Dicationic tellurium analogues of the classic N-heterocyclic carbene

Article abstract of DOI:10.1002/chem.201001447

The synthesis and comprehensive characterization of the first dicationic tellurium analogues of N-heterocyclic carbenes (NHCs) has been reported, in both the +2 and +4 oxidation states. For the +2 oxidation state, a base-stabilized form of TeCl₂ is used as the starting material. The dications are isolated by means of halide metathesis and the solid-state structures confirm the previously calculated diimine bonding arrangement. For Te^IV, a diamine is used in a high-yielding dehydrohalogen coupling reaction from TeCl₄. The dicationic NHC analogue is isolated in a base-stabilized form through halide abstraction and subsequent coordination by pyridine. Carbene analogues: Compounds based on an unsaturated diimine ligand bear two lone pairs of electrons about the central element and can be considered structural mimics of N-heterocyclic carbenes (NHCs; see figure). Heterocycles centered about a saturated diamine carry one stereochemically active lone pair of electrons on tellurium, and are therefore isovalent as well as isostructural to NHCs.

Full text of DOI:10.1002/chem.201001447

DOI: 10.1002/chem.201001447
Dicationic Tellurium Analogues of the Classic N-Heterocyclic Carbene
Jason L. Dutton and Paul J. Ragogna*[a]
Dedicated to Professor J. Peter Guthrie on the occasion of his retirement
Abstract: The synthesis and compre-
hensive characterization of the first di-
cationic tellurium analogues of N-het-
erocyclic carbenes (NHCs) has been
reported, in both the +2 and +4 oxi-
dation states. For the +2 oxidation
state, a base-stabilized form of TeCl2 is
used as the starting material. The dicat-
ions are isolated by means of halide
metathesis and the solid-state struc-
tures confirm the previously calculated
diimine bonding arrangement. For TeIV,
a diamine is used in a high-yielding de-
hydrohalogen coupling reaction from
TeCl4. The dicationic NHC analogue is
isolated in
a
base-stabilized form
Keywords: carbene analogues · het-
erocycles · N ligands · polycations ·
tellurium
through halide abstraction and subse-
quent coordination by pyridine.
Introduction
charge may be imposed on the
molecule,
negative
neutral
for
for
The discovery of an isolable N-heterocyclic carbene (NHC;
1) by Arduengo and co-workers has been recognized as one
of the most significant achieve-
group 13,[6–10]
group 14,[11–13] and cationic for
group 15.[14–17]
ments in synthetic chemistry
In all cases similar geome-
tries are calculated and ob-
served experimentally for the
within the past 20 years.[1,2]
N-heterocyclic carbenes are
renowned for being strong s-
donors for elements spanning
the periodic table and are
Scheme 1. Schematic diagram
of known NHC analogues
from groups 13–15.
N-heterocycles.[18] The E N
ꢀ
bonds are somewhat shortened
from single standard bond
lengths, with a decreasing dif-
known to act as ideal ligands for some of the most impor-
tant transition-metal catalysts (i.e. Grubbs 2nd-generation
olefin metathesis catalyst).[3–5] Following the discovery of
isolable NHCs, chemists have endeavored to synthesize p-
block analogues of NHCs featuring main-group elements
(other than carbon) at the central position. These com-
pounds have been reported for many of the elements from
groups 13–15 and have shown a rich chemistry complemen-
tary to and divergent from NHCs. Known examples are
shown schematically in Scheme 1, with the syntheses gener-
ally relying on a diazabutadiene (DAB) a-diimine ligand.
Depending on the periodic group of the central atom a
ference moving down the groups. This observation is usually
attributed as reflecting the occupancy of the formally empty
p-orbital by the lone pairs on the flanking nitrogen atom,
ꢀ
giving partial multiple-bond character to the E N bonds
and an octet of electrons about the central element.[19] Less
effective orbital overlap for the heavier congeners decreases
ꢀ
the importance of this effect. Therefore, short C C and long
ꢀ
N C endocyclic bonds are observed, consistent with double
and single-bond character, respectively (Scheme 2).
The optimized geometries and electronic structures of the
dicationic S, Se, and Te analogues display striking differen-
ꢀ
ces relative to those from groups 13–15. The N C bond
ꢀ
lengths are short, averaging 1.32 ꢀ, consistent with an N C
[a] J. L. Dutton, Prof. P. J. Ragogna
Department of Chemistry, The University of Western Ontario
1151 Richmond St. London, Ontario
N6A 5B7 (Canada)
ꢀ
double bond. The C C bond is longer, at 1.40 ꢀ, approach-
ing the distance observed in the free diazabutadiene (DAB)
ligands. These two pieces of data suggest a diimine frame-
work is retained. The Ch N bonds are not shortened from
typical single bond lengths. The electron localization func-
tion (ELF) for the chalcogen compounds reveals two mono-
Fax : (+1)519-661-3022
E-mail: pragogna@uwo.ca
ꢀ
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001447.
