A reductive rearrangement of iron acetylide hydride complexes

Article abstract of DOI:10.1039/a605846i

Iron acetylide hydride complexes of the type [Fe(H)(C≡CR)(dmpe)₂] [dmpe = 1,2-bis(dimethylphosphino)ethane] react with anions in a methanol solution to form substituted alkenyl complexes; the crystal structure of trans-FeN₃(CH=CHPh)(dmpe)₂] is presented.

Full text of DOI:10.1039/a605846i

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A reductive rearrangement of iron acetylide hydride complexes
Leslie D. Field,* Adrian V. George,* Elizabeth Y. Malouf, Trevor W. Hambley and Peter Turner
School of Chemistry, University of Sydney, NSW 2006, Australia
Iron acetylide hydride complexes of the type
1,2-bis(dimethylphos-
phino)ethane] react with anions in a methanol solution to
form substituted alkenyl complexes; the crystal structure of
trans-FeN3(CHNCHPh)(dmpe)2] is presented.
centre and the stereochemistry of the double bond is (Z). The
[Fe(H)(C·CR)(dmpe)2] [dmpe
=
three nitrogen atoms of the azide group are approximately
collinear (176°) and form an approximate angle of 146° with the
equatorial plane containing the metal and the four phosphorus
donors.
A likely reaction course for the reduction of the acetylide
complexes reported here, involves initial protonation of the s-
bound acetylide ligand at the carbon b to the metal centre to
form a vinylidene complex. Metal-to-carbon hydride migration
in the vinylidene complex with attack by the anion (Cl2, N32 or
SCN2) leads to the neutral complexes (Scheme 2).
Protonation (or the addition of other electrophiles) at the b
carbon of metal acetylide complexes is well known12 and the
intramolecular hydride migration from the metal to the a carbon
There is current interest in metal acetylide complexes as
materials which may be used as components in non-linear
optical materials,1,2 the construction of organometallic poly-
mers2–4 and as new liquid crystals.5 Metal s-acetylide com-
plexes have been prepared by a number of routes including
ligand exchange reactions,4,6 oxidative addition reactions7,8 and
by deprotonation of metal vinylidene complexes.9
Acetylide hydride complexes of iron [FeH(C·CR)L4] have
been formed by reaction of a terminal acetylene with a
coordinatively unsaturated iron complex [FeL4] (L is usually a
phosphine).8,10 The acetylide hydride complexes of [Fe(PP)2]
[PP = 1,2-bis(dimethylphosphino)ethane (dmpe) or 1,2-bis-
(diethylphosphino)ethane] are typically reactive, air-sensitive
materials but they can be handled and stored as solids under an
inert atmosphere.11 They can be converted to the more stable
acetylide chloride derivatives [FeCl(C·CR)(PP)2] by treatment
with dichloromethane or other alkyl halides.‡
C(6)
C(5)
C(7)
C(4)
C(8)
C(3)
Treatment of the iron acetylide hydride complexes with a
chloride, azide or thiocyanate salt in methanol solution, resulted
effectively in substitution of the hydride by Cl2, N32 or SCN2
with partial reduction of the bound acetylene. Treatment of
trans-[FeH(C·CPh)(dmpe)2] 1 with a methanol solution of
C(2)
C(9)
C(1)
sodium
azide
resulted
in
formation
of
trans-
C(19)
[FeN3(CHNCHPh)(dmpe)2] 2 (Scheme 1).
C(13)
The conversion of 1 to 2 was clean and quantitative and the
31P NMR spectrum indicated the reaction was complete within
30 min. Removal of the solvent and washing the residue with
methanol produced trans-[FeN3(CHNCHPh)(dmpe)2] 2 as an
P(1)
P(4)
C(15)
C(18)
C(17)
Fe(1)
C(12)
P(2)
P(3)
C(11)
1
orange solid. The H NMR spectrum exhibited two alkenyl
protons (at d 7.05 and 5.73) with a coupling (3JHH 16.7 Hz)
and this is consistent with a trans stereochemistry for the
alkene. The vinyl resonance at d 7.05 also showed coupling to
the four equivalent phosphorus ligands (3JPH 6.9 Hz). An
analogous reaction between 1 and chloride or thiocyanate
C(10)
N(1)
C(20)
C(16)
C(14)
N(2)
in
[FeCl(CHNCHPh)(dmpe)2]
(CHNCHPh)(dmpe)2] 4 respectively.
methanol
solvent
quantitatively
gave
trans-[FeSCN-
trans-
3
and
N(3)
The crystal structure of 2 (Fig. 1)† confirms that the azide and
alkenyl ligands adopt a trans configuration about the metal
Fig. 1 ORTEP20 plot of trans-[FeN3(CHNCHPh)(dmpe)2] 2
+
+
R
R
H
R
C
H
X
C
C
C
C
H
H+
P
P
P
P
P
P
P
P
H
C
Fe
C
Fe
C
X –
P
P
P
P
P
P
P
P
P
P
P
Fe
H
Fe
H
Fe
MeOH
H
H
C
P
C
X –
H
R
C
H
C
P
P
P
P
= dmpe
P
= N3–, Cl –, SCN –
P
Fe
X
X – = N3–, Cl –, SCN –
X –
Scheme 1
Scheme 2
Chem. Commun., 1997
133

