Metal-metal bond cleavage of carbene complexes by halogens: The crystal and molecular structures of ax-[Mn2(CO)9{C(OEt)2-thienyl}], [Mn(CO)4(I){C(OEt)2-thienyl}] and eq-[Mn2(CO) 9{C(NH2)2-thienyl}]

Article abstract of DOI:10.1016/j.jorganchem.2005.07.090

The metal-metal bond in [M₂(CO)₉{C(OEt)R}] (M = Mn (1), Re (2), R = 2-thienyl (a), 2-bithienyl (b)) is readily cleaved with halogens to afford cis-[M(CO)₄(X){C(OEt)R}] (M = Mn (3), X = I; M = Re (4), X = Br). In the binuclear manganese complex, the carbene ligand is found in an axial position due to steric reasons, whereas the electronically favoured equatorial position is found for the carbene ligands in the corresponding rhenium complexes and in [Mn₂(CO)₉{C(NH ₂)thienyl}] (5a), containing a sterically less demanding NH ₂-substituent.

Full text of DOI:10.1016/j.jorganchem.2005.07.090

Journal of Organometallic Chemistry 690 (2005) 5929–5937
www.elsevier.com/locate/jorganchem
Metal–metal bond cleavage of carbene complexes by halogens:
The crystal and molecular structures of
ax-[Mn2(CO)9{C(OEt)2-thienyl}], [Mn(CO)4(I)-
{C(OEt)2-thienyl}] and eq-[Mn2(CO)9{C(NH2)2-thienyl}]
a,
b
a
a
*
´
Simon Lotz , Marile Landman , Daniela I. Bezuidenhout , Andrew J. Olivier ,
a
a
David C. Liles , Petrus H. van Rooyen
a
Department of Chemistry, University of Pretoria, Pretoria 0002 Gauteng, South Africa
b
Department of Chemistry, University of South Africa, Pretoria 0003, South Africa
Received 10 June 2005; received in revised form 22 July 2005; accepted 22 July 2005
Available online 31 August 2005
Dedicated to Professor Helgard G. Raubenheimer.
Abstract
The metal–metal bond in [M2(CO)9{C(OEt)R}] (M = Mn (1), Re (2), R = 2-thienyl (a), 2-bithienyl (b)) is readily cleaved with
halogens to afford cis-[M(CO)4(X){C(OEt)R}] (M = Mn (3), X = I; M = Re (4), X = Br). In the binuclear manganese complex,
the carbene ligand is found in an axial position due to steric reasons, whereas the electronically favoured equatorial position is found
for the carbene ligands in the corresponding rhenium complexes and in [Mn2(CO)9{C(NH2)thienyl}] (5a), containing a sterically less
demanding NH2-substituent.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Carbene complexes; Manganese; Metal carbonyls; Rhenium; Thiophene derivatives
[Mn (CO) (COCH CH CH )]
, that was initially assigned
1. Introduction
2
9
2
2
2
an incorrect structure without a carbene ligand, [Mn2-
(CO)10(CH2)3] [4], sparked interest in the challenges
associated with the synthesis of [M(CO)4(carbene)X]
(M = Mn, Re; X = halogens) complexes. The isolation
of binuclear monocarbene complexes of group 7 transi-
tion metals, [Mn2(CO)9{C(OR)R}], (R = alkane) pre-
pared from Na[Mn(CO)5] and 1,3-dihaloalkanes was
reinvestigated and resulted in detailed mechanistic stud-
ies of the reactions of Na[M(CO)5] (M = Mn, Re) with
dihaloalkanes [5]. By using different dihaloalkanes, a
number of complexes similar to the King compound
containing a cyclic carbene ligand were synthesized and
characterized [6]. In a few instances, with more sophisti-
cated chloro precursors, cationic mononuclear carbene
complexes [Mn(CO)3(L)2(carbene)]+ (L = phosphine or
The chemistry of group 7 transition metals carbonyl
complexes containing Fischer carbene and halogen
ligands, [M(CO)4(carbene)X] (M = Mn, Re; X = halo-
gens), has been neglected due to difficulties associated
with the synthesis of such compounds [1]. On the other
hand, group 7 transition metal carbene complexes with
cyclopentadienyl ligands are readily accessible from
MCp(CO)3 and organolithium reagents [2]. Also, binu-
clear monocarbene complexes [M2(CO)9(carbene)]
(M = Mn, Re) are obtainable from M2(CO)10 and
organolithium precursors [3]. The King compound,
*
Corresponding author. Fax: +27 12 3625297.
E-mail address: slotz@scientia.up.ac.za (S. Lotz).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.07.090

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S. Lotz et al. / Journal of Organometallic Chemistry 690 (2005) 5929–5937
CO) could be obtained, which after subsequent treat-
ment with halides, afforded the desired neutral halo-
carbene complexes [M(CO)4(carbene)X] [7]. However,
the complexes [Mn(CO)3(L)2(carbene)]+ (L = a phos-
phine ligand or CO) could be afforded by employing
the alkylation of [Mn(CO)3(L)2{C(O)R}] with the strong
alkylating agent MeOSO2F [8]. A major contribution to
the carbene chemistry of group 7 transition metals came
from the group of Angelici [9,10] on the reaction of
strained 3-membered heterocyclic substrates with
M(CO)5X (M = Mn, Re; X = Br, Cl). The reactions of
aziridine and oxirane with these reagents afforded
5-membered cyclic aminooxy and dioxy carbene com-
pounds. The reaction could also be applied to M2-
(CO)10 (M = Mn, Re) for oxirane [9]. Largely unnoticed
in later work, the cleavage of the metal–metal bond of
the dioxy carbene complexes by bromine, was also
reported [9]. Nevertheless, very few examples exists
where the ligand in [M(CO)4{carbene}X] (M = Mn,
Re; X = halogen) complexes is not a cyclic alkoxy
carbene. Complexes of group 7 transition metals
[M(CO)4{C(R)R0}X] (M = Mn, Re; R = H, alkoxy,
etc., R0 = aryl; X = halogen) are attractive alternative
precursors to existing carbene complexes with potential
catalytic properties.
