β-trans-[N4{P(Ph)(C6H11N)} 4]4-; a tetraanionic ligand system exhibiting two separated tetradentate coordination sites of concave-shaped cavities

Article abstract of DOI:10.1039/a607055h

β-trans-[N₄{P(Ph)(C₆H₁₁NH)} ₄] can be deprotonated at all four N-H functions with BuLi in toluene yielding Li₄{β-trans-[N₄{P(Ph)(C₆H ₁₁NH)}₄]}, using an exc

Full text of DOI:10.1039/a607055h

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b-trans-[N4{P(Ph)(C6H11N)}4]42; a tetraanionic ligand system exhibiting two
separated tetradentate coordination sites of concave-shaped cavities
Alexander Steiner* and Dominic S. Wright
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, UK CB2 1EW
b-trans-[N4{P(Ph)(C6H11NH)} ] can be deprotonated at all
the reaction of excess BuLi with thf. However, this is a common
decomposition pathway of thf in the presence of strong bases
and inclusion of CH2NCHOLi, caused by fragmentation of thf,
has been reported recently.4
4
four N–H functions with BuLi in toluene yielding Li4{b-
trans-[N4{P(Ph)(C6H11NH)} ]}, using an excess of BuLi in
4
the presence of thf, the lithium enolate co-complex Li6{b-
trans-[N4{P(Ph)(C6H11N)} ](CH2NCHO)2}·4thf is formed,
The molecular structure of 3 exhibits Ci symmetry and is
composed of the centrally arranged tetraanionic ligand encapsu-
lated by two triangular Li3 faces, each capped by an O-donor
function of the monoanionic enolate moieties (Fig. 1). The
equatorial configuration of all four phenyl substituents and the
overall axial arrangement of the cyclohexylamido substituents
at the puckered chair-shaped P4N4 ring separate the donating
ligand surface into two coordination spheres. Both form
concave-shaped cavities, due to the shielding arrangement of
the surrounding organic substituents, and act in a tetradentate
way via two terminal and two ring-nitrogen centres. Fig. 2
illustrates the central core of 3, which approximately can be
described as three interlocked distorted hexagonal prisms, in
which the central one is surrounded by the P4N4 ring. Each N(r)-
function (r = ring) coordinates one and each N(t)-function
(t = terminal) coordinates two Li centres, resulting in three
different coordination patterns towards the three crystallo-
4
in
which
the
tetraanionic
ligand
b-trans-
[N4{P(Ph)(C6H11N)} ]42 exhibits two separated coordina-
4
tion sites; both act in a tetradentate way and form concave-
shaped cavities.
Multianionic ligand systems have scarcely been investigated in
metallo-organic coordination chemistry.1 Their high reactivity
and low solubility, due to their exceptional high polarity, limit
their synthetic applications in aprotic solvents. Recently we
discovered, that highly charged ligand systems can be stabilized
by introducing cyclophosphazene backbones into the ligand
cores as shown in the hexaanionic [N3P3(C6H11N)6]62
A
(C6H11 = cyclohexyl).2 Its structural and spectroscopic features
