Carbon-hydrogen bond activation, C-N bond coupling, and cycloaddition reactivity of a three-coordinate nickel complex featuring a terminal imido ligand

Article abstract of DOI:10.1021/ic5026153

The three-coordinate imidos (dtbpe)Ni=NR (dtbpe = ^tBu₂PCH₂CH₂P^tBu_2, R = 2,6-ⁱPr₂C₆H₃, 2,4,6-Me₃C₆H₂ (Mes), and 1-adamantyl (Ad)), which contain a legitimate Ni-N double bond as well as basic imido nitrogen based on theoretical analysis, readily deprotonate HC≡CPh to form the amide acetylide species (dtbpe)Ni{NH(Ar)}(C≡CPh). In the case of R = 2,6-ⁱPr₂C₆H₃, reductive carbonylation results in formation of the (dtbpe)Ni(CO)₂ along with the N-C coupled product keteneimine PhCH=C=N(2,6- ⁱPr₂C₆H₃). Given the ability of the Ni=N bond to have biradical character as suggested by theoretical analysis, H atom abstraction can also occur in (dtbpe)Ni=N{2,6-ⁱPr₂C₆H₃} when this species is treated with HSn(ⁿBu)₃. Likewise, the microscopic reverse reaction-conversion of the Ni(I) anilide (dtbpe)Ni{NH(2,6-ⁱPr₂C₆H₃)} to the imido (dtbpe)Ni=N{2,6-ⁱPr₂C₆H₃}-is promoted when using the radical MesO^? (Mes = 2,4,6-^tBu₃C₆H₂). Reactivity studies involving the imido complexes, in particular (dtbpe)Ni=N{2,6-ⁱPr₂C₆H₃}, are also reported with small, unsaturated molecules such as diphenylketene, benzylisocyanate, benzaldehyde, and carbon dioxide, including the formation of C-N and N-N bonds by coupling reactions. In addition to NMR spectroscopic data and combustion analysis, we also report structural studies for all the cycloaddition reactions involving the imido (dtbpe)Ni=N{2,6-ⁱPr₂C₆H₃}.

Full text of DOI:10.1021/ic5026153

Article
pubs.acs.org/IC
Carbon−Hydrogen Bond Activation, C−N Bond Coupling, and
Cycloaddition Reactivity of a Three-Coordinate Nickel Complex
Featuring a Terminal Imido Ligand
Daniel J. Mindiola,*,§,‡ Rory Waterman,§,⊥ Vlad M. Iluc,§,∥ Thomas R. Cundari,§,†
and Gregory L. Hillhouse§,#
§Department of Chemistry, The University of Chicago, Chicago, Illinois 60637, United States
†Department of Chemistry, Center for Advanced Scientific Computing and Modeling (CASCaM), University of North Texas,
Denton, Texas 76203, United States
S
* Supporting Information
ABSTRACT: The three-coordinate imidos (dtbpe)NiNR (dtbpe =
tBu2PCH2CH2PtBu2, R = 2,6-iPr2C6H3, 2,4,6-Me3C6H2 (Mes), and 1-
adamantyl (Ad)), which contain a legitimate Ni−N double bond as well as
basic imido nitrogen based on theoretical analysis, readily deprotonate
HCCPh to form the amide acetylide species (dtbpe)Ni{NH(Ar)}(C
CPh). In the case of R = 2,6-iPr2C6H3, reductive carbonylation results in
formation of the (dtbpe)Ni(CO)2 along with the N−C coupled product
keteneimine PhCHCN(2,6- iPr2C6H3). Given the ability of the NiN
bond to have biradical character as suggested by theoretical analysis, H atom
abstraction can also occur in (dtbpe)NiN{2,6-iPr2C6H3} when this
species is treated with HSn(nBu)3. Likewise, the microscopic reverse
reactionconversion of the Ni(I) anilide (dtbpe)Ni{NH(2,6-iPr2C6H3)} to the imido (dtbpe)NiN{2,6-iPr2C6H3}is
promoted when using the radical Mes*O• (Mes* = 2,4,6-tBu3C6H2). Reactivity studies involving the imido complexes, in
particular (dtbpe)NiN{2,6-iPr2C6H3}, are also reported with small, unsaturated molecules such as diphenylketene,
benzylisocyanate, benzaldehyde, and carbon dioxide, including the formation of C−N and N−N bonds by coupling reactions.
In addition to NMR spectroscopic data and combustion analysis, we also report structural studies for all the cycloaddition
reactions involving the imido (dtbpe)NiN{2,6-iPr2C6H3}.
chelating bis-N-heterocyclic carbene ligand (3,3-methylenebis-
(1-tert-butyl-4,5-dimethylimidazoliylidene, Scheme 1e).8 Ter-
minal imidos are not just restricted to three-coordinate
environments. Hillhouse reported examples of linear, two-
coordinate imidos using an impressively sterically demanding
N-heterocyclic carbene as well as a bulky dmp group on the
imido nitrogen (Scheme 1f).9
Apart from their fascinating coordination and electronic
properties,5 terminal imido complexes of nickel are also highly
reactive and effect H atom-abstraction,10 engage in C−H bond
activation chemistry such as ethylene amination,9 the amination
of allylic groups such as 1,4-cyclohexadiene, and even activation
of stronger C−H bonds in substrates such as indane,
ethylbenzene, and toluene.6,11 Warren and co-workers demon-
strated that C−H bond activation proceeds via hydrogen atom
abstraction and alkyl radical formation, followed by recombi-
nation of the latter with a second equivalent of Ni-imido to
form primary and secondary nickel(II) amides.11 The radical
nature of some Ni(III) imidos is also manifested in radical
recombination to form diphenoquinonediimine-type scaf-
INTRODUCTION
■
Since the discovery of a three-coordinate terminal nickel imido,
(dtbpe)NiNAr (dtbpe = tBu2PCH2CH2PtBu2, Ar =
2,6-iPr2C6H3, Scheme 1a) in 2001,1 this class of complexes
remains relatively scarce, and their reactivity remains underex-
plored.2 The low coordination geometry offered by the strong
σ-donating and sterically encumbered chelating bisphosphine
ligand allows for optimal σ and π bond formation in a trigonal
planar environment without causing π electron−electron
repulsion between the electron-rich late transition metal and
the π-loaded imido ligand.3 Three-coordinate nickel(II) imidos
can also be prepared with other sterically demanding groups on
the imido nitrogen, such as mesityl,4 1-adamantyl (Ad),4 and
dmp5 (dmp = 2,6-dimesitylphenyl, Scheme 1a). The use of a
bulky scaffold such as dmp allows the isolation of a three-
coordinate Ni(III) imido by virtue of a one-electron oxidation
process (Scheme 1b).5 Warren and co-workers have also found
that β-diketiminate ligands can stabilize Ni(III) complexes
possessing the terminal imido ligand (Scheme 1c),6 while
Limberg has recently expanded this approach to a diazenido
ligand using a more sterically encumbered chelating ligand
(Scheme 1d).7 Other examples of kinetically stabilized Ni(II)
imidos have been reported using a sterically encumbering and
Received: October 29, 2014
© XXXX American Chemical Society
A
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Scheme 1. Terminal Coordinated NiN-type Complexes of Nickel
folds.7,12−14 Lastly, nickel complexes with a terminal imido
ligand can also engage in nitrene group transfer to unsaturated
molecules such as alkenes,15 organic azides,16 CO, and C
NR.6,17,18 Surprisingly, the reactivity of nickel imidos toward
other unsaturated substrates including cumulenes, has not been
reported despite the rich chemistry from the similar species
(THF-d8), and CD2Cl2 were purchased, degassed, and dried over
CaH2 or activated 4 Å molecular sieves and vacuum transferred. Celite,
alumina, and 4 Å molecular sieves were activated under vacuum
overnight at a temperature above 180 °C. Anhydrous solvents such as
THF and diethyl ether were purchased from Acros Organics or
Fischer, stirred over sodium metal, and filtered through activated
alumina.. All other chemicals were used as received. OCCPh2,24
[(dtbpe)Ni(μ-Cl)]2,1,25 (dtbpe)Ni(COD),26,27 (dtbpe)Ni{NH-
(2,6-iPr2C6H3)},1 (dtbpe)NiN(2,6-iPr2C6H3) (1),1 (dtbpe)Ni
N(Mes) (2),4 Mes*O radical,28 MesNNMes,29 (dtbpe)NiCl2,27
and (dtbpe)NiN(1-Ad) (3)4 were prepared according to the
literature. For complex 1 an alternative, and optimized, procedure is
reported below. HCCPh was distilled under reduced pressure and
dried over molecular sieves. Infrared data (Fluorolube mulls or
solution, CaF2 plates or in Nujol mulls, KBr plates) were measured by
using a Nicolet 670-FT-IR instrument. Elemental analysis was
18
(dtbpe)NiCPh2 and (dtbpe)NiP[dmp].15,16,20
In this study, we describe reaction chemistry of the first
nickel(II) complexes having a terminally bound imido ligand,
(dtbpe)NiNR (R = 2,6-iPr2C6H3, 2,3,6-Me3C6H2 (Mes), and
Ad) with various small unsaturated molecules. These Ni(II)
imidos can abstract an H atom from a reductant such as
HSnnBu3,21 but are also basic enough to heterolytically activate
the C−H bond of a terminal alkyne. Other substrates, such as
carbon dioxide, isocyanate, aldehyde, and ketene functional
groups, engage in [2 + 2] cycloaddition, and sometimes
insertion chemistry, across the NiN bond to form stable and
crystalline 4- and 6-membered ring nickel metallacycles. In
addition to activating C−H bonds, it was also discovered that
CO can promote C−N coupling of anilide with an acetylide
to form a transient ynamine, which rearranges to the
keteneimine. Theoretical studies, employing both density
functional theory and higher-level ab initio simulations, were
applied to understand the bonding and reactivity of the nickel-
imido ligand.
