Reduction of terphenyl iron(ii) or cobalt(ii) halides in the presence of trimethylphosphine: An unusual triple dehydrogenation of an alkyl group

Article abstract of DOI:10.1039/b904647j

The reduction of {ArFeBr}₂ (Ar = terphenyl) with KC₈ in the presence of excess PMe₃ afforded the Fe(i) complex 3,5-Pr ⁱ₂-Ar′Fe(PMe₃) (1) (Ar′-3,5-Pr ⁱ₂ = C₆

Full text of DOI:10.1039/b904647j

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Reduction of terphenyl iron(II) or cobalt(II) halides in the presence of
trimethylphosphine: an unusual triple dehydrogenation of an alkyl group†
Chengbao Ni,a Bobby D. Ellis,a Troy A. Stich,a James C. Fettinger,a Gary J. Long,b R. David Britta and
Philip P. Power*a
Received 6th March 2009, Accepted 17th April 2009
First published as an Advance Article on the web 26th May 2009
DOI: 10.1039/b904647j
The reduction of {ArFeBr}2 (Ar = terphenyl) with KC8 in the presence of excess PMe3 afforded the
Fe(I) complex 3,5-Pri2-Ar¢Fe(PMe3) (1) (Ar¢-3,5-Pri2 = C6H-2,6-(C6H3-2,6-Pri2)-3,5-Pri2), which has a
structure very different from the previously reported, linear Cr(I) species 3,5-Pri2-Ar*Cr(PMe3)
6
(3,5-Pri2-Ar* = C6H-2,6-(C6H2–2,4,6-Pri3)2–3,5-Pri2) and features a strong Fe-h -aryl interaction with
the flanking aryl ring of the terphenyl ligand. In sharp contrast, the reduction of {ArCoCl}2 (Ar =
3
3,5-Pri2-Ar¢ and Ar¢) afforded the allyl complexes Co(h -{1-(H2C)2C-C6H3–2-(C6H2–2,4-Pri2–5-
3
(C6H3–2,6-Pri2))-3-Pri})(PMe3)3 (4) and Co(h -{1-(H2C)2C-C6H3–2-(C6H4–3-(C6H3–2,6-Pri2))-
3-Pri})(PMe3)3 (5) formed by an unusual triple dehydrogenation of an isopropyl group. It is proposed
that the reduction initially generates an intermediate 3,5-Pri2-Ar¢Co(PMe3), which is similar in structure
to 1, followed by 3,5-Pri2-Ar¢Co(PMe3) decomposition to a cobalt hydride intermediate and
dehydrogenation of the isopropyl group via remote C–H activation induced by PMe3 complexation.
Complexes 1, 4, and 5 were characterized by X-ray crystallography. In addition, 1 was studied by NMR
and EPR spectroscopy; 4 and 5 were characterized by NMR spectroscopy.
and characterization of complexes with very different structures.
Introduction
These are 3,5-Pri2-Ar¢Fe(PMe3) (1) (3,5-Pri2-Ar¢ = 3,5-Pri2-C6H-
2,6-(C6H3–2,6-Pri2)2), which although it has similar stoichiometry
to the corresponding two-coordinate Cr(I) complex, has a very
Recent work has shown that a series of dimeric transition metal(I)
complexes with the formula Ar¢MMAr¢ (M = Cr,1 Fe2 and Co2)
could be isolated and characterized with the use of the bulky
terphenyl ligand Ar¢ (Ar¢ = C6H3-2,6-(C6H3-2,6-Pri2)2). Unlike
the Ar¢CrCrAr¢ species, which has a quintuple bond between the
two Cr centers, the iron and cobalt derivatives have essentially
6
strong Fe-h -aryl interaction with a flanking ring from the ter-
3
phenyl ligand, and the allyl complexes Co(h -{1-(H2C)2C-C6H3-
3
2-(C6H2-2,4-Pri2-5-(C6H3-2,6-Pri2))-3-Pri})(PMe3)3 (4) and Co(h -
{1-(H2C)2C-C6H3-2-(C6H4-3-(C6H3-2,6-Pri2))-3-Pri})(PMe3)3 (5),
6
no metal–metal bonding and strong h interactions with one
in which the terphenyl ligands have been triply dehydrogenated
of the flanking –C6H3-2,6-Pri2 rings from the terphenyl ligand
attached to the neighbouring metal center. By modifying the
bulky terphenyl ligand on both the central ring and flanking rings,
3
at an Pri group from a flanking aryl ring to afford h -allyl cobalt
phosphine complexes.
