New pincer-type diphosphinito (POCOP) complexes of NiII and NiIII

Article abstract of DOI:10.1039/b613812h

This communication reports the synthesis and characterization of the new, pincer-type, square-planar, 16-electron compounds {2,6-(OPPrⁱ ₂)₂C₆H₃}Ni^IIBr, 1, and {(Prⁱ₂POCH₂)₂CH}Ni^IIBr, 2, and the square-pyramidal, 17-electron complex {(Prⁱ ₂POCH₂)₂CH}Ni^IIIBr₂, 3. The Royal Society of Chemistry.

Full text of DOI:10.1039/b613812h

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COMMUNICATION
www.rsc.org/chemcomm | ChemComm
II
III
New pincer-type diphosphinito (POCOP) complexes of Ni and Ni {
Valerica Pandarus and Davit Zargarian*
Received (in Berkeley, CA, USA) 21st September 2006, Accepted 14th November 2006
First published as an Advance Article on the web 12th December 2006
DOI: 10.1039/b613812h
This communication reports the synthesis and characterization
of the new, pincer-type, square-planar, 16-electron compounds
i
II
i
II
{
2,6-(OPPr ) C H }Ni Br, 1, and {(Pr POCH ) CH}Ni Br,
2 2 6 3 2 2 2
2
,
and the square-pyramidal, 17-electron complex
III
i
{(Pr POCH ) CH}Ni Br , 3.
2 2 2 2
Transition metal complexes featuring PCP or POCOP type pincer
1
ligands can promote unusual stoichiometric transformations and
2,3
exceptionally efficient catalytic processes. Curiously, most PCP
and POCOP complexes reported to date are based on 4d and 5d
metals, while the chemistry of analogous complexes based on 3d
metals remains underdeveloped. Our interest in the chemistry of
Scheme 1
4
organonickel complexes and the exciting results reported by van
5
Koten’s group on NCN–Ni compounds inspired us to investigate
The diamagnetic complexes 1 and 2 were identified readily on
1
the basis of their NMR spectra. For instance, their P{ H} NMR
31
the reactivities of the PCP–Ni and POCOP–Ni complexes. In
II
earlier reports, we have described the chemistry of the PCsp2P–Ni
spectra displayed singlet resonances at d 188 ppm (1) and 186 ppm
6
indenyl}NiCl and the PCsp
II
species {1,3-(Ph PCH
2
2
CH
2
)
2
3P–Ni
(
2) for two equivalent P nuclei, in accord with the trans disposition
i
of the Pr P moieties in these compounds. The H NMR spectrum
2
t
1
complexes {(Bu
t
2
PCH
CH)NiL] (L = NCCH
2
CH
2 2
+
) CH}NiX (X = Cl, Br, I, Me, H) and
7
[
(Bu
2
PCH CH
2
2
)
2
3
, NCCHLCH
2
). PCP–
of 1 showed only one signal for the four equivalent methyne
II
8
Ni complexes have also been reported by other groups.
As an extension to our earlier studies, we have set out to explore
the synthesis and reactivities of Ni complexes based on the
protons and two signals corresponding to the non-equivalent
i
methyl groups in each Pr moiety; these features are consistent with
the presence of a mirror plane encompassing the square plane and
the planar aromatic system of the ligand backbone. In complex 2,
on the other hand, the non-planar aliphatic linker system breaks
the symmetry relating the groups above and below the coordina-
tion plane, thus giving rise to two signals for the non-equivalent
methyne protons and four signals for the non-equivalent methyl
diphosphinito type POCsp2OP and POCsp3OP ligands (A and B in
Scheme 1) in order to probe the influence of ligand electronics on
the structures and reactivities of these closely related families of
pincer complexes. Herein we report our preliminary results on
the synthesis and full characterization of the compounds {2,6-
i
II
i
II
i
13
1
(OPPr
2 2 6 3 2 2 2
) C H }Ni Br (1) and {(Pr POCH ) CH}Ni Br (2), the
2
groups in each Pr moiety. The C{ H} NMR spectra of 1 and 2
oxidation of the latter to the pentacoordinated 17-electron species
i
were also consistent with these symmetry considerations.
Moreover, these spectra showed the characteristic virtual triplets
for C–P–O–C and for the metallated carbon nuclei.
III
{(Pr
2 2 2 2
POCH ) CH}Ni Br (3), and the promotion by 3 of the
II 9
Kharasch type addition of CCl
II 10
4
to olefins. Related POCOP–Ni
and PNCNP–Ni
complexes have also been reported recently.
The solid-state structures of 1 and 2 have been elucidated by
single-crystal X-ray diffraction studies (Fig. 1).{ The overall
Stirring a toluene solution of NiBr (THF) and ligand A at
2
2
room temperature for 1 h gave complex 1 as a yellow solid in 80%
yield. The yield of this reaction can be increased to 95% by adding
4-dimethylaminopyridine (DMAP) to the reaction mixture and
heating it to ca. 60 uC for 1 h (Scheme 1). The analogous reaction
of NiBr (THF) with ligand B in the presence of DMAP gave
2
2
complex 2 in 60–65% yield after a 5 h reflux; alternatively, 2 can be
obtained in 90–93% yields if NiBr (NCCH ) is used as the Ni
2 3 2
precursor.
