Metal-hydride bond activation and metal-metal interaction in dinuclear iron complexes with linking dinitriles: A synthetic, electrochemical, and theoretical study

Article abstract of DOI:10.1021/ic025835k

The dinuclear iron(II)-hydride complexes [{FeH(dppe)₂}₂(μ-LL)][BF₄]₂ (LL = NCCH=CHCN (1a), NCC₆H₄CN (1b), NCCH₂CH₂CN (1c); dppe = Ph₂PCH₂CH₂PPh₂) and the corresponding mononuclear ones, trans-[FeH(LL)-(dppe)₂][BF₄] (2a-c) were prepared by treatment of trans-[FeHCl(dppe)₂], in tetrahydrofuran (thf) and in the presence of TI[BF₄], with the appropriate dinitrile (in molar deficiency or excess, respectively). Metal-metal interaction was detected by cyclic voltammetry for 1a, which, upon single-electron reversible oxidation, forms the mixed valent Fe^II/Fe^II 1a⁺ complex. The latter either undergoes heterolytic Fe-H bond cleavage (loss of H⁺) or further oxidation, at a higher potential, also followed by hydride-proton evolution, according to ECECE or EECECEC mechanistic processes, respectively, which were established by digital simulation. Anodically induced Fe-H bond rupture was also observed for the other complexes and the detailed electrochemical behavior, as well as the metal-metal interaction (for 1a), were rationalized by ab initio calculations for model compounds and oxidized derivatives. These calculations were used to generate the structural parameters (full geometry optimization), the most stable isomeric forms, the ionization potentials, the effective atomic charges, and the molecular orbital diagrams, as well as to predict the nature of the other electron-transfer induced chemical steps, i.e. geometric isomerization and nucleophilic addition, by BF₄^-, to the unsaturated iron center resulting from hydride-proton loss. From the values of the oxidation potential of the complexes, the electrochemical P_L and E_L ligand parameters were also estimated for the dinitrile ligands (LL) and for their mononuclear complexes 2 considered as ligands toward a second binding metal center.

Full text of DOI:10.1021/ic025835k

Inorg. Chem. 2002, 41, 6456−6467
Metal−Hydride Bond Activation and Metal−Metal Interaction in Dinuclear
Iron Complexes with Linking Dinitriles: A Synthetic, Electrochemical,
and Theoretical Study
Ana I. F. Venaˆncio,† Maxim L. Kuznetsov,† M. Fa´tima C. Guedes da Silva,†,‡
,†
Lu´ısa M. D. R. S. Martins,†,§ Joa˜o J. R. Frau´sto da Silva,† and Armando J. L. Pombeiro*
Centro de Qu´ımica Estrutural, Complexo I, Instituto Superior Te´cnico, AV. RoVisco Pais,
1049-001 Lisboa, Portugal, UniVersidade Luso´fona de Humanidades e Tecnologias,
Campo Grande, 376, 1749-024 Lisboa, Portugal. and Secc¸a˜o de Qu´ımica Inorgaˆnica,
DEQ, ISEL, R. Conselheiro Em´ıdio NaVarro, 1949-014 Lisboa, Portugal
Received July 1, 2002
The dinuclear iron(II)−hydride complexes [{FeH(dppe)2}2(µ-LL)][BF4]2 (LL ) NCCHdCHCN (1a), NCC H CN
6
4
(1b), NCCH CH CN (1c); dppe ) Ph2PCH CH PPh2) and the corresponding mononuclear ones, trans-[FeH(LL)-
2
2
2
2
(dppe)2][BF ] (2a−c) were prepared by treatment of trans-[FeHCl(dppe)2], in tetrahydrofuran (thf) and in the presence
4
of Tl[BF ], with the appropriate dinitrile (in molar deficiency or excess, respectively). Metal−metal interaction was
4
detected by cyclic voltammetry for 1a, which, upon single-electron reversible oxidation, forms the mixed valent
II
III
+
Fe /Fe 1a+ complex. The latter either undergoes heterolytic Fe−H bond cleavage (loss of H ) or further oxidation,
at a higher potential, also followed by hydride-proton evolution, according to ECECE or EECECEC mechanistic
processes, respectively, which were established by digital simulation. Anodically induced Fe−H bond rupture was
also observed for the other complexes and the detailed electrochemical behavior, as well as the metal−metal
interaction (for 1a), were rationalized by ab initio calculations for model compounds and oxidized derivatives. These
calculations were used to generate the structural parameters (full geometry optimization), the most stable isomeric
forms, the ionization potentials, the effective atomic charges, and the molecular orbital diagrams, as well as to
predict the nature of the other electron-transfer induced chemical steps, i.e. geometric isomerization and nucleophilic
addition, by BF -, to the unsaturated iron center resulting from hydride-proton loss. From the values of the oxidation
4
potential of the complexes, the electrochemical P and E ligand parameters were also estimated for the dinitrile
L
L
ligands (LL) and for their mononuclear complexes 2 considered as ligands toward a second binding metal center.
Introduction
hydrogen bond cleavage but only very rarely have the
mechanisms been reported, in all the cases, to the best of
Transition metal-hydrogen bonds play a relevant role in
coordination chemistry, namely, in laboratorial, industrial,
and biological catalyses, and electrochemical methods pro-
vide convenient tools for the investigation of their activation
by electron transfer (ET). Nevertheless and in contrast with
the well-developed hydride chemistry,1 the electrochemistry
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of hydride complexes has been barely investigated.2-10
versatile reactivity can follow the ET-induced metal-
A
* E-mail: pombeiro@ist.utl.pt.
† Instituto Superior Te´cnico.
‡ Universidate Luso´fona de Humanidades e Tecnologias.
§ DEQ, ISEL.
(1) Recent AdVances in Hydride Chemistry; Peruzzini, M., Poli, R., Eds.;
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Elsevier: Amsterdam, 2001.
6456 Inorganic Chemistry, Vol. 41, No. 24, 2002
10.1021/ic025835k CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/31/2002

Dinuclear Iron Complexes with Linking Dinitriles
our knowledge, concerning starting mononuclear hydride
complexes. Moreover, no theoretical calculations appear to
have been applied to the detailed interpretation of the ET-
promoted processes in such systems. Hence, the electro-
chemical investigation of dinuclear dihydride complexes, and
the comparison of their behavior with that of the mononuclear
hydride constituents, as well as the rationalization of such
behaviors by theoretical studies, would be a matter of
considerable interest. The possibility of occurrence of a
metal-metal interaction via a suitable bridging ligand in the
former complexes would constitute a further motif of interest
in view of the significance of such interaction for the under-
standing of the electrochemical, magnetic, spectroscopic, and
semiconductor11a and nonlinear optical11b properties and in
the application of such complexes, e.g. in molecular elec-
tronics,12-15 biochemistry,16-20 multielectron redox reactions
of small molecules21 and as a support to the formation of
dendrimers and coordination polymers.22 The communication
between the two metal atoms has been studied with a variety
of linkage ligands22-36 (although dinitriles36 do not appear
to have been fully explored, despite their availability and
simplicity) and metals by electrochemical or spectroelec-
trochemical,25b-d,26-28,30a,31,32b,d,35b,c,37-41 ESR,25c,d,26-28,35a,c,41a,42
and other spectroscopic24,25b-d,27,28,32b,d,35c,38,39,41a methods and
also by theoretical quantum chemical investigations, mainly
by Hu¨ckel or extended Hu¨ckel (EHMO)23,31,32b,35a,37,43 (but
also INDO/1,39,41a,43 HF25c,29,44 or DFT25c,e,j,27,28,30a,33,42,45
methods.
)
By taking into consideration the above motives and that
we have previously investigated in detail the mechanism of
the anodic process of the hydride isocyanide complexes
trans-[FeH(CNR)(dppe)2]+ (R ) alkyl, aryl; dppe )
Ph2PCH2CH2PPh2), which proceeds via the initial ET-
induced Fe-H bond cleavage followed by further reactivity,
according to a curious ECEC (E ) ET step, C ) chemical
step) reaction mechanism, we decided to prepare and inves-
tigate by electrochemical and ab initio methods the following
compounds: the dinuclear complexes [{FeH(dppe)2}2(µ-LL)]-
[BF4]2 with the conjugated bridging fumaronitrile (LL )
NC-CHdCH-CN, 1a) or terephthalonitrile (1,4-dicyanoben-
zene) [LL ) NC(1,4-C6H4)NC, 1b] ligands or with the
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Inorganic Chemistry, Vol. 41, No. 24, 2002 6457

