Arene ci￡h amination at nickel in terphenyl-diphosphine complexes with labile metal-arene interactions

Article abstract of DOI:10.1002/chem.201302539

The meta-terphenyl diphosphine, m-P₂, 1, was utilized to support Ni centers in the oxidation states 0, I, and II. A series of complexes bearing different substituents or ligands at Ni was prepared to investigate the dependence of metal-arene interactions on oxidation state and substitution at the metal center. Complex (m-P₂)Ni (2) shows strong Ni ⁰-arene interactions involving the central arene ring of the terphenyl ligand both in solution and the solid state. These interactions are significantly less pronounced in Ni⁰ complexes bearing L-type ligands (2-L: L=CH₃CN, CO, Ph₂CN₂), Ni^IX complexes (3-X: X=Cl, BF₄, N₃, N₃B(C ₆F₅)₃), and [(m-P₂)Ni ^IICl₂] (4). Complex 2 reacts with substrates, such as diphenyldiazoalkane, sulfur ylides (Ph₂Si￡CH₂), organoazides (RN₃: R=para-C₆H₄OMe, para-C ₆H₄CF₃, 1-adamantyl), and N₂O with the locus of observed reactivity dependent on the nature of the substrate. These reactions led to isolation of an η¹-diphenyldiazoalkane adduct (2-Ph₂CN₂), methylidene insertion into a Nii￡P bond followed by rearrangement of a nickel-bound phosphorus ylide (5) to a benzylphosphine (6), Staudinger oxidation of the phosphine arms, and metal-mediated nitrene insertion into an arene Ci￡H bond of 1, all derived from the same compound (2). Hydrogen-atom abstraction from a Ni^I-amide (9) and the resulting nitrene transfer supports the viability of Ni-imide intermediates in the reaction of 1 with 1-azido-arenes. Put a ring on it: The utilization of a meta-terphenyl diphosphine ligand leads to labile metal-arene interactions in Ni complexes in various oxidation states and coordination environments. When these complexes are treated with group-transfer reagents, such as diazoalkanes, sulfur ylides, or azides, various types of ligand-centered reactivity were observed, including methylidene insertion into a Pi￡C bond and amination of an arene Ci￡H bond. Copyright

Full text of DOI:10.1002/chem.201302539

FULL PAPER
DOI: 10.1002/chem.201302539
À
Arene C H Amination at Nickel in Terphenyl–Diphosphine Complexes
with Labile Metal–Arene Interactions
David E. Herbert, Nadia C. Lara, and Theodor Agapie*[a]
Abstract: The meta-terphenyl diphos-
phine, m-P2, 1, was utilized to support
Ni centers in the oxidation states 0, I,
and II. A series of complexes bearing
different substituents or ligands at Ni
was prepared to investigate the de-
pendence of metal–arene interactions
on oxidation state and substitution at
the metal center. Complex (m-P2)Ni
(2) shows strong Ni0–arene interactions
involving the central arene ring of the
terphenyl ligand both in solution and
the solid state. These interactions are
significantly less pronounced in Ni0
complexes bearing L-type ligands (2-L:
L=CH3CN, CO, Ph2CN2), NiIX com-
plexes (3-X: X=Cl, BF4, N3, N3B-
led to isolation of an h1-diphenyldia-
zoalkane adduct (2-Ph2CN2), methyli-
(C6F5)3), and [(m-P2)NiIICl2] (4). Com-
dene insertion into a Ni P bond fol-
À
plex 2 reacts with substrates, such as di-
phenyldiazoalkane, sulfur ylides (Ph2S=
CH2), organoazides (RN3: R=para-
C6H4OMe, para-C6H4CF3, 1-adaman-
tyl), and N2O with the locus of ob-
served reactivity dependent on the
nature of the substrate. These reactions
lowed by rearrangement of a nickel-
bound phosphorus ylide (5) to a benzyl-
phosphine (6), Staudinger oxidation of
the phosphine arms, and metal-mediat-
À
ed nitrene insertion into an arene C H
bond of 1, all derived from the same
compound (2). Hydrogen-atom ab-
straction from a NiI–amide (9) and the
resulting nitrene transfer supports the
viability of Ni–imide intermediates in
the reaction of 1 with 1-azido-arenes.
