Cobalt half-sandwich, sandwich, and mixed sandwich complexes with soft tripodal ligands

Article abstract of DOI:10.1021/ic0496525

Reaction of sodium hydrotris(methimazolyl)borate (NaTm^Me) with cobalt halides leads to the formation of paramagnetic pseudotetrahedral [Co(Tm^Me)X] (X = CI, Br, I), of which the bromide has been crystallographically characterized. Mas

Full text of DOI:10.1021/ic0496525

Inorg. Chem. 2004, 43, 4927−4934
Cobalt Half-Sandwich, Sandwich, and Mixed Sandwich Complexes with
Soft Tripodal Ligands
†
†
‡
,†
Christopher A. Dodds, Mario-Alexander Lehmann, Jonathan F. Ojo, John Reglinski,* and
Mark D. Spicer*
,†
Department of Pure & Applied Chemistry, UniVersity of Strathclyde, 295 Cathedral Street,
Glasgow G1 1XL, U.K., and Department of Chemistry, Obafemi Awolowo UniVersity,
Ile-Ife, Nigeria
Received March 16, 2004
Me
Reaction of sodium hydrotris(methimazolyl)borate (NaTm ) with cobalt halides leads to the formation of paramagnetic
Me
pseudotetrahedral [Co(Tm )X] (X ) Cl, Br, I), of which the bromide has been crystallographically characterized.
Me
+
Me
+
Mass spectrometry reveals the presence of higher molecular weight fragments [Co(Tm )
2
] and [Co
2
(Tm )
] cation, which has
, Br , and I , salts. Attempts to prepare the mixed
sandwich complex, [Co(Cp)(Tm )] , resulted in ligand decomposition to yield [Co(mtH) I]I (mtH ) 1-methylimidazole-
2
X] in
Me
+
solution. Aerial oxidation in donor solvents (e.g. MeCN) leads to formation of the [Co(Tm )
2
been crystallographically characterized as the BF
4 4
-, ClO
-
-
-
Me
+
3
Me
Me
Me
2
-thione), but with the more electron donating methylcyclopentadienyl (Cp ) ligand, [Co(Cp )(Tm )]I was isolated
and characterized. Electrochemical measurements reveal that the cobalt(III) Tm complexes are consistently more
Me
difficult to reduce than their Tp and Cp congeners.
Introduction
Chart 1
The reactivity of metal-based reagents and catalysts is
modulated to a great extent by the nature of the ligands which
complete their coordination sphere. Typically this reactivity
is controlled by alteration of steric bulk and electron donor
properties. Although the chemical literature is rich in studies
which seek to design new ligand systems, many workers
regularly return to the ubiquitous cyclopentadienide (Cp) and
phosphine ligands as their basic chemical building blocks.
While phosphines are readily altered, both sterically and
electronically, such alteration for cyclopentadienyl ligands
1
is much more difficult to achieve. Consequently there has
been significant interest in generating facially capping ligands
that facilitate such tuning of the metal center. In 1966
2
Trofimenko introduced the somewhat harder facially capping
ligands, the polypyrazolylborate anions (Tp and pzTp; Chart
softer ligand systems such as the tetrakis{(methylthio)-
1), the metal complexes of which in many instances parallel
3
methyl}borate anion (Tt) and the hydrotris(methimazolyl)-
those formed with Cp. More recently broadly analogous
Me 4
borate anion (Tm ) have appeared. Concurrent with the
*
Authors to whom correspondence should be addressed. E-mail:
appearance of these soft tripodal ligands, Fehlhammer has
reported the synthesis of a much harder ligand motif (cf.
j.reglinski@strath.ac.uk (J.R.); m.d.spicer@strath.ac.uk (M.D.S.). Tel: +44
1
(
41 548 2349 (J.R.); +44 141 548 2800 (M.D.S.). Fax: +44 141 552 0876
J.R. and M.D.S.).
†
University of Strathclyde.
Obafemi Awolowo University.
(3) Ge, P.; Haggerty, B. S.; Rheingold, A. L.; Riordan, C. G. J. Am. Chem.
Soc. 1994, 16, 8406.
‡
(
1) Tolman, W. B. Chem. ReV. 1977, 77, 313.
(4) Cassidy, I.; Garner, M.; Kennedy, A. R.; Reglinski. J.; Spicer, M. D.
J. Chem. Soc., Chem. Commun. 1996, 1975.
(2) Trofimenko, S. J. Am. Chem. Soc. 1966, 88, 1842.
1
0.1021/ic0496525 CCC: $27.50 © 2004 American Chemical Society
Inorganic Chemistry, Vol. 43, No. 16, 2004 4927
Published on Web 07/08/2004

Dodds et al.
Tp) namely the tripodal N-heterocyclic carbene ligand,
Experimental Section
5
hydrotris(1-methyl-imidazol-2-ylidene)borate anion (Tim)
All chemicals were commercially obtained and used without
further purification. Sodium12 and thallium hydrotris(methima-
zolyl)borate, [CoCp(CO)I
by published methods. All NMR spectra were recorded on a Bruker
+
and its iron(III) and cobalt(III) complexes, [M(Tim)
2
] . Thus,
13
a great diversity of facially capping tripodal ligands is now
available.
], and [CoCpMe(CO)I
]
14
were prepared
2
2
Me
1
A systematic investigation of the chemistry of Tm , a
AMX 400 spectrometer operating at 400.1 MHz for H, 100 MHz
13
59
1
13
softer analogue of Cp and Tp, is ongoing in our laboratories.
