Near-IR luminescent neodymium complexes: Spectroscopic probes for hydroamination catalysis

Article abstract of DOI:10.1039/c3cc42923g

Neodymium complexes bearing the sensitising bis(oxazolinylphenyl)amine (BOPA) ligands have been prepared, and analysed spectroscopically under both catalytic and pseudo-catalytic conditions with respect to the intramolecular hydroamination of an aminoalke

Full text of DOI:10.1039/c3cc42923g

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Near-IR luminescent neodymium complexes:
spectroscopic probes for hydroamination catalysis†
Cite this: Chem. Commun., 2013,
49, 6072
Stacey D. Bennett, Simon J. A. Pope and Benjamin D. Ward*
Received 19th April 2013,
Accepted 24th May 2013
DOI: 10.1039/c3cc42923g
www.rsc.org/chemcomm
Neodymium complexes bearing the sensitising bis(oxazolinylphenyl)-
amine (BOPA) ligands have been prepared, and analysed spectroscopi-
cally under both catalytic and pseudo-catalytic conditions with respect
to the intramolecular hydroamination of an aminoalkene, providing a
direct means of monitoring binding events and relative space around
the metal centre.
Rare-earth metal complexes have many applications in catalysis,
including hydroamination.1 The presence of partially occupied, low-
lying 4f orbitals also gives them unique spectroscopic properties.
The formally forbidden f–f transitions are notably sharp compared
Scheme 1 Preparation of [Nd(R-BOPA){N(SiMe3)2}2] 1a–c.
to many other fluorescence systems, and the long near-IR wave- such that the resultant near-IR photophysical properties can be used
length provides an emission profile that is largely free from envir- to probe the complexes under conditions relevant to catalytic reac-
onmentally emissive ‘pollution’.2
tions. In their anionic form, these ligands possess conjugated amides
The spectroscopic fingerprint of a luminescent lanthanide ion coordinated to the lanthanide in an ideal position to aid investiga-
(i.e. spectral profile and lifetime of f-centred emission) is uniquely tions in to the luminescence properties of the species.
sensitive to the local coordination environment.3 In particular, the
The protio-ligands react with the neodymium precursor
lifetime provides information on the relative shielding of the ion [Nd{N(SiMe3)2}3] to afford the five-coordinate complexes
from bound and unbound proximate quenchers, such as O–H, N–H [Nd(R-BOPA){N(SiMe3)2}2] (R = iPr 1a, Ph 1b, Bn 1c) (Scheme 1).
and C–H oscillators,4 present in solvents, (bound) substrates, ligand The 1H NMR data of the paramagnetic complexes were found to give
architectures, and/or the proximity of stereodirecting groups. This relatively sharp signals with an expected reduction in fine structure
parameter may therefore provide reportage on the space at the metal and an increase in chemical shift range (ca. 20 to ꢀ40 ppm).7 The
centre in which catalytic reactions take place, and may provide overall molecular symmetry, number of resonances, and their
critical information relating to the short-lived species that lie beyond relative intensity are fully consistent with the proposed structures.
the detection limit of many other analytical techniques.5
The resonance attributed to the two equivalent N(SiMe3)2 ligands is a
Our goal in this study was to use the luminescent properties of distinctive, large singlet in the range of ꢀ5.96 to ꢀ5.93 ppm. The
Nd(III) to probe the intricate nature of chiral complexes, providing structure of [Nd(Ph-BOPA){N(SiMe3)2}2] 1b has been confirmed by
information that cannot be obtained using other spectroscopic X-ray crystallography; X-ray data are included in the ESI.†8
techniques, and to relate this information to their behaviour in
hydroamination/cyclisation catalysis.
Complexes 1a–c were employed in the catalytic hydroamination of
aminoalkenes (vide infra). The catalyst resting state for this reaction (i.e.