Venaˆncio et al.
Xe atoms. Nominal molecular masses were calculated using the
most abundant isotopes 56Fe (52%). However, further complexity
due to addition (from the matrix) or loss of hydrogen was usually
not taken into account. Mass calibration for data system acquisition
was achieved using CsI.
Syntheses. (a) Dinuclear Complexes [{FeH(dppe)2}2(µ-LL)]-
[BF4]2 (LL ) NCCHdCHCN (1a), NCC6H4CN (1b), NCCH2-
CH2CN (1c)). These complexes can be obtained either directly from
trans-[FeHCl(dppe)2] (method A) or, in a less straightforward way,
from the corresponding mononuclear complexes 2 (see below;
method B).
(1) Method A. A solution of trans-[FeHCl(dppe)2] (0.20 g, 0.22
mmol) in tetrahydrofuran (thf) (20 cm3) was stirred under dinitrogen,
at room temperature, for 15-30 min with Tl[BF4] (0.070 g, 0.25
mmol). The appropriate dinitrile (LL) was then added [0.009 g,
0.11 mmol (NCCHdCHCN); 0.014 g, 0.11 mmol (NCC6H4CN);
0.005 g, 0.006 mmol (NCCH2CH2CN)], and the mixture was stirred
at room temperature for ca. 20 h, whereafter it was taken to dryness
by evaporation of the solvent in vacuo. Extraction with CH2Cl2
(20 cm3) followed by filtration (removal of the thallium salts),
concentration in vacuo to ca. 10 cm3, and addition of diethyl ether
(ca. 5-7 cm3) led to the precipitation of the dinuclear product as
a blue (1a), red (1b) or yellow (1c) solid which was separated by
filtration, washed with Et2O, dried in vacuo, and recrystallized from
CH2Cl2/Et2O [0.16 g, 72% yield (1a); 0.18 g, 81% yield (1b); 0.054
g, 48% yield (1c)].
nonconjugated succinonitrile bridge (LL ) NC-CH2CH2--
CN, 1c), as well as, for comparative purposes, the corre-
sponding mononuclear compounds [FeH(LL)(dppe)2][BF4]
(2a-c). The investigation of the activation of the Fe-H bond
by ET also constitutes a matter of potential biological
significance toward the understanding of the enzymatic action
of iron-hydrogenases46 and nitrogenases.47 The study also
provides an opportunity to extend to dinuclear dinitrile
complexes the investigation of the electronic properties,
coordinating and activating abilities of N2-binding iron
phosphinic centers which we have been using, in mono-
nuclear complexes, for other small unsaturated substrates
such as organosilanes,8 cyanamides and cyanoguanidine,48
mononitriles,49 isocyanides,8,9,50 phosphaalkynes,51 and alkynes
and carbon disulfide.52
Experimental Section
All manipulations and reactions were carried out using standard
inert gas flow or high-vacuum techniques. The complex trans-
[FeHCl(dppe)2] was prepared by a published method;53 the dinitriles
were used as purchased from Aldrich. The solvents were purified
and dried by standard methods and freshly distilled under dinitrogen.
The IR spectra (400-4000 cm-1) were recorded on a Bio-Rad FTS
3000MX instrument in KBr pellets. NMR spectra (run in CD2Cl2
unless stated otherwise) on a Varian UNITY 300 spectrometer at
(2) Method B. The above dinuclear complexes 1 can also be
obtained by reacting the corresponding mononuclear complexes 2
(see below) with the stoichiometric amount of trans-[FeHCl(dppe)2].
As a typical example, the synthesis of 1a from 2a is given: A
solution of trans-[FeHCl(dppe)2] (0.027 g, 0.030 mmol) in thf (10
cm3) was stirred under dinitrogen, at room temperature, for 15 min
in the presence of Tl[BF4] (0.010 g, 0.033 mmol), whereafter a
CH2Cl2 solution (5 cm3) containing a stoichiometric amount of
trans-[FeH(NCCHdCHCN)(dppe)2][BF4] (2a, 0.031 g, 0.030 mmol)
was added. The color changed immediately to dark blue and was
kept with stirring for 2 h. It was then taken to dryness in vacuo.
The solid residue was dissolved in CH2Cl2, the solution filtered
and concentrated, and Et2O added (with stirring) to give a dark
blue precipitate of [{FeH(dppe)2}2(µ-NCCHdCHCN)][BF4]2 (1a,
0.033 g, 55% yield).
1
room temperature. H, 13C, 13C{1H}, and 31P{1H} chemical shifts
(δ) are reported in parts per million relative to tetramethylsilane
(TMS) and H3PO4, respectively. In the 13C NMR data, assignments
and coupling constants common to the 13C{1H} NMR spectra are
not reported. Abreviations: s ) singlet; d ) doublet; t ) triplet; q
) quartet; qnt ) quintet; tqnt ) triplet of quintet; m ) multiplet.
UV/vis spectra were recorded on a Perkin-Elmer Lambda-9
spectrophotometer. The spectral range between 230 and 850 nm
was covered by using quartz cells. Results are reported in terms of
ꢀ (molar adsorption coefficient) in M-1‚cm-1. C, H, and N elemental
analyses were carried out by the Microanalytical Service of the
Instituto Superior Te´cnico. Positive-ion FAB mass spectra were
obtained on a Trio 2000 instrument by bombarding 3-nitrobenzyl
alcohol matrices of the samples with 8 keV (ca. 1.28 × 10-15 J)
1
(3) Compound 1a. IR (KBr, cm-1): 2165 (νCN). H NMR: δ
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7.42-6.81 (m, 80H, (C6H5)2PCH2CH2P(C6H5)2), 5.16 (s, 2H,
CHdCH), 2.82 (m, 8H, 1/2Ph2PCH2CH2PPh2), 2.23 (m, 8H, 1/2Ph2-
PCH2CH2PPh2), -16.93 (qnt, JHP ) 48.0 Hz, 2H, Fe-H). 31P{1H}
NMR: δ 82.2 (s, Ph2PCH2CH2PPh2). 31C{1H} NMR: δ 33.72 (qnt,
JCP ) 12.1 Hz, Ph2PCH2CH2PPh2), 114.51 (s, NCCHdCHCN),
122.10 (s, NCCHdCHCN), 128.14 and 128.94 (s, Cm or Co from
dppe), 129.96 and 130.47 (s, Cp from dppe),132.71 and 133.38 (Co
or Cm from dppe), 134.58 (qnt, JCP ) 7.4 Hz, Ci from dppe), 135.40
(qnt, JCP ) 10.6 Hz, Ci from dppe). 31C NMR: δ 33.72 (tqnt, JCH
) 135.2 Hz), 114.51 (d, JCH ) 179.5 Hz, 122.10 (s), 128.14 (d,
JCH ) 164.4 Hz), 128.94 (d, JCH ) 161.3 Hz), 129.96 (d, JCH
)
160.7 Hz), 130.47 (d, JCH ) 162.5 Hz), 132.71 (d, JCH ) 160.6
Hz), 133.38 (d, JCH ) 161.3 Hz), 134.58 (m), 135.40 (m). Anal.
Calcd for C108H100B2F8N2P8Fe2: C, 65.8; H, 5.1; N, 1.4. Found:
C, 65.6; H, 5.4; N, 1.4%. FAB mass: m/z 932 (M - FeH(dppe)2)+.
UV/vis [λmax/nm (ꢀ/(M-1‚cm-1))]: 615 (14 376) and 265 (43 558).
(50) Baptista, M. B.; Lemos, M. A. N. D. A.; Frau´sto da Silva, J. J. R.;
Pombeiro, A. J. L. J. Organomet. Chem. 1992, 424, 49.
(51) Meidine, M. F.; Lemos, M. A. N. D. A.; Pombeiro, A. J. L.; Nixon,
J. F.; Hitchcock, P. B. J. Chem. Soc., Dalton Trans. 1998, 3319.
(52) Almeida, S. S. P.; Duarte, M. T.; Ribeiro, L. M. D.; Gormley, F.;
Galva˜o, A. M.; Frau´sto da Silva, J. J. R.; Pombeiro, A. J. L. J.
Organomet. Chem. 1996, 524, 63.
1
(4) Compound 1b. IR (KBr, cm-1): 2184 (νCN). H NMR: δ
7.49 (m, 16H, Ho from dppe), 7.37 (t, JHH ) 7.1 Hz, 8H, Hp from
dppe), 7.29 (t, JHH ) 7.0 Hz, 8H, Hp from dppe), 7.18 (t, JHH
)
7.4 Hz, 16H, Hm from dppe), 7.13 (t, JHH ) 7.5 Hz, 16H, Hm from
dppe), 6.82 (m, 16H, Ho from dppe), 6.72 (s, 4H, C6H4), 2.70 (m,
(53) Giannoccaro, P.; Sacco, A. Inorg. Synth. 1977, 17, 69.
6458 Inorganic Chemistry, Vol. 41, No. 24, 2002