A consistent theme in these studies has been the parallels
between the complexes of these three facially capping ligands
for C, and 94.94 MHz for Co. H and C spectra were referenced
5
9
to internal solvent peaks and thus to TMS, while Co spectra were
referenced to external 1.0 M K [Co(CN) ] in D O. Magnetic
measurements were determined at room temperature in the solid
state using an MSB1 magnetic susceptibility balance. UV-visible
spectra were recorded in solution (dichloromethane, acetonitrile,
3
6
2
(in particular their bonding and structures) in an attempt to
decipher the trends and divergent reaction profiles of these
popular ligand systems. Recent work on molybdenum and
300-850 nm) using a Perkin-Elmer Lambda 16 spectrometer and
6
tungsten carbonyl complexes has highlighted some aspects
by solid-state reflectance using a Photonics CCD array UV-visible
spectrometer (400-900 nm) and a Perkin-Elmer Lambda 19
spectrometer (900-2000 nm). Mass spectra were recorded at the
EPRSC facilities at University of Wales, Swansea, U.K., using a
Me
of the bonding in these species. Although Tm has been
identified as a soft analogue of Cp (which is in turn softer
than Tp), the effects of these ligands on the carbonyl
stretching frequencies in [W(L)(CO)
3
I] and in [Mo(L)(CO)
)] (L ) Cp, Tp, Tm ) places them in the order Cp
Tp < Tm in terms of their donor ability to the metal center.
2
-
Finnegan MAT95 or MAT900.
3
Me
(η -C
3
H
5
Preparation of [Co(TmMe)Cl]. CoCl ‚6H O (0.12 g, 0.5 mmol)
2
2
Me
<
was refluxed with TlTm (0.28 g, 0.5 mmol) in acetone (100 mL)
for 3 h. The thallium chloride precipitate was removed by filtration
and the solution reduced in volume. The product was precipitated
as an olive green solid by addition of excess hexane, collected by
filtration, washed twice with diethyl ether, and dried in vacuo.
Yield: 0.92 g, 41%.
Thus, the application of the terms “hard” and “soft” in these
systems is apparently not particularly helpful. Rather, the
trends would appear to relate to the bonding modes of the
Me
respective ligands. The Tm ligand may be considered a
σ-donor, π-donor, which results in greater electron density
at the metal center than for Tp (σ-donor only) and Cp (σ-
donor, π-acceptor). This varying electron donor ability also
Anal. Found: C, 29.72; H, 3.83; N, 16.76. Calcd for C12
16
H -
BClCoN ‚0.75CH Cl : C, 30.07; H, 3.46; N, 16.50. [Although
6
S
3
2
2
the microanalysis can be scaled using lattice solvent as presented,
the microanalysis data for the compounds obtained from these
reactions show great variation. Mass spectrometry clearly shows
modulates the structural characteristics of the allyl ligand in
3
[
Mo(L)(CO)
2
(η -C
3 5
H )], with clear trends in both the Mo-C
distances to and C-C distances within the allyl fragments
being observed.
This behavior prompted us to return to the analysis of
analogues of cyclopentadienyl complexes and, in particular,
those of iron and cobalt. The chemistry of iron with Tm
was found to be dominated by the formation of [Fe(Tm ) ],
2
at face value a direct analogue of ferrocene. However, unlike
ferrocene, it was found to be a high-spin iron(II) species
Me
+
trace amounts of the oxidized species [Co(Tm
and appreciable amounts of the dimeric species [(Co{Tm })
(X ) Cl, m/e 855.4; X ) Br, m/e ) 900.8; X ) I, m/e 947.4) are
2
) ] (m/e 761.4)
Me
+
2
X]
-
1
also present.] FTIR [ν/cm (KBr disk)]: 2435 (B-H). MS (FAB;
+
Me
+
7
Me
m/e): 410.0, [M - Cl]; 761.4, [Co(Tm ) ] ; 855.4, [Co -
2
2
Me
1
+
Me
(Tm
)
2
Cl] . µeff (298 K) ) 4.19 µ . UV-visible (solid state; Emax,
B
-
cm ): 4250 (sh), 4580 (sh), 4890, 5730, 7250, 14 330, 15 432
-
4
(
sh), 24 510, 32 470. UV-visible (CH
2
-1
Cl
2
solution, 5.2 × 10
-
3
-1
3
-1
mol dm , Emax/cm (ꢀ/dm mol cm )): 13 590 (568), 14 290
(again consistent with a π-donor ligand) which decomposed
(
713), 15 090 (491), 22 125 (sh, 268), 27 170 (2203).
Preparation of [Co(TmMe)Br]. TlTmMe (1.00 g, 1.8 mmol) was
under oxidizing conditions rather than forming a ferrocenium
ion analogue. Subsequently, our attention has turned to
cobalt. A series of cobalt(II) and cobalt(III) adducts with
2
suspended in acetone (50 mL) to which a solution of CoBr (0.40
g, 1.8 mmol) in acetone (30 mL) was added. The mixture was
refluxed while stirring for 3 h. The thallium bromide precipitate
was removed by filtration and the solution reduced in volume. The
product was precipitated by addition of excess diethyl ether. The
solid was collected by filtration, and the resulting olive green solid
was washed twice with ether and dried in vacuo. Yield: 0.63 g,
8
9
Ph 10
Me 10
the anionic face-capping ligands Tp, Cp, Tm , pzBm ,
1
1
5b
Tt, and Tim is well established, and thus, an extension
Me
to the soft face-capping ligand, Tm , was considered
desirable. The results of our studies are reported herein.
