Synthesis of 1,4,7-triphenyl-1,4,7-triphosphacyclononane: The first metal-free synthesis of a [9]-aneP3R3 ring

Article abstract of DOI:10.1021/ic100274m

The 1,4,7-triphenyl-1,4,7-triphosphacyclononane ([9]-aneP ₃Ph₃) macrocycle was synthesized through the reaction of lithium bis(2-phenylphosphidoethyl)phenylphosphine with 1,2-dichloroethane. [9]-aneP₃Ph₃ was subsequently coordinated to a Mo metal center and isolated as the fac-Mo([9]-aneP₃Ph₃)(CO) ₃ metal complex.

Full text of DOI:10.1021/ic100274m

4732 Inorg. Chem. 2010, 49, 4732–4734
DOI: 10.1021/ic100274m
Synthesis of 1,4,7-Triphenyl-1,4,7-triphosphacyclononane: The First Metal-Free
Synthesis of a [9]-aneP3R3 Ring
Daniel J. Lowry and Monte L. Helm*
Chemistry Department, Fort Lewis College, 1000 Rim Drive, Durango, Colorado 81301
Received February 9, 2010
The 1,4,7-triphenyl-1,4,7-triphosphacyclononane ([9]-aneP3Ph3)
macrocycle was synthesized through the reaction of lithium bis-
(2-phenylphosphidoethyl)phenylphosphine with 1,2-dichloroethane.
[9]-aneP3Ph3 was subsequently coordinated to a Mo0 metal center
and isolated as the fac-Mo([9]-aneP3Ph3)(CO)3 metal complex.
resulting in their kinetic and thermal stability, while creating
labile positions trans to the phosphorus coordination sites.
The ability to electronically and sterically tune the phos-
phorus donor atoms adds another degree of control that is
ideal for catalytic systems.
Early preparations of triphospha macrocycles were carried
out by Kyba et al. through direct solution methods; however,
only 11-membered rings containing benzyl backbones were
obtained.6 Norman et al. reported the first transition-metal
template synthesis of a [12]-aneP3H3 macrocycle in 1982.7
This was followed by the pioneering work of the Edwards
group in the template synthesis of numerous [12]-aneP3R3
derivatives that were successfully removed from the metal
template and used in subsequent metal coordination and
catalysis studies.8 Indeed, not only did the [12]-aneP3R3
macrocycles show interesting metal coordination properties,
they were also shown to be an entirely new class of homo-
geneous alkene polymerization and ROMP catalysts with
early transition metals.9
Herein we report the first direct synthesis of a tripho-
sphacyclononane macrocycle without the use of a transi-
tion-metal template. Cyclononane macrocycles containing
nitrogen (i.e., tacn, [9]-aneN3H3) and sulfur ([9]-aneS3) are
well-known and have a rich transition-metal coordination
chemistry.1,2 For example, transition-metal complexes of tacn
are used as functional models for metalloenzyme active sites
and in numerous catalytic organic conversions.3,4 The phos-
phorus derivatives of the cyclononane macrocyclic com-
pounds, however, have remained absent from study because
no effective route to their synthesis has been reported.
Given the widespread use of phosphorus ligands in transi-
tion-metal homogeneous catalysis, it can be expected that
triphosphacyclononane compounds hold substantial poten-
tial in this area.5 For example, the triphosphacyclononane
compounds have the ability to facially cap metal centers,
Although the [12]-aneP3R3 macrocycles showed usefulness
for metal coordination and catalysis, the larger ring size is not
ideal for a maximum metal-chelating stability. The [9]-aneP3R3
(6) Kyba, E. P.; John, A. M.; Brown, S. B.; Hudson, C. W.; McPhaul,
M. J.; Harding, A.; Larsen, K.; Niedzwiecki, S.; Davis, R. E. J. Am. Chem.
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(7) Diel, B. N.; Haltiwanger, R. C.; Norman, A. D. J. Am. Chem. Soc.
1982, 104, 4700–4701.
*To whom correspondence should be addressed. E-mail: helm_m@
fortlewis.edu.
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J. Chem. Soc., Dalton Trans. 1995, 1139–1145. Coles, S. J.; Edwards, P. G.;
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Trans. 1997, 193–197. Edwards, P. G.; Ingold, F.; Coles, S. J.; Hursthouse, M. B.
Chem. Commun. 1998, 545–546. Jones, D. J.; Edwards, P. G.; Tooze, R. P.;
Albers, T. J. Chem. Soc., Dalton Trans. 1999, 1045–1046. Edwards, P. G.;
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r
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Published on Web 05/04/2010
2010 American Chemical Society

