Syntheses, structures and magnetic properties of Mn(II) dimers [CpMn(μ-X)]2 (Cp = C5H5; X= RNH, R 1R2N, C≡CR)

Article abstract of DOI:10.1039/b409135c

Manganocene, Cp₂Mn, has been employed as a precursor in the synthesis of a range of Mn(II) dimers of the type [CpMn(μ-X)]₂ [X = 8-NHC₉H₆N (1), N(Ph)(C₅H₄N) (2), N(4-EtC₆H₄)(C₅H₄N) (3) and C≡CPh (4)] as well as the bis-adduct [Cp₂Mn{HN=C(NMe ₂)₂}₂] (5). The solid-state structures of 1-5 are reported. Variable-temperature magnetic measurements have been used to assess the extent of Mn(μ-X)Mn communication within the dimers of 1-4 as a function of the bridging ligands (X).

Full text of DOI:10.1039/b409135c

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F U L L P A P E R
Syntheses, structures and magnetic properties of Mn(II) dimers
[CpMn(l-X)]2 (Cp = C5H5; X = RNH, R1R2N, CCR)†
Carmen Soria Alvarez,a Sally R. Boss,a Jonathan C. Burley,a Simon M. Humphry,a
Richard A. Layfield,a Richard A. Kowenicki,a Mary McPartlin,b Jeremy M. Rawson,*a
Andrew E. H. Wheatley,*a Paul T. Wood*a and Dominic S. Wright*a
a
Chemistry Department, University of Cambridge, Lensfield Road, Cambridge, UK CB2 1EW.
E-mail: dsw1000@cus.cam.ac.uk; Fax: +0044 1223 336362
Department of Health and Biological Science, London Metropolitan University, London,
b
UK N7 8DB
Received 15th June 2004, Accepted 2nd September 2004
First published as an Advance Article on the web 21st September 2004
Manganocene, Cp2Mn, has been employed as a precursor in the synthesis of a range of Mn(II) dimers of the type
[CpMn(l-X)]2 [X = 8-NHC9H6N (1), N(Ph)(C5H4N) (2), N(4-EtC6H4)(C5H4N) (3) and CCPh (4)] as well as the
bis-adduct [Cp2Mn{HNC(NMe2)2}2] (5). The solid-state structures of 1–5 are reported. Variable-temperature
magnetic measurements have been used to assess the extent of Mn(l-X)Mn communication within the dimers of 1–4
as a function of the bridging ligands (X).
and cyano-compounds have been the focus of most investiga-
Introduction
tions to date.8 In tandem with our synthetic aims of applying
The polarity of Cp–metal bonds in a range of transition metal
transition-metal metallocenes as precursors, an additional
(M) metallocenes (Cp2M; Cp = C5H5) has been appreciated
aim in our work is the elucidation of the magnetic properties
for some time.1 This characteristic is in marked contrast to
of this extensive range of materials containing novel bridging
the classical behaviour exhibited by ferrocene (Cp2Fe), for
which the covalent Cp–Fe bonding and the stability of the
eighteen-electron configuration results in the maintenance
groups. We present here further studies of the synthetic utility of
Cp2Mn. In addition to the dimer [CpMn{l-8-NHC9H6N}]2 (1),
whose synthesis and structure were reported by us previously,6c
of metal–ligand bonding in reactions with electrophiles and
the new dimers [CpMn{l-N(Ph)(C5H4N)}]2 (2) and [CpMn{l-
N(4-EtC6H4)(C5H4N)}]2 (3) and [CpMn{l-CCPh}·thf]2 (4),
and the bis-adduct [Cp2Mn{HNC(NMe2)2}2] (5) have been
prepared and structurally characterised. Although a series of
dimers [MeCpMn(l-X)PEt3]2 (MeCp = MeC5H4, X = halide)
nucleophiles.2 The ionic character of Cp-metal bonding in
metallocenes like manganocene (Cp2Mn) results in three
distinct types of reactivity, (i) substitution of the Cp ligands
by nucleophiles,3 (ii) addition reactions of weaker nucleophiles
to the metal centre4 and (iii) basic character in reactions with
have been structurally characterised and their magnetic proper-
organic acids (e.g., eqns. (1)–(3)).5,6
ties investigated thoroughly,9 the dimers 1–4 are the first Mn(II)
compounds of this type containing bridging N and C groups.
They provide a unique opportunity to uncover fundamental
(1)
information concerning the mechanism and relative order of
magnetic exchange interactions within a series of closely related
(2)
species containing little-studied bridging metallo-organic or
organometallic groups.
(3)
These reaction characteristics, which are in many ways similar
to those exhibited by main group metallocenes,7 make polar
transition metal metallocenes attractive precursors to an
extensive range of molecular and supramolecular magnetic
materials.3–6
Results and discussion
The Mn(II) dimers [CpMn{l-8-NHC9H6N}]2 (1),6c [CpMn{l-
N(Ph)(C5H4N)}]2 (2) and [CpMn{l-N(4-EtC6H4)(C5H4N)}]2
(3) were obtained from the 1:1 stoichiometric reactions of the
corresponding amines with Cp2Mn (eqns. (4)–(6), respectively)
in yields of 41–65%.
Our interest in this area has so far focused on the preparation
of metallo-organic compounds3,4,6 and on extended organo-
metallic lattices of Mn(II).5 We found that the reactions of
Cp2Mn as a base with 2-aminopyrimidines (pmNH2) and
2-aminopyridines (pyNH2) provide easy access to poly-
nuclear amido (pyNH−) and imido (pmN2−) cages,6 e.g., in
the octanuclear imido Mn(II) cages [{Mn(Npm)}4{CpMn-
(NHpm)}4].6a,c The addition of alkali metal cyclopentadienyl
compounds to Cp2Mn gives extended layer structures
containing [Cp3Mn]− anions, e.g., as found in the graphite-like
arrangement of [{Cp3Mn}K]∞.5 The magnetic properties of
polynuclear cages and lattice arrangements containing Mn in
various oxidation states have been investigated extensively in
the past two decades.8 However, these studies have involved a
restricted range of ligands. In particular oxo/carboxylate cages
(4)
(5)
(6)
Owing to the lower reactivity of phenyl acetylene, the dimer
[CpMn{l-CCPh}·thf]2 (4) could only be obtained by the
nucleophilic substitution reaction of [PhCCLi] with Cp2Mn
(eqn. (7)).
† Electronic supplementary information (ESI) available: Derivation of
magnetic susceptibility of MnII dimers including biquadratic exchange.
See http://www.rsc.org/suppdata/dt/b4/b409135c/
(7)
T h i s j o u r n a l i s
©
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
D a l t o n T r a n s . , 2 0 0 4 , 3 4 8 1 – 3 4 8 7
3 4 8 1