1
performed by Desert Analytics (Tucson, AZ). H, 13C, 19F, and 31P
NMR spectra were recorded on Bruker 500 and 400 MHz NMR
spectrometers. 1H and 13C NMR are reported with reference to
solvent resonances (residual C6D5H in C6D6, 7.16 and 128.0 ppm;
residual CHDCl2 in CD2Cl2, 5.32 and 53.8 ppm; residual proteo THF
in THF-d8, 1.73 and 3.58 ppm, and 65.6 and 23.5 ppm). 19F NMR
spectra were reported with respect to external CCl3F (0 ppm). 31P
NMR spectra were reported with respect to external 85% H3PO4 (0
ppm). Magnetic susceptibilities were determined by the method of
Evans.30,31 X-ray diffraction data were collected on a Siemens Platform
goniometer with a charged coupled device (CCD) detector. Structures
were solved by direct or Patterson methods using the SHELXTL
(version 5.1) program library (G. Sheldrick, Bruker Analytical X-ray
Systems, Madison, WI).32
Improved Synthesis of 1. A mixture of (dtbpe)Ni{NH(2,6-
iPr2C6H3)}1 (55 mg, 0.1 mmol) and Mes*O• (28 mg, 0.1 mmol) in 10
mL of C6H6 was placed in a 20 mL scintillation vial and stirred for 12
h. After removal of volatiles under reduced pressure, the residual solids
were extracted with hexanes. The hexanes solution was concentrated,
filtered through a plug of Celite, and cooled to −35 °C to generate
EXPERIMENTAL SECTION
■
General Considerations. Unless otherwise stated, all operations
were performed in an MBraun Lab Master drybox under an
atmosphere of purified nitrogen or using high-vacuum and standard
Schlenk techniques under an argon atmosphere.22 Hexanes, petroleum
ether, benzene, and toluene were dried by passage through activated
alumina and Q-5 columns.23 Benzene-d6, deuterated tetrahydrofuran
B
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pure green crystals of 1 (50 mg, 90%). Spectroscopic characterization
matched the previously reported data.1
orange crystals of 6 (32 mg, 0.051 mmol, 86%). 1H NMR (22 °C, 500
MHz, C6D6): δ 7.47 (d, Ph, 2 H), 7.26 (t, Ph, 2 H), 7.03 (t, Ph, 1 H),
2.03 (s, C10H15, 6 H), 1.96 (s, C10H15, 3 H), 1.38 (d, C10H15, 3 H),
1.36 (d, C10H15, 3 H), 1.33 (m, CH2, 4 H), 1.25 (d, (CH3)3, 18 H),
1.10 (d, (CH3)3, 18 H), 13C{1H} NMR (22 °C, 125.8 MHz, C6D6): δ
132.6 (s, Ph), 131.6 (s, Ph), 129.6 (s, Ar), 127.1 (s, Ph), 114.7 (d,
NiCCPh, JCP = 21 Hz), 100.6 (dd, NiCCPh, JCP = 91, 34 Hz), 43.6 (t,
Ad, JPC = 6.8 Hz), 38.4 (s, Ad), 35.4 (m, C(CH3)3), 32.5, (s, Ad), 30.6
(d, (CH3)3, JPC = 6.3 Hz), 30.2 (d, (CH3)3, JPC = 6.3 Hz), 23.9 (m,
CH2). The NH resonance could not be located. 31P{1H} NMR (22 °C,
202.4 MHz, C6D6): δ 98.8 (d, JPP = 47.1 Hz), 90.2 (d, JPP = 47.1 Hz).
IR (Nujol, KBr): 2153(s νCC), 1589(s), 1307(w), 1260(w), 1179(s),
1095(m), 1067(w), 1019(m), 935(w), 850(w), 813(s), 755(s),
695(m), 673(m), 658(w) cm−1.
Synthesis of (dtbpe)Ni{NH(2,6- iPr2C6H3)} from 1 and
HSnnBu3. A mixture of 1 (82.8 mg, 0.15 mmol), HSnnBu3 (43.6
mg, 0.15 mmol), and C6H6 (10 mL) was placed in a Schlenk tube and
heated at 75 °C for 12 h. The mixture gradually turned color from
emerald green to dark red. After the mixture cooled to room
temperature, the volatiles were removed under reduced pressure, and
the solids were extracted with toluene. After filtration, crystallization
occurred by cooling a concentrated toluene solution stored at −35 °C
over a period of 1−2 d. Red crystals were separated by filtration and
dried under reduced pressure to yield analytically pure product,
(dtbpe)Ni{NH(2,6- iPr2C6H3)} (68 mg, 82%). Spectroscopic
characterization matched the previously reported data.1
Synthesis of (dtbpe)Ni{NH(2,6-iPr2C6H3)}(CCPh) (4). In a vial
was dissolved 1 (90 mg, 0.163 mmol) in 15 mL of Et2O, and the green
solution was cooled to −35 °C. To the cold solution was added
dropwise 5 mL of an Et2O solution containing HCCPh (17 mg,
0.167 mmol), which caused darkening of the solution over a period of
2 h after reaching room temperature. The reaction mixture was
allowed to stir for an additional 3 h at room temperature. The dark red
solution was filtered, concentrated, and cooled to −35 °C overnight to
afford (dtbpe)Ni{NH(2,6-iPr2C6H3)}(CCPh) (4) in two crops as dark
red crystals/powder, which was filtered, washed with cold petroleum
ether, and dried under reduced pressure (96 mg, 0.147 mmol, 90%
Carbonylation of 4 to Produce (dtbpe)Ni(CO)2 and
Keteneimine PhCHCN(2,6- iPr2C6H3). A 25 mL round-bottom
flask was charged with 4 (78 mg, 0.0141 mmol), attached to an
adapter, and evacuated. Approximately 6 mL of toluene was vacuum-
transferred into the flask. Carbon monoxide (1 atm) was introduced
into the vessel at −78 °C. Examination of the reaction mixture by 31P
17
NMR spectroscopy revealed clean formation of (dtbpe)Ni(CO)2
along with one major organic product (observed by 1H NMR
spectroscopy). After 30 min, the gas was evacuated, the flask was
opened in air, and the solvent was removed under reduced pressure.
The PhCHCN(2,6-iPr2C6H3) residue was passed through a short
column of silica with hexanes/EtOAc (4:1) as the eluent to provide a
1
yield). H NMR (22 °C, 500.1 MHz, CD2Cl2): δ 6.95 (m, aryl, 3 H),
1
6.80 (d, aryl, 2 H), 6.49 (m, aryl, 3 H), 4.01 (sept, CH(CH3)2, 2 H),
1.96 (br, NH, 1 H), 1.88 (m, CH2, 2 H), 1.73 (m, CH2, 2 H), 1.56 (d,
tBu, 36 H, JHP = 13 Hz), 1.29 (d, CH(CH3)2, 6 H), 1.16 (d,
CH(CH3)2, 6 H). 13C{1H} NMR (22 °C, 125.8 MHz, CD2Cl2): δ
158.21 (s, aryl), 139.12 (s, aryl), 131.04 (s, aryl), 129.59 (s, aryl),
127.49 (s, aryl), 124.35 (s, aryl), 121.83 (s, aryl), 114.41 (s, aryl),
viscous oil (19 mg, 0.68 mmol, 48%). H NMR (22 °C, 500.1 MHz,
CD2Cl2): δ 6.97 (s, aryl, 3 H), 5.65 (s, CH), 3.65 (sept, CH(CH3)2, 2
H), 1.70 (d, CH(CH3)2, 12 H). 13C{1H} NMR (22 °C, 125.8 MHz,
CD2Cl2): δ 178.2 (s, CCN), 141.2 (s, aryl), 134.1 (s, aryl), 127.3 (s,
aryl), 124.6 (s, aryl), 56.2 (s, CCN), 28.2 (s, CH(CH3)2), 23.5 (s,
CH(CH3)2). GC/MS (m/z): 214 (M+), 176, 130, 91.
