Stabilization of dimethyliron(II) and methylcobalt(I) complexes bearing a hemilabile 2-benzoylpyridine ligand in octahedral and square-pyramidal coordination

Article abstract of DOI:10.1016/j.ica.2008.09.021

Octahedral cis-Fe(CH₃)₂{2-(benzoyl)pyridyl-N,O}(PMe₃)₂ (1), square-pyramidal Co(CH₃){2-(benzoyl)pyridyl-N,O}(PMe₃)₂ (2), and triangular-planar Ni{2-(benzoyl)pyridyl-η²

Full text of DOI:10.1016/j.ica.2008.09.021

Inorganica Chimica Acta 362 (2009) 1984–1990
Contents lists available at ScienceDirect
Inorganica Chimica Acta
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Stabilization of dimethyliron(II) and methylcobalt(I)
complexes bearing a hemilabile 2-benzoylpyridine ligand in octahedral
and square-pyramidal coordination
b
a
Robert Beck a, , Ulrich Flörke , Hans-Friedrich Klein
*
a Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Technische Universität Darmstadt, Petersenstraße 18, D-64287 Darmstadt, Germany
b Anorganische und Analytische Chemie, Universität Paderborn, Warburger Straße 100, D-33098 Paderborn, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 July 2008
Received in revised form 3 September 2008
Accepted 10 September 2008
Available online 17 September 2008
Octahedral cis-Fe(CH3)2{2-(benzoyl)pyridyl-N,O}(PMe3)2 (1), square-pyramidal Co(CH3){2-(benzoyl)pyri-
dyl-N,O}(PMe3)2 (2), and triangular-planar Ni{2-(benzoyl)pyridyl-
sized by reaction of 2-benzoylpyridine with thermally labile Fe(CH3)2(PMe3)4 and Co(CH3)(PMe3)4
complexes. With Ni(CH3)2(PMe3)3, reductive elimination of ethane is observed when a
2-C,O-coordina-
g
2-C,O}(PMe3)2 (3) have been synthe-
g
tion is constituted. The complexes were investigated by NMR spectroscopic methods and the molecular
structures of 1 and 2 were determined by X-ray crystallography.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords:
2-Benzoylpyridine
Iron
Cobalt
Nickel
Trimethylphosphine
1. Introduction
substrates, assisted by [N, O, P] coordinating donor functions (D)
[5]. The reaction pathway for the newly formed M–C bond is irre-
The interest in complexes containing a 2-benzoylpyridine [N,O]
coordination is substantiated by their hemilability that facilitates
substrate binding and can stabilize reactive intermediates of tran-
sition metals [1].
As a rare example of an isolable product, a palladium center
with N,O-coordination has been described, where carbon monox-
ide/alkene insertion into the Pd–C bond is observed [2]. A rhodium
complex containing a related dipyridylketone backbone, by inser-
tion of ethene forms a five-membered oxametallacycle [3]. These
can be seen as model compounds for studying the growth of an al-
kyl-metal chain.
versible, since methane does not cleave a Fe–C or Co–C bond which
does occur with dihydrogen [6].
Our recent study of C–H activation was concerned with vinylic-
and aromatic C–H bonds of 1,4-CH/N exchanged azadiene systems
in 2-vinylphenylpyridine (A), benzophenoneimine (B) [7], and iso-
electronic benzophenone prechelating ligands (C) [8], containing a
set of hard (carbon)/soft (oxygen, nitrogen) donor atoms. Cyclo-
metalation was observed under mild conditions (ꢀ70 °C), with
high regioselectivity. The reactions proceeded by evolution of
methane forming stable five-membered [C,N]- or [C,O]-metallacy-
cles (Scheme 2).
Furthermore, in the case of 2-benzoylpyridine the interest in
neutral N,O ligands is based on the above-mentioned properties
and the relation with isoelectronic benzophenones and -imines
which Murai demonstrated at ruthenium centers for catalyzed
functionalization at the ortho position with 100% regioselectivity
[4].