suggest, that the negative charge is stabilised by delocalisation
from the terminal amido onto the central ring-nitrogen func-
tions. The dimeric lithium complex Li12[N3P3(CyN)6]2·4thf 1
(Cy = C6H11) is soluble in aprotic solvents and therefore a
potential transfer reagent for introducing this ligand system into
a wide range of diverse metal complexes. In systems such as A,
where there is an extensive surface of donating N-centres, a
variety of metal coordination modes is expected. This paper is
concerned with manipulating the ligand sphere of these systems
by partial substitution of donating centres with organic
substituents. The introduction of non-coordinating substituents
should provide steric control and therefore a specific design of
the ligand surface. Consequently the overall equatorial arrange-
ment of the phosphorus-bonded phenyl substituents in the novel
tetraanionic ligand system b-trans-[N4{P(Ph)(C6H11N)}4]42 B,
presented here, should provide a sterically controlled separation
of coordination sites within this ligand system.
O(3a)
O(2a)
O(1a)
Li(3a)
Li(2a)
N(3a)
Li(1a)
N(4a)
N(1a)
P(1a)
N(2a)
P(2a)
P(2)
The b-trans isomer of 2,4,6,8-tetraphenyl-2,4,6,8-tetra-
kis(cyclohexylamino)cyclotetraphosphazene 1 can be obtained
with retention of configuration by the aminolysis reaction of the
N(2)
P(1)
N(1)
N(4)
corresponding
nongeminal
tetrachlorotetraphenylcyclo-
Li(1)
N(3)
teraphosphazene3 with cyclohexylamine in the presence of
triethylamine in diethyl ether in 50% yield. Further reaction
with 4 equiv. of BuLi in toluene gives almost quantitatively the
tetrametallated derivative Li4{b-trans-[N4{P(Ph)(CyN)}4]} 2.
Carrying out the reaction in thf by using an excess of BuLi
yielded a colourless crystalline material, which was suitable for
X-ray structure analysis‡ and could be identified as Li6{b-
trans-[N4{P(Ph)(C6H11N)}4](CH2NCHO)2}·4thf 3. The pres-
ence of two equivalents of lithium enolate in 3 is derived from
Li(3)
Li(2)
O(1)
C(2)
C(1)
O(2)
O(3)
Fig. 1 Molecular structure of 3. Selected bond lengths (Å) and angles (°):
P(1)–N(1) 1.626(6), P(1)–N(2) 1.635(6), P(1)–N(3) 1.624(6), P(2)–N(2)
1.603(6), P(2)–N(4) 1.622(6), P(2)–N(1a) 1.660(6), Li(1)–N(2a) 2.07(1),
Li(1)–N(4) 2.10(1), Li(1)–O(1) 1.94(1), Li(2)–N(1) 2.08(1), Li(2)–N(3)
2.06(1), Li(2)–O(1) 1.91(1), Li(2)–O(2) 1.96(1), Li(3)–N(3) 2.09(1), Li(3)–
N(4) 2.08(1), Li(3)–O(1) 2.02(1), Li(3)–O(3) 2.06(1), O(1)–C(1) 1.339(9),
C(1)–C(2) 1.28(1); N(1)–P(1)–N(2) 112.9(3), N(1)–P(1)–N(3) 103.0(3),
N(2)–P(1)–N(3) 119.5(3), N(2)–P(2)–N(1a) 110.0(3), N(2)–P(2)–N(4)
111.5(3), N(4)–P(2)–N(1a) 115.9(3), P(1)–N(1)–P(2a) 124.0(3)
P(1)–N(2)–P(2) 127.6(4), O(1)–C(1)–C(2) 128.9(9). Atoms labelled a are
generated by a centre of inversion.
6–
4–
C6H11
Ph
C6H11
Ph
–
–
C6H11 C6H11
N
N
N
–
–
N
N
–
–
P
P
P
N
A
N
P
N
P
N
N
N
N
C6H11
C6H11
Ph
P
P
Ph
N
N
–
N
–
N
–
N
–
C6H11
C6H11
C6H11
C6H11
B
Chem. Commun., 1997
283

