Reactivity of Hydrides FeH2(CO)2P2 (P = Phosphites) with Aryldiazonium Cations: Preparation, Characterization, X-ray Crystal Structure, and Electrochemical Studies of Mono- and Binuclear Aryldiazenido Complexes

Article abstract of DOI:10.1021/ic980430e

Mono- and binuclear aryldiazenido complexes [Fe(ArN₂)(CO)₂P₂]BPh₄ (1-4) and [{Fe(CO)₂P₂}₂(μ-N₂Ar-ArN ₂)](BPh₄)₂ (5-8) [P = P(OEt)₃, PPh(OEt)₂, PPh₂OEt, P(OPh)₃; Ar = C₆H₅, 2-CH₃C₆H₄, 4-CH₃C₆H₄; Ar-Ar = 4,4′-C₆H₄-C₆H₄, 4,4′-(2-CH₃)C₆H₃-C₆H ₃(2-CH₃), 4,4′-C₆H₄-CH₂-C₆H₄] were prepared by allowing hydride species FeH₂(CO)₂P₂ to react with an excess of mono- (ArN₂)(BF₄) or bis-aryldiazonium (N₂Ar-ArN₂)(BF₄)₂ salts, respectively, at low temperature. A reaction path involving a hydride-aryldiazene intermediate [FeH-(ArN=NH)(CO)₂P₂]⁺, which, through the loss of H₂, affords the final aryldiazenido complexes 1-8, is proposed. The compounds were characterized by ¹H and ³¹P{¹H} NMR spectroscopy (including ¹⁵N isotopic substitution) and X-ray crystal structure determination. The complex [Fe(CO)₂{P(OEt)₃}₂{μ-4,4′-N ₂(2-CH₃)C₆H₃-C₆H ₃(2-CH₃)N₂}](BPh₄)₂ (5b) crystallizes in the space group P1^- with a = 15.008(4) A?, b = 17.094(5) A?, c = 10.553(3) A?, α = 99.56(1)°, β= 102.80(1)°, γ = 65.30(1)°, and Z = 1. The structure is centrosymmetric and consists of binuclear cations with the two iron atoms in a quite regular trigonal bipyramidal environment, with the two CO in the equatorial and the two phosphites in the apical position, respectively. Aryldiazenido complexes 1-8 react with strong acids HX (X = Cl, CF₃SO₃, CF₃CO₂) to give the corresponding aryldiazene derivatives, according to the equilibrium [Fe(ArN₂)(CO)₂P₂]⁺ + HX ? [FeX(ArN=NH)(CO)₂P₂]⁺. Electrochemical studies of both mono- (1-4) and binuclear (5-8) compounds were undertaken, and a mechanism for oxidation and reduction processes is proposed.

Full text of DOI:10.1021/ic980430e

5602
Inorg. Chem. 1998, 37, 5602-5610
Reactivity of Hydrides FeH2(CO)2P2 (P ) Phosphites) with Aryldiazonium Cations:
Preparation, Characterization, X-ray Crystal Structure, and Electrochemical Studies of
Mono- and Binuclear Aryldiazenido Complexes
Gabriele Albertin,*,† Stefano Antoniutti,† Alessia Bacchi,‡ Davide Barbera,†
Emilio Bordignon,† Giancarlo Pelizzi,‡ and Paolo Ugo§
Dipartimento di Chimica and Dipartimento di Chimica Fisica, Universita` Ca’ Foscari di Venezia,
Dorsoduro 2137, 30123 Venezia, Italy, and Dipartimento di Chimica Generale ed Inorganica,
Chimica Analitica, Chimica Fisica, Centro CNR di Strutturistica Diffrattometrica, Universita` di Parma,
Viale delle Scienze, 43100 Parma, Italy
ReceiVed April 16, 1998
Mono- and binuclear aryldiazenido complexes [Fe(ArN2)(CO)2P2]BPh4 (1-4) and [{Fe(CO)2P2}2(µ-N2Ar-ArN2)]-
(BPh4)2 (5-8) [P ) P(OEt)3, PPh(OEt)2, PPh2OEt, P(OPh)3; Ar ) C6H5, 2-CH3C6H4, 4-CH3C6H4; Ar-Ar )
4,4′-C6H4-C6H4, 4,4′-(2-CH3)C6H3-C6H3(2-CH3), 4,4′-C6H4-CH2-C6H4] were prepared by allowing hydride
species FeH2(CO)2P2 to react with an excess of mono- (ArN2)(BF4) or bis-aryldiazonium (N2Ar-ArN2)(BF4)2
salts, respectively, at low temperature. A reaction path involving a hydride-aryldiazene intermediate [FeH-
(ArNdNH)(CO)2P2]+, which, through the loss of H2, affords the final aryldiazenido complexes 1-8, is proposed.
1
The compounds were characterized by H and 31P{1H} NMR spectroscopy (including 15N isotopic substitution)
and X-ray crystal structure determination. The complex [Fe(CO)2{P(OEt)3}2{µ-4,4′-N2(2-CH3)C6H3-C6H3(2-
CH3)N2}](BPh4)2 (5b) crystallizes in the space group P1h with a ) 15.008(4) Å, b ) 17.094(5) Å, c ) 10.553(3)
Å, R ) 99.56(1)°, â ) 102.80(1)°, γ ) 65.30(1)°, and Z ) 1. The structure is centrosymmetric and consists of
binuclear cations with the two iron atoms in a quite regular trigonal bipyramidal environment, with the two CO
in the equatorial and the two phosphites in the apical position, respectively. Aryldiazenido complexes 1-8 react
with strong acids HX (X ) Cl, CF3SO3, CF3CO2) to give the corresponding aryldiazene derivatives, according
to the equilibrium [Fe(ArN2)(CO)2P2]+ + HX h [FeX(ArNdNH)(CO)2P2]+. Electrochemical studies of both
mono- (1-4) and binuclear (5-8) compounds were undertaken, and a mechanism for oxidation and reduction
processes is proposed.
Introduction
fixation process but also to their diverse reactivity modes and
structural properties, this paper reports the synthesis and
Previous reports1-5 from our laboratory have dealt with
studies on the reactions of metal(II) dihydrides MH2P4 (M )
Fe, Ru, Os; P ) phosphite) with aryldiazonium cations, which
proceed to give mono- and bis(aryldiazene) [MH(ArNdNH)-
P4]BPh4 and [M(ArNdNH)2P4](BPh4)2 derivatives through the
insertion of the ArN2+ cations into the M-H bonds. We have
now extended these studies to include the reactivity of iron
dihydrides FeH2(CO)2P2 with aryldiazonium cations and found,
instead of aryldiazene complexes, the formation of aryldiazenido
[Fe(ArN2)(CO)2P2]+ cations in good yields.
(9) Johnson, B. F. G.; Haymore, B. L.; Dilworth, J. R. In ComprehensiVe
Coordination Chemistry; Wilkinson, G., Gillard, R. D., McCleverty,
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Radiochem. 1983, 27, 197.
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(15) Sellmann, D. Angew. Chem., Int. Ed. Engl. 1993, 32, 64.
(16) Yan, X.; Batchelor, R. J.; Einstein, F. W. B.; Zhang, X.; Nagelkerke,
R.; Sutton, D. Inorg. Chem. 1997, 36, 1237.
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Am. Chem. Soc. 1997, 36, 8869.
(18) Hirsch-Kuchma, M.; Nicholson, T.; Davison, A.; Davis, W. M.; Jones,
A. G. Inorg. Chem. 1997, 36, 3237.
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(20) Rose, D. J.; Maresca, K. P.; Kettler, P. B.; Chang, Y. D.; Saghomo-
mian, V.; Chen, Q.; Abrams, M. J.; Larsen, S. K.; Zubieta, J. Inorg.
Chem. 1996, 35, 3548.
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D. Inorg. Chem. 1995, 34, 6163.
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35, 4038.
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Chem., Int. Ed. Engl. 1994, 33, 657.
In view of current interest6-29 in the chemistry of the “diazo”
complexes, due not only to their relevance to the nitrogen
† Dipartimento di Chimica, Universita` Ca’ Foscari di Venezia.
‡
Dipartimento di Chimica Generale ed Inorganica, Universita` di Parma.