The main feature from most previously reported reactions of
Co(CH3)(PMe3)4 and Fe(CH3)2(PMe3)4 (Scheme 1) containing ther-
mally labile methyl groups, is C–H activation of various organic
Our interest in the related 2-benzoylpyridine (D) is substanti-
ated by the combination of soft/soft ligand donor effects and the
presence of further possible coordination sites, with isoelectronic
constitution as in A, B, and C. Typically [N,O]-coordination is ob-
served, other activation sites appear likely (indicated by arrow)
where the question of preference in N/O donor or five-/six-mem-
bered ring coordination by C–H activation with Fe(CH3)2(PMe3)4,
Co(CH3)(PMe3)4, and Ni(CH3)2(PMe3)3 arises.
The [N,O] coordination chemistry of 2-benzyolpyridine has
been well described previously with many transition metals for
example with Zn [9], Mn [10], Cu [11], Ru [12], or Rh [13], typically
carrying additional sets of hard donor atoms [Cl, O, N]. Surprisingly
there are only few literature reports of organometallic compounds
bearing 2-benzoylpyridine, described for Rh [14], Pd [15], Ru [16],
* Corresponding author. Present address: School of Chemistry and Chemical
Engineering, Shandong University, Shanda Nanlu 27, 250100 Jinan, People’s
Republic of China.
E-mail address: metallacycle@gmail.com (R. Beck).
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.09.021

R. Beck et al. / Inorganica Chimica Acta 362 (2009) 1984–1990
1985
Scheme 1. C–H activation with methyliron and methylcobalt.
Scheme 2. 18 Electron-chelates of cobalt(I) and iron(II).
Fe [17], Re [18], whereas only one methylpalladium complex has
been described [19].
Here we report the results of our studies on the reactivity of
methyl(Fe, Co, Ni) complexes with 2-benzoylpyridine.
diethyl ether and the solution cooled to ꢀ27 °C to afford rhombic
crystals with brown faces of 1. Yield 723 mg (86%); m.p. 82–
84 °C (dec.). IR (Nujol):
C@O); 1192 w, 1173 w (ds Fe–CH3); 937 s (q1 PCH3). 1H NMR
m m m
= 1577 cmꢀ1 m ( C@C); 1492 cmꢀ1
(m
(500 MHz, [D8]THF, 293 K): d = 0.31 (s(br), 3H, Fe–CH3); 0.51
(s(br), 3H, Fe–CH3); 0.95 (s(br), 9H, PCH3), 1.25 (s(br), 9H, PCH3),
6.52–8.51 (m, 9H, CH). 13C{1H} NMR (125 MHz, [D8]THF, 293 K):
d = 17.3 (m, PCH3); 111.9 (s, CH); 121.3 (s, CH); 122.5 (s, CH);
123.8 (s, CH); 126.0 (s, CH); 126.3 (s, CH); 129.8 (s, CH); 146.4 (s,
C); 152.4 (s, CH); 154.1 (s, C@N); 160.1 (s, C@O). 31P{1H} NMR
2. Experimental
2.1. General considerations
2
All air-sensitive and volatile materials were handled using
standard vacuum techniques, und were kept under argon. Microa-
nalyses: Kolbe Microanylytical Laboratory, Mühlheim/Ruhr, Ger-
many. Melting points/decomposition temperatures: Sealed
capillaries, uncorrected values. Chemicals 2-benzoylpyridine
(Merck/Schuchardt) were used as purchased. Literature methods
were applied for the preparation of trimethylphosphine [41], [Fe-
Me2(PMe3)4] [42], [CoMe(PMe3)4] [43], and [NiMe2(PMe3)3] [44].
And IR: Nujol mulls between KBR discs, Bruker spectrophotometer
type FRA 106. 1H, 13C{1H}, and 31P{1H} NMR spectra (500 MHz,
125 MHz, and 202 MHz, respectively) were recorded with a Bruker
Bruker DRX 500 spectrometer. 13C{1H} and 31P{1H} resonances
were obtained with broad-band proton decoupling.
(202 MHz, [D8]THF, 203 K, ppm): d = ꢀ15.6 (d, JP,P = 56.2 Hz, 1P,
2
PCH3), 10.7 (d, JP,P = 56.2 Hz, 1P, PCH3). Anal. Calc. for C20H33Fe-
NOP2 (421.3): C, 57.02; H, 7.90; N, 3.32. Found: C, 56.97; H, 8.12;
N, 3.36%.
