Formation of a chiral NPPN ligand via metallation of acyclic NPNCN systems

Article abstract of DOI:10.1039/b413137a

A new type of bidentate N,N′-chelating ligand containing a chiral phosphorus centre has been synthesized via the metallation of an acyclic NPNCN species. The zwitterionic ligand backbone contains a phosphenium centre stabilised by an imido phosphine fragment.

Full text of DOI:10.1039/b413137a

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Formation of a chiral NPPN ligand via metallation of acyclic NPNCN
systems
Tristram Chivers,* May C. Copsey and Masood Parvez
Department of Chemistry, University of Calgary, Calgary, Alberta, Canada T2N 1N4.
E-mail: chivers@ucalgary.ca; Fax: 11 (403) 289-9488; Tel: 11 (403) 220-5741
Received (in Cambridge, UK) 26th August 2004, Accepted 20th September 2004
First published as an Advance Article on the web 9th November 2004
A new type of bidentate N,N’-chelating ligand containing a
chiral phosphorus centre has been synthesized via the
metallation of an acyclic NPNCN species. The zwitterionic
ligand backbone contains a phosphenium centre stabilised by
an imido phosphine fragment.
Interest in the coordination chemistry of b-diketiminato ligands has
been increasing significantly over the past few years.1,2 These
versatile ligands, using bulky N-substituents, are effective in
stabilizing low-coordinate complexes for both main group3,4 and
transition metal elements.5 The steric demands of the ligand can be
tuned with relative ease, by changing the organic substituents on
the nitrogen atoms. In comparison, relatively little work has been
done on the electronic influence engendered at the metal centre by
variation of the elements in the ligand backbone. Recent studies
from the groups of Stephan6 and Piers7 have focused on the
inclusion of phosphinimine donors in the ligand framework. We
report here investigations of the reactivity of novel, acyclic
NP(III)NCN systems which, unexpectedly, give rise to chelating
NPPN ligands containing a chiral phosphorus centre upon
metallation with BunLi or Me3Al.
Fig. 1 Thermal ellipsoid plot of Li[DippNPhP–P(Bun)PhNDipp]?OEt2 (2).
˚
Hydrogen atoms have been omitted for clarity. Selected bond lengths (A)
and angles (u): P(1)–N(1) 1.6522(15), P(2)–N(2) 1.6065(15), P(1)–P(2)
2.2436(7), N(1)–Li(1) 1.939(3), N(2)–Li(1) 1.965(4), N(1)–P(1)–C(1)
108.90(8), N(1)–P(1)–P(2) 97.83(6), C(1)–P(1)–P(2) 93.04(6).
Synthesis of neutral acyclic compounds of the type DippN(H)-
PhPNRCR’NR (Dipp ~ 2,6-PriC6H3) (1) was achieved in good
yield according to Scheme 1. Derivatives 1a and 1b have been
characterised by elemental analyses, multinuclear NMR (1H, 13C
and 31P) and X-ray crystallography.{
the three-coordinate phosphorus. This shortened bond length
suggests a larger degree of ionicity on the P2–N2 side of the ligand
and hence that resonance form 2a contributes significantly to the
structure. Consistently, the metal centre in both 2 and 3 is also
more strongly coordinated to N1 than to N2. The geometry at P2 is
distorted tetrahedral with bond angles in the range 103.4–114.5u (2)
and 101.5–115.7u (3), while P1 adopts a pyramidal geometry
reflecting the presence of a lone pair. A similar aluminium complex,
which contains an intramolecular P–P coordination, has been
reported by Burford et al.9 In that case however, the two
phosphorus centres are bridged by an NSiMe3 group to form a
three-membered aza-diphosphiridene ring.9
Deprotonation reactions of 1a using one equivalent of BunLi
gave an unexpected 31P NMR spectrum. Instead of a single
resonance corresponding to the expected lithium complex, two
mutually coupled doublets are observed at d ~ 26.9 and 59.8
(1JP–P ~ 298 Hz) suggesting the formation of a complex containing
two inequivalent phosphorus centres bonded together. Analysis by
X-ray crystallography showed the product to be Li[DippNPhP–
P(Bun)PhNDipp]?OEt2 (2) (Fig. 1).
A similar aluminium-
containing complex, Al(Me)2[DippNPhP–P(Me)PhNDipp] (3),
was isolated from the reaction of 1b with equimolar amounts of
Me3Al (Fig. 2). Complexes 2 and 3 have isostructural DippNPhP–
P(R)PhNDipp ligand backbones N,N’-chelated to the metal centre.
