Synthesis and Mossbauer spectroscopic studies of chemically oxidized ferrocenyl(phenyl)phosphines

Article abstract of DOI:10.1021/ic991147y

The electrochemical potentials of Fc(3-x)PPh(x), (1-3, x = 0-2) and (FcPPh)(n) (4) indicate that iodine should oxidize ferrocenyl(phenyl)phosphines. The molar conductivity of solutions of 1-3 increases sharply when the solutions are titrated with iodine, leveling off after the addition of > 2 equiv of oxidant, consistent with formation of 1:1 electrolytes. Diamagnetic salts 6-9 are observed upon addition of a benzene solution of iodine to a benzene solution of 1-4 at ambient temperature in ratios of I₂/metallocene ranging from 1:1 to 2:1. Well-resolved ¹H and ³¹P NMR spectra are obtained for 6-8. Absorptions assigned to the I₃^- anion dominate the UV-vis spectrum of 6-8, whereas characteristic absorptions for [Fc][I₃] are absent. Mossbauer spectra of 7-9 reveal isomer shifts consistent with low-spin iron(II) in ferrocene derivatives rather than those in ferricenium ions. Small amounts of low-spin Fe(III) appear to be present in 6. Taken together, the results suggest that 6-9 are iodophosphonium salts and not ferricenium salts. Diferrocenyl(phenyl)phosphine oxide (5) reacts with iodine to produce a diamagnetic, dark solid 10. Low-spin Fe(II) is observed at 77 and 293 K in the Mossbauer spectra of 10 with no evidence for oxidation of Fe(II) to Fe(III). Compound 10 is proposed to be a neutral complex between 5 and I₂. Reactions between 5 and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) yield [Fc₂P(=O)][DDQ]2 (11). Mossbauer spectroscopy of 11 indicates the presence of a mixture of low-spin Fe(II) and low-spin Fe(III) at 77 K, suggesting that some electron transfer occurs from 5 to DDQ. The fraction of low-spin Fe(III) increases at room temperature.

Full text of DOI:10.1021/ic991147y

3506
Inorg. Chem. 2000, 39, 3506-3514
Synthesis and Mossbauer Spectroscopic Studies of Chemically Oxidized
Ferrocenyl(phenyl)phosphines
Daniel A. Durfey,† Rein U. Kirss,*,† Christoph Frommen,† and William Feighery§
Department of Chemistry, Northeastern University, Boston, Massachusetts 02115 and
Department of Chemistry, Indiana UniversitysSouth Bend, South Bend, Indiana 46634
ReceiVed September 27, 1999
The electrochemical potentials of Fc3-xPPhx, (1-3, x ) 0-2) and (FcPPh)n (4) indicate that iodine should oxidize
ferrocenyl(phenyl)phosphines. The molar conductivity of solutions of 1-3 increases sharply when the solutions
are titrated with iodine, leveling off after the addition of >2 equiv of oxidant, consistent with formation of 1:1
electrolytes. Diamagnetic salts 6-9 are observed upon addition of a benzene solution of iodine to a benzene
solution of 1-4 at ambient temperature in ratios of I2/metallocene ranging from 1:1 to 2:1. Well-resolved 1H and
31P NMR spectra are obtained for 6-8. Absorptions assigned to the I3- anion dominate the UV-vis spectrum of
6-8, whereas characteristic absorptions for [Fc][I3] are absent. Mossbauer spectra of 7-9 reveal isomer shifts
consistent with low-spin iron(II) in ferrocene derivatives rather than those in ferricenium ions. Small amounts of
low-spin FeIII appear to be present in 6. Taken together, the results suggest that 6-9 are iodophosphonium salts
and not ferricenium salts. Diferrocenyl(phenyl)phosphine oxide (5) reacts with iodine to produce a diamagnetic,
dark solid 10. Low-spin FeII is observed at 77 and 293 K in the Mossbauer spectra of 10 with no evidence for
oxidation of FeII to FeIII. Compound 10 is proposed to be a neutral complex between 5 and I2. Reactions between
5 and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) yield [Fc2P(dO)][DDQ]2 (11). Mossbauer spectroscopy
of 11 indicates the presence of a mixture of low-spin FeII and low-spin FeIII at 77 K, suggesting that some electron
transfer occurs from 5 to DDQ. The fraction of low-spin FeIII increases at room temperature.
Introduction
studies of 1-3, the reported blue-green colors of the oxidized
solutions are consistent with the formation of ferricenium ions.4
Interpretation of the electrochemistry of bis(diphenylphosphino)-
ferrocene, however, is different. Oxidation of (η5-C5H4PPh2)2-
Fe is believed to occur initially at iron but is followed by a
rapid intramolecular transfer of one electron from phosphorus
to iron, thereby regenerating the FeII center and creating a
phosphonium ion radical.6 The latter decomposes to a phosphine
oxide by reaction with solvent. Similar explanations were
recently proposed for the electrochemical oxidation of 1 and
other ferrocene-substituted phosphines.7,8
To our knowledge, there is only a single report describing
the chemical oxidation of ferrocenyl(phenyl)phosphines.9 Reac-
tion of diferrocenyl(phenyl)phosphine with iodine yielded [Fc2-
PPh][I3] based on microanalysis. Mossbauer spectroscopy was
used to identify the product as a localized, mixed-valent FeII/
FeIII compound. The electrochemical and chemical oxidation
of FcPPh2Re(CO)4Cl, where the nonbonding electron pair on
phosphorus is coordinated to a transition metal, also produces
ferricenium ions.10 In this paper we report on the chemical
oxidation of 1-3 and poly(ferrocenylphenylphosphine) (4) with
iodine and characterization of the products using a combination
Ferrocene is an attractive functional group in the design of
electroactive ligands and polymers because of its readily
accessible electrochemical potential and the reversibility of its
one-electron oxidation.1 Ferrocenyl(diphenyl)phosphine (FcP-
Ph2, 1)2 undergoes a reversible one-electron oxidation in both
CH2Cl2 and acetonitrile.3,4 Two oxidations separated by 0.16
V are observed for diferrocenyl(phenyl)phosphine (2) whereas
three irreversible oxidations are reported for tris(ferrocenyl)-
phosphine (3) under the same conditions.3 The first oxidation
potentials for 1-3 are all more positive than those for ferrocene.
This has been interpreted in terms of the electronic effect of
the phosphorus substituent on the oxidation potential of the iron-
(II) center in 1.3,4 Additional ferrocene substituents in 2 and 3
appear to have little effect on the first oxidation potential. The
first oxidation potential of poly(n-butyl ferrocenyl(phenyl)-
phosphine), however, is essentially the same as that of ferrocene
with two subsequent oxidations at more positive potentials.5 The
electrochemistry of the polymer has been interpreted in terms
of cooperative interactions between metal centers.
At first glance these conclusions are reasonable. The oxidation
of triphenylphosphine occurs at potentials much more positive
than those of ferrocene. In controlled potential coulometry
(6) Pilloni, G.; Longato, B.; Corain, B. J. Organomet. Chem. 1991, 420,
57.
(7) Gref, A.; Diter, P.; Guillaneux, D.; Kagan, H. B. New J. Chem. 1997,
21, 1353.
(8) (a) Louati, A.; Gross, M.; Douce, L.; Matt, D. J. Organomet. Chem.
1992, 438, 167. (b) Masson-Szymczak, A.; Riant, O.; Gref, A.; Kagan,
H. B. J. Organomet. Chem. 1996, 511, 193. (c) Butler, I. R.; Kalaji,
M.; Nehrlich, L.; Hursthouse, M.; Karaulov, A. I.; Abdul-Malik, K.
M. J. Chem. Soc., Chem. Commun. 1995, 459.
† Northeastern University.
§ Indiana UniversitysSouth Bend.
(1) Togni, A., Hayashi, T., Eds. Ferrocenes; VCH Publishers: New York,
1995.
(2) Fc represents (η5C5H5)(η5C5H4)Fe.
(3) Kotz, J. C.; Nivert, C. A. J. Organomet. Chem. 1973, 52, 387.
(4) Kotz, J. C.; Nivert, C. A.; Lieber, J. M.; Reed, R. C. J. Organomet.
Chem. 1975, 91, 87.
(9) Wei, H.-H, Lin, C.-Y.; Chang, S.-J. Proc. Natl. Sci. Counc., Repub.
China, Part B: Life Sci. 1983, 7, 35.
