Rigid NON-donor pincer ligand complexes of lutetium and lanthanum: Synthesis and hydroamination catalysis

Article abstract of DOI:10.1039/c7ra04432a

Reaction of H₂XN₂ {4,5-bis(2,4,6-triisopropylanilino)-2,7-di-tert-butyl-9,9-dimethylxanthene} with [Lu(CH₂SiMe₃)₃(THF)₂], and crystallization from O(SiMe₃)₂, yielded [(XN₂)Lu(CH₂SiMe₃)(THF)]·(O(SiMe₃)₂)_1.5 (1·(O(SiMe₃)₂)_1.5). Lanthanum complexes of the XN₂ dianion were also prepared by salt metathesis; treatment of H₂XN₂ with excess KH in DME produced the dipotassium salt, [K₂(XN₂)(DME)_x] (x = 2-2.5), and subsequent reaction with [LaCl₃(THF)₃] afforded [{(XN₂)LaCl(THF)}_x]·(O(SiMe₃)₂)_0.25x (2·(O(SiMe₃)₂)_0.25x; x = 1 or 2) after crystallization from O(SiMe₃)₂. Compound 2 reacted with two equivalents of LiCH₂SiMe₃, to form the dialkyl-'ate' complex, [Li(THF)_x][(XN₂)La(CH₂SiMe₃)₂]·Toluene·LiCl (3·toluene·LiCl; x = 3). Both 1 and 3 (x = 4) were structurally characterized, and were evaluated as catalysts for intramolecular hydroamination; while 3 showed poor activity, 1 is highly active for both intramolecular hydroamination and more challenging intermolecular hydroamination. Reactions with unsymmetrical alkenes yielded Markovnikov products, and the activity of 1 surpassed that of the previously reported yttrium analogue in the reaction of diphenylacetylene with 4-tert-butylbenzylamine.

Full text of DOI:10.1039/c7ra04432a

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PAPER
Rigid NON-donor pincer ligand complexes of
lutetium and lanthanum: synthesis and
Cite this: RSC Adv., 2017, 7, 27938
hydroamination catalysis†
a
Kelly S. A. Motolko,a David J. H. Emslie
and James F. Brittenb
*
Reaction of H2XN2 {4,5-bis(2,4,6-triisopropylanilino)-2,7-di-tert-butyl-9,9-dimethylxanthene} with
[Lu(CH2SiMe3)3(THF)2], and crystallization from O(SiMe3)2, yielded [(XN2)Lu(CH2SiMe3)(THF)]$(O(SiMe3)2)1.5
(1$(O(SiMe3)2)1.5). Lanthanum complexes of the XN2 dianion were also prepared by salt metathesis;
treatment of H2XN2 with excess KH in DME produced the dipotassium salt, [K2(XN2)(DME)x] (x ¼ 2–2.5),
and
subsequent
reaction
with
[LaCl3(THF)3]
afforded
[{(XN2)LaCl(THF)}x]$(O(SiMe3)2)0.25x
(2$(O(SiMe3)2)0.25x; x ¼ 1 or 2) after crystallization from O(SiMe3)2. Compound 2 reacted with two
equivalents of LiCH2SiMe3, to form the dialkyl-‘ate’ complex, [Li(THF)x][(XN2)La(CH2SiMe3)2]$Toluene$LiCl
(3$toluene$LiCl; x ¼ 3). Both 1 and 3 (x ¼ 4) were structurally characterized, and were evaluated as
catalysts for intramolecular hydroamination; while 3 showed poor activity, 1 is highly active for both
intramolecular hydroamination and more challenging intermolecular hydroamination. Reactions with
unsymmetrical alkenes yielded Markovnikov products, and the activity of 1 surpassed that of the
previously reported yttrium analogue in the reaction of diphenylacetylene with 4-tert-butylbenzylamine.
Received 19th April 2017
Accepted 18th May 2017
DOI: 10.1039/c7ra04432a
rsc.li/rsc-advances
including rare earth ansa-cyclopentadienyl complexes (a in
Fig. 1) developed by Marks et. al.,6,7 yttrium complexes of both
bidentate and tridentate 1,10-binaphthalene-backbone ligands
1 Introduction
A variety of rare earth alkyl and amido complexes have
demonstrated high activity for alkene and alkyne hydro-
amination catalysis, typically exceeding that achievable with
transition metal catalysts.1 However, the ease with which
hydroamination reactions occur varies greatly as a function of
the substrate. In particular, intramolecular hydroamination
reactions are much more facile than intermolecular variants,
and intramolecular hydroamination reactions are generally
more favorable for (a) alkynes vs. alkenes, (b) substrates leading
to 5- vs. 6- vs. 7-membered ring formation, (c) 1-aminoalkenes
and 1-aminoalkynes di-substituted at the 2-position (Thorpe–
Ingold effect; bulkier phenyl substituents are more effective
than methyl substituents), and (d) aminoalkenes without
additional alkene substitution (e.g. H2NCH2CPh2CH2CR]CHR0
where R ¼ R0 ¼ H).1–5 Intermolecular reactions with unactivated
alkenes are particularly challenging, and only a handful of
effective catalysts have been reported for these substrates,
(b and c in Fig. 1) reported by Hultzsch,8,9 and an yttrium
complex of a rigid xanthene-backbone NON-donor pincer
ligand (d in Fig. 1) reported recently by the Emslie group.10
For trivalent rare earth catalyzed alkene hydroamination,
catalytic activity typically increases in parallel with metal ionic
radius. For example, in cyclization reactions with H2C]
CHCH2CMe2CH2NH2, the activity of [Cp*2Ln{CH(SiMe3)2}],
[{Me2Si(C5Me4)2}Ln{CH(SiMe3)2}] (b in Fig. 1) and [(L)Ln
aDepartment of Chemistry and Chemical Biology, McMaster University, 1280 Main
St. W., Hamilton, Ontario L8S 4M1, Canada. E-mail: emslied@mcmaster.ca; Fax:
+1-905-522-2509; Tel: +1-905-525-9140 extn 23307
bMcMaster Analytical X-Ray (MAX) Diffraction Facility, Department of Chemistry and
Chemical Biology, McMaster University, 1280 Main St. W., Hamilton, Ontario L8S
4M1, Canada
† Electronic supplementary information (ESI) available: NMR spectra for new
compounds, and NMR and mass spectra for hydroamination reaction products.
