Preparation and properties of carbonyliron complexes of 1-aza-4-oxa-1,3-butadiene (α-imino ester). X-ray crystal structures of Fe(CO)2(σ(N): σ(C)-i-PrN(H)C(H)C(O)OEt)2 and Fe(CO)2(σ(N): σ(C)-i-PrN(H)C(H)C(O)OMe)(i-PrN=C(H)C(O)O)

Article abstract of DOI:10.1021/om9910266

Reaction of Fe₂(CO)₉ at room temperature in THF or toluene with the α-imino esters (L) R¹N=C(R²)C(R³)=O (a-i; R¹ = alkyl, R² = H, R³ = O-alkyl), results in the formation of several mono- and binuclear complexes, depending on the bulkiness of the R¹ substituent. With all ligands employed, the known type of complex Fe₂(CO)₆(L) (6) is formed. Reaction of the less bulky R¹ substituted ligands a-e (R¹ = neo-Pe, i-Pr, 1-cyclohexylethyl) leads to the formation of Fe(CO)₂(σ(σ(N): σ(C)-i-PrN(H)C(H)C(O)OEt)₂ (10a-d/10′a-d) as a mixture of two noninterconverting isomers together with the known type of complexes Fe₂(CO)₆(L-L) (7). Complexes 7 contain two (former) α-imino ester ligands joined at the imine carbon atoms. In the presence of water, the reaction in THF leads to the formation of Fe(CO)₂(σ(N): σ(C)-i-PrN(H)C(H)C(O)OMe)(i-PrN=C(H)C(O)O) (11/11′). The unusual σ(N):σ(C) coordination of the newly formed ligands in 10 and 11 is confirmed by X-ray structure determinations. Thermal or photochemical activation of 7 in toluene leads to the formation of Fe₂(CO)₄(L-L) (9), and reaction of 7 with HCl leads to displacement of the N-protonated C-C coupled organic compounds 8. Reaction of 9 with CO (1 atm) at room temperature results in the re-formation of 7. Reaction with the bulky substituted R¹ ligands f-i (R¹ = t-Bu, 2-methyl-2-butyl) results in the formation of the new binuclear complexes Fe₂(CO)₅(L-L) (12) in which, like in 7, two α-imino ester ligands are C-C coupled on the imine carbon atoms. In contrast to 7, the C=O oxygen atoms in 12 are coordinated, and instead of a metal-metal bond a bridging CO is present. All complexes and the organic products have been characterized by spectroscopy (IR, ¹H and ¹³C NMR, FD/FAB-mass) and by elemental analyses. An explanation for the product distribution and the possible mechanisms for the formation of the complexes is discussed.

Full text of DOI:10.1021/om9910266

3016
Organometallics 2000, 19, 3016-3031
P r ep a r a tion a n d P r op er ties of Ca r bon ylir on Com p lexes
of 1-Aza -4-oxa -1,3-bu ta d ien e (r-Im in o Ester ). X-r a y
Cr ysta l Str u ctu r es of
F e(CO)2(σ(N): σ(C)-i-P r N(H)C(H)C(O)OEt)2 a n d
F e(CO)2(σ(N): σ(C)-i-P r N(H)C(H)C(O)OMe)(i-P r NdC(H)C(O)O)
Ron Siebenlist, Hans-Werner Fru¨hauf,*,† and Kees Vrieze
Inorganic Laboratory, Institute of Molecular Chemistry, University of Amsterdam,
Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands
Huub Kooijman, Wilberth J . J . Smeets, and Anthony L. Spek*
Bijvoet Center for Bimolecular Research, Crystal and Structural Chemistry,
Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands
Received December 22, 1999
Reaction of Fe2(CO)9 at room temperature in THF or toluene with the R-imino esters (L)
R1NdC(R2)C(R3)dO (a -i; R1 ) alkyl, R2 ) H, R3 ) O-alkyl), results in the formation of
several mono- and binuclear complexes, depending on the bulkiness of the R1 substituent.
With all ligands employed, the known type of complex Fe2(CO)6(L) (6) is formed. Reaction of
the less bulky R1 substituted ligands a -e (R1 ) neo-Pe, i-Pr, 1-cyclohexylethyl) leads to the
formation of Fe(CO)2(σ(N): σ(C)-i-PrN(H)C(H)C(O)OEt)2 (10a -d /10′a -d ) as a mixture of two
noninterconverting isomers together with the known type of complexes Fe2(CO)6(L-L) (7).
Complexes 7 contain two (former) R-imino ester ligands joined at the imine carbon atoms.
In the presence of water, the reaction in THF leads to the formation of Fe(CO)2(σ(N): σ(C)-
i-PrN(H)C(H)C(O)OMe)(i-PrNdC(H)C(O)O) (11/11′). The unusual σ(N):σ(C) coordination of
the newly formed ligands in 10 and 11 is confirmed by X-ray structure determinations.
Thermal or photochemical activation of 7 in toluene leads to the formation of Fe2(CO)4(L-
L) (9), and reaction of 7 with HCl leads to displacement of the N-protonated C-C coupled
organic compounds 8. Reaction of 9 with CO (1 atm) at room temperature results in the
re-formation of 7. Reaction with the bulky substituted R1 ligands f-i (R1 ) t-Bu, 2-methyl-
2-butyl) results in the formation of the new binuclear complexes Fe2(CO)5(L-L) (12) in which,
like in 7, two R-imino ester ligands are C-C coupled on the imine carbon atoms. In contrast
to 7, the CdO oxygen atoms in 12 are coordinated, and instead of a metal-metal bond a
bridging CO is present. All complexes and the organic products have been characterized by
spectroscopy (IR, 1H and 13C NMR, FD/FAB-mass) and by elemental analyses. An explanation
for the product distribution and the possible mechanisms for the formation of the complexes
is discussed.
In tr od u ction
Reaction of these ligands with metal carbonyls, espe-
cially of group 8, leads to the formation of several
mono-, di-, and polynuclear complexes.2-7 An interesting
aspect of the versatile coordination chemistry is the
activation of the imine carbon atoms toward C-C,7-10
C-H,11-13 C-N,12-14 and N-H15 coupling reactions
of R-DAB ligands in the coordination sphere of transi-
R-Diimine ligands R1NdCHCHdNR1 (R-DAB)1 and
C6H4N-2-CHdNR1 (R-Pyca)1 in transition-metal com-
plexes are known for their versatile coordination be-
havior, based on their ability to donate from two up to
eight electrons via the nitrogen lone pairs and the Cd
N π-electrons toward one or more metal centers.2-6
† E-mail: hwf@anorg.chem.uva.nl. Fax: +31-20-525 6456.
(1) The abbreviations used throughout this text are as follows:
R-DAB, 1,4-diaza-1,3-butadienes of formula R1NdC(H)C(H)dNR1;
R-Pyca, 1,4-diaza-1,3-butadienes of formula C5H4N-2-(C(H)dNR1);
DMAD, dimethyl acetylenedicarboxylate; MP, methyl propynoate; CE,
1-cyclohexylethyl; t-Am, 2-methyl-2-butyl; R-ADO, 1,6-di-R-1,6-diaza-
hexa-1,5-diene-3,4-dimethyl-3,4-diolato, R1NdC(H)(Me)C(O)(H)C(O)C-
(Me)dNR1; R1,R3-ODA, 1,6-dioxahexa-1,5-diene-2,5-di-R3-3,4-di-R1-
aminato, OdC(R3)(H)C(NR1)(H)C(NR1)C(R3)dO; 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, R1NdC(H)(H)C(NR1)(H)C(NR1)C(H)dNR1; R-APE,
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.
(2) Vrieze, K.; van Koten, G. Inorg. Chim. Acta 1985, 100, 79.
(3) Vrieze, K. J . Organomet. Chem. 1986, 300, 307.
(4) Zoet, R.; van Koten, G.; Stam, C. H.; Stufkens, D. J .; Vrieze, K.
Organometallics 1988, 7, 2118.
(5) Zoet, R.; J astrzebski, J . T. B. H.; van Koten, G.; Mahabiersing,
T.; Vrieze, K.; Stam, C. H.; Heijdenrijk, D. Organometallics 1988, 7,
2108.
(6) van Koten, G.; Vrieze, K. Adv. Organomet. Chem. 1982, 21, 151.
(7) Muller, F.; van Koten, G.; Polm, L. H.; Vrieze, K.; Zoutberg, M.
C.; Heijdenrijk, D.; Kragten, E.; Stam, C. H. Organometallics 1989, 8,
1340.
(8) Polm, L. H.; van Koten, G.; Vrieze, K.; Stam, C. H.; van Tunen,
W. C. J . J . Chem. Soc., Chem. Commun. 1983, 1177.
10.1021/om9910266 CCC: $19.00 © 2000 American Chemical Society
Publication on Web 07/06/2000

Carbonyliron Complexes of 1-Aza-4-oxa-1,3-butadiene
Organometallics, Vol. 19, No. 16, 2000 3017
Sch em e 1. Rea ction s of
F e(CO)3(R1NdC(H)C(R3)dX) (XdNR1, O) w ith
DMAD
tion metals with a wide variety of molecules such as
R-diimines,9,10,16-18 carbodiimides,19 sulfines,19 ketenes,8
and especially alkynes.14,20-22
In a series of previous papers23-34 we have reported
in detail on C-C coupling reactions of the chelate (1,4-
diaza-1,3-butadiene)Fe(CO)3 complexes 1 with the elec-
tron-deficient alkyne dimethyl acetylenedicarboxylate
(DMAD).1 The initial step in these reactions consists of
an oxidative 1,3-dipolar [3 + 2] cycloaddition reaction
of the electron-deficient alkyne (dipolarophile) across the
Fe-NdC fragment (the 1,3-dipole) to give intermediate
2. Insertion of CO into the Fe-N bond and subsequent
uptake of an additional ligand (L′), followed by reductive
elimination and recoordination of the double bond,
finally leads to the formation of (1,5-dihydropyrrol-2-
one)Fe(CO)3 complexes 3 (cf. Scheme 1; X ) NR1, R3 )
H).
To expand the potential of these organometallic 1,3-
dipolar cycloadditions, we also investigated the reactions
of the closely related σ(N):σ(O) chelate Fe(CO)3(R1Nd
(9) Polm, L. H.; Elsevier, C. J .; van Koten, G.; Ernsting, J . M.;
Stufkens, D. J .; Vrieze, K.; Andre´a, R. R.; Stam, C. H. Organometallics
1987, 6, 1096.
(10) Mul, W. P.; Elsevier, C. J .; Fru¨hauf, H.-W.; Vrieze, K.; Pein, I.;
Zoutberg, M. C.; Stam, C. H. Inorg. Chem. 1990, 7, 2336.
(11) Zoet, R.; van Wijnkoop, M.; Versloot, P.; van Koten, G.; Vrieze,
K.; Duineveld, C. A. A.; Elsevier, C. J .; Goubitz, K.; Heijdenrijk, D.;
Stam, C. H. Organometallics 1989, 8, 23.
Sch em e 2. Rea ction of F e2(CO)9 w ith
P h CH(Me)NdC(H)C(OEt)dO (L*)
(12) Muller, F.; van Koten, G.; Vrieze, K.; Heijdenrijk, D.; Krijnen,
B. B.; Stam, C. H. Organometallics 1989, 8, 41.
(13) Muller, F.; van Koten, G.; Vrieze, K.; Duineveld, K. A. A.;
Heijdenrijk, D.; Mak, A. N. S.; Stam, C. H. Organometallics 1989, 8,
1324.
(14) Muller, F.; van Koten, G.; Vrieze, K.; Heijdenrijk, D. Organo-
metallics 1989, 8, 33.
(15) Keijsper, K.; Mul, J .; van Koten, G.; Vrieze, K.; Ubbels, H. C.;
Stam, C. H. Organometallics 1984, 3, 1732.
(16) Staal, L. H.; Polm, L.; Balk, R. W.; van Koten, G.; Vrieze, K.;
Brouwer, A. M. F. Inorg. Chem. 1980, 19, 3343.
(17) Keijsper, J .; van Koten, G.; Vrieze, K.; Zoutberg, M. C.; Stam,
C. H. Organometallics 1985, 4, 1306.
(18) Keijsper, J .; Polm, L. H.; van Koten, G.; Vrieze, K.; Abbel, G.;
Stam, C. H. Inorg. Chem. 1984, 23, 2142.
(19) Keijsper, J .; Polm, L. H.; van Koten, G.; Vrieze, K.; Stam, C.
H.; Schagen, J . D. Inorg. Chim. Acta 1985, 103, 137.
(20) Muller, F.; van Koten, G.; Kraakman, M. J . A.; Vrieze, K.; Zoet,
R.; Duineveld, K. A. A.; Heijdenrijk, D.; Stam, C. H.; Zoutberg, M. C.
Organometallics 1989, 8, 982.
(21) Muller, F.; van Koten, G.; Kraakman, M. J . A.; Vrieze, K.;
Heijdenrijk, D.; Zoutberg, M. C. Organometallics 1989, 8, 1331.
(22) Muller, F.; Dijkhuis, D. I. P.; van Koten, G.; Vrieze, K.;
Heijdenrijk, D.; Rotteveel, M. A.; Stam, C. H.; Zoutberg, M. C. Inorg.
Chim. Acta 1989, 158, 99.
(23) van Wijnkoop, M.; de Lange, P. P. M.; Fru¨hauf, H.-W.; Vrieze,
K.; Wang, Y.; Goubitz, K.; Stam, C. H. Organometallics 1992, 11, 3607.
(24) van Wijnkoop, M.; Siebenlist, R.; Ernsting, J . M.; de Lange, P.
P. M.; Fru¨hauf, H.-W.; Horn, E.; Spek, A. L. J . Organomet. Chem. 1993,
482, 99.
(25) van Wijnkoop, M.; Siebenlist, R.; de Lange, P. P. M.; Fru¨hauf,
H.-W.; Vrieze, K.; Smeets, W. J . J .; Spek, A. L. Organometallics 1993,
12, 4172.
(26) Fru¨hauf, H.-W.; Seils, F.; Goddard, R. J .; Roma˜o, M. J . Angew.
Chem., Int. Ed. Engl. 1983, 22, 992.
(27) Fru¨hauf, H.-W.; Seils, F.; Goddard, R. J .; Roma˜o, M. J .
Organometallics 1985, 4, 948.
(28) Fru¨hauf, H.-W. J . Organomet. Chem. 1986, 302, 59.
(29) Fru¨hauf, H.-W.; Seils, F. J . Organomet. Chem. 1987, 323, 67.
(30) Fru¨hauf, H.-W.; Seils, F.; Stam, C. H. Organometallics 1989,
8, 2338.
(31) de Lange, P. P. M.; Fru¨hauf, H.-W.; van Wijnkoop, M.; Kraak-
man, M. J . A.; Kranenburg, M.; Groot, A. H. J . P.; Fraanje, J .; Wang,
Y.; Numan, M. Organometallics 1993, 12, 417.
(32) de Lange, P. P. M.; van Wijnkoop, M.; Fru¨hauf, H.-W.; Vrieze,
K.; Goubitz, K. Organometallics 1993, 12, 428.
(33) de Lange, P. P. M.; de Boer, R. P.; van Wijnkoop, M.; Ernsting,
J . M.; Fru¨hauf, H.-W.; Vrieze, K.; Smeets, W. J . J .; Spek, A. L.
Organometallics 1993, 12, 440.
(34) de Lange, P. P. M.; Alberts, E.; van Wijnkoop, M.; Fru¨hauf, H.-
W.; Vrieze, K.; Kooijman, H.; Spek, A. L. J . Organomet. Chem. 1994,
465, 241.
C(R2)C(R3)dO) (R-imino ketones, X ) O, R3 ) Me,
Ph;24,25 R-imino esters, X ) O, R3 ) O-alkyl35) toward
DMAD and MP1 and found an analogous reactivity.
During these investigations we found that these ligands
show interesting coordination behavior toward iron
carbonyl. In contrast to the R-diimine chemistry rela-
tively little is known about the coordination behavior
of R-imino ketones and especially esters toward d8
metals. R-Imino ketones and esters have been shown
to form several types of coordination complexes with Pt-
(Cl)2(PR3)2, ZnR2, AlR3, ZnCl2, and AlCl3, monodentate
trans-[PtCl2(PEt3)(R1NdC(R2)C(R3)dO],36 bridging bi-
dentate [(AlMe3)2(σ(N):σ(O)-MeNdC(Ph)C(Ph)dO)],37 and
the dimeric [EtZn(Et)(t-Bu)NC(H)dC(Me)O]238 complex.
Weiss et al.39 reported on the reaction of the R-imino
ester PhCH(Me)NdC(H)C(OEt)dO (L*) with iron car-
bonyl, which resulted in the formation of two binuclear
products, Fe2(CO)6(L*) (6) and the Fe2(CO)6(L*-L*) (7)
complex (cf. Scheme 2), in which the imine-C atoms of
two R-imino ketone ligands have been C-C coupled.
This C-C coupled ligand will be abbreviated as R1,R3-
ODA.1
(35) Siebenlist, R.; de Beurs, M.; Feiken, N.; Fru¨hauf, H.-W.; Vrieze,
K.; Kooijman, H.; Veldman, N.; Spek, A. L. Organometallics 2000, 19,
3032-3053.
(36) van Vliet, M. R. P.; van Koten, G.; van Beek, J . A. M.; Vrieze,
K.; Muller, F.; Stam, C. H. Inorg. Chem. Acta 1986, 112, 77.
(37) van Vliet, M. R. P.; van Koten, G.; Rotteveel, M. A.; Scharp,
M.; Vrieze, K.; Kojic-Prodic, B.; Spek, A. L.; Duisenberg, A. J . M.
Organometallics 1986, 5, 1389.
(38) van Vliet, M. R. P.; van Koten, G.; Buysingh, P.; J astrzebski,
J . T. B. H.; Spek, A. L. Organometallics 1987, 6, 537.
(39) de Cian, A.; Weiss, R. E.; Chauvin, Y.; Commereuc, D.; Hugo,
D. J . Chem. Soc. Chem. Commun. 1976, 249.

