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P.I. Richards et al. / Journal of Organometallic Chemistry 692 (2007) 2773–2779
Zn2O2 ring that is part of a Zn6O2 assembly of type f.
Zn
While in the 14-nuclear zinc complex 7 each hexaanionic
ligand hosts six ethylzinc groups to form a [(EtZn)6II] seg-
ment (Fig. 4), in the 13-nuclear zinc complex 6 each ligand
hosts only five ethylzinc groups in a monoanionic
[(EtZn)5II]À segment (Fig. 3). However, while 7 contains
a neutral Zn2O2 unit, the overall charge in 6 is restored
by accommodation of the formally dicationic [Zn3O2]2+
unit. Fig. 5 compares the coordination surfaces of segments
[(EtZn)3I] (in 2), [(EtZn)5II]À(in 6) and [(EtZn)6II] (in 7)
that are available for ZnO coordination. While [(EtZn)3I]
offers three basic N-sites and three acidic EtZn sites, there
are only three N-sites and two EtZn sites available at the
[(EtZn)5II]À segments in 6 and merely two N-sites and
two EtZn sites on offer at the segment [(EtZn)6II] in 7.
The restriction of the coordination surfaces in 6 and 7 is
due to the uptake of additional EtZn units per ligand. As
a result the size of the encapsulated ZnO moieties is
reduced to a [Zn3O2]2+ unit in 6 and a Zn2O2 unit in 7.
A closer look at the crystal structure of 6 reveals that the
phosphazenate ligands are still ‘sandwiching’ the zinc oxide
cluster, but are puckered into a half-chair conformation.
The out-of-plane N atom is bent towards the vacant site
of the [Zn3O2]2+ unit. In 7 the phosphazenate rings are
forced into a boat conformation and are bent away from
the central Zn2O2 unit, leading to a departure from the
‘sandwich’ mode observed in 2.
O
Zn
O
O
O
O
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
O
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
O
Zn
O
Zn
O
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
O
Zn
O
O
O
Zn
O
Zn
Zn
Zn
O
Zn
Zn
O
O
Zn
Zn
O
Zn
Zn
Zn
Zn
Fig. 1. Pictorial representation of oxide ions in various molecular Zn–O
assemblies.
hazenate ions II to encapsulate zinc oxide clusters. We have
combined the phosphazene (RNH)6P3N3with 1.5 equiva-
lents of H2O and 7.5 equivalents of Et2Zn, which is
required for both the full deprotonation of phosphazene
and the in situ hydrolysis of the excess Et2Zn (Scheme 2).
Although many of the reactions resulted in indistinct prod-
uct mixtures, we were able to isolate crystals suitable for X-
ray structure analysis from product mixtures in two
instances.
All Zn2+ ions in 6 and 7 adopt tetrahedral coordination
geometries. The[Zn3O2]2+ unit of 6 can be regarded as a
hexagon missing one vertex. This is reflected by its bond
angles which measure 128.44(1)° at Zn and 118.35(2)° at
O. Obviously, sharper angles are observed in the planar
four-membered Zn2O2 ring of 7, which measure 97.57(1)°
at Zn and 84.15(1)° at O. The Zn–O bond lengths involving
Zn2+ and oxide ions are somewhat shorter in 6 than in 7.
Within the [Zn3O2]2+ unit of 6, the central zinc ion is asso-
˚
In the first instance a hexane solution containing the
phosphazene (iBuNH)6P3N3, 1.5 equivalents of water and
7.5 equivalents of diethylzinc was refluxed for 12 h. In
the second instance a toluene solution of the phosphazene
(BzNH)6P3N3, 1.5 equivalents of water and 7.5 equivalents
of diethylzinc is refluxed for 12 h. The 31P NMR of both
reaction solutions showed a product mixture that con-
tained one major component as indicated by the appear-
ance of a predominant peak at 32.3 and 31.1 ppm,
respectively. Storage of the filtered solutions resulted in
the formation of single crystals among a white precipitate.
Crystal structures of both crystalline products revealed the
ciated with longer Zn–O bond distances (av. 1.98 A) com-
˚
pared to the two terminal zinc ions (av. 1.928 A). In
contrast, the planar four-membered Zn2O2 core of 7 exhib-
˚
its Zn–O bonds ranging from 2.015(1) to 2.077(1) A. On
the other hand 7 shows on average shorter Zn–O bonds
˚
towards EtZn units ranging from 1.917(0) to 1.940(0) A
compared to 6 where the EtZn–O bonds vary from
˚
1.946(5) to 2.007(4) A. These variations might be a conse-
quence of the different types of chelation of O-coordinating
EtZn units in 6 and 7. In compound 6 these are chelated
within a bidentate N(ring)–N(exo) site, while in 7 they
are solely coordinated to the ligand via one N(ring) site.
EtZn units not engaged in coordination to oxide ions in
6 and 7 are accommodated in either bidentate or tridentate
coordination sites resembling those already observed in the
previously described hexakis(ethylzinc) phosphazenate
complex, which exists as a monomer in the solid state
[12]. The variety of coordination modes and the large num-
ber of accommodated Zn centres per ligand leads to con-
siderable distortion of the ligand structure. For example
the P–N bond lengths in the P3N3 rings range by a wide
formation of the complexes Zn3O2[(iBuN)6P3N3(EtZn)5]
2
(6) and Zn2O2[(BzN)6P3N3(EtZn)6]2 (7), respectively (Table
1). Further data could not be obtained for both com-
pounds, since it proved impossible to separate a sufficient
amount of pure compound from the precipitate.
Both complexes contain zinc oxide clusters that are
encapsulated by two ethylzinc phosphazenate segments. 6
exhibits an acyclic dicationic [Zn3O2]2+ core, which is part
of a Zn7O2 assembly of type g, whereas 7 displays a neutral