CCDC 1544724 and 1544725. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c7ra04432a
Fig. 1 Highly active rare earth catalysts for intermolecular hydro-
amination of unactivated alkenes (Ln ¼ Nd; Ln0 ¼ Y).
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{C6H4(CH2NMe2)-o}] (c in Fig. 1; R ¼ SiPh3, R0 ¼ Me) increased
Crystals of 1$(C6H6)0.5 (lattice solvent is residual benzene
in the order (a) Lu < Sm < La,11,12 (b) Lu < Sm < Nd,13 and (c) Sc < from the synthesis) were grown by cooling a concentrated
Lu < Y,9 respectively. Similarly, with E-PhHC]CHCH2CPh2- O(SiMe3)2 solution to ꢁ30 ꢀC (Fig. 2). In the solid state, the XN2
CH2NH2 as the substrate, the activity of [{(Ind)(CH2)2N(o-C6H10
)
backbone is slightly bent with a 26ꢀ angle away from planarity,
NMe2}Ln{N(SiMe3)2}] (Ind ¼ 1-indenyl) increased in the order based on the orientation of the two xanthene aryl rings. This
Sc < Lu < Y < Sm,14 and for H2C]CHCH2CPh2CH2NH2 cycliza- orientation is mirrored in [(XN2)Y(CH2SiMe3)(THF)], which
tion at 60 ꢀC, [{OC6H3(o-tBu)(o-CH]NAr)}2Ln(CH2SiMe2Ph)] (Ar displayed a backbone angle of 25ꢀ and a very similar geometry at
i
¼ C6H3 Pr2-2,6) was inactive for Sc, but active for Y.15,16 By the rare earth metal.10 Lutetium is 5-coordinate with the three
contrast, for alkyne hydroamination, this trend is reversed, and anionic donors and coordinated THF arranged in an approxi-
higher activity is commonly observed for smaller rare earth mate tetrahedron around the metal center. The largest angle in
ions. As an example, the activity of [Cp*2Ln{CH(SiMe3)2}] for this approximate tetrahedron is the N(1)–Lu–N(2) angle of 130ꢀ,
HC^C(CH2)3NH2 cyclization increased in the order La < Nd < and the smallest is the O(2)–Lu–C(54) angle of 97ꢀ, while the
Sm < Lu.17 However, these general trends are not always fol- other angles are between 102ꢀ and 110ꢀ. The oxygen donor of
lowed; for H2C]CHCH2CMe2CH2NH2 cyclization, the activity the xanthene backbone is coordinated on the N(1)/N(2)/C(54)
of [Ln{N(SiMe3)2}3] graed onto partially hydroxylated periodic face of the tetrahedron closest to the nitrogen donors. Lute-
mesoporous silica increased in the order Nd < La < Y.18 Addi- tium lies 0.74 A out of the plane of the XN2 ligand donor atoms,
˚
tionally, for intermolecular hydroamination of Ph(CH2)2CH] leading to a 53ꢀ angle between the NON and the NLuN planes.
CH2 with benzylamine, yttrium and lutetium [(L)Ln{C6H4 The neutral oxygen donor on the xanthene backbone is located
t
(CH2NMe2)-o}] (c in Fig. 1; R ¼ R0 ¼ Bu) catalysts9 showed 0.5 A out of the N(1)/C(4)/C(5)/N(2) plane in order to coordinate
˚
comparable activity, while the lanthanum analogue was nearly to lutetium, with N–Lu–O(1) angles of 69 and 70ꢀ. Additionally,
inactive.19–29
the nitrogen donors on the XN2 ligand are bent towards lute-
As noted above, we previously described the synthesis tium, illustrated by the C(1)/C(8), C(4)/C(5) and N(1)/N(2)
˚
˚
˚
and hydroamination activity of [(XN2)Y(CH2SiMe3)(THF)] distances of 5.02 A, 4.57 A and 4.04 A respectively, which are
˚
(XN2
¼
4,5-bis(2,4,6-triisopropylanilido)-2,7-di-tert-butyl-9,9- comparable with those in [(XN2)Y(CH2SiMe3)(THF)] (4.98 A, 4.56
10
dimethylxanthene; d in Fig. 1),10 and in this work we report A and 4.06 A).
˚
˚
˚
˚
the synthesis of lutetium and lanthanum XN2 complexes. These
The Lu–N distances of 2.221(2) A and 2.228(2) A are slightly
10
˚
complexes were targeted in order to probe the effectiveness of shorter than those in [(XN2)Y(CH2SiMe3)(THF)] (2.252(3) A),
the rigid XN2 pincer ligand to support robust alkyl derivatives of consistent with the smaller ionic radius of LuIII compared to YIII
smaller and larger rare earth elements, and to assess the activity (0.861 A vs. 0.900 A).30 Additionally, the Lu–N distances in 1 fall
˚
˚
of the complexes for alkene and alkyne hydroamination.
within the range reported for related compounds such as [{(2-
i
ArN]CMe)(6-ArNCMe2)C5H3N}Lu(CH2SiMe3)2] (Ar ¼ C6H3 Pr2-
2,6; 2.188(4) A) and [{1,8-(PziPr)2Cz}Lu(CH2SiMe3)2] (PziPr ¼ 1-
31
˚
(3-isopropyl)pyrazolyl; Cz ¼ 3,6-dimethylcarbazole; 2.231(3)
2 Results and discussion
32
˚
˚
A). The Lu–C(54) distance of 2.326(2) A is shorter compared to
˚
The alkane elimination reaction between H2XN2 and [Lu(CH2-
that in [(XN2)Y(CH2SiMe3)(THF)] (2.364(3) A), also in keeping
SiMe3)3(THF)2]
yielded
[(XN2)Lu(CH2SiMe3)
with the relative sizes of yttrium and lutetium (vide supra).