2.1.2. Preparation of methyl-[2-(benzoylpyridine)-
j jO]bis(trimethylphosphine)cobalt(I) (2)
N,
2-Benzoylpyridine (324 mg, 1.77 mmol) in 50 mL of THF were
combined at ꢀ70 °C with CoMe(PMe3)4 (670 mg, 1.77 mmol) in
50 mL of THF with a color change from red to dark blue within
3 h, while the mixture is coming to r.t. After further stirring for
3 h at 20 °C, all volatiles were removed in vacuo and the solid res-
idue was extracted with two 80 mL portions of diethylether. From
the solution 460 mg of dark blue quadrangular crystals were ob-
tained at 4 °C which proved suitable for X-ray diffraction. Upon
cooling to ꢀ27 °C a second fraction of 2 was collected and com-
bined with the first. Yield 602 mg (83%); m.p. 91–93 °C (dec.). IR
2.1.1. Preparation of cis-dimethyl-[2-(benzoylpyridine)-
j jO]bis(trimethylphosphine)-iron(II) (1)
N,
2-Benzoylpyridine (366 mg, 1.99 mmol) in 30 mL of diethyl
(Nujol):
m m m C@O); 1172
= 1579 cmꢀ1 m ( C@C); 1485 cmꢀ1 m (
ether were combined at ꢀ70 °C with FeMe2(PMe3)4 (786 mg,
1.99 mmol) in 80 mL of diethyl ether. During warm-up evolution
of gas was noticed, and a red brown color was attained. The mix-
ture was kept at 20 °C for 3 h. The volatiles were removed in vacuo
and the solid residue was extracted with two 70 mL portions of
w (ds Co–CH3); 935 s (q1 PCH3). 1H NMR (500 MHz, [D8]THF,
293 K, ppm): d = 0.51 (s, 3H, Co–CH3); 1.27 (s(br), 18H, PCH3);
3
3
6.65 (t, JH,H = 7.3 Hz, 1H, Ar-H); 7.09 (t, JH,H = 7.5 Hz, 1H, Ar-H);
7.53 (dd, JH,H = 6.6 Hz, JH,H = 9.2 Hz, 1H, Ar-H); 7.84 (t,
3
3

1986
R. Beck et al. / Inorganica Chimica Acta 362 (2009) 1984–1990
3JH,H = 5.5 Hz, 1H, Ar-H); 8.02 (d, 3JH,H = 8.3 Hz, 1H, Ar-H); 8.12 (dd,
order of P(1)Me3 group accordingly, so the large a.d.p.’s of C17–C19
have no physical relevance.
3
4
3JH,H = 7.4 Hz, JP,H = 12.0 Hz, 2H, Ar-H); 8.44 (t, JP,H = 5.9 Hz, 1H,
Ar-H). 13C{1H} NMR (125 MHz, [D8]THF, 293 K, ppm): d = ꢀ29.3
1
(m, Co–CH3); 16.3 (d, JP,C = 16.2 Hz, PCH3); 111.9 (s, CH); 121.9
3. Results and discussion
(s, CH); 123.1 (s, CH); 124.3 (s, CH); 126.8 (s, CH); 130.3 (s, CH);
146.9 (s, C); 152.9 (s, CH); 154.7 (s, C@N); 160.8 (s, C@O).
31P{1H} NMR (202 MHz, [D8]THF, 203 K, ppm): d = ꢀ5.7 (s(br), 2P,
PCH3). Anal. Calc. for C19H30CoNOP2 (409.3): C, 55.75; H, 7.39; P,
15.13. Found: C, 55.60; H, 7.19; P, 15.28%.
When simply combining the starting materials Fe(CH3)2(PMe3)4
with 2-benzoylpyridine in diethyl ether (Eq. (1)) at ꢀ70 °C a slow
color change from red to brown is observed when the reaction
mixture is allowed to warm to 20 °C during 3 h.
