Preparation and properties of carbonyliron complexes of 1-aza-4-oxa-1,3- butadiene (α-imino ketone) - X-ray crystal structures of Fe(CO)3[MeN=C(Ph)C(Ph)=O] and Fe2(CO)4[tBuN=C(H)(Me)C(O)(O)C(Me)C(H)=NtBu]

Article abstract of DOI:10.1002/(SICI)1099-0682(200005)2000:5<907::AID-EJIC907>3.0.CO;2-N

The reaction of Fe₂(CO)₉ at room temperature in THF or toluene with the α-imino ketones R¹N=C(R²)-C(R³)=O (L) (R¹ = alkyl, R² = H, Me, Ph, R³ = Me, Ph) (a-k), initially results in the formation of the mononuclear chelate Fe(CO)₃(α-imino ketone) complexes 6 which can be isolated in moderate (6h) to high (6e-g) yield. Under these reaction conditions, complexes 6 subsequently react with [Fe(CO)₄] fragments or dimerise, to form the dinuclear complexes Fe₂(CO)₆(α-imino ketone) (7a-j) or Fe₂(CO)₄(L-L) (8a,b,k), respectively. The complexes 8 contain two α-imino ketone ligands C-C coupled at the ketone carbon atoms. Complexes 8a,b react with CO at elevated temperatures to quantitatively yield Fe(CO)₃(α-imino ketone) (6a,b). This reaction can be reversed photochemically. Irradiation of a solution of 6a,b in the low-energy band results in the reformation of 8a,b in almost quantitative yield. The extremely air-sensitive complexes 6 and the dinuclear complexes 7 and 8 have been characterised spectroscopically (IR, UV/Vis, ¹H and ¹³C NMR) and by elemental analysis. The solid state structures of complexes 6g and 8a have been determined by single-crystal X- ray diffraction. The molecular structure of 6g confirms the flat σ-O, σ-N chelate coordination of the α-imino ketone. The structure of 8a consists of two metal-metal-bonded Fe(CO)₂ units, bridged by a formally 10e-donating dianionic C-C coupled (tBu-ADO) ligand. A mechanism for the formation of complexes 8 is discussed.

Full text of DOI:10.1002/(SICI)1099-0682(200005)2000:5<907::AID-EJIC907>3.0.CO;2-N

FULL PAPER
Preparation and Properties of Carbonyliron Complexes of 1-Aza-4-oxa-1,3-
butadiene (α-Imino Ketone) – X-ray Crystal Structures of Fe(CO)3[MeN؍
C(Ph)C(Ph)؍O] and Fe2(CO)4[tBuN؍C(H)(Me)C(O)(O)C(Me)C(H)؍NtBu]
Ron Siebenlist,[a] Hans-Werner Frühauf,*[a] Kees Vrieze,[a] Wilberth J. J. Smeets,[b] and
Anthony L. Spek[b]
Keywords: Coordination chemistry / α-Imino ketones / CO complexes / Iron complexes
The reaction of Fe2(CO)9 at room temperature in THF or tolu-
ene with the α-imino ketones R1N=C(R2)–C(R3)=O (L) (R1
alkyl, R2 = H, Me, Ph, R3 = Me, Ph) (a–k), initially results in
versed photochemically. Irradiation of a solution of 6a,b in
the low-energy band results in the reformation of 8a,b in al-
most quantitative yield. The extremely air-sensitive com-
=
the formation of the mononuclear chelate Fe(CO)3(α-imino plexes 6 and the dinuclear complexes 7 and 8 have been
ketone) complexes 6 which can be isolated in moderate (6h)
characterised spectroscopically (IR, UV/Vis, 1H and 13C
to high (6e–g) yield. Under these reaction conditions, com- NMR) and by elemental analysis. The solid state structures
plexes 6 subsequently react with [Fe(CO)4] fragments or
dimerise, to form the dinuclear complexes Fe2(CO)6(α-imino
ketone) (7a–j) or Fe2(CO)4(L–L) (8a,b,k), respectively. The
of complexes 6g and 8a have been determined by single-
crystal X-ray diffraction. The molecular structure of 6g con-
firms the flat σ-O,σ-N chelate coordination of the α-imino ke-
tone. The structure of 8a consists of two metal–metal-bonded
Fe(CO)2 units, bridged by a formally 10e-donating dianionic
complexes
8 contain two α-imino ketone ligands C–C
coupled at the ketone carbon atoms. Complexes 8a,b react
with CO at elevated temperatures to quantitatively yield C–C coupled (tBu-ADO) ligand. A mechanism for the forma-
Fe(CO)3(α-imino ketone) (6a,b). This reaction can be re- tion of complexes 8 is discussed.
Introduction
polynuclear complexes.[1–5] The ability to donate the nitro-
gen lone pairs and the (CϭN) imine π-bonds results in a
flexible coordination behaviour in which the α-diimine li-
gand can donate from 2 to 8 electrons to one or more metal
centres. An interesting aspect of this versatile coordination
behaviour is the activation of the imine bond towards C–C,
C–H, C–N, and N–H coupling reactions of R-DAB ligands
in the coordination sphere of transition metals with a vari-
ety of unsaturated molecules, such as α-diimines[6–10] car-
bodiimides[11] sulfines,[11] and alkynes.[12–18]
The reaction of α-diimine ligands R1NϭC(H)–C(H)ϭ
NR1 (R-DAB) (cf. Table 1) and C5H4N-2-C(H)ϭNR1 (R-
Pyca) (cf. Table 1) with carbonylmetal compounds of the
iron triad leads to the formation of several mono-, di-, and
Table 1. Abbreviations used throughout this text
Abbrevia-
tion
Explana-
tion
In a series of previous papers[19–24] we have reported in
detail on the C–C coupling reaction of the [(R-
DAB)Fe(CO)3] complexes 1 (cf. Scheme 1; X ϭ NR1, R3 ϭ
H) with the electron-deficient alkynes dimethyl acetylened
icarboxylate (DMAD) (cf. Table 1) and methyl propynoate-
(MP) (cf. Table 1). The initial step in this reaction can be
described as an oxidative 1,3-dipolar [3ϩ2] cycloaddition,
R-DAB
R-Pyca
DMAD
MP
tAm
R-ADO
1,4-Diaza-1,3-butadienes of formula R1NϭC(H)–C(H)ϭNR1
1,4-Diaza-1,3-dienes of formula C5H4N-2-C(H)ϭNR1
Dimethyl acetylenedicarboxylate
Methyl propynoate
2-Methyl-2-butyl
1,6-Di-R-1,6-diazahexa-1,5-diene-3,4-dimethyl-3,4-diolato,
R1NϭC(H)(Me)C(O)HC(O)C(Me)ϭNR1
2,3-Diolato-2,3-bis(2-pyridyl)butane,
OPB
[C5H4N-2]CMe(O)CMe(O)[2-C5H4N]
R-ADA
1,6-Di-R-1,6-diazahexa-1,5-diene-3,4-di-R-aminato,
the reductively C–C coupled dimer of R-DAB;
R1NϭC(H)(H)C(NR1)(H)C(NR1)C(H)ϭNR1
1,2-Di-R-aminato-1,2-bis(2-pyridyl)ethane,
the reductively C–C coupled dimer of R-Pyca;
[C5H4N-2]CH(NR1)CH(NR1)[2-C5H4N]
R-APE
[a]
University of Amsterdam, Institute of Molecular Chemistry,
Department of Inorganic Chemistry,
Nieuwe Achtergracht 166, 1018 WV Amsterdam, The
Netherlands
Fax: (internat.) ϩ 31-20/525-6456
E-mail: hwf@anorg.chem.uva.nl
Utrecht University, Bijvoet Center of Biomolecular Research,
Crystal and Structural Chemistry,
[b]
Padualaan 8, 3548 CH Utrecht, The Netherlands
Supporting information for this article is available on the WWW
under http://www.wiley-vch.de/home/eurjic or from the author.
Scheme 1. Reactions of [Fe(CO)3{R1NϭC(H)–(R3)CϭX}] (Xϭ
NR1,O) with electron-deficient alkynes
Eur. J. Inorg. Chem. 2000, 907Ϫ919
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
1434Ϫ1948/00/0505Ϫ0907 $ 17.50ϩ.50/0
907

R. Siebenlist, H.-W. Frühauf, K. Vrieze, W. J. J. Smeets, A. L. Spek
FULL PAPER
in which the electron-deficient alkyne (dipolarophile) adds ketone carbon atoms. The influence of the substitution pat-
across the Fe–NϭC fragment (1,3-dipole) to give the bi- tern of the α-imino ketone on the formation of the com-
cyclo[2.2.1] intermediate 2. Subsequent insertion of CO and plexes is discussed. The X-ray crystal structures of
uptake of an additional ligand (LЈ), followed by reductive [Fe(CO)3{σ-N,σ-O-(MeNϭC(Ph)–C(Ph)ϭO)}] (6g) and of
elimination and recoordination of the double bond, results the newly formed dimeric complex [Fe2(CO)4(tBuNϭ
in the formation of the [(1,5-dihydropyrrol-2-one)Fe(CO)3] (H)C(Me)C(O)(O)C(Me)C(H)ϭNtBu)] (8a) are described.
complexes 3.
In order to broaden the synthetic potential of this reac-
Results and Discussion
tion we have investigated the closely related [Fe(CO)3(1-aza-
4-oxa-1,3-butadiene)] complexes 6 (cf. Scheme 1; X ϭ O,
R3 ϭ Me, Ph), in order to probe their reactivity towards
electron-deficient alkynes.[25,26] To this end we studied the
coordination behaviour of α-imino ketones towards car-
bonyliron compounds.
The α-imino ketones R1NϭC(R2)–C(R3)ϭO (L) and the
complexes discussed in this investigation are schematically
presented in Scheme 3. The type of complex is identified by
Arabic numbers. The different α-imino ketones are indi-
cated by the letters a–k.
In contrast to the α-diimine chemistry little is known
about the coordination behaviour of α-imino ketones to-
wards d8-metals. α-Imino ketones have been shown to form
monodentate trans-[PtCl2(PR3)(R1NϭC(R2)–C(R3)ϭO][27]
and bidentate [(AlMe3)2(σ,σ-N,O–MeNϭC(Ph)–C(Ph)ϭ
O)] complexes.[28] In these complexes, as well as in the free
ligand, the NϭC–CϭO moiety has a gauche conformation
with a torsion angle of approximately 90° around the cent-
ral C–C bond, which is attributed to a minimising of the
steric repulsion between the skeleton substituents. The reac-
tion of Fe2(CO)9 with the closely related α-imino ester
PhCH(Me)NϭC(H)C(OEt)ϭO (L*), as described by Weiss
et al.,[29] resulted in the formation of two dinuclear prod-
ucts, Fe2(CO)6(L*) (4) and the C–C coupled complex
Fe2(CO)6(L*–L*) (5) (cf. Scheme 2).
Scheme 3. Observed reaction sequence for [Fe2(CO)9] with the α-
imino ketones a–k
Scheme 2. Reaction of [Fe2(CO)9] with PhCH(Me)NϭC(H)–
C(OEt)ϭO (L*)
The reaction of [Fe2(CO)9] with the α-imino ketones a–k
In complex 5, a C–C coupling reaction occurs between at room temperature results in the formation of the monon-
the imine C atoms of two α-imino ester ligands. This type uclear chelate complexes [Fe(CO)3(α-imino ketone)] (6e–h),
of C–C coupling has also been observed in the reaction of the new dinuclear complexes [Fe2(CO)6(α-imino ketone)]
R-DAB and R-Pyca with carbonylruthenium compounds,[7] (7a–j), and the C–C-coupled dimers [Fe2(CO)4(R-ADO)]
leading to [Ru2(CO)4,5(R-ADA)] (cf. Table 1) and (8a,b) (cf. Table 1) and [Fe2(CO)4(R-OPB)] (8k) (cf.
[Ru2(CO)4,5(R-APE)] (cf. Table 1), respectively, but has Table 1). Throughout the text [Fe2(CO)4(L–L)] (8a,b,k) is
never been observed for iron.
used for these C–C-coupled dimers. The product formation
In this paper we report on the reactions of α-imino ke- and distribution depends strongly on the substitution pat-
tones (L) with Fe2(CO)9, which results in the formation of tern of the α-imino ketone and on the solvent.
the mononuclear chelate complexes [Fe(CO)3(α-imino ke-
In all cases the mononuclear chelate complexes
tone)] (6) and the dinuclear complexes [Fe2(CO)6(α-imino [Fe(CO)3(α-imino ketone)] (6) are initially formed. How-
ketone)] (7) and [Fe2(CO)4(L–L)] (8). Complexes 8 contain ever, only when the ligands e–h are used are complexes 6
two α-imino ketone ligands which are C–C-coupled at the stable enough to be isolated, giving 6h in moderate yield
908
Eur. J. Inorg. Chem. 2000, 907Ϫ919

