Synthesis of binuclear complexes bound in an enlarged tetraphosphamacrocycle: Two diphosphine metal units linked in front-to-front style

Article abstract of DOI:10.1021/ic9013224

A large-hole tetraphosphamacrocycle 2, with four phosphorus centers separated at the corners of a 3.7 A wide and 9.7 A long rectangle, was synthesized by a stepwise cyclization reaction between PCI-bridged [1.1]ferrocenophane and bisphenol A in a 2:2 rati

Full text of DOI:10.1021/ic9013224

7534 Inorg. Chem. 2009, 48, 7534–7536
DOI: 10.1021/ic9013224
Synthesis of Binuclear Complexes Bound in an Enlarged
Tetraphosphamacrocycle: Two Diphosphine Metal Units Linked in
Front-to-Front Style
Tsutomu Mizuta,* Yuta Inami, Kazuyuki Kubo, and Katsuhiko Miyoshi
Department of Chemistry, Graduate School of Science, Hiroshima University, Kagamiyama 1-3-1,
Higashi-Hiroshima, Hiroshima 739-8526, Japan
Received July 9, 2009
A large-hole tetraphosphamacrocycle 2, with four phosphorus
by-side, as shown in Figure 1a.3 However, if the active sites of
˚
˚
centers separated at the corners of a 3.7 A wide and 9.7 A long
rectangle, was synthesized by a stepwise cyclization reaction
between PCl-bridged [1.1]ferrocenophane and bisphenol A in a
2:2 ratio. The macrocycle 2 could incorporate two Agþ or Pt0
fragments in the hole to provide binuclear complexes, which were
identified as μ-2-[Ag(NCMe)2]2(BF4)2 (3) and μ-2-[Pt(PhCCPh)]2
(5), respectively, using X-ray and spectroscopic analysis. The
X-ray structure of 3 demonstrates that the macrocycle 2 serves as a
framework in which two diphosphine silver units are aligned in a
front-to-front style, while that of 5 indicates that 2 can also bind two
bulky Pt(PhCCPh) fragments by the flexible change of its con-
formation.
the two metal fragments face each other, with two dipho-
sphine metal fragments linked to form a macrocyclic frame-
work, as in Figure 1b, the two metal centers can be expected
to interact more directly and cooperatively with a substrate.
Such a front-to-front-type binuclear complex is known at the
active enzyme sites, and several large-azamacrocycle binuc-
lear complexes were prepared in order to mimic the front-to-
front-type functionality of such enzymes.4,5 However, a large
phosphamacrocycle that can incorporate two metal frag-
ments in its hole has not been reported to date. This is
probably largely because of the inherent air sensitivity of
most tervalent phosphorus species and the presence of many
possible stereoisomers, which clearly complicates the chem-
istry of the macrocycle complexes. On the other hand,
azamacrocycles readily invert the configuration of the nitro-
gen centers to give the most stable isomer. We report a novel
phosphamacrocycle that is designed to avoid the formation
of stereoisomers by the introduction of two doubly chelated
diphosphine units, as shown in Figure 1c.
Diphosphine ligands play a crucial role in organometallic
chemistry because of their successful application in homo-
geneous catalysis for organic synthesis and material science.1,2
On the other hand, there is also a growing interest in
polyphosphine ligands that can form binuclear organome-
tallic complexes and exhibit novel and efficient catalytic
activity. Most polyphosphine ligands are designed to form
a side-by-side-type binuclear complex, in which the two
neighboring diphosphine metal fragments are aligned side-
The phosphorus macrocycle was prepared according to the
route outlined in Scheme 1, where PCl-bridged
[1.1]ferrocenophane (1) was employed as the doubly chelated
diphosphine unit.6 Bisphonol A was first protected with
a TBS group [TBS = Si(t-Bu)Me2] at one end and then
attached to each phosphorus center of 1. After removal of the
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(b) Zalatan, J. G.; Fenn, T. D.; Herschlag, D. J. Mol. Biol. 2008, 384, 1174–1189.
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Dalton Trans. 2009, 2, 227–239. (d) Lu, Z.-L.; Liu, C. T.; Neverov, A. A.; Brown,
R. S. J. Am. Chem. Soc. 2007, 129, 11642–11652. (e) Sessler, J. L.; Tomat, E.;
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Warzeska, S.; Pritzkow, H.; Wadepohl, H.; Imhof, P.; Smith, J. C.; Kraemer, R.
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*To whom correspondence should be addressed. E-mail: mizuta@sci.
hiroshima-u.ac.jp.
(1) (a) Pignolet, L. H. Homogeneous Catalysis with Metal Phosphine
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r
pubs.acs.org/IC
Published on Web 07/14/2009
2009 American Chemical Society