Additionally, this distance falls within the range of Lu–C
distances reported in the literature. For example, the Lu–C
(THF)]$(O(SiMe3)2)1.5 (1$(O(SiMe3)2)1.5) in 59% yield, aer
recrystallization from O(SiMe3)2 (Scheme 1). By contrast, the
analogous reaction between H2XN2 and [Sc(CH2SiMe3)3(THF)2]
in benzene at 24 ꢀC was unsuccessful, resulting only in
unreacted proligand and [Sc(CH2SiMe3)3(THF)2] thermal
distances in the aforementioned alkyl complexes range from
31,32
˚
˚
2.329(6) A to 2.374(3) A,
and Lu–C in [{(2-NAr)(6-Xyl)C5H3-
i
N}2Lu(CH2SiMe3)(THF)] (Ar ¼ C6H3 Pr2-2,6; Xyl ¼ o-xylyl) is
1
33
˚
decomposition products. The H NMR spectrum of 1 is largely
2.323(14) A.
analogous to that of [XN2Y(CH2SiMe3)(THF)], which we previ-
ously reported,10 and is indicative of Cs symmetry in solution,
with two Ar–H, ortho-CHMe2 and CMe2 peaks. Compound 1 was
found to be fairlꢀy thermally stable, being only 25% decomposed
aer 24 h at 90 C.
[(XN2)Lu(CH2SiMe3)(THF)]$(O(SiMe3)2)1.5 (1$(O(SiMe3)2)1.5
)
ꢀ
was tested as an ethylene polymerization catalyst at 24 C and
80 ꢀC (toluene, 1 atm ethylene, 1 h) but exhibited negligible
activity. Compound 1 was also investigated as a catalyst for both
intra- and inter-molecular hydroamination and the results are
summarized in Tables 1 and 2. Compound 1 catalyzed intra-
molecular hydroamination of a range of substrates in benzene
at 24 ꢀC, proceeding to >99% completion in all cases. The time
required to reach >99% completion was slightly increased
compared to reactions catalyzed by the yttrium complex, [(XN2)
Y(CH2SiMe3)(THF)], which is consistent with the majority of
previous reports (vide supra), in which hydroamination activity
increases with increasing rare earth metal size.30 This is
particularly evident in entries 2 and 4 in Table 1, as [(XN2)
Y(CH2SiMe3)(THF)] achieved >99% conversion aer 1.5 h and
Scheme 1 Synthesis of lutetium complex 1 (R ¼ CH2SiMe3).
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Fig. 2 Two views of the X-ray crystal structure for 1$(C6H6)0.5. Ellipsoids are set to 50%. Hydrogen atoms and lattice solvent are omitted. The tert-
butyl groups are rotationally disordered over two positions, and only one is shown for clarity. In view B the 2,4,6-triisopropylphenyl groups are
depicted in wire-frame format. Selected bond lengths [A˚] and angles [ꢀ]: Lu–N(1) 2.221(2), Lu–N(2) 2.228(2), Lu–C(54) 2.326(2), Lu–O(1) 2.299(1),
Lu–O(2) 2.264(1), N(1)–Lu–N(2) 130.41(6), N(1)–Lu–C(54) 106.09(7), N(2)–Lu–C(54) 110.41(7), O(1)–Lu–C(54) 104.38(6), O(2)–Lu–C(54) 97.56(7),
O(2)–Lu–N(2) 105.39(6), O(2)–Lu–N(1) 101.76(6).
34 h,10 whereas 1 required 2.75 h and 48 h respectively. Never- former amines. The same trend was previously observed for
theless, the ability of 1 and the yttrium analogue to catalyze [(XN2)Y(CH2SiMe3)(THF)],10 and the ability of 1 to catalyze
these more challenging intramolecular hydroamination reac- intermolecular hydroamination of 1-octene (an unactivated
tions at room temperature stands these catalysts apart from alkene) places it in a select group of catalysts with this capability
most others.10
(vide supra). The intermolecular hydroamination activity of 1
Compound 1 also catalyzed intermolecular hydroamination closely mirrors that of the yttrium analogue, although for
with 4-tert-butylaniline, 4-tert-butylbenzylamine and octylamine entries 2 and 5 in Table 2, compound 1 afforded lower and
in combination with 1-octene and diphenylacetylene, and in all higher activities, respectively (Nt ¼ 0.35 vs. 0.40 and 0.42 vs.
reactions with 1-octene, the Markovnikov product was formed 0.33). The intermolecular reactions with the largest conversions
selectively. These reactions were performed in toluene at 110 ꢀC aer 24 h at 110 ꢀC (10 mol% catalyst) were those utilizing
and the degree of conversion was determined by GC-MS (Table diphenylacetylene (entries 4–6), as the amounts of unreacted 4-
2). Over a 24 h time period, the reaction of 1-octene with tert-butylaniline, 4-tert-butylbenzylamine and octylamine were
octylamine (entry 3) resulted in a turnover frequency (Nt) of 0.41 below the detection limit of the GC instrument.
hꢁ1, which is greater than that obtained for the reaction with 4-
In order to further explore the effectiveness of the XN2 ligand
tert-butylbenzylamine (entry 2, 0.35 hꢁ1), which in turn is for rare earth coordination, and the impact of metal ionic radius
signicantly greater than that obtained for the reaction with 4- on hydroamination activity, the synthesis of lanthanum XN2
tert-butylaniline (entry 1, 0.04 hꢁ1). These results are consistent complexes was undertaken. As the lanthanum trialkyl
with the increased donor ability and reduced steric bulk of the compound, [La(CH2SiMe3)3(THF)x] is not readily accessible,34
Table 1 Intramolecular hydroamination reactivity with 1 in C6D6
Entry
Reagent
Product
Mol%
1
Time
Temp. (ꢀC)
24
Product formationa
>99%
Ntb (hꢁ1
$600
)
1
<10 min
2
3
1
2.75 h
24
24
24
>99%
>99%
>99%
ꢂ36
$30
ꢂ0.2
10
10
<20 min
4
48 h
a
1
b
Conversion of reactant to product determined by H NMR spectroscopy. Turnover frequency.
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Table 2 Intermolecular hydroamination reactivity with 1 (10 mol%) in toluene
Temp.
