Nickel(II) complexes of bis(2-diphenylphosphinophenyl)amide

Article abstract of DOI:10.1021/om030237e

A series of o-phenylene derivatives were synthesized with an aim to develop new chelating amido phosphine ligands. A monoanionic tridentate bis(2-diphenylphosphinophenyl)amide ligand and its lithium and divalent nickel derivatives were prepared. The coordination chemistry of the synthesized products were studied. The alkyl complexes were found to be highly stable to β-elimination.

Full text of DOI:10.1021/om030237e

© Copyright 2003
Volume 22, Number 15, July 21, 2003
American Chemical Society
Communications
Nick el(II) Com p lexes of
Bis(2-d ip h en ylp h osp h in op h en yl)a m id e
Lan-Chang Liang,*,† J ia-Ming Lin,† and Chen-Hsiung Hung‡
Department of Chemistry, National Sun Yat-sen University, and Center for Nanoscience &
Nanotechnology, National Sun Yat-sen University, Kaohsiung 80424, Taiwan, and Department
of Chemistry, National Changhua University of Education, Changhua 50058, Taiwan
Received April 1, 2003
Ch a r t 1
Summary: A series of nickel(II) alkyl and aryl complexes
supported by the bis(2-diphenylphosphinophenyl)amide
ligand, [PNP]-, have been prepared, including those in
which the alkyl contains â-hydrogen atoms. The stability
of these compounds toward â-elimination is attributed
to the rigidity and robustness of the new tridentate
amido diphosphine ligand.
Metal complexes of “hybrid” ligands that contain both
soft and hard donors have received increasing attention
in recent years because of their potentials for generating
unusual chemical transformations.1 One remarkable
example in this aspect is the chelating amido phosphine
derivatives that contain the -SiMe2CH2- ligand back-
bone as depicted in Chart 1. The tridentate ligands of
this type have shown widespread reactivity with metals
across the periodic table.2-10 These ligands, however,
are prone to phosphine dissociation under certain
circumstances due to the flexibility of the backbone.11
With the silyl linker, the ligands may become reactive,
as cleavage of both N-Si12 and C-H13 bonds has been
observed. In an effort to develop new chelating amido
phosphine ligands that are potentially more rigid and
robust, we aim to synthesize a series of o-phenylene
derivatives.14 To this end, we have prepared the poten-
tially tridentate bis(2-diphenylphosphinophenyl)amide
ligand, [PNP]-, and explored the coordination chemistry
* Corresponding author. E-mail: lcliang@mail.nsysu.edu.tw. Fax:
+886-7-5253908.
† National Sun Yat-sen University.
‡ National Changhua University of Education.
(1) Fryzuk, M. D.; Love, J . B.; Rettig, S. J .; Young, V. G. Science
1997, 275, 1445.
(8) Fryzuk, M. D.; Giesbrecht, G. R.; Rettig, S. J . Organometallics
1997, 16, 725-736.
(9) Fryzuk, M. D.; Leznoff, D. B.; Ma, E. S. F.; Rettig, S. J .; Young,
V. G. Organometallics 1998, 17, 2313-2323.
(10) Fryzuk, M. D.; Leznoff, D. B.; Thompson, R. C.; Rettig, S. J . J .
Am. Chem. Soc. 1998, 120, 10126-10135.
(2) Fryzuk, M. D. Can. J . Chem. 1992, 70, 2839-2845.
(3) Fryzuk, M. D.; Haddad, T. S.; Rettig, S. J . Organometallics 1992,
11, 2967-2969.
(4) Fryzuk, M. D.; Leznoff, D. B.; Rettig, S. J .; Thompson, R. C.
Inorg. Chem. 1994, 33, 5528-5534.
(11) Fryzuk, M. D.; Carter, A.; Rettig, S. J . Organometallics 1992,
11, 469-472.
