observed for dtbpe in some cases.11 The one-electron
oxidations of 1 and 2 are also discussed. The oxidation of 1 led
to an internal rearrangement and the formation of a nickel(I)
alkyl [{κ2-P,C-di-tert-butylphosphino-di-tert-butyl-PCH(dmp)-
Scheme 1. Formation of (dtbpe)NiCH(dmp) (1)
ethane}Ni][BArF ] (4). The oxidation of 2 allowed the
4
isolation of an unrearranged product, formulated as the cationic
Received: March 1, 2014
Published: April 9, 2014
nickel(III) carbene complex [(dippn)NiCH(dmp)][BArF ]
4
© 2014 American Chemical Society
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characteristic νCN at 2058 cm−1 (cf. 2052 cm−1 in (dmp)-
CHN2) and νNN at 1518 cm−1 (cf. 1437 cm−1 in
(dmp)CHN2).12 The NMR spectra of 3 are consistent
with an unsymmetrical, planar geometry around the metal
center, with chemically nonequivalent environments for the two
a few minutes.17,18 Complex 1 was synthesized on a preparative
scale by using either a high-energy mercury UV lamp or a
commercial ”sun lamp”; no special quartz glassware was
required (no difference was observed in a control experiment
when employing both quartz and regular glassware vessels side
by side).
1
phosphorus nuclei (δ 91.9, 88.9; JPP = 63.2 Hz). In the H
1
NMR spectrum, the resonance for the unique hydrogen of the
diazo ligand was found as a doublet at 5.36 ppm, coupled to
one phosphine (4JHP = 6 Hz) and shifted by more than 1 ppm
from the resonance for free (dmp)CHN2 (δ = 4.04).
The solid-state molecular structure of 3, determined by
single-crystal X-ray diffraction (Figure 1), features a planar
The H NMR spectrum of 1 features a specific downfield
resonance for the α-hydrogen atom at 11.57 ppm, appearing as
a triplet due to the coupling with the two phosphorus nuclei
(JHP = 15.2 Hz). In the 31P NMR spectrum, the downfield
singlet at 117.91 ppm is characteristic of a trigonal nickel
carbene with the carbene substituents perpendicular to the
coordination plane. The 13C{1H} NMR spectrum features a
downfield-shifted carbene carbon at 205.9 ppm, as a triplet,
with coupling to the two phosphorus nuclei (JCP = 51.5 Hz); a
doublet of triplets, with the coupling constant JCH = 98.3 Hz
(Figure 2),19−24 was observed in the H-coupled-13C NMR
1
spectrum, as a consequence of additional coupling with the
hydrogen nucleus (shown in Figure 2).
Figure 2. Downfield region of the 13C NMR spectrum of 1.
Figure 1. Thermal-ellipsoid (50% probability) representation of
(dtbpe)Ni{η2-N2CH(dmp)} (3, irrelevant H atoms omitted for
clarity). Selected distances (Å) and angles (°): Ni−N(1) =
1.845(2), Ni−N(2) = 1.876(2), N(1)−N(2) = 1.257(2), Ni−P(1) =
2.1794(7), Ni−P(2) = 2.1655(6), N(1)−C(3) = 1.331(3), C(3)−
C(41) = 1.451(3) C(3)−H = 0.93(2); P(1)−Ni−P(2) = 93.66(3),
N(1)−Ni−N(2) = 39.46(7), P(1)−Ni−N(1) = 120.15(6), P(2)−Ni−
N(2) = 106.94(6), N(2)−N(1)−C(3) = 139.0(2), N(1)−C(3)−
C(41) = 124.2(2), N(1)−C(3)−H = 114.1(13), Ni−N(1)−C(3) =
149.12(15).
The solid-state molecular structure of 1 (Figure 3) supports
the solution structural assignments. In addition, it was found
that the new carbene has a shorter Ni−C distance (1.793(3) Å)
compared to that of (dtbpe)NiCPh2 (1.836(2) Å).3 The
bulky dmp substituent adopts a staggered conformation versus
nickel center (sum of angles at Ni = 360.2°). The N−N bond
of the diazo ligand lies in the P−Ni−P plane at a distance of
1.751 Å from Ni, with Ni−N(1) being 1.845(2) Å and Ni−
N(2) 1.876(2) Å. The interaction between the diazo ligand and
the metal center is not very strong: the N(1)−N(2) distance is
1.257(2) Å, in the range of a N−N double bond,13,14 indicating
the lack of significant backbonding from nickel to the NN
fragment. This interpretation is corroborated by the fact that
the νNN is shifted by only 81 cm−1 from the corresponding
frequency in the free diazo compound (see above).
Nitrogen extrusion from 3 was not straightforward. Complex
3 was stable to prolonged heating; even after a week at 80 °C
only a small amount was converted to the desired carbene,
(dtbpe)NiCH(dmp) (1). The use of Sm(OTf)3 as a
catalyst3,15,16 did not induce the formation of 1 either. It is
likely that the steric congestion in 3 prevents the coordination
of the Lewis acid to the nitrogen atom.
Since the thermal and Lewis acid treatments of 3 were not
successful, a photochemical approach was sought. After
exposing an NMR tube to a UV-light source (mercury lamp,
254 nm), the quantitative conversion of 3 to 1 was observed in
Figure 3. Thermal-ellipsoid (50% probability) representation of
(dtbpe)NiCH(dmp) (1, irrelevant H atoms omitted for clarity).
Selected distances (Å) and angles (°): Ni−C(3) = 1.793(3), Ni−P(1)
= 2.1715(9), Ni−P(2) = 2.1763(10), C(3)−C(41) = 1.454(4), C(3)−
H = 0.98(3); P(1)−Ni−P(2) = 91.60(4), P(1)−Ni−C(3) =
132.24(11), P(2)−Ni−C(3) = 129.84(11), Ni−C(3)−C(41) =
155.7(3), Ni−C(3)−H = 99.4(16), C(41)−C(3)−H = 105(2).
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the dtbpe ligand, with the central phenyl ring almost
perpendicular to the P(1)−Ni−P(2) plane (the dihedral
angle between the P(1)−Ni−P(2) plane and the C(46)−
C(41)−C(42) plane is 79.71°). The nickel center has a
trigonal-planar geometry, with the sum of the angles around
nickel being 353.68°, slightly lower than the expected 360°.