Communication
Inorganic Chemistry, Vol. 48, No. 16, 2009 7535
Figure 1. Three types ofbinuclear complexes: (a) side-by-sidetype(left);
(b) front-to-front type (middle); (c) front-to-front type supported by a
doubly chelated macrocycle.
Scheme 1
Figure 2. ORTEP diagram of macrocycle 2. Ellipsoids are shown at
50%. The solvent and hydrogen atoms are omitted for clarity.
TBS group, cyclization was carried out by reaction with 1.
This stepwise procedure was necessary because the direct 2:2
reaction of 1 and bisphenol A afforded a 1:1 cyclic product as
the major product with only a trace amount of the desired 2:2
product 2. The macrocycle 2 in Scheme 1 was purified as a
BH3 adduct using preparative-scale gel permeation chroma-
tography equipment, and isolated in 13% yield after removal
of BH3 by reaction with HNEt2. 1H, 13C{1H}, and 31P{1H}
NMR spectra of 2 indicated the absence of stereoisomers and
were consistent with the high symmetry expected for the
structure of 2; four Me and C6H4 groups, four phosphorus
atoms, and eight C5H4 groups are all observed as equivalent
moieties. For example, the 31P{1H} NMR spectrum has a
sharp singlet at 112.8 ppm, and 40 cyclopentadienyl carbons
provide only five signals, three of which are observed as
triplets, because of the through-bond and space couplings
with the two phosphorus centers.7 The molecular structure of
the macrocycle 2 was confirmed by X-ray analysis, as shown
in Figure 2, where the center of inversion determines eachhalf
unit as strictly equivalent, and four phosphorus atoms form
Figure 3. ORTEP diagram of Agþ binuclear complex 3. Ellipsoids are
shown at 50%. The solvent, counterion, and hydrogen atoms are omitted
for clarity.
Macrocycle 2, with a large hole in its center, was examined
to determine whether it could accommodate two metal
fragments. The AgI ion was selected as a small metal frag-
ment for this test. A reaction mixture of 2 and 2 equiv
of AgBF4 in acetonitrile yielded a product that exhibited
a slightly broad doublet in the 31P{1H} NMR spectrum
at ambient temperature, which became a sharp multiplet
at -40 °C with P-Ag couplings (J107Ag-P = 396 Hz and
J109Ag-P = 458 Hz). The molecular structure of the silver
complex μ-2-[Ag(NCMe)2]2(BF4)2 (3) was determined by
X-ray analysis, as shown in Figure 3. Macrocycle 2 incorpo-
rates two AgI ions as expected, with two diphosphinesilver
fragments aligned in a front-to-front style. Each AgI ion
forms pseudotetrahedral geometry with two coordinated
acetonitrile ligands, which are responsible for the dynamic
behavior observed in the 31P{1H} NMR spectra.
˚
˚
a rectangle approximately 3.7 A wide and 9.7 A long. The
unit cell of the crystal 2 was found to consist of two none-
In contrast, when macrocycle 2 was reacted with d8 metal
fragments, such as PtCl2(cod) and [RhCl(cod)]2 (cod = 1,
5-cyclooctadiene), insoluble precipitates were formed,
although they could not be characterized because of their
low solubility in common solvents. On the other hand, the
reaction of 2 with Pt0 fragments afforded a soluble product.
From the reaction with Pt(PPh3)4, the 31P{1H} NMR spec-
trum of the product after workup displayed a doublet and a
˚
˚
quivalent molecules, of which the other has a 3.6 A ꢀ 10.1 A
rectangular arrangement of the four phosphorus atoms, as
shown in Figure S1 in the Supporting Information.
(7) Hierso, J.-H.; Evrard, D.; Lucas, D.; Richard, P.; Cattey, H.; Hanque,
B.; Meunier, P. J. Organomet. Chem. 2008, 693, 574-578 and references cited
therein.

7536 Inorganic Chemistry, Vol. 48, No. 16, 2009
Mizuta et al.
triplet at 162.1 and 58.1 ppm, respectively, which coupled to
one another with JP-P = 147 Hz. The former signal can be
assigned to macrocycle 2 and the latter to PPh3. These signals
had satellite signals arising from 195Pt with J195Pt-P=5064 and
195Pt-P
4538 Hz, respectively. The large J
coupling constants
and the P-P coupling patterns indicate a three-coordinate
Pt0 complex with a PtP2P0 spin system, which is consistent
with the formula μ-2-[Pt(PPh3)]2 (4), in which two Pt(PPh3)
fragments coordinate to respective diphosphine units. Struc-
ture 4 is of interest in that PPh3 is so bulky that macrocycle 2
cannot accommodate two PtPPh3 fragments in its hole. An
X-ray structure was not available for the product because of
the thermal instability of the product; however, the structure
is expected to be similar to that of a diphenylacetylene
analogue of 4 (vide infra).
Diphenylacetylene is expected to serve as an electron-
withdrawing ligand that thermodynamically stabilizes the
Pt0 complex; therefore, the reaction of 2 with Pt(PhCCPh)-
(PPh3)2 in place of Pt(PPh3)4 was carried out to give
μ-2-[Pt(PhCCPh)]2 (5) after workup. Complex 5 had a
single 31P{1H} NMR signal at 133.6 ppm with 195Pt satellites
(J195Pt-P=4053 Hz) and was stable at ambient temperature
under a nitrogen atmosphere. The molecular structure of
5 determined by X-ray analysis is given in Figure 4, where
each diphosphine unit directs its binding site to above or
below the mean plane of the macrocyclic ring, so as to bind
the bulky Pt(PhCCPh) fragments. A similar conformational
deformation of 2 is expected for the PPh3 analogue 4.
In conclusion, a large tetraphosphorus macrocycle 2 was
designed so as to incorporate two metals and avoid the
formation of stereoisomers. The prepared macrocycle 2
provided a coordination space in which the front-to-front-
type alignment was realized for a small metal fragment such
as Ag(NCMe)2. Macrocycle 2 could also accommodate a
sterically bulky fragment by flexibly changing its conforma-
Figure 4. ORTEP diagram of Pt binuclear complex 5. Ellipsoids are
shown at 50%. The solvent and hydrogen atoms are omitted for clarity.
tion. The front-to-front alignment realized with 2 is currently
being applied to a new metal-substrate interaction that is
useful for the cleavage of a thermally inactive bond.
Acknowledgment. This work was supported by Grants-
in-Aid for Scientific Research (Grants 19350032 and
19550066) from the Ministry of Education, Culture,
Sports, Science, and Technology of Japan.
Supporting Information Available: Experimental details of the
syntheses and catalyses, ORTEP drawings, and crystallographic
data for 2, 3, and 5. This material is available free of charge via
the Internet at http://pubs.acs.org.