Bifunctional diphosphorus Lewis acids from cyclodiphosphadiazanes

Article abstract of DOI:10.1039/b710853b

The quantitative displacement of triflate groups in 1,3-ditriflato-2,4- bis(2,6-dimethylphenyl)cyclodiphospha-2,4-diazane by DMAP (4- dimethylaminopyridine) or Me₃P gives dicationic complexes containing bifunctional diphosphorus Lewis acceptors

Full text of DOI:10.1039/b710853b

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Bifunctional diphosphorus Lewis acids from cyclodiphosphadiazanes{
Reagan J. Davidson,a Jan J. Weigand,a Neil Burford,*a T. Stanley Cameron,a Andreas Deckenb and
Ulrike Werner-Zwanzigerc
Received (in Berkeley, CA, USA) 16th July 2007, Accepted 21st September 2007
First published as an Advance Article on the web 8th October 2007
DOI: 10.1039/b710853b
cyclophosphenium 6 and cyclodiphosphenium 7 frameworks that
represent dimers of phosphadiazonium complexes of type 2.
The quantitative displacement of triflate groups in 1,3-ditri-
flato-2,4-bis(2,6-dimethylphenyl)cyclodiphospha-2,4-diazane by
DMAP (4-dimethylaminopyridine) or Me3P gives dicationic
complexes containing bifunctional diphosphorus Lewis
acceptors.
Isolation of derivatives of
7 introduces the potential for
development of oligomeric and polymeric polycationic systems.
Solution 31P NMR spectra of reaction mixtures containing
DmpNH2 (Dmp = 2,6-dimethylphenyl) and PCl3 in the presence of
Et3N13 show two signals (intensity ratio y95 : 5) assigned to 1,3-
dichlorocyclodiphospha-2,4-diazane,14–16 5b, with the chlorine
substituents in a cis [d(31P) = 210.1 ppm] and trans [d(31P) =
295.4 ppm] configuration with respect to the approximate plane of
the P2N2 ring. Reaction of 5b with AgOTf in hexane gives an almost
quantitative yield of 1,3-ditriflato-2,4-bis(2,6-dimethylphenyl)-
cyclodiphospha-2,4-diazane 5c as a cis (98%) [d(31P) = 182.8 ppm]
and trans (2%) [d(31P) = 272.4 ppm] mixture. A crystalline sample of
the cis isomer 5c (Fig. 1a, Table 2) exhibits two 31P CP/MAS NMR
signals (Table 1), consistent with the observation of two crystal-
lographically non-equivalent phosphorus centers.
Cationic phosphine centers are readily prepared from phosphines1
bearing good anionic leaving groups that are displaced by neutral
ligands. Chlorophosphines (in the presence of Me3SiOTf) or
Mes*NPOTf (Mes* = 2,4,6-tri-tert-butylphenyl; OTf = trifluoro-
methanesulfonate)2 react with a wide variety of Lewis bases to give
phosphenium cationic complexes of type 13–8 and phosphadiazo-
nium complexes of type 2,6,9–12 respectively.
Solution 31P NMR spectra of a reaction mixture containing 5c
with one equivalent of DMAP show two doublets [d(31P) = 191.0
and 150.5 ppm, 2JPP = 48.7 Hz], consistent with a non-symmetric
monocation in 6a[OTf] (isolated 78%). Fig. 1b shows that cation
6a contains a covalently bound OTf substituent at one phosphorus
center that is cis-configured with a DMAP ligand at the other
phosphorus center.
Solution 31P NMR spectra of a mixture containing 5c with two
equivalents of DMAP show a single resonance [d(31P) = 149.1 ppm]
assigned to 7a[OTf]2 (isolated 89%). The symmetric cis-configured
dication 7a (Fig. 1c) contains two crystallographically different
phosphorus centers that are responsible for two resonances in the
31P{1H} CP/MAS NMR spectrum (Table 1).
Analogous complexes with two charges have been realized using
diphosphine and diamine ligands that tether two phosphorus cation
acceptors, generically represented by 3.4,10 Moreover, reductive
coupling of chloro- derivatives of 1 provide dications that behave as
complexes involving a bifunctional P–P diphosphenium acceptor,
generically represented by 4,3 demonstrating the ability of such
compounds to accommodate more than one charge.