Synthesis of (dtbpe)Ni{O,C:OCN(2,6-iPr2C6H3)CPh2} (7). In a
vial was dissolved 1 (100 mg, 0.181 mmol) in 10 mL of Et2O, and to
the green solution was added dropwise OCCPh2 in 5 mL of
Et2O, causing a rapid color change from green to an intense orange-
red. After 10 min red precipitate began to form, and after allowing the
reaction to stir for an additional 40 min, the mixture was cooled to
−35 °C for 20 min, filtered, and the solids washed with cold Et2O. The
red solid was dried under reduced pressure to afford crude
(dtbpe)Ni{O,C:OCN(2,6-iPr2C6H3)CPh2} (127 mg, 0.172 mmol,
95% yield). Analytically pure 7 was obtained by dissolving the solids in
a minimum of CH2Cl2, filtering, layering carefully with excess Et2O,
and cooling the solution to −35 °C for 1 d. Large dark red blocks of 7
were collected via filtration, washed with cold Et2O, and dried under
113.85 (d, NiCCPh, JCPtrans = 21 Hz), 106.80 (dd, NiCCPh, JCPtrans
=
96 Hz, JCPcis = 41 Hz), 36.75 (s, CH(CH3)2), 36.62 (s, CH(CH3)2),
36.11 (s, C(CH3)3), 36.04 (s, C(CH3)3), 30.91 (s, C(CH3)3), 30.70 (s,
C(CH3)3), 28.36 (s, CH(CH3)2), 25.04 (s, CH(CH3)2), 24.68 (t,
CH2CH2, JCP = 15 Hz), 22.74 (s, CH(CH3)2), 21.22 (t, CH2CH2, JCP
=
=
15 Hz). 31P{1H} NMR (22 °C, 202.4 MHz, CD2Cl2): δ 82.80 (d, JPP
26 Hz), 68.84 (d,, JPP = 26 Hz). Anal. Calcd for C38H64NNiP2: C,
69.73; H, 9.70; N, 2.14. Found: C, 68.83; H, 9.63; N, 2.00%.
Synthesis of (dtbpe)Ni{NH(Mes)}(CCPh) (5). A 25 mL round-
bottom flask was charged with 2 (83 mg, 0.163 mmol) and 12 mL of
Et2O, and then it was cooled to −35 °C. A similarly cold 2 mL
solution of HCCPh (17 mg, 0.163 mmol) was added dropwise to
the nickel solution causing a color change to red-orange. After it was
stirred for 45 min at room temperature, the solution was filtered and
then cooled to −35 °C overnight to give dark red crystals of 5 (84 mg,
1
vacuum (95 mg, 0.127 mmol, 70% yield). H NMR (22 °C, 500.1
MHz, CD2Cl2): δ 8.37 (d, aryl, 4 H), 7.24 (t, aryl, 4 H), 7.13 (t, aryl, 2
H), 6.85 (d, aryl, 2 H), 6.74 (t, aryl, 1 H), 2.87 (sept, CH(CH3)2, 2
1
0.132 mmol, 81%). H NMR (22 °C, 400 MHz, C6D6): δ 7.23 (t,
H), 1.80 (m, CH2, 2 H), 1.48 (m, CH2, 2 H), 1.44 (d, tBu, 18 H, JHP
=
C6H5, 2 H), 7.19 (s, CH2Me3, 2 H), 7.14 (d, C6H5, 2 H), 6.93 (t,
C6H5, 1 H), 2.82 (s, CH3, 6 H), 2.49 (s, CH3, 3 H), 1.44 (d, (CH3)3,
18 H), 1.35 (d, (CH3)3, 18 H), 1.13 (d, CH2, 4 H). 13C{1H} NMR (22
°C, 125.8 MHz, C6D6): δ 156.7 (t, NiCCPh, JCP = 4.2 Hz), 130.3 (s,
Ar), 128.4 (s, Ar), 127.6 (s, Ar), 127.4 (s, Ar), 127.1 (s, Ar), 126.6 (s,
Ar), 123.6 (s, Ar), 121.3 (s, Ar), 112.0 (dd, NiCCPh, JCP = 2, 23 Hz),
35.8, 34.5 (m, C(CH3)3, JPC = 16.3 Hz), 35.3 (m, C(CH3)3, JPC = 9.6
Hz), 29.9 (d, (CH3)3, JPC = 3 Hz), 29.7 (d, (CH3)3, JPC = 3.8 Hz), 23.7
(t, CH2, JPC = 15 Hz), 20.5 (s, CH3), 19.6 (s, CH3). The NH
resonance could not be located. 31P{1H} NMR (22 °C, 202.4 MHz,
C6D6): δ 81.0 (d, JPP = 25.2), 67.8 (d, JPP = 25.2). IR (Nujol, KBr):
2092(s νCC), 1591(m), 1294(w), 1236(s), 1178(m), 1151(m),
1020(m), 851(s), 812(w), 785(w), 755(s), 722(s), 693(m), 683(m),
603(w), 500(m), 454(w) cm−1. Anal. Calcd for C35H57NiNP2: C,
68.64; H, 9.38; N, 2.29. Found: C, 68.56; H, 9.49; N, 2.20%.
t
15 Hz), 1.08 (d, Bu, 18 H, JHP = 13 Hz), 0.99 (br, CH(CH3)2, 6 H),
0.86 (br, CH(CH3)2, 6 H). 13C{1H} NMR (22 °C, 125.8 MHz,
CD2Cl2): δ 175.7 (s, OCN(2,6-iPr2C6H3)), 150.2 (s, aryl), 145.2 (s,
aryl), 140.5 (s, aryl), 130.6 (s, aryl), 127.4 (s, aryl), 124.0 (s, aryl),
121.5 (s, aryl), 120.6 (s, aryl), 35.79 (CPh2), 35.69 (s, CH(CH3)2),
35.47 (br, C(CH3)3), 30.63 (s, C(CH3)3), 30.40 (s, C(CH3)3), 28.15
(s, CH(CH3)2), 25.24 (m, CH2CH2, JCP = 13 Hz), 23.50 (br,
CH(CH3)2), 18.05 (br, CH2CH2). 31P{1H} NMR (22 °C, 202.4 MHz,
CD2Cl2): δ 77.49 (d, JPP = 13 Hz), 66.21 (d, JPP = 13 Hz). IR (CaF2,
Fluorolube mull): 3047 (w), 2987 (w), 2953 (w), 2899 (w), 1603 (s,
νCN), 1577 (s), 1482 (m), 1468 (m), 1429 (m), 1391 (w), 1357 (w)
cm−1. Anal. Calcd for C44H67NNiP2O: C, 70.78; H, 9.04; N, 1.88.
Found: C, 70.38; H, 9.33; N, 1.92%.
Synthesis of (dtbpe)Ni{O,C:OCN(Mes)CPh2} (8). A 25 mL
round-bottom flask was charged with 2 (48 mg, 0.094 mmol) and 5
mL of Et2O, and then it was cooled to −35 °C. A similarly cold
solution of diphenylketene (18 mg, 0.094 mmol) in 4 mL of Et2O was
added dropwise to 2, causing a color change to red-orange. The
solution was filtered after 1.5 h of stirring at room temperature, then
concentrated to ca. 4 mL and cooled to give orange crystals of 8 (51
Synthesis of (dtbpe)Ni{NH(1-Ad)}(CCPh) (6). A 25 mL
round-bottom flask was charged with 3 (31 mg, 0.0589 mmol) and
10 mL of Et2O, and then it was cooled to −35 °C. A similarly cold 2
mL solution of HCCPh (6 mg, 0.0589 mmol) was added dropwise
to the nickel solution causing a color change to orange-yellow. After it
was stirred for 90 min at room temperature, the solution was filtered,
concentrated to 3 mL, and cooled to −35 °C overnight to give dark
1
mg, 0.068 mmol, 72%). H NMR (22 °C, 500 MHz, CD2Cl2): δ 8.22
C
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(d, Ph, 4 H), 7.13 (t, Ph, 4 H), 7.06 (t, Ph, 2 H), 6.58 (s, CH2Me3, 2
