Iron dinitrosyl complexes of TCNE: A synthetic, X-ray crystallographic, high field NMR and electrochemical study

Article abstract of DOI:10.1016/S0022-328X(98)00399-4

Treatment of Fe(NO)₂[PR₃](CO) with tetracyanoethylene (TCNE) in diethyl ether leads to the formation of Fe(NO)₂[PR₃](η²-TCNE), where PR₃=P(OCH₃)₃, 1, P(n-Bu)₃, 2, PMe₂Ph, 3, and PEt₂Ph, 4. An X-ray crystallographic study of 1 shows the iron to be situated in a nearly tetrahedral environment with a π-bonded tetracyanoethylene and two linearly bound nitrosyl groups. From the ambient-temperature NMR spectral data, it is evident that there exist two non-equivalent cyanocarbon environments, indicating that the rotation about the Fe-TCNE π-bond is slowed at room temperature; variable-temperature NMR studies on Fe(NO)₂[P(OMe)₃](η²-TCNE), 1, yielded an activation energy barrier of approximately 18.1±0.5 kcal mol^-1 for this rotational process. Electrochemical studies revealed that the neutral Fe(NO)₂[PR₃](η²-TCNE) complexes undergo irreversible reductions at positively shifted potentials, relative to the related Fe(NO)₂[PR₃](CO) complexes. Moreover, a trend toward cathodic shift of the reduction potentials with increasing phosphine pK_a has been observed. The high energy barrier for alkene rotation and the shift towards positive reduction potentials are rationalized in terms of a strong π-interaction between the iron center and TCNE.

Full text of DOI:10.1016/S0022-328X(98)00399-4

Journal of Organometallic Chemistry 558 (1998) 1–9
Iron dinitrosyl complexes of TCNE: a synthetic, X-ray
crystallographic, high field NMR and electrochemical study
Arne Ho¨rsken, Guodong Zheng, Mark Stradiotto, Christopher T.C. McCrory, Lijuan Li *
Department of Chemistry, McMaster Uni6ersity, Hamilton, Ontario, L8S 4M1, Canada
Received 11 April 1997; received in revised form 16 December 1997
Abstract
Treatment of Fe(NO)2[PR3](CO) with tetracyanoethylene (TCNE) in diethyl ether leads to the formation of Fe(NO)2[PR3](p2-
TCNE), where PR3=P(OCH3)3, 1, P(n-Bu)3, 2, PMe2Ph, 3, and PEt2Ph, 4. An X-ray crystallographic study of 1 shows the iron
to be situated in a nearly tetrahedral environment with a y-bonded tetracyanoethylene and two linearly bound nitrosyl groups.
From the ambient-temperature NMR spectral data, it is evident that there exist two non-equivalent cyanocarbon environments,
indicating that the rotation about the Fe-TCNE y-bond is slowed at room temperature; variable-temperature NMR studies on
Fe(NO)2[P(OMe)3](p2-TCNE), 1, yielded an activation energy barrier of approximately 18.190.5 kcal mol−1 for this rotational
process. Electrochemical studies revealed that the neutral Fe(NO)2[PR3](p2-TCNE) complexes undergo irreversible reductions at
positively shifted potentials, relative to the related Fe(NO)2[PR3](CO) complexes. Moreover, a trend toward cathodic shift of the
reduction potentials with increasing phosphine pKa has been observed. The high energy barrier for alkene rotation and the shift
towards positive reduction potentials are rationalized in terms of a strong y-interaction between the iron center and TCNE.
© 1998 Elsevier Science S.A. All rights reserved.
Keywords: NMR; Electrochemistry; Cyclic voltammetry; Iron dinitrosyl; Nitrosyls; TCNE
1. Introduction
and Eaton also rationalized the electronic structure of
these compounds as 17 electron complexes with a d9
electron configuration [7]. More recently, Atkinson and
co-workers have provided empirical evidence in support
of a catalytic active species possessing tetrahedral ge-
ometry, and a 17 electron structure [8]. Although y-
bound-olefin iron dinitrosyl intermediates have been
proposed in the catalytic action of iron nitrosyls, most
of these complexes have been only characterized by use
of in situ spectroscopic techniques, such as EPR spec-
troscopy and IR spectroscopy. It was not until 1994
that the isolation and structural characterization of a
stable compound of Fe(NO)2 bound to olefinic TCNE,
Fe(NO)2(PPh3)(p2-TCNE), was first reported [9]. Given
the potential of these olefinic-iron nitrosyl complexes as
catalytically-active intermediates in related polymeriza-
tion processes, the study of these compounds, both in
solution and in the solid state, is of fundamental inter-
The catalytic activity of iron dinitrosyl complexes has
been known for nearly two decades, and has been the
focus of several reports, including studies of the cata-
lytic activity of these complexes in the cyclodimeriza-
tion of dienes, and in the polymerization of styrene and
acrylonitrile [1–4]. In 1987, Gadd and coworkers pro-
posed that the dimerization of butadiene might occur
via the intermediacy of Fe(NO)2(p2-C4H6)(p4-C4H6),
where one nitrosyl ligand is linear and the other is bent
[5]. These olefin-bound metal nitrosyls were not stable
and decomposed at temperatures as low as −50°C.
Ballivet-Tkatchenko and co-workers [6] summarized
the catalytic activity of these complexes, while Bryar
* Corresponding author. Tel.: +1 905 5259140; fax: +1 905
5222509; e-mail: lijuanli@mcmaster.ca
0022-328X/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.
