Reaction of M(II) Diaryls (M = Mn or Fe) with ammonia to afford parent amido complexes

Article abstract of DOI:10.1021/om1000502

The reaction of the two-coordinate diaryls MAr′₂ (M = Mn or Fe; Ar′ = C₆H₃-2,6-(C₆H ₃-2,6-ⁱPr₂)₂) with excess NH ₃ below room temperature afforded the paren

Full text of DOI:10.1021/om1000502

1988 Organometallics 2010, 29, 1988–1991
DOI: 10.1021/om1000502
Reaction of M(II) Diaryls (M = Mn or Fe) with Ammonia to Afford
Parent Amido Complexes
Chengbao Ni, Hao Lei, and Philip P. Power*
Department of Chemistry, University of California, Davis, California 95616
Received January 21, 2010
Summary: The reaction of the two-coordinate diaryls MAr02
catalytic routes.22-24 Currently known metathetical exchange
reactions of NH3 with transition metal organometallic complexes
mainly involve derivatives of the early first-row metals. For
example, group 3-5 metal complexes, such as (η5-Cp*)2ScMe
(Cp* = C5Me5),7 {(η5-Cp)Ti(μ:η1,η5-Cp)}2 (Cp=C5H5),2 (η5-
Cp*)2MH2 (M=Zr or Hf),6 (η5-Cp*)TiMe3,9 and (tBuCH2)3-
TadCHtBu,10 react with NH3 to form bridging amido, imido, or
nitrido complexes with elimination of alkane or H2. NH3 has also
been shown to react with some late second- or third-row transi-
tion metal complexes. For example, the hydride cluster {(η5-
Cp*Ru)3(μ3-H)2(μ-H)3} shows reversible reactivity toward NH3
to form a μ3-imido cluster {(η5-Cp*Ru)3(μ-H)3(μ3-NH)} and
H2.15 Cooperative action of the multiple metal centers in metal
clusters such as {Os3(CO)11(L)} (L=cyclo-C6H8, CH3CN, or
NH3)1,3-5 effectively activates NH3 to form bridging amido
species. Milstein and Hartwig reported the oxidative addition
of NH3 to iridium complexes to afford μ2-NH2 species or
terminal amido/hydride complexes.8,12,14 Furthermore, oxidative
addition reactions of NH3 with low-valent main group metal
species have been reported.16,18,20 The mechanisms and thermo-
dynamics of NH3 reactions with transition metal complexes have
been investigated by spectroscopy13,25-30 and computational
studies.30-41 Nonetheless, little attention has been given to
(M = Mn or Fe; Ar0 = C6H3-2,6-(C6H3-2,6-iPr2)2) with excess
NH3 below room temperature afforded the parent amido com-
plexes {Ar0Mn(μ-NH2)(NH3)}2 (1) and {Ar0Fe(μ-NH2)}2 (2) in
good yields. The reactions were accompanied by elimination of the
arene Ar0H. Both complexes were obtained as dimers in which the
metals are bridged by two NH2 ligands. The complex 1 also
includes an ammonia (NH3) ligand bound to each manganese.
Ammonia complexation did not occur in 2, and the metals
remained three-coordinate. The metal electron configurations
are high-spin and antiferromagnetically coupled.
Cleavage of the N-H bond in ammonia under mild conditions
either by insertion of a transition metal atom or by metathetical
exchange with other ligands is attracting increasing interest1-21
because NH3 is an inexpensive potential precursor for a variety of
amino compounds that may be accessible by transition metal
*Corresponding author. E-mail: pppower@ucdavis.edu.
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r
pubs.acs.org/Organometallics
Published on Web 03/23/2010
2010 American Chemical Society

Note
Scheme 1. Proposed Reaction Pathway for the Formation of
1 and 2
Organometallics, Vol. 29, No. 8, 2010 1989
the reactions of first-row mid to late metal complexes, e.g.,
manganese, iron, cobalt, or nickel complexes with NH3, and
only limited spectroscopic studies, e.g., guided ion beam
tandem mass spectrometry of iron species, have been re-
ported.28,29 We now show that the reactions of two-coordi-
nate M(II) diaryls MAr02 (M = Mn or Fe;42 Ar0 = C6H3-
2,6-(C6H3-2,6-iPr2)2) with NH3 readily afford rare examples
of parent amido derivatives of manganese and iron as the
complexes {Ar0Mn(μ-NH2)(NH3)}2 (1) and {Ar0Fe(μ-NH2)}2
(2) with arene (Ar0H) elimination.