6
the monomeric half-sandwich arene complexes (h -C6H6)FeAr*-
Experimental
3
6
4
3,5-Pri2 and (h -C7H8)CoAr*-3,5-Pri2 (Ar*-3,5-Pri2 = C6H-
2,6-(C6H2-2,4,6-Pri3)2-3,5-Pri2) and the inverted sandwich Mn(I)
General procedures
6
6
complex (m-h :h -C7H8){MnAr*-3,5-Pri2}23 could be isolated. For
chromium, however, the corresponding arene complexes proved
to be unstable. Instead the two-coordinate chromium(I) Lewis
base complexes 3,5-Pri2Ar*Cr(L) (L = THF or PMe3)5 could be
obtained upon addition of THF or PMe3. We wished to isolate
similar Lewis base complexes of univalent later metals such as iron
and cobalt in order to obtain two-coordinate M(I) species of the
type Ar-M-L (L = donor molecule), which are currently unknown.
We now report that the attempted isolation of analogous two-
coordinate iron and cobalt complexes leads to the synthesis
All manipulations were carried out using modified Schlenk tech-
niques under an argon atmosphere or in a Vacuum Atmospheres
HE-43 drybox. All of the solvents were first dried by the method of
6
˚
Grubbs, followed by storage over 3 A molecular sieves overnight
and degassed three times (freeze-thaw) prior to use. {3,5-Pri2-
Ar¢FeBr}2, {Ar¢CoCl}2, and {3,5-Pri2-Ar¢CoCl}2, were prepared
according to a literature procedure.7 Melting points were recorded
in glass capillaries sealed under N2 and are uncorrected. UV-vis
data were recorded on a Hitachi-1200 spectrometer.
Preparation of 3,5-Pri2-Ar¢Fe(PMe3) (1). A pale pink solution
of {3,5-Pri2-Ar¢FeBr}2 (0.926 g, 0.75 mmol) and PMe3 (0.456 g,
6.00 mmol) in ca. 20 mL THF was added dropwise to a freshly
prepared suspension of KC8 (0.203 g, 1.50 mmol) in ca. 20 mL
THF at 0 ◦C. The solution turned orange red immediately and
was stirred for a further 24 h. The solvent was removed under
aDepartment of Chemistry, University of California, Davis, California,
95616. E-mail: pppower@ucdavis.edu
bDepartment of Chemistry, University of Missouri-Rolla, Rolla, Missouri,
65409-0010
† CCDC reference numbers 723169–723171. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b904647j
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reduced pressure and the resulting dark solid was extracted with
hexanes (ca. 40 mL). The solution was filtered and the reddish
brown filtrate was concentrated to ca.10 mL, which afforded
X-ray quality red-brown crystals of 1 after storage for three days at
-18 ◦C. Yield: 0.305 g (33.1%). Melting point: 175–177 ◦C. UV-vis
(hexane, nm [e, cm-1M-1]): 360 (1800), 426 (850). Anal. calcd For
C39H58FeP: C 76.33, H 9.53. Found: C 76.51, H 9.69.
