Oxidative addition of aryl halides: Routes to mono- and dimetallic nickel amino-bis-phosphinimine complexes

Article abstract of DOI:10.1039/c1dt10341e

Oxidative addition of an aryl-halide to Ni(COD)₂ in the presence of an equivalent of amino-bis-phosphinimine ligand affords complexes of the form [HN(CH₂CH₂NPPh₃)₂Ni-Ar][X] (Ar = C₆H₄F, C₆H₅, X = Cl, Br) while the analogous reactions with 2 equivalents of Ni yield the amido-bridged complexes N(CH₂CH₂NPPh₃)Ni₂Br₃ and N(1,2-C₆H₄NPPh₃)Ni₂Br₃. The Royal Society of Chemistry.

Full text of DOI:10.1039/c1dt10341e

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COMMUNICATION
Oxidative addition of aryl halides: routes to mono- and dimetallic nickel
amino-bis-phosphinimine complexes†
Renan Cariou, Todd W. Graham, Fatme Dahcheh and Douglas W. Stephan*
Received 28th February 2011, Accepted 31st March 2011
DOI: 10.1039/c1dt10341e
Oxidative addition of an aryl-halide to Ni(COD)2 in the
presence of an equivalent of amino-bis-phosphinimine ligand
affords complexes of the form [HN(CH2CH2N PPh3)2Ni-
Ar][X] (Ar = C6H4F, C6H5, X = Cl, Br) while the analo-
gous reactions with 2 equivalents of Ni yield the amido-
bridged complexes N(CH2CH2N PPh3)Ni2Br3 and N(1,2-
C6H4N PPh3)Ni2Br3.
ligands, HN(CH2CH2N PR3)2 (R = Ph 1, iPr 2) were combined
with Ni(COD)2 in THF. This gave only Ni metal. Similarly,
combination of Ni(COD)2 in THF and an aryl halide resulted in
immediate formation of an Ni mirror. However, the reaction of 1
with Ni(COD)2 in THF in the presence of 1,2-chlorofluorobenzene
afforded a new Ni(II) complex 3. Complex 3 displays a singlet
1
at 36.0 ppm in the 31P{ H} NMR spectrum and a 19F NMR
signal at -84.7 ppm. The aromatic protons of the fluoroben-
zene moiety were observed at 4.91 and 5.79 ppm in the 1H
spectrum. These data are consistent with the formulation of
3 as [HN(CH2CH2N PPh3)2Ni–C6H4F][Cl] 3 (Scheme 1). This
presumably results from the oxidative addition of the C–Cl bond of
1,2-fluorochlorobenzene to a transient bis-phosphinimine pincer
ligands Ni(0) complex. Salt methathesis of 3 with NaPF6 in CH2Cl2
gave the analogous [PF6] salt, 4. The spectroscopic signature
of 4 is similar to 3 with the expected additional resonances at
Tridentate or pincer ligand complexes have garnered much interest
as they provide access to a broad range of tunable reactivity
permitting applications as sensors, switches and catalysts.1 Of
the wide variety of such ligands,1–4 PNP type ligands offer the
electronic and steric features that accommodate a variety of
highly reactive metal bound fragments. For example, alkylidene,
phosphinidene,3 metal hydrides,4,5 and even three coordinate
complexes6,7 have been stabilized by PNP tridentate ligands. Such
PNP pincer complexes also display interesting reactivity including
oxidative addition of C–Halogen bonds,8,9 C–H bond activation,5
N2 activation and homolytic cleavage of H2.7
Much of this unique reactivity is attributable to the nature of
the phosphine donors which are strong sigma donors and provide
local steric shielding. While other authors have recently described
analogous carbene-based CNC ligand systems,10–17 it has only been
recently that analogs incorporating phosphinimine donors have
been reported.18–21 Such donors are generally strong sigma donors,
generally between phosphines and carbenes in terms of basicity.
However, the nature of the phosphinimine linkage provides steric
congestion that is removed from the metal center, thus providing
steric shielding without crowding the metal environment; indeed
the steric bulk is adjacent the metal binding coordination sphere.
In a recent paper, we have described the coordination of such
tridentate amino-bis-phosphinimine ligands to Ni(II) and Pd(II)
precursors.22 In this communication, we demonstrate that related
organometallic Ni complexes are accessible from Ni(0) synthons
via oxidative addition. Moreover, control of the stoichiometric
offers a facile route to amido-bridged bimetallic Ni complexes.