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Table 1 Selected bond lengths (Å) and angles (°) for the amido complexes [CpMn{l-8-NHC9H6N}]2 (1), [CpMn{l-N(Ph)(C5H4N)}]2 (2) and
[CpMn{l-N(4-EtC6H4)(C5H4N)}]2 (3)
Compound
1
2
3
Mn(1)–N(1)
Mn(1)–N(1A)
Mn(1)–N(2/2A)
Mn(1)–C(1)
Mn(1)–C(2)
Mn(1)–C(3)
Mn(1)–C(4)
Mn(1)–C(5)
Cpcentroid–Mn
2.143(3)
2.197(3)
2.219(2)
2.497(4)
2.515(4)
2.482(4)
2.429(4)
2.448(4)
2.18
2.285(2)
2.183(2)
2.213(2)
2.421(2)
2.431(2)
2.426(2)
2.431(2)
2.429(2)
2.12
2.170(2)
2.295(2)
2.242(2)
2.426(3)
2.391(2)
2.374(3)
2.437(3)
2.495(3)
2.13

C
C (Cp)
1.367(6)–1.416(7)
2.944(1)
1.398(3)–1.400(4)
3.0230(8)
1.375(4)–1.393(3)
3.0370(9)

Mn(1)Mn(1A)
N(1)–Mn(1)–N(1A)
Mn(1)–N(1)–Mn(1A)
N(2/2A)–Mn(1)–N(1/1A)
94.6(1)
85.4(1)
76.28(9)
94.89(6)
85.11(6)
60.52(6)
94.35(6)
85.65(6)
60.14(7)
a Symmetry transformations used to generate equivalent atoms A for 2: 2 − x, −y, 2 − z and 3: −x + 2, −y, −z.
Table 2 Selected bond lengths (Å) and angles (°) for the alkyne-bridged dimer [CpMn{l-CCPh}·thf]2 (4)
Mn(1)–C(11)
Mn(1)–C(21)
Mn(1)–O(1)
Mn(1)–C(1)
Mn(1)–C(2)
Mn(1)–C(3)
Mn(1)–C(4)
Mn(1)–C(5)
CpcentroidMn
2.162(3)
2.249(3)
2.207(2)
2.479(3)
2.478(3)
2.484(3)
2.472(3)
2.464(3)
Mn(2)–C(11)
Mn(2)–C(21)
Mn(2)–O(2)
Mn(2)–C(6)
Mn(2)–C(7)
Mn(2)–C(8)
Mn(2)–C(9)
Mn(2)–C(10)
CC
2.250(3)
2.158(3)
2.220(2)
2.478(3)
2.463(3)
2.453(3)
2.456(3)
2.474(3)
1.215(4)
2.17