§ Dipartimento di Chimica Fisica, Universita` Ca’ Foscari di Venezia.
(1) Albertin, G.; Antoniutti, S.; Pelizzi, G.; Vitali, F.; Bordignon, E. J.
Am. Chem. Soc. 1986, 108, 6627.
(2) Albertin, G.; Antoniutti, S.; Bordignon, E. Inorg. Chem. 1987, 26,
3416.
(3) Albertin, G.; Antoniutti, S.; Pelizzi, G.; Vitali, F.; Bordignon, E. Inorg.
Chem. 1988, 27, 829.
(4) Albertin, G.; Antoniutti, S.; Bordignon, E. J. Chem. Soc., Dalton Trans.
1989, 2353.
(5) Albertin, G.; Antoniutti, S.; Bacchi, A.; Bordignon, E.; Pelizzi, G.;
Ugo, P. Inorg. Chem. 1996, 35, 6245.
(6) Zollinger, H. Diazo Chemistry II; VCH: Weinheim, Germany, 1995.
(7) Sutton, D. Chem. ReV. 1993, 93, 995.
(8) Kisch, H.; Holzmeier, P. AdV. Organomet. Chem. 1992, 34, 67.
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32, 960.
10.1021/ic980430e CCC: $15.00 © 1998 American Chemical Society
Published on Web 10/02/1998

Reactivity of Hydrides FeH2(CO)2P2
Inorganic Chemistry, Vol. 37, No. 21, 1998 5603
a. FeH2(CO)2P2 [P ) PPh(OEt)2 and PPh2OEt]. These com-
plexes were prepared by reacting K[FeH(CO)4] with the appropriate
phosphite following the method reported for the related P(OEt)3 and
P(OPh)3 derivatives.34 A reaction time of 5 h was used, and the
complexes were extracted with a mixture of petroleum ether (40-60
°C) and benzene (4:1 ratio) giving, after crystallization, a light-green
oil at room temperature; yield g 85%. For P ) PPh(OEt)2: IR, 1998,
Chart 1
1
1954 [s, ν(CO)] cm-1; H NMR (CD3C6D5, 25 °C), δ 7.56-6.90 (m,
10 H, Ph), 3.64, 3.44 (m, 8 H, CH2), 0.84 (t, 12 H, CH3), -10.40 (t,
2 H, hydride); 31P{1H} NMR (CD3C6D5, 25 °C), δ 196.0 (s). For P )
1
PPh2OEt: IR, 1991, 1952 [s, ν(CO)] cm-1; H NMR (CD3C6D5, 25
°C), δ 7.60-6.70 (m, 20 H, Ph), 3.50 (m, 4 H, CH2), 0.82 (t, 6 H,
CH3), -9.83 (t, 2 H, hydride); 31P{1H} NMR (CD3C6D5, 25 °C), δ
171.6 (s).
b. [Fe(ArN2)(CO)2P2]BPh4 (1-4) [P ) P(OEt)3 (1), PPh(OEt)2
(2), PPh2OEt (3), P(OPh)3 (4); Ar ) C6H5 (a), 2-CH3C6H4 (b),
4-CH3C6H4 (c)]. A solution of the appropriate hydride FeH2(CO)2P2
(2 mmol) in 15 mL of CH2Cl2 was cooled to -80 °C and quickly
transferred by needle into a reaction flask containing an excess of the
aryldiazonium salt (5 mmol), previously cooled to -80 °C. The
reaction mixture was allowed to reach room temperature, stirred for
7-8 h, and then filtered to remove the unreacted diazonium salt. The
solvent was removed under reduced pressure, giving a brown oil, which
was treated with 5 mL of ethanol. The addition of an excess of NaBPh4
(4 mmol, 1.37 g) in 5 mL of ethanol to the resulting solution caused
the precipitation of a red solid, which was filtered and crystallized from
CH2Cl2 (3 mL) and ethanol (5 mL). In some cases, mainly with PPh-
(OEt)2 or PPh2OEt derivatives (2c, 3c), an oil was obtained after the
addition of NaBPh4 that was dissolved in acetone and chromatographed
through a silica gel column (20 cm × 2 cm) using acetone as eluent.
The eluate (60 mL) was evaporated to dryness, giving an oil, which
was triturated with 4 mL of ethanol. After the mixture was stirred for
2-3 h, a red solid separated out from the solution, which was filtered
and dried under vacuum; yield from 40 to 70%. Anal. Calcd for 1a:
C, 60.85; H, 6.38; N, 3.23. Found: C, 61.01; H, 6.40; N, 3.40. ΛM
) 49.3 Ω-1 mol-1 cm2. Anal. Calcd for 1b: C, 61.24; H, 6.51; N,
3.17. Found: C, 61.07; H, 6.42; N, 3.10. ΛM ) 51.6 Ω-1 mol-1 cm2.
Anal. Calcd for 1c: C, 61.24; H, 6.51; N, 3.17. Found: C, 61.35; H,
6.30; N, 3.08. ΛM ) 57.8 Ω-1 mol-1 cm2. Anal. Calcd for 2c: C,
characterization of new aryldiazenido complexes of iron,
together with detailed studies on the reaction course between
FeH2(CO)2P2 and ArN2+. The use of bis(aryldiazonium) cations
(N2Ar-ArN2)(BF4)2 as reagents (Chart 1) also yields binuclear
complexes with a bis(diazenido) bridging ligand whose first
X-ray crystal structure determination is also reported here,
together with an electrochemical study of the new aryldiazenido
derivatives.
Experimental Section
All synthetic work was carried out in appropriate atmospheres (Ar,
N2) using standard Schlenk techniques or a Vacuum Atmosphere
drybox. All solvents were dried over appropriate drying agents,
degassed on a vacuum line, and distilled into vacuum-tight storage
flasks. Triethyl phosphite and triphenyl phosphite were Aldrich
products; phosphines PPh(OEt)2 and PPh2OEt were prepared by the
method of Rabinowitz and Pellon.30 Diazonium salts were obtained
in the usual way.31 Related bis(diazonium) [N2Ar-ArN2](BF4)2 [Ar-
Ar ) 4,4′-C6H4-C6H4, A; 4,4′-(2-CH3)C6H3-C6H3(2-CH3), B; 4,4′-
C6H4-CH2-C6H4, C] salts were prepared by treating the amine
precursors H2NAr-ArNH2 with NaNO2, as described in the literature
for the common mono(diazonium) salts.31 Labeled diazonium tet-
rafluoroborates [C6H5Nt15N]BF4 and [4,4′-15NtNC6H4-C6H4Nt
15N](BF4)2 were prepared from Na15NO2 (99% enriched, CIL) and the
appropriate amine. p-Tolyl isocyanide was obtained by the phosgene
method of Ugi et al.32 Other reagents were purchased from commercial
sources in the highest available purity and used as received. Infrared
spectra were recorded on Digilab Bio-Rad FTS-40 or Nicolet Magna
750 FT-IR spectrophotometers. NMR spectra (1H, 13C, 31P, 15N) were
obtained on a Bruker AC200 spectrometer at temperatures varying
between +30 and -90 °C, unless otherwise noted. 1H and 13C spectra
refer to internal tetramethylsilane. 31P{1H} chemical shifts are reported
with respect to 85% H3PO4, with downfield shifts considered positive.
15N spectra refer to external CH315NO2, with downfield shifts considered
positive. The SwaN-MR software package33 has been used in treating
the NMR data. All spectroscopic data are summerized in Table 1. The
conductivities of 10-3 M solutions of the complexes in CH3NO2 at 25
°C were measured with a Radiometer CDM 83 instrument.
67.25; H, 6.07; N, 2.96. Found: C, 67.02; H, 6.20; N, 3.01. ΛM
)
58.2 Ω-1 mol-1 cm2. Anal. Calcd for 3c: C, 72.49; H, 5.86; N, 2.77.
Found: C, 72.65; H, 5.78; N, 2.64. ΛM ) 54.8 Ω-1 mol-1 cm2. Anal.
