Unusual iron(II) and cobalt(II) complexes derived from monodentate arylamido ligands

Article abstract of DOI:10.1021/ic700850e

The coordination chemistry of the N-substituted arylamido ligands [N(R)(C₆H₃R′₂-2,6)] [R = SiMe ₃, R′ = Me (L¹); R = CH₂Bu^t, R′ = Pr^j (L²)] toward Fe^II

Full text of DOI:10.1021/ic700850e

Inorg. Chem. 2007, 46, 7695−7697
Unusual Iron(II) and Cobalt(II) Complexes Derived from Monodentate
Arylamido Ligands#
Ho Yu Au-Yeung,† Chung Hei Lam,† Chi-Keung Lam,‡ Wai-Yeung Wong,§ and Hung Kay Lee*,†
Department of Chemistry, The Chinese UniVersity of Hong Kong, Shatin, New Territories, Hong
Kong SAR, People’s Republic of China, School of Chemistry and Chemical Engineering, Sun
Yat-Sen UniVersity, Guangzhou 51275, People’s Republic of China, and Department of Chemistry,
Hong Kong Baptist UniVersity, Waterloo Road, Kowloon, Hong Kong SAR, People’s Republic of
China
Received May 3, 2007
The coordination chemistry of the N-substituted arylamido ligands
[N(R)(C6H3R 2-2,6)] [R
SiMe3, R′ ) Me (L1); R CH2But, R
Pri (L2)] toward FeII and CoII ions was studied. The monoamido
complexes [M(L1)(Cl)(tmeda)] [M
Fe (1), Co (2)] react readily
with MeLi, affording the mononuclear, paramagnetic iron(II) and
CoII derivatives.8-10 Our recent work has been centered upon
the synthesis and reaction chemistry of metal amides derived
from chelating aryl- and alkylamido ligands.11-14 Earlier we
have reported on a bidentate monoanionic 2-pyridylamido
ligand system, [N(R)(2-C5H3N-6-Me)]- (R ) SiButMe2 or
SiButPh2), that displayed interesting complexation chemistry
with transition metals and the lanthanides.11,13,14 Continuing
our studies on low-valent transition-metal amides, we have
extended our work to the closely related monodentate
arylamido ligands [N(R)(C6H3R′2-2,6)]-, where R ) SiMe3
or CH2But and R′ ) Me or Pri. The latter ligand system has
been shown to be versatile in supporting a variety of main-
group, transition-metal, and lanthanide complexes.15 Nev-
ertheless, its chemistry with low-valent late transition metals
remains relatively less explored. Cummins and co-workers
have reported a few low-coordinate iron arylamido com-
plexes derived from N-(tert-hydrocarbyl)arylamido ligands.16
The reactivity of the latter complexes toward small mol-
ecules, such as NO, has also been investigated. In this
Communication, we report the initial results of our studies
on the complexation of the [N(R)(C6H3R′2-2,6)]- ligand
system with FeII and CoII ions. Noteworthy results of the
′
)
)
′
)
)
cobalt(II) methyl− )
arylamido complexes [M(L1)(Me)(tmeda)] [M
Fe (3), Co (4)]. Treatment of 2:1 [Li(L2)(THF)2]/FeCl2 affords the
unusual two-coordinate iron(II) bis(arylamide) [Fe(L2)2] (5).
Amide ligands [NR2]- belong to a class of versatile ligands
that form stable complexes with a wide spectrum of metals.1
Compared to the chemistry of early-transition-metal amides,
which has been extensively investigated, studies of amido
complexes of the later transition metals have received
relatively less attention.1-4 The bulky [N(SiMe3)2]- ligand
has been extensively studied and proved to be successful in
the isolation of divalent transition-metal amides with low
coordination numbers.5-7 Using sterically very crowded
silylamides [N(SiMenPh3-n)2]- (n ) 1, 2) and diarylboryl-
amides [NR(BR′2)]- (R ) C6H5, 2,4,6-Me3C6H2; R′ ) 2,4,6-
Me3C6H2, 2,6-Me2C6H3), Power and co-workers have suc-
cessfully prepared crystalline, two-coordinate MnII, FeII, and
(8) Power, P. P. Comments Inorg. Chem. 1989, 8, 177-202.