3018 Organometallics, Vol. 19, No. 16, 2000
Sch em e 3. Obser ved Rea ction s of th e r-Im in o Ester s a -i w ith F e2(CO)9
Siebenlist et al.
Recently, we reported on the coordination chemistry
of R-imino ketones toward iron carbonyl40 leading to Fe-
(CO)3(R-iminoketone), Fe2(CO)6(R-imino ketone), and
Fe2(CO)4(R-ADO).1 In the last complex a type of C-C
coupled ligand similar to that in 7 is formed; however,
in Fe2(CO)4(R-ADO) the C-C coupling has occurred
between the two ketone carbon atoms of the R-imino
ketone instead of the imine carbons.
mechanisms for the formation of the complexes will be
discussed.
Exp er im en ta l Section
Gen er a l Rem a r k s. 1H and 13C NMR spectra were recorded
on a Bruker AMX-300 spectrometer. IR spectra were recorded
on Perkin-Elmer 283 and Biorad FTIR-7 spectrophotometers.
Elemental analyses were carried out by Dornis & Kolbe,
Microanalytisches Laboratorium, Mu¨lheim a. d. Ruhr, Ger-
many. The solvents were carefully dried and distilled under
nitrogen before use. All preparations were carried out under
an atmosphere of dry nitrogen by standard Schlenk tech-
niques. The irradiations were carried out using the 514.5 nm
line of a SP 2025 argon ion laser or a Philips HPK 125 W high-
pressure mercury lamp equipped with the appropriate inter-
ference filter. Silica gel for column chromatography (Kieselgel
60, 70-230 mesh, E. Merck, Darmstadt, Germany) was dried
and activated prior to use by heating to 180 °C under vacuum
for 16 h. Fe(CO)5 (Strem Chemicals) and dimethyl, diethyl,
and diisopropyl L-tartrates (Aldrich) were used as com-
mercially obtained.
In this article we report on an investigation of the
reactions of R-imino esters R1NdC(H)C(R3)dO (R1
)
alkyl, R2 ) H, R3 ) O-alkyl; a -i) with Fe2(CO)9,
resulting in the formation of the two new mononuclear
complexes 10a -d /10′a -d and 11b,c/11′b,c and the new
C-C coupled dimeric complexes Fe2(CO)5(R,R3-ODA)1
(12f-i) together with the known types of complexes
6a -i and 7a -e39 (cf. Scheme 3). The product selectivity
strongly depends on the imine R1 substituent and on
the solvent. An explanation for this and the possible
(40) Siebenlist, R.; Fru¨hauf, H.-W.; Vrieze, K.; Smeets, W. J . J .;
Spek, A. L. Eur. J . Inorg. Chem., in press.
41
Syn th esis of Sta r tin g Ma ter ia ls. Fe2(CO)9 and the

Carbonyliron Complexes of 1-Aza-4-oxa-1,3-butadiene
Organometallics, Vol. 19, No. 16, 2000 3019
Ta ble 1. Syn th eses of Com p lexes 6a -i, 7a -e, 10a -d /10′a -d , 11b,c/11′b,c, a n d 12f-i
ligand substituents
compd
6a
6b
6c
R1
R3
amt of ligand (mmol)a
amt of Fe2(CO)9 (g (mmol))
elution ratiob
yieldc (%)
yieldd (%)
neo-Pe
i-Pr
i-Pr
i-Pr
CE
t-Bu
t-Bu
t-Bu
t-Am
neo-Pe
i-Pr
i-Pr
i-Pr
CE
neo-Pe
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
OMe
OMe
OEt
O-i-Pr
OMe
OMe
OEt
O-i-Pr
OMe
OMe
OMe
OEt
O-i-Pr
OMe
OMe
OMe
OEt
O-i-Pr
OMe
OEt
OMe
OEt
O-i-Pr
OMe
OMe/Et
3.18
3.87
3.49
3.18
2.53
3.49
3.18
2.92
3.18
3.18
3.87
3.49
3.18
2.53
3.18
3.87
3.49
3.18
3.87
3.49
3.49
3.18
2.92
3.18
2.31 (6.36)
2.82 (7.74)
2.54 (6.98)
2.31 (6.36)
1.84 (5.06)
2.54 (6.98)
2.31 (6.36)
2.13 (5.84)
2.31 (6.36)
2.31 (6.36)
2.82 (7.74)
2.54 (6.98)
2.31 (6.36)
1.84 (5.06)
1.15 (3.18)
1.41 (3.87)
1.27 (3.49)
1.15 (3.18)
1.41 (3.87)
1.27 (3.49)
2.54 (6.98)
2.31 (6.36)
2.13 (5.84)
2.31 (6.36)
2.31 (6.36)
15:1
15:1
15:1
15:1
15:1
9:1
9:1
9:1
9:1
9:1
9:1
9:1
9:1
9:1
1:1
1:1
1:1
1:1
16
13
13
10
10
10
7
13
11
11
10
11
10
11
9
10
4
5
5
5
6d
6e/6′ee
6f
6g
6h
6i
7a
7b
7c
7d
7e
10′a
10b/10′b
10c/10′c
10d /10′d
11b
11c
12f
12g
12h
12i
7
10
23
24
26
24
23
10
12
12
13
14
20
12
12
9
e
3
3
3
e
e
t-Bu
t-Bu
t-Bu
t-Am
t-Bu
3:1
3:1
5:1
3:1
3:1
32
33
38
32
11
12fg
a
b
On the basis of 0.5 g of the respective R-imino ketone. Ratio of pentane to Et2O for elution of 6, 7, and 10; ratio of Et2O to CH2Cl2
for the elution of 12. c Yield in toluene. Yield in THF. e Ratios of isomers: 6e:6e′ (3:1); 10:10′ (6:1); 11:11′ (15:1).
d
R-imino esters b, c, f, and g have been prepared according to
13C NMR (75.47 MHz, CDCl3, room temperature, δ in ppm
relative to TMS): a , 163.2 (CdO), 151.0 (CdN), 71.8 (NCH2),
53.1 (OCH3), 33.1 (C(CH3)3), 28.2 (C(CH3)3), d , 161.5 (CdO),
150.6 (CdN), 67.9 (OCH(CH3)2), 60.5 (NCH(CH3)2), 27.8 (OCH-
(CH3)2), 21.9 (NCH(CH3)2), e, 164.1 (CdO), 153.8 (CdN), 75.6
(NCCH3), 52.1 (OCH3), 42.7 (c-Hex CH), 29.7, 29.2, 26.2, 26.0,
25.8 (c-Hex CH2), 19.1(NCCH3); h , 161.4 (CdO), 147.6 (Cd
N), 67.2 (OCH(CH3)2), 57.6 (C(CH3)3), 27.8 (OCH(CH3)2), 20.5
(C(CH3)3); i, 163.0 (CdO), 148.0 (CdN), 60.7 (NC), 51.3 (OCH3),
34.2 (C(CH3)2), 25.1 (CH2CH3), 7.5 (CH2CH3).
published procedures.42
The new R-imino esters R1NdC(H)C(OMe)dO with R1
)
neopentyl (neo-Pe; a ), 1-cyclohexylethyl (CE; e), 2-methyl-2-
butyl (t-Am;1 i) and R1NdC(H)C(Oi-Pr)dO with R1 ) i-Pr (d ),
t-Bu (h ) were synthesized by reaction of methyl or isopropyl
glyoxylate,43 respectively, with the corresponding amines
according to ref 42. Yield: a , 30%; d , 50%; e, 70%; h , 70%; i,
70%.
IR (νmax in pentane, cm-1): a , 1761 m, 1735 s ν(CO2Me),
1653 w ν(CdN); b, 1765 m, 1739 s ν(CO2Me), 1652 w ν(CdN);
c, 1761 m, 1730 s ν(CO2Me), 1652 w ν(CdN); d , 1757 m, 1725
s ν(CO2Me), 1652 w ν(CdN); e, 1766 m, 1738 s ν(CO2Me), 1652
w ν(CdN); f, 1761 m, 1738 s ν(CO2Me), 1652 w ν(CdN); g,
1762 m, 1728 s ν(CO2Me), 1653 w ν(CdN); h , 1751 m, 1724 s
ν(CO2Me), 1652 w ν(CdN); i, 1766 m, 1735 s ν(CO2Me), 1652
w ν(CdN).
Syn th esis of Com p lexes. The general procedures for the
preparation of complexes 6a -i, 7a -e, 10a -d /10′a -d , 11b,c/
11′b,c, and 12f-i are given below; details are listed in Table
1.
Syn th esis of F e2(CO)6(L) (6a -e) a n d F e2(CO)6(R1,R3-
ODA) (7a -e). To a stirred suspension of Fe2(CO)9 in 50 mL
of toluene was added 0.5 g of the respective R-imino ester (cf.
Table 1), whereupon the solution turned dark purple. The
reaction mixture was stirred for 16 h, during which time the
color of the reaction mixture changed from deep purple to red-
brown. The reaction mixture was evaporated to dryness, the
residue was redissolved in a minimum of CH2Cl2, and the
products were separated by column chromatography on silica
gel (column 1 × 20 cm). Elution with pentane afforded a green
fraction, containing Fe3(CO)12. Further elution with pentane/
Et2O afforded a brown-orange fraction which after evaporation
of the solvent gave Fe2(CO)6(L) (6a -e) as an orange-brown
oil. Crystallization from pentane at -60 °C resulted in most
cases in red-brown crystalline products; the yields varied
between 10 and 16% (isolated product) depending on the ligand
used. Continued elution with pentane/Et2O afforded an orange
to yellow fraction which after evaporation of the solvent gave
Fe2(CO)6(R1,R3-ODA) (7a -e) as a yellow powder in 23-26%
yield depending on the ligand used. Analytically pure com-
plexes could be obtained by crystallization from pentane or
pentane/Et2O (10:1) at -60 °C, resulting in orange to orange-
brown crystals in almost quantitative yield.
1H NMR (300.13 MHz, CDCl3, room temperature, δ in ppm
relative to TMS): a , 7.65 (t, 1H, 1.3 Hz, NdCH), 3.86 (s, 3H,
OCH3), 3.39 (d, 2H, 1.3 Hz, CH2C(CH3)3), 0.95 (s, 9H, C(CH3)3);
d , 7.70 (s, 1H, NdCH), 5.20 (sept, 1H, 6.3 Hz, OCH(CH3)2),
3.61 (sept, 1H, 6.3 Hz, NCH(CH3)2), 1.34, 1.27 (2 × d, 12H,
6.3 Hz, 2 × CH(CH3)2); e, 7.63 (s, 1H, NdCH), 3.87 (s, 3H,
OCH3), 3.03 (dq, 1H, 6.8 Hz, NCH), 1.81-1.41 (m, 6H, c-Hex
H), 1.32-1.02 (m, 3H, c-Hex H), 1.21 (d, 3H, 6.4 Hz, NCCH3),
0.96-0.72 (m, 2H, c-Hex H); h , 7.64 (s, 1H, NdCH), 5.20 (sept,
1H, 6.3 Hz, CH(CH3)2), 1.34 (d, 6H, 6.3 Hz, CH(CH3)2), 1.28
(s, 9H, C(CH3)3); i, 7.60 (s, 1H, NdCH), 3.87 (s, 3H, OCH3),
1.63 (q, 2H, 7.5 Hz, CH2CH3), 1.20 (s, 6H, C(CH3)2), 0.77 (t,
3H, 7.5 Hz, CH2CH3).
(41) Fe2(CO)9 was prepared by a slightly modified literature proce-
dure (Braye, E. H.; Hu¨bel, W. 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 last compound was
added to prevent the mixture from containing too much water). A
Rayonet RS photochemical reactor (λmax ) 2500 Å) was used for
irradiation, and a continuous stream of air was used to cool the reaction
mixture. Filtering, washing with water, ethanol, and ether, and
subsequently drying in vacuo gave Fe2(CO)9 in usually more than 90%
yield.
Syn th esis of F e2(CO)6(L) (6f-i) a n d F e2(CO)5(R1,R3-
ODA) (12f-i). (a ) Rea ction in Tolu en e. To a stirred
suspension of Fe2(CO)9 in 50 mL of toluene, 0.5 g of the
respective R-imino ester was added (cf. Table 1), whereupon
(42) van der Poel, H.; van Koten, G. Synth. Commun. 1978, 8, 305.
(43) Kelly, R. T.; Schimdt, T. E.; Haggerty, J . G. Synthesis 1972,
544.