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graphically independent Li centres; while Li(1) and Li(3) can be
seen as parts of two different six-membered heterocycles
[Li(1)–N(r)–P–N(r)–P–N(t) and Li(3)–N(t)–P–N(r)–P–N(t)],
Li(2) is chelated by a N(r)–P–N(t) unit. Four- and six-
membered heterocyclic P–N ring-systems containing s-block
metals were observed also in the presence of monoanionic
linear phosphazene ligands.5 All Li centres in 3 are further
saturated by an enolate O-function, in addition Li(2) and Li(3)
are coordinated by thf donor molecules, resulting in trigonal-
planar [Li(1)] and tetrahedral [Li(2) and Li(3)] metal environ-
ments.
throughout the P–N cores and its influence on coordination
behaviour. There is also the possibility that anion B could
accommodate two mixed-valent metal centres in its separate
cavities, interacting electronically via the P–N core.6
We thank Dr J. M. Rawson for providing a sample of
[N4{P(Ph)Cl}4]. A. S. gratefully acknowledges the EU Human
Capital and Mobility Program for a research fellowship.
Footnotes
† Selected NMR data (25 °C): 1: 1H NMR (C6D6) d 0.8–1.6 (m, CH2, C6H11
,
P
40 H), 2.6 (m, CH, C6H11, 4 H), 7.0–7.4 (m, Ph, 12 H), 8.3 (m, Ph, 8 H); 31
Structural parameters of the central P4N8 unit are comparable
with those of the P3N9 unit in 1 and suggest an equal distribution
of the negative charge onto all eight N-functions. As observed
in 1 there is no significant difference in bond lengths from the
phosphorus atoms toward the ring [av. P–N(4) 1.63 Å] and
those toward the terminally located nitrogen centres [av. P–N(t)
1.62 Å]. On the other hand, neutral amino-substituted cyclo-
phosphazenes exhibit significantly shorter P–N-ring bond
distances compared to their terminal located counterparts, as
NMR (C6D6, relative to H3PO4) d 3.2. 2: 1H NMR (C6D6) d 1.0–2.0 (m,
CH2, C6H11, 40 H), 2.8 (m, CH, C6H11, 4 H), 7.1–7.5 (m, Ph, 12 H), 8.6 (m,
Ph, 8 H); 31P NMR (C6D6, relative to H3PO4 d 24.6. 3: 1H NMR (C6D6)
d 0.8–1.8 (m, CH2, C6H11, 40 H), 1.5 (m, thf, 16 H), 2.4 (m, CH, C6H11, 4
H), 3.6 (t, thf, 16 H), 3.7–4.0 (m, CH2, OCHNCH2, 4 H), 7.0 (dd, CH,
OCHNCH2, 2 H), 7.2–7.5 (m, Ph, 12 H), 8.6 (m, Ph, 8 H); 31P NMR (C6D6,
relative to H3PO4) d 24.3.
‡ Crystal data for 3: C68H102Li6N8O6P4, Mw = 1293.10, triclinic, space
–
group P1, a = 10.969(2), b = 13.022(3), c = 14.760(3) Å, a = 64.12(3),
b = 69.62(3), g = 86.52(3)°, U = 1767.7(6) Å3, Z = 1, l = 0.71073 Å,
illustrated
in
the
structurally
related
b-trans-
T = 153(2) K, Dc = 1.215 Mg m23, m(Mo-Ka) = 0.161 mm21. Data were
[N4{P(Ph)(MeNH)}4] [P–N(r) 1.60 Å, P–N(t) 1.68 Å (av.)].3b
Ligand systems such as A and B show that there is a vast
potential in multianionic ligand systems. Coordination mode
and charge can be varied by formal successive substitution of
donating centres with steric organic groups, thus introducing a
specific design of the ligand surface. It would be interesting to
investigate the nature of delocalisation of the negative charge
–
collected on a Siemens-Stoe AED diffractometer (q–w method, 8 @ 2q @
45°) using an oil-coated rapidly cooled crystal7 of dimensions 0.4 3 0.5 3
0.5 mm. Of a total of 5662 reflections, 4574 were independent (Rint = 0.14).
The structure was solved by direct methods8 and refined by full-matrix least
squares on F2 using all data to final values of R1 = 0.084 [F > 4sF (3471
reflections)], wR2
= 0.300 (all data). [R1 = SıFo 2 Fcı/SFo and
wR2 = {Sw(Fo 2 Fc2)/Sw(Fo2)2}0.5].9 Largest features in the final
difference map, 0.491 and 20.754 e Å23. Atomic coordinates, bond lengths
and angles, and thermal parameters have been deposited at the Cambridge
Crystallographic Data Centre (CCDC). See Information for Authors, Issue
No. 1. Any request to the CCDC for this material should quote the full
literature citation and the reference number 182/335.
2
O(1a)
Li(3a)
References
Li(2a)
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Li(2)
Li(3)
Fig. 2 Core structure of 3
Received, 16th October 1996; Com. 6/07055H
284
Chem. Commun., 1997