Carbonyliron Complexes of 1-Aza-4-oxa-1,3-butadiene (α-Imino Ketone)
FULL PAPER
and 6e–g in good to high yields. The complexes 6 react with be expected.[40,41] Judging from the angles around the iron
[Fe(CO)4] fragments forming [Fe2(CO)6(α-imino ketone)] atom (cf. Table 4) the coordination around the central iron
(7a–j) in moderate to good yield depending on the α-imino atom in 6g more closely resembles a distorted square-pyr-
ketone used. In complexes 7 the α-imino ketone is σ-O,µ2- amidal geometry (89% along the Berry pseudorotation co-
N,η2-CϭN-coordinated, donating six electrons to the two ordinate[42]) with the basal plane defined by N(1), C(16–
iron centres. When R3 ϭ Me, complexes 6 can also react 18), and with Fe(1) 0.353(1) A above that plane. An ideal
˚
with another equivalent of the α-imino ketone and the car- tbp or sqp geometry is not possible for these chelate com-
bonyliron compound leading to the new complexes plexes because of the small bite angle of the α-imino ketone
[Fe2(CO)4(L–L)] in moderate (8a,b) or low (8k) yield. In (81 ± 1°), a feature also observed in many R-DAB com-
complexes 8, the two α-imino ketones are reductively plexes.[40,41,43,44]
coupled through the ketone carbon atoms. The resulting
The geometries of several five-coordinated [Fe(1,4-dihet-
bridging ligand is formally dianionic and donates 10 elec- ero-1,3-butadiene)] complexes, determined by X-ray struc-
trons to the two iron centres. Complexes 8a,b react readily tures, change from approximately tbp to sqp with increasing
with carbon monoxide (1 atm) at 80 °C to quantitatively π-acceptor properties of the 1,4-dihetero-1,3-butadiene li-
regenerate 6a,b, which under these conditions are stable. Ir- gand.[44–47] The observed sqp geometry in 6g, in which the
radiation of a solution of 6a,b in the low-energy band re- α-imino ketone exhibits good π-acceptor properties, is in
sults in the reformation of complexes 8a–b in almost quant- agreement with this observation.
˚
itative yield. Irradiation of 6a in the presence of PPh3 re-
The NϭC–CϭO skeleton is planar within 0.010(3) A,
˚
sults in the formation of complex 10a and complex 8a is with the iron atom located 0.03(4) A above this plane. A
not formed. The formed complexes have been characterised similar planar conformation has been found in [Fe(CO)2(t-
1
by IR, UV/Vis, H- and 13C-NMR spectroscopy, elemental BuNC)(pTol-DAB)],[48]
and
[Fe(CO)3(MeNϭN–Nϭ
analysis, and X-ray crystal structure determinations for NMe)].[45] The present example is the first structure show-
complexes 6g and 8a. In the following section we will first ing an intact flat chelate NϭC–CϭO skeleton, i.e. a con-
discuss the structural data and spectroscopic properties of formation with maximum orbital overlap of the CϭN and
the relevant complexes and subsequently deal with the as- CϭO chromophores. From the crystal structure of the free
pects of their formation, their stability and their reactivity. α-imino ketone PhH(Me)CNϭC(Ph)–C(Ph)ϭO, it is
A mechanism for the formation of complexes 8 is proposed. known that the NϭC–CϭO moiety has a torsion angle of
[46]
97.7(2)°
around the central C–C bond. In monodentate
Molecular Structure of σ-N,σ-O-Fe(CO)3[MeN؍
C(Ph)C(Ph)؍O] (6g)
σ-N-coordinated
trans-[PtCl2(Et3)(tBuNϭC(H)C(Me)ϭ
O][27] and in the bidentate bridging [Al(Me3)2-σ,σЈ-N,O-
{MeNϭC(Ph)–C(Ph)ϭO}][28] the torsion angles are
89.0(4)° and 85.9(3)°, respectively (cf. Figure 2). On the ba-
sis of these data van Vliet et al.[28] concluded that in the
case of α-imino ketones a minimising of the steric repulsion
between the ligand substituents is predominant. In [Fe2(C-
O)6(PhCH(Me)NϭC(H)C(OEt)ϭO)][29] (5) (cf. Scheme 2)
an almost flat coordination of the NϭC–CϭO has been
reported; however, in this complex, the NϭC imine bond
has lost its double-bond character because of the µ2-N,η2-
CϭN coordination.
An ORTEP drawing of the molecular structure of 6g to-
gether with the atomic numbering scheme is shown in Fig-
ure 1. Selected bond lengths and angles are given in Table
4 (see Supporting Information). The asymmetric unit of the
unit cell contains two crystallographically independent mo-
lecules. Since these molecules are nearly identical, only one
is discussed here.
Figure 2. Torsion angles around the central C–C bond of coordin-
ated and free α-imino ketones
The observed planar conformation in 6g suggests that in
the present case the orbital overlap of the imine and ketone
moieties, i.e. the formation of a delocalised, low-lying π or-
bital, prevails over a minimising of the steric interactions
between the skeleton substituents. The π*-orbital of the Nϭ
C–CϭO system, which is antibonding in the CϭN and Cϭ
O bond and bonding in the central C–C bond, plays an
Figure 1. ORTEP drawing (50% probability level) of the molecular
structure of 6g
For a regular five-coordinate d8 complex, a trigonal-bi- important role for the π-back donation from the electron-
pyramidal (tbp) or square-pyramidal (sqp) structure might rich iron thus allowing the formation of a stable chelate.
Eur. J. Inorg. Chem. 2000, 907Ϫ919
909

