Functionalised saturated-backbone carbene ligands: Yttrium and uranyl alkoxy-carbene complexes and bicyclic carbene-alcohol adducts

Article abstract of DOI:10.1002/chem.200801321

A new and modular route to bidentate ligands that combines an alkoxide with a saturated backbone N-heterocyclic carbene (NHC) is presented. The bi(heterocyclic) compounds are formally the addition product of a saturated NHC and the alcohol group of the N-functionalised arm. Using these compounds, the synthesis and structural characterisation of the first electro-positive metal complexes of saturated N-heterocyclic carbenes has been achieved, and examples structurally characterised for the yttrium(III) and the uranyl [UO ₂]²⁺ cations.

Full text of DOI:10.1002/chem.200801321

FULL PAPER
DOI: 10.1002/chem.200801321
Functionalised Saturated-Backbone Carbene Ligands: Yttrium and Uranyl
Alkoxy–Carbene Complexes and ACHTNUTRGNEBUNG icyclic Carbene–Alcohol Adducts
Polly L. Arnold,* Ian J. Casely, Zoꢀ R. Turner, and Christopher D. Carmichael[a]
Abstract: A new and modular route to
bidentate ligands that combines an alk-
oxide with a saturated backbone N-het-
erocyclic carbene (NHC) is presented.
The bi(heterocyclic) compounds are
formally the addition product of a satu-
rated NHC and the alcohol group of
the N-functionalised arm. Using these
compounds, the synthesis and structur-
al characterisation of the first electro-
positive metal complexes of saturated
N-heterocyclic carbenes has been ach-
ieved, and examples structurally char-
acterised for the yttrium
ACHTUNGTRENUN(NG III) and the
uranyl [UO2]2+ cations.
Keywords: alkoxides · bicyclic · car-
benes · uranium · uranyl · yttrium
Introduction
work on the synthesis of metal carbene complexes from
electron-rich olefins, for example in the synthesis of trans-
N-Heterocyclic carbenes (NHCs) of the form A, Figure 1,
are undisputed as extremely strong s-donor ligands, and
now also as p-acceptors in appropriate situations.[1, 2] Owing
[PtCl2{CACTHNGUTERNNUG
(NPhCH2)2}PEt3].[11]
Nolan has examined the differences between saturated
and unsaturated NHCs in Group 10 complexes by analysing
the stretching frequency in [(NHC)Ni(CO)3] adducts. He
finds that within the group, the s-donor properties of unsa-
turated and saturated NHCs are almost the same.[12] Anoth-
er study showed that the saturated NHCs are better accept-
ors of p-back-donation than the unsaturated analogues in
PtII complexes, which contain a bonding contribution of
about 10% from back-bonding.[13] Most recently, studies on
an iridium system that allows for a direct comparison of a
Figure 1. N-Heterocyclic carbenes (NHCs) A and B.
wide range of monodentate carbenes, [IrACTHNUTRGEN(UNG NHC)(CO)2Cl],
to an appreciation of the former property, and a desire to
understand better the latter, they have been studied inten-
sively as ligands for homogeneous transition metal cataly-
sis.[3–8] They have also been proven as active organocatalysts
in their own right.[9, 10]
As a part of efforts to tune the catalytic properties, the sa-
turated analogue of the first generation NHC, B in Figure 1,
has become the focus of much interest for late metal homo-
geneous catalysis. It should be noted, however, that saturat-
ed NHC complexes were also studied in Lappertꢀs original
have again concluded that the differences between the two
types of NHC are very small, and the sterics of the ligands
probably provide the greatest contribution to differences in
the complexes formed.[14]
A number of synthetic routes to substituted imidazolini-
um proligands have been developed. These include a modu-
lar substituted diamine synthesis with subsequent ring clo-
sure,[15] multicomponent synthesis by using a combination of
amine, aldehyde, isocyanide and alkyl halide,[16] the intramo-
lecular “hydroamidiniumation” of alkenes,[17] and a template
synthesis with an azido isocyanide at a tungsten metal
centre.[18]
[a] Dr. P. L. Arnold, I. J. Casely, Z. R. Turner, Dr. C. D. Carmichael
School of Chemistry, University of Edinburgh
West Mains Road, Edinburgh, EH9 3JJ (UK)
Fax : (+44)131650 6453
Late metal complexes containing alcohol and phenol-
functionalised saturated NHC ligands have been reported,
see Figure 2: Seeking an analogy with salicylaldimine li-
gands, Grubbs reported the synthesis of o-hydroxyaryl-sub-
stituted NHCs and showed that the Pd adducts C were
stable.[19] Hoveyda has used the BINOL-derived proligand
E-mail: polly.arnold@ed.ac.uk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801321.
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secondary amine, during the synthesis of hydroxyl substitut-
ed polyamine ligands.[30] This result presented an opportuni-
ty for us to extend our straightforward modular unsaturated
ligand synthesis to a range of saturated variants. Thus, the
new imidazolinium salts [H2LR]X, [HOCR2R3CH
2ACHTUNGTRENNUNG(1-
CH{NCH2CH2NR})]Cl (R=iPr, denoted as P; R=
C6H2Me3-2,4,6-(Mes), denoted as M; R=C6H3iPr2-2,6-
(Dipp), denoted as D, X=Cl, I) are synthesised according
to Scheme 2, through nucleophilic attack by a mono N-sub-
Figure 2. Functionalised, saturated NHC complex and imidazolinium
salts C–F.