À
Keywords: amination · arenes · C
H activation · group transfer · inser-
tion reactions
Introduction
cursors.[12–14] Warren has examined analogous reactions at Ni
centers supported by b-diketiminate ligands.[9,10] In these
cases, divergent types of group transfer was observed (e.g.,
The design of ligands that support first-row-transition-metal
complexes that can react with molecules such as organoa-
zides, diazoalkanes, and nitrous oxide to form terminal mul-
tiply bound metal imides, carbenes, and oxo complexes
active in group-transfer reactions has received significant at-
tention recently as atom-efficient routes are sought for the
functionalization of unreactive moieties, such as olefins and
À
aziridination versus C H amination), depending on the co-
ordination number and oxidation state of the metal center.
À
Ni-mediated nitrene insertion into a strong aromatic C H
bond was not reported.[15]
We have investigated the chemistry of trans-spanning mul-
tidentate terphenyl diphosphine ligands wherein the central
arene ring can act as a multihapto binding site and exhibit
a range of versatile coordination and noninnocent behav-
ior.[16,17] In this report, we describe the ability of a meta-ter-
phenyl diphosphine (1) to support Ni centers in a variety of
oxidation states [Ni0, 2-L (L=CH3CN, CO, Ph2CN2); NiI, 3-
[1–4]
À
C H bonds.
Examples of isolated complexes bearing
imides (or nitrides) of late transition metals, such as Mn,[5]
Fe,[6] Co,[7] and Ni,[8–10] have been reported and their group-
transfer reactivity explored. Terminal copper nitrenes have
been proposed as key reactive intermediates in Cu-catalyzed
[11]
X (X=Cl, BF4, N3, N3-BACHTUNGETRNNUG
(C6F5)3); NiII, 4 (X=Cl2)] and
À
nitrene transfer/C H amination.
Notably, Hillhouse and co-workers have demonstrated
that Ni, supported by bulky donating 1,2-bis(di-tert-butyl-
phosphino)ethane (dtbpe) ligands, can form three-coordi-
nate imides and carbenes from azide and diazoalkane pre-
detail the reactivity of these complexes towards a range of
small molecules, such as azides, diazoalkanes, and nitrous
oxide. A variety of reactivity modes was observed, including
carbene coupling, Staudinger oxidation of phosphine arms,
À
methylidene insertion into a P C bond, and amination of an
À
aromatic C H bond. The role of the oxidation state, metal–
arene interactions, and the substrate in these reactions are
[a] Dr. D. E. Herbert, N. C. Lara, Prof. T. Agapie
Division of Chemistry and Chemical Engineering
California Institute of Technology–MC 127-72
1200 E California Boulevard, Pasadena, CA 91125 (USA)
Fax : (+1)626-568-8824
À
discussed in the context of functionalization of aromatic C
H bonds.
E-mail: agapie@caltech.edu
Results and Discussion
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201302539 and contains experi-
mental details, NMR spectra, and crystallographic information for all
compounds.
Ni complexes in oxidation states 0, I, and II were prepared
by reaction of
1
with the appropriate Ni precursor
16453
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À
À
À
(Scheme 1). Treatment of diphosphine 1 with one equivalent
of Ni0 in the form of [Ni
(cod)2] (cod=1,5-cyclooctadiene) in
THF gave the terphenyl-diphosphine Ni complex 2, which
was isolated as a red-brown solid. Treatment of 1 with
C(1) C(2) 1.425(2), C(2) C(3) 1.441(3), C(3) C(4)
À
À
À
G
1.371(3), C(4) C(5) 1.418(3), C(5) C(6) 1.373(3), C(6)
C(1) 1.435(2) ꢁ].[16–18] Binding a s-donor ligand, such as
CH3CN, to the Ni0 center in 2 leads to an elongation of the
À
À
[NiCl
(dme)] (dme=1,2-dimethoxyethane) provided the ter-
Ni CACTHNGUTERN(UNG arene) distances in the solid state [2-CH3CN: Ni(1)
À
phenyl-diphosphine Ni complex 4 as a purple solid. Orange-
C(1) 2.0892(9) ꢁ; Ni(1) C(2) 2.427(1) ꢁ; Figure 1]. The
binding of more-p-acidic ligands has a similar effect. For ex-
À
ample, in the CO adduct 2-CO, the Ni(1) C(1) distance is
increased to 2.256(2) ꢁ. Higher oxidation state complexes
À
show significantly longer Ni–arene distances [3-Cl: Ni(1)
C(1) 2.562(1) ꢁ;[17] 4: Ni(1) C(1) 2.775(1) ꢁ]. To accommo-
À
date the significant change in ligand conformation, the P-Ni-
P angle is more acute in the lower oxidation state com-
plexes, with shorter Ni–arene contacts [139.06(2)8 in 2,
155.11(1)8 in 3-Cl, and 169.935(8)8 in 4].