We sought to exploit oxidative addition to intercept Ni(II)
organometallic complexes with the use of tridentate amino-bis-
phosphinimine ligands. To that end, bis-phosphinimine pincer
1
-142.3 ppm and -71.0 ppm in the 31P{ H} and 19F NMR respec-
tively, attributable to the PF6 anion. The salt 4 was subsequently
crystallographically characterized (Fig. 1(a)). The geometry at the
Ni centre is best described as distorted square planar geometry
with angles about the Ni centre of 173.45(9)◦ and 166.06(8)◦
for N(2)–Ni–C(41) and N(1)–Ni–N(3), respectively. The Ni–C(41)
◦
˚
bond length is 1.900(2) A. The aryl ring is orthogonal (89.24 ) to
˚
˚
the ligand plane. Ni–N bonds (1.9126(17) A and 1.9264(18) A)
Department of Chemistry, 80 St. George St., University of Toronto, Toronto,
Ontario, Canada M5S 3H6. E-mail: dstephan@chem.utoronto.ca
† CCDC reference numbers 815361–815363. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c1dt10341e
Scheme 1 Synthesis of Ni complexes.
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Fig. 2 ORTEP drawings of 8. Hydrogen atoms are omitted for clarity.
phosphinimine-nitrogen bond lengths Ni(1)–N(1) and Ni(2)–N(3)
˚
˚
were determined to be 1.961(4) A and 1.965(4) A respectively. The
˚
bridging Ni–Br bond distances were observed to be 2.4879(8) A
˚
and 2.5054(8) A, significantly longer than the terminal Ni–Br
˚
˚
distances of 2.3580(8) A and 2.3445(9) A.
We have recently reported the analogous compound N(1,2-
C6H4N PPh3)Ni2Cl3 bearing ligand 6. This species was obtained
by deprotonating the ligand with a base prior to its reaction with
the Ni(II) source.22 The Ni–N bond lengths in 8 are shorter than
that of N(1,2-C6H4N PPh3)Ni2Cl3 and are consistent with ligand
2 being more electron donating as a result of the alkyl substituents
on P.
Fig. 1 ORTEP drawings of the cations (a) 4 and (b) 5. Hydrogen atoms
are omitted for clarity.
In each of the reactions affording 7–9, GC-MS analysis of the
reaction mixture indicated the formation of biphenyl (C12H10)
and benzene as the organic byproducts. It is also noteworthy
that addition of Ni(COD)2 to a solution of 5 in C6H5Br afford
the blue/purple dinuclear complex 7. These observations were
consistent with consecutive oxidative additions of C6H5Br to
Ni, suggesting a dinuclear Ni(II) intermediate that undergoes
subsequent loss of the biaryl and arene (Scheme 2).
are in the range of previously reported bond length Ni–N(2) bond
˚
length of 1.9796(19) A is slightly longer than that reported for
[HN(CH2CH2N PPh3)2NiCl][PF6] due to the trans influence of
the aryl ligand.22
In the same vein, reaction of 1 with Ni(COD)2 in bromobenzene
afforded the new species 5. This species exhibited NMR resonances
1
that parallel those of 3 and 4. A 31P{ H} NMR chemical shift was
seen at 35.2 ppm and protons resonances observed between 5.30
and 5.80 ppm were attributed to the Ni–C6H5 fragment. X-ray
quality crystals of 5, grown from acetonitrile, revealed the expected
square planar geometry at the Ni center (Fig. 1(b)). The metric
parameters are similar to those seen in 4, while the Ni–C(41) bond
˚
length of 1.8853(13) A. Additionally, the Ni–N(2) bond length
˚
in 5 of 2.0314(11) A is longer than the corresponding distance
in 4.
In marked contrast, use of two equivalents of Ni(COD)2
in reactions with the ligands 1, 2 and HN(1,2-C6H4N PPh3)
6 with bromobenzene afforded nearly quantitative yields of
new bimetallic complexes. In these cases, ligands 1, 2 and 6
afforded the blue/purple, blue and brown species formulated
as N(1,2-CH2CH2N PR3)2Ni2Br3 (R = Ph 7, R = iPr 8) and
N(1,2-C6H4N PPh3)Ni2Br3 9. These species exhibited magnetic
moments of 4.96, 4.43 BM and 5.02 BM, respectively. In the
case of the reaction of ligand 2, the paramagnetic complex 8
was unambiguously identified by X-ray crystallography (Fig. 2).
These data revealed that 8 is a bimetallic species in which two
Ni centers adopt pseudo-tetrahedral geometries and are bridged
by the central amide of the ligand and a bromide atom. In
addition, the coordination spheres of the Ni atoms are completed
by a single phosphinimine N and a terminal bromide atom.
The average bridging Ni–N(2) bond lengths were found to be
Scheme 2 Plausible intermediate en route to 7–9.