PII S0022-328X(98)00399-4

2
A. Ho¨rsken et al. / Journal of Organometallic Chemistry 558 (1998) 1–9
yield) was obtained. IR (CH2Cl2): wNO=1752, 1704
est. Herein we report the syntheses, structure, dynamics
1
cm−1; wCO=1995 cm−1. H-NMR (200 MHz, CDCl3):
and electrochemical behavior of
a
series of
Fe(NO)2(PR3)(p2-TCNE) complexes, where PR3=
P(OCH3)3, 1, P(n-Bu)3, 2, PMe2Ph, 3, and PEt2Ph, 4.
1.00 ppm (9H, m, CH3); 1.49 (12H, s, CH2CH2); 1.81
ppm (6H, m, CH2P).
2.4. Fe(NO)2[PMe2Ph](CO)
2. Experimental
Fe(NO)2(CO)2 (0.5 g, 2.91×10−3 mol) was added to
a solution of dimethylphenylphosphine (0.40 g, 2.90×
10−3 mol) in pentane (50 ml), a dark brown liquid
product Fe(NO)2[PMe2Ph](CO) (0.0.54 g, 1.92×10−3
All experiments were performed under a nitrogen
atmosphere either in a glove box or by using standard
Schlenk techniques. All solvents were dried according
to standard procedures [10]. The Fe(CO)5, P(OCH3)3,
P(n-Bu)3, PMe2Ph and PEt2Ph were obtained from
Aldrich and used without further purification.
Fe(NO)2(CO)2 was synthesized according to the proce-
mol, 66% yield) was obtained. IR (CH2Cl2): wNO
=
1754, 1708 cm−1; wCO=2004 cm−1 1H-NMR (200
.
MHz, CDCl3): 1.87 ppm (6H, d, JH–P=9.2 Hz, CH3);
7.82 ppm (5H, m, Ph).
dure described by Hieber and Beutner [11]. H-, 13C-
1
and 31P-NMR spectra were recorded on either a Bruker
DRX 500, an AC 300 or an AC 200 spectrometer; the
chemical shifts were referenced to the solvents and
reported relative to TMS, or 85% H3PO4 in D2O. IR
spectra were obtained on a Bio-Rad FTS-40 spectrome-
ter using either KBr pellets or solution cell with CaF2
window. The solution IR experiments were prepared in
a Glove Box with degassed CH2Cl2.
2.5. Fe(NO)2[PEt2Ph](CO)
Fe(NO)2(CO)2 (0.5 g, 2.91×10−3 mol) was added to
a solution of diethylphenylphosphine (0.48 g, 2.89×
10−3 mol) in pentane (50 ml), a dark brown liquid
product Fe(NO)2[PEt2Ph](CO) (0.8 g, 2.58×10−3 mol,
89% yield) was. IR (CH2Cl2): wNO=1755, 1706 cm−1
;
1
wCO=2004 cm−1. H-NMR (200 MHz, CDCl3): 1.09
ppm (m, 6H, CH3); 2.14 ppm (m, 4H, CH2); 7.82 ppm
(m, 5H, Ph).
2.1. General synthesis of carbonyl phosphines
2.6. General synthesis of the TCNE compounds
These complexes were synthesized by adopting a
similar
method
used
in
the
synthesis
of
To a round-bottom flask fitted with a rubber septum
was charged diethyl ether, Fe(NO)2[P(OMe)3](CO) and
TCNE (equimolar). The initially dark red solution
turned to light yellow after 1/2 h, and a yellow precipi-
tate was formed. After 3 h, the precipitate was filtered
off to give yellow powders, which were washed three
times with diethyl ether and dried in vacuo.
Fe(NO)2(PPh3)(CO) [12]. Fe(NO)2(CO)2 was added to a
solution of the appropriate phosphine/phosphite
(equimolar) in pentane in a round-bottom flask fitted
with a rubber septum. The carbon monoxide evolved
during the reaction was released through a needle in-
serted in the rubber stopper. After stirring overnight at
room temperature, the products Fe(NO)2[PR3](CO)
were obtained by removing solvent under reduced
pressure.
2.7. Fe(NO)2[P(OMe)3](p2-TCNE), (1)
In diethyl ether (150 ml), Fe(NO)2[P(OMe)3](CO)
(1.1 g, 4.1×10−3 mol) and TCNE (0.66 g, 5.2×10−3
mol) were allowed to react to give 1 (1.45 g, 3.96×10−
3 mol, 96% yield). Recrystallization from methylene
chloride yielded orange single crystals, suitable for X-
ray crystallographic analysis. IR (KBr): wNO=1843,
2.2. Fe(NO)2[P(OMe)3](CO)
Fe(NO)2(CO)2 (2.0 g, 1.16×10−2 mol) was added to
a solution of trimethyl phosphite (0.88 g, 1.16×10−2
mol) in pentane (30 ml), a dark brown liquid product
Fe(NO)2[P(OMe)3](CO) (2.95 g, 1.1×10−2 mol, 95%
1786 cm−1; wCN=2230 cm−1. IR (CH2Cl2): wNO
1843, 1797 cm−1; wCN=2233 cm−1 1H-NMR (500
=
yield) was obtained. IR: wNO=1770, 1722 cm−1; wCO
=
2018 cm−1. 1H-NMR (200 MHz, acetone-d6): 3.60 ppm
(d, JH–P=12.0 Hz, OCH3).
.