The reaction of 5c with two equivalents of PMe3 in benzene at
RT gives 7b[OTf]2 (isolated 91%) containing a dication with trans-
configuration of two PMe3 ligands at the two acceptor phosphorus
centers (Fig. 1d). Reaction mixtures containing equimolar amounts
of 5c and PMe3 give solution 31P NMR spectra showing a mixture
of 7b[OTf]2 and 5c.
Consistent with a Ci symmetric framework of 7b observed in the
solid state (Fig. 1d), the 31P{1H} NMR spectrum has been fitted as
an AA9XX9 spin system (Table 1, Fig. 2). The relatively large 4JXX9
and the high field resonance of PA is in agreement with a transoid
arrangement of the phosphorus lone pairs in the P2N2 ring. The
solid state CP/MAS 31P NMR spectrum of 7b[OTf]2 shows two
signal groups for the AA9XX9 pattern as poorly resolved doublets
(Fig. 2) that have been simulated using the J values obtained from
the liquid state NMR spectrum with adjustment of shift offset and
line width.
We have now exploited the well-established series of cyclo-
diphospha-2,4-diazanes 5 as synthetic origins for complexes of
aDepartment of Chemistry, Dalhousie University, Halifax, Nova Scotia,
Canada B3H 4J3. E-mail: neil.burford@dal.ca; Fax: +1 (902) 494-1310;
Tel: +1 (902) 494-3190
bDepartment of Chemistry, University of New Brunswick, Fredericton,
New Brunswick, Canada E3A 6E2
cDepartment of Chemistry, Atlantic Region Magnetic Resonance
Centre, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J3
{ Electronic supplementary information (ESI) available: Experimental
details and crystallographic data in CIF format for 5c, 6a[OTf], 7a[OTf]2
and 7b[OTf]2 (CCDC 650721, 650722, 650720 and 650811, respectively).
See DOI: 10.1039/b710853b
Selected structural parameters for 5c, 6a[OTf], 7a[OTf]2 and
7b[OTf]2 are presented in Table 2. The endocyclic interatomic
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Fig. 1 ORTEP plots of the solid state structures for (a) 5c, (b) the cation in 6a[OTf], (c) the dication in 7a[OTf]2 and (d) the dication in 7b[OTf]2. Thermal
ellipsoids with 50% probability at 173(2) K (hydrogen atoms are omitted) [symmetry code: (i) 2x + 1, 2y, 2z + 1].
distances and bond angles in the P2N2 ring are essentially
independent of the substituents or ligands attached to phosphorus,
and the distances are typical of those observed for neutral
diphosphadiazanes.19–22 For example, all of the P–N distances
phosphadiazonium complex cations of type 2. While the formula
Mes*NPOTf is observed as both a monomer and a dimer (5a) in
the solid state, only structural type 2 is observed in the presence of
Lewis bases (ligands). Retention of the P2N2 framework in the
ionic complexes 6 and 7 illustrates the significance of the sterically
bulky substituents (Mes*) in restricting dimerization of derivatives
of 2. The sterically mild Dmp substituent is insufficient to effect
dissociation of the dimeric N2P2-ring arrangement in solution and
in the solid state. Consequently, the accommodation of a
˚
within the P2N2 ring are within normal ranges (1.68–1.74 A). The
P–NDMAP bonds in the ligand-stabilized monocation 6a[OTf]
˚
(1.774(2) A) and dication 7a[OTf]2 (1.790(3) and 1.775(3) A) are
˚
6
consistent with that in [DMAP?PPh2][OTf] (1.789(1) A) and are
˚
significantly shorter than the two P–NDMAP distances (1.873(2)
10
˚
and 1.879(2) A) in [Mes*NLP(DMAP)2][OTf]. The dication in
7a[OTf]2 lies on a center of inversion (crystallographically
imposed), resulting in a planar P2N2 diphosphadiazanium frame-
work, while the cis-configurations of 5c, 6a[OTf] and 7a[OTf]2
impose puckering.