(5) Foucher, D. A.; Honeyman, C. H.; Nelson, J. M.; Tang, B. Z.; Manners,
I. Angew. Chem., Int. Ed. Engl. 1993, 32, 1709.
(10) The chemical oxidation of FcPPh2Re(CO)4Cl was reported. Miller,
T. M.; Ahmed, K. J.; Wrighton, M. S. Inorg. Chem. 1989, 28, 2347.
10.1021/ic991147y CCC: $19.00 © 2000 American Chemical Society
Published on Web 07/08/2000

Chemically Oxidized Ferrocenyl(phenyl)phosphines
Inorganic Chemistry, Vol. 39, No. 16, 2000 3507
Table 1. NMR Spectral Data for Compounds 1-5 in CD2Cl2
compound
FcPPh2 (1)
1H
31P
13C
4.09 s (5 H, Cp)
-17.8
69.4, 71.2, 73.4
76.1, 128.4,
133.8, 139.7
4.13, 4.40 br s (4 H, C5H4)
7.35, 7.39 br s (10 H, Ph)
3.98, 4.20, 4.28, 4.33 br m
(2 H each), 4.07 s (10 H, Cp)
4.04 s (15 H, Cp), 4.17 q (6 H)
4.30 t (6 H)
3.6-4.7 br m (10 H, Cp),
6.8-7.9 br, 5 H, Ph)
4.50, 4.15, 4.08, 4.00 br s (2 H),
4.19 br s (10 H, Cp),
Fc2PPh (2)
-29.8
-59.9
-31.5
+26.7
69.37, 70.31, 70.84
72.28, 73.19, 78.46, 128.2, 129.1, 134.3
69.3, 70.1, 73.1,
80.0
Fc3P (3)
[PhPFc]y (4)
Fc2P(dO)Ph (5)a
7.09, 7.95 m (5 H, PPh)
a In C6D6.
1H (CD2Cl2): δ 4.30 s (6 H, Cp + one proton of the unsubstituted
ring), 4.38 br m (1 H), 4.52 br m (1 H), 4.76 br m (1 H), 7.35 br m (4
H), 7. 39 br m (6 H).
of cyclic voltammetry, molar conductivity, NMR, UV-vis, and
Mossbauer spectroscopy methods. To examine the role of the
phosphorus lone pair on the chemical oxidation of ferrocenyl-
(phenyl)phosphines, the reactivity of ferrocenyl(phenyl)phos-
phine oxide (5) with iodine and 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ) is also described.
31P{1H} (CD2Cl2): δ -11.5 s.
IR (KBr): 420 w, 459 w, 493 s, 530 m, 570 m, 620 w, 692 s, 704
s, 727 w, 751 w, 825 w, 1108 w, 1123 m, 1184 w, 1437 m, 1542 m,
1626 s br, 2855 w br, 2930 w br, 3065 w br.
Using the procedure for 6a, 513 mg (1.99 mmol, 1.5 equiv) of I2
and 492 mg (0.133 mmol) of 1 were stirred for 16 h in benzene (60
mL) yielding 6b as a sticky brown solid in ≈50% yield. Accurate
microanalyses could not be obtained for this product in part because
of its tarlike consistency and because the product was a mixture of
compounds.
Experimental Section
All compounds described in this work were handled using Schlenck
techniques or in an M. I. Braun glovebox under purified argon or
nitrogen atmospheres.11 Ferrocenyl(diphenyl)phosphine (1), diferroce-
nyl(phenyl)phosphine (2), tris(ferrocenyl)phosphine (3),3,12 poly(ferro-
cenylphenylphosphine) (4),13 and diferrocenyl(phenyl)phosphine oxide
(5)3 were prepared by literature procedures. Iodine and 2,3-dichloro-
5,6-dicyano-1,4-benzoquinone were purchased from Aldrich Chemical
Co. and used as received. Solvents were purified by refluxing over
Na/benzophenone (toluene, benzene, tetrahydrofuran) or P2O5 (dichlo-
romethane, 1,2-dichloroethane, acetonitrile) and distilled prior to use.
Elemental analyses (C, H) were performed by Desert Analytics, Inc.
Melting points were determined on an Electrothermal 9100 apparatus
under argon in capillaries sealed with epoxy and are uncorrected.
NMR spectra were recorded at 300 MHz for 1H and 121.4 MHz for
31P{1H} on a Varian XL300 spectrometer in 5-mm tubes equipped with
a Teflon valve (Wilmad Glass, Inc.). Proton chemical shifts are relative
to residual protons in the solvent (CDHCl2 at δ 5.24 ppm). Carbon
chemical shifts are reported relative to 13C in the solvent (CD2Cl2 at δ
53.8 ppm). Phosphorus chemical shifts are reported relative to external
85% H3PO4 at δ 0.0 ppm. NMR data for 1-5 are summarized in Table
1.
UV-vis spectra were recorded in CH2Cl2 and CH3CN solution on
a Hewlett-Packard 8452A diode array spectrometer from 250 to 800
nm. Variable-temperature Mossbauer spectra were recorded in the solid
state on a standard constant-acceleration apparatus and fit to standard
independent Lorentzians.14 Infrared spectra were obtained as KBr pellets
on a Mattson Satellite FTIR interfaced with a Digital PC3000 computer.
Reaction of Ferrocenyl(diphenyl)phosphine (1) with Iodine. A
solution of 144 mg (0.57 mmol, 1.0 equiv) of I2 in 15 mL of benzene
was added dropwise to an orange solution of 210 mg (0.57 mmol) of
1 in 15 mL of benzene at ambient temperature. Formation of a brown
precipitate was observed after stirring for 20 min. The orange
supernatant was rapidly decanted and the precipitate was washed with
benzene (2 × 10 mL) until the washings were colorless. The brown
tar was dried under vacuum, yielding 240 mg (68% yield) of 6a.
Accurate microanalyses could not be obtained for this product in part
because of its tarlike consistency.
1H (CD2Cl2): δ 4.26 s (5 H, Cp), 4.46 s (2 H, Ha), 4.76 s (2 H, Hb),
7.36 s (3 H, C6H6), 7.6-7.8 m (10 H, PPh2).
31P{1H} (CD2Cl2): δ 6.9 s (major), 50.8 s (minor).
13C {1H} (CD2Cl2): δ 70.7 s, 73.0 d, 74.2 d, 128.5 s(C6H6), 129.6
d, 132.0 d, 134.5 s.
IR (KBr): 447 m, 473 s, 489 s, 518 m, 548 w, 567 m, 687 s, 703
m, 729 m, 745 m, 827 m, 870 w, 906 w, 924 w, 999 m, 1030 m, 1060
w, 1104 m, 1122 m, 1178 m, 1309 w, 1365 w, 1387 w, 1411 w, 1436
w, 1476 m, 2871 w, 2851 w, 2924 m, 2954 w, 3055 w, 3081 w.
UV-vis (CH2Cl2): 294, 364 nm.
Mossbauer: δ ) 0.551 m/s, ∆EQ ) 2.269 mm/s.
Using the procedure for 6a, 137 mg (0.54 mmol, 2.0 equiv) of I2
and 100 mg (0.27 mmol) of 1 were stirred for 2 h in benzene (10 mL)
at ambient temperature yielding 70 mg (30% yield) of 6c as a brown
solid (mp 137-140 °C). The product contained a mixture of products,
as determined by NMR, which could not be readily separated.
1H (CD2Cl2): δ 4.45 s (5 H, Cp), 4.60 q (2 H, J ) 3 Hz), 5.02 q (2
H, J ) 2 Hz), 7.6-7.8 m (10 H, PPh).
31P{1H} (CD2Cl2): δ 15.1 s.
UV-vis (CH2Cl2): λmax ) 300, 368 nm.
Mossbauer (77 K): δ ) 0.452 mm/s, ∆EQ ) 2.202 mm/s; δ ) 0.431
mm/s, ∆EQ ) 0 mm/s; δ ) 0.781 mm/s, ∆EQ ) 1.969 mm/s.
IR (KBr): 428 cm-1 w, 447 w, 477 s, 505 m, 616 w, 701, m, 723
m, 744 m, 776 m, 808 m, 822 m, 838 m, 1034 m, 1058 w, 1094 m,
1157 w, 1175 m, 1377 w, 1433 m, 1460 m, 2851 s, 2919 s, 2955 sh.
A second set of resonances accounting for 13% of the products (by
integration) is assigned to FcPPh2‚3/2I2 (vide supra).
Reaction of Diferrocenyl(phenyl)phosphine (2) with Iodine. Using
the procedure for 6a, 104 mg (0.41 mmol, 1.0 equiv) of I2 and 197 mg
(0.41 mmol) of 2 were stirred for 16 h in benzene (30 mL) yielding
260 mg (87% yield) of 7a (mp 165-168 °C). Anal. Calcd for C26H25-
Fe2I2P‚1/3C6H6: 44.34% C, 3.32% H. Found: 44.57% C, 3.45% H.
1H (CD2Cl2): δ 4.36 s (10 H, Cp), 4.39 m (2 H), 4.56 m (2 H), 4.75
m (2 H), 4.79 m (2 H), 7. 35 s (2 H, C6H6), 7.62 m (2 H), 7.69-7.8
m (3 H).
(11) Shriver, D. F. Manipulation of Air-SensitiVe Compounds; McGraw-
Hill: New York, 1969.
(12) Sollott, G. P.; Mertwoy, H. E.; Portnoy, S.; Snead, J. L. J. Org. Chem.
1963, 28, 1090.
(13) Honeyman, C. H.; Foucher, D. A.; Dahmen, F. Y.; Rulkens, R.; Lough,
A. J.; Manners, I. Organometallics 1995, 14, 5503.
(14) Cheng, C.; Reiff, W. M. Inorg. Chem. 1977, 16, 2092.