3020 Organometallics, Vol. 19, No. 16, 2000
Siebenlist et al.
the solution turned dark blue. The reaction mixture was
stirred for 16 h, during which time the color of the reaction
mixture changed from deep blue to red-brown. The reaction
mixture was evaporated to dryness, the residue was redis-
solved in a minimum of CH2Cl2, and the products were
separated by column chromatography on silica gel (column 1
× 20 cm). Elution with pentane afforded a green fraction,
containing Fe3(CO)12. Further elution with pentane/Et2O af-
forded a brown-orange fraction which after evaporation of the
solvent gave Fe2(CO)6(L) (6f-i) as a brown oil. Crystallization
from pentane at -80 °C resulted in most cases in red-brown
crystalline products, in yields of 7-10% (isolated product)
depending on the ligand used. Subsequent elution with Et2O/
CH2Cl2 afforded a dark blue fraction which after evaporation
of the solvent yielded Fe2(CO)5(R1,R3-ODA) (12f-i) as a blue
powder in 32-38% yield. Crystallization from Et2O/CH2Cl2 (4:
1) resulted in dark purple-blue diamond-shaped crystals of
12f-h , which due to twinning were unsuitable for determi-
nation of refined structures by X-ray diffraction. Crystalliza-
tion of 12i yielded a dark blue crystalline powder. Reaction of
equimolar quantities of ligands f and g resulted in formation
of the mixed Me/Et substituted complex 12fg. Crystallization
resulted in purple-blue diamond-shaped crystals of 12fg, which
were unfortunately again not suitable for X-ray diffraction.
(b) Rea ction in THF . The reaction in THF was performed
as in toluene, only for 3 instead of 16 h. The yields of 6f-i
varied from 9 to 11% and the yields for 12f-i between 9 and
12%.
Syn th esis of F e(CO)2(σ(N):σ(C)-R1NC(H)CR3)2 (10a -d /
10′a -d ). To a stirred suspension of Fe2(CO)9 in 25 mL of THF
was added 0.5 g of the respective R-imino ester (cf. Table 1),
whereupon the solution turned dark purple. The reaction
mixture was stirred for 3 h, during which time the color of
the reaction mixture changed from deep purple to red-brown.
The reaction mixture was evaporated to dryness, the residue
was redissolved in a minimum of CH2Cl2, and the products
were separated by column chromatography on silica gel
(column 1 × 15 cm). Elution with pentane afforded a green
fraction, containing Fe3(CO)12. Further elution with pentane/
Et2O (4:1) afforded a red-brown fraction containing small
amounts of Fe2(CO)6(L) (6) and Fe2(CO)6(R1,R3-ODA) (7).
Subsequent elution with Et2O afforded a yellow to brown
fraction. This fraction was evaporated to dryness and rechro-
matographed because the brown oily residue still contained
impurities. Elution with pentane/Et2O (5:1) afforded a small
orange fraction containing traces of Fe2(CO)6(R1,R3-ODA) (7).
Subsequent elution with pentane/Et2O (1:1) afforded one
yellow fraction for 10a and 10b /10′b. For 10c,d /10′c,d two
separate yellow fractions were collected; the first fraction
contained pure 10′c,d and the second fraction contained almost
pure 10c,d . The ratio of 10b-d to 10′b-d was approximately
1:6. After evaporation of the solvent 10a -d /10′a -d were
isolated as yellow powders in 10-13% yield. Analytically pure
complexes were obtained by crystallization from pentane or
pentane/CH2Cl2 (10:1), resulting in yellow to yellow-orange
crystals. Crystals of 10c were suitable for X-ray diffraction.
Syn t h esis of F e(CO)2(σ(N):σ(C)-i-P r NC(H)CR 3)(R 1Nd
C(H)C(O)O) (11b,c). To a stirred suspension of Fe2(CO)9 in
15 mL of THF were added 0.5 g of the respective R-imino ester
and 3 equiv of H2O (cf. Table 1), whereupon the solution turned
deep purple. The reaction mixture was stirred for 5 h, during
which time the color of the reaction mixture changed from deep
purple to red-brown. The reaction mixture was evaporated to
dryness, the residue was redissolved in a minimum of CH2-
Cl2, and the products were separated by column chromatog-
raphy on silica gel (column 1 × 15 cm). Elution with Et2O
afforded a red-brown fraction, containing traces of Fe2(CO)6-
(L) (6b,c) and Fe2(CO)6(R1,R3-ODA) (7b,c). Elution with CH2-
Cl2 yielded a fraction containing unknown paramagnetic
products. Subsequent elution with CH2Cl2/THF (1:1) afforded
a yellow-brown fraction which after evaporation of the solvent
gave 11b ,c/11′b,c as yellow powders in 14 and 17% yields,
respectively. Analytically pure complexes were obtained by
crystallization from Et2O/CH2Cl2 (5:1), resulting in yellow to
yellow-brown crystals. Crystals of 11b were suitable for X-ray
diffraction.
Rea ction of F e2(CO)6(R1,R3-ODA) (7b-e) w ith HCl. To
a solution of 200 mg of Fe2(CO)6(R1,R3-ODA) (7b-e) in 40 mL
of toluene was added 5 mL of concentrated aqueous HCl. When
the organic layer had become colorless, after about 2-4 days,
the excess of HCl was neutralized with NaHCO3. The aqueous
layer was separated and extracted twice with 50 mL of Et2O.
The combined organic layers were dried (Na2SO4) and evapo-
rated to dryness, the residue was redissolved in pentane, and
this solution was purified by column chromatography on silica
gel (column 1 × 15 cm). Elution with pentane/Et2O (3:1)
afforded a colorless fraction, which after evaporation of the
solvent gave 8b-e as colorless oils in 70% yield. Analytically
pure 8e was obtained by crystallization from pentane at -80
°C, resulting in white crystalline material in 50% yield.
Th er m a l a n d P h otoch em ica l Con ver sion of 7b -d to
F e2(CO)4(R1,R3-ODA) (9b-d ). Th er m olysis. When a solu-
tion of 100 mg of Fe2(CO)6(R1,R3-ODA) (7b-d ) in toluene (20
mL) was stirred at reflux, the color of the solution changed
from orange to deep green. The reaction was followed by IR
spectroscopy and revealed that a new product was formed with
CO stretching bands at about 1999, 1950, 1911, and 1665 cm-1
.
The reaction was stopped ((3 h) when the intensities of the
IR bands of 9 were at their maximum, at which time ap-
proximately 70-80% of 7b-d was consumed. Prolonged heat-
ing did not improve the yields of 9. After evaporation of the
solvent the green/brown residue was dissolved in 10 mL of
Et2O. The resulting deep green solution was filtered and gave
after evaporation of the solvent a green oily residue containing
a 2:3 mixture of 7 and 9. Unfortunately, due to decomposition
of 9 on silica no further purification was possible. Complex
1
9b has been characterized by IR, H and 13C NMR, and UV-
vis spectroscopy. Reaction of a toluene solution of a 9 (mixture
with 7) at room temperature under an atmosphere of CO leads
within 2 min to the re-formation of 7 in quantitative yield.
P h otolysis. Irradiation of toluene solutions of 7b-d with
a mercury lamp also leads to the formation of 9b-d . However,
this reaction is accompanied by extensive decomposition,
probably due to the photolability of 9 on irradiation into the
metal-metal bond σ-σ* transition (λ ) 395 nm), similar to
the photodecomposition of the isostructural Fe2(CO)4(R-
ADO).40
Th er m a l a n d P h otoch em ica l Rea ctivity of F e2(CO)5-
(R1,R3-ODA) (12f-i). Stirring a toluene solution of 12 under
an atmosphere of CO at room temperature or at reflux gave
no reaction. Refluxing a solution of 12 in toluene under N2,
i.e., without CO, did lead to a new complex (IR) in about 20%
yield. However, the new product could not be identified because
it decomposed upon purification on silica. Irradiation of 12 in
toluene or THF into the high-energy slope of the MLCT band
(12f; λ ) 589 nm) with the 514.5 nm line resulted in slow
decomposition, and irradiation with a mercury lamp resulted
in fast decomposition of 12f.
X-r a y Str u ctu r e Deter m in a tion of 10c a n d 11b. Orange
crystals of 10b and 11c suitable for X-ray diffraction were
glued to the tip of a Lindemann glass capillary and transferred
to an Enraf-Nonius CAD4-F diffractometer in a sealed tube
(10b) or into the cold nitrogen stream of an CAD4-T diffrac-
tometer on a rotating anode (11c). Accurate unit-cell param-
eters and an orientation matrix were determined by least-
squares treatment of the setting angles of 22 well-centered
reflections (SET4)44 in the ranges of 8.61° < θ < 15.32° and
9.86° < θ < 13.83° for 10b and 11c, respectively. The unit-
cell parameters were checked for the presence of higher lattice
(44) Boer, J . L.; Duisenberg, A. J . M. Acta Crystallogr. 1984, A40,
410.

Carbonyliron Complexes of 1-Aza-4-oxa-1,3-butadiene
Organometallics, Vol. 19, No. 16, 2000 3021
Ta ble 2. Cr ysta l Da ta a n d Deta ils of th e Str u ctu r e
Deter m in a tion of 10c a n d 11b
Ta ble 3. Com bin a tion s of Su bstitu en ts in r-Im in o
Ester Liga n d s R1NdC(H)C(R3)dO (a -i)
10c
11b
a
b
c
d
e
f
g
h
i
R1 neo-Pe i-Pr i-Pr i-Pr
R3 OMe
CE
t-Bu t-Bu t-Bu
t-Am
Crystal Data
16H28FeN2O6
400.25
formula
mol wt
C
C13H20FeN2O6
356.16
OMe OEt O-i-Pr OMe OMe OEt O-i-Pr OMe
cryst syst
space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/c (No. 14)
11.5890(10)
19.722(3)
10.1620(10)
114.950(10)
2105.9(5)
4
monoclinic
P21/c (No. 14)
10.0831(6)
14.7323(11)
13.6017(7)
124.77(5)
1659.7(10)
4
1.425
744
9.3
0.15 × 0.25 × 0.50
for the amine hydrogen atoms and the C-H hydrogen atoms
neighboring an amine moiety, which were located on a differ-
ence Fourier map. Their coordinates were included in the
refinement. The methyl groups of 11c were refined as rigid
groups, allowing for rotation around the C-C bond. Non-
hydrogen atoms were refined with anisotropic displacement
parameters. The hydrogen atoms of 10b were refined with
individual isotropic displacement parameters for the freely
refined atoms and the hydrogen atoms attached to a tertiary
carbon atom. One overall displacement parameter of 0.128(6)
Å2 was applied to all other hydrogens. The hydrogen atoms of
11c were refined with a fixed isotropic parameter related to
the value of the equivalent isotropic displacement parameter
of their carrier atoms by a factor amounting to 1.5 for the
methyl hydrogen atoms and 1.2 for the other hydrogen atoms,
respectively. Compound 10b was refined using neutral atom
scattering factors taken from Cromer and Mann50 amplified
with anomalous dispersion corrections from Cromer and
Liberman.51 For 11c neutral atom scattering factors and
anomalous dispersion corrections were taken from ref 52.
Geometrical calculations and illustrations were performed with
PLATON;53 all calculations were performed on a DEC-station
5000 cluster. Additional material available from the Cam-
bridge Crystallographic Data Center comprises H atom coor-
dinates, displacement parameters, and remaining bond lengths
and angles.
â (deg)
V (Å3)
Z
Dcalcd (g/cm3)
F(000)
1.262
848
7.4
µ(Mo KR) (cm-1
cryst size (mm)
)
0.30 × 0.25 × 0.30
Data Collection
298
1.03, 27.50
0.710 73 (Zr filtered) 0.710 73 (graphite
temp (K)
150
1.38, 27.50
θmin, θmax (deg)
wavelength
(Mo KR)(Å)
scan type
monochr)
ω/2θ
0.59 + 0.35 tan θ
3.00, 4.00
ω/2θ
0.53 + 0.35 tan θ
3.00, 4.00
∆ω (deg)
horiz and vert
aperture (mm)
X-ray exposure
time (s)
linear decay (%)
ref rflns
73
16
2
2
142h, 3h23h
253h, 025h, 123h
-13 to +13, -19 to 0,
-17 to +17
8275
data set
-15 to +13, -24 to
+13, -10 to +13
5958
total no. of data
total no. of
unique data
4820 (Rint ) 0.030)
3790 (Rint ) 0.030)
no. of obsd data
1861 (I > 2.5σ(I))
3790 (no observn
criterion applied)
DIFABS cor range
0.72, 1.10
Resu lts a n d Discu ssion
Refinement
244
no. of
213
The ligands and complexes discussed are schemati-
cally shown in Table 3 and Scheme 3, respectively. The
R-imino esters R1NdC(R2)C(R3)dO (R1 ) alkyl, R2 ) H,
R3 ) O-alkyl) (L) are indicated by the letters a -i. The
type of complex is identified with Arabic numbers.
Reaction of Fe2(CO)9 with the R-imino esters a -i at
room temperature leads to the formation of several
mononuclear and binuclear complexes. The product
formation and distribution strongly depend on the
nitrogen substituent R1 and on the solvent.
refined params
final R1a
0.054
0.043
0.037 (2906, I > 2σ(I))
b
final Rw
final wR2c
0.086
1.01
goodness of fit
1.39
weighting schemed
(σ2(F) +
(σ2(F2) +
(0.0335P)2)-1
0.000, -0.003
-0.44, 0.41
0.000102F2)-1
0.02, 0.08
-0.34, 0.55
(∆/σ)av, (∆/σ)max
min and max
resid density (e/Å3)
R1 ) ∑||Fo| - |Fc||/∑|Fo|. Rw ) [∑[w(||Fo| - |Fc||)2]/∑[w(Fo2)]
a
b
c wR2 ) [∑[w(Fo - Fc2)2]/∑[w(Fo2)2] ]1/2
1/2
2
d
]
.
.
P ) (Max(Fo2, 0) +
2Fc2)/3.
Gen er a l Rea ction s. With all ligands employed the
formation of the intense blue (5f-i) or purple (5a -e)
chelate Fe(CO)3(R-imino ester) complexes is observed.
However, complexes 5 are not stable under the reaction
conditions and react further with iron carbonyl in
toluene and THF to form the Fe2(CO)6(L) complexes
(6a -i) in low yield (7-16%). Complexes 5 are most
likely formed via intermediate 4, in which the R-imino
ester is σ-N coordinated via the nitrogen lone pair to
the Fe(CO)4 fragment due to the better σ-donative
capacity of nitrogen in comparison to oxygen.54,55
Rea ction w ith liga n d s a -e. Reaction of ligands
a -e, i.e., with the less bulky R1 imine substituents, in
toluene leads predominately to the binuclear Fe2(CO)6-
symmetry.45 Crystal data and details on data collection and
refinement are collected in Table 2. Data were corrected for
Lp effects and for the observed linear decay of the reference
reflections. An empirical absorption/extinction correction (DI-
FABS)46 was applied for compound 10b. The structures were
solved by automated Patterson methods and subsequent
difference Fourier techniques (DIRDIF-92).47 Refinement on
F was carried out by full-matrix least-squares techniques
(SHELX76)48 for 10b. Compound 11c was refined on F2 by full-
matrix least-squares techniques (SHELX-93);49 no observance
criterion was applied during refinement on F2. For both
compounds hydrogen atoms were included in the refinement
on calculated positions riding on their carrier atoms, except
(45) Spek, A. L. J . Appl. Crystallogr. 1988, 21, 578.
(46) Walker, N.; Stuart, D. Acta Crystallogr., Sect. A 1983, A39, 158.
(47) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.;
Garcia-Granda, S.; Gould, R. O.; Smits, J . M. M.; Smykalla, C. The
DIRDIF Program System; Technical Report of the Crystallography
Laboratory; University of Nijmegen, Nijmegen, The Netherlands, 1992.
(48) Sheldrick, G. M. SHELX76, Crystal Structure Analysis Pack-
age; University of Cambridge, Cambridge, England, 1976.
(50) Cromer, D. T.; Mann, J . B. Acta Crystallogr., Sect. A 1968, A24,
321.
(51) Cromer, D. T.; Liberman, D. J . Chem. Phys. 1970, 53, 1891.
(52) Wilson, A. J . C. International Tables for Crystallography;
Kluwer Academic: Dordrecht, The Netherlands, 1992.
(53) Spek, A. L. Acta Crystallogr., Sect. A 1990, A46, C34.
(54) Renk, I. W.; tom Dieck, H. Chem. Ber. 1972, 105, 1403.
(55) Friedel, H.; Renk, I. W.; tom Dieck, H. J . Organomet. Chem.
1971, 26, 247.
(49) Sheldrick, G. M. SHELXL-93, Program for Crystal Structure
Determination; University of Go¨ttingen, Go¨ttingen, Germany, 1993.