(5) Fryzuk, M. D.; Mylvaganam, M.; Zaworotko, M. J .; MacGillivray,
L. R. Organometallics 1996, 15, 1134-1138.
(6) Fryzuk, M. D.; Giesbrecht, G.; Rettig, S. J . Organometallics 1996,
15, 3329-3336.
(7) Fryzuk, M. D.; Giesbrecht, G. R.; Olovsson, G.; Rettig, S. J .
Organometallics 1996, 15, 4832-4841.
(12) Ozerov, O. V.; Ferard, H. F.; Watson, L. A.; Huffman, J . C.;
Caulton, K. G. Inorg. Chem. 2002, 41, 5615-5625.
(13) Fryzuk, M. D.; Haddad, T. S.; Rettig, S. J . Organometallics
1991, 10, 2026-2036.
(14) Liang, L.-C.; Lee, W.-Y.; Huang, M.-H.; Yin, C.-C.; Hung, C.-
H. Manuscript in preparation.
10.1021/om030237e CCC: $25.00 © 2003 American Chemical Society
Publication on Web 06/24/2003

3008 Organometallics, Vol. 22, No. 15, 2003
Communications
involving it. In this contribution, we present the syn-
thesis of H[PNP] and an account of our experiments
involving divalent nickel, which led to the discovery of
thermally stable alkyl derivatives including those con-
taining â-hydrogen atoms, a result that is markedly
different from that found in the silyl-derived [N(SiMe2-
CH2PR2)2]- system.15
The ligand synthesis takes two straightforward steps
from relatively inexpensive, commercially available
starting materials. As shown in eq 1, the palladium-
F igu r e 1. Molecular structure of [PNP]Li(THF)2. Selected
bond distances (Å) and angles (deg): Li(1)-O(2) 1.955(5),
Li(1)-O(1) 1.969(5), Li(1)-N(1) 2.039(5), Li(1)-P(2) 2.779-
(5), Li(1)-P(1) 2.824(5), O(2)-Li(1)-O(1) 102.3(2), O(2)-
Li(1)-N(1) 118.0(2), O(1)-Li(1)-N(1) 139.7(3), O(2)-Li-
(1)-P(2) 105.4(2), O(1)-Li(1)-P(2) 95.26(18), N(1)-Li(1)-
P(2) 73.35(15), O(2)-Li(1)-P(1) 94.92(18), O(1)-Li(1)-P(1)
106.85(19), N(1)-Li(1)-P(1) 73.09(15), P(2)-Li(1)-P(1)
146.01(18), C(18)-N(1)-C(19) 117.1(2), C(18)-N(1)-Li(1)
118.2(2), C(19)-N(1)-Li(1) 124.6(2).
catalyzed cross-coupling reaction16-18 of 2-fluoroaniline
with 2-bromofluorobenzene in the presence of sodium
tert-butoxide in refluxing toluene produces di(2-fluo-
rophenyl)amine quantitatively. After standard workup,
di(2-fluorophenyl)amine may be directly used for the
subsequent nucleophilic phosphanylation19,20 without
further purification. Treatment of di(2-fluorophenyl)-
amine with 2 equiv of KPPh2 in refluxing 1,4-dixoane
generates H[PNP], which can be readily purified by
recrystallization from dichloromethane or methanol.
Compound H[PNP] and the fluorine precursor have
been fully characterized by multinuclear NMR spec-
troscopy and elemental analysis. The solid-state struc-
ture of H[PNP] was determined by X-ray crystallogra-
phy (see Supporting Information).
