Rhodium complexes containing bidentate imidazolyl ligands: Synthesis and structure

Article abstract of DOI:10.1016/S0022-328X(99)00347-2

The preparation and characterisation of square planar cationic rhodium(I) dicarbonyl complexes {[Rh((mim)₂C=O)(CO)₂]⁺BPh₄ ^-] (1),{[Rh((mim)₂CH₂)(CO)₂]⁺BPh ₄^-} (2) and{[Rh((mBnzim)₂CH₂)(CO)₂]⁺BPh ₄^-} (3) [mim=N-methylimidazol-2-yl, mBnzim=N-methylbenzimidazol-2-yl] is reported. The carbonyl ligands in 2 and 3 are readily exchanged for triphenylphosphine to form{[Rh((mim)₂CH₂)(PPh₃)₂] ⁺BPh₄^-} (7) and {[Rh((mBnzim)₂CH₂)(PPh₃)₂] ⁺BPh₄^-} (8). Complexes 2 and 3 were characterised by multinuclear NMR spectroscopy as well as by X-ray crystallography. Structural characterisation by X-ray analysis confirmed that complexes 2 and 3 are essentially square planar.

Full text of DOI:10.1016/S0022-328X(99)00347-2

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 588 (1999) 69–77
Rhodium complexes containing bidentate imidazolyl ligands:
synthesis and structure
Sarah Elgafi, Leslie D. Field *, Barbara A. Messerle 1, Peter Turner,
Trevor W. Hambley
School of Chemistry, Uni6ersity of Sydney, Sydney, NSW 2006, Australia
Received 23 March 1999
Abstract
The preparation and characterisation of square planar cationic rhodium(I) dicarbonyl complexes {[Rh((mim)2CꢀO)-
(CO)2]+BPh4−] (1),{[Rh((mim)2CH2)(CO)2]+BPh4−} (2) and{[Rh((mBnzim)2CH2)(CO)2]+BPh4−} (3) [mim=N-methylimidazol-
2-yl, mBnzim=N-methylbenzimidazol-2-yl] is reported. The carbonyl ligands in 2 and 3 are readily exchanged for triphenylphos-
phine to form{[Rh((mim)2CH2)(PPh3)2]+BPh4−} (7) and {[Rh((mBnzim)2CH2)(PPh3)2]+BPh4−} (8). Complexes 2 and 3 were
characterised by multinuclear NMR spectroscopy as well as by X-ray crystallography. Structural characterisation by X-ray
analysis confirmed that complexes 2 and 3 are essentially square planar. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Bisimidazolylmethane; Carbonyl; Ligand exchange; NMR; Rhodium
1. Introduction
shown that rhodium and platinum complexes containing
these ligands catalysed the enantioselective hydrosilation
of ketones [7]. More recently, rhodium complexes con-
taining a range of polypyridine ligands have been pre-
pared and the catalytic activity of these complexes in the
water gas shift reaction has been explored [8].
To date, there have been few reports of the synthesis
of rhodium complexes containing imidazolyl ligands
and, in general, details of catalytic activity have not been
reported. One neutral complex containing an N-
methylimidazolyl ligand [9] {Rh(N-methylimida-
zolyl)(CO)2Cl} and analogues containing a variety of
N-donor heterocyclic ligands [RhL(CO)2Cl], L=
phenazine, quinoxaline and 1,2,4-triazole) have been
patented [9]. These complexes are active catalysts for
hydrosilation reactions in the formation of siloxane
polymer gels.
Bidentate nitrogen-based ligands containing sp2 hy-
bridized nitrogen donors have been used in a variety of
transition metal complexes [1–8], however, few Rh(I)
complexes are known. Some of the earliest work using
bidentate nitrogen donor ligands on rhodium involved
the aromatic compound 1,10-phenanthroline (PHEN) as
a ligand and Rh(I) complexes of PHEN have been tested
for their biological activity in interfering with cell divi-
sion in bacteria [1]. In the late 1970s and early 1980s
rhodium and iridium complexes of 2,2%-bipyridyl (BIPY)
and PHEN were prepared [2,3] and emerged as promis-
ing catalysts for a number of homogeneous reactions
such as transfer hydrogenation [4]. The success of simple
nitrogen donor ligands in rhodium and iridium com-
plexes has meant that complexes containing BIPY and
PHEN ligands and their chiral variants [5] have been
explored in some depth. In 1984 Brunner et al. [6].
reported the synthesis of an extensive range of bidentate
ligands derived from optically active primary amines,
amino acids and amino acid derivatives and it was
* Corresponding author. Tel.: +61-2-93576650.
E-mail address: l.field@chem.usyd.edu.au (L.D. Field)
1 Also corresponding author.
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00347-2

70
S. Elgafi et al. / Journal of Organometallic Chemistry 588 (1999) 69–77
In this paper we report the synthesis and characteri-
sation of the square planar cationic rhodium(I) dicar-
bonyl complexes {[Rh((mim)2CꢀO)(CO)2]+BPh4−} (1),
{[Rh((mim)2CH2)(CO)2]+BPh4−} (2) and {[Rh((mBn-
zim)2CH2)(CO)2]+BPh4−} (3) [mim=N-methylimid-
azol-2-yl, mBnzim=N-methylbenzimidazol-2-yl]. The
complexes were prepared by the reaction of the biden-
tate imidazolyl ligands (mim)2CꢀO (4), (mim)2CH2 (5)
and (mBnzim)2CH2 (6) with [Rh2(CO)4(Cl)2]. The
square planar dicarbonyl complexes 2 and 3 were char-
acterised by X-ray crystallography, and all complexes
1–3 were fully characterised by high-field NMR spec-
troscopy.