Dinuclear Iron Complexes with Linking Dinitriles
1
8H, 1/2Ph2PCH2CH2PPh2), 2.18 (m, 8H, 1/2Ph2PCH2CH2PPh2),
-17.93 (qnt, JHP ) 48.0 Hz, 2H, Fe-H). 31P{1H} NMR: δ 83.3 (s,
Ph2PCH2CH2PPh2). 31C{1H} NMR: δ 33.40 (qnt, JCP ) 11.8 Hz,
Ph2PCH2CH2PPh2), 115.93 (s, Ci from dinitrile), 124.67 (s,
NCC6H4CN), 128.65 (s, Cm or Co from dppe), 129.42 (s, Cm or Co
from dppe), 130.59 and 130.86 (s, Cp from dppe), 132.78 (s, Co
and Cm from dinitrile), 133.19 and 133.80 (s, Co or Cm from dppe),
134.49 (qnt, JCP ) 7.1 Hz, Ci from dppe), 135.59 (qnt, JCP ) 9.1
Hz, Ci from dppe). 31C NMR: δ 33.40 (tqnt, JCH ) 129.0 Hz),
115.93 (s), 124.67 (s), 128.65 (d, JCH ) 160.0 Hz), 129.42 (d, JCH
) 158.7 Hz), 130.59 (d, JCH ) 160.2 Hz), 130.86 (d, JCH ) 161.9
Hz), 132.78 (d, JCH ) 169.3 Hz), 133.19 (d, JCH ) 165.6 Hz),
133.80 (d, JCH ) 162.5 Hz), 134.49 (m), 135.59 (m). Anal. Calcd
for C112H102B2F8N2P8Fe2: C, 66.9; H, 5.1, N, 1.4. Found: C, 67.0;
H, 5.0; N, 1.4%. FAB mass: m/z 1923 (M + BF4)+, 982 (M -
FeH(dppe)2)+. UV/vis [λmax/nm (ꢀ/(M-1‚cm-1)]: 545 (12 282) and
265 (42 603).
(2) Compound 2b. IR (KBr, cm-1): 2182 (νCN). H NMR: δ
7.64 (d, JHH ) 8.4 Hz, 2H, Hm from dinitrile), 7.50 (m, 8H, Ho
from dppe), 7.28 (m, 4H, Hp from dppe), 7.26 (m, 4H, Hp from
dppe), 7.16 (t, JHH ) 7.4 Hz, 8H, Hm from dppe), 7.06 (t, JHH
)
7.8 Hz, 8H, Hm from dppe), 6.89 (d, JHH ) 8.7 Hz, 2H, Ho from
dinitrile), 6.80 (m, 8H, Ho from dppe), 2.80 (m, 4H, 1/2Ph2-
PCH2CH2PPh2), 2.21 (m, 4H, 1/2Ph2PCH2CH2PPh2), -18.15 (qnt,
JHP ) 47.2 Hz, 1H, Fe-H). 31P{1H} NMR: δ 81.6 (s, Ph2PCH2-
CH2PPh2). 31C{1H} NMR: δ 32.62 (qnt, JCP ) 12.1 Hz,
Ph2PCH2CH2PPh2), 115.47 (s, Ci from dinitrile), 116.96 (s, Cp from
dinitrile), 117.71 (s, -NCC6H4CN), 123.71 (s, -NCC6H4CN),
128.21 and 128.98 (s, Cm or Co from dppe), 130.28 and 130.42 (s,
Cp from dppe), 132.42 (s, Cm or Co from dinitrile), 132.61 (Co or
Cm from dppe), 132.97 (s, Co or Cm from dinitrile), 133.33 (s, Co
or Cm from dppe), 133.69 (qnt, JCP ) 7.2 Hz, Ci from dppe), 134.85
(qnt, JCP ) 10.6 Hz, Ci from dppe). 31C NMR: δ 32.62 (tqnt, JCH
) 134.3 Hz), 115.47 (t, JCH ) 8.7 Hz, 116.96 (t, JCH ) 8.7 Hz),
117.71 (s), 123.71 (s), 128.21 (d, JCH ) 161.3 Hz), 128.98 (d, JCH
) 164.3 Hz), 130.28 (d, JCH ) 155.8 Hz), 130.42 (d, JCH ) 167.4
Hz), 132.42 (d, JCH ) 176.8 Hz), 132.61 (d, JCH ) 164.3 Hz),
132.97 (d, JCH ) 169.9 Hz), 133.33 (d, JCH ) 160.0 Hz), 133.69
(m), 134.85 (m). Anal. Calcd for C60H53BF4N2P4Fe: C, 66.3; H,
4.9; N, 2.6. Found: C, 66.3; H, 5.2; N, 2.3%. FAB mass: m/z 982
(M)+. UV/vis [λmax/nm (ꢀ/(M-1‚cm-1))]: 490 (7 609) and 265 (34
855).
(5) Compound 1c. IR (KBr, cm-1): 2229 (νCN). H NMR: δ
1
7.35-6.75 (m, 80H, (C6H5)2PCH2CH2P(C6H5)2), 2.84 (m, 8H,
1/2Ph2PCH2CH2PPh2), 2.12 (m, 8H, 1/2Ph2PCH2CH2PPh2), 2.12
(s, 4H, CH2CH2), -20.45 (qnt, JHP ) 44.6 Hz, 2H, Fe-H).
31P{1H} NMR: δ 81.6 (s, Ph2PCH2CH2PPh2). 31C{1H} NMR: δ
16.71 (s, NCCH2CH2CN), 33.85 (qnt, JCP ) 12.4 Hz, Ph2PCH2CH2-
PPh2), 125.88 (s, NCCH2CH2CN), 128.01 and 128.80 (s, Cm or Co
from dppe), 129.82 and 130.31 (s, Cp from dppe),132.74 and 133.47
(Co or Cm from dppe), 135.78 (qnt, JCP ) 6.9 Hz, Ci from dppe),
136.13 (qnt, JCP ) 10.1 Hz, Ci from dppe). 31C NMR: δ 16.71 (t,
JCH ) 141.7 Hz), 33.85 (tqnt, JCH ) 134.0 Hz), 125.88 (s), 128.01
(d, JCH ) 163.7 Hz), 128.80 (d, JCH ) 160.5 Hz), 129.82 (d, JCH
) 161.3 Hz), 130.31 (d, JCH ) 161.3 Hz), 132.74 (d, JCH ) 160.6
Hz), 133.47 (d, JCH ) 160.0 Hz), 135.78 (m), 136.13 (m). Anal.
Calcd for C108H102B2F8N2P8Fe2: C, 64.0; H, 5.1; N, 1.4. Found:
C, 64.0; H, 5.1; N, 1.4%. FAB mass: m/z 934 (M - FeH(dppe)2)+.
UV/vis [λmax/nm (ꢀ/(M-1‚cm-1))]: 440 (1 199) and 260 (33 392).
(b) Mononuclear Complexes trans-[FeH(LL)(dppe)2][BF4]
(LL ) NCCHdCHCN (2a), NCC6H4CN (2b), NCCH2CH2CN
(2c)). A procedure identical to that described above for the synthesis
of the dinuclear complexes from trans-[FeHCl(dppe)2] was applied,
but by using a 5- to 10-fold molar excess of the appropriate dinitrile
(LL), i.e. 0.172 g (2.20 mmol, for 2a), 0.141 g (1.10 mmol, for
2b), or 0.176 g (2.20 mmol, for 2c). The products were also isolated
similarly as above, as a violet (2a, 0.16 g, 71% yield), red (2b,
0.19 g, 78% yield), or yellow (2c, 0.16 g, 69% yield) solid.
1
(3) Compound 2c. IR (KBr, cm-1): 2230 (νCN). H NMR: δ
7.47-6.76 (m, 40H, (C6H5)2PCH2CH2P(C6H5)2), 2.67 (m, 4H,
1/2Ph2PCH2CH2PPh2), 2.47 (t, JHH ) 6.0, 2H, -NCCH2CH2NC),
2.24 (t, JHH ) 6.4, 2H, -NCCH2CH2NC), 2.06 (m, 4H, 1/2Ph2-
PCH2CH2PPh2), -20.41 (qnt, JHP ) 46.9 Hz, 1H, Fe-H). 31P{1H}
NMR: δ 81.6 (s, Ph2PCH2CH2PPh2). 31C{1H} NMR: δ 14.26 (s,
-NCCH2CH2CN), 17.88 (s, -NCCH2CH2CN), 32.82 (qnt, JCP
)
11.9 Hz, Ph2PCH2CH2PPh2), 117.75 (s, -NCCH2CH2CN), 126.40
(s, -NCCH2CH2CN), 128.05 and 128.91 (s, Cm or Co from dppe),
130.22 (s, Cp from dppe),132.63 and 133.38 (Co or Cm from dppe),
134.44 (qnt, JCP ) 7.1 Hz, Ci from dppe), 135.59 (qnt, JCP ) 10.4
Hz, Ci from dppe). 31C NMR: δ 14.26 (t, JCH ) 138.3 Hz), 17.88
(t, JCH ) 139.6 Hz), 32.82 (tqnt, JCH ) 134.0 Hz), 117.75 (s),
126.40 (s), 128.05 (d, JCH ) 162.5 Hz), 128.91 (d, JCH ) 161.9
Hz), 130.22 (d, JCH ) 156.1 Hz), 132.63 (d, JCH ) 160.0 Hz),
133.38 (d, JCH ) 160.0 Hz), 134.44 (m), 135.59 (m). Anal. Calcd
for C56H53BF4N2P4Fe: C, 63.8; H, 5.1; N, 2.6. Found: C, 64.0; H,
5.2; N, 2.4%. FAB mass: m/z 934 (M)+. UV/vis [λmax/nm (ꢀ/(M-1
‚
cm-1))]: 440 (964) and 260 (18 932).
1
(1) Compound 2a. IR (KBr, cm-1): 2175 (νCN). H NMR: δ
7.49-6.66 (m, 40H, (C6H5)2PCH2CH2P(C6H5)2), 5.81 (d, JHH
)
Electrochemistry. The electrochemical experiments were per-
formed on an EG&G PARC 273 potentiostat/galvanostat connected
to a PC computer through a GPIB interface (National Instruments
PC-2A). Cyclic voltammetry (CV) experiments were undertaken
in a two-compartment three-electrode cell, at a platinum-disk (or
vitreous carbon disk) working electrode probed by a Luggin
capillary connected to a silver-wire pseudo-reference electrode; a
platinum auxiliary electrode was employed. Controlled potential
electrolyses (CPE) were carried out in a two-compartment three-
electrode cell with platinum-gauze working and counter electrodes
in compartments separated by a glass frit; a Luggin capillary,
probing the working electrode, was connected to a silver wire
pseudo-reference electrode. The electrochemical experiments were
performed in a N2 atmosphere at room temperature. The potentials
were measured by CV in 0.2 mol dm-3 [NBu4][BF4]/CH2Cl2, and
the values are quoted relative to the saturated calomel electrode
(SCE) by using the trans-[FeHCl(dppe)2]0/+ redox couple (E° )
-0.143 V vs SCE in CH2Cl2) as the internal standard. The IUPAC
recommended ferrocene/ferricinium redox couple was not used for
15.0 Hz, 1H, -NCCHdCHNC), 4.98 (d, JHH ) 15.3 Hz, 1H,
-NCCHdCHNC), 2.64 (m, 4H, 1/2Ph2PCH2CH2PPh2), 2.11 (m,
4H, 1/2Ph2PCH2CH2PPh2), -16.07 (qnt, JHP ) 47.6 Hz, 1H,
Fe-H). 31P{1H} NMR: δ 82.7 (s, Ph2PCH2CH2PPh2). 31C{1H}
NMR: δ 32.68 (qnt, JCP ) 12.2 Hz, Ph2PCH2CH2PPh2), 114.81
(s, -NCCHdCHCN), 116.05 (s, -NCCHdCHCN), 118.38 (s,
-NCCHdCHCN), 119.56 (s, -NCCHdCHCN), 128.28 and
129.04 (s, Cm or Co from dppe), 130.42 and 130.64 (s, Cp from
dppe),132.56 and 133.40 (Co or Cm from dppe), 133.28 (qnt, JCP
)
7.7 Hz, Ci from dppe), 134.42 (qnt, JCP ) 10.7 Hz, Ci from dppe).
31C NMR: δ 32.68 (tqnt, JCH ) 134.0 Hz), 114.81 (d, JCH ) 182.4
Hz), 116.05 (s), 118.38 (d, JCH ) 180.5 Hz), 119.56 (s), 128.28 (d,
JCH ) 161.9 Hz), 129.04 (d, JCH ) 161.9 Hz), 130.42 (d, JCH
)
163.1 Hz), 130.64 (d, JCH ) 168.7 Hz), 132.56 (d, JCH ) 149.5
Hz), 133.40 (d, JCH ) 160.0 Hz), 133.28 (m), 134.42 (m). Anal.
Calcd for C56H51BF4N2P4Fe: C, 65.0; H, 5.0; N, 2.7;.Found: C,
64.7; H, 4.9; N, 2.6%. FAB mass: m/z 932 (M)+. UV/vis [λmax
nm (ꢀ/(M-1‚cm-1))]: 550 (11 778) and 270 (47 861).
/
Inorganic Chemistry, Vol. 41, No. 24, 2002 6459

