Cyclophosphazene-supported tetranuclear copper assembly containing 15 contiguous inorganic rings

Article abstract of DOI:10.1021/ic702500n

The cyclophosphazene hydrazide gem-N₃P₃Ph ₂[N(Me)NH₂]₄ was reacted with o-hydroxybenzaldehyde to afford the multisite coordination ligand gem-N ₃P₃Ph₂[N(Me)N=CHC₆H ₄-2-OH]₄ (LH₄). The latter reacted with copper(II) salts to afford a novel tetranuclear copper assembly {N ₃P₃Ph₂[N(Me)N=CHC₆H ₄-2-O]₄Cu₂}₂, which contains, remarkably, 15 contiguous inorganic rings.

Full text of DOI:10.1021/ic702500n

Inorg. Chem. 2008, 47, 1922-1924
Cyclophosphazene-Supported Tetranuclear Copper Assembly Containing
15 Contiguous Inorganic Rings
Vadapalli Chandrasekhar,* Gurusamy Thangavelu Senthil Andavan, Ramachandran Azhakar,
and Balasubramanian Murugesa Pandian
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, India
Received December 27, 2007
The cyclophosphazene hydrazide gem-N3P3Ph2[N(Me)NH2]4 was
reacted with o-hydroxybenzaldehyde to afford the multisite coor-
dination ligand gem-N3P3Ph2[N(Me)NdCHC6H4-2-OH]4 (LH4). The
latter reacted with copper(II) salts to afford a novel tetranuclear
copper assembly {N3P3Ph2[N(Me)NdCHC6H4-2-O]4Cu2}2, which
contains, remarkably, 15 contiguous inorganic rings.
zones12 has encouraged us to apply such a methodology to
the cyclophosphazene ring systems. We had chosen cyclo-
phosphazene hydrazides for this purpose, and previously we
have reported the metalation of spiro-N3P3[O2C12H8]-
[N(Me)NH2]4.13 In all of the cases that we examined, we
were able to obtain only 2:1 (L:M) mononuclear complexes
where the central metal ion CoIII, NiII, ZnII, or CdII was bound
by two cyclophosphazene ligands, each as a non-gem-N3
donor.13 Anticipating a change in the coordination response
upon elaborating the tetrahydrazide, we prepared gem-
N3P3Ph2[N(Me)NdCHC6H4-2-OH]4 (LH4, 3) and examined
its metalation with copper(II) salts. In contrast to the
mononuclear complexes obtained in the reactions of the
parent hydrazide, metalation of LH4 with copper(II) salts
affords a novel tetranuclear copper(II) assembly, {N3P3Ph2
[N(Me)NdCHC6H4-2-O]4Cu2}2 (4). Additionally, 4 also is
structurally quite interesting because it represents a one-pot
synthesis of a polycyclic system where 15 contiguous
inorganic rings are attached to each other.
The utility of cyclophosphazenes as robust platforms for
the construction of multisite coordination ligands has been
recognized, and this area has been receiving attention in
recent years.1 This application stems from the rich nucleo-
philic substitution chemistry of chlorocyclophosphazenes.2,3
Accordingly, a large variety of ligands have been built by
appropriate substitution at the phosphorus centers of the
cyclophosphazene ring.4–11 The versatility of the cyclophos-
phazene-based ligands would be considerably enhanced if
an existing ligand can be further elaborated into a new
system. Our recent experience of acyclic phosphorus hydra-
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* To whom correspondence should be addressed. E-mail: vc@iitk.ac.in.
Fax: (+91)-512-2597436.
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1922 Inorganic Chemistry, Vol. 47, No. 6, 2008
10.1021/ic702500n CCC: $40.75
2008 American Chemical Society
Published on Web 02/14/2008