Scheme 1.
2. Discussion
{[Rh((mBnzim)2CH2)(PPh3)2]+BPh4−} (8), respectively
2.1. Preparation of (mim)2CꢀO (4), (mim)2CH2 (5) and
(mBnzim)2CH2 (6)
(Scheme 1).
The
1H-
and
31P{1H}-NMR
spectra
of
{[Rh(mim)2CH2(PPh3)2]+BPh4−} (7) and {[Rh((mBn-
zim)2CH2)(PPh3)2]+BPh4−} (8) contain broad reso-
nances at room temperature (r.t.).
The bidentate ligands (mim)2CꢀO (4) and (mim)2CH2
(5), used in the preparation of {[Rh((mim)2-
CꢀO)(CO)2]+BPh4−} (1) and {[Rh((mim)2CH2)-
(CO)2]+BPh4−} (2), were prepared by a previously re-
ported method [10]. The ligand (mBnzim)2CH2 (6) used
The 31P{1H}-NMR spectrum (acetone-d6, 300K) of 7
contains a broad singlet (W1/2=880 Hz). At low tem-
perature (233 K), the single 31P resonance appears as a
broad doublet with a phosphorus–rhodium coupling
in
the
preparation
of
{[Rh((mBnzim)2CH2)-
(CO)2]+BPh4−} (3) was prepared by two independent
methods: (i) by reduction of bis(N-methylbenzimidazol-
2-yl)ketone, (mBnzim)2CꢀO (9) with hydrazine under
basic conditions; (ii) by the condensation of diethyl-
malonate with N-methyl-1,2-phenylenediamine.
1
constant JRhꢁP=116 Hz. The 31P{1H}-NMR spectrum
(acetone-d6) of 8 contains a broad singlet at 32.2 ppm
(W1/2=430 Hz). At low temperature (243 K) the 31P
signal of 8 appears as a doublet at 32.2 ppm with a
phosphorus–rhodium coupling constant 1JRhꢁP=125
Hz.
The broadness of the resonances in the NMR spectra
of 7 and 8 at r.t. is probably due to an equilibrium
involving reversible dissociation of one of the
triphenylphosphine ligands from the metal centre. This
would form a three-coordinate species or a four-coordi-
nate species where a solvent molecule is coordinated in
place of a triphenylphosphine ligand (Scheme 2). This
type of equilibrium has been reported previously for a
number of related metal complexes [11].
The protons of the methylene group joining the
imidazolyl rings in 2, 3, 7 and 8 are readily exchanged
for deuterium in acetone-d6 or methanol-d4 once coor-
dinated to the metal centre. The exchange is probably
2.2. Preparation of rhodium complexes containing the
bidentate ligands (mim)2CꢀO (4), (mim)2CH2 (5) and
(mBnzim)2CH2 (6)
The rhodium(I) dicarbonyl complexes {[RhL-
(CO)2]+BPh4−} (where L=(mim)2CꢀO, (mim)2CH2
and (mBnzim)2CH2) were readily prepared by the addi-
tion of the bidentate ligands (mim)2CꢀO, (mim)2CH2
and (mBnzim)2CH2, respectively to methanol solutions
of [Rh2(CO)4(Cl)2]. Addition of NaBPh4 precipitated
the complexes {[Rh((mim)2CꢀO)(CO)2]+BPh4−} (1),
{[Rh((mim)2CH2)(CO)2]+BPh4−}
(2)
and
{[Rh((mBnzim)2CH2)(CO)2]+BPh4−} (3) in high yield.
When triphenylphosphine was added to a solution of
2 or 3 in acetone, bubbles of CO were immediately
observed. The carbonyl ligands in complexes 2 and 3
are labile and undergo facile exchange with PPh3 to
form {[Rh((mim)2CH2)(PPh3)2]+BPh4−} (7) and
Scheme 2.

S. Elgafi et al. / Journal of Organometallic Chemistry 588 (1999) 69–77
71
complexes and the phenyl residues of the associated
tetraphenylborate counter ions. In particular, the
N(2a)ꢁC(9a) imidazolyl residue of complex A appears
to have an offset stacking interaction with the
N(1b)ꢁC(5b) residue of complex B. The dihedral angle
between the least squares planes of the imidazolyl
residues of complexes A and B is 7°. Atom N(2b) is
Scheme 3.
,
,
only 3.319(4) A from C(9a) and C(3b) is 3.572(5) A
,
from C(5a), whereas C(6a) is 3.777(6) A from C(3b).
due to the facile, reversible, deprotonation of the
methylene bridge to give the neutral complex 10
(Scheme 3).
Presumably, the stacking interaction is responsible for
the pairing of the complexes in the solid state. There
appears to be weak hydrogen bonding between the
acetone solvate and the two independent complexes.
,
The O(3)ꢁC(8a) distance is 3.350(5)
A and the
,
O(3)ꢁC(11b) distance is 3.447(6) A.
2.3. X-ray structure of
{[Rh((mim)2CH2)(CO)2]+BPh−4 } (2)
2.4. X-ray structure of {[Rh((mBnzim)2CH2)(CO)2]+-
BPh−4 } (3)
The solvated complex {[Rh(mim)2CH2(CO)2]+-
BPh4−} (0.5 acetone) crystallised from acetone as yel-
low plates that were suitable for X-ray crystallographic
The complex {[Rh((mBnzim)2CH2)(CO)2]+BPh4−}
analysis.
