Organophosphazenes. 25. The synthesis and electrochemistry of the dicobalt hexacarbonyl complexes of phenylethynylcyclophosphazenes

Article abstract of DOI:10.1139/v02-138

The syntheses of the dicobalt hexacarbonyl complexes, N₃P₃F_6-nC≡CPhCo₂(CO) ₆)_n (n = 1 (3), n = 2 (4), is reported. The introduction of the organometallic fragment allows for simplification of the NMR spectra and separation of the isomers of the disubstituted (4) derivatives. Electrochemical studies show that 3 undergoes a reversible one-electron reduction. At 233 K, the geminal isomer of 4 undergoes two separate reversible one-electron reductions. The ESR spectra of the radical anions of 3 and 4 have been obtained and show the absence of delocalization of unpaired spin density from the organometallic cluster to the phosphazene.

Full text of DOI:10.1139/v02-138

1393
Organophosphazenes. 25. The synthesis and
electrochemistry of the dicobalt hexacarbonyl
complexes of phenylethynylcyclophosphazenes1
Maneesh Bahadur, Christopher W. Allen, William E. Geiger, and Adam Bridges
Abstract: The syntheses of the dicobalt hexacarbonyl complexes, N3P3F6–n(CϵCPhCo2(CO)6)n (n = 1 (3), n = 2 (4)), is
reported. The introduction of the organometallic fragment allows for simplification of the NMR spectra and separation
of the isomers of the disubstituted (4) derivatives. Electrochemical studies show that 3 undergoes a reversible one-
electron reduction. At 233 K, the geminal isomer of 4 undergoes two separate reversible one-electron reductions. The
ESR spectra of the radical anions of 3 and 4 have been obtained and show the absence of delocalization of unpaired
spin density from the organometallic cluster to the phosphazene.
Key words: cyclophosphazenes, cobalt–alkyne clusters, electrochemistry, ESR.
Résumé : On a réalisé la synthèse de complexes de dicobalt hexacarbonyle, N3P3F6–n(CϵCPhCo2(CO)6)n (n = 1 (3),
n = 2 (4)). L’introduction du fragment organométallique simplifie les spectres RMN et la séparation des isomères des
dérivés disubstitués (4). Des études électrochimiques montrent que le composé 3 subit une réduction réversible à un
électron. À 233 K, l’isomère géminé du composé 4 subit deux réactions séparées de réductions réversibles à un
électron. Les spectres RPE des anions radicaux des composés 3 et 4 ont été mesurés et ils mettent en évidence
l’absence de délocalisation de la densité de spin non pairée de l’agrégat organométallique vers le phosphazène.
Mots clés : cyclophosphazène, agrégats cobalt–alcyne, électrochimie, RPE.
[Traduit par la Rédaction] Bahadur et al. 1397
Introduction
alkylethynyl (11), and mixed aryl-phenylethynyl derivatives
(12). Indirect routes involve catalyzed rearrangements of
propargyl derivatives (13) and byproducts in the reaction of
lithiopropene with N3P3F6 (14). The electronic structure of
these materials has been probed by UV–photoelectron spec-
troscopy (15). In this paper, we present the results of a study
of the synthesis and redox processes of the dicobalt hexa-
carbonyl derivatives of phenylethynylfluorocyclotriphos-
phazenes. These investigations provide additional insights
into the nature of the interactions between unsaturated or-
ganic substituents and the cyclophosphazene ring. A prelimi-
nary report of certain aspects of this work has appeared (16).
The reactions of main group organometallic reagents with
halocyclophosphazenes has lead to the formation of a broad
array of organophosphazene derivatives (1, 2). Within this
class of compounds, the aryl derivatives have attracted the
most attention (1, 3–5). The ethenyl derivatives have been
widely exploited as monomers in olefin addition polymeriza-
tion reactions leading to carbon chain polymers with cyclo-
phosphazene substituents (6–8). By way of contrast,
significantly less interest has been shown in the corresponding
ethynyl derivatives, in which a direct phosphorus — ethynyl
carbon bond exists. The first examples of this class of com-
pounds, N3P3F6–n (CϵCPh)n (n = 1,2), and the dicobalt
hexacarbonyl derivative of N3P3F5CϵCPh were reported by
Chivers (9). Utilization of the reactions of lithioethynyl re-
agents with N3P3F6 also provides trimethylsilylethynyl (10),
Experimental
Materials
Received 26 January 2002. Published on the NRC Research
Press Web site at http://canjchem.nrc.ca on 23 September 2002.