Venaˆncio et al.
Scheme 1. Syntheses of the Dinitrile Complexes 1 and 2
ox
this purpose on account of the partial overlap of its wave (E1/2
)
chelate dppe ligand does not affect significantly the structural,
electronic, and energetic properties of the complexes. Thus, the
hypothetical dinuclear complexes [{FeH(PH3)4}2(µ-LL)]n+ (LL )
NC-CHdCH-CN and NC-CH2-CH2-CN, n ) 2, 3, 4) and the
corresponding deprotonated species, as well as the mononuclear
complexes [Fe(NCCH3)(PH3)4]2+ and [FeF(NCCH3)(PH3)4)]+ were
chosen as model compounds for our calculations instead of the real
complexes with two dppe ligands at each metal site. The choice of
the basis set and of the model of the complex took into account a
reasonable computational time required for geometry optimization.
0.525 V vs SCE in CH2Cl2) with the first ones of our complexes.
The CPE experiments were monitored regularly by CV to ensure
that no significant potential drift occurred along the electrolyses.
The acid-base potentiometric titrations were carried out by using
a standard solution of KOH in methanol. The results were corrected
for background effects by performing also the titration of a blank
solution of the electrolyte, which has been electrolyzed under
conditions identical to those used for the corresponding complex
solution. The corrected values are the following ones: 1H+ when
the CPE was performed at the oxidation wave of the mononuclear
complexes or at the first oxidation wave of the dinuclear complex
1a and 2H+ when the CPE was carried out at the second oxidation
wave of the latter compound.
The mechanism of the oxidation processes was investigated by
digital simulation (program ESP54) of the cyclic voltammograms
at different scan rates (in the 0.4-60 V s-1 range). The E° and khet
values for the electron-transfer processes were chosen in order to
allow a close correspondence between simulated and experimental
cyclic voltammograms for the entire range of scan rates of the CV
experiments.
Results and Discussion
Syntheses. Treatment at room temperature of a thf solution
of trans-[FeHCl(dppe)2], in the presence of Tl[BF4] as the
halide abstracting agent, with the appropriate dinitrile (LL)
in a molar defficiency (LL:complex molar ratio not higher
than 0.5:1), leads to the formation (Scheme 1 (eq 1)) of the
corresponding dinuclear complexes with bridging dinitriles
[{FeH(dppe)2}2(µ-LL)][BF4]2 (LL ) NCCHdCHCN (1a),
NCC6H4CN (1b), NCCH2CH2CN (1c)), which were isolated
as blue (1a), red (1b), or yellow (1c) solids in ca. 80-50%
yields. If the reaction is performed in similar experimental
conditions but using an excess of the dinitrile (5- to 10-fold
Theoretical Studies. The full geometry optimization of the
structures has been carried out in Cartesian coordinates using the
quasi-Newton-Raphson gradient method and the restricted (for
close-shell structures) or unrestricted (for open-shell structures)
Hartree-Fock approximation with help of the GAMESS55 and
Gaussian 9856 program packages. Symmetry operations were not
applied for all structures. The standard basis set of Gauss functions
6-31G57 was selected for all atoms. The single-point calculations
of the several structures have been performed at the MP2 level of
theory on the basis of the equilibrium Hartree-Fock geometries.
The solvent effects were taken into account for the mononuclear
complexes by using the polarizable continuum model58 in the
CPCM version59 with CH2Cl2 and thf as solvents.
(56) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,
V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;
Clifford, S.; Ochterski, J.; Peterson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts,
R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C.
Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.;
Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.9; Gaussian,
Inc.: Pittsburgh, PA, 1998.
As it was previously shown,60 the use for the calculations of the
model structures with two monodentate PH3 ligands instead of the
(54) Nervi, C. (nervi@lem.ch.unito.it). Electrochemical Simulation Package
(ESP, version 2.4); Dipartimento di Chimica IFM: Torino, Italy, 1994/
98.
(55) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon,
M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su,
S. J.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. J. Comput. Chem.
1993, 14, 1347.
(57) Rassolov, V.; Pople, J. A.; Ratner, M.; Windus, T. L. J. Chem. Phys.
1998, 109, 1223, and references therein.
(58) Tomasi, J.; Persico, M. Chem. ReV. 1997, 94, 2027.
(59) Barone, V.; Cossi, M. J. Phys. Chem. 1998, 102, 1995.
(60) Zhang, L.; Guedes da Silva, M. F. C.; Kuznetsov, M. L.; Gamasa, M.
P.; Gimeno, J.; Frau´sto da Silva, J. J. R.; Pombeiro, A. J. L.
Organometallics 2001, 20, 2782.
6460 Inorganic Chemistry, Vol. 41, No. 24, 2002