{C(OEt)thienyl}]. This reaction showed that it was
possible to eliminate a bromide from [Re(CO)5X] during
a
Fischer carbene synthesis-procedure (Li-thienyl/
Et3OBF4) and replace it with the isolobal fragment
Re(CO)5 to give [Re2(CO)9{C(OEt)thienyl}]. In con-
trast, reactions of lithiated thiophene with [Mn(CO)5Br]
afforded a number of unstable compounds that could
not be characterized unambiguously. We envisaged that
the reverse reaction of cleaving metal–metal bonds in
carbene complexes of group 7 transition metals with hal-
ogens could be applied more generally, provided the
carbene ligand is not affected by the oxidant. The
metal–metal bond in [M2(CO)10] (M = Mn, Re) is easily
cleaved reductively by sodium affording the anionic
sodium salt, Na[M(CO)5] [13] or oxidatively with halo-
gens to give neutral metal carbonyl halides,
[M(CO)5X] (M = Mn, Re; X = halogens) [14]. The
reductive cleavage of the Mn–Mn bond of [Mn2(CO)9-
{C(OR)Me}] with sodium in THF was attempted by
Fischer and co-workers, but only Mn2(CO)10 and
Na[Mn(CO)5] were observed to have formed [3a]. Oxi-
dation of the metal results either in the loss of the car-
bene ligand or the coordination of the products of the
oxidizing agent to the metal. Biscarbene complexes of
group 6 transition metals are readily oxidized by trace
amounts of oxygen to give monocarbene-ester products
[15] while diaminocarbene complexes of tungsten could
be oxidized with I2 to yield [W(CO)4(I)2(carbene)] [16].
Reaction of the lithiated [(g5-thiophene)Cr(CO)3]
or the analogous lithiated [(g6-benzene)Cr(CO)3 with
[Mn(CO)5Br] involved attack either on the metal centre
or on a carbonyl ligand with elimination of bromide and
yielded for the thiophene precursor the binuclear com-
plexes [{g1,g5-thienyl-Mn(CO)5}Cr(CO)3] and [{g1,g5-
thienyl-C(O)Mn(CO)5}Cr(CO)3] (Scheme 1) [11,12]. By
contrast, the corresponding reaction with [Re(CO)5Br]
involved attack on a carbonyl ligand without the
elimination of bromide. Subsequent alkylation of the
latter with [Et3OBF4] yielded the binuclear carbene
complex, [{g1,g5-thienyl-C(OEt)Re(CO)4Br}Cr(CO)3]
(Scheme 1).
This prompted us to investigate the reaction of [Re
(CO)5Br] with lithiated thiophene to assess the role, if
any, of the Cr(CO)3-fragment. Although not reacting
smoothly, it was possible, after subsequent alkylation
with [Et3OBF4], to isolate and characterize the mono-
carbene dirhenium nonacarbonyl complex, [Re2(CO)9-
2. Results and discussion
2.1. Synthesis of dinuclear carbene complexes
In a typical experiment (Scheme 2), the bimetallic car-
bene complexes 1a, 1b, 2a and 2b were prepared by
reacting the bimetallic decacarbonyl precursors with
lithiated thiophene or 2,20-bithiophene in THF at ꢀ78
ꢁC [17]. After removing the THF under reduced pres-
sure, the residue was dissolved in dichloromethane and
alkylated by Et3OBF4 at ꢀ30 ꢁC. Purification of the
product to remove unreacted metal decacarbonyl was
achieved by column chromatography. The bimetallic
monocarbene complexes were isolated in yields ranging
O
S
S
C
Mn(CO)5
Mn(CO)5
OEt
Mn(CO)5Br
S
+
Cr(CO)3
Cr(CO)3
OLi
Li
S
S
Cr(CO)3
C
C
Re(CO)5Br
Re(CO)4Br
Re(CO)4Br
Cr(CO)3
Cr(CO)3
Scheme 1.

S. Lotz et al. / Journal of Organometallic Chemistry 690 (2005) 5929–5937
5931
R
S
EtO
L
R
S
C
M
L
L
Li
L
+
Y
M2(CO)10
Y2
[M2(CO)9(carbene)]
L
L
M
L
Y
L
L
3, 4
1, 2
M = Mn (1, 3), Re (2, 4), R = H (a), thienyl (b); Y = I (Mn), Br (Re); L = CO
Scheme 2.
from 63–75%. IR spectroscopy, 1H and 13C NMR spec-
troscopy (Table 1) and mass spectrometry corroborated
the single crystal X-ray structure of 1a (Fig. 1).