The interaction between the Ni center and the central
arene is also apparent in solution by NMR spectroscopy.
À
The central arene C H ortho to both flanking arene rings
À
(denoted as C HNi) is shifted considerably upfield (dCH
=
À
5.18 ppm; C6D6, 228C) relative to other arene C H resonan-
ces. The strong shielding of this resonance by the Ni0 center
is consistent with a significant metal–arene interaction in so-
lution. In addition, the 13C{1H} NMR spectrum of 2 contains
À
a triplet at d=68.6 ppm (JCP =3.6 Hz) assigned to C HNi
that shows coupling to the two phosphorus nuclei, further
supporting an interaction between Ni and the arene in solu-
Scheme 1. Synthesis of Ni complexes 2, 3-X, and 4.
colored NiI-chloride 3-Cl was
prepared by comproportiona-
tion of NiII and Ni0 precursors
by the reaction of
0.5 equivalents of [Ni(cod)2] fol-
lowed by the addition of
0.5 equivalents of [NiCl2-
(dme)].[17] Salt metathesis of 3-
1 with
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
Cl provided access to structur-
ally-related species with anions
that have various electronic
properties. The central arene of
the terphenyl framework shows
variable interactions with the
metal center (in both solution
and the solid state) as a function
of the metal oxidation state and
other coordinated ligands.
The compounds described
have been characterized by
single-crystal X-ray diffraction.
The central arene is bound h2
Figure 1. ORTEPs of 2, 2-CH3CN, 2-CO, and 4 with thermal ellipsoids shown at 50% probability levels. Some
À
to the metal center in 2 [Ni(1)
À
hydrogen atoms have been omitted for clarity. Selected bond lengths [ꢁ] and angles [8]. 2: C(1) Ni(1)
À
C(1)
1.973(2);
Ni(1) C(2)
À
À
À
À
À
1.973(2), C(2) Ni(1) 2.133(2), Ni(1) P(1) 2.1777(5), Ni(1) P(2) 2.1825(5), C(1) C(2) 1.425(2), C(2) C(3)
2.133(2) ꢁ], resulting in a locali-
zation of electron density
within the central arene ring
with concomitant lengthening
of the distance between the two
À
À
À
À
1.441(3), C(3) C(4) 1.371(3), C(4) C(5) 1.418(3), C(5) C(6) 1.373(3), C(6) C(1) 1.435(2); P(1)-Ni(1)-P(2)
À
À
À
À
139.06(2). 2-CH3CN: Ni(1) C(1) 2.0892(9), Ni(1) C(2) 2.4272(10), Ni(1) N(1) 1.9131(9), Ni(1) P(1)
À
À
2.1887(3), Ni(1) P(2) 2.1854(3); P(1)-Ni(1)-P(2) 130.62(1), C(31)-N(1)-Ni(1) 164.70(9). 2-CO: C(1) Ni(1)
À
À
À
À
2.256(2), Ni(1) P(1) 2.2123(6), Ni(1) P(2) 2.1954(5), C(31) Ni(1) 1.754(1) C(31) O(1) 1.1565(2); P(1)-Ni(1)-
À
À
À
P(2) 125.94(2), O(1)-C(31)-Ni(1) 178.2(1). 4: Ni(1) P(1) 2.2576(2), Ni(1) P(2) 2.2424(2), Ni(1) Cl(1)
À
À
2.1879(2), Ni(1) Cl(2) 2.1805(2), C(1) Ni(1) 2.775(1); P(1)-Ni(1)-P(2) 169.935(8), Cl(1)-Ni(1)-Cl(2)
carbon atoms bound to Ni [2: 167.570(8).
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Chem. Eur. J. 2013, 19, 16453 – 16460

À
Arene C H Amination at Nickel in Terphenyl–Diphosphine Complexes
FULL PAPER
1
tion. Both the H- and 13C{1H} NMR spectra of 2 are consis-
nation requirements of various Ni complexes, the reactivity
with group-transfer reagents was investigated.
The addition of a stoichiometric amount of diphenyldia-
zoalkane to a solution of 2 in hexanes resulted in the isola-
tion of a 1:1 adduct (2-Ph2CN2; Scheme 2). Analysis of 2-
Ph2CN2 by NMR spectroscopy revealed that the diagnostic
tent with an average Cs-symmetric structure in solution at
room temperature, indicating fast exchange, with the Ni
center shuttling between the two sides of the pseudo mirror
plane relating the two phosphine moieties on the timescale
of the NMR experiments.