Dinuclear Iron Complexes with Linking Dinitriles
Table 1. Cyclic Voltammetric Dataa for [{FeH(dppe)2}2(µ-LL)][BF4]2
(1) and trans-[FeH(LL)(dppe)2][BF4] (2)
molar ratio), the corresponding mononuclear complexes
trans-[FeH(LL)(dppe)2][BF4] (LL ) NCCHdCHCN (2a),
NCC6H4CN (2b), NCCH2CH2CN (2c)) are the obtained
products [Scheme 1 (eq 2)], isolated as violet (2a), red (2b),
or yellow (1c) solids in ca. 78-70% yields. Further treatment
of the mononuclear complexes 2 with the starting trans-
[FeHCl(dppe)2] complex, also in thf and in the presence of
Tl[BF4], yields the corresponding dinuclear complexes 1
[Scheme 1 (3)] formed upon coordination of the hanging
nitrile function of the monodentate dinitrile ligand of 2 to
the added iron(II) center. However, this alternative route (2-
3) for the dinuclear complexes 1 is less convenient than the
above route (1) which is more direct, faster, and presents
higher yields.
II
complex
IE1/2ox/V
E
ox/V
E1/2red (Ep/2red)/V
p/2
1a
1b
1c
2a
2b
2c
0.68
0.69
0.64
0.72
0.70
0.63
0.86
0.98
1.30
(1.07)
1.29
-
a Potentials (half-wave potential E1/2 for the reversible processes or half-
height peak potential Ep/2 for the irreversible ones, in V ( 0.02 vs SCE)
measured in 0.2 mol dm-3 [NBu4][BF4]/CH2Cl2 at a scan rate of 0.4 V s-1
and at a Pt-disk (d ) 0.5 mm) electrode.
The complexes have been characterized by IR, UV/vis,
and multinuclear magnetic resonance spectroscopies, el-
emental analysis, FAB+-MS spectrometry, and electrochemi-
cal methods. In their IR spectra, ν(NtC) appears as bands
at ca. 2165-2175 (with a strong intensity, 1a or 2a), ca.
2183 (medium, 1b or 2b), or ca. 2230 cm-1 (weak, 1c or
2c), corresponding to coordination shifts ∆ν ) ν(NtC)ligand
- ν(NtC)free nitrile of ca. -65 to -75 (LL ) NCCHdCHNC),
ca. -48 (LL ) NCC6H4CN), or ca. -25 (LL ) NCCH2-
CH2CN) cm-1. The ν(NtC) shift to lower wavenumbers
upon coordination is indicative of a significant π-electron
effect in the decreasing order of NCCHdCHNC > NCC6H4-
CN > NCCH2CH2CN. The ν(NtC) values fall in the range
known61 for related mononitrile complexes (2205-2250
cm-1) and are higher than those observed for the isocyanide
complexes trans-[FeH(CNR)(dppe)2]+ (R ) H,8 alkyl,50 or
aryl50) or the related isocyanotriphenylborate complex trans-
[FeH(CNBPh3)(dppe)2],8 in accord with the expected62
stronger π-electron acceptor ability of the isocyanide com-
pared with the nitrile ligands.
The trans geometry of the complexes is assigned on the
basis of the singlet resonance observed in their 31P{1H} NMR
spectra, whereas the presence of the hydride ligand is
accounted for by the high-field (δ ca. -16 to -20) quintet
1
(2JHP ca. 45-48 Hz) resonance exhibited by the H NMR
spectra. In the 13C{1H} NMR spectra, the NC resonances of
the ligated dinitriles in the dinuclear complexes 1 appear as
one singlet at δ ca. 122-126, while in the mononuclear
compounds 2 two singlets are observed (e.g at δ 119.56 and
116.05 for 2a) on account of the nonequivalence of the two
NC groups of the η1-dinitrile ligands. All the other resonances
of the dinitrile and dppe ligands have also been assigned in
Figure 1. Cyclic voltammograms of complexes 2a (1.06 mM) (a), 1a
(1.35 mM) (b), and 1b (0.51 mM) (c), in CH2Cl2 with 0.2 M [NBu4][BF4]
at a platinum disk (d ) 0.5 mm) working electrode. Potentials are given in
1
both the H and the 13C NMR spectra, including those of
volts vs SCE. Scan rate: 0.2 V s-1
.
the ipso-, ortho-, meta-, and para-atoms of the phenyl rings.
Electrochemical Studies. The electrochemical behavior
of the mono- and dinuclear complexes in 0.2 mol dm-3
[NBu4][BF4]/CH2Cl2 was studied by cyclic voltammetry
(CV) and controlled potential electrolysis (CPE), at a Pt-
disk (or vitreous C disk) or a Pt-gauze electrode, respectively.
Relevant data are indicated in Table 1.
Mononuclear Complexes 2. The cyclic voltammograms
of complexes 2 display one single-electron reversible oxida-
ox
tion wave (Figure 1a for complex 2a) at a potential (E1/2
)
ranging from 0.63 to 0.72 V vs SCE, attributed to the
reversible FeII f FeIII oxidation, and one single-electron
red
partially reversible (E1/2 ) -1.29 V, 2b) or irreversible
(Ep/2red ) -1,07 V, 2a) reduction wave which is believed to
be ligand centered (the cyclic voltammograms, run at similar
experimental conditions, of free terephthalonitrile and fu-
(61) Giannoccaro, P.; Rossi, M.; Sacco, A., Coord. Chem. ReV. 1972, 8,
77.
(62) Pombeiro, A. J. L. New J. Chem. 1997, 21, 649.
Inorganic Chemistry, Vol. 41, No. 24, 2002 6461