Two
crystallographically
independent
(3) was readily recrystallised from acetone to give clear
molecules were located in the asymmetric unit and
ORTEP [12] plots of the cations {[Rh((mim)2-
CH2)(CO)2]+} are shown in Fig. 1 and selected bond
lengths and angles for the inner coordination sphere are
listed in Tables 1 and 2, respectively.
The two independent complexes are in close contact,
,
with a Rh1(a)ꢁRh(1b) separation of 3.5197(4) A. The
,
rhodium centre of complex B is 3.41 A above the
,
coordination plane of complex A, and Rh(1a) is 3.50 A
away from the coordination plane of complex B. The
N(1a)ꢁN(2a)ꢁC(1a)ꢁC(2a) and N(1b)ꢁN(2b)ꢁC(1b)ꢁ
C(2b) least-squares coordination planes are almost par-
allel, with an interplane dihedral angle of 7°. The
coordination planes of the two complexes are slightly
,
offset by approximately 0.7 A (assuming parallel planes
,
and an interplanar separation of 3.45 A). The two
complexes have relative rotation about the
a
Rh(1a)ꢁRh(1b) axis of 71.5(2), such that the carbonyl
,
carbon of complex A (C(2a)) is 3.530(6) A from the
carbonyl carbon of complex B (C(1b)), and the car-
,
bonyl C(2b) is 3.635(5) A away from the imidazolyl
nitrogen of complex A (N(1a)). Both complexes have
square planar coordination. Deviations from the least
squares plane defined by N(1a)ꢁN(2a)ꢁC(1a)ꢁC(2a)
,
range between 0.044(3) and 0.115(4) A, and the devia-
tions from the least squares plane defined by
N(1b)ꢁN(2b)ꢁC(1b)ꢁC(2b) range between 0.041(3) and
,
0.103(4) A. The rhodium metal centres are contained
within their respective coordination planes. The Rh1a
deviation from the N(1a)ꢁN(2a)ꢁC(1a)ꢁC(2a) plane is
,
0.04 A, and the Rh(1b) deviation from the
Fig. 1. ORTEP plot and crystal structure numbering of
{[Rh(mim)2CH2(CO)2]+, 25% thermal ellipsoids are shown for the
,
N(1b)ꢁN(2b)ꢁC(1b)ꢁC(2b) plane is 0.05 A.
There may be a stacking interaction between the
imidazolyl residues of the two molecules in the asym-
metric unit and also between imidazolyl residues of the
,
non-hydrogen atoms; hydrogen atoms have an arbitrary radius 0.1 A.
(a) Stacking of the two independent complexes viewed from above.
(b) A single cation with crystal structure numbering.

72
S. Elgafi et al. / Journal of Organometallic Chemistry 588 (1999) 69–77
Table 1
Selected bond distances (A) for {[Rh((mim)2CH2)(CO)2]+BPh4−} (2)
and {[Rh((mBnzim)2CH2)(CO)2]+BPh4−} (3) a
,
,
Atoms
Bond distances (A)
{[Rh((mim)2CH2)(CO)2]+BPh−4 } (2)
Rh(1a)ꢁN(1a)
Rh(1b)ꢁN(1b)
Rh(1a)ꢁN(2a)
Rh(1b)ꢁN(2b)
Rh(1a)ꢁC(1a)
Rh(1b)ꢁC(1b)
Rh(1a)ꢁC(2a)
Rh(1b)ꢁC(2b)
2.071(3)
2.055(3)
2.048(3)
2.063(3)
1.847(4)
1.840(4)
1.840(4)
1.854(4)
{[Rh((mBnzim)2CH2)(CO)2]+BPh−4 } (3)
Rh(1)ꢁN(1)
Rh(1)ꢁN(2)
Rh(1)ꢁC(1)
Rh(1)ꢁC(2)
2.069(6)
2.066(5)
1.79(1)
1.846(8)
a Estimated S.D.s in the least significant figure are given in paren-
theses.
Fig. 2. ORTEP plot and crystal structure numbering of
[Rh((mBnzim)2CH2)(CO)2]+, 25% thermal ellipsoids are shown for
the non-hydrogen atoms; hydrogen atoms have an arbitrary radius
,
0.1 A. The complex is viewed from above.
colourless plates of the solvated complex {[Rh-
((mBnzim)2CH2)(CO)2]+BPh4−} (1.0 acetone). An OR-
TEP plot of the cation [Rh((mBnzim)2CH2)(CO)2]+ is
shown in Fig. 2 and the bond lengths and angles for the
inner coordination sphere are listed in Tables 1 and 2,
respectively.
between the metal complex 3 and the solvent of crys-
tallisation (acetone) or between the imidazolyl residues
of the complexes and the phenyl residues of the associ-
ated tetraphenylborate counterion.
The solid state structures of complexes {[Rh((mim)2-
Complex 3 has square planar coordination. Devia-
tions from the least squares plane defined by
N(1)ꢁN(2)ꢁC(1)ꢁC(2) range between 0.35(5) and
CH2)(CO)2]+BPh4−}
(2)
and
{[Rh((mBnzim)2-
CH2)(CO)2]+BPh4−} (3) are very similar — both com-
plexes are essentially square planar about the metal
centre. In each molecule, there is a plane of symmetry
that bisects the ligand NꢁRhꢁN bite angle and the
metal centre. The bite angles of the bidentate imidazolyl
ligands are small, with the angle between the NꢁRhꢁN
,
0.097(8) A. The rhodium metal centre is contained
within its coordination plane, with a deviation of only
,
0.04 A from the plane N(1)ꢁN(2)ꢁC(1)ꢁC(2). In con-
trast to the structure of {[Rh((mim)2CH2)-
(CO)2]+BPh4−} (2), there appears to be no interaction
,
,
bonds of 87.6(1) A and 87.1(1) A in complex 2 and
85.8(2) A in complex 3.