In summary, we have shown that oxidative addition provides
a convenient route to both aryl-monometallic Ni complexes
containing amino-bis-phosphinimine ligands. These species are
readily converted to related bimetallic amido-bis-phosphinimine
complexes via reaction with additional arylhalide and Ni(COD)2.
An examination of the reactivity of these and related compounds
is the current focus of our efforts.
Financial support of NSERC of Canada is gratefully acknow-
ledged. DWS is grateful for the award of a Canada Research Chair
and a Killam Research Fellowship for 2009–2011.
◦
˚
1.981(4) A, while the Ni(1)–N(2)–Ni(2) angle is 99.75(17) . The Ni
5420 | Dalton Trans., 2011, 40, 5419–5422
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115.3 (d, CH, JPC = 21 Hz), 110.6 (d, C6H4F, JPC = 29 Hz), 50.6
(CH2), 50.1 (d, JPC = 13 Hz). Anal. Calcd. for C46H43FClN3NiP2:
C, 67.96; H, 5.33; N, 5.17. Found: C, 66.77; H, 5.61; N, 5.43.
Experimental
General remarks
1
All manipulations were carried out under an atmosphere of dry,
O2-free N2 employing an Innovative Technology glove box and
a Schlenk vacuum-line. Solvents were purified with a Grubbs-
type column system manufactured by Innovative Technology
and dispensed into thick-walled Schlenk glass flasks equipped
with Teflon-valve stopcocks (pentane, toluene, CH2Cl2), or were
dried over the appropriate agents and distilled. All solvents
were thoroughly degassed after purification (repeated freeze-
pump-thaw cycles). Deuterated solvents were dried over the
appropriate agents, vacuum-transferred into storage flasks with
Teflon stopcocks and degassed accordingly (CD2Cl2). Toluene and
4. Yield: 109 mg (96%). H NMR (400 MHz, CDCl3): 7.65
(m, 12H), 7.49 (m, 18H), 5.84 (m, 1H), 5.32 (dt, 2H), 4.84 (s br,
1H), 3.36 (m, 3H, CH2 and NH overlapping), 3.10 (m, 2H, CH2),
2.62 (m, 4H, CH2).31P NMR (162 MHz, CDCl3): 36.8. 19F NMR
(377 MHz, CDCl3): -71.0 (d, JPF = 714 Hz), -84.9. 13C{ H} NMR
1
1
(CDCl3): 167.8 (d, C6H4F, JCF = 223.6 Hz), 143.0 (d, C6H4F,
2JCF = 18.9 Hz), 139.0 (CH), 133.1 (d, CH, JCP = 9.8 Hz), 132.0
1
(Cq), 128.5 (d, CH, JCP = 12.6 Hz), 127.1 (d, ArC, JCP = 100.8
Hz), 123.8 (d, C6H4F, JCF = 7.4 Hz), 119.6 (br, C6H4F), 110.7 (d,
CH, JPC = 29.7 Hz), 50.0 (CH2), 49.9 (d, CH2, JPC = 13 Hz). Anal.
Calcd. for C46H43F7ClN3NiP2: C, 59.89; H, 4.70; N, 4.56. Found:
C, 58.86; H, 4.77; N, 4.29.
pentane were stored over potassium mirrors, while bromobenzene
1
˚
and dichloromethane were stored over 4 A molecular sieves. H,
5. Yield: 124 mg (84%) 1H NMR (400 MHz, CDCl3): 7.64 (t
br, 12H, ArH), 7.45 (m, 18H, ArH), 5.85 (t br, 1H, C6H5), 5.62 (s
br, 2H, C6H5), 5.30 (s br, 1H, NH + C6H5), 3.24 (m, 2H, CH2), 2.81
13C and 31P NMR spectra were recorded at 25 ◦C on Varian
400 MHz and Bruker 400 MHz spectrometers. Chemical shifts are
given relative to SiMe4 and referenced to the residue solvent signal
(1H, 13C) or relative to an external standard (31P: 85% H3PO4).
Chemical shifts are reported in ppm and coupling constants as
scalar values in Hz. Combustion analyses were performed in
house employing a Perkin-Elmer CHN Analyzer. Samples for GC-
MS were filtered through activated Al2O3 prior to injection. GC-
MS analysis were performed using an Agilent 7890A GC system
coupled to a Agilent 5974C Mass Spectrometer.