C
C (Cp)
1.375(5)–1.405(5)
2.9691(8)
2.96 (mean)

Mn(1)Mn(2)
C(12/22)Mn(1/2)
Mn(1)–C(11)–Mn(2)
Mn(1)–C(21)–Mn(2)
84.6(1)
84.7(1)
C(12)–C(11)–Mn(2)
C(22)–C(21)–Mn(1)
C(22)–C(21)–Mn(2)
O–Mn–l-C
115.2(2)
112.7(2)
162.7(2)
95.6 (mean)
C(11)–Mn(1)–C(21)
C(11)–Mn(2)–C(21)
C(12)–C(11)–Mn(1)
95.2(1)
95.3(1)
160.2(2)
Attempts to prepare an imido-bridged dimer [CpMn{l-
NC(NMe2)2}]2 (6)bytheroom-temperaturereactionof 1,1,3,3-
tetramethylguanidine [HNC(NMe2)2] with Cp2Mn resulted
only in formation of the bis-adduct [Cp2Mn{HNC(NMe2)2}2]
(5) (eqn. (8))
the room-temperature 1H NMR spectra of 4 and 5 were severely
broadened and no quantitative information could be obtained.
In each of compounds 1–5 the resonances for the Cp ligands
were very broad and therefore difficult to assign. The chemical
shifts of the Cp hydrogen atoms in these species were highly
variable, occurring in the region ca. d 6.47–3.50. The presence
of Cp ligands in all of the compounds is supported further
by the observation of a weak band at ca. 3080 cm−1 for the
cyclopentadienyl C–H stretch in their IR spectra.
(8)
The low-temperature diffraction X-ray structures of 2–5 have
been obtained. Selected bond lengths and angles for amido-
bridged dimers 2 and 3 are given in Table 1. This table also
includes the corresponding data for the previously reported
primary-amido complex 1 for comparison. Selected bond lengths
and angles for the alkyne-bridged dimer 4 and the bis-adduct 5
are given in Tables 2 and 3, respectively. Details of the data
collections and structure refinements of the new compounds
are to be found in the Experimental section in Table 5.
The structures of the primary- and secondary-amido
bridged dimers 1–3 (Figs. 1–3, respectively) are similar, having
centrosymmetric arrangements in which the Mn(II) centres of
their Mn2N2 cores are p-bonded to Cp ligands. In addition, the
Mn centres are chelated by the quinoline- or pyridyl-N centres
of the bridging organic groups, giving the Mn centres of each
dimer a nominal coordination number of four. The strained
chelate interactions in 2 and 3 result in a marked disruption of
Mn–N bonding within their Mn2N2 ring units. Thus, within each
core the longest Mn–N bonds correspond to those involved in
the chelate interactions [in 2 Mn(1)–N(1) 2.285(2), in 3 Mn(1)–
N(1A) 2.295(2) Å], with the remaining Mn–N bonds being
significantly shorter [in 2 Mn(1)–N(1A) 2.183(2), in 3 Mn(1)–
N(1) 2.170(2) Å]. This is opposite to the pattern of Mn–N bond
Repeatedattemptstoobtainthedesireddimer6fromnucleophilic
substitution reaction of Cp2Mn with [LiNC(NMe2)2] (1:1
equivalents) failed to produce isolable products. The synthesis
and structure of 1 have been reported by us previously6c so that
experimental and structural details of the complex will only be
described here by way of comparison with the new dimers 2, 3
and 4.
The paramagnetic nature of all of these compounds and
their high air-sensitivity made characterisation by spectroscopic
and analytical techniques difficult. Only in the case of 5 could
satisfactory elemental analysis be obtained. In addition, all
of the species exhibited low solubility in all organic solvents
once isolated, except for (polar) DMSO. Although the room-
temperature 1H NMR spectra of all of the compounds
exhibited line-broadening, in the cases of 2 and 3 spectra were
sufficiently well resolved to identify the [N(Ph)(C5H4N)]− and
[N(4-EtC6H4)(C5H4N)]− ligands as well-separated multiplets
1
with the correct relative integrals; the H resonances for the
C5H4N group in 2 being found in the region d 9.18–7.84 and
those in 3 being in the range d 9.92–7.50, with those due to the
Ph and 4-EtC6H4 groups being found in the regions d 7.71–6.69
and d 7.05–6.65, respectively. For 3, the 4-Et group is observed
as the expected triplet (d 1.13) and quartet (d 2.48). In contrast,
3 4 8 2
D a l t o n T r a n s . , 2 0 0 4 , 3 4 8 1 – 3 4 8 7