R. Siebenlist, H.-W. Frühauf, K. Vrieze, W. J. J. Smeets, A. L. Spek
FULL PAPER
[49]
˚
Consequently, the coordination of the α-imino ketone to A are normally observed for single Fe–Fe bonds.
The
the iron centre in 6g leads to a significant lengthening of Fe(1)–O(1)–Fe(2) and Fe(1)–O(2)–Fe(2) bond angles are
˚
both the imine [C(2)–N(1) ϭ 1.336(3) A] and the ketone 74.14(14) and 73.78(14)°, respectively, and the Fe–O(car-
˚
[C(3)–O(1) ϭ 1.295(3) A] bonds while the central [C(2)– boxylato) bond lengths vary between 1.944(4) and 2.002(4)
˚
˚
C(3) ϭ 1.410(3) A] bond is significantly shortened when A. These bond angles and lengths are comparable to the
compared to the CϭN, CϭO and the central C–C bonds Fe–X–Fe (X ϭ C, N, O) bond angles and Fe–X (X ϭ C,
in the free α-imino ketone PhH(Me)CNϭC(Ph)C(Ph)ϭO N, O) bond lengths found in complexes with similar (N,NЈ),
[1.286(2) A, 1.209(2) A and 1.530(2) A, respectively].[46] The (N,S) and (C,O) bridging ligands.[29,48,50,51] In these com-
˚
˚
˚
CϭO bond is significantly more elongated than the CϭN plexes comparably short Fe–Fe distances have also been
˚
bond (0.086 vs. 0.054 A), indicating that the chelate coor- found. The two intact imine moieties are bonded to the iron
dination exerts a larger effect on the electron density of the centre by normal Fe–NϭC σ-donor bonds [Fe(1)–N(1) ϭ
˚
˚
CϭO than of the CϭN bond. This effect is also observed 2.043(5) A and Fe(2)–N(2) ϭ 2.030(5) A]. The angles
in the 13C-NMR spectra of complexes 8 and might explain around the C–C connective bond [C(8)–C(6)–C(5) 107.9(4)°
the chemoselectivity for C–C coupling at the ketone carbon and C(6)–C(8)–C(10) 107.3(4)°] differ only slightly from the
atom leading to complexes 3 and 8. Comparable changes in optimum value of 109.7° for sp3-hybridised atoms. In the
bond lengths, although less pronounced, have been found related complex [Ru2(CO)5(iPr-ADA)], it was concluded
in [Fe(CO)2(tBuNC)(pTol-DAB)][47] and [Fe(CO)3(2,6- from several structural features that the iPr-ADA ligand is
iPr2C6H3-DAB)].[40] The Fe–C(16), Fe–C(17) and Fe–C(18) forced to coordinate to the ruthenium centres which might
˚
˚
˚
distances of 1.771(3) A, 1.799(3) A and 1.802(3) A, respect- explain the facile thermal rupture of the central C–C bond
ively, are within the range of normally observed Fe–CO in this complex.[7] From the observed bond angles and
bond lengths.
lengths in 8a it appears that the tBu-ADO ligand exhibits
no geometrical constraints that could explain the facile fis-
sion of the C(6)–C(8) bond under carbon monoxide. The
newly formed C–C connective bond C(6)–C(8) in 8a of
Molecular Structure of [Fe2(CO)4{tBuN؍
C(H)(Me)C(O)(O)C(Me)(H)C؍NtBu}] (8a)
˚
1.587(8) A is, however, significantly longer than a normal
An ORTEP drawing of the molecular structure of [Fe2(C-
O)4(tBu-ADO)] (8a) (cf. Table 1) together with the atomic
numbering scheme is shown in Figure 3. Table 5 (see Sup-
porting Information) gives selected bond lengths and bond
angles.The molecule consists of two identical Fe(CO)2 units
connected by a metal–metal bond and bridged by the newly
formed, formally dianionic 10-electron-donating tBu-ADO
ligand. The tBu-ADO ligand has been formed by a reduct-
ive C–C coupling of two tBuNϭC(H)–C(Me)ϭO ligands
through the ketone carbon atoms. As a result of this C–C
coupling between C(6)–C(8), the former keto groups C(6)–
O(1) and C(8)–O(2) have been reduced to anionic olato
groups. The molecule exhibits noncrystallographic pseudo
C2 symmetry with the C2 axis running through the centres
of the C(6)–C(8) and the Fe(1)–Fe(2) bonds.
C(sp3)–C(sp3) bond. Comparatively long bond lengths have
been reported earlier in complexes containing two C–C-
coupled R-DAB or α-imino ester ligands[8,29,52–55] and seem
to be characteristic for C–C bonds in N–C–C–N moieties
in which both nitrogen atoms are covalently coordinated
to the metal centres. On dissolution, the dimer [EtZn(tBu-
[53,54]
Pyca)]2
dissociates to a very small extent into mono-
mers, thus forming an equilibrium mixture showing the
weakness of the C–C connective bond. However,
[Mo2(CO)6(R-ADA)][55] complexes do not split under car-
bon monoxide.
In conclusion, we can say that the tBu-ADO ligand in 8a
exhibits no geometrical constraint upon coordination to the
iron centres, the newly formed central C(6)–C(8) bond
˚
[1.587(8) A] is longer than normal C–C bonds. Whether or
not the facile breaking of the central C(6)–C(8) bond, which
is an experimental fact, may be explained by the elongation
of C(6)–C(8) remains an open question.
IR, UV/Vis Spectroscopy and Elemental Analysis
The IR, UV/Vis and analytical data of the complexes 6,
7, 8 and 10a are listed in Table 6 (see Supporting Informa-
tion). The IR spectra of complexes 6 show three strong ter-
minal carbonyl absorptions of equal intensity in the ν(CO)
region of 2100–1900 cm–1. The CO absorptions of 6e–g,
and even more so of 6h, are found at higher frequencies
than those of 6a,b indicating that π-back bonding to the
terminal carbonyl ligands decreases on going from 6a to 6h.
This is a result of the better π-accepting capacity of the α-
imino ketones going from ligands a,b to e,f to g and h,
Figure 3. ORTEP drawing (50% probability level) of the molecular
structure of 8a
˚
The Fe(1)–Fe(2) bond length in 8a is 2.372(2) A, i.e. a which bear none, one, two or three electron-withdrawing
short Fe–Fe bond, since distances in the range of 2.50–2.70 aromatic groups, respectively.
910
Eur. J. Inorg. Chem. 2000, 907Ϫ919

Carbonyliron Complexes of 1-Aza-4-oxa-1,3-butadiene (α-Imino Ketone)
FULL PAPER
The six terminal carbonyl ligands of [Fe2(CO)6L] (7a–j) not analytically pure and the complex decomposes on crys-
give rise to six medium to strong absorptions in the ν(CO) tallisation.
region. The pattern of the absorptions is identical to that
of [M2(CO)6(R-DAB/R-Pyca)] (M ϭ Fe, Ru, Os and
FeRu).[6,56–59] This strongly suggests that these complexes
are isostructural, which is corroborated by the 1H- and 13C-
NMR- spectroscopic data. The substituents on the α-imino
ketone show only a marginal influence on the frequency of
the CO absorption bands of complexes 7.
NMR Spectroscopy
Tables 6 and 7 with the complete H- and 13C-NMR-
spectroscopic data of complexes 6, 7 and 8 are available as
Supporting Information.
1
The 1H- and 13C-NMR spectra of 6a–b were obtained by
splitting 8a–b under carbon monoxide pressure because,
due to the instability of 6a–b, it was not possible to prepare
a suitable NMR sample from the reaction mixture (see Ex-
perimental Section). The imine proton resonances (6a,b,e,f)
are observed between δ ϭ 8.74 and δ ϭ 7.61, whereas in
the uncoordinated α-imino ketones the imine proton (a–f)
resonances are observed between δ ϭ 8.1 and δ ϭ 7.2. The
imine and ketone carbon atom resonances are shifted by
approximately 12–15 and 20–28 ppm to lower frequency,
relative to those of the free ligands, as a result of π-back
donation into the low-lying delocalised LUMO of the α-
imino ketone. Comparable shifts have been observed for
[Fe(CO)3(R-DAB)].[64] The large low-frequency shift of the
ketone carbon atom in comparison with the imine carbon
atom suggests that the chelate coordination exerts a larger
effect on the electron density of the ketone carbon atom.
This might be the reason for the observed chemoselectivity
in the C–C-coupling reactions, i.e. only coupling at the ke-
tone carbon atom is observed resulting in the formation of
complexes 8 and 3 (cf. Scheme 1). Furthermore, it is evident
from the X-ray crystal structure of 6g that the CϭO bond
is significantly more elongated than the CϭN bond, which
also suggests a larger effect on the CϭO bond upon che-
late coordination.
The IR spectra of complexes 8 show three terminal car-
bonyl absorptions, two of high and one of medium intens-
ity. The pattern of the CO absorption bands strongly re-
sembles that of the closely related complex [Ru2(CO)4(R-
ADA)],[6,7] which contains a similarly coupled dimer of R-
DAB. The absorption bands are shifted to a higher fre-
quency (approximately 10–20 cm–1) than those of
[Ru2(CO)4(R-ADA)]. Similar shifts to higher frequency are
observed for [M(CO)3(R-DAB)] and [M2(CO)6(R-DAB)]
on going from iron to ruthenium.
Electronic Absorption Spectra of Complexes 6 and 8
All complexes 6 show an intense band at about 530 nm.
A second band is located at about 410 nm and for com-
plexes 6e–h a third band is present at approximately
348 nm. The pattern of the bands in the UV/Vis spectra of
complexes 6 closely resembles those found for the isostruc-
tural complex [Fe(CO)3(R-DAB)],[60] except for the shift of
approximately 20 nm to lower energy. The polarity of the
solvent has almost no influence on the positions of the ab-
sorption bands [λmax 6a; 524 nm (pentane), 526 nm (tolu-
ene), 527 nm (THF)]. This lack of solvatochromic behavi-
our is also found for [Fe(CO)3(R-DAB)][40,60,61] and
[W(CO)4(R-DAB)][61] and is attributed to a strong mixing
of the metal d-(HOMO) and α-diimine π*-(LUMO) or-
bitals. As a result these transitions hardly possess any
charge-transfer character, and therefore do not show solva-
tochromic behaviour. On the basis of these similarities, the
band at 530 nm is assigned to several electronic transitions
within the metallacycle[40,61] rather than to a charge transfer
transition from the metal to the α-imino ketone ligand
(MLCT). The band at 410 nm is assigned to a LF transition
as for [Fe(CO)3(R-DAB)]. The third band at 348 nm ob-
served in 6e–h is due to an intra-ligand transition.
In all complexes 6 the three terminal carbonyl ligands
give rise to a single resonance between δ ϭ 212.0 and δ ϭ
214.1, indicating that they rapidly interchange on the NMR
time scale at 263 K.
The imine proton and carbon atom resonances of com-
plexes 7 are observed between δ ϭ 4.78–3.77 and δ ϭ 65–
90, respectively. These large shifts to lower frequency of ap-
proximately 4 ppm and 80 ppm are typical for a µ2-N,η2-
CϭN coordination of the CϭN moiety of the α-imino ke-
tone and have also been observed in [M2(CO)6(R-DAB)]
(M ϭ Fe, Ru, Os and FeRu) complexes.[6,57–59] The reson-
ances of the ketone carbon atoms are only marginally
shifted relative to those of the free ligand.
Because of the asymmetric coordination of the α-imino
ketone ligand, the six terminal carbonyl ligands in com-
plexes 7 are inequivalent. In a rigid carbonylmetal skeleton
this should result in six independent resonances for the car-
bonyl groups. However, at 263 K complexes 7 show four
resonances with relative intensities of 1:3:1:1 going from
Complexes 8a,b possess a weak band at approximately
640 nm (MLCT) and a strong absorption band at 367 nm.
The latter is assigned to the σ–σ* transition of the metal–
metal bond. This is corroborated by the increase in intensity
of this band upon lowering the temperature, an effect which
has been observed before for σ–σ* transitions of metal–
metal bonds.[62,63]
Elemental Analyses
Except for 6g, complexes 6 have not been analysed due high to low frequency. This indicates a local scrambling of
to their instability and the fact that they cannot be crys- the carbonyl groups on one metal centre, which has also
tallised. No satisfactory elemental analyses could be ob- been observed in [Fe2(CO)6(R-DAB)] complexes.[65] This
tained for complex 7d because the isolated oil was not ana- exchange process has been demonstrated for complex 7f in
lytically pure and no crystals could be obtained. Complex CD2Cl2. At 183 K the six carbonyl ligands give rise to six
8k has not been analysed because the isolated powder was separate signals, showing that at this temperature the ter-
Eur. J. Inorg. Chem. 2000, 907Ϫ919
911