Table 2 Selected interatomic distances and bond angles for 5c,
6a[OTf] and 7a[OTf]2, and 7b[OTf]2
5c
6a[OTf]
7a[OTf]2
7b[OTf]2
X1 = O1 X1 = O1 X1 = N17 X1 = P2
X2 = O4 X2 = N17 X2 = N26
The dimeric N2P2 structures of the bis-donor–bis-phosphenium
dications in 7a[OTf]2 and 7b[OTf]2 contrast the monomeric
˚
[A]
P1–N1
1.704(2)
1.701(2)
1.697(2)
1.698(2)
2.556(1)
1.750(2)
1.749(2)
1.696(2)
1.702(2)
1.717(2)
1.724(2)
2.568(1)
1.799(2)
1.774(2)
1.735(3) 1.741(1)
1.730(3) 1.726(1)
1.717(3)
P1–N2(1i)
P2–N1
P2–N2
Table 1 31P NMR data (202.46 MHz, 300 K) for 5b, 5c, 6a[OTf],
7a[OTf]2 and 7b[OTf]2
1.740(3)
Ligand 31P NMR (solution)c
31P NMR (solid state)
P1–P2(1i)
P1–X1
P2–X2
2.559(2) 2.6101(4)
1.790(3) 2.2832(5)
1.775(3)
d
trans-5ba
cis-5ba
—
—
—
—
295.4 (A2)
210.1 (A2)
276.9 (A2)
182.8 (A2)
—
212.8, 207.8
—
[u]
d
trans-5ca
cis-5ca
P1–N1–P2(1i)
P1–N2–P2
N1–P1–N2(1i)
N1–P2–N2
97.4(1)
97.6(1)
97.1(1)
82.3(1)
81.1(1)
95.7(2)
95.0(2)
81.5(2)
81.7(2)
97.67(5)
82.33(5)
180.3, 178.3
193.6, 147.4
136.9, 132.5
97.53(9)
81.82(9)
82.11(9)
6a[OTf]a
7a[OTf]2
7b[OTf]2
a
DMAP 191.0, 150.5 (AX)f
DMAP 149.1 (A2)
a
b
Me3P
b
279.5, 17.5 (AA9XX9)e 280.5, 19.5 (AA9XX9)
P1–N1–P2(1i)–N2(1i) 28.07(9) 210.6(1)
218.5(2)
0.0
0.0
c
In CDCl3. In d3-MeCN. An Dn/J ¢ 10 (AX); 0 , Dn/J ,10
d
(AB); Dn/J A 0 (A2). Not observed in the solid state NMR
Puckering angle in [u]a 10.6(1)
14.02(9)
24.7(1)
e 1
2
3
experiment.
J
AX
=
1JA9X9 = 2474.2, JAA9 = 21.4, JAX9
=
3JXA9
41.2, JXX9 = 107.0 Hz (a negative value for JPP in agreement with
=
4
1
a
Puckering angle between the N1–N2(1i)–P1 and N1–N2(1i)–P2(1i)
planes; [symmetry code: (i) 2x + 1, 2y, 2z + 1].
other observations17,18).
= 48.7 Hz.
f 1
J
PP
4672 | Chem. Commun., 2007, 4671–4673
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Fig. 2 31P NMR spectra for 7b[OTf]2: experimental solution spectrum (exp.) in d3-MeCN at 202.46 MHz (25 uC, external H3PO4) and simulated solution
spectrum (sim.). 31P CP/MAS NMR spectrum, broad resonances indicated by Q are modeled as an AA9XX9 spin system (* spinning side bands, u
impurity).
8 N. Burford, T. S. Cameron, P. J. Ragogna, E. Ocando-Mavarez,
M. Gee, R. McDonald and R. E. Wasylishen, J. Am. Chem. Soc., 2001,
123, 7947–7948.
9 N. Burford, A. D. Phillips, H. A. Spinney, M. D. Lumsden, U. Werner-
Zwanziger, M. J. Ferguson and R. McDonald, J. Am. Chem. Soc.,
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dicationic charge in derivatives of 7 provides examples of
bifunctional phosphorus Lewis acceptors of type 4 and comple-
ments the previously reported dicationic complexes of a bifunc-
tional donor on two phosphorus acceptors, of type 3.4,10 In this
context, the new dications provide insight for the development of
extended systems towards cationic polymers.
10 N. Burford, H. A. Spinney, M. J. Ferguson and R. McDonald, Chem.
Commun., 2004, 2696–2697.
We thank J. C. Landry for preliminary experimental work, the
Natural Sciences and Engineering Research Council of Canada,
the Killam Foundation, the Canada Research Chairs Program and
the Alexander von Humboldt-Stiftung (J. J. W., Feodor Lynen
program) for funding.
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