H), 2.07 (s, CH3, 3 H), 1.79 (s, CH3, 6 H), 1.34 (m, CH2, 4 H), 1.23
(d, (CH3)3, 18 H), 1.01 (d, (CH3)3, 18 H). 13C{1H} NMR (22 °C,
125.8 MHz, CD2Cl2): δ 170.2 (s, CO), 133.8 (s, Ar), 130.3 (s, Ar),
129.8 (s, Ar), 127.3 (s, Ar), 127.2 (s, Ar), 123.7 (s, Ar), 122.6 (s, Ar),
121.3 (s, Ar), 38.1 (t, CPh2, JPC = 15 Hz), 35.4 (m, C(CH3)3), 30.4 (d,
(CH3)3, JPC = 4.6 Hz), 30.1 (d, (CH3)3, JPC = 3.7 Hz), 22.7 (m, CH2),
22.1 (s, CH3), 18.0 (s, CH3). 31P{1H} NMR (22 °C, 202.4 MHz,
CD2Cl2): δ 78.5 (d, JPP = 12.4 Hz), 66.9 (d, JPP = 12.4 Hz). IR (Nujol,
KBr): 1594(m), 1564(s, νCN), 1266(m), 1232(w), 1180(m), 1022(m),
859(w), 837(m), 799(w), 786(w), 738(w) 716(m), 694(s), 673(m),
651(w) cm−1. Anal. Calcd for C40H61NiNOP2: C, 69.37; H, 8.88; N,
2.02. Found: C, 68.87; H, 8.22; N, 2.01%.
Synthesis of (dtbpe)Ni{O,C:OCN(1-Ad)CPh2} (9). A 25 mL
round-bottom flask was charged with 3 (95 mg, 0.180 mmol) and 7
mL of Et2O, and then it was cooled to −35 °C. A similarly cold
solution of diphenylketene (35 mg, 0.180 mmol) in 2 mL of Et2O was
added dropwise to 3, causing little visible color change. The solution
was filtered after 1.5 h of stirring at room temperature, then
concentrated to ca. 4 mL and cooled to give dark red blocks of 9
(87 mg, 0.139 mmol, 77%). 1H NMR (22 °C, 500 MHz, C6D6): δ 8.55
(d, Ph, 4 H), 7.24 (t, Ph, 4 H), 7.10 (t, Ph, 2 H), 2.38 (s, C10H15, 6 H),
2.18 (s, C10H15, 3 H), 1.85 (d, C10H15, 3 H), 1.78 (d, C10H15, 3 H),
1.16 (d, (CH3)3, 18 H), 1.13 (m, CH2, 4 H), 0.88 (d, (CH3)3, 18 H),
13C{1H} NMR (22 °C, 125.8 MHz, C6D6): δ 179 (br, CO), 141.6 (s,
(11) (137 mg, 0.207 mmol, 83% yield), which was filtered and
washed with cold petroleum ether. Analytically pure 11 was obtained
from two consecutive recrystallizations from Et2O. H NMR (22 °C,
1
500.1 MHz, C6D6): δ 8.15 (d, aryl, 2 H), 7.39 (d, aryl, 1 H), 7.33 (t,
aryl, 1 H), 7.18 (m, aryl, 2 H), 7.01 (d, aryl, 1 H), 6.63 (d, aryl, 1 H),
4.27 (sept, CH(CH3)2, 2 H), 1.94 (d, CH(CH3)2, 3 H), 1.88 (d,
t
t
CH(CH3)2, 3 H), 1.57 (d, Bu, 9 H, JHP = 13 Hz), 1.45 (d, Bu, 9 H,
t
JHP = 12 Hz), 1.30 (d, Bu, 9 H, JHP = 12 Hz), 1.27 (d, CH(CH3)2, 3
H), 1.09 (br, CH2, 2 H), 0.88 (br, CH2, 2 H), 0.73 (d, tBu, 9 H, JHP
=
12 Hz), 0.47 (d, CH(CH3)2, 3 H). Note: The OCHPh proton could
not be located in the 1H NMR spectrum. 13C{1H} NMR (22 °C, 125.8
MHz, C6D6): δ 157.4 (s), 152.8 (s), 148.9 (s, aryl), 147.6 (s, aryl),
127.5 (s, aryl), 126.5 (s, aryl), 124.3 (s, aryl), 122.8 (s, aryl), 122.6 (s,
aryl), 114.3 (s, aryl), 35.32 (d, C(CH3)3, JCP = 6 Hz), 35.17 (d,
C(CH3)3, JCP = 6 Hz), 34.78 (d, C(CH3)3, JCP = 13 Hz), 34.37 (d,
C(CH3)3, JCP < 3 Hz), 31.02 (br), 30.47 (br), 30.24 (br), 29.92 (br),
29.79 (br), 27.46 (s, CH(CH3)2), 26.84 (s, CH(CH3)2), 25.50 (s,
CH(CH3)2), 23.84 (s, CH(CH3)2), 23.59 (m, CH2CH2), 19.59 (m,
CH2CH2). 31P{1H} NMR (22 °C, 202.4 MHz, C6D6): δ 75.45 (d, JPP
= 6 Hz), 69.29 (d, JPP = 7 Hz). Anal. Calcd for C37H63NNiP2O: C,
67.48; H, 9.64; N, 2.13. Found: C, 67.22; H, 9.78; N, 2.10%.
Synthesis of (dtbpe)Ni{O,C:OC(O)N(2,6-iPr2C6H3)} (12). In a
Schlenk flask equipped with a stir bar was dissolved 1 (126 mg, 0.228
mmol) in Et2O, and the solution was degassed and cooled to −78 °C.
To the cold green solution was added excess CO2 (25 °C, 1 atm) for
several minutes, and the solution was allowed to slowly reach room
temperature. Upon reaching 0 °C the solution changed slowly to a
pale yellow, and a pale yellow precipitate formed upon reaching room
temperature. The solution was allowed to stir for an additional 2 h at
room temperature, the solvent was removed under reduced pressure,
and the flask was taken into the box. The solids were filtered and
washed with cold Et2O/petroleum ether (2:1), and the solids were
dried under vacuum to afford crude (dtbpe)Ni{O,C:OC(O)N-
(2,6-iPr2C6H3)} (12) as a pale yellow solid (128 mg, 0.214 mmol,
94% yield). Analytically pure complex 12 can be obtained by dissolving
the crude product in a minimum of CH2Cl2, filtering the golden brown
solution, layering the filtrate carefully with excess Et2O, and cooling
the solution to −35 °C for 2 d (2 crops, 71 mg, 0.119 mmol, 52%
Ph), 131 (s, Ph), 127.0 (s, Ph), 126.3 (s, Ph), 48.6 (t, Ad, JPC = 6.2
Hz), 38.6 (s, Ad), 36.4 (t, CPh2, JPC = 12 Hz), 35.2 (m, C(CH3)3),
32.4, (s, Ad), 30.2 (d, (CH3)3, JPC = 6.5 Hz), 29.3 (d, (CH3)3, JPC = 6.7
Hz), 22.1 (m, CH2). 31P{1H} NMR (22 °C, 202.4 MHz, C6D6): δ 77.5
(d, JPP = 19.8 Hz), 62.4 (d, JPP = 19.8 Hz). IR (CaF2, Fluorolube):
1559(s), 1469(s), 1444(m), 1385(w), 1361(s) cm−1.
