Synthesis and electrochemical aspects of group 14 iron complexes of the type (η5-C5H5)Fe(L)2ER3, L2 = (CO)2, (Ph2P)2CH2; E = C, Si, Ge, Sn

Article abstract of DOI:10.1016/j.ica.2006.12.043

The dicarbonyl and diphosphine complexes of the type (η⁵-C₅H₅)Fe(L)₂ER₃ (L₂ = (CO)₂ (a), (Ph₂P)₂CH₂ (b); ER₃ = CH₃ (1a/b); SiMe₃ (2a/b), GeMe₃ (3a/b), SnMe₃ (4a/b)) were synthesized and studied electrochemically. Cyclic voltammetric studies on the dicarbonyl complexes 1a-4a revealed one electron irreversible oxidation processes whereas the same processes for the chelating phosphine series 1b-4b were reversible. The E_ox values found for the series 1a-4a were in the narrow range 1.3-1.5 V and in the order Si > Sn ≈ Ge > C; those for 1b-4b (involving replacement of the excellent retrodative π-accepting CO ligands by the superior σ-donor and poorer π-accepting phosphines) have much lower oxidation potentials in the sequence Sn > Si ≈ Ge > C. This latter oxidation potential pattern relates directly to the solution ³¹P NMR chemical shift data illustrating that stronger donation lowers the E_ox for the complexes; however, simple understanding of the trend must await the results of a current DFT analysis of the systems.

Full text of DOI:10.1016/j.ica.2006.12.043

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Inorganica Chimica Acta 361 (2008) 1793–1796
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Note
Synthesis and electrochemical aspects of group 14 iron complexes
of the type (g5-C5H5)Fe(L)2ER3, L2 = (CO)2, (Ph2P)2CH2;
E = C, Si, Ge, Sn
*
Mukesh Kumar, Enrique A. Reyes, Keith H. Pannell
Department of Chemistry, University of Texas at El Paso, Texas, USA
Received 3 November 2006; accepted 21 December 2006
Available online 13 January 2007
Dedicated to Prof. Piero Zanello.1
Abstract
The dicarbonyl and diphosphine complexes of the type (g5-C5H5)Fe(L)2ER3 (L2 = (CO)2 (a), (Ph2P)2CH2 (b); ER3 = CH3 (1a/b);
SiMe3 (2a/b), GeMe3 (3a/b), SnMe3 (4a/b)) were synthesized and studied electrochemically. Cyclic voltammetric studies on the dicar-
bonyl complexes 1a–4a revealed one electron irreversible oxidation processes whereas the same processes for the chelating phosphine
series 1b–4b were reversible. The Eox values found for the series 1a–4a were in the narrow range 1.3–1.5 V and in the order
Si > Sn ꢀ Ge > C; those for 1b–4b (involving replacement of the excellent retrodative p-accepting CO ligands by the superior r-donor
and poorer p-accepting phosphines) have much lower oxidation potentials in the sequence Sn > Si ꢀ Ge > C. This latter oxidation poten-
tial pattern relates directly to the solution 31P NMR chemical shift data illustrating that stronger donation lowers the Eox for the com-
plexes; however, simple understanding of the trend must await the results of a current DFT analysis of the systems.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Cyclic voltammetry; Diphosphine; Group 14 elements; Retrodative bonding
1. Introduction
with chelating diphosphino substituents has been
reported to exhibit a greater degree of reversibility [3].
Recently, (g5-C5Me5)Fe(dppe)X complexes (dppe = bis-
(diphenylphosphino)ethane, X = F, Cl, Br, I, H, CH3)
have been studied theoretically as well as experimentally
in terms of the Fe–X bond energy and the electrochemi-
cal oxidation [4]. Cyclic voltammetry of these complexes
revealed a reversible redox process and the electrode
potential for the [Fe–X]II/III couple decreased in the order
I > Br > Cl > H > F > CH3. This trend is the opposite of
that predicted on the basis of halide electronegativity
alone, and suggested the importance of p-donation from
halide to metal, a suggestion corroborated by DFT
calculations.
The varying elements of group 14, C, Si, Ge, Sn and
Pb, ranging from non-metal via metalloid to metal, rep-
resent an entertaining area of the Periodic Table. We are
interested in their properties when directly bonded to
transition metals and have found the metal substituent
(g5-C5H5)Fe(CO)2 (Fp) a rewarding source of new chem-
istry [1]. The electrochemistry of some complexes of this
type, FpER3, R = group 14 element are scattered in the
literature and in general Fp derivatives exhibit irrevers-
ible oxidations [2,3]. However, substitution of the car-
bonyl groups by phosphine ligands, especially those
In the present article, we report an electrochemical study
of the complexes (g5-C5H5)Fe(CO)2ER3 and (g5-C5H5)-
Fe(dppm)ER3, ER3 = CH3, SiMe3, GeMe3, SnMe3;
dppm = bis(diphenylphosphino)methane.
*
Corresponding author.
E-mail address: kpannell@utep.edu (K.H. Pannell).
Piero: Congratulations on your fine contributions. Cheers, Keith.
1
0020-1693/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2006.12.043