D to make ruthenium catalysts for asymmetric alkene meta-
thesis,[20] Mauduit reported a five step route from b-aminoal-
cohols to chiral alkoxy-imidazolinium salts E, which were
highly active for copper-catalysed conjugate addition,[21] and
Wilhelm has shown that F is a good proligand for the enan-
tioselective addition of ZnEt2 to aldehydes.[22]
We have previously reported syntheses of unsaturated al-
cohol-carbene ligands,[23,24] and their use in copper-catalysed
asymmetric conjugate addition,[25] lactide polymerisation,[26]
Scheme 2. Syntheses of saturated alkoxy-carbene proligands [H2LR]X
(X=Cl, I), 1R and 1aR.
29]
and Group 4[27] and f-block organometallic chemistry.[28,
We, like others, have found that the acidity of the protons of
the carbene heterocyclic backbone can lead to unanticipat-
ed, and sometimes unwanted, ligand rearrangements.
stituted ethylene diamine on a substituted epoxide. Subse-
quent acidification and heating in trimethyl orthoformate
furnishes the ring-closed alcohol-N-functionalised imidazoli-
nium chloride salts in yields of 60 to 74%, as a viscous
brown oil for [H2LP]Cl/I (1P/1aP) and as colourless—or
cream-coloured powders for [H2LM]Cl/I (1M/1aM) and
[H2LD]Cl/I (1D/1aD). This simple modular approach allows
for the incorporation of three different R groups, each close
to a ligand donor atom, to yield proligands with a variety of
steric and electronic profiles.
Herein we report
a modular synthesis of bidentate
alkoxy-functionalised saturated NHC ligands and the first
examples of early metal saturated carbene complexes; ad-
ducts of YIII and the uranyl dication.
Results and Discussion
Synthesis of proligands: Our previous synthesis of unsaturat-
ed alkoxy-carbene ligands proceeded by means of the ring
opening of an epoxide with either imidazole, followed by
subsequent quaternization of the remaining imidazole nitro-
gen with an alkyl halide, or with a substituted imidazole to
give a zwitterionic compound,[23, 25] Scheme 1.
The identities of the imidazolinium salts were confirmed
by multinuclear NMR spectroscopy and combustion analy-
ses (for all except 1P) of 1P, 1M, 1D; the iodide salts 1aP, 1aM
and 1aD are readily made by treatment of [H2LR]Cl with
1
NaI in acetone. The H NMR spectra of proligands 1R and
1aR show the characteristic highfield imidazolinium reso-
nance between d=8 and 10 ppm, which are similar to those
observed in the unsaturated analogues. In the 13C NMR
spectra, the imidazolinium CH resonances lie in the range
d=155 to 160 ppm, as would be expected for the saturated
cationic heterocycle, and lie approximately 20 ppm further
downfield than in the unsaturated examples (range d=135
to 141 ppm).
Hancock et al. previously reported that a primary amine
will selectively ring open an epoxide in the presence of a
In addition, the conformation of 1aP and 1D were deter-
mined by single crystal X-ray diffraction studies on crystals
grown from the dried crude product, which was an oil, and
an acetone solution, respectively. The molecular structures
Scheme 1. Synthesis of unsaturated alkoxy-carbene proligands.
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Yttrium and Uranyl Alkoxy-Carbenes
FULL PAPER
are shown in Figure 3 and selected distances and angles are
displayed in Table 1.
all are intermolecular, and involve the chloride counterion
and a partial (60% present) molecule of water in the lattice.
Thus the OH forms hydrogen bonds with a lattice water
molecule and the imidazolinium group forms a hydrogen
bond with a chloride.
Deprotonation chemistry: Treatment of the [H2LR]X salts
with a single equivalent of base does not form zwitterionic
or imidazolinium salts as for the unsaturated analogues of
these. Instead, [H2LR]X reacts with benzyl potassium in
THF, or with LinBu in hexanes/toluene to afford the bicyclic
products HLR (2R) in high yield (Scheme 3). Compounds
HLP (2P) and HLM (2M) are colourless, distillable oils (408C
at 10ꢀ1 mbar, 51% and 858C at 10ꢀ1 mbar, 73% respective-
ly); compound HLD (2D) was isolated as a colourless powder
and washed with hexanes, 97%.
Scheme 3. Bicyclic structure adopted by HLR products 2R.
Figure 3. Displacement ellipsoid drawings (50% probability) of [H2LP]I
1aP (top) and [H2LD]Cl 1D (bottom).
1
The H NMR spectra of these products confirm the pres-
Table 1. Selected distances (ꢂ) and angles (8) for the molecular struc-
ence of a single remaining acidic CH proton between d=
5.47 and 5.79 ppm, but the loss of the imidazolinium CH res-
onance between d=8 and 10 ppm. The spectra also show
magnetically inequivalent gem-dimethyl and CH2 arm and
backbone resonances, consistent with an asymmetric struc-
ture for 2R in each case, as drawn in Scheme 3. A similar bi-
cyclic structure G was recently reported from the annelation
of a 1-substituted benzimidazole using a,b-acetylenic-g-hy-
droxyacidnitriles (Figure 4).[31] This compound exhibits a
tures of proligands 1aP and 1D
.
