Iron-carbene functionalities supported by a macrocyclic ligand: Iron-carbon double bond stabilized by tetramethyldibenzotetraazaannulene

Article abstract of DOI:10.1039/a704736c

We report an unprecedented entry to the chemistry of the iron-carbene functionality supported by an easily available macrocycle.

Full text of DOI:10.1039/a704736c

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Iron–carbene functionalities supported by a macrocyclic ligand: iron–carbon
double bond stabilized by tetramethyldibenzotetraazaannulene
Alain Klose,a Euro Solari,a Carlo Floriani,*a Nazzareno Re,b Angiola Chiesi-Villac and Corrado Rizzolic
a Institut de Chimie Mine´rale et Analytique, BCH, Universite´ de Lausanne, CH-1015 Lausanne, Switzerland
b Dipartimento di Chimica, Universita` di Perugia, I-06100 Perugia, Italy
c Dipartimento di Chimica, Universita` di Parma, I-43100 Parma, Italy
We report an unprecedented entry to the chemistry of the
iron–carbene functionality supported by an easily available
macrocycle.
derived from the reaction of 2 with diazoalkanes is strongly
dependent on the substituents at the carbene carbon. In the case
of PhCHN2, the carbene derivative forms only at low tem-
perature. It decomposes at room temperature leading to the
starting material and trans-stilbene (81%) and cis-stilbene
(19%), while in the presence of dioxygen it forms PhCHO and
complex 4. Owing to its diamagnetism, 3 has been readily
characterized by 1H and 13C NMR spectroscopies and structural
details have been revealed by X-ray analysis‡ (see Fig. 1). The
coordination environment of iron is a distorted tetragonal
pyramid, unlike iron–prophyrin2a or osmium–porphyrin6b ex-
amples, where the metal is always six-coordinate. The metal is
displaced by 0.335(1) Å from the N4 average plane. The Fe–
C(23) vector is perpendicular to the N4 core, the dihedral angle
with the normal to the N4 plane being 1.0(1)°. The carbene plane
C(23)C(31)C(41) is almost parallel to the N(2)···N(4) vector,
the torsion angles C(31)–C(23)–Fe–N(4) and C(41)–C(23)–Fe–
N(2) being 215.5(3) and 215.2(3)°, respectively. The Fe–
C(23) bond distance [1.794(3) Å] is particularly short compared
to the only available iron–porphyrin [FeNCCl2 1.83(3) Å]2a or
organometallic derivatives of iron, where it ranges from
1.978(3) to 1.85(3) Å.9 The question whether the oxidation state
of iron in 3 is +2 or +4 can be reasonably answered, in the
absence of Mo¨ssbauer measurements, by considering the Fe
distance from the N4 plane. A correlation in tmtaa complexes
has been established between the out of plane distance of the
metal and its dn configuration.7b The value found in the present
case [0.335(1) Å] is expected for low-spin d6 five-coordinate
iron(ii), e.g. 0.29 Å in [Fe(tmtaa)(CO)].8c Much longer
distances would be found for a d4 configuration. The extended
The metal–carbene unit is a key functionality both in organome-
tallic and organic chemistry.1 It is usually bonded to conven-
tional ancillary ligands, with a few remarkable exceptions in
metal–porphyrin chemistry,2–6 and particularly in the case of
iron2 and ruthenium.2b,3,5 Despite of considerable effort in this
field, owing to the relevance of metal–porphyrin carbene
derivatives in chemistry and biology,2,3 some facts are partic-
ularly surprising: (i) there is an absence of any modeling
approach in macrocycle chemistry, i.e. the use of readily
available macrocycles; (ii) the potential of the metal–carbene
bonded to a macrocyclic structure is still to be explored; (iii)
very few (three, at present) metal–porphyrin carbenes have been
structurally characterized.2a,6
Here, we report a straightforward synthesis of iron–carbenes
stabilized by the easily accessible tetramethyldibenzo-
tetraazaannulene ligand (H2tmtaa)7 along with their structural
characterization in solution and in the solid state. The
availability of tetramethyldibenzotetraazaannulene encourages
the use of [Fe(tmtaa)] for developing iron–carbene chemistry
based on this and other macrocycles. The synthesis was
performed by reacting 28a with the corresponding diazoalkane
(Scheme 1).
The reaction of 2 with Ph2CN2 in THF at 230 °C, and then
at room temperature, gave high yields of 3 as a green crystalline
solid.† Complex 2 showed a very high thermal stability and
resistance to hydrolysis. The reaction of 3 with dioxygen gave,
almost quantitatively, benzophenone and the well known m-oxo
dimer 4.7a The stability and the nature of the final compound
C(35)
C(45)
C(34)
C(36)
C(46)
C(44)
NH
N
N
N
N
N
N
C(33)
LiBu
C(41)
C(31)
C(23)
Fe
FeCl2(thf)1.5
C(43)
C(32)
HN
C(9)
C(19)
C(20)
C(42)
C(10)
C(18)
H2tmtaa
[Fe(tmtaa)]
C(8)
C(11)
C(17)
C(22)
1
2
N(4)
C(7)
Fe
N(3)
C(21)
Ph2CN2
C(6)
Ph
Ph
N(2)
C
C(15)
N(1)
C(13)
C(12)
C(14)
C(16)
Fe
C(4)
C(2)
N
N
O2
C(3)
C(5)
C(1)
Ph2C=O + [(tmtaa)Fe–O–Fe(tmtaa)]
N
N
4
Fig. 1 ORTEP drawing of complex 3 (50% probability ellipsoids. Selected
interatomic distances (Å) and angles (°): Fe–N(1) 1.921(3), Fe–N(2)
1.926(2), Fe–N(3) 1.911(3), Fe–N(4) 1.920(2), Fe–C(23) 1.794(3),
C(23)–C(31) 1.487(5), C(23)–C(41) 1.472(4); N(3)–Fe–N(4) 94.6(1),
N(2)–Fe–N(4) 163.3(1), N(2)–Fe–N(3) 82.7(1), N(1)–Fe–N(4) 82.3(1),
N(1)–Fe–N(3) 156.4(1), N(1)–Fe–N(2) 93.6(1), Fe–C(23)–C(41) 122.6(2),
Fe–C(23)–C(31) 121.7(2), C(31)–C(23)–C(41) 115.7(3).
[(tmtaa)Fe=CPh2]
3
Scheme 1
Chem. Commun., 1997
2297