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Chem. Eur. J. 2010, 16, 12454 – 12461

FULL PAPER
by reduction and halide abstraction to give the dication. We
have previously demonstrated the coordination of the tert-
butyl-DAB ligand to TeBr4; in this case, reduction was spon-
taneous upon attempted halide abstraction giving a 1,2,5-tel-
luradiazolium cation, due to the facile elimination of tert-bu-
tylbromide.[38,39] Use of aryl or secondary alkyl-substituted
diazabutadiene ligands should reduce the propensity for
elimination of a side group. The isolation of the tellurium
NHC analogue has been identified as a challenge,[40] and in
this context we now report the synthesis of dicationic TeII N-
heterocycles, by using a coordination/halide abstraction
pathway, relying on a 2,2’-bipyridine sequestered source of
TeCl2. An alternate route, based on a diamine ligand al-
lowed for isolation of a TeIV N-heterocycle in a base-stabi-
lized form, representing the first isovalent and isostructural
dicationic analogue of an N-heterocyclic carbene.
Scheme 2. Resonance structures for NHCs and their p-block analogues
(charges omitted).
synapic basins about the central element, consistent with
two lone pairs of electrons, and assignment of the +2 oxida-
tion state, rather than +4 for the group 16 atoms. The ELF
for all other p-block analogues (except for Sb) gives the ex-
pected single lone pair about
the central atoms. Combined,
these findings indicate that
group 16 carbene analogues
(Scheme 3) of type 2 should be
considered as either chalcadia-
zolium dications (2a), or coor-
Scheme 3. Bonding representa-
dination complexes between a
tions for group 16 carbene an-
Results and Discussion
diazabutadiene ligand and
a
alogues.
Ch2+ dication (2b), and thus
are only structural mimics of
the ubiquitous NHC.
Synthesis of dicationic TeII carbene analogues: For telluri-
um, attempts to reduce TeCl4 concomitant with ligation by
Dipp2DAB (Dipp=2,6-diisoproylphenyl) were met with
abysmal failure, with large amounts of Te0 immediately pro-
duced. Therefore, initial coordination of the ligand to the
tetrahalide was attempted. The direct reaction of Dipp2DAB
with one stoichiometric equivalent of TeBr4 in CH2Cl2 re-
sulted in the precipitation of a dark-red solid over a period
of four hours. The powder was found to be highly insoluble
in a variety of organic solvents, but was sufficiently soluble
Our past efforts have resulted in the synthesis of the
sulfur and selenium derivatives with a coordination/halide
abstraction protocol (Scheme 4). The experimentally deter-
1
in CD3CN for H NMR spectroscopy, which revealed a set
of resonances consistent with a single product containing
intact Dipp2DAB. The diagnostic signals arising from the
“backbone” protons were found at d=9.84 ppm, significant-
ly downfield from Dipp2DAB (d=7.99 ppm), which is char-
acteristic for the generation of a highly charged compound.
Single crystals were grown from a CH3CN solution of the
bulk powder held at ꢀ308C, and subsequent X-ray diffrac-
tion studies revealed a TeII center in the N,N-chelate of the
Dipp2DAB ligand, bridging to a [TeBr6] dianion in the solid
state, giving an overall neutral molecule (3; Scheme 5).
Scheme 4. Synthesis of ChII NHC analogues through a coordination/
halide abstraction strategy.
mined metrical parameters agreed well with the calculated
values, confirming that the diimine bonding motif was re-
tained, and ligand exchange reactions were demonstrated,
which indicated that 2b is a reasonable bonding model.[20–23]
We have also synthesized a related tellurium derivative;
however, covalent bonds were observed between the telluri-
um center and the triflate counterions, giving a neutral com-
pound.[24]
The binary chlorides were used as the starting reagents;
however, SCl2 and SeCl2 are both unstable with respect to
disproportionation.[25] That said, SCl2 may be stored for
months at a low temperature (ꢀ308C), and SeCl2 may be
synthesized and stored for approximately one day, making
both materials relatively easy to use. The corresponding
binary halides for tellurium are only known as transient spe-
cies in the gas phase, and are essentially unobtainable for
practical synthetic purposes.[26,27] Coordination by Lewis
bases allows for TeX2 to be stabilized and stored.[28–34] Re-
cently a few groups have used this strategy to deliver this
otherwise unstable electrophilic TeII source.[35–37] Therefore,
the proposed synthetic route to the tellurium NHC ana-
logues is coordination by a DAB ligand to TeX4, followed
Scheme 5. Reaction of Dipp2DAB with TeBr4 to give compound 3.
Spontaneous reduction of chalcogen tetrahalides in the
presence of DAB ligands has been previously observed for
both selenium and tellurium.[38,39] Hexahalochalcogenide
anions are generally undesirable, as they are readily avail-
able sources of halides.[41] Unfortunately, all attempts at
Chem. Eur. J. 2010, 16, 12454 – 12461
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12455

P. J. Ragogna and J. L. Dutton
metathesis of [TeBr6]2ꢀ were unsuccessful, so a synthetic
route was sought avoiding its incorporation (Scheme 6).