D e´ partement de Chimie, Universit e´ de Montr e´ al, C.P. 6128, Succursale
Centre-Ville, Montr e´ al, QC, Canada H3C 3J7.
E-mail: zargarian.davit@umontreal.ca; Fax: 514-343-2468;
Tel: 514-343-2247
Fig. 1 ORTEP diagrams for complexes 1 and 2. Thermal ellipsoids are
shown at the 30% probability level. Methyl groups and hydrogens are
˚
omitted for clarity. Selected bond distances (A) and angles (u): Ni–C2
{
Electronic supplementary information (ESI) available: Complete
experimental details on the synthesis and characterization of complexes
, 2 and 3, the electrochemical studies, and the Kharasch additions. See
DOI: 10.1039/b613812h
1.885(3) (1), 1.964(3) (2); Ni–P1 2.1534(8) (1), 2.1574(8) (2); Ni–P2
2.1422(8) (1), 2.1527(8) (2); Ni–Br1 2.3231(5) (1), 2.3458(5) (2); C2–Ni–Br1
1
178.10(8) (1), 176.29(14) (2); P1–Ni–P2 164.92(4) (1), 166.50(3) (2).
9
78 | Chem. Commun., 2007, 978–980
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geometry around the Ni centre in both complexes is square planar,
the largest distortions arising from the P–Ni–P angles of ca. 166u.
˚
The Ni–P distances are fairly symmetrical in 1 and 2 (2.14–2.16 A)
and slightly shorter than the corresponding distances in the related
7
t
complexes {( Bu
t
2
PCH
2 2 2 2
CH ) CH}NiBr and (2,6-(NHPBu ) -
7
C H )NiCl (ca. 2.20–2.21 A). The Ni–C bonds also follow the
˚
6 3
˚
same trend, being somewhat shorter in 1 (ca. 1.89 A) vs. its
1
0
˚ ˚
NP analogue (ca. 1.91 A), and in 2 (ca. 1.96 A) vs.
PNCsp
2
7
˚
P analogue (ca. 1.97 A). The generally shorter Ni–L
its PCsp
3
distances in 1 and 2 relative to the analogous PCP– and PNCNP–
Ni complexes may be attributed to the increased p-acidity of the
2
OPR moieties in 1 and 2.
Complexes 1 and 2 are stable to atmospheric oxygen and
moisture in the solid state and thermally stable up to 200 uC in
DMF solutions. Cyclic voltammetry measurements showed,
however, that both complexes can be oxidized. Thus, 1 undergoes
a quasi-reversible single-electron oxidation (E = 1.17 V; Fig. 2),
Fig. 3 ORTEP diagram for 3 (POCsp3OP)NiBr
2
complex. Thermal
ellipsoids are shown at the 30% probability level. Methyl groups and
hydrogens are omitted for clarity. Selected bond distances ( A˚ ) and angles
(u): Ni–C2 2.011(5), Ni–P1 2.235(1), Ni–P2 2.251(1), Ni–Br1 2.3683(9),
Ni–Br2 2.436(1); C2–Ni–Br1 157.09(15), P1–Ni–P2 160.57(6).
1/2
III
implying that a Ni species derived from 1 should, in principle, be
accessible. The single-electron oxidation of 2 was even more facile
but irreversible (Eox = 0.88 V). Significantly, a second oxidation
An X-ray diffraction analysis of 3 showed that it is the square-
pyramidal pincer complex shown in Fig. 3.{ The Ni atom is
displaced out of the basal plane (defined by the atoms C2, P1, P2
IV
was also detected for this complex, implying that Ni species
might be accessible under certain conditions. We found that the
˚
and Br1) in the direction of the apical atom Br(2) by 0.0972 A. The
large-scale oxidation of 2 proceeds in nearly quantitative yield in
angular structural parameter t for the solid structure of 3 was
the presence of CuBr
as the pentacoordinated Ni complex (POCsp
Scheme 1). Unfortunately, however, our efforts at preparing d
POCsp
POCsp
2
to give a paramagnetic product identified
calculated to be 0.05,§ implying only a small degree of distortion
12
III
3
OP)NiBr2 (3)
towards a trigonal bipyramidal geometry. By comparison, the
III
7
(
2
NCN–Ni (I) complex reported by van Koten displayed a greater
5a
6
2
OP–Ni compounds (by one-electron oxidation of 1) or d
trigonal distortion (t y 0.25). The bond distances for Ni–C2
3
OP–Ni compounds (by two-electron oxidation of 2) were
˚ ˚ ˚
2.011(5) A) and Ni–P (average 2.243(1) A) in 3 are ca. 0.088 A
(
unsuccessful.
longer than the corresponding distances in the four-coordinate
Ni(II) complex 2, presumably reflecting the greater coordination
number of the metal center (5 vs. 4) and the partial population
The dark red, air-stable crystals of 3 are freely soluble in almost
all solvents but only sparingly soluble in hexane. That complex 2 is
thermodynamically more stable than 3 is inferred from the
observation that red solutions of the latter undergo a color change
to yellow over 2 days, forming the diamagnetic parent complex.