7
1%. Crystals suitable for X-ray analysis were obtained from
(
5) (a) Kernbach, U.; Ramm, M.; Luger, P.; Fehlhammer, W. P. Angew.
Chem., Int. Ed. Engl. 1996, 35, 310. (b) Fr a¨ nkel, R.; Kernbach, U.;
Bakola-Christianopoulou, M.; Plaia, U.; Suter, M.; Ponikwar, W.;
Moinet, C.; Fehlhammer, W. P. J. Organomet. Chem. 2001, 617-
acetone by slow solvent evaporation.
16
Anal. Found: C, 29.72; H, 3.61; N, 16.91. Calcd for C12H -
-
1
618, 530.
BBrCoN
6 3
S : C, 29.40; H, 3,29; N, 17.15. FTIR [ν/cm (KBr
+
+
(
(
(
6) Garner, M.; Lehmann, M.-A.; Reglinski, J.; Spicer, M. D. Organo-
disk)]: 2425 (B-H). MS (EI; m/e): 410.1, [M - Br]; 490.9, [M ];
metallics 2001, 20, 5233.
Me
+
Me
+
761.0, [Co(Tm
)
2
] ; 900.8, [Co
2 2
(Tm ) Br] . µeff (298 K) ) 4.91
7) Garner, M.; Lewinski, K.; Pattek-Janczyk, A.; Reglinski, J.; Sieklucka,
B.; Spicer, M. D.; Szaleniec, M. Dalton Trans. 2003, 1181.
8) (a) Fujihara, T.; Sch o¨ nherr, T.; Kaizaki, S. Inorg. Chim. Acta 1996,
. UV-visible (solid state; Emax, cm- ): 4150 (sh), 4590 (sh),
1
µ
B
4830, 5710, 7460 (sh), 1330 (sh), 14 290, 15 150, 25 000, 35 710.
UV-visible (CH
2
49, 135. (b) Hayashi, A.; Nakajima, K.; Nonoyama, M. Polyhedron
-3
-3
-1
2
Cl
2
solution, 1.0 × 10 mol dm , Emax/cm
1997, 16, 4087.
(9) E.g.: In ComprehensiVe Organometallic Chemistry II; Atwood, J. D.,
Vol. Ed.; Pergamon Press: London, 1995; Vol. 8, p 88.
(12) Armstrong, D. R.; Cassidy, I. D.; Kennedy, A. R.; Reglinski, J.; Slavin,
P. A.; Spicer, M. D. J. Chem. Soc., Dalton Trans. 1999, 2119.
(13) Kennedy, A. R.; Reglinski, J.; Slavin, P. A.; Spicer, M. D. J. Chem.
Soc., Dalton Trans. 2000, 239.
(
10) Kimblin, C.; Churchill, D. G.; Bridgewater, B. M.; Girard, J. N.;
Quarless, D. A.; Parkin, G. Polyhedron 2001, 20, 1891.
11) Ohrenberg, C.; Ge, P.; Schebler, P.; Riordan, C. G.; Yap, G. P. A.;
Rheingold, A. L. Inorg. Chem. 1996, 35, 749.
(
(14) King, R. B. Inorg. Chem. 1966, 5, 82.
4928 Inorganic Chemistry, Vol. 43, No. 16, 2004

Co Sandwich Complexes with Soft Tripodal Ligands
Table 1. Crystallographic Data
[
Co(TmMe)Br]
[Co(TmMe)2]BF4‚0.5H2O‚0.25THF
[Co(mtH)3I]I
empirical formula
fw
C12H16BBrCoN6S3
490.14
C25H35B3CoF4N12O0.75S6
875.37
C12H18CoI2N6S3
655.23
T/K
123
123
123
cryst system
space group
a/Å
b/Å
c/Å
monoclinic
P21/n (no. 14)
12.0655(6)
12.4897(7)
12.7057(7)
90
95.714(3)
90
4
monoclinic
C2/c (no. 15)
34.0403(7)
12.8344(3)
19.2489(5)
90
101.540(1)
90
8
orthorhombic
Pc21n (no. 33)
11.302(5)
16.167(5)
11.902(5)
90
90
90
4
R/deg
â/deg
γ/deg
Z
3
V/Å
1905.2(2)
3.334
980
8239.6(3)
1.411
3472
2174.7(15)
3.924
1252
-
1
µcalc/mm
F(000)
cryst size/mm
radiatn
0.2 × 0.2 × 0.2
0.4 × 0.15 × 0.04
0.4 × 0.2 × 0.2
Mo KR
4596
2410 (Rint ) 0.0275)
222
Mo KR
Mo KR
no. rflcns measd
no. unique reflcns
no. params
13 621
4210 (Rint ) 0.0694)
221
0.0462
0.1066
17 044
9437 (Rint ) 0.0359)
495
0.0559
0.1643
a
R (I > 2σ(I))
0.0293
0.0747
b
Rw (all reflcns)
GOF
1.023
1.038
1.061
a
R ) ∑||Fo| - |Fc||/∑|Fo|. Rw ) {∑[w(Fo2 - Fc ) ]/∑[w(Fo ) ]}1/2.
b
2 2
2 2
ꢀ/dm mol cm-1)): 13 640 (371, sh), 14 220 (459, sh), 14 810
3
-1
(ꢀ/dm mol cm-1)): 21 010 (sh, 8940), 25 000 (15 580), 40 980
3
-1
(
(
522), 16 390 (sh, 325), 21 410 (sh, 214), 27 250 (2797).
(sh, 15 180).