1aP Distance/angle
1
D Distance/angle
ꢀ
C1 N1
1.352(6)
1.402(6)
1.525(6)
1.653(7)
1.611(7)
3.462
3.771
–
110.6(4)
98.7(4)
1.305(3)
1.317(3)
1.438(3)
1.481(3)
1.527(3)
3.438
3.100
2.853
113.7(2)
102.9(2)
ꢀ
C1 N2
ꢀ
C2 N2
ꢀ
C5 N2
ꢀ
C5 C6
ꢀ
C1 Hal1*
ꢀ
O1 Hal1’*
ꢀ
O1 O1w’
N1-C1-N2
N2-C5-C6
* Hal=I (1aP), Cl (1D).
The structure of both 1aP and 1D show no unusual distan-
ces or angles. The imidazolinium NC(H)N group is charac-
ꢀ
terised by the C N distances of 1.305(3) to 1.402(6) ꢂ, con-
Figure 4. Alcohol adducts of NHCs.
sistent with the sp2 character of each atom. A noticeable
ꢀ
asymmetry is seen in the ring, with the C1 N1 bond next to
1
ꢀ
the alcohol arm shorter than the C1 N2 bond in each case.
characteristic CH resonance in the H NMR spectra as a sin-
The NCN angles of 110.6(4) and 113.7(2)8 are approximate-
ly seven degrees wider than the NCN angle in imidazolini-
um salts. The heterocycle rings in both are essentially planar.
The directionality towards lattice iodide ions of the alco-
hol and of the imidazolinium group in 1aP both suggest the
existence of hydrogen bonding interactions, but the distan-
glet at d=6.28 ppm, and in the 13C{1H} spectra at d=
109 ppm, respectively. These values are comparable to the
1
chemical shifts of the NC(H)N proton in the H NMR spec-
tra for 2R, considering the different arm substituents, at d=
5.47, 5.79 and 5.78 ppm and d=108–109 ppm in the
13C{1H} NMR spectra.
ces are long; O I=3.771 ꢂ in the former, and C I=
4.306 ꢂ in the latter.
The bicyclic structures formed by compounds 2R are
ꢀ
ꢀ
single-molecule analogues of the alcohol,[10,
fluoro-
32]
The alcohol and imidazolinium groups in 1D both partici-
pate in hydrogen-bonding networks within the structure, but
phenyl,[33] and chloroform[32–34] adducts that have been used
with great success as masked carbenes. For comparison, in
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10417

P. L. Arnold et al.
the compound (MeO)(H)CACHTNUGRTNEUNG(NMesCH2)2, H in Figure 4,
formed from the reaction between the free imidazolin-2-yli-
1
dines and methanol,[10] the H and 13C NMR chemical shifts
of the NC(H)N fragment are d=5.48 ppm and 104 ppm re-
spectively. In the zwitterionic unsaturated analogue
OCMe2CH2{1-CHNCHCHNiPr} reported by us,[24] the 1H
and 13C NMR chemical shifts of the NC(H)N fragment are
d=7.77 ppm and 199 ppm, respectively, suggesting that ad-
ducts 2R do not exist as zwitterionic imidazolinium alkox-
ides.
Scheme 5. Synthesis of the yttriumACHTUNGTRNEUNG
(III) complex 3D.
[Y(L){N
C(NCHCHNMes)}]2,[36] and d=194.0 ppm in [Y{N-
(SiHMe2)2}3{C
(NMeCH)2}2].[37]
The 1H NMR spectrum is simplified upon breaking the
A
L=NACHTGNUTRENU[NG CH2CH2ACHTUNGTRENNUNG{1-
The addition products (X)(H)CACHTNUTRGENUG(N NMesCH2)2 (X=OtBu,
OMe, C6F5, CCl3) can be thermally converted to the free
carbenes; the tert-butanol adduct spontaneously decomposes
at room temperature[32] and the methanol adduct reversibly
eliminates methanol at 258C.[10] Here, the bicyclic structure
of 2R renders them more stable; heating a benzene solution
of 2P to 708C in the NMR spectrometer does not show any
evidence of a dissociated carbene-alcohol, Scheme 4.
A
ACHTUNGTRENNUNG
doubly five-membered bicyclic ring system into the six-
membered ring-containing yttrium-alkoxide carbene; the
alkoxide arm CH2 protons are a singlet at d=3.25 ppm.
NMR spectroscopic-scale reactions between HLR and YN’’3
for R=iPr and Mes show that the analogous yttrium com-
plexes 3P and 3M can be made as readily. The one-bond yttri-
um-carbene coupling constants for these two complexes are
46 and 44 Hz respectively, seen in the carbene chemical
shifts of d=212.3 and 215.5 ppm respectively.
Single crystals of 3D suitable for X-ray structural analysis
were grown from a saturated toluene solution cooled to
ꢀ308C; the molecular structure is shown in Figure 5, and se-
lected distances and angles are collected in Table 2.
Scheme 4. Transient formation of alcohol-carbene compound from 2R.
Dissolution of 2P in CDCl3 does not form any choloro-
form addition product, but a slow H/D exchange of the oxa-
zolidine proton and the chloroform becomes apparent in the
solution after a few hours at room temperature. Since a
direct H/D exchange of the proton in the bicyclic structure
is presumed to be not possible under these reaction condi-
tions, the simplest exchange mechanism, shown also in
Scheme 4, involves the transient generation of the free car-
bene in an equilibrium with the inter and intra-molecular
addition product.