MHz, acetone-d6): 4.08 ppm (d, JH–P=12.3 Hz,
OCH3). 13C-NMR (125.7 MHz, CD2Cl2): 113.1 ppm (d,
J
C–P=4.0 Hz, CꢀN); 113.3 ppm (d, JC–P=5.6 Hz,
2.3. Fe(NO)2[P(n-Bu)3](CO)
CꢀN); 55.3 ppm (d, JC–P=6.5 Hz, OCH3). 31P-NMR
(121.4 MHz, CD2Cl2): 62.73 ppm. Anal: Calculated for
C9H9N6O5FeP: C, 29.37; H, 2.46; N, 22.84; Fe, 15.17;
P, 8.42. Found: C, 29.52; H, 2.61; N, 21.75; Fe, 14.81;
P, 7.90.
Fe(NO)2(CO)2 (0.5 g, 2.91×10−3 mol) was added to
a solution of Tri-n-butylphosphine (0.59 g, 2.92×10−3
mol) in pentane (50 ml), a dark brown solid product
Fe(NO)2[P(n-Bu)3](CO), (0.83 g, 2.4×10−3 mol, 82.5%

A. Ho¨rsken et al. / Journal of Organometallic Chemistry 558 (1998) 1–9
3
Table 1
List of IR frequencies of related iron dinitrosyl complexes
Complexes
CO stretch (cm−1
)
NO stretch (cm−1
)
CN stretch (cm−1
)
Reference
Fe(NO)2[P(OMe3)](CO)
Fe(NO)2[P(OMe)3](h2-TCNE), 1
Fe(NO)2[P(n-Bu)3](CO)
Fe(NO)2[P(n-Bu)3](h2-TCNE), 2
Fe(NO)2[PMe2Ph](CO)
2018
—
1770; 1722
1843; 1790 (1843, 1797)
(1752; 1704)
1828; 1778 (1824; 1785)
(1754; 1708)
1839; 1792 (1830; 1786)
(1755; 1706)
1812; 1755 (1827; 1790)
1766; 1718
1834; 1790
—
This work
This work
This work
This work
This work
This work
This work
This work
[7]
2230 (2233)
(1995)a
—
—
2229 (2230)
(2004)
—
Fe(NO)2[PMe2Ph](h2-TCNE), 3
Fe(NO)2[PEt2Ph](CO)
2219 (2226)
(2004)
—
2007
—
2225 (2231)
—
Fe(NO)2[PEt2Ph](h2-TCNE), 4
Fe(NO)2(PPh3)(CO)
Fe(NO)2(PPh3)(h2-TCNE)
Fe(NO)2(CO)2
—
2224
[9]
[7]
2090; 2040
1817; 1766
—
a The numbers shown in brackets were measured in CH2Cl2 solution.
2.8. Fe(NO)2[P(n-Bu)3](p2-TCNE), (2)
2.11. Crystal structure determinations
X-ray crystallographic data for 1 were collected from
a single crystal which was mounted on a glass fiber and
transferred to a P4 Siemens diffractometer, equipped
with a rotating anode and graphite-monochromated
In diethyl ether (50 ml), Fe(NO)2[P(n-Bu)3](CO) (0.27
g, 7.8×10−4 mol) and TCNE (0.10 g, 7.8×10−4 mol)
were allowed to react to give 2. (0.29 g, 6.5×10−4 mol,
83% yield). IR (CH2Cl2): wNO=1785, 1824 cm−1
;
;
˚
wCN=2230 cm−1; IR (KBr): wNO=1778, 1828 cm−1
Mo-Kh radiation (u=0.71073 A). Three standard
wCN=2229 cm−1. H-NMR (200 MHz, CDCl3): 0.98
ppm (9H, m, CH3); 1.51 (12H, s, CH2CH2); 2.23 ppm
(6H, m, CH2P). Anal: Calculated for C18H27N6O2FeP:
C, 48.43; H, 6.05; N, 18.83. Found: C, 47.55; H, 6.21;
N, 17.85.
1
reflections that were measured after every 97 reflections
showed neither instrument instability nor crystal decay.
The structures were solved by using the Direct Methods
procedure in the SHELXTL-PLUS program library [13],
and refined by full-matrix least-squares methods with
anisotropic thermal parameters for all non-hydrogen
atoms. All hydrogen atoms were included as fixed
contributors at calculated positions, with isotropic dis-
placement parameters based on the attached carbon
atom. Crystallographic collection parameters, atomic
coordinates and bond lengths for 1 are presented in
Tables 2–4, respectively. Additional data, including
anisotropic displacement coefficients, hydrogen atom
coordinates, and a list of observed and calculated struc-
ture factors are available from the authors.
2.9. Fe(NO)2[PMe2Ph](p2-TCNE), (3)
In diethyl ether (50 ml), Fe(NO)2[PMe2Ph](CO) (0.22
g, 7.8×10−4 mol) and TCNE (0.10 g, 7.8×10−4 mol)
were allowed to react to give 3. (0.22 g, 5.76×10−4
mol, 74% yield). IR (CH2Cl2): wNO=1786, 1830 cm−1
;
;
wCN=2226 cm−1; IR (KBr): wNO=1792, 1839 cm−1
1
wCN=2219 cm−1. H-NMR (200 MHz, CDCl3): 2.24
ppm (6H, d, JH–P=10.5 Hz, CH3); 7.64 ppm (5H, s,
Ph). Anal: Calculated for C14H11N6O2FeP: C, 43.98; H,
2.88; N, 21.99. Found: C, 44.36; H, 2.85; N, 21.97.