(m br, 4H, CH2), 2.69 (s br, 2H, CH2). 31P{ H} NMR (162 MHz,
1
CDCl3): 35.2. 13C{ H} NMR(101 MHz, CDCl3): 141.4 (Ni-C6H5),
1
138.6 (Ni-C6H5), 133.5 (d, ArC, JCP = 9.7 Hz), 131.7(ArC), 128.4
(d, ArC, JCP = 12.5 Hz), 127.7 (d, ArCq, 1JCP = 100.7 Hz), 122.9
(Ni-C6H5), 120.2 (Ni-C6H5), 50.4 (CH2), 49.9 (d, CH2, 2JCP = 12.3
Hz). Anal. Calcd. for C46H44BrN3NiP2: C, 65.82; H, 5.28; N, 5.01.
Found: C, 62.52;‡ H, 5.26; N, 4.84.
i
Synthesis of N(1,2-CH2CH2N PR3)2Ni2Br3R = Ph 7, R = Pr 8
and N(1,2-C6H4N PPh3)Ni2Br3 9
Synthesis of [HN(CH2CH2N PPh3)2Ni-C6H4F][X] X = Cl 3,
[PF6] 4 [HN(CH2CH2N PR3)2Ni-C6H5][Br] R = Ph 5
These compounds were prepared in a similar fashion, thus only
one preparation is detailed. Ni(COD)2 (0.100 g, 0.364 mmol) was
added to a solution of 1 (0.113 g, 0.182 mmol) in 3 ml C6H5Br
while stirring. A color change from yellow to red was observed
after a few hours and to brown when left stirring overnight. The
solution was filtered and cooled to -35 ◦C before adding cold ether
affording 6.
Compounds 3 and 5 were prepared in analogous manner, em-
ploying the reagents 1,2-chlorofluorobenzene and bromobenzene
respectively. Thus, only one of these preparations is detailed.
350 mg (0.481 mmol) of HN(CH2CH2N PPh3)2 was added to
132 mg (0.479 mmol) of Ni(cod)2 and 101 mg (0.773 mmol) of 1,2-
chlorofluorobenzene in ca. 5 ml of THF. The mixture was stirred
for one hour resulting in the formation of an orange precipitate.
The solvent was decanted and the solid was washed with ether (2 ¥
2 ml) and dried in vacuo. The solid was dissolved in CH2Cl2 (ca.
4 ml), filtered through Celite and then the solvent was removed
in vacuo. The orange solid was triturated in ether until a powder
formed which was then dried in vacuo. Yield 350 mg (77%). In
the case of 4, subsequent salt methathesis was performed using
NaPF6 (31 mg, 0.184 mmol) added to 3 (100 mg, 0.123 mmol) in
ca. 2 mL of CH2Cl2. The mixture was stirred overnight and was
then filtered through Celite. 10 mL of ether was added dropwise to
precipitate a purple solid that was triturated for The solvent was
decanted and the solid was washed with 2 ¥ 5 mL of ether and
dried in vacuo.
1
7. Blue/purple solid. Yield: 263 mg (74%) 31P{ H} NMR
(162 MHz, CD2Cl2) d: 50.1. meff = 4.96 BM. Anal. Calcd. for
C40H38Br3N3Ni2P2: C, 49.03; H, 3.91; N, 4.29. Found: C, 49.41;
H, 4.34; N 4.41.
1
8. Blue solid. Yield: 66 g (72%). 31P{ H} NMR (162 MHz,
CD2Cl2) d: 78.3. meff
=
4.43 BM. Anal. Calcd. for
C22H50N3P2Ni2Br3.C6H5Br: C, 36.06; H, 5.94; N, 4.51. Found: C,
35.74; H, 6.03; N, 4.28.
9. Brown solid. Yield: 96 mg (80%), Anal. Calcd. for
C48H38Br3N3Ni2P2: C, 53.59; H, 3.56; N, 3.91. Found: C, 53.43;
H, 3.86; N,3.43. meff = 5.02 BM.
3. Yield 350 mg (77%). 1H NMR (400 MHz, CDCl3) 7.70 (m,
12H, C6H5); 7.45 (m, 18 H, C6H5); 6.14 (broad s, 1H, NH); 5.79
(dd, 1H, C6H4F); 5.26 (t, 2H, 3JHH = 7.4 Hz, C6H4F); 4.91 (broad s,
1H, C6H4F); 3.25 (m, 2H, CH2); 2.80 (m, 4H, CH2); 2.61 (m, 2H,
CH2). 31P NMR (162 MHz, CDCl3): 36.0. 19F NMR (377 MHz,
Notes and references
‡ In this case, repeated attempts to obtain satisfactory analysis led to
consistently low analysis for C. This was attributed to the formation of
Ni2C during combustion.
1
1
CDCl3): -84.7. 13C{ H} NMR (CDCl3): 168.2 (d, C6H4F, JCF
=
=
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