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M. Kumar et al. / Inorganica Chimica Acta 361 (2008) 1793–1796
2. Results and discussion
Synthesis: The known compounds 1a–4a were prepared
from the direct reaction of Fp–Na and iodomethane (for
1a) and corresponding trimethylchlorosilane/germane/
stannane (for 2a–4a). In turn, as shown in Scheme 1, com-
pounds 1b–4b were synthesized in 70–80% yield by photo-
chemical treatment of 1a–4a with an equimolar amount of
dppm. The substitution of the carbonyl groups was moni-
tored by infrared and 31P NMR spectroscopy. We observed
that the first carbonyl group replacement takes 10–15 h,
while 40–50 h were taken to replace both the carbonyl
groups. Each of 1b–4b were isolated as crystalline solid
materials and characterized by 1H, 13C, 31P, 29Si, 119Sn
NMR and elemental analysis.
Fig. 1. A plot between Eox vs. EN (Pauling) for CpFe(CO)2ER3 (E = C,
R = H; E = Si, Ge, Sn, R = Me) [1a–4a].
decreasing the electronegativity of E in ER3, the oxidation
potential increases [EN (Pauling) = 2.55 (C), 1.90 (Si), 2.01
(Ge) and 1.96 (Sn)] [7], a counterintuitive result.
2.1. Spectroscopic characterization
Many (g5-C5H5)Fe(CO)2-X complexes have been stud-
ied by Mossbauer, IR, and multinuclear NMR spectros-
copy [8]. From these studies it has been suggested a small
component of retrodative p-bonding between the metal
atom and the silicon/tin exists in complexes of type
(g5-C5H5)Fe(CO)2SiR3 [8b] as well as in (g5-C5H5)Fe-
(LL0)2SnR3 [LL0 = CO, L = CO; L0 = PPh3] [8a,8c]. The
pattern in 1a–4a for oxidation potential data may be ana-
lyzed in terms of the variable presence of such retrodative
bonding. In the case of (g5-C5H5)Fe(CO)2CH3 retrodative
bonding by the alkyl group is non-existent and its strong
r-donor character, observed on the basis of Mossbauer
studies result in a lower Eox.
At a similar scan rate the chelating phosphine com-
plexes, 1b–4b exhibited a single one electron reversible oxi-
dation at much lower oxidation potentials in the greater
range ꢂ0.1–0.32 V, Table 1, Fig. 2. This change is antici-
pated for the replacement of the strongly retrodative
p-bonding carbonyl ligands by the less effective p-bonding,
and superior r-donating, diphosphine. Such a replacement
results in a build-up of electron-density at iron, facilitating
the oxidation process [2]. The oxidation potential sequence
of 1b–4b exhibits an order of Sn > Si > Ge > C.
NMR spectroscopic data authenticated the molecular
formulation of 1b–4b, and in the case of 1b being in agree-
ment with the literature values [5]. For example, 13C NMR
spectroscopic analysis revealed a significant upfield shift of
the g5-C5H5 carbons of all Nb complexes (Dd ꢁ 8 ppm
when compared to the corresponding dicarbonyl precur-
sors, Na). The 29Si NMR spectroscopic data for 2b exhib-
ited a triplet at 27.6 ppm (2JSi–P = 43.3 Hz) ꢁ 14 ppm
upfield relative to 2a (29Si d = 41.8 ppm) which is similar
2
to the published value d = 25.1 ppm, JSi–P = 46 Hz] for
(g5-C5H5)Fe(dppe)SiMe3 [6]. Similarly an upfield triplet
at 69.0 ppm [2JSn–P = 447 Hz] was observed in 119Sn
NMR of 4b, representing a Dd value of 73 ppm. The
uncoupled 31P NMR spectrum of each compound revealed
a singlet in the range of 37–50 ppm, downfield (48–70 ppm)
compared to the free ligand [d31P (dppm) = ꢂ21 ppm].
2.2. Cyclic voltammetric studies
Electrochemical study of 1a–4a and 1b–4b was con-
ducted in order to establish a correlation among the group
14 elements in the present class of compounds. At the scan
rate of 50 mV sꢂ1 1a–4a exhibited an irreversible oxidation
and Eox (V) was found as follows: 1.335 (1a), 1.421 (2a),
1.401 (3a) and 1.403 (4a). This is a narrow range in the
order of Si > Sn ꢀ Ge > C. Fig. 1 shows a plot between
electronegativity and Eox for 1a–4a which reveals that by
Table 1
Cyclic voltammetric data of 1a–4a and 1b–4b in CH2Cl2 (0.1 M
[Bu4N]BF4)a
Compound
Eox (V)
Ered (V)
ꢂ0.236
ꢂ0.007
ꢂ0.016
ꢂ0.011
E1/2 (V)
ꢂ0.173
0.083
DEp (V)
0.126
1a
1b
1.335
ꢂ0.110
1.421
0.174
hv, THF
Fe
ER3
ER3
dppm, -2CO
Fe
2a
2b
OC
0.181
Ph2P
CO
PPh2
3a
3b
1.401
0.163
0.073
0.179
[1a] E = C, R = H
[2a] E = Si, R = Me
[3a] E = Ge, R = Me
[4a] E = Sn, R = Me
[1b] E = C, R = H
[2b] E = Si, R = Me
[3b] E = Ge, R = Me
[4b] E = Sn, R = Me
4a
4b
1.403
0. 319
0.154
0.330
Sweep rate 50 mV sꢂ1
.
a
Scheme 1.
References with AgCl/Ag.