2.12. Electrochemistry
Electrochemical measurements were performed on a
PARC (Princeton Applied Research, Princeton, NJ)
263A potentiostat, controlled by a computer running
the electrochemistry software M270. The working elec-
trode used was a commercial gold electrode purchased
from Bioanalytical Systems and the counter electrode
was a platinum coil. The quasi-reference electrode was
a Ag wire and the potentials were reported with respect
to a ferrocenium-ferrocene standard. The CH2Cl2 solu-
tion containing the supporting electrolyte (n-
C6H13)4NPF6 (0.1 M) was prepared and each complex
(1.0×10−3 M) was added to the degassed solution and
degassed again before running CV experiments. During
2.10. Fe(NO)2[PEt2Ph](p2-TCNE), (4)
In diethyl ether (50 ml), Fe(NO)2[PEt2Ph](CO) (0.24
g, 7.8×10−4 mol) and TCNE (0.10 g, 7.8×10−4 mol)
were allowed to react to give 4. (0.25 g, 6.1×10−4 mol,
78% yield). IR (CH2Cl2): wNO=1790, 1827 cm−1
;
;
wCN=2231 cm−1; IR (KBr): wNO=1755, 1812 cm−1
1
wCN=2225 cm−1. H-NMR (200 MHz, CDCl3): 1.23
ppm (6H, td, JH–P=18.6 Hz, JH–H=7.5 Hz, CH3);
2.65 ppm (4H, m, CH2); 7.60 ppm (5H, m, Ph). Anal:
Calculated for C16H15N6O2FeP: C, 46.83; H, 3.66; N,
20.49. Found: C, 46.50; H, 3.60; N, 20.20.

4
A. Ho¨rsken et al. / Journal of Organometallic Chemistry 558 (1998) 1–9
these experiments, the electrochemical micro-cell, con-
taining 10 ml solution, was kept in a Smart Stir Faraday
Cage purchased from EG&G to prevent from any
interference from electromagnetic waves.
3. Results and discussion
3.1. Syntheses and Reactions
Complexes of Fe(NO)2(CO)(PR3), (R=OCH3, P(n-
Bu)3, PMe2Ph, PEt2Ph), were synthesized by adopting
similar synthetic methods used in the preparation of
Fe(NO)2(CO)(PPh3), and were monitored by IR and
NMR. These compounds reacted rapidly with TCNE to
afford the complexes Fe(NO)2[PR3](p2-TCNE), (PR3=
P(OCH3)3, 1, P(n-Bu)3, 2, PMe2Ph, 3, and PEt2Ph, 4.) in
good yield. These compounds proved to be soluble in
polar solvents such as CH2Cl2, THF, (CH3)2CO,
CH3CN and MeOH (but not in diethyl ether and water),
however complete decomposition occurs after a few
hours. When stored in the solid state at ambient temper-
ature under an atmosphere of nitrogen, similar decom-
position occurs, albeit much more slowly. The rapid
decomposition of these complexes in polar solvents can
be rationalized in terms of a nucleophile-induced dispro-
portionation reaction, as observed in the reaction of
[Fe(CO)3(PPh3)2]+ and pyridine [14], and in the conver-
sion of Fe(NO)2(PPh3)2+ to Fe(NO)2(PPh3)2 [8].
The rapid kinetics associated with the formation of
the p2-TCNE iron dinitrosyl complexes is evident in
comparing the substitution reaction of carbonyls by
phosphines, which requires about two days at elevated
temperatures [12]. A similar phenomenon has been
observed in the synthesis of Fe(NO)2[PPh3](p2-TCNE),
which is believed to proceed via an electron transfer
autocatalysis mechanism, through a 17-electron para-
magnetic intermediate [9].
Fig. 2. The molecular structure of Fe(NO)2P(OMe)3(p2-TCNE), 1,
determined by X-ray diffraction. Anisotropic thermal ellipsoids are
shown at 30%.
(X=Cl or I), are also interconvertable through a redox
system [8]. This allowed them to synthesize complexes of
the series [Fe(NO)2(PR3)2], in a few minutes in the
presence of reducing reagents.
3.2. IR data
The FT-IR data for the mono-substituted products,
Fe(NO)2(CO)(PR3), the TCNE adducts, Fe(NO)2(PR3)
(p2-TCNE) (1, 2, 3, and 4), and other related complexes
are listed in Table 1. The spectrum of the starting
material, Fe(NO)2(CO)2, exhibits two nitrosyl stretches
and two carbonyl stretches. Upon replacement of a
single carbonyl moiety by a phosphite or phosphine
group, the remaining CO absorbs around 1995–2018
cm−1. Replacement of the second carbonyl group by
TCNE gives rise to a peak ~2225 cm−1, attributable to
the nitrile functionalities.
Fe(NO)2PR3(CO)
“Fe(NO)2PR3(CO)+TC“NEFe(NO)2PR3(p2-TCNE)
On examination of the two nitrosyl IR stretches in
these complexes, a noticeable trend arises in the shifts of
these peaks when carbonyl groups are substituted by
other ligands. The phosphorus donor increases the elec-
tron density at the iron center, which in turn enhances
the back-bonding from the filled d-orbitals on the metal
to the vacant anti-bonding orbitals of the nitrosyls, with
concomitant weakening of the NꢀO bond. This results
in a shift of the nitrosyl stretches towards lower
wavenumbers, in the region of 1700–1770 cm−1. In
contrast, the electron-withdrawing effect of TCNE re-
duces back-bonding which in turn strengthens the nitro-
syl bond, resulting in a shift of the two nitrosyl peaks to
higher frequencies (1750 and 1850 cm−1). The magni-
tude of this high frequency shift completely compensates
for the bond weakening observed on initial incorpora-
tion of the phosphine or phosphite moiety.