Magnetic studies
For a typical measurement, 17.6 mg of 1 was dissolved in exactly
1.0 mL mixture of C6H6 and C6D6 and some solution was
transferred into an NMR tube. Into the NMR tube, a sealed
capillary that contained the C6H6–C6D6 solvent mixture was
placed. The NMR spectra were recorded on a Varian spectrometer
operating at 300.077 MHz at 292.75 K. Two peaks were identified
for C6H6 protons, which have a chemical shift difference of
0.231 ppm. Based on the theory of the Evans¢ method,8,9 the
magnetic susceptibility was calculated to be 1.92 ¥ 10-3 cm3mol-1,
which corresponds to an effective magnetic moment of 2.11 mB per
3,5-Pri2-Ar¢Fe(PMe3) molecule. EPR data were recorded at 8 K
on a Bruker EC 106 X-band Spectrometer using an ER-4116 DM
dual-mode cavity.
Preparation of Co(g3-{1-(H2C)2C-C6H3–2-(C6H2–2,4-Pri2–5-
(C6H3–2,6-Pri2))-3-Pri})(PMe3)3 (4). A dark blue solution of
{3,5-Pri2-Ar¢CoCl}2 (1.729 g, 1.50 mmol) and PMe3 (0.912 g,
12.00 mmol) in ca. 30 mL THF was added dropwise to a freshly
prepared suspension of KC8 (0.405 g, 3.00 mmol) in ca. 20 mL
THF at 0 ◦C. The solution turned orange immediately and stirring
was continued for 24 h. The solvent was removed under reduced
pressure and the resulting dark solid was extracted with hexanes
(ca. 60 mL). The solution was filtered and the reddish orange
filtrate was concentrated to ca. 30 mL and stored in a -18 ◦C
freezer to afford 4 as X-ray quality orange crystals which were
separated from 3,5-Pri2-Ar¢-H produced during reduction. Yield:
0.26◦9 g (11.7%). This compound decomposes to a black solid at
X-Ray crystallography
Suitable crystals of 1, 4 and 5 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
were collected on a Bruker SMART 1000 diffractometer at 90(2)
122 C. 1H NMR (600 M Hz, C6D6, 25 ◦C): d = 1.113 (d, 3JH–H
=
6.6 Hz, 6 H, CH(CH3)2), 1.132 (d, 3JH–H = 6.6 Hz, 6 H, CH(CH3)2),
1.147 (d, 3JH–H = 6.6 Hz, 6 H, CH(CH3)2), 1.162 (d, 3JH–H = 6.6 Hz,
6 H, CH(CH3)2), 1.176 (d, 3JH–H = 6.6 Hz, 6 H, CH(CH3)2), 1.841
(br. m. 9 H, P(CH3)3), 2.543 (s, 2 H, C(CH2)2), 2.620 (s, 2 H,
˚
K using Mo Ka radiation (l = 0.71073 A) or on a Bruker
SMART Apex II diffractometer at 90(2) K with Mo Ka radiation
˚
(l = 0.71073 A). Absorption corrections were applied using
SADABS.10 The structures were solved using direct methods and
refined by the full-matrix least-squares procedure in SHELX.11 All
of the non-hydrogen atoms were refined anisotropically. Hydrogen
atoms were placed at calculated positions and included in the
refinement using a riding model.
3
C(CH2)2), 2.729 (sept, JH–H = 6.6 Hz, 1 H, CH(CH3)2), 2.750
(sept, 3JH–H = 6.6 Hz, 1 H, CH(CH3)2), 2.780 (sept, 3JH–H = 6.6 Hz,
3
1 H, CH(CH3)2), 2.801 (sept, JH–H = 6.6 Hz, 1 H, CH(CH3)2),
3
2.852 (sept, JH–H = 6.6 Hz, 1 H, CH(CH3)2), 6.830 (s, 1 H,
C6H2), 7.140 (m, 1 H, p-C6H3), 7.172 (m, 1 H, p-C6H3), 7.311
(m, 2 H, m-C6H3), 7.292 (m, 2 H, m-C6H3), 7.541 (s, 1 H, o-C6H2).