The bis(oxazolinylphenyl)amine (BOPA) series of ligands6 are able a tethered amidoalkene) is well understood,9 but cannot be isolated
to efficiently sensitise the metal-centred excited states of neodymium, owing to the intramolecular nature of the reaction. Therefore model
complexes were prepared that suppress the catalytic turnover by
School of Chemistry, Cardiff University, Main Building, Park Place,
Cardiff CF10 3AT, UK. E-mail: WardBD@Cardiff.ac.uk; Fax: +44 (0)29 208 74030;
Tel: +44 (0)29 208 70302
removing the CQC from the substrate. The resulting complexes were
prepared using the pseudo substrate 2,20-diphenyl-aminopentane, to
afford [Nd(R-BOPA){NHCH2C(Ph2)Pr}2] (R = iPr 2a, Ph 2b, Bn 2c)
according to Scheme 2. The 1H NMR data of complexes 3a–c showed
similar features to the spectra of 1a–c in terms of the overall C2
symmetry of the complexes. In comparison to the data obtained for
† Electronic supplementary information (ESI) available: Full experimental proce-
dures, crystal, computational and characterising data for all new complexes.
CCDC 903609. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c3cc42923g
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Table 1 Lifetime measurements for [Nd(R-BOPA){N(SiMe3)2}2] and related com-
plexes (toluene, N2; lex = 355 nm; lem = 1055 nm)
Lifetimes (t)/ns
+2,2-Diphenyl-
aminopentane
+2,2-Diphenyl-
aminopentene
Compound
1a
1b
1c
148, 621 (70%)
129, 677 (90%)
41, 308 (80%)
111, 267 (59%) (2a)
71, 165 (90%) (2b)
41, 182 (93%) (2c)
—
—
81, 252 (85%)
Scheme 2 Preparation of [Nd(R-BOPA){NHCH2C(Ph)2Pr)}2] 2a–c.
complexes [Nd(R-BOPA){(N(SiMe3)2}2] (1a–c) exists in (at least) two
distinct environments. The fact that the analytical and NMR data
suggest a single pure complex suggests that these species are likely
to be interconverting isomers.
The lifetimes of the two species are remarkably different, and
suggest that the main component represents a well-shielded Nd(III)
centre, possibly representative of the X-ray structure. The second
component (10–30% relative weighting) has a significantly shorter
Scheme 3 Catalytic hydroamination using [Nd(R-BOPA){N(SiMe3)2}2].
complexes 1a–c the signal assigned to the silylamide co-ligands was lifetime and suggests a species that is more readily quenched (i.e. by
absent from the spectra of 2a–c. proximate C–H oscillators), possibly arising from a ligand conforma-
The spectroscopic analysis of these complexes can only provide tion that renders the metal more accessible to solvent molecules.
adequate information relating to their catalytic performance if sup- The lifetimes for the isopropyl and phenyl derivatives 1a and 1b are
ported by a clear demonstration of their implementation in the catalytic similar, suggesting a comparable environment around the Nd(III)
reaction being studied. Consequently, complexes 1a–c were employed centre, whereas the lifetimes for the benzyl derivative 1c are shorter,
in the hydroamination/cyclisation of 2,20-diphenyl-amino-pent-4-ene implying less effective shielding. This may originate in the greater
in toluene-d8 using 10 mol% catalyst at 25 1C (Scheme 3).10 All flexibility of a benzyl (at the methylene group) relative to phenyl and
reactions proceeded to quantitative conversion and with moderate isopropyl groups, which are generally more rigid.
enantioselectivities. The phenyl and benzyl derivatives 1b and 1c gave
Importantly, the relative intensity of the emission bands of Nd(III)
essentially identical enantiomeric excesses of 44–46%, whereas the are known to be sensitive to the local coordination environment,3,13
isopropyl congener 1a gave a lower level of selectivity (36%). These suggesting that changes in either ligand type or ligand arrangement will
moderate levels of selectivity can be reasonably explained by the modulate the emission profile of a luminescent Nd(III) complex. In
presence of two isomeric forms of the complexes (which the lumines- this context, addition of the saturated pseudo-substrate (2,20-diphenyl-
cence studies would suggest pertain during the catalytic reaction and aminopentane) to form 2a–c, subsequently showed a definitive and
that have been identified computationally, vide infra), particularly where measurable change in spectral emission profile (in terms of peak
one of the components moves the stereodirecting groups away from position, profile and integrated intensity, Fig. 1), as well as significant
the expected site of alkene insertion.