Preparation of [Co(TmMe)I]. A solution of CoI
mmol) in 100 mL of acetone was added to a suspension of TlTm
1.11 g, 2 mmol) in acetone (250 mL). The mixture was refluxed
under continuous stirring for 4 h. The initially blue solution of the
cobalt salt turned green. The filtered solution was treated with 500
mL of diethyl ether to precipitate a green solid. The solid was
collected by filtration, washed with diethyl ether, and dried in vacuo.
Yield: 0.33 g, 31%.
(0.626 g, 2
Preparation of [Co(TmMe)(CpMe)]I. [Co(CpMe)(CO)I
2
2
] (0.31 g,
Me
0.74 mmol) was placed in a Schlenk tube equipped with a stirring
bar. To this was added dry THF (25 mL) and the resulting mixture
(
Me
cooled to -40 °C. NaTm (0.276 g, 0.74 mmol) was added and
the mixture allowed to warm to room temperature with stirring.
The green solid which formed was collected and washed with
diethyl ether. A crystalline product was obtained from chloroform-
diethyl ether by vapor diffusion. Yield: 0.32 g, 70%.
Anal. Found: C, 30.15; H, 3.82; N, 13.80. Calcd for C12
H
16
-
Anal. Found: C, 34.65; H, 3.55; N, 13.01. Calcd for C H -
1
8
23
-1
BCoIN ‚Me CO: C, 30.26; H, 3.72; N, 14.12. [See comments
6
S
3
2
6 3
BCoIN S : C, 35.08; H, 3.73; N, 13.66. FTIR [ν/cm (KBr disk)]:
Me
-1
1
after analytical data for [Co(Tm )Cl]]. FTIR [ν/cm (KBr disk)]:
2469 (B-H). H NMR (400 MHz, acetone-d ; δ/ppm): 1.77 (s,
6
+
2
437 (B-H). MS (FAB; m/e): 410.0, [M - I]; 761.4,
3H, CH Cp); 3.89 (s, 3H, CH Tm); 5.23-5.43 (m, 4H, CH); 7.04
3
3
Me
+
Me
+
+
[
Co(Tm
)
2
] ; 947.0, [Co
2
(Tm
)
2
I] . µeff (298 K) ) 4.35 µ
B
. UV-
(d, 1H, CH); 7.41 (d, 1H, CH). MS (FAB; m/e): 489.2, [M ]; 410.1,
visible (solid state; Emax, cm- ): 4530 (sh), 4860, 5480, 7350 (sh),
1
+
Me
-5
-3
[M - Cp ]. UV-visible (solution, CH Cl , 4.2 × 10 mol dm ;
2
2
Emax/cm (ꢀ/dm3 mol cm )): 15 665 (2625), 26 040 (sh,
-1 -1 -1
1
2 2
3 040, 13 700, 14 560 (sh), 25 190, 34 840. UV-visible (CH Cl
-
4
-3
-1
3
-1
-1
solution, 4.1 × 10 mol dm , Emax/cm (ꢀ/dm mol cm )):
3 190 (691), 13 760 (602), 14 560 (513), 21 600 (sh, 2030), 25 580
5517).
Preparation of [Co(TmMe
14 597), 29 070 (31 896).
1
(
X-ray Crystallography. Crystals were coated in mineral oil and
mounted on glass fibers. Data were collected at 123 K on a Nonius
Kappa CCD diffractometer using graphite-monochromated Mo KR
radiation. The structures were solved by direct methods, and the
remaining atoms located in difference electron density maps. Full-
)
2
]BF
. TlTmMe (0.175 g, 1.6 mmol)
4
and NaBF
4
(0.18 g, 1.6 mmol) were added to a solution of [Co-
Me
(
Tm )Br] (0.200 g, 0.4 mmol) in acetonitrile (250 mL). The
solution was stirred for 4 days in a beaker covered with a watch
glass. The solvent losses through evaporation were replaced
regularly. The initially green solution slowly turned brown over
this period. Toward the end of this period, molecular sieve was
added to remove some of the water. The mixture was then filtered
and the filtrate taken to dryness in vacuo. The resulting brown solid
was redissolved in the minimum amount of dry dichloromethane,
cooled, and filtered through Celite. A dark red crystalline product
was obtained from dichloromethane solution by vapor diffusion
with THF. Yield: 0.13 g, 38%.
matrix least-squares refinement was based on F , with all non-
2
hydrogen atoms refined anisotropically where possible. While
hydrogen atoms were mostly observed in the difference maps, they
were placed in calculated positions riding on the parent atoms.
Me
Crystals of [Co(Tm
) ]BF
2
4
‚0.5H O‚0.25THF lost solvent rapidly.
2
As a result, the solvent molecule positions were partially occupied,
and the solvent atoms were thus refined isotropically. The solution
Me
Me
of the structure of [Co(Cp )(Tm )]I was problematic. The crystals
were obtained only with great difficulty and were far from ideal.
The structure was initially solved in space group C2/m, in which
the cobalt atom was apparently situated on a mirror plane, and thus
Analysis. Found: C, 31.67; H, 4.21; N, 18.44. Calcd for
Me
C
[
24
H
32
-
B
3
F
4
CoN12
S
6
‚3H
2
O: C, 31.95; H, 4.24; N, 18.63. FTIR
the Tm ligand was disordered over two sites. The observed ligand
1
1
ν/cm (KBr disk)]: 2433 (B-H). H NMR (400 MHz, CDCl
3
;
geometry was significantly distorted from that normally expected.