Synthesis of dicationic TeIV carbene analogues: Efforts to
generate a group 16 carbene analogue in the +4 oxidation
state (and therefore isostructural and isovalent to an NHC)
necessitated a different synthetic strategy. Fully saturated
amine ligands have been used to generate p-block NHC an-
alogues for groups 14–15, and the parent NHC is also
known with a saturated backbone (7–9).[42] While these are
no longer formally 6-p-electron aromatic compounds, only a
small drop in stability is observed. The critical p donation
from the flanking nitrogen atoms into the otherwise empty
p-orbital on the central atom remains for the fully saturated
analogues.
Scheme 6. A ligand exchange reaction transferring TeCl2 to Cy2DAB and
halide abstraction by using TMSOTf to give monocationic heterocycle 6.
The 2,2’-bipyridine chelate of TeCl2 (4) has been demon-
strated to be a stable and viable source of TeCl2.[35] Attempt-
ed ligand exchange with Dipp2DAB resulted in no reaction,
but the 1:1 stoichiometric reaction of the more Lewis basic
Cy2DAB (Cy =cyclohexyl) with 4 resulted in the immediate
deposition of a dark-yellow powder. The yellow powder was
found to be completely insoluble in organic solvents, pre-
cluding analysis through NMR spectroscopy, similar to what
had been found for the SeCl2-Cy2DAB adduct.[21] In that
case, addition of two stoichiometric equivalents of TMSOTf
(TMSOTf=trimethylsilyl trifluoromethanesulfonate) gave
the dicationic heterocycle. Under the assumption that the
TeCl2-Cy2DAB adduct (5) had been generated, a halide ab-
straction reaction was carried out with an excess of
TMSOTf, which after workup gave a bright-yellow powder.
Recently West and co-workers reported a facile synthesis
of a saturated derivative of the tBu2DAB ligand (10) by
means of a carbolithiation reaction with MeLi, followed by
an aqueous workup (Scheme 8).[43]
Scheme 8. Westꢂs synthesis of amine 10.
1
The H NMR spectrum of the redissolved powder showed
The diamine is isolated and used as a racemic mixture of
the R,R and S,S enantiomers, which is not represented in
the succeeding drawings for simplicity. The 1:1 stoichiomet-
ric reaction of 10 with TeCl4 in the presence of two equiva-
lents of NEt3 in THF at ꢀ788C resulted in the formation of
a yellow slurry. After warming to room temperature, the
solids were removed by centrifugation and the THF was re-
moved from the supernatant to give a yellow powder. The
1H NMR spectrum of the powder revealed a single set of
peaks, consistent with intact ligand. The diagnostic protons
are a quartet from the endocyclic backbone protons, which
were found at d=4.05 ppm (c.f. d=2.44 ppm for free 10).
One resonance was observed in the 125Te{1H} NMR spec-
trum at d=1557 ppm. Single crystals were grown from a
CH2Cl2 solution of the bulk powder through vapor diffusion
with n-hexane, and X-ray diffraction studies revealed a TeIV-
centered heterocycle with the two chlorine atoms bound to
the tellurium atom (11, Scheme 9).
that the diagnostic backbone resonances were d=9.68 ppm.
A mass spectrum of a sample of the bulk powder revealed
an isotope pattern consistent with the presence of tellurium
and chlorine ([M]+ =385). X-ray diffraction studies on
single crystals grown from a CH3CN solution of the bulk
powder through vapor diffusion of Et2O revealed a mono-
cationic tellurium-centered heterocycle bearing a single
chlorine atom paired with a triflate anion (6).
The use of the stronger halide abstraction reagent AgOTf
under identical conditions gives a compound containing a
1H NMR spectroscopic resonance for the backbone protons
signal at d=9.90 ppm, shifted downfield from the monocat-
ion. The synthesis of the dicationic tellurium 7 NHC ana-
logue was confirmed by X-ray crystallographic studies on
single crystals grown from a CH3CN solution of the bulk
powder (Scheme 7).
To generate the target dication, a methathesis reaction
was performed on 11 by using two stoichiometric equiva-
lents of AgOTf, which gave a yellow powder after workup.
1H NMR spectroscopy of a redissolved sample revealed a
distinct downfield shift for the diagnostic protons (d=
4.40 ppm). It is noteworthy that the corresponding signal for
the neutral silylene (d=2.81 ppm) is found at much higher
Scheme 7. Halide abstraction by using AgOTf to generate the dicationic
TeII NHC analogue 7.
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Chem. Eur. J. 2010, 16, 12454 – 12461

Dicationic Tellurium Analogues
FULL PAPER
rial as 13. Compound 13 can be considered a base-stabilized
dicationic TeIV analogue of an N-heterocyclic carbene with a
single stereochemically active lone pair of electrons.