Time (ꢀC)
Product
% Markovnikov
productc
Entry Amine
1
Alkene or alkynea Product
formationb,c
Ntd (hꢁ1
)
24 h 110
24 h 110
11%
83%
97
0.04
2
>99
0.35
3
4
24 h 110
24 h 110
98%
>99
N/A
0.41
0.42
>99%e
5
24 h 110
>99%e
>99%e
N/A
N/A
0.42
0.42
6
24 h 110
a
Alkene/alkyne present in 20 fold excess relative to the amine. b Conversion determined by product: unreacted amine ratio. c Determined by GC-
MS. d Turnover frequency. e In entry 4 the product is formed as a single isomer, whereas in entries 5 and 6 the products are formed as 1 : 0.35 and
1 : 0.24 mixtures of the E and Z isomers (based on literature assignments for similar compounds),35,36 respectively.
salt metathesis was employed for ligand attachment in the place
of alkane elimination. Stirring H2XN2 with excess KH in DME at
ꢀ
24 C produced the dipotassium salt of the 4,5-bis(2,4,6-triiso-
propylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene
ligand,
[K2(XN2)(DME)x] (x ¼ 2–2.5), as a beige solid in 80% isolated
yield. [K2(XN2)(DME)2] was reacted with [LaCl3(THF)3] in THF at
24 ꢀC, and aer recrystallization from O(SiMe3)2, [{(XN2)
LaCl(THF)}x]$(O(SiMe3)2)0.25x (2$(O(SiMe3)2)0.25x; x ¼ 1 or 2) was
obtained in 51% yield as an off-white solid (Scheme 2).
Attempts to synthesize a lanthanum monoalkyl complex
were undertaken via reactions of trimethylsilylmethyl lithium
and methyl lithium with [{(XN2)LaCl(THF)}x]$(O(SiMe3)2)0.25x
(2$(O(SiMe3)2)0.25x; x ¼ 1 or 2). NMR-scale reactions were per-
formed in d8-THF at 24 ꢀC and both resulted in the formation of
a dialkyl-‘ate’ complex, based on the integrations of the
respective alkyl peaks, regardless of whether one equivalent (per
La) or an excess of the alkali metal-alkyl reagent was added. The
reaction utilizing trimethylsilylmethyl lithium was pursued
Scheme 2 Synthesis of [K2(XN2)(DME)x] and complexes 2 and 3.
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˚
˚
further, as it provides a direct comparison with the lutetium and
The La–N distances of 2.462(5) A and 2.445(5) A are
yttrium trimethylsilylmethyl complexes of the XN2 ligand. substantially lengthened relative to those in [(XN2)Y(CH2-
10
˚
A)
[{(XN2)LaCl(THF)}x]$(O(SiMe3)2)0.25x (2$(O(SiMe3)2)0.25x; x ¼ 1 or SiMe3)(THF)]
(2.252(3)
and
[(XN2)Lu(CH2SiMe3)
˚
˚
2) reacted with 2 equivalents of trimethylsilylmethyl lithium in (THF)]$(O(SiMe3)2)1.5 (1) (2.221(2) A and 2.228(2) A), consistent
THF at 24 ꢀC, and aer removal of the salts by centrifugation in with the large ionic radius of lanthanum compared to yttrium
ꢀ
toluene and layering with hexanes at ꢁ30 C, [Li(THF)x][(XN2) and lutetium, combined with increased steric hindrance and an
La(CH2SiMe3)2]$toluene$LiCl (3$toluene$LiCl; x ¼ 3) was iso- overall negative charge in 3, resulting in a less electrophilic
lated as a pale yellow solid in 55% yield (Scheme 2). Compound metal centre. For analogous reasons, the La–C(54) and La–C(58)
˚
˚
3 is sparingly soluble in benzene and other non-polar solvents distances of 2.573(7) A and 2.613(7) A are also greatly elongated
10
˚
such as hexanes and pentane, so all characterization was compared to those in [(XN2)Y(CH2SiMe3)(THF)] (2.364(3) A)
carried out in d8-THF. The 1H NMR spectrum of 3 revealed the and 1 (2.326(2) A). However, both the La–N and La–C distances
˚
expected signals for the XN2 ligand backbone and only one set in 3 are signicantly shorter than those previously reported for
of signals for the two alkyl groups was observed between 25 and [{(R)-Binap(NCyp)2}La{(m-Me)2Li(THF)}{(m-Me)Li(THF)}2] (Binap
ꢁ80 ꢀC (a singlet with an integration of 18 for LaCH2SiMe3 and ¼ 2,20-disubstituted-1,10-binaphthyl; Cyp ¼ cyclopentyl; La–N ¼
29,37
˚
˚
a singlet with an integration of 4 for LaCH2SiMe3). Crystals of 2.626(7)–2.677(8) A; La–C ¼ 2.704(8)–2.832(11) A).
[Li(THF)4][(XN2)La(CH2SiMe3)2]$THF
were
grown
from
Reaction of [Li(THF)3][(XN2)La(CH2SiMe3)2]$toluene$LiCl
a concentrated THF solution of 3$toluene$LiCl (x ¼ 3), layered (3$toluene$LiCl)
with
[{(XN2)LaCl(THF)}x]$(O(SiMe3)2)0.25x
ꢀ
with pentane and cooled to ꢁ30 C (Fig. 3).
(2$(O(SiMe3)2)0.25x; x ¼ 1 or 2) did not provide access to the
In the solid state, lanthanum is 5-coordinate with the two neutral XN2 lanthanum alkyl; at 24 ꢀC no reaction was observed,
amido donors and two alkyl groups arranged in a distorted and heating to 70 ꢀC resulted only in thermal decomposition of
tetrahedron around the metal center. The largest angle in this 3$toluene$LiCl.