Communication
Inorganic Chemistry, Vol. 49, No. 11, 2010 4733
derivatives have a more idealized geometry for metal coordina-
tion, as illustrated in both aza- and thia-crown chemistry.
Consequently, where there are currently over 50 reports of
the [12]-aneS3 ligand in the literature, the [9]-aneS3 ligand
appears in over 160 papers.
To date, there are only four literature reports of 9-mem-
bered triphospha macrocyclic compounds, all of which were
synthesized using FeII as a templating metal center.10 Only
the 2000 Communication and 2006 full report include the
synthesis of triphosphacyclononane with a hydrocarbon
backbone for the 9-membered ring.10a,b In all cases, however,
the triphospha macrocyclic compounds could not be liber-
ated from the transition-metal template in a form useful for
subsequent metal coordination and catalysis studies.
Our method represents the first direct synthesis of a [9]-
aneP3R3 ligand without the use of a transition-metal tem-
plate. This synthesis allows for future metal coordination and
catalysis studies to be conducted on this important macro-
cycle. Subsequent metal coordination of the newly synthe-
sized [9]-aneP3Ph3 ligand to form a fac-Mo([9]-aneP3Ph3)-
(CO)3 metal complex illustrates the transition-metal coordi-
nating ability of this novel compound.
Figure 1. Isomers of the [9]aneP3Ph3 ring.
subsequent isolation of the metal complex were accom-
plished. The reaction of a [9]-aneP3Ph3 solution with (cht)-
Mo(CO)3 (cht = cycloheptatriene) resulted in the formation
of the fac-Mo([9]-aneP3Ph3)(CO)3 metal complex (eq 2). The
31P{1H} NMR of the reaction
The synthesis of [9]-aneP3Ph3 was accomplished through
the reaction of lithium bis(2-phenylphosphidoethyl)phenyl-
phosphine with 1,2-dichloroethane (eq 1).11 The reaction was
carried out under low-concentration and high-temperature
conditions, which are essential for obtaining the intramole-
cular ring-closed product over intermolecular oligomeric and
polymeric materials.
mixture shows the disappearance of the peaks attributed
to the [9]-aneP3Ph3 ring and the appearance of a new
singlet at 94.0 ppm. The downfield shift observed in the
31P{1H} NMR spectra is indicative of metal coordination
by the phosphorus atoms. After purification, fac-Mo([9]-
aneP3Ph3)(CO)3 was obtained in an overall 68% yield and
fully characterized through spectroscopic and single-
crystal X-ray analysis. Interestingly, the 31P{1H} NMR
integration and reaction yield indicate that both the syn-
syn and syn-anti isomers react to form the facially
capped metal complex. Apparently, the barrier for inver-
sion around the phosphorus center is low enough to be
overcome by the strength of metal chelation.
The X-ray structure of fac-Mo([9]-aneP3Ph3)(CO)3
(Figure 2) shows a distorted octahedral environment around
the molybdenum metal center consisting of the facially
capping [9]-aneP3Ph3 and three carbonyl ligands in mutually
cis positions. The structural distortion around the molybde-
num metal revolves mainly around the [9]-aneP3Ph3 ring,
which shows acute P-Mo-P bite angles that average
78.42(5)° and trans C-Mo-P angles that average 170.0(2)°.
The average C-Mo-C bond angles, however, are 92.9(2)°,
illustrating that the carbonyl ligands maintain close to an
idealized octahedral position around the metal center.
Synthesis of the [9]-aneP3Ph3 ring results in the formation
of two isomers, as shown in Figure 1. The syn-syn isomer
appears as a singlet at -15.9 ppm in the 31P{1H} NMR
spectrum, where the syn-anti isomer yields a double-triplet
(2:1) pairing (3JPP = 4.7 Hz) at -16.3 and -17.1 ppm,
respectively. Integration of the 31P{1H} NMR spectrum of
the reaction mixture shows that the [9]aneP3Ph3 ring is
formed in a 85% overall yield, with a 3:7 ratio of the syn-syn
to syn-anti isomers, respectively. The remaining 15% of
peaks in the 31P{1H} NMR spectrum appear as broad,
unresolved resonances, most likely because of the heavier
oligomer and polymer material.
Separation of the [9]aneP3Ph3 rings from the reaction
mixture was accomplished through solvent extraction, and
1
the [9]aneP3Ph3 ring was further characterized through H
and 13C{1H} NMR.
To obtain unequivocal structural characterization of the
[9]-aneP3Ph3 ring, coordination to a transition-metal and
(10) (a) Edwards, P. G.; Newman, P. G.; Malik, K. M. A. Angew. Chem.,
Int. Ed. 2000, 39, 2922–2924. (b) Edwards, P. G.; Haigh, R.; Li, D.; Newman,
P. D. J. Am. Chem. Soc. 2006, 128, 3818–3830. (c) Edwards, P. G.; Whatton,
M. L. Dalton Trans. 2006, 442–450. (d) Albers, T.; Edwards, P. G. Chem.
Commun. 2007, 858–860.
(11) Mason, L. J.; Moore, A. J.; Carr, A.; Helm, M. L. Heteroat. Chem.
2007, 18, 675–678.
Figure 2. X-ray structure of Mo([9]-aneP3Ph3)(CO)3.

