A rareether-bridged cobalt complex which gives rise to an unusual 'serpentine' metal-ligand binding motif

Article abstract of DOI:10.1039/b208221g

An unusual metal-ligand binding motif is found in dimeric cobalt(II) complexes coordinated by diamidoether ligands that bridge the metals in a 'serpentine' fashion through the ether donors of the ligand backbone rather than the amido groups.

Full text of DOI:10.1039/b208221g

A rare ether-bridged cobalt complex which gives rise to an unusual
‘serpentine’ metal–ligand binding motif
Garry Mund, Andrea J. Gabert, Raymond J. Batchelor, James F. Britten and Daniel B. Leznoff*
Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby, B.C, Canada V5A 1S6.
E-mail: dleznoff@sfu.ca; Fax: 1-604-291-3765; Tel: 1-604-291-4887
Received (in Purdue, IN, USA) 22nd August 2002, Accepted 25th October 2002
First published as an Advance Article on the web 13th November 2002
An unusual metal–ligand binding motif is found in dimeric
cobalt(II) complexes coordinated by diamidoether ligands
that bridge the metals in a ‘serpentine’ fashion through the
ether donors of the ligand backbone rather than the amido
groups.
diamidoether ligand that bridges the cobalt atoms of the dimer
rather than the more strongly basic amido group. Each cobalt
centre is four-coordinate and displays a distorted see-saw
geometry: each ligand binds via a terminal amido group, bridges
the cobalt atoms through the oxygen donor in the backbone and
finally swings around in a ‘serpentine’ fashion and binds to the
other cobalt atom through another amido group. The Co1–Co2
distance of 3.716(1) Å precludes any bonding interaction
between the metal centres.
The temperature (T) dependence of the magnetic susceptibil-
ity (cm) of 1–4 were measured from 2–300 K. Plots of meff vs. T
per cobalt atom for 1, 2 and 4 are shown in Fig. 3. Both 1 and
2 show a significant decrease in meff as T decreases, indicative
of antiferromagnetic coupling between the metal atoms of the
dimer.20 However, 4 shows a considerably smaller drop in meff
with temperature—there is much less coupling between the
cobalt atoms of the ether-bridged dimer. The greater coupling
Chelating diamidodonor ligands have been shown to be
excellent ligands for high-valent metal centres.1–8 This is not
only due to their ease of steric and electronic modification but
more importantly due to their strong p-donating ability. Neutral
donors that have been incorporated into the ligand backbone
include amino,1 phosphino,2 thio3,4 or weaker ether donors.3,5
In general, the amido donors chelate the metal centre with the
neutral donor playing an important electronic role.6 The few
reported bimetallic diamidodonor complexes all contain amido-
groups that bridge the metal centres as expected.8,9 The neutral
donor in the backbone has not been observed to do so. In
particular, weakly basic ether donors rarely bridge metal
centres.10 The bridging role for THF, one of the most utilized
ether-type ligands, is very uncommon in the solid state.11–14
[Co(acac)2(PhHgOHgPh)(THF)]2,12 [(C5H4Me)TiF3]2·THF,13