31P{1H} (CD2Cl2): δ -5.9 s.
IR (KBr): 420 cm-1 m, 434 m, 446 m, 470 s, 484 s, 554 m, 574 m,
617 m, 669 m, 684 m, 747 m, 826 m, 1027 m, 1106 m, 1181 m, 2855
w br, 2930 w br, 3065 w br.
UV-vis (CH2Cl2): 280, 355 nm.

3508 Inorganic Chemistry, Vol. 39, No. 16, 2000
Durfey et al.
Using the procedure for 6a, 164 mg (0.65 mmol, 1.5 equiv) of I2
and 207 mg (0.43 mmol) of 2 were stirred for 16 h in benzene (30
mL) yielding 350 g (88% yield) of 7b as an orange-red powder (mp
170-173 °C). Anal. Calcd for C26H25Fe2I3P‚1/2C6H6: 38.79% C, 2.92%
H. Found: 38.82% C, 2.96% H.
1H (CD2Cl2): δ 4.38 s (10 H, Cp), 4.47 m (2 H), 4.63 m (2 H), 4.84
m (2 H), 4.88 m (2 H), 7.35 s (3 H, C6H6), 7.62 m (2 H), 7.69-7.8 m
(3 H).
31P{1H} (CD2Cl2): δ 3.7 s.
13C{1H} (CD2Cl2): 66.9, 68.1, 71.1, 72.2 (Cp), 74.1 d, 74.6 d, 75.1,
128.9 (C6H6), 130.1 d, 132.8 d, 134.9.
IR (KBr): 469 cm-1 s, 490 m, 556 w, 615 w, 689 w, 749 w, 822 m,
1002 m, 1028 m, 1055 w, 1065 w, 1100 m, 1184 s, 303 w, 1365 w,
1390 w, 1409 m, 1433 m, 1475 w, 1571 w, 1627 w, 3050 w br, 3090
w br.
IR (KBr): 463 cm-1 s, 543 w, 606 w, 619 w, 651 w, 678 m, 694 sh
w, 830 m, 1003 m, 1027 m, 1057 m, 1107 m, 1179 s, 1242 w, 1304 w,
1365 w, 1388 w, 1411 w, 1634 br w, 2349 s, 2363 s, 3085 br w, 3428
br m.
Reaction of Poly(ferrocenylphenylphosphine) (4) with Iodine.
Using the procedure for 6a, 469 mg (1.8 mmol) of I2 and 360 mg (1.3
mmol) of 4 were stirred for 16 h in benzene (30 mL) yielding 684 mg
(82% yield) of 9a as a brown solid insoluble in benzene and
dichloromethane. The product loses I2 as purple vapor above 230 °C.
Anal. Calcd for C16H13FeI3P‚1/6C6H6: 29.75% C, 2.16% H. Found:
29.57% C, 2.12% H.
IR (KBr): 469 cm-1 s, 490 s, 509 w, 556 w, 616 w, 690 w, 708 w,
749 w, 822 m, 1003 w, 1028 w, 1055 w, 1065 w, 1100 w, 1184 m,
1308 w, 1365 w, 1388 w, 1409 w, 1434 m, 1475 w, 2854 w br, 2922
w br, 2962 w br, 2962 w br, 3051 w br, 3087 w br.
Using the procedure for 6a, 76.3 mg (0.30 mmol) of I2 and 43.9 mg
(0.15 mmol) of 4 were stirred for 16 h in benzene (30 mL) yielding 99
mg (82% yield) of 9b as a brown solid insoluble in benzene and
dichloromethane (mp 124-128 °C). Anal. Calcd for C16H13FeI4P:
24.03% C, 1.64% H. Found, 24.30% C, 1.65% H.
UV-vis (CH2Cl2): 294, 364 nm.
Mossbauer: δ ) 0.475 mm/s, ∆EQ ) 2.30 mm/s.
Using the procedure for 6a, 36 mg (0.14 mmol, 2.0 equiv) of I2 and
34 mg (0.071 mmol) of 2 were stirred for 2 h in benzene (10 mL)
yielding 51 mg (73% yield) of 7c as a brown powder (mp 100-104
°C). Anal. Calcd for C26H23Fe2I4P‚C6H6: 36.13% C, 2.75% H. Found:
35.90% C, 2.43% H.
1H NMR (DMSO-d6): 4.48 br (8 H), 7.51 br (5 H).
Mossbauer (77 K): δ 0.500 mm/s, ∆EQ ) 2.292 mm/s.
IR (KBr): 460 cm-1 s, 550 m, 576 m, 619 m, 686 m, 717 m, 743
m, 831 s, 879 w, 910 w, 996 sh w, 1059 s, 1094 s, 1261 s, 1305 w,
1357 w, 1369 w, 1414 w, 1434 m, 1627 br w, 3087 br w, 3418 br m.
Reaction of Diferrocenyl(phenyl)phosphine Oxide (5) with 2.0
Equiv of Iodine. Using the procedure for 6a, 107 mg (0.42 mmol) of
I2 and 105 mg (0.21 mmol) of 5 were stirred for 16 h in benzene (20
mL) yielding 89 mg (42% yield) of 10 as a black powder. Anal. Calcd
for C26H23FeI3OP‚1/2C6H6: 38.11% C, 2.80% H. Found: 38.22% C,
2.92% H.
1H (CD2Cl2): δ 4.44 s (10 H, Cp), 4.57 br m (2 H), 4.72 br m (2
H), 4.96 br m (2 H), 5.00 br m (2 H), 7. 35 s (6 H, C6H6), 7.69 t (3 H,
PPh), 7.77 t (2 H, PPh).
31P{1H} (CD2Cl2): δ 19.2 s.
UV-vis (CH2Cl2): λmax ) 300, 366 nm.
Mossbauer (77 K): δ 0.497 mm/s, ∆EQ ) 2.257 mm/s.
IR (KBr): 444 cm-1 w, 479 s, 537 w, 565 w, 577 w, 613 w, 684 m,
715 w, 748 w, 826 m, 1002 m, 1031 s, 1107 w, 1178 s, 3050 br w.
Reaction of Tris(ferrocenyl)phosphine (3) with Iodine. Using the
procedure for 6a, 102 mg (0.40 mmol, 1.0 equiv) of I2 and 235 mg
(0.40 mmol) of 3 were stirred for 16 h in benzene (50 mL) yielding
335 mg (99% yield) of 8a as an orange-red powder (mp 189-191 °C).
Anal. Calcd for C30H27Fe3I2P: 42.89% C, 3.24% H. Found: 43.05%
C, 3.14% H.
1H (CD2Cl2): 4.31 s (10 H, Cp), 4.42 br s (2 H), 4.6 br s (2 H), 4.7
br s (2 H), 4.8 br s (2 H), 7.3-7.8 br signals (8 H, C6H6, PPh).
31P{1H} (CD2Cl2): δ 51 br s.
UV-vis (CH2Cl2): λmax ) 294, 364 nm with a tail at ≈500 nm.
Mossbauer (77 K): δ ) 0.387 mm/s, ∆EQ ) 2.26 mm/s; (298 K) δ
) 0.468 mm/s, ∆EQ ) 2.371 mm/s.
IR (KBr): 480 cm-1 m, 575 m, 622 w, 695 w, 717 w, 751 w, 826
sh w, 870 br m, 1003 w, 1028 w, 1107 w, 1188 m, 1307 w, 1384 w,
1463 m, 2850 s, 2925 s, 2955 sh m.
1H (CD2Cl2): δ 4.37 s (6 H, Cp + one proton of the substituted
ring), 4.40 m (1 H), 4.63 m (1 H), 4.81 m (1 H).
31P{1H} (CD2Cl2): δ 0.
Reaction of Diferrocenyl(phenyl)phosphine Oxide (5) with 1.0
Equiv of 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). An
orange solution of 50.1 mg (0.122 mmol) of DDQ in benzene (20 mL)
was added dropwise to a yellow solution of 97.4 mg (0.20 mmol) of 5
in benzene (20 mL) at ambient temperature. Formation of a dark
precipitate was observed after several days. The orange supernatant
was decanted and the brown precipitate was washed with 2 × 10 mL
of benzene. The brown powder was dried under vacuum, yielding 47
mg (47% yield) of 11 as a brown solid (mp >280 °C, d). Anal. Calcd
for C42H23Cl4Fe2N4O5P: 53.21% C, 2.45% H, 5.91% N. Found: 52.36%
C, 2.34% H, 4.72% N.
IR (KBr): 420 cm-1 w, 446 w, 468 s, 484 m, 546 w, 575 w, 584 w,
593 w, 620 w, 640 w, 649 w, 659 w, 669 w, 688 m, 744 w, 808 w,
827 m, 1004 m, 1028 m, 1107 w, 1179 m, 2854 w br, 2929 w br, 3065
w br.
UV-vis (CH2Cl2): 270 sh, 340 nm.
Using the procedure for 6a, 160 mg (0.63 mmol, 1.5 equiv) of I2
and 246 mg (0.42 mmol) of 3 were stirred for 6 h in benzene (30 mL)
yielding 365 mg (90% yield) of 8b as an orange powder (mp 180-
183 °C). Anal. Calcd for C30H27Fe3I3P‚1/6C6H6: 37.97% C, 2.88% H.