◦
1
31P NMR (300 M Hz, C6D6, 25 C, H gated decoupled), d =
9.43 (br. 1 P, PMe3), -0.05 (br. 2 P, PMe3). UV-vis (hexane, nm
[e, cm-1M-1]): 387 (4900), 462 (1750).
Results and discussion
Synthesis
Like the reduction of {3,5-Pri2Ar*CrCl}2 in the presence of PMe3,5
the reduction of {3,5-Pri2-Ar¢FeBr}2 with KC8 in THF in the
presence of excess PMe3 (Scheme 1) afforded the Fe(I) complex 1,
which has an analogous stoichiometry to the two-coordinate Cr(I)
complex. Complex 1 was isolated from hexanes as paramagnetic,
air and moisture sensitive dark orange crystals in modest yield.
Preparation of Co(g3-{1-(H2C)2C-C6H3-2-(C6H4-3-(C6H3-2,6-
Pri2))-3-Pri})(PMe3)3 (5). A dark blue solution of [Ar¢CoCl]2
(1.476 g, 1.50 mmol) and PMe3 (0.912 g, 12.00 mmol) in ca. 30 mL
THF was added dropwise to a freshly prepared suspension of KC8
(0.405 g, 3.00 mmol) in ca. 20 mL THF at 0 ◦C. The solution
turned orange immediately and was stirred for a further 24 h. The
solvent was removed under reduced pressure and the resulting dark
solid was extracted with hexanes (ca. 60 mL). The solution was
filtered and the red orange filtrate was concentrated to ca. 15 mL
and stored in a -18 ◦C freezer. X-ray quality orange crystals were
◦
isolated after storage for ca. three days at -18 C. Yield 0.28◦4 g
(13.9%). This compound decomposes to a black solid at 117 C.
1H NMR (300 M Hz, C6D6, 25 ◦C): d = 1.124 (d, 3JH–H = 6.9 Hz,
6 H, CH(CH3)2), 1.136 (d, 3JH–H = 6.9 Hz, 6 H, CH(CH3)2), 1.140
(d, 3JH–H = 6.9 Hz, 6 H, CH(CH3)2), 1.837 (br. m. 9 H, P(CH3)3),
2.550 (s, 2 H, C(CH2)2), 2.640 (s, 2 H, C(CH2)2), 2.713 (sept,
Scheme 1 Reduction of {3,5-Pri2-Ar¢FeBr}2 with KC8 in the presence of
PMe3.
3
3JH–H = 6.9 Hz, 1 H, CH(CH3)2), 2.773 (sept, JH–H = 6.9 Hz, 1
H, CH(CH3)2), 2.903 (sept, 3JH–H = 6.9 Hz, 1 H, CH(CH3)2), 7.04
(s, 1 H, C6H4), 7.113 (m, 1 H, p-C6H3), 7.142 (m, 1 H, p-C6H3),
7.17 1(m, 2 H, m-C6H3), 7.192 (m, 2 H, m-C6H3), 7.316 (m, 3 H,
C6H4). 31P NMR (300 M Hz, C6D6, 25 ◦C, 1H gated decoupled),
d = 9.27 (br. 1 P, PMe3), -0.13 (br. 2 P, PMe3). UV-vis (hexane,
nm [e, cm-1M-1]): 385 (4800), 465 (1600).
In sharp contrast to 1, the reduction of {3,5-Pri2-Ar¢CoCl}2
(Scheme 2) under similar conditions did not afford a product
analogous to either 1 or the linear 3,5-Pri2Ar*Cr(PMe3);5 instead,
the highly unusual conversion of the terphenyl group into an
3
h -allyl ligand was observed and the unexpected product 4 was
isolated in low yield after the separation of the co-product
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Scheme 2 Reduction of {3,5-Pri2-Ar¢CoCl}2 with KC8 in the presence of
PMe3.
3,5-Pri2-Ar¢-H. The related product 5, which carries no Pri groups
on the central aryl ring, was isolated in a similar way.
Structures
The structures of compounds 1, 4 and 5 were determined by
X-ray crystallography. Important data collection and refinement
parameters for 1, 4 and 5 are provided in Table 1.
