Thermally stable rare earth dialkyl complexes supported by a novel bis(phosphinimine)pyrrole ligand

Article abstract of DOI:10.1039/c2dt12485h

A novel bis(phosphinimine)pyrrole based ligand (HL) and its synthesis are reported. Rare earth dialkyl complexes of the ligand, LLn(CH ₂SiMe₃)₂ (Ln = Er, Lu, Sc), have been prepared and found to exhibit high thermal stability in solution. The protio-ligand and dialkyl lanthanide complexes (Ln = Er, Lu) have been characterised by single crystal X-ray diffraction studies.

Full text of DOI:10.1039/c2dt12485h

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COMMUNICATION
Thermally stable rare earth dialkyl complexes supported by a novel
bis(phosphinimine)pyrrole ligand†
Kevin R. D. Johnson, Matt A. Hannon, Jamie S. Ritch and Paul G. Hayes*
Received 23rd December 2011, Accepted 29th January 2012
DOI: 10.1039/c2dt12485h
to a metal instead formed two five-membered rings, the ligand
A novel bis(phosphinimine)pyrrole based ligand (HL) and its
synthesis are reported. Rare earth dialkyl complexes of the
ligand, LLn(CH2SiMe3)2 (Ln = Er, Lu, Sc), have been pre-
pared and found to exhibit high thermal stability in solution.
The protio-ligand and dialkyl lanthanide complexes (Ln =
Er, Lu) have been characterised by single crystal X-ray dif-
fraction studies.
bite angle would be enlarged.7 To this end, we have incorporated
pyrrole into the ligand framework in place of carbazole.
Synthesis of the target bis(phosphinimine)pyrrole ligand was
achieved in high yield over 3 steps from the N-Boc protected
derivative of 2,5-dibromopyrrole, 3,8 as outlined in Scheme 2.
Lithium halogen exchange of 3 with nBuLi in THF,9 followed by
reaction with chlorodiphenylphosphine, afforded the Boc-pro-
tected bis(phosphine), 4. Thermal removal of the Boc group was
performed at 155 °C to liberate 2,5-bis(diphenylphosphino)-1H-
pyrrole, 5. Finally, the phosphinimine functionality was installed
via a Staudinger reaction between 5 and para-isopropylphenyl
(Pipp) azide, with concomitant loss of N2, to afford 6 in 60%
overall yield.
The last decade has been witness to a surge in the development
of non-carbocyclic scaffolds for supporting highly reactive rare
earth metal species.1 The novel ligands that have arisen through-
out this “post-metallocene” era have often proven to be attractive
frameworks for catalytic transformations due to their high degree
of tunability. This inherent property allows for rational and con-
venient adjustment of ligand steric and/or electronic properties
and, hence, reaction chemistry of the resultant metal complexes.
Of particular interest to us is the preparation of rare earth alkyl
complexes stabilized by non-carbocyclic ancillaries as they often
play prominent roles as catalysts or pre-catalysts in important
industrial transformations, such as olefin polymerisation,2
lactone polymerisation3 and hydroamination.4
Protio-ligand 6 contains chemically equivalent phosphorus
nuclei in solution and exhibits a single resonance in its 31P{1H}
1
NMR spectrum at δ −8.1 (benzene-d6). The H and 13C{1H}
We recently reported a family of bis(phosphinimine)carbazole
ligands and their utility in the synthesis of organolutetium com-
plexes, e.g. 1.5 Unfortunately, dialkyl lanthanide complexes sup-
ported by this ligand were found to be highly thermally sensitive
and, as a result, susceptible to a rapid intramolecular metalative
alkane elimination process. The resultant products of this metala-
tive degradation were lutetium complexes that were coordinated
by the ligand in a κ5 bonding mode via three nitrogen atoms and
two ortho-metalated P-phenyl rings, 2 (Scheme 1).5 Addition-
ally, we have recently demonstrated that this ligand framework is
susceptible to ortho-metalation of the N-aryl rings.6
Scheme 1 Double metalative alkane elimination reaction of 1.
In an attempt to mitigate the ortho-metalation difficulties
encountered with our bis(phosphinimine)carbazole pincer, we
elected to modify the geometry of the ligand. As seen in 1, tri-
dentate coordination of the original scaffold forms two six-mem-
bered chelate rings. As such, it was reasoned that if coordination
Department of Chemistry and Biochemistry, University of Lethbridge,
4401 University Drive, Lethbridge, AB, Canada, T1K 3M4. E-mail:
p.hayes@uleth.ca; Fax: +1 403 329 2057; Tel: +1 403 329 2313
†Electronic supplementary information (ESI) available: experimental
details pertaining to the crystallography, synthesis and characterisation
of all new compounds. CCDC 861266–861268. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c2dt12485h
Scheme 2 Synthesis of bis(phosphinimine)pyrrole ligand 6.