3022 Organometallics, Vol. 19, No. 16, 2000
Siebenlist et al.
The molecule consists of an Fe(II) center coordinated
by two newly formed anionic ligands and two carbonyl
ligands. The newly formed ligands are coordinated to
iron via both the amine nitrogen and the adjacent
carbon atom (η2(N,C) coordination), thus forming a
rather strained three-membered metallacycle. This type
of coordination has been observed only a few times with
late transition metals.56 In early-transition-metal chem-
istry this type of coordination is well-known for many
(η2-pyridyl)MCp1,2 (M ) Ti, Zr, Y, Sc, Lu) complexes.57,58
The nitrogen atoms N(1) and N(2) are donative σ(N)
coordinated to iron. The Fe-N(1) (1.983(4) Å) and Fe-
N(2) (1.983(5) Å) bond lengths are comparable with the
Fe-N(amine) distances reported in the literature.13,21
The Fe-C(4) (2.045(6) Å) and the Fe-C(11) (1.982(6)
Å) distances are shorter than those generally observed
for Fe-C(sp3) (2.07-2.15 Å) bonds.24,59-62 The shorten-
ing of these Fe-C bonds might be due to the coordina-
tion of the amine nitrogen, thus bringing the amine
carbons C(4) and C(11) closer to the iron center. The
amine bond distances (N(1)-C(4) ) 1.424(9) Å and
N(2)-C(11) ) 1.419(9) Å) are somewhat shorter than
the mean value of 1.47 Å normally observed for a single
N-C(sp3) bond.63 The Fe-C(15) and Fe-C(16) bond
distances of 1.764(7) and 1.722(7) Å are short for an
electron-poor Fe(II) center and indicate extensive π-back-
donation toward the carbonyl ligands, which is cor-
roborated by the high-frequency position of the CO
resonances in the 13C NMR and the low-frequency
position in the IR spectrum. The iron center in 10c has
a strongly distorted tetrahedral coordination geometry,
if one considers the two η2(N,C) coordinated N(1)-C(4)
and N(2)-C(11) moieties occupying each one coordina-
tion site. While normally an octahedral coordination
geometry would be expected for a regular six-coordinate
d6 complex, this rather strange coordination geometry
is forced onto the molecule by the η2(N,C) coordination
of the two newly formed ligands.
F igu r e 1. ORTEP drawing (30% probability level) of the
molecular structure of 10c.
(R1,R3-ODA)1 complexes 7a -e in moderate yield (21-
25%) together with small amounts (3%) of 10a -d /10′a -
d . In complexes 7a -e the two R-imino ester ligands are
reductively coupled via the imine carbon atoms acting
as a dianionic six-electron donor. Performing the same
reaction in THF leads predominantly to the new mono-
nuclear complexes Fe(CO)2(R1N(H)-C(H)-CR3)2 (10a -
d /10′a -d ) in low yield (12-15%) together with small
amounts (5%) of 7a -d . In the presence of water, the
reaction in THF leads to the formation of Fe(CO)2(i-PrN-
(H)C(H)-CR3)(i-PrNdC(H)C(O)O) (11b,c) in 14 and 20%
yields, respectively.
Rea ction w ith Liga n d s f-i. With the bulky R1
imine N-substituted ligands f-i, a totally different
reaction is observed. Reaction in toluene or THF leads
to the formation of the new dimeric complexes Fe2(CO)5-
(R1,R3-ODA) (12f-i) in moderate yield (32-38%) or low
yield (9-12%), respectively. As in complexes 7, the
newly formed ligand in 12 consists of two R-imino ester
ligands which are reductively coupled via the imine
carbon atoms. However, in contrast to 7, both of the Cd
O oxygen atoms in 12 are coordinated to iron and
instead of a metal-metal bond a bridging CO is present.
With these ligands complexes 7 and 11 and compound
8 are not observed.
Reaction of complexes 7b-e with aqueous HCl leads
to protonation of the bridging aminato nitrogen atoms
with formation of the organic products 8b -e in 70%
yield. Furthermore, refluxing a toluene solution of 7b-d
at 110 °C leads to the loss of two terminal CO ligands
under the formation of Fe2(CO)4(R1,R3-ODA) (9b-d ), in
which the C-C coupled ligand is now coordinated with
the oxygen of both the ester CdO groups. In the
following we will first discuss the structural and spec-
troscopic data of the relevant complexes and subse-
quently deal with the aspects of their formation and
their reactivity.
The crystal structure of 10c shows a complete in-
tramolecular bifurcated hydrogen bond from N(2) to-
ward O(1) (with N‚‚‚O ) 3.025(6) Å and N-H‚‚‚O )
143(3)°) and O(3) (with N‚‚‚O) ) 2.799(8) Å and N-H‚
‚‚O ) 109(4)°). N(1) is also involved in a bifurcated
hydrogen bond toward O(1) (intramolecular, with N‚‚‚
O ) 2.873(6) Å and N-H‚‚‚O ) 101(4)°) and O(1) [2 -
x, -y, 1 - z] (with N‚‚‚O ) 3.150(7) Å and N-H‚‚‚O )
156(5)°). The intermolecular hydrogen bond links 10c
into dimers. Both bifurcated hydrogen bonds are non-
planar; the sum of the angles involving the central
hydrogen atom amounts to 339(7) and 339(5)° for N(1)
and N(2), respectively.
Molecu lar Str u ctu r e of Fe(CO)2(σ(N):σ(C)-i-P r NC-
(H)C(O)OMe)(i-P r NC(H)C(O)O) (11b). A view of the
(56) Bouzid, M.; Pradere, J . P.; Palvadeu, P.; Ve´nien, J . P.; Toupet,
L. J . Organomet. Chem. 1989, 369, 205.
(57) Deelman, B.-J .; Stevels, W. M.; Teuben, J . H.; Lakin, M. T.;
Spek, A. L. Organometallics 1994, 13, 3881.
(58) Thompson, M. E.; Baker, S. M.; Bulls, A. M.; Burger, B. J .;
Nolan, C. N.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J . E. J . Am.
Chem. Soc. 1987, 109, 203.
(59) Lindner, E.; Schauss, E.; Hiller, W.; Fawzi, R. Angew. Chem.
1984, 96, 727.
(60) Kru¨ger, C.; Tsay, Y.-H. Cryst. Struct. Commun. 1975, 5, 215.
(61) Grevels, F.-W.; Feldhoff, U.; Leitich, J .; Kru¨ger, C. J . Orga-
nomet. Chem. 1974, 79.
(62) Eilbracht, P.; J elitte, R.; Walz, L. Chem. Ber. 1984, 117, 3473.
(63) Ladd, M. F. C.; Palmer, R. A. Structure Determination by X-ray
Crystallography, 2nd ed.; Plenum Press: New York, 1985.
Molecu la r Str u ctu r e of F e(CO)2(i-P r N(H)C(H)C-
(O)OEt)2 (10c). An ORTEP view of the molecular
structure of 10c together with the atomic numbering is
presented in Figure 1; selected bond distances and
angles of 10c are given in Table 4.

Carbonyliron Complexes of 1-Aza-4-oxa-1,3-butadiene
Organometallics, Vol. 19, No. 16, 2000 3023
Ta ble 4. Selected Bon d Dista n ces (Å) a n d An gles (d eg) for 10c (Esd ’s in P a r en th eses)
Fe-N(1)
Fe-N(2)
Fe-C(4)
Fe-C(11)
1.983(4)
1.983(5)
2.045(6)
1.982(6)
Fe-C(15)
Fe-C(16)
O(1)-C(5)
O(5)-C(15)
1.764(7)
1.722(7)
1.216(9)
1.145(8)
O(2)-C(5)
O(3)-C(12)
O(4)-C(12)
N(1)-C(4)
1.332(10)
1.207(9)
1.346(9)
1.424(9)
N(2)-C(11)
C(4)-C(5)
C(11)-C(12)
O(6)-C(16)
1.419(9)
1.462(9)
1.451(11)
1.158(9)
N(1)-Fe-N(2)
N(1)-Fe-C(4)
N(1)-Fe-C(11)
N(1)-Fe-C(15)
N(1)-Fe-C(16)
N(2)-Fe-C(4)
N(2)-Fe-C(11)
101.8(2)
N(2)-Fe-C(15)
N(2)-Fe-C(16)
C(4)-Fe-C(11)
C(4)-Fe-C(15)
C(4)-Fe-C(16)
C(11)-Fe-C(15)
C(11)-Fe-C(16)
96.6(3)
C(15)-Fe-C(16)
Fe-N(1)-C(1)
Fe-N(1)-C(4)
C(1)-N(1)-C(4)
Fe-N(2)-C(8)
Fe-N(2)-C(11)
C(8)-N(2)-C(11)
92.1(3)
122.8(4)
71.6(3)
118.5(5)
125.5(4)
69.0(3)
Fe-C(4)-N(1)
67.0(3)
120.5(4)
118.4(6)
69.1(3)
116.9(5)
116.0(6)
41.4(3)
137.5(2)
109.9(3)
104.5(3)
98.1(2)
147.5(3)
110.4(3)
150.1(3)
89.4(3)
98.0(3)
105.9(3)
Fe-C(4)-C(5)
N(1)-C(4)-C(5)
Fe-C(11)-N(2)
Fe-C(11)-C(12)
N(2)-C(11)-C(12)
41.9(3)
120.9(5)
The intermolecular hydrogen bond links 11b into infi-
nite one-dimensional chains parallel to the c axis.
IR Sp ectr oscop y. The IR data for the complexes 6a -
i, 7a -e, 8b-e, 9b-d , 10a -d /10′a -d , 11b,c, and 12f-i
are collected in Table 6, together with the FD and FAB
mass data and the elemental analyses.
Com p lexes 6a -i. The IR spectra of complexes 6
show five strong terminal carbonyl absorptions in the
expected ν(CO) region between 2100 and 1850 cm-1. The
pattern of the spectra closely resembles that of the
analogous Fe2(CO)6(R-imino ketone)39,40 and M2(CO)6-
(R-DAB) (M ) Fe, Ru, Os, FeRu)16,65-68 complexes. The
coordinated ester groups give rise to a weak absorption
around 1570 cm-1, a large coordination shift of ap-
proximately 160 cm-1 to lower frequency. Similar
coordination shifts are observed for the analogous Fe2-
(CO)6(R-imino ketone) complexes. Coordination of ke-
tone and ester O-donor sites to Fe2+, Fe3+ Zn2+, and
Al3+69-72 results in coordination shifts between 50 and
110 cm-1, which are mainly attributed to σ-donation.
In Fe(CO)3(R-imino ketone)25 a very large coordination
shift of 200 cm-1 is observed, due to σ-donation and
extensive π-back-donation from Fe(0) to the antibonding
CdO orbital. This suggests that, apart from σ-donation,
π-back-donation to the ester moiety also plays an
important role in 6.
F igu r e 2. ORTEP drawing (50% probability level) of the
molecular structure of 11b.
molecular structure of 11b together with the atomic
numbering is shown in Figure 2. Selected bond lengths
and angles are given in Table 5.
The molecular structure of 11b consists of an Fe(II)
center coordinated by two CO ligands, one η2(N,C)
coordinated ligand as in 10c, and a newly formed
anionic ligand. The intact imine function (N(2)-C(10)
) 1.271(3) Å) of the newly formed ligand is coordinated
to iron by a normal σ-donative bond Fe(1)-N(2) of
2.0068(17) Å.14,24,25,32 The bond distances of 1.229(3) and
1.280(3) Å for C(11)-O(3) and C(11)-O(4), respectively,
indicate a substantial delocalization within the cyclo-
metalated ester function and compare well with those
of Fe[(OC(O)-η3-allyl)(P(Me)3)2].64 The Fe(1)-O(4) σ-bond
distance of 1.9769(14) Å is consistent with comparable
Fe(1)-O-C(O) distances reported in the literature.32
The carbonyl ligands in 11b are, as in 10c, coordinated
to iron by relatively short bonds (Fe(1)-C(12) ) 1.775-
(3) Å and Fe(1)-C(13) ) 1.762(2) Å); however, these
distances are closer to the Fe-CO distances normally
observed for Fe(II)-carbonyl ligands. The iron center
in 11b shows a distorted-trigonal-bipyramidal coordina-
tion geometry with the η2(N,C)-coordinated N(1)-C(4)
moiety occupying one coordination site. An octahedral
coordination geometry, which might be expected for a
regular six-coordinate d6 complex, is not possible due
to the η2(N,C) coordination and the resulting acute N1-
Fe(1)-C4 angle of 41.75(12)°. The crystal structure of
11b shows a bifurcated hydrogen bond from N(1) toward
O(1) (intramolecular, with N‚‚‚O ) 2.853(2) Å and N-H‚
Com p lexes 7a -e. Complexes 7 give rise to four or
five medium to strong CO stretching bands in the
terminal ν(CO) region, and the carbonyl ester functions
show two weak absorption bands between 1755 and
1725 cm-1. These latter bands are only marginally
shifted compared to those of the free ligands (vide infra),
indicating that the ester groups are not involved in
coordination toward the iron centers.
Com p ou n d s 8b-e. The ester carbonyl groups in 8
give rise to one absorption between 1757 and 1744 cm-1
:
i.e., comparable to those of complexes 7. The new N-H
bond in 8 gives rise to one weak absorption between
3329 and 3336 cm-1, which is indicative of a strong
(65) Staal, L. H.; van Koten, G.; Vrieze, K. J . Organomet. Chem.
1981, 206, 99.
(66) Zoet, R.; van Koten, G.; Muller, G.; Vrieze, K.; van Wijnkoop,
M.; Goubitz, K.; van Halen, C. J . G.; Stam, C. H. Inorg. Chim. Acta
1988, 149, 193.
(67) Kraakman, M. J .; Elsevier: C. J .; Ernsting, J .-M.; Vrieze, K.;
Goubitz, K. Inorg. Chim. Acta 1993, 202, 129.
(68) Fru¨hauf, H.-W.; Landers, A.; Goddard, R. J .; Kru¨ger, C. Angew.
Chem. 1978, 90, 64.
(69) Torres, M. R.; Santos, A.; Ros, J .; Solans, X. Organometallics
1987, 6, 1091.
(70) van der Poel, H.; van Koten, G.; Vrieze, K. Inorg. Chem. 1980,
19, 1145.
(71) van Vliet, M. R. P.; van Koten, G.; Modder, J . F.; van Beek, J .
A. M.; Klaver, W. J .; Goubitz, K.; Stam, C. H. J . Organomet. Chem.
1987, 319, 285.
1
‚‚O ) 101(2)°) and O(3) [x, /2 - y, -1/2 + z] (with N‚‚‚O
) 2.894(3) Å and N-H‚‚‚O ) 159(2)°); the sum of the
angles involving the central hydrogen atom is 359(3)°.
(64) Hoberg, H.; J enni, K.; Kru¨ger, C.; Raabe, E. Angew. Chem., Int.
Ed. Engl. 1986, 25, 810.
(72) Lappert, M. F. J . Chem. Soc. 1961, 817.