The suggestion of transient carbene formation in these
systems has led us to explore their protonolysis chemistry
with representative early transition metal and f-block metal
complexes.
Figure 5. Displacement ellipsoid drawing (50% ellipsoid probability) of
3D. Hydrogen atoms and Si-bound methyl groups omitted for clarity.
Yttrium complexes: Treatment of YN030 (N’’=N
ACTHNUTRGEN(NUG SiMe3)2)
with an equivalent of 2D in THF affords a yellow-coloured
solution and, after recrystallisation from toluene, colourless
LDYN020 3D can be isolated in 51% yield (Scheme 5).
The yttrium cation in 3D is distorted away from pseudo-
tetrahedral, as the angle between the carbene and the two
silylamido groups is widened to accommodate the large N-
Dipp substituent, for example, C1-Y1-N4=122.55(6)8.
There are two silylamido silicon atoms relatively close to
Compound 3D is immediately identifiable by the yttrium-
1
carbene JYC coupling of 42 Hz for the carbene carbon reso-
nance at d=216.3 ppm in the 13C{1H} NMR spectrum. This
the yttrium cation, with Y Si distances of 3.1957(7) and
ꢀ
is at a higher frequency than observed for unsaturated yttri-
3.3268(7) ꢂ. The yttrium carbene distance is 2.599(2) ꢂ.
um-carbene complexes, and compares with d=186.3 (1JYC
=
This is long compared with Y C distances in unsaturated
ꢀ
53 Hz)
in
[Y(L){N
(SiMe3)2}2],
L=NtBu
[CH2CH
{1-
carbene complexes; in these the distances are 2.501(5) ꢂ in
C(NCHCHNtBu)}],[35] d=194.3 ppm (1JYC =48 Hz) in
[Y(L){N
G
L=NtBuACHTGNUTRENU[NG CH2CH2ACHTUNGTRENNUNG{1-
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Yttrium and Uranyl Alkoxy-Carbenes
FULL PAPER
Table 2. Selected distances (ꢂ) and angles (8) for the molecular struc-
tures of complexes 3D, 4M, and 4D
283.6 ppm respectively. To our knowledge, these are the
highest frequency carbene resonances exhibited by a metal
carbene (NHC) compound.
3D
4M
4D
ꢀ
ꢀ
C1 Y1
2.599(2)
U1 C1
2.580(4)
2.596(4)
1.7978(18)
2.161(3)
1.298(17)
1.343(6)
180.0(2)
85.81(12)
72.28(14)
89.16(11)
118.3(6)
132.3(3)
2.612(2)
A particularly high carbene chemical shift of d=
ꢀ
ꢀ
ꢀ
N3 Y1
2.2654(17)
2.2613(16)
2.0314(15)
1.331(3)
1.347(3)
1.432(3)
1.478(3)
1.515(3)
77.88(6)
112.40(14)
140.55(15)
122.55(6)
107.0(2)
U1 C1
255.5 ppm was reported for the open-chain bis(diiso-propyl)-
carbene (N,N,N’,N’-tetra iso-propylformamidinylidene), and
the high value attributed to the wide NCN bond angle. Like-
wise, its complexes exhibit chemical shifts at values d=30–
40 ppm higher frequency than the corresponding unsaturat-
ed NHC analogues.[39]
ꢀ
ꢀ
N4 Y1
U1 O1
1.7984(16)
2.1538(16)
1.336(3)
ꢀ
ꢀ
O1 Y1
U1 O2
ꢀ
ꢀ
C1 N1
C1 N1
ꢀ
ꢀ
C1 N2
C1 N2
1.338(3)
180.00(11)
89.83(7)
72.46(6)
87.58(7)
ꢀ
C2 N2
O1-U1-O1
O1-U1-O2
O2-U1-C1
O1-U1-C1
N1-C1-U1
N2-C1-U1
ꢀ
C5 N2
ꢀ
C5 C6
In carbene complexes, the highest chemical shifts yet re-
O1-Y1-C1
N1-C1-Y1
N2-C1-Y1
N4-Y1-C1
N1-C1-N2
N2-C5-C6
ported have been in uranyl systems. For example dcarbene
=
117.23(15)
132.38(15)
262.8 ppm for trans-
C(NCHCHNtBu)}.[40]
N
2ACHTUNGTRENNUNG{1-
N1-C1-N2
N2-C5-C6
108.9(7)
101.5(9)
108.0(2)
102.0(2)
The asymmetric OUO stretch, n˜asymmACHTGNUTRENNUNG
102.32(17)
in the FTIR spectrum at n˜ =851 cmꢀ1, indicating a relatively
strong binding of the two alkoxy-carbene ligands in the
equatorial plane, which serves to weaken the UO2 stretching
C(NCHCHNtBu)}],[35] 2.574(3) and 2.565(3) ꢂ in [Y(L){N-
(SiMe3)2}Cl], L=N[CH2CH2A
{1-C(NCHCHNMes)}]2,[36] and
2.560(9) and 2.55(1) ꢂ in [Y{N(SiHMe2)2}3{C-
(NMeCH)2}2].[37] Here, the Y C bond is only slightly longer
than the average Y C single bond in the CS Database (ver-
sion 1.10), which is 2.49 ꢂ, and compares with longer Y-
alkyl bonds, such as the average Y C distance in bridging
methyl groups of 2.55 ꢂ in [{Y
ligand bite angle is 77.88(6)8.