Dalton Trans., 2012, 41, 7873–7875 | 7873
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Scheme 3 The synthesis of dialkyl rare earth complexes.
Fig. 1 Thermal ellipsoid plot (50% probability) of HL (6) with hydro-
gen atoms (except H1A) omitted for clarity. Disordered atoms are
depicted as spheres of an arbitrary radius.
NMR spectra corroborate the expected ligand connectivity.
Single crystals of 6 suitable for an X-ray diffraction experiment
were obtained from a concentrated toluene solution at ambient
temperature and the molecular structure is depicted in Fig. 1 as a
thermal ellipsoid plot.10
Tridentate pincer ligand 6 has been designed to chelate metals
in a meridional fashion via N1, N2 and N3. As such, when coor-
dinated to a metal, it would be expected that the pincer arms (N2
and N3) would occupy a common plane with the pyrrole back-
bone. In the solid-state, N2 is close to the plane of the pyrrole
backbone (N2–P1–C1–N1 torsion angle = 23.3(3)°). Conversely,
N3 lies significantly further out of this plane with an N3–P2–
C4–N1 torsion angle of 54.6(3)°. This rotation is influenced by
intermolecular contacts in the crystal structure.11 Other metrical
parameters of interest in ligand 6 include the phosphinimine P–
N bond lengths of 1.562(2) Å (P(1)–N(2)) and 1.572(2) Å (P
(2)–N(3)). These values correlate well with other examples in
the literature whereby the phosphinimine functionality exhibits a
strong PvN double bond character.5,12
Protio-ligand 6 reacts readily with organometallic reagents at
ambient temperature. For example, dialkyl lanthanide complexes
of the ligand were prepared via an alkane elimination reaction of
6 (HL) with Ln(CH2SiMe3)3(THF)2 (Ln = Er, Lu, Sc;
Scheme 3). When these reactions were followed in situ on an
NMR tube scale in benzene-d6, they proceeded rapidly with for-
mation of the desired metal dialkyl complexes, one equivalent of
SiMe4 and two equivalents of liberated THF. Upon scale-up of
the reactions in a toluene solution, the dialkyl products,
LLn(CH2SiMe3)2 (Ln = Er, 7a; Lu, 7b; Sc, 7c), were obtained in
high yield (81%, 82% and 81%, respectively) after
recrystallization.
Fig. 2 A thermal ellipsoid plot (50% probability) of LLu(CH2SiMe3)2
(7b) with hydrogen atoms omitted for clarity. The solid-state structure of
LEr(CH2SiMe3)2 (7a) is isostructural to that of 7b. Disordered atoms are
depicted as spheres of an arbitrary radius.
methyl signals (δ 0.18, 7b; 0.11, 7c) as sharp singlets, integrating
as 4H and 18H, respectively.
Notably, the dialkyl rare earth complexes were all base-free
and the lutetium and scandium derivatives exhibited high
thermal stability in both the solid and solution states. For
example, complexes 7b and 7c can be left in benzene-d6 solution
at ambient temperature for at least one week with no apparent
decomposition (as evidenced by 1H and 31P NMR spectroscopy).
This is a dramatic improvement compared to the analogous car-
bazole-based dialkyl lutetium complex 1 that exhibited a half-
life at ambient temperature (22.6 °C) of only 1160 s.5 Perhaps
even more significant is the fact that preliminary experiments
indicate 7b is stable in solution at 60 °C with no sign of
decomposition over 4.5 hours.
In order to unambiguously establish the connectivity of the
rare earth dialkyl complexes, X-ray diffraction analyses were per-
formed. Single crystals of 7a and 7b were obtained from concen-
trated toluene–THF mixtures and their molecular structures were
determined.13 Complexes 7a and 7b were found to be isostruc-
tural; a representative thermal ellipsoid plot of 7b is depicted in
Fig. 2. While we did not obtain a solid-state structure of 7c, it is
reasonable to assume that it would possess an isostructural geo-
metry to that of the lutetium and erbium derivatives.