3024 Organometallics, Vol. 19, No. 16, 2000
Siebenlist et al.
Ta ble 5. Selected Bon d Dista n ces (Å) a n d An gles (d eg) of 11b (Molecu le 1, Esd ’s in P a r en th eses)
Fe(1)-O(4)
Fe(1)-N(1)
Fe(1)-N(2)
Fe(1)-C(4)
Fe(1)-C(12)
1.9769(14)
1.999(2)
2.0068(17)
1.972(2)
Fe(1)-C(13)
O(1)-C(5)
O(3)-C(11)
O(6)-C(13)
1.762(2)
1.205(3)
1.229(3)
1.146(3)
O(4)-C(11)
N(1)-C(1)
N(1)-C(4)
N(2)-C(7)
1.280(3)
1.491(3)
1.415(3)
1.488(3)
N(2)-C(10)
C(4)-C(5)
C(10)-C(11)
O(5)-C(12)
1.271(3)
1.468(3)
1.500(3)
1.140(4)
1.775(3)
O(4)-Fe(1)-N(1)
O(4)-Fe(1)-N(2)
O(4)-Fe(1)-C(4)
O(4)-Fe(1)-C(12)
87.29(9)
80.73(9)
86.53(9)
N(1)-Fe(1)-C(13)
N(2)-Fe(1)-C(4)
N(2)-Fe(1)-C(12)
92.89(13) Fe(1)-N(1)-C(1)
149.64(12) Fe(1)-N(1)-C(4)
99.95(13) C(1)-N(1)-C(4)
97.97(11) Fe(1)-N(2)-C(7)
126.53(17) N(1)-C(4)-C(5)
68.09(16) O(1)-C(5)-O(2)
118.69(18) O(1)-C(5)-C(4)
125.76(17) O(2)-C(5)-C(4)
118.3(2)
123.6(2)
125.4(2)
111.0(2)
88.63(11) N(2)-Fe(1)-C(13)
O(4)-Fe(1)-C(13) 178.66(10) C(4)-Fe(1)-C(12)
107.21(14) Fe(1)-N(2)-C(10) 113.65(16) N(2)-C(10)-C(11) 116.1(2)
N(1)-Fe(1)-N(2)
N(1)-Fe(1)-C(4)
N(1)-Fe(1)-C(12) 148.88(12) Fe(1)-O(4)-C(11)
109.78(10) C(4)-Fe(1)-C(13)
41.75(12) C(12)-Fe(1)-C(13)
94.49(12) C(7)-N(2)-C(10)
91.90(14) Fe(1)-C(4)-N(1)
115.46(14) Fe(1)-C(4)-C(5)
120.57(19) O(3)-C(11)-O(4)
70.16(13) O(3)-C(11)-C(10) 119.9(2)
121.31(18) O(4)-C(11)-C(10) 114.02(19)
126.0(2)
Ta ble 6. IR Da ta a , Elem en ta l An a lyses, a n d F D/F AB Ma ss Da ta b of 6,c 7,c 8,c 9,e 10/10′,c 11,d a n d 12d
elem anal. obsd (calcd) (%)
compd
IR ν(NH) and ν(CO) (cm-1
)
C
H
N
M+
6a
6b
6c
6d
6e/6′e
6f
6g
6h
6i
7a
7b
7c
7d
7e
8b
8c
8d
2068 s, 2016 vs, 1997 s, 1988 s, 1955 s, 1567 w
2070 s, 2018 vs, 1998 s, 1987 s, 1955 s, 1572 w
2068 s, 2018 vs, 1997 s, 1986 s, 1955 s, 1567 w
2068 s, 2017 vs, 1996 s, 1985 s, 1954 s, 1560 w
2068 s, 2016 vs, 1997 s, 1985 s, 1954 s, 1568 w
2069 s, 2018 vs, 1997 s, 1985 s, 1955 s, 1579 w
2068 s, 2017 vs, 1996 s, 1985 s, 1954 s, 1574 w
2067 s, 2016 vs, 1996 s, 1984 s, 1954 s, 1570 w
2068 s, 2017 vs, 1997 s, 1984 s, 1955 s, 1580 w
2065 m, 2030 s, 1986 s, 1980 s 1970 m (sh), 1755 w
2065 m, 2030 s, 1986 s, 1979 s 1969 m (sh), 1754 w, 1741 w
2065 m, 2031 s, 1986 s, 1978 s, 1749 w, 1733 w
2065 m, 2030 s, 1986 s, 1977 s, 1744 w, 1725 w
2063 m, 2029 s, 1985 s, 1976 s, 1753 w, 1737 w
3329 vw, 1744 m
38.49 (38.48)
35.32 (35.25)
3.46 (3.46)
2.79 (2.71)
not anal.
not anal.
not anal.
3.19 (3.10)
not anal.
not anal.
not anal.
5.07 (5.09)
4.16 (4.12)
4.66 (4.63)
not anal.
not anal.
not anal.
not anal.
not anal.
3.24 (3.21)
3.44 (3.43)
409 (409)
437 (437)
477 (477)
36.84 (36.92)
3.26 (3.31)
437 (437)
451 (451)
437 (437)
44.33 (44.47)
40.29 (40.18)
42.32 (42.43)
4.66 (4.71)
5.29 (5.21)
4.85 (4.95)
594 (594)
674 (674)
261 (260)
289 (288)
317 (316)
398 (397)
3331 vw, 1753 m
3332 vw, 1757 m
8e
3336 vw, 1754 m
9b
2000 s, 1951 s, 1912 s, 1672 w
not anal.
9c
1999 s, 1950 s, 1912 s, 1665 w
not anal.
9d
1997 s, 1948 s, 1910 s, 1658 w
3277 w, 1982 s, 1915 s, 1681 m
3269 w, 3223 w, 1981 s, 1912 s, 1706 m, 1669 m
3243 w, 1981 s, 1912 s, 1689 m
3268 w, 3225 w, 1980 s, 1911 s, 1701 m, 1663 m
3243 w, 1980 s, 1911 s, 1694 m
not anal.
7.54 (7.47)
not anal.
538 (538)
428 (428)
356 (356)
356 (356)
10′a
10b
10′b
10c
10′c
10d
10′d
11b
11c
12f
12g
12h
12i
12fg
50.47 (50.55)
6.54 (6.59)
not anal.
47.97 (47.97)
47.97 (47.97)
50.58 (50.47)
50.58 (50.47)
44.06 (43.84)
45.59 (45.42)
42.35 (42.41)
44.68 (44.55)
7.11 (7.06)
7.11 (7.06)
7.60 (7.54)
7.60 (7.54)
5.69 (5.66)
6.08 (5.99)
4.74 (4.87)
5.45 (5.34)
not anal.
6.83 (7.00)
6.83 (7.00)
6.49 (6.54)
6.49 (6.54)
7.78 (7.87)
7.44 (7.57)
5.18 (5.21)
4.88 (4.95)
3267 w, 3222 w, 1979 s, 1910 s, 1697 m, 1659 m
3244 w, 1979 s, 1912 s, 1679 m
3273 w, 2022 s, 1956 s, 1699 m, 1661 s
3271 w, 2021 s, 1955 s, 1693 m, 1661 s
2022 s, 1975 vs, 1936 s, 1759 w (b), 1700 m
2021 s, 1974 vs, 1935 s, 1757 w (b), 1696 m
2022 s, 1974 vs, 1934 s, 1752 w (b), 1687 m
2022 s, 1974 vs, 1935 s, 1757 w (b), 1700 m
2022 s, 1974 vs, 1935 s, 1758 w (b), 1697 m
594 (594)
566 (566)
not anal.
not anal.
a
Abbreviations: vs ) very strong, s ) strong, m ) medium, w ) weak, vw ) very weak, sh ) shoulder, b ) broad. b Observed (calculated)
FD masses of molecular ion M+ (m/e) of complexes 6, 7, and 9-12; M+ values are based upon 56Fe isotope; observed (calculated) FAB
d
masses of molecular ion M+ (m/e) of 8. c IR measured in pentane. IR measured in dichloromethane. e IR measured in toluene, derived
from mixture of 7 and 9.
N-H bond. This is also evident from the low-frequency
position of the N-H protons in the 1H NMR (see below).
Hydrolysis of EtZn[t-BuNC(H)C(NEt2)dO]2, an N,N′-
bridged C-C coupled dimer of the closely related
R-imino amide t-BuNdC(H)C(NEt2)dO, results in the
N-protonated organic compound 1,2-bis(N,N-diethylcar-
bonamide)-1,2-bis(tert-butylamine)ethane.73 The N-H
absorption of this compound is found at somewhat lower
frequency (3280 cm-1).
weak band at about 1665 cm-1, a shift of ca. 80 cm-1 to
lower wavenumber compared to those of 7, which
indicates Fe-OdC coordination. The pattern and posi-
tion of the observed CO stretching bands strongly
resemble those of Fe2(CO)4(R-ADO), suggesting that
complexes 9 are structurally analogous.
Com p lexes 10a -d /10′a -d a n d 11b,c. The sym-
metric and antisymmetric CO stretching bands (A1, B2)
of dicarbonyl-Fe(II) complexes are normally found at
or somewhat below 2040 cm-1 (A1) and 1980 cm-1
(B2).27,31,74 The two intense CO stretching bands of
complexes 11 are found at clearly lower frequencies
Com p lexes 9b-d . The IR spectra of complexes 9
show three terminal CO stretching bands around 1998,
1948, and 1910 cm-1. The ester carbonyls give rise to a
(73) Van Vliet, M. R. P. Ph.D. Thesis, University of Amsterdam,
1987.
(74) Breuer, J .; Fru¨hauf, H.-W.; Smeets, W. J . J .; Spek, A. L. Inorg.
Chim. Acta 1999, 291, 438.

Carbonyliron Complexes of 1-Aza-4-oxa-1,3-butadiene
Organometallics, Vol. 19, No. 16, 2000 3025
(around 2020 and 1956 cm-1), but those of complexes
10 are found at extremely low wavenumbers of about
1980 and 1912 cm-1. They indicate extensive π-back-
donation to the terminal carbonyls. From this one has
to infer a very strong donor capacity of the three-
electron-donating C-N fragments (σ(C), κ-(N)). In ad-
dition, the unusual coordination geometries (vide supra),
i.e., close to trigonal bipyramidal (11) and tetrahedral
(10), may also play a role.
inequivalent and should in a rigid metal carbonyl
skeleton give rise to six independent resonances. How-
ever, complexes 6 show only one signal at room tem-
perature, while at 243 K three separate signals are
observed. The signal at room temperature does not lie
at the weighted average ppm value of the signals at 243
K. This indicates local scrambling of the carbonyl groups
of one iron center at 243 K, while the carbonyl ligands
on the other iron center are in the limit of slow
exchange. A similar dynamic behavior has been reported
for Fe2(CO)6(R-DAB)76 and Fe2(CO)6(R-imino ketone).40
Individual assignment of CO resonances to either one
of the two Fe(CO)3 moieties can be deduced from
carbonyl resonances of FeRu(CO)6(R-Pyca),67 in which
the µ2(N),η2(CdN) coordination of the CdN moiety is
directed to iron. In the 13CO NMR of these complexes
recorded at 263 K the Fe-carbonyl ligands appear as
one signal, while the Ru-carbonyl ligands give rise to
three separate resonances. On this basis, the three CO
resonances observed at 243 K correspond to the µ2(N),σ-
(O) coordinated Fe1(CO)3 center and the resonance
observed at room temperature to the three CO ligands
on the µ2(N),η2(CdN) coordinated Fe2(CO)3 moiety (cf.
Scheme 3).
As expected, the ester carbonyl groups in the sym-
metric complexes 10′a -d give rise to only one stretching
band at about 1685 cm-1, and those in the asymmetric
complexes 10b-d give rise to two bands around 1700
and 1665 cm-1. The ester carbonyl stretching bands of
complexes 11 are found at 1695 cm-1. The bands are
shifted between 30 and 70 cm-1 to lower wavenumber
compared to those of the free ligand. These rather
pronounced shifts are ascribed to the presence of in-
tramolecular hydrogen bridges with the newly formed
N-H bonds, evident from the X-ray structures of 10c
and 11b, and to the anionic character of the ligand. The
cyclometalated ester carbonyl in 11 gives rise to a strong
carbonyl stretching band at 1661 cm-1. The new N-H
bonds in the symmetric complexes 10′ give rise to one
and those in the asymmetric complexes 10 to two
absorption bands in the ν(N-H) region between 3222
and 3277 cm-1. The N-H vibration of complexes 11 is
found at 3272 cm-1. These absorptions are generally
somewhat lower than the ν(N-H) absorptions (3275-
3305 cm-1) observed for FeM(CO)6[RNC(R)C(R)N(H)R]
(M ) Mn, Re) and Ru2(CO)6[RCC(H)C(H)N(H)R].15,75
For ligand e, containing a chiral 1-cyclohexylethyl
substituent on nitrogen, the two diastereomeric com-
plexes 6e and 6′e are formed in a 3:1 ratio due to the
imine carbon, which becomes chiral upon η2(CdN)
coordination. For 6e/6′e dynamic behavior is observed
to be similar to that for the other complexes 6.
Com p lexes 7. The 1H and 13C resonances of the
former imine protons and imine carbon atoms, observed
between 3.67 and 3.81 and 72.8-73.5 ppm, respectively,
are shifted ca. 4 and 80 ppm to lower frequency in
comparison with those in the free ligand. This clearly
shows that the former imine functions are reductively
coupled via the imine carbon atoms; i.e., they are now
sp3 hybridized, resulting in two anionic aminato nitro-
gen donor sites. The carboxyl carbon atoms of the
noncoordinating ester groups resonate around 172 ppm,
a coordination shift of ca. 11 ppm to higher frequency
compared to those of the free ligand. However, these δ
values are still normal for ester carboxyl carbon atoms
and compare well with those observed in compounds
8b-e.
Com p ou n d s 8b-e. The central carbon atoms appear
around 41.0 (8b-d ) and 42.7 (8e) ppm, respectively. The
shift of ca. 25-30 ppm to lower frequency compared to
the corresponding carbon atoms in complexes 7 is a
result of the protonation of the bridging anionic aminato
nitrogen atoms. The new N-H protons in compounds
8b-e resonate between 1.81 and 1.52 ppm. This is at
relatively low frequency and is indicative of an electron-
rich, strong N-H bond, consistent with the IR data. The
large 3J coupling in 8d between the N-H and the
central C-H proton (3J ) 9.7 Hz) indicates that the
N-H and C-H bonds are nearly coplanar. The dihedral
angle between the N-H bond and the i-Pr C-H bond
is probably close to 90°, as can be concluded from the
3J coupling of only 1.9 Hz (Karplus-Conroy relation)
between these protons. In 8b,c,e, however, the N-H
Com p lexes 12f-i. The IR spectra of complexes 12
give rise to three terminal CO stretching bands around
2022, 1974, and 1935 cm-1. The bridging carbonyl
ligands give rise to a weak and broad absorption band
around 1755 cm-1. The pattern of these CO stretching
bands strongly resembles that of Ru2(CO)5(R-ADA),1 a
similarly C-C coupled dimer of R-DAB.10,16 This makes
it reasonable to assume similar structures; i.e., the
oxygen atoms of the two CdO ester groups in 12 are
coordinated to the iron centers. The coordinated ester
groups give rise to an absorption band between 1700
and 1687 cm-1, a coordination shift of 40 cm-1. This
small shift in comparison with the coordination shift of
160 cm-1 in 6 suggests that in 12 the ester CdO donor
sites act purely as a σ-donor.
1
NMR Sp ectr oscop y. The H and 13C NMR data of
complexes 6, 7, 8, 9, 10/10′, 11/11′, and 12 are collected
in Tables 7 and 8, respectively.
Com p lexes 6. The 1H and 13C resonances of the
imine protons and the imine carbon atoms of complexes
6 are observed between 3.58 and 3.29 and 55.4-48.9
ppm, respectively. These shifts are typical for µ2(N),η2-
(CdN) coordination of the CdN moiety and agree with
those found in M2(CO)6(R-DAB or R-Pyca)16,65-67 and
Fe2(CO)6(R-imino ketone).39,40 The carboxyl carbon at-
oms show coordination shifts of 25-28 ppm to higher
frequency, indicating that the Fe-O bond still mainly
consists of σ(O) donation (see IR Spectroscopy).
Since the R-imino ester is asymmetrically coordinated,
the six terminal carbonyl ligands in complexes 6 are
(75) Mul, W. P.; Elsevier: C. J .; van Leijen, M.; Vrieze, K.; Smeets,
W. J . J .; Spek, A. L. Organometallics 1992, 11, 1877.
(76) Staal, L. H.; Keijsper, J .; Polm, L.; Vrieze, K. J . Organomet.
Chem. 1981, 204, 101.