and [Rh2(O2CCF3)4(THF)]14 are among the few known THF-
bridged transition-metal complexes. We have been exploring
diamido ligands incorporating a neutral ether donor, termed
[NON], as supporting ligands for paramagnetic first-row
transition metals.7,8 Through the use of a flexible [NON] ligand,
we hereby report an unusual dimeric cobalt(II) complex in
which the cobalt atoms of the dimer are bridged in a ‘serpentine’
fashion through the ether moieties of the ligand rather than the
stronger amido donors.
We have considered two [NON] ligand backbones: (1) a short
silicon-derived backbone, {[RN(SiMe2)]2O}22 (R = Bu,7–9
t
Fig. 1 Molecular structure of 1 (ORTEP view, 33% probability ellipsoids
are shown; methyl groups on aryl ring excluded for clarity). Selected bond
lengths (Å) and angles (°): Co1–Co1* 2.468(3), Co1–N1 1.912(7), Co1–N2
2.007(8), Si1–O1 1.645(8), Si1–N1 1.720(9), Si2–O1 1.617(7), Si2–N2
1.780(9); N1–Co1–N2 120.0(4), N1–Co1–N2* 139.3(4), N2–Co1–N2*
95.0(4), Si2–O1–Si1 145.6(6). * = 2x, y, 2z + 1/2.
2,4,6-Me3Ph,8 and 2,6-iPr2Ph8) and (2) a more flexible carbon-
based framework, {[RN(CH2CH2)]2O}22 (R = 2,4,6-Me3Ph,15
2,6-iPr2Ph3). Reaction of the appropriate dilithio diamidoether
ligands with CoCl2 at 278 °C resulted in an immediate colour
change from aqua blue to dark green. From these solutions, the
paramagnetic and air-sensitive {Co[Me3PhN(SiMe2)]2O}2 (1),8
{Co[tBuN(SiMe2)]2O}2 (2),8 {Co[Me3PhN(CH2CH2)]2O}2 (3)
and {Co[iPr2PhN(CH2CH2)]2O}2 (4) complexes were isolated
in moderate to high yield.16
The single-crystal X-ray structures of 1 and 4 are shown in
Figs. 1 and 2, respectively.17 In the dimeric cobalt(II) complex
(1) with the short silicon-containing [NON] ligand backbone,
(Fig. 1) the cobalt atoms have a roughly trigonal geometry: each
cobalt atom is bound by one terminal and two bridging amido
groups. The tert-butyl substituted amido system (2) has a
similar structure.8 Both bridging and terminal amido bond
lengths are comparable to other cobalt–amido systems such as
Co(CH2Ph)[N(SiMe2CH2PPh2)2]18 and {Co2[N(SiMe3)2]4}.19
The cobalt atoms of the dimer are held in close proximity by the
bridging amido ligands with Co1–Co1* distances of 2.468(3)
and 2.5682(13) Å in 1 and 2, respectively, suggesting metal–
metal interactions. The dimeric cobalt(II) complex (4) with the
longer and more flexible carbon-based [NON] ligand backbone
has a remarkably different ligand coordination environment
(Fig. 2). The molecule is non-centrosymmetric with approx-
imate D2 symmetry. In this case it is the ether atom of the
Fig. 2 Molecular structure of 4 (ORTEP view, 33% probability ellipsoids
are shown; isopropyl groups excluded for clarity). Selected bond lengths
(Å) and angles (°): Co1–Co2 3.716(1), Co1–N1 1.827(4), Co1–N2
1.832(4), Co2–N3 1.841(4), Co2–N4 1.850(4), Co1–O1 2.279(4), Co1–O2
2.423(4), Co2–O1 2.415(4), Co2–O2 2.306(4); N2–Co1–N1 162.83(17),
N2–Co1–O1 79.61(16), O1–Co1–O2 76.08(13), N1–Co1–O2 77.70(15),
N3–Co2–N4 158.42(16).
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This journal is © The Royal Society of Chemistry 2002