The Ni−C bond is bisecting the P(1)−Ni−P(2) angle, but is
shifted out of its plane by 26.97°. The α-hydrogen atom was
located in the electronic density map, attached to the carbene
carbon and at a distance of 0.98(3) Å from it, in agreement with
the high JCH coupling constant determined by 13C NMR
spectroscopy. The sum of the angles around the carbene carbon
is 360°. Notably, the value observed for the Ni−C(3)−C(41)
angle of 155.7° is high and falls between 120°, expected for an
sp2 hybridized carbon, and 180°, for an sp carbon. It is likely
that this value is a consequence of the steric requirements
imposed by the dmp substituent and the t-butyl groups: an
NBO analysis of 1 indicates a bond order of 2 for NiC.
DFT calculations offered insight into the electronic structure
of 1. The HOMO of the carbene model complex (dmpe)N
C(H)Ph (dmpe = 1,2-bis(dimethylphosphino)ethane) is a
corresponding to the σ and π Ni−C bond, respectively (Figure
4).
In order to determine whether a cationic nickel carbene is
accessible, the oxidation of 1 was attempted. The reaction of a
cold (−35 °C) diethyl ether solution of 1 with [Cp2Fe+][B-
(ArF)4 ] (Scheme 2) led to a new nickel cationic complex
−
a
Scheme 2. Oxidation of (dtbpe)NiCH(dmp) (1)
2
bonding orbital of π symmetry between the dx2−y orbital of
nickel and the px orbital of carbon (Figure 4). The other
frontier molecular orbitals also agree with the above bonding
description: HOMO−4 is constituted by the σ Ni−C bond,
while LUMO and LUMO+1 are the antibonding orbitals
a
A = B(ArF)4.
formulated as [{κ2-P,C-di-tert-butylphosphino-di-tert-butyl-
PCH(dmp)ethane}Ni][B(ArF)4 ] (4), in which one of the
−
phosphine donors migrated to the carbene fragment generating
an ylide during an intramolecular rearrangement. The light-
yellow complex 4 has a magnetic moment of 2.1 μB in solution
(measured by the Evans method)25,26 that corresponds to one
unpaired electron.
The single-crystal X-ray structure of 4 (Figure 5) revealed
that the oxidation took place with an internal rearrangement.
The Ni−C distance of 2.077(4) Å is longer than in 1 and in the
range of Ni(I)−C alkyl single bonds.27,28 One of the
phosphorus atoms of the dtbpe ligand is still attached to nickel
and one of the mesityl substituents from the dmp ligand is
coordinated in an η2 fashion. The C(321)−C(322) distance
(1.432(5) Å) of the phenyl ring coordinated to nickel is slightly
longer than a usual aromatic C−C distance (1.38 Å), but the
aromaticity is not disrupted. This is the longest C−C distance;
the other C−C distances of the phenyl ring vary between
1.371(5) and 1.424(5) Å. It is proposed that one of the
phosphine branches of the dtbpe ligand in 4a (Scheme 2)
migrated from nickel to the carbene carbon, generating a
phosphonium ion. The coordinative properties of ylides have
been studied but it is interesting to note that ylide complexes
are mostly synthesized from phosphonium ions by deprotona-
tion.29−32
The rearrangement induced by the redox process is
reversible. By treating 4 with a one-electron reducing agent
(KC8), the carbene 1 was obtained (Scheme 2). The reversible
nature of the 1-to-4 conversion is supported by cyclic
voltammetry data (Figure 6). A 10−2 M solution of 1 in 1 M
[nBu4N][PF6]/THF shows two quasi-reversible waves, at −0.61
and −2.76 V, and an irreversible wave, at −0.15 V (vs Fc/Fc+).
Figure 4. Frontier molecular orbitals for (dmpe)NC(H)Ph, a
computational model of 1.
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SMesO• even after extended periods of time at increased
temperatures. This lack of reactivity is likely not related to steric
factors, since the same reagent abstracts hydrogen from
(dtbpe)Ni−NH(dmp) and (dtbpe)Ni−PH(dmp),36 but to a
higher dissociation energy of the C−H compared to the N−H
and P−H bonds.37 Complex 1 also exhibits an unusual
chemical inertia toward strong bases; it did not react with
i
nBuLi, (Me3Si)2NNa, Pr2NLi, or CH3Li even at 80 °C.
It is possible that the internal redistribution of electrons to
give a Ni(I) complex is favored by the dissociation of one of the
phosphines. It was reasoned that replacing the ethylene
backbone with a 1,8-naphthylene spacer might prevent
phosphine dissociation. Although 6-member rings are consid-
ered to be less stable than 5-member rings, the formation of a
6-member nickel species may prevent the rearrangement
process, since the 1,8-naphthylene ligand would lead to the
formation of a 7-member ring, likely an unfavorable process.
The second factor that could prevent the rearrangement
identified during the oxidation of 1 is kinetic. An equilibrium
between the complex with the ligand coordinated in a chelating
fashion and the complex with the ligand coordinated in a
monodentate fashion exists in solution. The chelated species is
favored, but the equilibrium can be shifted away from it if
additional donors are present. The coordination of dtbpe as a
monodentate or bridging ligand was observed in several
instances.11 During the rearrangement of 4a, the migration of
the phosphine ligand from nickel to carbon might involve such
a process. Replacing the ethylene backbone with 1,8-
naphthylene would make this ligand rigid. Even in the extreme
case of monodentate coordination, the other phosphine arm is
forced to stay in close proximity. The rotation around the P−
Cnaphthalene bond is also hindered, making the switch between
monodentate and bidentate coordination unlikely.
Figure 5. Thermal-ellipsoid (50% probability) representation of [{κ2-
P,C-di-tert-butylphosphino-di-tert-butyl-PCH(dmp)ethane}Ni][B-
−
(ArF)4 ] (4, irrelevant H atoms omitted for clarity). Selected distances
(Å) and angles (°): Ni−C = 2.077(4), Ni−P(2) = 2.2275(12), Ni−
C(321) = 2.136(4), Ni−C(322) = 2.092(4), C−P(1) = 1.811(4), C−
C(311) = 1.538(5), C(321)−C(322) = 1.432(5); P(2)−Ni−C =
106.35(12), Ni−C−P(1) = 109.0(2), Ni−C−C(311) = 107.2(3),
P(1)−C−C(311) = 125.5(3), C(321)−Ni−C(322) = 39.57(14).