ð1Þ
2.1.3. Preparation of [2-(benzoylpyridine)-
(trimethylphosphine)nickel(0) (3)
g
2-C,O]bis
From concentrated ether solutions at ꢀ27 °C dark red-brown,
rhombic crystals of 1 were isolated which remained unchanged
under air for 3 h, although their solutions are decomposed within
a few seconds if air is admitted. High yields up to 86% of 1 were ob-
tained when crystallization was neglected and the product simply
washed with ice-cold pentane, which afforded an analytically pure
sample.
By IR a bathochromic coordination shift of the C@O stretching
frequency to 1490 cmꢀ1 is observed next to a less intensive band
at 1173 cmꢀ1 which is attributed to the deformation band of the
remaining methyl group in the product complex.
To a solution of NiMe2(PMe3)3 (837 mg, 2.64 mmol) in 50 mL
pentane (483 mg, 2.64 mmol) of 2-benzoylpyridine dissolved in
50 mL diethyl ether were added dropwise at ꢀ 70 °C. During warm
up the yellow solution darkens, accompanied with gas evolution
(ethane) and a yellow powder starts to precipitate. After 6 h the
mixture was filtered through a sinter disc (G3). The remaining solid
was washed with 50 mL ice-cold pentane affording an analytical
pure, highly sensitive sample of 3 in (593 mg) 57% yield; m.p.
91–94 °C (dec.). Efforts to get crystals suitable for X-ray diffraction
failed with solutions of pentane, ether, and THF or mixtures there-
By dynamic NMR spectroscopy mobility of ligands is recognized
by broad singlet resonances for the two methyl groups at 0.31 ppm
and 0.51 ppm caused by reversible dissociation of the trimethyl-
phosphine ligands.
of. IR (Nujol):
m m
= 1579 cmꢀ1 m ( C@C); 924 s (q1 PCH3). 1H NMR
(500 MHz, [D8]THF, 293 K, ppm): d = 1.15 (s(br), 18H, PCH3); 7.37
3
(m, 3H, Ar-H); 7.53 (t, JH,H = 8.1 Hz, 1H, Ar-H); 7.88 (t,
3JH,H = 8.1 Hz, 1H, Ar-H); 8.14 (m, 3H, Ar-H); 8.61 (d, JH,H = 7.5 Hz,
When the sample is cooled to ꢀ70 °C the 31P{1H} NMR spectrum
exhibits signals at ꢀ15.6 ppm as doublet (2JP,P = 56.2 Hz, 1P), and at
10.7 ppm as doublet (2JP,P = 56.2 Hz, 1P) for the trimethylphos-
3
1H, Ar-H). 13C{1H} NMR (125 MHz, [D8]THF, 293 K, ppm): d = 15.3
1
1
(d, JP,C = 19.2 Hz, PCH3); 16.3 (d, JP,C = 21.3 Hz, PCH3); 122.3 (s,
CH); 125.2 (s, CH); 127.1 (s, CH); 130.1 (s, CH); 132.8 (s, CH);
135.5 (s, CH); 147.1 (s, NiC@O); 154.7 (s, CN). 31P{1H} NMR
(202 MHz, [D8]THF, 293 K, ppm): d = 0.9 (s(br), 2P, PCH3). 31P{1H}
NMR (202 MHz, [D8]THF, 203 K, ppm): d = ꢀ12.4 (d, 2JP,P = 57.1 Hz,
Table 1
Crystal data for compounds 1 and 2
1
2
2
1P, PCH3), ꢀ15.3 (d, JP,P = 57.1 Hz, 1P, PCH3). Anal. Calc. for
Empirical formula
Molecular mass
Crystal size (mm)
Crystal system
Space group
a (Å)
C20H33FeNOP2
421.26
0.44 ꢁ 0.40 ꢁ 0.20
monoclinic
P21/n
7.9965(11)
12.866(2)
21.641(4)
96.253(13)
2213.3(6)
4
C19H30CoNOP2
409.31
0.50 ꢁ 0.45 ꢁ 0.45
monoclinic
P21/c
13.274(2)
7.409(2)
22.928(9)
106.190(13)
2165.5(9)
4
1.255
0.946
150(2)
1.60 6 H 6 27.07
ꢀ16 6 h 6 16
ꢀ9 6 k 6 9
ꢀ29 6 l 6 29
29068
C18H27NNiOP2 (394.06): C, 54.86; H, 6.91; N, 3.55; P, 15.72. Found:
C, 54.30; H, 7.31; N, 3.42; P, 15.18.