Found: 37.49% C, 2.45% H.
1H (CD2Cl2): δ 4.35 s (15 H, Cp), 4.60 q (6 H, J ) 1.9 Hz), 4.84 q
(6 H, J ) 1.8 Hz), 7.35 s, (1 H, C6H6).
UV-vis (CH2Cl2): λmax ) 254 nm (ꢀ ) 64 000), 338 (6800), 355
(6600), 582 (4120).
31P{1H} (CD2Cl2): δ 7.2 s.
IR (KBr): 425 cm-1 w, 454 w, 480 m, 542 s, 575 w, 697 s, 721 s,
753 m, 789 w, 822 w, 904 w, 999 m, 1033 m, 1072 m, 1095 sh, 1121,
1163 w, 1189 s, 1352 s, 1412 s, 1560 br w, 1627 br w, 2227 w, 3076
w, 3095 w, 3446 br m.
13C{1H} (CD2Cl2): δ 71.8 s, 73.8 d, 74.2 d, 80.0 d, 110 s.
IR (KBr): 467 cm-1 s, 487 m, 621 w, 695 w, 827 m, 1001 m, 1026
m, 1056 w, 1107 w, 1178 m, 1194 w, 1303 w, 1364 w, 1410 w, 1955
w br, 3056 w br, 3082 w br.
Mossbauer (77 K): δ ) 0.498 mm/s, ∆EQ ) 2.322 mm/s, δ ) 0.465
mm/s, ∆EQ ) 0.935 mm/s.
UV-vis (CH2Cl2): 294, 364 nm.
Reaction of Ferrocene with DDQ.15 Addition of a solution of 237
mg (1.0 mmol) of DDQ in benzene (8 mL) to a solution of 199 mg
(1.1 mmol) of ferrocene in benzene (8 mL) at ambient temperature
precipitates dark brown [Cp2Fe][DDQ] as described in the literature.
The product was collected by filtration, washed with hexane, and dried
under vacuum. The yield was nearly quantitative. The product is
essentially insoluble in all common solvents.
Mossbauer: δ ) 0.550 mm/s, ∆EQ ) 2.268 mm/s
Using the procedure for 6a, 59.2 mg (0.23 mmol, 2.0 equiv) of I2
and 68.4 mg (0.12 mmol) of 3 were stirred for 2 h in benzene (10 mL)
yielding 110 mg (85% yield) of 8c as an orange powder (mp 195-200
°C). Anal. Calcd for C30H27Fe3I4P‚1/6C6H6: 33.61% C, 2.55% H.
Found: 33.51% C, 2.59% H.
1H (CD2Cl2): δ 4.40 s (15 H, Cp), 4.66 q (6 H, J ) 3 Hz), 4.92 q
(6 H, J ) 3 Hz), 7.35 s (1 H, C6H6).
IR (KBr): 493 cm-1 w, 782 m, 848 m, 1041 m, 1182 m, 1416 w,
1457 w, 1543 s, 1580 s, 2209 m, 3079 m, 3116 m.
31P{1H} (CD2Cl2): δ 22.9 s.
UV-vis (CH2Cl2): λmax ) 280, 364 nm.
(15) Brandon, R. L.; Osiecki, J. H.; Ottenberg, A. J. Org. Chem. 1966, 31,
Mossbauer: (77 K): δ 0.482 mm/s, ∆EQ ) 2.288 mm/s.
1214.

Chemically Oxidized Ferrocenyl(phenyl)phosphines
Inorganic Chemistry, Vol. 39, No. 16, 2000 3509
Table 2. Electrochemical Data for 1-5
Reaction of Ferrocene with Iodine.30 A solution of 1280 mg (5.1
mmol) of I2 in benzene (35 mL) was added to a solution of 500 mg
(2.7 mmol) of ferrocene in benzene (15 mL). Ferricenium triiodide
precipitates immediately. The purple supernatant containing the excess
I2 was decanted and the solid was washed with benzene until the
washings were colorless. The product was dried under vacuum. Yield
was nearly quantitative.
compound
E1/2 (mV)a
solvent
Fc
92
196
CH3CN
CH3CN
CH3CN
FcPPh2 (1)
Fc2PPh (2)
Fc3P (3)
[FcPPh]x (4)
Fc2P(dO)Ph (5)
PPh3
180b
352b
366b
480b
1090
357
360
509
620b 782b CH2Cl2
731b 905b CH2Cl2
UV-vis (CH2Cl2): λmax ) 294, 364, and 500 nm.30
689b
CH3CN/CH2Cl2 (5%)
CH3CN
CH3CN
IR (KBr): 849 cm-1 s, 1006 m, 1110 w, 1412 m, 3098 m.
Reaction of Biferrocene with DDQ.39 Addition of a solution of 41
mg (0.18 mmol) of DDQ in benzene (15 mL) to a solution of 115 mg
(0.36 mmol) of biferrocene in benzene (15 mL) at ambient temperature
precipitates dark brown [CpFe(C5H4-C5H4)FeCp][DDQ] as described
in the literature. The solvent was evaporated under vacuum, and the
precipitate was washed with hexane and dried under vacuum. The yield
was nearly quantitative. The product is sparingly soluble (e0.1 mM)
in CH2Cl2.
I2
435
534
CH3CN/CH2Cl2 (5%)
CH3CN/CH2Cl2 (5%)
DDQ
a Experimental conditions: 2 mM solution containing 0.1-0.2 M
[nBu4N][ClO4] as the supporting electrolyte. Potentials relative to Ag/
Ag+ at a scan rate 100 mV/s. b Irreversible.
IR (KBr): 435 cm-1 w, 475 w, 782 w, 819 w, 850 w, 1002 w, 1046
w, 1110 w, 1190 w, 1250 w, 1410 w, 1420 w, 1493 m, 1508 m, 1542
m, 1576 s, 2212 m, 3115 m.
UV-vis (CH2Cl2): 292 nm (ꢀ ) 10 700), 348 (6300), 434 (3440),
458 (3460), 548 (3068), 586 (3234).
Electrochemical Measurements. Oxidation potentials of 1-5 were
determined on a BAS 100B electrochemical analyzer in CH2Cl2, CH3-
CN, and 5% CH2Cl2/CH3CN (v/v) solution (2 mM) containing 0.1-
0.2 M [nBu4N][ClO4] or [nBu4N][BF4] as the supporting electrolyte. A
1.2-mm Au or 3-mm Pt disk working electrode (BAS), Pt counter
electrode, and Ag/Ag+ reference electrodes (Ag wire in 10 mM AgNO3
and 0.1 M[NEt4][BF4] in CH3CN)10 were used. All experiments were
performed under a nitrogen or argon atmosphere at a scan rate of 100
mV/s.
Conductivity Experiments. Solutions of 1-5 (0.010 M in 1,2-
dichloroethane) were titrated with a solution of I2 (0.089 M in 1,2-
C2H4Cl2). The resistivity of the resulting solution was determined using
a Beckman Instruments model RC 16B2 conductivity bridge. Molar
conductivities (cm2/mol-Ω) of the solutions were calculated by literature
methods.16
Figure 1. Cyclic voltammogram of Fc2PPh, 2.
workers4 but differs from the more recent value reported by
Kagan et al.7 by 100 mV. Differences in the supporting
electrolyte (LiClO4 vs [nBu4N][ClO4]) and solvent may con-
tribute to this discrepancy.
Titration of 1 and 3 with Iodine. Solutions of 1, 3, and iodine
were prepared with ratios of I2 to ferrocenyl(phenyl)phosphine equaling
0.5, 1, 1.5, 2, and 4:1 under argon in 5-mm NMR tubes equipped with
Teflon valves in CH2Cl2. 31P NMR spectra were recorded within 2 h
of reaction and again after 24 h. The spectra were unchanged over this
time period. The same samples were then analyzed by Raman
spectroscopy on a SPEX-1877 triple spectrometer using a Coherent
INNOVA 302 Kr+ ion excitation source operating at 100 mW of power
and a liquid-nitrogen-cooled CCD detector (Princeton Instruments). A
Princeton Photonics Instruments Raman 101 spinner was used to spin
the samples.