X-ray crystallography: Compounds 3, 6, 7, and 11–13 have
been characterized by single-crystal X-ray diffraction stud-
ies. Views of the formula units are shown in Figures 1–6,
and refinement details may be found in Table 1.
ꢀ
Compound 3 (Figure 1) contains Te N bond lengths of
2.146(7) ꢀ, approximately 0.1 ꢀ longer than those calculat-
ꢀ
ed in the optimized structure with Ph nitrogen substitu-
ents.[18] The Te N bonds are much shorter than those found
ꢀ
in a related TeI2 chelate of a DAB ligand (2.40 ꢀ), reflecting
weaker interactions with the anion.[34] The geometry about
the tellurium center in the chelate is square planar
(S angles=359.98), consistent with an AX3E2 electron pair
Scheme 9. Synthetic pathway to base-stabilized TeIV NHC analogue 13.
field.[43] There was also a downfield shift relative to 11 for
the resonance in the 125Te{1H} NMR spectrum to d=
1823 ppm. These data pointed to a successful metathesis re-
action, which was confirmed by X-ray crystallography of
single crystals grown from the bulk powder. However,
rather than a dicationic heterocycle, the triflates were found
to be weakly bound to the Te center, giving an overall neu-
tral molecule (12).
To displace the triflates from the Te center, five stoichio-
metric equivalents of pyridine were added to 12. There was
an immediate upfield shift of the singlet in the
125Te{1H} NMR spectrum to d=1736 ppm, consistent with
coordination of a Lewis base to the tellurium center. A
sample of the isolated powder redissolved in CDCl3 for
1H NMR spectroscopy showed that the heterocycle was
intact; the diagnostic quartet was virtually unchanged from
12. The signals arising from pyridine were shifted slightly
downfield from free pyridine and integrated for two pyri-
dine molecules relative to the heterocycle. Single crystals
suitable for X-ray crystallography were grown from a con-
centrated pyridine/Et2O solution of the bulk powder held at
ꢀ308C overnight, which confirmed the identity of the mate-
Figure 1. Solid-state structure of 3: Ellipsoids are displayed to the 50%
probability level. Hydrogen atoms and MeCN solvate are omitted. Se-
ꢀ
ꢀ
lected bond lengths [ꢀ] and angles [8]:Te(1) N(1): 2.146(7), Te(1) Br(1):
ꢀ
ꢀ
ꢀ
2.8534(11), Te(2) Br(1): 2.9929(11), Te(2) Br(2) 2.5313(11), Te(2)
ꢀ
ꢀ
ꢀ
Br(3): 2.6828(15), Te(2) Br(4): 2.6742(15), N(1) C(2): 1.286(10), C(2)
C(2A): 1.421(16); N(1)-Te(1)-N(1A): 74.4(4).
Table 1. Single-crystal X-ray diffraction structural and refinement details.
3
6
7
11
12
13
empirical formula
Fw
crystal system
space group
a [ꢀ]
b [ꢀ]
c [ꢀ]
a [8]
b [8]
C36H51Br6N7Te2
1316.50
orthorhombic
Pnma
27.2439(9)
16.6918(5)
10.6181(3)
90
C15H24Cl1F3N2O3S1Te1
532.47
monoclinic
P21/n
6.757(1)
23.223(5)
13.129(3)
90
101.48(3)
90
C16H24F6N2O6S2Te1
646.09
Triclinic
Pꢀ1
C12H26Cl2N2Te1
396.85
monoclinic
P21/c
8.210(1)
16.395(3)
12.1987(3)
90
106.77(3)
90
C14H26F6N2O6S2Te1
624.09
orthorhombic
Pbca
8.4746(5)
16.5761(9)
33.566(2)
90
C35H41F6N5O6S2Te1
933.45
triclinic
Pꢀ1
14.640(3)
14.815(3)
22.623(5)
82.03(3)
79.33(3)
89.77(3)
4774.2(17)
1.798
14.971(3)
17.522(4)
18.034(4)
68.89(3)
81.73(3)
67.46(3)
4083(1)
1.518
90
90
90
90
g [8]
V [ꢀ3]
4828.6(3)
1.811
2019.0(7)
1.752
1673.9(6)
1.575
4715.2(5)
1.758
1calcd [mgmꢀ3
]
R
G
0.0648
0.1934
0.0328
0.1068
0.0808
0.1919
0.0488
0.1400
0.0667
0.1677
0.0505
0.1553
wR2(F2)[b]
[a] R(F
[I>2s(I)])=SjjjFo jꢀjFc jj/SjFo j ; wR
E
G
varied. [b] w=1/[s2(Fo2)+(aP)2 +bP] in which P=(Fo2 +2F2c)/3 and a and b are constants suggested by the refinement program.