Very recently, Atkinson and co-workers elegantly
demonstrated that two types of four-co-ordinate iron
dinitrosyl complex, [Fe(NO)2L2] and [Fe(NO)2LX]
Fig. 1. Molecular orbital illustration of the TCNE | and y bonding
to the metal. (a) TCNE |-bond. (b) TCNE y-bond.

A. Ho¨rsken et al. / Journal of Organometallic Chemistry 558 (1998) 1–9
5
Table 3
Typically, ferrocene reacts with TCNE to form
charge transfer complexes of the type (p5-C5Me5)2Fe+
(TCNE)−, in which the cyano stretching frequency
appears unchanged from free TCNE [15]. The |-
bonded TCNE (Fig. 1a) shows multiple CN absorp-
tions owing to the effective local Cs symmetry of the
coordinated TCNE [16]. The observation of only one
broad cyano stretching frequency in the IR spectrum of
1, 2, 3 and 4, in both solid state and in solution,
confirms that the TCNE moiety is in fact y-bonded to
the iron. This is supported by the observation of only
one strong wCꢀN absorption in other y-bonded TCNE
Atomic coordinates (×104) and equivalent isotropic displacement
parameters (A ×103) for 1
2
˚
x
y
z
U(eq)
41(1)
Fe(1)
P(1)
3899(1)
4383(1)
4295(2)
3707(2)
3514(3)
2561(3)
3772(3)
2848(3)
4562(2)
3600(2)
4684(4)
4773(3)
4130(4)
3295(2)
3405(2)
5032(6)
5007(7)
4065(6)
3400(3)
2879(3)
3609(3)
3095(2)
366(1)
765(1)
12(2)
2352(2)
4132(3)
1818(10)
640(10)
54(1)
59(2)
56(2)
82(2)
80(2)
75(2)
75(2)
92(2)
83(2)
186(6)
126(4)
176(6)
45(2)
41(1)
189(8)
256(14)
137(5)
55(2)
54(2)
46(2)
49(2)
N(1)
N(2)
N(11)
N(12)
N(21)
N(22)
O(1)
O(2)
O(3)
O(4)
O(5)
C(1)
C(2)
C(3)
C(4)
C(5)
C(11)
C(12)
C(21)
C(22)
680(2)
−711(2)
233(2)
−92(2)
865(2)
−233(2)
873(2)
511(3)
965(2)
1080(3)
33(2)
264(2)
573(5)
1174(4)
1445(3)
−387(2)
144(2)
64(2)
2925(13)
734(13)
7582(12)
5339(12)
1304(11)
complexes, such as Fe(NO)2(PPh3)(p2-TCNE) (wCꢀN
=
−694(10)
5598(18)
2889(16)
5256(19)
2876(10)
4606(10)
6888(28)
2054(32)
5569(24)
2916(11)
1686(11)
6300(11)
5023(11)
2224.3 cm−1) [9]; W(CO)5(p2-TCNE) (wCꢀN=2190
cm−1) [15], (PR3)2Pt(p2-TCNE) (wCꢀN=2222 cm−1);
Rh(PPh3)2(CO)Cl(p2-TCNE) (wCꢀN=2230 cm−1) [17];
Cr(CO)5(p2-TCNE) (wCꢀN=2203 cm−1
) [18]. The
cyano stretching frequency is shifted to lower
wavenumbers in comparison to the corresponding
stretching frequencies for free TCNE, and can be inter-
preted in terms of the efficient back-donation from the
filled metal d-orbitals on iron into the vacant y*-orbital
of TCNE. This back-donation results in a weakening of
the CꢀN bond, as depicted in Fig. 1b.
600(2)
U(eq) is defined as one third of the trace of the orthogonalized Uij
tensor.
3.3. X-ray crystallography
Table 2
Crystal data and structure refinement for 1
Using suitable single crystals grown from methylene
chloride, the crystal structure Fe(NO)2[P(OMe)3](p2-
TCNE), 1, was determined, as shown in Fig. 2. This
complex crystallizes in the space group Fdd2 and has
an orthorhombic unit cell with the following dimen-
sions: a=27.224 A, b=33.862 A and c=6.834 A. The
iron center exhibits distorted tetrahedral geometry and
is bound to two nitrosyl groups, one phosphorus ligand
and one y-bonded TCNE ligand. The trimethylphos-
phite group exhibits significant amplitude torsional mo-
tion, which is manifested in the large thermal ellipsoids.
The parameters resulting from the structural refinement
are listed in Table 2, while the atomic coordinates and
selected principal bond distances and angles are col-
lected in Tables 3 and 4, respectively.