Synthesis of (dtbpe)Ni{O,C:OCNCH2PhN(2,6-iPr2C6H3)}
(10). In a vial was dissolved 1 (80 mg, 0.145 mmol) in 8 mL of
toluene, and the green solution was cooled to −35 °C. To the cold
solution was added dropwise 3 mL of a toluene solution containing
OCNCH2Ph (21 mg, 0.158 mmol) causing darkening of the
solution over a period of 2 h at room temperature and with
precipitation of green solid. The reaction mixture was allowed to stir
for an additional 3 h at room temperature. The dark solution was
concentrated, cooled to −35 °C overnight, and then filtered; the green
solids were washed with cold Et2O and dried under vacuum to afford
crude (dtbpe)Ni{O,C:OCNCH2PhN(2,6-iPr2C6H3)} (10) (89 mg,
131 mmol, 90% yield). The solids were dissolved in a minimum of
CH2Cl2, filtered, layered carefully with excess Et2O, and cooled to −35
°C for 2 d. Large dark green blocks of 10 were collected via filtration,
washed with Et2O, and dried under vacuum (76 mg, 0.110 mmol, 76%
1
yield). H NMR (22 °C, 500.1 MHz, CD2Cl2): δ 6.97 (m, aryl, 3 H),
4.00 (sept, CH(CH3)2, 2 H), 1.70 (m, CH2, 4 H), 1.65 (d, tBu, 18 H,
t
JHP = 13 Hz), 1.25 (d, CH(CH3)2, 6 H), 1.21 (d, Bu, 18 H, JHP = 13
Hz), 1.18 (d, CH(CH3)2, 6 H). 13C{1H} NMR (22 °C, 125.8 MHz,
CD2Cl2): δ 170.5 (br, CO2), 147.1 (s, aryl), 144.2 (s, aryl), 124.3 (s,
aryl), 122.6 (s, aryl), 36.91 (d, C(CH3)3, JCP = 16 Hz), 35.65 (d,
C(CH3)3, JCP = 16 Hz), 30.35 (s, C(CH3)3), 29.48 (s, CH(CH3)2),
25.82 (s, CH(CH3)2), 24.24 (m, CH2CH2, JCP = 12 Hz), 22.86 (s,
CH(CH3)2), 21.23 (m, CH2CH2, JCP = 11 Hz). 31P{1H} NMR (22 °C,
202.4 MHz, CD2Cl2): δ 89.71 (d, JPP = 28 Hz), 85.67 (d, JPP = 28 Hz).
IR (CaF2, Fluorolube): 2955 (w), 1617 (s, νCO), 1588 (m), 1468 (m),
1431 (m), 1371 (w), 1360 (w), 1321 (w) cm−1. Elemental analysis and
single-crystal X-ray diffraction methods were consistent with complex
(dtbpe)Ni{κ2-OC(O)N(2,6-(CHMe2)2C6H3)} retaining one molecule
of CH2Cl2. Anal. Calcd for C32H53Cl2NNiP2: C, 56.41; H, 8.73; N,
2.06. Found: C, 57.72; H, 8.53; N, 2.05%.
1
yield). H NMR (22 °C, 500.1 MHz, CD2Cl2): δ 7.21 (d, aryl, 2 H),
7.14 (t, aryl, 2 H), 7.01 (t, aryl, 1 H), 6.93 (s, aryl, 3 H), 4.26 (sept,
CH(CH3)2, 2 H), 4.10 (s, CH2Ph, 2 H), 1.65 (m, CH2CH2, 4 H), 1.58
t
(d, tBu, 18 H, JHP = 13 Hz), 1.24 (d, CH(CH3)2, 12 H), 1.23 (d, Bu,
18 H, JHP = 13 Hz). 13C{1H} NMR (22 °C, 125.8 MHz, CD2Cl2): δ
173.4 (s, OCNCH2Ph), 147.2 (br, aryl), 146.8 (s, aryl), 128.5 (s,
aryl), 127.5 (s, aryl), 124.8 (s, aryl), 123.3 (s, aryl), 122.6 (s, aryl),
47.23 (s, CH2Ph), 36.01 (d, C(CH3)3, JCP = 15 Hz), 35.51 (d,
C(CH3)3, JCP = 15 Hz), 30.97 (s, CH(CH3)2), 30.40 (s, C(CH3)3),
29.23 (s, CH(CH3)2), 26.28 (s, CH(CH3)2), 24.24 (m, CH2CH2),
23.28 (s, CH(CH3)2), 21.05 (m, CH2CH2). 31P{1H} NMR (22 °C,
202.4 MHz, CD2Cl2): δ 86.01 (d, JPP = 27 Hz), 83.31 (d, JPP = 27 Hz).
Anal. Calcd for C38H64N2NiP2O: C, 66.57; H, 9.41; N, 4.09. Found: C,
66.17; H, 9.49; N, 3.71%.
Synthesis of (dtbpe)Ni{O,O:(OC(O))2N(1-Ad)} (13). A 50 mL
round-bottom flask was charged with 3 (80 mg, 0.152 mmol), attached
to an adapter, and evacuated. Petroleum ether (10 mL) was vacuum
transferred into the flask, and 1 atm CO2 was introduced. The solution
became slightly red then lightened to a pale yellow, and a small
amount of precipitate formed. After 30 min the CO2 was removed, and
the solution was dried under reduced pressure. The solids were
extracted with 10 mL of Et2O, filtered, and cooled to −35 °C to
provide yellow crystals of 13 (58 mg, 0.094 mmol, 62%). 1H NMR (22
°C, 500 MHz, CD2Cl2): δ 2.29 (d, C10H15, 3 H), 2.01 (s, C10H15, 6 H),
1.68 (d, C10H15, 3 H), 1.64 (d, C10H15, 3 H), 1.54 (d, (CH3)3, 36 H),
1.52 (d, CH2, 4 H). 13C{1H} NMR (22 °C, 125.8 MHz, CD2Cl2): δ
157.9 (s, CO2), 40.9 (s, Ad), 38.4 (s, Ad), 37.6 (t, C(CH3)3, JPC = 6.0
Hz), 31.8, (s, Ad), 31.1 (d, (CH3)3, JPC = 6.2 Hz), 23.1 (m, CH2).
31P{1H} NMR (22 °C, 202.4 MHz, CD2Cl2): δ 87.7 (s). IR (CaF2,
Synthesis of (dtbpe)Ni{O,C:OCHPhN(2,6-iPr2C6H3)} (11). In a
vial was dissolved 1 (138 mg, 0.250 mmol) in 10 mL of toluene, and
the solution was cooled to −35 °C. To the green solution was added
dropwise a similarly cold solution of PhHCO (32 mg, 0.300 mmol)
in 3 mL of toluene. The solution was stirred for 7 h at room
temperature, during which time the color gradually changed from
green to brown to finally a dark navy blue color. The dark solution was
filtered, dried under vacuum, extracted with 50 mL of Et2O, filtered,
concentrated, and cooled to −35 °C for 2 d to afford large blue
needles and powder of (dtbpe)Ni{O,C:OCHPhN(2,6-iPr2C6H3)}
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Scheme 2. Original Synthetic Protocols to Prepare the Imido Complexes 1−3
Ni(II) complexes, namely, (dtbpe)Ni(COD)27 and (dtbpe)-
NiCl2. Complex [(dtbpe)Ni(μ-Cl)]2 can be prepared quanti-
tatively21,26 from these two reagents (Scheme 2) akin to
Sigman’s reported N-heterocyclic carbene complex of Ni(I).40
Notably, the synthesis of [(dtbpe)Ni(μ-Cl)]2 from this reaction
does not require isolation or purification of the Ni(II) and
Ni(0) starting materials since it can be produced by simply
adding premixed solutions of Ni(COD)2 with dtbpe and NiCl2
with dtbpe over several hours.21 Likewise, to obviate the need
for separate oxidation and deprotonation steps, a H atom-
abstraction reaction with the radical Mes*O (Mes* =
2,4,6-tBu3C6H2) was used for the direct preparation of 1.21,26
Smith41 and Hillhouse10 have applied this strategy to prepare
cobalt imido and other nickel imido derivatives, respectively.
Consequently, a more convenient route to multigram quantities
of imido 1 is shown in Scheme 3 via a comproportionation
Fluorolube): 1667(s, νCO), 1624(s, νCO), 1481(w), 1465(s), 1377(s),
1352(s) cm−1.
Thermolysis of Complex 2. A Schlenk tube was charged with 2
(76 mg, 0.149 mmol) and 6 mL of benzene. The solution was
degassed and heated to 80 °C for 16 h. The tube was then opened in
air, and the solvent was removed under reduced pressure.
Azomesitylene was purified on a silica gel column with 5:1 hexanes/
1
EtOAc as eluent to give red crystals (16 mg, 0.060 mmol, 40%). H
NMR (22 °C, 500 MHz, CDCl3): δ 6.95 (s, C6Me3H2, 2 H), 2.40 (s,
C6Me3H2, 6 H), 2.32 (s, C6Me3H2, 3 H). 13C{1H} NMR (22 °C, 125.8
MHz, CDCl3): δ 130.3 (s, Mes), 129.8 (s, Mes), 128.2 (s, Mes), 127.6
(s, Mes), 22.4 (s, CH3), 21.7 (s, CH3). GC/MS (m/z): 266 (M+).