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12457

P. J. Ragogna and J. L. Dutton
geometry, and the assignment of the +2 oxidation state for
ꢀ
ꢀ
the central tellurium. The N C bonds are short and the C
C bond is long, confirming retention of the diimine bonding
motif.
ꢀ
The Te Br bonds from the tellurium in the chelate to the
bridging bromides are long (2.853(1) ꢀ) relative to a stan-
IV
ꢀ
dard Te Br bond (2.6 ꢀ), but are even further to the Te
center in the TeBr6 unit (2.993(1) ꢀ).[32] Therefore, the com-
pound can be considered as either a tellurium-centered di-
cation interacting strongly with a [TeBr6] dianion in the
solid state or as a TeBr2–DAB chelate in which the TeBr2
1
acts as Lewis base to TeBr4. Based on the H NMR spectro-
Figure 3. Solid-state structure of 7: Ellipsoids are displayed to the 50%
probability level. Hydrogen atoms are omitted, and one of four inde-
pendent molecules in the asymmetric unit is shown. Selected bond
sopcic data, the backbone protons are shifted too far down-
field for a ChX2–DAB chelate (c.f. SeBr2–Dipp2DAB d=
8.16 ppm).[35] However, the fact that the Te Br bonds to the
ꢀ
ꢀ
ꢀ
ꢀ
lengths [ꢀ]: Te(1) N(1): 2.074(6), Te(1) N(2): 2.079(6), N(1) C(1):
TeII center are shorter than those to the TeIV center indicates
that the complex cannot be considered a dication, and the
bromides are best described as simply bridging in the solid
state. Tellurium-125 NMR spectroscopy would be able to
determine the nature of the anion in solution, unfortunately
3 is not sufficiently soluble for 125Te{1H} NMR spectroscopic
studies, and decomposes if held in solution, precluding
lengthy acquisition time. The solid-state structure of 6
(Figure 2) reveals a T-shaped environment about the telluri-
um center, consistent with an AX3E2 electron pair geometry
and the assignment of the +2 oxidation state for tellurium.
ꢀ
ꢀ
ꢀ
1.285(9), C(1) C(2): 1.43(1), Te(1) O(1): 2.549(6), Te(1) O(2): 2.602(6).
ꢀ
Te O single bond length of 1.95 ꢀ; shorter distances of
2.42 ꢀ have been described as merely solid-state contacts.[44]
The nature of the compound in solution is revealed by the
19F{1H} NMR chemical shift of the CF3 group in the tri-
ꢀ
flate. For 7, one sharp resonance was found at d=
ꢀ78.5 ppm in MeCN. This can be compared with an ionic
standard ([NOct4]
G
lent standards (MeOTf: d=ꢀ75 ppm; TMSOTf: d=
ꢀ77.6 ppm (MeCN)), which indicates that 7 exists in an
ionic form in solution, driven by the strong trans effect of
the Cy2DAB ligand.
ꢀ
ꢀ
The Te N bond trans to the chloride (Te(1) N(1) 2.152 ꢀ)
ꢀ
is slightly longer than the Te(1) N(2) bond (2.090(3) ꢀ).
The structure of compound 6 also provides evidence for the
successful synthesis of the TeCl2–Cy2DAB complex 5, which
could not be confirmed by spectroscopic methods.
The solid-state structure of 11 (Figure 4) features a dis-
phenoidal AX4E bonding arrangement about the tellurium
center, consistent with the tellurium carrying one lone pair
Figure 2. Solid-state structure of 6: Ellipsoids are displayed to the 50%
probability level. Hydrogen atoms and the triflate anion are omitted. Se-
ꢀ
ꢀ
lected bond lengths [ꢀ] and angles [8]: Te(1) N(1): 2.152, Te(1) N(2):
ꢀ
2.090(3), Te(1) Cl(1): 2.553(1); N(1)-Te(1)-Cl(1): 165.35(8), N(2)-Te(1)-
N(1): 74.8(1), N(2)-Te(1)-Cl(1): 90.53(9).
Figure 4. Solid-state structure of 11: Ellipsoids are displayed to the 50%
probability level. Hydrogen atoms and MeCN solvate are omitted. Se-
ꢀ
ꢀ
lected bond lengths [ꢀ] and angles [8]: Te(1) N(1): 1.944(4), Te(1) N(2):
ꢀ
The Te N bonds in 7 (Figure 3) average 2.08 ꢀ, shorter
than those found in the monocation, and slightly longer than
ꢀ
ꢀ
1.947(4), Te(1) Cl(1): 2.590(2), Te(1) Cl(2): 2.613(2); Cl(1)-Te(1)-Cl(2):
164.6(5), N(1)-Te(1)-N(2): 84.3(2).
ꢀ
the calculated values for the idealized dication bearing Ph
ꢀ
ꢀ
N substituents. The C N and C C bonds are again consis-
tent with double and single-bond functionalities, respective-
ly. There are short contacts in the solid state between the
tellurium center and oxygen atoms from two of the triflate
of electrons and the assignment of the +4 oxidation state.