Calcd for 4b: C, 70.78; H, 4.91; N, 2.39. Found: C, 70.65; H, 4.83;
N, 2.31. ΛM ) 53.6 Ω-1 mol-1 cm2. Anal. Calcd for 4c: C, 70.78;
H, 4.91; N, 2.39. Found: C, 70.65; H, 4.77; N, 2.35. ΛM ) 52.9 Ω-1
mol-1 cm2.
c. [Fe(C6H5Nt15N)(CO)2{P(OEt)3}4]BPh4 (1a1). This compound
was prepared exactly like the related compound 1a using the labeled
[C6H5Nt15N]+BF4 aryldiazonium salt; yield g 60%.
-
d. [{Fe(CO)2P2}2(µ-N2Ar-ArN2)](BPh4)2 (5-8) [P ) P(OEt)3
(5), PPh(OEt)2 (6), PPh2OEt (7), P(OPh)3 (8); Ar-Ar ) 4,4′-C6H4-
C6H4 (a), 4,4′-(2-CH3)C6H3-C6H3(2-CH3) (b), 4,4′-C6H4-CH2-C6H4
(d)]. A solution of the appropriate hydride FeH2(CO)2P2 (2 mmol) in
30 mL of acetone was cooled to about -80 °C and quickly transferred
by needle into a 50 mL three-necked round-bottomed flask cooled to
-80 °C and containing an excess of the bis(aryldiazonium) salt (3
mmol). The reaction mixture was brought to room temperature, stirred
for about 12 h, and then filtered to separate the unreacted diazonium
salt. The resulting solution was evaporated to dryness under reduced
pressure, giving a red-brown oil, which was treated with 5 mL of
ethanol. The addition of an excess of NaBPh4 (4 mmol, 1.37 g) in 5
mL of ethanol caused the separation of a red solid in some cases and
of a red oil in others (6a, 7a, 7b). All the compounds, however, were
purified by chromatography on silica gel using a 30 cm × 2 cm column
and acetone as an eluent. The first eluate (about 80 mL) was evaporated
to dryness under reduced pressure, giving an oil, which was triturated
with 5 mL of ethanol. A red solid slowly separated out under vigorous
stirring, which was filtered and dried under vacuum; yield from 40 to
Synthesis of Complexes. Hydrides FeH2(CO)2[P(OEt)3]2 and FeH2-
(CO)2[P(OPh)3]2 were prepared as previously reported.34
(27) Glassman, T. E.; Vale, M. G.; Schrock, R. R. J. Am. Chem. Soc. 1992,
114, 8098.
(28) Kawano, M.; Hoshino, C.; Matsumoto, K. Inorg. Chem. 1992, 31,
5158.
(29) Vogel, S.; Barth, A.; Huttner, G.; Klein, T.; Zsolnai, L.; Kremer, R.
Angew. Chem., Int. Ed. Engl. 1991, 30, 303.
(30) Rabinowitz, R.; Pellon, J. J. Org. Chem. 1961, 26, 4623.
(31) Vogel, A. I. Practical Organic Chemistry, 3rd ed.; Longmans, Green
and Co.: New York, 1956.
(32) Ugi, I.; Fetzer, U.; Eholzer, W.; Knupfer, H.; Offermann, K. Angew.
Chem., Int. Ed. Engl. 1965, 4, 472.
(33) Balacco, G. J. Chem. Inf. Comput. Sci. 1994, 34, 1235.
(34) Brunet, J. J.; Kindela, F. B.; Neibecker, D. Inorg. Synth. 1992, 29,
156.

5604 Inorganic Chemistry, Vol. 37, No. 21, 1998
Table 1. Selected IR and NMR Data for Iron Complexes
Albertin et al.
IRb
1H NMRc,d
assgnt
31P{1H} NMRc,e
compda
ν, cm-1
assgnt
δ
δ
1a [Fe(C6H5N2)(CO)2{P(OEt)3}2]+
2045, s
1988, s
νCO
4.27, m
1.29, t
CH2
CH3
145.8, s
1743, m
2044, s
1988, s
1708, m
2040, s
1977, s
1751, m
2040, s
1984, s
1736, m
2033, s
1978, s
1734, m
2028, s
1972, s
1731, m
νN
2
1a1 [Fe(C6H5Nt15N)(CO)2{P(OEt)3}2]+
1b [Fe(2-CH3C6H4N2)(CO)2{P(OEt)3}2]+
1c [Fe(4-CH3C6H4N2)(CO)2{P(OEt)3}2]+
2c [Fe(4-CH3C6H4N2)(CO)2{PPh(OEt)2}2]+
3c [Fe(4-CH3C6H4N2(CO)2(PPh2OEt)2]+
νCO
4.27, m
1.28, t
CH2
CH3
145.8, d
2
15
J
NP ) 20 Hz
15
ν
νCO
NtN
4.26, m
2.35, s
1.29, t
4.26, m
2.43, s
1.29, t
4.23, m
2.35, s
1.35, t
4.05, m
3.57, m
2.32, s
1.29, t
1,12, t
2.19, s
CH2
CH3
CH3 phos
CH2
CH3
CH3 phos
CH2
CH3
CH3 phos
CH2
146.5, s
146.3, s
173.0, s
152.8, s
νN
2
νCO
νN
2
νCO
νN
2
νCO
νN
CH3
CH3 phos
2
4b [Fe(2-CH3C6H4N2)(CO)2{P(OPh)3}2]+
2056, s
1992, s
1779, m
2055, s
1987, s
1776, m
2046, s
1991, s
1733, m
2040, s
1992, s
1701, m
2042, s
1987, s
1743, m
2042, s
1988, s
1735, m
2036, s
1982, s
1720, m
2035, s
1980, s
1733, m
2030, s
1973, s
1723, m
νCO
CH3
147.0, s
146.7, s
νN
2
4c [Fe(4-CH3C6H4N2)(CO)2{P(OPh)3}2]+
νCO
2.46, s
CH3
νN
2
5a {[Fe(CO)2{P(OEt)3}2]2(µ-4,4′-N2C6H4-C6H4N2)}2+
5a1 {[Fe(CO)2{P(OEt)3}2]2(µ-4,4′-15NtNC6H4-C6H4Nt15N)}2+ g
5b {[Fe(CO)2{P(OEt)3}2]2[µ-4,4′-N2(2-CH3)C6H3-C6H3(2-CH3)N2]}2+
5d {[Fe(CO)2{P(OEt)3}2]2(µ-4,4′-N2C6H4-CH2-C6H4N2)}2+
6a {[Fe(CO)2{PPh(OEt)2}2]2(µ-4,4′-N2C6H4-C6H4N2)}2+
6b {[Fe(CO)2{PPh(OEt)2}2]2[µ-4,4′-N2(2-CH3)C6H3-C6H3(2-CH3)N2]}2+
7a [{Fe(CO)2(PPh2OEt)2}2(µ-4,4′-N2C6H4-C6H4N2)]2+
νCO
4.30, m
1.30, t
CH2
CH3
145.4, s
(147.6, s)f
νN
2
νCO
4.30, m
1.31, t
CH2
CH3
145.4, d
2
15
J
NP ) 20 Hz
15
ν
νCO
NtN
4.27, m
2.39, s
1.30, t
4.26, m
4.18, s
1.27, t
4.25, br
1.35, t
CH2
CH3
CH3 phos
CH2 phos
CH2 diazo
CH3
146.0, s
146.0, s
170.0, s
170.6, s
152.4, s
νN
2
νCO
νN
2
νCO
CH2
CH3
νN
2
νCO
4.28, m
2.27, s
1.39, t
4.10, m
3.57, m
1.33, t
1.13, t
4.08, m
3.58, m
2.40, s
1.31, t
1,13, t
CH2
CH3
CH3 phos
CH2
νN
2
νCO
νN
CH3
CH2
2
7b [{Fe(CO)2(PPh2OEt)2}2{µ-4,4′-N2(2-CH3)C6H3-C6H3(2-CH3)N2}]2+
2028, s
1972, s
1733, m
νCO
153.1, s
149.2, sf
νN
CH3
CH3 phos
2
8a {[Fe(CO)2{P(OPh)3}2]2(µ-4,4′-N2C6H4-C6H4N2)}2+
2054, s
2000, s
1759, m
νCO
νN
2
a All compounds are BPh4 salts. b In KBr pellets. c At room temperature in (CD3)2CO, unless otherwise noted. d Phenyl proton resonances are
-
g
15
2
15 31
omitted. e Positive shift downfield from 85% H3PO4. f At -80 °C.