Figure 1. Thermal ellipsoid (30%) plot of 1 at 190(2) K without
˚
H atoms (except N-H). Selected bond lengths (A) and angles
(deg): Mn1 Mn1A 3.0686(5), Mn1-C1 2.162(2), Mn1-N1
3 3 3
2.093(2), Mn1-N1A 2.093(2), Mn1-N2 2.241(2), Mn1-N1-
Mn1A 94.29(6), N1-Mn1-N1A 85.71(6), C1-Mn1-N1
126.33(6), C1-Mn1-N1A 129.81(5).
the low (three)-coordinate iron(II) species {HC(CMeNAr)2-
Fe}2(μ-S) (Ar = C6H3-2,6-iPr2), which forms the stable NH3
adduct {HC(CMeNAr)2Fe(NH3)}2(μ-S).46 The driving
force for the formation of 1 and 2 may be the greater strength
of the M-N bond versus the M-C bond together with the
elimination of the arene (Ar0H) molecule. A similar parent
amido complex formation, together with arene elimination,
is observed when the two-coordinate diarylstannylene
SnAr02 is treated with NH3.17
Results and Discussion
Exposure of hexane solutions of MAr02 (M = Mn or Fe) to
a dry NH3 atmosphere at ca. -78 °C afforded the product 1 or
2 in moderate to high yields. For MnAr02, the pink solution
gradually changed to pale yellow in ca. 30 min, and extremely
air- and moisture-sensitive crystals of 1 were obtained from
the solution. For iron, large pale yellow crystals were obtained
from hexanes in high yield (77%). Similar reactions were also
attempted with cobalt(II) diaryl species; however, no crystal-
line product was isolated in that case.
The mechanism for the formation of 1 and 2 is currently
unknown. It is probable (Scheme 1) that the first step may
involve generation of the Lewis base adduct Ar02M(NH3),
which is energetically favored according to theoretical stu-
dies (two-coordinate iron diaryls and diamides are also
known to form isolable complexes with Lewis bases such
as nitriles,43 pyridine,44 or phosphines45).38 The N-H bond
may then be cleaved in concert with elimination of an arene
molecule to afford the M(II) amido species Ar0MNH2, which
then dimerizes to form {Ar0M(μ-NH2)}2. For manganese,
the dimer {Ar0Mn(μ-NH2)}2 complexes a further NH3 at
each metal to form the adduct {Ar0Mn(μ-NH2)(NH3)}2 (1);
however, this complexation was not observed for iron. A
possible alternative, if less likely, pathway also involves the
initial generation of Ar02M(NH3), which may undergo oxi-
dative addition of NH3 to give the M(IV) amido hydride
species Ar02M(H)(NH2), followed by reductive elimination
of an arene (ArH) molecule. In both cases, the byproduct
Ar0H was confirmed by NMR spectroscopy. However, with-
out the identification of intermediates, other reaction path-
ways cannot be ruled out. The facile reaction of the MAr02
molecules with NH3 may be contrasted with the behavior of
Structures. The structure of 1 (Figure 1) shows that it is
dimerized through bridging of the two manganese atoms by two
NH2 groups. Each manganese is terminally bound to an Ar0
ligand as well as an NH3 molecule and thus has a distorted tetra-
hedral geometry. Complex 1 appears to be the first structurally
characterized manganese complex with parent amido (NH2)
ligands. It is also a rare example of a structurally authenticated
organometallic manganese-ammonia adduct.47-50 In 1, the
˚
Mn-C bond length (2.162(2) A) is somewhat longer than those
in the two-coordinate species Mn(C6H2-2,4,6-tBu3)2 (2.108(2)
48
#
A) and Ar0MnN(H)Ar (Ar# = C6H3-2,6-(C6H2-2,4,6-
˚
Me3)2; 2.095(2) A),52 probably due to the higher coordination
˚
number of the manganese atoms in 1. The Mn-N(NH2) bond
lengths (2.093(2) A) are essentially indistinguishable from that
˚
0
˚
(av 2.0938 A) in the analogous complex {Ar Mn(μ-NMe2)}2,
which was synthesized by salt elimination.53 The terminal
˚
Mn-N(NH3) bond length of 2.241(2) A is, as expected,
significantly longer than the Mn-N(NH2) distances and is
comparable to that in the fulleride salt [Mn(NH3)6]C60
(46) Vela, J.; Stoian, S.; Flaschenriem, C. J.; Munck, E.; Holland,
P. L. J. Am. Chem. Soc. 2004, 126, 4522.
(47) Kuroda, R.; Sasaki, Y. Acta Crystallogr. B: Struct. Sci. 1974,
B 30, 687.
