A Lewis-basic, dionio-substituted phosphane

Article abstract of DOI:10.1039/b710923g

Bis{guanidine} H₂C{hpp}₂ [hppH = 1,3,4,6,7,8- hexahydro-2H-pyrimido[1,2-a]pyrimidine] reacts with PPhCl₂ to generate the dionio-substituted phosphane, [H₂C{hpp} ₂PPh]²⁺[Cl]^-

Full text of DOI:10.1039/b710923g

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A Lewis-basic, dionio-substituted phosphane
Martyn P. Coles* and Peter B. Hitchcock
Received (in Cambridge, UK) 17th July 2007, Accepted 26th September 2007
First published as an Advance Article on the web 9th October 2007
DOI: 10.1039/b710923g
Bis{guanidine} H2C{hpp}2 [hppH 5 1,3,4,6,7,8-hexahydro-2H-
pyrimido[1,2-a]pyrimidine] reacts with PPhCl2 to generate the
dionio-substituted phosphane, [H2C{hpp}2PPh]2+[Cl]2 which,
2
despite the formally dicationic phosphorus centre, forms an
unprecedented coordination compound with platinum.
Formation of cationic phosphonium and phosphenium ions
readily occurs via phosphorus–halogen bond heterolysis, and has
been promoted by a range of metal and main-group reagents.1
Scheme 1
environments [d 120.4 and 110.7; 6.6 : 1 ratio at 25 uC], resonating
at slightly lower field than Bertrand’s dionio-compounds of type B
[range 5 109–98 ppm]. Given the flexibility inherent in the C3N4P
ring, (taking into account the planarity of the ‘N–C–N’ moiety),
and the different possible orientations of the annular methylene
groups, these resonances are assigned to different conformations
of the heterocycle in solution.12 The corresponding 1H NMR
spectrum{ of 2 shows that for each isomer, the protons of the
bridging methylene group resonate with an AB pattern (major: d
5.67 and 4.64, 2JHH 15.7 Hz; minor: d 4.88 and 4.58, 2JHH 24.4 Hz),
consistent with inequivalent magnetic environments caused by
different rigid conformations.
Over 10 years ago, Bertrand and co-workers reported an alterna-
tive protocol that exploited the nucleophilic activity of the bicyclic
amidine bases, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,5-
diazabicyclo[4.3.0]non-5-ene (DBN).2,3 Using this approach the
onio- and dionio-substituted phosphanes [P(DBN)(NiPr2)2]+ (A)
and [P(DBN)2(NCy2)]2+ (B) were isolated and structurally
characterized, providing insight into the mechanism of base-
induced dehydrohalogenation employing these amidine reagents.
A number of studies4 have subsequently been presented in
which the Lewis-acidic properties of cationic phosphorus centres
have been investigated, using predominantly nitrogen-5 and
phosphorus-based6 donor groups. Whilst the retention of a non-
bonding electron pair has been noted,7 and the potential for Lewis-
basic behaviour of a monocationic phosphorus compound has
been demonstrated,8 this remains an underdeveloped field of
study, where interest may be anticipated in the areas of coordina-
tion chemistry and catalysis with the formation of formally
cationic phosphane ligands. Exploiting the ability of amidines and
guanidines to disperse positive charge throughout their core, we
report herein our studies to ascertain whether sufficient Lewis-
basic character is retained in a novel dionio-substituted phosphane
(a Weiss-type compound9 with a formally dicationic charge at
phosphorus) to enable donor ligand behaviour at a metal centre.
We have recently synthesized a series of poly{guanidyl}
compounds based on the bicyclic guanidine 1,3,4,6,7,8-hexa-
hydro-2H-pyrimido[1,2-a]pyrimidine (hppH), and demonstrated
coordination at neutral10 and cationic11 metal fragments. Reaction
between the methylene linked derivative, H2C{hpp}2 (1), and
PPhCl2 proceeded via nucleophilic displacement of both
chlorides to afford the heterocyclic dionio-substituted phosphane,
[H2C{hpp}2PPh][Cl]2 (2), which crystallized as the dichloro-
methane solvate (Scheme 1). Symmetric chelation of the formally
neutral H2C{hpp}2 ligand to the dicationic [PPh]2+ fragment was
indicated by a single resonance for the quaternary CN3 atoms in
the 13C NMR spectrum{ [d 159.8] with a 2JPC coupling of 13 Hz.