N NMR (CD2Cl2, 25 °C): δ 19.7 t ( J
) 20 Hz).
N
P
70%. Anal. Calcd for 5a: C, 60.92; H, 6.27; N, 3.23. Found: C,
60.75; H, 6.31; N, 3.16. ΛM ) 123 Ω-1 mol-1 cm2. Anal. Calcd for
5b: C, 61.31; H, 6.40; N, 3.18. Found: C, 61.46; H, 6.47; N, 3.16.
ΛM ) 112 Ω-1 mol-1 cm2. Anal. Calcd for 5d: C, 61.12; H, 6.34;
N, 3.20. Found: C, 61.25; H, 6.21; N, 3.08. ΛM ) 118 Ω-1 mol-1
cm2. Anal. Calcd for 6a: C, 67.04; H, 5.84; N, 3.01. Found: C,
67.16; H, 6.02; N, 2.93. ΛM ) 124 Ω-1 mol-1 cm2. Anal. Calcd for
6b: C, 67.32; H, 5.97; N, 2.96. Found: C, 67.19; H, 6.10; N, 2.88.
ΛM ) 123 Ω-1 mol-1 cm2. Anal. Calcd for 7a: C, 72.38; H, 5.47;
N, 2.81. Found: C, 72.21; H, 5.41; N, 2.75. ΛM ) 136 Ω-1 mol-1
cm2. Anal. Calcd for 7b: C, 72.56; H, 5.59; N, 2.77. Found: C,
72.39; H, 5.50; N, 2.66. ΛM ) 129 Ω-1 mol-1 cm2. Anal. Calcd for
8a: C, 70.67; H, 4.71; N, 2.42. Found: C, 70.54; H, 4.63; N, 2.37.
ΛM ) 125 Ω-1 mol-1 cm2.
e. [{Fe(CO)2[P(OEt)3]2}2(µ-4,4′-15NtNC6H4-C6H4Nt15N)](BPh4)2
(5a1). This compound was prepared like the related 5a using the labeled
[4,4′-15NtNC6H4-C6H4Nt15N](BF4)2 bis(diazonium) salt; yield g
70%.
Protonation Reactions. The protonation reactions of aryldiazenido
complexes 1a, 1a1, 1c, and 5a were carried out both in a 25 mL three-
necked round-bottomed flask and in a 5-mm NMR tube. In all cases,
an excess of the appropriate acid (CF3SO3H, CF3COOH, or HCl 0.1
M in diethyl ether) was added to the solution of the complex cooled to
-80 °C and, after room temperature was reached, the progress of the

Reactivity of Hydrides FeH2(CO)2P2
Inorganic Chemistry, Vol. 37, No. 21, 1998 5605
Table 2. X-ray Diffraction Data Collection and Structure
Refinement for Compound [{Fe(CO)2[P(OEt)3]2}2-
{µ-4,4′-N2(2-CH3)C6H3-C6H3(2-CH3)N2}](BPh4)2 (5b)
were also constrained to rigid-body behavior in order to decrease the
number of refined parameters. The final difference density map was
essentially featureless. Neutral scattering factors were employed and
anomalous dispersion terms were included for non-hydrogen atoms.
Calculations were performed on an ENCORE91 computer. The
programs PARST9538 and ZORTEP39 were employed to analyze the
final geometry. Use was made of the Cambridge Structural Database40
facilities at the “Centro di Studio per la Strutturistica Diffrattometrica
del C.N.R.” in Parma. The complete list of atomic coordinates,
geometric parameters, and thermal displacement parameters have been
deposited in CIF format as Supporting Information.
chem formula:
C90H112B2Fe2N4O16P4
fw ) 1763.04
µ ) 0.43 mm-1
F(000) ) 930
crystal system: triclinic
space group: P1h
radiation: Mo KR
λ ) 0.710 69 Å
absorption Tmax/Tmin ) 1.11/0.83
scan mode ) θ-2θ
θ range ) 3-28°
index range ) -19 e h e 19,
-22 e k e 22, 0 e l e 13
a ) 15.008(4) Å
b ) 17.094(5) Å
c ) 10.553(3) Å
R ) 99.56(1)°
â ) 102.80(1)°
γ ) 65.30(1)°
V ) 2390(1) Å3
Z ) 1
Electrochemical Apparatus and Procedures. Voltammetric and
chronoamperometric experiments were carried out in a three-electrode
cell. The working electrode was a glassy carbon disk (diameter 0.5
cm) mirror-polished with graded alumina powder and surrounded by a
platinum spiral counter electrode. The potential of the working
electrode was probed by a Luggin capillary reference electrode
compartment. All potentials were measured and refer to an aqueous
KCl saturated Ag/AgCl reference electrode. The E1/2 value for the
ferrocene/ferricinium (Fc/Fc+) couple measured in the supporting
electrolyte used in this work was +520 mV vs Ag/AgCl. The specific
resistance of the electrolyte solution, measured with a Crison CM 2202
conductometer, was 1250 Ω cm. All measurements were carried out
at room temperature in a nitrogen atmosphere. The voltammetric
equipment used was an Amel model 552 potentiostat, with positive
feedback correction of uncompensated resistance, in conjunction with
an Amel model 568 digital logic function generator, a Yokogawa 3023
X-Y recorder, or an EG&G-PAR model 273 potentiostat controlled
by a personal computer via M270 software.
Digital simulations of the experimental voltammograms were carried
out using Digisim 2.0 (BAS Inc.), a cyclic voltammetric simulation
program, running on a HP 735/90 CPU Pentium computer. The
voltammograms of the binuclear compounds were simulated by the
following mechanism (Chart 2), where A is the starting binuclear
compound.
For instance, in the case of [{Fe(CO)2[P(OEt)3]2}2{µ-4,4′-N2C6H4-
C6H4N2}](BPh4)2 (5a), the fit between simulated and experimental data
was optimized by the following values of the simulation parameters:
measured reflections ) 11 524
independent reflections ) 11 524
observed reflections [I g 2σ(I)] ) 3117
parameters/restraints ) 492/234
∆Fmax,min ) 0.41/-0.36 e Å-1
R1a (observed data) ) 0.0717
wR2b (all data) ) 0.3003
Dcalcd ) 1.22 g cm-3
goodness of fit ) 0.875
a R1
)
∑||Fo|
-
|Fc||/∑|Fo|. b wR2
)
{∑[w(Fo
-
Fc2)2]/
2
∑[w(Fo )2]}1/2}; w ) 1/[σ2(Fo ) + (aP)2 + bP], P ) [2Fc2
+
2
2
2
max(Fo ,0)]/3.
1
reaction was monitored by both IR (using 0.2 mm KBr cells) and H
and 31P NMR spectra.
X-ray Analysis of [{Fe(CO)2[P(OEt)3]2}2{µ-4,4′-N2(2-CH3)C6H3-
C6H3(2-CH3)N2}](BPh4)2 (5b). Suitable crystals for X-ray analysis
were obtained by recrystallization from ethanol. Automatic peak search
and indexing procedures carried out on a Siemens AED diffractometer
yielded a triclinic primitive cell. Inspection of E statistics indicated
the space group as P1h. Data on structure determination are summarized
in Table 2. Unit-cell dimensions and their standard deviations were
determined and refined from the angular positions of 25 carefully
measured reflections. During data collection, the intensity of one
standard reflection was monitored to check crystal decomposition or
loss of alignment. No intensity decay was detected. Polarization and
Lorentz effects were considered for data reduction. Both direct methods
and the automatic Patterson procedure failed to identify the correct
solution, and the best phase sets found in both cases presented four
heavy collinear peaks spaced by approximately 2.2 Å in the E-map.
When these apparent heavy atoms were used to phase the structure, a
sort of dimeric molecule, consisting of the splitting and shift of the
expected structure, appeared but did not refine. The correct position
of the single Fe atom was finally located in the midpoint of the four
heavy atom substructure when merging of the two images was observed.
The two apparent heavy atoms in the initial solution were in fact at
the midpoints of the collinear Fe-P bonds in the complex. The phasing
power of the correctly placed Fe atom was sufficient to retrieve the
entire structure by inspection of the Fourier electron density map.