(9) Chen, H.; Bartlett, R. A.; Rasika Dias, H. V.; Olmstead, M. M.; Power,
P. P. J. Am. Chem. Soc. 1989, 111, 4338-4345.
# Dedicated to Prof. Thomas C. W. Mak on the occasion of his 71st
birthday.
* To whom correspondence should be addressed. E-mail: hklee@
cuhk.edu.hk.
(10) Chen, H.; Bartlett, R. A.; Olmstead, M. M.; Power, P. P.; Shoner, S.
C. J. Am. Chem. Soc. 1990, 112, 1048-1055.
† The Chinese University of Hong Kong.
(11) Lee, H. K.; Wong, Y.-L.; Zhou, Z.-Y.; Zhang, Z.-Y.; Ng, D. K. P.;
Mak, T. C. W. J. Chem. Soc., Dalton Trans. 2000, 539-544.
(12) Lee, H. K.; Peng, Y.; Kui, S. C. F.; Zhang, Z.-Y.; Zhou, Z.-Y.; Mak,
T. C. W. Eur. J. Inorg. Chem. 2000, 2159-2162.
(13) Lee, H. K.; Lam, C. H.; Li, S.-L.; Zhang, Z.-Y.; Mak, T. C. W. Inorg.
Chem. 2001, 40, 4691-4695.
(14) Kui, S. C. F.; Li, H.-W.; Lee, H. K. Inorg. Chem. 2003, 42, 2824-
2826.
‡ Sun Yat-Sen University.
§ Hong Kong Baptist University.
(1) Lappert, M. F.; Power, P. P.; Sanger, A. R.; Srivastava, R. C. Metal
and Metalloid Amides; Ellis Horwood: Chichester, U.K., 1980.
(2) Bryndza, H. E.; Tam, W. Chem. ReV. 1988, 88, 1163-1188 and
references cited therein.
(3) Fryzuk, M. D.; Montgomery, C. D. Coord. Chem. ReV. 1989, 95, 1-18
and references cited therein.
(15) For example, see: (a) Kennepohl, D. K.; Brooker, S.; Sheldrick, G.
M.; Roesky, H. W. Chem. Ber. 1991, 124, 2223-2225. (b) Schumann,
H.; Gottfriedsen, J.; Dechert, S.; Girgsdies, F. Z. Anorg. Allg. Chem.
2000, 626, 747-758. (c) Anwander, R. In Topics in Current
Chemistry; Herrmann, W. A., Ed.; Springer: Berlin, 1996; Vol. 179;
pp 33-112.
(4) Mayer, J. M. Comments Inorg. Chem. 1988, 8, 125-135.
(5) Bradley, D. C.; Fisher, K. J. J. Am. Chem. Soc. 1971, 93, 2058-
2059.
(6) Andersen, R. A.; Faegri, K.; Green, J. C.; Haaland, A.; Lappert, M.
F.; Leung, W.-P.; Rypdal, K. Inorg. Chem. 1988, 27, 1782-1786.
(7) Olmstead, M. M.; Power, P. P.; Shoner, S. C. Inorg. Chem. 1991, 30,
2547-2551.
(16) Stokes, S. L.; Davis, W. M.; Odom, A. L.; Cummins, C. C.
Organometallics 1996, 15, 4521-4530.
10.1021/ic700850e CCC: $37.00
Published on Web 08/17/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 19, 2007 7695

COMMUNICATION
Scheme 1
present work include the isolation of the mononuclear iron-
(II) and cobalt(II) methyl-arylamide complexes [M(L1)(Me)-
(tmeda)] [M ) Fe, Co; L1 ) N(SiMe3)(C6H3Me2-2,6)] and
the two-coordinate iron(II) bis(arylamide) [Fe(L2)2] [L2 )
N(CH2But)(C6H3Pri2-2,6)].
Figure 1. Molecular structure of 3 (30% thermal ellipsoid). Selected bond
lengths (Å) and angles (deg) with data for 4 in square brackets are as
follows: M ) Fe [Co], M-N1 1.965(4) [1.960(2)], M-C18 2.084(6)
[2.038(2)], N1-C1 1.423(6) [1.422(2)], N1-Si1 1.697(4) [1.715(2)]; M
) Fe [Co], C18-M-N1 127.4(2) [124.90(9)], C1-N1-Si1 118.7(3)
[118.5(1)], C1-N1-M 116.4(3) [116.4(1)], Si1-N1-M 122.6(2) [122.65-
(8)].