Slow decomposition of products with time was ob-
served in the 1H NMR spectra of 1a and 1b, but spectra
of high resolution could be obtained in either CDCl3 or
d6-acetone. The carbonyl resonances in the 13C NMR
spectra were broadened in some instances. Whereas
the infrared data in the carbonyl region for the rhenium
complexes 2a and 2b displayed the typical 9 or 8-band
pattern for equatorially substituted bimetal nonacar-
bonyl complexes (Cs symmetry), the corresponding
manganese complexes 1a and 1b gave a 5-band pattern
associated with the carbene ligand in an axial position
(C4v symmetry) [18]. A search of the Cambridge Struc-
tural Database revealed that 1a is the second axially
substituted carbene complex of manganese nonacar-
bonyl to be structurally characterized. The only other
Table 1
Spectral data of complexes 1–5
CH3CH2O
S
S
C1
5
2
[M]
3
4
Complex
IRa (m/cmꢀ1 in hexane)
1H NMR (d/ppm and J/Hz in CDCl3)
13C NMR (d/ppm in CDCl3)
1a
2090 (m), 2010 (m), 1996 (vs), 1975(m),
1946 (m)
8.35 (dd, 4.1, 0.8, H3), 7.42 (dd, 4.9, 4.1,
H4), 8.07 (dd, 5.0, 0.8, H5), MnCOEt:
5.32 (q, 7.0) 1.80 (t, 7.0)
306.5 (C1, carbene), 224.6 (Mn(CO)5,
trans), 208.9 (Mn(CO)5, cis), 155.4 (C2),
133.9, 128.9 (C3, C4), 138.9 (C5),
MnCOEt: 74.6, 15.1
1b
2089 (m), 2009 (m), 1996 (vs), 1975 (m),
1945 (m)
8.33 (d, 4.4, H3), 7.61 (d, 4.4, H4), 7.65
(dd, 3.6, 0.8, H7), 7.23 (dd, 5.0, 3.6, H8),
7.76 (d, 4.1, H9), MnCOEt: 5.31 (q, 7.0)
1.79 (t, 7.0)
309.9 (C1, carbene), 222.7 (Mn(CO)5,
trans), 211.0 (Mn(CO)5, cis), 162.1 (C2),
140.9, 134.4, 128.8, 127.8, 126.6, 125.3
(C3, C4, C6, C7, C8, C9), 146.8 (C5)
MnCOEt: 74.4, 15.5
2a
2b
2103 (w), 2071 (w), 2042 (m), 2015 (vs),
2002 (s), 1994 (s), 1977 (s), 1955 (m), 1945
(w)
2102 (w), 2041 (m), 2017 (s), 2001 (vs),
1994 (vs), 1977 (w), 1972 (w), 1954 (m),
1944 (w)
7.65 (d, 4.4 H3), 7.11 (dd, 4.7, 4.4, H4),
7.65 (d, 4.7, H5), ReCOEt: 4.51 (q, 7.0)
1.58 (t, 7.0)
7.73 (d, 4.1, H3), 7.18 (d, 4.1, H4), 7.35 (d,
4.4, H7), 7.05 (dd, 4.4, 1.3, H8), 7.35 (d,
1.3, H9), ReCOEt: 4.54 (q, 7.0) 1.60 (t,
7.0)
294.8 (C1, carbene), 192.8 (Re(CO)5),
157.9 (C2), 132.5, 127.6 (C3, C4), 132.8
(C5), ReCOEt: 77.3, 14.6
289.8 (C1, carbene), 196.0 (Re(CO)5
trans), 181.4 (Re(CO)5 cis), 155.9(C2),
136.2, 128.4, 128.0 127.0, 125.9, 124.1
(C3, C4, C6, C7, C8, C9), 145.6 (C5),
ReCOEt: 77.4, 14.6
3a
3b
2078 (m), 2024 (s), 2000 (vs), 1963 (s)
2078 (m), 2024 (s), 2000 (vs), 1963 (s)
8.74 (dd, 4.1, 1.1, H3), 7.49 (dd, 4.9, 4.1,
H4), 8.29 (dd, 4.9, 1.1 H5), MnCOEt:
5.67 (q, 7.0) 1.72 (t, 7.0)
8.53 (H3), 7.72 (H4), 7.80 (H7), 7.49 (H8),
7.90 (H9), MnCOEt: 5.53 (q) 2.02 (t)
296.4 (C1, carbene), 211.9 (Mn(CO)4),
144.9 (C2), 138.0, 129.4 (C3, C4), 143.6
(C5), MnCOEt: 79.6, 14.9
n.o.(C1, carbene), n.o. (Mn(CO)4),
162.3(C2), 134.4, 133.5, 131.3, 127.3,
125.6, 124.4 (C3, C4, C6, C7, C8, C9),
134.6(C5), MnCOEt: 76.2, 14.7
4a
4b
2071 (s), 2015 (s), 2004 (s), 1977 (s)
7.98 (dd, 4.9, 1.0, H3), 7.19 (dd, 4.5, 4.2,
H4), 7.79 (dd, 4.1, 1.0 H5), ReCOEt: 4.78
(q, 7.0) 1.68 (t, 7.0)
8.50 (d, 4.4, H3), 7.36 (d, 4.4, H4), 7.45
(dd, 5.2, 1.0 H7), 7.09 (dd, 3.6, 5.2, H8),
7.48 (dd, 3.6, 1.0, H9), ReCOEt: 5.28 (q,
7.0) 1.66 (t, 7.0)
260.4 (C1, carbene), 211.9 (Re(CO)4),
137.3 (C2), 134.7, 128.5 (C3, C4), 138.3
(C5), ReCOEt: 77.2, 14.5
268.0 (C1, carbene), 185.6 (Re(CO)4),
152.8 (C2), 150.8 (C5), 135.7 (C6), 148.6,
128.8, 127.6, 125.7 (C3, C4, C7,C8), 131.7
(C9), ReCOEt: 78.8, 15.1
2042 (m), 2000 (vs), 1995 (vs), 1956 (m)
5a
2083 (m), 2013 (vs), 1987 (vs), 1979 (s),
1969 (m), 1936 (s), N–H stretch: 3453 (w),
3328 (w)
7.79 ( H3), 7.23 (H4), 7.58 (H5), NH2:
9.76 (s)
267.6 (C1, carbene), 225.9 (Mn(CO)5,
trans), 222.1 (Mn(CO)5, cis), 154.3 (C2),
130.1 (C3), 129.6 (C4), 131.8 (C5)
a
Carbonyl region.