Hexachlorocyclotriphosphazene (N3P3Cl6) was converted
to N3P3F6 (17), which in turn was converted to
N3P3F5(CϵCPh) (1), N3P3F4(CϵCPh)2 (2) (9, 10), and
P3N3F5C6H4CMe=CH2 (5) by previously reported procedures.
Butyllithium (2.5 M in hexane) and phenylacetylene were
obtained from Aldrich Co. Dicobalt octacarbonyl (Johnson
Matthey) was sublimed and used immediately. All manipula-
tions were performed under nitrogen. All solvents were dis-
tilled from the appropriate drying agents prior to use, and
further purification of the solvents for electrochemical and
ESR experiments was achieved by three freeze–pump–thaw
cycles followed by vacuum distillation.
This paper is dedicated to Professor Tris Chivers who has
continually demonstrated the ability to produce outstanding
chemistry while retaining the highest standards of collegiality
and personal integrity.
M. Bahadur, C.W. Allen,2 W.E. Geiger, and A. Bridges.
The Department of Chemistry, University of Vermont,
Burlington, VT 05405–0125, U.S.A.
1Part 24. C.W. Allen and S.D. Worley. Inorg. Chem. 38,
5187 (2000).
2Corresponding author (email: cwallen@zoo.uvm.edu)
Can. J. Chem. 80: 1393–1397 (2002)
DOI: 10.1139/V02-138
© 2002 NRC Canada

1394
Can. J. Chem. Vol. 80, 2002
the material was chromatographed on silica gel with hexane
and methylene chloride (90:10) as the eluent. Two red frac-
tions were isolated, which were shown to be the geminal
isomer and the cis–trans non-geminal mixture. The pure
geminal isomer was obtained as dark red crystals from hex-
ane solution at –20°C (0.89 g, 40% yield). The melting point
of the geminal isomer was found to be greater than 225°C.
Attempts to crystallize or sublime either of the non-geminal
isomers were unsuccessful, nor could the isomers be sepa-
rated. A yield of 0.14 g (15%) was obtained. Anal. calcd. for
C28H10F4N3P3O12Co2: C 34.12, H 1.01, MW 985; found:
(geminal) C 34.21, H 0.92. Molecular ion M+ was not ob-
served in either EI- or CI-MS.
Measurements
Infrared spectra were recorded on a PerkinElmer 1430
spectrophotometer. Mass spectrometry was carried out on a
Finnigan 4610 spectrometer operating at 70 eV. NMR spec-
tra were recorded on a Bruker WM250 spectrometer operat-
ing at: 250.1 MHz (1H); 62.9 MHz (13C); 235.35 MHz (19F);
101.2 MHz (31P) and using CDCl3 for a lock compound.
Tetramethylsilane (TMS) (1H and 13C) was used as an inter-
nal reference, while 85% H3PO4 (31P) and CFCl3 (19F) were
used as external references. Chemical shifts upfield from the
1
reference are negative. Broad band H decoupling was used
for the 13C, 19F, and 31P spectra. ESR measurements were
performed on a modified Varian E-3 spectrometer with ex-
ternal diphenylpicrylhydrazide (DPPH) as a standard. Ele-
mental analyses were performed by Robertson Laboratory,
Inc. Electrochemical measurements were carried out using a
Princeton Applied Research Model 173 potentiostat with a
Hewlett-Packard 7004B x-y recorder as described in the lit-
erature (18). Solutions for cyclic voltammetry (CV) con-
tained 0.5 mM of the metal complex and also contained
0.1 M Bu4NPF6 as the supporting electrolyte in THF. Poten-
tials are reported vs. Ag/AgCl. Low-temperature CV solu-
tions contained 0.1 M Bu4NCF3SO3 as the supporting
electrolyte. Low temperature was achieved by immersing the
voltammetry cell in a dry ice – acetone slush bath. For rotat-
ing platinum electrode (RPE) measurements, a Sargent syn-
chronous rotor (1800 rpm) was employed.