,
Table 2
Only a small number of rhodium complexes contain-
ing bidentate nitrogen-based ligands with sp2 hy-
bridized donor atoms have been studied crystallo-
graphically [13]. The rhodium–nitrogen bond lengths of
complexes directly comparable to 2 and 3 range be-
tween 2.033(5) and 2.111(8) A. The RhꢁN bond lengths
are between 2.048(3) and 2.071(3) A in complex 2 and
2.066(6) and 2.069(6) A in complex 3 (Table 3). The
rhodium carbonyl bond lengths for complexes com-
parable to 2 and 3 that appear in the literature [13]
range between 1.848(5) A and 1.868(7) A (Table 3). The
RhꢁCO bond lengths in 2 are similar at 1.840(4) and
1.854(4) A, however, in complex 3, the carbonyl bond
lengths differ slightly with Rh(1)ꢁ(C1) and Rh(1)ꢁC(2)
being 1.78(1) and 1.846(8) A, respectively and
C(1)ꢁO(1) and C(2)ꢁO(2) are 1.17(1) and 1.130(8) A,
Selected bond angles (°) for {[Rh((mim)2CH2)(CO)2]+BPh4−} (2) and
{[Rh((mBnzim)2CH2)(CO)2]+BPh4−} (3) a
Atoms
Bond angles (°)
{[Rh((mim)2CH2)(CO)2]+BPh−4 } (2)
N(1a)ꢁRh(1a)ꢁN(2a)
N(1b)ꢁRh(1b)ꢁN(2b)
N(1a)ꢁRh(1a)ꢁC(2a)
N(1b)ꢁRh(1b)ꢁC(2b)
N(2a)ꢁRh(1a)ꢁC(1a)
N(2b)ꢁRh(1b)ꢁC(1b)
C(1a)ꢁRh(1a)ꢁC(2a)
,
87.6(1)
87.1(1)
93.5(1)
91.7(2)
90.1(1)
93.6(1)
89.1(2)
87.8(2)
,
,
,
,
C(1b)ꢁRh(1b)ꢁC(2b)
,
{[Rh((mBnzim)2CH2)(CO)2]+BPh−4 } (3)
N(1)ꢁRh(1)ꢁN(2)
N(1)ꢁRh(1)ꢁC(2)
N(2)ꢁRh(1)ꢁC(1)
C(2)ꢁRh(1)ꢁC(1)
85.8(2)
95.3(3)
92.9(3)
86.2(4)
,
,
respectively. The RhꢁN distances in 3 are equivalent
and there are no unusual contacts with C(1) that might
otherwise explain the slightly shorter RhꢁC(1) bond
a Estimated S.D.s in the least significant figure are given in paren-
theses.

S. Elgafi et al. / Journal of Organometallic Chemistry 588 (1999) 69–77
73
and the slightly longer RhꢁC(2) bond. The angle be-
tween the two carbonyl ligands is only slightly smaller
than 90° for both 2 and 3, being 89.1(2) and 87.8(2)° in
complex 2 and 86.2(4)° in complex 3. The smaller angle
in 3 is the result of carbonyl contact with the bulkier
benzimidazolyl ligand. The C(1) to C(16) distance is
and {[Rh((mBnzim)2CH2)(CO)2]+BPh4−} (3) have been
synthesised in good yield. Complexes 2 and 3 have been
structurally characterised by X-ray analysis, confirming
that the complexes are essentially square planar. The
protons of the methylene group joining the imidazolyl
rings in 2, 3, 7 and 8 readily undergo exchange in
deuterated solvents once coordinated to the metal cen-
tre. The methylene protons exchange readily for deu-
terium in methanol-d4 and acetone-d6.
,
,
3.24(1) A and the C(2) to C(4) distance is 3.22(1) A.
The coordination of the bidentate ligand to the metal is
not symmetrical with respect to the carbonyl ligands.
The N(2)ꢁRh(1)ꢁC(1) angle is 92.9(3)° and the
N(1)ꢁRh(1)ꢁC(2) angle is 95.3(3)°. C(1) and C(2) are on
opposite sides of the least squares coordination plane,
The carbonyl ligands in [RhL(CO)2]+ are labile and
can be exchanged for triphenylphosphine to form
[RhL(PPh3)2]+ 7 and 8. These complexes have broad
resonances in the 31P- and 1H-NMR spectra at r.t.,
which is probably due to reversible dissociation of one
of the triphenylphosphine ligands from the metal
centre.
,
with C(1) being displaced 0.087(8) A from the plane
away from the ligand and C(2) being displaced 0.096(8)
,
A towards the bidentate ligand.
3. Conclusions
The cationic rhodium complexes {[Rh(mim)2CꢀO-
(CO)2]+BPh4−} (1), {[Rh(mim)2CH2(CO)2]+BPh4−} (2)
4. Experimental
All manipulations of metal complexes and air-sensi-
tive reagents were carried out using standard Schlenk
or vacuum techniques [14], or in a Vacuum Atmo-
spheres argon-filled drybox. Rhodium(III) trichloride
hydrate was obtained from Aldrich or Johnson
Matthey and was used without further purification.
n-Butyllithium was used as a solution in hexane (ap-
proximately 2.4 M) as supplied by Aldrich and was
titrated immediately prior to use against 2,5-dimethoxy-
benzyl alcohol [15]. Tetrahydrofuran and toluene were
stored over sodium wire and were distilled under nitro-
gen immediately prior to use from sodium benzophe-
none ketyl. Acetone was dried over and distilled from
anhydrous calcium sulfate.