4734 Inorganic Chemistry, Vol. 49, No. 11, 2010
Lowry and Helm
The bond distances in the fac-Mo([9]-aneP3Ph3)(CO)3
structure show that [9]-aneP3Ph3 is bound more tightly to
the metal center than its [12]-aneP3R3 counterparts. The
average Mo-P and Mo-C bond distances in fac-Mo([9]-
1813 cm-1 12
.
The IR data for fac-Mo([9]-aneP3Ph3)(CO)3
shows ν(CO) frequencies of 1902 and 1795 cm-1. The
stronger bonding of the [9]-aneP3Ph3 ring (relative to [12]-
aneP3R3) results in better σ donation to the metal and, hence,
increased π-back-bonding to the carbonyl ligands, support-
ing the X-ray crystallographic data.
˚
aneP3Ph3)(CO)3 are 2.459(2) and 1.949(7) A, respectively. In
four previously reported fac-Mo([12]-aneP3R3)(CO)3 struc-
tures, the average Mo-P and Mo-C bond distances are
In summary, we have successfully synthesized the first [9]-
aneP3R3 phospha-crown compound without the use of a
transition-metal template. The [9]-aneP3Ph3 ring was bound
to a molybdenum metal center, which demonstrated greater
structural distortion and stronger binding than the [12]-
aneP3R3 analogues. Work continues in our laboratory to
separate the two [9]-aneP3Ph3 ring isomers and to conduct
additional metal coordination studies.
2.497(1) and 1.972(1) A, respectively.12 Further, the average
˚
C-O bond distances in fac-Mo([9]-aneP3Ph3)(CO)3 are
˚
1.169(6) A, whereas those in the fac-Mo([12]-aneP3R3)(CO)3
˚
structure average 1.156(1) A. The shorter Mo-P bond
distances in the [9]-aneP3Ph3 structure indicate that the ring
is more tightly bound than that in the [12]-aneP3R3 com-
plexes. This is additionally supported by the fact that our [9]-
aneP3Ph3 structure shows an increase in π-back-bonding to
the carbonyl ligands, illustrated by the shorter Mo-C and
longer C-O bonds observed in the fac-Mo([9]-aneP3Ph3)-
(CO)3 structure over the fac-Mo([12]-aneP3R3)(CO)3 coun-
terparts.
Acknowledgment. This work was supported by the
ACS-PRF (42268-GB3). The NMR instrument used for
this work was purchased from funding by the NSF-MRI
program (0520618). We thank Joseph R. Smyth in the
Geosciences Department at the University of Colorado,
Boulder, CO, for collecting single-crystal X-ray data.
The IR data for fac-Mo([9]-aneP3Ph3)(CO)3 also supports
the observation of increased π-back-bonding to the carbonyl
ligands relative to the [12]-aneP3R3 complexes. In 10 reported
[12]-aneP3R3Mo complexes, the ν(CO) frequencies range
between 1954 and 1910 cm-1 and between 1864 and
Supporting Information Available: Further details of the
synthesis and characterization data. This material is available
free of charge via the Internet at http://pubs.acs.org. CCDC
764783 contains the supplementary crystallographic data for
fac-Mo([9]-aneP3Ph3)(CO)3. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
http://www.ccdc.cam.ac.uk/data_request/cif.
(12) Diel, B. N.; Brandt, P. F.; Haltiwanger, R. C.; Hackney, M. L. J.;
Norman, A. D. Inorg. Chem. 1989, 28, 2811–2816. Coles, S. J.; Edwards, P. G.;
Fleming, J. S.; Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1995, 4091–
4097. Edwards, P. G.; Ingold, F.; Liyanage, S. S.; Newman, P. D.; Wong, W.-K.;
Chen, Y. Eur. J. Inorg. Chem. 2001, 2865–2869.