3026 Organometallics, Vol. 19, No. 16, 2000
Ta ble 7. 1H NMR Da ta a of Com p lexes 6, 7, 9, 10/10′,b 11/11′, a n d 12
Siebenlist et al.
compd
δ
6a
6b
6c
3.91, 3.50 (2 × 1H, d, 13.0 Hz, NCHH), 3.58 (3H, s, OCH3), 3.56 (1H, s, NdCH), 1.26 (9H, s, CH2(CH3)3)
3.56 (3H, s, OCH3), 3.55 (1H, sept, 6.4 Hz, CH(CH3)2), 3.36 (1H, s, NdCH), 1.63, 1.54 (2 × 3H, d, 6.4 Hz, CH(CH3)2)
3.98, 3.92 (2 × 1H, dq, 10.8 Hz, 6.8 Hz, OCHH), 3.54 (1H, sept, 6.0 Hz, CH(CH3)2), 3.33 (1H, s, NdCH), 1.62, 1.54 (2 ×
3H, d, 6.4 Hz, CH(CH3)2), 1.20 (3H, t, 6.8 Hz, CH2CH3)
6d
6e
6′e
4.61 (1H, sept, 6.1 Hz, O-i-Pr CH), 3.52 (1H, sept, 6.2 Hz, N-i-Pr CH), 3.29 (1H, s, NdCH), 1.61, 1.52 (2 × 3H, d, 6.2 Hz,
i-Pr CH3), 1.17, 1.15 (2 × 3H, d, 6.1 Hz, i-Pr CH3)
3.58 (3H, s, OCH3), 3.56 (1H, s, NdCH), 2.47 (1H, m, c-Hex CH), 1.86-1.10 (10H, m, c-Hex CH2), 1.32 (3H, d, 6.9 Hz,
CH(CH3))
3.59 (3H, s, OCH3), 3.34 (1H, s, NdCH), 2.75 (1H, m, c-Hex CH), 1.86-1.10 (10H, m, c-Hex CH2), 1.44 (3H, d, 6.9 Hz,
CH(CH3))
6f
3.57 (1H, s, NdCH), 3.32 (3H, s, OCH3), 1.48 (9H, s, C(CH3)3)
6g
4.02, 3.92 (2 × 1H, dq, 10.8 Hz, 7.1 Hz, OCHH), 3.31 (1H, s, NdCH), 1,21 (3H, t, 7.1 Hz, CH2CH3), 1.47 (9H, s,
C(CH3)3)
6h
6i
4.63 (1H, sept, 6.2 Hz, i-Pr CH), 3.28 (1H, s, NdCH), 1.47 (9H, s, C(CH3)3), 1.19, 1.17 (2 × 3H, d, 6.2 Hz, i-Pr CH3)
3.57 (3H, s, OCH3), 3.35 (1H, s, NdCH), 2.01, 1.77 (2 × 1H, dq, 13.4 Hz, 7.4 Hz, CHHCH3), 1.33, 1.32 (2 × 3H, s,
C(CH3)2), 1.08 (3H, t, 7.4 Hz, CH2CH3)
7a
7b
3.77 (6H, s, 2 × OCH3), 3.67 (2H, s, 2 × N-CH), 3.10, 2.80 (2 × 2H, d, 14.5 Hz, 4 × NCHH), 0.97 (18H, s, 2 × CH2(CH3)3)
3.81 (2H, s, 2 × N-CH), 3.72 (6H, s, 2 × OCH3), 3.25 (2H, sept, 6.8 Hz, 2 × N-i-Pr CH), 1.49, 1.08 (2 × 6H, d, 6.8 Hz, 4 ×
i-Pr CH3)
7c
7d
7e
8b
8c
8d
4.21, 4.14 (2 × 2H, dq, 11 Hz, 7.1 Hz, 4 × OCHH), 3.77 (2H, s, 2 × N-CH), 3.25 (2H, sept, 6.8 Hz, 2 × i-Pr CH), 1.49, 1.11
(2 × 6H, d, 6.8 Hz, 4 × i-Pr CH3), 1.28 (6H, t, 7.1 Hz, 2 × CH2CH3)
5.05 (2H, sept, 6.3 Hz, 2 × Oi-Pr-CH), 3.72 (2H, s, 2 × N-CH), 3.24 (2H, sept, 6.8 Hz, 2 × N-i-Pr CH), 1.49, 1.13 (2 × 6H,
d, 6.8 Hz, 4 × N-i-Pr CH3), 1.27, 1.21 (2 × 6H, d, 6.3 Hz, 4 × O-i-Pr CH3),
3.87 (6H, s, 2 × OCH3), 3.73 (2H, s, 2 × N-CH), 2.79 (2H, dq, 12.8, 6.9 Hz, 2 × CH(CH3)), 2.06 (2H, m, c-Hex CH),
1.75-0.70 (20H, m, c-Hex CH2), 1.45 (6H, d, 6.9 Hz, 2 × CH(CH3))
3.74 (6H, s, 2 × OCH3), 3.70 (2H, s, 2 × NCH), 2.67 (2H, sept, 6.2 Hz, 2 × i-Pr CH), 1.79 (2H, s, b, 2 × NH), 1.03, 0.88 (2
× 6H, d, 6.2 Ηz, 4 × i-Pr CH3)
4.19 (4H, q, 7.2 Hz, 2 × OCH2), 3.69 (2H, s, 2 × NCH), 3.24 (2H, sept, 6.2 Hz, 2 × i-Pr CH), 1.81 (2H, s, b, 2 × NH), 1.30
(6H, t, 7.2 Hz, 2 × CH2CH3), 1.03, 0.89 (2 × 6H, d, 6.2 Ηz, 4 × i-Pr CH3)
5.05 (2H, sept, 6.3 Hz, 2 × O-i-Pr CH), 3.62 (2H, d, 9.7 Hz, 2 × NCH), 2.65 (2H, dsept, 6.1, 1.9 Hz, 2 × N-i-Pr CH), 1.75
(2H, br d, 9.7 Hz, 1.9 Hz, 2 × NH), 1.28, 1.26 (2 × 6H, d, 6.3 Ηz, 2 × O-i-Pr CH3), 1.01, 0.90 (2 × 6H, d, 6.1 Ηz, 2 × N-i-Pr
CH3)
8e
3.73 (6H, s, 2 × OCH3), 3.71 (2H, s, 2 × NCH), 1.52 (2H, br s, 2 × NH), 1.74-1.53 (16H, m, c-Hex H, CH(CH3)), 1.26-1.06
(10H, m, c-Hex H), 0.94 (6H, d, 6.6 Hz, 2 × CH(CH3))
9b
4.49 (2H, s, 2 × N-CH), 3.79 (6H, s, 2 × OCH3), 3.49 (2H, sept, 6.8 Hz, 2 × N-i-Pr CH), 1.18, 1.07 (2 × 6H, d, 6.8 Hz, 4 ×
i-Pr CH3)
10′a
10b
3.96 (2Η, d, 7 Hz, 2 × NCH), 3.76 (6H, s, 2 × OCH3), 2.45 (2H, d, 13.1 Hz, 2 × NCHH), 2.14 (2H, pt, 11.1, 7 Hz, 2 × NH),
1.89 (2H, dd, 13.1, 11.1 Hz, 2 × NCHH), 0.94 (18H, s, 2 × CH2(CH3)3)
3.82 (3H, s, OCH3), 3.46 (1H, d, 5.3 Hz, NCH), 3.46 (3H, s, OCH3), 2.87 (1H, d, 7.9 Hz, NCH), 2.76 (1H, br, dd, 8.6, 7.9
Hz, NH), 2.71 (1H, br dd, 10.0, 5.3 Hz, NH), 1.91 (1H, ds, 7.1, 8.6 Hz, i-Pr CH), 1.71 (1H, ds, 10.0, 6.4 Hz, i-Pr CH),
1.02, 0.82 (2 × 3H, d, 6.4 Hz, i-Pr CH3), 0.92, 0.67 (2 × 3H, d, 7.1 Hz, i-Pr CH3)
10′b
3.73 (2H, d, 7.1 Hz, 2 × NCH), 3.45 (6H, s, 2 × OCH3), 2.13 (2H, br pt, 10.9, 7.1 Hz, 2 × NH), 1.64 (2H, ds, 10.9, 6.6 Hz,
2 × i-Pr CH), 0.89, 0.70 (2 × 6H, d, 6.4Ηz, 4 × i-Pr CH3)
10c
4.53, 4.33, 4.27, 3.94 (4 × 1H, dq, 10.7, 6.3 Hz, OCHH), 3.53 (1Η, d, 7.4 Hz, NCH), 2.85 (1Η, d, 7.8 Hz, NCH), 2.78 (1H,
br, dd, 9.6, 7.8 Hz, NH), 2.75 (1H, br, dd, 10.7, 7.4 Hz, NH), 1.91 (1H, ds, 10.7, 6.3 Hz, i-Pr CH), 1.73 (1H, ds, 11.6, 6.3
Hz, i-Pr CH), 1.24, 1.18 (2 × 3H, t, 7.1Ηz, CH2CH3), 1.03, 0.93, 0.81, 0.69 (4 × 3H, d, 6.3Ηz, i-Pr CH3)
4.20, 3.94 (2 × 2H, dq, 10.8, 7.2 Hz, 4 × OCHH), 3.74 (2Η, d, 7.1 Hz, 2 × NCH), 2.21 (2H, br dd, 10.8, 7.2 Hz, 2 × NH),
1.68 (2H, ds, 10.7, 6.3 Hz, 2 × i-Pr CH), 1.08 (6Η, t, 7.2 Ηz, 2 × CH2CH3), 0.95, 0.74 (2 × 6H, d, 6.3 Ηz, 4 × i-Pr CH3)
5.38 (1H, sept, 6.4 Hz, i-Pr CH), 5.11 (1H, sept, 6.4 Hz, i-Pr CH), 3.52 (1Η, d, 7.4 Hz, NCH), 2.85 (1H, br pt, 11.5, 7.4
Hz, NH), 2.72 (1H, br pt, 10.7, 7.7 Hz, NH), 2.71 (1H, d, 7.7 Hz, NCH), 1.92 (1H, ds, 9.5, 6.4 Hz, i-Pr CH), 1.75 (1H, ds,
10.7, 6.4 Hz, i-Pr CH), 1.46, 1.36, 1.30, 1.19, 1.03, 0.93, 0.82, 0.70 (10 × 3H, d, 6.4 Ηz, i-Pr CH3)
5.08 (2H, sept, 6.3 Hz, 2 × i-Pr CH), 3.69 (2Η, d, 7.2 Hz, 2 × NCH), 2.23 (2H, br pt, 10.4, 7.2 Hz, 2 × NH), 1.69 (2H, ds,
10.4, 6.3 Hz, 2 × i-Pr CH), 1.20, 1.15, 0.97, 0.75 (4 × 6H, d, 6.3 Ηz, 10 × i-Pr CH3)
7.77 (1H, s, NdCH), 4.31 (1H, sept, 6.5 Hz, i-Pr CH), 4.00 (1H, d, 8.8 Hz, NCH), 3.74 (3H, s, OCH3), 3.51 (1H, br dd,
11.0, 8.8 Hz, NH), 2.85 (1H, ds, 11, 6.6 Hz, i-Pr CH), 1.53, 1.52 (2 × 3H, d, 6.5Ηz, i-Pr CH3), 1.26, 1.01 (2 × 3H, d, 6.6Ηz,
i-Pr CH3)
10′c
10d
10′d
11b
11′bc
7.79 (1H, s, NdCH), 4.31 (1H, sept, 6.3 Hz, i-Pr CH), 3.80 (3H, s, OCH3), 3.63 (1H, d, 10.4 Hz, NCH), 3.15 (1H, br dd,
10.5, 10.4 Hz, NH), 2.49 (1H, ds, 10.5, 6.6 Hz, i-Pr CH), 1.51, 1.40 (2 × 3H, d, 6.3Ηz, i-Pr CH3), 1.26, 1.01 (2 × 3H, d,
6.6Ηz, i-Pr CH3)
7.77 (1H, s, NdCH), 4.33 (1H, sept, 6.7 Hz, i-Pr CH), 4.31, 4.14 (2 × 1H, dq, 10.9, 7.3 Hz, OCHH), 4.05 (1H, d, 10.9 Hz,
NCH), 3.40 (1H, br dd, 10.9, 8.9 Hz, NH), 2.88 (1H, ds, 10.9, 6.5 Hz, i-Pr CH), 1.55, 1.54, 1.28, 1.02 (2 × 3H, d, 6.5 Ηz,
i-Pr CH3), 1.29 (3H, t, 6.7 Hz, CH2CH3)
11c
12f
3.81 (6H, s, 2 × OCH3), 3.50 (2H, s, 2 × N-CH), 1.83, (6H, s, b, 2 × C(CH3), 1.27 (2 × 12H, br s, 4 × C(CH3)) (1.82, 1.29,
1.20)
12g
12h
12i
4.29, 4.22 (2 × 2H, dq, 10.9 Hz, 7.1 Hz, 4 × OCHH), 3.45 (2H, s, 2 × N-CH), 1.83, (6H, br s, 2 × C(CH3)),, 1.32 (2 × 6H, br
s, 4 × C(CH3)), 1.31 (6H, t, 7.1 Hz, 2 × CH2CH3) (1.79, 1.27, 1.18)
5.00 (2H, sept, 6.2 Hz, 2 × i-Pr CH), 3.36 (2H, s, 2 × N-CH), 1.82 (6H br s, 2 × C(CH3)), 1.28 (12H, s, br s, 4 × C(CH3)),
1.30, 1.28 (2 × 6H, d, 6.2 Hz, 4 × i-Pr CH3) (1.711, 1.26, 1.18)
3.80 (6H, s, 2 × OCH3), 3.58 (2H, s, 2 × NCH), 1.71 (6H, s, b, 2 × C(CH3), 1.68 (1H, m, b, CHHCH3), 1.52 (2H, m, b,
CH2CH3), 1.33 (1H, m, b, CHHCH3), 1.18 (6H, s, b, 2 × C(CH3)) 1.00 (6H, t, b, 2 × CH2CH3)
4.28, 4.22 (2 × 1H, dq, 10.7 Hz, 6.8 Hz, OCH2), 3.81 (3H, s, OCH3), 3.48, 3.46 (2 × 1H, d, 10.6 Hz, N-CH), 1.83 (6H, br
s, 2 × C(CH3)), 1.31 (12H, br s, 4 × C(CH3), 1.31 (3H, t, 7.1 Hz, CH2CH3)
12fg
a
Abbreviations: s ) singlet, d ) doublet, t ) triplet, q ) quartet, dq ) double quartet, sept ) septet, ds ) double septet, dd ) double
b
double. δ values are in ppm relative to TMS, measured in CDCl3 at 293 K and 300.13 MHz. Measured in C6D6 at 293 K and 300.13
MHz. c Derived from a mixture.