COMMUNICATION
Cu
Scheme 1
14.65 G, and A
) 29.3 G). The observed g values are
15
consistent with a dx –y ground state for the CuII ion. The
coordination environment around the two terminal Cu atoms
is comprised of two phenolate O atoms and two imino N
atoms (Figure 2a). The coordination geometry may be
described as distorted square planar, where the ligands are
arranged in a cis orientation (Figure 2). The average Cu-N
distance is 1.972(5) Å and is slightly longer than the average
Cu-O distance, 1.905(5) Å. Each of terminal Cu atoms is
part of three interconnected six-membered rings (Figure 1).
The central Cu atoms (Cu2 and Cu2*) are connected to each
other by the µ2-bridging phenolate atoms O3 and O3* (Figure
2b). The resulting Cu2O2 four-membered ring is nearly
planar.
2
2
The Cu2-O3* distance is 2.557(2) Å, and although it is
longer than a normal Cu-O distance, it is much shorter than
the sum of the van der Waals radii of Cu and O (∼2.90
Å).16 The formation of such M2O2 dimers, although not
documented in cyclophosphazene-metal complexes, is quite
well-known.17 The coordination environment around the
central Cu atoms is square-pyramidal, with the bridging O
atoms occupying the apical position.
Complexation with Cu causes the cyclophosphazene ring
to be puckered. The atoms P2 and N3 move out of the mean
plane of the N3P3 ring by 0.18 Å, while P3 and N1 are
similarly displaced in the opposite direction.
The reaction of gem-N3P3Ph2Cl4 (1) with N-methylhydra-
zine afforded gem-N3P3Ph2[N(Me)NH2]4 (2). The tetrahy-
drazide (2) is formed by a regiospecific reaction and contains
four reactive –NH2 groups.14 Condensation of 2 with
o-hydroxybenzaldehyde affords the tetrahydrazone 3 (see
Scheme S1 in the Supporting Information). Compounds 1-3
are characterized by an AX2 type of 31P{1H} NMR spectrum.
The 31P NMR chemical shift involving the reactive phos-
phorus center moves downfield and then again upfield upon
conversion of 1 into 2 and then into 3 [δ PCl2 (1) 18.3 ppm;
δ P[N(Me)NH2]2 (2) 28.1 ppm; δ P[N(Me)NdCHC6H4-2-
OH]2 (3) 13.1 ppm]. Both 2 and 3 show parent ion peaks in
their fast atom bombardment (FAB) mass spectra (see the
Supporting Information).
A remarkable feature of the tetranuclear assembly 4 is the
presence of 15 interconnected inorganic rings (all of the rings
are labeled in Figure 1). Of these, only two (the cyclophos-
phazene rings) are originally present and the rest are
simultaneously generated coincident with the assembly of
4. Of the 15 rings, one is four-membered (Cu2O2), which
has on either side of it seven six-membered rings (Figure
1). Except for the central four-membered ring, the rest are
nonplanar (see the Supporting Information). The overall
architecture of the polycyclic assembly is Z-shaped (Figure
1), with the end-to-end copper distance Cu1-Cu1* being
18.2 Å.
Variable-temperature magnetic measurements on 4 reveal
an antiferromagnetic behavior. At room temperature, the
observed µB (per Cu) is 1.92 µB. As the temperature is
lowered, a gradual decrease in ꢀMT is noticed, consistent with
antiferromagnetic behavior.
In conclusion, we have shown that modification of
cyclophosphazene ligands into more elaborate and complex
coordinating systems is feasible by an appropriate design.
In the current example, elaboration of a cyclophosphazene
tetrahydrazide affords a novel multisite coordinating ligand,
LH4, which readily generates a polycyclic tetranuclear
copper(II) assembly. The versatility and diversity of cyclo-
phosphazene-based ligands can be considerably enhanced by
The reaction of 3 with sodium hydride followed by
anhydrous CuCl2 afforded 4 in about 70% yield (Scheme
1). 4 is also formed in a direct reaction involving LH4 and
Cu(OAc)2 · H2O (see the Supporting Information). The for-
mation of 4 can be understood readily. The two hydrazone
arms present on either side of the cyclophosphazene ring
are deprotonated, and the resulting tetraanionic ligand [L]4-
encapsulates two CuII ions in a concerted coordination action
to afford LCu2, which is present in the solid state as a
centrosymmetric dimer, [LCu2]2 (Figure 1). The FAB mass
spectrum of 4 shows a peak at m/z 1009 corresponding to
[LCu2]+. The optical spectrum of 4 is characterized by the
presence of ligand absorption (230 and 266 nm), ligand
(phenolate oxygen)-metal charge transfer (383 nm), and
d-d absorption (610 nm).17a The two distinct copper sites
present in 4 are not resolved in the electron paramagnetic
resonance spectrum of 4 (recorded as CH2Cl2/toluene glass
at liquid-nitrogen temperature). An axial pattern is observed
(15) De Oliveira, M. C. B.; Scarpellini, M.; Neves, A.; Terenzi, H.;
Bortoluzzi, A. J.; Szpoganics, B.; Greatti, A.; Mangrich, A. S.; de
Souza, E. M.; Fernandez, P. M.; Soares, M. R. Inorg. Chem. 2005,
44, 921.
(16) Bondi, A. J. Phys. Chem. 1964, 68, 441.
Cu
N
with g|| > g (g|| ) 2.216, g ) 2.04, A ) 168 G, A
)
||
(17) (a) Bhadbhade, M. M.; Srinivas, D. Inorg. Chem. 1993, 32, 5458. (b)
Miyasaka, H.; Clérac, R.; Wersdorfer, W.; Lecren, L.; Bonhomme,
C.; Sugiura, K.-i.; Yamashita, M. Angew. Chem., Int. Ed. 2004, 43,
2801.
(14) Chandrasekhar, V.; Andavan, G. T. S.; Nagendran, S.; Krishnan, V.;
Azhakar, R.; Butcher, R. J. Organometallics 2003, 22, 976.
Inorganic Chemistry, Vol. 47, No. 6, 2008 1923