R. Siebenlist, H.-W. Frühauf, K. Vrieze, W. J. J. Smeets, A. L. Spek
FULL PAPER
minal carbonyl ligands on both metal centres are at the in the solid state except for 6g. Complexes 6 are soluble and
limit of slow exchange. The individual assignment of the stable in common solvents as hexane, Et2O, toluene and
CO resonances to either one of the two Fe(CO)3 moieties THF, but decompose in chlorinated solvents like CHCl3, as
is not clear from this dynamic behaviour but can reasonably is the case for [M(CO)3(R-DAB)] (M ϭ Fe, Ru).[7] Com-
be deduced from the 13CO-NMR-spectroscopic data of plexes 6a,b react photochemically to form the correspond-
[FeRu(CO)6(R-Pyca)] complexes. From the crystal structure ing complexes 8a,b. Irradiation of 6e–g, however, does not
of [FeRu(CO)6(iPr-Pyca)],[59] it is known that the µ2-N,η2- lead to the corresponding C–C coupled complexes 8 but
CϭN coordination of the CϭN moiety is directed towards results in slow decomposition of 6e–g.
the iron centre. In the 13CO-NMR spectra of these com-
plexes recorded at 263 K the Fe–CO signals appeared as Formation and Reactivity of [Fe2(CO)6(α-Imino Ketone)] (7)
one sharp resonance and the Ru–CO signals as three separ-
ate resonances. Consequently, in our case, the three signals
The [Fe2(CO)6(α-imino ketone)] complexes 7 are formed
in THF and toluene by reaction of [Fe(CO)3(α-imino ke-
tone)] (6) with Fe(CO)4 (cf. Scheme 4). Complexes 7 are
probably formed via the CO-bridged intermediate A, sim-
of equal intensity (263 K) correspond to the µ2-N,σ-O-co-
ordinated Fe1(CO)3 moiety and the broad resonance corre-
sponds to the three carbonyl groups of the Fe2(CO)3 moi-
ilar to the proposed intermediate for the formation of
ety, which is µ2-N,η2-CϭN-coordinated through the CϭN
[Fe2(CO)6(R-DAB)].[67] The formation of A can be de-
bond of the α-imino ketone (cf. Scheme 3).
scribed as an isolobal analogue[68.69] of the addition of an
olefin to a carbene giving a cyclopropane ring system. In
the present case, Fe(CO)4 is isolobal with a carbene and the
Fe–C bond of a terminal CO ligand in 6 is, in its resonance
In complexes 8a,b the imine-proton and carbon-atom res-
onances are observed between δ ϭ 7.55–7.80 and δ ϭ
169.5–174.0, respectively, indicating that the imine moiety
is σ-N-coordinated to the iron centre. The signals of the
structure (6*), isolobal with an olefin. Addition results in
reductively coupled ketone carbon atoms in 8 accordingly
the formation of intermediate A, and subsequent coordina-
tion of the CϭN π-bond with concomitant loss of CO com-
show a pronounced coordination shift (ca. 120 ppm) to
lower frequency and are observed around δ ϭ 80. The four
pletes the structure of complex 7.
terminal carbonyl ligands give rise to two resonances be-
tween δ ϭ 217 and δ ϭ 218, indicating pairwise exchange
at each iron centre.
Reactions of α-Imino Ketones with Carbonyliron
Complex Formation
The formation, stability and yield of complexes 6, 7 and
8 strongly depends on the R1, R2, and R3 substituents at-
Scheme 4. Proposed reaction mechanism for the formation of 7
tached to the imine nitrogen atom, the imine carbon, and
Complexes 7 are isolated as red-brown crystalline solids,
except for 7d, and are air-stable in crystalline form. The
stoichiometry of complexes 7 has been established by ele-
mental analyses. The α-imino ketone in complexes 7 is σ-
O,µ2-N,η2-CϭN-coordinated to the two iron centres and
donates six electrons, as can be concluded from the IR, 1H-
NMR, and 13C-NMR spectra. An analogous coordination
has been found for the closely related α-imino ester complex
[Fe2(CO)6{PhCH(Me)NϭC(H)C(OEt)ϭO}][29] (5) (cf.
Scheme 2). No formation of the alternative possible six-
electron coordination mode, i.e. σ-N,µ2-O,η2-CϭO, has
been observed. In the α-imino ketone 2-acetylpyridine (k),
the CϭN π-bond is not available for η2 coordination be-
cause it is part of an aromatic pyridine system. Therefore,
σ-N,µ2-O,η2-CϭO is the only accessible six-electron coor-
dination mode. However, with ketone k only complex 8k is
formed and the formation of a complex with a σ-N,µ2-O,η2-
CϭO coordination mode is not observed. With the closely
related α-imino ketones 2-benzoylpyridine and pyridine-2-
the ketone carbon atom, respectively. In the following sec-
tions we will first discuss the formation of complexes 6 and
7 and the factors that influence their stability. Subsequently,
the formation of complex 8 is discussed and a mechanism
is proposed.
Formation and Reactivity of [Fe(CO)3(α-imino ketone)] (6)
The reaction of [Fe2(CO)9] with the α-imino ketones a–k
at room temperature in THF or toluene results in the initial
formation of the deep purple chelate [Fe(CO)3(α-imino ke-
tone)] complexes 6, probably via a monodenate N-coordin-
ated [Fe(CO)4(α-imino ketone)] intermediate, as a result of
the better σ-coordinating properties of the CϭN moiety
than the CϭO moiety,[27,66] according to Equation 1.
[Fe2(CO)9] ϩ α-imino ketone Ǟ
[Fe(CO)3(α-imino ketone)] (6) ϩ [Fe(CO)5] ϩ CO (1)
In THF no formation of 6i was observed. In toluene, carboxaldehyde (i.e. bearing a phenyl group and a proton
however, low concentrations of 6i were formed which, un- on the ketone carbon atom, respectively) no isolable com-
der the reaction conditions, rapidly reacts further to 7i. plexes are formed. Since initially the mononuclear chelate
Analogous to [Fe(CO)3(R-DAB)], complexes 6 are intensely [Fe(CO)3{C5H4N-2-C(R3)ϭNR1}] complexes are formed,
coloured, highly air-sensitive and could not be isolated pure indicated by the deep blue colour of the reaction mixture,
912
Eur. J. Inorg. Chem. 2000, 907Ϫ919

Carbonyliron Complexes of 1-Aza-4-oxa-1,3-butadiene (α-Imino Ketone)
FULL PAPER
this means that either complexes with a σ-N,µ2-O,η2-CϭO α-imino ketones are weaker donors than diazabutadienes.
coordination of the α-imino ketone are not formed, or they The unoccupied orbitals of the iron centre are at still higher
decompose under the reaction conditions.
energy than the n-electrons at the nitrogen atom.[66,71] Con-
The yields of complexes 7 depend strongly on the ligand sequently, reinforcement of the dative bond by significant π-
used. Generally the yields are higher for the ligands with a back donation is required for the formation of stable chelate
phenyl substituent on the ketone carbon atom than for complexes 6.[66] The extent of π-back donation depends on
those bearing a methyl group. In the latter case the lowest the energy level of the π*-LUMO, which in turn can be
yields are found for ligands a and b (ca. 17%). With these tuned with the ligand substituents, especially the substituent
ligands, however, complexes 8a,b are formed in ca. 25% on the ketone carbon atom. As a consequence, electron-
yield leading to a total yield of binuclear complexes of 40 donating groups (like methyl) disfavour chelate coordina-
to 45%. Furthermore, it is apparent that when R1 (the imine tion, while electron-withdrawing groups (like phenyl) will
nitrogen substituent) is an aryl group the yield is much lead to more stable complexes. It should be noted that when
lower than for R1 ϭ alkyl. This observation can be ac- the level of the π*-LUMO is lowered too far in energy this
counted for in terms of the lower basicity of the imine can lead to a redox reaction in which the metal centre is
donor site for aryl vs. alkyl; if R1 is aryl, this will lead to a oxidised, leading to decomposition of the formed com-
less-efficient µ2-N,η2-CϭN interaction. As a consequence, plexes.[66]
under these conditions complexes 7d,h react with Fe(CO)n
Thirdly, the stability of complexes 6 depends on the pro-
fragments to give unknown products, which lowers the pensity of η2-CϭN coordination to a second metal frag-
yields. This has also been found in [Fe2(CO)6(Aryl-Pyca)] ment, giving rise to the formation of complexes 7. The pro-
complexes.[70]
pensity of the analogous R-DAB towards η2-CϭN coor-
dination has been studied by Staal et al.[17] and decreases on
going from RCH2 to R2CH to R3C for the imine nitrogen
substituent on R-DAB. The propensity towards η2-CϭN
coordination will also depend on the substituent on the im-
ine carbon atom and decreases in the order H Ͼ Me ϾϾ
Ph, as has been found in the reactivity of complexes
[{C5H4N-2-C(R3)ϭNR1}Fe(CO)3] (R3ϭ H, Me, Ph) to-
wards Fe(CO)4 fragments.[70]
Reaction With Free Ligand
Complexes 7 do not react thermally or photochemically
with an excess of free ligand. Consequently, 7 is not an in-
termediate in the formation of 8.
Reaction With CO
Treatment of [Fe2(CO)6(R-DAB)] with carbon monoxide
(40–50 bar) at elevated temperatures yielded the monomeric
complexes [Fe(CO)3(R-DAB)] and [Fe(CO)5].[43,70] This
route should also be convenient for the formation of the
monomeric [Fe(CO)3(α-imino ketone)] complexes 6. How-
ever, reaction of [Fe2(CO)6(α-imino ketone)] (7) with CO
leads only to decomposition, and the monomeric complexes
6 are not detected. The η2-CϭN coordination is probably
stronger than the σ-O coordination and, consequently, the
coordinated keto oxygen atom is substituted by CO, which
triggers the decomposition of 7.
Considering these factors it is understandable that for the
α-imino ketones e–g, and less so for h, the stable complexes
6e–h are formed. All three have an electron-withdrawing
phenyl substituent on the ketone carbon atom which fa-
vours the formation of a stable chelate. The α-imino ketones
g and h have relatively small substituents on the imine nitro-
gen atom and a bulky phenyl substituent on the imine car-
bon atom, while the α-imino ketones e,f have relatively
bulky iPr and tBu groups on the nitrogen but only a proton
on the imine carbon atom. This means that these α-imino
ketones can easily adopt a planar cis conformation and sub-
sequently form the stable complexes 6e–h. The reaction of
6e Ǟ 7e is more favourable than the reaction 6f Ǟ 7f be-
cause of the greater propensity for η2-CϭN coordination
Factors that Influence the Formation and Stability of
Complexes 6 and 7
It has been mentioned before that the NϭC–CϭO skel- (R2CH vs. R3C) of ligand e, which is corroborated by the
eton has a conformational preference for a near ortho- higher yields of 7e. For the α-imino ketone h, the steric in-
gonality of the NϭC and CϭO groups (cf. Figure 2), fluence of the pTol group is similar to that of a cyclohexyl,
caused by steric interactions of the ligand substituents, es- but electronic influences also play an important role as can
pecially on the 1,3-and 2,4-positions.[27,28,46]
be seen from the instability of 7d,h (see above). Complex 6h
The crystal structure of 6g confirms that the α-imino ke- slowly decomposes in solution, which could be due to the
tone is σ-N,σ-O chelating in a flat planar cis conformation. inferior σ-donating capacity of the nitrogen or to an in-
This planar coordination of an intact NϭC–CϭO moiety ternal redox reaction because of the low-lying π*-LUMO.
shown in 6g has never been observed before. So, the prim-
When relatively bulky substituents are attached to both
ary factor that influences the formation and stability of the imine nitrogen and carbon atoms (c,i) it is difficult for
complexes 6 is whether the α-imino ketone can sterically these ligands to adopt the planar cis conformation. This is
easily adopt a planar or near planar cis conformation. Fur- confirmed by the observation that only very low concentra-
thermore, the stability of complexes 6 depends on the σ- tions of 6c,i are formed during the reaction, i.e. no intensive
donor and π-accepting properties of the α-imino ketone li- colouring of the reaction mixture is observed. The reaction
gand upon coordination. Because the energy of the n-elec- of α-imino ketone i in THF resulted only in the formation
trons at the oxygen is much lower than at the nitrogen atom, of [Fe(CO)4(THF)] and free ligand. Performing the same
Eur. J. Inorg. Chem. 2000, 907Ϫ919
913