It would appear that this is an example of a “partial polar
ambiguity”, in which a polar structure is solved in a nonpolar space
δ/ppm): 3.70 (s, 3H, CH
UV-visible (solution, CH
3
); 6.96 (d, 1H, CH); 7.05 (d, 1H, CH).
-
5
-3
-1
2
Cl
2
, 9.4 × 10 mol dm ; Emax/cm
Inorganic Chemistry, Vol. 43, No. 16, 2004 4929

Dodds et al.
Scheme 1
group to a false minimum.15 This most commonly occurs, as in
this case, when centrosymmetrically disposed heavy atoms dominate
the phasing of the overall noncentrosymmetric structure. We have
reprocessed the data in the polar space group C2, and while this
was some improvement, we have been unable to fully resolve this
problem and refine the structure to a satisfactory (“true”) minimum.
Nevertheless, the other experimental data (NMR, MS, and elemental
analysis) associated with this compound all support the structure
found by crystallography. We thus present a pictorial representation
of the structure but have withheld the crystallographic and structural
parameters in the knowledge that they may not be fully reliable.
The structure solution and refinement used the program SHELX-
9
716 and the graphical interface WinGX.17 A summary of the
crystallographic parameters is given in Table 1. Crystallographic
data have been deposited with the Cambridge Crystallographic Data
Centre, CCDC Nos. 238056-238058. Copies of this information
may be obtained free of charge from The Director, CCDC, 12 Union
Road, Cambridge, CB2 1EZ U.K. (fax +44-1223-336033; e-mail
deposit@ccdc.cam.ac.uk; www http://www.ccdc.cam.ac.uk).
Figure 1. X-ray crystal structure of [Co(TmMe)Br].
As expected, there is close structural homology between
Results and Discussion
Me
this compound and the previously reported Tm adduct of
4
,18
Treatment of cobalt(II) halides with the hydrotris(methi-
mazolyl)borate anion (Tm ) at a variety of molar ratios in
zinc and the tetrakis{(methylthio)methyl}borate (Tt) ad-
Me
11
duct of cobalt(II) (Table 2). The metal-sulfur distances
noncoordinating solvents (e.g. CH
2
Cl
2
or acetone) quickly
for the two cobalt complexes are in agreement with those
found for the analogous [Zn(Tm )Br] complex once the
Me
leads only to the formation of complexes with the empirical
Me
formula [Co(Tm )X] (X ) Cl, Br, and I, Scheme 1). Of
small extension to the bond distances is included to
compensate for the slightly larger cation. The influence of
the three species prepared, the bromide was successfully
structurally characterized, revealing a pseudotetrahedral motif
around the cobalt (Figure 1).
Me
the eight-membered rings generated by Tm is evident in
these comparisons. In adopting a classic facially capping
Me
arrangement, the Tm ligand requires to tilt the methimazole
(15) Kuchta, M. C.; Parkin, G. New J. Chem. 1998, 523 and references
therein.
rings at an angle (37.9°(av)) to the main HB‚‚‚Co axis to
accommodate the larger ring size. This added flexibility at
(
16) Sheldrick, G. M. SHELXL-97, Programs for Crystal Structure Analysis,
release 97-2; Instit u¨ t f u¨ r Anorganische Chemie, der Universit a¨ t,
Tammanstrasse 4, D-3400 G o¨ ttingen, Germany, 1998.
17) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
(18) Cassidy, I.; Garner, M.; Kennedy, A. R.; Potts, G. B. S.; Reglinski, J.
(
R.; Slavin, P. A.; Spicer, M. D. Eur. J. Inorg. Chem. 2002, 1235.
4930 Inorganic Chemistry, Vol. 43, No. 16, 2004

Co Sandwich Complexes with Soft Tripodal Ligands
Table 2. Comparison of M-S Bond Distances and ∠ S-M-S Angles
for Tetrahedral Soft Tripodal Complexes of Cobalt(II) and Zinc(II)
[
Co(TmMe)Br]
[Zn(TmMe)Br]
[Co(PhTt)Cl]
d(M-S)/Å
2
2
2
.319
.328
.328
2.355
2.320
2.321
2.3256
∠
S-M-S/deg
105.35
1
1
1
09.9
05.4
07.9
99.1
98.4
100.5
the methimazolyl units forces the bite angle of TmMe to
somewhat larger values (∠ S-M-S ∼ 108°) than found for
the analogous Tt complex (∠ S-M-S ∼ 99°), thus facilitat-
ing the formation of a more regular tetrahedral environment
for the cobalt cation. This contrasts starkly with Parkin’s
Ph
10
Figure 2. X-ray crystal structure of [Co(TmMe)2]BF4.
2
[Co(Tm ) ], which adopts a highly distorted pseudoocta-
hedral geometry, in which each ligand coordinates through
two thione sulfur atoms and additionally via the B-H moiety.
It is not clear why the alternative geometry is adopted in
this case. While, on the surface, it might appear that steric
effects are the cause, on further inspection this seems
unlikely. In particular, steric effects would favor a 1:1
adducts indicating that the crystallographically characterized
structure is representative of the bulk structures of the other
two species prepared. Similarly the magnetic moments (µeff
)
calculated are consistent with this analysis. Significantly, the
band structure observed in the reflectance spectra is also
maintained in the solution spectra, thus confirming that the
Ph
complex, [Co(Tm )X], analogous to our complex. It seems
Me
major species in solution is also [Co(Tm )X].
more likely that this is electronic in nature, the electron-
withdrawing phenyl group lessening the electron-donating
ability of the thione.