Carbonyliron Complexes of 1-Aza-4-oxa-1,3-butadiene
Organometallics, Vol. 19, No. 16, 2000 3027
Ta ble 8. 13C NMR Da ta a of Com p lexes 6,b 7, 8, 9, 10/10′, 11/11′, a n d 12d
compd
δ
6a
6b
6c
216.2, (213.2), 212.5, 201.7 (CO), 185.4 (CdO), 81.7 (NCH2), 59.0 (OCH3), 55.4 (CdN), 36.0 (C(CH3)3), 29.5 (C(CH3)3)
216.4, (213.8), 211.0, 201.8 (CO), 187.1 (CdO), 66.3 (i-Pr CH), 55.2 (OCH3), 52.1 (CdN), 28.1, 27.2 (i-Pr CH3)
216.6, (213.4), 211.0, 201.9, (CO), 186.7 (CdO), 66.3 (i-Pr-CH), 64.7 (OCH2), 52.5 (CdN), 28.2, 27.1 (i-Pr-CH3), 14.5
(OCH2CH3)
6d
216.7, (213.9), 211.1, 202.1 (CO), 186.7 (CdO), 73.1 (i-Pr OCH), 66.3 (i-Pr NCH), 52.9 (CdN), 28.2, 27.1, 22.1, 21.8
(i-Pr CH3)
216.2, (214.0), 211.5, 202.1 (CO), 185.7 (CdO), 73.6 (NCH(CH3)), 55.3 (OCH3), 49.8 (CdN), 32.8 (c-Hex CH), 27.5,
27.0, 26.7, 25.7 (c-Hex CH2), 18.1 (CH(CH3))
216.2, (214.0), 211.5, 202.1 (CO), 186.5 (CdO), 75.7 (NCH(CH3)), 50.8 (OCH3), 49.4 (CdN), 32.3 (c-Hex CH), 27.5,
27.0, 26.7, 25.7 (c-Hex CH2), 18.1 (CH(CH3))
6ed
6′ed
6f
6g
216.6, (214.2), 210.7, 201.8 (CO), 188.1 (CdO), 64.0 (C(CH3)3), 55.7 (OCH3), 48.9 (CdN), 33.7 (C(CH3)3)
216.8, (214.3), 210.7, 201.9 (CO), 187.1 (CdO), 64.7 (OCH2), 64.0 (C(CH3)3), 49.2 (CdN), 33.3 (C(CH3)3), 14.6
(OCH2CH3)
6h
6i
217.0, (214.3), 210.8, 201.9 (CO), 187.3 (CdO), 73.1 (OCH), 64.0 (C(CH3)3), 49.5 (CdN), 33.6 (C(CH3)3), 22.1, 21.8
CH(CH3)2)
216.7, (214.3), 210.7, 201.6 (CO), 188.0 (CdO), 66.6 (NC), 55.2 (OCH3), 49.2 (CdN), 40.4 (CH2CH3), 31.2, 29.0
(C(CH3)2), 11.5 (CH2CH3)
7a
7b
7c
213.3, 212.6, 204.3 (CO), 170.4 (CdO), 76.1 (NCH2), 73.5 (NCH), 52.8 (OCH3), 35.1 (C(CH3)3), 30.4 (C(CH3)3)
213.2, 212.2, 204.1 (CO), 173.2 (CdO), 72.8 (NCH), 67.6 (i-Pr CH), 53.2 (OCH3), 26.2, 23.8 (i-Pr CH3)
213.4, 212.3, 204.2 (CO), 173.0 (CdO), 73.2 (NCH), 67.7 (i-Pr CH), 62.4 (OCH2), 26.6, 22.9 (i-Pr CH3), 14.6
(OCH2CH3)
7d
7e
213.4, 212.3, 204.2 (CO), 172.7 (CdO), 73.5 (NCH), 69.8 (i-Pr OCH), 67.7 (i-Pr NCH), 26.7, 24.0, 22.4, 22.0 (i-Pr CH3)
213.4, 212.1, 204.1 (CO), 173.1 (CdO), 77.6 (NCH(CH3)), 73.4 (NCH), 52.9 (OCH3), 45.9 (c-Hex CH), 34.0, 32.2,
27.7, 27.0, 26.8 (c-Hex CH2), 22.3 (NCH(CH3))
8b
8c
8d
8e
9b
10a
10b
174.4 (CdO), 62.3 (i-Pr CH), 52.7 (OCH3), 48.4 (NCH), 24.2, 22.8 (i-Pr CH3)
173.9 (CdO), 62.4 (i-Pr CH), 61.7 (OCH2), 48.4 (NCH), 24.2, 22.9 (i-Pr CH3), 15.0 (OCH2CH3)
173.4 (CdO), 69.3 (O-i-Pr CH), 62.3 (N-i-Pr CH), 48.4 (NCH), 24.2, 23.1, 22.7, 22.5 (i-Pr CH3)
174.6 (CdO), 63.6 (NCHCH3), 58.1 (OCH3), 42.7 (NCH), 2 × 27.6, 27.5, 27.3 (c-Hex CH2)), 17.6 (NCHCH3)
212.6, 212.5 (CO), 181.3 (CdO), 63.9 (NCH), 61.6 (i-Pr CH), 56.1 (OCH3), 22.8, 22.2 (i-Pr CH3)
218.8 (CO), 178.8 (CdO), 68.7 (NCH2), 52.5 (OCH3), 49.1 (NCH), 33.9 (C(CH3)3), 28.1 (C(CH3)3)
218.5, 215.5 (CO), 178.8, 177.3 (CdO), 59.4, 59.2 (i-Pr CH), 52.4, 52.3 (OCH3), 42.7, 41.3 (NCH), 25.3, 25.0, 23.1,
23.0 (i-Pr CH3)
10′b
10c
219.3 (CO), 178.8 (CdO), 59.3 (i-Pr CH), 52.5 (OCH3), 44.3 (NCH), 25.6, 21.3 (i-Pr CH3)
218.7, 215.7 (CO), 178.1, 176.9 (CdO), 61.3, 60.8 (OCH2CH3), 2 × 59.1 (i-Pr CH), 43.5, 41.5 (NCH), 25.2, 24.9, 23.0,
22.9 (i-Pr CH3), 14.9, 14.8 (OCH2CH3)
10′c
10d
219.2 (CO), 179.2 (CdO), 61.1 (OCH2CH3), 59.1 (i-Pr CH), 43.3 (NCH), 25.5, 21.7 (i-Pr CH3), 15.0 (OCH2CH3)
218.9, 215.8 (CO), 177.4, 176.7 (CdO), 69.1, 67.8 (i-Pr OCH), 2 × 59.0 (i-Pr NCH), 44.3, 41.7 (NCH), 25.1, 24.8, 3 ×
23.0, 22.7, 22.4, 21.9 (i-Pr CH3)
10′d
11b
219.5 (CO), 179.0 (CdO), 68.2 (i-Pr OCH), 59.3 (i-Pr NCH), 44.7 (NCH), 25.6, 23.0, 22.6, 21.4 (i-Pr CH3)
211.7, 211.2 (CO), 177.4, 171.4 (CdO), 163.3 (NdCH), 64.3 (i-Pr CH), 53.2 (OCH3), 48.8, 48.7 (NCH; i-Pr CH), 24.8,
24.1, 23.4, 23.0 (i-Pr CH3)
212.7, 211.6 (CO), 174.8, 172.0 (CdO), 163.7 (NdCH), 64.0 (i-Pr CH), 56.7 (OCH3), 52.9, 46.9 (NCH; i-Pr CH), 24.9,
24.3, 23.0, 22.7 (i-Pr CH3)
11′bd
11c
12f
211.8, 211.2 (CO), 176.8, 171.4 (CdO), 163.2 (NdCH), 64.5 (i-Pr CH), 61.9 (OCH2CH3), 49.5, 48.7 (NCH; i-Pr CH),
24.8, 24.0, 23.4, 22.6 (i-Pr CH3), 14.8 (OCH2CH3)
258.6 (bridging CO), 212.8, 210.9 (CO), 183.2 (CdO), 66.4 (NCH), 60.1 (C(CH3)3), 55.8 (OCH3), 34.7, 31.4, 29.3 (3 ×
CCH3)
12g
12h
12i
258.9 (bridging CO), 213.0, 211.0 (CO), 182.7 (CdO), 66.7 (NCH), 65.1 (OCH2), 60.1 (C(CH3)3), 34.8, 31.6, 29.4 (3 ×
CCH3), 14.6 (OCH2CH3)
261.3 (bridging CO), 213.0, 211.0 (CO), 182.2 (CdO), 73.4 (i-Pr CH), 66.7 (NCH), 60.0 (C(CH3)3), 34.7, 31.6, 29.3 (3
× CCH3), 22.1, 21.0 (i-Pr CH3)
258.9, 258.2 (bridging CO), 213.1, 212.9, 210.9, 210.5 (CO), 183.6, 183.3, 183.0, 182.7 (CdO), 66.3, 65.8, 65.7 (3 ×
NCH), 63.2, 62.8, 62.6 (3 × C(CH3)3), 55.7 (OCH3), 38.0, 36.9, 34.0 (3 × CH2CH3), 30.8, 30.7, 28.1, 27.9, 25.8, 24.7 (6
× NCCH3), 12.0, 11.7, 11.4 (3 × CH2CH3)
a
b
δ values in ppm relative to TMS, measured in CDCl3 at 263 K and 75.47 MHz. Measured at 243 K, ppm value in parentheses
measured at 293 K. c Measured at 293 K. Measured at 243 K.
d
resonances appear as a broad singlet, indicating that
in these compounds the N-H bond, the i-Pr C-H (8b,c)
or C5H10 C-H (8e) bond, and the central C-H bond are
nearly orthogonal. The observation that in 8d the N-H
proton couples with both the i-Pr C-H and less so with
the proton on the central carbon is probably due to the
increased bulkiness of the isopropyl ester, forcing 8d
into a different conformation than 8b,c,e.
212.6 ppm: i.e., analogous to the resonances observed
for the CO ligands in the isostructural Fe2(CO)4(R-
ADO).40
1
Com p lexes 10a -d /10′a -d . The H NMR spectra of
complexes 10 are reported in benzene-d6 because of
considerable overlap of the signals in CDCl3. As ex-
pected, the symmetric complexes 10′a -d show, both in
1
their H and 13C NMR spectra, only one set of signals,
Com p lex 9c. The ester carboxyl carbon atom reso-
nances (181.3 ppm) of 9c are shifted ca. 10 ppm to
higher frequency compared to those in 7c. The positions
of these resonances compare well with those observed
for the corresponding carbon atoms in 12, indicating
Fe-OdC coordination. The four terminal CO ligands
are observed as two closely spaced resonances at ca.
while the asymmetric complexes 10b-d give rise to two
separate sets of signals. The N-H protons in 10′a show
a large 3J coupling with one of the diastereotopic
methylene protons of the neopentyl group (11.1 Hz) and
in 10b-d /10′b-d with the isopropyl methine proton (ca.
12 Hz). Also, the additional 3J coupling with the proton
on the metalated carbon atom is relatively large (ca. 7

3028 Organometallics, Vol. 19, No. 16, 2000
Siebenlist et al.
Sch em e 4. P r op osed Rea ction Mech a n ism for th e
F or m a tion of 6
Hz). This indicates that the N-H bond is almost
coplanar with one of the methylene C-H bonds (10′a )
or the isopropyl methine C-H bond (10b-d /10′b-d )
and with the metalated C-H bond, which is consistent
with the solid-state structure of 10b (i.e., dihedral
angles of 175(4)° (HC(1)-N(1)-H) and 151(5)° (HC(4)-
N(1)-H)). For a formal Fe(II) complex, the resonances
of the terminal carbonyl carbon atoms are observed at
rather low field (219.3-215.5 ppm), indicating a large
degree of π-back-donation. The cyclometalated carbon
atoms in complexes 10/10′ resonate between 49 and 41
ppm: i.e., normal ppm values for sp3-cyclometalated
carbon atoms.59
Com p lexes 11b,c. The imine protons and the imine
carbon atom resonances of complexes 11 are observed
between 7.77 and 7.71 ppm and at ca. 163 ppm; i.e., no
significant coordination shift is observed. This indicates
that the coordination of the imine consists mainly of σ-
(N) donation, which is also evident from the CdN bond
length of 1.271(3) Å in the solid-state structure of 11b.
The N-H proton in 11b,c/11′b,c gives rise to a broad
double doublet (pseudotriplet) at 3.51 and 3.15 ppm,
respectively. Analogously to complexes 10, the N-H
protons show large coupling constants with both the
isopropyl methine proton (10 Hz) and the proton (10 Hz)
on the cyclometalated carbon atom. When 11c is syn-
thesized in the presence of D2O instead of H2O, the
signal is reduced to approximately 70% of its normal
intensity in the 1H NMR spectrum, while in the 2H
NMR spectrum a weak signal at 3.1 ppm is observed,
which proves that the proton on nitrogen originates from
water. The reduction of the signal by only 30% is
probably due to reexchange of deuterium for hydrogen
during workup on silica gel. The terminal carbonyls of
complexes 11 resonate between 213 and 211 ppm, a shift
of ca. 4 ppm to lower frequency in comparison with those
in complexes 10, reflecting a decrease in π-back-dona-
tion which is also observed in the IR data.
carbonyl ligand resonances are observed between 258
and 261 ppm, which are normal ppm values for bridging
CO ligands.77 The resonances of the coordinated car-
boxyl carbon atoms are observed at ca. 182-183 ppm:
i.e., a high-frequency coordination shift of approximately
20 ppm compared to those of the free ligand, indicating
that the ester CdO donor sites are coordinated and act
as σ-donors.
Com p lex F or m a tion /Rea ction s of r-Im in o Ester s
w ith Ca r bon ylir on . The formation of the complexes
described above depends strongly on the imine nitrogen
R1 substituent and the solvent. In the following we will
discuss the possible mechanisms for the formation of
complexes 6-12 and their reactivity.
F or m a tion of F e(CO)3(r-im in o ester ) (5). Reaction
of the R-imino esters a -i with Fe2(CO)9 at room tem-
perature in toluene or THF leads initially to the
formation of the monodentate N-coordinated intermedi-
ate Fe(CO)4[N(R1)dC(H)C(R3)dO] (4), as result of the
better σ-coordinating properties of nitrogen compared
with oxygen54,55 (cf. Scheme 3). With all ligands, inter-
mediate 4 reacts further to complexes 5: i.e., CO
substitution and coordination of the ester carbonyl. The
formation of the chelate complexes is indicated by the
deep purple (5a -e) or deep blue (5f-i) reaction mix-
tures, analogous to the intensely colored Fe(CO)3(R-
DAB) and Fe(CO)3(R-imino ketone) complexes. Unfor-
tunately, however, complexes 5 could not be isolated due
to their instability and reactivity, resulting in the
formation of complexes 6 and 12. The existence of
complexes 5 has been proven by in situ 1,3-dipolar
cycloaddition reactions of 5 with DMAD, resulting in
Fe(CO)3(butenolide) complexes,35 analogous to the for-
mation of complexes 3 (cf. Scheme 1: X ) O; R1 ) n-Pr,
i-Pr, t-Bu, t-Am; R3 ) OMe, OEt, O-i-Pr).
The 1H NMR of 11b, measured immediately after
column chromatography, revealed that a second isomer
was present 11′b, in approximately 6% yield. The given
1H and 13C spectral data of 11′b are derived from this
mixture.
Com p lexes 12. The 1H NMR spectra of complexes
12 show at room temperature very broad resonances for
the t-Bu and t-Am substituents on nitrogen, indicating
that rotation around the N-C bond is hindered, prob-
ably due to interaction with the bridging carbonyl.
Cooling solutions of complexes 12f-h to 243 K results
in three separate sharp signals for the tert-butyl methyl
groups, showing that at this temperature the rotation
of the t-Bu group is frozen in. The limit of fast exchange
could not be reached due to some decomposition at 363
K: i.e., formation of paramagnetic species. As a result
of the hindered rotation around the N-C bond, complex
12i (t-Am) is present as distinguishable isomers. The
13C NMR clearly shows three sets of resonances in
approximately a 1:3:5 ratio, and in the proton spectra
three isomers can also be distinguished, although the
assignment is difficult due to overlap of the signals. J ust
as in complexes 7, the former imine protons and imine
carbon atoms are shifted to lower frequency by ca. 3.3
and 80 ppm, respectively, showing that the former imine
carbon atoms are reductively coupled. The bridging
F or m a tion of F e2(CO)6(r-im in o ester ) (6a -i).
Further reaction of Fe(CO)3(R-imino ester) (5) with Fe-
(CO)4 in THF, toluene, or pentane results in the forma-
tion of the known type of binuclear Fe2(CO)6(R-imino
ester) complexes 6 (cf. Scheme 4).
Complexes 6 are probably formed via a binuclear CO-
bridged intermediate (A). A similar intermediate has
been proposed for the formation of Fe2(CO)6(R-DAB)78
and Fe2(CO)6(R-imino ketone).40 The formation of A can
(77) Fru¨hauf, H.-W. J . Chem. Res., Miniprint 1983, 2035.
(78) Fru¨hauf, H.-W.; Breuer, J . J . Organomet. Chem. 1984, 277, C13.