À
In comparison with the solid-state structure of 2, all of the
yellow or orange NiI complexes, 3-X [X=Cl, BF4, N3, N3-B-
À
central arene C H resonances were found at much lower
field (1H: d=8.29 ppm, 13C{1H}: d=108 ppm; C6D6, assigned
using the HSQC spectrum), suggesting weaker metal–arene
interactions. Indeed, the solid-state structure of 2-Ph2CN2
confirmed this change in bonding (Figure 3). The Ni–carbon
distance in 2-Ph2CN2 is considerably elongated compared
ACHTUNGTRENNUNG(C6F5)3], show longer metal–arene distances [Ni(1) C(1): 3-
Cl 2.562(1), 3-BF4 2.4976(15), 3-N3 2.6182(8), 3-B(N3)ACHTUNGRTNEUNG(C6F5)3
2.400(3) ꢁ]. Comparing 3-BF4 with 3-Cl and 3-N3, the less
À
coordinating anion results in a shorter Ni C
tance. Similarly, appending the strongly Lewis acidic tris(-
pentafluorophenyl)borane to the terminal nitrogen atom of
G
À
with that in 2 [Ni(1) C(1) 2.511(2) ꢁ]. The diazoalkane
moiety binds in an almost linear fashion [Ni(1)-N(1)-N(2)
À
the Ni-bound azide, by the addition of B
of 3-N3, renders the azido moiety a weaker donor, resulting
(C6F5)3 to a solution
160.5(2)8]. Interestingly, the Ni N distance is significantly
shorter [1.751(2) ꢁ] compared with that of the terminal ace-
À
À
in a shorter Ni(1) C(1) contact in 3-B(N3)
(C6F5)3 compared
tonitrile adduct 2-CH3CN [Ni(1) N(1) 1.913(1) ꢁ; Figure 1].
These features suggest contribution from a resonance struc-
ture involving multiple bonding between Ni and N. Notably,
with 3-N3 (Figure 2). Thus, the arene binds more strongly to
the electron-rich Ni0 center in 2 by virtue of stronger metal-
to-ligand backbonding compared with complexes of NiI and
NiII. The labile interaction with the central arene is also sen-
sitive to additional ligands, with more coordinating anions
leading to weaker arene binding. With a better understand-
ing of the ability of the pendant arene to satisfy the coordi-
À
the Ni N distance in 2-Ph2CN2 is only slightly longer than
that of a tricoordinate NiII–imide reported by Hillhouse
[1.702(2) ꢁ].[12] Partial oxidation of the metal center is con-
À
sistent with the deshielding of the C HNi proton sitting
below the metal, as observed by 1H NMR spectroscopy
(compare 2 with 4-Cl2).
With diazoalkane adduct 2-
Ph2CN2 in hand, thermal extru-
sion of dinitrogen was attempt-
ed in order to access a Ni car-
bene.[14] However, extensively
heating solutions of 2-Ph2CN2
in benzene or toluene (80–
1108C) over several days slowly
regenerated 2 with no observa-
ble intermediates. The diazoal-
kane fragment was converted
under these conditions into
three detectable products: tet-
raphenylethylene, 1,2-bis(diphe-
nylmethylene)hydrazine,
and
N-(diphenylmethylene)-1,1-di-
phenylmethanamine,
in
a 50:40:10 ratio (GC). Control
reactions that consisted of heat-
ing solutions of diphenyldiazo-
methane at equivalent concen-
trations for the same period of
time yielded the hydrazine ex-
clusively in our hands. Addition
of the Lewis acid, SmACTHNUTRGNENUG(OTf)3, to
Figure 2. ORTEPs of 3-BF4, 3-N3, and 3-B(N3)
ACHTNUGTRNE(NNUG C6F5)3 with thermal ellipsoids shown at 50% probability levels.
the reaction mixture in an at-
tempt to assist N2 extrusion[14]
increased the proportion of the
tetrasubstituted olefin to 60%
of the observed products.