The N-C-N angle of the coordinated carbene group is
now 107.0(2)8, which is smaller than the 113.7(2)8 angle in
the imidazolinium proligand 1D as would be expected for
the high degree of s-character in the Y Ccarbene bond. Other
distances and angles are within accepted ranges.
energy. For comparison, n˜asymm
uranyl complex cis-[UO2A(OtBu)
and in [UO2N’’2A
(OPPh3)2] it is n˜ =901 cmꢀ1.[41] This ligand
set also weakens the UO2 bonding more than in the com-
plexes [UO2(LN)2], in which the average n˜asymm
(OUO)=
ACHTUNGTRENNUNG
G
R
G
A
N
CHTUNGTRENNUNG
T
CHTUNGTRENNUNG
ꢀ
ACHTUNGTRENNUNG
ꢀ
ACHTUNGTRENNUNG
931 cmꢀ1. We can conclude from these comparisons that the
alkoxides are the dominating factor in the ability of the
equatorial donor set to weaken the UO2 stretch, but that the
carbenes are stronger ligands than phosphine oxides.
The molecular structures of 4M and 4D were determined
by single crystal X-ray diffraction and are shown in Fig-
ure 6a and b, respectively. Selected distances and angles are
in Table 2.
ꢀ
(C5Me5)
N
ꢀ
2+
The UO2 uranyl unit in each is rigorously linear as ex-
Uranyl complexes: Mixing benzene solutions of [UO2N’’2-
pected and the compounds are four-coordinate in the equa-
torial plane, in which the two bidentate OC ligands adopt a
perfectly symmetrical trans-geometry about the metal, en-
forced by a crystallographic C2 axis. Most uranyl compounds
are between five and seven-coordinate in the equatorial
plane. Additionally, in 4M there is a disorder in the crystal,
which superimposes two crystallographically independent
molecules; only one is shown in Figure 6a.
ACHTUNGTRENNUNG
(THF)2] and two equivalents of 2M results in the deposition
of pale yellow crystals of a new compound over a period of
minutes (Scheme 6). The compound was identified as the
poorly benzene-soluble trans-bis(LM) adduct [UO2(LM)2] 4M,
by elemental analysis, multinuclear NMR spectroscopy and
a single crystal X-ray diffraction study, and isolated in 52%
yield.
In both structures, the alkoxycarbene ligands twist out of
the equatorial plane to accommodate the bulky aryl groups,
with a ligand bite angle of 728, smaller than in 3D by approx-
imately 68. The carbene ring is tilted with respect to the
UC2O2 equatorial plane by 118 in 4M and 228 in 4D; this
brings the arene rings out of the ligand plane in the same di-
rection as the substituents on the alkoxide of the opposite
ligand.
Scheme 6. Synthesis of the uranyl complexes 4M and 4D.
The U Ccarbene distances of 2.580(4) ꢂ in 4M and
ꢀ
2.612(2) ꢂ in 4D are shorter than in the six-coordinate
An NMR spectroscopic-scale reaction between two equiv-
[UO2L2],
(2.640(5) ꢂ) and L=NtBu
(2.633(7) ꢂ),[40]
and
(2.626(7) ꢂ).[42] The U OC distances of 2.161(3) ꢂ in 4
and 2.154(2) ꢂ in 4D are comparable to that in cis-
[UO2-
(OtBu)2A
(OPPh3)2] in which the average is 2.149 ꢂ.[41] The N-
C-N angle of the coordinated carbene group is now
L=NtBu
[CH2CH
[UO2Cl2{C
G
2A
{1-C(NCHCHNMes)}]-
(NMesCH)2}2]
alents of HLD and [UO2N’’
2A
G
N
CHTUNGTRENNUNG
gous uranyl complex 4D is also straightforwardly accessible
(Scheme 6).
ACHTUNGTRENNUNG
M
ꢀ
The most interesting feature of complexes 4M and 4D is
the extraordinarily high chemical shift exhibited by the car-
benic carbon in the 13C{1H} NMR spectrum, of d=281.6 and
ACHTUNGTRENNUNG
CHTUNGTRENNUNG
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P. L. Arnold et al.
forded the first early transition metal, and the first f-block
saturated carbene complexes.
Experimental Section
See the Supplementary Information for general synthetic and crystallo-
graphic details.
Proligand synthesis
General procedure: Proligands were synthesised through a combination
and modification of literature procedures[30,43] under ambient-atmosphere
conditions. In a typical reaction, an N-substituted ethylenediamine was
heated with an epoxide in a melt reaction in a sealed ampoule at 908C
for two days. The resulting oil was dissolved in diethyl ether and cooled
to 08C, acidified with a solution of anhydrous HCl (2m) in diethyl ether
and stirred for 1 h at ambient temperature. Following filtration and
drying in vacuo, the resulting yellow solid was combined with trimethy-
lorthoformate in toluene and refluxed at 908C for 2 h. The final workup
procedure varied for each proligand. Iodide salts were formed by means
of a standard anion exchange reaction with NaI in acetone.