bipyramidal geometry about the lithium center, as
depicted in Figure 1. The tridentate ligand coordinates
to the lithium atom in a meridional fashion. The bond
distances between the lithium atom and the two phos-
phorus donors in [PNP]Li(THF)2 (2.80 Å average) are
slightly longer than those found in {[LiN(SiMe2CH2Pi-
Pr2)2]2}LiCl8 by ca. 0.20 Å. The Li(1)-N(1), Li(1)-O(1),
and Li(1)-O(2) distances are all unexceptional. The
bond angles about Li(1) are in the range 73.09(15)-
146.01(18)°, corresponding to N(1)-Li(1)-P(1) and P(1)-
Li(1)-P(2), respectively. The two oxygen atoms lie
perfectly on the equatorial plane with the three equato-
rial angels ranging from 102.3(2)° to 139.7(3)°. The two
phosphorus donors are both distorted from the ideal
tetrahedral geometry. The two phenyl groups on each
phosphorus atom are oriented such that they are
virtually perpendicular to each other, with one roughly
being axial and the other equatorial. The two phenylene
rings of the backbone are tilted with respect to the
coordination plane due to the steric repulsion between
the two CH groups ortho to the amido nitrogen atom.
The dihedral angle defined by the two phenylene planes
in [PNP]Li(THF)2 is 38.95°, which is larger than the
corresponding angle in H[PNP] by 7.20° (see Supporting
Information). This is perhaps the consequence of the
decrease in C-N-C angles of the ligand backbone from
125.5(3)° to 117.1(2)° upon lithiation of H[PNP]. With
the rigid framework of the ligand, the P(1)-Li(1)-P(2)
angle is therefore much smaller than the ideal angle of
180°.
Addition of n-butyllithium to H[PNP] in THF at -35
°C affords in 78% yield the lithium amide complex
[PNP]Li(THF)2 as a pale yellow crystalline solid, which
is stable under an inert atmosphere and can be easily
1
manipulated in the drybox. The H NMR spectrum of
the lithium complex indicates 2 equiv of coordinated
THF molecules. The phosphorus donors of [PNP]Li-
(THF)2 in C6D6 appear as a broad singlet resonance at
-14.44 ppm in the 31P{1H} NMR spectrum at room
temperature, a value relatively downfield as compared
to the protonated compound H[PNP] at -18.62 ppm and
the silyl-derived LiN(SiMe2CH2PPh2)2 at -23.20 ppm.15
The absence of internuclear coupling between phospho-
rus and lithium-7 (I ) 3/2, natural abundance 92.6%)
is consistent with no or extremely weak phosphine
7
coordination. Accordingly, the Li{1H} NMR spectrum
at room temperature reveals a broad singlet at 2.42 ppm
instead of the expected triplet resonance.
The solid-state structure of [PNP]Li(THF)2 confirms
the tridentate feature of this new amido diphosphine
ligand. An X-ray study of [PNP]Li(THF)2 showed it to
be a five-coordinate species with distorted trigonal
Lithium amides are convenient starting materials for
metathetical reactions with transition metal halides.
With the hybrid characteristic, the tridentate [PNP]-
ligand is expected to adopt a wide variety of metals.
Following the lead of the X-ray structure of [PNP]Li-
(THF)2, where the tridentate ligand is meridional, it is
anticipated that square-planar, monoalkyl complexes of
[PNP]- should impose the alkyl group to occupy a
position trans to the amido nitrogen donor, thereby
concomitantly experiencing the two chemically equiva-
lent phosphorus donors in the cis positions. Our experi-
(15) Fryzuk, M. D.; Macneil, P. A.; Rettig, S. J .; Secco, A. S.; Trotter,
J . Organometallics 1982, 1, 918-930.
(16) Wolfe, J . P.; Wagaw, S.; Marcoux, J . F.; Buchwald, S. L. Acc.
Chem. Res. 1998, 31, 805.
(17) Hartwig, J . F. Acc. Chem. Res. 1998, 31, 852.
(18) Sadighi, J . P.; Harris, M. C.; Buchwald, S. L. Tetrahedron Lett.
1998, 39, 5327.
(19) Haenel, M. W.; Oevers, S.; Bruckmann, J .; Kuhnigk, J .; Kruger,
C. Synlett 1998, 301.
(20) Hingst, M.; Tepper, M.; Stelzer, O. Eur. J . Inorg. Chem. 1998,
73.