2.1.4. X-ray structure determinations
Data collection was performed on a STOE IPDSII image plate
b (Å)
c (Å)
detector Mo Ka radiation (k = 0.71019 Å). Details of the crystal
structure are given in Table 1. Data collection: Stoe XAREA [44].
Cell refinement: Stoe X-AREA [45]. Data reduction: Stoe X-RED [45].
The structure was solved by direct methods using SHELXS-97 [46],
and anisotropic displacement parameters were applied to non-
hydrogen atoms in a full-matrix least-squares refinement based
on F2 using SHELXL-97 [46]. In the title compounds, the hydrogen
atoms on C were placed at calculated positions (C–H = 0.93 Å,
0.96 Å for methyl groups) and were allowed to ride on the parent
atom [Uiso(H) = 1.2U(C) for CH groups and Uiso(H) = 1.5U(C) for
methyl groups]. For (1) the disordered PMe3 group could be treated
with a split model with site occupation factors of 0.58(1) for C211–
C231 and 0.42(1) for C212–C232 each. The disordered P(2)Me3
group in (2) was similarly split into C141–C161 with
s.o.f = 0.61(1) and C142–C162 with s.o.f. = 0.39(1) but with isotro-
pic model. However, it was not possible to model the apparent dis-
b (°)
V (Å3)
Z
Dcalc. (g/cm3)
1.264
0.834
150(2)
l
(Mo K
Temperature (K)
-Range (°)
Index ranges
a
) (mmꢀ1
)
H
1.84 6
H
6 27.17
ꢀ9 6 h 6 10
ꢀ16 6 k 6 16
ꢀ27 6 l 6 27
31064
Number of reflections measured
Number of unique data (Rint
Parameters
)
4776 (0.0758)
255
4699 (0.0504)
221
Restraints
18
1.078
15
1.044
Goodness-of-fit on F2
R1 [I P 2
r(I)]
0.0338
0.0415
wR2 (all data)
0.0915
0.1160

R. Beck et al. / Inorganica Chimica Acta 362 (2009) 1984–1990
1987
phine ligands. The range of coupling constants corresponds with cis
3.1. Reaction with CoCH3(PMe3)4
disposed trimethylphosphine ligands [20].
These data establish the only probable geometry around iron as
octahedral, with two phosphines and the methyl groups in cis posi-
tions. From the NMR spectra, the single set of resonances indicates
that only one of two possible configurational isomers is present,
which could be methyl trans either to oxygen or nitrogen.
When combined with 2-benzoylpyridine in tetrahydrofuran, a
solution of CoCH3(PMe3)4 during warm up turns green-blue, indi-
cating the coordination the of the aromatic backbone forming com-
plex 2 according to (Eq. (2)).
ð2Þ
From concentrated pentane solutions at ꢀ27 °C crystals were
obtained which were suitable for X-ray diffraction. The molecular
structure (Fig. 1) displays an octahedrally coordinated iron(II) atom
with two cis-disposed methyl groups, two cis-disposed trimethyl-
phosphine ligands, and the neutral [N,O] chelating ligand with a
bite angle of (O1–Fe1–N1 = 80.46(6)°), completes the coordination
sphere. The sum of internal angles in the five-membered chelate
ring (539°) indicates planarity and donor positions along the three
main axes of the octahedron deviate by less than 12° from
linearity.
The slightly shorter Fe–C bond with (Fe1–C1 = 2.045(2) Å) lies
opposite to the softer trimethylphosphine donor P2 and the longer
(Fe1–C2 = 2.055(2) Å) is subject to the influence of the harder
oxygen donor atom of the 2-benzyolpyridine. The strong trans
influence of the C1 atom is observed by an elongated distance
(Fe1–P2 = 2.3097(5) Å) when compared with the weaker pyridyl
anchoring group (N1), trans to P1 (2.1950(6) Å) which is ca. 11
pm shorter. The Fe–N and Fe–O bond lengths are as observed in
other iron(II) complexes containing oxygen and nitrogen donor
atoms [21].