AHCTUNGTRENNUNG
[H2LiPr]Cl 1P: Brown oil (3.92 g, 60%). 1H NMR (CDCl3): d=9.71 (s,
1H; N-CH-N), 5.25 (s, 1H; OH), 4.33 (bm, 2H; N-CH2-CH2-N), 4.05
(bm, 2H; N-CH2-CH2-N), 4.05 (bm, 1H; N-CH-(CH3)2), 3.74 (s, 2H; N-
CH2-C), 1.46 (d, 3J=6 Hz, 6H; N-CH-(CH3)2), 1.36 ppm (s, 6H; C-
AHCTUNGTRENNUNG
(CH3)2); 13C NMR (CDCl3): d=158.5 ppm (N-CH-N).
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
1H; N-CH-N), 6.89 (s, 2H; Ar-CH), 5.20 (s, 1H; OH), 4.42 (bm, 2H; N-
CH2-CH2-N), 4.13 (bm, 2H; N-CH2-CH2-N), 3.89 (bs, 2H; N-CH2-C),
2.26 (bs, 9H; Ar-CH3), 1.27 ppm (s, 6H; CACTHNUTRGNEUNG
(CH3)2); 13C NMR (CDCl3):
d=160.8 ppm (N-CH-N); elemental analysis calcd (%) for C16H25ClN2O:
C 64.73, H 8.51, N 9.44; found: C 64.84, H 8.57, N 9.41.
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
[H2LDipp]Cl 1D: Colourless solid (3.89 g, 60%). 1H NMR (CDCl3): d=
9.47 (s, 1H; N-CH-N), 7.40 (t, 3J=7 Hz, 1H; 4-Ar-CH), 7.21 (d, 3J=
7 Hz, 2H; 3,5-Ar-CH), 5.18 (s, 1H; OH), 4.44, 4.13 (m, 2H; N-CH2-CH2-
3
N), 3.95 (s, 2H; N-CH2-C), 2.88 (sept, J=2 Hz, 2H; Ar-CH
G
(s, 6H; C
(CH3)2), 1.27 (d, 3J=2 Hz, 6H; Ar-CH
ACHUTGTNRENNUG CAHTUNGTRENNUNG
3J=2 Hz, 6H; Ar-CH
AHCTUNGTRENNUNG
N); elemental analysis calcd (%) for C19H31ClN2O: C 67.33, H 9.22, N
8.27; found: C 67.29, H 9.30, N 8.28.
[H2LDipp]I 1aD Colourless solid (1.00 g, 73%). Elemental analysis calcd
AHCTUNGTRENNUNG
Figure 6. Displacement ellipsoid plot for 4M (top) and 4D (bottom)(50%
probability). Hydrogen atoms omitted for clarity.
(%) for C19H31IN2O: C 53.03, H 7.26, N 6.57; found: C 53.07, H 7.28, N
6.59.
108.9(7)8 (4M) and 108.0(2)8 (4D). Other distances and
Bicyclic carbene-alcohol adduct synthesis
General procedure: To a Schlenk charged with a stirred mixture of
KCH2Ph and one equivalent of the appropriate proligand, at ꢀ788C, was
added THF. The resulting dark-orange solution/suspension was allowed
to warm slowly to ambient temperature overnight, furnishing a yellow-
green solution and a fine precipitate. After filtration, the volatiles were
removed and, in the case of HLiPr and HLMes, the residue was purified by
means of a short path distillation and the product isolated as a colourless
oil. Compound HLDipp was isolated as a green oily solid after removal of
the volatiles, from which a small amount of colourless crystalline solid
could be sublimed.
angles are within standard ranges.
Conclusion
A new and modular route to proligands for bidentate, satu-
rated backbone alkoxy-carbenes has been developed. The
removal of one of the two acidic protons leads to the forma-
tion of bicyclic compounds that are formally the addition
product of a saturated N-heterocyclic carbene (NHC) and
the alcohol group of the functionalised arm. This is in con-
trast to the zwitterionic structures found for the unsaturated
analogues. Despite this, the proligands are still reactive to-
wards loss of the remaining acidic proton. Protonolysis
chemistry with yttrium and uranyl amido complexes have af-
Alternatively, one equivalent of LinBu was added to a cooled (ꢀ788C)
suspension of the proligand in hexanes or toluene, and the mixture al-
lowed to warm to ambient temperature slowly overnight. The subsequent
workup was the same as that just described, although no green colour
was observed in these preparations.
[HLiPr] 2P: Colourless oil (0.91 g, 51%). H NMR (C6D6): d=5.47 (s, 1H;
1
N-C(O)H-N), 3.13, 2.85, 2.64, 2.63 (m, 1H each; N-CH2-CH2-N), 2.91,
2.45 (d, 2JH-H =10.5 Hz, 1H each; N-CH2-C), 2.85 (m, 1H; N-CH-
10420
www.chemeurj.org
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 10415 – 10422

Yttrium and Uranyl Alkoxy-Carbenes
(CH3)2), 1.24, 1.16 (s, 3H each; C-
FULL PAPER
Table 3. Selected crystallographic details
ACHTUNGTRENNUNG
(CH3)2), 1.14, 1.09 ppm (d, 3J=6 Hz,
1aP
1D
3D
4M
4D
3H each; N-CH-(CH3)2); 13C NMR
(C6D6): d=107.8 ppm (N-C(O)H-N);
elemental analysis calcd (%) for
C10H20N2O: C 65.16, H 10.96, N 15.20;
found: C 65.13, H 11.06, N 15.19.