(48) Nakano, T.; Miyoshi, T.; Iwamoto, T.; Sasaki, Y. Bull. Chem.
Soc. Jpn. 1967, 40, 1297.
(49) Herberhold, M.; Wehrmann, F.; Neugebauer, D.; Huttner, G.
J. Organomet. Chem. 1978, 152, 329.
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(51) Wehmschulte, R. J.; Power, P. P. Organometallics 1995, 14, 3264.
(52) Ni, C. B.; Fettinger, J. C.; Long, G. J.; Power, P. P. Inorg. Chem.
2009, 48, 2443.
(42) Ni, C. B.; Power, P. P. Chem. Commun. 2009, 5543.
(43) Ni, C.; Power, P. P. Unpublished work.
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P. P. Polyhedron 2003, 22, 67.
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1990 Organometallics, Vol. 29, No. 8, 2010
Ni et al.
central aryl rings of the terphenyl ligands.53 In 2, the Fe-C
˚
˚
(2.027(2) A) and the Fe-N (1.996(2) A) distances are similar
0
˚
to those observed in {Ar Fe(μ-NMe2)}2 (Fe-C=2.056(2) A;
Fe-N=2.028 A (av)).53 As expected, the Fe Fe separation
˚
3 3 3
˚
˚
in 2 (2.7334(7) A) is essentially the same as that of 2.7239(6) A
in {Ar0Fe(μ-NMe2)}2,53 but is significantly longer than that in
55
˚
the heavily folded species {Fe(CO)3(μ-NH2)}2 (2.402(6) A).
Similar to the antiferromagnetic coupling interactions be-
tween the metal atoms in {Ar0Fe(μ-NMe2)}2,53 complex 2
has a μeff of 4.41 μB/dimer at 293 K. The N-H stretches were
located at 3351 and 3247 cm-1 in the IR spectrum, corre-
sponding to the two stretching modes of the N-H bonds.
In summary, we have shown for the first time that
two-coordinate M(II) diaryls (M = Mn or Fe) readily cleave
the N-H bond of NH3 to afford the parent amido (NH2)
dimers 1 and 2. The formation of 1 and 2 emphasizes the
importance of coordination numbers in determining the
reactivity of metal centers. Complexes 1 and 2 are rare
examples of mid to late first-row transition metal complexes
with parent amido ligands.
Figure 2. Thermal ellipsoid (30%) plot of 2 at 90(2) K without
˚
H atoms (except N-H). Selected bond lengths (A) and angles
(deg): Fe1 Fe1A 2.7334(7), Fe1-C1 2.027(2), Fe1-N1
3 3 3
1.996(2), N1-Fe1-N1 93.57(9), N1-Fe1-C1 133.21(4),
Fe1-N1-Fe1A 86.43(9).
Experimental Details
(2.298(3) A).50 The Mn2(μ-N)2 core is planar with Mn-N-Mn
˚
General Procedures. All manipulations were carried out using
Schlenk techniques under an argon atmosphere or in a Vacuum
Atmospheres HE-43 drybox. All solvents were dried over an
and N-Mn-N angles of 94.29(6)° and 85.71(6)°. The
Mn Mn separation in 1 is 3.0686(4) A, which is ca. 0.1 A
ꢀ
˚
˚
3 3 3
longer than the 2.9479(3) A in {Ar Mn(μ-NMe2)}2,53 which is
probably a result of the complexation of an additional NH3
molecule at each manganese atom. The effective magnetic
moment (μeff) of 1 (5.62 μB/dimer at 293 K) is significantly
lower than the value of 8.36 μB expected for two isolated 3d5
Mn(II) ions, due to the antiferromagnetic coupling interactions
between the metal atoms. The N-H stretching bonds are
observed at 3605 and 3355 cm-1 in the IR spectrum.