Me
Recrystallization of [Co(Tm )Br] from donor solvents
(
e.g. acetonitrile) under aerobic conditions led to the forma-
Me
Me
2
tion of the red [Co(Tm ) ]Br in poor yield (Scheme 1). The
introduction of a second 1 equiv of NaTm into the reaction
mixture and the addition of a suitable counteranion ([BF
The full characterization of the [Co(Tm )X] species (X
Me
)
Cl, I) was problematic. This was particularly reflected in
-
4
] ,
widely differing values in the microanalysis and by a
tendency to slowly oxidize in solution. Indeed, mass
spectrometry clearly shows evidence for the presence of two
species with higher molecular weights in solution. A
species with m/e ) 761.4 was seen in the spectra of all
three compounds, indicative of the oxidized species
-
[PF
6
] ) during product isolation increased the yield substan-
1
tially. H NMR confirmed the low-spin, diamagnetic nature
of the complex and showed the usual singlet arising from
the N-methyl group and two doublets corresponding to the
ring protons. The Co NMR showed a single resonance at
δ +8654 ppm, a little to high frequency of the normal range
for cobalt ligated by six sulfur donors (δ +6500 to +7200
ppm in tris(dithiocarbamate) and thioether complexes).
The line width (140 Hz) is remarkably narrow given the
deviation from pure octahedral symmetry. The electronic
5
9
III
Me
+
[Co (Tm )
2
] (Scheme 1). However, more intriguing is the
presence of a second, heavier molecular ion, consistent with
2
0
21
Me
+
2 2
a dimeric complex of formula [Co (Tm ) X] (X ) Cl, Br,
I; see Figure 1). Rabinovich has observed a similar species
tBu
using Tm as ligand and has been successful in crystallo-
-
19
6
spectrum was also consistent with an S donor set. The lowest
graphically characterizing the PF
6
salt. In his complex both
-
1
tBu
energy bands are at 21 010 and 25 000 cm , which most
Tm ligands bridge between the two metal centers, but the
cobalt atoms have nonidentical coordination spheres; one
1
1
likely arises from the splitting of the
observed in pure O
environment at the metal. The energies of these bands are
A
1g- T1g band
h
symmetry by the lower symmetry local
4 2 3
cobalt has an S H donor set, while the other has an S Br
donor set.
-
1
similar to the value of 19 530 cm in the CoS
6
complex,
Although the X-ray crystal structure of the bromide
1
3+
1
Me
2 2
[Co(L ){MeS(CH ) SMe}]
(L ) 1,4,7,11-tetrathiacy-
complex [Co(Tm )Br] had been successfully determined,
2
1
clotetradecane).
The [Co(TmMe)
microanalyses of the bulk materials were variable and mass
spectra indicated the presence of a dimeric complex and
rearrangement products in solution (Scheme 1). The UV-
]+ complex can be considered as directly
2
analogous to the cobaltacenium cation, with two face-capping
ligands sandwiching the metal cation (Figure 2). It would
seem that this cation is particularly stable, as a variety of
Me
visible reflectance spectrum of the crystalline [Co(Tm )-
Br] used to obtain the structure (Figure 1) was recorded. This
displayed absorption bands consistent with a pseudotetra-
-
-
-
-
-
4 4
salts (Cl , Br , I , BF , and ClO ) have been crystallized
7
from a range of different reactions and characterized by X-ray
diffraction. [In this paper the X-ray crystal structure of [Co-
hedral d ion. A similar band profile was observed for the
powders obtained from the respective chloride and iodide
(
19) (a) Rabinovich, D. Personal communication. (b) Mihalcik, D. J.; White,
J. L.; Tanski, J. M.; Zakharov, L. N.; Yap, G. P. A.; Incarvito, C. D.;
Rheingold, A. L.; Rabinovich, D. Dalton Trans. 2004, Advance Article,
published on the web 7th Apr 2004.
(20) Bond, A. M.; Colton, R.; Moir, J. E.; Page, D. R. Inorg. Chem. 1985,
24, 1298.
(21) Jenkinson, J. J.; Levason, W.; Perry, R. J.; Spicer, M. D. J. Chem.
Soc., Dalton Trans. 1989, 453.
Inorganic Chemistry, Vol. 43, No. 16, 2004 4931

Dodds et al.
which readily forms a stable Co sandwich compound,
II
Table 3. Co-S Bond Distances and ∠ S-Co-S Angles for S6-Donor
Complexes of Cobalt(III)
8
[
Co(Tp)
2
], and Cp which forms cobaltacene, which is readily
[
Co(TmMe)2]+
[Co(ttcn)2]3+ a
[Co(Et2dtc)3]b
oxidized. Given that Tm places greater electron density at
the metal center than Cp, [Co(Tm ) ] should oxidize even
2
Me
6
Me
d(Co-S)/Å
2.258
2
2
2
.298
.303
.327
2.260
2.258
2.255
more readily than cobaltacene, which may explain our
inability to observe this complex.
2.253
2.249
The synthetic protocol adopted for the preparation of the
[Co(Tm ) ] cation suggested that it should be possible to
2
introduce an alternative facial ligand (e.g. Cp) into the
coordination sphere of the cobalt center during the oxidation
step. This would lead to the formation of mixed sandwich
intra ∠ S-Co-S/deg
Me
+
9
9
9
5.0
7.1
4.9
91.0
90.8
90.6
180
89.4
89.2
89.0
77.8
75.9 (×2)
179.8
166.2
96.3-96.4
84.9
85.3
82.7
Me
+
24
species, e.g. [Co(Tm )(Cp)] . A previous, sketchy report
+
of the formation of [Co(Tp)(Cp)] gave hope that such
species should be attainable. However, on repetition of the
reaction, with replacement of NaTm with 1 equiv of NaCp
a
Reference 22. b Reference 23.