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Fig. 3 Plots of magnetic moment (meff) vs. temperature (K) for 1, 2 and 4.
observed in amido-bridged 1 and 2 vs. ether-bridged 4 is likely
the result of the much shorter metal–metal distances that are
supported by amido-bridges. Importantly, 3 shows minimal
coupling, implying that this carbon-containing [NON] ligand
system still appears to form an ether-bridged dimer as opposed
to an amido-bridged one, despite having the same amido R-
group as 1. Below 20 K, the sharp drop in meff is attributable to
zero-field splitting (ZFS) effects common in Co(II) systems.
The simultaneous presence of ZFS and antiferromagnetic
coupling impeded accurate modeling of the data.21
Why do rather similar diamidoether ligands give rise to such
different metal–ligand binding motifs? The length and rigidity
alterations in the ligand backbone provide a plausible explana-
tion. The silicon-containing [NON] backbone consists of a
short, five-atom chain that is sterically hindered around the
silylether donor. Alternatively, the carbon-containing [NON]
system is two atoms longer and is sterically unhindered at the
diethyl ether donor, yielding a more flexible ligand that may be
more apt to bridge metal atoms through the ether donor. The
stronger Lewis basicity of the latter may also assist in ether-
bridging. The size of the resulting metallacycle that is formed
could also account for the unusual ether atom bridging motif.
The silylamido-bridged cobalt system gives rise to stable six-
membered rings. An amido-bridged system featuring the
carbon-containing [NON] ligand would give rise to less stable
eight-membered metallacycles; ether-bridging allows for more
stable five-membered rings to form.
15 The synthesis of {[RNH(CH2CH2)]2O} (R = 2,4,6-Me3Ph) follows the
procedure for R = 2,6-Me2Ph as described in ref 3. Yield: 1.41 g (76%).
Anal. Calc. for C22H32N2O: C: 77.60, H: 9.47, N: 8.23. Found: C: 77.48,
H: 9.23, N: 8.00%. 1H NMR (C6D6): d 6.78 (s, 4H, Haryl), 4.03 (br, 2H,
NH), 3.22 (t, 4H, OCH2), 3.01 (t, 4H, NCH2), 2.24 (s, 12H, 2,6-MeAr)
2.17 (s, 6H, 4-MeAr). MS: m/z 340 (M+).
16 Synthesis of 3: a white powder of {[(Me3PhNH(CH2CH2)]2O} (0.20 g,
0.59 mmol) was dissolved in 10 mL of THF and two equivalents of 1.6
M nBuLi in hexanes (0.12 mL, 1.18 mmol) were added dropwise at 278
°C. After stirring for 1 h at room temperature, the resulting solution was
added dropwise to anhydrous CoCl2 (0.76 g, 0.59 mmol) in 30 mL of
THF at 278 °C, yielding a dark green solution. After 1 h, the solvent
was removed in vacuo, the product was extracted with toluene and
filtered through Celite®. Removal of the toluene in vacuo gave dark
green {Co[Me3PhN(CH2CH2)]2O}2 (3). Yield: 0.12 g (56%). Anal.
Calc. for C44H60N4Co2O2: C: 66.49, H: 7.61, N: 7.05. Found: C: 66.58,
H: 7.51, N: 6.94%. UV-vis (C7H8): l = 579 nm (e = 118 M21 cm21).
MS: m/z 397 (M+, monomer unit). For 4, a similar procedure was used.
Yield: 1.2 g (80%). Single crystals suitable for X-ray analysis were
obtained by a slow evaporation of a toluene solution. Anal. Calc. for
C56H84N4Co2O2: C: 69.82, H: 8.80, N: 5.82. Found: C: 69.62, H: 8.68,
N: 5.62%. UV-vis (C7H8): l = 422 nm (e = 98 M21 cm21). MS: m/z
481 (M+, monomer unit).
In conclusion, a series of dinuclear cobalt complexes
containing diamidoether ligands has been reported, in which an
amido-bridged system is favoured by the short, rigid silicon-
[NON] backbone whereas ether-bridging is favoured by the
longer, more flexible carbon-[NON] backbone. Further in-
vestigation of these [NON]-ligand systems with other metal
centres is underway. This new binding motif may have
implications for the use of diamidodonor ligands in alkene
polymerization catalyst design.3,5,22
17 Crystal data for 1: C44H68Co2N4O2Si4, M = 915.24, monoclinic, space
group C2/c, a = 18.132(11), b = 13.972(8), c = 19.761(12) Å, b =
107.854(10)°, V = 4765(5) Å3, Z = 4, m(Mo-Ka) = 0.8 mm21, T =
153(2) K, 3068 unique reflections, 1756 observed (Io > 2.0s(Io)). The
final R(F2) = 0.0986 and wR(F2) = 0.1868 (observed data). The sample
was a multiple crystallite. Crystal data for 4: C56H84Co2N4O2, M =
¯
931.18, triclinic, space group P1, a = 10.899(2), b = 15.104(3), c =
18.012(2) Å, a = 88.90(1), b = 84.64(2), g = 69.22(2)°, V =
2759.8(9) Å3, Z = 2, m(Mo-Ka) = 0.6 mm21, T = 293 K, 9759 unique
reflections, 4528 observed (Io > 2.5s(Io)). The final RF = 0.050 and
RwF = 0.052 (observed data). Structure solution and refinement for 4
was performed using CRYSTALS (D. J. Watkin, C. K. Prout, J. R.
Carruthers, P. W. Betteridge and R. I. Cooper, CRYSTALS Issue 11,
Chemical Crystallography Laboratory, University of Oxford, Oxford,
England, 1999). CCDC 195025 and 195120. See http://www.rsc.org/
suppdata/cc/b2/b208221g/ for crystallographic data in CIF format.
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Juan, C. Gómez-Garcia, E. Coronado and K. R. Dunbar, Inorg. Chem.,
1999, 38, 5841; H. Sakiyama, R. Ito, H. Kumagai, K. Inoue, M.
Sakamoto, Y. Nishida and M. Yamasaki, Eur. J. Inorg. Chem., 2001,
2027.
We are grateful to NSERC of Canada (D. B. L.) and Simon
Fraser University for financial support.
Notes and references
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