Geminal
IR (KBr pellet) (cm–1): 2098, 2077, 2067, 2044, 2031,
2026, 2008 (ꢂterminal CO), 1562 (ꢂcϵc), 1476, 1439, 1242
(ꢂas PNP), 1184, 926, 915 (sh) (ꢂas PF), 811 ((ꢂsym PF); hexane
solution), 2018, 2032 (sh), 2040, 2071, 2097 (ꢂterminal CO).
13C NMR ꢁ: 99.5 (ϵP[CϵC C6H5Co2(CO)6]2), 136.5 (ϵP[C
ϵC-C6H5Co2(CO)6]2), 130.0, 129.0, 128.8 (C6H5), 197.3
(Co2(CO)6). 19F NMR ꢁ: 72.9 (d, JFP = 889 Hz, ϵPF2).
2
31P NMR ꢁ: 8.2 (t, JPF = 892 Hz, ϵPF2), 32.5 (t, JPP
=
50.1 Hz, ϵP[CϵCC6H5Co2(CO)6]2). MS (molecular ion
([M+]) not observed), m/e (%): 901 ([M – 3CO]+, 10), 845
([M – 5CO]+, 1.7), 817 ([M – 6CO]+, 6.6), 789 ([M –
7CO]+, 20.8), 761 ([M – 8CO]+, 26), 733 ([M – 9CO]+, 6),
705 ([M – 10CO]+, 15), 677 ([M – 11CO]+, 8), 649 ([M –
12CO]+, 100).
Syntheses of (dicobalt hexacarbonyl phenylethynyl)
pentafluorocyclotriphosphazene (N3P3F5CϵCPhCo2(CO)6)
(3)
Cis non-geminal
19F NMR ꢁ: 68.4 (d, JFP = 912 Hz, ϵPF2[trans]), 73.2 (d,
JFP = 912 Hz, ϵPF2[cis]), 46.5 (d, JFP = 962 Hz,
This preparation is a modification of a previously reported
procedure (9). (ꢀ-Phenylethynyl)pentafluorocyclotriphospha-
zene (N3P3F5CϵCPh, 1) (0.74 g, 2.25 mmol), and dicobalt
octacarbonyl (0.78 g, 2.25 mmol), were allowed to react in
hexane (40 mL) in a nitrogen filled flask connected to a gas
bubbler system. The reaction mixture was stirred at room
temperature. The reaction was monitored by IR spectroscopy
and found to be complete within 30 min. After the solvent
was removed, the material was chromatographed on silica
gel with hexane and methylene chloride (90:10 v/v) as the
eluent. The product eluted as a red band. The red compound
was isolated and crystallized from hexane as red micro-
crystalline material. MW: calcd. 616; found 617 (CI-MS).
13C NMR ꢁ: 99.5 ([CϵCC6H5·Co2(CO)6]2), 136.5
([CϵCC6H5Co2(CO)6]2), 130.0, 129.0, 128.8, (C6H5), 197.3
ϵPF(CϵCC6H5Co2(CO)6]). 31P NMR ꢁ: 6.2 (t of t, JPF
=
2
997 Hz, JPP = 78 Hz, ϵPF2), 36.1 (d of d, JPF = 979 Hz,
2JPP = 70 Hz, ϵPF[CϵCC6H5Co2(CO)6]). MS, cis-trans
mixture (molecular ion ([M+]) not observed), m/e (%): 901
([M – 3CO]+, 10), 845 ([M – 5CO]+, 1.7), 817 ([M – 6CO]+,
6.6), 789 ([M – 7CO]+, 20.8), 761 ([M – 8CO]+, 26), 733
([M – 9CO]+, 6), 705 ([M – 10CO]+, 15), 677 ([M –
11CO]+, 8), 649 ([M – 12CO]+, 100).