Our initial attempts to prepare iron(II) and cobalt(II) bis-
(amido) complexes of the L1 ligand via the reaction of 2
mol equiv of [Li(L1)(tmeda)] with anhydrous MCl2 (M )
Fe, Co) have been unsuccessful. In our hands, only the
monosubstituted iron(II) and cobalt(II) amides [M(L1)(Cl)-
(tmeda)] [M ) Fe (1), Co (2);17 (Scheme 1) were isolated,
despite the stoichiometry of the reagents and reaction
conditions (reaction temperature and solvent polarity) we
employed. Apparently, the steric stability of 1 and 2 prevents
them from undergoing further ligand metathesis reactions
to form the corresponding disubstituted derivatives.18 This
is consistent with our observations that both 1 and 2 were
unreactive toward the bulky [N(SiMe3)2]- ligand, notwith-
standing that they contain a terminal chloride ligand that may
allow further functionalization of the compounds. Accord-
ingly, we turned our attention to the reaction of 1 and 2 with
less hindered ligands. Toward this end, we chose a prototypi-
cal alkyl ligand in organometallic chemistry, namely, the least
hindered methyl ligand.
signals for the metal-bound methyl ligand could not be
observed, probably because of the broadness of the signals.
Figure 1 depicts the single-crystal X-ray structure of 3, which
is isomorphous with the CoII analogue 4 (see Figure S3 in
the Supporting Information).25 Complexes 3 and 4 exhibit a
distorted tetrahedral geometry. Compared to other four-
coordinate iron(II) and cobalt(II) methyl complexes, the Fe-
Me distance of 2.084(6) Å for 3 is comparable to that of
2.079(3) Å for [PhTptBu]FeMe20 but slightly longer than that
of 2.013(3) Å for [PhBPiPr3]FeMe.21 On the other hand, the
Co-Me distance of 2.038(2) Å for 4 is slightly shorter than
the corresponding distances of [TptBu]CoMe [Co-C ) 2.12-
(1) Å]22 and [PhTttBu]CoMe [Co-C ) 2.052(3) Å].23
However, they are longer than the Fe-Me and Co-Me
distances of 2.009(3) and 1.963(3) Å, respectively, in the
three-coordinate diketiminate complexes [LMMe] (L )
â-diketiminate; M ) Fe, Co).24 The M-N(amido) distances
of 1.965(4) and 1.960(2) Å in 3 and 4, respectively, are
slightly longer than the corresponding distances in the chloro
derivatives 1 [1.948(2) Å] and 2 [1.922(2) Å] (see the
Supporting Information). This structural difference may be
As shown in Scheme 1, treatment of 1 and 2 with
equimolar amounts of LiMe in toluene afforded the 14-
electron iron(II) and 15-electron cobalt(II) methyl-arylamide
complexes [M(L1)(Me)(tmeda)] [M ) Fe (3), Co (4)],
respectively. Both 3 and 4 are mononuclear high-spin
complexes with solution magnetic moments of 5.02 µB (for
3) and 4.12 µB (for 4).19 The 1H NMR spectra of the methyl
complexes showed well-resolved isotropically shifted reso-
nance signals due to L1 and tmeda (see the Supporting
Information). However, like other iron(II) alkyl complexes
recently reported in the literature (vide infra),20-24 resonance
(23) Schebler, P. J.; Mandimutsira, B. S.; Riordan, C. G.; Liable-Sands,
L. M.; Incarvito, C. D.; Rheingold, A. L. J. Am. Chem. Soc. 2001,
123, 331-332.
(24) Holland, P. L.; Cundari, T. R.; Perez, L. L.; Eckert, N. A.; Lachicotte,
R. J. J. Am. Chem. Soc. 2002, 124, 14416-14424.
(25) Data collection for 3: C18H37FeN3Si, Mr ) 379.45, crystal dimensions
0.4 × 0.3 × 0.2 mm3, triclinic (P1h), a ) 8.036(14) Å, b ) 10.296-
(16) Å, c ) 14.97(2) Å, R ) 75.74(3)°, â ) 76.18(3)°, γ ) 70.79-
(17) The molecular structures of 1 and 2 have been confirmed by X-ray
crystallography. See Figures S1 and S2 in the Supporting Information.
