M. Distaso, E. Quaranta / Journal of Catalysis 253 (2008) 278–288
283
1
by IR spectroscopy. The measured shift, ꢀν(C=O), ranges
of decrease of electron density around these nuclei upon η -
−
1
+
around −100 cm and slightly changes with the solvent used.
ν(C=O) is larger than that observed for coordination com-
pounds of ScX3 (X = NO3, Cl, Br, SCN, ClO4) salts with
O(C=O) coordination of the organic carbonate to Li [52,53].
ꢀ
However, the methoxyl shift (comparable with that of carbonyl)
has not been related merely to the change of net charge on the
−
1
1
+
lactames (ꢀν(C=O) = −39 to −47 cm ) [40] and amides
methoxyl C-atoms of DMC η -O(C=O)-coordinated to Li ,
but has been ascribed also to conformational changes during co-
ordination or to some direct contribution of coordination modes
involving the ester O-atoms [52,53]. In our case, despite the
down field shift observed for OCH3 carbons is even larger than
that of C=O group, the presence in solution of adducts like (A)
−
1
(
ꢀν(C=O) = −23 to −25 cm ) [39] or for adducts of ScCl3
1
−
with ketones (ꢀν(C=O) = −43 to −80 cm ) [38], and is
comparable with that measured for complexes of ScCl3 with es-
−
1
ters of carboxylic acids (ꢀν(C=O) = −90 to −140 cm ) [38].
The shift is also markedly larger than that ascribed to the for-
2
+
3+
mation of (C)-like adducts between DMC and Zn
(from
or (B) (Scheme 1, M = Sc ) can be considered very unlikely,
−
1
+
Zn(O2CCH3)2·2H2O; ν˜(C=O) = 1722 cm ) [22] or Na
as under experimental conditions analogous or comparable with
those used in the NMR experiments, we have never obtained
any IR evidence supporting DMC coordination to scandium
through the ester oxygens (ꢀν(C=O) > 0).
−
1
cations of NaY supercages (ν˜(C=O) = 1747 cm ) [41]. This
trend can be related to the diverse nature of the metal centers
+
2+
3+
(
M = Na , Zn , Sc ) and their different acceptor-strengths
and suggests a stronger donor–acceptor interaction for the cou-
ple Sc(III)–DMC.
3.4. Characterization of the Sc(III)–DMC adduct
3
.3. Coordination of DMC to Sc(OTf)3: 13C NMR studies
The Sc(III)–DMC adduct has been isolated from a DMC so-
lution of Sc(OTf)3, as described in Section 2, and its stoichiom-
13
1
Coordination of DMC to scandium ion modifies also the
etry (1:1) determined by means of quantitative NMR ( C, H,
19
electron density at the carbon nuclei of the organic carbonate
F) analysis.
13
and is expected to shift the C resonances of coordinated DMC
with respect to those of the free molecule. Therefore, the in-
teraction of the organic carbonate with the salt (Sc(OTf)3) has
been investigated also by 13C NMR spectroscopy. C NMR
studies on coordination of DMC or other organic carbonates to
metal centers are relatively few and deal with adducts of such
substrates with lithium ions [52,53].
Fig. 4 compares the IR spectra of the adduct and liquid
DMC (for a detailed analysis of the IR spectrum of liquid DMC
see [33]). The IR spectrum of (DMC)Sc(OTf)3 shows, in addi-
tion to the OTf absorptions (1344, 1238–1202, 1018, 637, 586
13
−
1
and 509 cm ) [42–45], other bands at 1638, 1508, 1458, 1431,
−
1
935, 881, 793 cm . Triflate anion (or ligand) does not absorb
in these regions [42–45] and, therefore, these absorptions can
be attributed to coordinated DMC without any doubt. The posi-
We have found that the 13C NMR (125 MHz, 293 K; ext.
ref.: C6D6) spectrum of a CH2Cl2 solution of Sc(OTf)3 and
DMC (DMC/Sc: 5.7 mol/mol) showed, in addition to the CF3
resonance of OTf, only two signals for DMC, at 157.41 (C=O)
and 56.32 (OCH3) ppm. Both DMC resonances were down
field shifted (ꢀδ = +0.89 and +1.45 ppm, respectively) with
respect to those (156.52 (C=O) and 54.87 ppm (OCH3)) ob-
served in the 13C NMR (125 MHz, 293 K; ext. ref.: C6D6)
spectrum of a CH2Cl2 solution of DMC of equal concentra-
tion. An analogous down field shift was observed for both
−
−
1
1
tions of the signals at 1638 (ν(C=O)) and 1508 (δ(CH3)) cm
agree well with those of the bands at 1646 and 1504 cm
observed in CH2Cl2 solution (spectrum (c) in Fig. 3). The in-
spection of Fig. 4 (see also [33]) allows to locate the signals
associated with the νasymm(O–CH3) and νsymm(O–CH3) vibra-
tional modes of the adduct, which are found, respectively, at
−1
1
935 and 881 cm . As expected (see Section 3.2), η -O(C=O)
coordination of the organic carbonate to the metal center shifts
−
1
to red not only ν(C=O) (1759 cm for liquid DMC), but also
13
DMC resonances (C=O and OCH3) also in the C NMR
both asymmetric and symmetric CH3–O stretching frequencies
−
1
(
75 MHz, 293 K; ext. ref.: acetone-d6) spectrum of a CH3CN
(respectively at 970 and 914 cm for liquid DMC). Unfortu-
nately, we cannot locate the absorptions due to the asymmetric
and symmetric ν(OCO) vibrations. These bands, which are ob-
served, respectively, at 1277 (vs) and 1117 (w) cm in the
spectrum of liquid DMC, are expected to be blue-shifted in the
adduct and, most likely, are masked by the strong absorptions
solution of Sc(OTf)3 and DMC (DMC/Sc: 11.9 mol/mol).
However, with respect to a CH3CN solution of DMC of
equal concentration, the magnitude of shifts was, in this case,
more modest (ꢀδ = +0.20 ppm (C=O) and +0.32 ppm
−1
(OCH3)).
The shifts observed for both the 13C signals of DMC in the
due to OTf groups at 1342 and 1238–1202 cm
−1
.
presence of Sc(OTf)3 further confirm the ability of the organic
carbonate to interact with the salt in the solvents used. The pres-
ence of a unique set of DMC resonances for both coordinated
and free DMC indicates that, under the working conditions
The complex is extremely hygroscopic and, as established
by IR spectroscopy, rapidly decomposes upon exposure to air,
even for short times (≈ 1 min). In solution, in the absence of
any excess of DMC, the stability of the adduct is very modest
as the complex easily looses coordinated DMC. For instance,
upon treating the complex with anhydrous CH2Cl2, a white
solid separated from the solution, and the IR spectrum of the
solution (similar to spectrum (c) reported in Fig. 3) showed,
(
293 K), free and coordinated DMC molecules exchange fast
with respect to the chemical shift time scale. The above findings
agree well with those reported in the literature for adducts of
DMC or other organic carbonates with lithium ions [52,53]. The
interaction of DMC with lithium ion deshields both carbonyl
and methoxyl carbon nuclei of DMC [52], mainly because
−
1
in addition to the bands at 1646 and 1505 cm , a strong ab-
−
1
sorption at 1752 cm due to free DMC. This behavior can