COMMUNICATION
Figure 1. Diamond picture of compound 4. H atoms are omitted for clarity.
appropriate elaboration strategies provided that they are not
detrimental to the stability of the inorganic ring. We are
exploring further applications of this paradigm.
Acknowledgment. We thank the Department of Science
and Technology (DST) and the Council of Scientific and
Industrial Research (CSIR), New Delhi, India, for financial
support. R.A. thanks IITK for the fellowship. B.M.P. thanks
CSIR, New Delhi, India, for a SRF. V.C. is thankful to DST
for a National J. C. Bose fellowship. V.C. is a Lalit Kapoor
Chair Professor.
Figure 2. (a) Coordination environment around the terminal atom Cu1.
(b) Coordination environment around the central atoms Cu2 and Cu2*.
Selected bond distances (Å) around Cu1 and Cu2: Cu1-O2 1.882(5);
Cu1-N5 1.927(5); Cu1-O1 1.928(4); Cu1-N7 2.016(5); Cu2-O4 1.915(4);
Cu2-N9 1.958(5); Cu2-O3 1.974(4); Cu2-N11 1.985(5); Cu2-O3*
2.553(5). Important bond angles (deg) around Cu1 and Cu2: O2-Cu1-N5
156.3(2); O2-Cu1-O1 89.6(2); N5-Cu1-O1 92.2(2); O2-Cu1-N7
94.2(2); N5-Cu1-N7 93.0(2); O1-Cu1-N7 157.79(19); O4-Cu2-N9
177.18(18); O4-Cu2-O3 88.69(19); N9-Cu2-O3 90.9(2); O4-Cu2-N11
90.8(2); N9-Cu2-N11 90.5(2); O3-Cu2-N11 160.65(18); N9-Cu2-O3*
89.01(2); N11-Cu2-O3* 116.37(2); O4-Cu2-O3* 88.17(2);
O3-Cu2-O3* 82.94(2).
Supporting Information Available: Experimental section,
crystal data, magnetic measurement data, and mean-plane informa-
tion. This material is available free of charge via the Internet at
http://pubs.acs.org.
IC702500N
1924 Inorganic Chemistry, Vol. 47, No. 6, 2008