The structure consists of discrete [{Fe(CO)2[P(OEt)3]2}2{µ-4,4′-N2-
(2-CH3)C6H3-C6H3(2-CH3)N2}]2+ cations and BPh4- anions. All non-
hydrogen atoms were initially refined by several cycles of full-matrix
least-squares using F2 by SHELXL96,35 with isotropic thermal displace-
ment parameters. At convergence, an empirical absorption correction
was applied on F2, following the method of Walker and Stuart.36 This
procedure produced a statistically significant R-factor drop, according
to Hamilton’s criteria.37 Anisotropic thermal displacement parameters
were then refined for all non-H atoms, and hydrogen atoms were placed
at calculated positions, riding on their carrier atoms. The critical low
ratio of “observed” to “measured” reflections prompted us to increase
the number of observations and reduce the number of parameters with
restraints and constraints in the refinement. Rigid-bond restraints were
applied to the thermal motion of terminal ethyl groups and to the whole
E°(1) ) 0.585 V, R(1) ) 0.1; ks(1) ) 1 × 10-5 cm/s
E°(2) ) 0.615 V, R(2) ) 0.1; ks(2) ) 5 × 10-6 cm/s
E°(3) ) -0.590 V, R(3) ) 0.9; ks(3) ) 1.2 × 10-5 cm/s
E°(4) ) -0.550 V, R(4) ) 0.7; ks(4) ) 5 × 10-6 cm/s
Keq(5) ) 1 × 109; kf (5) ) 0.3 s-1
E°(6) ) -0.685 V, R(6) ) 0.77; ks(6) ) 1.15 × 10-6 cm/s
DA ) DB ) DC ) DD ) DE ) DF ) 1.7 × 10-5 cm2/s
The voltammograms of the mononuclear compounds were simulated
by a similar mechanism from which reactions 2 and 4 were excluded
and reaction 5 became S ) F.
Although positive feedback correction of the ohmic drop of the
solution was used to obtain the experimental data, a good fit between
the experimental and simulated voltammograms was achieved only by
introducing in the simulations a residual uncompensated resistance of
700 Ω. The reliability of this value was confirmed by comparison
-
BPh4 anion, to prevent physically unreasonable modeling of these
potentially disordered or mobile groups. In the latter, phenyl groups
(35) Sheldrick, G. SHELXL96. Program for structure refinement, â-test
version; University of Goettingen: Germany, 1996.
(36) Walker, N.; Stuart, D. Acta Crystallogr. 1983, A39, 158.
(37) Hamilton, W. C. Statistics in physical science; The Ronald Press
Company: New York, 1964.
(38) Nardelli, M. J. Appl. Crystallogr. 1995, 28, 659.
(39) Zsolnai, L.; Pritzkow, H. ZORTEP. ORTEP original program modified
for PC; University of Heidelberg: Germany, 1994.
(40) Allen, F. H.; Kennard, O. Chemical Design Automation News 1993,
8, 1, and 31.

5606 Inorganic Chemistry, Vol. 37, No. 21, 1998
Albertin et al.
Chart 2
When the progress of the reaction between FeH2(CO)2-
-
[P(OPh)3]2 and 4-CH3C6H4N2+BF4 in (CD3)2CO is followed
by NMR spectroscopy, the initial appearance of a quite broad
1
signal at 11.57 ppm in the H spectra may be observed. This
is split into a doublet (1J
) 65 Hz) using the labeled
15NH
4-CH3C6H4Nt15N+BF4- salt and strongly suggests the forma-
tion of an aryldiazene intermediate. Although such a diazene
cannot be isolated, its formulation as a hydride-diazene
complex (D) is supported by the appearance of a new hydride
signal at -6.80 ppm (triplet) in the proton NMR spectra, besides
the resonance of the starting FeH2(CO)2P2 species at -11.18
ppm. A new singlet at 182.1 ppm in the 31P spectrum is also
observed. On these bases, it is reasonable to suppose that the
S
S
between experimental and simulated cyclic voltammograms obtained
at different scan rates in 2.5 mM solutions of ferrocene.
+
reaction between FeH2(CO)2P2 and ArN2 proceeds with the
initial insertion of the ArN2 group into the FeH bond, giving
+
Results and Discussion
the [FeH(ArNdNH)(CO)2P2]+ (D) cation. As the reaction
proceeds, the 1H and 31P spectra of the reaction mixture do not
show the formation of other iron complexes in detectable
amounts, except for the final aryldiazenido [Fe(ArN2)(CO)2P2]+
derivative. However, in the proton spectra, parallel with the
appearance of the signals of the [Fe(ArN2)(CO)2P2]+ cation,
there is also a small signal near 4.6 ppm, which decreases on
shaking the NMR tube and may be attributed41 to the signal of
free H2. The presence of molecular hydrogen in the reaction
mixture may be explained by taking into account the known
properties of both diazene and hydride ligands in iron(II)
complexes,1-5,42 i.e., the acidity of the diazene hydrogen atom
on one hand and the easy protonation of the hydride ligand on
the other, which potentially allow the formation of an η2-H2
ligand. Thus, an intramolecular acid-base reaction between
the diazene proton of ArNdNH and the Fe-H hydride may
give rise to a very unstable dihydrogen intermediate (H), which,
Hydride complexes FeH2(CO)2P2 quickly react both with
mono- (ArN2+) and bis-aryldiazonium (+N2Ar-ArN2+) cations
to give the aryldiazenido derivatives [Fe(ArN2)(CO)2P2]+ (1-
4) and [{Fe(CO)2P2}2(µ-N2Ar-ArN2)](BPh4)2 (5-8), respec-
tively, which were isolated as BPh4 salts and characterized
(Scheme 1).
When the reaction was carried out changing the solvent,
temperature, and the ratio between the hydride and the diazo-
nium salt, the aryldiazenido compound was always exclusively
the product but was obtained in varying yields. The formation
of an aryldiazenido compound from the reaction of a dihydride
species of Fe(II) with aryldiazonium cation was surprising on
the basis of previous results1-5 and prompted us to investigate
the reaction in detail. Results indicated the reaction path
reported in Scheme 2, involving the formation of hydride-
diazene intermediate D, which, through the loss of H2, affords
final aryldiazenido complexes 1-8.
-
+
by loss of H2 and rearrangement of the ArN2- ligand to ArN2
(see below), affords the final pentacoordinate iron(0) [Fe(ArN2)-
(CO)2P2]+ cation, in agreement with the proposed mechanism.
Scheme 1a
A comparison of these results with those previously reported
by us1-5,43 on the reactivity of iron(II) hydride of the type
FeH2P4 and [FeH(CO)P4]+ with aryldiazonium cations shows
that the formation, in the present case, of an aryldiazenido
compound is plausible but at the same time somewhat surprising.
+
The reaction of the dihydride species FeH2P4 with ArN2 was
reported1 to give first the monoaryldiazene [FeH(ArNdNH)-
P4]+ complex, which further reacts with ArN2+ to give the bis-
(aryldiazene) [Fe(ArNdNH)2P4]2+ derivative. Treatment of the
bis(aryldiazene) with base gives the [Fe(ArN2)P4]+ aryldiazenido
compound through deprotonation of one ArNdNH ligand and
concurrent dissociation of the other. The hydride-monoaryl-
diazene [FeH(ArNdNH)P4]+ derivative, instead, is practically
unreactive toward bases in every set of conditions. However,
the formation of an aryldiazenido complex from a monoaryl-
diazene complex of iron(II) has been observed42 to take place
a P ) P(OEt)3 (1, 5), PPh(OEt)2 (2, 6), PPh2OEt (3, 7), P(OPh)3 (4,
8); Ar ) C6H5 (a), 2-CH3C6H4 (b), 4-CH3C6H4 (c); Ar-Ar ) 4,4′-
C6H4-C6H4 (a), 4,4′-(2-CH3)C6H3-C6H3(2-CH3) (b), 4,4′-C6H4-CH2-
C6H4 (d).
Scheme 2
only in complexes of the type [Fe(ArNdNH)(RCN)P4]2+
,
containing the rather labile RCN ligand, the easy dissociation
of which allows the following reaction (Scheme 3) to take place.