The first oxidation wave for 1 in acetonitrile is reversible,
consistent with conditions reported in the literature.4 A second
oxidation wave is not observed in acetonitrile although an
irreversible oxidation at extremely positive potentials (+1400
mV vs SCE) is reported in acetonitrile containing 5% CH2Cl2.8
At the scan rates used in our studies, the first oxidation of 2 in
CH3CN is irreversible even when the potential is reversed
immediately after oxidation is detected (Figure 1). The second
oxidation of 2 appears to be reversible (∆Eia, ic ) 60 mV) but
the third oxidation is again irreversible. Others report4 that both
the first and second oxidations in 2 are reversible in CH2Cl2
but a third oxidation is not observed. Both 3 and 4 have
insufficient solubility in pure acetonitrile for cyclic voltammetry
studies. Three irreversible oxidations are observed for 3 in CH2-
Cl2 consistent with studies by Kotz et al.4 Our results and those
from Kagan’s group8 indicate a significantly larger separation
between the three oxidation steps than that reported in the Kotz
work.4 A clear fourth oxidation wave is not observed for 3, but
the peak current for the final oxidation was larger than those
for the first and second waves, suggesting that more than one
electron may be transferred in this step. The irreversible behavior
of 3 is consistent with adsorption of the oxidized product at the
electrode as previously reported.4 The cyclic voltammogram of
4 shows three irreversible oxidation waves, although the second
Results
Electrochemistry. The oxidation potentials of 1-4, triph-
enylphosphine, and iodine were determined by cyclic voltam-
metry in dichloromethane, acetonitrile, and 5% CH2Cl2/CH3CN
solution with [nBu4N][ClO4] or [nBu4N][BF4] as the supporting
electrolyte.17 The results are summarized in Table 2. These data
confirm that 1-3 are harder to oxidize than ferrocene (by ≈100
mV), but are considerably easier to oxidize than PPh3. Poly-
(ferrocenylphenylphosphine) 4 oxidizes at essentially the same
potential as 3 does in CH2Cl2. The electrochemical potentials
for 1 and 2 are consistent with literature data.4,7,10 Where
comparisons are possible, electrochemical potentials in meth-
ylene chloride are slightly more positive than potentials in
acetonitrile. A similar solvent dependence has been observed
for oxidation potentials of alkylferrocenes.18 The first oxidation
potential for 3 is identical to that reported by Kotz and co-
(16) Shoemaker, D. P.; Garland, C. W.; Steinfeld, J. I. Experiments In
Physical Chemistry; McGraw-Hill: New York, 1974.
(17) Subject to solubility considerations.
(18) Zanello, P.; Cinquantini, A.; Mangani, S.; Opromolla, G.; Pardi, L.;
Janiak, C.; Rausch, M. D. J. Organomet. Chem. 1994, 471, 171.

3510 Inorganic Chemistry, Vol. 39, No. 16, 2000
Durfey et al.
oxidation appears as a shoulder on the final oxidation wave.
Adsorption at the electrode again affects the electrochemistry.
Conductivity Studies. The molar conductivity of 1,2-
dichloroethane solutions of 1-3 increases sharply when titrated
with iodine leveling off after the addition of >2 equiv of oxidant.
The molar conductivity of the final solution (ΛM ≈ 20-25 cm2
Ω-1 mol-1) in each case is consistent with formation of a 1:1
electrolyte after comparison with the molar conductivity of
dichloroethane solutions of [NEt4][Br] at comparable concentra-
tions. The molar conductivities of the solutions are also similar
to that of [(FcPPh2)2Co(CO)3][BPh4] in nitromethane.3
A dark precipitate forms immediately upon addition of the
first aliquot of I2 to a solution of poly(ferrocenylphenylphos-
phine) (4) in dichloroethane. Although a small increase in molar
conductivity is observed, the presence of undissolved solids does
not allow for comparison with the data obtained for 1-3.
Titration of Ferrocenyl(phenyl)phosphines with Iodine.
The reaction between ferrocenyl(phenyl)phosphines and iodine
can be followed by 31P NMR spectroscopy. A single resonance
is observed at room temperature in each case. A steady
downfield shift in the 31P NMR resonance from -42 to 23 ppm
is observed up to an I2 to 3 ratio of 2:1. Reaction with additional
I2 up to a 4:1 ratio of I2 to 3 leads to a much smaller downfield
shift in the 31P resonance from 23 to 26 ppm. A similar pattern
is observed for titration of FcPPh2 (1) with I2. The results are
consistent with the formation of a 2:1 complex between iodine
and 1 or 3.
solidify after storage for several weeks in a glovebox and
contains a mixture of products by NMR. Attempts at separating
and purifying 6c proved unsuccessful. Polymer 4 was reacted
with iodine in dichloromethane because it is insoluble in
benzene. Evaporation of the solvent and washing the product
with benzene yields 9. Acceptable elemental analyses were
obtained for the triiodide salts 7-9. Analysis of compounds 7
and 8 indicated that they retained variable amounts of benzene
even after being dried under vacuum. The presence of benzene
1
in 7 and 8 is confirmed by H NMR spectroscopy.
1H NMR spectra of 6-8 (Table 3) consist of a singlet for
the five protons on the C5H5 ring and either two (6b,c and 8b,c),
three (6a and 8a), or four (7a-c) multiplets assigned to the
protons of the phosphorus-substituted cyclopentadienyl rings.
All of the resonances are shifted downfield relative to the starting
ferrocene compounds. A similar downfield shift for all reso-
nances was observed in phosphonium salts of the general
formula [FcxPPh3-xMe][I].3 Within a series of compounds
differing only in the ratio of iodine to iron (e.g., 7a-c) the
1
largest downfield shifts in the H resonances are observed for
the product with the greatest amount of iodine (e.g., 7c). By
analogy to NMR studies of 1-3, the furthest downfield
resonances of the substituted ring protons are assigned to protons
attached to the 3 and 4 positions of the ring (Scheme 1). Three
sets of resonances for the substituted ring protons in 6a and 8a
result from an overlap of the Cp proton with one of the CH
protons of the substituted rings.
Raman spectra of solutions containing a 1:1 ratio of I2:3
in CH2Cl2 have absorptions at 168 and 209 cm-1. Absorptions
in the 145-204 cm-1 range are assigned to P-I stretching in
iodine complexes of aryl and alkyl phosphines, suggesting
a similar assignment for the 168 cm-1 absorption in 3.19-22 The
absorption at 209 cm-1 is assigned to free iodine.22 An additional
absorption at 288 cm-1 remains unassigned. Two different
absorptions are observed at 226 and 337 cm-1 in solutions
containing a 2:1 ratio of I2 to 3. It is possible that one of these
peaks arises from ν(P-I) in [Fc3PI][I3]. The P-I stretch at 159
cm-1 in the Raman spectrum of Ph3P‚I2 is shifted to a higher
frequency in [Ph3PI][I3] where either the 184 or the 202 cm-1
absorption corresponds to ν(P-I).22
Synthesis and Characterization of Triiodide Salts. The
electrochemical data in Table 2 suggest that compounds 1-4
should all undergo chemical oxidation with iodine. Three ratios
of iodine to ferrocenyl(phenyl)phosphines 1-3 (1:1, 1.5:1, and
2:1) were investigated. Rapid precipitation of diamagnetic iodide
salts, 6-8, is observed upon addition of benzene solutions of
iodine to benzene solutions of 1-3 at ambient temperature for
the entire range of iodine-to-iron ratios. Benzene was chosen
as the solvent because of the ease of isolation of the products
by filtration. Products 7 and 8 are generally isolated in good
yields (75-90%) by decanting the supernatant, and washing
the product with benzene and drying it in a vacuum. Compounds
6a-c prove to be oils or sticky solids even after repeated
washings and drying at 80 mTorr contributing to the lower
isolated yields for this product. Compound 6c does appear to
The four resonances for the substituted ring protons in 7 are
the result of chemical inequivalence of the four protons of the
unsubstituted ring from slow inversion about the phosphorus.
The resonances for the substituted ring protons in 6-8 show a
more complicated splitting than the expected AB pattern.
Multiplets are observed for 6, whereas quartets are observed
for 7 and 8. Quartets can be rationalized by considering coupling
to the adjacent as well as coupling to the cross-ring protons as
proposed in the literature for 1-3.3
The 31P NMR spectra of 6-8 consist of singlet resonances
shifted downfield from the resonances of the starting materials.
The spectra for 6 again reveal the presence of a mixture of
products. The downfield progression of the 31P resonances
within a series of compounds (e.g., 8a-c) was discussed in the
previous section. Identical spectra are obtained for compounds
prepared from 1-3 and 2 equiv of iodine in dichloromethane.
Solutions of 6-8 in CH2Cl2 are yellow rather than the
characteristic blue-green color of the ferricenium ion in [Fc]-
[BF4]. An absorption in the visible region of the spectrum at
444 nm for 1-3 disappears upon reaction with iodine. Absorp-
tions at 620 and 498 nm observed for [Fc][BF4] and [Fc][I3],
respectively, are absent in the UV-vis spectrum. A strong
absorption in the UV at 254 nm is replaced by two absorptions
at 294 and 364 nm characteristic of the I3 anion in 6c-8c.23
-
The latter absorption has a broad “tail” extending out to 550
nm. The absorptions assigned to the I3- ion obscure the region
where the charge-transfer bands of ferricenium ion in [Fc][BF4]
(316 and 364 nm) are typically observed. These charge-transfer
bands are also not observed in [Fc][I3]. In products 7a,b and
8a,b two absorptions are observed at slightly higher energies
than those in 7c-8c (e.g., 280 and 355 nm in 7a,b). A similar
trend is seen in the UV-vis spectra of Ph3P‚I2.23 In 6a and b,
the absorptions at ≈270-290 nm are obscured by intenseab-
sorptions at 255 nm from the ferrocenyl(diphenyl)phosphine
moiety.