0
˚
˚
alumina column, stored over 3 A molecular sieves overnight,
and degassed three times (freeze-thaw) prior to use. MAr02
(M = Mn or Fe) were prepared by literature procedures.58
Liquid NH3 (ca. 10 mL) was transferred into a Schlenk flask
containing sodium metal with cooling to ca. -78 °C. The result-
ing deep blue solution was stirred at -78 °C for 30 min and then
slowly warmed to ca. -30 °C. The flask containing MAr02 solu-
tion at ca. -78 °C was evacuated and quickly refilled with NH3
vapor. The solution was kept under an NH3 atmosphere for at
least 30 min. Melting points were recorded in glass capillaries
sealed under N2 and are uncorrected. UV-vis data were re-
corded on a Hitachi-1200 spectrometer. Magnetic susceptibility
measurements by Evans’ method59,60 and the 1H NMR spectra
were recorded on a Bruker 300 MHz spectrometer at ca. 293 K.
{Ar0Mn(NH3)(μ-NH2)}2 (1). A pale pink solution of MnAr02
(0.345 g, 0.4 mmol) in ca. 40 mL of hexanes was cooled to ca.
-78 °C in a flask, and dry NH3 was introduced. The solution
was kept at ca. -78 °C for ca. 1 h, slowly warmed to ca. 25 °C,
and stirred for another 2 h. The pale orange solution was filtered
and concentrated to ca. 10 mL, which afforded colorless, X-ray
quality crystals of 1 after storage overnight at 7 °C. Yield: 0.106
g (54.4%). Mp: 119-121 °C. 1H NMR (300 MHz, C6D6, 20 °C):
δ 1.13 (s, br, 48H, CH(CH3)2), 2.91 (s, br, 8H, CH(CH3)2). IR in
Nujol mull (cm-1) in KBr: νN-H 3605, 3355. Anal. Calcd for
C60H84Mn2N4: C, 74.20; H, 8.72; N, 5.77. Found: C, 74.52; H,
9.07; N, 5.44. μeff = 5.6(3) μB/dimer at 293 K.
The structure of 2 (Figure 2) is also dimerized through
symmetrically bridging NH2 groups; however, no Fe-NH3
coordination was observed. The absence of NH3 complexa-
tion may be due to steric effects owing to the smaller ionic
radius of iron versus manganese.54 In addition, for three-
coordinate Fe2þ(d6), the dz2 orbital is probably doubly occu-
pied, which would discourage complex formation. Each iron
displays a Y-shaped coordination geometry with a relatively
narrow N(1)-Fe(1)-N(1A) angle of 93.57(9)° and two ca.
40° wider C(1)-Fe(1)-N angles (133.21(4)°). To our knowl-
edge, iron complexes with parent amido ligands are quite rare,
and structurally characterized examples are limited to the
higher coordinate Fe complexes, such as the NH2-bridged
dimer {Fe(CO)3(μ-NH2)}2,55 prepared according to the Hie-
ber-Beutner procedure,56 and the monomeric amido hydride
complex (dmpe)2Fe(H)(NH2) (dmpe=1,2-bis(dimethylphos-
phino)ethane),57 obtained by reaction of (dmpe)2Fe(H)(Cl)
with NaNH2. The Fe2(μ-N)2 four-membered ring in 2 is
strictly planar. This is in contrast to the structure of {Fe(CO)3-
{Ar0Fe(μ-NH2)}2 (2). A procedure similar to that of 1, employ-
ing FeAr02 (0.346 g, 0.40 mmol) instead of MnAr02, afforded large
pale yellow crystals of 2 from hexanes at 7 °C. Yield: 0.144 g
(76.8%). Mp: 137-139 °C. 1H NMR (300 MHz, C6D6, 20 °C): δ
-7.61 (s, br, 4H), 1.14 (s, br, 48H, CH(CH3)2), 1.90 (s, br, 4H),
2.92 (s, br, 8H, CH(CH3)2), 3.46 (s, br, 4H), 5.61 (s, br, 2H), 6.21
(s, br, 8H). IR in Nujol mull (cm-1) in KBr: νN-H 3351 and 3247.
Anal. Calcd for C60H78Fe2N2: C, 76.75; H, 8.37; N, 2.98. Found:
C, 77.12; H, 8.23; N, 2.79. μeff = 4.4(2) μB/dimer at 293 K.
In both cases, after the isolation of the desired products, the
mother liquors were concentrated to ca. 2 mL to afford a color-
less crystalline product, which was identified as Ar0H according
(μ-NH2)}2,55 which is folded along the Fe Fe vector by ca.