R. Siebenlist, H.-W. Frühauf, K. Vrieze, W. J. J. Smeets, A. L. Spek
FULL PAPER
reaction in toluene, a less coordinating solvent, resulted in
Irradiation of a toluene or THF solution of 6a,b at the
the formation of 7i in good yield by a fast reaction of 6i low-energy band (6a; λmax ϭ 526 nm) with the 514.5-nm
with Fe(CO)4. This demonstrates particularly well the poor line of an argon ion laser at room temperature leads to re-
chelating properties of α-imino ketones when they cannot formation of 8a,b in high yield (cf. Figure 4).
easily adopt a planar cis geometry.
The ligands a–c have a methyl substituent on the ketone
carbon, a proton on the imine carbon, and a big aliphatic
substituent on the imine nitrogen atom. In principle this set
of substituents appears sterically suitable for the formation
of complexes 6. However, complexes 6a–c cannot be isol-
ated in high yield. With ligand c, complex 6c is formed in
very low concentrations, i.e. no intense colouring of the re-
action mixture is observed. Apparently, the 2,4-dimethyl-3-
pentyl substituent on the imine nitrogen atom in ligand c is
so bulky that the planar cis conformation is strongly disfa-
voured, i.e 6c is formed in low concentrations and quickly
reacts further to form complex 7c in 38% yield. The reduced
yields of 7a,b (ca. 17%) have to be ascribed to the formation
Figure 4. IR-spectral changes in the terminal carbonyl region dur-
of 8a,b and the sterically disfavoured η2-CϭN coordination
ing the irradiation of [Fe(CO)3{tBuNϭC(H)–C(Me)ϭO}] (6a) in
due to the tertiary (CR3) substituent on the imine nitrogen
in a,b as opposed to the secondary substituent (CHR2) in
c. The methyl substituent on the ketone carbon atom also
toluene at 293 K (514.5 nm); formation of 8a
The irradiation times in THF are longer than those in
contributes to the overall low yields because it restricts the toluene. The reaction time can be shortened in both cases
coordination of the oxygen atom through reduced π-back by purging the solution with nitrogen gas, or degassing the
donation. Under the reaction conditions this probably leads solution under vacuum during irradiation. This observation
to a slow chelate formation and consequently decomposi- can be explained by assuming the occurrence of an interme-
tion of the initially formed [Fe(CO)4{R1NϭC(R2)–C(R3)ϭ diate (I in Scheme 3) with the composition [Fe2(CO)5(α-im-
O]}] complexes which are known to be very labile.[68,69]
ino ketone)2], which is formed by the loss of CO from 6a
and subsequent reaction of the highly reactive [Fe(CO)2(α-
imino ketone)] (9) with the parent complex 6a. For interme-
diate I, a metal–metal-bonded structure B, containing a
bridging CO and two chelating α-imino ketone ligands, can
be envisaged. A similar intermediate has been proposed for
the formation of [Ru2(CO)4,5(R-IAE)].[7] The formation of
B, like the formation of A, can be seen as an isolobal ana-
logue to the reaction between a carbene and an olefin (cf.
Scheme 5). In the present case, [Fe(CO)2(α-imino ketone)]
(9) is a d8-ML4 fragment, like Fe(CO)4, and is thus isolobal
with a carbene. Subsequent addition to the FeϭC bond in
6* (isolobal with an olefin, Scheme 4) leads to the forma-
tion of B which contains, as opposed to intermediate A,
two chelating α-imino ketones. Reductive C–C coupling of
the ketone carbon atoms with concomitant extrusion of CO
results in the formation of 8a,b,k. Further support for struc-
ture B stems from the formation of the isostructural com-
Formation and Reactivity of Fe2(CO)4(L-L) (8a,b,k)
Complexes 8a,b,k are synthesised in moderate (8a,b) or
low (8k) yield by reaction of the ligands a,b,k with
[Fe2(CO)9], according to Equation 2.
2 [Fe2(CO)9] ϩ 2 α-imino ketone Ǟ
[Fe2(CO)4(L–L)] (8) ϩ 2 [Fe(CO)5] ϩ 4 CO (2)
They are isolated as dark green powders and are air-
stable (8a,b) in their crystalline form. They are readily sol-
uble in toluene, acetone, CH2Cl2 and THF. In solution 8a,b
are slightly light-sensitive whereas 8k is very light-sensitive,
especially in chlorinated solvents. Irradiation of the isos-
tructural [Mo2(CO)6(tBu-ADA)] at the Mo–Mo-σ,σ* trans-
ition results in the formation of [Mo2(CO)6(tBu-DAB)2],[63]
i.e. fission of the C–C bond. Irradiation of 8a,b at the high-
energy slope (λ ϭ 350 nm) of the metal–metal bond σ,σ*
transition (8a; λmax ϭ 362 nm), however, leads to a fast
(CH2Cl2) or slow (THF) decomposition of the starting
complexes, and formation of [Fe2(CO)4(α-imino ketone)2] is
not observed.
plexes
[Fe2(CO)5(cHex-DAB)2][60]
and
[Ru2(CO)5-
{CH(iPr)2-DAB}2][72] upon irradiation of [Fe(CO)3(cHex-
DAB)] and [Ru(CO)3{CH(iPr)2-DAB}] in hexane at 150 K.
However, in these cases no C–C bond formation was ob-
served even upon warming the solution.
In order to obtain spectroscopic proof for the proposed
mechanism, low-temperature IR and UV/Vis experiments
were carried out. Irradiation of 6a in 2-MeTHF at –120 °C
leads initially to the formation of a single photoproduct and
upon longer irradiation in the formation of 8a. The new
photoproduct has two CO stretching frequencies at 1962
and 1881 cm–1, a shift of approximately 75 cm–1 to lower
(3) frequency with respect to those of 6a, which points to the
Heating a solution of 8a,b (0.21 mmol, 50 mL) in toluene
under CO at 80 °C for ca. 40 min causes fission of the cent-
ral C–C bond between the two α-imino ketone ligands and
of the metal–metal bond giving the mononuclear complexes
6a,b in quantitative yield, according to Equation 3.
[Fe2(CO)4(L-L)] ϩ 2 CO Ǟ 2 [Fe(CO)3(α-imino ketone)]
914
Eur. J. Inorg. Chem. 2000, 907Ϫ919

Carbonyliron Complexes of 1-Aza-4-oxa-1,3-butadiene (α-Imino Ketone)
FULL PAPER
tion pathway 6 Ǟ 8. The decoupling reaction of 8a,b Ǟ
6a,b has been followed by means of high-pressure 1H-NMR
spectroscopy (see Experimental Section). However, an in-
termediate has not been observed during the conversion
from 6 Ǟ 8. It seems likely, however, that an intermediate
like structure B should also be present in the thermal reac-
tion.
As shown earlier, the C–C-coupled complexes 8 are only
formed with ligands a,b and k which all bear a methyl sub-
stituent on the ketone carbon atom, whereas for the corres-
ponding phenyl-substituted ligands e and f C–C coupling is
Scheme 5. Proposed reaction mechanism for the formation of 8
formation of [Fe(CO)2(2-MeTHF){tBuNϭC(H)–C(Me)ϭ not observed. Apparently, the methyl substituent is of de-
O}] (9a 2-MeTHF), i.e. loss of CO. The low-frequency shift cisive importance for the formation of 8. This effect is most
is due to the good σ-donor capacity of 2-MeTHF at low probably due to electronic factors (see below), although
temperature. In the UV/Vis spectrum, a new intense band steric reasons cannot be excluded. The exclusive coupling
is observed at 496 nm, which means that the ML band is at the ketone carbon atom is in line with the 1,3-dipolar
shifted somewhat to higher energy on substitution of CO reactivity, i.e. exclusive addition at the Fe–OϭC fragment
for 2-MeTHF. Irradiation in the presence of PPh3 results, is observed (cf. Scheme 1). From this it might be concluded
under the same conditions (–120 °C) or at room temper- that the factors which determine the reactivity for 1,3-di-
ature, in the formation of the stable complex polar cycloadditions and the C–C coupling reactions dis-
[Fe(CO)2(PPh3){tBuNϭC(H)–C(Me)ϭO}] (10a) [IR: 1967, cussed here are possibly similar. It has been shown that the
1903 cm–1; UV: 560 nm (cf. Scheme 3)], and complex 8a is 1,3-dipolar reactivity[26,73–75] decreases upon exchange of an
not observed. Thermally (–120 °C), 9a reacts back to form electron-donating substituent for an electron-withdrawing
the parent complex 6a (95%) and, to a small extent, to 8a substituent (Me Ǟ Ph). So analogously, the reactivity for
(5%). Irradiation of 6a in toluene at –80 °C leads, as at C–C coupling to form 8 might decrease in this order. With
room temperature, to the formation of 8a accompanied by ligand j the corresponding C–C-coupling product 8j is not
some decomposition; no intermediates are observed. These observed which is probably due to steric interactions of the
observations firmly support the proposal that the first step methyl groups on both the ketone and the imine carbon
in the photochemical reaction is loss of CO. Depending on atoms.
the stabilising effect of the σ-donor ligand present in solu-
The yields of 8a,b in the thermal reaction are almost
tion, the initially formed labile 16e-[Fe(CO)2{tBuNϭC(H)– identical in THF and toluene. On first sight, lower yields
C(Me)ϭO}] intermediate (9a) can be trapped and isolated are expected in THF due to its better σ-donor capacity,
(PPh3), stabilised at low temperature (2-MeTHF) or al- thus stabilising [Fe(CO)2(α-imino ketone)], i.e. less-effective
lowed to react further to form 8a (toluene).
formation of intermediate B and accordingly 8. However,
Irradiation of 6a in hexane at –100 °C leads predomin- in THF higher concentrations of [Fe(CO)3(α-imino ketone)]
antly to 8a together with a new photoproduct in approxim- are present which results in a more effective formation of
ately 10% yield. Due to the limited solubility of 8a in hex- intermediate B. This is in agreement with the observation
ane at –100 °C, and probably also of the new photoproduct, that [Fe2(CO)5(cHex-DAB)2] is only formed at rather high
they both precipitate from the solution, thus giving rise to concentrations of [Fe(CO)3(cHex-DAB)]. Furthermore, this
rather weak and broad absorptions. However, it can still be is in line with the observation that 8c is not formed as 6c is
seen that the new photoproduct has a bridging CO ligand present in very low concentration. Interestingly, the reaction
(1780 cm–1) and three or four absorptions in the terminal of ligand k with [Fe2(CO)9] in THF does not lead to 8k,
region. This supports the formation of the CO-bridged while in toluene it is formed in low yield. Obviously, in this
structure B, which is furthermore corroborated by the ab- case, THF inhibits the formation of 8k.
sorption band at 797 nm in the UV/Vis spectrum. Similarly,
[Fe2(CO)5(cHex-DAB)2] has an absorption band at 740 nm.
It must be emphasised, however, that the conclusion that B
is the only intermediate in the C–C-coupling reaction is not
justified, since it was only observed at low concentrations.
Extended photochemical studies are needed to obtain more
detailed information about the reaction pathway 6 Ǟ 8.
The question as to whether or not structure B is also
an intermediate in the thermal C–C-coupling (6 Ǟ 8) or -
decoupling (8 Ǟ 6) reaction is difficult to answer. It was
mentioned earlier that complexes 6a,b are stable in the pure
form, i.e. they do not react back to their corresponding
complexes 8. As a consequence, under the reaction condi-
Experimental Section
General Remarks: 1H- and 13C-NMR spectra were recorded with a
Bruker AMX-300 spectrometer. Chemical shifts are reported relat-
ive to TMS (δ ϭ 0) in the deuterated solvents indicated. – The IR
spectra were recorded with a BioRad FTS-7 FTIR spectrophoto-
meter or with a Perkin–Elmer 283 IR spectrophotometer. – Elec-
tronic absorption spectra were measured with a Perkin–Elmer
Lambda 5 UV/Vis spectrophotometer or with a Varian Cary 4E
spectrophotometer. – For low-temperature IR and UV/Vis meas-
urements an Oxford Instruments DN 1704/54 liquid-nitrogen cryo-
stat was used. The irradiations were carried out using the 514.5-
tions, complexes other than 6 must play a role in the reac- nm line of an SP 2025 argon ion laser or a Philips HPK 125-W
Eur. J. Inorg. Chem. 2000, 907Ϫ919 915