approximate tetrahedron is the N(1)–La–N(2) angle of 118ꢀ, and
Rare earth alkyl-‘ate’ complexes have been reported to cata-
the smallest is the C(54ꢀ)–La–C(58) angle of 100ꢀ, while the other lyze intramolecular hydroamination as well as intermolecular
ꢀ
˚
angles are between 101 and 106 . Lanthanum lies 0.96 A out of hydroamination. A few examples include, [{(R)-Binap(NCyp)2}La
the plane of the XN2 ligand donor atoms, leading to a 50ꢀ angle {(m-Me)2Li(THF)}{(m-Me)Li(THF)}2] which catalyzed asymmetric
between the NON and NLaN planes. The XN2 backbone is bent intramolecular
hydroamination
of
amino-1,3-dienes,29
2,20-
with a 35ꢀ angle away from planarity, based on the orientation [Li(THF)4][{(R)-Binap(NCyp)2}Y(CH2SiMe3)2] (Binap
¼
of the two aryl rings of the xanthene backbone. The neutral disubstituted-1,10-binaphthyl; Cyp ¼ cyclopentyl) which cata-
˚
oxygen donor of the xanthene backbone is located 0.64 A out of lyzed intramolecular hydroamination of secondary amino-
the N(1)/C(4)/C(5)/N(2) plane in order to coordinate to alkenes,38 [Li(THF)4][{ArNC(Me)]C(Me)NAr}Y(CH2SiMe3)2] (Ar
i
lanthanum, resulting in N–La–O(1) angles of 63–64ꢀ. In addi- ¼ C6H3 Pr2-2,6) which catalyzed the intermolecular hydro-
tion, it is of note that the nitrogen donors on the ligand are not amination reaction of styrene and pyrrolidine,39 and [{(R)-
bent towards lanthanum to the extent that they are in 1, illus- Binap(NCyp)2}Y{(m-Me)2Li(THF)2}{(m-Me)Li(THF)}] (Binap
¼
trated by the C(1)/C(8), C(4)/C(5) and N(1)/N(2) distances in 2,20-disubstituted-1,10-binaphthyl; Cyp ¼ cyclopentyl) which
˚
˚
˚
˚
3 of 4.90 A, 4.57 A and 4.21 A respectively, compared to 5.02 A, catalyzed 1-amino-2,2-diphenyl-4-pentene cyclization, requiring
˚
˚
4.57 A and 4.04 A in 1.
1.9 h at 25 ꢀC with 6 mol% catalyst loading to reach 100%
Fig. 3 Two views of the X-ray crystal structure for 3$THF (x ¼ 4). Ellipsoids are set to 50%. Hydrogen atoms and lattice solvent are omitted. The
tert-butyl groups are rotationally disordered over two positions, and only one is shown for clarity. In view A the [Li(THF)4]+ cation is omitted for
clarity. In view B the 2,4,6-triisopropylphenyl groups and the THF molecules are depicted in wire-frame format. Selected bond lengths [A˚] and
angles [ꢀ]: La–N(1) 2.462(5), La–N(2) 2.445(5), La–C(54) 2.573(7), La–C(58) 2.613(7), La–O(1) 2.643(4), N(1)–La–N(2) 118.22(17), N(1)–La–C(54)
108.9(2), N(1)–La–C(58) 100.9(2), N(2)–La–C(54) 106.3(2), N(2)–La–C(58) 120.7(2), O(1)–La–C(54) 97.63(18), O(1)–La–C(58) 159.7(2), C(54)–La–
C(58) 100.0(2), N(1)–La–O(1) 63.59(15), N(2)–La–O(1) 62.72(15).
27942 | RSC Adv., 2017, 7, 27938–27945
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conversion.37 Complex 3$toluene$LiCl, (x ¼ 3) was tested as Et2O), nBuLi (1.6 M in hexanes), Br2, NaH, NaOtBu, Pd(OAc)2,
a catalyst for intramolecular hydroamination with 1-amino-2,2- DPEPhos, [bis{2-(diphenylphosphino)phenyl}ether], diphenylace-
diphenyl-4-pentene in d8-THF at 24 ꢀC. However, the time tylene and MgSO4 were purchased from Sigma-Aldrich. LuCl3 and
required to reach >99% completion (45 h) was signicantly LaCl3 were purchased from Strem Chemicals. Solid LiCH2SiMe3
increased compared to that required for [(XN2)Y(CH2SiMe3) and MeLi were obtained by removal of the solvent in vacuo, and
(THF)] and [(XN2)Lu(CH2SiMe3)(THF)] (1$(O(SiMe3)2)1.5) (<10 solid KH was obtained by ltration and washing with hexanes.
min) under analogous conditions in benzene (or in THF for the [LuCl3(THF)3] and [LaCl3(THF)3] were obtained by reuxing the
yttrium complex). Consequently, the catalytic activity of 3 was anhydrous lutetium/lanthanum trihalide in THF for 24 h followed
not further investigated.
by removal of the solvent in vacuo. 4-tert-Butyl-aniline, 4-tert-
butylbenzylamine, n-octylamine and 1-octene were purchased
from Sigma-Aldrich, dried over molecular sieves and distilled prior
to use. Argon (99.999% purity) and ethylene (99.999% purity) were
3 Summary and conclusion
The rigid NON-donor pincer ligand, XN2, has been employed for purchased from Praxair.
the synthesis of a lutetium monoalkyl complex (1), a lanthanum
Combustion elemental analyses were performed both at
chloride complex (2) and a lanthanum dialkyl-‘ate’ complex (3). McMaster University on a Thermo EA1112 CHNS/O analyzer and
Complex 3 was tested as a catalyst for intramolecular hydro- by Midwest Microlab, LLC, Indianapolis, IN, USA. NMR spectros-
amination, but showed low activity. By contrast, the neutral copy (1H, 13C{1H}, DEPT-Q, COSY, HSQC, HMBC) was performed
lutetium alkyl complex, 1, is highly active for both intra- and on Bruker DRX-500 and AV-600 spectrometers. All 1H NMR and 13
C
inter-molecular hydroamination with a variety of substrates. For NMR spectra were referenced relative to SiMe4 through a reso-
intramolecular alkene hydroamination, the time required to nance of the employed deuterated solvent or proteo impurity of the
reach >99% completion was slightly increased compared to the solvent; C6D6 (7.16 ppm), d8-Tol (2.08, 6.97, 7.01, 7.09 ppm), d8-
1
previously reported yttrium analogue. By contrast, the inter- THF (1.72, 3.58 ppm) for H NMR; and C6D6 (128.0 ppm), d8-Tol
molecular hydroamination reaction between 4-tert-butylben- (20.43, 125.13, 127.96, 128.87, 137.48 ppm), d8-THF (25.31, 67.21
zylamine and diphenylacetylene afforded a higher turnover ppm) for 13C NMR. Herein, numbered proton and carbon atoms
number than the yttrium analogue.
refer to the positions of the xanthene backbone, as shown in
Scheme 1. Inequivalent ortho isopropyl protons are labeled A and
B, while inequivalent aryl ring protons and inequivalent methyl
4 Experimental
General details
0
protons are labeled and 00, so that the corresponding carbon
resonances can be identied.