Fig. 1 Molecular structure of 1 with thermal ellipsoids presented at a 30%
probability level. All hydrogen atoms are not shown. Selected bond lengths
◦
˚
(A) and angles ( ): Fe1–C1 2.0426(13), Fe1–centroid 1.574(2), Fe1–P1
The structure of 1 is illustrated in Fig. 1. The iron is
coordinated to the ipso carbon of the aryl ligand and one
PMe3 molecule. Unlike the almost linear two-coordinate Cr(I)
complex 3,5-Pri2-Ar*Cr(PMe3),5 the Fe center in 1 has a strongly
2.2509(5), C2–C7 1.5001(18), C6–C13 1.5122(17) Fe–C (flanking ring)
2.0155(13), 2.1007(13), 2.1027(14), 2.1628(14), 2.1651(14), 2.1679(14),
C1–Fe1–P1 113.77(4), Fe1–C1–C6 95.13(8), C2–C1–C6 117.27(11),
C1–C6–C13 103.10(11), C1–C2–C7 121.56(12).
◦
6
bent geometry (C(1)–Fe(1)–P(1) = 113.77(4) ) with a strong h
interaction with one of the flanking –C6H3-2,6-Pri2 rings of the
interaction with the flanking aryl ring; for example, the Fe(1)–
C(1)–C(6) and Fe(1)–C(1)–C(2) angles differ by over 42◦, the
angles involving the flanking rings (C(1)–C(6)–C(13) 103.10(11)◦,
C(1)–C(2)–C(7) 121.56(12)◦) differ considerably, and the angle
between the C(6)–C(13) bond and the interacting aryl ring is ca.
143.02◦. The C–C bond distances of C(13) within the flanking
˚
terphenyl ligand. The Fe-centroid distance of 1.574(2) A, which
is slightly longer than the solvent dependent bis(imino)pyridine
i
=
iron complexes [2,6-(2,6-Pr 2-C6H3N CPh)2C5H3N]Fe (1.527(4)
12
6
1
˚
and 1.534(4) A), in which
a
similar h /h interaction
6
was observed. It is similar to those in [Fe(h -C10H8)(1,2-
bis(dicyclohexylphosphino)ethane)] (1.597(6) A)13 and in {(C6H11-
ring are on average ca. 0.03 A longer than the other C–C distance.
˚
˚
6
14
˚
=
N CH)2(h -C7H8)Fe} (1.542 A). However, it is significantly
The strong deviation of C(1)–Fe(1)–P(1) angle from linearity is
due to the tendency of the iron, which has a low number of
shorter than those in the terphenyl ligand stabilized monomeric
6
i
3
6
˚
complex {(h -C6H6)FeAr*-3,5-Pr 2} (1.6427(13) A) and in the
valence electrons (11 without the h -arene interaction), to complex
2
dimeric complex {Ar¢FeFeAr¢} (1.7333(18) A). The terphenyl
electron rich moieties.15 The Fe(1)–C(1) distance (2.0426(13) A)
˚
˚
6
ligand has a very distorted geometry because of the strong h
is essentially the same as those in the aryl Fe(I) complexes
Table 1 Selected crystallographic data and collection parameters for 1, 4, and 5
1
4
5
Formula
C39H58FeP
613.67
90(2)
C45H74CoP3
766.88
90(2)
C39H62CoP3
682.73
90(2)
0.71073
Orange block
Orthorhombic
Pbca
23.347(5)
12.992(3)
26.446(5)
90
90
90
8021(3)
8
Formula weight
T/K
˚
l/A
0.71073
Red plate
Monoclinic
P21/n
10.3962(13)
15.737(2)
22.311(3)
90
95.490(2)
90
3633.6(8)
4
0.71073
Colour, habit
Crystal system
Space group
Orange block
Monoclinic
P21/c
19.466(3)
14.368(3)
16.471(3)
90
106.466(3)
90
4417.7(13)
4
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Z
D
calcd/Mg m-3
1.122
2.75–27.50
0.483
1.153
2.60–25.25
0.525
1.131
2.81–27.49
0.571
1.052