The 31P NMR spectrum{, however, indicated two phosphorus
The molecular structure of 2 has been solved by single-crystal
X-ray diffraction{, confirming the formation of the cyclic
[H2C{hpp}2PPh]2+ dication (Fig. 1a). The chloride counter ions
22
…
…
are integrated into a hydrogen-bonded [Cl H–C(Cl)2–H Cl]
dianion, which is positioned with the chlorides located above the
central CN3 regions of the guanidyl groups (Fig. 1b). The dication
itself is monomeric, with the P-atom incorporated into an unusual
eight-membered C3N4P heterocycle, with a degree of pyramida-
lization13 at phosphorus of 51.7%, suggesting retention of a
Fig. 1 (a) Molecular structure of the dicationic component of
2
(ORTEP, ellipsoids at 30%, hydrogens except on the bridging methylene
˚
omitted). Selected bond lengths (A) and angles (u): P–N2 1.7326(17), P–N5
1.7351(16), P–C16 1.825(2), C1–N1 1.346(3), C1–N2 1.375(2), C1–N3
1.319(2), C9–N4 1.344(2), C9–N5 1.379(2), C9–N6 1.312(2); N2–P–N5
107.63(8), N2–P–C16 103.71(8), N5–P–C16 102.09(9). (b) Arrangement of
Department of Chemistry, University of Sussex, Falmer, Brighton, UK
BN1 9QJ. E-mail: m.p.coles@sussex.ac.uk; Fax: +44 (0)1273 677196;
Tel: +44 (0)1273 877339
…
…
…
the ion-pair in 2: H22a Cl2 2.45, H22b Cl1 2.46, Cl1 N6 3.46,
…
Cl1 C9 3.33, Cl2 N3 3.43, Cl1 C1 3.37.
…
…
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Scheme 2 Key resonance forms of dionio-substituted phosphane, 2.
non-bonded electron pair. The bridging methylene and PPh
groups are located on opposite sides of the approximate C2N4
plane generated by the guanidine rings, giving a pseudo-chair
conformation that accounts for the inequivalent proton environ-
ments in the NMR spectrum of the major isomer. The P–N
Fig. 3 (a) Molecular structure of
4
(ORTEP, ellipsoids at 30%,
hydrogens except on the bridging methylene and phenyl group, and
˚
MeCN solvate omitted). Selected bond lengths (A) and angles (u): P–Pt
2.2006(17), P–N2 1.684(6), P–N5 1.738(5), P–C16 1.813(6), Pt–Cl1
2.3068(19), Pt–Cl2 2.3524(18), Pt–Cl3 2.2961(19), C1–N1 1.321(8), C1–
N2 1.388(8), C1–N3 1.314(9), C9–N4 1.349(8), C9–N5 1.358(8), C9–N6
˚
bonding is essentially symmetrical [1.7326(17) A and 1.7351(16) A],
˚
being considerably shorter than in the dicationic DBN adduct B
2
…
1.301(8), H21 Cl4 2.65; N2–P–N5 104.7(3), N2–P–Pt 113.62(19), N5–P–
˚
[P–Namidine 5 1.766(5) A and 1.752(4) A].
˚
Pt 115.5(2), C16–P–Pt 114.8(2), N2–P–C16 105.5(3), N5–P–C16 101.3(3).
The carbon–nitrogen bond distances within the CN3 core of the
guanidyl groups suggest delocalization of the positive charge into
the bicyclic system. Significantly shorter bonds to N3/N6 (ave.
(b) View emphasizing the twisted heterocycle and anion interactions:
… …
Cl4 N6 3.31, Cl4 C9 3.52.