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Table 3 Selected bond lengths (Å) and angles (°) for the bis-adduct [Cp2Mn{HNC(NMe2)2}2] (5)
C(1)–Mn(1)
C(2)–Mn(1)
C(3–5)Mn(1)
N(1)–Mn(1)
2.558(4)
2.350(3)
2.871(4)–3.312(4)
2.109(3)
N(2)–Mn(1)
C(6)–Mn(1)
2.120(3)
2.356(4)
2.673(4)–3.397(4)
1.352(8)–1.411(5)
1.299(4)–1.305(4)
C(7–10)–Mn(1)

C
C (Cp)


N(1,2) C(11,12)

N(1)–Mn(1)–N(2)
C(6)–Mn(1)–N(1,2)
111.9(1)
104.8(2)–109.1(2)
C(1)–Mn(1)–C(2)
33.1(1)
lengths found in 1, in which the less strained chelate interactions
appear to re-enforce the associated Mn–N bonds within the
core [Mn(1)–N(1) 2.143(3), Mn(1)–N(1A) 2.197(3) Å]. Despite
this difference the Mn–N–Mn and N–Mn–N angles within
the ring units of 1, 2 and 3 are very similar [Mn–N–Mn range
85.11(6)–85.65(6), N–Mn–N range 94.35(6)–94.6(1)°]. The
C–Mn bonds in all three compounds fall in the range expected
for high-spin Mn(II) cyclopentadienides [2.429(4)–2.515(4) in 1,
2.421(2)–2.431(2) in 2, and 2.374(3)–2.495(3) Å in 3]. Although
the C–Mn bond lengths in these species vary greatly, the
hapticity of Cp–Mn bonding is known to be highly flexible in
high-spin manganocene derivatives (in which the metal–ligand
bonding has largely electrostatic character), and even crystal
packing forces have been shown to have a major effect on the
Cp-bonding mode.10 The presence of much longer coordinative
interactions in 3 [Mn(1)–N(2A) 2.242(2) Å] than are found in the
closely related dimer 2 [Mn(1)–N(2) 2.213(2) Å] may result from
electronic effects (possibly stemming from the electron-donating
effect of the remote para-Et substituent).
Fig. 3 Structure of the dimer 3; H-atoms have been omitted for
clarity.
terminal Cp ligands. In the absence of intramolecular ligation,
the nominally four-coordinate geometries of the Mn atoms are
now each completed by the coordination of a thf molecule.
Although the dimeric structure of 4 is non-centrosymmetric,
the bond lengths and angles associated with the two, crystallo-
graphically-independent Mn centres are very similar. The
Mn2C2 ring unit is rhombic in shape, having an alternating
pattern of long [Mn(1)–C(21)/Mn(2)–C(11) mean 2.25 Å] and
short [Mn(1)–C(11)/Mn(2)–C(21) 2.16 Å] Mn–C distances and
with the Mn–C–Mn [mean 84.6°] and C–Mn–C [mean 95.2°]
angles being strikingly similar to the Mn–N–Mn and N–Mn–N
angles found in the amido-bridged dimers 1–3. The shortest
Mn–C bonds correspond to the most favourable (more linear)
alignment of the sp hybrid orbitals of the Ph–CC– ligands
with respect to each of the Mn centres in 4 [i.e., CC–Mn(1/2)
162.7(2)–160.2(2)°], while the longer bonds occur where the axes
of the Ph–CC ligands are mis-aligned [i.e., CC–Mn(1/2)
112.7(2)–115.2(2)°]. Although there is a possibility of additional,
weak interactions between the a-carbon atoms of the bridging
[Ph–CC]− groups and the metal centres, the C(12)Mn(2)
and C(22)Mn(1) distances in 4 [mean 2.96 Å] are significantly
longer than those observed previously in the only example of a
Mn complex in which g2-type bonding of a Ph–CC– group
has been observed (the Mn(I) complex [Mn(CO)3(l-H)(l-
CC–Ph)(Ph2PCH2PPh2)], 2.415 Å).11 The Cp ligands in 4
adopt essentially g5-bonding modes, for which (like in 1–3) the
range of C–Mn bond lengths [2.456(3)–2.484(3) Å] is typical of
high-spin manganocenes.12 Complex 4 is isomorphous with the
The structure of alkyne-bridged dimer 4 (Fig. 4) is similar
in its overall structure to those found for 1–3, being composed
of a Mn2C2 ring in which the Mn(II) centres are coordinated by
Fig. 1 Structure of the dimer 1; H-atoms (except those attached to N)
have been omitted for clarity.
13
Mg counterpart [CpMg{l-CCPh}·thf]2 and closely related
to the Ca complex [Cp*Ca{l-CCPh}·thf]2.14 However, it is the
first alkynyl compound of Mn(II).
The structure of the bis-adduct 5 (Fig. 5) is largely as
expected on the basis of spectroscopic and analytical investi-
gations. The Mn(II) centre is coordinated by two g1- and g2-
bonded Cp ligands [Mn(1)–C(1) 2.558(4), Mn(1)–C(2) 2.350(3),
Mn(1)–C(6) 2.356(4) Å] and two (Me2N)2CNH ligands
[N(1)–Mn(1) 2.109(3), Mn(1)–N(2) 2.120(3) Å]. The location
of the N–H protons in the Fourier map of the imine ligands in