Nucleophilic attack by the respective metallating reagents has
occurred to create a four-coordinate, chiral P2 centre bonded to an
n-butyl group in 2 and a methyl group in 3. As illustrated in
Scheme 2, the new complexes 2 and 3 can be viewed in terms of two
resonance structures: a phosphine(P1)–phosphonium(P2) complex
2a or a phosphine-stabilised phosphenium cation 2b. The P–P bond
The 31P NMR spectra of complexes 2 and 3 provide some insight
˚
lengths of ca. 2.24 A are at the upper limit of the range of values
reported for phosphine–phosphenium cation complexes.8 In both
complexes, the P–N bond from the four-coordinate phosphorus
˚
centre (P2–N2) is ca. 0.05 A shorter than P1–N1, which involves
Fig. 2 Thermal ellipsoid plot of Al(Me)2[DippNPhP–P(Me)PhNDipp] (3).
˚
Hydrogen atoms have been omitted for clarity. Selected bond lengths (A)
and angles (u): P(1)–N(1) 1.673(2), P(2)–N(2) 1.634(2), P(1)–P(2)
2.2367(11), N(1)–Al(1) 1.892(2), N(2)–Al(1) 1.929(2), N(1)–P(1)–C(4)
108.05(12), N(1)–P(1)–P(2) 95.36(8), C(4)–P(1)–P(2) 98.37(9).
Scheme 1 Reagents and conditions: i Et2O, 278 uC, ii Li[CR’(NR)2] in
hexane, 278 uC (1a, R ~ But, R’ ~ Bun; 1b, R ~ Cy, R’ ~ But).
2 8 1 8
C h e m . C o m m u n . , 2 0 0 4 , 2 8 1 8 – 2 8 1 9
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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In summary, metallation reactions of NP(III)NCN systems have
uncovered a synthetic route to a new type of bidentate N,N’ ligand
containing a phosphine–phosphenium bond and a chiral phos-
phorus centre. The presence of a phosphenium centre in 2 and 3
affords the intriguing possibility of additional metal coordination
via this Lewis basic site.
We thank NSERC (Canada) for financial support.
Notes and references
n
{ Synthesis of 2: BuLi (0.8 mL, 2.5 M solution in hexane, 2 mmol) was
added dropwise to a solution of DippN(H)PhPNButCBunNBut (1a)
(0.99 g, 2 mmol) in n-hexane (20 mL) at 278 uC producing a clear yellow
solution. The solution was warmed to 25 uC and stirred for 1 h. The volume
was reduced in vacuo to approximately 10 mL and diethyl ether (5 mL) was
added to give a clear yellow solution. Concentration of the reaction mixture
to ca. 5 mL, and storage at 25 uC for 24 h, yielded colourless crystalline
blocks (0.17 g, 24% yield). Synthesis of 3: Me3Al (1 mL, 2.0 M solution in
Scheme 2 Proposed route of formation of 2.
1
into the nature of the bonding in the two complexes. The JP–P
couplings of 298 and 264 Hz, respectively, are consistent with a
phosphine–phosphenium interaction,8 however the coordination of
the ligand to two different metal centres markedly changes the
phosphorus chemical shifts. The two, well-separated, equal
intensity doublets at 59.8 and 26.9 ppm observed for complex 2
indicate two distinctly different chemical environments for the
phosphorus centres. However for complex 3, a second-order AB
type pattern is observed with chemical shifts of 45.9 and 40.2 ppm,
arising from two phosphorus centres which are in a more
comparable chemical environment.