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S. Lotz et al. / Journal of Organometallic Chemistry 690 (2005) 5929–5937
(88.2(4)%). The carbene ligand is in the trans-configura-
tion about the C–O bond (C11–C10–O10–C15
177.2(8)ꢁ). The metal–carbon(carbonyl) bond distance
trans to the Mn–Mn bond is significantly shorter than
the mean distances of the Mn–C(carbonyl) bond lengths
in the equatorial planes (Table 2) to compensate for the
weaker p-acceptor properties of the carbene carbon
compared to a carbonyl carbon on the other side of
the metal-metal bond. The Mn1–Mn2 distance of 1a is
longer, and the trans Mn–C(carbonyl) distance is shorter
˚
than the corresponding Mn–Mn (2.9038(6) A) and
˚
trans Mn–C(carbonyl) (1.811(3) A) distances reported
for [Mn2(CO)10] [22]. The Mn–C(carbene), Mn–Mn
and M–C(carbonyl) distances are similar to the corre-
sponding distances in the dimanganese nonacarbonyl
carbene complex, eq-[Mn2(CO)9{C(OEt)Ph}], where
the carbene ligand is cis to the metal–metal bond and
the carbene substituents are in the cis-configuration
about the carbene C–O bond. The Mn–C(carbene)–C
angles are identical in the two structures, however, the
other bond angles about the carbene carbon are signifi-
cantly different with Mn–C(carbene)–O 119.4(3)ꢁ and
C–C(carbene)–O 115.5(3)ꢁ in [Mn2(CO)9{C(OEt)Ph}]
[23]. The large Mn2–C10–O10 angle in 1a (131.1(6)ꢁ)
can be ascribed to the steric interactions between the
CH2 of the ethyl group and the adjacent equatorial car-
bonyl ligands.
Fig. 1. ORTEP [20] + POV-Ray [21] plot of the geometry of 1a.
Displacement ellipsoids are set at 50% probability.
example has the unusual carbene ligand, @C(NMe2)-
OAl2(NMe2)5 [19]. Most eq-[Mn2(CO)9(carbene)] com-
plexes studied crystallographically have cyclic carbene
ligands that are less bulky compared to carbene com-
plexes with two separate substituents [6]. We concluded
that, although the equatorial position is the electroni-
cally favoured position for the carbene ligand (i.e. trans
to a carbonyl ligand), steric effects can force the carbene
ligand to occupy an axial position.
Complex 1a crystallizes in the monoclinic system,
space group P21/c with four molecules in the unit cell.
The coordination environment of each manganese atom
is approximately octahedral with the two sets of the
equatorial carbonyl ligands being staggered. The car-
bene ligand is in the axial position and is therefore trans
to the metal–metal bond. The thienyl ring is close to
being coplanar with the plane of the bonding geometry
about the carbene carbon. The dihedral angle between
the least-squares planes through (S1, C11, C12, C13
and C14) and through (C10, C11, O10 and Mn2) is
7.6(9)ꢁ. The plane of the carbene carbon, thienyl ring
and oxygen atom is approximately perpendicular to
the equatorial plane of carbonyl ligands (C10–Mn2–
CO (91–96ꢁ)) and is in an intermediate position between
the carbonyl ligands around Mn2, as is evident from the
torsion angles of 45.5(10)ꢁ and ꢀ44.9(10)ꢁ for C6–Mn2–
C10–O10 and C7–Mn2–C10–O10, respectively. Some
disorder of the ethoxy-thien-2-yl-methylidene ligand
was observed with a minor component rotated approx-
imately 180ꢁ about the Mn–C(carbene) bond with re-
spect to the orientation of the major component, such
that the C11A, C12A and C13A atoms of the thienyl
ring of the minor component nearly coincide with the
O10, C15 and C16 atoms, respectively, of the ethoxy
group of the major component (O10A, C15A and
C16A also nearly coincide with C11, C12 and C13,
respectively). Fig. 1 shows the major orientation
2.2. Halogen cleavage of the dinuclear carbene complexes
The bimetallic carbene complexes were dissolved in a
small volume hexane and a stoichiometric amount of
bromine, also dissolved in a minimum volume of hex-
ane, was added dropwise under vigorous stirring. A
red-orange precipitate immediately started forming
and stirring was continued until the solution became
clear. The solvent and unreacted bromine were removed
under reduced pressure and the products were purified
by column chromatography on silica gel. In a typical
experiment for rhenium with bithiophene, 41% of 4b
and 45% [Re(CO)5Br] could be isolated. The analogous
reaction with manganese did not proceed so well and
only 21% 3b and 41% [Mn(CO)5Br] were isolated. By
changing the solvent to CS2, a solvent generally used
to cleave Mn2(CO)10 [12a], only reactions leading to
the decomposition of the complex and elimination of
the carbene ligand with the formation of [Mn2(CO)10]
were observed. Replacing the bromine with iodine and
using the minimum amount of hexane as solvent, gave
43% 3b after purification on aluminium oxide 90.