In the 31P{1H} NMR spectra (benzene-d6), a significant
downfield shift of the phosphinimine resonance was observed
upon complexation of 6 with diamagnetic rare earth metals (7b δ
25.0; 7c, δ 23.8). Despite the paramagnetic nature of erbium, the
broad 31P{1H} NMR resonance of 7a (δ −0.29, benzene-d6) also
proved to be a diagnostic indicator of ligand coordination. The
1H NMR spectra (benzene-d6) of 7b and 7c revealed the corre-
sponding –CH2SiMe3 methylene (δ −0.20, 7b; 0.55, 7c) and
The lanthanide complexes 7a and 7b are monomeric with
five-coordinate metal centres that are bound to the pincer ligand
in a κ3 fashion through the three nitrogen atoms. The geometry
is best described as distorted trigonal bipyramidal with the
7874 | Dalton Trans., 2012, 41, 7873–7875
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equatorial plane defined by N2 and the alkyl groups (C47 and
C51). The phosphinimine donors (N1 and N3) occupy the apical
sites. The bond angles about the equatorial plane in each
complex are close to the ideal value of 120° (N2–Er1–C47 =
115.8(1)°, N2–Er1–C51 = 117.0(1)°, C47–Er1–C51 = 127.3(2)°,
7a; N2–Lu1–C47 = 117.2(1)°, N2–Lu1–C51 = 116.0(1)°, C47–
Lu1–C51 = 126.8(1)°, 7b); however, the apical bond angle (N1–
Ln–N3) deviates significantly from 180° (142.4(1)°, 7a; 144.3
(1)°, 7b). The Ln–C–Si bond angles fall within the normal range
for rare earth trimethylsilylmethyl complexes (129.5(3)°, 136.6
(2)°, 7a; 130.7(2)°, 136.9(2)°, 7b). The Er–C bond lengths in
7a (2.375(6) Å and 2.397(5) Å) agree well with other recently
structurally characterised organoerbium complexes, such as
(nacnac)Er(CH2SiMe3)2 (2.342(3) Å and 2.380(2) Å)14 and
(Czx)Er(CH2SiMe3)2 (2.398(3) and 2.404(3) Å),15 where nacnac
= 2,6-iPr-C6H3 substituted β-diketiminate and Czx = carbazole-
bis(oxazoline). Complex 7b exhibits slightly shorter Lu–C bond
lengths (2.347(4) Å and 2.355(4) Å) than the corresponding con-
tacts in the erbium congener, but the distances fall within the
range expected for typical Lu–CH2SiMe3 bonds.16
In 7a and 7b, the metal sits in the centre of the ancillary
ligand binding pocket. Both of the complexes exhibit Ln–pyrrole
distances that are significantly shorter than the Ln–phosphini-
mine lengths (Er1–N2 = 2.338(3) Å cf. Er1–N1 = 2.360(3) Å
and Er1–N3 = 2.396(3) Å, 7a; Lu1–N2 = 2.297(2) Å cf. Lu1–
N3 = 2.332(3) Å and Lu1–N1 = 2.364(2) Å, 7b). The P–N
bonds in the dialkyl complexes (ranging from 1.606(4) to 1.610
(3) Å) are elongated by ca. 3% compared to those in the free
protio-ligand, suggesting strong donation from the phosphini-
mine functionality to the rare earth metal.
Preliminary reactivity studies of 7b (on an NMR tube scale in
benzene-d6) have revealed rich reaction chemistry. For instance,
addition of one equivalent of the oxonium acid, [H(OEt2)2]+
[B(C6F5)4]−, 17 to 7b proceeded at ambient temperature over
4.5 h to liberate the expected cationic species as a diethyl ether
adduct, [LLu(CH2SiMe3)(OEt2)2]+[B(C6F5)4]−, 8, with loss of
one equivalent of SiMe4. Alternatively, reaction of 7b with one
equivalent of Mes*NH2 (Mes* = 2,4,6-tBu3-C6H2) in the pres-
ence of 4-dimethylaminopyridine (DMAP) at 100 °C (over
1.5 h) resulted in the clean formation of the DMAP adduct of the
mixed alkyl–anilide complex, LLu(CH2SiMe3)(NHMes*)
(DMAP), 9. The neutral (7a–c) and cationic (8) complexes are
of interest to us as catalysts for various applications (e.g. lactone
and olefin polymerisation) and we are currently evaluating their
efficacy for mediating such processes. We are also interested in
exploring the chemistry of 9 as it may provide fundamental
insight into the structure and reactivity of lanthanide alkyl–
anilide complexes, as well as serving as a useful hydroamination
catalyst precursor.
this ligand in that it can readily be complexed to lanthanide
metals via an alkane elimination protocol to generate thermally
robust rare earth dialkyl species. Current efforts are underway to
investigate the small molecule reactivity of complexes 7a–c in
order to exploit the full range of their utility.