Empirical formula
Molecular weight
Description
C9H9Fe1N6O5P1
386.04
Orange plate
293(2)
(Mo-Kh) 0.71073
Orthorhombic
Fdd2
27.224(4)
33.862(7)
6.8340(9)
6300(2)
16
1.552
1.089
q-2q
2976
Temperature (K)
˚
Wavelength (A)
˚
˚
˚
Crystal system
Space group
˚
a (A)
˚
b (A)
˚
c (A)
3
˚
Volume (A )
Z
Calcd density (g cm−3
)
Abs coeff. (mm−1
Scan mode
F(000)
)
q-range for collection (q)
1.92–25.01
The two nitrosyl groups in 1 are nearly linear with
angles of 175.1(7) and 172.6(6)°, while for
Fe(NO)2(PPh3)(p2-TCNE), angles of 178.0(5) and
165.8(5)° were observed [9]. The Fe(NO)2 unit is in an
attracto conformation with O–Fe–O and N–Fe–N
angles of 116.2 and 120.9°, respectively. The average
Index ranges
−15h532
−15k540
−85l58
3021
No. reflections collected
No. independent reflections
R(int)
2779
0.0422
Refinement method
Full-matrix least-squares on F2
˚
N–O distance in 1 (1.158(8) A) is similar to the corre-
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices (I\2|(I))a
2779/1/200
1.020
R1=0.0576; wR2=0.1494
R1=0.0712; wR2=0.1589
0.509
sponding values found in Fe(NO)2(PPh3)(p2-TCNE)
˚
˚
(1.169(7) A) [9] and Fe(NO)2(CO)2 (1.171 A) [19], sug-
gesting that the electron-withdrawing effect of the
TCNE moiety is sufficient to counteract the corre-
sponding electron-donating strength of the phosphorus
ligand, as compared to the Fe(NO)2(CO)2 reference.
R indices (all data)a
−3
˚
˚
Largest diff. Peak (e A
Largest diff. Hole (e A
)
)
−3
−0.379
a R1=S(ꢀFoꢁ−ꢁFcꢀ)/SꢁFoꢁ; wR2=[S[w(F2o−Fc2)2]/S[w(F2o)2]]
.
0.5

6
A. Ho¨rsken et al. / Journal of Organometallic Chemistry 558 (1998) 1–9
Table 4
the metal to the alkene y* manifold; this phenomenon
is, of course, well established especially for ligands such
as C2F4 which possess highly electron-negative sub-
stituents [20]. The steric demands of the phosphorus
ligand in 1 are manifested in the asymmetric bonding of
the iron center to the ethylenic moiety. The lengthened
˚
Selected bond lengths (A) and angles (°) for 1
Fe(1)–N(1)
Fe(1)–N(2)
Fe(1)–C(1)
Fe(1)–C(2)
Fe(1)–P(1)
N(1)–O(1)
N(2)–O(2)
C(1)–C(2)
1.653(6)
1.667(7)
2.024(6)
2.074(7)
2.245(2)
1.157(8)
1.158(8)
1.448(9)
˚
Fe–C(2) bond (2.074(7) A) nearest the trimethylphos-
phite group, relative to the related Fe–C(1) distance
˚
(2.024(6) A) opposite the phosphorus moiety, can be
N(1)–Fe(1)–N(2)
N(1)–Fe(1)–C(1)
N(2)–Fe(1)–C(1)
N(1)–Fe(1)–C(2)
N(2)–Fe(1)–C(2)
C(1)–Fe(1)–C(2)
N(1)–Fe(1)–P(1)
N(2)–Fe(1)–P(1)
C(1)–Fe(1)–P(1)
C(2)–Fe(1)–P(1)
O(1)–N(1)–Fe(1)
O(2)–N(2)–Fe(1)
C(12)–C(1)–C(2)
C(12)–C(1)–C(11)
C(2)–C(1)–C(11)
C(12)–C(1)–Fe(1)
C(2)–C(1)–Fe(1)
C(11)–C(1)–Fe(1)
C(22)–C(2)–C(1)
C(22)–C(2)–C(21)
C(1)–C(2)–C(21)
C(22)–C(2)–Fe(1)
C(1)–C(2)–Fe(1)
C(21)–C(2)–Fe(1)
120.9(3)
rationalized in terms of unfavorable steric interactions
of the phosphite group with the C(21)–N(21) and
C(22)–N(22) nitriles. These steric effects are enhanced
in the Fe(NO)2(PR3)(p2-TCNE) series when even
bulkier phosphorus ligands such as R=Ph [9], or R=
cyclohexyl (M. Stradiotto, J.F. Britten, personal com-
munication), are employed. The relevance of steric
effects in TCNE-metal complexes was also demon-
strated by McGinnety and Ibers, who reported the
formation of an unexpected cis-diphosphine adduct to
Vaska’s Br-compound, IrBr(CO)(PPh3)2(p2-TCNE), at-
tributable to the steric demands of the phosphine and
the TCNE ligand [22].
Space-filling models depicting the crystal packing of
the complex viewed from the origin along the +c and
−c axes are presented in Fig. 3(a) and (b), respectively.
These demonstrate that in the solid state, the nitrosyl
groups are always oriented in the direction of the −c
axis specifically. This packing motif gives rise to alter-
nating layers of nitrosyl groups and tetracyanoethylene
groups along the c-axis. One may be able to exploit this
very interesting feature whereby, with nitrosyls on one
face and TCNE groups on the other, unique manipula-
tions may be performed on each surface preferentially.
We are currently investigating the magnetic and pho-
tolytic properties of this material.