M. Kumar et al. / Inorganica Chimica Acta 361 (2008) 1793–1796
1795
F, Cl, Br, I, H, CH3) on the basis of theoretical calculations
and the involvement of p-donation (p-dative bonding) in
the order F > Cl > Br > I [4].
We are engaged in DFT calculations on the complexes
1–4, and cyclic voltammetric analysis of other transition
metal systems containing direct bonds to C, Si, Ge and
Sn to provide a more concise set of arguments for our
observed experimental data. Whereas oxidation of the
dicarbonyl complexes 1a–4a can involve loss of an electron
from a Fe–E bonding HOMO [10], that from the com-
plexes 1b–4b can be from a corresponding r* orbital with
more metal d content.
2.3. Experimental procedure
Fig. 2. A representative cyclic voltammogram of 1b.
All experiments were performed under an atmosphere
of dry N2 using Schlenk techniques. THF and hexanes
were freshly distilled from sodium benzophenone ketyl
immediately prior to use. 1a–4a were synthesized accord-
ing to the literature method [11]. Other chemicals were pur-
chased from Aldrich and used as received. Cyclic
voltammetric measurements were performed using a Per-
kin–Elmer potentiostat/galvanostat CV (model # 263A)
analyzer. The electrochemical cell comprised a platinum
wire working electrode, a silver wire reference electrode,
and platinum wire counter electrode. All measurements
were made in dry N2 atmosphere. Eox/Ered was referenced
to Ag/AgCl couple. Infrared spectra were obtained in THF
A useful tool for evaluating electronic effects of substit-
uents is the Taft (r*) coefficient [9]. As may be seen from
Fig. 3, a plot of the various Eox values against the Taft
(r*) values results in another inverse linear relationship
but as with the electronegativity relationship noted above,
this relationship is opposite to that predicted, i.e. more
electron-donating substituents result in higher Eox values.
An interesting correlation between 31P NMR and Eox val-
ues in 1b–4b revealed that they are also linearly related.
Complex 1b exhibited the strongest coordination between
P and Fe (31P NMR Dd ꢁ 70 ppm) which directly relates
to the increased electron-density at the iron center, result-
ing in easier oxidation. For complexes 2b–4b there is appar-
ently weaker P coordination to the iron atom as noted by
the shifting 31P resonances (31P NMR Dd ꢁ 55–60 ppm)
and elevated Eox values. These data strongly suggest that
for the complexes 1b–4b the donor function of the diphos-
phine controls the Eox values.
1
solution on an ATI Mattson infinity series FTIR; H, 13C,
29Si, 119Sn and 31P NMR was recorded on a Bruker
DPX-300 at 300, 75.4, 59.6, 111.8 and 121.5 MHz respec-
tively. Elemental analysis was performed at Galbraith
Laboratories.
2.3.1. Synthesis of (g5-C5H5)Fe(dppm)Me (1b)
From both the series 1a–4a and 1b–4b a general pattern
of carbon giving the lowest oxidation potential an argu-
ment could be put forward that retrodative p-bonding is
important to differing extents for the Si, Ge and Sn substit-
uents and explains the inability of both simple electroneg-
ativity and Taft coefficients relationships to hold. Such an
explanation is analogous to that proposed for the related
diphosphine derivatives of (g5-C5Me5)Fe(dppe)X, (X =
To a 25 mL THF solution of 1a (0.5 g, 1.17 mmol) in a
quartz tube was added (0.45 g, 1.18 mmol) of dppm and
the tube was degassed twice. The solution was then irradi-
ated by a Hanovia 450 W medium pressure lamp at a dis-
tance of 4 cm for two days. The progress of the
photochemical reaction was monitored by infrared and
31P NMR spectroscopy. After completion of the reaction
THF was evaporated under vacuum. The solid material
obtained was washed with hexanes and filtered. The hexane
insoluble material was collected, dried and characterized as
1b. Recrystallization from benzene and hexanes (1:2) affor-
ded an analytically pure compound in 64% yield. Com-
plexes 2b–4b were prepared following the identical
procedure as described above for 1b in 60% (2b), 67%
(3b) and 75% (4b) yields, respectively. Spectroscopic data
for 1b was reported elsewhere [5] and compared well with
the present data.
2.3.2. (g5-C5H5)Fe(dppm)SiMe3 (2b)
Anal. Calc. for C33H36P2SiFe: C, 68.51; H, 6.22. Found:
1
Fig. 3. A plot between Eox vs Taft constant (r*) for CpFe(PP)ER3 (E = C,
R = H; E = Si, Ge, Sn, R = Me) [1b–4b].
C, 68.22; H, 6.03%. H NMR: d 0.36 (s, 9H, SiMe3), 4.41
(s, 5H, Cp), 3.90 (2H, CH2), 7.00–7.51 (20H, Ph). 13C