Venaˆncio et al.
maronitrile present reduction waves at E1/2red ) -1.59 V and
Ep/2 ) -1.11, respectively, thus revealing a significant
Table 2. Estimated PL and EL Ligand Parameters
red
ligand
PL/V
ELa/V
anodic shift of the ligand centered reduction processes upon
coordination). However, the anodic CPE at the oxidation
wave of any of the complexes consumes 2F/mol and leads
to proton extrusion (overall -2e-/-H+ process). The liber-
NCCHdCHCN
NCC6H4CN
NCCH2CH2CN
0.32
0.34
0.41
0.36
0.18
0.35
0.35
0.40
0.40
0.46
0.44
0.39
0.43
0.58
0.44
0.44
0.39
0.39
NCCHdCHCN{FeIIH(dppe)2}+ (2a)
NCCHdCHCN{FeIIIH(dppe)2}2+ (2a+)
NCC6H4CN{FeIIH(dppe)2}+ (2b)
NCC6H4CN{FeIIIH(dppe)2}2+ (2b+)
NCCH2CH2CN{FeIIH(dppe)2}+ (2c)
NCCH2CH2CN{FeIIIH(dppe)2}2+ (2c+)
red
ated proton was detected by its broad cathodic wave at Ep
ca. -0.3 V (which undergoes an extensive cathodic shift on
replacement of Pt by vitreous carbon as the working electrode
material) and by potentiometric titration of the electrolyzed
solution. Hence, in the extended time scale of the CPE,
anodically induced proton loss occurs upon Fe-H bond
cleavage, conceivably according to the process shown by
a In V vs NHE.
and polarizability (â ) 1.0) for their iron(II) binding site.
E1/2ox[MSL] ) ES + âPL
ox
reactions 1 (E2 e E1ox), in which Fe ) trans-{Fe(LL)-
(2)
(dppe)2}, as known for the related trans-[FeH(L)(dppe)2]n
(L ) CNR, n ) 1;9 L ) CN, n ) 08) complexes. The
heterolytic M-H bond cleavage results from the increase
of the acidity character upon metal oxidation, as corroborated
by theoretical studies (see below) and is also documented
in other hydride complexes.2-7
The thus obtained PL values (Table 2) show that (i) the
net electron acceptor/donor ability of the dinitrile ligands
follow the order NCCHdCHCN (PL ) -0.32 V) g
NCC6H4CN (PL ) -0.34 V) > NCCH2CH2CN (PL ) -0.41
V), (ii) neither the coordination of one of the cyano groups
of NCC6H4CN or NCCH2CH2CN to {FeH(dppe)2}+ nor the
oxidation of this metal center affects significantly the electron
acceptor/donor character of the other NC group, since no
appreciable variation of PL occurs [-0.35 or -0.40 V for
the ligands NCC6H4CN{FeH(dppe)2}n (n ) +1 (2b) and +2
(2b+)) or NCCH2CH2CN{FeH(dppe)2}n (n ) +1 (2c) and
+2 (2c+)), respectively], and (iii) for NCCHdCHCN its
coordination to {FeH(dppe)2}+ appears to lead to a slight
decrease of the π-electron acceptor/σ-donor character of the
other cyano group (PL slightly decreases from -0.32 for
NCCHdCHCN to -0.36 V for NCCHdCHCN{FeH-
(dppe)2}+ (2a)), whereas oxidation of the iron(II) center
results in a substantial increase of that character (PL increases
to -0.18 V for NCCHdCHCN{FeH(dppe)2}2+ (2a+)). These
observations indicate that NCCHdCHCN has a net π-elec-
tron acceptor/σ-donor ability identical to (or even marginally
stronger than) that of the aromatic NCC6H4CN nitrile and is
the only one that allows an electronic interaction to be noticed
between the two metal centers. In comparison with related
ligands,62 all the above dinitriles (PL in the range from -0.41
to -0.18 V) behave as more effective net electron acceptor/
donor ligands than acetonitrile (PL ) -0.58 V),63 isocyano-
ox
ox
(E1
-e
)
(E2
-e
)
+
-H
[FeII-H]+ y z [FeIII-H]2+
8 [FeI]+ y z [FeII]2+ (1)
Dinuclear Complexes 1. Complexes 1 exhibit, by CV, at
a scan rate of 0.2 V s-1, one (partially) reversible anodic
ox
wave at E1/2 ) 0.64-0.69 V vs SCE (wave I) which, for
compound 1a (Figure 1b), is followed, at a slightly higher
potential (IIEp/2 ) 0.86 V), by a second one (wave II) with
an irreversible character.
The observation of two anodic waves (assigned to the two
sequential FeII f FeIII oxidations) only for compound 1a
can be attributed to an interaction between the two iron
centers propagated throughout the orbitals of the conjugated
bridging NtC-CdC-CtN framework. In contrast, for
compounds 1b (Figure 1c) and 1c only one anodic wave is
detected for both FeII f FeIII oxidations, showing that
incorporating a phenyl ring or a saturated carbon group
between the two cyano units hampers such an interaction to
an extent that it is not detected. Other features of the anodic
behavior are described below.
Reversible ligand centered single-electron cathodic waves
are also detected at E1/2red ) -0.98 (1a) or -1.30 (1b) V, in
the latter case with the peak current intensity quite lower
than (less than half) that of the oxidation wave, corroborating
the involvement of both FeII centers in the oxidation process.
Estimate of Electrochemical Ligand Parameters. The
measured oxidation potentials (IE1/2ox) of the complexes 1
(also Ep/2ox for 1a) and 2, viewed as closed shell octahedral-
type complexes [MSL] with the ligand L ligating the 16-
electron {MS} ) trans-{FeH(dppe)2}+ site, allows one to
estimate the electrochemical PL ligand constant (a measure
of the net π-electron acceptor minus σ-donor character of a
-
triphenylborate CNBPh3 (PL ) -0.51 V)8 or cyanide (PL
) -1.0 V).63
The values of the EL ligand parameter (an additive
parameter based on an empirical redox potential parametriza-
tion method64 distinct from the above, but that reflects, like
PL, although in a different scale, the net electron acceptor/
donor character of a ligand) were also estimated (Table 2)
for the above ligands, by using the empirical relationship
PL ) 1.17EL - 0.86 64 between PL and that parameter. The
same conclusions on the relative ligand electron acceptor/
donor abilities as the above based on PL can be drawn from
comparisons of the EL values.
ox
ligand), by applying the linear relationship (2) between E1/2
and PL (for ligand L)63 to our complexes and considering
the known63 values of the electron richness (ES ) 1.04 V)
(64) Lever, A. B. P. Inorg. Chem. 1990, 29, 1271.
(65) Richardson, D. E.; Taube, H. Inorg. Chem. 1981, 20, 1278, and
references therein.
(63) Chatt, J.; Kan, C. T.; Leigh, G. J.; Pickett, C. J.; Stanley, D. R. J.
Chem Soc., Dalton Trans. 1980, 2032.
6462 Inorganic Chemistry, Vol. 41, No. 24, 2002