(19) Godfrey, S. M.; Kelly, D. G.; McAuliffe, C. A.; Mackie, A. G.;
Pritchard, R. G.; Watson, S. M. J. Chem. Soc., Chem. Commun. 1991,
1163.
(20) Bricklebank, N.; Godfrey, S. M.; Lane, H. P.; McAuliffe, C. A.;
Pritchard, R. G.; Moreno, J. M. J. Chem. Soc., Dalton Trans. 1995,
2421.
(21) Bricklebank, N.; Godfrey, S. M.; Mackie, A. G.; McAuliffe, C. A.;
Pritchard, R. G.; Kobryn, P. J. J. Chem. Soc., Dalton Trans. 1993,
101.
(22) Deplano, P.; Godfrey, S. M.; Isaia, F.; McAuliffe, C. A.; Mercuri, M.
L.; Trogu, E. F. Chem Ber./Recl. 1997, 130, 299.
(23) Gabes, W.; Stufkens, D. J. Spectrochim. Acta 1974, 30A, 1835.

Chemically Oxidized Ferrocenyl(phenyl)phosphines
Inorganic Chemistry, Vol. 39, No. 16, 2000 3511
Table 3. NMR Spectral Data for Compounds 6-8 and 10 in CD2Cl2
compound
1H
31P
FcPPh2I2 (6a)
4.30 s (6 H, Cp + 1 H), 4.38 br m (1 H),
4.52 br m (1 H), 4.76 br m (1 H),
7.35 br m (4 H), 7. 39 br m (6 H)
4.26 s (5H, Cp), 4.46 s (2H, H3,4),
4.76 s (2H, H2,5), 7.36 s (3H, C6H6),
7.6-7.8 m (10H, PPh2)
4.45 s (5 H, Cp), 4.60 q (2 H, H3,4
J23 ) 3 Hz, J34 ) 2 Hz), 5.02 q (2 H, H2,5
7.6-7.8 (10 H, Ph)
4.36 s (10 H, Cp), 4.39 m (2 H),
4.56 m (2 H), 4.75 m (2 H),
4.79 m (2 H), 7.62 m (2H), 7.69-7.8 m (3 H)
4.38 s (10 H, Cp), 4.47 m (2 H), 4.63 m (2 H),
4.84 m (2 H), 4.88 m (2 H), 7.35 s (3 H, C6H6)
7.62 m (2 H), 7.69-7.8 m (3 H)
4.44 s (10 H, Cp), 4.57 br m (2 H),
4.72 br m (2 H), 4.96 br m (2 H),
5.00 br m (2 H), 7.35 s (3 H, C6H6)
7.69 t (3 H, Ph), 7.77 t (2 H, Ph),
7.35 s (6 H, C6H6)
-11.5
FcPPh2I3/2 (6b)
6.9
[FcPPh2I][I3] (6c)
15.1
-5.9
3.7
)
Fc2PPhI2‚1/3 C6H6 (7a)
Fc2PPhI3/2‚1/2 C6H6 (7b)
[Fc2PPhI][I3]‚C6H6 (7c)
19.2
Fc3PI2 (8a)
4.37 s (6 H, Cp + 1 H), 4.40 m (1 H),
4.63 m (1 H), 4.81 m (1 H),
0
7.35 s (1 H, C6H6)
Fc3PI3/2‚1/6 C6H6 (8b)
4.35 s (15 H, Cp), 4.60 q (6 H, J ) 1.9 Hz),
4.84 q (6 H, J ) 1.8 Hz), 7.35 s (1 H, C6H6)
4.40 s (15 H, Cp), 4.66 q (6 H, H3
J35 ) 3 Hz, J23 ) 4.5 Hz), 4.92 q (6 H
7.2
[Fc3PI][I3]‚1/6 C6H6 (8c)
22.9
H2,5) 7.35 s (1 H, C6H6)
Fc2P(dO)PhI3‚1/2 C6H6 (10)
4.31 s (10 H, Cp), 4.42 br s (2 H), 4.7 br s
(2 H), 4.8 br s (2 H), 7.3-7.8 br (8 H, Ph, C6H6)
51
Scheme 1. Numbering of Phosphorus-Substituted Cp Rings
in Fc3-xPPhx
The electrochemistry of 5 shows two oxidations at 0.480 and
0.689 V in acetonitrile containing 5% CH2Cl2. The first
oxidation is reversible, but the second is irreversible. Compari-
son with the electrochemical potential of iodine (Table 1) under
the same conditions suggests that oxidation of 5 with I2 should
not occur but reaction between 5 and DDQ is favorable.
Addition of 1.5 equiv of iodine to a benzene solution of 5
nevertheless precipitates a dark solid 10 which analyzes for
1
Fc2P(dO)PhI3‚1/2C6H6. Room temperature H and 31P spectra
of 10 are broadened relative to those observed for 5-8. The
31P resonance for 10 is shifted upfield from 26 ppm in 5 to 51
1
ppm in 10. The H NMR spectrum of 10 consists of a singlet
for the unsubstituted Cp rings and four broad resonances for
the protons of the substituted rings. The peak width at half-
height for the Cp resonance increases from <2 Hz in 5 to 25
Hz in 10. As in 2 and 7, the two ferrocene groups are
inequivalent due to restricted rotation. These resonances are
shifted downfield relative to those of diferrocenyl(phenyl)-
phosphine oxide.
Mossbauer Spectroscopy. Mossbauer spectra of 6-9 salts
reveal isomer shifts (δ ≈ 0.45-0.50 mm/s) and quadrupole
splittings (∆EQ ≈ 2.30 mm/s) that are nearly identical to those
of the starting materials 1-4. Mossbauer spectra of low-spin
iron(II) in ferrocene derivatives consistently show isomer shifts
between 0.300 and 0.400 mm/s and quadrupole splits about
2.20-2.30 mm/s.24 “Ferrocenium-like”, low-spin iron(III) com-
pounds typically have isomer shifts similar to that of ferrocene
but with much smaller quadrupole splits (≈0.5 mm/s). For
example, ferricenium triiodide has an isomer shift of 0.417 mm/s
and a quadrupole split of 0.156 mm/s at 300 K.30 We find no
evidence for a species with an isomer shift of 0.380 mm/s and
near-zero quadrupole split in the Mossbauer spectrum of 7b as
reported earlier for [Fc2PPh][I3].9 Only for 6c are additional
species detected in the Mossbauer spectrum. A compound with
an isomer shift of 0.431 mm/s and zero quadrupole shift suggests
the presence of low-spin FeIII. A third compound with Moss-
bauer parameters typical for low-spin FeII (δ ) 0.781 mm/s
and ∆EQ ) 1.969 mm/s) is also observed.
Absorptions at 254 and 440 nm for 525 in the UV-vis
spectrum are replaced by absorptions at 294 and 364 nm in 10.
As in compounds 6-8, these absorptions are characteristic of
the I3- anion. The UV-vis spectrum of 10 does show a broad
feature centered at ≈500 nm. This “tail” on the 364 nm
absorption is observed at approximately the same wavelength
as the 500 nm absorption of [Fc][I3] but is not clearly resolved
from the former.
The infrared spectrum of 10 is very similar to the spectrum
of the starting material. A band at 1188 cm-1 (KBr) in 10
assigned to the PdO stretch is well within the range of
frequencies for PdO vibrations in triaryl and trialkyl phosphine
oxides.26 The PdO stretch in 5 has been reported as 1180
Reaction of Diferrocenyl(phenyl)phosphine Oxide (5) with
Iodine and 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ).
(25) The same UV-vis absorptions were reported previously: Neuse, E.
W. J. Organomet. Chem. 1967, 7, 349.
(24) Greenwood, N. N.; Gibbs, T. C. Mossbauer Spectroscopy; Chapman
(26) Gilheany D. G. In The Chemistry of Organophosphorus Compounds;
Hartley, F. R., Ed.; John Wiley: New York, 1992, Vol. 2, p 1.
& Hall: London, U.K., 1971.

3512 Inorganic Chemistry, Vol. 39, No. 16, 2000
Durfey et al.
are essentially unchanged in 11. The νCN at 2226 cm-1 is
observed at lower energy than it is in DDQ (νCN ) 2233 cm-1).
The intense CdO stretch at 1676 cm-1 and the ν(CdC) at 1550
cm-1 in DDQ are missing in 11. The change in νCN in 11 is
much smaller than that in [Cp2Fe][DDQ] (2209 cm-1) or [CpFe-
(C5H4-C5H4)FeCp][DDQ] (2212 cm-1). Strong aromatic νCd
of the DDQ•- radical anion at 1580 and 1543 cm-1 in
C
[Cp2Fe][DDQ]27 or 1576 and 1542 cm-1 in [CpFe(C5H4-C5H4)-
FeCp][DDQ] are absent in 11.