Computational Methods. Computational studies of complex 1
utilized density functional theory (specifically, ONIOM33(M0634/6-
311+G(d):35UFF36); geometry optimizations started from the
reported crystal structure,1 using the Gaussian 09 code.37 At this
optimized geometry, multiconfiguration self-consistent field
(MCSCF38) computations were performed utilizing the complete
active space (CAS) formalism within the GAMESS code.39
Scheme 3. Optimized Synthetic Protocol to Prepare
Complex 1
RESULTS AND DISCUSSION
■
Complex (dtbpe)NiN(2,6-iPr2C6H3) (1)1 is a green
crystalline species that possesses a Ni−N formal bond order
of two and should therefore react similar to the carbene
19
complex (dtbpe)NiCPh2 or phosphinidene (dtbpe)Ni
P(dmp).15,16,20 Likewise, the imido relatives (dtbpe)Ni
N(Mes) (2, turquoise)4 and (dtbpe)NiN(1-Ad) (3, red)4
can also be prepared, and their chemistry can be similarly
explored. Complexes 2 and 3 are obtained by a different route
than 1, using the corresponding organic azide and Ni(0)
precursor (Scheme 1). Complex 1 is synthesized via a one-
electron oxidation of the three-coordinate Ni(I) complex
(dtbpe)Ni{NH(2,6-iPr2C6H3)} with the weak oxidant [C7H7]-
[PF6] followed by deprotonation with Na{N(SiMe3)2}
(Scheme 2). Although this protocol is reliable, with each
reaction being high yielding, it has two disadvantages: (i) It is a
stepwise process to remove overall an H atom, and (ii) The
synthesis of (dtbpe)Ni{NH(2,6-iPr2C6H3)} involves the use of
the precursor [(dtbpe)Ni(μ-Cl)]2,1,25 a complex prepared in
27
moderate yield from one-electron reduction of (dtbpe)NiCl2
reaction to form [(dtbpe)Ni(μ-Cl)]2, followed by trans-
metalation with Li{NH(2,6-iPr2C6H3)} to yield (dtbpe)Ni-
{NH(2,6-iPr2C6H3)} in 92% yield, and then the protocol was
completed by H atom abstraction with Mes*O. Separation of 1
from the HOMes byproduct can be achieved by fractional
crystallization to provide 1 in 90% yield (Scheme 2).
with KC8 (Scheme 2).1,25 The use of a Ni(I) precursor is
required given that attempts to transmetallate and dehydroha-
logenate (dtbpe)NiCl2 with 2 equiv of Li{NH(2,6-iPr2C6H3)}
resulted in complicated mixtures that contained traces of 1 and
other species such as (dtbpe)Ni{NH(2,6-iPr2C6H3)}.1 To
improve the overall yield of [(dtbpe)Ni(μ-Cl)]2, we reported
a comproportionation reaction using easy-to-prepare Ni(0) and
Preliminary reactivity studies confirmed that the imido ligand
in 1 can be readily carbonylated with CO to form OC
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Figure 1. Computed frontier orbitals (π and π*) of complex 1 in the plane defined by the N and the two P atoms. Hydrogen atoms omitted from
figure for clarity.
Figure 2. Illustration of the π and π* orbitals in 1 perpendicular to the plane defined by the N and two P. Hydrogen atoms omitted from figure for
clarity.
N(2,6-iPr2C6H3) or with CNCH2Ph to form the asymmetric
carbodiimido PhCH2NCN(2,6-iPr2C6H3). Similar reac-
tions have been explored with 2 and 3.42 In both reactions, the
η2-isocyanate or η2-carbodiimido intermediate could be
isolated.16 In addition, [2 + 2] cycloaddition of ethylene across
the NiN bond in 1 has been shown to be a crucial step in the
formation of aziridines,15 via an elusive azametallacyclobutane
intermediate.43
end, the electronic structure of 1 was scrutinized anew using
theoretical methods not feasible on such a large complex when
the earlier reports were published.
The geometry of 1 was first optimized utilizing hybrid QM/
MM methods.33 The Pr and Bu substituents on the imidoaryl
and dtbpe ligands, respectively, were modeled with the
Universal Force Field,36 while the remainder of the complex
was described at the M06/6-311+G(d) level of theory.34,35 As
expected, good agreement with the reported crystal data was
obtained (coordinates in Supporting Information), and tests
with other functionalspure (BP86) and hybrid (B3LYP)
gave similar results. The density functional theory-optimized
geometry was then used as the basis to analyze the electronic
structure of 1 with MCSCF38,39 techniques employing the CAS
approximation with active spaces ranging from two-orbital/two-
electron (CAS(2,2)) to 14-orbital/14-electron (CAS(14,14)).
The salient features of the frontier natural orbitals are similar
among the various CAS calculations; therefore, focus is given to
the pertinent orbitals of the latter, largest active space MCSCF
simulations.
i
t
The 2001 report by Mindiola and Hillhouse of complex 11
foreshadowed a tremendous upsurge in interest in late
transition-metal multiply bonded complexes. The original
crystallographic report has several hallmarks of a complex
“engineered” to have sufficient stability to permit solid-state
analysis−a bulky bidentate supporting ligand, a sterically
hindering imido N substituent, and hints of further stabilization
of the π-loaded imido nitrogen via resonance with the aryl
substituent (cf. the short Nimido−Cipso distance of 1.355 Å).
However, as detailed above, these features belie a remarkable
diversity of reactivity for complex 1 encompassing both even-
and odd-electron processes as well as reactions involving
homolytic (radical) and heterolytic (acid/base) transforma-
tions. A previous computational study in 2008,44 inspired by
the original experimental reports by Mindiola and Hillhouse,1
indicated that subtle changes to the ligands and substituents
markedly affect the computed kinetics and thermodynamics for
highly desirable reactions such as C−H bond activation. To this
The first orbitals of interest are the Ni−Nimido π and π*
orbitals that lie within the plane defined by the NiP2N
coordination plane (Figure 1). The CA(14,14) calculations
assign a substantial population to the latter orbital, 0.24 e−, and
thus the correlating π has a natural orbital occupation number
(NOON) of 1.76 e−. These orbitals’ NOONs are those
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Figure 3. Computed natural orbitals showing delocalization of π-electron density from imido nitrogen of 1 onto the 2,6-iPr2C6H3 ring: NOON =
1.94 (left) and 0.06 e (right). Hydrogen atoms omitted from figure for clarity.
Scheme 4. C−H Bond Activation of HCCPh by Compounds 1−3 to Form 4−6, Respectively, and Subsequent Carbonylation
of 4 to Form the Ketenemine
computed to deviate most significantly from the archetypal
values of 2 and 0 e−, and thus the MCSCF calculations are
indicative of significant π-biradical character to the Ni-imido
moiety, consistent with the ability of 1 to abstract an H atom
from HSn(nBu)3. Additionally, the π-biradical nature of 1
implies a formal bond order of less than three despite the nearly
linear coordination of the imido functionality, which obviously
assists the [2 + 2] cycloaddition and 1,3-dipolar addition
chemistry of complex 1 and its congeners.1,8,43
The in-plane NiN π/π* orbitals just discussed may be
contrasted to the perpendicular π orbitals (Figure 2). The π
orbital (Figure 2, left; NOON = 1.97 e−) is heavily polarized
toward the imido N with small Ni character but delocalized
onto the imido aryl ring, {2,6-iPr2C6H3}. Electron correlation is
not of the bond/antibond variety (cf. Figure 1) but with an
orbital that has an additional radial node (Figure 2, right;
NOON = 0.03 e−). The additional radial node in the right
diagram in Figure 2 is clear; note that like its bonding
counterpart, there is delocalization of electron density onto the
imido 2,6-iPr2C6H3 ring for both members of this correlating
pair of orbitals. These MCSCF orbitals imply that in this plane
the Ni−N π-bond may be best viewed more as a Nimido lone
pair, with potentially high nucleophilic or basic reactivity. As
with the biradical character, the bonding analysis suggests a
further diminution of the nickel−nitrogen bond order below
the ideal value of three that a cursory glance at its linear
coordination mode might imply.
late metal aryl-imido moieties via the introduction of electron-
donating and -withdrawing groups onto the meta and para aryl
ring positions. As such, with reduced metal−nitrogen bond
orders and synthetic tunability, one may easily envisage
derivatives of 1 providing improved routes to C−H bond
amination that can complement known metal-catalyzed routes
to C−N bond formation.
The observation that the frontier π symmetry orbitals
displayed in Figures 1−3 are heavily composed of Nimido
character suggests that these systems should be quite basic.
Because the three-coordinate anilide cation [(dtbpe)Ni
NH(2,6-iPr2C6H3)][PF6]1 can be deprotonated by NaN-
(SiMe3)2, the pKa of the anilide can be coarsely estimated to
be less than 30.45 We thus examined phenylacetylene as a
substrate because its pKa is slightly less than 30 in dimethyl
sulfoxide.46 Accordingly, treatment of the imidos 1−3 with
HCCPh in Et2O results in rapid and clean formation of
analytically pure phenylacetylidene−amides, as red crystals of
(dtbpe)Ni{NH(2,6-iPr2C6H3)}(CCPh) (4), (dtbpe)Ni{NH-
(Mes)}(CCPh) (5), and orange crystals of (dtbpe)Ni(NH-
{1-Ad})(CCPh) (6), in 81%, 90%, and 86% isolated yields,
respectively (Scheme 4). These results imply that the anilide in
[(dtbpe)NiNH(2,6-iPr2C6H3)][PF6] has a pKa from 29 to
35. Since the imidos 2 and 3 are not prepared by amide
deprotonation we do not know their basicity; however,
complex 2 most likely has a similar pKb to 1. Complexes 4−
6 were thoroughly characterized spectroscopically in solution.