The C@O bond lengths with C8–O1 = 1.294(2) Å is elongated,
when compared with free benzoylpyridine (1.213 Å) [22], but is
still to be regarded as a double bond. Related 2-pyridylalcoholato
ligands in W, Zr, Ti, and Mo complexes show C–O single bond
length in the range of 1.386–1.427 Å [23]. A related structure of
1 is described with a cationic acylFe(II) complex bearing 2-benzo-
ylpyridine [17].
Most methyliron complexes described in literature carry an
additional set of stabilizing phosphine [24], and/or cyclopentadi-
ene [25], ligands, but to the best of our knowledge, iron–methyl
complexes containing oxygen donor atoms in the ligand sphere
have not been described. Recently, with nitrogen ligands of the
tris(pyrazolyl)borato-type [26], and dimine-pincer type [27],
methyliron complexes were studied.
To our surprise the methyl group remained attached to the me-
tal after reaction. The complex was obtained in 83% yield. Crystals
of 2 form large blue sticks with shiny surfaces that slowly decom-
pose in air although their solutions decompose within a few sec-
onds if air is admitted, similar to 1. Typically cobalt(I) complexes
in trigonal bipyramidal coordination with similar sets of donor
atoms attain an orange to red color. The unusual blue color of com-
plex 2 appears to be a direct consequence of a square-pyramidal
coordination and hemilabile bonding.
The infrared spectrum displays the expected bathochromic shift
of the carbonyl stretching frequency, 1485 cmꢀ1
(m C@O), with
weak intensity, similar as observed for 1 and ortho-metalated ben-
zophenone Co(I) (1484 cmꢀ1) [30], when coordinated to the metal.
The 31P{1H} NMR spectrum features a broad singlet at ꢀ5.7 ppm,
which is not resolved upon cooling the sample to 203 K. There
can be two reasons for this exchange broadening in pentacoordi-
nated systems: we have either dissociation of PMe3-groups or fast
exchange of ligand positions in a pseudorotation process involving
square-pyramidal and trigonal bipyramidal coordination geome-
tries. Similar, on the relatively slow 1H NMR timescale, the fast
reversible ligand mobility of the trimethylphosphine ligands, con-
duct a collapse of the 3J(P,H) coupling between the Co–CH3 group
and the phosphorus nuclei. Typically, 3J(P,H) couplings in the 1H
NMR spectrum are observed in more rigid, octahedral coordina-
tion, in the range between 3 and 8 Hz. The singlet resonance of
the Co–CH3 group (0.51 ppm) in the 1H NMR spectrum corresponds
with a high field shift in the 13C{1H} NMR at ꢀ29.3 ppm (Co–CH3),
observed as multiplet (nuclear spin I: Co = 7/2).
An X-ray diffraction confirmed for the solid state the constitu-
tion derived from solution NMR spectra. The ORTEP plot of 2 is
shown in Fig. 2. The molecular structure of 2 closely resembles a
pentacoordinated cobalt(I) center in square-pyramidal coordina-
tion surrounded by an apical trimethylphosphine ligand (P2), the
methyl group (C1) in trans configuration of the oxygen donor atom,
a trimethylphosphine ligand (P1) and the chelating [N,O] (2-ben-
zyolpyridine) ligand spanning the four basal sites.
Moreover, dimethyliron complexes are very rare, only three
have been structurally described to date [28].
The balanced N,O coordination of the 2-benzoylpyridine can
stabilize two thermally labile methyl functions of iron(II)
[Fe(CH3)2(PMe3)4], which are easily removed by other substrate
molecules through C–H activation, or by extended reaction periods
when reductive elimination is observed and zerovalent [Fe(PMe3)4]
is produced [29].
The essential feature of a the square-pyramidal coordination is
described by the four angles between the apical P2 donor atom and
the four equatorial ligand positions (P1, C1, O1 and N1) with
102.47(3)°, 95.06(10)°, 100.94(6)° and 100.20(6)°, respectively.