Trans non-geminal
19F NMR ꢁ: 72.3 (complex d, JFP = 888 Hz, ϵPF2), 52.8
(complex d, JFP = 937 Hz, ϵPF[CϵCC6H5Co2(CO)6]). 31P
NMR ꢁ: 6.2 (t, JPF = 997 Hz, ϵPF2), 34.8 (d of d, JPF =
2
(Co2(CO)6). 19F NMR ꢁ: 57.4 (d, JPF
= 974 Hz,
944 Hz, JPP = 70 Hz, ϵPF[CϵCC6H5Co2(CO)6]).
ϵPF[CϵCC6H5Co2(CO)6]), 72.9 (d, JPF = 919 Hz, PF2[cis]),
72.2 (d, JPF = 946 Hz, ϵPF2[trans]). 31P NMR ꢁ: 9.4 (t, JPF
=
Attempted reaction of the lithium salt of
hexacarbonyl(phenylethyne)dicobalt
2
929 Hz, ϵPF2), 38.0 (d of t, JPF = 975 Hz, JPP = 68.8 Hz,
The reaction of LiCϵCPhCo2(CO)6, generated from lith-
ium diisopropylamide and HCϵCPhCo2(CO)6, with an
equimolar quantity of N3P3F5C6H4CMe=CH2 in hexane, re-
sulted in a dark green solution, from which a green solid
was isolated. The absence of detectable 31P NMR resonances
indicated that the phosphazene derivative was not formed.
ϵPF[CϵC6H5Co2(CO)6]).
Synthesis of bis(dicobalt hexacarbonylphenylethynyl)-
tetrafluorocyclotriphosphazene (2,2-
N3P3F4(CϵCPhCo2(CO)6)2) (4)
Bis(phenylethynyl)tetrafluorocyclotriphosphazene (N3P3F4[C
ϵCPh]2) (0.7 g, 2.25 mmol), and dicobalt octacarbonyl
(1.55 g, 4.5 mmol), were allowed to react in hexane (60 mL)
in a nitrogen filled flask connected to a gas bubbler system.
The reaction mixture was stirred at room temperature for
30 min and monitored by IR. After the solvent was removed,
Chemical reduction of 3 and 4
The reduction was carried out on a vacuum line with
0.75 mmol of 3, or the geminal isomer of 4, and 0.18 mg
(0.75 mmol) of cobaltocene, in an ESR tube. The ESR tube
© 2002 NRC Canada

Bahadur et al.
1395
was connected to the vacuum line and evacuated. Approxi-
mately 0.5 mL of 2:1 tetrahydrofuran–dichloromethane (v/v)
was introduced by vacuum transfer, and subsequently the
ESR tube was sealed off. After thawing the sample, the reac-
tion was carried out at –65°C (in a dry ice – acetone slush
bath) for less than 5 min, and then the sample was frozen in
liquid nitrogen. The ESR spectra were then immediately re-
corded at liquid nitrogen temperature.
(10) to 3 showed that the alkynylphosphorus center 31P reso-
nance undergoes a 32 ppm downfield shift while the ϵPF2
centers only change by about 2 ppm. This leads to a consid-
erable simplification in the spectrum in 3, since in 1 the two
phosphorus shifts are only separated by 2 ppm. The intra-
ring phosphorus–phosphorus coupling constant (2JPP) de-
creased by 32 Hz. Similar changes were observed in a com-
parison of the geminal isomer of 2 to 4g, wherein the
corresponding bis(alkynyl)phosphorus resonance underwent
a 66 ppm downfield shift and 2JPP decreased by 41 ppm. The
NMR spectra of the 4ng isomers were also sufficiently sim-
plified so that the principle parameters could be measured
and reported. The factors controlling the chemical shifts in
cyclophosphazenes are complex and not well understood
(20). In the present case, a change of the exocyclic bond an-
gle at the phosphorus center or an introduction of a signifi-
cant paramagnetic term to the shift tensor from the transition
Results and discussion
The reaction of dicobalt octacarbonyl with 1 (9) and 2
lead cleanly to the respective dicobalt hexacarbonyl – alkyne
cluster derivatives.