For example, the 1H and 13C NMR spectra of complex 4 reveals
the amide (NH, 1.93 ppm) and acetylide moieties (CCPh,
107 and 114 ppm), respectively. The acetylide α-C reveals
The final pair of orbitals of interest from the MCSCF
computations are plotted below (Figure 3). These orbitals
highlight the substantial interaction between the imido N and
the π-ring of the aryl substituent. Much of the experimental
emphasis on late metal imidos has focused on the steric
protection of the metal−nitrogen active site by changing the
ortho substituents. The present computations suggest the great
potential to synthetically tune the reactivity of these and related
2
diagnostic JCP = 96 and 41 Hz, respectively, for trans and cis
coupling to dtbpe, which are much greater than the coupling
3
observed for the acetylide β-C JCP = 21 Hz to the trans
phosphorus. Because of the lack of C2 symmetry, the 31P NMR
spectrum shows a pair of doublets for the dtbpe ligand with the
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2
respective JPP values listed in Table 1. Table 1 also lists other
amide ligands (Scheme 4). Ynamines, especially those
containing a secondary amine as in the case of PhC
CNHAr, are known to tautomerize to keteneimines sponta-
neously.48 The 1H NMR spectrum of this new organic product
revealed a diagnostic vinylic singlet at δ 5.62 consistent with the
rearranged keteneimine PhCHCNAr being produced.
This compound was isolated in pure form as an oil in 48%
yield.
salient NMR spectroscopic features for complexes 4−6.
Table 1. Selected NMR Spectroscopic Data for Complexes
4−13
31P, Ni−P
(2JPP in Hz)
13C, Ni−C 13C, CO 13C, CN 13C, CC
4
82.8, 68.8 (26)
81.0, 67.8 (25)
98.8, 90.2 (47)
77.5, 66.2 (13)
78.5, 66.9 (12)
77.5, 62.4 (20)
86.0, 83.3 (27)
75.4, 69.3 (7)
89.7, 85.7 (28)
87.7
106.8
112.0
100.6
35.8
38.1
36.4
n/a
n/a
n/a
113.8
156.7
114.7
n/a
It has been shown that nickel complexes supporting a
terminal imido ligand can engage in some cycloaddition
chemistry, and in some cases, the imido can be completely
transferred to ethylene,15 CO, and CNR.16,18 Iron and
cobalt imidos have been also shown to be reactive with these
substrates.49 Consequently, we examined the reactivity of 4−6
with other unsaturated, small molecules. Accordingly, diphe-
nylketene (OCCPh2)24 smoothly reacted to afford the oxy
metallacyclobutane species (dtbpe)Ni{O,C:OC(CPh2)
N(R)} shown in Scheme 5 (R = 2,6-iPr2C6H3 (7), Mes (8),
1-Ad (9)). Formation of 7−9 is nearly quantitative, although
isolated yields can range from 95 to 72%. The 31P NMR spectra
reveal two inequivalent phosphorus nuclei with their
5
n/a
n/a
6
n/a
n/a
7
n/a
175.7
170.2
179.0
173.4
n/a
8
n/a
n/a
9
n/a
n/a
10
11
12
13
n/a
n/a
n/a
n/a
n/a
n/a
170.5
157.9
n/a
n/a
n/a
n/a
n/a
Infrared spectra of 5 and 6 further confirm a terminal acetylide
ligand with νCC = 2091 and 2153 cm−1, respectively. X-ray
crystallographic analysis of a single crystal of 4 grown from a
saturated Et2O solution cooled to −35 °C reveals a square-
planar Ni(II) complex with cis acetylide and anilide ligands
(Figure 4). The Ni−N bond length in 4 is significantly longer
2
corresponding JPP values between 13 and 20 Hz (Table 1).
Because the lowest unoccupied molecular orbital of OC
CPh2 is augmented mostly with O−C π* character,50 one
would expect [2 + 2]-cycloaddition to take place by OC
addition across the NiN to form an aza oxy metal-
lacyclobutane. Indeed, the 13C NMR spectra of 7−9 show a
highly deshielded resonance at ∼170 ppm in accord with a
carbon atom possessing electron-withdrawing groups (Table
1). To determine unambiguously the type of metallacycle
formed, a single-crystal X-ray diffraction study of 7 was
performed. To our surprise, the solid-state structure of 7
revealed a square planar oxy nickel cyclobutane species,
whereby the imido had virtually undergone insertion into the
electrophilic carbon of the ketene (Figure 5). The short C−N =
1.280(2) Å is consistent with a double bond, while all metrical
parameters in the metallacycle suggest single bonds in the
NiOC2 ring. Formation of 7−9 implies that cycloaddition
across the NiN took place, though it cannot be distinguished
whether initial OC versus CC bond addition took place
based on these data. Scheme 6 depicts some possible pathways
for formation of complexes 7−9. O,C-Cycloaddition of the
ketene group across NiN to form metallacycle A is the
preferred pathway given the frontier orbitals of OCCPh2.
From A, two routes can take place: (i) homo or heterolytic Ni−
N bond cleavage and rotation about the C−O bond or (ii)
retrocycloaddition to form a nickel−oxo B and keteneimine
RNCCPh2, which can add across the CC bond to form
the NiOC2 ring. The propensity of a terminal nickel−oxo
compound to form robust dimers argues against the second
pathway.51 Alternatively, ketene can add via the C−C π* bond
to form the azametallacyclobutane C, which, analogously to A,
can undergo two similar pathways, namely, Ni−N bond rupture
or retrocycloaddition (via the nickel carbene D), to ultimately
form the Ni−O bond.
Figure 4. Solid-state structural diagram of complex 4 (thermal
ellipsoids at 50% probability). One chemically equivalent but
crystallographically independent molecule along with two Et2O
molecules confined in the asymmetric unit were omitted for clarity.
at 1.933(3) Å than that observed in [(dtbpe)NiN(H)-
(2,6-iPr2C6H3)][PF6] (1.768(14) Å)1 because of the now filled
Ni d-orbital (of b2 symmetry) in an ideal square planar and C2
symmetric environment.3 The lone pair on the anilide nitrogen
in 4 can no longer donate to the metal, hence, the
pyramidalization. Table 2 depicts selected metrical parameters
for complex 4.
The coupling of amines with alkynes is a relatively difficult,
yet synthetically desirable, process for the construction of
versatile synthetic precursors.47 Complexes 4−6 could be
considered analogous to intermediates in a reaction coupling
these two organic fragments. Consequently, the reductive
elimination from complex 4 was investigated further.
Accordingly, reaction of 4 with CO (−78 °C, 1 atm)
completely converted this nickel species to the known
To isolate an aza oxy metallacyclobutane species such as A,
the reactivity of 1 with the less sterically hindered isocyanate
OCNCH2Ph was investigated. Accordingly, it was found
that the O,C-cycloaddition product (dtbpe)Ni{O,C:OC
NCH2PhN(2,6-iPr2C6H3)} (10) could be isolated in 90%
yield as green-colored crystals (Scheme 5). Salient spectro-
scopic data for 10 are shown in Table 1, with the most notable
feature being the CN resonance at 173.4 ppm in the 13C
17
biscarbonyl complex (dtbpe)Ni(CO)2 based on 31P NMR
spectroscopy. The mass spectrum of the major organic was
consistent with the reductive elimination of the alkyne and
H
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a
Table 2. Selected Metrical Parameters for Complexes 4, 7, and 10−12
4
7
10
11
12
Ni−N
Ni−C
Ni−O
Ni−P1
1.933(3)
1.858(4)
1.939(6)
1.924(4)
1.9334(15)
2.0786(17)
1.8548(11)
2.2457(5)
2.2127(5)
89.894(18)
1.864(5)
2.210(2)
2.190(2)
89.80(8)
1.842(3)
1.8912(13)
2.2008(7)
2.2223(7)
89.63(3)
2.1971(10)
2.2565(10)
89.46(4)
2.2313(14)
2.2151(14)
89.85(5)
Ni−P2
P1−Ni−P2
N−Ni−C
N−Ni−O
C−Ni−O
P1−Ni−N
P2−Ni−N
P1−Ni−C
P2−Ni−C
P1−Ni−O
P2−Ni−O
89.39(14)
69.5(2)
73.38(15)
69.24(6)
71.33(6)
173.44(10)
93.16(9)
111.47(18)
158.68(18)
106.34(11)
163.78(12)
160.46(5)
109.72(5)
89.07(11)
169.86(11)
158.90(5)
111.05(5)
88.43(4)
173.60(16)
89.24(15)
172.43(12)
90.43(10)
91.30(4)
170.57(4)
177.16(4)
a
Distances are reported in Å, and angles are in degrees. Solvent was excluded from 4, 10, 11, and 12.