Dinuclear Iron Complexes with Linking Dinitriles
Scheme 2. Proposed Mechanism for the Anodic Processes of the Complex [{FeH(dppe)2}2(µ-NCCHdCHCN)][BF4]2, 1aa
a The numbers in parentheses (1++, 1+++, etc.) are those of the model complexes of the theoretical studies. Only the metal oxidation states different
from II are indicated. X indicates an (anionic) nucleophile (see text)
the oxidized species, at both oxidation waves, with formation
of products that are further oxidized (an increase of the
number of electrons involved), and the relative extent of the
two wave processes is also dependent on that time. The cyclic
voltammetric behavior does not appear to depend on the
complex concentration, thus ruling out second-order pro-
cesses on that concentration, in contrast to what was
observed2c,e,10 in other metal-hydride systems. However,
first-order ET-induced M-H bond cleavage processes are
also known in other cases.2a,3b,c,4,8,9
CPE at the onset of the anodic wave I led to the
consumption of 2F/mol with liberation of 1H+/molecule as
indicated (see above) by CV experiments and acid-base
Figure 2. Experimental (symbols) and theoretical (line) variation of F )
I
ipII/ip (see text) as a function of log scan rate. The solid line corresponds
to the working curve for the mechanism described in Scheme 2.
titration of the electrolized solution and expected on the basis
of theoretical studies (see below). The final obtained product
presents a reversible anodic wave at E1/2 ) 0.77 V, as
ox
Mechanistic Study of the Oxidation Process of 1a. The
cyclic voltammetric behavior of dinuclear complex 1a,
denoted in a simplified way by [HFekFeH]2+, was studied
in detail, showing a marked dependence on the scan rate.
Hence, at sufficiently high scan rates there is no time for
the occurrence, at a considerable extent, of any chemical
reaction and each oxidation (I and II) tends to a single-
electron reversible oxidation. This limiting behavior corre-
sponds to the reversible FeII f FeIII oxidation of each metal
center to form [HFekFeH]3+ (wave I) and [HFekFeH]4+
(wave II, Scheme 2). However, upon lowering the scan rate,
(i) the anodic current function ipoxV-1/2C-1 (ipox ) anodic peak
current, V ) scan rate, and C ) concentration) increases for
both waves, mainly for wave II (by a factor of ca. 3 from V
) 60 V s-1 to 0.4 V s-1), until, for sufficiently low scan
rates, that of wave I increases faster than that of wave II, as
shown by the ratio of the peak current intensities of these
waves (F ) IIipox/Iipox), which (Figure 2, symbols) increases
with the decrease of the scan rate until reaching a maximum
and then lowering for the lowest scan rates. Moreover, the
reversible character of the waves, mainly wave II, also
decreases with lowering of the scan rate.
shown by the cyclic voltammogram run after the electrolysis.
Exhaustive anodic CPE at the oxidation wave II of com-
pounds 1a (or at the unique oxidation wave for any of the
other dinuclear complexes) led to the overall (waves I and
II) consumption of 4F/mol, with liberation of 2H+/molecule
as also indicated by CV and measured by acid-base titration
of the electrolyzed solution. Hence, an overall -2e-/-H+
process per each iron site occurs for any of the dinuclear
complexes, as observed (see above) for the mononuclear
compounds 2.
Anodically induced proton loss upon heterolytic FeIII-H
bond cleavage is not the only expected chemical reaction.
In fact, a concomitant FeIII f FeI reduction (the two electrons
of that bond are transferred to the metal) occurs and the
generated FeI center is expected (see theoretical studies) to
be oxidized at a potential not significantly higher than that
of the parent FeII f FeIII oxidation, to form an unsaturated
FeII tricationic species (Scheme 2). This should readily
undergo nucleophilic attack, e.g. by the electrolyte anion,
-
BF4 , as we have observed9 for the related isocyanide
complexes trans-[FeH(CNR)(dppe)2]+ (R ) alkyl or aryl)
which follow a complex oxidation process that can be
described by an ECEC-type reaction mechanism with the
Hence, the increase of the time of the experiment (decrease
of scan rate) allows the occurrence of chemical reactions of
Inorganic Chemistry, Vol. 41, No. 24, 2002 6463

Venaˆncio et al.
Figure 3. Experimental (left) and simulated (right) cyclic voltammograms of complex 1a (1.35 mM, in CH2Cl2, with 0.2 M [NBu4][BF4]) at a platinum
disk (d ) 0.5 mm) working electrode. Potentials are given in volts vs SCE. Scan rate: 0.4 (a) and 20 V s-1 (b).
first chemical (C) step consisting of H+ extrusion from the
oxidized original complex and the second one involving
attack of BF4 to the thus formed unstable iron(I) species.
of the experimental (symbols) and simulated (line) current
ratio F ) ipox/Iip as a function of log scan rate.
II
ox
-
By using the E° values obtained by simulation, the
comproportionation constant KC ) exp|n1E°1 - n2E°2|/25.69
(at 298 K, with E° in millivolts),65 i.e. the equilibrium
constant of reaction 3 (n1 ) n2 ) 1; |E°1 - E°2| ) 190 mV),
was calculated. It indicates the stabilization of the mixed-
valence FeII-FeIII state in 1a and reflects the interaction
between the metals. The obtained value of KC ) 1.6 × 103
is indicative of a class II mixed valence system (delocaliza-
tion takes place but the two types of site are distinguish-
able).66 It is much lower than that (1.6 × 1012, in CH2Cl2)
for [{Fe(Cp*)(dppe)}2(µ-CtCCtC)]32c (Cp* ) η5-C5Me5),
comparable to those of [{Cr(CO)3}2(biphenyl)]67 (2.5 × 103,
in CH2Cl2), [{Cr(CO)2(PPh3)}2(biphenyl)]67 (2.6 × 104, in
CH2Cl2), or [{Fe(Cp*)(dppe)}2(µ-CtC-C6H4-CtC)]32c (2.6
× 104) and higher than those for several weakly coupled
class II dipyridyl-bridge bis(pentaammineruthenium) bi-
nuclear complexes68 (KC ranging from 4 to 890, in water).
The obtained metal fluorinated product [FeF(CNR)(dppe)2]+
could be reversibly oxidized at a slightly higher potential.
In the present study, the nature of the final products could
not be established since attempts for their isolation and full
characterization have failed. Hence, the nucleophilic reagent
is denoted by X in Scheme 2, but theoretical calculations
-
(see below) suggest that BF4 is a rather plausible one.
Cyclic voltammetric simulations54 were used to validate
quantitatively the processes given in Scheme 2 (variations
thereof or other reaction schemes could not be simulated
successfully) and to obtain kinetic data for the deprotonation
and the nucleophilic attack reactions. A good fit was obtained
for the following optimized values of E° and chemical rate
constants: E°1 ) 0.61 V, E°2 ) 0.80 V, E°3 ) E°5 ) 0.58
V, E°4 ) 0.67 V, and E°6 ) 0.78 V; k1 ) 4.5 s-1, k2 ) 50
s-1, k3[X] ) 0.25 s-1, and k4[X] ) k5[X] ) 0.01 s-1 (i.e. k3
) 1.25 M-1 s-1 and k4 ) k5 ) 0.05 M-1 s-1, assuming that
-
(66) Robin, M. B.; Day, P. AdV. Inorg. Chem. Radiochem. 1967, 10, 247.
(67) Van Order, N., Jr.; Geiger, W. E.; Bitterwolf, T. E.; Rheingold, A. L.
J. Am. Chem. Soc. 1987, 109, 5680.
X ) BF4 ). The agreement between the experimental and
the simulated data is illustrated in Figure 3 for the scan rates
of 0.4 and 20 V s-1 and is also shown by the plot (Figure 2)
(68) Creutz, C. Prog. Inorg. Chem. 1983, 30, 1, and references therein.
6464 Inorganic Chemistry, Vol. 41, No. 24, 2002