formula
Mr
C10H21IN2O
312.19
C19H31ClN2O·H1.2O0.6 C31H65N4OSi4Y C44H58N4O4U C38H58N4O4U·C12H12
349.72
711.14
monoclinic
P21/c
944.97
1029.13
triclinic
P1
crystal system orthorhombic monoclinic
triclinic
¯
¯
space group
T [K]
a [ꢂ]
b [ꢂ]
c [ꢂ]
a [8]
Pna21
P21/c
P1
150(2)
9.916(3)
11.780(3)
11.387(3)
90.00
150(2)
150(2)
150(2)
150(2)
ACHTUNGTRENNUNG ] : Colourless oil (3.08 g,
[HLMes 2M
11.9968(3)
11.2624(3)
14.8437(4)
90.00
20.3348(16)
10.6423(9)
20.1193(16)
90.00
112.017(4)
90.00
7.7547(2)
12.0229(3)
12.3593(3)
68.6800(10)
75.4660(10)
83.0090(10)
1038.50(4)
1
10.1261(7)
10.4437(7)
11.7599(8)
93.023(3)
92.881(4)
101.057(3)
1216.56(14)
1
73%). This oil solidified slowly over
time. 1H NMR (C6D6): d=6.80 (s, 2H;
Ar-CH), 5.79 (s, 1H; N-C(O)H-N),
3.42, 3.21, 3.02, 2.94 (m, 1H each; N-
b [8]
90.00
90.00
94.715(2)
90.00
2
CH2-CH2-N), 2.90 (d, JH-H 10.5 Hz,
g [8]
2
1H; N-CHH-C
N
V [ꢂ3]
Z
1330.1(6)
4
1998.79(9)
4
4036.5(6)
4
10.5 Hz, 1H; N-CHH-C
N
(bs, 6H; Ar-ortho-CH3), 2.14 (s, 3H;
Ar-para-CH3), 1.30, 1.11 ppm (s, 3H
each; C
108.6 ppm (N-C(O)H-N); elemental
ACHTUNGTRENNUNG
(CH3)2); 13C NMR (C6D6): d=
analysis calcd (%) for C16H24N2O: C 73.79, H 9.31, N 10.76; found: C
73.90, H 9.36, N 10.63.
(bm, 2H; N-CH2-CH2-N), 3.64 (bm, 2H; N-CH2-CH2-N), 2.29 (bs, 6H;
Ar-ortho-CH3), 2.25 (s, 3H; Ar-para-CH3), 1.45 ppm (s, 6H; C(CH3)2).
13C NMR (C5D5N): d=281.6 ppm (N-C-N); IR(nujol): n˜ =851 cmꢀ1 (O=
U=O); elemental analysis calcd (%) for C32H46N4O4U: C 48.72, H 5.89,
N 7.10; found: C 48.74, H 6.55, N 6.52.
[UO2L2Dipp] 4D: NMR experiment: A solution of 2D (32.9 mg, 0.10 mmol)
in benzene (2 mL) was carefully layered onto a red solution of [UO2N2’’-
ACHTUNGTRENNUNG
[HLDipp] 2D: Colourless solid (5.60 g, 97%). 1H NMR (C6D6): d=7.28–
7.04 (overlapping m, 3H; 3,4,5-Ar-CH), 5.78 (s, 1H; N-C(O)H-N), 4.09
(sept, 3J=7 Hz, 2H; Ar-CH
(CH3)2), 3.53–2.95 (overlapping m, 4H; 2H;
N-CH2-CH2-N), 3.36 (sept, 3J=7 Hz, 2H; Ar-CH
(CH3)2), 2.91, 2.52 (d,
2JHH =11 Hz, 1H each; N-CH2-C), 1.42, 1.30 (d, 3J=7 Hz, 2H each; Ar-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
CH
CH
N
ACHTUNGTRENNUNG
AHCTUNGRTEG(NNNU THF)2] (40.0 mg, 0.05 mmol) in benzene (2 mL) and allowed to slowly
ACHTUNGTRENNUNG
diffuse overnight at room temperature in the glovebox. The solution
turned yellow and 4D formed as yellow needles which were washed with
benzene (3ꢃ1 mL) and dried in vacuo. 1H NMR (C5D5N): d=7.43 (t,
3J=8 Hz, 1H; 4-Ar-CH), 7.27 (d, 3J=8 Hz, 2H; 3,5-Ar-CH), 3.94, 3.87
(m, 2H each; N-CH2-CH2-N), 3.87 (s, 2H; N-CH2-C), 3.43 (sept, 3J=
C(O)H-N); elemental analysis calcd (%) for C19H31N2O: C 75.45, H
10.00, N 9.26; found: C 75.51, H 10.07, N 9.20.
[LiPrYN’’2] 3P: NMR experiment: In a Youngs tap NMR tube, solutions
of YN’’3 (46.4 mg, 0.08 mmol) and 2P (15.0 mg, 0.08 mmol) in C6D6 were
combined and mixed well. The solution turned pale yellow and was
heated to 858C for 24 h.
7 Hz, 2H; Ar-CH
7 Hz, 6H each; Ar-CH
N); IR
(nujol); n˜ =853 cmꢀ1 (asymm-UO2).