[1]
N3P3F5CϵCPh + Co2(CO)8 (1)
J N3P3F5(CϵCPhCo2(CO)6) (3)
N3P3F4(CϵCPh)2 + Co2(CO)8 (2)
J N3P3F4(CϵCPhCo2(CO)6)2 (4)
2
metal component are both viable proposals. The JPP values
[2]
in
cyclophosphazenes
are
controlled
by
the
electronegativities of the phosphorus substituents (20). The
carbon atoms of the phosphazene substituent change, from
approximate sp hydridization in 1 and 2, to a lower s charac-
ter in the organometallic derivatives, and hence the orbital
electronegativity decreases, leading to the observed decrease
An alternative route to the synthesis of dicobalt
hexacarbonyl – alkyne complexes, via the reactions of
LiCϵCPhCo2(CO)6 (19), was also attempted. All attempts to
isolate cyclophosphazene derivatives failed. The major prod-
uct was a green solid, which is believed to be an alkyne cou-
pling product. A similar product was observed in the reactions
of LiCϵCSiMe3Co2(CO)6 with electrophiles, and was shown
to be a dimer, Co2(CO)6Me3SiCϵC-CϵCSiMe3Co2(CO)6
(19).
2
in JPP.
The IR spectra for complexes 3 (9) and 4g showed five
CO bands between 2000 and 2110 cm–1. The IR spectrum
for 3 (9) showed a PN stretching band at 1269 cm–1, while
the PN stretching band for 4g appeared at 1242 cm–1. The
PN stretching frequency showed a steady decrease in going
from P3N3F6, to 3, to 4g, which reflects an increased elec-
tron donation to the phosphazene ring by the [CϵC-
C6H5Co2(CO)6] substituent, in comparison to the fluorine
atom (21). The change in the ꢄCϵC absorbance from 2
(2139 cm–1) to 4 (1562 cm–1) indicates a significant reduc-
tion in bond order, and hence in orbital electronegativity.
The mass spectrum of 3 showed the parent ion and the peaks
associated with the sequential loss of the six carbonyl groups.
By way of contrast, in both 4g and 4ng, the [M – 3CO]+
fragment is the highest mass peak, followed by the [M –
5CO]+ ion. The base peak was the [M – 12CO]+ ion.
The electrochemistry of alkyne – dicobalt hexacarbonyl
clusters (Co2(CO)6( -RC2R) (R = CF3, Bu, Ph)) has been
examined in some detail (22–24). This background provides
a frame of reference for understanding the electrochemical
behavior of 3 and 4. The cyclic voltammogram (CV) of 3
showed a reversible reduction at E° = ꢅ0.64 V (298 K). The
anodic to cathodic (ia/ic) current ratios were unity over a
scan rate range of 40ꢅ400 mV s–1. Comparison of CV peak
separations ( Ep), and ip/ꢄ1/2 with those of ferrocene, under
the same experimental conditions,3 established the process
as diffusion-controlled, leading to the formation of 3–. A
second irreversible reduction was observed at –1.67 V. If one
scans beyond the second reduction, the reverse scan reveals
an anodic peak at ꢅ0.98 V, presumably due to a decomposi-
tion product of 32–. Low temperature CV studies did not
show any change in the behavior of 3, except for shifts in the
E° values (E° (3) = –0.55 V at 233 K) and smaller diffusion
Previous NMR studies have shown that 2 consists of a
mixture of inseparable isomers, with the geminal (2,2-) spe-
cies being in large excess over the non-geminal (2,4-) com-
ponents (10). The addition of the dicobalt hexacarbonyl
fragment allowed for the chromatographic separation of the
geminal (4g) and non-geminal isomers (4ng). The cis and
trans isomers of 2,4-[(ꢃ-methylethenyl)phenylethynyl]tetra-
fluorocyclotriphosphazene have also been separated as their
dicobalt hexacarbonyl complexes (12). These studies show
the applicability of metal complex formation to the often in-
tractable problem of separation of cyclophosphazene regio-
and stereoisomers. The addition of the dicobalt hexacarbonyl
fragment to the alkyne also had a significant effect on the
31P NMR spectra. A comparison of the NMR spectra of 1
3 The variable scan rate data for 3 and 4, Tables D1 and D2, can be found in the supplementary material. Supplementary data may be pur-
chased from the Depository of Unpublished Data, Document Delivery, CISTI, National Research Council Canada, Ottawa, ON K1A 0S2,
Canada (http://www.nrc.ca/cisti/irm/unpub_e.shtml for information on ordering electronically).
© 2002 NRC Canada

1396
Can. J. Chem. Vol. 80, 2002
Fig. 1. Cyclic voltammogram of 4g at 233 K.