An argon-lled MBraun UNIlab glove box equipped with
X-ray crystallographic analyses were performed on suitable
a ꢁ30 ꢀC freezer was employed for the manipulation and crystals coated in Paratone oil and mounted on a SMART APEX
storage of all air-sensitive compounds, and reactions were per- II diffractometer with a 3 kW sealed tube Mo generator in the
formed on a double manifold high vacuum line using standard McMaster Analytical X-ray (MAX) Diffraction Facility. In all
techniques.40 Residual oxygen and moisture was removed from cases, non-hydrogen atoms were rened anisotropically and
the argon stream by passage through an Oxisorb-W scrubber hydrogen atoms were generated in ideal positions and then
from Matheson Gas Products. A Fisher Scientic Ultrasonic FS- updated with each cycle of renement. GC-MS analyses were
30 bath was used to sonicate reaction mixtures where indicated. performed using an Agilent 6890N gas chromatograph (Santa
A VWR Clinical 200 Large Capacity Centrifuge (with 28ꢀ xed- Clara, CA, USA), equipped with a DB-17ht column (30 m ꢃ 0.25
angle rotors that hold 12 ꢃ 15 mL or 6 ꢃ 50 mL tubes) in mm i.d. ꢃ 0.15 mm lm, J & W Scientic) and a retention gap
combination with 15 mL Kimble Chase glass centrifuge tubes (deactivated fused silica, 5 m ꢃ 0.53 mm i.d.), and coupled to an
was used when required (inside the glovebox). Diethyl ether, Agilent 5973 MSD single-quadrupole mass spectrometer. One
THF, toluene, benzene and hexanes were initially dried and microliter of sample was injected using Agilent 7683 autosam-
distilled at atmospheric pressure from Na/Ph2CO. Hexame- pler in splitless mode. The injector temperature was 230 ꢀC and
thyldisiloxane (O(SiMe3)2) was dried and distilled at atmo- carrier gas (helium) ow was 0.7 mL minꢁ1. The transfer line
spheric pressure from Na. Unless otherwise noted, all proteo was 280 ꢀC and the MS source temperature was 230 ꢀC. The
solvents were stored over an appropriate drying agent (pentane, column tꢁem1 perature started at 50 ꢀC and was raised to 300 ꢀC at
hexanes, hexamethyldisiloxane (O(TMS)2/O(SiMe3)2)
¼
Na/ 8 ꢀC min . It was then held at 300 ꢀC for 15 min to give a total
Ph2CO/tetra-glyme; Et2O ¼ Na/Ph2CO) and introduced to reac- run time of 46.25 min. Full scan mass spectra between m/z 50
tions via vacuum transfer with condensation at ꢁ78 ꢀC. The and 800 were acquired aer a ve minute solvent delay.
deuterated solvents (ACP Chemicals) C6D6, THF-d8 and toluene-
d8 were dried over Na/Ph2CO.
[(XN2)Lu(CH2SiMe3)(THF)]$(O(SiMe3)2)1.5 (1$(O(SiMe3)2)1.5).
H2XN2 (0.10 g, 0.132 mmol) was dissolved in 2 mL of benzene
The H2XN2,10 [Lu(CH2SiMe3)3(THF)2],41 and the commercially and added to [Lu(CH2SiMe3)3(THF)2] (0.084 g, 0.145 mmol)
unavailable intramolecular hydroamination reagents10,42 were which was then stirred at 24 ꢀC in the glove box for 4 weeks. The
prepared according to literature procedures. 1-Amino-5-hexene solvent was removed in vacuo and the yellow solid was recrys-
was purchased from GFS Chemicals, dried over CaH2 and tallized from O(SiMe3)2 at ꢁ30 ꢀC yielding 1$(O(SiMe3)2)1.5 as
distilled prior to use. 1,3,5-Triisopropylbenzene, xanthone, KH (30 a light yellow powder (0.096 g, 55%). 1H NMR (C6D6, 600 MHz):
wt% in mineral oil), LiCH2SiMe3 (1.0 M in pentane), MeLi (1.6 M in d 7.25 (s, 2H, Ar–H0), 7.14 (s, 2H, Ar–H00), 6.77 (br s, 2H, Xanth-
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CH1), 6.23 (br s, 2H, Xanth-CH3), 4.24 (sept, 2H, 3JH,H 6.73 Hz, A- 1H NMR (d8-THF, 600 MHz): d 7.08 (s, 4H, Ar–H), 6.57 (br. s, 2H,
3
ortho-CHMe2), 3.38 (sept, 2H, JH,H 6.78 Hz, B-ortho-CHMe2), Xanth-CH0), 5.81 (br. s, 2H, Xanth-CH00), 3.62 (s, 4H, 1 equiv.