Presently we are investigating methods of synthesis of
the corresponding bimetallic biscarbene complexes,
[{M(CO)4(carbene)}2] (M = Mn, Re), hoping to double
the yields of [M(CO)4(carbene)X] (M = Mn, Re; X = I
or Br) after cleaving the metal–metal bond. In spite of
starting with equatorially coordinated dirhenium carbene

S. Lotz et al. / Journal of Organometallic Chemistry 690 (2005) 5929–5937
5933
Table 2
Selected bond lengths and angles of 1a, 3a and 5a
1a
3a
5a
˚
Bond lengths (A)
Mn1–C1
Mean Mn1–Cx (x = 2,3,4,5)
Mn1–Mn2
Mean Mn2–Cx (x = 6,7,8,9)
Mn2–C10
C10–O10
C10–C11
C11–C12
C12–C13
C13–C14
1.798(7)
1.841(9)
2.9238(14)
1.833(7)
1.933(7)
1.312(7)
1.470(11)
1.384(10)
1.390(12)
1.313(19)
Mn1–C1
Mean Mn1–Cx (x = 2,4)
1.949(10)
1.857(6)
Mn1–C1
Mean Mn1–Cx (x = 2,3,4,5)
Mn1–Mn2
Mean Mn2–Cx (x = 7,9)
Mn2–C10
C10–N1
C10–C11
C11–C12
C12–C13
C13–C14
1.820(4)
1.843(4)
2.9280(6)
1.845(3)
2.019(3)
1.464(4)
1.315(4)
1.496(4)
1.431(5)
1.321(7)
1.790(3)
1.836(3)
Mn1–C5
C5–O5
C5–C6
C6–C7
C7–C8
C8–C9
Mn1–C3
Mn1–I1
1.986(8)
1.318(8)
1.437(9)
1.411(10)
1.414(13)
1.364(15)
1.784(6)
2.709(1)
Mn2–C6
Mn2–C8
Bond angles (ꢁ)
Mn2–C10–O10
O10–C10–C11
C11–C10–Mn2
131.1(6)
103.8(5)
125.1(6)
Mn1–C5–O5
O5–C5–C6
C6–C5–Mn1
132.8(6)
105.2(6)
122.0(6)
Mn2–C10–N1
N1–C10–C11
C11–C10–Mn2
125.7(2)
109.8(3)
124.4(2)
complexes and axially coordinated dimanganese car-
bene complexes, the cleaved product is always cis-
[M(CO)4(carbene)X]. Substitution kinetics of carbonyl
ligands in [Mn(CO)5halides] revealed that rates in-
creased with decreasing atomic number for the halides
[24] and competitive rates for dimerisation vary in the
same order with greater rates for Mn compared to Re
for a particular halide [25]. Based on these results we
concluded that the bromo-carbene complex of manga-
nese was less stable in the reaction medium compared
to the analogous iodo-carbene complex of manganese.
Compositions of products were established by spectro-
scopic methods and mass spectrometry and final confir-
mation of the structures came from a single crystal
X-ray diffraction study of 3a (Fig. 2).
the trans-configuration about the C–O bond (C(6)–
C(5)–O(5)–C(10) ꢀ173.8(6)ꢁ) – as expected when the
thienyl ring is close to being coplanar with the plane
of the bonding geometry about the carbene carbon as
this prevents a cis-configuration being adopted. The
metal carbon bond distance trans to the iodide ligand
is significantly shorter than the Mn–CO distances
opposite carbonyl ligands, reflecting the poor p-accep-
tor properties of an iodide ligand. The Mn–carbon dis-
tances observed for the carbene and the carbonyl
ligand trans to the carbene are almost the same
corresponding to a very long metal–carbon(carbonyl)
bond.
Complex 3a crystallizes in the tetragonal system,
space group I41/a with 16 molecules in the unit cell.
The coordination of the manganese atom is octahedral
and a cis-arrangement of non-carbonyl ligands, already
deduced from the infrared spectra of the molecule, is
confirmed. The thienyl ring is again close to being
coplanar with the plane of the bonding geometry about
the carbene carbon. The dihedral angle between the
least-squares planes through (S1, C6, C7, C8 and C9)
and through (C5, C6, O5 and Mn1) is 8.0(6)ꢁ. The
plane of the carbene carbon, thienyl ring and oxygen
atom is again approximately perpendicular to the
equatorial plane of ligands (C5–Mn1–CO 90–93ꢁ, C5–
Mn1–I1 95.4(3)ꢁ) and is approximately positioned
intermediately between the equatorial ligands around
Mn1, as is evident from the torsion angles of
ꢀ38.9(7)ꢁ and 51.4(7)ꢁ for C2–Mn1–C5–O5 and I1–
Mn1–C5–O5, respectively. The same disorder of the
ethoxy-thien-2-yl-methylidene ligand as that described
for 1a was also observed in 3a. Fig. 2 shows the major
orientation (85.9(5)%). The carbene ligand is again in
Fig. 2. ORTEP [20] + POV-Ray [21] plot of the geometry of 3a.
Displacement ellipsoids are set at 50% probability.