5 and the pattern of C N bonds found within their C( N)


3

skeletons [N(1/2) C(11/21) range 1.299(4)–1.305(4) Å; cf.
1.351(4)–1.376(4) Å for the other N C bonds] provides



strong support for the formulation of this complex as an
adduct on the basis of spectroscopic and analytical results. The
complex is related to other mono- and bis-coordinated donor
Fig. 2 Structure of the dimer 2; H-atoms have been omitted for
clarity.
D a l t o n T r a n s . , 2 0 0 4 , 3 4 8 1 – 3 4 8 7
3 4 8 3

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Table 4 Parameters utilised in the curve-fitting of the susceptibility
data for 1–4 to the expression (12). The residual R(v) is defined as
(1/n)∑|vo − vcalc|/vobs where n is the number of data points
Compound
J/K
a
TIP
R(v)
1
2
−21.0
−21.0
−17.0
−18.5
−20.5
−30.5
−30.5
0.14
0.25
0.08
0.072
0.13
0.04
0.11
−0.005
−0.0014
0.003
0.0018
0.000
0.000
0.000
0.125
0.103
0.022
0.020
0.131
0.063
0.095
3
4
T ) / 2kT
Ng2 β2 S (S +1)(2S +1)e−E(S
∑
T
T
T
χm =
(10)
3kT (2S +1)e−E(S
T ) / 2kT
∑
T
where ST is the total spin multiplicity of the complex which for a
dimeric MnII complex lies in the range from 0–5 and E(ST) is the
energy of each spin state which is given by −J[ST(ST + 1) − 2S-
(S + 1)]. An expansion of this expression yields:
Fig. 4 Structure of the alkyne-bridged dimer 4; H-atoms have been
omitted for clarity.
Ng2 β2 330e25J / 4kT +180e5J / 4kT
+
χm =
⋅
3kT
11e25J / 4kT +9e5J / 4kT
+
84e−11J / 4kT +30e−23J / 4kT +6e−31J / 4kT
complexes of Cp2Mn.6c,15 The hapticities of the Cp ligands in
these species appear to be largely influenced by the steric demands
of the Lewis base ligands, with greater steric congestion result-
ing in more significant distortion of the Cp ligands from ideal
g5-bonding. In each of the most closely related nitrogen-donor
adducts [Cp2Mn·TMEDA] (TMEDA = Me2NCH2CH2NMe2)15a
and [Cp2Mn·(PhCH2NHCH2)2]6c the Cp ligands adopt g1- and
g5-bonding modes. The adoption of g1- and g2-coordination by
the Cp ligands in 4 is similar to that observed in the bis-carbene
complex [(g1-Cp)(g2-Cp)Mn{CN(Me)(CH)2}2].16
(11)
7e−11J / 4kT +5e−23J / 4kT +3e−31J / 4kT + e−35J / 4kT
The g value was fixed to a value of 2.0 as expected for an
S = 5/2 ion with no orbital contribution. The temperature
dependence of vT for 1–4 showed a downturn upon cooling
indicative of antiferromagnetic interactions between spins.
However, a broad maximum in v anticipated for short-range
antiferromagnetic coupling was not observed and substantial
deviation between observed and calculated susceptibilities was
found for all samples when attempting to fit the data with a
single parameter, J. The curve-fit improved substantially if some
sampledecompositionwasassumed,generatingtwoindependent
non-interacting S = 5/2 ions. Some sample decomposition is not
unexpected given the extreme air and moisture sensitivity of 1–4.
Eqn. (12) represents the expression used to model the behaviour
of 1–4, taking into account some sample degradation (a) and
an additional contribution from temperature-independent
paramagnetism (TIP).17b
2Nαg2 β2 S(S+1)
′
χ =(1−α)χm =
+TIP
(12)
3k
This approach seems justified by the ability to curve fit two
independent samples of 1, 2 and 4 using the same exchange
interaction but with different degrees of decomposition. Curve-
fitting parameters for 1–4 are given in Table 4. The temperature
dependences of vT for 1–4 with their curve-fits are illustrated
in Fig. 6. The degree of degradation in some samples seems
particularly large (up to 25%) but repeated measurements on
further samples provide satisfactory fits with smaller degrees of
decomposition and the same J value. Attempts to measure the