Acknowledgements
This research was financially supported by the Natural Sciences
and Engineering Research Council (NSERC) of Canada and the
Canada Foundation for Innovation. The authors thank Dr. Craig
Wheaton and Mr. Amin Moazeni for performing elemental ana-
lyses of the new compounds.
Notes and references
1 (a) F. T. Edelmann, D. M. M. Freckmann and H. Schumann, Chem. Rev.,
2002, 102, 1851; (b) P. Mountford and B. D. Ward, Chem. Commun.,
2003, 1797; (c) W. E. Piers and D. J. H. Emslie, Coord. Chem. Rev.,
2002, 233–234, 131.
2 (a) J. Gromada, J.-F. Carpentier and A. Mortreux, Coord. Chem. Rev.,
2004, 248, 397; (b) Y. Nakayama and H. Yasuda, J. Organomet. Chem.,
2004, 689, 4489.
3 W. Gao, D. Cui, X. Liu, Y. Zhang and Y. Mu, Organometallics, 2008, 27,
5889.
4 (a) S. Hong and T. J. Marks, Acc. Chem. Res., 2004, 37, 673; (b) T.
E. Müller, K. C. Hultzsch, M. Yus, F. Foubelo and M. Tada, Chem. Rev.,
2008, 108, 3795.
5 K. R. D. Johnson and P. G. Hayes, Organometallics, 2009, 28, 6352.
6 K. R. D. Johnson and P. G. Hayes, Organometallics, 2011, 30, 58.
7 G. J. P. Britovsek, V. C. Gibson, O. D. Hoarau, S. K. Spitzmesser,
A. J. P. White and D. J. Williams, Inorg. Chem., 2003, 42, 3454.
8 L. Groenendaal, H. W. I. Peerlings, J. L. J. van Dongen, E. E. Havinga,
J. A. J. M. Vekemans and E. W. Meijer, Macromolecules, 1995, 28, 116.
9 W. Chen and M. P. Cava, Tetrahedron Lett., 1987, 28, 6025.
ˉ
10 Structure 6: C46H45N3P2, P1, a = 12.9263(2) Å, b = 14.5326(2) Å, c =
14.8331(3) Å, α = 112.587(1)°, β = 103.521(1)°, γ = 108.770(1)°, N =
36845, Nind 10 171, R1 = 0.0709 (I > 2σ(I)) and wR2 = 0.1723 (I >
2σ(I)), GOF = 1.045.
11 In the solid-state, 6 assembles into centrosymmetric hydrogen-bonded
pairs, with the pyrrole N–H on each molecule associating with an imine
nitrogen (N3) on the other (d(N⋯N) = 2.851(3) Å).
12 K. D. Conroy, W. E. Piers and M. Parvez, J. Organomet. Chem., 2008,
693, 834.
ˉ
13 Structure 7a: C54H66ErN3P2Si2, P1, a = 9.729(5) Å, b = 12.188(6) Å, c =
24.287(11) Å, α = 84.796(5)°, β = 78.920(5)°, γ = 69.550(5)°, N =
34897, Nind 10 778, R1 = 0.0365 (I > 2σ(I)) and wR2 = 0.0818 (I >
ˉ
2σ(I)), GOF = 1.022. Structure 7b: C54H66LuN3P2Si2, P1, a = 9.7163(6)
Å, b = 12.1266(7) Å, c = 24.2569(14) Å, α = 84.9530(10)°, β = 78.9220
(10)°, γ = 69.4640(10)°, N = 37480, Nind 11 526, R1 = 0.0300 (I > 2σ(I))
and wR2 = 0.0721 (I > 2σ(I)), GOF = 1.023.
14 K. R. D. Johnson, A. P. Côté and P. G. Hayes, J. Organomet. Chem.,
2010, 695, 2747.
15 J. Zou, D. J. Berg, D. Stuart, R. McDonald and B. Twamley, Organome-
tallics, 2011, 30, 4958.
16 An analysis of 47 entries in the Cambridge Structural Database (CSD
version 5.32, updated Aug. 2011) for neutral bis(trimethylsilylmethyl)
lutetium complexes of the generic form (L)nLu(CH2SiMe3)2 suggested an
average Lu–CH2SiMe3 bond length of 2.35 Å (range = 2.293–2.406 Å).
17 P. Jutzi, C. Müller, A. Stammler and H.-G. Stammler, Organometallics,
2000, 19, 1442.
In conclusion, we have prepared and characterised a new
ancillary ligand comprised of a modular bis(phosphinimine)
pyrrole framework (6). We have demonstrated the versatility of
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 7873–7875 | 7875