5934
S. Lotz et al. / Journal of Organometallic Chemistry 690 (2005) 5929–5937
2.3. Reaction of the dimanganese carbene complex (1a)
with ammonia
S
H2N
L
L
C
L
Mn
L
L
L
L
L
NH3
OEt
A slow stream of ammonia was bubbled through a
solution of 1a dissolved in ether. The colour of the solu-
tion changed quickly from red to orange. After TLC
confirmed that all the ethoxy carbene had been con-
verted, the solvent was removed and the amino carbene
complex, 5a, was purified by column chromatography
on aluminium oxide 90. The bimetallic carbene complex
formed quantitatively and its composition was con-
firmed by NMR and IR spectroscopy. Surprisingly,
the infrared data in the carbonyl region for the amino
carbene displayed the pattern for an equatorially substi-
tuted bimetallic nonacarbonyl complex and not the 5-
band pattern observed for the ethoxy precursor 1a
[18]. The single crystal X-ray structure of 5a (Fig. 3)
Mn
L
L
Mn
L
Mn
L
L
C
L
S
L
L
L
L
5a
L = CO
Scheme 3.
donation from the nitrogen lone pair to the electrophilic
carbene carbon [26]. This is also evident from the chem-
ical shift of the carbene carbon atom at 268 ppm com-
pared to 307 ppm for 1a. In contrast to 1a and 3a, the
thienyl ring is not close to being coplanar with the plane
of the bonding geometry about the carbene carbon. The
dihedral angle between the least-squares planes through
(S1, C11, C12, C13 and C14) and through (C10, C11,
N1 and Mn2) is 39.7(1)ꢁ. A deviation out of the plane
is necessary to fit the thienyl S between the two closest
carbonyl ligands C6–O6 and C7–O7. Unlike the previ-
ous two structures, no disorder was observed in the car-
bene ligand as the spatial similarity between the nearly
coplanar ethoxy group and thienyl ring in the carbene
ligand in both 1a and 3a is not present in the carbene li-
gand of 5a. The small NH2-group can be accommodated
adjacent to the carbonyl ligands C2–O2, C3–O3 and
C9–O9 in the equatorial plane. In 1a, with the carbene
ligand adopting a trans-configuration about the C–O
bond, the ethoxy group is probably too bulky to be
accommodated and the carbene ligand is forced to occupy
the axial position. On the other hand, the ethoxy car-
bene ligand is a better p-acceptor ligand compared to
the amino carbene and one could argue that the carbene
position is affected by the p-accepting ability of the two
types of carbene ligands. This is supported by the Mn–
C(carbene) bond distances since the ligand trans to the
Mn–Mn metal bond represents the best site for the
strongest p-acceptor ligand in [Mn2(CO)9(carbene)]
complexes. However, the more bulky amino substituent
NHEt in [Mn2(CO)9{C(NHEt)thienyl}] again displayed
the 5-band pattern in the carbonyl region of the infrared
spectrum of this compound, typical for an axial substi-
tuted carbene ligand [27]. The Mn–Mn bond in 5a can
be cleaved with iodine in a manner similar to that of
complexes 1a,b [27].
1
[17] corroborated these observations. In the H NMR
spectrum, the thiophene and amine protons are dupli-
cated in a major and minor isomer. We suspect that in
solution, a mixture of the equatorial and axial isomers
exists. It is unclear which set of signals belongs to which
isomer. The N–H stretch vibrations in the infrared spec-
trum of 5a are at 3453 and 3328 cmꢀ1 and two of the
bands in the carbonyl region of the spectrum are very
strong displaying band overlap. See Scheme 3.
Complex 5a crystallizes in the monoclinic system,
space group P21/c with four molecules in the unit cell.
The average Mn–CO and Mn–Mn bond lengths are very
similar to that of 1a, although the Mn–C(carbene) bond
˚
length for 5a (2.019(3) A) is significantly longer than the
˚
corresponding distance in 1a (1.933(7) A). A very short
˚
C(carbene)–N bond length (1.315(4) A) indicates double
bond character and implies a high degree of electron
In conclusion, we have made use of the fact that me-
tal–metal bonds of carbene complexes can be cleaved
oxidatively by halogens without affecting the carbene li-
gand. The products represent a novel class of carbonyl
complexes that display both a halogen and a carbene li-
gand with potential application in organic synthesis and
catalysis. These complexes are ideal to study electronic
and steric factors in the conformations of [M(CO)9(car-
bene)] complexes. Studies on the mechanistic detail of
axial to equatorial conversions of the carbene ligand
Fig. 3. ORTEP [20] + POV-Ray [21] plot of the geometry of 5a.
Displacement ellipsoids are set at 50% probability.

S. Lotz et al. / Journal of Organometallic Chemistry 690 (2005) 5929–5937
5935
during aminolysis in binuclear complexes as well as the
use of other cleaving reagents to break metal–metal
bonds in carbene complexes of group 7 transition met-
als, are currently underway. We plan to further extend
this approach to include other transition metal carbene
complexes such as dicobalt and diiron carbonyl
complexes.
on aluminium oxide 90 using 4:1 hexane/dichlorometh-
ane as eluent. Product 1a ax-[Mn2(CO)9{C(OEt)2-thie-
nyl}] (1.24 mmol, 0.62 g, 62% yield) was obtained.
Complexes 1b, 2a and 2b were prepared in a similar
fashion. However, the rhenium complexes were purified
using silica gel 60 column chromatography. The reaction
solutions were adsorbed directly onto silica gel 60, and
eluting with hexane isolated the products. It was found
that for the Mn complexes, adsorption onto aluminium
oxide 90 afforded better yields. Column chromatogra-
phy performed at low temperatures (ꢀ20 ꢁC) lead to a
further increase in yields. Product 1a was recrystallized
from dichloromethane/hexane mixtures.