Mechanisms involving dissociat-
ed and metal-bound diphenyl-
Some hydrogen atoms have been omitted for clarity. Selected bond lengths [ꢁ] and angles [8] for 3-BF4:
À
À
À
À
Ni(1) P(1) 2.2665(4), Ni(1) P(2) 2.2634(4), Ni(1) F(1) 2.1852(11), C(1) Ni(1) 2.4976(15); P(1)-Ni(1)-
À
À
À
P(2)158.487(15), B(1)-F(1)-Ni(1) 128.78(10). 3-N3: Ni(1) P(1) 2.2687(3), Ni(1) P(2) 2.2449(3), Ni(1) N(1)
À
À
À
1.9746(10), N(1) N(2) 1.1852(13), N(2) N(3) 1.1696(13), C(1) Ni(1) 2.6182(8); P(1)-Ni(1)-P(2) 155.207(10),
Ni(1)-N(1)-N(2) 140.29(8), N(1)-N(2)-N(3) 177.13(11). 3-B(N3)ACHTNUTRGNE(UGN C6F5)3: Ni(1) P(1) 2.2625(9), Ni(1) P(2)
À
À
À
À
À
À
À
2.2673(8), Ni(1) N(1) 1.970(3), N(1) N(2) 1.160(4), N(2) N(3) 1.191(4), C(1) Ni(1) 2.400(3), N(3) B(1)
1.591(4); P(1)-Ni(1)-P(2) 153.27(3), Ni(1)-N(1)-N(2) 153.8(3), N(1)-N(2)-N(3) 173.9(3), N(2)-N(3)-B(1)
124.8(2).
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16455

T. Agapie et al.
added to a solution of 2 in THF (Scheme 2). The 31P{1H}
NMR spectrum collected immediately upon warming the re-
sulting solution exhibited two doublets (dP =48.4, 39.0 ppm;
J
P, P =9 Hz), indicating inequivalent magnetic environments
for the phosphines of the ligand coupling weakly to one an-
1
other. The H NMR spectrum contained a resonance consis-
À
tent with insertion of a methylidene into a Ni P bond to
yield a phosphine/phosphine-ylide ligand environment about
Ni (5; dH =0.45 ppm; dd, J=18.4, 6.8 Hz).
Over time, these resonances disappeared while two new
31P doublets grew in, showing considerably larger coupling
(dP =58.4, 35.8 ppm; JP, P =86 Hz). These resonances were ac-
companied by the appearance of two multiplets in the
1H NMR spectrum assigned to benzylic protons coupling
both to one another and to the phosphorus nuclei of the
ligand (dH =3.00 ppm, 1H, dd, 2JHH =13.2, 2JHP =5.7 Hz;
2.85 ppm, 1H, ddd, 2JHH =13.2, 2,3JHP =9.9, 3.5 Hz). These
data suggest rearrangement of monoylide 5 to benzylphos-
phine 6. This rearrangement is reminiscent of the phospho-
rus analogue of the Stevens rearrangement,[20] of which a lim-
ited number of examples have been reported to proceed at
high temperatures[21] and mediated by low-valent metals,
such as Ni0.[22] This rearrangement could be reproduced by
adding an equivalent of [NiACTHNUTRGNE(UNG cod)2] to the independently pre-
pared monoylide of 1 (7, Scheme 2). Triphenylmethylene-
phosphorane does not transfer a methylidene fragment to 2
and rather is isomerized to benzyldiphenylphosphine. No re-
action was observed between diphosphine 1 and triphenyl-
methylenephosphorane in the absence of Ni. This suggests
that the Ni center is involved in the methylene-transfer reac-
tion, although the intermediacy of a Ni-methylidene adduct
may not be required. The observed attack of phosphine on
the methylene carbon atom suggests that the putative Ni-
methylidene (or Ni-methylidene/diphenylsulfide adduct) is
electrophilic.[23]
Scheme 2. Reactivity of 2 towards diphenyldiazoalkane and methylidene
transfer reagents.
This unusual rearrangement to generate 6 (Figure 4) sug-
gests that although initial binding may occur at the Ni
center, the final locus of reactivity in 2 can be the phosphine
arms, which may or may not dissociate in the process.
Indeed, when treated with an oxygen-atom-transfer agent,
Figure 3. ORTEP of 2-Ph2CN2 with thermal ellipsoids shown at 50%
probability levels. Some hydrogen atoms have been omitted for clarity.
À
À
Selected bond lengths [ꢁ] and angles [8]: C(1) Ni(1) 2.511(2), Ni(1)
À
À
À
P(2) 2.1947(6), Ni(1) P(1) 2.1975(6), Ni(1) N(1) 1.7514(17), N(1) N(2)
À
1.190(2), C(31) N(2) 1.327(2); P(2)-Ni-P(1) 122.43(2), N(2)-N(1)-Ni(1)
160.5(2), N(1)-N(2)-C(31) 152.7(2).
diazomethane moieties are feasible, and the generation of
short-lived metal-carbene species is not required for the ob-
served reactivity.