Table 3
Comparison of RhꢁN and RhꢁC (C of CO) bond lengths of previ-
ously reported Rh complexes with dinitrogen ligands a
RhꢁN bond
RhꢁC bond
,
,
lengths (A)
lengths (A)
2.111(8)
2.097(7)
All bulk compressed gases were obtained from
British Oxygen Company (BOC Gases). Argon (\
99.99%), nitrogen (\99.5%) were used as supplied
without further purification. Mass spectra of organic
compounds were recorded on a KRATOS MS9/MS50
double focussing mass spectrometer with an accelera-
tion voltage of 8000 V and using electron impact ionisa-
tion (EI) with a source temperature of 150–350°C and
an electron energy of 70 eV. Mass spectra of
organometallic complexes were recorded on a VG
Quattro mass spectrometer (VG Biotech, Altricham,
UK). Data are quoted in the form x(y), where x is the
mass/charge ratio and y is the percentage abundance
relative to the base peak. In the case of organometallic
complexes in which the overall mass spectrum was
dominated by that of the ligands, mass spectra were
recorded scanning mass ranges greater than that of the
free ligand, typically m/z=250. Peaks with low inten-
sity are not quoted unless deemed significant. Micro
analyses were carried out at the Micro Analysis Facility
at the University of New South Wales, Sydney. IR
2.070(4)
2.093(4)
1.848(5)
1.867(6)
2.103(5)
2.040(5)
1.866(7)
1.868(7)
2.094(5)
2.033(5)
1.848(6)
1.864(9)
a Ligands used: {[Rh(COD)(CH2(Pz)2)]ClO4·1/2C2H4Cl2 (I) [13a];
{[Rh{h2-HBPz2*(Pz*H)](CO)2}·{BF4} (II) [13b]; [Rh(CO)2(indolato)]
(III) [13c]; [Rh(CO)2(NH2bpt)][ClO4] (IV) [13d].

74
S. Elgafi et al. / Journal of Organometallic Chemistry 588 (1999) 69–77
Table 4
Crystallographic data for {[Rh((mim)2CH2)(CO)2]+BPh4−} (2) and {[Rh((mBnzim)2CH2)(CO)2]+BPh−4 } (3)
{[Rh((mim)2CH2)(CO)2]+BPh4−}·0.5 acetone (2)
{[Rh((mBnzim)2CH2)(CO)2]+BPh4−}·acetone (3)
Empirical formula
Formula weight (g mol−1
Crystal system
Space group
Unit cell dimensions
C
683.42
Monoclinic
P21/c (c14)
36.50H35.0BN4.0O2.50Rh
C46.0H42.0BN4.0O3.0Rh
812.58
Monoclinic
P21/n (c14)
)
,
a (A)
22.182(3)
11.362(2)
28.051(6)
112.04(2)
6553(2)
8
1.385
2816.00
13.104(3)
17.935(4)
17.365(4)
96.32(2)
4056(2)
4
1.330
1680.0
,
b (A)
,
c (A)
i (°)
V (A )
3
,
Z
Dcalc (g cm−3
F(000)
)
2qmax (°)
hkl range
49.9
44.9
−26“24, 0“13, 0“33
−14“14, −1“19, −1“18
Number of reflections measured (N)
Unique number of reflections (N0)
No. observations (I\2.50|\(I))
12091
11827 (Rmerge=0.018)
7938
5.60
0.036
0.022
0.95
6578
5843 (Rmerge=0.028)
3245
4.65
0.049
0.044
0.89
Wavelength MoꢁKa (cm−1
)
a
R
wR
a
Max. transmission
Min. transmission
Residual extrema (e A
0.99
−0.34, 0.38
0.97
−0.57, 0.54
−3
,
)
a R=S(ꢀFoꢀ−ꢀFcꢀ)2/SꢀFoꢀ; wR=(S w(ꢀFoꢀ−ꢀFcꢀ)2/S wꢀFoꢀ2)1/2;w=1/|2(Fo).
spectra were recorded on a Perkin–Elmer 1600 series
FTIR spectrophotometer. Melting points were deter-
mined using a Gallenkamp melting point apparatus and
are uncorrected.
Cromer and Waber [18]. Anomalous dispersion effects
were included in Fcalc [19], the values for f % and f %% were
those of Creagh and McAuley [20]. The values for the
mass attenuation coefficients are those of Creagh and
Hubbell [21].
CDCl3 was used as supplied, acetone-d6 was dried
over and distilled from P2O5. All NMR solvents used
on air sensitive compounds were degassed by at least
three freeze–pump–thaw cycles before being distilled
under high vacuum prior to use. 1H-, 31P- and 13C-
NMR, spectra were obtained on a Bruker AMX400 at
300K unless otherwise stated. 1H and 13C-NMR spectra
are referenced to residual solvent resonances. 31P-NMR
spectra are referenced to external neat trimethyl phos-
phite taken as 140.85 ppm at the temperature quoted.
(mim)2CH2 (5) was prepared by a method described
4.2. Synthesis of bis(N-methylbenzimidazol-2-yl)ketone,
(mBnzim)CꢀO, (9)
n-Butyllithium (54.0 mmol) was added to a stirred
solution of N-methylbenzimidazole (8.25 g, 62.5 mmol)
in THF (100 ml) at −78°C under nitrogen. The colour
of the solution became bright red on addition of the
base and the mixture was stirred at −78°C for 2 h.