Consequently, a Ni(0) precursor, (dippn)Ni(cod), was
synthesized and characterized.38 The complex (dippn)Ni(cod)
reacts cleanly (Scheme 3) and in high yield with (dmp)CHN2.
Scheme 3. Formation of (dippn)NiCH(dmp) (2)
Figure 6. Cyclic voltammogram for (dtbpe)NiCH(dmp) (1) at 100
mV/s, 10 mM in 1 M [nBu4N][PF6] in THF, Cp2Fe/Cp2Fe+
corrected.
The quasi-reversible waves were assigned to the [(dtbpe)Ni
CH(dmp)+]/(dtbpe)NiCH(dmp) (E1/2 = −0.61 V) and
(dtbpe)NiCH(dmp)/[(dtbpe)NiCH(dmp)−] (E1/2
=
−2.76 V) processes. These values are close to the
corresponding values observed for (dtbpe)NiN(dmp)
(−0.76 and −2.81 V, respectively) under similar conditions.39
The irreversible wave (at −0.15 V) is consistent with the
formation of a product resulting from the oxidation of
(dtbpe)NiCH(dmp) and present in solution along [(dtbpe)-
NiCH(dmp)+]. This wave, present at low-speed scanning
rates, loses intensity at scanning rates higher than 100 mV/s.
We tentatively assign this to the oxidation of 4, an intermediate
not observed at fast scanning rates.
The reaction was performed in the dark to avoid the
decomposition of the Ni(0) product, (dippn)Ni{N,N-η2-
N2CH(dmp)} (5). The complex 5 can be readily isolated
due to its insolubility in pentanes. Its infrared spectrum features
a νCN stretch at 2055 cm−1 and a νNN stretch at 1512 cm−1
(compared to 2052 cm−1 and, respectively, 1437 cm−1 in free
(dmp)CHN2), a good indication for the η2 coordination of the
NN double bond. The key structural parameters describing
Removal of the carbene hydrogen of 1 could, in principle,
generate a nickel carbyne species. The bulky, stable radical
1,3,5-tri-tert-butylphenoxyl33−35 (SMesO•) has been used
successfully in removing the hydrogen atom from both the
amide (dtbpe)Ni−NH(dmp) and the phosphide (dtbpe)Ni−
PH(dmp).36 The carbene 1, however, does not react with
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the geometry of the nickel center obtained from the solid-state
structure of 5 (Figure 7) are similar to those of 3. The nickel
Figure 8. Thermal-ellipsoid (50% probability) representation of
(dippn)NiCH(dmp) (2, irrelevant H atoms omitted for clarity).
Selected distances (Å) and angles (°): Ni−C = 1.797(3), Ni−P(1) =
2.1322(10), Ni−P(2) = 2.1344(10), C−C(61) = 1.457(4), C−H =
1.02(3); P(1)−Ni−P(2) = 96.52(4), P(1)−Ni−C = 126.12(10),
P(2)−Ni−C = 133.79(11), Ni−C−C(61) = 152.3(3), Ni−C−H =
97.5(16).
Figure 7. Thermal-ellipsoid (50% probability) representation of
(dippn)Ni{η2-N2CH(dmp)} (5, irrelevant H atoms omitted for
clarity). Selected distances (Å) and angles (°): Ni−N(2) =
1.883(5), Ni−N(1) = 1.909(5), Ni−P(1) = 2.124(2), Ni−P(2) =
2.1361(15), N(1)−N(2) = 1.212(6), N(1)−C = 1.359(7); P(1)−Ni−
P(2) = 99.98(7), N(1)−Ni−N(2) = 37.3(2), P(1)−Ni−N(2) =
104.68(15), P(2)−Ni−N(1) = 118.1(2), N(2)−N(1)−C = 142.0(6),
N(1)−C−C(61) = 131.7(6).
center is planar and the two phosphorus atoms, Ni, and the two
nitrogen atoms are coplanar. The imine hydrogen atom was
located in the electron density map at 0.82 Å from carbon.
By exposing a diethyl ether solution of 5 to ultraviolet light
(either a “sun-lamp” or a mercury lamp) for several days,
dinitrogen was lost and the new carbene (dippn)Ni
CH(dmp) (2) could be isolated in a quantitative yield as
bright emerald-green crystals (Scheme 3). The reaction is very
clean; no byproducts were observed and no decomposition
occurred when the reaction mixture was kept at room
temperature.
Figure 9. Space-filling models for 1 and 2.
This steric repulsion determines the magnetic nonequivalence
of the methyl groups of the iso-propyl substituents even in
1
solution, as observed in the H NMR spectra of 2.
The new carbene 2 is a robust complex and not very reactive;
short exposure of the crystalline material or its solution to air or
water does not affect its integrity. The chemical stability of 2 is
likely due to the extremely bulky ligands protecting the core of
the complex. Additionally, common strong bases (MeLi, n-
butyllithium, (Me3Si)2NNa, or KH) do not react with it, and
abstraction of the hydrogen atom as a radical using SMesO• was
not possible.
1
The H NMR spectrum of 2 indicated that the carbene
proton appeared downfield, at 12.21 ppm, as a triplet due to
coupling to the two phosphorus nuclei. The spectroscopic
resemblance between 2 and 1 was also observed in the solid-
state structure. The single-crystal data (Figure 8) revealed that
the Ni−C distance (1.797(3) Å) has, within errors, the same
value for both carbenes, while the Ni−C−C angle (152.3(3)°)
is slightly smaller for 2 than for 1. The metal center is
pseudotrigonal planar, with the sum of angles of 356.43°
around nickel, slightly off the planar requirements, possibly a
consequence of the small displacement of the Ni−C bond from
the phosphine ligand plane (by 23.1°). The carbene hydrogen
atom was located in the electron density map at 1.02(3) Å from
the carbon atom. Interestingly, the dmp substituent is in a
twisted conformation with respect to the dippn ligand (Figure
8). For the analogous carbene 1, the two mesityls are in a
staggered conformation with the dtbpe ligand (vide supra), but
in 2 the dihedral angle between the P(1)−Ni−P(2) plane and
the C(62)−C(11)−C(66) plane is 46.2° indicating a
conformation between staggered and eclipsed. Also noticeable
from the space-filling model (Figure 9) is the problematic
fitting of the iso-propyl substituents on the phosphine ligand.