We therefore turned to an alternative method for prepar-
ing metal carbenes. “Transylidation” of a carbenoid frag-
ment from in situ generated sulfur ylides was reported by
Milstein and co-workers who demonstrated this methodꢂs
utility in preparing a range of late-metal carbenes, such as
Grubbsꢂ catalyst.[19] Deprotonation of diphenylmethylsulfo-
nium tetraphenylborate with lithium hexamethyldisilazide at
À788C gave the methylidene-bearing sulfur ylide, which was
Figure 4. ORTEP of 6 with thermal ellipsoids shown at 50% probability
levels. Some hydrogen atoms have been omitted for clarity. Selected
À
À
bond lengths [ꢁ] and angles [8]: C(1) Ni(1) 1.9614(9), C(6) Ni(1)
À
À
À
2.0614(10), Ni(1) P(1) 2.1813(3), Ni(1) P(2) 2.1519(3), C(19) P(2)
À
1.865(1), C(1) C(6) 1.430(1); P(2)-Ni(1)-P(1) 129.07(1).
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Chem. Eur. J. 2013, 19, 16453 – 16460

À
Arene C H Amination at Nickel in Terphenyl–Diphosphine Complexes
FULL PAPER
such as N2O, oxidation of a phosphine arm and loss of the
metal was observed. Similarly, 2 reacted with arylazides 1-
azido-4-methoxybenzene and 1-azido-4-(trifluoromethyl)-
benzene to give Staudinger oxidation products (oxidation of
the phosphine arms to phosphoranimines) and loss of the
metal (Scheme 3). A mixture of bis-oxidized and mono-oxi-
Figure 5. ORTEP of 8 with thermal ellipsoids shown at 50% probability
levels. Some hydrogen atoms have been omitted for clarity. Selected
À
À
bond lengths [ꢁ] and angles [8]: C(1) Ni(1) 2.053(6), C(2) Ni(1)
À
À
À
2.198(7), Ni(1) P(1) 2.125(2), Ni(1) P(2) 2.216(2), C(1) N(1) 1.415(8);
P(1)-Ni(1)-P(2) 137.95(8).
allows the metal center to bind closer to the central arene
with a more acute P-Ni-P angle [129.073(12)8; compare with
2: 139.06(2)8].
Assembling these observations, we propose that, as ob-
served for the diazoalkane adduct 2-Ph2CN2, azide reagents
may initially bind to the metal center on the opposite side
Scheme 3. Reactivity of 2 towards organoazides.
À
dized products was observed when one equivalent of aryla-
zide was used, even when the reaction was carried out at
low temperature (À358C). Different modes of reactivity
have been described stemming from the activation of organ-
ic azides by a metal center, including formal fragment inser-
tion into metal–phosphine bonds.[24–26]
to the activated C H bond and above the central arene
(Scheme 3, 2-N3R). Direct isomerization at Ni or isomeriza-
tion via dissociation of a phosphine arm allows the azide (2-
N3R) or putative imide (2-NR) to move to the coordination
À
site proximal to C HNi. This process could lead to oxidation
of a phosphine arm to give 1-NR or 1-(NR)2 when the re-
agent is sufficiently oxidizing (e.g., 1-azido-arenes or N2O).
Dissociation of a phosphine arm may be involved in the
conversion of 2 to 6 (Scheme 2). In this case, attack of
a phosphine arm on the proposed Ni-methylidene could be
responsible for the formation of the thermally unstable
ylide-stabilized Ni0 intermediate observed spectroscopically.
When the azide reagent is not sufficiently oxidizing to react
with the phosphine arm, as in the case of N3Ad, conversion
of the organoazide, likely to a Ni–imido species (2-NR),
As the oxidation of 1 by 1-azidoarenes proceeds in the ab-
sence of Ni to yield a bis(phosphorane), the less reactive 1-
azidoadamantane (N3Ad) was investigated, as it does not
oxidize 1.[27] The reactivity of 2 with 1-azido-adamantane
was different from that observed with 1-azido-arenes
(Scheme 3). Treatment of 2 with 1-azido-adamantane at
À358C led to only a slight shift of the 31P{1H} resonance of 2
from d=40.8 to 40.4 ppm, consistent with the phosphines re-
1
maining bound to a Ni0 center. H NMR spectroscopic anal-
À
ysis indicated complete consumption of 1-azido-adamantane
and also the disappearance of the resonance assigned to the
leads to nitrene insertion into the well-positioned C H
bond to generate 8. Terminal metal–imido species have simi-
À
À
central arene C HNi ortho to both flanking aryl rings of the
terphenyl backbone. The solid-state structure of the product
larly been proposed in examples of intramolecular C H
amination mediated by iron[15a] and cobalt.[28] The reactivity
observed with N3Ad may also result from Ni-templated ex-
trusion of N2 to generate a reactive nitrene that is not
bound to Ni, a species that can then insert into the Ni-acti-
8 revealed that an azide-derived adamantylnitrene had in-
À
serted into the central C H bond of 2 (Figure 5). The ami-
nated Ni0 product 8 retains close metal–arene contacts
[25]
À
À
À
[Ni(1) C(1) 2.053(6); Ni(1) C(2) 2.198(7) ꢁ].
vated arene C H bond.