G
(CH3)2), 1.28 1.17 ppm (d, 3J=
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
1H NMR (C6D6): d=4.37 (sept, 1H; 3J=7 Hz, N-CH-(CH3)2), 2.97 (s,
Crystallography: Crystallographic X-ray data were collected by using
MoKa radiation (l=0.71073 ꢂ) on a Bruker Smart APEX CCD area de-
tector diffractometer using w, or w and f scans (Table 3). Structure solu-
tion and refinement was carried out using the SIR program, WinGX, and
the SHELXTL suite of programs, and graphics generated using Ortep-3.
The ADPs for C(3) and C(4) (on the iso-propyl group) in 1aP were re-
strained to account for an unmodelled disorder, which results in close
contacts between the hydrogen atoms placed upon these carbons. This is
independent of the method used to place them (geometrically or by elec-
tron density). The hydroxyl hydrogen in 1aP was found in the electron
2H; N-CH2-C), 2.74 (m, 2H; N-CH2-CH2-N), 2.53 (m, 2H; N-CH2-CH2-
N), 1.14 (s, 6H;
CACHTUNGTRENNUNG
(CH3)2), 0.96 (d, 6H; 3J=7 Hz, N-CH-(CH3)2),
0.40 ppm (s, 36H; N(SiACTHNUTRGNEUNG
{CH3}3)2). 13C NMR (C6D6): d=212.3 ppm (d,
1JYC =46 Hz, N-C-N).
[LMesYN’’2] 3M: NMR experiment: In a Youngs tap NMR tube, C6D6 sol-
utions of YN’’3 (43.8 mg, 0.08 mmol) and 2M (20.0 mg, 0.08 mmol) were
combined and mixed well. The resulting solution turned pale yellow after
5–10 minutes. 1H NMR (C6D6): d=6.75 (s, 2H; Ar-CH), 3.10 (s, 2H; N-
CH2-C
2.14 (s, 3H; Ar-para-CH3), 1.22 (s, 6H; C
N(Si
{CH3}3)2); 13C NMR (C6D6): d=215.5 ppm (d, 1JYC =44 Hz, N-C-N).
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ꢀ
difference map and restrained to lie approximately on the O(1) I(1) axis.
ACHTUNGTRENNUNG
Compound 1D contains 0.6 molecules of water in the lattice. Compound
4M crystallised as a racemic mixture of the two enantiomers, with half of
the molecule, and one molecule of benzene, present in the asymmetric
unit. There is a superpositional disorder of the two enantiomers, which
appear superimposed in the asymmetric unit; no higher symmetry or cell-
doubling was found. The superpositional disorder has been modelled
[LDippYN’’2] 3D: To a solution of YN’’3 (0.47 g, 0.82 mmol) in THF (2 mL)
was added a solution of 2D (0.25 g, 0.82 mmol) in THF (2 mL). The reac-
tion mixture was stirred overnight at room temperature to afford a pale
yellow solution. The volatiles were removed in vacuo and slow cooling of
a toluene solution (ꢁ3 mL) to ꢀ308C afforded 3D, as colourless plates
suitable for an X-ray diffraction study (0.30 g, 51%). 1H NMR (C6D6):
d=7.18 (m, 1H; 4-Ar-CH), 7.04 (m, 2H; 3,5-Ar-CH), 3.25 (s, 2H; N-
CH2-C), 3.15 (m, 2H; N-CH2-CH2-N), 3.09 (sept, 3J=7 Hz, 2H; Ar-CH-
with a trans-LD geometry. Several anisotropic displacement parameter
2
restraints, both spatial and rigid-rotor, were required to account for the
superpositional disorder and also a disorder in the mesityl groups and co-
crystallised benzene. There are several close crystallographic contacts be-
tween some hydrogen atoms as a result, and several carbon atoms still
retain higher than desired thermal displacement parameters. Complex 4D
contains two molecules of benzene in the unit cell, disordered about a
special position. This disorder was not modelled, however, resulting in
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
mental analysis calcd (%) for C31H65N4OSi4Y: C 52.36, H 9.21, N 7.88;
found: C 52.27, H 9.28, N 7.73.
ACHTUNGTRENNUNG
ꢀ
short C C bonds across the special position.
AHCTUNGTRENNUNG
0.06 mmol) in benzene (2 mL) and allowed to diffuse slowly overnight at
room temperature. The resulting solution turned yellow and 4M formed
as yellow needles which were washed with benzene (3ꢃ1 mL) and dried
in vacuo (24.0 mg, 52%). Bi-refringent yellow-green crystals suitable for
an X-ray diffraction study were grown from a 5:1 pyridine/benzene mix-
ture by slow cooling of a hot solution to room temperature overnight.
1H NMR (C5D5N): d=6.82 (s, 2H; Ar-CH), 3.84 (bs, 2H; N-CH2-C), 3.75
Acknowledgement
We thank Dr. Ronan Bellabarba and Dr. Robert Tooze of Sasol Technol-
ogy UK Ltd, Purdie Building, North Haugh, St Andrews, Fife, KY16 9ST
Chem. Eur. J. 2008, 14, 10415 – 10422
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10421

P. L. Arnold et al.
(studentship for Z.R.T.), the Royal Society (fellowship for C.D.C), the
EPSRC, the Alexander von Humboldt Foundation, and the University of
Edinburgh for funding. We also thank Dr. Manuel Volpe and Dr. Anna
Collins for help with crystal structure solutions, and Prof. Simon Parsons
for the use of the diffractometer.
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Received: July 1, 2008
Published online: October 1, 2008
10422
www.chemeurj.org
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 10415 – 10422