alterations in Nd(III) lifetime. Such observations are clearly indicative of
Solutions of 1a–c (toluene, 10ꢀ4 M, N2 atmosphere) were inves- changes in the coordination environment and thus consistent with the
tigated from a photophysical standpoint and are described as dual bis(trimethylsilyl)amide ligands exchanging with the 2,20-diphenyl-
emissive. Irradiation (lex = 485 nm) induced visible emission from amino-pentane ligands, establishing that coordination to the Nd(III)
the complexes, typically characterised by a broad, unstructured must occur via the terminal amino group of the pseudo-substrate. The
luminescence band peaking between 500–510 nm with a small dominant lifetime components for 2a–c are significantly shortened
associated Stokes’ shift (ca. 1000 cmꢀ1).11 The precise positioning compared to 1a–c and consistent with a ligand substitution that would
of the band and the fluorescence lifetime (5.0, 4.6 and 4.5 ns, for introduce N–H oscillators into the coordination sphere and reduce the
i
R = Pr, Ph and Bn respectively) were subtly dependent on the steric crowding at Nd(III).
specific composition of the ligand. The diphenylamine anion is
During the catalytic hydroamination reaction, the proposed
known to be non-emissive,12 and therefore the visible fluores- cyclised intermediate is coordinated to Nd(III) via a methylene unit.9
cence was attributed to a ligand-centred excited state with a Therefore the actual catalytic substrate, 2,20-diphenyl-amino-pent-4-
contributing charge transfer (N-to-p*) component originating ene, was also used with 2c and, again, revealed modulation of the
from the deprotonated/coordinated amide bridge-head.
emission profile and shortening of the dominant lifetime compo-
Irradiation of solutions 1a–c at 445 nm also induced sensitised nent to 252 ns. Again, these observations are consistent with a
near-IR emission from each of the Nd(III) complexes, with character- substrate binding event, but one that is distinct from the pseudo-
4
4
4
istic bands attributed to the F3/2–4I9/2, F3/2–4I11/2 and F3/2–4I13/2 substrate. The lifetime data for this mixture can therefore be
transitions. The corresponding lifetime measurements were also differentiated from both the free complex (1c) and the speciation
obtained (lem = 1055 nm) and are shown in Table 1; the magnitude of the saturated substrate analogue (2c).
of the values are consistent with Nd(III)-centred emission, and each
The identity of the two components of 2a–c (and by
profile fitted reasonably well to a bi-exponential decay yielding two extension 1a–c) was probed by calculating the structures of
distinct lifetime values. These data suggest that each of the the La(III) congeners; the structures of La-2b are shown in Fig. 2.
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supports this theory; however this does not contradict the effects
of varying the size of the stereodirecting group. Rather, these
studies show evidence of additional processes taking place at the
molecular level that contribute to the overall effect of controlling
stereoselectivity in catalysis.
The photophysics of luminescent neodymium complexes have
been used to directly probe the metal species in the catalytic system,
with emission profiles before, during, and after giving distinct
luminescence signatures. The luminescence studies indicate the
presence of different conformations of the complex in solution that
are not apparent using NMR spectroscopy and X-ray crystallography.
We propose that the presence of these conformations is a key aspect
for explaining the relative stereocontrol offered by the different
catalyst derivatives.
Fig. 1 Near-IR luminescence spectra (10ꢀ4 M toluene, lex 445 nm). Main: 1c (green),
2c (orange), 1c + 2,20-diphenyl-amino-pentene (grey). Inset: comparison 1a/2a (light
blue/dark blue) and 1b/2b (red/brown).
We thank Cardiff University (Endowment Fellowship to SDB
and access to computing facilities ‘‘ARCCA’’), the Leverhulme
Trust, and the EPSRC crystallographic service for support.
Notes and references
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Fig. 2 Calculated exo (left) and endo (right) isomers of La-2b.
Calculations suggest that each of the complexes exist in two
energetically accessible forms, each with a different sense of
helical twist in the diphenyl backbone of the BOPA ligands, giving
both exo and endo isomers with respect to the orientation of the
stereodirecting groups. The exo isomer is consistent with the X-ray
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and therefore is expected to give less energetic distinction between
the isomeric forms, as observed experimentally by luminescence
spectroscopy (Table 1).
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6074 Chem. Commun., 2013, 49, 6072--6074
This journal is The Royal Society of Chemistry 2013