ꢀ
The Te N bonds are 1.94 ꢀ, significantly shorter than those
in 3, 6, and 7, reflective of the stronger bonding with the for-
ꢀ
ꢀ
anions. The Te O contacts average 2.57 ꢀ, with a range be-
tween 2.49–2.66 ꢀ, significantly longer than the standard
mally dianionic ligand. The Te Cl bonds (2.60 ꢀ) are typical
ꢀ
of Te Cl single bonds.
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Chem. Eur. J. 2010, 16, 12454 – 12461

Dicationic Tellurium Analogues
FULL PAPER
ꢀ
ꢀ
ꢀ
The Te N bonds in compound 12 (Figure 5) are only
slightly shorter than those in 11, unlike that which is ob-
2.333(4) and 2.393(4) ꢀ for Te(1) N(3) and Te(1) N(4), re-
ꢀ
spectively, longer than the endocyclic Te N bonds, which
served for the diimine complexes. This is due to the lack of
are virtually unchanged from the other two derivatives at
1.96 ꢀ. The disphenoidal geometry about tellurium is re-
tained for the coordination complex. There is little to com-
pare the pyridine–tellurium bonds with, as this is only the
fourth example of any monodentate pyridine-type donor
bound to any tellurium, and the first for TeIV. Two of the
other reports involve hemilabile complexes of TeII thiolates
with bridging 4,4’-bipyridine ligands containing very long
[45,46]
ꢀ
Te N contacts of ~2.8 ꢀ.
The other example is a dicat-
ionic homoleptic TeII complex with four 4-dimethylamino-
pyridine ligands in a square-planar arrangement, featuring
[24]
ꢀ
Te N bonds of ~2.3 ꢀ.
Conclusion
Synthetic, structural, and spectroscopic studies of tellurium
NHC analogues have been conducted. For the unsaturated
derivatives, the solid-state structures confirmed theoretical
predictions, in that the chalcogen atoms adopt the +2 oxida-
tion state, and the diimine framework of the ligand is re-
tained. Use of a nonredox active diamine ligand allowed for
the isolation of a TeIV N-heterocycle, and the generation of
the first dicationic ChIV NHC analogue, which was stabilized
by coordination of a Lewis base (pyridine).
Figure 5. Solid-state structure of 12: Ellipsoids are displayed to the 50%
probability level, methyl groups and hydrogen atoms are omitted. One
portion of the disordered NCCN backbone portion is shown. Selected
ꢀ
ꢀ
bond lengths [ꢀ] and angles [8]: Te(1) N(1): 1.900(8), Te(1) N(2):
ꢀ
ꢀ
1.908(9), Te(1) O(1): 2.358(7), Te(1) O(2): 2.332(7); O(2)-Te(1)-O(1):
166.2(3), N(1)-Te(1)-N(2): 86.1(4).
any substituents trans to the nitrogen atoms, within the dis-
ꢀ
phenoidal bonding arrangement. The Te O bonds average
ꢀ
2.34 ꢀ and are much longer than a typical Te O bond, re-
flecting the weak nucleophilicty of the OTf anions, and the
ease with which they are displaced by pyridine. Despite the
weak bond, the Te–O interactions are retained in solution,
based on the clear difference in the 19F{1H} NMR chemical
Experimental Section
General: Manipulations were performed in an N2-filled MBraun Labmas-
ter 130 glove box in 4 dr vials affixed with Teflon lined screw caps or by
using standard Schlenk techniques. Dichloromethane, THF, Et2O,
CH3CN, n-pentane, and n-hexane were obtained from Caledon Laborato-
ries and dried by using an MBraun SPS with appropriate drying agents.
Pyridine was obtained from Caledon, dried over CaH2, and distilled. The
dried solvents were stored in Strauss flasks under an N2 atmosphere or
over 4 ꢀ molecular sieves in a glove box. Solvents for NMR spectroscopy
were purchased from Cambridge Isotope Laboratories, dried by stirring
for 3 days over CaH2, distilled prior to use, and stored in the glove box
over 4 ꢀ molecular sieves. Tellurium tetrachloride, TeBr4, TMSOTf,
AgOTf, and 2,2’-bipyridine were purchased from Alfa Aeasar and used
as received. The Dipp2DAB, Cy2DAB, and diamine 10 were prepared ac-
cording to literature procedures.[42, 43,47] NMR spectra were recorded on
an INOVA 400 MHz spectrometer. Tellurium-125 NMR spectra were ref-
erenced to Me2Te (d=0.00 ppm by using H2TeO4 in D2O; d=712 ppm).