3
2.83 (sept, 2H, 3JH,H 6.74 Hz, para-CHMe2), 2.73 (s, 4H, 1 equiv. THF-C2,5H2), 3.15 (sept, 4H, JH,H 6.74 Hz, ortho-CHMe2), 2.88
THF-C2,5H2), 1.88 (s, 3H, CMe02), 1.72 (s, 3H, CMe020), 1.49 (d, 6H, (sept, 2H, 3JH,H 6.86 Hz, para-CHMe2), 1.77 (s, 4H, 1 equiv. THF-
3
3JH,H 6.73 Hz, A-orthoCHMe02-), 1.47 (d, 6H, JH,H 6.73 Hz, A- C3,4H2), 1.72 (s, 6H, CMe2), 1.23 (d, 12H, 3JH,H 6.75 Hz, A-ortho-
ortho-CHMe020), 1.26 (s, 18H, CMe3), 1.24 (d, 6H, 3JH,H 6.78 Hz, B- CHMe2), 1.21 (d, 12H, 3JH,H 6.85 Hz, para-CHMe2), 1.15 (s, 18H,
3
ortho-CHMe02), 1.21 (m, 12H, para-CHMe2), 1.01 (d, 6H, JH,H CMe3), 1.01 (d, 12H, 3JH,H 6.76 Hz, B-ortho-CHMe2). 13C NMR (d8-
6.78 Hz, B-ortho-CHMe020), 0.83 (s, 4H, 1 equiv. THF-C3,4H2), 0.35 THF, 126 MHz): d 146.81 (Xanth-C2), 146.17 (ortho-CCHMe2),
(s, 9H, LuCH2SiMe3), ꢁ0.40 (s, 2H, LuCH2SiMe3). 13C NMR 145.26 (para-CCHMe2), 140.45 (Ar-Cipso), 128.34 (Xanth-C10),
(C6D6, 126 MHz): d 147.83 (Xanth-C4), 147.72 (Xanth-C2) 147.38 122.57 (Ar-CH), 108.43 (Xanth-C00H), 107.41 (Xanth-C0H), 67.22
(A-ortho-CCHMe2), 146.17 (B-ortho-CCHMe2), 145.33 (para- (THF-C2,5H2), 35.07 (CMe3), 34.96 (para-CHMe2), 34.85 (Xanth-
CCHMe2), 141.82 (Ar-Cipso), 141.30 (Xanth-C11), 130.56 (Xanth- C9Me2), 33.05 (CMe2), 31.81 (CMe3), 29.19 (ortho-CHMe2), 25.84
C10), 122.20 (Ar-CH0), 121.85 (Ar-CH00), 109.35 (Xanth-C3H), (B-ortho-CHMe2), 25.25 (THF-C3,4H2), 24.52 (para-CHMe2), 24.16
106.47 (Xanth-C1H), 70.85 (THF-C2,5H2), 37.99 (LuCH2SiMe3), (A-ortho-CHMe2).
Anal.
calcd
for
[(XN2)
35.57 (Xanth-C9Me2), 35.25 (CMe20 ), 35.05 (CMe3), 34.50 (para- LaCl(THF)]$(O(SiMe3)2)0.25, C58.5H86.5N2O2.25Si0.5LaCl: C, 67.41;
CHMe2), 31.91 (CMe3), 28.39 (B-ortho-CHMe2), 27.67 (A-ortho- H, 8.36; N, 2.68%. Found: C, 66.89; H, 8.83; N, 2.60%.
CHMe2), 27.12 (A-ortho-CHMe020), 25.85 (B-ortho-CHMe020), 25.48
[Li(THF)x][(XN2)La(CH2SiMe3)2] (3$toluene$LiCl). [{(XN2)
(B-ortho-CHMe02), 25.04 (CMe200), 24.85 (THF-C3,4H2), 24.62 (A- LaCl(THF)}x]$(O(SiMe3)2)0.25x (x ¼ 1 or 2) (0.086 g, 0.082 mmol
ortho-CHMe02), 24.51 (para-CHMe2), 4.13 (LuCH2SiMe3). Anal. of La) was dissolved in 4 mL of THF and added to LiCH2SiMe2
calcd for C70H120N2O3.5LuSi4: C, 63.07; H, 9.07; N, 2.10%. (0.015 g, 0.165 mmol), which was stirred at 24 ꢀC for 5 days. The
Found: C, 63.03; H, 8.61; N, 2.30%.
solvent was removed in vacuo and the yellow solid was dissolved
[K2(XN2)(DME)x] (x ¼ 2–2.5). H2XN2 (0.50 g, 0.66 mmol) and in toluene (1 mL), centrifuged and the mother liquor was
ꢀ
KH (0.106 g, 2.64 mmol) were stirred in DME (40 mL) at 24 C layered with hexanes and stored at ꢁ30 ꢀC. Very small pale
for 72 h, yielding a cloudy off-white solution, which was ltered yellow crystals of [Li(THF)3][(XN2)La(CH2SiMe3)2]$toluene$LiCl
and the solvent was removed in vacuo. The resulting off-white were obtained (0.064 g, 55%). X-ray quality crystals of [Li(THF)4]
solid was dissolved in hexanes (10 mL), centrifuged, and the [(XN2)La(CH2SiMe3)2]$THF were grown from a concentrated
ꢀ
solvent was removed in vacuo yielding [K2(XN2)(DME)x] (x ¼ 2– THF solution of 3, layered with pentane and cooled to ꢁ30 C.