R. Siebenlist, H.-W. Frühauf, K. Vrieze, W. J. J. Smeets, A. L. Spek
FULL PAPER
high pressure mercury lamp equipped with the appropriate interfer-
(6e) or 90% (6f) yield. Due to their instability these complexes are
ence (λ ϭ 500 nm) filter. – Elemental analyses were carried out by best used directly after preparation or stored in pentane solution
Dornis und Kolbe, Microanalytisches Laboratorium, Mülheim a.
d. Ruhr, Germany. All solvents were carefully dried and distilled
under nitrogen before use. All preparations were carried out under
dry nitrogen by conventional Schlenk techniques. – Silica gel for
column chromatography (Kieselgel 60, 70–230 mesh, E. Merck,
Darmstadt, Germany) was dried and activated prior to use by heat-
ing to 180 °C under vacuum for 16 h. Fe2(CO)9[30] and the α-imino
ketones a,b,d–i[31,33] were prepared according to published proced-
ures. Fe(CO)5 (Strem Chemicals) and 2-acetylpyridine (k) (Janssen)
were used as commercially obtained.
at –30 °C. Cooling a pentane solution of 6g to –20 °C resulted in
dark purple lustrous crystals in about 35% yield, which were suit-
able for X-ray diffraction. Subsequent crystallisation from a con-
centrated mother liquor at –30 °C led to a second batch of crystals
of 6g (35%), giving a total yield of 70%. The second batch of crys-
tals often contained small amounts of [Fe2(CO)6(L)] (7g, Ͻ 5%),
also formed under these reaction conditions. Cooling the pentane/
Et2O solution of 6h to –30 °C resulted in a red voluminous precipit-
ate, and 6h was isolated as a pink-red powder in about 30% yield
containing small amounts of [Fe2(CO)6(L)] (9h, Ͻ5%). Complex 6h
decomposes within 1 d at room temperature and should be stored
at temperatures below –20 °C. Due to the instability of 6e,f,h, and
the fact that they cannot be crystallised, no elemental analyses
could be obtained.
The new α-imino ketone c, having R1 ϭ 2,4-dimethyl-3-pentyl
[CH(iPr)2] (cf. Table 1), R2 ϭ H, R3 ϭ Me, was synthesised accord-
ing to the literature procedure for ligands a,b.[33] Yield 65%. – IR
(pentane, cm–1): ν˜ ϭ 1708 (vs) ν(CϭO), 1650 (m) ν(CϭN). – 1H
NMR (CDCl3, room temp.): δ ϭ 7.46 (s, 1 H, NϭCH), 2.51 (t,
J ϭ 6.0 Hz, 1 H, N–CH), 2.39 (s, 3 H, CH3), 2.06 (dsept, J ϭ
6.0 Hz, 6.9 Hz, 1 H, CH), 0.83, 0.82 [d, J ϭ 6.9 Hz, 2 ϫ 6 H,
CH(CH3)2]. – 13C NMR (CDCl3, room temp.): δ ϭ 199.1 (Cϭ
O), 159.5 (CϭN), 83.2 (N–CH), 29.6 (C–CH), 25.03 (CH3), 20.5,
18.4 [CH(CH3)2].
Synthesis of [Fe(CO)3{tAmN؍C(H)–C(Me)؍O}] (6b): Solutions of
6b to be used in situ for further reactions were synthesised accord-
ing to the literature procedure for 6a (cf. Table 1).[25,26]
Synthesis of [Fe2(CO)6(L)] (7a–j) and [Fe2(CO)4(L–L)] (8a,b,k): To
a stirred suspension of [Fe2(CO)9] in 40 mL of THF or toluene was
added 0.5 g of the respective α-imino ketone (cf. Table 2), where-
upon the solution turned dark purple (a,b) or blue (k). The reaction
mixture was stirred for 3 h (THF) or 16 h (toluene) during which
time the colour of the reaction mixture changed from dark purple/
blue to red-brown. After evaporation of the solvent, the residue
was redissolved in a minimum of CH2Cl2 and the products were
The explanations for the acronyms used in this paper are given
in Table 1.
The general procedures for the preparation of complexes 6e–h, 7a–
j and 8a,b,k are given below, details are listed in Table 2.
Table 2. Syntheses of complexes 6e–h, 7a–j and 8a,b,k
Ligand substituents
Ligand
Fe2(CO)9
[g, (mmol)]
Solvent
Elution
ratio[c]
Yield
(%)[d]
(mmol)[a]
(reaction time)[b]
Compd.
6e
R1
R2
R3
iPr
H
Ph
Ph
Ph
Ph
Me
Me
Me
Ph
Ph
Ph
Ph
Ph
Ph
Me
Me
Me
Me
2.65
2.64
2.24
1.67
3.73
3.74
2.75
2.24
2.65
2.64
2.24
1.67
1.77
2.65
3.73
3.54
4.13
1.3 (3.60)
1.2 (3.30)
2.5 (6.96)
1.3 (3.60)
3.1 (6.52)
3.0 (6.24)
2.6 (7.14)
2.0 (5.50)
2.1 (5.70)
3.0 (6.24)
2.5 (6.67)
1.6 (4.37)
2.2 (6.00)
2.6 (7.70)
3.1 (6.52)
3.0 (6.24)
3.0 (6.24)
THF (1)
THF (1)
THF (2)
THF (0.5)
THF (3)
THF (3)
THF (3)
THF (2)
THF (3)
THF (3)
Tol (14)
Tol (14)
Tol (14)
THF (3)
THF (3)
THF (3)
Tol (14)
75
90
70
30
16
16
36
17
63
57
75
25
64
44
25
27
4
6f
tBu
H
6g
Me
Ph
Ph
H
6h
pTol
tBu
7a
7:1
7:1
7:1
7:1
7:1
7:1
6:2
7:1
6:2
6:4
2:3
2:3
3:2
7b
tAm
CHiPr2
pTol
iPr
H
7c
H
7d[e]
7e
H
H
7f
tBu
H
7g[f]
7h[e]
7i[e]
7j
Me
Ph
Ph
Ph
Me
H
pTol
iPr
Benzyl
tBu
8a
8b
tAm
2-Pyridine
H
8k[g]
[a]
[b]
Based on 0.5 g of the respective α-imino ketone. –
Solvent used, THF (tetrahydrofuran) or Tol (toluene) and the reaction time in
[d]
[c]
hours. –
Ratio of pentane/Et2O for elution of 7a–i and the ratio of Et2O/CH2Cl2 for elution of 8a,b,k. –
Yield based on α-imino
[e]
ketone consumed. –
After the reaction was stopped air was bubbled through the solution for 5 min to destroy the mononuclear
[f]
[Fe(CO)3(α-imino ketone)] complex present. –
Green-brown fraction, crystallisation from pentane/Et2O (5:1) yields a green brown
[g]
micro crystalline precipitate. – Reaction mixture turned intensively blue.
Synthesis of [Fe(CO)3(α-imino ketone)] (6e–h): To a stirred suspen-
sion of [Fe2(CO)9] in 40 mL of THF was added 0.5 g of the respect-
ive α-imino ketone (cf. Table 2), whereupon the solution immedi-
purified by column chromatography on silica gel (column 1 ϫ
15 cm). Elution with pentane afforded a green fraction, containing
[Fe3(CO)12]. Further elution with pentane/Et2O (ratio cf. Table 2)
ately turned dark purple. After all the free ligand had been con- afforded a red-brown fraction which, after evaporation of the solv-
sumed [monitored by the decrease of the ν(CϭO) band of the li- ent, gave [Fe2(CO)6(L)] (7a–j) as a red-brown oil or powder. Crys-
gand], the solution was filtered to remove the unchanged tallisation from pentane at –60 °C resulted in most cases in red-
[Fe2(CO)9]. The solution was concentrated to dryness and the res-
idue was redissolved in 60 mL of pentane (6e–g) or 30 mL of pent-
ane/5 mL Et2O (6h) and filtered again. For 6e,f the solution was
concentrated to dryness, yielding 6e,f as deep purple oils in 80%
brown crystalline products, the yields varied between 15 and 75%
(isolated product) depending on the ligand. Subsequent elution
with Et2O/CH2Cl2 (ratio cf. Table 2) afforded a dark green fraction
which, after evaporation of the solvent, yielded [Fe2(CO)4(L–L)]
916
Eur. J. Inorg. Chem. 2000, 907Ϫ919