Fluorine-19 NMR spectra were referenced to CFCl3 (d=0.00 ppm by
using C6H5CF3 d=ꢀ63.7 ppm) 1H and 13C{1H} NMR spectra were refer-
enced relative to Me4Si by using the resonances from within the NMR
spectroscopic solvents.[48] Samples for FT-Raman spectroscopy were
packed in capillary tubes, flame-sealed, and data were collected by using
a Bruker RFS 100/S spectrometer, with a resolution of 4 cmꢀ1. FTIR
spectra were collected on samples as KBr pellets by using a Bruker
Tensor 27 spectrometer, with a resolution of 4 cmꢀ1. Decomposition/melt-
ing points were recorded in flame-sealed capillary tubes by using a Gal-
lenkamp Variable Heater. All mass spectrometry measurements were
collected by Mr. Doug Hairsine by using an electrospray ionization Mi-
cromass LCT spectrometer. Suitable single crystals for X-ray diffraction
studies were individually selected under oil (Paratone-N), mounted on
nylon loops, and immediately placed in a cold stream of N2 (150 K). Data
were collected on a Bruker Nonius Kappa CCD X-ray diffractometer or
shift from ionic triflate (d=ꢀ77.6 ppm, c.f. [NOct4]
ACHTUNGTRNE[NUNG OTf]:
d=ꢀ79.0 ppm (CH2Cl2)), 12 cannot be represented as a di-
cationic compound.
The pyridine ligands replace the OTf anions in compound
ꢀ
13 (Figure 6). The Te N bonds from the pyridine are
Figure 6. Solid-state structure of 13: Ellipsoids are drawn to the 50%
probability level, methyl groups on the tert-butyl groups, hydrogen atoms,
pyridine solvate, and triflate anions are omitted. . Selected bond lengths
ꢀ
ꢀ
ꢀ
[ꢀ] and angles [8]: Te(1) N(1): 1.958(4), Te(1) N(2): 1.955(4), Te(1)
ꢀ
N(3): 2.333(4), Te(1) N(4): 2.393(4); N(2)-Te(1)-N(4): 172.8(1), N(2)-
Te(1)-N(1): 86.4(2).
Chem. Eur. J. 2010, 16, 12454 – 12461
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12459

P. J. Ragogna and J. L. Dutton
a Bruker APEX II CCD X-ray diffractometer by using graphite-mono-
chromated MoKa radiation (l=0.71073 ꢀ). The solution and subsequent
refinement of the data were performed by using the SHELXTL suite of
programs.[49] CCDC-625896 (3), -775711 (6), -775712 (7), -775713 (11),
-775714 (12), and -775715 (13) contain the supplementary crystallograph-
ic data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/
data_request/cif. Decomposition is observed within a few days if these
compounds are allowed to stand in the solid state at room temperature
under a N2 atmosphere. No decomposition was observed when they were
stored at ꢀ308C.
13C{1H} NMR (CH2Cl2): d=70.1, 61.8, 30.8, 21.9 ppm; 125Te{1H} NMR
(CH2Cl2): d=1823 ppm; 19F{1H} NMR (CH2Cl2): d=ꢀ77.6 ppm.
Synthesis of 13: Pyridine (90 uL, 1.12 mmol) was added to a solution of
12 (0.150 g, 0.240 mmol; CH2Cl2 5 mL), which resulted in a color change
from bright yellow to pale yellow. Diethyl ether (5 mL) and n-pentane
(5 mL) were then added to give a yellow precipitate. The mixture was
cooled to ꢀ308C for 1 h. The solution was then decanted, the solid
washed with Et2O (2ꢃ5 mL), and then dried in vacuo to give 13 as a
light-yellow powder. Yield: 0.098 g, 52%; d.p. 1108C; 1H NMR (CDCl3):
d=8.65 (d, 3JHꢀH =4.4 Hz, 4H), 7.73 (t, 3JHꢀH =8.0 Hz, 2H), 7.35 (d,
3JHꢀH =8.0 Hz, 4H), 4.40 (q, 3JHꢀH =6.6 Hz, 2H), 1.62 (s, 18H), 1.46 ppm
(d, 3JHꢀH =6.6 Hz, 6H); 13C{1H} NMR (CDCl3): d=148.6, 138.2, 124.9,
70.5, 61.9, 31.2, 22.1 ppm; 125Te{1H} NMR (CH2Cl2): d=1736 ppm;
19F{1H} NMR (CH2Cl2): d=ꢀ78.7 ppm.