Diethyl carbonate (3 ml, 25 mmol) was added and the
colour of the solution changed to yellow. The solution
was allowed to warm to −40°C over several hours,
quenched by addition of solid carbon dioxide and then
allowed to warm to r.t. Water (100 ml) was added and
the solution was extracted into ethyl acetate (600 ml) in
a continuous liquid–liquid extractor for 16 h. The
solvent was removed under vacuum to give a cream
coloured residue which was extracted into chloroform.
The solvent was removed under vacuum to give a
brown oil which crystallised on addition of acetone (ca.
10 ml). The ketone (mBnzim)2CꢀO (9) was obtained as
cream coloured needles (1.07 g, 26%). M.p. 167–169°C
(see Ref. [22]. 190–191°C). 1H-NMR (400 MHz,
previously
[10].
Tetracarbonyldichlorodirhodium
(Rh2(CO)4Cl2) was synthesised by the procedure de-
scribed by Cleverty and Wilkinson [16].
4.1. X-ray data collection
All measurements were made at 2191°C on an
Enraf–Nonius CAD4 diffractometer with graphite
monochromated MoꢁKa radiation. The crystallo-
graphic data are given in Table 4. All calculations were
performed using the TEXSAN [17] crystallographic soft-
ware package of Molecular Structure Corporation.
Neutral atom scattering factors were taken from

S. Elgafi et al. / Journal of Organometallic Chemistry 588 (1999) 69–77
75
CDCl3): l 7.99–7.97 (m, 1H, H6), 7.48–7.44 (m, 2H,
H4 and H7), 7.39–7.35 (m, 1H, H5), 4.14 (s, 3H,
N–CH3). 13C{1H}-NMR (100 MHz, CDCl3): l 179.6
(CꢀO), 147.5, 143.1, 137.5 (C2, C3a and C7a), 126.9,
124.4, 123.6, 110.9 (C4, C5, C6 and C7), 32. 7
(NꢁCH3). MS m/z (%): 291 (20, (M+1+), 290 (77,
M+), 275 (11), 173 (10), 160 (10), 159 (30), 149 (14),
148 (13), 147 (14), 146 (100), 145 (25), 133 (14), 132
(40), 133 (33), 104 (11), 77 (24), 57 (19), 51 (11), 45 (23),
43 (30), 41 (16).
Rh2(CO)4Cl2 (0.16 g, 0.42 mmol) in methanol (30 ml) at
r.t. The solution darkened immediately on addition of
4. The mixture was stirred for 2 h, after which time an
excess of NaBPh4 in methanol (5 ml) was added. The
mixture was stirred for several minutes and the orange
precipitate that formed was isolated by filtration and
washed with methanol (20 ml). {[Rh((mim)2-
CꢀO)(CO)2]+BPh4−} (1) was recrystallised from ace-
tone as red prisms (0.51 g, 92%). M.p. 178°C (decom-
posed without melting). 1H-NMR (400 MHz,
acetone-d6): l 7.84 (s, 2H, H5), 7.70 (s, 2H, H4), 7.40
(m, 8H, BPh4), 6.97 (m, 8H, BPh4), 6.82 (m, 4H, BPh4),
4.24 (s, 6H, NꢁCH3).13C{1H}-NMR (100MHz, acetone-
d6): l 183.0 (d, 1JRhꢁCO=68.7 Hz, RhꢁCO), 166.6 (CO),
163.9 (m, BꢁC), 138.8 (C2), 136.0 (BPh4), 133.4 (C5 or
C4), 130.0 (C4 or C5), 125.0 (BPh4), 121.2 (BPh4), 37.8
(NꢁCH3). IR (Nujol, cm−1): 2097 (m, RhꢁCO), 2024
(m, RhꢁCO), 1478(s, ligand CO). Anal. Calc. for
C35H30N4O2BRh: C, 62.90; H, 4.52; N, 8.38. Found: C,
62.8; H,4.6; N, 8.4
4.3. Synthesis of bis(N-methylbenzimidazol-2-yl)meth-
ane, (mBnzim)2CH2 (6)
4.3.1. Method I
The compound (mBnzim)CH2 (6) was prepared using
a method analogous to that of Byers and Canty for the
synthesis of (mim)CH2 [23]. The ketone (mBnzim)2CꢀO
9 (2.0 g, 15 mmol) was placed in a glass-sleeved reac-
tion bomb with hydrazine hydrate [24] (4.0 ml, 77
mmol) and sodium hydroxide (0.60 g, 15 mmol). The
vessel was sealed and heated to 140°C for 4 h, after
which time the vessel was cooled to r.t. and opened
carefully. The product was extracted into acetone and
the solvent removed to yield (mBnzim)CH2 (6) as a
cream solid (0.37 g, 19%). M.p. 206.5–208.5°C.