1796
M. Kumar et al. / Inorganica Chimica Acta 361 (2008) 1793–1796
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[2] N.G. Connelly, W.E. Geiger, in: F.G.A. Stone, Robert West (Eds.),
Advances in Organometallic Chemistry, vol. 23, Academic Press,
1984, p. 1.
NMR: d 8.89 (SiMe3), 76.0 (Cp), 43.8 (CH2, JP–C = 18 Hz)
128.9, 130.8, 130.9, 139.3 (Ph). 31P NMR: d 40.5.
2
29SiNMR: d 27.6 (t, JSi–P = 43.3 Hz).
2.3.3. (g5-C5H5)Fe(dppm)GeMe3 (3b)
[3] (a) P.M. Treichel, D.C. Molzahn, K.P. Wagner, J. Organomet.
Chem. 174 (1979) 191;
Anal. Calc. for C33H36P2GeFe: C, 63.56; H, 5.77.
Found: C, 63.57; H, 5.77%. 1H NMR: d 0.32 (s, 9H,
GeMe3), 3.90 (q, 2H, CH2), 4.35 (s, 5H, Cp), 7.00–7.46
(20H, Ph). 13C NMR: d 7.28 (GeMe3), 44.0 (t, CH2,
JP–C = 18 Hz), 75.2 (Cp), 129.1, 131.0, 139.5, 142.0 (Ph).
31P NMR: d 40.3.
(b) P.M. Treichel, D.A. Komer, J. Organomet. Chem. 206 (1981) 77.
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R. Benn, A. Rufinska, Organometallics 9 (1990) 1071.
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3rd ed., Wiley, New York, 1995, p. 821.
[8] (a) M.J. Mays, P.L. Sears, J. Chem. Soc. Dalton (1973) 1873;
(b) K.H. Pannell, J.J. Wu, G.J. Long, J. Organomet. Chem. 186
(1980) 85;
(c) W.R. Cullen, J.R. Sams, J.A.J. Thompson, Inorg. Chem. 10
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[9] (a) T.G. Traylor, W. Hanstein, H.J. Berwin, N.A. Clinton, R.S.
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2.3.4. (g5-C5H5)Fe(dppm)SnMe3 (4b)
Anal. Calc. for C33H36P2SnFe: C, 59.10; H, 5.37. Found:
1
C, 58.63; H, 5.36%. H NMR: d 0.13 (s, 9H, SnMe3), 3.90
(q, 2H, CH2), 4.33 (s, 5H, Cp), 7.03–7.43 (20H, Ph). 13C
NMR: d ꢂ4.2 (SnMe3), 45.2 (CH2, JP–C = 18 Hz), 73.3
(Cp), 128.2, 131.3, 140.1, 142.3 (Ph). 31P NMR: d 37.8.
2
119SnNMR: d 69.0 (t, JSn–P = 447 Hz).
(b) D.D. Davis, J. Organomet. Chem. 206 (1981) 21;
(c) L. Majis, Kimijas Serija 2 (1968) 253;
Acknowledgements
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We thank the Robert Welch Foundation, Houston
Texas (Grant # AH-546) for support of this study.
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