˚
˚
1.32 A) and N1/N4 (ave. 1.34 A) compared with those to the
resonances in the 31P NMR spectrum{ at d 101.8 (1JPtP 2950 Hz,
˚
phosphorus bound nitrogen (ave. 1.38 A), are consistent with a
2
H2C{hpp}2PPh) and d 15.9 (1JPtP 2530 Hz, PEt3) with a JPP of
large contribution from resonance forms II and III (Scheme 2).
This contrasts with the observed trend in the aluminocenium ion
[H2C{hpp}2AlMe2][BPh4], in which it is suggested that I and II are
the major contributors to the overall bonding.11
570 Hz are consistent with the formation of the trans-product.
Gentle heating of the sample (50 uC, 24 h) led to the formation
of a new product (4), indicated by a single phosphorus resonance
at d 72.5 (1JPtP 4877 Hz), significantly shifted from that of the
The isolated dication has been examined using the B3LYP
density functional theory and the 6-31g(d) basis set, implemented
by Gaussian 03.14 The NBO charges (Fig. 2) are consistent with
the phosphorus atom retaining cationic character (+1.286), with
the remaining positive charge delocalized into the guanidyl
fragments. The relative charges at nitrogen are in agreement
with resonance forms II and III dominating the bonding, as
described above. Interestingly, the HOMO is representative of
considerable non-bonding electron density at phosphorus, with
several lower energy MOs indicative of delocalized electron density
within p-symmetry orbitals across the CN3 component of the
guanidyl rings.
1
trans-diphosphane, 3. The H NMR spectrum of 4 contained a
widely spaced AB pattern for the bridging methylene protons
2
(d 7.05 and 4.66, JHH 15.7 Hz), confirming the presence of
the cationic phosphane and the lack of resonances for the ethyl
groups led us to conclude that PEt3 had been displaced from the
metal coordination sphere. In agreement with the NMR data,
crystals of 4 were analyzed by X-ray crystallography and
shown to correspond to the monocationic platinum compound,
[PtCl3(H2C{hpp}2PPh-kP)][Cl] (Fig. 3), generated by the phos-
phane displacement from platinum by chloride (Scheme 3).
The molecular structure is comprised of the metallated C3N4P
heterocycle in which a PtCl3 unit occupies the fourth vertex of a
distorted tetrahedral phosphorus atom [angles in the range
101.3(3)u–115.5(2)u]. Similar to the ion-packing arrangement in 2,
the chloride is located with closest contacts to the relatively short
Given the lone-pair characteristics calculated for the HOMO of
2, we wished to ascertain whether the phosphorus atom would
exhibit Lewis-basic properties, despite its formally positive charge.
No reaction was detected by NMR spectroscopy upon mixing a
sample of 2 with elemental selenium, despite prolonged heating
and sonication. This is attributed to the heterogeneity of the system
as the reaction with [PtCl(PEt3)(m-Cl)]2, indicated rapid formation
of [PtCl2(H2C{hpp}2PPh-kP)(PEt3)][Cl]2 (3) (y50% conversion),
in which the dionio-phosphane was coordinated to platinum. New
˚
C9–N6 bond [1.301(8) A], with additional hydrogen bonding to an
ortho-proton (H21) of the phenyl substituent in this instance.
Upon coordination of the platinum trichloride fragment, the
heterocycle distorts from the symmetrical chair-like conformation
in 2, to what is best described as a pseudo-twist-boat (Fig. 3b). A
˚
significant difference in the P–N distances is noted (DPN 5 0.054 A),
reflected in different C–N bond lengths within each guanidyl
˚
component, indicated by |DCN| values of 0.067 A (N1–C1–N2) and
Fig. 2 Calculated NBO charges and HOMO of the isolated dionio-
substituted phosphane, 2.
Scheme 3
5230 | Chem. Commun., 2007, 5229–5231
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4 N. Burford and P. J. Ragogna, J. Chem. Soc., Dalton Trans., 2002,
4307; B. D. Ellis and C. L. B. Macdonald, Coord. Chem. Rev., 2007,
251, 936.