Fig. 2. Frozen solution ESR spectrum of the radical anion of 3.
currents.3 The room temperature CV for 4g showed two
closely spaced reduction waves in the potential range where
only one was observed for 3. The first process (E° = ꢅ0.69 V)
is reversible whereas the second (E = –1.08 V) is irrevers-
ible. The second process gives a follow-up product with an
irreversible oxidation peak at 0.20 V, which was assigned to
phenyl group are counterbalanced to produce radicals of in-
termediate stability. The first reduction of 4g takes place at a
more negative potential compared with 3, and the radical an-
ion 3– is more stable than 4g–. Both of these observations
are a reflection of the decrease in the group electronegativity
of the phosphazene substituent as a result of the replacement
of a fluorine atom with the more electron donating alkyne –
cobalt carbonyl cluster on going from 3 to 4g.
The moderate stability of the radical anions allows for
their characterization by ESR spectroscopy in frozen solu-
tions. Chemical reduction was accomplished using cobaltocene
(Cp2Co) as the reductant. The ESR spectra of both 3– and
4g– showed 15 lines from hyperfine coupling to the two
equivalent cobalt (I = 7/2) nuclei (Fig. 2). Hyperfine cou-
pling to phosphorus was not observed. A first-order analysis
yielded values of g1 = 2.04 and a1 = 67.8 G. These values
are higher than those obtained from similar complexes that
were interpreted in terms of anisotropic g and cobalt
hyperfine tensors (27). The absence of phosphorus hyperfine
coupling indicates that the unpaired electron is localized on
the organometallic fragment and not delocalized to the phos-
phazene. The ultraviolet photoelectron spectra of alkynyl
phosphazenes have also been interpreted as showing absence
of ꢆ donation of the alkyne to the phosphazene (15). Thus,
alkynylphosphazenes and their organometallic complexes are
similar to alkenyl (15, 28) and arylphosphosphazenes (5, 28,
29) in that the carbonꢅcarbon ꢆ electron density is localized
on the organic exocyclic substituent.
–/0
Co(CO)4 (25). Other features in the potential range of 1.0
to –2.0 V are equivalent to those observed for 3. There are,
however, differences in the stabilities and formal potentials
for formation of 3– and 4g–. For 4g, the ia/ic ratio increased
from 0.66 at 40 mV s–1 to 0.96 at 400 mV s–1. This indicates
that the 4g0/1– redox process is only chemically reversible at
the higher scan rates. Assuming that the decomposition of
4g– is a first-order process, a half-life of approximately 6 s
can be calculated. The dianion, 4g2–, has an even smaller
half-life. The decomposition step of 4g2– can be avoided by
a reduction in temperature. At 233 K, the CV of 4g showed
two reversible reduction steps, at ꢅ0.61 V and ꢅ0.94 V, for
the 4g0/– and 4g–/2– steps, respectively (Fig. 1). Scans of the
waves with a rotating platinum electrode established that
they are both one-electron processes. Traditional diagnostic
criteria (26) were used to demonstrate diffusion control
through CV scans (Supplementary material, Tables D.1,
D.2).3 Within the limits of experimental error, the ip/ꢄ1/2 ra-
tios for 3 or 4g are the same (0.65 vs. 0.61) over the scan
rate of 0.1 to 0.4 V s–1. Thus, the following reactions
(eq. [3]) govern the reductive behavior of the species under
investigation. Previous investigations (22–24) have es-
tablished that the lifetime of the radical anion derived from
[3]
3 + e–
3–; E° = –0.55 V
4g–; E° = –0.61 V
W
Acknowledgements
4g + e–
W
The support of this work by the National Science Founda-
tion (NSF CHE 0092702) and the Vermont NSF EPSCoR
program is gratefully acknowledged.
4g– + e–
4g2–; E° = –0.94 V
W
dicobalt hexacarbonyl – alkyne clusters decreases as the
electronegativity of the alkyne substituents decrease. For ex-
ample, the room temperature lifetime of [(CF3)2C2Co2(CO)6]–
is greater than 1 h, whereas that of [Ph2C2Co(CO)6]– is
17 ms (27). In 3 and 4g, the effect of the strongly electron-
withdrawing phosphazene (15) and the electron-donating
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