3 3 3
86°. Notably, the Fe2(μ-N)2 plane is coplanar with the two
central rings of the terphenyl ligands (torsion angle=ca. 8°).
Such coplanarity is in sharp contrast to the geometry of the
analogous amido complex {Ar0Fe(μ-NMe2)}2, in which the
Fe2(μ-N)2 plane has a torsion angle of ca. 57.2° to the two
(54) Shannon, R. D. Acta Crystallogr. A 1976, 32, 751.
(55) Dahl, L. F.; Costello, W. R.; King, R. B. J. Am. Chem. Soc. 1968,
90, 5422.
(56) Hieber, W.; Beutner, H. Z. Anorg. Allg. Chem. 1962, 317, 63.
(57) Fox, D. J.; Bergman, R. G. J. Am. Chem. Soc. 2003, 125, 8984.
(58) Ni, C. B.; Power, P. P. Chem. Commun. 2009, 5543.
(59) Evans, D. F. J. Chem. Soc. 1959, 2003.
(60) Schubert, E. M. J. Chem. Educ. 1992, 69, 62.

Note
Table 1. Selected Crystallographic Data and Collection Para-
meters for 1 and 2
Organometallics, Vol. 29, No. 8, 2010 1991
12H), 7.09 (s, C6H4, 1H), 7.12 (m, p-C6H4 and p-C6H3, 3H),
7.14 (d, 3JH-H = 7.2 Hz, m-C6H3, 4H), 7.31 (t, 3JH-H = 7.2 Hz,
m-C6H4, 2H).
1
2 C6H14
3
X-ray Crystallographic Studies. Crystals of 1 and 2 were
selected and covered with a layer of hydrocarbon oil under a
rapid flow of argon. They were mounted on a glass fiber
attached to a copper pin and placed in the cold N2 stream on
the diffractometer. X-ray data for 1 were collected on a Bruker
SMART Apex II diffractometer at 190(2) K using Mo KR
formula
fw
color
C60H84Mn2N4
C66H92Fe2N2
1025.12
pale yellow
plate
971.19
colorless
plate
habit
cryst syst
Pbca
I2/m
ꢀ
˚
a, A
17.7138(7)
16.2752(7)
19.9105(8)
90
90
90
5740.1(4)
4
1.124
2.62-27.50
0.478
5006
11.963(2)
20.350(3)
12.583(2)
90
99.002(2)
90
3025.7(9)
2
1.125
1.92-27.5
0.518
3034
˚
radiation (λ = 0.71073 A). For 2, the data were collected on a
ꢀ
˚
b, A
Bruker SMART 1000 diffractometer at 90(2) K using Mo KR
ꢀ
˚
c, A
˚
radiation (λ = 0.71073 A). Absorption corrections were applied
R, deg
β, deg
using SADABS.61 The structures were solved using direct
methods and refined by the full-matrix least-squares procedure
in SHELXL.62 All of the non-hydrogen atoms were refined
anisotropically. The N-hydrogen atoms in 1 were located by a
Fourier difference map. All other hydrogen atoms in both
structures were placed at calculated positions and included in
the refinement using a riding model.
γ, deg
3
ꢀ
˚
V, A
Z
dcalcd, Mg/m3
θ range, deg
μ, mm-1
no. of obsd data, I > 2σ(I)
R1 (obsd data)
wR2 (all data)
0.0350
0.0961
0.0434
0.1240
Acknowledgment. We thank the National Science
Foundation (CHE-0948417) for financial support and
Dr. James C. Fettinger for crystallographic assistance.
to 1H NMR spectroscopy. 1H NMR (300 MHz, C6D6, 20 °C):
=
δ 1.12 (d, 3JH-H=6.9 Hz, CH(CH3)2, 12H), 1.13 (d, 3JH-H
6.9 Hz, CH(CH3)2, 12H), 2.91 (sept, 3JH-H=6.9 Hz, CH(CH3)2,
Supporting Information Available: Crystallographic informa-
tion files (CIFs); H NMR and IR spectra for 1 and 2. These
1
(61) SADABS, version 5.0 package; an empirical absorption correction
program from the SAINTPlus NT; Bruker AXS: Madison, WI, 1998.
(62) SHELXL, version 5.1; Bruker AXS: Madison, WI, 1998.
materials are available free of charge via the Internet at http://
pubs.acs.org.