5 N. Burford, P. Losier, A. D. Phillips, P. J. Ragogna and T. S. Cameron,
Inorg. Chem., 2003, 42, 1087; N. Burford, H. A. Spinney, M. J. Ferguson
and R. McDonald, Chem. Commun., 2004, 2696; E. Rivard, K. Huynh,
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Zwanziger, M. J. Ferguson and R. McDonald, J. Am. Chem. Soc.,
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6 C. A. Dyker, N. Burford, M. D. Lumsden and A. Decken, J. Am.
Chem. Soc., 2006, 128, 9632; N. Burford, T. S. Cameron, J. A. C.
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2003, 125, 14404.
˚
0.009 A (N4–C9–N5). These metrical parameters demonstrate
the high degree of flexibility within the dionio-substituted
phosphane 2, regarding both conformational changes within the
eight-membered heterocycle and, perhaps more importantly,
with respect to the distribution of electron density within the
guanidyl units.
Further studies on the coordinating ability of this and other
related onio- and dionio-substituted phosphanes are underway in
our laboratory. We wish to acknowledge the University of Sussex
for financial support and Prof. J. F. Nixon for helpful discussion
and the loan of [PtCl(PEt3)(m-Cl)]2.
Notes and references
7 D. Gudat, Coord. Chem. Rev., 1997, 163, 71.
8 N. Burford, P. Losier, S. V. Sereda, T. S. Cameron and G. Wu, J. Am.
Chem. Soc., 1994, 116, 6474.
9 R. Weiss and S. Engel, Synthesis, 1991, 1077; R. Weiss and S. Engel,
Angew. Chem., Int. Ed. Engl., 1992, 31, 216.
10 S. H. Oakley, M. P. Coles and P. B. Hitchcock, Inorg. Chem., 2004, 43,
7564; S. H. Oakley, M. P. Coles and P. B. Hitchcock, Dalton Trans.,
2004, 1113; M. P. Coles, Dalton Trans., 2006, 985.
{ Selected analytical data: 2, elemental analysis calcd (%) for
C22H33Cl4N6P: C 47.67, H 6.00, N 15.16; found: C 47.90, H 6.30, N
14.86. 31P NMR (CD3CN, 121.4 MHz, 298 K): d 120.4 (major), 110.7
(minor). 1H NMR (CD3CN, 300.1 MHz, 298 K, coupling constants in Hz):
3
d 7.65 (m, 2H, o-C6H5), 7.54 (m, 2H, m-C6H5), 7.17 (t, JHH 6.8, 1H,
p-C6H5), 5.67 (d, 2JHH 15.7, 1H, H2C{hpp}2 major), 5.46 (s, 2H, CH2Cl2),
4.88 (d, 2JHH 24.5, H2C{hpp}2 minor), 4.64 (d, 2JHH 15.7, 1H, H2C{hpp}2
major), 4.58 (d, JHH 24.3, H2C{hpp}2 minor), 3.94 (m, 6H, hpp-CH2),
2
11 P. J. Arago´n Sa´ez, S. H. Oakley, M. P. Coles and P. B. Hitchcock,
Chem. Commun., 2007, 816.
3.47 (m, 8H, hpp-CH2), 2.74 (m, 2H, hpp-CH2), 2.23–1.77 (m, 8H, hpp-
CH2). 13C NMR (CD3CN, 75.5 MHz, 298 K, coupling constants in Hz): d
159.8 (d, JPC 13, CN3), 133.9 (d, JPC 14, C6H5), 131.6 (d, JPC 2, C6H5),
12 Analysis of the dicationic component [H2C{hpp}2PPh]2+ computation-
ally, by means of geometry optimizations at a B3LYP/6-31+G** level of
theory, have identified several minima in the potential hypersurface.