4.5. Synthesis of {[Rh((mim)2CH2)(CO)2]+BPh−4 } (2)
A solution of (mim)2CH2 (5) (0.22 g, 1.3 mmol) in
methanol (5 ml) was added to a stirred solution of
Rh2(CO)4Cl2 (0.20 g, 0.52 mmol) in methanol (30 ml) at
r.t. A yellow precipitate formed initially, and disap-
peared as the reaction proceeded giving a clear yellow
solution which was stirred for 1.5 h. An excess of
NaBPh4 in methanol (5 ml) was added, the mixture was
stirred for several minutes and the pale yellow precipi-
tate that formed was isolated by filtration and washed
with methanol (20 ml). {[Rh((mim)2CH2)(CO)2]+
BPh4−} (2) was recrystallised from acetone as bright
yellow prisms (0.62 g, 91%). M.p. 175°C (decomposed
4.3.2. Method II
(mBnzim)CH2 (6) was synthesised using a modifica-
tion of the method described by Arnold et al. [25]. A
solution of N-methyl-1,2-phenylenediamine (11.3 g,
92.6 mmol) in 1,6-dichlorotoluene (50 ml) was heated
to 170°C in a flask equipped with an azeotroping
(Dean–Stark) head. Diethyl malonate (7.40 g, 46.3
mmol) was added drop-wise over 90 min. The tempera-
ture gradually increased to 185°C during the addition
and was maintained at 185–190°C for 2 h, during
which time volatile materials (ethanol and water) were
collected. The mixture was allowed to cool to r.t., and
the precipitated solid was isolated by filtration, washed
with benzene (225 ml) and methanol (50 ml) and dried
under vacuum. The product (mBnzim)2CH2 (6) was
obtained as a beige solid (7.62 g, 60%). M.p. 206.5–
1
without melting, darkens from 146°C). H-NMR (400
3
MHz, acetone-d6, 233 K): l 7.50 (d, 2H, JH5ꢁH4=1.7
3
3
Hz, H5), 7.42 (dd, 2H, JH4ꢁH5=1.7 Hz, JH4ꢁRh=0.8
Hz, H4), 7.37–7.32 (m, 8H, BPh4), 6.96 (t, 8H, 3JHꢁH
=
7.4 Hz, BPh4), 6.84−6.79 (m, 4H, BPh4), 4.43 (s, 2H,
CH2), 3.85 (s, 6H, NꢁCH3).13C{1H}-NMR (100MHz,
acetone-d6): l 186.0 (d, 1JRhꢁCO=67.6 Hz, RhꢁCO),
166.2–164.8 (m, BꢁC), 144.3 (C2), 137.6 (BPh4), 131.5
(C5 or C4), 126.6 (BPh4), 124.9 (C4 or C5), 122.9
(BPh4), 35.3 (NꢁCH3), 24.9(CH2). Electrospray MS m/
1
208.5°C. H-NMR (400 MHz, CDCl3): l 7.76–7.71 (m,
2H), 7.33–7.22 (m, 6H), 4.69 (s, 2H, CH2), 3.90 (s, 6H,
NꢁCH3). 13C{1H}-NMR (100MHz, CDCl3): l 149.8,
142.9, 136.7 (C2, C3a and C7a), 123.3, 122.7, 120.0,
109.9 (C4, C5, C6 and C7), 31.0 (NꢁCH3), 29.1 (CH2).
MS m/z (%): 277 (22, (M+1+), 275 (19), 216 (12), 148
(14), 147 (11), 146 (56), 145 (76), 138 (10), 131 (10), 145
(76), 144 (17), 138 (10), 131 (31), 119 (15), 77 (15).
z:
(ES+)
335.3
(100,
M+),
307.2
(47,
[Rh((mim)2CH2)(CO)]+). (ES−) 318.7 (77, BPh4). IR
(nujol, cm−1): 2076 (m, RhꢁCO), 2004 (m, RhꢁCO).
Anal. Calc. for C35H32N4.0O2BRh·acetone: C, 64.15; H,
5.16; N, 8.20. Found: C, 64.3; H,5.4; N, 8.0.
Crystals of 2 that were suitable for X-ray crystal
structure analysis were obtained by slow evaporation of
solvent from an acetone solution. The atom numbering
scheme is given in Fig. 1. Selected bond lengths and
angles are given in Tables 1 and 2.
4.4. Synthesis of {[Rh((mim)2CꢀO)(CO)2]+BPh−4 } (1)
A solution of (mim)2CꢀO (4) (0.18 g, 0.95 mmol) in
methanol (10 ml) was added to a stirred solution of

76
S. Elgafi et al. / Journal of Organometallic Chemistry 588 (1999) 69–77
6H, NꢁCH3 ). 13C{1H}-NMR (100MHz, acetone-d6):
2
4.6. Synthesis of {[Rh((mBnzim)2CH2)(CO)2]+BPh−4 }
(3)
164.5 (m, BꢁC), 141.5 (C2), 136.7 (BPh4), 134.8 (PPh3),
133.1 (broad s, ipso-PPh3), 131.4 (C5 or C4), 130.1 (C4
or C5),129.2 (PPh3), 126.1 (m, BPh4),122.3 (BPh4), 33.2
(NꢁCH3)2. 31P{1H}-NMR (162 MHz, acetone-d6, 233
K): l 45.2 (d, 1JRhꢁP=116 Hz). Anal. Calc. for
C69H62N4P2BRh·2H2O: C, 71.51; H, 5.74; N, 4.83.
Found: C, 71.2; H, 5.9; N, 4.6.
A solution of (mBnzim)2CH2 (6) (0.40 g, 1.4 mmol)
in methanol (5 ml) was added to a stirred solution of
Rh2(CO)4Cl2 (0.22 g, 0.57 mmol) in methanol (30 ml) at
r.t. A cream precipitate formed initially and disap-
peared as the reaction progressed, giving eventually a
clear pale orange solution. The mixture was stirred for
1.5 h, after which time an excess of NaBPh4 in
methanol (5 ml) was added. The mixture was stirred for
several minutes and the cream precipitate that formed
was isolated by filtration and washed with methanol (20
ml). {[Rh((mBnzim)2CH2)(CO)2]+BPh4−} (3) was re-
crystallised from acetone to yield pale yellow prisms
(0.83 g, 97%). M.p. 202°C (decomposed without melt-
4.8. Synthesis of {[Rh((mBnzim)2CH2)(PPh3)2]+BPh−4 }
(8)
A solution of triphenylphosphine (0.10 g, 0.38 mmol)
in acetone (5 ml) was added to a stirred solution of
{[Rh((mBnzim)2CH2)(CO)2]+BPh4−} (3) (0.10 g, 0.13
mmol) in acetone (20 ml) at r.t. The solution was
stirred for 4 h at r.t., followed by addition of hexane to
cause precipitation of the crude product. The solid was
isolated by filtration and washed with hexane (20 ml).