q range/◦
m/mm-1
Goodness-of-fit on (GOF) F2
Final R indices [I > 2s(I)]
1.047
0.982
R1 = 0.0344 wR2 = 0.0930
R1 = 0.0585 wR2 = 0.1582
R1 = 0.0293 wR2 = 0.0774
-3
˚
Maximum peak/hole/e A
0.639/-0.429
0.976/-0.604
0.474/-0.260
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6
3
[(h -C6H6)FeAr*-3,5-Pri2] (2.049(4) A) and {Ar¢FeFeAr¢}
˚
carbon of the central ring of the aryl ligand. Instead it becomes
coordinated to three carbon atoms that were originally part of
an isopropyl group from a flanking aryl ring. The coordination
geometry at the central carbon of the allyl group is planar
2
˚
˚
(2.028(4) and 2.048(4) A). The Fe–P distance of 2.2509(5) A lies
in the range 2.157 A to 2.471 A, observed in numerous iron
phosphine complexes. The strong h -arene interaction exhibited
16
˚
˚
6
˚
and the C–C distances lie in the range 1.420(5)-1.437(2) A,
by 1 and other putatively low-coordinate late transition metal
complexes is supported by calculations.17
consistent with a C–C bond order near 1.5. These distances
are slightly longer than bonds in other Co-allyl complexes,
The structures of 4 and 5 are very similar and are shown
in Fig. 2 and 3. The cobalt is no longer bound to the ipso
18
˚
such as {[(CH3O)3P](CO)2Co[(CH2)2CCH2]}2(CO) (1.408 A)
5
3
19
˚
and (h -C5H5)[h -2-(CH2CH)CH2CCH2]CoBr (1.394 A). This
may be due to the greater electron density at the metal because
of the good s-donor properties of the PMe3 co-ligand, which
may result in more back donation of electron density into
the p* orbital of the terphenyl ligand. The Co–C distances
˚
˚
in 4 and 5 are in the range 1.974(4) A to 2.0357(16) A,
which is also similar to that observed in the allyl complexes
mentioned above and are within the range of Co–C distance
20
˚
of 1.911 to 2.11 A in allyl cobalt complexes in general. The
three Co–P distances are uniform with an average Co–P value
˚
2.158(4) A, which is similar to those in the related complexes [anti-
5
3
21
˚
1,2,3,9,10-h :4,5,6-h -azulene]Co2(PMe3)5 (2.190(3) A, average)
and [Co(PhCCC5H11)(PMe3)3] [BPh4] (2.161(2) A average).
+
-
22
˚
We propose that the formation of 4 involves intermediates 2 or
3 (3a or 3b) (see Scheme 3). The similarities of the reduction of
{Ar¢MX}2 (M = Fe and Co) in THF2 and the reduction of {3,5-
Pri2-Ar*MX}2 in aromatic solvents3,4 suggest the initial formation
of 2. This molecule, although it has an 18-electron configuration, is
expected to have a geometry more strained than that of 1 owing to
the smaller size of cobalt. The strain could lead to cleavage of the
Co-C s-bond of 2 with formation of an alkene/hydride complex
3a via replacement of the h -PMe3 ligand by the h -alkene group
and concomitant abstraction of two hydrogen atoms from an Pri
group. Under the influence of excess PMe3, this intermediate could
rearrange to give 4 with elimination of H2. This dehydrogenation
of inert alkyl groups via remote C–H activation has been observed
in Pd(OAc)2 mediated systems,23 but not for cobalt complexes.