The presence of two strong π-acceptor CO ligands in our [FeH-
(ArNdNH)(CO)2P2]+ (D) monoaryldiazene intermediate prob-
ably makes the diazene hydrogen atom so acidic that an
intramolecular acid-base reaction with the hydride ligand
affords H2 and the [Fe(ArN2)(CO)2P2]+ aryldiazenido derivative.
(41) Crabtree, R. H.; Lavin, M.; Bonneviot, L. J. Am. Chem. Soc. 1986,
108, 4032.
(42) Albertin, G.; Antoniutti, S.; Bordignon, E. J. Am. Chem. Soc. 1989,
111, 2072.

Reactivity of Hydrides FeH2(CO)2P2
Inorganic Chemistry, Vol. 37, No. 21, 1998 5607
Scheme 3a
a R ) 4-CH3C6H4; P ) P(OEt)3.
Chart 3
Figure 1. ORTEP view of dimeric cation [{Fe(CO)2[P(OEt)3]2}2{µ-
4,4′-N2(2-CH3)C6H3-C6H3(2-CH3)N2}]2+ (5b) with 50% probability
thermal ellipsoids. Ethoxy groups have been omitted for clarity.
Molecule is centrosymmetric around midpoint of bond C6-C6A.
Scheme 4a
Both mono- (1-4) and binuclear (5-8) aryldiazenido com-
plexes are red or orange solids stable in air and in solutions of
polar organic solvents, where they behave as 1:1 or 2:1
electrolytes.44 Elemental analyses and spectroscopic properties
(Table 1) confirm the proposed formulation. The IR spectra
show two ν(CO) bands at 2056-1972 cm-1, in agreement with
two carbonyl ligands in a mutually cis position. The ν(NN)
band of the aryldiazenido ligand is also present in the 1779-
1720 cm-1 region and is shifted to lower frequencies (about 35
cm-1) in the labeled ArNt15N compound, according to the
proposed assignment. These values for the N-N stretching
frequency are indicative45 of a singly bent aryldiazenido ligand,
and this statement was shown to be correct by X-ray structure
determination of 5b, discussed below. The 15N NMR spectra
also fit the presence of a singly bent ArN2 ligand,46,47 showing
a HX ) CF3SO3H, CF3COOH, HCl.
Aryldiazenido complexes 1-8 react with an excess of strong
acid containing a good coordinating anion such as CF3SO3H,
CF3COOH, or HCl to give a yellow solution whose IR and
NMR spectra indicate the presence of an aryldiazene complex,
as shown in Scheme 4. The addition of HX to 1a, 1c, or 5b
does cause the appearance of a diazene proton signal (a doublet
using the labeled ArNt15N compound) near 13.0 ppm in the
1H NMR spectra, while a new singlet, in addition to the signal
of the starting [Fe(ArN2)(CO)2P2]+ cation, appears in the 31P
spectra. The 15N NMR spectra of the [FeCl(C6H5Nd15NH)-
a triplet at 19.7 ppm (2J
) 20 Hz) due to coupling with
15N31
P
two magnetically equivalent phosphorus nuclei for the [Fe-
(C6H5Nt15N)(CO)2{P(OEt)3}2]BPh4 (1a1) derivative.
In the temperature range between +30 and -90 °C, the 31P-
{1H} NMR spectra appear as a sharp singlet, suggesting the
presence of two magnetically equivalent phosphine ligands.
Furthermore, a complicated multiplet is present in the methylene
proton region of the 1H NMR spectra, suggesting the presence
of two phosphites in a mutually trans position. On these bases,
a trigonal bipyramidal geometry of the type shown in I and II
(Chart 3) and similar to that determined in the solid state for
5b (Figure 1) can be proposed in solution for both mono- (1-
4) and binuclear (5-8) aryldiazenido compounds.
(CO)2{P(OEt)3}2]+ cation was also detected and showed the
signal at -2.7 ppm (in CD2Cl2 at -80 °C) with a J
2
15N31
P
of
about 12 Hz. The disappearance of the ν(N2) absorption is also
observed in the IR spectra, together with the appearance of two
new ν(CO) bands due to the diazene complex. However,
spectroscopic studies on the reaction course show that a large
excess of acid HX must be added to the aryldiazenido complex
in order to obtain the corresponding aryldiazene and that the
removal of HX gives back the [Fe(ArN2)(CO)2P2]+ derivative.
The equilibrium of the reaction of Scheme 4 also prevents
aryldiazene derivatives from being obtained in pure form, as
the aryldiazenido complex is the predominant species in the
resulting solid.
Apart from reactions with acid, aryldiazenido 1-8 are very
robust complexes, which are inert to substitution by several
ligands such as CO, phosphites, ArN2+, and halogenide ions,
as well as to oxidation with Br2 or I2. Only with p-tolyl
isocyanide was the slow substitution of the carbonyl ligand
observed, although the result was intractable oils, which were
not characterized.
A comparison of the spectroscopic properties of ArN2
complexes 1-8 shows, first of all, that there is practically no
difference between mono- and binuclear complexes containing
the same phosphite ligand. Instead, variation of the ν(N2)
frequency on changing the π-acceptor properties of the phosphite
ligand was observed in all compounds, with the highest ν(N2)
value of 1779 cm-1 being observed in the [Fe(2-CH3C6H4N2)-
(CO)2{P(OPh)3}2]BPh4 complex, containing the larger48,49 π-ac-
ceptor phosphite ligand.
(43) Albertin, G.; Antoniutti, S.; Lanfranchi, M.; Pelizzi, G.; Bordignon,
E. Inorg. Chem. 1986, 25, 950.
(44) Geary, W. J. Coord. Chem. ReV. 1971, 7, 81.
(45) Haymore, B. L.; Ibers, J. A. Inorg. Chem. 1975, 14, 3060.
(46) Haymore, B. L.; Hughes, M.; Mason, J.; Richards, R. L. J. Chem.
Soc., Dalton Trans. 1988, 2935.
X-ray Crystal Structure of [{Fe(CO)2[P(OEt)3]2}2-{µ-4,4′-
N2(2-CH3)C6H3-C6H3(2-CH3)N2}](BPh4)2 (5b). The cation
consists of a binuclear complex in which the 4,4′-N2(2-CH3)-
(47) Dilworth, J. R.; Kan, C. T.; Richards, R. L.; Mason, J.; Stenhouse, I.
J. Organomet. Chem. 1980, 201, C24.
(48) Tolman, C. A. Chem. ReV. 1977, 77, 313.
(49) Rahman, M. M.; Liu, H.-Y.; Eriks, K.; Prock, A.; Giering, W. P.
Organometallics 1989, 8, 1.

5608 Inorganic Chemistry, Vol. 37, No. 21, 1998
Albertin et al.
Table 3. Most Relevant Bond Distances (Å) and Angles (deg)
with Uncertainties in Parentheses (C6A at -x + 1, -y, -z + 1) for
Compound 5b
Distances
Fe-P1
Fe-P2
Fe-N1
Fe-C1
Fe-C2
O4 -C1
O5-C2
N1-N2
N2-C3
2.223(3)
2.208(3)
1.702(5)
1.758(7)
1.786(6)
1.144(10)
1.143(8)
1.189(7)
1.451(7)
C3-C4
C3-C8
C4-C5
C4-C9
C5-C6
C6-C7
C7-C8
C6-C6A
1.381(9)
1.368(9)
1.376(8)
1.500(10)
1.385(9)
1.398(8)
1.351(8)
1.499(7)
Angles
C1-Fe-C2
N1-Fe-C2
N1-Fe-C1
P2-Fe-C2
P2-Fe-C1
P2-Fe-N1
P1-Fe-C2
P1-Fe-C1
P1-Fe-N1
P1-Fe-P2
Fe-N1-N2
N1-N2-C3
Fe-C1-O4
101.1(3)
136.3(3)
122.6(3)
91.6(3)
88.8(3)
88.0(2)
90.5(3)
90.2(3)
90.9(2)
177.8(1)
174.8(5)
123.6(5)
178.9(7)
Fe-C2-O5
N2-C3-C8
N2-C3-C4
C4-C3-C8
C3-C4-C9
C3-C4-C5
C5-C4-C9
C4-C5-C6
C5-C6-C7
C6-C7-C8
C3-C8-C7
C5-C6-C6A
C7-C6-C6A
179.9(6)
120.4(5)
117.4(5)
122.1(6)
122.1(5)
116.2(5)
121.8(6)
124.0(6)
116.4(5)
121.2(6)
120.1(6)
121.8(5)
121.8(5)
C6H3-C6H3(2-CH3)N2 ligand bridges two Fe atoms by means
of the two terminal diazo groups. Figure 1 shows the molecular
structure of the cation, together with the labeling scheme. Table
3 lists the most important geometric parameters of the cation.