{[Rh((mBnzim)2CH2)(PPh3)2]+BPh4−} (8) was obtained
as a pale yellow solid (0.14 g, 83%). M.p. 196–198.5°C.
1H-NMR (400 MHz, acetone-d6, 243 K): l 7.65–7.21
(m, 46H, PPh3, BPh4, mBnzim), 6.99 (m, 8H, BPh4),
6.83 (m, 4H, BPh4), 4.52 (CH2(CHD)), 3.77 (s, 6H,
NꢁCH3). 13C{1H}-NMR (100 MHz, acetone-d6): l
167.7 (m, BꢁC), 150.9, 142.9 (C2 and C3a or C7a),
139.1 (BPh4), 138.4 (C3a or C7a), 136.8 (PPh3), 134.0
(br s, ipso-PPh3), 133.8 (PPh3), 131.4 (PPh3), 128.0 (m,
BPh4), 125.9 (CH(mBnzim)), 125.2 (CH(mBnzim)),
1
ing). H-NMR (400MHz, acetone-d6): l 8.08–8.01 (m,
1H, H5 or H4), 7.87–7.81 (m, 1H, H7), 7.65–7.58 (m,
2H, H6 and H4 or H5), 7.41–7.36 (m, 4H, BPh4),
6.97–6.91 (m, 4H, BPh4), 6.82–6.76 (m, 2H, BPh4),
5.11 (s, 1H, CH2), 4.22 (s, 3H, NꢁCH3). 13C{1H}-NMR
(100MHz, acetone-d6): l 189.8 (d, 1JRhꢁC=68.1 Hz,
RhꢁCO), 169.8 (m, BꢁC), 149.7, 139.9, 13 4.4 (C2, C3a,
C7a), 136.1 (BPh4), 125.0 (BPh4), 121.2 (BPh4), 125.1,
124.8, 117.0 (C4, C5, C6), 111.8 (C7), 31.0 (NꢁCH3),
30.9 (CH2). Electrospray MS m/z: (ES+) 435.1(62,
M+), 407.3 (100, [Rh((mBnzim)2CH2)(CO)]+). (ES−)
319.5 (47, BPh4). IR (Nujol, cm−1): 2080 (s, RhꢁCO),
2012 (s, RhꢁCO).
Crystals of 3 that were suitable for X-ray crystal
structure analysis were obtained by slow evaporation of
solvent from an acetone solution. The atom numbering
scheme is given in Fig. 2. Selected bond lengths and
angles are given in Tables 1 and 2.
124.3
(BPh4),
121.3
(CH(mBnzim)),
113.1
(CH(mBnzim)), 33.3 (NꢁCH3)3. 31P{1H}-NMR (162
MHz, acetone-d6, 243 K): 32.2 (d, 1JRhꢁP=125Hz,
PPh3). Anal. Calc. for C77H66N4P2BRh·2H2O C, 73.45;
H, 5.60; N, 4.45. Found: C, 73.4; H, 5.7; N, 4.4.
5. Supplementary material
4.7. Synthesis of {[Rh((mim)2CH2)(PPh3)2]+BPh−4 } (7)
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC nos. 114536 for 2 and 114537 for
3. Copies of this information may be obtained free of
charge from: The Director, CCDC, 12 Union Road,
Cambridge, CB2 1EZ, UK (Fax: +44-1223-336-033;
email: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk). Listings of atom coordinates, an-
isotropic thermal parameters, torsion angles, details of
least squares planes calculations (34 pages) for 2 and 3
attached.
A solution of triphenylphosphine (0.10 g, 0.38 mmol)
in acetone (5 ml) was added to a stirred solution of
{[Rh((mim)2CH2)(CO)2]+BPh4−} (2) (90 mg, 0.14
mmol) in acetone (20 ml) at r.t. The solution began to
effervesce immediately and the colour lightened to a
golden–yellow. The solution was stirred for 4 h at r.t.,
followed by addition of hexane to cause precipitation of
the crude product. The solid was isolated by filtration
and washed with hexane (310 ml). The residue was
recrystallised from acetone to give {[Rh((mim)2CH2)-
(PPh3)2]+BPh4−} (7) as a yellow crystalline solid (0.15
1
g, 97%). M.p.165.9–169°C. H-NMR (400 MHz, ace-
2 The CH2 resonance of this complex was not observed in either the
1H or 13C{1H}-NMR spectra due to exchange of the CH2 to CD2 in
acetone-d6 solvent.
3 The CH2 of this complex was not observed in the 13C{1H}-NMR
spectrum due to exchange of the CH2 to CD2 in acetone-d6 solvent.
tone-d6, 233K): l 7.71–7.47 (broad multiplet, 30H,
PPh3), 7.38–7.34 (m, 8H, BPh4), 7.23 (broad s, 2H, H5
or H4), 7.00 (broad s, 2H, H4 or H5), 6.97 (m, 8H,
7.7Hz, BPh4), 6.83–6.80 (m, 4H, BPh4), 3.36 (broad s,

S. Elgafi et al. / Journal of Organometallic Chemistry 588 (1999) 69–77
77
[12] C.K. Johnson, ORTEP report ORNL-3794, Oak Ridge National
Laboratory, TN, 1965.
Acknowledgements
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We gratefully acknowledge financial support from
the Australian Research Council and we thank Johnson
Matthey Pty. Ltd. for the generous loan of rhodium
salts.
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