Alternatively, the putative Ni–
À
No reaction was observed between N3Ad and 3-Cl or 3-
BF4, indicating that the NiI center is not reactive enough,
owing either to steric constraints or to the metal center not
being sufficiently reducing. Furthermore, no reaction was
observed between N3Ad and the benzyl phosphine Ni0 com-
plex, 6, likely as a consequence of the increased steric
crowding of the metal center in 6 compared with that in 2,
as the increased flexibility of the benzylphosphine arm
imido species could undergo 1,2-addition of the arene C H
bond, reactivity reminiscent of that of early-transition-metal
imido complexes,[29] followed by reductive elimination.
To further probe the formation of a Ni–imide intermedi-
ate in either ligand oxidation or nitrene insertion
(Scheme 3), we attempted an independent synthesis of
imide complexes of 2 (Scheme 4). Hillhouse has demonstrat-
ed that phosphine-supported NiII imides (and phosphini-
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16457

T. Agapie et al.
Ph2CN2, consistent with multiple-bonding character in that
system (see above).
Despite displaying an electrochemically reversible oxida-
tion (À0.80 V versus ferrocene/ferrocenium), the oxidation
of 9-Dipp with ferrocenium triflate led to significant decom-
position, with 1 being the only identifiable phosphorus-con-
taining species (full conversion of the oxidant to ferrocene
was observed by 1H NMR spectroscopy). Treatment of 9-
Dipp with the stable 2,4,6-tri-tert-butylphenoxyl radical led
to the same result, with none of the desired reactivity dis-
cerned in the extracted equivalents of 1 and 2,4,6-tri-tert-bu-
tylphenol observed by 1H NMR spectroscopy. Addition of
one equivalent of 2,4,6-tri-tert-butylphenoxyl radical to
a pentane solution of 9-Ph at À788C led to a color change
from deep blue to red. Upon warming to room temperature,
31P{1H} and 1H NMR spectroscopic analysis revealed full
conversion to the phenol and generation of approximately
60% of the mono-oxidized Staudinger product, the phos-
Scheme 4. Hydrogen atom abstraction from 9 to generate 1-NPh.
denes) are accessible by either sequential[12,30] or concert-
ed[31] removal of a proton and an electron from a NiI anilide.
Treating 3-Cl with an equivalent of LiNHAd under a variety
of conditions, however, led to reduction of the NiI complex
and conversion to 2, despite a relatively negative NiI/0
couple for 3-Cl (E1/2 =À1.39 V versus ferrocene/ferroceni-
um).
Salt metathesis of 3-Cl with either LiNH
(dipp) (dipp=
phoranimine 1-NPh, and approximately 30% of
2
2,6-diisopropylphenyl) or LiNH(C6H5) proved possible in
N
(Scheme 4).
ether/toluene mixtures at À358C and yielded the NiI anilides
9-Dipp and 9-Ph, respectively, as bright blue solids [9-Dipp:
lmax (e): 592 nm (5450 cmÀ1 mÀ1) in high yields. EPR spec-
troscopy (toluene glass, 77 K) revealed a nearly axial signal
with g values (2.313, 2.095, 2.079) consistent with a metal-
centered radical coupled to two phosphorus nuclei, one ni-
trogen nucleus, and one hydrogen nucleus.[32] The solid-state
structures of 9-Dipp and 9-Ph (Figure 6) confirmed the in-
The oxidation of the ligand to generate the phosphorani-
mine product 1-NPh upon abstraction of a hydrogen atom
from 9-Ph supports the viability of a putative Ni–imido spe-
cies (2-NR) in facilitating the chemistry observed in reac-
tions of 2 with 1-azidoarenes. The mixture of products
(mono- and bis-Staudinger oxidation products 1-NR and 1-
(NR)2) observed in treating 2 with 1-azido-4-methoxyben-
zene, for example, may result from a fast reaction between 2
and the azide to generate the mono-oxidized ligand, which
loses Ni as a consequence of an unfavorable bite angle, and
then is prone to metal-free oxidation by a second equivalent
of azide. As the metal-free pathway is unavailable upon in
situ hydrogen-atom abstraction, only the mono-Staudinger
product is observed (Scheme 4). Alternate reactivity of the
imide fragment, possibly through coupling or nitrene disso-
ciation is likely responsible for
À
stallation of an amido moiety, with Ni(1) N(1) bond lengths
of 1.890(2) and 1.889(2) ꢁ and Ni(1)-N(1)-C(31) angles of
141.5(1) and 128.6(2)8, respectively, compared with a Ni–N
distance of 1.881(2) ꢁ and Ni-N-C angle of 134.6(2)8 in Hill-
houseꢂs three-coordinate terminal NiI–amide, [(dtbpe)-
NiNH(2,6-diisopropylphenyl)].[12] Notably, the Ni N bonds
À
are more than 0.1 ꢁ longer than in diazoalkane adduct 2-
the regeneration of 2, although
the fate of the remaining ni-
À
trene “N Ph” moiety (that
must dissociate to yield 2) has
eluded characterization to date.