The basal angles vary between 88° and 94° with the smallest for
the [N,O] bite angle at 82.05(8)°. The dimensions in the chelate ring

1988
R. Beck et al. / Inorganica Chimica Acta 362 (2009) 1984–1990
Fig. 1. Molecular structure of 1. Anisotropic displacement ellipsoids are plotted at the 50% probability level. Minor part of disordered P(1)Me3 group omitted. Selected
distances [Å] and angles [°]: Fe1–C1 2.045(2), Fe1–C2 2.055(2), Fe1–N1 1.9860(15), Fe1–O1 1.9071(13), Fe1–P1 2.1950(6), Fe1–P2 2.3097(5), C8–O1 1.294(2); O1–Fe1–N1
80.46(5), O1–Fe1–C2 175.75(7), N1–Fe1–P1 168.77(4), C1–Fe1–P2 171.84(6), C1–Fe1–C2 86.69(8), P1–Fe1–P2 96.57(2).
Fig. 2. Molecular structure of 2. Anisotropic displacement ellipsoids are plotted at the 50% probability level. Minor part of disordered P(2)Me3 group omitted. Selected
distances [Å] and angles [°]: Co1–C1 1.995(3), Co1–N1 1.918(2), Co1–O1 1.9207(17), Co1–P1 2.1736(8), Co1–P2 2.1735(10), C7–O1 1.307(3); O1–Co1–N1 82.05(8), O1–Co1–
C1 163.98(11), N1–Co1–P1 156.86(6), N1–Co1–C1 94.36(11), O1–Co1–P1 89.10(6), P2–Co1–P1 102.47(3), C1–Co1–P2 95.06(10), O1–Co1–P2 100.94(6), N1–Co1–P2
100.20(6).
are normal, and the bite angle of the chelating 2-benzoylpyridine
error equals those of the monoanionic benzophenone [C,O] coordi-
[N,O] ligand with (O1–Co1–N1 = 82.05(8)°), within experimental
nation (83.49(7)°) or monoanionic [C,N] coordination of 2-vinylpy-

R. Beck et al. / Inorganica Chimica Acta 362 (2009) 1984–1990
1989
Scheme 3.
ripdine ligands (80.5(1)°) [7,30], only 1.5° more augmented than in
1 and the sum of internal angles (539.1) is as expected for an al-
most planar five-membered metallacycle.
Similar ranges for Co–CH3 bond lengths (Co1–C1 = 1.995 Å) are
also observed for other cobalt compounds with a comparable set of
donor atoms, independent of the oxidation state. The cobalt–phos-
phorus distances, Co1–P1 = 2.1736(8) Å and Co1–P2 = 2.1735(10) Å
are equal and are well in the range of Co–P distances observed in
pentacoordinated Co(I)–PR3 complexes [31], indicating a balanced
trans influence in this coordination mode.
The sigma coordinated C@O-group (C7–O1 = 1.307(3) Å) is
again only slightly elongated compared with free 2-benzoylpyri-
dine, but much shorter as described for 2-pyridylalcoholato com-
plexes as aforementioned in the iron case.
Typically the methyl group of Co(CH3)(PMe3)4 is thermally la-
bile and the basic behavior easily activates C–H bonds of various
organic substrates with release of methane. However, previously
The two methyl groups are easily eliminated by release of eth-
ane in a reductive elimination process forming the zerovalent nick-
el complex 3 prior to an alternative sigma-N,O coordination of 2-
benzyolpyridine. Similar reductive elimination of dimethyl nickel
has been described in reactions with PPh2-donor ligands [38]. We
propose from NMR spectra a bonding mode as in a zerovalent
nickel complex of (g
2-C@O)benzophenone, which was previously
described without detection of the C@O resonance in 13C{1H}
NMR [39]. Additionally, in a related study with Ni(CH3)2(PMe3)3
and thiobenzophenones, we observed reductive elimination by re-
lease of ethane. The X-ray structure reveals a three coordinated
nickel(0) center with two trimethylphosphine ligands, almost
coplanar to the
p-coordinated g
2C@S fragment. The phenylrings
of the coordinated thiobenzophenone are in fan type conformation
and the closest aromatic ortho carbon atoms with 3.2 and 3.6 Å, are
far away from bonding interactions [40]. It appears a similar bond-
ing situation in 3, for 2-benzoylpyridine attached to the nickel
atom, without incorporating the N-donor atom.
it was only possible in reactions with strong
p-acceptor ligands
to keep the methyl group attached to the metal in a low oxidation
[32,33].