Me
Me
(
Tm )
2
]BF
4
is reported. The data for this cation with other
Me
+
2
in the second step, the [Co(Tm ) ] cation was isolated once
counteranions have been deposited at the Cambridge Crystal-
lographic Data Centre and may be accessed free of charge
as described in the Experimental Section quoting the fol-
more. This suggested that reaction is driven not by the
additional ligand but by a species already present. A likely
Me
+
candidate is the dimeric species [{CoTm }
), which it is thought may facilitate O
2
X] (Scheme
-
-
lowing deposition numbers: 238059 (Br ); 238060 (I );
19
1
2
oxidation of the
-
2
38061 (ClO
4
).]
metal center (possibly by coordinating molecular oxygen)
The environment around the cobalt center (Table 3) is
similar to that observed in other metal complexes,
M(Tm ) ] . The ∠ S-M-S angles are all close to 90°,
2
with the values for the intraligand (bite) angles being slightly
larger than the interligand angles. The six cobalt(III) to sulfur
distances all similar to one another (2.298-2.327 Å) and as
Me
and also providing the second Tm ligand. It is curious that
the oxidation process is accelerated in the presence of donor
solvents (e.g. CH CN), although this acceleration may result
3
Me
n+
[
either from perturbation of the solution equilibrium to favor
the dimer or by stabilization of the metal fragment which
would be necessarily ejected from the dimer to form the
expected are slightly smaller than those found in the parent
Co(Tm )Br] complex, principally due to the smaller
Me
+
[
Co(Tm )
2
] cation.
Me
[
O’Hare et al.25 have recently prepared [Co(Cp)(Tp)]I by
diameter of the Co(III) ion. It is informative to note that the
an alternative route, commencing from CpCo(CO)I . With
2
6
geometric preference of the smaller d cation for octahedral
Me
employment of this route using NaTm in place of NaTp a
green material was isolated. This analyzed poorly for
the desired mixed sandwich product, the presence of
coordination geometry is readily accommodated by this
ligand system. Rather than hinder the formation of octahedral
geometry, the Tm ligand reduces the twist of the methima-
zolyl rings relative to the HB-Co axis (33.3°(av) vs 37.9°
Me
+
[
Co(Cp)(Tm )] was not confirmed by mass spectrometry,
and furthermore, the sample was paramagnetic. After many
attempts to crystallize the material a small number of green
crystals were obtained. Crystallographic analysis revealed
these to be iodotris(methimazole)cobalt(II) iodide (Figure 3).
Me
in the tetrahedral [Co(Tm )Br]) such that it can interlock
the pendant methyl groups. It is also notable that interligand
S-Co-S angles are found to be smaller than 90°.
A comparison with other structurally characterized CoS
systems (Table 3) including the analogous complexes with
trithiacyclononane (ttcn)22 and diethyldithiocarbamate
dtc) ligands indicates that cobalt sulfur distances are
environment but at the higher end of
the range. Considerable variation is again seen in the ∠ S-
M-S angles. The bite angles of the Tm ligands are greater
than 90° indicative of highly flexible donor groups.
Curiously, although the structure of [Co (Tm )
cates that there is no steric barrier to the formation of
Co (Tm ) ], we have been unable to obtain such a species.
2
Attempts at direct preparation using a range of ligand transfer
agents and a variety of metal precursors and reaction
conditions were all unsuccessful. Furthermore, neither chemi-
cal nor electrochemical reduction of the Co analogue gave
any indication of such a species. This contrasts with Tp,
6
Me
It appears that the Tm anion is oxidized in the presence
III
of the Co precursor, presumably via the B-H moiety. This
results in fission of the B-N bonds thus releasing the three
methimazoles, which coordinate to the metal. Unconfined
by the apical boron, they are free to adopt their preferred
orientation and distance from the metal center. The ∠ S-
M-S angles move to values expected for a tetrahedral
geometry (108.52-110.55°). The M-S distances (2.331-
2
3
(
Et
2
typical for the CoS
6
Me
III
Me
]+ indi-
2
2
.336 Å) are now marginally larger than found in [Co(Tm)-
Br] (Table 2) most probably due to the loss of the partial
charge on the donors. Of some surprise is the orientation of
the methimazoles, which invert with respect to the parent
complexes, internalizing the metal center. It is tempting to
invoke a weak π-interaction between the coordinated iodide
and the methimazole rings, but with an average I to ring
centroid distance of 3.947 Å, this seems unlikely. The
uncoordinated iodide ion is located on the Co-I bond axis,
II
Me
[
III
(
22) Kuppers, H.-J.; Neves, A.; Pomp, C.; Venture, D.; Wieghardt, K.;
Nuber, B.; Weiss, J. Inorg. Chem. 1985, 25, 2400.
23) Healy, P. C.; Connor, J. W.; Skelton, B. W.; White, A. H. Aust. J.
Chem. 1990, 43, 1083.
(
(24) O’Sullivan, D. J.; Lalor, F. J. J. Organomet. Chem. 1973, 57, C58.
(25) Brunker, T. J.; Barlow, S.; O’Hare, D. Chem. Commun. 2001, 2052.
4932 Inorganic Chemistry, Vol. 43, No. 16, 2004

Co Sandwich Complexes with Soft Tripodal Ligands
Figure 3. X-ray crystal structure of [Co(mtH)3I]I.
at a distance of 4.11 Å from the cobalt. Inspection of the
Cambridge Crystallographic Database revealed no analogous
structurally characterized species.