The cyclic voltammogram of 2 (Figure 10), measured for a
0.01 M THF solution, features two quasi-reversible events at
−0.75 V and −3.85 V (vs Fc/Fc+) that were assigned to the
[(dippn)NiCH(dmp)+]/ (dippn)NiCH(dmp) and
(dippn)NiCH(dmp)/[(dippn)NiCH(dmp)−] redox cou-
ples. No irreversible features, as found for 1, were detected at
low scan rates for 2. Accordingly, the oxidation of 2 using
[Cp2Fe+][B(ArF)4 ] led to the isolation of a new paramagnetic
−
complex, formulated as the cationic nickel(III) carbene
−
[(dippn)Ni{CH(dmp)}+][B(ArF)4 ] (6), as a light-pink
powder (Scheme 4). Unfortunately, all the attempts to grow
single crystals of 6 suitable for X-ray diffraction were not
successful.
Complex 6 is a one-electron paramagnet as indicated by its
room-temperature μeff of 2.0 μB, measured in a CD2Cl2 solution
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Figure 11. Solid-state magnetization data for [(dippn)NiCH-
−
(dmp)+][B(ArF)4 ] (6) as a function of temperature (5−300 K,
Figure 10. Cyclic voltammogram for (dippn)NiCH(dmp) (2) at
100 mV/s, 10 mM in 1 M [nBu4N][PF6] in THF, Cp2Fe/Cp2Fe+
corrected.
5000 G).
Scheme 4. Formation of
a
−
[(dippn)Ni{CH(dmp)}+][B(ArF)4 ] (6)
Figure 12. EPR spectrum (1 mM in CH2Cl2, 5 K) of [(dippn)Ni
−
CH(dmp)+][B(ArF)4 ] (6).
comparison indicates that [(dippn)NiCH(dmp)+] (6) is
more stable than the presumed rearranged product [6a] by
−22.8 kcal/mol. These values are in agreement with the fact
that 6 features a more rigid ligand than 4 and is less likely to
rearrange to a nickel(I) alkyl phosphonium product, as was
observed for 4 (Figure 13).
CONCLUSIONS
a
A = B(ArF)4.
■
In conclusion, two new nickel carbenes, (dtbpe)NiCH(dmp)
(1) and (dippn)NiCH(dmp) (2), were isolated and
characterized. To stabilize these species, dmp (2,6-dimesityl-
by the Evans method, and variable-temperature solid-state
measurements (using a superconducting quantum interference
device). Although two electron configurations are possible for a
d7 trigonal-planar ion, high spin with three unpaired electrons
and low spin with one unpaired electron, both the solution and
the solid-state magnetization measurements indicate the
presence of one unpaired electron (Figure 11). Similar results
were obtained for the cationic nickel imide [(dtbpe)Ni
39
N(dmp)+][B(ArF)4 ], which has a low-spin configuration.
−
Complex 6 was characterized further by EPR spectroscopy
(Figure 12). The EPR X-band spectrum of 6 (1 mM in
CH2Cl2) at 5 K is also consistent with the presence of an
unpaired electron on the nickel center.40 The spectrum exhibits
a rhombic band, with unresolved hyperfine structure likely due
to 31P delocalization and is centered around g = 2.2. Overall,
the EPR spectrum of 6 shows similar features to that of
39
−
[(dtbpe)NiN(dmp)+][B(ArF)4 ].
DFT calculations support the assignment of 6 as [(dippn)-
NiCH(dmp)+][B(ArF)4 ] as follows. When the total
−
bonding energy of the putative carbene [(dtbpe)NiCH-
(dmp)+] ([4a]) was compared to that of 4, it was found that 4
is more stable than [4a] by −8.4 kcal/mol. The same
Figure 13. Calculated free energies (kcal/mol) for the rearrangement
of 6 to [6a] (top) and [4a] to 4 (bottom).
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The resulting yellow precipitate was suspended in 5 mL n-pentane,
filtered, washed 3 times with 1 mL of n-pentane, and dried under
reduced pressure to yield 131.7 mg (0.18 mmol, 90%) of pure 3 as a
yellow crystalline powder. Single crystals suitable for X-ray diffraction
analysis were obtained by the slow crystallization of a diluted solution
phenyl) was employed to offer the necessary steric protection
to the metal core. The other carbene substituent is hydrogen.
Structurally, 1 and 2 were characterized by short Ni−C
distances and Ni−C−C angles at the carbene carbon that are
intermediate between 120° and 180° (155.7° and 152.3(3)°,
respectively). Attempts to oxidize 1 and/or to abstract its
carbene hydrogen did not lead to the formation of a carbene or
carbyne moiety. Instead, phosphorus migration from nickel to
the carbene carbon occurred, leading to the formation of a
Ni(I) ylide complex, 4. In contrast, the oxidation of 2 led to a
1
of 3 in n-pentane at −35 °C. For 3: H NMR (20 °C, 500.13 MHz,
C6D6): δ 7.16 (m, 3H, C6H3), 6.93 (s, 4H, C6H2), 5.37 (d, JHP = 6.0
Hz, 1H, CHN), 2.40 (s, 12H, CH3), 2.32 (s, 6H, CH3), 1.08 (m,
4H, CH2), 1.03 (d, JHP = 12 Hz, 18H, C(CH3)3), 0.88 (d, JHP = 12 Hz,
18H, C(CH3)3). 31P{1H} NMR (20 °C, 202.47 MHz, C6D6): δ 91.91
(d, JPP = 63.2 Hz), 88.98 (d, JPP = 63.2 Hz). 13C{1H} NMR (20 °C,
125.77 MHz, C6D6): δ 140.69 (s, CAr), 137.28 (s, CAr), 136.70 (s, CAr),
136.45 (s, CAr), 134.51 (s, CAr), 129.21 (s, CAr), 128.59 (s, CAr), 123.09
(s, CAr), 85.50 (t, JCP = 9.7 Hz, CHN), 34.21 (dd, JCP = 12.0 Hz, JCP
= 12.0 Hz, CH2), 33.81 (dd, JCP = 12.0 Hz, JCP = 12.0 Hz, CH2), 30.16
(t, JCP = 7.8 Hz, C(CH3)3), 22.74 (t, JCP = 15.9 Hz, C(CH3)3), 21.87
(t, JCP = 15.9 Hz, C(CH3)3), 21.49 (s, CH3), 21.35 (s, CH3). IR (CaF2,
Fluorolube): 2997 (m), 2974 (m), 2943 (m), 2912 (m), 2863 (m),
2058 (w, νCN), 1519 (s, νNN), 1477 (m), 1420 (m) cm−1.