Scheme 5. [2 + 2]-Cycloaddition and Insertion Chemistry Involving the NiN Bond in Complexes 1−3
NMR spectrum. Like complexes 4−9, compound 10 also lacks
spectroscopic features of 11 are similar to those of complex
10, and the solid-state structure confirms a square planar aza
oxy nickel cyclobutane scaffold with Ni−N = 1.924(4) Å and
Ni−O = 1.842(3) Å distances that compare favorably to those
of 10 (Figure 6).
Exposure of a cold Et2O solution of 1 to 1 atm CO2 resulted
in the gradual precipitation of a pale yellow solid containing
(dtbpe)Ni{O,C:OC(O)N(2,6-iPr2C6H3)} (12) (Scheme 5).
The reaction is quantitative by 31P NMR spectroscopy, and
yellow solids of 12 can be isolated in 94% yield. In addition to
displaying two inequivalent phosphine resonances (2JPP = 28
Hz) akin to the other nickel metallacycles, the most salient
spectroscopic feature in 12 is the presence of a broad 13C NMR
resonance at 170.5 ppm in accord with a [2 + 2]-cycloadded
CO2 carbon. Likewise, νCO = 1617 cm−1 in the infrared region
corroborates the presence of this carbonyl functionality. A
solid-state structure of 12 is shown in Figure 6 and again
2
C2 symmetry but has a stronger JPP value of 27 Hz when
compared to complex 7, as a result of the phosphines of the
dtbpe ligand not being pushed back due to the sterically
congested CPh2 group. Therefore, one would expect less of a
distortion from a square planar geometry. A solid-state
structure of 10 confirms formation of a rare example of an
aza oxy nickel cyclobutane square planar complex with an
elongated Ni−N bond (1.939(6) Å) as compared to the imido
precursor 1 (Figure 5). Examples of N,O-bound ureates
resulting from isocyanate cycloaddition across early transition
metals having a terminal imido ligand have been reported.52
The Ni−O bond length (1.864(5) Å) is comparable to those
observed in oxy nickel metallacycles.53 Similar cycloaddition
chemistry of 1 was observed with benzaldehyde, which gave the
aza oxy nickel cyclobutane complex (dtbpe)Ni{O,C:OCHPhN-
(2,6-iPr2C6H3)} (11) in 83% yield. Overall, the NMR
I
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Article
Figure 5. Solid-state structural diagram of complexes 7 (left) and 10 (right) with thermal ellipsoids at 50% probability. One Et2O molecule in the
asymmetric unit was omitted from the structure of 10.
parameters, the terminal oxygen (CO(2) = 1.229(2) Å) is
Scheme 6. Proposed Mechanism for Formation of
Complexes 7−9
part of a carbonyl functionality, while the “other” half of the
CO2 has been reduced to a single bond, C−O(1) = 1.340(2) Å
as part of the metallacycle.
When exploring the chemistry of the more electron-rich
imido complex 3, a different outcome is observed. Treating 3
with a bed of CO2 resulted in quick formation of the yellow
complex (dtbpe)Ni{O,O:(OC(O))2N(1-Ad)} (13) in 62%
isolated yield (Scheme 5). Formation of 13 most likely involves
a [2 + 2]-cycloaddition intermediate (dtbpe)Ni{O,C:OC(O)-
N(1-Ad)}, like 12, followed by CO2 insertion into the more
electron-rich and less-hindered Ni−N bond. The reactivity of
CO2 with 3 mirrors that of (dtbpe)NiCPh2, which resulted
in six-membered ring formation observed in (dtbpe)Ni{O,O:
(OC(O))2CPh2)}.19 NMR spectra of 13 are indicative of a C2v-
symmetric complex, while the IR spectrum shows two carbonyl
stretches νCO = 1667 and 1624 cm−1. Attempts to prepare the
(dtbpe)Ni{O,C:OC(O)N(1-Ad)} using 1 equiv of CO2
resulted in formation of 13 along with unreacted starting
material. Mountford has observed contrasting modes of
reactivity between titanium imido complexes and CO2. For
example, electron-rich TiN bonds tend to undergo meta-
thesis with CO2 to form the titanium oxo, while aryl-substituted
depicts a square planar nickel complex possessing a metal-
lalactam or carbamate framework due to [2 + 2] cycloaddition
of CO2 across the NiN bond with distances of Ni−O =
1.9334(15) Å and Ni−N = 1.8912(13) Å. From the metrical
Figure 6. Solid-state structural diagram (thermal ellipsoids at 50% probability) of complexes 11 (left) and 12 (right). One Et2O molecule for 11 and
one CH2Cl2 molecule for 12, found in the asymmetric unit, are excluded for clarity.
J
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Inorganic Chemistry
Article
imido complexes such as Cp*TiN{2,6-Me2C6H3}{MeC-
(NiPr)2} can undergo cyloaddition of 1 equiv of CO2 to
form the N,O-bound carbamate, which can further react with
another equivalent of CO2 to form the azadicarboxylate ligand
[(OC(O))2N(2,6-Me2C6H3)]2−.52
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: mindiola@sas.upenn.edu.
Present Addresses
‡Department of Chemistry, University of Pennsylvania,
Philadelphia, PA 47405.
Lastly, we explored the thermal stability of imido 2 because it
is known that (2,2′-bipyridine)NiEt2 reacts with excess
mesitylazide to give azomesitylene (MesNNMes).54 It is
proposed in such reactions that a transient “(2,2-bipyridine)-
NiNMes” is a key species en route to the formation of the
azomesitylene, a hypothesis that partially prompted the
synthesis of 1. Hence, heating a benzene solution of 3 revealed
formation of azomesitylene, which can be isolated as red
crystals in ∼40% yield (Scheme 5). The thermolysis of 3
demonstrates that the nitrene fragments from a nickel imido
can couple to give azoarenes and provides some support to the
idea of an imido intermediate in the formal coupling of
mesitylazide by (2,2-bipyridine)NiEt2. This thermolysis is also
related to the report that Fe2(CO)9 can decompose phenyl-
azide to azobenzene.55
⊥Department of Chemistry, University of Vermont, Burlington,
VT 05405.
∥Department of Chemistry and Biochemistry, University of
Notre Dame, Notre Dame, IN 46556.
Notes
The authors declare no competing financial interest.
#Deceased March 6, 2014.
ACKNOWLEDGMENTS
■
This paper is dedicated to the memory of Professor Gregory
Lee Hillhouse. We thank the National Science Foundation for
financial support, and Dr. I. Steele for some assistance with X-
ray crystallography. D.J.M. acknowledges postdoctoral fellow-
ship support from the Ford Foundation and the National
Institutes of Health. R.W. acknowledges a GAANN Fellowship.
Dr. S. M. Baldwin and C. Hansen are also thanked for some
assistance with the crystallographic data. T.R.C. acknowledges
the U.S. Department of Energy, Office of Basic Energy
Sciences, for funding via Grant No. DE-FG02-03ER15387.
CONCLUSIONS
■
In this work we presented three synthetic pathways to
mononuclear nickel imido complexes having aliphatic or
aromatic groups and reported reactivity involving the NiN
motif. A reinvestigation utilizing high-level ab initio quantum
chemistry techniques is given for the original Ni-imido complex
reported in 2001. The computational analysis of 1, coupled
with the newly disclosed reactivity studies reported herein,
paint a picture of this late metal imido complex as being able to
effect both one- (i.e., radical) and two-electron (e.g.,
deprotonation of terminal alkynes) transformations given the
biradical and highly Nimido-localized nature of the frontier
orbitals of 1. Moreover, delocalization of these same frontier
orbitals from the π/π* orbitals of the NiNimido active site to the
aryl−imido substituent suggests considerable potential to tune
the reactivity of 1 and other late metal imido complexes among
these disparate reactivity manifolds through judicious manip-
ulation of the chemical environment about the active site.
In addition to C−H activation reactions (both homolytically
and presumably heterolytically), we include examples of
cycloaddition chemistry of 1 with various electrophiles
including some cumulenes. Taking advantage of the acidic
C−H bond in HCCPh, it is shown that imido group in 1 can
couple to acetylide to ultimately form a keteneimine. The imido
moiety can also engage in cycloaddition and subsequent
insertion or isomerization pathways to form unusual nickel
metallacycles. While 1 seems to only cycloadd small molecules
such as ketenes, isocyanates, aldehydes, and CO2, more
electron-rich alkyl−imidos such as 3 can undergo further Ni−
N insertion chemistry. The delocalization of the frontier
orbitals between NiN and NAr π/π* orbitals is also consistent
with the greater reactivity of 3 than 1. In contrast, reducing
steric bulk on the arylimido group can allow for reductive
coupling.
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