Fig. 2 Molecular structure of 4 with thermal ellipsoids presented at
a 30% probability level. All hydrogen (except hydrogen of the central
˚
aryl ring) atoms are not shown. Selected bond lengths (A) and angles
1
2
(◦): Co1–C34 1.974(4), Co1–C35 2.034(4), Co1–C36 2.037(4), Co1–P
2.1405(12), 2.1564(12), 2.1756(12), C18–C34 1.492(5), C14–C31 1.523(5),
C31–C33 1.531(5), C34–C36 1.420(5), C34–C35 1.426(5), P1–Co1–P2
108.28(5), P1–Co1–P3 98.70(5), P2–Co1–P3 99.65(5), C18–C34–C36
124.1(4), C18–C34–C35 124.7(3), C35–C34–C36 111.2(3).
Scheme 3 Proposed mechanisms of the formation of 4.
Fig. 3 Molecular structure of 5 with thermal ellipsoids presented at a
30% probability level. All hydrogen (except hydrogen of the central aryl
A possible alternative mechanism involves the formation of 2
initially. However, it does not involve decomplexation of the PMe3
ligand. Under the influence of excess PMe3, the cobalt center could
move to the central carbon atom of the isopropyl group and 3b
could be generated. Upon complexation of a PMe3 molecule and
elimination of H2 gas, 4 could be formed.
◦
˚
ring) atoms are not shown. Selected bond lengths (A) and angles ( ):
Co1–C28 1.9829(15), Co1–C29 2.0395(16), Co1–C30 2.0537(16), Co1–P
2.1546(6), 2.1637(6), 2.1735(6), C18–C28 1.490(2), C28–C29 1.434(2),
C28–C30 1.437(2), P1–Co1–P2 106.596(18), P1–Co1–P3 99.174(18),
P2–Co1–P3 100.064(19), C18–C28–C29 125.28(14), C18–C28–C30
124.23(14), C29–C28–C30 110.48(13).
5404 | Dalton Trans., 2009, 5401–5405
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6
Similarly, reduction of the less bulky ligand stabilized cobalt
species {Ar¢CoCl}2 with KC8 in the presence of excess PMe3
afforded a similar compound 5; however, when the reduction was
attempted with the significantly smaller Ar# (Ar# = 2,6-(2,4,6-
Me3-C6H2)-C6H2) substituent, which does not have isopropyl
groups on the flanking rings, a brown solution was obtained and no
product was isolated. Due to the multiple intermediates involved
in the reaction, 4 and 5 were isolated in low but reproducible
yield.
Fe atom in 1 has an h interaction with the flanking aryl ring,
which yielded a very distorted geometry due to the tendency of
Fe to form arene complexes.17 In contrast, reduction of 3,5-Pri2-
Ar¢CoCl and Ar¢CoCl in the presence of excess PMe3 afforded
the diamagnetic allyl complexes 4 and 5. The highly unusual
formation of 4 and 5 may involve cobalt hydride intermediates and
the dehydrogenation of the inert isopropyl group via remote C–H
activation. The labilities of arene ring and PMe3 coordination
make these compounds attractive substrates for small molecule
activation. The study of their chemistry is under way.
Magnetic properties of 1
Acknowledgements
Both 4 and 5 have 18 electron configurations, but 1 is param-
agnetic. According to the Evans’ method, the effective magnetic
moment is 2.11mB, which is somewhat larger than the expected
spin only value of 1.73 mB for one unpaired electron. This may be
due to orbital contributions.
We thank the National Science Foundation (CHE-06401020) for
financial support.
References
EPR studies of 1 were undertaken to throw further light on its
bonding. The spectrum in Fig. 4 is illustrative of a low-spin d7
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2
2
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orbital.
-y
2
This is consistent with the need for the 3dz orbital to be empty in
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n = 9.6898 GHz, T = 8.0 K, power = 0.19 mW, MF = 100 kHz, MA =
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Conclusion
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In summary, the Fe(I) complex (1) with a geometry very different
from its Cr analogue and the allyl Co(I) complexes (4) and
(5) were isolated and characterized. Unlike the two-coordinate
chromium species Ar*-3,5-Pri2Cr(L) (L = THF or PMe3),5 the
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