Carbonyliron Complexes of 1-Aza-4-oxa-1,3-butadiene
Organometallics, Vol. 19, No. 16, 2000 3029
be described as isolobally related79,80 to the addition of
a carbene to an olefin giving a cyclopropane ring system;
i.e., in the present case Fe(CO)4 is isolobal with a
carbene and adds to the FedC double bond (isolobal
with an olefin) in resonance structure 5*, which results
in the formation of intermediate A. Subsequent π-coor-
dination of the CdN double bond, i.e., formation of the
µ2(N),η2(CdN) bridge, with concomitant extrusion of CO
results in the formation of complexes 6. Complexes 6
are isolated as red-brown slightly air-sensitive crystal-
line solids. The R-imino esters in complexes 6 are µ2-
(N),σ(O):η2(CdN) coordinated to the two iron centers
and donate six electrons, analogous to the coordination
of R-imino ketones in Fe2(CO)6(R-imino ketone).39,40
Complexes 6 are formed in low yields and in general
the yields decrease somewhat on going from ligand a
via b-e to ligands f-i. This is in agreement with the
decreasing propensity for η2(CdN) coordination on
increasing bulkiness of the R1-imine nitrogen substitu-
ent, a (RCH2), b-e (R2CH), f-i (R3C), a trend also
observed for R-DAB81 and R-imino ketones.40 The yields
of complexes 6 in THF and toluene are almost identical,
while the reaction in pentane leads to slightly lower
yields. With chiral ligand e two diastereomeric com-
plexes 6e and 6e′ are formed in a 3:1 ratio due to the
imine carbon, which becomes chiral upon η2-CdN
coordination.
Rea ctivity of Com p lexes 6. Complexes 6 do not
react thermally or photochemically with an excess of
free ligand. Consequently, we can exclude 6 as an
intermediate in the formation of the binuclear com-
plexes 7 and 12. Furthermore, complexes 6 do not react
with activated alkynes such as DMAD. This is in
contrast with the rich chemistry of M2(CO)6(R-DAB) (M
) Fe, Ru) with activated alkynes and other unsaturated
substrates, leading to many interesting C-C (Ru) and
C-N (Fe) coupled products.19,21,82 Reaction of complexes
6 with CO (1-50 atm) at room temperature or at
elevated temperatures, leads to decomposition of 6, and
the monomeric complexes 5 are not detected, as is the
case for Fe2(CO)6(R-DAB).83 The η2(CdN) coordination
is probably stronger than the σ(O) coordination, and as
a result, the coordinated oxygen atom is substituted by
CO, which triggers the decomposition of 6.
Sch em e 5. Obser ved In ter con ver sion s of 7, 9, a n d
12
of 12f-i, however, leads to decomposition of 12f-i and
complexes 9f-i are not observed.
The different reactivities show that complexes 7 and
12 are most likely formed via different reaction paths,
although they contain the same C-C coupled ligand.
Complexes 7 are exclusively formed with the less bulky
R1-substituted ligands a -e, and complexes 12 are
exclusively formed with the more bulky R1-substituted
ligands f-i. Apparently, the R1 substituent has a
decisive influence on the involved intermediates, thus
giving rise to the observed product selectivity.
Upon reaction of the bulky N-R1 substituted ligands
f-i with Fe2(CO)9 the reaction mixture becomes colored
more quickly and much more intensely than with the
less bulky N-R1 substituted ligands a -e. This is
indicative of a faster formation, and of higher concen-
trations, of the chelate complexes 5f-i in comparison
with 5a -e. This is also reflected in the much higher
yields obtained for the in situ 1,3-dipolar cycloaddition
reaction of 5f-i (54-68%) than for 5a -d (18-30%).35
The faster reaction from 4 to 5 (chelation) for ligands
f-i can possibly be explained by steric interactions.
From the crystal structure of Fe(CO)4[MeNdC(H)C(H)d
84
C(H)Ph]
it is known that the N-methyl substituent
interacts sterically with one of the equatorial CO
ligands. For the bulky N-R1 substituted ligands f-i this
will lead to even stronger interactions between the
equatorial CO ligands and the R1 substituent, forcing
the ester carbonyl toward the iron, thus resulting in a
faster reaction to the chelate complexes 5f-i.
F or m a tion a n d Str u ctu r e of 12. The fast formation
of the chelate complexes 5f-i results in high stationary
concentrations of 5f-i, and the fact that the carbonyl
ester functions are coordinated in 12f-i strongly points
to a reaction mechanism in which the chelate complex
5 plays an important role. Therefore, complexes 12 are
suggested to be formed via intermediate B, similar to
the proposed reaction mechanisms for the formation of
the isostructural Ru2(CO)4,5(R-ADA)10 and Fe2(CO)4(R-
ADO)40 (cf. Scheme 6). J ust as for the formation of
intermediate A (cf. Scheme 4), the formation of inter-
mediate B can be seen as an isolobal analogue to the
reaction between a carbene and an olefin. In the present
case, dissociation of one of the terminal CO ligands in
F or m a t ion of F e2(CO)6(R 1,R 3-ODA) (7a -e) a n d
F e2(CO)5(R1,R3-ODA) (12f-i). When heated at reflux
(120 °C) or irradiated in toluene at room temperature,
complexes 7b-d lose two CO ligands with exclusive
formation of 9b-d (cf. Scheme 5) and complexes 12b-d
are not detected. Furthermore, complexes 12f-i do not
react to the corresponding complexes 7f-i when heated
to 110 °C in toluene under an atmosphere of CO (1 bar).
Refluxing a toluene solution of 12f-i under N2 leads to
decomposition (see Experimental Section), without for-
mation of the corresponding complexes 9f-i: i.e., loss
of CO in 12. The latter reaction has been reported for
Ru2(CO)5(R-ADA) (R ) t-Bu, i-Pr, p-Tol, c-Hex),10 re-
sulting in Ru2(CO)4(R-ADA). For R ) t-Bu, this reaction
proceeds only photochemically in good yield. Irradiation
(79) Hoffmann, R. Science 1981, 211, 995.
(80) Hoffmann, R. Angew. Chem. 1982, 94, 725.
(81) Staal, L. H.; Polm, L.; Vrieze, K.; Ploeger, F.; Stam, C. H. Inorg.
Chem. 1981, 20, 3590.
(82) Staal, L. H.; van Koten, G.; Vrieze, K.; van Santen, B.; Stam,
C. H. Inorg. Chem. 1981, 20, 3598.
(83) Fru¨hauf, H.-W.; Wolmersha¨user, G. Chem. Ber. 1982, 115, 1070.
(84) Nesmeyanov, A. N.; Rybin, L. V.; Stelzer, N. A.; Struchkov, Y.
T.; Batsanov, A. S.; Rybinskaya, M. I. J . Organomet. Chem. 1979, 182,
399.

3030 Organometallics, Vol. 19, No. 16, 2000
Siebenlist et al.
Sch em e 6. P r op osed Rea ction Mech a n ism for th e
F or m a tion of 12
Sch em e 7. P ossible Mech a n ism for th e F or m a tion
of 7
5f-i leads to Fe(CO)2(R-imino ester), a d8 ML4 fragment
like Fe(CO)4 and thus isolobal with a carbene. Subse-
quent addition to the FedC bond in 5* (isolobal with
an olefin; Scheme 4) gives intermediate B, in which, in
contrast to intermediate A, two σ(N):σ(O) coordinated
R-imino ester ligands are present. Reductive C-C
coupling of the imine carbons results, after breakage of
the metal-metal bond, in the formation of 12.
Though it is rather unusual to have a bridging
carbonyl between two formally unbonded metal atoms,
we are fairly confident in the structure of 12 as shown
in Scheme 3 for the following reasons. The X-ray
diffraction data sets of two measured (twinned) com-
plexes 12 do confirm the topology, which was based on
the IR and NMR spectroscopic data, and indicate an
Fe-Fe bond length of 2.60 Å. This is indeed a distance
normally found for bonding interactions. However, in
the isostructural complex Ru2(CO)4(R-ADA),10 which
also has the CO-bridged ruthenium atoms at a normally
bonding distance of 2.873(2) Å, it could be shown by
temperature-dependent UV-vis spectroscopy that no
direct metal-metal bond is present.
no intermediates can be observed. Complexes 7 could,
in line with the proposed mechanisms for the formation
of 6 and 12, be formed via a similar type of reaction
mechanism (cf. Scheme 7). In this case, Fe(CO)3[(R1)Nd
C(H)-C(R3)dO] (d8 ML4 fragment) is isolobal with a
carbene and adds to the FedC double bond (isolobal
with an olefin) in resonance structure 4*, which results
in the formation of intermediate C.
During the formation of complexes 7 the two prochiral
imine carbon atoms become chiral centers. De Cian et
al.39 have shown that the chiral centers are coupled and
only the R,R and S,S combinations are formed. Like-
wise, reaction with ligand e, bearing a chiral (R)-
cyclohexylethyl substituent on nitrogen, may lead to two
diastereoisomers, only one of which is observed. Similar
high diastereomeric ratios are obtained with N-cyclo-
hexylethyl-substituted ligands in 1,3-dipolar cycloaddi-
tions.86
For m ation of Fe(CO)2(R1N(H)C(H)C(O)R3)2 (10a-
d /10′a -d ). Complexes 10 are formed in low yield on
reaction of the R-imino esters a -d with Fe2(CO)9 at
room temperature in THF. In toluene 10 is formed only
in traces (3%), and when pentane is used as a solvent,
it is not observed. It is obvious that some electron-
transfer step has taken place and the imine double bond
has been hydrogenated, presumably by hydrogen ab-
straction from the solvent, because the yields parallel
the hydrogen donor properties of the solvent.87,88 To
confirm this, a small-scale reaction was carried out in
THF-d8 (1 mL). The result of this experiment tends to
support the assumption that THF is the hydrogen
donor, since the N-H resonance in the 1H NMR
spectrum is reduced to 80% of the intensity, although
reservations have to be made because considerable line
broadening hampered the interpretation. The partial
exchange of deuterium for a proton on nitrogen is
probably caused by workup on silica gel.
Interestingly, with the ligands a -e complexes 12a -e
are not formed while the mononuclear chelate complexes
5a -e are definitely present, although in lower station-
ary concentrations. Apparently, a high concentration of
the chelate 5 is necessary to induce the formation of 12.
This is in agreement with the observation that Fe2(CO)5-
(c-Hex-DAB)2, i.e., isostructural to intermediate B, is
only formed at rather high concentrations of Fe(CO)3-
(c-Hex-DAB)85 upon irradiation (λ < 488 nm) in pentane
at 150 K, while at low concentrations decomposition is
observed. Furthermore, this explains the lower yields
observed for the formation of 12 in toluene; i.e., due to
the limited solubility of Fe2(CO)9 in toluene compared
to THF, lower concentrations of 5f-i are produced and
accordingly complexes 12 are formed in low yield. Upon
heating, Fe2(CO)5(c-Hex-DAB)2 reacts back to Fe(CO)3-
(c-Hex-DAB), indicating that in this case the imine
carbon atoms are apparently not sufficiently activated
to undergo C-C coupling.
Like complexes 7, complexes 10 are only formed with
the less bulky ligands a -d . Therefore, they are probably
F or m a tion of 7. Complexes 7a -e are most likely
formed starting from the monodentate N-coordinated
R-imino ester intermediate complex 4, which has, like
7, a pendant ester moiety. It is, however, difficult to
propose a mechanism for the formation of 7a -e, because
(86) Feiken, N.; Schreuder, P.; Siebenlist, R.; Fru¨hauf, H.-W.; Vrieze,
K.; Kooijman, H.; Veldman, N.; Spek, A. L.; Fraanje, J .; Goubitz, K.
Organometallics 1996, 15, 2169.
(87) van Wijnkoop, M. Ph.D. Thesis, University of Amsterdam, 1992.
(88) Kidd, D. R.; Cheng, D. P.; Brown, T. L. J . Am. Chem. Soc. 1978,
100, 4103.
(85) van Dijk, H. K.; Stufkens, D. J .; Oskam, A. J . Am. Chem. Soc.
1989, 111, 541.

Carbonyliron Complexes of 1-Aza-4-oxa-1,3-butadiene
Organometallics, Vol. 19, No. 16, 2000 3031
also formed via intermediate 4. However, it is unknown
via which mechanism complexes 10 are formed, except
that at some stage of the reaction an electron-transfer
step is involved.
line with the 1,3-dipolar reactivity of the chelate Fe-
(CO)3(R-imino ketone); i.e., exclusive addition over the
Fe-OdC fragment is observed. From this it might be
concluded that the reactivities for 1,3-dipolar cycload-
ditions and the C-C coupling reactions discussed here
are possibly determined by the same factors. This is
corroborated by the enhanced reactivity of Ru(CO)3(R-
DAB) in both 1,3-dipolar cycloaddition23 and C-C
coupling reactions. In both types of reactions, steric
interactions also play an important role. When one of
the two imine C atoms in R-DAB bears a methyl
substituent, R1NdC(Me)C(H)dNR1, either type of reac-
tion exclusively takes place at the unsubstituted imine
carbon atom.
Complexes 10 are obtained as two isomers, an asym-
metric (10) and a symmetric (10′) isomer, in a ratio of
ca. 6:1 except for 10′a , which is formed as the symmetric
isomer exclusively. The ratio of the two isomers does
not change on heating to 60 °C, while further heating
results in decomposition. The observation that with
ligand a exclusively the symmetric isomer is formed,
and with the ligands b-d the asymmetric isomer is
favored, has to be ascribed to the steric requirements
of the substituent R1 on nitrogen. In the symmetric
complex the ester group of one of the ligands and the
nitrogen R1 substituent of the other ligand are pointing
toward each other; i.e., they sterically interact. For the
neopentyl-substituted ligand a this interaction will be
minimal because the neopentyl group can easily bend
away. For the more bulky isopropyl-substituted ligands
b-d , however, the steric interaction is apparently such
that they preferentially form the asymmetric complexes
10b-d .
Complexes 10/10′ are isolated as yellow powders and
are slightly air sensitive in crystalline form. Reaction
of 10/10′ with H2O results in decomposition, and forma-
tion of 11 is not observed. Complexes 10 do not insert
unsaturated molecules such as alkenes or alkynes into
the strained M-C bond, as is observed for the (η2-
pyridyl)MCp1,2 (M ) Ti, Zr, Y, Sc, Lu) complexes.57,58
F or m a tion of F e(CO)2(σ(N):σ(C)-R1NC(H)C(O)-
R3)(R1NdC(H)C(O)O) (11). Reaction of the R-imino
esters b and c with Fe2(CO)9 at room temperature in
THF in the presence of H2O leads to the formation of
11b/11′b (12:1) and 11c in 14 and 20% yields, respec-
tively. Since there are no observable intermediates, a
detailed reaction mechanism cannot be given. However,
it is plausible that 11 is formed via a route similar to
that for 10, with H2O in this case as hydrogen donor.
This was confirmed by the reaction with D2O (see NMR
Spectroscopy).
The observation that with the R-imino ketones R1Nd
C(H)C(Ph)dO C-C coupling of the ketone-C atoms is
not observed is probably due to electronic reasons as
well as steric reasons. It is known that the 1,3-dipolar
reactivity of the ketone carbon atom in Fe(CO)3(R-imino
ketone) is lowered on substitution with an electron-
withdrawing substituent (R3 ) Ph vs Me).89,90 Therefore,
the introduction of the electron-withdrawing alkoxy
substituents (R3 ) O-alkyl) on the ketone carbon atom
might be the reason for the change in reactivity of
R-imino esters compared to that of R-imino ketones: i.e.,
coupling of the imine or ketone carbon atoms. This is
corroborated by the in situ cycloaddition reactions of
5a -i, which result in cycloaddition across both the Fe-
35
OdC and Fe-NdC fragments.
It must be empha-
sized, however, that the Fe-OdC fragment is still more
reactive in this case. Furthermore, steric interactions
of the O-alkyl substituents could prevent C-C coupling
of the carboxyl carbon atoms.
However, why in the case of iron C-C coupling of the
imine-C atoms is not observed for R-DAB, R-Pyca, and
R1NdC(H)C(Ph)dO is not clear. Apparently, the imine
carbon atoms in these systems are not activated suf-
ficiently for C-C coupling, whereas they are in the
R-imino ester system.
Ack n ow led gm en t. The investigation was supported
in part (A.L.S.) by The Netherlands Foundation for
Chemical Research (SON) with financial aid from The
Netherlands Organization for Scientific Research (NWO).
Com p a r ison of Differ en t Ir on Ca r bon yl/1,4-Di-
h eter oa tom -1,3-bu ta d ien e System s in C-C Cou -
p lin g Rea ction s. We have shown above that with
R-imino esters C-C coupling is only observed between
the imine carbon atoms of two R-imino esters. For the
closely related R-imino ketones R1NdC(H)C(R3)dO and
C5H4N-2-C(R3)dO (R1 ) t-Bu, t-Am; R3 ) Me, Ph),
however, exclusive coupling of the methyl-substituted
ketone carbon atoms is observed.40 In the case of R-DAB
and R-Pyca, C-C coupling is not observed at all for Fe,
while for Ru9,10,18 these types of C-C coupling reactions
occur readily, resulting in either Ru2(CO)4,5(R-ADA) or
Ru2(CO)4,5(R-APE).1
Su p p or tin g In for m a tion Ava ila ble: Listings of crystal-
lographic data for the structure determinations of compounds
10c and 11b, including final coordinates of non-hydrogen
atoms, hydrogen atom positions, (an)isotropic thermal param-
eters, bond distances and angles, and torsion angles. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OM9910266
(89) Houk, K. N.; Sims, J .; Watts, C. R.; Luskus, L. J . J . Am. Chem.
Soc. 1973, 95, 7301.
(90) Houk, K. N.; Sims, J .; Duke, R. E.; Strozier, R. W.; George, J .
K. J . Am. Chem. Soc. 1973, 95, 7287.
The observation that the R-imino ketones exclusively
couple on the methyl-substituted ketone-C atoms is in