The UV-vis spectra of 11 in acetonitrile and dichloromethane
are dominated by absorptions by the DDQ portions of the
molecules. Four absorptions at λmax ) 258, 346, 434, and 546
nm and the corresponding extinction coefficients are similar to
those observed for [CpFe(C5H4-C5H4)FeCp][DDQ] and other
ferrocenium complexes containing the DDQ•- radical anion.27
Significantly, the Mossbauer spectra (Figure 2b) at 77 K
reveal that 11 contains primarily low-spin FeII with an isomer
shift of 0.498 mm/s and a quadrupole split of 2.322 mm/s. There
is clear evidence for another iron species with an isomer shift
of 0.465 mm/s and a quadrupole split of 0.935 mm/s. The value
of the quadrupole split is smaller than that in the valence
detrapped, mixed valent FeII/III species (“Fe2.5”) observed in
[Cp2*Fe2(pentalene)][BF4] (δ ) 0.41 mm/s, ∆EQ ) 1.30 mm/s
at 4.2 K) and [Cp2*Fe2(s-indacene)][BF4] (δ ) 0.55 mm/s, ∆EQ
) 1.67 mm/s at 4.8 K), but larger than that in low-spin FeIII in
[Cp2*Fe2(pentalene)][BF4]2 (δ ) 0.54 mm/s, ∆EQ ) 0.80 mm/s
at 4.2 K).28 The Mossbauer parameters for 11 are very different
from those in valence trapped [Cp2*Fe2(as-indacene)][BF4] (δ
) 0.49 mm/s, ∆EQ ) 0.43 mm/s and δ ) 0.50 mm/s, ∆EQ )
2.29 mm/s at 77 K). The fraction of mixed valent FeII/III species
increases at ambient temperature.
Figure 2. Mossbauer spectra of (a) Fc2P(dO)Ph‚I2 (10) and (b.) Fc2P-
(dO)Ph‚DDQ (11) at 77 K.
cm-1 in Nujol.12 We observe two strong absorptions at 1192
and 1178 cm-1 in KBr, either of which could be assigned to
the PdO stretch in Fc2P(dO)Ph. The lower energy absorption
is also seen in Fc2PPh (2) and in 7 suggesting that the absorption
at 1192 cm-1 is due to PdO stretching. The out-of-plane bend
for ferrocene C-H bonds at 826 cm-1 in 10 is essentially
unchanged from that of 5. Absorptions assigned to ferrocene-
phosphorus, phosphorus-phenyl, and phenyl group vibrations
are observed at energies in the range reported for 5 and
monosubstituted ferrocenes.12 Mossbauer spectra of 10 at both
77 and 293 K clearly show the presence of low-spin iron(II) (δ
Discussion
Taken together, the results of our molar conductivity, NMR,
UV-vis, and Mossbauer spectroscopy studies demonstrate that
the final product of reactions between ferrocenyl(phenyl)-
phosphines and iodine are iodophosphonium salts and not
ferricenium salts. The observation of well-resolved NMR spectra
is consistent with diamagnetic, low-spin FeII complexes rather
than paramagnetic, low-spin, FeIII ferricenium complexes such
as ferricenium triiodide, [Fc+][I3-].29 Mossbauer spectroscopy
confirms that the products contain low-spin FeII.
) 0.387 mm/s, ∆EQ ) 2.26 mm/s; δ ) 0.468 mm/s, ∆EQ
2.371 mm/s, respectively, in Figure 2a).
)
Titration of 5 with iodine in dichloroethane leads to an
increase in the molar conductivity but the titration profile and
final molar conductivity are quite different from those of
reactions between I2 and 1-3. The molar conductivity increases
linearly to a value of 9 cm2/mol-Ω at an I2/5 ratio of 3:1. This
is different from the sharp rise in Λ upon addition of the second
equivalent of I2 to 2. A slower increase to ≈12 cm2/mol-Ω is
observed as the I2 to 5 ratio reaches 9:1, a value 25% lower
than that for a similar ratio of I2 to 2.
Addition of an orange benzene solution of DDQ to a yellow
solution of 5 precipitates a paramagnetic, dark brown solid 11
after being stirred at room temperature for several days. For
comparison, the reaction between ferrocene and DDQ to form
[Cp2Fe][DDQ] is instantaneous in benzene. Microanalysis of
11 indicates a 1:2 stoichiometry of 5 to DDQ, [Fc2P(dO)Ph]-
[DDQ]2. Solid 11 is sparingly soluble in 1,2-dichloroethane and
other common solvents, thereby preventing measurement of
molar conductivity. The infrared spectrum of 11 has a sharp,
strong absorption at 1189 cm-1 which is assigned to the PdO
stretch. The PdO stretch occurs at slightly lower energy than
that in Fc2P(dO)Ph. Bands assigned to ferrocene-phosphine,
phenyl-phosphine, ferrocene C-C stretching, and C-H out-
of-plane bending for 5 in the literature and in our measurements
Compounds 7-9 closely resemble the iodophosphonium salt
isolated from reactions between triphenylphosphine and iodine
(eq 1).22 Reaction of PPh3 with 1 equiv of iodine produces a
I2
I2
Ph3P
98 [Ph3PI][I]
98 [Ph3PI][I3]
(1)
charge-transfer adduct between I- and Ph3PI+ which reacts with
additional iodine to form iodotriphenylphosphonium triiodide.22
Like [Ph3PI][I3], 7 and 8 are 1:1 electrolytes in dichloroethane
solution and have UV spectra consistent with the presence of
triiodide ions. Based on Mossbauer evidence, it seems likely
that poly(ferrocenylphenylphosphine) 4 also forms an iodo-
(ferrocenylphenylphosphonium) polymer similar to poly-
[{ferrocenyl(phenyl) (methyl)phosphonium}triflate].30 Proposed
structures for 6-9 are shown in Figure 3. Not surprisingly,
(27) Miller, J. S.; Krusic, P. J.; Dixon, D. A.; Reiff, W. M.; Zhang, J. H.;
Anderson, E. C.; Epstein, A. J. J. Am. Chem. Soc. 1986, 108, 4459.
(28) Pal, S. K.; Alagesan, K.; Samuelson, A. G.; Pebler, J. J. Organomet.
Chem. 1999, 575, 108.
(29) Manriquez, J. M.; Ward, M. D.; Reiff, W. M.; Calabrese, J. C.; Jones,
N. L.; Carroll, P. J.; Bunel, E. E.; Miller, J. S. J. Am. Chem. Soc.
1995, 117, 6182.
(30) Dong, T.-Y.; Lee, S.-H. J. Organomet. Chem. 1995, 487, 77.

Chemically Oxidized Ferrocenyl(phenyl)phosphines
Inorganic Chemistry, Vol. 39, No. 16, 2000 3513
Figure 4. Structures for (a) Ph3PdS‚I2 and (b) (Ph3PdS‚I2)2(µ-I2), and
(c) one possible structure for 10.
Figure 3. Proposed structures for iodo(ferrocenylphenyl)phosphonium
salts 6-9 with [In-] anions (n ) 1, 3).
cally characterized and contain monomeric Ph3PdE-I-I units
with a linear E-I-I linkage (Figure 4a). The structure of a 2:3
adduct between Ph3PdS and I2 (Figure 4b) was determined
earlier and found to contain two molecules of Ph3PdS‚I2
linked by a third I2 corresponding to an empirical formula Ph3Pd
SI3.34
compounds 7 and 8 resemble the diamagnetic, methyl iodide
salts of 2 and 3 reported over two decades ago.3 The first
member of the series, 6, is always isolated as a mixture of
compounds. In many respects, 6 is similar to 7 and 8 in its
NMR spectroscopy, molar conductivity, UV-vis, and IR
spectroscopy. The major component of 6 is likely to have a
structure similar to those shown for 7-9 in Figure 3. The
Mossbauer spectrum of 6, however, reveals the presence of a
mixture of iron compounds. The major component is a low-
spin FeII compound similar to 7-9. There is evidence for low-
spin FeIII as well as a second low-spin FeII species with a
different isomer shift and quadrupole split. Further characteriza-
tion of these three complexes has been thwarted by our inability
to separate and purify 6.
The reported isolation of [Fc2PPh][I3]9 prompted the synthesis
of 6b-9b. Our results for the reaction between ferrocenyl-
(phenyl)phosphines 1-4 and 2 equiv of iodine are different from
those reported in the literature. Whereas we find no evidence
for low-spin FeIII in the Mossbauer spectrum of our samples of
7b, Wei and co-workers report that reaction of 2 with 1.5 equiv
of I2 produces a compound containing at least some low-spin
FeIII. The percent of oxidation at iron is not reported.