1 and 2 are isotypic, monomeric complexes with a distorted tetrahedral
geometry around the high-spin metal center (µeff ) 4.79 µB for 1 and
3.77 µB for 219). Detailed structural and spectroscopic characterization
of the complexes will be reported in a full article.
(18) In order to verify that 1 and 2 are not kinetic products of the reactions,
we have also examined the reaction of MCl2 (M ) Fe or Co) with 2
mol equiv of [Li(L1)(tmeda)] at a higher temperature (80 °C). Only
the monoamido complexes 1 and 2 were isolated under the latter
reaction conditions.
(19) Magnetic moments of the complexes were measured by the Evans
method in toluene at 298 K. Evans, D. F. J. Chem. Soc. 1959, 2003-
2005.
(20) Kisko, J. L.; Hascall, T.; Parkin, G. J. Am. Chem. Soc. 1998, 120,
10561-10562.
(21) Daida, E. J.; Peters, J. C. Inorg. Chem. 2004, 43, 7474-7485.
(22) Jewson, J. D.; Liable-Sands, L. M.; Yap, G. P. A.; Rheingold, A. L.;
Theopold, K. H. Organometallics 1999, 18, 300-305.
(3)°, V ) 1114(3) Å3, Z ) 2, Fcalcd ) 1.131 g/cm3, µ ) 0.734 mm-1
,
Mo KR radiation λ ) 0.710 73 Å, T ) 273(2) K, reflections measured
) 6817, independent reflections ) 4788 (Rint ) 0.0506), observed
reflections ) 2522 [I > 2σ(I)], final R indices R1 ) 0.0615 and wR2
) 0.1514. Data collection for 4: C18H37CoN3Si, Mr ) 382.53, crystal
dimensions 0.3 × 0.2 × 0.18 mm3, triclinic (P1h), a ) 8.0103(5) Å, b
) 10.2402(6) Å, c ) 15.1224(9) Å, R ) 76.1290(10)°, â ) 76.1650-
(10)°, γ ) 70.2830(10)°, V ) 1116.54(12) Å3, Z ) 2, Fcalcd ) 1.138
g/cm3, µ ) 0.826 mm-1, Mo KR radiation λ ) 0.710 73 Å, T ) 273-
(2) K, reflections measured ) 6019, independent reflections ) 4266
(Rint ) 0.0132), observed reflections ) 3786 [I > 2σ(I)], final R indices
R1 ) 0.0331 and wR2 ) 0.0945. Data collection for 5: C34H56FeN2,
Mr ) 548.66, crystal dimensions 0.4 × 0.3 × 0.2 mm3, monoclinic
(C2/c), a ) 20.236(3) Å, b ) 17.716(2) Å, c ) 9.7692(12) Å, R )
90°, â ) 100.340(3)°, γ ) 90°, V ) 3445.4(8) Å3, Z ) 4, Fcalcd
)
1.058 g/cm3, µ ) 0.459 mm-1, Mo KR radiation λ ) 0.710 73 Å, T
) 293(2) K, reflections measured ) 11 620, independent reflections
) 4163 (Rint ) 0.0663), observed reflections ) 1663 [I > 2σ(I)], final
R indices R1 ) 0.0528 and wR2 ) 0.1115.
7696 Inorganic Chemistry, Vol. 46, No. 19, 2007

COMMUNICATION
Scheme 2
attributed to a stronger methyl ligand as compared to a
chloride ligand.
The majority of alkyl complexes of FeII and CoII, which
have been well-characterized, contain cyclopentadienyl
ligands or π-acid supporting ligands (such as CO, PR3, or
bipyridine).26 To our knowledge, the simple monodentate
arylamido ligand system has not been reported to be effective
in the stabilization of iron(II) and cobalt(II) methyl com-
plexes. Mixed methyl-arylamide complexes of FeII and CoII
are particularly rare. Recently, a number of coordinatively
and electronically unsaturated iron(II) and cobalt(II) methyl
complexes supported by “tetrahedral-enforcing” borato ligands
have been reported by several research groups led by
Parkin,20 Peters,21 Theopold,22 and Riordan,23 respectively.
Holland and co-workers have also reported three-coordinate
methyl complexes of FeII and CoII, which were supported
by a sterically demanding bidentate diketiminate ligand.24 It
is noteworthy that the bulky “tetrahedral-enforcing” borato
and chelating diketiminate supporting ligands can provide a
rigid structural framework, which may enhance the stability
of the corresponding methyl complexes.