Elemental analysis for C43H66N2NiP2: C% 70.59, H% 9.09 N% 3.83;
found C% 71.13, H% 8.82, N% 3.84.
−
product formulated as [(dippn)NiCH(dmp)+][B(ArF)4 ]
(6). Complex 6 was characterized by solution and solid-state
magnetization studies and EPR spectroscopy, which indicate
that its electronic configuration is similar to that of the cationic
nickel(III) imide [(dtbpe)NiN(dmp)+][B(ArF)4 ]. DFT
−
calculations supported that assignment. The formation of the
cationic nickel(III) carbene species took advantage of the steric
characteristics of the dippn supporting ligand, featuring a rigid,
1,8-naphthylene backbone, which does not allow its rearrange-
ment, as observed for the dtbpe ligand.
Synthesis of (dtbpe)NiCH(dmp) (1). A 50 mL airtight Pyrex
glass flask was charged with 146.3 mg (0.2 mmol) of (dtbpe)Ni{η2-
N2CH(dmp)} (3) and 10 mL of hexanes, cooled to −78 °C, and
evacuated to 1 mmHg. The flask was exposed for 12 h to a 275 W
General Electric sun lamp with continuous air cooling to keep the
mixture at room temperature. The resulting emerald-green solution
was concentrated to dryness under reduced pressure to yield
analytically pure 1 (140.73 mg, 0.2 mmol, 100%) as a dark-green
EXPERIMENTAL SECTION
■
Unless noted otherwise, all operations were performed under a
purified nitrogen atmosphere in a standard MBraun Lab Master 130
drybox or under an argon atmosphere using high-vacuum and Schlenk
techniques.41 Hexanes, petroleum ether, and toluene were dried by
passage through activated alumina and Q-5 columns under nitrogen.42
CH2Cl2 was dried by distillation from CaH2 or passed twice through
activated alumina columns. THF was distilled from a dark-purple THF
solution of sodium benzophenone ketyl. Anhydrous diethyl ether was
stirred for 24 h over metallic sodium and filtered under nitrogen
through an activated alumina plug. Benzene was refluxed under
nitrogen over CaH2 and distilled. CD2Cl2 and C6D6 were purchased
from Cambridge Isotope Laboratory, degassed, dried over CaH2,
transferred under vacuum, and stored over 4 Å molecular sieves. Four
Å molecular sieves, alumina, silica, and Celite were dried under
dynamic vacuum overnight at 180 °C. Q-5 was activated by heating at
200 °C under a 5% H2 in N2 atmosphere. Unless noted, chemicals
were purchased from commercial sources and used without further
purification. KC8 ,43 (dmp)C(H)N2 ,12 [Cp2 Fe][B(3,5-
(CF3)2C6H3)4],44 [(dtbpe)Ni]2(C6H6),45 (dippn)Ni(COD),38 and
SMesO33−35 were prepared according to the literature. All NMR
spectra were recorded using a Bruker DRX-400 or DRX-500
1
crystalline powder. For 1: H NMR (20 °C, 400.13 MHz, C6D6): δ
11.57 (t, JHP = 15.2 Hz, 1H, NiCH(dmp)), 7.89 (tt, JHH = 7.4 Hz,
JHP = 3.7 Hz, 1H, p-C6H3), 6.95 (s, 4H, C6H2), 6.89 (d, JHH = 7.4 Hz,
2H, m-C6H3), 2.31 (s, 6H, CH3), 2.26 (s, 12H, CH3), 1.90 (d, JHP
=
11.7 Hz, 18H, C(CH3)3), 1.21 (m, 2H, CH2), 0.98 (d, JHP = 11.7 Hz,
18H, C(CH3)3), 0.87 (m, 2H, CH2). 31P{1H} NMR (20 °C, 161.98
MHz, C6D6): δ 117.91 (s). 13C{1H} NMR (20 °C, 100.62 MHz,
C6D6): δ 205.93 (t, JCP = 51.5 Hz, NiCH(dmp)) 158.30 (d, JCP
=
7.5 Hz, CAr), 144.22 (s, CAr), 140.40 (m, CAr), 134.87 (s, CAr), 134.54
(s, CAr), 131.12 (d, JCP = 7.2 Hz, CAr), 129.29 (s, CAr), 124.36 (s, CAr),
34.09 (m, C(CH3)3), 33.41 (m, C(CH3)3), 31.06 (s, C(CH3)3), 30.48
(s, C(CH3)3), 22.38 (m, CH2), 21.5 (s, CH3), 21.37 (s, CH3). 13C
NMR (20 °C, 125.77 MHz, C6D6): δ 205.9 (dt, JCP = 51.5 Hz, JCH
=
98.3 Hz, NiCH(dmp)). Elemental analysis for C43H66NiP2: C%
73.40; H% 9.45; found: C% 73.24, H% 9.26.