Despite the structure determined from the crystals, there
was evidence (including a parent ion in the mass spectrum)
that the desired species was present to some extent in the
bulk materials but that it was not stable for extended periods.
It was believed that the CpCo fragment was too strongly
oxidizing for the complex to be stable. Therefore, by an
increase of the electron density at the cobalt center, it was
envisaged that the oxidizing potential would be reduced
Figure 4. X-ray crystal structure of [Co(CpMe)(TmMe)]I.
Me
Me
+
at -670 mV vs Ag/AgCl for [Co(Tm )(Cp )] , some 200
+
27
mV more negative than for [Co(Tp)(Cp)] , reflecting the
Me
greater electron donor ability of the Tm ligand compared
Me
+
2
to Tp. Interestingly, for [Co(Tm ) ] , no reduction was
observed within the limits of solvent stability, suggesting
that it is even less readily reduced than the pentamethylco-
rendering the oxidative ligand decomposition less favorable.
27
baltacenium cation (-1970 mV). An irreversible oxidation
was observed at +1.12 V (vs Ag/AgCl), which may arise
-
Thus, Cp was replaced by the methyl analogue (C
Cp ). Reaction of [Co(Cp )(CO)I
5
Me
H
4
Me ,
Me
Me
2
] with NaTm resulted
III
either from oxidative ligand decomposition or from the Co /
in the successful isolation and characterization of the green,
IV
Co couple. It is our experience that ligand decomposition
Me
Me
+
air-stable cation [Co(Cp )(Tm )] (Figure 4) as the iodide
salt.
usually occurs at lower voltages (ca. +0.5 V) and believe
this to be a metal-based oxidation.
The X-ray crystal structure determination for this complex
was problematic (vide supra) resulting in the TmMe ligand
being unsymmetrically disposed about the cobalt center. This
is clearly at variance with previous observations of the
Concluding Remarks
Our study again reveals a considerable degree of structural
homology between the cobalt complexes of Cp, Tp, and
Me
+
complexation of this ligand. In [Co(Tm )
2
] and in the only
Me
other structurally characterized mixed sandwich compound
Tm but also highlights the electronic differences between
Me
Me + 26
II
involving the Tm ligand, [Ru(p-cymene)(Tm )] , the
ligand geometry and disposition is symmetrical, as would
be expected. Despite the problems with the metrical param-
eters, we are confident of the connectivity observed. Spec-
troscopic and analytical data for this compound are consistent
with the observed structure. FAB mass spectrometry reveals
the parent ion at m/e ) 489.2 amu and fragmentation with
the ligands. It is observed that Co has a marked propensity
Me
for formation of a half-sandwich complex with the Tm
2
ligand. By contrast, the sandwich compound [Co(Tp) ] is
II
formed with Co halides in the presence of the Tp anion.
t
Only in the case of very bulky Tp ligands (e.g. with Bu
groups in the 3 position) is the tetrahedral geometry
2
8
enforced. While Cp tends to the sandwich compound
(cobaltacene), examples with the more strongly electron
donating Cp* do show some tendency to form half-sandwich
Me
1
loss of Cp . The H NMR spectrum shows resonances for
Me
methylcyclopentadiene and Tm in a 1:1 ratio, but we have
59
29
30
been unable to locate Co NMR resonances either for [Co-
2
motifs in [Co(Cp*)(py)Cl] and [Co(Cp*)(µ-Cl)] , where
Me
Me
+
+
(
Tm )(Cp )] or for [Co(Tp)(Cp)] . Presumably the sym-
coordination expansion is also observed. It would appear that
metry in these species is sufficiently low for quadrupole
broadening to render the resonances unobservable. Electro-
chemical reduction in CH Cl showed an irreversible wave
2 2
(27) Brunker, T. J.; Cowley, A. R.; O’Hare, D. Organometallics 2002, 21,
3
123.
28) Trofimenko, S.; Calabrese, J. C.; Thompson, J. S. Inorg. Chem. 1987,
6, 1507.
(
2
(
26) Bailey, P. J.; Lorono-Gonzales, D. J.; McCormack, C.; Parsons, S.;
(29) Raabe, E.; Koelle, U. J. Organomet. Chem. 1985, 279, C29.
(30) Koelle, U.; Fuss, B. Angew. Chem., Int. Ed. Engl. 1982, 21, 132.
Price, M. Inorg. Chim. Acta 2003, 354, 61.
Inorganic Chemistry, Vol. 43, No. 16, 2004 4933

Dodds et al.
the structures obtained in these systems are driven by the
electron donor capacity of the ligands, and this assertion is
Acknowledgment. We acknowledge the EPSRC Mass
Spectrometry Service at the University of Wales, Swansea,
U.K., and thank the EPSRC for funding (C.A.D). We also
thank Michelle K. Taylor for undertaking the electrochemical
measurements.
supported by the electrochemical data. The reduction po-
+
tentials of [Co(L)
2
] follow the expected pattern, with ease
of reduction in the order
Me
Tp > TpCp > TpCp* > Cp > Cp* > Tm
2
2
2
2
Supporting Information Available: X-ray crystallographic data
in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
In particular, this is consistent with our inability to observe
II
Me
2
the species [Co (Tm ) ] and explains the propensity to form
Me
the tetrahedral [Co(Tm )X] species with cobalt(II).
IC0496525
4934 Inorganic Chemistry, Vol. 43, No. 16, 2004