The characterization of 3 was as follows. Consistent with its
of the antibonding d
z
orbital (SOMO). A similar lengthening of
2
III
Ni–Lbasal bonds was also observed for van Koten’s NCN–Ni (I)
2
5a
complex. The much longer Ni–Br distance for the apical Br
˚
2.44 vs. 2.37 A) is consistent with a similar observation in the
(
3
1
(
formal) 17-electron count, complex 3 displayed no P NMR
13
structure of NiBr (PPhMe
3
2
)
2
,
whereas the two Ni–I bond
III
1
signal and its H NMR spectrum showed significantly broadened
distances in van Koten’s NCN–Ni (I)
5a
2
complex are fairly similar
signals. The paramagnetism of 3 was also confirmed by the Evans
˚
2.61 and 2.63 A).
(
2
3
NMR method: analysis of a 10 M CDCl
3
sample of 3 gave an
In order to compare the reactivities of 3 to van Koten’s NCN–
III 5e,f
Ni species, we have evaluated the effectiveness of complex 3
approximate value of 1.73 meff, corresponding to one unpaired
11
electron per molecule at 23 uC.
for promoting the addition of CCl to alkenes (Kharasch addition,
4
eqn (1)).
ð1Þ
The reaction of 0.1 mol% of 3 with CCl4 and styrene,
4-methylstyrene, or methyl methacrylate in refluxing acetonitrile
gave 95–97% isolated yields of the addition product 4; significantly,
5
f
no telomerisation or polymeric products were detected. Lower
yields were obtained for the addition to acrolein (85%), methyl
acrylate (80%) and acrylonitrile (65%).
As observed in the Kharasch additions promoted by the NCN–
III
Ni species, the additions promoted by 3 had to be carried out in
the absence of O to prevent the quenching of the intermediate
2
organic radicals. Moreover, complex 3 could be generated in situ
II
from the Ni species 2 in air; the addition reaction was then carried
2
3
Fig. 2 Cyclic voltammetry scans of 10 M solutions of 1 (a) and 2 (b) at
21
a Pt electrode in acetone (0.1 M Bu NPF
4
6
, scan rate 0.20 V s ).
out under anaerobic conditions. In contrast to the case of the
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III
NCN–Ni systems, the Kharasch additions promoted by complex
do not proceed at room temperature, presumably because of the
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3
2
greater steric bulk of the phosphinite moieties in 3.
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In conclusion, complexes 1 and 2 can be prepared via simple
C–H bond activation reactions, and the facile oxidation of 2 to 3
III
4
(a) D. Gareau, C. Sui-Seng, L. F. Groux, F. Brisse and D. Zargarian,
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gives access to the first Ni derivative of POCOP type pincer
complexes. The easy access to 3 and its effectiveness in promoting
the Kharasch addition bode well for further developments in the
chemistry of POCOP–Ni complexes.
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The authors gratefully acknowledge NSERC of Canada for
financial assistance of these studies.
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Notes and references
2
2
2
003, 204–205, 325; (k) D. Zargarian, Coord. Chem. Rev., 2002, 233–
34, 157; (l) R. Wang, L. F. Groux and D. Zargarian, Organometallics,
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{
triclinic, space group P1, a = 12.9840(3), b = 13.0363(3), c = 13.4366(3) A,
2 2
Crystal data for complexes 1–3. 1: C18H31BrNiO P , M = 479.99,
¯
˚
3
˚
a = 78.494(2), b = 77.305(1), c = 88.467u, V = 2173.79(9) A , T = 100 K,
Z = 4, m(Cu-Ka) = 4.889 mm , 26 424 reflections measured, 8305 unique
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2
1
(
R
int = 0.034), final R indices [I . 2s(I)]: R1 = 0.0358, wR2 = 0.084; R
indices (all data): R1 = 0.0482, wR2 = 0.0875. CCDC 620341. 2:
33BrNiO , M = 445.97, orthorhombic, space group P2 , a =
.7081(1), b = 13.9372(2), c = 17.1214(2) A, V = 2077.97(5) A , T = 100 K,
C
8
15
H
2
P
2
1 1 1
2 2
3
˚
˚
2
1
Z = 4, m(Cu-Ka) = 5.062 mm , 25 133 reflections measured, 4116 unique
int = 0.044), final R indices [I . 2s(I)]: R1 = 0.0298, wR2 = 0.0698, R
indices (all data): R1 = 0.0322, wR2 = 0.0709. CCDC 620342. 3:
33Br NiO M = 525.88, monoclinic, space group C2/c, a =
4.4889(8), b = 7.0423(2), c = 22.1302(5) A, b = 127.664(1)u, V =
(R
15
C H
2
2 2
P
˚
3
5
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21
˚
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§
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respectively.
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80 | Chem. Commun., 2007, 978–980
This journal is ß The Royal Society of Chemistry 2007