3. Experimental
3.1. General remarks
All reactions were performed in an inert atmosphere
of either nitrogen or argon with dry solvents by using
standard Schlenk and vacuum-line techniques. Column
chromatography was carried out under a nitrogen atmo-
sphere using silica gel (particle size 0.063–0.200 nm) and
aluminium oxide 90 as stationary phase. Solvents were
dried and distilled under an atmosphere of nitrogen be-
fore use. Diethyl ether, tetrahydrofuran and hexane
were distilled from sodium metal, with benzophenone
as indicator in the case of the etheral solvents. Dichloro-
methane was distilled from phosphorus pentoxide. All
other reagents were purchased from commercial suppli-
ers and used without further purification. All NMR
spectra were recorded in degassed deuterated chloro-
form on a Bruker ARX-300 spectrometer. Chemical
shifts were referenced to chloroform. 1H and 13C
NMR spectra were measured at 300.133 and 75.469
MHz, respectively. Infrared spectra were recorded on a
Perkin–Elmer Spectrum RX1 FT-IR spectrophotometer
with a NaCl cell using hexane as solvent. FAB-MS spec-
tra were recorded on a VG 70SEQ Mass Spectrometer
with the resolution for FAB = 1000 in a field of 8 kV.
Nitrobenzyl alcohol was used as solvent. Melting points
were recorded on a hot stage Gallenkamp melting point
apparatus and are uncorrected.
1a: Orange solid; m.p.: 103–106 ꢁC. Anal. Calc. for
Mn2C16H8O10S (502.18): C, 38.27; H, 1.61. Found: C,
38.51; H, 1.74%. MS (EI): m/z 502 [M+].
1b: Orange solid; m.p.: 103–106 ꢁC. Anal. Calc. for
Mn2C20H10O10S2 (584.30): C, 41.11; H, 1.73. Found:
C. 41.56; H, 1.81. Yield: 0.43 g (74%). MS (EI): m/z
584 [M+].
2a: Orange solid; m.p.: 112–113 ꢁC. Anal. Calc. for
Re2C16H8O10S (764.72): C, 25.13; H, 1.06. Found: C,
25.55; H, 1.17%. Yield: 0.49 g (64%). MS (EI): m/z
764 [M+].
2b: Orange solid; m.p.: 117–119 ꢁC. Anal. Calc. for
Re2C20H10O10S2 (846.84): C, 28.36; H, 1.19. Found: C,
28.54; H, 1.23%. Yield: 0.65 g (77%). MS (EI): m/z
847 [M+].
3.2.2. Preparation of [Mn(CO)4(I){C(OEt)2-thienyl}]
(3a)
To a minimum volume of hexane solvent was added
3a (1 mmol, 0.50 g) and I2 (1 mmol, 0.26 g) while stirring
at room temperature under an inert N2 atmosphere until
reaction was complete. The unreacted I2 was separated
on a aluminium oxide 90 column with a hexane eluent.
Products 3a [Mn(CO)4(I){C(OEt)2-thienyl}] (0.43 mmol,
0.185 g, 43% yield) and [Mn(CO)5I] (0.48 mmol, 0.155 g,
48%) were obtained. Complexes 3b, 4a and 4b were pre-
pared in a similar fashion. For the rhenium complexes a
solution of Br2 (1.1 mmol, 0.18 g) in hexane was used
and unreacted Br2 was removed during the removal of
solvent under reduced pressure. Again, the rhenium
complexes were purified using silica gel 60 column chro-
matography. Product 3a was recrystallized from dichlo-
romethane/hexane mixtures.
3a: Orange solid; m.p.: 103–106 ꢁC. Anal. Calc. for
MnC11H8O5SI (434.09): C, 30.43; H, 1.86. Found: C,
30.81; H, 1.98. MS (EI): m/z 434 [M+].
3b: Orange solid; m.p.: 103–106 ꢁC. Anal. Calc. for
MnC15H10O5S2I (516.21): C, 34.90; H, 1.96. Found: C,
35.08; H, 2.08%. Yield: 0.16 g (37%). MS (EI): m/z
516 [M+].