Dinuclear Iron Complexes with Linking Dinitriles
KC
occupied MOs, in particular the two degenerated HOMOs
(Figure 5) (mainly localized at the hydride ligands) or the
next eight MOs which are based on P atoms.
[HFekFeH]2+ + [HFeIIIkFeIIIH]4+ y z [HFeIIIkFeH]3+ (3)
\
Theoretical Studies. [{FeH(PH3)4}2(µ-NtC-CHdCH-
CtN)]2+ (1++) and [{FeH(PH3)4}2(µ-NtC-CH2-CH2-
CtN)]2+ (2++), as models for complexes 1a and 1c,
respectively, as well as their oxidized forms and products
of deprotonation have been investigated by theoretical
methods. For the starting ones the geometry optimization of
both isomeric forms of the bridging ligand (i.e. E or Z for
NtCCHdCHCtN and anti or syn for NtCCH2-CH2CtN)
has been performed. Each model complex is referenced
according to the bridge isomer followed by a number and
the charge.
Equilibrium Structure, MO Composition, Oxidation
Potential, and Interaction between the Metals. For all
equilibrium structures, the coordination sphere is of the
octahedral type with linear HFeNCC fragments (Figure 4).
Both isomers of 1++, i.e. E-1++ ([HFekFeH]2+ in Scheme
2) and Z-1++, and anti-2++ have planar HFeNCCCCNFeH
fragments, while for syn-2++ the CCCC torsion angle is
-81.5°. For each particular structure, both {FeH(PH3)4NCC}
moieties have the same structural parameters and effective
atomic charges. The central C-C bond lengths of E-1++
and Z-1++ correspond to a CdC bond, whereas for anti-
2++ and syn-2++ they represent a single C-C bond. The
other C-C bonds are shorter for 1++ than for 2++ due to
π-conjugation for the former case.
The E-1++ and anti-2++ isomers are more stable (by
5.47-5.94 and 3.36-3.47 kcal/mol, respectively) than the
corresponding Z-1++ and syn-2++ ones (thus, only the E-
and anti-isomers, as the most stable ones, will be further
discussed) what is in accord with the conceivable isolation
of such isomers [a single IR ν(NtC) band]. The calculated
vertical ionization potentials (IP) suggest that 1++ should
be oxidized at slightly higher potentials than 2++, what
agrees with the measured E1/2 values for complexes 1a
and 1c, although there is not necessarily a direct correlation
between the calculated data for isolated molecules and the
experimental ones measured for their solutions.
The stronger metal-metal communication for the complex
with the unsaturated dinitrile bridge can be rationalized in
terms of an electronic interaction involving frontier MOs of
the bridging ligand. It was shown23,37 that the compropor-
tionation constant Kc (reaction 3) correlates with the electron
density at the “coordination centers in the LUMO of a
bridging π-ligand”. Our preliminary calculations on the
NCCHdCHCN and NCCH2CH2CN ligands show that (i) the
contribution of AOs of the coordinating N atoms in the
LUMO is higher for the former than for the latter dinitrile
(32.2 and 26.2%, respectively) and (ii) the LUMO energy
for the former ligand is by 3.62 eV lower than for the latter.
Moreover, there is an extensive contribution of the bridging
ligand orbitals in some of the occupied frontier MOs of
E-1++ (104 and 108, Scheme 3), whereas for anti-2++ the
corresponding MOs are mostly centered at the two metal
atoms without a significant involvement of orbitals from that
ligand. However, the metal-metal interaction in E-1++
cannot be accounted for by the other higher energy lying
Interpretation of the Mechanism of Anodic Oxidation.
(a) Wave I. The HOMOs of E-1++ and anti-2++ (Figure
5) represent the bonding combinations of Fe and hydride
orbitals, with a predominant contribution of the latter. Hence,
their first oxidation should be hydride centered and lead to
a weakening (eventual cleavage) of the Fe-H bond. This is
confirmed by the full geometry optimization of the most
stable E-isomer of the mono-oxidized complex trans-[{FeH-
(PH3)4}2(µ-NtC-CHdCH-CtN)]3+ (E-1+++), which
indicates that one of the hydride hydrogens moves away from
the respective iron atom to a distance of 6.53 Å in the
equilibrium structure, what in fact corresponds to the
cleavage of the Fe-H bond. The other Fe-H bond is
preserved. A similar behavior is found for anti-2++. These
results agree with the experimental data (Scheme 2).
The full geometry optimization of the deprotonated
complex trans-[{FeH(PH3)4}(µ-NtC-CHdCH-CtN){Fe-
(PH3)4}]2+ (E-3++, Figure 4, Scheme 2) shows the occur-
rence of a structural rearrangement of the {Fe(PH3)4}+
moiety, which now exhibits the vacant site in cis position to
the bridging ligand. The HOMO of E-3++ (Figure 5) is
ligand bridge centered and thus its oxidation should not re-
sult in loss of the second proton, in accord with Scheme 2.
The general conformation of the complex is also preserved.
The vertical and adiabatic IPs calculated for E-1++ and
E-3++ (10.94 and 9.79 eV, respectively, for the adia-
batic IP) suggest that the former complex should be oxi-
dized at higher potential than the latter one and this is in
agreement with the experimental results (E°5 is marginally
lower than E°1).
The coordinatively unsaturated E-3+++ (as well as
E-4+++ formed at wave II) is expected (Scheme 2) to un-
-
I
ox
dergo nucleophilic attack, in particular by BF4 as is
known9 in the oxidation of the isocyanide complexes
trans-[FeH(CNR)(dppe)2][BF4]. The calculations on the
model [Fe(NCCH3)(PH3)4]2+ 5++ allowed the location of
two minima corresponding to the trans- and cis-isomers
(trans-5++ and cis-5++, Figure 4), the latter being more
stable than the former by 0.09 eV, what correlates with the
above isomerization in the oxidation of E-1++. The
-
theoretical examination of the nucleophilic addition of BF4
to trans-5++ and cis-5++, and the search of the potential
surface for these systems indicate that it should proceed
without overcoming a potential barrier. The calculations at
both HF and CPCM-HF levels (for the latter case the solvent
effect was taken into account) showed the formation of trans-
or cis-[Fe(η1-FBF3)(NCCH3)(PH3)4]+ (trans-6+ or cis-6+,
Figure 4) with weak bonds between the coordinated F and
both Fe and B atoms. For the real solution, the cleavage of
the F3B‚‚‚F bond in 6+ may well occur to give a fluorinated
complex and the solvated BF3.
(b) Wave II. The overall two-electron oxidation can be
considered to correspond to the removal of both electrons
from the HOMO of E-1++ (Figure 5). Thus it should result
in weakening of both Fe-H bonds what is confirmed by
Inorganic Chemistry, Vol. 41, No. 24, 2002 6465

Venaˆncio et al.
Figure 4. General view of the equilibrium geometries of the calculated structures.
the geometry optimization of E-1++++′ (with the same
structure as the parent E-1++) which shows a significant
elongation of the Fe-H bonds, consistent with the proposed
deprotonation (Scheme 2).
of the M-H bond toward heterolytic cleavage (upon increase
of the Bro¨nsted acidity) with proton loss that corresponds
(the electrons of the bond are transferred to the metal) to a
reductive elimination reaction which thus is triggered by an
oxidation. This EC-type process can be represented as
follows: M(dn)-H - e f [M(dn-1)-H]+ f M(dn+1) + H+.
Final Remarks
The oxidation can promote not only proton-transfer reac-
tions but also the generation of a reactive coordinatively
Single-electron oxidation of a metal-hydride center, as
shown in this work, constitutes a simple mode of activation
6466 Inorganic Chemistry, Vol. 41, No. 24, 2002

Dinuclear Iron Complexes with Linking Dinitriles
Scheme 3. MO Diagrams for the Interaction of the {HFe(PH3)4}+ Fragment (a) with {NtCCHdCHCtN} To Give E-1++ and (b) with
{NtCCH2CH2CtN} To Give anti-2++
other ExCy combinations, providing a multitude of ET-
induced reactions in series of electrogenerated reactive
species. Such potentialities are exemplified by this work and
deserve further exploration in synthesis.
Theoretical studies can provide a valuable contribution
toward the understanding of the various steps involved in
such complex behaviors and can assist experimental methods,
like cyclic voltammetry and digital simulation, for the
establishment of mechanistic details.
Acknowledgment. The work has been partially supported
by the Fundac¸a˜o para a Cieˆncia e a Tecnologia (FCT) and
by the PRAXIS XXI and POCTI Programs. A. I. F. V. is
very grateful to Lic. So´nia Cunha for her great help for
teaching how to operate the NMR spectrometer as well as
in preliminary electrochemical assays. We also thank Dr.
C. Nervi (University of Torino) for making available the
electrochemical simulation program, Dr. M. Caˆndida Vaz
(Instituto Superior Te´cnico) for the elemental analysis
services and Mr. Indale´cio Marques (Centro de Qu´ımica
Estrutural) for running the FAB-MS spectra.
Figure 5. Plots of the HOMOs of E-1++, anti-2++, and E-3++. Only
one HOMO for the pair of degenerate ones (for E-1++ and anti-2++) is
shown.
unsaturated and reduced metal center (with the metal in an
unusual oxidation state) which is prone to undergo further
ET (e.g. being oxidized at a potential that is not higher than
that of the starting hydride complex) or chemical reactions
(like addition of a nucleophile). Moreover, if the starting
molecule comprises more than one metal-hydride center and
these centers communicate electronically in a way that mixed
valent complexes with a significant stability are formed,
reaction sequences of the above types can occur at distinct
oxidation potentials, for instance according to (EC)n (n )
number of M-H centers) processes (as in Scheme 2) or to
Supporting Information Available: Table listing selected bond
lengths for the calculated structures and plots of selected MOs of
E-1++, anti-2++, and E-3++. This material is available free of
charge via the Internet at http://pubs.acs.org.
IC025835K
Inorganic Chemistry, Vol. 41, No. 24, 2002 6467