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of the vinylidene complex is analogous to the mechanism
3
proposed for formation of h -butenyne complexes from
metal
bis(acetylide)s.13
The
complex
trans-
[FeH(C·CC6H4C·CH)(dmpe)2] 5 which has both metal- and
non-metal-bound acetylides reacts with a methanol solution of
sodium azide, to give trans-[FeN3(CHNCHC6H4C·CH)-
(dmpe)2] 6 as the sole product. Exclusive formation of the vinyl
metal product 6 from 5 supports the proposal that an
intramolecular metal-to-carbon hydride migration is a key step
in the mechanism.
Treatment of analogous ruthenium acetylide hydride com-
plexes with a methanol solution of azide or chloride produces
ruthenium alkenyl complexes, presumably by a similar mech-
anism.14
The partial hydrogenation of an acetylene by metal hydride
complexes has previously been reported.15 Reaction of excess
phenylacetylene with [FeH2(dmpe)2] in methanol solution
forms a bis(acetylide) complex (via an acetylide hydride
complex) and 2 equiv. of styrene are produced in the reaction.16
A reasonable intermediate in the conversion of the acetylide
hydride complex to the bis(acetylide) is an acetylide styryl iron
complex, [Fe(C·CPh)(CHNCHPh)(dmpe)2], which would un-
dergo substitution of the styryl group in the presence of excess
phenylacetylene to form the bis(acetylide) and free styrene.
The metal-to-carbon hydride migration reported here pro-
vides a novel method of reducing metal acetylides to vinyl metal
complexes in protic solvents. The reduction is stereospecific
and provides an alternative approach for the introduction of the
vinyl group into metal complexes.
8 S. D. Ittel, C. A. Tolman, A. D. English and J. P. Jesson, J. Am. Chem.
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We gratefully acknowledge financial support from the
Australian Research Council.
Footnote
† Crystal data for trans-[FeN3(CHNCHPh)(dmpe)2] 2: C20H39N3FeP4,
M = 501.29, monoclinic, space group P21/c, a = 23.888(3), b = 9.112(2),
c = 35.680(3) Å, b = 95.568(8)°, Z = 12 (three independent complexes),
R = 0.055 [3624 F, I > 1.50s(I)]. Orange acicular crystals of trans-
[FeN3(CHNCHPh)(dmpe)2] 2 suitable for X-ray diffraction studies were
grown by the slow evaporation of a methanol–tetrahydrofuran solution of
the complex. A crystal was mounted in a glass capillary, on a Rigaku
AFC7R diffractometer equipped with a graphite monochromator. All
calculations were performed using the teXsan crystallographic software
package of Molecular Structure Corporation.17 Data were reduced and
Lorentz, polarisation and absorption corrections (psi scan) were applied.
The structure was solved by direct methods18 and expanded using Fourier
techniques.19
13 L. D. Field, A. V. George, G. R. Purches and I. H. M. Slip,
Organometallics, 1992, 11, 3019.
Atomic coordinates, bond lengths and angles, and thermal parameters
have been deposited at the Cambridge Crystallographic Data Centre
(CCDC). See Information for Authors, Issue No. 1. Any request to the
CCDC for this material should quote the full literature citation and the
reference number 182/303.
14 L. D. Field, A. V. George and G. R. Purches, unpublished work.
15 J. Lo´pez, A. Romero, A. Santos, A. Vegas, A. M. Echavarren and
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Received, 22nd August 1996; Com. 6/05846I
134
Chem. Commun., 1997