Carbonyliron Complexes of 1-Aza-4-oxa-1,3-butadiene (α-Imino Ketone)
Table 3. Crystallographic data for 6g and 8a
FULL PAPER
Compound
6g
8a
Empirical formula
Molecular mass
Space group
C18H13FeNO4
363.15
C18H26Fe2N2O6
478.11
¯
¯
P1 (no. 2)
P1 (no. 2)
Crystal system
triclinic
triclinic
7.974(2)
11.377(14)
13.525(10)
74.24(9)
78.95(4)
69.98(7)
1104(2)
2
˚
a [A]
10.6110(14),
12.1544(13),
14.5221(11)
88.96(1)
75.20(1)
67.75(1)
1669.6(4)
4
˚
b [A]
˚
c [A]
α [°]
β [°]
γ [°]
3
˚
V [A ]
Z
Dcalcd. [g cm–3]
F(000)
1.445(3)
744.0
1.439(2)
496.0
µ [cm–1]
9.2
13.5
Crystal size [mm]
Data collection
Temperature [K]
θmin/θmax [°]
Radiation (monochromated)
∆ω [°]
Horizontal and vertical aperture [mm]
X-ray exposure time [h]
Linear decay (%)
Reference reflections
Index ranges
0.52 ϫ 0.63 ϫ 0.20
0.10 ϫ 0.20 ϫ 0.75
295
1.5, 27.50
295
1.57, 27.50
˚
˚
Mo-Kα (graphite), 0.71073 A
Mo-Kα (graphite), 0.71073 A
0.83 ϩ 0.35tanθ
2.95, 4.0
0.53 ϩ 0.35tanθ
3.0, 4.0
51
1.0
35
1.0
–2 3 0, 4 2 5, 5 3 2
h: –13 – 13
k: –15 – 15
l: –18 – 18
8495
2 2 3, 3 2 –2, –2 4 2
h: 0 – 10
k: –13 – 14
l: –17 – 17
5445
Total data
Unique data
7610
6062 [I Ͼ 2.5σ(I)]
5077
3706 [I Ͼ 2.5σ(I)]
Observed data
Refinement
No. of refl. and params.
Weighting scheme
Final R, wR, S
6062, 441
3706, 278
w ϭ 1.0/[σ2(F)]
0.0380, 0.0418, 0.80
0.075(2)/0.095(5) for CH/CH3
0.0222
w ϭ 1.0/[σ2(F) ϩ 0.000699 F2]
0.0546, 0.0791, 1.10
0.081(4)
2
˚
Isotr. therm. par. H atoms [A ]
(∆/σ)av in final cycle
0.0142
–0.83, 0.95
3
˚
min./max. resd. density [e/A ]
–0.45, 0.45
(8a,b,k) as a green powder. Crystallisation from pentane/CH2Cl2
(2:3) resulted in dark green needles of 8a,b, in 25 and 27% yield,
respectively. Crystals of 8a were suitable for X-ray diffraction.
Complex 8k was formed in only 4% yield and decomposed on crys-
tallisation.
after evaporation of the solvent, yielded [Fe2(CO)4(L–L)] (8a,b) as
a green powder in 90% yield.
X-ray Crystal Structure Determination of 6g and 8a: Dark opaque
crystals were mounted under nitrogen in a Lindemann glass capil-
lary (6g) or glued on a glass fibre (8a) and transferred to an Enraf–
Nonius CAD-4T rotating-anode diffractometer for data collection.
Unit-cell parameters were determined from least-squares treatment
of the SET4 setting angles of 25 reflections and were checked for
the presence of higher lattice symmetry.[34] The crystals of 6g and
8a show broad reflection profiles. All data were collected with ω/
2θ scan mode, and were corrected for Lp and for the observed
linear decay of the intensity control reflections; redundant data
were merged into a unique data set. An empirical absorption cor-
rection was applied for both compounds with the DIFABS[34]
method (correction range 0.646–1.440 for 6g and 0.614–1.180 for
8a). The structures were solved with either standard Patterson
methods (8a) or direct methods (6g) (SHELXS86)[35] and sub-
sequent difference Fourier syntheses. Refinement on F was carried
out by full-matrix least-squares techniques. A small cavity at 0,0,1/
1
Preparation of Solutions of 6b for H- and 13C-NMR Spectroscopy:
A solution of ca. 80 mg 8b in 2.5 mL of [D6]acetone was placed in
a high-pressure NMR tube and kept at 313 K for 5 h under 35 bar
carbon monoxide. This resulted in a complete conversion into 6b.
Reaction of [Fe2(CO)4(L–L)] (8a,b) with Carbon Monoxide. –
Formation of 6a,b: Compound 8a (0.100 g, 0.21 mmol) or 8b
(0.106 g, 0.21 mmol) was dissolved in 50 mL of toluene and stirred
under carbon monoxide for 40 min at 80 °C. During this time the
colour of the solution turned from green to intense purple. The
reaction was followed by IR spectroscopy and showed that 8a,b
had been quantitatively converted into 6a–b.
Photochemical conversion of [Fe(CO)3(L)] (6a,b) in [Fe2(CO)4(L–L)]
(8a,b): A toluene solution containing 6a,b (0.21 mmol, 50 mL) was
irradiated into the high-energy slope of the low-energy band
3
˚
2 of 49 A was found in the unit cell of 8a. However, no significant
(λmax ϭ 526 nm) with the 514.5-nm line of an argon ion laser at residual density could be detected in that area (PLATON).[36] Hy-
room temperature. The reaction was followed by IR spectroscopy.
Complete conversion of 6 to 8 took approximately 1 h, during
which time the colour of the solution gradually changed from in-
tense purple to green/brown. Due to some decomposition of 6a,b,
the formed products 8a,b had to be purified by column chromato-
graphy. Elution with CH2Cl2 afforded a dark green fraction which,
drogen atoms were introduced at calculated positions [C–H ϭ 0.98
A] and included in the refinement riding on their carrier atoms. All
non-H atoms were refined with anisotropic thermal parameters,
H atoms were refined with common isotropic thermal parameters.
Weights were introduced in the final refinement cycles. Neutral
atom scattering factors were taken from[37] and corrected for anom-
˚
Eur. J. Inorg. Chem. 2000, 907Ϫ919
917