Figure 2. Molecular structure of 5 (30% thermal ellipsoid). Only one of
the two orientations of the disordered neopentyl group is shown for clarity,
Selected bond lengths (Å) and angles (deg): Fe1-N1 1.842(2), N1-C1
1.437(4), N1-C13 1.46(1), N1-Fe1-N1A 168.8(2), C1-N1-C13 111.1-
(5), C1-N1-Fe1 112.0(2), C13-N1-Fe1 136.1(4).
to a pair of monodentate L2 ligands. It is noteworthy that
the FeII center in 5 is protected from the surroundings by
the neopentyl and isopropyl groups of the two L2 ligands.
The Fe-N distance of 1.842(2) Å in 5 is slightly shorter
than those reported for other iron(II) bis(amide) complexes,
namely, 1.896(2) and 1.916(2) Å for [Fe{N(SiMenPh3-n)2}2]9
and 1.938(2) Å for [Fe{N(Mes)(BMes2)}2] (Mes ) 2,4,6-
Me3C6H2).10 The next-closest atoms to the FeII center in 5
are C1 and C1A, which are located at ca. 2.728 Å from the
metal center. The N-Fe-N angle [168.8(2)°] in 5 has a
minor deviation from linearity, which is similar to those
observed for [Fe{N(SiMenPh3-n)2}2]9 and [Fe{N(Mes)-
(BMes2)}2].10
Apart from the trimethylsilyl-substituted L1 ligand, we
have also investigated the coordination chemistry of the
closely related N-alkylated [N(CH2But)(C6H3Pri2-2,6)]- (L2)
ligand toward FeII and CoII ions.27 Because tmeda forms part
of the steric protection around the metal center in the
monoamido complexes 1 and 2, it was expected that the
absence of tmeda in the reaction mixture may facilitate a
complete anion metathesis reaction of 1:2 MCl2/lithium
amide. Therefore, in an effort to synthesize bis(amido)
derivatives, we treated anhydrous FeCl2 with 2 equiv of the
tmeda-free lithium salt [Li(L2)(THF)2] in toluene (Scheme
2).
In summary, the monodentate arylamido ligands [N(R)-
(C6H3R′2-2,6)]- [R ) SiMe3, R′ ) Me (L1); R ) CH2But,
R′ ) Pri (L2)] have been shown to be promising candidates
for the stabilization of electronically and coordinatively
unsaturated FeII and CoII complexes, namely, the methyl-
arylamide complexes [M(L1)(Me)(tmeda)] [M ) Fe (3), Co
(4)] and the two-coordinate iron(II) bis(arylamide) [Fe(L2)2]
(5). Further studies on the coordination chemistry of the
present arylamido ligand system with other late-transition-
metal ions and a detailed investigation on the reaction
chemistry of the corresponding metal complexes are currently
in progress in our laboratory.
Interestingly, a red crystalline complex was isolated, and
both X-ray data and elemental analysis indicated that the
product is the two-coordinate iron(II) bis(arylamide) 5
(Figure 2).25 Complex 5 conforms closely to an idealized
C2 molecular symmetry, with the FeII ion being coordinated
Acknowledgment. We thank The Chinese University of
Hong Kong for financial support of this work in the form of
a Direct Grant (A/C 2060253) and a postgraduate studentship
to H.Y.A.-Y. We thank George Fai Wong and Lai Fong
Yeung for their kind assistance.
Supporting Information Available: Detailed synthetic proce-
dures for 1-5, ORTEP drawings of 1, 2, and 4, and tables (in CIF
and PDF formats) listing selected X-ray crystallographic data for
1-5. This material is available free of charge via the Internet at
http://pubs.acs.org.
(26) (a) Kerber, R. C. In ComprehensiVe Organometallic Chemistry II; Abel,
E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: New York,
1995; Vol. 7, Chapter 2. (b) Sweany, R. L. In ComprehensiVe
Organometallic Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson,
G., Eds.; Pergamon: New York, 1995; Vol. 8, Chapter 1.
(27) We anticipated that different electronic properties of the SiMe3 and
CH2But substituents on L1 and L2, respectively, may lead to a variation
in their coordination properties.
IC700850E
Inorganic Chemistry, Vol. 46, No. 19, 2007 7697