4. Conclusion
To the best of our knowledge, a square-pyramidal configuration
of low-valent Co(I) has not been described so far which is typical
for Cobalt(II) with tri- or tetradentate ligands [34]. Most pentaco-
ordinated cobalt(I) complexes attain a trigonal bipyramidal coordi-
nation, and only a few methylcobalt complexes are known in +II
and +III oxidation states. Low-valent methylcobalt(I) complexes
were recently studied with pincer-type ligands of bis-iminopyri-
dines [35], and bis-N-heterocyclic carbenes [36], or with strong
A regiospecific displacement of two trimethylphosphine ligands
is achieved with a neutral hemilabile [N,O]-coordinating 2-benzo-
ylpyridine ligand in reaction with Fe(CH3)2(PMe3)4 and
Co(CH3)(PMe3)4. In both cases the methyl groups remain attached
to the metal after reaction. The N,O coordination is capable of sta-
bilizing these thermally labile methyl groups of iron(II) (1) in an
octahedral, and for cobalt(I) (2) in a square-pyramidal environ-
ment. By contrast, in a reaction with Ni(CH3)2(PMe3)3 reductive
elimination of ethane is initiated and a zerovalent nickel complex
3 is formed.
Surprisingly, in all reactions described here there was no evi-
dence for a C–H activation of available aromatic C–H bonds, which
was previously observed for isoelectronic systems. As the metals
retain their low oxidation states with an attached methyl group,
these compounds can be seen as model substances for studying a
growing alkyl-metal chain by subsequent insertion of ethene or
carbon monoxide in the metal–carbon bond. Experiments with this
aim are currently under way.
p-acceptor ligands as diazo-cylopentadienes and fulvenes
[32,33]. To date the longest Co–CH3 distance (2.081 Å) has been re-
ported for a methyltricarbonyl(tricyclohexylphosphine)cobalt(I)
[37].
3.1.1. Reaction with Ni(CH3)2(PMe3)3
At variance with reactions of 1 and 2 under similar conditions,
reductive elimination is observed during 6 h, when Ni(CH3)2
(PMe3)3 reacts with 2-benzoylpyridine in pentane/diethyl ether
(1:1). (Scheme 3).
From pentane the light yellow powder of 3 is deposited ꢀ27 °C,
that is extremely air-sensitive and melts at 91–94 °C under decom-
position. Crystallization from different solvents to get suitable
crystals for X-ray diffraction proved unsuccessful.
Acknowledgment
The loss of both methyl substituents can be derived from the 1H
NMR data for 3, and the multiplicity of the signals indicates mirror
symmetry of the molecule.
Financial support of this work by Fonds der Chemischen Indust-
rie is gratefully acknowledged.
The 31P{1H} NMR spectrum in THF at 25 °C, consists of singlet
resonance at 0.9 ppm, indicating dissociation of for the trimethyl-
phosphine ligands, which is split into doublet (2JP,P = 57.1 Hz) and
separated into two peaks accompanied by high field shift
(ꢀ12.4 ppm and ꢀ15.3 ppm) when the sample is cooled to
Appendix A. Supplementary material
CCDC 681203 and 681204 contain the supplementary crystallo-
graphic data for 1 and 2. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc
.cam.ac.uk/data_request/cif. Supplementary data associated with
this article can be found, in the online version, at doi:10.1016/
j.ica.2008.09.021.
ꢀ70 °C. An essential feature is observed in the
g
2-C@O resonance
of the 13C{1H} NMR spectrum by high field shift to 147.1 ppm when
compared with 1 and 2 (1: 160.1 ppm, 2: 160.8 ppm) and the free
2-benzoylpyridine (193.7 ppm, C@O).

1990
R. Beck et al. / Inorganica Chimica Acta 362 (2009) 1984–1990
(b) S. Doherty, R.J. Errington, A.P. Jarvis, S. Collins, W. Clegg, M.R.J. Elsegood,
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