2.5) as a beige solid (0.535 g, 80%). Multiple syntheses of K2XN2 1H NMR (d8-THF, 600 MHz): d 7.00 (s, 4H, Ar–H), 6.28 (d, 2H,
4
revealed that either 2 or 2.5 equivalents of DME were present in 4JH,H 2.16 Hz, Xanth-CH3), 5.62 (d, 2H, JH,H 2.18 Hz, Xanth-
the sample. The NMR characterization and elemental analysis CH1), 3.55 (sept, 4H, 3JH,H 6.84 Hz, ortho-CHMe2), 2.86 (sept, 2H,
is reported for a sample which contained 2.5 equivalents of 3JH,H 6.93 Hz, para-CHMe2), 1.61 (s, 6H, CMe2), 1.25 (d, 12H,
3
DME. 1H NMR (C6D6, 600 MHz): d 7.24 (s, 4H, Ar–H), 6.50, 6.16 3JH,H 6.9 Hz, para-CHMe2), 1.24 (d, 12H, JH,H 6.8 Hz, A-ortho-
4
(d, 2 ꢃ 2H, JH,H 2.12 Hz, Xanth-CH1 and Xanth-CH3), 3.28 (s, CHMe2), 1.10 (s, 18H, CMe3), 1.00 (d, 12H, 3JH,H 6.8 Hz, B-ortho-
3
10H, 2.5 equiv. DME-CH2), 3.10 (sept, 4H, JH,H 6.91 Hz, ortho- CHMe2), ꢁ0.39 (s, 18H, LaCH2SiMe3), ꢁ1.17 (br. s, 4H, LaCH2-
CHMe2), 3.08 (s, 15H, 2.5 equiv. DME-CH3), 2.97 (sept, 2H, 3JH,H SiMe3). 13C NMR (d8-THF, 126 MHz): d 149.03 (Xanth-C4), 146.12
3
6.86 Hz, para-CHMe2), 1.93 (s, 6H, CMe2), 1.40 (d, 12H, JH,H (ortho-CCHMe2), 145.20 (Xanth-C2), 143.09 (para-CCHMe2),
3
6.78 Hz, ortho-CHMe2), 1.37 (s, 18H, CMe3), 1.35 (d, 12H, JH,H 142.71 (Ar-Cipso), 142.38 (Xanth-C11), 129.52 (Xanth-C10), 121.70
6.86 Hz, para-CHMe2), 1.06 (d, 12H, 3JH,H 7.05 Hz, ortho-CHMe2). (Ar-CH), 107.97 (Xanth-C1H), 102.90 (Xanth-C3H), 48.35
13C NMR (C6D6, 126 MHz): d 151.32 (Ar-Cipso), 146.76 (Xanth-C2), (LaCH2SiMe3), 35.30 (CMe3), 35.07 (para-CHMe2), 34.93 (Xanth-
142.26 (ortho-CCHMe2), 139.34 (para-CCHMe2), 137.38 (Xanth- C9Me2), 32.04 (CMe3), 30.56 (CMe2), 28.21 (ortho-CHMe2), 26.39
C11), 130.91 (Xanth-C10), 121.36 (Ar-CH), 107.97, 100.19 (Xanth- (B-ortho-CHMe2), 25.58 (A-ortho-CHMe2), 24.71 (para-CHMe2),
C1H and Xanth-C3H), 72.07 (DME-CH2), 58.63 (DME-CH3), 35.65 4.51 (LaCH2SiMe3). Anal calcd for [Li(THF)3][(XN2)La(CH2-
(Xanth-C9Me2), 34.97 (CMe3), 34.64 (para-CHMe2), 32.28 (CMe3), SiMe3)2]$toluene$LiCl, C80H128N2O4Si2LaLi2Cl: C, 67.36; H,
31.71 (CMe2), 28.02 (ortho-CHMe2), 25.25 (ortho-CHMe2), 24.98 9.04; N, 2.24%. Found: C, 66.98; H, 8.63; N, 2.53%.
(para-CHMe2), 24.38 (ortho-CHMe2). Anal. calcd for
[K2(XN2)(DME)2.5], C63H99N2O6K2: C, 71.47; H, 9.42; N, 2.64%.
Found: C, 71.25; H, 9.39; N, 2.56%.
General procedure for intramolecular hydroamination
In the glove box, the appropriate amounts of 1 or 3 and the
hydroamination substrate were weighed into separate vials,
dissolved in C6D6 or d8-THF and placed in a teon-valved J-
young NMR tube. The reactions were monitored at 24 ꢀC by
1H NMR spectroscopy and the expected products were
conrmed by their agreement to reported literature spectra.42
[{(XN2)LaCl(THF)}x]$(O(SiMe3)2)0.25x (x
(2$(O(SiMe3)2)0.25x). K2(XN2)(DME)2 (0.53 g, 0.523 mmol) and
¼
1
or 2)
[LaCl3(THF)3] (0.244 g, 0.528 mmol) were dissolved in 40 mL of
ꢀ
THF and stirred at 24 C for 4 days. The cloudy beige solution
was centrifuged and the mother liquor solvent was removed in
vacuo. The resulting solid was dissolved in toluene (60 mL),
followed by centrifugation and removal of the mother liquor
solvent in vacuo to yield a beige solid. Recrystallization from
General procedure for intermolecular hydroamination
O(SiMe3)2
at
ꢁ30
ꢀC
yielded
[{(XN2)LaCl(THF)}x]- In the glove box, the appropriate amounts of 1, amine, and the
$(O(SiMe3)2)0.25x (x ¼ 1 or 2) as an off-white solid (0.28 g, 51%). alkene/alkyne were weighed into separate vials, dissolved in d8-
27944 | RSC Adv., 2017, 7, 27938–27945
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´
´
toluene, placed in a Teon-valved J-young NMR tube and then 22 A. Otero, A. Lara-Sanchez, J. A. Castro-Osma, I. Marquez-
placed into a preheated oil bath at 110 ꢀC. Aer heating for the
designated amount of time, NMR spectra were obtained and the
sample was submitted for analysis by GC-MS.
Segovia,
C.
Alonso-Moreno,
L. F. Sanchez-Barba and A. M. Rodrıguezc, New J. Chem.,
J.
´
´
Fernandez-Baeza,
´
2015, 39, 7672.
23 K. Huynh, B. K. Anderson and T. Livinghouse, Tetrahedron
Lett., 2015, 56, 3658.
24 K. Huynh, T. Livinghouse and H. M. Lovick, Synlett, 2014, 25,
1721.
25 A. G. Trambitas, D. Melcher, L. Hartenstein, P. W. Roesky,
C. Daniliuc, P. G. Jones and M. Tamm, Inorg. Chem., 2012,
51, 6753.
Acknowledgements
D. J. H. E. thanks NSERC of Canada for a Discovery Grant and an
NSERC Strategic Grant in collaboration with NOVA Chemicals.
K. S. A. M. thanks the Government of Ontario for an OGS –
Queen Elizabeth II Graduate Scholarship in Science and Tech-
nology (QEII GSST) scholarship. The authors would like to
thank Dr Kirk Green and Dr Fan Fei from the McMaster
Regional Centre of Mass Spectrometry (MRCMS) for acquisition
and analysis of all the mass spectra.
´
´
´
26 A. Otero, A. Lara-Sanchez, C. Najera, J. Fernandez-Baeza,
´
´
I. Marquez-Segovia, J. A. Castro-Osma, J. Martınez,
´
´
L. F. Sanchez-Barba and A. M. Rodrıguez, Organometallics,
2012, 31, 2244.
27 P. Benndorf, J. Jenter, L. Zielke and P. W. Roesky, Chem.
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28 D. V. Vitanova, F. Hampel and K. C. Hultzsch, J. Organomet.
Chem., 2011, 696, 321.
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R. Guillot, J. Hannedouche and A. Trifonov,
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20 T. Spallek and R. Anwander, Dalton Trans., 2016, 45, 16393.
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¨
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