Communications
Organometallics, Vol. 22, No. 15, 2003 3009
Ta ble 1. Selected NMR Da ta of [P NP ]NiXa
ments on divalent nickel chemistry provide such evi-
dence. The reaction of [PNP]Li(THF)2 with NiCl2(DME)
in THF at -35 °C produces in 94% yield the diamagnetic
[PNP]NiCl (eq 2). The chloride compound is a green,
δ
J
X
HR
CR
P
PCR
Cl
Me
Et
18.77
27.61
26.42
27.06
26.80
23.90
24.45
0.13
1.07
1.07
1.02
-0.08
-15.37
-2.80
5.24
18.53
-12.97
151.00
21.87
19.48
19.10
19.54
18.83
28.15
n-Bu
i-Bu
CH2SiMe3
Ph
a
All spectra were recorded in C6D6, chemical shifts in ppm,
coupling constants in Hz.
crystalline solid that is both air and water stable.
Treatment of 1 equiv of Grignard reagents to [PNP]NiCl
in THF at -35 °C generates quantitatively the corre-
sponding alkyl and aryl complexes, [PNP]NiR (R ) Me,
Et, n-Bu, i-Bu, CH2SiMe3, Ph). These hydrocarbyl
compounds are diamagnetic, red solids that are ther-
mally stable in the absence of air. The stability of the
ethyl, n-butyl, and isobutyl complexes toward â-elimina-
tion is particularly surprising, as the synthesis of the
corresponding compounds supported by the closely
related [N(SiMe2CH2PPh2)2]- ligand was not success-
ful.15 In particular, a solution of [PNP]NiEt in C6D6 (11
mM) did not decompose at 70 °C over a course of several
hours, as indicated by the 1H and 31P{1H} NMR
spectroscopy. We tentatively ascribe this unusual result
to the rigid and robust feature of the o-phenylene-
derived [PNP]- ligand, in which the phosphine dissocia-
tion, if any, is not significant.
[PNP]NiMe and [PNP]NiCH2SiMe3. The 31P{1H} NMR
spectra show a sharp singlet resonance in all cases at
ca. 25 ppm, a relative downfield resonance as compared
to [PNP]NiCl that gives a signal at 18.77 ppm.
In summary, we have developed a protocol to prepare
the new monoanionic tridentate bis(2-diphenylphosphi-
nophenyl)amide ligand and its lithium and divalent
nickel derivatives. The nickel(II) chemistry established
in this study is notably different from that found in the
[N(SiMe2CH2PPh2)2]- system, likely as a consequence
of the rigidity and robustness of the ligand employed
in the former. The stability of the alkyl complexes
toward â-elimination is particularly remarkable. Studies
directed to delineate the reactivity of the metal com-
plexes described here and the chemistry involving the
heavier members of group 10 metals will be the subjects
of further reports.
Solution NMR studies of these nickel(II) complexes
are all consistent with a square-planar geometry, with
the [PNP]- ligand being in a meridional coordination
mode. The diagnostic evidence is the presence of virtual
triplet21,22 resonances for the aromatic carbons of the
ligand backbone in the 13C{1H} NMR spectroscopy. The
CR atom in all alkyl complexes appears as a triplet
Ack n ow led gm en t. We thank the National Science
Council (NSC 91-2113-M-110-019-) of Taiwan and
NSYSU for financial support of this work.
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental details, ORTEP diagram of H[PNP], and all structural
parameters for both H[PNP] (CCDC No. 197884) and [PNP]-
Li(THF)2 (CCDC No. 197883). This material is available free
of charge via the Internet at http://pubs.acs.org.
2
resonance with a J CP of ca. 20 Hz in the 13C{1H} NMR
spectroscopy (Table 1). The 1H NMR spectra also reveal
the expected triplet resonances for the HR atoms in
(21) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals, 3rd ed.; Wiley-Interscience: New York, 2001; pp 260-262.
(22) Brookes, P. R.; Shaw, B. L. J . Chem. Soc. A 1967, 1079.
OM030237E