The two iron atoms are 13.87 Å apart. The molecule lies on a
crystallographic center of symmetry, located at the midpoint of
the C6-C6A bond. The metal binds one nitrogen belonging
to the 4,4′-N2(2-CH3)C6H3-C6H3(2-CH3)N2 ligand, two car-
bonyls, and two P(OEt)3 groups. The coordination polyhedron
is a quite regular trigonal bipyramid, with the two phosphites
at the apical positions, and the P-O vectors are staggered with
respect to the equatorial C-O groups. The greatest distortion
from ideal geometry is in the equatorial plane, where the angle
N1-Fe-C2 becomes wider (136.3(3)°) at the expense of C1-
Fe-C2, which narrows (101.1(3)°). This distortion is due to
the short repulsive interaction O5‚‚‚O5 (-x, 1 - y, -z) ) 3.161-
(8) Å, which is the only significant contact found in the crystal
packing of the compound. The bond geometry for the diazo
group is linear at N1 (Fe-N1-N2 ) 174.8(5)°) and trigonal at
N2 (N1-N2-C3 ) 123.6(5)°). The metal complex is located
trans to the o-methyl group (C4-C3-N2-N1 ) 167.9(6)°).
Despite the slight rotation of the o-toluyl group around the C3-
N2 bond, indicated by the torsion angle C8-C3-N2-N1 )
-15(1)°, the entire ligand in the dimer is coplanar within 0.5
Å to the equatorial plane of the complex trigonal bipyramids.
This dimeric structure is similar to those of the pentacoor-
Figure 2. Experimental (full lines) and simulated (dotted lines) cyclic
voltammograms obtained in 2.5 mM [{Fe(CO)2[P(OEt)3]2}2{µ-4,4′-
N2C6H4-C6H4N2}]2+ (5a) at 20 mV/s (a) and 200 mV/s (b). Supporting
electrolyte: 0.1 M tetrabutylammonium hexafluorophosphate (TBAH)/
dichloroethane (DCE). Working electrode: glassy carbon. Direct
oxidation scan: initial potential ) 0.0 V; vertex potential ) 1.4 V;
final potential ) 0.0 V. Direct reduction scan: initial potential ) 0.0
V; vertex potential ) -1.4 V; final potential ) 0.4 V.
sphere, which expands to octahedral. For the protonated ligand,
distances Fe-N1 and N1-N2 are much longer (1.913 and 1.273
Å, respectively) than those in the present case, while N2-C3
is not significantly affected (1.415 Å). The linear geometry
for N1 is also lost (N2-N1-Fe ) 132°), and the torsion angle
C8-C3-N2-N1 ) -37° shows that the rotation of the
aromatic plane around the C3-N2 bond is more pronounced
for the protonated ligand.
Electrochemical Studies. Figure 2a (full line) shows the
cyclic voltammogram of 2.5 mM [{Fe(CO)2[P(OEt)3]2}2{µ-4,4′-
N2C6H4-C6H4N2}](BPh4)2 (5a) in 0.1 M TBAH/DCE, recorded
at 20 mV/s. The dotted line shows the simulated voltammogram
resulting from mechanisms 1-6 (Chart 2). The direct oxidation
scan is characterized by an irreversible oxidation peak, A. In
the direct reduction scan, two peaks are observed: one main
peak, B, followed by a smaller peak, C. Relevant peak potential
values are reported in Table 4. As shown in Figure 2b, these
peak values change slightly when the scan rate is increased.
The satisfactory fit with the simulated data indicates that such
shifts are fully matched by mechanisms 1-6 (Chart 2), even if
it is necessary to take into account also some residual ohmic
drop resistance not fully compensated by the use of positive
feedback (see Experimental Section).
50
dinated monomeric complexes [Fe(CO)2(PPh3)2(C6H5N2)]BF4
and [Fe(4-CH3C6H4N2){P(OEt)3}4]BPh4.1 The geometry of the
Fe-N-N-C system is well preserved, and the average dimen-
sions for the three compounds are Fe-N ) 1.69(1), N-N )
1.20(1), and N-C ) 1.44(2) Å. The bond geometry of carbonyl
coordination is in agreement with that observed in [Fe(CO)2-
(PPh3)2(C6H5N2)]BF4,50 where narrowing of the C-Fe-C angle
is also found (108°).
The crystal structure of a symmetric dimeric iron complex,
[{FeH[P(OEt)3]4}2{µ-4,4′-HNdN(2-CH3)C6H3-C6H3(2-CH3)Nd
NH}](BPh4)2, is known,5 in which the ligand is protonated on
N1 at both ends and a hydride enters the metal coordination
Peaks A and B are irreversible when the scan rate is increased
to 1 V/s (not shown in figure). The currents of peaks A and B
(50) Haymore, B. L.; Ibers, J. A. Inorg. Chem. 1975, 14, 1369.

Reactivity of Hydrides FeH2(CO)2P2
Inorganic Chemistry, Vol. 37, No. 21, 1998 5609
Table 4. Selected Electrochemical Data for Iron Complexes
(Ep
)
(Ip )B/
(Ep
)
(Ep
)
A
ox
(Ip )A/
B
C
red
red
red
ox
compd
(mV)a
CV1/2 c
nb
(mV)a
(mV)a
CV1/2 c
nb
1c [Fe(4-CH3C6H4N2)(CO)2{P(OEt3}2]+
-940
-950
-910
-1000
-920
106
156
216
184
163
1
2
2
2
2
-1030
-1090
-1030
-1090
-1040
+860
+920
+930
+890
+920
113
230
260
230
235
1
2
2
2
2
5a {[Fe(CO)2{P(OEt)3}2]2(µ-4,4′-N2C6H4-C6H4N2)}2+
5b {[Fe(CO)2{P(OEt)3}2]2[µ-4,4′-N2(2-CH3)C6H3-C6H3(2-CH3)N2]}2+
6b {[Fe(CO)2{PPh(OEt)2}2]2[µ-4,4′-N2(2-CH3)C6H3-C6H3(2-CH3)N2]}2+
5d {[Fe(CO)2{P(OEt)3}2]2(µ-4,4′-N2C6H4-CH2-C6H4N2)}2+
a Voltammetric peak potential values measured at 20 mV/s in 0.1 M TBAH/DCE containing 2.5 mM of examined compound. b Calculated from
ratio of slopes of Ic vs t-1/2 plots obtained by chronoamperometric experiments carried out on equimolar solutions of ferrocene and the compound
under study. c Peak currents normalized over solution concentration, C (mM), of the compound and square root of scan rate (slope of Ip (µA) vs
V1/2 (V1/2 s-1/2) linear plots).
increase linearly with the square root of the scan rate, indicating
diffusive control for the processes at both peaks.
The peak current of peak A is roughly equal to the current
of peak B and is about 1.7 times higher than the oxidation peak
current recorded on a 2.5 mM ferrocene solution in the same
electrolyte. It is well-known51,52 that ferrocene undergoes a one-
electron reversible oxidation process; however, since the
electrochemical processes under study are irreversible, no final
information on the number of electrons exchanged in the
electrode reaction can be obtained by the simple comparison
of voltammetric peak currents. To this aim, it is more
appropriate to compare the slope of chronoamperometric current,
Ic, with the reciprocal of the square root of time. This slope is
in fact directly proportional to the number of electrons ex-
changed, n, independently of the reversibility degree of the
process.53 Assuming that the ratio between the square roots of
Figure 3. Cyclic voltammograms obtained in 2.8 mM [Fe(4-
CH3C6H4N2)(CO)2{P(OEt)3}2]+ (1c) at 20 mV/s (full line) and 200 mV/s
(broken line). Other experimental conditions are as in Figure 2. Direct
oxidation scan: initial potential ) 0.0 V; vertex potential ) 1.4 V;
final potential ) 0.0 V. Direct reduction scan: initial potential ) 0.0
V; vertex potential ) -1.4 V; final potential ) 0.0 V.
the diffusion coefficients of 5a and ferrocene is 1, comparison
of the slopes of Ic vs t-1/2 plots for the two compounds gives a
value of n ) 2, both for oxidation at peak A and reduction at
peak B of 5a. The voltammogram shown in Figure 2b indicates
that the current of peak C decreases progressively as the scan
rate is increased, showing that peak C has a kinetic character.