R. Siebenlist, H.-W. Frühauf, K. Vrieze, W. J. J. Smeets, A. L. Spek
FULL PAPER
[21]
alous dispersion.[38] All calculations were performed with
SHELX76[39] and the PLATON package[36] (geometrical calcula-
tions and illustrations) on a DEC-5000 cluster. Crystal data and
numerical details of the structure determinations are given in
Table 3.
H.-W. Frühauf, F. Seils, J. Organomet. Chem. 1986, 302, 59–64.
H.-W. Frühauf, F. Seils, J. Organomet. Chem. 1987, 323, 67–76.
H.-W. Frühauf, F. Seils, C. H. Stam, Organometallics 1989, 8,
2338–2343.
[22]
[23]
[24]
P. P. M. de Lange, H.-W. Frühauf, M. van Wijnkoop, M. J. A.
Kraakman, M. Kranenburg, A. H. J. P. Groot, J. Fraanje, Y.
Wang, M. Numan, Organometallics 1993, 12, 417–427.
Supporting information for this article (Tables 4 and 5 with selected
bond lengths and angles for 6g and 8a, Table 6 with IR, UV/Vis,
[25]
M. van Wijnkoop, R. Siebenlist, P. P. M. de Lange, H.-W.
Frühauf, K. Vrieze, W. J. J. Smeets, A. L. Spek, Organometallics
and analytical data, and Tables 7 and 8 with H- and 13C-NMR-
1
1993, 12, 4172–4181.
spectroscopic data for compounds 6, 7, and 8) is available on the
WWW under http://www.wiley-vch.de/home/eurjic or from the au-
thor. Crystallographic data (excluding structure factors) for the
structures of 6g and 8a have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication nos.
CCDC-137484 (8a), and -137485 (6g). Copies of the data can be
obtained free of charge on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK [Fax: (internat.) ϩ 44-1223/336-033; E-
mail: deposit@ccdc.cam.ac.uk].
[26]
M. van Wijnkoop, R. Siebenlist, J. M. Ernsting, P. P. M. de
Lange, H.-W. Frühauf, E. Horn, A. L. Spek, J. Organomet.
Chem. 1993, 482, 99–109; and references cited therein.
M. R. P. van Vliet, G. van Koten, J. A. M. van Beek, K. Vrieze,
F. Muller, C. H. Stam, Inorg. Chim. Acta 1986, 112, 77–85.
[27]
[28]
M. R. P. van Vliet, G. van Koten, M. A. Rotteveel, M. Scharp,
K. Vrieze, B. Kojic-Prodic, A. L. Spek, A. J. M. Duisenberg,
Organometallics 1986, 5, 1389–1394.
[29]
A. de Cian, R. E. Weiss, Y. Chauvin, D. Commereuc, D. Hugo,
J. Chem. Soc., Chem. Commun. 1976, 249–250.
[30]
[Fe2(CO)9] was prepared by a slightly modified literature pro-
cedure (E. H. Braye, W. Hübel, Inorg. Synth. 1966, 8, 178) using
a quartz Schlenk tube and starting with 25 mL of Fe(CO)5,
150 mL of glacial acetic acid and 10 mL of acetic anhydride
(the latter was added to prevent the mixture containing too
Acknowledgments
This investigation was in part supported (W. J. J. S., A. L. S.) by
the Netherlands Foundation for Chemical Research (SON) with
financial aid from the Netherlands Organisation for Scientific Re-
search (NWO). We are indebted to J.-M. Ernsting for practical as-
sistance with recording NMR data and to B. D. Rossenaar for
practical assistance with recording LT-IR and UV/Vis data.
much water). A rayonet RS photochemical reactor (λmax
ϭ
˚
2500 A) was used for irradiation and a continuous stream of
air was used to cool the reaction mixture. Filtration, washing
with water, ethanol, ether, and subsequently drying in vacuo
gave [Fe2(CO)9] usually in more than 90% yield.
[31]
´
B. Alcaide, G. Escobar, R. Perez-Ossario, J. Plumet, D. Sanz,
J. Chem. Research (M) 1984, 1466–1488; J. Chem. Research
(S) 1984, 144–145.
[1]
G. van Koten, K. Vrieze, Adv. Organomet. Chem. 1982, 21,
[32]
[33]
[34]
[35]
W. B. Wheatley, W. E. Fitzgibbon, L. C. Cheney, J. Org. Chem.
1953, 18, 1564–1571.
H. van der Poel, G. van Koten, Synth. Commun. 1978, 8,
305–313.
N. Walker, D. Stuart, Acta Crystallogr., Sect. A 1983, A39,
158–166.
G. M. Sheldrick, SHELXS86, Program for crystal structure de-
termination, Univ. of Göttingen, Germany, 1986.
A. L. Spek, Acta Crystallogr., Sect. A 1990, A46, C34.
D. T. Cromer, J. B. Mann, Acta Crystallogr., Sect. A 1968, A24,
321–324.
D. T. Cromer, D. Liberman, J. Chem. Phys. 1970, 53, 1891–
1898.
G. M. Sheldrick, SHELX76, Crystal structure analysis package,
Univ. of Cambridge: Cambridge, England, 1976.
M. Kokkes, D. J. Stufkens, A. Oskam, J. Chem. Soc., Dalton
Trans. 1983, 439–445.
H.-W. Frühauf, I. Pein, F. Seils, Organometallics 1987, 6,
1613–1620.
R. R. Holmes, in Progress in Inorganic Chemistry (Ed.: S. J.
Lippard), Wiley, Cambridge, Massachusetts, 1984, vol. 32, pp.
119–236.
H.-W. Frühauf, G. Wolmershäuser, Chem. Ber. 1982, 115,
1070–1082.
M. A. De Paoli, H.-W. Frühauf, F.-W. Grevels, E. A. Koerner
von Gustorf, W. Riemer, C. J. Krüger, J. Organomet. Chem.
1977, 136, 219–233.
R. J. Doedens, J. Chem. Soc., Chem. Commun. 1968, 1271–
1272.
I. Fonseca, S. Martinez-Carrera, S. Garcia-Blanco, Acta Crys-
tallogr., Sect. B 1979, B35, 2643–2646.
P. P. M. de Lange, M. J. A. Kraakman, M. van Wijnkoop, H.-
W. Frühauf, K. Vrieze, W. J. J. Smeets, A. L. Spek, Inorg. Chim.
Acta 1992, 196, 151–160.
151–239.
[2]
K. Vrieze, G. van Koten, Inorg. Chim. Acta 1985, 100, 79–96.
[3]
K. Vrieze, J. Organomet. Chem. 1986, 300, 307–326.
[4]
R. Zoet, G. van Koten, C. H. Stam, D. J. Stufkens, K. Vrieze,
Organometallics 1988, 7, 2118–2123.
[5]
R. Zoet, J. T. B. H. Jastrzebski, G. van Koten, T. Mahabiersing,
K. Vrieze, C. H. Stam, D. Heijdenrijk, Organometallics 1988,
7, 2108–2117.
[36]
[37]
[6]
L. H. Staal, L. Polm, R. W. Balk, G. van Koten, K. Vrieze, A.
M. F. Brouwer, Inorg. Chem. 1980, 19, 3343–3351.
[7]
W. P. Mul, C. J. Elsevier, H.-W. Frühauf, K. Vrieze, I. Pein, M.
[38]
[39]
[40]
[41]
[42]
C. Zoutberg, C. H. Stam, Inorg. Chem. 1990, 29, 2336–2345.
[8]
J. Keijsper, G. van Koten, K. Vrieze, M. C. Zoutberg, C. H.
Stam, Organometallics 1985, 4, 1306–1308.
[9]
J. Keijsper, L. H. Polm, G. van Koten, K. Vrieze, G. Abbel, C.
H. Stam, Inorg. Chem. 1984, 23, 2142–2148.
[10]
L. H. Polm, C. J. Elsevier, G. van Koten, J. M. Ernsting, D. J.
´
Stufkens, K. Vrieze, R. R. Andrea, C. H. Stam, Organometal-
lics 1987, 6, 1096–1104.
[11]
[12]
[13]
J. Keijsper, L. H. Polm, G. van Koten, K. Vrieze, C. H. Stam,
J. D. Schagen, Inorg. Chim. Acta 1985, 103, 137–149.
F. Muller, G. van Koten, K. Vrieze, D. Heijdenrijk, B. B.
Krijnen, C. H. Stam, Organometallics 1989, 8, 41–48.
F. Muller, G. van Koten, M. J. A. Kraakman, K. Vrieze, R.
Zoet, K. A. A. Duineveld, D. Heijdenrijk, C. H. Stam, M. C.
Zoutberg, Organometallics 1989, 8, 982–991.
[43]
[44]
[14]
[15]
F. Muller, G. van Koten, K. Vrieze, D. Heijdenrijk, Organomet-
allics 1989, 8, 33–40.
F. Muller, G. van Koten, K. Vrieze, K. A. A. Duineveld, D.
Heijdenrijk, A. N. S. Mak, C. H. Stam, Organometallics 1989,
8, 1324–1330.
[45]
[46]
[47]
[16]
[17]
[18]
[19]
F. Muller, G. van Koten, K. Vrieze, D. Heijdenrijk, Inorg.
Chim. Acta 1989, 158, 69–79.
L. H. Staal, L. Polm, K. Vrieze, F. Ploeger, C. H. Stam, Inorg.
Chem. 1981, 20, 3590–3597.
L. H. Staal, G. van Koten, K. Vrieze, B. van Santen, C. H.
Stam, Inorg. Chem. 1981, 20, 3598–3606.
H.-W. Frühauf, F. Seils, R. J. Goddard, M. J. Roma˜o, Angew.
Chem. Int. Ed. Engl. 1983, 22, 992–993; Angew. Chem. 1983,
95, 1014–1415; Angew. Chem. Suppl. 1983, 1435–1450.
H.-W. Frühauf, F. Seils, R. J. Goddard, M. J. Roma˜o, Organo-
metallics 1985, 4, 948–949.
[48]
[49]
[50]
[51]
S. Aime, L. Milone, E. Sappa, A. Tirpicchio, M. Tirpicchio
Camellini, J. Chem. Soc., Dalton Trans. 1979, 1155–1159.
J. Meunier-Piret, P. Piret, M. van Meersche, Acta Crystallogr.
1965, 19, 85–91.
H. Kisch, P. Reiser, F. Knoch, Chem. Ber. 1991, 124, 1143–
1148.
P. G. Le Borgne, D. Grandjean, Acta Crystallogr., Sect. B 1973,
B29, 1040–1046.
[20]
918
Eur. J. Inorg. Chem. 2000, 907Ϫ919

Carbonyliron Complexes of 1-Aza-4-oxa-1,3-butadiene (α-Imino Ketone)
FULL PAPER
D. Leibfritz, H. tom Dieck, J. Organomet. Chem. 1976, 105,
[52]
[53]
[64]
[65]
[66]
[67]
[68]
A. L. Spek, J. T. B. H. Jastrzebski, K. Vrieze, Acta Crystallogr.,
Sect. C 1987, 43, 2006–2007.
255–261.
E. Wissing, S. van der Linden, E. Rijnberg, J. Boersma, W. J.
J. Smeets, A. L. Spek, G. van Koten, Organometallics 1994, 13,
2602–2608.
L. H. Staal, J. Keijsper, L. Polm, K. Vrieze, J. Organomet.
Chem. 1981, 204, 101–113.
H. Friedel, I. W. Renk, H. tom Dieck, J. Organomet. Chem.
1971, 26, 247–259.
H.-W. Frühauf, J. Breuer, J. Organomet. Chem. 1984, 277,
[54]
[55]
[56]
G. van Koten, J. B. T. H. Jastrzebski, K. Vrieze, J. Organomet.
Chem. 1983, 250, 49–61.
L. H. Staal, A. Oskam, K. Vrieze, E. Roosendaal, H. Schenk,
Inorg. Chem. 1979, 6, 1634–1640.
C13–C17.
R. Rosenthal, I. Wender, in Organic Syntheses via Metal Car-
bonyls (Eds.: I. Wender, P. Pino), Interscience Publishers, New
York, 1968, vol. 1; pp. 405–466.
M. Kilner, in Advances in Organometallic Chemistry (Eds.: F.
G. A. Stone, R. West), Academic Press, Inc., New York, 1972,
vol. 10, pp. 115–198.
H.-W. Frühauf, A. Landers, R. J. Goddard, C. Krüger, Angew.
Chem. 1978, 90, 56–57; Angew. Chem. Int. Ed. Engl. 1978, 17,
64–65.
[69]
[70]
[57]
[58]
L. H. Staal, G. van Koten, K. Vrieze, J. Organomet. Chem.
1981, 206, 99–108.
R. Zoet, G. van Koten, G. Muller, K. Vrieze, M. van Wijnkoop,
K. Goubitz, C. J. G. van Halen, C. H. Stam, Inorg. Chim. Acta
1988, 149, 193–208.
M. J. Kraakman, C. J. Elsevier, J.-M. Ernsting, K. Vrieze, K.
Goubitz, Inorg. Chim. Acta 1993, 203, 129–143.
H. K. van Dijk, D. J. Stufkens, A. Oskam, J. Am. Chem. Soc.
1989, 111, 541–547.
D. J. Stufkens, Coord. Chem. Rev. 1990, 104, 39–112; and refs.
cited therein.
H.-W. Frühauf, F.-W. Grevels, A. Landers, J. Organomet.
Chem. 1979, 178, 349–360.
[71]
[72]
I. W. Renk, H. tom Dieck, Chem. Ber. 1972, 105, 1403–1418.
H. K. van Dijk, J. J. Kok, D. J. Stufkens, A. Oskam, J. Or-
[59]
[60]
[61]
[62]
ganomet. Chem. 1989, 362, 163–177.
[73]
[74]
[75]
K. N. Houk, J. Sims, R. E. Duke, R. W. Strozier, J. K. George,
J. Am. Chem. Soc. 1973, 95, 7287–7301.
K. N. Houk, J. Sims, C. R. Watts, L. J. Luskus, J. Am. Chem.
Soc. 1973, 95, 7301–7315.
H. B. Abrahamson, C. C. Frazier, D. S. Ginley, H. B. Gray, J.
Lilienthal, D. R. Tyler, M. S. Wrighton, Inorg. Chem. 1977, 16,
1554–1556.
K. N. Houk, Acc. Chem. Res. 1975, 8, 361–369.
Received October 6, 1999
[I99351]
[63]
´
R. R. Andrea, H. E. de Jager, D. J. Stufkens, A. Oskam, J.
Organomet. Chem. 1986, 316, C24–C28.
Eur. J. Inorg. Chem. 2000, 907Ϫ919
919