Synthesis of [{κ2-P,C-di-tert-butylphosphino-di-tert-butyl-
PCH(dmp)ethane}Ni][BArF ] (4). A cold (−35 °C) solution of
4
−
[Cp2Fe+][B(ArF)4 ] (52.46 mg, 0.05 mmol in 5 mL of Et2O) was
1
spectrometer. H NMR spectra were referenced to solvent residual
peaks at δ 7.16 for C6D6, and δ 5.32 for CD2Cl2. 31P NMR spectra
were measured using a 250 ppm window and offset referenced to an
external standard, 85% phosphoric acid (δ 0.00 ppm). 13C NMR
spectra were acquired using the pulse-Fourier technique. Proton-
decoupled spectra were collected with Walts decoupling. 13C NMR
spectra were referenced to solvent residual peaks, δ 128.0 for C6D6 and
δ 53.8 for CD2Cl2. Infrared spectra were measured as Fluorolube-S20
or Nujol mulls between CaF2 or KBr plates using a Nicolet 20-SXB
spectometer with TGS detector. Solution magnetic susceptibilities
were calculated using the Evans method25,26 using trimethylsilyl ether
as an internal standard. Elemental analyses were performed by
Columbia Analytics (Tucson, AZ) or by Midwest Microlab (Indian-
apolis, IN). Cyclic voltametry experiments were carried out on an Eco-
Chemie Autolab potentiostat. EPR spectra were collected on a
BrukerEMX EPR spectrometer. Magnetic susceptibility measurements
were carried out on batches obtained independently until at least two
different experiments gave superimposable results. Magnetic suscept-
ibility measurements were recorded using a SQUID magnetometer at
5000 G.
added dropwise to a solution of 1 (35.2 mg, 0.05 mmol in 5 mL of
Et2O) at −35 °C. The mixture was stirred at room temperature for 30
min. After removing the volatiles under reduced pressure, the residue
was triturated 5 times with 5 mL of n-pentane. Dissolving the residue
in 5 mL of Et2O and layering the solution with n-pentane at −35 °C
induced crystallization. Subsequent filtration led to pure 4 (65.80 mg,
0.042 mmol, 84%). For 4: 1H NMR (20 °C, 400.13 MHz, CD2Cl2): δ
20.91 (b, Δ1/2 = 250 Hz, 18H, C(CH3)3), 8.72 (br, Δ1/2 = 230 Hz,
18H, C(CH3)3), 7.74 (s, 8H, C6H2(CF3)2), 7.57 (s, 4H, C6H2(CF3)2),
6.34 (br, Δ1/2 = 120 Hz, 4H, C2H4), 6.25 (br, Δ1/2 =70 Hz, 3H,
C6H2(CH3)3), 5.33 (b, Δ1/2 = 70 Hz, 6H, C6H2(CH3)3), 1.72 (br, Δ1/2
= 50 Hz, 6H, C6H2(CH3)3), 1.15 (br, Δ1/2 = 50 Hz, 3H,
C6H2(CH3)3), 0.95 (br, Δ1/2 = 80 Hz, 3H, C6H2(CH3)3), 0.09 (br,
Δ
1/2 = 50 Hz, 1H, CH(dmp)), 2.4 (br, Δ1/2 = 180 Hz, 4H, C6H3). IR
(CaF2, Fluorolube): 3177 (m), 2959 (m), 1611 (w), 1479 (m), 1354
(s) cm−1. Magnetic moment (Evans, CD2Cl2, 20 °C): μeff = 2.1 μB.
Elemental analysis for C75H78NiP2BF24: C% 57.49; H% 5.02; found: C
% 56.46, H% 5.36.
Reduction of [{κ2-P,C-di-tert-butylphosphino-di-tert-butyl-
PCH(dmp)ethane}Ni][BArF ] (4). A cold (−35 °C) suspension of
Synthesis of (dtbpe)Ni{η2-N2CH(dmp)} (3). To a suspension of
83.4 mg (0.1 mmol) of [(dtbpe)Ni]2(μ-C6H6) in 5 mL of n-pentane
was added a solution of 70.9 mg (0.2 mmol) of (dmp)C(H)N2 in 5
mL of n-pentane. The mixture was stirred for 2 h at room temperature.
4
KC8 (5 mg, 0.037 mmol) in 2 mL of Et2O was added dropwise to a
solution of [{κ2-P,C-di-tert-butylphosphino-di-tert-butyl-PCH(dmp)-
ethane}Ni][BArF ] (4, 58 mg, 0.037 mmol in 5 mL Et2O) at −35 °C.
4
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The mixture was stirred at room temperature for 30 min. The solution
was concentrated to dryness and the black residue was suspended in
10 mL of hexanes. The black suspension was filtered through a plug of
Celite. Cooling the solution at −35 °C induced the crystallization of 1.
The emerald-green crystals of (dtbpe)NiCH(dmp) (1, 25 mg, 0.035
mmol 95%) were isolated by decanting the mother liquor and dried
under reduced pressure.
(s, 4H, C6H2(CF3)2), 4.12 (br, Δ1/2 = 210 Hz, 12H), 1.51 (br, Δ1/2 =
30 Hz, 12 H), 0.32 (br, Δ1/2 = 40 Hz, 1H, CH(dmp)), −2.23 (br, Δ1/2
= 60 Hz, 6 H), −4.18 (br, Δ1/2 = 105 Hz, 12 H). μeff = 2.0 μB (Evans).
Elemental analysis for C79H72NiP2BF24: C% 58.98; H% 4.51; found: C
% 59.22, H% 4.20.
Reduction of [(dippn)NiCHdmp][B(ArF)4] (6). A cold (−35
°C) suspension of KC8 (5 mg, 0.037 mmol) in 2 mL of Et2O was
added dropwise to a solution of [(dippn)NiCHdmp][B(ArF)4] (6,
60 mg, 0.037 mmol in 5 mL of Et2O) at −35 °C. The mixture was
stirred at room temperature for 30 min. The solution was concentrated
to dryness and the black residue was suspended in 10 mL of hexanes.
The black suspension was filtered through a plug of Celite. Cooling
the solution at −35 °C induced the crystallization of 2. The emerald-
green crystals of (dippn)NiCH(dmp) (2, 23 mg, 0.030 mmol, 85%)
were separated by decanting the mother liquor and dried under
reduced pressure.
Synthesis of (dippn)Ni{η2-N2CH(dmp)} (5). To a suspension of
105 mg (0.2 mmol) of (dippn)Ni(COD) in 5 mL of n-pentane was
added a solution of 71 mg (0.2 mmol) of (dmp)C(H)N2 in 5 mL of n-
pentane. The mixture was stirred for 2 h at ambient temperature. The
resulting dark-yellow precipitate was filtered, washed 3 times with 1
mL of n-pentane, and dried under reduced pressure to yield 125 mg
(0.16 mmol, 81%) of pure 5 as a yellow, crystalline powder. For 5: 1H
NMR (298 K, 500.13 MHz, C6H6): δ 7.45 (m, 2H, C6H8), 7.35 (t, JHH
= 8 Hz, 1H, C6H8), 7.11 (m, 3H, C6H3), 7.05 (t, JHH = 8 Hz, 1H,
C6H8), 7.02 (t, JHH = 8 Hz, 2H, C6H8), 6.92 (s, 4H, C6H2(CH3)3),
5.49 (d, JHP = 5 Hz, 1H, CH(dmp)), 2.39 (s, 12H, C6H2(CH3)3), 2.28
(s, 6H, C6H2(CH3)3), 2.05 (m, 2H, CH(CH3)2), 1.85 (m, 2H,
CH(CH3)2), 1.20 (dd, JHP = 15 Hz, JHH = 7 Hz, 6H, CH(CH3)2), 0.83
(m, 12H, CH(CH3)2), 0.69 (dd, JHP = 15 Hz, JHH = 7 Hz, 6H,
CH(CH3)2); 31P{1H} NMR (298 K, 202.5 MHz, C6H6): δ 46.47 (d.