One possible explanation we considered for the disparity in
the results is the reaction of small amounts of diferrocenyl-
(phenyl)phosphine oxide, 5, present as an impurity in com-
mercial samples of 2, with iodine. In principle, the reaction at
the phosphorus seen in 2 is avoided by oxidation of the
phosphorus(III) to phosphorus(V). Reaction of 5 with I2
produces a dark solid 10 with a formula Fc2P(dO)Ph‚I3
containing only low-spin iron(II) at both ambient temperature
and 77 K by Mossbauer spectroscopy. Clearly, the ferrocene
has not been oxidized to ferricenium. It is highly unlikely that
further reaction occurs at the phosphorus in 5 because there is
only one documented example of anodic oxidation of a formally
phosphorus(V) compound.31 Reaction of triphenylphosphine
chalcogenides Ph3PdE (E ) O, S, Se) with iodine, however,
yields products with a 1:1 stoichiometry between phosphine and
iodine.32,33 The Ph3PdO‚I2 complex readily loses iodine, but
the sulfur and selenium derivatives have been crystallographi-
The similarity in reaction chemistry of Fc3-xPPhx (1-3) and
triphenylphosphine with iodine makes it tempting to draw
similar conclusions for the corresponding phosphine oxides 5
and Ph3PdO. In the absence of an X-ray structure, structural
assignments for 10 must rely on spectroscopy. The ν(PdO) for
Ph3PdO‚I2 occurs at lower energy than for the parent Ph3PdO
by 15 cm-1. A strong absorption corresponding to the PdO
stretch in 5 is observed at 1192 cm-1 in KBr. In compound 10
a less intense absorption at 1188 cm-1 is consistent with the
donation of electron density from O to the iodine. The UV-
35
vis spectrum of Ph3PdO‚I2 showed two absorptions typical
of the I3- anion and identical to those observed for 10. Further
-
support for the presence of I3 in 10 is found in the molar
conductivity data (Λ ≈ 9 cm2/mol-Ω). The similarity of the
spectroscopy for 10 and Ph3PdO‚I2 leads us to propose the
structure for 10 in Figure 4c as a charge-transfer complex
between iodine and the oxygen of 5. The stoichiometry of 10
(Fc2P(dO)Ph‚I3) further suggests the dimeric structure seen for
(Ph3PdS)2(I2)3.
Ferrocene and alkylferrocenes readily form π-complexes
with p-benzoquinone, chloranil, tetracyanoethylene, and per-
fluorophenanthrene.36,37 In cases where the electrochemical
potentials of the acceptors do not allow for electron transfer
from ferrocene, neutral complexes are isolated. Stronger oxidants
such as DDQ oxidize ferrocene,15 biferrocene,38 and decam-
ethylferrocene27 yielding ionic, charge-transfer complexes con-
taining low-spin FeIII and the DDQ•- radical anion. The
crystallographically characterized [Cp*2Fe•+][DDQ•-] adopts the
(33) Apperley, D. C.; Bricklebank, N.; Burns, S. L.; Hibbs, D. E.;
Hursthouse, M. B.; Abdul Malik, K. M. J. Chem. Soc., Dalton Trans.
1998, 1289.
(34) Godfrey, S. M.; Jackson, S. L.; McAuliffe, C. A.; Pritchard, R. G. J.
Chem. Soc., Dalton Trans. 1997, 4499.
(35) Bransford, J. W.; Meyers, E. A. Cryst. Struct. Commun. 1978, 7, 697.
(36) Zingaro, R. A.; McGlothlin, R. E.; Meyers, E. A. J. Phys. Chem. 1962,
66, 2579.
(37) Brandon, R. L.; Osiecki, J. H.; Ottenberg, A. J. Org. Chem. 1966, 31,
1214.
(31) Peckham, T. J.; Lough, A. J.; Manners, I. Organometallics 1999, 18,
1030.
(32) Electrochemical oxidation of a triphenylphosphinephenylimine has
been suggested: Worner, C. J.; Gulick, W. M., Jr. Electrochim. Acta
1977, 22, 445.
(38) Beck, C. M.; Burdeneiuc, J.; Crabtree, R. H.; Rheingold, A. L.; Yap,
G. A. P. Inorg. Chim. Acta 1998, 270, 559.

3514 Inorganic Chemistry, Vol. 39, No. 16, 2000
Durfey et al.
complex 11 contains segregated chains of diferrocenyl(phenyl)-
phosphine oxide radical cations and chains of DDQ units (Figure
5).
The Mossbauer spectra of 11 confirm the presence of two
types of iron. The values for the quadrupole split in 11 bear a
strong resemblance to valence-averaged “Fe2.5+” found in other
dinuclear ferrocenes. One interpretation of the Mossbauer
spectrum is that 11 is delocalized on the Mossbauer time scale
(10-6 s). The difference between the first and second oxidation
potential for 5 (209 mV) is less than the typical value for
delocalized biferrocenes, but is consistent with communication
between metal centers on the electrochemical time scale (10-3
s). The observation of intervalence charge-transfer bands in the
near-IR spectrum of electrochemically generated 5+ has been
cited42 as evidence for delocalization in 5+.
The Mossbauer spectra of 11 also indicate that electron
transfer is incomplete at 77 K. The percentage of oxidized
species is less than 50% but appears to increase as the
temperature increases. These observations may simply reflect
differences in the recoil-free fraction as a function of temperature
or a temperature-dependent solid-state redox process in 11 which
favors electron transfer from iron to DDQ at higher temperatures
and reduction of FeIII at low temperature. Similar spectral
changes have been reported for [Cp*Fe]2(µ-pentalene) oxygen
complexes43 and recently for polyiodide salts of N,N′-bis-
(ferrocenylmethylidene)-p-phenylene diamine.28 Further inves-
tigations of the temperature-dependent redox process in 11 by
magnetic susceptibility and ESR spectroscopy are in progress.
Figure 5. Proposed segregated stack Structures for complex 11.
“mixed stack” structure of alternating donor and acceptor
molecules arranged in linear ‚‚‚‚D+A-D+A-D+A-D+A-‚‚‚
chains. Diferrocenyl(phenyl)phosphine oxide 5 is harder to
oxidize than ferrocene but the electrochemical potentials of 5
and DDQ indicate that electron transfer is favorable. Unlike
the typical 1:1 ratio in metallocene DDQ salts, reaction of 5
with DDQ precipitates 11 containing a 1:2 ratio of 5 to DDQ.
The same stoichiometry is observed in a related ferrocene
donor-acceptor complex, [Fe(C5H4Me)2][TCNQ]2.3939 The lat-
ter compound has been structurally characterized and adopts a
segregated stack structure containing parallel chains of donor
cations and acceptor anions.
Conclusions
The insolubility of 11 has prevented the growth of single
crystals for X-ray structure determination. Some insight into
the structure of 11 is obtained from spectroscopy. UV-vis
spectra of 11 are consistent with the presence of DDQ•- radical
anions as in other metallocene DDQ salts.27,28 The absence of
the carbonyl stretch in 11 is consistent with electron transfer
from iron to DDQ. Electrochemical potentials suggest that
oxidation of only one iron center in 5 by DDQ is favorable. As
a result, both of the DDQs in [Fc2P(dO)Ph][DDQ]2 (11) are
unlikely to be present as DDQ•- radical anions. Interactions
between the two DDQ molecules in 11 may be responsible for
the differences in the IR spectra of 11 and other DDQ salts of
metallocenes. Dimerization of anions in metallocene salts of
organic acceptors (e.g., TCNE, TCNQ) is frequently observed
in segregated stack structures consisting of parallel ‚‚‚
D+D+D+D+‚‚‚ and ‚‚‚A-A-A-A-‚‚‚ chains.40 Although con-
firmation of segregated stack structures for metallocene salts
of DDQ by X-ray diffraction has not been reported, the
dimerization of DDQ•- has been confirmed in a tetrathiaful-
valene salt.41 On this basis, we propose that the structure of
A combination of NMR, UV-vis, and Mossbauer spectros-
copy methods demonstrates that iodine reacts with ferrocenyl-
(phenyl)phosphines yielding iodo(ferrocenyl)(phenyl)phospho-
nium salts and not ferricenium salts. The correlation of our
results with the electrochemistry of ferrocenyl(phenyl)phos-
phines is difficult because the potential oxidant has an alternative
pathway available. Reactions of ferrocenyl(phenyl)phosphines
with oxidants lacking coordinating anions are in progress. The
results of these and related spectroelectrochemical studies on
ferrocenyl(phenyl)phosphines will be reported in a separate
paper. In cases where the phosphorus lone pair is unavailable
and electron transfer is feasible, charge-transfer complexes are
observed.
Acknowledgment. The authors thank Professor William M.
Reiff for assistance in obtaining the Mossbauer spectra.
Supporting Information Available: Mossbauer and NMR spectra
of 3 and 8c, Mossbauer spectra of 6c, plots of molar conductivity, and
31P chemical shifts as a function of I2:Fc3-xPPhx (x ) 0-2). This
material is available free of charge via the Internet at http://pubs.acs.org.
IC991147Y
(39) Wilson, S. R.; Corvan, P. J.; Seiders, R.; Hodgson, D. J.; Brookhart,
M.; Hatfield, W. E.; Miller, J. S.; Reis, A. H.; Rogan, P. K.; Gebert,
E.; Epstein, A. J. NATO Conf. Ser. 1978, 6, 407.
(42) Delgado-Pena, F.; Talham, D. R.; Cowan, D. O. J. Organomet. Chem.
1983, 253, C43.
(40) Miller, J. S.; Zhang, J. H.; Reiff, W. M. Inorg. Chem. 1987, 26, 600.
(41) Mayerle, J. J.; Torrance, J. B. Bull. Chem. Soc. Jpn. 1981, 54, 3170.
(43) Reiff, W. M.; Kreisz, J.; Kirss, R. U. Hyperfine Interact. 1994, 93,
1605.