Similar patterns and peak potential values are also obtained for
all binuclear compounds 5b, 5d, and 6b containing P(OEt)3 or
PPh(OEt)2 as ligands; the relevant electrochemical data are listed
in Table 4.
Figure 3 shows cyclic voltammograms recorded on a 2.8 mM
solution of the parent mononuclear compound [Fe(4-CH3C6H4N2)-
(CO)2{P(OEt)3}2]BPh4 (1c). Now, the currents of peaks A and
B (normalized with respect to the concentration) are about half
of those recorded for [{Fe(CO)2[P(OEt)3]2}2{µ-4,4′-N2C6H4-
C6H4N2}](BPh4)2 (5a) and the other binuclear compounds.
Interestingly, the shapes of the voltammograms, in particular
the difference between the peak and half-peak potentials of 1c
and 5a, remain unchanged.
All the above experimental evidence indicates that, for the
binuclear compounds studied here, although both reduction and
oxidation are electron-transfer processes involving two electrons,
they are really composed of two separate one-electron processes.
The observed voltammetric behavior is typical of molecules
containing two localized redox centers.54 For reversible pro-
cesses, it was shown that, when the difference in formal potential
∆E°) 35.6 mV,55,56 the two centers do not interact. In these
conditions, the peak parameters are the same as those of a one-
electron-transfer process, the only difference being an increase
in peak current. The satisfactory fit between simulated and
experimental data indicates that this situation also applies to
the processes under study, even if they are irreversible.
The scheme of the reactions that take place at the electrode/
solution interface may be represented as follows:
[Fe-Fe]2+ f [Fe-Fe]3+ + e-
[Fe-Fe]3+ f [Fe-Fe]4+ + e-
(7)
(8)
[Fe-Fe]2+ + e- f [Fe-Fe]+
[Fe-Fe]+ + e- f [Fe-Fe]0
(9)
(10)
where [Fe-Fe]2+ indicates a generic binuclear complex 5a, 5b,
5d, or 6b.
Since E°(8) is only a few tens of a millivolt more positive
than E°(7) (see E°(1) and E°(2) in the simulations), only one
oxidation peak A is observed; the same, with opposite signs,
holds for reactions 9 and 10 and peak B.
The zero-charged reduction product is unstable and reacts to
give the unknown product P
(51) Kadish, K. M.; Ding, J. Q.; Malinski, T. Anal. Chem. 1984, 56, 1741.
(52) Bond, A. M.; Henderson, T. L. E.; Mann, D. R.; Mann, T. F.; Thorman,
W.; Zoski, C. G. Anal. Chem. 1988, 60, 1878.
[Fe-Fe]0 f P
(11)
(53) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New
York, 1980.
(54) Bott, A. W. Curr. Sep. 1997, 16, 61.
(55) Flanagan, J. B.; Margel, S.; Bard, A. J.; Anson, F. C. J. Am. Chem.
Soc. 1978, 100, 4248.
(56) Ammar, F.; Saveant, J. M. J. Electroanal. Chem. 1973, 47, 115.
which is reduced at peak C to generate the unknown reduced
product Q.
P + e- f Q
(12)

5610 Inorganic Chemistry, Vol. 37, No. 21, 1998
Albertin et al.
In the case of mononuclear compound 1c, both oxidation and
reduction are one-electron irreversible processes.
tion steps, which take place at well-separated potential values;
when only the conjugation is hindered (as with the HNd
NC6H4-CH2-C6H4NdNH ligand), the difference in potential
for the two consecutive reductions decreases dramatically and
the two electrons are exchanged roughly at the same potential
value. On the contrary, for all the bis(diazenido) iron com-
pounds Z, the reduction proceeds via two independent one-
electron processes, which give rise to only one voltammetric
peak.
The electrochemical behavior of our binuclear compounds 5
and 6 is similar to that observed for the reduction of [{MHP4}2-
(µ-HNdNC6H4-CH2-C6H4NdNH)]2+ cations, in which con-
jugation between the two metal centers is hindered and the lack
of any interaction always appears to be operative, even when
the possibility of conjugation between the two iron centers is
not interrupted by the introduction of interposed saturated
functional groups, as in 5d. This different electrochemical
behavior for the binuclear complexes of types T and Z may be
attributed to the differences in the oxidation number (two for
T and zero for Z), geometry, and electronic configuration of
the metal center, as well as to the properties of the bridging
ligand, which may behave as an insulator in bis(aryldiazenido)
iron derivatives. However, other binuclear complexes with both
bis(aryldiazenido) and bis(aryldiazene) as bridging ligand must
be prepared and their properties studied for better understanding
of the properties of these new classes of derivatives.
[Fe]+ f [Fe]2+ + e-(peak A)
[Fe]+ + e- f [Fe]0 (peak B)
(13)
(14)
where [Fe]+ indicates mononuclear complex 1c.
Reaction 14 is followed by a chemical and electrochemical
reaction equivalent to reactions 11 and 12 shown for the
binuclear compounds. This evidence, together with the obser-
vation that peak potentials are almost the same for all the
examined compounds (both mono- and binuclear), suggests that
each iron center exchanges one electron, behaving independently
of the oxidation state of the other metal center. Similar behavior
has been observed for some bridged biferrocenes;57 when the
bridge was -C(CH3)2-C(CH3)2-, -Hg-, or -CHdCH-
C6H4-CHdCH-, the two iron centers did not interact and the
biferrocenes exhibited one irreversible two-electron oxidation
wave.
Chart 4a
Conclusions
Binuclear complexes with bis(aryldiazenido) bridging units
of the type [{Fe(CO)2P2}2{µ-4,4′-N2Ar-ArN2}](BPh4)2 (5-8)
may easily be prepared by reacting dihydride species FeH2(CO)2P2
with the bis(aryldiazonium) cations (N2Ar-ArN2}](BF4)2. The
related mononuclear complexes [Fe(ArN2)(CO)2P2]BPh4 were
also prepared, and the first structural parameters for binuclear
bis(aryldiazenido) complexes are reported. Among the proper-
ties shown by these complexes is the electrochemical behavior,
which shows that both iron centers can be oxidized or reduced
via one-electron processes for each iron, with standard potentials
roughly equal for both centers. Furthermore, aryldiazene [FeX-
(ArNdNH)(CO)2P2]+ cations can be obtained in solution by a
protonation reaction with strong acids of aryldiazenido deriva-
tives 1-8.
a M ) Ru, Fe; Ar-Ar ) 4,4′-C6H4-C6H4, 4,4′-(2-CH3)C6H3-
C6H3(2-CH3), 4,4′-C6H4-CH2-C6H4.
A comparison of the electrochemical results of bis(aryldia-
zenido) iron compounds 5 and 6 (Z, Chart 4) with those
previously obtained by us5 on iron and ruthenium binuclear
complexes of the type [{MHP4}2(µ-HNdNArArNdNH)]-
(BPh4)2 (T, Chart 4), containing a bis(aryldiazene) as the
bridging unit, shows the unexpectedly different behavior of the
two classes of compounds. When conjugation between the
metal centers is possible, the reduction of binuclear bis-
(aryldiazene) T proceeds via two separated one-electron reduc-
Acknowledgment. The financial support of MURST and
the CNR, Rome, is gratefully acknowledged. We thank Dr. G.
Balacco (Menarini Ricerche S.p.A.) for the license of use of
his NMR software SwaN-MR. We thank D. Baldan for technical
assistance.
Supporting Information Available: One X-ray crystallographic
file, in CIF format, is available on the Internet only. Access information
is given on any current masthead page.
(57) Morrison, W. H.; Krogsrud, S.; Hendrickson, D. N. Inorg. Chem. 1973,
12, 1998.
IC980430E