JPP = 9 Hz), 43.78 (d, JPP = 9 Hz); 13C{1H} NMR (298 K, 125.77
MHz, C6H6): δ 140.78 (s, CAr), 140.31 (s, CAr), 140.54 (s, CAr), 137.07
(s, CAr), 136.36 (s, CAr), 136.12 (m, CAr), 135,71 (m, CAr), 134.36 (s,
CAr), 132.31 (s, CAr), 132.14 (s, CAr), 132.02 (s, CAr), 129.03 (s, CAr),
DFT Calculations. Gaussian 03 (revision D.02) was used for all
reported calculations.46 The B3LYP (DFT) method was used to carry
out the geometry optimizations on the model compounds specified in
text using the LANL2DZ basis set. The validity of the true minima was
checked by the absence of negative frequencies in the energy Hessian.
Tables with atomic coordinates of the optimized geometries can be
found in the Supporting Information.
X-ray Diffraction. Data were collected on a Siemens platform
goniometer with a charged coupled device (CCD) detector. Structures
were solved by direct methods using the SHELXTL (version 5.1)
program library (G. Sheldrick, Bruker Analytical X-ray Systems,
Madison, WI).47 All atoms were refined anisotropically and hydrogen
atoms were placed in calculated positions unless specified otherwise.
Tables with atomic coordinates and equivalent isotropic displacement
parameters are listed in the cifsin the Supporting Information. The
crystals were coated with oil (STP Oil Treatment) on a glass slide,
which was brought outside the glovebox.
X-ray Crystal Structure of 3. X-ray quality crystals were obtained
by slow crystallization at −35 °C from a concentrated pentanes
solution. A 0.03 × 0.03 × 0.01 mm3, dark-yellow block was chosen and
mounted on the diffractometer. A total of 26091 reflections (−11 ≤ h
≤ 11, −29 ≤ k ≤ 22, −25 ≤ l ≤ 26) was collected at T = 100(2) K
with θmax = 28.26°, of which 9703 were unique (Rint = 0.0430). The
residual peak and hole−electron density were 0.258 and −0.105 eÅ−3.
The least-squares refinement converged normally with residuals of R1
= 0.0503 (I > 2σ(I)) and GOF of 1.076. Crystal and refinement data
for 3: C43H66NiN2P2, space group P21/n, a = 8.8867(18) Å, b =
22.452(5) Å, c = 20.673(4) Å, α = γ = 90°, β = 93.025(4)°, V =
4119.1(14) Å3, Z = 4, μ = 0.579 mm−1, F(000) = 1584, R1 = 0.0607,
wR2 = 0.1236 (based on all data).
X-ray Crystal Structure of 1. X-ray quality crystals were obtained
by slow crystallization at −35 °C from a concentrated pentanes
solution. A 0.5 × 0.3 × 0.2 mm3, emerald-green block was chosen and
mounted on the diffractometer. A total of 25363 reflections (−11 ≤ h
≤ 15, −15 ≤ k ≤ 15, −40 ≤ l ≤ 30) was collected at T = 100(2) K
with θmax = 28.28°, of which 9467 were unique (Rint = 0.0717). The
residual peak and hole−electron density were 0.204 and −0.515 eÅ−3.
The least-squares refinement converged normally with residuals of R1
= 0.0574 (I > 2σ(I)) and GOF of 0.947. Crystal and refinement data
for 1: C43H66NiP2, space group P212121, a = 11.3623(16) Å, b =
11.4984(16) Å, c = 30.630(4) Å, α = β = γ = 90°, V = 4001.7(9) Å3, Z
= 4, μ = 0.592 mm−1, F(000) = 1528, R1 = 0.0812, wR2 = 0.0912
(based on all data).
127.97 (s, CAr), 123.88 (d, 4.5 Hz, CAr), 122.93 (s, CAr), 87.68 (t, JCP
=
9.8 Hz, N2CH(dmp)), 28.04 (d, JCP = 14.5 Hz, CH(CH3)2), 27.55 (d,
JCP = 20.4 Hz, CH(CH3)2), 21.08 (s, C6H2(CH3)2), 21.03 (s,
C6H2(CH3)2), 19.42 (d, JCP = 9.8 Hz, CH(CH3)2), 19.24 (d, JCP = 3.8
Hz, CH(CH3)2), 19.02 (d, JCP = 3.9 Hz, CH(CH3)2), 18.23 (d, JCP
=
5.8 Hz, CH(CH3)2); IR (CaF2, fluorolube): 2055 (w, νCN), 1512 (s,
νNN) cm−1. Elemental analysis for C47H60N2Ni: C% 72.57; H% 7.82;
N% 3.62 found: C% 72.33, H% 8.04, N% 3.50.