Attempts to intercept the puta-
tive imide fragment with sub-
À
strates bearing weak C H
bonds (e.g., cyclohexene and
9,10-dihydroanthracene) were
unsuccessful.
In comparison to the intra-
À
molecular arene C H amina-
tion observed here, Hillhouseꢂs
dtbpe-supported Ni–imido spe-
cies (10, Figure 7), with a more
Figure 6. ORTEP of 9-Dipp and 9-Ph shown with thermal ellipsoids at 50% probability levels. Some hydrogen
acute cis arrangement of phos-
À
atoms have been omitted for clarity. Selected bond lengths [ꢁ] and angles [8]. 9-Dipp: C(1) Ni(1) 2.621(3),
phorus
ligands
(P-Ni-P:
À
À
À
À
Ni(1) P(1) 2.2312(6), Ni(1) P(2) 2.2438(5), N(1) Ni(1) 1.8905(17), N(1) H(43) 0.78; P(1)-Ni(1)-P(2)
91.12(5)8)[13] reacted with ethyl-
ene to give aziridination prod-
ucts (instead of insertion into
À
117.40(2), C(31)-N(1)-Ni(1) 141.52(14), N(1)-Ni(1)-P(1) 129.26(5), N(1)-Ni(1)-P(2) 113.24(5). 9-Ph: C(1)
À
À
À
À
Ni(1) 2.399(2), Ni(1) P(1) 2.2397(8), Ni(1) P(2) 2.2389(7), N(1) Ni(1) 1.889(2), N(1) H(43) 0.71; P(1)-Ni(1)-
P(2) 120.85(3), C(31)-N(1)-Ni(1) 128.6(2), N(1)-Ni(1)-P(1) 110.50(7), N(1)-Ni(1)-P(2) 124.22(7).
16458
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Chem. Eur. J. 2013, 19, 16453 – 16460

À
Arene C H Amination at Nickel in Terphenyl–Diphosphine Complexes
FULL PAPER
dene transfer agent was also recorded. These results suggest
À
that modification of the ligand at the central C H position,
as well as the substituents at phosphorus, to prevent such in-
tramolecular pathways may be useful in diverting reactivity
À
towards productive intermolecular group transfer and C H
functionalization.
Acknowledgements
The authors thank Larry M. Henling for invaluable crystallographic assis-
tance, the late Dr. Michael W. Day for solving the crystal structure of 4,
and Caltech for generous funding and support. The Bruker KAPPA
APEXII X-Ray diffractometer was purchased via an NSF CRIF:MU
award to Caltech (CHE-0639094). The Gordon and Betty Moore Foun-
dation, the Beckman Institute and the Sanofi-Aventis BRP are thanked
for their support of the Molecular Observatory at Caltech and the SSRL
beamline, operated for the DOE and supported by its Office of Biologi-
cal and Environmental Research and by the NIH and NIGMS
(P41 M103393) and the NCRR (P41RR001209).
Figure 7. Divergent reactivity for reported Ni imides.
[3a]
À
a C H bond).
DFT studies supported a mechanism
À
wherein dissociation of a phosphine arm allows for C N
bond-forming reductive elimination from a three-coordinate
T-shaped azametallacyclobutane intermediate.[33] The two-
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À
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À
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À
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Conclusion
The use of meta-terphenyl diphosphine 1 as a ligand for en-
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À
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À
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Received: July 2, 2013
Published online: October 14, 2013
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Chem. Eur. J. 2013, 19, 16453 – 16460