degree of decomposition directly using X-ray powder diffraction
studies proved unsuccessful, although it was clear for these
studies that prolonged storage of samples led to an increase
in amorphous materials. No significant improvement to the fit
could be achieved by the inclusion of a higher-order biquadratic
term into the Hamiltonian:19
Fig. 5 Structure of the bis-adduct 5; H-atoms except those attached to
N have been omitted for clarity.
Magnetic studies on 1–4
Samples of 1–4 were examined on a SQUID magnetometer
between 5 and 300 K in an applied field of 100 Oe. In the
cases of 1, 2 and 4, measurements were made in duplicate on
independent samples in order to confirm reproducibilty. The
molar susceptibilities were corrected for diamagnetism using
Pascal’s constants.17a
H = −2J{ { − j({ ·{ )2
(13)
1
2
1
2
The magnetic behaviour of dimeric complexes can be
described using the simple Heisenberg spin Hamiltonian,
This biquadratic term introduces a perturbation to the energy
level diagram in which the energy gap between spin states is
determined by the Landé interval rule. The energy levels for a
dimer of S = 5/2 ions including the biquadratic term is provided
in the ESI.†
H = −2J{ {
(9)
1
2
which is appropriate for isotropic S = 5/2 ions with a 6A ground
term. The general expression for the susceptibility of homo-
metallic dimeric species in small applied fields can then be
written as:18
The magnetic properties of numerous binuclear MnII
complexes have been studied, particularly those containing
3 4 8 4
D a l t o n T r a n s . , 2 0 0 4 , 3 4 8 1 – 3 4 8 7

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Experimental
General experimental
Manganocene and the new compounds 2–5 are highly air- and
moisture-sensitive. They were handled on a vacuum line using
standard inert-atmosphere techniques and under dry/oxygen-
free argon. N-2-Anilinopyridine and 4-ethyl-N-anilinopyridine
were prepared by the reaction of 2-bromopyridine with the
corresponding amines in the presence of ZnCl2 and Zn metal.22
Phenylacetylene and 1,1,2,2-tetramethylguanidine starting
materials were used as supplied (Aldrich). Toluene and thf
were dried by distillation over sodium/benzophenone prior to
the reactions. Manganocene was prepared using the method of
Wilkinson et al.23 Complexes 2–5 were isolated and character-
ised with the aid of a nitrogen-filled glove box fitted with a Belle
Technology O2 and H2O internal recirculation system. Melting
points (uncorrected) were determined by using a conventional
apparatus and sealing samples in capillaries under nitrogen. IR
spectra were recorded as Nujol mulls using NaCl plates and were
run on a Perkin-Elmer Paragon 1000 FTIR spectrophotometer.
Elemental analyses were performed by first sealing the samples
under nitrogen in air-tight aluminium boats (1–2 mg) and C, H
and N content was analysed using an Exeter Analytical CE-440.
1H NMR spectra were recorded on a Bruker Advance 400 MHz
FT spectrometer in dry deuterated d6-DMSO (using the solvent
resonances as the internal reference).
Synthesis of 2. A solution of N-2-anilinopyridine (0.085 g,
0.5 mmol) in toluene (12 ml) was added dropwise to a solution
of Cp2Mn (0.092 g, 0.5 mmol) in toluene (8 ml) at −78 °C. The
reaction mixture was stirred (5 min.) then allowed to warm to
room temperature and stirred (1 h). The precipitate that formed
was redissolved by gentle heating. The solution was then filtered
to remove a small amount of undissolved material. The solvent
was reduced under vacuum until a precipitate formed. This was
redissolved by gentle heating and the solution stored at room
temperature (24 h.) to give orange–yellow crystals of 2. Yield
0.06 g (41%). Decomp. 259–260 °C to dark brown solid. IR
−1