3.2. Synthesis
3.2.1. Preparation of ax-[Mn2(CO)9{C(OEt)2-
thienyl}] (1a)
Thiophene (2.2 mmol, 0.185 g, 0.175 mL), was stirred
while adding BuLi (2.2 mmol, 1.6M, 1.4 mL) in 40mL
THF at ꢀ30 ꢁC under an inert N2 atmosphere. Stirring
was continued for 30 minutes. Yellow Mn2(CO)10 (2
mmol, 0.78 g) was added to the reaction mixture at
ꢀ78ꢁC, resulting in a colour change of the reaction mix-
ture to orange-red while stirring for 1 h. Stirring was
then continued for an additional 30 min at room tem-
perature. THF solvent was evaporated under reduced
pressure. Et3OBF4 (2.2 mmol, 0.42 g) in dichlorometh-
ane was added to the reaction mixture at ꢀ30 ꢁC and
stirred until reaction completion. LiBF4 salts were re-
moved by filtering, and reaction products were purified
4a: Orange solid; m.p.: 117–119 ꢁC. Anal. Calc. for
ReC11H8O5SBr (518.36): C, 25.49; H, 1.56. Found: C,
25.66; H, 1.67%. Yield: 0.20 g (39%). MS (EI): m/z

5936
S. Lotz et al. / Journal of Organometallic Chemistry 690 (2005) 5929–5937
Table 3
Crystal data and structure refinement of 1a, 3a and 5a
1a
3a
5a
Colour
Red
Dark-red
Orange
Habit
Rectangular prism
0.40 · 0.30 · 0.18
Irregular fragment
0.58 · 0.54 · 0.42
C11H8IO5MnS
434.07
Hexagonal prism
0.32 · 0.28 · 0.20
Crystal size (mm3)
Chemical formula
Formula weight
C16H8Mn2O10
S
C16H8Mn2NO10
S
502.16
0.71073
473.13
0.71073
˚
Wavelength (A)
0.71073
Unit cell dimensions
˚
a (A)
7.920(4)
22.236(11)
11.548(5)
95.591(7)
2024.0(16)
Monoclinic
P21/c
21.438(8)
14.4298(8)
9.1653(5)
14.7785(8)
113.7790(10)
1788.58(17)
Monoclinic
P21/n
˚
b (A)
˚
c (A)
12.895(10)
b (ꢁ)
Volume (A )
Crystal system
Space group
3
˚
5926(5)
Tetragonal
I41/a
Z
4
16
3.125
4
Absorption coefficient (mmꢀ1
)
1.400
1.576
Theta range for data collection (ꢁ)
Index ranges
2.55–26.30
ꢀ5 6 h 6 9;
2.65–26.43
ꢀ25 6 h 6 21;
ꢀ20 6 k 6 25;
ꢀ15 6 l 6 10
11926
2.68–26.54
ꢀ13 6 h 6 17;
ꢀ11 6 k 6 11;
ꢀ14 6 l 6 17
9351
ꢀ24 6 k 6 27;
ꢀ10 6 l 6 3
6741
Reflections measured
Unique reflections [Rint
Completeness to theta = 25.0ꢁ
Absorption correction [28]
Minimum and maximum transmission
Refinement method [29]
]
3324 [0.0390]
0.893
Multi-scan
0.476 and 0.777
Full-matrix least-squares on F2
0.0582
2678 [0.0339]
0.931
Multi-scan
0.188 and 0.269
Full-matrix least-squares on F2
0.0409
3345 [0.0260]
0.992
Multi-scan
0.549 and 0.730
Full-matrix least-squares on F2
R1 = RiFo| ꢀ |Fci/R|Fo| for Fo > 4r(Fo)
0.0397
0.1263
2
2
1=2
wR2 ¼ fR½wðF 2o ꢀ F c2Þ ꢁ=R½wðF 2oÞ ꢁg
0.1431
0.1147
518 [M+]. On heating, the orange solid darkens around
118 ꢁC with the formation of Re2(CO)10 as a white crys-
talline material on the outside and leaving a dark residue
in the middle.
4b: Orange solid; m.p.: 117–119 ꢁC. Anal. Calc. for
ReC15H10O5S2Br (600.48): C, 30.00; H, 1.68. Found:
C, 30.26; H, 1.77%. Yield: 0.25 g (41%). MS (EI): m/z
518 [M+].
P4 diffractometer, fitted with a 1 K CCCD detector,
using graphite-monochromated, Mo Ka radiation. In
the structure refinements all hydrogen atoms were added
in calculated positions and treated as riding on the atom
to which they are attached. All non-hydrogen atoms (ex-
cept those refined as rigid groups) were refined with
anisotropic displacement parameters, all isotropic dis-
placement parameters for hydrogen atoms were calcu-
lated as X · Ueq of the atom to which they are
attached, X = 1.5 for the methyl hydrogens and 1.2 for
all other hydrogens. In structures 1a and 3a the carbene
ligand showed some disorder with a minor component
(11.8(4)% and 14.1(5)%, respectively) rotated approxi-
mately 180ꢁ about the Mn–C(carbene) bond with re-
spect to the major component. The parameters for the
major orientations were refined freely. The minor orien-
tations were refined as rigid bodies with geometries de-
rived from those of the major orientations. Site
occupation factors for the major and minor orientations
were refined but constrained to sum to 1.0. Crystal data
and other experimental procedures and refinement
parameters are given in Table 3.
3.2.3. Preparation of eq-[Mn2(CO)9{C(NH2)2-
thienyl}] (5a)
1a (2 mmol, 1.00 g) was dissolved in diethyl ether at
room temperature, and a slow stream of NH3 was bub-
bled through the reaction solution until the color of the
reaction mixture changed from red to orange. After
evaporation of the solvent under reduced pressure and
purification on aluminium oxide 90, product 5a eq-
[Mn2(CO)9{C(NH2)2-thienyl}] (1.80 mmol, 0.84 g,
89%) was obtained. The product was recrystallized from
dichloromethane/hexane mixtures.
5a: Red solid; m.p.: 103–106 ꢁC. Anal. Calc. for
Mn2C14H5O9SN (473.14): C, 35.54; H, 1.07. Found: C,
35.81; H, 1.16%. MS (EI): m/z 473 [M+].
3.3. X-ray crystallography
4. Supplementary material
Data were collected at 20 ꢁC by means of a combina-
tion of phi and omega scans on a Bruker (SIEMENS)
Crystallographic data (excluding structure factors)
for the structures reported in this paper have been

S. Lotz et al. / Journal of Organometallic Chemistry 690 (2005) 5929–5937
5937
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deposited with Cambridge Crystallographic Data Cen-
ter as supplementary crystallographic data for 1a, 3a
and 5a, CCDC – 267350, CCDC – 267351 and CCDC
– 267352, respectively, can be obtained free of charge
from The Director, Cambridge Crystallographic Data
Center, 12 Union Road, Cambridge CB2 1EZ, UK,
fax: +44 1223 336 033, e-mail: deposit@ccdc.cam.ac.uk
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The National Research Foundation of South Africa
by Grant No. 2047221 supported this work. Mr E. Pal-
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