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Chemistry, ed. G. Wilkinson, F. G. A. Stone and E. W. Abel, Pergamon,
Oxford, vol. 12, 1995, ch. 5.1–5.5.
Hu¨ckel calculations further support the +2 oxidation state for
iron in 3 and the use of carbene rather than alkylidene
terminology for the present complexes. The tmtaa, which shows
the usual saddle shape conformation,7 has structural parameters
quite close to those reported for the other iron complexes.8
We thank the ‘Fonds National Suisse de la Recherche
Scientifique’ (Grant No. 20-46’590.96), Ciba Speciality Chem-
icals (Basel, Switzerland), and ‘Fondation Herbette’ (Univer-
sity of Lausanne, Switzerland) for financial support.
2 (a) D. Mansuy, M. Lange, J. C. Chottard, J. F. Bartoli, B. Chevrier and
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7 Some leading references to the use of [tmtaa]22 as an ancillary ligand in
the organometallic chemistry of the early transition metals are: (a)
C. H. Jang, J. A. Ladd and V. L. Goedken, J. Coord. Chem., 1988, 18,
317; (b) S. De Angelis, E. Solari, E. Gallo, C. Floriani, A. Chiesi-Villa
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Footnotes and References
* E-mail: carlo.floriani@icma.unil.ch
† Preparation of 3: To a cooled (230 °C) THF solution (140 ml) of
[Fe(tmtaa)]·thf8a (1.79 g, 3.80 mmol) was added dropwise a THF solution
(40 ml) of diphenyldiazomethane (0.78 g, 4.0 mmol). When warmed to
room temperature the solution became red. Nitrogen gas was slowly given
off. After stirring for 2 d, the green solution was concentrated in vacuo (20
ml) and then hexane (15 ml) added. A green crystalline solid was obtained
(1.70 g, 75%). Crystals suitable for X-ray analysis were grown in toluene
(Found: C, 74.53; H, 5.77; N, 9.91. C35H32FeN4 requires C, 74.47; H, 5.71;
N, 9.92%). 1H NMR (200 MHz, C6D6, 25 °C): d 6.88–6.76 (m, 8 H),
6.35–6.33 (m, 10 H), 5.13 (s, 2 H), 2.12 (s, 12 H). 13C NMR (100.6 MHz,
C6D6, 25 °C): d 313.2 (C carbene), 166.6, 157.9, 149.3, 127.1, 124.8, 120.9,
120.5, 119.7, 107.3, 67.8, 25.8, 23.2.
‡ Crystal data for 3: C35H32FeN4·C7H8, M = 656.7, triclinic, space group
¯
P1, a = 11.670(3), b = 13.593(4), c = 11.234(3) Å, a = 106.72(2),
b = 96.21(2), g = 77.62(2)°, U = 1665.0(8) Å3, Z = 2, Dc = 1.310 g
cm23, F(000) = 692, Mo-Ka radiation (l = 0.7109 Å), m(Mo-Ka) = 4.87
cm21: crystal dimensions 0.08 3 0.72 3 0.85 mm. The structure was solved
by the heavy atom method and anisotropically refined for all the non-H
atoms. All the hydrogen atoms were located from difference Fourier maps
and introduced as fixed contributors in the last stage of refinement
(Uiso = 0.05 Å2). For 4428 unique observed reflections [I > 2s(I)]
collected at T = 133 K on a Rigaku AFC6S diffractometer (5 < 2q < 50°)
and corrected for absorption the final R is 0.059 (wR2 = 0.163 for the 5293
reflections having I > 0 used in the refinement). All calculations were
carried out on a QUANSAN Personal Computer equipped with an INTEL
PENTIUM processor. CCDC 182/644.
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Received in Basel, Switzerland, 4th July 1997; 7/04736C
2298
Chem. Commun., 1997