Synthesis of (dippn)NiCH(dmp) (2). A solution of 5 (155 mg,
0.2 mmol) in 50 mL of hexanes was exposed for 72 h to a UV light
source (250 W, GE Sun-Lamp) with constant air cooling in an airtight
100 mL glass bomb. Several aliquots were analyzed by NMR
spectroscopy to determine the reaction completion. The resulting
emerald-green solution was transferred to a flask in the drybox, filtered
through a plug of Celite, and the volatiles were removed under
reduced pressure. The dark-green, crude product was analytically pure
2. Isolated yield: 140 mg, 94%. For 2: 1H NMR (298 K, 500.13 MHz,
C6H6): δ 12.21 (t, JHP = 16 Hz, 1H, CH(dmp)), 7.66 (m, 1H,
C6H3Mes2), 7.47 (d, JHH = 8 Hz, 2H, C6H3Mes2), 7.63 (t, JHH = 7 Hz,
2H, C10H8), 7.06 (t, JHH = 7 Hz, 2H, C10H8), 6.98 (d, JHH = 7 Hz, 2H,
C10H8), 6.93 (s, 4H, C6H2(CH3)3), 2.35 (s, 12H, C6H2(CH3)3), 2.21
(s, 6H, C6H2(CH3)3), 1.96 (m, 2H, CH(CH3)2), 1.84 (m, 2H,
CH(CH3)2), 0.77 (m, 24 H, CH(CH3)2); 31P{1H} NMR (298 K,
202.5 MHz, C6H6): δ 61.49 (s); 13C{1H} NMR (298 K, 125.77 MHz,
C6H6): δ 215.82 (t, JCP = 62 Hz, CH(dmp)), 143.25 (s, CAr), 140.09
(s, CAr), 138.71 (s, CAr), 135.36 (s, CAr), 145.17 (s, CAr), 134.87 (s,
CAr), 131.50 (s, CAr), 131.22 (s, CAr), 130.68 (s, CAr), 130.64 (s, CAr),
130.61 (s, CAr), 125.21 (s, CAr), 124.53 (s, CAr), 124.48 (s, CAr), 124.44
(s, CAr), 123.90 (s, CAr), 26.00 (d, 8 Hz, CH(CH3)2), 25.17 (d, JCP = 8
Hz, CH(CH3)2), 21.85 (s, C6H2(CH3)2), 21.0 (s, C6H2(CH3)2), 19.98
(s, CH(CH3)2), 19.46 (s, CH(CH3)2), 18.81 (s, CH(CH3)2), 17.99 (s,
CH(CH3)2). Elemental analysis for C47H60NiP2: C% 75.71; H% 8.11;
found: C% 75.59, H% 8.03.
Synthesis of [(dippn)NiCHdmp][B(ArF)4] (6). A cold (−35
X-ray Crystal Structure of 4. X-ray quality crystals were obtained
by slow crystallization at −35 °C from a concentrated ether solution
layered with pentanes. A 0.1 × 0.08 × 0.05 mm3, yellow block was
chosen and mounted on the diffractometer. A total of 33275
reflections (−14 ≤ h ≤ 14, −17 ≤ k ≤ 17, −47 ≤ l ≤ 47) was
collected at T = 100(2) K with θmax = 25.00°, of which 12721 were
unique (Rint = 0.0638). The residual peak and hole−electron density
were 0.685 and −0.423 eÅ−3. The least-squares refinement converged
normally with residuals of R1 = 0.0504 (I > 2σ(I)) and GOF of 0.915.
Crystal and refinement data for 4: C75H78BF24NiP2, space group P21/
n, a = 12.7248(17) Å, b = 14.8784(19) Å, c = 39.541(5) Å, α = γ =
°C) solution of [FeCp2][BArF ] (105 mg, 0.1 mmol in 5 mL of Et2O)
4
was added to a solution of 2 (75 mg, 0.1 mmol in 5 mL of Et2O) at
−35 °C. The mixture was stirred at ambient temperature for 30 min.
After removing the volatiles under reduced pressure, the residue was
triturated 3 times with 5 mL of n-pentane. Dissolving the residue in 5
mL of Et2O and layering the solution with n-pentane at −35 °C
induced crystallization. Subsequent filtration led to analytically pure 6
1
as light-pink crystals (120 mg, 74%). For 6: H NMR (298 K, 400.13
MHz, C6H6): δ 15.11 (br, Δ1/2 = 90 Hz, 4H), 11.58 (br, Δ1/2 = 100
Hz, 6H), 8.23 (br, Δ1/2 = 35 Hz, 7 H), 7.48 (s, 8H, C6H2(CF3)2), 7.31
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90°, β = 93.228(3)°, V = 7474.1(17) Å3, Z = 4, μ = 0.404 mm−1,
F(000) = 3228, R1 = 0.0768, wR2 = 0.1225 (based on all data).
X-ray Crystal Structure of 5. X-ray quality crystals were obtained
by slow crystallization at −35 °C from a concentrated pentanes
solution. A 0.05 × 0.02 × 0.02 mm3, dark yellow block was chosen and
mounted on the diffractometer. A total of 21087 reflections (−29 ≤ h
≤ 28, −24 ≤ k ≤ 20, −9 ≤ l ≤ 9) was collected at T = 100(2) K with
θmax = 25.00°, of which 7296 were unique (Rint = 0.0794). The residual
peak and hole−electron density were 0.932 and −0.479 eÅ−3. The
least-squares refinement converged normally with residuals of R1 =
0.0593 (I > 2σ(I)) and GOF of 0.863. Crystal and refinement data for
5: C47H60N2NiP2, space group Pna21, a = 24.967(4) Å, b = 20.773(3)
Å, c = 8.0830(12) Å, α = γ = 90°, β = 90.907(2)°, V = 4192.1(11) Å3,
Z = 4, μ = 0.573 mm−1, F(000) = 1656, R1 = 0.0593, wR2 = 0.1231
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X-ray Crystal Structure of 2. X-ray quality crystals were obtained
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The least-squares refinement converged normally with residuals of R1
= 0.0486 (I > 2σ(I)) and GOF of 0.907. Crystal and refinement data
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̅
47 60
2
13.0889(11) Å, c = 39.706(3) Å, β = 90.907(2)°, V = 7409.5(11)
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details for compound characterizations, DFT
calculation details, full crystallographic descriptions (as cif), and
complete reference 46. This material is available free of charge
via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: viluc@nd.edu
■
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Present Address
Vlad M. Iluc, Department of Chemistry and Biochemistry,
University of Notre Dame, Notre Dame, IN 46556.
Notes
The authors declare no competing financial interest.
Gregory L. Hillhouse was deceased March 6, 2014.
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ACKNOWLEDGMENTS
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■
The authors thank Marisa Monreal (UCLA) for help with EPR
and SQUID measurements. This work was supported by the
National Science Foundation through grants CHE-1266281
and CHE-0957816 (to G.L.H.), and CHE-1048528 (CRIF MU
instrumentation). Dedicated to the memory of Gregory L.
Hillhouse.
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