99.5(3)
102.9(3)
117.8(3)
115.0(3)
41.4(3)
100.0(2)
100.4(2)
135.7(2)
94.5(2)
175.1(7)
172.6(6)
118.7(6)
114.8(6)
118.2(6)
113.1(5)
71.2(4)
112.9(5)
117.7(6)
115.7(6)
118.6(6)
113.3(5)
67.5(4)
114.9(5)
The dihedral angle between the plane containing
C(1)ꢁC(2), that is perpendicular to the Fe–C(1)–C(2)
plane and the plane containing C(1)–C(12)–N(12), and
C(11)–N(11) is 15.6°. This loss of planarity presumably
results from the back-donation of electron density from
Fig. 3. The crystal packing diagram of 1 (a) viewed from the origin down the +c axis. (b) viewed from the origin down the −c axis. The atomic
radii have been altered for clarity and the oxygen and nitrogen of the nitrosyl group are presented in black and dark grey, respectively.

A. Ho¨rsken et al. / Journal of Organometallic Chemistry 558 (1998) 1–9
7
along Fe-TCNE y-bond is restricted on the NMR
time-scale.
The variable-temperature 13C NMR spectra (in
CD3CN), shown in Fig. 5, shows peak coalescence at
about 70°C. Unfortunately, sample decomposition
above 80°C prevented the acquisition of data in the fast
exchange region, so a complete line-shape analysis was
not possible. Nevertheless, the Gutowsky-Holm ap-
proximation yields a value for DG‡343 of 18.190.5 kcal
mol−1. This barrier is rather high compared to the
published barriers for the rotation of coordinated
olefins. Typically, Cramer and coworkers reported bar-
riers for C2H4 rotation in the complexes (C5H5)Rh(p2-
C2H4)2, (C5H5)Rh(p2-C2H4)(p2-C2F4), and (C5H5)Rh
(p2-C2H4)SO2 of 15.0, 13.6, and 12.2 kcal mol−1, re-
spectively. However, it was not possible to observe
rotation about the Rh-C2F4 axis in (C5H5)Rh(p2-
C2H4)(p2-C2F4) [24]. Kaneshima et al. have also studied
the rotational processes of TCNE ligand in a series of
Fig. 4. The molecular structure of 1 showing 50% thermal ellipsoids,
viewed from the centroid of the C(1)–C(2) bond to the iron atom.
The OCH3 groups have been omitted for clarity.
3.4. NMR studies
Complex 1 is sufficiently stable in CD2Cl2 in a vac-
uum-sealed NMR tube to allow for a detailed NMR
study. The 125 MHz 13C-NMR spectrum of 1 in
CD2Cl2 at room temperature showed pairs of cyano
peaks at 113.1 ppm (JC–P=4.0 Hz) and 113.3 ppm
(JC–P=5.6 Hz), in addition to the expected methoxy
carbon at 55.3 ppm (JC–P=6.5 Hz). A very weak peak
at 29.0 ppm, corresponding to the ethylene carbons,
was also observed. The low intensity of the aforemen-
tioned peak may be attributable to its relatively long
relaxation time. Because of coordination to the iron
center, the ethylene carbons are significantly shielded
compared to those of free TCNE (112.6 ppm). This
phenomenon was also observed in the 13C-NMR spec-
trum of Fe(NO)2(PPh3)(p2-TCNE) [9]. The crystallo-
graphic data support these observations, especially the
lengthening of the C(1)–C(2) distance in the y-bonded
TCNE moiety relative to free TCNE. These chemical
shifts are indicative of significant sp3-character at the
olefinic carbons of the tetracyanoethylene moiety. It
should also be pointed out that in Pt(PPh3)2(p2-CH3–
CF=CF2), the 19F-NMR spectrum shows a geminal
2JFF coupling constant of 200 Hz, which is of the order
observed in saturated fluorocarbon systems [23]. Similar
trends were also observed in the Rh complex of
(C5H5)Rh(p2-C2F4)(p2-C2H4) [24].
The crystallographically-determined structure of 1, as
viewed down the centroid of C(1)–C(2) bond to the
iron atom, is presented in Fig. 4. From this perspective
it is evident that 1 adopts an interesting geometry, in
which two of the nitrile groups are oriented directly
above the two nitrosyl ligands and are different from
the others. The two sets of cyanocarbon peaks in the
13C-NMR spectrum at room temperature indicate that,
in agreement with the solid-state structure, rotation
Fig. 5. Variable-temperature 13C-NMR spectra of 1 recorded a
Bruker AC 300 operating at 75.47 MHz at a magnetic field strength
of 7.05 T in CD3CN.

8
A. Ho¨rsken et al. / Journal of Organometallic Chemistry 558 (1998) 1–9
Table 5
Electrochemical potentials of iron dinitrosyl complexes vs FeCp+2 /FeCp2 at scan rate of 100 mV s−1 and the pKa values for the phosphines and
phosphite
Complexes
Epc (V)
Epa(P) (V) E° (DE)a (V)
NO stretch (cm−1
)
NO stretches
pKa (Phosphine)
[28]
(Average) (cm−1
)
Fe(NO)2[P(OMe)3](h2-TCNE), 1
Fe(NO)2[PEt2Ph](h2-TCNE), 4
Fe(NO)2[PMe2Ph](h2-TCNE), 3
Fe(NO)2[P(n-Bu)3](h2-TCNE), 2
−0.990
−1.120
−1.123
−1.146
−0.398
−0.604
−0.607
−0.652
1843; 1790 (1843,
1797)b
1812; 1755 (1827;
1790)
1839; 1792 (1830;
1786)
1828; 1778 (1824;
1785)
1820
2.6
1808.5
1808
6.25
6.5
1804.5
8.43
Fe(NO)2[P(OMe3)](CO)
Fe(NO)2[PEt2Ph](CO)
Fe(NO)2[PMe2Ph](CO)
Fe(NO)2[P(n-Bu)3](CO)
−1.96 (125 mV)
−2.108 (131 mV)
−2.101 (268 mV)
−2.150 (130 mV)
1770; 1722
(1755; 1706)
(1754; 1708)
(1752; 1704)
1746
1730.5
1731
1728
2.6
6.25
6.5
8.43
a These are one electron reductions by comparison with equimolar ferrocene oxidation reactions.
b The numbers shown in brackets were measured in CH2Cl2 solution.