(Nujol, NaCl), m/cm : major bands at 1590(m) (C N str.),

1261(s), 1208(w), 1151(w), 1095(s), 1021(s), 801(s), 760(w),
1
694(w). H NMR (400.13 MHz, +25 °C, d6-DMSO), d 9.18 (s,
Ar, 1H), 8.13 (s, Ar, 1H), 7.84 (d, J = 6.7 Hz, Ar, 2H), 7.71 (t,
J = 6.7 Hz, Ar, 1H), 7.38 (dm, Jmajor = ca. 30 Hz, Ar, 2H), 6.89
(t, J = ca. 3 Hz, Ar, 1H), 6.69 (d, J = ca. 30 Hz, Ar, 1H), ca. 3.50
(br s, Cp).
Fig. 6 Temperature dependence of vT for (a) 1, (b) 2, (c) 3 and
(d) 4. The solid lines correspond to the best fit to eqn. (12) using the
parameters given in Table 4.
harder ligands such as alkoxides and halides.20 These ligands
typicallyexhibitexchangeinteractionsintheregion |J|  5 cm−1.9
Of particular relevance to the current studies are reports by
Köhler9 on the magnetic properties of dimeric cyclopentadienyl
MnII complexes of formula [CpMnLX]2 where L is a neutral
monodentate ligand, Cp a substituted cyclopentadienyl ligand
and X is a l2-bridging halide. Here the exchange interactions
were observed to be −4.8, −5.0 and −4.7 cm−1 for X = Cl−, Br−
and I−, respectively. Whilst magnetic exchange interactions
are strongly dependent on M–(l-X)–M bridge angle21 and
any folding around the XX vector,17 it is apparent that the
exchange interactions in the current series are substantially
larger than those reported previously. This most likely reflects
considerably greater metal–ligand covalency which enhances
the magnetic exchange pathway.17 It is noteworthy in this
regard that the l2-C bridged dimer 4 has an appreciably higher
absolute value of J (−30.5 cm−1) than the l2-N bridged dimers
1–3 (J = −17.0 to −21.0 cm−1), as expected on the basis of the
greater covalency of the bridge in 4. Despite the enhanced
air-sensitivity the development of larger clusters based upon
bridging amido or acetylide units appears an attractive goal for
designing spin systems with thermally well-isolated spin ground
states. The extension of these approaches to higher nuclearity
clusters is being actively pursued.6
Synthesis of 3. A solution of 4-ethyl-N-anilinopyridine
(0.098 g, 0.5 mmol) in toluene (9 ml) was added dropwise to
a solution of Cp2Mn (0.092 g, 0.5 mmol) in toluene (11 ml)
at −78 °C. The reaction mixture was stirred (5 min) then allowed
to warm to room temperature and stirred (30 min). The solution
was then filtered to remove a small amount of undissolved
material. The solvent was reduced under vacuum until a
precipitate formed. This was redissolved by gentle heating and
the solution stored at room temperature (48 h) to give crystals
of 3. Yield 0.10 g (65%). Decomp. 195 °C to dark brown solid.
−1

IR (Nujol, NaCl), m/cm : major bands at 1599(ms) (C N str.),

1261(s), 1217 (w), 1157(w), 1098(s), 1022(s), 802(s), 768(w),
1
730(w). H NMR (400.13 MHz, +25 °C, d6-DMSO), d 9.92 (s,
Ar, 1H), 8.08 (s, Ar, 1H), 7.50 (m, Ar, 2H), 7.05 (d, J = 7.0 Hz,
Ar, 2H), 6.75 (d, Jmajor = ca. 7.0 Hz, Ar, 1H), 6.65 (s, Ar, 1H),
ca. 6.40 (vbr s, Cp), 2.48 (q, J = 7.0 Hz, partially obscured by
DMSO solvent peak, 2H, CH3CH2), 1.13 (t, J = 7.0 Hz, 3H,
CH3CH2).
n
Synthesis of 4. BuLi (0.62 ml, 1.0 mmol) was added drop-
wise to a solution of phenylacetylene (0.11 ml, 1.0 mml) in
thf (10 ml) at −78 °C. The mixture was stirred (2 h). The
yellow solution produced was added dropwise to a solution of
Cp2Mn (0.185 g, 1.0 mmol) in thf (10 ml) at −78 °C and stirred
D a l t o n T r a n s . , 2 0 0 4 , 3 4 8 1 – 3 4 8 7
3 4 8 5