Synthesis of 3: Dipp2DAB (0.0504 g, 0.134 mmol) in CH2Cl2 (5 mL) was
added dropwise to a stirred slurry of TeBr4 (0.060 g, 0.134 mmol) in
CH2Cl2 (5 mL) at room temperature, resulting in the formation of a red/
brown slurry. The reaction mixture was allowed to stir for 3.5 h over
which time a red precipitate formed. The reaction mixture was centri-
fuged and the precipitate washed with CH2Cl2 (2ꢃ10 mL). The superna-
tant was decanted and the volatiles were removed in vacuo to give 3 as a
dark-red powder. Yield: 0.037 g, 50%; d.p. 1618C; 1H NMR (CD3CN):
d=9.91 (s, 2H), 7.54 (t, 3JHꢀH =6.80 Hz, 2H), 7.45 (d, 3JHꢀH =7.20 Hz,
4H), 2.60 (sept, 3JHꢀH =6.80 Hz, 4H), 1.31 (d, 3JHꢀH =6.80 Hz, 9H),
Acknowledgements
The authors thank the Natural Sciences and Engineering Research Coun-
cil of Canada (NSERC), the Canada Foundation for Innovation, and the
University of Western Ontario for their generous funding. We also thank
V. Colfari and B. T. Rookie for useful discussions, Dr. M. C. Jennings for
the collection of X-ray crystallographic data (compounds 3, 11, and 12),
Mr. D. Hairsine for acquisition of mass spectral data, and Mr. R. Tabeshi
for initial synthesis of 3.
3
1.25 ppm (d, JHꢀH =6.80 Hz, 9H); ESI-MS: m/z: 585 [M+Br]+.
Synthesis of 5: A solution of Cy2DAB (0.046 g, 0.211 mmol; THF 5 mL)
was added to a slurry of 4 (0.075 g, 0.211 mmol; THF 5 mL), resulting in
a color change of the slurry from green to orange. After five minutes, the
mixture was centrifuged and the supernatant decanted. The solids were
washed with Et2O (3ꢃ5 mL) and dried in vacuo to give 5 as a dark-
yellow powder. Yield: 0.086 g, 97%; d.p. 1708C.
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Synthesis of 6: Neat TMSOTf (60 uL, 0.332 mmol) was added to a slurry
of 5 (0.064 g, 0.153 mmol; CH3CN 5 mL) and stirred for 30 min. Diethyl
ether (15 mL) was added to give a yellow precipitate. The supernatant
was decanted, the precipitate washed with Et2O (2ꢃ10 mL), and dried in
vacuo to give 6 as a bright-yellow powder. Yield: 0.05 g, 61%; d.p. solid
turns black 120–1458C; 1H NMR (CD3CN): 9.69 (s, 2H), 4.86 (m, 2H, cy-
clohexyl C-H), 2.17–1.09 ppm (cyclohexyl CH2); 19F{1H} NMR (CH3CN):
ꢀ78.5 ppm; ESI-MS m/z: [M]+ 385.
Synthesis of 7: A slurry of 5 (0.086 g, 0.207 mmol; CH3CN 10 mL) was
added to solid AgOTf (0.400 g, 1.56 mmol) and stirred for 2 h. The mix-
ture was centrifuged and gave a yellow solution over a white solid. The
supernatant was decanted, and then Et2O was added to the supernatant
to give a light-yellow precipitate. The solution was decanted and the
solids washed with Et2O (3ꢃ10 mL) and then dried in vacuo to give 7 as
a light-yellow powder. Yield: 0.125 g, 93%; d.p. 195–2008C;1H NMR
ꢀ
(CD3CN): d=9.84 (s, 2H), 4.95 (m, 2H, cyclohexyl C H), 2.20–1.26 ppm
(cyclohexyl CH2); 13C{1H} NMR (CD3CN): d=160.0, 70.0, 36.5, 26.0,
25.0 ppm; 19F{1H} NMR (CH3CN):
d
=ꢀ78.5 ppm; ESI-MS: m/z:
[MOTf2ꢀH]+ 645, [MꢀC6H11]+ 264.
Synthesis of 11: A solution of 10 (0.470 g, 2.35 mmol; THF 10 mL) and
NEt3 (0.66 mL, 4.7 mmol) was added to a stirred ꢀ658C solution of TeCl4
(0.632 g, 2.35 mmol; THF 25 mL) to give a yellow slurry. After 1 h, the
solution was allowed to warm to room temperature and filtered to give a
yellow solution. The solvent was removed in vacuo and gave a dark-
yellow powder. The material was redissolved in CH2Cl2 (10 mL) and n-
pentane (5 mL) was added. The mixture was centrifuged and the super-
natant decanted. The solvent was removed from the supernatant in vacuo
to give 11 as a yellow powder. Yield: 0.705 g, 76%; d.p. solid browns at
1008C; 1H NMR (CDCl3): d=4.05 (q, 3JHꢀH =6.6 Hz, 2H), 1.58 (s, 18H),
1.53 ppm (d, 3JHꢀH =6.6 Hz, 6H); 13C{1H} NMR (CDCl3): d=69.7, 59.5,
31.5, 22.5 ppm; 125Te{1H} NMR (CH2Cl2): d=1557 ppm.
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was added to an aluminum-foil-wrapped vial containing solid AgOTf
(.305 g, 1.18 mmol). The vial was then carefully wrapped in additional
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ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 12454 – 12461

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Received: May 25, 2010
Published online: September 16, 2010
Chem. Eur. J. 2010, 16, 12454 – 12461
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12461