[Rh(RNC)4−n(PPh3)n(p2-TCNE)]X
complexes,
in
observed at much higher potential relative to the reduc-
tion, listed as Epa(P) in Table 5, is attributable specifi-
cally to the oxidation of the product P. This was
confirmed by the absence of this peak when the poten-
tial was switched off before the potential required to
reduce 1–4 (the potential range was set from 0.50 to
−0.96 V for 1; and from 0.50 to −1.12 V for 2–4) was
reached.
which, slowed rotation of the TCNE ligand was only
observed when n=0, however, no barrier was obtained
[25,26]. Clearly, the increased barrier to TCNE rotation
is a reflection of the enhanced back-donation from
metal d-orbitals into y* of the alkene; a similar ratio-
nale has been proposed by Wheelock et al. for plat-
inum(0) and acetylene and olefin complexes [27]. The
orientation shown in Fig. 4 may be a result of favorable
y-orbital interactions between the NꢀO and CꢀN y-sys-
tems, which would be broken upon rotation of the
TCNE group. These y-interactions may contribute in
part to the observed increase in the rotational activa-
tion energy barrier, in comparison to related metal-
olefin systems.
The complexes 1, 2, 3, and 4 [Epc=(−0.990)–(−
1.146) V] are harder to reduce than the free TCNE
ligand (E1/2= −0.207 V), but are easier to reduce than
the
corresponding
carbonyl
compound,
Fe(NO)2(PR3)(CO) [E°=(−1.96)–(−2.15) V]. The
coordination of TCNE leads to a shift in the reduction
potentials to a more negative value compared to free
TCNE, and to a more positive value in comparison to
Fe(NO)2(PR3)(CO). This indicates that the back-bond-
ing from the iron center to the TCNE ligand is stronger
than the characteristic covalent bonding arising from
|-donation by the TCNE ligand. This back-donation to
the TCNE ligand renders the iron atom partially posi-
tive, and thereby easier to reduce. The reduction is
presumed to occur at the iron center rather than on the
TCNE ligand since the basicity of the phosphorus
moiety has an impact on the electrochemical behavior
of these compounds. Upon evaluation of the data
presented in Table 5, it is apparent that a cathodic shift
in potential for both the E° values and the Epc and
Epa(P) values occurs for an increase in the phosphine or
phosphite pKa. This observation can be qualitatively
rationalized in that with increasing electron density
being donated to the iron center, it is rendered less
prone to reduction. It is well established that, in transi-
tion metal carbonyl complexes, the carbonyl stretching
3.5. Electrochemical studies
Cyclic voltammograms of the complexes 1–4 and its
precursors, Fe(NO)2(PR3)(CO), were recorded in
CH2Cl2 containing 0.1 M electrolyte, and the data are
collected
in
Table
5.
The
compounds,
Fe(NO)2(PR3)(CO), demonstrated a quasi-reversible re-
duction potential E° from −1.96 to −2.15 V with
large peak-to-peak separations. Upon replacement of
the carbonyl by TCNE, the reduction becomes irre-
versible. Table 5 lists the reduction potentials, Epc, of
complexes 1, 2, 3 and 4 at a scan rate of 100 mV s−1
.
This reduction has no corresponding oxidation peak,
even at a scan rate of 1000 mV s−1, which indicates
that the reduction is chemically irreversible at room
temperature. Thus, the radical anion, [Fe(NO)2PR3(p2-
TCNE)]−, decomposes rapidly to yield a decomposi-
tion product, P. The irreversible oxidation peak

A. Ho¨rsken et al. / Journal of Organometallic Chemistry 558 (1998) 1–9
9
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donating or withdrawing character. However, a correla-
tion between the pKa values and reduction potentials
has recently been observed for a series of phosphine-
substituted metal carbonyls [28]. Table 5 also lists the
averaged IR stretching frequencies for the nitrosyls,
which demonstrates the trend of decreasing IR frequen-
cies with increasing pKa values.
In conclusion, the IR and X-ray crystal structure of
Fe(NO)2[P(OMe)3](p2-TCNE), 1, indicate that both of
the nitrosyl groups are linearly bound to the iron
center, and the TCNE moiety is attached to the iron
center in an p2 fashion, through the CꢁC double bond.
High resolution solution NMR spectra data, in which
the cyanocarbons appear in pairs and the ethylene
carbons show sp3-hybridized character, are entirely
consistent with the crystal structure determination. The
slowed rotation along iron-olefin bond on the NMR
time-scale and the negative reduction potential for 1, as
compared to the free TCNE ligand, can be attributed to
the strong y-back bonding from the filled transition
metal d-orbital to the empty y*-orbital of the TCNE
ligand. This phenomenon is also reflected in the positive
shift in reduction potentials of the TCNE complexes,
relative to their carbonyl-possessing precursors.
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We wish to thank the Natural Sciences and Engineer-
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Women’s Faculty Award (L.L.), and a PGS-B scholar-
ship (M.S.). We also thank the Imperial Oil Research
Center Canada for their donation of some laboratory
equipment.
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