View Article Online
Table 5 Details of the data collections and structure solutions of [CpMn{l-N(Ph)(C5H4N)}]2 (2) and [CpMn{l-N(4-EtC6H4)(C5H4N)}]2 (3),
[CpMn{l-CCPh}·thf]2 (4) and [Cp2M{HNC(NMe2)2}2] (5)a
Compound
2
3
4
5
Empirical formula
M
C32H28Mn2N4
578.46
C36H36Mn2N4
634.57
C34H36Mn2O2
586.51
C20H36MnN6
415.49
T/K
Crystal system
Space group
180(2)
Monoclinic
P21/c
180(2)
Monoclinic
P21/c
200(2)
Triclinic
P1
180(2)
Monoclinic
P21/c
a/Å
b/Å
c/Å
a/°
9.0480(18)
17.276(4)
9.776(2)
—
116.32(3)
—
1369.7(5)
2
1.403
9.1976(18)
15.030(3)
11.495(2)
—
99.82(3)
—
1565.7(5)
2
1.346
9.8868(3)
11.1402(4)
13.8546(6)
86.83(3)
83.66(3)
73.50(3)
1453.70(9)
2
1.340
0.897
15869
6532
13.6426(3)
12.6271(3)
13.6115(3)
—
96.849(2)
—
2328.07(9)
4
1.185
b/°
c/°
U/Å3
Z
Dc/Mg m−3
l(Mo-Ka)/mm−1
Refl. collected
Unique refl. (Rint)
0.950
13024
3131
0.837
7984
3544
0.582
12058
2365
0.055
0.031
0.047
0.034
R indices [I > 2r(I)]
R indices (all data)
R1 = 0.036
wR2 = 0.075
R1 = 0.061
wR2 = 0.084
R1 = 0.039
wR2 = 0.100
R1 = 0.059
wR2 = 0.111
R1 = 0.052
wR2 = 0.101
R1 = 0.082
wR2 = 0.114
R1 = 0.038
wR2 = 0.098
R1 = 0.042
wR2 = 0.098
a Data in common, k = 0.71073 Å.
(5 min) before being allowed to warm to room temperature.
The mixture turned bright orange and a precipitate formed
that redissolved at room temperature. After filtration (Celite,
P3), the solvent was reduced under vacuum until precipitation
occurred. The precipitate was redissolved by heating gently
and storage at room temperature (24 h) gave crystals of 4.
Yield 0.11 g (36%). Decomp. 131 °C to black solid. IR (Nujol,
NaCl), m/cm−1: 3080 (C–H str. Cp), 1592 (CC str.) other bands:
Acknowledgements
We gratefully acknowledge the EPSRC (C. S. A., S. R. B.,
S. M. H., M. McP., J. M. R., A. E. H. W., P. T. W., D. S. W.)
and The States of Guernsey and The Domestic and Millennium
Fund (R. A. K.), Clare College Cambridge (Denman–Baynes
Research Fellowship for R. A. L.) and Jesus College Cambridge
(Fellowship for J. C. B.) for financial support. We thank Dr. J. E.
Davies for collecting X-ray diffraction data on 2–5.
1
1094(m), 1032(s), 919(w), 876(w), 801(m), 754(s), 694(m). H
NMR (400.13 MHz, +25 °C, d6-DMSO), d = 7.34 (s, Ph), 6.46
(apparent 1:1 d, Cp), 3.54 (s, thf), 1.70 (s, thf) (the resonances
are very broad and integrals were meaningless).
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X-Ray crystallographic studies of 2–5
Crystals of the new compounds 2–5 were mounted directly from
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diffractometer. The structures were solved by direct methods
and refinement carried out using full-matrix least squares on
F 2.24 Details of the data collections and structure solutions are
given in Table 5.
CCDC reference numbers 207480 and 241879–241882.
See http://www.rsc.org/suppdata/dt/b4/b409135c/ for crystal-
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D a l t o n T r a n s . , 2 0 0 4 , 3 4 8 1 – 3 4 8 7

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D a l t o n T r a n s . , 2 0 0 4 , 3 4 8 1 – 3 4 8 7
3 4 8 7