Two stable conformations have been identified as corresponding to
approximate ‘chair’ and ‘twist-boat’: M. P. Coles and G. Estiu,
unpublished results. Furthermore, variable temperature NMR studies
indicate a shift in the relative integrals of the two species upon heating
[25 uC, major : minor 5 6.6 : 1; 50 uC, major : minor 5 4.2 : 1] consistent
with a shift in the position of the equilibrium between the two
conformations.
2
130.9 (d, JPC 3, C6H5), 128.9 (d, JPC 18, C6H5), 72.5 (H2C{hpp}2), 55.2
(CH2Cl2), 50.5 (d, JPC 47, hpp-CH2), 49.2, 48.8, 47.5 (hpp-CH2), 23.7 (d,
JPC 3, hpp-CH2), 21.7 (hpp-CH2). 3, 31P NMR (CD3CN, 121.4 MHz,
2
1
298 K, coupling constants in Hz): d 101.8 (d, JPP 5 570, JPtP 5 2950,
2
1
P{hpp}2Ph), 15.9 (d, JPP 5 570, JPtP 5 2530) PEt3. 4, 31P NMR
(CD3CN, 121.4 MHz, 298 K, coupling constants in Hz): d 72.5
(1JPtP 5 4877, P{hpp}2Ph). 1H NMR (CD3CN, 300.1 MHz, 298 K,
3
3
coupling constants in Hz): d 8.61 (dd, JPH 5 14.7, JHH 5 7.4, 2H,
o-C6H5), 7.83 (m, 1H, p-C6H5), 7.73 (m, 2H, m-C6H5), 7.05 (d, 2JHH 15.7,
1H, H2C{hpp}2), 4.66 (d, 2JHH 15.7, 1H, H2C{hpp}2), 3.70–3.27 (m, 14H,
hpp-CH2), 2.21–1.91 (m, 10H, hpp-CH2).
´ ˆ ´
13 Z. B. Maksic and B. Kovacevic, J. Chem. Soc., Perkin Trans. 2, 1999,
2623.
{ Crystallographic data: 2, C21H31Cl2N6P?CH2Cl2, Mr 5 554.31, crystal
dimensions 0.40 6 0.40 6 0.40 mm, orthorhombic, space group
14 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N. Kudin,
J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone,
B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson,
H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene,
X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo,
J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
R. Cammi, C. Pomelli, J. Ochterski, P. Y. Ayala, K. Morokuma,
G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski,
S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui,
A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu,
A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox,
T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara,
M. Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen,
M. W. Wong, C. Gonzalez and J. A. Pople, GAUSSIAN 03
(Revision C.02), Gaussian, Inc., Wallingford, CT, 2004. The
input coordinates were taken from the X-ray data and the true
energy minimum was confirmed by the absence of any imaginary
frequencies.
˚
Pna21 (No. 33), a 5 13.5675(3), b 5 15.3805(2), c 5 12.3987(3) A, V 5
3
2587.30(9) A , Z 5 4, rcalcd 5 1.42 Mg m23, m 5 0.54 mm21; of 25557
˚
reflections collected (3.42 , h , 26.01u), 5042 were independent
(Rint 5 0.047), R1 5 0.028, wR2 5 0.080 (data for 4845 reflections with
I
. 2s(I)), R1 5 0.030, wR2 5 0.082 (all data). 4,
C21H31Cl4N6PPt?C2H3N, Mr 5 776.43, crystal dimensions 0.15 6
0.10 6 0.10 mm, monoclinic, space group P21/c (No. 14), a 5 14.0180(5),
3
˚
˚
b 5 14.4145(4), c 5 15.2158(5) A, V 5 2811.39(16) A , Z 5 4, rcalcd
5
1.83 Mg m23, m 5 5.46 mm21; of 38517 reflections collected (3.48 , h ,
26.04u), 5528 were independent (Rint 5 0.077), R1 5 0.044, wR2 5 0.107
(data for 4763 reflections with I . 2s(I)), R1 5 0.055, wR2 5 0.112 (all
data). CCDC 644406 and 644407. For crystallographic data in CIF format,
see DOI: 10.1039/b710923g
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This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 5229–5231 | 5231