Stabilised phosphenium ions were first reported over thirty years
ago and advances in this area were reviewed extensively by Cowley
et al. in 1985.10 A recent structurally characterised example of an
amidophosphenium ion is the complex [{N(Dipp)CH2CH2N-
(Dipp)}P–PMe3]OTf11 in which the cation is stabilised by a
classical phosphine donor. Complexes 2 and 3 can be viewed as
examples of zwitterionic imido phosphenium species; stabilisation
by a diorganophosphorus imido moiety results in monoanionic
ligands that chelate to the metal. A dianionic NPPN ligand
involving two chemically equivalent, three-coordinate phosphorus
centres has been characterized by spectroscopic methods in a PhB
complex.12
Formation of 2 and 3 from the metallation reactions of 1a and
1b must involve the elimination of the corresponding metal
amidinate (Scheme 2). This is supported by the isolation of
crystalline (Me)2Al[CyNCButNCy] from the reaction of 1b with
Me3Al.13 The loss of amidinate will be accompanied by the
formation of an iminophosphane RPLNR’ which are known to
form readily by thermal elimination of lithium salts at low
temperatures.14 Iminophosphanes undergo cyclodimerization
which, in the case of P-aryl and -alkyl derivatives, involves a
reversible [2 1 1] cycloaddition to give the corresponding l3,l5-
azadiphosphiridine (4) (Scheme 2).14,15 In such examples, [2 1 2]
cycloaddition to give the symmetric dimer is not observed.15 While
this cannot be completely ruled out for the iminophosphane
DippNLPPh, a [2 1 1] cycloaddition would account for the
observed P–P bond formation. Nucleophilic attack of a further
equivalent of BunLi on the exocyclic PLN bond of 4 would then
afford 2. When the reaction is carried out using a 3 : 2
stoichiometry (BunLi : 1a), as required by Scheme 2, complex 2
is produced quantitatively by 31P NMR spectroscopy.
Iminophosphanes are known to form readily by LiX elimination
(X ~ halide),14 suggesting the possibility of a more direct route to
the novel ligand in 2. Preliminary investigations have shown that
the reactions of PhP(Cl)NHDipp with RLi reagents (R ~ Bun,
Me) in the ratio 2 : 3 afford 2 and the related P-methylated
derivative in ca. 60% isolated yields. Details of this straightforward,
general synthesis together with the characterization of the
protonated NP(III)NCN reagents 1 will be given in a full account
of this work.
heptane,
2 mmol) was added dropwise to a stirred solution of
DippN(H)PhPNCyCButNCy (1b) (1.094 g, 2 mmol) in n-hexane (30 mL)
at 278 uC resulting in a pale yellow solution. The solution was stirred for
2 h at 25 uC. The volume of solvent was reduced to approximately 10 mL
and the solution was stored at 215 uC for 24 h. This yielded colourless
crystalline blocks which were identified as Al(Me)2[CyNCButNCy] by 1H
NMR spectroscopy and elemental analysis (0.19 g, 30% yield). The yellow
solution was decanted and further reduced in volume to a yellow oil.
Storage at 215 uC for 24 h yielded colourless crystalline blocks (0.23 g, 48%
yield). Crystal data for 2: M ~ 704.84, monoclinic, P21/c, a ~ 11.712(2),
3
˚
˚
b ~ 19.656(3), c ~ 18.528(2) A, b ~ 97.821(10)u, V ~ 4225.7(11) A , Z ~
4, T ~ 173(2) K, D ~ 1.108 g cm23, m(Mo–Ka) ~ 0.136 mm21, R ~
0.0493 and wR ~ 0.1452 (all data). CCDC 247957. Anal calcd. for
C44H63LiN2OP2: C 74.97, H 9.01, N 3.97; found C 74.67, H 9.15, N 4.28.
Selected NMR data: 31P{1H} (C6D6, 25 uC, 161.821 MHz): d ~ 59.8 (d,
1JP–P ~ 298 Hz), 26.9 (d, JP–P ~ 298 Hz); 7Li (C6D6, 25 uC,
1
155.459 MHz): d ~ 2.17 (s). Crystal data for 3: M ~ 638.75,
˚
orthorhombic, P212121, a ~ 9.5496(19), b ~ 13.510(3) c ~ 29.730(6) A,
23
3
˚
V ~ 3835.5(13) A , Z ~ 4, T ~ 173(2) K, D ~ 1.106 g cm
,
m(Mo–Ka) ~ 0.164 mm21, R ~ 0.0515 and wR ~ 0.1359 (all data).
CCDC 247958. Anal calcd. for C39H53AlN2P2: C 73.33, H 8.36, N 4.39;
found C 73.15 H 8.60, N 4.22. Selected NMR data: 31P{1H} (C6D6, 25 uC,
161.821 MHz): d ~ 45.9 (d, 1JP–P ~ 264 Hz), 40.2 (d, 1JP–P ~ 264 Hz). See
http://www.rsc.org/suppdata/cc/b4/b413137a/ for crystallographic data
in .cif or other electronic format.
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C h e m . C o m m u n . , 2 0 0 4 , 2 8 1 8 – 2 8 1 9
2 8 1 9